Nanocellulose-Based Polymer Nanocomposite: Isolation, Characterization and Applications 273 H

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ISBN 978-1-118-87190-4

Printed in the United States of America

10 987654321 To my parents and teachers who helped me become what I am today

Vijay Kumar Th akur

Contents

Preface xvii

Part 1: SYNTHESIS AND CHARACTERIZATION OF NANOCELLULOSE-BASED POLYMER NANOCOMPOSITES

1 Nanocellulose-Based Polymer Nanocomposites: An Introduction 3 Manju Kumari Th akur, Vijay Kumar Th akur and Raghavan Prasanth 1.1 Introduction 3 1.2 Nanocellulose: Source, Structure, Synthesis and Applications 5 1.3 Conclusions 12 References 13

2 Bacterial Cellulose-Based Nanocomposites: Roadmap for Innovative Materials 17 Ana R. P. Figueiredo, Carla Vilela, Carlos Pascoal Neto, Armando J. D. Silvestre and Carmen S. R. Freire 2.1 Introduction 17 2.2 Bacterial Cellulose Production, Properties and Applications 18 2.2.1 Bacterial Cellulose Production 18 2.2.2 Bacterial Cellulose Properties and Applications 25 2.3 Bacterial Cellulose-Based Polymer Nanocomposites 28 2.3.1 BC/Natural Polymers Nanocomposites 28 2.3.2 BC/Water-Soluble Synthetic Polymer Nanocomposites 35 2.3.3 BC/ Th ermoplastic (and Th ermosetting) Nanocomposites 36 2.3.4 BC-Based Electroconductive Polymer Nanocomposites 41 2.4 Bacterial Cellulose-Based Hybrid Nanocomposite Materials 41 2.4.1 Bacterial Cellulose Hybrids with Silver Nanoparticles (BC/Ag NPs) 42 2.4.2 Bacterial Cellulose Hybrids with Miscellaneous Metallic Nanoparticles 44

2.4.3 Bacterial Cellulose Hybrids with Silica Nanoparticles (BC/SiO2 NPs) 45 2.4.4 Bacterial Cellulose Hybrids with Titanium Oxide Nanoparticles

(BC/TiO2 NPs) 47

2.4.5 Bacterial Cellulose Hybrids with Iron Oxides (BC/FexOy NPs) 48 2.4.6 Bacterial Cellulose Hybrids with Hydroxyapatite (BC/HAp NPs) 50

vii viii Contents

2.4.7 Bacterial Cellulose Hybrids with Carbon Allotropes 51 2.4.8 Miscellaneous Bacterial Cellulose Hybrids 53 2.4.9 Final Remarks and Future Perspectives 54 2.5 Acknowledgements 55 References 55

3 Polyurethanes Reinforced with Cellulose 65 María L. Auad, Mirna A. Mosiewicki and Norma E. Marcovich 3.1 Introduction 65 3.2. Conventional Polyurethanes Reinforced with Nanocellulose Fibers 67 3.3. Waterborne Polyurethanes Reinforced with Nanocellulose Fibers 76 3.4. Biobased Polyurethanes Reinforced with Nanocellulose Fibers 78 3.4.1 Biobased Composites Obtained by Using Organic Solvents 78 3.4.2 Biobased Composites Obtained by Using Water as a Solvent 83 3.5 Conclusions and Final Remarks 84 References 85

4 Bacterial Cellulose and Its Use in Renewable Composites 89 Dianne R. Ruka, George P. Simon and Katherine M. Dean 4.1 Introduction 89 4.2 Cellulose Properties and Production 91 4.2.1 Introduction to Cellulose 91 4.2.2 Bacterial Cellulose 92 4.3 Tailor-Designing Bacterial Cellulose 105 4.3.1 Modifying the Properties of Bacterial Cellulose 105 4.3.2 In-Situ Modifi cations 106 4.3.3 Post Modifi cations 108 4.4 Bacterial Cellulose Composites 114 4.4.1 Introduction 114 4.4.2 Renewable Matrix Polymers 115 4.4.3 Bacterial Cellulose Composites 115 4.5 Biodegradability 121 4.6 Conclusions 123 References 123

5 Nanocellulose-Reinforced Polymer Matrix Composites Fabricated by In-Situ Polymerization Technique 131 Dipa Ray and Sunanda Sain 5.1 Introduction 131 5.2 Cellulose as Filler in Polymer Matrix Composites 132 5.2.1 Source 132 5.2.2 Structure 133 5.2.3 Properties 133 5.2.4 Cellulose Nanofi llers 133 5.2.5 Extraction of Cellulose Nanofi llers 134 Contents ix

5.2.6 Advantages and Disadvantages of Cellulose Nanofi llers 136 5.2.7 Surface Modifi cation of Cellulose Nanofi llers 137 5.3 Cellulose Nanocomposites 138 5.4 In-Situ Polymerized Cellulose Nanocomposites 138 5.5 Novel Materials with Wide Application Potential 140 5.5.1 Bone Defect Repair and Bone Tissue Engineering 140 5.5.2 Electrically Active Paper 142 5.5.3 Nanostructured Porous Materials for Drug Delivery or as Bioactive Compounds 146 5.5.4 Surface Coating Applications 148 5.5.5 Biobased Green Nanocomposites 152 5.6 Eff ect of In-Situ Polymerization on Biodegradation Behavior of Cellulose Nanocomposites 154 5.7 Future of Cellulose Nanocomposites 157 References 159

6 Multifunctional Ternary Polymeric Nanocomposites Based on Cellulosic Nanoreinforcements 163 D. Puglia, E. Fortunati, C. Santulli and J. M. Kenny 6.1 Introduction 163 6.2 Cellulosic Reinforcements (CR) 166 6.2.1 Microfi brillated Cellulose (MFC) 167 6.2.2 Nanocrystalline Cellulose (NCC) 168 6.2.3 Bacterial Cellulose (BC) 170 6.3 Interaction of CNR with Diff erent Nanoreinforcements 171 6.3.1 CNR and Metallic Nanoparticles 172 6.3.2 CNR and Ceramic Nanoparticles 175 6.3.3 CNR and Carbon-Based Nanoparticles 176 6.3.4 CNR and Biological Nanoreinforcements 177 6.4 Ternary Polymeric Systems Based on CNR 179 6.4.1 Th ermoplastic Matrices and CNR-Based Systems 180 6.4.2 Th ermosetting Matrices and CNR-Based Systems 186 6.5 Conclusions 190 Acknowledgments 191 References 191

7 Eff ect of Fiber Length on Th ermal and Mechanical Properties of Polypropylene Nanobiocomposites Reinforced with Kenaf Fiber and Nanoclay 199 Na Sim and Seong Ok Han 7.1 Introduction 199 7.2 Experimental 200 7.2.1 Materials 200 7.2.2 Fabrication of Nanobiocomposites 201 7.2.3 Analysis 201 x Contents

7.3 Results and Discussion 202 7.3.1 Th ermal Properties (TGA) 202 7.3.2 Th ermomechanical Properties (TMA) 203 7.3.3 Dynamic Mechanical Analysis (DMA) 205 7.3.4 Tensile Properties 206 7.3.5 Flexural Properties 207 7.3.6 Impact Properties 208 7.3.7 SEM and EDX Observation 209 7.4 Conclusions 211 References 211

8 Cellulose-Based Liquid Crystalline Composite Systems 215 J. P. Borges, J. P. Canejo, S. N. Fernandes and M. H. Godinho 8.1 Introduction 215 8.2 Liquid Crystalline Phases of Cellulose and Its Derivatives 216 8.2.1 All-Cellulosic-Based Biomimetic Composite Systems 219 8.2.2 Liquid Crystalline Electrospun Fibers 227 8.3 Conclusion 232 Acknowledgements 232 References 232

9 Recent Advances in Nanocomposites Based on Biodegradable Polymers and Nanocellulose 237 J. I. Morán, L. N. Ludueña and V. A. Alvarez 9.1 Introduction 237 9.1.1 Bioplastics Classifi cation and Current Status 238 9.1.2 Nanocellulose for Bionanocomposites 239 9.2 Cellulose Bionanocomposites Incorporation of Cellulose Nanofi bers into Biodegradable Polymers: General Eff ect on the Properties 243 9.2.1 Bioplastics-Based Nanocellulosic Composites 244 9.2.2 Treatment of CNW: Improvement of Cellulose Nanofi bers/ Biodegradable Matrix Compatibility 248 9.2.3 Processing of Cellulose-Based Bionanocomposites 248 9.3 Future Perspectives and Concluding Remarks 249 References 250

Part 2: PROCESSING AND APPLICATIONS NANOCELLULOSE- BASED POLYMER NANOCOMPOSITES

10 Cellulose Nano/Microfi bers-Reinforced Polymer Composites: Processing Aspects 257 K. Priya Dasan and A. Sonia 10.1 Introduction 257 10.2 Th e Role of Isolation Methods on Composite Properties 260 Contents xi

10.3 Pretreatment of Fibers and Its Role in Composite Performance 262 10.4 Diff erent Processing Methodologies in Cellulose Nanocomposites and Th eir Eff ect on Final Properties 264 10.5 Conclusion 268 References 268

11 Nanocellulose-Based Polymer Nanocomposite: Isolation, Characterization and Applications 273 H. P. S. Abdul Khalil, Y. Davoudpour, N. A. Sri Aprilia, Asniza Mustapha, Md. Nazrul Islam and Rudi Dungani 11.1 Introduction 274 11.2 Cellulose and Nanocellulose 274 11.3 Isolation of Nanocellulose 276 11.3.2 Ultrasonication 278 11.3.3 Electrospinning 279 11.3.4 Acid Hydrolysis 281 11.3.5 Steam Explosion 283 11.4 Characterization of Nanocellulose 283 11.4.1 Physical Properties 283 11.4.3 Th ermal Properties 286 11.4.4 Morphological Properties 288 11.5 Drying of Nanocellulose 289 11.6 Modifi cations of Nanocellulose 290 11.6.1 Acetylation 291 11.6.2 Silylation 291 11.6.3 Application of Coupling Agents 292 11.6.4 Graft ing 293 11.7 Nanocellulose-Based Polymer Nanocomposites 295 11.7.1 Th ermoplastic Polymer-Nanocellulose Nanocomposites 296 11.7.2 Th ermoset Polymer-Nanocellulose Nanocomposites 298 11.7.3 Application of Nanocomposites Based on Nanocellulose 301 11.8 Conclusion 302 Acknowledgement 303 References 303

12 Electrospinning of Cellulose: Process and Applications 311 Raghavan Prasanth, Shubha Nageswaran, Vijay Kumar Th akur and Jou-Hyeon Ahn 12.1 Cellulosic Fibers 311 12.2 Crystalline Structure of Electrospun Cellulose 312 12.3 Applications of Cellulose 313 12.4 Electrospinning 313 12.4.1 Processing – Fundamental Aspects 316 12.5 Electrospinning of Cellulose 317 xii Contents

12.6 Solvents for Electrospinning of Cellulose 318 12.6.1 Room Temperature Ionic Liquids 320 12.6.2 N-methyl morpholine-N-oxide 325 12.6.3 Lithium Chloride/N,N-Dimethylacetamide 329 12.7 Cellulose Composite Fibers 333 12.8 Conclusions 336 Abbreviations 336 Symbols 336 References 337

13 Eff ect of Kenaf Cellulose Whiskers on Cellulose Acetate Butyrate Nanocomposites Properties 341 Lukmanul Hakim Zaini, M. T. Paridah, M. Jawaid, Alothman Y. Othman and A. H. Juliana 13.1 Introduction 341 13.2 Experimental 342 13.2.1 Materials 342 13.2.2 Whisker Isolation 343 13.2.3 Nanocomposite Preparation 343 13.3 Characterization 344 13.3.1 Fourier Transform Infrared Spectroscopy (FTIR) 344 13.3.2 Th ermogravimetric Analysis (TGA) 344 13.3.3 Diff erential Scanning Calorimetry (DSC) 344 13.3.4 Dynamic Mechanical Properties (DMA) 344 13.4 Result and Discussion 345 13.4.1 Fourier Transform Infrared Spectroscopy (FTIR) 345 13.4.2 Th ermogravimetric Analysis 346 13.4.3 Diff erential Scanning Calorimetry Analysis 347 13.4.4 Dynamic Mechanical Analysis 350 13.5 Conclusions 352 Acknowledgements 353 References 353

14 Processes in Cellulose Derivative Structures 355 Mihaela Dorina Onofrei, Adina Maria Dobos and Silvia Ioan 14.1 Introduction 355 14.1.1 Liquid Crystalline Polymers 357 14.1.2 Liquid Crystal Dispersed in a Polymer Matrix 359 14.1.3 Techniques for Obtaining Liquid Crystals Dispersed into a Polymeric Matrix 360 14.1.4 Some Methods to Characterize the Liquid Crystal State 360 14.1.5 Liquid Crystal State of Cellulose and Cellulose Derivatives in Solution 364 14.1.6 Cellulose Derivatives/Polymers Systems 373 14.2 Conclusions 383 References 384 Contents xiii

15 Cellulose Nanocrystals: Nanostrength for Industrial and Biomedical Applications 393 Anuj Kumar andYuvraj Singh Negi 15.1 Introduction 393 15.2 Cellulose and Its Sources 394 15.3 Nanocellulose 396 15.4 Cellulose Nanocrystals 398 15.4.1 Extraction of CNCs 399 15.4.2 Overview of CNCs Production by Acid Hydrolysis 401 15.4.3 Characterization Methods 404 15.4.4 Properties and Behavior of CNCs 405 15.5 Aqueous Suspension and Drying of CNCs 408 15.6 Functionalization of CNCs 410 15.6.1 Oxidation 410 15.6.2 Polymer Graft ing 411 15.6.3 Cationic Functionalization 412 15.6.4 Acetylation 412 15.6.5 Silylation 413 15.7 Processing of CNCs for Biocomposites 414 15.7.1 Solution Casting 414 15.7.2 Melt Compounding 414 15.7.3 Partial Dissolution 415 15.7.4 Electrospinning 415 15.7.5 Layer-by-Layer Assembly 415 15.8 Applications of CNCs-Reinforced Biocomposites 416 15.8.1 Industrial Applications 416 15.8.2 Photocatalytic Materials 416 15.8.3 Printed Electronics Applications 417 15.8.4 Lithium-Ion Batteries (LIBs) 417 15.8.5 Other Studies 419 15.9 Biomedical Applications 421 15.9.1 Drug Delivery Systems 421 15.9.2 Tissue Engineering 422 15.9.3 Hydrogels 425 15.9.4 Bioimaging 426 15.9.5 pH-Sensing Materials 427 15.10 Conclusion 427 Acknowledgements 428 References 428

16 Medical Applications of Cellulose and Its Derivatives: Present and Future 437 Karthika Ammini Sindhu, Raghavan Prasanth and Vijay Kumar Th akur 16.1 Historical Overview 438 16.2 Use of Cellulose for Treatment of Renal Failure 439 16.2.1 Types of Dialyzers 441 16.2.2 Performance of Hollow-Fiber Dialyzers 443 1 1 Nanocellulose-Based Polymer Nanocomposite: Isolation, Characterization and Applications

H. P. S. Abdul Khalil *, 1,2 , Y. Davoudpour1 , N. A. Sri Aprilia 1 , Asniza Mustapha 1 , Md. Sohrab Hossain1 , Md. Nazrul Islam 1,3 and Rudi Dungani 1,4 1 School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia 2 Cluster for Polymer Composites (CPC), Science and Engineering Research Centre (SERC), Universiti Sains Malaysia, Pulau Pinang, Malaysia 3 Life Science School, Khulna University, Khulna, Bangladesh 4 School of Life Sciences and Technology, Institut Teknologi Bandung, Gedung Labtex XI, Bandung, West Java-Indonesia

Abstract New and advanced properties and functions, including uniformity, durability, biodegradability and sustainability, are required for the next generation of cellulose-based products and their engineering applications. Nanosize cellulosic materials, a good candidate for the preparation of polymer nanocomposites, have been gaining a lot of attention due to their low cost, biodegradability, abundance, high strength, renewability and some other excellent properties. Advantages in the use of nanosize cellulosic materials are related not only to these properties; in fact, its dimensions in the nanometer scale, open a wide range of possible properties to be discovered. Nanosize cellulosic materials can be isolated from a variety of cellulosic sources including plants, animals (tunicates), bacteria and algae, and in principle could be extracted from almost any cellulosic material by using diff erent procedures. However, the main challenges in the eldfi are related to an effi cient separation of nanosize cellulosic materials from natural resources. Th e noncompatible nature of nanocellulose with most polymers is also a crucial issue for its application in composites. In addition, the drying process of nanocellulose for application in polymer composite is another challenge. Last but not least, a process needs to be found for obtaining higher yield in nanocellulose isolation. All these challenges and drawbacks have become a strong driving force for discovering more effi cient processes and technologies to produce nanocelluloses for application in nanocomposites, and for inventing new applications as well. Th is chapter will concentrate on the isolation of nanocellulose from various sources and its utiliza- tion for fabrication methods, its characterization, drying processes and modifi cation. We will also concentrate our discussion on the application of nanoscale cellulosic materials in polymer nanocomposites . Keywords: Cellulose, isolation, nanofi ber, nanocrystal, characterization, nanocomposite

*Corresponding author: [email protected]

Vijay Kumar Th akur, Nanocellulose Polymer Nanocomposites, (273–310) 2015 © Scrivener Publishing LLC

273 274 Nanocellulose Polymer Nanocomposites

11.1 Introduction Nanotechnology is now recognized as one of the most promising areas for techno- logical development in the 21st century. In line with the development of nanotechnology and recent concern about environmental issues [1] , more attention have been paid to utilize biobased materials. In this regard, natural fi bers have been gaining much more interest because of their promising characteristics such as biodegradable nature, renewability and lower price [2] . Among these natural fi bers, cellulose as the most plentiful biopolymer which exists in a wide variety of natural fi bers such as kenaf [3] , cotton [4] , banana [5] , wood [6] , fl ax [7] , oil plam [8] , bamboo [9] and ani- mal species like tunicates [10] , etc., have been the subject of much research in nanotechnology. Cellulose is a linear biopolymer, having β-D-glucopyranose repeating units [11] in both crystalline and amorphous region [12] . In the case of application of cellulose in nanotechnology, two general types of nanocelluloses are recognized, namely cellulose nanocrystal and cellulose nanofi ber. Th ese two nanocelluloses can be distinguished by their diff erent production processes and structures. Cellulose nanofi ber, having a high aspect ratio in both amorphous and crystalline regions, can be produce using some techniques such as electrospinning, refi ning [13] , homogenization, grinding, cryocrushing [14] , ultrasonication [15] and steam explosion [16] . While cellulose nanocrystal is the whisker form of nanocellulose, its amorphous part can be completely removed by acid hydrolysis either H 2SO 4 or HCL [17] . Th ese nanocellulose structures have attracted attention as a potential material in reinforced nanocomposites. By inserting these nanoscale compounds into polymers even in small quantities, the properties of polymers improve; however, it depends on the type of nanocellulose used in the applications [18] . In order to utilize nanocellulose as reinforcement in nanocomposites, the strong hydrogen bonds between nanocellulose, which make it hydrophilic, must be broken down for good dispersion in the polymers with hydrophobic nature. Surface modifi cation is the most common way to make the surface of nanocellulose hydrophobic and to incorporate it homog- enously in diff erent polymers, including by graft ing, silylation, acetylation, etc. [19] . Apart from the modifi cation processes, drying of nanocellulose is another important issue which should be considered for adding nanocellulose in polymers. Th is is due to the change of size of these nanomaterials aft er drying, which may aff ect their unique properties. In this chapter, fi rst we will discuss the cellulose and nanocellulose structures. Later, isolation and characterization of cellulose nanofi ber and nanocrystal will be addressed. Drying and modifi cation will also be presented in the chapter. At the end, nanocomposite production from nanocellulose with thermoplastic and thermoset polymers will be discussed.

11.2 Cellulose and Nanocellulose Cellulose is the most abundant renewable natural biopolymer on earth. It is present in a wide variety of living species including plants, animals, and some bacteria [20] . Nanocellulose-Based Polymer Nanocomposite 275

11.2.1 Architecture of Cellulose As the skeletal component in all plants, cellulose is organized in a cellular hierarchical structure. Th e cell walls of plants are divided by a middle lamella from each other, followed by the primary cell wall layer. Cellulose is predominantly located in the second- ary wall and consists of roughly 6,000 glucose units in the primary cell wall [21-23] . Th is linear polymer is composed of repeating β-1, 4 linked anhydroglucopyranose units that are covalently linked through acetal functions between the equatorial OH group of C4 and the C1 carbon atom forming bundles of fi brils (also called microfi brillar aggre- gates), which allows the creation of highly ordered regions (i.e., crystalline phases) alternate with disordered domains (i.e., amorphous phases) [24, 25]. Figure 11.1 shows a schematic wood hierarchical structure from biomass to cellulose nanocrystal (CNC) and nanofi brillated cellulose (NFC) [26] .

11.2.2. Nanocellulosic Materials Structures and Size Cellulose nanofi bers are divided into two main families, diff eringin their size and crystallinity, which are cellulose nanocrystal (CNC) and nanofi brillated cellulose (NFC). Th e CNC, also known as nanowhiskers [27-31] , nanorods [24, 32] and rod- like cellulose crystals [33] , which is usually isolated from cellulose fi bers through acid

Figure 11.1 A schematic of wood hierarchical structure from biomass to CNC and NFC: (A) Biomass, (B) single-fi ber network, (C) microfi bril, (D) cellulose nanocrystal, and (E) nanofi brillated cellulose [26]. 276 Nanocellulose Polymer Nanocomposites hydrolysis [34, 35] . It possesses a relatively low aspect ratio and has a typical diameter of 2–20 nm and wide length distribution from 100 to 600 nm [36-38] . On the other hand, nanofi brillar cellulose [39, 40], cellulose nanofi ber [34, 41]and cellulose nanofi bril [39, 42] are the terms used for microfi brillated cellulose in the literature. Being the small- est structural units of plant fi ber, NFC consists of a bundle of stretched cellulose chain molecules [43] with long, fl exible and entangled cellulose nanofi bers of approximately 1–100 nm in size [44] . Figure 11.2 shows the morphology of CNC [45] and NFC [34] . As can be seen in this fi gure, compared to the rod-like crystalline structure of CNC, NFC has a long and fi brillar structure.

11.3 Isolation of Nanocellulose Nanocellulose in the form of NFC or CNC can be extracted by various methods. Th is section briefl y describes the processes for producing nanocellulose, their advantages and disadvantages, as well as some important issues regarding these methods.

11.3.1 Homogenization One of the mechanical processes which can be used for the production of NFC is homogenization. Beside homogenization, refi ning [46, 47] , cryocrushing [48] , grinding [49, 50] and microfl uidization [51, 52], which are similar to homogenizer, can also be considered as other mechanical approaches to reduce the size of cellulosic fi bers from micro- to nanoscale. Because of refi ning, cryocrushing and grinding are mostly used as combination processes with homogenization, it is therefore this section describes homogenization as a main process. In a high pressure homogenizer instrument, cellulose suspension passes through a small nozzle with high pressure. Eff ective parameters in diminishing the size of the fi bers to nanoscale in this process include high shear and impact forces along with high pressure and velocity on fl uid, which generate shear on the stream [53] . Some of the most important parameters which aff ect the properties of obtained nanofi bers are pressure, diff erent passing times through the machine, concentration of suspension and temperature. In 1983, Herrick and Turbak applied this method to isolate NFC from wood fi ber for the fi rst time [54, 55]. Although homogenization is a very simple process without the need for organic solvents [56] ,clogging is one of the most important issues

Figure 11.2 Transmission electron microscopoy (TEM) micrographs of CNC [45] and NFC [34] Nanocellulose-Based Polymer Nanocomposite 277 related to application of this instrument because of its small orifi ce. To overcome this problem, researchers have used various pretreatments like refi ning, cryocrushing [57] , and milling [58] in order to reduce the size of the fi bers. Another main drawback of homogenization is the high energy consumption. In this regard, application of some pretreatments including enzyme [59-61] , alkaline [34, 62] and ionic liquids [63] have been suggested by researchers to decrease the amount of energy for production of cellulose nanofi ber. As Siro and Plackett [14] mentioned, by using chemical or enzymatic pretreatments energy consumption in mechanical processes can be diminished from 20,000 to 30,000 kWh/tonne, which is common in these kinds of methods, to 1000 kWh/tonne. Figure 11.4 shows the morphology of NFC with various treatments. Th e diameter of wheat straw aft er alkaline treatment was 9 μm and aft er mechanical isolation it reached 30–40 nm (Figure 4a) [62] . Figure 11.4b displays long and distinct NFC with diameter around 5 nm aft er enzyme treatment (size of original fi bers was 10 μm) [64] . Circular shape nanoparticle with a diameter in the range of 10 to 20 nm

Figure 11.3 High pressure homogenizer (HPH) instrument.

Figure 11.4 TEM image of nanocellulose from (a) alkaline-treated wheat straw [62] , (b) enzyme-treated soft wood pulp [64] and (c) ionic liquid-treated sugarcane bagasse [63]. 278 Nanocellulose Polymer Nanocomposites was produced from ionic liquid treatment of sugarcane bagasse, as presented in Figure 11.4c [63 ]. Apart from the production process of NFC by homogenization, their fi nal application is a critical issue. Because of the hydrophilic nature of cellulose nanofi bers, their incorporation and dispersion with common polymers, which are hydrophobic, are very critical issues [38] . Low interfacial adhesion between these two parts in composite leads to reduction in the mechanical and other properties of the fi nal product. Th us, a wide variety of modifi cations like carboxymethylation [65] , 2,2,6,6-tetramethylpiperdine- 1-oxyl (TEMPO) oxidation [66, 67], acetylation [68, 69], and silylation [70, 71] have been designed to overcome this problem. Th e modifi cation strategies of cellulose nanosize materials are discussed in Section 11.6.

11.3.2 Ultrasonication High-intensity ultrasonication can be considered as a mechanical method for producing cellulose nanofi bers with hydrodynamic forces [72] . In this process, ultrasonic waves create strong mechanical stress because of cavitations, and therefore, cause the disaggregation of cellulosic fi ber to nanofi bers [73] . Several attempts have been made to isolate cellulose nanofi ber by ultrasonication from various cellulose sources such as microcrystalline cellulose, regenerated and pure cellulose fi bers [72] , kraft pulp [74] , fl ax, wood, wheat straw and bamboo [75] (Figure 11.5), para rubberwood sawdust [76] , and poplar wood powders [15] . Th e well-individualized, web-like structure and long entangled fi laments of NFC from wood (Figure 5a), bamboo (Figure 11.5b) and wheat straw (Figure 11.5c) with

Figure 11.5 Field emission scanning electron microscopoy (FESEM) images of NFC isolated from (a) wood, (b) bamboo, (c) wheat straw and (d) fl ax [75]. Nanocellulose-Based Polymer Nanocomposite 279

3 5 7

4 6 1

Figure 11.6 Schematic of ultrasonic setup, (1) Power, (2) Piezoelectric converter, (3) Ultrasonic probe, (4) Sample suspension, (5) Double-walled glass beaker, (6) Ice water inlet and (7) outlet [74]. diameters around 10–20 nm, 10–40 nm and 15–35 nm, respectively, can be easily distinguished. While because of the high cellulose content in fl ax fi ber which leads to strong H-bond and diffi culty in the fi brillation process, non-uniform NFC with a 15 to 100 nm width is produced ( Figure 11.5d ). Th e schematic of ultrasonic setup can be seen in Figure 11.6 [74 ]. Th e effi ciency of defi brillation in the ultrasonic process is dependent on power, concentration, temperature, size of fi bers, time and distance from probe tip to collector [77] . In some cases, researchers have been used a combination of ultrasonication with other methods to increase fi brillation of nanoscale cellulose. For example, Li et al. [78] prepared nanocrystalline cellulose by ultrasonication and acid hydrolysis with H2 SO4 from bleached soft wood pulp. Th ey found that ultrasonication led to folding and erosion of the cellulose surface, and thus provided more reactive site to penetrate acid and prepare high-crystalline and small-size nanocellulose. Furthermore, Wang and Chen [77] reported that a combination of ultrasonication and homogenization boosts uniformity and fi brillation of cellulose nanofi ber in comparison to ultrasonication solely. In addition, when compared to mechanical blender, ultrasonic bath and ultrasonic probe, Mishra et al. [79] concluded that TEMPO-oxidized fi ber treatment with ultrasonic probe was more effi cient for nanocellulose production than the other three methods.

11.3.3 Electrospinning Electrospinning is a versatile and simple process for formation of nanofi bers by electri- cal force from various sources such as cellulose. In 1930, Formhal patented this method [80] . Th e basic parts of the electrospinning instrument include high voltage supply, a syringe to carry polymer solution and a target to collect nanofi bers [81] . Figure 11.7 shows the basic electrospinning apparatus [82] . In this process, nanofi bers form from polymer solution between two electrodes with opposite polarity, one electrode con- nected to a syringe and the other one to a collector [83] . At a critical voltage, a conical shape droplet known as “Taylor cone” is held at the capillary tip due to surface tension [84, 85]. When the electric force which is created at the surface of polymer solution 280 Nanocellulose Polymer Nanocomposites

Spinneret

Syringe Pump

Rotating drum

High voltage Power Supply

Figure 11.7 Schematic of electrospinning apparatus [82]. overcomes surface tension of solution, an electrically charged jet emerges and electrospinning occurs [86, 87]. When the jet moves in a whipping motion between needle and collector, the solvent of polymer solution evaporates and dry nanofi ber in the form of nonwoven mat forms on the collector [88, 89]. Th e parameters impacting the electrospinning process can be categorized into solution parameters (surface tension, concentration, viscosity and conductivity), processing conditions (voltage, distance from needle to collector, type of collector, fl ow rate) and ambient conditions (humidity, pressure and temperature) [90, 91]. Based on the interaction of all these factors, the morphology and size of resultant nanofi bers can be changed. As stated above, a polymer solution should be prepared for electrospinning process at the beginning. However, processing of cellulose via electrospinning is a big challenge because of its limited solubility in common solvents as well as its tendency to agglomerate [92] . Nevertheless, several direct solvent systems including N-methyl-morpholine oxide/water (NMMO/ H2 O) [93, 94] , lithium chloride/dimethyl acetamide (LiCl/ DMAc) [82, 95, 96] , ionic liquids (ILs) [97-99] and trifl uoroacetic acid (TFA) [100, 101] have been established. However, removing solvent between needle to collector from three solvent systems including NMMO/ H 2O, LiCl/ DMAc and ILs is diffi cult [102] . So, one of the solutions to tackle this problem is applying cellulose derivatives such as cellulose acetate [103-105] , ethyl cellulose [106, 107] and other derivatives. It is worth noting that new types of materials known as composite or hybrid nanofi ber using electrospinning of CNC and diff erent polymers such as polyethylene oxide (PEO) [108] , polyvinyl alcohol (PVA) [109] and polymethyl methacrylate (PMMA) [45] have been fabricated. Uniform, smooth and continuous nonwoven mat with controllable diameter at all CNC loading were formed ( Figure 11.8 ) [45] . Th e author stated that using 17% CNC content, the storage modulus of composite nanofi ber increased 17% as well. Th e diameter of the PMMA nanofi bers was 459 nm (Figure 11.8a). Th e width of PMMA-CNC nanofi bers with 5% CNC (474 nm), 9% CNC (450 nm), 17% CNC (431 nm), 23% CNC (280 nm), 33% CNC (269 nm) and 41% CNC (182 nm) decreased with increasing CNC content (Figure 11.8b–g). Th e key point for incorporation of CNC with various Nanocellulose-Based Polymer Nanocomposite 281

Figure 11.8 Scanning electron microscopy (SEM) images of (a) PMMA and PMMA reinforced with various CNC loading, (b) 5%, (c) 9%, (d) 17%, (e) 23%, (f) 33% and (g) 41% [45]. polymers for electrospinning is to improve the mechanical properties of materials and to give new functionality to these kinds of electrospun nanofi bers [110] .

11.3.4 Acid Hydrolysis Th e main chemical process to produce nanocrystalline cellulose is acid hydrolysis by either sulphuric acid (H 2 SO4 ) [111] , hydrochloric acid (HCL) [112] or a combination of these two acids [113] at various concentrations. In the case of CNC from H2 SO4 hydrolysis, the surface of the material becomes negatively charged with sulfonate ester groups, causing the easy dispersion of CNC in aqueous solvents, whereas those produced using HCL have weaker charge density and show a higher tendency of fl occula- tion in organic solvents [114] . Native cellulose consists of crystalline and amorphous regions, and when cellulosic fi bers were subjected to insensitive acid treatment, the amorphous parts break up and just the individual crystallites remain. So, the characterization of CNC is dependent on various parameters, such as reaction time, cellulose sources, type of acid and reaction temperature [115] . Th e pathway of acid hydrolysis with H 2 SO4 on cotton linter can be seen in Figure 11.9 [116] .

In 1951, a process to degrade cellulosic fi bers with H 2 SO4 was introduced by Ränby for the fi rst time [111] . Since then a series of attempts have been taken to prepare CNC from various cellulosic fi bers such as curaua fi bers [117] , coconut husk [118] , cotton and tunicates [119] , sugarcane bagasse [120] , and wood [121] . In the acid hydrolysis process a suspension with specifi c acid concentration for a particular time and temperature based on various sources of fi bers is mechanically stirred. Th en the reaction is quenched with cold water. Subsequently, the washing process is conducted 282 Nanocellulose Polymer Nanocomposites

Cotton linter

Step one 50% sulfuric acid

Step two

55% sulfuric acid Step three

Step four 60% sulfuric acid

Figure 11.9 Pathway of H2 SO4 hydrolysis process on cotton linter [116].

Figure 11.10 (a) TEM image of CNC with 12 nm diameter from kenaf bast at optimum conditions [123] , (b) Atomic force microscopy (AFM) image of OPEFB with 2.05 ± 0.89 nm thickness at optimum conditions [8]. by centrifuging and each step of centrifuging the acidic supernatant is removed and again water is added into the suspension. Aft er several centrifuge steps, the suspension dialyzes against distilled water to obtain constant pH. Finally the ultrasonication is to be done in order to disperse CNC. Th e dimensions of CNC are dependent on the hydrolysis condition and the source of cellulose fi bers [122] . For example, the optimum acid concentration, temperature and hydrolysis time for H 2 SO4 hydrolysis of kenaf bast fi bers were reported as 65%, 45°C and 40 min, respectively (Figure 11.10a) [123] , whereas for oil palm empty fruit bunch fi bers (OPEFB) they were 64%, 45°C and 1 hour, respectively (Figure 11.10b) [8] . Furthermore, in HCL hydrolysis various conditions have been employed based on the raw material. For instance, CNC from MCC was produced by 2.5N HCL for 45 min [124] and from Whatman fi lter paper using 1.5N HCL for 4 hours at 100°C [125] . Nanocellulose-Based Polymer Nanocomposite 283

Figure 11.11 NFC from pineapple leaf: (a) TEM and (b) AFM [128].

11.3.5 Steam Explosion Steam explosion is a thermomechanical process. At high pressure, steam penetrates to cellulose fi ber through diff usion and when the pressure suddenly releases, creates shear force, hydrolysis of the glycosidic and hydrogen bonds and leads to formation of nanofi bers [126] . In 1927, Mason introduced the steam explosion method to defi bril- late wood to fi ber for board production [127] . Th e eff ective parameters of this process are pressure, temperature and time of material being autoclaved. Th e steam explosion process can be used solely or in combination with other processes. For instance, cellulose nanofi bers from banana at 20 lb pressure, 110–120°C, for 1 hour [16] and from pineapple at 20 lb pressure [128] were produced just using steam explosion. TEM and AFM images of Figure 11.11 [128 ] confi rm individualization of NFC from cell wall using steam explosion process. Only a little association happened between adjacent NFC. Th e author estimated the aspect ratio of 50 through TEM and diameter of around 5 to 60 nm through AFM for NFC. In another study, banana, jute and pineapple leaf nanofi bers were extracted with steam explosion along with mild chemical treatment [5] . Also, steam explosion was used along with homogenization to increase defi brillation of nanofi ber from wheat straw by Kaushik and Sing [129] .

11.4 Characterization of Nanocellulose

Basically, characterization of cellulosic materials in nanoscale is a crucial issue in order to explore their physical, chemical, thermal and morphological properties at various treatment stages. Regarding this, this section discusses some of these properties and their evaluation techniques.

11.4.1 Physical Properties Physical characterizations of nanocellulose include particle size analysis, surface charge, contact angle, etc. Particle size analysis of nanocellulose can be done using dynamic light scattering (DLS) and surface charge, which can be measure by zeta potential [130] . 284 Nanocellulose Polymer Nanocomposites

6 Count [%] Count 4

0 1 10 100 1000 10000 (a) Particle size [nm] 8

4 Count [%] Count 2

0 1 10 100 1000 10000 b ( ) Particle size [nm]

6 Count [%] 4

0 1 10 100 1000 10000 c ( ) Particle size [nm] Figure 11.12 Measurment of average particle size of nanocellulose from (a) acid hydrolysis, (b) TEMPO-oxidized nanocellulose and (c) ultrasonication using DLS [130].

Zhou et al. [130] stated that zeta potential can be estimated by following the moving rate of charged particle (negative or positive charge) across an electric fi eld. Generally, a value smaller than −15mV shows the start of agglomeration, whereas higher than −30 mV represents enough bilateral repulsion and colloidal stability. As the authors mentioned, the zeta potential values of the nanocellulose suspensions was −38.2 mV for nanocellulose using acid hydrolysis (because of sulfonate groups), −46.5 mV for TEMPO-oxidized nanocellulose (due to carboxyl groups) and −23.1 mV for ultrasoni- cated nanocellulose (as a result of its natural hydroxyl groups). Nanocellulose-Based Polymer Nanocomposite 285

Based on Figure 11.12 [130], the mean particle size of nanocellulose from acid hydrolysis, TEMPO-oxidized nanocellulose and ultrasonication was calculated to be 115, 210 and 623 nm, respectively. Th e contact angle measurement provides information about the degree of hydrophilicity or hydrophobicity of the nanocellulose surface. In order to measure the contact angle of nanocellulose using the sessile drop method, fi rst a dry network of nanocellulose should be prepared, then a droplet of water is deposited on the network and contact angle is measured [131] . A contact angle higher than 90° means that the surface is non-wetted (hydrophobic) and lower than 90°represents wet- ting characteristic (hydrophilic) of the surface [132] . For example, the contact angle of kenaf fi ber, acetylated fi ber, NFC and acetylated NFC aft er 60s was 0, 113 ± 2, 0 and 114 ± 2°, respectively [68] .

11.4.2 Chemical Properties Regarding chemical analysis of nanocellulose, some of the properties including chemical composition, crystallinity and functional group analysis can be measured. Chemical composition analysis can be utilized to measure the amount of lignin, hemicellu- loses and cellulose in nanocellulose. Cellulose, lignin and hemicellulose contents are measured according to TAPPI standards [34] . Table 11.1 shows an example of measur- ing chemical composition of NFC and CNC before and aft er each process measured by TAPPI standards. Changing the crystallinity of nanocellulose can be evaluated by X-ray diff raction patterns. In this test method the sample is scanned by CuK radiation (wavelength = 0.154 nm) with the diff raction angle in the ranged of 4 to 40° [135] . Th e crystallinity of fi bers can be calculated through the following equation: II− Crystallinity(%) =×200 AM 100 (1) I 200

In this equation, IAM (2θ = 18°) shows amorphous cellulose intensity and I200 (2θ = 22.5°) represents intensity of crystalline cellulose [63] . For instance, the degree of crystallinity for CNC from cotton linter was more than cotton linters [136] , while this characteristic for NFC from sugarcane bagasse was 36% [63] . Fourier Transform Infrared Spectrometer (FTIR) is an instrument for studying the change of the functional groups

Table 11.1 Chemical composition of NFC from hemp and CNC from kenaf bast. α-cellulose Lignin Hemicellulose Reference Hemp fi ber Raw 75.56 6.61 10.66 [133] Acid-alkaline 89.78 4.93 3.04 treated Bleached 93.87 3.18 1.85 NFC 94.53 2.71 1.59 Kenaf bast Raw 45.95 19.10 29.88 [134] fi ber Retted 92.27 0.24 1.95 Bleached 95.19 0 0.22 CNC 100 0 0 286 Nanocellulose Polymer Nanocomposites of nanocellulose before and aft er their production process. In this test, dried and pow- dered form samples are blended with KBr and then compressed. Th e spectra of samples are recorded in the range of 4000 to 400 cm−1 [63] . For example, the FTIR spectra of NFC from cotton linter can be seen in Figure 11.13 and its peak analysis is summarized in Table 11.2 [47 ].

11.4.2 Th ermal Properties Th ermal decomposition properties of nanocellulose are determined by thermogravi- metric analysis (TGA). In general, around 5 mg of sample is placed in a platinum pan and heated with rate of 10°C/min from 20 to 600° C [137] . Figure 11.14a shows the TGA curves of CNC from jute fi ber [137] . As the authors mentioned, the degradation of untreated jute fi ber, alkali-treated fi ber and TEMPO-oxidized CNC started at 270, 270 and 200°C, respectively. Th ey described that TEMPO oxidation causes a reduction in thermal degradation due to generation of sodium carboxylate groups. Figure 11.14b

0.8 Abs

0.6 After 30 passes 1059.93 1112.97 0.4 1163.13 3322.53 616.28 1371.45 1317.44 1429.31 0.2 403.14 900.90 2900.10

Control fibre 1634.74

4000 3500 3000 2500 2000 1500 1000 500 1/cm Wavenumber (cm–1) Figure 11.13 FTIR spectra of NFC from cotton linter [47].

Table 11.2 FTIR peak analysis of NFC from cotton linter [47]. Peak (cm -1) Assigning to NFC from cotton 4000-3000 hydrogen-bonded OH stretching fi ber 2900 CH stretching 1635 OH bending of adsorbed water 1429 HCH and OCH bending vibrations 1371 CH deformation vibration 1265 -C-O-C- bond 900 COC, CCO, and CCH deformation and stretching vibrations Nanocellulose-Based Polymer Nanocomposite 287 illustrates the TGA curves of carboxymethylated (c) NFC from refi ned bleached beech pulp (RBP) (RBP-c), a mechanical disintegrated one (RBP-m) and a combination of these two methods with various sequences including RBP-mc and RBP-cm [65] . Th ey stated that carboxymethylation led to a drop of cellulose degradation temperature from 300 to 200°C. D i ff erential scanning calorimetry (DSC) can be considered as another thermal test method to measure the glass transition temperature (Tg ) of nanocellulose samples. In this context, nanocellulose samples are heated from room temperature to 600°C under nitrogen fl ow with a rate of 10°C/min [138] . Figure 11.15 illustrates the DSC curve of bleached kenaf core, unsulfated and sulphated CNC [138] . Th ey found that the loss of water shown by the fi rst endothermic peak was around 32°C to 130°C for bleached kenaf and 32°C to 140°C for unsulfated and sulfated CNC. Th ey also stated that the second endothermic peak which represents degradation temperature was 350°C, 298°C and 200°C for bleached kenaf, unsulfated CNC and sulfated CNC, respectively. Th e authors attributed the low degradation temperature of sulfated CNC to the remaining

100 100

80 80 RBP-mc RBP-c 60 60 RBP-cm 40 40 c mass [%] 20 RBP- Weight percent (%) percent Weight 20 RBP-m b 0 a 0 100 200 300 400 500 600 100 200 300 400 500 600 700 a b Temperature (°C) ( ) Temperature (°C) ( ) Figure 11.14 TGA curves of (A) CNC from jute fi ber: (a) untreated, (b) alkali-treated and (c) CNC [137] (B) NFC from RBP-c, RBP-m, RBP-cm, RBP-mc [65] .

0 mW

–5 Bleached Kenaf Core Sulfated NCC Unsulfated NCC –10 100 200 300 400 500 800 Temperature (°C) Figure 11.15 DSC curve of bleached kenaf core fi ber and CNC [138] . 288 Nanocellulose Polymer Nanocomposites sulfate groups at the surface of CNC, which play a fl ame retardant role and lead to decreased degradation temperature.

11.4.3 Morphological Properties Th e atomic force microscopic (AFM) test is good for analyzing and evaluating the surface characteristic of nanocellulose. Normally, aft er preparation of nanocellulose suspension, a drop of this suspension is deposited onto cleaved mica, dried, and then the image is recorded at room temperature and in tapping mode [139] . Figure 11.16 shows the AFM image of NFC from soft wood [64] and CNC from MCC [140] . Th e AFM image (Figure 11.16a) exhibits a network structure, interconnected, entangled and in coiled form with width of 20 to 30 nm. As the AFM picture of CNC ( Figure 11.16b) displays, very low agglomeration occurred and the diameter of CNC was lower than 10 nm. Th e dimension and structure of the nanocellulose can be studied using transmission electron microscopy (TEM). Generally, a drop of diluted nanocellulose suspension is deposited on carbon-coated grid and is dried at room temperature. Size measurements can be done by image analyzer program [3] . Figure 11.17 illustrates the TEM image of

CNC from tunicate using H2 SO4 acid hydrolysis [115] and NFC from potato pulp [141] .

Figure 11.16 AFM images of (a) NFC from soft wood pulp [64] and (b) CNC from microcrystalline cellulose [140] .

Figure 11.17 TEM image of (a) CNC from tunicate [115] and (b) NFC from potato pulp [141] . Nanocellulose-Based Polymer Nanocomposite 289

Th e diameter of CNC is 10–20 nm and its length is a few microns, whereas the NFC diameter is around 5 nm with much higher length. Surface morphology of nanocellulose fi lms can be studied by scanning electron microscopy (SEM) analysis. In this case, samples are sputter coated with gold and then are analyzed in the instruments to avoid an increase of electrostatic charge [71] . Figure 11.18 shows the FESEM images of NFC from dissolved pulp fi ber aft er 15 passing times through grinder (Figure 11.18a) [33] , and CNC from MCC which produced from 60%

H2 SO4 hydrolysis for 2 h and at 40°C combined with HPH (Figure 11.18b) [142] . Th e diameter, length and aspect ratio of CNC were 11 nm, 199 nm and 18 nm, respectively, with uniform orientation, while NFC had 20 to 50 nm width and more than 1 μm length with aggregate structure.

11.5 Drying of Nanocellulose

One of the most important challenges related to application of nanocellulose, either NFC or CNC, is their drying process because nanocellulose is hydrophilic and tends to agglomerate. Why is the drying process of nanocellulose a considerable issue in this fi eld? To answer this question, Peng et al. [143] stated two main reasons: (1) maintain the nanosize of material for application, (2) reduce transportation cost of nanocellulose in aqueous form. Due to the hydrophilic nature of cellulose materials, during drying of nanocellulose hydrogen bonds can be generated and lead to irreversible agglomeration known as hornifi cation [65, 144], which can change the size of nanocellulosic materials as well as their unique characteristics. To tackle this drawback, a wide variety of drying methods have been employed and compared to each other by researchers. For instance, solvent exchange of CNC from water to nonaqueous solvent with lower surface tension such as acetone has been done by Ayuk et al. [145] . Sanchez-Garcia and Lagaron [146] compared freeze drying and solvent exchange of CNC and concluded that freeze-dried CNC showed better dispersion, transparency and morphology than solvent exchanged counterparts. Also, producing CNC with ionic charge using H2 SO4 hydrolysis can be considered as another solution which leads to relatively easy dispersion of this material aft er drying in aqueous media [147] . In the case of NFC, Abe et al. [41] applied undried fi bers to produce NFC and to reduce their agglomeration. Figure 11.19 displays four various drying process steps.

Figure 11.18 FESEM image of (a) NFC from dissolved pulp [33] and (b) CNC from MCC [142] . 290 Nanocellulose Polymer Nanocomposites

Freeze drying (FD)

• Frozen suspension in very low temperature (around-65°C) • Transfer suspension to freeze dryer for lyophilisation (Beck et al., 2012)

Supercritical drying (SCD) • Dehydration of suspension with non-aqueousmedia like ethanol • Repacement of non-aqueousm with liquid CO2 • Pressurizing and heating of liquid C02 and cellulose mixture to the suspercritical conditions • Elimination of liquid CO2 by decompression it to the atmosphere (peng et al., 2012)

Atomization drying (AD)

• Spraying suspension • passing material through a nozzle • drying by hot air flow. (Quievy et al, 2010)

Spray drying (SD)

• Pre-concentration of initial liquid to appropriate viscosity • Pumping liquid through atomizer • Dehydration process in stream of hot gas • Powder separation • Cooking • Packaging (Peng et al., 2012) Figure 11.19 Various nanocellulose drying processes steps.

Th e particular feature of crystallization at temperatures lower than the freezing temperature of water increases the rate of freezing, thus preventing aggregation [148] . Flow rate of suspension and temperature of hot air are some of the key factors in this context [149] . In addition, parameters such as concentration and feed rate of liquid as well as gas fl ow rate have eff ects on this process [143] . Oven drying, freeze drying and atomization process [149] , spray drying and supercritical drying [150] have been utilized by researchers to evaluate their eff ects on properties of resultant NFC.

11.6 Modifi cations of Nanocellulose

Modifi cation of NFC has received signifi cant interest from the scientifi c community. It is envisaged to improve the hydrophilic nature of cellulose in polar and nonpolar environments, thus, increasing compatibility with a wider variety of matrices. Numbers of reactions have been performed to modify the surface properties of the cellulose [151, 152] including corona or plasma discharges [153] , surface derivatization [154] , graft copolymerization [155] or application of surfactant [156, 157] . Some approaches aiming to hydrophobize nanocellulosic materials are briefl y discussed in the following sections. Nanocellulose-Based Polymer Nanocomposite 291

11.6.1 Acetylation Kim et al. [158] reported that cellulose was partially acetylated to modify its physical properties while preserving the microfi brillar morphology in which material properties were crucially infl uenced by the degree of acetyl substitution [42] . According to Ifuku et al. [159] , transparency and hygroscopicity of cellulose/acrylic resin composite materials were improved and reduced by acetylation, respectively, though the composites exhibited an optimum degree of substitution and were reduced in properties with excessive acetylation. A study by Nogi et al. [160] found that acetylation improved the thermal degradation resistance of cellulosic fi bers. Th e eff ect of biological exposure (Figure 11.20) upon the properties of acetylated and surface-treated plant fi ber-based polyester composites was studied by Abdul Khalil and Ismail [161] . It was found that acetylation exhibited superior bioresistance followed by silane, as well as cast resin and glass fi ber composites, in soil tests up to 12 months exposure. In other research of Abdul Khalil et al. [162] , modifi ed fi bers were shown to have a smoother surface compared to the unmodifi ed ones, which was believed to be a factor in improving the fi ber-matrix adhesion.

11.6.2 Silylation Isopropyl dimethylchlorosilane was used by Goussé et al. [163] for surface silylation of cellulose microfi brils resulting from the homogenization of parenchymal cell walls. Th ese authors claimed that microfi brils retained their morphology under mild silylation

Figure 11.20 SEM micrographs of unmodifi ed EFB composite: (A) severe degradation of unmodifi ed EFB, (B) acetylated EFB composite and (C) slight degradation of acetylated EFB composite (D) [161] . 292 Nanocellulose Polymer Nanocomposites conditions and could be dispersed in a non-fl occulating manner into organic solvents. Andresen et al. [70] reported that hydrophobizing of MFC via partial surface silylation using the same silylation agent resulted in partial interfacial of MFC and loss of nano- structure when silylation conditions were too harsh. Films prepared from the modifi ed cellulose by solution casting showed a very high water contact angle (117–146°). It is probable that in addition to decreased surface energy, higher surface roughness as a result of modifi cation could contribute to increased hydrophobicity. Moreover, a study by Andresen and Stenius [164] showed that hydrophobized MFC could be used for stabilization of water-in-oil type emulsions. Figure 11.21 compares the morphology of MFC samples silylated with isopropyl dimethylchlorosilane (IPDMSiCl) per glucose unit [70] .

11.6.3 Application of Coupling Agents Th e adhesion between microfi brils and epoxy resin polymer matrix is successfully enhanced by applying three diff erent coupling agents, which are 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, and a titanate coupling agent, Lica 38. Th e surface modifi cation changed the character of MFC from hydrophilic to hydrophobic, while the crystalline structure of the cellulose microfi brils remained intact. Lica 38 gave the most hydrophobic surface among the tested coupling agents, possibly due to the lower polarity of the titanate modifi er alkyl chain. Unlike silane coupling, titanate

Figure 11.21 TEM micrograph of MFC before and aft er silylation: (a) initial suspension of homogenized suspension of MFC from sugar beet; (b) as in (a) but aft er 16 h of reaction with IPDMSiCl – the molar ratio of reagent to surface AGU was of 2; (c) as in (a) and (b), but the molar ratio of reagent to surface AGU was of 4. Scale bar: 0.5 mm [70] . Nanocellulose-Based Polymer Nanocomposite 293 coupling is thought to occur via alcoholysis, surface chelation or coordination exchange. Th e monoalkoxy- and neoalkoxy-type titanium-derived coupling agents react with the hydroxyl groups present on the surface of the substrate to form a monomolecular layer [165, 166] . Nair and his partners [167] used phenyl isocyanates and alkenyl succinic anhydride to improve the quality of the interface between natural rubber and chitin whiskers with presence of 3-isopropenyl-R, R¢-dimethylbenzyl isocyanate. Th e expected chemical reactions that occur in the alternative chemical modifi cations are given in Figure 22 [167 ].

11.6.4 Graft ing Th ere are three methods reported by Stenstad et al. [40] for modifi cation of MFC by heterogeneous reactions in both water and organic solvents to produce cellulose nanofi bers with a surface layer of moderate hydrophobicity. Epoxy functionality was introduced onto the MFC surface by oxidation with cerium (IV) followed by graft ing with glycidyl methacrylate. Reactive epoxy groups served as a starting point for further functionalization with ligands, which typically do not react with the surface hydrox- yls present in native MFC. As the reaction is conducted in aqueous media, the use of organic solvents and laborious solvent exchange procedures can be avoided, which is the major advantage of this technique. In the same research conducted by these authors, graft ing of hexamethylene diisocyanate followed by reaction with amines yield a far more hydrophobic MFC surface. Succinic and maleic acid groups can be introduced directly onto the MFC surface as a monolayer by a reaction between the corresponding anhydrides and the surface hydroxyl groups of the MFC. Also, noctadecyl isocyanate

(C18 H37 NCO) has been applied as the graft ing agent in order to improve MFC compatibility with polycaprolactone [168] . Apart from this, fi ve diff erent chemicals, ethylene acrylic acid, styrene maleic anhydride, guanidine hydrochloride, and Kelcoloids HVF and LVF stabilizers (propylene glycol alginate), were used to prepare bionanocomposites from PLA and PHB as matrices by Wang and Sain [169] in order to explore the potential use of chemically coated

O OH NH O HO O HO O O NH OH n O

O O NCO O R R

O O R R O NH CH COOH O O O 2 NH O NH O HO O HO O HO O HO O O O NH n NH OH OH n O O

Figure 11.22 Chemical reactions that occur in the alternative chemical modifi cations of chitin whiskers with phenyl isocyanate, alkenyl succinic anhydride and 3-isopropenyl-R, R¢-dimethylbenzyl isocyanate [167] . 294 Nanocellulose Polymer Nanocomposites hemp nanofi bers as reinforcing agents for biocomposites. Nanofi bers were only partially dispersed in the polymers and therefore resulted in low mechanical properties compared to those predicted by theoretical calculations. Morphological analyses of sisal whiskers by Siqueira and his coworkers [168] by using N-Octadecyl isocyanate

(C18 H37 NCO) as the graft ing agent show the homogeneity and nanometric dimensions of sisal whiskers ( Figure 11.23 ). Besides enhancing compatibility of nanocellulose with nonpolar polymers and improving mechanical properties, the purpose of chemical modifi cation is to add extra functionality to nanocellulosic materials. For instance, Th omas et al. [170] reported that positively charged amine-functionalized MFC is said to be antimicrobially active in biomedical applications. Andresen and Stenius [164] also added extra functionality to MFC fi lm by covalently graft ing the cellulose with octadecyldimethyl(3-trime- thoxysilylpropyl)ammonium chloride (ODDMAC). When the atomic concentration of ODDMAC nitrogen on the fi lm surface was 0.14% or higher, the surface-modifi ed MFC fi lms showed antibacterial activity against both Gram-positive and Gram-negative bacteria, even at very low concentrations of antimicrobial agent on the surface, killing more than 99% of E. Coli and S. Aureus . Th e chronological order of events for various modifi cations of nanocellulose can be seen in Table 11.3 .

Figure 11.23 SEM image of sisal MFC: (A) optical microscopy image of sisal MFC and (B) TEM of sisal whiskers (C and D) [168] . Nanocellulose-Based Polymer Nanocomposite 295

Table 11.3 Chronological order of modifi cation of nanocellulose. Year Progress References 1989 Surface pretreatment of cellulosic fi bers, processing time and [153] temperature of cellulose-containing polypropylene 1998 Chemical modiﬁcation of coir, oil palm fi ber, fl ax, and jute [166] fi bers using acetic anhydride 2000 Stable dispersion of cellulose microcrystals in nonpolar [157] solvents by using surfactants as stabilizing agents 2001 Chemical modifi cation of oil palm empty fruit bunch and coir [162] fi ber using non-catalyzed acetic anhydride 2002 Surface acetylation of bacterial cellulose [158] 2003 Chitin whiskers surface modifi cation by using three diff erent [167] chemical coupling agents 2004 Surface silylation of cellulose microfi brils [163] 2005 Nanofi bers from natural and inorganic polymers via [170] electrospinning 2006 Individualized microfi brils from TEMPO- catalyzed oxidation [66] of native cellulose 2007 Surface modifi cation of bacterial cellulose nanofi bers by [159] dependence on acetyl-group DS 2008 Sisal-oil palm fi bers treated with varying concentrations of [152] sodium hydroxide solution and diff erent silane coupling agents 2009 Alteration of bacterial nanocellulose structure by in situ [171] modifi cation using polyethylene glycol and carbohydrate additives 2010 Modifi cation of acetylated cellulose nanofi bers from kenaf [68] using acetic anhydride 2011 Heterogeneous modifi cation of various celluloses with fatty [172] acids by an esterifi cation reaction 2012 Liquid crystal of nanocellulose whiskers graft ed with [173] acrylamide 2013 Modifi cation of native cellulose nanofi bers by functionalized- [49] few-walled carbon nanotubes for hybrid nanofi ber/nano- tube aerogels

11.7 Nanocellulose-Based Polymer Nanocomposites

Th e development of polymer nanocomposites is rapidly emerging as a multidisci- plinary research activity whose results could broaden the applications of polymers to the great benefi t of many industries. Polymer nanocomposites fi lled with nanocellulose represent a new class of material alternative to conventional fi lled polymers and possess some extremely interesting properties such as high strength and stiff ness combined with low weight, biodegradability and renewability [14] . 296 Nanocellulose Polymer Nanocomposites

During the last two decades micro/nanocellulose-reinforced composites have been the subject of intensive research and a number of review papers have appeared covering this work [14, 17, 19, 24, 53, 173, 174]. Nanocellulose either in CNC or NFC form will result in varying reinforcement of nanocomposites. Also, diff erent types of nanocellulose can be used in various forms of reinforcement, including distributed reinforcements, planar reinforcements, or continuous networked structures. Beside the above advantages of nanocellulose as reinforcement in nanocomposites, they present some disadvantages, for instance, high moisture absorption, poor wetabil- ity, incompatibility with most polymeric matrices and limitation of processing temperature. Indeed, lignocellulosic materials start to degrade near 220°C and this character restricts the type of matrix which can be used with natural fi llers [17] . To fully utilize the potential of nanocellulose as reinforcement in composite materials, the hydrophilic nature of cellulose should be altered to make it more compatible with organic solvents and nonpolar polymer matrices. Th is changing improves both the incorporation of cellulose into the composite materials, which results in more homogeneous composites, and the interfacial adhesion between nanocellulose and matrix in the fi nal composite. Th is section focuses on thermoplastic and thermoset polymer nanocomposites based on nanocelluse, their production processes, characterization and application.

11.7.1 Th ermoplastic Polymer-Nanocellulose Nanocomposites Many thermoplastic polymers have been used as matrix with nanocellulose as reinforcement such as poly(vinyl alcohol) (PVA) [130, 175] , polyurethane [176, 177] , polypyrrolle [178] , polypropylene [179] , poly(latic acid) (PLA) [180] , hydroxypropylcellulose (HPC) [74] , polyacrylamide [181] , etc. Figures 11.24 [182] and 11.25 [178] are examples of thermoplastics reinforced by nanocellulose. Poly(vinyl alcohol), which is one of the thermoplastic polymers, is a water soluble synthetic polymer, has excellent fi lm forming and emulsifying properties [183] , is interfacial, easy to process, has good physical and chemical properties [184] and is inexpensive [185] . It also has high tensile strength and fl exibility [130] . Because of these interesting properties, PVA has been the subject of much research in this fi eld. For example, Cho and Park [183] investigated the mechanical and thermal properties of PVA-based nanocomposites reinforced with CNC isolated from MCC. Th ey cast nanocellulose suspension-PVA on a Tefl on-coated petri dish. Th eir results showed that the tensile modulus decreased at 1 wt% CNC loading, and then increased with an increase

LDPE + Whiskers LDPE + modified LDPE 90: 10 Whiskers C18 90: 10

Figure 11.24 Photographs of neat fi lm and nanocomposite fi lm (LDPE) based on ramie CNC [182] . Nanocellulose-Based Polymer Nanocomposite 297

Figure 11.25 Photographs of MFC paper (a) and a MFC- polypyrrolle composite (b) [179] .

5 100

4 80

3 60

2 40 Tensile modulus (GPa) Tensile

1 (MPa) strength Tensile 20

0 0 01357 01357 Nano cellulose content (wt%) Nano cellulose content (wt%) Figure 11.26 Tensile modulus and tensile strength of PVA nanocomposites as function of CNC content [183] . in the CNC content up to 5 wt%, followed by a leveling off at higher CNC content ( Figure 11.26 ) [183] . B u l o t a et al. [186] prepared nanocomposite fi lm from NFC-PVA at various NFC loading. Th ey stated that the highest modulus was obtained at a solution concentration of 4 up to 5% (w/w) of NFC. As they stated, reinforcement eff ects and an increase in the dispersion viscosity were obvious at 5% NFC loading, implying that the percolation phenomenon took place at this loading. Furthermore, many other thermoplastic polymers have been used with CNC to produce nanocomposite. Th e functionalized CNC from ramie, which was graft ed by organic acid chlorides, reinforced low-density polyethylene (LDPE) nanocomposites studied by Junior de Menezes et al. [182] . Native and surface-trimethylsilylated CNC were employed as the particulate phase in nanocomposites with a cellulose acetate butyrate matrix to improve the mechanical properties of polymers and to enhance adhesion between the particulate and matrix phase in composites [187] . Poly(oxyethylene)-based polymer electrolytes should be used above their melting temperature to display appropriate conductivity. Unfortunately, at this temperature the mechanical properties were very poor. In this regard, Azizi Samir et al. [188] evaluated the eff ect of CNC extracted from tunicate to improve the mechanical properties of poly(oxyethylene) (PEO)-based nanocomposite electrolytes above its melting temperature. Th e SEM fracture surface of CNC-PEO 298 Nanocellulose Polymer Nanocomposites

Figure 11.27 SEM image for CNC extracted from tunicate of poly(oxyethylene) composites: (a) unfi lled POE matrix and related composites fi lled with (b) 3 wt% and (c) 6 wt% tunicin [188] . composite is illustrated in Figure 11.27 [188]. As the SEM images show, some holes can be seen which are attributed to entrapped air within the fi lm during the water evaporation step despite degassing of the suspension. In addition, NFC can be utilized to reinforce thermoplastic polymers. For instance, Nyström et al. [6] produced a nanocomposite from NFC isolated from wood and poly- pyrrole. Th ey demonstrated that it is possible to coat the individual NFC with poly- pyrrole using in-situ chemical polymerization to obtain an electrically conducting continuous high-surface-area nanocomposite. Johnson et al. [74] studied a new biobased nanocomposite using TEMPO-oxidized NFC through high-intensity ultrasonication in hydroxypropylcellulose (HPC) matrix. In bionanocomposites, the polymer matrix also should be biodegradable. In this context, Poly(ε-caprolactone) (PCL) as semicrystalline biopolymer with a glass transition temperature around −60°C and a melting temperature around 60°C can be considered to manufacture bionanocomposite. Th erefore, NFC covalently graft ed with PCL via ring-opening polymerization of ε-caprolactone was introduced by Lonnberg et al. [189] . Th ey evaluated the eff ect of PCL graft length and ring-opening polymerization on the mechanical properties of the bionanocomposite in diff erent molecular weights of the graft s. Qu et al. [190] added poly(ethylene glycol-1000) (PEG) as a compatibilizer to PLA in order to improve the interfacial interaction between the hydrophobic PLA and the hydrophilic NFC. Th eir results illustrated that when the PEG was added to the blend of PLA and NFC, the composites showed signifi cant improvements in tensile strength and elongations.

11.7.2 Th ermoset Polymer-Nanocellulose Nanocomposites Th ermosetting composites can be cured with low or no heat, which can be advanta- geous for limited thermal stability of nanocellulose. Th ermosetting plastics are polymer materials that irreversibly cure. Th e curing may be done through heating, radiation or a chemical reaction (e.g., a two-part epoxy). Once the curing is complete, the thermoset cannot be melted into a liquid form. Several thermosets, including epoxy, formalde- hydes and polyester, have been investigated for use with nanocellulose in nanocomposites. A high-strength elastomeric nanocomposite has successfully been prepared by dispersing microcrystalline cellulose in a polyurethane matrix [176, 177]. Th e SEM and TEM micrographs show the use of CNC with water-bond polyurethane as a matrix in Figures 11.28 [177 ] and 11.29 [176]. Epoxy-based nanocellulose composites have the potential for wide application due to their high mechanical properties. Epoxy is a thermosetting copolymer, also known Nanocellulose-Based Polymer Nanocomposite 299

Figure 11.28 SEM images of water-bond polyurethane CNC nanocomposite with diff erent loading [178].

Figure 11.29 TEM image of polyurethane nanocomposite with CNC [177] . as polyepoxide, formed from the reaction of an epoxide resin with polyamine hardener. Masoodi et al. [191] compared traditional epoxy and biobased epoxy reinforced with NFC. Th e wet layup process was employed to manufacture the double cantilever beam specimens. Th ey found that the biobased epoxy had similar performance characteristics for fracture toughness compared to the standard epoxy and also did not show reduc- tions at room temperature test conditions. Lu et al. [165] reported surface treatments of NFC with three diff erent coupling agents including 3-aminopropyltriethoxysilane (APS), 3-glycidoxypropyl trimethoxysilane, and a titanate coupling agent reinforced epoxy resin using acetone solvent. Th e eff ect of silane-treated NFC in unsaturated polyester and epoxy resin matrices was studied by Abdelmouleh et al. [192] . Th ey found that the large loss of mechanical properties was related to insuffi cient silane treatment to prevent NFC from water absorption. Epoxy and polyester manufacturing of nanocomposite fi lm from TEMPO-oxidized NFC and water-soluble phenol formaldehyde was also the subject of research by Qing et al. [193] . Th e SEM micrographs of the fi lm clearly presented the lamellar structure of NFC in cross-section of the neat fi lm as well as the composite. Th e mechanical properties of NFC-reinforced themoset polymer matrices are presented in Table 11.4 . Nanocelluloses have been mixed or dispersed in various resins using a wide variety of processing techniques. Gong et al. [195] prepared composite from both NFC and CNC in polyvinyl acetate using a master batch followed by melt extrution. Yang et al. 300 Nanocellulose Polymer Nanocomposites

Table 11.4 Mechanical properties of thermoset polymer matrices-fi lled NFC. Resin Nano- Strength Elastic Strain to Ref. cellulose (Mpa) modulus failure (%) form (Gpa) Epoxy NFC fi lm 1.5–2.9 [191] (0–5% fi ber content) Phenol MFC fi lm 201-216 4.16 – 4.48 12.6 – 14.7 [193] formaldehyde (5–20% wt fi bers) Melamin NFC (13% wt 142 16.6 0.81 [194] formaldehyde fi ber)

[196] used three general processes, viz., melt blending, grinding, and injection mold- ing, to produce CNC-polypropylene nanocomposites. Agarwal et al. [197] prepared the nanocomposite from polypropylene reinforced with CNC by extrusion method. While techniques for preparation of nanocellulose-reinforced nanocomposite are diff erent in complexity, they typically involve physically mixing and dispersing the nanocellulose and resin in a solvent system. In many cases, solvent exchange techniques are used, oft en along with surface modifi cation of nanocellulose to make it compatible with organic solvents and/or the resin system. In this context, nanocomposite fi lms from nanocellulose generally are prepared through three various techniques as below:

1. by casting on Tefl on or propylene dishes followed by water evaporation at moderate temperatures; 2. by freeze-drying and hot-pressing; or 3. by freeze-drying, extruding, and hot-pressing the mixture.

Sehaqui et al. [ 198 ] reported manufacturing NFC-reinforced hydroxyethylcellu- lose (HEC) fi lm in a polystyrene Petri dish under air atmosphere at room temperature with a thickness of 65–80 mm. Th e preparation process of the nanocomposite fi lm are illustrated in Figure 11.30 [198 ]. Gray [199] has studied the transcrystallization of polypropylene at CNC surface. Th ere is a resurgence of interest in composite materials incorporating cellulose as fi brous reinforcement in semicrystalline melt-processed polymers. Potential natural cellulose sources range from fl ax and ramie fi bers down to whiskers and nanocrystals isolated from bacteria ( Figure 11.31 ) [199] . Processing techniques have an important infl uence on the fi nal properties of the nanocomposites based on nanocellulose. Th erefore, the achievement of superior strength in the properties of nanocomposite based on nanocellulose can be used for many applications. However, the use of nanocellulose as a reinforcement is in its infancy, and the full reinforcing potential of nanocomposites has yet to be realized partly because of issues related to scaling up of the manufacturing processes. Nanocellulose becomes very important because incorporation of nano-reinforcement has been related to improvement in overall performance of nanocomposites. Nanocellulose-Based Polymer Nanocomposite 301

Cellulose fibrils suspended HEC water solution In water Vaccum filtration and drying for NFC nanopaper preparation

Magnetic stirrer

ration Vaccum filt and drying

HEC/NFC nanostructed Cellulose fibrils suspended biocomposites In HEC solution Figure 11.30 Preparation scheme for NFC nanopaper (arrow to the left ) and NFC/HEC biocomposites [198].

Microscope Transcrystalline layer at edge of film

Cover glass

Polypropylene melt Nano crystal film Pitch Spherulite

Microscope slide

Figure 11.31 Schematic cross-section of a sample of CNC fi lm embedded in crystallizing polypopylene melt [199].

Over the decades, this particular fi eld of study has become more interesting, leading to the advancement of nanocellulose characteristics. Th is is because many researchers have found that the properties of nanocellulose play an important role in nanocomposite. A better understanding of organic and polymer chemistries enables the current research to look deeper into the interaction between polymer matrix and nanocellulose, hence, leading to the advancement of nanocellulose values

11.7.3 Application of Nanocomposites Based on Nanocellulose Th e development and the application of polymeric composite materials fi lled with nanosized rigid particles (essentially inorganic) has attracted both scientifi c and industrial interest. Th e development of new polymer electrolytes is needed for many kinds of electrochemical applications such as separators in high-energy density lithium batteries. Poly(oxyethylene) (POE)-based polymer electrolytes are the most commonly studied, due to their cationic solvation ability [188] . Reinforcement using nanofi ber is nowadays dispersed in many fi elds not only in hard composite but in thin fi lm too. Consequently, its application could cover pretty much in 302 Nanocellulose Polymer Nanocomposites

EFB Kenaf Coconut Wood Bagasse

Cellulosic Materials

Product application: Nanocellulose Polymers Paper & Packaging Constraction CNC Thermoplastic Processing Automotive NFC Thermoset Furniture Electronic Pharmacy cosmetics

Figure 11.32 Graphic depicting the application of nanocomposites based on nanocellulose. every industry from material reinforcement for construction to food packaging. In the beginning of composite development, natural fi ber was mixed with petroleum-based polymer to create composite. Other than that, the petroleum-based polymer also was combined with natural bio-derived polymer to create green composite. Th e purpose of introducing biobased material into the petroleum-based polymer is to enhance biodegradability. As time goes by, more advanced material was created by disintegration of natural fi ber into nanosize fi ber, which was fi nally mixed with nanofi ber to create nanocomposite. Fully green nanocomposite is thereforeemerging, as green technology has become serious business and the research about this particular fi eld is to be studied [174] . Many products can be made from NFC- and NCC-reinforced polymer matrices such as products from CNF, i.e., fi lm and fl exible packaging, compostable replacement for plastic fi lm (trash bags and grocery bags), high-quality paper and board products, super-strong pulp, mineral paper, strong tissue products, wood fi ber composites, wood panels. Iridescent NCC fi lms; such as deposition on glass, plastics, applications in cosmetics and architectural industries, security paper, inks, varnishes and coatings, IR refl ectance, thermal barriers, UV refl ectance, and UV barriers. Applications of all the above nanocellulose-reinforced nanocomposites with thermoplastic and thermoset polymers are mainly considered to be in paper and packaging products, construction materials, automobiles, furniture, electronics, pharmaceuticals, and cosmetics. Figure 11.32 shows the application of nanobiocomposites.

11.8 Conclusion

Th is chapter has focused on the isolation techniques and fundamental properties of nanocellulose that have been developed so far, and ultimately the application of this nanocellulose in composites. Natural fi bers are commonly used to produce nanocelluloses which contain both crystalline and amorphous regions at varying proportions depending on the species. Th us, the characteristics of nanocellulosic materials depend Nanocellulose-Based Polymer Nanocomposite 303 largely on the raw materials. A cellulose nanofi ber has more than 200 times the surface area of isolated soft wood cellulose and possesses higher water-holding capacity, higher crystallinity, higher tensile strength, and a fi ner web-like network. In combination with a suitable matrix polymer, cellulose nanofi ber networks show considerable potential as eff ective reinforcement for high-quality specialty application of biobased composites. However, the use of cellulose nanofi bers as nano-reinforcement in composites still has to overcome some issues: 1) all the works related to bionanocomposites are still in labo- ratory scale, mass production technique is unknown, 2) the nanofi ber isolation process consumes a large amount of energy, water and chemicals, and 3) because of the higher density of –OH groups on the surface of the nanofi bers, they are mostly restricted to water-soluble polymers. Scientists are working to overcome these problems by applying diff erent types of pretreatments and surface modifi cation treatments. All these treatments improve the process of isolation and properties of nanocellulose signifi cantly. Th ese cellulose nanofi ber-reinforced composites can be used in medical devices, nanopaper, construction materials, automobiles, sports equipment, electronics, pharmaceuticals, cosmetics, packaging and so on. However, further research would improve the properties of nanocelluloses as well as the properties of bionanocomposites.

Acknowledgement

Th e authors would like to thanks to Universiti Sains Malaysia (USM), Penang, Malaysia, for providing Research Grant No. RU-1001/PTEKIND/811195 and RU-I 1001/ PTEKIND/814133 for completing this review.

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