The Effect of TiO2 Nanoparticles on the Surface Chemistry, Structure and Fouling Performance of Polymeric Membranes
By
Amir Razmjou Chaharmahali
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy
School of Chemical Engineering
The University of New South Wales
Sydney, Australia
September 2012
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ABSTRACT
Over the last decades, the sharp increase in population growth and also the industrialization of developing countries have resulted in a deterioration of water resource quality and quantity. This led to a significant increase in the number of membrane installations in water treatment plants worldwide. However, one of the primary challenges in any membrane process is fouling which has different types of organic and colloidal fouling, scaling and biofouling. The issue of fouling can be minimized or removed via operating conditions optimization, feed pretreatment and hydraulic and chemical cleaning. These techniques often increase the total operational cost of the membranes as well as shorten their life time. Recently, the development of new materials or the surface modification of current membrane to make them less prone to fouling has become a point of interest for both researchers and industry.
The inclusion of inorganic nanoparticles such as SiO2, Al2O3, Fe3O4, ZrO2 and TiO2 into polymeric membranes has been identified as a method to improve the separation performance of membranes and enhance thermal and mechanical characteristics. Among them, TiO2 has received the most attention because of its good physical and chemical properties, availability as well as its potential antifouling abilities. TiO2 nanoparticles can be incorporated into polymeric membranes through mixing them into the membranes or depositing them onto the surface of the membranes. The primary method has the disadvantage of difficulty in achieving a reasonable level of particles dispersion and limit in the loading of inorganic particles. The latter approach, on the other hand, suffers from the drawback of instability in coating layer and also it may result in the non-uniform distribution of nanoparticles on the membrane surface. Therefore, it is critical to modify the current methods or develop new techniques for the fabrication of nanocomposite membranes.
This study has two main parts of TiO2 blending and coating. In the blending part, a technique was introduced to minimize the nanoparticles agglomeration via mechanically and chemically modifications of commercial TiO2 nanoparticles for both flat sheet and hollow fibre membranes. In addition, the mechanism by which the TiO2 nanoparticles agglomeration size can affect surface chemistry and morphology as well as fouling performance of membranes was discussed.
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In the second part, a low temperature hydrothermal (LTH) process of TiO2 nanoparticles (synthesized by sol gel technology) coating was implemented to change the wettability of membrane surface through engineering surface morphology and roughness and also altering surface chemistry. Using this method, a thin robust mesoporous coating of TiO2 nanoparticles on the membrane surface was generated. The surface exhibited a dual level hierarchical roughness which shifted the surface wettability towards superhydrophilicity without the need for continuous UV illumination. The effect of such engineered surfaces on the fouling performance of membranes was also discussed. The coated TiO2 nanoparticles by LTH process itself was also used for further functionalization to impart superhydrophobicity for a membrane distillation application.
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ACKNOWLEDGEMENTS
First and foremost, I would like to thank God for blessing me with the strength and faith, and helping me understand myself by looking at his manifestations.
I am also deeply indebted to my supervisor Professor Vicki Chen for steering me through this gruelling journey with her encouragement, advice and support. This PhD project would not have been completed without her virtuous guidance. I sincerely appreciate her support and encouragement to attend national and international conferences as well as writing courses, which have increased my confidence and improved my presentation and writing skills.
It is with immense gratitude that I acknowledge Dr Jaleh Mansouri my cosupervisor for her fundamental and invaluable direction, guidance and assistance. I have learned from her not only how to perform and interpret experiments but also how to think and move the project forward. Her exceptional insights into engineering have immensely helped me to enrich my knowledge.
Special appreciation is given to Professor Rose Amal and Dr May Lim for their brilliant ideas and sharing their knowledge of nanotechnology with me. I’m honoured to have this opportunity to work with Rose and her team.
I would also like to thank Professor Robert Burford for the access to the polymer laboratories and also special thanks to Dr Rohan L. Holmes for his help and guidance with TGA, DSC and DMA testing.
My Special gratitude goes to our lab manager Dr Deyan Guang for his help in organizing the chemicals and materials for this project. I also wish to thank Associate Professor Greg Leslie, Dr Pierre Le-Clech and Dr Hongyu Li for their guidance and advice during my annual progress reviews; Ik Ling Lau for her help in all the administrative work; to honour students Ellen Arifin and Adhikara Resosudarmo for their valuable input in to this work. Special thanks to postdoctoral research associates Guangxi Dong, Alice Antony, Yun Ye and Shane Cox for their friendly assistance and advice in the lab.
I wish to also thank my fellow friends in the membrane family Anusha, Jingwei, Barun, William, Gustavo, Zenah, Tao, Sun Lin, Eileen, Angayar, Nashida, Hao, Brous, Erik, and Nui. Special appreciation is given to my Persian fellows Mojgan, Shima, Ebrahim and Ali.
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I would like to acknowledge financial support from the ministry of science, research and technology of Iran in the form of a scholarship and also Australian research council for their support with this project.
I would like to extend my sincerest thanks and appreciation to my parents, my brothers and sisters for giving me inspiration and encouragement.
Lastly, and most importantly, my overwhelming thanks goes to my wife, Fatemeh, for her endless love without expecting any reward and providing her support by standing shoulder to shoulder with me to get through all the difficulties experienced as an international student. This work is dedicated to her.
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PUBLICATIONS
Journal Papers
1. A. Razmjou, J. Mansouri, V. Chen, “The effects of mechanical and chemical
modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes” Journal of Membrane Science, Volume 378, Issues 1-2, 15 August 2011, Pages 73-84
2. A. Razmjou, J. Mansouri, V. Chen, “Titania nanocomposite polyethersulfone ultrafiltration membranes fabricated using a low temperature hydrothermal coating process” Journal of Membrane Science, Volume 380, Issues 1-2, 15 September 2011, Pages 98-113
3. A. Razmjou, A. Resosudarmo, R. L. Holmes , H. Li, J. Mansouri, V. Chen, “The
effect of Modified TiO2 nanoparticles on the PES ultrafiltration hollow fiber membranes”, Desalination, Volume 287, February 2012, Pages 271-280
4. A. Razmjou, E. Arifin, G. Dong, J. Mansouri and V. Chen “Superhydrophobic
modification of TiO2 nanocomposite PVDF membranes for the application in membrane distillation” , Journal of Membrane Science, 2012, Volume 415–416, 1 October 2012, Pages 850-863 Conference Proceedings/Presentations
1. A. Razmjou, J. Mansouri, V. Chen “Performance of PES-TiO2 hybrid membranes; Optimization of PES Membrane Fabrication Using Experimental Design and Statistical Analysis.” Oral presentation at MSA Student Symposium 2010 at The University of Wollongong, Australia, 18-20 February
2. A. Razmjou, J. Mansouri, V. Chen “Low temperature synthesis of TiO2 coating layer on UF polyether sulphone membrane” Oral presentation at North American Membrane Society (NAMS), Washington, DC, USA, July 17-22, 2010
3. A. Razmjou, J. Mansouri, V. Chen “The effect of mechanical and chemical
modifications of TiO2 nanoparticles on PES UF membranes” Oral presentation at North American Membrane Society (NAMS), Washington, DC, USA, July 17-22, 2010
4. M. Lim, M. Ng, A. Razmjou, J, Mansouri, V. Chen, R. Amal “Low Temperature Synthesis of Titanium Dioxide Coatings on PVDF Membrane” Oral presentation Chemeca 2010 ,26–29 September 2010 Hilton Adelaide
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5. A. Razmjou, J. Mansouri, V. Chen “The Effect of Mechanical and Chemical
Modifications of TiO2 Nanoparticles on the Surface Chemistry, Structure and Fouling Performance of PES Ultrafiltration Membrane” Oral presentation at Chemeca 2010, 26–29 September 2010 Hilton Adelaide
6. A. Razmjou, J. Mansouri, V. Chen “Blending titania nanoparticles with an effective approach for the modification of PES UF membranes” Oral presentation at AMS6/IMSTEC10 Conference, Sydney, 22–26 November 2010
7. A. Razmjou, J. Mansouri, V. Chen “Increasing the hydrophilicity of UF PES
membranes by synthesising a TiO2 coating layer at Low temperature” Poster presentation at AMS6/IMSTEC10 Conference, Sydney, 22–26 November 2010
8. A. Razmjou, J. Mansouri, V. Chen “Performance of TiO2 coated PES UF membranes by a low temperature hydrothermal (LTH) process” Oral presentation at ICOM 2011 Conference, Amsterdam, 23–29 July 2011
9. A. Razmjou, J. Mansouri, V. Chen, M. Lim, R. Amal “The Effect of Coating
Parameters of TiO2 Nanoparticles on Characteristics and Performance of PES UF Membranes” Oral presentation at Chemeca 2011, Sydney, Australia, September 18- 21, 2011
10. A. Resosudarmo, A. Razmjou, A. Gunawa, H. Li, J. Mansouri,V. Chen “Ultrafiltration Hollow Fiber Membrane Permeability and Fouling Resistance
Improvement by Addition of Modified TiO2 Nanoparticles” Oral presentation at Chemeca 2011, Sydney, Australia, September 18-21, 2011
11. A. Razmjou, J. Mansouri, V. Chen “TiO2 PES nanocomposite ultrafiltration membrane” Oral presentation at MSA Student Symposium 2011 at Glenelg in South Australia, Adelide, Nov 23-25
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TABLE OF CONTENTS
ABSTRACT i ACKNOWLEDGEMENTS iii PUBLICATIONS v TABLE OF CONTENTS vii LIST of FIGURES xii LIST of TABLES xviii CHAPTER 1 INTRODUCTION 2
1 Overview 2 1.1 Research Objectives 3 1.2 Dissertation Outline 4 CHAPTER 2 LITERATURE REVIEW 7
2 Introduction 7 2.1 Literature Review Introduction 7 2.1.1 Polymer nanocomposites 8 2.1.2 Membrane 10 2.1.3 Nanocomposites membrane elements 18 2.2 Membrane Nanocomposites Preparation Approaches 22 2.2.1 Blending 22 2.2.2 Coating 25
2.3 Titanium Dioxide (TiO2) 29
2.3.1 TiO2 applications and its main surface property 30
2.3.2 Effect of incorporation of TiO2 on the polymer properties 31
2.3.3 TiO2 nanocomposites membranes 34 2.4 Sol-Gel Chemistry 45 2.4.1 Introduction 45
2.4.2 TiO2 sol gel 46 2.4.3 Sol gel parameters 47 2.5 Surface Wettability 51 2.5.1 Wetting state of a smooth surface 51 2.5.2 Wetting state of a rough surface 52 2.5.3 Hierarchical structures and multilevel roughness 54
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2.6 Membrane Distillation (MD) 56 2.6.1 Introduction to membrane distillation 56 2.6.2 Operational parameters affecting MD process 57 2.6.3 Challenges in membrane distillation 59 2.7 Summary 62 CHAPTER 3 MATERIALS AND METHODOLOGY 64
3 Abstract 64 3.1 Materials 64 3.2 Preparation of Nanocomposite PES Ultrafiltration Membranes (Blending) 65
3.2.1 Modification of TiO2 65 3.2.2 Preparation of UF PES membrane (flat sheet) 65 3.2.3 Preparation of UF PES membrane (hollow fiber) 66 3.3 Preparation of Nanocomposite PES Ultrafiltration Membranes (Coating) 67
3.3.1 Preparation of TiO2 precursor sol for coating 67
3.3.2 Coating of TiO2 nanoparticles onto PES membrane by a low temperature hydrothermal (LTH) process 67 3.4 Superhydrophobic Modification of Microporous PVDF Membranes 68
3.4.1 TiO2 coating on the surface of PVDF membranes 68
3.4.2 Fluorosilanization of TiO2-PVDF membrane surface 68 3.5 Membrane Characterization 69 3.5.1 Scanning electron microscopy, energy dispersion of X-ray analysis and transmission electron microscopy (TEM) 69 3.5.2 Particle size distributions 71 3.5.3 Atomic force microscopy (AFM) 71 3.5.4 Surface area and porosity 71 3.5.5 Capillary flow porometry 72 3.5.6 Molecular weight cut-off 72 3.5.7 Contact angle measurement 73 3.5.8 Surface free energy of membrane 73 3.5.9 Static protein absorption 74 3.5.10 Streaming potential measurements 75 3.5.11 Surface chemistry (XPS) 76 3.5.12 Fourier transform infrared spectroscopy (FTIR) 76 3.5.13 Thermal analysis (TGA and DSC) 77
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3.5.14 Dynamic mechanical analysis (DMA) 78 3.5.15 Membrane tensile strength test 79 3.5.16 Membrane Performance 79 CHAPTER 4 BLENDING (FLAT SHEET NANOCOMPOSITE MEMBRANES) 87
4 Abstract 87 4.1 Introduction 88 4.2 Experimental 91 4.2.1 Materials 91
4.2.2 Modification of TiO2 and Preparation of Membrane 91 4.2.3 Membrane Characterization 91 4.3 Results and Discussions 92
4.3.1 TiO2 Nanoparticles Modifications 92
4.3.2 Dispersion of TiO2 Particles (TGA and EDAX) 94
4.3.3 Effect of TiO2 Modification on Cross-Section Micro-Structure 98
4.3.4 Effect of TiO2 Modification on Glass Transition Temperature 99
4.3.5 Effect of TiO2 Modification on Surface Pore Size 100
4.3.6 Effect of TiO2 Modification on Surface Roughness 101
4.3.7 Effect of TiO2 Modification on Membrane Tensile Strength 102
4.3.8 Effect of TiO2 Modification on Surface Chemistry: Contact Angle, Protein Absorption, Surface Free Energy and Stream Potential 104
4.3.9 Effect of TiO2 Modification on Membrane Performance 106 4.4 Conclusion 110 CHAPTER 5 BLENDING (HOLLOW FIBER NANOCOMPOSITE MEMBRANES) 113
5 Abstract 113 5.1 Introduction 114 5.2 Experimental 115 5.2.1 Materials 115
5.2.2 Modification of TiO2 and the Preparation of PES Hollow Fiber Membrane 116 5.3 Membrane Characterization 116 5.4 Results and Discussions 116
5.4.1 Dispersion of TiO2 Particles in the Hollow Fiber PES Membranes 116
5.4.2 Effect of TiO2 Modification on Cross-Section Micro-Structure 120
5.4.3 Effect of TiO2 Modification on Glass Transition Temperature, Loss and Storage Modulus 120
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5.4.4 Effect of TiO2 Modification on Surface Pore Size 124
5.4.5 Effect of TiO2 Modification on Membrane Tensile Strength 125
5.4.6 Effect of TiO2 Modification on Membrane hydrophilicity 126
5.4.7 Effect of TiO2 Modification on the Membrane Performance 126 5.5 Conclusion 129 CHAPTER 6 COATING (SUPERHYDROPHILIC MODIFICATIONS) 131
6 Abstract 131 6.1 Introduction 132 6.2 Experimental 134 6.2.1 Materials 134
6.2.2 Preparation of In-house PES Membrane and TiO2 Precursor Sol for Coating 134
6.2.3 Coating of TiO2 Nanoparticles onto PES Membrane by LTH Process 134 6.3 Membrane Characterization 135 6.4 Results and Discussion 136 6.4.1 Dip-Coating Parameters 136 6.4.2 Characterization of Coating 136 6.4.3 Membrane Tensile Strength 143 6.4.4 Surface Morphology 143 6.4.5 Surface Chemistry 145 6.4.6 Assessment of Hydrophilicity 147 6.4.7 Effect of Templating Agent 150 6.4.8 Effect of Coating Cycles and Heat Treatment 153
6.4.9 Effect of TiO2 Coating on the Membrane Performance 157 6.5 Conclusion 162 CHAPTER 7 COATING (SUPERHYDROPHOBIC MODIFICATION)-MD APPLICATION 164
7 Abstract 164 7.1 Introduction 165 7.2 Experimental 167 7.2.1 Materials 167
7.2.2 TiO2 Coating and Fluorosilanization of PVDF Membranes 167 7.3 Membrane Characterization 167 7.4 Results and Discussions 168
7.4.1 Evidence of TiO2 and FTCS on the Membrane Surface 168 7.4.2 Assessment of Hydrophobicity 168
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7.4.3 Thermomechanical Stability 172 7.4.4 Surface Morphology 172 7.4.5 Effect of Modifications on the Pore Size and Permeability 176 7.4.6 Effect of Modifications on the LEP 177
7.4.7 Role of TiO2 in the Surface Modification 178
7.4.8 Durability, Stability and Photo-catalytic Activity of TiO2 -PVDF Membrane 180 7.5 Membrane Performance 181 7.5.1 Effect of Feed Temperature and Pressure on the Flux 181 7.5.2 Effect of Superhydrophobic Modification on Wetting Resistance 183 7.5.3 Direct Contact Membrane Distillation 184 7.6 Conclusion 188 CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 190 7.1 Conclusions 190
7.1.1 Blending of TiO2 nanoparticles into membrane matrix (Chapter 4 and 5) 190
7.1.2 Coating of TiO2 nanoparticles on the membrane surface (Chapter 6 and 7) 191 7.2 Recommendations 192 7.2.1 Recommendations for blending the nanoparticles into membrane matrix 192 7.2.2 Recommendations for coating of the nanoparticles on the membrane surface 193 REFERENCES 195 APPENDIX 209
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LIST of FIGURES
Figure 2-1 Schematic diagram of a membrane separation process...... 10
Figure 2-2 Fouling schematics of pore narrowing or constriction, pore plugging and gel or cake layer formation ...... 14
Figure 2-3 Number of published papers vs. year on nanocomposite membranes ...... 17
Figure 2-4 Molecular structure of PES and PVDF polymers ...... 20
Figure 2-5 In-situ grafting polymerization approaches (grafting to and grafting from) ...... 25
Figure 2-6 Different coating techniques (a) solution adsorption (b) vacuum filtration (c) spin- coating (d) dip-coating ...... 27
Figure 2-7 Time temperature transformation diagram for the formation of crystalline titanium dioxide [184] ...... 48
Figure 2-8 Effect of templating agent on the inorganic sol gel derived structure (a) hydrophilic [208] (b) amphiphilic [207] agent ...... 50
Figure 2- 9 Contact angle θ and the interfacial forces of a liquid droplet on a flat smooth surface ...... 51
Figure 2-10 Wetting state of (a) Wenzel and (b) Cassie-Baxter and (c) the intermediate state of Wenzel and Cassie-Baxter ...... 53
Figure 2-11 Effect of roughness on the water contact angle, (a) maximum achievable water contact angle of 120o after fluorination of the surface, (b) contact angle between 120o and 150o after micro-scale roughening and fluorination and (c) superhydrophobic surface with water contact angle above 150o...... 54
Figure 2-12 Schematics of different types of membrane distillation MD configurations [237] ...... 56
Figure 3-1 Low temperature hydrothermal process of TiO2 sol-gel synthesized coating on the polymeric membranes ...... 68
Figure 3-2 Coating of TiO2 nanoparticles on the membrane by vacuum filtration technique . 69
Figure 3-3 Sample preparation of SEM S4500 and S3400-I: a) before chromium coating (left), after chromium coating (middle) and sandwiched membranes by using double sided sticky tapes after chromium coating (right) for cross section imaging, and b) sample preparation of SEM S900 after chromium coating, and c) transmission electron microscopy (TEM) copper grids...... 70
Figure 3-4 Capillary flow porometer for measuring pore size and distribution of microfiltration membranes ...... 72
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Figure 3-5 Calibration curve for absorbance versus mass of BSA (mg) ...... 75
Figure 3-6 Clamping cell for stream potential measurement...... 76
Figure 3-7 Fourier transform infrared spectroscopy used in this study ...... 77
Figure 3-8 TGA (a) and DSC (b) instruments used in this study ...... 78
Figure 3-9 Flat sheet membranes clamped in a uniaxial tensile test machine ...... 79
Figure 3-10 Dead end filtration set up which was set to work at both constant pressure and flux mode (a) and dead end cell details (b) ...... 80
Figure 3-11 Cross flow set up for filtration performance test for hollow fiber membranes .... 82
Figure 3-12 Schematic diagram (a) and lab scale set up (b) of Direct Contact Membrane Distillation...... 85
Figure 4-1 FTIR spectra of unmodified P25, Chemically modified TiO2 with APTES and Pure APTES (a) from 2300 cm-1 to 3550 cm-1 (b) from 650 cm-1 to 1700 cm-1 ...... 93
Figure 4-2 Top surface EDS spectra of control and TiO2 blend membrane (2 wt.%) ...... 94
Figure 4-3 A typical TGA for a 2 wt.% unmodified TiO2 flat sheet blend membrane (10cm×20cm) ...... 96
Figure 4-4 Effect of various TiO2 concentrations on the initial weight loss ...... 96
Figure 4-5 EDAX mapping coupled with SEM images for (a) control, (b) mechanically modified, (c) mechanically and Chemically modified and (d) unmodified TiO2 PES membrane ...... 97
Figure 4-6 Effect of unmodified and different modified TiO2 concentrations on PES membrane structure (300-500x) ...... 98
Figure 4-7 Effect of different modified TiO2 concentration on PES membrane structure (1- 1.5K) ...... 99
Figure 4-8 Molecular weight cut-off for modified, unmodified and control membrane ...... 101
Figure 4-9 Control PES membrane prepared from DMAC as solvent and coagulating into mixtures of water and isopropanol (concentration of isopropanol indicated, wt. %) ...... 101
Figure 4-10 Three dimensional AFM images for (a) modified, (b) unmodified TiO2 blend and (c) control membrane ...... 102
Figure 4-11 Maximum load at breaking point for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes (means and standard deviations are indicated)...... 103
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Figure 4-12 Extension at breaking point for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes (means and standard deviations are indicated)...... 103
Figure 4-13 Average contact angles on the top surface of the PES membranes with different modified and unmodified TiO2 content (means and standard deviations are indicated) ...... 104
Figure 4-14 Absorbed protein by Lowry method for control, unmodified and modified TiO2 blend membranes (2 wt.%) on the base of nominal surface area (means and standard deviations are indicated) ...... 105
Figure 4-15 Surface free energy of the membranes calculated by acid-base (van Oss) approach (means and standard deviations are indicated) ...... 105
Figure 4-16 Zeta potential versus pH for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes ...... 106
Figure 4-17 Effect of modification of TiO2 nanoparticles on the pure water flux of control and different TiO2 content ...... 107
Figure 4-18 Initial water flux measured at one bar for control, unmodified and mechanically
(and chemically) modified TiO2 blend PES membranes and 300MWCO UF PES membrane from Millipore ...... 107
Figure 4-19 TMP vs. time for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes and 300 MWCO UF PES membrane from Millipore...... 108
Figure 5-1 TiO2 nanoparticles before (left) and after 10 min sonication in ethanol (right) ... 117
Figure 5-2 Top surface EDX spectra of control and chemically and mechanically modified
TiO2 blend hollow fiber membranes...... 117
Figure 5-3 Weight loss versus temperature for 3 different pieces of (a) mechanically and chemically (b) mechanically modified TiO2 PES Hollow fiber membrane, the inset image in b is the TGA for a different sample of mechanically modified TiO2 PES hollow fiber membrane ...... 118
Figure 5-4 EDX mapping coupled with SEM images for (a) mechanically and chemically modified and (b) only mechanically TiO2 PES hollow fiber membrane (arrows point the agglomeration of TiO2 nanoparticles)...... 119
Figure 5-5 The effect of TiO2 modification on PES membrane structure (a) Control (b) mechanically (c) chemically and mechanically modified TiO2 PES hollow fiber membrane...... 121
Figure 5-6 Storage modulus (a) and loss modulus (b) versus temperature for control and modified membranes ...... 123
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Figure 5-7 Effect of TiO2 modifications on the initial weight loss ...... 124
Figure 5-8 Molecular weighs cut-off for control and modified PES hollow fiber membranes ...... 125
Figure 5-9 Average contact angles on the PES membranes for control and modified PES hollow fiber membranes (means and standard deviations are indicated) ...... 126
Figure 5-10 Effect of modification of TiO2 nanoparticles on the pure water flux of membranes (means and standard deviations are indicated) ...... 127
Figure 6-1 TEM images of TiO2 nanoparticles (a) from Sol-Gel solution (b) Degussa P25 . 135
Figure 6-2 SEM images of TiO2 layer after heating PES membrane coated with (a) TiO2 sol (LTH process) and (b) Degussa P25 solution up to 800oC...... 137
Figure 6-3 AFM phase images of (a) coated membranes with TiO2 sol (LTH process) (b) membranes coated with P25 and (c) control membranes ...... 139
Figure 6-4 Effect of sonication on mechanical stability of TiO2 coated in-house PES membranes ...... 139
Figure 6-5 Effect of UV irradiation on the color change of KI solution in the presence of (a) control and (b) TiO2 coated membranes ...... 140
Figure 6-6 UV-Vis absorption spectroscopy for (a) KI solution of TiO2 coated membrane and (b) absorbance of the KI solution at 351 nm and 288 nm after exposure of the TiO2 coated PES in-house membrane by LTH process to 6 h of UV irradiation for 5 cycles ...... 141
Figure 6-7 Cross section SEM images of (a) coated (triple) in-house membrane with LTH process and (b) control membrane ...... 142
Figure 6-8 Three dimensional AFM topography of membranes and their corresponding roughness images (a) control (b) coated with P25 and (c) coated with TiO2 sol (LTH process) ...... 142
Figure 6-9 SEM images of coated PES membrane with (a) P25 solution and (b) TiO2 sol (triple, LTH process) ...... 145
Figure 6-10 Surface composition of control PES and TiO2 coated (LTH process) PES membranes (Pall Corporation, 100kDa MWCO) with 1 to 3 cycles of coating derived from XPS results ...... 146
Figure 6-11 Contact angle measurements vs. time for PES (Pall corporation, 100 kDa MWCO) coated by LTH process using F127 as an additive for different coating cycles (Single: one cycle of coating, Double: two cycles of coating and Triple: three cycles of coating) ...... 148
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Figure 6-12 Contact angle monitoring during 6 weeks for PES (Pall Corporation, 100 kDa MWCO) membrane coated by LTH process within 3 cycles of coating (the membrane irradiated with UV only one time and then were kept at ambient conditions and laboratory environment) ...... 149
Figure 6-13 SEM images of PES (Pall Corporation, 100 kDa MWCO) coated membranes by LTH process (Triple) at different magnifications ...... 149
Figure 6-14 PES (Pall Corporation, 100 kDa MWCO) membranes coated by LTH process without F127 ...... 151
Figure 6-15 SEM images for PES membrane (Pall Corporation, 100 kDa MWCO) coated by LTH process (triple) (a) with F127 (b) without F127 ...... 152
Figure 6-16 Effect of templating agent on the surface structure of TiO2 coated membranes (a) F127 and (b) PEG ...... 153
Figure 6-17 Contact angles of membrane (Pall Corporation, 100kDa MWCO) coated by LTH process in 1 (single) and 3 (triple) cycles with different heat treatment stages ...... 154
Figure 6-18 Contact angles at (5th sec) during 3 cycles of coating by LTH process for membrane (Pall Corporation, 100kDa MWCO) coated without F127 ...... 155
Figure 6-19 SEM images for membrane (Pall Corporation, 100 kDa MWCO) coated by LTH process without F127 ...... 156
Figure 6-20 SEM images (a) after coating, (b) before coating and (c) contact angle measurements for commercial PES (Millipore, 500kDa MWCO) membrane by LTH process ...... 157
Figure 6-21 Fouling behaviour (0.1 wt.% HA, pH 9.4) of control and coated (LTH process) PES (Millipore, 500 kDa MWCO) membranes at constant pressure of 100kPa for 2 h ...... 159
Figure 6-22 SEM images of triple coated (LTH process) PES (Millipore, 500 kDa MWCO) membrane after fouling and cleaning ...... 160
Figure 6-23 Fouling behaviour (0.1 wt.% HA, pH 9.4) of control and coated (LTH process) PES (Millipore, 500 kDa MWCO) membranes at constant flux of 85 Lm-2h-1 for 2 h, the initial feed side pressure was set to 40kPa and the experiment was run at room temperature. The stirrer speed was at fixed speed of 600 rpm...... 161
Figure 7-1 Surface composition (wt.%) of virgin PVDF, TiO2–PVDF and FTCS–TiO2–PVDF membranes...... 168
Figure 7-2 Effect of superhydrophobic modifications on the PVDF membranes (a) before and after modification (withe: MQ water, brown: humic acid solution, pink, blue and green: buffer solutions with pH of 4, 7 and 14), and (b) imparting self-cleaning property ...... 170
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Figure 7-3 Contact angle measurements of virgin and modified (FTCS-TiO2-PVDF) membrane using different concentration of NaCl and humic acid ...... 171
Figure 7-4 SEM images of (a) virgin, (b) TiO2-dip coated, (c) FTCS-vacuum filtered and (d) TiO2-dip coated and FTCS-vacuum filtered PVDF membrane at 5K magnification ...... 173
Figure 7-5 Effect of superhydrophobic modification on the PVDF membranes (a) virgin
PVDF (b) FTCS-TiO2-PVDF membrane (insets at top right corner of (a) and (b) are the SEM images at high resolution and at top left corner of (b) is the schematic side view of clusters assumed in a series of cylinders aligned horizontally)...... 174
Figure 7-6 Mean pore diameter (microns) for virgin, TiO2 coated, fluorinated and TiO2 coated followed by fluoro-silanized PVDF membranes ...... 177
Figure 7-7 Proposed scheme for the silanization of the PVDF membranes (a) hydrolyzation of FTCS (b) interaction with the surface of TiO2 (c) in-plane reticulation (d) condensation of trisilanols in the solution in the absence of TiO2 coating, and (e) surface SEM image of FTCS–PVDF membranes...... 179
Figure 7-8 Absorbance of the KI solution at 351 nm and 288 nm after exposure of the TiO2- PVDF membrane to 6 h of UV irradiation for 4 cycles ...... 181
Figure 7-9 Effect of feed inlet temperature on permeate flux of both virgin and modified
(FTCS-TiO2-PVDF) membrane...... 182
Figure 7-10 Effect of feed inlet pressure on the permeates flux ...... 183
Figure 7-11 Pore wetting resistance behaviour of virgin (PVDF) and modified (TiO2-FTCS- PVDF) membranes...... 184
Figure 7-12 Flux and permeate conductivity vs. time for virgin and FTCS-TiO2-PVDF membrane with 3.5 %wt. NaCl; bulk feed temperature 70°C; bulk permeate temperature 25°C; feed inlet pressure 20 kPa...... 185
Figure 7-13 Flux reduction (instantaneous flux J over the average pure water flux Jo) vs. time for virgin and FTCS-TiO2-PVDF membrane with 150 mg/L humic acid with/out 3.775 CaCl2 at pH of 7; bulk feed temperature 70°C; bulk permeate temperature 25°C; feed flow rate 300 ...... 186
Figure 7-14 Virgin (a) and FTCS-TiO2-PVDF (b) PVDF membranes after fouling by 150 mg/L HA and 3.775 mM CaCl2 solution for 20 h and cleaning by 0.2 wt.% NaOH for 15 min recirculation (insets are water droplets on the fouled and cleaned membranes) ...... 187
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LIST of TABLES
Table 2-1 Characteristics of membrane process [2, 3, 29] ...... 11
Table 2-2 Different types of fouling and their corresponded foulant and common fouling cleaning and controlling approaches ...... 13
Table 2-3 Effects of cleaning and operating strategies on membrane fouling [6]* ...... 15
Table 2-4 Most common nanoparticles used in nanocomposite membrane preparation for liquid separation ...... 19
Table 2-5 Properties of the most three common phases of TiO2 i.e. rutile, anatase and brookite ...... 29
Table 2-6 Four different categories of TiO2 nanomaterials applications ...... 30
Table 2-7 TiO2 nanocomposite with different polymer matrix and synthesis method ...... 32
Table 2-8 A summary on the selected TiO2 nanocomposite membrane prepared for liquid separation and water purification ...... 41
Table 3-1 Parameters of acid-base (van Oss) approach ...... 74
Table 3-2 Operating parameters for DCMD experiments ...... 84
Table 4-1 Incorporation of TiO2 nanoparticles into in-house PES UF flat sheet membranes (means and standard deviations are indicated) ...... 92
Table 4-2 Expected and experimental residual values (wt. %) for different concentrations of
TiO2 ...... 94
Table 4-3 Average value of glass transition temperature for each type of membrane measured by DSC ...... 100
Table 4-4 Filtration resistances of control and unmodified TiO2 blend membranes (the experiment repeated three times and the average was reported) ...... 109
Table 4-5 Flux recoveries and BSA rejections for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes and 300MWCO PES membrane from Millipore ...... 110
Table 5-1 Average value of glass transition temperature, tensile strength and elongation at break for each type of membrane measured...... 122
Table 5-2 Filtration resistances and TGA residues for control and modified TiO2 PES hollow fiber membranes...... 128
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Table 5-3 Flux recoveries within 3 cycles of fouling-cleaning experiments for control and modified TiO2 hollow fiber PES membranes...... 128
Table 6-1 Contact angles of in-house PES membranes were dip-coated at different conditions. Treated control is the membrane which was dipped in a coating solution without
TiO2 followed by heat treatments...... 135
Table 6-2 BET Surface areas (m²/g), the mean of BJH adsorption and desorption average pore diameter (nm), porosity for control PES (Pall Corporation, 100kDaMWCO) membranes and coated membranes at different cycles of coating by LTH process. Tensile strength and elongation were measured for in-house PES membranes coated by LTH process...... 143
Table 6-3 Initial water flux, flux recoveries after physical, chemical cleaning and backwashing, humic acid rejections, membrane intrinsic resistance and membrane resistance ratios for control and coated membranes during 3 cycles of coating at constant pressure of 100 kPa...... 159
Table 6-4 Initial water flux, flux recoveries after each stage of cleaning, rejections, membrane intrinsic resistance and membrane resistance ratios for control and coated membranes by LTH process during 3 cycles of coating at constant flux of 85 Lm-2h-1 ...... 161
Table 7-1 Contact angles for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition ...... 169
Table 7-2 Surface free energy and, bubble point pressure, liquid entry pressure values and fluxes for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition...... 171
Chapter 1 Introduction 1
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 1 Introduction 2
CHAPTER 1 INTRODUCTION
1 Overview Membrane is a thin interface between two phases, which is impermeable to specific particles, molecules, or substances [1]. Over the last few decades, the applications of membranes have broadened considerably and they are now employed in a great variety of industries for gas and liquid separation. This wide range of applications is due to their benefits and special features such as small footprint for large membrane surface area, compactness, the ease of fabrications, operation and module design [2]. However, there are a few drawbacks which limit the membrane applications [2, 3]. Fouling which refers to the blockage of membrane pores during filtration is one of the most important obstacles to membrane development as it worsens membrane performance and shortens membrane life [4]. There are two main techniques to remove or minimize fouling: the optimization of operating conditions and hydraulic and chemical cleaning [5, 6]. However, developing or modifying membrane materials to make them less prone to fouling has shown a great potential to control and mitigate fouling [7].
Recently, the incorporation of inorganic nanoparticles such as SiO2, Al2O3, Fe3O4, ZrO2 and
TiO2 into polymeric membranes has been shown a potential to improve the separation performance of membranes [7, 8]. Among them, TiO2 has received the most attention because of its good physical and chemical properties, availability as well as its potential antifouling abilities [9-11]. There are two main approaches to prepare TiO2 nanocomposite membranes: blending TiO2 nanoparticles into the membranes [12-14] and coating them onto the surface of the membrane [15-17]. The first approach suffers from the limits in the loading of inorganic particulates and difficulty in achieving uniform distribution of nanoparticles due to agglomerations. The coating approach, on the other hand, has the drawback that it may result in the non-uniform distribution of the TiO2 nanoparticles on the membrane surface and the instability of coating layer. Therefore, a significant consideration should be given to modify the conventional approaches or implement new techniques for the inclusion of nanoparticles into the polymeric membranes. In addition, the effect of nanoparticles addition on the surface chemistry and morphology of the membranes should be elucidated to
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 1 Introduction 3 understand the mechanisms by which the nanoparticle additions could improve the fouling performance of membranes.
1.1 Research Objectives
In this research, TiO2 nanoparticles are incorporated into ultrafiltration (UF) and microfiltration (MF) polymeric membranes via blending and coating techniques. The properties of these nanocomposite membranes will be characterised with the following aims:
¾ Investigating the effect of TiO2 nanoparticles agglomeration and the level of dispersion on the surface chemistry, structure and fouling performance of flat sheet polymeric membranes.
¾ Optimizing the level of TiO2 inclusion into flat sheet membranes after mechanical and chemical modifications of commercial titania nanoparticles
¾ Blending the TiO2 nanoparticles into hollow fiber membranes using the techniques implemented for the flat sheet membranes
¾ Studying the effect of TiO2 nanoparticles inclusion into hollow fiber membranes
¾ Engineering the architecture and chemistry of membrane surface via coating the TiO2
nanoparticles (synthesised by sol-gel technology and functionalized) by a low temperature hydrothermal process
¾ Studying the effect of various parameters during TiO2 coating on the surface chemistry, morphology and fouling performance of the UF membranes ¾ Changing the wettability of UF membrane surface toward superhydrophilicity through
engineered TiO2 coating without continuous UV irradiation and studying its consequences on the fouling performance of the membranes ¾ Changing the wettability of MF membrane surface toward superhydrophobicity by
reducing the surface free energy of roughened TiO2 coated membranes for a membrane distillation application
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 1 Introduction 4
1.2 Dissertation Outline The structure of the thesis is outlined in 7 chapters as follows:
Chapter 2 which consists of six major sections has tried to provide sufficient backgrounds and theories on polymer nanocomposites and nanoparticle synthesis approaches with regards to their application in membrane technology. This chapter also gives information about the principles of membrane separation technology and different fouling types with current fouling mitigation strategies. A comprehensive review in the present chapter covers the latest works and gaps on the TiO2 nanocomposite membranes. Sol gel chemistry of TiO2 and current theories on the wetting action of a smooth and rough surface are studied and how nanoparticles deposition can change the wettability of the surface is also discussed. Membrane distillation as one of the uprising areas in membrane technology and its practical challenges are finally addressed.
Chapter 3 provides information on the materials used, preparation procedures, experimental set-ups, protocols and characterization techniques involvedthroughout this research.
Chapter 4 introduces a technique to reduce the commercial TiO2 nanoparticles (Degussa P25) agglomerations by mechanical and chemical modifications of the particles. The effects of modifications on the surface chemistry and morphology as well as fouling performance of the UF flat sheet membranes will also be argued.
Chapter 5 shows an effort to use the implemented techniques in chapter 4 to incorporate TiO2 nanoparticles into hollow fiber membranes and compares the properties of fabricated hollow fiber membranes with its flat sheet counterpart.
Chapter 6 presents an implementation of a low temperature hydrothermal (LTH) coating process of TiO2 nanoparticles on the surface of in-house and commercial UF membranes with various pore sizes. Dip-coating parameters such as drying and holding time, dipping and withdrawal velocities, and the number of coating cycles are varied to optimise the microstructure and surface properties of the coating. The present chapter shows an attempt to engineer the architecture and chemistry of the membrane surface during coating to shift the surface wettability toward superhydrophilicity. Comprehensive characterization techniques were used to understand the effect of TiO2 coating on the surface chemistry and morphology as well as fouling performance of the membranes.
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 1 Introduction 5
Chapter 7 outlines how the surface wettability can be shifted toward superhydrophobicity after roughening the membrane surface by coating TiO2 nanoparticles using the technique introduced in chapter 6 and then reducing the surface free energy of the roughened surface. The performance of fabricated superhydrophobic membranes was examined in a direct contact membrane distillation process.
Chapter 8 summarizes the conclusions of this research and provides recommendations for future work.
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 6
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 7
CHAPTER 2 LITERATURE REVIEW
2 Introduction
Since this PhD study is about the incorporation of TiO2 nanoparticles into polymeric membranes and is an overlaps of polymer and membrane technology as well as nanotechnology, the literature review which is consisted of six major sections has tried to provide sufficient backgrounds and theories on polymer nanocomposites and nanoparticle synthesis approaches with regards to their application in membrane technology. However, it is not possible to cover every aspect of nanocomposites due to depth and breadth of the area and only the most important areas to this work have been considered.
2.1 Literature Review Introduction In the first section, nanocomposite with its elements was defined. The basic principles of membrane separation processes and fouling as a main drawback of membrane technology and its mitigation strategies were also discussed. The second section ( 2.2) is focused on the membrane nanocomposite synthesis approaches. Two common synthesis approaches of blending and coating were reviewed. The following section ( 2.3) mainly concentrated on the
TiO2 in terms of its crystal structures, main surface properties and applications, and also its effect on the polymer matrix. The last part of the section consists of a comprehensive review on the TiO2 nanocomposite membranes. Sol gel technology and parameters were studied in the section 2.4 as it is one of the most commonly used approaches for creating nanoparticles at modest temperature and pressure. In order to understand the effect of deposited nanoparticles on the wettability of surfaces, current theories on the wetting action of smooth and rough surfaces were studied in section 2.5. The effect of hierarchical structure in shifting the wettability of surfaces towards the extreme conditions of superhydrophilicity and superhydrophobicity were also explored. The last part of the present chapter (section 2.6) covered membrane distillation (MD) as one of the attractive applications of membrane separation technology. Current issues in MD and how superhydrophobic surface modification could contribute to resolving those issues were also argued.
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Chapter 2 Literature Review 8
2.1.1 Polymer nanocomposites
Nanocomposite materials consist of two or more dissimilar phases fabricated via mixing or coating the inorganic phase into or on the surface of organic phase at nanometer scale to develop new materials with superior performance [18-21].
In conventional polymer composite materials (macrocomposites), the inorganic micrometer- scale fillers are incorporated into polymer matrix to reinforce them and increase their life time. Moreover, the incorporation results in a higher strength and stiffness [22, 23]. For decades, those fillers have been used in a variety of industries such as construction, automobile and consumer products. However, achieving those properties in traditional composite materials involve compromises between desired performance, mechanical properties, cost, and processibility.
Recently, with the development in nanotechnology, the issues in macrocomposites can be overcome by taking the advantage of nano-scale inorganic fillers to fabricate nanocomposites and hybrid materials. In fact, the fundamental properties of materials such as thermomechanical, morphological, electrical and even their optical and colour properties can be manipulated by recreating them at nanometer-scale without changing their original chemical compositions. These fundamental changes are possible via influencing the quantum mechanical properties of electrons and atomic interactions inside matter by materials variations on the nanometer (nm) scale [21].
In macrocomposites materials the filler length scale is in micrometers whereas in nanocomposites materials the scale is between 1-100 nanometers (nm) [21]. According to Jordan et al. [24], the current micromechanics theory explains that the final properties of macrocomposite materials such as Young's modulus are rely on the volume fraction of each components and the properties of each individual elements used such as shape and the arrangement of inclusions, and matrix-inclusion interface. The final properties are independent of the size of inclusions. However, the theories may not true for nanocomposites materials. Indeed, the nanofillers have an ultralarge interfacial area per volume with the size of the same order of magnitude as the polymer coils. This results in a molecular interaction between the polymer and the nanoparticles which gives the inorganic-organic hybrids unique properties that conventional polymers do not possess [21].
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 9
Based on the nature of molecular interaction between polymer and inorganic fillers, the nanocomposites or hybrids were classified into two categories: class one and class two. In class one hybrids, there are weak secondary forces and interactions such as electrostatic interactions, van der Waals forces and hydrogen bonding. On the contrary, class two hybrids corresponds to materials with strong interactions for instance coordination, ionic, and covalent bonding [25].
There are different synthetic approaches for class one hybrids which mostly based on the dispersion of inorganic moieties into polymeric matrix such that in macroscopic scale it is uniform, but in microscopic scale the nanocomposite phases are separated or weakly bonded. Examples of class one hybrids are [26, 27] melt mixing, solvent mixing and interpenetrating networks which will be discussed in section 2.2.1. It should be pointed out here that the class one hybrids are fabricated mostly by sequential preparation techniques whereas the class two hybrids are most of the time based on the simultaneous formation of inorganic and organic phases by using complex techniques such as sol-gel technology and in-situ grafting polymerization (see section 2.2.1) [20, 26].
In membrane technology, nanocomposite membranes have recently shown a significant potential for improving the performance of the membranes, which is addressed in the next section.
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 10
2.1.2 Membrane
2.1.2.1 The basic principle of membrane separation process As can be seen in Figure 2-1, membrane is a permselective barrier or interface between two phases (liquid or gas), which is impermeable to specific particles, molecules, or substances [1]. The Driving force between the two phases could be pressure (∆P), temperature (∆T), concentration (∆C) or electrical potential (∆E) gradient [28]. Membrane separations can be categorized based on the size of retained material, application, driving forces or type of membrane [2, 3, 29] (see Table 2-1 ). The membrane separation process is a clean and high efficient technology which is significantly advantageous when compared to the conventional separation techniques such as sedimentation, centrifugation, distillation, evaporation, crystallization, precipitation, absorption, adsorption, and condensation. Separation and concentration without the use of heat is one of the most important characteristics of the membrane process particularly when it is considered as an alternative for distillation, evaporation and freeze concentration. There are more benefits and special features for membrane processes which make it attractive for industrial applications. Those features are small footprint for large membrane surface area, compactness, ease of fabrications, operation and modular design [2]. However, there are a few drawbacks which limit the membrane applications: membrane fouling, upper solid limit (particularly in reverse osmosis membrane process when the concentration of feed side increases), low mass transfer, high viscosity of retentate which makes pumping difficult, operating cost due to occasional replacement, fouling and poor clean-ability [2, 3]. Membranes can also be classified based on their material: organic (polymeric), inorganic (ceramic) and biological membranes [1].
Driving force: ∆P, T, C, E
Phase 1 Phase 2
(Feed) (Permeate)
Membrane
Figure 2-1 Schematic diagram of a membrane separation process
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 11
Table 2-1 Characteristics of membrane process [2, 3, 29]
Size of Type of membrane / Process materials Driving force application retained Porous / 0.1 - 10 μm Pressure difference Microfiltration Separation of bacteria and microparticles (0.5 - 2 bar) cells from solutions Microporous / 1 - 100 nm Pressure difference Separation of proteins and Ultrafiltration macromolecules (1 - 10 bar) virus, concentration of oil-in-water emulsions Microporous / 0.5 - 5 nm Pressure difference Separation of dye and Nanofiltration molecules (10 - 70 bar) sugar, water softening Nonporous / Reverse < 1 nm Pressure difference Desalination of sea and Osmosis molecules (10 - 100 bar) brackish water, process water purification Nonporous or < 1 nm Concentration microporous / Dialysis molecules difference Purification of blood (artificial kidney) Nonporous or < 1 nm Electrical potential microporous / Electrodialysis molecules difference Separation of electrolytes from nonelectrolytes Nonporous / Concentration Pervaporation - Dehydration of ethanol difference and organic solvents Nonporous / Partial pressure Hydrogen recovery from Gas - difference process gas streams, Permeation (1 - 100 bar) dehydration and separation of air Microporous / Membrane Temperature - Water purification and Distillation difference desalination
Organic membranes are further classified as microporous, asymmetric (skinned) and thin film composite membrane [2]. Microporous membranes are similar to conventional filters but with extremely small pores (0.01-10 μm). A microporous membrane is isotropic if the pore size is uniform throughout the membrane. However, it is anisotropic if the pore size changes from one surface to the other one. The asymmetric membrane has a thin skin (0.1-1 μm) on the top
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 12 surface of the membrane. The skin layer is supported by marovoids which has less effect on the separation characteristics. If the skin and supported layers cast together during phase inversion process, the resulting membrane is integrally skinned membrane. On the other hand, if the skin layer is deposited on the supported pores from the same material, it is non- integrally skinned membrane. Thin film composite membranes consist of a thin dense polymer skin formed over a microporous support film usually from different material.
In contrast to organic membrane, the inorganic membrane can be operated at elevated temperature (500-800ºC for metallic membrane and over 1000ºC for ceramic membrane) and harsh conditions with high resistance to corrosive liquids and gases [2, 30]. However, inorganic membrane fabrication is more expensive and complicated. As a result, they are used in those aggressive situations (chemical or thermal) where the polymeric membrane cannot or do not perform well. Module design limitations, large footprint due to high surface- to-volume ratio and sealing difficulty at high temperature are the most current problems with the inorganic membranes [30]. Hybrid or composite membranes are a new class of membrane which usually consist of an inorganic species into an organic matrix, which was discussed in section 2.1.2.3.
Biological membrane (biomembrane) is a selective barrier, within or around a cell in a living organism. The biomembrane is able to recognize what is necessarily for cell to receive or block for its survival. On the contrary to aforementioned artificial organic and inorganic membranes, the biomembranes cannot meet the industrial requirements because of thermomechanical stability and productivity [31].
2.1.2.2 Fouling One of the most important obstacles to membrane development is fouling as it worsens membrane performance and shortens membrane life. The term of “Fouling” refers to the blockage of membrane pores during filtration, which causes a flux decline over time. Fouling is a result of the combination of sieving and the adsorption of particulates and compounds onto the membrane surface or within the membrane pores [4]. According to the type of fouling material, membrane fouling can be classified into four major groups including (a) inorganic fouling (scaling) due to exceeding the concentration of one or more inorganic species beyond their solubility limits and their precipitation onto the membrane surface [5], (b) microbial or biological fouling (Biofouling) because of the dynamic process of microbial colonization and growth and the formation of microbial biofilms [7, 32], (c) organic fouling
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 13 as a results of the adsorption of natural organic maters (NOM) [33] and (d) particulate or colloidal fouling due to the accumulation of particulates and formation of a cack layer [4]. Table 2-2 lists the fouling types and materials (foulant) along with the common cleaning and controlling approaches used in the most reported membrane processes. It should be pointed out here that this is only a rough classification and there are overlaps between fouling types and membrane processes. For example, the classification of organic fouling overlaps those of colloidal fouling and biofouling, or for another example, although particulate or colloidal fouling mostly occurs in MF and UF, it could also occur in RO [34] or NF [35].
Table 2-2 Different types of fouling and their corresponded foulant and common fouling cleaning and controlling approaches
membrane Common cleaning Selected Fouling type Fouling materials process (mostly and controlling references reported) approaches The addition of acid [4, 5, 36] or scale inhibitors / CaCO , CaSO , Inorganic 3 4 Operating at Ca (PO ) , BaSO , RO and NF (Scaling) 3 4 2 4 conditions lower SrSO , CaF , SiO 4 2 2 than the critical solubility limits Released Biocide dosing [5, 7, 37, biopolymers (Chlorination), UV 38] (polysaccharides, treatment, feed pre- Microbial or proteins, and amino treatment to reduce biological RO and NF sugars) due to nutrient availability, (Biofouling) microbial activity of effective cleaning biofilms (bacterial, procedures algal, or fungal) Biological treatment, [5, 33] Natural organic oxidation, material (NOM) adsorption, Organic such as humic acids, MF and UF coagulation, protein and backwashing and carbohydrate chemically enhanced backwashing Algae, bacteria, and Backwashing, [4, 39, 40] Particulate or some natural organic MF and UF increasing shear rate, colloidal matters using cross flow According to the pore size, Belfort et. al [41] introduced three different fouling mechanisms (Figure 2-2 ): pore narrowing/constriction, pore plugging and gel/cake layer formation. In the case of pore narrowing, the foulant size (d) is much smaller than the pore size (dp) and the foulants could adsorb into the pores, and could possibly close smaller pores. For the pore plugging, the foulant size (d) is at the same order of magnitude as the pore size (dp) and the
Amir Razmjou, PhD Thesis, The University of New South Wales
Chapter 2 Literature Review 14 foulants could plug the pores in the absence of adsorption. For the cake layer formation, the foulant size (d) is much bigger than the pore size (dp) and the foulants are not able to enters the pores thereby particles deposit on the surface of membrane to form a loose gel or a cake layer.
FOULING SCHEMATICS
Pore size dp Foulant size dp
PORE NARROWING / CONSTRCTION
d < PORE PLUGGING d ~dp Blockage GEL/CAKE LAYER FORMATION d >>dp Deposition Figure 2-2 Fouling schematics of pore narrowing or constriction, pore plugging and gel or cake layer formation 2.1.2.2.1 Ultrafiltration fouling control strategies There are three main strategies to control the fouling in membrane process including the optimization of operating conditions, hydraulic and chemical cleaning and developing or modifying materials to make them less prone to fouling [7]. Operating parameters and procedures have a direct impact on the fouling of membranes. The parameters have to be set to minimize fouling and concentration polarization. In order to overcome the fouling and concentration polarization, both design parameters such as module Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 15 arrangements and operational parameters such as flow rate, flux, temperature, pH and pressure should be designed and controlled properly [5]. On the basis of the nature of the fouling (reversible or irreversible), all or part of it can be removed by hydraulic or physical and/or chemical cleaning. Certain foulants can be removed only be hydraulic (backwashing) cleaning but most of them can be cleaned by chemical cleaning (cleaning in-place (CIP), off-line chemical cleaning (or soaking) or a combination of both such as enhanced backwash (EBW)). In the backwashing the product water (permeate) flows reversely across the membrane to remove the accumulated or plugged foulants at the membrane surface or into the pores. EBW involves the circulation of backwash water with the addition of a cleaning chemical for a short period of time. In CIP, the membrane modules are cleaned in the installation plant without removing whereas in the off-line chemical cleaning the modules were soaked in the chemicals out of the plant [4]. Table 2-3 shows the effects of various cleaning and operating strategies against different types of fouling [6]. Table 2-3 Effects of cleaning and operating strategies on membrane fouling [6]* Hydraulic Feed Feed Chemical Type of Fouling Cleaning/ Chlorination Acidification Cleaning Backwashing Inorganic (Scaling) - - ++ ++ Particulate or colloidal ++ - - ++ together with Microbial or biological + ++ feed chlorination ++ + Organic fouling - + - ++ * ‘-‘ No effects/negative effects; ‘+’, some positive effects; ‘++’, Positive effects The common chemicals used in the cleaning of membranes are categorized into five groups of caustic, oxidants or disinfectants, acids, chelating agents and surfactants. The typical chemical of caustic is NaOH which is often used to clean organic and microbial fouling through the function of hydrolysis and solubilisation. Sodium hypochlorite and hydrogen peroxide are the most common oxidants which function through oxidation and disinfection. Acids and chelating agents such as citric, nitric, hydrochloric acid are very efficient for scaling and metal dioxides removal. Because of the amphiphilic nature (hydrophilic and hydrophobic structures) of surfactants, they are able to clean the membranes by the formation of micelles with certain foulants such as fat, oil, and proteins in water [4, 6]. As mention before, many water treatment and membrane processes frequently require exposure to physical and chemical cleaning. This increases the total operational cost of the membranes as Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 16 well as shortens their life time. Recently, the development of new materials or the surface modification of current membrane to make them less prone to fouling has become a point of interest for both researchers and industry. These new antifouling membranes can be generated through either anti-adhesion or anti-microbial approaches. In the anti-adhesion approaches, the surface architecture of membranes is changed to prevent or reduce the adsorption of foulants via reducing the roughness, increasing the hydrophilicity or incorporating fouling release agents. On the other hand, the anti-microbial approaches are based on killing the organisms, and suppress the biofilm formation by disrupting bacterial colonisations [7]. Surprisingly, new developed TiO2 nanocomposite membranes have recently shown the properties of both anti-adhesion via imparting hydrophilicity and anti-microbial (under UV irradiation) [42] approaches, which has been discussed in section 2.3.3. 2.1.2.3 Nanocomposite membranes The application of nanocomposite coatings and materials are one of the fastest-growing areas of membrane technology and application. The keyword of “nanocomposite membrane” were searched in the ScienceDirect and limited to the published papers. As can be seen in Figure 2-3 , there is a dramatic increase on the number of papers published in this field, which represents an enormous time and effort devoted by researchers during last few years. This significant attention is because the new developed nanocomposite membranes often present new properties such as higher permselectivity, antifouling performance, mechanical, thermal and chemical resistance. Such properties are usually absent in its individual elements used. The new characteristics can not only be altered by changing the individual components used but also by changing the morphology and surface chemistry. It is worth to mention that the term of mixed-matrix membrane is frequently used for hybrid membranes in the field of gas separation processes while the term of nanocomposite membrane is mostly used in the liquid and water separation processes [4, 43]. The idea of using mixed-matrix membrane in gas separation is to increase its selectivity without compromising the permeability [44]. Based on the Robeson upper bound [45], there is an upper limit for the selectivity of a polymeric membrane [46]. However, inorganic dense membranes such as carbon molecular sieves, ceramics and zeolites have shown a potential to surpass the Robeson upper bound [47-53]. As mentioned before, there are a few issues which could severely hinder inorganic membrane applications. Since mid-1980s [54, 55], researcher Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 17 have tried to combine the high permeability and processability of polymeric membranes with superior separation performance of inorganic membrane. The fabrication of defect free surface is one of the major challenges in mixed-matrix membranes [43]. 1,600 1,400 1,200 1,000 800 600 The number of papers 400 200 0 1990 1995 2000 2005 2010 Year Figure 2-3 Number of published papers vs. year on nanocomposite membranes In the liquid separation (ultrafiltration [9] , nanofiltration [56] , membrane bioreactor [57] and reveres osmose [58]), the incorporation of nanofillers into polymeric membranes has been applied to not only target the thermomechanical stability of the membrane but also to increase the antifouling properties of the membranes. There are many approaches to mitigate the fouling of polymeric membrane in filtration applications and most of them are based on the increasing membrane hydrophilicity and reducing microscale roughness, either by grafting or coating with hydrophilic organic moieties such as polyethylene glycol (PEG) [7]. More recently, the addition of nanomaterials have also been shown to improve the permeability, impart self-cleaning and/or antifouling properties, reduce compaction, and alter the morphology of the membrane [7, 8]. A variety of nanoparticles such as SiO2, Al2O3, Fe3O4, ZrO2 and TiO2 [12, 59-62] have been used for surface modifications and coating. The focus Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 18 of this review is on the incorporation of nanoparticles on the polymeric membranes with the application in liquid separation and water purifications. 2.1.3 Nanocomposites membrane elements Nanocomposites membrane consists of a polymeric material and a nanoscale material (nanoparticle). In order to achieve a desirable nanocomposite membrane, a careful selection of material constituents and a proper processing and fabrication method to gain uniform dispersibility are necessarily. In the following sections, the most commonly used nanoparticles and polymers for the fabrication of nanocomposite membranes were reviewed. 2.1.3.1 Inorganic materials Inorganic nanofillers are classified based on their geometry: a) One dimensional (1D) nanotubes or nanoribbons fillers with cylindrical nanostructure with one diameter less than 100 nm [63]; b) two dimensional (2D) known also as layered materials with a thickness on the order of 1nm [64]; c) three dimensional (3D) nanoparticles, which are equi-axed particles with less than 100 nm in their largest dimension, and are the focus of this study. Nanoparticles can be categorized into metals (Pt, Au, Cu, Pd, etc.), metal oxides (SiO2, Al2O3, TiO2, ZrO2, Fe2O3, etc.) and semiconductors (ZnS, CdS, CdSe, etc.). Since the nanoparticles have a high surface energy, they tend to agglomerate into micro-powders or even macroscopic materials. Researchers have tried to use a variety of synthetic approaches to control the nanoparticle size and the degree of agglomerations. Basically, there are two main nanoparticles fabrication routes: gas phase [65-67] and liquid phase [68, 69] method. The gas phase synthetic approaches include flame hydrolysis, gas condensation, chemical vapour condensation and laser ablation. On the other hand, the liquid phase consists of co- precipitation, sol-gel processing, micro-emulsion, hydrothermal and solvothermal processing and templated synthesis. In order to provide a desired effect in an application, a careful selection of nanoparticle is necessarily. An appropriate selection of nanoparticles type and quantity along with a proper dispersion could result in a substantial improvement in the mechanical properties (tensile strength, stiffness and toughness), gas barrier, dimensional stability, thermal expansion, thermal conductivity, ablation and chemical resistance and antifouling properties. On the contrary, an inappropriate selection of nanoparticle type, quantity and poor dispersion may result in viscosity increase (limits processability), optical and sedimentation issues as well as mechanical defects [21]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 19 Table 2-4 Most common nanoparticles used in nanocomposite membrane preparation for liquid separation Nanoparticle Size Selected Polymer used as matrix Application Type (nm) references Polyethersulfone (PES), Polysulfone (PSF) Polyacrylonitrile (PAN), Ultrafiltration, TiO2 20-30 Polyvinylidene fluoride Microfiltration, [56, 70-72] (PVDF), Polystyrene-alt- Reverse osmosis maleic anhydride (SMA), Polyamide (PA) Polyvinylidene fluoride Al2O3 10-48 (PVDF), Polyethersulfone Ultrafiltration [61, 73-76] (PES) Polysulfone (PSF), SiO2 3-16 Polyamide (PA), Ultrafiltration [77-80] Polyethersulfone (PES) Polysulfone (PSF), Polyvinylidene fluoride ZrO >100 Ultrafiltration [59, 81, 82] 2 (PVDF), Polyethersulfone (PES) Polysulfone (PSF), Ultrafiltration, Ag 1–70 Polyamide (PA), [83-86] Nanofiltration Cellulose acetate (CA) zeolites 50-200 Polyamide (PA), Reverse osmosis [87, 88] Polysulfone (PSF), Ultrafiltration, Carbon 10-20 Polyvinylidene fluoride Microfiltration [89-92] nanotube (PVDF) Pervaporation 2.1.3.1.1 Nanoparticles in membrane technology A variety of nanoparticles have been used in membrane technology to mitigate fouling and improve the thermomechanical properties of membranes as well as its filtration performances including higher flux, permeability, rejection and life time [93]. Table 2-4 shows the most common nanoparticles with their applications used for the fabrication of nanocomposite membranes. Antibacterial properties of metallic nanoparticles particularly silver (Ag) were used to reduce the fouling [83-86]. The interesting catalytic properties of metal oxides nanoparticles particularly TiO2 were also applied to mitigate membrane fouling by giving the composite membranes a built-in oxidative functionality [93]. However, there are some reports that metal oxides can improve membrane performance and the fouling properties of membranes by affecting the membrane skin layer morphology and microstructure as well as its surface chemistry. For morphology and microstructure, metal oxides could affect both the porosity, pore size and roughness of the skin layer and also improve the membranes Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 20 compaction behaviour through altering the macrovoids [94]. For the surface chemistry, the nanoparticles can change the wettability, surface charge and protein adsorption [73]. The focus of this research is on the incorporation of commercial and sol-gel synthesised TiO2 nanoparticles into in-house and commercial polymeric membranes. TiO2 nanoparticles were reviewed in section 2.3 in terms of properties, applications as well as synthetic approaches. 2.1.3.2 Polymer materials As can be seen from Table 2-4 , a variety of materials is used for polymeric membranes and can be categorized roughly into two groups: hydrophilic and hydrophobic polymers. The membranes which are categorized by hydrophilicity include cellulose based, polyacrylonitrile, hydrophilized polyethylene, hydrophilized poly (ether) sulfone, and so forth. The latter group consist of polyvinylidene fluoride, polypropylene, polytetrafluoroethylene and so forth [4]. In the first group, the goal is to achieve a stable and high level of filtration by avoiding the fouling relying on the hydrophilicity of the raw materials. On the other hand, the second group materials can be used to achieve a stable and high level of filtration by intensifying chemical cleaning as this class of materials show a high chemical resistance and mechanical strength over a long period. Considering the mass production and cost benefit analysis, it is very rare to find a polymer which satisfies both requirements. This is the main reason that the nanocomposite membranes with the properties of both groups have recently become a point of interest. H F SO2 O2 C C n n H F (PVDF) (PES) Figure 2-4 Molecular structure of PES and PVDF polymers In this thesis, polyethersulfone (PES) and polyvinylidene fluoride (PVDF) were chosen as the main polymers for superhydrophilic and superhydrophobic nanocomposite membrane fabrication, respectively (see Figure 2-4 for molecular structure). Polyethersulfone (PES) is one of the polymeric materials which commonly used in microfiltration [95-97], ultrafiltration [98, 99] as well as nanofiltration membranes [100]. Its wide application is a Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 21 result of good chemical and thermal resistance, easy processing and environmental endurance. However, its poor antifouling properties affect its application and usage life [15]. The incorporation of TiO2 nanoparticles into PES membrane could be a solution to increase its fouling performances. PVDF microporous membranes are intrinsically hydrophobic due to fluorine in the back bone of polymer chains and have application in the supported liquid membrane (SLM), membrane distillation (MD) and membrane contactors (MC) [1]. Although PVDF has a high mechanical and thermal resistance, its wettability is not considered in the range of superhydrophobicity and its application is limited. The fabrication of superhydrophobic PVDF nanocomposite membrane could potentially increase the performance of membrane distillation through increasing the air-gap thickness and reducing the risk of pore wetting and also increasing the antifouling property of the membrane [101]. In the next section, different techniques for the fabrication of nanocomposite membrane were reviewed. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 22 2.2 Membrane Nanocomposites Preparation Approaches As mentioned before, the incorporation of nanoparticles into polymeric membranes falls into two categories: blending (melt, solvent, sol-gel mixing and in-situ grafting) and coating (direct deposition and low temperature hydrothermal (LTH) process). In the former technique, the nanoparticles are dispersed into the flesh of the membranes. Achieving a homogenous uniform composite wherein the nanoparticles are dispersed uniformly is the major challenge in the blending approach. In the latter approach, the nanoparticles are deposited and coated on the top surface of the membranes. The most important issue is achieving a uniform coating with high stability and durability of the coated particles. The following sections tried to address the both approaches. 2.2.1 Blending 2.2.1.1 Melt mixing In this approach, the nanoparticles are mixed into the molten polymer and if the nanoparticles surfaces are sufficiently compatible with the selected polymer, a uniform nanocomposite could be achieved. Picard et al. used the technique for the preparation of Polyamide 6- montmorillonites nanocomposite membranes [102]. In their work, the permeation properties were related to the clay content and quality of dispersion. Polyethylene-aluminophosphate [103] and nafion-montmorillonite [104] nanocomposite membranes were also prepared by melt mixing technique. The technique suffers from the drawback of particles agglomerations as nanopowders have a very strong tendency to aggregate. To overcome this issue the surface modification of nanoparticles is suggested [105]. In addition to that the processing is energy intensive, as high shear forces are required to overcome the high viscosity of the molten mixture. 2.2.1.2 Solvent mixing Solvent mixing is a well suited technique for polymer materials which are difficult to melt. The approach is applicable if the polymer and inorganic powders are able to be dissolved and dispersed. Solvent mixing has been widely used with water-soluble polymers such as polyvinylalcohol (PVOH), polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), and polyacrylic acid (PAA) [21]. However most of the common polymers used in the membrane fabrication listed in Table 2-4 are dissolved in the organic solvent such as N-Methyl-2- pyrrolidone (NMP), Dimethylacetamide (DMAc), N,N-dimethylformamide (DMF) and, Tetrahydrofuran (THF). Although solvent mixing is the most common method for the fabrication of nanocomposite membranes [56, 70-72, 77-80], the particles agglomeration is Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 23 still an issue, but, in contrast to melt mixing the surface modifications of particles without drying to minimize agglomeration is conceivable [106]. After dispersion, the polymer/inorganic solution also called dope/casting solution can be casted on to solid surfaces such as glasses following by the extraction of solvent using phase separation process which was widely used for the preparation of polymeric membranes. Phase separation methods are categorized by four major techniques; thermal induced phase separation (solvent evaporation), vapour induced phase separation, control evaporation and immersion- precipitation or non-solvent induced phase separation [107]. In immersion-precipitation which was applied in this PhD theses, a solution of polymer and organic solvent is cast on a substrate and immersed in a coagulation bath to form the membrane [108, 109]. 2.2.1.3 Sol-gel mixing In the direct mixing approaches (melt and solvent mixing), the commercial or beforehand prepared nanoparticles (using gas or liquid synthesis methods which were introduced in section 2.1.3.1) are added to the polymer. These nanoparticles have a high surface energy and thereby high tendency to agglomeration. As mentioned before, the surface modification of nanoparticles can be useful to minimize the agglomeration which is the main obstacle to make a homogenous nanocomposite. Alternatively, researchers have tried to use the in-situ formation of nanoparticles within the polymer matrix by sol-gel technology [14]. The sol-gel mixing nanocomposite fabrication is an ideal procedure at the milder temperature for the formation of interpenetrated networks of organic-inorganic composites. This can be achieved via the reaction of an inorganic alkoxide directly with an organic polymer or an oligomer which have the appropriate functional groups [110, 111] resulting in a chemical bond or just physical mixing between the inorganic and organic networks [105]. Sol gel processing has two main consecutive stages: hydrolysis and condensation, which are explained by detail in section 2.4. Controlling the sol gel processing parameters such as hydrolysis and condensation reaction rates, the type of solvent, water to alkoxide ratio, the type of catalyst, reaction temperature and organic additives (templating agent) could result in different nanoparticle sizes and structure [102, 112]. There are two main strategies for the fabrication of nanocomposite materials through the sol gel processing: the consecutive [70, 103] and simultaneous [104, 105] formation of inorganic-organic networks. The consecutive approach can be performed in different ways: The simplest way is preparing the inorganic network first using sol gel and then adding it to the dissolved polymer to interact via strong covalent bonds or weak secondary forces (there is Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 24 not any more hydrolysis and condensation after the addition). The second way is polymerization of organic functional groups from a preformed sol–gel network [105, 110]. In the third way, the sol–gel hydrolysis and condensation of a precursor is carried out starting from a preformed functional organic polymer [111, 113]. McCarthy et al. used the combination of second and third ways to prepare polydimethylsiloxane (PDMS)-silica nanocomposites [114]. In the simultaneous synthesis approaches also known as in-situ techniques, the inorganic and organic networks form simultaneously through the sol-gel reaction and the polymerisation reaction occur at the same time. Using this technique, a strong chemical reaction between organic and inorganic can be carried out at lesser amount of solvent. Although this technique has gained widespread use, there are a few disadvantages. For instance, during polymerizations the sol needs to be extremely stable to prevent agglomeration particularly at higher temperatures commonly used for initiation. A careful tuning between sol-gel and polymerisation kinetics is vital to form uniform nanocomposites. If the hydrolysis and condensation reactions proceed much faster than organic polymerization, it may result in the large agglomeration and nonhomogeneous composite. In addition to that the inappropriate selection of solvents and chemicals may result in undesired side reactions [26, 115, 116]. 2.2.1.4 In-situ grafting In the in-situ graft polymerization, at first the nanoparticles were dispersed in the monomer or monomer solution and then the resulting mixture is polymerized by conventional polymerization methods. In-situ polymerization has the advantage of controlling the agglomeration of nanoparticles via the layer of polymer bonded to the nanoparticle. As can be seen in Figure 2-5 , the grafting approaches can be conducted by the means of “grafting to” or “grafting from” polymerizations [26]. In “grafting to” approaches, the polymerizable groups are immobilized directly to the nanoparticles surfaces whereas in “grafting from” approaches an initiator is grafted to the nanoparticle surface first and then the polymerizable group is immobilized through the initiator. Although the “grafting to” approach is simpler than the “grafting from” one, it has not gained widespread use due to difficulty of reaching the high grafting density. Moreover, the grafting density being limited by space constraints around the active sites and the situation becomes worst as the layer thickness increases consequently the process becomes self-limiting [117, 118]. On the other hand, the “grafting from” approach has received significant attention for the grafting of polymers on the inorganic building blocks. In this approach, the appropriate surface density can be achieved Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 25 through choosing the initiator and anchor groups rightly along with a precise control of polymerization process. It should point out here that the in-situ grafting approaches for the preparation of inorganic-organic nano-composite membrane for liquid separation and water purification applications has not widely been reported. Inorganic-organic core shell Inorganic particles nanoparticles A Grafting to A IN IN IN IN Grafting From ININ A: Anchoring group IN: Initiating group Figure 2-5 In-situ grafting polymerization approaches (grafting to and grafting from) 2.2.2 Coating Basically there are two main approaches for the fabrication of nanocomposite membranes: blending the nanoparticles into the membrane as it was described in section 2.2.1, and coating the nanoparticles onto the surface of the membrane [15-17]. As mentioned before, the blending approach can be easily implemented but limits the loading of inorganic particulates and may alter the membrane morphology during the phase inversion casting process. Due to the agglomeration, uniform distribution would be difficult to achieve in the absence of any surface treatment of nanoparticles. During the membrane process the first point where foulants reach and interact is the membrane surface thereby changing the interaction between the foulant and surface via coating a layer of inorganic nanoparticles is an alternative approach to mitigate fouling. However, the main challenge in the coating approach is it may result in the non-uniform distribution of the nanoparticles on the membrane surface and instability of coating layer. Coating technology is a broad method which has been widely used for different applications and it may apply as liquid, gas or solid. A variety of coating techniques for membrane surface modification has been addressed in the literature such as chemical vapour deposition (CVD) [119, 120], physical vapour deposition (PVD) [121], Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 26 chemical (solution adsorption and sol-gel) [15-17] and plasma spraying [122] coatings. The techniques are mostly used for the modification of ceramic membranes with the gas separation application. CVD, PVD and plasma spraying have a few drawbacks which limit their application for polymeric membrane fabrication. Those drawbacks are they are expensive due to the need for a high vacuum, require high energy usage, need to work at high temperature and pressure, and are limited in the geometry of support to a flat substrate [121]. However, the chemical routs are more applicable for polymeric membranes since they are feasible to apply at low temperature and pressure and also consume far less energy. In this review, the focus is on coating the inorganic nanoparticles on the polymeric UF and MF membranes using chemical routs. Membrane nanocomposite fabrication via chemical coating rout has two main parts: preparing nanoparticles coating solution and depositing the prepared nanoparticles on the membrane surface. The common way of coating approach is preparing nanoparticles coating solution by dispersing the commercial or preformed nanoparticles with and without surface functionalization into water or alcohol base solvent. In the next stage, the nanoparticles are deposited on the surface of the membrane through different coating approaches such as solution adsorption [17], vacuum filtration [123, 124], dip-coating [125] and spin-coating [125, 126]. In the solution adsorption coating (Figure 2-6 a), after the coating solution is prepared, the membranes are immersed in the coating solution for a limited period of time. In this way the nanoparticles were absorbed to the membrane surface and pores due to chemical potential gradient between bulk and membrane surface. The main problems in this method are difficulty in controlling the coating thickness and achieving uniform stable coverage. For vacuum filtration coating, the membrane is placed within a Buchner funnel that is attached to a conical flask-vacuum pump setup as shown in Figure 2-6b and then the nanoparticle coating solution is filtered through the membrane. This technique is easy to implement and provides more control on coating thickness. However, the technique required a careful selection of particles and membrane pore size. If the pore sizes being much bigger than the particle sizes, the particles will pass through the wide channels by hydraulic flow without the chance to adsorb to the surface. On the other hand, if the pores be too small, a more vacuum power is required and also coating may turns into a loose cake layer rather than a robust film. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 27 Membrane Membrane (a) (b) Membrane To vacuum (c) (d) Figure 2-6 Different coating techniques (a) solution adsorption (b) vacuum filtration (c) spin-coating (d) dip-coating Spin coating (Figure 2-6c) is a procedure for coating a thin layer on the flat surfaces by taking the advantage of centrifugal and viscous force where they acting against each other [127]. The detail mechanism of the flow of a Newtonian liquid on a rotating disks was explained by Emslie et al. [127].They tried to describe the importance of spin speed, liquid viscosity, and spin time on the film thickness. A typical spin-coating process consists of a dispense step in which the nanoparticle coating solution is deposited onto the substrate surface, a high speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. In this method, the main disadvantage is the difficulty of using porous substrates such as membranes particularly at larger pore sizes. Moreover, vacuum power is usually used for Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 28 sticking the substrate on the stage and for porous substrates there is a risk of suctioning the coating solution into the pores rather than spreading them on the surface. Dip-coating is a very common approach for creating thin films. As can be seen in Figure 2-6 d, the coating process consists of three stages: (a) immersion (b) withdrawal (c) evaporation. During the immersion step where the effect of capillary-filtration dominates, the membrane is immersed into the coating solution. Due to the capillary forces, the solvent sucks into the pores and causes particles to accumulate and deposit on the membrane surface. After immersion, the membrane usually holds in the solution for a while to assure pores are filled with particles solution. In withdrawal step, the effect of film coating is dominated and the drag force exerted by the membrane during withdrawal results in a layer of particle on the surface of the membrane. The faster the substrate is withdrawn, the thicker the film deposited [127]. During the withdrawal step, the tangential flow of the suspension against the membrane sweeps away the loosely attached particles and also wicks the particles into some unfilled pores [128]. Finally, during the evaporation, the solvent evaporates to leave behind a thin layer of inorganic coating. In the case of volatile solvents, such as alcohols, evaporation could start from withdrawal step. In order to have a uniform coating and enough coverage, the dip-coating procedures often need to be repeated. TiO2 nanoparticles have been coated on the surface of the polymeric membrane using the conventional coating approaches [15-17, 127, 129], which is the deposition of commercial nanoparticles (mostly Degussa P25) on the surface of the membranes. However, achieving uniform coating with high stability is still one of the main issues. An alternative approach which may potentially result in a more comprehensive and uniform coverage of TiO2 on the membrane surface is the coating of titania sol followed by heat treatments to generate a crystalline TiO2 film. Further details of this technique were explained in section 2.3.3. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 29 2.3 Titanium Dioxide (TiO2) Titanium dioxide (TiO2), also called titanium (IV) oxide or titania, was initially discovered in 1821 but commercialized in 1916 as white pigment [129]. There are at least 8 titanium dioxide structures: four naturally polymorphs (i.e. rutile, anatase, brookite, TiO2 (B)) and four high pressure laboratory synthesized (TiO2 (II), TiO2 (H), baddelleyite and cotunnite) [118]. Among them, rutile, anatase and to some extend brookite are the most common structure and manufactured worldwide. Thermodynamically at room temperature, rutile is more stable than brookite or anatase, but for ultrafine particle (smaller than 15nm) anatase has been found to be the most stable since stability is particle size dependant [130]. This may be the reason that the sol gel processing is very efficient in the preparation of anatase phase [131]. Since the preparation of highly pure brookite is difficult, most of the studies have been focused on the rutile and anatase [116]. Table 2-5 summarizes the main properties of rutile, anatase and brookite form of TiO2. Table 2-5 Properties of the most three common phases of TiO2 i.e. rutile, anatase and brookite After TiO Main property and Selected 2 Molecular structure heating Stability phase application reference converts to Tetragonal structure Highest refractive with two edges of The index (2.65-2.95) the octahedra [132, Rutile - most mostly used as being shared to form 133] stable pigment and in zigzag chains and optical device shares corner oxygen Tetragonal structure, The most which shares four photoactive form of Anatase edges of the Rutile Stable TiO2, used as a [133] octahedra and no catalyst or corner oxygen catalyst support The largest cell Orthorhombic volume (8 TiO2 structure and three The groups per unit cell, [134, Brookite edges are shared Anatase least 4 for anatase and 2 135] between stable for rutile), mixing neighbouring units with anatase form for catalytic applications Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 30 2.3.1 TiO2 applications and its main surface property Titanium dioxide is the most widely used white pigment with the annual world consumption of above 4.4 million tons [136] due to its brightness and very high refractive index. Its applications are ranged from paints, plastics, paper and cosmetic products to photocatalyst, gas sensor and electrical and photochromic devices. As can be seen in Table 2-6, the application of TiO2 nanomaterials are classified into four groups according to the base properties of TiO2 nanomaterials [129]. Table 2-6 Four different categories of TiO2 nanomaterials applications TiO property Selected 2 Application used reference Optical UV-protections: sunglasses, window [137-139] Environmental protections: decomposition of organic/inorganic pollutant in the air and water, for out- [140] and in-door air and water purification Self-sterilizing materials for hospital and indoor wall [141] Photocatalytic application As photocatalyst for cancer therapy [142] As photocatalyst for water-splitting and hydrogen [143] production As active electrode for dye-sensitized solar cell and [144] photochromic applications Superhydrophilic/ Self-cleaning and anti-fogging materials for building, [145-147] superhydrophobic window and mirrors Electric Sensors for gases and humility [148, 149] These broad applications are based on the nature of TiO2 as a semiconductor material. In semiconductors, the energy band gaps are not too large and if the semiconductor absorbed light or photon with energy equal or greater than the band gap, electrons will jump from valance band (VB) into the conduction band (CB) and generate holes (electron deficiencies) [150, 151]. These pair of electrons and holes (charge carriers) can be recombined by releasing heat. Alternatively, these charge carriers can be presented at the semiconductor surface to carry out photo-oxidation or photo-reduction reactions with chemical species [152]. The photocatalytic ability of TiO2 coatings has been applied for the generation of self- cleaning and antifouling surfaces by turning the titania film into superhydrophilic surface. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 31 When the surface becomes superhydrophilic, it absorbs more water molecules rather than impurities and organic matters and resists adsorption [153-155]. It has been widely reported that TiO2 films will turn into superhydrophilic surfaces when expose to UV light. This phenomenon has been termed photo-induced superhydrophilicity (PSH) [156-159]. The PSH is initiated by the photo-generation of electrons and holes and their migration to the surface. However, the exact mechanism of this photo-induced change has been remained controversial. Basically, there are two main theories: photocatalytic oxidative (PCO) process and photo-induced surface reorganisation (PISR). In the PCO model, it is believed that the TiO2 inherently is hydrophilic [160] but adsorbed organic hydrophobic matters make the surface hydrophobic [153]. The photo generated holes degrade the adsorbed organic species by the formation of reactive oxygen species (ROS) and bring the surface back to its intrinsic superhydrophilic surface via the following reaction: ୟ୬ୢ୧మ ଶሻ 2-1 ଶ ሱۛۛۛۛۛۛۛۛሮ ሺǤ Ǥ ǡ ଶ In PISR, the dissociation of water at surface defects which were created by photo-generated holes causes an increase in the number of titanol groups (Ti-OH) and results in superhydrophilic domains and leaves the rest of the surface area hydrophobic or oleophilic [161]. According to Feng and Jiang [162], the position of hydrophilic domains are higher than that of hydrophobic areas and these hydrophilic and hydrophobic walls form channels with 0.4-6nm in width for water to flow, which is similar to a 2D capillary phenomena [163]. By switching the UV light and in the presence of O2, the surface defects will be removed by substituting the chemisorbed hydroxyl group with oxygen and therefore bringing the surface back to its initial hydrophobic nature, which is in contrast with the PCO model [153, 158]. The photo-induced surface reorganisation model is shown as below. ౚోమ ؠ୧ି ሱۛۛۛۛۛۛۛۛሮ ؠെ ଶ ርۛۛۛۛۛۛۛۛሲ 2-2 ؠ୧ି ౨ౡ ؠെీ 2.3.2 Effect of incorporation of TiO2 on the polymer properties TiO2 nanocomposite can be fabricated through different methods of melt, solvent, sol-gel mixing and in-situ grafting as explained in section 2.2 and a few examples were summarized in Table 2-7. In all of the mentioned techniques, the most important factor to consider is the requirement of good TiO2 dispersion in the polymer matrix. The quality of dispersion plays a major role in the ultimate properties of nanocomposite. In literature, there has not been yet a consensus on how TiO2 nanoparticles could affect the polymer properties. This is partly Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 32 because of the novelty of the area and lack of systematic experimental data as well as scarcity of theoretical backgrounds. However, some of the properties such as thermomechanical, optical, morphological and electrical have been recently studied in depth, which are reviewed selectively as follow. Table 2-7 TiO2 nanocomposite with different polymer matrix and synthesis method Selected Method Polymer references Melt mixing low-density polyethylene (LDPE) [127] poly-3-hydroxy-2,3-dimethylacrylic acid (PDAA), Solvent Polystyrene and polycarbonate, polyvinyl chloride (PVC), [8, 127, 129] mixing polyethylene glycol (PEG), and polyethylene oxide (PEO) poly(ethylene oxide), poly(p-phenylenevinylene) (PPV), [131, 132, Sol-gel mixing Poly(methyl methacrylate) 134] In-situ grafting styrene [130] 2.3.2.1 Effect of TiO2 on the thermomechanical properties of nanocomposites The effect of TiO2 nanomaterials on the thermomechanical properties of polyimide (PI) has been investigated by Chiang et al. [133]. They found that the incorporation of small amount of TiO2 nanoparticles enhances the mechanical properties of the polyimide composite at both low and high temperature. In order to examine the thermomechanical properties, a series of parameters such as elastic modulus, elongation, yield stress, thermal stability, and glass transition temperature (Tg) were investigated in their work. Elastic modulus and elongation coefficient increased and decreased respectively with increasing the volume fraction of TiO2 additions. Yield stress increased slightly followed by a steady drop as filler content increased. Tg is also increased to the higher temperature but thermal stability reduced marginally. The effect of TiO2 on the thermomechanical properties of polybenzoxazine has been investigated in another study by Agag et al. [135]. Viscoelastic properties and thermal stability (in contrast to the previous study) have been improved significantly after the small addition of TiO2. An increase in storage modulus and Tg due to the restriction in the micro-Brownian motion of polybenzoxazine segments leading to reinforced polymer matrix were also reported in their work. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 33 2.3.2.2 Effect of TiO2 on the optical properties of nanocomposites The great optical properties of TiO2 nanocomposites come not only from high refractive index of TiO2 and strong UV absorption up to visual wavelength but also from not scattering light significantly and transparency at the visible wavelength. Yoshida et al. [136] reported that the incorporation of TiO2 at micron size into polyamide resulted in a non-penetrating light composite due to scattering light by the large TiO2 particles. However, if the TiO2 size reduces into nanometre range, the nanocomposite exhibited an optical propagation loss of 1.41db/cm similar to that of neat polyamide and also an increase in the refractive index from 1.55 to 1.56, which functions greatly for optical waveguide application. Similar to Yoshida et al.’s results, transparency and an increase in the refractive index from the addition of TiO2 nanoparticles were also reported for other polymer matrix such as polymethyl methacrylate [132], polybenzoxazine hybrid [135], poly(3-hydroxy-2,3-dimethylacrylic acid) [8] and poly(p-phenylenevinylene) [131]. 2.3.2.3 Effect of TiO2 on the morphological properties of nanocomposites The morphological properties of polymers have been related to the degree of crystallinity, intermolecular regularity, the homogeneity of the organic-inorganic phases and also interaction between the nanoparticle surface and the surrounding polymer. Ma et al. has investigated the effect of different agglomeration sizes of TiO2 nanoparticles on the crystalline structure of low-density polyethylene (LDPE) [127]. Their results did not reveal any relation between the different agglomeration of nanoparticles with/out surface modification and the degree of LDPE crystallinity, the unit cell dimensions, the average lamellar thickness, or the average spherulite size (spherical semicrystalline regions inside non-branched linear polymers). However, it was observed that the addition of nanoparticles did cause internal spherulite disorder which means the organization of lamellae (or bundles) within the spherulites was affected by the presence of the nanoparticles in the interfibrillar regions. It was also found that the larger the agglomeration, the higher the internal disorder. On the other hand, in another study reported by Chiang et al., the x-ray diffraction spectroscopy (XRD) results showed that the incorporation of TiO2 nanoparticles disrupted the intermolecular regularity of polyimide films leading to a disappearance of a highly ordered crystalline phase [133]. Morphology of the polymer can be affected by the degree of dispersion. Agag et al. managed to prepare a homogenous TiO2-polybenzoxazine hybrid film by the incorporation of 2 wt.% of TiO2 with particle size of less than 100nm [135]. Further increase in the TiO2 loading resulted in a large agglomeration on the surface of the composite. The interaction between the nanoparticle surface and the surrounding polymer is Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 34 an important parameter that dominates the polymer morphology. The dispersed nanoparticles could enhance or restrict the chain mobility near the particle surface and consequently affect the morphology of the nanocomposite [127]. 2.3.2.4 Effect of TiO2 on the electrical properties of nanocomposites One of the optical properties that can be altered by the inclusion of TiO2 is dielectric constant which is an indication of polarizability of the molecules and their ability to store electric energy. Chiang et al. managed to increase the dielectric constants of polyamide from above 3.25 K for neat polymer to below 4 K for 9 wt.% TiO2 loading polyamide nanocomposite with a reverse trend for surface and volume resistivity [133]. Another electrical parameter is the electrical breakdown strength which means an electric-field intensity that causes the “insulator to conductor” transition in a material. The impact of TiO2 inclusion on the breakdown strength of low-density polyethylene (LDPE) nanocomposites was investigated by Ma et al. [164]. They found that the breakdown strength is directly influenced by the surface chemistry, such as the presence of surface water or surface silane. It was found that the breakdown strengths for non-water absorbed (dried) and silanized TiO2 nanocomposite are 50% and 40%, respectively, higher than that for the samples filled with as-received nanometre scale TiO2. The effect of TiO2 on the nanocomposites electrochemical properties such as ionic conductivity, Li- transference number, and stability window have also been studied for battery applications. Liu et al. prepared TiO2-poly (ethylene oxide) nanocomposite with the addition of lithium tetrafluoroborate (LiBF4) as the lithium salt and studied the effect of TiO2 on its electrochemical properties [134]. It was found that the ionic conductivity of nanocomposite is significantly higher than that of neat polymer (an order of - magnitude). A high Li transference number of 0.51 was observed, and the 10 wt.% TiO2 loaded nanocomposite electrolyte was found to be electrochemically stable up to 4.5 V versus Li-/Li. Similar study was conducted by Yan-Jie Wang and Dukjoon Kim for in-situ formed PVDF- LiClO4-TiO2 nanocomposite polymer electrolytes [165]. 2.3.3 TiO2 nanocomposites membranes In recent years, the incorporation of inorganic additives into polymeric materials has expanded markedly for filtration and gas separation membranes. As noted before, a variety of nanoparticles have been introduced to modify organic membranes, such as SiO2, Al2O3, Fe3O4, ZrO2 and TiO2 [12, 59-61]. Among them, TiO2 has received the most attention because of its good physical and chemical properties, availability as well as its potential antifouling abilities [9-11]. As mentioned before, there are two main approaches for the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 35 fabrication of TiO2 nanocomposite membranes: (1) blending the nanoparticles into the membrane [12-14] and (2) coating the nanoparticles onto the surface of the membrane [15- 17]. The blending approach can be easily implemented but limits the loading of inorganic particulates and may alter the membrane morphology during the phase inversion casting process. Uniform distribution would be difficult to achieve in the absence of any surface treatment of nanoparticles. The coating approach, on the other hand, has the drawback that it may result in the non-uniform distribution of the TiO2 nanoparticles on the membrane surface and the instability of coating layer. In this section, the incorporation of TiO2 nanoparticles into polymeric membranes with the view of application in the liquid separation and water purifications was reviewed. In addition, an attempt has been made to address the main issues, lack of knowledge in the literature and non-investigated areas in the preparation and application of TiO2 nanocomposite membranes. Polymeric materials such as polyethersulfone (PES) and polyvinylidene fluoride (PVDF) are commonly used in microfiltration [95-97], ultrafiltration [98, 99] as well as nanofiltration membranes [100]. Their wide application is a result of good chemical and thermal resistance, easy processing and environmental endurance. However, their poor antifouling properties affect its application and usage life [15]. The incorporation of widely available commercial titania powders into polymeric membranes is one of the strategies to improve antifouling performance of the membranes. Compare to coating approach this method is simpler since the particles are added to the membrane casting solution. Furthermore, coating of membranes can lead to some significant undesirable changes in membrane permeability due to pore narrowing or plugging. The effect is more critical in the case of UF membranes due to smaller pore size. There is also potential of delamination of a coating layer. In blending approach, as explained in section 2.2.1, the TiO2 nanoparticles are dispersed in a casting solution and then the membranes are cast by phase separation method which was widely used for the preparation of polymeric membranes. Although the primary particle size of commercial TiO2 nanoparticles such as Degussa P25 is about 20nm, its particle size as powder or in dispersion is in the range of hundreds of nanometres due to agglomeration. This agglomeration leads to not only the uneven distribution in the membrane but also potential reduction in antifouling abilities of TiO2 particles by changing parameters such as membrane topography and hydrophilicity as well as self-cleaning properties of particles [166]. The presence of hills and valleys on the surface of a rough membrane increases the sites which favour the attachment of foulants on the surface of the membrane [167]. It is also evident that Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 36 membranes with higher hydrophilicity are less prone to fouling than hydrophobic membranes since it absorbs more water molecules than foulant molecules. It was shown that TiO2 nanoparticles can effectively degrade organic materials under UV light. The agglomeration of particles reduces active sites for the degradation of organic materials [168]. All of these deficiencies caused by agglomeration negatively affect the effective utilization of TiO2 nanoparticles in the nanocomposite membranes. To overcome agglomeration, researchers have tried to use the in-situ formation of TiO2 within the PES matrix by sol-gel technology [14]. Using this technique the nanoparticle size can be controlled by varying various parameters including the choice of organic additives [112]. However, agglomeration still exists even for the in-situ formation of particles due to the poor interfacial interactions between the hydrophilic nanoparticles and hydrophobic polymer and high surface energy of particles [169]. In addition, due to the difficulty and complexity of sol-gel reactions and also cost issues, the potential of using commercial TiO2 nanoparticles to improve PES membrane properties provides an attractive alternative. In order to avoid agglomeration of commercial nanoparticles into membranes, two methods have been generally tried: the dispersion of nanoparticles by conventional methods such as sonication and grinding (mechanical modification) and surface pre-treatment approaches for nanoparticles (chemical modification). Most researchers applied the former approach that is based on the shear forces provided by conventional mixers and/or normal sonicators [10, 17]. Since the intra-nanoparticles interaction are very strong, it is hard to break the intra-particle interactions with conventional mixing [170]. Thus, it seems crucial to have a stronger dispersion technique for the fabrication of nanocomposite membranes. In general, surface modifications as a complementary approach to mechanical modification of particles has been used for minimizing particle/particle interaction in the preparation of polymer composite with microfillers [169]. Recently, with development in nanotechnology, the incorporation of chemically modified or functionalized nanoparticles into polymeric materials has become a topic of great interest. In this approach, the inorganic particles are coated with organic coatings by physical and/or chemical interactions between the particles and organic modifiers. In physical treatment, the reaction between organic surfactant or polymer and particles results a weak secondary forces such as van der Waals, hydrogen and electrostatic forces while in the chemical treatment there is a strong covalent attachment. However, the resultant bond is often a combination of chemical and physical reactions. One common method to apply technique for chemical modification of inorganic particles is treatment by Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 37 silane coupling agents such as 3-methacryloyloxypropyl trimethoxysilane (MAPTMS), 3- aminopropyltriethoxysilane (APTES), and methacryloyloxy methylenemethyl diethoxysilane (MMDES) [171]. Silane coupling agents can effectively reduce the hydrophilic nature and surface energy of the particles consequently reducing agglomerations and increasing matrix interactions [169, 171]. Li et al. investigated the effect of incorporation of an unmodified Degussa P25 (21nm particle size, 80% anatase ) on the PES microfiltration membrane in terms of surface chemistry and morphology [95]. They found that the addition of TiO2 could produce membranes with higher hydrophilicity, better thermal and mechanical stability as well as better permeation performance. Although the maximum flux and pore size were obtained when the TiO2 content was 4–5 wt.%, they recommended 1–2 wt.% TiO2 nanoparticles in the casting solutions as the optimum level. At higher concentrations the agglomeration of particles led to a lower stability in membrane performance. Consequently, they had to compromise the higher flux in the favour of stability as a result of agglomeration. The fouling performance of the TiO2 blend membrane was not, however, investigated in their work. The effect of unmodified Degussa P25 TiO2 nanoparticles on the performance of PES ultrafiltration membrane for the milk industry was also investigated by Rahimpour et al. [17]. They incorporated the inorganic particles into a PES membrane by two approaches: blending and coating. In their blending approach, contrary to the results presented by J. Li e. al., the initial pure water flux of TiO2 blend membranes decreased; however, the membrane antifouling property and long term flux stability were enhanced for up to 4% TiO2 content and a further increase of TiO2 concentration did not change the antifouling properties of the membranes. The authors believe that during immersion the precipitation of TiO2 nanoparticles could plug pores at higher concentrations and consequently hinder the interaction between PES and solvent molecules. The addition of TiO2 to membranes also increased the hydrophilicity of their membranes. However the effects were not significantly changed by varying the inorganic content. The quality of dispersion and the effect of TiO2 on the surface chemistry of the membranes were not investigated in their work. TiO2 rutile nanoparticles (30 nm), modified with J-aminopropyl triethoxysilane, were used for modification of PES ultrafiltration membrane by Wu et al. [13]. The addition of TiO2 nanoparticles improved the hydrophilicity, thermal stability, mechanical strength and anti- fouling ability of membrane. The best performance was achieved at 0.5 wt.% TiO2 content Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 38 while at higher TiO2 content, the defective pore structure of the membranes and the decline of the performances were observed. Rahimpour et al. and Li et al. both reported that Degussa P25 with 80% anatase phase could change the structure of the membrane significantly, while Wu et. al.’s results showed that the 30nm rutile TiO2 nanoparticles did not affect the structure of the membranes significantly. The effect of different TiO2 phase and size of rutile (primary diameter 26–30nm) and anatase nanoparticle (average diameter10nm) on the ultrafiltration PVDF membranes was investigated by Cao et al. [10]. It was found that the anatase TiO2 nanoparticles with smaller nanoparticles performed better during BSA filtration. According to their results, the TiO2 nanocomposite membrane with smaller nanoparticles had smaller mean pore size, smoother surfaces and more apertures inside the membrane. Bae and Tak [9] prepared two types of TiO2 incorporated ultrafiltration membranes (TiO2 blended and coated membranes) in order to evaluate TiO2 fouling mitigation effect on the activated sludge MBR filtration. In their work three different polymers including PSf (polysulfone), PVDF (polyvinylidenefluoride) and PAN (polyacrylonitrile) were used. They did find that regardless of polymeric materials, the blending of TiO2 nanoparticles led to a lower flux decline and better fouling mitigation compared to that of neat polymeric membrane. Also, the addition of TiO2 content up to a certain level, for example TiO2-PSf ratio of 0.3, can change the membrane fouling performance. Coating of TiO2 showed greater fouling mitigation effect compared to that of TiO2 blended membrane, since the higher concentration of nanoparticle was on the membrane surface. However, the stability issue of coating of TiO2 nanoparticles was addressed as a main issue in their work and remained for the further investigation. Incorporation of TiO2 into PSF membrane has also been investigated by Yang et al. [172]. Their results showed that the inclusion of TiO2 nanoparticles could improve membrane performance and fouling mitigation at 2 wt.% content. However, nanoparticles agglomeration and a decline in performance were observed at higher TiO2 content (than 2 wt.%). TiO2 nanoparticles were coated or self-assembled (term used by authors) on thin film composite membranes (TFC) (Kim et al. [42], Kwak et al. [72] and Madaeni and Ghaemi et al. [173] ). A significant antifouling property particularly under UV irradiation was observed for the filtration of E-Coli and Whey. In their work, the antifouling strength for biofouling was proposed as below: Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 39 TiO2-Membrane +UV >> Membrane+ UV > TiO2/Membrane > Membrane Only TiO2 nanotubes were grafted in the channels of alumina microfiltration (MF) membrane for HA (humic acid) filtration by Zhang [174]. Their results showed that under UV irradiation not only HA were rejected and photodegraded, but also the membrane fouling was improved significantly. They also tried to fabricate TiO2 nanowire membrane via hydrothermal synthesis-filtration method [175]. The fabricated nanowire membranes presented a substantial performance on the concurrent filtration and photocatalytic degradation of HA in water. Hollow fiber TiO2 nanocomposite membrane The majority of studies have been focused on the flat sheet membranes while there have been much fewer investigations on the nanocomposite hollow fibre membranes. Hollow fiber membranes are desirable because of the larger membrane area per volume of module, high flexibility as well as ease of handling in the module fabrication [176]. Recent studies on the TiO2 hollow fiber nanocomposite membranes have been directed mostly on PVDF [177-180] and polyimide [181] composite membranes rather than PES membranes. Yuliwati et al.[177] fabricated PVDF ultrafiltration (UF) nanocomposite hollow fiber membranes for the treatment of refinery waste water. They found that the 1.95 wt.% of TiO2 content leads to the optimal performance when it is blended with 0.98 wt.% LiCl.H2O. At this TiO2 concentration, a small pore size, high porosity and hydrophilicity were observed. However, the effect of TiO2 without LiCl.H2O has not been investigated in their work. To prepare UF nanocomposite hollow fiber membranes, different concentrations of TiO2 nanoparticles along with SiO2 and Al2O3 were blended in PVDF by Han et al. [178]. The highest rejection (approximately 100%) of BSA was observed for 1 wt.% of TiO2 (1 wt.% SiO2 and Al2O3) content membranes whereas the highest water permeation flux of 352 L.m-2.h-1.bar-1 was reported for 2 wt.% of TiO2 (1 wt.% Al2O3 ) content membranes. The effects of TiO2 alone were not investigated in their work. Yun et al. [180] compared the sol-gel method versus blending method for preparation of the TiO2-PVDF nanocomposite UF hollow fiber membranes. Sol-gel methods resulted in a better dispersion of particles and a stronger interaction between TiO2 and polymeric matrix. Although an increase in the hydrophilicity and permeability were observed by the increase of TiO2, the retention property of their membranes did not changed significantly. The best performance and mechanical property were reported at 1 wt.% of TiO2 for both methods. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 40 It seems that the focus of the previous studies on hollow fiber composite membranes was mostly on the improvement of permeation and water flux rather than improving antifouling performance of composite membranes. It is also unclear whether particle aggregation was a serious concern as it could be the main source of inconsistencies in outcomes. The effect of TiO2 addition and concentration on the chemistry and structure of the composites has not also been studied in depth particularly during phase inversion and membrane casting. Given that the main weaknesses of hollow fibre membranes relative to other membrane types are their difficulty in preparation and vulnerability to fouling, any possibility of improving their preparation techniques and the membranes antifouling performance will definitely be of great interest for future research and development. TiO2 nanocomposite ceramic membranes In the literature there are a limited number of papers on the incorporation of TiO2 nanoparticles into ceramic membranes for liquid separation and water purifications. Choi et al. [182] reported that the photocatalytic TiO2-Al2O3 composite membranes exhibited high water permeability, reliable organic retention, and anti-biofouling properties while Syafei et al. reported that TiO2 coated membrane did not enhance flux and the removal of humic substances compared to unmodified membrane [183]. In reviewing the previous work it is clear that the adverse effect of particle agglomeration has been overlooked in many studies. In fact, an ideal nanocomposite membrane is a membrane which is free of any agglomerations. The mechanism for improved fouling performance also has yet to be elucidated. In addition, the effect of particle size distribution, the mechanical and chemical modification of particles on the membrane structure and surface chemistry (hydrophilicity, roughness and protein absorption) in conjunction with fouling performance has not been investigated. In the case of TiO2 coating nanocomposite membranes, the issue of long term particle stability and sustainable antifouling performance have remained to be resolved. The above results were summarized in Table 2-8 with regards to particle type, membrane matrix, foulant, preparation method and main outcomes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 41 Table 2-8 A summary on the selected TiO2 nanocomposite membrane prepared for liquid separation and water purification Author/s and Particle Membrane foulant Preparation method Results and conclusion reference type matrix Rahimpour et TiO2 PES non-skim Blending (solvent mixing) and x TiO2-coated membrane shows better al. [17] anatase and (polyethersu milk coating (dip coating of performance compared to TiO2-blended. rutile 3 to fone) membrane into TiO2 dispersed x Optimum conditions: 0.03 wt. % TiO2 1(25 nm) solution) colloidal suspension concentration, 15 min immersion and 15 min UV irradiation with 160W lamp x Long term coating stability and durability was not investigated Wu et al. [13] TiO2 rutile PES Bovine Blending (solvent mixing) x Excellent performance & anti-fouling 30nm serum ability at 0.5 wt.% TiO2 content albumin x Higher TiO2 content (than 0.5 wt.%) (BSA) resulted in a defective pore structure and decline of the performance Li et al. [95] TiO2 PES Pure Blending (solvent mixing) x Cross section structure changed as the anatase water amount of TiO2 increased. 21nm x Better thermal & mechanical stability because of higher interaction between TiO2 and PES matrix x Increase in TiO2 content resulted in a serious loose agglomerations x Hydrophilicity increases by increasing the TiO2 content x The membranes with maximum flux and pore size were obtained when the TiO2 was 4wt.% to 5wt.% but stable performance was observed at 1-2 wt.% TiO2 Luo et al. TiO2 PES polyethyl Coating (self-assembly of TiO2 x A significant increase in hydrophilicity and [15] anatase 5- ene surface hydroxyl group and the anti-fouling performance Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 42 42 nm glycol- sulfone group and 5000 ether bond in PES structure via MW coordination and hydrogen bond interaction) Luo et al.[70] TiO2 PES No Blending (Sol- gel mixing) x Comparing with the pure PES polymer, the Nanosized permeati PES/TiO2 composites exhibited higher (not exactly on test glass transition temperature, an increase specified) and flattening of the rubbery plateau modulus and an increase in hydrophilicity x No permeation results Bae et al. TiO2 5nm SPES MBR Coating (self-assembly x The nanocomposite membranes had a [57] (Sulfonated sludge between TiO2 nanoparticles structure similar to the PES membranes, polyethersul and sulfonic but pore size and permeability were slightly fone) acid groups on the membrane reduced due to the nanoparticles coating on surface) the membrane surface. x Better antifouling performance Cao et al. TiO2 rutile PVDF BSA Blending (solvent mixing) x Smaller sized TiO2 (anatase)/PVDF [10] 26–30 nm, (Polyvinylid (Blood membrane showed denser skins as well as anatase ene luoride) serum smoother surface. The lower the fouling, TiO2 10 nm albumin) the higher antifouling capability x Anatase shows better performance than Rutile Bae et al. [9] TiO2 20 nm PSf, PVDF Activate Blending (solvent mixing) and x TiO2-coated membrane shows better and PAN d sludge coating (dip coating of performance than TiO2 blending. However (polyacrylo membrane into TiO2 dispersed it has a drawback of particle stability nitrile) solution and pressurizing) x fouling mitigation effect was gradually improved until TiO2/PSf ratio reached to 0.3 Yang et al. TiO2 Haina PSF Emulsifi- Blending (solvent mixing) x Excellent performance & anti-fouling [172] 20–30 nm ed oil ability at 2 wt.% TiO2 content Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 43 waste x Higher TiO2 content (than 2 wt.%) resulted water in nanoparticles aggregation and a decline of performance x After 15 day of immersion in water bath, the hydrophilicity of membrane is highly constant with the same trend, suggesting the long-term stability of filler in the skin layer. x The rheological properties of casting solution were changed from Newtonian fluid to non-Newtonian fluid by TiO2 x Viscosities of casting solutions increased with the increase of TiO2 content x The mechanical strength of membrane was enhanced through adding inorganic fillers Kim et al. TiO2 TFC E. Coli Coating (self-assembly of the x XPS shows that TiO2 particles were tightly [42] anatase (aromatic TiO2 through coordination and self-assembled with a sufficient bonding and 10 nm polyamide H-bonding interaction with the strength Kwak et al. thin-film COOH functional group of x Antifouling strength: [72] layer) aromatic polyamide thin-film TiO2/Mem.+UV >> Mem.+ UV > layer) TiO2/Mem > Mem. Only Madaeni and TiO2 20 nm TFC Whey Coating (Self-assembly of x Using TiO2 without UV not only does not Ghaemi from feta TiO2 particles through create self-cleaning property but also [173] cheese coordinance bonds with OH decrease the whey flux functional groups of polymer x Excellent performance & anti-fouling on the membrane surface) ability at 0.003 wt.% of TiO2 content,10 min UV,15 min immersion in water before x Addition of SiO2 to the TiO2/TFC membrane resulted in an increase in surface acidity, hydrophilicity, photocatalytic and Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 44 self-cleaning property of the membrane. Zhang et al. TiO2 Alumina Humic Using TiF4 solution through x The TiO2 nanotube membrane presented [174] nanotube, membrane acid liquid-phase grafting on good potential in practical application due inner (MF) alumina MF membrane. TiO2 to its multifunctional properties, separation, diameter 5- Nanotubes were grafted into photocatalytic oxidation and anti-fouling. 100 nm the channels of the alumina membrane template. Zhang et al. TiO2 Accumulati Humic Hydrothermal-filtration x Similar photocatalytic activity as [175] nanowire, on of TiO2 acid method commercial TiO2 powder (P25) on inner Nanowire degradation of humic acid in water. diameter 20-100 nm Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 45 2.4 Sol-Gel Chemistry 2.4.1 Introduction The sol-gel processing is an ideal procedure for the formation of nanoparticles (typically metal oxides) at mild reaction conditions (ambient temperature and pressure) with the advantage of the manipulation of reaction conditions via the use of chemical additives. The sol gel process consists of a colloidal solution called “sol” that acts as the precursor for an integrated network known as “gel” of either discrete particles or network polymers. The precursors are usually metal alkoxides ܯሺܱܴሻ௭ or halides ܯሺ݈ܿሻ௭ which undergo various forms of hydrolysis and condensation reactions. M could be Ti, Si, Al, Zn, Si and so forth. Since the OR groups are electronegative, the metal is highly prone to nucleophilic attack. The metal alkoxides react with water to form the amorphous hydroxides and hydrous oxides. They can then be converted to crystalline oxide by post heat treatment [184]. In the hydrolysis step, the precursor is hydrolysed to form alcohol or HCl and introduces the highly reactive hydroxyl sites (OH) on the precursor. ௬ௗ௦ 3-2 ܪሻ௭ ݖܴܱܪሺܱܯଶܱ ሱۛۛۛሮܪሺܱܴሻ௭ ݖܯ ௬ௗ௦ 4-2 ݈ܿܪሻ௭ ݖܪሺܱܯଶܱ ሱۛۛۛሮܪሺ݈ܿሻ௭ ݖܯ The condensation step proceeds with the elimination of the small molecules similar to organic polycondensation reaction and the production of alcohol or water as shown below [116]: ௬ௗ௦ 5-2 ܪؠሱۛۛۛሮؠ െ െ ؠ ܴܱܯെܱܪെܱܴܯؠ ௬ௗ௦ ଶܱ 2-6ܪؠሱۛۛۛሮؠ െ െ ؠ ܯെܱܪܪെܱܯؠ Ideally, the condensation step should begin after the complete hydrolysis of metal alkoxides or halide to avoid the incorporated of alkyl groups into the structure and sterically hinder the formation of ordered structures [185]. In addition to that, the condensation step should proceed slowly to prevent an amorphous or metastable structure. A faster reaction rate does not let the molecules to crystallize into an equilibrium structure. In order to tailor the certain desired structural characteristics [186] (compositional homogeneity, grain size, particle morphology and porosity) of the sol gel derived inorganic network, the kinetics of the reactions that form the molecular structure can be controlled via varying different parameters during hydrolysis and condensation steps [187]. These Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 46 parameters consist of the type of metal alkoxide (alkyl groups in the alkoxide), pH (catalyst) , the amount of water (R value = H2O/metal alkoxide mole ratio), the concentration of metal alkoxides (molecular separation by dilution), aging and the reaction temperature [188]. 2.4.2 TiO2 sol gel A variety of precursors for TiO2 including titanium halides (Titanium tetrachloride, TiCl4) and titanium alkoxides (methoxide, ethoxide, isopropoxide and tert-butoxide) can be used for the preparation of TiO2 nanoparticles [129]. TiO2 nanoparticle formation with the hydrolysis of TiCl4 is the easiest and widely used technique as there is no large organic leaving group during hydrolyses reaction. TiCl4 is in liquid form and very sensitive to air so requires a dry inert atmosphere. The size of the substituent groups in alkoxide precursors affects the reactivity of the precursor, which influence the kinetics of the sol-gel process for TiO2 preparation particularly for methoxide which is the most reactive form among the others. The typical sol gel reaction for TiO2 preparation using titanium alkoxides has two main steps of hydrolyse and condensation. The following reaction occurs in the case of full hydrolyse reaction which leads to what is sometimes referred to as the “titanol molecule” [189]: ுା ܶ݅ሺܱܴሻସ Ͷܪଶܱ ሱሮܶ݅ሺܱܪሻସ Ͷܴܱܪ 2-7 ுା ଶܱ 2-8ܪെܶ݅ؠሱሮؠെെؠܱܪܪؠܶ݅െܱ However, practically the hydrolysis and condensation reaction occur under three competitive sub-reactions [190]: Ǥ ுگ 9-2 ܪோ ՜ؠ െ െؠܴܱڰܱ ڮ ݅ܶ ؠഥܪOlation: ؠܶ݅െܱ ܪ 10-2 ܪؠܶ݅െܱܴ՜ؠ െ െ ؠ ܴܱܪAlcoxolation: ؠܶ݅െܱ 11-2 ܪܱܪ՜ؠ െ െ ؠ ܪؠܶ݅െܱܪOxolation: ؠܶ݅െܱ Particle growth and size are important to control during the sol gel reaction as the large particle size and agglomerations are undesired since they negatively affect certain properties such as optical transparency and homogeneity. At the beginning of the condensation reaction, the formation of dimers (two structurally similar subunits joined by strong or weak bond) and trimmers (three structurally similar subunits joined together) increases the size of the particles and follows by the formation of cyclic oligomers. The coarsening or Ostwald ripening could also occur during sol-gel reactions. In the Ostwald ripening, small crystals or sol particles Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 47 dissolve in the solution and redeposit onto larger crystals or sol particle to increase their size and form insoluble agglomerations. The driving force of this phenomenon is the minimisation of surface free energy as the surface to volume ratio of small particles is much greater than that of the larger particles and as a result the system favours the formation of larger particles and agglomerations [191]. 2.4.3 Sol gel parameters An appropriate adjustment of sol gel parameters such as water content, pH and temperature in conjunction with a careful selection of solvent, organic additive or templating agent as well as modification of precursor can be employed to alter the TiO2 thin film properties and structure. 2.4.3.1 Water content One of the critical parameters in controlling the hydrolysis reaction kinetics and the resulting TiO2 morphology is water content which is adjusted based on its molar ratio to TiO2. A mono-dispersed particle size between 0.2 and 1μm can be prepared by setting the water to titanium ratio (r) to less than or equal 10 (r ≤ 10) [192, 193]. Further increase in r-value leads to an extremely fast particulate nucleation and growth causing the particles precipitation and the formation of large agglomerations which can then break up into very small agglomerations (<20 nm) and primary particles by peptization at elevated temperature in the presence of chloride or nitrate [194]. 2.4.3.2 pH By controlling the pH of solution not only one could control the condensation reaction and particle size but also the stability of sol. At pH of above 3 the precipitation of TiO2 occurs. Lowering the pH increases the sol stability and postpones the gelation time. It is reported that the most stable TiO2 sol is approximately at around pH 1.3. Further reduction in pH shortens the gelation time and reduces the sol stability [195]. Acidic condition (low pH) is used to have a complete hydrolysis and slow condensation step. At low pH, the rate of hydrolysis enhances, because the OR groups attached to the metal (M) are protonated by + H3O and making the charge of OR groups more positive. Therefore, the metal ions repel the OR group due to the fact that the metal ions are positively charged. This shifts the metal ions towards the OH groups, thus promoting hydrolysis step. The rate of condensation also reduces in the acidic solution due to the reduction in the interaction of protonated species [102]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 48 2.4.3.3 Temperature The effect of temperature has been investigated during and after sol gel reaction. Since the hydrolysis reaction for silica is slow, an elevated temperature is required to facilitate diffusion, thus leading to the formation of larger particles with higher oxide contents [188]. However, high temperature for sol-gel derived TiO2 is not critical as the nature of hydrolysis reaction is exothermic and leads to temperature increases for the sol. Post heat treatment and calcination at high temperature (above 500oC) after gelation is often applied to form a crystalline product [196, 197]. Traditionally, high annealing temperature is only used for preparing TiO2 coating on thermal resistant substrates such as ceramics and metallic surface, and is not applicable to thermally fragile substrates for instance polymeric substrates. The surface structure alteration of sol gel derived TiO2 can be induced at low temperature either by a conventional dry treatment or by a humid environment [184, 195]. Figure 2-7 which is introduced by Gopal et al. is an approximate time temperature transformation diagram for the formation of crystalline titanium dioxide, where the lower and upper lines are the beginning and the end of precipitation process respectively and the black circles correspond to experimentally determined points [184]. As can be seen, different TiO2 crystallinity can be achieved by controlling time and temperature under 100o C. Figure 2-7 Time temperature transformation diagram for the formation of crystalline titanium dioxide [184] Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 49 2.4.3.4 Solvent selection Since solvent participate in the sol-gel reaction, it can affect the structure of the resultant material. Solvents such as the ones with pairs of electrons can tailor the gelation time via slowing down the condensation rates [116]. Solvents also can affect the diffusion rates of the reactants i.e. water and hydrolysed species. As a result of this, TiO2 sol gel derived products in lower alcohols have higher equivalent oxide content (≡Ti-O-), indicating a greater extent of condensation. For instance, when titanium (IV) ethoxide is used in isopropanol (C2H5OH) and dried at 100°C, the oxide content of the gel by weight is approximately 83-84% whereas this value for butanol (C4H9OH) is about 73%. The volume of the solvent can also affect the nature of product. Decreasing the concentration of reactants by diluting the solution could separate the molecules and favours hydrolyse reactions which depend less on the diffusion of larger species. Therefore, oxide content of gel and particle sizes are reduced [188]. 2.4.3.5 Precursor modification In order to prepare the TiO2 nanoparticles, controlling the sol gel reactions are critical as uncontrolled reactions lead to large agglomerations and precipitate structures. For instance, a direct addition of titanium alkoxides to water resulted in a vigorous reaction and the formation of large white precipitates. The rationale behind the chemical modification of precursor is to reduce the activity of alkoxide precursors in order to obtain sols with desirable properties. The precursor can be modified via the addition of chelating ligands for example β-diketones, β-ketoesters, carboxylic acids or other complex ligands [198] to produce new precursor. Bidentate ligands with two lone pairs of electrons are commonly used to interact with the titanium to reduce reactivity through limiting the sites available for condensation step [199]. 2.4.3.6 Templating agent Incorporation of organic additives to precursor solution in sol-gel technology is an approach which has been widely applied to precisely control the nanostructure architecture of inorganic films [200-204]. The organic additive mostly poly(ethylene glycol) has been used for producing a network of highly accessible pores on the TiO2 films [200, 203]. The three- dimensionally interconnected porous structure of TiO2 films is related to high superhydrophilic wetting behaviour of TiO2 [203, 205]. The mechanism of formation of porous network is based on the migration of TiO2 nanoparticles into the additive chains which acts as structure-directing or templating agent (Figure 2-8 a). The migration is due to hydrogen bonding which forms an inorganic-polymer composite. The organic additives are Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 50 removed from the composite by high temperature calcination decomposition or hot water leaching treatment to leave a porous network behind [206]. Surfactants such as Pluronic F127 can also be used as structure-directing agent to order mesostructure architectures. However the mechanism by which these amphiphilic materials direct the structure is different from their hydrophilic counterpart such as PEG. In the aqueous solutions, an inner core (micelle) forms due to the inward and outward orientation of hydrophobic PPO and hydrophilic PEO chains of the amphiphilic block copolymers, respectively (see Figure 2-8b). Under hydrothermal conditions, the supersaturated M (OH) z starts to nucleate in the macromolecular micelles. Afterward, the growth of crystals proceeds under the restriction of hydrophobic cores to form homocentric bundle structures. It is also reported that the size of the products controls by the size of the macromolecular micelles which act as nanoreactor that induces larger or smaller particle growth [207]. (a) (b) Figure 2-8 Effect of templating agent on the inorganic sol gel derived structure (a) hydrophilic [208] (b) amphiphilic [207] agent Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 51 2.5 Surface Wettability The hydrophilic or hydrophobic quality of a smooth and clean surface arises from its chemical make-up. These qualities are enhanced by the roughness of the surface; due to the trapping of air in the cavities between the drop and a rough hydrophobic surface, and by capillary wicking (nano-wicking) on a rough and structured hydrophilic surface [209]. The physicochemical principle of wetting of a surface by a liquid has been well established [210- 213]. As shown in Figure 2-9, when a water droplet rests on a solid surface, three interfacial forces of liquid-vapour surface tension (ɀ୪୴ሻ, solid-vapour interfacial tension (ɀୱ୴ሻ and solid- liquid interfacial tension (ɀୱ୪ሻ are in a thermodynamic balance [210, 214]. This balance forces determines whether the droplet spreads into a film or reaches to an equilibrium shape. The surface solid wettability is governed by both chemistry and geometrical structure of the surface. Surface chemistry determines the surface tensions at microscopic level but geometrical structure governs how these forces act upon the liquid [215]. Therefore, surface wettability can be tuned by dynamically varying one of these two parameters [162]. γLV θ γ SV γSL Figure 2-9 Contact angle θ and the interfacial forces of a liquid droplet on a flat smooth surface 2.5.1 Wetting state of a smooth surface Laplace’s law states that when a drop of one component liquid is in equilibrium with its vapour, it will spontaneously assume to form a sphere in order to minimise the necessary "wall tension" of the surface layer in the absence of any external forces. Work must be done on the drop to increase its surface area as surface molecules are at higher free energy state than those in bulk [210, 216]. The change of free energy associated with the reversible, isothermal formation of a liquid surface is then defined as surface free energy. Surface free energy also determines the wettability of solid surfaces, which is an important fundamental Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 52 property to most natural systems and technical applications such as painting, coating, windshields design, waterproof clothing, and other consumer products. The progress of wetting is determined by the net change in total surface energy of each interface. Wetting is a thermodynamic process and its spontaneity and rate are determined by the magnitude of the free energy change. A drop of water on horizontal solid (Figure 2-9) surface may spontaneously spread if wetted area has a lower specific energy than the surrounding. The net energy difference between the consumption of the energy by the free liquid surface and the release of energy by the wetted area under the drop determines wetting speed and characteristics of the solid surface [217]. The theory of wetting illustrated by Young’s equation [216] relates the thermodynamic equilibrium contact angle ߠ of a droplet on an ideal solid surface to the specific energies of solid-liquid (ߛ௦), solid-gas (ߛ௦௩), and liquid-gas (ߛ୪୴) interfaces: ࢽ࢜ࢉ࢙ࣂࢅ ൌࢽ࢙࢜ െࢽ࢙ 2-12 The assumption of ideal solid surface refers to the neglect of roughness and chemical heterogeneity of the surface. Swelling, surface reconstruction and dissolution are also neglected. By rearranging the Young’s equation, contact angles are calculated to range between 0° (complete wetting or super wetting) and 180° (complete dewetting). Young’s equation can only be applied to systems in thermodynamic equilibrium and is difficult to be realised experimentally due to non-equilibrium processes such as evaporation and condensation [216]. 2.5.2 Wetting state of a rough surface When a water droplet is placed on the surface, it can follows the contours and roughness of the surface as explained by Wenzel model [218] (Figure 2-10 a). According to this model, the effect of roughness should be to amplify the wettability of the surface toward its intrinsic tendency of either roll-up or film formation of the liquid. In other words, if the surface is intrinsically hydrophilic with water contact angle of less than 90o, roughening makes it superhydrophilic but if it is hydrophobic with water contact angle greater than 90o roughening turns it into superhydrophobic surface. The Wenzel equation describes the effect of surface chemistry and morphology on the water droplet contact angle: ܥݏߠ ൌ ݎܥݏߠ 2-13 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 53 where θ is apparent contact angle and θe is the contact angle on the flat smooth surface, and the roughness factor “r” is the ratio of the actual solid/liquid contact area to its vertical projection. According to Wenzel, morphology lies within r whilst the surface chemistry represents by θe. (a) (b) (c) Figure 2-10 Wetting state of (a) Wenzel and (b) Cassie-Baxter and (c) the intermediate state of Wenzel and Cassie-Baxter The Wenzel assumption can be breached if the liquid finds it difficult to penetrate into the roughness or texture of the surfacer due to capillary forces as it is explained by Cassie-Baxter model [219] (Figure 2-10b). As a result, liquid energetically favours to bridge across the top of surface protrusions such that the water droplet sits upon the patchwork of solids and air gaps and does not follow the surface contours. The Cassie-Baxter equation for solid-liquid-air composite interface (porous) considers the effect of air gaps under the droplets: ܥݏߠ ൌ ݂௦ሺܥݏߠ ͳሻെͳ 2-14 In the equation, if fs which is solid surface fraction decreases, the apparent contact angle (ߠ) o increases and in limit approaches 180 when fs approaches zero. That means increase in roughness reduces the area of contact between the liquid and solid and resulted in a reduction in the adhesion of a droplet to the solid surface. Wenzel and Cassie-Baxter have sometimes been considered to a more general form of intermediate state between two models to cover the case when the contacting areas themselves are not flat (Figure 2-10 c): ܥݏߠ ൌ ݎ݂௦ሺܥݏߠ ͳሻെͳ 2-15 Equation 2-15 is valid only if the drop size is large relative to the size of the roughness protrusion [216]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 54 2.5.3 Hierarchical structures and multilevel roughness Hierarchical structure with multilevel roughness is necessarily to approach the extreme wettability of superhydrophilicity or superhydrophobicity [220, 221]. 2.5.3.1 Superhydrophobicity Superhydrophobicity can be achieved by combining low-energy surface and multi-level roughness by either coating a rough surface with a low surface energy material or roughening a low-energy surface. It is reported that the water contact angle of 120o is the highset achievable contact angle on a flat smooth surface, which is not considered a superhydrophobic surface (water contact angle above 150o [215]). This can be achieved by saturating the surface with CF3 and CF2 groups which reduces the surface free energy to the lowest available energy of 6.7 mJ/m2 [222, 223] (Figure 2-11 a). Increasing the roughness of the surface into micro-scale following by fluorination barely could increase the roughness to the level where meet the superhydrophobic definition (Figure 2-11b). However, the hierarchical structure with multilevel roughness in micro-nano scale in conjunction with the fluorination of the surface could increase the contact angle above 150o (Figure 2-11c) [224]. Such high water contact angle and resists to wetting can be ascribed to a dramatic increase in the number of narrow widths and sharp tips in nano-scale contacting liquid droplet. According to Cassie-Baxter model, that significant reduction in solid surface fraction (fs) results in a substantial increase in air gaps which shifts the surface wettability toward superhydrophobicity [221, 224, 225]. μm size nm size μm size (a) (b) (c) Figure 2-11 Effect of roughness on the water contact angle, (a) maximum achievable water contact angle of 120o after fluorination of the surface, (b) contact angle between 120o and 150o after micro-scale roughening and fluorination and (c) superhydrophobic surface with water contact angle above 150o TiO2 superhydrophobic surface modifications have been recently reported on solid high thermal resistance substrates with and without UV irradiation [221, 226-230]. These techniques are mostly based on the creating a hierarchical multilevel roughness using TiO2 nanostructure coatings and then reducing the surface free energy of the roughened surface by a low energy materials. However, there is a lack in the literature for the generation of Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 55 superhydrophobic surfaces using TiO2 nanostructures on the low thermal resistance materials such as polymeric membranes. 2.5.3.2 Superhydrophilicity The definition of superhydrophilicity is still unclear as to exactly what type of surfaces is being dealt with for example some researchers claiming that if a surfaces being roughened, achieving zero degree contact angle is sufficient to have a superhydrophilic surfaces [231]. Other researchers have considered contact angles smaller than 5o as a superhydrophilic surfaces [229]. There are also some researches who narrowed down the term of superhydrophilicity to those surfaces that could reach to contact angle smaller than 5o within 5 seconds [201] or even less than 0.5 second [203]. Recently, TiO2 surface shows superhydrophilicity as a result of 3D capillary phenomena rather than 2D [162, 232]. In fact, 2D capillary effect can be employed for the generation of TiO2 superhydrophilic surfaces by taking advantage of its surface photosensitivity (section 2.3.1) while in 3D capillary phenomena, multi-level roughnesses plays a more important role for changing the wettability of a surface. In 3D capillary effect, the imbibitions of water will fill the inner pores below the droplets and increases the spreading of water on the surface [233], which corresponds to Wenzel mode. Practically, in most situations the 2D and 3D capillary effects work together to make a super-wetting surfaces specially for TiO2 multilevel roughness surfaces [162]. Bai et al. show that coupling the hierarchically assembled TiO2 nano-thorn sphere coated on polymeric membranes (cellulose acetate) with concurrent photocatalytic oxidation process, can lead to superhydrophilic membranes with less fouling, higher flux and water quality [234]. However, from a practical standpoint, it is highly desirable to make the hydrophilic TiO2 film without the need for continuous UV illumination. Gan et al. showed that superhydrophilicity could be generated without UV activation on TiO2 with PEG templated nanoparticle coatings [204]. Mane et al. also were able to generate a superhydrophilic rutile TiO2 thin film without using UV light in their work [235]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 56 2.6 Membrane Distillation (MD) 2.6.1 Introduction to membrane distillation Membrane distillation (MD) is an emerging technology for separations that are conventionally performed by distillation or reverse osmosis such as water purification, brine management, heavy metal removal, food industry and purification of pharmaceutical products. Membrane distillation was first introduced in 1976 with diminishing popularity due to its low production as compared to reverse osmosis technique. The increasing interest was only seen in the early of 1980s due to the availability of novel membranes and modules with better characteristics as well as the fact that MD is capable of using low-grade waste and/or alternative energy sources, such as solar and geothermal energy [236]. MD is a thermally driven process which uses microporous hydrophobic membrane as a barrier to only allow vapour molecules to be transported across from feed to permeate side. The hot liquid feed to be treated must be in direct contact with one side of the membrane and does not penetrate through dry membrane pores due to the hydrophobic nature of membrane and surface tension forces of liquid solutions. This phenomenon forms liquid/vapour interfaces at the entrances of membrane pores. The temperature difference between the hot feed and cold permeate forms a trans-membrane vapour pressure as the main driving force of MD, which can be maintained by different configuration of MD on the permeate side. The four possible MD configurations are Direct Contact Membrane Distillation (DCMD), Air Gap Membrane Distillation (AGMD), Sweeping Gas Membrane Distillation (SGMD), and Vacuum Membrane Distillation (VMD). The difference between configurations is illustrated in Figure 2-12 [237]. Figure 2-12 Schematics of different types of membrane distillation MD configurations [237] Both hollow fibres and flat-sheets membrane can be used for membrane distillation processes and they are normally made of polymers polypropylene (PP), poly vinylidene fluoride (PVDF), and poly tetrafluoroethylene (PTFE, Teflon). However, MD processes mostly use PVDF membrane due to its high melting point and good resistance towards temperature, solvents, and abrasion. The membrane used for MD is not necessarily a selective barrier. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 57 They should not be wetted and should allow only vapour and non-condensable gases to pass through the pores. Therefore, membranes should be fabricated by highly hydrophobic materials (i.e. high water contact angle) with small maximum pore size [237]. The major advantage of these processes is the ability to use low-grade energy to drive the mass transport as the feed does not have to reach boiling point (typically 50o–90oC). The advantage of MD process over reverse osmosis (RO) is the concentration and water recovery process at high solute concentrations. When MD is operated at high concentrations, the permeate flux is highly dependent on viscosity and vapour pressure suppression. In MD process, suppression of driving force due to high concentrations is modest, while in RO process, the operating pressure must be significantly elevated to overcome osmotic pressure in the concentrate processing. Moreover, MD can be operated under reasonable fluxes at moderate temperatures with high solute concentrations [238]. However, MD has not been commercially implemented for industrial use and a viable separation technology due to factors such as its relatively low permeate flux compared to other techniques, permeate flux decay resulting from concentration and temperature polarizations, membrane fouling and total or partial pore wetting, membrane and module design for MD, and economic concern due to high thermal energy consumption for different MD configurations and applications [237]. 2.6.2 Operational parameters affecting MD process 2.6.2.1 Feed temperature Increase of feed temperature exponentially increases the trans-membrane vapour pressure (i.e. driving force), thus increasing the MD permeate flux [236]. Higher feed temperature is preferred to maintain high internal evaporation efficiency (the ratio of the heat that contributes to evaporation and the total heat exchanged from the feed to permeate side) while realising that increases in feed temperature contributes to temperature polarisation. In the case of an aqueous solution containing a volatile component, the exponential increase in permeates flux due to increase in feed temperature may be hindered by the reduction in selectivity as the two components compete each other while passing through the pores [237]. 2.6.2.2 Feed inlet concentration The increase in feed inlet concentration does not significantly decrease the flux [236]. However, in the case of highly concentrated solutions (i.e. non-volatile solute), the addition Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 58 of non-volatile solute to water reduces partial vapour pressure and consequently reduces the driving force of MD process, thus decreases permeate flux regardless of which configuration is being used. Increase in feed inlet concentration leads to concentration polarization and most likely, temperature polarization. On the other hand, increases in volatile solute concentration on the feed side causes an increase in the trans-membrane partial pressure of the volatile component, thus increasing permeate flux [237]. 2.6.2.3 Feed side velocity The optimisation of feed circulation velocity and stirring rate is to increase heat transfer coefficient in the feed side of the membrane module and to minimize temperature and concentration polarisation effects for obtaining higher permeate flux. Higher permeate flux can be achieved through operation under turbulent flow regime by increasing circulation velocity. By doing so, the temperature at membrane surface becomes closer to bulk feed temperature and increases trans-membrane temperature difference. However, permeate flux will only increase to a maximum and then will start to decrease [236]. The effects of changing the feed circulation velocity and stirring rate are observed differently for different MD configurations. DCMD, AGMD, and VMD shows increases of permeate flux with feed flow rate to asymptotic level. On the other hand, SGMD shows negligible changes in the presence of non-volatile solutes in the feed, which indicates that the effect of temperature polarisation in the feed side of membrane module is insignificant as it is localised in the permeate side. Selectivity of MD is directly affected by the feed solution flow velocity with the present of volatile component [237]. 2.6.2.4 Permeate inlet temperature The increase of permeate temperature will decrease the trans-membrane vapour pressure as far as the feed temperature is kept constant, thus decreasing permeate flux. The effect of permeate inlet temperature is 2-fold smaller than the effect of feed inlet temperature for the same temperature difference [236]. The effect of decreasing permeate temperature have different effects for different MD configurations. Decreases in permeate temperature increase permeate flux for DCMD [239], however it shows negligible effect in AGMD as the heat transfer coefficient in the air gap dominates the overall heat transfer coefficient. Negligible effect on permeate flux has also been seen in SGMD as the temperature of the sweeping gas in the permeate side increases very fast from the inlet to the outlet of the membrane module [237].Obtaining higher MD flux is preferably done by increasing the feed side temperature rather than to decrease the permeate side temperature. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 59 2.6.2.5 Permeate flow velocity The effect of permeate flow velocity is only observed in DCMD and SGMD configurations. In the presence of volatile components, increasing the permeate flow velocity reduces the temperature and concentration polarisation effect, thus increasing the heat transfer coefficient in the permeate side of the membrane module. The increase of heat transfer coefficient in the permeate side causes an increase of temperature in the bulk permeate side and consequently the driving force as well as the permeate flux. The extent of velocity effect is more pronounced for SGMD due to physico-chemical and thermal conductivity properties of both liquids (water) and gases (air). When non-volatile components present in the feed, temperature polarisation is localised at the permeate side and after exceeding the optimum permeate velocity, any increase may cause flux decline [237]. 2.6.3 Challenges in membrane distillation Practically, sustaining a stable flux and controlling heat and mass transfer throughout the module is the main obstacle to commercial implementation of MD processes. Sustained operations are compromised by pore wetting, fouling (scaling, biofilm, particulate and colloidal fouling), temperature and concentration polarization at the membrane interface, and heat loss via conduction through the membrane [237]. 2.6.3.1 Pore wetting phenomenon Pore wetting occurs when the feed liquid penetrates or floods the pores partially or totally and consequently contaminates the product water and reduces permeate flux. Increasing the hydrophobicity and decreasing the maximum pore size between 10 nm and 1 μm can reduce the risk of pore wetting. Microporous polymeric membranes such as polyvinylidene fluoride (PVDF), polypropylene (PP), polytetrafluoroethylene (PTFE) and polyethylene (PE) were widely been used for MD since they are intrinsically hydrophobic [240-243]. However, pore wetting may occur even for these membranes if the membrane surface being in direct contact with solutions with surface active components or if the transmembrane pressure (TMP) goes beyond the liquid entry pressure (LEP) [237]. LEP which is also called pore entry pressure is the point that liquid will penetrate or flood the pores and its magnitude is affected by hydrophobicity, pore size and pore shape [244]. Shifting the hydrophobicity toward superhydrophobicity helps the surface to lift a water drop above the microstructure by an air gap as explained by Cassie state [245]. This air gap provides an opportunity to increase the allowable pore sizes much larger than that of normal membranes if the operating pressure were the same, accordingly allowing higher mass flux [246]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 60 2.6.3.2 Membrane fouling Fouling the membrane by contaminant species in the feed water by organic and inorganic species (scaling) may reduce the permeate flux and process efficiency by depositing on the membrane surface and plugging the pores. In addition to the flux decline effect, fouling and pore plugging could reduce the size of the small pores and causes an increase in the pressure drop. This increase in the pressure drop may eventually lead to the point where hydrostatic pressure exceeds the LEP of the membrane pores [237]. To control fouling researcher have tried to use feed pretreatment, increasing feed flow rate, hydraulic and chemical cleaning, changing hydrophilicity, roughnesses, surface charges, adding polymer barriers [236, 247- 249]. Since these techniques impose additional cost and energy usage, a safer and more efficient way of fouling mitigation is greatly needed. Shifting the wettability of the surface toward superhydrophobicity could reduce the interaction between feed water solution and membrane surface thereby reducing the risk of fouling. Zhang et al. investigated the anti- biofouling properties of superhydrophobic coating for marine application [250]. They observed no microorganism trace in the first week of immersion whereas unmodified substrate fouling was occurred within a day. However, the superhydrophobic coating showed limited long term antifouling performance in the complicated real sea environment. Benjamin et al. synthesized a superhydrophobic xerogel coating with a mixture of nanostructured fluorinated silica colloids, fluoroalkoxysilane, and a backbone silane, which exhibited a significant reduction in the bacterial adhesion of Staphylococcus aureus and Pseudomonas aeruginosa [251]. Zeyu et al. [246] prepared a narrowed pore size superhydrophobic glass membrane with the integrated arrays of nanospiked microchannels for membrane distillation applications, which retarded fouling during operation and performed much better than the polymeric membrane used at the same conditions. Similar results were observed by Hendren et al. where alumina Anodisc™ ceramic membranes were made hydrophobic via the surface functionalization of different fluorosilane chemicals [244]. The modified ceramic membranes showed better performance in comparison to its counterpart polymeric membrane which was tested under the similar conditions. Although inorganic membranes can be operated at elevated temperature and harsh conditions with high resistance to corrosive liquids and gases [2, 30], there are serious issues which make inorganic membranes less attractive in comparison with their polymeric counterpart (see section 2.1.2.1). Considering the fact that the operating temperature in MD is usually below 90oC, therefore, improving the antifouling performance of polymeric membranes via Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 61 superhydrophobic modification at low temperature without compromising the pore size and permeate flux seems crucial for membrane distillation commercial implementation. 2.6.3.3 Conduction heat loss The issue of heat loss through conduction tends to decrease the evaporation driving force thus compromising vapour flux. Increasing the membrane thickness reduces the heat conduction. However, there is a trade-off between heat conduction and vapour diffusion, as increasing the thickness may result in a large diffusion resistance of vapour molecules. Since the thermal conductivity of air/gases is an order of magnitude lower than that of polymeric membranes, increasing the porosity and void volume [237] or having an air gap by means of superhydrophobic modification (see section 2.5.3.1) could minimize the heat loss by conduction across the membrane. 2.6.3.4 Temperature and concentration polarization Temperature polarization which causes a difference between the membrane surface temperature and bulk temperature of the feed can reduce evaporation driving force for mass transfer up to 80% [237, 243]. If the concentration of feed increases, concentration polarization along with temperature polarization may occur and result in a driving force reduction and so the mass flux. High shear rates and baffling in the conventional shell and tube configuration can be used to reduce boundary layer thickness and polarization, but the high feed pressure across the module increases the potential for pore wetting (exceeding the LEP) due to liquid intrusion into the pores as transmembrane pressure increases [237]. Superhydrophobic modification may raise the LEP [244], thus higher feed pressure and shear rates can be applied to minimize the polarizations. 2.6.3.5 TiO2 coating and membrane distillation Although TiO2 superhydrophobic surface modifications have been recently reported on solid high thermal resistance substrates [221, 226-230], there are a very limited number of reports on the superhydrophobic modifications of polymeric membranes using nanoparticles such as TiO2. Sun et al. prepared a superhydrophobic microporous polyethersulphone (PES) via a surface silica sol treatment followed by fluoroalkyl silanization [252]. The contact angle for modified membrane was 154o whereas it was 75o for original PES membranes. However, membrane performance and thermomechanical modification stabilities were not investigated. The wettabilities of PES and two other different macroporous polymeric membranes were also changed by Urmenyi et al. [62]. Firstly, they coated homogeneously the inside of Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 2 Literature Review 62 microporous membranes by silica via alkaline hydrolysis of tetraethoxysilane. The hydroxyl groups on the deposited silica allowed further functionalization with hydrophilic (aminated) and hydrophobic (octadecyl or fluorinated) silylating agents. After the fluorination of silica- PES membranes, the contact angle increased from 62o to 142o with a significant high measured LEP of 3 bars. Although the authors believe that the agents are covalently bonded to the surface, there are not any results on the thermomechanical stabilities of the silylating agents. In addition, the performances of the modified membranes were not investigated. In this PhD study, macrospores polymeric membranes were modified such that the surface wettability shifts toward superhydrophobicity via TiO2 coating by LTH process to change the structures of the surface and then the surface energy was reduced by surface fluorosilanization. 2.7 Summary This chapter covers a comprehensive literature review which encompasses all areas related to this study and objectives mentioned in chapter one section 1.1.The area of nanocomposite membranes is a broad field that lies at the intersection of three research disciplines of nanotechnology, polymer and membrane technology. It is impossible to cover comprehensively all aspects of this field in this thesis due to its broadness and therefore those prior works that deemed essential was explored. Nanocomposite terminology and its preparation techniques, the principle of membrane separation technology and different kinds of fouling with current fouling mitigations strategies were introduced. TiO2 nanocomposite membranes were comprehensively reviewed and the gaps were identified. TiO2 nanoparticles as one of the main element of nanocomposites were individually and in the polymer matrix were studied. Sol gel chemistry of TiO2 also was reviewed. The effect of nanoparticle deposition on the surface chemistry and morphology were also argued. The current theories on the wetting action of a smooth and rough surface were studied and how nanoparticles deposition can change the wettability of the surface was also discussed. Membrane distillation as one of the uprising areas in membrane technology and its practical challenges were explored. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 63 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 64 CHAPTER 3 MATERIALS AND METHODOLOGY 3 Abstract In this chapter, materials used during this study and their providers were presented. The procedure of the preparation of flat sheet and hollow fiber membranes was explained in details. The technique by which the membranes were coated by TiO2 nanoparticles was also introduced. Fluorosilanization of the TiO2 coated membranes for superhydrophobic modification was presented. Finally, a variety of characterization techniques which were used to study surface chemistry, structure and fouling performance of membranes were explained in details. 3.1 Materials Polyethersulfone (PES, 58000 g/mol) as polymer was purchased from BASF Co. Ltd. Polyvinylpyrrolidone (PVP, 40000 g/mol) as pore former and hydrophilic additive, Pluronic F127, a commercially available tri-block copolymer (EO106-PO70-EO106), polyethylene glycol (PEG) with molecular weight of 1000 g/mol, titanium (IV) iso-propoxide (TTIP) (97%) as TiO2 precursor, and humic acid (HA) were purchased from Sigma-Aldrich. TiO2 nanoparticles (anatase/rutile mixture, Aeroxide P25, previously known as Degussa P25) were obtained from Degussa. Aminopropyltriethoxysilane (APTES) from Sigma Aldrich was used for surface modification of TiO2 nanoparticles. Dimethylacetamid (DMAC) and N-Methyl-2- pyrrolidone (NMP) as solvent was supplied by ScharlavChemie S.A. Bovin serum albumin (BSA, reagent grade, pH 5) was purchased from Morrgate Biotech. Toluene and absolute ethanol as solvent were supplied by Ajax Finechem. Perchloric acid (70%), sodium chloride (NaCl) and calcium chloride (CaCl2) were purchased from Chem-Supply. 2,4-pentanedione was supplied by Lancaster. Perchloric acid (70%) was supplied by G. Frederick Smith Chemical Co. Commercially available PES membranes with different molecular weight cut off of 500kDa and 100kDa were obtained from Millipore and Pall Corporation respectively. Commercial hydrophobic flat-sheet membrane HVHP (nominal pore size: 0.45 μm, porosity: 75%) made of polyvinylidene fluoride (PVDF) polymer was purchased from Millipore. 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FTCS) from Sigma-Aldrich was used for surface modification of PVDF membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 65 3.2 Preparation of Nanocomposite PES Ultrafiltration Membranes (Blending) 3.2.1 Modification of TiO2 To avoid agglomeration of nanoparticles in membrane and also to increase the stability of particles in the casting solution, the modification of TiO2 nanoparticles via mechanical and chemical modifications was carried out as described below. Mechanical modification At first, the original Degussa P25 was ground by using a mortar and pestle to increase the density of the bulk and also to reduce the number of large agglomerations. For this purpose, 5g of Degussa P25 with the volume of 50 cm3 was ground to fine powder until its volume decreased to 12.5 cm3. Afterward, the powder was dispersed in DMAC by sonication in bath (Transconic 460 35 kHz, Germany) for 15 min followed by 10 min sonication by probe (Misonic sonicators S-4000, USA) at amplitude of 20. Chemical modification To decrease the agglomerations and also to increase the stability of the particles in the casting solution, the chemical modification of TiO2 was also carried out by surface modification of TiO2 nanoparticles with APTES as silane coupling agent. 2.5g of mechanically modified TiO2 was added to the pure ethanol under nitrogen purging followed by 30 and 10min sonication in bath and by probe, respectively. Different levels of APTES (2, 20, 50, 80 wt.%) o were added drop-wise to the mixture under N2 atmosphere. After stirring for 2 hours at 65 C, the particles were separated from the solution by centrifuging at 10000 RPM for 10 min. o Finally, the TiO2 particles were dried in an oven for 24 hours at 50 C and then were ground to fine powder again. In this study, modified particles refer to both mechanical and chemical being applied unless otherwise indicated. 3.2.2 Preparation of UF PES membrane (flat sheet) Three types of membranes were prepared: control membranes with no nanoparticles, membranes with mechanically modified nanoparticles, and membranes with both chemically and mechanically modified nanoparticles. Phase inversion induced by immersion precipitation technique was applied for the preparation of control and blend PES membranes with modified TiO2 nanoparticles at different concentrations (0 (control), 2, 4 and 6 wt. %). 4 wt.% of PVP was added to DMAC first and the solution was heated to 60°C. Different concentrations of modified TiO2 were added to the PVP solution. After 2 h of mechanical Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 66 stirring at 150 rpm, PES (16 wt%) was added and the solution was stirred until PES was completely dissolved. The solution was left for 4 h for degassing. Afterward, the membranes were cast by casting machine (Sheen 1133N automatic film applicator) at speed of 60 mm per second at room temperature and 50% humidity. The phase inversion was done in a coagulation bath with a mixture of water and isoporopanol (70/30 by volume). After the membranes were immersed for 19 h in the coagulation bath, they were kept in glycerol for 4 h. In this way, pores can be better preserved, and therefore the pore structure will not change significantly after drying. Finally, the membranes were dried in air by keeping them between tissue papers. 3.2.3 Preparation of UF PES membrane (hollow fiber) The PES hollow fibers were fabricated with dry-jet wet spin technique using equipment described by Dong et al. [253]. Similar to flat sheet membrane preparation method, phase inversion induced by immersion precipitation was the main process of the hollow fiber membrane formation. The concentration of TiO2 in dope was 2 wt.% and was the same for membranes with chemically and mechanically modified nanoparticles and conventionally (only mechanical) modified nanoparticles. Based on the literature [98, 254] and experimental design, the polymer (dope) solution which was chosen for the fabrication of UF PES control membrane is composed of 23 wt.% PES, 65 wt.% NMP, and 12 wt.% PVP. Membranes with TiO2 nanoparticles contain 63 wt.% NMP. The nonsolvent (bore) solution used was a 50/50 (volume %) water and NMP mixture. Similar to flat sheet membranes, three types of membranes were prepared. Each type of membrane was fabricated at bore and dope flow rates of 1.0 ml/min and 1.4 ml/min, respectively. Drum take up speed was set at 2.5 m/min for control and only mechanically modified membranes and lowered to 2.1 m/min for mechanically and chemically modified TiO2 membranes. The nascent membranes were submerged in MilliQ water for 3 days to allow solvent exchange with non-solvent. Following that, the membranes were submerged in a 40/60 volume % glycerol/water solution for 4 h to prevent pore collapse. Membrane drying was done at room temperature for 3 days before transferring to an airtight container for preservation. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 67 3.3 Preparation of Nanocomposite PES Ultrafiltration Membranes (Coating) 3.3.1 Preparation of TiO2 precursor sol for coating TiO2 precursor sol for coating purpose was prepared by mixing 0.504 g Pluronic F127 as amphiphilic templating agent in anhydrous ethanol at room temperature. A second solution was prepared by mixing anhydrous ethanol with 2, 4-pentanedione as a precursor modifier, perchloric acid as catalyst, and titanium (IV) iso-propoxide (TTIP) as precursor and ultra- pure Milli-Q water also at room temperature. After the solutions were stirred separately for 1 h with a magnetic stirrer (IKA°RCT Basic, In Vitro Technology), they were mixed and stirred for another h (Figure 3-1). A stable sol formed with pH of 1.2. The molar ratios of each component in the resulting sol were TTIP: Pluronic F127: 2,4-pentanedione: HClO4:H2O: Ethanol = 1:0.004:0.5:0.5:0.45:4.76. PEG (1000 g/mol) is also substituted with the Pluronic F127 to see the effect of hydrophilic templating agent on the coating films. 3.3.2 Coating of TiO2 nanoparticles onto PES membrane by a low temperature hydrothermal (LTH) process After the preparation of TiO2 precursor sol, TiO2 thin film was coated onto the in-house PES membrane (21mm in diameter, 250-270 kDa MWCO) by dip-coating (see Figure 3-1). The optimal coating speed and holding time was determined to be 0.2 mm·s-1 and 8 seconds, respectively. The coated membranes were then transferred into an uncovered glass Petri dish and dried in a vacuum oven at 120°C for 16 h. Afterward, the membranes were placed inside a 30ml vial filled with ultra-pure Milli-Q water and heated at 90°C in a hot water bath for 24 h . At the end of the heat treatment step, the membranes were rinsed three times with distilled water and irradiated with UV light for 12h in order to remove any organic residual leftover from the coating process. The number of times the coat-dry-heat cycle was carried out and the number of times the membrane was coated in each cycle was varied to determine if they have any effects on the properties of the coating produced. For comparison purposes, a second set of TiO2 coated PES membranes were prepared by dipping the membranes into a solution containing 0.05 wt.% of Degussa P25 TiO2 nanoparticles 20-30 nm, using the same coating speed and holding time with the titania sol previously. Commercial UF PES membranes were also coated similar to the in-house membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 68 12 hr UV Figure 3-1 Low temperature hydrothermal process of TiO2 sol-gel synthesized coating on the polymeric membranes 3.4 Superhydrophobic Modification of Microporous PVDF Membranes 3.4.1 TiO2 coating on the surface of PVDF membranes TiO2 nanoparticles were coated on the commercial flat-sheet PVDF membrane (HVHP, Millipore, nominal pore size: 0.45 μm, porosity: 75%) similar to UF PES membranes explained in section 3.3.2. These membranes are labelled as “TiO2-PVDF” membranes. 3.4.2 Fluorosilanization of TiO2-PVDF membrane surface FTCS solution preparation After the toluene was initially purged in a 250 ml container using N2 for 1 h, it was quenched down to a temperature between 0°C and -5°C by using an ice bath and/or fridge of below 0°C to avoid reaching the transition temperature (Tc) of FTCS (0°C). Keeping the temperature of reaction below transition temperature leads to a highly packed ordered phase with minimal surface energy [255] whereas reaction above Tc causes a disordered phase [256]. While keeping the container in the ice bath, FTCS (0.5 wt.%) was mixed with toluene until it is completely dissolved using a magnetic stirrer. NaCl added to ice bath to keep the temperature as low as possible during stirring. The container was also sealed and the experiment was performed in the fume cupboard under a continuous air stream to avoid the absorption of moisture into the solution. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 69 Fluorination of TiO2 coated PVDF membranes by a vacuum filtration The pre-dried TiO2 coated membranes (in ambient condition) were fixed to a Buchner funnel that is attached to a conical flask-vacuum pump as shown in Figure 3-2. The membranes were initially filtered with the minimum volume of toluene for wetting purpose. The prepared FTCS solution was then filtered through the membranes at a sufficient amount. A minimum amount of vacuum was applied to ensure uniform coating on the surface. At the end of the filtration, the membranes were removed and rinsed with 10 ml of fresh toluene to remove any residuals. The fluorinated membranes were then transferred into a covered petri-dish and were placed in the oven at 120°C for 2 h. Finally, backwashing with ethanol (30% vol.) was performed for 5 minutes at a pressure of slightly higher than the LEP of ethanol solution to open the pores plugged by residuals from fluorosilanization step. These membranes are labelled FTCS -TiO2-PVDF membranes. Figure 3-2 Coating of TiO2 nanoparticles on the membrane by vacuum filtration technique 3.5 Membrane Characterization Comprehensive characterization techniques were used to analyse and study the prepared nanocomposite membrane morphology and chemistry. Those techniques can be related to the morphological (SEM, TEM, AFM, DLS, molecular weight cut-off measurement and porosimetry), surface chemical (stream potential, EDAX, BET, Lowry method for protein adsorption, contact angle and surface free energy measurement, FTIR and XPS), and thermomechanical (TGA, DSC, DMA and tensile strength test) properties. 3.5.1 Scanning electron microscopy, energy dispersion of X-ray analysis and transmission electron microscopy (TEM) The morphology of membranes was characterized by FESEM (Hitachi S4500II or Hitachi S900) operating at 15 kV and 4 kV, respectively. Membrane samples were prepared for Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 70 imaging by drying under vacuum and sputtered with a thin layer of chromium to render them conductive (Figure 3-3 ). For cross section imaging, the membrane was first frozen in liquid nitrogen and then fractured prior to the sample preparation and imaging steps. The existence and coverage of TiO2 on the surface of membranes were determined by EDAX (Energy dispersion of X-ray, Hitachi S3400). The Philips CM200 field emission gun transmission electron microscope (FETEM) was used to investigate the effect of modifications on the agglomerations of TiO2 nanoparticles and also to measure the particles crystal sizes. For TEM imaging, the nanoparticles were first dispersed in ethanol by sonication for a few minutes and then a droplet of the prepared solution was dropped on a copper grid and dried at atmospheric conditions. (a) (b) (c) Figure 3-3 Sample preparation of SEM S4500 and S3400-I: a) before chromium coating (left), after chromium coating (middle) and sandwiched membranes by using double sided sticky tapes after chromium coating (right) for cross section imaging, and b) sample preparation of SEM S900 after chromium coating, and c) transmission electron microscopy (TEM) copper grids Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 71 3.5.2 Particle size distributions The Malvern Mastersizer S (Brookhaven Instrument Corporation) which works based on dynamic light scattering (DLS) was used to measure the particle size distribution and polydispersity of TiO2 particles. Before any measurement, the particles were dispersed in ethanol or DMAC and sonicated for 1 minute. 3.5.3 Atomic force microscopy (AFM) The surface morphology and roughness of the membranes were characterized by atomic force microscopy (Digital Instrument 3000). The samples were cut into pieces of 3cm by 5cm and 5 different parts of each sample were scanned in tapping mode at 20μm×20μm. Data scales and scan rates for TiO2 blend membranes (Figure 4-10 ) were 0.029Hz and 500nm and for TiO2 coated membranes (Figure 6-3) were 0.38 Hz and 990nm, respectively. The number of samples in each scan for all of the samples was set to 256. 3.5.4 Surface area and porosity The surface area and porosity of a solid material can be obtained by using physical adsorption and capillary condensation principles. From the nitrogen adsorption and desorption isotherms, information about the surface and internal pore characteristics of the membranes can be obtained. The method of Brunauer, Emmett, and Teller (BET) is employed to determine surface area on the isotherm of adsorption [257]. At low pressure, N2 molecules begin to adsorb on the isolated sites of the sample and then the pressure increases to increase the coverage and form a monolayer of N2 molecules, at which point the BET equation is used to calculate the surface area. Further increase in the pressure will result in the formation of multilayer coverage and filling the smaller pores in the sample at the beginning. Still further increase in the pressure will fill the larger pores and causes complete coverage. After saturation, reducing the pressure evaporates the condensed gas and forms the desorption isotherms. Evaluation of the adsorption and desorption isotherms reveals information about the characteristic of pores such as size, volume and shape. In this study, the method of Barrett, Joyner and Halenda (BJH) is employed for calculating pore size average from the isotherms using the Kelvin model of pore filling [258]. The Micromeritics Tri Star 3000 Analyzer (using Brunauer, Emmett, and Teller (BET) and Barrett, Joyner and Halenda (BJH) models) was used for measuring the surface area and porosity of the TiO2 coated and uncoated UF membranes. Each membrane was completely dried in a vacuum oven for 2 days at room temperature and then cut into small pieces. The Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 72 small pieces were put in the machine chamber and were vacuum dried again at 120ºC for 3h prior to analysis. At least 0.2 g of each sample was loaded into the instrument chamber. 3.5.5 Capillary flow porometry A capillary flow porometer (model no: CFP-1200-AEXL) from PMI Porous Materials Inc. shown in Figure 3-4 was used to conduct the "bubble-point" test in order to measure the mean flow pore size and the bubble pressure point of microporous membrane. Wet-up/dry-down mode was used for the porometry test on flat sheet membranes with the diameter of 22.25mm. In order to wet the samples, before running the experiments, one side of the membranes was filled with GalWick liquid with surface tension of 15.9Ȁ. Each experiment was repeated at least 3 times and the average was reported. A 22.25 mm in diameter membrane Figure 3-4 Capillary flow porometer for measuring pore size and distribution of microfiltration membranes 3.5.6 Molecular weight cut-off The molecular weight cut-off (MWCO) of membranes was characterized by measuring the rejection of dextran at different molecular weights. A total organic carbon analyser V-CSH (Shimadzu TOC-VCSH & Auto-sampler ASI-V) was used to calculate the rejections. The membranes were soaked in MilliQ water for 24 h and then the MilliQ water was filtered to Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 73 remove any organic residue (solvent and glycerol) from the membranes. The concentration of dextran solution was 2 g.L-1 and the solution was stirred during dextran filtration. 3.5.7 Contact Angle Measurement The contact angles formed by water droplet (2.5-3 ul) on the membranes surface were measured by the sessile drop technique (KSV Cam 200 instrument, Finland). The total frame number was set to 60 with a 1 or 0.1s interval. The average of at least 5 measurements was reported. For hollow fiber membranes the angle between the membrane surface and the meniscus formed by the water was measured. 3.5.7.1 Advancing, receding and hysteresis contact angle The advancing and receding contact angles were calculated in two independent methods of tilting base and add/remove volume method [216, 259]. In the first method, once the sessile drop is placed on the horizontal surface with a zero degree angle, the surface is tilted from 0° to 90° to form downhill and uphill sides with the state of imminent wetting and dewetting respectively. The tilt angle increases to the point where the droplet releases, at which point the downhill contact angle represents advancing contact angle while the uphill contact angle represents the receding contact angle. The difference between the advancing and receding contact angles is the contact angle hysteresis. In the add/remove volume method, the syringe tip remained attached to the droplet after it being in contact with the membrane surface. The volume of the droplet increases to the maximum until the interfacial area increases. The formed angle at the maximum volume is advancing contact angle. In order to measure the receding contact angle, the volume of the liquid decreases to the point where the interfacial area starts to decrease. At that point, the receding contact angle is measured. Similar with the tilting base method, the difference between these contact angles is the contact angle hysteresis. 3.5.8 Surface Free Energy of Membrane To calculate the surface free energy of membranes, acid-base (van Oss) approach was applied. In this method, contact angles against at least three liquids with known parameters are measured and the surface free energy of the membrane was calculated from a set of three first order linear equations [260]. In this work, Milli-Q water, glycerol and di-iodomethane were used as the three liquids. Table 3-1 shows the parameter of acid-base (van Oss) approach used in this study [261]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 74 Table 3-1 Parameters of acid-base (van Oss) approach γTOT γd γ+ γ- MQ water 72.80 21.80 25.50 25.50 Glycerol 64.00 34.00 03.92 57.40 Di-iodomethane 50.80 50.80 00.00 00.00 “γ” refers to surface tension (surface free energy) and the superscripts d, + and - refers to dispersive, acid and base components respectively. 3.5.9 Static Protein Absorption To investigate the protein resistance characteristics, the Lowry Assay Analysis was used. The amount of protein extracted were then analysed by UV visible spectrometry (Cary 300 Scan) [262]. Details of Lowry method is come below: In order to use Lowry Assay analysis four different solutions needs to be prepared beforehand: for solution “A”, 0.2 g of sodium tartrate and 10 g of sodium carbonate was dissolved in 55 ml of 1N sodium hydroxide and volume increased to 100 ml with Milli-Q water. 2g of sodium tartrate and 1 g of anhydrous copper sulphate was dissolved in 90 ml of Milli-Q water and 10 ml of 1N sodium hydroxide to prepare solution ”B”. A daily solution of “C” was prepared by mixing 1 part of Folin-Ciocalteu reagent with 2 parts of water. The last solution “D” was prepared by dissolving 5 g of sodium docedyl sulfate (SDS) in 90 ml of 0.5N sodium hydroxide and then was heated to 60oC and stirred slowly with a magnetic stirring until all SDS had dissolved, and then volume increased to 100 ml with Milli-Q water. A four cm2 sample of each membrane was soaked in a 0.5 wt.% BSA (pH ~ 4.8) solution for 4 h and then cut into small pieces and soaked in 2 ml of solution D and constantly agitated on a rotary shaker for 1 hour. Three 150 μl samples of the solution were transferred into separate sampling tubes before adding 100 μl of 2N NaOH and 180 μl of solution A to each sampling tube. The tubes then were shaken briefly and allowed to stand at room temperature for half an hour. Afterward, 20 μl of solution B was added to each tube, which was shaken briefly and allowed to stand for another 20 min. Two portions of 300 μl (600 μl) of solution C was then added to each sampling tube and was shaken briefly and then allowed to stand at room temperature for about 60 min or until there was no color change. The final bluish solution in each tube was then transferred into a microcuvette to measure its absorbance against a blank Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 75 solution at wavelengths of 650 nm and 750 nm using a UV-Visible spectrophotometer. It should be pointed out here that the blank solution is similar to the previous solution but without fouled membrane pieces and was prepared from solution D with the same procedure as described above with double the amount of solutions. The calibration curve was used to determine the mass of protein (mg) on the membrane at 750 nm (shown in Figure 3-5 ) based on standard BSA solutions, as it was more accurate. The final determined mass was divided by 4 cm2 to report the amount of absorbed mass per initial sample area (mg/cm2). Calibration Curve (BSA) 1.5 y = 27.681x 1.2 R2 = 0.9981 0.9 0.6 Absorbance y = 21.521x 0.3 R2 = 0.9882 0.0 0 0.01 0.02 0.03 0.04 0.05 Mass of BSA (mg) 650 nm 750 nm Figure 3-5 Calibration curve for absorbance versus mass of BSA (mg) 3.5.10 Streaming potential measurements The Sur PASS electro kinetic analyzer (Anoton Paar Corporation) was used in surface analysis to investigate the zeta potential of membranes based on a streaming potential and streaming current measurement. The zeta potential is related to the surface charge at a solid/liquid interface and is a powerful indicator for the surface chemistry (pH titration) and liquid phase adsorption processes. The membrane has a dimension of 2cm by 5cm and was placed in the clamping cell with two spacers (Figure 3-6 ). The 500mL of 1mM potassium chloride solution, prepared with pure water, was supplied to cell as background electrolyte and 0.25M HCl (acid) and 0.1M KOH were chosen for titration. The acid and base volume increments were set to 0.1mL. The experiment was started from pH of around 5 up to 9.5 (for basic range) and then after two times rinsing with MQ water from pH down to 2 (for acidic range). For each pH, the zeta potential measurement was repeated 4 times (2 measurements from left to right and another 2 from right to left) and average was taken. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 76 Clamping cell Membrane Membrane holders Spacers Figure 3-6 Clamping cell for stream potential measurement 3.5.11 Surface Chemistry (XPS) X-ray photoelectron spectroscopy (XPS) was performed with a VG Scientific (UK) surface analysis system. The X-ray radiation source was monochromated Al Kalpha, 1486.6eV, 200W. Data was recorded in the “Fix Analysis Transmittance” (FAT) mode. For wide scans, pass energy of 100 eV and for region scans pass energy of 20 eV was used. C1s at 285.0eV was used for binding energy reference. Sample is irradiated with soft x-rays to eject electrons from the core energy levels of the atoms present. Those generated with characteristic low energies within the top few atomic layers of the surface can escape into the high vacuum spectrometer and be energy analyzed to form a spectrum. The peak energy position provides an elemental analysis, and precise chemical specification can be obtained from the detailed peak shapes using curve resolving procedures. The un-treated samples were mounted on the copper sample stubs by means of double-sided adhesive tape. Under these conditions, no charging effect was observed for samples. The detection depth of XPS is typically about 5-10 nm. 3.5.12 Fourier transform infrared spectroscopy (FTIR) The surface functionalization of TiO2 nanoparticles with APTES was examined by using Fourier Transform Infrared Spectroscopy shown in Figure 3-7 (FTIR, NICOLET 5700 FT-IR, Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 77 Thermo Electron Corporation). Dried KBr was mixed with TiO2 particles to produce more visible spectra. Figure 3-7 Fourier transform infrared spectroscopy used in this study 3.5.13 Thermal analysis (TGA and DSC) To investigate the quality of dispersion of TiO2 nanoparticles in membrane, and also membrane liquid absorption ability, thermogravimetric analysis (TGA5000, TA instrument) at a heating rate of 20o C per minute up to 800o C under air atmosphere was used. Membrane samples were cut into small pieces and were placed in a high thermal resistance tray (Figure 3-8 ). Differential scanning calorimetry (DSC2010, TA instrument) at a heating rate of 10o C/min up to 300o C was used to observe the effect of particle modifications on glass transition temperature (Tg). To enhance the DSC signal by removing the thermal history in samples, samples were heated to 260oC at the same heating rate and then were cooled to room temperature and heated again to 300º C (the Tg was determined from the second run). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 78 (a) (b) Figure 3-8 TGA (a) and DSC (b) instruments used in this study 3.5.14 Dynamic mechanical analysis (DMA) Dynamic mechanical analysis is a useful technique for measuring the glass transition temperature as well as the viscoelastic properties of the membranes. DMA was used for hollow fibre membranes which were fastened in the tension clamps of a TA Instruments Q800 dynamic mechanical analyser. Samples were heated at 2°C/min from ambient temperature up to a final temperature of 280°C whilst oscillated at a frequency of 1Hz at amplitude of 10μm. Glass transition temperature values were determined from the peak of the loss modulus trace. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 79 3.5.15 Membrane tensile strength test Mechanical properties (Tensile strength and Elongation at break) of the control and modified membranes were obtained from a uniaxial tensile test machine as shown in Figure 3-9 (Instron 5866). A cross-head speed of 50 mm/min and static load cell of 10N was used. Measurements were performed at room temperature (25ºC) with film specimens. The flat sheet dimensions were 10 mm in width, 50 mm in length and 0.14 mm in thickness while the hollow fiber membranes dimensions were approximately 7 cm in length, 1.1-1.3 mm in diameter and 0.11-0.18 mm in thickness. Clamped membrane Figure 3-9 Flat sheet membranes clamped in a uniaxial tensile test machine 3.5.16 Membrane Performance 3.5.16.1 Membrane performance after blending the TiO2 nanoparticles into PES UF flat sheet membranes Water flux and flux recovery were measured using a dead end cell filtration set up (Figure 3-10 ) at fixed speed of 600 rpm. The flux measurement of pure water was done at 1 bar and room temperature with the effective membrane area of 0.0014 m2. It should be noted that the fluxes decreased gradually due to compaction and then reached to a constant flux Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 80 after about 30 min. The membranes were soaked in ethanol for wetting for 10 mins before experiment. Pressure transducer Dead-end cell Digital balance Stirrer Pump Feed reservoir (a) Feed inlet To pressure transducer Permeate out let Magnetic stirrer Membrane (b) Figure 3-10 Dead end filtration set up which was set to work at both constant pressure and flux mode (a) and dead end cell details (b) The clean water fluxes were measured at constant pressure of 1 bar while the membranes were fouled by filtration of BSA solution (0.5 wt.%) at constant flux of 50 L/m2h for 2 h. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 81 Constant flux was chosen to provide constant hydraulic conditions for membranes with different permeabilities. At the beginning of the experiment, a 40kPa pressure was provided and pump speed was increased gently to reach 50 L/m2h flux. During the experiment the pump speed and feed side pressure was adjusted to keep the flux constant. The fouled membranes were hydraulically and chemically cleaned. For hydraulic cleaning, the membranes were rinsed 2 times and then 60 ml of Milli-Q water was added to the dead end cell and stirred for 5 minutes followed by another 2 times rinsing. For chemical cleaning, 100 ml of sodium hydroxide solution (2 g/L, pH=12) was added to the cell and stirred for 20 minutes and then rinsed 2 times. In order to evaluate the fouling performance of membranes, flux recovery (FR) and resistance of membranes was calculated as follows: ሺΨሻ ൌ ఽూ ൈ ͳͲͲ 3-1 ాూ JBF and JAF are the pure water flux of membrane before fouling and after physical and chemical cleaning, respectively. Fouling behaviour can be demonstrated by estimation of resistance of membrane as shown below: Intrinsic or membrane resistance ( Rm) ୫ ൌ 3-2 ஜൈాూ where TMP is transmembrane pressure (100 kPa) and μ is permeate viscosity. Irreversible resistance (Rir) ୧୰ ൌ െ୫ 3-3 ஜൈఽూ The JAF is measured at 100 kPa. Reversible resistance (Rr) ୰ ൌ െ୫ െ୧୰ 3-4 ஜൈూ 2 Where, JF is the BSA filtration flux which is set at 50 L/m h in our experiment (TMP was considered after 2h filtration). Total resistance (Rt) ୲ ൌ୫ ୰Ǧ୧୰ 3-5 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 82 3.5.16.2 Membrane performance after blending the TiO2 nanoparticles into PES hollow fiber membranes Water flux and flux recovery were measured using a cross flow system shown in Figure 3-11 and run at fixed cross flow speed of 0.05 m.s-1 and flow rate of 75 mL.min-1. The membranes were soaked in ethanol for wetting for 10 mins before experiment. Each module was made with 6 hollow fiber membranes with 15cm effective length. The inside module pressure was controlled by adjusting a valve on the retentate side and feed solution was on the shell side. The flux measurement of pure water was undertaken at 100 kPa and room temperature (25oC). The membranes were fouled by filtration of BSA solution (2 g.L-1) at constant pressure of 100 kPa for 2 hours. BSA concentration was estimated using UV-Visible spectrophotometry (Cary 300 Scan) for rejection. The fouled membranes were hydraulically and chemically cleaned. For hydraulic cleaning, the membranes were rinsed 2 times by keeping the valve on the retentate side fully opened and adjusting the pump flow rate to 360 mL.min-1 (5 times the cross flow rate used during filtration). For chemical cleaning, membranes were rinsed by 500 ml of sodium hydroxide solution (2 g/L, pH=12) for 20 min (pump flow rate of 360 mL.min-1 with full-open valve). Three cycles of fouling-cleaning were repeated consecutively for each membrane module and at the beginning of first cycle membranes were wetted by rinsing/filtering with ethanol. In order to evaluate the fouling performance of membranes, flux recovery (FR) and intrinsic membrane resistance ( Rm) were calculated using equation 3-1 and 3-2 while the total and fouling membrane resistance were calculated as follow: ൌ 3-6 Ɋൈ ൌ െ 3-7 Figure 3-11 Cross flow set up for filtration performance test for hollow fiber membranes Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 83 3.5.16.3 Membrane performance after coating the TiO2 nanoparticles on the PES UF membranes Similar to previous section, water flux and flux recovery were measured in a dead end cell with the effective membrane area of 0.0014m2. Pure water flux was measured at 1 bar and room temperature and the flux value after 30 min compaction was reported. The membranes were soaked in ethanol for wetting for 10 mins before experiment. The membranes were fouled by filtration of humic acid solution (0.1wt. %, pH~9.4) at a constant pressure of 1 bar for 2 h. In a separate set of experiments, fouling performance of the membranes was also investigated by the filtration of 0.1 wt.% humic acid (HA) at constant flux (85 L/m2 h) for 2 h. At the beginning of the experiment, a 40kPa pressure was provided and pump speed was increased gently to reach 50 L/m2h flux. During the experiment the pump speed and feed side pressure was adjusted to keep the flux constant. The fouled membranes were hydraulically and chemically cleaned. For hydraulic cleaning, the membranes were rinsed 2 times and then 100 ml of Milli-Q water was added to the dead end cell and stirred for 5 min followed by another 2 times rinsing. Then, the membranes were flipped over in the cell for backwashing for 5 min at 1 bar. For chemical cleaning, 100ml of sodium hydroxide solution (2 g/L, pH=12) was added to cell and stirred for 20min. Equation 3-1 and 3-2 were used for calculation of flux recovery (FR) and intrinsic membrane resistance (Rm). Different membranes resistances were calculated as follow: ୦୷ୱ୧ୡୟ୪ ൌ 3-8 ஜൈౌ౯౩ౙౢ େ୦ୣ୫୧ୡୟ୪ ൌ 3-9 ஜൈిౣౙౢ ୟୡ୩୵ୟୱ୦୧୬ ൌ 3-10 ஜൈాౙౡ౭౩ౝ ୦୷ୱ୧ୡୟ୪is membrane resistance after physical cleaning and to calculate that pure water flux after physical cleaning ( ୷ୱ୧ୡୟ୪ ) is used. Membranes resistances after chemical cleaning and backwashing can be calculated similar to calculation of ୦୷ୱ୧ୡୟ୪ (Equation 3-9 and 3-10). 3.5.16.4 Membrane Performance in a Direct Contact Membrane Distillation (DCMD) Process The MD performance of virgin and modified flat sheet membranes with the approximate contact area of 14.4 cm2 was examined in a direct contact membrane distillation (DCMD) set up (Figure 3-12). A horizontal configuration and counter current flow was implemented to achieve the highest temperature driving force through the module. Table 3-2 shows the operating parameters used during the MD experiments. The two synthetic feed solutions used Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 84 in this experiment were sodium chloride solution (3.5 wt. % [263]) to model seawater and humic acid solution to model natural organic foulant. In the membrane distillation process the concentration of humic acid for the fouling experiment is usually between 20 to 100 mg/L [264]. In this study high concentration of humic acid, 150 mg/L with and without 3.775 mM CaCl2, was used to potentially speed up the fouling process Table 3-2 Operating parameters for DCMD experiments Foulant Sodium chloride Humic acid (150 mg/L (3.5 wt. %) with/out CaCl2 3.775 mM) Feed inlet temperatures1 70o C 70o C Permeate inlet temperatures1 25o C 25o C Feed inlet Pressure 20 kPa 5 kPa Permeate inlet pressure 20 kPa 15 kPa Feed flow rate* 720 mL/min 300 mL/min Feed cross flow velocity 0.95 m/s 0.39 m/s Permeate flow rate** 700 mL/min 460 mL/min Permeate cross flow velocity 0.93 m/s 0.61 m/s Feed volume 1.3 L 2 L Reynolds number at feed side* 2727 1136 Reynolds number at permeate side ** 1191 783 1 the difference between feed inlet and out let temperature was around 3 o C while the difference between permeate outlet and inlet was 1.5 o C * data were measured at 70o C * * data were measured at 25o C Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 3 Materials and Methodologies 85 (a) (b) Figure 3-12 Schematic diagram (a) and lab scale set up (b) of Direct Contact Membrane Distillation Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 86 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 87 CHAPTER 4 BLENDING (FLAT SHEET NANOCOMPOSITE MEMBRANES) 4 Abstract As mentioned in literature review, TiO2 nanoparticles blended within polymeric membranes have shown to provide improvements in fouling performance. However, the agglomeration of nanoparticles remains as one of the major obstacles for generating a uniform surface, and also the mechanisms for improved fouling performance has yet to be elucidated. In this chapter, mechanical and chemical modifications approaches were adapted using Degussa P25 TiO2 nanoparticles to improve their dispersion. Afterward, the modified TiO2 nanoparticles were incorporated into polyethersulfone based in-house membranes and their effect on microstructure, surface chemistry, and fouling performance were investigated. Different techniques explained in chapter 3 were applied to characterize and explore the effect of different factors on fouling performance. The results showed that a good dispersion of nanoparticles in the membrane was achieved after both the chemical and mechanical modifications of particles, as a result of less agglomeration. The combination of chemical and mechanical modifications was found to have significant effects on surface free energy, roughness, surface pore size and protein absorption resistance as well as hydrophilicity. While previous researchers believe that the increase in hydrophilicity is the most likely reason for improvement in fouling performance, these other parameters such as changes in membrane morphology and local surface modifications may contribute just as much to greater fouling resistance when the effects of unmodified and modified TiO2 were compared. This chapter is based on: “The effects of mechanical and chemical modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes” by A. Razmjou, J. Mansouri and V. Chen. 2011, published in Journal of Membrane Science 378 (1-2), 73–84. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 88 4.1 Introduction In recent years, the incorporation of inorganic additives into polymeric materials has expanded markedly for filtration and gas separation membranes. A variety of nanoparticles have been introduced to modify organic membranes, such as SiO2, Al2O3, Fe3O4, ZrO2 and TiO2 [12, 59-61]. Among them, TiO2 has received the most attention because of its good physical and chemical properties, availability as well as its potential antifouling abilities [9- 11]. Polyethersulfone (PES) is one of the polymeric materials which commonly used in microfiltration [95-97], ultrafiltration [98, 99] as well as nanofiltration membranes [100]. Its wide application is a result of good chemical and thermal resistance, easy processing and environmental endurance. However, its poor antifouling properties affect its application and usage life [15]. Several studies investigated the incorporation of TiO2 into PES membranes. There are two main approaches for making nanocomposite TiO2 PES membranes: blending the nanoparticles into the membrane [13, 14, 95] and coating the nanoparticles on the surface of the membrane [15-17]. The focus of this chapter is on the former approach with the view of improving membrane performance by improving the dispersion of widely available commercial TiO2 powders in to flat sheet membranes. The coating approach will be discussed in chapter 6. In this approach, TiO2 nanoparticles are dispersed in a casting solution and then membranes are cast by immersion-precipitation or non-solvent induced phase separation method which was widely used for the preparation of polymeric membranes [107]. In immersion- precipitation, a solution of polymer and organic solvent is cast on a substrate and immersed in a coagulation bath to form the membrane [108, 109]. Although the primary particle size of commercial TiO2 nanoparticles such as Degussa P25 is about 20nm, its particle size as powder or in dispersion is in the range of hundreds of nanometres due to agglomeration. This agglomeration leads to not only an uneven distribution in the membrane but also potential reduction in antifouling abilities of TiO2 particles by changing parameters such as membrane topography and hydrophilicity as well as self-cleaning properties of particles [166]. The presence of hills and valleys on the surface of a rough membrane increases the sites which favour the attachment of foulants on the surface of the membrane [167]. It is also evident that membranes with higher hydrophilicity are less prone to fouling than hydrophobic membranes since it absorbs more water molecules than Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 89 foulant molecules. It was shown that TiO2 nanoparticles can effectively degrade organic materials under UV light. The agglomeration of particles reduces active sites for the degradation of organic materials [168]. All of these deficiencies caused by agglomeration negatively affect the effective utilization of TiO2 nanoparticles in the nanocomposite membranes. To overcome agglomeration, researchers have tried to use the in-situ formation of TiO2 within the PES matrix by sol-gel technology [14]. Using this technique, the nanoparticle size can be controlled by varying various parameters including the choice of organic additives [112]. However, agglomeration still exists even for the in-situ formation of particles due to the poor interfacial interactions between the hydrophilic nanoparticles and hydrophobic polymer and high surface energy of particles [169]. In addition, due to the difficulty and complexity of sol-gel reactions and also cost issues, the potential of using commercial TiO2 nanoparticles to improve PES membrane properties provides an attractive alternative. In order to avoid agglomeration, two methods have been generally tried: the dispersion of nanoparticles by conventional methods such as sonication and grinding (mechanical modification) and surface pre-treatment approaches for nanoparticles (chemical modification). Most researchers applied the former approach that is based on the shear forces provided by conventional mixers and/or normal sonicators [10, 17]. Since the intra- nanoparticles interaction are very strong, it is hard to break the intra-particle interactions with conventional mixing [170]. Thus, it seems crucial to have a stronger dispersion technique for the fabrication of nanocomposite membranes. In general, surface modifications as a complementary approach to mechanical modification of particles has been used for minimizing particle/particle interaction in the preparation of polymer composite with microfillers [169]. Recently, with the development in nanotechnology, the incorporation of chemically modified or functionalized nanoparticles into polymeric materials has become a topic of great interest. In this approach, the inorganic particles are coated with organic coatings by physical and/or chemical interactions between the particles and organic modifiers. In physical treatment, the reaction between organic surfactant or polymer and particles results a weak secondary forces such as van der Waals, hydrogen and electrostatic forces while in the chemical treatment there is a strong covalent attachment. However, the resultant bond is often a combination of chemical and physical reactions. One common method to apply technique for chemical modification of inorganic particles is treatment by silane coupling agents [171]. Silane coupling agents can effectively reduce the hydrophilic Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 90 nature and surface energy of the particles consequently reducing agglomerations and increasing matrix interactions [169, 171]. Silanization treatment by APTES was targeted in this study for chemical modification of TiO2 particles. In reviewing the previous work (section 2.3.3 of literature review), it is clear that the adverse effect of particle agglomeration has been overlooked in many studies. In fact, an ideal nanocomposite membrane is a membrane which is free of any agglomerations. The mechanism for improved fouling performance also has yet to be elucidated. In addition, the effect of particle size distribution, mechanical and chemical modification of particles on the membrane structure and surface chemistry (hydrophilicity, roughness and protein absorption) in conjunction with fouling performance has not been investigated. In this chapter, an effective mechanical modification using different dispersion techniques coupled with a chemical modification of particles (silanization treatment by APTES) was applied to reduce agglomeration of Degussa P25 TiO2 nanoparticles. The effect of modifications on the surface chemistry, membrane structure and fouling performance of ultrafiltration PES membrane was investigated and the contributing factors which have the most influence on the fouling mitigation were determined. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 91 4.2 Experimental 4.2.1 Materials Polymeric materials used in this chapter were: polyethersulfone (58000 g/mol) as polymer and polyvinylpyrrolidone (PVP, 40000 g/mol) as pore former and hydrophilic additive from BASF Co. Ltd and Sigma Aldrich, respectively. TiO2 nanoparticles (P25, 20nm) were also provided by Degussa. Dimethylacetamid (DMAC) as solvent and isopropanol as nonsolvent were supplied by Scharlav Chemie S.A. and Ajax Finechem Pty Ltd., respectively. Aminopropyltriethoxysilane (APTES) from Sigma Aldrich was used for surface modification of TiO2 nanoparticles. The bovine serum albumin (BSA) was purchased from Morrgate Biotech (reagent grade, pH 5). 4.2.2 Modification of TiO2 and Preparation of Membrane To avoid agglomeration of nanoparticles in membrane and also to increase the stability of particles in the casting solution, the modification of TiO2 nanoparticles via mechanical and chemical modifications was carried out, which have been explained in section 3.2.1 of materials and methodology chapter. The procedure of preparation of control and blend PES membrane with/out modified TiO2 nanoparticles was explained in section 3.2.2. In this chapter, modified particles refer to both mechanical and chemical being applied unless otherwise indicated. 4.2.3 Membrane Characterization Different techniques such as SEM, EDX, TGA, DSC, AFM, FTIR, contact angle goniometry, molecular weight cut-off, static protein absorption, stream potential and surface free energy measurement were applied to characterize and explore the effect of different factors on fouling performance. These techniques were explained in detail in chapter 3. Fouling performance of the membranes was also investigated by the filtration of BSA solution (0.5 wt.%) at constant flux of 50 L/m2h for 2 hours, for details see section 3.5.16.1 of chapter 3. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 92 4.3 Results and Discussions 4.3.1 TiO2 Nanoparticles Modifications The results in Table 4-1 show that the TiO2 average particles size decreased from around 380nm down to below 100nm after mechanical and chemical modifications. TGA results of unmodified and APTES coated TiO2 presented in Appendix (Figure A4) do not reveal any information about the quantity of APTES coating due to the limitation of the instrument of detecting molecular coating layers. To investigate that the chemically modification of TiO2 nanoparticles with APTES has been done successfully, the infrared adsorption of TiO2 nanoparticles was measured. This provides some information about the chemical bonding to the modified TiO2 particles. The modification of TiO2 with APTES is expected to occur by the reaction of surface hydroxyl groups of TiO2 with the APTES functionalities of silane. The formation of covalent bonds in this reaction can be observed by FTIR. Figure 4-1 shows the spectra of unmodified, chemically modified TiO2 with APTES and pure APTES. In the -1 spectral of original unmodified TiO2, the sharp band at around 1600 cm and the broad band -1 at around 3300 cm are due to absorbed water on TiO2 surface [265-267]. The comparison between the spectral of modified and unmodified TiO2 exhibits some new characteristics absorption peaks of APTES, which means that the particles were successfully modified by - APTES. In the spectra of modified TiO2 with APTES, the bands at around 2926 and 2973 cm 1 -1 being to alkyl groups [-(CH)n-] [268, 269]. Also, the bands at around 1600 cm are assigned to the vibration of primary amine groups and the other vibration amine bands are around 3471 cm-1 which overlapped with the strong water band at around 3300 cm-1 [268-270]. In addition to the alkyl and amine groups, the Si-O-C group appeared as a double peak in the spectrum; at around 1075 and 1104 cm-1 [268]. There is also a weak band at around 950 cm-1 which is assigned to Ti-O-Si [270]. Table 4-1 Incorporation of TiO2 nanoparticles into in-house PES UF flat sheet membranes (means and standard deviations are indicated) Average particle size(nm) Polydispersity Unmodified P25 387±7.2 0.32±0.05 Mechanically modified 160±1.8 0.26+0.02 Mechanically and chemically modified 84.2±10 0.19±0.01 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 93 (a) (b) -1 Figure 4-1 FTIR spectra of unmodified P25, Chemically modified TiO2 with APTES and Pure APTES (a) from 2300 cm to 3550 cm-1 (b) from 650 cm-1 to 1700 cm-1 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 94 4.3.2 Dispersion of TiO2 Particles (TGA and EDAX) The presence of TiO2 was investigated by EDAX analysis which confirms the existence of TiO2 on the top surface of the nanocomposite membrane (Figure 4-2 ). A peak observed around 4.5KeV belongs to Ti and the peak around 2.5KeV belongs to sulphur which comes from PES. To examine if TiO2 is well dispersed in the membrane matrix, thermogravimetric analysis (TGA) test was done on control membrane and three different pieces of a flat sheet membrane with 2 wt.% TiO2 (10 cm×20cm). The TGA results which were summarised in Table 4-2 show that the residue for all three different samples of 2 wt.% TiO2 is 3%. This indicates that the TiO2 nanoparticles were well dispersed with the minimum of agglomeration size throughout the membrane. 3 2.5 Control membrane 2 1.5 1 Counts (cps/ev) Counts (cps/ev) 0.5 0 Energy (ke V) 4 3 S Blend membrane 2 C Ti 1 O 0 Counts (cps/ev) 0246 Energy (keV) Figure 4-2 Top surface EDS spectra of control and TiO2 blend membrane (2 wt.%) Table 4-2 Expected and experimental residual values (wt. %) for different concentrations of TiO2 Experimental TiO content Expected TiO 2 2 Residual from TiO in (wt. %) in in membrane 2 TGA (wt.%) membrane casting solution (wt.%) (wt.%) Mechanically and 2% 10-12.5 3%±0 11-12 chemically 4% 16-20 7%±0 18-20 modified 6% 23-27 10%±0 25-27 Unmodified 2% 10-12.5 6%±1 20-25 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 95 The casting solution contained 2g TiO2, 16 g PES and 4g PVP, so the final amount of TiO2 in the membrane should be between 10% and 12.5%, taking into account the fact that part or all of the PVP may leach out during the fabrication process. The decomposition temperature of PES membrane reported between 350ºC and 480ºC [13, 271], so the weight loss between 350ºC and 480ºC is around 25%-27%, which includes PES and at least part of the PVP (see the control curve in Figure 4-4). The ratio of the residual weight (3%) to the organic content of 25%-27% yields an estimate of 11 to 12 wt.% TiO2 in original membrane which is in the expected range. This confirms not only a good dispersion of TiO2 but also shows that the loss of TiO2 nanoparticles during fabrication process was insignificant. However, the ratio of average residual for unmodified TiO2 blend membrane (5%) to 24%-29% organic content gives 17-20% which is not in the expected range (10%-12.5%), see Figure 4-3. This suggests inhomogeneity of membrane in regard to TiO2 dispersion. The same sets of experiments were done for 4 wt.% and 6 wt.% TiO2 loaded membrane (see Table 4-2 ). The results indicate that the modification of particles could provide a good dispersion of TiO2 in membranes regardless of its concentration. However, there are some differences in the weight loss prior to polymer degradation. Figure 4-4 shows the TGA for control and different blend membranes. Higher weight loss below 400ºC may be an indication of higher percentage of volatile residues including water, solvent, isoporopanol and glycerol. Higher absorption may be due to larger pore size or higher porosity. In Figure 4-4, the 2 wt.% mechanically and chemically modified TiO2 loaded membrane lost 74% of its weight which is around 14% greater than the control and 20% greater than the other blend membranes, in average. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 96 Figure 4-3 A typical TGA for a 2 wt.% unmodified TiO2 flat sheet blend membrane (10cm×20cm) Figure 4-4 Effect of various TiO2 concentrations on the initial weight loss Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 97 TiO2 nanoparticles distribution across the membranes The distribution of nanoparticles were characterized by FESEM (Hitachi S3400) equipped with EDAX gun. In Figure 4-5, the TiO2 nanoparticles were shown in red color after EDAX mapping. The figures confirm a uniform distribution of nanoparticles across the membrane after mechanical (and chemical) modifications of TiO2 nanoparticles (Figure 4-5b and c) while incorporation without any modifications caused particles to agglomerate across the membrane (Figure 4-5d). It should point out here that the technique is not the best way to pick up an accurate distribution since the interaction volume of EDAX beam for 15kV is around 2-7 microns, which cannot point exactly the location of nanoparticles. Besides, the cross sections of the membranes do have microvoids which make it difficult for detector to locate the received X-ray emissions. However, the method is suitable to gain a rough idea of large agglomerations. (a) (b) (c) (d) Figure 4-5 EDAX mapping coupled with SEM images for (a) control, (b) mechanically modified, (c) mechanically and Chemically modified and (d) unmodified TiO2 PES membrane Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 98 4.3.3 Effect of TiO2 Modification on Cross-Section Micro-Structure In Figure 4-6 and Figure 4-7 the overall porosity for 2 wt.% TiO2 appears greater than the others although the porosity between pores wall were higher for membranes with higher than 2 wt.% TiO2. The Figures show that the addition of TiO2 increased the length of microvoids and also makes the pore wall more granular up to 4 wt.% and then the granularity decreased. The higher magnification images of walls between the voids showed that the structure shifts from granular to nodular by increasing the TiO2 content. By introducing TiO2, it seems that the finger-like microvoids has elongated across the thickness, and become wider close to the back side of membrane (Figure 4-6 ). The widths of microvoids seem larger for membrane with 2 wt.% modified TiO2 and also there are more circular voids close to the back-side of this membrane. On the other hand, the unmodified TiO2 had no significant effect on the membrane microstructure (Figure 4-6b) and it only caused the microvoids to grow at the upper parts of the membrane, consequently increasing in porosity, while kept the large microvoids at the lower parts of the membrane similar to those found in control membranes. a (0% TiO2) b (2% Unmodified TiO2 ) c (2% Modified TiO2) d (4% Modified TiO2) e (6 % Modified TiO2) Figure 4-6 Effect of unmodified and different modified TiO2 concentrations on PES membrane structure (300-500x) Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 99 a (0% TiO2) b (2% TiO2) c (4% TiO2) d (6 % TiO2) Figure 4-7 Effect of different modified TiO2 concentration on PES membrane structure (1-1.5K) 4.3.4 Effect of TiO2 Modification on Glass Transition Temperature Glass transition temperature (Tg) is very sensitive to chain mobility and may be related to the quality of dispersion. The quality of interaction between the nanoparticles and polymer molecules could affect the chain mobility and region of free volume which consequently resulted in a reduction or increase in Tg [272, 273]. A differential scanning calorimetry (DSC) was used to measure glass transition temperature (Tg). Since the TiO2 nanoparticles are highly hydrophilic and PES molecules are hydrophobic, during phase inversion there is a repulsive interaction resulted in a higher chain mobility and region of free volume. Therefore, the Tg would be expected to decrease specially after the mechanical modification of particles (Table 4-3 ). However, the result for chemical modification shows a significant increase in Tg, which is perhaps due to changing the nature of interaction between inorganic nanoparticles and polymer molecule from repulsive to attractive interactions [272, 274]. The silane coupling agent could reduce the hydrophilicity of nanoparticles and consequently reduce the chain mobility and region of free volume in nanocomposite membranes [169, 272]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 100 Table 4-3 Average value of glass transition temperature for each type of membrane measured by DSC Glass transition temperature (Tg), °C Control Membrane 215±1 Unmodified TiO2 blend membrane 220±5 Mechanically modified TiO2 blend membrane 203±1 Mechanically and chemically modified TiO2 blend membrane 230±2 4.3.5 Effect of TiO2 Modification on Surface Pore Size Molecular Weight Cut-Off and Cross-Section Micro-Structure Molecular weight cut-off curves for unmodified and modified TiO2 blend membrane as well as control membrane were shown in Figure 4-8 . Results show that the MWCO at the rejection of 90% shifts from 100 kDa to around 240 kDa for modified TiO2 blend membrane while the membrane with unmodified particles has the same MWCO as the control membrane. This implies that the modification on TiO2 has led to membranes with larger surface pore size. This result along with the TGA and SEM data demonstrated that the modification of TiO2 nanoparticles increased the pore size, especially the surface pore size, and porosity of the modified membrane in comparison with those of the control and unmodified TiO2 blend membranes. During the phase inversion process, the higher counter diffusion velocity of solvent and nonsolvent resulted membranes with the higher porosity and pore size especially at the upper (top) side of membrane. To explore this, different concentrations of isopropanol in coagulation bath was used to decrease the counter diffusion velocity, see Figure 4-9. As can be seen, by increasing the isopropanol content in the coagulation bath and consequently reduction in counter diffusion velocity, the micropores or voids have been suppressed and the morphology of the top layer changed from porous to dense [275]. The static contact angle of o TiO2 nanoparticles has been reported to be around 10 indicating a very hydrophilic surface [276]. Thus the higher affinity of TiO2 to water than PES could increase the diffusion velocity of nonsolvent in the nascent membrane during phase inversion. This behaviour was also reported by other researchers [9, 95, 277]. However, the role of particle size has not been explored in their work. The first advantage of modifications of particles is an increase in the dispersion of particles throughout of the membrane, which resulted in a higher interaction between PES molecules and nanoparticles than solvent molecules. Therefore, solvent molecules can diffuse more easily from polymer structure and increase the counter diffusion velocity in the phase inversion process. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 101 100 98 Control 96 Unmodified Modified 94 92 90 88 Rejection (%) 86 84 82 0 100 200 300 400 500 MW of Dextran, kDa Figure 4-8 Molecular weight cut-off for modified, unmodified and control membrane (0%) (50%) (70%) (100%) Figure 4-9 Control PES membrane prepared from DMAC as solvent and coagulating into mixtures of water and isopropanol (concentration of isopropanol indicated, wt. %) 4.3.6 Effect of TiO2 Modification on Surface Roughness Atomic Force Microscopy (AFM) The effect of modifications on roughness was shown in Figure 4-10 . The images indicate that the modifications reduced the roughness of the membrane. The mean roughness (Ra) of the membrane was reduced from 50.76nm for control to 39.77nm for modified blend membrane. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 102 It is well established that membrane with lower roughness and surface energy has stronger antifouling abilities. Furthermore, foulants are likely to be absorbed in the valleys of membrane with coarser surfaces, resulting in clogging of the valleys [10, 249, 278]. Therefore, it is important to fabricate membrane with less surface energy and roughness to improve antifouling ability and performance of membrane. The above results along with the following fouling experiments suggest the utility of particle modifications as a new approach for making membrane with improved antifouling performance. (a) (b) (c) Figure 4-10 Three dimensional AFM images for (a) modified, (b) unmodified TiO2 blend and (c) control membrane 4.3.7 Effect of TiO2 Modification on Membrane Tensile Strength The mechanical properties of membranes such as elastic properties and maximum strength can be quantified by tensile strength test. The loading force and length of the membrane at the breaking point were recorded at Figure 4-11 and Figure 4-12 . From the graphs, 57% increase in max load (N) and 40% increase in extension at breaking point (mm) were observed after chemically and mechanically modifications of TiO2 nanoparticles. Comparison between control and only mechanically modified TiO2 PES membranes showed that only mechanically modification reduces the maximum load and extension at breaking point Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 103 whereas the one step further in modification by functionalization of APTES molecules could significantly enhance the mechanical properties of membranes. This behavior can be explained by change in the nature of interaction of nanoparticles and polymer matrix which was discussed earlier. 7 57% increase in Max Load (N) after chemically and 6 mechanically modification of Titania nanoparticles 5 4 3 Max Load (N) (N) Load Max 2 1 0 Control Unmodified Titania Mechanically Chemically and PES membrane Modified Titania Mechanichally PES membrane Modified Titania PES membrane Figure 4-11 Maximum load at breaking point for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes (means and standard deviations are indicated) 16 40% increase in extension at breaking point (mm) after chemically and mechanically modification of Titania 14 nanoparticles 12 10 8 6 4 2 0 Extension at breaking point (mm) (mm) point breaking at Extension Control Unmodified Titania Mechanically Chemically and PES membrane Modified Titania PES Mechanichally membrane Modified Titania PES membrane Figure 4-12 Extension at breaking point for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes (means and standard deviations are indicated) Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 104 4.3.8 Effect of TiO2 Modification on surface chemistry: Contact Angle, Protein Absorption, Surface Free Energy and Stream Potential In order to investigate the effect of TiO2 nanoparticles on the hydrophilicity or hydrophobicity of the membranes, the angle between a small droplet of water and the flat horizontal surface of the membrane was measured by Contact Angle Goniometric instrument. Results shown in Figure 4-13 indicate that the addition of TiO2 itself could increase the hydrophilicity by around 18% regardless of modifications or changing the amount of nanoparticles in membrane. 80 70 60 50 40 30 20 Contact Angle Contact (º) 10 0 0 wt.% TiO2 2 wt.% TiO2 2 wt. % TiO2 4 wt.% TiO2 6 wt.% TiO2 control unmodified modified modified modified Figure 4-13 Average contact angles on the top surface of the PES membranes with different modified and unmodified TiO2 content (means and standard deviations are indicated) Figure 4-14 shows the amount of absorbed BSA on the surface of the membranes measured by Lowry method. As can be seen, the 2 wt.% modified TiO2 blend membrane showed more resistance against the absorption of protein. In fact, the control and unmodified blend membranes absorbed around 1.6 times greater than that of modified membrane (this is based on nominal surface area). The surface free energy of different membranes calculated by acid- base (van Oss) approach is shown in Figure 4-15 which shows that the modifications led to membranes with slightly lower free surface energy. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 105 45 40 ) 2 35 30 μ gr/cm 25 20 15 10 Absorbed BSA ( 5 0 Control Unmodified 2% Modified Figure 4-14 Absorbed protein by Lowry method for control, unmodified and modified TiO2 blend membranes (2 wt.%) on the base of nominal surface area (means and standard deviations are indicated) 40 38 36 34 32 Surface free Energy (mN/m) 30 Control Unmodified 2 wt.% Modified Figure 4-15 Surface free energy of the membranes calculated by acid-base (van Oss) approach (means and standard deviations are indicated) An electro kinetic analyzer was used to investigate the zeta potential of membranes based on a streaming potential and streaming current measurement. The zeta potential is related to the surface charge at a solid/liquid interface and is a powerful indicator for the surface chemistry (pH titration) and liquid phase adsorption processes. The zeta potential (ZP) versus pH were demonstrated in Figure 4-16 for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes. The graph shows that the incorporation of TiO2 nanoparticles makes the surface less negative on the basic range and less positive on acidic range and also shifts the isoelectric point toward left. In addition, the surface becomes less Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 106 sensitive to pH changes while the incorporation of TiO2 nanoparticles without any modification shifts the isoelectric point toward right. 15 10 5 pH 0 0246810 -5 ZP (mV) -10 Control -15 Mechanically Modified TiO2 PES membrane Chemically and Mechanically Modified TiO2 PES Membrane -20 Umodified TiO2 PES membrane -25 Figure 4-16 Zeta potential versus pH for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes 4.3.9 Effect of TiO2 Modification on Membrane Performance Figure 4-17 shows the average pure water fluxes (measured for three samples) for unmodified and modified TiO2 blend membrane with different TiO2 content. Figure 4-18 shows the effect of different modification steps and also the flux of a commercial membrane with the similar MWCO. As expected, the 2 wt.% modified TiO2 blend membranes showed the highest water flux among the in-house membranes, which may be due to the combination effects of increase in pore size (Figure 4-8), porosity and hydrophilicity (Figure 4-13). However, the role of pore size seems more dominant than the hydrophilicity which is in good agreement with Li et. al. [95]. This finding is in contrast with the other researchers who believe that the hydrophilicity plays the major role [14, 17]. Other contributing factors could be skin layer and porosity. It is well known that thinner skin layer and higher porosity may lead to a higher pure water flux. Figure 4-6 shows that the inclusion of TiO2 resulted in a reduction of skin layer regardless of TiO2 modifications or change in the concentration. As discussed in section 4.3.5, higher porosity and pore size was observed after addition of modified TiO2 nanoparticles particularly for 2 wt.% TiO2 content membrane. The commercial membrane showed significantly higher flux as its porosity is substantially higher than its in-house counterpart. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 107 500 450 ) -1 400 h -2 350 300 250 200 150 100 Initial Water Flux (Lm Initial Water 50 0 Control 2 wt.% 2 wt.% 4 wt.% Modified 6 wt.% Modified Unmodified Modified Figure 4-17 Effect of modification of TiO2 nanoparticles on the pure water flux of control and different TiO2 content 2500 2,000 ) -1 2000 h -2 1500 1000 510 460 500 365 345 Initial Water Flux (Lm Initial Water 0 Control 2 wt.% 2 wt.% 2 wt.% 300 MWCO Unmodified Mechanically Chemically and Millipore UF PES Titania Modified Titania Mechanically Membrane Modified Titania Figure 4-18 Initial water flux measured at one bar for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes and 300MWCO UF PES membrane from Millipore Figure 4-19 shows the variation of transmembrane pressure, TMP, during 2 hours BSA 2 filtration at constant flux of 50 L/m h. Incorporation of TiO2 without any modification has not changed the fouling behaviour initially; however, after 90 min of filtration, the TMP increased up to 110 kPa which is 20 kPa greater than that of control. On the other hand, membranes with the modified TiO2 retained a positive effect on the fouling behaviours. In fact, the TMP which is a representative of total resistance of membrane remained low and reaching approximately 50 kPa after 120 min which is 30 kPa lower than that of control. These findings were reproducible among 3 independent experiments. It should point out here Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 108 that the rejection for all of the membranes were above 99%, thus differences in transmission was not a contributory factor in the decrease in fouling. 140 300 MWCO Millipore UF PES Membrane 120 2% Chemically and Mechanically Modified Titania 2% Mechanically Modified Titania 100 control 80 2% Unmodified Titania TMP (kPa) 60 40 20 0 0 20406080100120140 Time (min) Figure 4-19 TMP vs. time for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes and 300 MWCO UF PES membrane from Millipore. To quantitatively investigate the membrane fouling, flux recovery (FR%), intrinsic membrane resistance (Rm) which is due to factors related to membrane properties, reversible resistance (Rreversible) due to loose attachment of foulants on the surface of the membrane, irreversible resistance (Rirreversible) because of the adsorption of foulants on membrane pore wall or surface and total filtration resistance (Rt) were calculated by using formulation introduced in section 3.5.16.1 of materials and methodology chapter. The results in Figure 4-19 also show that the final TMP of mechanically modified TiO2 PES membrane is about 50 kPa which is about similar to mechanically and chemically modified TiO2 PES membrane. However, the flux recovery of mechanically and chemically modified TiO2 PES membrane is 18% higher than that of mechanically modified TiO2 PES membrane. The flux recovery of mechanically and chemically modified TiO2 PES membrane is also higher than the one for 300 MWCO Millipore PES membrane although the final TMP of 300 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 109 MWCO Millipore PES membrane is significantly lower than the others (26kPa). It should point out here that the initial pure water flux of the commercial membrane from Millipore was much higher than the in-house membranes (see Figure 4-18 ). It could be seen from Table 4-4 that the FR value of 2 wt.% modified TiO2 blend PES membrane was highest among the others. The intrinsic resistance results indicated that the modification of TiO2 nanoparticles for 2 wt.% TiO2 could reduce the Rm by 20 wt.% while no improvement was found when unmodified TiO2 was used. This modification also could decrease the irreversible foulant resistance by around 5 × 1011 m−1. This could be due to the positive effect of both low surface free energy and roughness in comparison with unmodified and control membranes. However, the major fouling was due to formation of cake layers on the surface of membranes which could easily be removed by physical cleaning (see R reversible data in Table 4-4 ). The results also show that the total resistance significantly decreased by the addition of TiO2 in casting solution up to 4 wt%. Further addition of TiO2 in the casting solution resulted in an increase of total resistance to values higher than that of control. This might be as a result of precipitation of TiO2 nanoparticles during the casting and phase inversion process which causes pore plugging and provides extra hydraulic resistance [9, 17]. The resulting higher flux recoveries compare favorably to that found previously by other researchers who achieved an increase from 42% for control membrane to 66% for modified with 2 wt.% TiO2 in their studies [13, 17]. Table 4-4 Filtration resistances of control and unmodified TiO2 blend membranes (the experiment repeated three times and the average was reported) Flux Rm R irreversible R reversible Rt Membranes Recovery 11 −1 (FR%) × 10 m Control 60 9.86±0.7 6.50 ±1.2 37.44 ±2.8 53.80±4.7 2 wt.% Unmodified TiO2 57 10.43±0.5 7.57±2.0 30.40±1.7 48.40±4.2 2 wt.% Modified TiO2 88 7.83±0.3 1.48±0.5 23.10±2.2 32.40±3.0 4 wt.% Modified TiO2 74 12.63±1.1 4.35±1.2 24.02±2.0 41.00±4.34 6 wt.% Modified TiO2 61 18.70±1.6 11.68±2.2 26.42±0.5 56.80± 4.3 The performance of 2 wt.% TiO2 blend membranes within 3 cycles of fouling and cleaning was evaluated and results are shown in Table 4-5 . In each cycle the membranes were fouled by BSA and then physically and chemically cleaned. The results showed that the FR decreases by increasing the number of cycles for both control and blend membranes however rd at the end of the 3 cycle, the FR for 2 wt.% chemically and mechanically modified TiO2 membrane was still highest among the others even higher than the commercial one. Silane Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 110 groups (from APTES) which can form local low energy hydrophobic surfaces could be an important contributing factor in antifouling properties of modified membranes. The application of low surface energy coating materials such as siloxane, fluoropolymers, and fluorosiloxanes as alternatives for paints containing lead and other toxic materials in marine applications is well documented. For example, poly (dimethyl siloxane) (PDMS) elastomers are widely used in commercial marine foul-release coatings because of their combination of properties such as low surface energies, low micro-roughness, and low modulus [279, 280]. Table 4-5 Flux recoveries and BSA rejections for control, unmodified and mechanically (and chemically) modified TiO2 blend PES membranes and 300MWCO PES membrane from Millipore Flux Recoveries (%) BSA Cycle 1 Cycle 2 Cycle 3 Rejections Control 60 50 45 >99% 2 wt.% Unmodified TiO2 57 - - >99% 2 wt.% Mechanically Modified TiO2 70 49 30 >99% 2 wt.% Chemically and Mechanically 88 66 58 >99% Modified TiO2 300 MWCO Millipore UF PES 75 50 40 >99% 4.4 Conclusion To avoid agglomerations and also to improve nanocomposite antifouling properties, commercial TiO2 nanoparticles were successfully modified by a combination of chemical and mechanical methods for blending into ultrafiltration membranes. The incorporation of modified nanoparticles into PES ultrafiltration membranes showed a significant improvement in fouling behaviour. The flux recovery increased from around 60% for control and 57% for unmodified nanoparticles to 84% for chemically and mechanically modified particles at a 2 wt.% TiO2 loading. This was accompanied by much lower TMP increase during filtration with bovine serum albumin. The degree of dispersion was found to have significant effects on the surface free energy, roughness, charge, pore size and protein absorption resistance as well as hydrophilicity. Upon modifications, the surface free energy and roughness decreased whereas surface pore size and porosity increased. Hydrophilicity also was improved by around 18% regardless of type of modification or changing the content of TiO2 particles in membrane. While the chemical and mechanical modified particles showed increased antifouling performance at 2 wt% TiO2, at higher concentrations, the modifications may not be sufficient to prevent agglomerations as evidenced by the poorer performance. While previous researchers believe that the increase in hydrophilicity is the most likely reason for improvement in fouling performance, other parameters discussed in this work may contribute Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 4 Blending (Flat Sheet Nanocomposite Membranes) 111 just as much to greater fouling resistance. Other contributing factor in antifouling properties of modified membranes might be the fouling release effect of silane groups (from APTES) which can form local low energy hydrophobic surfaces. The generation of local low energy surfaces using functionalized nanoparticles may contribute to a potentially interesting strategy for increasing antifouling performance in nanocomposite membranes and is recommended for further investigation. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 112 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 113 CHAPTER 5 BLENDING (HOLLOW FIBER NANOCOMPOSITE MEMBRANES) 5 Abstract* In chapter 4, we have demonstrated that the mechanical and chemical modifications of TiO2 nanoparticles have shown to provide improvements in reducing the nanoparticles agglomeration and fouling in flat sheet membranes. In this study, hollow fibre PES membranes with 2 wt.% mechanically and chemically modified TiO2 were fabricated. The effect of chemical and mechanical modification on the titania particles was compared with the effect of mechanical modification only and membranes without nanoparticle additives. The results showed that the migration of nanoparticles toward outer layer only occurred after mechanical modifications, whereas the migration and size of agglomerations reduced significantly after both chemical and mechanical modifications of particles. Higher thermal resistance, stiffness and lower elasticity were observed in fibers made with chemically and mechanically modified particles. Enhancement in initial pure water flux due to lower intrinsic membrane resistance and bigger pore size was also observed. While the rejection of BSA remained similar and lower overall ultrafiltration resistance was observed, the benefits of particles modifications on flux recovery after cleaning of hollow fiber membranes was not observed, possibly due to a significant change in pore structure and initial permeabilities. This chapter is based on: “The effect of modified TiO2 nanoparticles on the polyethersulfone ultrafiltration hollow fiber membranes” by A. Razmjou, A. Resosudarmo, R. L. Holmes, H. Li J. Mansouri and V. Chen, published in Desalination Volume 287, 15 February 2012, Pages 271-280. *The results that were jointly obtained by me and Adhikara Resosudarmo are: the fabrication of hollow fibres with and without TiO2 nanoparticles and also water contact angle measurements (Figure 5-9 ). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 114 5.1 Introduction One of the biggest issues with TiO2 nanocomposite membranes is the agglomeration of nanoparticles during the fabrication process. Agglomeration causes an uneven distribution of particles in the membrane. This may lead to potential reduction in the antifouling ability of TiO2 particles parameters such as membrane topography and hydrophilicity as well as the self-cleaning property of particles as it was discussed in chapter 4. Review of the literature indicated a limited investigation of the adverse effects of particle agglomeration on hollow fiber membranes (see section 2.3.3 of literature review). Recent studies on the TiO2 hollow fiber nanocomposite membranes have been directed mostly on PVDF [177-180] and polyimide [181] composite membranes rather than PES membranes. The mechanism for improved fouling performance after the incorporation of TiO2 into the polymer matrix also has not been elucidated. The means by which the particle size distribution, mechanical and chemical modification of particles can affect the membrane structure, surface chemistry and their consequences on fouling performance have not been investigated. The majority of studies have been focused on the flat sheet membranes while there has been much fewer investigations on nanocomposite hollow fibre membranes. Hollow fiber membranes are desirable because of the larger membrane area per volume of module, high flexibility as well as ease of handling in the module fabrication [176]. In chapter 4, an effective mechanical and chemical modification (silanization treatment by APTES) was introduced to reduce agglomeration of Degussa P25 TiO2 nanoparticles. The surface chemistry, membrane structure and fouling performance of ultrafiltration PES flat sheet membrane before and after the modification of particles were also investigated. Although many researchers believe that the increase in hydrophilicity is the most likely reason for improvement in fouling performance, the morphological factors may contribute just as much to greater fouling resistance. A significant improvement in fouling behaviour for chemically and mechanically modified particles at a 2 wt.% TiO2 loading was observed. The flux recovery increased by 24% while TMP was much lower during filtration with bovine serum albumin. Upon chemical and mechanical modifications, surface pore size and porosity increased whereas the surface free energy and roughness decreased. An increase in hydrophilicity of approximately 18% was found regardless of the type of modification or the concentration of TiO2 particles in membrane. The potential influence of fouling release effect of silane groups (from APTES) which can form local low energy hydrophobic surfaces was also discussed. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 115 In general, a significant improvement in membrane performance was achieved by controlling the concentration, size and distribution of TiO2 nanoparticles in the PES flat sheet membranes. However, due to the difference in the phase inversion process, the effects of chemical and mechanical modifications on the composite TiO2 hollow fibre membranes may differ from the flat sheet counterparts as solvent exchange occurs both at the shell and lumen side and the difference in shear rate may alter the distribution of particles. An important issue related to hollow fibre fabrication with TiO2 additive involves its influence on the viscosity and thus extrusion of the membrane casting solution. In this chapter, an attempt has been made to utilize the strategy used for flat sheet membranes in chapter 4 for hollow fiber PES composite membranes. The effects of TiO2 nanoparticles and their modifications on the PES hollow fiber structure are compared to previous work with flat sheet membranes. Surface functionalization through silanization by APTES was chosen for the chemical modification of TiO2 particles (Degussa P25). A variety of characterization techniques were applied to study the effect of modifications on the surface chemistry, thermal and mechanical resistances, membrane structure and fouling performance of ultrafiltration PES hollow fiber membrane. 5.2 Experimental 5.2.1 Materials Polyethersulfone (PES, 58000 g/mol) as polymer was purchased from BASF Co Ltd. and polyvinylpyrrolidone (PVP, 40000 g/mol) as pore former and hydrophilic additive were supplied by Sigma Aldrich. TiO2 nanoparticles (P25, 20nm) were provided by Degussa. N- Methyl-2-Pyrrolidone (NMP) as solvent was obtained from Scharlau. The Aminopropyltriethoxysilane (APTES) was used for nanoparticle surface pre-treatment and was supplied by Sigma Aldrich. Isopropanol for wetting membranes and sodium hydroxide pellets for chemical cleaning were supplied by Ajax Finechem Pty Ltd. Bovine Serum Albumin (BSA, reagent grade, pH 5) for membrane performance test was obtained from Morrgate Biotech. To estimate the surface pore size of membranes, their molecular weight cut-off (MWCO) was characterized by measuring the rejection of dextrans (20 kDa from MP Biomedicals, 50KDa and 250KDa from Pharmacosmos Co., 580 KDa from Sigma Chemical). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 116 5.2.2 Modification of TiO2 and the Preparation of PES hollow fiber Membrane The commercial nanoparticles were altered via mechanical and chemical modifications, which have been explained in section 3.2.1 of chapter 3. The procedure of preparation of control and blend PES hollow fiber membrane with/out modified TiO2 nanoparticles was explained in section 3.2.3 of Materials and Methodology chapter. In this chapter, modified particles refer to both mechanical and chemical being applied unless otherwise indicated. 5.3 Membrane Characterization A variety of techniques such as SEM, EDX, TGA, DSC, DMA, contact angle goniometry, molecular weight cut-off, were applied to characterize and explore the effect of different factors on fouling performance of hollow fiber membranes. These characterization techniques were explained in detail in chapter 3. Water flux and flux recovery were measured using a cross flow system at fixed cross flow speed of 0.05 m.s-1 and flow rate of 75 mL/min, for details see section 3.5.16.2 of materials and methodology chapter. 5.4 Results and Discussions 5.4.1 Dispersion of TiO2 Particles in the Hollow Fiber PES Membranes Effect of modification on particles agglomeration Figure 5-1 shows the effect of 10 min probe sonication of particles in ethanol on the size of agglomeration. From the TEM images, the agglomeration of Degussa P25 nanoparticles can be reduced to around 200 nm through the mechanical modification. Therefore, the produced shear forces during mechanical modifications are not strong enough to bring the agglomeration size under 100 nm. The surface functionalization or chemical modification of nanoparticles with APTES is able to reduce the agglomeration size of the nanoparticles to less than a hundred nanometers as explained in chapter 4. Evidence of TiO2 on the surface (EDAX) Figure 5-2 confirms the existence of TiO2 nanoparticles detected by EDAX analysis. The test was done on the top surface and cross section of the mechanically (and chemically) modified TiO2 PES hollow fiber membranes. Similar to the flat sheet nanocomposite membranes, the observed peak at 4.5KeV belongs to Ti and the one at 2.5 KeV belongs to S which comes from polyethersulfone. The Ti peak was not detected for control membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 117 100nm 100nm Figure 5-1 TiO2 nanoparticles before (left) and after 10 min sonication in ethanol (right) 4 Control membrane Blend membrane 3 S 2 C Ti 1 O Counts (cps/ev) Counts (cps/ev) 0 0123456 Energy (keV) Figure 5-2 Top surface EDX spectra of control and chemically and mechanically modified TiO2 blend hollow fiber membranes. Sample homogeneity (TGA) In order to examine whether TiO2 nanoparticles were well dispersed in the membrane matrix, the TGA test was done on three 20mm pieces of each hollow fiber membrane (control and 2 wt.% TiO2). The TGA results in Figure 5-3 a show that the residue for all three different samples of mechanically and chemically modified TiO2 PES hollow fiber membrane is 2 wt.%. This indicates that the TiO2 nanoparticles were well dispersed throughout the membrane with the minimum agglomeration sizes. Since the casting solution contained 2g TiO2, 23g PES and 12g PVP, the final amount of TiO2 in the membrane should be between 5.7 wt.% and 8.7 wt.%, taking into account the fact that part or all of the PVP may leach out during the fabrication process. The PES membrane has a decomposition temperature between 350ºC and 500ºC [13, 95], so the weight loss between 350ºC and 500ºC is around 25 wt.%-30 wt.%, which includes PES and at least part of the PVP. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 118 25%-30% 2.00% (a) 100 80 60 Weight (%) 40 20 2.32% 2.326% 0 0 200 400 600 800 Temperature (°C) Universal V4.2E TA I 33% (b) Figure 5-3 Weight loss versus temperature for 3 different pieces of (a) mechanically and chemically (b) mechanically modified TiO2 PES Hollow fiber membrane, the inset image in b is the TGA for a different sample of mechanically modified TiO2 PES hollow fiber membrane The ratio of the residual weight (2 wt.%) to the organic content of 25 wt.%-30 wt.% yielded an estimate of 6.6 to 8 wt.% TiO2 in the original membrane, which is in the expected range. This confirms not only the good dispersion of TiO2 but also shows that the loss of TiO2 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 119 during fabrication processes was insignificant. However, the ratio of average residual for only mechanically modified TiO2 PES hollow fiber membrane (3 wt.%) to 33 wt.% organic content gives 9 wt.% which is slightly outside the expected range (5.7 wt.%-8.7 wt.%), see Figure 5-3 b. In addition, a different residual content was observed (inset image in Figure 5-3b) when another sample of mechanically modified TiO2 PES hollow fiber membrane was examined (2.32 wt.% TiO2 and 29 wt.% PES). This suggests a potential inhomogeneity of the membrane in regard to TiO2 dispersion. In the TiO2 PES flat sheet membranes, this variation was observed with neither the mechanical and chemical modified TiO2 nor the only mechanically modified particles. TiO2 nanoparticles distribution across the membranes The distribution of TiO2 nanoparticles in the membrane cross sections was examined by EDX mapping coupled with SEM images from the inner shell to the outer shell of each membrane. As can be seen in Figure 5-4 b, the TiO2 agglomerations migrated toward skin layer during spinning for only the mechanically modified TiO2 PES hollow fiber membrane. The concentration of nanoparticles to the outer layer of dual-layer asymmetric hollow fiber membranes was also observed by Jiang et al. [176]. For chemically and mechanically modified TiO2 PES hollow fiber membrane, not only the size and number of agglomerations were reduced but also their migrations were limited. Reduction in TiO2 mobility could be due to surface functionalization which changes the nature of interaction between the inorganic nanoparticles and polymer molecule from repulsive to attractive [272, 274]. In flat sheet membranes the agglomerations across membranes were observed only for unmodified TiO2 PES membranes (section 4.3.2). (a) (b) Figure 5-4 EDX mapping coupled with SEM images for (a) mechanically and chemically modified and (b) only mechanically TiO2 PES hollow fiber membrane (arrows point the agglomeration of TiO2 nanoparticles). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 120 5.4.2 Effect of TiO2 Modification on Cross-Section Micro-Structure SEM images in Figure 5-5 reveal that the membranes structure for all of membranes consists of a skin layer (at the outer shell), finger-like microvoids, circular microvoids and porous lumen. From the images, the addition of TiO2 with only mechanical modifications increases the number of circular microvoids whereas the addition of chemically and mechanically modified TiO2 nanoparticles causes the finger-like microvoids to elongate across the membrane thickness. From higher magnification images (insets), a dense skin layer and nodular structure back side were observed for all of membranes. Therefore, there is no significant micro-structural change due to the addition of TiO2. The observation is different to those obtained with flat sheet membranes (section 4.3.3) where the different sizes of membrane macrovoids can be clearly distinguished. This could be attributed to the higher viscosity of dope due to the greater concentration of polymer (23 wt.% versus 16 wt.% for flat sheet membranes) used for hollow fiber spinning. 5.4.3 Effect of TiO2 Modification on Glass Transition Temperature, Loss and Storage Modulus Differential scanning calorimetry (DSC) As explained in chapter 4, glass transition temperature (Tg) is very sensitive to chain mobility and could be related to the quality of dispersion. Attractive or repulsive interaction between the nanoparticles and polymer molecules could affect the chain mobility and region of free volume, which consequently resulted in increase or reduction in Tg [272, 273]. A reduction in the available free volume for segmental motion can lead to an increase in the Tg [281]. From the DSC results in Table 5-1 and taking into account the standard deviations, the Tg was increased after the addition of TiO2. Thus the nanocomposites have higher thermal resistance than control membranes. This result is not in agreement with Han et al.’s results [178] where they have observed that the addition of inorganic nanoparticles into PVDF hollow fiber membrane reduces the thermal resistance of the membranes. The effect of chemical and mechanical modification of TiO2 on Tg was more significant on the flat sheet membranes (230o±2oC) rather than hollow fiber membranes (218o±6.16oC). This might be due to the increased polymer concentrations of dope solution as well as different fabrication process, which resulted in a lower dispersion and lower Tg. In addition, a level of agglomerations can still be seen even after chemical and mechanical modification of TiO2 (Figure 5-4 a). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 121 Back side Skin layer (a) Back Side Skin layer (b) Skin layer Back side (c) Figure 5-5 The effect of TiO2 modification on PES membrane structure (a) Control (b) mechanically (c) chemically and mechanically modified TiO2 PES hollow fiber membrane. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 122 Table 5-1 Average value of glass transition temperature, tensile strength and elongation at break for each type of membrane measured. Average Average Tensile membrane strength PES Hollow fiber membrane Tg (°C) Tg (°C) Elongation outer N/mm2 membrane thickness from DSC from DMA at break, % diameter (mm) (mm) (MPa) Control 0.12 1.19 213±3.79 223±4.4 11.1 ±0.64 47.78±6.01 Mechanically 0.12 1.2 225 ±1.15 243±1.0 9.66 ±0.64 38.15±4.93 modified TiO2 Mechanically and chemically 0.16 1.26 218±6.16 240±1.5 7.53 ±0.68 21.00±7.90 modified TiO2 Dynamic mechanical analysis (DMA) From the storage modulus traces (Figure 5-6 a), there is an increase in the glass transition temperature of approximately 20°C for the modified membranes in comparison with the control sample. For the chemically and mechanically modified sample, there is an increase in the storage modulus values, which may be due to the chemical modification of the titania particles leading to a better compatibility between the titania and the polymer. The organic modification of the titania nanoparticles may also allow for stronger chemical interactions with the polymer as it discussed in chapter 4. The loss modulus trace (Figure 5-6 b) for a material measures the energy dissipated into the sample through heat from molecular relaxations. It is also a good measure of the molecular relaxations such as the α-relaxation which is the glass transition temperature and β-relaxations which describe secondary relaxations such as that for a large side group. For the control membrane, the glass-transition value was determined as 223°±4.4°C (Table 5-1 ). The two modified membranes had significantly higher glass-transition temperatures which were: 243°±1.0°C for the mechanically modified sample and 240°±1.5°C for the chemically and mechanically modified sample. This reflects the trends that measured from DSC, where the added TiO2 lead to an increase in the glass transition temperature. However the magnitude of the trend measured from DSC was less. These slight variations between samples are common as two different techniques with slightly different definitions of glass transition temperature were used; the measurement frequency may also influence the position of Tg. There is a second peak (~150°C) in the loss modulus which can be associated with remaining PVP in the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 123 membrane. This is a reasonable match for PVP which has glass transition temperatures reported in the literature ranging from 150°C [282] to 180°C [283]. The above dynamic mechanical analysis also shows that the modified samples are stiffer than the control. It should be pointed out here that the chemically and mechanically modified TiO2 PES membranes appeared stiffer during handling. 140 Control Mechanically & Chemically Modif. Mechanically modified 120 100 80 60 Loss Modulus (MPa) 40 20 0 50 100 150 200 250 Temperature (°C) Universal V4.2E TA Instruments (a) 800 Control Mechanically & Chemcially modifi Mechanically modified 600 400 Storage Modulus (MPa) 200 0 40 90 140 190 240 Temperature (°C) Universal V4.2E TA Instruments (b) Figure 5-6 Storage modulus (a) and loss modulus (b) versus temperature for control and modified membranes Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 124 5.4.4 Effect of TiO2 Modification on Surface Pore Size Molecular Weight Cut-Off Figure 5-7 shows the TGA for control and chemically (and mechanically) modified TiO2 PES hollow fiber membrane. Higher weight loss below 400°C may be an indication of higher percentage of volatile residues including water, solvent and glycerol. Higher absorption of these species may be due to a larger pore size or higher porosity. In the Figure, the mechanically and chemically modified TiO2 loaded membrane lost above 75% of its weight which is about 10% greater than that of control and only mechanically modified TiO2 PES hollow fiber membrane. Figure 5-8 shows the molecular weight cut-off curves for control and modified PES hollow fiber membranes. The molecular weight cut off at 90% rejection shifts from approximately100 kDa (for the control and mechanically modified TiO2 membranes) to between 300-400 kDa for the mechanically and chemically modified PES hollow fiber membranes. This suggests that the surface functionalization led to a membrane with larger surface pore size. Similar shift in molecular weight cut off was observed for flat sheet membranes (section 4.3.5). Figure 5-7 Effect of TiO2 modifications on the initial weight loss Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 125 100 95 90 85 80 Control Rejection (%) Mechanically modified 75 Mechanically and chemically modified 70 0 100 200 300 400 500 600 700 Dextran Molecular Weight, kDa Figure 5-8 Molecular weighs cut-off for control and modified PES hollow fiber membranes 5.4.5 Effect of TiO2 Modification on Membrane Tensile Strength The tensile strength (N/m2) and elongation at break (%) of membranes were presented in Table 5-1 . The results showed that there are some small changes in tensile strength between control and nanocomposite membranes, whereas the elongation at break reduced for composite membranes significantly. The reduction in elongation at break was also observed for TiO2-Al2O3-SiO2-PVDF nanocomposite hollow fiber membranes previously [178]. It should point out here that a greater reduction in elongation at break (%) after surface functionalization (chemical modification) could be an indication of better dispersion of particles in the polymer matrix. Blackwood et al. reported that the occurrence of large areas of densified polymer around TiO2 particles leads to antiplasticization effect, higher Tg and reduction in mechanical properties [281]. The change of the fracture mechanisms, from the ductile fracture to the brittle fracture, was also observed in another study [284]. It is mentioned in the literature [281] that the incorporation of TiO2 nanoparticles at a low level can often improve the general mechanical properties. However, increasing the loading of TiO2 could significantly reduce the impact strength and related mechanical properties. Reducing the TiO2 concentration in the spinning dope (below 2 wt.%) may improve the elasticity behaviour of nanocomposites membranes. Nevertheless, the chemically and mechanically modified TiO2 PES hollow fiber membranes remained flexible and robust during handling and potting. They also appear more resistant to deformation compared to the purely polymeric membranes during the filtration and cleaning process. These results are not comparable with the flat sheet membranes as they have different polymer composition, Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 126 geometry and quality of dispersion which may significantly affect the mechanical properties of membranes. 5.4.6 Effect of TiO2 Modification on Membrane hydrophilicity The membrane contact angle results in Figure 5-9 show that the addition of TiO2 resulted in membranes with lower contact angles, indicating better hydrophilic properties. Membranes with the mechanically and chemically modified titania were observed to be the most hydrophilic. 5.4.7 Effect of TiO2 Modification on the Membrane Performance Figure 5-10 shows the effect of modifications of nanoparticles on the average pure water fluxes (measured for three samples) of membranes. As expected, the mechanically and chemically modified TiO2 PES hollow fiber membranes showed the highest water flux. This may be because of the combination effects of increase in porosity and pore size (Figure 5-8 and Figure 5-7 ) and hydrophilicity (Figure 5-9). However, the role of pore size seems more dominant than the modest hydrophilicity increase which is in good agreement with Li et. al. [95] and our previous work on flat sheet membranes. This finding is in contrast with the other researchers who believe that the hydrophilicity contribution is the most important factor [14, 17]. 80 70 ) o 60 50 40 30 Contact Angles Contact Angles ( 20 10 0 Control Mechanically modified Mechanically and TiO2 PES Hollow fiber Chemically modified TiO2 membrane PES Hollow fiber membrane Figure 5-9 Average contact angles on the PES membranes for control and modified PES hollow fiber membranes (means and standard deviations are indicated) Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 127 70 60 50 .h) 2 40 30 Flux (L/m Flux 20 10 0 Mechanically and Mechanically modified Control Chemically modified TiO2 PES Hollow fiber TiO2 PES Hollow fiber membrane membrane Figure 5-10 Effect of modification of TiO2 nanoparticles on the pure water flux of membranes (means and standard deviations are indicated) In order to quantitatively analyse the membrane fouling performance, intrinsic (Rm), fouling (Rf) and total (Rt) membrane resistances were calculated by formulation introduced in section 3.5.16.2. From the results in Table 5-2, the intrinsic resistance values indicate that the modification of TiO2 nanoparticles could reduce the Rm by 58 and 68% for only mechanically and mechanically and chemically modified TiO2 PES hollow fiber membranes, respectively. The hollow fiber PES membranes showed a greater reduction in Rm after the incorporation of TiO2 nanoparticles in comparison with a 20% reduction in Rm for 2 wt.% chemically and mechanically modified TiO2 PES flat sheet membranes, see section 4.3.9. Mechanical and chemical modifications also showed a reduction of around 9% in fouling resistance. The reduction in fouling resistance for flat sheet membranes was more prominent (around 77% reduction in irreversible resistance for 2 wt.% chemically and mechanically modified TiO2).It should point out here that the rejections for all of the samples were above 97%. To investigate whether the TiO2 nanoparticles could leach out during filtration experiment, the TGA test was done on the samples before and after fouling experiments. As can be seen in Table 5-2 , the ratios of TiO2 to PES remained almost in a same range, which means the particles were strongly attached to the polymer matrix. The performance of membranes within 3 cycles of fouling and cleaning was evaluated and results are shown in Table 5-3. In each cycle the membranes were fouled by BSA and then physically and chemically cleaned. The results showed that the flux recovery (FR) decreases for increasing the number of cycles for both control and modified TiO2 PES hollow fiber membrane. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 128 Table 5-2 Filtration resistances and TGA residues for control and modified TiO2 PES hollow fiber membranes Membrane resistances TGA residue ୧ (×1015 m−1) ሺ మ Ψ) PES Hollow fiber membrane ୗ Before After R R R m f t fouling fouling Control 20.35 4.07 24.42 - - Mechanically modified TiO2 9.04 8.40 17.44 9±1 10.5±1 Mechanically and Chemically 6.57 3.71 10.28 7.7±0.5 7±0.7 modified TiO2 However at the end of the 3rd cycle the FR reduction for mechanically and chemically modified TiO2 PES hollow fiber membrane was higher than that of control. In general, the impact of controlling the size and distribution of nanoparticles on the fouling performance of hollow fiber PES nanocomposite membranes was not as significant as it was on flat sheet membrane. This might be due to this fact that control membranes have a denser skin layer and tighter pores resulting in lower molecular weight cut off, lower initial pure water flux and higher Rm. Consequently the dominant resistance is reversible and cleaning procedure is more efficient whereas for nanocomposite membrane, the fouling mechanism may have a higher contribution of pore plugging which is more difficult to remove. In addition, 2 wt.% TiO2 in casting solution in hollow fiber PES membranes may not be optimal, despite the fact that it showed the highest performance in flat sheet PES membranes. Reducing the concentration of TiO2 in the spinning dope may improve the performance of the hollow fiber membranes in terms of the mechanical properties and is recommended for future work. Table 5-3 Flux recoveries within 3 cycles of fouling-cleaning experiments for control and modified TiO2 hollow fiber PES membranes. Flux Recovery (FR, %) PES Hollow Fiber Membrane Cycle I Cycle II Cycle III Control 83.33 66.67 58.33 Mechanically modified TiO2 51.85 44.44 37.04 Mechanically and chemically modified 61.54 53.85 46.15 TiO2 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 5 Blending (Hollow Fiber Nanocomposite Membranes) 129 5.5 Conclusion Commercial TiO2 (Degussa P25) nanoparticles were successfully modified to reduced agglomerations and also to improve nanocomposite hollow fiber PES membranes performance. Initial pure water flux of mechanically and chemically modified TiO2 PES hollow fiber membrane was enhanced significantly while there is a small improvement in its fouling performance in comparison with control. Poor dispersion and migration of TiO2 nanoparticles toward outer layer were observed for only mechanically modified TiO2 nanoparticles while better dispersion was achieved after both chemically and mechanically modified TiO2 nanoparticles. The FR reduction for mechanically and chemically modified TiO2 PES hollow fiber membrane was higher than that of control under constant pressure filtration. Further investigation using constant flux or larger molecular weight solutes may elucidate whether the high initial flux or pore size contributed to the lower flux recovery. Upon the incorporation of mechanically and chemically modified TiO2 nanoparticles, the glass transition temperature, membrane porosity and pore size, stiffness and hydrophilicity increased whereas tensile strength and elongation at break decreased. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 130 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 131 CHAPTER 6 COATING (SUPERHYDROPHILIC MODIFICATIONS) 6 Abstract Thin mesoporous coatings of TiO2 nanoparticles were developed for in-house and commercial PES membranes of varying pore sizes using a low temperature hydrothermal (LTH) approach. Titania sol-gel particles were dip-coated onto membrane substrates, followed by heat and UV treatments to extract the residual organic templates. Dip-coating parameters such as drying and holding time, dipping and withdrawal velocities, and the number of coating cycles were varied to optimise the microstructure and surface properties of the coating. Coated membranes exhibit a dual level hierarchical roughness and superhydrophilicity which was maintained without continuous UV illumination. The organic templating agent (Pluronics F127) enhanced the adhesion of the particles; however the heat treatment collapsed some pores in the tighter ultrafiltration membranes. Surface architecture was changed from cauliflower to rose structure by changing the templating agent from amphiphilic (Pluronics F127) to hydrophilic (polyethylene glycol) compound. Results showed that the coatings were mechanically robust and photoactive. Passive protein adsorption was reduced significantly on the TiO2 coated surfaces. Filtration performance of coated and uncoated 500 kDa membranes was also investigated with humic acid as a model foulant, and an increase in flux recovery was observed during multiple fouling and cleaning cycles with the titania coated membrane. The LTH approach provides a platform for further surface functionalization of polymeric membranes to generate photoactive coatings, tuneable hydrophilicity, low fouling surfaces and novel surface architecture. This chapter is based on: “Titania nanocomposite polyethersulfone ultrafiltration membranes fabricated using a low temperature hydrothermal coating process” by A. Razmjou, J. Mansouri, V. Chen, May Lim and Rose Amal.2011, published in Journal of Membrane Science 380 (1-2), 98–113. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 132 6.1 Introduction There are many approaches to mitigate the fouling of polymeric membrane in filtration applications and most of them are based on increasing membrane hydrophilicity and reducing microscale roughness, either by grafting or coating with hydrophilic organic moieties such as polyethylene glycol (PEG). More recently, the addition of nanomaterials have also been shown to improve the permeability, impart self-cleaning and/or antifouling properties, reduce compaction, and alter the morphology of the membrane [7]. A variety of nanoparticles such as SiO2, Al2O3, Fe3O4, ZrO2 and TiO2 [12, 59-62] have been used for surface modifications and coating. Among them, the addition of nano-sized TiO2 had received the greatest attention because of its photocatalytic properties, physical and chemical stability, and wide availability. The photocatalytic properties arise when the TiO2 is illuminated with light of energy equal or greater than its bandgap energy, causing electron to be excited from the valance band (VB) to the conduction band (CB), leaving behind a hole (electron deficiencies) [150, 151]. These electron/hole pairs (charge carriers) can either recombine or undergo oxidation/reduction reactions with chemical species on the semiconductor surface, as well as in the bulk solution [152]. There are two main approaches for the fabrication of TiO2 nanocomposite membranes: (1) blending the nanoparticles into the membrane [12-14], which was the subject of chapter 4 and 5, and (2) depositing nanoparticles onto the surface of the membrane [15-17]. The blending approach can be easily implemented but limits the loading of inorganic particulates and may alter the membrane morphology during the phase inversion casting process. As explained in chapter 4 and 5, a uniform distribution would be difficult to achieve in the absence of any surface treatment of nanoparticles. The deposition approach, on the other hand, has the drawback that it may result in a non-uniform distribution of the TiO2 nanoparticles on the membrane surface and an instable coating layer. A third approach which may potentially result in a more comprehensive and uniform coverage of TiO2 on the membrane surface is the coating of titanium sol followed by heat treatment to generate a crystalline TiO2 film. Traditionally, this technique is only used for preparing TiO2 coating on thermal resistant substrates such as ceramics and metallic surface, and is not applicable to polymeric membrane due to the high annealing temperature (>400 °C) that is required [196, 197]. Moreover, the hydrophilicity of TiO2 is frequently only activated by the presence of UV illumination. Thus, an alternative process to generate long Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 133 term hydrophilic surfaces in the absence of UV light and the use of lower temperature processes would be highly advantageous for coating polymer substrates. Recent advancement in low temperature hydrothermal (LTH) processes for generating nano- composite coatings, offers new ways to tailor the surface chemistry, hierarchical surface structure, and hydrophilicity on polymeric surfaces. Instead of annealing the coating at high temperature, TiO2 is crystallised by immersing the coating in hot water. The topography and porosity of the surface can be varied by changing thermal treatment, humidity, withdrawal speed, concentration and type of organic structural directing agents (see section 2.4.3 of literature review). Matsuda et al. and Yang et al. showed that transparent anatase nanocomposite films can be produced on polymeric films using this method [202, 285]. The LTH process was employed by Lam et al. to deposit TiO2 on polycarbonate film to generate self-cleaning surfaces [201]. The technique was also employed by Mane et al. to produce superhydrophilic rutile TiO2 thin films on indium tin oxide surfaces [235]. In textile industry, low temperature coatings of TiO2 photocatalytic thin film on cotton fabrics have been applied as well [286]. In order to overcome the drawback of hydrophilicity dependent on continuous UV illumination, hydrophilicity via multilevel roughness and 3D capillary phenomena rather than 2D capillary phenomena has been recently reported (see section 2.5.3.2 of literature review). In spite of the considerable interest in LTH coatings, these techniques have been mainly focussed on solid (nonporous) surfaces or fibers, and have not been fully explored for the surface modification of porous substrates such as filtration membranes. The modest heat treatment offers a more facile route to form inorganic, nanostructure coating with hierarchical surface structures and to generate hydrophilicity without continuous UV irradiation. However, the coatings must have significant mechanical and chemical robustness to withstand the aggressive shear forces and chemical cleaning environment commonly encountered in filtration processes. There is also little information in the literature on the effect of these hierarchical surface structures and novel surface chemistry on the deposition of biopolymers and bioadhesion. In this Chapter, the LTH approach was used to generate mesoporous TiO2 coatings on ultrafiltration membranes. TiO2 nanoparticles (synthesized by sol-gel technology with the addition of an organic as templating agent) were dip-coated onto the surface of in-house and commercial polyethersulfone ultrafiltration (UF) membranes, and the coating was stabilised Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 134 in a low temperature heat treatment processes. Comprehensive characterization techniques were applied to investigate the properties of coating in terms of photocatalytic activity, surface chemistry, chemical and mechanical robustness, and microstructure. Fouling and cleaning performance were also assessed to determine the potential use of the coating as an antifouling surface. 6.2 Experimental 6.2.1 Materials Polyethersulfone (PES, 58000 g/mol) as polymer was purchased from BASFCo. Ltd. Polyvinylpyrrolidone (PVP, 40000 g/mol) as pore former and hydrophilic additive, Pluronic F127 as templating agent, titanium (IV) iso-propoxide (TTIP) (97%) as TiO2 precursor, and humic acid (HA) were purchased from Sigma-Aldrich.TiO2 nanoparticles (anatase/rutile mixture, Aeroxide P25, previously known as Degussa P25) were obtained from Degussa. Dimethylacetamid (DMAC) as solvent was supplied by ScharlavChemie S.A. Bovin serum albumin (BSA, reagent grade, pH 5) was purchased from Morrgate Biotech. Anhydrous ethanol was supplied by Ajax Chemicals. 2,4-pentanedione was supplied by Lancaster. Perchloric acid (70%) was supplied by G. Frederick Smith Chemical Co.Commercially available PES membranes with different molecular weight cut off of 500 kDa and 100 kDa were obtained from Millipore and Pall Corporation respectively. 6.2.2 Preparation of In-house PES Membrane and TiO2 Precursor Sol for Coating Phase inversion induced by immersion precipitation technique was used to prepare the PES control membranes (for more details see section 3.2.2 of chapter 3). A stable TiO2 precursor sol with pH of 1.2 for coating purpose was prepared by using sol gel technology as introduced in section 3.3.1 of chapter 3. 6.2.3 Coating of TiO2 Nanoparticles onto PES Membrane by LTH Process After preparation of TiO2 precursor sol, TiO2 thin film was coated onto the in-house PES membrane (250-270 kDa MWCO) by dip-coating (membranes were cut in circles with diameter of 21mm).The membranes were then hydrothermally treated at low temperature and were irradiated by UV light to remove any organic residual leftover from the coating process (see section 3.3.2 of chapter 3). The number of times the coat-dry-heat cycle was carried out and the number of time the membrane was coated in each cycle was varied to determine if they have any effects on the properties of the coating produced (see Table 6-1 ). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 135 The TEM results showed that the crystal size of TiO2 was around 10nm whereas the average particle size from dynamic light scattering (DLS) in the sol solution was about 52nm due to agglomeration (Figure 6-1a). For comparison purposes, a second set of TiO2 coated PES membranes were prepared by dipping the membranes into a solution containing 0.05 wt.% of Degussa P25 TiO2 nanoparticles 20-30nm (Figure 6-1b), using the same coating speed and holding time with the titania sol previously. (a) (b) Figure 6-1 TEM images of TiO2 nanoparticles (a) from Sol-Gel solution (b) Degussa P25 Table 6-1 Contact angles of in-house PES membranes were dip-coated at different conditions. Treated control is the membrane which was dipped in a coating solution without TiO2 followed by heat treatments. Dip-coating Dip-coating Evaporation Membrane Contact Angle (o) cycle times (times) time (sec) 1 control - - 73 ± 4 2 Treated Control - - 59 ± 5 5 1 0 56 ± 7 6 0 57 ± 2 7 2 8 50 ± 2 Single 8 30 45 ± 2 9 0 56 ± 1 10 4 8 44 ± 3 11 30 49 ± 3 12 Double 1 0 36 ± 1 13 Triple 1 0 18 ± 2 6.3 Membrane Characterization Different techniques including SEM, EDX, TEM, XPS, TGA, DSC, AFM, BET, contact angle goniometry, molecular weight cut-off, static protein absorption, stream potential, surface free energy, tensile strength and elongation at break measurement were applied to Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 136 characterize and explore the effect of TiO2 coating by LTH process on surface chemistry, structure and fouling performance of membranes. These techniques were explained in detail in chapter 3. Fouling performance of the membranes was also investigated by the filtration of 0.1 wt.% humic acid at (HA) both constant flux (85 L/m2 h) and pressure (1 bar) modes for 2 h, for details see section 3.5.16.3 of chapter 3. 6.4 Results and Discussion 6.4.1 Dip-Coating Parameters Table 6-1 shows the contact angle for coated membranes with different coating parameters such as dip-coating time and evaporation time. There is a reduction in contact angle around 10° to 15° which could be as a result of either solvent (ethanol) in coating solution which makes the surface of membranes slightly hydrophilic or heat treatments. However, membranes coated 2 times with 30 second evaporation time and 4 times with 8 sec evaporation time showed lower hydrophilicity around 45°.Therefore, one cycle (single) of coating is not enough for achieving hydrophilic surfaces even by increasing the number of dip-coating times. Increasing the number of coating cycles rather than coating times showed a substantial improvement in hydrophilicity which consist of 35° and 18° for double and triple coating of in-house membranes. 6.4.2 Characterization of Coating 6.4.2.1 Evidence of TiO2 on the Surface The presence of TiO2 was investigated by energy dispersion of X-ray analysis (EDAX Point and Line analysis) which confirms the existence of TiO2 on the top surface of the coated membrane. A peak observed around 4.5keV belongs to Ti and the peak around 2.5keV belongs to sulphur which comes from PES. Different areas were investigated to see whether Ti peak exists or not. Ti peak was observed on different areas of modified sample.The line scan EDAX analysis over the membrane also confirmed that the coating layer is homogenous (see Appendix Figure A1). 6.4.2.2 Coverage and Sample Homogeneity Thermogravimetric Analysis (TGA) To examine the TiO2 loading of membranes and homogeneity of coating, thermogravimetric analysis (TGA) was carried out. Different areas of membrane were tested. Residue for the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 137 triple coated in-house membrane was 10±a0.2 wt.%, in contrast, membrane coated with P25 had only 2±a1.1 wt.% residue. Low standard deviation for membranes coated with sol-gel method is an indication of homogeneity of these samples. Residues after first and second coatings were 2 and 5.8 wt.%, respectively. On the other hand the residues for 100 MWCO PES membranes coated by LTH process were 2, 3 and 5 wt.% after one, two and three cycles of coating, respectively. Integrity of the coating The residues from TGA analysis test were examined visually and also by FESEM. Samples were heated up to 800°C under air atmosphere to remove the organic components. As Figure 6-2 shows membrane prepared by sol-gel method forms a free-standing inorganic residue whereas the P25 coated membranes forms powdery residue. The thickness of the TiO2 layer is around 20 to 30 μm (Figure 6-2a), which most probably is due to the expansion as a result of the change of the TiO2 from mostly amorphous phase to anatase or rutile after heating the sample up to 800°C [262, 273]. Since similar findings were achieved for membranes coated by TiO2 sol but without templating agent of Pluronic F127, high integrity is less likely due to the effect of additive but probably as a result of in-situ formation of nanoparticles during LTH process [14]. In blending of nanoparticles in polymer matrix, functionalization of Degussa P25 nanoparticles by low energy additives such as silane coupling agents have been shown to improve the dispersion of particles in the casting solutions [123]. (a) (b) Figure 6-2 SEM images of TiO2 layer after heating PES membrane coated with (a) TiO2 sol (LTH process) and (b) Degussa P25 solution up to 800oC. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 138 6.4.2.3 Hardness (AFM Phase Imaging) Phase imaging in AFM is one of the powerful techniques for mapping variations in sample properties such as variation of the mechanical (friction, hardness) and adhesive properties [287]. Since PES is an organic matter while TiO2 is an inorganic, there would be a difference between their hardness to some extent. Actually, an ideal coated membrane should have only one phase which is due to either a stiff uniform inorganic coated layer on top or a homogenous nanocomposite surface, therefore, different phase might be an indication of lack of uniform coating and stiffness. Figure 6-3 shows the phase image of coated membrane with TiO2 sol, control and P25. As the images clearly show, the membrane coated by Degussa P25 has a few defects which might be due to agglomerations. Comparing the phase images of control and coated membrane by LTH process shows that the coating has not changed the hardness of the membrane surfaces. 6.4.2.4 Stability of Coating 6.4.2.5 Contact Angle Measurements Stability of the coating layer has a critical role in maintaining the performance of membrane. Membranes are under severe mechanical conditions due to the hydraulic flow of liquid through pores which could detach the loosely attached particles. To evaluate the stability of the coating layer in the composite membranes, they were sonicated in Milli-Q water for 5 and 30 min and then their contact angles were measured. The results shown in Figure 6-4 confirm that the membrane coated by LTH processes has a very good stability even after 30 min sonication whereas the contact angle of the P25 coated membranes increased by 87% (from 31o to 59o) and became close to the contact angle of control membrane after sonication. Comparison between the standard deviations of membrane coated by P25 (31o±18o) and LTH process (27o±7o) before sonication might suggest a non-uniformity in the coating layer for membrane coated by P25. However, decrease in the variations after sonication (59o±5ofor membrane coated by P25) implies this fact that there should have been some large agglomerates of P25 TiO2 particles with very loose attachment to the surface. This was also confirmed by SEM images which are shown in Figure 6-9 . Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 139 (b) (a) (c) Figure 6-3 AFM phase images of (a) coated membranes with TiO2 sol (LTH process) (b) membranes coated with P25 and (c) control membranes 90 80 Membrane coated by P25 70 ) Membrane coated by LTH process o 60 50 40 30 Contact angle( 20 10 0 Befor After 5 min After 30 min Control Sonication Sonication Sonication Figure 6-4 Effect of sonication on mechanical stability of TiO2 coated in-house PES membranes Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 140 6.4.2.6 Durability, Stability and Photo-Catalytic Activity of TiO2 Coated Membrane by Iodide Oxidation Test As photocatalytically active TiO2 coated membranes have some specialized practical applications in membrane technology, the photo-catalytic activity of TiO2 coated membranes were investigated by iodide oxidation test [288]. The test also has been used to investigate the durability of the TiO2 nanoparticles deposited on the membranes. The TiO2 coated membranes were immersed in 30 ml of 0.1 M potassium iodide (KI) solution which then irradiated by UV lamp (T10 backlight blue lamp, 20 W) for 6 h. If the TiO2 coated membrane - being photo-catalytically active, it oxidizes iodide (I ) to iodine (I2) and changes the KI solution from colorless to light yellow (Figure 6-5 ). The mechanism of oxidation process from iodide to iodine is straightforward. After UV irradiation of TiO2 surfaces, the photo- + - generated holes (h ) react with iodide (I ) to form iodine (I2) and also complex of tri-iodide - ions (I3 ) as shown below. - + I + h → ½I2 (6-1) - - I2 + I → I3 (6-2) Apart from observed color change, the peaks of these species were detected by UV-Vis - absorption spectroscopy at 288 nm and 351 nm for I2 and I3 , respectively (Figure 6-6a). The color change and peaks were not observed for control membrane after 6 h of UV irradiation. To investigate the durability of the photocatalytic activity of TiO2 coated membranes the KI test was repeated during different cycles of UV irradiations. As can be seen in Figure 6-6 b the absorbance at the end of each cycle remained relatively unchanged for both species. The insignificant effect of UV light on polymeric substrate nature after TiO2 coating has been investigated by Lim et al.[289]. UV UV (a) (b) Figure 6-5 Effect of UV irradiation on the color change of KI solution in the presence of (a) control and (b) TiO2 coated membranes Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 141 2 1.5 1 0.5 Absorbance 0 250 270 290 310 330 350 370 390 Wavelength(nm) (a) 1.8 1.6 351 nm 1.4 288 nm 1.2 1 0.8 0.6 Absorbance (nm) 0.4 0.2 0 Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 (b) Figure 6-6 UV-Vis absorption spectroscopy for (a) KI solution of TiO2 coated membrane and (b) absorbance of the KI solution at 351 nm and 288 nm after exposure of the TiO2 coated PES in-house membrane by LTH process to 6 h of UV irradiation for 5 cycles 6.4.2.7 Coating Thickness Figure 6-7 is the cross section of coated and control membranes. As can be seen, due to the low sensitivity of FESEM method it is not possible to detect and measure the thickness of coated layer on the membranes. Since 10 wt.% of residuals from TGA test is a significant value, it was expected to see the coating layer on the surface of the membrane. The reason is that the in-house membranes do not have a support layer and during the dip-coating both surfaces of the membranes are exposed to the TTIP solution and were coated. In addition, the membranes are relatively rough and porous, and the coating fills the roughness of the membranes and pores. Therefore, the loading covers much more than the nominal surface area, which consequently increases the amount of loaded TiO2 nanoparticles. SEM images of membrane cross-section (Figure 6-7) show similar structure for both control and coated membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 142 (a) (b) Figure 6-7 Cross section SEM images of (a) coated (triple) in-house membrane with LTH process and (b) control membrane (a) (b) (c) Figure 6-8 Three dimensional AFM topography of membranes and their corresponding roughness images (a) control (b) coated with P25 and (c) coated with TiO2 sol (LTH process) Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 143 The difference in calculated average roughness (Ra) from AFM could be helpful to have a rough estimation of the coating thickness. In AFM, the Ra is calculated based on the vertical distance (z-direction) of each point on the sample to a baseline or reference line in a Cartesian coordinate system. Figure 6-8 shows the three dimensional images of different membranes. The images shows that the morphology of coated membrane with P25 has not changed substantially although its average roughness decreased from 58 nm to 47 nm, while the Ra for membrane coated by LTH processes has undertaken a significant reduction from 58nm to 15nm. By assuming that the reduction in roughness is only due to filling the valleys by particles and it is not affected by heat treatment, the 44 nm reduction in roughness could be the maximum achievable thickness of the coating layer within 3 cycles of coating. 6.4.3 Membrane Tensile Strength The tensile strength (N/m2) and elongation at break (%) of membranes were presented in Table 6-2 . The results showed that there are some small changes in tensile strength between control and membranes, whereas the elongation at break reduced for coated membranes. Decrease in elongation at break (ductility) of membrane after coating is expected due to the titania coating as well as thermal stress caused by post heat treatments and mismatch in the thermal expansion of TiO2 and PES layers [290]. Table 6-2 BET Surface areas (m²/g), the mean of BJH adsorption and desorption average pore diameter (nm), porosity for control PES (Pall Corporation, 100kDaMWCO) membranes and coated membranes at different cycles of coating by LTH process. Tensile strength and elongation were measured for in-house PES membranes coated by LTH process. The mean of BJH adsorption Tensile Elongation BET Surface Porosity Membrane and desorption average pore strength at break area (m²/g) (%) diameter (4V/A),nm (N/m2) (%) Control 3.30 ±0.16 27.72 ±3.05 0.5 0.45±0.04 84±16 1 cycle 3.57 ±0.17 26.52 ±1.91 0.6 0.49±0.04 65±12 2 cycles 3.33 ±0.19 22.23 ±1.88 0.5 0.39±0.03 40±7 Coated 3 cycles 4.54 ± 0.18 23.84 ±1.52 1 0.42±0.07 54±13 6.4.4 Surface Morphology 6.4.4.1 Scanning Electron Microscopy (SEM) Figure 6-9 shows the SEM images of coated membrane with P25 and TiO2 sol. As images show the coating of P25 has led to non-uniform surfaces, whereas membrane coated with TiO2 sol showed uniform coating. However, coating with TiO2 sol showed some cracks on the surface of dried membrane. This might be due to thermal treatment and difference in dimensional changes of membrane and TiO2 film and fast drying of TiO2 layer [197], the evaporation of solvent and surfactant (Pluronic F127), which cause a large mass change Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 144 might have also contributed [285]. To examine whether these cracks are due to TiO2 and/or heat treatment, the control membranes were dipped in a solution similar to TiO2 sol but without titanium (IV) iso-propoxide and then membranes were heat-treated similar to coated membranes. The results showed that, the cracks were not observed (See Appendix Figure A2). The optimization of LTH process for further controlling of drying speed could minimize the cracking, which is suggested for future work. 6.4.4.2 Atomic Force Microscopy (AFM) As discussed in the previous section, the AFM images in Figure 6-8 showed a significant change in the average roughness by around 74% decrease from 58 to 15 nm, which makes the coated membrane by LTH process smooth in macro-scale. 6.4.4.3 Surface Area, Pore Size and Porosity Measurement Using BET and BJH model The surface area and pore size of a solid material can be estimated by using physical adsorption and capillary condensation principles. From the nitrogen adsorption and desorption isotherms, information about the surface and internal pore characteristics of the membranes can be obtained. In this work, the method of Barrett, Joyner and Halenda (BJH) was employed for calculating average pore size from the isotherms using the Kelvin model of pore filling [258].The BET results in Table 6-2 show a significant increase (37.5%) in the surface area (m²/g) and porosity (100%) in 3rd cycle of coating. The average pore diameter of membranes calculated from BJH model shows that there is a small reduction in pore size (14%) within 3 cycles of coating, which is most likely due to the collapse of pores during heat treatment. The issue of pore collapse in polymeric substrates because of hydrothermal processes has also flagged previously [231, 291]. The actual reduction in surface pore size was expected to be more than that of calculated by BJH model since the commercial PES (100kDa MWCO) membranes does have a microporous support, and the BJH gives the total pore size which includes the skin layer and support. In fact, the PES skin layer is more sensitive to heat treatments than polyolefin nonwoven support with large macrovoids. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 145 (a) (b) Figure 6-9 SEM images of coated PES membrane with (a) P25 solution and (b) TiO2 sol (triple, LTH process) 6.4.5 Surface Chemistry 6.4.5.1 XPS Surface chemistry of control and coated PES membrane (100 kDa MWCO) by LTH process were examined by XPS. Results are demonstrated in Figure 6-10 . Presence of coating is confirmed by increasing the level of Ti, the lower level of S and C, and higher level of N. The lower level of sulphur and the increasing level of Ti after coating is an indication of thicker coating after each cycle. Increase in N might also be due to the leaching out of PVP from PES membrane by solvents/chemicals used in the preparation of sol and during dip coating process. Table 6-1 for treated in-house control membrane showed about 15 degree reduction in contact angle which is in agreement with the higher level of PVP at the surface of membrane, similar reduction in contact angle was observed for treated commercial (100 kDa MWCO) PES control membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 146 70 60 50 O1s 40 Ti2p wt% 30 C1s 20 S2p 10 N1s 0 Control Cycle 1 Cycle 2 Cycle 3 Figure 6-10 Surface composition of control PES and TiO2 coated (LTH process) PES membranes (Pall Corporation, 100kDa MWCO) with 1 to 3 cycles of coating derived from XPS results 6.4.5.2 Surface Charge (Streaming Potential) The zeta potential (ZP) measurements for control and coated membranes revealed that at pH of around 9 (where the humic acid fouling test was performed), the surface charges were - 15.5 mV and -5.9 mV for control and coated membranes respectively. Although the surface is still negative during fouling performance experiment, the TiO2 deposition made the surface less negative in the basic pH range. The measurement is also carried out at pH of around 5 where the protein (BSA) adsorption test was done. For control and coated membranes the zeta potential were -6.8 mV and -5 mV respectively, therefore; surface charge has an insignificant contribution in protein adsorption resistance after coating. It is worth to mention that the isoelectric points for both membranes were between pH of 3.5 and 4.8. 6.4.5.3 X-ray Diffraction (XRD) The membrane XRD patterns were recorded on a Philips Xpert Materials Research diffractometer (MRD) (Japan) equipped with graphite monochromated Cu Kα radiation (λ =0.15405 nm) operated at 40 mA, 45 kV from 6° to 65° for 2θ. The XRD analysis on the PES (100 kDa MWCO, triple coating) membrane shows that the majority of the crystals are in the anatase phase and rutile in the 2nd position (See Appendix Figure A3). 6.4.5.4 Surface Free Energy (SFE) On the base of acid-base (van Oss) approach, there is an insignificant difference between the surface free energy of the control and coated (triple) PES membranes, which were 44mN/m and 43mN/m, respectively. It should point out here that the surface free energy of the membranes were calculated based on the contact angles at 5th seconds while the acid-base (van Oss) approach formulations work well on smooth surface at equilibrium contact angles. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 147 Therefore, there might be deviations from actual surface free energy. However, this approach could give a rough idea of surface free energy changes upon coating. 6.4.5.5 Protein Adsorption The amounts of adsorbed BSA on the surface of the membranes measured by Lowry method showed up to 70% lower protein adsorption for the TiO2 sol coated membranes (15 and 4.5 μg/cm2 adsorbed BSA for control and coated membranes, respectively). Since the BET results (Table 6-2 ) showed that the coated membranes have larger surface area than control, the change in wettability and surface chemistry play important role in higher protein adsorption resistance. 6.4.6 Assessment of Hydrophilicity The hydrophilic or hydrophobic quality of a smooth and clean surface arises from its chemical make-up. These qualities are enhanced by the roughness of the surface, due to the trapping of air in the cavities between the drop and a rough hydrophobic surface, and by capillary wicking (nano-wicking) on a rough and structured hydrophilic surface [209]. The definition of superhydrophilicity is still unclear as to exactly what type of surfaces is being dealt with; for example some researchers have claimed that for a rough surface contact angle of zero is required to have a superhydrophilic surface [231]. Others have considered contact angles lower than 5o a superhydrophilic surface [229]. There are also some researchers who limited the term of superhydrophilicity to those surfaces that could reach to contact angle lower than 5o within 5 seconds [201] or in 0.5 second [203]. Achieving zero contact angles within 5 seconds is a broad definition of superhydrophilicity which is mostly referred in the literature. Zero contact angles have been achieved within around 12 seconds for commercial PES (100 kDa MWCO) membranes coated for 3 cycles (Figure 6-11). Although the coated membrane may not be considered superhydrophilic based on the above mentioned definitions, Figure 6-11 clearly shows the significant change in surface properties of membranes coated for 3 cycles with the ones coated for one or two cycles. The triple-coated membrane hydrophilicity had shifted toward superhydrophilic behaviour. The immediate contact angles (at 0 sec) for all of the coated membranes were below 60o while the uncoated membranes showed higher initial contact angle (below 80o). This might not be entirely due to increase in the coverage of membrane with TiO2 since a significant reduction in initial contact angle was observed for membranes which were dipped in a coating solution without TiO2 precursor followed by heat treatments. Therefore, the effect of chemicals and heat treatment on the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 148 migration of PVP from bulk of membrane to the surface which was shown earlier in XPS results might also be contributed to the lower contact angle. 120 Control 100 Single ) o Double 80 Triple 60 40 Contact Contact Angle ( 20 0 010203040506070 Time (sec) Figure 6-11 Contact angle measurements vs. time for PES (Pall corporation, 100 kDa MWCO) coated by LTH process using F127 as an additive for different coating cycles (Single: one cycle of coating, Double: two cycles of coating and Triple: three cycles of coating) The hydrophilicity of the coated membranes by LTH process was monitored over a period of 6 weeks in the absence of UV light and in the ambient conditions. As can be seen in Figure 6-12 , there was no increase in contact angle of membrane stored in the absence of UV irradiation. Therefore, the mechanism of hydrophilicity of the nano-composite membranes is probably different from what is governed in the inorganic TiO2 thin film which shows hydrophilicity due to 2D capillary phenomena. As mentioned earlier, TiO2 surface recently shows hydrophilicity as a result of 3D capillary phenomena, in which the multi-level roughness plays a more important role for changing the wettability of a surface [162, 232]. In 3D capillary effect, the imbibition of water will fill the inner pores below the droplets and increases the spreading of water on the surface [233]. Although AFM results showed that the overall roughness for coated membrane has been reduced in large scale (20μm × 20μm), a dual-scale roughness was found by locally investigation of the surface by SEM. Figure 6-13 shows that a surface composed of clusters demonstrating hierarchical submicron with two level roughnesses has formed. The size of cluster is between 70-110nm, formed from smaller nano-particles or clusters of diameter varying from 15-30nm. The porosity of the membrane could be related to the peripheral cracks around the clusters as well. Practically, in most situations the 2D and 3D capillary Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 149 effects work together to make super-wetting surfaces specially for TiO2 multilevel roughness surfaces [162]. 60 50 Week 0 ) o Week 2 40 Week 4 30 Week 6 20 Contact Angle Contact Angle ( 10 0 0 5 10 15 20 Time (sec) Figure 6-12 Contact angle monitoring during 6 weeks for PES (Pall Corporation, 100 kDa MWCO) membrane coated by LTH process within 3 cycles of coating (the membrane irradiated with UV only one time and then were kept at ambient conditions and laboratory environment) Figure 6-13 SEM images of PES (Pall Corporation, 100 kDa MWCO) coated membranes by LTH process (Triple) at different magnifications The means by which roughness and 3D capillary could change the wettability of the coated membranes surface has been discussed earlier in section 2.5.2 of literature review. It was mentioned that there are two options for a droplet to take place on a surface: either the contact line of the liquid follows the contours and roughness of the surface or the droplet does Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 150 not follow the roughness and bridges across the top of surface protrusions. The former one is known as the Wenzel model [218] and the latter one called the Cassie-Baxter model [219]. A straightforward experiment of tilting the surface and looking whether the droplet rolls-off at low or high tilt angle can determine which model is followed by the surface. If high tilt angle is required, the surface most likely follows the Wenzel model but if low tilt angle is required, the surface follows the Cassie-Baxter model [214]. For coated membrane a high tilt angle was required indicating that the membranes most likely follow the Wenzel model. Based on this model, increase in roughness amplifies the wettability of the surface toward its intrinsic tendency. It means, if the surface is originally hydrophilic (contact angle <90o), roughening makes it superhydrophilic but if it is hydrophobic (contact angle > 90o) roughening turns it into superhydrophobic surface [218]. In the Wenzel model, the effect of surface chemistry and morphology on wettability is given by the equation 6-3: ܥݏߠ௪ ൌ ݎܥݏߠ (6-3) where the roughness factor r is the ratio of the actual surface area of a solid to its geometrical projection and θe and θw are the contact angles on the surface before and after roughening, respectively. In the equation, the surface chemistry represents by θe whilst morphology lies within r. If we apply this equation for the single and triple in-house coated membranes with contact angles of 55 and 18°, respectively the value of the roughness factor r would be 1.65. This increase in roughness means there would be more areas exposed to the water resulting in a greater decrease in net energy to induce spreading and the rough area is wetted the more rapidly [218]. 6.4.7 Effect of Templating Agent As mentioned in section 2.4.3.6 of literature review, the addition of organic additives to precursor solution in sol-gel technology is an approach which has been widely applied to produce a network of highly accessible pores on the TiO2 films. In this work, Pluronic F127 was dissolved in ethanol and added to the TiO2 precursor solution before coating the membranes by dip-coating procedure. To see the effect of F127 on the wettability of the coating, the contact angles of membranes after coating with and without out F127 were measured, Figure 6-11 and Figure 6-14 show that the hydrophilicity was achieved within 3 cycles of coating regardless of the use of additive or not. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 151 60 50 ) o 40 30 Triple Cycles 20 Double Cycles Contact angle Contact angle ( 10 Single Cycle 0 010203040 Time (sec) Figure 6-14 PES (Pall Corporation, 100 kDa MWCO) membranes coated by LTH process without F127 However it was found that membrane with F127 had higher initial contact angle than membrane with no F127. Higher initial contact angle might be due to the residual amount of F127 and possible migration of the hydrophobic (PPO) segment to the surface of membrane. It is also possible that the membrane without F127 was smoother as will be discussed later. Similar results were observed for PEG modified TiO2 coating on glass substrate [204]. To investigate the durability of TiO2 coating with and without addition of F127, membranes were sonicated for 30 min and the change in contact angle at 5th sec was estimated. The results showed that after sonication the contact angle for membrane coated without F127 increased by 10% which might suggest that some of the particles have been detached from membrane. However, the contact angles remained almost constant after sonication for the membranes coated with TiO2 treated with F127, which might show that F127 has improved the adhesion between substrate and coating due to the bridge formation effect of F127. Similar behavior was observed by Yang et al., where they found that the use of additive could enhance the interfacial adherence between TiO2 films and polymeric substrates [285]. The morphology of membranes coated with and without F127 was shown in Figure 6-15 . From the SEM images, the membrane coated without F127 has larger surface clusters resulted in a higher surface roughness. Therefore, the surface roughness of coated membrane has been compromised in favour of the improvement in nanoparticles adhesion to the substrate. More efficient removal of residual F127 or the use of other types of the templating agent may further improve the surface hydrophilicity of membrane [202]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 152 (a) (b) Figure 6-15 SEM images for PES membrane (Pall Corporation, 100 kDa MWCO) coated by LTH process (triple) (a) with F127 (b) without F127 As highlighted in section 2.4.3.6 of literature review, different templating agents may result in different porous structures of coating film. Hydrophilic agents particularly PEG has been used widely as structure-directing agent. The mechanism of formation of porous network is based on the migration of TiO2 nanoparticles into the linear PEG chains which acts as structure-directing or templating agent. The organic additives are then removed from the composite by high temperature calcination decomposition or hot water leaching treatment to leave a porous network behind [206]. However the mechanism by which amphiphilic materials such as F127 direct the structure is different from their hydrophilic counterpart. Amphiphilic templating agents such as F127 can order the coating structure via the formation of micelles [207]. In the aqueous solutions, an inner core (micelle) forms due to the inward and outward orientation of hydrophobic PPO and hydrophilic PEO chains of the amphiphilic block copolymers, respectively. Under hydrothermal conditions, the supersaturated Ti(OH)4 starts to nucleate in the macromolecular micelles. Afterward, the growth of crystals proceeds under the restriction of hydrophobic cores to form homocentric bundle structures (see Figure 1-8 of literature review). In order to see the effect of changing the templating agent from amphiphilic to hydrophilic on the surface morphology of the membranes, PEG (2gr/100 mL of sol) was substituted by F127 (4.26 gr/100 mL of sol) in the ethanol solution. As can be seen in Figure 6-16 , the surface morphology was changed from cauliflower to rose structure by changing the templating agent from F127 to PEG. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 153 (a) (b) Figure 6-16 Effect of templating agent on the surface structure of TiO2 coated membranes (a) F127 and (b) PEG 6.4.8 Effect of Coating Cycles and Heat Treatment As demonstrated earlier, the strong hydrophilicity was achieved during 3 cycles of coating. To find out whether this phenomenon is due to TiO2 coverage or crystallization of underlying TiO2 layers, two sets of membranes were coated separately. For the first set, membranes were coated once followed by 3 times heat treatment and for the second set, the membranes coated 3 times with only one stage heat treatment at the end. For complete removal of solvent for the second set, membranes were kept in vacuum oven at ambient temperature for 24h after each coating. Although it is well known that the crystallinity and therefore hydrophilicity of TiO2 can be improved by post heat treatment [292, 293], the contact angle results of the membranes (Figure 6-17) showed that the crystallization of underlying TiO2 layers is unlikely the reason for the higher hydrophilicity, since membrane with 3 cycles of heat treatment is less hydrophilic than membrane which was coated 3 times with TiO2. This experiment supports the idea that TiO2 coverage during 3 cycles of coating plays an important role to achieve hydrophilicity. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 154 45 40 35 ) o 30 25 20 3 cycles of coating with one stage heat treatment 15 1 cycle of coating with 3 stages heat treatment Contact Angle Contact Angle ( 10 5 0 0 102030405060 Time (sec) Figure 6-17 Contact angles of membrane (Pall Corporation, 100kDa MWCO) coated by LTH process in 1 (single) and 3 (triple) cycles with different heat treatment stages Examining the standard deviations (STD) in Figure 6-18 and assuming that the errors come mainly from patchy (incomplete) coating on substrate, one could conclude that after second cycle there is a significant reduction in the surface variations, which may suggest that a good coverage was achieved after cycle 2. However, there is not a significant reduction in STD of cycle 3 compare to cycle 2. Therefore, we suggest that the increased wettability after duplicate coating may be related to morphological changes rather than chemistry and coverage. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 155 60 50 ) o 40 30 20 Contact Angle Contact Angle ( 10 0 1 Cycle 2 Cycles 3 Cycles Figure 6-18 Contact angles at (5th sec) during 3 cycles of coating by LTH process for membrane (Pall Corporation, 100kDa MWCO) coated without F127 As mentioned before, the increase in roughness (Wenzel model) is important contributing factor in achieving superhydrophilicity. In fact, increase in roughness from cycle 2 to 3, could push the wettability of surface toward extreme conditions. SEM images in Figure 6-19 are useful for better understanding of the roughness effect. As can be seen, the increased width of peripheral cracks around the clusters during coating cycles especially from cycle 2 to 3 causes an increase in nanoscale roughness and porosity. This increase in the width of peripheral grooves could be due to projecting the clusters out as coating cycles increase. These results are consistent with the BET results (Table 6-2 ), which showed that the surface area increases significantly from cycle two to three. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 156 coverage 2 Cycle 1 (From Cycle 1 to 2) to 2) 1 Cycle (From Increase in hydrophilicity due to TiO due hydrophilicity in Increase Cycle 2 (From Cycle 2 to 3) to 3) 2 Cycle (From Cycle 3 Increase in hydrophilicity due to increase in roughness in roughness increase to due hydrophilicity in Increase Figure 6-19 SEM images for membrane (Pall Corporation, 100 kDa MWCO) coated by LTH process without F127 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 157 6.4.9 Effect of TiO2 Coating on the Membrane Performance To quantitatively investigate the membrane fouling performance, flux recovery (FR%) and intrinsic membrane resistance (Rm) which is due to factors related to membrane properties and different resistance ratios were calculated by formulation introduced in section 3.5.16.3. The performance of coated membranes was investigated in both modes of constant pressure and constant flux. Commercial membranes with higher permeability (500 kDa MWCO) were chosen in this study in order to sustain reasonable flux after coating. The LTH process was applied on the higher flux membranes and increased hydrophilicity and dual scale roughness hierarchical structure similar to (100 kDa MWCO) membranes were observed (Figure 6-20). (a) (b) 70 Control 60 ) o Coated 50 40 30 20 Contact Angle Contact Angle ( 10 0 0246810 Time (sec) (c) Figure 6-20 SEM images (a) after coating, (b) before coating and (c) contact angle measurements for commercial PES (Millipore, 500kDa MWCO) membrane by LTH process Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 158 6.4.9.1 Constant Pressure Fouling Experiment As can be seen in Figure 6-21, the fouling behaviour (flux profile) of control and coated PES membranes are very similar during 2 hours filtration of 0.1 wt.% humic acid (HA) at constant pressure of 1 bar. These findings were reproducible among 3 independent experiments. Table 6-3 shows the flux recoveries of control and coated PES (500 kDa MWCO) membranes during 3 subsequent fouling cycles followed by 3 cleaning stages (physical and chemical cleaning and backwashing) between each cycle. There is not a large difference in flux recovery of control and coated membranes after first cycle of fouling and cleaning although the flux recovery of coated membrane is relatively higher than that of control. However, a significant flux recovery differences were observed during fouling and cleaning for cycle 2 and 3 in a way that at the end of cycle 3 the flux recovery of control was 15% whereas for the coated membrane it was 81%. The membrane resistance ratios (which are the ratio of membrane resistance after each stage of cleaning to its corresponded intrinsic resistance) were calculated and shown in Table 6-3 . The ideal membrane is one on which no foulants can absorb irreversibly or if there is fouling, the membrane could achieve complete flux recovery and membrane resistance ratio of one after cleaning. Deviations from ratio of one (>1) means that there are still resistance against liquid flow through the membranes, which is due to physical or chemical adsorption of foulants on the membrane surface. From the table, significant reductions in ratios in each cycle were achieved after chemical cleaning and backwashing for coated membranes. This means that the coating could reduce the interaction between HA molecules and the surface of the membrane since the chemical cleaning did not decrease the resistance ratio and increase the FR for coated membrane significantly. In contrast, chemical cleaning changed the FR and resistances ratios for control membranes considerably. Moreover, the coating causes membrane to show greater resistance against adsorption, which is in agreement with the previous results found by Lowry method for 100 kDa membrane substrates. The backwashing was able to significantly increase the FR for coated membrane while it did not maintain the same effectiveness in mitigating the fouling of the control membrane. The intrinsic membrane resistance (Rm) during fouling cycles has increased for control membranes more significantly than the coated membranes. SEM images of coated membranes before and after fouling and cleaning showed that there were not significant changes in morphology of the membranes at nanoscale (see the inset images in Figure 6-22 ). Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 159 400 350 Coated 300 Control h) h) 250 2 200 150 Flux (L/m Flux 100 50 0 0 20406080100120140 Time (min) Figure 6-21 Fouling behaviour (0.1 wt.% HA, pH 9.4) of control and coated (LTH process) PES (Millipore, 500 kDa MWCO) membranes at constant pressure of 100kPa for 2 h Table 6-3 Initial water flux, flux recoveries after physical, chemical cleaning and backwashing, humic acid rejections, membrane intrinsic resistance and membrane resistance ratios for control and coated membranes during 3 cycles of coating at constant pressure of 100 kPa Membrane resistance ratios after Initial Humic acid Flux recovery each stage of cleaning Water Flux Fouling average R m -2 -1 after cleaning -15 -1 (Lm h ) Cycle rejection × 10 (m ) ୦୷ୱ୧ୡୟ୪ େ୦ୣ୫୧ୡୟ୪ ୟୡ୩୵ୟୱ୦୧୬ FR (%) Membrane (±200) (%) ୫ ୫ ୫ I 76 0.08 2.02 1.11 1.21 4707 II 39 81 0.10 2.05 1.89 1.84 Control Control III 15 0.19 3.08 2.17 2.10 I 93 0.33 2.00 1.71 1.08 1080 II 84 76 0.37 1.70 1.62 1.07 Coated III 81 0.40 1.51 1.51 1.04 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 160 Figure 6-22 SEM images of triple coated (LTH process) PES (Millipore, 500 kDa MWCO) membrane after fouling and cleaning 6.4.9.2 Constant Flux Fouling Experiment In order to exclude the contribution of differing initial water fluxes to the fouling results, the hydraulic conditions was kept constant for membrane with different permeabilities by performing the experiments in constant flux mode. Figure 6-23 shows the variation of transmembrane pressure, TMP, during 2 h of 0.1 wt.% humic acid (HA) filtration at constant flux of 85 L/m2 h for control and coated PES (500 kDa MWCO) membranes. As can be seen, at the end of 2 h filtration, the final TMP for the control was twice as high than that of coated membrane. In the Table 6-4 , the flux recoveries of control and coated PES (500 kDa MWCO) membranes during 3 subsequent fouling cycles followed by 3 cleaning stages (physical, backwashing and chemical cleaning) between each cycle are given. From the results, the flux recoveries of coated membranes are significantly higher than that of control (an average of 86% and 43% for coated and control membranes, respectively). In addition, the FR% reduction after 3 cycles of fouling for control was around 20% whereas it was 7% for coated membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 161 35 Coated 30 Control 25 20 15 10 TMP increase (kPa) TMP 5 0 050100150 Time (min) Figure 6-23 Fouling behaviour (0.1 wt.% HA, pH 9.4) of control and coated (LTH process) PES (Millipore, 500 kDa MWCO) membranes at constant flux of 85 Lm-2h-1 for 2 h, the initial feed side pressure was set to 40kPa and the experiment was run at room temperature. The stirrer speed was at fixed speed of 600 rpm. Table 6-4 Initial water flux, flux recoveries after each stage of cleaning, rejections, membrane intrinsic resistance and membrane resistance ratios for control and coated membranes by LTH process during 3 cycles of coating at constant flux of 85 Lm-2h-1 Membrane resistance ratios and FR% after Initial each stage of cleaning Average R m water flux Fouling rejection × 10-15 (Lm-2h-1) cycle FR FR (%) (m-1) ୦୷ୱ୧ୡୟ୪Ƭ௪௦ େ୦ୣ୫୧ୡୟ୪ Membrane Membrane (±80) ୫ (%) ୫ (%) I 0.12 2.73 37 1.88 53 3000 II 73 0.23 1.52 35 1.20 44 Control Control III 0.27 1.56 28 1.33 33 I 0.48 1.25 80 1.12 89 750 II 70 0.54 1.04 86 1.02 88 Coated Coated III 0.55 1.05 84 1.06 82 From Table 6-4 , similar to constant pressure results, the reductions in membrane resistance ratios after chemical cleaning for control is more obvious than coated membranes. The physical cleaning and backwashing significantly could increase the FR for coated membrane while it has not retained a positive effect on the fouling performance of the control Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 6 Coating (Superhydrophilic Modifications) 162 membrane. This supports the previous results that the nature of interaction between HA molecules and the surface of the membrane has been changed. It should point out here that there is an inevitable trade-off loss of initial water flux due to the coating. As it is shown in Table 6-4 , the initial water flux of control is almost 4 times greater than that of coated membranes and even after 3 cycles of fouling, the initial water flux of control was 990 L/m2h (33% flux recovery) which is comparable with that of coated membranes. Nevertheless, the coated membrane showed superior performance over more fouling cycles. Using larger pore membrane substrate and optimization and controlling the rate of drying in heat treatments could minimize the issue of initial water flux reduction, which are suggested for future work. The membranes rejection presented in Table 6-3 and Table 6-4 shows that the rejection of control membrane is slightly higher than that of coated. Thus the improvement in fouling performance is not strongly linked to a large change in the solute rejection. 6.5 Conclusion PES membranes were coated with TiO2 by the application of dip-coating and LTH process. The comprehensive characterization techniques confirmed a uniform coating layer with good stability and durability as well as low protein adsorption compared with the control membrane. The coatings exhibit sustained photocatalytic activity over repeated cycles of use. On the other hand, long term hydrophilicity without continuous UV light irradiation was achieved after 3 cycles of coating and was attributed to the surface architecture and dual scale roughness generated by the nanoparticle coating. While there was a substantial decrease in initial water flux due to the coating, a significant improvement in fouling performance of coated membranes was observed. While dip-coating was utilized in this work to coat flat sheet membranes, the process is amendable to coating hollow fibers and other membrane configurations by alternative methods such as microfiltration as was previously shown for titania coating on PVDF membranes [289]. Further work to optimize coating thickness, structure and substrate selection is suggested. The formation of a robust hierarchically rough surface provides the potential to form both superhydrophilic and superhydrophobic microporous membranes with further surface functionalization. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 163 AmirAmir Razmjou, PhD Thesis, TheThe University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 164 CHAPTER 7 COATING (SUPERHYDROPHOBIC MODIFICATION)-MD APPLICATION 7 Abstract* Superhydrophobic membrane for the application in membrane distillation was generated by creating a hierarchical structure with multilevel roughness via depositing TiO2 nanoparticles (10nm) on microfiltration (0.45 μm) PVDF membranes by means of a low temperature hydrothermal (LTH) process. The TiO2 coated membranes were then fluorosilanized by using a low surface energy material i.e. H,1H,2H,2H-perfluorododecyltrichlorosilane. The anti- fouling performance of virgin and modified membranes were examined in a direct contact membrane distillation (DCMD) process using sodium chloride and humic acid solution as model foulants. Results showed that the modification was mechanically and thermally robust and photoactive. The liquid entry pressure (LEP) and and water contact angle were increased o o from 120 kPa and 125 to 190 kPa and 166 , respectively. The fluorosilanization of TiO2 nanocomposite PVDF membranes did not compromise the mean pore size. The experimental results also showed that the pure water flux of the modified membrane was lower than that of virgin membrane particularly at higher temperature. However, the sodium chloride DCMD test showed that the permeate conductivity of virgin membrane was increased sharply whereas it was not changed for the modified membrane over the period of the experiment. The 20 h fouling experiment of humic acid DCMD did not show a reduction in flux for virgin and modified membranes. However, the addition of 3.775 mM CaCl2 into the solution significantly increased the flux reduction due to the formation of complexes with humic acids and consequent particles coagulation and precipitation on the membrane surface. Although both virgin and modified membranes showed similar fouling behaviours, a significantly higher flux recovery was found for modified membrane compared to the virgin membrane. This chapter is based on: “Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation” by A. Razmjou, E. Arifin, G. Dong, J. Mansouri and V. Chen, Journal of Membrane Science, 2012, Volume 415–416, 2012, Pages 850-863 *The results that were jointly obtained by me and Ellen Arifin in this chapter are: Optimization of FTCS coating, water contact angle and LEP measurements (Figure 7-3, Table 7-1), thermo- mechanical stability tests, Figure 7-9 Figure 7-10 and Figure 7-12. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 165 7.1 Introduction Recently, a considerable attention has been devoted to the superhydrophobic and superoleophobic surfaces which have a water or oil contact angle greater than 150o [215]. This remarkable property of extreme liquid wettability has been widely applied in both fundamental research and practical applications [210, 231]. Superhydrophobic surfaces in the nature have prompted many researchers to mimic the micro or nano scale structures of the superhydrophobic biosurfaces [216]. A variety of techniques such as plasma treatment [294], lithography [295], sol gel technology [296], nanoparticle deposition on smooth or roughened substrates [297], fluoroalkylsilane coatings [298] and phase separation of a multi-component mixture [299] were used to fabricate superhydrophobic surfaces. These routes can generate superhydrophobic surfaces via reducing the surface free energy of a rough surface or roughening a low surface energy material or a combination of both [210, 259, 300]. As explained in section 2.5 of literature review, the surface solid wettability is governed by both chemistry and geometrical structure of the surface. Surface chemistry determines the surface tensions at microscopic level but geometrical structure governs how these forces act upon the liquid [215]. Therefore, surface wettability can be tuned by dynamically varying one of these two parameters [162]. Recently, reducing the surface free energy by functionalization of low surface energy materials particularly fluorosilanes was mainly used for the generation of superhydrophobic surfaces. Alternatively, researchers have tried to generate hierarchical nanostructure surface morphology with multilevel surface roughness in order to modulate surface wettability toward extremes [220, 221]. Hydrophobic membranes can be used in membrane distillation (MD) for water purification, brine management, heavy metal removal, food industry and purification of pharmaceutical products [237]. The major advantage of MD is the ability to use low-grade energy to drive the mass transport as the feed does not have to reach boiling point (typically 50o– 90oC). In membrane distillation (MD), vapor pressure difference as a result of temperature gradient transports only vapor molecules from the high temperature feed (typically a saline solution) across the porous hydrophobic membrane to produce purified condensed liquid on the low temperature permeate side while quarantining the liquid on the feed side of the membrane As noted in section 2.6.3 of literature review, the main obstacle to commercial implementation of MD processes is sustaining a stable flux and controlling heat and mass transfer throughout the module. Sustained operations are compromised by pore wetting, Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 166 fouling, temperature and concentration polarization at the membrane interface, and heat loss via conduction through the membrane. A variety of techniques have been employed to minimize these issues (see section 2.6.3.1-4). However, superhydrophobic modification is an alternative approach which can be used to minimize the aforementioned issues. Shifting the hydrophobicity toward superhydrophobicity helps to introduce a gap between water/oil drop and the surface by an air gap as explained by Cassie state (section 2.5.2). This air gap provides an opportunity to increase the allowable pore sizes much larger than that of normal membranes if the operating pressure were the same, accordingly allowing higher mass flux without pore wetting [246]. The air gap can also minimize the issue of heat loss through conduction because the thermal conductivity of air/gases is an order of magnitude lower than that of polymeric membranes (section 2.6.3.3). Superhydrophobic modification could also reduce the interaction between feed water solution and membrane surface thereby reducing the risk of fouling (section 2.6.3.2). Temperature and concentration polarization can be lessened by raising feed pressure and shear rate, but the high feed pressure increases the potential for pore wetting (exceeding the LEP) due to liquid intrusion into the pores as transmembrane pressure increases [237]. Superhydrophobic modification may raise the LEP [244], thus higher feed pressure and shear rates can be applied to minimize the polarizations. Titanium dioxide is one of the most widely used semiconductor with the annual world consumption of above 4.4 million tons [136] because it is stable, nontoxic and abundant. In previous chapters, it was shown the surface architecture and chemistry of the TiO2 deposited nanocomposite membranes can be tuned via coating by a low temperature hydrothermal (LTH) process. It was mentioned that the hierarchical structure with multilevel roughness generated during TiO2 coating by LTH process can change the wettability of the membrane surface toward the extremes of superhydrophilic or superhydrophobic (see chapter 6). Although TiO2 superhydrophobic surface modifications have been recently reported on solid high thermal resistance substrates [221, 226-230], there are a very limited number of reports on the superhydrophobic modifications of polymeric membranes using nanoparticles such as TiO2. As it was reviewed in section 2.6.3.5, there is a potential for employing TiO2 nanoparticles for the generation of superhydrophobic surfaces. In this chapter, microfiltration PVDF membranes were modified such that the surface wettability shifts toward superhydrophobicity via increasing the surface roughness by TiO2 coating by LTH process and subsequently reducing the surface energy by surface Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 167 fluorosilanization. Surface chemistry, hydrophobicity and surface structure of modified membranes were examined by XPS, contact angle and SEM techniques, respectively. Stability of coating was also investigated. The performance of the membranes in direct membrane distillation process before and after modification was investigated using NaCl, humic acid and calcium chloride as model foulants. 7.2 Experimental 7.2.1 Materials Commercial polyvinylidene fluoride (PVDF) flat-sheet membrane HVHP (Millipore, nominal pore size 0.45 μm, porosity 75%) was used in this study. Pluronic F127, humic acid (HA), and 1H,1H,2H,2H-Perfluorododecyltrichlorosilane (FTCS) were purchased from Sigma-Aldrich. Titanium (IV) iso-propoxide (TTIP) (98%) as TiO2 precursor and 2,4- Pentanedione were purchased from Acros Organics. Toluene and absolute ethanol as solvent were supplied by Ajax Finechem. Perchloric acid (70%), sodium chloride (NaCl) and calcium chloride (CaCl2) were purchased from Chem-Supply. 7.2.2 TiO2 Coating and Fluorosilanization of PVDF Membranes The superhydrophobic modification of the PVDF membranes has two main steps: coating of TiO2 nanoparticles and fluorosilanization of the TiO2 coated membranes. As explained in detail in section 3.4, the TiO2 nanoparticles were first coated on the surface by using the low temperature hydrothermal process which has been implemented in chapter 5 and then a solution of fluorosilane was filtered through the membranes. 7.3 Membrane Characterization Different techniques explained in chapter 3 such as SEM, XPS, contact angle goniometry, capillary flow porometry, surface free energy and liquid entry pressure measurement were applied to characterize and study the effect of superhydrophobic modification on surface chemistry and structure. The MD performance of virgin and modified flat sheet membranes with the approximate contact area of 14.4 cm2 was examined in a direct contact membrane distillation process (see section 3.5.16.4 for more details). Two synthetic feed solutions used in this experiment were sodium chloride solution (3.5 wt.%) to model seawater and humic acid solution (150 mg/L with and without 3.775 mM CaCl2) to model natural organic foulant. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 168 7.4 Results and Discussions 7.4.1 Evidence of TiO2 and FTCS on the Membrane Surface The evidence of the presence of TiO2 nanoparticles and FTCS molecule was examined by XPS analysis of membranes (Figure 7-1 ). Ti was detected for membrane coated with TiO2 and its amount was decreased after coating with FTCS as was expected. Appearance of N in FTCS -TiO2- PVDF membrane might be due to the leaching out of PVP from the commercial PVDF membrane by the chemicals contained in the sol and during dip coating process as it was discussed in section 6.4.5.1. The increase in the percentage of O might also be an indication of the increase in the OH functional group from TiO2 coating. The significant increase of weight percentage of F and the presence of Si on the membrane coated with TiO2 and FTCS confirm the presence of FTCS on the membrane. FTCS-Titania-PVDF C O Titania-PVDF N Si Ti F Virgin PVDF 0 102030405060 At.% Figure 7-1 Surface composition (wt.%) of virgin PVDF, TiO2–PVDF and FTCS–TiO2–PVDF membranes. 7.4.2 Assessment of Hydrophobicity The pure water contact angle measurement has shown that the contact angle of the membrane was increased from 125°±1° for virgin PVDF membranes to 163°±3° for TiO2-FTCS-PVDF membranes, which indicate that the modified membrane is superhydrophobic (Table 7-1 and Figure 7-2 a). An increase in water contact angle was observed for FTCS-PVDF membranes due to significant reduction in surface free energy (Table 7-2) and increase in roughness which will be discussed in section 7.4.4. From Table 7-1 , a sharp decrease in contact angle hysteresis was observed from 29° for virgin membrane to 2° for FTCS-PVDF membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 169 However, the hydrophilicity and contact angle hysteresis of the virgin membrane increased significantly after coating with TiO2 nanoparticles compared with the virgin membrane due to increase in roughness as it was shown in chapter 6. Water droplets on superhydrophobic surface with low contact angle hysteresis are almost spherical and can be easily rolled off at low tilt angle and clean the surface (self-cleaning property) at low tilt angle. As can be seen in Figure 7-2 b, the virgin and TiO2 coated PVDF membranes did not show self-cleaning property even at high tilt angle whereas it was observed at low tilt angle for FTCS-TiO2 PVDF membrane. For FTCS-PVDF membranes, as soon as the droplet released from the syringe (time = t2), it formed spherical shape but the droplet did not roll off at low tilt angle to show self-cleaning property. It should point out here that the droplet rolled off at high tilt angle for the membrane. Table 7-1 Contact angles for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition Contac angle (deg) Contact angle Membrane 30% (vol) hysteresis (deg) Water Glycerol Mono-ethanol amine (MEA) PVDF (virgin) 125°±1° 99° ±6° 115°±3° 29° ± 3° TiO2 + PVDF 98°±13° 101°±5° 82°±16° 47° ± 9° FTCS + PVDF 146°±5° 140°±10° 137°±5° 17° ± 8° FTCS + TiO2 + PVDF 163°±3° 166°±1° 150°±3° 2 ° ± 1° As can be seen in Figure 7-2a, the FTCS-TiO2-PVDF membrane showed superhydrophobicity behaviour not only for pure water but also for other solutions with different pH and also humic acid solution. It is reported in the literature that the addition of capillary-inactive substances such as salts into pure water increases its surface tension and causes a negative adsorption in the surface layer [301]. On the other hand, the addition of surfactant such as humic acid or Bovine Serum Albumin (BSA) could reduce the surface tension of solution up to a certain level. Further increase in the concentration of humic acid will not change the surface tension of solution due the formation of micelles [302, 303]. Contact angle measurements were carried out using different concentrations of model fouling solutions of sodium chloride, humic acid and BSA. Contact angle measurements in Figure 7-3 shows that the hydrophobicity of both virgin and the FTCS-TiO2-PVDF membrane have not reduced Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 170 against any of these solutions. Similar results were observed for BSA (150°±4.9° and 154°±7° for 1 and 5 gr/L BSA, respectively). Before After (a) PVDF (virgin) TiO2 PVDF t1 t2 t3 t4 FTCS PVDF t1 t2 t3 t4 FTCS TiO2 PVDF (b) Figure 7-2 Effect of superhydrophobic modifications on the PVDF membranes (a) before and after modification (withe: MQ water, brown: humic acid solution, pink, blue and green: buffer solutions with pH of 4, 7 and 14), and (b) imparting self-cleaning property As mentioned in section 2.5 of literature review, liquid surface tension is one of the three interfacial forces that may influence the wettability of the surface [210, 214]. In addition to that, morphology plays an important role in the wetting action of a solid [215]. Therefore, the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 171 negative effect of humic acid or BSA on the hydrophobicity of the membrane surface could be insignificant in spite of reducing the surface tension of the feed solution. Increasing the concentration of sodium chloride increases the surface tension of water, while increasing the concentration of humic acid decreases the surface tension of water. Thus, similar superhydrophobicity towards liquids of different surface tensions had proven that the high contact angle was mainly due to the change in the surface chemistry of the membrane and not due to the surface tension of liquid. Table 7-2 Surface free energy and, bubble point pressure, liquid entry pressure values and fluxes for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition. 2 Surface free Liquid (water) Flux (L/m .h) at LEP* Bubble point Membrane energy entry pressure pressure, kPa Pure 30% (vol) (mN/m) (LEP), kPa Water Ethanol PVDF (virgin) 14 ± 0.4 55 120 450±10 670±10 TiO2 + PVDF 13 ± 1.6 54 90 210±15 - FTCS + PVDF 0.5 ± 0.14 55 130 40± 8 - FTCS + TiO2 + PVDF 0.1 ± 0.01 55 190 30± 3 690±5 *The flux for 30% (vol) ethanol solution was measured at its corresponded LEP 180 Modified Control 160 140 ) ° 120 100 80 60 Contact Angle Contact Angle ( 40 20 0 Ultra-pure 10 g/L NaCl 35 g/L NaCl 70 g/L NaCl 25 mg/L 50 mg/L 100 mg/L water Humic Acid Humic Acid Humic Acid Figure 7-3 Contact angle measurements of virgin and modified (FTCS-TiO2-PVDF) membrane using different concentration of NaCl and humic acid Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 172 The potential application of the superhydrophobic modification for CO2 and H2S removal from flue gas was examined by measuring the 30% (vol.) mono-ethanol amine (MEA) contact angles on membrane surfaces. MEA is used as an absorption liquid in membrane contactors where the membrane acts as an interface between the feed gas and MEA [304]. As can be seen in Table 7-1 , the MEA contact angle was increased from 115° for virgin membrane to 150° for FTCS-TiO2-PVDF membranes. From the Table, the FTCS-TiO2- PVDF membranes also showed superoleophobic behaviour to relatively high surface tension oils such as glycerol; the glycerol contact angle was increased from 99o to 166o. This demonstrates another importance of this modification route as the superoleophobic coatings can be used in a wide range of applications such as bio-fouling and stain resistant materials, fluid power systems and also can be used for elastomeric seals and O-rings to prevent swelling [220, 305] in organic environments. In addition, the introduced modification technique is easy to implement and requires moderate temperate whereas the current techniques to create superoleophobic surfaces are much more tedious and costly [305]. 7.4.3 Thermomechanical Stability The stability of coating layers plays an important role in maintaining the performance of membrane. In Direct Contact Membrane Distillation (DCMD) applications, FTCS-TiO2- PVDF membranes are exposed to different projected temperatures and cross-flow velocities which could detach the loosely attached particles. To evaluate the mechanical stability of coatings, membranes were immersed in Milli-Q water and were sonicated at room temperature for 15 min and the contact angle was measured. The contact angle of the FTCS- TiO2-PVDF membrane was 160°±1°. By comparing the standard deviation of initial contact angle (163°±3°), the coating was considered to be mechanically stable. Thermal stability of the coated membranes was analysed by immersing the membrane in Milli-Q water at 90°C for 15 min to ensure that the membrane still maintains its superhydrophobicity even when exposed to a much extreme environment. The contact angle of membrane was measured to be 156°±2° which shows a marginal decrease in contact angle but still superhydrophobic. An increase in the temperature reduces the surface tension of water as a result of reduction in the cohesive forces and enhancement in the molecular thermal activity and consequently surface hydrophobicity [306]. 7.4.4 Surface Morphology The effect of different modification steps on the structure of the membranes in macro-scale was presented in Figure 7-4. As can be seen in Figure 7-4 b, the deposition of TiO2 on the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 173 PVDF membrane did not change significantly the surface structure, porosity and pore size of the membrane in macro-scale as there is a very thin layer of TiO2 on the surface (see section 6.4.2.7). However, the FTCS modification (Figure 7-4 c) without TiO2 coating resulted in a loose cake layer of fluorosilane which can be detached from the surface by backwashing or cross flow velocity. (a) (b) (c) (d) Figure 7-4 SEM images of (a) virgin, (b) TiO2-dip coated, (c) FTCS-vacuum filtered and (d) TiO2-dip coated and FTCS- vacuum filtered PVDF membrane at 5K magnification Figure 7-4 d shows the surface structure of PVDF membranes after TiO2 and FTCS modification. From the image, the overall structure and average pore size of the FTCS-TiO2- PVDF membrane is similar to the virgin membrane although there are some large residues loosely attached to the surface. Despite the little detectable change in the surface of the membrane at macro scale after superhydrophobic modification, its surface structure and pore walls were altered dramatically at nano scale as presented in Figure 7-5. The images showed that after the modification there is a hierarchical structure with multilevel roughness which plays an important role in shifting the wettability toward superhydrophobicity [220, 221]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 174 (a) (b) Figure 7-5 Effect of superhydrophobic modification on the PVDF membranes (a) virgin PVDF (b) FTCS-TiO2-PVDF membrane (insets at top right corner of (a) and (b) are the SEM images at high resolution and at top left corner of (b) is the schematic side view of clusters assumed in a series of cylinders aligned horizontally). Figure 7-5 b shows that the FTCS-TiO2-PVDF membrane surface composed of clusters which is demonstrating hierarchical submicron with two level roughnesses. The size of clusters is approximately around 85 nm in length and 57 nm in width and formed from smaller nano- particles or clusters of diameter around 20 nm. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 175 The mechanism of superhydrophobic modification Wenzel model and Cassie-Baxter model are used to understand the mechanism of superhydrophobic modification. These two models have been well established and reviewed in section 2.5. According to Wenzel model, the degree of roughness should be to amplify the wettability of the surface toward its intrinsic tendency of either superhydrophilic or superhydrophobic film formation. As introduced in section 2.5.2, the Wenzel equation describes the effect of surface chemistry and morphology on the water droplet contact angle: ܥݏߠ ൌ ݎܥݏߠ 7-1 where θ is apparent contact angle and θe is the contact angle on the flat surface, and the roughness factor “r” is the ratio of the actual solid/liquid contact area to its vertical projection. According to Wenzel, morphology lies within r (higher roughness value means higher “r” value) whilst the surface chemistry represents by θe (higher surface energy results in lower contact angles and vice versa). Since the PVDF membranes were coated by FTCS, it can be considered as a new surface which is saturated by fluorinated methyl groups. The maximum contact angle that can be o reached on a flat solid surface saturated by CF3 groups is 120 which corresponds to a surface free energy of 6.7 mJ/m2 [222, 223]. Taking into account the apparent contact angle of o fluorinated TiO2-PVDF membrane as 163° and 120 for flat surface of FTCS saturated by CF3 groups, the r-value will be 1.91. Using the methodology used by Xiu et al. [224] and by assuming the clusters as a series of cylinders (85 nm in length and 57 nm in diameter) aligned in a horizontal orientation (top left corner of inset image in Figure 7-5b), the calculated roughness factor “r” would be 1.57 (π/2) if water contacts entire top half of each cylinder. Therefore, the apparent contact angle would be around 140o which cannot be considered as superhydrophobic surface. These deviations from experimental values may come from the fact that the clusters are aligned randomly and the modified surface is more complex than the models with simple flat-topped surface protrusions. It was reported that the complex topography is often more effective at generating superhydrophobic surfaces [210]. In addition to surface complexity, the Wenzel assumption can be breached if the liquid finds it difficult to penetrate into the roughness or texture of the surfacer due to capillary forces as it is explained by Cassie-Baxter model [219]. As a result, liquid energetically favours to bridge across the top of surface protrusions such that the water droplets sit upon the patchwork of solids and air gaps and does not follow the surface contours. The Cassie-Baxter equation for Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 176 solid-liquid-air composite interface (porous) considers the effect of air gaps under the droplets: ܥݏߠ ൌ ݂௦ሺܥݏߠ ͳሻെͳ 7-2 In the equation if fs which is solid surface fraction decreases, the apparent contact angle (ߠ) o increases and in limit approaches 180 when fs approaches zero. That means increase in roughness reduces the area of contact between the liquid and solid and resulted in a reduction in the adhesion of a droplet to the solid surface. The effect of multilevel roughness on hydrophobicity As mentioned in section 2.5.3, a multi-level roughness (micro and nano) is required to achieve superhydrophobic surfaces. As can be seen in Figure 7-4a, the virgin PVDF membranes consists of pores in microscale with water contact angle of 125o. Therefore, microscale roughness is not sufficient enough to gain superhydrophobicity. However, after surface modifications shown in Figure 7-5 b, the hierarchical structure with micro and nano roughness leads to a superhydrophobic surface with water contact angle of 163o. Such high water contact angle and wetting resistance can be ascribed to a dramatic increase in the number of narrow and sharp nanosize protrusion in contact with liquid droplet. According to ିଵ Cassie-Baxter model, the sharp reduction in solid surface fractionሺ౩՜ஶ ሺɅሻ ൌ ͳͺͲ୭ሻ resulted in a substantial increase in air gaps which shifts the surface wettability toward superhydrophobicity [221, 224, 225]. 7.4.5 Effect of modifications on the pore size and permeability One of the concerns of any membrane modification is the possibility of pore plugging or constriction [244]. To verify that the modifications have not altered the membrane porosity and pore size, bubble point test was carried out and the results (shown in Figure 7-6) revealed that the mean pore sizes were not changed, the original porosity remained intact after dip- coating of the TiO2 nanoparticles as the TiO2 layer is very thin (see section 6.4.2.7). This can also be seen by comparing the SEM images presented in Figure 7-4a and Figure 7-4b. However, a 15% reduction in mean pore size was observed after fluorosilanization of the surface in the absence of TiO2 coated layer (FTCS-PVDF membrane). Although the SEM image in Figure 7-4 d showed some large residues on the surface, the fluorosilanization did not significantly changed the mean pore size of the membranes which were firstly coated by Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 177 TiO2 (Figure 7-6 ). It should point out here that the similar bubble point pressure was obtained for all of the membranes (Table 7-2). 0.50 0.45 0.40 0.35 0.30 0.25 0.20 (Microns) (Microns) 0.15 Mean Pore Diameter Pore Diameter Mean 0.10 0.05 0.00 PVDF TiO2+PVDF FTCS+PVDF TiO2+PVDF+FTCS Figure 7-6 Mean pore diameter (microns) for virgin, TiO2 coated, fluorinated and TiO2 coated followed by fluoro- silanized PVDF membranes Superhydrophobic modifications resulted in a drastic reduction in pure water flux of PVDF membrane at LEP point from 450 L/m2.h for virgin membrane to 30 L/m2.h for modified membranes (Table 7-2 ). However, when the surface tension of water was reduced from 72 to 36 mN/m by the addition of 30% (vol.) ethanol, the flux of modified membrane was at the same order of magnitude as that of virgin membrane. 7.4.6 Effect of modifications on the LEP As noted before, one of the critical membrane characteristics in MD is liquid entry pressure (LEP) above which pore wetting occurs and results in the contamination of permeate and a significant reduction in the flux [237]. According to Laplace (Cantor) equation, the LEP is directly proportional to the liquid surface tension (ߛሻ, cosines of liquid–solid contact angleሺߠሻ and geometric factor ሺܤሻ and is reversely proportional to the largest pore radiusሺݎ௫ሻ as below [240]: ିଶఊ௦ሺఏሻ οܲ௧௬ ൌ ൏ܮܧܲ (7-3) ೌೣ where ΔPentry is entry pressure difference. Increasing the Reynolds number can reduce the temperature and concentration polarization but increases the chance of exceeding the LEP. Liquid surface tension (ߛሻ increases by temperature and reduces at a high salt concentration. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 178 Geometric factor ሺܤሻ (one for cylindrical pores and less than one for non-cylindrical pores [307]) and ݎ௫ are membrane characteristics which can be altered to improve MD performance [244]. As can be seen in Table 7-2 , the superhydrophobic modifications raised the LEP values from 120 kPa for virgin membrane to 190 kPa for FTCS-TiO2-PVDF membranes. This is most likely due to change in surface chemistry ሺߛܿݏߠሻrather than geometry. From the bubble point test results, the maximum pore radius was not changed significantly for all of the membranes and was around 0.4 μm. The geometric factor ሺܤ ൌ ݎ௫ ൈ ȀͶߛሻ [307] is also remained approximately unchanged (0.35). It should point out here that the Equation 7-3 is based on the assumptions such as pore shape that may result in substantial errors. Franken et al. suggested that the LEP must be calculated experimentally [308] rather than using Laplace equation and according to Lawson and Lloyd the equation should only be used as a model to interpret the experimental results [240]. From the Table 7-2 , the LEP value for TiO2-PVDF membranes was reduced significantly to 90 kPa as TiO2 deposition reduces the hydrophobicity of the membranes (see Chapter 6). An increase of 10o in water contact angle was observed for FTCS-PVDF membranes due to reduction in surface free energy after fluorosilanization. 7.4.7 Role of TiO2 in the surface modification As noted before in section 2.5.3, a hierarchical structure with multilevel roughness is necessary to approach extreme wettability of either superhydrophobicity or superhydrophilicity. In previous chapter, we have shown that a hierarchical morphology can be generated by the deposition of TiO2 nanoparticles using templating agents and a low temperature hydrothermal coating process. In addition to engineering the surface architecture, the deposition of TiO2 nanostructure might also provide sites for fluorosilanization via covalent bonding between hydrolysed silane coupling agents and hydroxyl groups (OH) available on the surface of TiO2 coated layer. The increase of OH functional group was confirmed by XPS results (Figure 7-1 ) which showed the increased level of O1s on the TiO2 coated membrane. The coating mechanism of self-assembled monolayer (SAMs) of organosilane coupling agents is based on a series of hydrolysis and condensation reactions and affinity for the wet surface [309]. The proposed mechanism of the fluorosilanization on the surface of PVDF membranes with and without TiO2 was summarized in Figure 7-7. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 179 CL OH (n) Si CLL + 3H2O (n) Si OHH + 3HcL CL OH (a) H2O H O H O H2O 2 2 H2O H2O (n) (n) (n) (n) (n) (n) (n) (n) Si Si OH Si Si OH SiS Si Si Si OH OH O O OH O OHO OH OHO OHO O O O O OH O O O O Ti Ti Ti TiT Ti Ti Ti Ti O O O O O O O O PVDF PVDF (c) (b) H2O H2O (n) (n) (n) Si Si Si OH O OHO OH O OH OHO PVDF (d) (e) Figure 7-7 Proposed scheme for the silanization of the PVDF membranes (a) hydrolyzation of FTCS (b) interaction with the surface of TiO2 (c) in-plane reticulation (d) condensation of trisilanols in the solution in the absence of TiO2 coating, and (e) surface SEM image of FTCS–PVDF membranes. FTCS molecule consists of hydrophilic trichlorosilane anchor and fluorinated hydrophobic carbon chain. As illustrated in Figure 7-7 a, in the presence of water, the hydrophilic trichlorosilane head can hydrolyse to form hydroxilanes or trisilanols. Brownian motion helps the long-chain silanes to have an in-plane lateral mobility which leads to an in-plane reorganization and the formation of densely packed monolayer of vertical chains [255]. In the Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 180 next stage, the hydroxyl groups in the trisilanol heads can form hydrogen bond to the surface hydroxyl groups of TiO2 and eventually the covalent bonds of Si-O-Ti can take place (Figure 7-7 b) [309, 310]. Under the proper conditions such as favourable distance and orientation, the intermolecular cross linking between the trisilanol anchors can also occur which leads to a 2D network of polysiloxane (Figure 7-7c) [255]. These interactions of hydroxysilanes laterally and to the surface of TiO2-PVDF membranes may lead to a dense, robust and water repellent composite. Post heat treatment at 120o could lead to a significant reduction in the density of surface silanol groups (Si-OH) in the favour of Si-O-Ti bonds [311]. According to the silica sol-gel chemistry (section 2.4 of literature review) and in the absence of Ti-OH groups, the hydrolysed FTCS molecules can condense and form a network and agglomerations of Si-O-Si in the solution, which limits the formation of covalent bond to the substrate (Figure 7-7 d). The cloudiness of the coupling agent solution with time could be due to the formation of these agglomerations which can be formed from micelles or other extended structures such as lamellar, hexagonal, and cubic phases [312]. As can be seen in Figure 7-7 e, the fluorosilanization of PVDF membranes in the absence of TiO2 nanoparticles coating resulted in an extended toothed walls structure which are physically absorbed and can be easily detached from the PVDF substrate. 7.4.8 Durability, Stability and Photo-catalytic Activity of TiO2 -PVDF Membrane Iodide oxidation test which is introduced in section 6.4.2.6 was used in order to investigate the photo-catalytic activity of TiO2 coated PVDF membrane. The KI test was also used to examine the durability of the TiO2 nanoparticles coated on the PVDF membranes. If the - coated membrane is photo-catalytically active, it can oxidize iodide (I ) to iodine (I2) and changes the KI solution from colorless to yellow. Although the observed color change is qualitatively an indication of photo activity, the peaks of these species were detected by UV- - Vis absorption spectroscopy at 288 and 351 nm for I2 and I3 , respectively, to quantitatively investigate the photoactivity of the surface. The KI test can be repeated during different cycles of UV irradiations to investigate the durability of the photocatalytic activity of TiO2 coated membranes. The virgin membrane did not show the color change and peaks were not observed after 6 h of UV irradiation whereas the yellow color and peaks were observed for TiO2 coated membranes. As presented in Figure 7-8, the absorbance at the end of each cycle remained relatively unchanged for both species, which shows the stability and durability of TiO2 coating as well as its photo-catalytic activity. Long term operation of modified Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 181 membrane may reduce the number of FTCS molecules on the surface. Photo-activity of the modified membranes also facilitates the re-application of FTCS coating due to the possibility of generation of OH groups at the membrane surface by UV radiation. 1.8 351 nm 1.6 288 nm 1.4 1.2 1 0.8 Absorbance (nm) 0.6 0.4 0.2 0 1234 Cycle Figure 7-8 Absorbance of the KI solution at 351 nm and 288 nm after exposure of the TiO2-PVDF membrane to 6 h of UV irradiation for 4 cycles 7.5 Membrane Performance 7.5.1 Effect of feed temperature and pressure on the flux Prior to running the membrane performance test using DCMD set-up, a preliminary analysis was carried out to determine the effect of feed inlet temperatures and pressures on flux. There is a general consensus that permeate flux increases exponentially with temperature as the vapour pressure generated from temperature difference between the feed and permeate exponentially increases with temperature [237]. As can be seen in Figure 7-9, this behaviour was observed for both virgin and FTCS-TiO2-PVDF membranes, which is in a good agreement with the theoretical and experimental results published by Schofield et al. [313] and Banat and Simandl [314] who used similar PVDF (0.4 μm) flat sheet membranes. Results furthermore demonstrated that the FTCS-TiO2-PVDF membrane has lower average permeate flux. This corresponds to contact angle measurement which shows that the FTCS-TiO2-PVDF membrane is more hydrophobic than the virgin membrane. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 182 100 PVDF Membrane 90 FTCS+TiO2+PVDF membrane 80 Expon. (PVDF Membrane ) ) 70 Expon. (FTCS+TiO2+PVDF membrane) -1 .h -2 60 50 40 Flux (L.m 30 20 10 0 50 60 70 80 90 100 Temperature (oC) Figure 7-9 Effect of feed inlet temperature on permeate flux of both virgin and modified (FTCS-TiO2-PVDF) membrane. The increase in feed flow resulted in an increase in feed pressure whose effect on flux is shown in Figure 7-10 . The figure shows that permeate flux increases to an asymptotic level with feed pressure for DCMD configuration which is also in agreement with the literature [236, 314]. The increase in the flow rate may switch the flow into the turbulent flow regime which reduces the thickness of the thermal boundary layer on the membrane surface. This causes the temperature at the membrane surface to be closer to the bulk feed temperature, resulting in an increase of vapour pressure difference across the membrane, thus increasing the permeate flux [237]. However, the increase in feed inlet pressure may also results in exceeding the liquid entry pressure. Therefore, there is a trade-off between the high flow rate and permeate quality. To avoid these potentially occurring phenomena, a low feed inlet pressure was applied during the membrane fouling experiment. In determining the flow regime, Reynolds Number (Re) was calculated using the following formula [315]: ఘ௩ ସఘொ ܴ݁ ൌ ൌ (7-4) ఓ గఓ Where ߩ is the density of feed solution (i.e. water), Q is the flow rate (ml/s), μ is the dynamic water viscosity which reduces by temperature, and D is the hydraulic diameter of the cross flow channel. The calculated Re (Table 2-3) of feed side was categorised under the turbulent flow regime. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 183 25 20 .h) 2 15 10 Flux (L/m Flux 5 0 45 50 55 60 65 70 75 80 Pressure (kPa) Figure 7-10 Effect of feed inlet pressure on the permeates flux 7.5.2 Effect of superhydrophobic modification on wetting resistance The wetting resistance of membranes was investigated by the injection of ethanol (15 wt.%) during a pure water direct contact membrane distillation process. In order to see the effect of wetting, the permeate side pressure was set at a higher pressure (15kPa) than the feed side (5kPa). As can be seen in Figure 7-11, after the injection of ethanol, the flux of virgin membrane decreased sharply and became negative. This means water flows from permeate to feed side and hence the predominant driving force was pressure difference rather than temperature gradient. The sharp reduction and negative flux were not observed for the FTCS- TiO2-PVDF membrane, which is an indication of a higher wetting resistance for this membrane. However, a gradual reduction in flux was observed, which is due to a continuous increase of ethanol content on the permeate side where did not have modification. In addition, the water vapour molecules and ethanol as a volatile organic compound were competing with each other to pass through the membrane pores, which could reduce the flux [237]. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 184 40 Ethanol injected (15 wt.%) 30 20 ) -1 10 h -2 0 02468 Flux (L.m Flux -10 Virgin PVDF membrane -20 FTCS-TiO2-PVDF membrane -30 -40 Time (hr) Figure 7-11 Pore wetting resistance behaviour of virgin (PVDF) and modified (TiO2-FTCS-PVDF) membranes 7.5.3 Direct contact membrane distillation The first membrane distillation experiment was performed using 1.3 L of 3.5 wt.% sodium chloride solution (Λ: 58.6 mS/cm). At the end of 7 h, 367 g of permeate was collected using virgin membrane, while 323 g was collected using FTCS-TiO2-PVDF membrane. The difference in the quantity of permeate obtained indicates that the FTCS-TiO2-PVDF membrane is more hydrophobic. Although the FTCS-TiO2-PVDF membrane did not show enhancement in permeability, unlike modified (TiO2 + FTCS) PVDF membranes, the conductivity of permeate for virgin membrane significantly increased after 4 h through the experiment (Figure 7-12). This could be due to partial pore wetting which facilitates salt transportation and was also observed elsewhere [237, 242, 314, 316]. Another contributory factor is the relatively higher LEP for FTCS-TiO2-PVDF membrane as compared to the virgin one, such that the increase in conductivity due to local transmembrane pressure exceeding of the LEP point was not observed throughout the duration of experiment. The second membrane distillation experiment was performed using 2 L of humic acid solution with and without CaCl2. Figure 7-13 shows the flux reduction (J/J0) for virgin and FTCS-TiO2-PVDF membranes as a function of time for 150 mg/L humic acid solution with and without 3.775 mM CaCl2 at pH of 7. From the figure, no flux reduction was observed in the absence of salt throughout 20 h of operation. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 185 45 700 40 600 35 500 30 ) -1 400 .h 25 -2 PVDF (Flux) μ S/cm) 20 300 FTCS + TiO2 + PVDF (Flux) 15 Flux (L.m PVDF (Conductivity) 200 10 FTCS + TiO2 + PVDF (Conductivity) 100 ( Conductivity 5 0 0 02468 Time (h) Figure 7-12 Flux and permeate conductivity vs. time for virgin and FTCS-TiO2-PVDF membrane with 3.5 %wt. NaCl; bulk feed temperature 70°C; bulk permeate temperature 25°C; feed inlet pressure 20 kPa. The experiment was continued for 97 h and the flux reduction was not again seen. However, the addition of divalent cation, Ca2+, in HA solution resulted in a significant flux reduction from J/J0 of less than one to 0.52 and 0.48 for virgin and FTCS-TiO2-PVDF membranes, respectively. The negatively charged carboxyl functional groups of humic acids bind together by Ca2+ cations to form bigger molecules and causes coagulation and particle precipitation [317]. Charge screening could also enhance the fouling [247]. The flux reduction in membrane distillation could be due to both effects of first blocking the pore entrances and reducing the available surface area for vaporization and second reducing the heat transfer driving force [237, 247]. The fouled membranes were cleaned by 15 min of recirculation of NaOH solution (0.2 wt.%, pH 12) at 25oC on feed side and then the pure water flux was measured to calculate flux recovery. The flux recovery of FTCS-TiO2-PVDF membranes was 94% whereas it was 59% for virgin membranes. As can be seen in Figure 7-14 , the brownish precipitate of HA was not observed on FTCS-TiO2-PVDF membrane after chemical cleaning while it was there for virgin membrane, which means the modification increases the antifouling performance of the membrane. It should point out here that a more efficient Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 186 cleaning method such as longer recirculation time or backwashing may increase the flux recovery of the virgin membrane and remove the precipitates as it was conducted by Srisurichan et al.[247]. 1.2 Flux Recovery after chemical cleaning: PVDF membrane = 59% 1 FTCS-TiO2-PVDF Membrane = 94% 0.8 0 0.6 J/J 0.4 PVDF membrane (150 mg/L HA and 3.77 mM CaCl2) 0.2 FTCS-TiO2-PVDF membrane (150 mg/L HA and 3.77 mM CaCl2) PVDF membrane (150 mg/L HA) 0 0 5 10 15 20 Time (h) Figure 7-13 Flux reduction (instantaneous flux J over the average pure water flux Jo) vs. time for virgin and FTCS-TiO2- PVDF membrane with 150 mg/L humic acid with/out 3.775 CaCl2 at pH of 7; bulk feed temperature 70°C; bulk permeate temperature 25°C; feed flow rate 300 From the inset images of Figure 7-14, the superhydrophobicity of the FTCS-TiO2-PVDF membrane has been preserved over the course of 20 h operation, which means modification was robust. However, from industrial point of view a longer experiment is necessary to find out how the modification stands. Since the superhydrophobic modification introduced in this study was achieved at low temperature, it is practically feasible to recover the superhydrophobicity of the surface by filtering the FTCS solution and heating the membrane module at moderate temperature without the need of dismantling it from the industrial plant. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 187 (a) (b) Figure 7-14 Virgin (a) and FTCS-TiO2-PVDF (b) PVDF membranes after fouling by 150 mg/L HA and 3.775 mM CaCl2 solution for 20 h and cleaning by 0.2 wt.% NaOH for 15 min recirculation (insets are water droplets on the fouled and cleaned membranes) Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 7 Coating (Superhydrophobic Modifications) 188 7.6 Conclusion A superhydrophobic PVDF membrane with water contact angle of 163°±3° was successfully prepared by both generating multilevel roughness and reducing the surface free energy of the membranes via TiO2 coating by a low temperature hydrothermal (LTH) process followed by fluorosilanization of the surface with 1H, 1H, 2H, 2H-Perfluorododecyltrichlorosilane (FTCS). The modifications led to membranes with not only superhydrophobic behaviour but also superoleophobicity to glycerol. The FTCS-TiO2-PVDF membranes had also shown a good thermal and mechanical resistance which is essential for the membrane application in membrane distillation. The liquid entry pressure (LEP) of water increased significantly without compromising the mean pore size from 120 kPa to 190 kPa due to the change in the surface chemistry of the membrane. The TiO2 coating provided hierarchical structure and also OH functional group to ensure uniform functionalisation of FTCS. The coating of FTCS on the rough membrane allows its hydrophilic end to be hydrolysed onto the Titania coated membrane and leaving its hydrophobic fluorinated carbon chain to be exposed. The contact angle measurement had shown that membrane still maintained its superhydrophobicity towards high concentration of model fouling solution. The FTCS-TiO2- PVDF membrane was also relatively superhydrophobic towards different concentration of both sodium chloride and humic acid solution. Membrane performance test on direct contact membrane distillation (DCMD) using 3.5 wt.% sodium chloride solution did not show a significant improvement in flux and anti-fouling property. However, the permeate conductivity of the virgin membrane was observed to increase significantly due to partial pore wetting, thus proving pathways for sodium chloride to permeate through the membrane. Pores wetting was expected to occur with the virgin membrane since it has lower LEP point than the modified one. In the experiment of DCMD using 150 mg/L humic acid solution, the virgin membrane showed insignificant flux reduction toward the end of experiment. However, the addition of Ca2+ to the HA solution resulted in a significant flux decline for both virgin and modified membranes due to coagulation of HA molecules. A significant increase in flux recovery after chemical cleaning for FTCS-TiO2-PVDF membrane was found, which is an indication of enhancement in the antifouling property of membranes. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 189 Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 190 CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 7.1 Conclusions This dissertation has investigated the effect of the incorporation of TiO2 nanoparticles into ultrafiltration and microfiltration in-house and commercial polymeric membranes. The thesis has focused on the development of two different techniques of making TiO2 nanocomposite membranes: blending and coating. The effect of incorporated TiO2 nanoparticles on the surface chemistry, morphology and fouling performance of nanocomposite membranes fabricated by each technique was separately explored. The following conclusions can be drawn from each developed technique: 7.1.1 Blending of TiO2 nanoparticles into membrane matrix (Chapter 4 and 5) In chapter 4, commercial TiO2 nanoparticles were successfully modified by a combination of chemical and mechanical methods for blending into ultrafiltration flat sheet membranes. The modifications resulted in a high level of dispersion with the minimal agglomerations and also a considerable improvement in nanocomposite antifouling properties. A 40% increase in flux recovery was observed after the chemical and mechanical modification of particles at a 2 wt.% TiO2 loading, which was accompanied by much lower TMP increase during filtration with bovine serum albumin. A higher TiO2 content did not show better performance even after modifications. The most obvious finding to emerge from this chapter is that the degree of dispersion was found to have significant effect on surface free energy, roughness, surface charge, surface pore size and protein absorption resistance as well as hydrophilicity. The chemically and mechanically modifications caused a reduction in the surface free energy and roughness whereas it resulted in an enhancement in surface pore size and porosity. The addition of TiO2 regardless of type of modification or TiO2 particles content led to 18% improvement in hydrophilicity. While previous researchers believed that the increase in hydrophilicity is the most likely reason for improvement in fouling performance, it was shown that other parameters discussed in present work may contribute just as much to greater fouling resistance. The fouling release effect of silane groups (from APTES) could be another contributing factor in antifouling properties of modified membranes, which can form local low energy hydrophobic surfaces. Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 191 In chapter 5, the modification technique introduced in previous chapter was utilized to incorporate the TiO2 nanoparticles (2 wt.%) into hollow fiber UF membranes. The purpose of the current chapter was to determine whether the effects of chemical and mechanical modifications of TiO2 nanoparticles on the composite hollow fibre membranes are different from the flat sheet counterparts as solvent exchange occurs both at the shell and lumen side and the difference in shear rate may alter the distribution of particles. The results showed that the initial pure water flux of mechanically and chemically modified TiO2 PES hollow fiber membrane was enhanced significantly, but there was a small improvement in its fouling performance. It was observed that only mechanical modification resulted in poor dispersion and migration of TiO2 nanoparticles toward outer layer while better dispersion was achieved after addition of both chemically and mechanically modified TiO2 nanoparticles. Upon the incorporation of mechanically and chemically modified TiO2 nanoparticles, the glass transition temperature, membrane porosity and pore size, stiffness and hydrophilicity increased whereas tensile strength and elongation at break decreased. The findings of this chapter suggest that the strategy used for the preparation of TiO2 nanocomposite flat sheet membranes did not enhance the performance of hollow fiber nanocomposite membranes to the level that it did for flat sheet membranes. 7.1.2 Coating of TiO2 nanoparticles on the membrane surface (Chapter 6 and 7) In chapter 6, a low temperature hydrothermal (LTH) approach was introduced to generate thin robust photoactive mesoporous coatings of TiO2 nanoparticles on the in-house and commercial PES membranes with various pore sizes. The wettability of the membrane surfaces shifted toward superhydrophilicity without the need for continuous UV illumination. Such a long term superhydrophilicity of TiO2 coated membrane was related to the dual level hierarchical roughness of the coated surfaces. It was observed that the organic templating agent (Pluronics F127) enhanced the adhesion of the particles. Surface architecture was changed from cauliflower to rose structure by changing the templating agent from amphiphilic (Pluronics F127) to hydrophilic (polyethylene glycol) compound. Passive protein adsorption was reduced significantly on the TiO2 coated surfaces. Filtration performance of coated and uncoated 500 kDa membranes was also investigated with humic acid as a model foulant, and an increase in flux recovery was observed during multiple fouling and cleaning cycles with the TiO2 coated membrane. The most important limitation of LTH coating process lies in the fact that heat treatment could collapse some pores in the tighter ultrafiltration membranes. Therefore, further optimization on the techniques is necessary Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 192 when membrane pore sizes are small. This research will serve as a platform for further surface functionalization of polymeric membranes to generate photoactive coatings, novel surface architecture, low fouling surfaces and tuneable hydrophilicity. Chapter 7 shows that the TiO2 incorporated nanoparticles by LTH coating process (implemented in chapter 6) itself can be used for further functionalization to improve antifouling properties of membranes via greater scope in the variation of surface chemistry and structure. In this chapter, the incorporated TiO2 nanoparticles on microporous PVDF membranes were fluorosilanized to impart superhydrophobicity for membrane distillation application. Results showed that the modification was mechanically and thermally robust and photoactive, which resulted in an increase in the LEP and water contact angle from 120kPa and 125o to 190 kPa and 166o, respectively. The bubble point pressure test showed that the modifications resulted in a superhydrophobic surface without compromising the mean pore size. It was also revealed that the TiO2 coating not only contributes in engineering the hierarchical structure but also provides sites (OH functional groups) for hydrolyzed silane coupling agent to be anchored on the surface and forms a uniform robust water repellent film. The 20 h fouling experiment of humic acid in DCMD process did not show a reduction in flux for virgin and modified membranes. However, the addition of CaCl2 into the solution significantly increased the flux reduction due to the formation of complexes with humic acids and consequently particles coagulation and precipitation on the membrane surface. Although both virgin and modified membranes showed similar fouling behaviour, a significantly better flux recovery was found for the modified membrane. 7.2 Recommendations 7.2.1 Recommendations for blending the nanoparticles into membrane matrix It was found that the generation of local low energy surfaces using functionalized nanoparticles may contribute to a potentially interesting strategy for increasing antifouling performance in nanocomposite membranes. It is recommended to use other silicone-based fouling release agents for the functionalization of nanoparticles to not only increase the dispersion of particles but also enhance antifouling performance of membranes. Using more efficient fouling release agents, may result in a higher dispersion at a lower TiO2 concentration than 2 wt.% where found the optimum level for flat sheet nanocomposite membranes. It was also observed that the 2 wt.% concentration of TiO2 in the flat sheet membranes was not the optimum level for hollow fiber membranes. Lower concentration of Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 193 TiO2 in conjunction with the optimization of modification (mechanical) technique for TiO2 nanoparticles and more control on the parameters involved during the fabrication of hollow fibres are suggested. 7.2.2 Recommendations for coating of the nanoparticles on the membrane surface For the LTH coating process demonstrated in chapter 6, further work to optimize coating thickness, structure and substrate selection is suggested. The technique has the limitation of pore collapsing for tight UF membranes due to post heat treatments. Therefore, optimization of heat treatment process or using other indirect heat treatment recourses such as microwaves or LED light is recommended. The preliminarily results revealed that changing the templating agent can change the membrane surface structure, and thus further work needs to be done to establish the effect of different templating agents (amphiphilic, hydrophilic and hydrophobic) on the surface chemistry, structure and fouling performance membranes. It is expected that choosing the suitable templating agent might change the surface architecture and the level of TiO2 crystallinity. This might result in a shorter heat treatment period and consequently minimizing the pore collapsing issue for tight UF membranes. A further study could assess the effect of various parameters during TiO2 coating process as summarized below: 1. changing the temperature and viscosity of TiO2 sol 2. varying humidity of dip-coating chamber or using other coating techniques 3. choosing different chelating ligands for instance β-diketones, β-ketoesters, carboxylic acids or other complex ligands to reduce or increase the activity of TiO2 precursor during coating 4. varying the titania particle size in the sol by increasing or decreasing the water content of sol along with changing the pH of solution For the superhydrophobic modification implemented in chapter 7 the following recommendation can be drawn: 1. optimizing the modification technique by using different solvents, adding co- solvents in the FTCS solution and also varying the FTCS concentration to minimize the agglomerations of Si-O-Si particles which is not chemically bounded to the substrate Amir Razmjou, PhD Thesis, The University of New South Wales Chapter 8 Conclusions and Recommendations 194 2. investigating the effect of scaling salts to see if the surface resist crystal attachment 3. using the superhydrophobic modification techniques for hollow fiber membranes with larger surface area 4. calculating the energy efficiency of the membrane distillation before and after modifications using mathematical formulation and modelling 5. converting the feed side face of the MF membrane into superhydrophobic surface while making the permeate side superhydrophilic using the techniques implemented in chapter 6 and 7 6. modelling of the energy efficiency of membrane distillation for modified and unmodified membranes The findings of chapter 7 have a number of interesting implications for future practice. 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Amir Razmjou, PhD Thesis, The University of New South Wales Appendix 209 APPENDIX 4 3 Coated S 2 C 1 Ti Counts (cps/ev) Counts (cps/ev) O 0 3 2.5 2 Control 1.5 1 Counts (cps/ev) Counts (cps/ev) 0.5 0 0123456 Energy (Ke V) (a) (b) Figure A1 top surface EDAX spectra of control and TiO2 In-house coated (triple) membranes (a) EDAX point and (b) EDAX line Amir Razmjou, PhD Thesis, The University of New South Wales Appendix 210 Figure A2 SEM images of treated control: membrane dipped in a coating solution similar to TiO2 sol but without TiO2 followed by heat treatments Counts coatedTiO2-08-2 10000 5000 0 10 20 30 40 50 60 Position [°2Theta] (Copper (Cu)) Figure A3 XRD patterns for PES 100MWCO (Millipore) membrane coated with TiO2 (3 cycles) by LTH process Amir Razmjou, PhD Thesis, The University of New South Wales Appendix 211 Figure A4 TGA curves for the TiO2 (Degussa P25) and the APTES coated TiO2 Amir Razmjou, PhD Thesis, The University of New South Wales