Nanocellulose from fundamentals to advanced materials 9783527342693, 3527342699, 9783527807437, 3527807438, 9783527807444, 3527807446, 9783527807468, 3527807462

Table of contents :
Content: Preface xiii Acknowledgments xv 1 Introduction to Nanocellulose 1Jin Huang, Xiaozhou Ma, Guang Yang, and Dufresne Alain 1.1 Introduction 1 1.2 Preparation of Nanocellulose 2 1.2.1 Cellulose Nanocrystals 2 1.2.2 Cellulose Nanofibers 3 1.2.3 Bacterial Nanocellulose 4 1.3 Surface Modification of Nanocellulose 4 1.3.1 Esterification 7 1.3.2 Oxidation 7 1.3.3 Etherification 8 1.3.4 Amidation 8 1.3.5 Other Chemical Methods 8 1.3.6 Physical Interaction 9 1.4 Nanocellulose-Based Materials and Applications 9 1.5 Conclusions and Prospects 13 References 15 2 Structure and Properties of Cellulose Nanocrystals 21Chunyu Chang, Junjun Hou, Peter R. Chang, and Jin Huang 2.1 Introduction 21 2.2 Extraction of Cellulose Nanocrystals 21 2.2.1 Extraction of Cellulose Nanocrystals by Acid Hydrolysis 21 2.2.2 Pretreatments of Cellulose Before Acid Hydrolysis 27 2.2.3 Other Methods of Preparing Cellulose Nanocrystals 31 2.3 Structures and Properties of Cellulose Nanocrystals 32 2.3.1 Physical Properties of Cellulose Nanocrystals 32 2.3.2 Properties of Cellulose Nanocrystal Suspension 39 References 45 3 Structure and Properties of Cellulose Nanofibrils 53Pei Huang, Chao Wang, Yong Huang, and Min Wu 3.1 Production of CNF 53 3.1.1 Chemical Bleaching 54 3.1.2 Mechanical Disintegration 54 3.1.2.1 Homogenization 54 3.1.2.2 Grinding 58 3.1.2.3 Ball-milling 59 3.1.2.4 Ultrasonication 59 3.1.2.5 Steam Explosion 61 3.1.2.6 Aqueous Counter Collision 61 3.1.2.7 Refining 62 3.1.2.8 Cryocrushing 62 3.1.2.9 Twin-Screw Extrusion 62 3.1.2.10 Other Methods 63 3.1.3 Pretreatment 63 3.2 Features and Properties 64 3.2.1 Morphology of CNF 64 3.2.2 Rheology 64 3.2.3 CNF in Different Forms 65 3.2.3.1 Suspensions 65 3.2.3.2 Powders 66 3.2.3.3 Films 67 3.2.3.4 Hydrogels 70 3.2.3.5 Aerogels CNF 72 3.3 Conclusion 72 References 74 4 Synthesis, Structure, and Properties of Bacterial Cellulose 81Muhammad Wajid Ullah, Sehrish Manan, Sabella J. Kiprono, Mazhar Ul-Islam, and Guang Yang 4.1 Introduction 81 4.2 Biogenesis of Bacterial Cellulose 83 4.2.1 Biochemistry of BC Synthesis 83 4.2.2 Biochemical Pathway of BC Production 85 4.2.3 Molecular Regulation of BC Synthesis 87 4.3 Structure and Exciting Features of Bacterial Cellulose 88 4.3.1 Chemical Structure and Properties 89 4.3.2 Physiological Features 89 4.3.3 Self-assembly and Crystallization 90 4.3.4 Ultrafine Thin Fibrous Structure 90 4.3.5 Macrostructure Control and Orientation 91 4.3.6 Porosity and Materials Absorption Potential of BC for Composite Synthesis 91 4.3.7 Biocompatibility 92 4.3.8 Biodegradability 92 4.4 Production of Bacterial Cellulose: Synthesis Approaches 93 4.4.1 Static Fermentative Cultivation: Production of BC Membrane, Film, or Sheet 93 4.4.2 Shaking Fermentative Cultivation: Production of BC Pellets 94 4.4.3 Agitation Fermentative Cultivation: Production of BC Granules 94 4.4.3.1 Rotating Disk Reactor 95 4.4.3.2 Trickling Bed Reactor 95 4.5 Additives to Enhance BC Production 95 4.5.1 Carboxymethylcellulose 97 4.5.2 Organic Acids 97 4.5.3 Vitamin C 97 4.5.4 Sodium Alginate 99 4.5.5 Alcohols 99 4.5.6 SSGO 99 4.5.7 Lignosulfate 100 4.5.8 Agar and Xanthan 100 4.5.9 Thin Stillage 100 4.6 Strategies Toward Low-Cost BC Production 101 4.6.1 Fruit Juices 101 4.6.2 Sugarcane Molasses 101 4.6.3 Agricultural and Industrial Wastes 103 4.6.4 Food Wastes 104 4.7 Conclusions and Future Prospects 105 Acknowledgment 105 References 106 5 Surface Chemistry of Nanocellulose 115Ge Zhu and Ning Lin 5.1 Brief Introduction to Nanocellulose Family 115 5.1.1 Cellulose Nanocrystals (CNCs) 115 5.1.2 Cellulose Nanofibrils (CNFs) 117 5.1.3 Bacterial Cellulose (BC) 117 5.2 Surface Modification of Nanocellulose 119 5.2.1 Physical Adsorption of Surfactants 119 5.2.2 Sulfonation 121 5.2.3 TEMPO-oxidation 122 5.2.4 Esterification 123 5.2.5 Silylation 125 5.2.6 Grafting Onto 126 5.2.7 Grafting From 131 5.2.7.1 Ring-Opening Polymerization (ROP) 132 5.2.7.2 Living Radical Polymerization (LRP) 134 5.2.8 Chemical Modification from End Hemiacetal 137 5.3 Advanced Functional Modifications 139 5.3.1 Fluorescent and Dye Molecules 139 5.3.2 Amino Acid and DNA 142 5.3.3 Self-cross-linking of Nanocrystals 144 References 145 6 Current Status of Nanocellulose-Based Nanocomposites 155Xiaozhou Ma, Yuhuan Wang, Yang Shen, Jin Huang, and Alain Dufresne 6.1 Introduction 155 6.2 Cellulose Nanocrystal-Filled Nanocomposites 156 6.2.1 Polyolefin-Based Nanocomposites 156 6.2.2 Rubber-Based Nanocomposites 161 6.2.3 Polyester-Based Nanocomposites 164 6.2.4 Polyurethane- and Waterborne Polyurethane-Based Nanocomposites 167 6.2.5 Epoxy- and Waterborne Epoxy-Based Nanocomposites 169 6.2.6 Natural Polymer-Based Nanocomposites 171 6.3 Fibrillated Cellulose-Filled Nanocomposites 172 6.3.1 Polyolefin-Based Nanocomposites 172 6.3.2 Rubber-Based Nanocomposites 176 6.3.3 Polyester-Based Nanocomposites 178 6.3.4 Polyurethane- andWaterborne Polyurethane-Based Nanocomposites 180 6.3.5 Natural Polymer-Based Nanocomposites 182 6.3.6 Other Polymer Nanocomposites Filled with Fibrillated Cellulose 184 6.4 Conclusion and Prospect 186 References 186 7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites 201Yaoyao Chen, Lin Gan, Jin Huang, and Alain Dufresne 7.1 Percolation Approach 201 7.1.1 Mean-Field Theory 202 7.1.2 Percolation Model 204 7.1.3 Factors Influencing the Percolation Network Formation 208 7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix 211 7.2.1 Effect of Functional Groups on CNC Surface on Interfacial Interaction 211 7.2.2 Effect of Segmental Entanglement Mediated with Grafted Chains on CNC Surface 225 7.2.3 Role of Co-continuous Structure Derived from Chemical Coupling of Filler/Matrix 229 7.2.3.1 Thiol ene Coupling Process Between Modified Cellulose Nanocrystals (CNCs) and Matrix 230 7.2.3.2 Huisgen Cycloaddition Click Chemistry Between Modified CNCs and Matrices 232 7.2.3.3 Schiff's Base Reaction Between Cellulose Nanocrystals (CNCs) and Matrix 233 7.2.3.4 Esterification Reaction Between CNCs and The Matrix 237 7.2.3.5 Chemical Coupling Between Hydroxyl Groups of Matrix and Aldehyded CNCs or Modified CNCs 237 7.3 Conclusions 242 References 243 8 Role of Cellulose Nanofibrils in Polymer Nanocomposites 251Thiago H. S. Maia, Marilia Calazans, Vitor Lima, Francys K. V.Moreira, and Alessandra de Almeida Lucas 8.1 Introduction 251 8.2 Characteristics of Cellulose Nanofibrils 252 8.3 Mechanical Properties of CNF Polymer Nanocomposites 253 8.3.1 Thermoset Resins 254 8.3.2 Thermoplastics 255 8.3.3 Waterborne Polymer Systems 257 8.4 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites 258 8.5 Effect of Fiber Size and Lignin Presence 264 8.6 Multifunctionality: Optical and Barrier Properties of CNF Nanocomposites 267 8.7 Outlooks in CNF Nanocomposites 269 References 269 9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals 277Lin Gan, Siyuan Liu, Dong Li, and Jin Huang 9.1 Self-assembly Structure of CNCs 277 9.1.1 Structure of CNC Liquid Crystals 278 9.1.2 Components of CNC Self-assembly 279 9.1.3 Form of CNC Self-assembly Products 279 9.2 Self-assembly Methods and Materials 281 9.2.1 Casting Method and Spin Coating Method 281 9.2.2 Vacuum-Assisted Self-assembly 283 9.2.3 Evaporation-Induced Self-assembly 284 9.3 Structural Adjustment of CNC Self-assembly 284 9.3.1 Cholesteric Structure of Neat CNC Films 284 9.3.2 Cholesteric Structure and Cross-linking Structure in Gel 286 9.3.3 Cholesteric Structure in Bulk Materials of CNC Composite Self-assembly 288 9.3.4 Nematic Structure 290 9.4 Modifying Surface Chemical Structure of CNC 291 9.5 Properties of CNC Self-assembly 295 9.5.1 Mechanical Properties 295 9.5.1.1 Mechanical Properties of CNC Films 295 9.5.1.2 Mechanical Properties of CNC Composite Films 295 9.5.2 Iridescent Color 298 9.5.2.1 Iridescent Color Control of CNC Films 298 9.5.2.2 Iridescent Color Control of CNC Composite Materials 300 9.5.2.3 Optical Control of CNC Self-assembly Gels 302 9.5.3 Plasmonic Properties of CNC 304 9.6 Potential Applications 305 9.6.1 Oil/Water Separation 305 9.6.2 Application of Optical Materials 306 9.6.2.1 Optical Application of CNC Films 306 9.6.2.2 Optical Application of CNC Composite Films 306 9.6.3 Sensors 307 References 309 10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals 315Shiyu Fu and Linxin Zhong 10.1 Colloidal Properties of CNC and Applications in Functional Materials 315 10.2 Nanocellulose for Paper and Packaging 324 10.2.1 Nanocellulose for Paper Coating 326 10.2.2 Microfibrillated Cellulose Coated Paper for Delivery System 328 10.2.3 Water-Resistant Nanopaper Based on Modified Nanocellulose 329 10.2.4 Effect of Chemical Composition on Microfibrillar Cellulose Film 334 10.2.5 Antimicrobial Diffusion Films Based on Microfibrillated Cellulose 336 10.3 Nanocellulose for Wood Coatings 339 References 341 11 Strategies to Explore Biomedical Application of Nanocellulose 349Yanjie Zhang, Peter R. Chang, Xiaozhou Ma, Ning Lin, and Jin Huang 11.1 Introduction 349 11.2 Research on Biological Toxicity of Nanocellulose 349 11.3 Application of Nanocellulose for Immobilization and Recognition of Biological Macromolecules 355 11.4 Application of Nanocellulose for Cell Imaging 360 11.5 Application of Nanocellulose for Cell Scaffolds 361 11.6 Application of Nanocellulose in Tissue Engineering 366 11.6.1 Tissue Repairing, Regeneration, and Healing 366 11.6.1.1 Skin Tissue Repairing 368 11.6.1.2 Bone Tissue Regeneration 370 11.6.2 Tissue Replacement 371 11.6.2.1 Artificial Blood Vessels 371 11.6.2.2 Soft Tissues, Meniscus, and Cartilage 373 11.6.2.3 Nucleus Pulposus Replacement 375 11.7 Application of Nanocellulose in Drug Carrier and Delivery 375 11.8 Application of Nanocellulose as Biomedical Materials 382 11.8.1 Antimicrobial Nanomaterials 382 11.8.1.1 Nanocellulose Incorporated with Inorganic Antimicrobial Agents 385 11.8.1.2 Nanocellulose Incorporated with Organic Antimicrobial Agents 386 11.8.2 Medical Composite Material 388 11.9 Summary 389 References 389 12 Application of Nanocellulose in Energy Materials and Devices 397Gang Chen and Zhiqiang Fang 12.1 Introduction 397 12.2 Nanocellulose for Lithium Ion Batteries (LIBs) 398 12.2.1 Nanocellulose-Based Electrodes 398 12.2.2 Nanocellulose-Based Separators 401 12.2.3 Nanocellulose-Based Electrolytes 403 12.2.4 Nanocellulose-Based Binders 403 12.3 Nanocellulose for Supercapacitors 404 12.3.1 Nanocellulose As a Substrate 405 12.3.2 Nanocellulose As a Nano-template 406 12.3.3 Nanocellulose As a Mesoporous Membrane 410 12.4 Nanocellulose for Other Energy Devices 411 12.4.1 Fuel Cells 411 12.4.2 Solar Cells 412 12.4.3 Nanogenerators 414 12.5 Conclusion and Outlook 415 References 416 13 Exploration of Other High-Value Applications of Nanocellulose 423Ruitao Cha, Xiaonan Hao, Kaiwen Mou, Keying Long, Juanjuan Li, and Xingyu Jiang 13.1 Fire Resistant Materials 423 13.1.1 Introduction 423 13.1.2 Flame Retardant Additives 424 13.1.2.1 Halogenated Flame Retardants 424 13.1.2.2 Phosphorus-Based Flame Retardants 424 13.1.2.3 Nitrogen-Based Flame Retardants 424 13.1.2.4 Silicon-Based Flame Retardants 424 13.1.2.5 Mineral Flame Retardants 425 13.1.2.6 Nanoparticles 425 13.1.3 Fire Resistance of Clay Nanopaper Based on Nanocellulose 425 13.1.4 Conclusion 432 13.2 Thermal Insulation Materials 432 13.2.1 Introduction 432 13.2.2 Thermal Building Insulation Materials 432 13.2.2.1 Mineral Wool 433 13.2.2.2 Expanded Polystyrene (EPS) 433 13.2.2.3 Polyurethane (PUR) 433 13.2.2.4 Aerogel 433 13.2.3 Thermal Insulation Performance of Nanocellulose-Based Materials 434 13.2.4 Conclusion 437 13.3 The Templated Materials 438 13.3.1 Introduction 438 13.3.2 Synthesis of Magnetic Composite Aerogels 442 13.3.3 Synthesis of Inorganic Hollow Nanotube Aerogels 454 13.3.4 The Self-assembled CNC Templates 458 13.3.5 Conclusion 464 References 464 Index 475

Citation preview

Nanocellulose From Fundamentals to Advanced Materials

Edited by Jin Huang, Alain Dufresne, and Ning Lin

Editors Prof. Jin Huang

Southwest University School of Chemistry and Chemical Engineering, and Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing Tiansheng Road 2 Beibei District 400715 Chongqing China Prof. Alain Dufresne

Grenoble INP-Pagora International School of...Lab. Génie des 461 rue de la Papeterie 38402 Saint Martin d’Hères cedex France Dr. Ning Lin

Wuhan University of Technology School of Chemistry, Chemical Engineering and Life Sciences 122 Luoshi Road 430070 Wuhan China

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10 9 8 7 6 5 4 3 2 1

v

Contents Preface xiii Acknowledgments xv 1

Introduction to Nanocellulose 1 Jin Huang, Xiaozhou Ma, Guang Yang, and Dufresne Alain

1.1 1.2 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5 1.3.6 1.4 1.5

Introduction 1 Preparation of Nanocellulose 2 Cellulose Nanocrystals 2 Cellulose Nanofibers 3 Bacterial Nanocellulose 4 Surface Modification of Nanocellulose 4 Esterification 7 Oxidation 7 Etherification 8 Amidation 8 Other Chemical Methods 8 Physical Interaction 9 Nanocellulose-Based Materials and Applications 9 Conclusions and Prospects 13 References 15

2

Structure and Properties of Cellulose Nanocrystals 21 Chunyu Chang, Junjun Hou, Peter R. Chang, and Jin Huang

2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.3.1 2.3.2

Introduction 21 Extraction of Cellulose Nanocrystals 21 Extraction of Cellulose Nanocrystals by Acid Hydrolysis 21 Pretreatments of Cellulose Before Acid Hydrolysis 27 Other Methods of Preparing Cellulose Nanocrystals 31 Structures and Properties of Cellulose Nanocrystals 32 Physical Properties of Cellulose Nanocrystals 32 Properties of Cellulose Nanocrystal Suspension 39 References 45

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Structure and Properties of Cellulose Nanofibrils 53 Pei Huang, Chao Wang, Yong Huang, and Min Wu

3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6 3.1.2.7 3.1.2.8 3.1.2.9 3.1.2.10 3.1.3 3.2 3.2.1 3.2.2 3.2.3 3.2.3.1 3.2.3.2 3.2.3.3 3.2.3.4 3.2.3.5 3.3

Production of CNF 53 Chemical Bleaching 54 Mechanical Disintegration 54 Homogenization 54 Grinding 58 Ball-milling 59 Ultrasonication 59 Steam Explosion 61 Aqueous Counter Collision 61 Refining 62 Cryocrushing 62 Twin-Screw Extrusion 62 Other Methods 63 Pretreatment 63 Features and Properties 64 Morphology of CNF 64 Rheology 64 CNF in Different Forms 65 Suspensions 65 Powders 66 Films 67 Hydrogels 70 Aerogels CNF 72 Conclusion 72 References 74

4

Synthesis, Structure, and Properties of Bacterial Cellulose 81 Muhammad Wajid Ullah, Sehrish Manan, Sabella J. Kiprono, Mazhar Ul-Islam, and Guang Yang

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6

Introduction 81 Biogenesis of Bacterial Cellulose 83 Biochemistry of BC Synthesis 83 Biochemical Pathway of BC Production 85 Molecular Regulation of BC Synthesis 87 Structure and Exciting Features of Bacterial Cellulose 88 Chemical Structure and Properties 89 Physiological Features 89 Self-assembly and Crystallization 90 Ultrafine Thin Fibrous Structure 90 Macrostructure Control and Orientation 91 Porosity and Materials Absorption Potential of BC for Composite Synthesis 91 Biocompatibility 92 Biodegradability 92 Production of Bacterial Cellulose: Synthesis Approaches 93

4.3.7 4.3.8 4.4

Contents

4.4.1 4.4.2 4.4.3 4.4.3.1 4.4.3.2 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5 4.5.6 4.5.7 4.5.8 4.5.9 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.7

Static Fermentative Cultivation: Production of BC Membrane, Film, or Sheet 93 Shaking Fermentative Cultivation: Production of BC Pellets 94 Agitation Fermentative Cultivation: Production of BC Granules 94 Rotating Disk Reactor 95 Trickling Bed Reactor 95 Additives to Enhance BC Production 95 Carboxymethylcellulose 97 Organic Acids 97 Vitamin C 97 Sodium Alginate 99 Alcohols 99 SSGO 99 Lignosulfate 100 Agar and Xanthan 100 Thin Stillage 100 Strategies Toward Low-Cost BC Production 101 Fruit Juices 101 Sugarcane Molasses 101 Agricultural and Industrial Wastes 103 Food Wastes 104 Conclusions and Future Prospects 105 Acknowledgment 105 References 106

5

Surface Chemistry of Nanocellulose 115 Ge Zhu and Ning Lin

5.1 5.1.1 5.1.2 5.1.3 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.2.6 5.2.7 5.2.7.1 5.2.7.2 5.2.8 5.3 5.3.1 5.3.2 5.3.3

Brief Introduction to Nanocellulose Family 115 Cellulose Nanocrystals (CNCs) 115 Cellulose Nanofibrils (CNFs) 117 Bacterial Cellulose (BC) 117 Surface Modification of Nanocellulose 119 Physical Adsorption of Surfactants 119 Sulfonation 121 TEMPO-oxidation 122 Esterification 123 Silylation 125 Grafting Onto 126 Grafting From 131 Ring-Opening Polymerization (ROP) 132 Living Radical Polymerization (LRP) 134 Chemical Modification from End Hemiacetal 137 Advanced Functional Modifications 139 Fluorescent and Dye Molecules 139 Amino Acid and DNA 142 Self-cross-linking of Nanocrystals 144 References 145

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Current Status of Nanocellulose-Based Nanocomposites 155 Xiaozhou Ma, Yuhuan Wang, Yang Shen, Jin Huang, and Alain Dufresne

6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.4

Introduction 155 Cellulose Nanocrystal-Filled Nanocomposites 156 Polyolefin-Based Nanocomposites 156 Rubber-Based Nanocomposites 161 Polyester-Based Nanocomposites 164 Polyurethane- and Waterborne Polyurethane-Based Nanocomposites 167 Epoxy- and Waterborne Epoxy-Based Nanocomposites 169 Natural Polymer-Based Nanocomposites 171 Fibrillated Cellulose-Filled Nanocomposites 172 Polyolefin-Based Nanocomposites 172 Rubber-Based Nanocomposites 176 Polyester-Based Nanocomposites 178 Polyurethane- and Waterborne Polyurethane-Based Nanocomposites 180 Natural Polymer-Based Nanocomposites 182 Other Polymer Nanocomposites Filled with Fibrillated Cellulose 184 Conclusion and Prospect 186 References 186

6.2.5 6.2.6 6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6 6.4

7

Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites 201 Yaoyao Chen, Lin Gan, Jin Huang, and Alain Dufresne

7.1 7.1.1 7.1.2 7.1.3 7.2

Percolation Approach 201 Mean-Field Theory 202 Percolation Model 204 Factors Influencing the Percolation Network Formation 208 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix 211 Effect of Functional Groups on CNC Surface on Interfacial Interaction 211 Effect of Segmental Entanglement Mediated with Grafted Chains on CNC Surface 225 Role of Co-continuous Structure Derived from Chemical Coupling of Filler/Matrix 229 Thiol−ene Coupling Process Between Modified Cellulose Nanocrystals (CNCs) and Matrix 230 Huisgen Cycloaddition Click Chemistry Between Modified CNCs and Matrices 232 Schiff’s Base Reaction Between Cellulose Nanocrystals (CNCs) and Matrix 233 Esterification Reaction Between CNCs and The Matrix 237 Chemical Coupling Between Hydroxyl Groups of Matrix and Aldehyded CNCs or Modified CNCs 237

7.2.1 7.2.2 7.2.3 7.2.3.1 7.2.3.2 7.2.3.3 7.2.3.4 7.2.3.5

Contents

7.3

Conclusions 242 References 243

8

Role of Cellulose Nanofibrils in Polymer Nanocomposites 251 Thiago H. S. Maia, Marília Calazans, Vitor Lima, Francys K. V. Moreira, and Alessandra de Almeida Lucas

8.1 8.2 8.3 8.3.1 8.3.2 8.3.3 8.4

Introduction 251 Characteristics of Cellulose Nanofibrils 252 Mechanical Properties of CNF Polymer Nanocomposites 253 Thermoset Resins 254 Thermoplastics 255 Waterborne Polymer Systems 257 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites 258 Effect of Fiber Size and Lignin Presence 264 Multifunctionality: Optical and Barrier Properties of CNF Nanocomposites 267 Outlooks in CNF Nanocomposites 269 References 269

8.5 8.6 8.7

9

Advanced Materials Based on Self-assembly of Cellulose Nanocrystals 277 Lin Gan, Siyuan Liu, Dong Li, and Jin Huang

9.1 9.1.1 9.1.2 9.1.3 9.2 9.2.1 9.2.2 9.2.3 9.3 9.3.1 9.3.2 9.3.3

Self-assembly Structure of CNCs 277 Structure of CNC Liquid Crystals 278 Components of CNC Self-assembly 279 Form of CNC Self-assembly Products 279 Self-assembly Methods and Materials 281 Casting Method and Spin Coating Method 281 Vacuum-Assisted Self-assembly 283 Evaporation-Induced Self-assembly 284 Structural Adjustment of CNC Self-assembly 284 Cholesteric Structure of Neat CNC Films 284 Cholesteric Structure and Cross-linking Structure in Gel 286 Cholesteric Structure in Bulk Materials of CNC Composite Self-assembly 288 Nematic Structure 290 Modifying Surface Chemical Structure of CNC 291 Properties of CNC Self-assembly 295 Mechanical Properties 295 Mechanical Properties of CNC Films 295 Mechanical Properties of CNC Composite Films 295 Iridescent Color 298 Iridescent Color Control of CNC Films 298 Iridescent Color Control of CNC Composite Materials 300 Optical Control of CNC Self-assembly Gels 302 Plasmonic Properties of CNC 304

9.3.4 9.4 9.5 9.5.1 9.5.1.1 9.5.1.2 9.5.2 9.5.2.1 9.5.2.2 9.5.2.3 9.5.3

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9.6 9.6.1 9.6.2 9.6.2.1 9.6.2.2 9.6.3

Potential Applications 305 Oil/Water Separation 305 Application of Optical Materials 306 Optical Application of CNC Films 306 Optical Application of CNC Composite Films 306 Sensors 307 References 309

10

Potential Application Based on Colloidal Properties of Cellulose Nanocrystals 315 Shiyu Fu and Linxin Zhong

10.1

Colloidal Properties of CNC and Applications in Functional Materials 315 Nanocellulose for Paper and Packaging 324 Nanocellulose for Paper Coating 326 Microfibrillated Cellulose Coated Paper for Delivery System 328 Water-Resistant Nanopaper Based on Modified Nanocellulose 329 Effect of Chemical Composition on Microfibrillar Cellulose Film 334 Antimicrobial Diffusion Films Based on Microfibrillated Cellulose 336 Nanocellulose for Wood Coatings 339 References 341

10.2 10.2.1 10.2.2 10.2.3 10.2.4 10.2.5 10.3

11

Strategies to Explore Biomedical Application of Nanocellulose 349 Yanjie Zhang, Peter R. Chang, Xiaozhou Ma, Ning Lin, and Jin Huang

11.1 11.2 11.3

Introduction 349 Research on Biological Toxicity of Nanocellulose 349 Application of Nanocellulose for Immobilization and Recognition of Biological Macromolecules 355 Application of Nanocellulose for Cell Imaging 360 Application of Nanocellulose for Cell Scaffolds 361 Application of Nanocellulose in Tissue Engineering 366 Tissue Repairing, Regeneration, and Healing 366 Skin Tissue Repairing 368 Bone Tissue Regeneration 370 Tissue Replacement 371 Artificial Blood Vessels 371 Soft Tissues, Meniscus, and Cartilage 373 Nucleus Pulposus Replacement 375 Application of Nanocellulose in Drug Carrier and Delivery 375 Application of Nanocellulose as Biomedical Materials 382 Antimicrobial Nanomaterials 382 Nanocellulose Incorporated with Inorganic Antimicrobial Agents 385 Nanocellulose Incorporated with Organic Antimicrobial Agents 386

11.4 11.5 11.6 11.6.1 11.6.1.1 11.6.1.2 11.6.2 11.6.2.1 11.6.2.2 11.6.2.3 11.7 11.8 11.8.1 11.8.1.1 11.8.1.2

Contents

11.8.2 11.9

Medical Composite Material 388 Summary 389 References 389

12

Application of Nanocellulose in Energy Materials and Devices 397 Gang Chen and Zhiqiang Fang

12.1 12.2 12.2.1 12.2.2 12.2.3 12.2.4 12.3 12.3.1 12.3.2 12.3.3 12.4 12.4.1 12.4.2 12.4.3 12.5

Introduction 397 Nanocellulose for Lithium Ion Batteries (LIBs) 398 Nanocellulose-Based Electrodes 398 Nanocellulose-Based Separators 401 Nanocellulose-Based Electrolytes 403 Nanocellulose-Based Binders 403 Nanocellulose for Supercapacitors 404 Nanocellulose As a Substrate 405 Nanocellulose As a Nano-template 406 Nanocellulose As a Mesoporous Membrane 410 Nanocellulose for Other Energy Devices 411 Fuel Cells 411 Solar Cells 412 Nanogenerators 414 Conclusion and Outlook 415 References 416

13

Exploration of Other High-Value Applications of Nanocellulose 423 Ruitao Cha, Xiaonan Hao, Kaiwen Mou, Keying Long, Juanjuan Li, and Xingyu Jiang

13.1 13.1.1 13.1.2 13.1.2.1 13.1.2.2 13.1.2.3 13.1.2.4 13.1.2.5 13.1.2.6 13.1.3 13.1.4 13.2 13.2.1 13.2.2 13.2.2.1 13.2.2.2 13.2.2.3 13.2.2.4

Fire Resistant Materials 423 Introduction 423 Flame Retardant Additives 424 Halogenated Flame Retardants 424 Phosphorus-Based Flame Retardants 424 Nitrogen-Based Flame Retardants 424 Silicon-Based Flame Retardants 424 Mineral Flame Retardants 425 Nanoparticles 425 Fire Resistance of Clay Nanopaper Based on Nanocellulose 425 Conclusion 432 Thermal Insulation Materials 432 Introduction 432 Thermal Building Insulation Materials 432 Mineral Wool 433 Expanded Polystyrene (EPS) 433 Polyurethane (PUR) 433 Aerogel 433

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13.2.3 13.2.4 13.3 13.3.1 13.3.2 13.3.3 13.3.4 13.3.5

Thermal Insulation Performance of Nanocellulose-Based Materials 434 Conclusion 437 The Templated Materials 438 Introduction 438 Synthesis of Magnetic Composite Aerogels 442 Synthesis of Inorganic Hollow Nanotube Aerogels 454 The Self-assembled CNC Templates 458 Conclusion 464 References 464 Index 475

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Preface Recently, nanocellulose has been considered as a promising and ideal candidate for the development of high-performance functional composite materials. Nanocellulose is a kind of biomass nanomaterial that is constructed by cellulose and can be extracted from most plants, some animals, and microorganisms. Its hydroxyl group-rich surface makes nanocellulose hydrophilic and easily modifiable. Meanwhile, as a polysaccharide-based material, nanocellulose has proved to be nontoxic, biocompatible, renewable, and degradable. Thus, nanocellulose-based nontoxic drug carriers, wound coverage and contrast agents have been reported and have shown a great potential application in clinical medical care. On the other hand, as nanocellulose contains a large amount of cellulose crystalline regions, its mechanical strength is extremely high. In addition, because of the capability of nanocellulose to form networks cross-linked by hydrogen bonds in the suspension/matrix, it has also been widely used as a green reinforcing agent in polymer materials to prepare high-performance materials. Nanocellulose mainly consists of three kinds of nanomaterials – cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial nanocellulose (BC). CNC is the pure nano-sized cellulose monocrystal, and occurs as rod-like nanoparticle with a highly hydrophilic surface. CNF and BC are nanofibers containing both crystalline and amorphous regions. These three nanomaterials can form networks in liquid medium or matrix via hydrogen bonds or chain entanglements. However, the main problem with these nanomaterials is to keep the balance between the compatibility with the matrix ensuring their dispersibility and the capability to form hydrogen-bonded network providing high mechanical stiffness. Appropriate surface modification of the nanomaterial would allow the development of stiff materials with interesting functions. On the other hand, CNC itself also has special self-assembling behavior. The CNCs can form cholesteric liquid crystal phase in aqueous suspensions, and thus can be used to make some interesting optical-tuning materials and so on. For the abovementioned interesting properties, nanocellulose has drawn much attention and has been given more importance in the development of advanced materials. Nanocellulose has been profoundly studied and widely applied in the development of high-performance and functional materials. In this book, Nanocellulose: From Fundamentals to Advanced Materials, we have tried to provide a full overview of the structure and preparation of nanocellulose and its applications.

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We first introduce the structure and strategies for the extraction of CNC, CNF, and BC, including physical and chemical approaches. Then, as a fundamental step for the preparation of nanocellulose-based composites, a variety of approaches for the surface modification of nanocellulose are exposed. The following chapters focus on the current status of nanocellulose-based nanocomposites and the mechanisms involved in their reinforcing capability; we discuss the applications of these high-performance nanocomposites. The application of the colloidal characteristics and self-assembling behavior of CNC for the development of functional nanomaterials is highlighted for their huge potential to make promising optical materials, and so on. Finally, in the last three chapters, the use of nanocellulose for biomedical applications, energy materials and devices, and other high-value use is introduced. October 6, 2018

Jin Huang Lanhua Building, Southwest University

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Acknowledgments We are thankful for the contribution of all writers of this book, especially the chief editors Jin Huang, Alain Dufresne, and Ning Lin, who did a lot of work to ensure the high quality of this book. The first and corresponding authors of every chapter, i.e. Xiaozhou Ma, Chunyu Chang, Peter R. Chang, Pei Huang, Min Wu, Muhammad Wajid Ullah, Guang Yang, Ge Zhu, Yuhuan Wang, Yaoyao Chen, Thiago Henrique Silveira Maia, Alessandra de Almeida Lucas, Lin Gan, Shiyu Fu, Yanjie Zhang, Gang Chen, Zhiqiang Fang, Ruitao Cha, and Xingyu Jiang (according to the order of the chapters), and all the other coauthors have also provided their input in different areas to successfully complete this book. We also thank Muhammad Wajid Ullah for his great proofreading work during the preparation of this book. Lastly, we thank the grant of the National Natural Science Foundation of China (51373131, 51873164, 51603171, 31570569, 51733009, 21574050, 31700508, 21774039, and 21875050), and the Talent Project of Southwest University (SWU115034).

1

1 Introduction to Nanocellulose Jin Huang 1 , Xiaozhou Ma 1 , Guang Yang 2 , and Dufresne Alain 3 1 Southwest University, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, School of Chemistry and Chemical Engineering, Tiansheng Road 2, Chongqing, 400715, China 2 Huazhong University of Science and Technology, College of Life Science and Technology, Luoyu Road 1037, Wuhan 430074, China 3 University Grenoble Alpes, CNRS, LGP2, Grenoble INP, 38000 Grenoble, France

1.1 Introduction Cellulose is the most abundant polymer on earth produced by plants, microorganisms, and cell-free systems. Chemically, it is composed of repeated β-d-glucose monomers linked together through β-(1,4) glycoside linkage. The morphology of natural cellulose is usually fibrous with intermittent crystalline and amorphous sections [1]. Separation of fibers results in nanoscale cellulose substances known as nanocellulose, which exists in different morphologies such as cellulose nanocrystals (CNCs) or cellulose nanowhiskers (CNWs) as another name, and cellulose nanofibers (CNFs) (Figure 1.1). Compared to the plant cellulose containing lignin and hemicelluloses, microbial cellulose, termed as bacterial nanocellulose (BNC) and cell-free cellulose, represents the purest form of cellulose. Nanocellulose exhibits distinctive structural and physicochemical, mechanical, and biological characteristics, including reticulate fibrous three-dimensional web-shaped structure, high crystallinity, good mechanical strength, biocompatibility, biodegradability, optical transparency, high specific surface area, polyfunctionality, hydrophilicity, and moldability into 3D structures [2–4]. As a sustainable material, nanocellulose could be extracted from the disintegration of plant and animal cellulose [5], synthesized by different microbial strains [6], and cell-free systems [7]. As a tunable material, nanocellulose both alone and in composite form with other materials is immensely used in various fields such as wound dressing, textiles and clothing, food, cosmetics, regenerative medicines, tissue engineering, energy, optoelectronics, bioprinting, environmental remediation, and so on [2, 8–13]. Nanocellulose exhibits superior structural characteristics than microcrystalline cellulose (MCC), and possesses high mechanical strength and can be facilely surface-modified via different strategies. For example, the longitudinal modulus of CNC extracted from tunicate can reach as high as 151 GPa, while the Young’s modulus of CNF varies between 58 and 180 GPa and its tensile Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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1 Introduction to Nanocellulose

Cellulose fibers

Woods

Cellulose nanofibers

H OH H O O HO

H

O OH

H

Cellulose nanocrystals H

Cellulose

Figure 1.1 Schematic illustration for the preparation of cellulose nanofibers and cellulose nanocrystals using the wood resource.

strength can reach up to 22 GPa according to the type of raw material and the preparation method [14, 15]. Nanocellulose contains greater number of hydroxyl (OH) groups, which make it highly hydrophilic and can be modified via different chemical and physical strategies [16]. Thus, considering its high biocompatibility, high mechanical strength, renewability, and low cost, nanocellulose has received immense consideration as an ideal nanostructure to make new high-value nanomaterials.

1.2 Preparation of Nanocellulose Depending on the type of nanocellulose and its source, different strategies are employed for its production. For example, destructuring strategies involving high-pressure homogenization, grinding, and chemical or enzymatic treatments, are used for the isolation of nanocrystalline cellulose (NCC). In contrast, BNC is naturally produced by different microorganisms. The following section briefly overviews different preparation methods of nanocellulose (see Chapters 2, 3 and 4 for details). 1.2.1

Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are the mostly commonly used nanocellulose, which are mainly produced through hydrolysis of the amorphous section of cellulose fibers [17]. This process consists of two steps: the pretreatment of the raw material followed by its hydrolysis into CNCs. The raw material contains different impurities in the form of esters, wax, hemicelluloses, and lignin, which are removed by treating with alkaline (NaOH) solution or applying a bleaching method before hydrolysis. Thereafter, the purified raw material is heat-treated in the acidic environment for around 45 min in general or longer time up to several hours to hydrolyze the amorphous section of cellulose fibers. The obtained suspension is then centrifuged and dialyzed to achieve the purified CNCs. The CNCs produced through this strategy commonly show a rod-like morphology when observed under transmission electron microscopy (TEM)

1.2 Preparation of Nanocellulose

Figure 1.2 TEM micrograph of cotton linter-derivate CNC obtained through acid hydrolysis (scale bar = 200 nm).

(Figure 1.2). The diameter and length of CNCs obtained through acid hydrolysis vary depending on raw material type, acid type, and hydrolysis temperature and time. For example, CNCs with diameter of 10–30 nm and length 200–300 nm were obtained through acid hydrolysis of cotton fibers at 65 ∘ C using H2 SO4 . In contrast, CNCs with the length more than 1 μm were obtained from ascidian under the same experimental conditions [18, 19]. Similarly, CNCs with length 100–200 nm and diameter 10 nm were obtained through acid hydrolysis using HCl under the analogous experimental conditions [20]. Besides acid hydrolysis, other reactants including oxidants such as tetramethyl-piperidin-1-oxyl (TEMPO) or ammonium persulfate (APS), and some bio-enzymes, are also used for the production of CNCs. For instance, TEMPO, NaBr, and NaClO could be together used to directly produce TEMPO-oxidized CNC (TOCNC) from cellulose fibers [21]. During the preparation process, the reaction is carried out in an alkaline environment by constantly adding NaOH solution to the reaction mixture until a constant pH is kept. The TOCNC in the mixture could be purified by centrifugation and dialysis. Similarly, Satyamurthy et al. used the cellulolytic fungus to hydrolyze MCC. The yield of this strategy could be as high as 20%; however, its high cost strongly limits its large-scale application [22]. 1.2.2

Cellulose Nanofibers

Compared to CNCs, the preparation of cellulose nanofibers (CNFs) is much simpler as it does not require severe chemical cleavage in the molecular structure of the cellulose chain. Generally, CNFs are prepared by the physical separation of cellulose fibers, such as grinding, homogenization, and ultrasonication [23–26].

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Besides the strategies of mechanical peeling and dissection, CNFs can also be prepared by chemical methods. For example, oxidation of wood raw material by TEMPO under gentle stirring results in fine individual CNFs [27]. Sometimes, both mechanical and chemical strategies could be employed together to produce individual CNFs. For example, carboxymethylation and high-pressure homogenization can be applied simultaneously to prepare uniformly distributed CNFs [28]. For the preparation of CNFs, the most commonly used raw materials are wood, however, cotton fibers, tunicate, and other raw materials are also attempted to be used. Depending on the type of raw materials, the diameter of CNFs usually varies from 2 to 50 nm [29] (Figure 1.3). CNF usually has a high aspect ratio, which makes it an ideal positive ingredient for the enhancement of polymer materials (see Chapter 3, 6 and 8 for details). 1.2.3

Bacterial Nanocellulose

Bacterial nanocellulose (BNC) is naturally produced by several bacterial genera including Acetobacter, Rhizobium, Agrobacterium, Aerobacter, Achromobacter, Azotobacter, Salmonella, Escherichia, and Sarcina [30] and cell-free systems [7]. It is produced in the form of a hydrogel at the air–medium interface. During the synthesis, β-1,4-glucan chains are produced in the interior of the bacterial cell, which is then excreted across the cell membrane as protofibrils that crystallize to form ribbon-shaped microfibrils followed by formation of pellicle [31] (Figure 1.4). The structural characteristics such as diameter (20–100 nm), arrangement of fibers, and physico-mechanical properties of BNC strictly depend on the microbial strain type, the synthesis method (cell and cell-free systems), and the culture conditions including carbon and nitrogen source [33, 34]. BNC has been largely used for various biomedical applications such as tissue engineering, regenerative medicines, enzyme immobilization, drug delivery, and 3D printing biomaterials [2, 35–37]. However, the large-scale applications of BNC are hampered by its high production cost [38]. Different strategies have been developed for high-yield and cost-effective production of BNC with superior structural and physico-mechanical features. These include the advanced fermentation approaches, genetic engineering strategies, and strain improvement for high-yield BNC production, and use of different industrial wastes such as fruit juices, sugarcane molasses and agricultural wastes for low-cost production [39–43] (see Chapter 4 for details).

1.3 Surface Modification of Nanocellulose The hydroxyl groups present on the surface of nanocellulose make it highly hydrophilic and favorable for surface chemical modification or physical interaction [16]. The chemical and physical properties of nanocellulose can be easily controlled by different strategies (see Chapter 5 for details), and some chemical methods are summarized in Figure 1.5 [44]. In order to guarantee the quality and efficiency of surface modification of nanocellulose, the uniform dispersibility of nanocellulose in the suspension medium is the principle factor to reactive

Source (a)

Cell wall

Microstructure

(b) S3

Chemical purified fiber (d)

(c)

Nanofiber character (e)

Wood

S2 S1 P (g)

(h)

5 μm

(i)

200 nm

(j)

(k)

Bamboo

(f)

100 μm

200 μm

(l)

(m)

(n)

100 μm

200 nm

200 nm

(p)

(o)

Cotton

S2 R P

C

S1

100 μm

10 μm

Figure 1.3 Preparation of cellulose nanofibers from different raw materials. Source: Chen et al. 2018 [29]. Reproduced with permission of RSC.

200 nm

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1 Introduction to Nanocellulose

Bacterial cellulose fiber

Bundle

β-1,4-glucan

Cellulose I Microorganisms

Figure 1.4 Preparation of bacterial cellulose. Source: Lai et al. 2013 [32]. Reproduced with permission of Springer Nature. O

O R

O O

O

N H

O

R

OH

N H

H N

O

R N

O OH

RSiCl3

C

O O

OH

OH

N

O

O

O

O

Si O

Si O

R

R

SO3H

SO3H O

O

O

O R

N

TEMPO, NaOCl Cl

R

(d)

COOH

O O

COOH

(b)

O

R

COOH

(c) R

SO3H

(a)

OH

N

O

O

O H2SO4

N

R Si O

(g)

O O

R Si O

O

(e) O

O

(f)

O

OH O O

R

O O

OH O HO

OH

OH HO O

O

OH

OH

= n

OH

OH

Figure 1.5 Schematic illustration for surface chemical modification of CNCs by various methods including sulfonation (a), oxidation (b), nucleophilic substitution (c), etherification (d), esterification (e), carbamation (f ), and silylation (g).

1.3 Surface Modification of Nanocellulose

conjugation and physical attachment of the nanocellulose surface. Especially when using an organic solvent as the suspension medium, due to the hydrophilicity of the nanocellulose surface, the solvent replacement from aqueous medium to organic solvent is employed as an effective way while the intensified dispersion techniques, such as ultrasonication, high-speed shear and so on, are essential to homogenize the nanocellulose suspension. 1.3.1

Esterification

The most common strategy used to modify nanocellulose is by esterification, which offers a high substitution ratio of OH groups on nanocellulose surface [45–47]. According to the chemical structure of cellulose, there are three OH groups on each glucoside unit, which are C2-OH, C3-OH, and C6-OH. In general, the order of activity of these OH groups is C6-OH > C3-OH > C2-OH (Figure 1.5). During the chemical reaction, only one moiety for C3-OH and C2-OH might be modified due to the steric hindrance effect of nanoscale CNC surface. Another advantage of this esterification method is that the reaction conditions are mild; thus, a high substitution ratio can be achieved without compromising the crystalline structure of nanocellulose [48–50]. Researchers have successfully used this method to make hydrophobic nanocellulose, which can be easily used to enhance the mechanical properties of hydrophobic polymer matrix. In addition, when using ethylenediaminetetraacetic dianhydride (EDTAD) to decorate carboxyl groups onto the CNC surface, high carboxylation degree of the CNC surface was achieved. Furthermore, in comparison with the TEMPO-oxided CNC with an equivalent carboxylation degree, the surface integrality of EDTAD-esterified CNC was kept [51]. 1.3.2

Oxidation

Oxidation is the most commonly used method for making carboxylated nanocellulose because the C6-OH on nanocellulose surface can be easily oxidized into a carboxyl group. This oxidation reaction usually takes place in the presence of TEMPO [27, 32, 52]. Additionally, TEMPO can selectively oxidize C6-OH to a carboxyl group in the presence of NaClO and NaBr under alkaline environment. However, high TEMPO concentration and long oxidation time may break the chemical bonds between glucoside units, and thus compromise the crystalline structure and surface integrality of nanocellulose [21]. It was also reported that such phenomenon could be used in making carboxylated CNC directly from the cellulose-containing raw materials. Another oxidant that can be used in nanocellulose oxidation is APS. However, due to its high oxidation potential, APS seriously damages the crystalline structure of cellulose; thus the morphology of the CNC produced is obviously shorter and thinner than the one produced through traditional acid hydrolysis [53]. By using oxidation strategy, aldehyde groups can also be formed onto the nanocellulose surface. Usually, sodium periodate (NaIO4 ) is used to oxidize the 2,3-diol structure of the glucoside, and thus can make two aldehyde groups, which can be widely used in Schiff base reaction-based further modification [54]. Besides, a study has also reported

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that some of the glucoside units on CNC innately present aldehyde groups, and thus the amino- or diamine-containing fluorescent molecules could be directly conjugated on the CNC surface without any additional pretreatment [55]. 1.3.3

Etherification

Etherification is a highly efficient modification method for cellulose, which usually uses an epoxylated molecule as the modification agent together with organic solvent-containing heating system [56–58]. The efficiency of this process is largely compromised by the polymerization; nevertheless, it offers a high substitution ratio. As well-known, the epoxy group reacts with the OH group of the nanocellulose surface and forms ether bonds and an OH at its β position. The OH group formed further reacts with the epoxy groups; hence, polymerization takes place on the nanocellulose surface. This issue can be partially resolved by controlling the reaction conditions; however, polymerization cannot be completely avoided. 1.3.4

Amidation

As most biomolecules possess amine groups, amidation is considered as a mild and efficient way to modify the nanocellulose surface with these biomolecules. The amidation reaction can take place both in organic solvents (e.g. dimethylformamide, abbreviated as DMF) and aqueous solutions to conjugate two molecules that have carboxyl groups and amine groups, respectively. Since nanocellulose generally has no carboxyl groups or amine groups on its surface, the carboxyl groups or amine groups should be decorated on the nanocellulose surface before the amidation, for example using TEMPO oxidation. In the aqueous reaction system, the carboxylated nanocellulose is thereafter reacted with N-hydroxylsuccinimide (NHS), which can improve the reactivity of carboxyl groups on nanocellulose surface with the amide groups. The final step of amidation modification is to mix the NHS-activated carboxylated nanocellulose with the amide-containing molecules. The amine groups of the molecules can directly react with NHS-activated carboxylated nanocellulose, and conjugate the molecules onto the nanocellulose surface with the linkage of amide bonds [59, 60]. However, if organic solvent (e.g. DMF) is used instead of aqueous medium, the amine-containing molecules can be directly conjugated onto the carboxylated surface of nanocellulose. 1.3.5

Other Chemical Methods

Other chemical methods, such as nucleophilic substitution, carbamation and so on, have also been reported to modify the nanocellulose surface, especially in making hydrophobic nanocellulose for nanocomposite enhancement or fluorescent nanocellulose for biomedical applications [61, 62]. In addition, long chain molecules and polymers have been attempted to be grafted on the nanocellulose surface. In this case, the “graft onto” strategy has been employed via the abovementioned chemical methods to covalently linked long chains on the nanocellulose surface, but is obviously subject to the steric hindrance of nanoscale surface;

1.4 Nanocellulose-Based Materials and Applications

meanwhile, the “graft from” strategy could produce higher grafting density and longer polymer chains using various polymerization methods, which are initiated by hydroxyl groups and newly formed corresponding functional groups on the nanocellulose surface (see Chapter 5 for details). All these methods mentioned above generally require organic solvents and heating condition during the reaction. As a result, such chemical modification methods are quite suitable to hydrophobic surface modification of nanocellulose; however, these methods are limited for the modification of many hydrophilic molecules on the nanocellulose surface due to the mismatching of solvent in the reaction system. 1.3.6

Physical Interaction

Physical interaction is another way to modify the nanocellulose surface; however, the physical interactions are commonly relatively weaker than the covalent bonds. Generally, physical interactions include hydrogen bonding, electrostatic interaction, hydrophilic/hydrophobic interaction, and π–π stacking. The rich OH groups on nanocellulose surface can directly interact with the electron-rich groups containing oxygen or nitrogen atom, other hydroxyl groups, and carboxyl groups to form hydrogen bonds. The hydrogen bonding potential of nanocellulose might improve the association with the pre-designated modifiers; and then the hydrophobized nanocellulose after physical modification has been frequently used to enhance its dispersibility in nonpolar polymer matrix of composites. For example, a simple method that can improve the dispersion of nanocellulose in nonpolar matrix is to introduce surfactants or amphiphilic polymers containing both polar and nonpolar moieties, which can interact with both hydrophilic nanocellulose surface in physical pre-modification stage and the nonpolar matrix in material compounding process, and can act as a bridge to improve the compatibility between nanocellulose and the matrix [63, 64]. Also, the carboxylated surface enables nanocellulose to strongly interact with the molecules containing positive charges (e.g. ammonium-containing molecules). Such interaction can also be used in the surface modification of nanocellulose and thus allows compounding with nonpolar polymers or hydrogels to produce nanocellulose-based composites with excellent properties. The surface modification strategy based on physical interaction of nanocellulose with other substances offers several advantages, including the protection of nanocellulose crystalline structure, facile and cost-effective preparation process, and so on. However, it is also noteworthy that the physical interaction between the components in most cases is not as strong as covalent bonds, and the integrity preservation of nanocellulose and modifiers in applications is a key issue (see Chapter 5 and 6 for details).

1.4 Nanocellulose-Based Materials and Applications Based on the chemical and physical properties of nanocellulose and its surface modification strategies related with various methods, varieties of nanocellulose-based composite materials have been developed and widely

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applied in electronic and energy devices, biomedical diagnosis and treatment, and other high-value fields. Nanocellulose generally possesses high mechanical properties; and, for example, the longitudinal modulus of CNC extracted from tunicate can reach as high as about 151 GPa, which is equivalent to that of steel as about 200 GPa. Such high mechanical strength with highly reactive surface, renewability and degradability, makes nanocellulose an ideal sustainable candidate for material enhancement. Similarly, when cellulose raw material is hydrolyzed into CNC, the resultant suspension can perform some interesting optical properties. The as-prepared CNC in the suspension can form a chiral nematic liquid crystal phase. The pitch and twist angle of this liquid crystal structure strongly depend upon the cellulose resources and the concentration of CNC in suspension, and also are affected by some environmental conditions, such as salt concentration, temperature, pH and so on. Furthermore, this kind of chiral nematic CNC arrangement could be transplated into the solid-state materials, and even the matrix exhibit stimuli-response characteristics and contribute to produce thermo-, pH-, and other stimuli-sensitive composite materials (see Chapter 9 for details). In addition, without any material as matrix, the CNC could arrange vertically to the suspension plane by a facile evaporation-induced method, and produce the film-form materials with a structural color of monochromatic light as specifically limited in the ultraviolet region, which is attributed to scattering enhancement of uniaxial periodical CNC arrays. It is worthy of note that this assembly strategy removes the usual chirality and iridescence of the traditional optical materials derived from the free assembly of CNC mentioned above, and prevents the iridescence-based information from being misread and shows an application potential as information-hiding and anti-counterfeiting materials [65]. The following section overviews various preparation strategies, performances and potential applications of nanocellulose-based materials (detailed in Chapter 6-13 of this book). Firstly, the major issue associated with the fabrication of nanocellulose-based composite material is the compatibility and dispersion of the nanofiller in the matrix. Generally, the nanocellulose reinforcer is blended with the hydrophobic polymer matrix, such as polypropylene (PP) or poly(butylene succinate) (PBS). However, its hydrophilic nature makes it hard to uniformly disperse into the hydrophobic material, thus seriously affecting the performance of as-prepared composite material. As described earlier, the hydrophobic moieties can be conjugated to the nanocellulose surface to turn the particles or fibers hydrophobic together with polarity matching. An effective way is to conjugate the acetyl groups onto the nanocellulose surface via different chemical reactions. A study has shown that modifying the acetyl groups onto CNC surface via esterification of acetic anhydride improved the dispersibility of CNC in organic solvents as a blending medium, which ultimately enhanced the performance of the composite material [49, 66]. On the other hand, various processing technologies and compatibilizers (via in situ chemical reactions or physical interaction) can also be used to improve the compatibility of nanocellulose with nonpolar and hydrophobic matrix [50, 67]. For example, CNFs can be blended into polypropylene and polyethylene by twin-screw extrusion with maleic anhydride as compatibilizer. Maleic anhydride forms hydrogen bonds or esterifies the hydroxyl groups of CNF

1.4 Nanocellulose-Based Materials and Applications

during the blending process and thus can adjust the hydrophobicity and polarity of the nanocellulose surface; it facilitates the uniform dispersion of CNF in the matrix. The results showed that the Young’s modulus of polyolefin-based composites was improved about six times after blending with 50 wt% CNF. The situation becomes more complex when BNC is used to blend with nonpolar substances [68]. As BNC is synthesized by microorganisms in the culture medium, the achieved BNC usually presents in gel state, [69] while all the fibers are entangled with each other and hard to be separated. Thus, blending BNC with nonpolar polymers requires surface pre-modification towards BNC as that of CNF (see Chapter 4, 5 and 8 for details). In contrast, it is relatively easier to enhance the polar polymers such as poly(lactic acid) (PLA) using nanocellulose as filler due to its innate polarity nature mainly ascribed to surface OH groups. The nanocellulose or modified nanocellulose has been directly blended with various types of matrix by different processing technologies, and showed excellent dispersion, mainly attributed to the matching surface characteristics of nanocellulose. Another important application of nanocellulose in material field is the unique self-assembling behavior of CNCs. The CNCs self-assemble into cholesteric liquid crystal under higher concentration in aqueous media and demonstrate interesting optical phenomenon [70], which is utilized in the preparation of optical materials and devices (see Chapter 9 for details). According to previous report [71], the morphology of the films made by the CNC self-assembly is intimately related to the ionic strength and evaporation speed of the system. The ionic strength in CNC suspension affects the pitch of the liquid crystal structure formed by CNC, while the evaporation speed also exerts some effect on the integrity and pitch of the CNC film. High ionic strength and fast evaporation are unfavorable to the pitch of ordered structure and make a disordered CNC film, while low ionic strength and slow evaporation make the CNC film more uniform with good optical properties. It is believed that the ions in the system and evaporation speed affect the formation of hydrogen bonds and the electrostatic interaction between the CNCs [71]. Based on these basic principles, the composite materials, which inherited optical properties of CNC assembly, can be prepared. For example, by in situ polymerization of the suspension containing monomers and 3% CNCs, the ordered spacial arrangement of CNC could be kept in the newly formed polymer matrix, and the as-prepared composite inherited the optical properties of the CNC-based liquid crystal structure [72]. Furthermore, the CNC also enhanced the mechanical strength of the polymer materials [72]. In another work, the CNC with a higher loading-level was used to prepare tough composite film with high mechanical strength. The mass fraction of CNC in the composite material could reach 50–90%, while the mechanical strength of the composite material could be as high as about 12 GPa [73]. Currently, various kinds of monomers, such as organosilica monomer of tetramethoxysilane, have been attempted to fabricate the supporting matrix of chiral nematic-arranged CNCs [74]. Moreover, the silicon oxide-based mesoporous chiral materials were derived by removing the chiral nematic phase of CNC, which inherited the unique optical properties [74], and further acted as a removable hard-template to produce the mesoporous titanium oxide-based replica films as the candidates of

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energy and sensor materials [75]. In addition, one more facile method has been carried out to controllably prepare the chiral plasmonic films via simple mixing and subsequent evaporation of the gold rod and CNC suspension, and showed a potential of the scalable and cost-effective manufacturing [76]. (see Chapter 9 for details). Nanocellulose-based materials and composites possess high biocompatibility, improved mechanical properties, good degradability, and special optical properties. These materials and composites have found potential applications in various fields. For example, fluorescent cellulose nanocrystal (fCNC) can be used in in vivo bioimaging. Studies have shown that the Alexa Fluor 633-decorated fCNC can be injected into living mice, where the fluorescence can be held in the body for more than seven days and the fCNC can be selectively enriched at the limb bones of mice; furthermore, these fCNCs can also be potentially used as a biocompatible luminous reporter of bone disease [55]. As described earlier, the CNF can also be used as filler in food packaging materials where it is blended with polypropylene and polyethylene after being modified. The addition of CNF can improve both the mechanical strength and oxygen barrier properties while maintaining good optical transparency. The high mechanical strength and biocompatibility of nanocellulose make it can be used in the fabrication of transparent wound dressing in various forms such as hydrogel. The material was produced by polymerization of 2-hydroxyethyl methacrylate and in situ nanoparticlation of silver nitrate reduction with the incorporation of bacterial cellulose nanocrystals (BCNCs). The presence of BCNC improved the flexibility and water absorption of the material while silver nanoparticle could provide good antibacterial activity. The high optical transmittance could facilitate the observation of the wound healing while antibacterial activity makes it become a promising wound coverage material. [77]. In addition, BNC aerogels are used to separate oil/water mixture [78]. The hydrophilic surface of BNC enables these aerogels to show a high affinity for water. With rational modification, BNC aerogels could be used to selectively absorb water molecules in oil, and thus showed an application potential in the petrochemical industry. Chapters 10–13 provide a detailed description of the application of nanocellulose-based materials and composites in biomedicine, energy, and other high-value fields, respectively. Interestedly, it was found that highly crystalline CNC could be carbonized into well-shaped carbon nanorod under high temperature. This kind of one-dimensional carbon nanorods was more uniform with less structural deficiencies, and was believed to fabricate carbon-based energy materials with high specific-surface-area and porosity. For example, along with the chiral self-assembling behavior of CNC, the as-prepared silicon-based materials containing chiral nematic CNC phase were treated at 900 ∘ C under nitrogen atmosphere, and the CNC was converted into carbon nanorods with a chiral nematic arrangement in the silicon-based matrix. Such derived mesoporous carbon materials exhibited high conductivity of 1.3 × 10−2 S/cm and the specific capacitance as high as ca. 170 F/g, which was mainly ascribed to their high porosity (0.3 - 1.22 cm3 /g) [79]. If doping nitrogen atoms into the CNC-derived carbon, the capacitance could be further improved. After coating the CNC with melamine formaldehyde, the N-doping carbon nanorod prepared under

1.5 Conclusions and Prospects

800-1000 ∘ C for 5 min showed the capacitance up to 352 F/g [80]. Depending upon high capacitance and conductivity, this kind of CNC-derived carbon materials could have a great potential as a high-performance electrode of batteries. Besides the direct carbonization of CNC for energy application, high mechanical performances of nanocellulose also contribute to develop conductive films by compounding carbon nanoparticles [81–83] or conductive polymers [84] with nanocellulose (especially for the CNF and BNC with an entanglement network structure). The mechanical properties of conductive films are extremely critical to the wearable devices, which can improve the durability of the devices and make it suitable for sensing and monitoring during vigorous sport exercise. In this case, nanocellulose was usually mixed with carbon nanomaterials such as carbon nanotube (CNT) [81] or graphene oxide (GO) [82], and then processed into fibers (via electrospinning or wet-spinning) and films (via casting/evaporation method). In addition, by mechanical compressing CNF and graphene nanosheets (GN), high-mechanical strength conductive film could be produced [83]. When the GN content was 30%, the Young’s modulus and tensile strength could be as high as about 7.5 GPa and about 140 MPa, respectively. Moreover, the conductance of the CNF/GN film could be 1 × 10−1 S/cm [83]. These nanocellulose-derived materials and nanocellulose-based composites are promising in energy and electronics applications, which will be discussed in detail in Chapter 12.

1.5 Conclusions and Prospects Nanocellulose is an interesting material that has received immense consideration in various academic and application fields owing to its unique structural, physicochemical, mechanical and biological properties, and availability to being modified by various methods. However, its wide applications have been overshadowed by high production cost and a few limitations associated with tuning of its structural features through surface modification. The high production cost of nanocellulose is mainly attributed to cost of energy consumption and generated waste post-treatment during the preparation process [85]. Extensive efforts have been devoted to the cost-effective production of nanocellulose from many low-cost raw materials; nevertheless, the structural parameters and mechanical properties of nanocellulose produced from those low-cost raw materials need to be further improved. Similarly, several physical approaches have been developed for the preparation of nanocellulose to circumvent the energy consumption issue; however, these approaches compromise its crystalline structure and other physical properties. To address the issues of low-cost and energy-saving, cell-free enzyme strategies are also used; however, these are also compromised by extended production time to be applicable at an industrial scale. The good mechanical properties of nanocellulose make it possible to enhance polymer materials. However, the hydrophilic surface of nanocellulose results in a poor dispersion in most of non-polar polymer matrices. As a result, in order to improve the compatibility and dispersibility of nanocellulose in the composite system, the principal issues involve the approaching of

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hydrophilicity/hydrophobicity and the matching of interaction groups between nanocellulose and matrix. In this case, surface modification towards nanocellulose is the common strategy. Many chemical and physical methods have been developed, and especially the chemical methods abide by the concepts of small-molecules conjugation, and polymer “graft onto” and “graft from”. Further, it has also been found that the addition of nanocellulose may improve the crystallinity of the polymer matrix, which may ultimately increase the rigidity of composite materials. Furthermore, nanocellulose fillers with an optimal loading-level could form a network associated with hydrogen bonds, which can significantly improve the final mechanical performances (especially for strength and modulus) of the composite material. These findings indicate that in order to continuously improve the performance of the composite material, the regulation mechanism of interaction between the nanocellulose and the matrix should be further explored. Some recent reports indicate that the mode of association between nanocellulose and modifiers could also affect the performance of the composite materials filled with modified nanocellulose. For instance, studies have compared the differences of the composite materials when CNCs interacted with two kinds of modifiers via either hydrogen bonding or electrostatic interaction. The results showed that strong interaction might lead to a tougher composite material [86]. However, to answer how the surface modification strategy affects the mechanical properties, extensive research is still required. On the whole, the authors believe that the composite material is always the field in the most extensive application and the most consumption of nanocellulose. As the reinforcing fillers for composite materials, three main types of nanocellulose, i.e. CNC, CNF and BNC, provide abundant options of short/rigidity and corresponding long/flexibility, adjacention and homologous entanglement percolation, and so on. It is critical to select the proper type of nanocellulose for the given polymer materials and match the appropriate processing techniques. Furthermore, in order to improve the compatibility and dispersibility of nanocellulose in the matrix together with achieving the optimal mechanical performances, the key issue is to balance the interaction intensities between two of at least three components (i.e. nanocellulose and its modifier, and matrix) and self-agglomeration force of each component. Herein, the employed association modes of nanocellulose/modifier and modified-nanocellulose/matrix are very important, and the related theoretical prediction should be established. Although nanocomposites are commonly believed to be the most popular field in terms of the demanded quantity and application range of nanocellulose, it is currently constrained by high production cost mentioned above. Therefore, how to use nanocellulose to develop high-value functional materials has attracted much attention, and is greatly expected to impulse the current application of nanocellulose. To date, several nanocellulose-based functional materials have also been developed. Studies have shown that one-dimension rigid structure of CNC can prevent the fluorescent molecules from self-quenching and produced a solid-state luminescent material of fluorescent molecules-modified CNC [51]. In this case, the controlled chemical reaction together with steric effect of rigid CNC support is the key. In addition, CNC could assemble to give the ordered structures of 2D uniaxial orientational alignment and 3D chiral nematic

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2 Structure and Properties of Cellulose Nanocrystals Chunyu Chang 1 , Junjun Hou 2 , Peter R. Chang 3 , and Jin Huang 2,4 1 Wuhan University, College of Chemistry and Molecular Science, Bayi Road 299, Wuhan 430072, China 2 Wuhan University of Technology, College of Chemistry, Chemical Engineering and Life Sciences, Luoshi Road 122, Wuhan 430070, China 3 Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon S7N 0X2, Canada 4 Southwest University, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, School of Chemistry and Chemical Engineering, Tiansheng Road 2, Chongqing 400715, China

2.1 Introduction Being a typical nanostructured biopolymer, nanocellulose contains cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). In this chapter, the preparation, structure, and properties of CNCs will be mainly introduced. Compared with traditional inorganic nanoparticles, CNCs exhibit good characteristics such as renewability, low density, biocompatibility, and biodegradability. In addition, their surfaces are covered with a number of hydroxyl groups that can be chemically modified. The geometrical dimensions of CNCs can vary widely, with diameter in the range of 4–25 nm and length in the range of 100–1000 nm [1]. The dimensions and crystallinity of CNCs depend on the cellulose source and extraction conditions. In the following sections, the different modern methods for the extraction of CNCs are described, and the effects of structural characteristics of CNCs on their physical and chemical properties are discussed.

2.2 Extraction of Cellulose Nanocrystals 2.2.1

Extraction of Cellulose Nanocrystals by Acid Hydrolysis

CNCs are crystalline regions of macromolecules that are tightly entangled by hydrogen bonding interactions in cellulose. Acid hydrolysis is the main strategy for the isolation of CNCs from cellulose. In the 1950s, Rånby reported for the first time that CNCs could be obtained from cellulose fibers by utilizing sulfuric acid hydrolysis [2]. Amorphous and paracrystalline regions of cellulose are preferentially hydrolyzed, whereas crystalline regions that have a higher resistance to acid attack remain intact [3, 4]. Therefore, rod-like CNCs are produced by acid Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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2 Structure and Properties of Cellulose Nanocrystals (a)

(b)

(c)

0.4 μm (d)

(e)

500 nm

1 μm

(f)

1 μm

Figure 2.1 TEM images of cellulose nanocrystals derived from cotton linter (a), cotton (b), sisal (c), ramie (d), tunicate (e), and bacteria (f ) [5–8].

treatment of cellulose. The crystallinity of cellulose raw material and CNCs is strongly dependent on the source of cellulose. The crystallinity of CNCs is also affected by the preparation technique, pretreatment, and the type of acid [3]. The dimensions of CNCs from various cellulose sources are different, with distinctive sizes and aspect ratios, as shown in Figure 2.1. For example, CNCs from tunicates are much longer and have a much higher aspect ratio than others. On the other hand, variations in the conditions of acid hydrolysis (e.g. acid type and concentration, hydrolysis time, and temperature) will also impact the physical attributes/properties of the CNCs prepared (i.e. surface charge, size, yield, and birefringence) [9]. Conventionally, sulfuric acid is one of the most popular acids that are used to fabricate CNCs. For a typical procedure, the CNCs were obtained using 64–65% (w/w) sulfuric acid aqueous solution with the ratio of 1 : 8 to 1 : 20 (g/mL) of raw material containing cellulose (weight) vs. acid (volume) at 45 ∘ C [10–13]. The cellulose fibers from different plants and animals were pre-mixed with sulfuric acid and then stirred for a given time (i.e. 60 minutes) [13, 14]. After the hydrolysis of amorphous cellulose was completed, the suspension was diluted with plenty of deionized water to end the reaction and was followed by centrifuging for several minutes to remove the excess acid solution. The precipitates obtained were washed with deionized water and the suspension was centrifuged again. This process was repeated several times and then the suspension was dialyzed with deionized water until the pH value reached neutral (alternatively, ammonia or NaOH aqueous solution can be used for neutralization prior to dialysis) [14, 15]. Afterwards, in order to obtain better dispersible and more homogeneous CNC suspension, mechanical treatment was used for the suspension of CNCs by a high-speed homogenizer [12] or the suspension was treated by ultrasonic to facilitate the CNCs to disperse in water with better

2.2 Extraction of Cellulose Nanocrystals

uniformity and stability [10]. Finally, freeze-drying technology was employed to yield dried CNCs [13] or small amounts of toluene or chloroform added into the CNCs suspension to avoid bacterial growth, and then directly stored in cold [16, 17]. In addition, the CNCs were also transferred into organic solvents (such as methylbenzene, DMF, or DMSO) from the aqueous suspension through the approach of ultracentrifugation–redispersion (generally using acetone as an intermediate solvent). The mechanism for sulfuric acid hydrolysis of native cellulose is as shown in Figure 2.2a [13]. In sulfuric acid solution, acid hydrolysis of native cellulose follows two paths of rapid protonation: the protonation of glucosidic oxygen or cyclic oxygen by protons from sulfuric acid, followed by water induction to break the glucosidic bonds (see Figure 2.2a). Then two shorter chain fragments with the basic backbone structure of cellulose are yielded during the hydrolysis process. Moreover, besides chain scission, using sulfuric acid to hydrolyze the native H+

OH (2)

OH O

HO O (1)

O HO t as

m

F

OH

(1)

O

OH Cellulose

OH

riu

lib

i qu

E

n

(2)

OH

OH O

OH +

+ HO

OH

H O

O HO

O

O

n

HO

OH H

O OH

O

OH

H –H

H

+

O H

n

OH

–H + OH

OH

O O HO

+

HO HO O

OH

(a)

OH O

HO O

OH

O S

O

O

H

H +

OH2

SO3H OH

O O HO

O OH

(b)

OH O

HO O

OH

n

O HO

HO O

+ H O 2 O

OH Byproduct

n

OH

Figure 2.2 Acid hydrolysis mechanism (a) and esterification (b) of cellulose nanocrystal surfaces. Source: Lu and Hsieh 2010 [13]. Reproduced with permission of Elsevier.

23

24

2 Structure and Properties of Cellulose Nanocrystals

cellulose also involves esterification of the hydroxyl groups (Figure 2.2b). Acid half-esters (so-called “cellulose sulfate”) were produced with the esterification reaction. The resulting CNCs obtained by sulfuric acid hydrolysis contain sulfonic acid groups on the surface, which could contribute to the negative charge of CNCs. This negative charge was very efficient in preventing the aggregation of CNCs by electrostatic repulsion. In general, ultrasonic treatment conducted in an ice bath will be able to avoid possible desulfation of the sulfate groups on the surface of CNCs [13]. On the other hand, hydrochloric acid could also be used to extract CNCs, and its hydrolysis process is similar to that of sulfuric acid, but there are some differences in acid concentration, solid (weight) vs. acid (volume) ratio, and reaction time. CNCs are extracted using 2.5–6 M hydrochloric acid (HCl) aqueous solutions with a certain ratio of cellulose fibers vs. acid (e.g. the ratio of 1 g : 30 mL for the Whatman No. 1 filter paper vs. 4N HCl aqueous solution [18]) at 70–110 ∘ C [18–21]. For example, commercial microcrystalline cellulose (MCC) was added into 6 M HCl solution with the ratio of 1 : 60 (g/mL) of MCC vs. HCl. The suspension obtained was subsequently treated with ultrasonic for 10 minutes in an ice-water bath. Then the mixture was transferred to a hydrothermal reaction kettle and placed in an oven at 110 ∘ C for three hours and was cooled down to room temperature. Then, the resultant mixture was diluted with deionized water and neutralized. Finally, the dry CNC powders were obtained by freeze-drying [21]. Stable turbid colloidal suspensions obtained by hydrolyzing the cellulose material with hydrochloric acid or sulfuric acid are compared in Figure 2.3. TEM images showed that the individual nanocrystals of CNCs were c. 3.5 nm in width and 180 ± 75 nm in length. The structure and morphology of the CNCs isolated by sulfuric acid were similar to those of CNCs obtained by hydrochloric acid. The dispersion of CNCs hydrolyzed by hydrochloric acid was poor and the aggregation could be observed in the TEM image [22]. On the other hand, sulfuric acid could introduce sulfate groups onto the CNCs surface during the hydrolysis

(a)

(b)

Figure 2.3 TEM images of cellulose nanocrystals extracted by (a) HCl; (b) H2 SO4 treatments. Typical individual nanocrystals are shown by arrowheads. Scale bar is 500 nm. Source: Araki et al. 1998 [22]. Reproduced with permission of Elsevier.

2.2 Extraction of Cellulose Nanocrystals Acid hydrolysis

Centrifugation

Centrifugation/purification Reaction mixture

Cellulose fiber

Sediment I

Freeze dry

Unreacted cellulose

1500 g Suspension I Centrifugation 15 000 g

Suspension II

Discarded

Freeze dry Sediment II

Cellulose nanocrystal

Figure 2.4 Schematic representation of the HBr hydrolysis preparation of cellulose nanocrystals (left) and TEM image of cellulose nanocrystals (right). Source: Sadeghifar et al. 2011 [24]. Reproduced with permission of Springer Nature.

process, leading to good dispersion of CNCs in water. However, the introduction of sulfate groups resulted in lower degradation temperature of CNCs [23]. Furthermore, hydrobromic acid, a mineral acid that is stronger than hydrochloric acid, can be used for the preparation of CNCs. A simplified version of the process for the fabrication of CNCs by using hydrobromic acid is shown in Figure 2.4 [24]. Typically, cellulose pulp obtained from Whatman-1 filter paper was hydrolyzed by using 100 mL of 2.5 M HBr at 100 ∘ C for three hours. After the ultrasonication was applied at room temperature, the turbid suspension was diluted with deionized water and centrifuged at 1500 × g for 10 minutes for five cycles to remove excess hydrobromic acid and water-soluble fragments. The cellulose nanoparticles were dispersed in the aqueous solution with pH 4. The turbid supernatant was further treated with centrifugation at 15 000 × g for 45 minutes and the resultant precipitate was freeze-dried to obtain the CNC powders. The rod-like CNCs with approximately 100–200 nm length could be observed in Figure 2.4 [24], where CNCs tended to aggregate in the suspensions. Additionally, phosphoric acid could also be used to extract CNCs. First, the filter paper was soaked in deionized water for 15 minutes, and then vigorously stirred to obtain a pulp-like slurry. The blend was transferred to a beaker and treated for 15 minutes in an ice bath, followed by the addition of phosphoric acid (85% v/v) (keeping below 30 ∘ C) slowly using a dropping funnel until a certain concentration of phosphoric acid was reached. After the addition of acid, the reaction vessel was put to a preheated 100 ∘ C oil bath, and stirred for a preset time. The resulting yellow reaction mixture was slightly cooled with an ice bath to room temperature. Then, the CNCs were separated from the mixture by centrifugation, and washed with deionized water. This process was repeated until the supernatant became colorless. After centrifugation, the CNCs obtained were dispersed in deionized water, and dialyzed against deionized water, with the water being changed every day, until the pH value was 7. Finally, the suspension was treated by ultrasonication and freeze-dried to obtain the CNC powders [25], where the yield of CNCs was 76–80%. It is noteworthy that the yield of CNCs largely depended on the reaction temperature and concentration of phosphoric acid. As shown in Figure 2.5 (Left), at low concentrations of phosphoric acid (7.8 M), regardless of the reaction time, the hydrolysis was incomplete and inhomogeneous. When the concentration of phosphoric acid reached 10.7 M, the hydrolysis reaction was rapid and homogeneous. After 90 minutes of

25

2 Structure and Properties of Cellulose Nanocrystals

300

Over hydrolyzed 240

Time (min)

26

2 μm

180

120

Pulp

60

0

500 nm

500 μm

8

9 Concentration (M)

11

Figure 2.5 TEM images of phosphoric acid hydrolyzed cotton as a function of the hydrolysis time and phosphoric acid concentration (left); TEM (top) and AFM height (bottom) images of CNCs isolated from cotton after 90 minutes of hydrolysis with a 10.7 M phosphoric acid solution at 100 ∘ C (right). Source: Camarero et al. 2013 [25]. Reproduced with permission of ACS.

hydrolysis, the CNCs were obtained. Therefore, CNCs with an average width of 31 ± 14 nm, length of 316 ± 127 nm, and aspect ratio of 11 ± 1.5 were obtained under optimized hydrolysis conditions (at 100 ∘ C with an acid concentration of 10.7 M reacted for 90 minutes), as shown in TEM and atomic force microscopy (AFM) images (Figure 2.5) [25]. Although acid hydrolysis is widely used for the production of CNCs, certain problems must be overcome, such as high consumption of energy and chemicals, acidic corrosion of equipment, and health and environmental hazards. Recently, a number of studies have focused on hydrolysis parameter optimization, corrosion prevention, and waste reduction [26]. An interesting proposal has been made, where it has been suggested that the strong liquid acid used in this process should be replaced by a solid acid. For example, a hydrolysis procedure using an acidic cation exchange resin as a solid catalyst in combination with high-power disintegration has been reported to produce cellulose particles with a yield of ∼50%, and the solid acid can be regenerated using a posttreatment procedure [27]. Recently, Liu et al. [26] reported the preparation of cellulose nanoparticles with 15–40 nm in diameter and hundreds of nanometers in length using the hydrolysis of bleached pulp with solid phosphor-tungsten acid (H3 PW12 O40 ). They found that the resultant CNCs exhibited higher thermal stability than the CNCs prepared using hydrolysis with sulfuric acid. In addition, the solid acid could be easily recovered and recycled through extraction with diethyl ether.

2.2 Extraction of Cellulose Nanocrystals

The major advantages of hydrolysis with a solid acid are easy recovery of the solid acid, low equipment corrosion, and a relatively safe working environment. Moreover, the recovered acid can be reused several times for further cellulose hydrolysis without loss of acid activity or reduction of the final product yield. The primary shortcomings of this hydrolysis method are the very high cost of solid acid, prolonged hydrolysis time, low productivity, the heterogeneity of the hydrolysis process, and wide particle size distribution, which are caused by limited contact between the solid acid granules and the cellulose feedstock. However, Hamid et al. [28] reported that sonication in combination with solid phosphor-tungsten acid dramatically reduced the time of operation from 30 hours to 10 minutes by using an optimum sonication power of 225 W. The CNCs obtained were 15–35 nm in diameter and 150–300 nm in length, and showed crystallinity of ∼88% and yield of 85%. To obtain a higher yield of CNCs, gaseous acid is also used in the preparation of CNCs. In this method, wet cellulose with a moisture content of up to 80% is hydrolyzed in the presence of an acidic gas. The gaseous acid is absorbed by the cellulose fibers, leading to a high local acid concentration and a high hydrolysis rate of the amorphous domains. Various types of gaseous acids can be used in this procedure, such as hydrochloric acid, nitric acid, and trifluoroacetic acid. This strategy can allow several environmentally harmful and time-consuming steps that are required for classical acid hydrolysis. For example, the acid recycling is easier, and the dialysis step can be omitted. Because of lower cellulose feedstock loss during the gaseous hydrolysis process, the CNC yield is higher [29]. 2.2.2

Pretreatments of Cellulose Before Acid Hydrolysis

To improve the acid hydrolysis, some techniques are mainly used for the pretreatment of cellulose, including ball-milling and treatment with various solvents (N-methylmorpholine-N-oxide (NMMO) or ionic liquid (IL)) and metal inorganic salt solution. According to Siró and Plackett [30], an efficient pretreatment helps to reduce energy consumption by 20- to 30-fold. It is worth noting that appropriate pretreatment of cellulose fibers promotes accessibility, increases the inner surface, alters crystallinity, breaks hydrogen bonds, and boosts the reactivity of the cellulose. Thus, it decreases the energy demand and facilitates the process of CNC production [31, 32]. Theoretically, cellulose fibers can be refined by ball-milling to increase the contact area between cellulose fibers and acid and facilitate the extraction efficiency. However, research has shown that the crystallinity of ball-milling cellulose fibers dramatically decreases and the cellulose polymorph may be easily transformed in the subsequent extraction process. For example, when cotton linter was ball-milled at 1000 rpm for 24 hours [33], the crystallinity of ball-milled cellulose greatly decreased to 21.9%, resulting in the increase of accessibility for reagents to cellulose, whereas no noticeable transformation of the cellulose polymorph was observed. However, after sulfuric acid hydrolysis of the ball-milled cellulose, polymorph of CNCs slightly changed from cellulose crystal I to II [34]. The CNCs obtained were sphere-like with diameters of 100–200 nm, and their crystallinities and yields were ∼68.4% and ∼7.7%, respectively.

27

28

2 Structure and Properties of Cellulose Nanocrystals

For NMMO pretreatment, the cotton linters were suspended in 90 wt% NMMO solution with a ratio of 1 : 10 (g/mL) of cotton linter vs. NMMO solution at 80 ∘ C for 30 minutes to dissolve fully and regenerate by excess deionized water, and then dried in an oven [33]. The obtained cellulose polymorph transformed from cellulose I to cellulose II, and the crystallinity decreased to 40.1%, due to the de-crystallization of cellulose during the NMMO dissolution and water regeneration process [35]. Sphere-like CNCs with 50–100 nm diameters were obtained after sulfuric acid hydrolysis for two hours [33], where the yield and crystallinity of the CNCs declined to 3.5% and 29.7%, respectively. Additionally, IL could also be widely used to pretreat cellulose fibers. Currently, imidazolium-based acidic ILs, such as 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-butyl-3-methylimidazolium acetate ([BMIM]OAc), and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO4 ), are considered as the most interesting and common solvents for cellulose. For example, cotton linter was treated by using [BMIM]Cl with a concentration of 2 wt% and stirred at 300 rpm at 105 ∘ C for six hours. Excess ethanol as an anti-solvent was added gradually into the above solution. The dissolved/regenerated cellulose was washed thoroughly with deionized water and dried in an oven [33]. The crystallinity of the obtained cellulose decreased significantly to 19.7%. Owing to the breakage of inter- and intramolecular hydrogen bonds during dissolution of cellulose in ionic liquid, the original crystalline structures of cellulose were destroyed, and recrystallization of cellulose in the regeneration process was strongly limited [36]. Another work of Tan et al. [37] can be highlighted as well, where [BMIM]HSO4 was investigated as a solvent and acid catalyst. Treatment of MCC in [BMIM]HSO4 at 70–100 ∘ C for 1.5 hours was utilized to prepare rod-like CNCs. The authors mentioned that the basic cellulose I structure was preserved in CNCs during the catalytic conversion process and the degree of crystallinity of 95.8% was found to be higher compared to the MCC. Recently, Abushammala et al. [38] have reported for the first time a direct extraction of CNCs from wood by means of [BMIM]OAc treatment. They demonstrated that the CNCs obtained present a high crystallinity of 75% and high aspect ratio of 65 with a yield of 44%. Moreover, metal inorganic salts can also increase the hydrolysis rate of cellulose during the hydrolysis process. The metal inorganic salts in the trivalent (FeCl3 , Fe2 (SO4 )3 , Al(NO3 )3 ), divalent (CaCl2 , FeCl2 , FeSO4 ), and monovalent (NaCl, KCl) categories have been demonstrated by many researchers for enhancing the hydrolysis efficiency of cellulose [39–42] and preparation of micro- or nanocrystalline cellulose [43–45]. Transition metal salts-assisted dilute sulfuric acid hydrolysis is an efficient method for the preparation of nanocellulose from native cellulose. However, different transition metal ions exhibited different effects on the physicochemical properties of treated nanocellulose, such as crystallinity, thickness of crystal, surface morphology, dimensional profile, and thermal stability. Chen et al. [46, 47] have reported that nanostructured cellulose was successfully prepared from native cellulose using H2 SO4 hydrolysis pathway in the presence of Cr (III)-, Fe (III)- and Mn (II)-, and Co (II)- and Ni (II) transition metal salts as the co-catalyst. The dimensional profile of the nanocellulose prepared was further studied via TEM analysis (Figures 2.6 and 2.7).

2.2 Extraction of Cellulose Nanocrystals

(a)

35

Count (%)

30 25 20 15 10 5 0

5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45

d = 31.6 ± 4.14 nm

Diameter range (nm)

(b)

35

Count (%)

30 25 20 15 10 5 0 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45

d = 23.5 ± 9.1 nm

Diameter range (nm)

(c)

35

Count (%)

30 25 20 15 10 5 0 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45

d = 18.5 ± 6.2 nm

Diameter range (nm)

(d)

25

Count (%)

20 15 10 5 0 80

Diameter range (nm)

Figure 2.6 TEM images of nanocellulose treated by (a) Co(II)-, (b) Ni(II)-, (c) Fe(III)-transition metal salt catalyst, and (d) dilute sulfuric acid [46].

29

2 Structure and Properties of Cellulose Nanocrystals

(b)

(a)

100 nm

200 nm

0.5 μm Average diameter = 58.4 ± 15.3 nm

10 5

< 10 11–19 20–29 30–39 40–49 50–59 60–69 70–79 80–89 90–99 >100

0

Diameter range (nm)

20 15

Average diameter = 44.7 ± 13.2 nm

10 5 0

< 10 11–19 20–29 30–39 40–49 50–59 60–69 70–79 80–89 90–99 >100

15

0.5 μm

Frequency (%)

20 Frequency (%)

30

Diameter range (nm)

Figure 2.7 TEM micrographs of (a) Mn(II)- and (b) Cr(III)-hydrolyzed nanocellulose [47].

Under controlled hydrolysis reactions, the amorphous regions of cellulose were degraded successfully by metal ion catalyst to different extents. As compared to sulfuric acid hydrolysis under the same reaction conditions, the results showed that transition metal salts-treated nanocellulose rendered relatively higher crystallinity and narrower diameter range of dimension. Transition metal salts-assisted hydrolysis reaction rendered better simplicity in operation and rapid reaction to yield higher crystallinity nanocellulose and extraction yield. Effect of transition metal ions on acid hydrolysis efficiency showed a similar acid hydrolysis mechanism as with inorganic acid during the depolymerization of cellulose. The metal ions were generated from transition metal salts capable of attaching to the highly electronegative oxygen atoms in β-1,4-glycosidic bonds of the cellulose chain, which resulted in weakening of the bond strength between pyranose rings in the amorphous region, leading to the easy rupture of the cellulose chain into smaller fiber fragments. As shown in Eq. (2.1), the transition metal ions can form coordination covalent bonds with six water molecules to form a metal–ligand complex in the presence of water. Afterwards, the metal–ligand complex ions were deprotonated to yield more H+ ions in the solution. This deprotonation process increased the concentration of H+ ions in the solution, which hydrolytically cleaved the glycosidic bond between the glucose units along the cellulose chain effectively, resulting in the degradation of glycosidic linkages. The process continued until the metal complex became stable. Therefore, the oxidation state of transition metal cations played an important role in generating acidic condition. When the metal ion possessed the higher

2.2 Extraction of Cellulose Nanocrystals

oxidation state, more H3 O+ would be generated in order to produce a stable complex. Therefore, a higher rate of cellulose hydrolysis to the nanocellulose product can be achieved by using higher oxidation state transition metal ions. [Fe(H2 O)6 ]3+ + 3H2 O → [Fe(H2 O)3 (OH)3 ] + 3H3 O+ [Co(H2 O)6 ]2+ + 2H2 O → [Co(H2 O)4 (OH)2 ] + 2H3 O+ [Ni(H2 O)6 ]2+ + 2H2 O → [Ni(H2 O)4 (OH)2 ] + 2H3 O+ [Cr(H2 O)6 ]3+ + 2H2 O → [Cr(H2 O)3 (OH)3 + 3H3 O+ [Mn(H2 O)6 ]2+ + 2H2 O → [Mn(H2 O)4 (OH)2 ] + 2H3 O+

(2.1)

Reaction mechanism of Fe(III)-,Co(II)-, Ni(II) and Cr(III)-, and Mn(II)- transition metal salt catalyst [46, 47]. 2.2.3

Other Methods of Preparing Cellulose Nanocrystals

2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO) is a well-known reagent that is widely used for pretreatment of cellulose materials in the laboratory to reduce the energy required for mechanical disintegration. The TEMPO-meditated oxidation method has also been applied to cellulose fibers to produce CNCs (as shown in Figure 2.8). TEMPO-oxidized CNCs were synthesized according to the methods reported in the literature [48]. TEMPO (10 mg, 0.065 mmol), NaBr (0.21 g, 2 mmol), and NaClO (1.76 M, 0.5 mL, 0.88 mmol) were stirred in 10 mL of water until complete dissolution. This solution was then added to the cellulose suspension, which was mechanically stirred and maintained at 20 ∘ C. The pH value of reaction was monitored using a pH meter and the pH value maintained at 10 by incrementally adding 0.5 M NaOH. When the pH value was stable, the reaction was considered to be finished. About 5 mL of methanol was then added to react and quench with the extra oxidant. After adjusting the pH value to 7 by adding 0.5 M HCl, the TEMPO-oxidized product was washed with distilled water OH +N O NaCIO

NaBr

O

O HO

OH

N O O NaCl

NaBrO

O N OH

HO

ONa O OH

Figure 2.8 Simplified oxidation scheme. Source: Perez et al. 2003 [48]. Reproduced with permission of ACS.

31

32

2 Structure and Properties of Cellulose Nanocrystals

by centrifugation, and further purified by dialysis against distilled water for two days. The solid products were obtained by freeze-drying. CNCs exhibited superior dispersity in water after TEMPO treatment, because of the incorporation of a higher number of carboxylate groups in the cellulose. The oxidized nanoparticles exhibit smaller sizes, improved transmittance, higher shear stress, and higher viscosity, compared to CNCs obtained using the conventional hydrolysis method [49]. Recently, Carlsson et al. [50] prepared highly crystalline CNCs from Cladophora algae via co-oxidant-free TEMPO oxidation. They demonstrated that the same degree of oxidation could be achieved within approximately the same time by replacing the co-oxidants with TEMPO+ electro-generation in a bulk electrolysis setup. It was revealed that the oxidation did not affect the morphology, specific surface area, and pore characteristics of the nanoparticles obtained. As compared to the traditional acid hydrolysis method, enzymatic fabrication of CNCs is a feasible alternative preparation technique that avoids using harsh chemical reaction conditions and high energy consumption of mechanical fibrillation and heating. Furthermore, enzymes that selectively degrade the amorphous domains of cellulose fibers do not considerably digest the crystalline areas, which allows for easier chemical manipulation and an expanded commercial potential. Satyamurthy et al. [51] have produced CNCs using a controlled microbial hydrolysis of MCC with the cellulolytic fungus Trichoderma reesei. The reported yield of production was 22%. However, this technique is hindered by economical (i.e. high cost of cellulose enzyme) and technical (rate-limiting step of cellulose degradation with a long processing period) constraints.

2.3 Structures and Properties of Cellulose Nanocrystals 2.3.1

Physical Properties of Cellulose Nanocrystals

It is well known that the structure parameters of the rod-like CNCs, the length and width, vary widely mainly depending on the source and the hydrolysis conditions of cellulose. To reduce the size distribution of CNCs extracted from a given cellulose source, some strategies were employed such as filtration [8], differential centrifugation [52], and ultra-centrifugation [53]. The source and extraction method also influence the size, morphology (with particular reference to the rod-like structure), and crystallinity of CNCs. Typical characteristics (structure parameters, crystallinity, and morphology) of CNCs derived from various cellulose sources and extracted by different methods are summarized in Table 2.1. The width of CNCs obtained is usually only a few nanometers, but the length of CNCs has a very broad distribution, ranging from tens of nanometers to several micrometers, and the degree of crystallinity of CNCs is greater than 70%. Owing to the high crystallization of CNCs, they also show high rigidity. Although the axial elastic modulus of CNCs from different sources has been investigated for many years [63–66, 73, 84–90], there are rare reports on the transverse elastic modulus of CNCs. Early studies using X-ray diffraction determined the longitudinal modulus of CNCs to be in the range of 90–138 GPa

2.3 Structures and Properties of Cellulose Nanocrystals

33

Table 2.1 The fundamental properties of CNCs from various sources obtained by different techniques. Structural parameters Sources

Extraction method L (nm)

Cotton

HCl

100–200 5–10

H2 SO4

200–300 8

w (nm)

Moduli (GPa) Crystallinity (%)

H2 SO4

—

17.7 ± 5.0 89.3 [55] [54]

[56]

90.5 [55]

[57]

70–170

∼7

[5]

255

15

[58] [33]

100–200 10–20

25–320

91 [34]

6–70

35–265

[18]

[8]

300–500 15–30 MCC

References

ET

150–210 5–11 Cotton linter

TEM images

EA

[59]

3–48

84.3 [24] 87.3 [24]

[8]

H2 SO4

200–400 10

[60]

HCl

250–270 23

[61]

∼500

10

Ramie

H2 SO4

150–250 6–8

Sisal

H2 SO4

100–500 3–5

[62] 90–138 [63–67]

88 [68]

134 ± 59 10.8 ± 4.5

[7] 81 [68]

150–280 3.5–6.5 Soft wood HCl

H2 SO4

100–200 3–4

100–150 4–5

[69]

[70]

[6] —

24.8 ± 7.0 [54]

[20, 71]

[72] (Continued)

34

2 Structure and Properties of Cellulose Nanocrystals

Table 2.1 (Continued) Structural parameters

Moduli (GPa) Crystalli- TEM nity (%) images References

Extraction Sources method L (nm)

w (nm)

Tunicate H2 SO4

8.8–18.2 143 [31] 2–25 [74]

[75]

500–1000

16

[58]

1000–3000

10

151 ± 29

[11]

100–1000

15–30

[73]

[57]

1160

EA

ET

1073

15

Bacterial H2 SO4

100–1000

5–50

72

[9, 23, 76]

Hemp

H2 SO4

158.4 ± 63.6

13.2

81.8

[77]

Hard wood

H2 SO4

140–150

4–5

Jute

Tempo

100–200

3–10

69.72

[78]

Rice straw

H2 SO4

50–700

10–65

86

[79]

40–260

4–26

91.2

Soy hull H2 SO4

122.66 ± 39.40 2.77 ± 0.67

[10]

[72]

73.5

[80]

(Continued)

2.3 Structures and Properties of Cellulose Nanocrystals

35

Table 2.1 (Continued) Structural parameters Sources

Extraction method L (nm)

Mengkuang H2 SO4 leaves

∼200

Sugarcane bagasse

H2 SO4

Bamboo

H2 SO4

w (nm)

10–20

Moduli (GPa) EA

ET

Crystallinity (%)

TEM images

References

69.5

[81]

255 ± 55 2–4

87.5

[82]

100 ± 28 8 ± 3

87

[83]

EA , elastic modulus in the axial direction; ET , elastic modulus in transverse direction; L, length; w, width.

[63–67]. In 1968, Jaswon et al. [85] utilized a theoretical model of cellulose I to estimate the values of 76, 51, and 57 GPa for the longitudinal and two transverse moduli, respectively. In the same way, the longitudinal and two transverse elastic moduli were predicated to have elastic modulus values of 167, 11, and 50 GPa, respectively, in 1991 [87]. Furthermore, inelastic X-ray scattering, Raman spectroscopy, and AFM were also used to measure the elastic modulus of CNCs. For example, the transverse modulus of CNCs determined by X-ray scattering was 15 GPa, which was much lower than the value of 220 GPa for the longitudinal modulus [91]. Raman spectroscopy was also used to determine the elastic modulus of CNCs obtained from tunicate (143 GPa), which was consistent with the calculated value from the theoretical chain structure (145 GPa) [1], and the elastic modulus of CNCs from plant was 105 GPa [92]. AFM results indicated that the longitudinal modulus of CNCs from tunicate was 151 ± 29 GPa [73], and the transverse modulus of wood, cotton, and tunicate CNCs were in the ranges of 18–50 GPa [93], 17.7 ± 5.0 GPa [54], and 2–25 GPa [74], respectively. The modulus of CNCs obtained from various cellulose sources by different methods is also summarized in Table 2.1. Furthermore, the modulus change of the CNCs to the composites as a filler will be discussed in Chapter 6. Since the conventional thermal processing temperature for polymeric materials often exceeds 200 ∘ C, the thermal stability of the CNCs becomes very important, especially for high temperature matching [94]. In general, the thermal decomposition temperature of cellulose is about 300 ∘ C and meets the requirements of the thermal processing of composite materials. However, the normal extraction method uses sulfuric acid for hydrolysis, which makes it easy to introduce the sulfate ester groups on the surface of the CNCs obtained, leading to diminishment of the thermal stability and the thermal decomposition onset temperature of CNCs. According to the literature (Table 2.2), sulfuric acid-hydrolyzed CNCs showed a

36

2 Structure and Properties of Cellulose Nanocrystals

Table 2.2 Thermal properties of nanocrystals from different sources.

Materials

Onset of degradation (∘ C)

Main degradation step (∘ C)

Jute

[78]

Untreated

270

Alkaline treated

270

Oxidized cellulose nanocrystals

200

Industrial bio-residues Raw

References

[95] 202

270

Cellulose

248

290

Sonified cellulose

260

317

Homogenized cellulose

258

314

Hydrolyzed cellulose

122, 253

133, 283

Initially (raw)

190

327

Treated (after the purification)

240

342

Crystals

170

294

Raw bast

177

321

NaOH-treated fibers

256

368

Bleached fibers

220

346

Sulfuric acid crystals

171

317

HCl crystals

256

358

Soy hulls

[80]

Kenaf

[96]

Mengkuang leaves

[81]

Raw

370

Alkali–treated

250

370

Bleached

250

350

Sugarcane bagasse

[82]

Bleached

270

Crystals (30 min hydrolyzed)

255

Crystals (75 min hydrolyzed)

210

two-stage degradation with an initial onset of degradation around 120 ∘ C and a second degradation around 225 ∘ C. Until now, two methods have been used to improve the thermal stability of CNCs: desulfurization to reduce the sulfate ester groups on the surface of CNCs, and neutralizing the sulfuric ester groups with alkaline solution. Solvolytic desulfation [97] is an effective protocol to significantly reduce the sulfur content of H2 SO4 -hydrolyzed CNCs. However, solvolytic desulfation increased the crystallite dimensions slightly, where the reason might be that the removal of the sulfate groups of H2 SO4 -hydrolyzed CNCs led to aggregation of the particles. The most important thing was that purification of desulfated CNC with solvents (butanol)

2.3 Structures and Properties of Cellulose Nanocrystals

did not impact the thermal stability or degradation. This result was very important for potential utilization of (desulfated) CNC in thermoplastic processing, where temperatures upward of 200 ∘ C were typically encountered. The pH value of solution could be adjusted by sodium hydroxide to neutralize the CNCs sulfate ester groups until neutral and increase the thermal stability of CNCs [77]. Thermogravimetric analysis (TGA) curves of CNCs and neutralized CNCs are shown in Figure 2.9 [77, 98]. Under nitrogen atmosphere, the CNCs were heated from room temperature to 600 ∘ C at a rate of 10 ∘ C/min. The results of TGA analysis obviously displayed a shift in degradation temperature, increasing from 120 to 280 ∘ C, and the neutralized CNCs exhibited a different degradation pattern,

Weight of residues (%)

(a) 100 80

60 40

20

Neutralized CNC – 40 min CNC – 40 min

0 0

100

200

300

400

500

600

Temperature (°C) (b) 100 pH 7

Weight of initial (%)

98

96

94

92 pH 2.5 90 0

10

20 Time (min)

30

40

Figure 2.9 TGA curves of CNCs and neutralized CNCs: (a) the thermal stability of CNCs with increase of temperature. Source: Kargarzadeh et al. 2012 [77]. Reproduced with permission of Springer Nature. (b) The thermal stability of CNCs at 180 ∘ C with the prolongation of time. Source: Bondeson and Oksman 2007 [98]. Reproduced with permission of Taylor & Francis.

37

38

2 Structure and Properties of Cellulose Nanocrystals

which involves only one pyrolysis process that is typical of cellulose (Figure 2.9a) [77]. After neutralizing CNCs using 0.25 M NaOH, they exhibited a higher degradation temperature than 180 ∘ C, whereas the CNCs without NaOH neutralization gradually decomposed at 180 ∘ C [98]. Meanwhile, when hydrochloric acid was used instead of sulfuric acid to hydrolyze the native cellulose, the thermal stability of the CNCs enhanced significantly and the thermal degradation onset temperature increased from 234 to 316 ∘ C, due to absence of the sulfate ester groups on the surface of CNCs [20]. This result showed that the thermal stability of CNCs depended on the type of acid used. On the other hand, the dispersity of CNCs isolated by hydrochloric acid was very poor and the CNCs were easy to aggregate, compared to that extracted by sulfuric acid. Because the surface of CNCs presents abundant hydroxyl groups, CNCs with high hydrophilicity can be stably dispersed in water to form a relatively homogeneous suspension. The hydrophilicity of CNCs could be mainly characterized by water contact angles. Generally, the lower the water contact angle of CNCs, the higher the hydrophilicity of CNCs. The water contact angles of CNCs prepared from different cellulose sources and extraction methods are presented in Table 2.3. Owing to the difference in the surface roughness and liquid–surface interactions between the pellet and film of CNCs [102], the water contact angles for the CNC pellets was 44.7∘ [13], but that of the CNC film was 10–15∘ [18, 103]. Surface energy (𝛾 S ) involved the dispersive surface energy (𝛾 d ) and polar surface energy (𝛾 p ). Generally, the 𝛾 S of CNCs can be calculated by measuring the different contact angles in different solvents, following the equation √ √ p p 𝛾L (1 + cos 𝜃) = 2 𝛾Ld 𝛾Sd + 2 𝛾L 𝛾S where the subscripts L and S referred to the liquid drop and the solid surface, respectively, and 𝜃 expressed the contact angle between the solid substrate and the liquid drop. For example, the contact angle values of CNCs for water, diiodomethane, and ethylene glycol were 44.60∘ , 19.65∘ , and 17.90∘ , respectively, p and according to the above equation the calculated dispersive (𝛾Sd ) and polar (𝛾S ) surface energy values were 39.2 and 21.5 MJ/m2 , respectively. So the total surface p energy (𝛾 S ) was equal to the 𝛾Sd plus the 𝛾S , 60.7 MJ/m2 [99]. The surface energy of CNCs extracted from the different cellulose sources and extraction methods is Table 2.3 Contact angles for water and total surface energy of CNCs derived from different sources and extraction methods.

Sources

Extraction method

Cotton linter MCC

𝜽 (∘ )

𝜸sd (mJ/m2 )

𝜸sp (mJ/m2 )

𝜸 s (mJ/m2 )

References

H2 SO4

44.6

39.2

21.5

60.7

[99]

HCl

30

Ramie

H2 SO4

35

Cotton

H2 SO4

34

Soft wood

HCl

12

Tunicin

H2 SO4

24

[100] 35.2

25.3

60.5

[68] [101] [18]

4.9 × 10−6

94.9

94.9

[75]

2.3 Structures and Properties of Cellulose Nanocrystals

summarized in Table 2.3. Moreover, the water contact angle and surface energy of CNCs are critical properties for determining the performance of polymer/CNCs nanocomposites. The interfacial compatibility between hydrophilic CNCs and hydrophobic polymers is relatively poor, and not conducive to the preparation of nanocomposites based on hydrophobic polymers. Therefore, hydrophobic modification should be undertaken for the CNCs. For example, surface acetylation of CNCs can enhance their dispersity in organic solvents; the contact angle for water on the acetylated CNCs surface is 78∘ and the total surface energy is 50 MJ/m2 [13]. 2.3.2

Properties of Cellulose Nanocrystal Suspension

Generally, CNCs obtained by sulfuric acid hydrolysis of the native cellulose in suspension could be arranged spontaneously from the isotropic phase to anisotropic phase above critical concentrations, and the anisotropic phase was composed of stacked planes of rod-like CNCs aligned along a vector, where the orientation of CNCs in each plane was rotated at a slight angle around the perpendicular axis from one plane to the next [3, 104] (Figure 2.10). Owing to the arrangement of CNCs in suspension, the CNCs exhibited some distinctive structures and properties. One of the most typical features is that the suspension of CNCs could display cholesteric liquid crystalline property and flow birefringence phenomenon, where the flow birefringence property of the CNCs suspension is associated with the concentration of CNCs. For example, at a relatively low concentration, CNCs suspension only exhibited a slight birefringence under polarized light (Figure 2.11a) [105]. With the increased concentration of CNCs, more birefringence in CNCs colloids suspension was observed. As shown in Figure 2.11b [105], it was observed clearly that the isotropic and anisotropic (nematic) phases coexisted in CNCs colloids suspension with a CNCs concentration of 2.03%. When the CNCs concentration reached 3.17%, the colloids suspension exhibited birefringence with

P/2

(a)

Isotropic phase

(b)

Anisotropic phase

Figure 2.10 Schematic representation of rod-like CNCs orientation in both the (a) isotropic and (b) anisotropic (chiral nematic) phases. Source: Yang et al. 2013 [33]. Reproduced with permission of Springer Nature.

39

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2 Structure and Properties of Cellulose Nanocrystals

(a)

(b)

50 μm

50 μm

(c)

50 μm

Figure 2.11 Polarized optical micrographs of CNC colloids at concentrations of 0.91% (a), 2.03% (b), and 3.17% (c). Source: Liu et al. 2011 [105]. Reproduced with permission of Elsevier. (a)

(b)

200 μm

Figure 2.12 (a) Chiral nematic texture of the anisotropic phase of a CNCs suspension, and (b) solid film of CNCs observed between cross-polarizers. Source: (a) Derek and Mu 2016 [108]. Reproduced with permission of ACS. (b) de Souza Lima and Borsali 2004 [109]. Reproduced with permission of John Wiley & Sons.

2.3 Structures and Properties of Cellulose Nanocrystals

colors and a clear micro-fingerprint texture (Figure 2.11c) [105], indicating the existence of cholesteric liquid crystalline phases. It is noteworthy that this micro-fingerprint texture was not aligned along a vector but was twisting [105–107]. Moreover, CNCs could spontaneously arrange or self-assemble into a highly ordered structure and subsequently form an anisotropy region when a critical concentration of CNCs in the colloids suspension was reached. For example, the CNCs suspensions exhibited shear birefringence. After long-time standing, they could spontaneously separate into an upper isotropic and a lower anisotropic phase with the CNCs concentration exceeding the critical concentration of chiral nematic phase formation [3] (Figure 2.12) [76]. This interesting phenomenon is dependent on the surface charge, aspect ratio, and length distribution of CNCs. Above the critical concentration, the occurrence of the CNCs spontaneous phenomenon was ascribed to the entropically driven self-orientation phenomenon of rod-like CNCs to form nematic order structure. Compared to the disordered phase, the nematic order phase eliminated the volume interactions, resulting in higher packing entropy [3]. However, the suspensions of CNCs prepared by hydrochloric acid did not generate such chiral nematic order due to no charges of CNCs. In the suspension the existence of negative charges on the surface of CNCs was beneficial to phase stability [110]. The threshold/critical concentration for sulfated CNCs to form ordered nematic phases in electrolyte-free aqueous suspensions is largely dependent upon the charge density and is typically in the ranges of 1–10% (w/w). With increasing concentration of CNCs, the obtained chiral nematic anisotropic phase’s pitch decreased in the range of 20–80 μm [3]. As stated previously, when CNCs suspensions reached a critical concentration, the CNCs spontaneously displayed an ordered phase, showing interesting liquid crystalline properties (chiral nematic). This phenomenon was common in non-agglomerated colloid suspensions of other rod-like nanoparticles such as the chitin nanocrystals. The polarized optical microscope (POM) can be used to investigate the self-organization of CNCs suspension. The existence of nematic phase transition will give a special “fingerprint texture structure” of the rod-like CNCs suspensions, which indicated the ordered structure of the chiral nematic (Figure 2.12a) [108]. The rod-like morphology and the spiral stacked structure of CNCs brought about these particular properties. After complete evaporation of water from the suspension, the chiral nematic structure can be preserved, and an iridescent film was obtained, as shown in Figure 2.13b [109]. The dispersion and self-ordering property of CNCs are restricted to aqueous suspensions or a few organic solvents with high dielectric constants, such as dimethylsulfoxide (DMSO), DMF, and ethylene glycol. In a polar organic solvent, the inefficient electrostatic repulsion and the strong hydrogen bonds between CNCs led to rapid aggregation of the CNCs in suspension. In fact, the electrostatic repulsion that stabilized the CNCs in suspension can induce stronger chiral interactions among CNCs [56]. For example, the dispersion of CNCs with different aspect ratios in cyclohexane has been studied, and it was found that the critical concentration of CNCs suspension in cyclohexane was higher than that in water. The suspensions of CNCs with the highest aspect ratio did not exhibit the phase separation phenomenon, but at relatively high concentrations

41

42

2 Structure and Properties of Cellulose Nanocrystals

(a)

(b)

200 μm

200 μm

Figure 2.13 Polarized optical micrographs of birefringent gel phases in CNC suspensions with high aspect ratios in cyclohexane at total concentrations of 26.3 wt% (a) and 30.8 wt% (b). Source: Elazzouzi-Hafraoui et al. 2009 [111]. Reproduced with permission of ACS.

of CNCs (26.3 and 30.8 wt%), the suspension exhibited an anisotropic gel phase (Figure 2.13) [111]. Owing to the presence of stronger chiral interactions in the polar medium, the chiral nematic pitch was much lower than that measured in water, in general as small as 2 μm [111]. Moreover, there are plenty of hydroxyl

(a)

(b)

Figure 2.14 Aqueous 0.53% (w/v) suspensions of CNCs observed between crossed polarizers after production by HCl-catalyzed hydrolysis (a), and after their oxidation via TEMPO-mediated reactions (b). Source: Habibi et al. 2006 [118]. Reproduced with permission of Springer Nature.

2.3 Structures and Properties of Cellulose Nanocrystals

Figure 2.15 Polarized optical micrographs of the silylated CN suspension in THF. Source: Goussé et al. 2002 [114]. Reproduced with permission of Elsevier.

groups on the CNCs surface, so the various chemical methods have been essayed to modify CNCs, involving esterification [112], etherification, oxidation [113], silylation [114, 115], and polymer grafting [116, 117]. Some of the chemical modifications of CNCs can still maintain the birefringence. For example, TEMPO oxidation or carboxymethylation of CNCs aqueous suspensions also exhibited birefringence property (Figure 2.14) [118] as did silylated CNCs, which can stably disperse in tetrahydrofuran (THF), and also exhibit birefringence property, as shown in Figure 2.15. Under shearing or shaking, the modified CNCs THF suspension would appear as bright and black areas [114]. The suspensions of CNCs have the unique property of liquid crystal phase transition as well as distinctive rheological properties [119]. CNCs displayed different rheological properties under different CNCs concentrations and shear rates. At the beginning, the behavior was mainly shear thinning and it was thought that at this stage the network, constructed by strong hydrogen or ionic bonding interactions among CNCs, was broken by increasing shear strength. When the critical shear rates and concentrations were exceeded, the viscosity of suspensions increased with increasing shear rates and concentrations. At this stage, the CNCs rearranged to form an ordered network, thus leading to a sudden increase in viscosity, which was typical rheological behavior of liquid crystalline polymers [123].

43

2 Structure and Properties of Cellulose Nanocrystals

1000 0.91% 2.03% 3.17%

100 Viscosity (Pa s)

44

10

1

0.1 10

1

100

Shear rate (s–1)

Figure 2.16 Viscosity as a function of shear rate for CNC suspensions of different concentrations. Source: Liu et al. 2011 [105]. Reproduced with permission of Elsevier.

70 °C

0.5

1.0

2.0

3.0

Figure 2.17 Inverted sample tubes containing 0.5, 1.0, 2.0, and 3.0 wt% CNC gel in glycerol under evaporation at 70 ∘ C. Source: Dorris and Gray 2012 [119]. Reproduced with permission of Springer Nature.

At room temperature, the viscosity (𝜂) of CNCs suspensions of various concentrations as a function of shear rate (d𝛾∕dt) is shown in Figure 2.16 [105]. It was clearly observed that the viscosity of CNCs suspension increased with increase in CNCs concentration, and at lower shear rates, the viscosity of CNCs suspension linearly decreased when the shear rates further increased. This result indicated

References

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104 Beck, S., Bouchard, J., Chauve, G., and Berry, R. (2013). Controlled produc-

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tion of patterns in iridescent solid films of cellulose nanocrystals. Cellulose 20 (3): 1401–1411. Liu, D., Chen, X., Yue, Y. et al. (2011). Structure and rheology of nanocrystalline cellulose. Carbohydrate Polymers 84 (1): 316–322. Majoinen, J., Kontturi, E., Ikkala, O., and Gray, D.G. (2012). SEM imaging of chiral nematic films cast from cellulose nanocrystal suspensions. Cellulose 19 (5): 1599–1605. Roman, M. and Gray, D.G. (2005). Parabolic focal conics in self-assembled solid films of cellulose nanocrystals. Langmuir 21 (12): 5555–5561. Derek, G.G. and Mu, X.Y. (2016). Twist-bend stage in the relaxation of sheared chiral nematic suspensions of cellulose nanocrystals. ACS Omega 1: 212–219. de Souza Lima, M.M. and Borsali, R. (2004). Rodlike cellulose microcrystals: structure, properties, and applications. Macromolecular Rapid Communications 25 (7): 771–787. Revol, J.F., Bradford, H., Giasson, J. et al. (1992). Helicoidal self-ordering of cellulose microfibrils in aqueous suspension. International Journal of Biological Macromolecules 14 (3): 170–172. Elazzouzi-Hafraoui, S., Putaux, J.L., and Heux, L. (2009). Self-assembling and chiral nematic properties of organophilic cellulose nanocrystals. Journal of Physical Chemistry B 113 (32): 11069–11075. Sassi, J.F. and Chanzy, H. (1995). Ultrastructural aspects of the acetylation of cellulose. Cellulose 2 (2): 111–127. Montanari, S., Roumani, M., Heux, L., and Vignon, M.R. (2005). Topochemistry of carboxylated cellulose nanocrystals resulting from TEMPO-mediated oxidation. Macromolecules 38 (5): 1665–1671. Goussé, C., Chanzy, H., Excoffier, G. et al. (2002). Stable suspensions of partially silylated cellulose whiskers dispersed in organic solvents. Polymer 43 (9): 2645–2651. de Oliveira Taipina, M., Ferrarezi, M.M.F., Yoshida, I.V.P., and do Carmo Gonçalves, M. (2013). Surface modification of cotton nanocrystals with a silane agent. Cellulose 20 (1): 217–226. Zhao, B. and Brittain, W.J. (2000). Polymer brushes: surface-immobilized macromolecules. Progress in Polymer Science 25 (5): 677–710. Ljungberg, N., Bonini, C., Bortolussi, F. et al. (2005). New nanocomposite materials reinforced with cellulose whiskers in atactic polypropylene: effect of surface and dispersion characteristics. Biomacromolecules 6 (5): 2732–2739. Habibi, Y., Chanzy, H., and Vignon, M.R. (2006). TEMPO-mediated surface oxidation of cellulose whiskers. Cellulose 13 (6): 679–687. Dorris, A. and Gray, D.G. (2012). Gelation of cellulose nanocrystal suspensions in glycerol. Cellulose 19 (3): 687–694. Urena-Benavides, E.E., Ao, G., Davis, V.A., and Kitchens, C.L. (2011). Rheology and phase behavior of lyotropic cellulose nanocrystal suspensions. Macromolecules 44 (22): 8990–8998. Bercea, M. and Navard, P. (2000). Shear dynamics of aqueous suspensions of cellulose whiskers. Macromolecules 33 (16): 6011–6016.

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3 Structure and Properties of Cellulose Nanofibrils Pei Huang 1, 2 , Chao Wang 1 , Yong Huang 1 , and Min Wu 1 1 Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Beijing 100190, China 2 Chongqing University, College of Aerospace Engineering, Chongqing 400044, China

Cellulose, the most abundant biopolymer on earth, has attracted much attention as a cheap, renewable, and biodegradable material. This material is a semicrystalline polymer comprising crystalline and amorphous regions, where the individual cellulose molecule is considered to pass through several crystalline and amorphous parts according to fringe micelle theory (Figure 3.1) [2]. Owing to the excellent mechanical properties, low thermal conductivity, chemical tunability, and renewable and biodegradable character, nanocellulose has been widely used in the fields of the reinforcement of nanocomposites [3, 4], enzyme immobilization [5], drug delivery [6], bioimaging [7], and biosensing [8]. Depending on the production condition and the dimensions, there are mainly two types of nanocellulose: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) (also known as nanofibrillated cellulose (NFC), microfibrillated cellulose, or cellulose nanofiber). However, because of the intense inter- and intramolecular hydrogen bonds, excessive energy is required to isolate highly crystallized nanocellulose from each other. Generally, chemical approaches, such as acid hydrolysis and oxidation, destroy the amorphous regions and release CNCs with 5–50 nm width and several hundred nanometers in length. In contrast, mechanical approaches seldom destroy the intermolecular hydrogen bonding and cleave cellulose fibers into CNFs with a length of up to a few micrometers. Generally, CNFs produced by mechanical treatments have a yield up to 70%. These features promote mechanical disintegration as a highly promising strategy for the upscale production of CNFs. So far, various types of resources, including wood, fruit, cane, straw, leaf, and bast, have been studied to produce CNFs via mechanical treatments (Table 3.1).

3.1 Production of CNF Prior to mechanical disintegration, the removal of matrix by various bleaching methods, which are similar to those used in papermaking industry, is essential Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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3 Structure and Properties of Cellulose Nanofibrils

< 5 μm Elementary fibril (3–7 nm) Gap Microfiber (26 ± 9 μm)

Gap Nanofibril (3–40 nm)

Figure 3.1 A schematic model of the nanostructure of cellulose nanofibrils. Source: Yousefi et al. 2011 [1]. Reproduced with permission of ACS.

to obtain purified cellulose pulp and to improve disintegration efficiency. Studies also indicate the residual of lignin and hemicellulose likely benefits for the receiving of finer CNF, which is probably caused by a dual mechanism consisting of increased swelling caused by hemicelluloses and formation of mechanoradicals stabilized by residual lignin [25]. 3.1.1

Chemical Bleaching

Typically, a de-waxing step is carried out by boiling at first. Then, lignin is removed by using an acidified solution, such as sodium chlorite, chlorine dioxide, and sodium hypochlorite. Subsequently, the samples are treated by alkali solution in order to leach hemicellulose, residual starch, and pectin. After a series of chemical treatments, the samples are washed with distilled water until the residues are neutralized. To avoid generating strong hydrogen bonding among nanofibers after matrix removal, the samples are kept in a water-swollen state for further disintegration process [12]. The bleached cellulose pulp is further subjected to mechanical treatments, including homogenization, ball-milling, cryocrushing, grinding, and ultrasonic, for the production of CNFs. 3.1.2 3.1.2.1

Mechanical Disintegration Homogenization

Homogenization, an emerging technology widely used in food, cosmetics, pharmacy, and biotechnology, was first introduced by Turbak and Herrick for the production of individualized CNFs [26, 27]. That is, a cellulose pulp suspension is pumped by pressure intensifiers, forcing it to flow at high pressure through a narrow gap, where an abrupt pressure drop occurs (from 100 MPa down to 0.1 MPa) with consequent fluid acceleration, cavitation effects, turbulence, and high shear stress [28]. Those effects cause the fibrillation of cellulose fibers in aqueous suspension. The morphology of CNFs can be manipulated in a well-controlled manner by alternating the number of passes and chamber sizes within the homogenizer [29]. With decrement or increment in the number of passes, thicker or thinner nanofibers are produced [30].

Table 3.1 Cellulose nanofibrils derived from different sources. Original materials

Processing methods

Diameter

Length

Crystallinity (%)

Wood

Chemical bleaching and sonication

5–20

Microns

Potato tuber

Chemical bleaching and homogenization

Wheat straw

Chemical bleaching, cryocrushing, and homogenization

Soy hulls

Rice straw

Potato tuber

Chemical bleaching and grinding

Morphology

Yield (%)

References

69

—

[9]

5

—

—

[10]

10–80

57.5

—

[11]

20–120

59.8

—

12–35

76

—

12–55

82

—

[12]

(Continued)

Table 3.1 (Continued) Original materials

Processing methods

Diameter

Sugar beet

Chemical bleaching and homogenization

70

—

[16]

Hyacinth weeds

Chemical bleaching, ball-milling, cryocrushing, and sonication

25

—

—

[17]

Pineapple leaf

Chemical bleaching and steam explosion

5–15

—

69

[18]

Areca nut husk

Chemical bleaching and homogenization

3–5

73

70

[19]

Banana peel

Chemical bleaching and homogenization

7.6–10.9

73.9

—

[20]

Canola straw

Chemical bleaching and grinding

32 ± 10

62

—

[21]

Bacterial cellulose

Chemical bleaching and aqueous counter collision

20–40

70

c. 100

[22, 23]

Eucalyptus bleached kraft pulp

Enzymatic hydrolysis, twin-screw extrusion

5

80

80

[24]

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3 Structure and Properties of Cellulose Nanofibrils

There are mainly two types of equipment, including homogenizers and microfluidizers, applied to conduct high-pressure homogenization (Figure 3.2). Basically, a high-pressure homogenizer consists of a high-pressure generator and a homogenizing valve assembly designed for this specific high-pressure application. The processed liquid in any type of homogenizer valve passes under high pressure through a convergent section called the “homogenizing gap” and then expands [31]. As an alternative, microfluidizer is another instrument used to produce CNFs, which includes an intensifier pump and chamber with a specific geometry, e.g. Z- or Y-shape, with diameter between 75 and 200 μm [32]. By applying high pressure, strong shear and impact forces against colliding streams and the channel walls accelerate the defibrillation of cellulose fibers. 3.1.2.2

Grinding

Modified commercial grinders with specially designed disks have been used by some researchers in order to fibrillate CNFs. In such equipment, the cellulose suspension is passed between a static grind stone and a rotating grind stone (Figure 3.2c). The surfaces of these disks are fitted with grooves and bars against which the pulp is exposed to sequential cyclic stresses. The hydrogen bonds between CNFs are broken down by the shearing forces generated by the grinding stones, thereby individualizing the CNFs. To increase the fibrillation efficiency, the gap between the grinding stones is reduced from zero position to −100 μm of motion [33]. Owing to the presence of pulp, there is no direct contact between the two grinding stones even at a negative setting of disk position. Although increasing the grinding time brings about finer CNFs, it leads to a decrease in the degree of polymerization, crystallinity, and length of CNFs (Figure 3.3) [35].

Homogenization Homogenizer

Microfluidizer

Grinding

Inlet suspension Impact ring Valve seat

Inlet suspension

Valve Inlet suspension

Outlet suspension

Outlet suspension

Static stone Outlet suspension Rotative stone

Figure 3.2 High-pressure homogenizer(a), microfluidizer (b), and grinder (c) for the CNF production. Source: Floury et al. 2004 [31]. Reproduced with permission of Elsevier.

3.1 Production of CNF

(a)

(b)

(c)

(d)

500 μm (e)

(f)

(g)

(h)

Figure 3.3 Scanning electron micrographs of (a) the original pulp; the pulp agitated by the blender for (b) 1, (c) 3, (d) 5, (e) 10, (f ) 20, and (g) 30 minutes; and (h) the pulp fibrillated by one pass through the grinder. Source: Uetani et al. 2011 [34]. Reproduced with permission of ACS.

3.1.2.3

Ball-milling

Ball-milling is nowadays widely used for the preparation of nanoparticles because of its simple operation, use of relatively inexpensive equipment, and its applicability to essentially all classes of materials. In this method, a cellulose suspension is placed in a cylinder loaded with balls (e.g. ceramic, zirconia, or metal). While the container rotates, the high-energy collision between the balls weakens the hydrogen bonding between CNFs, and thus facilitates the disintegration of cellulose. Control of the processing parameters, such as ball-to-cellulose mass ratio, milling time, and ball size, is necessary to prevent cellulose decrystallization and to produce CNFs rather than short particles [16]. It is worth noting that sealed ball-milling condition facilitates the production efficiency and functionalization of CNFs. For instance, milling in sodium carbonate solutions leads to decrease of CNF length due to the selective dissolution and removal of the amorphous area. Ball-milling cellulose in organic solvents promotes the simultaneous application of mechanical disintegration and surface modification. Thus, the surface property of CNFs can be finely controlled by the choice of solvent media and esterifying agents (Figure 3.4) [36, 37]. Compared with the unmodified CNF, this hexanoyl chloride-modified CNF can be well dispersed in organic solvents, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), and tetrahydrofuran (THF), while the succinic anhydride-modified CNF forms stable suspension in water or DMSO. This method greatly simplifies the production of CNFs, which may take the industrial application of CNFs further. 3.1.2.4

Ultrasonication

Ultrasonication is a simple approach for the isolation of CNFs by the hydrodynamic forces of ultrasound. During this process, the formation, growth, and implosive collapse of microscopic gas bubbles results in a high temperature of up to 5000 ∘ C and a high pressure of more than 500 atm at the implosion sites. The energy generated by cavitation is approximately 10–100 kJ/mol, which is within

59

60

3 Structure and Properties of Cellulose Nanofibrils

(a)

(b) C5H11COCl Hydrophobic OH

O O

Ball mill

(c)

(d)

2 μm

(e)

1 μm

(f)

1 μm

400 nm

Figure 3.4 (a) Schematic illustration of the production process for chloride-modified CNFs. (b) Stability of ball-milled CNFs in DMF suspension (left: 8 hours, right: 16 hours). (c) TEM images of acetyl chloride-modified CNFs. (d) Hexanoyl chloride-modified CNFs ball-milled for 8 hours. (e) Centrifuged part and (f ) supernant part of 16 h-ball-milled CNFs modified by hexanoyl chloride (Inset: top right: nanofiber suspension viewed through crossed polarizers, bottom left: dispersion of chloride-modified CNFs in DMF). Source: Reprinted with permission from Ref; Copyright (2013) Springer and (2012) Wiley-VCH Verlag GmbH & Co. KGaA.

the hydrogen bond energy scale, and therefore, promotes the fibrillation of CNFs [9, 38]. The higher power output and higher temperature benefit the manufacture of uniform CNFs. Studies also indicate that the longer cellulose materials, the higher concentration, and larger distance from probe to beaker were not beneficial for fibrillation [9, 39].

3.1 Production of CNF

3.1.2.5

Steam Explosion

Steam explosion is a process where high-pressure steaming suffers a rapid decompression. During the steam explosion process, cellulose pulp is exposed to pressurized steam for short periods of time. Further sudden release of pressure generates a shear force, which breaks hydrogen bonds between the glucose chains, leading to the rupture of the cellulose fiber. This treatment also results in the hydrolysis of significant amounts of hemicellulose to elementary sugars and water-soluble oligomers and in the depolymerization of some lignin, and therefore, is also used as a pulping process for the extraction of cellulose fibers from lignocellulosic materials [40]. The advantages of steam explosion include significantly lower environmental impact, energy consumption, and capital investment. 3.1.2.6

Aqueous Counter Collision

Aqueous counter collision (ACC) allows to cleave interfacial interactions among cellulose molecules using dual high-speed water jets. In this method, the cellulose suspension was transferred to the sample tank, pumped by a feed pump to the two-hydraulic high pressure boosters that generated a pressure of 200 MPa, and then fed to a collision chamber that has two diamond nozzles where two water jets of feeding cellulose suspension collide each other. Owing to heat generation during the collision, a cooling process was followed after the collision (Figure 3.5). The CNF can be more downsized by repeating the collision and increasing the ejecting pressure. Interestingly, ACC treatment onto bacterial cellulose transformed cellulose I α crystalline phase into cellulose I β phase [22]. The shear stress due to the collision energy of water at a high speed in ACC treatment enhanced sliding of cellulose molecules in the cellulose I α phase to be rearranged into the cellulose I β phase, which was induced on the fiber surface. It is noted that at >40 pass, the transformation rate of cellulose I α was reduced gradually with increase in pass when compared with that in the range from 0 to 40 pass [41, 42]. Figure 3.5 The dual water jet aqueous counter collision (ACC) system. Source: Kondo et al. 2014 [23]. Reproduced with permission of Elsevier.

Nozzle a

Ejecting aqueous suspension Pr

es

su

re

θ ure

Nozzle b

Counter collision

s es Pr

Nozzle Chamber θ

Plunger

Circulated system Sample tank Aqueous suspension

Cooler

61

62

3 Structure and Properties of Cellulose Nanofibrils

3.1.2.7

Refining

Refining is also used to produce CNF. In a disk refiner, cellulose pulp suspension is passed through a gap that has a static grind stone and a rotating grind stone. The surfaces of these disks are fitted with grooves and bars against which the pulp is exposed to sequential cyclic stresses. The hydrogen bonds between CNFs are broken down by the shearing forces generated by the grinding stones and then nano-sized fibers are individualized from the pulp. To obtain finer CNFs, several times of refining is required. Meanwhile, the gap needs to be narrowed. 3.1.2.8

Cryocrushing

Cryocrushing is another method for mechanical disintegration of cellulose. In this process, cellulose fibers swollen by water are frozen in liquid nitrogen and subsequently crushed by mortar and pestle [43]. The high impact forces to the frozen cellulose fibers lead to rupture of the cell wall due to the pressure exerted by ice crystals, thus liberating nanofibers [44]. Generally, the samples are further subjected to a mechanical treatment using high shear, high energy transfer, and high impact for uniform CNFs [45]. 3.1.2.9

Twin-Screw Extrusion

Although all of the above-reported processes are extensively studied for the nanofibrillation of cellulose, CNFs are produced at a solid content between 0.5 and 5 wt% at maximum, which greatly increases the transport cost. Twin-screw extrusion is a process where cellulose pulp is fibrillated by two co-rotating, intermeshing screws mounted in a closed barrel (Figure 3.6). The output of CNFs has a high solid content (25–40 wt%) even up to c. 50 wt%, which is beneficial for transportation and storage. Moreover, twin-screw extrusion is traditionally used for polymers and composites; the availability and popularity of twin-screw extrusion makes continuous production of CNF composite feasible [46, 47]. However, there is a challenge for twin-screw extrusion that the NFC can form irreversible agglomerations when CNF is redispersed in solvents in order to be used in further industrial applications [48].

Mixing compartment with 15 ml volume Recirculation channel

Conical mixing screws

Figure 3.6 Twin-screw mini extruder used for the disintegration of fibers.

3.1 Production of CNF

3.1.2.10

Other Methods

Electrospinning is a bottom-up strategy for producing uniform CNFs. However, the low productivity, poor crystallinity, and the use of toxic solvents and high voltage limit their applications. Bacterial CNFs via biosynthesis have narrow diameter distributions, high aspect ratios, and high crystallinity, but have limitations such as the requirement for strict and costly production conditions, poor reproducibility between bacteria of different generations, and complex posttreatment purification procedures. The hybrids of two or more mechanical treatments can yield a thorough cellulose suspension as well as cellulose fibrils that are more uniform than those treated by one method only. For instance, the integration of high-shear homogenization and high-pressure homogenization resulted in CNF with the diameters of 5–28 nm and crystallinity index of 44–46% [49]. The combination of refining and microfluidization processes leads to producing uniform CNFs whereas individual methods could not yield good results. A sequence of steaming treatment, chemical bleaching, and milling accelerated the transformation of discarded chopsticks into CNFs [50]. 3.1.3

Pretreatment

All these mechanical methods require high energy between 5000 and 30 000 kWh/t depending on the cellulose source, the mechanical process, and the CNF quality [51, 52]. In terms of homogenization, a large number of passes is required to obtain finer CNFs, inevitably leading to energy consumption up to 28 300 and 3000 kWh/t for the homogenizer and microfluidizer, respectively [53]. Meanwhile, the clogging of the system, especially the homogenizer and microfluidizer, is the main challenge for upscaling of such a process. Efforts have been devoted to increase the disintegration efficiency and decrease the energy consumption by employing pretreatments, e.g. alkali treatment and enzymatic and charge introduction, which allow one to decrease the energy consumption from around 20 000 to 1000–2000 kWh/t [54–56]. Alkali treatment is traditionally used to extract hemicellulose and other impurities from cellulose, giving a positive effect on pulp disintegration. Pretreated by 5% sodium hydroxide and further grinded or homogenized, CNFs derived from bleached pulp exhibited higher mechanical property, which is attributed to the higher cellulose content and crystallinity [57, 58]. Since sodium hydroxide partially dissolves cellulose, milder alkalis, including urea and ethylenediamine, were also applied to pretreat cellulose pulp, achieving energy savings of up to 55% and 32% upon microfluidization [59]. Deep eutectic solvents (DES), e.g. choline chloride–urea, provided a non-hydrolytic and environment-friendly method to pretreat cellulose fibers to enhance the fibrillation. The degree of polymerization of the cellulose remained at the same level as the original cellulose pulp after DES treatment [60]. The use of enzymes, i.e. monocomponent endoglucanase and fungus OS1, allows the selective hydrolysis of the noncrystalline cellulose, and thus increases swelling of cellulosic pulp fibers in water suspension [61–63]. A small addition of the monocomponent endoglucanase enzyme promoted cellulose disintegration and prevented the blocking of the homogenizer [64]. This is believed to enhance

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the effect of the enzyme treatment by creating a controllable amount of weak points within the fiber cell wall that facilitates mechanical disintegration. The introduction of charged groups can improve the ease of individualization of CNFs through electrostatic repulsion effect [65]. Such modifications will make it possible to loosen the adhesion between CNFs by preventing the formation of strong interfibril hydrogen bonds. Classically, this is achieved by treating cellulose with sulfuric acid to obtain stable aqueous dispersions of cellulose microcrystals that are surface charged by sulfate groups. However, this treatment is hydrolytic and yields a dramatic decrease in fiber length down to about 100−200 nm. In comparison, 2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation is a milder method [55]. However, TEMPO-mediated oxidation leads to a significant decrease in the thermal degradation point and reduces the mechanical strength of the CNF [66]. In addition, periodate–chlorite oxidation, periodate-bisulfite sulfonation, and carboxymethylation were used to introduce negative charges onto the cellulose surface to weaken the inter-fibril bonding [52, 67–69].

3.2 Features and Properties 3.2.1

Morphology of CNF

CNF morphology depends strongly on the cellulose source and the production process (Table 3.1). CNFs with a diameter of 3–5 nm can be produced using periodate–chlorite oxidation, followed by homogenization [70]. Similar diameter was also measured from transmission electron microscopy (TEM) analysis of CNF from bleached sulfite wood pulp and cotton, produced by TEMPO-mediated oxidation and blending [71]. Thus, nanofibrils with a diameter of 3–5 nm may relate to the elementary fibrils, while the thicker ones may represent the bundles of elementary fibrils and microfibrils. CNF produced from corn stover by the processes of alkali treatment and delignification resulted in >93% purity. The particle size of the extracted cellulose was reduced by mechanical shearing through high-pressure homogenization. The diameters of the CNFs ranged between 5 and 50 nm, and the lengths were in microns [72]. Moreover, slightly different observations can be obtained when using different microscopic techniques, e.g. TEM [72], field emission scanning electron microscopy (FE-SEM) or atomic force microscopy (AFM), as well as the way of sample preparation for the analysis. For instance, CNFs can aggregate to some extent during some dehydration procedures, e.g. freeze-drying. In situ cryo-TEM observation indicates that CNFs produced by quaternization and homogenization have a diameter of 2.6–3.0 nm [73]. 3.2.2

Rheology

Rheology is a key characteristic of CNF suspensions, since it can reflect the behavior of this material in different industrial processes, e.g. in mixing, pumping, or coating. The high viscosity at low nanocellulose concentrations makes

3.2 Features and Properties

nanocellulose very interesting as a non-caloric stabilizer and gellant in food applications, the major field explored by the early investigators. The dynamic rheological properties revealed that the storage and loss modulus were independent of the angular frequency at all nanocellulose concentrations between 0.125% and 5.9% [64, 74]. There is a particularly strong concentration dependence as the storage modulus increases 5 orders of magnitude if the concentration is increased from 0.125% to 5.9%. Nanocellulose gels are also highly shear thinning and therefore the viscosity is lost upon the introduction of the shear forces. The shear-thinning behavior is particularly useful in a range of different coating applications. CNF suspensions are thixotropic, i.e. the longer they undergo shear stress, the lower their viscosity, and it takes a certain time to attain an equilibrium level. Viscosity recovers as the shearing forces are removed. Thus, shear stress and viscosity of CNF suspensions are commonly measured at equilibrium by applying a stepped shear rate. It was demonstrated that viscosity of CNF suspensions increases when the pH is lowered [75]. This is a result of reducing electrostatic repulsion between the nanofibrils due to neutralization of negative charges by hydrogen ions, which leads to higher interfibrillar interactions. Such behavior was explained by the continuous disintegration of cellulose fibers into CNFs. At higher mechanical treatment, less residual microscopic fibers were detected using microscopic observations. Thus, an increase in viscosity was associated with an increase of CNF concentration in water. An increase of viscoelastic properties followed by progressive decrease was observed when passing the CNF suspension, produced from corn cobs, through a homogenizer [76]. Similar behavior was observed from CNF prepared from date palm tree, when the oxidation level was increased [77]. The reduction of suspension stiffness was assigned to a decrease in aspect ratio and an increase in the electrostatic repulsion of the nanofibrils. Although the rheological behavior of CNF suspensions still requires further investigation, the above observations suggest that rheometry can be used as a tool to control the CNF production process. It can be efficient to ensure CNF isolation while preserving its further deconstruction and the reduction of the aspect ratio. Furthermore, CNF suspensions have a complex rheology, which is justified by its heterogeneous flow behavior. 3.2.3

CNF in Different Forms

CNFs prepared by different processes can exist in various forms, such as suspension, powder, aerogel, hydrogel, and film (Figure 3.7), to satisfy the requirements of different applications. 3.2.3.1

Suspensions

As mentioned above, depending on the production process, the CNF aqueous suspensions obtained have different properties. Rheological behavior is one of the key parameters used to characterize CNF suspensions. Regardless of the production methods, as well as biological or chemical pretreatments, all of them possess gel-like, shear thinning and thixotropic behavior at low solid content. However, the suspensions can be further converted to other CNF products, e.g. hydrogels,

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Figure 3.7 CNF in different forms (suspension, powder, aerogel, hydrogel, and film).

aerogels, powders, or films. The production of various types of CNF products is briefly reviewed hereafter. 3.2.3.2

Powders

The hydrophilic nature of cellulosic fibers limits their applications such as coated products or within composites, due to two main drawbacks: (i) they are generally a highly viscous aqueous suspension at low solid content and (ii) they undergo irreversible aggregation once dried (films or powder). This is mainly due to the capacity of NFC to form hydrogen bonds between these high specific area nanofibers. Moreover, hornification phenomenon during drying reveals the formation of additional hydrogen bonds between amorphous parts of the cellulosic nanofibers, thus contributing to the aggregate’s irreversible formation. In addition, more stable hydrogen bonds are formed during drying and cannot be “broken” after rewetting [78]. Thus, one of the challenges is to produce dry CNF powder with preserved nanoscale structure and to ensure its redispersibility. This can provide advantages in CNF storage, transportation, and industrial applications. A number of methods were applied for CNF drying, e.g. oven drying, spray drying, freeze-drying, or supercritical drying. However, all of them led to aggregation or formation of strong highly networked structures. Still, a water-redispersible CNF in powder form can be prepared from bleached hardwood pulp using carboxymethylation and mechanical disintegration, followed by oven drying (Figure 3.8). SEM measurements of freeze-dried suspensions from redispersed CNF confirmed that carboxymethylation prevented hornification during drying [48]. A water-redispersible CNF can be achieved by adsorbed carboxymethyl cellulose [79]. It was shown that above a critical threshold of carboxymethyl cellulose adsorption of 2.3 wt%, the oven dried CNF sample was fully redispersible in water

3.2 Features and Properties

(A)

a

b

c

(B)

d

e

f

(C)

g

0 h (D) h

(D′)

3 h (E)

(E′)

24 h (F)

(F′)

(G)

(G′)

Figure 3.8 (A) Sedimentation test for 0.2 wt% water suspensions of never dried CNFs (a) and redispersed oven dried CNFs (b), CNF–CMC–HT-20 (c), CNF–CMC–HT-50 (d), CNF–CMC–HT-100 (e), CNF–CMC–RT-100 (f ), CNF–CMC–RT-200 (g), and CNF–CMC–RT-300 (h). (B) Photographs of oven dried and (C) water-redispersed CNF–CMC–HT-100 sample with a solid content of 5 wt%. FE-SEM images of suspensions of never dried CNFs (D, D’) and water-redispersed CNFs (E), CNF–CMC–HT-10 (E’), CNF–CMC–HT-20 (F, F’), and CNF–CMC–HT-100 (G, G’) observed at a magnification of 92 000 and 920 000.

(Figure 3.9). Moreover, after the redispersion, the initial CNF properties were preserved. Another way to produce water-redispersible CNF without any chemical surface modification or adsorption was realized by exchange of cellulose counterion from H+ to Na+ by sodium chloride, which decreases significantly the intermolecular hydrogen bonding [80]. It was shown that the freeze-dried CNF with added sodium chloride can be easily redispersed in water and produces similar suspensions compared to those prepared from non-dried CNF. 3.2.3.3

Films

CNF suspensions can be formed into films, also known as nanopaper. Several methods can be applied to produce such films, e.g. air drying, pressurized filtration followed by hot pressing, or spray coating. CNF films typically possess high level of transparency (80–90%) in the visible wavelength spectrum, since the small nanofiber dimensions and their dense packing significantly reduce light scattering (Figure 3.7). Apart from the residual microscopic fiber fragments, some lack of transparency is associated with surface light scattering. The transparency of CNF films can be enhanced by polishing of their surface, e.g. by lamination or deposition of transparent resins/plastics (Figure 3.10) [81].

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NFC 1 pH 4 pH 6 pH 8 pH 10

Unstable 30 seconds

NFC 0 Freeze-drying

NFC 2 Neat NFC

pH 4

pH 6 pH 8 pH 10

Dialysis

NFC 3 (a)

Stable after 3 months

pH 4

200 nm

pH 6

Grenoble-INP-CMTC

pH 8

200 nm

200 nm

Grenoble-INP-CMTC

pH 10

Grenoble-INP-CMTC

200 nm

Grenoble-INP-CMTC

(b)

Figure 3.9 (a) Pictures of the different samples obtained for each sample. (b) FE-SEM pictures characterizing samples from sample NFC3 after dialysis. (c) Ion–dipole interactions between nanofibrillated cellulose and NaCl salt acting as “hydrogen bond blocker.”

The elastic modulus of CNF films may approach 20 GPa and their strength can reach 240 MPa [44]. However, in general, values of 10 GPa and 100 MPa are obtained for the elastic modulus and tensile strength, respectively, with a strain at break of 5% [82].

3.2 Features and Properties

2δ–

Na+

Na+

O H – δ+ Cl

COO–

OH

O

HO

COO–

OH

COO– COO– COO– OHNa

COO–

+

O HO

O

OH

Na+ Cl–

Cl–

Cl–

HO

OH

Cl– Na+

Cl–

O

O

OH

OH

HO O

OH

Na+

OH

HO O

OH

OH

OH

HO O

OH

Na+

Cl– O

HO OH

O

Na+ OH

HO O

O OH

(c)

Figure 3.9 (Continued)

Polished

(a)

Unpolished

(b)

Figure 3.10 (a) CNF nanopaper and (b) the traditional cellulose paper polished with acrylic resin (left) and unpolished (right) CNF films. Source: Adapted from Nogi et al. 2009 [81]. Reproduced with permissions from John Wiley & Sons.

CNF films were also found to be good barriers for oxygen gas, varying widely from 0.003 to 0.03 mL μm/m2 /d/kPa at 0% relative humidity depending on the conditions used to prepare the films [83]. The lowest oxygen permeability is achieved for the alkali/urea (AU) film prepared from 6 wt% cellulose solution by regeneration with acetone at 0 ∘ C [84]. The oxygen permeabilities of the AU cellulose films are negatively correlated with their densities, and AU films prepared from solutions with high cellulose concentrations by regeneration in a solvent at low temperatures generally have low oxygen permeabilities (Figure 3.11). The AU cellulose films are, therefore, promising bio-based packaging materials with high oxygen barrier properties.

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Oxygen permeability at 0% RH: 0.003 ml μm/m2/d/kPa Water vapor permeability at 50% RH: 50 g μm/m2/d/kPa Density: 1.55 g/cm3 Crystallinity index: ∼52% Light transmittance at 600 nm: 90% Tensile strength: 150 MPa

Figure 3.11 The oxygen permeabilities of the AU cellulose films [84].

3.2.3.4

Hydrogels

Hydrogel CNF suspensions at low cellulose concentrations appear as viscous fluids. They can be converted to hydrogels that can hold a large amount of water while maintaining their shape. Polysaccharide-based hydrogels have multiple advantages because of their inherent biocompatibility, biodegradability, and nontoxic properties. The feasibility of using polysaccharide-based hydrogels could be improved if they could simultaneously fulfill the mechanical property and cell compatibility requirements for practical applications. Cellulose hydrogels constructed by double-cross-linked (DC) cellulose hydrogels using sequential chemical and physical cross-linking exhibits mechanical superiority to single-cross-linked cellulose hydrogels [85]. The formation and spatial distribution of chemically cross-linked domains and physically cross-linked domains within the DC cellulose hydrogels are demonstrated in Figure 3.12. The molar ratio of epichlorohydrin (ECH) to anhydroglucose units of cellulose and the concentration of the aqueous ethanol solution are two critical parameters for obtaining mechanically strong and tough DC cellulose hydrogels. The mechanical properties of the DC cellulose hydrogels under loading–unloading cycles are described using compression and tension models. The possible toughening mechanism of double cross-linking is discussed. Two hydrogel types with different crystal forms, namely, cellulose I and cellulose II, were prepared by treating mechanically disintegrated CNF aqueous suspensions with alkali, followed by neutralization using sodium hydroxide below and above a specific concentration of c. 12 wt%, respectively [87, 88]. Above this concentration, the hydrogels were formed due to cellulose shrinkage and coalescence due to mercerization, while in weaker alkaline media, before the mercerization threshold, the formation of hydrogels was suggested to occur due to some swelling, and hence the enhancement of CNF entanglement and, additionally, some aggregation. Such hydrogels have a great potential for application in

3.2 Features and Properties

(i)

(a)

(ii)

Covalent cross-linking by ECH

Hydrogen bonds between cellulose chains

Chain entanglements

Crystallite hydrates of cellulose II

(i)

(iv)

(ii)

(iii)

(v)

(vi)

(b)

Figure 3.12 (a) Preparation of DC cellulose hydrogels. (i) Chemical cross-linking with ECH and (ii) physical cross-linking from an aqueous ethanol solution. Illustration of the network structure of the DC cellulose hydrogel. (b) (i–iii) Photographs of the cellulose hydrogels: (i) DC cellulose hydrogel, (ii) chemically cross-linked cellulose hydrogel, and (iii) chemically cross-linked cellulose hydrogel under bending. (iv) The compression behavior of the physically cross-linked cellulose hydrogel (white), DC cellulose hydrogel (red), and chemically cross-linked cellulose hydrogel (blue) before [86], during (middle), and immediately after compression (bottom). (v) Photograph of the DC cellulose hydrogel flowers. (vi) Macroscopic views of a DC cellulose hydrogel ball [85].

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such fields as tissue engineering, drug delivery, sorbents, sensors, contact lenses, and purification. 3.2.3.5

Aerogels CNF

Aerogels are porous sponge-like materials produced by replacing the liquid medium in suspensions or hydrogels with air. Thus, by preserving the CNF three-dimensional (3D) network during dehydration, aerogels with high specific surface area, low density, and high porosity can be obtained [89]. However, the presence of water during freeze-drying results in significant CNF aggregation during sublimation (Figure 3.13). One solution is to exchange the solvent to tertbutanol prior for further freeze-drying. Another route is that CNF aqueous suspensions were converted to hydrogels, which were afterwards solvent exchanged to tertbutanol and freeze-dried in order to obtain aerogels. Additionally, CNF aerogels can be produced using 3D bioprinting [90]. Therefore, CNF aerogels have a potential for use as adsorbents, carriers for catalysts and drug release, and separator for energy storage devices. A solid-state electric double-layer capacitor (EDLC) using activated carbon as electrodes and the OH-saturated mCel-membrane as a polymer electrolyte exhibits a high capacitance of 110 F/g at 1.0 A/g, and long cycling life of 10 000 cycles with 84.7% capacitance retention [91]. Moreover, a highly integrated planar-type micro-supercapacitor (MSC) fabricated by directly depositing the electrode materials on the mCel-membrane-based polymer electrolyte without using complicated devices also exhibits a high areal capacitance of 153.34 mF/cm2 and volumetric capacitance of 191.66 F/cm3 at 10 mV/s, representing one of the highest values among all-carbon-based MSC devices. Cellulose aerogel membranes (CAMs) used as separator for the fabrication of lithium ion batteries (LIBs) show superior thermal stability [92]. The presence of high porosity, nanoporous network structure, and numerous polar hydroxyl groups benefits the quick absorption of liquid electrolytes for gelation of the CAMs and improves the ionic conductivity of the gelled CAMs. LIBs assembled with the gelled CAMs display excellent electrochemical performance at room temperature, and more importantly, the intrinsic thermal resistance of cellulose allows the LIBs to run stably for at least 30 minutes at working temperatures as high as 120 ∘ C. CAMs, with their excellent thermal stability, are promising for the development of highly safe, cost-effective, and high-performance LIBs. These findings suggest that the developed renewable, flexible, mesoporous cellulose membrane holds great promise in the practical applications of flexible, solid-state, portable energy storage devices that are not limited to supercapacitors.

3.3 Conclusion CNFs are composed of crystallized and amorphous regions, and exhibit 5–100 nm width and several micrometers length. Because of the larger length, CNF produced by mechanical treatment possesses advantages upon reinforcement in composite, enrichment in resource, and higher yield over CNC. This chapter discussed various mechanical treatments, including homogenization,

3.3 Conclusion

Substrate Cel/IL gel

PTFE mask

With the mask

Active material

Brush coating

MSC

Dipping in water

Hydrogel Drying

(a) 15 Current (mA)

10 0°

90°

10 mV/s 20 mV/s 50 mV/s

100 mV/s 200 mV/s 300 mV/s

5 0 –5 –10 0.0

0.2

(c)

125 100

MSC

75 50 25 0 0

(d)

160 140 120 100 80 60 40 20 0

200 400 600 800 1000 Scan rate (mV/s)

100 90 80 70 60 50 40 30 20 10 0

5

1 st 500 th 1000 th

0 –5

200 mV/s

–10

0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

200 400 600 800 1000 Cycles

Liquid electrolyte

Processing

1.0

110 100 90 80 70 60 50 40 30 20 10 0

Areal capacitance Vol. capacitance

0

(e)

0.4 0.6 0.8 Voltage (V )

10 Current (mA)

Areal capacitance Vol. capacitance 180

150

Areal capacitance (mF/cm2)

200

175

Vol. capacitance (F/cm3)

Areal capacitance (mF/cm2)

(b)

Vol. capacitance (F/cm3)

–15

180°

+ Assembly

–

Instantaneous gelation Cellulose aerogel membrane (CAM)

Cathode

Gelled CAM

Potential (V)

Cellulose

200 nm

(f)

Gelled cellulose Liquid nanofibrils electrolyte

3.75 3.00 2.25 1.50 0.75 0.00

Gelled CAM Anode

LIB Gelled CAM (EC/PC electrolyte) Gelled CAM (EC/EMC/DMC electrolyte)

Open-circuit voltage testing at 120 °C Celgard 2400 (EC/EMC/DMC electrolyte)

0

5

10 15 20 25 30 Time (min)

Figure 3.13 Electrochemical properties of the mCel-membrane-based MSC. (a) Schematic diagram of the preparation. (b) Optical photographs of MSCs with different configuration patterns, and in bent states. (c) Cyclic voltammetry (CV) curves measured at scan rates of 10–300 mV/s. (d) Areal and volumetric capacitances measured at different scan rates. (e) Cycling performance measured at a scan rate of 200 mV/s. (f ) Schematic diagram of the preparation process of cellulose aerogel membranes and the SEM images of 3D structure as well as the open-circuit voltage curves of cells employing CAM-4 and Celgard 2400 separator at 120 ∘ C.

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grinding method, ball milling, twin-screw extrusion, cryocrushing, blending, and ACC, which are promising for the upscale production of CNF. Meanwhile, pretreatments to reduce energy consumption of mechanical disintegration have also been presented. In addition, the features and properties of CNF in different forms have been presented to explore its potential applications.

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29 Nakagaito, A.N. and Yano, H. (2004). The effect of morphological changes

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4 Synthesis, Structure, and Properties of Bacterial Cellulose Muhammad Wajid Ullah 1,2 , Sehrish Manan 3 , Sabella J. Kiprono 1,2 , Mazhar Ul-Islam 4 , and Guang Yang 1,2 1 Huazhong University of Science and Technology, Department of Biomedical Engineering, College of Life Science and Technology, Wuhan 430074, China 2 Huazhong University of Science and Technology, National Engineering Research Center for Nano-Medicine, Wuhan 430074, China 3 Huazhong Agricultural University, National Key Laboratory of Crop Genetic Improvement, College of Plant Sciences and Technology, Wuhan 430070, China 4 Dhofar University, Department of Chemical Engineering, College of Engineering, Salalah 211, Oman

4.1 Introduction Bacterial cellulose (BC) is produced by several microbial genera belonging to Acetobacter, Rhizobium, Agrobacterium, Aerobacter, Achromobacter, Azotobacter, Salmonella, Escherichia, and Sarcina [1, 2], and a cell-free system [3–6]. It represents the purest form of cellulose compared to plant cellulose, which contains impurities in the form of lignin and hemicellulose. It possesses unique features such as high water holding capacity (WHC), slow water release rate (WRR), greater tensile properties, high crystallinity, better thermal and mechanical properties, ultrafine fiber network, hydrophilicity, polyfunctionality, transparency, nontoxicity, and moldability into three-dimensional structures [7–9]. These features make BC a preferred choice than plant cellulose for various applications such as carrier in drug delivery systems and enzyme immobilization and two-dimensional (2D) and three-dimensional (3D) scaffolds for tissue engineering, wound dressing materials, artificial skin burns, vascular grafts, tissue regeneration, artificial blood vessels, biological films, and biosensors [10–14]. However, its wide range applications are limited by several inadequacies associated with it, such as the lack of antibacterial, antifungal, antioxidant, conducting, and magnetic properties. Nevertheless, BC has the potential to form composites with different materials ranging from organic to inorganic metals, nanoparticles, and biocompatible polymers, which not only improve its existing properties but also impart it additional features [8, 9]. During BC production, the microbial cells polymerize various carbon sources into single liner β-1,4-glucan chains that protrude out through the pores located on the cell membrane, termed as terminal complexes (TCs) [5, 15]. The successively synthesized β-1,4-glucan chains get assembled in the culture medium Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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4 Synthesis, Structure, and Properties of Bacterial Cellulose

and form protofibrils, which further crystallize into micro- and macro-fibrils, bundles, and ultimately form ribbons-like structures [16]. A single ribbon is composed of about 1000 individual β-1,4-glucan chains [8]. Ultimately, BC is produced in the form of a thick gelatinous membrane at the air–medium interface in a static cultivation [5]. The unique physiological, chemical, mechanical, thermal, and biological properties of BC can be attributed to its unique structural features. BC fibers possess a high aspect ratio with a diameter of 20–100 nm and a high surface area, which confers it a high materials adsorption capability. Electron microscopy of BC demonstrates a well-distributed 3D structure of nanofibers with a high surface area and porosity [17]. These nanofibers are interconnected through inter- and intramolecular hydrogen bonding that stabilizes the reticulate structure of BC and are 100 times smaller than those of plant cellulose [18]. The nanofibers are loosely arranged and contain empty spaces in the form of pores, which increase the overall surface area endowing BC with a highly porous matrix [19]. Any structural variation in BC affects its physico-mechanical and thermal properties: for example, strong, stable, and compact fibers with high density offer better resistance to the applied force while a highly porous structure bestows it with a high WHC, usually 100–200 times more than its dry weight in water [20, 21], and can accommodate different types of materials, thus showing the potential to form composites [22]. These structural variations in BC are attributed to the type of microbial strain, composition of culture medium, variation in culture time and conditions, amount of inoculum, and carbon source [23, 24]. An economically feasible production has always been the main aim of BC research; therefore, several strategies have been developed for improved BC production such as conventional fermentation approaches, genetic engineering strategies, and strain improvement approaches, among others [25]. However, BC production from chemically defined medium is very expensive; thus, researchers are exploring cheap media sources in addition to developing new strategies for cost-effective, efficient, and high yields of BC. This includes the use of various carbon and nitrogen sources, industrial wastes, isolation of high rate production strains, in situ pH control, controlled side product formation, and supplementation with additional substrates [26–28]. However, high yield of BC is overshadowed by several discrepancies associated with microbial production due to the utilization of a large fraction of medium components for growth and proliferation, formation of by-products, formation of negative strain, and risk of contamination, thus making the process ineffective for large-scale production [29–31]. Further, the benefits of whole-cell fermentation are overshadowed by inhibition of microbial cell growth and viability due to membrane fluidity [32]. Together, these factors limit the large-scale production of BC. A few of these limitations can be potentially overcome by using thermo-tolerant microbial strains and immobilized microbial cell systems, which offer improved stability and resistance, low production cost, and high-purity products [33]. Further, the limitations of whole-cell fermentation can be overcome by using a cell-free system for BC production, which offers improved yield [4]. Additionally, this system produces BC with better structural and physico-mechanical features [5] and offers a one-pot in situ composite synthesis method using bactericidal

4.2 Biogenesis of Bacterial Cellulose

materials, which are otherwise likely to kill the microbial cells [34]. Therefore, a major objective behind BC research is to improve its production by improving the efficacy of the process, development of advanced fermenters and cultivation strategies, exploring cheap media sources, and supplementation of medium with other components for improved structural features. This chapter overviews the biogenesis of BC at biochemical and molecular levels and details its various structural, physico-mechanical, and biological features. It further describes various cultivation strategies for BC production with different shapes, sizes, quantity, quality, and physico-mechanical properties. The role of different additives in BC production medium and their effects on various structural features are discussed. Finally, the efforts of different researchers to explore the potential of different raw materials for low-cost BC production are summarized.

4.2 Biogenesis of Bacterial Cellulose Biosynthesis of BC is a complex process that is regulated by a large number of specific enzymes and regulatory proteins. It is an aerobic process and is directly linked with cellular catabolism; however, it does not interfere with other anabolic processes such as protein synthesis in bacterial cells. In addition to aerobic synthesis of BC by microbial cells, it is also produced anaerobically by a cell-free system using cellulose-producing enzymes and cofactors [4]. The synthesis mechanism of uridine-di-phosphoglucose (UDP-glucose) is well understood; however, this process of BC synthesis involving glucose polymerization is yet to be completely explored. 4.2.1

Biochemistry of BC Synthesis

Sutherland described BC synthesis by microbial cells as a four-step process involving (i) activation of monosaccharides through formation of glucose nucleotides, (ii) polymerization of glucose repeating units through their sequential addition, (iii) simultaneous addition of acyl groups (if present) to individual glucose units, and (iv) excretion of cellulose fibers through the wall/membrane complex into the extracellular environment [35]. BC synthesis involves the production of UDP-glucose, which serves as a precursor for BC synthesis. This process is followed by the polymerization of individual glucose units to form β-1→4 glucan chains. Individual chains are excreted through TCs and form ribbon-like structures composed of hundreds and thousands of individual chains. These ribbon-like structures in turn form fibrils [35]. Each biochemical step in BC synthesis pathway is regulated by the availability of specific enzymes, substrates, and a cofactor (if required), which otherwise results in the abolishment of the metabolic pathway. The synthesis of cellulose fibrils in microbial cells, their transport across TCs into extracellular environment, and the formation of bundles and ribbons are shown in Figure 4.1. During the first step of BC synthesis, sugar nucleotides provide monosaccharides by means of interconversion through epimerization, dehydrogenation, and

83

84

4 Synthesis, Structure, and Properties of Bacterial Cellulose

Bundles

Ribbon

Cellulose microfibrils

Cellulose I Cellulose II

Cellulose synthase β-1, 4-glucan Cytoplasmic membrane Lipopolysaccharide envelope

Figure 4.1 Illustration of formation of cellulose chains in microbial cells, their secretion across the cell wall through TCs, and formation of micro- and macro fibrils, bundles, and ribbons. Source: Ul-Islam et al. 2015 [14]. Reproduced with permission of Springer.

decarboxylation reactions, each of which is catalyzed by specific enzymes [35]. The cellulose backbone is formed in the second step through the sequential addition of d-glucose-1-phosphate and UDP-glucose; this process is carried out by UDP-glucose pyrophosphorylase and cellulose synthase, respectively. It is followed by the acylation of cellulose during which the acyl group is transferred to form acetyl Co-A; the process is catalyzed by specific sugar transferases such as 1-acyl-sn-glycerol-3-phosphate acyltransferase. The acetyl Co-A formed here serves as a precursor for the tricarboxylic acid (TCA) cycle. It is worth mentioning here that the structure of BC is determined by the sequential transfer of glucose monomers and acyl groups from their respective donors, regulated by highly specific sugar transferases. In the last step, β-1→4 glucan chains produced inside the cytoplasm are crystallized in or near the outer membrane and extruded across the cell membrane through TCs arranged in an orderly manner to the extracellular environment in an energy-dependent process regulated by cellular adenosine triphosphates (ATPs) [35, 36]. This demonstrates that a well-organized system must be present on the microbial cell surface to ensure efficient excretion of cellulose fibrils. Compared to microbial cell systems, a cell-free system bypasses the complicated process of β-1,4-glucan chain extrusion to the extracellular environment since it does not contain any barrier in the form of a cell wall or a membrane [4]. Further, it bypasses the energy utilization and internalization of cellulose within the periplasm, thus avoiding any lethal effects.

4.2 Biogenesis of Bacterial Cellulose

4.2.2

Biochemical Pathway of BC Production

Liquid chromatography–mass spectrometry/mass spectrometry linear trap quadrupole (LC–MS/MS LTQ) Orbitrap analysis of the crude cell-free lysate of G. hansenii PJK indicated the presence of several key enzymes involved in the cellulose metabolism (Table 4.1). Herein, the glucokinase, phosphoglucomutase, UDP-glucose pyrophosphorylase, and cellulose synthase are directly involved in BC synthesis (Figure 4.2). Likewise, glucose-6-phosphate (G6P) serves as a common intermediate for the principal cellulose synthesis pathway, pentose phosphate pathway (PPP), and TCA cycle. The flux of phosphorylated glucose through the two possible routes determines the BC synthesis level. UDP-glucose also serves as a common substrate for UDP-glucose dehydrogenase and cellulose synthase. Again, the flux of UDP-glucose through the two possible routes determines the BC synthesis level in either a microbial or a cell-free system. A lower concentration of nicotinamide adenine dinucleotide (NAD) in the cell-free lysate may partially inhibit the UDP-glucose dehydrogenase activity, and thus, Table 4.1 Illustration of the G. hansenii PJK enzymes in the cell-free extract involved in bio-cellulose synthesis. The enzymes were analyzed by LC–MS/MS LTQ Orbitrap using the Mascot algorithm.

Accession no.

GI no.

Description

Taxonomy

Mass (Da)

PI

6.51

EFG85049.1

gi|295978312

Glucokinase

G. hansenii

34 282

EFG84192.1

gi|295977434

Phosphoglucomutase

G. hansenii

59 762

6.03

EFG85649.1

gi|295978924

UDP-glucose pyrophosphorylase

G. hansenii

22 755

4.36

EFG83224.1

gi|295976446

Cellulose synthase catalytic subunit (UDP-forming)

G. hansenii

175 801

6.94

EFG84542.1

gi|295977790

UDP-D-glucose dehydrogenase

G. hansenii

47 825

5.61

EFG83101.1

gi|295976316

1-Acyl-sn-glycerol-3phosphate acyltransferase

G. hansenii

41 281

11.15

EFG83324.1

gi|295976549

Diguanylate cyclase/ phosphodiesterase

G. hansenii

84 871

6.11

EFG83841.1

gi|295977078

Glucose dehydrogenase

G. hansenii

84 604

5.73

WP003620002.1

gi|489715879

Aldolase

G. hansenii

29 824

6.42

EFG84043.1

gi|295977283

Triosephosphate isomerase

G. hansenii

25 791

4.96

EFG82935.1

gi|295976147

Pyruvate dehydrogenase (acetyl-transferring)

G. hansenii

34 832

5.35

EFG85628.1

gi|295978903

Glucose-6-phosphate dehydrogenase

G. hansenii

57 043

6.04

Source: Ullah et al. 2015 [4]. Reproduced with permission of Elsevier.

85

Cellulose production [40]

Glucose

Cellulose synthase activation [37]

Glucokinase

Byproducts formation [35, 38–41]

Glucose-6-phosphate

Glucose-1-phosphate

2Pi

Diguanylate cyclase

PPi

Mg2+

2GTP

pppGpG

PDE A

2Pi Ca

Cellulose

PDE B

UDP-glucose

2+

PPi

pGpG 2 5′ GMP pGpG

UDP-glucose pyrophosphorylase

2NAD NADH

UDP-glucose dehydrogenase

pGpG UDP-glucoronic acid Glucoronide

UDP Glucoronic acid

Glucoronyl transferase

Glucoronidase

Inhibition

Cellulose synthase (active)

Cellulose synthase (inactive)

Gluconate

Glucose phosphate dehydrogenase

Aldolase, triosephosphate isomerase

Phosphoglucomutase

Glucose dehydrogenase

ATP TCA cycle

6-Phospho-gluconate

ED pathway

PP pathway

Pyruvate

Glyceraldehyde3-phosphate

Pyruvate dehydrogenase Acetyl Co-A

Acetate

CO2

Glucoronic acid oligomer

Figure 4.2 Schematic representation of bio-cellulose production by the cell-free system through the principal glucose pathway, and other pathways interconnected through the activation of cellulose synthase. The scheme was developed based on literature review and the results of LC-MS/MS LTQ Orbitrap analysis. Source: Ullah et al. 2015 [4]. Reproduced with permission of Elsevier.

4.2 Biogenesis of Bacterial Cellulose

may favor high level of cellulose synthesis (Figure 4.2). Importantly, the presence of diguanylic cyclase and two phosphordiesterases (PDE-A and PDE-B) serves to regulate cellulose synthase activity through a specific activator bi-(3′ →5′ )-cyclic diguanylic acid. Notably, PDE-A and PDE-B are involved in the synthesis and degradation of the activator, respectively [37]. It may aid in accelerating the polymerization of UDP-glucose into cellulose. Similarly, glucose can also enter other pathways in the microbial metabolism [38]. A small portion of triose sugar produced by the Entner Doudorouff (ED) and PP pathways can be converted into pyruvate [42], which may subsequently be converted into acetyl-CoA by pyruvate dehydrogenase (Table 4.1, Figure 4.2). Acetyl-CoA serves to produce bulk ATPs to enhance the efficacy of BC synthesis [39]. Generally, a high concentration of ATP (Table 4.1) inhibits G6P dehydrogenase, and hence, reduces its activity by several folds. Thus, it may retard the PP pathway and favor BC synthesis through the principal pathway in either a microbial or a cell-free system (Figure 4.2). 4.2.3

Molecular Regulation of BC Synthesis

BC production and its extrusion to the extracellular environment is a complex process that is readily effected by several mutations in the genes regulating the enzymes involved in its synthesis. Any mutations in key genes lead to the accumulation of cellulose in the periplasm with inevitable lethal effects [35]. BC production by microbial cells is regulated by at least four different genes, axcess A, axcess B, axcess C, and axcess D, which are organized in the form of an operon (Figure 4.3) and encode specific proteins, AxCESA, AxCESB, AxCESC, and AxCESD, respectively, which perform specific functions [15]. Among these, AxCESA and AxCESB catalyze and regulate the polymerization of individual β-1,4-glucan chains. Specifically, AxCESA contains amino acid motifs suggestive of processive β-glycosyltransferase and binds the UDP-glucose [43]. Similarly, the AxCES subunit reversibly binds large quantities of cyclic di-guanosine monophosphate (c-di-GMP), which in turn activates AxCESB [15, 44]. During this process, c-di-GMP is released and auto-inhibits the state of enzyme by breaking the salt bridge, which otherwise tethers a conserved gating loop that controls the access to and substrate coordination at the active site. The disruption of salt bridge by mutagenesis generates a constitutively active cellulose synthase. It has also been demonstrated that the c-di-GMP-activated BcsA-B complex contains a nascent cellulose polymer whose terminal glucose unit resides at a new location above BcsA’s active site where it is positioned for catalysis. The other two subunits AxCESC and AxCESD have been proposed to mediate extrusion of β-1,4-glucan chains and crystallization of cellulose sub-elementary fibrils (SEF), respectively, and are considered to be the rate-limiting steps during cellulose assembly [45, 46]. AxCESD is reported to be a cylinder-shaped protein formed by an octamer as a functional unit [47]. All N-termini of the octamer are positioned inside the AxCeSD cylinder and create four passageways from which four β-1,4-glucan chains are extruded individually along the dimer interface in a twisted manner to the extracellular environment. This suggests that AxCESD protein resides at the extracellular side of the TC. These four subunits and perhaps other components from the cellulose-producing complexes can be viewed

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Cellulose AxCESD AxCESC Outer membrane AxCESA

Intermembrane space

AxCESB

Cytoplasmic membrane

UDP-Glc

DGC

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UDP-Glc

PDEA

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pGpG

Figure 4.3 Cellulose synthesis in A. xylinum. AxCESA, with 8 to 10 putative transmembrane regions (TMDs), is located in the cytoplasmic membrane and binds to UDP-Glc. Free c-di-GMP allosterically activates the AxCESA. The majority of the cellular c-di-GMP is bound to AxCESB, which is in close proximity to AxCESA. The diguanylate cyclase (DGC) catalyzes the synthesis of c-di-GMP, whereas phosphodiesterase A (PDEA) degrades the molecule. AxCESC is located in the outer membrane, whereas AxCESD might be located in the intermembrane space or extracellularly. Source: Endler et al. 2010 [15]. Reproduced with permission of Nature Publishing Group.

as pores (i.e. TCs) at the cell surface. These complexes assist the extrusion of the nascent β-1,4-glucan chains that aggregate to form the twisted SEF, which later crystallize into ribbons. This unique molecular configuration gives microbial cellulose different features compared to plant cellulose, such as high crystallinity and lower degree of polymerization despite its chemical structure being similar to that of plant cellulose.

4.3 Structure and Exciting Features of Bacterial Cellulose BC has received immense consideration due to its purity, unique structural, physicochemical, mechanical, and biological properties. These unique features and its potential to form composites with a wide range of materials, including biocompatible polymers, bactericidal elements, and conducting materials, provide BC with a high potential to find broad spectrum applications in different areas. The following sections describe the exciting features of BC.

4.3 Structure and Exciting Features of Bacterial Cellulose

4.3.1

Chemical Structure and Properties

BC possesses the same chemical structure as plant cellulose and is a linear homopolymer of glucose monomers linked by β-(1→4) glycosidic linkage with the chemical formula (C6 H10 O5 )n . However, it possesses different macromolecular structure and properties than plant cellulose. The two successive monomers in BC are linked in such a way that the former glucose unit is rotated at 180∘ with reference to the preceding. BC represents the purest form of cellulose; however, its degree of polymerization only ranges between 2000 and 6000 compared to plant cellulose, which lies in the range of 13 000–14 000 [48]. The repeated glucose monomers in BC form a continuous long unbranched polymer chain. Several cellulose chains in BC are held together through strong intra- and intermolecular hydrogen bonds that form a sheet. BC is produced at the air–medium interface in static cultivation where its shape is maintained by the hydrophilic interactions [5, 49]. The crystalline structure of BC results from the hydrogen bonding between the cellulose sheets. The thickness of cellulose fibers varies from species to species beside the culture conditions. For example, BC fibers produced by Gluconacetobacter xylinum consist of microfibril ribbons that are 3–4 nm thick and 70–80 nm wide. Chemical structure analysis by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) demonstrated slight variations in BC produced by different methods (e.g. static and shaking cultivation) and obtained from different sources (e.g. microbial and the cell-free system) [5, 50]. Structural analysis by NMR revealed that cellulose produced by microbial and cell-free systems demonstrated cellulose I and cellulose II polymorphic structures, respectively. X-ray diffraction (XRD) analysis of cellulose produced by microbial and the cell-free systems demonstrated cellulose I and cellulose II polymorphic structures, respectively [5]. Surface analysis of BC by scanning electron microscopy (SEM) reveals random distribution of fibers while the cross section shows layers of clustered fibers [19]. CP/MAS 13 C NMR spectroscopy, wide-angle X-ray diffractometry, and transmission electron microscopy (TEM) have been used to study the solid-phase nitration and acetylation of BC. The relative reactivity of OH groups in BC was found to be in the order of 6′ OH > 2′ OH > 3′ OH [51]. 4.3.2

Physiological Features

The broad spectrum applications of BC are highly dependent on its physiological features such as WHC, WRR, thermal properties, and mechanical features. These properties of BC are associated with its structural features including the arrangement of fibrils and the conformation of porous matrix, which are in turn associated with the microbial strain, synthesis method, chemical composition of culture medium, variation in culture time and conditions, amount of inoculum, and carbon source [23, 24]. Appropriate moisture content is required for a wound dressing material for the effective adsorption of bioactive substances into the BC matrix, which in turn offers several advantages, including easy and painless wound dressing, accelerated wound healing, and prevention of any damage to the newly formed skin

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tissues [6, 24]. Therefore, owing to the high WHC and slow WRR in addition to its hydrophilic nature, BC has received immense consideration as a wound dressing material. According to an estimate, BC can accommodate 100–200 times its dry weight in water [20, 21]. Highly porous BC favors a high WHC where the water molecules remain within the porous matrix by binding to the cellulose fibrils via hydrogen bonding [52]. Any structural variations in BC account for its altered physico-mechanical and thermal properties. For example, the fiber density is improved by prolonged BC production through supplementation of an additional carbon source or by extending its availability, which allows the newly formed β-1,4-glucan chains to be continuously added to the preexisting fibrils [6]. These strong and stable fibrils account for improved resistance to the applied force [53]. Further, a compact structure also imparts thermal stability to BC, which is an important feature for its commercial applications. 4.3.3

Self-assembly and Crystallization

The unique structural and physico-mechanical features of BC are attributed to the extrusion and self-assembly of β-1,4-glucan chains in the extracellular environment. The synthesized fibrils after extrusion through TCs subsequently form 2–4 nm protofibrils, bundles, and finally 80 × 4 nm ribbon-shaped microfibrils [54]. Electron micrographs of cell envelope revealed the presence of about 50–80 pore-like TCs along the axis of a microbial cell and in combination with the extracellular ribbon [15]. These TCs are assumed to be the sites for the extrusion of β-1,4-glucan chains, which form the initial assembly of cellulose in the form of aggregates. The formation of such aggregates suggests that simultaneous synthesis of several β-1,4-glucan chains is a common feature of the assembly of cellulose microfibrils in both higher and lower organisms [55]. This mutual orientation and association of β-1,4-glucan chains, their aggregation, and formation of microfibrils, bundles, and ribbons are governed by the original pattern of TCs while the process of self-assembly and crystallization of cellulose is cell directed [56]. Specifically, G. xylinum synthesizes two distinct physical forms of cellulose, the ribbon-like cellulose I and the thermodynamically more stable amorphous cellulose II [57]. The β-1,4-glucan chains of cellulose I are aligned in parallel and arranged uniaxially whereas those of cellulose II are arranged in a random manner. The microfibrillar arrangements of fibers and their extreme purity compared to plant cellulose mainly contribute to the exciting and unique features of BC such as greater tensile strength, better crystallinity, high WHC, slow WRR rate, and improved thermal stability. 4.3.4

Ultrafine Thin Fibrous Structure

Compared to plant cellulose, pure BC possesses a 100 times thin ultrafine fibrous structure that bestows it with unique features and provides the base for its broad spectrum applications [18, 49]. Its fibers are well distributed and form a reticulate web-shaped structure, giving a firm appearance to BC. This arrangement of fibers accounts for higher crystallinity, better mechanical features, and high

4.3 Structure and Exciting Features of Bacterial Cellulose

thermal properties of BC. Further, this arrangement of fibers plays a vital role in the development of BC-based composites with different types of materials for various biomedical and optoelectronic applications [58, 59]. 4.3.5

Macrostructure Control and Orientation

The structural features of BC are exciting both in dry (powder or sheet) and wet (hydrogel) forms and their applications vary accordingly [60]. The physical structure of BC is flexible enough to be controlled at the macro, micro, and nano scales. Further, BC can be produced in different shapes (sheets, pellets, or granules) by varying the culture conditions (static, shaking, or agitation) and microbial strains [58]. Further, several of its characteristic features such as surface chemistry, porosity, and fiber arrangement and orientation can also be controlled at the micro, macro, and nano scales. This microstructure control offers advantages for its broad-spectrum applications [8, 61]. In nature, BC demonstrates anisotropic patterns in the culturing plane. The movement of microbial strains in the culture medium accounts for the randomness of BC fibers. The arrangement of fibers can be controlled by controlling the movement of microbial cells. For example, Uraki et al. obtained a honeycomb pattern of BC by using honeycomb-patterned microgrooves in an agarose film [62]. In another study, Wang et al. controlled microbial cell movement by using microfluidic channels and successfully obtained aligned BC fibers [63]. Shi et al. used a magnetic field to control the movement of G. xylinum for the production of patterned BC fibers [64]. Despite the natural random behavior of BC fibers, its overall structure is quite compact and mechanically strong. BC can achieve comparable isotropic alignment and prove useful for a wide range of applications. 4.3.6 Porosity and Materials Absorption Potential of BC for Composite Synthesis BC is porous in nature and its level of porosity varies to a great extent depending upon its synthesis method and pre- and post-synthesis treatment processes. Its porous geometry provides an ideal environment for the adsorption of different types of materials such as solid particles and liquids, including water, cells, solutions, medium components, wound exudates, nanoparticles, and polymer solutions. The small natural porosity of BC is advantageous in that it prevents the invasion of microbial cells; however, it is a limitation from the medical perspective as it prevents the penetration of larger particles and mammalian cells deep into its matrix [65]. Currently, efforts have been devoted toward the development of artificially porous BC by using different materials such as salts, paraffin particles, ice crystals, gelatin, and various sugars, collectively termed as “porogens.” A porogen is any material that is first incorporated as a space holder, followed by its removal in a careful manner to avoid any damage or interference to the geometry of the pores formed [61, 66]. Porogens are adsorbed only physically and not chemically into the BC matrix for different time intervals depending upon the objectives of the study. The sizes and shapes of the porogens are also varied according to the

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requirements of the culturing cells or other materials to be incorporated into the BC matrix. Bäckdahl et al. reported the use of potato starch and paraffin wax particles as porogens for the development of BC tubes containing pores of various sizes, which were interconnected with each other. Further, they also effectively controlled the pore size and created partial particle fusion through heat treatment of initial paraffin wax particles at specific temperatures. The structures formed effectively supported the growth of smooth muscle cells inside the pores [61]. 4.3.7

Biocompatibility

Biocompatibility of BC refers to its ability to remain in contact with the living tissue without causing any toxic or allergic side effects [14, 67]. Biocompatibility of BC is directly linked with its higher water and other liquid absorbing capabilities to support the sustained growth and proliferation of living cells and tissues. To date, BC has been reported to show moderate to high levels of biocompatible behavior toward different types of mammalian cells and tissues [18, 68, 69]. For example, in vivo study in rate by implanting BC subcutaneously has shown biocompatibility up to 12 weeks. Microscopic observation demonstrated the complete absence of any fibrotic capsule or giant cells, indicating no foreign body reaction. Further, implantation of BC avoided any redness, swelling, and formation or accumulation of exudates around the BC implant [70]. In a recent study, Khan et al. reported the fabrication of highly biocompatible microporous BC scaffold for skin tissue regeneration application, which demonstrated in vitro adhesion and proliferation of human keratinocytes (HaCaT) and complete regeneration of skin tissues within two weeks in experimental mice [71]. However, the biocompatibility of BC in certain cases is not up to the desired levels and thus requires further improvement. One possible solution is to form composites with other biocompatible materials to improve its biocompatibility. For example, development of composites with various materials has been reported to enhance the biocompatibility, cell adhesion, and proliferation for wound dressing application and development of scaffold in tissue engineering [72, 73]. 4.3.8

Biodegradability

The potentials of biodegradable polymers have long been recognized [74]. A biomaterial must be biodegradable: it must be capable of eliciting an appropriate host response in a specific application. This response can be described in terms of chemical, physical, and biological properties of the materials to the shape and structure of the implant. Biodegradation is an exciting and rather the most difficult feature of BC to meet the requirements for various biomedical applications. Difficulty in biodegradation of BC is attributed to its high degree of crystallinity and compact structure. Lack of cellulase enzyme in mammalian cells or tissues necessitates the development of alternative strategies for degradation of BC implants. To this end, Li et al. reported an enhanced degradation of BC in vivo through periodate oxidation, which showed improved degradation in

4.4 Production of Bacterial Cellulose: Synthesis Approaches

water, phosphate buffered saline (PBS), and simulated body fluid (SBC) without disturbing its original fibrous network [75].

4.4 Production of Bacterial Cellulose: Synthesis Approaches The chemically defined medium for BC production contains several components including yeast extract, glucose, peptone, disodium phosphate, and citric acid. BC production is carried out in acidic conditions (pH 5–6); however, variation in other growth conditions such as pH, temperature, and carbon source and its concentration optimizing BC production eventually affect both its quality and quantity. In addition, different cultivation methods lead to the production of BC with different structures and properties [76]. Therefore, efforts have been made to develop a low-cost and efficient method for BC production with high yield and improved properties. The following sections overview the commonly used fermentation strategies including static, shaking, and agitation fermentation for BC production with different structural and physiological properties and yields [77]. 4.4.1 Static Fermentative Cultivation: Production of BC Membrane, Film, or Sheet Static fermentative cultivation is the most commonly used method for BC production. During static cultivation, the cellulose-producing microbial cells are inoculated into the culture medium at optimum pH and incubated at appropriate temperatures. Undisturbed incubation is carried out for 5–10 days during which the microbial cells grow and proliferate by partly utilizing the available carbon source and other medium components and produce BC. In static cultivation, BC is produced across the surface at the air–medium interface as an assembly of reticulated crystalline ribbons that forms a gel and ultimately a membrane, film, or sheet with increasing cultivation time [5, 78]. The BC pellicle grows downward until all microbial cells are entrapped inside the pellicle and become inactive or die due to depletion of nutrients or oxygen deficit [58]. Most of the BC used for biomedical applications is obtained by static cultivation because it forms a sheet with 3D interconnected reticulate structure, which makes it suitable for preparing different types of scaffolds [77, 79]. Unfortunately, the low yield and longer incubation time of the static cultivation approach limits its large-scale production and hence restricts its commercial applications. Several efforts have been made to overcome the limitations associated with static fermentative cultivation. For example, Jung et al. developed a novel technique for static cultivation that utilized an intermittent feeding strategy for BC production. Since the BC pellicles become separated from the culture medium due to depletion of nutrients and limited oxygen availability in static cultivation, the thickness of BC produced after 7 and 10 days cultivation is generally around 2 mm. However, using intermittent feeding, the thickness of BC produced after 30 days of cultivation was significantly increased to 30 mm using the same culture conditions [31].

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4.4.2

Shaking Fermentative Cultivation: Production of BC Pellets

Shaking fermentative cultivation refers to BC production in an incubator containing a rotor or shaker that can be easily differentiated from agitation, which is the production of BC in a reactor that offers controlled speed [78]. During shaking cultivation, the cellulose-producing microbial cells are inoculated into the culture medium at optimum pH and incubated at appropriate temperature under shaking conditions. Shaking is generally measured in terms of revolutions per minute (rpm). This type of cultivation is generally carried out for 24–36 hours during which microbial cells grow and proliferate by partly utilizing the available carbon source and other medium components and produce BC. In shaking cultivation, BC is produced within the culture medium in the form of small pellets. The shape of the pellet varies according to the type of microbial strain, incubation period, and shaking speed. A study by Hu and Catchmark reported that the shape of the BC pellet was sphere-like when the shaking speed was kept at 125 rpm. With further increase in the shaking speed, the pellets exhibited a tail-like feature that appeared to grow in size as the particle size decreased or the shaking speed increased. At further high rotational speeds, the tail-like BC pellets were found interconnected with each other [80].

4.4.3

Agitation Fermentative Cultivation: Production of BC Granules

In agitation cultivation, BC is produced in the form of granules. A comprehensive model for BC production by agitation cultivation has been developed by optimizing various factors such as bioreaction (substrate consumption and product formation), transport of oxygen and carbon sources, and removal of cellulose fibrils. BC production by agitation cultivation takes place at a much faster rate compared to static and shaking fermentative cultivation. The high cell density and better contact with available oxygen result in very high volumetric productivity in agitated cultures. Further, the improved productivity can also be correlated to the high density of microbial cells. The fermenters offer controlled pH, temperature, supply of nutrients, availability of oxygen, and prevent the formation of foam. Despite the high productivity of BC by agitation cultivation, there are several limitations associated with such cultivation strategies. For example, high supply of power is required, which usually produces cellulose pellets instead of pellicles under submerged and aerated cultivation, which limits its application spectrum [79–81]. Moreover, BC produced through agitated cultivation possesses a lower degree of polymerization, mechanical strength, and crystallinity than those produced in static or shaking cultivations. Sometimes, irregular or sphere-like cellulose particles (SCPs) are also produced due to agitation or stirring [77], which are less crystalline, possess low mechanical strength, and demonstrate low degree of polymerization [82]. Further, the BC granules cannot be utilized directly for biomedical and several other applications. Furthermore, continuous agitation converts the cellulose-producing microbial strains into more enriched cellulose-negative (Cel− ) mutants, which grow much faster compared to the wild type and result in lower BC production [83].

4.5 Additives to Enhance BC Production

Several modified reactors such as rotating disk bioreactor, cylindrical silicone membrane vessel, and a direct oxygen and glucose feeding reactor have been developed to improve microbial cellulose production (Figure 4.4). Some of these are briefly described in the following sections. 4.4.3.1

Rotating Disk Reactor

This is a designed bioreactor that enhances BC production by ensuring that one half of its disk remains inside the culture medium while the other half is present at the air surface. During BC production, the cells present on the surface of the disk are able to obtain the nutrients when they come in direct contact with the medium and are exposed to oxygen in the atmosphere [84]. Previous reports indicate that using horizontal fermenters with rotating discs effectively improved the culture conditions and BC production. G. xylinum ATCC 700178 were able to attach on the plastic composite surface (PCS) and increased BC production (0.24 g/l/day) in a 5-day cultivation while retaining its important structural features [85, 86]. 4.4.3.2

Trickling Bed Reactor

Generally, this kind of reactor is used for vinegar production and is composed of inlets that help in air circulation. In such a reactor, the fermentation liquid from the collection reservoir is circulated to the top of the tank by a pump until the desired product is obtained. Such a reactor has also been successfully employed for BC production. For this purpose, cellulose-producing microbial cells such as G. xylinum are adsorbed on to packings such as husks of rice or corncobs and exposed to the fermentation medium and air space. The oxygen supply to the fermenter further enhances the capability of the system. This cultivation method increases the provision of oxygen and hence reduces the sheer force, which in turn increases BC production [87]. In such reactors, BC films grow among gaps of husk or are fixed to the surface of the corncobs. The presence of plenty of packings in the reactor provides a larger surface to volume ratio, which enhances the oxygen supply to an adequate level; thus bacteria can easily attach and grow in the semisolid and liquid–solid microenvironments [87]. BC produced by trickling bed reactors appeared to be irregular sheet films with different thicknesses ranging between 1 and 5 mm. BC produced by such reactors possesses better degree of polymerization, purity, WHC, porosity, and thermal stability compared to that produced in static and shaking cultivations. Thus, BC produced by trickling reactors paves the way for various industrial applications.

4.5 Additives to Enhance BC Production The main idea behind the supplementation of culture medium with additives in BC production is to improve its morphology in one way or another. For this purpose, the effect of different additives has been evaluated for improved BC production with enhanced features. The following sections describe a few tested additives.

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Bacterial cellulose production in laboratory

Bacterial cellulose production in reactors (static conditions)

Bacterial cellulose production in reactors (agitated conditions)

Turbine

Max-blend

Helical ribbon

Screw with Gate with draft tube turbine

Production in flask

Horizontal lift reactor

Agitated configuration in bacterial cellulose production

Mats

Aerosol bioreactor

Various shapes of bacterial cellulose Agitator shaft PCS tubes

Side view

Pellets

Rotary disc reactor

Top view

Plastic composite support biofilm reactor

Figure 4.4 Various bioreactors designed for enhanced production and productivity of BC. Source: Ul-Islam et al. 2015 [14]. Reproduced with permission of Springer.

4.5 Additives to Enhance BC Production

4.5.1

Carboxymethylcellulose

Addition of carboxymethylcellulose (CMC) to the culture medium significantly improves BC production. Chang et al. reported that the addition of 1% CMC increased BC production from 1.3 to 8.2 g/l. Besides the yield, its addition to the culture medium also increased the crystallinity and crystal size of the produced BC by attaching onto the cellulose microfibrils during the fermentation process. Moreover, the addition of CMC-supplemented BC retained its fibrous structure and accounted for enhanced WHC. However, the mechanical strength of CMC-supplemented BC was reduced to a significant level [88]. Another study showed that the supplementation of 1.5% CMC into the G. xylinum culture enhanced BC production by 1.7 folds; however, the crystallinity of CMC-modified BC was much lower, with a much smaller pore size compared to pristine BC [85]. The addition of CMC increases the viscosity of the culture medium, which lowers the shear stress to the microbial cells. It was also reported that BC pellicles are transformed into pellet when modified by CMC in a biofilm reactor [85]. Carboxymethylated BC (CM-BC) is used for biomedical applications as a hemostatic material and has been established to enhance its functionality among minor cuts and less in larger wounds due to its solubility in water with a high degree of substitution. 4.5.2

Organic Acids

The effect of different organic acids as additive on BC production has been investigated. It was noted that oxalic acid and tartaric acid inhibit BC synthesis. For example, BC production was 0.16 and 0.17 g/l, respectively upon the addition of oxalic and tartaric acids, which were lower than those achieved without the addition of any acid to the culture medium (1.48 g/l). In contrast, BC production levels with malic acid, pyruvic acid, and citric acid were found to be 2.83, 2.34, and 2.27 g/l, respectively, which were higher than for the ethanol-supplemented sample (1.93 g/l). Similarly, the addition of lactic acid, acetic acid, and succinic acid to the culture media significantly improved BC production to 1.67, 1.85, and 1.49 g/l, respectively. All this could be due to a synergistic effect on BC production; therefore, the accumulation of oxalic acid and tartaric acid resulted in low BC production while citric and acetic acid accumulation improved its production (Figure 4.5) [89]. 4.5.3

Vitamin C

The addition of vitamin C to Hestrin and Schramm (HS) medium for BC production resulted in lowering the crystallinity of BC due to decreased inter-hydrogen bonding between cellulose sheets because it interferes with the cellulose planes. The resulting interference could be attributed to high water solubility, low molecular weight, and less steric hindrance of vitamin C. However, BC production was increased in the presence of vitamin C and reached 0.47/30 ml compared to 0.25/30 ml in its absence. Being an antioxidant, vitamin C decreased the production of gluconic acid produced by G. xylinus during BC production. Thus, the addition of vitamin C resulted in overall increased production of BC [94].

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Figure 4.5 Effect of different additives on BC production. Figure modified from Refs. [27, 89–93].

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4.5 Additives to Enhance BC Production

4.5.4

Sodium Alginate

Sodium alginate is an anionic polysaccharide made up of a linear unbranched copolymer of 1,4-linked β-d-mannuronate (M) and α-d-guluronate (G) and is mainly extracted from a brown marine algae [95]. It offers favorable characteristic features such as nontoxicity, gelled structure, transparency, biocompatibility, and biodegradability [96]. The addition of sodium alginate to the BC culture medium significantly improves BC production. Zhou et al. reported that the addition of 0.04% (w/v) sodium alginate enhanced BC production from 3.7 to 6.0 g/l by G. xylinum NUST4.1 in a stirred-tank reactor. Moreover, it changed the morphology of BC from irregular clumps and fibrous masses entangled in the internals to discrete masses dispersed in the broth. This demonstrates that sodium alginate functions to prevent the formation of large BC clumps and improves its production. Moreover, sodium alginate forms hydrogen bonds with BC, as confirmed by structural morphology (SEM) and chemical structure analysis (FTIR). However, the incorporation of sodium alginate to the BC matrix lowers its crystallinity and crystal size [97]. 4.5.5

Alcohols

Ethanol, on addition to the BC production medium, functions as an energy source for ATP generation at the early stage of fermentation [98, 99]. Further, it reduces glycerol production in the hexose monophosphate pathway, thereby improving overall BC production [100]. Investigations on the addition of different types of alcohols to the culture medium showed improved BC production to different levels. For example, the addition of 1.0% (v/v) methanol to the HS medium accounted for 21.8% improved BC production compared to the control group. Similarly, the addition of 0.5% ethylene glycol to the culture medium resulted in 24.1% improved BC production compared to the control group. A 13.4% improvement in BC production was observed when 0.5% of n-propanol was added to the culture medium. Further, the addition of 3.0% glycerol accounted for 27.4% improved BC production compared to the control group. Similarly, the addition of 0.5% n-butanol and 4.0% of mannitol to the culture media accounted for 56.0% and 47.3% improved BC production, respectively, compared to the control group. These observations suggest that the addition of alcohol seems to have a stimulatory effect at the later stage of fermentation, which enhanced BC production [90] (Figure 4.5). 4.5.6

SSGO

The production of BC follows a biosynthetic pathway that does not completely convert glucose into BC but also forms oligomers. A study by Ha et al. reported that the addition of single sugar-linked glucuronic acid-based oligosaccharide (SSGO) into the culture medium inhibits the glucuronic acid oligomers synthesis during BC production where these also serve as alternative source of glucose [38]. BC produced by the addition of SSGO showed improved mechanical properties; for instance, the fibrils became thicker and denser, which

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further accounted for improved crystallinity [49]. In another study, Ha and Park reported that the addition of 1.0% of SSGO to the chemically defined medium resulted in increased BC production up to 89.3% and 52.3% by G. xylinum and G. hansenii after 15 days of cultivation under static condition. Therefore, it is definite that the addition of SSGO to the culture medium successfully improves BC production [91] (Figure 4.5). 4.5.7

Lignosulfate

Lignosulfonate is an antioxidant containing polyphenolic compounds. Keshk and Sameshima reported that the addition of lignosulfate to the HS medium resulted in lowering the production of gluconic acid, which is commonly produced in the hexose monophosphate pathway of BC production and led to overall increased BC production. Further, the addition of lignosulfonate to the culture medium enhanced the crystallinity of BC. These findings can be justified by the presence of hydrogen bonding in cellulose, which is not affected by the lignosulfonate, and therefore does not interfere with cellulose planes due to its high water solubility and steric hindrance [101]. 4.5.8

Agar and Xanthan

Studies have shown that the addition of both agar and xanthan as additives to the culture media resulted in increased BC production. Microbial strains producing BC are always sensitive to shear force; therefore, it should be minimized to the lowest possible level for efficient BC production. Agar has been identified as one of the components that can be added to the culture medium to lower the sheer force for yield of BC. Agar as a polymeric compound tends to increase the viscosity of the culture medium, which in turn improves the yield of BC. The effect of addition of agar into the culture medium on BC production was further enhanced when agar concentration was increased to 0.4%. Thus, it can be concluded that soluble viscosity-inducing materials such as agar have rather more effect on BC productivity due to its mass production; therefore, such soluble polymers are recommended for enhanced BC production [102]. Chao et al. showed that the addition of 0.1% (w/v) agar and 0.06% (w/v) xanthan to the culture medium increased BC production from 6.3 to 8.7 g/l and 7.2 g/l, respectively, using G. xylinum in batch cultivation with an airlift reactor. BC was produced in the form of pellets, with the size decreasing as productivity of BC increased, indicating that an increase in relative viscosity with the addition of polysaccharides hindered the formation of large clumps [103]. 4.5.9

Thin Stillage

Thin stillage (TS) is a wastewater from rice wine distillery, which has been used as a supplement to the traditional HS medium for enhanced BC production because it is rich in organic acid. The addition of TS (100% TS–HS medium) enhanced BC production by 3.4-folds to 10.38 g/l, which is about 2.5-folds higher than that obtained in HS-only medium. This could be attributed to the reduction

4.6 Strategies Toward Low-Cost BC Production

of sugar consumption rate by G. xylinus, leading to higher BC production. Use of TS as a supplement is also a way of reducing the pollution caused by wine industry and employing cheap materials for enhanced BC production [92] (Table 4.2, Figure 4.5).

4.6 Strategies Toward Low-Cost BC Production The different chemically defined media used for BC production are generally very expensive. A typical BC production medium containing glucose as the carbon source and other nutrient sources increases the overall BC production cost, thus limiting its use in value-added applications. Therefore, finding a low-cost medium for BC production has always been the target of research right from its discovery. The use of raw and cheap carbon sources is one such strategy that can lower the overall BC production cost [26, 27]. The following sections summarize a few low-cost BC production media (Figure 4.6). 4.6.1

Fruit Juices

Fruits are used as a routine food. However, these usually have short shelf-life and often become rotten if not consumed in time. When bad quality fruits are not shipped or processed in time, these must be essentially discarded, which results in wastes. However, fruits are known to contain a large amount of simple sugars, especially fructose, glucose, and sucrose, which can be used for the production of useful products, in addition to being used as a carbon source in BC production [27]. It has been reported that various types of sugars are present in different levels in various fruit juices; for example, sucrose is found highest in orange and pineapple juices while grape, apple, and Japanese pear juices contain high amounts of fructose. Various juices were reportedly utilized as media for production of BC by G. xylinum NBRC 13693. Orange and Japanese pear juices yielded the highest BC production, thus proving to be the most suitable medium for BC production. The BC yield could be further enhanced by adding nitrogen to the fruit juices [27]. 4.6.2

Sugarcane Molasses

Molasses is a low-cost sugar industrial product that is often used in microbial fermentation. It is mainly composed of glucose, fructose, and sucrose sugars, which are readily biodegradable. Besides, it contains some nitrogen and vitamins, which can stimulate BC production by microbial cells. Molasses is pretreated by two different methods: chemical and physical. Chemically treated molasses usually gives a high BC yield compared to the physically treated molasses. However, the production cost of physically treated molasses is much lower than that of chemically treated molasses because of the low concentration. A dense fibrils-web was clearly seen during the molasses medium production compared with HS and M1A05P5 media [115]. Pretreatment of molasses with acid helps in the production of high quantity of sugars such as fructose, glucose, and sucrose, which could aid in BC production. It further helps in the removal of coloring materials and

101

Table 4.2 Production of BC in static and agitation cultures with a variety of BC-producing strains, carbon sources, and supplementary materials.

Microorganism

Carbon source

Supplementary materials

Culture time (days)

Yield (g/l)

Cultivation mode

References

G. xylinus, Trichoderma reesei

Glucose

Fiber sludge

14

6.23

Static

[104]

G. xylinus

Glucose

Cellulosic fabrics

14

10.80

Static

[105] [106]

G. medellensis

Glucose

None

14

4.50

Static

G. hansenii PJK (KCTC 10505 BP)

Glucose

SSGO

10

7.4

Static

[38]

G. xylinus (PTCC, 1734)

Glucose

Date syrup

14

40.35

Static

[107]

G. persimmonis (GH-2)

Glucose

Fructose, beef extract

14

5.14

Static

[108]

G. xylinus (ATCC 53524)

Sucrose

None

4

3.83

Static

[109]

G. hansenii PJK (KCTC 10505 BP)

Waste from beer culture

None

14

8.6

Static

[110]

G. xylinus (K3)

Mannitol

Green tea

7

3.34

Static

[111]

G. xylinus (IFO 13773)

Sugar cane molasses

None

7

5.76

Static

[101]

G. xylinum (ATCC 700178)

CSL-Fru

Carboxymethyl cellulose

5

13.00

Agitated

[88]

G. xylinus

CSL-Fru

Sodium alginate, agar, CMC

5

7.05

Agitated

[85]

Gluconacetobacter sp. (RKY5)

Glycerol

None

6

5.63

Agitated

[112]

G. xylinus (BPR2001)

Molasses

None

3

7.80

Agitated

[113]

G. xylinus (BPR2001)

Fructose

Agar/oxygen

3

14.10

Agitated

[114]

G. hansenii PJK (KCTC 10505 BP)

Glucose

Ethanol

3

2.50

Agitated

[28]

Source: Wang et al. 1991 [42]. Reproduced with permission of Elsevier.

4.6 Strategies Toward Low-Cost BC Production

(a)

(b)

(e)

(c)

(d

)

Figure 4.6 Cost-effective production of BC from various waste sources including (a) fruit juices, (b) brewery waste, (c) sugarcane molasses, (d) agricultural and food wastes, and (e) waste from beer fermentation broth.

heavy metals in molasses and enhances BC production. Sulfuric acid pretreated molasses yielded high BC production with relatively better mechanical properties compared to the one obtained from glucose [116]. Khattak et al. reported BC production from sugarcane jaggery (gurr), obtained from brown sugar industry and identified to contain a high quantity of glucose. The BC produced from sugarcane jaggery demonstrated better structural properties and possessed high mechanical and thermal properties. Moreover, the BC produced showed better biocompatibility properties and supported the adhesion and growth of skin cells, which demonstrates its usefulness in medical application [117]. 4.6.3

Agricultural and Industrial Wastes

Most of the organic wastes from agro industries are available but are never used due to their poor quality although these are a rich source of carbon such

103

104

4 Synthesis, Structure, and Properties of Bacterial Cellulose

as glucose, fructose, and sucrose. These waste materials can be effectively utilized for BC production. Such waste materials cannot only serve as a cheap carbon source but can also prove to be an environment cleaning step [118]. For example, wastewater from candied Jujube (WWCJ) processing industry as a low-cost and an economical product was studied as agro-industrial waste for BC production in fruit-producing areas in China. The wastewater contained low level of glucose, glucan, and other carbohydrates. Therefore, hydrolysis by acid treatment was performed to increase the glucose concentration, which increased to 58% compared to unhydrolyzed WWCJ. The pretreated medium produced BC with improved yield (0.375 g/l/d) after 6 days of cultivation, although with a much lower crystallinity [93]. In another study, a high yield of BC was produced by K. sucrofermentans DSM 15973 when fermented in the presence of by-product streams from oil seed-based biodiesel industries and waste streams from confectionery industries as the sole source of nutrients [119]. A carbon source for production of BC by G. sacchari was employed by using a residue of olive oil production industry, which also proved to be a good source that showed improved BC production [120]. This concludes that most of the agro-industrial wastes provide all the nutrients required for bacterial growth and BC production, mostly with improved properties. 4.6.4

Food Wastes

Most of the flour-rich waste (FRW) streams produced in the manufacturing industries (bread and confectionery industries) contain high levels of starch, proteins, and micronutrients, which are useful in the fermentation processes for production of other industrial products, thus reducing environmental pollution [119]. Most of the available food wastes used for BC production are fruit peels such as rinds or skins of various foods. The use of such food wastes as substrates for BC production helps in reducing the production cost and hence proves very economical. BC production by using pineapple (PA) and watermelon (WM) peels as culture media was analyzed and compared with the HS medium. The wet weight of BC obtained from pineapple medium was highest (i.e. 12.5 g/100 ml) as compared to watermelon medium (i.e. 10 g/100 ml) and HS medium (3 g/100 ml). The fibers in BC produced by pineapple peel medium (PA-BC) were highly disordered, showing an irregular arrangement. Similarly, the ribbons were shorter in length for WM-BC and PA-BC and longer and uniform in HS-BC. In contrast, the BC fibers produced by PA were thicker compared to those produced by WM and HS after seven days. The texture for PA-BC was hard, adhesive, and cohesive followed by WM-BC, and lowest for HS-BC [121]. In another study, food wastes of rotten fruits and whey milk were evaluated for BC production, which showed high BC production among all the other culture media used due to the high percentage of carbon in the rotten fruits. BC production by whey milk was limited by the presence of lactose, which is not favorable for BC production and thus results in lowering the overall yield. In contrast, the culture medium containing rotten fruits blended with whey milk accounted for improved bacterial cell growth and BC production

Acknowledgment

due to the presence of rich nutrients. This demonstrates that a combination of different culture media of different food wastes can be useful for improved BC production. These findings show that food waste medium can be a potential substrate for low-cost BC production for different applications [122]. Besides, the recycling of food wastes helps in reducing environmental pollution and increasing ecological awareness.

4.7 Conclusions and Future Prospects BC is an important natural biopolymer produced by various microbial strains and a cell-free system. It possesses unique structural and physicochemical, mechanical, and biological features, which make it an attractive material for various applications. Different methods of synthesis of BC greatly affect its yield and structural and physico-mechanical properties. The broad spectrum applications of BC have been limited by its low yield and high production cost, mainly due to the expensive medium components. Therefore, extensive efforts have been made to minimize BC production cost including (i) the development of novel production strategies (static, shaking, and agitation cultivation), (ii) designing of new bioreactors, (iii) replacement of common carbon sources with new and cheaper sources, (iv) exploration of novel and cheap waste sources, (v) discovery of new BC-producing microbial species and genetic modification of known bacterial strains, and (vi) the production of BC through cell-free systems, which ideally convert all available substrate to product. BC yield and its properties have been significantly improved by adding different supplements to the culture medium. The utilization of various wastes as nutrient sources has significantly reduced the overall production cost. Efforts are required to further increase the BC yield and explore and utilize cheap media sources. A mixture of different waste materials from different sources could lead to improved productivity. This can be an important aspect of future research for improved BC production with better characteristic features for various applications. Development of advanced low-cost BC production strategies will open the gateways for industrial-scale applications. Nevertheless, the most important future aspects of BC research are to identify cheap, easily available, and renewable media resources and microbial strains for cost-effective BC production. With increased BC production, we can expect the development of new useful BC-based materials in the near future.

Acknowledgment This work was supported by National Natural Science Foundation of China (31270150, 51603079, 21774039), China Postdoctoral Science Foundation (2016M602291), Fundamental Research Funds for the Central Universities, Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, and Chinese Academy of Sciences.

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4 Synthesis, Structure, and Properties of Bacterial Cellulose

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cellulose-producing bacterium, Rhodococcus sp. MI 2: screening and optimization of culture conditions. Carbohydrate Polymers 92 (1): 421–428. Ruka, D.R., Simon, G.P., and Dean, K.M. (2014). Bacterial cellulose and its use in renewable composites. In: Nanocellulose Polymer Nanocomposites: Fundamentals and Applications (ed. V.K. Thakur), 89–130. Shezad, O., Khan, S., Khan, T., and Park, J.K. (2010). Physicochemical and mechanical characterization of bacterial cellulose produced with an excellent productivity in static conditions using a simple fed-batch cultivation strategy. Carbohydrate Polymers 82 (1): 173–180. Hu, Y. and Catchmark, J.M. (2010). Formation and characterization of spherelike bacterial cellulose particles produced by Acetobacter xylinum JCM 9730 strain. Biomacromolecules 11 (7): 1727–1734. Kralisch, D., Hessler, N., Klemm, D. et al. (2010). White biotechnology for cellulose manufacturing – the HoLiR concept. Biotechnology and Bioengineering 105 (4): 740–747. Shi, Q.-S., Feng, J., Li, W.-R. et al. (2013). Effect of different conditions on the average degree of polymerization of bacterial cellulose produced by Gluconacetobacter intermedius bc-41. Cellulose Chemistry and Technology 47: 503–508. Kim, Y.J., Kim, J.N., Wee, Y.J. et al. (2007). Bacterial cellulose production by Gluconacetobacter sp. PKY5 in a rotary biofilm contactor. Applied Biochemistry and Biotechnology 137–140 (1–12): 529–537. Chawla, P.R., Bajaj, I.B., Survase, S.A., and Singhal, R.S. (2009). Microbial cellulose: fermentative production and applications (review). Food Technology and Biotechnology 47 (2): 107–124. Cheng, K.C., Catchmark, J.M., and Demirci, A. (2011). Effects of CMC addition on bacterial cellulose production in a biofilm reactor and its paper sheets analysis. Biomacromolecules 12 (3): 730–736. Lin, S.P., Hsieh, S.C., Chen, K.I. et al. (2014). Semi-continuous bacterial cellulose production in a rotating disk bioreactor and its materials properties analysis. Cellulose 21 (1): 835–844. Lu, H. and Jiang, X. (2014). Structure and properties of bacterial cellulose produced using a trickling bed reactor. Applied Biochemistry and Biotechnology 172 (8): 3844–3861. Cheng, K.C., Catchmark, J.M., and Demirci, A. (2009). Effect of different additives on bacterial cellulose production by Acetobacter xylinum and analysis of material property. Cellulose 16 (6): 1033–1045. Lu, H., Jia, Q., Chen, L., and Zhang, L. (2015). Effect of organic acids on bacterial cellulose produced by Acetobacter xylinum. Journal of Microbiology and Biotechnology 5 (2): 30–37. Lu, Z., Zhang, Y., Chi, Y. et al. (2011). Effects of alcohols on bacterial cellulose production by Acetobacter xylinum 186. World Journal of Microbiology and Biotechnology 27 (10): 2281–2285. Ha, J.H. and Park, J.K. (2012). Improvement of bacterial cellulose production in Acetobacter xylinum using byproduct produced by Gluconacetobacter hansenii. Korean Journal of Chemical Engineering 29 (5): 563–566.

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enhances bacterial cellulose production by Gluconacetobacter xylinus. Carbohydrate Polymers 90 (1): 116–121. Li, Z., Wang, L., Hua, J. et al. (2015). Production of nano bacterial cellulose from waste water of candied jujube-processing industry using Acetobacter xylinum. Carbohydrate Polymers 120: 115–119. Keshk, S.M.A.S. (2014). Vitamin C enhances bacterial cellulose production in Gluconacetobacter xylinus. Carbohydrate Polymers 99: 98–100. Aljohani, W., Li, W., Ullah, M.W. et al. (2017). Application of sodium alginate hydrogel. IOSR Journal of Biotechnology and Biochemistry 03 (3): 19–31. Lee, K.Y. and Mooney, D.J. (2013). Alginate: properties and biomedical applications. Progress in Colloid and Polymer Science 37 (1): 106–126. Zhou, L.L., Sun, D.P., Hu, L.Y. et al. (2007). Effect of addition of sodium alginate on bacterial cellulose production by Acetobacter xylinum. Journal of Industrial Microbiology and Biotechnology 34 (7): 483–489. Ullah, M.W., Khattak, W.A., Ul-Islam, M. et al. (2014). Bio-ethanol production through simultaneous saccharification and fermentation using an encapsulated reconstituted cell-free enzyme system. Biochemical Engineering Journal 91: 110–119. Ullah, M.W., Khattak, W.A., Ul-Islam, M. et al. (2015). Encapsulated yeast cell-free system: a strategy for cost-effective and sustainable production of bio-ethanol in consecutive batches. Biotechnology and Bioprocess Engineering 20 (3). Li, Y., Tian, C., Tian, H. et al. (2012). Improvement of bacterial cellulose production by manipulating the metabolic pathways in which ethanol and sodium citrate involved. Applied Microbiology and Biotechnology 96 (6): 1479–1487. Keshk, S. and Sameshima, K. (2006). Influence of lignosulfonate on crystal structure and productivity of bacterial cellulose in a static culture. Enzyme and Microbial Technology 40 (1): 4–8. Kim, S.J., Li, H., Oh, I.K. et al. (2012). Effect of viscosity-inducing factors on oxygen transfer in production culture of bacterial cellulose. Korean Journal of Chemical Engineering 29 (6): 792–797. Chao, Y., Mitarai, M., Sugano, Y., and Shoda, M. (2001). Effect of addition of water-soluble polysaccharides on bacterial cellulose production in a 50-L airlift reactor. Biotechnology Progress 17 (4): 781–785. Cavka, A., Guo, X., Tang, S.J. et al. (2013). Production of bacterial cellulose and enzyme from waste fiber sludge. Biotechnology for Biofuels 6 (1). Feng, Y., Zhang, X., Shen, Y. et al. (2012). A mechanically strong, flexible and conductive film based on bacterial cellulose/graphene nanocomposite. Carbohydrate Polymers 87 (1): 644–649. Castro, C., Zuluaga, R., Álvarez, C. et al. (2012). Bacterial cellulose produced by a new acid-resistant strain of Gluconacetobacter genus. Carbohydrate Polymers 89 (4): 1033–1037. Moosavi-Nasab, M. and Yousefi, A. (2011). Biotechnological production of cellulose by Gluconacetobacter xylinus from agricultural waste. Iranian Journal of Biotechnology 9 (2): 94–101.

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ter persimmonis GH-2 using dual and cheaper carbon sources. Journal of Microbial and Biochemical Technology 05 (02). Mikkelsen, D., Flanagan, B.M., Dykes, G.A., and Gidley, M.J. (2009). Influence of different carbon sources on bacterial cellulose production by Gluconacetobacter xylinus strain ATCC 53524. Journal of Applied Microbiology 107 (2): 576–583. Ha, J.H., Shehzad, O., Khan, S. et al. (2008). Production of bacterial cellulose by a static cultivation using the waste from beer culture broth. Korean Journal of Chemical Engineering 25 (4): 812–815. Nguyen, V.T., Flanagan, B., Gidley, M.J., and Dykes, G.A. (2008). Characterization of cellulose production by a Gluconacetobacter xylinus strain from Kombucha. Current Microbiology 57 (5): 449–453. Kim, S.Y., Kim, J.N., Wee, Y.J. et al. (2006). Production of bacterial cellulose by Gluconacetobacter sp. RKY5 isolated from persimmon vinegar. Applied Biochemistry and Biotechnology 131 (1–3): 705–715. Bae, S. and Shoda, M. (2004). Bacterial cellulose production by fed-batch fermentation in molasses medium. Biotechnology Progress 20 (5): 1366–1371. Bae, S., Sugano, Y., and Shoda, M. (2004). Improvement of bacterial cellulose production by addition of agar in a jar fermentor. Journal of Bioscience and Bioengineering 97 (1): 33–38. Çakar, F., Kati, A., Özer, I. et al. (2014). Newly developed medium and strategy for bacterial cellulose production. Biochemical Engineering Journal 92: 35–40. Tyagi, N. and Suresh, S. (2016). Production of cellulose from sugarcane molasses using Gluconacetobacter intermedius SNT-1: optimization and characterization. Journal of Cleaner Production 112: 71–80. Khattak, W.A., Khan, T., Ul-Islam, M. et al. (2015). Production, characterization and biological features of bacterial cellulose from scum obtained during preparation of sugarcane jaggery (gur). Journal of Food Science and Technology 52 (12). Castro, C., Zuluaga, R., Putaux, J.L. et al. (2011). Structural characterization of bacterial cellulose produced by Gluconacetobacter swingsii sp. from Colombian agroindustrial wastes. Carbohydrate Polymers 84 (1): 96–102. Tsouko, E., Kourmentza, C., Ladakis, D. et al. (2015). Bacterial cellulose production from industrial waste and by-product streams. International Journal of Molecular Sciences 16 (7): 14832–14849. Gomes, F.P., Silva, N.H.C.S., Trovatti, E. et al. (2013). Production of bacterial cellulose by Gluconacetobacter sacchari using dry olive mill residue. Biomass and Bioenergy 55: 205–211. Kumbhar, J.V., Rajwade, J.M., and Paknikar, K.M. (2015). Fruit peels support higher yield and superior quality bacterial cellulose production. Applied Microbiology and Biotechnology 99 (16): 6677–6691. Jozala, A.F., Pértile, R.A.N., dos Santos, C.A. et al. (2014). Bacterial cellulose production by Gluconacetobacter xylinus by employing alternative culture media. Applied Microbiology and Biotechnology 99 (3): 1181–1190.

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5 Surface Chemistry of Nanocellulose Ge Zhu and Ning Lin Wuhan University of Technology, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan 430070, China

Nanocelluloses, including cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial cellulose (BC), have become fascinating building blocks for the design of new biomaterials. Hundreds of scientific articles or patents have been published on the research on nanocellulose during the last 20 years, even if most of the studies focus on their basic physical and chemical properties such as the reinforcing phase and liquid crystalline self-ordering property. Yet, owing to their hydrophilic nature, their application is commonly limited to those involving hydrophilic or polar media or matrices, which restrict their development. With the presence of a large number of chemical functionalities, mainly hydroxyl groups, within the structure of nanocellulose these novel nanoparticles provide a unique platform for significant surface modification with the purpose of changing surface properties. These chemical modifications are a prerequisite, sometimes unavoidable, to adapting the interfacial properties of nanocellulose substrates or adjusting their hydrophilic–hydrophobic balance. In addition, surface modification can introduce the functional groups of molecules on the surface of nanocellulose contributing to the specific properties, further extending their use in highly sophisticated applications.

5.1 Brief Introduction to Nanocellulose Family 5.1.1

Cellulose Nanocrystals (CNCs)

Cellulose microfibrils formed during biosynthesis are 2–20 nm in diameter, depending on the source, and can be several micrometers in length. Each microfibril consists of crystalline domains interspersed with disordered amorphous regions. The preparation of CNCs involves a chemical hydrolysis process to dissolve amorphous chains and release crystalline domains from cellulose fibers. Although a few examples of producing CNCs through processes involving enzymatic hydrolysis were reported [1, 2], the main process for the isolation of CNCs from cellulose fibers is based on strong acid hydrolysis, such as sulfuric Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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acid (H2 SO4 ), hydrochloric acid (HCl), and phosphoric acid (H3 PO4 ). The first attempt to hydrolyze cellulose fibers with strong acids (HCl/H2 SO4 ) appeared in 1947 as reported by Nikerson and Habrle [3]. With diverse hydrolyzing agents, CNCs possessing different surface groups and surface chemistry can be prepared. Hydrolysis with hydrochloric acid preserves the hydroxyl groups of native cellulose but leads to less stable aqueous suspensions [4]. With the combination of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and hypochlorous sodium, surface hydroxyl groups of CNCs can be selectively transformed into carboxyl groups, which may be useful for subsequent modification [5]. The resulting CNC possesses the crystalline structure similarly to the original cellulose fibers. Typical procedures currently employed for the extraction of CNCs consist of subjecting pure cellulosic material to strong acid hydrolysis under strictly controlled conditions of temperature, agitation, and time. The nature of the acid and source of cellulose are extremely important to the properties of the CNCs prepared (as shown in Figure 5.1). Attributed to good dispersion and high stability of CNCs in water, water is a preferred processing medium for liquid compounding especially for those

(a)

(b)

500 nm

(c)

1 μm (d)

200 nm

Figure 5.1 Transmission electron micrographs from a dilute suspension of CNCs obtained from different sources: (a) tunicate, (b) bacterial, (c) ramie, and (d) eucalyptus wood. Source: (a) Elazzouzi-Hafraoui et al. 2008 [6]. Reproduced with permission of ACS. (b) Winter et al. 2010 [7]. Reproduced with permission of ACS. (c) Habibi et al. 2008 [8]. Reproduced with permission of RSC. (d) De Mesquita et al. 2010 [9]. Reproduced with permission of ACS.

5.1 Brief Introduction to Nanocellulose Family

CNCs prepared using sulfuric acid [10]. Consequently, CNCs can easily be used as fillers to reinforce hydrophilic polymers; however, when organic solvents are used as the blending media and the matrix is a hydrophobic polymer the use of CNCs is greatly restricted. Furthermore, the sulfide groups on the surface of CNCs, derived from hydrolysis with sulfuric acid, usually result in a mismatch between the thermal stability of CNCs and the thermoprocessing temperature. At this time, physical and chemical modification is considered to regulate the chemical structure and hydrophilic/hydrophobic properties on the surface of CNCs. 5.1.2

Cellulose Nanofibrils (CNFs)

As introduced by Turbak et al. in the 1983 [11], the process for isolating CNFs consists of disintegration of cellulose fibers along their long axis. Simple mechanical methods in combination with enzymatic or chemical pretreatments are included in the preparation of CNFs. Cellulosic fibers are disintegrated into their sub-structural fibrils having lengths in the micron scale and widths ranging from 10 to a few hundred nanometers, depending on the nature of the native cellulose. The resulting aqueous suspensions exhibit gel-like characteristics in water with pseudoplastic and thixotropic properties even at low solid content. Three main technologies, namely, homogenization, microfluidization, and microgrinding, are widely used for the mechanical treatment. Other methods were also developed but they are not widely accepted and are far from being amenable to scale up, for example, high-speed blending, cryocrushing, high-intensity ultrasonication [12], and steam explosion [13]. Using the abovementioned technologies, CNFs were prepared from numerous cellulosic sources including soft and hard woods [14], sugar beet pulp [15], banana [16], Opuntia ficus-indica [17], potato [18], wheat straw [19], bamboo [20], and Luffa cylindrica [21], and also from some seaweed [22]. The morphological characteristics of the resulting CNFs depend mainly on those of the original fibers. Generally, CNFs obtained from primary cell wall fibers are longer and thinner than those obtained from secondary cell wall fibers. Examples of the TEM (transmission electron microscopy) images of the extracted CNFs using different pretreatments are given in Figure 5.2. 5.1.3

Bacterial Cellulose (BC)

Besides being a dominant component of cell walls in plants, cellulose also exists extracellularly as synthesized cellulose fibers by several bacterial species [26]. Bacterial cellulose is mainly produced by the bacteria of Acetobacter G. xylinus species by cultivation in aqueous culture media containing carbon and nitrogen sources within a period of days. The resulting cellulosic network structure is in the form of a pellicle of randomly assembled ribbon-shaped fibrils less than 100 nm wide, which are in turn composed of a bundle of much finer nanofibrils (2–4 nm in diameter). These bundles are relatively straight, continuous, and dimensionally uniform (Figure 5.3).

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(a)

(b)

200 nm

(c)

100 nm

(d)

1 μm

Figure 5.2 Transmission electron micrographs from a dilute suspension of CNFs obtained from wood pulp using different pretreatments (a) enzymatic, (b) TEMPO-mediated oxidation, (c) carboxymethylation, and (d) Opuntia ficus-indica. Source: (a) Pääkkö et al. 2007 [23]. Reproduced with permission of ACS. (b) Saito et al. 2007 [24]. Reproduced with permission of ACS. (c) Wagberg et al. 2008 [25]. Reproduced with permission of ACS. (d) Malainine et al. 2005 [17]. Reproduced with permission of Elsevier. Figure 5.3 Scanning electron micrographs of a bacterial cellulose pellicle. Source: Nakagaito et al. 2005 [27]. Reproduced with permission of Springer Nature.

2 μm

5.2 Surface Modification of Nanocellulose

5.2 Surface Modification of Nanocellulose 5.2.1

Physical Adsorption of Surfactants

Non-covalent surface modifications of nanocelluloses are typically achieved via adsorption of surfactants, oppositely charged entities, or polyelectrolytes. Thus, these interactions with the nanocellulose substrate are ensured through hydrophilic affinity, electrostatic attractions, hydrogen bonds, or van der Walls forces. In order to obtain non-flocculated dispersion of CNCs in nonpolar solvents, the easiest approach is to cover the surface of nanocrystals with surfactants bearing polar heads and long hydrophobic tails. With the simple mixing in solvent, non-covalent physical absorption of surfactants on the nanocellulose will be a much more controllable and easily operated process than delicate chemical modification. Surfactants are usually amphiphilic organic compounds, containing both a water-insoluble (oil soluble) component for hydrophobic groups (so-called tails) and a water-soluble component for hydrophilic groups (so-called heads). The hydrophilic end of the surfactant molecule may adhere on the surface of nanocellulose whereas the hydrophobic end may extend out providing a nonpolar surface and lowering the surface tension of the nanoparticles. With this process, the ensuing surfactant-coated nanoparticles display reduced surface energy and improved dispersibility or compatibility with nonpolar organic media. Since the first report on the use of surfactant Beycostat NA (BNA) as stabilizing agent to prepare stable suspensions of CNCs in nonpolar solvents [28], there were more studies focusing on the influence and property of BNA-modified CNCs. BNA is a commercial anionic surfactant, and its chemical structure involves a phosphoric ester of polyoxyethylene nonylphenyl ether. The mixing ratio of the surfactant and CNCs should be cautiously controlled. A low amount of surfactant is not sufficient to obtain good dispersion in organic solvents, whereas too high an amount of surfactant may induce self-aggregation of nanoparticles during the mixing process. Meanwhile, using small angle neutron scattering, it was observed that a surfactant layer that is 15 Å thick coated the surface of CNCs, which indicated the effective adsorption of BNA and folded conformation of the surfactant molecule on the surface of nanocrystals [29]. In further studies, the properties of BNA-adsorbed CNCs, such as morphology, self-assembling, and chiral nematic property, were investigated with various techniques including atomic force microscope (AFM), TEM, and field emission scanning electron microscopy (FESEM) [30]. Particularly, the chiral nematic ordering of BNA-adsorbed CNC suspensions in cyclohexane with different concentrations was observed and discussed [31]. When using these surfactant-modified CNCs as nanofillers to reinforce the polymeric matrix (such as poly(lactic acid), PLA or polypropylene, PP), the presence of the surfactant on the surface of nanocrystals was found to facilitate the dispersion of the nanofiller in the matrix and improve the nucleation effect of the matrix for the ensuing materials [32–34].

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Unlike the hydrophilic–hydrophobic mechanism for the absorption of common surfactants, cationic surfactants (such as dimethyldioctadecylammonium bromide [DODA-Br]) can be adsorbed on the surface of CNCs through strong electrostatic interactions between the negative charge of sulfate groups on H2 SO4 -hydrolyzed nanocrystals and the cationic charge of the surfactant. DODA is a cationic surfactant that has been widely used in films to investigate adsorption and binding processes involving anionic polyelectrolytes at the air/water interface. Rojas and coworkers applied cationic surfactant DODA to modify the surface of CNCs and used this surfactant as the carrier to cross-link cellulose nanoparticles [35]. With Langmuir–Schaeffer technique, the precursor CNC–DODA complexes were subsequently used to prepare monolayer films. A nonionic surfactant, such as sorbitan monostearate, was also used to improve the dispersion of CNCs in organic solvents and to prevent their self-aggregation [36]. From the results of a turbidity experiment performed on 0.3 and 0.6 wt% nanocrystals suspension in tetrahydrofuran (THF), the surfactant concentration determined the stability of modified CNCs in the solvent. With high surfactant concentrations, the surfactant molecules self-aggregated and probably did not adhere to the surface of nanocrystals, which probably caused the weak stress transfer effect when using CNCs as the reinforcing filler. In another study, as a small surfactant molecule, tert-butanol was used to modify the surface of CNCs [37]. It was reported that tert-butanol was able to limit the aggregation of nanocrystals during the freeze-drying process and promote more loose condition and homogeneous dispersion in PLA matrix (Table 5.1).

Table 5.1 Reactive conditions of adsorption of anionic, cationic, and nonionic surfactants (𝛤 = W surfactant /W cellulose ). Source

Surfactant

Solvent

Conditions

𝜞

References

Cotton

BNA

Toluene

BNA:CNC = 4 : 1, pH = 9

0.7

[28]

Tunicate

BNA

Toluene

BNA:CNC = 4 : 1, pH = 9

1.5

[28]

Tunicate

PEPNP

Toluene

—

0.2–1.5

[29]

MCC

Beycostat AB09

Chloroform

60 ∘ C, 6 h

—

[30]

Cotton

BNA

Cyclohexane

BNA:CNC = 4 : 1, pH = 8

1.1

[31]

MCC

Beycostat AB09

H2 O

pH = 7.5

—

[32]

Tunicate

BNA

Toluene

—

[34]

Cotton/ sisal/ramie

DODA-Br

H2 O

BNA:CNC = 4 : 1, pH = 8 20 ∘ C, 15 min

0.2–0.8

[35]

Whatman filter

Sorbitan monostearate

THF

24 h

—

[36]

MCC

tert-Butanol

Chloroform

Solvent exchange

—

[37]

5.2 Surface Modification of Nanocellulose

5.2.2

Sulfonation

Sulfonation of CNCs occurs during the course of sulfuric acid-catalyzed hydrolysis through the esterification of the hydroxyl groups. However, the esterification levels depend highly on several factors such as hydrolysis time, temperature, and acid concentration for the precise control of the amount of sulfate groups (as shown in Table 5.2). It is also worth noting that the quantification of the resulting ester groups remains challenging [38]. To alleviate this issue, post-treatment of CNCs generated by hydrochloric acid hydrolysis with sulfuric acid has been suggested to introduce, in a controlled manner, sulfate moieties on their surfaces [44]. CNCs generated from hydrochloric acid hydrolysis and subsequently treated with sulfuric acid solution had the same particle size as those directly obtained from sulfuric acid hydrolysis; however, the surface charge density could be tuned to regimes that are accessed by sulfuric acid hydrolysis. Note that desulfonation of CNCs, prepared by sulfuric acid hydrolysis, was also used to adjust the sulfate content on the surface of CNCs [39]. With respect to the morphology of the particles, a combination of both sulfuric and hydrochloric acids during hydrolysis steps appears to generate spherical CNCs instead of rod-like nanocrystals when carried out under ultrasonic conditions [40]. These spherical CNCs demonstrated better thermal stability mainly because they possess less number of sulfate groups on their surfaces compared to those generated solely from sulfuric acid hydrolysis [41]. Besides the direct sulfonation from sulfuric acid hydrolysis, post-sulfonation modification of the CNCs can be achieved via chemical reactions between the surface hydroxyl groups and sulfonated agents, as shown in Figure 5.4. Lin and Dufresne reported the strategy of introducing a gradient of sulfate groups on the surface of CNCs through regulation of reactive conditions for postsulfation (with chlorosulfonic acid) and desulfation, and further investigated the effect of the sulfation degree on the surface chemistry, morphology, dimensions, and physical properties of the modified CNCs [45]. Another strategy of the sulfonation modification was performed on the surface of CNFs, which involves a two-step process from the periodate oxidation and sulfonation. The resulting sulfonated CNFs, Table 5.2 Reactive conditions of sulfonation and sulfur contents for the prepared cellulose nanocrystals from sulfuric acid hydrolysis (L, length; W, width; D, diameter).

Sources

H2 SO4 concentration

Temperature

Duration

S contents (wt%)

References

Whatman filter

64 wt%

45 ∘ C

45 min

0.62

[38]

Softwood pulp

64 wt%

45 ∘ C

60 min

0.57

[39]

MCC

H2 SO4 :HCl:H2 O = 3 : 1 : 6

RT

2, 4, 10 h

—

[40]

MCC

H2 SO4 :HCl:H2 O = 3 : 1 : 6

10 h

0.053

[41]

90 min

0.35

[42]

30 min

0.49

[43]

Cotton

63.5 wt%

68 ∘ C 45 ∘ C

Cotton

61 wt%

72 ∘ C

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122

5 Surface Chemistry of Nanocellulose OSO3H O

H2SO4 O HO

O HO

OH O OH

CISO3H

Cl

OH

SO3H Cell

Cell

n

O

O

SO3H

H

Cl

OH O

NaIO4 O NaIO3, H2O

n

O

O

O HO3SO

2NaHSO3 O n

HO +

Na–O3S

OSO3H O OSO3H n OH O OH n SO3–Na+

Figure 5.4 Three approaches of the sulfonation modification on nanocellulose.

having typical widths of 10–60 nm, were obtained from sulfonated celluloses with very low charges (0.18–0.51 mmol/g) [46]. Although the oxidation and consecutively the sulfonation occur at the vicinal hydroxyl groups of the glucose unit at positions 2 and 3 inducing the opening of the glucopyranose ring, CNFs retained their native crystalline structure even with high sulfate with this modification. 5.2.3

TEMPO-oxidation

TEMPO-mediated oxidation has been used to convert the hydroxymethyl groups on the cellulose chains into the carboxylic form. The use of this technique has been the subject of a number of reports since its introduction by de Nooy et al. [47], who were the first to show that only the hydroxymethyl groups of polysaccharides were oxidized while the secondary hydroxyls remained unaffected. Figure 5.5 depicts the TEMPO-oxidation mechanism for cellulose molecules. The procedure for modification of CNCs is carried out from the TEMPO/NaClO/NaBr system, with control of the pH = 10 condition to preserve high oxidation efficiency. Modification of TEMPO-mediated oxidation of CNCs, obtained from HCl hydrolysis of cellulose fibers, was first reported by Araki et al. [48, 49]. The authors demonstrated that after TEMPO-mediated oxidation, the CNCs maintained their initial morphological integrity and formed a homogeneous suspension when dispersed in water. The presence of carboxyl groups imparted negative charges at the CNC surface and thus induced electrostatic stabilization. Montanari et al. [50] systematically investigated the degree and size influence of the TEMPO-mediated oxidation of cotton linter CNCs. The degree of oxidation (DO) was determined by conductometric titration as 0.24. During TEMPO-mediated oxidation, a decrease in the crystal size occurred, and the introduction of negative charges on the surface of the crystalline domains induced better individualization and dispersion of the crystallites. Habibi et al. [51] performed TEMPO-mediated oxidation of CNCs obtained from HCl hydrolysis of tunicate cellulose fibers and showed that TEMPO-mediated oxidation did not compromise the morphological integrity of CNCs nor their

5.2 Surface Modification of Nanocellulose

COOH O OH

COONa O O OH

NaOH O

OH

OH NaCl

NaBrO

N OH CHO O OH

N O.TEMPO

O

OH NaClO

NaBr

N+ O

CH2OH O O OH OH

Figure 5.5 Scheme of the TEMPO-mediated oxidation mechanism of cellulose.

native crystallinity. Based on the supramolecular structure, morphological attributes, and crystallographic parameters of the oxidized CNCs, these studies demonstrated that various degrees of oxidation could be predicted and achieved by using specific amounts of the primary oxidizing agent ratios. In the case of CNFs, TEMPO-mediated oxidation is mostly used as a cost-effective chemical treatment prior to the mechanical processing in order to facilitate the individualization of the fibers. Indeed, after impairing their structural potential and softening their rigid structure by breaking the strong intra-fiber hydrogen bond networks, the TEMPO-oxidized cellulose fibers can be converted very easily utilizing the mechanical shearing discussed before (high-speed blending, ultrasonication, high mechanical shearing, etc.) into highly crystalline individual nanofibers. This method has been reported to be employed for a variety of cellulosic fibers [52]. Isogai’s group carried out pioneering work in this area as reported recently in a concise review [53]. With the most common TEMPO derivative, the optimal conditions reported are a pH of 10 and temperature ranging between 0 and 10 ∘ C. These operating conditions usually yield cellulose nanofibers that are 3–20 nm in width and a few microns in length, depending on the cellulosic raw material used. 5.2.4

Esterification

Owing to its ease and straightforwardness, modification of hydroxyl groups present at the surface of nanocellulose through esterification is widely used. Among esterification reactions, acetylation of nanocellulose is the most widely studied. The use of pyridine as a solvent diluent in the heterogeneous acetylation of nanocellulose, using acetic anhydride and sulfuric acid as catalyst, seems to impact only slightly their morphological and crystalline structure even at 80 ∘ C for 5 hours [54]. The simultaneous occurrence of cellulose hydrolysis and

123

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5 Surface Chemistry of Nanocellulose

acetylation of hydroxyl groups has also been reported. Fischer esterification of hydroxyl groups simultaneously with the hydrolysis of amorphous cellulose chains has been introduced as a viable one-pot reaction methodology that allows isolation of acetylated CNCs in a single-step process [55]. However, strict control of the esterification degree using this method is quite tough. In a similar approach, Herrick et al. [56] have proposed in the early development of nanocellulose the simultaneous acetylation of CNFs during the mechanical shearing using a mixture of acetic acid and acetic anhydride as processing medium with sulfuric acid as catalyst. A similar one-step procedure aiming to esterify and defibrillate cellulose fibers was recently developed by Huang et al. [57]. The procedure involves ball milling of the fibers in organic solvents such as dimethylformamide (DMF) or THF containing esterifying agents such as chloride or anhydride derivatives, leading to CNFs with tunable properties. It is worth mentioning that acetylation was also suggested as a chemical pretreatment step prior to the mechanical processing to access CNFs [58]. A novel and straightforward method for the surface acetylation of CNCs by transesterification of vinyl acetate was proposed by Cetin et al. [59]. The reaction was conducted in DMF at 95 ∘ C using potassium carbonate as catalyst. Analysis of the results indicated that during the first stage of the reaction, only the surface of CNCs was modified, while their dimensions and crystallinity remained intact. With increasing reaction duration, diffusion mechanisms controlled the rate, leading to higher levels of acetylation, smaller dimensions, and lower crystallinity of the modified CNCs. In another study, similar transesterification of CNFs obtained through carboxymethylation to substitute carboxymethyl groups by hexyl moieties was performed by using 1-hexanol as reactant and reaction medium as well as in combination with sulfuric acid as catalyst [60]. The reaction was carried out at 105 ∘ C for 72 hours but did not show any regioselectivity because all hydroxyl groups were modified (Figure 5.6). An environment friendly esterification modification route to confer high hydrophobicity to CNCs was developed by Yuan et al. [61]. The method used alkenyl succinic anhydride aqueous emulsions, which were simply mixed with CNC aqueous suspensions and freeze dried, as a template and the resulting solid was heated to 105 ∘ C. With low reagent consumption and simple treatment procedures, modified CNCs with highly hydrophobic character can be obtained. The acylated CNCs can disperse in low-polarity solvents, i.e. dimethyl sulfoxide (DMSO) (Relative permittivity = 46.45) and 1,4-dioxane (Relative permittivity = 2.209). O

O HO

OH O OH

Cl

O– Na+ O

EtOH/isopropanol NaOH

RO

O R = H or

OR O

1-Hexanol O H2SO4

OR O

O– Na+

R′ = H or

R′O

OR′ O OR′

O OH

Figure 5.6 Transesterification modification of nanocellulose.

or

O

(CH2)5CH3

5.2 Surface Modification of Nanocellulose

Menezes et al. [62] have reported the esterification of CNCs by grafting organic acid chlorides presenting different lengths of the aliphatic chain. The reaction was conducted under reflux for four hours using triethylamine (TEA) as catalyst and neutralizing agent for the in situ formed hydrochloric acid. The crystallinity of the particles was not altered by the chain grafting, but it was shown that covalently grafted chains were able to crystallize at the cellulose surface when using C18 agent. Berlioz et al. [63] have reported a highly efficient solvent-free synthetic method allowing almost complete surface esterification of CNCs, extracted from tunicate, leading to highly substituted CNC esters. The reaction of fatty acids such as palmitoyl chloride was carried out at reduced pressure and temperature ranging from 160 to 190 ∘ C on CNC substrates either freeze-dried or dried via the supercritical carbon dioxide process. It has been shown by SEM (scanning electron microscope) and X-ray diffraction analyses that the esterification proceeded from the surface of the substrate to the crystal core. 5.2.5

Silylation

Silylation is another commonly used reaction for conjugating small molecules on the nanocellulose surface, as shown in Figure 5.7. A series of alkyl dimethylchlorosilanes (alkyl-DMSiCl) with alkyl moieties of isopropyl, n-butyl, n-octyl, and n-dodecyl can be reacted on the surface of CNCs in toluene. The surface of CNCs was only partially silylated, and gave different degrees of surface substitution (DSs). It was found that the moderate extent of silylation kept the integrity and slender morphology, but made the CNCs stick together with slight swelling and thus showed wavy contours. Furthermore, surface silylation also resulted in the ready dispersibility of CNCs in low-polarity solvents such as THF. The appearance of the silylated CNC suspensions in THF was relevant to the DS, and is shown as flocculated and stable/cloudy states, respectively, for DS 0.4 and 0.6 of modified CNCs with isopropyl-DMSiCl. Meanwhile, the stable and homogeneous suspension of isopropyl-DMSiCl modified CNCs with a DS of 0.6 was birefringent when observed between crossed polarizers. This indicated that the dispersibility of silylated CNCs increased with a higher degree of surface silylation substitution [64]. Figure 5.7 Two commonly used approaches for the silylation of nanocellulose.

CH3 Cell-OH

Cl Si

CH3 R

Cell O Si

CH3

R

CH3

(a) R: i-C3H7, n-C4H9, n-C8H17, n-C12H25

Si O Cell-OH

O

Si O

O R

Cell O Si O Si

(b)

R

125

126

5 Surface Chemistry of Nanocellulose

A mild silylation protocol was adapted to surface silylate CNFs extracted from parenchymal cell cellulose with isopropyl dimethylchlorosilane. The silylated CNFs maintained the same morphological features as those of the unmodified sample and were homogeneously dispersible in nonpolar solvents, providing stable suspensions that did not flocculate [65]. The rheological properties of their suspension in methyl oleate showed a marked shear thinning effect. Thus, by silylation, the CNFs acquired an inherent flexibility with the result that their suspensions presented rheological behavior equivalent to that of polymer solutions. Similarly, Andresen et al. [66] have reported the successful surface silylation of CNFs with chlorodimethyl isopropylsilane (CDMIPS). CNFs obtained by disintegration of bleached softwood sulphite pulp in a homogenizer was hydrophobically modified by surface silylation with CDMIPS. At moderate DS (between 0.6 and 1), hydrophobized silylated CNFs retained their morphological integrity and were able to stabilize water-in-oil emulsions [67]. Silylation was also attempted in order to tune the wetting properties toward nonpolar liquids of preformed CNF-based aerogels. Perfluorodecyltrichlorosilane [68] and octyltrichlorosilane [69] were grafted merely by chemical vapor deposition method at an elevated temperature. Owing to its simplicity silylation was also exploited as an intermediate step for further functionalization. Aminosilanes were first grafted onto CNCs and served as reactive sites to covalently attach fluorescent moieties [70]. More recently, alkene thiol-functionalized silanes were also covalently grafted on CNF-based films and consequently clicked with appropriate moieties through thiol–ene click chemistry shown in Figure 5.8 [71].

5.2.6

Grafting Onto

The “grafting onto” method involves attachment of pre-synthesized polymer chains, carrying reactive end groups, onto (modified) hydroxyl groups of the cellulose surface. Yet, steric hindrance can prevent optimal attachment during the grafting reaction because the polymer chains have to diffuse through the layer of already attached brushes to reach the available reactive sites at the surface; hence, an unfavorable reduced surface grafting density is often obtained by using the “grafting onto” method. The case of poly(ethylene glycol) (PEG) or polyoxyethylene (PEO) grafted onto the surface of CNCs was widely studied because of the similar hydrophilic property and simple chemical structure of PEG. Three approaches were reported to combine the PEG (or PEO) with CNCs via the “grafting onto” strategy, as summarized in Figure 5.9. Araki et al. first reported the steric stabilization of TEMPO-mediated oxidized CNC suspensions by amine-terminated PEG grafting using a carbodiimide-catalyzed amidation procedure in water at room temperature [72]. By grafting PEG at a ratio of 0.2–0.3 g/g on CNCs, the modified CNCs showed drastically enhanced dispersion stability and the ability to redisperse into either water or chloroform from the freeze-dried state. On the basis of the chemical reaction between the hydroxyl and isocyanate groups, the polymeric chains of PEG can also be grafted onto the surface of CNCs (through covalent linkage with the isocyanate modified PEG agent).

O

Si O Cell-OH

O

EtOH/H2O

Si

pH = 3.5

HS

O Cell O Si

O Cell O Si

hv, DMPA

O Si

O

Si O

CNF-Vi-Sx O

Si O Cell-OH

O

EtOH/H2O

Si

SH

pH = 3.5

O Cell O Si

O

O

O Si

CNF-Vi

(a)

S

SH

O Si

O

O

O

Cell O Si hv, DMPA

S

O

O Cell-S-ABu

(b)

Cell-SH O

O O O

Si O

O HS

O

hv, DMPA O

O

S

OMe

Cell-OH

S

OMe

Cell O Si

Si O

O

EtOH/H2O

O

(c)

Figure 5.8 Scheme for the silylation of nanocellulose with vinyltrimethoxysilane (a), 3-mercaptopropyltrimethoxysilane (b), and trialkoxysilane (c).

128

5 Surface Chemistry of Nanocellulose O TEMPO-mediated

COOH

EDC, NHS

C

H O N

NaBr, NaClO

COOH

PEG-NH2

C

N

O H N

O OCN

H N

OH

C O

O

C

CH3 Op

N H H N

O O

OH

H N

O

cpoxy, methoxy-terminated PEO NaOH

O

O

O

n

O

n

H

C O

C O

O

m

OH OH

m

O

CH Op 3

O

CH Op 3

OMe

OH

O

O

OMe

OH

Figure 5.9 Different methods of introducing PEG/PEO chains on the surface of nanocellulose.

Kloser and Gray reported a novel method to graft PEO with the high molecular weight on the surface of CNCs, involving the reaction between epoxy and hydroxyl groups under alkaline conditions [73]. Figure 5.10 depicts this approach from the two steps of the desulfation reaction and epoxy-terminated PEO grafting under alkaline conditions for grafting PEO onto the CNC surface. The CNCs prepared by sulfuric acid hydrolysis were desulfated with sodium hydroxide, and then nucleophilic attack by the surface hydroxyl groups under strongly alkaline reaction conditions opened the epoxide ring of epoxy-terminated PEO to form a covalent ether linkage between the CNCs and the PEO chains. AFM images indicated that the whole process did not change the morphology or dimensions of CNCs, and the grafted PEO chains successfully contributed to steric stabilization for good dispersion of CNCs. The resultant PEO-grafted CNC suspension was stable for several months and no precipitation was observed. Besides the hydrophilic PEG, several hydrophobic polymers were also reported to be grafted onto the surface of CNCs, as summarized in Table 5.3. The “grafting onto” approach was used by Ljungberg et al. [74] to graft maleated polypropylene (PP-g-MA), bearing 7.5 wt% of maleic anhydride groups, onto the surface of tunicate-extracted CNCs. This esterification was carried out in toluene at 100 ∘ C for 5 minutes. The resulting grafted nanocrystals showed very good compatibility and high adhesion when dispersed in the polypropylene matrix. Harrisson et al. [75] and Azzam et al. [76] adopted the same approach using DMF as reaction medium to graft amine-terminated poly(styrene) (PS) or poly(tert-butyl acrylate) (poly(TBA)) and Jeffamine copolymers respectively and the grafting efficiency seemed to be greater than that in water. Isocyanation is the most popular method for termination and functionalization of end groups in polymer chains, and hence provides good precursors for surface

5.2 Surface Modification of Nanocellulose 5.0

OSO3–

HO

O

OH

4.5 4.0

O O

HO

OH

3.5

OH

3.0 2.5 2.0

Electrostatic stabilization

1.5 1.0 0.5

(a)

0.0 0.0 0.5 1.0

OH

3.0 3.5 4.0 4.5 5.0

5.0

HO

OH

O O

4.5 4.0

O HO

1 μm 1.5 2.0 2.5

OH

OH

3.5 3.0 2.5

Pristine cellulose nanocrystals

2.0 1.5 1.0 0.5

(b)

0.0 0.0 0.5

OMe O n

1 μm 1.0

1.5

2.0 2.5 3.0 3.5 4.0 4.5 5.0

1.0

1.5

2.0 2.5 3.0 3.5 4.0 4.5 5.0

5.0 4.5

O

4.0

OH HO

O

OH

O

3.0

O HO

O OH

3.5

OH

2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5

1 μm

Steric stabilization

Figure 5.10 Schematic illustration of the grafting of PEO onto the CNC surface via desulfation reaction (a) and epoxy-terminated PEO grafting under alkaline condition (b), and the AFM images corresponding to the initial sulfated CNC and all the products (the scale in the AFM images is 1 μm).

129

Table 5.3 Reactive conditions and grafting efficiency for the preparation of polymeric chain grafted onto CNCs/CNFs. Types of nanocellulose

CNC

Source of cellulose

Polymeric chain

Coupling agent

Solvent

Condition

GE (%)

Whatman filter paper

PEG

Carbodiimide

Water

RT, 24 h, pH = 7.5–8.0

20

[72]

Tunicate

PPgMA

—

Toluene

100 ∘ C, 5 min

—

[74]

Whatman filter paper

PS poly(TBA)

—

DMF

RT, 16 h

56

[75]

RT, 13 h

60

Cotton

Jeffamine

—

Water

RT, 24 h pH = 7.5–8.0

DMF

CNF

References

30.1

[76]

53.5

Cotton

PEO

—

Water

WPU

—

DMF

65 ∘ C , 6.5 h alkaline conditions 80 ∘ C, 2 h, N

20–40

Cotton

45–57

[78]

Ramie fibers

PCL10000

2,4-TDI

Toluene

80 ∘ C, N2 , 7 d

56.3

[79]

PCL42500

2

53

[77]

5.2 Surface Modification of Nanocellulose

Step 1 O

O O

NCO

H

O

O H

n OH

nO

NH

Pl-PCL Step 2

O NH

NCO Pl-PCL +

nO

O

NH

Pl-PCL-TDI

NCO

NCO

O

O

Step 3 OH

OH

CNC-g-PCL

O NH

Pl-PCL-TDI

O

HN O

O

O nO

NH

Figure 5.11 Three-step routine for the PCL-grafting on the surface of CNCs using the “grafting onto” strategy.

“grafting onto.” The grafting of polycaprolactone (PCL) with different molecular weights on the surface of CNCs has been achieved by using isocyanate-mediated coupling, which involves a three-step process as shown in Figure 5.11 [77]. The first step requires the reaction of phenylisocyanate with one functionality of the polymer. The second step requires the reaction of the modified polymer, having thus one protected functionality, with one isocyanate functionality of 2,4-tolylene diisocyanate (2,4-TDI). During the third step, the unreacted second isocyanate functionality of 2,4-TDI is then able to react with the surface hydroxyl groups of the CNCs to graft the polymer chain onto the nanoparticle [79]. The grafting density reached was high enough that the grafted PCL chains were able to crystallize at the surface of CNCs. Nevertheless, to reach the high grafting density the reaction was conducted for 7 days at 80 ∘ C with continuous addition of the coupling agent and the catalyst as well as by using a high volume of toluene to ensure good dispersion. The same approach was implemented by Cao et al. [78] who reported on the isocyanate-catalyzed grafting of pre-synthesized waterborne polyurethane (WPU) polymers via a one-pot process using DMF as the reaction medium. These grafted-WPU chains were able to form a crystalline structure on the surface of CNCs and thus induce the crystallization of the matrix, which created a co-continuous phase. The same approach was followed by Pei et al. [80] to prepare polyurethane–CNC nanocomposites with ultrahigh tensile strength and strain-to-failure with strongly improved modulus. 5.2.7

Grafting From

The “grafting from” approach was chosen to increase the grafting density of the polymer brushes on the surface and to ensure their stability in different

131

132

5 Surface Chemistry of Nanocellulose

application conditions. In this method, polymer brushes can be grown in situ from nanocellulose directly using the surface hydroxyl groups serving as initiating sites for ring-opening polymerization (ROP) or the surface can be modified to introduce different initiator sites needed for controlled polymerization techniques such as atom-transfer radical polymerization (ATRP) and other radical polymerization techniques. 5.2.7.1

Ring-Opening Polymerization (ROP)

ROP, which was used to graft and polymerize cyclic monomers, mainly lactones, was first applied in the modification of CNCs by Habibi et al. [8]. The ROP grafted PCL on the surface of CNCs using stannous octoate (Sn(Oct)2 ) as catalyst and the reaction was conducted at 95 ∘ C in toluene. Under these conditions, the CNCs kept their original crystalline structure and morphology as confirmed during the modification reaction [81]. With a similar strategy, Chen et al. [82] and Lin et al. [83] conducted the grafting reactions under microwave irradiation to enhance the grafting efficiency, and the CNC-g-PCL was incorporated into the PCL matrix by thermoforming. In this case, the long grafted PCL chains entangled with each other in the melting process of thermoforming and, hence, produced molded sheets with good mechanical strength. In order to overcome the use of metal-based catalysts, Labet and Thielemans [84] recently used citric acid, a benign naturally available organic acid, as the catalyst. The possibility to exert control over the SI-ROP of PCL from CNCs using a benign catalyst is a major step toward truly green materials as the incorporation of potentially harmful metal catalysts in polymers is a continuous concern. Reaction conditions and grafting efficiency reported in the literature for the preparation of PCL-grafted CNCs are summarized in Table 5.4. Recently, Carlsson et al. reported the monitoring of CNC surface grafting with PCL by quartz crystal microbalance, which provided a tool to regulate the polymerization reaction [91]. Lonnberg et al. have successfully achieved the surface grafting of CNFs with PCL via ROP using Sn(Oct)2 as the catalyst with a two-step procedure [89]: (i) CNFs were dispersed in the monomer and the reaction was conducted at 95 ∘ C without any solvent, and (ii) the reaction was performed in toluene where CNFs were previously dispersed [90]. The good reinforcing ability derived from the characteristic dimensions of CNFs must be preserved during the grafting reaction. Great improvement in the dispersibility of the CNFs in organic solvent can be achieved after PCL grafting, and it was shown that longer graft lengths better improved the stability of the suspensions. Similar to the strategy followed in the case of grafting of PCL at the surface of CNCs, Goffin et al. [85] reported the PLA chains grafting from the surface of CNCs by ROP. However, the grafted chains were suspected to be very short according to their crystallization behavior. Therefore, in order to achieve the high molecular weight of grafted PLA chains, Braun et al. [86] suggested the use of partially acetylated CNCs, obtained through the Fischer esterification pathway, to control the grafting density. This well-designed approach to produce the materials is used to balance competing effects; blocking a fraction of surface hydroxyl groups available for ROP initiation leads to high-molecular-weight chains covalently attached to the CNC surface. Significant enhancement of the mechanical

Table 5.4 Reactive conditions and grafting efficiency (GE %) for the surface grafting of the poly(caprolactone) (PCL) or poly(lactic acid) (PLA) on the nanocellulose. Nanocellulose

CNC

CNF

Source

Grafted polymer

Catalyst

Solvent

Reactive condition

GE (%)

References

85

[81]

Ramie

PCL

Sn(Oct)2

Toluene

95 ∘ C, 24 h

Native linter

PCL

Sn(Oct)2

—

85

[82, 83]

Cotton wool

PCL

Citric acid

—

Microwave irradiation, 5 min 120 ∘ C, 24 h

58

[84]

80 ∘ C, 24 h, N2 90 ∘ C, 68 h, argon flow

83

[85]

60–70

[86] [87]

Ramie

PLLA

Sn(Oct)2

Toluene

—

PLA

Sn(Oct)2

Toluene

Ramie

PDLA

Sn(Oct)2

Toluene

PCL/PLA

Sn(Oct)2

Toluene

80 ∘ C, 24 h, N2 80 ∘ C, 48 h, N

80

Ramie

83

[88]

Wood pulp

PCL300

Sn(Oct)2

—

95 ∘ C, 18–20 h, argon flow

98

[89]

2

PCL600

99

PCL1200 Softwood

PCL-short

95 Sn(Oct)2

Toluene

110 ∘ C, 3–5 h, argon flow

22

PCL-medium

70

PCL-long

78

[90]

134

5 Surface Chemistry of Nanocellulose

performances was also obtained, most probably induced by the percolation of CNC-g-PLA network stabilized by stereocomplexation. These results and a thorough comparison with their counterparts filled with PLA chains converge toward the contribution of the tiny stereocomplexed PLA structure surrounding CNCs. Efforts toward tuning this surface stereocomplexation are underway to increase the size and amount of stereocomplexed PLA around CNCs [87]. In a particular work, Goffin et al. [88] employed ROP catalyzed by Sn(Oct)2 to graft successively PCL and then PLA from the surface of CNCs (Figure 5.12). Despite the prolonged reaction time under metal-catalyzed ROP conditions, the morphology and crystalline structure of the CNCs were preserved. These CNCs grafted with PCL-b-PLA copolymer showed distinct crystallization behavior and dispersion ability when blended with a mixture of PCL and PLA. 5.2.7.2

Living Radical Polymerization (LRP)

Living radical polymerizations (LRPs), such as ATRP and single electron transfer-living radical polymerization (SET-LR), can also be used for surface polymer grafting on nanocellulose. Commonly, a two-step reaction is involved, which is the formation of initiating sites for LRP with the immobilized initiator on CNCs, and the reaction of the initiator-modified nanoparticles with a monomer to induce the polymerization. It has been proved that LRP is a versatile technique to synthesize well-defined polymers with high surface grafting density and low polydispersity on the surface of CNCs. Copper-mediated LRP is generally chosen for its versatility with respect to monomer choice and ease of synthesis. Generally, two steps are involved in the chemical modification

PCL O OH

OH

O

n

OH

Nanocellulose

OH

Sn(Oct)2

Nanocellulose

(a) O H3C OH

OH

OH

PLA O

m O O

Nanocellulose

CH3

Sn(Oct)2

OH Nanocellulose

(b) PLA O H 3C

O OH

OH

OH

Nanocellulose

n

O

Sn(Oct)2

OH Nanocellulose

m O

O CH3

O Sn(Oct)2

PCL OH Nanocellulose

(c)

Figure 5.12 Chemical routes of surface grafting of PCL (a), PLA (b), and PCL-b-PLA (c) on the nanocellulose from the ROP reaction.

5.2 Surface Modification of Nanocellulose

OH

OH

O

OH Br

Nanocellulose

Br

THF, TEA, DMAP

O

Br

Br

O

Br

n

O

n

Br

R

R OH

O

Nanocellulose

O

O R O

OH

O

CuX/HMTETA Nanocellulose

Figure 5.13 Common synthetic routes of grafting polyolefin chains on CNCs with the technique of ATRP reaction.

of grafting polymeric chains on CNCs with LRP techniques, as shown in Figure 5.13. The first step is the initial formation of initiating sites for LRP, in which an initiator is immobilized on the nanoparticles, and a macromolecular initiator (commonly used as CNC-Br) can be obtained. It should be pointed out that 2-bromoisobutyryl bromide (BriB) is always chosen for the esterification of nanocrystals, and provides the initiating sites. To prevent the destruction of the crystalline structure of CNCs, this reaction should be carefully performed at low temperature (sometimes in the ice bath), which can dissipate superfluous heat. The second step involves the reaction of the initiator-modified nanoparticles with monomers (C=C alkene molecules) to induce the polymerization. Early reports on using SI-ATRP to graft polyolefin polymeric chains on CNC surface were published by Zhang and coworkers concerning polystyrene (PS) [92], poly[2-(N,N-dimethyl amino) ethyl methacrylate] (PDMAEMA) [93], and poly{6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate} (PMMAZO) [94]. In these studies, the authors used 2-bromo-2-methylpropionyl bromide (Br-MPBr) as the initiator agent, which was attached to the hydroxyl groups of CNCs by esterification in THF in the presence of 4-dimethylaminopyridine (DMAP) and TEA. The polymerization was then carried out in the presence of Cu(I)Br and 1,1,4,7,10,10-hexamethyltriethylenetetramine. All resulting polymer-grafted CNCs exhibited interesting liquid crystalline behaviors, which are in some cases thermally sensitive. For example, CNCs grafted with azo-polymer poly(MMAZO) exhibited both types of liquid crystal formation, thermotropic and lyotropic, as they showed smectic-to-nematic transition at 95 ∘ C and nematic-to-isotropic transition at 135 ∘ C, and exhibited analogous lyotropic liquid-crystalline phase behavior above 135 ∘ C. Another PDMAEMA-grafted CNC exhibited chiral nematic structure in the lyotropic state. The results are consistent with the hypothesis of chiral interaction arising from the shape of the rods and not from the chiral character of the cellulose chain. The temperature-induced fingerprint texture changes of PDMAEMA-grafted CNC aqueous suspensions were investigated at various temperatures. It was a pity that the amount of bromine in the initiator-modified CNCs was very low (only 0.6 wt%) in these studies, which restricted the subsequent grafting reaction. On the other hand, the issue of grafting polymeric chains with different lengths, which implied the controllable feature of ATRP, was not investigated. By altering the extent of initiator surface modification through polymerization control, Morandi et al. [95] produced polystyrene(PS)-grafted CNCs with varied grafting densities and different polymer brush lengths. Various grafting densities of PS on the surface of CNCs were realized by controlling the final content

135

5 Surface Chemistry of Nanocellulose

of the initiating sites under different conditions (duration and temperature of BriB/CNC reactions). In a further study, a method of grafting a photosensitive moiety bearing initiating sites onto the surface of CNCs was reported, followed by grafting PS chains with ATRP. Majoinen et al. [96] reported an effective method of chemical vapor deposition (CVD) and continued esterification to improve the initiator density on CNC surface. The simple CVD pretreatment of BriB for CNCs was conducted for 24 hours at room temperature (5 wt% bromine content), which promoted the subsequent esterification to obtain more initiating sites (15 wt% bromine content). Whereafter, Zoppe et al. [97] reported the CNCs were grafted with thermoresponsive poly(N-isopropylacrylamide) brushes via SI-SET-LRP under various conditions at room temperature as shown in Figure 5.14. Accordingly, Cu(I) was rapidly disproportionated to Cu(0) and Cu(II), resulting in simple catalyst removal. It was determined that with increased initiator loading increased molecular masses of polymer brushes were obtained. This effect could be explained by local heterogeneities shifting the SET-LRP equilibrium to the active state. Likewise, with increased monomer loading, increased molecular masses of the resulting polymer brushes were obtained. Finally, the results presented are expected to provide the basis for the development of temperature-responsive materials based on CNCs. Examples can be found in the literature reporting on the grafting of polymer brushes, i.e. poly(butyl acrylate) (PBA), from the surface of CNFs through SI-ATRP [98, 99]. The SI-ATRP of butyl acrylate (BA) on CNFs was conducted to create controllable hydrophobic chains on CNFs. The macroinitiator, CNF-Br, was prepared by the esterification of hydroxyl groups on CNFs with 2-bromoisobutyrylbromide (BiBB), followed by the ATRP of BA using two catalyst systems, Cu(I)Br/2,2′ -bipyridine (BPY) and Cu(I)Br/pentamethyl-diethylenetriamine (PMDETA). The results indicated that the PMDETA system exhibited relatively poor control over the ATRP, whereas the BPY system produced PBA with tailored chain lengths and relatively narrow

Br

Step 1: OH

OH

O

OH

Br

Br CNC

THF, TEA, DMAP

O O

Br OH

O O

CNC

Step 2: O O

Br OH CNC

HN

O O

Cu(I)Br PMDETA H2O : MeOH

O

Br

HN

OH

O

O

O

O N H

Br

n

Br

n

136

CNC

Figure 5.14 Synthetic routes of grafting polyolefin chains on CNCs with the technique of SET-LRP.

5.2 Surface Modification of Nanocellulose

polydispersities but experienced a rather slow polymerization process. To optimize the polymerization with the Cu(I)Br/PMDETA system, several influencing factors were investigated, including the type of solvents, reaction temperature, and the use of co-catalyst Cu(II)Br2 . Some studies exploited other approaches to graft polyolefin on the surface of CNCs. Harrisson et al. [75] grafted PS and poly(t-butyl acrylate) (PtBA) chains on CNCs via the carbodiimide-mediated amidation reaction between the terminal amine groups from polyolefin polymers and carboxylic acid groups on the surface of oxidized CNCs. With the method of HCl hydrolysis or saponification, grafted polyolefin chains can be cleaved from the surface of CNCs, and the properties of grafted polymers, such as molecular weight (Mn) and polydispersity, can be carefully investigated by gel permeation chromatography (GPC) or size exclusion chromatography (SEC). Diverse monomers, reactive conditions, and properties of grafted polyolefin chains on CNCs are summarized in Table 5.5. Despite the advantages of controllability and high grafting density of surface-initiated LRP, there are some shortcomings for this technique, which are the inconvenience of experimental operation (with three freeze–pump–thaw or vacuum–nitrogen cycles) and rigorous reactive conditions of anaerobic circumstance.

5.2.8

Chemical Modification from End Hemiacetal

It is well known that the unidirectional parallel orientation of cellulose chains within the elementary fibrils, occurring during biosynthesis and deposition, induces the formation of crystals having one side face with hydroxyl pendant groups known as the non-reducing end and another front face with reducing end groups bearing hemiacetal functionality. The latter can be selectively exploited for chemical modification. In a pioneering work, Sipahi-Saglam et al. [100] have demonstrated the feasibility of performing groups or molecules grafting on the hemiacetal groups of CNC end as sketched in Figure 5.15.

O

OH OH H HO

(a)

NH2NH2

OH OH H

CNC O

O

HO

R1

COOH

CNC

CNC

NHS-EDC

N NH2

R1

OH

O

HO

C

O

OH OH H

CNC O

O

HO

(b) OH OH O HO

(c)

NaClO2 H

O

OH OH

CNC

R3

OH

O HO

NH2

OH NHS-EDC C

N N H

O

R2

O

NH2

NHS-EDC

CNC

CNC

R2

CNC

CNC

OH NH2 HN OH H

R3

Figure 5.15 Three traditional end modification approaches on the hemiacetal groups of CNCs.

137

Table 5.5 Reactive conditions and properties of various grafted polymeric chains on the nanocellulose from the LRP reactions.

Polymeric chain

Monomer

H C

O O

O O

1.21

[92]

Cu(I)Br/ HMTETA

—

110 ∘ C,

68

ATRP

Cu(I)Br/ PMDETA

Toluene

30/70 ∘ C, 16/24 h

24–40 2 800– 17 800

1.09– 1.13

[95]

PDMAEMA ATRP

Cu(I)Br/ HMTETA

CH3 OH

55 ∘ C, 12 h

53

10 200

1.38

[93]

PMMAZO

ATRP

Cu(I)Br/ HMTETA

Chlorobenzene 90 ∘ C, 24 h

74.6

—

—

[94]

PtBA/PAA

ATRP

Cu(I)Br/ PMDETA

DMF

75 ∘ C

—

6 200– 36 300

1.10

[96]

P(NiPAAm) SET-LRP

Cu(I)Br/ PMDETA

H2 O/MeOH

25 ∘ C, 24 h

—

10 630– 42 860

1.20

[97]

PBA

ATRP

Cu(I)Br/ PMDETA

Toluene

90 ∘ C, 24 h

83

9 600

—

[98]

ATRP

Cu(I)Br/ PMDETA

DMF/toluene/ 1-butanol

70/90/ 110 ∘ C, 2–6 h

57

—

—

[99]

ATRP

Cu(I)Br/BPY DMF/toluene/ 1-butanol

70/90/ 110 ∘ C, 2–6 h

—

—

—

[99]

N N

74 700

Condition

N

O

References

Solvent

ATRP

Catalyst/ ligand

Grafted polymer (Mn ) PDI

GE (%)

PS

Reactive mechanism

12 h

O CH3

PMMAZO O O O N H O O

5.3 Advanced Functional Modifications

Before the end modification reaction on CNCs, these crystallites were chemically activated via hydrazone linkages at the reducing ends. Macromolecules, carrying reactive end groups, were attached to the reactive cellulose crystallites by coupling reactions. Amino-terminated polyethylene glycols and polydimethylsiloxanes can be coupled to CNCs carrying phenyl carboxylic functions at their front faces. The cellulose-block-polydimethylsiloxane copolymer exhibits a pronounced tendency to form specific superstructures in slurries and films made from it. Alternatively, polyacrylamide was tethered from these hemiacetal units, after being modified with reactive azo initiators, by radical polymerization. Diverse reactive conditions and grafted molecules on the basis of the end hemiacetal of the CNCs are summarized in Table 5.6. Loganathan et al. [101, 102] introduced a new method by which CNCs were topochemically thiolated at their reducing ends by using a simple procedure involving initial carboxylation of the reducing end aldehyde groups followed by NHS-EDC-mediated activation and reaction with nucleophilic amine molecules carrying thiol termini, as shown in Figure 5.16. The end modification was performed in the water with the critical conditions of pH 9.2 and specific reducing agent (sodium triacetoxyborohydride). The 6-amino-1-hexanethiol hydrochloride (NH2 (CH2 )6 SH⋅HCl) and sodium triacetoxyborohydride (as the reducing agent) were added in three steps over a period of three days accompanied by stirring. After the reaction, the modified CNCs should be treated in the 2 M KCl solution for incubation. In order to prove the end modification, the CNC-SH suspension was added to silver nitrate solution, followed by addition of sodium borohydride, which can be observed by cryo-TEM based on selective attachment of silver nanoparticles to the CNC-SH ends to form colloidal rod-sphere adducts (Figure 5.16). Further, the promoted binding of CNC-SH on gold surfaces is shown by atomic force microscopy and quartz crystal microbalance, where the high dissipation suggests pronounced orientational mobility due to flexible joints at one rod end onto the surfaces.

5.3 Advanced Functional Modifications Recently, more and more studies have led to the emergence of special applications of CNCs with diverse potential as advanced functional nanomaterials, which are closely associated with their modification using common derivatization or polymer grafting through the introduction of advanced functional groups or molecules [105]. 5.3.1

Fluorescent and Dye Molecules

Through covalent conjunction with nanoparticles, fluorophores can be used to label these nanoparticles not only for fluorescence bioassay and bioimaging applications but also for the investigation of bioeffects and safety of nanoparticles inside cells or humans. For the first time, Dong and Roman introduced a fluorescent molecule (FITC) on the surface of CNCs via a three-step approach

139

Table 5.6 Reactive conditions and grafted molecules on the basis of the end hemiacetal of the CNCs. Pretreatment

Grafted molecules

Catalyst

Solvent

Reactive condition

References

[100]

4-Hydrazinobenzoic acid

O,O-Bis(2-aminoethyl)-polyethylene glycol

Carbodiimide

H2 O

35 ∘ C, 48 h, pH = 9, RT

4-Hydrazinobenzoic acid

3-Aminopropyl-terminated polydimethylsiloxanes

Carbodiimide

H2 O

35 ∘ C, 48 h, pH = 9

[100]

4-Cyanopentanoic dihydrazide

Acrylamide

—

H2 O

60 ∘ C, 48 h, N2

[100]

4-Hydrazinobenzoic acid

Acrylamide

—

H2 O

Sodium chlorite acetic acid

6-Amino-1-hexanethiol hydrochloride (NH2 –R–SH)

EDC, NHS (N-hydroxysuccinimide)

H2 O

70 ∘ C, 48 h, N2 70 ∘ C, 72 h, pH = 9.2

[101]

—

NH2 (CH2 )6 SH⋅HCl

Sodium triacetoxyborohydride

H2 O

70 ∘ C, 72 h, pH = 9.2

[102]

Bis(triethylammonium) salt, acetic acid

Alexa Fluor 633

—

H2 O

RT, 24 h, in the dark

[103]

—

7-Hydrazino-4-methylcoumarin (HMC)

—

H2 O

7-Amino-4-methylcoumarin (AMC)

—

H2 O

25 ∘ C, 24 h, in the dark 25 ∘ C, 24 h, in the dark

[104]

—

[100]

[104]

5.3 Advanced Functional Modifications

OH OH

CNC

H

O HO

O 50 nm

aB

H

4

NH2(CH2)6SH.HCl

HO

Gold (CH2)6 SH

SSS

S SS S SS

CN C

50–500 nm

CN C

CN C

CN C CN C

S SS S S S

S SS

H N

O

CN C

CN C

OH OH

CNC

CN C

Ag N O

3,

N

70 °C pH = 9.2 Ac

S SS

SSS

Au

surface

Figure 5.16 Common synthetic routes of topochemically functionalizing nanorods of CNCs with thiol end groups. Source: Lokanathan et al. 2013 [102]. Reproduced with permission of ACS. H2N

O

HO Cl

OH OH OH

OH OH O

O

OH OH O

NH4OH

CNC

CNC

CNC O

O

O

OH

O

OH HOOC

HOOC

S HN C N C S

NH

HO OH OH O

(a)

CNC O HN C

NH2 O C OH OH OH

CNC

MeO MeO Si MeO

NH2

O

O

CNC

O

O

Si O

N O

Si O

O

O

CNC

(b)

Figure 5.17 Reaction pathways of attaching FITC (a) and pyrene (b) moieties on the surface of CNC.

as shown in Figure 5.17 [106] First, the surface of the nanocrystal was decorated with epoxy functional groups via the reaction with epichlorohydrin. Then, the epoxy ring was opened with ammonium hydroxide to introduce primary amino groups on CNCs. Finally, the primary amino group was reacted with the isothiocyanate group of FITC, in which covalent association between CNCs

141

142

5 Surface Chemistry of Nanocellulose

and FITC was realized via the thiourea groups formed. It was shown that the unlabeled suspension was colorless and slightly opaque, whereas FITC-labeled CNC suspension appeared clear and yellow. Meanwhile, according to the results of UV/vis spectroscopy, FITC-labeled nanocrystals showed absorption maxima of both the dianionic (490 nm) and the anionic (453 and 472 nm) form of FITC, whereas unlabeled CNCs did not show any absorption peaks in the wavelength range 200–600 nm. The simple method of covalently attaching fluorescent FITC molecules to the surface of CNCs enables the use of fluorescence techniques, such as spectrofluorometry, fluorescence microscopy, and flow cytometry, to study the interaction of CNCs with cells and the biodistribution of CNCs in vivo. In another study, with a similar method, two fluorescent molecules (FITC and RBITC) were introduced on the surface of CNCs, which was used to investigate the effects of cellular uptake and cytotoxicity for different fluorescent CNCs [107]. CNC-RBITC was taken up by HEK cells without affecting the cell membrane integrity, whereas no significant internalization of CNC-FITC was noted at physiological pH. CNC-RBITC exhibited no noticeable cytotoxicity at the studied concentrations on the two cell lines under investigation. In Table 5.7, diverse reactive conditions and optical spectrum of the fluorescent molecule modifying CNCs are summarized. The grafting of fluorescent terpyridine molecules [110] and pyrene molecules [108] on the surface of CNCs was also reported. The fluorescent chemosensor based on pyrene fluorophore, Py-CNC, was synthesized and evaluated for its sensing ability toward metal ions. The fluorescence emission of pyrene was enhanced after the modification to CNCs. Py-CNC exhibited high selectivity toward Fe3+ among other screened metal ions with good discrimination between different iron oxidation states (Fe2+ and Fe3+ ). Recently, Ryley and Thielemans [109] developed synthetic approaches to introduce an imidazolium salt on CNCs using copper(I)-catalyzed azide–alkyne cycloaddition, and the bromide anion was successfully exchanged for bistriflimide and an anionic dye, providing an opportunity to synthesize a wide variety of ion exchange systems or catalysts using CNCs as a support medium. 5.3.2

Amino Acid and DNA

As the natural nanomaterial, nanocellulose is expected to be a good candidate for the construction of biocompatible and biologically active nanomaterials, such as drug carrier and gene delivery. The anchoring of amino acid or DNA molecules on the surface of CNCs allows the creation of a binding site to which drugs or targeting molecules can be attached. The chemical modification of L-leucine on CNCs was performed through a two-step process involving the reaction between Fmoc-protected L-leucine and thereafter the removal of Fmoc-protecting group. The first step consisted in the reaction between CNCs and Fmoc–L-leucine using DMAP as the catalyst and EDC⋅HCl as the coupling agent. The removal of the protecting Fmoc group was performed by stirring Fmoc–L-leucine CNCs in piperidine/DMF solution. This strategy provides a direct, facile way to merge the properties of these biocompatible materials and offers the possibility to introduce biologically active building

Table 5.7 Reactive conditions and optical spectrum of the fluorescent molecule modifying CNCs. Optical spectrum (nm)

References

RT, 24 h, in the dark

632.8

[103]

452

[104]

H2 O

25 ∘ C, 24 h, in the dark 25 ∘ C, 24 h, in the dark

Sodium borate buffer

24 h in the dark

490

—

Sodium borate buffer

[107]

DMSO

24 h 60 ∘ C, 24 h, N

528/560

NaOH

453/478

[108]

Fluorescent molecule

Emitting color

Catalyst

Solvent

Condition

Alexa Fluor 633

Blue

—

H2 O

7-Hydrazino-4-methylcoumarin (HMC)

Blue

—

H2 O

7-Amino-4-methylcoumarin (AMC)

Blue

—

440

[104]

FITC

Yellow

—

[106]

RBITC

Amaranthine

Pyrene

Blue

Pyrene

Blue

NaOH

H2 O

[108]

Orange

Cu(I)

H2 O

60 ∘ C, 24 h, N2 70 ∘ C, 48 h

376/386/397

MPIM

484

[109]

PSE

Blue

—

DMF

RT, 19 h

338

[70]

2

144

5 Surface Chemistry of Nanocellulose

blocks in CNCs [111]. Mangalam et al. developed the synthetic route of grafting different single-stranded oligonucleotides with an amino modifier on CNCs, and subsequently duplexing complementary oligonucleotides formed with individual modified CNCs via the molecular recognition ability of the oligomeric base pairs [112]. The grafting reaction of amino-modified ssDNA on CNCs was realized using the carboxylation–amidation procedure. Two kinds of oligonucleotide-modified CNCs, both strand A- and strand B-modified CNCs, were mixed together in water for the preparation of duplexed DNA-modified CNCs. It was shown that complementary strands of DNA bonded to separate populations of CNCs hybridized under suitable conditions to form bonded structures. 5.3.3

Self-cross-linking of Nanocrystals

Contrary to the traditional application of CNCs as the reinforcing filler, some studies apply CNCs themselves to be the matrix for the construction of self-cross-linked materials. Goetz et al. developed the self-cross-linking of CNCs extracted from MCC with poly(methyl vinyl ether comaleic acid) (PMVEMA) and PEG as the cross-linking agents by reacting varying amounts of CNCs (0–100%) [113]. The cross-linking of CNCs with PMVEMA and PEG was anticipated to occur via an esterification reaction among the hydroxyl groups of cellulose, terminal hydroxyl groups of PEG, and the carboxylic acid groups of PMVEMA as shown in Figure 5.18a. The cross-linking between three components trapped the nanocrystals in the cross-linked network, preventing aggregation and subsequently producing CNC self-cross-linked nanomaterials through intramolecular and interchain reactions. In another study, a three-step route of click chemistry was utilized for the synthesis and formation of cellulose nanoplatelet gels with the regular self-cross-linking of CNCs as shown in Figure 5.18b [114]. Initially the primary hydroxyl groups on the surface of CNCs were selectively activated by converting them to carboxylic acids using TEMPO-mediated oxidation. Further reactions were carried out via carbodiimide-mediated formation of an amide linkage between precursors carrying the amine groups and the carboxylic acid groups on the surface of oxidized CNCs. Finally, with two kinds of modified CNCs containing the azide derivative or alkyne derivative on their surface, the click chemistry reaction of Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between the azide and the alkyne was performed on CNCs, bringing together the nanocrystalline materials in a unique, regularly packed arrangement, demonstrating a degree of molecular control for creating self-cross-linked CNC nanomaterials. Inspired by the water-enhanced mechanical gradient character of the squid beak, Fox et al. recently reported a mechanical gradient nanocomposite made from modified tunicate CNCs by nanocrystal–nanocrystal cross-linking in PVA matrix. CNCs were first modified by the introduction of allyl moieties on the surface. With the addition of a small molecule tetra-thiol cross-linker and radical photoinitiator, modified CNCs self-cross-linked under exposure to UV irradiation via photoinduced thiol–ene chemistry reaction. With the control of UV

References Na+ –

O

HO

O

HOOC

O

O

O O– Na+

HO

OH

OH

HOOC CNC

COOH

CHOOH2

H2O pH = 2

COOH

HOOC

OMe

MeO

COOH

O n

COOH

COOH

O O

OMe COOHOMe

OMe CHOOH2

COOHOMe COOH

O O

HOOC

OMe O O OMe

O

O

n

MeO O MeO

COOH HOOC

COOH

CNC

HO

MeO

OH

O

COOH

COOH

HOOC

OMe

(a) Na+–O

HO

O

+

Na –O

OH

O

O

O O– Na+

HO

OH

O O

OH

CNC-AZ CuSO4

(b)

O O

O

O

N

O O CNC-PR

H N O O

CNC

Ascorbic acid

N N O

N3

H N

MES buffer, EDC/NHS

CNC

3

O

CNC-AZ

H2N

H N

O

O

MES buffer, EDC/NHS

O O–Na+

. 5H O 2

O

O

CNC

HO

H N

N3

O

O

O

2

O

CNC

HO

O

O

O

1

H2N

OH

O

CNC-PR

Figure 5.18 Two reactive routines of the self-cross-linking between the CNC, PR, AZ (a) and the CNC, PMVEMA, PEG (b).

exposure duration, the degree of cross-linking of the nanocrystals can be regulated and mechanical gradient nanocomposites fabricated [115].

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6 Current Status of Nanocellulose-Based Nanocomposites Xiaozhou Ma 1 , Yuhuan Wang 1 , Yang Shen 1 , Jin Huang 1 , and Alain Dufresne 2 1 Southwest University, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, School of Chemistry and Chemical Engineering, Tiansheng Road 2, Chongqing 400715, China 2 Université Grenoble Alpes, CNRS, LGP2, Grenoble INP, 38000 Grenoble, France

6.1 Introduction Nanocellulose, an easily accessible nanofiller for the fabrication of novel nanocomposites, consists of cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), and bacterial celluloses (BCs). With unique morphology, high surface-tovolume ratio, and high modulus and strength, the rigid rod-like CNCs are frequently applied as nanofillers to reinforce polymeric materials which are similar to other inorganic nanofillers, including layered silicates [1] and nanoclay [2], spherical nanosilica [3] and carbon black [4], rod-like attapulgite [5], and so on. Moreover, with a relatively low loading level of CNCs, a rigid percolation network can formed, and therefore, the mechanical properties of nanocomposites could be significantly enhanced [6]. Compared with CNCs, both CNFs and BCs show much higher aspect ratio and relatively flexible structure, and easily present more entanglements with each other in the nanocomposites to form a three-dimensional structure [7, 8], which is similar to carbon nanotubes (CNTs) [9]. Besides, the entanglement of CNF and BC improves the interaction between the filler and the matrix, thereby effectively improving the mechanical properties of the nanocomposites [10, 11]. Generally, unlike inorganic fillers, nanocellulose enjoys exclusive properties of high aspect ratio, high specific modulus, relatively good surface reactivity, and nontoxicity as well as the fact that it is easily available, renewable, and biodegradable [12]. Thus, it has been widely applied as filler in polyolefin, polyester, rubber, polyurethane and waterborne polyurethane (WPU), epoxy resin and waterborne epoxy resin, natural polymers, and other polymer matrices. Since nonpolar matrices are poor in interfacial compatibility and show weak interaction with polar nanocellulose, various methods, e.g. the modification of either fillers or matrices, or some processing techniques, have been proposed to tune the compatibility between the filler and the matrix [13–15]; on the other hand, because of the abundance of hydroxyl groups on the surface of nanocellulose, polar nanocellulose shows much better compatibility and adhesion with Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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the polar matrix. Then, various typical nanocellulose-filled nanocomposites currently proposed are reviewed in categories, and various methods solutions are also discussed for better dispersion and compatibility of the nanocellulose in matrices of various types. We have only focused on the topic of BC as a filler to fabricate nanocomposites, and merged them into the section of fibrillated cellulose for discussion.

6.2 Cellulose Nanocrystal-Filled Nanocomposites 6.2.1

Polyolefin-Based Nanocomposites

Polyolefins including polystyrene (PS), polypropylene (PP), polyethylene (PE), and poly(vinyl alcohol) (PVA) are the most widely used class of plastics, because of their versatility, chemical stability, excellent mechanical properties, non-toxicity, energy-saving and low cost as well as easy availability of raw materials [16]. Furthermore, Polyolefin can be categorized into polar PVA and nonpolar PP, PE, PS, etc. To further improve their mechanical properties, the use of CNCs as filler to fabricate nanocomposites based on various polyolefins has been reported because of their impressive characteristics such as high modulus (close to 150 GPa), high aspect ratio (10–70), and high specific surface area (few 100 m2 /g) [17]. However the abundant hydroxyl groups (–OH) on the CNC surface together with possible hydrogen bonding among the CNCs usually result in poor compatibility to nonpolar polyolefin, and heterogeneous dispersion and even obvious agglomeration within the matrix, leading to a significant deterioration in the mechanical properties of nanocomposites. Polar matrices, such as PVA, often show a relatively good compatibility with CNCs, due to the similarity in polarity. To cope with such challenges of nancomposite compatibilzation and related high performances, various methods of the CNC modifications and the nanocomposite processing are devised. The nanocomposite processing and the CNC modification methods of the CNC/polyolefin nanocomposites are summarized in Table 6.1. Among these processing methods, the method of solution mixing and subsequent casting/evaporation are prevalent. The key of this method is to disperse CNCs in the polymer solution under external force field, which will contribute many advantages to the preparation of composites. Firstly, CNCs with a good dispersibility in the solvent can be stably dispersed in some polymer solutions. Secondly, CNCs have sufficient degrees of freedom to adjust themselves in the solution to give a homogeneous distribution. In addition, during evaporation, the CNCs have enough availability to form percolation networks. Therefore, the nanocomposites produced by solution mixing method present relatively high mechanical reinforcement offered by CNCs. Despite such advantages, there are still some problems with this method. Solution mixing requires that CNCs should be dispersed stably in the polymer solution; however, due to the polarity disparity it is difficult to disperse CNCs stably in most organic solvents, only resulting in agglomeration in the subsequent preparation process. As reported, CNCs can become more stable in some organic solvents through solvent

6.2 Cellulose Nanocrystal-Filled Nanocomposites

Table 6.1 The nanocomposite processing methods and the CNC modification methods commonly used in the preparation of CNCs-modified polyolefin.

Matrix

Filler source

Fillers fraction (wt%)

LDPE

Cotton

Processing method

Modification method

Reference

0–10

Extrusion/ hot-pressing

CNC

[18]

Ramie

0–15

Extrusion/ hot-pressing

CNCa)-g-organic acid chlorides

[19]

Sisal

1–5

Solution casting/melt mixing/hot pressing

CNCa)-g-vinyl triethoxy silane

[20]

—

0–20

In situ polymerization

CNC-g-10undecenoyl chloride

[21]

UV-treat PP —

0–6

Spinning coating

CNC

[22]

PP

Softwood Kraft pulp

0–5

Extrusion/ injection molding

CNC

[23]

iPP

Tunicate

0–6

Casting/ evaporation

Surfactantmodified CNC

[24]

PS

—

0–6

Extrusion/ compression molding

Poly[(styrene)-co(2-ethylhexyl acrylate)] latex-modified CNC

[25]

Cotton

0–20

Extrusion

CNC-g-poly (ethylene glycol)/ polyoxyethylene

[26]

Soft wood craft

0–10

Solution mixing

Quaternary ammoniumcontaining ionomersmodified CNC

[27]

PE

EOCb)

a) This substance is actually cellulose nanocrystal, referenced as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [19, 20]. b) The full name of EOC is ethylene−olefin copolymer.

exchange [8], normally from water to acetone, and subsequently to a suitable organic solvent, such as N,N-dimethylformamide (DMF) [28], pyridine [29], toluene [30], and chloroform [31]. However, the solvent exchange method cannot be widely applied due to the limitation of the type of solvents. Therefore, the solution mixing method generally requires chemical or physical pre-modification of CNCs to help them homogeneously disperse and become compatible with the polymer matrices. As is known, the main reason for the poor dispersion of CNCs in the matrix is ascribed to the fact that the CNCs can form hydrogen bonds through the hydroxyl groups on the surface to form agglomeration, and so the addition of surfactants can effectively reduce the original associations among the

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CNC. For instance, isotactic polypropylene (i-PP)-based nanocomposite films were prepared with the incorporation of bare CNCs, surfactant-modified CNCs and maleated polypropylene-grafted CNCs. (this substance is actually cellulose nanocrystal, referenced as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [24]). Compared with either bare or grafted CNCs, such surfactant modification imposed a high dispersion level of CNCs in nonpolar iPP. In addition, compared with neat iPP, the tensile strength of both two nanocomposite films filled with modified CNCs was significantly increased, but the tensile strength and elongation at break of the nanocomposites were decreased when filled with bare CNCs. Interestedly, surfactant-modified CNC-filled nanocomposites exhibited the maximum elongation at break. This is ascribed to the fact that surfactant-modified CNC could be better dispersed in the matrix, and form an effective stress transfer from the matrix to rigid filler and present the plasticity promotion of the modifying moieties. Moreover, the difference in the tensile behaviors of three nanocomposites filled with bare and modified CNCs can also prove that the mechanical properties of nanocomposites depend not only on the interaction between the fillers and matrices but also on the dispersion quality of the rigid fillers and the synergistic effect of the modifying moieties. of the fillers in the matrix [24]. However, the surfactant to coating CNC is required to be enough high in quantity, and is susceptible to desorption because of its weak non-covalent interactions; in some situations, the addition of surfactant-modified CNCs may even reduce the mechanical properties of the resulting composites [10]. Moreover, the interaction between fillers, such as hydrogen bonding, may be inhibited and reduced due to the shielding of a high quantity of the surfactant. Another feasible way to improving the effects of physical modification can be realized through ionic binding. The positively charged quaternary ammonium-containing ionomers, for instance, can be ionic-bound onto the negatively charged surface of CNC. Compared with CNCs, modified CNC dispersed much better in the hydrophobic ethylene–olefin copolymer (EOC) matrix and showed improvement in thermal stability. Tensile tests showed that after being filled with modified CNC, the tensile modulus and elongation at break of the nanocomposite materials were effectively improved in contrast to those filled with unmodified CNC [27]. In addition to applying the above physical modification methods, chemical modification methods have also been reported to further improve the poor dispersion of CNC in nonpolar hydrophobic polyolefin solution and final polyolefin-based nanocomposites. For instance, CNC (this substance is actually cellulose nanocrystal, referred as “cellulose nanowhisker,” and abbreviated “CNW” in Ref. [32]) grafted with polyolefin elastomer (POE-CNC) was synthesized through covalent linkage. The ∘ contact angle of CNC was 24 ;chemically modified CNC showed a contact angle ∘ of 106.8 .The increase in contact angle indicated a significant improvement in the hydrophobicity of CNC. Then, the nanocomposites were prepared by blending POE-CNC with PP. Through such modification, the polarity and hydrophilicity of the CNC surface can be dramatically reduced; in return, the compatibility between POE-CNC and the nonpolar hydrophobic PP matrix can be improved. According to mechanical tests, the tensile strength, Young’s modulus, and impact strength of the nanocomposites were significantly improved [32].

6.2 Cellulose Nanocrystal-Filled Nanocomposites

However, the solution mixing method is normally both nonindustrial and noneconomic, and is generally used for the preparation of films. In addition, volatilization of the solvent has an undesirable impact on the environment and its slow speed also hinders it from large-scale production. Melt compounding and molding techniques, such as successive extrusion or injection molding, are probably the most convenient processing techniques for the industrial production of CNC-filled nanocomposites. The melt compounding and molding method as a production route for nanocellulose-filled nanocomposites mainly depends on high temperature to approach and reach the melting temperature of the polymer; CNCs are added into the polymer melt and mixed uniformly under the assistance of strong shear force. Due to no solvents (especially organic solvents) involved, this kind of method might be thought as a green process and is beneficial to large-scale production and economical availability [19, 33, 34]. Moreover, polyolefin as the thermoplastic matrix is suitable to these thermoprocessing and thermoforming techniques. Nevertheless, in most scientific studies, these conventional melt compouding and molding techniques are infrequently employed for the preparation of CNC-filled nanocomposites. This is ascribed to the issues of low thermal stability of CNCs and inherent incompatibility between polar cellulose and most nonpolar polyolefin matrices [35]. Especially, CNCs produced by sulfuric acid hydrolysis are more sensitive to high temperature because of the presence of sulfate groups on the CNC surface resulting from sulfuric acid hydrolysis. As a result, CNCs present inferior thermal stability with degradation temperature lower than ca. 150 ∘ C. But, most polymeric matrices derived from petrochemical synthesis are melted and processed at ca. 200 ∘ C or above [36], which requires the higher thermal stability of CNCs. Fortunately, this problem can be alleviated through some supplementary modifications towards the surface of CNCs, such as physical coating, chemical derivation or grafting hydrophobic moieties. It has been demonstrated in Figure 6.1 that CNCs adsorbed by hydrophilic polyoxyethylene (PEO) showed significant improvement of both thermal stability and dispersibility during the processing of nanocomposites due to the shielding and wrapping of PEO. The neat low-density polyethylene (LDPE) film was obviously translucent; the nanocomposite film containing 3 wt% CNC was homogeneously dark. This can be explained by the fact that sulfuric acid-treated CNCs have a lower thermal stability due to the sulfonic acid groups on the surface [37]. However, the nanocomposite filled with PEO-modified CNC did not show any blackening after the extruding operation. This proved that the thermal stability of CNCs was improved by physical adsorption. Indeed, the binding force between PEO and CNCs was still poor, which inevitably led to the agglomeration of CNCs, microphase separation, etc. Moreover, it is reported that CNC can be modified with poly[(styrene)-co-(2-ethylhexyl acrylate)] latex through ionic interaction, and the sulfate acid groups on the surface of CNC can be then concealed. Compared with unmodified CNC, the thermal stability of modified CNC was found to be clearly improved. Such modification not only achieve a better dispersion of CNCs in the matrix, but also improves the compatibility between the CNCs and the PS matrix. Nanocomposites were successfully prepared by the extrusion method, and SEM showed that modified CNC presented less

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6 Current Status of Nanocellulose-Based Nanocomposites

LDPE 6 wt% CNC/PEO5M

3 wt% CNC

6 wt% CNC/PEO35,000 9 wt% CNC/PEO5M

9 wt% CNC

6 wt% CNC/PEO35,000

Figure 6.1 Pictures of the extruded films: unfilled polyethylene (LDPE) and LDPE reinforced with neat CNC and PEO-adsorbed CNC.

agglomeration in the matrix compared to pure CNC [25]. However, the ionic bond or physical adsorption force is still weak between CNC and the modifier, and can be easily destroyed by external force. For chemical modification, the association between the filler and the modifier is obviously fastened because of covalently bonding as the linkages of the filler and the modifier. For example, the carboxylated surface of CNC could be grafted with poly(ethylene glycol) (PEG) by amidation; then, the long PEO chains (molecular weight 5 × 106 g/mol) were absorbed on the surface of CNC-g-PEG nanoparticles. Because of the similar chemical structure and hydrophilicity, the affinity between PEO and PEG was ensured, and the interaction between PEO and PEG could be further improved by chain entanglement. Meanwhile, the modified layers on the CNC surface could serve as a compatibilizer to improve the interface interaction between CNC and the nonpolar PS matrix. With rheological analysis, the PEO was better absorbed in the CNC-g-PEG than in pristine CNC. Atomic force microscope (AFM) and SEM showed that modified CNCs were homogeneously dispersed in the PS matrix. Thermal gravimetric analysis (TGA) showed that the CNC-g-PEG/PEO could avoid the degradation of nanoparticles despite the high temperature of 200 ∘ C [26]. In addition to the above methods, in situ polymerization has also been reported in the preparation of polyolefin-based nanocomposites [21]. This method refers to combining the reactive monomers with the nanoparticles and allowing them to polymerize between particles. For instance, CNC could be firstly modified by 10-undecenoyl chloride and added into ethylene with

6.2 Cellulose Nanocrystal-Filled Nanocomposites

a constrained geometry catalyst, and then the nanocomposite was prepared through in situ polymerization. After chemical modification, the alkyl chains on the CNC surface had similar chemical structure with PE; so that the compatibility between CNC and PE could be effectively improved, leading to a better dispersion of CNC in the PE matrix [21]. As a commonly used polar polyolefin, PVA has many advantages such as being biodegradable, water soluble, and easy to process. The most promising virtue of PVA is its biocompatibility [38]. However, due to its hydrophilic properties, PVA tends to being plasticized in humidity [39]. Therefore, there are many polymers or fillers used to improve its performance and barrier properties [40, 41]. CNC as a popular filler is commonly used to improve the overall properties of PVA [42, 43]. As both PVA and CNCs are water soluble, the final nanocomposites are commonly prepared through solution mixing method. For example, nanocomposite films were produced by blending PVA solution and CNC suspensions. Because of the similar polarity and hydrophilicity between CNC and PVA, the PVA can form hydrogen bonds with the hydroxyl groups on the surface of CNC. As a result, CNC can be uniformly dispersed in the PVA matrix and have a good interfacial interaction with the matrix. According to the water sorption measurement, with the increase of CNC content, the water sorption percentage of PVA-based nanocomposites reduced from 93% to 75%; the decrease in the moisture content of the nanocomposite films further verified the formation of new intermolecular hydrogen bonds between CNC and PVA [44]. In the process of using CNC to modify polar hydrophilic PVA, apart from the solution mixing method, electrospinning technology is also involved. For instance, Peresin et al. [45] have successfully produced electrospun PVA/CNC fiber mats. Mechanical analysis found that the tensile strength of neat PVA fiber mats was reduced from 1.5 to 0.4 MPa with the surrounding humidity changing from 10% to 70% RH. However, when CNCs were incorporated into PVA, at the same range in relative humidity, the tensile strength was reduced from 2 to 0.8 MPa. The reduction of the tensile strength was ascribed to the plasticizing effect of water molecules, which penetrated into the PVA-based films. But CNC-filled nanocomposites show less reduction in the tensile strength; it may be ascribed to the fact that CNC can form a strong hydrogen bond with PVA, thereby reducing the interaction between the nanocomposite components and the water molecules, and hindering the penetration and plasticization of water molecules toward the nanocomposite films. 6.2.2

Rubber-Based Nanocomposites

Rubber, an important and extensively applied elastomer, is excellent in elasticity, strength, flexibility, resistance, and anti-aging properties. In order to further improve its mechanical properties, various fillers, such as carbon black [46] and silica [47], are frequently utilized. Besides, in the past few years, researchers are looking for alternative biomass-derived fillers to replace the traditional fillers. CNCs, an environment-friendly filler with high specific strength and modulus, are also widely used in the field of rubber modification [48, 49]. Traditionally, the rubber latex is usually used to prepare CNC-modified nanocomposites via emulsion blending of CNCs fillers and rubber latex. CNCs usually act as the

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reinforcement phase that has been incorporated into various kinds of rubber, including natural rubber (NR), eucommia ulmoides gum (EUG), poly(butadiene) (PBD), cariflex-isoprene (IR), and carboxylated butadiene-styrene rubber (XSBR). Sometimes, in order to meet the requirement of preparation process, such as the involvement of chemical reactions, the solution systems of rubber and organic solvent are also chosen to prepare nanocomposites. Table 6.2 summarizes the rubber-based nanocomposites and their mechanical properties; with a relatively low content of CNCs, the modulus of the nanocomposites can be obviously enhanced. More recently, CNC isolated from maize (Zea mays) husk with a high aspect ratio (157) was incorporated into NR latex to fabricate nanocomposites. In this work, ultrasound was used to better disperse the CNC in the aqueous solution, and help CNC uniformly disperse in the rubber latex. With incorporating only 2 wt% of such high-aspect-ratio CNC, the Young’s modulus was increased from 0.89 ± 0.15 MPa to 1.98 ± 0.73 MPa [60]. To further improve compatibility and interfacial interaction between CNCs and rubber, chemical or physical modification of CNCs is also employed for the preparation of rubber-based nanocomposites. For example, 11-mercaptoundecanoic acid could be modified onto the CNC surface via esterification reaction to make the thiolated CNC (m-CNC). The nanocomposites were prepared by blending m-CNC with NR solution. Particularly, with UV irradiation, the covalent bond could be formed between the m-CNC surface and the NR chains via the thiol-ene reaction, so as to achieve the purpose of cross-linking at the m-CNC/NR interface. It is worth noting that m-CNC not only served as the reinforcer but also as the cross-linking agent in the NR-based nanocomposite. The establishment of covalent bonds between the thiol-terminated stretching chains of m-CNC and the rubber chains can effectively improve the compatibility and interfacial interaction between the matrix and the filler. Therefore, compared with the nanocomposite reinforced with unmodified CNCs, NR/m-CNC (10 wt% of m-CNC in NR) showed a 2.4-fold increase in tensile strength, 1.6-fold increase in strain-to-failure, and 2.9-fold increase in work-of-fracture [54]. Although the chemical modification method can effectively improve the compatibility and interfacial adhesion between the filler and the matrix, sometimes such modification may further hinder the formation of the percolation network of CNCs, resulting in relatively low enhancement effect. Therefore, instead of modifying the CNC surface, some researchers have presented an alternative method of modifying the rubber matrix. For instance, epoxidation reaction was used to modify natural rubber so as to obtain the epoxidized natural rubber (ENR). Through simple latex mixing of ENR with CNC, nanocomposites with high mechanical properties and self-healing function were obtained. Compared with pure NR, the nanocomposites benefited from enhanced interfacial adhesion between CNC and the rubber matrix, due to the formation of dynamic hydrogen bonding between the epoxy groups of ENR and the hydroxyl groups on the CNC surface. In addition, the high-aspect-ratio (80) of CNC used in this work enabled it to easily form a percolation network, which can further improve the mechanical properties of the nanocomposites [55, 61]. Moreover, another rubber-based nanocomposite with a tunable water responsiveness was also prepared through ENR reactivated

Table 6.2 Effect of the CNC filler on rubber-based nanocomposites. Elastomer matrix

Solid NR NR latex

c)NR

solution

ENR latex

SBR latex

Fillers fraction (wt%)

Elastomer modulus (Em) (MPa)

Composite modulus (Ec) (MPa)

a)Ec/b)Em

CNC L/d

Processing method

Reference

10

1.7

3.8

2.3

—

Extrusion/compressing molding

[48]

5

0.108

0.423

3.9

8.6

Casting and evaporation

[50]

2

0.89

1.40

1.6

157

Casting and evaporation

[51]

10

0.64

165.3

258

67

Casting and evaporation

[49]

10

—

6.3

—

13

Casting and evaporation

[52]

20

1.3

635.3

488.7

80

Casting and evaporation

[53]

10

1.01

1.86

1.8

10

Casting and evaporation

[54]

10

4.2

10.7

2.6

75

Emulsion blending/extrusion/compression molding

[55]

15

—

—

—

—

Casting and evaporation

[51]

5

45

76

1.7

8

Casting and evaporation/casting and compression-molding/the template approach

[56]

[57]

IR latex

6

4.8

24.6

1.8

5.1

Casting and evaporation

EUG solution

4

97.3

120.7

1.2

1.6

Casting and evaporation

[58]

PBD solution

80

—

—

—

23

Casting and evaporation

[59]

a) Ec: composite modulus. b) Em: matrix modulus. c) Only this method has used mercaptoundecanoy-CNC (m-CNC).

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with CNC [51]. To sum up, compared with adding pure CNCs or modified CNCs into the rubber latex, epoxidation of rubber can effectively improve compatibility by promoting the associations between the CNC filler and the rubber matrix. In addition to enhancing NR, CNCs have also been used to modify the EUG, which is an isomer of natural rubber with a molecular structure of trans-1,4-polyisoprene. Compared with NR, EUG has a lower plasticization temperature, and can crystallize at room temperature, which makes it usable only as a hard plastic. To further expand its application, CNC was used to reinforce EUG to obtain nanocomposite films through the simple casting/evaporation method. Compared with pure EUG, with the addition of 4 wt% CNC, the tensile strength and Young’s modulus of the nanocomposite were increased by 1.9 and 1.2 times respectively, and the crystallinity of the composites was also increased 1.5 times. These results indicated that the addition of rigid CNC can significantly improve the mechanical properties of the composites, and CNC can act as a nucleating agent to increase the crystallinity of the matrix [58]. This green bio-based EUG/CNC film could potentially be applied in the packaging industry. 6.2.3

Polyester-Based Nanocomposites

Polyester, a general term for polymers typically produced either from petroleumbased monomers or from renewable resources, is a class of engineering plastics with excellent properties and versatility. Their molecular structure contains ester bonds, which are polar and hydrophobic substrates compared with the polyolefin and rubbers mentioned above. Moreover, polyester derived from renewable resources generally exhibits superior biodegradability, biocompatibility, and processability. Thus, coupled with the increasing social concern for climate change and deterioration of the environment, those renewable bio-polyester is increasingly applied as matrices in the development of bio-nanocomposites, especially when CNCs are used as the filler. Therefore, as biomass-derived nanoparticles, CNCs have been incorporated into various kinds of polyesters, such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), polyhydroxyakanoate (PHA), poly(ε-caprolactone) (PCL), polyethylene terephthalate (PET), etc. In the preparation process of the CNC-modified polyester composites, many methods, such as solution mixing, melt compounding, and in situ polymerization approach, are involved. In order to meet the needs for material molding, the matched molding techniques are required on the basis of the above blending methods – film formation via solution evaporation, complex shapes and excellent surface smoothness of material via thermoforming (hot pressing, injection molding and extrusion molding), and nanofibers formation via electrospinning technology. For example, Uniform PLA-based fibrous mats, which were composed of either random or aligned fibers reinforced with CNCs, were successfully produced by two different electrospinning processes. The thermal properties and mechanical performances of the fibrous mats were enhanced, and the mechanical enhancement was ascribed to a good stress transfer from the PLA matrix to the rigid CNC fillers [62]. In another work, PBS/CNC-foamed nanocomposite was prepared via the melt-compression method. Mechanical analysis showed

6.2 Cellulose Nanocrystal-Filled Nanocomposites

that compared with the neat foamed material, the flexural strength and modulus of the nanocomposite foams containing 5 wt% CNCs were increased by 50% and 62.9% respectively. Theoretically, due to the introduction of gas molecules in the foaming process, the mechanical properties of the foamed material were lower than those of the un-foamed material. Here, the foamed nanocomposites were reinforced by rigid CNCs; CNC served as the stress transferring phase that endowed the foamed material with significant mechanical properties. Moreover, due to the nucleation of the CNC in the matrix, the crystallinity of the matrix was also increased. At the same time, the introduction of CNC significantly increases the cell density, and the cell size was homogeneous during the foaming process [63]. Overall, the melt-compounding and thermal molding method can help to prepare biodegradable foamed composites. CNC was also used as filler to prepare nanocomposites by in situ polymerization method. In this work, l-lactide was selected as the monomer, and the hydroxyl groups on the surface of CNC were used as the ring-opening polymerization initiator; CNC-modified PLA composite was successfully obtained by in situ polymerization. During in situ polymerization, the interfacial interaction between CNC and PLA was enhanced by covalent linkage. Through TGA, compared to pure PLA, it was clearly concluded that bio-nanocomposites showed a remarkable improvement in thermal stability. From the data of differential scanning calorimetry (DSC), the addition of CNC reduced the crystallinity of the nanocomposites. Compared with other CNC-modified PLA nanocomposites, CNC could generally act as a nucleating agent to increase the crystallinity of the matrix [64]. Here, This reduction could be ascribed to the fact that the polymerization took place on the surface of CNC, and thus reduced the motion freedom of the polymer chain, resulting in the hindrance to the crystallization process of the polymer chains. In addition, the storage modulus of the nanocomposite had been significantly improved [65]. Generally, casting and evaporation method is matched with the most commonly used solution mixing method to prepare CNC-reinforced nanocomposites. However, due to polarity difference, CNCs are difficult to stably disperse in most organic solvents, and agglomeration is inevitable in the process of solvent volatilization. The solution coagulation method can effectively reduce the agglomeration of CNCs due to the absence of solvent evaporation steps. For example, PBS-based nanocomposites were prepared by solution coagulation and subsequent hot-press. SEM showed uniform dispersion of the CNCs in the matrix and interfacial adhesion with the PBS matrix. With the addition of small loading of CNCs (0.5%), the crystallinity of the nanocomposite had been improved obviously. Tensile tests showed that the tensile strength and Young’s modulus of the composites gradually increased with increasing CNC content [66]. Nevertheless, CNC-filled polyesters are still hampered by the interfacial incompatibility problem that the polyester is hydrophobic while the CNCs are hydrophilic. To alleviate this problem, CNCs can be modified with small molecules or be grafted with long-chain polymers. Previous studies have grafted polymers onto the CNC surface to reduce the surface hydrophilicity, and this modification can effectively improve the dispersion uniformity of CNCs in the hydrophilic polyester [67–69]. For example, CNC can be modified by surface acetylation. Through such modification, some hydroxyl groups on the surface

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of CNC were replaced by hydrophobic acetyl groups. Both the substituents on the surface of CNC and polyester matrix have ester groups; thereby the compatibility between CNC and PLA matrix can be improved. When 6 wt% acetylation-modified cellulose nanocrystals (ACNCs) were added into the matrix, the tensile strength of the nanocomposite increased by 61.3% and the Young’s modulus increased 1.5 times compared with that of pure PLA. The improvement in mechanical properties of the nanocomposites can be ascribed to the rigid ACNC served as the reinforcing phase together with enhanced interfacial interaction between ACNC and PLA matrix, which favored stress transferring and enduring. Furthermore, this kind of nanocomposites showed higher thermal stability than the pure PLA [70]. Similar to this method, ACNC was also used in the PBS polymeric matrix for the development of biodegradable foamed nanocomposites [71]. The surface acetylation of CNC has an important effect on the mechanical reinforcement of hydrophobic polyester-based composites, but the interfacial properties still need to be further optimized. Therefore, a series of surface-acetylated CNCs was prepared by controlling the degree of surface acetylation. The hydrophobicity of the acetylated CNCs increased along with the degree of surface acetylation. Besides, the ACNCs with different acetyl substitution degree were incorporated into the typical hydrophobic polyester of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHB). Tensile tests showed that the tensile strength and elongation at break were increased by 43.3% and 12.6% when surface-acetyl degree increased from 0% to 62.9%. Moreover, the dispersibility of CNC in the PHB matrix gradually increased by increasing the degree of acetylayion [72]. However, the interfacial interaction between matrix and filler is still expected to further strengthened due to the fact that the above strategy fails to provide more associations between matrix and filler only depending on the small moieties on the CNC surface. When small molecule-modified CNC fails to perform its intended function [73], the grafting of long chains onto the surface of CNC can effectively improve the surface properties of CNCs and further enhance the interaction between the filler and the hydrophilic polyester matrix through the entanglement of chains and even multi-location interactions [74]. It is mainly based on the surface hydroxyl groups of CNC as the initiating points and the ring opening polymerization of lactone. In addition, this strategy not only guarantees the structural integrity of CNCs, but also contribute the advantages of high graft densities and controllability of grafted chain length to some extent [13]. For example, Lin et al. [75] used microwave-assisted ring-opening polymerization to graft PCL on the surface of CNC (this substance is actually cellulose nanocrystal, referred as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [75]) and produced cellulose nanocrystal-graft-polycaprolactone (CNC-g-PCL) nanoparticles. Then, the resulting CNC-g-PCL nanoparticles were added into PLA by solution mixing method, and exhibited superior functions for enhancing the mechanical properties of the PLA-based materials. When 8 wt% CNC-g-PCL was added, the tensile strength and modulus increased by 1.9- and 10.7-fold respectively compared with pure PLA materials. It can be ascribed to the good miscibility between the grafted PCL chains and the PLA matrix, and improvement in the association between the CNC-g-PCL filler and the PLA

6.2 Cellulose Nanocrystal-Filled Nanocomposites

matrix, thereby facilitating the transfer of stress to the rigid nanoparticles. In another study, cellulose nanocrystal-graft-Poly(L-Lactic acid) (CNC-g-PLLA) was successfully obtained from CNC and l-lactide through the abovementioned ring-opening polymerization method, and the CNC-g-PLLA was then added to the PLA matrix by extrusion process. A well-uniform nanocomposite was obtained. Compared with pure PLA, the Young’s modulus and elongation at break of the nanocomposites were obviously improved [76]. A similar modification of CNCs was performed by surface-grafting PBS through in situ polymerization, and the CNC-g-PBS was then incorporated into the PLA/PBS matrix by melt-compounding method [77]. In short, grafted long polymer chains on the surface of CNC can be used as a compatibilizer to increase the interaction between the filler and matrix, and facilitate the homogeneous effect of the fillers in the matrix, and, at the same time, prevent aggregation of CNC through hydrogen bonding among the fillers before and during the compounding process, which in turn improves the overall properties of the nanocomposites. 6.2.4 Polyurethane- and Waterborne Polyurethane-Based Nanocomposites Polyurethane (PU), a linear block copolymer consisting of alternative soft and hard segments, is used in a variety of products, such as coatings, adhesives, flexible and rigid foams, elastomers, and so forth. To further expand its application, CNCs have been usually incorporated into polyurethane to enhance the mechanical performance and thermal stability [71, 78, 79]. For example, polyurethane-based nanocomposites with strong reinforcement effect were prepared by adding CNC during the preparation of the prepolymer. Because there are many hydroxyl groups on the surface of CNC, the CNCs can be covalently bonded and specifically associated with the hard PU microdomains. Dynamic mechanical analysis found that the storage modulus of the nanocomposites were significantly improved. This was due to the fact that CNC could effectively reinforce the soft matrix of PU and increase the cross-linking density of the elastomer network depending upon the molecular-level interaction of CNC and PU. In addition, the tensile strength and strain-to-failure of nanocomposites were both improved. Such high strength indicated that the addition of a small amount of rigid CNCs to the PU matrix can stiffen and strengthen the material without sacrificing the deformation ability [80]. In the polyurethane industry, rigid polyurethane foams (RPUF), due to their special structural characteristics, such as good heat insulation, lightweight, high specific strength and convenient construction, was widely used in many fields. For heat insulation application of RPUF, in order to further reduce thermal conductivity and enhance mechanical properties, a very low fraction of CNCs were incorporated into the RPUF matrix through a solvent-free ultrasonication method. With 0.4 wt% of CNC incorporated, the thermal conductivity of RPUF reduced by approximately 5%, which has almost double the effect of any other unmodified nanoparticulate nucleating agent reported so far. This reduction in thermal conductivity can be explained by the better compatibility of CNC with the polyol together with

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6 Current Status of Nanocellulose-Based Nanocomposites

smaller cell size and higher closed cell fraction of nanocomposite RPUF [81]. In another work, maleic anhydride (MA)-modified CNC was also used to reinforce PU to prepare polyurethane-based nanocomposites. Compared with untreated CNC, chemically modified CNC with anhydride showed changes in polarity and dimensions, which could weaken the interaction of the adjacent aggregation, and further enhanced the miscibility between the nanoparticle and the matrix. Tensile strength and thermal stability of the nanocomposites based on modified CNC were improved when compared with the PU and PU/CNC [82]. Yet, in spite of the attractive qualities mentioned above, conventional polyurethane products usually contain a significant amount of organic solvents and occasionally free isocyanate monomers [83]. Therefore, they have been gradually replaced by WPUs in the past decades in health and environmental considerations. However, the thermal stability, insolubility, and mechanical properties of the WPU are still lower than those of the organic solvent-borne PU, and need to be improved. Therefore, considering the remarkable enhancement function of CNC to PU, CNCs are also incorporated into the WPU matrix to improve the properties mentioned above. Moreover, CNCs are relatively not bothered with the dispersion problems since they are as hydrophilic as WPUs while the hydrogen bonding interactions can arise between filler/filler and filler/matrix simultaneously. Thus, much work has been done in this respect to analyze the reinforcing effect of CNC in the WPU matrix. For example, CNCs were used to modify WPU for the preparation of nanocomposite materials by the mixing/casting method. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and SEM showed that CNCs were homogeneously dispersed in the WPU matrix and the adhesion in the interfacial area was increased, which was attributed to the formation of strong hydrogen bonding between the filler and the matrix. In terms of thermodynamics, theoretically, the presence of CNCs can influence the T g of the PU-based nanocomposites in two opposing ways. First, since the CNCs can form hydrogen bonding in the interfacial area, the movement of the WPU chains can be restricted. It can result in an increase in the T g . Conversely, the T g of the nanocomposites might decrease if the original interactions between hard-and soft-segments are hampered with the addition of CNCs, which results in an improvement in the microphase separation in WPU. Here, the CNC-filled nanocomposite films showed a decrease in T g , which indicated that the interaction between the filler and the soft-segment of matrix was far less than the disassociations between soft-and hard-segments, as a result of an improvement of microphase separation between soft and hard segment in WPU matrix. On the other hand, as mechanical tests indicated, the Young’s modulus and tensile strength of the nanocomposite films increased with the addition of CNC (0–30 wt%) from 0.5 to 344 MPa and from 4.3 to 14.9 MPa. This could be ascribed to the formation of three-dimensional networks among CNCs, and the interfacial interaction between CNC and WPU, mainly depending upon hydrogen bonding of particle-particle and particle-molecule [84]. More recently, waterborne polyurethane–urea (WBPUU)/CNC composites with different CNC contents (from 0.5 to 5 wt%) were prepared by mixing/casting method. The CNCs were homogeneously dispersed in the WPU. Besides, thermal analysis revealed that CNCs could act as nucleating agents, and, meanwhile, provide

6.2 Cellulose Nanocrystal-Filled Nanocomposites

more chain mobility as reflected by a lower T g . In addition, enhancement in thermomechanical properties was also observed [85]. However, in most previous studies, the CNC-modified nanocomposites often showed reduction of elongation at break when the strength and modulus increased. Therefore, to prepare high-performance WPU-based nanocomposites materials, Cao et al. [86] had successfully grafted the pre-synthesized WPU chains onto the CNC surface, through the reaction between the isocyanates of WPU prepolymers and the hydroxyl of CNC. The nanocomposites were finally produced through casting and evaporation. These grafted WPU chains were able to yield a crystalline structure on the surface of CNC, and thus induce the co-crystallization together with the matrix, creating a co-continuous phase associated the filler and the WPU matrix. Moreover, the good dispersion of CNCs and strong interfacial adhesion with WPU were achieved and consequently contributed to an enhancement in thermal stability and mechanical properties of the as-prepared WPU-based nanocomposites. Dynamic mechanical analysis found that the T g of nanocomposites was increased. This was because the co-crystallization might restrict the mobility of the WPU chains neighboring CNC and enhance the interaction between soft-and hard-segments. Overall, the ductility of the final nanocomposite was slightly reduced, but the Young’s modulus and strength were significantly improved. 6.2.5

Epoxy- and Waterborne Epoxy-Based Nanocomposites

Epoxy resin has many unique properties such as excellent thermal resistance, high processability, and perfect adhesion to many substrates. With such outstanding advantages, epoxy resin has gained great interest in extensive applications, such as transparent adhesive, surface protective coating material [87, 88], and so on. Moreover, the incorporation of CNC into epoxy resin can further endow epoxy resin with many excellent features, i.e. high strength and, stiffness, and satisfied fatigue resistance, and make it quite promising to be applied in the industry [89]. Therefore, there are many reports on the reinforcement of epoxy resin using CNCs. For example, Liu et al. [90] successfully prepared a sandwiched nanocomposite film from epoxy resin and graphene/CNCs (this substance is actually cellulose nanocrystal, referred as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [90]). Compared with the neat epoxy resin, this kind of sandwiched films exhibited an enhanced tensile strength by over two times and enhanced modulus by 300 times. Besides, the T g of the sandwiched films increased, but at the same time, the conductivity of the nanocomposites did not change. These unique properties showed the promising potential of the graphene/CNC-modified epoxy composites in the application as an anti-static, electromagnetic interference shielding material. However, since CNCs were hydrophilic and epoxy was hydrophobic, it is difficult to obtain the homogeneous nanocomposites. Therefore, physical and chemical modifications are involved in the preparation of CNC-modified epoxy resins. For instance, the surfactant of amphiphilic block copolymer was used to modify CNCs (this substance is actually cellulose nanocrystal, referred as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [91]) to obtain surfactant-treated CNCs. The hydrophilic

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portion of the surfactants can interact with the hydroxyl and the carboxyl groups of CNC surface through non-covalent bonding. Therefore, the hydrophilicity of the surfactant-treated CNC was decreased. Then, those surfactant-treated CNC was directly added into epoxy through high-speed mixing method. Compared with those CNC-filled nanocomposites, surfactant-treated CNC showed better dispersion in the nanocomposite system and improved interfacial interaction between CNC and the epoxy matrix. Moreover, the glass-rubbery transition temperature of the surfactant-treated CNC/epoxy nanocomposite was increased by appropriately 10 ∘ C; such increase might lead to potential application at higher service temperature of the nanocomposites. In another work, chemically modified CNC was also used to enhance the overall performance of the epoxy resin. The CNC (this substance is actually cellulose nanocrystal, referred as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [92]) was chemically modified with dodecenyl succinic anhydride (DCNC), the surface-modified moiety of which was relatively long and flexible. Through such modification, the CNC became hydrophobic, but still could not achieve the homogeneous dispersion within the hydrophobic matrix and could improve the compatibility between the filler and the matrix. In addition, the long alkyl chains on the surface of CNC were supposed to easily entangle with the epoxy matrix and then improve the interaction with the matrix. SEM found that the DCNC was homogeneously dispersed in the epoxymatric. As a result, the nanocomposite showed an obvious increase in T g by about 30 ∘ C; simultaneously, the Young’s modulus, tensile strength, and stain at break were also increased [92]. The preparation of waterborne epoxy-based nanocomposites containing CNCs has also been reported. CNCs are water-dispersible fillers, and waterborne epoxy resins are also a water-stable matrix; therefore, in this system, the problem of heterogeneous dispersion of hydrophilic CNCs in the hydrophobic matrix might be solved to some extent. Besides, the additional chemical or physical functionalization towards the CNCs will also be avoided or eliminated. For instance, waterborne epoxy composites containing CNC were prepared through casting method. Compared with the neat epoxy, the storage modulus, T g , and tensile strength of the nanocomposites were increased. It could be attributed to the fact that CNC and water epoxy shared some similar physicochemcial characteristics, and hence improved the compatibility of filler and matrix. In addition, the increase in T g indicated that the CNC in the epoxy network further prevented polymer chain motion by the physical/chemical interactions and the steric hindrance of rigid particles. It is noteworthy that absorbing water can effectively influence the mechanical properties of the waterborne epoxy. As a kind of hydrophilic filler, CNCs can easily impact the water content of the composites. However, here, the water content of the nanocomposites was slightly decreased. The unchange of water content suggested that the reaction or interaction of CNC with the epoxy matrix might block access and even hinder possible interaction between water molecules and the hydrophilic components in the nanocomposites, and, meanwhile, the good compatibility between the filler and the matrix was reflected [93]. In another study, a novel biomass-based thermosetting nanocomposite was prepared from a waterborne terpene-maleic ester type epoxy resin (WTME) and CNC (this substance is actually cellulose

6.2 Cellulose Nanocrystal-Filled Nanocomposites

nanocrystal, referred as “cellulose nanowhisker,” and abbreviated as “CNW” in Ref. [94]). The Young’s modulus and tensile strength of the nanocomposites were significantly improved by the incorporation of CNC. These results indicated that the addition of CNCs into epoxy had an excellent reinforcement effect, due to the formation of hydrogen bonds and the improvement of interfacial interaction between CNCs and the epoxy [94].

6.2.6

Natural Polymer-Based Nanocomposites

Among natural polymers, starch is the most promising material for its inherent biodegradability, high abundance, low price, and renewability [95]. However, its high hydrophilicity renders starch strongly sensitive to the degree of moisture, especially considering that moisture reforms the hydrogen bonds in the amorphous thermoplastic starch (TPS) and causes the recrystallization of TPS together material brittleness [96]. One traditional facile way of solving this disadvantage is to mix starch with synthetic polymers [97, 98] or natural polymers [99, 100]. However, such composites often suffer from the decrease of mechanical properties, due to the poor phase compatibility and weak adhesion between the hydrophilic starch and the common hydrophobic synthetic polymer. Alternatively, another method is to use CNCs to modify TPS, and it is the key to improve the compatibility between fillers and the matrix [101, 102]. Compared with other methods, this method of introducing CNCs has two advantages: CNCs and TPS are both polar and hydrophilic; and they enjoy identical chemical structures. Therefore, it is easy for the CNCs to be well dispersed in the matrix and establish a good interaction with the matrix. It is found that CNCs can effectively improve the tensile strength as well as Young’s modulus of the starch-based nanocomposites [103]. By solution mixing and casting/evaporation method, CNC was used to modify starch to prepare nanocomposite films, and mechanical analysis showed that both tensile strength and Young’s modulus were obviously increased. This was because there are abundant hydroxyl groups on the CNC to form hydrogen bonding with TPS and in return improve the compatibility of two components. Moreover, since TPS and CNC were derived from renewable resources, the resulting nanocomposite was a biodegradable substitute to traditional food packaging materials [104]. Another work also produced plasticized starch nanocomposite films filled with homogenously dispersed CNC by casting and evaporation method. In this work, the tensile strength and Young’s modulus of the nanocomposite films showed a significant increase from 3.9 to 11.9 MPa as the content of CNC increased from 0 wt% to 30 wt%). At the same time, with the addition of CNC, water resistance showed a consistent upward increase. As indicated above, these improved characteristics might be ascribed to their identical chemical structures and hydrogen bonding associations [105]. Such influence of CNC on the mechanical properties of starch materials can be further confirmed when CNC was used to reinforce low-density PE/TPS composites through extrusion method, and mechanical test showed that the tensile strength, elasticity modulus, and hardness were considerably improved [106].

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Protein is another important natural polymer with thermoplastic characteristics. Similar to starch, protein-based composites have also gained much attention in recent years because of their biodegradability, low cost, low toxicity, and good gas barrier property [107]. However, protein-based materials have also been criticized for their poor mechanical properties and high sensitivities to moisture [108, 109]. CNCs have been also incorporated into proteins to produce bio-nanocomposites. As is reported, the incorporation of CNC obviously improved the mechanical properties of protein-based nanocomposites, especially when the proportion of CNC was on the level of 15 wt%, which could significantly reduce the influence of moisture [110]. It was because the protein would establish hydrogen bonds with the incorporated CNC rather than with the possible penetrated water molecules [111].

6.3 Fibrillated Cellulose-Filled Nanocomposites 6.3.1

Polyolefin-Based Nanocomposites

Cellulose nanofibers (CNFs), a kind of biomass nanofillers with a larger aspect ratio than CNCs, can interweave in the matrix to form an osmotic network structure, which can significantly improve the mechanical properties of nanocomposites [10, 112, 113]. However, there are a great number of polar and hydrophilic hydroxyl groups on the surface of CNF, usually resulting in mutual exclusion with the nonpolar and/or hydrophobic polymer matrices. This mutual exclusion leads to poor dispersion of polar CNF in nonpolar matrices, which reduces the mechanical properties of nanocomposites. To solve the key issue on poor dispersion of the CNF fillers in nonpolar polyolefin matrices, CNF can be treated by various physical and chemical ways to improve the interfacial adhesion between fillers and matrices, so that CNF can be well dispersed and hence enhance the mechanical properties of the nanocomposites. Physical treatment is mainly dominated by the introduced reagents with polar/nonpolar and hydrophilic/hydrophobic counterparts, such as surfactants [114, 115] or amphiphilic block copolymers [116], and the modifier is usually used by directly mixing with the CNF suspension. The hydrophilic polar part of the reagent can be approach and interact with the CNF surface, but the hydrophobic part is away from the CNF surface and forms a hydrophobic coating onto the CNFs. Therefore, these reagents can reduce the surface polarity of CNF and improve the compatibility of modified CNF with nonpolar matrices. For example, CNF was firstly dispersed in the surfactant emulsion prepared from PEO sorbitan monoester, polyethylene glycol tert-octylphenyl ether and n-pentanol, and then the PS toluene solution was added and mixed sufficiently to produce a W/O/W double emulsion system. Finally, the nanocomposite films were prepared by solution casting. The uniform dispersion of CNF in the emulsion could enhance interfacial interaction between the oil and water components, and led to a compatible system with a homogeneous distribution. SEM showed that CNF was uniformly distributed in the films and had good compatibility with the PS matrix [114]. The diblock copolymer with polar and nonpolar

6.3 Fibrillated Cellulose-Filled Nanocomposites

counterparts can be adsorbed on the CNF surface to reduce the polarity of CNF, so that the modified CNF can be more uniformly dispersed in the nonpolar matrix. For example, a stable oil-in-water emulsion was prepared by using a diblock copolymer of poly(lauryl methacrylate)-block-poly(2-hydroxyethyl methacrylate) (PLAM-b-PHEMA), and then the CNF suspension was added into the above emulsion. The mixed emulsion was sufficiently treated with a homogenizer at 7000 rpm for 30 min, so that CNF could be dispersed uniformly while PLAM-b-PHEMA could be well adsorbed on the surface of CNF. Finally, CNF/HDPE nanocomposites were prepared from modified CNF powder and HDPE melt by a twin-screw extruder. X-ray CT imaging of the nanocomposites showed that CNFs were uniformly dispersed and had few aggregates in the nanocomposites. Therefore, compared with pure HDPE, the Young’s modulus and tensile strength of the nanocomposite containing 10 wt% CNF were increased by 140% and 84% respectively [116]. Chemical modification of CNF mainly involves introducing different functional groups in the forms of small moieties or long chains on the surface of CNF through covalent linkage, and, adjusting the hydrophilicity/hydrophobicity of CNF; and it makes the CNFs better dispersed in the solvents or the solid matrix. Chemical modification methods of CNFs include TEMPO-oxidation, esterification, silanization, grafting of long polymer chains, and so on. It needs to be noted that the TEMPO-oxidation method mainly refers to the addition of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) oxidant in the preparation process of CNF to obtain the CNFs with finer diameter and surface carboxyl groups, which can be abbreviated as TOCNF. The TOCNF with carboxyl group can be ionized in aqueous solution and showed a negative surface charge. Mutual repulsion of static electricity can inhibit the aggregation of CNF and improve its dispersibility during solution mixing with other substances. For example, based on casting and evaporation method, the nanocomposite films were prepared with PS and TOCNF. Transmission electron microscope (TEM) image showed that TOCNF was evenly dispersed and formed networks in the PS matrix. The Young’s modulus and relative storage modulus of the nanocomposite film containing 10 wt% CNF were three orders of magnitude higher and 10 times those of pure PS, respectively [117]. The introduction of nonpolar molecular moieties on the surface of CNF through esterification and silylation can reduce the surface polarity of CNF, and increase the compatibility of CNF with nonpolar polyolefins. For example, alkyl succinic anhydride (ASA) was introduced on the surface of CNF by esterification. The compatibility of modified CNF with nonpolar HDPE was improved because of the alkene chain of ASA. Therefore, compared with pure HDPE, the Young’s modulus and tensile strength of nanocomposites (containing 8 wt% modified CNF) were increased by 100% and 86% respectively. The effect of methyl silylation on CNF is similar to that of esterification, and the nonpolar molecular moiety can also be introduced on the surface of CNF [118]. For example, alkoxy silane was firstly introduced at the end of the PE chain, and then reacted with the carboxyl group of TOCNF to produce polyethylene-grafted cellulose nanofiber (PE-CNF). The modified CNF was compatible with the linear low-density polyethylene (LLDPE) matrix. Thus, the mechanical properties of PE-CNF/LLDPE nanocomposites

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had been significantly improved. The Young’s modulus and tensile strength of the nanocomposite containing 5 wt% modified CNF increased by 106% and 56%, respectively [119]. Grafting hydrophobic polymer onto the surface of CNFs can also improve the adhesiveness and compatibility between CNF and nonpolar matrices. For example, polymethyl methacrylate-cellulose nanofiber (PMMA-CNF) was synthesized by grafting polymethyl methacrylate (PMMA) onto the CNF surface, and then the PP-based nanocomposites were prepared by melt-compounding PMMA-CNF with PP on a twin-screw extruder. The elongation at break of the nanocomposites was significantly improved, and the Young’s modulus and tensile strength of the composite containing 7 wt% of modified CNF were increased by 108% and 50%, respectively, compared to pure PP [120]. The compatibility between CNF and polyolefin matrices can also be improved by in situ polymerization or by adding compatibilizers during thermal processing. The homogeneous distribution of CNF can be promoted by in situ polymerization. Owing to the entanglement between CNFs, uniformly dispersed CNF will form network structures in the matrices, so that the improvement of mechanical properties of nanocomposites is more obvious. However, CNF is not easy to be dispersed in the aqueous solution. In order to achieve a well-dispersed CNF aqueous suspension, the help of external forces (such as the vibration force of ultrasonic machine and the high-speed shear of homogenizer) are required. For example, CNF could be firstly uniformly dispersed in an aqueous solution using a homogenizer, and then added to the styrene monomer and sonicated again to form a stable Pickering emulsion. Solid particles were obtained by evaporating the solvent in the emulsion, and subsequently the nanocomposite films were prepared from the solid particles by hot pressing. The mechanical properties and thermal stability of the nanocomposite films were significantly enhanced, and the films also had high light transmittance [121]. Compatibilizers such as PVA [122] and MAPP(maleic anhydride-modified polypropylene) [123] can be added during the melt blending process to promote the compatibility between the fillers and the matrices. Among them, the olefin chain in the PVA can interact with the nonpolar polyolefin and the polar hydroxyl group can interact with CNF, thereby increasing the compatibility of the CNF filler with the nonpolar matrix. After adding 5 wt% CNF and PVA to the PE matrix, the elastic modulus and tensile strength of the nanocomposite were increased by 40% and 25%, respectively, compared to pure PE [122]. The effect of adding MAPP was similar to that of PVA. After adding 6 wt% CNF and MAPP to PP, the tensile modulus and strength of the nanocomposites could be increased by 36% and 11%, respectively [123]. Table 6.3, summarizes the mechanical properties and processing strategies of CNF-filled composites with different polyolefin matrices. Bacterial cellulose (BC) with high crystallinity, high specific modulus, and high aspect ratio can also obviously improve the mechanical properties of polyolefin-based nanocomposites. Similarly, BC can form a unique network structure in nanocomposites, which can effectively transfer stress and improve the mechanical properties of composites [11, 128, 129]. However, the poor interfacial compatibility between polar hydrophilic BC and nonpolar hydrophobic polyolefin can lead to uneven dispersion of the BC fillers, which can reduce

Table 6.3 Mechanical properties and processing strategies of CNF-filled composites.

Matrix

PP

HDPE

LDPE

PP/PS blends

CNF source

CNF surface treatment/processing aids

Processing Technology

Proportion of CNF (wt%)

Young’s modulus (GPa)

Tensile strength (MPa)

Reference

Cotton

Surfactant

Extrusion/hot pressing or injection molding

0–10

1.23

29

[115]

Cotton

MAPP-modified CNF

Extrusion/hot pressing

0–8

1.94

3.82

[123]

Cotton

PLMA-b-PHEMA

Extrusion/injection molding

0–10

1.54

35

[116]

Cotton

ASA-modified CNF

Extrusion/injection molding

0–10

1.97

43.4

[118]

Cotton

PDCPMA-block-PHEMA

Extrusion/injection molding

0–10

2.7

39

[124]

Cotton

PEO

Extrusion/ injection molding

10–50

0.14

6.3

[125]

Cotton

PVA

Melt-compounding/ injection molding

0–5

—

17.4

[122]

Cotton

PMMA-modified CNF

Extrusion/injection molding

0–5

0.41

15.1

[120]

Carrot

—

Extrusion/injection molding

0–5

0.06

7.4

[126]

Wheat stalks

—

Extrusion/injection molding

6–14

0.4

5.52

[102]

Sisal

—

Intensive mixing/injection molding

0.25–5

1.5

31.5

[127]

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6 Current Status of Nanocellulose-Based Nanocomposites

the mechanical properties of nanocomposites. As described in the previous section, the introduction of nonpolar groups on the surface of BC could reduce the polarity of BC, thereby improving compatibility with nonpolar substances. For example, the polar hydroxyl groups on the surface of BC were decorated by esterification to introduce hydrophobic octane groups onto the BC surface. The modified BC improved the compatibility with the nonpolar PP matrix, which made it be better dispersed in the matrix and improved the mechanical properties of the nanocomposites. The tensile strength, impact strength, and tensile modulus of BC/PP nanocomposite containing 2 wt% modified BC were increased by 9.9%, 7.8%, and 15.6%, respectively, compared to pure PP. [130] In addition, MAPP was added in the system of BC and PP as a compatibilizer to improve interfacial compatibility between the filler and matrix. For example, MAPP was added to prepare a BC/PP nanocomposite via a twin-screw extrusion process. The addition of BC and MAPP could increase the crystallization rate of PP and the crystallinity of nanocomposites, and improve the mechanical properties of the composites [131]. As well-known, BC has a unique three-dimensional network structure, which leads to much interconnection of pores in the BC and provides a space to accommodating polymer. The BC/PS nanocomposites could be prepared by filling PS in the porous BC framework by in situ polymerization. In the process of in situ polymerization, there was no chemical reaction between PS and BC, and the filling of PS had no effect on the thermal stability of BC. The PS/BC nanocomposite showed good mechanical properties, such as the stress and strain at break of nanocomposite as about 60 MPa and 1.5%, which were respectively much better than those of pure PS (about 12 MPa and 0.6%) [129]. 6.3.2

Rubber-Based Nanocomposites

CNFs have been filled into a highly elastic natural rubber (NR) as matrix to give a structure of interpenetrating networks, which consists of a molecular-level cross-linked NR and an entangled nanoscale CNFs. Interpenetration of these two networks promotes the association between the hydrophobic NR phase and the hydrophilic CNF phase, thereby inhibiting the phase separation of the two phases [132]. Thus, the mechanical properties of NR-based nanocomposites are significantly improved after CNF is added. Based on emulsion blending and vulcanization method, the CNF/NR nanocomposites were prepared from NR latex and CNF aqueous suspension. Before emulsion blending, CNF was uniformly dispersed in aqueous solution using ultrasound or homogenization, so that CNF could be more uniformly dispersed in the NR latex during subsequent emulsion blending. For example, the suspension of jute-derived CNF with a diameter of about 50 nm was ultrasonically treated, and then added into the NR latex and vulcanized to obtain the CNF/NR nanocomposite. The increase of CNF content in the NR matrix led to a sharp increase in the mechanical properties of the nanocomposites, which might be ascribed to the network structure formed by CNF. Specifically, the Young’s modulus and tensile strength of the nanocomposites increased significantly, and the nanocomposites had good thermal properties. Unfortunately, CNF fillers reduced the elongation at break of the materials [133]. NR-based nanocomposites were also prepared by

6.3 Fibrillated Cellulose-Filled Nanocomposites

blending banana-derived CNF with NR latex [134]. Through mechanical analysis of nanocomposites, similar conclusions as above are obtained. CNF with high remnant hemicellulose content has good hydrophobicity and flexibility, and can reduce the adverse effects of filler in the matrix, such as the hardening and tensile behavior of the matrix. Therefore, the researcher isolated the spinifex CNF with the remaining hemicellulose content up to 42% from the spinifex herb via mechanical treatment, and then added pre-vulcanized natural rubber latex with a CNF content of 0.5 wt%, followed by vulcanization to get the CNF/rubber composite. The tensile strength of the as-prepared nanocomposite could be increased by 11% compared to the pure pre-vulcanized NR [135]. Although the tensile strength of NR can be reinforced by unmodified CNF, the elongation at break and the toughness of the composite cannot be significantly improved using the above strategy. Thus, surface modification of CNF is applied to intensify the interaction with the NR matrix and increase the compatibility of the two phases. For example, after introducing cationic surfactants into the NR latex, TEMPO-oxidized CNF could perform a good interfacial compatibility with the positively charged NR latex via electrostatic interaction. Subsequently, the nanocomposite with improved mechanical properties was obtained by solvent casting. Owing to the attraction of static electricity, the CNF filler could be well dispersed and construct a network in the matrix, and as a result the elastic modulus and ultimate strength of the composite could be well improved [136]. In addition, small molecules can be immobilized on the CNF surface by chemical reaction. The introduction of small molecules can increase the interaction between the modified CNF and the NR, and achieve the effect of enhancing the mechanical properties of the nanocomposite. For example, oleic acid could be esterified onto the CNF surface to build hydrophobic CNF; the modified CNF and NR were then dispersed in a toluene solution. The unsaturated double bonds in oleic acid and NR molecules could be crosslinked by vulcanizing agent, and improved the interaction between the filler and the matrix. After removing the toluene solution, the transparent CNF/NR nanocomposites were prepared via hot pressing to achieve the purpose of vulcanization and molding. The nanocomposite containing 5 wt% oleic acid-modified CNF exhibited a Young’s modulus of 27.7 MPa, which was 15 times higher than that of pure rubber [137]. Like the entanglement mechanism of CNF fillers, BC fibers can also form a network structure in the NR matrix and enhance the NR-based materials. Similarly, the BC/NR nanocomposites can also be prepared by casting/evaporating the blending emulsion of the NR latex and BC fillers. In this way, with an increase in the amount of the BC filler, the dispersibility of BC will not be reduced in nanocomposites, as NR latex can be filled in the gaps of the BC network. Therefore, BC as filler can significantly improve the mechanical properties of NR-based nanocomposites. The Young’s modulus and tensile strength of BC/NR nanocomposite with 80 wt% BC were increased to 4128.4 MPa and 75.1 MPa, respectively, which were ca. 2580 and 94 times higher than those of pure NR films [138]. In addition, the decrease of the BC polarity can increase its compatibility with NR, and can further improve the mechanical properties of the composite. For example, the PS-coated BC (PS-BC) was used to prepare the NR-based nanocomposite containing different PS-BC content by casting method. The results showed

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that the increase in the BC content led to a significant enhancement of mechanical properties, and for the composites containing 1–7 wt% PS-BC, the Young’s modulus could be increased from 600 to 6000 MPa while the tensile strength could be increased from 2000 to 6000 MPa [139]. 6.3.3

Polyester-Based Nanocomposites

CNFs are used as reinforcing fillers for bio-polyesters to improve the mechanical properties of polyester-based nanocomposites, which benefits from the high modulus of CNF. CNF can act as nucleating agent to the polyester matrices and also form network structures in the matrix. For example, the Young’s modulus and tensile strength of PLA were increased by 40% and 25% respectively by adding 10 wt% CNF [140]. Moreover, the introduction of CNF can also increase the storage modulus of composites. The storage modulus of 20 wt% CNF-filled PLA nanocomposite could reach 1000 MPa at 120 ∘ C [141]. However, the polar hydrophilic CNF can easily agglomerate and exhibit poor dispersibility in the polyester matrices due to relatively low interfacial compatibility, which may adversely affect the mechanical properties of the nanocomposites. Therefore, it is necessary to modify CNF to increase its dispersibility and interfacial adhesion in polyester matrix and to further improve the mechanical properties of the nanocomposites. The oxidation of CNF with TEMPO can provide more negative charges on the fiber surface, and thus the electrostatic repulsion effect of the TOCNFs results in their better dispersibility in the solvent or the solid matrix. Therefore, the well-dispersed TOCNF can be used to prepare polyester composite films by solution casting. The as-prepared nanocomposite films exhibited a light transmittance of more than 80%, Young’s modulus of 9800 MPa, and tensile strength of 266 MPa, together with an enhanced performance of oxygen barrier in contrast to neat polyester [142]. In addition, the surface of CNF can be modified by esterification, silanization, amidation, polymer grafting, and so on. Modification of CNF can increase compatibility with the polyester material and improve the mechanical properties of the composite. For example, by esterification, the oleic acid could be grafted onto the CNF surface to increase the hydrophobicity of CNF. The oleic acid-modified CNF could improve the compatibility with PLA matrix and increase the crystallinity of nanocomposites, together resulting in the significant enhancement of mechanical properties of the nanocomposites [143]. Silanization is also a good method to improve the surface hydrophobicity of CNF, and the hydrophobic moiety of silane can improve the compatibility of CNF with polyester matrix. Therefore, by adding as less as 3 wt% of silanized CNF into the PLA matrix, the tensile strength and impact strength of the composite could be increased by 22.7% and 56.1%, respectively [144]. Grafting a hydrophobic polymer onto the surface of CNF is also an effective method for elevating the hydrophobicity of CNF. The hydrophobic polymer modification of CNF surface can not only have a better adhesion to the polyester matrix, but also increase the density of the CNF network in the matrix. For example, the PLA-grafted CNF could be prepared by adding l-lactide monomers and initiating their ring-open polymerization from the CNF surface in the suspension system. The polymerization could decorate PLA chains onto

6.3 Fibrillated Cellulose-Filled Nanocomposites

the CNF surface, which would dramatically improve the compatibility of CNF to the PLA matrix. Therefore, by adding the PLA-grafted CNF into the PLA matrix, the tensile strength and impact strength of the as-prepared composites could be increased by 28% and 63%, respectively [145]. In the process of preparing polyester-based nanocomposites, re-dispersion of CNF in the polyester matrix can be achieved by different processing methods such as solution mixing and melt blending, so that nanocomposites with various excellent properties can be obtained. The solution mixing method is to homogenize CNF in a solution and then mix with polyester solution to form a uniform suspension. In the solution mixing process, a mechanical agitation, an ultrasonic machine, or a homogenizer is required to uniformly disperse the CNF in the liquid-state medium. Then, the mixture is poured onto a petri dish to evaporate the solvent for preparing CNF/polyester nanocomposite films. The crystallinity and light transmittance of the polyester-based nanocomposite film prepared by solution mixing/casting method can be improved by the addition of CNF. At the same time, the mechanical properties of CNF/polyester nanocomposites prepared by melt blending method have also been verified to significantly improve. For example, when 5 wt% CNF is added, the Young’s modulus and tensile strength of CNF/PLA nanocomposites were increased by 24.1% and 22.4% respectively, compared to pure PLA [146]. However, due to the hydrogen bonding effect between the surface hydroxyl groups of CNF, dried CNF is easily aggregated and is hard to re-disperse in the solvent. Thus, a nonhomogeneous mixture is generally produced if CNF is directly blended with polyester matrix, which will have adverse effects on the polyester-based nanocomposites. This problem can be solved by pre-mixing the CNF filler and polyester matrix in the solvent and subsequently evaporating to obtain polyester-supported CNF filler. For example, PLA and CNF were completely dissolved and uniformly dispersed in a mixture solvent of acetone and chloroform respectively, followed by solvent displacement and solution casting to obtain dried CNF/PLA composite particles. Then, the dried solid particles were thermoprocessed using a twin-screw extruder at a screw speed of 120 rpm and a temperature range from 165 to 200 ∘ C. By this way, the mechanical and thermal properties of the composites were obviously improved. The modulus and tensile strength of the 5 wt% CNF-filled composite increased by 24% and 21%, respectively [147]. On the other hand, when chemically modified CNF is used as filler, the mechanical properties of the composite can be improved. For example, after adding 5 wt% of acetylated CNF into the polyester matrix, the storage modulus of the composite could be increased by 2800% [146]. High-strength and high-modulus BC fibers can also be introduced into the PLA matrices to improve the mechanical properties and keep transparency for the PLA-based nanocomposites [148]. The high-aspect-ratio BC can form a unique network structure inside the matrix, so that the nanocomposites can have stronger impact resistance [149]. Owing to the hydrogen bonds formed between BCs, BC fillers, like CNFs, also tend to agglomerate and disperse unevenly in the matrix. Chemical or physical modification of the BC surface can reduce the effect of hydrogen bonds between the BCs and the BC fillers can be homogeneously dispersed in the PLA matrix to achieve better mechanical properties. For example, the BC/PLA nanocomposites prepared by

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6 Current Status of Nanocellulose-Based Nanocomposites

melt-blending PLA with acetylated BC exhibited higher distribution uniformity and dispersibility of the fillers, and the thermal and mechanical properties as well as water sensitivity could be significantly improved. The tensile strength of the composites increased by approximately 100% when loaded with 6 wt% acetylated BC fillers [150]. At the same time, the acetylated BC/PLA nanocomposites could also be prepared by compression molding, and the corresponding performance of PLA-based nanocomposites could be significantly improved [151]. In addition, various organic acids (i.e. acetic acid, hexanoic acid, and dodecanoic acid) can be used to chemically modify the BC surface to obtain the BCs with various functional groups (f-BC) as the fillers of PLA matrix. Herein, a key method of temperature-induced phase separation was used to obtain the precipitates of f-BC/PLA blends by dropwise adding the mixing solution of f-BC/PLA into a liquid nitrogen bath, and then freeze-dried to produce porous composite microspheres as the dry pre-extrusion substances. Finally, the f-BC/PLA composite film was prepared by a thermal process of screw extrusion and subsequent compression-molding. In comparison to pure PLA film, the mechanical properties, thermal stability and viscoelastic behavior of the f-BC/PLA nanocomposite films were all significantly enhanced [152]. PHB is also a biodegradable thermoplastic polyester, and has an obvious semi-crystalline characteristics with a crystallinity degree up to 60% [153]. The incorporation of BC can accelerate the crystallization rate of the semi-crystalline polymer matrix. Therefore, the introduction of high strength, high modulus and high crystallinity BC into the PHB matrix can not only promote the crystallization behavior of the matrix, but also further improve the mechanical properties of PHB-based nanocomposites. Herein, BC was firstly dispersed in a suspension of PHB; and then the BC/PHB nanocomposite film was prepared by solvent evaporation. BC filler as nucleating agent could accelerate the crystallization rate of the PHB matrix. Owing to the good dispersion of BCs and their interaction with polymer matrix, the water vapor permeability, transparency, and thermal and mechanical properties of the PHB-based nanocomposites could be wholly improved [154, 155]. 6.3.4 Polyurethane- and Waterborne Polyurethane-Based Nanocomposites It is known that high-aspect-ratio CNFs could form a network structure in the matrix due to mutual entanglement, thereby significantly enhancing the mechanical properties of the nanocomposite materials. When 16.5 wt% of CNF was added to the PU matrix, the tensile strength and modulus of the CNF/PU nanocomposite could be increased by nearly 500% and 3000%, respectively, compared to pure PU [156, 157]. The addition of CNF having a high density of hydroxyl groups on the surface can increase the hydrogen bond density with the PU matrix, and further enhance the mechanical properties of the PU-based materials. For example, the suspension of CNF in DMF was first ultrasonically treated and added into thermoplastic polyurethane (TPU), and then the mixture was ultrasonicated again to disperse CNF in the TPU. After drying the mixed suspension, the CNF/TPU nanocomposite films could be obtained by hot pressing.

6.3 Fibrillated Cellulose-Filled Nanocomposites

Owing to the good dispersion of CNF in the matrix, the nanocomposite films exhibited enhanced stress and elongation at break as well as improved thermal stability [158]. CNF fillers can also improve the foaming structure of PU-based foams. For example, adding different nano-scale CNFs might yield the cells with different scales and uniform distribution, and increase the cell density of the material. On the other hand, CNF as a filler is able to change the foaming model of PU, i.e. from homogeneous nucleation to heterogeneous nucleation [159, 160]. In addition, the uniform distribution of CNF in the matrix might improve the compressive strength and modulus of the nanocomposite foam. For example, foam compression strength and modulus prepared by adding 0.5 wt% CNF into PU were increased by 27% and 63%, respectively, compared to conventional PU foam. Moreover, this nanocomposite foam had a very low bulk density ( T g is unusual for such a low CNC concentrations. In order to further understand this unusual effect, a comparison between the experimental results and different mechanical approaches has been built. The establishment of an interconnected network has been put forward as a reasonable mechanism. This is because the theoretical model has been

7.1 Percolation Approach

Figure 7.2 Schematic representation of a “quasi isotropic”composite. Source: Halpin and Kardos 1972 [6]. Reproduced with permission of AIP.

used extensively to predict the elastic shear modulus G of short fiber reinforced composites. Its aim is then to explain the unusual enhancement in modulus of CNC-reinforced nanocomposites since it has exceeded the value predicted by a mean-field approach (e.g. Halpin–Kardos model) [6]. In the Halpin–Kardos model, fibers are assumed to be embedded in the matrix to form a homogeneous continuum. The modulus, the mechanical anisotropy, and the geometry of the fibers are accounted for, but one assumes that there is no interaction between the fibers. The mean-field approach is based on another concept that material is mechanically equivalent to the superposition of four plies. That concept is used in materials made of short fibers, which are homogeneously dispersed in a continuous matrix. The composite is then considered to be a “quasi isotropic” material with four layers of oriented plies (at O∘ , 45∘ , 90∘ , −45∘ ), as shown in Figure 7.2. The mechanical properties of each ply can be derived from the micromechanic equations of the Halpin–Tsai (“self-consistent” approach): [1, 7] (1 + 𝜉ii )Eiif + 𝜉ii (1 − 𝜈R )Em Eii = (i = 1, 2) Em (1 − 𝜈R )Eiif + (𝜉ii + νR )Em

(7.1)

(1 + 𝜈R )Gf + (1 − 𝜈R )Gm G12 = Gm (1 − νR )Gf + (1 + 𝜈R )Gm

(7.2)

where Eii is the stiffness in the fiber direction of a unidirectional ply, E22 is the stiffness estimate perpendicular to the fiber direction, G12 is the in-plane shear modulus estimate, and 𝜈 R is the volume fraction of fibers. The subscripts “m” and “f” refer to the matrix and the filler, respectively. The geometry of the filler is involved through the 𝜉 11 parameters, which are calculated by Eq. (7.3). L, l, and e are the length, width, and thickness of the fibers, respectively. In our case, l = e, and it is the diameter of the whiskers, which is as follows [6]: L L =2 e d l l =2 =2 =2 e d

𝜉11 = 2 𝜉22

(7.3)

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7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

The engineering constant characteristics of the unidirectional plies are then given by Eii (i = 1, 2) 1 − v12 v21 Qii = v12 Q22 = v21 Q11

Qii =

(7.4)

Q66 = G12 which leads to the following expressions for the invariant terms:

1 (3Q11 + 3Q22 + 2Q12 + 4Q66 ) 8 1 U2 = (Q11 + Q22 − 2Q12 + 4Q66 ) (7.5) 8 The tensile modulus of the “quasi-isotropic” laminate, which is assumed to be close to that of the short fiber composite, is then given by U1 =

EC =

4U2 (U1 − U2 ) U1

(7.6)

The Poisson’s ratio ν12 is approximately given by a mixing rule 𝜈12 = 𝜈m 𝜈m + 𝜈f 𝜈R =

Q11 ν Q22 21

(7.7)

The predicted modulus values agree generally well with the experimental data in the glassy state but fail to describe the rubbery modulus [3, 8–11]. Above T g of the matrix, the observed reinforcing effect on the modulus is found to be much higher than possible for a composite only reinforced with discrete fibers, as shown in Figure 7.3. The assumption of “no filler/filler interaction” thus seems to be wrong, although this model displayed limits when the stiffness of the reinforcing phase is far above the continuous phase [12]. 7.1.2

Percolation Model

An acceptable fit of experimental data could be obtained by modifying the geometrical (aspect ratio) and/or stiffness (modulus) properties of the cellulosic fillers [13]. However, the former result from experimental microscopic observations and the latter are not easily questionable. The topological arrangement of the fillers and their interactions should thus be taken into account to describe the behavior of cellulosic whisker filled composites. The numerous hydroxyl groups on whisker surface suggest a hydrogen bonding potential for filler/filler interactions. The high specific surface area of whiskers also suggests the possible formation of a filler network within the host matrix. Those features led to the idea of a percolation type approach, which could be applied in cellulose whiskers reinforced polymeric composites. The term percolation for the statistical geometry model was first introduced in 1957 by Hammersley [14]. It is a statistical theory that can be applied to any system involving a great number of species likely to be connected. The aim of the statistical theory is to forecast the behavior of a non-completely connected set

7.1 Percolation Approach

9 Experimental results Halpin–Kardos model Percolation model

log(Gc/Pa)

8

7

6

5 T = 325 K 4 0.02

0

0.04

0.06

0.08

0.1

Volume fraction of CNC

Figure 7.3 Logarithm of the shear modulus, taken at 325 K, as a function of the whisker volume fraction. Comparisons among the experimental results, data calculated by the Halpin–Kardos model(the shear moduli of the matrix and the whiskers are respectively taken equal to 0.5 MPa according to the experiment data and to 5 GPa according to [10], the aspect ratio was taken equal to 100), and the percolation model. Source: Chazeau et al. 1999 [10]. Reproduced with permission of John Wiley & Sons.

of objects. This approach allows the transition from a local to an infinite “communication” state to be described by varying the number of connections. The percolation threshold is defined as the critical volume fraction separating these two states. The value of the percolation threshold can be modified by various parameters, such as particle interactions [15], orientation [16], or aspect ratio [17]. This approach was used to describe and predict the mechanical behavior of cellulosic whisker based composites. The results suggested that the formation of a rigid network of whiskers could be responsible for the unusual reinforcing effect observed at high temperatures. The modeling consists of three important steps: (i) First, the calculation of the percolation threshold (νRc ) should be carried out. The volume fraction of whiskers required to achieve geometrical percolation was calculated using a statistical percolation theory for cylindrical shaped particles according to their aspect ratio. The effective skeleton of whiskers was also estimated [18]. The skeleton corresponds to the infinite length branch of whiskers connecting the sample ends. Favier et al. [18] used computer simulation and found that about 0.75 vol% tunicin whiskers (assuming L/d = 100) are needed to get a 3D geometrical percolation. The authors calculated the effective skeleton by eliminating the finite length branches. The following relation was found between the percolation threshold (𝜈 Rc ) and the aspect ratio of rod-like particles [2]:

𝜈Rc =

0.7 L∕d

(7.8)

205

206

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

Table 7.1 Geometrical characteristics of some cellulose nanocrystals: average length (L), average diameter (d), aspect ratio (L/d), and percolation threshold (𝜈 Rc )of rod-like particles. Nature

Source

L

d

L/d

𝝂 Rc (vol%)

Cellulose nanocrystals

Tunicin

1000

15

67

1.0

Wheat straw

220

5

45

1.6

Cotton

170

15

10

7.0

Sugar beet pulp

210

5

40

1.8

a) Calculated from Eq. (7.8) [2]

Based on this equation, percolation threshold values of CNCs from different sources are calculated and reported in Table 7.1. Flandin et al. [19] performed DC electrical conductivity measurements to evaluate the tunicin whisker percolation threshold. In this study, the surface of the cellulosic filler was covered with conductive polypyrrole before incorporation in a latex matrix. The coated whiskers were characterized by scanning electron microscopy (SEM). The final sticks have an average length of about 2 μm and a diameter of 160 nm. The percolation threshold was determined to be about 3 vol% in agreement with the percolation theory and experimental data [20] for an aspect ratio close to 15. The electrical percolation threshold is generally lower than the geometrical one, which provides the mechanical stiffness. The presence of the continuous path can be enough to make the system conducting but is not necessarily sufficient to ensure the rigidity of the whole system. (ii) The second step is the estimation of the modulus of the percolating filler network. It is obviously different from that of individual whiskers and depends on the origin of cellulose, preparation procedure of the whiskers, and obviously the nature and strength of inter-whisker interactions. This modulus can be assumed to be that of a paper sheet for which the hydrogen bonding forces provide the basis of its stiffness. Experimental tensile tests were performed on films prepared from the evaporation of a suspension of cellulose whiskers in a Teflon mold. For tunicin [21] and wheat straw cellulose whiskers [9], the tensile modulus was around 15 and 6 GPa, respectively. The apparent tensile modulus of a cellulose whisker network can be calculated by a 3D finite elements simulation [22]. The linking elements were considered as beams with adjustable stiffness. All of the calculated values were lower than 1 GPa. For link modulus values below 1 GPa, the network modulus was found to increase with increasing whisker concentration and seemed to increase linearly with the link modulus. For a higher linking modulus, the modulus of the percolating network tends toward the value for totally rigid links. (iii) The description of the composite requires the use of a model involving three different phases, viz., the matrix, the filler percolating network, and

7.1 Percolation Approach

R

R

S ψ

Figure 7.4 Schematic representation of the series–parallel model. R and S refer to the rigid (cellulosic filler) and soft (polymeric matrix) phases, respectively, and 𝜓 is the volume fraction of the percolating rigid phase. Source: Takayanagi et al. 1964 [24]. Reproduced with permission of John Wiley & Sons.

the non-percolating filler phase. The simplest model consists of two parallel phases, namely, the effective whiskers skeleton and the rest of the sample. Ouali et al. [23] extended the classical phenomenological series–parallel model of Takayanagi et al. [24] and proposed a model in which the percolating filler network is set in parallel with a series part composed of the matrix and the non-percolating filler phase (Figure 7.4). In this approach, the elastic tensile modulus EC of the composite is given by the following equation: EC =

(1 − 2𝜑 + 𝜑𝜈R )ES ER + (1 − 𝜈R )𝜑ER 2 (1 − 𝜈R )ER + (𝜈R − 𝜑)ES

(7.9)

The subscripts S and R refer to the soft and rigid phases, respectively. The adjustable parameter, 𝜓, involved in the Takayanagi et al. model corresponds in the Ouali et al. prediction to the volume fraction of the percolating rigid phase. With b being the critical percolation exponent, 𝜓 can be written as 𝜓 = 0 for vR < vRc ( ) vR − vRc b 𝜓 = vR for vR > vRc 1 − vRc

(7.10)

where b = 0.4 [25, 26] for a 3D network. This percolation approach was found to fit satisfactorily the experimental data, especially for high filler loading [1–3, 11, 27, 28]. At higher temperatures when the polymeric matrix could be assumed to have a negligible stiffness, the calculated stiffness of the composites is simply the product of that of the percolating filler network and the volume fraction of percolating filler phase: Ec = 𝜓ER

(7.11)

Figure 7.5 shows the comparison between experimental and predicted data from both the Halpin–Kardos model and the percolation approach. In this figure,

207

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

4 3.5 log Ec′Tg + 50 K/Em′Tg + 50 K

208

3 2.5 2 1.5 1 0.5 0 –0.5 0

0.05

0.1

0.15

0.2

Whiskers volume fraction

Figure 7.5 Logarithm of the relative storage tensile modulus measured at T g + 50 K (log E c ′ T g + 50 K/E m ′ T g + 50 K, where E c ′ T g + 50 K and E m ′ T g + 50 K correspond to the value measured at T g + 50 K for the composite and the matrix, respectively), vs. volume fraction of tunicin whiskers: comparison between the experimental data for poly(S-co-BUA)(⚫), PHO(○), and glycerol-plasticized starch-based systems conditioned at 35% RH (▴), 43% RH (△), 58% RH (⧫), and 75% RH (⋄) and predicted data from the Halpin–Kardos model (-) or from the percolation approach (---). Source: Favier Barlier 1995 [2].

the logarithm of the relative storage tensile modulus measured at T g + 50 K (log Ec ′ T g + 50 K/Em ′ T g + 50 K, where Ec ′ T g + 50 K and Em ′ T g + 50 K correspond to the values measured at T g + 50 K for the composite and the matrix, respectively) is plotted as a function of tunicin whisker content expressed in volume fraction. Experimental data were obtained for tunicin whisker reinforced poly(S-co-BuA) [2], amorphous poly 3-hydroxyoctanoate (PHO) [27], and glycerol-plasticized starch conditioned in different moisture conditions. For poly(S-co-BuA)-based systems, the shear modulus values from Ref. [2] were converted into tensile moduli assuming a mixing rule for the Poisson’s ratio. The various parameters used for plotting Figure 7.5 are collected in Table 7.1. For the glycerol-plasticized starch-based systems, it is worth noting that the matrix appeared as a complex heterogeneous system composed of glycerol-rich domains dispersed in an amylopectin-rich continuous phase. Each phase exhibited its own glass–rubber transition, for which the temperature decreases as the moisture content increases owing to the plasticizing effect of water. The values of T g reported in Table 7.1 correspond to the amylopectin-rich domains. The calculated curves are similar whatever the nature of the matrix because the relative moduli are reported. 7.1.3

Factors Influencing the Percolation Network Formation

Many factors have been found to affect the formation of the CNC percolation network and then the mechanical performances of the composites. The aspect ratio of the filler is an important factor since it determines the percolation threshold value. For instance, the rubbery storage tensile modulus measured

7.1 Percolation Approach

at T g + 50 K is systematically lower for wheat straw whiskers/poly(S-co-BuA) composites (Figure 7.5, open circles) than for tunicin-whiskers-based materials. In addition, for the former system, a gradient of whisker concentration between the upper and lower faces of the composite film was reported and evidenced by SEM, wide angle X-ray scattering (WAXS), and DMA [11]. It was ascribed to a translational motion of the nanocrystals, leading to their sedimentation during the evaporation step. The mechanical behavior of wheat straw cellulose based composites was well described by using a multilayered model consisting of layers parallel to the film surface. On the contrary, the modulus of tunicin-whisker-reinforced amorphous PHO latex (Figure 7.5, filled squares) displays a higher reinforcing effect than that of poly(S-co-BuA)-based composites. This effect was ascribed to the latex particle size that could affect the whisker network formation [27]. The latex particle size was around 150 nm and 1 μm for poly(S-co-BuA) and PHO, respectively. Indeed, the particles act as impenetrable domains to whiskers during the film formation due to their high viscosity. Increasing latex particle size leads to an increase in the excluded volume and to a decrease in the percolation threshold. When using a semicrystalline PHO as the matrix (Figure 7.5, open squares), the modulus of the unfilled material is substantially higher than for amorphous PHO. The semicrystalline PHO displayed a melting temperature around 55 ∘ C [27]. It is well known that the crystalline regions of PHO, or any semicrystalline polymer, act as physical cross-links for the elastomer. In this physically cross-linked system, the crystalline regions would also act as filler particles due to their finite size, which would increase the modulus. For low filler content composites (below 1 vol%), the modulus of tunicin-whisker-filled semicrystalline PHO is higher than that of tunicin-whisker-filled amorphous PHO. This indicates that the mechanical behavior of the composite is matrix dominated for this filler loading level. At higher whisker content, the differences between both kinds of the matrices are reduced and the modulus of composites is similar. This indicates that the mechanical behavior of the material becomes whiskers dominated, because of the formation of a percolating cellulosic network. The modulus of the rubbery matrix does not play any role, whatever its crystallinity state may be. However, a significant difference was reported at higher temperatures [28]. It was suspected that in semicrystalline PHO-based composites the cellulose whiskers network originates from the formation of inter-whisker links through transcrystalline layers grown from the surface of the nanoparticles. This resulted in a disastrous decrease in the mechanical properties of the semicrystalline PHO composites filled with tunicin whiskers, as soon as the melting temperature (around 57 ∘ C) of the matrix was reached. However, it was reported that suitable thermal treatment allowed restoring the cellulose whiskers network through hydrogen bonds [28]. Another semicrystalline polymer, polyolefin elastomer (POE), was also used as a matrix for the processing of tunicin-whisker-reinforced composites. For these systems, the reinforcing effect observed between T g and the melting point was low because of the high degree of crystallinity (around 60%) of POE [29]. However, the main effect of the filler was a thermal stabilization of the storage tensile modulus for the composites above the melting temperature T m of the POE matrix. It was attributed to the formation of an internal cellulosic network

209

210

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

through interparticle hydrogen bonds. In Figure 7.5, experimental data for the modulus of tunicin whisker/POE composites measured above T m are reported vs. filler content. The experimental data are very close to those reported for tunicin-whiskers-filled poly(S-co-BuA). This indicated that the formation of the percolating cellulosic network is not affected by the matrix crystallization process and cellulose/POE interactions that were evidenced. High-performance lithium-conducting nanocomposite polymer electrolytes were also prepared using tunicin whiskers and POE [30–32]. Glycerol-plasticized starch-based composites were processed using waxy maize. It was shown that the matrix was a complex heterogeneous system composed of glycerol and amylopectin-rich domains. Each domain exhibited its own glass–rubber transition. Figure 7.5 shows the evolution of the storage tensile modulus at T g + 50 K for such systems conditioned at 35% RH (relative humidity) (Figure 7.5, filled triangles), 43% RH (open triangles), 58% RH (filled lozenges), and 75% RH (open lozenges). It is worth noting that in Figure 7.5 and in Table 7.2 T g was taken as the amylopectin-rich glass–rubber transition temperature. The reinforcing effect is very low and the modulus remains roughly constant regardless of the composite composition. The reasons for this observation are the competitive interaction between each component and the accumulation of plasticizer at the cellulose/amylopectin interface [4]. This plasticizer accumulation phenomenon, enhanced in moist conditions, could most probably interfere with hydrogen bonding forces that are likely to hold the percolating cellulose whisker network within the matrix. In highly moist conditions, a possible Table 7.2 Parameters used for plotting Figure 7.5. Matrix

𝝆

m

𝝂 m a)

(g/cm3 )

T g

(K)

Poly(S-co-BuA)

1.1

0.5

273

PHO

1.019

0.5

250

Starch 35% RH

1.1

0.5

300

Starch 43% RH

1.1

0.5

274

Starch 58% RH

1.1

0.5

271

Starch 75%RH

1.1

0.5

260

Tunicin whiskers

𝜌

3 f (g/cm )

1.58 Percolation approach

a) b) c) d) e)

𝜈 f b)

L/dc)

E11f (GPa)d)

E22f (GPa)d)

Gf (GPa)d)

0.3

67

150

15

5

𝜈 Rc (vol%) e)

b

ER (GPa)

1

0.4

15

The matrix being in the rubbery state at T g + 50 K. Cellulose being in the crystalline state. Estimated from TEM. Average value from literature [93–98]. Calculated from Eq. (7.8).

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

transcrystallization phenomenon on the surface of the whiskers was reported to originate from the amylopectin chains located in the glycerol-rich domains. The coating of the cellulose whiskers by soft plasticizer-rich interphase hindered the stress transfer at the filler/matrix interface when the material was submitted to high strain tensile test [4]. The cross-shaped symbol in Figure 7.5 refers to a composite based on poly(S-co-BuA) and reinforced with 6 wt% of cellulose whiskers obtained from sugar beet pulp. Its mechanical behavior is surprisingly close to that of tunicin whiskers despite the lower aspect ratio of the filler.

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix In these nanocomposite materials, interfacial phenomena are expected to be important owing to both the high specific area and the relatively reactive surface of CNCs. For tunicin whiskers, the specific area is around 170 m2 /g and for a 6 wt% tunicin-whiskers-filled composite, the filler surface is of the order of 100 000 cm2 /cm3 . In the non-percolating network complex system, the interaction between the matrix and filler will be the dominant factor in the enhancement mechanism of nanoparticles. For example, using CNCs to reinforce polyethylene–vinyl acetate (EVA) matrixes, the vinyl acetate content of EVA copolymer can be changed to control the polarity of the polymer, and thus to study the interfacial interaction between CNCs and different substrates. From the studies, we can draw the conclusion that the stronger the polarity of the polymer matrix, the higher the storage modulus of the composite material. 7.2.1 Effect of Functional Groups on CNC Surface on Interfacial Interaction The interaction between nanofillers and matrix can be controlled by appropriate surface modification of the nanoparticles. Surface modification method is widely used to improve the compatibility between CNCs and the matrix. For example, introducing hydrophobic groups onto the surface of CNCs, together with “grafting onto” or “grafting from” a polymer with similar properties to the polymer matrix, can be used. Thereby, it is necessary to enhance the interaction between the filler and the matrix to improve the mechanical performance of the composites. Yet, while surface modification can improve the compatibility and interface bonding between the two, it also weakens the hydrogen bonding interaction, which would influence the formation of the rigid percolation network. CNC is widely used in poly(vinyl alcohol), abbreviated as PVA or PVOH. CNCs can disperse and interface well within the PVA matrix without the necessity to functionalize their surface. The high compatibility stems from the fact that both PVA and CNCs have hydroxyl-rich surfaces. In the PVA matrix, surface functionalization of CNCs would rather be detrimental to create an exfoliated nanocomposite. In this vein, Paralikar et al. [33] examined the effect of dispersing carboxylated CNCs – obtained using surface tetramethyl-piperdine-1-oxyl

211

212

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

(TEMPO) oxidation as outlined in Araki et al. [34] on the performance of the resulting CNC–PVA nanocomposites. Carboxylated CNCs can help increase the cross-linking within the polymer. However, they interfere with the formation of the CNC percolated network. In a percolated network, interactions between the reinforcing elements are critical, and, above a critical threshold concentration, the CNCs connect to each other to form a rigid network within the polymer [23]. As such, surface-modified CNCs can have no notable improvement in the mechanical properties of CNC–PVA nanocomposites. On the contrary, CNC aggregation resulting from surface modification contributes to reducing the stretching ability and toughness of the resulting nanocomposite compared to systems containing neat CNCs and PVA [34]. The reinforcing potential of neat CNCs in “perfectly” compatible polymer systems such as PVA – and following a preparation method described in Hamad and Su [35] – can be gleaned from examining the dynamic mechanical data, measured in tensile mode, for CNC–PVA nanocomposites. Evidently, CNCs increase the rubbery modulus of the PVA matrix at all loading levels, and regardless of the moisture content [23, 36]. The remarkable improvements in the dynamic mechanical properties are attributed to the compatibility between OH-rich CNCs and PVA. This compatibility results in (i) strong interactions between OH-rich CNCs and PVA and (ii) a mechanically percolated network of CNCs linked by strong hydrogen bonding interactions. This percolation phenomenon and concomitant improvements in mechanical performance have been well documented for other “perfectly” compatible nanocomposite systems, for example, CNCs in aqueous latexes [1] and CNCs in aqueous epoxy [37]. Mariko Ago et al. [38] prepared non-defect electrospun fibers from aqueous dispersions of lignin, PVA, and CNCs, which were used to enhance nanoparticles. The thermomechanical properties of lignin-based electrospun fibers and spin-coated films are improved when CNCs are embedded. Isochronal DMA was used to assess the viscoelastic properties of lignin: PVA electrospun fiber mats loaded with CNCs. DMA revealed that the relaxation process became less prominent with an increased lignin content, an effect that correlated with the loss tangent (tan 𝛿 = E′′ /E′ ) and a peak (T g ) that shifted to higher temperatures. This can be attributed to the suppression of strong intermolecular interactions on the segmental motion of PVA in the amorphous region. Figure 7.6 shows tan 𝛿 profiles as a function of temperature for the 20 : 80/% CNC system. The temperature position of the peak corresponding to the leathery transition shifted to higher values, from 116 to 159 ∘ C, and the magnitude of the peak decreased with CNC loading up to 10%. This effects related to the presence of strong interactions, i.e. hydrogen bonding between hydroxyl groups of the polymeric matrix and CNCs, which causes restricted molecular mobility. As discussed previously, intermolecular interaction via hydrogen bonds between the matrix and the CNCs in multicomponent systems is an important factor contributing to the improved mechanical and thermal properties. The reinforcing effect of CNCs and the composition dependence are consistent with the effective stress transfer occurring between the matrix and CNC. When 15 wt% CNC was present in the composite fiber, the temperature position of tan 𝛿 peak slightly decreased to 104 ∘ C. This observation can be related to (i) filler–filler slippage

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

20 : 80/CNC

tan δ and storage modulus (MPa)

100

75 : 25/CNC

10%

Storage modulus

CNC, 0% 5% 15%

15%

CNC, 0% 5%

10

1

tan δ β

β

0.1

–50 (a)

0

50

100

Temperature (°C)

150

200 –50 (b)

0

50

100

150

200

Temperature (°C)

Figure 7.6 Storage tensile modulus E’ and loss angle tangent, tanδ, as a function of temperature for lignin: PVA/CNC electrospun fiber mats: 20 : 80/%CNC (a) and 75 : 25%CNC (b). The DMA experiments were conducted at a frequency of 1 Hz in the elastic region. The data were not normalized, and three repetitions were run for each condition, all showing similar profiles. Source: Ago et al. 2013 [38]. Reproduced with permission of ACS.

or friction, (ii) particle–polymer motion at the filler interface, and (iii) a change in the properties of the polymer by adsorption onto the reinforcing particles. It is of interest to compare the performance of the lignin: PVA/CNC fibers to the data predicted from a model based on the percolation concept. As such, they calculated that the modulus of electrospun fibers reinforced with CNCs was much higher than the values calculated by the Halpin–Kardos model [6] after considering negligible interactions between fillers. This observation provides further support to the hypothesis that the enhancement of the mechanical strength of the electrospun fiber mats is the result of the formation of a network between CNCs at the given loading, as shown in Figure 7.7. Worarin Meesorn et al. [39] reported the effect of the addition of a small amount of a judiciously selected polymeric additive designed to act as a dispersant and to simultaneously contribute to the stress transfer. In a systematic study, they employed poly(ethylene oxide-co-epichlorohydrin) (EO-EPI) as the polymeric matrix, low-aspect-ratio CNCs isolated from cotton, and a small amount of PVA as an additive (Figure 7.8). Previous work in their group had shown that EO-EPI provides an ideal framework to monitor CNC-induced reinforcement on account of its low modulus and solubility in dimethyl formamide, which is also a good dispersing agent for CNCs [40–43]. Interestingly, the reinforcement achieved with high-aspect-ratio CNCs was much higher

213

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

1000 Percolation model Storage modulus (MPa)

214

Experimental at 120 °C

100

Halpin-Kardos model

10 0.00

0.05

0.10

0.15

Figure 7.7 Comparison of between predicted data and experimental storage moduli for lignin: PVA/CNC at various CNC volume fractions. Experimental results were obtained above the glass transition temperature at 120 ∘ C, and the solid lines are the percolation model used in refs [1, 2] and Halpin–Kardos model [6]. Source: Ago et al. 2013 [38]. Reproduced with permission of ACS.

0.20

Volume fraction of CNC

EO-EPI/CNC

EO-EPI/PVA/CNC

Figure 7.8 Schematic representation of the proposed mechanism for the enhancement of the mechanical properties of EO-EPI/CNC nanocomposites upon addition of PVA. Through hydrogen bonding, PVA leads to better dispersion of CNCs and supports the formation of a hydrogen-bonded percolating network within the EO-EPI matrix. Source: Meesorn et al. 2017 [39]. Reproduced with permission of ACS.

than that realized with low-aspect-ratio CNCs [44, 45], which points to CNC aggregation in the latter case [46]. They surmised that the addition of PVA to composites of CNCs and EO-EPI would (i) improve the CNC dispersion, as both PVA and CNCs offer many hydroxyl groups that are able to establish hydrogen bonds and (ii) possibly also act as a binder, enhancing CNC–CNC stress transfer on account of hydrogen bonding. Indeed, remarkable improvements in the stiffness and strength were observed at a PVA content of 1–5% w/w. Similar effects were observed for CNC nanocomposites made with polyurethane (PU) or poly(methyl acrylate) matrices, demonstrating that the approach is broadly exploitable. The mechanical properties of EO-EPI/CNC and EO-EPI/PVA/CNC nanocomposite films were also studied with DMA as shown in Figure 7.9, and to better understand the mechanism through which PVA reinforces the CNC nanocomposites investigated here, they analyzed the experimental E′ values of PVA-free EO-EPI/CNC nanocomposites and the 5% w/w PVA containing EO-EPI/PVA/CNC nanocomposites cast from dimethyl sulfoxide (DMSO) (which were all made with CNCs isolated from cotton) in the framework

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

Storage modulus (MPa)

104

103

102

101

100

10–1

neat EOEPI EOEPI-1%PVA EOEPI-3%PVA EOEPI-5%PVA EOEPI-CNC EOEPI-1%PVA-CNC EOEPI-3%PVA-CNC EOEPI-5%PVA-CNC

–50 (a)

0 Temperature (°C)

50

100

50

100

104

Storage modulus (MPa)

103

102

101

100

10–1

EOEPI-5%PVA-1%CNC EOEPI-5%PVA-5%CNC EOEPI-5%PVA-10%CNC EOEPI-5%PVA-15%CNC EOEPI-5%PVA-20%CNC

–50 (b)

0 Temperature (°C)

Figure 7.9 Representative DMA traces for (a) neat EO-EPI, EO-EPI/PVA blends with 1%, 3%, and 5% w/w PVA (filled symbols), and EO-EPI/PVA/CNC nanocomposites containing 10% w/w CNCs (open symbols) and 1%, 3%, and 5% w/w PVA and (b) EO-EPI/PVA/CNC nanocomposite containing 5% w/w PVA and 1%, 5%, 10%, 15%, and 20% w/w CNCs. Source: Meesorn et al. 2017 [39]. Reproduced with permission of ACS.

of a percolation model that is commonly used to describe the stiffness of nanocomposites with a high-aspect-ratio nanofiller (Figure 7.10). Because of their nanoscale, spindle-like geometry, and functionalizable surface, CNCs also can potentially act as effective nucleating agents in semicrystalline matrices (such as polyhydroxyalkanoate [PHA], polyvinylidene fluoride [PVDF], glycerol-plasticized starch, and so on). On the contrary, sometimes, they may be anti-nucleating agents to impede the nucleate crystallization of the polymer matrix.

215

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

109

108 E′ (Pa)

216

107

106 0.00

cCNC (DMF) cCNC (DMSO) cCNC+PVA (DMSO) cCNC Theoretical cCNC Model FL (DMSO) cCNC+PVA Model FL (DMSO) ICNC Theoretical

0.05

0.10

0.15

0.20

Volume fraction of CNC

Figure 7.10 Experimental data (symbols) and values predicted by a percolation model (lines, see text for details) for the tensile storage modulus (E′ ) of EO-EPI/CNC nanocomposites made with CNCs isolated from cotton. Shown are the values for PVA-free nanocomposites that were cast from DMF (red line, squares, taken from Ref. [46]) or DMSO (black line, circles) and 5% w/w PVA composites cast from DMSO (pink line, triangles). In addition to these data/fits a corresponding fit of the E ′ values of nanocomposites made with CNCs isolated from tunicates is shown for reference purposes (green line, fit from Ref. [46]). Source: Meesorn et al. 2017 [39]. Reproduced with permission of ACS.

Dufresne et al. [47] reported cellulose whiskers obtained from sea animals (tunicates) as nanofillers to improve the mechanical properties of medium-chain-length poly(hydroxyalkanoate) (Mcl-PHA) latex. The author used differential scanning calorimetry (DSC) measurements to determine the melting temperature (T m ) and heat of fusion (ΔH m ) of the thermoplastic matrix. X-ray diffraction was also used as a technique to elucidate the eventual modifications in the crystalline structure of the matrix after the addition of whiskers. They can draw the conclusion that cellulose whiskers probably acted as a nucleating agent for PHA by analysis and observation, producing a transcrystalline region around the cellulose whisker. This resulted in a disastrous decrease in the mechanical properties of the semicrystalline Mcl-PHA composites filled with tunicin whiskers, as soon as the melting point of the matrix is reached. It is ascribed to the breakup of crystalline domains and, at the same time, to the breakup of the cellulose network formed through transcrystalline domains. This phenomenon occurs, regardless of the whiskers content, up to 6 wt%, probably. Dufresne and coworker [48] also reported that tunicate whiskers can act as a nucleating agent in glycerol-plasticized starch (amylopectin)-based composites. For a low loading level (up to 3.2 wt%), a classical plasticization effect of water was reported. However, an anti-plasticization phenomenon was observed for higher whiskers contents (6.2 wt% and above). These observations were discussed according to the possible interactions between hydroxyl groups on the cellulosic

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

surface and amylopectin, the selective partitioning of glycerol and water in the bulk amylopectin matrix or at the whisker surface, and the restriction of amorphous amylopectin chains mobility in the vicinity of the amylopectin crystallite coated filler surface. Furthermore, for glycerol-plasticized starch-based systems, the formation of the transcrystalline zone around the whiskers was assumed to be due to the accumulation of plasticizer in the cellulose/amylopectin interfacial zones, improving the ability of amylopectin chains to crystallize. These specific crystallization conditions were evidenced at high moisture content and high whiskers content (>16.7 wt%) by DSC and WAXS. It was displayed through a shoulder on the low-temperature side of the melting endotherm and the observation of a new peak in the X-ray diffraction pattern. This transcrystalline zone could originate from a glycerol–starch V structure. In addition, the inherent restricted mobility of amylopectin chains was put forward to explain the lower water uptake of cellulose/starch composites for increasing filler contents. Tunicate cellulose whiskers were also used in reinforcing sorbitol-plasticized waxy maize starch [49]. The unfilled sorbitol-plasticized starch matrix exhibits a single glass–rubber transition temperature, which decreases with increasing moisture content due to the plasticizing effect of water. This plasticizing effect induces the crystallization of amylopectin chains in a moist atmosphere. Compared to the previous study the main difference was the presence of a single glass–rubber transition for sorbitol-plasticized materials instead of two distinct ones as in the case of glycerol. It seems that the unfilled plasticized material was more homogeneous when using sorbitol. When tunicin whiskers are added to the plasticized starch matrix, they get homogeneously dispersed in the system. The overall crystallinity of the system increases continuously upon the addition of tunicin whiskers. This phenomenon results most probably from a nucleating effect of the filler. The T g of the plasticized starch matrix was found to increase slightly up to about 15% whiskers loading. This was ascribed to the presence of stiff crystalline whiskers and to the increase in crystallinity upon whisker addition, both resulting in a restriction of the mobility of amorphous amylopectin chains. At higher whisker content a decrease of T g was observed. It is most probably due to an increase in the concentration of the plasticizer in amorphous domains, resulting from the crystallization of starch. Both DSC and WAXS measurements (Figures 7.11 and 7.12) showed no evidence of preferential migration of the plasticizers toward the cellulose and transcrystallization of amylopectin on cellulose surface contrary to what was reported for glycerol-plasticized systems. The same nucleating effect of neat CNCs was also reported for POE-based composites [29]. For isotactic polypropylene (iPP) filled nanocomposites, the effect was observed for cellulosic whiskers coated with a surfactant [50]. In these systems, the nucleating effect was supported by optical polarized light observations of composites during their isothermal crystallization. When whiskers are coated with the surfactant, they are very good nucleating agents for iPP. On the contrary, untreated whiskers do not modify the crystallization of iPP. When whiskers are grafted by maleated iPP, cellulose acts as an anti-nucleating agent [50]. It seemed that the nucleating effect of cellulosic whiskers was mainly governed by surface chemical considerations. Indeed, Bonini [50] did not observe any

217

ENDO

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

Figure 7.11 DSC thermograms for 15% filled tunicin whiskers/sorbitol-plasticized waxy maize starch composites for different moisture contents. The relative humidity conditions are indicated in the figure. Source: Mathew and Dufresne 2002 [49]. Reproduced with permission of ACS.

98% RH

75% RH 58% RH

EXO

218

43% RH 31% RH 0% RH

–100

100 200 0 Temperature (°C)

Figure 7.12 Wide-angle X-ray diffraction patterns for 75% RH conditioned tunicin whiskers/sorbitol-plasticized waxy maize starch composites. The tunicin whisker contents are indicated in the figure. Source: Mathew and Dufresne 2002 [49]. Reproduced with permission of ACS.

25 wt%

20 wt%

15 wt% 10 wt% 5 wt%

0 wt%

5

10

15

20 2θ (°)

25

30

35

nucleating effect of the iPP matrix when using either untreated or anhydride maleic grafted iPP tunicin whiskers. Grunert and Winter [51] assumed from DSC measurements that native bacterial filler impedes the crystallization of the cellulose acetate butyrate (CAB) matrix, whereas silylated ones help to nucleate the crystallization. Neat CNCs are polar and hydrophilic, which makes them difficult to disperse well in nonpolar, hydrophobic thermoplastic polymers. Furthermore, due to its large specific surface area or high surface energy, CNC tends to aggregate. However, it is usually accepted that the dispersion state of CNCs in the polymer

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

matrix, the interface interaction between CNCs and polymer matrix, as well as the formation of CNC percolating network within the polymer matrix, are the three most critical factors determining the reinforcement of CNCs in the polymer matrix [1, 52]. It is only when the CNCs are homogeneously dispersed within the polymer matrix that their advantages, i.e. large aspect ratio, high modulus, and large surface area to interact with the polymer matrix, can be fully realized. In addition, the strong adhesion occurring at the CNC–polymer interface and the creation of a rigid CNC percolating network within the polymer matrix would facilitate an effective stress transfer from the soft polymer matrix to rigid CNCs, leading to improvement in the strength and toughness of polymer nanocomposites. One important and easy method to maximally improve the dispersion state and construct strong interfacial adhesion is to change the chemical groups on the surface of CNCs. A cationic surfactant dodecyltrimethylammonium chloride was used to hydrophobically modify CNCs by ionic interaction between the negatively charged sulfate groups on the CNCs and the positively charged ammonium groups from the surfactant [53]. They found that modified CNCs exhibited superior reinforcement in poly(vinyl acetate) matrix over CNCs due to the accelerated physical aging process as well as the improved dispersion state and interfacial interaction. As shown in Figure 7.13, the tensile strength and the modulus increased dramatically as small amounts of modified CNCs are 35

PVAc 1% 3% 5% 10% 20%

modCNC/PVAc

30 25 20

Tensile stress (MPa)

Tensile stress (MPa)

35

15 10 5 10

20

(a)

30

40

50

20 15 10 5 0 0

60

35

3.0 2.5 2.0 1.5 Predicted modCNC/PVAc CNC/PVAc

1.0 0.5

(c)

15 10 200 300 400 500 Tensile strain (%)

Young’s modulus

3.5

Tensile strength (MPa)

Young’s modulus (GPa)

4.0

0.0

5

(b)

Tensile strain (%)

PVAc 1% 3% 5% 10%

25

0 0

CNC/PVAc

30

25 20 15 10

5

10

15

CNC content (wt%)

20

(d)

modCNC/PVAc

5 0

0

Tensile strength

30

CNC/PVAc

0

5

10

15

20

CNC content (wt%)

Figure 7.13 Typical stress–strain curves for (a) modCNC/PVAc nanocomposites and (b) unmodCNC/PVAc composites, (c) Young’s modulus, and (d) the tensile strength for CNC/PVAc and modCNC/PVAc as a function of cellulose nanocrystal content. Source: Ansari et al. 2015 [53]. Reproduced with permission of ACS.

219

220

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

added (Figure 7.17b). The modulus increased from 0.24 to 0.92 GPa and the strength from 4.6 to 15 MPa as 1 wt% modified-CNC was added. The addition of unmodified CNC also had a positive effect on strength and modulus, but the increase was much lower. Cationically modified CNCs were synthesized through an alkaline activated nucleophilic reaction between CNCs and a cationic surfactant, i.e. (2,3-epoxypropyl)trimethylammonium chloride (EPTMAC) [54]. The presence of positively charged EPTMAC substituents on the surface of cationically modified CNCs (mCNCs) was designed to (i) increase their dispersion state through electrostatic repulsion and (ii) further enhance interfacial interaction through the construction of electrostatic attraction between the positively charged EPTMAC substituents of mCNCs and the negatively charged carboxylate group of carboxymethyl cellulose (CMC), as shown in Figure 7.14 [55, 56]. Mechanical results indicated that cCNCs (mCNC prepared via concentrated reaction systems) exhibited superior reinforcing capacity over CNCs and dCNCs (mCNC prepared via diluted methods) at any loadings due to the improved dispersion state and interfacial bonding by electrostatic attraction between positively charged EPTMAC groups of cCNCs and negatively charged CMC molecular chains. However, dCNCs had worse reinforcing capacity than CNCs at high loadings due to the poor dispersion of dCNCs and the limited EPTMAC groups available for binding CMC molecular chains by electrostatic attraction, as shown in Figure 7.15.

–

OOCH2C

O

HO O

OH

Hydrogen bond

n

OH

EPTMAC substituent

O

HO

cCNC

OH

CMC

O O

HO

dCNC

OH

O HO

CH2COO–

dCNC

CMC

HO

OH

OH

OH

OH

HO OH HO OH OH HO OH HO HO OH HO OH HO OH HO OH OH HO OH

HO

HO

HO

HO

HO

CNC

n

OH

– CH2COO OH O

HO

OH

O

(c)

dCNC

OH

O

HO

OH HO O

HO

OH

CMC

O

(b)

HO

O

– OOCH2C

(a)

Electrostatic attraction

Figure 7.14 Schematic illustration on the reinforcing mechanisms for (a) CNC/CMC, (b) DCNC/CMC, and (c) cCNC/CMC films (left). Source: Li et al. 2016 [54]. Reproduced with permission of Elsevier.

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

60

Stress (MPa)

40 30 20 10

CNC/CMC dCNC/CMC cCNC/CMC

55 Tensile strength (MPa)

10 wt% 5 wt% 3 wt% 1 wt% 0 wt%

cCNC/CMC 50

0

50 45 40 35 30

0

10

20 30 40 Strain (%)

(a)

50

60

0 1 3 5 10 (b) Concentration of CNC or mCNC (wt%)

700 60 Elongation at break (%)

Young’s modulus (MPa)

600 500 400 300 200 100

40 30 20 10 0

0 (c)

50

0

1

3

5

10

(d)

0

1

3

5

10

Figure 7.15 Mechanical properties of CNC/CMC and mCNC/CMC films: (a) Typical stress–strain curves for cCNC/CMC films with different cCNC concentrations; and comparative study on the mechanical properties of CNC/CMC, dCNC/CMC, and cCNC/CMC films: (a) tensile strength, (b) Young’s modulus, and (c) elongation (right). Source: Li et al. 2016 [54]. Reproduced with permission of Elsevier.

It is well known that enhancements in the mechanical and thermal performances of biopolymer-based nanocomposites can be tailored by interactions between the functional groups of CNC and the groups of biopolymer. Especially, the more hydroxyl groups CNCs have, the more hydrogen bonds between the nanofiller and matrix form [57–69]. Nevertheless, structural design of CNCs for modulating the formation of hydrogen bonds in nanocomposites is still poorly investigated. Therefore, Yu et al. designed some ways to endow CNCs with different numbers of hydroxyl groups via HCOOH/HCl and C6 H8 O7 /HCl hydrolysis of commercial microcrystalline cellulose (MCC) according to a method previously described in their literature [70–72], as shown in Figure 7.16. In order to evaluate the impact of hydrogen bonding interactions on the properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanocomposites, CNC samples with different amounts of hydroxyl groups were prepared. Bionanocomposites composed of unmodified CNCs, cellulose nanocrystal formates (CNeF), and cellulose nanocrystal citrates (CNeC) in a PHBV matrix

221

222

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

O O CH

CH3 CH CH2 C

O

OH

CH2 CH2 CH2 C

O

O

OH

HB

HV

O

OH O OH O

HO

PHBV

O

CN

O O

HO O

OH

OH O

O O

OH

O CH O OH

HO

OH

O

CN-F

O

O O CH CH3

O O CH

HO

O

O

HO

OH

O

CN-C

O OH

O

OH O HO

(a) O

CNPs

PHBV

O

OH

OH O

O HO

O

Hydrogen bonding interaction

O

OH O

O O HO O O

O O

O O

HO

OCH O

HO HCO O CH OH

O

OCH

HO

O

OH O

HO OH

O HO OH

HO O C HO O O OH O

OH

OH

O

HO

OH

OH

O

OH

HO O C

O

HO O

10% CN/PHBV

O

OH

O

HO O C O

10% CN-F/PHBV

O

O C

O

O

O OH OH

OH

O OH O O OH

O

O OH O

10% CN-C/PHBV

(b)

Figure 7.16 (a) For PHBV and different nanocrystals (CN, CNeF, and CNeC) and the nanocomposites, (b) schematic representation for possible formation of hydrogen bonding interactions in 10% CN/PHBV, 10% CNeF/PHBV, and 10% CNeC/PHBV. Source: Yu and Yao 2016 [70]. Reproduced with permission of Elsevier.

were produced via simple solution casting. CNCs with different surface groups (CNeF, CN, and CNeC) were successfully prepared and used as reinforcement materials for PHBV nanocomposites. It confirms that at good dispersion state of nanofillers, CNeC with more polar groups could form more intermolecular hydrogen bonds with PHBV matrix. As a result, at the same loading level, greater improvements in the mechanical and thermal properties were found compared to other nanocomposites. Compared with neat PHBV, tensile strengths and T max of 10% CNeC/PHBV were improved by 187% and 48.1 ∘ C, respectively, as shown in Figure 7.17. The nanocomposites showed a 64% reduction in water vapor permeability (WVP), indicating stronger reinforcing capability on PHBV matrix than CNCs and CNeF. This could be due to stronger interactions (more hydrogen bonds or hydrogen bond network) and good nanoparticle dispersion.

100

Tensile strength (MPa)

30

(a)

BV Elongation to break (%) PH -C/ CN 10% V HB -F/P CN % 0 1 V HB N/P C 10% BV PH

80 60 40 20 0 100

0

270 CN-F CN CN-C

240

10% CN-F/PHBV 10% CN/PHBV 10% CN-C/PHBV PHBV

200

(b)

WVP*10

–0.4

(c)

(Kg m/m/s/Pa)

9

–0.8

6

CN-F CN CN-C 10% CN-F/PHBV 10% CN/PHBV 10% CN-C/PHBV PHBV

–1.2 –1.6 –4

–2

0

θ (T–Ts)

2

3

BV PH C 10%

V HB N/P 10

BV /PH N-F %C

BV /PH N-C %C

Isooctane Ethanol 10%

160

120

BV PH 80

/PH CN

BV

10%

/PH

-F CN

BV

10% 40

10

4

BV /PH

400

Water uptake (%) 14

In[In(W0 /WT)]

BV PH

-C CN 10% V BV B H H /P -F/P CN CN 10% 10%

12

0.0

(d)

Tmax (°C)

210

300 Temperature (°C)

Overall migration (μg/kg)

200 60

T0 (°C)

300

Young’s modulus (MPa)

Weight loss (%)

250

C 10%

BV /PH N-C

0

(e)

(f)

Figure 7.17 Tensile strength, Young’s modulus, and elongation at break (a), TGA curves (b), T 0 and T max (c), and the plot of ln [ln(W 0 /W T )] vs. q for the main thermal degradation stage (d), water uptake and water vapor permeability (e), overall migration data in ethanol 10% (v/v) and isooctane (f ) for neat PHBV and the nanocomposites reinforced with different CNCs. Source: Yu and Yao 2016 [70]. Reproduced with permission of Elsevier.

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

OH

OH

O

OH

CH(CH2)8CH3

+ O OH

OH

R

R

R

DMAP heat

OH

R

O

CNC

DDSA

R

R

DDSA-CNC O

R = OH or

CH(CH2)8CH3 O OH O

Figure 7.18 Grafting reaction of DDSA on CNC surface. Source: Miao and Hamad 2016 [73]. Reproduced with permission of Elsevier.

Grafting of alkenyl chains on the surface of CNCs was achieved by the reaction between succinic anhydride and the CNCs’ hydroxyl groups [73]; (2-dodecen-1-yl) succinic anhydride (DDSA) was chosen as the modal anhydride, as shown in Figure 7.18. Moreover, nanocomposites of PU and ethylene propylene dienemonomer (EPDM) reinforced with DDSA–CNC were prepared. Percolation theory has been used to describe improvements in the mechanical modulus of primarily compatible CNC-polymer composite systems [1, 74], whereby the performance is governed by efficient stress transfer owing to good dispersion of the nanoparticles in a compatible matrix above a critical percolation threshold. In this case, CNC had an aspect ratio of 11, and the corresponding percolation threshold would be 6 vol% CNCs. However, the results clearly indicated significant improvement in the tensile strength and Young’s modulus of the nanocomposites at CNCs’ volume fraction 1/4 of 2.0% and 4.1%, as shown in Figure 7.19, well below the percolation threshold. It was 10000 CNC volume fractions

1000 Storage modulus (MPa)

224

8.3% 100

4.1% 2%

10 1

0%

Sample broke

0.1 0.01 –100

–50

50 0 Temperature (°C)

100

150

Figure 7.19 Storage moduli for PU and DDSA-CNC-PU nanocomposites at different CNC volume fractions. Source: Miao and Hamad 2016 [73]. Reproduced with permission of Elsevier.

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

(a)

(b)

(c)

Figure 7.20 (a) The gel-like appearance of 3 w/v% DDSA-CNCs suspension in MEK; (b) disruption of the gel structure by shaking; and (c) gel re-formation after settling. Source: Miao and Hamad 2016 [73]. Reproduced with permission of Elsevier.

proposed that a significant increase in tensile strength and modulus occurred with DDSA-CNCs-PU nanocomposites at fairly low CNC volume fractions because of the appreciable interactions between DDSA-CNC and PU through the formation of hydrogen bonding [75]. It was evident that the abundant carboxylic and hydroxyl groups on the DDSA-CNCs surfaces had strong interactions leading to the formation of a gel-like structure in organic solvents (Figure 7.20). Given the large quantity of N and O atoms in PU molecules, it is highly likely that hydrogen bonds can form between DDSA-CNCs and PU. Hence, the superior performance of DDSA-CNCs-PU nanocomposites can be attributed to both the good dispersion of DDSA-CNCs in the PU matrix and the excellent interfacial properties (i.e. compatibility) between them. 7.2.2 Effect of Segmental Entanglement Mediated with Grafted Chains on CNC Surface The chemical grafting of polymer chains of chemical structure similar to that of the matrix on the surface of nanocrystals is expected to create a near-perfect interface when using these nanoparticles to process nanocomposites using a matrix with similar chemical nature. Habibi and Dufresne [68] prepared nanocomposite films using unmodified and polycaprolactone (PCL)-grafted CNC nanoparticles as filler and PCL as the matrix, and they found that PCL-grafted nanoparticles were easily dispersed when compared to the unmodified system. They demonstrated the transformation of CNC nanoparticles into a co-continuous material through long-chain surface chemical modification. The mechanical properties of PCL-based nanocomposites reinforced with either unmodified or PCL-grafted cellulose have been investigated in the linear (DMA) and nonlinear range (tensile tests). The addition of PCL-grafted ramie whiskers

225

226

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

Table 7.3 Rubbery tensile storage modulus estimated at 0 ∘ C (E’) and high tensile mechanical properties of cellulose nanocrystals/PCL nanocomposite films.

Filler

None cellulose nanocrystals

Filler modification

Filler content (wt%)

E′0 (MPa)

E (MPa)

𝛔b (MPa)

𝛆b (%)

None

0

518

231 ± 25

22.0 ± 2.0

637 ± 40

10

776

33 ± 30

16.7 ± 1.5

395 ± 27

20

660

30 ± 27

13.9 ± 1.7

8.0 ± 1.0

30

524

25 ± 18

7.64 ± 1.1

4.0 ± 0.6

10

690

252 ± 15

20.2 ± 2.0

531 ± 25

20

847

345 ± 20

21.6 ± 1.6

420 ± 23

PCL10000

PCL24500

30

879

356 ± 22

20.0 ± 1.2

185 ± 12

40

889

372 ± 20

18.3 ± 1.8

64.0 ± 3.0

50

1340

380 ± 17

16.8 ± 1.3

28.0 ± 1.7

10

605

255 ± 16

20.0 ± 1.9

598 ± 34

20

671

294 ± 18

15.1 ± 1.5

200 ± 23

30

982

328 ± 21

15.7 ± 1.5

13.5 ± 1.5

40

1019

390 ± 29

17.6 ± 2.0

12.0 ± .2

50

1028

442 ± 31

18.7 ± 2.5

8.60 ± 0.8

results in a continuous increase in the tensile modulus in agreement with DMA experiments, as shown in Table 7.3. It has been shown that the grafted polymeric chains along with unbounded polymeric chains from the matrix form a co-continuous phase at the interface through physical entanglement or co-crystallization. Consequently, the physical performances of the final nanocomposites were greatly enhanced upon the incorporation of CNW-based nanoparticles. In Goffin and Habibi ‘s work [65, 68], CNC was subjected to different surface chemical modifications including the grafting of poly(lactic acid) (PLA) and PCL homopolyesters as well as P(LA-b-CL) diblock copolymers (Figure 7.21). The resulting substrates were incorporated in a PLA/PCL blend having an equivalent weight ratio of both components where extreme immiscibility was expected. As shown in Figure 7.22, the addition of unmodified CNW or their modified counterparts in a quantity as tiny as 2 wt% significantly modifies the rheological behavior because the modulus of the polymer blend largely increases in the entire frequency range. Such an effect finds its explanation in the previously suggested interfacial compatibilization. Interestingly, the addition of polyester-grafted CNW further enhances the low-frequency modulus. Indeed, when the CNWs are grafted by either homopolyester (PCL or PLA) or by the P(CL-b-LA) diblock copolymer, the G′ values recorded at low frequency increases by about 1–2 orders of magnitude with respect to the PCL/PLA blend loaded with unmodified CNW. This “solid-like” behavior can be accredited to the appearance of strong interactions between grafted polyester chains and the components of the blend, inducing

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

PCL O OH

OH

OH

CNW

CNW

Toluene, Sn(oct)2 95 °C, 24 h

OH

OH

m

PLA

O

H3C OH

OH

O

n

O O

CNW

OH

CH3

O

CNW

Toluene, Sn(oct)2 80 °C, 24 h

PLA

PCL O OH

OH CNW

OH

n

O

H3C OH

O

m

O O

CNW

PCL O

OH CH3 CNW

Toluene, Sn(oct)2 95 °C, 24 h

Toluene, Sn(oct)2 95 °C, 24 h

Figure 7.21 Sketch of the synthesis of the CNC-g-polymers as initiated from the CNC surface. Source: Goffin et al. 2012 [65]. Reproduced with permission of ACS.

100 000 10 000

G′ (MPa)

1000 100 10 1 0.1 0.01 0.001 0.0001 0.01

0.1

1 Frequency (Hz)

10

Figure 7.22 Dynamic storage modulus versus frequency as recorded for (⚫) PCL/PLA binary and different ternary blends: (◽) PCL/PLA/unmodified CNW, (◼) PCL/PLA/CNW-g-PCL/CNWg-PLA, and (▴) PCL/PLA/CNW-g-P(CL-b-LA). Source: Goffin et al. 2012 [65]. Reproduced with permission of ACS.

227

228

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites O O

+ OH

H

O

O

OH

SnCl2 O

140 °C, 6 h

O

O

H

O

HO

n

Om

H

n

PEG

PCL

CON

80 °C, 3 h

NCO

CH2

MDI O N H 2 CH

O

O O

O O

H N

O O

m

n

CH

2 CH

2

N

CO

OH HO O

O O HO OH

OH

NCO

H

O

+

N H

O

O

n

N

O

CH

2

N

CO

NCO

O OH n

PCL PEG CNC

Figure 7.23 Synthetic route of the PEG-PCL-CNC nanocomposite network. Source: Liu et al. 2015 [76]. Reproduced with permission of ACS.

a strong adhesion between them. This effect is even more pronounced for the PCL/PLA/CNW-g-PCL/CNW-g-PLA nanocomposite. One type of thermo-responsive and water-responsive shape-memory nanocomposite network with PCL and poly(ethylene glycol) (PEG) as soft segments and CNC nanofiller as cross-linkers was successfully prepared, as shown in Figure 7.23 [76]. The chemical cross-linking of 10% CNC with both PCL and PEG with low molecular weights could significantly improve the mechanical strength of the nanocomposite network. Simultaneously, the PEG [54]−PCL[36]−CNC[10] nanocomposite exhibited an excellent thermo-responsive and water-responsive shape-memory effect; as the PEG component increased, the storage modulus of the nanocomposites significantly increased (in Figure 7.24). Furthermore, good cytocompatibility was maintained upon the introduction of CNC into the biocompatible PCL and PEG polymer. A strategy involving two PEG/polyoxyethylene (PEO) layers on the surface of CNCs using chemical grafting and physical adsorption was investigated [77]. These modifications did not affect the crystallinity of the nanoparticles and improved their thermal stability. The basic idea is consistent with the possibility of entanglements and co-crystallization between short PEG chains and long PEO chains, which can closely wrap and protect the surface of the nanocrystals. Meanwhile, the chemical and physical compatibilization imparted by PEG and PEO layers promoted interfacial interactions between cellulosic nanoparticles and apolar matrix. Preliminary results showed the possibility of processing

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix PEG[50]-PCL[50]-CNC[10] PEG[60]-PCL[40]-CNC[10] PEG[70]-PCL[30]-CNC[10]

800

0.30

PEG[50]-PCL[50]-CNC[10] PEG[60]-PCL[40]-CNC[10] PEG[70]-PCL[30]-CNC[10]

0.25 Tanδ

Storage modulus (MPa)

1000

600

0.20

400

0.15

200 0.10 0 –10

(a)

0

10

20 30 40 50 Temperature (°C)

60

70

20

(b)

30

40 50 60 70 Temperature (°C)

80

90

Figure 7.24 The storage modulus and tan 𝛿 of PEG-PCL-CNC nanocomposite network. Source: Liu et al. 2015 [76]. Reproduced with permission of ACS. 100

Weight loss (%)

80

60

40

MPEG-NH2 PEO5M CN CN-g-PEG CN/PEO CN-g-PEG/PEO

20

0 50

150

250 350 Temperature (°C)

450

550

Figure 7.25 TGA thermograms for pure MPEG-NH2, PEO5M; pristine CN, grafted CN-g-PEG; and CN/PEO and CN-g-PEG/PEO complexes. Source: Lin 2013 [77]. Reproduced with permission of ACS.

modified nanocrystals (CN-g-PEG/PEO) at high temperature (200 ∘ C), avoiding the degradation of the nanoparticle and providing good dispersion and compatibility between modified nanocrystals and the matrix, as shown in Figure 7.25. Furthermore, the surface modifications of nanocrystals did not block the interactions between the nanoparticles, and kept at least partially the three-dimensional network architecture, which is beneficial for the improvement of mechanical and barrier properties of extruded nanocomposites (in Figure 7.26). 7.2.3 Role of Co-continuous Structure Derived from Chemical Coupling of Filler/Matrix Modification, or more accurately functionalization, of CNC surfaces is not only one of the most effective ways to change the compatibility between CNC and the matrix, but it can also introduce special chemical groups that can react with

229

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

6 × 106 CN-PS CN/PEO-PS CN-g-PEG-PS CN-g-PEG/PEO-PS

5 × 106 Storage modulus E′ (Pa)

230

4 × 106

WRc = 7.49 wt%

3 × 106 2 × 106 1 × 106 0 0

2

4

6 8 10 12 14 16 Nanocrystal content (wt%)

18

20

22

Figure 7.26 Storage modulus at Tg + 10 ∘ C (389 K) and fitted curves for extruded nanocomposites vs. nanocrystal content. Source: Lin 2013 [77]. Reproduced with permission of ACS.

the matrix; thus the mechanical properties of composites can greatly improve. In the following part, we briefly present some special modifications that can build chemical coupling between CNCs and the matrix. 7.2.3.1 Thiol-ene Coupling Process Between Modified Cellulose Nanocrystals (CNCs) and Matrix

CNCs and rubber matrix are incompatible, and the molecular scale interactions between them are not sufficient to produce a satisfactory enhancement in properties. Moreover, cellulose nanoparticle agglomerates lead to poor dispersion in the rubber matrix and the nanoparticles act as points of stress concentration, resulting in low strain to failure of the composites. The improvement in the mechanical properties of the composites by the addition of CNCs was not very high, as expected due to insufficient interfacial adhesion and interaction between the reinforcements and the matrix. For improving the compatibility between the nanocellulose and the rubbery matrix, the surface of CNCs was modified with a hydrocarbon chain with a double bond at the chain end [78], as shown in Figure 7.27. A bifunctional dithiol cross-linker was then used with a UV-initiator to create covalent bonds at the interface between modified CNCs and the polybutadiene (PBD) matrix by means of a thiol-ene reaction [79] triggered by UV radiation. The resulting composites showed increased mechanical properties owing to the intercalated structures of alternating reinforcing modified CNCs and rubbery PBD, as shown in Figure 7.28. Indeed, internal trisubstituted double bonds have been demonstrated to retain enough reactivity for the thiol-ene coupling process via a free-radical mechanism [80–82]. The potential of using CNCs not only as the reinforcing filler but also as the cross-linking reagent in biobased natural rubber matrix was explored [83]. To achieve a synergistic effect of reinforcement and cross-linking at the interface,

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

Poly(butadiene)

Cross-linker

mCNC

HS

SH 7

O

n

n

Mixing UV-light

100 nm

(a) mCNC/PBD 80/20 w/w

200 nm

1 μm mCNC/PBD 20/80 w/w

Intercalated domain

200 nm

(b)

1 μm

Figure 7.27 A scheme of the mCNC/PBD composites with structures at two length scales. (a) Composite films were prepared from mCNCs, a dithiol cross-linker, PBD, and a UV-initiator by mixing and cross-linking with UV-light. (b) Tightly packed self-assembled aligned domains are formed by intercalating reinforcing mCNCs and rubbery PBD, forming a periodicity of c. 40 nm based on TEM. Such tightly packed domains are separated by thicker layers of PBD. The TEM micrographs are for the compositions mCNC/PBD 80/20 w/w and 20/80 w/w. Source: Rosilo et al. 2013 [78]. Reproduced with permission of ACS. 250

150 100

10

Strain (%)

15 σ (MPa)

E (MPa)

200

50

5

50 0

0 0.0

(a)

100

20

0.5

1.0 (b)

0.0 0.5 1.0 mCNC fraction (w/w) (c)

0 0.0

0.5

1.0

Figure 7.28 The tensile mechanical properties for cross-linked mCNC/PBD composites, showing a percolation transition in the range of 30–35 wt% of mCNCs due to space filling of the intercalated mCNC/PBD domains. The percolation is qualitatively indicated by the arrows. Source: Rosilo et al. 2013 [78]. Reproduced with permission of ACS.

231

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites Mercaptoundecanoyl cellulose nanocrystals (m-CNCs) O O O O

H3C

NR/m-CNCs nanocomposite

SH SH

H3C

S UV irradiation

CH3 CH3

Photoinitiator Irgacure 651

Natural rubber (NR)

S

CH3

H3C

Figure 7.29 Illustration of the structure for the NR/m-CNCs nanocomposites. Source: Kanoth et al. 2015 [83]. Reproduced with permission of ACS. 12 NR NR/CNCs-5 NR/CNCs-10 NR/m-CNCs-5 NR/m-CNCs-10

10 Tensile stress (MPa)

232

8

6

4 2

0 0

200

400

600 800 1000 Tensile strain (%)

1200 1400

1600

Figure 7.30 Tensile stress–strain curves of pure NR, NR/CNCs, and NR/m-CNCs. The designations 5 and 10 refer to weight fraction CNCs. Source: Kanoth et al. 2015 [83]. Reproduced with permission of ACS.

the CNC’s surface was modified by surface-grafted brushes using a long hydrocarbon chain with thiol (−SH) groups at the end, as shown in Figure 7.29. Nanocomposites were prepared by solution casting with modified CNCs dispersed in natural rubber. Covalent cross-links were formed between the double bonds of natural rubber molecules and the thiol groups by a photochemically initiated thiol−ene reaction. It was observed that the addition of unmodified CNCs and m-CNCs in the NR matrix resulted in an increase in modulus and tensile strength as compared to the neat NR, while the strain-to-failure values of NR/CNCs composites were similar to that for the neat NR (Figure 7.30). 7.2.3.2 Huisgen Cycloaddition Click Chemistry Between Modified CNCs and Matrices

Alkynylated cellulose nanocrystals (ACNC) were synthesized from the Huisgen click chemistry (in Figure 7.31), and ACNC was introduced as a green

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

Synthesis of 4-oxo-4-(prop-2-yn-1-yloxy)butanoic anhydride

OH +

O

O

O

DMAP

O

OH

O DCM, RT, 36 h

O O

O DCC

OH

O

O O

O

DCM, RT, 24 h

O

O

O O

(a) Synthesis of alkynyl-modified cellulose nanocrystals

4 6

*

OH O

O +

O HO

3

2

OH

1

O

O O

*

OR

80 °C, 5 h

O O

g c

O RO

DMAP, pyridine O

OR O

* (b)

O

5

*

R=

a

d

b

f

O e

or H

O

Figure 7.31 Chemical structures of 4-oxo-4-(prop-2-yn-1-yloxy) butanoic anhydride (a), and alkynyl-modified cellulose nanocrystals (b). Source: Chen et al. 2015 [84]. Reproduced with permission of RSC.

bionanofiller in the reactive glycidyl azide polymer (GAP)/propargyl-terminated polybutadiene (PTPB) polymeric matrix for the preparation of nanocomposites [84]. Enabled by the surface alkynyl groups, the ACNC nanoparticles can participate into the Huisgen cycloaddition click reaction of the GAP and PTPB components. Significantly, the presence of 1.0 wt% ACNC played a promisingly nano-reinforcing effect on the mechanical properties of the nanocomposite (GP2/ACNC-1.0), which showed the highest cross-linking density (0.74%) and a 103.3%, 100.0%, and 12.4% increase in the tensile strength, Young’s modulus, and elongation at break, respectively, compared to the neat GP2 material, as shown in Figure 7.32. The simultaneous enhancement of the strength, modulus, and toughness of the nanocomposites can be attributed to the strong interfacial adhesion and stress transferring from the chemical linkage between the nanofiller and the polymeric matrix. 7.2.3.3 Schiff’s Base Reaction Between Cellulose Nanocrystals (CNCs) and Matrix

Dash et al. [85] reported the first successful study on the synthesis and characterization of gelatin hydrogels chemically cross-linked by dialdehyde CNWs containing varying amounts of aldehyde groups. The aldehyde groups could act as a potential cross-linker since they will react with free amine groups of gelatin through Schiff’s base formation (Figure 7.33.). The increase in aldehyde groups resulted in an increase in the degree of cross-linking leading to the formation

233

50

8

40

6

30

4

20 Tensile strength Young’s modulus Breaking elongation

2

10

0

E (MPa)

10

εb (%)

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

σb (MPa)

234

0 0

1

2 3 4 ACNC content (wt%)

5

Figure 7.32 Mechanical properties of the GP2/ACNC nanocomposites involving tensile strength (𝜎 b ), Young’s modulus (E), and breaking elongation (εb ) affected by the various loading levels of ACNC. Source: Chen et al. 2015 [84]. Reproduced with permission of RSC.

NH2

NH2

H C +

H2N

O

N

CH NH2 Gelatin

NH2

O Dialdehyde cellulose nanowhiskers

N

H2N

H C N C H C H

NH2 NH2

N

C H Gelatin cross-linked with nanowhiskers

Figure 7.33 Schematic representation of the cross-linked hydrogels. Source: Dash et al. 2013 [85]. Reproduced with permission of Elsevier.

of a rigid dense network, observed by T 2 nuclear magnetic resonance (NMR) experiments, which reduced the water uptake ability of the hydrogels. Further, the increase in the degree of cross-linking improved the mechanical properties of hydrogels by 150% and increased the thermal stability of the gels, as the gels did not degrade until 50 ∘ C. There was a significant increase in the storage modulus of chemically cross-linked hydrogels compared to the physical gel, and the increase in the degree of cross-linking led to an increase in the storage modulus of the cross-linked gels as shown in Figure 7.34b, which is also reflected in the tan 𝛿 plot (Figure 7.34c). This is mainly attributed to the influence of chemical interaction between gelatin and dialdehyde nanowhiskers. A new class of CNC-reinforced nanocomposites based on injectable polysaccharide hydrogels filled with CNCs was presented, which can act as simple fillers (unmodified CNCs) or as chemical cross-linkers (aldehyde-functionalized CNCs) [86, 87], as shown in Figure 7.35. The CNC-cross-linked hydrogels exhibited significantly higher elastic moduli (>140% increase at peak strength) relative to unfilled hydrogels without significantly impacting the pore structure of the hydrogels. Figure 7.36 shows the compressive stress–strain behavior

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

Storage modulus (G′) and Loss modulus (G″) (Pa)

10000 Storage modulus (G′) Loss modulus (G″)

1000

100 0

10

(a)

20 30 40 Frequency (rad/s)

50

60

Storage modulus (Pa)

32000 28000 24000 Gelatin 0.14% 4.32% 13.02% 17.30%

20000 16000 12000 0

10

(b) 0.040

20 30 40 Frequency (rad/s)

50

60

30 40 20 Frequency (rad/s)

50

60

Gelatin 0.14% 4.32% 13.02% 17.30%

0.035

Tan δ

0.030 0.025 0.020 0.015 0.010 0 (c)

10

Figure 7.34 (a) Dynamic rheological observations of the gelatin gels. (b) Effect of chemical cross-linking on the storage modulus of the gelatin gels. (c) tan 𝛿 value of the hydrogels. Source: Dash et al. 2013 [85]. Reproduced with permission of Elsevier.

235

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

(a) H

HO

2 NH

N

NH N OH H2

OH HO O H C CH O OH

OH

NHNH2-CNC + CHO-CNC

(b)

(c)

R-NHN= CH-R′

Ice crystal

Freeze

(d)

Solvent exchange & critical point dry

Figure 7.35 Schematic representation of chemically cross-linked CNC aerogels prepared by a sol–gel process: (a) formation of initial sol from NHNH2 -CNCs and CHO-CNCs, (b) sol suspension, (c) gel formation with the growth of ice crystals, and (d) aerogel formation after solvent exchanging and critical point drying. Source: Yang et al. 2014 [86]. Reproduced with permission of ACS.

50

120

Water

100

Compressive stress (kPa)

Air Compressive stress (kPa)

236

2.0 wt% CNC 1.5 wt% CNC

80

1.0 wt% CNC 0.5 wt% CNC

60 40 20

1.5 wt% CNC 1.0 wt% CNC

30

0.5 wt% CNC

20 10 0

0 0 (a)

2.0 wt% CNC

40

20 40 60 80 Compressive strain (%)

100

0 (b)

20 40 60 80 Compressive strain (%)

100

Figure 7.36 Compressive stress–strain curves of aerogels prepared from 0.5, 1.0, 1.5, and 2.0 wt% CNC suspensions, from 0 to 95% strain in (a) air and (b) water. Source: Yang et al. 2014 [86]. Reproduced with permission of ACS.

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

CMC-NHNH2

dextran-CHO

CNCs or CHO-CNCs

200 nm

Figure 7.37 Schematic representation of injectable hydrogels reinforced with cellulose nanocrystals (CNCs), prepared using a double-barrel syringe. The cross-linking hydrogel components include hydrazide-functionalized carboxymethyl cellulose (CMC-NHNH2 ), aldehyde-functionalized dextran (dextran-CHO), and either unmodified CNCs or aldehyde-modified CNCs (CHO-CNCs). Source: Yang et al. 2013 [87]. Reproduced with permission of ACS.

of aerogels in air, where two regions are apparent: a slowly increasing stress response below 80% strain and an exponentially increasing stress response above 80% strain. Moreover, they presented a new type of chemically cross-linked “all CNC” aerogel based on hydrazone cross-links between NHNH2 -CNCs and CHO-CNCs (Figure 7.37). The final aerogel was ultralight and highly porous, surpassing other CNC aerogels reported to date. Chemically cross-linked CNC aerogels exhibit enhanced mechanical properties and shape recovery, especially on water, compared with previously reported physically cross-linked CNC aerogels. Specifically, aerogel shapes recover more than 85% after 80% compression, even after 20 compression and release cycles. The overall trend at low CNC loadings was for the storage modulus to increase relative to the unfilled hydrogels with increasing CNC concentration, followed by a decrease in modulus above a critical CNC concentration (Figure 7.38). 7.2.3.4

Esterification Reaction Between CNCs and The Matrix

Using CNCs as the cross-linking agent, Goetz et al. [88, 89] developed cross-linked cellulose whisker nanocomposites using poly(methyl vinylether-comaleic acid)−polyethylene glycol as the matrix (Figure 7.39). The cross-linking via an esterification reaction between cellulose and the matrix prevented nanowhisker aggregation by trapping the nanowhiskers in the cross-linked network and produced nanocomposites with unique mechanical behavior. 7.2.3.5 Chemical Coupling Between Hydroxyl Groups of Matrix and Aldehyded CNCs or Modified CNCs

Bifunctional reactive cellulose nanocrystals (RCNCs) with carboxyl and aldehyde functionalities were produced using sequential periodate and partial chlorite

237

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

8000

Figure 7.38 Dynamic storage modulus (G′ ) and loss modulus (G′′ ) of injectable hydrogels at different CNC or CHO-CNC loading concentrations, measured by parallel plate rheometry at a frequency of 75 rad/s. Confidence intervals are reported for N = 5 repeats with 95% confidence. Source: Yang et al. 2013 [87]. Reproduced with permission of ACS.

G′- CHO-CNCs G′- CNCs G″- CHO-CNCs

7000

G″- CNCs

6000

G′, G″ (Pa)

5000 4000 3000 2000 1000 0 0.0

HO

COOH

O

1.0

OH

OH HO O O OH

O

HO O O OH

OH O O HO O

O O OH

COOH

COOH

COOH OMe COOH OMe O

OH

OMe COOH O

COOH OMe

COOH HO O O HO

HO O O OH

O

O

O2 n

OMe

0.2 0.4 0.6 0.8 Loading concentration (wt%)

O

238

OMe COOH OMe

COOH OH O O

O

HO HO

COOH OMe

COOH

O HO O O OH HO

OH O

Figure 7.39 Schematic representation of cross-linked nanocomposites. Source: Goetz et al. 2009 [88]. Reproduced with permission of Elsevier.

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

OH

OH

OH

OH OH

O

O

RCNC

O

OH

–2H2O

RCNC

O OH OH

OH

OH

O

O

OH

OH

Figure 7.40 Schematic illustration of the acetal cross-linking of PVA with the aldehyde groups of RCNCs. Source: Sirviö et al. 2015 [90]. Reproduced with permission of ACS.

oxidations of wood cellulose fibers. Owing to the ability of aldehydes to form stable cyclic acetal bonds with hydroxyl groups of PVA, RCNCs were studied by Sirvio et al. [90] as cross-linking reinforcing agents for PVA (Figure 7.40). A notable increase in the tensile strength and in Young’s modulus was achieved with the addition of only 0.5% RCNCs to the PVA film. Aldehyde groups were found to be crucial in the reinforcement of PVA films: the use of reference CNCs without reactive aldehydes led to a significantly lower tensile strength and modulus (in Figure 7.41). The RCNCs produced were found to be efficient reinforcing agents for PVA films. CNCs were successfully modified by isophorone diisocyante (IPDI) monomer. In the IPDI molecule, the secondary isocyanate (the −NCO substituent directly connected to the ring) has a higher reactivity compared to the primary group (the −CH2 NCO group), which is attributed to the primary group being more sterically hindered by the neighboring methyl group, and the surviving pendant isocyanate group is then available for reaction with additional monomers of interest, such as polyols commonly used in polyurethane formulations (as shown in Figure 7.42) [91]. This chemistry can facilitate covalent bonding between CNC and the polymer matrix, a feature that could offer desirable mechanical properties and new functionalities to CNC composites. Girouard et al. [91] used modified CNCs as filler to reinforce polyurethane. They found that the tensile properties of composites significantly improved at 5 wt% m-CNC compared to the neat matrix, as shown in Figure 7.43. Increasing environmental awareness has promoted the development of eco-friendly materials incorporating renewable raw materials and using green synthesis routes such as waterborne dispersion, avoiding the employment of organic solvents and thus reducing the generation of volatile organic compounds. In addition, the hydrophilic waterborne polyurethanes (WBPUs) can also be composited with CNC for its ability to disperse in aqueous solution, which

239

7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

60

3500 RCNC1 RCNC2 CNC-Ref

55 50

RCNC1 RCNC2 CNC-Ref

3000 Modulus (MPa)

Tensile strength (MPa)

45 40 35 30

2500 2000 1500

25 1000

20

(a)

0%

0.5%

1% 1.5% 5% Amount of CNC

Strain (%)

240

60 55 50 45 40 35 30 25 20 15 10 5 0

(c)

10%

(b)

0%

0.5%

1% 1.5% 5% Amount of CNC

10%

RCNC1 RCNC2 CNC-Ref

0%

0.5%

1% 1.5% 5% Amount of CNC

10%

Figure 7.41 Mechanical properties of CNC-reinforced PVA films: (a) tensile strength, (b) modulus, and (c) strain (error bars represents standard error). Source: Sirviö et al. 2015 [90]. Reproduced with permission of ACS. NCO

OH OH + NCO

O HO

HO O O

OH OH

O

n

OH OH O HO

HO O O

OH

O

OH n

HNOC

NCO

Figure 7.42 Illustration of IPDI/CNC reaction with the secondary NCO group on IPDL. Source: Girouard et al. 2015 [91]. Reproduced with permission of ACS.

16

1.4

14

1.2

12 0%

10

1% um-CNC

8

1% m-CNC

6

5% um-CNC 5% m-CNC

4

Work of fracture (J/cm3)

Tensile strength (MPa)

7.2 Interfacial Behaviors Between Cellulose Nanocrystals and Matrix

1

0% 1% um-CNC

0.8

1% m-CNC

0.6

5% um-CNC 5% m-CNC

0.4

2

0.2

0

0

Figure 7.43 Tensile strength and work of fracture for um- and m-CNC composites and neat polyurethane. Source: Girouard et al. 2015 [91]. Reproduced with permission of ACS.

TgSS

WBPU1.05

Heat flow (a.u.)

Endo

TmSS

WBPU1.2 TmHS1

TmHS2 WBPU1.05-3 WBPU1.2-3

–50

–25

0

25 50 75 100 Temperature (°C)

125

150

175

Figure 7.44 DSC thermograms of WBPU1.05 and WBPU1.2 and their nanocomposites with 3 wt% CNC content. Source: Santamaria-Echart et al. 2016 [92]. Reproduced with permission of Springer Nature.

represents a suitable candidate for preparation of nanocomposites. Various WBPUs were synthesized at NCO/OH group ratios of 1.05, 1.2, and 1.4 [92], and it was observed that the microstructure of polyurethane changed only by changing the ratio of NCO/OH, resulting in different ordered structures. Soft ordered domains are obtained when NCO/OH ratio is low, and hard ordered domains are obtained when NCO/OH ratio is high, as shown in Figure 7.44. These different microstructures of the matrix induce different behaviors of CNC reinforcements, both as crystal growth inhibitors and as nucleating agents (see Figure 7.45).

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7 Reinforcing Mechanism of Cellulose Nanocrystals in Nanocomposites

WBPU1.05 WBPU1.2

–44

–48 50

8

40

4 6

TmSS (°C)

ΔHmSS (J/g)

12 (b)

30 120

(c)

100 4 80 2

TmHS (°C)

TgSS (°C)

(a)

ΔHmHS (J/g)

242

60 0

1

3 2 CNC content (%)

4

5

Figure 7.45 Thermal properties of WBPU1.05 (solid lines) and WBPU1.2 (dashed lines) series: (a) T gSS (filled circle), (b) T mSS (filled square) and ΔHmSS (open square), and (c) T mHS1 (filled triangle), ΔHmHS1 (open triangle), T mHS2 (filled inverted triangle), and ΔHmHS2 (open inverted triangle). Source: Santamaria-Echart et al. 2016 [92]. Reproduced with permission of Springer Nature.

7.3 Conclusions In conclusion, in this chapter, we reviewed the mechanisms of the enhancement of CNC in nanocomposites. Briefly, with respect to the hydrophilic matrices, CNC as a hydrophilic nanoparticle with a highly hydroxylated surface can form the internal network in the composite via interparticle hydrogen bonds; in addition to its high mechanical strength, the CNC network can efficiently improve the mechanical properties of the matrices. On the other hand, with respect to the hydrophobic matrices, as the compatibility of CNC and the matrices is low, surface modification of CNC is usually required. Via the modification, the decoration of functional groups on CNC can change the polarity and hydrophilicity of the particle, and thus can improve the dispersibility of CNC in the hydrophobic matrices, and even enhance the interaction between CNC fillers and matrices. In this way, the mechanical strength of the matrices can be dramatically improved. Meanwhile, when the polymers were grafted onto the CNC surface, the chain entanglement of the polymers can further strengthen the CNC network and thus enhance the matrices better. The formation of covalent bonds between CNC fillers and matrices is also an efficient way to develop high-performance nanocomposites. The covalent bonds formed can transport the mechanical strength of CNC to the matrices, distract the external force, and thus improve the mechanical properties of the matrices.

References

As mentioned earlier, there are four factors that should be considered while preparing the high-performance nanocomposites. CNC has a high mechanical strength and a hydrophilic and facile-modifiable surface, which makes it an ideal candidate for the enhancement of polymer materials. As CNC is also nontoxic, and degradable, it is believed that with the development of CNC-based nanocomposites, more high-performance, safe, and environment-friendly materials can be prepared and applied in every aspect of our daily life.

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14 Hammersley, J.M. (1957). Percolation processes. II. The connective con-

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites Thiago H. S. Maia, Marília Calazans, Vitor Lima, Francys K. V. Moreira, and Alessandra de Almeida Lucas Universidade Federal de São Carlos, Departmento de Engenharia de Materiais, Rodovia Washington Luis, km 235, CEP 13565-905, São Carlos, São Paulo, Brazil

8.1 Introduction The search for bio-based, sustainable, environment-friendly and carbon-neutral raw materials has intensified research on applications of cellulose nanofibrils (CNFs) in nanocomposites. The number of scientific papers on development of CNF nanocomposites for structural, semi-structural, barrier, and optical applications has increased exponentially over recent years. CNFs have also been called microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), cellulose nanofibers, and cellulose microfibrils, among other terms. They are multifunctional materials for many added value applications, which is mainly because of their very high mechanical properties, large surface area, reactivity, low density and thermal expansion coefficient, low abrasiveness, high oxygen barrier, abundance, and low cost. Unlike other composites, the properties of CNF-based nanocomposites rely on individual features of each phase (matrix and filler), composition, morphology (dispersion, distribution, dimensions, crystallinity degree, orientation) and filler/matrix interface. Most of these aspects are determined by the processing route as well as by the CNF–CNF and CNF–matrix interactions. The main challenges in using CNF as a filler in polymer nanocomposites are the irreversible agglomeration of CNFs upon drying and their hydrophilic nature, because of the large number of hydroxyl groups at the cellulose backbone. The high polarity and hydrophilic nature of CNFs limit their compatibility with nonpolar, hydrophobic polymers. The agglomeration upon drying makes the processing of these nanocomposites by conventional routes a big challenge, since most of the industrial polymer melt processing techniques require dried materials, and the extensional and shearing forces seems to not be intense enough to re-disperse the CNF agglomerates. Research on CNF nanocomposites is very recent, but significant outcomes have already been achieved so far. Scientific literature now offers excellent reviews on CNF extraction, reinforcement and its mechanisms, processing routes, and barrier properties of CNF nanocomposites, as well as comparisons Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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with other nanoparticles and nanocomposites [1–11]. The purpose of this chapter is not to review all articles published to date, but to discuss the main aspects involved in the development of polymer nanocomposites based on this promising nanomaterial.

8.2 Characteristics of Cellulose Nanofibrils CNFs have been previously described in earlier chapters of this book. In this section, important aspects concerning the role of CNF as fillers in polymer nanocomposites are highlighted. Structurally speaking, CNFs consist of a web-like, hairy, and semiflexible network of semicrystalline cellulose fibrils that occur both in individual and agglomerate forms. The diameters of cellulose fibrils range from 5 to 50 nm, while their length reaches several micrometers, thereby leading to a very high aspect ratio (L/d), as depicted in Figure 8.1a. Because of their long and flexible structure, cellulose fibrils can self-associate through entanglements, which imparts a further reinforcing effect in nanocomposites, in addition to hydrogen bonding linking each fibril, similar to the well-known percolated network mechanism observed for cellulose nanocrystals (CNCs) [12, 13]. There are two approaches for obtaining CNFs, namely, bottom-up or top-down. The first one consists of biosynthesis of CNFs by bacteria from Acetobacter species, the so-called bacterial cellulose (BC). It is made up of pure cellulose with a homogeneous morphology. The top-down process involves the mechanical disintegration of larger bundles of cellulose fibers using high-pressure homogenization, ultrasound, grinding, or microfluidization [1, 2, 10]. CNFs might contain hemicelluloses depending on the bleaching process and pre and posttreatments. Pretreatments are generally performed to facilitate the disintegration process, decreasing the amount of input energy, while posttreatments are used to enhance the compatibility between CNFs and polymer matrices. Damage of cellulose fibrils during the disintegration process must be avoided in order to preserve their polymerization degree, crystallinity, and high aspect ratio. Since less mechanical energy and chemicals are needed

(a)

(b)

Figure 8.1 Typical morphologies of (a) cellulose nanofibrils, CNFs, and (b) lignocellulosic nanofibrils, LCNFs.

8.3 Mechanical Properties of CNF Polymer Nanocomposites

for producing CNFs in comparison with CNCs, a much lower cost is expected, turning CNF into an eco-friendly and inexpensive nanoreinforcement. CNFs possess outstanding mechanical properties, typically a Young’s modulus of around 58–180 GPa and a tensile strength ranging from 0.3 to 22 GPa, depending on the measuring technique, stress mode, and failure mechanism [2]. These values are many orders of magnitude greater than the mechanical stiffness and strength of most polymers, making CNF a very good candidate to reinforce them. More recently, lignocellulosic nanofibers (LCNFs) were developed and tested as reinforcement in polymer nanocomposites [14, 15]. In this kind of CNF, lignin and hemicellulose are partially removed and the fibrils are obtained after hot water treatment in autoclave followed by grinding in the wet state. Figure 8.1b shows the morphology of a LCNF produced by Suzano Papel & Celulose in Brazil. It can be seen that cellulose fibrils are slightly thicker in LCNFs than in CNFs due to the presence of hemicellulose and lignin, but they are still long, flexible, and entangled, and usually present a brown color similar to kraft pulp. One of the advantages of using LCNF relies on the presence of lignin, which can additionally increase the stiffness and strength of nanocomposites. Lignin can also provide some antioxidant effect, enhancing the thermal stability of nanocomposites, which is opposite to the thermal effect imparted by hemicellulose. Lignin can still improve chemical interactions between LCNFs and hydrophobic polymer matrices, also avoiding re-agglomeration of CNFs over melt processing. The web-like structure of CNFs plays a fundamental role in the mechanical, barrier, and optical properties of nanocomposites. The random plane orientation is beneficial for some applications, but modification of the orientation of CNFs is also possible depending on the processing route [16, 17]. Vainio and Paulapuro [18] prepared resistant nanopapers, which were further impregnated with liquid resins. According to these authors, during preparation of nanopapers it is crucial to avoid their shrinkage because it increases flexibility while decreasing the load bearing capability of the nanopapers. Drying of CNFs is a particular concern and agglomeration represents a large-scale challenge that must be avoided as much as possible. Never dried CNF gels and polymer adsorbed CNF can be used to overcome this challenge, but moisture-sensitive polymers can hydrolyze at high temperatures when steam is formed. The presence of hemicellulose and lignin can also hinder the irreversible re-agglomeration of CNF upon drying [19], but while lignin would enhance the strength of nanocomposites, hemicellulose would decrease their mechanical properties due its low molecular weight.

8.3 Mechanical Properties of CNF Polymer Nanocomposites In order to better explore the reinforcing potential of CNFs the relative increase in Young’s modulus (Ecomp /Ematrix ) and tensile strength (𝜎 comp /𝜎 matrix ) were calculated from reported data. The idea was to eliminate the influence of polymer type, processing route, and variability of CNFs (method of extraction, plant source, bacterial, or plant cellulose, etc.). The graphics of Ecomp /Ematrix and 𝜎 comp /𝜎 matrix were plotted as a function of CNF content for different groups: thermosets,

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thermoplastics (including polyolefins and biodegradable polymers), and water-based systems (including water-soluble polymers, water dispersible, or latex). Values higher than 1 indicate enhancement of the properties. 8.3.1

Thermoset Resins

The properties of CNF-based polymer nanocomposites for structural and semi-structural applications can be mainly achieved by using thermoset resins [20–31]. Highest values of Young’s modulus and strength at break upon tensile and bending modes were achieved for these systems. The evident enhancement in tensile properties imparted by CNFs can be observed from Figure 8.2. Young's modulus – thermoset polymers 16 14

ECOMP/EMATRIX

12 10 8 6 4 2 0 0

10

20

30

(a)

40 50 wt% CNF

60

70

80

Tensile strength – thermoset polymers 16 14 12 σCOMP/σMATRIX

254

10 8 6 4 2 0 0

(b)

10

20

30

40 50 wt.% CNF

60

70

80

Figure 8.2 Relative Young’s modulus (a) and tensile strength (b) for thermoset resins as a function of CNF content [20–31].

8.3 Mechanical Properties of CNF Polymer Nanocomposites

Table 8.1 Bending properties of CNF-based thermoset resin nanocomposites. E (GPa)

𝝈 b (MPa)

Resin

CNF content (wt%)

Reference

Phenolic

70–90

19–20

390

[20]

Phenolic

92.6

18–19

370

[21]

Phenolic

88

28

425

[22]

According to Lee et al. [2], there are no homeopathic doses of CNFs when their polymer nanocomposites are intended for structural and semi-structural applications. Effective reinforcing effects are achieved only when CNF contents larger than 30 vol% are used. It was not possible to perform comparisons for bending properties owing to the lack of data for pure resins; however, very high values have also been reported in the literature [20–22], as shown in Table 8.1. These values are similar to those of synthetic-fiber-reinforced polymer composites, which are traditionally used for semi-structural and structural applications. CNF/thermoset resin nanocomposites compete against AZ91 T6-treated magnesium alloys in terms of mechanical strength, but exhibit smaller density. Liquid mixing or embedding CNF with thermoset resins is quite favorable to form strongly connected 3-D networks. According to Yano and coworkers [20–22], an intense disintegration of cellulose fibers is needed to obtain nanocomposites with superior mechanical properties, but there is an optimal number of passes through a homogenizer for attaining it, because CNFs degrade under excessive disintegration. Retegi et al. [23] developed acetylated BC/ESO (epoxidized soybean oil) nanocomposites containing up to 75 wt% BC. Stiff, tensile resistant, ductile, and transparent films were obtained. Young’s modulus from 0.45 to 5.9 GPa and tensile strength from 5.5 to 81 MPa were achieved. Lee et al. [24] used vacuum-assisted resin impregnation (VARI) to produce nanocomposites based on low-viscosity epoxy resin and nanopapers of BC and NFC, and compared the reinforcing efficiency of these two nanomaterials. Both types of nanocomposites presented similar and excellent mechanical properties, for example, Young‘s modulus of c. 8 GPa and tensile strength of c. 100 MPa, at BC/CNF loadings of 60 vol%. 8.3.2

Thermoplastics

Thermoplastic polymers/CNF nanocomposites have been obtained mainly by two processing techniques: solvent casting and melt processing. Although solvent casting has low yield and environmental and health concerns, it can be used as a model process for exploring the high reinforcing potential of CNFs on thermoplastic matrices. The low solvent evaporation rate is favorable for the establishment of a stiff 3-D skeleton, made up of an entangled and percolated cellulose fibrils network [12, 13, 32].

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Melt extrusion is the most viable industrial compounding process since it can be used to fabricate plastic products at large scales. Additionally, it is a greener process when compared with solvent casting precisely because it is solvent free. Processing cellulose polymer nanocomposites by melt extrusion is challenging due to the moderately low degradation temperatures of cellulosic materials, especially in the presence of hemicellulose, chemical incompatibility between CNFs and hydrophobic matrices, and agglomeration of cellulose fibrils upon drying. These challenges have been overcome by using polymers with low melting point, chemical modifications of CNFs, and never dried approaches, respectively. One of the most interesting applications of CNFs is their use as reinforcement for biodegradable polymers. A fully biodegradable material is then obtained when CNF is the disperse phase. The most studied biodegradable polymers are poly(lactic acid) (PLA), its blends and copolymers [33–46]. PLA exhibits low thermal stability at temperatures above its Tg , which is close to 60–70 ∘ C. A three order of magnitude increase in the storage modulus of PLA evaluated by dynamic mechanical thermal analysis (DMTA) was achieved, which remained constant up to 120 ∘ C, as reported by Iwataki, Nogi, and Yano [38]. The graphs displayed in Figures 8.3 and 8.4 show that solvent casting of PLA/CNF nanocomposites results in substantial increases on both tensile modulus and strength at break, as expected. This increase is not easily achieved when PLA/CNF nanocomposites and other rigid biodegradable polymers are produced by melt extrusion due to the aforementioned challenges. Another important category of thermoplastic polymer/CNF nanocomposites is that based on polyolefins, because this group of polymers is largely used in packaging. CNFs could be promising as a renewable filler for enhancing the mechanical and O2 -barrier properties of polyolefins while retaining their transparency. Owing to the nonpolar character of polyethylene (PE) and polypropylene (PP), special strategies must be adopted for dispersing CNF, since the usual twin-screw extrusion processing in the presence of maleic anhydride as a compatibilizer has not resulted in significant reinforcements due to inefficient redispersion of CNF. Figure 8.5 summarizes the results of different types of polyolefin/CNF nanocomposites, by which significant reinforcements have been obtained. Recent advances in polyolefin/LCNF and in situ fibrillated nanocomposites processing successfully led to good improvement in mechanical properties, with the solid-state shear pulverization process (sssp or S3P) and its modifications being the most promising techniques [15, 47, 48] and in which the LCNF never drying approach was more effective than the freeze-drying of PP/CNF mixtures [15]. S3P consists of pulverizing the polymeric material added to the filler on a special twin-screw extruder at low, ambient, and sub-ambient temperatures. Because of the intense shear and compression forces, the filler particles are de-agglomerated and, ideally, individual particles get well attached to the pulverized polymer. The filler particles can even react with the polymer, since high shear forces induce in situ formation of reactive sites, enhancing interfacial compatibility in the composite or nanocomposite [15, 47, 48].

8.3 Mechanical Properties of CNF Polymer Nanocomposites

Young's modulus – PLA (casting) 5

ECOMP /EMATRIX

4

3

2

1

0 0

10

20

(a)

30 wt.% CNF

40

50

Young's modulus – PLA (melting) 5

ECOMP /EMATRIX

4

3

2

1

0 0 (b)

5

10 15 wt.% CNF

20

25

Figure 8.3 Relative Young’s modulus of PLA and other biodegradable thermoplastic polymers obtained by casting (a) and melt extrusion (b) as a function of CNF content [33–46].

8.3.3

Waterborne Polymer Systems

Waterborne polymers can be successfully employed to produce CNF-reinforced polymer nanocomposites because CNF is originally obtained in aqueous medium. The most common waterborne polymer systems used in this regard are polyvinyl alcohol (PVOH) [58, 59], polyurethanes (PUs) [60–64] , starches including water-assisted twin-screw extrusion [65–71] , natural rubber latex [72–77], and aqueous PE dispersions [78–80]. These systems can be considered as models for evaluating the reinforcement potential of nanocellulose, since they exhibit very low elastic modulus and are relatively weak, in comparison with

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites

Tensile strength – PLA (casting) 5

σCOMP/σMATRIX

4

3

2

1

0 0

10

20

(a)

30 wt.% CNF

40

50

Tensile strength – PLA (melting) 5

4 σCOMP/σMATRIX

258

3

2

1

0 0 (b)

2

4

6

8

10 12 wt.% CNF

14

16

18

20

22

Figure 8.4 Relative tensile strength of PLA and other biodegradable thermoplastic polymers obtained by casting (a) and melt extrusion (b) as a function of CNF content [33–46].

CNFs and CNCs. Huge increases in elastic modulus and tensile strength have been observed (Figure 8.6).

8.4 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites Recently, an aqueous dispersion of polyethylene-co-acrylic acid copolymer was employed to produce PE/CNF nanocomposites and a very significant improvement in mechanical properties was observed, around four times the tensile

8.4 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites

Young's modulus – polyolefins 9 8

ECOMP/EMATRIX

7 6 5 4 3 2 1 0 0

10

20

(a)

30 wt.% CNF

40

50

Tensile strength – polyolefins 9 8

σCOMP/σMATRIX

7 6 5 4 3 2 1 0 (b)

10

20

30 40 wt.% CNF

50

60

Figure 8.5 Relative tensile strength for polyolefins-based nanocomposites as a function of CNF content [15, 47–57].

strength and a hundred times the Young’s modulus [78–80]. The nanocomposite formulation was dried and then hot pressed, which resulted in a biphasic morphology comprised of PE-rich and CNF-rich domains, with a good dispersion but poor distribution of CNFs within the PE matrix [78]. The melt processing of these nanocomposites on a twin-screw extruder was further investigated using screw rotation speeds of 100 and 200 rpm [80]. Figure 8.7 shows the Young’s modulus and tensile strength of PE/CNF nanocomposites before and after melt extrusion. Young’s modulus was found to diminish after extrusion at both screw rotation speeds regardless of the nanocomposite composition, while the properties of the pure PE were not

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites

Young’s modulus – waterborne polymer systems 100

ECOMP/EMATRIX

80 60 40

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

5 10 15 20 25 30 35 40 45

20 0 0

20

(a)

40 60 wt.% CNF

80

100

Tensile strength – waterborne polymer systems 6.0 5.5 5.0 4.5 σCOMP/σMATRIX

260

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

(b)

20

40 60 wt.% CNF

80

100

Figure 8.6 Relative Young’s modulus and tensile strength of water-soluble polymers as a function of CNF content [58–80].

altered. On the other hand, tensile strength did not vary significantly taking into account the high standard deviations. Melt extrusion can enhance the dispersion and distribution of CNFs, as well as their further fibrillation; thus an increment of both tensile properties would be expected. Nevertheless, shear and elongational forces taking place within the twin-screw extruder could also degrade both CNFs and the PE copolymer, as well induce re-agglomeration of CNFs, increasing their diameters, decreasing their aspect ratio, and consequently lessening their reinforcing efficiency. The morphologies of PE/CNF nanocomposites loaded with 10 wt% CNFs before and after melt extrusion at 100 rpm are illustrated in Figure 8.8. Although the distribution of CNFs is improved after extrusion, the re-agglomeration

Young’s modulus (MPa)

8.4 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites

850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

PE/F hot pressed PE/CNF extruded at 100 rpm PE/CNF extruded at 200 rpm

–4

0

4

8

12 16 20 wt.% CNF

(a)

28

32

PE/CNF hot pressed PE/CNF extruded at 100 rpm PE/CNF extruded at 200 rpm

18

Tensile strength (MPa)

24

16 14 12 10 8 0

(b)

5

10

15 20 wt.% CNF

25

30

Figure 8.7 Young modulus (a) and tensile strength (b) as a function of CNF content of PE/CNF nanocomposites before and after extrusion at 100 and 200 rpm.

of cellulose fibrils becomes evident. Some CNFs can still be seen, but the presence of micro-sized fibrous structures is evident. In this case, CNF acted as a micrometric cellulosic filler rather than a nanoreinforcement. This behavior suggests that the attraction forces between cellulose fibrils are much stronger than the shearing and elongational forces imposed by the extrusion process. Although the chemical interaction of acrylic-acid-grafted PE and cellulose is assumed to be good, hydrogen bonding in CNF is stronger, leading the cellulose fibrils to bond to each other when they are approximated upon flowing. The morphological changes after extrusion are corroborated by the evolution of the optical properties, as shown in Figure 8.9. Since there is a considerable number of the micro-sized fibrils throughout the matrix after extrusion, the

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites

Cross-sectional view of PE + 10 wt.% CNF

10 μm

10 μm

5 μm

5 μm

2 μm

1 μm

Thermo pressed at 100 °C

2 μm

1 μm

Thermo pressed at 100 °C after extrusion at 100 °C, 100 rpm

Figure 8.8 Morphologies of the PE/CNF (10 wt%) nanocomposite before and after melt extrusion at 100 rpm.

scattering of light becomes more intense, diminishing clarity and transmittance and increasing the haze of nanocomposite films. Figure 8.10 resumes the effects of processing on PE/CNF morphologies before and after melt extrusion.

8.4 Effects of Extrusion on Mechanical Properties of PE/CNF Nanocomposites

Clarity

Transmittance

Haze

10 wt.% 30 wt.% 100

5 wt.%

50

0 d

se

res op

erm

Th

0 10

d d m m rpm 0 rpm esse 0 rp 0 rp esse 0 rpm 0 rpm 0 rpm r p 10 20 10 10 20 opr 20 o erm erm Th Th

Figure 8.9 Optical properties of PE/CNF nanocomposites before and after melt extrusion at two different screw rotation speeds.

PE/NFC nanocomposite

After hot pressing at 100 °C

After extrusion (shearing and extensional forces)

30 view

30 view

Dispersion Distribution Lower aspect ratio

Dispersion Distribution Higher aspect ratio

˂ NFC-rich phase PE copolymer-rich ˂ phase Cross-sectional view

NFC agglomerate ˃ PE copolymer-rich phase ˃ Cross-sectional view

Figure 8.10 Schematic of melt extrusion effect on PE/CNF nanocomposites morphology.

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites

8.5 Effect of Fiber Size and Lignin Presence The same PE copolymer was used to evaluate the influence of fiber size and lignin on the mechanical and thermal properties of the PE nanocomposites comprised of conventional bleached micrometric cellulose pulp fibers (CFs), CNF, and LCNF. All these fillers were added at 3 wt% to water before mixing with aqueous PE copolymer dispersion. The formulations were stirred, dried, and then hot pressed as performed in [78]. Figure 8.11 reveals a twofold increase in the Young’s modulus of nanocomposite films containing 10 and 30 wt% LCNF. The modulus values of PE/CF composites are similar to those of PE/CNF nanocomposites. Furthermore, the tensile strength is considerably greater for PE/LCNF samples than for PE/CF and PE/CNF nanocomposites. As 1400

CNF LCNF CF

Young’s modulus (MPa)

1200 1000 800 600 400 200 0 0

5

10

(a)

15 wt.% filler

20

25

30

20

25

30

CNF LCNF CF

20 18 Tensile strength (MPa)

264

16 14 12 10 8 0

(b)

5

10

15 wt.% filler

Figure 8.11 Effect of fiber size (CNF vs. CF) and composition (CNF vs. LCNF) on the tensile mechanical properties of PE-cellulose films.

8.5 Effect of Fiber Size and Lignin Presence

expected, the presence of lignin is advantageous for tensile properties, while PE/CNF nanocomposites exhibit a superior mechanical performance compared with PE/CF composites. Figures 8.12 and 8.13 illustrate the cross-sectional morphology of the PE/LCNF and PE/CF samples, respectively, after tensile tests. The same fibrillation phenomenon observed for PE/CNF nanocomposites (see Figure 8.12 left) also occurred for PE/LCNF nanocomposites. PE/CF presented micro-scale fibers in its microstructure, as expected. Naturally, the optical properties of PE/LCNF nanocomposites and PE/CF composites are quite inferior to those of the PE/CNF samples (Figure 8.14). Lignin makes films more opaque, and micro-sized fibers of CNF scatter light to a greater extent.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 8.12 Morphology of PE/LCNF samples (a) and (b) 5 wt% LCNF, (c) and (d) 10 wt% LCNF, (e) and (f ) 30 wt% LCNF.

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8 Role of Cellulose Nanofibrils in Polymer Nanocomposites

(a)

(b)

(c)

(b)

Figure 8.13 Morphology of PE/CF samples. (a) Pure CF, (b) 5 wt% CF, (c) 10 wt% CF, and (d) 30 wt% CF.

110 100 90 Haze, transmittance (%)

266

80 70 60 50 40 30

Haze PE/CNF Transmittance PE/CNF Haze PE/CF Transmittance PE/CF

20 10 0 0

5

10

15

20

25

30

wt.% CNF, CF

Figure 8.14 Comparison between the optical properties of PE/CNF and PE/CF samples.

8.6 Multifunctionality: Optical and Barrier Properties of CNF Nanocomposites

8.6 Multifunctionality: Optical and Barrier Properties of CNF Nanocomposites CNFs have been excessively investigated as a nanoreinforcement in polymer nanocomposites, but they can also add some multifunctionality to polymer nanocomposites. Good optical properties of CNF nanocomposites, such as elevated light transmittance, require effectual dispersion of cellulose fibrils at nanoscale to prevent particles with sizes comparable to the visible range wavelengths from scattering light. BC exhibits a very homogeneous morphology, fulfilling perfectly the requirements for application in optoelectronic devices in substitution to rigid glasses as substrates and as flat panel displays, while plant-sourced NFC needs further chemical treatments in order to improve its optical properties [81–91]. The basic idea is to increase the mechanical properties of polymer films without sacrificing its transparency [82]. After acetylation the optical properties at elevated temperatures can be even increased [83]. Furthermore, due their high crystallinity and dense and flexible network, BC and CNFs also form nanocomposites with lower coefficient of thermal expansion (CTE) [84, 90], good thermal conductivity [85, 87], and high flexibility, with their nanocomposites being good candidates for flexible displays and organic light emission diodes (OLEDs) [88–90]. CNFs also exhibit an enormous potential to extend the O2 barrier properties of polymer films. Because their dense web-like architecture is strongly connected by hydrogen bonding, CNFs act as a physical barrier against O2 diffusion through the polymer matrix. In this case, a well-dispersed CNF network within the polymer matrix should be attained, besides good interfacial CNF/polymer compatibility. With sizes in the nanometric scale, the pores resulting from the densification of CNF web-like morphology would serve as a major path for molecular diffusion, meaning that CNF could form a more tortuous path for diffusion of gases and other substances [10, 92]. The flexibility of CNFs makes the densification process more efficient in comparison with CNC networks [92]. CNFs can also increase barrier properties against oils and greases, which is a technological requirement for many food packaging materials. On the other hand, since CNFs have a hydrophilic character, their water vapor transmission rate (WVTR) is typically elevated. When relative humidity is as large as 90%, CNFs tend to swell and be plasticized by water molecules, which in turn significantly increases both O2 permeability and WVTR [10, 92]. Lignin can also be used to increase the hydrophobic character of the resulting films, which is usually confirmed by high water contact angles. However, this strategy would lead to a less compacted morphology due to hindering effect over hydrogen bonding interactions [93, 94], potentially increasing WVTR again. The most common polymers used to increase O2 barrier properties of food packaging materials are PVOH, ethylene vinyl alcohol (EVOH), and PVDc as an internal layer in polyethylene-poly(ethylene terephthalate) (PE-PET) or polyethylene-polyamide (PE-PA) external layers and internal layers [95, 96]. Figure 8.15 presents a graphical comparison between the O2 -permeability of CNF films, its nanocomposites, and as a barrier layer with the aforementioned

267

2250

120 OTR (cm3.μm)/(m2.day.atm)

OTR (cm3.μm)/(m2.day.atm)

2200 2150 2100 2050 2000 500

100 80 60 40 20 0

0 0

20

40

60

80

0

Relative Humidity (%)

20

40

60

80

Relative Humidity (%)

Bio-HDPE/bio-LDPE

CNF

Bio-HDPE/Al2O3/bio-LDPE

PLA PVOH Amylose HDPE

Acetylated NFC EVOH Cellophane Bio-LDPE/CNF/bio-HDPE

PLA coated with CNF_PE Amylopectin PP Bio-LDPE/CNF/PET

HDPE cotaed with CNF

Figure 8.15 A comparison between oxygen transmission rate (OTR) of CNF, its nanocomposites, and multilayer assemblies vs. traditional and other promising packaging materials [92–97].

References

packaging materials, revealing that CNF is quite competitive in this aspect. CNF is advantageous because its use results in lighter and thinner packaging plastic films because of the low density and a reinforcing effect of cellulose fibrils. Considering water-soluble polymers, the excellent reinforcing effect and barrier potential of CNFs make this nanomaterial very promising for coatings and layer films in food packaging applications, especially if the resulting films are transparent. Packaging is one of the growing markets and CNF nanocomposites can find many applications in this area as sustainable, eco-efficient, bio-based, and even biodegradable multilayer films [92–97].

8.7 Outlooks in CNF Nanocomposites It is well accepted that petroleum-based products will become less available whereas their cost will increase over years. This makes CNFs a very promising class of innovative nanomaterials that could potentially replace petroleum-based plastic in future. Intensive research in this field still needs to be performed to comprehend the full potential of CNFs in structural, semi-structural, packaging, and other applications that certainly demand for transparency, flexibility, and high mechanical performance, such as in optoelectronics: flexible displays and OLEDs, among many others. There is an urgent demand for establishment of reproducible and efficient routes for producing morphologically homogeneous CNFs from plants. BC fulfills this requirement, but industrialization of CNFs from plant sources seems to be closer to the market. Refined CNF morphologies play a crucial role in barrier and optical properties, while for mechanical properties a less homogeneous disintegration of the cellulose fibrils is acceptable. Challenges that must also be overcome include production and market logistics, considering issues of drying and transportation of CNFs. Also, it is necessary to investigate new nanocomposite processing techniques that could result in well-dispersed, distributed, and adequately oriented CNFs within polymer matrices. The compatibility between nonpolar polymers and CNFs is another issue that still needs to be solved. Academic and Industrial Research should work together in order to achieve a cost-effective process for using this very promising green nanomaterial in polymer nanocomposites for replacing oil-based plastics in future.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals Lin Gan, Siyuan Liu, Dong Li, and Jin Huang Southwest University, School of Chemistry and Chemical Engineering, and Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, Tiansheng Road 2, Chongqing 400715, China

In 1951, Randy et al. first prepared cellulose nanocrystals (CNCs) by extracting the crystalline regions of cellulose microfibers with sulfuric acid, and such a method is still in use today. A few sulfonic groups existed on the surface of the CNCs prepared by that method [1], so these nanocrystals had a certain negative charge and could be steadily dispersed in aqueous systems due to the effect of electrostatic repulsion. In 1959, Marchessault et al. first discovered the lyotropic crystallization behavior of CNC suspensions [2]. In 1992, Revol et al. [3] found that 3 wt% CNC suspensions had a cholesteric (also called chiral nematic) phase, proving the existence of the cholesteric structure of CNC as a liquid crystal unit. After that, Gray and his colleagues reported that the cholesteric phase of CNCs can be immobilized into the film by evaporation-induced self-assembly (EISA), which opened up a new world for the application of CNCs in the field of optics. The self-assembly behavior of CNCs is induced by the helical structure of CNC chains [4]. Since CNCs are rigid one-dimensional rod-like nanocrystals with a length of 200–1000 nm and a diameter of 5–15 nm, the liquid crystal structure of CNCs can be adjusted. The optical properties of CNC self-assembly can thus be controlled. Their high specific surface area and reactivity also render their chemical structure highly adjustable and thus suitable for many matrices. The high crystallinity of CNCs is also conducive to preparing CNC self-assembly films with high mechanical properties.

9.1 Self-assembly Structure of CNCs The CNC is rod-like, and usually negatively charged due to the sulfonate groups and hydroxyl groups on its surface. The CNC can thus assemble itself with a cholesteric in solvents with a density similar to CNC [5]. CNC self-assembly materials are mainly prepared from CNC liquid crystals [6a]. These materials can be classified based on three factors: (i) the structure of the original liquid crystals, (ii) the components, and (iii) the form of products.

Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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9.1.1

Structure of CNC Liquid Crystals

There are two structural arrangements of CNCs in liquid crystals: cholesteric and nematic structures. The cholesteric structure is also the main structure of CNC self-assembly because the nematic structure is not stable for the rod-like nanoparticles [7]. By electrostatic force, CNC liquid crystal can be easily obtained (Figure 9.1a) when the CNC concentration is high enough. In 1992, Revol et al. [3] proved that this CNC liquid crystal is of cholesteric structure by observing the planar and fingerprint structure of the liquid crystals with a polarization optical microscope (Figure 9.1b). The planar structure indicates that the screw axis of the CNC liquid crystal is perpendicular to the substrate, leading to a birefringence phenomenon (Figure 9.1c) [6a]. The fingerprint structure also indicates that the axis is parallel to the substrate to form alternatively bright (a)

(b)

Pitch (P)

100 μm

(c)

(d)

Figure 9.1 (a) Schematic of cholesteric liquid crystals of CNCs. (b) POM (polarization optical microscope) image of cholesteric CNC film. (c) Optical microscopic image (crossed polars) of bulk CNC film (scale bar 40 μm). (d) Optical microscopic image (crossed polars) of rod-like nanocrystals with planar texture. Source: (a) Meseck et al. 2017 [8]. Reproduced with permission of Elsevier. (b) Revol et al. 1992 [3]. Reproduced with permission of Elsevier. (c) Majoinen et al. 2012 [6a]. Reproduced with permission of Springer Nature. (d) de Souza Lima and Redouane 2004 [6b]. Reproduced with permission of John Wiley & Sons.

9.1 Self-assembly Structure of CNCs

and dark stripes because the perpendicular one would lead to a planar texture (Figure 9.1d) [6b]. 9.1.2

Components of CNC Self-assembly

CNC self-assembly materials can be classified by their components into bulk materials and composite materials. The CNCs in bulk materials mainly connect through electrostatic force, so the modulus of these materials is high. However, they are brittle because they are highly crystalline and short of adhesive phase to dissipate the impact energy. Water-soluble polymers, such as polyethylene glycol [9], should thus be introduced into the CNC suspension to form interlayers in CNC self-assembly (Figure 9.2). These layers can dissipate energy and increase the toughness of CNC self-assembly materials. Decreasing their crystalline degree by introducing NaOH can also increase the toughness [10]. On the other hand, introducing glycerol as an adhesive (Figure 9.3a) can increase the interaction among CNCs [11]. In another kind of self-assembly of CNC-based composites, the CNC self-assembly is used as the template. When the CNC is removed, mesoporous materials can be obtained (Figure 9.3b) [12]. 9.1.3

Form of CNC Self-assembly Products

Three forms of CNC self-assembly have been prepared: liquid crystal, gel, and film. The CNCs in liquid crystal system are dispersed in solvents. When polyacrylamide or other gel-type polymers are introduced, the CNC liquid crystal Cellulose nanocrystals O S OH HO

O HO O OH

OH O O HO OH

O

O

O O HO O OH

OH O OH

P/2

+

O H

OH n

Polyethylene glycol

Figure 9.2 Schematic of cellulose nanocrystalline/polyethylene glycol composites. Source: Yao et al. 2017 [9]. Reproduced with permission of John Wiley & Sons.

279

OSO3– O

Δ Pitch HO

C

Glycerol

OH

N C

OH

HO OH

OH OH HO

HO

HO

O OSO3–

O

HO

HO

OSO3– O

OSO3– O

HO HO O

HO HO O

OH

O OSO3–

OH OH OSO3– O O HO

OH

(a) CNC + TMOS

CNMS

CNC/Silica composite

1. Stir at RT 2. EISA

3. Calcination or acid hydrolysis

(b)

Figure 9.3 Schematic of (a) cellulose nanocrystalline/glycerol composites and (b) cellulose nanocrystalline/silica composites. Source: (a) He et al. 2018 [11]. Reproduced with permission of ACS. (b) Shopsowitz et al. 2014 [12]. Reproduced with permission of John Wiley & Sons.

9.2 Self-assembly Methods and Materials

(a)

(b)

500 μm

Figure 9.4 (a) POM image of a PAAm (poly acrylamide) nanocomposite prepared with high acrylamide loading (10 wt% CNC) swollen in water (inset: a photograph of the swollen transparent hydrogel). (b) Optical image of the dry state iridescent CNC film. Source: (a) Kelly et al. 2013 [13]. Reproduced with permission of John Wiley & Sons. (b) Zhang et al. 2013 [14]. Reproduced with permission of Elsevier.

can convert to CNC self-assembly gels [13] (Figure 9.4a). In these gels, the CNCs arrange into a variable cholesteric structure, which brings pH-response and temperature-response ability to these gels. In contrast, when the solvents are removed, the CNC liquid crystals can convert to CNC self-assembly films (Figure 9.4b). These films have unique properties and usually need toughening.

9.2 Self-assembly Methods and Materials Many methods have been used to prepare specific self-assembly materials of CNC. For example, casting is a simple and effective method to prepare CNC liquid crystal films [15]. Some studies have also used spin-coating to prepare the liquid crystal films of CNC [16]. Self-assembly CNC composites have been reported to be prepared by a vacuum filtration method in a form of a membrane [17]. On the other hand, the EISA method is often used to prepare mesoporous materials, such as mesoporous SiO2 [18], with CNC self-assembly as templates. A centrifugal method has been used in the TiO2 mesoporous system of CNC self-assembly [19]. These two are efficient methods to preserve the cholesteric structure of CNC. 9.2.1

Casting Method and Spin Coating Method

The casting method is often used to prepare CNC bulk liquid crystal films (Figure 9.5) [15]. This is a simple method but its process takes much time. By pouring a certain concentration of CNC dispersion into the mold, the casting film can be directly obtained by slowly drying the solvent (Figure 9.6a) [15]. However, this method may lead to uneven concentration distribution and a certain aggregation of CNCs due to the slow process [20] (Figure 9.6b). Moreover, the pitch, P, of the CNC liquid crystal changes in the casting process, which leads to the formation of uneven films.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

Figure 9.5 The drying process of the casting method to prepare CNC bulk film [15].

25 mm

(a)

(b)

Figure 9.6 (a) The CNC-film casting setup of the casting method to prepare CNC bulk film. (b) Optical image of a CNC film dried on a 25 mm diameter glass slide from a 4.8 wt% CNC suspension. Source: (a) Natarajan et al. 2017 [15]. Reproduced with permission of ACS. (b) Park et al. 2014 [20]. Reproduced with permission of John Wiley & Sons.

(a)

(b)

2 mm

2 μm

Figure 9.7 Optical image of a multilayer film of PAH/cellulose prepared by spin-coating on glass and (b) polarized optical microscopic image of the area outlined in the black box. Source: Cranston and Gray 2008 [16]. Reproduced with permission of Elsevier.

The spin-coating method is also used to prepare CNC bulk film [16] (Figure 9.7). Its process only takes tens of seconds and it is often used in layer-by-layer self-assembly of CNC. The controllable arrangement and P can be obtained by this method at high concentration of CNC suspension. With a radial order of the process, the birefringence of the CNC self-assembly materials is very close to that of pure crystalline cellulose, which can hardly be achieved by the casting method.

9.2 Self-assembly Methods and Materials

9.2.2

Vacuum-Assisted Self-assembly

Vacuum-assisted self-assembly (VASA) is a simple and rapid method to prepare self-assembly thin films of CNCs. The high aspect ratio and good dispersity of CNCs ensure high orientation of the CNC liquid crystal film that is prepared by the VASA method (Figure 9.8a) [17a]. This method can also be used to prepare CNC composite membrane materials (Figure 9.8b) [17b]. The structure of this composite material is mainly influenced by the concentration of CNC, ultrasonic time, and vacuum in the preparation process. The long ultrasonic time has little effect on the final assembly results.

e

tim

n

io at

c

ni

So

14 h

16 h

18 h

Solution volume

VASA

Va

Sonication pretreatment

cu

um

23 nL

21 nL

17.5 nL

14 nL

de

gr

ee

0.07 MPa 0.06 MPa 0.05 MPa 0.04 MPa

(a)

Sonication

VASA

+ GO suspension

CNC suspension

(b)

Stable solution

Non-iridescence film

+ Sonication

GO powder

VASA

Metastable solution

Iridescence film

Figure 9.8 Schematic for the preparation process of (a) CNC iridescent films via vacuum-assisted self-assembly and (b) GO (graphene oxide)/CNC hybrid films with or without iridescence. Source: (a) Chen et al. 2014 [17a]. Reproduced with permission of ACS. (b) Chen et al. 2014 [17b]. Reproduced with permission of RSC.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

Nanostructured composite

Liquid-crystalline template Selfassembly

Ordered mesoporous material Removal of the template

Precursor

Figure 9.9 Schematic of the EISA method with CNC as the template to prepare ordered mesoporous materials. Source: Giese et al. 2015 [18]. Reproduced with permission of John Wiley & Sons.

9.2.3

Evaporation-Induced Self-assembly

The EISA is a method to cast an appropriate amount of CNCs and polymer into thin films by slowly evaporating their co-solvents. If the CNC is removed, mesoporous materials can be obtained (Figure 9.9) [18]. This is a one-step method, which can control the properties of the films by using different polymers, and the cholesteric structure of CNC can be maintained. However, the used CNCs usually have a high negative charge and can only be dispersed in water or solvents with high dielectric constant. This method is thus not suitable for hydrophobic or oil-soluble polymer matrices.

9.3 Structural Adjustment of CNC Self-assembly With optimized structures, the CNC self-assembly can be widely used to prepare new devices such as sensors and optical thin films, which will be discussed in detail in Section 9.6. Here, we first focus on the methods to adjust the structure of CNC self-assembly. The adjustment of the CNC self-assembly structure is mainly to control the cholesteric structure of CNC liquid crystal or its derivate products. 9.3.1

Cholesteric Structure of Neat CNC Films

The cholesteric structure of CNC can be used to prepare one-dimensional photonic crystals [21]. A photonic crystal is a material with periodically varying refractive index in one, two, or three dimensions, whose period is of visible light [21]. They can selectively diffract light with certain wavelengths. In the case of CNC self-assembly, the reflected wavelength depends on the P of the CNC cholesteric structure since the refractive index of CNC is fixed [18]. Therefore, by adjusting the P and angle, the liquid crystal structure of CNC self-assembly can be directly controlled, and then different types of liquid crystal film materials can be obtained. Recent studies have shown that during the preparation of CNC liquid crystal films, ionic strength, temperature, suspension concentration, and magnetic field have great effects on the P and the angle [22]. For example, the P decreases linearly with the increase in ion strength (Figure 9.10a), and the reciprocal P (1/P)

9.3 Structural Adjustment of CNC Self-assembly

400 380 360 Pitch (nm)

340 320 300 280 260 240 220 200 0

0.1

0.3

0.4

0.5

0.6

Salt concentration (mM)

(a)

Reciprocal pitch (nm–1)

0.2

0.0046 0.0044 0.0042 0.004 0.0038 0.0036 0.0034 0.0032 0.003 0.0028 0.0026 0.0028

0.003

0.0032

0.0034

0.0036

0.0038

Reciprocal temperature (K–1)

(b) –1.5 –5.3

–1

–0.5

0

0.5

1

1.5

–5.4

In(1/P)

–5.5 –5.6 –5.7 –5.8 –5.9 –6 (c)

In(c)

Figure 9.10 Relationships between (a) the P and salt concentration, (b) reciprocal P and reciprocal temperature, and (c) the P and the concentration of CNC. Source: Pan et al. 2010 [22]. Reproduced with permission of ACS.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

decreases linearly with the reciprocal temperature (Figure 9.10b). The concentration is also positively correlated with the logarithm of P [22] (Figure 9.10c). CNC could be self-assembled quickly at 11.7 T magnetic field [23], and the CNC suspensions usually display a negative diamagnetic susceptibility while the cholesteric axis aligns parallel to the magnetic field [24]. Meanwhile, the P decreases with increasing magnetic field intensity. In addition, toluene can be used as solvent and surfactant to adjust the P under electric fields. The frequency of AC (alternating current) electric field and the aspect ratio of CNC can affect the self-assembly behavior of CNC, which can control the P [25]. CNC suspensions can show increasing birefringence as the electric field strength increases, and display interference Newton colors [26]. When the electric field intensity increases, the self-assembly structure of CNCs can change from cholesteric orientation to an unwinding one (Figure 9.11a,b). This method can not only adjust the P accurately but also realize the dynamic control of the product uniformity and structure color at the macro scale (Figure 9.11c) [27]. The shear flow can also alter the P and make the film uniform [20]. The effect of lower shear flow can lead to a vertical helical orientation of the whole film (Figure 9.12), thus controlling the optical properties of CNC self-assembly. In contrast, a high shear flow can destroy the uniformity of the film and lead to loss of optical properties. On the other hand, a higher concentration of CNC should be applied with more massive shear stress. 9.3.2

Cholesteric Structure and Cross-linking Structure in Gel

CNC self-assembly can be introduced into specific matrices, but the most important problem is to maintain and control the cholesteric structure of CNC. The matrices are usually polymers. The monomer of polymers and CNCs should be first dispersed in a solvent together, and then the solvent is evaporated to ensure that the concentration of the suspension reaches a certain value. Finally, an in situ photo-initiated polymerization is used to form the gel of the polymer/CNC composites (Figure 9.13) [28]. In this process, temperature and pH are usually very important factors because the P of CNC cholesteric liquid crystal and the cross-linking density of polymer hydrogel are related to them. For example, a high concentration of CNC tends to form a helical P [13]. Under the condition of high AAM content (10 wt% CNC), the surface of PAAm composite is smooth and layered in the SEM image (Figure 9.14a). And the pitch can be even clearly seen at higher magnification (Figure 9.14b). When the nanocomposite contains 66 wt% CNC, it shows a P of hundreds of nanometers (Figure 9.14c,d). Different concentrations of CNC and the ratio of CNC to monomer can lead to different iridescent structures. The isotropic or anisotropic CNC arrangement can also be induced by different concentrations during the preparation process. The cation in solvents also has a great influence on the structure of a gel network and the structure of CNC self-assembly composite materials. The PAAm/CNC nanocomposites can have an enhanced swelling structure as the size and hydrophobicity of the cation increases [13]. The change in swelling is likely due to the influence of the cation on H-bonds between CNC and PAAm.

9.3 Structural Adjustment of CNC Self-assembly

(a)

0 V/cm

155 V/cm

354 V/cm 464 V/cm 508 V/cm 552 V/cm 663 V/cm

(b)

EAC

P0 /2

P/2

P0 /2

P/2

P/2

Unwinding

Orientation (c)

P/2

4

Pitch (μm)

3.5

3

2.5

2 0

2

4

6

0

2

4

6

8

10

12

14

8

10

12

14

EAC (V/cm)

600 400 200 0 Time (s)

Figure 9.11 (a) Relationship between the intensity of the electric field and the pattern of the monochromatic laser diffraction from the CNC self-assembly films. (b) Schematic of the sequential cholesteric orientation and unwinding as electric field increases. (c) Function of the P and iridescent colors as the electric field treatment is modulated. Source: Frka-Petesic et al. 2017 [27]. Reproduced with permission of John Wiley & Sons.

However, due to the effect of polymerization, these polymer/CNC composite materials are short of dense CNC filling, and the content of CNC is also low. The modulus of these composites is thus low, and the mechanical properties decrease. In order to solve this problem, researchers have tried to modify the CNC surface

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals Tactoids in a still sample with phase coexistence.

(a)

Tactoids in a sample with phase coexistence with a horizontal shear flow.

(b)

Figure 9.12 Helix orientation of CNC in cholesteric liquid crystalline (a) with and (b) without a horizontal shear flow. Source: Park et al. 2014 [20]. Reproduced with permission of John Wiley & Sons.

Polymer precursor

1. Thermal curing at 75 °C, 24 h 2. NaOH (aq.) at 70 °C

CNC

CNC-composite polymer (CP)

Mesoporous polymer (MP)

Figure 9.13 Schematic of the mesoporous cholesteric phenol–formaldehyde resin. Source: Khan et al. 2013 [28]. Reproduced with permission of John Wiley & Sons.

and then self-assemble CNC in oil-soluble polymers or nonpolar solvents, which will be discussed in later sections. 9.3.3 Cholesteric Structure in Bulk Materials of CNC Composite Self-assembly The bulk materials of CNC composite self-assembly can be divided into integrated bulk ones and the materials whose CNC-assembly template is removed. Regardless of the kind of bulk materials of CNC composite self-assembly, the introduction of other components can directly affect the P of CNC arrangement. For example, the introduction of polyethylene glycol (PEG) increases the P [9]. With the change in PEG content, the structure and color of the CNC film material can change accordingly (Figure 9.15) [9]. Highly oriented materials of CNC composite self-assembly can also be obtained by applying a magnetic field in the

9.3 Structural Adjustment of CNC Self-assembly

(a)

(b)

50 μm

(c)

10 μm

(d)

2 μm

500 nm

Figure 9.14 SEM images of PAAm nanocomposites with (a) 10 wt% CNC and (b) the corresponding higher resolution image; and with 66 wt% CNC and (d) the corresponding higher resolution image. Source: Kelly et al. 2013 [13]. Reproduced with permission of John Wiley & Sons.

CNC

C

CNC/PEG (9/1)

CNC/PEG (8/2)

CNC/PEG (7/3)

B

A

(a)

(b)

(c)

Figure 9.15 (a) Optical images, (b) side SEM images, and (c) POM images of CNC and CNC/PEG films with different mass proportion. The scale bars in (b) and (c) are 2 and 200 μm, respectively. Source: Yao et al. 2017 [9]. Reproduced with permission of John Wiley & Sons.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

100 Normalized (%T)

290

90 80 70 60

CNC 400

C1

C2

600

C4 800

C6

C7

1000

1200

C8 1400

1600

Wavelength (nm)

Figure 9.16 Reflective spectra of CNC and CNC/silica composite films [12]. The composites of C1, C2, C4, C6, C7, and C8 own 71, 68, 60, 51, 47, and 42 wt% CNC, respectively.

EISA method. CNCs in PVA (polyvinyl alcohol)/CNC system can orient perpendicularly to the direction of the magnetic field [29]. Inorganic materials with CNC self-assembly as a template can be used to prepare mesoporous thin films. In the SiO2 system, the aqueous CNC suspension should be mixed with the solution of pre-hydrolyzed tetramethoxysilane (TMOS) [30]. CNC can act as a liquid crystal template and be used to produce birefringent silica/CNC composites through EISA (Figure 9.3b). However, no long-range chiral alignment can be observed in this material. In contrast, through the hydrolysis and condensation of TMOS and TEOS (tetraethoxysilane) in 3 wt% CNCs aqueous solution, cholesteric mesoporous silica films with long-range cholesteric order from CNC self-assembly can be obtained [12]. In addition, by adjusting the amount of silane precursor added in the system of CNC/silica composite, the P of the material can be controlled, and the helical P increases as the ratio of silica to CNC increases. The increased P expands the light reflective wavelength from the visible region to ultraviolet and even infrared regions (Figure 9.16) [12]. Cholesteric TiO2 films can also be obtained by using the hard template of cholesteric mesoporous silica [19]. The hard template allows the transfer of cholesteric structure from CNC liquid crystal template to a material with poor compatibility to the aqueous CNC suspension. TiO2 /silica composites can be prepared by reloading TiCl4 into the pores of cholesteric mesoporous silica films. They are then annealed, calcined, and exposed to sodium hydroxide to remove silica carriers and obtain TiO2 films (Figure 9.17). The pore size of the silicon template directly affects the size of the final TiO2 film. Also, the pore size can be used to adjust the P to obtain TiO2 films with different optical properties. 9.3.4

Nematic Structure

In addition, there is an example of columnar one-dimensional photonic crystals. After placing CNC suspensions in a clean glass bottle and placing a hydrophilic substrate into the container, the CNC can self-assemble vertically with the solvent evaporating. The obtained self-assembly structure is nematic instead of

9.4 Modifying Surface Chemical Structure of CNC

Mesoporous silica

Mesoporous Titania

1. TiCl4 infiltration 2. Calcination 3. Silica etching

Figure 9.17 Schematic for preparing cholesteric mesoporous titania from mesoporous silica. Source: Shopsowitz et al. 2012 [19]. Reproduced with permission of John Wiley & Sons. Figure 9.18 The AFM image of CNC vertical-assembly film prepared by the evaporation-induced method.

being the usual cholesteric. This new structure can be seen in the AFM diagram (Figure 9.18).

9.4 Modifying Surface Chemical Structure of CNC The surface chemical structure plays an important role in preparing stable CNC self-assembly materials. For example, when the CNC is prepared by acidolysis with sulfuric acid, sulfonic groups can be introduced on the CNC surface (Figure 9.19) [18], which can increase the surface charge of the CNC in

291

Intra-and intermolecular hydrogen bonding of cellulose Cellulose microfibrils

O

O

Crystalline region

Amorphous region

Crystalline region

HO

O

O

O H

H

H

H O H O

O H

O O

O H O

O

O

O O

H O

O

O H

H

O

O H O O

H O

H

OH

HO O

O

O H O

O H O

O H O H

O O H

OH O

O H

H

H2SO4 (64 wt%) 45 °C

≈100–250 nm

EISA

O ≈5–30 nm

H OSO3H HO HO

H H

OH O

H n

Cellulose nanocrystals (CNCs)

Figure 9.19 Schematic for isolation of rod-like CNC from trees and the self-assembly of CNC via EISA. Source: Giese et al. 2015 [18]. Reproduced with permission of John Wiley & Sons.

9.4 Modifying Surface Chemical Structure of CNC Fibril surface

NaClO

NaCl

NaBr

NaBrO

COONa

CH2OH Cellulose fibril +

CH2OH

N O

N OH

COONa

TEMPO-mediated oxidation CH2OH

COONa

CH2OH

COONa

Figure 9.20 Schematic for oxidation of C6 primary hydroxyl of CNC by TEMPO/NaClO/NaBr system. Source: Okita et al. 2010 [32]. Reproduced with permission of ACS.

suspension. In this way, the CNC suspension can be more stable, and thus the properties of CNC liquid crystals can be more controllable. Chemical modification has also been used in CNC self-assembly to increase the number of functional groups on CNC surface. Habibi et al. [31] have developed a method based on TEMPO oxidation to introduce carboxyl groups on the surface of the CNC that is prepared with hydrochloric acid. With this method, the substitution degree from primary hydroxyl to carboxyl on the CNC surface can be adjusted from 0 to 0.03 mmol/g (COOH content) [31], and the water dispersity of the CNC can thus be improved. The suspension of the CNC can exert a clear birefringence effect (Figure 9.20), indicating that the suspension has liquid crystal behavior. The carboxyl modification degree can also be further enhanced. For example, Castro-Guerrero et al. [33] have reported a method of ammonium persulfate oxidation for preparing CNC liquid crystals. With this method, the carboxyl content on the CNC surface [33] (0.9 mmol/g) is much higher than with the TEMPO oxidation method. The significant fingerprint texture can then be observed with only 5 wt% CNC in suspension. However, the yield of modified CNC that is prepared by ammonium persulfate oxidation is low, which limits its application. CNCs can also be ionized. When the acidic form of CNCs is treated by alkali metal or quaternary ammonium hydroxide, the neutralization form of CNCs (CNC–X, X = Li+ , Na+ , K+ , NH4 + , NMe4 + , NBu4 + ) can be obtained after freeze-drying [34]. This kind of CNC–X can actually increase the concentration of CNCs in suspension and expand its application range. For example, the CNC–X is easily dispersed in polar non-proton organic solvents such as dimethyl sulfoxide (DMSO), formamide, N-methylformamide (NMF), and dimethylformamide (DMF). The improved dispersion is attributed to the reduction in H-bond between CNCs and the increase in solvent–particle H-bond interaction due to the neutralization of surface sulfate.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

In the CNC self-assembly structure, chemical modification can also increase the effective diameter of CNCs as rods. The combination of the surface charge and the opposite charge in dispersion forms an electric double layer on CNCs, which increases the effective diameter of CNCs, and thereby affects the CNC cholesteric structure [35]. Also, the polymer-modified CNC has a bigger effective diameter and a higher stability in the solvent than the neat CNC. For example, Araki et al. [36] have grafted a single amino-terminated PEG (PEG-NH2 ) onto Tempo-oxidized CNC via amidation to prepare a sterically stable rod-shaped CNC suspension in water. The PEG-modified CNC can even disperse stably in a 2 M NaCl aqueous solution. The liquid crystal structure of the modified CNC suspension has been confirmed by its clean fingerprint texture. In addition to hydrophilic groups, a hydrophobized poly(ethylene oxide) (PEO) has also been grafted onto CNC [37] by an alkaline epoxide ring-opening strategy to prepare a sterically stabilized CNC (Figure 9.21a). This CNC can stably 18 15 Pitch (μm)

294

p/2

12 9

p/2

6 3 4 5 6 7 3 Cellulose volume fraction (vol. %)

2 (a)

(b) S-CNC

M2070-g-CNC

TO-CNC

OH OH CH2 CH2

Oxidation

CH2 TEMPO, NaCIO, NaBr CH2 OH pH 10 – 10.5 OH

8

OH OH C O C O C O CO OH OH

Peptidic coupling NH2, EDAC, NHS pH 7.5 – 8

NH C O

OH C O

C OC O OH NH

OH O HO

Reactions in water at room temperature

O OH n

CH3

NH2 =

(c)

O

H 3C O

x

y

NH2

For Jeffamine* polyetheramine M2070, x = 29 and y = 6

Figure 9.21 (a) Optical images of dispersion of (left) PEO-grafted CNC and (right) desulfated CNC. Cellulose content is 0.44% w/w for both. (b) Schematic for preparing TEMPO-oxidized and Jeffamine polyetheramine M2070-grafted CNC by modifying sulfated CNC. (c) Relationship between the P and the volume fraction of cellulose in the Jeffamine polyetheramine M2070-grafted CNC. Source: (a) Kloser and Gray 2010 [37]. Reproduced with permission of ACS. (b) Azzam et al. 2016 [38]. Reproduced with permission of ACS. (c)Azzam et al. 2016 [38]. Reproduced with permission of ACS.

9.5 Properties of CNC Self-assembly

suspend with a cholesteric structure under base condition. On the other hand, the hydrophilized polyether can be grafted onto CNC (Figure 9.21b) to control the cholesteric structure of the CNC liquid crystal (Figure 9.21c) [38].

9.5 Properties of CNC Self-assembly 9.5.1 9.5.1.1

Mechanical Properties Mechanical Properties of CNC Films

The interaction between CNC particles is mainly through H-bonds, which allow CNCs to self-assemble as films. However, the H-bonds are not as strong as chemical bonds, so the impact energy can seldom be resisted in the CNC self-assembly films. This feature results in low toughness and thus low mechanical properties of the CNC films. Modification to increase the interaction between CNC particles is thus necessary to improve the toughness of CNC self-assembly films. Adsorbing ionic surfactants onto the CNC surface by ionic linkages can increase the interaction between CNC particles [39]. For example, dimethy-lmyristylammonio propanesulfonate (DMAPS) [40], a zwitterionic surfactant, has been introduced in the CNC self-assembly films. It interacts with the sulfonic acid groups on the CNC surface via ionic bonding forces, which yields nanometric CNC–DMAPS complexes. The DMAPS can act as small springs to dissipate impact energy (Figure 9.22). Such CNC self-assembly films are thus flexible enough that the CNC suspension with DMAPS can be sprayed or coated onto any substrates to form tough films. On the other hand, the toughness of the CNC films can also be improved by reducing the crystallinity of the CNC. For example, the CNC can be transformed from a higher crystallinity of form 1 to a lower crystallinity of form 2 by NaOH treatment (Figure 9.23) [10]. In this case, the CNC self-assembly film must be prepared by a vacuum-assisted method. The modified CNC film exhibits 92.5 times increased toughness. Meanwhile, the sulfate groups on CNC surface can also be removed, increasing the thermal degradation temperature from 142 to 263 ∘ C. 9.5.1.2

Mechanical Properties of CNC Composite Films

In addition to the grafting on neat CNC, toughening of CNC self-assembly film materials can be achieved by compounding with other kinds of polymers. These polymers are usually water soluble and contain hydroxyl groups, which makes them able to interact strongly with CNC. Owning a soft polymer layer, the composites have more unconstrained movement to dissipate impact energy [41]. This increases the elongation at break of the modified materials and even allows the CNC films to yield (Figure 9.24a) [9]. Importantly, the introduced component should have good interfacial adhesion on CNCs to transfer stress. Poly(2-hydroxyethyl methacrylate) (PHEMA), PVA, and PEG are thus used as toughening components due to their hydroxyl groups and high polarity. For example, CNC/PVA nanocomposites can self-assemble in water and be casted into a composite film with good optical and mechanical

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

NaCNC041 NaCNC1 NaCNC01

60

NaCNC1

Stress (MPa)

296

40

20

0 0

0.5

1

1.5

Strain (%)

Figure 9.22 Stress–strain curves of CNC/DMAPS composites with CNC:DMAPS mass ratio of 1 : 0.41 (NaCNC041), 1 : 1 (NaCNC1), and 1 : 0.1 (NaCNC01). The inset is the optical image of the NaCNC1 film. Source: Guidetti et al. 2016 [40]. Reproduced with permission of John Wiley & Sons. i

(a)

P/2

ii

500 nm

iii

500 nm

500 nm

P/2

P/2 Short time

Long time

Short time

Long time

(b)

(c)

Figure 9.23 (a) SEM images and (b) schematics of CNC iridescent films treated by 16 wt% NaOH solution for (i) 0, (ii) 3, and (iii) 12 hours. (c) Schematic for morphological change in single CNC due to the NaOH treatment. Source: Nan et al. 2017 [10]. Reproduced with permission of ACS.

9.5 Properties of CNC Self-assembly

60 CNC-2/PEG (90/10) CNC-2/PEG (80/20) CNC-2/PEG (70/30) CNC-2/PEG (60/40)

50

Stress (MPa)

40 30 20 10 0 0

0.5

1.0

(a)

1.5

2.5

2.0

3

Strain (%) 80 90/10 80/20 70/30 60/40

70

Stress (MPa)

60

55/45 50/50

50

CNC/PVA = 40/60 w/w

40 30

Pure PVA

20 10 0 0 (b)

1

2

3

4

5

6

48

50

Strain (%)

Figure 9.24 Stress–strain curves of (a) CNC/PEG and (b) CNC/PVA composites with different mass proportions. The inset is an optical image of CNC/PEG composite with a CNC/PEG mass proportion of 80/20. Source: (a) Yao et al. 2017 [9]. Reproduced with permission of John Wiley & Sons. (b) Wang and Walther 2015 [41]. Reproduced with permission of ACS.

properties. In the casting process, PVA chains can coat on the CNC surface by H-bonds. Although the strength of the CNC/PVA composite film decreases with the introduction of PVA, its elongation at break still increases significantly. At a 1 : 1 ratio of CNC/PVA, the elongation at break of the composite film can reach 6% (Figure 9.24b) [41]. Also, the dynamic modulus of the nanocomposite was around 2 GPa, referring to a great dynamic mechanical property [42]. The preparation method of CNC/PEG composite films is similar to that of CNC/PVA films. The introduction of PEG can also enhance the toughness of

297

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

the material. The elongation at break of the composite can reach about 2.5% by adjusting the proportion of CNC/PEG [9]. The addition of PEG also enhances the pull-off adhesion strength of the material [42], which means that PEG can strongly adhere to CNCs. In the case of CNC/PHEMA composite films, HEMA monomer should be introduced into the CNC suspension. Polymerization was then initiated to obtain a flexible CNC/PHEMA composite film. This kind of composite film has an increased strength and elongation at break [42]. On the other hand, specific micromolecules have been introduced as energy-dissipating components into the CNC films. For example, Fei Song and coworkers [11] have introduced glycerol into the CNC suspension and the toughness of the obtained CNC film was greatly improved. The H-bonds between CNC and glycerin have been observed from the FT-IR spectra of CNC/glycerol films. According to the different amounts of glycerol added, the flexibility of the CNC films can also be controlled. 9.5.2 9.5.2.1

Iridescent Color Iridescent Color Control of CNC Films

CNC self-assembly cholesteric films usually have an attractive iridescence (Figure 9.4b) [14]. This is mainly because the P of CNC cholesteric structure is in the wavelength range of visible light. Although the length/diameter ratio of the CNCs from different sources affects the cholesteric structure, the iridescent color properties of the CNC films mainly depend on the cholesteric P. The concentration and surface charge of CNC in suspension, and the evaporation of the solvent, which affect the P, thus play important roles in controlling the optical properties of the CNC iridescent films [15, 22]. The P increases as the concentration increases, and the dominant color of iridescence becomes clearer. Reducing the particle size and surface charge density of CNC by sonication treatment can also control the iridescence of the CNC films (Figure 9.25) [43]. The length and diameter of CNCs decrease with prolonging sonication time, so the total surface area of CNCs increases and their surface charge density decreases. This results in Red shift of the dominant color. A blue shift in the dominant color of CNC self-assembly films can be achieved by accelerating evaporation conditions. Mark J. MacLachlan et al. [44] have reported that the rainbow colors of CNC self-assembly films can be controlled by tuning the evaporation time of the solvent in the CNC suspension. This strategy can be used in the covered evaporation method, diluted evaporation method, and sealed evaporation method. Longer evaporation time of solvent results in a short reflection wavelength (blueshift) (Figure 9.26). On the other hand, the optical properties of CNC self-assembly films can also be controlled by the external field. For example, the orientation of the CNC cholesteric phase can be well controlled in an electric field. The iridescence of the CNC films can be adjusted by accurately controlling the electric field size [27]. With an increase in electric field intensity, the reflection wavelengths can redshift, and the iridescent color can even disappear (Figure 9.27).

9.5 Properties of CNC Self-assembly

(a)

(b)

(c)

(e)

(f)

1 cm

(d)

Figure 9.25 CNC films cast from the suspensions sonicated for (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f ) 50 minutes, respectively. Source: Liu et al. 2014 [43]. Reproduced with permission of RSC.

Normalized ellipticity

1.0

1 day 3 days 6 days 11 days 15 days

0.5

0.0 400

500

600

700

800

Wavelength (nm)

Figure 9.26 Circular dichroism spectra of the CNC films prepared by covered evaporation method over 1–15 days. Source: Tran et al. 2018 [44]. Reproduced with permission of ACS.

Niobium (NdFeB) magnets have also been used to control the orientation of the CNC and the optical properties of the obtained CNC iridescent films [45]. Meanwhile, the spiral orientation of CNC cholesteric films can be controlled by shearing force [20].

299

300

9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

0 V/cm 303 V/cm 390 V/cm 433 V/cm 477 V/cm 520 V/cm 542 V/cm 715 V/cm 2.2 kV/cm

Figure 9.27 Relationship between the intensity of the electric field and the iridescent colors of the CNC self-assembly films. Source: Frka-Petesic et al. 2017 [27]. Reproduced with permission of John Wiley & Sons.

9.5.2.2

Iridescent Color Control of CNC Composite Materials

In CNC composite films, the components of the composites have a great influence on the optical properties of the CNC composite films. For example, in the composite films of CNC/PEG, the P increases with the increasing PEG content [9]. With an addition of 10% PEG, the composite shows iridescence [41]. In contrast, the increase in the proportion of PVA in the PVA/CNC composite system can lead to a decrease in P. Moreover, the decreasing effect of PVA on the P is greater than the effect of introducing PEG. The iridescence of the CNC composite films with inorganic nanoparticles can be influenced by the dispersion state of the nanoparticles. For example, in the conductive CNC/graphene oxide (GO) composite films that were formed by VASA [17b], the optical properties of the films mainly depended on the initial state of GO (Figure 9.28a,b). When powdered GO and CNC were mixed in the solvent by ultrasonic, they would form a metastable dispersion. In contrast, the direct use of GO dispersion with CNC would form a stable dispersion. The metastable dispersion can yield films with iridescence, while the stable dispersion cannot. However, the conductivity of the latter is much better than that of the former. Gold nanorods (NRs) can alter the optical properties of CNC self-assembly composite films in a different way. In the work of Eugenia Kumacheva’s group [46], all of the cast CNC films with gold NRs showed a brown-reddish color, which was caused by gold NRs (Figure 9.28c). The increase in gold NR concentration led (a)

(c) I

II +

(b) I

II

Colloidal solution of CNCs

Water evaporation

P/2

Colloidal solution of gold NRs

Figure 9.28 (a) Optical images of CNC/GO film prepared by adding (I) GO dispersion and (II) GO into the CNC suspension, respectively. The yellow and black parts are unreduced and reduced regions, respectively. (b) Optical images of the reductive films of the CNC/GO composites. (c) Schematic for preparing the composite chiroptical plasmonic film by mixing aqueous suspensions of CNCs and gold NRs. Source: (a) Chen et al. 2014 [17b]. Reproduced with permission of RSC. (b) Chen et al. 2014 [17b]. Reproduced with permission of RSC. (c) Querejeta-Fernandez et al. 2014 [46]. Reproduced with permission of ACS.

9.5 Properties of CNC Self-assembly

Chrial nematic CNCs matrix with left-handed AgNWs distribution CNCs AgNWs

Weak

Electrostatic repulsion induced self-assembly

CNCs/AgNWs mixed suspension

Strong

Chiral nematic CNCs matrix with right-handed AgNWs distribution

Figure 9.29 Schematic for the self-assembly of CNC/AgNW composites with electrostatic repulsion-induced realignment. Source: Chu et al. 2015 [47]. Reproduced with permission of ACS.

to a decrease in the P until the chiral nematic structure disappeared. The addition of a small molecular stabilizer, cetyltrimethylammonium bromide (CTAB), also led to the disappearance of iridescence. Ionic strength can also affect the optical properties of CNC composite films. For example, in the thin film of long-range ordered silver nanowires with CNC [47], the electrostatic repulsion inducing realignment can be adjusted by adding NaCl. NaCl can further affect the interaction intensity between silver nanowires and CNC during self-assembly, which influences the optical properties of the film. When less salt is added, the electrostatic repulsion between CNC and silver nanowires decreases, resulting in red iridescence. In contrast, when excess salt is added, the electrostatic repulsive force increases, which leads to the transition of the film from cholesteric to nematic ordering (Figure 9.29). Also, the pH can control the iridescent property of CNC self-assembly composite when the composite is in the form of a gel. With PAAc as the matrix, the CNC/PAAc hydrogel can show a redshift of its iridescence with increasing pH from 8 to 13 [13]. By immersing the PAAc/CNC nanocomposites in aqueous solutions, their iridescence at pH 9.5 and pH 7 can be differentiated within about 200 seconds. Besides the above direct composites, nanostructured materials with CNC-based templates also have adjustable optical properties. The most

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

0 wt%

8 wt%

16 wt%

28 wt%

Figure 9.30 Optical images of silica films prepared with different content of glucose. Source: Kelly et al. 2013 [48]. Reproduced with permission of John Wiley & Sons.

common examples are silicon dioxide films and titanium dioxide films. For example, after mixing TMOS with CNC suspension, the films with structural color can be obtained by the EISA method [12]. However, the brittleness of the composite films increases greatly at the same time. After removing CNC with strong acid, the remaining silica films also have an iridescence. The change in the iridescence is mainly determined by the ratio of TMOS to CNC, and also by the acid used to remove CNC. pH also plays an important role in controlling the optical property of CNC self-assembly films. Too high or too low a pH can lead to the destruction of cholesteric structures. In the CNC/TMOS system, films with iridescence can be obtained at pH 2.4. When the initial pH is adjusted to 2, the final film is transparent and has no cholesteric structure. If the pH is adjusted to 7 with NaOH, the film is opaque. When the pH reaches 12.5, the film also becomes transparent because of the disruption of CNC self-assembly ability in the material [12]. In addition, because the film is prone to rupture during drying (Figure 9.30), polyols, such as glucose, are often added to CNC suspensions to reduce the cracking of the final silica films [48]. For example, by adding glucose before EISA, the flexibility of the final membrane can be greatly improved. With the increase in glucose content, the flexibility of the film increases with maintained iridescence [48]. In the above example, we know that based on the template of CNC self-assembly, porous silica thin films with cholesteric structure can be obtained by removing CNC. Similarly, porous titanium dioxide thin films with cholesteric structure can be obtained by adding titanium tetrachloride and removing silica. The optical properties of this material are mainly determined by the etching of CNC templates. Removing the templates by strong acid is better than by calcination to obtain films of iridescence [19]. 9.5.2.3

Optical Control of CNC Self-assembly Gels

CNC-based hydrogels had been studied for a long time, which generally have no cholesteric structures. This means that they may not have iridescence and other

9.5 Properties of CNC Self-assembly

(a)

(b)

25 mm

Figure 9.31 Optical images of an iridescent film of the PAAm/CNC nanocomposite (a) as swelled in water and (a inset) before being swelled in water. (b) Optical images of PAAm composite hydrogels with increasing ionic strength from left to right. Source: Kelly et al. 2013 [13]. Reproduced with permission of John Wiley & Sons.

optical properties [27]. However, when Kelly et al. mixed AAm with CNC [13], the CNC self-assembly hydrogel with cholesteric structure could be obtained by EISA with the cross-link agent and photo-initiator (Figure 9.31a). Some other monomers and polymers have also been used to prepare CNC-based gel, such as N-isopropylacrylamide (NIPAm), acrylic acid (AAc), 2-hydroxyethylmethacrylate (HEMa) polyethylene glycol methacrylate (PEGMa), N,N ′ -methylenebisacrylamide (bis), and polyethylene glycol dimethacrylate (DiPEGMa). It has been proved that CNC self-assembly hydrogels with special optical properties can also be prepared by specific methods. These hydrogels can be iridescent, whose optical properties are mainly controlled by ionic strength. The increase in ionic strength can even lead to a blueshift of the reflectance across the visible regions (Figure 9.31b). Melamine-ureaformaldehyde (MUF)/CNC composites can also be used to prepare iridescent hydrogels. The MUF/CNC hydrogel has a bright color change from red to blue as the ionic strength increases. The applied pressure has the same effect on the ionic strength. But once cured, the material no longer has this pressure effect [49]. On the other hand, the mesoporous gels of other polymers with CNC self-assembly as the template can also own cholesteric structure. Khan et al. have prepared a mesoporous phenol–formaldehyde (PF) resin after removing the CNC template, and the ratio of CNC to PF affects the color of the material [28]. The type of solvent used in swelling material can also change its color [50] by changing the cross-link degree and thus the P. The CNC self-assembly structure can also be introduced into hydrophobic matrices after the CNC surface is chemically modified. With hydrophobic modification, the CNC can disperse and self-assemble in organic solvents such as DMF. By this way, CNC can be compounded with some polymers such as polystyrene (PS), poly(methyl methacrylate) (PMMA), polycarbonate (PC), and poly(9-vinylcarbazole) (PVK). The colors of the composites are mainly controlled by the type of modifying group on the CNC surface, the ratio of CNC to the polymer, and the ionic strength [34].

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9.5.3

Plasmonic Properties of CNC

In recent years, because the new composite materials can greatly expand the physical and chemical properties of CNC, the assembly of noble metal nanoparticles into CNC photonic crystals has attracted much attention. In this section, we focus on the introduction of metal nanoparticles to change the plasmonic properties of CNC composites. Chu et al. introduced negatively charged gold nanoparticles (GNPs) into a CNC chiral liquid crystal system (Figure 9.32a,b) to prepare a novel self-supporting photonic crystal film with chiral plasmon resonance effect [51]. These GNPs dispersed between the layers of CNC chiral liquid crystal film and distributed in a disorderly manner. By adjusting the P of the CNC matrix, the photonic band gap and the plasma resonance peak of the GNPs can be adjusted, and then the resonance coupling can be realized in the CNC matrix. In addition, this material also shows an angular-dependent plasma resonance peak displacement phenomenon. The position of the plasmon resonance peak of GNPs in cholesteric CNC films will blueshift significantly with the decrease of incident light angle. The length of the blueshift is related to the resonance coupling degree between the photonic band gap and the plasmon resonance peak. Kumacheva and coworkers also found that gold NRs have a similar influence on the plasma properties of CNC composites. Both the concentration of CNC and the amount of NaCl added will affect this kind of property [46]. CNC/silver nanowires (AgNWs) composite films also have plasmon resonance properties [47]. The intensity of the plasmon resonance is directly related to the concentration of AgNWs. The change in the CNC matrix structure also has a strong effect on the SPCD (surface plasmon-induced circular dichroism) signal of the composite films. As mentioned above, the ionic strength can affect the interaction between CNC and AgNWs, which finally leads to a change in the 1.0 0.8 Extinction (a.u.)

304

Angle-dependent SPR spectra in chiral photonic crystals

0.6 0.4 0.2 0.0 200

(a)

(b)

400

600 Wavelength (nm)

800

1000

Figure 9.32 (a) Schematic of the self-assembly of CNC/Au nanoparticle mixture. (b) Extinction spectra of the CNC film before (black) and after (red) being soaked in alcohol/water mixture. Source: Chu et al. 2015 [51]. Reproduced with permission of ACS.

9.6 Potential Applications

CNC self-assembly structure. On the other hand, when CNC forms a cholesteric structure, the CNC/noble metal nanoparticle system has a strong linear birefringence, but in the nematic form, the linear birefringence disappears.

9.6 Potential Applications 9.6.1

Oil/Water Separation

CNC self-assembly can also be used in oil/water separation by mixing with other materials. Silica aerogels have good adhesive properties due to their large surface area and high porosity, making them a great oil/water separating material. However, due to the existence of Si–OH groups, the hydrophobicity and mechanical properties of aerogels are not good enough, leading to an adsorption of water, which seriously affects its oil/water separating application [52]. When the CNC is introduced, this rod-like nanoparticle could self-assemble to form a three-dimensional network structure, which greatly enhances the mechanical properties of aerogels. When He et al. chose methyltriethoxysilane (MTES) and CNC to prepare aerogels [53], the great number of Me–O–Si groups improved the hydrophobicity. The aerogel had a water contact angle of 152∘ (Figure 9.33a), which was able to withstand a compressive strain of 80% and restore the original shape after stress release. The quality factor of this material can stay at 11 even under extreme conditional treatment down to −200 ∘ C and up to 300 ∘ C, which shows great potential for the application of oil/water separation. Another example is the aerogel prepared by Hao-Yang Mi et al. with fluorinated hybrid aerogel (FHA) as matrix and GO, CNC, and silica nanoparticles as filler. The structure formed by CNC self-assembly can prevent the volume 170 t = 10 s

150°

Contact angle (°)

165 Water

t = 40 s t = 70 s

160

155

150 0 (a)

(b)

10

20

30 40 50 Time (s)

60

70

80

Figure 9.33 (a) Contact angle test image for the hydrogel prepared by MTES and CNC. (b) Relationship between the contact angle of CNC/PVA/GO carbon aerogels and the absorption time. The inset images are the corresponding contact angle test images. Source: (a) He et al. 2018 [53]. Reproduced with permission of Elsevier. (b) Xu et al. 2018 [54]. https://creativecommons.org/licenses/by/4.0/.

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9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals

shrinkage of aerogels. This aerogel also had a large water contact angle and surface area. The contact angle hysteresis (CAH) of the aerogels can even be less than 1∘ (Figure 9.33b), which indicates that this aerogel was super waterproof [54]. 9.6.2

Application of Optical Materials

9.6.2.1

Optical Application of CNC Films

The fascinating iridescent color of CNC self-assembly films can be applied in coatings, cosmetics, and safety labeling [45]. Controlling the orientation of the cholesteric structure by external fields, such as magnetic fields [24], electric fields [25] or shear forces [20], directly affects the visual appearance of the films. For example, there is a noticeable color difference in the resulting CNC films under different magnetic fields. The controllable visual appearance provides CNC self-assembly with the possibility for the applications in the film in optics. CNC films can be used as iridescent pigment with controllable colors. However, the sulfate ester groups on CNC surface make the films susceptible to thermal degradation and redispersion in water, limiting the application of CNC films in the pigment [55]. Bras and coworkers [55] thus removed sulfur from CNCs with vacuum over-drying and screened the negative charges on CNCs with NaCl. Finally, the CNC iridescent engineered pigment was obtained after 75% of the sulfate ester groups were removed, and the pigment can be soaked in water for a long time (Figure 9.34a). This CNC film’s pigment can thus be used in many application areas such as food, pharmaceutical, and cosmetic packaging. Also, Zhang et al. [57] have reported that CNC films can possess some safety features by controlling the addition of the optical brightener (OBA) TINOPAL to adjust the P and zeta potential. The safety features include obvious safety (iridescence) and covert security (selective circular reflections and fluorescence), and public encryption as an anti-counterfeiting measure has become a potential possibility. 9.6.2.2

Optical Application of CNC Composite Films

Besides CNC neat films, CNC composite films also have many optical applications. Owning other components, these composite materials have been widely used in security features, polarizer, micro-optical devices, and so on. For example, the CNC/PEG thin film that was prepared by Gu et al. was visibly iridescent, and it had strong adhesion and good mechanical properties [58]. Besides, Kumacheva and co-workers added latex nanoparticles to the CNC system. The composites have close-to-uniform fluorescence, birefringence, and circular dichroism properties [59]. The nanoparticles can help adjust the P of CNC liquid crystal and form cholesteric structures themselves so that the films have the potential to prepare polarizers. In contrast, Khan et al. used CNC self-assembly as a template and added it to phenol–formaldehyde resins to obtain polymer composites under the guidance of CNC self-assembly. After the CNC was removed, porous films were obtained. By coating acid or formaldehyde in the various regions of the film, potential images can appear when the film swells (Figure 9.34b) [50]. With inkjet printing, text and

9.6 Potential Applications (a) 100

A

Mass loss (%)

90 80

in deionzed water

70

in 25 mM NaCl solution in 25 mM NaCl solution

60 50

B

(b)

40 Wet

30 C

20

Dry

10 0 0

1

2

3

4

5

6

7

8

9

7 mm

10

Immersion time (h)

(c) EISA

CNC/TMOS

CNC/SIO2 composite

Base

1. HAuCl4

or acid

2. NaBH4

CNMC

Au@CNMC

Figure 9.34 (a) Evolution of the mass loss of the iridescent pigment with immersion time in different solvents. The inset images are optical images of the corresponding CNC-based pigment suspensions. (b) Optical images of mesoporous phenol–formaldehyde resin prepared with CNC assembly as the template. (c) Schematic for preparing gold-functionalized chiral nematic mesoporous cellulose (CNMC). Source: (a) Bardet et al. 2015 [55]. Reproduced with permission of Elsevier. (b) Khan et al. 2015 [50]. Reproduced with permission of John Wiley & Sons. (c) Schlesinger et al. 2015 [56]. Reproduced with permission of RSC.

images can be printed as higher resolution photonic patterns, so this material can be used as a security material. Similarly, Schlesinger et al. used a CNC/TMOS system to obtain CNC/SiO2 composites by EISA method, and then removed CNC and introduced gold NRs [56]. The interaction between surface plasmon resonance of gold NRs and CNC chiral nematic assembly resulted in a tunable cholesteric structure (Figure 9.34c). This material also has a good application prospect in the field of security features. Mark et al. obtained the cholesteric structure of CNC in polymer microspheres by the growth of CNC tactoids in water microdroplets of the reverse emulsion system. They extended this method to silica microspheres with cholesteric structure and obtained materials with large surface area and mesoporosity, which had the potential to prepare micro-optical devices [60]. 9.6.3

Sensors

The iridescent color of CNC self-assembly films has been found to be able to change upon exposure to liquid water and high relative humidity [14]. A reversible transition from dry blue-green to wet red-orange iridescence was observed due to the water-induced P change (Figure 9.35a). Therefore, the CNC self-assembly film can be used as humidity sensors. The thickness of the

307

9 Advanced Materials Based on Self-assembly of Cellulose Nanocrystals Schematic representation of SEM image

Helicoid axis

P/2

90 Transmission (%)

308

H2O

80 70 60 50 40 30 350

550

750

Wavelength (nm) 1

(a)

2

3

4

(b) HCI

Pristine

CH2O

Dried in air W/E 10/90 W/E 50/50 W/E 100/0 5 mm

(c)

Figure 9.35 (a) Schematic of the proposed correspondence between CNC assembly orientation in one domain of the cholesteric phase solid film. (b) UV–visible transmission spectra of different color amino-formaldehyde-cellulose films of 1 (red), 2 (orange), 3 (yellow-green), and 4 (blue). (c) Optical images of mesoporous phenol–formaldehyde resin films treated by immersion in HCl, pristine, and CH2 O and drying; and then immersed in water/ethanol binary solvent mixtures with different proportions, respectively. Source: (a) Zhang et al. 2013 [14]. Reproduced with permission of Elsevier. (b) Giese et al. 2013 [49]. Reproduced with permission of ACS. (c) Khan et al. 2015 [50]. Reproduced with permission of John Wiley & Sons.

CNC film has an important influence on the color change. The smaller the film thickness, the faster the color change. The color transition was observed in less than two seconds by reducing the thickness of the films, greatly improving the efficiency and value of the sensors. In addition to the sensing applications of CNC films, the CNC composite films also have good responsiveness. By introducing different components or matrix, CNC composite films have wider responsive and more sensitive sensing effect, which can be responsive to solvent [61], pH [13], pressure [49], and other stimulations [13, 17b]. For example, the iridescence of hydrogels with PAAm matrix that was prepared by Kelly’s groups changed greatly before and after water absorption [13].

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Meanwhile, the reflected light color of hydrogel films changed in different solvents. The wavelength of reflected light was also different at different pH values. The main reason for these changes was the change in P caused by the change in the degree of polymer cross-linking in the hydrogel. The MUF/CNC composites that were prepared by MacLachlan and his fellows had the ability to act as a pressure sensor [49]. The P of MUF-CNC composites decreased with increase in pressure. Macroscopically, the CD spectrum of the material then showed a blueshift (Figure 9.35b). Therefore, the change in pressure is related to the change in material color. Zhang and coworkers have prepared CNC/GO composite films and obtained a CNC/RGO material by reducing GO, which can have both iridescence and conductivity [17b]. This special property ensures that the CNC composite films can be used in electrically conductive sensors. Khan and coworkers used urea formaldehyde (UF) as matrix and CNC as the template to prepare mesoporous photonic cellulose (MPC) films. The obtained CNC/UF composite film had a high tensile strength. When immersed in different concentrations of alcohol solution, it can show different colors (Figure 9.35c), which meant that the material can be used as a solvent sensor [61].

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals Shiyu Fu and Linxin Zhong South China University of Technology, State Key Laboratory of Pulp and Paper Engineering, 381 Wushan Road, Tianhe District, Guangzhou 510641, China

Many natural structures from plants and animals exhibiting special colors are of great interest; for example, polysaccharides, DNA, collagen, and protein are intriguing platforms for mimicking solid replicas with photonic properties [1–3]. Scientists are trying to mimic their unique properties for photonic technologies [4–7]. Cellulose nanocrystals (CNCs), which are biomass-derived nanomaterials, attract great interest in applications in a wide range of scientific and commercial fields [8]. The colloidal properties of CNC from various sources or by different methods may be different.

10.1 Colloidal Properties of CNC and Applications in Functional Materials CNCs extracted from native cellulose sources by controlled acid hydrolysis generally have a rod-like shape [9]. Sulfuric acid imparts negatively charged acidic sulfate ester groups to the CNC surfaces during hydrolysis. CNCs could be well dispersed in distilled water due to the electrostatic repulsion among negatively charged sulfate ester groups. Such colloid stability of CNCs in water is very sensitive to electrolytes. When exposed to inorganic cations (e.g. Na+ and Ca2+ ), negative charges of CNCs are shielded, thus reducing the absolute value of the zeta potential and the electrostatic repulsion [10]. Increasing the concentration resulted in CNC aggregation. The divalent cation Ca2+ has a more significant impact on the colloid stability of CNCs than the monovalent cation Na+ solution. The organic low-molecular-weight electrolyte sodium dodecyl sulfate favored the stability of CNC suspension, whereas organic high-molecular-weight electrolytes such as sodium carboxymethyl cellulose (CMC) easily induced the aggregation of CNCs due to the intermolecular bridging interaction or entanglement. Cationic polyacrylamide (CPAM) caused a serious aggregation of CNC particles even at low concentrations [10]. The rod-like shape and negative surface charge of CNCs in water result in an upper random phase and a lower ordered phase at CNC concentrations above a critical value in water [11, 12] The ordered phase is a Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Figure 10.1 Schematic illustration of the organization of the isotropic and chiral nematic phases of a biphasic CNC suspension at equilibrium. Half the chiral nematic pitch, P/2, is shown. Source: Reprinted with permission from Beck et al. [13]. Copyright 2013, Springer Science+Business Media Dordrecht.

Isotropic phase

CNC

Director rotation Chiral nematic phase

P/2

Cholesteric axis

chiral nematic liquid crystal and the CNCs are arranged in pseudo-planes [13], as shown in Figure 10.1. The average CNC axis direction in each plane (the director) is rotated at a small angle to the planes above and below it, producing a helical arrangement of the directors about a line perpendicular to the planes (the cholesteric axis). The pitch P of the helix is the distance required for the average director to make one full rotation about the cholesteric axis. The formation and characteristics of chiral nematic ordered domains in suspension are closely related to the size, polydispersity, physical dimension, surface charge of the CNCs, and the ionic strength of the suspension. The volume fraction of the chiral nematic phase had a minimum value at a NaCl concentration of ∼1.0 mM [14]. With NaCl concentrations of 2.0–5.0 mM, the suspensions became entirely liquid crystalline, instead of separating into two phases. When the NaCl concentration increased from 0 to 2.75 mM, the size of the ordered domains in the anisotropic phase decreased. At a NaCl concentration of 2.75 mM, only tactoids were observed, while chiral nematic domains could not be observed at 5.0 mM. The chiral nematic pitch decreased with NaCl concentration and reached a minimum value at ∼0.75 mM, which was followed by a sharp increase with NaCl concentration up to 2.0 mM. It was found that the average separation distances range from 51 nm at the onset of the anisotropic phase formation to 25 nm above 6 vol% (fully liquid crystalline phase), as CNC concentration increases from 2.5 up to 6.5 vol%; however, the average pitch varies from 15 μm down to 2 μm [15]. As CNC concentration increased from 2.5 up to 6.5 vol%, the twist angle between neighboring CNCs increased from about 1∘ up to 4∘ . Beck-Candanedo et al. [16] examined the effect of size on the properties of CNC suspension and found that shorter CNCs slightly increased the critical concentration for anisotropic phase formation.

10.1 Colloidal Properties of CNC and Applications in Functional Materials

CNCs were commonly dispersed in water. Bruckner et al. [17] found that protic solvents with high dielectric permittivity can significantly speed up the self-assembly (from days to hours) of CNC at high mass fraction and reduce the concentration dependence of the helix period (variation reducing from more than 30 μm to less than 1 μm). Computer simulations indicated that increasing the dielectric permittivity of protic solvents can promote the degree of CNC order, leading to shorter pitch and a reduced threshold for liquid crystallinity. Therefore, the behavior of CNC suspensions can be controlled to obtain novel cellulose-based materials by choosing solvents with the desirable parameters. Cotton-derived CNC with various aspect ratios can be successfully dispersed in cyclohexane in the presence of surfactants [18]. The CNC spontaneously separated into a chiral nematic phase above a critical concentration that was higher than that in water. The CNC with the highest aspect ratio showed an anisotropic gel phase at high concentration, instead of phase separation. The liquid-crystalline properties of CNC suspensions make the CNC response to a magnetic field above and below the critical concentration C*. CNCs below C* would orient linearly perpendicular to the magnetic field in a strong magnetic field, giving long-range uniaxial (nematic) order [19]. Above C*, the anisotropic phase contains CNCs arranged in chiral nematic tactoids (small local regions with order) [20]. In a magnetic field, the chiral nematic tactoids are preferentially reoriented with the helical chiral nematic director parallel to the magnetic field, again leading to the orientation of CNCs perpendicular to the field [21]. Yager and coworkers [22] found that the alignment of CNCs in magnetic field occurs in two stages: an initial partial alignment occurring within minutes and a slower subsequent reorientation of chiral nematic tactoids (Figure 10.2). Dilute CNC suspension did not order appreciably when exposed to a magnetic field. The success of alignment under relatively weak magnetic fields will open up a new way to design and fabricate novel functional nanomaterials with enhanced mechanical strength, tunable optical properties, and unique microstructures. This magnetic field-induced alignment technique opens a new way to produce functional materials. For example, CNCs could be incorporated into a dissolved cellulose matrix and alignment induced with a magnetic field [23]. The CNCs orientation in a magnetic field led to improved stiffness and strength of the composites, but not to the level that is theoretically predicted for a fully aligned system. Decreasing the CNC volume fraction allowed a more readily oriented alignment in the magnetic field, leading to more significant increases in the mechanical properties. The all-cellulose composites have a domain structure, with some domains showing pronounced orientation of CNCs and others where no preferred orientation occurs. The chiral nematic order of the liquid crystalline phase can be retained by evaporating the aqueous CNC suspensions to produce solid semi-translucent CNC films with unique optical properties. During water evaporation, the chiral nematic pitch shrinks and the film’s pitch depends on the specific CNC suspension properties and the film formation conditions. Chiral nematic CNC films reflect left-handed circularly polarized light in a wavelength band. The reflected wavelength becomes shorter at oblique viewing angles, producing visible iridescence colors [13]. The periodic layer structure arises from the helical twist axis

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

0.10

qz (Å–1)

318

P/2

0.05 0.00

–0.05 –0.10

x-ray

B

–0.1

0.0 qx (Å –1 0.1 )

y

d0 (a)

(b)

S

→

z

q→

x

d0

z

ξ0 y (c)

x

Figure 10.2 (a) Schematic of the chiral nematic liquid-crystalline texture that CNCs adopt in suspension above C*; one-half pitch (P) is depicted. (b) CNC suspensions were loaded into a capillary centered between permanent magnets. The X-ray beam (red) probes orthogonal to the magnetic field direction (blue, along the x axis). Anisotropic scattering is collected on an area detector. CNCs orient perpendicular to the magnetic field direction (degenerate orientations shown at bottom). (c) CNCs pack and align with neighbors into tactoids. Because of the antialignment of CNCs with respect to the field (blue), ordered CNC tactoids are also antialigned. Ordering can be characterized by the packing distance (d0 ), correlation length (𝜉 0 ), and orientation order parameter (S). Source: Reprinted with permission from France et al. [22]. Copyright 2016, American Chemical Society.

of the chiral nematic mesophase film (Figure 10.3) [24]. In effect, the film comprises multi-domain Bragg reflectors. Interestingly, on exposure to liquid water and high relative humidity, a reversible shift in the film iridescence from dry state blue-green to wet state red-orange occurred because the sorption of water causes the pitch of the Bragg reflector to enlarge, and this leads to a redshift in the iridescence. The photonic properties of these films are of interest for coloration, reflectors, sensor, and security features [25]. In recent years, there has been increasing interest in CNCs as a promising key component to designing new materials exhibiting optical functionality or mechanical high performance, in view of CNC’s attractions such as the nanoscale dimension with a high aspect ratio and the inherent high stiffness. Heating temperature showed significant influence on the optical performance. Placing materials at different temperatures beneath an evaporating CNC suspension resulted in watermark-like patterns of different reflection wavelength incor-

10.1 Colloidal Properties of CNC and Applications in Functional Materials

Figure 10.3 Proposed correspondence between CNC rigid rod assembly orientation in one domain of the chiral nematic phase solid film and the periodic structure. The pitch P increases and decreases reversibly on sorption and desorption of water, with a concomitant change in color. Source: Reprinted with permission from Zhang et al. [24]. Copyright 2013, Elsevier.

Schematic representation of SEM image

Helicoid axis

P/2 H2O

porated within the final film structure. CNC film obtained from different temperatures results in watermark-like patterns with different wavelength reflection (Figure 10.4), which is believed to be caused by different evaporation rates and thermal motion of CNC [13]. It was found that the pattern formation occurred in the final stages of film casting, and thus it is possible to control the reflection wavelength of CNC films by a film forming process. By locally monitoring and controlling water evaporation via humidity adjustment, the self-assembly structure and optical properties of CNC film can be further tailored. These indicate that the main cause of the color fluctuations in such films is due to a nonuniform helical pitch, and not to the misalignment of the chiral nematic director. It is possible to fine-tune the color of the self-assembled film by carefully adjusting the evaporation conditions [26]. A simple methodology to address this problem and to promote a uniform helix orientation perpendicular to the film plane is presented. By raising the CNC concentration in the initial suspension to the fully liquid crystalline range, a vertical helix orientation was promoted. Further control

23 °C

35 °C

44 °C

Figure 10.4 CNC films formed by heating a Na-CNC suspension in a Petri dish on a metal ring at increasing casting temperatures. The redshift of the patterned area increases with increasing temperature. Source: Reprinted with permission from Beck et al. [13]. Copyright 2013, Springer Science+Business Media Dordrecht.

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of the helix orientation was achieved by subjecting the suspension to a circular shear flow during drying. The starting CNC suspension that had a sufficiently high concentration made it fully liquid crystalline. Introducing a circular shear flow by drying the suspension on an orbital mixer led to vertical helix orientation throughout the films with consequently much improved optical properties. A chiral nematic structure of sulfated CNCs would be more stable, but partial desulfation of preassembled CNC films by thermal treatment was found to make the films less susceptible to redispersion in water [27, 28]. During water evaporation, CNCs first form chiral nematic liquid crystals above a first critical concentration and then form gels above a second critical concentration [29]. Sulfation of CNC had significant impact on the rheological properties of CNC suspension and thus on the critical concentrations at which transitions from isotropic to liquid crystal and liquid crystal to gel occurred. In fact, most dried films exhibit a characteristic multi-domain mosaic pattern. To promote a uniform helix orientation perpendicular to the film plane, Park et al. proposed a simple strategy by using high CNC concentration in the starting suspension to ensure complete liquid crystallinity [26]. It is found that the high concentration of the starting CNC suspension rendered CNC fully liquid crystalline, thus avoiding the internal interfaces at the tactoid boundaries that are characteristic of the phase coexistence regime. A circular shear flow during the drying process breaks the symmetry and the helix develops vertically even within tactoids forming from an initially isotropic suspension as the water evaporates, leading to vertical helix orientation throughout the films with consequently much improved optical properties. By following this strategy, with some further optimization of initial drop volume, initial CNC concentration, and strength of the orbital shear flow, possibly combined with dynamic control of the ambient humidity throughout the drying process, solid CNC films with control of the helix pitch and thus excellent optical properties can be produced. Generally, static solution casting is the most widely used method to prepare CNC iridescent films. The CNC iridescent films show a polydomain structure with the helical axes of different chiral nematic domains pointing in different directions. The films tend to crack during the last stages of casting due to the significant capillary pressure gradients generated during evaporation, which is a major limitation to broadening these materials in applications. To overcome these disadvantages for fabricating highly oriented, large area, smooth, and structurally homogeneous CNC iridescent films, Chen et al. [30] proposed a vacuum-assisted self-assembly technique (VASA, vacuum filtration and subsequent evaporation). It was found that a long ultrasonic pretreatment is required to obtain CNC iridescent films via VASA (Figure 10.5). Furthermore, the iridescent color of the CNC films can be tuned by the sonication time, suspension volume, and the degree of vacuum [31]. Dispersing CNCs in organic media is necessary to expand the applications of CNCs. It was shown that the neutralized form of CNCs (CNC–X, X = Li, Na, K, etc.) obtained by treating the acidic form of CNCs (CNC–H) with an appropriate quantity of base and then freeze-drying can be readily dispersed in polar organic media and form chiral nematic phases during evaporation, producing solid films with chiral nematic

10.1 Colloidal Properties of CNC and Applications in Functional Materials

e

n

tio

ca

ni

So

VASA Sonication pretreatment

tim

14 h

16 h

18 h

Solution volume

Va c

28 nL

uu

m

21 nL

17.5 nL

14 nL

de

gr ee

0.07 MPa 0.06 MPa 0.05 MPa 0.04 MPa

Figure 10.5 Tuning the iridescence of CNC chiral nematic films with a vacuum-assisted self-assembly technique. Source: Reprinted with permission from Chen et al. [30]. Copyright 2014, American Chemical Society.

photonic properties. Preparing chiral nematic phases under nonaqueous conditions will significantly enlarge the scope of photonic materials of CNC, for example, iridescent polymeric composites. Isotropic and chiral nematic suspensions of CNC are stable, but interactions with ionic and polymeric species may result in flocculation or (non-covalent) gel formation. The reflection wavelength of the CNC films depends on the pitch (the distance over which the helical orientation of the CNCs undergoes a complete turn). The value of the pitch depends on many factors, for example, the ionic strength of the suspension, the presence of additives, or the evaporation rate of water from the suspension. The reflected colors of iridescent CNC films can be shifted toward shorter wavelengths when the electrolyte concentration of CNC suspension increases prior to film casting or toward longer wavelengths if the suspension is exposed to high-energy sonication [12]. The CNCs are organized in left-handed helicoids with pitch values typically around 1–2 μm, just above the pitch length where the reflection of visible light is expected. However, the addition of electrolyte to chiral nematic CNC suspensions is known to move the pitch to smaller values [32]. The electrolyte will screen the surface negative charges of CNCs to some degree [10], thus decreasing the electrostatic repulsion among CNC particles and shortening the pitch in a predictable manner. By adding small quantities of NaCl to the suspension before film casting, the pitch values are reduced and colored iridescent films of CNC obtained. Sonicating CNC suspensions can increase the chiral nematic pitch and shift the peak reflection wavelength of films cast from these suspensions toward longer wavelengths in a controllable manner [33]. Thus, iridescent films of any desired wavelength can be prepared from CNC suspensions by combining electrolyte addition with ultrasonication. Two distinct stages in the P change during evaporation can be observed when glucose is added to CNC suspension [34]. The first stage involves the decrease in P with CNC concentration in the chiral nematic suspension. In a second stage, CNC concentration is reached where the formation of ordered

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

gels and glasses prevents further change in P. The addition of glucose lowers the CNC concentration at which this occurs, leading to an increase in P and hence an overall shift to the red end of the spectrum in the final film. Pure CNC iridescent films self-assembled from a concentrated suspension under specific drying condition are usually very brittle, which makes them difficult to handle in practical applications. To improve the flexibility and to modulate the coloration of iridescent CNC films, polymers can be incorporated. For instance, the addition of 10 wt% polyethylene glycol (PEG) makes films much more flexible while preserving the coloration, while anionic sodium polyacrylate (PAAS) can be used for tuning the coloration within CNC films, as shown in Figure 10.6 [35]. It causes a stronger, narrower coloration in the visible spectrum with a well-pronounced fingerprint texture. Such results open up the prospect of bio-based materials dedicated to decorative, security, and anti-counterfeit flexible films following a biomimetic approach. The use of liquid-crystal templates to embed structures has emerged as a promising bottom-up approach to synthesize photonic materials. Self-assembly of colloidal particles has been widely explored and used to prepare patterned photonic structures through magnetic alignment [36], extrusion and compression molding [37], and capillary force-induced infiltration [38, 39]. Responsive photonic hydrogels based on CNC can be prepared by mixing CNC and acrylamide (AAm) in aqueous solution and subsequent evaporation [40]. It was easy to form a chiral nematic phase during evaporation, with an AAm/CNC ratio of 2.7 : 1 by weight. Nanocomposite hydrogels with iridescence can be prepared by allowing the dispersion to evaporate to dryness before polymerization (for example, to a final composition of 64.4 wt% CNC, 33.5 wt% AAm, and 2.1 wt% cross-linker) (Figure 10.7). Increasing the ionic strength of the dispersion by adding salts such as NaCl, which is known to decrease the helical pitch of CNC chiral nematic phases, produces nanocomposite hydrogels with reflected color.

Flexibility with nonionic polymer

Self-assembly of cellulose nanocrystals

Tunable coloration with anionic polymer

Figure 10.6 FE-SEM images of a fracture surface across an iridescent CNC film and various iridescent films. Source: Reprinted with permission from Bardet et al. [35]. Copyright 2015, American Chemical Society.

10.1 Colloidal Properties of CNC and Applications in Functional Materials

(a)

(b)

250 μm

500 μm

(d)

(c)

15 mm

25 mm

Figure 10.7 Formation of chiral nematic structure in nanocomposite hydrogels at varying composition. (a) Polarized optical microscopy image of a CNC/AAm dispersion during evaporation. (b) POM of a PAAm nanocomposite prepared with high AAm loading (10 wt% CNC) swollen in water (inset: a photograph of the swollen transparent hydrogel). (c) Photographs of iridescent PAAm nanocomposite hydrogels (66 wt% CNC) and varying amounts of NaCl; increasing the ionic strength blueshifts the reflectance across the visible region. (d) Photograph of an iridescent photopatterned PAAm nanocomposite as the film swells in water. The masked region swells at a faster rate, producing a latent image (a photograph of the patterned film before swelling is given in the inset). Source: Reprinted with permission from Kelly et al. [40]. Copyright 2013), John Wiley and Sons.

Tatsumi et al. [41] synthesized novel composites consisting of poly(2hydroxyethyl methacrylate) (PHEMA) and CNC from a suspension of CNC and 2-hydroxyethyl methacrylate (HEMA) monomer solution. The starting suspension containing 5 wt% CNC separated into an isotropic upper phase and an anisotropic bottom phase during quiescent standing. Three polymer composites, PHEMA-CNCiso, PHEMA-CNCaniso (Figure 10.8), and PHEMA-CNCmix, can be obtained from the isotropic phase, anisotropic phase, and embryonic non-separating mixture via polymerization of HEMA in different phase situations of the suspensions, respectively. All the composites were transparent and exhibited birefringence under a polarized optical microscope. A fingerprint texture typical of cholesteric liquid crystals of longer pitch spread widely in PHEMA-CNCaniso but appeared rather locally in PHEMA-CNCiso. CNC improved the original thermal and mechanical properties of the amorphous PHEMA matrix.

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

10 9 Log E′ (Pa)

324

7 6

50 μm

PHEMA-CNCaniso PHEMA

5

PHEMA-CNCaniso Composite (a)

8

0 (b)

50

100 150 200 Temperature (°C)

250

Figure 10.8 (a) FE-SEM images of the fracture surfaces of polymer composites; (b) temperature dependence of the storage modulus (E ′ ). Source: Reprinted with permission from Tatsumi et al. [41]. Copyright 2012, American Chemical Society.

Copolymer ureidopyrimidinone (UPy) motifs can also be incorporated into CNCs to produce a film with characteristic photonic stop bands, as shown in Figure 10.9 [42]. The dimensions of the helical pitch can be regulated by the ratio of polymer to CNC. When exposing the biomimetic hybrids to water, the film shows photonic response performance due to the regulation of the swelling by supramolecular motifs. Moreover, by engineering UPy molecules films with the highest strain of up to ∼13% and the highest stiffness (E) of ∼15 GPa can be obtained. By incorporating poly(vinyl alcohol) (PVA) into CNCs, an iridescent film with cholesteric liquid-crystal structure can be obtained, as shown in Figure 10.10 [43]. Different helical pitches and photonic bandgaps can be realized by varying the CNC/PVA ratio. Transition from a cholesteric to a disordered structure or stiffness to ductility can be observed at a critical PVA concentration. Different extents of crack deflection, layered delamination, ligament bridging, and constrained microcracking can also be observed. Drawing of highly plasticized films sheds light on the mechanistic details of the transition from a cholesteric/chiral nematic to a nematic structure.

10.2 Nanocellulose for Paper and Packaging Packaging plays an important role in driving modern economy. The packaging industry is poised to grow rapidly by the emergence of innovative packaging materials and the rising flexible packaging market. Food and beverages occupy the largest share in the total packaging sector, accounting for 85%, followed by the plastic packaging market, which is expanding rapidly with a growth of 20–25% per annum. Global packaging market revenues totaled nearly $42.5 billion in 2014. It is estimated this market will increase from $43.3 billion in 2015 to nearly $48.3 billion by 2020 at a compound annual growth rate of 2.2% through 2020. The global biodegradable polymer market should reach $7.1 billion by 2021 from

10.2 Nanocellulose for Paper and Packaging

CO

n

O

O

m

O H N O

O N HN

O HN

O 8.5 O

O

NH

N H N

NH

N H O

NH

HN N

(a)

1 μm

(b)

Figure 10.9 Self-assembly of tailored mixtures of EGUPyX polymers modified with different fractions of fourfold hydrogen-bonding 2-ureido-4-pyrimidinone (UPy) segments and cellulose nanocrystals (CNCs) into cholesteric phases during solution casting (a) and long-range cholesteric order of UPy/CNC nanocomposites (b). Source: Reprinted with permission from Zhu et al. [42]. Copyright 2016, American Chemical Society.

OSO3H O HO

O OH n 10 μm

n

2 μm

OH

(a)

(b)

Figure 10.10 Preparation of cholesteric, crustacean-mimetic CNC/PVA nanocomposites (a) and SEM images of fractured cross sections of nanocomposites (b). Source: Reprinted with permission from Wang and Walther [43]. Copyright 2015, American Chemical Society.

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$3.1 billion in 2016 at a compound annual growth rate of 18.0%, from 2016 to 2021 [44]. However, the poor physical properties of bio-based materials limit its application in food packaging. The incorporation of nanocellulose into these materials may improve those properties. In the foreseeable future, the packaging for food or high-end products can be manufactured from natural biopolymers, which substitute plastic materials (Figure 10.11). Cellulose nanostructures have been widely studied as components of materials for a variety of applications including food packaging [45, 46]. They are incorporated as a reinforcement phase in nanocomposites (as CNCs or cellulose nanofibrils). Nanocellulose combined with bioplastics or biosynthesis polymers can drive the target to put into practice. Microfibrillated cellulose (MFC) was essentially used in nanocomposites as a reinforcing-structure agent [47]. Its nanoscale dimension and its ability to form a strong entangled nanoporous network have encouraged the emergence of new high-value applications. The presence of 5% Cellulose nanofibers (CNFs) in the polylactic acid (PLA) matrix, a bioplastic, led to increase in tensile strength, Young’s modulus, and improved viscoelastic behavior. This underlines the success of the melt compounding procedure to prepare cellulose nanocomposites in consideration of the intended application in food packaging [48].

10.2.1

Nanocellulose for Paper Coating

Coating treatment is an important technology to improve the mechanical strength and some functional properties of paper. In general, the coating ingredient is constituted of polymers and some fine fillers. Pure nanocellulose cannot be used as coating on paper because there is an abundance of hydroxyl groups on the surface, which endow the substrates, such as paper and films, with poor moisture resistance. Most of the work for the application of nanocellulose includes strength enhancement, medicine delivery, enhanced oxygen and water vapor barrier properties, etc. Commercial available paperboards coated with thin layers of nanocellulose can improve their moisture barrier properties [49, 50]. This is because the nanocellulose coating induces a surface smoothening effect on the coated sheets. The moisture-protective layer of renewable alkyd resins is deposited on the nanocellulose pre-coated sheets using a waterborne dispersion coating process or lithographic printing. The applied alkyd resins are transformed into moisture sealant layers by an auto-oxidation process. The water vapor barrier properties of the nanocellulose pre-coated substrates were significantly improved by thin layers of renewable alkyd resins. The nanocellulose coating had a notable effect on the homogeneity and barrier performance of the alkyd resin layers and those alkyd resin layers that were applied by printing. The concept is environmentally friendly, energy-efficient, and economic. With this method, large-scale renewable coatings applicable for sustainable packaging are foreseen. A nanocellulose coat weight of 3 g/m2 resulted in a continuous film formation of the coating layer and a hardly visible fiber structure in the base paper (paperboard). The nanocellulose coating on both single-coated substrates did not change surface

200 nm

Figure 10.11 Nanocellulose combined with paper or film for food packaging.

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

textures. On top of the nanocellulose pre-coated paperboards alkyd resin coating can form a highly uniform and featureless layer. MFC had a strong synergistic effect on linting propensity with anionic starch so that the tinting and dusting tendency can be held out in the process of newsprint manufacturing [51]. 10.2.2

Microfibrillated Cellulose Coated Paper for Delivery System

Papers coated with MFC can be used to develop a high-performance delivery system that is a controlled release system combined with the drug delivery agent, β-cyclodextrin (βCD) [47, 52]. Antibacterial substances, such as chlorhexidine digluconate (CHX) or carvacrol, were mixed with a suspension of MFC or a βCD solution or a mixture of both before coating onto a cellulosic substrate. Intermittent diffusion of medicines was conducted in an aqueous medium, and the release from βCD was processed more gradually and over a longer period of time compared to MFC. βCD can form an inclusion complex with CHX or carvacrol, which cannot be released fast. MFC mainly affected the burst effect, while βCD primarily controlled the amount of medicines released over time. Two controlled release systems are proposed, and the use of βCD alone would release low amounts of active molecules over time (slow delivery), whereas the combination of βCD and MFC would be more suitable for the release of higher amounts of active molecules over time. CHX mixed with MFC was used as coating on paperboard [53]. Five layers of MFC were successively deposited onto the surface of the paperboard to ensure a homogeneous coat. Besides improving the bending stiffness, MFC was also used as a sustained release system. The nanoporous network of MFC plays a major role in the controlled release of CHX. It did not only release the CHX molecules more slowly, but released them more gradually, as seen in Figure 10.12. Bacterial nanocellulose (BNC) is also studied to construct a drug delivery system for proteins. The overlay of diffusion- and swelling-controlled processes was confirmed by Ritger–Peppas equation. The integrity and biological activity of proteins could be retained during the loading and release processes [55]. BNC membranes can be used to fabricate nanostructured transdermal delivery systems for diclofenac sodium salt (a typical non-steroidal anti-inflammatory drug). The membranes with good flexibility and very high swelling behavior are similar to human epidermal membranes, and showed good permeation rates in conducting diffusion test in vitro with Franz cells. There are enormous potentialities of using NBC membranes for transdermal administration of diclofenac [56]. Control of drug action through formulation is a vital and very challenging topic within pharmaceutical sciences. Cellulose nanofibers (CNF) are an excipient candidate in pharmaceutical formulations. CNF combined with surfactants can be used to create very stable air bubbles and dry foams. It is possible to modify the release kinetics of the model drug riboflavin in a facile way. The drug was suspended in the wet-stable foams followed by a drying step to obtain dry foams. The drug was released from the solid foams in a diffusion-controlled, sustained manner in the presence of intact air bubbles, which imparted a tortuous diffusion path. By changing the dimensions of dry foams while keeping drug load and total

10.2 Nanocellulose for Paper and Packaging

Scenario 1

Release of the CHX and of the excess of cyclodextrins

Cellulosic fiber

Scenario 2

Release of the cyclodextrin followed by the release of the CHX

Cellulosic fiber

Scenario 3

Release of the inclusion complex

Cellulosic fiber

Figure 10.12 The release mechanism between the CHX, the cyclodextrins, and the CHX-βCD inclusion complexes. Source: Reprinted with permission from Lavoine et al. [54]. Copyright 2014, Elsevier.

weight constant, the drug release kinetics could be modified; e.g. a rectangular box-shaped foam of 8 mm thickness released only 59% of the drug after 24 hours whereas a thinner foam sample (0.6 mm) released 78% of its drug content within eight hours. The drug release from films (0.009 mm, with the same total mass and an outer surface area comparable to the thinner foam) was much faster, amounting to 72% of the drug within one hour. The entrapped air bubbles in the foam also induced positive buoyancy, which is interesting from the perspective of gastroretentive drug delivery [57] 10.2.3

Water-Resistant Nanopaper Based on Modified Nanocellulose

Nanocellulose films (nanopaper) under hydrated or even fully wet conditions may lose their strength. It is one of the major challenges for applications of nanocelluloses in high-value products to maintain high mechanical properties under such environment. Modifications, such as covalent cross-linking or surface hydrophobization, are viable approaches. However, cross-linking method may hamper processability, while the surface hydrophobization may block the

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interfibrillar bonding. Toivonen et al. [58] provided a concept based on physical cross-linking of cellulose nanofibers with chitosan for the aqueous preparation of films with high mechanical strength in the wet state. Transparency (∼70% to 90% in the range 400–800 nm) is also achieved by lessening the aggregation of CNFs. UV−vis spectra show that pure CNF films have the highest transparency over the whole spectrum, while hybrid films have slightly lower transparencies. However, the hybrid films have increasing transparency with increasing chitosan loading (Figure 10.13). Chitosan dissolves in aqueous medium at low pH and under these conditions the CNF/chitosan mixtures form easily processable hydrogels. Increasing pH leading to reduced hydration of chitosan can promote multivalent physical interactions between CNF and chitosan over those with water, resulting effectively in cross-linking (Figure 10.14). In optimal pH, both components remain stable, which may be very good for the transparency of nanopaper (Figure 10.14b), while in high pH, the chitosan aggregates in the system (Figure 10.14c). Wet water-soaked films of CNF/chitosan 80/20 w/w have excellent mechanical properties, with an ultimate wet strength of 100 MPa (with corresponding maximum strain of 28%) and a tensile modulus of 4 and 14 GPa at low (0.5%) and large (16%) strains, respectively (Figure 10.15). The maximum strain per cycle was increased with small steps at low strain, which is highlighted in the inset in Figure 10.16. Already in the first strain cycle the behavior seems to be highly plastic as the stress during the second strain cycle begins to rise at a strain only slightly lower than the maximum strain of the previous cycle. After the first cycle, the sample begins to demonstrate some elastic behavior also as roughly half of the strain is recovered. This behavior continues also to larger deformations until fracture. The material clearly demonstrates strain hardening also in the wet state as the stress needed to cause further plastic deformation increases. 100

80 Transmittance (%)

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CNF/chitosan 100/0 95/5 90/10 85/15 80/20 75/25 50/50 25/75

60

40

20

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500 600 Wavelength (nm)

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Figure 10.13 UV–vis spectra of thin films for different CNF/chitosan. Source: Reprinted with permission from Toivonen et al. [58]. Copyright 2015, American Chemical Society.

10.2 Nanocellulose for Paper and Packaging

(a)

(b)

(c)

Figure 10.14 Schematic representation of the colloidal behavior upon mixing of CNF/chitosan. (a) Low pH CNF flocculates due to the surface-bound heteropolysaccharides; (b) optimal pH where both components remain stable; (c) high pH where chitosan aggregates. Source: Reprinted with permission from Toivonen et al. [58]. Copyright 2015, American Chemical Society.

Wet CNF/chitosan 80/20

Stress (MPa)

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0

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Strain (%)

Figure 10.15 Wet tensile properties for base-treated and not base-treated CNF. Source: Reprinted with permission from Toivonen et al. [58]. Copyright 2015, American Chemical Society.

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals 3

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Strain (%)

Figure 10.16 Cyclic tensile tests of the base-treated CNF/chitosan 80/20 w/w composition in the wet state. The inset shows short cycles at low stress and strain. Source: Reprinted with permission from Toivonen et al. [58]. Copyright 2015, American Chemical Society.

More dry films of similar composition display strength of 200 MPa with maximum strain of 8% at 50% relative humidity. The tensile stress–strain behavior of base-treated CNF/chitosan 80/20 w/w films of high strength was retained in a more dry state (relative humidity 50%, temperature 20 ∘ C, film not wetted) very similar to that of the neat CNF films. The CNF/chitosan 80/20 w/w film is slightly softer and more extensible than the neat CNF film. The films made of MFCs, despite having sufficient mechanical property for packaging applications in dry condition, do not have water vapor barrier property similar to petroleum-based plastics. These properties can be mended by adding mineral fillers within the film structure. Research indicated that addition of fillers, such as wax, or surface agents, resulted in films with lower densities, as well as lower water vapor transmission rates (WVTRs). The reason is that the water vapor solubility decreased in the films after coating. Interestingly, coating with beeswax, paraffin, and cooked starch resulted in MFC films with WVTRs lower than those of low density polyethylene (LDPE) (Figure 10.17). These coatings were modeled with a three-layer model, which determined that coatings were more effective in reducing WVTR [59]. Cellulose nanofibril (CNF) in rod shape with a diameter of 30 nm and length of 200 nm was used as a filler to reinforce sodium CMC film. The rod CNF was evenly distributed in the CMC matrix to form smooth and flexible films to increase the tensile strength and elastic modulus of CMC films. The important feature is that the water vapor permeability (WVP) of CMC film decreased at low content of CNF, as shown in Table 10.1 [60]. When 1 wt% CNF was added in the composite, the WVP decreased significantly up to 5 wt%. The decrease in the WVP of composite films was attributed to the impermeable CNF, which was well distributed through the polymer matrix

10.2 Nanocellulose for Paper and Packaging

2.5

WVTR * 100 ((g/m2*d)/m)

MFC

2 Beeswax Paraffin Cooked starch MFC LDPE

1.5

1 LDPE

0.5

0 0

5

10

15

20

25

30

Coating weight (g/m2)

Figure 10.17 WVTR of Nanopaper coated with fillers. Source: Reprinted with permission from Spence et al. [59]. Copyright 2011, BioResources. Table 10.1 Water vapor permeability (WVP) and contact angle (CA) of CMC and CMC/CNF films.

Film

WVP (×10−9 g m/m2 Pa s)

CA (∘ )

CMC control

1.40 ± 0.03

39.2 ± 1.8

CMC/CNF1%

1.27 ± 0.06

28.1 ± 1.4

CMC/CNF3%

1.33 ± 0.03

27.1 ± 0.7

CMC/CNF5%

1.36 ± 0.04

26.4 ± 0.9

CMC/CNF10%

1.44 ± 0.07

23.9 ± 1.7

Source: Reprinted with permission from Oun and Rhim [60]. Copyright 2015, Elsevier.

to form a tortuous path for water vapor diffusion and to increase the effective diffusion path length. However, the WVP increased beyond that of the pure CMC film when the content of CNF was over 10 wt%. The water contact angle (CA) is a measure of hydrophobicity of the surface of packaging films. In general, the highly hydrophilic material has CA less than 65∘ . The CA measure of the films is listed in Table 10.1. The CA of the CMC film was 39.2∘ , which indicates the high hydrophilic surface of the CMC film. The CA of the CMC film decreased significantly (p < 0.05) after addition of cotton linter CNF, which reveals that CNF is more hydrophilic. The CMC is listed in the GRAS (generally recognized as safe) materials and widely used in various sectors relating to food or medicines. Although nanocrystalline cellulose is not on the GRAS list, there is no information to suggest that such forms of cellulose have significantly different biological properties from those currently considered as GRAS.

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

The CMC/CNF composite films may have a high potential to be applied as an edible coating or packaging film for the extension of shelf life of fresh and minimally processed fruits and vegetables. However, further research on the health problems of these packaging films and the environmental safety is still required. Soybean cellulosic microfibrils (SMF) and soybean microparticles (SMP) extracted from soybean hulls can be used in films and combined with wood-based micro and nanofibrillar cellulose (MNFC) in hybrid systems [61, 62]. The hybrid films displayed strength (elastic modulus and strength at rupture) and barrier performance, offering an option for reduced cost while maintaining performance through synergistic contributions of the components. 10.2.4

Effect of Chemical Composition on Microfibrillar Cellulose Film

MFCs produced by homogenizer from pulps containing extractives, lignin, and hemicelluloses to some extent were cast to form MFC films. The presence of lignin significantly increased film toughness, tensile index, and elastic modulus [63–65]. Regardless of chemical composition, the process of converting macrofibrils to microfibrils resulted in a decrease in water adsorption and WVTR (Figure 10.18). Water adsorption and WVTR are important properties for food packaging materials. MFCs with high lignin content had a higher WVTR, even with a higher initial contact angle, hypothesized to be due to large hydrophobic pores in the film. Only a small amount of paraffin wax in the materials, such as less than 10%, can reduce the WVTR of cellulose-based films to a similar value as LDPE. Hard-to-remove water content correlated with specific surface area up to approximately 50 m2 /g but not with water retention value. The drying rate of the MFCs increased with the specific surface area (Figure 10.19). All cellulose multilayer films were self-assembled by sequential addition of oppositely charged cellulose I nanoparticles [66]. The layer-by-layer adsorption 12 Water adsorption (g/g)

334

R2 = 0.6447

10 8 6 4

R2 = 0.0161

2

R2 = 0.0058

0

0

5

10

15

Lignin content (%)

Figure 10.18 Water adsorption vs. lignin content for MFC films, original pulps (diamond), pretreated materials (square), and homogenized samples (triangle). Source: Reprinted with permission from Spence et al. [64]. Copyright 2010, Elsevier.

10.2 Nanocellulose for Paper and Packaging

Initial contact angle (°)

100 80

R2 = 0.919

60 40 R2 = 0.8338 20 0 0

5 10 Lignin content (%)

15

Figure 10.19 Initial contact angle vs. lignin content for film samples produced from MFC before extraction (diamond) and after extraction (square). Source: Reprinted with permission from Spence et al. [64]. Copyright 2010, Elsevier.

of cationic modified cellulose nanofibrils (CNFs) and anionic short CNC was used to build up films. It is found that cat CNF charge was significantly dependent on the pH of the system for both direct surface interactions and layer properties. Lower adsorption of the first cat CNF layer onto SiO2 crystal was found at pH 4.5 compared to pH 8.3 (Figure 10.20). The corresponding viscoelastic response of the layers during the formation at pH 4.5 is also lower compared to the layer built up at higher pH. The underlying cellulose layer in multilayer films influenced the surface forces, especially at lower pH, where the cat CNF chain extensions were facilitated and overall charge was affected by the cationic counterpart within the layers. In Figure 10.21a, cat (CNF/CNCs)6 multilayers at pH 4.5 were built up with CNCs as the capping layer and with cat CNF as the capping layer (Figure 10.21b). The underlying cat CNF possessed higher overall positive charge at this pH, and influenced the forces. Multilayer systems with CNCs as the capping layer at SiO2 0

In situ adsorption

1

–20

1

–40 ΔF (Hz)

Figure 10.20 Frequency and dissipation changes upon LbL build-up of all cellulosic structures by sequential adsorption of cationic CNF and anionic CNCs at pH 4.5 and pH 8.3. Source: Reprinted with permission from Olszewska et al. [66]. Copyright 2013, Elsevier.

2 3

Cat NFC

–60

pH 4.5

–80 CNCs

–100

2

–120 –140

Cat NFC

–160 0

5

10

15 20 25 Time (min)

3 30

pH 8.3 35

40

335

10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals 0.020

0.020

0.015

0.015 F/R (mN/m)

0.010

F/R (mN/m)

336

pH 8.3

0.005 0.000 –0.005

pH 8.3

0.005 0.000

–0.005

pH 4.5

pH 4.5

–0.010

–0.010 0

(a)

0.010

10 Separation (nm)

20

0

(b)

5

10

15

20

25

Separation (nm)

Figure 10.21 Effect of capping layer on normalized force versus separation on approach between a cellulose sphere and (CNF/CNC)6 multilayers system with CNC (a) or cat CNF (b) capping layer at pH 8.3 (closed symbols) and pH 4.5 (open symbols). Source: Reprinted with permission from Olszewska et al. [66]. Copyright 2013, Elsevier.

both pH 4.5 and pH 8.3 present more attraction on approach compared to one single layer of CNCs, further demonstrating the effect of the underlying layer. The role of charge and structure on the interaction between these nanoparticles is significant for designing novel materials based on nanocellulose. Hemicellulose in fiber may affect the properties of subsequently prepared CNFs. CNFs from holocellulose have a pure cellulose fibril core, with a hemicellulose coating that contains anionic charge. These CNFs are used to prepare honeycomb and foam structures by freeze-drying from their dilute hydrocolloidal suspensions. The honeycomb structures show superior out-of-plane properties compared with the more isotropic foam structures. Honeycombs based on holocellulose CNFs showed better properties than sulfite pulp CNF honeycombs, since the cellular structure contained less defects. This is related to better stability of holocellulose CNFs in colloidal suspension [67]. Application of nanocellulose leads to improvement in overall performance of polymer composites by improving their mechanical, thermal, and barrier properties, usually even at very low content. Thus, nanocellulose plays an important role in improving the feasibility of use of polymer composites for food-packaging sector by reducing the packaging waste of processed foods and improving the preservation of packaged foods by extending their shelf life. Nanocellulose with extremely high surface area and tunable surface chemistry gives nanocellulose-based materials great potential to enhance oxygen and water vapor barrier properties when used as coating, fillers in composites, and as self-standing thin films [68]. CNF and CNC composites and their coating reduce the oxygen permeability that enhances the shelf life of packed food. Actually, the oxygen permeability of pure CNF and CNC films is highly competitive and even comparable with commercial synthetic polymers [69]. 10.2.5 Antimicrobial Diffusion Films Based on Microfibrillated Cellulose For food packaging, antimicrobial diffusion films (ADFs) are significant products with antibacterial activity besides good mechanical strength and barrier

10.2 Nanocellulose for Paper and Packaging

properties. The film for food packaging with nanocellulose from biomass is often called bio-based nanocomposite. A bio-based nanocomposite was developed by incorporation of cellulose nanoparticles (CNs) obtained from sulfuric acid hydrolysis into alginate biopolymer using the solution casting method [70]. The tensile strength value of the composite films increased with increasing NC content. A biohybrid film can be formed by combining vermiculite nanoplatelets with nanocellulose [71]. The resulting hybrid films were stiff (tensile modulus of 17.3 GPa), strong (strength up to 257 MPa), and transparent with good oxygen barrier (0.07 cm3 μm m−2 d−1 kPa−1 ) at 50% relative humidity and water vapor barrier properties. The ADFs for food packaging were composed of two external layers of polycaprolactone (PCL) and one internal layer of CNC-reinforced methylcellulose (MC) matrix (PCL-MC-PCL). The packaging in the presence of ADFs showed good performance against Salmonella Typhimurium (Figure 10.22, [73]). The middle layer was composed of methylcellulose and nanocellulose absorbed natural antibacterial agents (formulate A and B). MC-control did not inhibit any growth of tested pathogenic bacteria. Notable inhibitory zones were detected for MC-A and -B against S. Typhimurium. MC-A and MC-B induced similar inhibitory zones against Listeria monocytogenes and Escherichia coli (p > 0.05). Those antimicrobial results were generated via the diffusion of volatile compounds and their effectiveness from the film to the package headspace, in relation with the concentration of antimicrobials in the closed atmosphere as a function of time, that is, the difference of diffusion through the polymeric matrix. The role of nanocellulose may control the diffusion of volatile compounds in the films. Paper made from bagasse pulp have been modified using nanocellulose and the antibacterial agent chitosan or by surface coating. Air permeability decreased Figure 10.22 Inhibitory zones of S. Typhimurium growth on a bacterial plate induced by trilayer ADF-control, ADF-A, and ADF-B. Source: Reprinted with permission from Boumail et al. [72]. Copyright 2013, American Chemical Society. ADF-A

ADF-B

12.8 mm

13.6 mm

ADF-control

337

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

with addition of 8% NC to the paper matrix. The presence of chitosan as an additive or in a coating formulation in the paper sheets enhanced the paper resistance to different microorganisms, especially those causing food poisoning [74]. Immobilizing nisin, a peptide, on CNF also displayed antimicrobial activity against different gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) with significant 3.5 log reduction. CNF can strongly interact with nisin to form coating layers on papers. This kind of paper is potential to apply for food packaging due to their antimicrobial and gas barrier properties [75]. Nisin was immobilized on the surface of CNC/chitosan nanocomposite films by using genipin as a cross-linking agent. The genipin cross-linked films demonstrated high antimicrobial activity against both the bacteria at the end of 35 days of storage at 37 ∘ C, showing an inhibition zone of 27.1 mm for E. coli and 27.7 mm for L. monocytogenes as compared to 23.4 and 23.8 mm for the same respective bacteria at day 1. The films restricted the growth of psychrotrophs, mesophiles, and Lactobacillus spp. (LAB) in fresh pork loin meats and increased the microbiological shelf-life of meat sample by more than five weeks. The films also reduced the count of E. coli and L. monocytogenes in meat samples by 4.4 and 5.7 log CFU/g, respectively, after 35 days of storage. The use of nanocellulose can permit reinforcing the films and can improve their physicochemical properties [76]. Nanocellulose can be mixed with chitosan to form composite with antimicrobial activity. The CNFs were subjected to periodate oxidation to obtain nanocellulose dialdehyde (CDA). The aldehyde groups of CDA were reacted with the amino groups of chitosan to form Schiff base. The resulting CDA/chitosan composite fibers were then cast into films binding with cellulose acetate. The composite films showed excellent antimicrobial properties against S. aureus and E. coli. The maximum antimicrobial activity of the film was 96.5% and 78.75% against E. coli and S. aureus, respectively. The antimicrobial activity was sustained over a period of six months. These nanocomposite films can thus find potential applications in packaging material [77]. CNFs combined with chitosan and S-nitroso-N-acetyl-d-penicillamine (SNAP) can form antimicrobial composite membranes for food packaging. The fabricated membranes had a uniform dispersion of chitosan and SNAP within the CNFs. Antimicrobial property evaluation of SNAP-incorporated membranes showed an effective zone of inhibition against bacterial strains of Enterococcus faecalis, S. aureus, and L. monocytogenes and demonstrated their potential applications for food packaging [78]. Chitosan-nanocellulose biocomposites were prepared from chitosan having molecular weight of 600–800 kDa, nanocellulose with 20–50 nm diameters, and glycerol. Chitosan-nanocellulose nanocomposites showed a high T g range of 115–124 ∘ C and were able to keep their solid state until the temperature (T m ) range of 97–99 ∘ C. The nancomposite had inhibitory effects against both gram-positive (S. aureus) and gram-negative (E. coli and Salmonella enterica) bacteria through its contact area. Application of chitosan-nanocellulose nanocomposite on the ground meat decreased lactic acid bacteria population compared with nylon packaged samples up to 1.3 and 3.1 logarithmic cycles at 3 and 25 ∘ C after six days of storage, respectively [79].

10.3 Nanocellulose for Wood Coatings

10.3 Nanocellulose for Wood Coatings Many wood products, such as furniture and woody arts, need surface protection with film-forming wood coatings over a long period of time. In ancient China, Chinese wood oil (Tung oil) has been widely implemented for the protection of woody stuff and ornaments, for example, agricultural tools, floors, carved beams, and painted rafters, by painting with pigments. The oil-based coatings impart appealing optical and haptic properties to the wood surface, but lack sufficient protection against water and mechanical influences. Veigel et al. [80] developed a method to improve the performance of linseed oil coating by the addition of CNF. The CNF was chemically modified with acetic anhydride (AC) and (2-dodecen-1-yl)succinic anhydride (DDSA), respectively, using propylene carbonate (PC) as a solvent. Modified CNF mixed with linseed oil varnish was homogenized by 20 passes in a high-pressure homogenizer operated at 500 bars. The oil formulations were applied to beech wood substrates by compressed air spraying in a quantity of 55 g/m2 . The finished wood samples were conditioned at 20 ∘ C and 65% relative humidity for three weeks before further processing. Nanocellulose used as additives for a waterborne acrylate/polyurethane-based wood coating can improve the mechanical resistance of coated wood surfaces. When the wood coatings contain up to 5 wt% nanocellulose as matting agent, the coatings showed significantly lower levels of gloss than the unmodified coating. Scratch and abrasion resistance improved consistently with increasing nanocellulose addition [81]. When CNF is mixed with different additives, the surface gloss was found to be considerably different for the individual variants. Apart from cellulose nanofibrils-(2-dodecen-1-yl)succinic anhydride (CNF-DDSA) (Figure 10.23), other cellulose-modified coatings appeared clearly less glossy than the unmodified oil. This matting effect of CNF was primarily attributed to an increased surface roughness induced by the addition of CNF. Surface gloss is usually determined by the refractive index of the material, the incident angle of light, 50

Ra x 10 (μm)

45

Gloss 85° (GU)

40 35 30 25 20 15 10 5 0 Oil

CNF-AC

CNF-PC

CNF-DDSA

CNF-AA

Figure 10.23 Arithmetic mean roughness (Ra ) and 85∘ gloss level of beech wood surfaces coated with linseed oil varnish containing 1 wt% CNF.

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10 Potential Application Based on Colloidal Properties of Cellulose Nanocrystals

and the surface topography. Since the incident angle was the same for all measurements and surface roughness was similar for all variants, the higher gloss level of CNF-DDSA may be explained by a change in the refractive index of CNF introduced by DDSA modification (Figure 10.23). The wear resistance of CNF/oil-coated wood surfaces was evaluated by a test combining abrasive loading on a Taber Abraser and subsequent wettability measurements (Figure 10.24). A higher contact angle after a given number of abrasion cycles indicates a higher wear resistance. Before the first abrasion cycle, all specimens show similar wettability with water as expressed by a contact angle of approximately 100∘ . With increasing number of abrasion cycles on the surface, these specimens lose their hydrophobic properties because oil and oil-impregnated wood cells were gradually removed from the surface. It is interesting that CNF without chemical modification suspended in propylene carbonate had the best improvement on the coating’s wear resistance. This was primarily attributed to the loose network structure of this CNF variant, which effectively prevents the oil from penetration into the wood surface, thus forming a protective CNF/oil composite layer on the wood surface. The CNF is an ideal reinforced filler for waterborne coatings. CNF was dispersed in the polymer matrix by mixing with gamma-aminopropyltriethoxysilane (APS). The APS in 0.16% achieved a superior stability of CNFs in the aqueous solution. The APS (0.16%)-modified CNFs were distributed uniformly in the waterborne acrylic coating. The prepared coatings retain high light transmittance around 90%, improve Young’s modulus by 500% and hardness by two levels, and reduce abrasion loss by 35% as compared with those of neat coating [82]. CNC was modified using alkyl quaternary ammonium bromides or acryloyl chloride to form CNC derivatives, dispersed well in acrylic wood coatings. The coatings including various CNC derivatives were applied for surface protection on sugar maple wood, a much-appreciated material as indoor timber or wooden furniture. The modified CNCs confer a higher scratch resistance, with an improvement from 24% to 38% for coatings containing CNC derivatives over those with unmodified CNC [83]. The mechanical strength of UV-curable 120

Oil CNF-AC CNF-PC CNF-DDSA CNF-AA

100 Contact angle (°)

340

(a)

80 60 40 20

(b)

0 0

20

40 60 80 100 120 140 160 Rotations

® Abraser

Figure 10.24 Wear resistance of oiled wood surfaces containing 1 wt% CNF. Taber (a) and water contact angle in the abraded area (b).

35

35

30

30

Tensile strength (MPa)

Tensile strength (MPa)

References

25 20 15 10 5 0

(a)

25 20 15 10 5 0

0% CNC 1% CNC 3% CNC CNC content (%)

0% CNC (b)

1% CNC 3% CNC CNC content (%)

Figure 10.25 MOE and tensile strength for pure coating (a) and nanocomposite coatings (b). Source: Reprinted with permission from Kaboorani et al. [84]. Copyright 2017, Elsevier.

acrylic coating can be improved through the addition of CNC by mixing with the cationic surfactant, hexadecyltrimethylammonium bromide. Tensile strength and modulus of elasticity (MOE) of coating films were affected positively by the addition of CNC. Hardness was found to increase as CNC loading increased in the coatings. Mass loss due to abrasion resistance tests was reduced by the addition of CNC [84, 85]. The coating MOE increased following CNC addition. The extent of the improvement in MOE was proportional to CNC loading (Figure 10.25b). The same trend was also observed for tensile strength (Figure 10.25a).

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55 Mueller, A., Ni, Z. et al. (2013). The biopolymer bacterial nanocellulose

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as drug delivery system: investigation of drug loading and release using the model protein albumin. Journal of Pharmaceutical Sciences 102 (2): 579–592. Silva, N.H.C.S., Rodrigues, A.F. et al. (2014). Bacterial cellulose membranes as transdermal delivery systems for diclofenac: In vitro dissolution and permeation studies. Carbohydrate Polymers 106: 264–269. Svagan, A.J., Benjamins, J. et al. (2016). Solid cellulose nanofiber based foams – towards facile design of sustained drug delivery systems. Journal of Controlled Release 244 (A): 74–82. Toivonen, M.S., Kurki-Suonio, S., Schacher, F.H. et al. (2015). Water-resistant, transparent hybrid nanopaper by physical cross-linking with chitosan. Biomacromolecules 16 (3): 1062–1071. Spence, K.L., Venditti, R.A., Rojas, O.J. et al. (2011). Water vapor barrier properties of coated and filled microfibrillated cellulose composite films. BioResources 6 (4): 4370–4388. Oun, A.A. and Rhim, J. (2015). Effect of post-treatments and concentration of cotton linter cellulose nanocrystals on the properties of agar-based nanocomposite films. Carbohydrate Polymers 134: 20–29. Ferrer, A., Salas, C., and Rojas, O.J. (2015). Dewatering of MNFC containing microfibrils and microparticles from soybean hulls: mechanical and transport properties of hybrid films. Cellulose 22 (6): 3919–3928. Ferrer, A., Salas, C., and Rojas, O.J. (2016). Physical, thermal, chemical and rheological characterization of cellulosic microfibrils and microparticles produced from soybean hulls. Industrial Crops and Products 84: 337–343. Savadekar, N.R., Karande, V.S., Vigneshwaran, N. et al. (2012). Preparation of nano cellulose fibers and its application in kappa-carrageenan based film. International Journal of Biological Macromolecules 51 (5): 1008–1013. Spence, K.L., Venditti, R.A., Habibi, Y. et al. (2010). The effect of chemical composition on microfibrillar cellulose films from wood pulps: mechanical processing and physical properties. Bioresource Technology 101 (15): 5961–5968. Spence, K.L., Venditti, R.A., Rojas, O.J. et al. (2010). The effect of chemical composition on microfibrillar cellulose films from wood pulps: water interactions and physical properties for packaging applications. Cellulose 17 (4): 835–848. Olszewska, A.M., Kontturi, E., Laine, J., and Österberg, M. (2013). All-cellulose multilayers: long nanofibrils assembled with short nanocrystals. Cellulose 20 (4): 1777–1789. Prakobna, K., Berthold, F., Medina, L. et al. (2016). Mechanical performance and architecture of biocomposite honeycombs and foams from core-shell holocellulose nanofibers. Composites Part A: Applied Science and Manufacturing 88: 116–122. Ferrer, A., Pal, L., and Hubbe, M. (2017). Nanocellulose in packaging: advances in barrier layer technologies. Industrial Crops and Products 95: 574–582.

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69 Bharimalla, A.K., Deshmukh, S.P., Vigneshwaran, N. et al. (2017).

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Nanocellulose-polymer composites for applications in food packaging: current status, future prospects and challenges. Polymer-Plastics Technology and Engineering 56 (8): 805–823. Abdollahi, M., Alboofetileh, M., Behrooz, R. et al. (2013). Reducing water sensitivity of alginate bio-nanocomposite film using cellulose nanoparticles. International Journal of Biological Macromolecules 54: 166–173. Aulin, C., Salazar-Alvarez, G., and Lindstrom, T. (2012). High strength, flexible and transparent nanofibrillated cellulose–nanoclay biohybrid films with tunable oxygen and water vapor permeability. Nanoscale 4: 6622–6628. Boumail, A., Salmieri, S., Klimas, E. et al. (2013). Physico-chemical properties of antimicrobial film based on polycaprolactone and nanocellulose and their capacity to inhibit salmonella typhimurium on vegetables. Journal of Science & Technology for Forest Products and Processes 3 (1): 45–49. Boumail, A., Salmieri, S., Klimas, E. et al. (2013). Characterization of trilayer antimicrobial diffusion films (ADFs) based on methylcellulose-polycaprolactone composites. Journal of Agricultural and Food Chemistry 61 (4): 811–821. El-Samahy, M.A., Mohamed, S.A.A. et al. (2017). Synthesis of hybrid paper sheets with enhanced air barrier and antimicrobial properties for food packaging. Carbohydrate Polymers 168: 212–219. Saini, S., Sillard, C. et al. (2016). Nisin anchored cellulose nanofibers for long term antimicrobial active food packaging. RSC Advances 6 (15): 12437–12445. Khan, A., Gallah, H. et al. (2016). Genipin cross-linked antimicrobial nanocomposite films and gamma irradiation to prevent the surface growth of bacteria in fresh meats. Innovative Food Science & Emerging Technologies 35: 96–102. Bansal, M., Chauhan, G.S. et al. (2016). Extraction and functionalization of bagasse cellulose nanofibres to Schiff-base based antimicrobial membranes. International Journal of Biological Macromolecules 91: 887–894. Sundaram, J., Pant, J. et al. (2016). Antimicrobial and physicochemical characterization of biodegradable, nitric oxide-releasing nanocellulose-chitosan packaging membranes. Journal of Agricultural and Food Chemistry 64 (25): 5260–5266. Dehnad, D., Mirzaei, H. et al. (2014). Thermal and antimicrobial properties of chitosan-nanocellulose films for extending shelf life of ground meat. Carbohydrate Polymers 109: 148–154. Veigel, S., Lems, E.-M., Grüll, G. et al. (2017). Simple green route to performance improvement of fully bio-based linseed oil coating using nanofibrillated cellulose. Polymers 9: 425. Veigel, S., Gruell, G., Pinkl, S. et al. (2014). Improving the mechanical resistance of waterborne wood coatings by adding cellulose nanofibres. Reactive & Functional Polymers 85 (SI): 214–220. Tan, Y., Liu, Y., Chen, W. et al. (2016). Homogeneous dispersion of cellulose nanofibers in waterborne acrylic coatings with improved properties and

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11 Strategies to Explore Biomedical Application of Nanocellulose Yanjie Zhang 1 , Peter R. Chang 2 , Xiaozhou Ma 3 , Ning Lin 1 , and Jin Huang 1,3 1 Wuhan University of Technology, College of Chemistry, Chemical Engineering and Life Sciences, Luoshi Road 122, Wuhan 430070, China 2 Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, S7N 0X2, Canada 3 Southwest University, Chongqing Key Laboratory of Soft-Matter Material Chemistry and Function Manufacturing, School of Chemistry and Chemical Engineering, Tiansheng Road 2, Chongqing 400715, China

11.1 Introduction Nanocellulose is a kind of unique and promising material extracted from native cellulose, which has gained much attention for its remarkable physical properties and special surface chemistry [1]. Cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial cellulose (BC) are three main categories of the nanocelluloses. The outstanding properties and characteristics of nanocelluloses lead to their potential for wide applications in biomedical materials; for example, biocompatibility, which is the ability of a foreign material implanted in the body to exist in harmony with tissue without causing deleterious changes, is an essential requirement for biomedical materials [2]. The common hydrophilic and polar surface of nanocelluloses however may limit their applications in some aspects, and many works have been published regarding modification of their surface properties to accommodate various environments. Herein, the biomedical applications of nanocelluloses are reviewed including drug delivery systems, tissue engineering materials, and other nanocellulose-based biomaterials. The toxicity of nanocellulose and the interactions of nanocellulose with biomacromolecules or cells are also discussed.

11.2 Research on Biological Toxicity of Nanocellulose Many studies have reported that nanocelluloses have low or no toxicity (comparable to that of table salt) when used in biomedical materials; however, the toxicology and safety concerns of these natural nanomaterials should be further emphasized. Nanotoxicology research has built a comprehensive assessment system for metallic nanoparticles (Au, Ag, quantum dots (QDs), Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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11 Strategies to Explore Biomedical Application of Nanocellulose

etc.) and carbon nanotubes; however, the toxicology study of nanocellulose and nanocellulose-based biocomposites is still at a preliminary stage (mainly cytotoxicity) [1]. Table 11.1 summarizes recent reports on toxicology experiments and conclusions for nanocellulose; most studies have shown that nanocelluloses have low and even non-toxicity; however, the inhalation of nanocellulose (especially CNC) may induce pulmonary inflammation due to easy self-aggregation and non-degradation of nanocellulose in the body. Kovacs et al. initially studied the inherent ecotoxicology of CNCs with aquatic organisms (i.e. different species of fish) [3]. Rainbow trout hepatocytes were selected as the model cells, and a toxicity monitoring program, as well as an in-depth toxicity assessment component, was included in the toxicity testing strategy. Ecotoxicological characterization showed that CNCs had a low toxicity potential and environmental risk, and showed no harm to aquatic organisms at concentrations that would occur in receiving waters. In another report, the cytotoxicity of CNCs against nine different cell lines was determined both by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assay and lactate dehydrogenase (LDH) assay. No cytotoxic effects from CNCs against any of these cell lines in the concentration range and exposure time studied (0–50 μg/ml and 48 hours) were reported [7]. However, recently it was reported that CNCs may induce some slight dose-dependent cytotoxic and inflammatory effects on human lung cells, associated with exposure to and inhalation of high concentrations of CNC powders [4]. There were no inflammatory or cytotoxic effects associated with CNFs on mouse and human macrophages, and only low acute environmental toxicity (assessed using a kinetic test with luminescent bacteria) has been reported [8]. The VTT Technical Research Centre in Finland proposed an evaluation report for the systemic study of CNF in vitro cytotoxicity, genotoxicity, immunotoxicity, and neurotoxicity, together with pharyngeal aspiration studies on mice. These results revealed low cytotoxicity and no DNA or chromosome damage from CNF, but pulmonary inflammation, possibly induced by the particulate/bacteria from CNF, was noted in the mice [10, 11]. Pereira et al. evaluated the in vitro cytotoxicity and the effects on gene expression of CNFs on fibroblasts cells. It was reported that low concentrations of CNF (100 μg/ml) had no obvious toxicity, whereas high concentrations (2000 and 5000 μg/ml) caused a sharp decrease in cell viability and affected the expression of stress- and apoptosis-associated molecular markers [13]. Alexandrescu et al. compared the cytotoxicity of pure CNFs and CNFs that had been surface modified with the cross-linking agent polyethyleneimine (PEI) or the surfactant cetyl-trimethylammonium bromide (CTAB) on fibroblast cells. Compared to the pure CNFs that showed no acute toxicity, both modified CNF samples caused a significant reduction in cell viability and proliferation [12]. Interestingly, in another recent study, cationic-modified CNF (trimethylammonium-CNF) was reported to have better cytocompatibility than unmodified and anionic-modified CNF (carboxymethylated CNF) [14]. Bacterial cellulose is commonly regarded as one of the most biocompatible nanocelluloses. As shown in Table 11.1, there was no cytotoxicity detected for

Table 11.1 Toxicological evaluations of nanocellulose. Type

Toxicity and allergenicity

Environmental risk

Proof methods

References

CNC

LC50 > 1 g/l

Low

[3]

Can lead to cellular damage via the release of cytosolic enzyme lactate dehydrogenase (concentration >0.005 mg/ml)

—

Animal test used: Daphnia, Rainbow trout, Ceriodaphnia (i) Acute lethal test (ii) Multi-trophic assays (iii) In vitro rainbow trout hepatocyte assay Respiratory toxicity of aerosolized CNC on the human airway with a co-culture of human monocyte-derived macrophages, dendritic cells, and a bronchial epithelial cell line (i) In vitro RNA inhibition test (ii) In vitro Ames test (iii) In vivo nematode test (iv) In vitro the highest tolerated dose test

No sublethal effects No genotoxicity No systemic effects; No cytotoxic effects Low cytotoxicity (concentration between 0.01 and 0.20 wt%)

—

No cytotoxic effects (0–50 μg/ml, 48 h)

CNF

No diminishing effect on cell viability (0.3 g/ml) No evidence of inflammatory effects or cytotoxicity (0.3 g/ml)

None

[4]

[5]

Cell line used: L929 fibroblast cells (i) MTT assay

[6]

Cell line used: HBMEC, bEnd.3, RAW 264.7, MCF-10A, MDA-MB-468, KB, PC-3 and C6 cell lines (i) MTT assay

[7]

Cell lines used: human monocyte and mouse macrophages Microorganisms used: D. magna and Vibrio fischeri (i) MTT assay (ii) Inflammation-related cytokines (iii) Kinetic luminescent bacteria test for acute environmental toxicity

[8]

(Continued)

Table 11.1 (Continued) Type

Toxicity and allergenicity

Environmental risk

Proof methods

References

No significant cytotoxicity (9.5–950 μg/cm2 for 4, 24, and 48 h)

—

Cell line used: human bronchial epithelial BEAS 2B cells Propidium iodine staining and luminometric assay

[9]

In vitro genotoxicity with enzyme comet assay

No significant DNA damage (950 μg/cm2 for 24 h) —

Cell line used: human macrophages, human bronchial epithelial BEAS 2B cells Microorganisms used: Caenorhabditis elegans Animal used: mice (i) Neurotoxicity and systemic effects with a nematode model (ii) In vitro pharyngeal aspiration study for pulmonary immunotoxicity and genotoxicity with mice

[10, 11]

No cytotoxicity for pure CNF Cytotoxicity for modified-CNF (with PEI or CTAB surface modification)

—

Cell line used: 3T3 fibroblast cells (i) Neutral red test (ii) XTT test (iii) BrdU test

[12]

Low cytotoxicity at low concentration (0.02–100 μg/ml) Cytotoxic at high concentration (2000–5000 μg/ml) and may induce cell stress and apoptosis

—

Cell line used: bovine fibroblast cells (i) Flow cytometric cytotoxicity assay

[13]

Low or no cytotoxicity (0.3 mg/ml) No DNA and chromosome damage (0.25 mg/cm2 ) Pulmonary inflammation (20, 40, 80, and 200 μg/ml)

BC

Non-cytotoxic effect of both CNF and EPTMAC-modified CNF

—

Cell line used: human dermal fibroblasts (hDF) (i) Trypan blue staining (ii) Cytocompatibility test was carried out in compliance with the ISO-10993-5 procedure

[14]

Non-cytotoxic effect

—

Cell line used: osteoblast cells and L929 fibroblast cells (i) MTT assay

[15]

Nontoxic effect in vitro and in vivo

—

Cell line used: human umbilical vein endothelial cells Animal test used: C57/Bl6 male mouse (i) MTT assay (ii) Inject PBS dispersed BC suspension for 7 days, and analyzing the serum concentrations of albumin, total cholesterol, aspartate aminotransferase, alanine transaminase, creatinine, and triglyceride.

[16]

Non-genetic and cell toxicity and non-immunogenicity (50 μg/kg, 24 h)

—

Cell line used: human umbilical vein endothelial cells Animal test used: BALB/c male mice (i) MTT assay (ii) RNA isolation and RT-PCR (iii) Enzyme-linked immunosorbent assay (iv) Western blot analysis (v) Apoptosis assay (vi) Intraperitoneal injection for 24 h

[17]

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11 Strategies to Explore Biomedical Application of Nanocellulose

BC in the evaluation on osteoblasts and endothelial cells, and in mouse feeding experiments [15–17]. Although studies conducted on nanocellulose so far have reported the absence of serious environmental and biological concerns, research and systematic assessment of the ecotoxicology of nanocellulose still needs deeper investigation, especially aimed at the effects and mechanisms of nanoparticle aggregation in the body and at long-term in vivo toxicity evaluation of nanocellulose. Moreover, toxic effects induced by the incorporation of nanocellulose are another important issue and indicator of the ecotoxicology of nanocellulose-based materials that needs to be studied. Despite there being no significant cytotoxicity of nanocellulose-based materials (generally hydrogels) in many studies [18–21], there have been reports of negative effects on biocompatibility for nanocellulose-based composites [22]. Recently, a lucifer yellow derivative was labeled onto modified CNFs through an amidation reaction, and the fluorescent CNF was proved to be a viable biomarker without any toxic effect in the specimen of Daphnia magna [23]. Live juvenile daphnids were exposed to the luminescent CNF (1 g/l) for 3 hours, and no dead species were observed during this period. As shown in the Figure 11.1, the bright field images revealed the presence of green algae in the alimentary canal tissues, while the fluorescent images revealed the presence of the luminescent CNF throughout the alimentary canal (Figure 11.1, top). No fluorescence signal was detected in the control specimen (Figure 11.1, bottom). Thus, the D. magna ingested the luminescent CNFs, which then dissipated within the algae in the guts of the daphnids, demonstrating the nontoxicity of our material for the viability of the species.

0.2 cm

0.2 cm

0.2 cm

0.2 cm

Figure 11.1 Bright field (left) and fluorescence (right) images of Daphnia magna in (bottom) control experiment and (top) exposed to luminescent CNFs. Source: Navarro et al. 2016 [23]. Reproduced with permission of ACS.

11.3 Application of Nanocellulose for Immobilization and Recognition

11.3 Application of Nanocellulose for Immobilization and Recognition of Biological Macromolecules The study of enzymes and proteins has attracted much scientific research, including the research into carriers that can be used to immobilize or identify the enzyme or protein. An ideal carrier material for enzyme or protein immobilization should be biocompatible without compromising the protein structure and biological activity. Furthermore, this carrier material should be easily processed to enhance enzyme and protein loading and activity, as well as stability in both operation and storage. As a material that is nontoxic, noncarcinogenic, biocompatible, and in no way injurious in the biological environment, nanocellulose meets the rigid medical requirements of a suitable carrier for the immobilization of enzyme or protein. Available hydroxyl groups and negative charges (CNCs and CNFs) on the surface of nanocellulose provide the possibility of enzyme or protein immobilization on the basis of chemical conjunction and electrostatic adsorption. Table 11.2 summarizes recent studies on surface immobilization of enzymes and proteins on nanocellulose on the basis of different strategies (chemical binding or physical adsorption). Covalent immobilization of enzymes or proteins on nanocellulose can provide significantly higher loading and excellent stability, but the effect obtained is always treated with complicated chemistry procedure. A physical approach is simple, cheap, and allows better preservation of the original structure, but has limited loading and efficiency of immobilization. The availability of hydroxyl groups, negative surface charges, excellent biocompatibility, high crystallinity, and the mechanical tensile strength of CNCs enable enzyme or protein immobilization based on chemical conjugation and electrostatic adsorption. There has been much research on this topic; for example, Edwards et al. reported on a colorimetric approach for the detection of human neutrophil elastase (HNE) using peptide-conjugated CNCs [29]. In order to enhance the immobilization of enzyme or protein on CNFs, some studies attempted to modify the surface of CNFs before entrapping enzyme or protein molecules using methods such as oxidation and activation pretreatments [39] or surface polymeric grafting [40]. Based on a similar strategy of immobilization, labeled DNA or enzyme was immobilized onto CNCs as a bioprobe, and used for the identification or recognition of target DNA sequences and enzyme molecules, or as a platform for immunoassays and diagnostics [48, 49]. Magnetic nanoparticles (MNPs) have been used in enzyme and protein support in biotechnological and biomedical fields to facilitate substrate and product recovery [50]. There are several reports on the synthesis of magnetic core–shell nanoparticles with a magnetite (Fe3 O4 ) core and a metal or metal oxide shell. In particular, Au nanoparticles (AuNPs) are promising materials for protecting Fe3 O4 nanoparticles because of their simple reductive preparation, reliable chemical stability, biocompatibility, and versatility in surface modification [51]. The use of a nanocomposite consisting of magnetite nanoparticles (Fe3 O4 NPs) and AuNPs embedded on CNCs as a magnetic support for the covalent conjugation of papain and facilitated recovery of this immobilized enzyme were reported [30].

355

Table 11.2 Surface immobilization of enzyme or protein on nanocellulose.

Strategy

Chemical conjunction

Type of nanocellulose

CNC

Processing methods

Enzyme or protein model

Potential application

References

(i) Esterified by Fmoc-glycine to decorate amino groups (ii) Amidated by enzyme

Chemical conjugation with lysozyme

Biofilm-resistant materials Bioremediation etc.

[24]

(i) Carbamation to make amide groups or cyclic imidocarbonate to make hydrazone by the activation of CN-Br (ii) Coupling with protein

Chemical conjugation with peroxidase

Elimination of peroxide

[25]

(i) Surface-deposited gold nanoparticles and thioctic acid form carboxyl groups (ii) Carbodiimide coupling with enzyme

Chemical conjugation with alcohol oxidase or CGTase

Studies of enzymes Multifunctional nanomaterial

[26]

(i) Polyetherimide surface cationization (ii) Deposited Au nanoparticles via electrostatic interaction (iii) Decorate gold nanoparticles by 3-mercaptopropionic acid or 11-mercaptoundecanoic acid (iv) Amidation with enzyme

Physical adsorption with Au nanoparticles Chemical conjugation with glucose oxidase

Studies of enzymes

[27]

(i) Surface TEMPO-oxidation to form carboxyl groups (ii) Amidation with peptides

Chemical conjugation with tryptophan-based peptides

Fast production of enzyme-functional CNCs

[28]

(i) Esterification with glycine to decorate amino groups (ii) Amidation with elastase (i) Embedment of Fe3 O4 nanoparticles and Au nanoparticles on the surface (ii) Surface activation with thioctic acid to decorate carboxyl groups (iii) Amidation with enzyme

Chemical conjugation with human neutrophil elastase

Chronic wounds healing assistance

[29]

Chemical conjugation with Fe3 O4 nanoparticles

The development of recyclable enzyme-functional CNCs

[30]

Multifunctional nanomaterial

Physical adsorption with Au nanoparticles Chemical conjugation with papain enzyme (i) Azide modification on CNC reducing ends (ii) Acetylene modification on β-casein (iii) Click reaction for the coupling of protein

Chemical conjugation with β-casein

(i) Amine/epoxy/carboxylic acid modification (ii) Coupling with protein

Chemical conjugation with alkaline phosphatase and anti-hydrocortisone antibody

Printing or coating technologies

[32]

(i) Surface TEMPO-oxidation to form carboxyl groups (ii) Amidation with protein Cross-linked enzyme aggregates

Chemical conjugation with avidins

Immunological diagnosis

[33]

Lipase

—

[34]

Adsorption of peptide via electrostatic interaction

Physical adsorption with heptapeptide

Development of artificially assembled bio-nanocomposites

[35]

(i) Copolymerization of diblock proteins (ii) Adsorption of proteins

Physical adsorption with diblock protein (elastin-co-cartilage oligomeric matrix)

Development of artificially assembled bio-nanocomposites

[36]

Physical adsorption of pancreatic serine protease trypsin

Elimination of proteins

[38]

Development of novel functional self-assembled nanomaterials

[31]

Drug delivery CNF

Physical adsorption

CNC

CNF

Adsorption of glucose oxidase (GOx) (i) Copolymerization of glycidylmethacrylate and ethyleneglycoldimethacrylate to decorate epoxyl groups (ii) Modification of poly(acrylic acid) to decorate carboxyl groups (iii) Adsorption of proteins

[37]

(Continued)

Table 11.2 (Continued)

Strategy

Type of nanocellulose

BC

Processing methods

Enzyme or protein model

Potential application

References

(i) Oxidation to decorate carboxyl groups and activation by NHS (ii) Amidation with antibodies (iii) Adsorption of protein

Chemical conjugation with antihuman IgG antibody Physical adsorption with human IgG

Immunological diagnosis

[39]

(i) Surface grafting from poly(AMAco-HEMA) (ii) Adsorption of modified peptide (acetylated-HWRGWVA) via electrostatic interaction (iii) Coupling with protein Adsorption of enzyme

Physical adsorption with human immunoglobulin G (IgG)

Immunological diagnosis

[40]

Physical adsorption with laccase

Textile effluent decoloration

[41]

Adsorption of enzyme

Physical adsorption with lipase

Elimination of oil pollution

[42]

(i) Surface phosphorylation (ii) Adsorption of protein

Physical adsorption with hemoglobin; myoglobin; albumin; lysozyme

Immunological diagnosis

[43]

(i) Surface quaternary ammonium (ii) Adsorption of protein

Physical adsorption with hemoglobin

Immunological diagnosis

[44]

(i) Preactivation by glutamate (ii) Adsorption of enzyme

Physical adsorption with glutamate decarboxylase

—

[45]

(i) Nucleophilic substitution by Green 5 dye (ii) Adsorption of enzyme

Physical adsorption with Urease

Separation and purification of biomolecules

[46]

Cross-linked by glutaraldehyde

Physical adsorption with urease

Separation and purification of biomolecules

[47]

11.3 Application of Nanocellulose for Immobilization and Recognition

S S

S S

O

(i)

O

O

O

O

S S

S S

S S

S S

S S

S S

S S

S S

O

O

O

O

(iii)

O

O

CNC/Fe3O4NPs/AuNPs CNC/Fe3O4NPs/AuNPs/papain

CNC/Fe3O4NPs

CNC

100 Residual activity (100%)

100 Residual activity (100%)

O

(ii)

(A)

CO O H

COOH

d

CO NH

c

b

CONH

a

80 60 40 20

80 60 40 20 0

0 0

(B)

5

10

15

20

25

Storage time (d)

30

35

0

(C)

1

2

3

4

5

6

Storage time (h)

Figure 11.2 (A) Process for synthesis of CNC/Fe3 O4 NP/AuNP/papain and a photograph of (a) pristine CNC; (b) CNC/Fe3 O4 NPs; (c) CNC/Fe3 O4 NPs/AuNPs; and (d) CNC/Fe3 O4 NPs/AuNPs attracted to the side wall with a permanent magnet. (B) The effect of storage time on the activity of immobilized (black) and free (light gray) papain. CNC/Fe3 O4 NPs/AuNPs/papain was prepared based on 9.6 mg/ml initial papain loaded on 50 mg/ml CNC/Fe3 O4 NPs /AuNPs. (C) The effect of storage temperature on the activity of free (◼) and immobilized (⚫) papain at 70 ∘ C. The total duration of the experiment was five hours. Source: Mahmoud et al. 2013 [30]. Reproduced with permission of ACS.

The study, which describes the applicability of the embedded CNC/Fe3 O4 NPs for immobilization and separation of papain from a solution, found the optimal enzyme loading level to be 186 mg protein/g CNC/Fe3 O4 NPs/AuNPs, significantly higher than the value reported in the literature. The immobilized enzyme retained 95% of its initial activity after 35 days of storage at 4 ∘ C, compared to 41% for its free form counterpart (Figure 11.2B). The immobilized and free enzyme still retained 78% and 19% activity of the original value after three hours, respectively (Figure 11.2C). In addition, the enzyme is usually immobilized by bacterial cellulose through adsorption. Pesaran et al. [47] used glutaraldehyde cross-linked with urease to entrap urease in bacterial cellulose nanofibers. The immobilized enzyme showed improved storage stability (20 days) and retained almost 81% and 68% of the initial activity after 15 and 20 cycles of reuse, respectively. Recently, methyl cellulose (MC) nanofibrous mats were cross-linked with glutaraldehyde to increase the stability of the mats in water. Lipase was directly immobilized

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onto the mats, 34.82 μg lipase/mg support, without a further activation step. Furthermore, enzyme reusability was also enhanced as more than 90% of the initial activity was retained after reusing seven times [34]. Immobilization of DNA on a cellulose substrate by irradiation with ultraviolet light was a novel approach [52], where approximately 90% of DNA (calf thymus DNA) was retained on cellulose after irradiation. The immobilization of double-stranded DNA onto nonwoven cellulose fabric by the thymin-thymin crosslinking under UV irradiation and further utilization of DNA-immobilized cloth for accumulating endocrine disrupters, harmful DNA intercalating pollutants, and heavy metal ions were examined respectively [53–55]. Another simple and efficient method for coupling DNA to cellulose was described by Moss et al. [56]. In order to activate cellulose and subsequently bind calf thymus DNA onto epoxy-activated cellulose, 1,4-butanediol diglycidyl ether, a bifunctional oxirane, was utilized. Two separate populations of complementary single-stranded DNA were grafted to carboxylated CNCs and the populations were then combined to hybridize the DNA and bond the CNCs [57]. Recently, Demirci et al. [58] reported that electrospun cellulose acetate (CA) nanofibers that were surface modified by cationic polymer brushes (poly[(ar-vinylbenzyl)trimethylammonium chloride]), ([poly(VBTAC)]) could be used for DNA adsorption. The membrane of [poly(VBTAC)]-grafted CA showed affinity to DNA, and the adsorption capacity was determined as 23.51 μg/mg from the Langmuir isotherm. In conclusion, cationic polymer brush-modified CA nanofibers can be suitable as membrane materials for DNA filtration, purification or separation.

11.4 Application of Nanocellulose for Cell Imaging Bioinspired composite systems with molecular-scale interactions can be tailor made, as we have discussed earlier, with specific functionalities, coupled with nontoxicity, and cellulose can be used in cell imaging. In such applications, nanocellulose is distinguished for its inertness and biocompatibility in humans. The labeling of nanocellulose with fluorescent probes is of great interest in biomarkers and sensor applications [29, 59]. With standard fluorescence spectroscopy and microscopy, Mahmoud et al. [60] monitored the in situ cellular uptake of fluorescently labeled CNCs and evaluated its cytotoxicity. Ulrica Edlund and coworkers [23] used CNFs as the matrix, and subsequent fluorescence correlation spectroscopy (FCS) confirmed the successful modification of CNFs. Fluorescent CNFs proved to be viable biomarkers and can be applied for fluorescence-based optical detection of CNF uptake and distribution in organisms such as crustaceans. Carboxymethyl cellulose/ZnCdS fluorescent quantum dot nanoconjugates could be used in cancer cell bioimaging [61]. The application of sodium carboxymethyl cellulose (CMCel) as a multifunctional biocompatible material for the direct synthesis of fluorescent alloyed-ZnCdS QD nanoconjugates via aqueous process at room temperature has been reported. These nanoconjugates have a high cytocompatibility and are luminescent for the detection of human osteosarcoma cancer cells. Thus, these fluorescent bioconjugates offer promising perspectives as nanoplatform for cancer

11.5 Application of Nanocellulose for Cell Scaffolds

diagnosis. Both cellulose and poly(p-dioxanone) have been widely applied in biomedicine due to their good mechanical properties and high biocompatibility and bioresorbability [62, 63]. It was reported that novel microcrystalline cellulose-graft-poly(p-dioxanone) (MCC-g-PPDO) nanomicelles were used as carriers for the liposoluble fluorescent conjugated polymers (FCPs) to develop water-phase bioprobes with high brightness, which can be applied in tumor cell imaging. Meanwhile, MCC-g-PPDO was also synthesized by a homogeneous ring-opening polymerization (ROP) of PDO with cellulose. Mingqian Tan and coworkers [64] developed a kind of cellulose-multicolor carbon nanoparticles as fluorescent probes for cell imaging. Besides, Erik M. Shapiro and coworkers [65] designed a kind of magnetic poly(lactide-co-glycolide) and CNC for magnetic resonance imaging (MRI)-based cell tracking. Laura Colombo et al. [66] studied the organ distribution and bone tropism of CNCs with a fluorescent dye in living mice. The results showed no specific signal related to CNCs after 6 hours of incubation (data not showed), while a mild internalization of CNCs was detectable after 24 hours of incubation (Figure 11.3a, left column). This limited uptake could be likely due to the negative 𝜁 -potential or the elongated shape: both factors may influence the interaction with cell membrane and the endosomal vesicle formation [67, 68]. After 48 hours of incubation, CNCs progressively penetrated in cells, in a concentration-dependent manner (Figure 11.3a, middle column). The merge between the fluorescent signals and the differential interference contrast (Nomarsky), which enabled us to visualize the shape of the cells in a 3D-like manner, showed spotted red circles exclusively confined to the cytoplasm. This peculiar spotted staining strongly suggested CNCs clustering into endocytic vesicles (Figure 11.3a, right column), which proved that CNCs were able to penetrate in the cytoplasm of cancer cells without inducing material-related detrimental effects in terms of cell survival. Based on the interaction between fluorescence CNCs and cells, a longitudinal pattern of CNC biodistribution with changes of time is shown in Figure 11.3b. It revealed a CNC-related signal corresponding to the hind limbs (here the work focused on the hind legs, and the forelegs were not in the region of scanning), that is, CNCs had a peculiar and transient tropism to the limb bones that was likely related to the interaction with the Ca2+ deposits in the bone matrix. All these results make CNCs a very promising candidate for potential development in the field of theranostic, in particular toward bone diseases with particular emphasis on bone tumors, as demonstrated by the ability to internalize in cancer cells.

11.5 Application of Nanocellulose for Cell Scaffolds Because of the properties of biocompatibility and the right mechanical properties similar to natural tissue, nanocellulose-based materials can be used as special tissue bioscaffold. Diverse cell species cultured on nanocellulose-based biomaterials have been reported; among them, BC seems to be the most prevalent choice for the medium of cell culture because of its low cytotoxicity and high porosity [1]. Jian Wu et al. [69] used dialdehyde to modify BC and

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Figure 11.3 (a) Representative confocal microscopic images showing internalization of different concentrations of CNC-NPs (red signal) in HeLa cells (nuclei, blue signal) after 24 and 48 hours of incubation; (b) optical imaging scans acquired before and at 1 hour, 24 hours, and 7 days, respectively, after CNCs intravenous injection (a single dose of 120 μl at a concentration of 35 μg/ml). Source: (a) Ten et al. 2014 [67]. Reproduced with permission of ACS. (b) Petros and DeSimone 2010 [68]. Reproduced with permission of Springer Nature.

11.5 Application of Nanocellulose for Cell Scaffolds

obtained C2, 3-oxidized dialdehyde BC (DBC); both DBC and BC have a nanofiber network topology structure that is similar to the extracellular matrix, but the data of cultivation of epidermal cells showed that DBC membranes had a better adhesion, proliferation and proliferate effects in cultivated cells than BC membranes. Moreover, the reaction between cell-surface proteins and aldehydes of DBC is conducive to the adhesion and proliferation of cells within the DBC networks. Lin Jin et al. [70] used a bacterial (Gluconacetobacter intermedius BC 41) reduction method to produce a kind of bacterial cellulose nanofiber/graphene oxide (BC-RGO) film that maintained excellent hydrophilicity as also improved the electrical properties and biocompatibility. Human marrow mesenchymal stem cells (hMSCs) were cultured well on the film compared to other films. hMSCs cultured on the BC-RGO film displayed much better attachment and retention (see Figure 11.4) compared to that on Nuclei

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RGO films, and were even better than that on the tissue culture plates (TCPs). The morphology of hMSCs on BC-RGO film (Figure 11.4B(c)) showed that the cells adhered tightly on the surface of bacterial-functionalized films and formed integrated cell–film constructs, so it was difficult to visually distinguish the cell morphology between single hMSCs and the BC-RGO film, which reveals that BC nanofibers of the BC-RGO films enhanced hMSCs adhesion and spreading, which also implied a positive interaction among hMSCs and with the surface of the BC-RGO films. Another work presented a cellular building unit made from microstrand-shaped BC covered with mammalian cells. By folding or reeling the building unit, the multiple shapes of millimeter-scale cell constructs (coiled and ball-of-yarn-shaped structures) were investigated. Histological analysis of the cell constructs indicated that the BC microstrand served as a pathway of nutrition and oxygen to feed the cells in the central region [71]. Recently, the fabrication of a nanofibrillar patch by using BC and its application as a wound-healing platform for traumatic tympanic membrane (ear drum) perforation was reported. The nanostructured surface, biocompatibility, transparency, and appropriate mechanical properties were expected to meet the requirements of an ideal wound-healing platform for tympanic membrane perforation. The tympanic membrane cells were found to adhere well and proliferate on the BC nanofibrillar patch, and in vitro the growth and migration of cells were promoted under the guidance of the BC patch. Specific effects of BC patch materials on the regeneration and healing of tympanic membrane tissues were investigated through in vivo animal study (12 weeks Sprague-Dawley rats). It was reported that the presence of BC patch materials significantly increased the tympanic membrane healing rate as well as recovered the function of tympanic membrane better than spontaneous healing [72]. BC was reported to be developed as biomaterial for the reconstruction of damaged peripheral nerves via cellulosic guidance channels. In vivo experiments were conducted on the femoral nerve of Wistar rats for three months. Evaluation of results from histological analysis and postoperative observation of motor recovery showed that BC neurotubes can effectively prevent the formation of neuromas, while allowing the accumulation of neurotrophic factors inside, and facilitating the process of nerve regeneration [73]. In recent years, three-dimensional (3D) cell cultures have attracted many researchers. It was considered an emerging tool in cell biology, regenerative medicine, cell therapy, chemical testing and drug discovery, and so on. However, it must meet the following requirements for better applications. Firstly, the ideal materials of 3D cell cultures should have excellent biocompatibility that is similar to the in vivo tissue. Secondly, they also should possess good mechanical properties that can support cell growth and maintenance with biochemical signals, and yield a framework for transfer of nutrients, waste metabolites, and intercellular chemical signaling. In essence, the scaffold should maintain the cellular functions the same as the native-state cell type. Various materials have been introduced as potential 3D cell culture scaffolds. These include protein extracts, peptide amphiphiles, and synthetic polymers. The bacterial cellulose scaffolds have already proved to be suitable for the tissue engineering of hard tissues such as bone and cartilage. Hydrogel scaffolds without human

11.5 Application of Nanocellulose for Cell Scaffolds

or animal-borne components or added bioactive components are preferred from the immunological point of view. Many researchers have proved that in hydrogels, a network of interconnected pores enables retention of high water content, and efficient transport of oxygen, nutrients, and waste products [74]. Hydrogels from both synthetic and natural sources have been used for 3D cell culture. Madhushree Bhattacharya et al. [75] used nanofibrillar cellulose (NFC) hydrogel to produce a kind of three-dimensional cell scaffolds. Their properties were evaluated using hepatocyte lines. As shown in Figure 11.5, the morphology of NFC was long and thin, similar to the spider silk mesh, which provided the possibility for 3D scaffolds (Figure 11.5a). The viscosity of the NFC network was very high at low concentrations even at rest (at low levels of shear stress) (Figure 11.5b), which was a necessary property to keep the cells in 3D environment. This kind of shear-thinning behavior of the NFC hydrogel is beneficial, because it allows mixing cells into the gel and dispensing the hydrogel cell cultures easily. Both hepatic progenitor HepaRG cells and human hepatocellular

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carcinoma HepG2 cells grew well in NFC hydrogel (Figure 11.5c). Meanwhile, CNCs modified with maleic anhydride (MAH) grafted poly(lactic acid) (PLA) and then combined with electrospun technology used in bone tissue engineering (BTE) were reported [76]. The electrospun fibrous bio-nanocomposite scaffolds of MAH grafted PLA/CNC (i.e. MPLA/CNC) showed excellent properties, such as biodegradability, cytocompatibility, useful mechanical properties, and so on. Moreover, the MPLA/CNC bio-nanocomposite scaffolds were nontoxic; this character was proved by human adult adipose-derived mesenchymal stem cells (hASCs, which were used as a cell source for bone regeneration because of its osteogenesis). The growth and proliferation of hASCs on the scaffolds were very good. As in Figure. 11.6A, when the CNC content was 5 wt%, the tensile strength of the scaffolds was the best. Figure 11.6B shows fluorescence images of hASCs cultured on PLA/CNC and MPLA/CNC nanofibrous scaffolds. Live cells (stained green) cultured on the MPLA/CNC-5 nanofibrous scaffolds were obviously more numerous than those on the PLA/CNC-5 nanofibrous scaffolds, and showed a spindle-like morphology. The results showed that MPLA/CNC could support cell proliferation, and could be potentially suitable in the BTE field. This is attributed to the formation of nanofibrous morphology in MPLA/CNC scaffolds resulting from the greatly decreased diameter of nanofibers, promoting cell–matrix interactions by providing more binding sites for cell adhesion and proliferation [77–79]. Another study reported the application of all-cellulose scaffold materials (CNC/cellulose) to culture cells, and the influence of CNC orientation in scaffolds on cell growth was investigated. It was shown that CNCs can be well dispersed in electrospun scaffolds and achieve considerable orientation along the long axis direction. Cultured cells can proliferate rapidly not only on the surface but also deep inside the scaffolds. More interestingly, the aligned nanofibers of CNC/cellulose exhibited a strong effect on directing cellular organization [80].

11.6 Application of Nanocellulose in Tissue Engineering Nanocellulose has an important application value in biological macromolecule and cell levels. Based on these studies, some researchers have promoted the research fields of nanocellulose and organism to the organization levels and explored the potential values of nanocellulose in tissue repair and tissue replacement. 11.6.1

Tissue Repairing, Regeneration, and Healing

Tissue repair and regeneration is the process of renewal, restoration, and growth that makes the function of diseased and damaged cells, organs, and tissues resilient to natural fluctuations. From small animals to humans, all species have a specific ability of tissue repair and regeneration. Unlike the effects of substitute implants, the behavior of tissue repairing and regeneration for organisms inherently originates from the individual self. Nanocellulose can provide a nontoxic

11.6 Application of Nanocellulose in Tissue Engineering

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Figure 11.6 (A) Typical tensile stress–strain curves of electrospun MPLA and MPLA/CNC fibrous scaffolds; (B) fluorescence micrographs of stained cells consisting of live (green) and dead (red) cells for PLA (a), PLA/CNC-5 (b), MPLA (c), and MPLA/CNC-5 (d) scaffolds; scale bar represents 75 μm. Source: Zhou et al. 2013 [76]. Reproduced with permission of ACS.

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and biocompatible platform to cover growth factors or cells, which will activate and accelerate the process of tissue repair and regeneration. Recently, most studies are focused on nanocellulose-based biomaterials for tissue repairing, regeneration, and healing. 11.6.1.1

Skin Tissue Repairing

Absorbing exudate during the dressing process and its removal from a wound surface after recovery are important characteristics of skin repair materials. The drawback of traditional skin tissue repair materials, e.g. gauze, is their strong permeability, which will cause the tight adhesion of repair materials on the desiccated wound surface and thus induce new trauma on removal [81]. In the studies of clinical effects of BC in skin tissue repair, clinical trials were conducted on 34 patients suffering from severe thermal burns covering 9–18% of the total body surface area, in which 22 of the patients received BC as the testing group. It was reported that the adherence of BC membrane to the wound surface was excellent in avoiding dead spaces, which indicated that the application of BC dressing in the treatment of partial thickness burns promotes a favorable environment for fast wound cleansing and rapid healing [82]. On the basis of fundamental research on the development of BC-based skin repair materials, some companies have launched several commercial products in wound healing systems. BioFill Produtos Bioetecnologicos (Curitiba, PR Brazil) developed a series of products based on BC, including Biofill and Bioprocess (used in the therapy of burns, ulcers as temporary artificial skin), and Gengiflex (applied in treatment of periodontal diseases). Another company, Xylos Corporation in the United States, has developed several medical devices using BC since 1996. The XCell family offered by Xylos Corporation has been marketed in the United States since 2003. Unlike BC dressings manufactured by Biofill , the Xcell product is claimed to have a dual function of both hydration and absorption to maintain the ideal moisture balance [83]. Recently, evaluation of the effect of the structure of bacterial cellulose on full thickness skin wound repair on a microfluidic chip was reported [84]. However, although BC is known as a good candidate for wound dressing, there is no detailed report about which side is better for skin repair nor are there reported studies based on microfluidics to evaluate the BC-based wound dressing for skin repair so far. To verify the reliability of the in vitro results, the performance of BC film in vivo was evaluated, by setting the traditional wound dressing (gauze) as the control [84]. Round full-thickness wounds with diameters of 2.0 cm were prepared on the dorsal side of Wistar rats. On each rat, the authors covered three same-sized wounds by gauze, top side of BC film, and bottom side of BC film, respectively (Figure 11.7a). They fixed the dressings to the wounds by suturing to surrounding healthy skin by using a skin stapler (Manipler AZ) designed for wound closure (Figure 11.7a). Photographs of the wound sizes on day 0, day 7, and day 14 (Figure 11.7b) were taken and the wound areas were measured by Image-Pro Plus software and plotted by Origin Pro 8.0 (Figure 11.7c). All the wound sizes were significantly reduced and none were infected during the experimental procedure (Figure 11.7b). Reduction of the area of the wound was the fastest in the BC-bottom group, and BC-top group was faster than the gauze group (Figure 11.7b,c). The wound covered by the bottom of BC film had the

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smallest wound size on day 7 (26% ± 1%) and day 14 (13% ± 1%), and the top of the BC film had a smaller wound size (day 7, 39% ± 1%, day 14, 18% ± 1%) than gauze (day 7, 61% ± 2%, day 14, 45% ± 2%) (Figure 11.7b,c). The wound healing rate revealed that the wounds treated with the bottom of BC film healed faster than that treated with the top of BC film and the gauze on day 7 and day 14, respectively (Figure 11.7d). The reason for this may be the better hygroscopicity of the BC film and its complete contact with the wound. So, the BC film was more effective than gauze in enhancing wound healing. 11.6.1.2

Bone Tissue Regeneration

Developing effective bone regeneration therapy is a long-term attractive clinical topic. Bone loss caused by trauma, neoplasia, reconstructive surgery, congenital defects, or periodontal disease is a health problem worldwide. As mentioned above, nanocellulose and its biocomposites have been proved to be promising scaffolds for the culture of various cells, including osteoblast and chondroblast, which indicates that nanocellulose-based materials have the potential for bone tissue regeneration and healing. However, studies on nanocellulose for bone tissue regeneration and healing applications are still at the fundamental stage, and only a few publications have reported the practical effects on animal experiments [1]. Some studies have been conducted for biomaterials as an alternative to grafts in BTE due to their nontoxic and biocompatible properties [85]. Collagen, mineralized by hydroxyapatite (HAp), constitutes a major part of bone matrix [86]. Guided bone regeneration (GBR) is a medical practice that prompts in vivo regrowth of bone tissue by the use of osteopromoting fillers and membranes. Being a collagen-like material, BC fibers were used as a basis for biomimetic HAp growth with the ultimate aim of fabricating filler materials for GBR [87]. BC fibers were phosphorylated with phosphoric acid and preconditioned with calcium (Ca2+ ) for nucleation of HAp. Simulated body fluid (SBF) enhanced the growth of HAp. Over a period of 14 days, BC-HAp maintained a Ca-to-P ratio as high as 1.45 ± 0.92, covering the standard of 1.67 for HAp. Higher Ca-to-P ratios were detected on the pellicle surface suggesting the deposition of HAp crystals. The study indicated the potential for formation of 3D samples and a basis for further optimization of BC-HAp scaffold for GBR. One of the biomimetic mineralization pathways of BC has been depicted in Figure 11.8 [88]. In addition, goat bone apatite was reported to be introduced in BC for the fabrication of novel bone repair biomaterials, which can stimulate bone cell proliferation and promote cell differentiation. However, no in vivo experiment was reported in this study [89]. Recently, sericin covalent immobilization onto cellulose acetate membrane for biomedical applications such as in the osseointegration field has been reported [90]. Initially, the surface of the cellulose acetate membrane was immobilized with the amino-propyl-triethoxysilane (APTS) functional group, while the protein was immobilized through glutaraldehyde that was used as a linker between APTS and sericin, and then preosteoblasts were cultivated on the membranes. Furthermore, cellular viability and the proliferation potential of MC3T3-E1 preosteoblasts grown onto membrane surfaces were monitored at 24 and 72 hours post-seeding by combining a qualitative method (cell labeling

11.6 Application of Nanocellulose in Tissue Engineering

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Figure 11.8 FE-SEM images of (a) BC, (b) BC-PVP, (c) 5-PVP-HAp-BC, and (d) 7-PVP-HAp-BC. Source: Na et al. 2011 [88]. Reproduced with permission of Elsevier.

with calcein AM and EthD-1) with a quantitative one (MTT colorimetric assay). Fluorescence micrographs of representative fields of membranes incubated with calcein AM/EthD-1 indicate that both analyzed membranes support preosteoblast adhesion and growth. 11.6.2

Tissue Replacement

Nanocellulose also could be used as a substitute/medical biomaterial because of its excellent biocompatibility, mechanical properties, nontoxicity to cells, and so on. Artificial blood vessels, soft tissue ligament, meniscus and cartilage replacements, nucleus pulposus replacement, and other medical biomaterials are all included. 11.6.2.1

Artificial Blood Vessels

One of the most common treatments to cardiovascular disease is the coronary bypass graft surgery, which is performed to supply blood to the heart tissue with a suitable blood vessel replacement. Because of its good mechanical strength (a burst pressure of up to 880 mmHg) and blood biocompatibility, it is possible to develop nanocellulose (especially for BC) as a material for artificial tubes used as potential replacement of small (6 mm) size vascular grafts. Bodin et al. first made BC tubes by fermenting Acetobacter xylinum on top of silicon tubes [91]. The increase in wall thickness of BC tubes was easily obtained by increasing the oxygen ratio during fermentation. The fermentation method used by these researchers enabled it to produce branched BC tubes

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(a)

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Figure 11.9 (a) BC tubes with different sizes and shapes for applications: BC tubes with different inner diameters: 1.5, 2.4, 3.0, 4.0, and 6.0 mm; (b) branched BC tube fermented on a branched silicone tube. Source: Bodin et al. 2007 [91]. Reproduced with permission of John Wiley & Sons.

with unlimited length and inner diameters. Such BC tubes are shown in Figure 11.9 [91]. Figure 11.9a shows BC tubes with different inner diameters while Figure 11.9b shows a branched BC tube fermented on a branched silicon tube. The biocompatible BC tubes showed excellent mechanical properties capable of being utilized as vascular grafts for large animals [91]. In addition, BC tubes can have different dimensions [92]. The small and short tubes are useful as blood vessel substitutes in microsurgery; the larger ones represent novel types of cardiovascular implants. One study reported that long BC tubes of inner diameters 85% over 16 000 cycles at 5 A/g. Fabricating additive-free electrodes with high mass loading is an ideal method to prepare SCs with high areal capacitance. BC is a suitable nanoscale template

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for conductive polymers to construct continuous networks with high electrical conductivity and mechanical flexibility. Shen and coworkers demonstrated highly conductive self-standing BC-polypyrrole nanofibers/MWCNTs hybrid electrodes for SCs without using any binders and current collectors [48]. The hybrid electrode showed a highest capacitance of 2.43 F/cm2 at a mass loading of 11.23 mg/cm2 . The SC fabricated with two hybrid electrodes presented high areal capacitance of 590 mF/cm2 and outstanding cycling ability (94.5% retention after 5000 cycles). Nanocellulose also plays a significant role in hybrid PANI electrodes for high-performance flexible SCs. Na and coworkers presented hybrid PANI/NFC/graphene nanoplatelet (GNP) electrodes for all-solid-state SCs [49]. Nanocellulose was mixed with GNP to form NFC/GNP composite substrates by vacuum filtration, and then the hybrid electrodes were prepared by in situ polymerization of aniline on the surface of NFC/GNP substrates. Nanocellulose in these hybrid electrodes can not only act as a binder to improve their mechanical properties and flexibility but also mediate their morphology and structure, which imparts them with enhanced specific capacitance, superior rate capability, and good balance between energy and power density due to the improvement of ion transportation across solid/liquid interfaces and redox degree in PANI nanorods. Hybrid electrodes with 20 wt% nanocellulose loading exhibited a maximum specific capacitance of 421.5 F/g at a current density of 1 A/g. 3D nanocellulose aerogels have been widely used in the fields of energy storage devices, sensors, tissue engineering, and oil recovery due to their high porosity and mechanical flexibility. These excellent properties enable them to serve as universal substrates for various nanomaterials. Cranston and coworkers presented a one-step method to prepare cellulose nanocrystal (CNC) aerogels with a variety of capacitive nanoparticles, such as PPy nanofibers, PPy-CNT, and MnO2 -NP, for flexible energy storage devices (Figure 12.4a). Figure 12.4b displays the sol–gel procedure used to prepare aerogels including different capacitive materials, and various hybrid aerogels are displayed in Figure 12.4c. This sol–gel processing involves in situ incorporation of functional nanomaterials into the CNC network during assembly, which avoids the problem caused by posttreatments of CNC aerogels such as impregnation and coating. Therefore, the 3D hybrid aerogels have the advantages of lightweight, porous microstructure, strong mechanical properties, shape recovery abilities, and high loading weight of active materials, leading to lightweight and flexible SCs with excellent capacitance retention at high charge/discharge rates [50]. It is well known that SCs suffer from a low energy density that restricts their use in applications where both high power and energy density are required. Designing a Faradic electrode material and selecting a suitable capacitive electrode material are the effective approaches to achieving SCs with both high power and energy density. Recent reports indicated that using interconnected carbon nanofibers from BC pellicle as an electrode material has the ability to improve the energy density of SCs. Yu and coworkers domenstrated an asymmetric SC with excellent electrochemical performances by using BC-derived carbon nanofibers coated with MnO2 and nitrogen-doped carbon nanofibers as electrode materials [51].

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Figure 12.4 (a) Functionalized CNCs and capacitive materials for 3D lightweight substrates for supercapacitors. (b) Preparation procedure for the composite aerogels. (c) Photographs of hybrid aerogels with different capacitive materials. Source: Yang et al. 2015 [50]. Reproduced with permission of John Wiley & Sons.

A 3D BC nanofiber network was first annealed at 1000 ∘ C in N2 to obtain a nanofibrous carbon network that was further treated with KMnO4 /K2 SO4 aqueous solution to produce BC@MnO2 nanocomposites acting as a positive electrode. Additionally, nitrogen-doped BC carbon nanofiber network was synthesized by hydrothermal reduction with BC pellicle and aqueous urea at 180 ∘ C, which served as a negative electrode material. The binder-free asymmetric SC exhibited a high energy density of 32.9 Wh/kg and a maximum power density of 284.6 kW/kg while possessing a superior cycling stability with a capacity retention of approximately 95.4%. A one-step, low energy consumption, scalable hydrothermal reaction between activated BC derived from pyrolysis and aqueous ammonia was utilized to prepare 3D nitrogen-doped activated nanofiber network electrodes for high-performance, flexible, and all-solid-state SCs [52]. The SCs reached a power density of 390.5 kW/kg and possessed a long cycling performance (95.9% retention after 5000 cycles). To further improve the performance of SCs with carbonized nanofiber electrodes derived from nanocellulose, Li et al. proposed a novel strategy to fabricate freestanding high-performance electrodes for SCs by carbonization of hybrid NFC/CNC film [53]. Figure 12.5a displays the preparation procedure of carbonized hybrid nanocellulose films. Firstly, CNC with large specific surface area was blended with NFC to prepare a hybrid film with hierarchical porous structure. After that, an atomic layer deposition (ALD) was utilized

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12 Application of Nanocellulose in Energy Materials and Devices

(a)

Al2O3

Cellulose

Carbon Carbon

Hybrid

ALD Carbonize

HCl e–

CNF

(b)

CNC

Carbonized hybrid nanocellulose

CNF/CNC film

(c) 150 Capacitance (F/g)

410

CNF/CNC-900

100 Reference AC 50 CNF-900 0 1

10 Current density (A/g)

Figure 12.5 (a) Schematic showing the preparation of electrodes with carbonized hybrid nanocellulose. (b) Hybrid nanocellulose film before and after carbonization. (c) Specific capacitance vs. the charge/discharge current density of the carbonized hybrid nanocellulose film. Source: Li et al. 2017 [53]. Reproduced with permission of Springer Nature.

to conformally deposit a thin layer of aluminum oxide on the surface of nanocellulose to overcome the CNC aggregation. The hybrid nanocellulose film before and after carbonization is shown in Figure 12.5b. Consequently, a carbon nanofiber film with a maximum specific surface area of 1244 m2 /g was achieved, leading to high ion-transport efficiency. The hierarchical porous carbon film presented a specific capacitance of 170 F/g and outstanding electrochemical performance at high current densities (as shown in Figure 12.5c). 12.3.3

Nanocellulose As a Mesoporous Membrane

Incorporating renewable and biodegradable nanocellulose into membrane for SCs has been received much attention in past decade. Yu and coworkers developed a scalable and facile solution-phase inversion approach to prepare a flexible and transparent mesoporous cellulose membrane [54]. The membrane exhibited uniform mesopores of ∼24.7 nm and high porosity of 71.8%. Symmetric solid-state SCs with cellulose mesoporous membrane/KOH polymer electrolyte and active carbon electrode presented exceptional electrochemical performance such as high capacitance (120.6 F/g), high rate capability,

12.4 Nanocellulose for Other Energy Devices

and excellent cyclability with 84.7% retention after 10 000 cycles at a current density of 1.0 A/g. In comparison to EDLC with commercial membranes, the cellulose-mesoporous-membrane-based EDLC has a higher energy density.

12.4 Nanocellulose for Other Energy Devices In addition to energy storage devices such as SCs and LIBs, nanocellulose has been used in energy conversion or harvesting devices such as fuel cells, solar cells, and generators. In the following, we will discuss the roles played by nanocellulose in these emerging fields. 12.4.1

Fuel Cells

Fuel cells have a high efficiency (80%) of converting chemical energy into electricity and are expected to be the most promising environmentally friendly power sources for transportation and stationary applications. Proton membranes with high ion conductivity and low methanol permeability are highly desirable for high-performance fuel cells. Nafion made of poly(perfluorosulfonic acid) is the most widely used membrane for fuel cells owing to its superior thermal, mechanical, and proton-conductive properties. However, this type of membrane has several disadvantages such as expensiveness, toxicity, and loss of proton conductivity when operating at high temperature (>100 ∘ C). Nanocellulose shows the potential to be an alternative material for the fabrication of novel, biodegradable, and inexpensive ionomer membrane for both polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) due to the good gas barrier properties, mechanical properties, biodegradability, low cost, and acidic oxygen functional groups. Many efforts have been devoted to enhancing the performance of nanocellulose membranes over the past decade. Jiang et al. reported highly proton-conductive, low cost, and stable BC membranes for fuel cells by acid doping [55]. BC films were immersed into H3 PO4 or phytic acid and the H3 PO4 -treated membranes exhibited a maximum proton conductivity of 0.15 S/cm. Lin et al. attempted to modify the BC with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) using ultraviolet light-induced grafting polymerization. The methanol permeability of the modified membrane was approximately half of that of Nafion 115 (5.64 × 10−7 cm2 /s) [56]. Bayer et al. reported the strong dependence of proton conductivity on relative humidity, temperature, and the preparation methods of nanocellulose [57]. Figure 12.6a shows the schematic structure of fuel cells with nanocellulose ionomer membranes. They were quite stable when operating at 80 ∘ C and 95% RH. CNCs and NFCs were separately utilized to prepare nanocellulose ionomer membranes for fuel cells. The possible proton conduction mechanism for nanocellulose membranes was illustrated in Figure 12.6b. CNC films presented a maximum conductivity of 4.6 mS/cm at 120 ∘ C (100% RH), but only 0.05 mS/cm was achieved for NFC films at 100 ∘ C. Increased number of charge carriers

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and hydrophilicity/acidity of the sulfuric acid groups introduced during acid hydrolysis primarily contributed to the higher proton conductivity of CNC films. Figure 12.6c,d shows the photographs of fuel cells with a CNC membrane and an NFC membrane, respectively. Fuel cells with CNC membranes displayed better performance (17 mW/cm2 ) than those with NFC (0.8 mW/cm2 ). However, hydrogen permeability through the nanocellulose membranes was 3 orders of magnitude lower than that of commercial Nafion . Hence further optimization of nanocellulose membrane is required to compete with commercial Nafion. Nanocellulose has also been blended with Nafion aiming to enhance the performance of proton-conducting membranes. Jiang et al. blended BC with Nafion to prepare composite membranes for both PEFCs and DMFCs [58]. In contrast with Nafion membrane, incorporating BC into Nafion not only reduced the water uptake and swelling ratio, but also increased the mechanical properties and thermal stability. In addition, it revealed that the annealing process had a positive effect on the performance of the composite membrane. PEFCs with annealed hybrid membranes exhibited improved power density of 106 mW/cm2 . Gadim et al. demonstrated a similar BC/Nafion composite membrane by impregnating BC film into Nafion solution [59]. The obtained hybrid membrane displayed a conductivity of 0.14 S/cm at 94 ∘ C (98%) and much lower fuel cell power density. Hasani-Sadrabadi et al. reported the exploration of nanocellulose (cellulose whisker) as an effective nanostructure to regulate the microstructure of Nafion membrane even at high loading ratios [60]. By adding nanocellulose into Nafion , the proton conductivity increased remarkably while a superior conductivity was well maintained at a temperature of >100 ∘ C. Furthermore, the hybrid membrane provided considerably suppressed methanol permeability. Direct methanol–air single fuel cells with a nanocomposite membrane exhibited a maximum power density of 91 mW/cm2 , which was much higher than that of fuel cells with Nafion membrane (47 mW/cm2 ).

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12.4.2

Solar Cells

Solar cells can convert clean and inexhaustible solar energy into electricity or heat that powers our society. However, it is still challenging to rapidly increase the worldwide consumption of solar energy, which is partly due to the high material costs of solar cells. Substrates account for 25–60% of total material costs in solar cells [61, 62]. Therefore, adopting an inexpensive substrate to prepare solar cells is a promising way to overcome the aforementioned challenge. Nanocellulose is considered a desirable starting material for the preparation of novel substrates for solar cells due to its outstanding performance and potential low cost. Nanocellulose film can function either as a substrate or as an active layer due to its exceptional mechanical properties, tunable optical properties, nanoscale surface roughness, and so on [63–73]. Cui’s group first demonstrated the application of nanocellulose in organic solar cells as a novel supporting substrate [74]. Nanocellulose disintegrated from wood pulp was used to prepare a transparent nanopaper substrate by vacuum filtration for organic solar cells showing a power conversion efficiency (PCE) of 0.4%. Since

12.4 Nanocellulose for Other Energy Devices

(a)

H2

O2

Nanocellulose ionomer membrane (b)

H3O+ H+

Vehicle mechanism (c)

Grotthuss mechanism

Hopping via oxygen groups

(d)

Figure 12.6 (a) Schematic representation of fuel cells with nanocellulose ionomer membrane. (b) A schematic to illustrate the possible proton conduction mechanisms and pathways in nanocellulose. (c) Fuel cells with an NFC membrane and (d) a CNC membrane. Source: Bayer et al. 2016 [57]. Reproduced with permission of ACS.

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then, much research has been focused on the fabrication of various solar cells on these novel cellulose substrates by rationally optimizing nanopaper substrates and processing technology for cell fabrications, designing novel device structures, and using new functional materials [75–78]. The maximum PCE of organic solar cells on transparent nanopaper substrate is approximately 4% [78]. The application of nanocellulose in solar cells could go beyond substrates. Gerbaldi et al. demonstrate efficient quasi-solid dye-sensitized solar cells (DSSCs) by incorporating nanocellulose into polymer electrolyte membranes [79]. The nanocellulose not only acted as light-scattering and light-reflecting agents that contributed to the enhanced PCE of DSSCs but also served as a reinforcing filler that led to an improvement in both the mechanical properties of the membranes and the durability of the devices. As a result, the DSSCs with nanocellulose as filler exhibited a PCE of 7.03% and superior long-term durability (i.e. above 95% efficiency retention after 500 hours of extreme aging conditions). 12.4.3

Nanogenerators

The unique and sustainable mechanical energy can be converted into electricity by nanogenerators, which is quite suitable for serving as an alternative power source for portable electronics and unattended devices. Generally, according to the principles of electricity generation, it can be categorized into two types of nanogenerators: triboelectric nanogenerators (TENGs) and piezoelectric nanogenerators (PENGs). Herein, we review the most recent technology evolution in nanocellulose-based TENGs and PENGs. Natural cellulose is a slightly triboelectric positive material that shows the potential to be a good candidate for TENGs. In addition, it brings unprecedented natural biodegradability, recyclability, and biocompatibility to TENGs. Yao et al. developed a flexible and transparent nanocellulose-based TENG by pairing a mesoporous nanocellulose film acting as a triboelectric material and a fluorinated ethylene propylene (FEP) serving as a negative material [80]. Figure 12.7a presents the structure and photograph of the NFC-based TENG. The nanocellulose films had a nanoscale surface roughness of ∼300 nm, providing a large surface area for contact and electrostatic charge generation. The assembled TENG presented a performance (0.56 mW at 1 MΩ) comparable to the reported TENGs built on synthetic polymers. Figure 12.7b shows typical open-circuit voltage (V oc ) measured from this TENG. Kim et al. continued this work by using BC to fabricate BC-based TENG that consisted of a Cu/BC composite film and a Cu foil [81]. The TENG demonstrated a similar triboelectric output as the previous report because BC has almost the same dielectric and triboelectric polarizations as wood cellulose. As mentioned above, synthetic polymers and/or other electrode materials were usually used in nanocellulose-based TENGs in an effort to improve charge separation due to the weak polarization tendency of natural cellulose [9]. Therefore, the electric properties of nanocellulose should be modulated to realize all-nanocellulose TENGs with the advantages of environmental friendliness, low cost, and recyclability. Natural cellulose is rich in hydroxyl groups that facilitate its functionalization by facile wet-chemistry reactions. Yao et al. presented a high-performance all-nanocellulose TENG by introducing functional groups

12.5 Conclusion and Outlook

6

PET ITO

4

FEP CNF

Voc (V)

Spacer

2 0

–2 –4 0.0 (a)

0.5

1.0

1.5 2.0 Time (s)

2.5

3.0

(b)

Figure 12.7 (a) Schematic structure and an image of an NFC-based TENG. (b) Open circuit voltage (V oc ) from the NFC film-based TENG. Source: Yao et al. 2017 [80]. Reproduced with permission of Elsevier Ltd

into nanocellulose [82]. Nitro groups and methyl groups were separately introduced into nanocellulose without deteriorating its physical properties by employing chemical reactions, aiming to manipulate the triboelectric polarity of cellulose. The nanocellulose with nitro groups exhibited a negative surface charge density of 85.8 μC/m2 , while the nanocellulose with methyl groups showed a positive surface charge density of 62.5 μC/m2 . When the two types of nanocellulose were paired together, the output of as-prepared all-nanocellulose TENG showed an average voltage and current value of 8 V and 9 μA, respectively, which demonstrated a comparable output with TENG using FEP. Crystalline cellulose is a well-known piezoelectric material. The highly ordered structure of the glucose units through glycosidic linkage linearly along the C2 monoclinic lattice contributes to the piezoelectric polarity of crystalline cellulose. However, there is no report on the use of crystalline cellulose for PENG due to its relatively weak piezoelectric effect. Piezoelectric composite paper, which consisted of ferroelectric BaTO3 nanoparticles and BC, exhibited excellent piezoelectric output with a peak voltage of 14 V and a peak current density of 190 nA/cm2 , and a maximum power density of 0.64 μW/cm2 [83]. Note that the output of nanocellulose-based PENG is much lower than that of nanocellulose-based TENG devices.

12.5 Conclusion and Outlook Nanocellulose has been exploited as a promising candidate material for a variety of energy devices (LIBs, SCs, solar cells, fuel cells, and nanogenerators) by rationally serving as substrate, template, binder, and separator over the past decade due to its exceptional physical and chemical properties (low density, high strength, transparency, large aspect ratio, etc.). It can not only improve the mechanical properties and device integrity, but also enhance device performance. We believe that incorporating nanocellulose into advanced energy devices may bring a new paradigm of material innovation although the energy applications of nanocellulose are still in their infancy.

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lulose materials for effective triboelectric nanogenerator development. Advanced Functional Materials 27: 1700794. 83 Zhao, Y., Liao, Q., Zhang, G. et al. (2015). High output piezoelectric nanocomposite generators composed of oriented BaTiO3 NPs@PVDF. Nano Energy 11: 719–727.

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13 Exploration of Other High-Value Applications of Nanocellulose Ruitao Cha 1 , Xiaonan Hao 2 , Kaiwen Mou 3 , Keying Long 1 , Juanjuan Li 1 , and Xingyu Jiang 1,4,5 1 National Center for NanoScience and Technology, CAS Center for Excellence in Nanoscience, Beijing Engineering Research Center for BioNanotechnology and CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, No. 11 Zhongguancun Beiyitiao, Beijing 100190, China 2 Tianjin University of Science and Technology, School of Chemical Engineering and Material Science, No. 29 Thirteenth Street, TBDA, Tianjin 300457, China 3 Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Science, CAS Key Laboratory of Bio-based Materials, No. 189 Songling Road, Qingdao 266101, China 4 Southern University of Science and Technology, Department of Biomedical Engineering, No. 1088 Xueyuan Road, Shenzhen, Guangdong 518055, China 5 University of Chinese Academy of Sciences, Sino-Danish College, 19 A Yuquan Road, Beijing 100049, China

13.1 Fire Resistant Materials 13.1.1

Introduction

Nowadays, wood is one of the most widely used biological materials in building industries, which is due to its special properties such as easy processability, low thermal conductivity, sustainability, and environmental friendliness [1]. However, flammability related to wood limits its application; for instance, its use in structures and furniture found in homes, schools, and offices can pose hazards during fire [2]. Therefore, enhancing fire resistance of wood has attracted substantial attention from many researchers. The objective of fireproofing treatments is to increase the time of igniting wood, reduce the heat release rate during burning, and limit the spreading of flames on the wood surface. Fire retardant additives play fairly important roles in achieving the fire protection of wood, commonly by being either deposited as coatings or impregnated into the wood structure [2]. In the present chapter, fire retardant additives are proposed as coating painted or sprayed on the wood surface. This kind of surface treatment method is obviously advantageous to improve fire protection, because it is not only simple, efficient, and economical but also retains the inherent feature of the materials [3, 4].

Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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13.1.2

Flame Retardant Additives

Several kinds of flame retardants are briefly reviewed in the following sections. 13.1.2.1

Halogenated Flame Retardants

The effectiveness of halogenated flame retardants primarily relies on the type of halogen. Fluorine- and iodine-based compounds are not used as flame retardants because they do not participate in polymer combustion process [5]. Bromine and chlorine are widely used as flame retardants because they can easily be released and interfere in the combustion process due to their low bonding energy with carbon atoms [6]. Nevertheless, halogen and particularly bromine-based flame retardants are increasingly recognized as environmentally unfriendly because they may present serious safety hazards. For example, halogen-based flame retardants have been found in the food chain and animal bodies, which can cause bioaccumulation in human body and adverse health effects in children [7]. 13.1.2.2

Phosphorus-Based Flame Retardants

With an extensive range, phosphorus-based flame retardant products include phosphates, phosphonates, phosphinates, phosphine oxides, and red phosphorus. These agents are either used as additives or incorporated into the polymer chain during its synthesis [6]. Phosphorated flame retardants are remarkably effective in polymers consisting of nitrogen or oxygen atoms, as previously reported. Consequently, it is apparently crucial to have oxygen or nitrogen atoms in the polymer [8]. Furthermore, phosphorus-based fire retardants for cellulose-containing materials are known to improve char formation and sometimes stop the cycle of free radical production in the flame [9]. 13.1.2.3

Nitrogen-Based Flame Retardants

For nitrogen-based flame retardants, it is worth pointing out that melamine contains 67% by weight of nitrogen atoms. The melting point of melamine is as high as 345∘ and it is therefore recognized as a thermally stable crystalline product [5]. At high temperature, melamine decomposes, accompanied by the release of ammonia, which can dilute oxygen and flammable gases and result in the generation of thermally stable condensates such as melam, melem, and melon, as shown in Figure 13.1 [10]. The formation of melam, melem, and melon has an important influence on flame retardancy. 13.1.2.4

Silicon-Based Flame Retardants

Silicon-based compounds are commonly used as a relatively hard, smooth coating on the materials surface to protect materials against heat, fire, abrasion, and 2C3H6N6 Melamine

–NH3

C6H9N11 Melam

–NH3

C6H6N10 Melem

–NH3

C6H3N9 Melon

Figure 13.1 Thermal decomposition of melamine and related products. Source: Costa et al. 1990 [10]. Reproduced with permission of ACS.

13.1 Fire Resistant Materials

corrosion. Moreover, they also can be impregnated into polymers [6, 11]. The presence of a small amount of silicon-based compounds in polymer has been reported to dramatically promote flame resistant properties. 13.1.2.5

Mineral Flame Retardants

Mineral flame retardants can influence the response of polymers to fire due to their behavior at high temperature. The normally used mineral flame retardants include hydroxycarbonates, zinc borates, aluminum tri-hydroxide, and magnesium di-hydroxide. When the temperature rises, they decompose with the release of H2 O and CO2 , which dilute the oxygen and flammable gases and improve the production of a protective ceramic or vitreous layer. In addition, a large amount of energy is absorbed during the reaction; hence, the temperature is further reduced [6]. 13.1.2.6

Nanoparticles

Nowadays, application of advanced nanotechnologies for fire retardancy has attracted substantial attention from many researchers [12–16]. Nanotechnologies are studied for developing environmentally friendly coatings for polymeric engineering materials, which include wood and polymer composites [16]. Furthermore, nanoparticles can also be dispersed in polymer matrices and contribute to improving performances in addition to fire resistance, such as thermal and mechanical. More specifically, the effect of each type of nanoparticles on fire retardant properties varies, which mainly depends on the respective chemical structure. The nanoparticles used for fire resistance are based on layered materials, fibrous materials, and particulate materials [6]. 13.1.3

Fire Resistance of Clay Nanopaper Based on Nanocellulose

Nanocellulose/clay hybrid nanocomposites (clay nanopaper) are proposed as fire protection coating for wood. Clay in the form of sodium montmorillonite (MTM) and nanocellulose in the form of cellulose nanofiber (CNF) are studied. CNF is from a renewable resource, with diameter of 3–15 nm and length of 0.7–3 μm. Compared with other nanocellulose such as cellulose nanocrystal (CNC) and bacterial nanocellulose (BNC) [17, 18], wood-based CNF has potential in large-scale industrial application and is widely available at much lower cost. Bio-based clay nanopaper would meet the increasing needs of green and sustainable materials. The MTM nanoplatelets as the inorganic phase are strongly oriented in the plane parallel to the nanopaper surface. In colloidal mixtures of CNF/MTM, the MTM platelets are embedded in CNF networks, which form a continuous matrix phase. CNF can impart strength, toughness, and ductility to the nanocomposites and allow inorganic contents as high as 90% by weight [12]. Colloidal mixtures are prepared by filtration processing similar to the paper-making approach. This water-based procedure is environmentally friendly and much simpler than traditional layer-by-layer deposition approach because it mainly depends on physical mixing of nanoparticles. Consequently, water-based

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Clay nanopaper Glue Pinewood

Figure 13.2 Sketch of the procedure adopted for the production of clay nanopaper-coated wood. Source: Carosio et al. 2016 [9]. Reproduced with permission of ACS.

Coated wood

MTM nanoplatelets suspension

Highly oriented ‘‘brick and mortar’’ structure

Cellulose nanofibrils suspension

Filtration and drying Colloidal suspension Clay nanopaper

Figure 13.3 Schematic representation of clay nanopaper preparation and the oriented structure. Source: Carosio et al. 2016 [9]. Reproduced with permission of ACS.

processing has been employed to produce large and flat clay nanopaper nanocomposites. The demands for security are recently becoming more and more important in terms of the materials’ response to fire and their fireproof performances. Hence, the study of structures for fire retardant composites in building interiors is highly worthwhile. Improving the fire resistance of materials is one of the major challenges for extending their application. For clay nanopaper based on nanocellulose, fire resistance performances and the corresponding mechanisms are mainly introduced in the following sections. Clay nanopaper consisting of CNF and MTM is obtained, which acts as a fire-protective coating for wood as reported in Figure 13.2 [9]. Figure 13.3 [9] shows how CNF/clay nanocomposites are produced by filtration and drying, eventually forming highly oriented “brick and mortar” structure, in which MTM nanoplatelets represent “bricks” and CNF network represents “mortar.” Carosio et al. [9] investigated the capability of the fire protection of CNF/MTM nanocomposites by cone calorimetry in order to simulate a real fire scenario to observe the burning behavior of a material (Figure 13.4a) [19]. The exposure to heat flux triggered degradation of CNF, leading to gas formation so that the clay nanopaper formed expanded the protective structure in the out-of-plane

13.1 Fire Resistant Materials

Transmitted heat flux (kW/m2)

Unshielded system Clay nanopaper shielded

Temperature monitoring

1 mm stainless steel thermocouple 2 mm below surface (a)

35 30 25 20 15 10 5 0

35 kW/m2

81% reduction in transmitted heat flux 6.8 kW/m2

(b)

Temperature (°C)

600 500 400 300 Ignition 200 100

150 °C

0

Ignition

Wood Coated 0

(c)

500 600 100 200 300 400 Time (s)

Expanded coating reduces transmitted heat flux and prevents release of combustible volatiles

Figure 13.4 (a) Temperature measurement set-up for cone calorimetry; (b) heat flux data obtained with a flux meter positioned under the conical heater. Data are collected by measuring 35 kW/m2 heat flux and then shielding the system with a clay nanopaper sample observing the reduction in transmitted heat; (c) temperature profile during cone calorimetry, measured 2 mm under the surface of coated (clay nanopaper coated wood) and wood (uncoated wood) samples. Source: Carosio et al. 2016 [9]. Reproduced with permission of ACS.

direction [16]. The heat flux transmitted to the wood was decreased by as much as 81% as shown in Figure 13.4b. Owing to thermal shielding of the expanded structure, coated wood exhibited a strong increase in time to ignition (TTI) and a 46% reduction in maximum average rate of heat emission (MARHE), commonly recognized as a measure of the propensity for fire development under real scale conditions [20]. Furthermore, a 33% reduction in the total heat release (THR) also demonstrated improved fire resistance of coated wood. In order to quantify the effectiveness of the expanded clay nanopaper structure, temperature measurements were conducted on both coated wood and uncoated wood using cone calorimetry. When subjected to the same heat flux, as displayed in Figure 13.4c, the temperature of coated wood remained below 150 ∘ C for more than five minutes and increased very slowly even during combustion compared to uncoated wood. Compared to the abovementioned favorable performance, clay nanopaper layer also had gas barrier function preventing the emission of combustible volatiles from the wood substrate. Therefore, the fire protection of wood was achieved by two means, both thermal shielding and gas barrier. These greatly improved the

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13 Exploration of Other High-Value Applications of Nanocellulose

50% MTM

Intensity (cps) 500 nm

Intensity (cps)

30% MTM

25 000 20 000 15 000 10 000 5000 0

Intensity (cps)

500 nm

8000 6000 4000 2000 0

20 000 15 000 10 000 5000 0

Intensity (cps)

10% MTM

25 000 20 000 15 000 10 000 5000 0

Intensity (cps)

428

20 000 15 000 10 000 5000 0

MTM

2

2

6

8

10 12 14 16 18 20 22 24 26 28 30

4

6

8

10 12 14 16 18 20 22 24 26 28 30

8

10 12 14 16 18 20 22 24 26 28 30

8

10 12 14 16 18 20 22 24 26 28 30

8

10 12 14 16 18 20 22 24 26 28 30

20% MTM

2

4

6

30% MTM

2

4

6

50% MTM

2 500 nm

4

10% MTM

4

6

2θ (°)

Figure 13.5 SEM micrographs of microtome cut cross sections and XRD spectra of CNF/MTM nanocomposites with weight fractions of MTM content according to the designations. Source: Carosio et al. 2015 [16]. Reproduced with permission of ACS.

capability of fire protection by delaying the degradation of wood and the release of combustible volatile gases. CNF/MTM nanocomposites were thus considered as promising and effective fire-protective coating for wood. In contrast to the clay nanopaper in the previous sections, Carosio et al. [16] measured the fire retardant features of MTM/CNF hybrid nanocomposites with different content of MTM. As could be observed from Figure 13.5, [16] the morphology of nanocomposites was studied by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM micrographs showed ordered nanostructures for clay nanopaper where MTM nanoplatelets oriented in the plane. Especially for 30% MTM content, the in-plane orientation was obvious. XRD measurements indicated that the characteristic peak of nanocomposites is centered around 6.0∘ compared with that of neat MTM at 9.0∘ , which suggested that the values of average MTM interlayer distance had increased. This peak increased in intensity with increased MTM content, which demonstrated that the structure of clay nanopaper is highly organized and MTM nanoplatelets strongly oriented in the plane. The thermal degradation of neat CNF and clay-containing nanocomposites was measured by thermogravimetric analysis in nitrogen and air, respectively. See the thermal gravimetric (TG) curves presented in Figure 13.6 [16]; the degradation

13.1 Fire Resistant Materials

Weight loss (%)

100 80

CNF 10% MTM 20% MTM 30% MTM 50% MTM

60 40 20 0

200

300 400 500 Temperature (°C)

600

700

800

100

200

300 400 500 Temperature (°C)

600

700

800

Derivate weight loss (%/°C)

100

Figure 13.6 TG and dTG plots of CNF and CNF/MTM nanocomposites in nitrogen. Source: Carosio et al. 2015 [16]. Reproduced with permission of ACS.

rate of CNF/MTM nanocomposites was much slower than that of pure CNF. Furthermore, the dTG curves in Figure 13.6 showed that the peak height represents maximum weight loss rate, which reduced with increase in clay content. These effects were owing to oriented MTM nanoplatelets that served as thermal barrier, which reduced the heat transfer rate and degradation rate [21]. As reported in Figure 13.6, the amount of CNF residue of nanocomposites was more than that of pure CNF, which was partly because MTM functioned as a thermal insulation shield. Another reason for this was that Na+ ions as catalysts on the MTM surface could trigger char formation of CNF [22, 23]. TG and dTG curves in air were employed to assess thermo-oxidative stability. The following analysis results were similar to the research of Liu et al. [12]. As shown in Figure 13.7 [16], the thermal oxidation of pure CNF occurred in two steps. However, for MTM/CNF nanocomposites, only one degradation step was shown in the range between 300 and 400 ∘ C. Then continued oxidative degradation proceeded at a slow rate. The corresponding mechanism similar to the abovementioned theory was a thermal insulation shield resulting from MTM nanoplatelets and catalytic effect of Na+ on the MTM surface, which facilitated char formation and further reduced the thermal oxidation rate in the second step. Moreover, high orientation of clay nanoplatelets parallel to the film surface enhanced oxygen barrier properties; CNF/clay nanocomposites strongly reduced oxygen diffusion and thus slowed down the continued oxidation [24]. MTM as a barrier against gas diffusion attracts substantial attention [25]. Liu et al. [12] measured oxygen permeability features of clay nanopaper under different conditions, as listed in Table 13.1 [12]. The oxygen transmission rate (OTR) of dry 50 N/50 M clay nanopaper (represents the weight ratio of nanofibrillated cellulose (NFC) to clay of 1 : 1) was lower than 0.001 cm3 mm/m2 /d/atm. When the

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Weight loss (%)

100 80

CNF 10% MTM 20% MTM 30% MTM 50% MTM

60 40 20 0 100

200

300 400 500 Temperature (°C)

600

700

800

100

200

300 400 500 Temperature (°C)

600

700

800

Derivate weight loss (%/°C)

430

Figure 13.7 TG and dTG plots of CNF and CNF/clay nanocomposites in air. Source: Carosio et al. 2015 [16]. Reproduced with permission of ACS. Table 13.1 ORT (cm3 /mm/m2 /d/atm) of NFC and 50 N/50 M clay nanopaper under 0%, 50%, and 95% relative humidity (RH) and 100% oxygen conditions. Samples

0% RHa)

50% RH

95% RH

NFC

—

0.048

17.8

50 N/50 M

—

0.045

3.5

a) OTR at 0% RH and 100% O2 for these materials was below detection limit. Source: Liu et al. 2011 [12]. Reproduced with permission of ACS.

relative humidity was increased up to 50%, the OTR of clay nanopaper increased to 0.045 cm3 mm/m2 /d/atm. However, this was still better than other biopolymers, for instance, polylactic acid (PLA). Particularly the ORT of pure NFC at 90% RH was strongly increased to 17.8 cm3 mm/m2 /d/atm, nearly exponentially. However, as could be observed from Table 13.1, the ORT of clay nanopaper at 95% RH was much lower than that of pure CNF. Therefore, MTM had a remarkably important effect on oxygen barrier characteristics. Moreover, NFC also displayed low oxygen permeability due to a high order of molecular arrangement [26]. The layered structure of clay nanopaper and high orientation of MTM nanoplatelets in the plane created a tortuous path [24, 27], which increased oxygen transport length and thus enhanced the fire resistance of materials. Flammability test [16] was performed on CNF/MTM hybrid nanocomposites in order to measure the response to a direct flame and evaluate the capability of fire protection of clay nanopaper. Pure CNF ignited rapidly and burned vigorously and completely. An afterglow phenomenon was observed as listed

13.1 Fire Resistant Materials

Table 13.2 Flammability data from vertical flammability test of CNF and CNF/MTM nanocomposites.a)

Sample

Total burning time (s)

Afterglow time (s)

CNF

9

15

10% MTM

8

Residue (%)

0 25

20% MTM

16

55

30% MTM

5

76

50% MTM

2

95

a) Residue is in wt% of total mass. Source: Carosio et al. 2015 [16]. Reproduced with permission of ACS.

Figure 13.8 Residues collected after vertical flammability test of CNF/clay nanocomposites. Source: Carosio et al. 2015 [16]. Reproduced with permission of ACS.

10% MTM

20% MTM

30% MTM

50% MTM

in Table 13.2 [16] and it could be a fire hazard. For clay nanopaper with 20% by weight of MTM, the increase in total burning time was due to the reduced flame spreading rate exerted by MTM. Both the 30% and 50% MTM samples exhibited self-extinguishing behavior as observed in Figure 13.8 [16]. After ignition, the flame was gradually reduced in size and speed of spreading and therefore narrowed down to a smaller field in the sample. Eventually, the flame vanished. Hence, it could be seen that the structural ordering of MTM nanoplatelets dramatically promoted flammability behavior. Meanwhile, high clay content was obviously advantageous to enhance the properties. Compared with traditional clay nanocomposites with low clay content [28, 29], the present green and nontoxic nanocellulose-based clay nanopaper is much better in terms of capability of fire protection. The mechanisms related to the fire resistant feature are assessed on the basis of thermogravimetric analysis, flammability, and cone calorimetry data. The effects of different clay content have also been studied.

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13.1.4

Conclusion

The primary mechanisms can be summarized in the following: favorable clay orientation in the plane results in reduced thermal conductivity, hence delaying degradation of CNF. Na+ -rich MTM surface as catalyst facilitates the formation of thermal stable char. Because oxygen barrier property, highly organized MTM, and the layered structure increase oxygen diffusion length according to the tortuous-path model [27], as a consequence, oxygen transport rate is reduced and thus thermal degradation rate is also reduced. Additionally, the clay barrier hinders CNF volatiles diffusion, resulting in microscale voids formation so that thermal conductivity is further decreased. Oriented MTM provides barrier properties and low thermal conductivity while CNF imparts favorable charring. The reported mechanisms clearly point out that clay nanopaper based on nanocellulose is a promising, sustainable, and efficient alternative to current fire retardant materials.

13.2 Thermal Insulation Materials 13.2.1

Introduction

The problems of energy consumption are beginning to appear with the rapid development of industry. Energy usage in the building sector contributes to major global energy usage. Buildings emitting carbon dioxide each year account for a significant part of greenhouse gas emissions [30]. In the efforts to control energy consumption of buildings, thermal insulation materials play an important role [31, 32]. Therefore, traditional insulation materials in thicker walls were and are being used [30, 33–35], which generate a series of issues such as floor area, architectural restrictions, material usage, and economic cost [31]. Therefore, it is essential to develop high-performance thermal insulation materials. Thermal conductivity is a key property of thermal building insulation materials. In general, the objective of research is to decrease thermal conductivity. The total overall thermal conductivity 𝜆tot [35], i.e. the thickness of a material divided by its thermal resistance, is in principle made up of several contributions: 𝜆tot = 𝜆solid + 𝜆gas + 𝜆rad + 𝜆conv + 𝜆coupling + 𝜆leak where 𝜆tot is the total overall thermal conductivity, 𝜆solid is the solid-state thermal conductivity, 𝜆gas is the gas thermal conductivity, 𝜆rad is the radiation thermal conductivity, 𝜆conv is the convection thermal conductivity, 𝜆coupling is the thermal conductivity term accounting for second order effects between the various thermal conductivities, and 𝜆leak is the leakage thermal conductivity. The best way to obtain materials with good heat insulation performance is to minimize all of these thermal conductivity contributions. 13.2.2

Thermal Building Insulation Materials

A simple overview of several kinds of thermal building insulation materials is given [35].

13.2 Thermal Insulation Materials

13.2.2.1

Mineral Wool

Mineral wool includes glass wool (fiber glass) and rock wool, which is usually produced as mats and boards applied in the building field, but occasionally also as filling material to fill various cavities and spaces. The thermal conductivity of mineral wool is between 30 and 40 mW/m/K, and it varies with temperature, moisture content, and mass density. Mineral wool products may be perforated, and also cut and adjusted at the building site, without any loss of thermal resistance. 13.2.2.2

Expanded Polystyrene (EPS)

Expanded polystyrene (EPS) is produced from small spheres of polystyrene (from crude oil) containing an expansion agent, which can be cast as boards. Thermal conductivity of EPS is between 30 and 40 mW/m/K, and it changes with temperature, moisture content, and mass density. 13.2.2.3

Polyurethane (PUR)

Polyurethane (PUR) is prepared from a mixture of isocyanates and polyols (alcohols containing multiple hydroxyl groups). During the expansion process the closed pores are filled with an expansion gas, HFC, CO2, or C6 H12 . PUR is produced as board and may be used as expanding foams at building sites. The thermal conductivity of PUR is between 20 and 30 mW/m/K, and it changes with temperature, moisture content, and mass density. 13.2.2.4

Aerogel

Aerogel is a kind of material prepared by replacing the liquid solvent in a gel by air without substantially altering the network structure or the volume of the gel body [36, 37]. Aerogels have a high potential to be used in many applications such as particle filters, particle trappers, heat insulators, and catalyst supports due to its lightness, high porosity, large surface area, and low thermal conductivity [38– 44]. Especially, the large commercial market for aerogels is primarily driven by thermal and acoustic insulation sector. Aerogels can be either inorganic or organic. For instance, silica aerogels with low thermal conductivity are produced by liquid-phase processes accompanied by supercritical drying [45]. However, silica aerogels are too fragile to handle without any damage, and the production cost of materials is very high [46]. In addition, organic aerogel is made of toxic molecules [47]. Therefore, researchers have laid emphasis on developing super-insulator aerogels from bio-resources. With industry expansion and population boom, increasing paper consumption has led to a large amount of paper-related waste, which causes a series of issues such as environmental population and forest destruction [31]. Cellulose fibers from paper waste, which are cheap and widely abundant, play an important role in dealing with these problems. According to statistics, recycling one ton of paper saves roughly 17 trees, 3.3 cubic yards of landfill space, 360 gallons of water, 100 gallons of gasoline, 60 pounds of air pollutants, and 10 401 kW of electricity [48]. Moreover, cellulose fibers with low thermal insulation can be combined with highly porous aerogel structure to form cellulose aerogel, which greatly improved thermal insulation [30, 33, 35, 49, 50]. Cellulose aerogel will have a promising future in the thermal insulation building sector.

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13.2.3 Thermal Insulation Performance of Nanocellulose-Based Materials The normal focus in nanotechnology is to control the dimension of particles between 0.1 and 100 nm. However, as far as nanotechnology is applied for making high-performance thermal insulation materials, the emphasis often shifts onto pore size in the nanoscale [35]. The view is shown in Figure 13.9. CNFs can be obtained from agricultural residues, which reduce the environmental load. In addition, due to their abundance in nature CNFs have a high potential to be used in many different areas. To the best of our knowledge, Silva et al. [51] and Yuan et al. [52] investigated the thermal insulating features of NFC-based aerogels. A short description of CNF combined with other materials such as thermal insulator is given. Bendahou et al. [53] combined CNFs and nanozeolite particles to prepare nanoporous hybrid aerogels by freeze-drying. The NFC films interacted with each other to form three-dimensional network structure with cavities. Nanozeolites were well dispersed on the surface of the NFC. No nanozeolite aggregation can be observed on hybrid materials. The combination of inter-cellulose films mesoporosity and closed nanozeolite pores can reach pore sizes lower than 100 nm. Therefore, thermal conductivity as low as 18 mW/m/K was obtained for hybrid materials based on nanocellulose. The values of thermal conductivity of NFC-nanozeolite aerogels were almost close to that of polymeric aerogels known for their low thermal conductivity and application in thermal insulation materials. This work confirmed a possible way to prepare nanocellulose-based hybrid aerogels as potential candidates for future thermal super-insulators. Hayase et al. [45] prepared a composite aerogel with high thermal insulation by compositing polymethylsilsesquioxane (PMSQ) with nanocellulose. In addition, their group previously reported PMSQ aerogels produced from methyltrimethoxysilane (MTMS). As listed in Table 13.3, five groups of composite foams were obtained with a varied amount of MTMS and two pure PMSQ aerogel groups. In Table 13.3, CTAC represents surfactant n-hexadecyltrimethylammonium chloride. The nanocellulose solution was diluted to 0.18 w/w% in water to homogeneously disperse CNFs and avoid aggregation of PMSQ nanoparticles Nanotechnology: technology for controlling matter of dimensions between 0.1 and 100 nm

0.1 – 100 nm

Nanoparticles

0.1 – 100 nm

Nanopores

0.1 – 100 nm

For comparison: Solar radiation: 30 – 3000 nm Atomic diameters: Hydrogen: 0.16 nm Carbon: 0.18 nm Gold: 0.36 nm Molecular length: Stearic acid: 2.48 nm (C17H35COOH) Nanotechnology: technology for controlling matter at an atomic and molecular scale.

Figure 13.9 Nanotechnology and its application in high-performance thermal insulation materials. Source: Jelle 2011 [35]. Reproduced with permission of Elsevier.

13.2 Thermal Insulation Materials

Table 13.3 Starting compositions and properties of typical aerogels obtained in the present study. CNFs in 𝝀a) MTMS 5 Mm Urea CTAC HOAc 𝝆b b) SBET c) (aq./wt%) (mW/m/K) (g/cm3 ) (m2 /g) WCd)(∘ ) Materials (ml) HOAc (ml) (g) (g)

C5

5

100

30

0.40

0.18

24.3

0.020

657

152.7

C10

10

100

30

0.80

0.18

21.7

0.039

550

152.4

C25

25

100

30

2.00

0.18

18.8

0.097

732

154.3

C50

50

100

30

4.00

0.18

15.3

0.142

525

152.7

C75

75

100

30

6.00

0.18

16.2

0.186

560

145.4

P5

5

100

30

0.40

0

22.5

0.040

601

152.5

P50

50

100

30

4.00

0

14.9

0.135

631

150.6

a) Thermal conductivity at an ambient condition. b) Bulk density. c) BET specific surface area obtained from nitrogen sorption. d) Contact angle of water. Source: Hayase et al. 2014 [45]. Reproduced with permission of ACS.

on the surface of nanocelluloses. Composite aerogels possessed super hydrophobicity with a water contact angle greater than 150∘ and could float on water at least for one month without any additional hydrophobizing treatments. Gas-phase conduction and solid-phase conduction were considered as two main contributions to the thermal conductivity of composite aerogels. As shown in Figure 13.10, thermal conductivity of the gas phase in C50 was lower than that of C5, C10, and C25 because of larger pores in C5, C10, and C25, which was longer than the mean free path of the gas molecules in air at ambient pressure. Besides, solid-state thermal conductivity of C50 was lower than that of C75 due

(a)

(b)

200 nm

200 nm (d)

(c)

(e)

200 nm

200 nm (d′)

200 nm

200 nm

Figure 13.10 SEM images of PMSQ–CNF composite aerogels and pure PMSQ aerogel without CNFs; (a) C5, (b) C10, (c) C25, (d) C50, (e) C75, and (d’) P50. Source: Hayase et al. 2014 [45]. Reproduced with permission of ACS.

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13 Exploration of Other High-Value Applications of Nanocellulose

to a larger amount of solid phase in C75. Thermal conductivity of hybrid aerogels was as low as 15 mW/m/K, which was the same as the value for nanocomposite foams based on CNFs, graphene oxide (GO), and sepiolite nanorods (SEPs) [54]. Most aerogels applied for thermal insulation materials exhibit thermal conductivity higher than 20 mW/m/K [47, 55]. Therefore, the present work will certainly open up a new way for the development of nanocellulose-based aerogels as thermal insulating materials. Wicklein et al. [54] successfully obtained thermally insulating materials by freeze-casting a suspension of CNFs, GO, and SEPs. Eventually, nanocomposite foams consisting of anisotropic ice crystals surrounded by walls of the dispersed nanoparticles were produced using freeze-casting, which could be used to prepare highly anisotropic porous materials. As was shown in Figure 13.11, nanocomposite foams were characterized by aligned tubular pores structure parallel to the freezing direction. Pore density decreased along the ice growth direction (Figure 13.11d,e), which indicated that (a)

(b)

(c)

50 μm

250 μm

(d)

(e)

250 μm

500 μm

(f)

1 cm

Ice growth

5 μm

25 μm

Figure 13.11 Microstructure of freeze-cast nanocomposite foams. (a) SEM cross-section image of a freeze-cast nanocomposite foam containing cellulose nanofibers (CNF), graphene oxide (GO), sepiolite (SEP), and boric acid (BA). (b) Three-dimensional reconstruction of the tubular pore structure of the nanocomposite foam derived from X-ray microtomography. (c) X-ray microtomography image showing that the tubular pores are straight and several millimeters long in nanocomposite foam with a composition of 77% CNF/10% GO/10% SEP/3% BA (in wt%). (d, e) X-ray microtomography cross sections of a nanocomposite foam, taken through upper and lower parts, respectively (scale bars, 100 μm). (f ) HRSEM image of a foam wall, where the yellow dotted line indicates a section of the tubular cell. Inset: Distributed SEP nanorods within the cell wall. The nanomaterials are homogeneously distributed in the cell walls, forming an anisotropic tubular pore structure as a result of the unidirectional freeze-casting process. Source: Buratti and Moretti 2012 [54]. Reproduced with permission of Elsevier.

13.2 Thermal Insulation Materials

Heat transfer: Conduction Convection Radiation

Pore channels Weight

Cell wall: Qradial

Mullite

Qradial

Heat source

(a) GO

SEP Pores

Thermal conductivity (mW/mK–1)

180 160

Heat source and sensor

(c)

Radial Axial

30

Air

20 Mullite

10 0

EP

S

EP

F CN

–B

GO

– NF

(b)

C

Heat source

S A–

70 65 60 55 50 45 40 35 30 25 20 (°C) 70 65 60 55 50 45 40 35 30 25 20 (°C)

(d)

Figure 13.12 Thermal transport properties of anisotropic nanocomposite foams. (a) Schematic illustration of contributions to thermal conductivity in the radial and axial directions of a foam with oriented pores (not drawn to scale). (b) Thermal conductivity values in the axial and radial directions of CNF and CNF–GO–BA–SEP foams, compared with expanded polystyrene (EPS). The thermal conductivity normal to the tubular pores is significantly lower than the value for air. (c, d) Schematic (left) and thermographic (right) images of CNF–GO–BA–SEP nanocomposite foam with the tubular pores oriented normal to the heat source (c) and a closed-cell EPS foam (d). The heated volume of the CNF–GO–BA–SEP nanocomposite foam is smaller and more homogeneous than that of the EPS foam. The colors in the thermographic images show the temperature distribution on the surface of the foams. The rectangular nanocomposite foam in (c) (right image) was pressed onto the heat source with a weight made of zirconia. Source: Wicklein et al. 2015 [54]. Reproduced with permission of Springer Nature.

the pore structure was homogeneous. The SEPs were well dispersed in the cell wall and no CNF or graphene oxide aggregation can be seen. Thermal transport properties of anisotropic nanocomposite foams are shown in Figure 13.12 [54]. Figure 13.12b shows that the thermal conductivity in the radial direction of nanocomposite foams is significantly lower than that in the axial direction and below the value of pure CNF and EPS. Figure 13.12c,d shows excellent heat insulating property in the radial direction of nanocomposite foams compared to EPS. In conclusion, the radial heat insulation properties of nanocomposite foams are related to nanosized composition and microstructure in the pore walls. The thermal conductivity of 15 mW/m/K in nanocomposite materials indicates that nanoscaled technology is a promising way to produce highly thermal insulating performance materials. 13.2.4

Conclusion

Nanocellulose-based thermally insulating materials, such as NFC-zeolites, NFC–PMSQ, and NFC–GO–SEP, show low thermal conductivity. The low

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13 Exploration of Other High-Value Applications of Nanocellulose

thermal conductivity of NFC-zeolites as organic–inorganic hybrid aerogels is attributed to adjunction of nanozeolites to CNFs by reaching pore sizes lower than 100 nm. NFC–PMSQ biocomposite aerogels also show, besides low thermal conductivity, low density, bending flexibility, and super-hydrophobicity. They could be used in outdoor environments, such as dirt, humidity, rain, UV irradiation, and temperature change, and do not change their properties, because both PMSQ and CNF are stable under these conditions. The anisotropic and nanoporous composite foams consisting of nanocellulose, graphene oxide, and SEPs also exhibit significantly low thermal conductivity (15 mW/m/K). The synthesis of these thermal insulating materials based on renewable and widely abundant resources provides potential for the improvement of energy efficiency and reduction of the environmental impact of buildings.

13.3 The Templated Materials 13.3.1

Introduction

Cellulose is considered as the most abundant renewable biopolymer and is present in a wide variety of living species, such as animals, plants, and bacteria. From the given cellulose source, the fibrils can be typically separated into amorphous and crystalline components [56], yielding cellulose nanofibrils (CNF) or CNCs. CNC can be also called nanocrystalline cellulose (NCC). Compared with CNF including both amorphous and crystalline cellulosic regions, CNC is obtained from fibers or fibrils through acid hydrolysis that could degrade amorphous regions, thus generating highly crystalline cellulose nanoparticles, as shown in Figure 13.13 [58]. General procedures for the preparation of NCC are described in the scheme in Figure 13.14 [59]. The excellent properties of nanocellulose such as sustainability, biodegradability, high strength and stiffness, and high aspect ratio have attracted much attention [17, 56, 60–64]. Moreover, because of the large amount of −OH groups on the surface of cellulose, resulting in the formation of hydrogen bonding, nanocellulose has been proved to be an attractive templating material. In this context, nanocellulose, especially CNC, is used as template or scaffold to direct the deposition and patterning of inorganic materials to form nanoparticles, nanowires, or nanotubes with improved properties. This concept opens new ways to make good use of inorganic materials in combination with nanocellulose. As templates for the synthesis of inorganic nanomaterials, CNCs usually function as reducing agent, structure-directing agent, and stabilizer, which is ascribed to the hydroxyl groups at the nanocrystal surface used for stabilizing [65]. Synthesis of silver (Ag) nanoparticles is accomplished by using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyradical) to oxidize CNCs as the scaffold and using NaBH4 to reduce metallic cations. Furthermore, the carboxyl and hydroxyl groups of carboxylated CNCs contribute to adsorbing metallic cations and reducing Ag+ to Ag0 , as observed in Figure 13.15a. Namely, under mild alkaline

13.3 The Templated Materials

Non-crystalline 20–60 μm

Crystalline Macroscopic fibers

Figure 13.13 Schematic illustration of CNF and CNC production from fiber cell walls by mechanical and chemical treatments, respectively. Source: Adapted with permission from Salas et al. [57]. Copyright 2014, The Royal Society of Chemistry.

Pulping

Bleaching

Chipping Cellulose

Lignocellulosic biomass

Milling

Steam explosion

Fractionation Acid hydrolysis

Glucose Sonication

Dialysis

Centrifugation

NCC Acid

NCC shape and dimensions

NCC suspension

Figure 13.14 Scheme of main steps needed to prepare NCC from lignocellulosic biomass. Source: Brinchi et al. 2013 [59]. Reproduced with permission of Elsevier.

conditions, CNCs subjected to periodate oxidation yield aldehyde functions to reduce Ag+ for the synthesis of Ag nanoparticles [77]. In addition to the method described above, the synthesis of Ag nanoparticles, using bacterial CNCs as templates, is carried out by an ion-exchange reaction between sodium and the silver salt as well as by its thermal reduction. As

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13 Exploration of Other High-Value Applications of Nanocellulose

(a)

(b)

(c)

(d)

100 nm

100 nm Ag nanoparticle (D < 10 nm)

(e)

50 nm

Ag (D = 10–20 nm)

(f)

20 nm

Ag (D < 5 nm)

Au–Ag nanoparticle (D = 3–7 nm)

(g)

(h)

5 nm

10 nm Ni nanocrystal (D = 5–15 nm)

Pt nanoparticle (D = 2 nm)

(i)

(j)

Se nanoparticle (D = 10–20 nm)

Pd (D = 3.6 ± 0.8 nm)

(k)

400 nm

5 nm

200 nm PbS nanocube

100 nm

TiO2 nanocube (D = 200 nm) CaCO3 (D = 11.3 ± 0.5 nm)

Figure 13.15 Morphologies and dimensions of inorganic nanoparticle-templated synthesis using cellulose nanocrystals. (a) Ag nanoparticles with TEMPO-oxidation cotton CN [66], (b) Ag nanoparticles with TEMPO-oxidation bacterial CN [67], (c) Au nanoparticles [68], (d) Au–Ag alloy nanoparticles with Au and Ag content of 25% and 75%, respectively [69], (e) Ni nanoparticles [70], (f ) Pt nanoparticles [71], (g) Se nanoparticles [72], (h) Pd nanoparticles [73], (i) PbS nanoparticles with tunicate CN [74], (j) TiO2 nanocubes [75], and (k) CaCO3 nanoparticles [76]. (The inset image indicates the homogeneous hybrid nanomaterial containing 30 wt% CaCO3 and cellulose nanocrystals.) Source: Reprinted with permission of Refs. [66] and [74]. Copyright 2011, Springer; Refs. [67] and [69], Copyright The American Chemical Society; Refs. [68, 71, 73, 76], Copyright The Royal Society of Chemistry; Refs. [70, 72, 75], Copyright Elsevier.

presented in Figure 13.15b, the Ag nanoparticles exhibit a controlled size distribution and high density on the CNCs surface [67]. Gold (Au) nanoparticles are prepared on the surface of crystalline CNFs via reduction reaction (Figure 13.15c) [65]. Additionally, Au–Ag alloy nanoparticles (Figure 13.15d) as well as Ag–Pd alloy nanoparticles, using CNCs and carboxylated CNCs as the template respectively, are obtained through co-reduction of the corresponding metal ions. The morphology and dimension of both alloy nanoparticles are similar [69, 78]. Using a hydrothermal process (400–500 ∘ C), nickel (Ni) metal ions can be firstly deposited and stabilized on the surface of CNCs and form Ni nanoparticles by a thermal reduction method (Figure 13.15e) [70]. Platinum (Pt) nanoparticles are synthesized by heating aqueous solution containing hexachloroplatinic acid and CNCs as reducing agents at a relatively low

13.3 The Templated Materials

temperature of 80 ∘ C [79]. Periodate oxidation of nanocellulose, yielding reducing aldehyde groups on the surface of nanocellulose, enhances the reducing properties of CNCs. With a similar method of hydrothermal process, selenium (Se) nanoparticles are obtained at 120–160 ∘ C by using CNCs as the template [72]. The reduction reaction of homogeneous colloidal CNCs and palladium (Pd) nanoparticles can lead to the formation of Pd nanoparticles (Figure 13.15h). In addition to the reduction process, the synthesis of cadmium sulfide (CdS), zinc sulfide (ZnS), and lead sulfide (PbS) nanoparticles on CNC templates can be carried out by adding a cationic surfactant, which can stabilize nanoparticles, enhance their reaction with CNCs, and control the dimensions of nanoparticles [74, 80]. As a consequence of the chiral characteristics and structuring ability from CNCs, they can be utilized to facilitate alignment of inorganic particles. By applying sol–gel mineralization, chiral nematic concentrated suspensions of CNCs are used as template to create mesoporous silica (Si) nanoparticles with aligned cylindrical pores structure [81–84]. Furthermore, the chiral imprint of the helically ordered rod-like CNCs is exerted on the silica structure, breaking new ground for applications such as color display, protein analysis, and chiral separation [85]. The blue-light-emitting silica nanowires and nanotubes can be fabricated via sol–gel mineralization of the dilute CNCs, followed by removal of organic templates through calcination [84]. With a similar approach, core–shell titania (TiO2 ) nanoparticles with tunable aspect ratio and cavity shape can be prepared by using titanium tetrabutoxide precursors [86], which are utilized in photovoltaic devices and photocatalysis [57]. The synthesis of zinc oxide (ZnO), TiO2 , and aluminum oxide (Al2 O3 ) hollow nanotube aerogels is performed by depositing nanoparticles on the preformed cellulose aerogels by atomic layer deposition (ALD) followed by removal of organic templates by calcination [87]. But beyond that, by applying a solution plasma process (SPP) without adding the reducing agent, the ZnO nanoparticles are still successfully produced [88]. The values 10.00 and 20.00 observed in Figure 13.16 represent different concentrations of Zn2+ solution (10.00% and 20.00% (w/v) in methanol). The templated ZnO obtained through two methods by using NH4 OH as a reducing agent and SPP shows different particle shapes as seen in Figure 13.16. The shapes of the formed ZnO particles are varied with different Zn2+ precursors such as Zn(NO3 )2 and ZnAct. Moreover, the reducing agents also have a significant effect on the morphology of ZnO [89]. SPP can facilitate the penetration of Zn2+ ions from the Zn2+ precursor solution into BC templates. The presence of numerous reactive species yielded by SPP facilitates the transformation of Zn2+ into ZnO, where ZnO can cover the surface of BC nanofibers throughout. Moreover, the ZnO-deposited BC composites display significant antibacterial activity without a photocatalytic reaction against both Staphylococcus aureus and Escherichia coli, according to Zn2+ releasing mechanism. Although calcium carbonate (CaCO3 ) nanoparticles are not synthesized by using CNCs as templates, Figure 13.15k indicates a uniform distribution of organic–inorganic hybrid nanomaterials including CaCO3 nanoparticles and CNCs [76].

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13 Exploration of Other High-Value Applications of Nanocellulose

(a)

(b)

(c)

(d)

Figure 13.16 SEM images of the surface morphology of ZnO/BC composites at a magnification of 10 000× for 10.00 Zn(NO3 )/NH4 OH/BC (a), 10.00 Zn(NO3 )/SPP/BC (b), 20.00 Zn(NO3 )/SPP/BC (c), and cross-sectional morphology of 10.00 ZnAct/SPP/BC at the same magnification (d). Source: Janpetch et al. 2016 [88]. Reproduced with permission of Elsevier.

A straightforward and versatile template processing scheme for the preparation of nanocellulose/polymer composites has been reported [90]. The method follows two steps: first, the formation of a three-dimensional network template of well-individualized nanofibers, and then the template is filled with any polymer of choice. In the previous approach, it is difficult to mix unmodified cellulose whiskers with highly polar polymers or nonpolar polymers before surface modifications are made or surfactants are used [91, 92]. On the other hand, the surface-modified whiskers limit mechanical properties of nanocomposites [91, 92]. The template process reported here can solve the problem and break new ground to create other inaccessible nanocomposites with immiscible components. Figure 13.17 exhibits the synthesis pathway of polymer nanocomposites. 13.3.2

Synthesis of Magnetic Composite Aerogels

Bacterial cellulose nanofibrils possess a native cellulose crystal structure [93] with high Young’s modulus of 150 GPa, and can be readily disintegrated from plant cells; as a result, they can be available in large quantities. Olsson et al. [94] used bacterial cellulose nanofibril aerogels as the template to synthesize highly flexible magnetic aerogels and subsequently compact into a magnetic nanopaper, as observed in Figure 13.18b,c.

13.3 The Templated Materials

(i)

(ii)

(iii)

(v)

(iv)

Figure 13.17 Nanocomposite preparation by a template approach. Schematic of this template approach to well-dispersed polymer/nanofiber composites. (i) A non-solvent is added to a nanofiber dispersion in the absence of any polymer. (ii) Solvent exchange promotes the self-assembly of a nanofiber gel. (iii) The gelled nanofiber scaffold is imbibed with a polymer by immersion in a polymer solution before the nanocomposite is dried (iv) and compacted (v). Source: Capadona et al. 2007 [90]. Reproduced with permission of Springer Nature.

In view of the problem of nanoparticles aggregation in the classic synthesis method of magnetic nanoparticles-polymer composites, the aim of the process is to reduce nanoparticles aggregation and control the assemblies at high concentrations with tunable mechanical properties [95–97]. Olsson et al. used 20–70 nm thick bacterial cellulose nanofibril aerogels [93, 98] as templates for the non-aggregation growth of 40–120 nm diameter cobalt ferrite nanoparticles to form ferromagnetic inorganic/organic hybrid materials, allowing the elastic modulus to be adjusted over several orders of magnitude and high nanoparticle content in the materials if aerogels were compacted into solid films. Figure 13.18 depicts the synthesized routes. As per data listed in Table 13.4, control over the total concentration of the FeSO4 /CoCl2 salts with a fixed molar ratio could significantly affect the nanoparticle structure. Figure 13.19a exhibits that with increase in concentration of FeSO4 /CoCl2 salts the distribution of nanoparticles located on the bacterial cellulose nanofibril surfaces varied from the loose array of sample C1 to the dense array of sample C2. Simultaneously, the volume fraction and porosity of the nanoparticles changed with the concentration of FeSO4 /CoCl2 salts. Similarly, nanoparticles size increased from 40 to 60 nm and finally to 120 nm, respectively, for sample C1 to sample C3, as described in Figures 13.19b and 13.20. XRD data for samples C1, C2, and C3 in Figure 13.19b suggest the spinel phase, while at the same time high-resolution transmission electron microscopy (HRTEM) images in Figure 13.19c taken at the particle edges indicate inverse spinel lattice spacing. Additionally, Figure 13.19d shows that nanocomposites had large coercivities (μ0 hc = 95, 130, and 70 mT for C1, C2, and C3, respectively) and relatively strong saturation magnetization per ferrite volume (Ms = 400, 350, and 300 kA/m for C1, C2, and C3, respectively).

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13 Exploration of Other High-Value Applications of Nanocellulose (1) Bacterial cellulose hydrogel

(2) Dried aerogel

(3) Precursors onto template

aq.

(4) Magnetic nanoparticles aq.

aq. FeSO4/CoCl2

Freeze-drying

NaOH KNO3

Heating 2.5 μm

(a) (5) Ultraflexible magnetic aerogel

(6) Stiff magnetic nanopaper

1 μm 120 nm

Compaction

10 μm

(b)

50 μm

(c)

Figure 13.18 Synthesis of elastic aerogel magnets and stiff magnetic nanopaper. (a) Schematic showing the synthetic steps. (1) Bacterial cellulose hydrogel (1 vol%) is produced by Acetobacter xylinum FF-88. (2) Photograph, scanning electron microscopy (SEM) image, and schematic of a cellulose aerogel after freeze-drying. (3) Immersion of the dry aerogel in aqueous FeSO4 /CoCl2 solution for 15 minutes followed by heating to 90 ∘ C for 3 hours transforms soluble Fe/Co hydroxides into insoluble complexes. (4) Cellulose networks subjected to NaOH/KNO3 solutions at 90 ∘ C immediately change in color from red to orange to black as nanoparticles precipitate on the cellulose nanofibrils. (b) Representative SEM image of a 98% porous magnetic aerogel containing cobalt ferrite nanoparticles after freeze-drying. Right inset: nanoparticles surrounding the nanofibrils. Left insets: photograph and schematic of the aerogel. (c) SEM image of a stiff magnetic nanopaper obtained after drying and compression. Inset: higher magnification image. Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

In addition, tunability of the mechanical properties, large-strain magnetic actuation, and behavior of water release and deformability for hybrid nanocomposites are shown in detail in Figures 13.21 and 13.22. Therefore, it could be concluded that magnetic nanoparticles were deposited on the bacterial cellulose aerogels yielding magnetic composite aerogels, which were highly porous, flexible, and could support high particle loading when transformed into solid film. Furthermore, they could be magnetically actuated and adsorb water and release it upon compression. Owing to these unique properties, composite aerogels are promising materials to be applied in many fields requiring multifunctional characteristics. Similarly, Liu et al. [99] also proposed a facile method for the synthesis of magnetic composite aerogel, namely, in situ synthesis of CoFe2 O4 in the cellulose matrix. The preparation process involved two steps, and the first step was the preparation of regenerated cellulose hydrogel membranes. Aqueous LiOH/urea solvent was developed for cellulose dissolved using the freeze–thaw approach.

13.3 The Templated Materials

Table 13.4 Characteristics of the cobalt ferrite-based nanocomposites with different precursor concentrations. Sample

C1

C2

C3

CoCl2 (mol/dm3 )

0.033

0.055

0.066

FeSO4 (mol/dm3 )

0.066

0.110

0.132

CoCl2 + FeSO4 (mol/dm3 )

0.1

0.165

0.2

DXRD a) (nm)

43 ± 1

62 ± 2

120 ± 4

DV TEM b) (nm)

—

73 ± 15

130 ± 80

Fe:Coc) (atom%)

2.07

2.04

2.01

Fe:Cod) (atom%)

2.20

2.08

2.08

Inorganic fraction (wt%)

70.9

78.9

93.6

59

63

85

CoFe2 O4

fractione) (vol%)

a) Particle size determined from a full XRD pattern refinement. b) Particle size as determined from fitting a volume-weighted size distribution with a lognormal distribution function based on TEM micrographs. c) Particle composition as determined from energy-dispersive spectrometry. d) Particle composition as determined from inductively coupled plasma spectrometry. e) Fraction of CoFe2 O4 in the total inorganic content. Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

Note that cellulose films in wet state showed uniform porous structure, which not only acts as nano-reacting sites [100–105] but also could stabilize nanoparticles and provide channel for migration of the reactants and by-products [99]. The second step was the deposition of CoFe2 O4 nanoparticles on the surface of cellulose templates. The cellulose hydrogel films were immersed in aqueous FeCl3 /CoCl2 solution, and in the meantime, inorganic ions were impregnated into cellulose matrix through pores and interacted with cellulose. Subsequently, the addition of NaOH solution changed the color of samples from red/orange into brown/black, confirming that CoFe2 O4 nanoparticles had been successfully synthesized on the cellulose matrix. In order to investigate the effects of inorganic nanoparticles content on magnetic aerogel microstructures, composite aerogels with CoCl2 salts concentrations of 0.01, 0.03, 0.06, and 0.1 mol/l (RCF-001, RCF-003, RCF-006, and RCF-01) were prepared, and the aerogels obtained from regenerated cellulose membranes with the same method were coded as RC. As shown in Figure 13.23, RC hydrogel membranes possessed even macroporous structure; the distribution of pore size was in the mesopore and macropore scales and the SBET of cellulose aerogels was around 270 m2 /g. Compared with images of RC, Figure 13.24 exhibits the surface morphologies of the composite aerogels. CoFe2 O4 nanoparticles were deposited on the cellulose nanofibril surfaces and it could be observed that a small amount of nanoparticles played critical roles in the microstructure of the aerogels. Transmission electron microscopy (TEM) images (Figure 13.25) of the composite aerogels further revealed that large quantities of inorganic nanoparticles with small particle size were homogeneously dispersed in the cellulose aerogels. Owing to electrostatic interactions between the electron-rich oxygen atoms in

445

C2: 80 wt%

C1: 70 wt%

C3: 95 wt%

(a)

70 wt%

0.48 nm

M (kA/m)

Intersity (a.u.)

200

80 wt%

0

20

40

5 nm

(c)

–400 –4.0

–1.5 –1.0 –0.5 0.0 0.5 μ0H (T)

100

Scattering angle 2θ (°)

(b)

0 –200

–400 80

400 200

–200

BC

60

70 wt% 80 wt% 95 wt%

400

95 wt%

T = 200 °C

0.0

1.0

0.4

1.5

(d)

Figure 13.19 Magnetic aerogels at different loadings of cobalt ferrite nanoparticles. (a) SEM images (from left to right): sample C1 (70 wt% of particles), sample C2 (80 wt% of particles), and sample C3 (95 wt% of particles). Scale bars, 4 mm. (b) XRD patterns for the different C1, C2, and C3 compositions. Unmodified bacterial cellulose (BC) is shown as a reference. The vertical lines at the bottom mark the positions of the cobalt ferrite (above) and cellulose (below) peaks (JCPDS nos. 22-1086 and 03-0289, respectively). (c) HRTEM image of a single particle from sample C3 showing the lattice fringes corresponding to the reflections of the spinel structure, and the corresponding distance. The image was fast Fourier transform (FFT) filtered for clarity. (d) Magnetic hysteresis loops of cobalt-ferrite-based aerogels. Inset: hysteresis loop of cobalt ferrite-based C2 at T = 200 ∘ C. Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

13.3 The Templated Materials 0.3

0.5

Frequency

Frequency

〈D〉 = 67 nm 0.4 σ = 9 nm 0.3 0.2

〈D〉 = 91 nm σ = 21 nm

0.2

0.1

0.1 0.0 10 (a)

(c)

〈D〉 = 140 nm σ = 45 nm

0.05

100 D (nm)

100 D (nm)

1000

〈D〉 = 130 nm 0.10 σ = 80 nm

0.10

0.00 10 (c)

0.0 10

1000

Frequency

Frequency

0.15

100 D (nm)

1000

0.05

0.00 10 (d)

100 D (nm)

1000

Figure 13.20 Particle size histograms from electron micrographs. Estimated number-weighted particle sizes distributions for samples (a) C1, (b) C2, and (c) C3 obtained from SEM micrographs. The volume-weighted particle size distribution histograms for sample C3 obtained from TEM micrographs is demonstrated in (d). Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

the cellulose molecules and the electropositive transition metal cations, CoFe2 O4 nanoparticles were stabilized on cellulose nanofibrils, resulting in uniform distribution on the cellulose templates. The XRD results for cellulose aerogels and composite aerogels are presented in Figure 13.26. In contrast to RC, the diffraction peak of RCF shows significant line broadening. This result is in agreement with that of TEM images, which further clarifies that the particle size of CoFe2 O4 nanoparticles obtained by this process is small because of low crystallinity. From the data listed in Table 13.5, it can be found that as a consequence of the variation of the surface areas, porosities, densities, pore volumes, and pore sizes, CoFe2 O4 nanoparticles had strong effects on the microstructure of the cellulose aerogels. Room-temperature magnetization hysteresis curves of samples are exhibited in Figure 13.27. Nanoparticles featured superparamagnetic behavior due to the lack of hysteresis and coercivity [106, 107], which also demonstrates synthesized nanoparticles with small particle size. Figure 13.28 shows the stress–strain curves measured with compression test for RC and RCF. It is obvious that Young’ modulus as well as the maximum compression strength and the corresponding strain increased, further evidencing that

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13 Exploration of Other High-Value Applications of Nanocellulose

Magnetic aerogel

(a)

30

0

Mechanical properties Strain (%) 2 4 Magnetic nanopaper

6

0.3

0.2

20

10

0.1 Magnetic aerogel

0 0 (b)

20

40 60 Strain (%)

80

Stress (MPa)

Stress (MPa)

448

0 100

Aerogel actuation

No magnet

Water

Magnet up

(c) Water absorption Magnet down

No magnet water removed

(d)

Figure 13.21 Tunability of the mechanical properties and large-strain magnetic actuation. (a) Demonstration of the flexibility of the magnetic aerogel in the dried state. (b) Stress–strain curves for magnetic aerogels (compression) and magnetic nanopaper (tension) made based on the C2 composition. (c) A piece of magnetic aerogel is held using tweezers (left panel). Applying a small household magnet bends the magnetic aerogel upwards (right panel). (d) The magnetic aerogel bends downwards in response to the magnet and absorbs the water droplet below (left panel). The aerogel recovers its original shape upon removal of the magnet (right panel). Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

13.3 The Templated Materials

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 13.22 Water release and deformability. (a–d) Magnetic aerogel (dimensions 10 mm × 10 mm × 3 mm) with 0.3 cm3 absorbed water content (a) can be compressed (arrows in b) by a pair of tweezers. More than 95% of the water content (shown by arrows) is released upon compression (c, d). (e–g) Deformability of a magnetic aerogel into a 2–3 mm diameter ball. (h) Demonstrating the water retention capacity of the material. A sample of aerogel weighing 60 mg holds ∼1 g of water. The ball (left) was initially of the same dimension as the plate (right) before its formation. Source: Olsson et al. 2010 [94]. Reproduced with permission of Springer Nature.

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13 Exploration of Other High-Value Applications of Nanocellulose

500 nm

(a)

(b)

1500

0.030

1200

0.025 Absorption Desorption

900 600 300

0

0.0

(c)

dVR/dV(log dR)

Quantivy adsorbed (cm3/g)

450

0.020 0.015 0.010 0.005 0.000

0.2

0.4

0.6

0.8

1.0

0

(d)

Relative pressure (P/P0)

20

40

60

80

100

Radius (nm)

Figure 13.23 SEM (a) and TEM (b) images of cellulose xerogel after being freeze-dried with liquid nitrogen, and nitrogen adsorption–desorption isotherm of the cellulose xerogel (c) with the BJH pore size distribution of the cellulose xerogel (d). Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

500 nm

(a)

500 nm

(b)

500 nm

(c)

500 nm

(d)

Figure 13.24 SEM images of the composite aerogels; (a–d) were for RCF-001, RCF-003, RCF-006, and RCF-01, respectively. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

13.3 The Templated Materials

200 nm

200 nm

(a)

(b)

200 nm

200 nm

(c)

(d)

Figure 13.25 TEM images of the composite aerogels; (a–d) were for RCF-001, RCF-003, RCF-006, and RCF-01, respectively. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

Figure 13.26 XRD of cellulose and magnetic composite aerogels. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

RCF-01

Intersity (a.u.)

RCF-006

RCF-003

RCF-001 RC 10

20

30

40 2θ (°)

50

60

70

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13 Exploration of Other High-Value Applications of Nanocellulose

Table 13.5 Properties of the cellulose and magnetic composite aerogels. Sample

RC

RCF-001

RCF-003

RCF-006

RCF-01

CoFe2 O4 content (wt%)a)

—

2.30

3.89

5.35

10.36

Density (g/cm3 )

0.218 ± 0.38 0.254 ± 0.23

0.262 ± 0.46

0.311 ± 0.39

0.390 ± 0.41

Porosity (%)

82.21 ± 3.27 78.34 ± 4.06

72.78 ± 4.74

63.49 ± 3.98

52.13 ± 4.31

SBET (m2 /g)b)

270.42

298.15

317.37

305.45

323.93

Pore size (nm)c)

31.72

14.38

20.32

12.16

20.56

Pore volume (cm3 /g)c)

2.71

1.72

1.78

1.88

2.37

Young’s modulus (MPa)d)

85.23 ± 9.38 133.34 ± 11.44 166.67 ± 10.76 171.48 ± 8.27 180.76 ± 11.71

𝜆 (mW/m/K)e) 38.44 ± 1.16 34.91 ± 1.34

46.63 ± 2.08

48.55 ± 2.56

43.17 ± 2.44

a) Data from TG analysis. b) Data obtained from BET with a relative vapor pressure of 0.05–0.3 of desorption branch. c) Data from BJH desorption branch. d) Data obtained from compression test. e) Thermal conductivity values obtained at ambient conditions with thickness about 1 mm. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

0.3

0.2

0.1

Magnetization (emu/g)

0.04

Magnetization (emu/g)

452

0.02

RCF006 RCF01

0.00 –0.02 –0.04 –1000

0.0

RCF001 RCF003

RCF001 RCF003 RCF006 RCF01

–500 0 500 Applied field (Oe)

1000

–0.1

–0.2

–0.3 –10 000

–5000

0

5000

10 000

Applied field (Oe)

Figure 13.27 Hysteresis cycles of the composite aerogels at 298 K; the inset shows the initial magnetization curve as a function of applied magnetic field. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

13.3 The Templated Materials

RCF001 3.0 RCF003 2.5

Stress (MPa)

RCF01 2.0

RC RCF006

1.5 1.0 0.5 0.0 0

10

20

30 40 Strain (%)

50

60

70

Figure 13.28 Compression stress–strain curves of cellulose and composite aerogels. Source: Liu et al. 2012 [99]. Reproduced with permission of Elsevier.

even small quantities of inorganic nanoparticles facilitated improvement of the mechanical properties of cellulose aerogels. In general, cellulose nanofibrils served as suitable templates in constructing magnetic composite aerogels with improved mechanical properties and superparamagnetic behavior. As the concentration of CoFe2 O4 precursor increased, CoFe2 O4 nanoparticles content increased, but the size of particle hardly changed. Moreover, the incorporated nanoparticles had a significant influence on the cellulose aerogel microstructure. Magnetic composite aerogels were flexible, highly porous, and with large specific surface area, and hence they could be used as functional materials in many applications such as microfluidics devices and electronic actuators. In addition, Menchaca-Nal et al. [108] also used BNC as template to synthesize cobalt ferrite nanotubes. The nanocellulose templates reported here were in the form of freeze-dried BC nanoribbons, which had high affinity for water. The combination of metal precursors consisting of Fe3+ and Co2+ ions with BC is attributed to electrostatic interactions between transition metal cations and electron-rich oxygen atoms existing in the hydroxyl groups on the BC surface [99]. Subsequently, the oxyhydroxides formed functioned as nucleation points for the growth of nanoparticles, which further formed nanotubes. In this procedure, uniform nanotubes were formed at comparatively low temperature. The freeze-dried BC with three-dimensional network is shown in Figure 13.29a,c–f, clearly exhibiting the morphology of BC-templated CoFe2 Co4 nanotubes. Compared with pure BC structure, BC-templated CoFe2 Co4 nanotubes showed a much denser network with decreased porosity. It was observed that BC nanofibrils could interconnect the inorganic hollow nanotubes (Figure 13.29f ). Furthermore, BC-templated nanotubes had magnetic behavior

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13 Exploration of Other High-Value Applications of Nanocellulose

(a)

(b)

200 nm

1 μm

(c)

(d)

200 nm

200 nm

(e)

(f)

200 nm

100 nm

Figure 13.29 FE-SEM images of freeze-dried BC (a) and different representative regions of BC-templated-CoFe2 O4 nanotubes (b–f ). Source: Menchaca-Nal et al. 2016 [108]. Reproduced with permission of Elsevier.

at room temperature, and the magnetic properties were affected by a fraction of nanoparticles in the superparamagnetic state. 13.3.3

Synthesis of Inorganic Hollow Nanotube Aerogels

Korhonen et al. [87] proposed a simple method for the preparation of TiO2 , ZnO, and Al2 O3 nanotube aerogels on highly porous nanocellulose aerogel templates using ALD, followed by calcination (Figure 13.30). In the case of ALD, first the gaseous ALD precursors are released into a sample chamber one at a time to react with the surface groups of the sample, and then the second precursors are introduced and steps are repeated until the expected membrane thickness is achieved. The number of cycles rather than the deposition time can control the thickness deposited [109–112]. The advantage of ALD is that it can form homogenous films [113–119].

Overview of preparation process

Different aerogel preparation methods Liquid nitrogen freeze-drying

Liquid propane freeze-drying

Freeze-drying or supercritical drying Nanocellulose hydrogel

Nanocellulose fibril

5 μm

500 nm

Supercritical CO2 drying

5 μm

(b)

(a)

Inorganic nanotube aerogel

500 nm

Atomic layer deposition

Nanocellulose aerogel

Core–shell fibrous aerogel

5 μm

Ambient drying (reference)

Calcination at 450 °C

Inorganic–organic core-shell fiber

Inorganic nanotube

500 nm

5 μm

Photographs of aerogel samples

(1)

(2)

(4) (3)

(c)

Figure 13.30 (a) Schematic representation of the preparation processes. First, the nanocellulose hydrogel is dried to aerogel, which is then coated with inorganic oxides using ALD to form composite organic/inorganic nanofibers, and finally calcinated to inorganic hollow nanotubes. (b) SEM images demonstrating the effect of the different drying methods: freeze-drying by freezing nanocellulose hydrogel in liquid nitrogen followed by sublimation of ice in vacuum leads to aerogels with sheet-like aggregates; freeze-drying in liquid propane leads to a fibrillar aerogel with suppressed aggregation, provided the sample is sufficiently thin; supercritical drying leads to fibrillar aerogels essentially without aggregates even in thick samples; drying in ambient conditions leads to collapse of the structure. (c) Photographs of the aerogel samples: (1) Liquid propane freeze-dried aerogel of 2 mm thickness; (2) supercritically dried sample with c. 12 mm diameter and 10 mm height; (3) atmospherically dried sample, which has collapsed completely. Wet dimensions were the same as in the supercritically dried sample; (4) supercritically dried aerogel after ALD shows a slight yellow color on the surface. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

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Table 13.6 Comparison among different aerogel preparation methods. Method

Structure

Aggregation

Observations

a

Freeze-drying by immersion in liquid nitrogen and ice sublimation

Aerogel

Sheet-like aggregates

b

Freeze-drying by immersion in liquid propane and ice sublimation

Aerogel

No aggregation in thin samples

Aggregation can occur in thick samples

c

Supercritical CO2 drying from acetone organogel

Aerogel

No aggregation

No aggregation even in thick samples

d

Drying under ambient conditions

Collapsed

Sheets parallel to substrate

Paper- or plastic-like appearance

Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

The preparation of the nanocellulose aerogel templates was performed by using several kinds of drying methods. A comparison among different approaches is listed in Table 13.6, and SEM results of structures are presented in Figure 13.30. Using ALD method, uniform oxide layers were readily deposited on templates, leading to the formation of inorganic–organic aerogels, as observed in Figure 13.31. SEM and TEM images of composite aerogels exhibited diverse structures between different oxide layers. In contrast to the smooth and uniform layers of TiO2 and Al2 O3 , ZnO layers were rough owing to crystalline formation (Figure 13.32). In all cases, the fibers had not fused together to form sheets or other larger aggregation (Figure 13.32), but they represented a coated replica of the original aerogel structure [87]. The cellulose cores were removed by calcination at 450 ∘ C under air to form inorganic self-supporting nanotube aerogels. The hollow structures are shown in the SEM images of Figure 13.31. After calcination, smooth and uniform Al2 O3 layers remained (Figures 13.31 and 13.34), and ZnO layers became rougher than those after the ALD process (Figure 13.33). In addition, the TiO2 -coated fibers crystallized during the calcination (Figures 13.31 and 13.34). The inorganic nanotube aerogels could be dispersed by grinding, for instance, in ethanol to obtain a slurry of hollow nanotubes, where aggregation of nanotubes occurred. TiO2 nanotube films were cast on fluorine tin oxide (FTO)-coated glass substrates (Figure 13.35). Energy-dispersive X-ray (EDX) spectrum results (Figure 13.36) confirmed that the organic core had been thoroughly removed, while only the inorganic aerogel remained. Furthermore, it could be inferred from XRD used to evaluate the crystalline structure of the TiO2 nanotube that there was primarily anatase, which was desired after calcination.

13.3 The Templated Materials

(a)

ZnO initial layer

250 nm

(d)

ZnO nanotubes

(e)

250 nm

ZnO 50x ALD

250 nm

(c)

250 nm

(b)

50 nm

(f)

TiO2 50x ALD

250 nm

Al2O3 50x ALD

Figure 13.31 SEM (a–f ) and TEM (g–i) of ALD coated aerogels. The number of ALD cycles is indicated in each figure. (a) A thin uniform ZnO layer formed on nanocellulose fibrils after initial exposure to the zinc precursor. (b) ZnO layer thickness is increased upon the ALD process (here 50 cycles). (c) Calcinated, hollow ZnO nanotubes are visibly rough. (d) Close-ups on ZnO nanotubes show that they are hollow. (e) TiO2 nanotube aerogel. (f ) Hollow Al2 O3 nanotube aerogel. (g) A hollow TiO2 nanotube, showing some roughness due to crystallization. (h) A hollow Al2 O3 nanotube demonstrating smooth uniform coating. (i) A hollow ZnO nanotube has undergone crystallization, making the hollow tube interior difficult to resolve. The inset shows a ZnO-coated fibril before calcination. (j) An intensity profile across a hollow TiO2 nanotube. (k) An intensity profile across a hollow Al2 O3 nanotube. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

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13 Exploration of Other High-Value Applications of Nanocellulose

(g)

TiO2 50x ALD

(h)

Al2O3 50x ALD

100 nm

100 nm

(j)

(i)

14 nm 4 nm

(k) 43 nm

100 nm

ZnO 50x ALD

26 nm

Figure 13.31 (Continued)

13.3.4

The Self-assembled CNC Templates

CNCs and their suspensions have received increasing attention, not only because of being used as renewable nanomaterials but also because of the capacity for self-assembly. The rod-shaped CNC suspensions can self-assemble into chiral nematic liquid crystalline phase, namely, cholesteric phases that are distinguished by long-range order and helical modulation of the arrangement direction [120]. Iridescent structures could be observed when the CNC suspensions were coated on a solid support to produce solid films, due to the chiral nematic order of particles (Figure 13.37) [12, 122]. Reflected light of CNCs and the orientation of CNCs in the films would vary due to the helical pitch of chiral nematic structure [123]. The pitch is decreased while changing from fluid to solid state, leading to Bragg reflection of visible light from solid films; in other words, the brilliant iridescent color arises from a photonic bandgap in the materials [124]. The characteristics of CNCs such as cylindrical shape, rigidity, aspect ratio, and chiral ordering, among others, can cause optical effects in the resultant templated films or solids.

13.3 The Templated Materials

ZnO CVD

ZnO CVD+50ALD

ZnO CVD+150ALD

ZnO CVD+250ALD

Figure 13.32 Scanning electron microscopy micrographs of ZnO coated aerogels prior to calcination. Here “CVD” refers to the initial layer. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

ZnO CVD+50ALD prior to burning

ZnO CVD+50ALD after burning

Figure 13.33 Effect of calcination on ZnO-coated aerogels demonstrated by scanning electron microscopy. The surface of the fibers becomes rougher and the hollow cores are exposed after calcination. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

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13 Exploration of Other High-Value Applications of Nanocellulose

500 nm

Figure 13.34 SEM micrographs of Al2 O3 (on left) and TiO2 (on right) coated hollow aerogels freeze-dried with liquid nitrogen containing sheet-like aggregates. Sheets, which are formed of fibers, have been coated into plates and TiO2 has additionally crystallized into spherical objects and the fibers are like pearl necklaces. Empty fibers can be seen on the Al2 O3 coated sample. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

8 5 500 μm

(a)

(b)

50 μm

200 nm

(c)

(d) Ti Lα O Kα

Sn Lα

Si Kα

Ti Kα

Sn Lβ

Mg Kα

Ti Kβ cps

460

S Kα

C Kα

(e)

0

Sn

Sn

Na Kα 1

2

3

4

5

6

7

E (keV)

Figure 13.35 Films cast from a crushed hollow TiO2 nanotube dispersion in ethanol. (a) Photographs of 15 (left) and 25 μm (right) thick films. Film dimensions are 5–8 mm. (b) Tilted SEM image shows that the TiO2 film (c. 50 μm thick) with only a few cracks has formed on the substrate. (c) Larger magnification shows the edge of the film. The film consists of granules of micrometer dimensions. (d) High-magnification image shows that the granules are formed of networks of TiO2 tubes. (e) Energy dispersive X-ray spectrum shows that there is only very little carbon in the film and the TiO2 content is high. Other peaks originate from the FTO glass. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

13.3 The Templated Materials

10

Scattering angle, 2θ (°) 30 40 50

20

Intensity

Anatase (101) Rutile (110)

60

70

Anatase (200) (105) (211) Rutile (101)

Anatase (103) (004) TiO2 nanotube film

Substrate

0.5

1.0

1.5

2.5 2.0 3.0 3.5 Scattering vector, q (Å–1)

4.0

4.5

5.0

Figure 13.36 X-ray diffraction profile of a TiO2 nanotube film deposited on Si (100). Anatase is the dominant crystalline form. The pattern of the nanotubes and the pattern of the Si substrate are offset by an arbitrary amount and are not on the same scale. Source: Korhonen et al. 2011 [87]. Reproduced with permission of ACS.

The synthesis of inorganic films with an internal left-handed helical structure is carried out by employing self-assembly of CNCs as templates. MacLachlan and coworkers [125–129] first made inorganic sol–gel precursors, and cholesteric CNC suspensions became dry, and after solidification and calcination, CNCs were removed with the formation of inorganic materials, which possessed CNC-templated internal helical structure, as schematically described in Figure 13.38. The CNC-inorganic composite film showed no color before the removal of nanocellulose, but after calcination, the pure inorganic film exhibited Bragg reflection of visible light with emergence of iridescent color. It was reported that the removal of the CNCs caused reduction in helical pitch, and hence the photonic bandgap fell in the visible range. The templating concept transferred the helical structure into organic materials; moreover, there were differences in the optical response between the templated inorganic film and the pure dried CNC film. If the voids of inorganic replica film were filled with refractive index matching fluid, the reflection of light could be turned off, and by evaporating the liquid, the index matching disappeared so that the inorganic film showed color [127, 128]. Compared with the templated inorganic film, for the pure dried CNC film also with porous structures, such an ON/OFF switching for light reflex did not occur because index matching liquid did not exist for the birefringent matrix materials. As shown in Figure 13.38, the templated helical mesoporous carbon did not have interesting photonic

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13 Exploration of Other High-Value Applications of Nanocellulose

(a)

(b)

20 μm (c)

(d)

200 nm

20 μm

Figure 13.37 Optical microscope image (a, crossed polars) of a CNC film (scale bar 40 μm). SEM images of a fracture surface across the film are shown in (b), (c), and (d). In (c) and (d) the film is oriented horizontally. The white arrows in (c) and (d) indicate examples of nanocrystal bundles pulled above the fracture surface. Source: Majoinen et al. 2012 [121]. Adapted with permission of Springer Nature..

Drying with added inorganic precursor

Fluid CNC suspension Drying (slow evaporation of solvent)

Removal of CNC via calcination

Helically arranged CNC surrounded by solid inorganic matrix

Inorganic material with CNCtemplated internal helical structure

Pyrolysis of CNC

Removal of inorganic component

CNC helix from chiral nematic self-assembly

Helically arranged graphitized CNC surrounded by solid inorganic matrix

Helical mesoporous carbon

Figure 13.38 Schematic illustration of the CNC-templating of inorganic materials (as explored by MacLachlan and coworkers [125–129]) compared with the preparation of dried CNC films. The starting point is always a fluid CNC suspension (top left), which may be liquid crystalline or isotropic. By drying this without additives, a dried film with left-handed, helically arranged CNC rods is produced (bottom left). If an inorganic precursor is added before drying, the CNC helix is contained in an inorganic solid matrix (top middle). By calcinating the sample, the CNCs are removed, leaving the inorganic material with a CNC-templated internal helical structure (top right). If, instead, the CNCs are pyrolyzed into carbon (downwards track, middle) followed by the removal of the inorganic component, a helically arranged mesoporous carbon structure results (bottom right).

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13 Exploration of Other High-Value Applications of Nanocellulose

crystal properties because carbon could absorb light in the visible wavelength scales [124]. CNC-templated nanostructured films have photonic crystal properties and Bragg reflection of visible light determined by helical pitch; this has become a hot research topic. In addition, the preparation of inorganic film with internal helical structure using the templated self-assembly of CNCs offers a versatile platform for creating mesoporous inorganic materials. This general and straightforward method for producing inorganic materials could be applied to a variety of fields, especially for their optical properties. 13.3.5

Conclusion

Nanocellulose can form ductile and tough networks that are suitable as templates to synthesize materials with functional properties. Furthermore, they can direct the patterning and deposition of materials to form nanoparticles, nanowires, and nanotubes, which show unique optical and catalytic properties. The magnetic composite aerogels are highly porous, lightweight, and flexible and could be used in microfluidics and as electronic actuators. Inorganic hollow nanotube aerogels have spurred research toward drug release, sensors, and optoelectronics. Cholesteric CNC suspensions have been used as templates to prepare inorganic films with an internal left-handed helical structure. Additionally, a versatile templating route could be applied to fabricate multifunctional mesoporous materials with photonic crystal properties and very large surface areas.

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Index a absorption rate, of BC-based biomaterials 388 acetylated BC/ESO 255 acetylation-modified cellulose nanocrystals (ACNCs) 166 acetyl-CoA 87 acid half-esters 24 acid hydrolysis cellulose pretreatments 27–31 extraction of cellulose nanocrystals 21–27 aerogels 72, 433, 442–458 agar 100 agitation fermentative cultivation, BC granules 94, 95 alkali treatment 63, 64 alkynylated cellulose nanocrystals (ACNCs) 232 α-d-guluronate (G) 99 3-aminopropyl trimethoxysilane (APMS) grafting procedure 387 aminosilanes 126 ammonium persulfate (APS) 3, 185, 293 anisotropic gel phase 42, 317 anisotropic nanocomposite foams, thermal transport properties of 437 antimicrobial diffusion films (ADFs) 336–338 antimicrobial nanomaterials 382 inorganic antimicrobial agents 385 medical composite material 388

organic antibacterial agents 386–388 aqueous counter collision (ACC) 61 artificial blood vessels 81, 371–373 atomic force microscopy (AFM) 26, 64, 139 atomic layer deposition (ALD) 409, 441, 454 AU cellulose films, oxygen permeabilities 69, 70 A. xylinum 88

b bacterial cellulose 252 BC/ICPs nanocomposites 185 MnO2 nanocomposites 409 bacterial cellulose (BC) 117–118, 174, 350 additives carboxymethyl cellulose (CMC) 97 lignosulfonate 100 organic acids 97, 98 sodium alginate 99 alcohols 99 SSGO 99–100 vitamin C 97 biocompatibility 92, 372 biodegradability 92–93 biomimetic mineralization pathways 370 biosynthesis biochemical pathway 85–87 biochemistry 83–84

Nanocellulose: From Fundamentals to Advanced Materials, First Edition. Edited by Jin Huang, Alain Dufresne, and Ning Lin. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Index

bacterial cellulose (BC) (contd.) molecular regulation 87–88 chemical structure and properties 89 for dental root canal treatment 388 macrostructure control and orientation 91 microbial genera 81 nanofibrillar patch fabrication 364 physiological features 89–90 porosity and materials 91–92 production by cell-free system 86 static fermentative cultivation 93 ribbons-like structures 82 self-assembly and crystallization 90 shaking fermentative cultivation, pellets 94 for skin tissue repair 368–370 strategies agricultural and industrial wastes 103–104 food wastes 104–105 fruit juices 101 sugarcane molasses 101–103 ultrafine thin fibrous structure 90–91 for wound dressing 368 bacterial cellulose nanofibers/graphene oxide (BC-RGO) film 363 bacterial cellulose nanofibril aerogels 442, 443 bacterial cellulose scaffolds 364 bacterial nanocellulose (BNC) 1, 4, 328, 425 ball-milling 27, 54, 59, 60, 74, 124 BC/chitosan nanocomposite 184 BC/polyacrylamide (PAAm) gels 373 BC/starch nanocomposites 183 BC-templated-CoFe2 Co4 nanotubes 453 β-1,4-glucan chains 4, 81–84, 87, 88, 90 β-(1,4) glycoside linkage 1 β-1,4-glycosidic bonds 30 β-glycosyltransferase 87

bending properties, of CNF-based thermoset resin nanocomposites 255 Beycostat NA (BNA) 119 biocompatible BC tubes 372 biological macromolecules, immobilization and recognition of 355–360 biomass-based thermosetting nanocomposite 170 bio-nanocomposites 164, 165, 172, 337 birefringence 22, 39, 41, 43, 278, 282, 286, 293, 305, 306 bone tissue regeneration 370–371 bromine-based flame retardants 424 2-bromoisobutyryl bromide (BriB) 135

c carboxylate nanocellulose 7, 8 carboxymethylated BC (CM-BC) 97 carboxymethylation 4, 43, 64, 66, 118, 124 carboxymethyl cellulose (CMC) 97 carboxymethyl cellulose/ZnCdS fluorescent quantum dot nanoconjugates 360 casting method 168, 170, 178, 179, 182, 281–282, 337 cell imaging 360–361 cell scaffolds 361–366 cellulose aerogel 433 stress–strain curves 453 XRD results 447 cellulose aerogel membranes (CAMs) 72, 73 cellulose fibers 2, 3, 21, 22, 24, 25, 27, 28, 31, 32, 53, 54, 58, 61–63, 65, 83, 89, 115–117, 122–124, 201, 239, 252, 255, 433 cellulose nanocrystal-graftpolycaprolactone (CNC-g-PCL) nanoparticles 166, 167 cellulose nanocrystalline/glycerol composites1 280 cellulose nanocrystalline/polyethylene glycol composites 279

Index

cellulose nanocrystalline/silica composites 280 cellulose nanocrystals (CNC) 1 21, 115, 315 amino acid and DNA 142–144 aminosilanes 126 anisotropic phases 39 characteristics 458 chiral nematic phase 41 colloidal properties 315–324 cotton linter-derivate 3 esterification of 124 fabrication of 32 from fiber cell walls 439 fluorescent and dye molecules on 139–142 gold (Au) nanoparticle 440 H2 SO4 -hydrolyzed 36 hydrolysis, cellulose fibers 2 ecotoxicology 350 geometrical characteristics 206 iridescent films 320 inorganic nanoparticle templated synthesis 438, 440 isotropic phase 39 from lignocellulosic biomass 439 mesoporous silica (Si) nanoparticles 441 non-flocculated dispersion 119 organ distribution and bone tropism 361 percolation network formation, factors affecting 208–211 platinum (Pt) nanoparticle synthesis 440 preparation of, acid hydrolysis 21, 22 freeze-drying technology 23 HBr hydrolysis 25 microcrystalline cellulose (MCC) 24 sulfuric acid 22, 23 pretreatments of cellulose, before acid hydrolysis 27–31 self-assemble structure of 277–281 self-cross-linking 144–145

silver (Ag) nanoparticle 438, 439 silylation of 125–126 structure and properties cellulose nanocrystal suspension 39–45 physical properties 32–39 sulfonation of 121–122 CNC-templated nanostructured films 464 TEMPO-mediated oxidation mechanism of 122 TEMPO-oxidized CNC (TOCNC) 3 theranostic field 361 ureidopyrimidinone (UPy) motifs 324 vertical-assembly film 291 cellulose nanofibril (CNF) 1, 4, 21, 53, 117, 251 aqueous counter collision (ACC) 61 aerogels 72 bottom-up process 252 characteristics of 252–253 chemical bleaching 54 cryocrushing 62 from different sources 55–57 drying 253 features and properties morphology of 64 rheology 64–65 from fiber cell walls 439 as fillers, challenges of 252 films 67–70 fluorescence correlation spectroscopy (FCS) 360 food packaging applications 267, 269 grafting in water 387 from holocellulose 336 hydrogels 70–72 industrialization 269 mechanical disintegration aqueous counter collision (ACC) 61 ball-milling 59, 60 cryocrushing 62

477

478

Index

cellulose nanofibril (CNF) (contd.) grinding 58 homogenization 54–58 other methods 63 refining 62 steam explosion 61 twin-screw extrusion 62 ultrasonication 59–60 morphology 252 nanocomposites 267–269 nanostructure of 54 optical and barrier properties of 267–269 powders 66–67 pretreatment 63, 64 random plane orientation 253 sedimentation test 67 suspensions 65, 66 TEMPO-mediated oxidation 123 tensile strength 253 top-down process 252 water vapor transmission rate 267 Young’s modulus 253 cellulose nanowhisker (CNW) 1, 157, 158, 166, 169–171, 201 cellulose-negative (Cel- ) mutants 94 cellulose raw material 10, 22 cellulose sulfate 24 cellulose synthase 84–87 cellulose xerogel 450 chemically cross-linked CNC aerogels 234, 236, 237 chitin nanocrystals 41, 387 chitosan (CH) 183 chitosan-nanocellulose biocomposites 338 chlorhexidine digluconate (CHX) 328 chlorine dioxide 54 cholesteric liquid crystals 11, 39, 41, 278, 286, 288, 323, 324 cholesteric structure of CNC films 284–286 and crosslinking structure in gel 286–288 cholesteric TiO2 films 290 circular dichroism spectra, of CNC films 299

Cladophora 32, 406, 407 clay nanopaper nanocomposites 426 CNC/silver nanowires (AgNWs) composite films 304 CNF/PA nanocomposites 185 CNF/thermoplastic starch nanocomposites 182 cobalt ferrite nanoparticles 443, 444, 446 composite aerogels 435 stress– strain curves 453 XRD results 447 contact angle (CA) of carboxymethyl cellulose films 333 vs. lignin content, of MFC films 335 copper-mediated LRP 134 Cr(III)-hydrolyzed nanocellulose 30 cryocrushing 54, 62, 74, 117 crystal thickness 28

d Daphnia magna 354 DC cellulose hydrogels 70, 71 deep eutectic solvents (EDS) 63 defibrillation, of cellulose fibers 58 degradation, of glycosidic linkages 30 depolymerization of cellulose 30 deprotonation process 30 diguanylate cyclase (DGC) 85, 88 diguanylic cyclase 87 disk diffusion method 385 dimethylsulfoxide (DMSO) 41, 59 dispersive surface energy (γd ) 38 distinctive rheological properties 43 DNA immobilization 360 dodecenyl succinic anhydride-modified CNC (DCNC) 170 (2-dodecen-1-yl) succinic anhydride-CNC-PU nanocomposites 225 double-cross-linked (DC) cellulose hydrogels 70 drug carrier systems 376, 377 drug delivery system 81, 328, 349, 375, 381 bacterial nanocellulose (BNC) 328

Index

forms of 375 MFC coated paper for 328–329 drug release studies 376

e elastic aerogel magnets 444 electric double layer capacitors (EDLCs) 72, 404 electrochemical capacitors 404 electrodes, LIBs 399 electrolytes, LIBs 403 electronically conducting polymers (ECPs) 406 Entner Doudorouff (ED) 87 enzyme and protein immobilization 355, 356 epichlorohydrin (ECH) 70, 141, 185, 213 (2,3-epoxypropyl)trimethylammonium chloride (EPTMAC) 220 epoxy resin nanocomposites 169, 184 esterification reaction, CNC-polymer matrix 124, 237 Eucommia ulmoides gum (EUG) 162 evaporation-induced self-assembly (EISA) method 277, 284 expanded polystyrene (EPS) 433, 437

f FeSO4 /CoCl2 salts 443 fibrillated cellulose-filled nanocomposites natural polymer-based 182–184 polyester-based nanocomposites 178–180 polyolefin-based 172–176 polyurethane-based 180–182 rubber-based 176–178 waterborne polyurethane-based 180–182 field emission scanning electron microscopy (FE-SEM) 64, 119 fingerprint texture structure 41 fireproofing treatments 423 fire resistance

of clay nanopaper nanocomposites 426 nanoparticles for 425 FITC-labeled nanocrystals 142 flame retardant additives 424 halogenated 424 mineral 425 nanoparticles 425 nitrogen based 424–425 phosphorus-based 424 silicon-based 424 flour-rich waste (FRW) 104 fluorescence correlation spectroscopy (FCS) 360 fluorescent molecule-modified CNC (fCNC) 12 fossil fuels 397 Fourier transform infrared spectroscopy (FTIR) 168 freeze-cast nanocomposite foams 436 freeze-dried BC 453, 454 freeze-drying technology 23 fuel cells 411–413, 415

g gamma-aminopropyltriethoxysilane (APS) 340 gaseous acid 27 G. hansenii 85, 100, 102 global packaging market revenues 324 glucokinase 85 glucose-6-phosphate (G6P) 85 glucosidic bonds 23 glycerol plasticized starch based composites 210 grafting from approach 131 end modification reaction 139 living radical polymerization (LRP) 134–137 ring opening polymerization (ROP) 132–134 grafting onto method 126 graphene/CNC modified epoxy composites 169 guided bone regeneration (GBR) 370

479

480

Index

h halogenated flame retardants 424 Halpin–Kardos model 203, 205, 207, 208, 213, 214 HBr hydrolysis 25 HCl-catalyzed hydrolysis 42 Hestrin and Schramm (HS) medium 97 high-pressure homogenization 2, 4, 58, 63, 64, 252 high-pressure homogenizer 58, 339 hornification 66 Huisgen cycloaddition click chemistry 232–233 human keratinocytes (HaCaT) 92 human marrow mesenchymal stem cells (hMSCs) 363, 364 hydrobromic acid 25 hydrogel scaffolds 364 hydroxyl (OH) groups 2

i inorganic antimicrobial agents 382, 385 inorganic hollow nanotube aerogels, synthesis of 454–458 in situ polymerization 11, 157, 160, 161, 164, 165, 167, 174, 176, 185, 405, 406, 408 interfacial behaviors, CNC-polymer matrix 211 chemical coupling 237–242 CNC-poly (vinyl alcohol) nanocomposites 211, 212 EO-EPI/PVA/CNC nanocomposites 214 esterification reaction 237 functional groups effect 211–225 Huisgen cycloaddition click chemistry 232–233 Schiff’s base reaction 233–237 segmental entanglement mediated with grafted chains 225–229 surface modification method 211 thiol-ene coupling process 230–232

inter-molecular hydrogen bonding 53, 67, 89, 97, 161, 222 intramolecular hydrogen bonds 28, 53, 82 intrinsically conductive polymers (ICPs) 185 inverted sample tubes 44 ionic liquid (IL) 27, 28, 405 iridescent CNC films fracture surface across 322 poly (vinyl alcohol) incorporation 324 reflection wavelength 321 static solution casting 320 vacuum-assisted self-assembly technique 321 iridescent color control of CNC composite materials 300–302 of CNC films 298–300 isocyanation 128 isophorone diisocyante (IPDI) monomer 239

k K. sucrofermentans 104

l Langmuir–Schaeffer technique 120 lignocellulosic nanofibers (LCNF) morphology 252, 253 polymer nanocomposites 253 lignosulfonate 100 1,4-linked β-d-mannuronate (M) 99 liquid crystal phase transition 43 liquid crystal structure, of CNCs 277, 284, 294, 324 lithium ion batteries (LIBs) 72, 398 nanocellulose-based binders 403–404 nanocellulose-based electrodes 398–400 nanocellulose-based electrolytes 403 nanocellulose-based separators 401–403

Index

Li4 Ti5 O12 /CNT/NFC hybrid film 400 living radical polymerization (LRP) 134–137 luminescent CNF 354

m magnetic composite aerogels, synthesis of 442–454 maleic anhydride (MAH) grafted poly(lactic acid)/CNC bio-nanocomposite scaffolds 366 mean-field theory 202–204 mechanical properties BC/PP nanocomposite 176 CNF/chitosan films 330, 332 of CNC/CMC and mCNC/CMC films 221 of CNC composite films 295–298 of CNC/PCL nanocomposite films 225 of CNC/PEG composite films 297 of CNC/poly(2-hydroxyethyl methacrylate) composite films 298 of CNC self-assemble films 295 of CNF/polyester nanocomposites 179 of CNF/thermoset resin nanocomposites 255 for cross-linked mCNC/PBD composites 231 EO-EPI/CNC and EO-EPI/PVA/CNC nanocomposite films 214 of PVA films, CNC-reinforced 240 of thermoplastic polymers/CNF nanocomposites 255–257 semicrystalline PHO based composites 209 of waterborne polymer nanocomposites 257–258 melamine, thermal decomposition of 424 melamine-ureaformaldehyde (MUF)/CNC composites 303 melt-compounding method 159 melt extrusion process 256–257

polyethylene/CNF nanocomposites 259 mesoporous cholesteric phenol–formaldehyde resin 288 metal inorganic salts 27, 28 methyl cellulose (MC) nanofibrous mats 359 methyl silylation effect 173 microbial strains 1, 4, 82, 89, 91, 94, 100, 105 microcrystalline cellulose (MCC) 1, 24, 221, 361 microcrystalline cellulose-graft-poly (p-dioxanone) (MCC-graft-PPDO) nanomicelles 361 microfibrillated cellulose (MFC) 53, 251, 326 antimicrobial diffusion films 336–338 coated paper for delivery system 328–329 film, chemical composition effect on 334–336 microfluidization processes 63 micro-supercapacitor (MSC) 72 mineral flame retardants 425 mineral wool 433 Mn(III)-hydrolyzed nanocellulose 30 modified CNCs (mCNCs) composites 220 monocomponent endoglucanase enzyme 63 morphology-tailored BC/PPy hybrid composites 406

n nanocellulose 1 chemical modification amidation 8 esterification 6, 7 etherification 8 other chemical methods 8–9 oxidation 7, 8 physical interaction 9 ECP composite electrodes 407

481

482

Index

nanocellulose (contd.) nanocellulose-based composite materials 9–12 synthesis bacterial nanocellulose (BNC) 4 cellulose nanocrystals (CNC) 2–3 cellulose nanofibers (CNF) 3–4 nanocellulose-based separators 401–403 nanocellulose/MCNT electrode preparation 399 nanocrystalline cellulose (NCC) 2, 28, 201, 333, 372, 438 nanocrystals, thermal properties of 36 nanofibrillated cellulose 53, 68, 251, 399, 429 nanofibrillated cellulose/MoS2 /CNT film 400 nanofibrillated cellulose-nanozeolite aerogels 434 nanogenerators 414–415 nanopaper 67, 69, 253, 255, 412, 414, 432 fire resistant 425–431 magnetic 442, 444, 448 water-resistant 329–334 nanoparticles, for fire resistance 425 nanotechnology for fire retardancy 425 high-performance thermal insulation materials 434 nanotoxicology research 349 natural polymer-based nanocomposites 171–172, 182–184 neat CNC 160, 212, 218, 284–286, 294, 295 NFC based aerogels 434 NFC-based TENG 415 N-hydroxylsuccinimide (NHS) 8 niobium (NdFeB) magnets 299 nisin 338 nitrogen based flame retardants 424 N-methylmorpholine-N-oxide (NMMO) 27 non-agglomerated colloid suspensions 41

nuclear magnetic resonance (NMR) 89, 234 nucleus pulposus replacement 371, 375

o O2 barrier properties, polymers used 267 oil/water separation, CNC self-assembly 305 oligonucleotide-modified CNC 144 O2 -permeability, of CNF films 267 optical applications of CNC composites films 306–307 of CNC films 306 optical control, of CNC self-assemble gels 302–303 organic antibacterial agents 386

p PAAm/CNC nanocomposites 286, 303 packaging industry 164, 324 paper coating 326–328 PBS/CNC-foamed nanocomposite 164 PDMAEMA-grafted CNC aqueous suspensions 135 pellicle formation 4 pentose phosphate pathway (PPP) 85 percolation model 204–208, 214–216 percolation network 155, 156, 162, 201, 208–211 percolation threshold 181, 205, 206, 208, 209, 224 periodate-bisulfite sulfonation 64 periodate– chlorite oxidation 64 phosphate buffered saline (PBS) 93 phosphodiesterase A (PDEA) 88 phosphoglucomutase 85 phosphordiesterases (PDE-A and PDE-B) 87 phosphoric acid hydrolyzed cotton 26 phosphorus-based flame retardants 424 photonic crystal 284, 290, 304, 464 physical adsorption, of surfactants 119–120

Index

piezoelectric nanogenerators (PENGs) 415 pineapple peel medium (PA-BC) 104 PLA/CNF nanocomposites 256 plasmonic properties, of CNCs 304 plastic composite surface (PCS) 95 polar surface energy (γp ) 38 polyamide (PA) polymers 184, 267 polyaniline/NFC/graphene nanoplatelet (GNP) electrodes 408 polyester-based nanocomposites 164–167, 178–180 polyethylene/cellulose pulp fiber (PE/CF) nanocomposites cross-sectional morphology 265, 266 modulus values 264 optical properties 265, 266 polyethylene/CNF nanocomposites fiber size effect and lignin presence 264 morphologies 260, 262 optical properties 261, 263 Young’s modulus and tensile strength 259, 261 polyethylene/lignocellulosic nanofibers (PE/LCNF) nanocomposites cross-sectional morphology after tensile tests 265 optical properties 265, 266 tensile strength 264 polyethylenimine-grafted cellulose nanofibril aerogels 381 poly(lactic acid) (PLA) 11 polymer grafting 43, 134, 139, 178, 186 polymethylsilsesquioxane (PMSQ) aerogels 434 polymethylsilsesquioxane–cellulose nanofiber (PMSQ–CNF) composite aerogels 435 polyolefin-based nanocomposites 156 fibrillated cellulose-filled 172 in situ polymerization 160 poly (vinyl alcohol) advantages 161 processing and modification of CNCs methods 156

solution mixing method 156, 157, 159, 161 polyolefin/CNF nanocomposites 256, 259 polypropylene (PP) 10 nanocellulose reinforced 3D PPy electrodes 405 nanocellulose composite electrodes 406 polypyrrole-Cladophora cellulose composite electrodes 407 poly(S-co-BuA)-based nanocomposite films 202 polystyrene(PS)-grafted CNC 135 polystyrene sulfonate sodium (PSSNa)/nanocellulose composite 406 polyurethane (PUR) 433 polyurethane-based nanocomposites 167 fibrillated cellulose-filled 180 poly (vinyl alcohol)/CNC electrospun fiber mats 212, 213 CNC-reinforced 239 porous CNF/polypyrrole composite 388 post-sulfonation modification, of CNC 121 pressured extrusion method 400 protein-based composites 172 protofibrils 4, 82, 90 pseudocapacitors 404 pyrene fluorophore (Py-CNC) 142

q quanternary ammonium-functionalized nanocellulose/graphene oxide solid-state electrolyte 403 quasi isotropic composite 203

r reactive cellulose nanocrystals (RCNCs) 237 refining 62, 63

483

484

Index

reinforcing effect, of cellulose whiskers in poly(S-co-BuA) matrix 202 ring-opening polymerization 132–134, 165–167, 184, 361 rod-like morphology 2, 41 rod-shaped CNC suspensions 294, 458 rotating disk reactor 95 rubber-based nanocomposites 161 chemical/physical CNC modification 162 fibrillated cellulose-filled 176 fillers effect 162

s Schiff base reaction 7, 233 self-assembly, of CNCs bulk materials of 288 casting method 281–282 components of 279 evaporation-induced self-assembly 284 forms of 279–281 iridescent color properties 298–302 mechanical properties of 295–298 modifying surface chemical structure 291–295 oil/water separation 305–306 optical applications 306–307 plasmonic properties 304–305 sensor applications 307–309 spin-coating method 282 structural adjustment 284 templates 458–464 vacuum assisted self-assembly (VASA) 283–284 self-cross-linking, of CNC 144–145 semicrystalline PHO based composites 209 semicrystalline polymer 53 sensor applications 307–309 separator, LIBs 401–403 shaking fermentative cultivation 94 silica aerogels 433 silicon-based flame retardants 424 silver nanoparticle/bacterial cellulose gel membranes 385

silver sulfadiazine loaded bacterial cellulose/sodium alginate (BC/ SA-AgSD) composite films 385 silylated CN suspension 43 silylation, of CNC 126 simulated body fluid (SBC) 93, 370 single sugar-linked glucuronic acid-based oligosaccharide (SSGO) 99 skin repair materials 368 skin tissue repairing 368–370 slow water release rate (WRR) 81 small-diameter replacement vascular graft (SDRVG) 372 sodium alginate 99, 376, 385, 386 sodium alginate/CNF antibacterial composites 386 sodium chlorite 54 sodium hypochlorite 54 sodium montmorillonite-cellulose nanofiber (MTM-CNF) hybrid composites filtration processing 425 fire protection 426, 428 fire retardant features 428 flammability test 430, 431 oxygen transmission rate 429 thermal degradation 428 thermal oxidation 429 wood, fire protective coating for 426 sodium periodate (NaIO4 ) 7 sodium salicylate (NaSA) loading, of CNFs-PEI aerogels 381 solar cells 411–415 solid phosphor-tungsten acid 26, 27 solid-state shear pulverization process (sssp/S3P) 256 solution mixing method 156, 157, 159, 161, 165, 166, 179 solution plasma process (SPP) 441 solvent casting 177, 255, 256 solvolytic desulfation 36 sorbitan monostearate 120 special tissue bioscaffold 361 sphere-like cellulose particles (SCPs) 94

Index

spin-coating method 281, 282 stable turbid colloidal suspensions 24 static fermentative cultivation 93 static solution casting 320 steam explosion 57, 61, 117 stiff magnetic nanopaper synthesis 444 sub-elementary fibrils (SEF) 87 sulfonation, of CNC 121–122 sulfuric acid hydrolysis 21, 23, 24, 27, 28, 30, 39, 121, 128, 159, 337 supercapacitors (SCs) with carbonized nanofiber electrodes 409 cellulose mesoporous membrane 410–411 description 404 electric double layer capacitors 404 nano-templates 405 nanocellulose 405–406 pseudocapacitors 404 specific energy density 404 surface acetylation of CNC 39, 124, 166 surface energy (γS ) 38 surface immobilization, of enzyme/protein 355, 356 surface silylation, of CNF 125, 126 surfactants, physical adsorption of 119

t tail-like BC pellets 94 temperature-sensitive polymer-modified CNF cryogel microspheres 382 template processing scheme, for nanocellulose/polymer composites 442 TEMPO-mediated oxidation 64, 118, 122–123, 144 TEMPO-oxidized CNC (TOCN) 3, 31, 177, 294 tensile modulus of percolating filler network 206 of tunicin whisker reinforced amorphous PHO latex 209 of tunicin whisker/POE composites 210

terminal complexes (TCs) 81 tert-butanol 120 tetrahydrofuran (THF) 43, 59, 120 tetramethyl-piperidin-1-oxyl (TEMPO) 3 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) 31, 64, 116, 400, 438 2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation 64 thermal building insulation materials 432–433 thermal conductivity 432 of expanded polystyrene 433 of mineral wool 433 of NFC-zeolites 438 of PMSQ-CNF composite aerogels 435 of polyurethane 167, 433 thermal decomposition, of melamine 424 thermal stability 26, 28, 35–38, 72, 90, 95, 117, 121, 158, 159, 165–169, 174, 176, 180–186, 202, 228, 234, 253, 256, 399, 401, 403, 412 thermoplastic starch (TPS)-based composites 171, 183 thin stillage (TS) 100, 101 thiol-ene coupling process, CNC-polymer matrix 230–232 three-dimensional (3D) cell cultures 364, 365 3D hybrid CNC aerogels 408 three-dimensional (3D) network 72 three-dimensional (3D) scaffolds 81 TiO2 nanotubes dispersion in ethanol 460 film deposition on Si (100) 461 TiO2 /silica composites 290 tissue engineering 4, 72, 81, 92, 349, 364, 366–382 tissue repair and regeneration bone tissue regeneration 370–371 skin tissue repairing 368–370 tissue replacement artificial blood vessels 371–373

485

486

Index

tissue replacement (contd.) nucleus pulposus replacement 375 soft tissues, meniscus and cartilage 373–375 TOCNFs 173, 178, 181 transesterification, of CNF 124 transmembrane regions (TMDs) 88 transmission electron microscopy (TEM) 64, 89 transmittance electron microscopy (TEM) 2 treated-nanocellulose 28, 30 triboelectric nanogenerators (TENGs) 414 tricarboxylic acid (TCA) 84, 85 Trichoderma reesei 32, 102 trickling bed reactor 95 trifluoroacetic acid 27 tunicin whisker/glycerol plasticized starch (amylopectin) based composites 216 tunicin whisker percolation threshold 206 tunicin whisker reinforced amorphous PHO latex 209 tunicin whisker/sorbitol plasticized waxy maize starch composites 217, 218 twin-screw extrusion 11, 62, 74, 176, 183, 256, 257 two-dimensional (2D) scaffolds 81

u UDP-glucose pyrophosphorylase 84, 85 ultracapacitors 404 ultracentrifugation–redispersion 23 ultrafine thin fibrous structure 90–91 ultrasonication 59–60 uridine-di-phosphoglucose (UDP-glucose) 83 UV–vis spectra, of CNF/chitosan thin films 330

v vacuum-assisted resin impregnation (VARI) 255

vacuum assisted self-assembly (VASA) method 283–284, 320, 321 vacuum filtration 281, 320, 399, 400, 403, 408, 412

w wastewater from candied Jujube (WWCJ) 104 water adsorption vs. lignin content, for MFC films 334, 335 waterborne acrylate/polyurethane-based wood coating 339 waterborne epoxy resin-based nanocomposites 155, 170 waterborne polymer systems 257–258, 260 waterborne polyurethane 131, 155, 167–169, 180–182, 239 water contact angle (CA) 38, 39, 267, 305, 306, 333, 340, 435 water holding capacity (WHC) 81 water-resistant nanopaper 329–334 water-soluble fragments 25 water vapor barrier properties 326, 332, 336, 337 water vapor transmission rates (WVTR) 267, 332 wear resistance, of CNF/oil coated wood surfaces 340 wood MTM-CNF fire protective coating for 426 properties 423 wood coatings 339–341

x X-ray diffraction (XRD) 32, 89, 125, 216–218, 428, 461 xanthan 100

z ZnO-coated aerogels 459 ZnO-deposited BC composites 442

441,