New 3D Printable Polymeric Materials for Fused Filament Fabrication (FFF)

NEW 3D PRINTABLE POLYMERIC MATERIALS FOR

FUSED FILAMENT FABRICATION (FFF)

Gayan Adikari Appuhamillage

APPROVED BY SUPERVISORY COMMITTEE:

______Ronald A. Smaldone, Chair

______John P. Ferraris

______Walter E. Voit

______Mihaela C. Stefan

Gayan Adikari Appuhamillage

To my family and friends

NEW 3D PRINTABLE POLYMERIC MATERIALS FOR

FUSED FILAMENT FABRICATION (FFF)

GAYAN ADIKARI APPUHAMILLAGE, BS, MS

DISSERTATION

Presented to the Faculty of

The University of Texas at Dallas

in Partial Fulfillment

of the Requirements

for the Degree of

DOCTOR OF PHILOSOPHY IN

CHEMISTRY

THE UNIVERSITY OF TEXAS AT DALLAS

May 2018

ACKNOWLEDGMENTS

It was a wonderful experience for me to work in Dr. Smaldone lab for the past five years. I would like to acknowledge my research advisor Dr. Ronald A. Smaldone, for letting me carry out my graduate research work under his supervision. I sincerely appreciate all of his support, guidance, and immense encouragement throughout my graduate studies. I would like to acknowledge the members of my advisory committee- Dr. John P. Ferraris, Dr. Walter E. Voit, and Dr. Mihaela C.

Stefan for their valuable suggestions and support.

I also appreciate all the valuable help and training given to me by Dr. Christina Thompson for organic synthesis and reaction mechanisms; Dr. Hien Nguyen for nuclear magnetic resonance

(NMR) spectroscopy and inductively coupled plasma (ICP) analysis; Dr. Benjamin Batchelor for

Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA); Dr. Faisal Mahmood for gel permeation chromatography (GPC); Dr.

Layne Winston for the Universal Testing Machine-Instron, and finally, thanks to the clean-room staff.

I acknowledge Dr. Walter E. Voit for his productive collaborations and great support, his past graduate students, Dr. Jonathan Reeder, Dr. Gregory Ellson, Dr. Radu Reit, and Dr. Kejia Yang, and his past lab manager, Dr. Benjamin Lund, for helping me in various ways during collaborations. I acknowledge Dr. Jeremiah Gassensmith and his graduate students, Candace

Benjamin, Michael Luzuriaga, and Madushani Dharmarwardana, for their valuable collaborations.

I thank the Department of Chemistry and Biochemistry staff for their help- Betty Maldonado,

Linda Heard, Kelli Lewis, Lydia Selvidge, Dr. Pathum Panapitiya, Dr. Sumudu Wijenayake and

Dr. George McDonald.

I thank my past lab members, Dr. Arosha Karunathilake, Joshua Davidson, Xie Yinhuan, Fei Li, and Victoria Cameron, and the current lab members Sampath Alahakoon, Grant Sturgeon, Danielle

Berry, Alejandra Silva, and Shashini Diwakara, and also all the undergraduates for all of their valuable help and friendship. I thank my undergraduates, John Reagan and Sina Khorsandi, for their valuable help for research. Also I thank the Sri Lankan Student Association (SLSA) and all of my Sri Lankan friends for their support in various situations.

Finally, I would like to thank my loving parents, Thilakarathna Adikari Appuhamillage and

Pathmalatha Jayasooriya, my loving uncle, Chandrarathna Adikari Appuhamillage, my in-laws, my loving wife, Sankalya Ambagaspitiye Gedara, and beloved daughter, Oshadhi Imaya Adikari, for their immense love, support, and endless encouragement during this journey.

April 2018

NEW 3D PRINTABLE POLYMERIC MATERIALS FOR

FUSED FILAMENT FABRICATION (FFF)

Gayan Adikari Appuhamillage, PhD The University of Texas at Dallas, 2018 ABSTRACT

Supervising Professor: Ronald A. Smaldone

3D printing, also known as additive manufacturing, is an advancing technology for fabricating three dimensional objects of simple to complex architectures using computer aided design (CAD) models. Fused Filament Fabrication (FFF) is a relatively cost effective, straightforward 3D printing technique which relies upon utilization of relatively cheap, commercially available thermoplastics like polylactic acid (PLA) and acrylonitrile-butadiene-styrene (ABS) in a coil form to fabricate desired objects in layer-by-layer fashion on a print bed. In spite of the growing popularity of FFF, the technique suffers from poor mechanical strength of 3D printed parts due to weak inter-layer adhesion at interfilamentous junctions. To address these issues related to the materials used for FFF 3D printing, for the first time, we introduce a polymer blending strategy that utilizes Diels-Alder (DA) dynamic covalent chemistry which plays the model role for remending PLA. A partially cross-linked terpolymer having furan-maleimide DA linkages both in the main chain and at cross-linking junctions is used as the mending agent (MA). Results indicate dramatic improvements in both ultimate strength and toughness along the z-print axis for

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remendable PLA than pristine PLA. As a follow-up, we increase the cross-linking density of the

MA polymer to yield remendable PLA with isotropic mechanical properties.

Finally we are introducing utilization of hydrogels, prepared from a cheap, abundant, biopolymer and 3D printed via direct-write 3D printing technique based on FFF principles, for the removal of toxic heavy metal ions from contaminated water. With promising results in mechanical strength, metal adsorption efficiency, and recyclability, the strategy lays a foundation for future design and development of cheap, safe, and durable 3D printable materials for water purification.

Chapter 1 provides a general introduction on 3D printing techniques, issues related to FFF, our approaches to address the issues, and utilizing hydrogel materials in direct-write 3D printing.

Chapter 2 describes a design paradigm utilizing reversible Diels-Alder reactions to enhance the mechanical properties of 3D printed materials.

Chapter 3 describes 3D printed remendable polylactic acid blends with uniform mechanical strength enabled by a dynamic Diels-Alder reaction.

Chapter 4 describes 3D printable hydrogels for toxic heavy metal adsorption from water.

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TABLE OF CONTENTS

ACKNOWLEDGMENTS ...... v

ABSTRACT ...... vii

LIST OF FIGURES ...... xii

LIST OF TABLES ...... xvii

CHAPTER 1 INTRODUCTION ...... 1 1.1 Background ...... 1 1.2 3D printing technologies ...... 1 1.3 Problems associated with FFF-3D printing ...... 8 1.4 Previous research work to improve interlayer adhesion ...... 9 1.5 Challenges encounter using fmDA chemistry in FFF-3D printing ...... 15 1.6 Our approach to enhance the mechanical properties of FFF-3D printed materials .....16 1.7 FDM-based 3D printing of soft materials for environmental remediation ...... 16 References ...... 18

CHAPTER 2 DESIGN PARADIGM UTILIZING REVERSIBLE DIELS−ALDER REACTIONS TO ENHANCE THE MECHANICAL PROPERTIES OF 3D PRINTED MATERIALS1 ...... 28 2.1 Abstract ...... 29 2.2 Introduction ...... 29 2.3 Results ...... 32 2.4 Discussion ...... 38 2.5 Conclusion ...... 39 2.6 Experimental Section ...... 40 References ...... 42

CHAPTER 3 3D PRINTED REMENDABLE POLYLACTIC ACID BLENDS WITH UNIFORM MECHANICAL STRENGTH ENABLED BY A DYNAMIC DIELS-ALDER REACTION1 ...... 46 3.1 Abstract ...... 47 3.2 Introduction ...... 47

3.3 Experimental Section ...... 50 3.3.1 Materials ...... 50 3.3.2 Synthesis of fmDA-polymers ...... 50 3.3.3 Preparation of fmDA-polymer/PLA blends ...... 50 3.3.4 Preparation of 3D Printable Filaments ...... 51 3.3.5 FFF 3D Printing ...... 51 3.3.6 Characterization ...... 51 3.3.7 Healing Experiments ...... 52 3.4 Results and discussion ...... 52 3.5 Conclusions ...... 58 References ...... 59

CHAPTER 4 3D PRINTABLE HYDROGELS FOR TOXIC HEAVY METAL ADSORPTION FROM WATER ...... 62 4.1 Abstract ...... 63 4.2 Introduction ...... 63 4.3 Results and Discussion ...... 65 4.3.1 Evaluation of Hydrogel Mechanical Properties ...... 68 4.3.2 Maximum Metal Adsorption Tests of DAP and cDAP Sheets ...... 68 4.3.3 Metal Adsorption Tests of DAP and cDAP Cups ...... 70 4.3.4 Metal Leaching Tests of DAP and cDAP Cups ...... 71 4.3.5 Pb2+ Adsorption Kinetics vs. Hydrogel Surface Area...... 72 4.3.6 Reusability Tests ...... 73 4.4 Conclusions ...... 74 4.5 Experimental Section ...... 75 References ...... 80

APPENDIX A SUPPORTING INFORMATION FOR CHAPTER 2 ...... 84 A.1 Synthetic Procedures ...... 84 A.2 Thermal Analysis of Polymers ...... 87 A.3 Mechanical Testing of Polymers – Quantitative Data ...... 87 References ...... 88

APPENDIX B SUPPORTING INFORMATION FOR CHAPTER 3 ...... 89 B.1 Synthetic Procedures ...... 89 B.2 Thermal Analysis of Polymer Blends ...... 90 B.3 Mechanical Testing of Polymer Blends ...... 91 B.4 Thermal Stability Tests ...... 95

APPENDIX C SUPPORTING INFORMATION FOR CHAPTER 4 ...... 97 C.1 1H NMR- diacrylated pluronic F-127...... 97 C.2 3D Printing Parameters ...... 98 C.3 Evaluation of Hydrogel Mechanical Properties ...... 101 C.4 Maximum Metal Adsorption Tests of DAP and cDAP Sheets ...... 102 C.5 Metal Adsorption Tests of DAP and cDAP Cups ...... 103 C.6 Metal Leaching Tests of DAP and cDAP Cups ...... 105 C.7 Adsorption Kinetics vs. Hydrogel Surface Area ...... 105 C.8 Reusability Tests ...... 106 References ...... 106

BIOGRAPHICAL SKETCH ...... 107

CURRICULUM VITAE ...... 108

LIST OF FIGURES

Figure 1.1. Schematic diagram of SLA 3D printing technology. Adapted and reprinted from Stansbury, J. W.; Idacavage, M. J. 3D Printing with Polymers: Challenges Among Expanding Options and Opportunities. Dent. Mater. J. 2016, 32, 54-64., Copyright (2016), with permission from Elsevier...... 2

Figure 1.2. Schematic diagram of SLS 3D printing technology. Adapted and reprinted from Stansbury, J. W.; Idacavage, M. J. 3D Printing with Polymers: Challenges Among Expanding Options and Opportunities. Dent. Mater. J. 2016, 32, 54-64., Copyright (2016), with permission from Elsevier...... 3

Figure 1.3. Schematic diagram of SLM 3D printing technology. Adapted from Ref 22 with permission of The Royal Society of Chemistry...... 3

Figure 1.4. Schematic diagram of DLP 3D printing technology. Adapted and reprinted from Mu, Q.; Wang, L.; Dunn, C. K.; Kuang, X.; Duan, F.; Zhang, Z.; Qi, H. J.; Wang, T. Digital Light Processing 3D Printing of Conductive Complex Structures. Addit. Manuf. 2017, 18, 74-83., Copyright (2017), with permission from Elsevier...... 4

Figure 1.5. Schematic diagram of EBM 3D printing technology-metal powder-based system. Adapted and reprinted from Heinl, P.; Müller, L.; Körner, C.; Singer, R. F.; Müller, F. A. Cellular Ti–6Al–4V Structures with Interconnected Macro Porosity for Bone Implants Fabricated by Selective Electron Beam Melting. Acta Biomater. 2008, 4, 1536-1544., Copyright (2008), with permission from Elsevier...... 5

Figure 1.6. Schematic diagram of CLIP 3D printing technology. Adapted and reprinted from Stansbury, J. W.; Idacavage, M. J. 3D Printing with Polymers: Challenges Among Expanding Options and Opportunities. Dent. Mater. J. 2016, 32, 54-64., Copyright (2016), with permission from Elsevier...... 6

Figure 1.7. Schematic diagram of LOM 3D printing technology. Adapted from Ref 22 with permission of The Royal Society of Chemistry...... 7

Figure 1.8. Schematic diagram of FFF-3D printing. Adapted and reprinted from Stansbury, J. W.; Idacavage, M. J. 3D Printing with Polymers: Challenges Among Expanding Options and Opportunities. Dent. Mater. J. 2016, 32, 54-64., Copyright (2016), with permission from Elsevier...... 7

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McLouth, T.; Patel, D. N.; Schmitt, K.; Nokes, J. P. Influence of Processing and Orientation Print Effects on the Mechanical and Thermal Behavior of 3D-Printed ULTEM® 9085 Material. Addit. Manuf. 2017, 13, 71-80., Copyright (2017), with permission from Elsevier. Black regions represent interfilamentous void spaces...... 9

Figure 1.10. Diastereomeric DA adducts resulting after a DA reaction between furan and maleimide...... 13

Figure 1.11. (A) Maleimide homopolymerization. (B) Aromatization of a furan DA adduct...... 15

Figure 1.12. Schematic diagram of the direct-write printing technique...... 17

Figure 2.1. (A) Synthesis of cross-linked mending agent. (B) Reversible reaction of Diels-Alder mending agent. (C) Forward reaction of Diels-Alder mending agent. (D) Structure and composition of a reversible fmDA polymer at different stages of the FFF printing process...... 32

Figure 2.2. (A) Synthesis of the mending agent polymer used in this study. (B) Filament fabrication and mechanical testing process used for the remendable PLA blends...... 33

Figure 2.3. 1H-NMR spectra of (bottom to top); 2F, 3F, 2M, and the mending agent...... 33

Figure 2.4. The Diels-Alder adduct proton signals of the mending agent a,b,c,d, and e are shown in the top spectrum. Note that the relative intensities of c,d, and e signals are reduced and the appearance of the maleimide proton signal (f, g) ~ 6.65 ppm in the bottom spectrum after the retro-fmDA reaction. The two peaks ~ 6.25 ppm (h) and 6.35 ppm (i) in the bottom spectrum are due to the furan protons of the monomers (usually these appear very close to the b and a Diels-Alder adduct proton signals respectively). The retro-fmDA reaction was monitored by heating the mending agent at 110 °C in toluene for 30 min (50 mg/mL). The mending agent solution (0.2 mL) was diluted into CDCl3 (2 mL) and the resulting mixture 1 was immediately analyzed using H-NMR. The NMR solution was prepared in CDCl3 with gentle heating. This heating is used to detach enough crosslinks to allow the polymer to be soluble. The remaining main-chain fmDA linkages are observed to undergo retro-DA reactions after heating in toluene...... 34

Figure 2.5. Differential scanning calorimetry of PLA, 10% MA/PLA, 25% MA/PLA and MA. PLA (blue) shows a Tg of 61.8 °C, 10% MA/PLA (pink) 55.2 °C, 25% MA/PLA (black) 48.2 °C, The MA (orange) 5.7 °C and an endothermic thermal transition at near 120 °C (retro-DA)...... 35

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remendable PLA dogbones for each printing pattern. (D) Representative stress vs. strain plots for each polymer blend and printing pattern tested in this study...... 36

Figure 2.7. Illustrations and images of 3D-printed PLA and PLA blended with mending agent. (A) Illustration of 3D-printed PLA filaments (blue), (a’) inter-filament junctions. (B) Illustration of PLA blended with mending agent (purple). (C) Microscopic image of 3D- printed PLA filaments. (D) Microscopic image of 3D printed remendable PLA...... 37

Figure 3.1. Illustration of the thermoreversibility of MA-polymers during the FFF 3D printing process...... 49

Figure 3.2. Synthesis of the Mending Agent (MA) polymer series used in this study...... 53

Figure 3.3. Filament fabrication, 3D printing, and tensile testing of the MA-PLA blends...... 54

Figure 3.4. (A) Illustration of the three representative print orientations of dogbones; X (blue), Y (red), and Z (green) along with their infill patterns. (B) Ultimate strength and (C) toughness graphs of PLA and MA-PLA blends with varying cross-link density as dogbones for each print pattern. (D) Representative stress vs. strain plots of PLA and MA-PLA-100. The stress vs. strain plots of the rest of the blends is shown in the Appendix B...... 55

Figure 3.5. Ultimate strength (A) and toughness (B) ratios of print patterns Z to Y (purple) and Z to X (orange) for PLA, MA-PLA-0, and MA-PLA-100. The rest of the plots of the other MA-PLA blends are shown in the Appendix B...... 56

Figure 3.6. (A) The healing process carried out for the in the study. Illustration of surface optical images of the Z print dogbones of (B) MA-PLA-100 and (C) pristine PLA; after applying the mechanical load for a complete rupture before healing is performed, after healing, and after applying the mechanical load again on healed sample. (D) Ultimate strength values obtained for each material before and after healing tests along with the tabulated ultimate strength recovery...... 57

Figure 4.1. Schematic presentation of preparing chitosan/pluronic diacrylate hydrogels. At low temperatures (~ 4 °C), chitosan and DAP molecules are evenly distributed forming a homogeneous solution, the ‘sol’ state. When the temperature of the ‘sol’ is increased above the critical gelation temperature,27 the DAP molecules arrange into micelles in which the hydrophobic component is composed of poly(propylene oxide) (PPO) groups of the pluronic polyether backbone, while poly(ethylene oxide) (PEO) groups form the hydrophilic component...... 66

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increasing wt % of chitosan via disrupted prints of hydrogel cups containing 5, 10 wt% chitosan with 25 wt% DAP. (C) 3D printed and UV cured solid cubes and shape A, shape B with internal structure containing cDAP-1 and the control DAP. Used CAD models are shown for each shape respectively. All the 3D printed dimensions are given in the Experimental Section...... 67

Figure 4.3. Maximum heavy metal adsorptions of the three thin hydrogel sheets cDAP-1, cDAP- 5, CDAP-10, and the control DAP...... 69

Figure 4.4. Comparison of remaining heavy metal ion concentrations in solution for cDAP-1 and the control DAP. EPA recommended action levels are shown in dotted lines as 15, 1300, 5, and 2 ppb for Pb2+, Cu2+, Cd2+, and Hg2+ respectively. Contact time 0 min represents the initial heavy metal ion concentration in the stock solution...... 71

Figure 4.5. Heavy metal leaching (%) for cDAP-1 and the control DAP after a contact time 10 h...... 71

Figure 4.6. (A) Comparison of the remaining Pb2+ concentrations after 10 min for the solid cubes and shapes A, B with internal structure prepared from the cDAP-1 and the control DAP. Stock solution represents the initial Pb2+ concentration in each case. EPA action level of 15 ppb for Pb2+ is shown by the dotted line. (B) CAD models used to 3D print the solid cubes and shape A and B with internal structure. Tabulated is the estimated surface area of each of these geometries calculated using the CAD models...... 72

Figure 4.7. (A) Pb2+ removal efficiency (%) for cDAP-1-Shape B hydrogel during 5 regeneration cycles with a contact time of 10 min in each. (B) Illustration of the adsorbent shape before reusing and after the 5th regeneration cycle...... 74

Figure A.1. 1H NMR spectrum of 2F...... 85

Figure A.2. Thermogravimetric Analysis of PLA, 10% remendable PLA, 25% remendable PLA and the mending agent alone. PLA (blue), 10% remendable PLA (black), 25% remendable PLA (red) and the mending agent (orange). As the mending agent is partially crosslinked, there is likely to be char material left over after the TGA as indicated by the remaining ~18% mass after analysis...... 87

Figure B.1. Differential scanning calorimetry analysis of MA-Polymer (16.7% cross-link density), MA-PLA blends, and PLA followed by their Tg values tabulated...... 90

Figure B.2. Thermogravimetric Analysis of PLA and MA-PLA blends. PLA (black), MA-PLA- 100 (blue), MA-PLA-83 (brown), MA-PLA-67 (purple), MA-PLA-17 (green), and MA- PLA-0 (red)...... 91

Figure B.3. Ultimate Strength and toughness ratios of print patterns Z to Y (purple) and Z to X (orange) for PLA and MA-PLA blends with varying crosslink density...... 93

Figure B.4. Stress vs. strain curves of PLA and MA-PLA blends as dogbones for each print pattern X (blue), Y (red), and Z (green)...... 94

Figure B.5. Ultimate strength (A) and toughness (B) values for PLA and MA-PLA blends heated under 65 °C/15 min conditions. Related stress vs. strain curves (C)...... 95

Figure C.1. 1H NMR of diacrylated pluronic F-1271 showing acrylation protons via a,b, and c. .97

Figure C.2. Illustration of (A) representative stress vs. strain plots, (B) ultimate strength, and (C) toughness graphs for the hydrogel cups containing 1 wt % chitosan with 25 wt % DAP (cDAP-1) and the control DAP. (D) Young’s moduli tabulated along with an illustration of the two materials under compressive stress...... 101

Figure C.3. Showing adsorption (%) of each heavy metal ion (A) Pb2+, (B) Cu2+, (C) Cd2+, and (D) Hg2+ from the three thin hydrogel sheets containing 1, 5, 10 wt % chitosan with 9 wt% DAP (cDAP-1, cDAP-5, CDAP-10 respectively) and the control DAP after each time interval using an initial stock solution of 3000 ppb for each heavy metal...... 103

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LIST OF TABLES

Table A.1. Ultimate strength values for each polymer blend and print pattern...... 87

Table A.2. Toughness values for each polymer blend and print pattern...... 88

Table B.1. Ultimate strength (MPa) values for PLA and MA-PLA blends along each print direction...... 91

Table B.2. Toughness (MJ/m3) values for PLA and MA-PLA blends along each print direction...... 91

Table B.3. Comparison of the %increase of ultimate strength along Z print axis for each of the MA-PLA blend vs. PLA...... 92

Table B.4. Comparison of the %increase of Toughness along Z print axis for each of the MA-PLA blend vs. PLA...... 92

Table B.5. Comparison of ultimate strength ratios along Z print direction to Y and Z print direction to X for PLA and MA-PLA blends...... 93

Table B.6. Comparison of Toughness ratios along Z print direction to Y and Z print direction to X for PLA and MA-PLA blends...... 94

Table B.7. Ultimate Strength (MPa) values for PLA and MA-PLA blends along each print direction after heating under 65 °C/15 min conditions...... 95

Table B.8. Toughness values (MJ/m3) for PLA and MA-PLA blends along each print direction after heating under 65 °C/15 min conditions...... 96

Table C.1. Direct-write 3D printing parameters used to print thin hydrogel sheets using cDAP-1, cDAP-5, cDAP-10, and the control DAP with dimensions (l × w × h) 15.0 mm × 15.0 mm × 1.0 mm with four inner circles each of 4.0 mm diameter...... 98

Table C.2. Direct-write 3D printing parameters used to print cylindrical cups using cDAP-1 and the control DAP hydrogels with dimensions (outer diameter-d × height-h × wall thickness- t) 25.0 mm × 20.0 mm × 2.0 mm and a base height of 1.5 mm...... 99

Table C.3. Direct-write 3D printing parameters used to print solid cubes and shapes with internal structures using cDAP-1 and the control DAP hydrogels. Solid cubes with dimensions (l × w × h) 14 mm × 14 mm × 14 mm, estimated surface area 1176 mm2. Shape A with dimensions (l × w × h) 25 mm × 10 mm × 10 mm, cylindrical pore diameter (d) 5 mm, estimated surface area 1554 mm2. Shape B with dimensions (l × w × h) 17.5 mm × 10.1

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mm × 17.5 mm, cylindrical pore diameter (d) 5 mm, estimated surface area 1791 mm2...... 100

Table C.4. Ultimate strength (kPa) values for cDAP-1 and the control DAP...... 101

Table C.5. Toughness (kJ/m3) values for cDAP-1 and the control DAP...... 101

Table C.6. Resulted maximum adsorptions from solution for the four heavy metal ions Pb2+, Cu2+, Cd2+, and Hg2+ for the three hydrogel membrane compositions and the control DAP....102

Table C.7. Remaining Pb2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 31.28 ppb...... 103

Table C.8. Remaining Cu2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 1522.28 ppb...... 104

Table C.9. Remaining Cd2+ concentration in solution for cDAP-1 and the control DAP after 30 60, 90, and 120 min of contact time starting with an initial concentration of 30.56 ppb...... 104

Table C.10. Remaining Hg2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 10.26 ppb...... 104

Table C.11. Heavy metal leaching (%) of cDAP-1 and the control DAP after 10 h...... 105

Table C.12. Remaining Pb2+ concentration in solution for each of the solid cubes and shapes with internal structure after 10 min contact time starting with an initial concentration of 31.28 ppb...... 105

Table C.13. Pb2+ removal efficiency (%) for cDAP-1-Shape B and the control-DAP-Shape B after 10 min contact time up to 5 regeneration cycles, starting with an initial concentration of 31.28 ppb for each cycle...... 106

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CHAPTER 1

INTRODUCTION

1.1 Background

3D Printing is an additive manufacturing technology which drives a controlled addition of materials layer-by-layer to build objects of simple to complex geometries from 3D model data.

Due to the ability of the 3D printing technology to custom produce printed parts with desired internal geometries, it has a spectrum of industrial and commercial applications covering transportation,1,2 defense,3 biomedical,4,5 apparel,6 cosmetics,7 food,8 and energy9 related disciplines. This internal part complexity uplifts 3D printing over the traditional plastic processing techniques such as molding,10 extrusion, casting, and other forms of thermoplastic or thermoset processing where complex internal geometries are challenging or almost impossible to achieve. In fact, according to some cost estimates, the 3D printing technology draws a huge potential demand to a $ 1 trillion market by 2030, if the related techniques are improved to print parts with uniform mechanical strength and toughness relative to the print ‘axis’ (mechanical isotropy).11

1.2 3D printing technologies

There are several 3D printing techniques in the current market tailored towards specific applications. They vary basically on the type of material, processing, and printing technique used.

Among these, stereolithography (SLA),12-15 fused deposition modeling (FDM),6,11,16 and selective laser sintering (SLS)15,17-20 are more popular over the other techniques such as selective laser melting (SLM),21-25 digital light processing (DLP),26,27 electronic beam melting (EBM),22 continuous liquid interface production (CLIP),22 and laminated object manufacturing (LOM).22

SLA utilizes photopolymerization of a liquid resin into desired geometries defined by the 3D data model used. A light-emitting device (usually a laser) is selectively illuminates the transparent bottom of the photopolymerizable resin containing tank and the solidified 3D object is progressively dragged up by a lifting platform (Figure 1.1). SLA 3D printing is characterized by its high resolution. The technique is fast, capable of building any complex geometry but the instrumentation is relatively expensive.

SLS builds a 3D object by binding a powder material (nylon, polyamide etc.) via directing a laser source along a defined pathway/pattern. The laser selectively fuses the powdered material by scanning the cross-sections defined by the 3D model data (Figure 1.2). This relatively new technology thus far has been used for rapid prototyping and production of component parts in low- volume. The technology doesn’t require support material and hence more complex geometries can be achieved. However, it suffers from high power consumption and requirement of sophisticated heat controls.

SLM (Figure 1.3), a much similar method to SLS but using a higher power density laser to melt and fuse metallic powders together rather than sintering. The ability of the laser to fully melt the powder grains provides stronger 3D printed parts. The technology is only feasible with a single metallic powder.

Figure 1.3. Schematic diagram of SLM 3D printing technology. Adapted from Ref 22 with permission of The Royal Society of Chemistry.

DLP 3D printing technology is much similar to SLA. But it uses a digital light projector (DLP) as the light source to cure the photo-reactive material used (Figure 1.4).26 This DLP screen flashes a single image of each layer of the desired object across the entire platform at once rather than drawing by a laser in SLA. This produces faster prints but with tradeoffs in resolution and surface finish.

EBM (Figure 1.5) technology relies on using an electron beam in a high vacuum as the heat source to melt (weld) metallic powder or wire materials. There are basically two modes of operation: metal powder-based systems and metal wire-based systems. Metal powder-based systems work similar to SLM while the latter resembles FDM printing due to the fact that its metallic feedstock is a wire/filament. This particular 3D printing technology has high part building rates due to high energy density of electron beam and the scanning method used. The technology is versatile for 3D printing of metallic alloys. On the other hand, this EBM 3D printing technology is restricted to metallic materials.

CLIP is a relatively new 3D printing technology that uses photopolymerizable resins to build smooth-sided 3D objects with a variety of shapes. A UV light which shines under the transparent bottom of the building platform illuminates the desired cross-sections of the target object causing the resin to solidify (Figure 1.6). This is a continuous process capable of printing the objects at a rapid speed. This particular 3D printing technology is capable of 3D printing rubbery and flexible materials.

In LOM, adhesive-coated plastic, paper, or metallic sheet material is supplied onto a building platform under the control of a heated roller. A laser traces the desired prototype on each layer and these are successively glued together to build the required full model (Figure 1.7). Basically a low cost technology due to cheap and abundant raw material and suitable to print relatively large parts.

However, the disadvantage of this technology is that the prints have relatively less dimensional accuracy than in SLA or SLS.

Figure 1.7. Schematic diagram of LOM 3D printing technology. Adapted from Ref 22 with permission of The Royal Society of Chemistry.

One of the main techniques used in current 3D printers especially in consumer oriented models is fused filament fabrication (FFF) or FDM. This special plastic extrusion method was developed in

1988 by Scott Crump, commercialized by Stratasys to market the first FFF machine in 1992. In

FFF, a continuous thermoplastic filament is fed through a heated print head where the material is at molten state, forced out via a nozzle to be deposited on a growing workpiece layer-by-layer fashion (Figure 1.8).

1.3 Problems associated with FFF-3D printing

Owing to the diverse advantages of the FFF 3D printing technique such as cost effectiveness, simplicity, broad material compatibility, high prototyping speed, and low environmental impact, it is dominant in several industries with an increasing popularity also in domestic applications for its ability to print materials like plastic ornaments and kids’ toys. However, despite of this versatility, FFF inherently suffers mainly in mechanical weakness of the final print parts because of the poor interfilamentous adhesion caused by lack of strong chemical interactions like covalent bonds bridging adjacent printed layers and limited molecular chain entanglements across printed layers.6 Usually the mechanical properties are weakest along the perpendicular axis to the printed layers. This phenomenon, anisotropy which is defined as lack of uniform mechanical properties along print axises has been a major challenge in additive manufacturing. In fact, according to literature reports, the average effect of anisotropy on tensile properties is ~ 85%,28 compression ~

20%,28 and impact strength ~ 90%.29 According to Shaffer et al., polylactic acid (PLA), an abundant, popular FFF-3D printing material showed ~65% tensile toughness reduction upon deformation in the building orientation. Anh et al. reported reduction of tensile properties up to

~85% and compression properties ~20% along the perpendicular axis to the printed layers for acrylonitrile-butadiene-styrene (ABS),28 the other common candidate in FFF-3D printing. Zaldivar et al. reported significant variations in tensile strength, failure strain, and modulus as a function of

FFF build orientation for ULTEM, a high engineering polyetherimide thermoplastic blend.30

Figure 1.9 illustrates the presence of void spaces at interfilamentous junctions for 3D printed PLA,

ABS, and ULTEM materials. Mainly this inherent anisotropy factor has limited the current use of

the FFF-3D printing technique for prototyping and other applications where the mechanical strength is of less importance.

Figure 1.9. (A) Microscopic image of a cross-section of 3D printed PLA filaments. (B) Scanning electron microscope (SEM) image of ABS fracture surface. Adapted from Ref 28, Copyright (2002), with permission from MCB UP Limited. (C) In-plane cross-sectional view of 3D printed ULTEM. Adapted and reprinted from Zaldivar, R. J.; Witkin, D. B.; McLouth, T.; Patel, D. N.; Schmitt, K.; Nokes, J. P. Influence of Processing and Orientation Print Effects on the Mechanical and Thermal Behavior of 3D-Printed ULTEM® 9085 Material. Addit. Manuf. 2017, 13, 71-80., Copyright (2017), with permission from Elsevier. Black regions represent interfilamentous void spaces.

1.4 Previous research work to improve interlayer adhesion

Liquid-crystalline polymers

In the context of addressing the issues related to poor interlayer adhesion of thermoplastic materials, important previous research efforts are reported. Owing to the excellent tensile properties (moduli ~ 50-100 GPa) and fibril forming nature, thermotropic liquid crystalline polymers (TLCPs) had been used as additives to reinforce thermoplastics. Gray et al. reported the use of a TLCP material Vectra A950 (Hoechst Celanese) to reinforce polypropylene (PP) and ended up with an increase of the tensile moduli ~ 100%, ~150% for the Vectra A/PP (40/60 wt %) composite than that of neat ABS and PP respectively.31 Krishnaswamy and Baird reported a hydroquinone-based liquid crystalline polyester (DuPont TLCP, HX8000) reinforcing nylon-11.32

The composite nylon-11 fibers having HX8000 (~35 wt %) showed ~ 5.5 times higher modulus

and ~2.5 times higher strength than those of neat nylon-11. Crevecoeur and Groeninckx used

Vectra A950, B950 TLCPs (Hoechst Celanese) to effectively reinforce brittle polystyrene (PS) and a ductile poly-2,6-dimethyl-1,4-phenylene ether (PPE)/PS material respectively. However,

TLCPs encounter two disadvantages when forming composites with thermoplastics. Most TLCPs are processed near or above the degradation temperature of most of the commodity plastics which prevents their reinforcement. On the other hand, the dominant shear flow present at the FDM extrusion head retards interlayer fibril formation by TLCPs which leads to a low degree of reinforcement in the composites.

Organoclay

Use of organically modified clay to form composites had been another method to enhance the mechanical strength of thermoplastics like ABS for the development of FDM 3D printing. Weng et al. reported use of montmorillonite clay modified with an organic modifier benzyldimethylhexadecylammonium chloride (HDBAC) to form nanocomposites with ABS via extrusion.10 The FDM 3D printed nanocomposites containing 5 wt% of organically modified montmorillonite (OMMT) showed an increase in tensile strength of ABS by 43% vs. only a 28.9% increase by injection molding. Moreover, the introduction of OMMT into ABS has significantly increased the mechanical properties: tensile modulus, flexural strength, flexural modulus, and dynamic mechanical storage modulus while also improving thermal properties: linear thermal expansion ratio and thermal stability. Fornes et al. reported preparation of organoclay nanocomposites with three different molecular weight grades of nylon-6. Superior tensile properties were observed for the nanocomposites with high Mw nylon-6 resulting highest stiffness,

tensile modulus, yield strength, elongation at break, and a slight loss of ductility in comparison with the medium Mw and low Mw counterparts.33

Cross-linking

Introducing cross-linking between print layers, another strategy to reinforce the mechanical strength is challenging due to formation of thermosets which lack melt-processability and hence incompatible with FFF 3D printing. Shaffer et al. reported a solution to this problem by introducing chemical crosslinking within PLA matrix via ionizing radiation as a post-printing process.34

Blending PLA with a radiation sensitizer triallyisocyanurate (TAIC) followed by subjecting printed materials to γ irradiation they were able to achieve a 1.7× increase in toughness for 10 wt%

TAIC samples with respect to neat PLA. However the cross-linking density was insufficient to reduce the anisotropy efficiently.

Use of dynamic covalent chemistry

The use of dynamic covalent chemistry (DCC) to enhance the interlayer adhesion has been a recent material approach targeted towards improving the mechanical properties of 3D printed parts. In

DCC, thermally reversible covalent bonds are formed to yield complex but yet thermodynamically stable moieties. DCC has been intensely applied in the fields of supramolecular chemistry35-40 to construct molecular assemblies such as catenanes,41,42 rotaxanes,43-45 suitanes,46 Borromean rings,47-49 and Solomon knots50 via imine bond formation, boronic acid condensation51,52 for 2D macrocycles, 3D cages, and covalent organic frameworks (COFs) and disulfide exchange,53,54 to yield macrocycles and trefoil knots. Importantly, DCC is utilized in self-healing materials55-61 focusing on damage repair mechanisms.

Healable polymer materials are categorized under two main classes: autonomically healable materials and rehealable/remendable materials. Autonomically healable materials, once fractured are exempt of an external stimulus to regain physical properties of the pristine material. In contrast, remendable materials require a specific external stimulus such as heat and or light to return into the virgin properties. However, regardless of the requirement of an external stimulus, a healable material should possess required components in its structure to be exploited in forming new bonding interactions in and around the fractured zone. Burattini et al. presents four main research efforts to address this challenge: (1) via encapsulated-monomers,62-67 (2) utilizing DCC to form reversible covalent bonds, (3) irreversible covalent bond formation,68 and (4) via supramolecular self-assemblies.69-73

During the healing process via DCC, the bulk polymer contains reversible covalent bonds which can undergo bond breaking and forming dynamic reactions allowing an intrinsically dynamic exchange of molecular components which help the polymer to regain pristine properties during the healing process after a fracture. Herein the Diels-Alder (DA) reaction appears to play an eminent role in the development of healable materials.

The DA reaction, cofounded by the Nobel laureates Otto Diels and Kurt Alder in 1928, is a [4+2] cycloaddition reaction between a conjugated diene and a substituted alkene, the dienophile forming a substituted cyclohexene derivative. The reaction has an important synthetic value to reliably form a six membered product with a good control over regio- and stereo selectivity. The resulting diastereomers are called endo and exo (Figure 1.10). The DA reaction is thermally reversible, the

reversible reaction is known as the retro-DA (rDA)40,74 which predominates usually after a temperature > ~ 120 °C.

Figure 1.10. Diastereomeric DA adducts resulting after a DA reaction between furan and maleimide.

A significant number of research work has been published up to date based on the thermoreversibility of the Diels-Alder reaction utilizing furan functionality as the diene and maleimide as the dienophile in self-healing and damage repair studies,59,61,75-82 cross-linking,77,79,80,83 and step- growth polymerizations.84,85 Stevens and Jenkins reported the first thermo-reversible crosslinked polymer network based on furan-maleimide Diels-Alder (fmDA) chemistry by reacting maleimide functionalized polystyrene with furfuryl alcohol to form the cross-links.86 McElhanon and Wheeler reported the first fmDA based thermally labile remendable dendrimer which exhibited ~ 40% breakage of DA linkages after 1 h at 110 °C and a 82% reassembly to the original structure upon cooling at 65 °C for 5 days.87 Chen et al. reported the synthesis of a transparent polymer crosslinked via fmDA chemistry by utilizing a tris-maleimide and a tetra-furan to obtain a 57% healing efficiency for the remendable polymer after fracture tests.75 After a slight change of the polymer synthesis by replacing the tris-maleimide with a bis-maleimide monomer, they were able to increase the healing efficiency up to 87%.88 Afterwards literature reports research efforts on developing remendable epoxy resins via fmDA chemistry.89 However most of these systems showed reversibility at elevated temperatures above ~ 90 °C and achieving multiple fracture-repair

cycles had been a challenge. At a temperature above ~ 120 °C, around 30% of DA linkages disconnect and reconnect upon cooling the system demonstrating the remendability.58 Liu et al. reported synthesis of a series of DA cross-linked polymers with high glass transition (Tg) temperatures and thermo-reversible polyamide gels using maleimide containing polyamides and a tris-functional furan.90 Also, Liu and Chen reported a series of thermo-reversible DA cross-linked polymers with high-toughness and self-repairing ability using maleimide and furan functionalized aromatic polyamides in which fmDA adduct formation occurred at 50 °C and the rDA at 150 °C.91

Rivero et al. recently reported a one-pot thermo-remendable shape memory polyurethanes based on fmDA chemistry.92

Even though most of the DA systems show reversibility at higher temperatures as mentioned above, self-healing DA dynamers under ambient conditions have also been reported. Boul et al. described development of such a system by reacting substituted fulvenes as the dienes with dicyano and tricyano-ethylene caroboxyesters as the dienophiles under ambient conditions.

According to 1H NMR studies, these polymer systems have undergone a reversible DA reaction at

25 °C.93 Reutenauer et al. reported a self-healing DA dynamer by reacting bis(dicyanofumarate) and bis(fulvene) terminated polyethylene substituted diene at room temperature. The yielded soft stretchy polymer film, once cut into two pieces and overlapped via gentle pressing to ensure contact showed self-healing behavior at room temperature within ~ 10 seconds.94 Another two examples for room temperature reversible DA systems were obtained by reacting anthracene derivatives (dienes) together with cyanofumarates and N-phenyltriazolinedione (dienophiles).95,96

Solvent assisted transfer of the healing agent at the cracked surface and subsequent healing via fmDA dynamics were also reported.97,98

Recently, fmDA chemistry has been utilized to develop recyclable thermosets. Polgar et al. reported the use of fmDA chemistry for thermoreversible cross-linking of rubbers in which the newly formed fmDA cross-links were broken at temperatures > 150 °C and re-formed upon thermal annealing ~ 50-70 °C.99 Trovatti et al. reported an application of fmDA chemistry in reversible cross-linking of poly(butadiene), a platform for producing recyclable tires.100

1.5 Challenges encounter using fmDA chemistry in FFF-3D printing

However, the use of DA chemistry as a self-healing strategy to develop materials in 3D printing, especially in FFF is a challenge mainly because of the processing complexities arise at elevated temperatures. When using fmDA moieties, higher temperatures lead to side reactions like furfuryl ring opening,101,102 maleimide homopolymerization103 (Figure 1.11 a) and aromatization of the DA adducts101,104 (Figure 1.11 b) which lead to polymer degradation and breakdown of cross-linker reversibility accounting for an overall loss of processability of the polymers.6

Figure 1.11. (A) Maleimide homopolymerization. (B) Aromatization of a furan DA adduct.

Consequently these processing complexities in 3D printable polymers have limited the use of self- healing chemistry to techniques like photolithography,76,83 micro contact printing,105 and direct- write printing of soft hydrogel materials.4,106,107 Although research efforts in these fields have contributed to a significant progress in the 3D printing field, manufacturing of 3D printed parts in large-scale and with mechanical soundness remain challenging.

1.6 Our approach to enhance the mechanical properties of FFF-3D printed materials

To address these issues, for the first time we introduced a design paradigm to enhance the mechanical properties of FFF 3D printed materials utilizing reversible DA chemistry which is described in detail in the second chapter. This work was based on using a partially cross-linked terpolymer having fmDA linkages which was blended with commercially available PLA to yield a remendable PLA material with improved mechanical properties with respect to neat PLA. In spite of improving mechanical strength the material was still suffering from mechanical anisotropy, having significantly different mechanical properties on the print direction. To address this issue we were able to successfully increase the cross-linker density of the mending agent polymer, resulting a polymer ink with nearly uniform mechanical strength in all print directions.

The third chapter demonstrates this work.

1.7 FDM-based 3D printing of soft materials for environmental remediation

The fourth chapter describes about utilizing direct-write 3D printing, another 3D printing technique which is based on FDM principles, to 3D print hydrogel materials prepared using chitosan and diacrylated pluronic F-127 to adsorb toxic heavy metal ions. Direct-write printing or direct ink writing (DIW), first developed in USA in 1996, is also an extrusion based additive manufacturing technique in which a paste (resembles a filament in FFF) is extruded through a moving nozzle onto a print bed (Figure 1.11). Hence the desired object is written on the print bed layer-by-layer manner under the control of a 3D computer aided design (CAD) model.

Figure 1.12. Schematic diagram of the direct-write printing technique.

The technique is basically designed for printing soft materials like polymeric hydrogels,108,109 colloidal suspensions,110 and ceramics111 but has also been used with materials like calcium phosphate, hydroxyapatite, molten glasses, alloys, and slurries. To be printable via DIW, the printing ink should have “shear thinning” properties, which is defined as decrease of viscosity under shear strain and immediate shape recovery after removal of the applied load. A wide variety of geometries is printable, from solid monolithic parts to intricate microscale scaffolds. Up to date, most of the research work based on the DIW technique has been focused on tissue engineering applications like manufacturing biologically compatible implants112-114, tissue constructs115-117 and also in developing sensor devices,108 regenerative medicine & diagnostics.118,119 Herein, 3D printing of chitosan based hydrogel materials for toxic heavy metal adsorption, the research work presented in the fourth chapter, represents the first use of a biomaterial processed via 3D printing to be used in environmental remediation applications.

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CHAPTER 2

DESIGN PARADIGM UTILIZING REVERSIBLE DIELS−ALDER REACTIONS TO

ENHANCE THE MECHANICAL PROPERTIES OF 3D PRINTED MATERIALS1

Authors- Joshua R. Davidson,‡,† Gayan A. Appuhamillage,‡,† Christina M. Thompson,‡ Walter Voit,*,§,⊥ and Ronald A. Smaldone*,‡

‡Department of Chemistry and Biochemistry, BSB13

§Department of Mechanical Engineering, EC38

⊥Department of Materials Science and Engineering, RL10

The University of Texas at Dallas

800 West Campbell Road

Richardson, Texas 75080-3021

†Joshua R. Davidson and Gayan A. Appuhamillage contributed equally to this work.

1Reprinted and adapted with permission from (Davidson, J. R.; Appuhamillage, G. A.; Thompson, C. M.; Voit, W.; Smaldone, R. A. Design Paradigm Utilizing Reversible Diels-Alder Reactions to Enhance the Mechanical Properties of 3D Printed Materials. ACS Appl. Mater. Interfaces 2016, 8, 16961-16966). Copyright (2016) American Chemical Society.

2.1 Abstract

A design paradigm is demonstrated that enables new functional 3D printed materials made by fused ﬁlament fabrication (FFF) utilizing a thermally reversible dynamic covalent Diels−Alder reaction to dramatically improve both strength and toughness via self-healing mechanisms. To achieve this, we used as a mending agent a partially cross-linked terpolymer consisting of furan- maleimide Diels− Alder (fmDA) adducts that exhibit reversibility at temperatures typically used for FFF printing. When this mending agent is blended with commercially available polylactic acid

(PLA) and printed, the resulting materials demonstrate an increase in the interﬁlament adhesion strength along the z-axis of up to 130%, with ultimate tensile strength increasing from 10 MPa in neat PLA to 24 MPa in fmDA-enhanced PLA. Toughness in the z-axis aligned prints increases by up to 460% from 0.05 MJ/m3 for unmodiﬁed PLA to 0.28 MJ/m3 for the remendable PLA.

Importantly, it is demonstrated that a thermally reversible cross-linking paradigm based on the furan-maleimide Diels−Alder (fmDA) reaction can be more broadly applied to engineer property enhancements and remending abilities to a host of other 3D printable materials with superior mechanical properties.

2.2 Introduction

Improvements in 3D printing technology are projected to drive new innovations in the areas of medical devices and prosthetics1-5 as well as myriad industrial and commercial sectors6-9 where specially designed materials are needed either on demand or in geometries not conducive for conventional plastics processing like blow molding, injection molding, extrusion, or vacuum- assisted resin transfer molding. 3D printing, also known as additive manufacturing,10 will

supplement and perhaps surpass some of these conventional plastics processing techniques over the next two decades. Some estimates point to a potential emerging $1 trillion market by 2030 for

3D printed parts, if techniques can be developed to print parts with uniform strength and toughness relative to the axis of orientation11 of the part in the 3D printer. In fused filament fabrication (FFF), one of the most versatile 3D printing methods, plastic filaments are heated above their melting points, deposited onto a printing bed, and built into the desired shape through subsequent filaments adhering to neighboring filaments to produce a physical object from a computer generated model.

Today, the resulting interfacial strength is poor at interfilamentous junctions and the mechanical properties of parts are not competitive with those parts made via injection molding or milling.12

The lack of uniform strength in printed materials has limited the uses of FFF-based 3D printing to prototyping and applications where mechanical strength has not been a primary consideration.

To address these issues with FFF, we have created 3D printable polymeric materials that address filament adhesion at a molecular level utilizing dynamic covalent chemical functionality.

Dynamic covalent chemistry utilizes reversible covalent bond forming reactions to form complex, but thermodynamically stable assemblies. These reactions have been used to develop highly ordered supramolecular arrangements13-16 as well as self-healing materials.17-20 Improving adhesion between two separate polymer surfaces (typically a fracture) has been a major area of research for the development of self-healing polymers.21-23 One of the most effective strategies for repairing polymers has been through the rearrangement or creation of new covalent bonds within the polymer structure via either an autonomic response,24 or an external stimulus such as heat.25

Processing complexities involved with developing and handling printable polymers have limited the use of self-healing moieties in custom manufacturing techniques to photolithography,26,27

micro contact printing28 and methods that use soft materials like hydrogels.3,5,26,29-31 This prior work has contributed significant progress to the field at large, but unsolved problems, namely the inability to fabricate large-scale, mechanically robust 3D printed parts, remain.

We hypothesized that the interfilamentous adhesion in FFF printing could be significantly improved by engineering a polymer system that thermally depolymerizes during the print process followed by re-polymerization during cooling to improve the fusion at the filament interface and therefore the strength of the printed object. Our strategy for developing an improved polymer system for mechanically robust FFF 3D printing involves the use of a thermally reversible furan- maleimide Diels-Alder (fmDA) reaction.32 The fmDA reaction has been used in a variety of polymer systems as a method to promote self-healing and damage repair.21,23,25,27,33-38 It is particularly desirable for these types of applications owing to its temperature controllable dynamics, and stability to external factors such as light, water, and oxygen. Polymers with fmDA functional groups can be made with the dynamic components incorporated as pendant groups on polymer backbones for crosslinking26,33,35,36 or in the main chain in step-growth polymers.39-41 At low temperatures (~40-50 ºC) this dynamic reaction favors the fmDA adduct, but at elevated temperatures (~100-120 ºC) the retro Diels-Alder reaction is favored.32,42 Herein, we report the first use of a dynamic covalent chemical reaction for the purpose of improving the strength and toughness of FFF printed materials. For this study, we synthesized a crosslinked polymer containing fmDA functional groups that can be depolymerized at temperatures suitable for FFF printing, and then re-polymerized upon cooling, creating new covalent bonds between filament layers (Figure 2.1). To our knowledge this work represents the first mechanically robust, 3D

printable material and offers a design paradigm with promise for engineering even tougher remendable thermoplastics.

2.3 Results

The polymer filament used in this study is comprised of commercially available PLA and a novel synthetic polymer that contains fmDA adducts in the main chain. PLA was chosen as the bulk printing material as it is inexpensive and readily compatible with most 3D printers currently available. The fmDA polymer was synthesized by mixing monomers 2F, 2M,43 and 3F44 in a 15 to 18 to 2 molar ratio (~5% crosslink density) in ethyl acetate at 75 ºC for 24 h and heated under vacuum at 60 ºC (Figure 2.2 a) to remove excess solvent. As-synthesized mending agent was blended in ratios of 10 and 25 weight percent with PLA in dioxane at 80 ºC and the solvent was removed under vacuum at 60 ºC. The filaments of 3 mm in diameter were fabricated using a

Filabot™ extruder using pellets of the remendable PLA blend and extruding them at 160 ºC. This filament was used to print dogbone samples for mechanical testing (Figure 2.2 b).

Figure 2.2. (A) Synthesis of the mending agent polymer used in this study. (B) Filament fabrication and mechanical testing process used for the remendable PLA blends.

1H NMR studies were performed on the mending agent to confirm the formation of the fmDA adduct and are shown in Figure 2.3 and 2.4.

Figure 2.3. 1H-NMR spectra of (bottom to top); 2F, 3F, 2M, and the mending agent.

The proton signals at 2.85, 2.95, 5.25, 6.23, and 6.33 ppm represent the Diels-Alder adduct peaks.

The retro Diels-Alder reaction was monitored at 110 °C in toluene for 30 min and an NMR spectrum was collected immediately afterwards. The fmDA adduct proton signals at 2.85, 2.95, and 5.25 ppm are reduced. The proton signals of the furan slightly shift to 6.25 and 6.35 ppm, and the maleimide vinyl peak reappears at 6.65 ppm (Figure 2.4).

Figure 2.4. The Diels-Alder adduct proton signals of the mending agent a,b,c,d, and e are shown in the top spectrum. Note that the relative intensities of c,d, and e signals are reduced and the appearance of the maleimide proton signal (f, g) ~ 6.65 ppm in the bottom spectrum after the retro-fmDA reaction. The two peaks ~ 6.25 ppm (h) and 6.35 ppm (i) in the bottom spectrum are due to the furan protons of the monomers (usually these appear very close to the b and a Diels- Alder adduct proton signals respectively). The retro-fmDA reaction was monitored by heating the mending agent at 110 °C in toluene for 30 min (50 mg/mL). The mending agent solution (0.2 mL) was diluted into CDCl3 (2 mL) and the resulting mixture was immediately analyzed using 1 H-NMR. The NMR solution was prepared in CDCl3 with gentle heating. This heating is used to detach enough crosslinks to allow the polymer to be soluble. The remaining main-chain fmDA linkages are observed to undergo retro-DA reactions after heating in toluene.

Differential scanning calorimetry (DSC) was used to study thermal transitions of the mending agent alone, PLA, and the remendable PLA blends (Figure 2.5) to determine miscibility of the polymer system components. The mending agent shows two endothermic transitions beginning at

5.7 ºC and 120 ºC. The transition at 5.7 ºC is the Tg and the second value, 120 ºC, is indicative of the retro-fmDA reaction. PLA exhibits three transitions: a Tg at 61.8 ºC, an exothermic crystallization beginning at 100 ºC, and a Tm at 145 ºC. The blend ratios of 10% mending agent and 25% mending agent in PLA exhibit Tg values of 55.2 °C and 48.2 °C. The presence of a single

Tg in the polymer blends demonstrates that the two materials are miscible and do not phase separate at elevated temperatures.

The strengths of 3D-printed PLA and the 10% and 25% remendable PLA blends were determined by tensile testing (Figure 2.6).

Figure 2.6. (A) Printing patterns used to print each dogbone type for this study. (B) Ultimate strength values for neat PLA, and PLA with 10% and 25% mending agent as dogbones for each printing pattern. (C) Toughness values for the PLA, 10% remendable PLA, and 25% remendable PLA dogbones for each printing pattern. (D) Representative stress vs. strain plots for each polymer blend and printing pattern tested in this study.

Three print patterns (X, Y, Z), shown in Figure 2.6. a, were tested to represent structural features inherent in FFF parts. The print variation X has a 45º alternating infill pattern, whereas print variations Y and Z are linear patterns printed parallel to (Y), or perpendicular to (Z), the direction of mechanical stress on the printed material. Objects printed using FFF techniques will rarely be comprised entirely of one of these patterns, but any printed object will contain some combination of these. Tensile tests demonstrated that the ultimate strength (Figure 2.6 b) and toughness (Figure

2.6 c) of the materials were largely unchanged with the alternating infill pattern (X) but showed improvements in both the Y and Z designs. Ultimate strength improved 27% for the Y printing pattern for both 10 and 25% remendable PLA blends. When printed in the Z direction the ultimate strength was improved 88% and 130% for the 10 and 25% remendable PLA blends respectively.

Toughness measurements for each polymer blend are shown in Figure 2.6 c. These values were

determined through the integration of the stress vs. strain plots (Figure 2.6 d). Toughness values for the X pattern were again statistically similar for each polymer sample. The Y pattern however, demonstrated improvements in toughness of 70% for the 10% remendable PLA and 109% for the

25% remendable PLA. The Z design showed even more dramatic increases of 260 and 460% for the 10 and 25% blends, respectively.

Cross-sectional optical images of a printed PLA material and the 25% remendable PLA blend printing using pattern Y are shown in Figure 2.7. The cross-section of a pure PLA material (Figure

2.7 c) shows that the filament layers have a consistent, uniform packing arrangement with a clear, observable void space present at the inter-filament junctions. However, the filaments shown in the remendable PLA blend image (Figure 2.7 d) are bound together with fewer observable voids and interfilamentous boundaries.

Figure 2.7. Illustrations and images of 3D-printed PLA and PLA blended with mending agent. (A) Illustration of 3D-printed PLA filaments (blue), (a’) inter-filament junctions. (B) Illustration of PLA blended with mending agent (purple). (C) Microscopic image of 3D-printed PLA filaments. (D) Microscopic image of 3D printed remendable PLA.

2.4 Discussion

As illustrated in Figure 2.1, the polymer mending agent can influence the printing process through several mechanisms. Upon heating, thermal reversal of the fmDA crosslinks will allow the mending agent network to exhibit thermoplastic behavior. The retro Diels-Alder reactions within the linear portions of mending agent will reduce its overall molecular weight and plasticize the remendable PLA melt thereby improving its ability to mix with the previously added polymer filaments on the print bed. Upon cooling below ~70º C, the fmDA reaction will favor formation of the adducts and regenerate the mending agent network within the PLA blend. The reversibility of this reaction was confirmed by 1H NMR studies (Figure 2.4). The polymer mending agent alone is a soft, malleable material at room temperature and lacks the mechanical strength of bulk PLA or engineering thermoplastics. Blends of 10% and 25% mending agent with PLA were chosen to balance the remendable capability of the fmDA functionalized-polymer with the mechanical properties of PLA.

In order to evaluate the effect that the mending agent had on PLA, three print variations were designed to capture different print orientations of a FFF printed part. The three print patterns, shown in Figure 4a, exhibit significant mechanical variations in the PLA samples. Due to the low

Tg of pure mending agent, we would expect to see a slight plasticizing effect when blended with

PLA resulting in lower ultimate strength. This effect was observed in the Tg of the blends (Figure

2.5); however, the strain capacity of remendable PLA blends was not compromised as evidenced in Figure 2.6 b-d. In fact, the X printing patterns show equivalent strength compared to bulk PLA.

We hypothesize that any potential loss in strength resulting from plasticization is offset by the crosslinked structure of the mending agent. The Y print design is used as a model to evaluate a

raster infill pattern in which an applied tensile force is exerted parallel to the filament grain.

Improvements in the observed mechanical properties of the remendable PLA blends could be attributed to the crosslinked network of the mending agent. Additionally, cross-sectional images of the printed materials (Figure 2.7 c-d) show that there are observable void spaces between each filament junction in the PLA sample, whereas the 25% remendable PLA blend displays essentially no void space and the filaments appear to be adhered to one another. The Z print pattern was designed to represent situations where mechanical stress is applied perpendicular to the print grain of the printed object. Under these conditions, which are the greatest source of mechanical anisotropy in FFF 3D printing, the printed material relies entirely on interfilamentous adhesion for strength. Mechanical tests of the Z pattern materials demonstrated the largest increases in both strength and toughness for any of the print directions. This indicates that the improvements observed cannot be completely attributed to the use of a stronger bulk polymer, but rather to the increased interfilamentous adhesion arising from a combination of improved polymer filament adhesion between the previously deposited filament and the melted filaments added subsequently, combined with the ability of the mending agent to repolymerize owing to the dynamic covalent nature of the fmDA functionalized polymer network.

2.5 Conclusion

In conclusion, we have synthesized a printable PLA blend material that demonstrates improved mechanical strength and toughness for FFF 3D printing applications without the need to chemically modify the bulk polymer, in this case, PLA. This was achieved through the use of a dynamic furan-maleimide Diels-Alder reaction that allows for the improved remending of interfilamentous junctions that are created as part of the FFF process. The concepts demonstrated

in this study are generally applicable for the development of FFF printable materials that reduce the impact of mechanical defects introduced by the printing process.

2.6 Experimental Section

Jeffamine D-400 (diamine, weight-average molecular weight = 430 g/mol) was supplied by

Huntsman and was used as received. Maleic anhydride and sodium bicarbonate were purchased from Alfa Aesar and were used as received. Tris(2-aminoethyl)amine, p-phenylenediamine, ethyl acetate, acetone, methylene chloride, and methanol were purchased from Sigma-Aldrich and were used as received. Triethylamine, 2-furaldehyde, and sodium borohydride were supplied by Acros

Organics and were used as received. Acetic anhydride, magnesium sulfate, and 1,4-dioxane were supplied by Fisher Scientific and were used as received. PLA plastic in fine resin pellet form was produced by IC3D printers LLC. Dublin, Ohio. PLA plastic in high-grade 3D-printing filament form was purchased from ExcelFilTM TECH, Voltivo Group Ltd., Taiwan. The polymer blends 10

(wt%), 25 (wt%) fmDA-polymer/PLA were prepared according to the procedure in the Appendix

A (Synthetic Procedures section). These blends were then ground into small pieces followed by extrusion into 3 mm diameter filaments using a single-screw extruder (Filabot). Extrusion temperatures were 160 °C and 155 °C, respectively. Neat PLA was extruded using the same stock pellets (ExcelFilTM TECH) at 165 °C as the control. Extruded filaments of PLA, 10 (wt%) remendable PLA, and 25 (wt%) remendable PLA, were printed using a Taz-5 3D-printer from

Lulzbot using the open-source MatterControl software. Print-head temperatures for PLA, 10 (wt%) remendable PLA, and 25 (wt%) remendable PLA were maintained at 205 °C, 200 °C, and 190 °C, respectively while print-bed temperatures were 60 °C, 55 °C, and 48 °C respectively. The materials were printed into rectangular prisms with the following dimensions (l x w x h) 64.5 mm x 10.5

mm x 1.0 mm using a print resolution of 0.1 mm in each direction. The rectangular prisms were cut into American Society for Testing and Materials (ASTM) D638 Type V, standard dogbones using a Gravograph LS100 CO2 laser to ensure consistency in the samples and minimize error caused by print resolution. 1H-NMR and 13C NMR spectra were obtained using an Avance-500

500 MHz spectrometer (Bruker, Switzerland) with deuterated chloroform (CDCl3) as the solvent in order to monitor both fmDA bond formation and retro-fmDA reaction. DSC analysis was performed with DSC 1 STARe System (Mettler Toledo AG, Analytical-Switzerland). All the samples were subjected to three heating cycles from -40 °C to 200 °C at a heating rate of 10

°C/min. Tensile tests were carried out according to ASTM D638-04 using a universal testing machine (Instron 5848 MicroTester) at a loading speed of 3 mm/min and a temperature of 25 °C.

Three dogbones were tested in each print/blend variation and the mean and standard deviation were calculated based on these and used to generate the error bars for each measurement. Cross- sectional areas of the printed dogbones were imaged using a versatile upright microscope (Leica

DM4000 M LED) using 5X as the magnification to demonstrate mending between printed layers.

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CHAPTER 3

3D PRINTED REMENDABLE POLYLACTIC ACID BLENDS WITH UNIFORM

MECHANICAL STRENGTH ENABLED BY A DYNAMIC DIELS-ALDER REACTION1

Authors- Gayan A. Appuhamillage,a John C. Reagan,b Sina Khorsandi,a Joshua R. Davidson,a Walter Voit,*c,d and Ronald A. Smaldone*a

aDepartment of Chemistry and Biochemistry, BSB13

bDepartment of Biomedical Engineering, EC31

cDepartment of Mechanical Engineering, EC38

dDepartment of Materials Science and Engineering, RL10

The University of Texas at Dallas

800 West Campbell Road

Richardson, Texas 75080-3021

1Appuhamillage, G. A.; Reagan, J. C.; Khorsandi, S.; Davidson, J. R.; Voit, W.; Smaldone, R. A. 3D Printed Remendable Polylactic Acid Blends with Uniform mechanical Strength Enabled by a Dynamic Diels-Alder Reaction. Polym. Chem. 2017, 8, 2087-2092- Adapted by permission of The Royal Society of Chemistry.

3.1 Abstract

Here we report a 3D printable polymer that retains uniform mechanical strength after printing and can be used with a conventional fused filament fabrication (FFF) printer. To achieve this, a synthetic polymer containing dynamic Diels-Alder functionality was blended with commercially available polylactic acid (PLA). This new polymer contains cross-links that are reversible at the temperatures typically used for FFF 3D printers. By increasing the cross-link density of the polymer system, we were able to dramatically improve both ultimate strength and toughness along the interfilament junctions of the printed material up to ~290% and ~ 1150% respectively. The final achieved ultimate strength and toughness values for the optimized system are isotropic within error along the three representative print directions X, Y, and Z. Self-healing studies on the Z print direction of the optimized blend showed a 77% recovery of the ultimate strength vs. control PLA having only a 6% recovery, further proving the advanced interfilamentous adhesion via the fmDA dynamics.

3.2 Introduction

Additive manufacturing (AM),1,2 also known as 3D printing, is a promising technology with the capability to manufacture 3D objects from a computer-aided design (CAD) model. The ability to custom produce objects on demand has launched AM into a diverse spectrum of applications including automotive,3 aerospace,4 defense,5 biomedical6,7 and energy8 industries. A variety of 3D printing techniques have been developed to date such as fused filament fabrication (FFF),2,9,10 stereolithography (SLA),11,12 and selective laser sintering (SLS)11,13 among others. Of these, FFF is one of the most versatile and cost effective owing to the simplicity of the technique and broad

compatibility with a wide variety of materials. In FFF, the thermoplastic filament (typically PLA or ABS) is heated to a molten state inside a print head, extruded from a nozzle, and deposited layer by layer to form the printed part. FFF printing is extremely versatile and inexpensive. However, the main disadvantage of this method is a lack of adhesion at the interfilamentous junctions, resulting in non-uniform mechanical strength (i.e., mechanical anisotropy).10,14,15

To date, there have been several reports of methods that aim to improve the mechanical quality of

FFF 3D printed materials. Thermotropic liquid crystalline polymers (TLCPs)16,17 and organically modified clays9,18,19 have been used as additives for FFF polymers aimed at improving their overall strength. Post-printing processes such as ionizing radiation10,20 have also been developed to improve layer-to-layer adhesion. Despite these efforts, large scale production of mechanically robust 3D printable parts that can compete with those fabricated via conventional plastic processing techniques like injection molding,15,19 still remains a challenge. Currently there are no commercial materials that can be printed via FFF without significant loss in overall mechanical strength relative to the neat mechanical properties.

To address these limitations we previously reported2 a design approach to enhance both strength and toughness of FFF 3D printable materials utilizing Diels-Alder based dynamic covalent chemistry (DCC).21-23 The reversible furan maleimide Diels-Alder (fmDA) cycloaddition reaction24-26 has been shown to form thermally reversible cross-links in a wide range of healable polymer systems27-30 as a damage repair mechanism. By designing partially cross-linked terpolymers with fmDA functionality, we can produce systems which undergo depolymerization during the FFF 3D printing process at elevated temperatures (> 120 °C) due to the retro-DA reaction,23,31 followed by repolymerization upon cooling at low temperatures (~ 40-50 °C) via

fmDA adduct formation (Figure 3.1). This thermocycling, inherent in all FFF printing, allows for the formation of new covalent bonds between interfilamentous junctions, strengthening the final printed material.2 One limitation of our previous work was that, while the interfilament strengths were improved, there was still a large difference in mechanical strength depending on the direction of print. In this work we describe a novel polymer ink that can be processed with a conventional

FFF 3D printer to create a polymeric object with nearly uniform mechanical strength in all directions. Furthermore, we also show that these materials can retain the remendability properties endowed by the reversible fmDA functionality after printing, demonstrating that these materials could be healed after suffering damage or fracture from external forces or wear.

Figure 3.1. Illustration of the thermoreversibility of MA-polymers during the FFF 3D printing process.

3.3 Experimental Section

3.3.1 Materials

All chemicals were used as received without further purification. Jeffamine D-400 diamine (Mw

=430 g/mol) was supplied from Huntsman corporation, Woodlands, TX. Maleic anhydride and sodium carbonate were supplied by Alfa Aesar. p-Phenylenediamine, tris-(2-aminoethyl)amine, ethyl acetate, methylene chloride, acetone, and methanol were purchased from Sigma-Aldrich.

Sodium borohydride, triethylamine, and 2-furaldehyde were purchased from Acros Organics.

Magnesium sulfate, acetic anhydride, and 1,4-dioxane were purchased from Fisher Scientific. The synthetic procedures were carried out as described in our previous work.2

3.3.2 Synthesis of fmDA-polymers

The fmDA-polymers were synthesized varying the mole ratio of the monomers (2M32: 2F: 3F33) to obtain five fmDA-polymers with 0, 17, 67, 83, and 100% cross-link density as given in the

Appendix B. The cross-linked fmDA- polymeric material is denoted as the mending agent (MA).

3.3.3 Preparation of fmDA-polymer/PLA blends

PLA plastic in fine resin pellet form was purchased from IC3D printers LLC. Dublin, Ohio and was used as received. Five different polymer blends of 25 wt% fmDA-polymer/PLA (denoted as

MA-PLA) were prepared using each of the above synthesized fmDA-polymers according to the procedure in the Appendix B.

3.3.4 Preparation of 3D Printable Filaments

These blends were powdered using a grinder and then extruded through a 3 mm diameter die into filaments using a Filabot single-screw extruder. Extrusion temperature was set to 150 °C. Pristine

PLA was extruded using the same process using commercially purchased pellets (IC3D printers) at 160 °C and used as the control.

3.3.5 FFF 3D Printing

These extruded filaments of pristine PLA and MA-PLA blends were 3D printed using a Lulzbot

Taz-5 FFF 3D printer using the open source software Cura. Print-head temperatures for pristine

PLA and MA-PLA blends were maintained at 205 °C and 200 °C respectively while print-bed temperatures were at 60 °C and 50 °C respectively. These materials were first printed into rectangular prisms having the dimensions (l × w × h) 64.5 mm × 10.5 mm × 1.0 mm with a print resolution of 0.38 mm in each direction and then were laser cut using a Gravograph LS100 CO2 laser to meet American Society for Testing and Materials (ASTM) D638 Type V- standard dogbone dimensions. This strategy was followed in order to ensure the consistency in the samples, minimizing errors of print resolution.

3.3.6 Characterization

1H NMR and 13C NMR spectra of monomers and MA-PLA, DSC experiments, and tensile tests were carried out according to the procedures mentioned in our previous work.2 TGA analysis was performed on a TGA/DSC 1 STARe System (Mettler Toledo AG) with a temperature range of 30-

800 °C and a ramp rate of 10 °C/min under an N2 atmosphere.

3.3.7 Healing Experiments

These were carried out using the MA-PLA-100 blend as follows. The prepared dogbones of the Z- print direction of this blend were first subjected to tensile tests to mark the break point. Then the two broken ends were heated at 120 °C for 5 s on a hot plate and immediately afterwards the two hot ends were stuck together. Then these dogbones were cured at 65 °C for 15 min in an oven.

Finally the tensile tests were carried out on the healed dogbones in order to calculate the total recovery of ultimate strength. The same procedure was carried out on pristine PLA dogbone samples as the control. Three dogbones were tested for each procedure and the mean and standard deviation were calculated based on these measurements and used to generate the error bars of ultimate strength graphs.

3.4 Results and discussion

The printable filaments used in this study consist of a series of Mending agent (MA) polymers with varying crosslink density achieved by tuning the molar ratios between linear monomers (2M,

2F) and the cross-linker (3F) as shown in Figure 3.2.

Figure 3.2. Synthesis of the Mending Agent (MA) polymer series used in this study.

Following their synthesis, each of the MA polymers were separately blended with PLA to give 25 wt% MA/PLA blends (MA-PLA 0-100) using dioxane as a solvent to obtain homogeneous blending. After removal of the solvent under vacuum, the resulting blends were powdered by mechanical grinding. These powder samples were subjected to melt extrusion in a single screw extruder at 150 °C to form filaments of 3 mm in diameter which were then printed into dogbone samples for tensile testing (Figure 3.3). PLA samples printed from filaments extruded using pellets under the same conditions were used as the control. The blending ratio of 25 wt% of the MA polymers to PLA was chosen in this study because this blending ratio gave the best compromise of print quality and mechanical strength based on our previous work.2

Figure 3.3. Filament fabrication, 3D printing, and tensile testing of the MA-PLA blends.

Tensile tests were carried out for the printed dogbones of MA-PLA blends and PLA to determine their ultimate strength and toughness. Any FFF 3D printed part can be considered as combinations of or directly related to three basic print patterns X, Y, and Z where the X print pattern consists of

45° alternating infill layer pattern while Y and Z consist of linear layer patterns (Figure 3.4 a).

Tensile testing results showed that ultimate strength values improved with increasing crosslink density in all the three print patterns with respect to unfunctionalized PLA (Figure 3.4 b). The ultimate strength has been improved ~ 290 % in the Z print direction for the blend with MA-PLA-

100. Toughness also improved with increasing cross-link density with respect to pristine PLA

(Figure 3.4 c) achieving a remarkable improvement ~ 1150% along the Z axis for the most crosslinked blend. Representative stress vs. strain plots of PLA and the optimized MA-PLA-100 blend further illustrates the improvement of mechanical strength upon increasing the cross-linking density of MA-PLA blends (Figure 3.4 d).

The ratios of ultimate strength and toughness along the Z print axis compared to that of Y and X print axes were calculated as a measure of the loss in strength due to printing (Figure 3.5). Neat

PLA retains only ~25% of its mechanical strength between its interfilament junctions, compared with the more mechanically robust X print pattern and only ~40% compared to the Y pattern

(Figure 3.5 a). In contrast, the ratios nearly reach unity with MA-PLA-100 indicating that the material is equally strong no matter the print pattern or orientation. The plotted toughness ratios

(Figure 3.5 b) show a similar trend, although only ~80% the toughness is retained in the MA-PLA blend.

The increases in strength and toughness for the MA-PLA blends can come from two potential mechanisms. At temperatures greater than 120 °C the retro-DA reaction results in the breaking of cross-links between the MA polymer chains in the blend allowing the melted polymer to flow freely. In addition to breaking the interchain crosslinks, the retro-DA reaction also results in the breaking of linear polymer chains which lowers the molecular weight and decreases the melt viscosity. Both of these processes allow for better mixing of the molten polymer being printed with the material that has already been deposited. Upon cooling to lower temperatures (< 60 °C) the DA adduct formation is favored thereby regenerating the extended MA polymer network

within the PLA matrix. During this process new covalent bonds are formed between the filament layers which strengthen their adhesion to one another.

Thermal transitions were determined with differential scanning calorimetry (DSC) (Figure B.1).

The thermal transition resulting from the retro-fmDA reaction can be observed at 120 °C in the neat MA-PLA-17 sample. The Tg values of the MA-PLA blends show slight increases with increasing crosslink density. All the blends display a single Tg indicating a minimal level of phase separation and good blending between the MA and PLA components. Thermogravimetric analysis

(TGA) was performed on each of the MA-PLA blends and neat PLA (Figure B.2). The decomposition temperature of these blends increased slightly with increasing crosslink density.

We also carried out healing experiments to determine the ability of the printed materials to be remended even after experiencing several thermal cycles during the process of filament extrusion and printing. To do this we used the Z print dogbones of MA-PLA-100 and neat PLA as the control (Figure 3.6).

Here the printed dogbones were first broken using the tensile tester. The two broken ends were heated at 120 °C for 5 s, followed by immediate touching of the heated ends and cured at 65 °C/15 min. After this process, tensile tests were repeated. These tests showed that MA-PLA-100 recovers

77% of its ultimate strength while the control PLA retains only 6%. These data indicate that the fmDA linkages are still active after many thermal cycles and that the printed parts retain the thermal healing properties of the neat polymers. While neat PLA is known to lose mechanical strength with exposure to higher temperatures, we have not observed this with the MA-PLA blends.

In fact, the MA-PLA polymers of all crosslinking densities appear to maintain their mechanical strength after being heated under ambient conditions to 65 °C for short periods of time (Figure

B.5). These tests demonstrate that the MA polymer does appear to have some capability to prevent the loss of mechanical strength under exposure to mild heating– a property that neat PLA does not have.34

3.5 Conclusions

In conclusion, we have developed a 3D printable polymer ink that can be processed using a conventional FFF 3D printer without extensive loss in its mechanical strength after printing. This material utilizes reversible furan-maleimide Diels-Alder chemistry to achieve these remarkably improved mechanical properties. Along with this improved strength, the polymers retain their remendability properties after printing. This is an important step towards the development of 3D printable materials that have comparable mechanical properties to commercially produced parts made via conventional manufacturing methods such as injection molding.

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CHAPTER 4

3D PRINTABLE HYDROGELS FOR TOXIC HEAVY METAL

ADSORPTION FROM WATER

Authors- Gayan A. Appuhamillage, Danielle R. Berry, Candace E. Benjamin, Michael A. Luzuriaga, John C. Reagan, Jeremiah J. Gassensmith,* and Ronald A. Smaldone*

Department of Chemistry and Biochemistry, BSB13

The University of Texas at Dallas

800 West Campbell Road

Richardson, Texas 75080-3021

4.1 Abstract

Herein, a toxic heavy metal remediation strategy demonstrated with 3D printed designs using chitosan, a cheap, abundant, biopolymer. To achieve this, hydrogels with shear thinning properties were prepared by mixing chitosan with diacrylated Pluronic F-127 (F127-DA) followed by 3D printing and UV curing to form mechanically enhanced structures. Chitosan/ F127-DA hydrogels were tested for adsorption of common metal pollutants Pb2+, Hg2+, Cd2+, and Cu2+. Metal adsorption from stock solutions showed superior remediation efficiencies for chitosan/F127-DA hydrogels compared with the F127-DA control gel with ~ 95% Pb2+ removal within 30 min. The hydrogels were also printed in various shapes via direct-write 3D printing. 3D printed sheet structures demonstrate affinity for in the order Pb2+> Cu2+> Cd2+> Hg2+ within a range of ~ 120-

105 ppm/g of the dry hydrogel representing potential use in highly polluted water. Leaching tests prove the sustainability of the chitosan/F127-DA hydrogels in metal contaminated water.

Chitosan/F127-DA cups show improved mechanical properties than the control. Increase of surface area with complex internal structures improve Pb2+ removal efficiency. Recycling tests show ~98% recovery of the original Pb2+ removal even after five cycles. The technology offers a great platform for future design and development of mechanically sound materials for water filtration.

4.2 Introduction

The production of fresh water is critical for the needs of both the individual as well as commercial industry. As the population grows, sources of fresh water have come under stress, not only from increased usage, but also pollution that originates from industrial waste or decaying urban

infrastructure. The cost for cleanup can be prohibitively high or politically difficult, leaving few viable options for those in poor or remote locations. Water contaminants often associated with industrial pollution are toxic heavy metal ions include Pb2+, Hg2+, Cd2+, and Cu2+. These metals can have severe and persistent effects on the neurological,1-3 reproductive,3,4 gastrointestinal,3 endocrine,3,4 and immune1-3,5 systems of those who have been exposed to these contaminants; therefore, the efficient remediation of these toxins from drinking water is of great importance to the global community.

A wide variety of water purification techniques have been employed such as filtration, distillation, and chemical purification techniques. Many of these processes are optimized for large scale purification at water treatment plants, making them difficult to adapt for consumer use.

Furthermore, when the infrastructure that carries the water to consumers is the source of the pollution as is the case in Flint, Michigan, these industrial scale processes are ineffective.

According to some cost estimates, the most popular water filtration systems can cost up to $10k or more for a household 6 whereas an industrial wastewater treatment system could be in a range of $200k to over $1 million.7 In low-income areas where this type of pollution often persists, these costs are a major barrier to clean water.

In order to address these challenges, we report a shear-thinning hydrogel composition made from chitosan and diacrylate Pluronic-F127 (DAP), a photocrosslinkable polyether, that can be molded or 3D printed into custom shapes. Chitosan, a polysaccharide derived from the main component found in crustacean shell wall chitin, is a biocompatible, biodegradable, non-toxic, and cost effective material.5,8-10 Due to these appealing features, it has found applications in the biomedical,11,12 regenerative engineering,13,14 clinical,15 and pharmaceutical16 fields. Its structure

contains lone electron pairs on the amine nitrogens and hydroxyl groups, which can act as chelating sites for heavy metal ions.17 Hence, the use of an affordable and sustainable material like chitosan

(~$15-20 /kg)18 could potentially reduce the cost of water purification systems. Previous work has used modified chitosan with cross-linking agents, grafting, and composite formation for the removal of dyes and metal ions from water sources.5,9,19-25 Even though these modifications could increase the heavy metal adsorption capacities of chitosan, these previous examples suffered from poor processability due to low solubility, which also led to poor reusability.26 While chitosan possesses the functionality necessary for metal chelation, it is not easily processed into functional materials on its own. By combining chitosan with DAP, we can render it compatible with inexpensive extrusion-based 3D printing techniques (~$1000 U.S.). Unlike injection molding, 3D printing allows for objects with complex internal structures to be produced that have increased surface area and improved metal absorption kinetics, and water flux rate. Herein, we describe a cost effective, reusable, and 3D printable adsorbent material for removal of four major toxic heavy metal ions Pb2+, Hg2+, Cd2+, and Cu2+ from contaminated water.

4.3 Results and Discussion

In shear-thinning hydrogels, the viscosity of the gel decreases under shear strain. This property can be exploited in extrusion-based 3D printing applications to produce free standing objects without the need for heating or additional chemical reactions. While smooth extrusion under pressure is required for gel printing, the extruded material must also adhere to the previously printed gels. Additives (like chitosan) to DAP could affect the shear thinning and printing properties of the gel. In order to determine the optimal chitosan-DAP (cDAP) mixture for good

printability, we prepared three hydrogel compositions containing 1, 5, and 10 wt% chitosan with

9 wt% DAP and also 1 wt% chitosan with 25 wt% DAP in water as shown in Table 4.1.

Table 4.1. Different cDAP hydrogel compositions and the control DAP prepared.

Hydrogel Chitosan DAP Ultra-pure water (wt %) (wt %) (wt %) containing acetic acid 0.5 (v/v %)

cDAP-1 1 9 90

cDAP-5 5 9 86

cDAP-10 10 9 81

cDAP-1 1 25 74

Control 0 26 74

Figure 4.1 illustrates how we hypothesize that the chitosan polymers can blend with DAP. The hydrophilic chitosan molecules arrange themselves to be embedded in the DAP matrix in contact with its hydrophilic poly(ethylene)oxide component via hydrogen bonding interactions, resulting stable hydrogels.28

Each of the cDAP compositions were then 3D printed into various shapes (illustrated in Figure

4.2) via direct-write 3D printing,29,30 which is a cheap and versatile layer-by-layer fabrication method like fused filament fabrication (FFF)31-34 .

Figure 4.2. (A) 3D printed thin hydrogel sheets containing 1, 5, and 10 wt% chitosan with 9 wt% DAP (cDAP-1, cDAP-5, cDAP-10 respectively) and the control DAP under UV curing. (B) 3D printed hydrogel cups containing 1 wt% chitosan with 25 wt% DAP (cDAP-1) and the control DAP under UV curing. Also (B) illustrates the limitation on Z-height with increasing wt % of chitosan via disrupted prints of hydrogel cups containing 5, 10 wt% chitosan with 25 wt% DAP. (C) 3D printed and UV cured solid cubes and shape A, shape B with internal structure containing cDAP-1 and the control DAP. Used CAD models are shown for each shape respectively. All the 3D printed dimensions are given in the Experimental Section.

Initially we printed sheets consisting of single layers of gel deposition (Figure 4.2 a). We observed increasing surface roughness and inhomogeneity as the amount of chitosan was increased in the blend. As we printed more complex shapes, it became clear that the cDAP-5 and cDAP-10 mixtures could not be printed into shapes with more than one layer of height as the adhesion between printed filaments was poor (Figure 4.2 b). As such, cDAP-1 was identified as the most printable formulation for filter fabrication. In fact, cDAP-1 containing 25 wt% DAP was useful for 3D printing cup shapes with a longer z-height than the sheets (Figure 4.2 b). Using this cDAP-

1 formulation, we were also able to print several other designs, including a solid cube and other shapes, including objects that have internal structure (Figure 4.2 c).

4.3.1 Evaluation of Hydrogel Mechanical Properties

Compression tests were carried out for the hydrogel cups using a universal testing machine

(Experimental Section). All the resulting mechanical properties; ultimate strength, toughness, and

Young’s modulus had been improved in the cDAP-1 compared with the control DAP cups (Figure

C.2). This could be attributed to the stabilization of the resulting hydrogel via inter-and

28 intramolecular H-bonds formed at –OH and –NH2 groups in chitosan structure.

4.3.2 Maximum Metal Adsorption Tests of DAP and cDAP Sheets

Adsorption experiments were carried out in order to determine the maximum concentration of each heavy metal ion that can be adsorbed from solution onto the hydrogel adsorbents (Figure 4.3).

Figure 4.3. Maximum heavy metal adsorptions of the three thin hydrogel sheets cDAP-1, cDAP- 5, CDAP-10, and the control DAP.

After equilibrating the thin hydrogel sheets with each of the metal ion for the desired times (see

Experimental Section), the used adsorbents were separated and washed with acid solution after which the adsorbed metal ions desorbed back into solution due to high competition from concentrated protons to the active chelating sites. Under experimental conditions, all the three thin hydrogel sheets of chitosan (1, 5, and 10 wt %) show a trend in maximum adsorptions as Pb2+>

Cu2+> Cd2+> Hg2+ given the units in ppm of the metal adsorbed from solution per gram of the dry hydrogel. All the experiments were carried out at an initial pH 7 where the cations Pb2+,35 Cu2+,35 and Cd2+ 36 are mainly found as free ions in aqueous solution and can be adsorbed onto chitosan

2+ 35 via chelation. However, at neutral pH, Hg mainly form hydroxy-complex Hg(OH)2 and hence

adsorption % as Hg2+ can be low as resulted above. Overall, the cDAP-10 sheets show maximum adsorptions of the four heavy metals from solution, in the range of ~ 120-105 ppm/g of the dry hydrogel which is 18 times greater than the DAP under the experimental conditions used here.

4.3.3 Metal Adsorption Tests of DAP and cDAP Cups

The main goal in this work was to test the heavy metal chelation ability of the resulting 3D printed and UV cured hydrogels and evaluate the purity of treated water against the U.S. Environmental

Protection Agency (EPA) recommended action levels for the selected heavy metal ions. An ‘action level’37 is defined as the critical metal concentration to enforce laws, regulations on public supplies and improve toxicant removal methods to achieve safe water for human consumption. Here we have equilibrated the hydrogel cups with initial heavy metal ion concentrations representing moderately polluted water bodies resulting from public supplies, industrial effluents, and irrigation etc.38,39 Moreover, these 3D printed hydrogel cups structures represent similar customized objects that can be 3D printed using the technology for water filtration purposes. According to the results as in Figure 4.4, cDAP-1 hydrogel cups showed significantly high metal ion adsorptions with respect to the control DAP, bringing down the toxic initial concentrations below the EPA action levels for all the four heavy metal ions after the designated contact times. In fact, cDAP-1 was able to remove ~ 94% of Pb2+ within 30 min passing down the EPA level, an efficiency ~6 times greater than the control DAP (Figure 4.4 a).

4.3.4 Metal Leaching Tests of DAP and cDAP Cups

As given in the results (Figure 4.5), cDAP-1 cups also showed significantly low leaching (%) for all the four metal ions than the control DAP after a contact time of 10 h.

Figure 4.5. Heavy metal leaching (%) for cDAP-1 and the control DAP after a contact time 10 h.

Overall, the remarkable metal adsorption capability, low % leaching, and high mechanical properties ensure a huge promise of using these hydrogel cups as novel, customized 3D printed objects for water filtration, especially for domestic use to remove Pb2+ in tap water.

4.3.5 Pb2+ Adsorption Kinetics vs. Hydrogel Surface Area

In order to further evaluate the Pb2+ adsorption kinetics with surface area, solid cubes and shapes with internal structures were used. Figure 4.6 a indicates the remaining Pb2+ concentrations in solution after equilibrating 10 min with the 3D printed cubes and shapes A, B with internal structure.

According to the results, Pb2+ removal efficiency has been improved with increasing surface area for the chitosan 1 (wt %) hydrogels than that of the control. The cDAP-1-shape-B with the highest estimated surface area (~1791 mm2) show ~ 74% Pb2+ removal after 10 min, ~ 4 times higher than that of the control DAP counterpart. Pb2+ usually leaches from plumbing systems of public supplies37 to water bodies is the most common toxicant out of the four tested metals in drinking water. Therefore, the Pb2+ removal efficiency achieved here is remarkable considering the toxicity and the abundance of Pb2+.

Moreover these structures confirm the ability to print objects with complex internal geometries via direct-write 3D printing using these hydrogel materials which is unattainable with other competing molding techniques like injection molding or blow molding.

4.3.6 Reusability Tests

The used adsorbents of cDAP-1-shape B and the control DAP-shape B of the above kinetics experiments were then subjected to reusability tests for 5 regeneration cycles. Only the results of cDAP-1-shape B are shown (Figure 4.7 a) since the Pb2+ adsorption of the control DAP-shape B was negligible with respect to the cDAP-1-shape B.

As the results indicate, ~ 98% recovery of the original Pb2+ removal efficiency could be achieved even after 5 cycles. Also Figure 7b illustrates how the cDAP-1-shape-B retains its internal structure after undergoing several surface washes during the regeneration cycles vs. the control. Overall, these results highlight the inherent sustainability of chitosan-DAP composite hydrogels as adsorbent materials, which is imparted mainly by 3D printing.

4.4 Conclusions

In conclusion, for the first time we have introduced recyclable 3D printed filtration designs originated from a cheap, abundant biopolymer for the remediation of toxic heavy metal ions from contaminated water. Using this biopolymer, we were able to successfully prepare shear-thinning hydrogels which were then developed into more mechanically robust 3D printed thin hydrogel sheets and cylindrical cup structures for efficient heavy metal adsorption. Further, objects with internal structures unattainable via conventional molding techniques were 3D printed using these hydrogels which demonstrated improved Pb2+ adsorption efficiencies and reusability. Moreover,

this novel, cost effective, robust, and efficient approach to remediate health and environmental hazards lays foundation for a new direction towards fabricating cheap and safe water filtration devices in future.

4.5 Experimental Section

Materials: All chemicals were used as received without further purification. Chitosan powder

(85% deacetylated, viscosity average molecular weight ~ 100-300 kDa), acryloyl chloride, and sodium bicarbonate were purchased from Alfa Aesar. Pluronic F127 (average molecular weight =

12.5 kDa) was supplied by Spectrum Chemical Corporation, New Brunswick, NJ. Irgacure-754 was purchased from BASF Corporation, Kaisten, Switzerland. ICP-MS standards for Cu, Cd, Pb, and Hg (1000 ppm in nitric acid), Cu(II) nitrate.trihydrate, Pb(II) nitrate, and Hg(II) nitrate.monohydrate were purchased from Sigma-Aldrich. Cd(II) nitrate.tetrahydrate was supplied by Fisher Scientific. Acetic acid/glacial, dichloromethane (DCM), and anhydrous diethyl ether were supplied by Fisher Chemical. Triethyl amine (TEA) was purchased from Acros Organics.

Synthesis of pluronic diacrylate (F127-DA)-(DAP):40 Pluronic F127 (1 mmol) was dissolved in ice-cold anhydrous DCM (50 mL). To this solution in ice-bath, TEA (6 mmol) was added followed by acryloyl chloride (6 mmol) dropwise. The reaction was allowed to stir for 24 h in ice- bath. The reaction mixture was then mixed with DCM and washed with de-ionized water, saturated sodium bicarbonate solution, and again de-ionized water. The final organic layer was collected and solvent was removed in vacuum to obtain a crude solid. This was dissolved in a minimal amount of DCM and anhydrous diethyl ether was added in an ice-bath under vigorous stirring to precipitate the final product as a white solid. This was collected via vacuum filtration and dried

1 for 24 h under vacuum. Yield (add mass) 80%. H NMR (Figure C.1) (600 MHz, DMSO-d6, δ):

6.34 (d, J = 12 Hz, H; CH2), 6.2 (dd, J = 9, 12 Hz, H; CH), 5.97 (d, J = 9 Hz, H; CH2).

Synthesis of hydrogels: Hydrogel compositions containing 1, 5, and 10 wt% chitosan with 9 wt% DAP (cDAP-1, cDAP-5, CDAP-10 respectively) were prepared as follows. Chitosan powder was added to an acetic acid solution (0.5 v/v %) in ultrapure water. The mixture was heated at 50

°C for 15 min to gain a homogenous solution. To this, DAP was added. After adding Irgacure-754

(1 µL) the mixture was gently stirred at 4 °C for 5 h to obtain a homogenous solution. This was then incubated at 37 °C for 5 h to promote gelation. The hydrogel containing 1 wt% chitosan with

25 wt% DAP (cDAP-1) was also prepared in a similar manner. The control, DAP (26 wt %) was prepared as follows. DAP powder as synthesized was added to acetic acid solution (0.5 v/v %) in ultrapure water. Irgacure-754 (1 µL) was added and the mixture was gently stirred at 4 °C for 5 h to obtain a homogenous solution. This was then incubated at 37 °C for 5 h to promote gelation.

3D printing: For 3D printing of thin hydrogel sheets, the three hydrogels cDAP-1, cDAP-5, and cDAP-10 were used along with the control DAP. These were kept again at 4 °C for 1 h to obtain the solution state. Each was then separately poured into a plastic loading syringe (60 mL) of a direct-write 3D printer (Printrbot), equipped with a 0.9 mm diameter needle. Gelation state was reobtained by incubating at 37 °C for 1 h. After confirming a homogeneous hydrogel with no trapped air bubbles, the syringe was capped, loaded into the 3D printer, and the hydrogels were

3D printed into thin sheets having the dimensions (length-l × width-w × height-h) 15.0 mm × 15.0 mm × 1.0 mm with four inner circles each of 4.0 mm diameter (Figure 2a). For 3D printing of hydrogel cups, cDAP-1 and the control DAP were used. 3D cylindrical cups with dimensions

(outer diameter-d × height-h × wall thickness-t) 25.0 mm × 20.0 mm × 2.0 mm and a base height

of 1.5 mm were 3D printed as above (Figure 2b). Solid cubes and shapes with internal structure of same weight but varying surface area were also 3D printed using the cDAP-1 and the control DAP hydrogels. Solid cubes with dimensions (l × w × h) 14 mm × 14 mm × 14 mm, estimated surface area 1176 mm2 ; complex shape-A having dimensions (l × w × h) 25 mm × 10 mm × 10 mm, cylindrical pore diameter (d) 5 mm, estimated surface area 1554 mm2; and complex shape-B with dimensions (l × w × h) 17.5 mm × 10.1 mm × 17.5 mm, cylindrical pore diameter (d) 5 mm, estimated surface area 1791 mm2 were 3D printed (Figure 2c). The detailed printing conditions for all the structures are given in Appendix C (Table C.1-C.3).

UV curing: Each of the above 3D printed structures were then subjected to UV curing at long wave of 365 nm in a UV oven for 1 h.

Evaluation of Hydrogel Mechanical Properties: Compression tests were carried out for the 3D printed and UV cured hydrogel cups, using a universal testing machine (Instron 5848 MicroTester) at a compression rate of 5 mm/min and a temperature of 25 °C (Figure C.2). Ultimate strength, toughness, and Young’s modulus were determined (Table C.4-C.5).

Maximum Metal Adsorption Tests of DAP and cDAP Sheets: Before performing adsorption tests, the 3D printed and UV cured thin hydrogel sheets were immersed separately in ultrapure water until a constant weight, in order to make sure no more water adsorption occurs during metal adsorptions.[19]. 3000 ppb stock solutions (5 mL) were prepared using the nitrate salts of the heavy metals in ultrapure water. Plastic vials (20 mL) were used as containers for all the experiments.

The thin hydrogel sheets were equilibrated in these solutions with shaking at a rate of 60 rpm, at pH 7.0, on an orbital shaker for 1, 3, 5, 7, and 9 h at room temperature. Triplicates were used for each time interval for each heavy metal. After each time interval, the adsorbent hydrogels were

each separated from the solutions by filtration and the adsorbents were shaken with a nitric acid (1

M) solution (5 mL) for 24 h to release all adsorbed metals from hydrogel sheet surfaces into solution. Each of these solutions were diluted × 10 with ultrapure water. Then the metal concentrations were analyzed via an Agilent 7900 Inductively Coupled Plasma-Mass Spectrometer

(ICP-MS) under Helium mode. Adsorptions were calculated as percentages (Figure C.3). Based on these values, maximum adsorption of each heavy metal was calculated as ppm of heavy metal adsorbed from solution per gram of dry hydrogel.[19]

Metal Adsorption Tests of DAP and cDAP Cups: Before performing adsorption tests, the 3D printed and UV cured hydrogel cups were also immersed separately in ultrapure water until a constant weight, in order to make sure no more water adsorption occurs during metal adsorptions.

For detection of Cu2+ levels, the cups were equilibrated separately in a 1500 ppb stock solution (5 mL) prepared using the Cu2+ nitrate salt in ultrapure water with shaking as before, at pH 7.0, at room temperature for 30, 60, 90, and 120 min. After each time period, the adsorbent hydrogel cups were separated from the solutions and the remaining heavy metal concentrations in solutions were analyzed via ICP-MS under Helium mode using 2% nitric acid as the matrix. Triplicates were used. To detect Cd2+ and Pb2+ levels, the same procedure was carried out but using 30 ppb stock solutions, Hg2+ levels with a 10 ppb stock solution, prepared using the corresponding nitrate salt.

Metal Leaching Tests of DAP and cDAP Cups: Here, the adsorbent cups of the above section were used. These cups were then equilibrated in ultrapure water (5 mL) separately and were shaken as before for 10 h. After removing the solutions from cups, the remaining heavy metal concentrations in solutions were analyzed as before via ICP-MS using 2% nitric acid as the matrix.

Leached amounts of each heavy metal from each hydrogel composition was calculated as a percentage.

Pb2+ Adsorption Kinetics vs. Hydrogel Surface Area: First, the 3D printed and UV cured solid cubes and shapes A, B with internal structure were immersed separately in ultrapure water until a constant weight, in order to make sure no more water adsorption occurs during metal adsorptions.

These were then equilibrated separately in 30 ppb stock solutions (5 mL) prepared using the Pb2+ nitrate salt in ultrapure water with shaking as before for a contact time of 10 min. Remaining Pb2+ levels in solutions were detected as before.

Reusability tests: These tests were carried out with the used adsorbents cDAP-1-shape B and the control DAP-shape B of the above kinetic experiments. These were first washed with HCl

(1M) solution followed by regenerating the adsorbents with NaOH (0.1 M) solution wash and a subsequent wash with ultrapure water until pH 7.0.[19] The refreshed adsorbents were subjected to

Pb2+ adsorption again following the same procedure in the kinetic experiment and this whole procedure was repeated for 5 regeneration cycles.

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(29) Basu, A.; Saha, A.; Goodman, C.; Shafranek, R. T.; Nelson, A. Catalytically Initiated Gel-in- Gel Printing of Composite Hydrogels. ACS Appl. Mater. Interfaces 2017, 9, 40898-40904.

(30) Lin, Q.; Hou, X.; Ke, C. Ring Shuttling Controls Macroscopic Motion in a Three-Dimensional Printed Polyrotaxane Monolith. Angew. Chem. Int. Ed. 2017, 56, 4452-4457.

(31) Appuhamillage, G. A.; Reagan, J. C.; Khorsandi, S.; Davidson, J. R.; Voit, W.; Smaldone, R. A. 3D Printed Remendable Polylactic Acid Blends with Uniform Mechanical Strength Enabled by a Dynamic Diels-Alder Reaction. Polym. Chem. 2017, 8, 2087-2092.

(32) Davidson, J. R.; Appuhamillage, G. A.; Thompson, C. M.; Voit, W.; Smaldone, R. A. Design Paradigm Utilizing Reversible Diels-Alder Reactions to Enhance the Mechanical Properties of 3D Printed Materials. ACS Appl. Mater. Interfaces 2016, 8, 16961-16966.

(33) Yang, K.; Grant, J. C.; Lamey, P.; Joshi-Imre, A.; Lund, B. R.; Smaldone, R. A.; Voit, W. Diels–Alder Reversible Thermoset 3D Printing: Isotropic Thermoset Polymers via Fused Filament Fabrication. Adv. Funct. Mater. 2017, 27, 1700318-n/a.

(34) Xu, W.; Pranovich, A.; Uppstu, P.; Wang, X.; Kronlund, D.; Hemming, J.; Öblom, H.; Moritz, N.; Preis, M.; Sandler, N.; Willför, S.; Xu, C. Novel Biorenewable Composite of Wood Polysaccharide and Polylactic acid for Three Dimensional Printing. Carbohydr. Polym. 2018, 187, 51-58.

(35) Benavente, M.; Moreno, L.; Martinez, J. Sorption of Heavy Metals from Gold Mining Wastewater using Chitosan. J. Taiwan. Inst. Chem. Eng. 2011, 42, 976-988.

(36) Srivastava, P.; Singh, B.; Angove, M. J. In Competitive adsorption of cadmium onto kaolinite as affected by pH, 3rd Australian New Zealand Soil Conferences; University of Sydney, Australia, December, 2004.

(37) O. Tarragó, M. J. Brown Case Studies in Environmental Medicine (CSEM), Lead Toxicity. https://www.atsdr.cdc.gov/csem/lead/docs/CSEM-Lead_toxicity_508.pdf (accessed January, 2018).

(38) Zhang, Z.; Wang, J.; Ali, A.; Delaune, R. Heavy Metal Distribution and Water Quality Characterization of Water Bodies in Louisianas Lake Pontchartrain Basin, USA. Environ. Monit. Assess. 2016, 188, 628.

(39) Das, T.; Haldar, D. Mopping up the Oil, Metal, and Fluoride Ions from Water. ACS Omega 2017, 2, 6878-6887.

(40) Yoo, H. S. Photo-Cross-Linkable and Thermo-Responsive Hydrogels Containing Chitosan and Pluronic for Sustained release of Human Growth Hormone (HGH). J. Biomater. Sci. Polym. Ed. 2007, 18, 1429-1441.

APPENDIX A

SUPPORTING INFORMATION FOR CHAPTER 2

A.1 Synthetic Procedures

Monomer 2M – Monomer 2M was prepared according to a previously reported protocol.1 Maleic anhydride (4.84 g, 49.2 mmol) was dissolved in acetone (12 mL). To this solution, D-400 (10.32 g, 24 mmol) diluted with acetone (10 mL), was added dropwise in an ice bath with stirring. During the addition, the temperature was maintained below 35 °C. The ice bath was removed after 30 min and the reaction mixture was heated at 40 °C for 2 h after which the intermediate D-400-maleamic acid was obtained. This solution was cooled to room temperature and triethylamine (2.5 g, 24 mmol) was added dropwise with stirring. The reaction mixture was then heated to 55 °C and acetic anhydride (11.36 mL, 120 mmol) was added dropwise. The final reaction mixture was maintained at 55 °C for 24 h. The organic layer was extracted into methylene chloride (3×50 mL) and washed several times with water and aqueous 5% sodium bicarbonate. The organic layer was dried over anhydrous magnesium sulfate and the solvent was removed by rotary evaporation. 2M was dried under dynamic vacuum to 12.0 g (85%). 1H NMR spectra matched those reported previously.

Monomer 2F – p-Phenylenediamine (8.0 g, 74.0 mmol) and 2-furaldehyde (12.3 mL, 148.3 mmol) were dissolved in methanol (70 mL) and the solution was stirred at rt overnight. NaBH4 (8.4 g,

222.0 mmol) was added to the mixture in a water-ice bath while stirring. The solution was again stirred at room temperature for 24 h. The resulting brownish-yellow precipitate was collected by filtration and washed with water (50 mL x 2). 2F was obtained and used without further

1 purification 18.0 g (91%). H NMR: (500 MHz, CDCl3) δ 7.38 (d, J= 1.0 Hz, 2H), 6.68 (s, 4H),

6.33 (dd, J= 3.0 Hz, 1.0 Hz, 2H), 6.26 (d, J= 3.0 Hz, 2H), 4.29 (s, 4H). 13C NMR: (400 MHz,

CDCl3) δ 153.3, 141.8, 140.5, 115.1, 110.3, 106.8, 42.7. HR MS: Predicted 269.1285, Found

269.1284.

Figure A.1. 1H NMR spectrum of 2F.

Synthesis of 3F-3F was prepared according to a previously reported procedure.2 Tris(2- aminoethyl)amine (0.5 mL, 3.34 mmol) and furfural (0.83 mL, 10.01 mmol) were mixed with methanol (25 mL) and stirred overnight at room temperature. NaBH4 was added in excess (505 mg, 13.4 mmol) in ice with stirring for 2h. HCl (1M, 10 mL) was added to the solution to react with any excess of NaBH4. The product was extracted into methylene chloride (2×20 mL). The solution was dried over anhydrous MgSO4 and the solvent was removed under vacuum. The final product 1.1 g (84%) was obtained after further drying on vacuum. 1H NMR spectra matched those reported previously.

Preparation of Polymer Blends MA – 2M (14.9 g, 25.2 mmol), 2F (5.6 g, 21.0 mmol), and 3F

(1.082 g, 2.8 mmol), were mixed in ethyl acetate (5 mL) and the mixture was heated at 75 °C for

24 h. Then the solvent was removed at 60 °C in a vacuum oven (< 1 mmHg) overnight. The resulting polymer film was used for the formation of the remendable PLA blends.

A.2 Thermal Analysis of Polymers

120

100

80 60

40 (%) Weight 20

0 30 230 430 630 830 Temperature (°C)

A.3 Mechanical Testing of Polymers – Quantitative Data

Table A.1. Ultimate strength values for each polymer blend and print pattern.

Print PLA 10% Remendable 25% Remendable Direction (MPa) PLA (MPa) PLA (MPa) X 39.98±1.63 40.29±2.40 39.68±1.39 Y 26.63±0.78 33.84±3.52 33.73±0.87 Z 10.30±0.87 19.38±0.53 23.64±1.81

Table A.2. Toughness values for each polymer blend and print pattern.

10% Remendable 25% Remendable PLA Print PLA PLA (MJ/m3) Direction (MJ/m3) (MJ/m3) X 0.49±0.06 0.70±0.15 0.50±0.07 Y 0.23±0.03 0.39±0.09 0.48±0.05 Z 0.05±0.02 0.18±0.08 0.28±0.02

References

(1) Zhong, Y.; Wang, X.; Zheng, Z.; Du, P. Polyether–Maleimide-Based Crosslinked Self- Healing Polyurethane with Diels–Alder Bonds. J. Appl. Polym. Sci. 2015, 132, n/a-n/a.

(2) Gandini, A.; Coelho, D.; Gomes, M.; Reis, B.; Silvestre, A. Materials from Renewable Resources Based on Furan Monomers and Furan Chemistry: Work in Progress. J. Mater. Chem. 2009, 19, 8656-8664

APPENDIX B

SUPPORTING INFORMATION FOR CHAPTER 3

B.1 Synthetic Procedures

Synthesis of Mending Agent Polymers. In order to synthesize the MA-PLA blends with 0, 16.7,

66.7, 83.3, and 100% cross-link density, the monomers 2M, 2F, and 3F were reacted together according to the molar ratios given in shown table. Ethyl acetate (5 mL) was used as the solvent and the mixtures were heated at 75 °C for 24 h. Then the solvent was removed at 60 °C in a vacuum oven (< 1 mmHg) for 24 h. The crosslink density is calculated based on the contribution of the

furan groups from monomer 3F to form the fmDA adducts (cross-links), to the total available furan groups in each MA-PLA blend.

Preparation of MA/PLA blends. Each of these synthesized MA polymers were then mixed with

PLA in 1,4-dioxane at 80 °C to obtain a series of 25 wt% MA/PLA blends with 0, 16.7, 66.7, 83.3, and 100% cross-link density. The solvent was removed under vacuum at 60 °C for 24 h.

B.2 Thermal Analysis of Polymer Blends

Figure B.1. Differential scanning calorimetry analysis of MA-Polymer (16.7% cross-link density), MA-PLA blends, and PLA followed by their Tg values tabulated.

120

100

Weight (%) Weight 40

0 30 230 430 630 830 Temperature (°C)

Figure B.2. Thermogravimetric Analysis of PLA and MA-PLA blends. PLA (black), MA-PLA- 100 (blue), MA-PLA-83 (brown), MA-PLA-67 (purple), MA-PLA-17 (green), and MA-PLA-0 (red).

B.3 Mechanical Testing of Polymer Blends

Table B.1. Ultimate strength (MPa) values for PLA and MA-PLA blends along each print direction. MA-PLA blends

Print PLA MA-PLA-0 MA-PLA-17 MA-PLA-67 MA-PLA-83 MA-PLA-100 Direction (MPa) (MPa) (MPa) (MPa) (MPa) (MPa) X 41.41 ± 1.18 36.36 ± 3.10 39.73 ± 1.13 35.07 ± 0.52 36.01 ± 0.17 47.61 ± 1.62 Y 29.94 ± 3.35 28.83 ± 0.07 33.74 ± 0.79 34.15 ± 1.92 32.72 ± 1.57 48.04 ± 4.82 Z 11.44 ± 1.26 17.59 ± 1.68 23.89 ± 1.07 23.46 ± 1.44 27.77 ± 1.26 44.62 ± 1.24

Table B.2. Toughness (MJ/m3) values for PLA and MA-PLA blends along each print direction.

MA-PLA blends

Print PLA MA-PLA-0 MA-PLA-17 MA-PLA-67 MA-PLA-83 MA-PLA-100 Direction (MJ/m3) (MJ/m3) (MJ/m3) (MJ/m3) (MJ/m3) (MJ/m3) X 0.43 ± 0.02 0.39 ± 0.04 0.44 ± 0.04 0.43 ± 0.06 0.40 ±0.07 0.60 ± 0.03 Y 0.27 ± 0.02 0.29 ± 0.02 0.35 ± 0.03 0.40 ± 0.03 0.39 ±0.03 0.69 ± 0.06 Z 0.04 ± 0.02 0.13 ± 0.02 0.19 ± 0.03 0.21 ± 0.02 0.25 ±0.02 0.50 ± 0.03

Table B.3. Comparison of the %increase of ultimate strength along Z print axis for each of the MA-PLA blend vs. PLA.

%Increase of Ultimate Strength Material along Z axis vs. PLA MA-PLA-0 53.76 MA-PLA-17 108.83 MA-PLA-67 105.07 MA-PLA-83 142.74 MA-PLA-100 290.03

Table B.4. Comparison of the %increase of Toughness along Z print axis for each of the MA- PLA blend vs. PLA.

%Increase of Toughness Material along Z axis vs. PLA MA-PLA-0 225 MA-PLA-17 375 MA-PLA-67 425 MA-PLA-83 525 MA-PLA-100 1150

Figure B.3. Ultimate Strength and toughness ratios of print patterns Z to Y (purple) and Z to X (orange) for PLA and MA-PLA blends with varying crosslink density.

Table B.5. Comparison of ultimate strength ratios along Z print direction to Y and Z print direction to X for PLA and MA-PLA blends.

Material Ultimate Strength Ultimate Strength

ratio of Z to Y ratio of Z to X

PLA 0.39 ± 0.06 0.28 ± 0.02

MA-PLA-0 0.61 ± 0.06 0.48 ± 0.06

MA-PLA-17 0.71 ± 0.05 0.60 ± 0.01

MA-PLA-67 0.69 ± 0.07 0.67 ± 0.05

MA-PLA-83 0.85 ± 0.07 0.77 ± 0.04

MA-PLA-100 0.93 ± 0.06 0.94 ± 0.05

Table B.6. Comparison of Toughness ratios along Z print direction to Y and Z print direction to X for PLA and MA-PLA blends.

Toughness ratio of Z Toughness ratio of Z Material to Y to X PLA 0.16 ± 0.05 0.09 ± 0.04 MA-PLA-0 0.44 ± 0.07 0.33 ± 0.07 MA-PLA-17 0.54 ± 0.04 0.43 ± 0.03 MA-PLA-67 0.53 ± 0.06 0.49 ± 0.06 MA-PLA-83 0.63 ± 0.03 0.63 ± 0.05 MA-PLA-100 0.73 ± 0.03 0.83 ± 0.04

Figure B.4. Stress vs. strain curves of PLA and MA-PLA blends as dogbones for each print pattern X (blue), Y (red), and Z (green).

B.4 Thermal Stability Tests

Figure B.5. Ultimate strength (A) and toughness (B) values for PLA and MA-PLA blends heated under 65 °C/15 min conditions. Related stress vs. strain curves (C).

Table B.7. Ultimate Strength (MPa) values for PLA and MA-PLA blends along each print direction after heating under 65 °C/15 min conditions.

MA-PLA blends

MA-PLA-0 MA-PLA-17 MA-PLA-67 M A-PLA-83 MA-PLA-100 Print PLA (MPa) (MPa) (MPa) (MPa) (MPa) Direction (MPa)

X 38.24 ± 1.80 42.41 ± 1.59 42.55 ± 1.13 34.84 ± 3.13 35.98 ± 2.70 53.86 ± 3.51 Y 23.81 ± 0.79 36.01 ± 3.37 33.90 ± 0.79 34.10 ± 1.87 34.77 ± 0.92 52.72 ± 1.10 Z 7.42 ± 0.45 19.76 ± 1.12 29.89 ± 1.07 25.44 ± 1.07 33.86 ± 1.56 52.07 ± 1.29

Table B.8. Toughness values (MJ/m3) for PLA and MA-PLA blends along each print direction after heating under 65 °C/15 min conditions.

MA-PLA blends

Print PLA MA-PLA-0 MA-PLA-17 MA-PLA-67 MA-PLA-83 MA-PLA-100 3 3 3 3 3 3 Direction (MJ/m ) (MJ/m ) (MJ/m ) (MJ/m ) (MJ/m ) (MJ/m )

X 0.42 ± 0.03 0.50 ± 0.04 0.52 ± 0.05 0.47 ± 0.07 0.43 ± 0.06 0.62 ± 0.05 Y 0.15 ± 0.02 0.37 ± 0.07 0.37 ± 0.08 0.39 ± 0.07 0.39 ± 0.03 0.73 ± 0.02 Z 0.02 ± 0.01 0.25 ± 0.05 0.31 ± 0.03 0.31 ± 0.07 0.40 ± 0.04 0.57 ± 0.02

APPENDIX C

SUPPORTING INFORMATION FOR CHAPTER 4

C.1 1H NMR- diacrylated pluronic F-127

Figure C.1. 1H NMR of diacrylated pluronic F-1271 showing acrylation protons via a,b, and c.

C.2 3D Printing Parameters

Thin Hydrogel Sheets

cDAP-1 cDAP-5 cDAP-10 Control

Nozzle size (mm) 0.9 0.9 0.9 0.9

Head temperature (°C) 37 36 35 30

Bed temperature (°C) 40 40 40 35

Layer height (mm) 0.5 0.5 0.5 0.5

Shell thickness (mm) 1.8 1.8 1.8 1.8

Initial layer thickness (mm) 0.3 0.3 0.3 0.3

Initial layer line width (%) 100 100 100 100

Print speed (mm/s) 2 2 2 2

Travel speed (mm/s) 25 25 25 25

Bottom layer speed (mm/s) 10 10 10 10

Hydrogel Cups

Table C.2. Direct-write 3D printing parameters used to print cylindrical cups using cDAP-1 and the control DAP hydrogels with dimensions (outer diameter-d × height-h × wall thickness-t) 25.0 mm × 20.0 mm × 2.0 mm and a base height of 1.5 mm. Parameter Hydrogel Cup Composition

cDAP-1 Control

Nozzle size (mm) 0.9 0.9

Head temperature (°C) 25 25

Bed temperature (°C) 27 27

Layer height (mm) 0.5 0.5

Shell thickness (mm) 1.8 1.8

Initial layer thickness (mm) 0.3 0.3

Initial layer line width (%) 100 100

Print speed (mm/s) 2 2

Travel speed (mm/s) 25 25

Bottom layer speed (mm/s) 10 10

Solid cubes and shapes with internal structure

cDAP-1 Control

Nozzle size (mm) 0.9 0.9

Head temperature (°C) 25 25

Bed temperature (°C) 28 28

Layer height (mm) 0.6 0.6

Shell thickness (mm) 1.8 1.8

Initial layer thickness (mm) 0.3 0.3

Initial layer line width (%) 100 100

Print speed (mm/s) 5 5

Travel speed (mm/s) 25 25

Bottom layer speed (mm/s) 5 5

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C.3 Evaluation of Hydrogel Mechanical Properties

Table C.4. Ultimate strength (kPa) values for cDAP-1 and the control DAP.

Material Ultimate Strength (kPa)

cDAP-1 23.72 ± 3.09

Control 11.86 ± 2.18

Table C.5. Toughness (kJ/m3) values for cDAP-1 and the control DAP.

Material Toughness (kJ/m3)

cDAP-1 207.32 ± 19.58

Control 107.19 ± 23.21

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C.4 Maximum Metal Adsorption Tests of DAP and cDAP Sheets

Table C.6. Resulted maximum adsorptions from solution for the four heavy metal ions Pb2+, Cu2+, Cd2+, and Hg2+ for the three hydrogel membrane compositions and the control DAP.

Maximum heavy metal adsorption from solution Material (ppm/g of dry hydrogel)

Pb2+ Cu2+ Cd2+ Hg2+

cDAP-1 86.34 ± 1.46 78.43 ± 1.23 76.46 ± 0.52 70.45 ± 1.19

cDAP-5 106.96 ± 1.57 95.32 ± 1.14 90.12 ± 1.50 87.85 ± 0.97

cDAP-10 122.20 ± 0.93 115.50 ± 0.85 109.40 ± 0.84 104.19 ± 0.88

Control 8.12 ± 1.03 5.87 ± 1.62 6.21 ± 1.19 5.28 ± 0.57

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C.5 Metal Adsorption Tests of DAP and cDAP Cups

Comparison with EPA Levels

Table C.7. Remaining Pb2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 31.28 ppb.

Remaining Pb2+ concentration (ppb) Material 30 min 60 min 90 min 120 min

cDAP-1 1.70 ± 0.18 - - -

Control 26.47 ± 0.69 23.76 ± 0.41 21.48 ± 1.12 20.33 ± 0.92

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Table C.8. Remaining Cu2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 1522.28 ppb.

Remaining Cu2+ concentration (ppb) Material 30 min 60 min 90 min 120 min

cDAP-1 1159.80 ± 3.11 804.87 ± 5.61 440.89 ± 2.98 99.70 ± 4.08

Control 1507.08 ± 17.31 1495.21 ± 15.91 1485.91 ± 13.73 1469.41 ± 10.83

Table C.9. Remaining Cd2+ concentration in solution for cDAP-1 and the control DAP after 30 60, 90, and 120 min of contact time starting with an initial concentration of 30.56 ppb.

Remaining Cd2+ concentration (ppb) Material 30 min 60 min 90 min 120 min

cDAP-1 3.63 ± 0.27 - - -

Control 25.80 ± 1.07 23.15 ± 0.91 21.33 ± 0.48 20.14 ± 0.59

Table C.10. Remaining Hg2+ concentration in solution for cDAP-1 and the control DAP after 30, 60, 90, and 120 min of contact time starting with an initial concentration of 10.26 ppb.

Remaining Hg2+ concentration (ppb) Material 30 min 60 min 90 min 120 min

cDAP-1 1.39 ± 0.16 - - -

Control 8.87 ± 0.42 8.08 ± 0.71 7.58 ± 0.81 7.34 ± 0.52

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C.6 Metal Leaching Tests of DAP and cDAP Cups

Table C.11. Heavy metal leaching (%) of cDAP-1 and the control DAP after 10 h.

Heavy metal leaching (%) after 10 h Material Pb2+ Cu2+ Cd2+ Hg2+

cDAP-1 2.19 ± 0.12 2.76 ± 0.39 3.79 ± 0.24 5.85 ± 0.36 Control 31.95 ± 3.28 23.34 ± 1.30 33.47 ± 4.09 36.83 ± 3.53

C.7 Adsorption Kinetics vs. Hydrogel Surface Area

Table C.12. Remaining Pb2+ concentration in solution for each of the solid cubes and shapes with internal structure after 10 min contact time starting with an initial concentration of 31.28 ppb.

Estimated Remaining Pb2+ concentration Material surface area (mm2) (ppb)

cDAP-1-Cube 26.07 ± 0.75 1176 Control-Cube 30.71 ± 0.70

cDAP-1-Shape A 17.10 ± 2.04 1554 Control-Shape A 28.13 ± 0.27

cDAP-1-Shape B 8.29 ± 0.70 1791 Control-Shape B 25.60 1.34

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C.8 Reusability Tests

Regeneration Pb2+ removal efficiency Material cycle (%)

cDAP-1-Shape B 73.50 ± 2.23 1 Control-Shape B 18.15 ± 4.27

cDAP-1-Shape B 73.68 ± 0.70 2 Control-Shape B 13.62 ± 0.78

cDAP-1-Shape B 73.56 ± 0.72 3 Control-Shape B 12.95 ± 0.71

cDAP-1-Shape B 73.13 ± 0.68 4 Control-Shape B 10.90 ± 0.73

cDAP-1-Shape B 72.32 ± 0.64 5 Control-Shape B 9.12 ± 1.26

References

(1) Yoo, H. S. Photo-Cross-Linkable and Thermo-Responsive Hydrogels Containing Chitosan and Pluronic for Sustained release of Human Growth Hormone (HGH). J. Biomater. Sci. Polym. Ed. 2007, 18, 1429-1441.

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BIOGRAPHICAL SKETCH

Gayan Adikari Appuhamillage was born in Colombo, Sri Lanka in 1984. He graduated from

Ananda College, Colombo in 2004. He started his Bachelor of Science (BS) degree at the

University of Kelaniya and was selected for specialization in Chemistry in 2007, graduating with a BS in Chemistry in 2009. He was then appointed as a graduate teaching assistant (GTA) in the

Department of Chemistry, University of Kelaniya. Afterwards he was admitted to the department of Chemistry at Sam Houston State University (SHSU), Texas in 2011 in the Chemistry MS program. Working as a GTA, he earned his MS in Chemistry in 2013 at SHSU. Then in 2013, he was admitted to The University of Texas at Dallas for the Chemistry PhD program and later joined

Dr. Ronald A. Smaldone’s research group.

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CURRICULUM VITAE

Gayan Adikari Appuhamillage Department of Chemistry and Biochemistry The University of Texas at Dallas 800 W. Campbell Rd., BE 26, Richardson, TX 75080 [email protected] linkedin.com/in/gayan-adikari-appuhamillage-b1ba20152 EDUCATION

Ph.D., Chemistry, The University of Texas at Dallas, Richardson, TX 2018 Thesis title: New 3D Printable Polymeric Materials for Fused Filament Fabrication (FFF). Advisor: Prof. Ronald A. Smaldone

M.S., Chemistry, Sam Houston State University, Huntsville, TX 2013 Thesis title: Resistance of Escherichia coli towards toxic metalloidal oxyanions and study of gshA, gshB genes in their glutathione biosynthetic pathway, and an effort to produce an Escherichia coli strain with a higher glutathione content. Advisor: Prof. Thomas G. Chasteen

B.S., Chemistry, University of Kelaniya, Kelaniya, Sri Lanka 2009 Thesis title: Study of heavy metal adsorption capacities of clays and development of recovery methods to effectively remove and to convert the adsorbed metals into reusable forms. Advisor: Dr. Russel C. L. de Silva

HONORS, AWARDS, AND ACHIEVEMENTS

Ph.D. Research Travel Grant Award 2017 Department of Chemistry, The University of Texas at Dallas Ph.D. Research Travel Grant Award 2016 Department of Chemistry, The University of Texas at Dallas Presentation Award in Graduate Research Exchange 2013 Graduate Studies and College of Education, Sam Houston State University Special Graduate Scholarship Award 2012-2013 College of Sciences, Sam Houston State University Graduate Student Fellowship 2012-2013 Department of Chemistry, Sam Houston State University

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PATENTS AND RESEARCH PUBLICATIONS

Patents: Appuhamillage, Gayan A.; Berry, Danielle R.; Benjamin, Candace E.; Luzuriaga, Michael A.; Reagan, John C.; Gassensmith, Jeremiah J.; Smaldone, Ronald A. 3D Printable Hydrogels for Toxic Heavy Metal Adsorption from Water, U.S. Patent Application ID 62/642,935; March-2018, Patent pending.

Peer-Reviewed Journal Articles:

1. Appuhamillage, Gayan A.; Reagan, John C.; Khorsandi, Sina; Davidson, Joshua R.; Voit, Walter; Smaldone, Ronald A. 3D Printed Remendable Polylactic Acid Blends with Uniform Mechanical Strength Enabled by a Dynamic Diels-Alder Reaction, Polym. Chem. 2017, 8, 2087. 2. Davidson, Joshua R.; Appuhamillage, Gayan A.; Thompson, Christina M.; Voit, Walter; Smaldone, Ronald A. Design Paradigm Utilizing Reversible Diels-Alder Reactions to Enhance the Mechanical Properties of 3D Printed Materials, ACS Appl. Mater. Inter. 2016, 8, 16961. (co-first author) 3. Diaz-Vasquez, Waldo A.; Abarca-Lagunas, Maria J.; Arenas, Felipe A.; Pinto, Camilo A.; Cornejo, Fabian A.; Wansapura, Poorna T.; Appuhamillage, Gayan A.; Chasteen, Thomas G.; Vasquez, Claudio C. Tellurite Reduction by Escherichia coli NDH-II Dehydrogenase Results in Superoxide Production in Membranes of Toxicant-Exposed Cells, BioMetals 2014, 27, 237. 4. Appuhamillage, Gayan A.; De-Silva, Russel C. L. Study of Heavy Metal Adsorption Capacities of Clays and Development of Recovery Methods to Effectively Remove and to Convert the Adsorbed Metals into Reusable Forms, Proc. Sri Lanka Assoc. Adv. Sci. 2009, 65, 137.

Manuscripts in Preparation:

1. Appuhamillage, Gayan A.; Berry, Danielle R.; Benjamin, Candace E.; Luzuriaga, Michael A.; Reagan, John C.; Gassensmith, Jeremiah J.; Smaldone, Ronald A. 3D Printable Hydrogels for Toxic Heavy Metal Adsorption from Water, In Preparation. 2. Dharmarwardana, M.; Bhargav, A. S.; Luzuriaga, M. A.; Lee, H.; McCandless, G. T.; Appuhamillage, G. A.; Smaldone, R. A.; Gassensmith, J. J. Thermochromic, Thermo- mechanical, and Colossal Anisotropic Thermal Expansion Behavior of Alkoxyphenyl N- substituted Naphthalene Diimdies, Invited article for Cryst. Eng. Comm. 2018: New Talent themed issue, In Preparation.

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CONFERENCE PRESENTATIONS

1. “Chitosan-pluronic diacrylate hydrogels as 3D printable material for heavy metal ion adsorption”-Oral, 255th American Chemical Society (ACS) National Meeting 2018, New Orleans, LA. 2. “Chitosan-Pluronic Diacrylate Hydrogels as 3D Printable Material for Heavy Metal Ion Adsorption and Potential Candidates for Cell Growth Applications”- Poster, National Nuclear Security Administration (NNSA)/ Honeywell Conference 2017, The University of Texas at Dallas, TX. 3. “3D Printable Diels-Alder Polymers as Potential Candidates for Electrical Insulating Materials”- Oral, 50th Annual Meeting-in-Miniature, American Chemical Society Conference 2017, Fort Worth, TX. 4. “Self-Healing Design Paradigm Utilizing Reversible Diels-alder Reactions to Enhance Mechanical Properties of 3D Printed Materials”-Poster, 252nd ACS National Meeting 2016, Philadelphia, PA. 5. “Design Paradigm and Post Curing Studies Utilizing Reversible Diels-Alder Reactions to Enhance Mechanical Properties of 3D Printed Materials”-Oral, 72nd Southwest American Chemical Society Conference 2016, Galveston, TX. 6. “Polyimide-Polyether Block Copolymers as Adhesives and Potential Candidates for Electrochromic Devices”-Oral. 49th Annual Meeting-in-Miniature, American Chemical Society Conference 2016, Denton, TX. 7. “3D Printable Polymer Blends via Diels-Alder Dynamic Covalent Chemistry”-Oral, 5th Texas Soft Matter Conference 2016, The University of Texas at Dallas, TX. 8. “Bioremediation of Toxic Oxyanions Using Genetically Modified Escherichia coli Strains and Study of Their Glutathione Biosynthetic Pathway”-Oral, Graduate Research Exchange Conference 2013, Sam Houston State University, Huntsville, TX.

RESEARCH EXPERIENCE

Graduate Research, The University of Texas at Dallas, Richardson, TX 2013-2018 Synthesized and characterized novel polymeric materials based on furan-maleimide Diels- Alder chemistry for FFF 3D printing. Engaged in preparation, thermo-mechanical characterization, and extrusion of thermoplastic polymer blends via reversible Diels-Alder chemistry to achieve enhanced mechanical properties upon 3D printing. Developed 3D printable Diels-Alder reversible thermoset polymer blends and post-curing studies to achieve isotropic mechanical properties. Carried out synthesis, characterization, and 3D printing (using direct-write printing) of hydrogels based on chitosan as a biopolymer for toxic heavy metal adsorption in waste-water systems and as potential materials for cell-growth applications.

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Supervised laboratory work of less experienced graduate students and undergraduates. Managed laboratory chemical waste ensuring compliances with environmental health and safety.

Graduate Research, Sam Houston State University, Huntsville, TX 2011-2013 2- 2- Studied the reducing power of selected E. coli strains for toxic oxyanions SeO3 , SeO4 , and 2- TeO3 against a wild type E. coli in terms of their relative ability to produce intracellular glutathione (GSH). Engaged in modifying an E. coli strain with a better reducing power for bioremediation of toxic oxyanions via gene engineering.

Undergraduate Research, University of Kelaniya, Kelaniya, Sri Lanka 2007-2009 Studied heavy metal (Pb, Cd, and Cr) adsorption capacities of selected environmental clay types with varying pH, temperature, contact time, and interfering cationic species. Introduced ion-exchange techniques to remove adsorbed metals from clay surfaces. Used 3-electrode electrodeposition technique to recover heavy metals.

TEACHING AND MENTORING EXPERIENCE

Graduate Teaching Assistant, The University of Texas at Dallas, TX 2013-2016 Organic chemistry I and II.

Graduate Teaching Assistant, Sam Houston State University, TX 2011-2013 Instrumental analytical chemistry, quantitative analytical chemistry, inorganic chemistry, and general chemistry (laboratory and tutoring).

Teaching Assistant, University of Kelaniya, Kelaniya, Sri Lanka 2009-2011 Organic chemistry, inorganic chemistry, analytical chemistry, and physical chemistry.

TECHNICAL PROFICIENCY

Chemical Methods: Organic synthesis, polymerization methods, purification techniques, characterization methods.

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Instrumentation Methods: Fused filament fabrication (FFF) 3D printing, Stereolithography (SLA) 3D printing, filament extrusion, paste extrusion, tensile testing (extension mode, compression mode), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), Gel Permeation Chromatography (GPC), flash column chromatography, ion exchange chromatography, NMR spectroscopy, FT-IR spectroscopy, UV-Vis spectroscopy, Inductively Coupled Plasma/Mass Spectroscopy (ICP/MS), Gas Chromatography/Mass Spectrometry (GC/MS), Flame Atomic Absorption Spectroscopy (Flame AAS), fluorescence spectroscopy, oxygen plasma asher operation, Polymerase Chain Reaction (PCR) gene sequencing, gel electrophoresis, electrodeposition techniques.

Computer/IT: Computer-aided Design-(CAD) software (Autodesk, SolidWorks, Blender 3D modeling) Analytical software (Origin) Molecular modeling software (Avogadro) Microsoft Office Suit (Word, Excel, Powerpoint, Access, Outlook) ChemOffice Suit (ChemDraw)

PROFESSIONAL AFFLIATIONS/MEMBERSHIPS

American Chemical Society 2014-present

REFERENCES

Prof. Ronald A. Smaldone Prof. Walter E. Voit Prof. T. G. Chasteen Assistant Professor Associate Professor Professor University of Texas at Dallas University of Texas at Dallas Sam Houston State University, TX Office: 972-883-6342 Office: 972-883-5788 Office: 936-294-1533 E-Mail: [email protected] E-Mail: [email protected] E-Mail: [email protected]

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