Handbook of Chitin and Chitosan: Volume 2: Composites and Nanocomposites from Chitin and Chitosan, Manufacturing and Characterisations [2, 1 ed.] 0128179686, 9780128179680

Table of contents :
Handbook of Chitin and Chitosan
Copyright
Contents
List of contributors
1 Polymer blends, composites and nanocomposites from chitin and chitosan; manufacturing, characterization and applications
1.1 Introduction
1.2 Processing of chitin and chitosan blends and composites
1.2.1 Simple blending
1.2.2 Lyophilization
1.2.3 Spray drying and freeze spray drying
1.2.4 Electrospinning
1.2.5 Solution/solvent casting method
1.2.6 Melt extrusion method
1.2.7 Coprecipitation method
1.2.8 Polymerization
1.2.9 Phase inversion/separation process
1.3 Important applications of chitin and chitosan
1.3.1 Application of chitin and chitosan composites in food packing
1.3.2 Application of chitin and chitosan composites in antimicrobial activities
1.3.3 Application of chitin and chitosan composites in wound-healing activities
1.3.4 Application of chitin and chitosan composites in water treatment process
1.3.4.1 Heavy metals removal
1.3.4.2 Dye removal
1.3.4.3 Arsenic and mercury removal
1.3.4.4 Phosphate, nitrate, and humic acid removal
1.3.4.5 Fluoride removal
1.3.4.6 Nanofiltration and antibacterial process
1.3.5 Tissue engineering
1.3.6 Application of chitin and chitosan composites in drug release
1.3.7 Application of chitin and chitosan composites in bleeding control
1.4 Conclusions
Acknowledgment
References
2 Processing techniques of chitin-based gels, blends, and composites using ionic liquids
2.1 Introduction
2.2 Dissolution and gelation of chitin with ionic liquids
2.3 Fabrication of chitin-based blend and composite materials using ionic liquids
2.4 Conclusion
Acknowledgment
References
3 Processing techniques of chitosan-based interpenetrating polymer networks, gels, blends, composites and nanocomposites
3.1 Introduction
3.2 Chitosan
3.3 Types of chitosan-based materials
3.3.1 Chitosan-based interpenetrating polymer networks
3.3.1.1 Interpenetrating polymer network based on chitosan and synthetic ionic matrices
3.3.1.2 Interpenetrating polymer network based on chitosan and synthetic nonionic matrices
3.3.1.3 Interpenetrating polymer network based on chitosan and natural matrices
3.3.2 Chitosan-based gels
3.3.2.1 Chitosan gels without external cross-linkers
3.3.2.2 Ionic chitosan macrogels
3.3.2.3 Ionic chitosan micro- and nanogels
3.3.2.4 Thermosensitive gels
3.3.3 Chitosan-based blends
3.3.3.1 Synthetic polymer blended chitosan
3.3.3.2 Natural polymer blended chitosan
3.3.4 Chitosan-based composites and nanocomposites
3.3.4.1 Synthetic filler/chitosan-based composites and nanocomposites
3.3.4.2 Natural filler/chitosan-based composites and nanocomposites
3.4 Processing techniques for chitosan-based interpenetrating polymer networks and gels
3.4.1 Photopolymerization
3.4.2 Cross-linking
3.4.3 Physical interaction
3.5 Processing techniques for chitosan-based blends
3.5.1 Solution blending
3.5.2 Melt blending
3.5.3 Processing techniques of chitosan-based nanocomposites
3.5.3.1 Mechanical stirring
3.5.3.2 Solvent casting
3.5.3.3 Freeze-drying
3.5.3.4 Layer-by-layer
3.5.3.5 Electrospinning
3.6 Conclusions
References
4 Microscopic studies on chitin and chitosan-based interpenetrating polymer networks, gels, blends, composites, and nanocom...
4.1 Introduction
4.1.1 Biopolymers
4.1.2 Chitosan and chitin
4.2 Chitin and chitosan-based gels, interpenetrating polymer network, blends, and composites
4.2.1 Features of chitin and chitosan-based interpenetrating polymer network
4.2.2 Properties of chitin and chitosan-based gels
4.2.3 Characteristics of chitin and chitosan blends
4.2.4 Study of chitin and chitosan composites
4.3 Microscopic study
4.3.1 General
4.3.2 Scanning electron microscopy
4.3.3 Transmission electron microscopy
4.3.4 Optical microscopy
4.3.5 Atomic force microscope
4.4 Applications and future outlook
4.4.1 Applications of chitin and chitosan-based products
4.4.2 Future outlook
4.5 Conclusion
Acknowledgment
References
5 Thermal degradation characteristics of chitin, chitosan, Al2O3/chitosan, and benonite/chitosan nanocomposites
5.1 Introduction
5.2 Preparation of chitin, chitosan, bentonite/chitosan, and Al2O3/chitosan nanocomposites
5.2.1 Preparation of chitin and chitosan
5.2.2 Preparation of the nanocomposite of 5%bentonite/chitosan and 10%Al2O3/chitosan
5.3 Characterization of chitin, chitosan, Al2O3/chitosan, and bentonite/chitosan nanocomposites
5.3.1 Fourier-transform infrared spectroscopy
5.3.2 X-ray diffraction
5.3.2.1 Scanning electron microscopy
5.3.3 Differential scanning calorimetry
5.4 Kinetics of thermal degradation of chitin, chitosan, Al2O3/chitin, and bentonite/chitosan nanocomposites
5.4.1 Theoretical background
5.4.2 Estimation of the degradation mechanism
5.4.3 Thermogravimetric analyses
5.4.3.1 Chitin and chitosan
5.4.3.2 Nanocomposites of 5%bentonite/chitosan and 10%Al2O3/chitosan
5.4.4 Thermal degradation kinetics
5.4.4.1 Chitin and chitosan
5.4.4.2 Nanocomposites of 5%bentonite/chitosan and 10%Al2O3/chitosan
5.4.5 Degradation model
5.5 Conclusions
References
6 Barrier properties, antimicrobial and antifungal activities of chitin and chitosan-based IPNs, gels, blends, composites, ...
6.1 Introduction
6.1.1 Insect sources
6.1.2 Marine sources
6.2 Barrier properties of chitin and chitosan
6.2.1 Parameters affecting barrier properties
6.2.1.1 Chemical structures
6.2.1.2 Cohesive energy density
6.2.1.3 Free volume
6.2.1.4 Crystallinity
6.2.1.5 Orientation
6.2.1.6 Cross-linking
6.2.1.7 Degree of dispersion of the platelets
6.2.2 Permeability tests
6.2.3 Development of barrier properties of chitin and chitosan nanocomposites
6.3 Antimicrobial properties of the chitin and chitosan
6.3.1 Parameters affecting antimicrobial properties
6.3.1.1 Degree of deacetylation
6.3.1.2 Gram-positive and Gram-negative bacteria
6.3.1.3 Surface charge density of bacteria membrane
6.3.1.4 Abiotic factors
6.3.1.5 Chitosan derivatives
6.3.1.6 Molecular weights
6.3.1.7 Sources
6.3.2 Antimicrobial mechanisms
6.3.2.1 Stimulation of secondary cellular effects that lead to destabilization of the wall
6.3.2.2 Binding of low molecular weight of chitosan with DNA and RNA
6.3.2.3 Metal chelation by amine groups in chitosan chain
6.4 Antioxidant properties of chitin and chitosan
6.4.1 Parameters affecting antioxidant properties
6.4.1.1 Deacetylation period
6.4.1.2 Molecular weight
6.4.2 Mechanism of antioxidant properties
6.4.3 Antioxidant assays
6.5 Applications of chitin and chitosan
6.5.1 Packaging applications
6.5.1.1 Type of packaging
6.5.1.1.1 Active packaging
6.5.1.1.2 Smart packaging
6.5.1.2 Packaging technologies
6.5.1.2.1 Coating technology
6.5.1.2.2 Electrospinning technology
6.5.2 Dietary supplement applications
6.5.3 Agricultural applications
6.5.4 Cosmeceutical applications
6.6 Conclusions
References
7 Chitin and chitosan-based polyurethanes
7.1 General considerations
7.2 Chitin and chitosan
7.3 Polyurethanes
7.3.1 Basic reactions to obtain polyurethanes
7.3.2 Isocyanates
7.3.3 Polyols
7.4 Development methods for chitin/chitosan-based polyurethanes
7.5 Chitin and chitosan-based polyurethanes materials: characterization and applications
7.6 Concluding remarks
References
8 Chitin and chitosan-based blends, composites, and nanocomposites for packaging applications
8.1 Introduction
8.2 Biodegradable film production methods
8.2.1 Combination of biodegradable materials with synthetic polymers
8.2.1.1 Application of biopolymers as filling materials
8.2.1.2 Application of biopolymers as one of the combined base materials
8.2.2 Application of only biodegradable materials to produce polymers
8.2.3 Methods of biopackaging production
8.3 Functional properties of films
8.3.1 Mechanical properties of films
8.3.1.1 Effective factors on mechanical behavior of films
8.3.1.2 Measuring mechanical properties of films
8.3.2 Barrier properties of the films
8.3.2.1 Permeability to gases
8.3.2.1.1 Effect of different factors on gas permeability of the films
8.3.2.2 Water vapor permeability of the films
8.3.2.3 Water vapor permeability measuring method
8.3.3 Thermal properties of the films
8.3.3.1 Methods of determination thermal properties of the films
8.3.3.2 Dynamic mechanical thermal analysis
8.3.3.2.1 Differential scanning calorimetry
8.3.4 Contact angle of water drop on film surface
8.3.5 Antimicrobial properties of films
8.3.6 Antioxidant property of the films
8.4 Conclusion
References
9 (Bio)composites of chitin/chitosan with natural fibers
9.1 Introduction
9.2 Fundamentals on natural fibers
9.2.1 Physical methods of fiber treatment
9.2.2 Chemical methods of fiber treatment
9.3 Chitin/chitosan (bio)composites with natural fibers
9.3.1 Plant fibers
9.3.1.1 Flax
9.3.1.2 Kenaf
9.3.1.3 Jute
9.3.1.4 Sunflower
9.3.1.5 Rice
9.3.1.6 Carnauba
9.3.1.7 Barley
9.3.1.8 Oil palm
9.3.1.9 Sisal
9.3.1.10 Date palm
9.3.1.11 Pineapple
9.3.1.12 Cotton
9.3.1.13 Banana
9.3.1.14 Durian
9.3.1.15 Bamboo
9.3.1.16 Sugarcane
9.3.1.17 Wood
9.3.1.18 Paper
9.3.1.19 Lignocellulose
9.3.2 Animal fibers
9.3.2.1 Feathers
9.3.2.2 Wool
9.3.2.3 Silk fibroin
9.4 Conclusion
References
Index

Citation preview

HANDBOOK OF CHITIN AND CHITOSAN

HANDBOOK OF CHITIN AND CHITOSAN COMPOSITES AND NANOCOMPOSITES FROM CHITIN AND CHITOSAN, MANUFACTURING AND CHARACTERIZATIONS VOLUME 2 Edited by

SREERAG GOPI Center for Innovations and Technologies (CIT), ADSO Naturals Private Limited, Bangalore, India

SABU THOMAS Mahatma Gandhi University, Kottayam, India

ANITHA PIUS The Gandhigram Rural Institute (Deemed University), Dindigul, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-817968-0 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Editorial Project Manager: Kelsey Connors Production Project Manager: Sujatha Thirugnana Sambandam Cover Designer: Christian J. Bilbow Typeset by MPS Limited, Chennai, India

Contents

List of contributors

ix

1. Polymer blends, composites and nanocomposites from chitin and chitosan; manufacturing, characterization and applications

1

AUGUSTINE AMALRAJ, SHINTU JUDE AND SREERAJ GOPI

1.1 Introduction 1.2 Processing of chitin and chitosan blends and composites 1.3 Important applications of chitin and chitosan 1.4 Conclusions Acknowledgment References

2 4 16 36 36 36

2. Processing techniques of chitin-based gels, blends, and composites using ionic liquids

47

JUN-ICHI KADOKAWA

2.1 Introduction 2.2 Dissolution and gelation of chitin with ionic liquids 2.3 Fabrication of chitin-based blend and composite materials using ionic liquids 2.4 Conclusion Acknowledgment References

47 48 52 56 57 57

3. Processing techniques of chitosan-based interpenetrating polymer networks, gels, blends, composites and nanocomposites 61 M. MEHEDI HASAN, MD. LAWSHAN HABIB, MD. ANWARUZZAMAN, MD. KAMRUZZAMAN, M. NURUZZAMAN KHAN AND MOHAMMED MIZANUR RAHMAN

3.1 3.2 3.3 3.4

Introduction Chitosan Types of chitosan-based materials Processing techniques for chitosan-based interpenetrating polymer networks and gels 3.5 Processing techniques for chitosan-based blends 3.6 Conclusions References

v

62 63 64 73 75 83 83

vi

CONTENTS

4. Microscopic studies on chitin and chitosan-based interpenetrating polymer networks, gels, blends, composites, and nanocomposites 95 K. JAYARAJ, SREERAG GOPI, A. RAJESWARI, E. JACKCINA STOBEL CHRISTY AND ANITHA PIUS

4.1 Introduction 4.2 Chitin and chitosan-based gels, interpenetrating polymer network, blends, and composites 4.3 Microscopic study 4.4 Applications and future outlook 4.5 Conclusion Acknowledgment References

96 102 112 125 129 129 130

5. Thermal degradation characteristics of chitin, chitosan, Al2O3/chitosan, and benonite/chitosan nanocomposites

139

HAMOU MOUSSOUT, MUSTAPHA AAZZA AND HAMMOU AHLAFI

5.1 Introduction 5.2 Preparation of chitin, chitosan, bentonite/chitosan, and Al2O3/chitosan nanocomposites 5.3 Characterization of chitin, chitosan, Al2O3/chitosan, and bentonite/chitosan nanocomposites 5.4 Kinetics of thermal degradation of chitin, chitosan, Al2O3/chitin, and bentonite/chitosan nanocomposites 5.5 Conclusions References

6. Barrier properties, antimicrobial and antifungal activities of chitin and chitosan-based IPNs, gels, blends, composites, and nanocomposites

140 142 151 155 170 170

175

KHALINA BINTI ABDAN, SOON CHU YONG, ERIC CHAN WEI CHIANG, ROSNITA A. TALIB, TAN CHOON HUI AND LEE CHING HAO

6.1 Introduction 6.2 Barrier properties of chitin and chitosan 6.3 Antimicrobial properties of the chitin and chitosan 6.4 Antioxidant properties of chitin and chitosan 6.5 Applications of chitin and chitosan 6.6 Conclusions References

7. Chitin and chitosan-based polyurethanes

176 179 189 202 205 214 215

229

REJIANE DA ROSA SCHIO, EVANDRO STOFFELS MALLMANN AND GUILHERME LUIZ DOTTO

7.1 General considerations 7.2 Chitin and chitosan

229 230

CONTENTS

7.3 Polyurethanes 7.4 Development methods for chitin/chitosan-based polyurethanes 7.5 Chitin and chitosan-based polyurethanes materials: characterization and applications 7.6 Concluding remarks References

8. Chitin and chitosan-based blends, composites, and nanocomposites for packaging applications

vii 231 236 240 240 242

247

SAMAR SAHRAEE AND JAFAR M. MILANI

8.1 Introduction 8.2 Biodegradable film production methods 8.3 Functional properties of films 8.4 Conclusion References

9. (Bio)composites of chitin/chitosan with natural fibers

248 248 251 268 269

273

´ RIO COSTA, LIRIAN FERREIRA ROSA PEREIRA BOM, CAROLINA GREGO CRISTIANE REIS MARTINS, CLASSIUS FERREIRA DA SILVA AND MARIANA AGOSTINI DE MORAES

9.1 Introduction 9.2 Fundamentals on natural fibers 9.3 Chitin/chitosan (bio)composites with natural fibers 9.4 Conclusion References

Index

273 277 280 294 294

299

List of contributors Mustapha Aazza Laboratory of Chemistry/Biology Applied to the Environment, Faculty of Sciences, Moulay Ismaı¨l University, Meknes, Morocco Khalina Binti Abdan Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia; Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia Hammou Ahlafi Laboratory of Chemistry/Biology Applied to the Environment, Faculty of Sciences, Moulay Ismaı¨l University, Meknes, Morocco Augustine Amalraj Cochin, India

R&D Centre, Aurea Biolabs Private Limited, Kolenchery,

Md. Anwaruzzaman Department of Applied Chemistry and Chemical Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalgonj, Bangladesh Lirian Ferreira Rosa Pereira Bom Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil Eric Chan Wei Chiang Department of Food Science with Nutrition, Faculty of Applied Sciences, UCSI University, Cheras, Kuala Lumpur, Malaysia Carolina Grego´rio Costa Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil Rejiane da Rosa Schio Chemical Engineering Department, Federal University of Santa Maria, Santa Maria, Brazil Classius Ferreira da Silva Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil Mariana Agostini de Moraes Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil Guilherme Luiz Dotto Chemical Engineering Department, Federal University of Santa Maria, Santa Maria, Brazil Sreerag Gopi Department of Chemistry, The Gandhigram Rural Institute— Deemed to be University, Dindigul, India

ix

x Sreeraj Gopi India

List of contributors

R&D Centre, Aurea Biolabs Private Limited, Kolenchery, Cochin,

Md. Lawshan Habib Department of Applied Chemistry and Chemical Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalgonj, Bangladesh Lee Ching Hao Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia M. Mehedi Hasan Department of Applied Chemistry and Chemical Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalgonj, Bangladesh Tan Choon Hui Department of Food Science with Nutrition, Faculty of Applied Sciences, UCSI University, Cheras, Kuala Lumpur, Malaysia E. Jackcina Stobel Christy Department of Chemistry, The Gandhigram Rural Institute—Deemed to be University, Dindigul, India K. Jayaraj Department of Chemistry, The Gandhigram Rural Institute— Deemed to be University, Dindigul, India Shintu Jude India

R&D Centre, Aurea Biolabs Private Limited, Kolenchery, Cochin,

Jun-ichi Kadokawa Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan Md. Kamruzzaman Department of Applied Chemistry and Chemical Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalgonj, Bangladesh M. Nuruzzaman Khan Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh Evandro Stoffels Mallmann Chemical Engineering Department, Federal University of Santa Maria, Santa Maria, Brazil Cristiane Reis Martins Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil Jafar M. Milani Department of Food Science and Technology, Agricultural Sciences and Natural Resources University, Sari, Iran

Sari

Hamou Moussout Laboratory of Chemistry/Biology Applied to the Environment, Faculty of Sciences, Moulay Ismaı¨l University, Meknes, Morocco Anitha Pius Department of Chemistry, The Gandhigram Rural Institute— Deemed to be University, Dindigul, India Mohammed Mizanur Rahman Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh

List of contributors

xi

A. Rajeswari Department of Chemistry, The Gandhigram Rural Institute— Deemed to be University, Dindigul, India Samar Sahraee Department of Food Science and Technology, Sari Agricultural Sciences and Natural Resources University, Sari, Iran Rosnita A. Talib Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia Soon Chu Yong Department of Food Science with Nutrition, Faculty of Applied Sciences, UCSI University, Cheras, Kuala Lumpur, Malaysia

C H A P T E R

1 Polymer blends, composites and nanocomposites from chitin and chitosan; manufacturing, characterization and applications Augustine Amalraj, Shintu Jude and Sreeraj Gopi R&D Centre, Aurea Biolabs Private Limited, Kolenchery, Cochin, India

O U T L I N E 1.1 Introduction

2

1.2 Processing of chitin and chitosan blends and composites 1.2.1 Simple blending 1.2.2 Lyophilization 1.2.3 Spray drying and freeze spray drying 1.2.4 Electrospinning 1.2.5 Solution/solvent casting method 1.2.6 Melt extrusion method 1.2.7 Coprecipitation method 1.2.8 Polymerization 1.2.9 Phase inversion/separation process

4 4 10 11 11 12 14 15 15 16

1.3 Important applications of chitin and chitosan 1.3.1 Application of chitin and chitosan composites in food packing 1.3.2 Application of chitin and chitosan composites in antimicrobial activities 1.3.3 Application of chitin and chitosan composites in wound-healing activities

16 17

Handbook of Chitin and Chitosan DOI: https://doi.org/10.1016/B978-0-12-817968-0.00001-9

1

23 24

© 2020 Elsevier Inc. All rights reserved.

2

1. Introduction

1.3.4 Application of chitin and chitosan composites in water treatment process 1.3.5 Tissue engineering 1.3.6 Application of chitin and chitosan composites in drug release 1.3.7 Application of chitin and chitosan composites in bleeding control

26 31 35 35

1.4 Conclusions

36

Acknowledgment

36

References

36

1.1 Introduction Recently, the emphasis on environment-friendly technology has stimulated interest in biopolymers and biobased polymers, due to their functionality and greater biodegradability than the synthetic polymer counterpart. The biobased polymers have received increasing attention because of their outstanding physical and biological properties. Chitin (CN) is a polysaccharide and abundant organic compound, being the second most common biopolymer in nature in the world after cellulose. CN is commonly found in invertebrates, as crustacean shells or insect cuticles, but is also present in the vast majority of fungi, some mushrooms envelopes, green algae, cell walls, and yeasts [1]. Several million tons of marine waste are produced every year, which are very hazardous for the environment due to their high biological and chemical oxygen demand, pathogens, organic matter, etc. The waste produced by the marine foods and marine food industries, the shells of shrimp, crab, and other crustaceans, is the major commercial source of CN. CN is a linear aminopolysaccharide consisting mainly of β-(1-4)-linked 2-acetamido-2-deoxy-β-D-glucopyranose units and partially of β-(1-4)-linked 2-amino-2-deoxy-β-D-glucopyranose [1,2]. The structures of CN and chitosan (CS) are depicted in Fig. 1.1. CN is insoluble in common solvents because of its high crystalline structure and hydrogen bonding between carbonyl, hydroxyl, and acetamide groups. CN has a strong intra- and intermolecular hydrogen bonding network, which provides the insoluble property of this polymer in common organic and inorganic solvents [1,3]. However, it is soluble in highly concentrated inorganic acids, such as hydrochloric acid, sulfuric acid, and phosphoric acid. CS is the most important derivative of CN and is prepared by the alkaline deacetylation of CN [4]. CS’s structure comprises β-1,4-linked 2-amino-2deoxy-β-D-glucose (deacetylated D-glucosamine) and N-acetyl-D-glucosamine

Handbook of Chitin and Chitosan

1.1 Introduction

3

FIGURE 1.1 Sources and extraction of chitosan.

units [5]. CS is insoluble in water and in most organic solvents; in contrast, it is soluble in most aqueous acid solutions such as acetic, citric, formic, lactic acids, etc., below its pKa (pH 5 6.5), and in some other solvents such as dimethylsulfoxide, p-toulene sulfonic acid, and 10-camphorsulfonic acid [1,6]. CS is mainly characterized by the deacetylation degree—when the acetylation degree is less than 50%, it is named CS [7,8]—and the molecular weight. These parameters strongly affect many physicochemical and biological properties of CS, such as solubility, hydrophilicity, crystallinity, and cell response. Each D-glucosamine unit contains a free amino group, and these groups can take on a positive charge which gives the important properties of CS such as solubility and antimicrobial property. These groups make a good chelating ligand capable of binding to a variety of metal ions and can adsorb the dye anions by electrostatic attraction. Additionally, these amino groups might be protonated providing solubility of CS in diluted acidic solutions [9,10]. The presence of multiple functional groups in this polymer, such as hydroxyl and amino groups, on its polysaccharide chain provides the flexibility for preparing molecularly imprinted polymers and for structural modifications [11]. According to the US Food and Drug Administration (USFDA), it is a GRAS (Generally Recognized as Safe) material and therefore it has found a wide range of pharmaceutical and biomedical applications [12]. CS has been extensively studied for various applications because of its biocompatibility, biodegradability, mucoadhesiveness, and derivability from abundant and inexpensive biomass [13]. It also possess antimicrobial activity, wound-healing properties, and hemostatic activity, which make CS-based composites very useful in the biomedical field. Moreover, it has also been used in agriculture as a fertilizer, in food as a stabilizer and thickener, and in water treatment as an ion exchanger [14]. In addition. CS is readily processed into nanofibers, sponges, nanoparticles (NPs), gel, beads, scaffolds, membranes, and standalone films

Handbook of Chitin and Chitosan

4

1. Introduction

[15]. The main advantage of CN and CS is the ease with which they can be processed into different forms like beads, gels, microparticles, NPs, nanofibers, scaffolds, etc. [16].

1.2 Processing of chitin and chitosan blends and composites CH and CS have many applications, thus various forms and structures are needed. As CS is soluble in aqueous acidic media, it is able to form films and scaffold by the solution casting method and simple blending techniques and it is also used in food industries and nutraceutical and biomedical fields, Therefore the spray drying/freeze-drying method has also been used. The different techniques for processing CN and CS blends and composites are shown in Fig. 1.2 and summarized in Table 1.1.

1.2.1 Simple blending A simple blending method is widely used which to combine two or more polymers by solution blending or extrusion blending. The binary polycaprolactone (PCL)/CS blends were fabricated by experimentation and a dissipative particle dynamics simulation with homogeneous morphology and excellent ductility, which indicated that the combination of experiments and computer simulation can be an effective way to design and develop polymer blends with tailored properties for advanced

FIGURE 1.2 Various techniques for processing chitin and chitosan blends and composites.

Handbook of Chitin and Chitosan

TABLE 1.1

Various techniques for processing chitin and chitosan blends and composites and their applications.

Handbook of Chitin and Chitosan

Methods

Formulations

Applications

References

Simple blending

PCL CS

Technology applications

[17]

CS agarose

Biomedical

[18]

CS PVA

Biomedical

[19]

CS microcapsules/starch

Drug delivery system

[20]

CS PEGME

Biomedical applications

[21]

Controlled drug delivery Zinc ion cross-linked alginate NSC

Therapeutic applications

[22]

Drug delivery system CHC LMW HA

Antiapoptosis injectable system

[23]

CS COL HA HAp

Bone scaffolds

[24]

Corneal epithelial cells/CMC /gelatin (GL)/HA

Tissue engineering

[25]

CS PEG

Seed germination

[26]

Carboxymethyl cellulose CS

Food packaging

[27]

CS sodium caseinate

Food preservation

[28]

Rice starch CMC

Biodegradable edible films

[29]

CS eggshell membrane GL

Packing material

[30]

CS poly(allylamine)

Industrial CO2 separation

[31] (Continued)

TABLE 1.1

(Continued)

Methods

Lyophilization Handbook of Chitin and Chitosan

Formulations

Applications

References

CS guar gum AgNPs

Environmental

[32]

CS zeom PVA PEG

Edible films, compatibility and functionality enhancer

[33]

Buccal CS composite sponges

Drug delivery

[34]

CS COL HA

Cartilage tissue damage treatment

[35]

CS aconitic acid chloramphenicol and CS trimellitic anhydride chloramphenicol

Topical wound delivery of chloramphenicol

[36]

HAp CS poly

Bone tissue regeneration

[37]

D

L lactide co glycolide

Spray dying/spray freezedrying

CS PVA PLA

Food packaging, textile, medical

[38]

Electrospinning

PLA PCL CS

Tissue regeneration and drug delivery

[39]

CS GL

3D tissue engineering

[40]

CS GL PCL

Skin regeneration

[41]

CS arginine

Wound healing

[42]

CS PEO

Packaging material

[43]

CS polyacrylamide

General applications

[44]

CS PEO AuNPs

Analysis

[45]

CS PEG

Controlled insulin release

[46]

POC CS

Tissue engineering

[47]

Solution/solvent casting

Handbook of Chitin and Chitosan Melt extrusion

CS poly(N vinyl 2 pyrrolidone)

Wound healing

[48]

CS eggshell membrane

Antibacterial

[49]

CS CL glycosaminoglycans

Biomedical

[50]

HTCC PVA sodium carboxymethyl cellulose

Biomaterials for medical applications

[51]

CS PVA

Medical applications

[52]

CMC PVA Cu

Packaging and biomedical materials

[53]

CS pectin

Medicine, agriculture, food packaging

[54]

COL HA CS

Packaging

[55,56]

CS GL

Food packaging

[57]

Curcumin CS

Food packaging, agricultural

[58]

CS gallic acid PVA

Food packing

[59]

CS PVA

Membrane bioreactors

[60]

Battery fabrication

[61]

CS PEO

Electric applications

[62]

CS starch

Electrochemical

[63]

CS corn starch

Packaging

[64]

CS PLA PVA

Packaging

[65]

Starch PE maleic anhydride CS

Packaging

[66]

PET CS

Packaging

[67] (Continued)

TABLE 1.1 (Continued) Methods Handbook of Chitin and Chitosan

Coprecipitation Polymerization

Phase inversion/separation techniques

Formulations

Applications

References

Lignin CS

Organic pollutant removal

[68]

CS HAp

Tissue engineering

[69]

Polyurethane CS curcumin

Biomedical

[70]

CS PVA

Drug delivery, tissue engineering

[71]

COL CS HA

Antibacterial

[33]

CS ethylene diamine functionalized polymers

Fluoride removal

[72]

PVA CS

Biomedical and electronic

[73]

CS PVDF

Environmental applications

[74]

CS PVA

Antibacterial, air filtration

[75]

1.2 Processing of chitin and chitosan blends and composites

9

technology applications [17]. Highly porous CS agarose blend scaffolds were prepared for use in biomedical applications like soft tissue repair due to the enhancement of their mechanical and swelling performance by the interaction between agarose and CS via hydrogen bonding [18]. CS was cospun with polyvinyl alcohol (PVA) as a nanofiber to use as a delivery system for genistein due to its reduced degradation rate of nanofibers and controlled/sustained genistein release from nanofibers and nontoxicity on normal human fibroblast cells making it safe for use in biomedical applications [19]. CS-microcapsules/starch blend films for antofloxacin delivery were prepared with improved thermostability, mechanical and morphological properties. The improved performance of the films and drug release behavior indicated that the CS-microcapsules/ starch blend films could be a promising candidate for a drug delivery system [20]. Smart drug release films were prepared by blending CS with polyethylene glycol methyl ether (PEGME) for controlled drug release applications. The blended films were tested for Metformin hydrochloride release ability, which indicated that there was good compatibility between the drug and film matrix, thus showing that the prepared blends are suitable candidates and can be employed for controlled drug release and other biomedical applications [21]. Zinc ion cross-linked alginate/N-succinyl CS (NSC) blend microspheres were developed for the codelivery of zinc and 5-aminosalicylic acid for synergistic therapy of colitis, which showed a superior treatment effect in alleviating inflammation of colitis rats without systemic toxicity. The blends might be employed as a safe and effective carrier for the clinical treatment of inflammatory bowel diseases [22]. An in situ forming gel based on simply blending carboxymethyl hexanoyl CS (CHC) with low-molecular-weight hyaluronic acid (LMWHA) was developed, without needing cross-linking, photopolymerization, or thermal treatments. Berberine, a naturally occurring antiapoptotic and antiarthritis compound, was encapsulated within the CHC with LMWHA gels, which had a marked protective effect against the apoptosis of chondrocytes caused by sodium nitroprusside, making it an ideal injectable system for inhibiting chondrocytes apoptosis and preventing the progression of cartilage degeneration [23]. Three-dimensional (3D) porous composites based on the blend of CS, collagen (COL), and HA with the addition of nanohydroxyapatite (HAp) were prepared with improved mechanical and thermal properties with interconnected pores; they can be used in scaffold application in the treatment of bone defects [24]. The combination treatment of corneal epithelial cells/carboxymethyl CS (CMC)/gelatin (GL)/HA blended membranes could improve corneal wound healing and restore normal structure in the rabbit model of a corneal alkali burn wound, providing an opportunity for developing a new approach to corneal epithelial reconstruction and treatment based on tissue engineering methods [25].

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1. Introduction

The CS polyethylene glycol (PEG) blended filmogenic solution with plasticizer can be used as a carrier for Trichoderma strains to be applied as a seed-coating material, which was effective in enhancing seed germination and plant growth of castor [26]. Carboxymethyl cellulose incorporated films based on quaternized CS were developed by simple mixing of a coating to preserve food to improve the microbiological safety. Bananas coated in quaternized CS/carboxymethyl jhmyucellulose blend films had a longer shelf life than uncoated bananas, which suggested that the films can be used as food packaging materials [27]. The CS and sodium caseinate blend films were prepared with different concentrations to investigate the effects of solid concentration on structure and properties. The blend films showed improved mechanical properties with pseudoplastic behavior due to the interaction between the CS and sodium caseinate. These blends can be used as food coating materials for food preservation [28]. Rice starch-based blend films that incorporated with CMC had good mechanical properties, transparency, and thermal stability due to the interaction between the -OH group of rice starch and -COO groups of CMC, which indicated that the rice starch/CMC films possess potential as biodegradable edible films [29]. Composite edible films were prepared by eggshell membrane GL and CS with good mechanical, thermal, and barrier properties, which demonstrated that these high-potential properties can be used in packaging materials to improve food quality [30]. CS poly(allylamine) active layers were prepared for facilitated CO2 transport due to their superior CO2 permeability and CO2/N2 selectivity, which can be used for industrial CO2 separation [31]. CS guar gum blend silver NP (AgNPs) bionanocomposite was synthesized by incorporating palm shell extract-capped AgNPs during the formation of the blend. The biocomposite was highly facile for catalytic degradation of environmental pollutants such as single and binary mixture of dyes as well as nitrophenol [32]. Different edible films were prepared by blending zein, CS, PVA, and PEG using the Box Behnken design. These films showed good thermal and mechanical properties due to potential interactions through hydrogen bonding between the polar groups of zein and hydroxyl groups of PVA and CS, suggesting the effectiveness of the blending technique in improving the compatibility of biopolymers and the overall functionality of edible films [33].

1.2.2 Lyophilization Lyophilization or freeze-drying is a dehydration process, where the material is subjected to freezing, followed by sublimation under lower

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1.2 Processing of chitin and chitosan blends and composites

11

pressure. Lyophilization is important in many fields, mainly due to the exact retention of the product properties even after storage. Sixteen CS-based composite sponges were prepared by a lyophilization technique with blending of five different polymers—hydroxyl propyl methyl cellulose, sodium carboxymethylcellulose, carbopol, sodium alginate, and HA—with three different CS polymer ratios. They showed high hardness values, porosity, swelling index, and enhanced mucoadhesion and could be considered promising platforms for buccal delivery of drugs [34]. 3D porous composites based on blends of CS, COL, and HA were prepared through the lyophilization process, which can be used in tissue engineering, especially for treating cartilage tissue damage, due to their elasticity with good mechanical, thermal, and swelling properties and improved cell attachment [35]. Superhydrophilic and high-strength chloramphenicol-loaded foams were prepared by a freezedrying method based on CS blends with modified CS grafted with transaconitic acid and trimellitic anhydride for topical wound delivery of chloramphenicol with increased mechanical and swelling ability and adequate antibacterial activity [36]. Utilizing the solvent/nonsolvent precipitation method, along with freeze-drying processing, HAp-NPs were synthesized and subsequently coated with CS and CS poly-D-L-lactide-co-glycolide polymer blend. Coating HAp with a polymeric blend composed of CS and poly-D-L-lactideco-glycolide led to a decrease in the reactivity and antimicrobial activity of the composite particles, and to an increase in the quality of the newly formed bone tissue in the reconstructed defect area [37].

1.2.3 Spray drying and freeze spray drying By spray drying, powder form of the product can be obtained from a slurry or solution, which is atomized and directed to a drying chamber where it is encapsulated with hot gas. The dry powdered form of product obtained and consistent particle size distribution etc. have made it a popular technique in different industries. In the case of thermosensitive compounds, the atomized droplets are sprayed into a cryogenic medium and are frozen rapidly. These frozen droplets are subjected to lyophilization to obtain the product, and the technique is termed as freeze spray drying. Thermoplastic blends of PVA-CS in a polylactic acid (PLA) matrix were produced by spray and freeze-drying techniques to contribute to increasing and extending the applications of CS in various fields, such as food packaging, medicine, and textiles [38].

1.2.4 Electrospinning The electrospinning technique is also a widely used technique, in which a polymer melt or solution is charged under electric field and

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12

1. Introduction

ejected through a spinneret to form ultrafine fibers. Homogeneous electrospun polylactide-co-PCL (PLA-PCL)/CS blend nanofibers were prepared by an electrospinning technique for tissue regeneration and drug delivery with improved hydrophilicity, cell attachment, and proliferation without losing the entangled nanofibers structure and stability [39]. GL-blended CS nanofiber mats with enhanced hemostatic properties were fabricated using a versatile electrospinning method, and are promising candidates for use as 3D tissue engineering scaffolds as well as hemostatic wound dressing due to their synergistic effect [40]. Nanofibrous scaffolds were prepared by electrospinning blends of CS, GL, and PCL and possess the most promising physicochemical and biological properties regarding their envisaged use in skin regeneration [41]. An electrospun membrane comprising deacetylated/arginine modified CS was produced by an electrospinning technique and used as a wound dressing; it has a highly hydrophilic, nontoxic, and porous 3D nanofibrous network similar to that found in human native extracellular matrix. The membrane significantly improved antibacterial and wound-healing activities by tissue regeneration of full thickness wounds [42]. Electrospun nanofibers were produced by the blending of CS and polyethylene oxide (PEO) via an electrospinning method and they exhibited potential antibacterial activity and high thermal stability that therefore allowed their application as an active packaging material [43].

1.2.5 Solution/solvent casting method The solution or solvent casting method is one of the most widely used methods to form various blends and composites, particularly in CS processes. Solvent casting produces high-quality films or composites by dissolving different polymers into a solution and particles/molds with specific dimensions are added to this solution. The solvent is evaporated to obtain a composite/scaffold with the required specifications. CS/polyacrylamide and CS/partially hydrolyzed polyacrylamide blends were prepared by a casting technique. These blends had a good miscible property and mechanical strength due to the intermolecular interactions between the amide groups of the polyacrylamide and hydrolyzed polyacrylamide and the hydroxyl or amino groups of CS [44]. Binary blends of CS and PEO containing gold NPs (AuNPs) were prepared by a solution casting method and had a smooth and homogeneous surface due to the interaction between acetyl and hydroxyl groups and Au atoms in the CS/PEO/AuNPs [45]. A biodegradable thiolated CS-PEG blend incorporated with medical clay, montmorillonite composites for the oral delivery of insulin was formulated by a solution casting method; the insulin release was much

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1.2 Processing of chitin and chitosan blends and composites

13

more pronounced in the basic medium than in the acidic medium, moreover, the composite material posed no threat to the body’s systems, thereby making the material a potential candidate for the controlled release of insulin [46]. Biocompatible blend films of poly(1,8-octanediol citrate) (POC)/CS were prepared by a solution casting method with a homogenous surface, good miscibility, and improved mechanical and thermal properties. The POC/CS blends well supported human dermal fibroblast cells attachment and proliferation and thus can be used for a range of tissue engineering applications [47]. Biopolymer-based stimuli-responsive CS and poly(N-vinyl-2-pyrrolidone) blend hydrogels were prepared by a solvent casing technique in the presence of neutralized polyacrylic acid for wound-healing applications; they showed good and antibacterial activity and enhanced responsive swelling behavior and also exhibited controlled release of drugs [48]. CS and eggshell membrane blend films were prepared using the film casting method for wound care dressing and cell and animal experiments due to their improved water resistance, wound fluid and protein absorption capacity, and antibacterial activity [49]. Thin films based on CS and COL with the addition of glycosaminoglycans isolated from fish skin were prepared by a solvent evaporation technique with good mechanical and thermal properties and resistant to degradation, which revealed that the films can be used to create biocompatible coatings for biomedical applications [50]. Blends of N-(2hydroxyl) propyl-3-trimethylammonium CS chloride (HTCC), PVA, and sodium carboxymethylcellulose were prepared by the solution casting method with improved strength, flexibility, enhanced water absorption capacity, improved swelling ratio, appropriate moisture permeability, and good antibacterial activities, revealing that these blends have the potential to be used as biomaterials in medical applications [51]. Binary blends of PVA and CS were prepared by a solution casting method followed by exposure to gamma radiation to enhance medical strength and plastic effect by inter- and intramolecular hydrogen bonding and the adhesive nature of the blend, which can be used for various medical applications due to its good blood compatibility [52]. Antibacterial quaternized CMC/PVA/Cu blend film was prepared by quaternary ammonium salt modified CMC, PVA, and copper sulfate pentahydrate via the process of solution casting and ion adsorption. These blends had acceptable cell viability, biocompatibility, and good antibacterial activity, which indicated that these blend films had potential applications in packaging and biomedical materials [53]. Biobased CS/pectin blend films were prepared by a solution casting method, which showed enhanced water resistance and thermomechanical properties due to the electrostatic interaction of CS/pectin which can be used for specific applications in medicine, agriculture, and food packaging [54].

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1. Introduction

Thin films of COL/HA/CS blends prepared by a casting technique had good hydrophilicity and enhanced mechanical and thermal stability due to the cross-linking interaction of COL with HA and CS [55,56]. Blend and bilayer biobased active films were developed by a solvent casting technique, using CS and GL as biopolymers, glycerol as a plasticizer, and lauroyl arginate ethyl as an antimicrobial compound; they had high tensile strength and elastic modulus and low water vapor permeability and good antibacterial activity. These blend and bilayer films can be used as a good potential material in the development of biodegradable and renewable packaging with an additional bioactive function to ensure food safety and to extend the shelf life of foods [57]. Curcumin CS blend films were prepared by solution casting with excellent tensile strength and antibacterial activity for food packaging and agricultural products storage [58]. Active blend films from CS gallic acid and PVA were prepared via a simple mixing and casting methods through the addition of citric acid as a plasticizer; they have potential for use as food packing materials due to their antibacterial activities [59]. The blend membranes with varying weight ratios of CS/PVA were prepared using a solvent casting method and were evaluated for their potential application in a single-use membrane bioreactor. The CS/PVA blend membrane showed enhanced membrane flexibility, reduced water uptake, less protein sorption as a potential surface for adhesion and proliferation with possible application in single-use membrane bioreactors [60]. High conductivity, plasticized CS-PVA blend polymer electrolytes were prepared by the solution casting technique, and can be used to fabricate several batteries [61]. Solid polymer electrolytes based on CS and PEO with ammonium iodide as an ion donor were prepared by the solution casting method. Blending CS with PEO increased the flexibility of electrolytes and ion mobility thus leading to an increase in conductivity [62]. Biodegradable polymer electrolyte comprising the blend of CS and starch, plasticized with glycerol as the host polymer and lithium perchlorate as a dopant was prepared by a solution casting method; it showed good electrochemical performance, such as high energy density and specific capacitance [63].

1.2.6 Melt extrusion method One or more polymer particles are subjected to temperature and pressure to melt them and passed through an extrusion orifice. This technique produces products with uniform shape and density. Blends of thermoplastic corn starch and CS were produced by the melt extrusion method; they had good thermal stability due to the addition of CS, enhanced elongation at break, and decreased tensile strength and

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1.2 Processing of chitin and chitosan blends and composites

15

elastic modulus, revealing that the blends have potential for packaging applications [64]. CS was blended with PLA and PVA without any chemical modification by the melt process. The process allows the production of bioactive blends of CS with water-insoluble polymers under conventional processing conditions used for thermoplastic materials [65]. Thermoplastic starch/polyethylene-graft-maleic anhydride/CS blends were prepared by the melt-blending method and they can be applied as packaging materials due to their enhanced miscibility and improved mechanical properties [66]. Blends of commercial and recycled polyethylene terephthalate (PET)/PLA and PET/CS were prepared by the extrusion process with good thermal properties by the interaction between PET matrixes and CS via hydrogen bridges or electrostatic forces; these were comparable to commercial bottles of BioPET, which used higher amounts of biopolymer materials [67]. Activated lignin CS extruded pellets were prepared by extrusion and a thermal activation method with controlled particle size distribution, offering great potential for removing cationic organic pollutants like methylene blue [68].

1.2.7 Coprecipitation method Coprecipitation method is a technique for simultaneous precipitation of a soluble compound along with another component from a solution, by forming a composite. CS-wrapped HAp nanorods were in situ synthesized by a coprecipitation method with enhanced tensile properties. The biostability of the thermoplastic polyurethane polydimethylsiloxane blend matrix together with the osteoconductive CS-HAp assured a better performance in bone and cartilage tissue engineering application by the synergistic effect of physicochemical interaction and superior crystallization behavior [69].

1.2.8 Polymerization Polymerization is the reaction where monomer units combine together to form a larger chain or network polymers. Hexamethylene diisocyanate and hyroxylterminated polybutadiene-based polyurethanes were prepared following step-growth polymerization by the introduction of varying mole ratios of CS and curcumin. These polyurethanes-based natural polymer blends showed good miscibility, improved thermal and mechanical stability, and antibacterial activity, revealing that these blends can be used for prospective biomedical applications [70]. CS/PVA blends were fabricated by the cross-linking of tetraethoxy silane and showed good viscoelastic, thermal, and biomechanical properties. The CS/PVA blends can be employed for drug delivery systems, tissue engineering, and other biomedical applications [71]. Polymeric blends based on COL, CS, and HA in the form of thin

Handbook of Chitin and Chitosan

16

1. Introduction

films with the addition of gentamicin sulfate were obtained, which inhibited the growth of both Gram-negative and Gram-positive bacteria [33]. CS blended with ethylenediamine functionalized synthetic polymer, that is, acrylonitrile/divinylbenzene/vinylbenzylchloride and styrene/divinylbenzene/vinylbenzyl chloride were synthesized by the suspension polymerization technique. The prepared polymeric blends potentially adsorbed fluoride ions by electrostatic attraction via hydrogen bonding [72]. Polymer blends of PVA and CS were used to prepare sustainable and degradable polymer films which show promise for potential application as substrates for transient bioelectronics. The mechanical and thermal performance and programmable degradation of the composite films are superior due to the physical interaction of the PVA and CS providing a sustainable platform for wide application in transient biomedical and electronic devices [73].

1.2.9 Phase inversion/separation process Phase inversion is carried out by the removal of the solvent from a liquid polymer solution to form a solid membrane. Sometimes, instead of evaporation, it is performed by casting the polymer solution on a suitable support, followed by immersing it in a coagulation medium containing a nonsolvent. While dealing with emulsions, phase inversion refers to the conversion of the oil-in-water emulsion to the water-in-oil emulsion. CS particles were mixed with polyvinylidene fluoride (PVDF) dope in order to prepare flat sheet membranes through increased PVDF membrane hydrophilicity, surface free energy, work of adhesion along with higher pore size and surface roughness via phase inversion process. The impact of CS on the fabrication of the membrane can facilitate the fine-tuning of the membrane properties to achieve higher efficiencies for various applications particularly different environmental applications [74]. CS/PVA was prepared via a nonsolvent-induced phase separation method through the interaction of the intermolecular hydrogen bond between them for air filtration. The thickness of the membrane is closely related to the filtration performance and the direct interception on the membrane surface was the dominant mechanism for removing NaCl aerosol particles. The membrane also displayed high antibacterial activity, which opens a new avenue to tailor an antibacterial and environment-friendly blend membrane for air filtration [75].

1.3 Important applications of chitin and chitosan CH and CS has been used in a wide range of applications in different industrial and medicinal fields such as food packaging, antimicrobial

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1.3 Important applications of chitin and chitosan

17

wound healing, water treatment, tissue engineering, drug release, and bleeding control, which are shown schematically in Fig. 1.3 and various studies are highlighted in Table 1.2.

1.3.1 Application of chitin and chitosan composites in food packing Transparent and colorless citric acid-incorporated fish GL/CS composite films were prepared by the solution casting method, which slowed down the swelling rate and swelling values of the films and maintained their integrity, resulting in flexible hydrated films with a reduction in the growth of Escherichia coli, indicating the combined effect of citric acid and CS as natural antimicrobial compounds. The composite films showed good ultraviolet barrier properties and hydrophobic surfaces, essential properties for food packaging applications, highlighting the potential use of these films as active packaging [76]. Three different types of sulfur NPs were prepared by the solution casting method and used to prepare functional CS/sulfur NPs composite films. Sulfur NPs capped with CS composite film exhibited the strongest antimicrobial activity against food-borne pathogenic E. coli and Listeria monocytogenes bacteria with enhanced hydrophobicity, mechanical strength, and water vapor barrier property. The CS/sulfur NPs

FIGURE 1.3 Important applications of chitin/chitosan and their blends and composites.

Handbook of Chitin and Chitosan

18

1. Introduction

TABLE 1.2 Various applications of chitin/chitosan and their blends and composites. Applications

Formulations

Methods

References

Food packing

Citric acid fish GL/CS

Solution casting

[76]

CS sulfur NPs

Solution casting

[77]

Microcrystalline cellulose urea CS

Solution casting

[78]

PLA/carbon nanotubes/CS

Electrospinning

[79]

Zein CS

[80]

Mahua oil polyurethane CS nano zinc oxide

Antimicrobial

Epoxidation

Rapeseed protein CS

[82]

PLA tea polyphenol CS

[83]

Cellulose PLA CS

[84]

Cellulose spheres CS

[85]

Carvacrol nanoemulsion loaded CMC

[86]

Mango leaf extract CS

[87]

CS TiO2

[88]

AgNPs CS

[89] Self-assembly

[90]

CS zinc oxide Neem seed

Chemical precipitation

[91]

Chitosan ZnO

In situ precipitation

[92]

CS chitosan oligosaccharide Wound healing

[81]

[93]

CS hydroxybutyl chitosan

Freeze-drying

[94]

CS COL alginate

Paint coating and freeze-drying method

[95]

Hap β tricalciumphosphate CS

Freeze-drying

[96]

CS Aloe vera extract hydrochloride

Freeze-drying

[97]

CS silver sulfadiazine

Freeze-drying

[98]

CS COL organomontmorillonite Callicarpa nudiflora

Solution mixing

[99]

tetracycline

(Continued)

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19

1.3 Important applications of chitin and chitosan

TABLE 1.2 Applications

(Continued) Formulations

Methods

References

CS poly(vinyl pyrrolidone) nanocellulose

Solution casting

[100]

CS Ag ZnO

Lyophilization and immersion

[101]

COL chitosan gel cellpenetrating peptide

[102]

Lecithin I CMC sodium alginate

Microwave drying

[103]

CS polyacrylic acid sodium

Electrospinning

[104]

Phosphorylated magnetic CS

Hydrothermal

[108]

Magnetic CS graphene oxide

Hummers method

[109]

Hydroxysodalite CS

Hydrothermal

[110]

CS carbon

Sol gel method

[111]

Thiolated quaternized CS

Chemical coprecipitation method

[112]

Heavy metals Water treatment

Fe3O4 CS@bentonite

[113]

Polyethylenimine CS

Freeze-drying

[114]

CS silica

Sol gel method

[115]

CS coating attapulgite

Self-assembly

[116]

Poly(amidoxime) grafted CS bentonite

In situ intercalative polymerization

[117]

Dyes CPAC CS sodium dodecyl sulfate Poly (methacrylic acid) CS oxide CdS quantum dots

[118] zinc

Cross-linking

[119]

NanoZnO CS

Polymerization

[120]

Dialdehyde microfibrillated cellulose CS

Solvent-casting

[122]

CS polyethylenimine

One-pot synthesis

[123]

CS clay

[124]

Polyurethane/CS foam

One-shot process

[125] (Continued)

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1. Introduction

TABLE 1.2 (Continued) Applications

Formulations

Methods

[126]

CS polyaniline CS magnetic microsphere Zirconium

References

Inverse suspension method

CS perlite

[127] [128]

CS/GNPs

Syringe dropping method

[129]

CS/kaolin

Solvent casting and evaporation process

[130]

SiO2 CS

Hydrothermal

[131]

Polyethylenimine CMC

One-step cross-linking reaction

[132]

As and Hg

CS nano HAp

[133]

Phosphate, nitrate, and humic acid PEG/CS

[134]

PVA/CS

[135]

Carbon CS

[136]

CS Al2O3 Fe3O4

Electrospinning

[137]

Bentonite CS

[138]

Cerium CS

[139]

Iron hydrotalcite CS

[140]

Fluoride

Nanofiltration and antibacterial

Tissue engineering

PVDF/CS

Phase inversion

[141]

Graphene oxide/CS

Phase inversion

[142]

Chitosan chloride graphene oxide quartz sand

One-step solution blending

[143]

Freeze-drying

[144]

Zirconium-based bioceramics nanopowder CS

Freeze-drying

[145]

CS CH

Dispersion-based freeze-dry approach

[146]

CS

Sr21 montmorillonite

(Continued)

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21

1.3 Important applications of chitin and chitosan

TABLE 1.2 Applications

(Continued) Formulations

Methods

References

GL CS HAp β-tricalcium phosphate 58s bioactive glass

Freeze-drying

[147]

Ti6Al4V CS

Freeze-drying

[148]

CS diatomite

Solvent casting

[149]

Hydroxypropyl CS soy protein

Cross-linking, solution casting, and evaporation process

[150]

HAp GL CS

Coaxial electrospinning technique

[151]

PLA

Electrospinning

[152]

Magnesium oxide PCL CS

Electrospinning

[153]

HAp hydroxypropyl CS

Coprecipitation

[154]

Ti

Potentiostatic method

[155]

CS montmorillonite HAp

Direct agglomeration method

[156]

CS HAp

Coagulation

[157]

SiO2 CaO P2O5

Sol gel method

[158]

45S5 bioactive glass CS

Dip-coating method

[159]

CS Aloe poly(lactic co glycolic acid)@curcumin

Ultrasonic emulsification and a tape-casting

[160]

fibroin CMC ascorbic acid CSNPs

Ionic gelation process.

[161]

CS

polypyrrole CS

Halloysite nanotubes CS alkali urea

[162]

Polyglycerol sebacate CS GL

[163]

TiO2 NPs

[164]

PEG CS

COL CS

[165]

Calcium phosphate CS

[166]

CS bioglasss TiO2

Electrophoretic deposition

[167]

CS silk fibroin fiber

[168]

Cs Gl glass NPs

[169] (Continued)

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1. Introduction

TABLE 1.2 (Continued) Applications

Formulations

Drug release

CS PVA

Methods

[170]

Fe3O4@C-carboxymethyl cellulose CS

Solution casting method, self-assembly technique

Gentamicin sulfate CS COL Bleeding control

References

[171]

[172]

Nanobioglass CS

Sol gel method

[173]

CS dopamine diatom biosilica

Alkylation precipitation

[174]

composite films can be used as antimicrobial food packaging films or wound dressings in biomedical applications [77]. Microcrystalline cellulose was surface-modified with urea under microwave irradiation and was then added as filler to a CS matrix to prepare a fully natural polymer composite material through casting the mixture slurry followed by coagulation; the results showed improved mechanical properties, reduced water vapor permeability, and satisfactory visible light transparency, which indicate promising potential as a packaging material [78]. PLA/carbon nanotubes/CS composite fibers containing different CS contents were fabricated by the electrospinning technique and their ability in strawberry preservation was examined. These composite fibers showed significant mechanical performance, water solubility and swelling ratio, and exhibited high antibacterial activity. The PLA/carbon nanotubes/CS composite fiber preserved strawberries at room temperature, which can be used for fruit and vegetable preservation applications [79]. Zein films were modified to improve mechanical, thermal, and barrier properties by compositing with CS followed by exposure to cold plasma, which showed potential improvement as a packing material [80]. Mahua oil-based polyurethane composite films with CS and nanozinc oxide were fabricated by the epoxidation method followed by hydroxylation; they showed enhanced thermal stability, tensile strength, antibacterial resistance, oxygen barrier ability, and surface wettability due to the synergistic interactions of the film containing components, leading to the composite film being a prospective packing material [81]. The composite films were prepared by mixing rapeseed protein hydrolysate with CS. The addition of CS enhances the mechanical properties of

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the rapeseed protein films due to its hydrogen bonding, the main force between the two components that contributed to good compatibility. The enhanced antibacterial properties of the composite film created the possibility for the practical application of rapeseed protein CS composite film in food packaging [82]. PLA, tea polyphenol, and CS composite membranes have the best preservation effect on cherries due to their significant reduction of rotting rate and mass loss rate; they delayed the consumption of soluble solids and vitamin C, maintained the quality of the cherries, and extended the shelf life, thus proving their potential for application in food packaging [83]. PLA/CS-based composite films reinforced with functionalized cellulose for potential food packaging were developed with significant mechanical properties. The reinforcement of functionalized cellulose/PLA/CS composite film exhibited excellent antimicrobial performance against E. coli and Bacillus subtilis, which was attributed to the synergistic antimicrobial effect of CS and rosin [84]. The introduction of cellulose spheres and its derivatives into CS film was performed to obtain composites with enhanced mechanical properties, glass transition, and moisture barrier properties due to its Schiff base structure allowing the best interfacial interaction. It also possessed good transparency in the visible light region, as well as barrier properties against ultraviolet light. Due to their promising features the composite films can be used in various applications such as edible films or food coatings and food packaging materials [85]. Carvacrol nanoemulsions loaded CMC is an excellent candidate for active packaging due to its satisfactory antioxidant activity, good antibacterial activity against Staphylococcus aureus and E. coli, and excellent ability to extend the shelf life of wheat bread [86]. The incorporation of the hydrophobic mango leaf extract into CS polymer demonstrated the conversion of mango leaves waste biomass into a primary antioxidant ingredient for active food packaging films, which resulted in films with better performance for cashew nuts’ storage than commercial polyamide/polyethylene film. The mango leaf extract impregnated CS films seem to be a promising and better alternative for the food packaging films [87]. By the incorporation of TiO2 nanopowder in CS, a CS-TiO2 composite film was prepared with efficient antimicrobial activity against food-borne pathogenic microbes showing promise as a food packaging material which could successfully protect red grapes from microbial infection and extend their shelf life [88].

1.3.2 Application of chitin and chitosan composites in antimicrobial activities A microfluidic approach for single-step and in situ synthesis of AgNPs-loaded CS microparticles showed enhanced antibacterial ability

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because of the properties of AgNPs and CS. The synthesized composite microparticles can be used in several future potential applications, such as bactericidal agents for water disinfection, antipathogens, and surface plasma resonance enhancers [89]. The self-assembly approach was used for the synthesis of an AgNPs organized CS nanopolymer which provided a monolayer film with good antimicrobial activity against pathogens, that when applied to a liquid medium or surfaces of edible food products can prevent the growth of microbes [90]. CS-zinc oxide Neem seed hybrid composites were synthesized by the chemical precipitation method, and showed enhanced antibacterial activity; these may be a promising potential component for application in the biomedical field [91]. Flexible CS/ZnO nanocomposite films were prepared by a green and facile method through in situ precipitation of nano-ZnO in the CS film which formed a micro nano-binary hierarchical structure, mimicking the structure of a lotus leaf. The CS/ZnO nanocomposites films can be used in the field of antibacterial packaging and dressing due to their good biocompatibility and nontoxicity [92]. A CS-based tissue conditioner formulated via hydrolysis under reflux conditions by CS and CS oligosaccharide provided a safe and alternative therapy to conventional synthetic antifungals for the treatment and prevention of denture stomatitis [93].

1.3.3 Application of chitin and chitosan composites in woundhealing activities Hydrophilic and macroporous composite hydroxybutyl CS sponge was developed via the incorporation of CS into hydroxybutyl CS through the vacuum freeze-drying method. The composite sponge showed high porosity, great water absorption, good softness, and low blood-clotting index without cytotoxicity. The composite sponge had a better ability to promote wound healing and helped faster formation of skin glands and reepithelialization, which encourages the use of this composite sponge for wound dressings [94]. CS COL alginate composite dressing was prepared by paint coating and freeze-drying method, and it is exhibited great porosity, excellent swelling ability, and can quickly absorb wound exudate and maintain a relatively moist environment of the wound site. In addition, CS COL alginate composite dressing composed an antiseawater immersion PU membrane, and it prevented the wound from seawater immersion for at least 4 h and exhibited no cytotoxicity and good hemocompatibility, which indicate that CCA composite dressing is a potential dressing for wound healing, and would specially satisfy the needs of people who work at sea [95]. Highly porous HAp/β-tricalcium phosphate/CS composite scaffolds

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were fabricated using a freeze-drying technique. These composites exhibited better mechanical and thermal properties, slower degradation and lower water swelling and retention, and had a stimulatory effect on cell growth, which suggested that the CS-based bone-like composites could be potential candidates for biomedical applications [96]. Porous, microfibrous structure composite sponges were prepared by a freezedrying method with a fungal CS-reinforced Aloe vera extract and associated with tetracycline hydrochloride. They exhibited good antibacterial activity and showed augmented cell viability, which suggested that they could be used as a cost-effective, potential wound-dressing material [97]. CS/silver sulfadiazine composite sponges were prepared via the freeze-drying method by incorporating silver sulfadiazine particles into a CS matrix in order to develop novel biomaterials for wound healing; they had high porosity and excellent swelling behaviors. The CS/ silver sulfadiazine sponges displayed excellent antibacterial performances against E. coli, Candida albicans, S. aureus, and B. subtilis and they had biocompatibility which demonstrated that the CS/silver sulfadiazine composite sponges have potential applications in antimicrobial wound-dressing materials [98]. A CS-COL/organomontmorillonite loaded with Callicarpa nudiflora composite membrane was prepared by a solution mixing method as a wound dressing, combining the excellent properties of each material, such as the biocompatibility and hemostasis effect of CS, the low inflammation of COL, and the antibacterial activity of the leaves of C. nudiflora. The CS-COL/organomontmorillonite membrane loaded with C. nudiflora has the potential to become a good wound dressing [99]. Biocompatible CS/poly (vinyl pyrrolidone)/nanocellulose composites were successfully prepared by the solution casting method and showed enhanced swelling, blood compatibility, and antibacterial activity. The CS/poly(vinyl pyrrolidone)/nanocellulose composites can be a potential candidate as a wound-healing material for biomedical application [100]. The nano-Ag/ ZnO-loaded CS composite dressing was prepared by the lyophilization and immersion method. The prepared CS-Ag/ZnO composite dressing has potential application for wound care because of its high porosity, swelling ratio, moisture retention time, and enhanced blood clotting capacity and antibacterial activities [101]. A COL/CS gel composite supplemented with a cell-penetrating peptide (Oligoarginine, R8) was capable of inhibiting S. aureus growth and had a good ability to heal wounds, with a high healing rate. The COL/ CS/cell-penetrating peptide could promote cutaneous wound healing through enhancing granulation tissue formation, increasing COL deposition, and promoting angiogenesis in the wound tissue. The COL/CS/ cell-penetrating peptide gel composite which has antibacterial activity renders a high therapeutic efficiency to heal wounds [102]. Iodine was

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complexed with hydroxylated lecithin to improve its stability and complexing efficiency for the composition with CMC/sodium alginate with the aid of microwave drying. The high wound repairing and healing efficiency of this novel antibacterial hydroxylated lecithin-I/CMC/ sodium alginate composite membrane suggest that it can be potentially used as a high-performance membrane dressing for treating and repairing open and infected traumas due to the good bioactivity of the membrane matrix, and the high stability, controllable release, and antibacterial properties of its activated iodine component [103].

1.3.4 Application of chitin and chitosan composites in water treatment process 1.3.4.1 Heavy metals removal Uniform CS nanofiber mats have been prepared by the incorporation of polyacrylic acid sodium into CS through the electrospinning method; they showed significantly improved solvent resistance, mechanical strength, and chelating abilities. The CS/polyacrylic acid sodium composite nanofiber mats can effectively adsorb Cr(VI) ions from dilute aqueous solution, and the higher adsorption performance has been attributed to abundant CS chelating sites, the large surface area of thee nanofiber mats, and the higher stability in acidic water solution. The adsorption abilities of these cross-linked CS/polyacrylic acid sodium nanofiber mats are dependent on the CS contents as well as the N-atom basicity of the CS chain. The prepared nanofiber mats could also be fabricated into a membrane reactor for continuous applications to remove heavy metal pollutants from wastewater [104]. A novel green composite of bentonite/CS@Co3O4 was synthesized through support of a bentonite/CS composite by green Co3O4 NPs using green tea extract. The composite was applied as an eco-friendly adsorbent in the decontamination of organic Congo red pollutants and inorganic Cr(VI) ions [105]. Activated carbon from oil palm empty fruit bunch has been produced and used as a filler in PEG diglycidyl ether cross-linked CS/activated carbon composite film preparation. The film showed high adsorption potential for the adsorption of Cd21 due to a good adhesion of the matrix and filler interface [106]. CS composite films reinforced with cellulose isolated from oil palm empty fruit bunch were successfully prepared with improved mechanical properties due to the formation of hydrogen bonds between CS and cellulose particles in the composite film, leading to good adhesion of the matrix filler interface and a homogeneous structure of composite film. It was applied for the removal of cadmium ions from aqueous solutions. The CS/cellulose composite film has a high potential as a low-cost adsorbent with high performance for Cd21 removal from

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polluted water [107]. Phosphorylated magnetic CS composite was hydrothermally synthesized to adsorb Pb(II) and Cd(II) from aqueous solutions. The good adsorption performance for Pb(II) and Cd(II) of phosphorylated magnetic CS composite was mainly attributed to the complexation interaction and ion exchange between metal ions and functional groups. The excellent magnetic separation and regeneration performance was beneficial for saving cost, which indicated that phosphorylated magnetic CS composite would be a promising material for the efficient treatment of wastewater containing heavy metal ions [108]. The magnetic CS/graphene oxide composite material was synthesized via a modified Hummers method for the removal of lead metal from aqueous solution. The magnetic CS/graphene oxide composite material has shown a high potential and adsorption capacity for the removal of Pb(II) ions from aqueous solutions [109]. Hydroxysodalite/CS composites were synthesized from an aluminum waste utilizing a hydrothermal method and effectively applied to purify polluted water from the Ni(II) and Pb(II) ions [110]. A cost-effective magnetic CS composite adsorbent was prepared with magnetic macroparticles and highly porous activated carbon carrier using the sol gel method for the removal of Cu(II). The magnetic CS composite adsorbent is a promising adsorbent for removing Cu(II) due to its good reusability and convenient magnetic separability [111]. Magnetic thiolated/quaternized CS composites were prepared by a chemical coprecipitation method, which showed high removal efficiency for various heavy metal ions, such as As(V), As(III), Cu21, Hg21, Zn21, Cd21, and Pb21 under neutral conditions due to the multifunctional groups modification and polymer ion oxide combination [112]. Magnetic Fe3O4-CS@bentonite composites using natural materials were synthesized and used to remediate acid mine drainage for heavy metal removal due to their good magnetism, high stability, and good performance in the removal of heavy metals [113]. The polyethylenimine-coated biomass CS composite fibers prepared using freeze-dried conditions were useful for treating industrial influents and recovering precious Ru in metallic form [114]. A mesoporous CS silica composite was prepared by a sol gel reaction that holds great potential in the application for Re(VII) recovery from wastewater containing certain metal ions and also possesses good stability and reusability for adsorption-related fields [115]. The CS-coated attapulgite was prepared by a self-assembly method and applied for U(VI) removal from aqueous solutions. The CS-coated attapulgite composite is a potential and suitable candidate for the preconcentration and separation of U(VI) from contaminated natural wastewater and aquifers due to its wide availability, simple synthesis procedure, efficient sorption ability, and good bioacceptability [116]. A novel amidoxime functionalized adsorbent, poly(amidoxime)-grafted-CS/bentonite composite was prepared by in situ intercalative polymerization of acrylonitrile

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and 3-hexenedinitrile onto CS/bentonite composite using ethylene glycol dimethacrylate as a cross-linking agent and potassium peroxy disulfate as a free radical initiator for the recovery of U(VI) from seawater. Nitrile groups from two monomers converted to amidoxime groups and therefore increased the adsorption efficiency of U(VI) from seawater, showing that poly(amidoxime)-grafted-CS/bentonite composite is a promising adsorbent for the removal of U(VI) from seawater [117]. 1.3.4.2 Dye removal Coir pith activated carbon (CPAC), CS, and sodium dodecyl sulfate composites were prepared and used as an effective low-cost adsorbent for the removal of malachite green from wastewater [118]. An efficient photocatalyst, poly(methacrylic acid) cross-linked CS@zinc oxide/CdS quantum dots, was synthesized under microwave irradiation by crosslinking of poly(methacrylic acid) with CS followed by incorporation of ZnO and CdS quantum dots on the cross-linked polymer surface. It showed excellent photocatalytic efficiency toward the degradation of toxic cationic dyes (malachite green and safranin) as well as toxic pollutant 2,4-dichloro phenol in the presence of sunlight. An antibacterial study reveals that the developed composite is worth being considered as an excellent antibacterial agent toward E. coli and B. subtilis. Finally, the nanocomposite is highly photosensitive toward sunlight and probably would be worthy as a plasmonic photocatalyst [119]. Nano-ZnO/CS composite beads were prepared by polymerization in the presence of nano-ZnO and CS, and they were used for the removal of reactive black 5 (RB 5) from aqueous solution [120]. CS/montmorillonite intercalated composite showed excellent absorption performance for the removal of reactive red 136, due to the involvement of functional groups such as OH, CONH2, and NH2 of CS and montmorillonite in the adsorption process [121]. Dialdehyde microfibrillated cellulose/CS composite films were prepared by solvent casting, and they acted as good adsorbents to adsorb Congo red from aqueous solution. The composites can be promising candidates for environment-friendly and economical bioadsorbents not only for the removal of Congo dye but also for the adsorption of other anionic dyes [122]. A porous magnetic CS polyethylenimine polymer composite was synthesized by a cross-linking CS with polyethylenimine using a facile one-pot synthesis approach and showed an enhanced high-adsorption capacity for Congo red removal in aqueous solutions. The magnetic CS polyethylenimine can be a promising adsorbent with high adsorption performance for the efficient removal of dye pollutants from wastewater [123]. CS/clay composite could effectively remove Rose FRN dye from aqueous solutions due to the involvement of characteristic functional

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group of CS like amine in the adsorption process. This would be an effective, precise, and inexpensive material for the treatment of textile wastewater [124]. A novel biobased polyurethane/CS foam was synthesized by the one-shot process using a polyol derived from castor oil and applied to remove Food Red 17 dye (FR17) from aqueous solutions; the foam has high potential for the removal of anionic dyes from aqueous media due to the capability of polyurethane foams to support CS to generate composites as adsorbents with better mechanical stability [125]. CS polyaniline composite is a promising material as an adsorbent for tartrazine removal from aqueous solutions due to the availability of amine and imine groups in the composite [126]. A magnetic microsphere containing CS were prepared by inverse suspension method and used for the methyl orange removal from aqueous solutions [127]. An adsorbent, zirconium (IV) doped immobilized cross-linked CS/perlite composite, was synthesized and characterized for acid orange II adsorption, and had a high adsorption capacity [128]. A simple and effective method was developed to prepare a composite sphere adsorbent, CS/GNPs, based on the syringe dropping method and using glutaraldehyde as the cross-linking agent. It showed high adsorption abilities for two azo acid dyes, methyl orange and acid red 1. CS/grapheme nanoplates are promising adsorbents for dye removal that are nontoxic, efficient, low-cost, and easy to prepare [129]. CS/kaolin composite porous membranes were successfully prepared by solvent casting and evaporation process to lead the enhancement of water permeability and to improve the resistance to water washout. They could be prospective membranes for future applications in wastewater treatments like water purification or the treatment of industrial effluents [130]. 1.3.4.3 Arsenic and mercury removal A novel biomimetic SiO2@CS composite prepared by a hydrothermal method exhibited a high adsorption performance toward heavy metal ions As(V) and Hg(II) in solution due to the availability of many functional groups including amino and hydroxyl groups for adsorbing heavy metal ions [131]. A novel polymer-based adsorbent of hyperbranched polyethylenimine functionalized CMC semiinterpenetrating network composite was fabricated through a facile one-step cross-linking reaction, which exhibited excellent selectivity for removing Hg(II) ions by the interaction between Hg(II) ions and nitrogen functional groups, such as amine and imine groups. This adsorbent holds great potential in remediating water polluted with Hg(II) ions [132]. CS/ nano-HAp composites based on scallop shells were prepared with different CS:nano-HAp ratios for the removal of Hg(II) which effectively adsorbed Hg(II) [133].

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1.3.4.4 Phosphate, nitrate, and humic acid removal PEG/CS and PVA/CS were effectively utilized for the removal of phosphate and nitrate due to good affinity of phosphate and nitrate ions toward PEG/CS and PVA/CS composites [134,135]. A granular CSbased sorbent was fabricated from carbon waste for enhancing removal of nitrate and phosphate. Ion exchange, electrostatic interaction, and hydrogen bonds were the potential sorption mechanisms [136]. CS/Al2O3/Fe3O4 composite nanofibrous adsorbent was prepared by an electrospinning process which has high potential for the removal of nitrate and phosphate [137]. A modified bentonite and CS composite was a promising adsorbent for the removal of undesirable humic acid from aqueous solutions due to its significant removal efficiency, natural abundance, and low cost; it may be an alternative to more costly materials available [138]. 1.3.4.5 Fluoride removal A cerium immobilized CS composite adsorbent was prepared for the removal of fluoride from water. This composite has great potential to remove fluoride by the mechanism of the electrostatic attraction among 31/41 2NO1 with F2 ions, the ligand exchange between NO2 3 and Ce 3 and 2 F , and the formation of the CTS-Ce-F complexation [139]. Magnetic iron oxide fabricated hydrotalcite/CS composite was synthesized and utilized for fluoride sorption studies; it was governed by adsorption, ion-exchange, and complexation and could be an economical and eco-friendly composite for the development of defluoridation technology [140]. 1.3.4.6 Nanofiltration and antibacterial process The nontoxic, biocompatible PVDF/CS composite was synthesized by an nonsolvent induced phase inversion process to make fouling-resistant nanofiltration membranes which exhibited excellent mechanical and thermal stability and flux recovery ratio [141]. Superfunctionalized composite graphene oxide/CS nanoplates were prepared by the surface modification of graphene oxide nanoplates with CS through phase inversion induced by an immersion precipitation method to investigate the effect of the modification on the dispersion quality. The nanofiltration performance was examined by measuring the Na2SO4 and CrSO4 rejection. The rejection is increased sharply due to the porosity reduction and the presence of more active sites for adsorption. Thus functionalization of graphene oxide with CS is an excellent way to overcome agglomeration at high concentrations in the preparation of a mixed matrix membranes due to better dispersion [142]. In order to control bacterial pollution in water treatment, it is necessary to prepare efficient antibacterial materials. Novel CS chloride graphene

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oxide composites were prepared via a one-step solution blending method and modified with quartz sand filter media (CSCl@GO/QS) to control bacterial pollution in water treatment in the secondary effluent of domestic sewage. E. coli and S. aureus were completely inactivated after treatment [143].

1.3.5 Tissue engineering CS and Sr21-modified montmorillonite composite was fabricated into porous three-dimensional scaffolds via a freeze-drying method, and it displayed desirable physicochemical and biological characteristics as a bone tissue engineering construct. This construct may be an ideal biomimetic template for the repair of defective bone with osteoblasts [144]. Zirconium-based bioceramics nanopowder CS scaffolds were prepared by a freeze-drying method and utilized for bone tissue engineering application due to their great mechanical strength, cell proliferation nature, cell spreading on the scaffolds, and sufficient water absorption capacities and porosities [145]. The CS/CH nanocrystals composite scaffolds were prepared utilizing a dispersion-based freeze-dried approach and exhibited a significant enhancement in compressive strength and modulus compared with pure CS scaffold both in dry and wet state. The composite scaffolds are successfully applied as scaffolds for MC3T3-E1 osteoblast cells, showing their excellent biocompatibility and low cytotoxicity. The biocompatible composite scaffolds with enhanced mechanical properties have potential application in bone tissue engineering [146]. GL and CS-based composite scaffolds reinforced with 30 wt.% HAp, β-tricalcium phosphate, and 58s bioactive glass were prepared using a freeze-drying technique and had high porosity that significantly improved bone-forming efficiency. Bioactive 58s glass-reinforced GL-CS scaffold offers promising opportunities in the form of 3D porous bioactive scaffolds with synergistically improved physicochemical and biological properties for improved bone tissue engineering [147]. A novel composite scaffold consisting of porous Ti6Al4V part-filled with CS sponge was fabricated using a combination of electron beam melting and freeze-drying method. The composite scaffolds can be considered as potential biomaterials for load-bearing applications because of their significantly superior porous architectures, mechanical properties, and osteoblast responses [148]. CS diatomite composite membranes were prepared with a solvent casting technique for bone tissue engineering applications and as a possible bone regeneration membrane. This composite improved the surface area and roughness, swelling properties, and protein adsorption capacities of membranes without cytotoxicity, which are important

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factors for osteoblast adhesion and proliferation [149]. A series of hydroxypropyl CS/soy protein isolate composite films with different soy protein isolate contents were developed via cross-linking, solution casting, and evaporation process. The HCSFs support L929 cells attachment and proliferation without obvious hemolysis, indicating good cytocompatibility and hemocompatibility. The hydroxypropyl CS/soy protein isolate composite films had a fast healing speed and good skin regeneration efficiency and may be potential candidates as wound dressings [150]. Biomimetic HAp/ GL-CS core shell nanofibers composite scaffolds were fabricated to mimic both the specific structure and the chemical composition of natural bone by the coaxial electrospinning technique, followed by the wet chemical method. The biomimetic composite scaffolds improved the mineralization efficiency of HAp and formed a homogeneous HAp deposit and also enhanced osteoblast cell proliferation, which indicated that the scaffold could be a promising material to promote osteoblast cell growth in bone tissue engineering [151]. Fibrous scaffolds with PLA and CS were fabricated by a conventional electrospinning method with higher mechanical strength and biocompatibility. The scaffolds were found to support cardiomyocyte viability, elicit cell elongation, and enhance production of sarcomeric α-actinin and troponin I, which indicated that composite scaffolds consisting of PLA/CS fibers have great potential for engineering cardiac tissue, and for accelerating the regeneration of myocardia [152]. Composite nanofibers of magnesium oxide, PCL, and CS were fabricated by the electrospinning method. PCL-CS/magnesium oxide-based nanofibers can be used as a novel biomaterial scaffold for use in tissue engineering applications, such as wound healing, bone regeneration, drug delivery, and regenerative medicine, due to their significant mechanical strength and cell viability without toxicity [153]. Potentially bioactive HAp/hydroxypropyl CS nanocomposites were synthesized by the incorporation of synthesized HAp into hydroxypropyl CS solution, followed by cross-linking of the HAp/hydroxypropyl CS blends with genipin through a coprecipitation method to obtain the nanocomposite hydrogels and scaffolds. Additionally, an oestoconductive and osteogenic marine algae polysaccharide fucoidan was adsorbed to the HAp/hydroxypropyl CS composite scaffolds, which increased the alkaline phosphatase activity in 7F2 osteoblast cells and promoted their mineralization. The fucoidan-adsorbed HAp/hydroxypropyl CS composite scaffolds can be a potential biomaterial for bone tissue engineering applications [154]. A Ti-coated polypyrrole/CS composite was prepared by the potentiostatic method, which improved the corrosion resistant and ensured the potential for bone implants with biocompatibility [155]. Macrosphere-based CS/montmorillonite/HAp composite scaffolds were developed by the direct agglomeration method with enhanced mechanical properties for load-bearing bone tissue engineering applications. They

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exhibited a highly hemocompatible nature and provided a most favorable microenvironment for osteoblast cell proliferation. The montmorillonite/ Hap-reinforced CS macrospheric scaffolds are potential candidates for bone tissue engineering applications [156]. A CS-HAp composite fiber as a scaffold for ligament regeneration was made using the coagulation method. It improved bone bonding ability by the orientation of CS and the interaction between CS and HAp [157]. An injectable SiO2-CaO-P2O5based bioglass composite was prepared in two different powder/liquid ratios using a sol gel-derived bioglass constituent for bone regeneration applications. It formed apatite in simulated body fluid and a high rate of new bone regeneration in a rat animal model due to better osteoconductivity [158]. The 45S5 bioactive glass CS composite was used to improve the osteointegration of polyetheretherketone spinal fusions by a dipcoating method. The bonding behavior of the 45S5 bioactive glass-CS composite coating on the desirable wetting polyetheretherketone was effectively enhanced by the surface roughness rather than the surface polar energy. The in vitro biocompatibilities of polyetheretherketone, including cell adhesion, cell proliferation, differentiation, and bioactivity in the stimulated body fluid, were enhanced by the presence of 45S5 bioactive glass CS composite coatings, which also suggested that this composite coating method could provide an effective solution for the weak polyetheretherketone bone integration and would be prospectively used for spinal or other orthopedic applications [159]. A shape-controllable, CS/Aloe film uniformly embedded with curcumin-loaded poly(lactic-coglycolic acid) microspheres was developed with a high-power ultrasonic emulsification and a tape-casting process to improve wound healing and skin tissue regeneration. The CS/Aloe poly(lactic-co-glycolic acid)@curcumin films dressing showed suitable physicochemical properties and flexibility which were suitable for clinical applications and achieved promising antibacterial activity against both Gram-positive bacteria and Gram-negative bacteria. In addition, the CS/Aloe poly(lactic-co-glycolic acid)@curcumin films showed higher fibroblasts proliferation and antiinflammatory properties, consequently promoting skin tissue regeneration. Therefore, this CS/Aloe poly(lactic-co-glycolic acid)@curcumin films dressing may serve as an effective and translational tool for skin tissue regeneration [160]. A set of biomimetic and bioactive scaffolds for bone regeneration based on silk fibroin CMC incorporated with ascorbic acidloaded CSNPs were prepared by the ionic gelation process. These composites had a biphasic release profile with the initial burst release followed by controlled release. Furthermore, these composites enhanced the formation of both brushite and apatitic calcium phosphates, promoted the proliferation, spreading, and differentiation of the seeded MG-63 and enhancing the osteoconductivity and osteoinductivity of silk fibroin scaffolds by the addition of both ascorbic acid and CMC [161].

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CS composites hydrogels were prepared by the addition of halloysite nanotubes in the CS alkali/urea solution with significant mechanical properties and antideformation ability. The composite hydrogels have a good ability to support the growth of MC3T3-E1 cells, indicating their excellent biocompatibility. The CS/halloysite nanotubes composite hydrogels show promising potential in biomaterials, such as drug-loaded wound-healing materials or tissue engineering scaffolds [162]. A semicrystalline polyglycerol sebacate/CS/GL produced fibers with an average diameter of about 80 nm with great porosity, which exhibited the maximum proliferation of the PC12 cells and the formation of nerve tissue layer. The polyglycerol sebacate/CS/GL nanocomposite scaffolds are promising structures for the nerve tissue engineering applications [163]. TiO2-NPs-fabricated PEG/CS composite hydrogels were developed as an advanced cardiac material to enhance functional activity for cardiac tissue repair. The results of cardiac markers (troponin I, sarcomeric α-actinin) exhibited the formation of interconnected cardiac layers and the formation of cell hydrogel matrix interactions and healthy and synchronous activity. The TiO2-NPs-incorporated PEG/CS hydrogel has admirable properties and provides a suitable material for cardiac repair applications [164]. Degradable COL-CS composite materials have been used to fabricate tissue engineered heart valves. The tissue engineered heart valves stained positively for both smooth muscle actin and endothelial cell factor VIII, suggesting that the seeded cells were in fact smooth muscle cells, fibroblasts, and endothelial cells which had shapes similar to the morphology of smooth muscle cells, fibroblasts, and endothelial cells [165]. A biomimetic bone-like nanostructured calcium phosphate/CS composite was successfully grown as a film on CS-coated Ti in modified simulated body fluid. The CS surface allowed the growth of calcium phosphate and contributed to a uniform distribution of the homogenous calcium phosphate/CS nanostructure across the surface. The calcium phosphate/CS composites showed better cell viability, adhesion, and differentiation of MG-63 cells compared to the HAp/CS composites, which indicated that the calcium phosphate/CS composites prepared on commercially pure Ti could be potential candidates for application in bone tissue engineering [166]. Homogeneous CS/Bioglasss/TiO2 composite coatings on 316 L stainless steel substrates were fabricated by an electrophoretic deposition method from aqueous suspensions by applying 1.5 V for 2 min; they showed improved mechanical properties. The microstructure and morphology of CS/Bioglasss/TiO2 coatings were reproducible and the electrophoretic deposition method can thus be considered a convenient lowcost method to produce CS/Bioglasss/TiO2 composite coatings of variable mechanical properties for biomedical applications [167]. A N,N-dimethylacetamide/10% LiBr solvent system was prepared for the processing of nonmulberry silk fibroin (Antheraea mylitta) and then micron-sized silk fibroin fiber-reinforced CS-based composite scaffolds were successfully fabricated

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which had optimal hydrophilicity, optimal pore size, swelling property, and structural stability in wet conditions. The silk fibroin fiber-reinforced CS scaffold showed enhanced glycosaminoglycan deposition, COL type II, and aggrecan expression under a chondrogenic microenvironment. Thereby the CS/silk fibroin fiber scaffold facilitated an enhanced extracellular matrix deposition and thus promoted human mesenchymal stem cells to proliferate, colonize, and differentiate toward the chondrogenic lineage. Thus the developed CS/silk fibroin fiber composite scaffold might be a suitable template for cartilage tissue engineering applications [168]. In situ forming composite hydrogels were synthesized based on CS and GL biopolymers associated with bioactive glass NPs; they can be a promising temporary injectable matrix for bone tissue engineering due to their smooth and cytocompatible nature [169].

1.3.6 Application of chitin and chitosan composites in drug release A CS/PVA-based hydrogel composite was prepared using tetraethyl orthosilicate as a cross-linking agent via wet conventional synthesis. The CS/PVA-based hydrogel composite exhibited good properties as a drug delivery system, because its release profile of gallic acid showed high antioxidant activities [170]. Ultrastrong composite film of CS and silicacoated graphene oxide sheets were prepared by a simple solution casting method to utilize more potential applications in biomedical fields. Fe3O4@C NPs were incorporated into carboxymethyl cellulose matrix and coated with a CS layer via a self-assembly technique to form core shell polyelectrolyte complexes, which showed a high swelling ratio and prevented a sudden release in phosphate buffer. The Fe3O4@C/ carboxymethyl cellulose/CS composite beads in proper proportion can be an excellent and directed controlled release drug carrier [171]. Gentamicin sulfate was incorporated into CS and COL composites for controlled drug release, because the blending of two or even more natural polymers and the incorporation of drugs into the mixture can be a new way to prepare new materials for controlled drug delivery in the place of implantation of biopolymeric materials [172].

1.3.7 Application of chitin and chitosan composites in bleeding control Incorporation of nanobioglass with silica, calcium and phosphate ions into CS hydrogel by sol-gel method, which is acted simultaneously on mechanisms involved in hemostasis and bring about effective bleeding control might be due to the synergistic effect of CS and nanobioglass

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when it contact with blood. This composite hydrogel might have good potential in achieving effective bleeding control during major surgeries and traumatic conditions [173]. CS/dopamine/diatom biosilica composite beads were prepared via an alkylation precipitation method using a combination of CS and diatom biosilica with dopamine as bioglue for rapid hemostasis with good biocompatibility [174].

1.4 Conclusions With the growing importance of green chemistry and the use of biopolymers, CN and CS, the most abundant and renewable polysaccharide, have gained more attention from many researchers due to the presence of reactive functional groups, that is, amino and hydroxyl groups, as well as the polysaccharide nature of CS, which allows for various modifications. The CN and CS are very attractive for various fields and the wide panel of CS properties and processed materials suggest their promising future as biomaterials due to the results in improved solubility in water and organic solvents, which favor the continuous development of their applications as new functional biomaterials. This chapter has reviewed the preparation methods of CN and CS blends and composites such as simple blending, lyophilization, spray and freeze spray drying, electrospinning, solution/solvent casting, melt extrusion, coprecipitation, polymerization, and phase inversion process, their advantages, and the various applications in different fields, such as food packaging, antimicrobials, wound healing, water treatment, tissue engineering, drug release, and bleeding control.

Acknowledgment The authors would like to thank the management of Plant Lipids Private Limited, Cochin, India for their support and encouragement.

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[111] J. Li, B. Jiang, Y. Liu, C. Qiu, J. Hu, G. Qian, et al., Preparation and adsorption properties of magnetic chitosan composite adsorbent for Cu21 removal, J. Clean. Prod. 158 (2017) 51 58. [112] X. Song, L. Li, L. Zhou, P. Chen, Magnetic thiolated/quaternized-chitosan composites design and application for various heavy metal ions removal, including cation and anion, Chem. Eng. Res. Des. 136 (2018) 581 592. [113] G. Feng, J. Ma, X. Zhang, Q. Zhang, Y. Xiao, Q. Ma, et al., Magnetic natural composite Fe3O4-chitosan@bentonite for removal of heavy metals from acid mine drainage, J. Colloid Interf. Sci. 538 (2019) 132 141. [114] M.-H. Song, J.A. Kim, W. Wei, S. Kim, Y.-S. Yun, Polyethylenimine-coated biomasschitosan composite fibers for recovery of ruthenium from industrial effluents: effects of chitosan molecular weight and drying method, Hydrometallurgy 182 (2018) 114 120. [115] W. Shan, D. Zhang, X. Wang, D. Wang, Z. Xing, Y. Xiong, et al., One-pot synthesis of mesoporous chitosan-silica composite from sodium silicate for application in rhenium(VII) adsorption, Micropor. Mesopor. Mat. 278 (2019) 44 53. [116] D. Pan, Q. Fan, F. Fan, Y. Tang, Y. Zhang, W. Wu, Removal of uranium contaminant from aqueous solution by chitosan@attapulgite composite, Sep. Purif. Technol. 177 (2017) 86 93. [117] T.S. Anirudhan, G.S. Lekshmi, F. Shainy, Synthesis and characterization of amidoxime modified chitosan/bentonite composite for the adsorptive removal and recovery of uranium from seawater, J. Colloid Interf. Sci. 534 (2019) 248 261. [118] T.K. Arumugam, P. Krishnamoorthy, N.R. Rajagopalan, S. Nanthini, D. Vasudevan, Removal of malachite green from aqueous solutions using a modified chitosan composite, Int. J. Biol. Macromol. 128 (2019) 655 664. [119] L. Midya, A.S. Patra, C. Banerjee, A.B. Panda, S. Pal, Novel nanocomposite derived from ZnO/CdS QDs embedded crosslinked chitosan: an efficient photocatalyst and effective antibacterial agent, J. Hazard. Mater. 369 (2019) 398 407. ¨ .H. Kaynar, T. Aydemir, S.C. Kaynar, M. Ayvacikli, An efficient removal [120] S. Cinar, U of RB5 from aqueous solution by adsorption onto nano-ZnO/Chitosan composite beads, Inter. J. Biol. Macromol. 96 (2017) 459 465. [121] J. Li, J. Cai, L. Zhong, H. Cheng, H. Wang, Q. Ma, Adsorption of reactive red 136 onto chitosan/montmorillonite intercalated composite from aqueous solution, Appl. Clay Sci. 167 (2019) 9 22. [122] X. Zheng, X. Li, J. Li, L. Wang, W. Jin, J. Liu, et al., Efficient removal of anionic dye (Congo red) by dialdehyde microfibrillated cellulose/chitosan composite film with significantly improved stability in dye solution, Int. J. Biol. Macromol. 107 (2018) 283 289. [123] L. You, C. Huang, F. Lu, A. Wang, X. Liu, Q. Zhang, Facile synthesis of high performance porous magnetic chitosan -polyethylenimine polymer composite for Congo red removal, Int. J. Biol. Macromol. 107 (2018) 1620 1628. [124] A. Kausar, K. Naeem, M. Tariq, Z.-I.-H. Nazli, H.N. Bhatti, F. Jubeen, et al., Preparation and characterization of chitosan/clay composite for direct Rose FRN dye removal from aqueous media: comparison of linear and non-linear regression methods, J. Mater. Res. Technol. 8 (2019) 1161 1174. [125] R.R. da Schio, B.C. da Rosa, J.O. Gonc¸alves, L.A.A. Pinto, E.S. Mallmann, G.L. Dotto, Synthesis of a bio based polyurethane/chitosan composite foam using ricinoleic acid for the adsorption of food red 17 dye, Int. J. Biol. Macromol. 121 (2019) 373 380. [126] S. Sahnoun, M. Boutahala, Adsorption removal of tartrazine by chitosan/polyaniline composite: kinetics and equilibrium studies, Int. J. Biol. Macromol. 114 (2018) 1345 1353.

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[127] B. Zhao, X. Sun, L. Wang, L. Zhao, Z. Zhang, J. Li, Adsorption of methyl orange from aqueous solution by composite magnetic microspheres of chitosan and quaternary ammonium chitosan derivative, Chin. J. Chem. Eng. (2018). Available from: https://doi.org/10.1016/j.cjche.2018.12.014. [128] P. Demirc¸ivi, Synthesis and characterization of Zr (IV) doped immobilized crosslinked chitosan/perlite composite for acid orange II adsorption, Int. J. Biol. Macromol. 118 (2018) 340 346. [129] C. Zhang, Z. Chen, W. Guo, C. Zhu, Y. Zou, Simple fabrication of chitosan/graphene nanoplates composite spheres for efficient adsorption of acid dyes from aqueous solution, Int. J. Biol. Macromol. 112 (2018) 1048 1054. [130] S.B. Rekik, S. Gassara, J. Bouaziz, A. Deratani, S. Baklouti, Development and characterization of porous membranes based on kaolin/chitosan composite, Appl. Clay Sci. 143 (2017) 1 9. [131] J. Liu, Y. Chen, T. Han, M. Cheng, W. Zhang, J. Long, et al., A biomimetic SiO2@chitosan composite as highly-efficient adsorbent for removing heavy metal ions in drinking water, Chemosphere 214 (2019) 738 742. [132] H. Zeng, L. Wang, D. Zhang, P. Yan, J. Nie, V.K. Sharma, et al., Highly efficient and selective removal of mercury ions using hyperbranched polyethylenimine functionalized carboxymethyl chitosan composite adsorbent, Chem. Eng. J. 358 (2019) 253 263. [133] A.F. Hassan, R. Hrdina, Chitosan/nanohydroxyapatite composite based scallop shells as an efficient adsorbent for mercuric ions: static and dynamic adsorption studies, Int. J. Biol. Macromol. 109 (2018) 507 516. [134] A. Rajeswari, A. Amalraj, A. Pius, Removal of phosphate using chitosan-polymer composites, J. Environ. Chem. Eng. 3 (2015) 2331 2341. [135] A. Rajeswari, A. Amalraj, A. Pius, Adsorption studies for the removal of nitrate using chitosan/PEG and chitosan/PVA polymer composites, J. Water Process. Eng. 9 (2016) 123 134. [136] X. Cui, H. Li, Z. Yao, Y. Shen, Z. He, X. Yang, et al., Removal of nitrate and phosphate by chitosan composited beads derived from crude oil refinery waste: sorption and cost-benefit analysis, J. Clean. Prod. 207 (2019) 846 856. [137] F. Bozorgpour, H.F. Ramandi, P. Jafari, S. Samadid, S.S. Yazd, M. Aliabadi, Removal of nitrate and phosphate using chitosan/Al2O3/Fe3O4 composite nanofibrous adsorbent: comparison with chitosan/Al2O3/Fe3O4 beads, Int. J. Biol. Macromol. 93 (2016) 557 565. [138] M.H. Dehghani, A. Zarei, A. Mesdaghinia, R. Nabizadeh, M. Alimohammadi, M. Afsharnia, et al., Production and application of a treated bentonite chitosan composite for the efficient removal of humic acid from aqueous solution, J. Clean. Prod. 140 (2018) 102 115. [139] T. Zhu, T. Zhu, J. Gao, L. Zhang, W. Zhang, Enhanced adsorption of fluoride by cerium immobilized cross-linked chitosan composite, J. Fluor. Chem. 194 (2017) 80 88. [140] K. Pandi, S. Periyasamy, N. Viswanathan, Remediation of fluoride from drinking water using magnetic iron oxide coated hydrotalcite/chitosan composite, Int. J. Biol. Macromol. 104 (2017) 1569 1577. [141] K. Ekambaram, M. Doraisamy, Fouling resistant PVDF/Carboxymethyl chitosan composite nanofiltration membranes for humic acid removal, Carbohydr. Polym. 173 (2017) 431 440. [142] E. Bagheripour, A.R. Moghadassi, S.M. Hosseini, B. Van der Bruggen, F. Parvizian, Novel composite graphene oxide/chitosan nanoplates incorporated into PES based nanofiltration membrane: chromium removal and antifouling enhancement, J. Ind. Eng. Chem. 62 (2018) 311 320.

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[161] M. Moaddab, J. Nourmohammadi, A.H. Rezayan, Bioactive composite scaffolds of carboxymethyl chitosan-silk fibroin containing chitosan nanoparticles for sustained release of ascorbic acid, Eur. Polym. J. 103 (2018) 40 50. [162] B. Huang, M. Liu, C. Zhou, Chitosan composite hydrogels reinforced with natural clay nanotubes, Carbohydr. Polym. 175 (2017) 689 698. [163] S. Saravani, M. Ebrahimian-Hosseinabadi, D. Mohebbi-Kalhori, Polyglycerol sebacate/chitosan/gelatin nano-composite scaffolds for engineering neural construct, Mater. Chem. Phys. 222 (2019) 147 151. [164] N. Liu, J. Chen, J. Zhuang, P. Zhu, Fabrication of engineered nanoparticles on biological macromolecular (PEGylated chitosan) composite for bio-active hydrogel system in cardiac repair applications, Int. J. Biol. Macromol. 117 (2018) 553 558. [165] J.H. Fu, M. Zhao, Y.R. Lin, X.D. Tian, Y.D. Wang, Z.X. Wang, et al., Degradable chitosan-collagen composites seeded with cells as tissue engineered heart valves, Heart Lung Circ. 26 (2017) 94 100. [166] K.H. Park, S.-J. Kim, Y.-H. Jeong, H.-J. Moon, H.-J. Song, Y.-J. Park, Fabrication and biological properties of calcium phosphate/chitosan composite coating on titanium in modified SBF, Mater. Sci. Eng. C 90 (2018) 113 118. [167] S. Clavijo, F. Membrives, G. Quiroga, A.R. Boccaccini, M.J. Santilla´n, Electrophoretic deposition of chitosan/bioglasss and chitosan/bioglasss/TiO2 composite coatings for bioimplants, Ceram. Int. 42 (2016) 14206 14213. [168] B.N. Singh, K. Pramanik, Fabrication and evaluation of non-mulberry silk fibroin fiber reinforced chitosan based porous composite scaffold for cartilage tissue engineering, Tissue Cell 55 (2018) 83 90. [169] C.D.F. Moreira, S.M. Carvalho, R.G. Sousa, H.S. Mansur, M.M. Pereira, Nanostructured chitosan/gelatin/bioactive glass in situ forming hydrogel composites as a potential injectable matrix for bone tissue engineering, Mater. Chem. Phys. 218 (2018) 304 316. [170] T. Thanyacharoen, P. Chuysinuan, S. Techasakul, P. Nooeaid, S. Ummartyotin, Development of a gallic acid-loaded chitosan and polyvinyl alcohol hydrogel composite: release characteristics and antioxidant activity, Int. J. Biol. Macromol. 107 (2018) 363 370. [171] X. Sun, J. Shen, D. Yu, X.-K. Ouyang, Preparation of pH-sensitive Fe3O4@C/carboxymethyl cellulose/chitosan composite beads for diclofenac sodium delivery, Int. J. Biol. Macromol. 127 (2019) 594 605. [172] A. Sionkowska, B. Kaczmarek, R. Gadzala-Kopciuch, Gentamicin release from chitosan and collagen composites, J. Drug. Deliv. Sci. Technol. 35 (2016) 353 359. [173] M.N. Sundaram, S. Amirthalingam, U. Mony, P.K. Varma, R. Jayakumar, Injectable chitosan-nano bioglass composite hemostatic hydrogel for effective bleeding control, Int. J. Biol. Macromol. 129 (2019) 936 943. [174] Y. Wang, Y. Fu, J. Li, Y. Mu, X. Zhang, K. Zhang, et al., Multifunctional chitosan/ dopamine/diatom-biosilica composite beads for rapid blood coagulation, Carbohydr. Polym. 200 (2018) 6 14.

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C H A P T E R

2 Processing techniques of chitin-based gels, blends, and composites using ionic liquids Jun-ichi Kadokawa Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan

O U T L I N E 2.1 Introduction

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2.2 Dissolution and gelation of chitin with ionic liquids

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2.3 Fabrication of chitin-based blend and composite materials using ionic liquids

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2.4 Conclusion

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Acknowledgment

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References

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2.1 Introduction Chitin, a natural aminopolysaccharide, can be identified as a very important biomass resource because it is the second most abundant organic substrate on the Earth after cellulose [1
3]. It is composed of the β-(1-4)-linked chain structure similar to that of cellulose, but the hydroxy groups at position 2 in the glucose repeating units in cellulose

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© 2020 Elsevier Inc. All rights reserved.

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2. Processing techniques of chitin-based gels, blends, and composites using ionic liquids

FIGURE 2.1 Structure of chitin.

are replaced by acetamido groups in N-acetyl-D-glucosamine repeating units in chitin (Fig. 2.1). Different from cellulose, however, chitin still remains mostly as an unutilized biomass resource, primarily due to its insolubility in water and common organic solvents, leading to difficulties in efficient processability. The insolubility is principally owing to its highly crystalline fibrous structure and stiff chain packing by the formation of numerous intra- and intermolecular hydrogen bonds. There are two representative crystalline structures of chitin, that is, α- and β-chitins with stable antiparallel and metastable parallel chain alignments, furthermore, the former fibril shows lower solubility than the latter one [2,3]. Therefore the research on its processing to new chitinbased functional materials through proper dissolution processes has attracted much attention even in recent years. In the limited solvents systems for chitin, 5%
7% LiCl/N,N-dimethylacetamide (DMAc) [4] and CaCl2 2H2O-saturated methanol [5
8] have been extensively used for the dissolution of chitin. Furthermore, the preparation of chitinbased functional materials, such as fiber, film, beads, and hydrogel, has been reported through dissolution in such solvent systems [7,9
13]. Ionic liquids, which are molten salts melted at temperatures below the boiling point of water, are identified as good solvents for polysaccharides [14
23], since Rogers et al. discovered the dissolution of cellulose in an ionic liquid, 1-butyl-3-methylimidazolium chloride (BMIMCl) [24]. Recently, some ionic liquids have been found to dissolve chitin in certain concentrations [20
23,25,26]. Accordingly, research concerning the fabrication of chitin-based functional materials through the dissolution or gelation of chitin with the ionic liquids have been the object of increasing attention based on the viewpoint of the efficient use of chitin as a biomass resource. On the basis of the above background, this chapter deals with the processing techniques of chitin-based blend and composite materials through dissolution and gelation with the ionic liquids.



2.2 Dissolution and gelation of chitin with ionic liquids It was only a little over 10 years ago that ionic liquids were discovered to show an ability to dissolve chitin. In 2008 an ionic liquid,

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FIGURE 2.2 Structures of imidazolium acetates and alkanoates.

1-butyl-3-methylimidazolium acetate (BMIMOAc, Fig. 2.2), was reported to dissolve both α- and β-chitin with different molecular weights at relatively low temperature [27]. This is probably the first reliable report on the dissolution of chitin in ionic liquids in certain concentrations. By cooling the chitin/BMIMOAc solutions to ambient temperature, corresponding chitin/BMIMOAc gels were formed, which were further converted into chitin sponge and film materials by regeneration using water or methanol coagulants. When the dissolution behaviors of chitin with a series of alkylimidazolium chloride and dimethyl phosphate and 1-allyl3-methylimidazolium acetate (AMIMOAc, Fig. 2.2) were investigated, the former two series of the ionic liquids did not dissolve certain amounts of chitin (less than 1.5 wt.%), while the latter ionic liquid showed an ability for the dissolution of chitin in 5 wt.% [21]. Furthermore, the dissolution behavior was affected by the degree of deacetylation, the degree of crystallinity, and the molecular weight. 1-Ethyl-3-methylimidazolium acetate (EMIMOAc, Fig. 2.2) was also found to dissolve chitin in certain concentrations [21,27,28]. Extraction of chitin from raw crustacean shells, such as shrimp shells, was achieved using EMIMOAc. In the following studies, EMIMOAc has been used for the formation of gels, films, and fibers from chitin [29
32]. For example, the preparation of chitin hydrogels was investigated through solution molding with EMIMOAc, followed by water coagulation and washing [30]. More recently, some 1-ethyl-3methylimidazolium alkanoates (Fig. 2.2) and tetrabutylphosphonium amino acid salts were found to dissolve chitin [33]. The dissolution of chitin with deep eutectic solvents (DESs), as ionic liquid analogs, composed of mixtures of choline halide
urea, chlorocholine chloride
urea, and choline chloride
thiourea were also investigated [34]. Consequently, the highest concentration on the chitin dissolution (9% w/w) was obtained in the choline chloride
thiourea system.

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2. Processing techniques of chitin-based gels, blends, and composites using ionic liquids

In 2009 it was reported that another ionic liquid, 1-ally-3methylimidazolium bromide (AMIMBr), dissolved chitin in concentrations up to 4.8 wt.% by heating at 100 C (Fig. 2.3A) [35]. Furthermore, it was also revealed that even the presence of a small amount of bromide anion in 1-allyl-3-methylimidazolium chloride, which was generated by in situ anion exchange with 2-bromoethyl acetate as a bromide generator, enhanced the dissolution of chitin [36]. The facile production of chitin from crab shells was achieved by direct extraction using AMIMBr, followed by demineralization using citric acid [37]. In contrast, other imidazolium bromides, for example, 1-methyl-3-propylimidazolium and 1-butyl-3-methylimidazolium bromides, do not dissolve chitin at all, suggesting that in addition to the bromide anion, the allyl substituent in AMIMBr strongly affects the dissolution ability, although the reason as to why AMIMBr specifically dissolves chitin has not yet been made clear. From a mixture of the larger amounts of chitin (6.5
10.7 wt.%) with AMIMBr, ion gels were successively formed by standing at room temperature, followed by heating at 100 C (Fig. 2.3B). Interestingly, the oscillatory shear rheometrical measurement showed that both the 4.8 and 6.5 wt.% liquids of chitin with AMIMBr behaved as the weak gels. From the 9.1
10.7 wt.% chitin ion gels with AMIMBr, nanofiber dispersions were obtained by soaking in methanol at room temperature for 24 h to slowly regenerate chitin, followed by sonication (Fig. 2.3C) [38]. The SEM image of the diluted dispersion with methanol observed the

FIGURE 2.3 (A) Dissolution and (B) gelation of chitin with 1-allyl-3methylimidazolium bromide (AMIMBr) and preparation of (C) self-assembled chitin nanofiber dispersion/film and (D) chitin nanofiber/poly(vinyl alcohol) (PVA) composite film.

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nanofiber morphology with a width of c. 20
60 nm and several hundred nm in length, supporting the self-assembling formation of the chitin nanofibers by the regeneration approach from the ion gel. The resulting nanofibers constructed a film through filtration of the dispersion (Fig. 2.3C). The SEM image of the resulting film showed the highly entangled nanofiber morphology. Such an entangled structure from the nanofibers probably contributed to the film formation. It was also found that the regeneration from the chitin ion gels using calcium halide
2H2O/methanol solutions affected the morphologies of selfassembled chitin nanofibers [39]. The self-assembling to the nanofibers was not achieved by the regeneration from the ion gel using CaCl2
2H2O or CaBr2
2H2O/methanol solution at high concentration. On the other hand, the self-assembled chitin nanofibers with higher aspect ratio were fabricated by the regeneration using CaBr2
2H2O/ methanol solution in lower concentration. The self-assembled chitin nanofibers were also fabricated by gelation of chitin with the choline chloride
thiourea DES system (10% w/w), followed by dilution with water [40]. The self-assembled chitin nanofibers, fabricated using the DES, were used to prepare calcium alginate bionanocomposite gel beads with enhanced elasticity [40]. A chitin/cellulose binary ion gel was fabricated from a solution with the two ionic liquids, AMIMBr and BMIMCl (Fig. 2.4A) [41]. In this

FIGURE 2.4 Preparation of (A) chitin/cellulose binary ion gel and (B) chitin/cellulose blend film using ionic liquids.

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study, a 4.8 wt.% chitin solution with AMIMBr and a 9.1 wt.% cellulose solution with BMIMCl were first mixed at 100 C to obtain a homogeneous solution. From the resulting solution, the chitin/cellulose binary ion gel was formed, excluding the excess ionic liquids by standing at room temperature, followed by washing with ethanol, which was employed as a novel electrolyte for an electric double layer capacitor. The binary ion gel was first converted into the corresponding acidic gel by treating with 2.0 mol/L H2SO4 aqueous solution [42
44]. Electrochemical characteristics of the resulting acidic gel electrolyte were evaluated by galvanostatic charge
discharge measurements. The test cell with the acidic binary gel electrolyte exhibited a specific capacitance of 162 F/g at room temperature, which was higher than that for a cell with a standard H2SO4 electrolyte (155 F/g). The acidic binary gel electrolyte showed an excellent high-rate discharge capability in a wide range of current densities, as well as in H2SO4 aqueous solution. In addition, the discharge capacitance of the test cell retained over 80% of its initial value through 105 cycles even at a high current density of 5000 mA/g.

2.3 Fabrication of chitin-based blend and composite materials using ionic liquids From the abovementioned homogeneous solution of chitin/cellulose with AMIMBr/BMIMCl, cast in a glass plate, a cellulose/chitin blend film was fabricated by coagulation with ethanol and water (Fig. 2.4B) [41,45]. A chitin/cellulose blend yarn was fabricated by the continuous wet spinning process of a solution in 1-ethyl-3-methylimidazolium propionate [46]. Chitin was also blended with poly(L-lactic acid) (PLLA) by codissolution in EMIMOAc, followed by extrusion into a water coagulation bath [47]. Similarly, blend fibers of chitin with calcium alginate were fabricated by codissolution of chitin with alginic acid in EMIMOAc, followed by extrusion into a calcium chloride aqueous solution [48]. The resulting fibers, used as wound healing patches, showed excellent biocompatibility, fast reepithelization, and early healing ability. This process was extended to the electrospinning of crustacean biomass solution, chitin, and chitin composites with other polymers, such as lignin, cellulose, and PLLA [49,50]. As one of the possible applications of the abovementioned selfassembled chitin nanofibers, attempts have been made to fabricate composite materials with other polymers. For example, the following coregeneration technique was achieved to fabricate the self-assembled chitin nanofiber/poly(vinyl alcohol) (PVA) composite films (Fig. 2.3D) [38]. After a solution of PVA (DP 5 c. 4300) with a small amount of hot water was mixed with the 9.1 wt.% chitin ion gel with AMIMBr (feed weight

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ratio of chitin to PVA 5 1:0.3), the coregeneration of the two polymers was carried out by soaking the mixture in methanol as a poor solvent for both the two polymers. The subsequent filtration and Soxhlet extraction with methanol gave the self-assembled chitin nanofiber/PVA composite film. The SEM image of the composite film showed that the nanofiber morphology remained, suggesting the relative immiscibility of the two polymers. However, the DSC result of the composite film indicated that chitin and PVA might partially be miscible at the interfacial area on the fibers by hydrogen bonding between the two polymers. Tensile testing of the composite film supported enhancement of both tensile strength and elongation at break compared with the selfassembled chitin nanofiber film. The self-assembled chitin nanofibers were used as stabilizers for Pickering emulsion polymerization of styrene (Fig. 2.5) [51]. Pickering emulsions are those of any type, either oil-in water, water-in-oil, or even multiple, stabilized by solid particles or other types of solid materials, in place of surfactants in general emulsions. Prior to the emulsion polymerization, anionic carboxylate groups were introduced onto the chitin nanofibers by reaction with maleic anhydride in the presence of perchloric acid, and dispersed well in ammonia. Radical polymerization by potassium persulfate as an initiator was then conducted at 70 C in the emulsion, in which styrene droplets were stably surrounded by the nanofibers, to fabricate the composite particles. The particle sizes were changed in accordance with the nanofiber/styrene feed ratios. Furthermore, the composite particles were facilely converted into the chitin nanofiber-based hollow particles by solubilizing out the styrene core with toluene. The self-assembled chitin nanofibers have been used as a reinforcing agent through composition with other polymers. Because chitin has been regarded as a cationic polysaccharide owing to the presence of

FIGURE 2.5 Pickering emulsion polymerization of styrene using maleylated chitin nanofiber film to produce composite particles and conversion into hollow particles.

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several percent of free amino groups in total repeating units by deacetylation of acetamido groups, the self-assembled chitin nanofibers were used as a reinforcing agent for an anionic polysaccharide, that is, carboxymethyl cellulose (CMC), by electrostatic interaction [52]. CMC films, prepared by a casting technique, were immersed in the selfassembled chitin nanofiber methanol dispersions with the different contents, followed by centrifugation and drying, to produce the chitin nanofiber-reinforced films. Self-assembled chitin nanofiber-reinforced cellulose films were also fabricated (Fig. 2.6) [53]. When the cellulose ion gels with BMIMCl were soaked in the chitin nanofiber methanol dispersions with the different contents, two polysaccharides were composited through regeneration of cellulose to fabricate the chitin nanofiber-reinforced cellulose films. The unit ratios of chitin nanofiber to cellulose in the films increased in accordance with chitin nanofiber contents in the dispersions. The SEM images of cross-sectional areas of the films showed the tips of the nanofibers extending from the solid, suggesting that the chitin nanofibers were present not only on the surface but also inside the film. The amount of chitin nanofibers in the CMC and cellulose composite films strongly affected the enhancement of the mechanical properties under tensile mode. Self-assembled chitin nanofiber-reinforced natural rubber (NR) sheets were also fabricated [54]. The self-assembled chitin nanofiber dispersion with ammonia was mixed with NR latex stabilized with ammonia, followed by drying under reduced pressure to obtain the chitin nanofiberreinforced NR sheets. The tensile testing of the sheets indicated the

FIGURE 2.6 Preparation of self-assembled chitin nanofiber-reinforced cellulose film.

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reinforcing effect of the nanofibers in the sheets. When the above dispersions were heated to evaporate ammonia, followed by lyophilization, porous materials were fabricated. By evaporating ammonia stabilizer, the nanofibers were aggregated with NR, which were then agglomerated, forming spaces between them to construct the porous morphology (Fig. 2.7). The self-assembled chitin nanofiber-graft-synthetic polymer composite films were obtained by surface-initiated graft polymerization approach (Fig. 2.8) [55]. For example, chitin nanofiber-graft-biodegradable polyester and -polypeptide composite films were prepared by

FIGURE 2.7 Plausible process for formation of self-assembled chitin nanofiber/natural rubber (NR) porous material.

FIGURE 2.8 Surface-initiated graft ring-opening (co)polymerization and atom transfer radical (ATRP) polymerization from appropriate initiating sites on self-assembled chitin nanofibers.

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surface-initiated graft ring-opening polymerization of the corresponding cyclic monomers, that is, L-lactide (LA)/ε-caprolactone (CL) and γ-benzyl L-glutamate-NCA (BLG-NCA), initiated from hydroxy and amino groups, respectively, present on the chitin nanofiber films (NCA 5 N-carboxyanhydride) [56,57]. Surface-initiated graft atom transfer radical polymerization (ATRP) from chitin nanofiber macroinitiators was also conducted. ATRP is a versatile living radical polymerization technique, which has been employed to synthesize a wide range of well-defined polymeric materials [58,59]. Because ATRP is initiated by α-haloalkylacyl groups, the chitin nanofiber macroinitiator film with the initiating groups was prepared by acylation of the hydroxy groups on the self-assembled chitin nanofibers with α-bromoisobutyryl bromide. The surface-initiated ATRP of 2-hydroxyethyl acrylate (HEA) from the resulting macroinitiator film was then carried out in the presence of CuBr (catalyst)/2,20 bipyridine (ligand) in 3 wt.% LiCl/DMAc at 60 C to produce chitin nanofiber-graft-polyHEA films [60]. The stress
strain curves of the produced films under tensile mode indicated the larger elongation values at break compared with the original chitin nanofiber film, which increased in accordance with the monomer conversions. Such enhancement of flexibility was explained by grafting the longer polyHEA chains on the chitin nanofiber film. Surface-initiated graft ATRP of methyl methacrylate (MMA) on the chitin nanofibers under dispersion conditions was also attempted, followed by entanglement of the products to obtain chitin nanofiber-graft-polyMMA films [61]. The SEM images of the resulting films showed the nanofiber morphologies. The SEM image of the abovementioned chitin nanofiber-graft-polyHEA film, which was prepared by surface-initiated graft ATRP on the film, did not show nanofiber morphology. The polymerization occurred on the surfaces of the individual nanofibers during the surface-initiated graft ATRP of MMA under dispersion conditions. Then, the nanofibers were entangled during isolation, resulting in the nanofiber morphology.

2.4 Conclusion This chapter has provided an overview of the research focusing on the dissolution, gelation, and fabrication of blend and composite materials of chitin using ionic liquids. In particular, the chapter has been largely concerned with the author’s studies on the research topics using AMIMBr. Chitin-based materials, for example, films, self-assembled nanofibers, and composites, have been efficiently fabricated mainly by regeneration approaches through the dissolution and gelation with AMIMBr. Surface-initiated graft polymerization on the nanofibers has

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been revealed as an efficient approach to produce composite materials. Chitin is one of the most abundant organic substances, comparable to cellulose, and accordingly is expected to be used as a component in new functional bioactive and tissue materials because of its biocompatibility, biodegradability, and renewability. The studies on the fabrication of chitin-based materials using ionic liquids have developed significantly and will increasingly attract attention in the application fields related to the medicinal, pharmaceutical, and environmental industries in the future.

Acknowledgment The author is indebted to the coworkers, whose names are found in references from his papers, for their enthusiastic collaborations.

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347.

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C H A P T E R

3 Processing techniques of chitosan-based interpenetrating polymer networks, gels, blends, composites and nanocomposites M. Mehedi Hasan 1 , Md. Lawshan Habib 1 , Md. Anwaruzzaman1 , Md. Kamruzzaman 1 , M. Nuruzzaman Khan 2 and Mohammed Mizanur Rahman 2 1

Department of Applied Chemistry and Chemical Engineering, Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalgonj, Bangladesh, 2Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering and Technology, University of Dhaka, Dhaka, Bangladesh

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3.4 Processing techniques for chitosan-based interpenetrating polymer networks and gels 3.4.1 Photopolymerization 3.4.2 Cross-linking 3.4.3 Physical interaction

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3.5 Processing techniques for chitosan-based blends 3.5.1 Solution blending 3.5.2 Melt blending 3.5.3 Processing techniques of chitosan-based nanocomposites

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3.1 Introduction About a half of the whole world’s biodiversity is represented by the marine environment, which covers nearly the 70% of the Earth’s surface. As it is a chiefly unexplored terrain, marine organisms represent a great source of novel compounds that include both small molecules and macromolecules [1]. The principal marine origin materials can be classified broadly in three main groups, namely polysaccharides, proteins, and lipids. Among many marine polysaccharides, chitin is the one that stands out due to its availability, as it is the second most plentiful natural polymer after cellulose. Approximately 100 billion tons of chitin are produced on Earth by crustaceans, insects, mollusks, fungi, and similar organisms every year. It is the major structural component in the exoskeleton of various marine invertebrates [2]. Chitin or poly(β-(1-4)-N-acetyl-D-glucosamine) is a natural polysaccharide, first identified in 1884. In this form, chitin is insoluble in water and common organic solvents, dissolving only in specific solvents such as N,N-dimethylacetamide (DMAc)-LiCl, hexafluoroacetone or hexafluoro-2-propanol [3]. Structurally, three different polymorphs of chitin can be found: (1) α-chitin, which corresponds to a firmly compacted orthorhombic cell formed by alternated sheets of parallel and antiparallel chains; (2) β-chitin, in which the polysaccharide chains are arranged in a parallel fashion and the intermolecular forces are much weaker than those between the chains of α-chitin; and (3) γ-chitin, with an arrangement of two parallel and one antiparallel sheet. Of all these structures, most natural chitins have an α-type structure [4]. When the degree of N-acetylation (defined as the average number of N-acetyl-Dglucosamine units per 100 monomers expressed as a percentage) is less than 50%, chitin becomes soluble in aqueous acidic solutions (pH , 6.0) Handbook of Chitin and Chitosan

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and is then called chitosan [5]. Thus chitosan is a combined name for a group of fully and partially deacetylated chitins, but a rigid nomenclature with respect to the degree of N-deacetylation between chitin and chitosan has not been defined yet. Presently, chitin is extracted from marine shell waste streams at the industrial level, usually using chemical methods. The most common industrial process applied for chitin extraction consists of three main steps: deproteinization of the raw material by the addition of an alkaline solution, demineralization by the treatment with an acidic solution, and finally discoloration of the obtained product by treatment with an alkaline solution. Despite chitin being abundant and having exceptional functional features, such as biocompatibility, bioactivity, biodegradability, and high mechanical strength, it has limited utility due to its poor solubility [6]. This limits the performance of chitin and shifts attention toward CS, which is the primary derivative of chitin. CS is considered one of the most valuable polymers for biomedical and pharmaceutical applications due to its biodegradable, biocompatible, antimicrobial, nontoxic, and antitumor properties. Nanoparticles, microspheres, IPNs, hydrogels, films, nanocomposites, and fibers are typical CS-based forms for a miscellany of applications. Examples of such applications include, but are not limited to, biomedical, pharmaceuticals, antimicrobial, growth promoter, scaffolds, and water treatment.

3.2 Chitosan CS is a random copolymer obtained from the alkaline deacetylation of chitin, formed by D-glucosamine and N-acetyl-D-glucosamine units, linked by α-1,4-glycosidic linkages. The ratio between the two units is considered as the degree of deacetylation. When the deacetylation degree of CS reaches about 50%, it becomes soluble in aqueous acidic media [7]. The amino groups (pKa from 6.2 to 7.0) are completely protonated in acids with pKa smaller than 6.2 making CS soluble. CS is insoluble in water, organic solvents, and aqueous bases and it is soluble after stirring in acids, such as acetic, nitric, hydrochloric, perchloric, and phosphoric acids. When CS is dissolved in an acidic environment, the amino groups in the chain protonate and the polymer becomes cationic, allowing it to interact with diverse types of molecules, thus turning CS into the only cationic marine polysaccharide. This positive charge is thought to be responsible for its antimicrobial activity, via the interaction with the negatively charged cell membranes of microorganisms [7]. Being considered to be materials of great future potential with immense possibilities for structural modifications to impart desired properties and functions, research and development works on chitin and CS have reached a status of intense activity in many parts of the world. Handbook of Chitin and Chitosan

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3.3 Types of chitosan-based materials CS is the second most abundant next to cellulose, and is a naturally occurring amino polysaccharide derived as a deacetylated form of chitin. Its nontoxic, biocompatible, antibacterial, and biodegradable properties have led to significant research into its biomedical and pharmaceutical applications, such as drug delivery, tissue engineering, and wound-healing dressings. The types of CS-based materials are discussed below.

3.3.1 Chitosan-based interpenetrating polymer networks An interpenetrating polymer network (IPN) is defined as a blend of two or more polymers in a network with at least one of the systems synthesized in the presence of another [8]. This results in the formation of a physically cross-linked network when polymer chains of the second system are entangled with or penetrate the network formed by the first polymer. Each individual network retains its individual properties so synergistic improvements in properties like strength or toughness can be seen [9]. An IPN can be distinguished from a polymer blend in the way that an IPN swells but does not dissolve in solvents and creep and flow are suppressed [10]. They are also different from graft copolymers and polymer complexes that involve either chemical bonds and/or a low degree of cross-linking. From this point of view only, IPN can be generally named “polymer alloys” through which polymer blends can be made chemically compatible to achieve the desired phase morphology [11]. IPN can be distinguished from the other multiple systems through their bicontinuous structure, ideally formed by cross-linking of two polymers that are in intimate contact but without any chemical contact, and yields a material with improved properties depending on the composition and degree of cross-linking. 3.3.1.1 Interpenetrating polymer network based on chitosan and synthetic ionic matrices CS and its derivatives have been used as components in the formation of IPN composites with various ionic polymers containing carboxylic groups like poly(acrylic acid) (PAA) [12,13], copolymers of acrylic acid [14
17], poly(methacrylic acid) (PMAA) [18
20], poly(N-acryloylglycine) [21], or cationic centers like quaternary ammonium groups [22] and amine groups [23
25]. The synthesis of semi-IPN has been carried out either by selective cross-linking of CS in the presence of a preformed polyelectrolyte [13,18,23] or by the synthesis of the cross-linked polyelectrolyte in the presence of CS [12,14,15,19
21,24]. Full-IPNs have

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been also prepared by the postcross-linking of CS entrapped in a polyelectrolyte matrix [16]. In the case of the composite IPN of CS and polyelectrolytes containing carboxylic groups, the ionic interactions between 2 2NH1 from the anionic polyelectrolyte, 3 groups of CS and 2 COO which were identified in a certain range of pH, contributed to the increase of the mechanical properties of the gels and to the decrease of the degree of swelling because they contribute to the relative increase of the cross-linking density of the gel [13,18,19]. However, the ionic crosslinks allow the gels to be reversible in response to variations in the solution’s pH and ionic strength [12,17,18]. The interest in the preparation of IPN composites based on CS and polyanions containing carboxylic groups has been motivated by the finding of more efficient systems for the sustained release of proteins [14
16,20] and drugs [15]. Guo and coworkers have reported an interesting approach to obtain thermoand pH-responsive semi-IPN polyampholyte hydrogels based on carboxymethyl CS and poly(dimethylaminoethyl methacrylate) (PDMAEM) [24]. The semi-IPN hydrogel shrunk most at the isoelectric point (IEP) and swelled when pH deviated from the IEP. In the presence of PDMAEM, which presents a lower critical solution temperature (LCST), the swelling ratio of the composite gel dramatically decreased between 30 C and 50 C at pH 6.8. The key advantage of this composite hydrogel is that the release rate of coenzyme A could be modulated as a function of temperature, being higher at 50 C than at 37 C and 25 C at pH 6.8, affording the semi-IPN hydrogel great promise in pH/temperature responsive drug delivery systems. 3.3.1.2 Interpenetrating polymer network based on chitosan and synthetic nonionic matrices Numerous IPN composite hydrogels have been prepared by crosslinking polymerization of nonionic monomers in the presence of CS, the most employed monomers being acrylamide (AAm) [26
32], N-isopropylacrylamide (NIPAAm) [33,34], N,N-dimethylacrylamide [35], and 2-hydroxyethyl methacrylate (HEMA) [36
39]. Currently, modifications of the mechanical properties and the water content of hydrogels by the preparation of the abovementioned IPN gels are expected; one main purpose being their use in biomedical applications such as in controlled release systems and as scaffolds in tissue engineering. Kim et al. described an interesting approach for the preparation of semi-IPN composed of CS and poloxamer [40]. Their strategy consisting of photocross-linking the poloxamer macromer in the presence of CS coupled with freeze-drying to obtain sponge-type hydrogels. These IPN composite hydrogels demonstrated rapid water adsorption, high mechanical strength, and interconnected pores, which recommend them for wound-dressing application. Other strategies for the synthesis of

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IPN composite hydrogels consist of the blending of CS with preformed synthetic polymers like poly(acrylamide) (PAAm) [26], polyacrylonitrile (PAN) [41,42], poly(ethylene glycol) (PEG) [43
45], poly(vinyl alcohol) (PVA) [46
48], poly(vinyl pyrrolidone) (PVP) [49
51], poly (dimethylsiloxane)
PEG copolymer [52] followed by the selective crosslinking of CS. pH and temperature responsive semi-IPN hydrogels have been obtained by the cross-linking of CS in the presence of PAN [41,42]. 3.3.1.3 Interpenetrating polymer network based on chitosan and natural matrices A semi-IPN (semi-IPN) composed of cross-linked CS with glutaraldehyde and silk fibroin was prepared by Chen et al. [53]. According to them the CS and silk fibroin had a strong hydrogen bond interaction and formed an interpolymer complex. The semi-IPN showed good pH sensitivity and ion sensitivity. IPN hydrogels based on CS and gelatin using genipin as the cross-linker were also prepared by Cui et al. [54]. Swelling results revealed that the IPN hydrogels are pH-sensitive, exhibit reversibility, and have rather rapid swelling in response to pH changes.

3.3.2 Chitosan-based gels The term “gel” is usually associated with highly hydrated networks in which two components are present in different proportions, that is, the solvent—the one prevailing in mass—and the polymeric solute; typically, the latter are either natural or synthetic macromolecules able to properly retain a large amount of the former component. Due to CS’s low-toxicity, high abundance in nature, and being a positively charged biopolymer, gels of CS have been widely accepted for bioengineering applications. 3.3.2.1 Chitosan gels without external cross-linkers One of the simplest ways to produce physical gels of CS—without the use of any external gelling agent—is to precisely modulate the polymer chemical composition. Hirano et al. reported the formation of a true physical CS gel via controlled re-N-acylation processes of an almost completely deacetylated CS [5]. Specifically, CS solutions were treated with large amounts of acetic anhydride as an acetyl group donor. Authors claimed that the excess of acetic anhydride was pivotal for the gel formation, leading to an acetylation of both amino and hydroxyl groups present in CS. The final result was a colorless, transparent, and rigid gel composed of highly acetylated polymer chains, showing a degree of substitution of 2.36 per monosaccharide unit. Molecular

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aggregation between acetylated polysaccharide chains was proposed as the gelation mechanism. The same gelling system was further studied by Domard and coworkers [55]. They identified some critical parameters allowing for gel formation. For instance, a minimum molar ratio R between the anhydride over D sugars of 1.3—yielding CS with FA around 0.8—was found necessary for the complete gelation. The same authors proposed the application of such a gelling system as injectable material for periodontal surgery purposes [56]. Physical CS gels were also formed without the use of any external cross-linker in the presence of an aqueous ammonia solution [57]. In order to allow for gelation, three key conditions had to be met: (1) the initial concentration of solutions had to be over the critical concentration of chain entanglements, C*; (2) the hydrophobic/hydrophilic balance had to achieve a critical value; and (3) an interphase, corresponding to a bidimensional sol
gel transition, had to be created uniformly. The latter condition was achieved by the use of gaseous alkaline ammonia instead of an alkaline solution, since a heterogeneous gelation was noticed when CS was dialyzed against an aqueous ammonia solution. Recently Fiamingo et al. exploited gaseous ammonia for gelling CS solutions at different polymer concentrations. The resulting gels were used in vivo to evaluate the potential regeneration of infarcted myocardium, showing that such materials were well incorporated onto the epicardial surface of the heart, with a general lack of toxicity [58]. 3.3.2.2 Ionic chitosan macrogels Sacco et al. reported the fabrication of cylindrical CS gels through a controlled external gelation using tripolyphosphate (TPP) as a crosslinker. TPP is a multivalent anion, which shows from one to five negative charges depending on pH. A typical gelation requires that CS is acid solubilized in order to protonate all D sugars. The resulting CS solution was cast into a mold, and subsequently placed in a gelling bath containing TPP [59]. This approach avoids the instantaneous gelation of CS and allows for a uniform distribution of the cross-linker throughout the solution. A rapid ionic gelation of CS using a 6-phosphogluconic trisodium salt (6-PG-Na1) was recently reported [60]. The formation of a macroscopic gel was demonstrated by rheometry. From the biological point of view, such gels did not cause toxicity and dermal irritation, with potential application as a vehicle for topical administration or as a wound dressing. Va˚rum and coworkers reported on the fabrication of macroscopic ionic CS gels using mannuronan oligomers as cross-linkers [61]. Ionic CS gels could be obtained also using a metallic anion based on Mo(VI) [62]. In the case reported by Draget et al. a dispersion of MoO3 is added to a CS solution and the sol
gel transition was verified by rheometry. The gelation mechanism is based on the gradual ionic

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interaction under acidic conditions between molybdate polyoxyanion (Mo7O2462) and D residues on CS. The resulting gels showed lower mechanical properties if compared with Ca21
alginate gels produced via internal gelation using similar experimental conditions, due to the different binding mode of Mo7O2462 to CS with respect to Ca21 in alginate. Finally, the authors demonstrated that Mo-containing CS gels swelled/deswelled depending on the ionic strength of the incubation medium. 3.3.2.3 Ionic chitosan micro- and nanogels CS forms polyelectrolyte complexes with negatively charged small molecules or macromolecules if the concentration of polymer is below or close to C*. Different polyanions have been extensively investigated to form micro- and nanogels with CS, including inorganic ions such as TPP, polysaccharides such as hyaluronan or alginate, and synthetic polymers [63,64]. The ionotropic micro- or nanogelation of CS usually requires a single injection or a dropwise titration of the polyanion into the CS solution at room temperature until a turbid system is obtained [65,66], thus indicating the onset of coacervation. The final outcome is a phase separation where a more viscous polymer-rich phase, the “coacervate,” is noticed [67]. The first evidence of the possibility to form nanogels based on CS was reported by Calvo and coworkers in 1997 [65], with the use of TPP as the cross-linker. The authors evaluated the influence of CS and TPP concentrations on the physicochemical properties of the resulting nanogels. Three different results were observed: (1) solutions at high CS concentration and low TPP amounts; (2) insoluble aggregates at low CS concentration; and (3) nanogels at intermediate CS and TPP concentrations. Lapitsky and coworkers extensively investigated the formation mechanism of CS/TPP microgels and their properties by tuning different parameters, such as FA, the molecular weight of CS, pH, and the ionic strength and concentrations of polymer and crosslinker [68
72]. The kinetics of gelation was dramatically slowed down by simply tuning the TPP and monovalent salt (NaCl) amounts [68]. CS-based nanogels usually tend to aggregate after lyophilization and spray-drying [73]. Rampino et al. investigated the possibility to improve nanoparticle stability after drying in the presence of cryoprotectants such as trehalose, PEG, and mannitol [74]. One of the most interesting polyelectrolyte CS-based complexes is that formed with hyaluronic acid (hyaluronan, HA) [75]. CS
hyaluronan nanogels are typically obtained by dropping hyaluronan into a CS solution [66,76]. Alginate is another widely used polysaccharide to form CS-based nanogels [64]. Stokke and coworkers investigated the possibility to synthetize coacervates using polymers with different physical/chemical composition and by varying the modality of mixing of the two polysaccharides [77].

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3.3.2.4 Thermosensitive gels Thermosensitive gelling systems consist of stable, low viscosity, aqueous solutions that turn into a gel state upon heating. The sol
gel transition usually occurs at neutral pH and takes a few minutes [78,79]. The first evidence of the possibility to synthesize thermosensitive gels based on CS was reported in 2000 by Chenite and collaborators [80]. Thermosensitive gelling systems were devised by combining CS and polyol-phosphates, polyol-bearing molecules, or inorganic phosphate molecules. At first, β-glycerophosphate (β-GP) was used as a gelling agent [80], then the attention moved also toward other polyol-phosphates such as glucose-1-phosphate (G1-P) and glucose-6-phosphate (G6-P) [81]. As polyol molecules, 1,3-propanediol, 1,2-propanediol, mannitol, trehalose, and mannitol were used as well. As inorganic gelling agents, hydrogen phosphate salts—ammonium, sodium or potassium—were also used [79]. Thermosensitive gels based on CS and β-GP were studied as materials for cartilage and bone regeneration. Hoemann and coworkers described the utility of such materials as arthroscopically injectable vehicles for cell-assisted cartilage repair [82]. Different approaches for obtaining thermosensitive gels based on CS were undertaken using inorganic phosphate: hydrogen phosphate salts. Nair and coworkers reported that by adding different concentrations of ammonium hydrogen phosphate to CS solution it was possible to tune the gelling time from 5 min to 30 h at 37 C [83]. These gels did not show any toxicity toward osteoblast-like cells. Moreover, they were suitable as stem cell and payload carriers.

3.3.3 Chitosan-based blends Polymeric blends are inexpensive and enable materials with a full set of tailored properties and improved specific properties. The combination of CS derivatives with other natural or synthetic polymers results in mitigation of inherent disadvantages, for example, the poor mechanical stability of collagen can be enriched with the addition of CS by blending. 3.3.3.1 Synthetic polymer blended chitosan Studies on the preparation and the potential physicochemical properties of CS with poly(vinyl alcohol) (PVA) have been reported [84,85]. Incorporation of PVA into a CS blend produced a reduction of tensile strength, improvement of elongation at break, increment of moisture content, and reduction of crystallinity in the CS network in the blend [86]. Poly(glutamic acid) (PGA) was chosen to modify CS matrices by enhancing the hydrophilic and cytocompatibility of CS-based

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biomaterials [87
89]. PGA has many carboxyl groups and is therefore considered as a polyanionic biopolymer [90]. As CS is a positively charged polyelectrolyte in an acidic environment, when a negatively charged polyelectrolyte PGA was introduced, a polyion complex can be formed by electrostatic attraction at the interfaces [91
93]. This phenomenon was the main reason for the increase of the mechanical properties of CS matrices up to 25%
50% due to the incorporation of 0%
20% PGA [94]. Poly-ε-caprolactone (PCL) has been widely used as a copolymer in CS blend materials [95
97]. The hydrophobicity as well as the lack of functional groups of PCL is reduced by incorporation of CS matrix [98]. This strategy suggests that the hydrophobicity of PCL makes it possible to produce a blended material with good biocompatibility, full biodegradability, adequate mechanical strength, and that is available for biofunctions [99]. PAN is one of the most important filmforming polymers with some disadvantages, such as strong hydrophobicity and electrostatic accumulation [100]. It is expected that the antistatic properties and antimicrobial activity of hydrophobic PAN can be improved by introducing CS as a copolymer blend [101]. PEG has been widely used blended with CS because of its biocompatibility and minimal toxicity and desirable properties in water or other common solvents [102,103]. 3.3.3.2 Natural polymer blended chitosan Collagen-based biomaterials have been widely used due to their excellent binding capacities [104]. Recently, research groups have reported that CS/collagen blended could improve the regeneration of tissue in vitro of the human body, including bone, cartilage, cardiovascular tissue, nerves, and bladder, and minimize scars in regenerated tissue as human cells could attach and organize well around the blended material [105,106]. Besides collagen, the incorporation of gelatin with CS polymer was able to stimulate the production of a loose collagen network in skin wounds and reduced the risk of hypertrophic scarring by maintaining a condition of high hydration that favors the tissue repair process [107
110]. It was found that the complex between CS and gelatin was formed mainly through hydrogen bonding [111] but the size of structure was also affected by electrostatic repulsions [112,113]. CS/cellulose derivative blend is uses different kinds of cellulose derivatives, such as sisal, bacterial cellulose, alginate, methylcellulose, ethyl cellulose, carboxymethyl cellulose, cellulose acetate, oxycellulose, and hydroxylethyl cellulose [114], as the copolymer with CS. Since the chemical structure of the CS backbone is very similar to that of cellulose, it is expected that CS could be miscible with cellulose and the blending might improve the chemical, physical, mechanical, and biological properties of developed films [115
117]. One type of fast growing CS blend

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that affects the elastic properties is made by blending natural rubber (NBR) and CS in definite proportions. Characteristically, the blend consisting of a rubber soft segment gave rise to elastic properties [118], while the crystalline hard segment was contributed by CS polymers [119]. The CS/NBR blends were prepared with dicumyl peroxide as a cross-linking agent and it was found that a gradual transition from rubbery to plastic nature resulted as the CS content increased, the chain flexibility of the system was highly restricted, and the elongation was drastically reduced [120
122].

3.3.4 Chitosan-based composites and nanocomposites Composite and nanocomposites of CS have numerous applications in different fields. Composites with polymer, both natural and synthetic, and inorganic materials have been reported in many studies for applications ranging from tissue engineering to drug delivery to environmental remediation. 3.3.4.1 Synthetic filler/chitosan-based composites and nanocomposites Composite fibers composed of CS and carbon nanofibers (CNs) have been fabricated with several difference techniques, such as the wet spinning method, wet casting process, spray layer-by-layer, solution intercalation, electrodeposition, microwave irradiation, and in situ gel-forming method [123]. Additionally, the dispersion state of single carbon nanotubes (CNTs) in a CS matrix plays a major role in deciding the ultimate properties of the composites. Several approaches have been used to achieve the optimum dispersion of CNTs including ultrasonication and acid functionalization [124,125]. It was found that good dispersion of CNTs produced nanocomposites with a dramatically increased Young’s modulus and tensile strength compared with native CS film. The formation of an ionic interaction between anions of a metal nanoparticle and cationic CS has inspired many works that have focused on metal or CS/metal oxide composites. Researchers have developed a biosensor based on the gold nanoparticles (Au)
CS sol
gel composites to detect various types of a specific electrode. The large quantities of OH groups provided by CS matrix form strong hydrogen bonds, which interact with the immobilized enzymes, stopping them from leaking out of the film [126,127]. The proposed methods have shown potential applications to investigate new biosensors against specific DNA, biological tissue, H2O2, cancer cells, carbohydrate derivatives, antigen and protein derivatives, and lipids [128]. Composite materials based on CS nanoparticles encapsulating silver (Ag) have been prepared by several researchers [129
131]. The advantage of Ag/CS composites over

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Au/CS composite is the antibiotic or antimicrobial properties of Ag particles [132,133]. Arias et al. [134] prepared magnetite (Fe3O4)/CS nanocomposite for intravenous supply of the anticancer nucleoside analogue gemcitabine. The magnetic responsiveness was examined by recording the hysteresis cycle via exposure to a 1.1.T permanent magnet. The entrapment of the drug into the polymeric shell gives higher drug loading along with a slower drug release profile. 3.3.4.2 Natural filler/chitosan-based composites and nanocomposites An approach in developing mineral filler such as hydroxyapatite (HA) or zeolite (ZE) has been investigated. It was expected that CS/HA composites could provide a combination of biocompatibility, antimicrobial activity, and other important properties for application in biomedical implants [135
137]. The HA powders were prepared by a chemical precipitation method and used for the fabrication of CS/HA through the electrodeposition process [138
140]. Other Cs-based composites material using HA were reported by other researchers [141,142]. The properties of α-chitin whisker-reinforced CS nanocomposite films were analyzed by several studies [143
145]. The addition of α-chitin whiskers did not have much effect on the thermal stability and did not show any apparent degree of crystallinity of the CS matrix. The tensile strength of α-chitin whisker-reinforced CS films was increased from that of the pure CS film with the initial increase in the whisker content (2.96 wt.%) and decreased gradually with further increase in the whisker content, while the percentage of elongation at break showed an inverse trend at the similar compositions of α-chitin whisker [143]. It is worth noting that similar behavior to that observed by the above researchers was also observed by other researchers when studying the properties of starch-filled CS film [146
148]. Regardless of starch type, the tensile strength and elasticity of composite film first increased and then decreased with the addition of starch. The concept of fabricating CS-based nanocomposites was recently applied to develop CS
montmorillonite (MMT) nanocomposites. The idea was based on the arrangement of CS polymer as a bilayer when the amount of the intercalated CS was formed through cationic exchange and formation of hydrogen bonding via the cationic site of CS with the sodium site (Na) of MMT [149,150]. In particular, research on the interaction of CS with wood-based products or wood-based components, which are very useful biomass resources, has become of interest in recent years. The effect of the addition of kenaf dust into CS matrix was studied with respect to the resulting mechanical, thermal, and degradable properties [128,151,152]. It was clarified that the CS matrix markedly modified the mentioned properties with the increase of the amount of kenaf dust up

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to 28 wt.% by the formation of inter- and intra-hydrogen bonding of composite components.

3.4 Processing techniques for chitosan-based interpenetrating polymer networks and gels CS, being a nontoxic, biocompatible, and biodegradable material, attracts more intention for its use in IPN hydrogels—complex polymers composed of two kinds of polymers, and gels for biomedical applications [153
155]. Several processing techniques have been reported over the last few years.

3.4.1 Photopolymerization Photopolymerization is one of the most promising techniques. This can be done either in the presence or absence of a photoinitiator (PI). The use of PIs is a growing concern with regards to their incomplete usage in the polymerization process. This sometimes results in undesirable impurities, which exist as unreacted fragments formed from their photolysis process. These fragments, trapped in the polymer, can be leached out over time [156]. That is why PI free polymerization process has been practiced recently. The mechanism for the initiation of the photopolymerization process without PI involves a donor/acceptor pair in the presence of UV light [157]. This mechanism involves strong interactions between a donor and acceptor when both species are exposed to a UV source, leading to the formation of an intermediate. This intermediate species then breaks down to form free radicals, which initiate polymerization. A significant advantage of using a donor/acceptor pair in the initiation process is that it has the dual function of acting as an initiator as well as forming a copolymer. For example, photopolymerization of N-vinylpyrrolidinone (NVP) within the CS matrix can be initiated by a donor/acceptor pair with NVP as the donor and N-hydroxymethylmaleimide (HMMI) and 2-hydroxyethyl methacrylate (HEMA) as the acceptors, under the influence of a UV source. HMMI and HEMA each possess an electronwithdrawing group adjacent to the double bond in their structures, thus contributing to the electron-accepting property in these monomers [50]. In the presence of a strong hydrogen donor the rate of polymerization is found to be increased [158]. Hydrogen donors are compounds which contain abstractable hydrogens located adjacent to heteroatoms such as oxygen, nitrogen, and sulfur. When N-substituted maleimides (MI) (an acceptor) and vinyl ethers (donors) are subjected to UV radiation in the presence of a hydrogen donor, the excited state MI can undergo intermolecular hydrogen abstraction. After hydrogen abstraction, these donors

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bear free radicals that are capable of initiating polymerization. Thus the hydrogen abstraction process involving intermolecular abstraction will enhance the rate of polymerization to a greater degree as a result of the production of more free radical species [157].

3.4.2 Cross-linking Increased interest in chemically cross-linked gels in the current era is due to their good mechanical strength. Following are the different methods used to synthesize chemically cross-linked hydrogels. Hydrophilic polymers contain different hydrophilic functional groups, like
COOH,
NH2,
OH, etc., which help to formulate hydrogels. Reactions such as an amine
carboxylic acid or isocyanate
OH/ NH2 reactions or Schiff base formation may be used to recognize covalent linkages between polymer chains. Hydrophilic polymers with
OH groups, for example, polyvinyl alcohol, may be cross-linked through glutaraldehyde [159]. To establish cross-linking, tight conditions are applied (low pH, methanol added as a quencher, high temperature). Alternatively, polymers with amine groups may be cross-linked by the use of the same cross-linker under mild conditions in which Schiff bases are formed. This was specially designed for the cross-linked protein synthesis, for example, gelatin and albumin and the amine containing polysaccharides. While physically bonded hydrogels have the advantage of gel formation without the use of cross-linking entities, they have limitations. It is difficult to precisely control the physical gel-pore size, chemical functionalization, and degradation or dissolution, leading to inconsistent performance in vivo. IPNs and semi-IPNs based on CS can be obtained by a chemical crosslinking process. Stimuli responsive gels based on an interpenetrating network of CS and PVP can be obtained by this technique. In that case CS and PVP are mixed together and gluteraldehyde can be used as a crosslinker. A semiinterpenetrate network will be obtained which may be used to prepare membranes with swelling properties in a wide pH range [49].

3.4.3 Physical interaction Physical interactions like electrostatic interactions, hydrophobic interactions, or hydrogen bonding can also be utilized to form different hydrogels based on CS. These interactions are dependent on various parameters, such as concentration, pH, and temperature. Gels formed by physical interactions are not very stable, rather they can perform reversible gelation. The swelling properties of these hydrogels can be modified by changing the nature and amount of raw elements.

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CS-based hydrogels can also be formed by blending CS with nonionic water-soluble polymers, such as PVA [160]. Thermosensitive hydrogels can be obtained by blending CS with polyol salts, such as glycerol phosphate disodium salt [80]. CS’s structure can also be modified to form physical hydrogels: CS-g-PEG graft copolymer is capable of self-organization, depending on the temperature, to form stable hydrogels [161].

3.5 Processing techniques for chitosan-based blends Polymer blending is a method that is commonly used for making desirable polymeric materials with combined properties for particular applications [162]. Blending is done to enhance the specific properties of a specific polymer. Modification of CS by means of blending is an attractive method that has been extensively used for providing new desirable characteristics to CS [162,163]. This is mainly due to its simplicity, availability of a wide range of synthetic and natural polymers for blending, and effectiveness for practical utilization. The selection of polymers to be blended with CS depends on the end product’s application demands, such as enhanced hydrophilicity, enhanced mechanical properties, improved blood compatibility, reduction in production costing, and enhanced antibacterial properties [164]. This improvement of the base CS can be done by blending CS with various natural and synthetic polymers, such as sodium alginate [165], tropocollagen [166], cellulose, sodium hyaluronate, sodium heparin, sodium chondroitin sulfate, and poly(acrylic acid) [167]. For example, hydrophilicity of the CS can be increased by blending with PEG and PVA; mechanical properties can be enhanced by blending with silk fibroin, cellulose, and polyamides; faster burn healing films can be prepared from the blending of CS and minocycline hydrochloride [168].

3.5.1 Solution blending In this method, CS is dissolved in a slightly dilute acidic solvent and a second desired polymer is dissolved in the solvent properly for a long time. After proper homogenous mixing of the polymers, the blended mixture is spread over the Petri dish followed by evaporation, which results in formation of a film. The mechanical properties of the blended mixture can be increased by adding cross-linking agents such as glutaraldehyde [169]. Physicochemical properties of CS/PVA blends have been studied in the presence of plasticizers like sorbitol and sucrose. The melting point of the product had lower melting and heat of fusion temperature compared with pure CS. The presence of plasticizers affected the mechanical properties by decreasing the tensile strength and modulus [170]. An

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increase in elongation was possible by blending CS with PEG [171]. In another study, cellular biocompatible nanofibers of CS and poly ethylene oxides were prepared by solution blending. Blended mats of nanofibers were found to promote the attachment of human cells, while preserving their morphology and viability [1,172]. The addition of ultrahigh molecular weight PEO favored the preparation of fibers ranging from less than a hundred nanometers to a few tens of micrometers [173]. The diameter of CS nanofiber can further be decreased by adding polyvinyl pyrrolidone (PVP) polymers [174]. One of the major problems regarding the synthesis of nanofibers is the formation of beads in the fibers. In the case of CS, due to its high cationic nature, nanofibers of CS are formed with a very high content of beads. This problem can be resolved by blending PVA in the CS solution. Blending of PET with CS formed nanofibers with enhanced biocompatibility and antibacterial properties compared to pure PET [173]. These blends thus offer great prospects for designing tissue engineering scaffolds [175].

3.5.2 Melt blending CS was melt blended with PCL, poly(butylene succinate) (PBS), poly (lactic acid) (PLA), poly(butylene terephthalate adipate) (PBTA), and poly(butylene succinate adipate) (PBSA) [176]. Depression in melting temperature had been observed in the case of CS/PBS or PBSA blend. The addition of CS in PCL, PBS, and PBSA resulted in lower crystalline products. In all cases, the addition of CS decreased the tensile strength of the polyesters but increased their tensile modulus. Microscopic evaluation of the materials revealed that the addition of high Tg polymers like PLA in CS resulted in brittle-like fractures on the surface and a fibrous appearance at the surface, as obtained a by blend of CS/PCL. In some studies, CS is plasticized with a high content of polyols, water, and acetic acid. Despite a reduction in size of the dispersed CS phase, the irregularly in geometry of the dispersed phase indicated the ineffective and improper melting of CS. To resolve this problem a new approach has been proposed by Grande et al. [153]. The process involves the use of spray- and freeze-drying techniques to produce acid-free blends of PVA/CS, which is then incorporated into a PLA matrix by extrusion. SEM analysis showed that oven-dried blend materials had a wide size distribution and irregular geometry. The particles had dimensions in the range of 30
300 μm [153].

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than 100 nm [177]. High specific interfacial area and controlled stress transfer across the interface are the two essential key aspects responsible for the successful development of nanocomposites. The first key factor is of major importance concerning the properties of nanocomposites. The large specific area of the nanoparticles provides a high surface to volume ratio, resulting in extensive binding between the polymer and the nanofiller [178], CS is a remarkable naturally available polymer with versatile applications in numerous fields. CS nanocomposites have gained much attention in recent years. These nanocomposites have been extensively studied and applied in many fields, such as water purification, cosmetics, pollutant detection, dye adsorption, and heavy metal removal. CS has antimicrobial activity which makes it perfect for application in wound dressing and tissue engineering. Scaffolds of CS incorporated with nanomaterials have been reported to have promising application in bone treatment and drug delivery. Nanocomposites of CS can be prepared by following many methods. The selection of the method influences the purity, characteristics, and field of application to a significant level. 3.5.3.1 Mechanical stirring Composites of polymers can be prepared easily by mixing two or more polymers at specific weight ratios. But in the case of CS it is not easy to obtain a homogenous system because of its insolubility in many common solvents. It is quite difficult to find a solvent in which both the CS and the subjected polymer dissolve. For example, Levengood and Zhang reported that a homogenous system of CS and polyester was difficult because of the lack of cosolvents that can accommodate both polymers [2]. In some studies, mechanical stirring was utilized to fabricate a homogenous suspension of milled CS microparticles in a solution of PLGA in methylene chloride. After that microsphere composites of CS and PLGA were formed by aid of solvent evaporation and microparticles were fused to yield scaffolds [179]. 3.5.3.2 Solvent casting Solvent casting is one of the most widely used techniques for the preparation of films with nanocomposite materials. Prepared films have versatile applications ranging from biomedical to water purification. The lack of the need for very expensive equipment like freeze dryers or high processing temperatures are the main reasons behind the adaptability of this method [6,180]. Generally, in this method polymer is dissolved in a suitable moderately volatile solvent and then cast onto a casting surface, such as a Petri dish or aluminum foil. Then solvent is allowed to evaporate at room temperature or in an air oven, which results in the formation of dry films [181]. Keeping the advantages of this technique in mind many researchers have used this cheap Handbook of Chitin and Chitosan

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and easy method to prepare nanocomposite films and membranes of CS with various nanomaterials [6,178]. CS hydroxyapatite (HA) nanocomposites have been prepared by utilizing the solvent casting method [182]. In this study the effect of formaldehyde on the mechanical properties of the nanostructured composite films were examined. During the synthesis mediation of HA was done by the incorporation of CS in the reaction. Both Young’s modulus and ultimate tensile strength, with the same HA content, was found to be significantly higher for nanocomposite films with formaldehyde compared with pristine films of CS and HA, due to the strong interaction with CS and HA. Rhimet al. used an unmodified montmorillonite (Na-MMT), an organically modified montmorillonite (Cloisite 30B), a nanosilver, and a Ag-zeolite (AgIon) to prepare four different types of CS CS-based nanocomposite films using the solvent casting method to explore the effects of these nanoparticles on mechanical properties, water permeability, and antimicrobial activity of the CS [183]. Studies showed that nanomaterials were uniformly dispersed and intercalated to certain degrees throughout the films. Due to the intercalation mechanical and barrier properties were affected. Tensile strength increased by 7%
16% while water permeability decreased by 25%
30%, depending on the nanomaterial tested. Moreover, all the CS-based nanocomposite films showed a promising range of antimicrobial activity, especially the films containing nano-Ag. Genipin cross-linked CS-based reduced graphene oxide (rGO) nanocomposite films fabricated by the solvent casting method have been reported to have higher tensile and thermal stability compared to pure CS films. Tensile strength of the films was increased because of the interaction of CS macromolecules and functional groups of rGO sheets. In addition, an increase of rGO in the films increased the wet stability by reducing the swelling [184]. In an another study, nanocomposite membranes of CS and bioglass were prepared by the solvent casting method [185]. Bioactivity and viscoelasticity were investigated and it was found that the immersion of nanocomposite films in SBF induced the development of an apatite-like structure. The study of the viscoelasticity of the membranes by dynamic mechanical analysis revealed that a continuous decrease in storage modulus was present which tended to approach to the storage modulus of the pure CS membranes. This loss of stiffness indicated the possibility of the dissolution of the BG particles from the membranes. In spite of many advantages of the solvent casting due to some unavoidable limitations, this method cannot be used for some sophisticated purposes. Possibility of retention of residual solvent may lead to toxicity effects arising for cell transplantation and inflammation around the wound [186]. Another drawback of this technique is the limitation in shaping the final products. Only simple common shapes can be given to the

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films, whereas complex structure scaffolds are required for biomedical application [180]. 3.5.3.3 Freeze-drying Freeze-drying is one of the most extensively used methods for the preparation of highly porous scaffolds by means of thermal phase separation. In this process, polymer is dissolved in a solvent, generally water based, and other ingredients are dispersed in the solution. After proper mixing the solution is frozen and frozen particles of solvent are formed. Then, the frozen solvent, through sublimation, leaves the polymeric structure forming a pore. Morphology and other properties of the resulting porous scaffolds can be controlled by controlling the polymer and its concentrations [4]. A composite of CS/graphene oxide and HAp nanoparticles was synthesized by the freeze-drying method by Mohandes and coworkers [187]. In this study nano-HAp was prepared by a simple precipitation method with the aid of a capping agent. Bioactivity of the composite was evaluated by immersing it in SBF for several days. Results showed that the composite showed a higher amount of Ca and P release compared to pure HA. Moreover, due to the strong interaction between GO and CS, the elasticity modulus was found to be 200 times higher than pure CS. In an another study of CS and HA, TiO2 was used as an auxiliary component to fabricate porous nanocomposite bone scaffolds [188]. In here, CS-graft-poly(AAc)/nano HA was prepared by the freezedrying method. Prepared nanocomposite was porous and the pores were interconnected. Their bioactivity and mechanical properties were evaluated. The presence of poly(AAc) overcame the brittle nature of HA and gave the scaffold a significantly higher elastic nature. The data showed that the composite had a compressive modulus of 6.5 MPa, which is similar to the natural cancellous bone of human, and excellent bioactivity with no cytotoxic effect. Freeze-dried macroporous cyrogels were prepared from an aqueous colloidal suspension of CS/xanthan gum/Na1
montmorillonite nanoclay (MMT) by Liu et al. [189]. In order to investigate the effect of experimental conditions different rates of freezing were applied. It was found that a slow rate of cooling during freezing step led to the formation of larger pores and a more aligned and tilted pore distribution. This tilted structure of slowly cooled composite influenced the mechanical properties by increasing the hardness value compared to rapidly cooled scaffolds. Small-angle X-ray diffraction data indicated that the polymeric networks were modified by the exfoliated MMT which resulted in enhancement of the mechanical strength of the composites due to its well-known reinforcement effect as a nanofiller. Antimicrobial sponges of CS, hyaluronic acid, and Ag NPs were prepared by Anisha et al. by the freeze-drying method for wound

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dressing [190]. A homogenous mixture of components followed by lyophilization resulted in a flexible and porous structure. Prepared sponges were composed of drug-resistant bacteria. A sponge-like blend scaffold of CS and anionic, biocompatible, highly hydrophilic, and biodegradable biopolymer alginate was prepared by a freeze-drying method. The prepared sponge scaffold was biodegradable. In vitro analysis, using lysozymes, showed extended drug release over 20 days and in vivo animal testing showed that the sponge had a better effect than cotton gauze. The healing effect of the sponge is further enhanced by the addition of curcumin [191,192]. A comparative study between freeze-drying and solvent casting of CS and CNT nanocomposite was done by Sun and coworkers [193]. They had found that the tensile stress and ductility of films by solvent casting are higher than by freeze-drying, probably due to the porosity. 3.5.3.4 Layer-by-layer In 1996 Ilher proposed a novel technique capable of making polymeric nanocomposites and films with highly ordered and modified surfaces [194], named layer-by-layer (LbL) assembly, later on popularized by Decher [195], This simple, reproducible, and flexible method is based on the sequential adsorption of different macromolecular components, which are attracted to each other due to electrostatic interactions, hydrogen bonding, van der Waals forces, and electron exchange, among others [196]. Different LBL approaches such as dip coating, spin coating, and spray coating can be used to build up a multilayer film or membrane [197]. Any nature, size, shape, and chemical composition and assembly can be assured for the resulting final product by LBL methods because of the great availability of the building blocks, such as CNT, clays, NPs, and polymers [198]. In addition, the properties of multilayered devices can be tuned through solution pH, temperature, or ionic strength [196]. Using LBL, Mesquita et al. fabricated a novel biodegradable nanocomposite of CS and eucalyptus wood cellulose nanowhiskers (CNW) [199]. The CS/CNWs multilayers were prepared on substrates with a negative excess of charge, such as glass or quartz slides. Hydrogen bonds and electrostatic interactions between the negatively charged sulfate groups on the whisker surface and the ammonium groups of CS were the driving forces for the growth of the multilayered films. Studies showed that nanowhciskers were distributed evenly all over the layers of CS and it was possible to achieve a uniform bilayer thickness of 7 nm. LBL assembly has been used to fabricate bioglass nanoparticle/ CS/alginate scaffolds by Yang et al. A decline in porosity was observed when bioglass scaffolds were immersed in the solutions of CS and alginate. After three cycles of immersion in both polymers, due to the

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infiltration of polymers porosity was 1.8% less compared with pristine bioglass scaffolds. This continuous assembly of polymer also results in the increase of elastic modulus of 80 MPa and a compressive modulus four times higher than the BG scaffolds [200]. In an another study, by utilizing the polycationic nature of CS a multilayer composite film with poly(γ-glutamic acid) (γ-PGA) was prepared by the LBL method [201]. In this study, low immunogenic and low cytotoxicity γ-PGA was microbially produced, and it is known for its polyanionic character. Results showed that with the increase of layer number surface roughness also increased and FTIR analysis revealed the presence of both CS and γ-PGA in the self-assembled structures and highlighted the electrostatic interaction between the polyions. Cytotoxic evaluation reveals that this composite can be successfully used in biomedical applications. Nanostructured CS/MWNTs films were built using the LBL method by Pavinatto and coworkers [202]. They prepared this nanostructured material to study the detection of an environmental pollutant, 17αethinylestradiol. Charge resistance of the detector was found to decrease with the increase of MWNTs content in LbL fims. They found faster electron transfer kinetics and a good detection limit of 0.09 μmol/L. 3.5.3.5 Electrospinning Recently nanofiber technology has gained much attention from researchers around the globe. Nanofibers of different polymers can exhibit many different properties, which can further be enhanced either by dipping the nanofiber in other nanomaterial solutions or by mixing the nanomaterial directly in the polymeric mother solution. Electrospinning is a more elegant and easy way to produce and reproduce uniform nanofibers with diameters in the nm
μm length scale, compared to any other methods to date [203,204]. It allows fabrication of a fine, dense, and tailorable network of any given polymer fibers directly from their solution in the presence of a high electric field [203]. In addition, morphology, porosity, and composition of the final product can be controlled using relatively unsophisticated equipment. Typically, an electric field between polymer solution and collector is developed and the generation of internal repulsive forces in the polymer solution causes the expulsion of nanofibers [205]. Dry electrospinning, wet spinning, and coaxial spinning are the most common types of electrospinning methods. In dry spinning, a volatile solvent is used to dissolve the subjected polymer and it evaporates during the spinning of the fiber over the collector. In the case of wet spinning, a nonvolatile solvent is used for the dissolution of the polymer and another second solvent is used as the collector. This second collector solvent washes out the nonvolatile solvent of the polymer and results in pure nanofibers [6]. Coaxial spinning is different from the previously mentioned methods.

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This process is used for the fabrication of core
shell-based nanofibers. Two different materials are spun at a same time. This cost-effective and facile technique has been widely studied for nanocomposite production for wound dressings, medical implants, scaffolds, and environmental pollutant remediation [206]. Rheological properties and chain configuration of a given specific prespun polymer solution influence the morphology of the postspun fiber [207]. In the solution state, hydrogen bonding between CS molecules drives the formation of microfibrils, depending on the concentrations. These characteristics of CS inspire the methodology for successful electrospinning of this material [208]. However, achieving high yield and quality fiber formation from neat CS solutions is a challenging task. This is mainly due to the very rigid structure of CS chains, which does not promote entanglements that are required for the formation of the Taylor cone, which in turn generates nanofibers [209]. This problem can be overcome by using various plasticizers or the addition of neutral salt that reduce the electrostatic interaction between the polyelectrolyte molecules, resulting in the decreased critical concentration for the electrospinning procedure [210]. Bai et al. [211] made a membrane of CS nanofiber to explore its ability as a virus-removal from wastewater application. In this study CS was functionalized with a quaternary amine, N-[2-(2-hydroxy-3trimethylammonium)propyl] CS chloride (HTCC), to adsorb and reduce virus presence. To overcome the difficulties of spinning HTCC alone, graphite was used as an additive. Good interaction between graphite and the charged surface of the functionalized CS allowed the synthesis of nanofibers. The study revealed that the hydrophobicity of graphene and the high charge of the HTCC create a system that is capable of achieving 95% reduction in porcine parvovirus due to the higher amount of HTCC in the fiber mat [211]. Electrospun nanofiber of PAV and CS was prepared by Paipitak et al. The effect of polymeric solution concentration on the morphology of the spun fiber was examined and reported. SEM and AFM studies showed that the morphology changed gradually from a more beads structure to a uniform fiber structure with the increase in concentration of the solution from 3 to 5 wt.%. The maximum fiber yield was found at a concentration of 5 wt.% [212]. Naja fabadi et al. [213] studied the ability of CS and GO to absorb metal ions such as Cu21, Pb21, and Cr61. Their study showed that nanocomposites were able to remove such ions from the aqueous solutions at different optimum pH of 6, 6, and 3, respectively. But with the increase of pH (above 6) adsorption starts to fall. They have found that due to the nanofiber formation it was possible to achieve a high surface area, which provided more spaces to the ions to bind on the surface of the fibers. In addition, the adsorbent could be used up to five times

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without losing its initial adsorption capacity. Fathollahipour et al. successfully fabricated core
sheath nanocomposite nanofibers of PVA/ CS/lidocaine hydrochloride blends containing erythromycin-loaded gelatin nanoparticles (GNPs) through electrospinning [214]. GNPs were produced via the nanoprecipitation method. Duel release studies revealed that the core
sheath structure effectively reduced the burst effect of erythromycin from the core part. The antibacterial studies indicated that the nonwoven mats of PVA/CS blends containing drugloaded GNPs had excellent antibactericidal activity against microbial strains of Staphylococcus aureus and Pseudomonas aeruginosa. Therefore this composite with erythromycin can be used simultaneously to provide antibacterial activity and pain reduction in wound areas.

3.6 Conclusions Chitin and CS are natural amino-terminated polysaccharides with unique structures and fascinating properties, such as biocompatibility and biodegradability. They are nontoxic, have a wide range of potential applications, and the diverse raw material sources for their production are unlimited. Chemical modification of these polymers results in improved solubility in water or organic solvents, which enhances their biological activities and favors the continuous development of their applications as new functional biomaterials with excellent potential in various fields. In order to acquire a deeper understanding of the mechanism of these properties and processing techniques, it is necessary for chitin/CS and their derivatives to be structurally and physicochemically well characterized. Knowledge of the microstructure of these compounds is essential for an understanding of structure
property
activity relationships.

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4 Microscopic studies on chitin and chitosan-based interpenetrating polymer networks, gels, blends, composites, and nanocomposites K. Jayaraj, Sreerag Gopi, A. Rajeswari, E. Jackcina Stobel Christy and Anitha Pius Department of Chemistry, The Gandhigram Rural Institute—Deemed to be University, Dindigul, India

O U T L I N E 4.1 Introduction 4.1.1 Biopolymers 4.1.2 Chitosan and chitin

96 97 98

4.2 Chitin and chitosan-based gels, interpenetrating polymer network, blends, and composites 102 4.2.1 Features of chitin and chitosan-based interpenetrating polymer network 102 4.2.2 Properties of chitin and chitosan-based gels 104 4.2.3 Characteristics of chitin and chitosan blends 107 4.2.4 Study of chitin and chitosan composites 110 4.3 Microscopic study 4.3.1 General 4.3.2 Scanning electron microscopy

Handbook of Chitin and Chitosan DOI: https://doi.org/10.1016/B978-0-12-817968-0.00004-4

95

112 112 113

© 2020 Elsevier Inc. All rights reserved.

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4.3.3 Transmission electron microscopy 4.3.4 Optical microscopy 4.3.5 Atomic force microscope

116 117 122

4.4 Applications and future outlook 4.4.1 Applications of chitin and chitosan-based products 4.4.2 Future outlook

125 125 127

4.5 Conclusion

129

Acknowledgment

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References

130

4.1 Introduction The essence of life is polymers, and polymers like proteins, carbohydrates, and nucleic acids are the most important components of living cells, which are used by nature for construction. Polymers are also a part of the highly complex cell machinery. Polymers are macromolecules which contain a very large number of atoms including synthetic polymer. Polymers have unique properties, depending on the type of molecules being bonded and how they are bonded. Some polymers bend and stretch, like rubber and polyester. Others are hard and tough, like epoxies and glass. Thus polymers touch almost every aspect of modern life. These macromolecules compose structural units of monomers when connected with covalent chemical bonds and the reaction to form polymers is known as polymerization reaction. Polymerization is the method of creating synthetic polymers by combining smaller molecules, called monomers, into a chain held together by covalent bonds. These chains of monomers are also called macromolecules. Most polymer chains have a string of carbon atoms as a backbone. A single macromolecule can consist of hundreds of thousands of monomers, it is this process that gave birth to various types of polymers which are divided into two wide areas called natural and synthetic. Proteins, chitin, chitosan, collagen, silk, keratin, carbohydrates, starch, and glycogen are natural polymers used as conventional coating material to control active ingredient release. They are also used as drug delivery systems because they are easily available. In addition, they are chemically inert, nontoxic, less expensive, biodegradable, and eco-friendly [1].

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Polymers are a very special kind of compounds, which don’t behave like small molecules do. And they don’t not only because of their big molecular size. Precisely that “big size” sets off unique properties like molecular weight and entanglement, crystallinity, and cross-linking. All these properties determine the diverse states of macromolecular aggregation that polymers show. Polymer molecules constitute long chains with a large number of segments, forming tightly folded coils which are even entangled with each other. Numerous cohesive and attractive, both intra- and intermolecular forces hold these coils together, such as dispersion, dipole dipole interaction, induction, and hydrogen bonding. When crystalline, hydrogen bonded, or highly cross-linked substances are involved, where polymer polymer interactions are strong enough, the process does stop at this first stage, giving a swollen gel as a result. If on the contrary, the polymer solvent interactions are still strong enough, the solvation unfolding swelling process will continue until all segments are solvated. Generally, polymers are not seen in gaseous state. At high temperatures one would no longer get polymers, but a big mass of decomposed or carbonized monomers. Even the liquid state is rarely observed in polymers. Most of the time the compound exhibits a rubber-like consistency, and becomes viscous when temperature is gradually increased. Solid polymers usually exist as amorphous glasses. However, when a certain order in their chain structure is present, such polymers can crystallize, like fibers, polyketones, or syndiotactic polystyrene. All solid polymers display a high state of aggregation, unlike macromolecular solutions, particularly the dilute ones. That’s why specific studies of the shape and size of each polymer chain have been carried out.

4.1.1 Biopolymers Polymers biologically synthesized by nature are popularly known as biopolymers, in which polysaccharides are included. They are formed by simple monosaccharide, i.e., sugar, molecules which are connected with ether linkages which give a high molecular weight to polymers. The chemistry of the polysaccharides has received great attention for the development of new methods of isolation, extraction, separation, chemical, and enzymatic modification with the pairing of sensitive and powerful instrumental analysis techniques in the second half of the 20th century [2]. Polysaccharides, like cellulose, starch, dextran, guar gum, and chitosan, are abundant and readily available from renewable sources like the algal and plant kingdoms and cultures of microbial genetic variants. They are also easily available through recombinant DNA techniques.

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The production of polysaccharides brings out various sorts which are cheaper because of the easy methods and processes of production [3]. We can see that the chief advantages of polymeric biomaterials compared to metal or ceramic materials are the three given below: • Polymeric biomaterials are easy to manufacture to produce various shapes. • They give an ease of secondary processability. • They are readily available at a reasonable cost with mechanical and physical properties as desired. Biodegradable polymers have a very long history although the exact time of their discovery and use cannot be traced back because many of them are natural products. Another definition of biopolymers is that they are organic polymers delivered by any living creatures [4]. A natural polymer delivered normally by living beings is also termed as a biopolymer [5]. The classification of biopolymers in accordance with their source is given in Fig. 4.1

4.1.2 Chitosan and chitin A biopolymer derived by deacetylation of chitin is known as chitosan. (Fig. 4.2). The exoskeleton of arthropods like insects, crabs, shrimps,

FIGURE 4.1 Classification of biopolymers.

FIGURE 4.2 Structure of chitosan.

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FIGURE 4.3 Structure of chitin.

and lobsters are enriched with chitin (Fig. 4.3). The waste of the seafood industry pave the way for financially practical creation of chitosan. It is used as a potential material for food packaging, especially edible films and coatings because of its film-forming properties. But one of the main drawbacks of chitosan is that it bears, to a certain extent, a poor mechanical quality. Chitosan is mixed with different polymers and cross-linked most of the time to increase the quality of their utilitarian properties by initiating inter- or intraatomic cross-linking in the polymer when the receptive amino and hydroxyl functional groups are taken into consideration. This intensifies their utilitarian properties. This is done by initiating between interatomic or intraatomic cross-linking in the polymer [6 13]. Chitosan is a unique basic polysaccharide obtained by N-deacetylation [14,15] of chitin in an alkaline medium. This alkaline consists mainly of β-(1-4)-2-acetamido-2-deoxy-D-glucose units. Besides, it is the second most abundant biopolymer on Earth after cellulose that can be found in crustacean shells and the cell walls of fungi [16,17]. Another feature of chitosan is that it is a copolymer of N-acetyl-D-glucosamine and D-glucosamine. Protonation of the NH2 functional group on the C-2 position of the D-glucosamine repeating unit makes it easily soluble in aqueous acidic media and it results in the conversion of the polysaccharide to a polyelectrolyte in acidic media. Chitosan shows more potential than chitin for use in different applications as a result of the presence of its NH2 groups. It has good properties including biodegradability, biocompatibility, and antibacterial activity. Chitosan has abundant applications in different fields because it is the one and only pseudonatural cationic polymer [18,19]. Generally it is prepared by deacetylation of α-chitin using 40% 50% aqueous alkali solution at 100 C 160 C. This preparation takes a few hours. As a result of this, newly derived chitosan has a degree of deacetylation (DD) up to 0.95. We must remember that both the chitin and chitosan are accompanied by many problems while in use. One problem is that they won’t dissolve in water and neutral pH range. The chemical modifications

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also bring changes to the fundamental framework of chitin and chitosans. The genuine physicochemical and biochemical activities are lost when chitin and chitosan are modified. Although the basic structure is changed, the chemical modifications of chitin and chitosan have an advantage: the modification with a hydrophilic reagent would be expected to result in hydrophilic chitin or chitosan while keeping the basic structure with any hardly damage. To improve the affinity to water or organic solvents, technically there are some approaches for the graft reaction of hydrophilic reagents onto chitin and chitosan according to research reports [20]. The amino groups of chitosan are weak bases which are predominantly protonated when pH , 6.5, leading to the solubilization of the polymer only in acid dilute solutions. However, the poor solubility of chitosan above pH . 6.5 is a serious drawback in many of its potential applications. It is a biocompatible, pH-dependent cationic polymer, which is soluble in water up to a pH value of 6.2. Basification of chitosan aqueous solutions above this pH leads to the formation of a hydrated gel-like precipitate. Phase separation results from the neutralization of chitosan amine groups and the consequent elimination of repulsive interchain electrostatic forces, subsequently allowing for extensive hydrogen bonding and hydrophobic interactions between chains [21]. Chitosan is provided with high adsorption capacity and selectivity by many NH2 and OH groups that can chelate heavy metal ions [22]. The intermolecular and intramolecular hydrogen bonds are numerous in chitosan molecules. The packing structure of chitosan in the three unit cell directions [23] stabilizes strongly and gives the power to chitosan not to melt at all. Chitosan dissolves only in certain organic acids, like formic, acetic, propionic, lactic, citric, and succinic acids, and in some inorganic solvents, like hydrochloric, phosphoric, and nitric acid [24]. Chitosan can also be prepared in a variety of forms, namely, hydrogels and xerogels, powders, beads, films, tablets, capsules, microspheres, microparticles, nanofibrils, textile fibers, and inorganic composites. Today in advanced fields chitosan is a protagonist. For example it gives a high performance for nonviral vectors for DNA and gene delivery [25]. To modulate the general properties of chitosan and to last long enough to deliver drugs over a desired period of time chitosan needs to be linked, like all other biodegradable polymers. For the cross-linking of chitosan certain reagents like glutaraldehyde, tripolyphosphate, ethylene glycol, diglycidyl ether, and diisocyanate [26,27] are used. Many naturally occurring polysaccharides such as pectin, agarose, agar, cellulose, dextran, carrageenan, and alginic acid have either

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neutral or acidic features, while chitosan is a basic polysaccharide. The ability to form films and polyoxy salts is the unique characteristic of chitosan. It also bears many optical properties [28]. One of the main parameters that affect its properties is the N-deacetylation degree which is the ratio of the 2-acetamido-2-deoxy-D-glucopyranose to the 2-amino-2-deoxy-D-glucopyranose structural units and it has a remarkable effect on chitosan solubility and its other physical properties. Both chitin and chitosan degrade before melting and this is a typical feature for polysaccharides with extensive hydrogen bonding. Because of this special feature, to impart functionality first of all it is necessary to dissolve both of them in an appropriate solvent system. Besides this, we must know the effect of some parameters like pH, temperature, and polymer concentration on the viscosity of the solution [29]. Depending on the DD of chitosan, its nitrogen percentage changes, and most of the nitrogen content is in the form of primary aliphatic amino groups. Thus chitosan is counted as a reactive polymer which undergoes reactions of amines very easily like N-acetylene and Schiff base (imine) formation. It shows a good chelating ability toward metal ions (Fig. 4.4) because of the reactivity mentioned before and its derivatives are obtained under mild conditions [30] (Fig. 4.5).

FIGURE 4.4 Characteristics of chitosan.

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FIGURE 4.5 Properties of chitosan.

4.2 Chitin and chitosan-based gels, interpenetrating polymer network, blends, and composites 4.2.1 Features of chitin and chitosan-based interpenetrating polymer network The invention of interpenetrating polymer network (IPN) has had far reaching, profound, and allusive or oblique consequences for the whole pharmaceutical industry and indeed medicine. The IPN can render biocompatible and biodegradable materials. A multicomponent material consisting of two or more cross-linked networks in which at least one is cross-linked in the presence of another is referred to as IPN. Topologically they are interlaced and entangled polymer networks, but they not at all covalently bonded to each other. This feature gives IPN the advantage of retaining the properties of each network. We can see that it is undoubtedly proved that the proportion of each network can be varied independently [31]. Porosity, elasticity, degree of swelling, and responsive behavior to a stimulus are the properties of IPN. By the appropriate choice of the network-forming polymers and suitable crosslinking agent and their proportions, these properties can be tuned or adjusted or adapted to a particular purpose or situation of when and where IPN is needed [32 35] (Fig. 4.6). IPN carries certain physical characteristics noted on account of its excellent biocompatibility and safety: it imparts stability of the drug in

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FIGURE 4.6 Types of IPN.

the formulations, improves solubility of hydrophobic drugs, and provides excellent swelling capacity. It also has first rate biological characteristics like biodegradability, imparts bioavailability, is a great drug targeting in a specific tissue, and has very weak antigenicity. Both these physical and biological characteristics made IPN useful in drug delivery systems, particularly for controlled release drug delivery systems. IPN was also used in tissue engineering including bone substitutes, stationary phase, and cartilage scaffolds very easily. There following are the ideal characteristics of an IPN: an ideal IPN can suppress creep and flow; can swell in solvents hardly dissolving; can keep the separate phases together; is always distinguishable from blends, block copolymers, and graft copolymers. It is to be noted that in terms of both properties and performance, a homopolymer alone cannot meet the divergent demands. On such situations the better choice is an ideal and composite IPN of two or three different polymers [36]. Heterogeneous systems comprising one rubbery phase and one glassy phase are the most ideal IPNs. They produce a synergistic effect yielding either high impact strength or reinforcement. Both of them are dependent on phase continuity [37]. Research scientists have been giving great attention to the development of IPNs from natural biodegradable and biocompatible polymeric materials recently [38]. They have found out that chitosan is a biopolymer with plenty of structural possibilities for chemical and mechanical

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modification to generate fully new properties [39]. There are abundant methods of modifying the original structures of polymers.

4.2.2 Properties of chitin and chitosan-based gels “Gel” is a word related with highly hydrated networks, in which there are two components of different fractions or proportions. They are the solvents prevalent in mass and the polymeric solute. In these the polymeric solutes are natural or synthetic macromolecules, and they can properly retain a large amount of the solvents. The simple description of gel is generally valid. But when we do a deep study we can understand that it is quite a task to define gel in detail. When we assess the mechanical behavior of gels, precisely passing through the system to make an unusually effort under changing conditions for a large range of frequencies with rheometry as the analytical tool, we can understand that it is the safest way to categorize pure viscous liquids from strong elastic gels [40]. In the biomedicine sector the use of gel is widely growing [41 43]. Cellular anchoring, colonization, and metabolic activity are highly benefitted with the macrogel fabrication networks endowed with peculiar composition and mechanical behavior. On top of this, particularly for drug delivery purposes the application of micro- and nanogels is genuinely attractive. It is for certain that this type of material is expected to be biocompatible. Generally speaking it is also biodegradable in due time, if implanted or administered, without eliciting any immune response. Chitosan represents a very desirable biopolymer for use in both tissue engineering and drug delivery [44,45]. One of the simplest ways to produce physical gels of chitosan is to modulate the polymer chemical composition, without any external agent [46]. Thus the gelation process is very quick with the increase of temperature, anhydride, and polymer concentrations [47]. There was also an in-depth study on the influence of the nature of cosolvents, that is, alcohols in connection with chitosan gels without cross-linkers. Here for the gel formation, despite the prevalence of different kinetics, both ethanol and 1,2-propanediol were suitable [48]. The fact is that the presence of cosolvents like methanol was found to be very fundamental to prevent O-acetylation, to generally decrease the dielectric constant of the media, and to foster the N-acetylation process [49]. The gelation of chitosan using hydroalcoholic media has been exploited for the fabrication of bioinspired bilayer physical gels for use in the treatment of fullthickness burn injuries, showing that such chitosan-based materials were well tolerated in vivo, promoting tissue regeneration [50]. When we talk about ionic chitosan macrogels, we can say that when a polycation like chitosan is fully charged it will be well below its pKa.

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Metallic anions based on Mo(VI) also can give ionic chitosan gels [51]. Draget and others have reported that a dispersion of MoO3 is added to a chitosan solution for the sol gel transition. It was verified by rheometry. It is the gradual ionic interaction under acidic conditions between molybdate polyoxyanion (Mo 7 O 246 2 ) and D residues on chitosan on which the gelation mechanism is based. There are various bindings made of Mo 7 O 246 to chitosan in respect of Ca21 in alginate which results in gel formation. But comparing with Ca21 alginate gel produced via internal gelation using similar experimental conditions, it shows lower mechanical properties. Eventually, the authors showed that Mocontaining chitosan gels swelled/deswelled depending on the ionic strength of the incubation medium. When the polymeric concentration comes below or close to C*, the complexes of polyelectrolytes with negatively charged macromolecules are formed by chitosan. Very deep and extensive investigations were done on various polyanions to know how to form micro- and nanogels with chitosan, including inorganic ions such as TPP, polysaccharides such as hyaluronan or alginate, and synthetic polymers [52,53]. One single injection or a dropwise titration of the polyanions into the chitosan solution, shows the sign of coacervation beginning, which is required by the ionotropic micro- or nanogelation of chitosan until a turbid system is obtained [54,55]. This should be done at room temperature. If the electrostatic interactions with negatively charged polysaccharides are mixed in the right proportions, it will result in the formation of chitosan-based complexes. Hyaluronic, alginate, dextran sulfate, and carrageenan are the most widely used anionic polysaccharides [53]. They are polyelectrolyte complexes, popularly used as the drug delivery systems in strictly nonidentical fields of application. Their uses extend from the delivery of genes to that of proteins [56]. Hyaluronic acid (HA) is one of the most interesting polyelectrolyte chitosan-based complexes [57]. Usually, when we drop hyaluronan into a chitosan solution chitosan hyaluronan nanogels are derived [55,58]. We can see that the driving forces for self-assembly of nanogels are profited by entropy. This is gained because of the release of hydromolecules and counterions from both polysaccharides together. They also have electrostatic interactions between the two oppositely charged polyelectrolytes [59] (Fig. 4.7). For the formation of chitosan-based nanogels alginate is widely used. Alginate is another polysaccharide. Then the size of nanogels is influenced by the mixing-order. Despite this, the net charge ratio between chitosan and alginate and the molecular weight of both polymers affect both the particle size and surface charge. We can change the charge density and the swelling degree of the microgels by tuning the alginate content [60] (Fig. 4.8).

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FIGURE 4.7 Structure of haluronic acid.

FIGURE 4.8 Structure of alginate.

FIGURE 4.9 Structure of dextran sulfate.

By simply tuning the ionic strength of the system via dialysis, nanogels are formed and dissolved. We can see that colloidal gels are exclusively obtained in the presence of an excess of polycation. It is found that the spontaneous gelation provides macrogels with different homogeneity at low salt concentrations [61]. For an antigenic protein, useful as vaccine carriers, nanogels based on chitosan and dextran sulfate are potential delivery systems [62] (Fig. 4.9). Nanogels based on chitosan carrageenan are highly promising protein carriers for pulmonary and nasal trans-mucosal delivery [63] (Fig. 4.10). To overcome the inadequate mechanical strength of thermoresponsive gels based on chitosan for osteochondral repair, construction of a solid-supported thermogel comprising a chitosan gel system and

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FIGURE 4.10 Structure of carrageenan.

FIGURE 4.11 Purpose of chitosan blending.

demineralized bone matrix are introduced [64]. On the basis of chitosan, different approaches for obtaining thermosensitive gels are undertaken using inorganic phosphate hydrogen phosphate salts [65].

4.2.3 Characteristics of chitin and chitosan blends When we want to provide desirable polymeric materials with combined properties for certain particular applications, polymer blending is a great method commonly used. In recent times blends of natural polymers are significant because they have strong potential to replace synthetic polymers in many applications. Among the natural polymers, chitosan and its blends occupy a special position on account of its versatility and suitability to a large number of applications, as discussed earlier (Fig. 4.11). There are very different strategies adopted for improving the properties and diversifying the applications of chitosan, like cross-linking, graft copolymerization, complexation, chemical modifications, and

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blending [66 85]. Here, the modification of chitosan through blending process has been widely used to provide new desirable characteristics to chitosan, because it is an attractive and easier method than many others in this regard [86 90]. The main reasons for this are its simplicity and the availability of a wide range of synthetic and natural polymers for blending. Besides, its effectiveness for practical utilization is also genuine and very high. Although chitin can be completely acetylated, completely deacetylated, and partially deacetylated, a complete deacetylation is achieved only in rare cases. It is in terms of the average molecular weight and the DD that chitosan is often described. The fact is that generally chitin is considered as chitosan when its DD is 70% or above. There are two parameters on which the physical, chemical, and biological properties of chitin and chitosan depend: they are DD and molecular weight distribution. It is by the chitin sources and the method of preparation that these parameters are dictated. The property to be conferred or boosted can be altered depending upon the selection of the polymers to blend with the chitosan. We can take the hydrophilic property of chitosan as an example: it is modified by blending with polymers such as PEG and PVA [91,92]. But to enhance mechanical properties [93 98], chitosan has also been blended with several polymers such as polyamides, poly (acrylic acid), gelatin, silk fibroin, and cellulose [99]. Polymer blends, with or without any chemical bonding between two or more polymers, are physical mixtures of two or more polymers. Polymer blending is one of the practically and commercially viable providing products with unique properties, at lower cost than other techniques. The properties of single-component homopolymers are inferior to those of polymer blends. Attractive opportunities for reuse and recycling of polymer wastes are also provided by blending technology. The phase structure of the resulting material can be either miscible or immiscible as and when two or more polymers are blended. On account of their high molar mass, the entropy of mixing of polymers is comparatively low. As a result of this, specific interactions are necessary to obtain blends, which are either miscible or homogeneous on a molecular scale. There are two demanding structural parameters for these immiscible systems, in which the overall physical and mechanical behavior critically depend on proper interfacial tension, leading to a phase size small enough to allow the material to be considered as macroscopically homogeneous and interphase adhesion strong enough to assimilate stresses and strains without disruption of the established morphology. Basically there are three different types of blends depending on the miscibility (Fig. 4.12). Completely miscible blends have ΔHm ,0, because of the specific interactions. Here, at least, on a nanometer scale, if not on the molecular

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Classification of blends.

FIGURE 4.13 Another classification of blends.

level, homogeneity is observed. Only one glass transition temperature (Tg) is shown by this sort of blend. It is inbetween the glass transition temperatures of the blend components in a close relation to the blend composition. A small part of one of the blend components is always dissolved in the other part in partially miscible blends. This type of blend is referred to as compatible. This blend exhibits fine phase morphology and satisfactory properties. Both phases of blends are homogeneous. They have their own Tg. For the pure blend components toward the Tg of the blend component, both Tgs are shifted from the values. We can see that the fully immiscible blends have coarse morphology, sharp interface, and poor adhesion between the blend phases. Because of these behaviors, these blends are not useful without compatibilization. Different Tgs corresponding to the Tg of the component polymers are unfailingly exhibited by these blends (Fig. 4.13). Again polymer blends are classified into four types: Homologous polymer blend is a mixture of two or more fractions of the same polymer. Each of these have different molecular weight distributions. Isomorphic polymer blends contain two or more different semicrystalline polymers. They are miscible in the molten state. They are also miscible in the crystalline state. Compatible polymer blends are immiscible blends. They are called compatible only if it is a useful blend in which the homogeneity resulting from different phases is on a small scale which is not at all apparent in use. The blends that are miscible in a certain useful range of composition and temperature but immiscible

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in others are also called compatible polymer blends. Most of the compatible blends are immiscible. They can be made compatible only by a variety of compatibilization techniques. A polymer alloy is obtained when both the interface and the morphology of an immiscible blend are modified. Polymer alloy is an immiscible, compatibilized polymer blend with a modified interface and morphology. When the unique functional properties of the chitosan were found useful in the development of various blends with other polymeric systems, it was another step towards the progress of polymer chemistry. If the fragile nature and membrane permeability of the material is expected to be improved, chitosan could be used as a biomaterial. When chitosan is blended with other polymers the functional groups of chitosan can be modified. A high degree of improvement in physical and mechanical properties was there as a result of this sort of blending. This could be utilized and destined for biomedical applications, as well as plenty of different short-term applications like packaging, agriculture, and hygiene devices.

4.2.4 Study of chitin and chitosan composites Chitosan occupies a unique set of properties. This attribute of chitosan, a first rate polymer, makes it a great high-level candidate for varieties of applications. The low acid stability, poor mechanical properties, low thermal stability, and resistance to mass transfer, low porosity, and surface areas [100,101] altogether make it weak preventing it from being properly functional. The next issue was how to overcome this weakness of chitosan. For this great efforts have been devoted to the development of physicochemical modification methods in order to include different types of functionalization in the polymer. There are many chemical modifications, such as oligomerization, alkylation, acylation, quaternization, hydroxyl alkylation, carboxy alkylation, thiolation, sulfation, phosphorylation, enzymatic modifications, and graft copolymerization that are carried out to obtain modified properties for specifically used applications in a large variety of fields. The mechanical properties of chitosan must be boosted up to a level of acceptability in order to use it as an alternative to synthetic polymers. The shortcomings of biopolymers inherent in it can be improved by the use of reinforcements and the preparation of composites [102]. It is well-known to the researchers that bionanocomposites consist of welldispersed nanofillers with a minimum of one dimension within the nanometer range in a biopolymeric matrix. Nanoparticles with excellent mechanical and thermal properties can interact with polymer chains at

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the nanoscale when they are fillers in the biopolymers. As a result of their high surface area and high aspect ratio, their interaction will improve the properties of the resulting nanocomposites in a significant manner. Because of this improved interaction, the keys for the preparation of composite films with satisfying performances are the homogeneous dispersion of nanofillers in the matrix and the strong interface interactions. There are several research efforts to improve the physical and mechanical properties of chitosan film by incorporating reinforcing nanofillers. These nanofillers are cellulose nanofibers [103,104], cellulose nanowhiskers and nanocrystals [105], chitin nanoparticles [106], lignin and polylactide nanoparticles [107], graphene oxide nanosheets [108], montmorillonite, etc. [109]. In the field of biomedicines, polymer nanoparticles are extensively applied as the tools in the diagnosis and treatment of diseases [110], because polymer nanoparticles can adsorb multiple drugs or loads and control the release of drugs to be more effective carriers. On account of the catatonic nature of the polymer, chitosan solutions have bactericidal and bacteriological properties. Another thing is that the positive charge on the polymer chain will adhere to bacterial surfaces. This adherence induces changes in the permeability of the membrane wall and prevents microbial growth [111]. It is also found that the low DD and low pH chitosan have better antibacterial activity. When the molecular weight is reduced, the antibacterial activities against Gram-negative bacteria are increased, simultaneously decreasing the activities against Gram-positive bacteria. However, chitosan has broad antimicrobial activities against both Gram-positive and Gram-negative bacteria. This broad extent has a high killing rate, both through the interaction between chitosan and its derivatives and the bacterial cell wall [112]. The ability of chitosan to adhere to surfaces helps to adsorb molecules that have no affinity for mucus also [113]. On account of this feature, chitosan speeds up the rate of woundhealing through interactions between platelet and amino groups on chitosan [114]. Chitosan-based nanoparticles possess a large number of lone-pair electrons. They have high binding power with material which has empty orbitals, which is used in drugs and gene delivery, in biosensors, and in fractionated images. Always the size of chitosan-based nanoparticle affects the amount of antigen adsorption and distribution. This affects the immune effect. Above all, the function of microspheres is affected by their structure, the size of surface micropores, and the release rate of antigen [115 118]. It is through the processes of emulsion cross-linking, ionic cross-linking, solvent evaporation, spray drying, precipitation, or flocculation and chitosan solution coating, that the chitosan nanoparticles are obtained.

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4.3 Microscopic study 4.3.1 General There are plenty of methods for the measurement of particle size. Microscopy is one of these methods as it gives the actual size of particles. To view samples and objects that cannot be observed with the naked eye, this technique uses microscopes because microscopes can magnify the particles [119]. The shape, size, and physical morphology of the specimen can be observed very clearly. For this only a very small quantity of the samples will do. There are four main instrumentation categories into which microscopic methods can be divided: 1. 2. 3. 4.

Optical microscopy Scanning electron microscope (SEM) Transmission electron microscope Atomic force microscope

Using an optical microscope is called optical microscopy, which is an old technique. The use of visible light lenses is involved in this technique. There are six parts to the microscope, which are listed below: 1. 2. 3. 4. 5. 6.

Stage with a slide holder Objective lenses Illuminator Eye pieces of 10 3 , 15 3 or larger magnification Condenser lens Diaphragm

For surface microscopic studies SEM is used [120]. With the help of a beam of high energy the image of a solid specimen is taken. The image seen here will have details like surface morphology, and crystallographic information [121]. It is used in many fields, including medical and materials research. But the use of electrons is involved in the process of taking images in the transition electron microscopy (TEM). Here, when the electrons interact with the specimen and the electrons are transmitted through it, the image is obtained. The samples with all the details, even the small ones, can be visualized by the TEM right down to the atomic levels. On account of these features, TEM is an important tool in medical, biological, and material sciences. Besides, it has been significantly considered in the use of environmental geochemistry research [122]. The principle of SEM is very similar to that of TEM. The significant difference between those electron microscopy techniques is that TEM electrons transmit though the specimen, while the SEM electron beams

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scan across the specimen. Environmental SEM allows to avoid vacuum and drying connected problems, but the resolution is lower [123]. More advanced electron microscopy techniques, such as cryo-SEM [124], have been developed that do not require sample dehydration, but necessitate a delicate process of cooling rate that can easily alter the internal structure of a hydrogel. Nowadays SEM is still very often described in the literature to study morphological details of hydrogel, but such information is restricted to the location where the analysis is performed.

4.3.2 Scanning electron microscopy SEM is one of three types of electron microscopes and is used in many fields, including medical and materials research. The principle of SEM is very similar to that of TEM. The significant difference between those electron microscopy techniques is that TEM electrons transmit though the specimen, while the SEM electron beams scan across the specimen. Environmental SEM allows to avoid vacuum and drying connected problems, but resolution is lower [123]. More advanced electron microscopy techniques, such as cryo-SEM [124], have been developed, which do not require sample dehydration, but necessitate a delicate process of cooling rate that can easily alter the internal structure of a hydrogel. Nowadays SEM is still very often described in the literature to study morphological details of hydrogel, but such information is restricted to the location where the analysis is performed. Fig. 4.14 shows the SEM images of raw chitin and chitosan. It is used to measure the shape and morphological characteristics. It consists of

FIGURE 4.14

SEM images of raw chitin and chitosan.

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the determination of particle size and distributions. SEM uses electrons transmitted from the surface of the sample. SEM images clearly indicate the formation of interconnected pores, capillary channels, and crosslinked chitosan molecules around the peripheries of pores. SEM also shows large numbers of pores, indicating that the formation of hydrogel would not destroy the superporous structure. It also indicates the difference in morphology of chitosan and how chitosan is distributed in the polymer to form a three-dimensional network. Fig. 4.15 shows that chitosan aerogel consists of a porous structure. The aerogel presents a form of scaffolds on fibers or leaves disorderly distributed in layers showing the highly porous structure consisting of interconnected pores. The presence of microfibrils in the morphologic structure of the aerogel suggests that the aerogel has characteristics that make it a good candidate for fiber spinning [125]. Nanoparticle morphology was also investigated by SEM in both the secondary and the backscattered electron modes using a LEO 1450 VP microscope coupled with an EDX microanalysis system INCA 300 to obtain the elemental analysis of chitosan DNA nanoparticles. Fig. 4.16 reports the SEM images of chitosan/DNA nanoparticles at two different magnifications. Macroscopically (Fig. 4.16, left) chitosan/DNA particles

FIGURE 4.15 SEM images of chitosan gel. (A1 and A2) Aerogel: chitosan (batch-112) gelled by NaOH, [chitosan] 5 1.5%. (B) Xerogel: chitosan (batch-112) gelled by NaOH, [chitosan] 5 1.5%. (C) Xerogel: chitosan (batch-112) gelled by NaOH, [chitosan] 5 2.3%. (D1 and D2) Xerogel: chitosan (batch-111) gelled by NaOH, [chitosan] 5 1.5%. (E1 and E2) Xerogel: chitosan (batch-112) gelled by SDS, [chitosan] 5 1.5%.

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FIGURE 4.16 SEM images of chitosan/DNA nanoparticles obtained with H2O/EtOH solvent/nonsolvent couple at two different magnifications.

FIGURE 4.17 Scanning electron microscope micrograph of chitosan gel synthesized using 5% of chitosan, at 20.00 kV.

appear as a long chain of interacting particles but at a higher magnification (Fig. 4.16, right) these chains appear to be composed of small nanoparticles with a calculated diameter of 45 ( 6 10) nm. Fiver percent chitosan gel has the potential to be developed as an effective burn wound-healing agent in comparison with existing treatment options. After processing, hydrogel samples showed a highly porous structure with varying pore size. In addition, a fibrous network was observed with the diameter of 40 45 nm, as shown in Fig. 4.17. While making organic inorganic coatings on chitosan with different silanes, it is necessary that we should make sure that silica is homogeneously distributed with chitosan. Fig. 4.18A and B shows needle-like structures. Therefore it is evident that these hybrid materials, which are made of chitosan and GPTMS and TEOS, have an acerose surface with interspaces for less inorganic SiO2. The dentritic structure is greater in chitosan TEOS compared with chitosan (GPTMS-TEOS) it can be considered that TEOS is forming this structure. Chitosan VTMS exhibits a firm and imporous surface. The homogenous distribution of silica in chitosan hybrid is evident from the bright spots representing the silicon element in Fig. 4.18C.

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FIGURE 4.18 SEM Images of chitosan siloxane hybrids: (A) chitosan (GPTMSTEOS): (B) chitosan TEOS; (C) chitosan VTMS hybrid coatings.

FIGURE 4.19 HR-TEM images of the synthesized silver nanoparticle inside chitosan PVA hydrogel (CP-50). Inset shows the size distribution of nanoparticle.

When acrylic acid monomer is directly grafted on chitosan using potassium persulfate as an initiator and methylene bis acryl amide as a cross-linking agent under an inert atmosphere, the hydrogel has a porous structure. It is supposed that these pores are the regions of water permeation and interaction sites of external stimuli with the hydrophilic groups of the graft copolymers [126].

4.3.3 Transmission electron microscopy Several polymeric materials for incorporation of silver nanoparticles have been investigated in the form of microparticles, multilayered films, nanofibers, and hydrogel structures [127 130]. Inside the hydrogel matrix, silver ions make favorable interactions with the functional moieties present in the polymeric chain and thus form a uniform distribution inside the whole network [131,132]. HR-TEM study provides valuable information regarding size and distribution of AgNPs within hydrogel nanocomposites. Fig. 4.19 indicates that the average size of Ag NPs was 13.3 6 3.8 nm with size distribution in the range of 8 21 nm (inset). No aggregates of nanoparticles were observed.

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FIGURE 4.20 TEM micrographs of CMCh/PVA nanogels: (A) CMCh/PVA (2.1); (B) CMCh/PVA (1:1); (C) CMCh/PVA (1:2).

Another study reveals the morphology of PVA in water and water/ acetone was investigated by TEM. Images of CMCh/PVA nanoparticles that formed via the polymerization of CMCs in 1% PVA water/acetone solution are represented in Fig. 4.20A C according to the different applied concentrations of CMCs, namely, 1:2, 1:1, and 2:1 g/100 mL. The morphology of the nanoparticles was greatly affected by the initial CMCs concentration in solution. In Fig. 4.3A,B, the polymerized particles (0.5 and 1 g/100 mL PVA) show sphere-shaped particles with an average diameter of 15 and 20 nm. Fig. 4.20C with a higher concentration of CMCs (2 g/100 mL) resulted in nanogels with diameters of 10 nm. This shows that the increase in CMCh concentration decreases CMCs/PVA nanogels particle size up to the 2 g/100 mL concentration of CMCs.

4.3.4 Optical microscopy Electron microscopy remains in use for high-resolution imaging of hydrogel topology where quantitative results are not necessary, but its limitations have led to the development of optical imaging techniques for measurement of fiber structure of fully hydrated, unmodified hydrogels. These include two closely related techniques, two-photon fluorescence (TPF) and second-harmonic generation (SHG), and confocal fluorescence microscopy, all of which have become popular modalities for recording fibers’ structures, as they can be used to obtain images of gels in the hydrated state [131]. TPF and SHG are

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both based on two-photon laser excitation and therefore have the advantages associated with near-infrared illumination, i.e., high penetration depth and low phototoxicity. For these reasons, confocal microscopy, although somewhat noisier than TPF or SHG, is more commonly used, most likely because of the availability of necessary equipment [132]. For all three optical modalities, the distance of the focal plane into the hydrogel is a critical parameter. The microscopic examination of the films based on polyvinyl alcohol and chitosan in different ratios gives information about the homogeneity of the samples, highlighting any phase transformation. Fig. 4.21A and B provides the microscopic images of the PVA/CS film blends and the corresponding curves of the particle size distribution of the

FIGURE 4.21 Microscopic image and distribution curve of the dispersed phase of PVA/CS.

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dispersed phase. From the analysis of optical microscopy images and the corresponding distribution curves, it can be seen that the chitosan particle distribution in the PVA matrix is uniform and the chitosan particles present various sizes and shapes, usually as globules. Each technique offers advantages and disadvantages associated with sample preparation and resolution. It is by using field emission SEM (FESEM), the latest technology, that the high miscibility between chitosan and agar is confirmed. The blended films displayed homogenous and smooth surface properties compared with the individual pure components. This was also found out during the research works of scientists of polymers chemistry. The SEM micrographs of surfaces of chitosan/agar blend films contain different proportions of agar together with the corresponding ones of pure chitosan and agar films. This is shown in Fig. 4.22. As can be seen from the figure, along with some strips not like that of pure agar, which exhibits a rougher surface, chitosan film shows a smooth and homogeneous surface. From this we can understand that the surfaces of the blend films of chitosan and agar seem to have no interface layer. It is more homogeneous than that of pure chitosan and agar films. Thus it is confirmed that chitosan and agar are highly compatible. The interactions of hydrogen bonds between the functional groups of the blend component ( OH and NH2 groups in chitosan and OH groups in agar) mostly cause the formation of homogeneous blends of chitosan and agar. The SEM images of noncross-linked and cross-linked chitosanbased nanofibers containing Ag and Fe NPs and f-MWCNT are shown in Fig. 4.23. An addition of Ag1, Fe31 ions, and MWCNTs to the optimized concentration of blended chitosan solution did not have a significant effect on the integrity of the nanofibers as per the SEM images shown in Fig. 4.25. Associated to this was the concentration of the charge carriers (Ag1 and Fe31) added in the optimized concentration of the polymers. We can understand that the size of the crosslinked nanofibers has been increased from 471 6 139 nm (Fig. 4.25A) to 627 6 284 nm (Fig. 4.25B). This was regarded as the swelling behavior of the CS-based nanofibers as a result of the absorption of the moisture during cross-linking. In Fig. 4.24 TEM images show the successful settlement of the Ag and Ag/Fe NPs on the surface of the nanofibers. After photochemical reduction of the respective metal ions—Ag and Ag/Fe—of these NPs, they were found growing on the surface of the nanofibers, when they were under observation. In comparison with the well-distributed Ag/Fe NPs, the Ag NPs were collected into a mass on the surface of the nanofibers. The presence of the Fe NPs assisted in the distribution of the Ag

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FIGURE 4.22 SEM micrographs of chitosan/agar blend films: (A) 100/0; (B) 0/100; (C) 90/10; (D) 80/20; (E) 70/30; (F) 60/40; and (G) 50/50.

FIGURE 4.23

SEM Images of chitosan-based nanofibers: (A) noncross-linked nanofiber and (B) cross-linked nanofiber.

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FIGURE 4.24 TEM Images showing the distribution of the Ag and Ag/Fe NPs on the surface of the nanofiber: (A) Ag NPs and (B) Ag/Fe NPs.

NPs on account of their high electrical conductivity nature. The presence of Ag and Fe NPs on the surface of the nanofibers has also been indicated by the TEM results. From the above microscopic studies it can be confirmed that the addition of PIP was found to greatly reduce the swelling of chitosan-based nanofibers. It is also confirmed that their morphologies were improved and they are worth the attention of researchers and scholars. As per the indications shown in SEM and atomic force microscopy (AFM) images, nanosized fibers were completely embedded in the chitosan matrix. Chitosan as the polymer and tripolyphosphate as the cross-linking agent were used for nanoparticles prepared by the emulsion crosslinking method. Thus by the reaction between the negative groups of sodium tripolyphosphate and the positively charged amino ( NH2) groups on chitosan, chitosan nanoparticles are produced. SEM photomicrographs are presented in Fig. 4.4A in connection with the morphology of the nanoparticles modified through emulsion crosslinking. Here the nanoparticles shown are regular spherical-shaped and narrowly distributed. However, these nanoparticles have greatly improved properties, which are stability and prolonged drug release time. The spray drying method was used to prepare chitosan nanoparticles. The drug and chitosan were dissolved together in a solvent. Then this solution was sprayed through a nozzle into a drying chamber. When this was done, small droplets formed. Hot air evaporates the water, as well as the volatile organic solvents in the droplets, to obtain the nanoparticles. When utilized as a carrier for neurotrophic factors, nanoparticles are produced from a complex of ethyl cellulose. On the other hand, chitosan is produced by the spray drying method. SEM photomicrographs in Fig. 4.4D show the morphology of the nanoparticles. The nanoparticles have a uniform and spherical shape. In the

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treatment of neurodegenerative disorders and pulmonary tuberculosis, these nanoparticles, carrying the ability of sustained release, could play a significant role. Precipitation or flocculation using sodium sulfate as the precipitating agent will produce nanoparticles. It is on the concentration of sodium sulfate that the extent of precipitation depends [133]. SEM photomicrographs presented in Fig. 4.4E refer to the morphology of the nanoparticles modified through precipitation [134]. The nanoparticles are not uniform in their sizes. One of the possible explanations for these findings is that they have undergone the freeze-drying process for sample preparation of the SEM. The production of chitosan solution coating is done by adding the existing nanoparticles to a suitable concentration of chitosan solution. A moderate shell of chitosan covers the nanoparticles on account of the adhesiveness of chitosan and the presence of lone-pair electrons. SEM photomicrographs of the morphology of the chitosan alginate nanoparticles modified through chitosan solution coating are shown in Fig. 4.4F [135]. The nanoparticles have a smooth surface and good shape with particle sizes ranging between 75 and 85 nm. Additionally, these nanoparticles have good absorption and good target-controlled release performance.

4.3.5 Atomic force microscope AFM is a form of scanning probe microscope developed in the mid1980s. It works by scanning an extremely fine probe on the end of a cantilever across the surface of a material, profiling the surface by measuring the deflection of the cantilever. This allows a 3D profile of the surface to be produced at magnifications over one million times, giving much more topographical information than optical or SEMs. Its limitation is that the surface to be observed needs to be very flat or the tip will crash into the “hills” as it is scanned. Invented in 1986 by Binning et al., AFM has undergone much development. The first AFMs operated in contact mode. The microscope can run in two modes, contact and close contact. Contact mode scans the probe across the surface, keeping a constant force between tip and sample, maintained by a feedback control. The amount of movement required to keep the constant force is then used to create the image. Close contact mode, often called tapping mode, uses a vibrating cantilever. Biological samples are difficult to scan using contact mode because they are often soft and weakly bound to the surface and therefore can be damaged easily.

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Noncontact mode was first introduced in 1987 and was developed in an effort to more accurately image soft biological samples. In noncontact mode the cantilever oscillates close to its resonant frequency at a small distance (1 10 nm) above the surface. Long-range attractive forces induce changes in the amplitude, frequency, and phase of the cantilever and maintain a constant distance during scanning. Because the forces on the sample are much lower than in contact mode, even the softest samples can be imaged without damage. Imaging in attractive mode is also possible. The Optical AFM Version 4 was also developed in 1989. This fourth prototype of the AFM generated interest in the commercial development and implementation of AFMs. Microfabricated tips were developed in 1991. In 1993 tapping mode was first introduced. In this mode, the cantilever oscillates at its resonant frequency, but unlike noncontact mode, the cantilever gently taps the surface during scanning, greatly reducing damaging lateral forces. Tapping mode in fluids was introduced in 1994. In the first implementation of tapping mode in fluids, the sample, which sits on a piezoelectric scanner, oscillates up and down and taps the tip at the apex of each oscillation cycle. The amplitude of the piezoelectric is set manually at the beginning of the run, and the tapping force is held constant by a feedback loop. Smaller cantilevers were developed in 1996, allowing higher resolution and smaller scanning times. The Hansma group began developing a new generation of AFMs that would utilize these smaller, lighter cantilevers. For biological samples, desirable cantilevers have a higher resonant frequency (and therefore a higher scanning speed) and a low spring constant. This is most easily achieved by decreasing the mass of a cantilever. Recently cantilevers have been fabricated on the order of 9 40 μ in length with resonant frequencies an order of magnitude higher than commercially available cantilevers. The next prototype was a small cantilever AFM Version 5. The major differences between this AFM and previous AFMs lie in the optics design of the microscope. This prototype has a much smaller laser spot allowing for smaller cantilevers to be used. It also includes an integrated illumination source, essentially combining an optical microscope and an atomic force microscope in the same piece of equipment. Improvements in instrumentation and understanding of the AFM have led to its wide use in many fields in engineering, materials science, and biology. Now AFM is an invaluable tool not only to obtain high-resolution topographical images, but also to determine certain physical properties of specimens, such as their mechanical properties and composition. The versatility of this technique is also reflected in the wide range of sizes of

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the sample that can dealt with, such as atoms, molecules, molecular aggregates, and cells. Indeed, this technique enables biological problems to be tackled from the single-molecule point of view and it allows not only to see but also to touch the material under study (i.e., mechanical manipulation at the nanoscale), a fundamental source of information for its characterization. AFM differs from other types of nonoptical microscopy in that it can image samples under natural conditions—in air or water— without the samples being placed under destructive artificial conditions, such as drying, coating with metal, vacuum, or freezing. It is therefore especially useful for biological applications, and as AFM performance and range of time and spatial resolution improves, increasingly smaller biological samples (such as molecules and molecular assemblies) can be imaged at increasingly faster speeds, producing videos of biological processes at very sharp resolutions (down to several nanometers). The importance of the AFM being developed to allow researchers to track biological processes in real time cannot be overstated—such a capability will allow researchers to actually see, at both spatial (down to the atomic scale) and time resolutions, how disease develops, and how the healthy body functions. One question that AFM can help answer is how proteins pathologically misfold in the human body, producing diseases such as Alzheimer’s and Parkinson’s. AFM studies shows that smooth films can be made by chitosan with roughness approximately 0.9 and 0.3 nm in Fig. 4.25

FIGURE 4.25 Showing AFM images of chitosan with different formulations measuring 300 nm 3 300 nm.

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AFM of chitosan nanoparticles.

Another study confirmed the size of the nanoparticle by AFM analysis. The average diameters for the three nanoparticles were found to be 40 50, 90 100, and 80 85 nm, respectively (Fig. 4.26). In short, nanoparticle size can be exclusively studied by AFM images.

4.4 Applications and future outlook 4.4.1 Applications of chitin and chitosan-based products There are numerous areas where chitin and chitosan find application (Fig. 4.27). They have been widely used in immobilizing enzymes. Both in medicine and pharmacy as wound-dressing and slow drug release materials chitin films and fiber are used. For the preparation of affinity chromatography columns to isolate lectins and for determining their structure chitin is used. In the adsorptive removal of heavy metals for water treatment chitin-based materials are applied [19]. For the treatment of effluents contaminated with mixed heavy metals or the recovery of valuable metals, the high sorption capacities of modified chitosan for metal ions have been evaluated in connection with the selectivity and removal performance. chitosan derivatives including new functional groups on the chitosan backbone are obtained with the aim of adsorbing metal ions. When the new functional groups are incorporated into chitosan, one could increase the density of sorption, change pH range for metal sorption, and increase selectivity of sorption [136]. The

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FIGURE 4.27 Applications of chitosan-based products.

ability to adsorb cationic dyes from aqueous solutions can be given to chitosan, by grafting carboxyl groups onto chitosan. It also helps for chitosan to serve as an electron donor in an alkaline environment. In the adsorption of cationic dyes like crystal violet (CV) and Bismarck brown Y (BB.) [137] highly modified chitosan gel beads with phenol derivatives are greatly effective. We can see that even in the 1980s chitosan had a rich history of being researched for applications in agriculture, biology, and horticulture. It can be used as a seed treatment or seed coating, on cotton, corn, seed potatoes, soybeans, sugar beets, tomatoes, wheat, and many other seeds. It is found to have an innate immunity response in developing roots which destroys parasitic cyst nematodes without bringing harms to beneficial nematodes and organisms by chitosan [138]. Chitosan has many other remarkable applications in industries like cosmetics, hydroengineering, paper industry, textile industry, food processing, photography, chromatographic separations, solid-state batteries (Fig. 4.26). Besides these applications of chitosan, its gel form can be highly and dependably used in light-emitting devices (LEDs) [139] and environment-friendly and biodegradable flexible organic thin-film transistors [140]. Drug delivery systems are designed to deliver drugs at a particular rate with minimum fluctuation for a desired period of time on the basis of IPN. Currently, for improved delivery of therapeutic products like sheets, films, hydrogel, calcifiable

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matrix, sponges, tables, capsules, transdermal patches, microspheres, nanoparticles, etc., IPN-based porous sponges are very useful in the treatment of severe burns. They also can be used as a dressing for many types of wounds, such as pressure sores, donor sites, leg ulcers, and decubitus ulcers. This can be achieved by forming a semi-IPN composed of chitosan and poloxamer [141]. There are various physical forms of chitosan blends. They are microparticles, tablets, films, beads, and gels [133,142 145]. For various drug release applications these chitosan blends were proposed. They were also proposed as an intensifier of absorption for nasal and oral drug delivery [146 149]. Above all, chitosan is also a useful polymer for colon-specific and oral administered drug delivery. It is the tissue engineering field, another biomedical field where chitosan blends and other forms have found growing interest. In different physical forms, including porous scaffolds and gels [150 153], chitosan-based materials are reported for tissue engineering. Chitosan is a well-known biopolymer due to its ability to accelerate the healing of wounds in humans [154] as an antimicrobial agent. It is also reported that the antimicrobial activity of chitosan is well-known against a variety of bacteria and fungi owing to its polycationic nature [155]. Chitosan can bind to DNA. It also prevents DNA from being degraded by nucleases. Thus in the gastrointestinal tract, it increases the residence time of DNA [156 158]. Chitosan is also compacted with potential adjuvant properties like the promotion of endocytosis and increased immune response. The primary amine group is responsible for use as an auxillary agent, which is a promising feature of chitosan. Technically speaking, polymer hydrogels are effective for adsorption studies. It is because of their chemical stability, selectivity, and porous structure [159 161]. Hydrogels of chitin or chitin-derivatives are employed in the adsorption and removal of uranyl [162], cadmium [163], lead [164], manganese [165], mercury [166], zinc ions, and dye molecules [167] from aqueous solutions. For many decades, the application of chitosan in dentistry has been the subject of research because it features characteristics and possibilities to create a complex in various forms. Chitosan is applied to the treatment of chronic periodontitis and canker sores in the form of gels and hydrogels. Endodontic cements reduce inflammation and support bone regeneration, if they are based on chitosan [168].

4.4.2 Future outlook There is an acute scarcity of usable freshwater in Asia-Pacific caused by the immeasurable consumption of ground water level, rapidly

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growing urban population, and industrialization. Water treatment activities chiefly in Asia-Pacific has risen to a level to boost product demand. There are great impacts on the overall chitosan market size succeeding these activities even in nearest future. As a flocculent to treat water, this product is widely preferred and used. This product is also used as a highly effective coagulant both in organic and inorganic compounds present in the water. It is also a chelating agent that binds highly toxic heavy metals frequently present in water discharged from industries. It is also used as an absorption medium for many industrial chemicals, especially phenol and PCB that exist in wastewater. The overall market size of chitosan is disrupted by the high production cost in the course of the forecast time frame. There are many processes to pass through for the production of chitosan, which include the derivation of commercial-grade chitosan from crustacean shells, mechanical grinding, washing, decalcification of shells, and deproteination. When these processes arrive at chitin, further deacetylation is needed to get chitosan. It is the DD of chitin that gives first rate quality to chitosan. These are very long and time-consuming processes which also need close monitoring. Despite all these laborious efforts, there are suitable opportunities for the chitosan market size in future, because of its biodegradability and nontoxic nature, and applications across several industry actually use chitosan. Besides these, there are some more processes to generate chitin, which involve demineralization of shell using hydrochloric acid or deproteinization using sodium hydroxide. It is this chitin which undergoes deacetylation to produce chitosan. This chitosan has widespread applications among many end-user industries, in addition to water treatment, cosmetics, food and beverage, and agriculture. Chitosan’s market size is growing due partly to its application in the manufacturing of antiaging creams. Retinol is a great example for this achievement, because it is the main active ingredient in antiaging cosmetics and it is extremely unstable under the influence of light, especially at elevated temperatures. It is stable and more effective than any other commercial retinol delivery system, when introduced into chitosan matrix. For HA, which is used widely in antiaging cosmetics, chitosan is a commercially viable substitute. This product is also used in the manufacture of hair care and oral care products, as well as skincare products. To give softness and physical strength to hair fiber, it forms an elastic film on hair. It is also used in many other cosmetics like shampoos, styling gels, hair sprays, and hair colorants. Besides this, it is also used in the manufacturing of toothpastes, mouthwashes, and chewing gums because it has antiplaque and antidecay properties. To ascertain other future novel applications of chitosan, some wide and extensive research and development initiatives continue to be undertaken.

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4.5 Conclusion Biodegradable polymers represent an inspiring route for creating new and innovative materials. Chitin and chitosan have many important applications in engineering, food technology, biomedicine, water engineering, agriculture, cosmetics, biotechnology, textile and paper industry, photography, solid-state batteries, chromatographic separations, and ophthalmic technology. Chitin has more applications when it is partially deacetylated under alkaline conditions. The biopolymer can be used in many different applications on its raw form or included in the preparation of a large variety of materials such as hydrogel beads, nanoparticles, films, membranes, and composites. In conclusion, the area of bionanocomposites as packaging materials still needs scientific research and improvement in order to develop the shelf life, quality, and marketability of diverse packaging materials. Look at nature’s examples of packaging, skins, structures with specific processes, and imagine if we could make synthetic equivalents. Use of chitosan-based nanoparticles in the agriculture field is still in a budding phase. Significant outcomes have been reported in in vitro and a few in vivo studies in plant growth and protection by chitosan-based nanomaterials. Use of nanochitosan for the delivery of agrochemicals (pesticides, micronutrients, fertilizers, and plant growth hormones) would be the most promising field in the coming times for nanotechnology application in agriculture. Apart from chitosan nanocomposites, there is a great opportunity to develop chitosan-based nanobioconjugates (NBCs). NBCs can be defined as a nanosize complex of two or more biomolecules or an encapsulated nanocomplex of two or more biomolecules which having high surface area with unique biological activity. The involvement of two or more compounds in a single complex allows it to perform multiple tasks in plants. Before application, it is very important that the surface properties should be studied in detail. Microscopic studies help to reveal the compatible nature, surface smoothness, and bond formation with the NH2 group of chitosan. These studies reveal the agglomeration of the nanoparticles, the swelling nature of chitosan, and also indicate the differences in morphology of chitosan when treated with other compounds. Hence microscopic studies are having direct action on the applications of chitosan. Chitosan can be effectively used as a biopolymer of the era.

Acknowledgment The authors would like to thank the authorities of Gandhigram Rural Institute Deemed to be University for the encouragement.

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C H A P T E R

5 Thermal degradation characteristics of chitin, chitosan, Al2O3/chitosan, and benonite/chitosan nanocomposites Hamou Moussout, Mustapha Aazza and Hammou Ahlafi Laboratory of Chemistry/Biology Applied to the Environment, Faculty of Sciences, Moulay Ismaı¨l University, Meknes, Morocco

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5.2 Preparation of chitin, chitosan, bentonite/chitosan, and Al2O3/ chitosan nanocomposites 5.2.1 Preparation of chitin and chitosan 5.2.2 Preparation of the nanocomposite of 5%bentonite/chitosan and 10%Al2O3/chitosan 5.3 Characterization of chitin, chitosan, Al2O3/chitosan, and bentonite/chitosan nanocomposites 5.3.1 Fourier-transform infrared spectroscopy 5.3.2 X-ray diffraction 5.3.3 Differential scanning calorimetry

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5.4 Kinetics of thermal degradation of chitin, chitosan, Al2O3/chitin, and bentonite/chitosan nanocomposites 5.4.1 Theoretical background 5.4.2 Estimation of the degradation mechanism 5.4.3 Thermogravimetric analyses 5.4.4 Thermal degradation kinetics 5.4.5 Degradation model

155 156 158 158 161 166

5.5 Conclusions

170

References

170

5.1 Introduction Chitin (CT) and chitosan (CS) (Fig. 5.1) are two natural biopolymers, which have unique structures that offer them a wide range of applications in biomedicine, protection of the environment, and other industrial areas, because of their biodegradability and bioabsorbability. The presence of multiple functionalities in the CS chain make it to be of more interest than CT [1
3]. The first applications have been developed in food and nutrition, material science, pharmacology, cosmetology, medicine, agriculture, and water treatment [4
6,55,69,70]. Indeed, CT is a polysaccharide corresponding to linear copolymers of 2-amino2-deoxy-D-glucan and 2-acetamido-2-deoxy-D-glucan linked by a β-(1-4), which can be found in the exoskeletons of crustaceans and insects and in the cell wall of fungi and microorganisms [7]. Its main derivative, CS, obtained from a deacetylation of CT, consists of the random distribution of β-(1
4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). CT samples contain low amounts of 2-amino-2-deoxyglucose (Fig. 5.1) and hence it is less

FIGURE 5.1 Chemical structure of (A) CT and (B) CS.

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soluble in acidic solvents, whereas CS samples contain a lesser number of N-acetyl2-amino-2-deoxy-D-glucose (Fig. 5.1) and hence it is soluble in acidic solvents [8
10]. However, it has been shown that these applications and the effectiveness of the two biopolymers depend mainly on the acetylation (DA) or deacetylation (DD) levels of CT, so a DD of CT greater than 50% can be considered as CS. Several studies have focused on optimizing the extraction parameters of CT from its various natural sources [11], followed by its deacetylation to obtain CS with high DD [12
14]. Although various methods used to remove the minerals and the proteins from the shell can be found in the literature, adverse effects on DA and DD cannot be avoided with any of these extraction methods. Therefore there is still a great interest in optimizing the reaction parameters for the CT extraction and the steps of its deacetylation reaction to get the DD to a satisfactory value for specific applications. The deacetylation process consists of the removal of the acetyl groups from the CT molecular chain, leaving behind a complete amino group (
NH2). The properties of CS are closely linked to the chemical reactivity of these amino groups, for example, their protonation in an acidic medium leads to antifungal or antimicrobial activities, since cations can bend to the anionic sites in proteins. Therefore it is essential to characterize CS by determining its DD prior to use. The degree of deacetylation depends mainly on the purification method and the experimental conditions of the deacetylation reaction. Depending on the method used, the main parameters involved in the chemical process are the temperature, the duration of the deacetylation reaction, and the concentration of the reagents [12]. For the enzymatic treatment, the parameters are the temperature, the concentration of enzymes, and the size of the raw material fragments [15]. However, this alternative method is limited in industrial applications due to the high enzyme costs [16,17]. Whatever the method used, these polymers have a certain limitation in many engineering applications due to their low mechanical properties, moisture resistance, and thermal stability. These limitations are overcome by the recent development of a new class of materials, named biocomposite/bionanocomposite materials, in which these biopolymers have been used as a matrix [18]. Unlike other nanofillers, biopolymer/clay biocomposites have been widely studied and applied in several research fields, because clays are available, cheaper, and their intercalation chemistry is well-known [19,20]. CS/MMT (montmorillonite) is the most studied one [21,22], and it has been shown that the nanocomposites have improved properties, such as mechanical properties, gas barrier, and thermal stability, compared to pure CS [23]. This was related to the excellent dispersion of the nanoclay and its strong interaction with the CS matrix. Considering the wide fields of applications of these biopolymers and

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their composites, the precise knowledge of their thermal degradation seems to be of great importance in otder to define the applications of such materials. Thermal behavior can be achieved by using certain physicochemical techniques, including DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), DTG (differential thermal analysis), DMA (dynamic mechanical analysis), and other thermal analysis methods. Thermogravimetric analysis (TGA/DTA) has been widely used by many authors to study the kinetics of thermal degradation under isothermal and dynamic conditions, both in air and in nitrogen. In most cases, the data can be used to determine the degradation mechanism and kinetic triplet evaluation (Ea: activation energy, A: preexponential factor, conversion function). In this chapter, an attempt is made to resume some of our recent published [12,17,23] works related to: 1. The optimization of the experimental extraction conditions of CT from shrimp shells, collected in Mekne`s city, Morocco. 2. The kinetics of deacetylation reaction to obtain the biopolymer CS with different DD. 3. The preparation of Bt/CS (Bt) and Al2O3/CS nanocomposites. 4. The characterization of the as obtained biopolymers (CT and CS) and their nanocomposites Bt/CS (Bt) and Al2O3/CS by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential DSC techniques. 5. The studies of the thermal stability of these samples using ATG/ ATD, in dynamic conditions and under air atmosphere.

5.2 Preparation of chitin, chitosan, bentonite/chitosan, and Al2O3/chitosan nanocomposites 5.2.1 Preparation of chitin and chitosan
CT: CT biopolymer was extracted from the shrimp shells collected from fishmongers in the city of Meknes, in the northeast of Morocco. The CT extraction process was carried out by the chemical treatment of shrimp shells in two successive stages, according to the general procedure shown in Fig. 5.2. The first step concerns the demineralization reaction with HCl (1 N, 24 h, 25 C) of the dried shrimp shells and the second step corresponds to the deproteinization/decolorization of the resulting sample with NaOH (1 N) for 24 h at T 5 25 C [24,25]. The spectrum in Fig. 5.3 shows the absorption FTIR bands of the CT extracted from shrimp skeletons at room temperature, successively

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FIGURE 5.2 Extraction procedure of CT and its deacetylation to form CS.

FIGURE 5.3 FTIR spectrum of CT.

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under NaOH (1 M) and HCl (1 M). The attributions of the observed bands are summarized in Table 5.1. The most important bands are those which make it possible to distinguish between α- and β-CT and those which are used to determine the DA or DD values. The bands between 1600 and 1660/cm of the carbonyl (C~O) groups in the amides are of great importance because they make it possible to distinguish between the α and β structures of CT. The two bands at 1660 and 1626/cm, corresponding to Amide I, are characteristics of α-CT, while that at 1640/cm corresponds to β-CT. These two bands are originated, respectively, from intermolecular (CO-N-H) and intramolecular (CO-H-OCH2) hydrogen bonds [12]. The band of Amide II is unique and is located at 1556/cm for α-CT and at 1560/cm for β-CT [12]. These differences are due to the conditions of preparation, pretreatment, and the origin of the raw material. However, it is still difficult to decide between the two structures, because the two bands are very close and sometimes they are not well resolved. Nevertheless, careful observation of the spectrum (Fig. 5.3) of CT shows the existence of the band

TABLE 5.1 Attributions of the FTIR absorption bands of CT. Bands (/cm)

Attributions [26]

3441 and 3111 (as), 3256 (s)

νOsH in the hydrogen bonds, νNsH stretching

3436

νOsH Stretching

2930 and 2961 (as), 2890 (s)

νCsH Stretching in CH3 and CH2

1625 and 1654 (shoulder)

νC~O Amide I in secondary amine α-Chitin, β-chitin

1577

νC~O Amide II in secondary amine

1416

δCsH Deformation

1382

δ CH2 and CH3

1324

νCsN in secondary amine (amide III)

1261

δNsH Stretching

1156 (as) and 1075 (s)

νCsOsC (glucose linkage)

1116

νCsOsC, νCsO Stretching

1023

C-O-C Stretching

952, 895

Deformation of CH3 off plan, C-O-C bridge

752, 701

Deformation of NsH off plan

699

Deformation of OsH off plan

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at 1660/cm in the form of a sharp peak and that at 1626/cm in the form of a shoulder, which allows to match this spectrum to α-CT.
CS: Once the CT is extracted, its transformation into CS is monitored under alkaline medium (NaOH, 12 M) at different temperature T, according to the following overall reaction: R 2 NHCOCH3 1 NaOH-R 2 NH2 1 CH3 COO2 Na1 for which the DD is calculated using the FTIR technique [12,27]. The determination of the DA or DD by IR spectroscopy is a useful method, because several measuring absorption ratios may be used. The DD value of CT is controlled by the following parameters: concentration of the base (NaOH), reaction temperature, reaction time, and the number of reaction cycles. The effect of each parameter on the reaction was studied in order to optimize the value of DD. The follow-up of the different reactions is carried out by FTIR by choosing the evolution of the absorbances of the 1660 and 1626/cm bands (A1655/A3450), characteristics of the acetyl group of CT (Amide I) and the band at 1556/cm (Amide II),
HN
COCH3. Thus the following equation was used to calculate the DD under different reaction conditions: [28]
 A1655 100 (5.1) DDð%Þ 5 100 2 3 1; 33 A3450 where A1655 is the absorbance of primary amide (probe band), A3450 the absorbance of hydroxyl group (reference band), and 1.33 is the factor which represents the ratio (A1655/A3450) for fully N-acetylated CT. The deacetylation was studied at T 5 25 C, 80 C, and 120 C, using the same initial mass of CT (m 5 2 g) for different concentrations of NaOH (CNaOH 5 8, 10, and 12 N). The FTIR spectra of Fig. 5.4 is an example of the evolution of the CT bands during its deacetylation in basic medium (CNaOH 5 12 N) at different temperatures (25 C, 80 C, and 120 C). At T 5 25 C, the band at 1555/cm (Amide II) decreased slightly and the Amide I bands at 1659 and 1626/cm appeared at equal intensities, indicating that the deacetylation of CT is low at T 5 25 C. For T 5 80 C, the comparison of these spectra with those obtained at T 5 25 C indicates a clear difference with regard to the intensities of the bands at 1660, 1626, and 1556/cm. Indeed, the intensity of the first band increases at the expense of the other two bands as the treatment time under NaOH increases. It can also be noticed that the 1659 and 1626/ cm bands have decreased in intensity, indicating that the N-H bonds involved in intermolecular (CO-N-H) and intramolecular hydrogen bonds in (CO—H-O-CH2) have partially broken down. Finally, the spectra recorded for the T 5 120 C show a significant evolution of the bands

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FIGURE 5.4 FTIR spectra of samples taken for different times during deacetylation reaction of CT (CNaOH 5 12 N, T 5 25 C, 80 C, 120 C).

relating to Amide I and Amide II. For Amide I, as the deacetylation time increases, the intensity of the band at 1660/cm also increases, and that at its right, initially detected at 1626/cm, decreases in intensity and disappears completely. For longer treatment times t 5 240 and 300 min, the band initially located at 1555/cm (Amide II) increases in intensity with the treatment time and undergoes a slight displacement (Δυ 5 5/ cm) from its initial position to υ 5 1600/cm for t 5 300 min. At the same time, it can be noted that there is a significant elimination of other groups from the CT chain. This displacement can be attributed to the reduction of the effects created by the intra- and intermolecular hydrogen bonds in the acetyl groups. The evolution of DD (%) values, determined from these FTIR spectra and those recorded for CNaOH 5 8 and 10 N (spectra not shown), using Eq. 5.1, are shown in Fig. 5.5. It can be observed that the rate of the deacetylation reaction increases as the temperature and the concentration of

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FIGURE 5.5 DD (%) versus time (T 5 25 C, 80 C, and 120 C; CNaOH 5 8, 10, and 12 N).

NaOH increases. For T 5 120 C and CNaOH 5 12 N, the DD value exceeds 70%, while at T 5 25 C, the DD value does not exceed 55% for CNaOH 5 12 N. To confirm the formation of CS at T 5 120 C, acetic acid was added to the resulted solid, and it was observed that a large part of the solid was solubilized, since CT is insoluble in most solvents. For T 5 80 C, the deacetylation rate reaches a maximum of 63% for the concentrated NaOH solution (CNaOH 5 12 N) in 40 min of reaction. This rate is slightly higher than that obtained for CNaOH 5 12 N at T 5 25 C. It can be determined that a small amount of acetyl groups is hydrolyzed with NaOH at T 5 80 C. The addition of HCl (0.1 N) for 5 h did not lead to its total dissolution, which means that the content of the acetyl groups is still high, unlike in a highly deacetylated CT, which solubilizes almost instantaneously in HCl or 0.05 N acetic acid. From these results, it can be concluded that the increase of the temperature and NaOH concentration leads to the increase of the rate of the deacetylation reaction and consequently the DD value. This can be largely related to the breakage of hydrogen bonds, which prevent the attack of acetyl groups at these experimental conditions. To improve the value of DD, the sample previously obtained (DD 5 70%) was treated again in the same conditions (CNaOH 5 12 N, T 5 120 C). The FTIR spectra of the samples taken during the reprocessing time are shown in Fig. 5.6a. The progressive increase and displacement of Amide II band at 1590/cm, initially present as a single peak at 1559/cm for t 5 30 min, indicates the formation of additional NH2 groups, in the CS chain. This can be confirmed by the coverage of the O-H and N-H bands, which gives a wide band, centered at 3400/cm, which is due to the displacement of the bands of N-H group (3100 and 3200/cm) of the amide toward the high frequencies.

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FIGURE 5.6 (A) FTIR spectra of samples taken after the second treatment; (B) DD (%) versus time (CNaOH(12 N) at T 5 120 C).

The values obtained for DD (%) during the reprocessing of CS (Fig. 5.6b) show that the DD value reaches a maximum (83%) for t 5 5 h.
Kinetics of CT deacetylation: The kinetic parameters, such as the rate constant k and the activation energy Ea of the deacetylation reaction of CT were determined from the curves in Fig. 5.5, which represent the evolution of DD versus the temperature treatment of CT. The deacetylation reaction of CT is written as follows: k

R-NHCOCH3 1 NaOH
! R-NH2 1 CH3 COONa 1 t0 ; t:

N0

N0 2 N

0 N

where N0 represents the initial number of acetyl groups contained in CT at t 5 0, which can be converted in (N) NH2 groups, k is the rate constant, given by the Arrhenius equation: k 5 A 3 exp R23ETa , where A is frequency factor (/min), R is gas constant, Ea is activation energy (kJ/mol), and T is temperature (K). The rate of the disappearance of the acetyl groups is given as follows: 2

dN 5k3N dt

(5.2)

The integration of this equation at the boundary conditions (t 5 0, N0) and [t, (N0 2 N)], leads to the following equation: 2 ln (N0 2 N)/N0 5 k 3 t

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Taking into account that the ratio (N/N0) corresponds to the degree of deacetylation (DD), this equation becomes: 2lnð1 2 DDÞ 5 k 3 t 1 constant According to this equation, Fig. 5.7 shows the result obtained for the deacetylation reaction carried out at different temperatures for CNaOH 5 12 N (Fig. 5.5). For all the temperatures studied (25 C, 80 C, and 120 C), two lines with different slopes were obtained. Table 5.2 summarizes the values of the calculated rate constants k. The values of k calculated from the first slope are greater than that obtained from the second slope. This indicates that the deacetylation rate is faster at the

FIGURE 5.7
ln(1
DD) 5 f(t) for the different temperatures studied, CNaOH 5 12 N. TABLE 5.2 Rate constant ki for the deacetylation reaction at different temperatures, CNaOH 5 12 N. k1(/min)

k2(/min)

T( C)

First slope

Second slope

25

0.013

1.704 3 1025

80

0.002

2.226 3 1024

120

0.015

22.000 3 1024



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beginning of the reaction, whereas at longer reaction times, the deacetylation become slow, which indicates the existence of a limiting step. At low treatment times (t , 60 min), the order of the reaction is greater than 2 (rapid reaction). This result is in good accordance with those reported by other authors [29]. The activation energy is estimated from the plot of ln(k2) versus 1/T in the Arrhenius relationship:
 Ea 1 3 lnk2 5 lnA 2 (5.3a) T R This leads to a value of the activation energy in the order of 48.76 kJ/ mol, which is of the same order of magnitude as those mentioned in the literature [29] for the deacetylation reaction, carried out on solid catalysts between T 5 40 C and 120 C. On the other hand, other authors [30], working at very low temperature, found that the values of k are ranging between 35.56 and 57.74 kJ/mol. This value has been linked to the presence of the intermolecular and intramolecular hydrogen bonds present in the CT structure and to the diffusion phenomenon. In fact, the surface acetyl groups are very accessible at the beginning of the

FIGURE 5.8 Procedure of nanocomposites preparation.

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reaction and as the deacetylation time increases, the deacetylation reaction becomes slow, which is consistent with the decrease in the values of k after t 5 60 min of the reaction.

5.2.2 Preparation of the nanocomposite of 5%bentonite/ chitosan and 10%Al2O3/chitosan The nanocomposites 5%Bt/CS and 10%Al2O3/CS are elaborated, using the CS obtained above with DD . 80%. Fig. 5.8 summarizes the procedure for the preparation of these nanocomposites. Indeed, before the development of nanocomposites, a CS matrix solution was dissolved in a 5% (v/v) solution of acetic acid, and the filler (Bt or Al2O3) was swollen by H2O and stirred under ultrasonic radiation for 15 min. The suspension of the Bt or Al2O3 was then added to the CS solution, and the mixture was stirred for 24 h at 60 C, after which the mixture was filtered. The obtained nanocomposite was washed, dried, and ground.

5.3 Characterization of chitin, chitosan, Al2O3/chitosan, and bentonite/chitosan nanocomposites The characterization and the structural changes of the samples were made by FTIR, XRD, SEM, and thermal analyses techniques, such as TGA, DTA, and DSC.

5.3.1 Fourier-transform infrared spectroscopy FTIR spectra were collected in the range of 400
4000/cm, using a JASCO 4100 FTIR spectrometer with a resolution of 4/cm and accumulation of at least 64 scans. The samples were prepared using KBr pellets. FTIR spectra of prepared CT, CS, Bt, Al2O3, 5%Bt/CS, and 10%Al2O3/ CS are shown in Fig. 5.9. CT and CS IR bands (Fig. 5.9, A) were previously presented in Table 5.1. • Bt and 5%Bt/CS (spectrum B, Fig. 5.9): The FTIR spectrum of Bt shows the presence of characteristic bands of montmorillonite. The band at 3453/cm is attributed to the (OH) groups and the band at 1620/cm is attributed to the O-H deformation vibration of the physisorbed water. The bands between 900 and 1045/cm correspond to the elongation vibrations of the Si-O bond in the Bt. The bands below 900/cm are characteristic of the deformation vibrations of Al-Al-OH, Al-Fe-OH, Al-Mg-OH Al-O-Si, Si-O-Si type bonds in clays. Some of the original bands of Bt and

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FIGURE 5.9 FTIR spectra of CT, CS, Bt, Al2O3, 5%Bt/CS, and 10%Al2O3/CS.

those related to CS have undergone slight displacements in the nanocomposite 5%Bt/CS (Fig. 5.9B). For example, the 3440/cm (OH) and 1080/cm (Si-O-Si) bands in Bt have shifted to lower frequencies. The same behavior is observed for the 2920 (CH) and 1657/cm (Amide I) vibration bands characteristic of CS. These displacements are due to the interactions between these functional groups, following the intercalation of the CS in the sheets of Bt, confirming the formation of the considered nanocomposite. • Al2O3 and 10% Al2O3/CS (spectrum C, Fig. 5.9): The two broad and intense bands detected between 1100 and 400/ cm in the Al2O3 spectrum are characteristic of Al-O bond vibrations in O-Al-O and Al-O-Al of pure alumina. The metal
oxygen elongation vibration was observed at 590/cm. For the 10%Al2O3/CS sample, the IR spectrum shows that all of the initial bands of CS are present (Fig. 5.9). However, a decrease in the intensity of the bands of Amide I and an increase in the intensity of the bands of Amide II are accompanied by its displacement toward the low wave number. These observations indicate that the functional groups such as
NH2,
OH, and
CO
are involved in the formation of the nanocomposites of 10% Al2O3/CS.

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5.3.2 X-ray diffraction The XRD patterns were recorded using an X’PERT MPD-PRO wideangle X-ray powder diffractometer provided with a diffracted beam ˚ ). The 2θ monochromator and Ni-filtered CuKα radiation (λ 5 1.5406 A   angle was scanned between 4 and 80 range with a counting time of 2.0 s at steps of 0.02 . XRD diffractograms of the samples are shown in Fig. 5.10. ˚ • CT and CS (Fig. 5.10A): spectra show two main peaks at d 5 9.28 A ˚ , which correspond to the α-CT structure [31]. and at d 5 4.57 A • Bt and 5%Bt/CS (Fig. 5.10B): XRD spectrum of Bt shows a reflection ˚ , characteristic of its basal spacing (d001) [23]. This peak at d 5 12.32 A peak disappears after the incorporation of 5%(w/w) of Bt in CS, while the two peaks of CS are still present on the diffractogram of 5% Bt/CS. The absence of this peak is due to the increase of the d(001) distance, which results from the intercalation of CS in the interfoliar space of the Bt, thus widening the basal spacing d(001) to which the layered structure of Bt becomes partially or completely exfoliated. Similar results were also observed by Monvisade and Siriphannon [32,33], where the displacement of the d(001) of MMT was attributed

FIGURE 5.10

XRD patterns of CT, CS, Bt, Al2O3, 5%Bt/CS, and 10%Al2O3/CS.

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to the intercalation of two layers of CS in the clay sheets following different ways: horizontal, tilted, or vertical. • Al2O3 and 10%Al2O3/CS (Fig. 5.10C): for Al2O3, there are five peaks ˚, located at d 5 2.39, d 5 2.27, d 5 2.12, d 5 1.98, and d 5 1.39 A corresponding to the planes (220), (311), (222), (400), and (440), respectively, which are assigned to the γ-Al2O3 structure [34]. However, in the diffractogram of 10%Al2O3/CS, three intense peaks ˚ . The two first peaks are observed at d 5 9.38, d 5 4.52, and d 5 3.39 A correspond to CS, which has undergone a slight displacement ˚ accompanied by an increase in their intensities. The peak at d 5 1.39 A is that of the starting alumina which has decreased in intensity. The ˚ , which is not present on the initial appearance of the peak at d 5 3.39 A solids, indicates the formation of a new phase. These observations confirm the formation of the nanocomposite of 10%Al2O3/CS. 5.3.2.1 Scanning electron microscopy The surface morphology of the samples was investigated using SEM Brand EIFQuanta 200 apparatus and the resulting images are presented in Fig. 5.11. The SEM images of CT and CS are composed of lamellar fibers and have a rough and thick surface morphology. The surface of the Bt is uniformly sized grains; however, the 5% Bt/CS SEM image shows a compact and rough surface, which looks different from the initial materials (Bt and CS). It can be seen that the grains of Bt are dispersed at the surface of CS matrix. For 10%Al2O3/CS, the image shows a porous structure, different from that of Al2O3 whose structure appears in the form of regular hexagons.

FIGURE 5.11 SEM images of CT, CS, Bt, Al2O3, 5%Bt/CS, and 10%Al2O3/CS.

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FIGURE 5.12

155

DSC curves of CT, CS, Bt, Al2O3, 5%Bt/CS, and 10%Al2O3/CS.

5.3.3 Differential scanning calorimetry DSC analysis was carried out with DSC 131Evo instrument. Fig. 5.12 shows the DSC thermograms of the samples studied. In the case of Bt, only one endothermic peak is detected at T 5 152 C, whereas for CS and CT samples, three peaks are observed. The first endothermic peak corresponds to the desorption of the physisorbed water and the second exothermic one is due to the depolymerization/decomposition of biopolymers [35,36]. The third peak observed at T . 350 C is due to their oxidation. The positions of the latter peaks shift to lower temperatures in 5%Bt/CS and 10%Al2O3/CS biocomposites, indicating that the thermal stability of CS was lowered when it was combined with Bt or Al2O3.

5.4 Kinetics of thermal degradation of chitin, chitosan, Al2O3/ chitin, and bentonite/chitosan nanocomposites Thermal degradation studies were performed in dynamic conditions with a TA60 SHIMADZU, simultaneous TGA/DTA analyses. Samples were placed in the balance system and heated linearly (T 5 T0 1 β 3 t) from ambient to a final temperature at the desired heating rate β (5, 10, Handbook of Chitin and Chitosan

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15, 20K/min) in an air atmosphere. The loss of weight was monitored, allowing the calculation of the extension of conversion α as a function of the reaction time. The DTG curves were used to determine the rates of degradation versus the extent of conversion α.

5.4.1 Theoretical background Differential thermogravimetric and thermal analyses (TGA/DTA) experiments were carried out to compare the thermal stability of the samples studied with each other, and to determine the kinetic parameters involved in their degradation process, such as the activation energy (Ea) of degradation, the frequency factor (A), and the degradation mechanism, known as the kinetics triplet. The rate of the degradation process of a sample during its heating can be related to its mass loss according to the following relationship [37]: K

Solid
! Degradation products dα 5 f ðαÞkðT Þ dt

(5.3b)

where f(α) represents the kinetic model that takes into account the reaction mechanism of the degradation process [38]. There are numerous heterogeneous reaction models that can be classified as a reaction order and in the autocatalytic model of SB [39] α is the conversion fraction given by: m0 2 mt α5m 0 2 mf

(5.4)

where m0 is initial mass, mt is mass at time t, mf is final mass, and k(T) is the rate constant, which is represented by the Arrhenius law:
 Ea kðTÞ 5 Aexp 2 (5.5) RT where A is the preexponential factor (/min), E is the activation energy (J/mol), and R (J/mol/K), is the gas constant. Under dynamic conditions, dT/dt 5 β, the Eq. (5.2) becomes:
 dα 1 Ea 5 Aexp 2 dT (5.6) f ðαÞ β RT A and Ea are then calculated using the experimental data from TGA or DTG experiments. The mathematical resolution of Eq. 5.6 is done by two main methods: the “integral” method and the “differential” method. The first is more

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advantageous because it does not introduce significant errors into the calculation of the kinetic parameters. Thus on integrating the Eq. 5.6 and applying an initial condition (α 5 0 at T 5 T0), the following expression of integral conversion function g(α) is obtained:
 ð ðα dα A3E T Ea A3E 5 pð xÞ (5.7) exp 2 gð α Þ 5 dT 5 f ð α Þ β 3 R β3R RT 0 T0 where x 5 Ea/RT and pð xÞ 5

ðN x0

expð 2xÞ dx x2

(5.8)

The variables given in the Eq. (5.7) may be separated and integrated to give p(x) in logarithmic form.


   AE E log gðαÞ 5 log 2 logðβ Þ 1 log p (5.9) R RT So, p(x) does not have an analytical solution. OFW [40] uses the Doyle’s approximation: ln((p(E/RT)) 5
2.315
0.456 3 E/RT, which assumes that the conversion function f(α) does not depend on the heating program. Then Eq. (5.9) becomes: logβ 5 log

AEα Ea 2 2:315 2 0:4567 gðαÞR RT

(5.10)

Therefore the activation energy (Ea) can be obtained from the plot of log β against 1/T for a fixed degree of conversion, since the slope of such line is given by
0.4567Ea/R. The preexponential factor, A, can be obtained from the equation:

 Ea Ea A5β exp (5.11) 2 RT RT Another method is that proposed by KIS [38], which showed that the simple first-order decomposition reaction temperature at the maximum rate of decomposition (Tm) varies with heating rate. The value of (Tm)is obtained at the maxima of the first derivative  of DTG curves, where the maximum mass loss occurs at: dðdα=dtÞ 5 0. The derivative of the dt Eq. (5.6) gives:
 β AR Ea (5.12) ln 2 5 ln 2 Tm Ea RTm Ea can be calculated   from the slope obtained from the linear least squares plot of ln Tβ2 versus 1/Tm. m

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5. Thermal degradation characteristics of chitin, chitosan, and etc.

These methods presuppose an Arrhenius-type relationship but do not require any hypothesis on the reaction scheme. It is possible to define a kinetic model when the activation energy is constant in the studied temperature range.

5.4.2 Estimation of the degradation mechanism Once activation energy has been determined, it is possible to determine the kinetic model that gives the best description of the experimental data. Indeed, two functions are easily obtained by the transformation of the experimental data [41,42]. For experiments in dynamic mode, these functions are:
 dα 2 (5.13) ZðαÞ  T 5 f ðαÞgðαÞ dT

 dα Ea YðαÞ 5 exp 5 Af ðαÞ (5.14) dT RT The values of α at a maximum of Z(α) and Y(α), αz and αy , are characteristic of the kinetic model. With the shape of Y(α) being formally identical to the kinetic model, the resulting experimental values of Y(α) and Z(α) are plotted as a function of α and compared against theoretical Y(α) and Z(α) master plots.

5.4.3 Thermogravimetric analyses 5.4.3.1 Chitin and chitosan The TGA curves of CT and CS recorded at different heating rates (β) from room temperature to 600 C, in air atmosphere are shown in Fig. dα 5.13, while the corresponding DTG curves (derived from TGA: dt Tm ) are shown in the insert. The TGA curves show three temperature ranges where the samples undergo weight losses. The first stage in the range 317K
375K, corresponding to a weight loss of 10%
15%, is linked to desorption of physisorbed water on the samples [43]. The second weight loss, which occurs in the range of 523K
673K, corresponds to 65% and 50% for CT and CS, respectively, and is caused by deacetylation and depolymerization of the polymer chains via the degradation of the glycosidic bond (C-O-C) [44]. The last stage, which occurs at a temperature above 673K, is 10% and 15% weight loss for CT and CS, respectively. These losses correspond to the thermal destruction of the pyranose ring of the two polymers and the decomposition of carbon residues [43,45]. These results are in agreement

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159

with the results found by other authors. Corazzari et al. [43]. studied the degradation of these polymers under N2 on a coupled TGA/FTIR and TGA/CGSM apparatus, and showed that the degradation products corresponding to the total mass loss consist of the following species: H2O, NH3, CO, CO2, CH4, and CH3COOH. The amino groups in the CT and CS structures were released in two different ways: by liberation of ammonia and random disruption of C-O-C backbones [46,47]. On the other hand, the DTG curves of CT and CS (inserted into the Fig. 5.13) show that the maximum temperatures shifted to higher temperatures as the heating rate increases from 5 C/min to 20 C/min, and their profiles are similar, suggesting that the degradation kinetics of these polymers are the same in the temperature range studied. The displacement of the maximum of the peak DTG with the heating rate β is related to the heat transfer during the rise in temperature. However, for a given heating rate, the maximum degradation peak is different. For example, for β 5 5K/min, the degradation peak for CS is at Tm 5 595K, whereas that for CT is at Tm 5 603K, which indicates that the thermal stability of the two polymers are not the same due to the difference in their degree of deacetylation (DD) and the difference of their activation energy. Indeed, it is indicated [35,48] that the decomposition processes of these polymers are linked to the acetyl groups (GlcNAc) of the CT units and to the amine groups (GlcN) and acetyl groups (GlcNAc) in the case of CS. These results are in good agreement with those obtained by other authors using commercial CT and CS [45,49,50].

FIGURE 5.13

TGA/DTG curves of CT and CS at different heating rates [17].

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5. Thermal degradation characteristics of chitin, chitosan, and etc.

5.4.3.2 Nanocomposites of 5%bentonite/chitosan and 10%Al2O3/ chitosan Fig. 5.14 shows the TGA and DTG curves of 5% Bt/CS and 10% Al2O3/CS recorded at four different heating rates (β 5 5, 10, 15, and 20K/min) from room temperature to 873K, under ambient air. It can be noted that the profiles of the TGA and DTG curves of these nanocomposites are different from that obtained for CS. The appearance of more than one peak in the DTG curves indicates that at least two stages are involved in the thermal degradation of the nanocomposites. The first common weight loss (10%), between 303K and 413K, corresponds to the desorption of physically adsorbed water on the sample surfaces, however, the second predominant stages are ranged between 500K
650K during which 31.87% and 34.5% of mass loss were observed for 5%Bt/ CS and 10%Al2O3/CS, respectively. These stages were only due to the thermal degradation of CS, since Bt and alumina are stable in this temperature range. They are attributed to further deacetylation, cleavage of glycosidic bonds, and the subsequent oxidation of the residues [51,52]. The third mass loss of about 15%, recorded for T . 650K, which is not significant with respect to the second step, corresponds to the CS decomposition reactions and the formation of volatile compounds formed by oxidation of carbon residues [52]. However, the most important information to make from Fig. 5.14 is that the temperature Tm of maximum degradation rate in DTG curves of 5%Bt/CS and 10%Al2O3/

FIGURE 5.14 TGA/DTG curves of 5%Bt/CS and 10%Al2O3/CS at different heating rates.

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161

CS are different compared to those of CS, reflecting a net difference in their thermal stabilities and degradation mechanisms. The existence of multisteps in the thermal degradation curves of nanocomposites indicates: (1) the thermal stability of CS in nanocomposites depends on the interactions of CS with Bt and alumina in the CS matrix [45,53
55], and (2) the thermal degradation of CS in nanocomposites follows a complex reaction. Hence, it is known that the incorporation of negatively charged filler is expected to strongly interact with the positively charged NH31 groups of CS.

5.4.4 Thermal degradation kinetics In order to obtain a complete description of thermal degradation of CT, CS, 5%Bt/CS, and 10%Al2O3/CS, in the range of 373K and 873K, the kinetic triplet—activation energy (Ea), the frequency factor (A), and the functions f(α) and g(α)—describing the corresponding thermal degradation kinetics and mechanisms, should be determined. The most isoconversional methods used are those of OFW and KIS [38,40]. 5.4.4.1 Chitin and chitosan Based on the equation of OFW and KIS described in Section 5.4.1, the  plots of logβ as a function of 1/T within α of 0.1 , α , 0.9 and ln Tβ2 m versus 1000 Tm (Tm: maximum of DTG peak) for a given value of heating rate β 5 5 C, 10 C, 15 C, and 20 C give straight lines, as shown in Fig. 5.15. The values of Ea and the frequency factor A, determined for CT and CS are summarized in Table 5.3. As can be seen, the values determined using the two models are different, especially in the case of CS. These differences can be attributed to the different approximations used in each model. These values are almost constants for the extent conversion α , 0.6, which indicate that a single mechanism is involved in the thermal degradation of both polymers. The average values of Ea suggest that CT is less stable than CS in the studied α-conversion range. These values are in good agreement with those found in the literature for commercial CT and CS (Table 5.3) [56] (Ea 5 149 kJ/mol) [57], (Ea 5 147
166 kJ/mol) [48], and (Ea 5 149 kJ/mol) [58]. 5.4.4.2 Nanocomposites of 5%bentonite/chitosan and 10%Al2O3/ chitosan Based on the above results, it is established that the largest proportion of mass loss (50%) of 5%Bt/CS, occurring in the second stages of degradation, is due to the thermal degradation of CS, while the first and third steps are not. Thus this predominant step is chosen for the

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5. Thermal degradation characteristics of chitin, chitosan, and etc.

FIGURE 5.15 (A) Plots of log β versus 1000/T for the thermal of CT and  degradation 

CS at different conversion rates α (FWO method). (B) Plots of Ln CS (KIS method).

β 2 Tm

versus 1000 Tm of CT and

estimation of the kinetic parameters Ea, A, and f(α). Within the corresponding temperature range 500K
650K, the presence of two peaks in the DTG curves of 5%Bt/CS indicates that the interactions sites of Bt and Al2O3 with CS are different. Thus in the case of 5%Bt/CS, this stage

Handbook of Chitin and Chitosan

TABLE 5.3

Kinetic parameters obtained for CT and CS using FWO and KIS methods. FWO

KIS

CT α

CS 2

A (/min)

R

125.15

1.36 3 1010

0.998

123.44

9.66 3 10

0.2 0.3

Ea (kJ/mol) 0.1 0.15

09

0.999

CT

Ea (kJ/mol)

2

A (/min)

R

134.83

2.09 3 1011

0.999

126.89

4.08 3 10

0.999

10

10

121.53

9

6.58 3 10

0.999

127.49

4.62 3 10

0.999

123.12

9.06 3 109

0.999

133.26

1.52 3 1011

0.998

0.4



















0.5

125.6

1.49 3 1010

0.998

139.83

5.85 3 1011

0.998

0.6



















0.7

109.63

5.95 3 108

0.994

216.69

3.81 3 1013

0.967

0.8



















0.9



















Average

121.41 6 4

9.07 3 109




146.50 6 23

6.52 3 1012




Ea (kJ/mol) 119.44

CS 2

R

0.999

Ea (kJ/mol) 191.61

R2 0.981

164

5. Thermal degradation characteristics of chitin, chitosan, and etc.

can be divided into two intervals (Fig. 5.14), unlike the case of 10% Al2O3/CS where only one peak was obtained. Fig. 5.16 shows the lines obtained according to the FWO and KIS methods, applied to the experimental data in the range of 0.1 , α , 9, for the two nanocomposites. Activation energies and the corresponding frequencies factors issued from these models are given in Table 5.4. For 5% Bt/CS, the Ea values determined by the two methods (FWO and KIS) are not close to each other, because of the wide range of approximations used in each model [42]. For each step, the Ea values deduced from the KIS model are lower than those determined for CS and those found using the FWO model [59], because this method has certain limitations, as highlighted by the study presented by the International Confederation for Thermal Analysis and Calorimetry (ICTAC) [60]. Moreover, by analyzing the activation energies obtained by OFW, it can be seen for each stage that the average values of Ea 5 210.5 and 348.28 kJ/mol, are almost constants in the corresponding range of α, respectively. These values of Ea prove that the addition of Bt to the CS

FIGURE 5.16 Plots of log β versus 1000/T for the thermal degradation of (A) 5%Bt/CS   and (C) 10%Al2O3/CS at different conversion rates α (FWO method); and plots of ln

versus 1000/Tm of (B) 5%Bt/CS and (D) 10%Al2O3/CS (KIS method).

Handbook of Chitin and Chitosan

β 2 Tm

TABLE 5.4

Kinetic parameters obtained for 5%Bt/CS and 10%Al2O3/CS using FWO and KIS methods.

FWO

KIS

5%Bt/CS 1st step

2nd step

10%Al2O3/CS

5%Bt/CS

α

Ea (kJ/ mol)

A (/min)

R2

Ea (kJ/ mol)

A (/min)

R2

Ea (kJ/ mol)

A (/min)

R2

0.1

229.12

5.43 3 1021

0.982










177.48

4.23 3 1015

0.993

0.15



















133.49

3.78 3 1011

1.000

0.2

183.96

2.12 3 1017

0.977










130.54

2.01 3 1011

0.999

0.3

12

0.4

180.51

16

9.74 3 10

0.979

202.31

1.31 3 1019

0.962

0.5

256.77

2.72 3 10

0.936

0.6










24

392.49

32

4.57 3 10

0.982

139.46

1.34 3 10

0.996

346.41

5.73 3 1028

0.979

193.06

1.14 3 1017

0.990

323.42

6.49 3 10

182.77

1.30 3 10

0.984

315.89

1.49 3 10

89.87

3.37 3 10

0.988

26 26

0.976 0.976

16 07

0.7










321.21

4.21 3 10

0.975

72.60

8.03 3 10

0.991

0.8










372.28

8.87 3 1030

0.972










0.9

























Average

210.53 6 4

5.46 3 10




345.28 6 5

7.77 3 10




140.97 6 36

1.88 3 10

23

26

31

05


17




10%Al2O3/CS Ea (kJ/ mol)

R2

Ea (kJ/ mol)

R2

1st step

100.63

0.990

110.10

0.998

2nd step

168.93

0.979

166

5. Thermal degradation characteristics of chitin, chitosan, and etc.

matrix contributes to the improvement of its thermal stability [71]. In general, it has been shown, that with only a few percent of clay filler (#5% by weight), the nanocomposites often have significantly improved thermal, mechanical, and barrier properties compared to CS, due to a good dispersion of clay in the CS matrix. Other authors [52,53,55,61,62], using the FWO method, found that the weight loss of the composites based on CS matrix is done according to three stages of degradation, whose activation energies increase slightly from 140 to 190 kJ/mol along with the increase of the degree of conversion. It is stated that the addition of filler to the CS matrix acts as a barrier to the volatile compounds generated in the previous steps, leading to the increase of the thermal proprieties of the nanocomposites. For 10%Al2O3/CS, the largest DTG peak observed in the temperature range of 500K
650K corresponds to the largest weight loss of CS. This means that the overall degradation mechanism of the CS in this nanocomposite is governed by one step. The corresponding average activation energy values determined in the conversion range of α (0.3
0.6) by OFW and KIS methods are 135 and 110 kJ/mol, respectively. Neither the OFW nor KIS model gives a lower activation energy value than that found for CS alone, showing that CS alone is more stable than 10% Al2O3/CS.

5.4.5 Degradation model Most of the established models for describing the thermal degradation reactions of polymers are based on the plot of the model functions Z(α) and Y(α) as a function of α, using nonisothermal experimental data (dα dt ). Fig. 5.17 shows that for all samples the experimental curves of Z(α) and Y (α) functions are almost identical and do not depend on the heating rate β. The maximum of the functions Z(α) and Y(α) are ðαz 5 0:50;αy 5 0:47Þ for CT, ðαz 5 0:46; αy 5 0:40Þ for CS, ðαz 5 0:55; αy 5 0:52Þ for 5%Bt/CS, and ðαz 5 0:26; αy 5 0:24Þ for 10%Al2O3/CS. However, for the values of α , α , the shape of the curves obtained for each sample is different. This means that the degradation rates of CT, CS, 5%Bt/CS, and 10%Al2O3/CS are different, which is probably due to the difference in the side chain functional groups present in each sample that may affect the temperature value of the onset of thermal degradation, rather than only a difference in the degradation mechanism. This can also be related to the difference in stability between these solids. For values of α . α , the same profile becomes dominant because of the completion of the degradation process. Britto et al. [50] obtained the same profile in the case of CS for 0 , αy , αz , and found that the appropriate model to describe the kinetics of thermal degradation of CS is the autocatalytic model, introduced by Sestak and Berggren

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167

FIGURE 5.17 Experimental functions Y(α) and Z(α) of the thermal degradation in dynamic mode of CT, CS, 5%Bt/CS, and 10Al2O3/CS.

(SB (m, n)) [39]. This model is described by the conversion function fðαÞ 5 αm ð12αÞn (m and n are variable parameters). The values of the paraα meters m and n in the function fðαÞ are given by p 5 mn with p 5 1 2yα . y Thus Eq. 5.6 can be expressed in the form: [38] 
 dα Ea exp ln (5.15) 5 lnA 1 nln½αp ð1 2 αÞ dt RT (Table 5.4) Using the values of Ea determined

 Ebythe OFW method p a ½  leads to for 0.2 , α , 0.8, the plot of ln dα exp ð1 2 αÞ versus ln α RT dt straight lines with a slope equal to n. Table 5.5 shows the values found. The values of n tend to increase with the heating rate with values of m , n, for CT, CS, 5%Bt/CS, and 10%Al2O3/CS. Recently, Pe´rez-Maqueda et al. [63] introduced a new model by modifying that of SB. The new form of the model is as follows: fðαÞ 5 cð12αÞn αm (c, n, and m are parameters). The advantage of this method is that the determination of the degradation reaction mechanism is not limited to the usual kinetic models, commonly used by many researchers [64
67]. It also gives the possibility to overcome the

Handbook of Chitin and Chitosan

TABLE 5.5 Al2O3/CS.

Values of kinetic parameters m, n, and αm in dynamic mode for the thermal degradation of CT, CS, 5%Bt/CS, and 10% CT

CS

5%Bt/CS

10%Al2O3/CS

β (K/min)

m

n

αm (exp.)

αm (sim)

M

n

αm (exp.)

αm (sim)

m

n

αm

αm (sim)

m

n

αm

αm (sim)

5

1.45

1.57

0.48

0.38

0.85

0.96

0.47

0.37

6.10

6.95

0.49

0.50

3

12

0.22

0.22

10

1.61

1.74

0.48

0.38

1.03

1.32

0.43

0.34

4.44

5.67

0.54

0.56

5

17

0.23

0.23

15

1.63

1.80

0.47

0.37

2.22

2.87

0.44

0.34

4.47

5.78

0.60

0.62

6

19

0.24

0.24

1.98

1.92

0.51

0.40

1.42

2.06

0.41

0.31

3.42

4.96

0.62

0.63

8

24

0.25

0.25

20

5.4 Kinetics of thermal degradation of chitin, chitosan, Al2O3/chitin, and bentonite/chitosan

169

problems due to some factors, such as the heterogeneous distribution of particle size [68]. Therefore experimental data from the dynamic mode are generally submitted to the modified SB model. Thus the simulations were done from the function fðαÞ 5 cð12αÞn αm by plotting the theoretical function Y(α), to identify the probable reaction mechanism for the thermal degradation of CT, CS, 5%Bt/CS, and 10%Al2O3/CS. Fig. 5.18 shows the simulations of the Y(α) functions, plotted using the parameters m and n, determined experimentally. A perfect agreement between the experimental (Fig. 5.18) and simulated curves is obtained with a regression coefficient (R2 . 0.9985). On the other hand, according to the mathematical expression of f(α), the maximum of the function must be located at αm 5 m/m 1 n. The different values of αm, which appear in the different simulated curves for CT, CS, 5%Bt/CS, and 10% Al2O3/CS, are also close to those determined experimentally, which validates the kinetics parameters and function f(α) used in the different simulations. Moreover, the presence of a maximum in the function Y(α), indicates that the models suggested in the literature are those of Avami
Erofeev (A2 or A3) or the modified model SB(m, n). These models correspond to the kinetics of nucleation and growth with a random

FIGURE 5.18 Simulated (lines) and experimental (symbols) Y(α) for the dynamic degradation of CT, CS, 5%Bt/CS, and 10%Al2O3/CS.

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5. Thermal degradation characteristics of chitin, chitosan, and etc.

division of polymer chains (L2 and L8), initiated at the level of weakly bound molecules [64,66]. However, the experimental values αy corresponding to the maximum of Y(α) are practically identical to those found in the case of the Avrami
Erofeev model, which allows the validity of this model.

5.5 Conclusions The CT extracted from shrimp shells collected in the city of Meknes in Morocco was deacetylated at the optimal experimental condition: T 5 120 C, CNaOH 5 12 N, and t 5 300 min for the second treatment cycle, to obtain a high DD (83%). The deacetylation reaction occurs in two stages with different rate constants obeying a first-order kinetics. This may be due to the equilibrium reaction, diffusion, or the inaccessibility of the acetyl group. XRD analyses demonstrate that the CS has been intercalated within the layered structure of Bt, thus widening the basal spacing d(001) to which the layered structure becomes partially or completely exfoliated. The nonisothermal degradation process of the samples is studied by the TGA/DTG technique at different heating rates β (5, 10, 15, and 20K/min), under an air atmosphere. It was found that for CT the value of activation energy is Ea 5 121.41 kJ/mol, for CS Ea 5 146.50 kJ/mol, for Bt/CS Ea (step1) 5 210 kJ/mol and Ea (step2) 5 345 kJ/mol, and for Al2O3/CS Ea 5 131 kJ/mol. It was found that the modified SB (f(α) 5 cαm(1 2 α)n) model is the most adequate one to describe the decomposition kinetics in the air of the studied samples at various heating rates of CT, CS, and Bt/CS. It was established that this kinetics model could be used for a quantitative description of the nonisothermal decomposition process of samples, which corresponds to a random scission mechanism.

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[26] M. Kaya, O. Seyyar, T. Baran, T. Turkes, Bat guano as new and attractive chitin and chitosan source, Front. Zool. 11 (2014) 1
10. [27] G. Ca´rdenas, G. Cabrera, E. Taboada, S.P. Miranda, Chitin characterization by SEM, FTIR, XRD, and13C cross polarization/mass angle spinning NMR, J. Appl. Polym. Sci. 93 (2004) 1876
1885. [28] M.L. Duarte, M.C. Ferreira, M.R. Marva˜o, J. Rocha, An optimised method to determine the degree of acetylation of chitin and chitosan by FTIR spectroscopy, Int. J. Biol. Macromol. 31 (2002) 1
8. [29] T.G. Liu, B. Li, W. Huang, B. Lv, J. Chen, J.X. Zhang, et al., Effects and kinetics of a novel temperature cycling treatment on the N-deacetylation of chitin in alkaline solution, Carbohydr. Polym. 77 (2009) 110
117. [30] P. Methacanon, M. Prasitsilp, T. Pothsree, J. Pattaraarchachai, Heterogeneous Ndeacetylation of squid chitin in alkaline solution, Carbohydr. Polym 52 (2003) 119
123. [31] M. Kaya, F. Dudakli, M. Asan-Ozusaglam, Y.S. Cakmak, T. Baran, A. Mentes, et al., Porous and Nanofiber α-chitosan Obtained from Blue Crab (Callinectes sapidus) Tested for Antimicrobial and Antioxidant Activities, Elsevier Ltd., 2016. [32] M. Darder, M. Colilla, E. Ruiz-Hitzky, Biopolymer-clay nanocomposites based on chitosan intercalated in montmorillonite, Chem. Mater. 15 (2003) 3774
3780. [33] P. Monvisade, P. Siriphannon, Chitosan intercalated montmorillonite: preparation, characterization and cationic dye adsorption, Appl. Clay Sci. 42 (2009) 427
431. [34] M. Aazza, H. Ahlafi, H. Moussout, H. Maghat, Ortho-nitro-phenol adsorption onto alumina and surfactant modified alumina: kinetic, isotherm and mechanism, J. Environ. Chem. Eng. 5 (2017) 3418
3428. [35] L.S. Guinesi, E´.T.G. Cavalheiro, The use of DSC curves to determine the acetylation degree of chitin/chitosan samples, Thermochim. Acta. 444 (2006) 128
133. [36] Y.S. Nam, W.H. Park, D. Ihm, S.M. Hudson, Effect of the degree of deacetylation on the thermal decomposition of chitin and chitosan nanofibers, Carbohydr. Polym. 80 (2010) 291
295. [37] A. Khawam, D.R. Flanagan, Solid-state kinetic models: basics and mathematical fundamentals, J. Phys. Chem. B 110 (2006) 17315
17328. [38] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Anal. Chem. 29 (1957) 1702
1706. ˇ ´ k, G. Berggren, Study of the kinetics of the mechanism of solid-state reactions [39] J. Sesta at increasing temperatures, Thermochim. Acta. 3 (1971) 1
12. [40] T. Ozawa, A new method of analyzing thermogravimetric data, Bull. Chem. Soc. Jpn. 38 (1965) 1881
1886. [41] S. Montserrat, J. Ma´lek, P. Colomer, Thermal degradation kinetics of epoxy
anhydride resins: I, Thermochim. Acta. 313 (1998) 83
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176. [43] I. Corazzari, R. Nistico`, F. Turci, M.G. Faga, F. Franzoso, S. Tabasso, et al., Advanced physico-chemical characterization of chitosan by means of TGA coupled on-line with FTIR and GCMS: thermal degradation and water adsorption capacity, Polym. Degrad. Stab. 112 (2015) 1
9. [44] M. Ziegler-Borowska, D. Chełminiak, H. Kaczmarek, Thermal stability of magnetic nanoparticles coated by blends of modified chitosan and poly(quaternary ammonium) salt, J. Therm. Anal. Calorim. 119 (2015) 499
506. [45] J.B. Marroquin, K.Y. Rhee, S.J. Park, Chitosan nanocomposite films: enhanced electrical conductivity, thermal stability, and mechanical properties, Carbohydr. Polym. 92 (2013) 1783
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[46] P. Avetta, R. Nistico`, M.G. Faga, D. D’Angelo, E.A. Boot, R. Lamberti, et al., Herniarepair prosthetic devices functionalised with chitosan and ciprofloxacin coating: controlled release and antibacterial activity, J. Mater. Chem. B 2 (2014) 5287. [47] M.K. Jang, B.G. Kong, Y.I. Jeong, C.H. Lee, J.W. Nah, Physicochemical characterization of α-chitin, β-chitin, and γ-chitin separated from natural resources, J. Polym. Sci. Part A Polym. Chem. 42 (2004) 3423
3432. [48] T. Wanjun, W. Cunxin, C. Donghua, Kinetic studies on the pyrolysis of chitin and chitosan, Polym. Degrad. Stab. 87 (2005) 389
394. [49] A. Al-Mulla, F. Al-Sagheer, Determination of kinetic parameters for the degradation of chitosan/silica hybrid nano composites, J. Polym. Environ. 21 (2013) 504
511. [50] D. de Britto, S.P. Campana-Filho, Kinetics of the thermal degradation of chitosan, Thermochim. Acta. 465 (2007) 73
82. [51] D. Depan, A.P. Kumar, R.P. Singh, Preparation and characterization of novel hybrid of chitosan-g-lactic acid and montmorillonite, J. Biomed. Mater. Res. Part A 78A (2006) 372
382. [52] S.F. Wang, L. Shen, Y.J. Tong, L. Chen, I.Y. Phang, P.Q. Lim, et al., Biopolymer chitosan/montmorillonite nanocomposites: preparation and characterization, Polym. Degrad. Stab. 90 (2005) 123
131. [53] C.-Y. Ou, C.-H. Zhang, S.-D. Li, L. Yang, J.-J. Dong, X.-L. Mo, et al., Thermal degradation kinetics of chitosan
cobalt complex as studied by thermogravimetric analysis, Carbohydr. Polym. 82 (2010) 1284
1289. [54] M.R. Ricciardi, V. Antonucci, M. Giordano, M. Zarrelli, Thermal decomposition and fire behavior of glass fiber
reinforced polyester resin composites containing phosphate-based fire-retardant additives, J. Fire Sci. 30 (2012) 318
330. [55] S. Sinha Ray, Thermal stability and flammability of environmentally friendly polymer nanocomposites using biodegradable polymer matrices and clay/carbon nanotube (CNT) reinforcements, Environmental Friendly Polymers Nanocomposites, 2013, pp. 295
327. [56] K.L.B. Chang, G. Tsai, J. Lee, W.R. Fu, Heterogeneous N-deacetylation of chitin in alkaline solution, Carbohydr. Res. 303 (1997) 327
332. [57] A. Pawlak, M. Mucha, Thermogravimetric and FTIR studies of chitosan blends, Thermochim. Acta. 396 (2003) 153
166. [58] K. Muraleedharan, P. Alikutty, V.M. Abdul Mujeeb, K. Sarada, Kinetic studies on the thermal dehydration and degradation of chitosan and citralidene cChitosan, J. Polym. Environ. 23 (2015) 1
10. [59] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Pe´rez-Maqueda, C. Popescu, N. Sbirrazzuoli, ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data, Thermochim. Acta. 520 (2011) 1
19. [60] D.C. Johnson, E. Witschi, Endocrinology of ovarian tumor formation in parabiotic rats, Cancer Res. 21 (1961) 783. [61] S. Wang, L. Chen, Y. Tong, Structure-property relationship in chitosan-based biopolymer/montmorillonite nanocomposites, J. Polym. Sci. Part A Polym. Chem. 44 (2006) 686
696. [62] Y. Xu, X. Ren, M.A. Hanna, Chitosan/clay nanocomposite film preparation and characterization, J. Appl. Polym. Sci 99 (2006) 1684
1691. [63] L.A. Pe´rez-Maqueda, J.M. Criado, P.E. Sa´nchez-Jime´nez, Combined kinetic analysis of solid-state reactions: a powerful tool for the simultaneous determination of kinetic parameters and the kinetic model without previous assumptions on the reaction mechanism, J. Phys. Chem. A 110 (2006) 12456
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[65] P.E. Sa´nchez-Jime´nez, L.A. Pe´rez-Maqueda, A. Perejo´n, J.M. Criado, A new model for the kinetic analysis of thermal degradation of polymers driven by random scission, Polym. Degrad. Stab. 95 (2010) 733
739. [66] L. Vlaev, N. Nedelchev, K. Gyurova, M. Zagorcheva, A comparative study of nonisothermal kinetics of decomposition of calcium oxalate monohydrate, J. Anal. Appl. Pyrolysis. 81 (2008) 253
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812. [68] Q.L. Yan, S. Zeman, J.G. Zhang, X.F. Qi, T. Li, T. Musil, Multistep thermolysis mechanisms of azido-s-triazine derivatives and kinetic compensation effects for the rate-limiting processes, J. Phys. Chem. C 119 (2015) 14861
14872. [69] Moussout, Hamou, et al., Performances of local chitosan and its nanocomposite 5% Bentonite/Chitosan in the removal of chromium ions (Cr (VI)) from wastewater, International journal of biological macromolecules 108 (2018) 1063
1073. [70] Moussout, Hamou, et al., Adsorption studies of Cu (II) onto biopolymer chitosan and its nanocomposite 5% bentonite/chitosan, Water Science and Technology 73 (9) (2016) 2199
2210. [71] Moussou, Hamou, et al., Bentonite/chitosan nanocomposite: preparation, characterization and kinetic study of its thermal degradation, Thermochimica Acta 659 (2018) 191
202.

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C H A P T E R

6 Barrier properties, antimicrobial and antifungal activities of chitin and chitosan-based IPNs, gels, blends, composites, and nanocomposites Khalina Binti Abdan1,2, Soon Chu Yong3, Eric Chan Wei Chiang3, Rosnita A. Talib4, Tan Choon Hui3 and Lee Ching Hao1 1

Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia, 2Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia, 3Department of Food Science with Nutrition, Faculty of Applied Sciences, UCSI University, Cheras, Kuala Lumpur, Malaysia, 4Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia

O U T L I N E 6.1 Introduction 6.1.1 Insect sources 6.1.2 Marine sources

176 177 177

6.2 Barrier properties of chitin and chitosan 6.2.1 Parameters affecting barrier properties

179 181

Handbook of Chitin and Chitosan DOI: https://doi.org/10.1016/B978-0-12-817968-0.00006-8

175

© 2020 Elsevier Inc. All rights reserved.

176

6. Barrier properties, antimicrobial and antifungal activities

6.2.2 Permeability tests 6.2.3 Development of barrier properties of chitin and chitosan nanocomposites

185 186

6.3 Antimicrobial properties of the chitin and chitosan 6.3.1 Parameters affecting antimicrobial properties 6.3.2 Antimicrobial mechanisms

189 196 201

6.4 Antioxidant properties of chitin and chitosan 6.4.1 Parameters affecting antioxidant properties 6.4.2 Mechanism of antioxidant properties 6.4.3 Antioxidant assays

202 203 204 204

6.5 Applications of chitin and chitosan 6.5.1 Packaging applications 6.5.2 Dietary supplement applications 6.5.3 Agricultural applications 6.5.4 Cosmeceutical applications

205 205 210 211 213

6.6 Conclusions

214

References

215

6.1 Introduction Chitin was first characterized and described in 1884 [1]. Chitin can be derived from a wide range of sustainable sources. It is a major product of multiple proposed biorefineries designed to convert waste biomass into usable biomaterials. This has economic sustainability as well as ecological sustainability as it reduces the amount of waste to be disposed of as well as the associated carbon footprint when biomaterials are converted back into CO2 [2,3]. In this handbook, only marine and insect chitin sources will be discussed. For information on chitin extraction from fungi cell wall one can refer to previous works [4
6]. Chitin presents as semicrystalline microfibrils which form structural components mainly in the exoskeleton of arthropods or in the cell walls of fungi and yeast. Chitin is a linear polysaccharide and composed of 10 wt.% of D-glucosamine and 90 wt.% of Nacetyl-D-glucosamine (Fig. 6.1) [8]. Due to the high acetyl content, chitin is hydrophobic in nature. Deacetylation of chitin into chitosan transforms it into a more water and organic solvents friendly material, influencing the performance of chitosan [9]. The deacetylation of chitin (chitosan) will not be discussed in this handbook, but has been reviewed by Yuan [10].

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FIGURE 6.1 Structure of chitosan [7].

Chitin and chitosan are widely applied in packing, diet supplements, agriculture, and cosmetic industries due to their attractive properties (promising barrier, antimicrobial, and antioxidant properties) [11
16]. In addition, the antimicrobial ability is enhanced when chitin is deacetylated into chitosan, a more water-soluble polymer [17].

6.1.1 Insect sources The main hurdle for deriving chitin and chitosan from insects is that they are just not widely cultivated [18]. In the cuticle of insects, the abundant chitin fibers are covered with catechol and a mineral-rich protein matrix. This matrix is much less complex than that of crustacean shells, which is one of the marine sources. Additionally, chitin in insects is much less sclerotized than crustaceans, in which mineralization with calcium carbonate can reach up to 95% w/w. The biosynthesis of chitin in insects has been reviewed previously [19,20]. Chitin can be extracted from insects in much simpler conditions. The demineralization of insect cuticles requires only 1.0 M of HCl compared to 10% w/v (3.2 M) [21]. Chitin extraction from crustaceans typically uses NaOH at concentrations of up to 5.0 M, temperatures up to 160 C, and complete deprotenization can take days [1]. Soon et al. reported high yields of chitin from insect larvae using only 0.5 M of NaOH at 80 C, overnight [21]. Table 6.1 shows the recent investigations of chitin and chitosan polymers biosynthesized from insects.

6.1.2 Marine sources Shrimp, crab, and lobster shells have been proposed as a biorefinery feedstock to derive chitin, protein, and calcium carbonate [32]. An

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TABLE 6.1 Recent investigations of chitin and chitosan polymers biosynthesized from insects. Extract from insect source

Highlights

References

Bombyx mori

Analyzing biochemical characterization of three midgut chitin deacetylases

[22]

Palomena prasina

Comparison between chitin extracted from Plumatella repens (Bryozoa) and Fomes fomentarius (Fungi)

[23]

Magicicada septendecim, Tibicens tibicens, Drosophila melanogaster

SEM characterization of insect and arthropod cuticles chitin

[24]




The applications of magnetic resonance spectroscopy have been reviewed on insect cuticles chitin

[25]

Leiperia cincinnalis Sambon

Characterization of worn chitin

[26]




The chapter discussed on recent synthesis, structure, physical state, modifications and degradation of chitin in insect tissues

[27]

Omophlus sp.

Bovine serum albumin absorption (key protein in blood) comparison between α-chitin (Omophlus sp.) and β-chitin (Sepia sp.)

[28]

M. sexta

Genome-wide search for genes encoding proteins with ChtBD2-type (peritrophin A-type) chitin-binding domains

[29]

Oryctes nasicornis L.

Physicochemical comparison of natural and synthetic chitin films

[30]




Review on cellular basis of chitin synthesis in insects and fungi

[31]

estimated 6
8 million tons of crustacean shells wastes are discarded annually in the world with about 1.5 million tons in Southeast Asia alone [32]. Unlike finfish, where 75% of the mass is consumed, only about 40% of the mass of shellfish is consumed. This creates a huge amount of waste which incurs a huge disposal cost and developing countries that cannot afford to properly finance disposal simply dump them into landfills or the sea. Although crustacean shell biorefineries have vast economic and ecological benefits (Fig. 6.2), there are various chemical challenges to overcome. The use of sodium hydroxide and hydrochloric acid to extract chitin creates another source of waste to be disposed of. Deacetylation of chitin into chitosan uses a very high concentration of sodium

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FIGURE 6.2 Shell biorefinery structure and its uses [32].

hydroxide solution and requires a large amounts of water to filter and wash until pH 7, causing severe water pollution [33]. Recently, better production methods have been developed [32]. These include the use of lactic acid fermentation to release chitin from its protein and calcium carbonate matrix, designing ionic liquids that can dissolve carbohydrate polymers, and solvent-free methods for physically separating chitin. Hong et al. has innovated one-step extraction of chitin with high purity from a lobster shell [34]. The parameters of the reaction temperature and acid concentration were found to be most important in varying the molecular weight (Mw) of chitin. This has been confirmed by other research works using a deep eutectic solvent prepared from choline chloride and four organic acids for faster extraction of chitin from the lobster shell [35]. Table 6.2 shows recent investigations of chitin and chitosan polymers biosynthesized from marine sources.

6.2 Barrier properties of chitin and chitosan The barrier properties of a polymer refer to the permeability of a gas or liquid through a polymeric layer. The permeation process of the polymer barrier involves four steps: absorption of the permeating species into the polymer surface, solubility into the polymer matrix, diffusion through the wall along a concentration gradient, and finally desorption from the outer surface [50]. The history and development of various theories in polymeric diffusion systems have been discussed in previous works [51].

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TABLE 6.2 Recent investigations of chitin and chitosan polymers biosynthesized from marine sources. Extracts from marine source

Highlights

Reference

Carb shells

Characterization of microfibrillated chitin/gelatin composites

[36]

Conus inscriptus

Extraction and characterization of chitin from sea snail C. inscriptus

[37]

Suberea clavata

Isolation and identification of chitin from S. clavate marine demosponge

[38]

Ianthella basta

Novel chitin scaffolds for tissue engineering

[39]

Marine chitin

A recyclable pollutant, nanocatalysts

[40]

Shrimp shells

Efficient enzymatic hydrolysis and dissolution mechanism on chitin

[41]

Pseudoceratina purpurea

New source of fibrous chitin

[42]

Cyclotella sp.

Chitin nanofiber CO2 permeability

[43]

Cyclotella sp.

Light intensity parameter on chitin nanofiber productions

[44]

Aplysina fistularis

Identification of α-chitin in skeleton of A. fistularis

[45]

Aplysina aerophoba

Chitin-based sponge for uranium absorption

[46]

Ernstilla lacunosa

Novel Dendrilla-like sponge

[47]

Prawn shell

Characterization of chitin nanocrystals and application for stabilization of Pickering emulsions

[48]

Shrimp waste, crab shells, and cuttlefish

Structural differences between chitin and chitosan extracted from three different marine sources

[49]

The general theory of gas or liquid permeation within the polymer matrix follows the mass transport mechanism ruled by Fick’s law (Eq. 6.1); in the x-direction, Fx, is proportional to the concentration gradient (δc/δx) [52].   ϑc (6.1) Fx 5 2 D δx where D is the diffusion coefficient. In most theoretical treatises, the composite is considered to consist of a permeable phase (polymer matrix) in which nonpermeable nanoplatelets are dispersed. Chitin and chitosan are hydrophilic in nature. However, chitin does not dissolve in both polar and nonpolar solvents while chitosan only dissolves in acidified solution. The presence of chitin and chitosan could enhance the

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barrier properties of the matrix toward permeates. The common permeates that have been extensively studied are oxygen gas and water vapor.

6.2.1 Parameters affecting barrier properties Chitin and chitosan are natural biopolymers that have been widely studied and applied in the packaging system due to their biocompatibility and excellent biodegradable properties. Meanwhile, chitosan has the added value of antimicrobial, antifungal, and antioxidant properties, which can enhance the ability of food preservation. Miller and Krochta reviewed several factors that affect the permeability of the composite [53]. Fig. 6.3 shows the parameters affecting the barrier properties of chitin and chitosan. 6.2.1.1 Chemical structures Different chemical structures on a polymer can affect the bonding effects in the composite and the mass transport mechanism. The types of the substituent groups present in a polymer can have a tremendous

FIGURE 6.3 Parameters affecting the barrier properties of chitin and chitosan.

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6. Barrier properties, antimicrobial and antifungal activities

effect on the variability of the permeability coefficient by influencing the two main factors: the intermolecular space distance, and the free volume or voids that exist between the polymer chains. The chemical structure has direct effects on the cohesive energy density. 6.2.1.2 Cohesive energy density Cohesive energy density (CED) is a measure of the polarity and binding energy of the polymers. In general, the higher the polymer’s cohesive energy density, the more difficult it is for the polymer chains to allow molecule permeation. Chitin and chitosan contain the major hydroxyl group (CED: 220 cal/cm3) and amine group (CED: 180 cal/cm3) in the polymer pyranose backbone. These chemical structures are high in polarity and greatly decrease the oxygen permeability, suited well to enhancing the barrier properties in the packaging. However, the polar water molecules that exist in water vapor do not rely on the polymer chains’ cohesive energy density. 6.2.1.3 Free volume Free volume is a measurement of the degree of interstitial space between the molecules in a polymer. Wenmu [54] has proven the fractional free volume (FFV) of polymer has a positive correlation with the gas permeability (Table 6.3) [54]. The diffusion coefficient and the permeability coefficient decrease with the decrease in free volume unit. Gas permeation in a membrane involves two main stages: adsorption of permeates on the surface of the membrane and diffusion through the membrane. During adsorption, permeates position themselves in the free volume of the surface and then diffuse into the neighboring free volume due to the concentration gradient. Thus it depends on the TABLE 6.3 Permeability coefficients measured at 30 C and 1 atm [54]. Polymer ratio

Permeability, 10210 cm3 (STP) cm/(cm2 s cmHg)

Polymer

aa

bb

PH2

PO2

PN2

PCO2

PCH4

FFV

PI




1

27.37

2.99

0.40

14.16

0.36

0.1974

PI-90

1

9

26.03

3.01

0.59

14.64

0.39

0.2081

PI-75

1

3

23.71

2.65

0.49

13.45

0.36

0.1907

PI-50

1

1

16.66

1.66

0.30

8.06

0.26

0.1753

PI-25

3

1

11.04

0.87

0.14

4.00

0.14

0.1537

PI-10

9

1

9.98

0.78

0.12

3.61

0.13

0.1251

a

2,5-Dichlorobenzophenone. 5-Chloro-2-[4-chloro-2-(trifluoromethyle)phenyl] isoindoline-1,3-dione.

b

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FIGURE 6.4 Correlation of average diffusion coefficient of CO2 in various polymers with antiplasticizers with free volume. PC, Polycarbonate; PPO, poly(phenylene oxide); PS, polystyrene; PSF, polysulfone [56].

number and the size of these holes (static free volume) and on the frequency of the jumps (dynamic free volume) [55]. Some practices have been taken to reduce the free volume of the packaging, such as the addition of antiplasticizers [56]. Fig. 6.4 shows the correlation of the average diffusion coefficient of CO2 in various polymer with antiplasticizers with free volume. As the free volume decreases, the permeability of the nonpolar gases decreases as well. Stiff-chained polymers that have a high glass transition temperature generally have a low gas permeability due to the strong cohesive energy density and low free volume. 6.2.1.4 Crystallinity Crystallinity is a measurement of the degree of structural order of the molecules in a polymer. It has an inverse relationship with the permeability of polymer composites. The mass transfer of a gas or aroma in a semicrystalline polymer is primarily a function of the amorphous phase, resulting in the permeation of the gas or aroma. Meanwhile, the crystalline phase is assumed to be impermeable. Guinault et al. has concluded that better oxygen or helium barrier ability was found with higher crystallinity [57]. Chitin is a semicrystalline polymer that has a crystallinity index up to 70%, subject to the purity of the chitin [21]. The incorporation of

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FIGURE 6.5 The water absorption for different crystallinity index for (A) chitin (B) chitosan [59].

higher crystallinity reinforcement fillers such as cellulose nanocrystals into chitosan further decreases the water vapor permeability of composite film since water vapor more favorably diffuse though the amorphous region of composite films [58]. This has been agreed by Ioelovich who investigated the relationship between the water absorption ability and crystallinity of chitin and chitosan [59]. The results revealed that a lower crystallinity index has a better water absorption ability (Fig. 6.5). Besides, chitin polymer containing hydrophobic acetyl groups has a lower absorption ability than the chitosan containing only hydrophilic hydroxyl and amine groups. 6.2.1.5 Orientation Orientation refers to the alignment of the polymer chains in the plane of the polymer backbone and is a byproduct of the processing operation. The decrease in the fractional free volume of the amorphous region with orientation correlates well with the decrease in permeability, solubility, and diffusivity coefficients. Nielsen proposed a simple permeability model based on the parallel alignment of polymer [60]. The orientation is perpendicular to the diffusion direction. However, the addition of filler platelets acts as an impermeable barrier to the diffusing molecules, forcing them to follow a longer and more tortuous paths during diffusion within the membrane [55]. Liang has confirmed there is a dramatic increase of pore size in cellulose/chitin membrane when increasing the chitin contents which can be further confirmed by swelling testing [61]. Apart from this, environmental scanning electron microscopy (ESEM) micrograph of pure cellulose membrane and 44.4 wt.% of chitin insertion cellulose membrane show different sizes of voids (Fig. 6.6). Nessa has confirmed that the period of the deacetylation process for prawn shell waste chitosan does affect the permeability, solubility, and degree of deacetylation (DD) [62].

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FIGURE 6.6 ESEM micrograph of pure cellulose membrane (A) and 44.4 wt.% of chitin insertion cellulose membrane (B) [61].

6.2.1.6 Cross-linking Cross-linking in polymer is the formation of intermolecular bonds among the chains of a polymer. Several cross-link inducing methods have been proposed, such as heat curing, irradiation, enzymatic treatment, and chemical cross-linking agent. Regardless of the biobased and synthetic polymer, the structuring and controlling of the polymer chains is a huge challenge. Nevertheless, the cross-linking treated polymer was found to have a significant decrease in free volume size, hence reducing the permeability. 6.2.1.7 Degree of dispersion of the platelets The degree of dispersion of the platelets determines the degree of delamination of the composite. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite and it is much more effective for use in barrier membranes for gases. Meanwhile, exfoliation, which implies the complete breakage of the initial layer stacking order and homogeneous dispersion of the layers in the polymer matrix, is one of the current challenges in enhancing the barrier properties of the composite.

6.2.2 Permeability tests There are several approaches to identify the permeability of the polymer film. A film with lower permeability to permeate renders higher barrier properties which is usually desired to be used as a food packaging.

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6. Barrier properties, antimicrobial and antifungal activities

The transfer of gases and water vapor from an external to internal environment will cause the food to deteriorate, hence shortening the shelf life. However, there are some applications, such as wound dressing, that prefer high permeability to allow the rapid transfer of material but small enough to prevent the invasion of bacteria. All of the related standards for permeability tests are listed in Table 6.4.

6.2.3 Development of barrier properties of chitin and chitosan nanocomposites Chitin and chitosan in recent studies have been noted to be in the nanorange size, whose polymer length is ideally from 100 to 1000 nm. They can be the main components in the polymeric matrix, or regarded as the fillers to enhance the mechanical, thermal, optical, biodegradation, and barrier properties. Nanochitin is very often studied as a filler in polymers. Due to its light weight and polydispersal properties, the loading of the nanochitin is usually not more than 10% of dry weight in the composite. High loading of the nanochitin in the polymer matrix will lead to aggregation, resulting in decreased delamination of nanochitin filler composition and facilitating permeate diffusion [63]. The selection of a secondary filler will influence the effective chitin filler content. The addition of the ZnO-Ag nanoparticles into the carboxymethyl cellulose (CMC) and chitin nanowhisker film increases the WVP. This is possible due to the decreased self-assembling of chitin in its dry state, which might be caused by the adhered nanoparticles on the surface of the chitin nanowhisker within the composite [64]. Fig. 6.7 shows the morphology of chitin, chitin nanowhiskers (ChNW), and nanochitin with ZnO/Ag hollow spheres. Chitin is generally chemically stable and only disperses well in water solvent after homogenization. When it is used as a filler, it forms a semicrystalline and dense network within the laminated matrix by strong interfacial interaction [65]. So, the impermeable nanochitin is often mixed with water-soluble gel films, such as bovine gelatin and carrageenan [63,66,67]. However, it contains the free hydrophilic groups (
OH,
NH) in the biopolymer matrix through which the polymer composite can be further improved by the addition of nanoparticles or hydrophobic oil-based materials. Nanoparticles can fill the free volume spaces between polymer chains and interact with them. However, there is a challenge in balancing the ratio between the oil-based filler and chitosan fillers to allow the increase of hydrophobicity of the film toward the permeability of water while not causing phase separation of the polymer matrix due to high oil-based filler loading [67].

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TABLE 6.4 List of standards related to gas, oxygen gas, and water vapor transmission or permeability tests. Code

Title

ASTM D1434

Standard test method for determining gas permeability characteristics of plastic film and sheeting

ASTM D3985

Oxygen gas transmission rate through plastic film and sheeting using a coulometric sensor

ASTM E398

Standard test method for water vapor transmission rate of sheet materials using dynamic relative humidity measurement

ASTM E96/ E96M

Standard test methods for water vapor transmission of materials

ASTM F1249

Standard test method for water vapor transmission rate through plastic film and sheeting using a modulated infrared sensor

ASTM F1307

Standard test method for oxygen transmission rate through dry packages using a coulometric sensor

ASTM F1927

Standard test method for determination of oxygen gas transmission rate, permeability and permeance at controlled relative humidity through barrier materials using a coulometric detector

ASTM F2622

Standard test method for oxygen gas transmission rate through plastic film and sheeting using various sensors

ISO 15105-1

Plastics—Film and sheeting—Determination of gas-transmission rate— Part 1: Differential-pressure methods

ISO 15105-2

Plastics—Film and sheeting—Determination of gas-transmission rate— Part 2: Equal-pressure method

ISO 15106-1

Plastics—Film and sheeting—Determination of water vapor transmission rate—Part 1: Humidity detection sensor method

ISO 15106-2

Plastics—Film and sheeting—Determination of water vapor transmission rate—Part 2: Infrared detection sensor method

ISO 15106-3

Plastics—Film and sheeting—Determination of water vapor transmission rate—Part 3: Electrolytic detection sensor method

ISO 2528

Sheet materials—Determination of water vapor transmission rate (WVTR)—Gravimetric (dish) method

ISO 2556

Plastics—Determination of the gas transmission rate of films and thin sheets under atmospheric pressure—Manometric method

TAPPI T448 OM

Water vapor transmission rate of paper and paperboard at 23 C and 50% RH

TAPPI T464 OM

Water vapor transmission rate of paper and paperboard at high temperature and humidity

TAPPI T557 WD

Water vapor transmission rate through plastic film and sheeting using a modulated infrared sensor

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6. Barrier properties, antimicrobial and antifungal activities

FIGURE 6.7 Morphology of chitin (1), chitin nanowhiskers (2) and nanochitin with ZnO/Ag hollow spheres (3 and 4) [65].

Chitin fillers have better barrier properties in thermoplastic corn starch as compared to chitosan [68]. However, chitosan is mostly studied as the main polymer matrix to which fillers are added. Recent findings have

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189

increased the interest in loading lipid, essential oil, and phenolic compounds into the chitosan matrix. In fact, the presence of oil not only leads to a strong association with chitosan but also entraps part of the plasticizer, avoiding its loss in an aqueous medium. Due to the high DD of the chitosan, electrostatic interaction predominates between the amino group of the glucosamine units (
NH31) and the acid group (
O2) of nonpolar free fatty acids, forming an insoluble salt. Therefore fatty acid chains anchor chitosan through electrostatic forces to form a Pickering emulsion which provides hydrophobicity and thus reduces the adsorption of water molecules [58]. Nanoparticles such as zinc oxide, silver nanoparticles, graphene oxide, and calcium carbonate are mixed with chitosan film [69
72]. Nano ZnO is a good filler that can enhance the barrier properties of a chitosan composite. It interacts with free D-glucosamine groups in chitosan to reduce the hydrogen bonding formation of chitosan with the water vapor permeate. Meanwhile, the addition of silver nanoparticles in chitosan
starch film causes the reduction of OTR due to the elongation of diffusion path length through the polymeric matrix. Recent studies on chitin and chitosan derivatives are also focused on wound dressing due to their excellent antimicrobial properties. The prevention of the microorganism infection can minimize the deterioration of the wound. Wound dressings from chitosan such as Kyotocel, Chitoderm, CRABYON, and Bac-Shield have been commercialized. An ideal wound dressing should have a good swelling property and a loose-porous structure to absorb exudates from wounds and speed up the diffusion of nutrients, which are beneficial for promoting cell proliferation and accelerating wound healing. Meanwhile, other aspects to consider in biomaterial production are the ability to trap and maintain the moisture content on the wound to prevent it drying out. For normal skin, the inherent evaporation rate of water is about 204 6 12 g/m2/day. The evaporation rate of water increased to 279 6 26 and 5138 6 202 g/m2/day for the injured skin and a burn wound. Hence, the water vapor permeation rate of an ideal wound dressing is at 2000
2500 g/m2/day to retain the moisture of the wounded skin. Water vapor transfer rate (WVTR) and oxygen transfer rate (OTR) from the literature are listed in Table 6.5.

6.3 Antimicrobial properties of the chitin and chitosan Recent studies of antimicrobial properties involve chitosan and chitosan derivatives. To date, no study has been reported on the positive antimicrobial effect of chitin [81]. Chitosan as an environment-friendly antibacterial and antifungal agent is receiving increasing attention due

Handbook of Chitin and Chitosan

TABLE 6.5

Water vapor transfer rate (WVTR) and oxygen transfer rate (OTR) of various film. Addition of fillers

Main component

Filler 1

Filler 2

WVTR

OTR

References







8.89 6 0.10 3 1010 g/m s pa

n.d.

[66]

3% nanochitin




8.09 6 0.09 3 10 g/m s pa

n.d.

5% nanochitin




7.89 6 0.12 3 10 g/m s pa

n.d.

Chitin as filler in gel and film Bovine gelatin

Bovine gelatin

Carrageenan

Carboxymethyl cellulose

10 10

10% nanochitin




7.51 6 0.20 3 10 g/m s pa

n.d.







9.68 6 0.45 3 1010 g/m s pa

n.d.

5% nanochitin




8.89 6 0.10 3 10 g/m s pa

n.d.

5% nanochitin

10% corn oil

7.86 6 0.12 3 10 g/m s pa

n.d.

5% nanochitin

20% corn oil

7.81 6 0.07 3 1010 g/m s pa

n.d.

5% nanochitin

30% corn oil

7.68 6 0.75 3 10 g/m s pa

n.d.







1.91 6 0.03 3 10 g/m s pa

n.d.

10

10 10

10 9

3% chitin nanofiber




1.63 6 0.01 3 10 g/m s pa

n.d.

5% chitin nanofiber




1.54 6 0.01 3 109 g/m s pa

n.d.

10% chitin nanofiber




1.71 6 0.02 3 10 g/m s pa

n.d.







1.97 6 0.01 3 10 g/m s pa

n.d.

9

9 9

1% nanochitin




1.78 6 0.01 3 10 g/m s pa

n.d.

5% nanochitin




1.85 6 0.01 3 109 g/m s pa

n.d.

10% nanochitin




1.83 6 0.02 3 10 g/m s pa

n.d.

9

9

[67]

[63]

[65]

Thermoplastic corn starch







1.33 6 0.09 3 109 g/m s pa

n.d.

5% chitosan




1.19 6 0.09 3 109 g/m s pa

n.d.

10% chitosan




0.87 6 0.04 3 109 g/m s pa

n.d.

10% chitin




0.59 6 0.02 3 10 g/m s pa

n.d.

Chitosan with lipids, essential oil, polyphenolic compounds Chitosan

Chitosan

Chitosan

9







2.63 6 0.05 3 1025 g mm/m2 Pa

n.d.

5% C. copticum essential oil




2.08 6 0.08 3 1025 g mm/m2 Pa

n.d.

5% C. copticum essential oil

4% cellulose nanofiber

1.53 6 0.11 3 1025 g mm/m2 Pa

n.d.

5% C. copticum essential oil

4% lignocellulose nanofibers

1.47 6 0.03 3 1025 g mm/m2 Pa

n.d.







1.57 mg/cm2 h

n.d.

0.1% ZnO




0.3% ZnO




0.5% ZnO




0.5% ZnO: neem oil




0.41 mg/cm h

n.d.







2.4 6 0.03 3 10 g/m s pa

n.d.

2

n.d.

2

1.08 mg/cm h

n.d.

0.69 mg/cm2 h

n.d.

1.23 mg/cm h

2

8

0.5% cellulose nanofiber




2.4 6 0.04 3 10 g/m s pa

n.d.

1.0% cellulose nanofiber




2.3 6 0.03 3 108 g/m s pa

n.d.

3.0% cellulose nanofiber




1.0 6 0.04 3 10 g/m s pa

n.d.

5.0% cellulose nanofiber




1.3 6 0.03 3 10 g/m s pa

n.d.

8

8 8

[68]

[73]

[70]

[74]

(Continued)

TABLE 6.5

(Continued) Addition of fillers

Main component Chitosan

Filler 1

Filler 2

WVTR

OTR

References

Zein

0.071 6 0.007 g mm/cm 24 h

n.d.

[75]

0.5% adipic acid 0.5% adipic acid

Zein and ferulic acid

0.022 6 0.003 g mm/cm 24 h

n.d.

Zein and gallic acid

0.022 6 0.004 g mm/cm2 24 h

n.d.

Zein

0.085 6 0.003 g mm/cm 24 h

n.d.

0.5% succinic acid

Zein and ferulic acid

0.054 6 0.007 g mm/cm 24 h

n.d.

0.5% succinic acid

Zein and gallic acid

0.036 6 0.008 g mm/cm2 24 h

n.d.







12 6 1 3 10

14.4 6 0.5 cm3 μm/ m2 day kPa

0.01 resveratrol




12 6 2 3 1010 g/m s pa

16 6 1 cm3 μm/ m2 day kPa

0.1 resveratrol




11.9 6 0.5 3 1010 g/m s pa

11.2 6 0.4 cm3 μm/ m2 day kPa







0.210 6 0.02 g mm/m2 kPa h

n.d.

Tannic acid




0.216 6 0.04 g mm/m kPa h

n.d.

0.5% adipic acid

0.5% succinic acid

Chitosan

Chitosan

Chitosan with glycerol

Chitosan with glycerol

2 2

2 2

10

g/m s pa

2

2.5% L. sativum phenolic extract




0.200 6 0.07 g mm/m kPa h

n.d.

5.0% L. sativum phenolic extract




0.187 6 0.05 g mm/m2 kPa h

n.d.

7.5% L. sativum phenolic extract




0.175 6 0.02 g mm/m kPa h

n.d.







1.35 6 0.66 3 10 g/m s pa

n.d.

3% nanocellulose




1.09 6 0.05 3 10 g/m s pa

n.d.

2

2

11 11

[76]

[77]

[58]

5% nanocellulose




1.01 6 0.14 3 1011 g/m s pa

n.d.

7% nanocellulose




0.90 6 0.12 3 1011 g/m s pa

n.d.

12% nanocellulose




0.83 6 0.11 3 10 g/m s pa

n.d.

10% olive oil

0.94 6 0.15 3 10 g/m s pa

n.d.

1% nanocellulose

10% olive oil

0.80 6 0.05 3 10 g/m s pa

n.d.

2% nanocellulose

10% olive oil

0.81 6 0.04 3 1011 g/m s pa

n.d.

3% nanocellulose

10% olive oil

0.60 6 0.02 3 10 g/m s pa

n.d.

100% gelatin




0.826 6 0.047 g mm/kPa h m2

n.d.

80% gelatin

20% chitosan

0.707 6 0.052 g mm/kPa h m

2

n.d.

70% gelatin

30% chitosan

0.602 6 0.034 g mm/kPa h m2

n.d.

40% chitosan

0.410 6 0.034 g mm/kPa h m

2

n.d.

100% chitosan

0.367 6 0.004 g mm/kPa h m

2

n.d.




11 11 11

11

Chitosan in gel film Gelatin
Chitosan

60% gelatin
Kefiran film

211

100% kefiran




7.94 6 0.06 3 10

g/m s pa

n.d.

68% kefiran

32% chitosan

7.83 6 0.02 3 10211 g/m s pa

n.d.

50% kefiran

50% chitosan

4.70 6 0.01 3 10211 g/m s pa

n.d.

32% kefiran

68% chitosan

Chitosan with irradiation treatment, nanoparticles, and coating
106.4 mL 0.5% w/v 250 mL 1.2% w/v γ-ray irradiated chitosan chitosan 75 mL 2.0% w/v rice starch solution 75 mL 2.0% w/v waxy starch rice solution 1.2 g glycerol

211

3.52 6 0.06 3 10

g/m s pa

47.60 6 0.36 g/m2 day

[78]

[79]

n.d. 2.39 6 0.02 cm3/ m2 day

[72]

(Continued)

TABLE 6.5

(Continued) Addition of fillers

Main component

Filler 1 26.6 mL 0.5% w/v γ-ray irradiated chitosan 1 silver nanoparticle

Filler 2 79.8 mL 0.5% w/v γ-ray irradiated chitosan

WVTR

OTR

47.75 6 1.18 g/m day

References

1.97 6 0.01 cm / m2 day

2

3

53.2 mL 0.5% w/v γ-ray irradiated chitosan 1 silver nanoparticle

53.2 mL 0.5% w/v γ-ray irradiated chitosan

55.76 6 2.75 g/m2 day

1.60 6 0.01 cm3/ m2 day

106.4 mL 0.5% w/v γ-ray irradiated chitosan 1 silver nanoparticle




59.21 6 2.10 g/m2 day

1.48 6 0.03 cm3/ m2 day







111 6 0.2 3 107 (K/L) g/pa s m2

n.d.

0.5% graphene oxide




108 6 0.1 3 10 (K/L) g/pa s m

2

n.d.

1.0% graphene oxide




102 6 0.5 3 10 (K/L) g/pa s m

2

n.d.

1.5% graphene oxide




103 6 0.6 3 10 (K/L) g/pa s m

2

n.d.

2.0 % graphene oxide




92 6 0.8 3 107 (K/L) g/pa s m2

n.d.

Chitosan coating noncomposited CaCO3

41.01 6 2.39 g/h m

2

2.19 6 0.42 mL mm/ kPa cm2

Unmodified CaCO3—chitosan composite coating

40.27 6 1.95 g/h m2

2.69 6 0.06 mL mm/ kPa cm2

37.89 6 2.28 g/h m2

1.32 6 0.38 mL mm/ kPa cm2

Chitosan

CaCO3—chitosan composite coating in situ modified by

Sodium strearate




7 7 7

[69]

[71]

Poly(lactic acid)

a

Sodium polyacrylate




38.51 6 4.19 g/h m2

Citric acid

Sodium strearate

40.05 6 0.96 g/h m2

3.79 6 0.36 mL mm/ kPa cm2

Sodium citrate




40.23 6 0.83 g/h m2

8.02 6 0.25 mL mm/ kPa cm2




15.94 g/m2 day

n.d.







20.0 wt.% ATBC

a

19.8 wt.% ATBC

a

19.4 wt.% ATBC

a

3 wt.% chitosan

56.67 g/m day

n.d.

19.0 wt.% ATBCa

5 wt.% chitosan

48.30 g/m2 day

n.d.

1 wt.% chitosan

Tributyl o-acetyl citrate (ATBC) (molecular weight of 406 g/mol and purity (GC) of 99.35%).

n.d., not determined.

2

n.d.

2

n.d.

2

42.50 g/m day 43.91 g/m day

[80]

196

6. Barrier properties, antimicrobial and antifungal activities

to its biocompatible, biodegradable, and nontoxic properties and its high abundance in nature. It is well suited wide range of applications that require sanitation such as packaging, wound healing, pharmaceutical drugs, cosmetics, nutraceutical food, agriculture and aquaculture, wastewater treatment, and so on [82].

6.3.1 Parameters affecting antimicrobial properties The factors that affect the antimicrobial properties of chitosan are mainly the deacetylation degree (DD), Mw, abiotic factors, and chitosan derivatives. However, based on the literature, the antimicrobial efficacy of chitosan and chitosan oligomer is also influenced by the specific behavior of the Gram-positive or Gram-negative bacteria. After all, chitosan is said to be bacteriostatic (prevents the growth of bacteria) rather than bactericidal (kills the viable bacteria) [83]. Besides, the type of microorganism and sources of chitosan have been discussed in the previous review papers as factors influencing the antimicrobial activity of chitosan [84]. Fig. 6.8 shows the parameters for the antimicrobial properties of chitin and chitosan. 6.3.1.1 Degree of deacetylation Generally, DD of chitosan can be categorized into low DD (47%
53%), medium DD (74%
76%), and high DD (95%
98%) groups. The antimicrobial activities showed that the higher the DD of chitosan, the lower the minimal lethal concentration (MLC). The chitosan with high DD has good antimicrobial action against both Gram-positive bacteria such as Bacillus cereus CCRC 10250, Listeria monocytogenes LM-LM, and Staphylococcus aureus CCRC 12652; and Gram (2) bacteria such as Escherichia coli CCRD 10674, Vibrio parahaemolyticus CCRC 1806, Vibrio chlolerea CCRC 13860, Shigella dysenteriae CCR 13983, and Pseudomonas aeruginosa CCRC 10944. However, the antimicrobial action is not efficient against Salmonella typhimurium CCRC 10746 (MCL 5 1500 ppm). The same test was conducted on fungi species. The MCL of high DD chitosan to yeast (Candida albicans CCRC 20511) is low but is weak against filamentous fungi such as Fusarium oxysporum CCRC 32121, Aspergillus fumigatus CCRC 30502, and Aspergillus parasiticus CCRC 30117. The antimicrobial activity of chitosan is greatly affected by the number of protonated D-glucosamine groups and the negative charge on the microbial surface. The increase of DD in chitosan allows more protonated D-glucosamine to bind on to the bacterial surface and allows a higher efficacy of antimicrobial activity [85].

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6.3 Antimicrobial properties of the chitin and chitosan

197

FIGURE 6.8 Parameters for the antimicrobial properties of chitin and chitosan.

6.3.1.2 Gram-positive and Gram-negative bacteria Generally, antimicrobial activity of chitosan is greater against Gramnegative bacteria due to the opposite charge. However, in the inhibition zone test, P. aeruginosa ATCC 27853 is totally resistant to chitosan in any DD and Mw of chitosan. As the DD increased, the increase in antimicrobial efficacy in terms of inhibition zone and MIC were observed in both Gram-positive and Gram-negative bacteria [86]. 6.3.1.3 Surface charge density of bacteria membrane Chitosan can be separated into chitosan chain (Mw more than 22 kDa) and chitosan oligomer (Mw less than 22 kDa). Chitosan showed better antibacterial activity than chitosan oligomers but not to all bacteria. Fig. 6.9 shows the antimicrobial effects of different Mws of chitosans and chitooligosaccharides on two bacteria. The antimicrobial efficacy remained low against some Gram-negative bacteria, such as E. coli

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198

6. Barrier properties, antimicrobial and antifungal activities

FIGURE 6.9 Minimum inhibitory concentrations (white column) and minimum lethal concentrations (black column) of chitosans of various MWs and chitooligosaccharides, COS, upon (A) E. coli and (B) S. aureus [87].

ATCC 11775, S. typhimuruim ATCC 14082, and V. parahaemolyticus ATCC 17802, with any Mw of chitosan. The antimicrobial activity against P. fluorescens ATCC 14028 and Lactobacillus bulgaricus IFO 3533 showed huge differences in efficacy across the chitosan Mw range [88]. Surprisingly, Fernandes et al. proved the strong association between Mw and antimicrobial activity. Of the tested sample, the MIC and MLC of Gram-negative E. coli NCTC 9001 increased when the Mw of chitosan increased, but the opposite trend was observed in Gram-positive S. aureus NCTC 8532. This suggested that the Mw of chitosan has different impacts on different groups of bacteria [87]. 6.3.1.4 Abiotic factors Apart from the characteristics of chitosan, abiotic factors such as pH, type of acids, ionic strength, and presence of metal ions can affect the antimicrobial properties. Chitosan only dissolves in dilute acidic solution that is below its pKa (about 6.3) [83]. This will lead to the formation of protonated D-glucosamine chitosan (
NH31) and hence creates intermolecular electric repulsion, concomitant to longer persistence length, as well as creating surface interference with negatively charged bacterial cell membrane for the bactericidal properties. In neutral pH, chitosan has a miniature antimicrobial effect [89]. It is a general practice to prepare a mild acid as a chitosan solvent using acetic acid. However, other organic acids such as lactic, formic, propionic, and ascorbic acid should be considered. The antibacterial properties are better when chitosan is dissolved in organic acids than in strong acids (HCl) [89]. Within the aforementioned organic acids, high carbon organic acids (propionic and ascorbic acids) are less effective in inhibiting bacterial growth [88]. Huang has studied the capability of chitosan derivatives on antibacterial and antifungal activity [90]. He concluded that the chitosan works well

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199

in growth inhibition of microorganism strains tested at a wide range of pH values. Similar findings by Goy have confirmed that the antibacterial activity of chitosan derivatives is strongly dependent on the concentration [91]. Meanwhile, high ionic strength can increase the solubility of chitosan due to the formation of intermolecular hydrogen bonds, leading to greater antimicrobial efficacy. Since ionic salts coexist with natural water, the addition of chitosan into mineral salt-rich water such as Mg (II), Ca(II), Ba(II), and Zn(II) can further decrease viable bacteria cell count. 6.3.1.5 Chitosan derivatives Macramin (N-poly-methylated chitosan) is a type of chitosan derivative that was discovered at the early 1950s. It has N-trimethyl iodide at the C2 of monomer. Macramin appears as a pale yellow to brown solid powder and is soluble in water. It was tested with a wide range of Gram-positive and Gram-negative bacteria in both broth and synthetic or semisynthetic media. The antibacterial activity of macramin is better in Gram-positive bacteria than Gram-negative bacteria and is greater than sulfonamides as control when tested in media broth or serum broth. In synthetic or semisynthetic media, the bacteria inhibition effect by macramin is weaker than sulfathiazole [92]. Next, the researcher further tested the hemagglutination effect of macramin by influenza virus. Macramin as the positive control on the negative colloid ion to potassium salt of polyvinyl alcohol sulfate (PVS-K) and potassium salt of cellulose sulfate (CS-K) was added to the different dilution of B (Lee strain) virus solution. The virus solution was prepared by erythrocytes suspension and allantoic fluid of embryonated chicken eggs. Macramin with a positive charged agglutinated red blood cells with a negative charge. The agglutinated erythrocytes showed sediments at the bottom of tube. However, diluted macramin solution (less than 1026) had no effect on the agglutination by B virus [93]. This compound was later restudied in the form of N,N,N-trimethyl chitosan chloride salt (TMC). TMC is produced in the process of methylation of chitosan. In brief, the chitosan is reacted with basic dimethyl sulfate in the presence of salt to maintain the dispersion of chitosan. The precipitation was washed with acetone and formed salt crystalline powder. The addition of 7.5 and 20 mg/L TMC into the E. coli broth showed the reduction of cell colony formation unit (c.f.u.) at the stationary phase and quickened the entry of the decline phase of bacteria [94]. Apart from TCM, other quaternary N-alkyl chitosan derivatives such as N
N-propyl-N,N-dimethyl chitosan and N-furfuryl-N,N-dimethyl chitosan are prepared by employing the reaction of Schiff’s base with methyl iodide. It is noted that all of the N-alkyl chitosan derivatives

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200

6. Barrier properties, antimicrobial and antifungal activities

have better antimicrobial activities than chitosan. The antibacterial activity of N,N-propyl-N,N-dimethyl chitosan against E. coli is 20 times that of chitosan, indicating that the chitosan derivatives with the cationic charge of ammonium salt exhibit particularly high activity [95]. N,O-carboxymethylated chitosan (N,O-CM-chitosan) and O-carboxymethylated chitosan (O-CM-chitosan) are prepared using a different method. To obtain N,O-CM-chitosan, chitosan is suspended in alkaline isopropanol and further reacted with monochloroacetic acid in an elevated temperature. O-CM-chitosan is prepared by the reaction of chitosan and monochloroacetate at a low temperature. In N,O-CM-chitosan, the
NH2 group and
OH group are substituted to
CH2COOH but in O-CM-chitosan, only
OH is substituted to the CH2COOH group whereas
NH2 remains unchanged. The alteration of the functional group of chitosan increases the number of negative charges of chitosan. However, O-CM-chitosan has better antimicrobial action against Gramnegative E. coli than chitosan and N,O-CM-chitosan. This might be explained by the presence of both positively charged groups (
NH31) and negatively charged groups (
CH3COO2) that can attach to negatively charged E. coli more easily, rendering enhanced antibacterial properties [96]. The selection of chitin and chitosan derivatives as antimicrobial agents is still thrilling, especially in medical research. The current challenge will be useful in nanotechnology to permit more promising applications (i.e., intelligent nanosystems with specified drug delivery method) [97]. 6.3.1.6 Molecular weights There are no firm conclusions regarding the best antimicrobial activity of chitosan according to its Mw. Some of the studies show better antimicrobial activity while others say high-Mw chitosan was more effective in inhibiting the growth of bacteria. Low-Mw chitosan has been tested effectively on yeast bacteria and fungi by Tikhonov et al. [98]. Liu et al. agreed, showing that antibacterial activity of low-Mw chitosan is higher than high-Mw samples [99]. 6.3.1.7 Sources Chitin and chitosan have been known as “natural antibiotics” due to their efficient antimicrobial effects against many types of microorganisms. However, different types of chitin extraction sources provide various antimicrobial levels to the microorganism. Fig. 6.10 shows the antibacterial activity from different types of carbs. D. dehaani exhibited the highest antimicrobial activities [100]. However, the figure also points out that not every bacteria is effectively inhibited by D. dehaani.

Handbook of Chitin and Chitosan

6.3 Antimicrobial properties of the chitin and chitosan

FIGURE 6.10

201

Antimicrobial activity for selected crab species toward 10 types of micro-

organisms [100].

6.3.2 Antimicrobial mechanisms 6.3.2.1 Stimulation of secondary cellular effects that lead to destabilization of the wall Chitin and chitosan have been used as an antimicrobial material against a wide range of harmful organisms like yeast, algae, bacteria, and fungi. Generally, the chitosan is considered as a bacteriocide which kills live bacteria or bacteriostatic, hindering the growth of bacteria but not killing the bacteria. The model of interaction between positively charged chitin/chitosan molecules and negatively charged microbial cell membranes by using electrostatic forces between the protonated NH13 groups and negative residues is most widely accepted [101]. Raafat has done extra work on the model, observing the ultrastructural hinges of S. simulans 22 cells when exposed to positively charged chitosan [102]. The chitosan attached on the bacteria cell surfaces show “vacuole-like” structures below the bacteria’s cell wall. After that, the detachment decreases the internal bacteria pressure. Effective membrane lysis has been reported by a previous study on Gram-negative and Gram-positive bacteria [103
105]. It suggests that the greater the number of the cationized animes, the higher the antimicrobial activity in the chitosan. Protonated chitosan binds and weakens the rigidity of the cell membrane and hence reduces its barrier properties. A weak membrane barrier promotes the transfer of substances across the membrane. The permeability of a weakened bacterial cell membrane can be tested by measuring the uptake of 1-N-phenylnaph-thylamine (NPN), a hydrophobic probe into the cell. As compared with 1 mM EDTA (a strong chelating ion), the uptake of NPN induced by chitosan is

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possible when 100 and 250 ppm chitosan is added into the culture broth [105]. 6.3.2.2 Binding of low molecular weight of chitosan with DNA and RNA Another experiment of SDS sensitization was conducted on E. coli ATCC 11775, P. aeruginosa, and S. typhimurium. In this experiment, the chitosan showed a remarkable cell lytic effect in the presence of anionic detergent SDS. This suggests that the action of chitosan that alters the cell membrane permeability might further cause cell lysis on Gramnegative bacteria [105]. 6.3.2.3 Metal chelation by amine groups in chitosan chain DNA and RNA are built up by the negatively charged phosphate
sugar component in their backbone. This allows the interaction of protonated chitosan to bind with microbial DNA and RNA. This happens when low-Mw chitosan (chitosan oligomers) penetrates through the bacterial cell wall and cell membrane and inhibits the protein translation by binding the RNA of the active site of chitosan [106].

6.4 Antioxidant properties of chitin and chitosan The antioxidant activity of chitin, chitosan, and their derivatives has been demonstrated in vitro and in vivo in terms of free radical scavenging [107]. Free radicals comprise many important biological macromolecules including DNA, proteins, and lipids. Cellular damage by free radicals has been associated with various disorders, including cardiovascular, inflammatory, aging, diabetes mellitus neurodegenerative, and cancer. Oxidation in foods or deterioration of food quality leads to rancidity and shortening of shelf life. The biocompatibility of chitin, chitosan, and their derivatives, coupled with their ability to scavenge free radicals, makes them potential functional ingredients in food formulations and functional materials for pharmaceutical and biomedical applications. The inhibitory activity was calculated in the following manner (Eq. 6.2) [108], I50 ð%Þ 5 100

Ao 2 AI Ao

(6.2)

where Ao is the absorbance of the control group, AI is the absorbance of the extract, and I50 is the extract concentration providing 50% of inhibition.

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Free radicals in humans are produced in the form of hydrogen peroxide (H2O2), superoxide anion (O2•2), and hydroxyl (•OH) radicals [109]. These are collectively known as reactive oxygen species (ROS). ROS play an important role in wound healing which requires a fine balance between the positive and deleterious effects of these molecules. A balanced ROS response will debride and disinfect a tissue and stimulate a healthy tissue turnover; a suppressed ROS will result in infection and an elevation in ROS will destroy otherwise healthy stromal tissue. ROS levels can be elevated by various causes, such as inflammation from stress or a high-glucose environment from diabetes [109]. In particular, an elevated ROS from diabetes has been shown to enhance interleukin-8 (IL-8) production and neutrophil infiltration that contribute to impaired wound healing [110]. The antioxidant properties of biopolymers such as chitin and chitosan are therefore important considerations in biomedical applications. Furthermore, accumulation of these radicals in biological tissue causes oxidative stress that can lead to inadvertent enzyme activation and oxidative damage to cellular systems. Lipid autoxidation is of particular importance in food [111]. Polyunsaturated fatty acids are very susceptible to lipid autoxidation. The autoxidation process is initiated by the removal of a hydrogen atom from an unsaturated carbon by ROS and this results in the generation of a lipid radical (L•) which quickly reacts with atmospheric oxygen to form a peroxy-radical (LOO•). The latter abstracts hydrogen from another acyl chain to form lipid hydroperoxides (LOOH) and a new radical L that propagates until radicals are removed by reaction with either another radical or with an antioxidant. These radicals not only directly cause rancidity of fats and oils but can also oxidize carbohydrates and proteins. Antioxidant food additives are therefore important for prolonging the shelf life of foods.

6.4.1 Parameters affecting antioxidant properties 6.4.1.1 Deacetylation period Chitosan obtained from the deacetylation of chitin is an even more potent antioxidant. Yen reported that the antioxidant activity of chitosan increased by 2.5-fold when deacetylation time was increased from 60 to 120 min [112]. This shows that antioxidant activity of chitosan is correlated with the degree of deacetylation. Amines can function as a hydrogen donor and can be oxidized into an oxime. Amines are generally more potent antioxidants than amides. Furthermore, amines are also better ferrous ion chelators, a secondary antioxidant activity.

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6.4.1.2 Molecular weight Mw of chitin and chitosan molecules affects solubility and hence the ability to interact with free radicals [107]. Chitin oligomers have a much better aqueous solubility than chitin polymers and while chitosan is in general a soluble polymer, its reduced Mw has been shown to improve its antioxidant properties. Ngo et al. showed that chitin oligomers (1
3 and ,1 kDa) inhibited DNA and protein oxidation in RAW 264.7 cells [113]. The oligomers also elevated intracellular GSH level and direct intracellular radical scavenging. Interestingly, 1
3 kDa were shown to be more effective than ,1 kDa oligomers in inhibiting protein oxidation and scavenging intracellular radicals. Both oligomers were equally effective in inhibiting DNA oxidations at the concentrations studied. Low-Mw chitosan has also been shown to interact with cellular systems and exhibit a range of bioactive properties associated with antioxidants. Low-Mw chitosan (2.8 kDa) was shown to inhibit neutrophil activation and oxidation of serum albumin [114]. Oxidation of serum albumin is commonly observed in hemodialysis patients and is associated with uremia. Chitosans with different Mw (30, 90, and 120 kDa) inhibited lipid oxidation in salmon, a fish rich in unsaturated omega-3 fatty acids, with 30 kDa exhibiting the best inhibition [115].

6.4.2 Mechanism of antioxidant properties Chitin is the second most abundant biopolymer on earth after cellulose, comprising β(1
4)-linked N-acetylglucosamine units, and it is widely distributed in the exoskeletons of crustaceans and insects and the cell wall of most fungi [107]. The antioxidant of chitin can be attributed to its acetylamide group and secondary hydroxyl groups at C-2, C-3, and C-6 positions [116]. The C-2, C-3 hydroxyl groups are able to act as hydrogen donors while the C-6 hydroxyl group can act as a reducing agent and be oxidized into a carboxyl group. The oxygen in the acetylamide group is a Lewis base which can act as an electron pair donor.

6.4.3 Antioxidant assays Generally, antioxidant activities for an in vitro model can be classified into two basic mechanisms: hydrogen atom transfer (HAT) and single electron transfer (SET). The HAT mechanism is the ability to quench free radicals by hydrogen donation, which can be measured by various assays such as oxygen radical absorbance (ORAC) [117], total radicaltrapping antioxidant parameter (TRAP) [118], inhibition of induced low-density lipoprotein (LDL) oxidation [9,119], and total oxyradical

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scavenging capacity assay [120]. On the contrary, SET mechanism study one electron transferring ability to reduce metals, carbonyl and radicals, which changes in color as a result. Trolox equivalence capacity (TEAC) assay, ferric ion reducing antioxidant power (FRAP) [121], and 2,2diphenyl-1-picrylhydrazyl radical (DPPH) scavenging [122], hydrogen peroxide scavenging [123], and singlet oxygen quenching [28,97] can record the scavenging ability for oxidants

6.5 Applications of chitin and chitosan 6.5.1 Packaging applications Changes in consumer preferences for safe food have led to innovations in packaging technologies. Food packaging serves as a barrier to the external and internal environment of a food product. The external environment refers to ambient variables such as humidity, heat, light (UV), oxygen and carbon dioxide partial pressures, microorganism contents, and exposure to miniature particles from dust or smoke. The internal environment refers to the space between the inner packaging of the product. Traditional food packaging is meant for protection, communication, convenience, and containment [124]. • Containment. This is the most basic function of a package and it is important for its ease of transportation or handling. • Protection. This function is as important as the previous one; indeed, food packaging keeps food products in a reduced volume, preventing them from leaking or breaking up and protecting them against possible contaminations and changes. • Convenience. Food should be freely enjoyed by consumers, at their convenience. For this aim, food packages should be designed on the basis of individual lifestyles, taking care, as an example, of portability and availability of multiple/single portions. • Communication. Food packaging should provide the consumers with important information about the contained food product and its nutritional content, together with guidelines about preparation and preservation. Very often, these traditional food packaging are designed to have enhanced mechanical properties with the minimal consideration of food spoilage retention. Some of the aspects that may cause food product spoilage are: • Oxidation phenomena: rancidification of fats and oils and consequent emergence of off-odors and off-flavors, loss or change of colors, and

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the loss of oxygen-sensitive nutrients (vitamins A, C, E, unsaturated fatty acids, etc.) • Growth of aerobic and anaerobic microorganisms. • Metabolism of food: the volatile amines are formed due to the protein breakdown in fish muscle. • Improper external conditions, such as storage temperature, humidity, gas atmosphere, light, and packaging material selection. 6.5.1.1 Type of packaging 6.5.1.1.1 Active packaging

In view of the traditional food packaging system that does not render a prolonged shelf life of food product by protecting it from spoilage, active packaging has been introduced to enhance the beneficial value of packaging. Active packaging acts as a system where the product is interacting with its package and the environment to prolong the shelf life. Redefined active packaging is a type of packaging that alters the packaging conditions to improve the shelf life and maintain the quality. Typically, active packaging is a composite polymer that is engineered to use the functional properties of incorporated components to extend shelf life or to maintain or improve the condition of packaged food in accordance with regulation 1935/2004/EC and 450/2009/EC [125]. There are several methods to produce composite packaging, including solvent casting. This method does not require heat and is able to preserve the reactivity of active compounds such as lipids, essential oil, and polyphenolic compounds. The solvent casting of chitosan and ellagic acid (polyphenol compound) showed moderate water vapor permeability and antioxidant properties. However, this biocomposite showed growth inhibition to food-borne pathogens (Salmonella aureus and P. aeruginosa) and UV radiation (UVA and UVB) [126]. In a comparison between melted animal fat, melted butter, sunflower oil, corn oil, and olive oil, chitosan
olive oil film showed better surface morphology, higher thermal stability, and excellent antimicrobial properties compared with the films with other unsaturated oils [127]. Rui introduced a new functional film fabricated by using gallic acid and grafted chitosan in the presence of 1-ethyl-3-(30 -dimethylaminopropyl) carbodiimide and hydroxybenzotriazole under room temperature [128]. The grafted chitosan composites had a uniform distribution of gallic acid that enhanced the water vapor permeability barrier, antimicrobial and antioxidant activities. In another study, chitosan and gelatin formed a compact network in composite films, showing better water vapor barrier properties than pure chitosan film due to the hydrophilic nature of chitosan. Meanwhile, the water vapor permeability decreased with the addition of amphiphilic β-cyclodextrin into chitosan
gelatin-encapsulated

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gallic acid film but increased at a higher concentration of Tween 20. This film was designed to allow the slow release of the active compound, gallic acid, when reacted with the fatty food stimulant [129]. Abdollahi incorporated montmorillonite (MMT) nanoclay and rosemary essential oil into chitosan film to improve the barrier, antimicrobial, and antioxidant properties of film [130]. Reinforcement with mMT showed an improvement in the barrier property to extend the shelf life of poultry meat, resulting in high interest in it as a food packaging application. Supplementation with EOs such as ginger and rosemary essential oil was also able to slow down the oxidative processes of poultry meat, but it failed to improve the antimicrobial barrier [131]. Chitosan/MMT nanocomposite was also applied as an oxygen barrier film in multilayers on the ceramic separator of the microbial fuel cell. Seven bilayers of nanocomposites reduced the oxygen diffusion coefficient to one-sixth yet insignificant improvement was recorded for further layer increases [132]. Nanoclay has a tortuous path due to its ordered nanostructure and provides a higher barrier to water vapor compared to chitosan films without any addition [133]. However, Rekik discovered that the addition of up to 5% (w/v) kaolin into chitosan composite created a porous structure with high water permeability [134]. The ethylene
vinyl alcohol (EVOH) copolymers were blends with chitosonium
acetate-based solvent cast at elevated working temperature. The chitosan with low-Mw grade showed better transparency, water barrier properties, phase morphology, antimicrobial activity, and reduced water permeability up to 86% [135]. On the other hand, silicon carbide insertion in a chitosan composite showed higher tensile strength and thermal stability but lower oxygen permeability and resistance to alkali. This resulted in potential materials for packaging applications [136]. Aside from being a reinforcement active filler, chitosan could be a potential plasticizer for thermoplastic starch with better extrusion processibility by the blown film extrusion method. The addition of chitosan reduced water adsorption and surface stickiness on untreated thermoplastic starch [137]. Another study on thermoplastic starch
chitosan composite was illustrated by Zhao et al. using subcritical water technology [138]. The insertion of chitosan reduced the film water vapor permeability; this is because of the formation of hydrogen bonding, ester bonding, and electrostatic interactions among chitosan, gallic acid, and starch. Antimicrobial activity of films on spoilage microorganisms of preinoculated ham showed a prolonged ham shelf life of up to 25 days compared to the control (7 days). Chitosan is generally edible. Edible films that contains polyphenolic extract of Nigella sativa seedcake on a cross-linked chitosan base was found to affect the buffered release of total polyphenols and

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antioxidants from the films [139]. In addition, chitosan edible films blended with allyl isothiocyanate (AIT) and barley straw arabinoxylan (BSAX) were developed. This study revealed the higher bacterial population reduction rates [140]. Coatings and edible films not only work as barriers of gases and volatile or water vapor compounds but also function as carriers of functional ingredients. Chitosan edible film composites with zein, dicarboxylic acids, and gallic acid have shown promising microbial inhibition [75]. 6.5.1.1.2 Smart packaging

Smart packaging is similar to active packaging but with only slight differences. According to EC/450/2009, smart materials represent the ability to monitor the condition and environment of food. Smart packaging systems allow the user to access the condition of the food in realtime conditions including its environment (pH and temperature). This is an innovative communication function of traditional packaging based on its ability to record the environmental changes [141]. Chitosan is highly employed for smart packaging making it a food freshness indicator. The metabolites from the microbial growth such as n-butyrate, Llactic acid, D-lactate, acetic acid, and carbon dioxide are responsible for the pH drop in food. Meanwhile, the volatile amines produced from seafood products increase the pH, rendering pH a convenient variable to capture the freshness of food. Cavallo et al. introduced a colorimetric sensor for milk spoilage by adsorption of methylene blue onto a grafted polypropylene with acryl acid and chitosan [142]. The microorganisms from spoilt milk generate reducing substances that decompose the colored methylene blue into colorless form. Pereira et al. mixed chitosan hydrogel and anthocyanin extract from red cabbage into PVA hydrogel and cast it in petri dishes [143]. Stefani further integrated the previous work by adding sodium tripolyphosphate into the hydrogel mixture to promote cross-linking [144]. This combination resulted in time
temperature indicator films by changing the color at different pH levels that resembled food spoilage. Anthocyanins such as cyanidin-3-rutinoside and cyanidin-3-glucoside pose antioxidant properties in chitosan. However, both temperature and pH have drastic impacts on the anthocyanin’s stability, resulting in a skeptical view toward the selection of anthocyanin as a color changing component in smart packaging [145]. 6.5.1.2 Packaging technologies 6.5.1.2.1 Coating technology

Coating is a method to preserve the quality of food products. Edible coating fabricated by biopolymers and other additives such as

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antimicrobials, antioxidants, and plasticizer agents have recorded a higher probability to preserve the quality of food products when preventing physiochemical and biological deterioration and extending shelf life [146]. Recently, Zhang found that a sample of bamboo vinegar with chitosan has a higher effect on food microbial inhibition [147]. This combination provides an active coating solution treatment without any bad effects on the original sensory attributes of the sample. Moreover, the color and odor of the food sample were improved by chitosan
bamboo vinegar coating. Coating eggs with soybean emulsion and chitosan
soybean emulsion could extend the egg quality from AA grade to B grade from 1 week (noncoated eggs) to 5 weeks (emulsion-coated eggs) at 25 C [148]. In a chilled temperature (4 C), the emulsion-coated eggs could be kept up to 15 weeks with only a slight quality deterioration from AA grade to A grade. The preparation of chitosan
soybean emulsion was facile by homogenizing chitosan and soybean oil in a 40:60 ratio with the addition of Tween 80 [148]. Chitosan, which is well known as an antifungal material, was used as a coating on fruits and vegetables as well. For example, the emulsion of chitosan and Mentha piperita L. (MPEO) or Mentha 3 villosa huds (MVEO) essential oils could control the growth of postharvest mold on cherry tomato without affecting its quality [149]. This coating greatly increased the shelf life of cherry tomato from 4 days (control) to 12 days (coated) at room temperature and from 6 days (control) to more than 24 days at low temperature (12 C) [149]. 6.5.1.2.2 Electrospinning technology

The electrospinnability of chitosan/poly(ethylene oxide) (PEO) antimicrobial electrospun membrane increased. The hydrophobic
hydrophilic segments of the polymer form a typical surfactant micelle due to its high hydration enthalpy. Besides, insertion of micelles into a nanofiber was found to affect the modulation of the molecular structure as well as the spinnability of the solution, thus influencing fiber morphology [150]. Another antibacterial electrospun nanofiber was studied using chitosan with liposomes immobilized releasing gentamicin. Gentamicin in the form of cream is used clinically in the treatment of infected skin cysts, ulcers, burns, infected insect bites and stings, infected lacerations and wounds. In that study, the biocompatible chitosan served as a nanofiber mesh to entrap antibiotic-loaded liposome immobilized at the polymer surface, preventing drug degradation and promoting antibacterial activity [151]. Regardless of the ability for the electrospinning technique to prepare films with porous structure, the control of stable electrospinnability remains a challenge. The addition of 20 wt.% of nanochitin into

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chitosan-based composite formed cluster structures, facilitating the formation of nanofibers in the electric field and significantly reducing the amount of defects [152].

6.5.2 Dietary supplement applications Food supplements are not a new thing in the current world. Almost everyone has consumed food supplements. They are a concentrated source of nutrients (minerals, vitamins) used to compensate for nutrient deficiencies in the daily diet and or as an extra boost of nutrients needed during a specific period, such as pregnancy. However, dietary supplements can never replace a balanced diet or treatments for diseases. Dietary fiber is one of the most famous supplement uses for chitosan. It can absorb extra fats in the body and extract them out of the body since fiber is difficult to digest; charges between fiber (positive charges) and lipid molecules (negative charges) that form an electrostatic force to bond them tightly. Therefore calories from excess lipid substances will not be accumulated in our body but will be extracted. In addition, lower levels of harmful triglycerides and cholesterol were found in the blood due to gastrointestinal absorption of exogenous cholesterol, creating a healthier body [153]. Besides, consuming chitosan fibers gives a sense of satiety and prevents overeating. This is because the fibers swell in the stomach when they come into contact with water, regulating the sense of hunger. The safety of consuming chitosan dietary supplements is the most important issue to the public. A 2 g of chitosan supplement consumed each day for a long period of time has been proven to not cause any harm to the body [154]. In addition, the contents of fat-soluble minerals and vitamins in the body were not affected. A study was conducted by Cornelli involving numerous volunteers (between 25 and 70 years old) in a weight reduction program. Participants were split into two batches and ordered to consume pasta with and without chitosan as a substitution for cereals [155]. A successful outcome of weight reduction was achieved by both batches. Interestingly, a drop of cholesterol and triglycerides levels were found in the subjects consuming chitosan, proving the effectiveness of chitosan in the dietary field. The insertion of chitosan reduces the glycemic index of the pasta and its effect depends on the chitosan fiber content. Hence, a significant meal reduction in a daily diet is not needed for a losing weight. A human trial found a 3.3 kg greater weight loss [156]. On the other hand, various studies have disagreed on weight loss effects from chitosan as they could not find any direct relation between chitosan dose and weight reduction [105]. A clinical test on 250 overweight adults showed no significant loss of body weight after a 24-week

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trial [157]. In addition major clinical trials show allergic reactions by 2%
3% of participants, who are mainly allergic to seafood. About 20% of participants suffered constipation due to the high amount of chitosan fiber consumption [158]. A lot of factors were involved for effective weight reduction, such as participants’ quality, trail periods, chitosan concentration, and degree of deacetylation [159]. However, one thing that cannot be denied by all researchers is that chitosan dietary supplements must not be considered as a balanced diet replacement or a cure for obesity. Chitosan dietary supplements only help to regulate lipid absorption and achieve the reduction of cholesterol and triglycerides level as well as weight loss [160,161]. Apart from this, soluble chitosan fiber repairs the intestinal microflora destroyed by inflammatory diseases and drug ingestion. An investigation into chitosan bifidogenic effect found a high amount of bifidobacteric and lactobacilli in the experimental rats [162]. Chitosan also has been found to be effective in protecting the liver. It can remove the heavy metals and free radicals. The multiple highly reactive molecule sides of chitosan allow it to undergo numerous types of chemical reactions such as enzymatic degradation, hydrolysis, and oxidation. Therefore its high biodegradability and biocompatibility grant it bioactivity. Drugs carrying chitosan delivered an effective drug transport inside the body and retained the nontoxic elements. A wide degree range of acetylation and Mw of chitosan mean that it is difficult to obtain a consistent mass during production. The use of chitosan as a biomaterial has been developing over the past 20 years. This is because chitosan biopolymer is a waste product of fish with zero value. Value-added chitosan increases the revenue and most importantly reduces landfill pollution as the disposal of waste shells from fish, shrimp, and shellfish causes concern in coastal areas. The famous use of chitosan as a dietary supplement is because of its ability to form electrostatic bonding with fats and lipids. Thus it is applied in weight loss schemes, to reduce the absorption of fat and reduce the weight. In addition, the swelling of chitosan fiber when in contact with water gives a feeling of satiety, further reducing calorie intake. Although there is no scientific confirmation of weight loss by using chitosan, it is strongly believed that chitosan supplementation with a balanced diet and regular exercise would produce an effective healthy weight loss scheme.

6.5.3 Agricultural applications Chitosan and chitosan derivatives have attracted huge attention due to their promising antimicrobial effects against bacteria, viruses,

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and fungi, and as plants’ biochemical defense systems activators [163,164]. In the mid-1920s, researchers noticed the applications of chitin on plants had reduced the cases of fungal attack. Phytopathogenic fungi and bacteria possess a wide range of enzymes destroying the cell walls of plants [165] and cause severe damage to the plants, thus affecting agricultural crops [166]. Chemical fungicides have been applied to control the diseases yet soil and environmental pollutions were found at the same time [167,168]. Hence, the use of chitin and chitosan in the agricultural field in order to control fungal pathogen is of great interest. The application of chitosan in agricultural was found to improve food productivity without any negative impact to the environment [169]. However, the insolubility of the bulk chitosan molecular structure in an acidic aqueous medium limits its use as an antimicrobial agent. Therefore much effort has been made to increase its dispersity for higher bioactivity [170]. Chemical modifications and the incorporation of metal ions were done on chitosan to yield better solubility and antimicrobial activity. Besides, nanoscale chitosan shows greater properties due to its higher surface-to-volume ratio. Previous works have introduced a simple chemical modification method to produce highly soluble chitosan (50% N-acetylated), since the acetyl group is insoluble, by degrading the chitosan followed by N-acetylation of chitosan [171]. It is obviously shows that the high ionic strength of N-acetylated chitosan dissolves well in aqueous acetic acid. On the other hand, the lower the Mw of chitosan, the lower the intermolecular attraction forces, leading to better water solubility. Badawy and Rabea have summarized the potential of chitosan against plant pathogens, presenting an overview of antimicrobial effects, mechanisms, and applications in agriculture [172]. Incorporation of metal ions (silver, copper, and zinc) leads to better antifungal properties [173]. A strong synergistic effect between chitosan and copper on controlling F. gramimearum growth was found [174]. The selection of copper was attributed to its redox properties, which enable it to produce highly reactive hydroxyl radicals to destroy proteins, DNA, and the lipids of fungus [175]. Therefore the antifungal activity of a copper(II)/chitosan spherical gel as a biobased pesticide is to be found reliable. This finding was agreed by Sahran, who found that copper
chitosan nanoparticles provide high antifungal activity against A. alternata, M. phaseolina, and R. solani [170]. On the other hand, chitosan
silver nanoparticles were studied by Anusuya on chickpea seeds [176]. It is confirmed that 0.1% (w/v) of chitosan resulted in a remarkable growth effect on seed germination, height and weight of the plant. An increase in enzymatic activity was observed. Chandra studied the antifungal activity of chitosan nanoparticles on plant leaves [177]. Significant improvement of Camellia sinensis leaves’

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immune response was due to the induction of defense enzyme activity by chitosan. The application of chitosan shows increased activities for all tested defense enzymes. Besides, fusarium wilt of tomato was caused by F. oxysporum sp. Lycopersici, one of the largest economic losses in agriculture [178].

6.5.4 Cosmeceutical applications A cosmetic product is any substance or preparation intended to be applied on the external parts of human body (lips, external genital organs, epidermis, hair, or nails) or on the teeth and the oral cavity with a purpose to correct body odors, cleaning, perfuming, changing appearance, protecting, or keeping the applied body parts in good condition [179]. Therefore a cosmetic is only to be applied outside the body without any treatment against any specific disease. Cosmetic products are strictly guarded by many governmental agencies due to the harmful chemicals that may be added by into cosmetics by producers to reduce the production costs. The term cosmeceutical was established by R.E. Reed in 1962 [180]. There are four main ideas which are still applied by the current cosmetic industry, as shown in Table 6.6. Chitosan is a natural cationic gum, unlike the other hydrocolloids, which are polyanions. It becomes viscous when blended with acid. Therefore it interacts with common integuments (skin covers) and hair. The chitosan in the size of 1
10 μm is commonly used in cosmetics [181]. It absorbs the harmful UV radiation and protects skin effectively. Besides, two advantages that make it a potential material for skin care are its great antimicrobial activity owing to its positive electrical charge (cationic gum) and its high Mw inhibiting UV penetration through the skin, making it safe to apply on skin or as an effective skin moisturizer. On the other hand, chitosan contains antiallergic, antioxidant, and antiinflammatory substances. Sonat Co., United States has used chitin content products in three areas of cosmetics: hair care, skin care, and TABLE 6.6

Four main ideas in Reed’s cosmeceutical definition [180].

No.

Idea

1

A cosmeceutical is a scientifically designed product intended for external application to the human body

2

A cosmeceutical produces a useful and desired result

3

A cosmeceutical has desirable aesthetic properties

4

A cosmeceutical meets rigid chemical, physical, and medical standards

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oral care. Sionkowska et al. prepared a blend of collage, chitosan, and hyaluronic acids to test the strength of hair after coating by the blends [182]. The blends are more stable in aqueous conditions and provide an improvement in the general appearance and condition of the covered hair. In addition, chitin and chitosan have been applied in moisturizers and conditioners during the preparation of cosmetic creams with hyaluronic acid. Nontoxic chitosan can be used in cosmetic applications and its film-forming ability provides a pleasant feeling to the skin and avoids it being damaged from bad working conditions and the consequences of detergent use. Hydroxypropyl
chitosan nail lacquer is an innovative medical device to relieve nail dystrophy. The nail lacquer was applied on a batch of healthy woman subjects and compared with nail lacquer without chitosan contents for 4 weeks [183]. The product with chitosan content improved the nail structure, appearance, and reduced lamellar splitting. The free hydroxypropyl groups in hydroxypropyl
chitosan interact with keratin by hydrogen bonding. There was a similar finding on hydroxypropyl
chitosan nail solution by Ghannoum et al., who showed it was proven to effectively strengthen nails, improve natural barriers, provide better drug penetration to the nail bed, and establish recurring fungal nail infection [184]. Several in vivo and in vitro studies have proved the long-term efficacy for effective antiplaque activity of chitosan-containing mouth rinse and 2% of chitosan have been established as the gold standard for bacteriostatic action over 12 h [185]. Higher concentrations were studied and details of their side effects have been reported [186]. On the other hand, Husain et al. have reviewed chitosan as a potential material for oral and dental applications [187]. The authors discussed in detail guided tissue regeneration, oral drug delivery, modifications of dentifrices, adhesion, dentine bonding and tooth enamel repair materials, modification of dental restorative materials, coating dental implants, and stem-based regenerative therapeutics. There are still very limited clinical data available regarding chitosan-based dental application materials as a few challenging tasks have to be figured out in chitosan extraction and reproducibility.

6.6 Conclusions Chitin or is a semicrystalline polysaccharide found abundantly in nature with a wide range of uses due to its attractive properties (promising barrier, antimicrobial, and antioxidant properties). Deacetylation of chitin into chitosan provides a better and friendlier solubility in water and organic solvents. Barrier properties of chitin and chitosan refer to

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the permeability of a gas or liquid through its polymeric layer. The permeability is potentially influenced by several parameters: chemical structures, cohesive energy density, free volume, crystallinity, orientation, cross-linking, and degree of dispersion of the platelets. Nanoscale chitin composites provide attractive barrier properties due to the low volume to high surface area ratio. On the other hand, deacetylation degree, Gram types, surface charge density, abiotic factors, chitosan derivatives, Mw, and chitin extraction sources result in different degrees of antimicrobial properties. The free radicals directly cause rancidity of fats and oils while oxidizing carbohydrates and proteins. Oxidation in foods leads to rancidity and shortening of shelf life. The biocompatibility of chitin, chitosan, and their derivatives coupled with their ability to scavenge free radicals make them potential functional ingredients in food formulations and functional materials for pharmaceutical and biomedical applications. The applications of chitin and chitosan in packaging, dietary supplements, agriculture, and cosmetics show that chitin is safe to be applied outside and inside the body due to its natural nontoxic nature. In the future, it is believed that more conventional products will apply chitin as the main ingredient to enhance product barrier properties and antimicrobial and antioxidant abilities.

References [1] I. Younes, M. Rinaudo, Chitin and chitosan preparation from marine sources: structure, properties and applications, Mar. Drugs 13 (2015) 1133
1174. Available from: https://doi.org/10.3390/md13031133. [2] S. Das, N. Chilukoti, S. Pvsrn, M. Jogi, P.N. Pallinti, M. Kaur, et al., Microbial chitinases for chitin waste management, Microorganisms in Environmental Management. (2012) 135
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Handbook of Chitin and Chitosan

C H A P T E R

7 Chitin and chitosan-based polyurethanes Rejiane da Rosa Schio, Evandro Stoffels Mallmann and Guilherme Luiz Dotto Chemical Engineering Department, Federal University of Santa Maria, Santa Maria, Brazil

O U T L I N E 7.1 General considerations

229

7.2 Chitin and chitosan

230

7.3 Polyurethanes 7.3.1 Basic reactions to obtain polyurethanes 7.3.2 Isocyanates 7.3.3 Polyols

231 233 234 236

7.4 Development methods for chitin/chitosan-based polyurethanes

236

7.5 Chitin and chitosan-based polyurethanes materials: characterization and applications

240

7.6 Concluding remarks

240

References

242

7.1 General considerations Nature contains various materials that can be obtained from animal and vegetable sources. Chitin and cellulose, for example, are abundant, Handbook of Chitin and Chitosan DOI: https://doi.org/10.1016/B978-0-12-817968-0.00007-X

229

© 2020 Elsevier Inc. All rights reserved.

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7. Chitin and chitosan-based polyurethanes

renewable, and biodegradable biopolymers. The structural difference between these biopolymers is due to the hydroxyl group, which is found in the 2-position for cellulose and is replaced by the acetamido group in the case of chitin [1]. Chitin is the second most abundant biopolymer (cellulose is the first) and is a prominent precursor to chitosan. Both chitin and chitosan are natural products of great economic and social importance [2,3]. When combined with other synthetic polymers, such as polyurethanes, they offer an innovative and potentially attractive proposition for diverse uses. Polyurethanes are considered exclusive materials due to their unique properties and applications. They can be adapted or customized for any type of usage by the correct choice of the constituent monomers. The chemical, mechanical, and morphological properties of polyurethanes provide the basis for their broad applications [4,5]. In this chapter, we will review conceptual aspects regarding chitin and chitosan biopolymers, as well as polyurethanes. In addition, the preparation, characterization, and application areas of the chitin/chitosan-based polyurethanes will be approached.

7.2 Chitin and chitosan Chitin and chitosan are natural, nontoxic, biodegradable, and biocompatible polymers. When modified, they demonstrate unique properties, like antimicrobial activity, film-, foam-, and fiber-forming capacities, as well as adsorption of heavy metals and dyes. Due to their attractive properties, these biopolymers have been exploited in different application areas for many years. The chemical structures of chitin and chitosan can be observed in Fig. 7.1. Chitin, β-(1-4)-2-acetamide-2deoxy-D-glucose (N-acetylglucosamine), differs from chitosan mainly in relation to its chemical structure and solubility [6,7]. From the chemical viewpoint, chitin differs from chitosan in the 2-position of the ring. For chitin, the majority of monomers contain an acetamido group in the 2position, while for chitosan an amino group is present in the majority [1,8] (Fig. 7.1). As for solubility, chitin is insoluble in water, organic

FIGURE 7.1 Chemical structure of (A) chitin and (B) chitosan.

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7.3 Polyurethanes

231

solvents, dilute acids and alkalis, while chitosan is soluble in solutions of organic and inorganic acids [6]. Chitin is the second most abundant fiber in nature, being surpassed only by cellulose [3,9]. It can be found in the exoskeleton of crustaceans and arthropods, in the cell wall of fungi, and in other biological materials. Normally, the isolation of chitin consists of the steps of pretreatment, demineralization, deproteinization, deodorization, and drying [10,11]. Chitin can adopt polymorphic structures described as α-chitin, β-chitin, and γ-chitin. Such denominations vary in terms of polymer chain structure and crystallinity. The α-chitin is considered the most common form and is also the most stable among the three structures, while β-chitin is more reactive and has higher affinity for solvents [12]. Chitosan is a cationic polysaccharide, generally produced by the alkaline deacetylation of chitin [3]. In its structure, it has the amino reactive groups, which are available for chemical reactions and salt formation with acids; and the hydroxyl groups at the C-6 (primary) and C-3 (secondary) positions which may also be used in the preparation of derivatives [1,6] (Fig. 7.1). Due to the presence of free amino groups, chitosan has become a very attractive substance with a broad range of applications [6,13]. Commercial chitosan is normally used in the form of powder or flakes and differs in the following characteristics: purity, color, deacetylation degree, molecular weight, particle size, and crystallinity [2]. Chitin and chitosan are highly functional products and therefore, in some cases, they are added to the synthesis of polyurethanes in order to improve the mechanical and biological properties of the material. In materials with biomedical potential, these polysaccharides improve biocompatibility, reduce cytotoxicity, and possess antimicrobial and antifungal activity [14]. Regarding adsorption in aqueous solutions, besides improving the mechanical properties, they provide additional adsorption sites, increasing the adsorption potential [15]. In the textile sector, they have potential when used as dye fixing agents, thickeners, binders, and also improve the firmness properties of the dyed fabrics [16].

7.3 Polyurethanes In 1937, in Germany, Otto Bayer (1902
82) and collaborators developed a process to obtain products made from hexane-1,6-diisocyanate (HDI) and hexa-1,6-diamine (HDA). This process gave origin to the publication of the patent DRP 728,981, on November 13, 1937, entitled “Process for the production of polyurethane and polyurea” [17]. However, the obtained material was infusible and highly hydrophilic, reducing its application. Meanwhile, the use of diols instead of diamines produced polyurethanes with interesting properties in the

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mentioned aspects. Other diisocyanates have also been used, for example, tetramethylene and octamethylene diisocyanate, in reaction with other polyhydroxylated compounds, such as cellulose [18,19]. Polyurethane (PU) is a type of polymer comprising repeated urethane units, characterized by the (
NH
CO
O
) linkage as shown in Fig. 7.2. The polyurethane synthesis reaction is a step-polymerization with isocyanate as reactant and a polyol, or other reagents containing two or more reactive groups, to obtain branched or cross-linked materials. It is also possible to synthesize urethane bonds without the use of isocyanates. In this case, oligomers with cyclic carbonate groups should be used. These cyclic carbonate groups must react with diamines, or there should be reactions involving vinyl carbonates with amines [20]. Polyurethanes belong to a family of polymers capable of presenting a wide range of properties, depending on their molecular composition. They can be prepared in different forms, including foam, fiber, film, tube, resin, and elastomers. and therefore find applications in multiple areas. The extreme versatility of polyurethanes, made this class of polymers present in different segments of science and technology, from the automotive sector, to medicine and industry [21]. They can be found as paints, adhesives, acoustic and thermal insulation, household utensils, toys, coatings, packaging, and human implants [22
26]. Due to the variety of groups constituting the PUs and the possibility of controlled polymerization, it is possible to adapt the process and the composition to obtain materials with different application requirements, from the soles of shoes to surgical implants, since a PU can contain aromatic groups, aliphatic, cyclic, amides, urea, esters, ethers, among others [27]. In addition, other chemicals may be added to control or modify both the formation reaction of the polyurethanes and their final properties. Catalysts, curing agents, blowing agents, surfactants, chain extenders, fillers, mold release agents, flame retardants, dyes, pigments, and antiaging agents are examples of substances that can be added in the process in order to control or modify it [28]. Nowadays, there are a variety of PU forms. This diversity is a result of improvements in the discoveries of Otto Bayer. Table 7.1 exemplifies the most important types of PUs and their applications.

FIGURE 7.2 Urethane group.

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7.3.1 Basic reactions to obtain polyurethanes Usually, PU is represented by its major monomer units, the diisocyanate and the polyol. Therefore not all types of bonds which can effectively be a part of the structure of the polymer are represented. The general structure that forms the chemistry basis of these polymers is a urethane bonding, as shown in Fig. 7.3. The main PU synthesis reactions involves stepwise polymerization between an isocyanate and a compound containing active hydrogen, usually a polyol, responsible for the formation of carbamic acid esters (urethane bonds), as shown in Fig. 7.4. Polyurethanes are usually produced by the reaction of di- or polyfunctional isocyanates with di- or polyhydroxyl (polyol) compounds.

TABLE 7.1

Main types of PUs and their applications.

Types of polyurethanes

Application

Rigid

Thermal and sound insulators

Flexible

Automotive interior parts, packaging, biomedicine, nanocomposites, carpet underlays, furniture, bedding, and cushion materials

Polyurethane ionomer

Artificial hearts, connector tubing for heart pacemakers and hemodialysis tubes

Thermoplastic

Keyboard protector for laptop, outer cases of mobile electronic devices, automotive instrument panels, caster wheels, power tools, sporting goods, medical devices, drive belts, footwear, inflatable rafts, and a variety of extruded film, sheet, and profile applications

Waterborne

Coatings, adhesives, sealants, and binders

FIGURE 7.3 General structure of urethane formation.

FIGURE 7.4 Scheme of the conventional synthesis of a polyurethane.

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7. Chitin and chitosan-based polyurethanes

Since the chemical functionality of the isocyanate or the hydroxylcontaining reagent (polyol) can be varied, a wide range of branched or cross-linked polymers with different properties can be obtained.

7.3.2 Isocyanates Isocyanates are organic compounds that contain the group (
N 5 C 5 O) in their chemical structure. NCO groups readily react with compounds that have active hydrogen atoms available and also have, at least, two functional groups. The reaction of the isocyanate group occurs when the sufficiently active hydrogen is bound to atoms with an available pair of electrons, as in nitrogen and oxygen, and this reaction is denominated the urethane reaction, when the hydrogen is connected to the hydroxyl oxygen. In the chemical structure of the isocyanate, the electron density is lower in the carbon atom, and higher in the oxygen one. Therefore most of the chemical reactions occur with the break of the double bond (C 5 N) and the addition of the nucleophilic atom to the nitrogen. The high reactivity of the isocyanate group (
N 5 C 5 O) is due to the positive charge of the carbon atom caused by the resonant sequence of the double bonds between nitrogen, carbon, and oxygen (Fig. 7.5) [29]. These groups react with a great number of compounds, such as amines, alcohols, carboxylic acids, or even with each other [30,31]. In addition to the main reaction between isocyanate and hydroxyl, during the polymerization of the polyurethanes, secondary reactions also occur. The major chemical reactions that may occur with the isocyanate group are outlined in Figs. 7.6
7.11 [29].

FIGURE 7.5 Resonance structure of the isocyanate group.

FIGURE 7.6 Representation of the reaction between isocyanate and alcohols.

FIGURE 7.7 Representation of the reaction between isocyanate and amines.

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FIGURE 7.8 Representation of the reaction between isocyanate and water.

FIGURE 7.9 Representation of the reaction between isocyanate and carboxylic acid.

FIGURE 7.10 Representation of the reaction between isocyanate and urethane.

FIGURE 7.11

Representation of the reaction between isocyanate and urea.

The reaction between isocyanate and alcohols is a moderate velocity reaction, which generates urethane and can be catalyzed by bases, tertiary, and organometallic amines, as in Fig. 7.6. When isocyanate is reacted with amine, urea is obtained. It is a high speed reaction, thus not requiring catalysts (Fig. 7.7). When reacting isocyanate with water (Fig. 7.8) carbamic acid is obtained, which is an unstable compound, which decomposes into carbon dioxide and the corresponding amine. The carbon dioxide generated is the compound responsible for the expansion of polyurethane foams. The velocity of this reaction is similar to the velocity rate of the alcohols reaction. When isocyanate is reacted with carboxylic acid (Fig. 7.9), an unstable compound is again obtained which decomposes into amide and carbon dioxide.

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The reaction between the isocyanate group and urethane (Fig. 7.10) enables allophanate cross-linking, which is a slow speed reaction. When isocyanate and urea are reacted (Fig. 7.11) cross-links are also obtained, however, this time they are biurethane cross-links, which is also a reaction that occurs at a low rate. When necessary, the reaction rate and cell size are controlled by appropriate catalysts and surfactants, respectively [21]. The isocyanates may have aromatic and aliphatic groups in their composition. Aromatic isocyanates are widely used in the production of coating polyurethane, under the forms of rigid and flexible foams, adhesives, elastomers, and fibers. The PUs obtained from aromatic diisocyanates undergo slow oxidation in the presence of air and light, causing discoloration. The reactivity of isocyanates depends on their chemical structures, thus aromatic isocyanates are more reactive than aliphatic ones [32]. Aliphatic isocyanates are generally used in the production of polyurethanes that are expected to exhibit stable colors, although it is necessary to add antioxidants and UV stabilizers to the formulation to maintain the physical properties over time [33].

7.3.3 Polyols Responsible for many of the properties of polyurethanes, polyols have great importance in the versatility of these polymers and their consequent application in several areas of science. By involving a large and diverse amount of compounds containing hydroxyl groups, the polyols are capable of reacting with the most diverse isocyanates to form the PUs [34]. Polyether polyols are the most commonly used in the synthesis of flexible foams and are usually derived from poly(propylene oxide) glycol and poly(propylene/ethylene) glycols copolymers. In addition to polyether polyols, there are the aliphatic polyester polyols used in highperformance applications, the aromatic polyester polyols used in rigid foams, and the natural polyols, which replace the polyester polyols in the synthesis of PUs [27]. PUs based on polyester polyols (Fig. 7.12) may undergo rapid hydrolysis when implanted into the human body. Therefore they are widely used in tissue engineering applications in the preparation of biodegradable materials [27,35].

7.4 Development methods for chitin/chitosan-based polyurethanes There are many ways to classify polymers; it all depends on the interest. They can be classified according to the way they are obtained (e.g.,

Handbook of Chitin and Chitosan

7.4 Development methods for chitin/chitosan-based polyurethanes

FIGURE 7.12

237

Chemical structure of polyester polyols.

natural or synthetic polymers), chemical structure (e.g., polyamide, polyolefin), form of application (e.g., fibers, films) among others. An important classification is related to the preparation method of the polymers, which can be based on polymerization mechanisms. Polymerization is the chemical reaction in which a large number of monomer molecules combine to form a macromolecule. Then, the polymerization mechanisms can be classified as step-growth processes or chain-growth processes [36,37]. Polyurethane is a polymer formed by a step-growth reaction between an isocyanate (NCO) and a polyol (OH). Step-growth consists of successive condensation between the functional groups, increasing the size of the molecules to the size of a polymer chain. This reaction mechanism only happens if the monomers are at least bifunctional. The step-growth polymerization reaction may be carried out in one or more steps [21]. In the one-step process, the reactants (polyol, isocyanate, chitin/chitosan, surfactants, catalysts, among others) are mixed all at once and form a resin. The polymer mass gets saturated with carbon dioxide and the diffusion of this gas forms the porous structure of the material. In the more-than-one-step process, the first step of the synthesis involves mixing a polyol with isocyanate in excess under inert atmosphere, leading to the formation of a high molar mass prepolymer with isocyanate groups at its ends. In the second step, a mixture of low-molecularweight polyol (chain extender), chitin/chitosan, surfactants, catalysts, and so on, are added to the prepolymer, and the polyurethane is then finished. The most commonly used isocyanate in the synthesis of chitin/chitosan-based polyurethanes is diphenylmethane 4,4-diisocyanate (MDI). Table 7.2 shows the aromatic and aliphatic isocyanates used in the synthesis of chitin/chitosan-based polyurethanes. The most used polyol in the synthesis of chitin/chitosan-based polyurethanes is poly(caprolactone) diol (PCL). Table 7.3 presents the polyols used in the synthesis of chitin/chitosan-based polyurethanes. For the synthesis of chitin/chitosan-based polyurethanes, besides the use of isocyanates, polyols, and chitin/chitosan itself, some studies use chain extenders such as 1,4-butanediol (BDO) [16,38
48,53
55,59
61], neutralization agents such as triethylamine (TEA) [16,38
48,61,63], viscosity reducers such as methyl ethyl ketone (MEK) [61], catalysts such as dimethylol propionic acid (DMPA) [38
48,61,63] and dibutyltin

Handbook of Chitin and Chitosan

TABLE 7.2 Isocyanates used in the synthesis of chitin/chitosan-based polyurethanes.

Handbook of Chitin and Chitosan

Chemical structure

References

4,40 -Diphenylmethane diisocyanate (MDI)

[38
52]

2,4 -Toluene diisocyanate (TDI)

[23,53
58]

Hexamethylene diisocyanate (HDI)

[59
61]

Isophorone diisocyanate (IPDI)

[16,62]

4,40 -Methylene dicyclohexyl diisocyanate (H12MDI)

[63]

Hexamethylene diisocyanate biuret (HDB)

[15]

TABLE 7.3 Polyols used in the synthesis of chitin/chitosan-based polyurethanes.

Handbook of Chitin and Chitosan

Chemical structure

References

Polycaprolactone diol (PCL)

[16,23,38
48,50
52]

Hydroxyl-terminated polybutadiene (HTPB)

[53
55,59,60]

Polyethylene glycol (PEG)

[56,57,61,63]

Ricinoleic acid

[15]

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7. Chitin and chitosan-based polyurethanes

FIGURE 7.13 Image of the polyurethane/chitosan composite foam.

dilaurate (DBTDL) [16,61
63], and also cross-linking agents such as glutaraldehyde. Recently, Schio et al. [15] prepared a foam composed of polyurethane and chitosan for application in the dye adsorption of aqueous solutions. The material was synthesized from HDB and from a natural polyol derived from castor oil, ricinoleic acid. The chitosan cross-linking was performed with glutaraldehyde. The results showed that the synthesized material, besides offering a simple procedure using alternative and renewable precursors, also presented promising characteristics in the area. Fig. 7.13 shows an image of the foam composed of polyurethane and chitosan.

7.5 Chitin and chitosan-based polyurethanes materials: characterization and applications Materials prepared from chitin and chitosan-based polyurethanes require different characterization techniques depending on their application. Table 7.4 presents some approaches found in the literature that have been studied using polyurethanes based on chitin and chitosan with different characterization techniques and forms of applications.

7.6 Concluding remarks Chitin and chitosan are natural polymers with diverse attractive properties and applicability that when combined with polyurethanes, which also offer a wide variety of compositions, properties, and structures, form materials with definite great potential in different areas of application. Studies show that this is a promising approach, and that this is a field that continues to grow with the development of new research and advances. Chitin and chitosan have brought the researchers’ attention to the use of their exceptional properties and are being

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7.6 Concluding remarks

TABLE 7.4 Some materials based on chitin and chitosan with different characterization techniques and their respective applications. Materials

Characterization techniques

Applications

References

Chitin-based polyurethane

Contact angle measurements, surface free energy, total work of water adhesion to polymer and equilibrium degree of swelling

Shape memory polyurethanes

[38]

Chitin-based polyurethane bionanocomposites

XRD, OM, DSC, dynamic mechanical measurements

Formulation of bionanocomposites

[50]

Polyurethane based on chitin/1,4-butane diol blends

FT-IR, 1H NMR, 13C NMR

Biomedical material (nonabsorbable suture)

[43]

Waterborne polyurethane-carboxy methyl chitin blend

FT-IR, SEM, TGA

Pharmacy, agriculture

[64]

Chitin-based polyurethane

FT-IR, DSC

Biomedical material

[45]

Chitin-based polyurethane elastomers

FT-IR, 1H NMR, 13C NMR

Biomedical applications with tunable mechanical properties

[42]

Chitosan
polyurethane

FT-IR, bending length/ stiffness, crease recovery angle, air permeability, pilling characterization, tear strength, tensile strength

Textile finishes

[16]

Chitin/curcuminblended polyurethane

FT-IR, SEM, TGA, XRD, DSC

Biomedical material (skin treatment)

[60]

Chitosan/ montmorillonite claybased polyurethane bionanocomposites

FT-IR, 1H NMR, SEM, TGA

Biomedical material

[55]

Chitin-based polyurethane copolymer

1

Pharmacy (smart material)

[65]

H NMR, FT-IR, DSC

(Continued)

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7. Chitin and chitosan-based polyurethanes

TABLE 7.4 (Continued) Materials

Characterization techniques

Applications

References

Polyurethane
chitosan hydrogel scaffolds

FT-IR, SEM, AFM

Biomedical material (wound healing)

[66]

Polyurethane/chitosan composite foam

FT-IR, SEM, XRD, color, mechanical properties

Dye adsorption

[15]

AFM, Atomic force microscopy; 13C NMR, 13C-nuclear magnetic resonance; DSC, differential scanning calorimetry; FT-IR, Fourier transform infrared spectroscopy; 1H NMR, 1H-nuclear magnetic resonance; OM, optical microscopy; SEM, scanning electron microscopy; TGA, thermogravimetric analysis XRD, X-ray diffraction.

used to the maximum with the combination of research in multidisciplinary areas.

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II: effect of diisocyanate structure, Int. J. Biol. Macromol. 44 (2009) 23
28. [43] M.K. Zia, M. Barikani, M. Zuber, I.A. Bhatti, M. Barmar, Surface characteristics of polyurethane elastomers based on chitin/1,4
butane diol blends, Int. J. Biol. Macromol. 44 (2009) 182
185. [44] K.M. Zia, M. Barikani, M. Zuber, I.A. Bhatti, M.A. Sheikh, Molecular engineering of chitin based polyurethane elastomers, Carbohydr. Polym. 74 (2008) 140
158. [45] M. Barikani, K.M. Zia, I.A. Bhatti, M. Zuber, H.N. Bhatti, Molecular engineering and properties of chitin based shape memory polyurethanes, Carbohydr. Polym. 74 (3) (2008) 621
626. [46] K.M. Zia, I.A. Bhatti, M. Barikani, M. Zuber, H.N. Bhatti, XRD studies of polyurethane elastomers based on chitin/1,4
butane diol blends, Carbohydr. Polym. 76 (2009) 183
187. [47] K.M. Zia, M. Barikani, I.A. Bhatti, M. Zuber, M. Barmar, XRD studies of UV
irradiated chitin based polyurethane elastomers, Carbohydr. Polym. 77 (2009) 54
58. [48] K.M. Zia, M. Barikani, A.M. Khalid, H. Honarakar, E. Haq, Surface characteristics of UV
irradiated chitin
based polyurethane elastomers, Carbohydr. Polym. 77 (2009) 621
627. [49] X.X. Li, J. Li, X.J. Sun, L.Y. Cai, Y.C. Li, X. Tian, et al., Preparation and malachite green adsorption behavior of polyurethane/chitosan composite foam, J. Cell. Plast. 51 (4) (2015) 373
386. [50] K.M. Zia, M. Zuber, M. Barikani, A. Jabbar, M.K. Khosa, XRD pattern of chitin based polyurethane bio
nanocomposites, Carbohydr. Polym. 80 (2010) 539
543. [51] M. Zuber, K.M. Zia, S. Mahboob, M. Hassan, I.A. Bhatti, Synthesis of chitin
bentonite clay based polyurethane bio
nanocomposites, Int. J. Biol. Macromol. 47 (2010) 196
200. [52] K.M. Zia, M. Zuber, M. Barikani, R. Hussain, T. Jamil, S. Anjum, Cytotoxicity and mechanical behavior of chitin
bentonite clay based polyurethane bio
nanocomposites, Int. J. Biol. Macromol. 49 (2011) 1131
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319. [54] K.M. Zia, K. Mahmood, M. Zuber, T. Jamil, M. Shafiq, Chitin based polyurethane using hydroxyl terminated polybutadiene, Part I: molecular engineering, Int. J. Biol. Macromol. 59 (2013) 320
327. [55] M.A. Javaid, R.A. Khera, K.M. Zia, K. Saito, I.A. Bhatti, M. Asghar, Synthesis and characterization of chitosan modified polyurethane bio
nanocomposites with biomedical potential, J. Biol. Macromol. 115 (2018) 375
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coated as a low
cost adsorbent in the effluent treatment, J. Water Process. Eng. 20 (2017) 201
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curcumin blends derived polyurethanes, J. Biol. Macromol. 105 (2017) 1180
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143.

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C H A P T E R

8 Chitin and chitosan-based blends, composites, and nanocomposites for packaging applications Samar Sahraee and Jafar M. Milani Department of Food Science and Technology, Sari Agricultural Sciences and Natural Resources University, Sari, Iran

O U T L I N E 8.1 Introduction

248

8.2 Biodegradable film production methods 248 8.2.1 Combination of biodegradable materials with synthetic polymers 248 8.2.2 Application of only biodegradable materials to produce polymers 250 8.2.3 Methods of biopackaging production 250 8.3 Functional properties of films 8.3.1 Mechanical properties of films 8.3.2 Barrier properties of the films 8.3.3 Thermal properties of the films 8.3.4 Contact angle of water drop on film surface 8.3.5 Antimicrobial properties of films 8.3.6 Antioxidant property of the films

251 251 255 259 263 264 267

8.4 Conclusion

268

References

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8.1 Introduction Chitin and its derivative chitosan are two biopolymers that have been known since 200 years ago and are the most abundant polysaccharides after cellulose in nature. Chitosan is a Greek word that means an armor or cover protecting the invertebrates [1]. The most important sources of chitin and its derivatives are sea creatures, specially shrimps and crustaceans. Recently, chitin and chitosan biopolymers have been considered because of their unique properties, as follows: 1. 2. 3. 4. 5.

high flexibility and tensile strength (TS); biocompatibility, biodegradability, and compostability in nature; antibacterial and antifungal properties; gelation and stabilizing effect; and nontoxic and nonallergic.

These interesting properties caused researchers to attempt to use chitin and chitosan in the packaging, medical, pharmaceutical, agricultural, and food industries [2]. The chemical structure of chitin is (C6NO5H13)n in which chitin monomers bind to each other through (1
4)β bonds. As can be seen, the structure of chitin is similar to cellulose except that the OH group on C2 is replaced by an acetamide (NHCOCH3) group. During deacetylation of chitin in hot and high concentration alkali, chitosan is produced. Chitin and chitosan are positively charged polysaccharides and have unique physicochemical properties because of their acetamide, amine, and OH groups on the C3 and C6 of their structure. Chitin is insoluble in most solvents because of its high crystallization degree and the hydrogen bonds between acetamide groups. But chitosan with (C6H11NO5)n structure is produced by omitting the acetyl groups of chitin and is soluble in diluted acidic solutions.

8.2 Biodegradable film production methods Generally biodegradable materials have been used for packaging films in two ways: 1. combination of biodegradable materials with synthetic polymers and 2. application of only biodegradable materials to produce polymers.

8.2.1 Combination of biodegradable materials with synthetic polymers The reason for the combination of degradable materials with synthetic ones to produce films for packaging is to increase the polymers’

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degradability. Biomaterials can be added to polymer formulation in two ways: (1) as a filling agent and (2) as one of the combined base materials. 8.2.1.1 Application of biopolymers as filling materials The first biodegradable polymers were the result of the addition of starch granules (5%
20%) to synthetic polymers. In this case starch granules were distributed in the polymer matrix by an extrusion method without any interaction with the base polymer (polyethylene). Faster degradation of starch granules in the films would lead to more degradability of these kinds of polymers in exposure with oxygen and water. This phenomenon was named “biofragmentation.” Isa and Mohamed applied net chitosan in polyethylene films. They aimed to produce an antimicrobial degradable film. The Fouriertransform infrared analysis (FTIR) results showed that blended chitosan in polyethylene had no interaction with its monomers and subsequently chitosan’s active groups for antimicrobial property were free. But in order to inhibit the reduction of the mechanical and thermal properties of polymer, just 5% chitosan could be added to polyethylene [3]. Makarios-Laham and Lee compared the biodegradability of polyethylene films containing 5% chitin or chitosan with commercial degradable films containing corn starch. They buried these films in a soil medium containing Serratia marcescens, Pseudomonas aeruginosa, and Beauveria bassiana, which are capable of degrading chitin and chitosan. The results showed that after 6 months, 73.4% of chitosan and 84.7% of chitin became degraded but only 46.5% of starch in commercial films was degraded. Thus according to the results of this study, the preparation of degradable films with chitin and chitosan causes faster biodegradability than starch films [4]. 8.2.1.2 Application of biopolymers as one of the combined base materials In this method the biomaterial is modified to be compatible with the base polymer and can interact with it. For example, gelatinized starch is added to polyethylene films accompanied with a compatibilizer like acrylic acid, vinyl alcohol, polyvinyl alcohol, and vinyl acetate. In this method, the biomaterial interacts with the base matrix and it is not just a filling agent. Quiroz-Castillo et al. tried to use chitosan in polyethylene films. Because of the hydrophilic property of chitosan, they needed to modify it to be compatible with a hydrophobic polymer. In this regard they used the method of chemical modification of chitosan with polylactic acid. They produced polyethylene and chitosan
poly lactic acid by an extrusion method. Moreover they applied polyethylene-graft-maleic anhydride

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as a compatibilizer. By this method they could use up to 30% chitosan in polyethylene film without decreasing the packaging properties [5]. One way to modify chitin and chitosan for application in synthetic films is chemical and radiation graft copolymerization. Graft copolymers include a polymer as the main chain and the connected polymers as side chains. Side chains can add new properties to the main polymer while preserving its initial properties [6].

8.2.2 Application of only biodegradable materials to produce polymers It is possible to apply biopolymers alone for packaging material production with no need to combine them with synthetic materials. Packaging polymers made from biomaterials are more degradable than combined synthetic
biomaterial ones. Biopolymers can be classified into two kinds: edible and nonedible ones. Because of the weak mechanical, thermal, and barrier properties of these films against gases and water vapor, it is necessary to use a combination of several materials, different layers, or extruding methods to improve their properties.

8.2.3 Methods of biopackaging production There are two methods of biofilm production: dry and wet methods. The dry method is called the thermoplastic method in which heating, molding, and extrusion is used for creating packaging polymers. In this method, the base material of the film formulation should have thermoplastic properties. Pelissari et al. prepared starch
chitosan films incorporated with oregano essential oil by an extrusion method. They used a laboratory extruder with a screw diameter of 25 mm. The formulation of 77% starch, 5% chitosan, and 18% glycerol was applied to produce films. First, the mixture was extruded and pelletized in a temperature range of 110 C
120 C and screw speed of 35 rpm. Following that the pellets were extruded again to form the films by the blow technique [7]. In the wet or casting method the base materials are dissolved in appropriate solvents. Then the solvent from the film matrix can be separated through evaporation or sedimentation of solid materials by changing the pH, or changing the solvent polarization. Rubentheren et al. made chitosan films reinforced with nanochitin and tannic acid as cross-linking agents by a casting method. The results showed that the addition of nanochitin caused an increase in mechanical properties and a decrease in water solubility [8].

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8.3 Functional properties of films Mechanical properties, barrier properties against gases and water vapor, thermal properties, etc. are very important factors for determining the film acceptability of a packaging polymer.

8.3.1 Mechanical properties of films Improving mechanical properties of degradable films is important because high mechanical resistance of films leads to the inhibition of perforation and permeability of water and gases through the packaging. Similarly, the flexibility of films increases the application possibilities of film for different shapes of foods without fracture. Higher mechanical properties of a packaging polymer can protect the food from stresses during the transformation process. 8.3.1.1 Effective factors on mechanical behavior of films • Nature of biopolymer The chain length of biopolymer, molecular weight, and the nature of functional groups on chains are important factors on the interaction between polymer chains. In order to have cohesive and improved mechanical properties in films we should have more interaction between polymer molecules. Chitin, poly[β-(1
4)-N-acetyl-D-glucosamine], is a natural polysaccharide which is identical to cellulose but there is acetamide group instead of hydroxyl group on the α-carbon. Thus the molecules of chitin can interact via hydrogen, peptide, van der Waals, and electrostatic bonds with other materials. Chitosan is formed by the deacetylation of chitin and can be used as a basic material for polymer production. Chitosan chains have polycationic properties that can be used to form a gel or matrix as the result of the evaporation of solvent and creation of hydrophilic, hydrogen, electrostatic, and ionic interactions. • The concentration of biopolymer in solvent As the biopolymer concentration increases in the solvent, the thickness of the film increases, and the mechanical strength of it enhances too. Cerqueira et al. studied the interaction effect of chitosan molecules and glycerol on film’s mechanical properties. They found that increasing chitosan concentration can increase polymer matrix strength against external stress [9]. • The solvent type and pH The solvent pH can impact on some ionized polymer materials like gelatin, soy protein, and chitosan. Generally chitosan can be

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dissolved in acetic solvents like formic acid and acetic acid. While chitin is insoluble in most of the industrial solvents, such as water, the production of nanochitin made this material applicable in the polymer industry. Accordingly, nanochitin can disperse well in water and can be used in order to produce degradable films. Kim et al. dissolved two different deacetylated chitosans in four kinds of acids: acetic, lactic, propionic, and formic acids. They adjusted the pH of the film solution to 3, 4, and 5 and investigated the mechanical properties of films, such as TS and elongation at break (EB). Results have shown that films made from low deacetylation degree (DA) chitosan had more TS than high DA ones. Increasing the pH led to a reduction of a film’s TS and dissolving film in acetic acid and propionic acid improved the film’s mechanical properties [10]. • The type and concentration of plasticizer Incorporation of a plasticizer usually causes lower mechanical strength and more flexibility of films, while appropriate amounts of plasticizer may improve the mechanical properties of polymers. The mechanical properties of films should be adjusted to have good TS and enough flexibility to inhibit fragility. Therefore choosing a proper plasticizer for each polymer to achieve this goal is very important. For example, Suyatma et al. studied the plasticizing effect of four hydrophilic compounds: glycerol (GLY), ethylene glycol (EG), polyethylene glycol (PEG), and propylene glycol (PG), on mechanical properties of chitosan films. The addition of plasticizers increased chitosan film’s ductility except for PG, which didn’t show any plasticization effect. This study revealed that GLY and PEG are appropriate plasticizers for chitosan films in terms of inducing flexibility and storage stability [11]. Thakhiew et al. plasticized chitosan films by four glycerol concentrations (0%, 25%, 75%, and 125%) in order to inhibit the rigidity and brittleness of them. They concluded that increasing the plasticizer content in the films caused a reduction of TS and enhancement of EB of the films. Since glycerol could penetrate between the chains of the base polymer and decrease the intermolecular interaction, the resultant polymer is more mobile and flexible. • Film production method The film production method has an effect on the films’ mechanical properties. For instance, chitosan films made by the extrusion method are more mechanically resistant than the ones made by the casting method. Thakhiew et al. prepared chitosan films by three drying methods: hot air drying (40 C), vacuum drying, and low-pressure superheated steam

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drying (LPSSD) (90 C, 10 kPa). The results have shown that films prepared by vacuum drying or LPSSD method are more compact than the ones prepared by hot-air drying. Therefore the mechanical properties of these films are more acceptable than the ones made by hot air drying. This might be due to higher inter- and intramolecular interaction between chitosan molecules and more crystallinity of films dried via vacuum or LPSSD methods that resulted in the preparation of tough films [12]. 8.3.1.2 Measuring mechanical properties of films The most common method to investigate mechanical properties of films is the tensile test. Fig. 8.1 shows a schematic of the tensile testing machine. Different factors that are investigated by this method are: • Tensile strength Ultimate TS (UTS) is the maximum stress that a material can resist without getting permanent strain. UTS is affected by the polymer chains interaction and the measuring unit is pascal (Pa) or megapascal (MPa).

FIGURE 8.1 Tensile testing machine schematic. Source: Available from: ,http://www. engineeringarchives.com/les_mom_tensiletest.html., accessed December 20, 2018.

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Rubentheren et al. determined the TS of chitosan films using the Shimadzu AGS-X series. The films’ pieces (size of 80 mm by 8.5 mm) were fixed between device’s clips and TS, EB, and elastic modulus (EM) of the films were measured. The results indicated that the TS of net chitosan films and chitosan films containing 30% N-chitin were respectively 22.5 and 52 MPa. • Elongation at break This factor shows the ultimate elongation of the films affected by tensile stress up to tearing and is usually stated by millimeter. This factor indicates the flexibility of the films. Chang et al. incorporated nanochitin in starch films and tested its effect on the mechanical properties of the films. They concluded that increasing the nanochitin concentration in the films enhanced the TS of the films but decreased the EB of the films. They explained that the compatibility of chitin and starch caused enough interaction between filler and film matrix and improved the mechanical properties of the films [13]. • Strain to break Similar to EB, this factor shows the flexibility of the film but it is determined by the ratio of EB to the first length. Thus it does not have any unit and usually is stated in percent. Researchers have investigated chitosan films’ mechanical properties in order to use it in shopping bags. The results showed that the TS of the films was sufficient for being applied as pouches but the SB of the films was very low. It means that the ductility of the films was not sufficient to be applied for pouches formation [14]. • Young modulus or elastic modulus Elastic modulus of a film is measured by slope of the stress
strain curve in the elastic region and is an indicator of a material’s resistance against being deformed elastically. Lewandowska et al. studied the mechanical properties of chitosan
nanoclay composite films with a Zwick & Roell 0.5 testing machine. During stretching of the film, the machine measured the force exerting on the film and then through dividing it by the cross-sectional area, it could calculate the stress. The increase in stress continues linearly up to the sample fail. The stress vs. elongation curve was drawn and the slope of the linear region was determined to be composite film’s elastic modulus. The results showed that the addition of nanoclay improves the mechanical properties of the chitosan films through filling the free spaces between the chains [15]. The most typical device used for tensile test is Instron. Samples are cut into a dumbbell shape and are fixed between two grips. The test conditions, like separation initial grip or normal gage length, loading

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rate or cross head speed, and width and height are determined based on ASTM standards. The tensile stress is recorded in terms of strain and its curve is drawn. For example, chitosan film has a high tensile stress but low elongation; in contrast protein films have medium TS but high flexibility. This difference in mechanical properties can be due to the diversity in molecular structure. Polysaccharide films usually have a simple linear structure but protein films have complicated structures with a variety of inter- and intramolecular interactions [1]. For biodegradable films, the relative moisture (RH) percent of the environment during the tensile test is very important. The water molecules have a plasticizer effect and by increasing their content the RH% TS of the films reduces and the EB increases. In order to make a definite RH%, different saturated salts solutions can be used. For instance, calcium nitrate can induce 50% RH in the desiccated environment [16].

8.3.2 Barrier properties of the films The most important roles of packaging for food products are as follows: • Maintaining internal atmosphere of the packaging: fruits and vegetables continue to breathe after harvest, which is accompanied by physiological and biochemical changes affecting their freshness. In modified atmosphere packaging (MAP) the concentration of gases like oxygen, carbon dioxide, and nitrogen is altered in order to decrease the rate of breathing, microbial growth, and oxidative reaction. • Inhibiting the oxidation of food components like lipids and vitamins. • Preventing moisture loss or absorption. • Avoiding loss of aroma and flavoring agents. Because of the abovementioned reasons biodegradable films like chitin and chitosan films that are substituted for synthetic ones should have sufficient barrier properties against gases, water vapor, aroma, and flavoring agents. 8.3.2.1 Permeability to gases Zhong and Xia [17] studied the physicochemical and barrier properties of chitosan, cassava starch, and gelatin films. They measured the permeability of the films to O2 and CO2 at 25 C via designed stainless steel cells in the gas testing instrument (CYES-2, Shenyang, China). The mechanism of measuring gas permeability in this method is based on the concentration or pressure difference of the gas through the two parts

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of the film. This difference causes diffusion of the gases through the film. The gas permeability is measured according to Eq. (8.1): P5

mb ðx2 2 x1 Þ AðP2 2 P1 Þ

(8.1)

where P is the permeability of the films, mb is the volumetric or mass velocity, (x2 2 x1) is the film thickness (P2 2P1) is the pressure difference across both sides of films. The permeability of gases through the films is affected by three factors: 1. the nature of the polymer, 2. the nature of the penetrating gas, and 3. the interaction between the polymer and penetrating gas. Regarding O2, CO2, and N2, there would be no interaction between the gas and the polymer, because they are independent gases, and in almost all polymers the permeability of CO2 is four to six times faster than O2 and 24 times faster than N2. The results of the mentioned study showed that the addition of starch and gelatin to chitosan films improved the films’ barrier property against gases. Incorporation of cassava starch into chitosan films increased the hydrogen bonds between OH2 groups and the addition of gelatin to the films enhanced the hydrogen bonds between NH41 and OH2, therefore the structure of the films got more compact and their permeability decreased [17]. 8.3.2.1.1 Effect of different factors on gas permeability of the films

Park et al. studied the impact of molecular weight of chitosan on OP of films. They found that oxygen permeability (OP) of the films made of low-molecular-weight chitosan was lower than the ones formed from high molecular weight [18]. Furthermore, the type of solvent can influence the permeability of chitosan films. In this case, studies showed that chitosan films made by malic acid solvent had less OP. On the other hand, chitosan films formed by citric acid as solvent had weak oxygen barrier properties [18]. The effect of temperature on films’ permeability was studied, too. Research indicated that at low temperatures, like 4 C or less, the free volume of the chitosan films’ structure increased and molecular conformation changed. This alteration at low temperatures led to a greater permeability of the film to the gases [19]. Two different concentrations of polyethylene glycol as plasticizer were incorporated in chitosan films and their effects on mechanical, WVP, and OP were studied. Results showed that increasing plasticizer concentration increases OP through the films [20].

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8.3.2.2 Water vapor permeability of the films The water vapor permeability of the packaging films can cause the following problems for foods: • Hygroscopic food like cereals, powder, etc. will absorb water and lose their organoleptic quality. • Some foods containing higher water vapor pressure than the environment’s relative humidity will lose their moisture and freshness. The examples are fresh fruits and vegetables. • The water vapor absorption of the foods cause appropriate aw for microbiological, chemical, and enzymatic activities. • Losing water from some kinds of food products will lead to weight loss and consequently reduce their economic value. As can be seen, barrier property to water vapor is very important for packaging material. The mechanism of water vapor diffusion through the films is similar to that explained for gas diffusion. WVP 5

JΔx p1 2 p2

(8.2)

WVP is water vapor permeability, J is the water vapor transmission rate (WVTR), and p1 2 p2 is the water pressure difference of the both sides of the film. The WVTR is calculated according to Eq. (8.3): J 5 WVTR 5

Δw t3A

(8.3)

where Δw is the amount of water vapor permeated through the film expressed in volumes, masses, or moles; t is the time of water vapor transmission; and A is the film surface area.

8.3.2.3 Water vapor permeability measuring method The water vapor permeability of the biodegradable films is measured according to the ASTM E96-95 method. In this method distilled water is poured into glass vials with specific surface area. Biodegradable films are used to cover the glass vials and they are put in a desiccator containing a definite salt saturated solution. The RH of the internal part of the glasses is about 97% affected by distilled water and the RH of the outside of the glasses is affected by the salt solution. For example saturated solution of calcium nitrate causes 50% RH. In specific time intervals, the weight of glass vials is measured. The weight loss of the glass vial is equivalent to the amount of water vapor transmitted through the film. The related curve of weight difference versus time is drawn and

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the slope of the curve is calculated and is divided to the glass vial surface to measure WVTR, and finally the WVP can be calculated. Chitin and chitosan have a hydrophilic nature and their films and coatings have a high permeability to water vapor. Thus recent studies aimed to overcome this problem by different approaches like applying a hydrophobic layer to chitosan films using waxes and lipids, the addition of hydrophobic substances to the film formulation, the incorporation of nanomaterials, etc. Despond et al. applied a chitosan layer on cellulose polymer in order to decrease the gas permeability of the polymer. In anhydrous condition, chitosan coating caused a great decrease of CO2 permeability through the film, but this effect was abolished by hydrating the polymer. In order to overcome this problem, a layer of carnauba wax was added to the polymer on the chitosan side to decrease the hydrophilic property of the films. In this case, the CO2 and O2 permeability coefficient decreased to 0.5 cm/s Hg [21]. The recent studies regarding the effect of plasticizers on the WVP of chitosan
gelatin films showed that there was a direct relationship between plasticizer concentration and WVP. Increasing the plasticizer content in film enhanced the mobility and space between the chains, subsequently, water vapor permeation increased [22]. Casariego et al. produced chitosan/nanoclay films and studied their morphological, physical, and barrier properties. They found that the incorporation of nanoclay into chitosan film can reduce its water vapor permeability by inducing a tortuous pathway to the water molecules [23]. Bourtoom et al. produced films containing chitosan/rice starch films (50%:50%) by mixing 100 mL rice starch solution (2%) with 100 mL chitosan solution (2% in acetic acid). Furthermore, sorbitol was added as a plasticizer (50% of the total solid weight). They studied the effect of the addition of oils containing palm oil, oleic acid, and margarine at different concentrations (10%
50% of total solid weight). Results have shown that the WVP of the films depends on the type and concentration of lipids. Increasing the fatty acid chain length and saturation led to a reduction of the WVP. However, solid lipids are harder to dissolve in film formulation than the liquid ones and cause structural deficiencies. In this study, at the same concentrations oleic acid induced a higher water vapor barrier property than palm oil and margarine [24]. Cazon et al. produced a novel composite film containing nanocellulose in chitosan
polyvinyl alcohol films and studied its mechanical and barrier properties. They conclude that the addition of nanoparticles to the films could improve the mechanical properties of the films’ comparable synthetic polymers. But still the water vapor barrier properties of the film was lower than synthetic ones [25].

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Chitosan
thymol nanoparticles were prepared by the addition of 1.9 g citric acid to 100 mL of thymol solution (1 mg thymol/ 100 mL water). Next, 300 mg chitosan was added and stirred overnight. Then, the mixture was pumped to the sodium tripolyphosphate solution and centrifuged to separate the nanoparticles. These nanoparticles were added to the chitosan
quinoa protein films to study their antimicrobial and barrier properties. The results showed that films containing chitosan
thymol nanoparticles had better water vapor barrier properties than films containing sunflower oil or control films. Nanocomposite polymer was then used as an internal layer of polyethylene terephthalate (PET) containers in order to store blueberries and tomato cherries. Applying the nanocomposite film led to lower weight loss of fruits during storage at 7 C for 10 days. Therefore, chitosan films cannot prepare a sufficient polymer to be used alone as a packaging film, but applying it as a second layer can provide different advantages such as enhanced antimicrobial and water vapor barrier properties [26]. Nataraj et al. tried to prepare commercial chitosan polymers by studying three kinds of treatment: uncross-linked, cross-linked, and alkali treatments. In this study, they made chitosan solutions in acetic acid, then added citric acid as a cross-linking agent and applied 150 C heating for cross-linking treatment. Also some of the films were immersed in 0.5 N NaOH solution for alkali treatment. Alkali treatment led to a decrease in water sorption of the films from 1466% for untreated films to 100%
250% for treated ones. Also modified chitosan films could be molded to different shapes and had the potential to substitute for plastic films [27].

8.3.3 Thermal properties of the films One of the most important properties of biopolymers is the glass transition temperature (Tg), which is the temperature of changing the material from a solid and glassy phase to a soft and rubbery phase. In temperatures higher than Tg, the permeability of the films increases. Thus polymers with higher Tg can act as a good coating and inhibitor in a larger temperature range. 8.3.3.1 Methods of determination thermal properties of the films According to phase changes of the material via thermal treatment, different methods were developed to investigate thermal properties of the films. Differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), and dynamic mechanical thermal analysis (DMTA) are used for determining melting point, crystallization, and glass Tg.

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8.3.3.2 Dynamic mechanical thermal analysis The basic principles of this method are similar to oscillating rheometery for determining viscoelastic properties. In this method, the sample is exposed to very low strains and the resulting stresses are determined. The reason for applying low strains is to make sure that the test is done in the linear viscoelastic region. Lazaridou et al. assessed the thermomechanical properties of chitosan, starch
chitosan, and pullulan
chitosan films by DMTA and tensile testing method. DMTA was done using a Mark III analyzer (Polymer Laboratory, Loughborough, United Kingdom) with a heating rate of 2 C/min, frequencies of 1, 3, and 10 Hz, and strain 3 2. In this method the Tg was determined through measuring the peak in tan δ curves. Incorporation of sorbitol (10% and 30%) and moisture led to a decrease in the Tg of films as the result of its plasticization effect [28]. Al-sagheer et al. investigated the thermal properties of chitosan
silica hybrid films by the DMTA method. They measured the Tg of the composite films from the α-transition curves. The measurements were done by applying DMA Q-800 (TA, United States). The temperature range in this method was 50 C
210 C and the heating rate was 2 C/min under inert atmosphere. Increasing the silica content led to an increase in Tg and storage modulus. The optimum concentration of silica in chitosan films was 30 wt.% which increased the Tg up to 159.37 C [29]. Chang et al. studied physicochemical properties of starch/nanochitin (S/NC) composite films. They assessed the thermal properties of the films by the DMTA method. They used Netzsch DMA 242 analyzer at frequency of 1 Hz and temperature range
80 C to 180 C with a heating rate of 3 C/min. The storage modulus of S/NC as a function of temperature is shown in Fig. 8.2A. The storage modulus is an indicator of the polymer stiffness. By increasing the nanochitin concentration the storage modulus increased. Correspondingly the curve of the loss factor (tan δ) can be seen in Fig. 8.2B. Tan δ is dependent on molecular movement and can be referred to Tg. In control films, there were two Tg which related to coherent and weak structure phases. The Tg of the starch-rich phase was higher than the starch-poor phase. By incorporation of nanochitin into starch composite films, both Tg were at higher temperatures. It can be concluded that nanochitin particles act as filler as well as cross-linking agents between starch chains which leads to less molecular motion and so higher Tg [13]. Another factor that can be understood from DMTA curves is phase separation in a film’s structure. For example, storage modulus curves of S/NC nanocomposite with 5% nanochitin concentration showed two peaks referring to the aggregation of nanochitin particles at high concentrations in starch films [13].

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FIGURE 8.2 (A) Storage modulus and (B) tan δ of starch films containing different concentrations of nanochitin [13]. 8.3.3.2.1 Differential scanning calorimetry

DSC measures the energy exchange of materials during physical or chemical interactions. The word “differential” refers to comparing the sample heat exchange with the reference material such as indium. The word “scanning” refers to the increasing or decreasing of the sample temperature during test.

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In this test, the temperature changes in a specific rate (for example: 10 C/min) in a programmed range (
150 C to 1200 C). In order to take both sample and the reference material in the same temperature, it may need different heating energies. The reason is that physical or chemical interactions like phase changing in the sample may occur and these interactions may be exothermic or endothermic reactions, and therefore different heating energies are needed for sample and reference material. The DSC thermograms include the heating energy versus temperature diagrams. Tg, crystallization, melting point, and compatibility of the ingredients of the polymer can be understood from DSC thermograms. Ma et al. studied the effect of (10%, 30%, 50%, and 70% based on dry material) sorbitol as a plasticizer on chitosan’s thermal properties. They used Q 2000 DSC (TA, United States). Since the moisture content of the films can affect the Tg, first the films were heated at 110 C for 3 min to remove the water molecules from the film structure. DSC analysis was done at a temperature range
80 C to 1180 C at a rate of 10 C/min. In this work the Tg of neat chitosan films was reported as 102 C but different literature states different Tg for chitosan films. The reason may be because of different DA and molecular weights of chitosan. Also the preparation methods, pH, and type of solvents affect the Tg of films [30]. From the DSC diagrams of films it can be understood that only 10% sorbitol in the film formulation disperses well and higher concentration led to phase separation (two Tg were shown). Increasing the plasticizer content caused a lower Tg of the films because in low concentrations the plasticizers act as cross-linking agents and help to form a coherent structure but high concentration led to phase separation. In phaseseparated films the Tg of the chitosan-rich phase was near to Tg of neat chitosan film and the Tg of sorbitol-rich phase was at 0 C [30]. Reiad et al. studied the thermal properties of chitosan/nanosilver (C/Ag) by the DSC method. In this regard, they used DSC-H50, Shimadzu instrument at a heating rate of 10K/min and a temperature range 23 C
400 C. The test was done under a nitrogen atmosphere at a flow rate of 10 mL/min. The results showed that incorporation of nanoparticles (100 mg in 2% chitosan solution in acetic acid) increased the melting point of chitosan films. Furthermore increasing the chitosan concentration (2%, 4%, and 8%) increased the thermal stability of the films [31]. Qin et al. produced maize starch films using 7 g of maize starch and 3 g of glycerol as a plasticizer. They also added (0%, 0.5%, 1%, 2%, or 5% based on dry matter) chitin nanoparticles to study their effect on the physicochemical properties of the films. The thermal properties of the films were studied using the DCS method. About 3 mg of films were

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FIGURE 8.3 DCS curves showing the Tg of the films. Source: S. Otles, S. Otles, Glass transition in food industry-characteristic properties of glass transition and determination techniques. Electron. J. Pol. Agric. Univ. 8 (4) (2005). ,http://www.ejpau.media.pl/volume8/issue4/art69.html. (accessed 20.12.18) [32].

placed in the instrument and heated up in the range 25 C to 250 C at a heating rate of 10 C/min. The Tg of the films is not shown as a sharp peak in the DSC thermograms and usually contains an onset (To) and midpoint (Tm) temperature (Fig. 8.3). In this literature, the To and Tm of the films containing different concentrations of the nanochitin were compared. Increasing the nanochitin concentration in film formulation improved the thermal properties of the films and To and Tm increased. Another parameter that can be calculated from the DSC is material enthalpy (ΔH). The incorporation of nanoparticles increased the enthalpy of them, too. The reason may be due to the interaction of nanochitin with maize starch chains, increasing the overall crystallinity of the films [33].

8.3.4 Contact angle of water drop on film surface Protein and carbohydrate polymers usually have a hydrophilic property and the contact angle of the water drop on their surface can be an indicator of their degree of hydrophilicity. The contact angle (θ) is the angle of the tangent line of the water drop and the surface of the film. When a drop is put on the film’s surface, one of two conditions will occur: (1) the drop is spread over the surface of the film or (2) the drop stays as a separate drop on the surface. According to the θ it can understood that the film is wettable or not. For example, if the θ is less than 30 degrees the film is completely wettable. At θ between 30 degrees to 89 degrees the film is partially hydrophilic and if θ is more than 90 degrees the film is hydrophobic. Guerrero et al. introduced a new method for chitosan film production that does not take too much time and solvent. They mixed 1 g chitosan powder, 10% or 20% citric acid powder, 3 mL deionized water, and 15% glycerol and after conditioning for 1 day, they put the mixture in a hot

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press at 125 C and 2.5 MPa for 2 min. Then, they studied the water contact angle of films surface using a Dataphysics Contact angle OCA 20 (DataPhysics Instrument GmbH, Filderstadt, Germany). The results indicated that chitosan films produced by this method had high contact angles, which means that the hydrophilic property of the films decreased. The contact angle of films containing 0%, 10%, and 20% citric acid as a cross-linking agent was 98.4, 96.3, and 81.4, respectively. It can be deduced that increasing the acid content decreased the contact angle and therefore increased the hydrophilic property of the films. The reason may be the polar carboxyl groups of citric acid. Also some of these groups interacted with chitosan amine groups, but some of them stayed free and increased the polarity of the films [34]. Shankar and Rhim investigated the effect of the addition of sulfur nanoparticles to the chitosan films. Three kinds of sulfur nanoparticles: sulfur nanoparticles without capping agent (SNP1), sulfur nanoparticles capped with sodium dodecyl benzene sulfonate (SDBS) (SNP2), and sulfur nanoparticles capped with chitosan (SNP3) were used in this study. The hydrophobicity of the films was determined through the contact angle method. In this case, square pieces of film samples were fixed in a contact angle analyzer (Phoenix 150, Surface Electro Optics Co., Korea) and a 10 μL water drop was put on them. The neat chitosan films had 64.5 degrees contact angle but the addition of SNP2 decreased this amount to 61.5 degrees. On the other hand, the addition of SNP1 and SNP3 increased the hydrophobicity with a contact angle of 67.8 degrees and 68.6 degrees. They stated that the increase in hydrophobicity of the chitosan films was due to hydrophobicity of SNP1 and SNP3 and the increase of the hydrophilic property of chitosan
SNP2 was due to the hydrophilic property of SDBS [35].

8.3.5 Antimicrobial properties of films The active groups of chitin and chitosan have functional properties relating to the amino group on C-6 and hydroxyl groups on C-6 and C3 positions. The difference between chitin and chitosan is due to the content of different amino groups, which is responsible for the functional properties of their films. There are different mechanisms for the antimicrobial properties of chitin and chitosan films (Fig. 8.4): 1. The positive amino groups of chitosan can react with negative groups of the microbial cell membrane and this interaction causes the transformation of the cell membrane. These changes in the microbial cell cause different permeability to vital components for living cells and therefore can lead to a decrease in microbial growth or its death.

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FIGURE 8.4 The schematic of chitosan antimicrobial functionality [36].

2. If chitosan can diffuse across the cell membrane, it can react with microbial DNA and inhibit RNA and protein production process. 3. Negative groups of chitosan can react with metal ions and take them out of reach of the microbial cells. 4. Chitosan can cover the microbial cell and prevent nutrients and gas permeability to the cells. Different factors are important for antimicrobial properties of chitosan films such as molecular weight, concentration, degree of deacetylation, source of chitosan, incorporation of active components in chitosan films, food components and pH, and microorganism type [36]. Soni et al. investigated the physicochemical and antimicrobial properties of chitosan/TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) cellulose nanofibers (CNF) nanocomposite films. Chitosan was extracted from shrimp exoskeletons. The films were prepared through a casting method with the following formulation: 100% chitosan/0% TEMPO, 85% chitosan/15% TEMPO, 75% chitosan/25% TEMPO, and 0% chitosan/100% TEMPO. The disk diffusion method was used for evaluating the antimicrobial properties of the films. In this regard, 100 μL strain mixtures of Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes were inoculated on their specific agar mediums. Then chitosan/TEMPO-CNF film samples and TEMPO-CNF films (as control) were cut into disks (8 mm diameter) and sterilized using X-ray irradiation. These film samples were put on the inoculated agars and incubated at 37 C for 24 h. The zone under and around film disks where no growth of bacteria was seen was assumed to be the inhibitory zone [36]. All film samples containing chitosan showed antimicrobial properties against positive and negative bacterial strains studied in this research,

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but control films (TEMPO-CNFs) did not show any antibacterial properties. Decreasing the chitosan concentration in the film formulation reduced the antimicrobial properties of the films. The reason owes to the amino groups of chitosan which are responsible for the antimicrobial activity of this component [37]. Xu et al. evaluated the effect of the addition of cinnamon oil in chitosan
gum Arabic films on their antimicrobial properties. They dissolved 1.5% w/v chitosan in 1% v/v acetic acid and added 20% (based on total solid) glycerol as plasticizer. On the other hand, they prepared gum Arabic (GA) solution in ultrapure water and dispersed 8% cinnamon essential oil (CEO) in it. The final formulation of the films were 1:0, 1:0.25, 1:0.5, 1:1, 1:2, and 1:4 ratios of chitosan solution to GA solution. The antibacterial properties of the films were studied against E. coli ATCC 25922 through a liquid culture test. Initially, the films were immersed in nutrient broth to swell and then they were transferred to a nutrient broth inoculated with 60 μL E. coli (108 CFU/mL). During incubation at 37 C, 150 μL of the mixture was taken every 1 h to transfer to the microplate reader (SpectraMax-M2e, Molecular Devices, China) in order to measure optical density. The same bacterial suspension in nutrient broth was assumed to be blank and broth without bacterial inoculation was the reference [38]. Increasing the bacterial colony forming units in the liquid medium increases its turbidity. When the antimicrobial films were immersed in the medium the antibacterial agent diffused in it and decreased the turbidity. Pure chitosan films had weak antimicrobial properties, but the addition of CEO significantly increased the antimicrobial activity of the films. Increasing the chitosan to GA ratio from 1:0 to 1:2 increased the antimicrobial properties, too. GA helped the gradual release of CEO to the medium and longer antimicrobial functionality of the films. The mechanism of antibacterial properties of CEO is related to the interaction with the bacterial cell membrane and the alteration of its permeability [38]. Ahmed et al. prepared chitin nanowhiskers/nano-ZnO/nano-Ag (CNW/ZnO/Ag) by dissolving 0.25 gr ZnO in ethanol and adding this solution to CNW (0.75, 1.0, and 1.5 gr) solution. They increased the alkalinity of the solution up to 10 by 1 M NaOH. Finally they added 0.125, 0.25, and 1.5 gr silver nitrate to the solution and heated it up to 80 C. The ratios of zinc acetate to silver nitrate in the solution were 1:0.5, 1:1, and 1:2 w/w. Subsequently, 5% (based on total solid) CNW or CNW/ ZnO/Ag was added to carboxymethyl cellulose (CMC) films containing 2% w/v CMC and 30% glycerol. The antibacterial properties of CMC nanocomposite films against Gram-negative (E. coli) and Gram-positive (L. monocytogenes) bacteria were studied using the colony count method.

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The results showed that CMC-CNW had little antibacterial activity against E. coli and no antibacterial activity against L. monocytogenes. But metallic nanocomposite films (i.e., CMC-CNW/ZnO/Ag) had antimicrobial property against both Gram-negative and -positive bacteria. In this regard, CNW/ZnO/Ag with ZnO: Ag ratio of (1.0:0.5) had the highest antibacterial property. The researchers presumed that increasing the Ag concentration increased the hybrid nanoparticle size and limited the permeability and reaction of nanoparticles with the DNA and the cell membrane of the bacteria [39].

8.3.6 Antioxidant property of the films Chitin and chitosan have good antioxidant property, specially scavenging activity of hydroxyl groups and chelating of metal ions. Researchers stated that low-molecular-weight chitosan had higher hydroxyl, superoxide, alkali, and DPPH radical scavenging activity than its high-molecular-weight counterpart. The antioxidant property of chitosan is related to the hydroxyl group of C6 and the amino group of C2 in its structure which can act as hydrogen donators or cationic agents [40]. Lee et al. prepared chitosan films containing halloysite nanotubes (HNT) of clay to carry clove essential oil (CEO). In this study, 10 g of chitosan was dissolved in 1 L of distilled water and 35% glycerol was added as a plasticizer. Then various amounts of HNT (0%, 5%, 10%, 15%, 20%, 25%, and 30% based on total solid) was incorporated to the solution. After vigorous stirring overnight, 1% CEO was added and after ultrasonic and homogenization treatments the film solutions were cast and dried at ambient temperature. The antioxidant property of the films was evaluated by determining total phenolic content, DPPH radical scavenging activity, and reducing power. Neat chitosan films did not have any phenolic content, but the addition of CEO increased the phenolic content of the films significantly. Incorporation of HNT to chitosan films increased the phenolic content due to stabilizing the essential oils and preventing evaporation from the film surface. Increasing the HNT content up to 15% enhanced radical scavenging activity of the films but further HNT increase, reduced the antioxidant properties of the films. The researchers explained that the high concentrations of HNT aggregated in the film structure, therefore caused cracks on the film surface and removed the CEO from the film’s matrix [41]. Lopez-Mata et al. prepared (1% and 2% w/v) chitosan films containing 0.25%, 0.5%, and 1% v/v cinnamaldehyde (CNE) and studied their mechanical, barrier, and antioxidant properties. For evaluating

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antioxidant property, 50 mg of film was weighed and homogenized in 80% methanol and the resulted extract was used for DPPH, ABTS, and hemolysis assays. They found that chitosan films without CNE have an antioxidant property which was related to the ammonium group on C2 of chitosan’s structure. The hydrogen of ammonium group can react with free radicals and bind them. Besides, it is stated that the antioxidant activity of chitosan is dependent on its molecular weight and 30 kDa chitosan indicated higher antioxidant activity than 90 and 120 kDa. Another method of antioxidant property assay that was studied in this research was hemolysis assay. In this method, AAPH was utilized to create peroxyl/alkoxyl radicals which are responsible for biological damage. Films containing 1% and 2% chitosan alone and with 1% CNE exhibited higher hemolysis inhibition effect. They stated that chitosan with higher DA had higher peroxyl radical scavenging activity. Chitosan molecules were protonated in acidic solution during film preparation. These cations can react with negatively charged molecules on the erythrocytes which are aggregated on the cell membrane and protect them from peroxyl radical hemolysis [42].

8.4 Conclusion Recently, the most important reason for developing degradable packaging was environmental concerns because the population is growing and recycling, degrading, and composting the growing piles of waste will be very difficult. In this regard, the methods of investigation of degradable film properties, such as chemical structure, mechanical, thermal, barrier, antimicrobial, and antioxidant properties, are professionally improved and help the researchers to assess different factors’ effects on physicochemical properties of films accurately. Chitin and chitosan, especially chitosan have acceptable film-forming properties. Several studies have approved mechanical, thermal, and barrier properties of chitosan films in order for them to be applied for food packaging in dry conditions. In addition, the functional properties of these materials, such as high antimicrobial and antioxidant properties, led to their recognition as potential substitutes for synthetic polymers in the food and pharmaceutical industries. Fortunately, several approaches have been developed to improve chitosan films’ properties, such as the addition of proper plasticizers, cross-linking agents, fillers, nanoparticles and essential oils, application of different film-forming methods, production of multilayer films, and copolymerization.

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C H A P T E R

9 (Bio)composites of chitin/ chitosan with natural fibers Carolina Grego´rio Costa, Lirian Ferreira Rosa Pereira Bom, Cristiane Reis Martins, Classius Ferreira da Silva and Mariana Agostini de Moraes Department of Chemical Engineering, Institute of Environmental, Chemical and Pharmaceutical Sciences, Federal University of Sao Paulo (UNIFESP), Diadema, Brazil

O U T L I N E 9.1 Introduction

273

9.2 Fundamentals on natural fibers 9.2.1 Physical methods of fiber treatment 9.2.2 Chemical methods of fiber treatment

277 279 279

9.3 Chitin/chitosan (bio)composites with natural fibers 9.3.1 Plant fibers 9.3.2 Animal fibers

280 281 290

9.4 Conclusion

294

References

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9.1 Introduction Composites are materials formed by a continuous phase (matrix)— polymer, metal, or ceramic—and a filler (dispersed or discontinuous

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phase)—fibers or particles—that aggregate physicochemically. Biocomposites are defined as composite materials of a biodegradable polymeric matrix, synthetic or natural, and biodegradable natural fibers or particles as fillers. The field of biocomposites is recent, even though they are extensively found in nature, in structures like bamboo, bone, nacre, and wood. Typically, the main reason to develop a (bio)composite is to improve the mechanical properties of a specific matrix; however, despite the mechanical properties, better physical and barrier properties can also be achieved. The scientific literature on composites has grown substantially in the last 20 years. To illustrate that increase, we performed an in-depth search of the Web of Science database by Clarivate Analytics. Initially, the word “composite*” was used to search only scientific papers in journals and proceeding papers in English between 1999 and 2018. The “*” was used as a wildcard to find inflections of the same word, that is, “composite” (singular) or “composites” (plural). The number of published papers increased from 12,242 in 1999 to 72,254 in 2018, which is about six times higher after 20 years. Even in 1999, the number of published papers was incredibly huge. The total number of publications in this period was 685,781. When we refined the search by including the word “chit*” (considering inflections like “chitosan” and “chitin”), we found 11,329 papers during the evaluated period. The number of published papers jumped from 23 in 1999 to 1699 in 2018, increasing about 74 times. However, the subject of this chapter is on biocomposites of chitin and chitosan, so we proceeded with the same search by using the same strategy. Initially, the search was assessed by using the word “biocomposite*,” reaching a total number of publications of 7148. This number was smaller than that of the search with “composite*”; however, the increase over time was higher, 24 times in the last 20 years, jumping from 40 papers in 1999 to 964 papers in 2018. Also refining the search by including the word “chit*,” we found 575 papers about chitin or chitosan biocomposites, but no paper was found in 1999 and 2000. The first (and the only) was in 2001, but 98 papers were found in 2018, that is 98 times higher and even higher than the 74-fold increase for chitin and chitosan composites. China, India, and the United States stand out among the countries that most present publications about chitosan/chitin biocomposites (Fig. 9.1). China alone presented about 37% of the total publications and these three countries together developed more than a half of the total of publications. A curious fact is the frequent use of the term composite, even when it comes to the development of a biocomposite material, which demonstrates that the term “biocomposite” still needs to be more widely

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disseminated in the scientific community. Thus for the greater comprehensiveness of the results, in the following searches, we chose the expression “composite or biocomposite” of “chitin or chitosan.” Fig. 9.2 illustrates the evolution of the number of publications involving this broader search of all (chitin/chitosan and biocomposite/composite). It is also important to note that the word “composite” is also frequently used to refer to a material composed of two different raw materials, not necessarily a matrix and a filler. Thus we have focused in this chapter on reviewing only the publications that have developed a material in which chitosan was used as the matrix and natural fibers as fillers.

FIGURE 9.1 Percentage of publications on chitin/chitosan (bio)composites by country (1999
2018). Source: Data extracted from the Web of Science database.

FIGURE 9.2

Number of publications on “chitin” (or “chitosan”) and “biocomposites” (or “composite”) between 1999 and 2018. Source: Data extracted from Web of Science database.

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FIGURE 9.3 Other biopolymers added to the chitin/chitosan (bio)composites (1999
2018). Source: Data extracted from the Web of Science database.

We further refine the search by including the words “fiber*” and “particle*” with the intent of finding the words “fiber” or “fibers” and “particle” or “particles.” We found 1385 and 1791 papers, respectively, for fiber(s) and particle(s). On the other hand, when we go to the nanoscale, the number of papers about nanoparticle* is 3752, while nanofiber* is 926 within the 20 years. Although the number of publications with biocomposites containing particles (or nanoparticles) is larger than the number of articles with fibers (or nanofibers), fillers as particles are not covered in this chapter. No less important are the biocomposites with nanotubes (1480 papers) and (nano)whiskers (181 papers). Concerning the incorporation of other biopolymers in the composition of these chitin/chitosan (bio)composites, Fig. 9.3 shows that cellulose stands out above the others, followed by alginate and starch. The biocomposites can present fillers from different sources: vegetable, animal, and mineral. In this chapter, we will focus just on vegetable and animal sources. There were 554 papers found about chitin/chitosan (bio) composites with fillers from a vegetable source. Cotton, wood, soy, and rice were among the most frequent (Fig. 9.4). On the other hand, animal sources in chitin/chitosan (bio)composites are practically unusual, just 45 papers. It was found 21, 17, and 7 papers using wool, hair, and chicken feather, respectively. Chitin/chitosan (bio)composites with fibroin (171 papers) and sericin (16 papers), which are animal-based biopolymers, were also found. It is also essential to check the most common characterizations available in such publications. Four techniques predominate in the papers about chitin/chitosan (bio)composites: scanning electron microscopy— SEM (4007 papers), infrared spectroscopy—IR (3590 papers), mechanical properties (2926 papers), and X-ray diffraction—XRD (2684 papers). Enhancements of the mechanical properties are usually studied in the composite development and this is the reason why it is one of the most

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FIGURE 9.4 Number of publications about chitin/chitosan (bio)composites with vegetable fillers (1999
2018). Source: Data extracted from the Web of Science database.

evaluated properties. The SEM analysis allows visualization of the homogeneity and morphology of the composite as well as the interaction between chitosan and the fillers. The IR analysis permit the study of chemical interactions between the composite compounds, for example, chitosan and the fillers or other added biopolymers. Finally, XRD analysis is also evaluated, although the biopolymers like chitosan are considered semicrystalline material, the processing of the biopolymers and the filler may modify the crystallinity of the material, and consequently other properties like mechanical properties. In the opposite direction, some techniques are unusual, that is, less than a hundred papers presented such techniques and they were included in the “others characterization”: melting temperature, dielectric spectroscopy, micro Xray computed tomography, flammability and scanning tunneling microscopy. Fig. 9.5 may help to choose the most suitable techniques for the development of chitin/chitosan (bio)composites, but for each application, the most important properties should be established for the choice of appropriate techniques.

9.2 Fundamentals on natural fibers The interest in eco-friendly materials has grown significantly worldwide during the last decades, due to environmental and sustainability concerns. The use of biobased materials represents an alternative to fossil-based resources and has a positive impact on agriculture. Natural fibers have good strength and stiffness, present a low-cost/low-density

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Others characterizations Dynamic mechanical analysis Raman spectroscopy Nuclear magnetic ressonance Biodegradability Water absorption Atomic force microscopy Optical properties X-ray photoelectron spectroscopy Cytotoxity Differential scanning calorimetry Density Thermogravimetric analysis X-ray diffraction Mechanical properties Infrared spectroscopy Scanning electron microscopy

0

1000 2000 3000 4000 Number of publications

5000

FIGURE 9.5 The main characterization techniques presented in publications about chitin/chitosan (bio)composites (1999
2018). Source: Data extracted from the Web of Science database.

ratio, are biodegradable and represent an environment-friendly alternative to glass and carbon fibers. Natural fibers are divided into groups based on their origins: plant, animal, or mineral. Animal fibers such as hair, wool and silk, and mineral fibers have not been widely used as reinforcement fibers. Plantbased fibers are the most common and can be classified based on the part of the plant from which they are extracted. Bast fibers, such as jute and ramie, are withdrawn from the stem of the plant and are most generally employed as reinforcements because they have the most extended length and highest stiffness and strength. Stalks can also yield fibers such as bamboo and wood (soft and hard). It is also possible to obtain reinforcements from leaf fibers such as banana and sisal. Seeds, such as cotton and cereal straw, can also provide fibers to reinforce composites. Plant-based fibers (also called lignocellulosic fibers) have complex structures, with three main constituents: cellulose, hemicellulose, and lignin. Some minor constituents such as pectin, waxes, proteins, lipids, ash, pigments, and extractive compounds may also be present [1]. The quantity of these components in a fiber change from plant to plant due to species, age, growth environment, and extraction conditions, among others [2]. The cellulose is the strongest and stiffest component of the fiber, which is a long chain polysaccharide formed by units of β-glycose. The presence of hydroxyl groups in each repetitive unit of cellulose results in a hydrophilic character [2]. Hemicellulose consists of a group of polysaccharides that remain associated with cellulose after lignin has been removed. It has chain branching and the degree of polymerization is 10

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to 100 times lower than that of cellulose [3]. Lignin is a highly crosslinked complex with an amorphous structure containing aliphatic and aromatic constituents [3]. Lignin is also responsible for the rigid property of the plant cell wall and acts as a permanent agent to cells’ connections, producing a structure resistant to impact and compression. Most of the plant-based fibers can be isolated, treated, and functionalized before being used in biocomposites. The plant fibers are extracted by a process, called retting, that separates the fiber bundles from a central stem, which loosens the fibers from the woody tissue of the fiber crops. There are several retting processes, classified as biological, mechanical, physical, and chemical, which were reviewed by Ramamoorthy et al. [2]. The performance of a polymer biocomposite depends not only on the selection of its components, but also on the adhesion between them. The poor fiber/matrix adhesion in polymer composites is, in fact, the main disadvantage of the use of natural fibers. All natural fibers are strongly hydrophilic owing to the presence of hydroxyl groups in the cellulose molecules. This hydrophilic nature of cellulose fiber is a potential cause for incompatibility and dispersion problems with hydrophobic polymer matrices. To eliminate this problem chemical or physical methods can be used to optimize natural fiber interfaces [1].

9.2.1 Physical methods of fiber treatment Physical treatments change the structural and surface properties of the fiber and influence their mechanical bonding to polymers. The most used physical method for plant-based fibers is corona treatment, used for surface oxidation activation, which changes the surface energy of the fibers and increases the number of aldehyde groups. Other physical methods, such as cold plasma treatment, stretching, calandering, thermal treatment, among others, can be successfully used [3].

9.2.2 Chemical methods of fiber treatment Chemical methods activate hydroxyl groups or introduce new moieties that will interact with the polymer matrix. Usually, the chemical coupling agents react with hydroxyl groups of cellulose and with the functional groups of the matrix [1]. There are several methods to induce chemical modifications of natural fibers such as acetylation, alkali treatment, silane treatment, and peroxide treatment with chemical coupling agents. Alkali treatment is the most common method to produce highquality fibers to reinforce polymeric matrices. Mercerization is the most traditional alkali treatment, which is based on sodium hydroxide. This

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treatment reduces amorphous hydroxyl groups from fibers (FiberOH 1 NaOH - Fiber-ONa 1 H2O) and results in the reduction of the hydrophilic nature of fibers. It also depends on some parameters such as temperature and concentration of the alkaline solution and time of treatment [4]. Acetylation of fibers is another chemical treatment, also known as the esterification method, causing plastification of cellulosic fibers. The reaction involves the substitution of cellulose hydroxyl groups by acetyl groups, adding a hydrophobic character to the fibers, as well as reducing moisture uptake. The process involves soaking the fibers in acetic acid, followed by acetic anhydride and, finally, washing the fibers with water. Acetylation decreases the hydrophilic nature of natural fibers and increases their mechanical strength [1,5]. Silanization is a chemical treatment, which uses organosilanes as coupling agents. Various silane coupling agents can be represented by formula R-(CH2)n-Si (OR0 )3, where n 5 0
3, OR0 is a hydrolyzable alkoxy group, and R the functional organic group [3]. These reactive alkoxy groups chemically bond with the fiber surface, which contains hydroxyl groups and the formation of polysiloxane structures can take place. Other chemical treatments, such as benzoylation, acrylation and acrylonitrile grafting, maleated coupling agents, permanganate treatment, peroxide treatment, and isocyanate treatment, can be applied for surface modification of plant-based fibers and were deeply reviewed by Li et al. [1].

9.3 Chitin/chitosan (bio)composites with natural fibers The use of natural fibers, especially lignocellulosic fibers, as fillers on chitosan biocomposites is attractive due to the chemical similarities in the polysaccharide structure. The possibility of hydrogen bond formation between chitosan and cellulose leads to an excellent fiber
matrix adhesion, usually avoiding the need for surface modification treatments [6]. In this session, we will provide an overview of the main published papers on biocomposites of chitin and chitosan-containing natural fibers. The session is divided by the source of the fibers, plant-based or animal-based. In each source, plant-based or animal-based, the state-ofthe-art found in the literature of chitin/chitosan (bio)composites was grouped based on the fiber raw material, such as jute, flax, sisal, bamboo, cotton, wool, and silk (Fig. 9.6). Biocomposites containing commercial cellulose and/or nanocellulose, either cellulose nanowhiskers or cellulose nanofibers, were not reviewed. For the most interested readers on nanocellulose, we suggest the reading of the review paper published

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FIGURE 9.6 Schematic representation of fiber classification for raw materials used on (bio)composites of chitin/chitosan.

by Khalil et al. [7] on chitosan blends and biocomposites with nanocellulose. Finally, we present our unpublished results on chitosan biocomposites reinforced with silk fibroin (SF), an animal fiber extracted from silkworm cocoons.

9.3.1 Plant fibers 9.3.1.1 Flax Prabhakar and Song [8] studied a starch
chitosan
flax fabric composite. The amount of chitosan was varied as 3, 6, and 9 wt.% and a starch
flax fabric composite was used as control. The incorporation of chitosan improved the tensile strength of the composites and the best result was found for the composite with 6 wt.% of chitosan. There was a good interaction between the matrix and the reinforcement. In addition, all composites were biodegradable and had the potential to substitute petroleum-based plastic. Thinking of eco-friendly materials, Mujtaba et al. [9] also analyzed chitosan matrix reinforced with flax cellulose nanocrystals incorporated in different concentrations (5, 10, 20, and 30 wt.%). The presence of the reinforcement improved the tensile strength and the Young’s modulus

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of the chitosan matrix and the best result was found for the composite with 20 wt.% of flax cellulose nanocrystals. The reinforcement and the matrix had a good interaction due to the anionic nature of cellulose nanocrystals and cationic nature of chitosan. In addition, the presence of flax cellulose nanocrystals also improved the antimicrobial activity of the chitosan matrix. Thus flax cellulose nanocrystals were shown to be the right candidate for reinforcement of the chitosan matrix. 9.3.1.2 Kenaf Julkapli and Akil [10] studied the thermal properties of kenaf-filled chitosan biocomposites varying the kenaf dust content (0, 7, 14, 21, and 28 wt.%). Results showed that the incorporation of kenaf dust did not change the thermal stability of the chitosan matrix. The degradability of kenaf-filled chitosan biocomposites in distilled water, alkaline, and acidic solutions was also elucidated [11]. The degradation period of chitosan composites increased with the addition of the kenaf dust into the chitosan matrix. The authors suggested that this improvement in hydrolysis was due to the formation of inter- and intrahydrogen bonds between amine groups of chitosan and hydroxyl methyl groups of kenaf. The mechanical and chemical properties of kenaf-filled chitosan biocomposites were also reported [12]. The kenaf dust acted as reinforcement in chitosan matrix since the tensile strength, elongation at break, tensile modulus, and toughness improved with the addition of kenaf. The best result was found for the biocomposite containing 28 wt. % of kenaf dust and a good interaction between kenaf and chitosan was found, showing it to be a successful eco-friendly biocomposite. Finally, the influence of two different plasticizers (glycerol and di-hydroxyl stearic acid) at various concentrations (0.2%, 0.4%, and 0.6%) on the mechanical properties of biocomposites of chitosan filled with kenaf dust was investigated [13]. The tensile strength and Young’s modulus of films without plasticizer presented the highest values and the results observed for films plasticized with di-hydroxyl stearic acid were higher than those of the films plasticized with glycerol. However, the elongation at break of the films increased with the presence of the plasticizers and the highest values were found for the films plasticized with dihydroxyl stearic acid, which showed better results as a plasticizer of the biocomposites studied. 9.3.1.3 Jute Kavitha and Rajarajeswari [14] developed a blended film of chitosan and jute fiber with or without surface modification of the fiber by an acetylation treatment. They found that the tensile strength and the hydrophobicity of the chitosan-acetylated jute film increased when

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compared to the chitosan-raw jute film. In addition, the film with treated jute fibers showed bioactivity and suitability for bone repair application. El-Shafei et al. [15] studied the curing of jute fabric with chitosan and phosphorylated nanocellulose composite with different fractions of both. The tensile strength of jute fabric decreased after curing because the treatment of the fiber under acidic conditions affected the fiber strength. However, at constant fractions of phosphorylated nanocellulose, the tensile strength improved with the increase of the chitosan content. In addition, the curing of the jute fabric with chitosan improved its thermal stability and antibacterial activity. 9.3.1.4 Sunflower Mati-Baouche et al. [16] reported biocomposites of chitosan with sunflower stalk particles. The sizes of the particles were between 1.6 and 6.3 mm and the ratios of chitosan/sunflower particles were between 0.04 and 0.15 g/g. Biocomposites with optimum mechanical and thermal properties were those with sunflower stalk particles size greater than 3.1 mm. Also, the increase of chitosan/sunflower particles ratio and compaction pressure improved the mechanical strength of the composites. The authors concluded that the mechanical and thermal properties found for the composites were similar to other insulating biobased materials available on the market. However, the performance in acoustical absorption was poor. Thus another study from the same research group focused on studying the acoustical properties of the composite without changing its mechanical rigidity [17]. The authors developed a composite with higher porosity and obtained an improvement in its acoustical properties. In addition, a formulation of chitosan cross-linked with genipin mixed with alginate, guar gum, and starch was developed, aiming to achieve mechanical properties competitive with other industrial nonbiobased composites [18]. Sun et al. [19] studied the mechanical response of biocomposites of chitosan reinforced with sunflower stem chips (bark and pith chips) with a size between 3 and 5 mm. The mechanical properties were more significant for biocomposites with the highest chitosan concentration (6.25% mass fraction of chitosan). Furthermore, the strain level was higher in pith than in bark. 9.3.1.5 Rice D’Angelo et al. [20] developed biocomposites from chitosan and rice straw to replace plastic shopping bags. Compared to pure chitosan, the addition of rice straw increased the resistance and Young’s modulus. Emadi et al. [21] prepared biocomposites with rice husk to reinforce wheat gluten
glycerol/chitosan
polyethylene glycol (Gly/Glut-CP).

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They showed that rice husk increases hydrophilic character, absorbing more water compared to biocomposite without this reinforcement. However, the biocomposites without chitosan had the highest water absorption. On the other hand, chitosan showed a 183% improvement in tensile strength. Xu et al. [22] used nanocrystalline cellulose (NCC) from rice straw to improve the tensile strength of chitosan, at an optimal content of 5 wt.%. They also observed an increase in thermal stability and water absorption up to 20 wt.% of NCC. Nanocomposites of chitosan reinforced with rice straw nanofibers were studied by Hassan et al. [23]. The authors found a significant improvement in wet and dry tensile strength of the chitosan matrix, with the higher value found for the nanocomposite with 20% of nanofibers. Rodrigues et al. [24] developed hydrogel composites of chitosan and poly(acrylic acid) reinforced with rice husk ash previously calcinated at 400 C or 900 C. The water uptake capacity at the equilibrium of the hydrogel composite increased with the addition of the rice husk ash and the highest value was found for the composite with rice husk ash calcinated at 900 C. Salt solution and pH also influenced the responsive behavior of the hydrogel composites, showing that high-performance materials could be obtained with the incorporation of calcinated rice husk ash in the hydrogel composites. The use of chitosan/poly(acrylic acid)/rice husk ash composites for removal of methylene blue from contaminated wastewater was proposed by Vaz et al. [25], resulting in high removal efficiency. The initial methylene blue concentration, solution pH, and contact time in the adsorption of methylene blue influenced the adsorption process, with the optimal condition reached for the composite with 5 wt.% of rice husk ash. 9.3.1.6 Carnauba Pereira et al. [26] evaluated biocomposites of chitosan reinforced with carnauba straw powder, at concentrations of 10 and 50 wt.%. The powder used had an average particle size between 0.15 and 0.10 mm and presented cellulose, hemicellulose, and lignin contents. The samples were submitted to two different treatments: alkaline treatment using sodium hydroxide and treatment with hexane. It was possible to observe that the addition of carnauba powder caused a decrease in tensile and strain at break of biocomposites when compared to the results of the chitosan matrix. Alkali treatment of powder contributed positively to the mechanical properties, since they removed surface impurity and hemicellulose partially and, for this reason, caused the improvement in strength. The authors concluded that chitosan biocomposites

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reinforced with carnauba straw powder could be an alternative to chitosan film with cost reduction. 9.3.1.7 Barley Fortunati et al. [27] studied poly(vinyl alcohol) and chitosan nanocomposite films reinforced with cellulose nanocrystals obtained by acidic hydrolysis of chemically or enzymatically pretreated barley straw and husks. The addition of chitosan into the poly(vinyl alcohol) film reduced its mechanical properties, but the presence of the nanocrystals acted as reinforcement. The higher results for mechanical properties were found for the nanocomposites with cellulose nanocrystals from enzymatically pretreated barley straw. Also, the nanocomposites presented inhibitions on fungal and bacterial development, showing its potential to be used in industrial, biomedicine, and tissue engineering sectors. 9.3.1.8 Oil palm Thinking of cadmium ions removal from aqueous solutions, Rahmi et al. [6] investigated chitosan reinforced with cellulose particles isolated from oil palm empty fruit bunch, at different concentrations (10, 30, and 50 wt.%). The highest value for the tensile strength was found for the composite with 10 wt.% of cellulose particles, after this concentration of particles the tensile strength decreased. Additionally, the authors studied the influence of contact time and pH on adsorption capacity of the composite and they concluded that the chitosan/cellulose composite film has high performance for cadmium ions removal from polluted water. 9.3.1.9 Sisal Almeida et al. [28] evaluated biocomposites made of chitosan and sisal as a renewable source. They observed a good adhesion and interaction between chitosan matrix and cellulose by scanning electron microscopy. The crystallinity index increased with the addition of sisal component, showing that the biocomposite is less amorphous compared to the pure chitosan matrix. 9.3.1.10 Date palm Adel et al. [29] studied chitosan bionanocomposite films reinforced with oxidized nanocellulose from date palm sheath fibers. The influence of the cellulose I and cellulose II polymorphs on the properties of the bionanocomposites were analyzed. Furthermore, the authors examined the importance of the reaction parameters on the nanocellulose extraction process and the bionanocomposites were made at the optimum conditions. The presence of the oxidized nanocellulose enhanced the

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mechanical properties of the chitosan matrix, and the bionanocomposite with oxidized nanocellulose I presented the highest mechanical properties. However, the bionanocomposite with oxidized nanocellulose II showed the highest thermal stability. The final material has the potential to be used as packaging material. 9.3.1.11 Pineapple Ninjiaranai [30] analyzed the influence of pineapple leaf fibers with 5 mm length treated with an alkaline solution on biopolymer films of chitosan and polyethylene glycol 6000 (used as plasticizer). Biopolymer films reinforced with fibers presented better results for moisture content and tensile strength but the lowest value of elongation at break. The developed material has potential use in food packaging. Wongkom and Jimtaisong [31] investigated a biocomposite of carboxymethyl chitosan and pineapple peel carboxymethylcellulose as a sunscreen carrier. Titanium dioxide (TiO2) and phenylbenzimidazole sulfonic acid (PBSA) were used as sunscreen agents. The ferulic acid was used as a cross-linker and the optimal ratio of carboxymethyl chitosan: carboxymethylcellulose: ferulic acid was found to be 2:1:4 wt.%. The authors suggested that the cross-linking could occur at the amine group of chitosan and the carboxyl group of ferulic acid. The best results were found for the ratio of TiO2:PBSA at 2:1 wt.% as it provided an SPF value of 2.00 and higher UV protection. Thus the authors concluded that the biocomposite obtained could be used as a hydrophilic sunscreen carrier. 9.3.1.12 Cotton Bierhalz and Moraes [32] studied composites of alginate and chitosan reinforced with cotton or linen (0.4 g/g of polymeric solution) in two different geometrical orientations: crossed (long fibers) and at random orientation (short fibers), aiming at wound coatings application with improved mechanical properties. Additionally, the authors studied dense composites and porous composites. For all the composites, a reduction of tensile strength, vapor transmission rates, and capacity for liquid uptake was found. However, for the composites with cotton or linen in the crossed orientation, the elongation at break increased significantly. Besides, composites with linen showed the lowest toxicity to human fibroblasts and the presence of epidermal growth factor increased cell proliferation. Raza et al. [33] evaluated cotton fabric impregnated with chitosan/ zinc oxide nanocomposites. It was found that after coating, air permeability and tensile strength decreased. On the other hand, UVA and UVB radiation blocking was better for coated specimens. Moreover, there was no bacterial growth under the contact area in coated cotton

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specimens. Busila et al. [34] also investigated the application of composite coatings on textile fabrics. The coating composite contained ZnO, Ag, and Ag:ZnO in chitosan matrix and they were applied on plainweave fabrics made of scoured and bleached cotton and on blended polyester/cotton. It was observed that all samples presented antimicrobial activity, in particular Ag:ZnO/chitosan composite coating, showing potential applications in medical materials and in the food industry as packaging. Li et al. [35] reported the properties of nanocomposite films of chitosan reinforced with cellulose whiskers obtained from cotton linter pulp, at contents of 5, 10, 15, 20, 25, and 30 wt.%. The authors observed that the incorporation of cellulose whiskers up to 15
20 wt.% in the chitosan matrix improved the mechanical properties of the nanocomposites. The inclusion of fillers improved the water resistivity of nanocomposites as well as their thermal stability, due to the formation of strong filler
matrix interactions. The chitosan
cellulose whiskers are reported to have potential applications in the field of food packaging. Soni et al. [36] developed bionanocomposite films of chitosan matrix reinforced with 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized (TEMPOoxidized) cellulose nanofibers, isolated from cotton stalks, at several contents (0, 5, 10, 15, 20, and 25 wt.%). The addition of nanofibers reduced the moisture and oxygen transmission properties of the films. The Young’s modulus and tensile strength of the nanocomposites increased with the increasing content of nanofibers, with the highest values found for the nanocomposite reinforced with 25 wt.% of nanofibers. Additionally, physicochemical and antimicrobial properties of the bionanocomposite films with TEMPO-oxidized cellulose nanofibers were studied [37]. The biocomposites presented higher thermal stability when compared to chitosan matrix. Moreover, the biocomposites showed antimicrobial activity against food pathogenic bacteria and antioxidant activity. The authors obtained flexible bionanocomposites with potential in food packaging applications. 9.3.1.13 Banana Kamel et al. [38] reported nanocomposites of chitosan reinforced with banana peel powder with different concentrations (0, 2, 5, and 10 wt.%) with particle size in the range of 30
40 nm in length and 3
10 nm in width. Nanocomposites with 5 and 10 wt.% of powder presented irregular distribution in the matrix and filler aggregates. The addition of banana peel powder decreased the swelling behavior of chitosan, but synergistic action had the highest antimicrobial activity at 10 wt.% of banana peel powder in chitosan matrix, showing potential for wound dressing application.

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Prasetyo et al. [39] studied a biocomposite of waste polypropylene filled with pseudostem banana fiber and montmorillonite. They studied biocomposites without chitosan, with untreated chitosan and with chitosan treated with glycidyloxypropyltrimethoxysilane. Flammability was higher in biocomposites without chitosan and with untreated chitosan when compared to treated chitosan. On the other hand, treated chitosan biocomposites showed the best tensile strength. The addition of coupling agents improved the fiber
polymer matrix interfacial adhesion. Also, treated biocomposites had a lower degradation rate. Therefore the authors concluded that the addition of glycidyloxypropyltrimethoxysilane on chitosan presented a positive influence on the mechanical and thermal properties of the biocomposites. 9.3.1.14 Durian Wong and Chan [40] investigated the biocomposite films of chitosan reinforced with durian husk cellulose. Different contents of cellulose were analyzed as well as a bleaching treatment using hydrogen peroxide. Biocomposites with bleached cellulose had better mechanical and thermal properties than those with unbleached cellulose. Also, elongation at break of the chitosan matrix was higher than the values presented by the biocomposites, but the modulus of elasticity of the biocomposites was higher than chitosan matrix’s modulus. The developed material could be an alternative to replace petroleum-based polymer. 9.3.1.15 Bamboo Luo et al. [41] analyzed a composite adsorbent of dissolved fibers of bamboo shoot shell blended with chitosan. The authors found that the developed composite presented better Cu21 adsorption than chitosan films and that after five times of use, the adsorption capacity of the composite did not change significantly, showing potential use as a sorbent for the removal of Cu21 from aqueous solution. Composite films of bamboo fiber and different N,O-carboxymethyl chitosan content (1, 3 and 5 wt.%) were developed by Zheng et al. [42]. The roughness and the antibacterial activity of the composites increased with the increasing of the N,O-carboxymethyl chitosan content, presenting promising characteristics for applications in antibacterial material. Liu and Xu [43] also investigated the antibacterial activity of chitosan cross-linked bamboo pulp fabric composite. The material showed antibacterial properties but lower thermal stability and decreased mechanical strength than original bamboo pulp fabric. Llanos and Tadini [44] studied chitosan nanocomposite films reinforced with bamboo nanofibers in concentrations of 0.5 and 1.0 g/100 g of polymer. The tensile strength and the elastic modulus of the nanocomposites decreased with the presence of the nanofibers, but the

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elongation at break increased. Furthermore, water vapor permeability of chitosan films was not significantly affected by the presence of the nanofibers. 9.3.1.16 Sugarcane Sharma and Bajpai [45] prepared microwave-assisted nanocomposite of chitin and sugarcane bagasse in the presence of montmorillonite clay. They studied the optimum reaction conditions and the nanocomposite showed antimicrobial activity and superabsorbent behavior. The material exhibits a higher swelling degree after each cycle and could be used in areas of water scarcity, serving as an irrigation source. Polyvinyl alcohol and chitosan bionanocomposite films reinforced with different cellulose nanocrystals contents (0.5, 2.5 and 5 wt.%) extracted from sugarcane bagasse, were developed by El Miri et al. [46]. The presence of the nanocrystals improved Young’s modulus, tensile strength, toughness, and thermal stability of the nanocomposite films, but decreased elongation at break. The highest values were found for the bionanocomposite reinforced with 5 wt.% of cellulose nanocrystals. Bansal et al. [47] studied chitosan matrix reinforced with cellulose nanofibers extracted from bagasse. Nanofibers were grafted onto chitosan through a Schiff base reaction and cellulose acetate was used as a binder. The Young’s modulus of the film decreased, but the elongation at break increased when compared to the chitosan matrix. Additionally, the composite presented antimicrobial activity that was sustained over 6 months, showing that the final material has potential in packaging material applications. 9.3.1.17 Wood Ji and Guo [48] prepared chitosan
lignin adhesives mixed with wood fibers resulting in a medium density fiberboard (MDF) made by hot-press. The chitosan
lignin adhesive content varied from 2 to 10 wt. % (by weight based on the solid content of the MDF specimen) and lignin/chitosan weight ratios of 1:2, 1:1, and 2:1 were assessed. The presence of chitosan improved the mechanical properties and the water resistance of the adhesive, and the optimal formulation was found to be 6% of chitosan
lignin content and lignin/chitosan weight ratio of 1:2. The authors obtained environment-friendly adhesives with improved properties. A similar study was conducted by Ji et al. [49] with chitosan and wood fibers, reaching a material with specific bonding strength and water resistance superior to the commercially available product. 9.3.1.18 Paper Guibal et al. [50] developed sponge-like structures of chitosan and cellulose fibers (extracted from Ahlstrom raw paper substrate) for the

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binding of silver ions. The cellulose fibers acted as a reinforcing structure since their presence prevented chitosan foam collapse. Also, there was an improvement in the sorption properties for silver ions on the composites with the presence of the fibers, which resulted in excellent antibacterial properties. The sponge-like composites could be used for biomedical or water disinfection applications. 9.3.1.19 Lignocellulose Biocomposites of chitosan cross-linked with glutaraldehyde, containing lignocellulose and multiwalled carbon nanotubes were investigated by Wang et al. [51]. The results showed that the final composite presented improved mechanical strength, dimensional stability, and fire retardancy when compared to the results given by the lignocellulose composite itself, being a promising candidate for green wood-based composite materials. Zhou et al. [52] reported a magnetized chitosan-coated lignocellulose fiber intending to remove acidic azo colorants from the water environment. It was found that the mass of chitosan deposited in lignocellulose fibers was about 48%. The biocomposite presented a good adsorption capacity reaching saturation quickly and it could be reused without losing its adsorption capacity. Thus the developed biocomposite showed the potential to be used on the removal of acidic azo colorant from wastewater. Solikhin et al. [53] studied poly(vinyl alcohol)/chitosan nanocomposites reinforced with several concentrations of amorphous lignocellulose nanofibers (0, 0.5, 1, 2.5, 5, 7.5, and 10 wt.%), aiming at food packaging applications. The nanocomposites reinforced with fiber concentration higher than 0.5 wt.% were rough and irregular due to aggregated fibers because of their hydrophilic character. On the other hand, the addition of 0.5 wt.% of fibers produced materials with a smooth external surface and enhanced the thermal stability and the tensile strength of the films.

9.3.2 Animal fibers 9.3.2.1 Feathers Flores-Herna´ndez et al. [54] produced biocomposites of chitosan
starch reinforced with keratin from feathers (long fibers, short fibers, and ground quill), at contents of 5, 10, 15, or 20 wt.%, by casting. It was noted that short fibers and ground quill were well dispersed in the matrix, but long fibers in composites exhibited nonhomogenous distribution and saturation appearance. Short fibers/long fibers/ground quill and matrix had a good interaction since they were fractured with the matrix and it was possible to note that

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thermal behavior of the composites was improved with ground quill reinforcement. The influence of the keratin treatment with sodium hydroxide was also investigated [55]. Storage modulus for composites with treated keratin material was improved, with the composite with 10 wt.% of chemically treated short fiber presenting the highest storage modulus. In addition, the composites reinforced with treated keratin material showed excellent biodegradable properties, offering the potential to be used as films in food packaging. Flores-Hernandez et al. [56] also developed starch and chitosan matrix reinforced with 5 or 10 wt.% of short keratin biofibers or ground quill, treated or not with sodium hydroxide produced by extrusion. The water solubility of the films decreased with the increase of keratin concentration and the lowest weight loss was found for the composite reinforced with 10 wt.% of the treated quill. The highest values of elastic modulus and maximum strength were found for the composites reinforced with 10 wt.% of ground quill and the effect of the treatment on the mechanical properties of the composites was evident for the composites with 5 wt.% of reinforcement. Spiridon et al. [57] studied polylactic acid and chitosan matrix reinforced with keratin fibers (average diameter of 50 μm and length ratios of 0.1
0.2 cm) varying the composition of the fiber (2% and 4%). The presence of keratin fibers decreased the mechanical properties of the composites but increased their thermal stability. Upon accelerated weathering, the presence of the fibers in the composites increased their elongation at break. 9.3.2.2 Wool Yu et al. [58] reported polyaniline and chitosan deposited onto the surface of wool fabric. Oxygen plasma pretreatment was used to improve the binding force among the components and to produce a more uniform conductive layer. The composite showed high conductivity and antibacterial properties even after 10 washes, indicating that the functionalities of wool fabric were improved. Shi et al. [59] analyzed wool fabric treated with biocomposite emulsion of chitosan and polyurethane. The molecular weight of chitosan and the effect of the concentration of chitosan in the composite polymer mixture affected the area shrinkage of wool fabric, showing that the dose of polyurethane could be reduced. Chitosan and polyurethane films presented higher stress at break and better degradability than pure polyurethane films, showing the reinforcing effect by the addition of chitosan. Chitosan was also used to treat alkali-damaged wool fabric [60], reducing the hydrophilicity of the alkali-treated fiber surface, creating a protective hydrophobic surface. The presence of chitosan was responsible for

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improving the tensile strength of the material; however, the wool fabric without any modification presented the higher value. Niu et al. [61] studied wool fibers grafted with chitosan-coated Agloading nano-SiO2 composite under UV irradiation. The force, tensile strength, and elongation at break of the final material were higher than the results found for the original wool. Furthermore, the authors studied the antibacterial and washable properties of the wool with and without the presence of the chitosan in the composite. After repeated washing, the durability of the material with the presence of the chitosan was superior to that without chitosan, also showing antibacterial properties, proving the importance of the presence of chitosan. Additionally, wool fibers modified by UV irradiation and reacted with chitosancoated Ag-loading nano-SiO2 composite had better dyeing properties than the original and UV-irradiated wool fibers [62]. The UV irradiation process was essential to form active radicals that reacted with the composite, improving the antibacterial properties of the wool fiber. Fan and Yu [63] prepared a cortical cells/chitosan biocomposite film by casting the homogeneous mixture of cortical cells extracted from wool fiber and chitosan
acetic acid solution varying the content of cortical cells. The addition of cortical cells resulted in an increased decomposition temperature of the biocomposite and a higher tensile strength due to a proper surface adhesion between cortical cells and chitosan. The authors considered that the results obtained were effectively improved by cortical cells and concluded that the film could be used in several fields of tissue engineering. 9.3.2.3 Silk fibroin Silk fibroin (SF) is a fibrous protein produced by the silkworm. It is possible to obtain up to 1000 m of single-stranded fibroin fibers by removing sericin by immersion in boiling water or alkaline solution [64]. Also, fibroin has excellent mechanical properties, high tensile strength, high elongation, elasticity, and biocompatibility, which allows its application in the area of biocomposites. Chitosan composite scaffolds for cartilage tissue engineering were developed with SF microfibers [65] and electrospun silk fibers [66] as reinforcement. It was found that the presence of the fibers decreased the water uptake capacity but increased the mechanical properties of the scaffolds. Also, the scaffolds were not cytotoxic to cells, showing that they could be used for cartilage tissue engineering. A chitosan matrix reinforced with short silk fibers of 3 mm length was studied by Ramya and Sudha [67] for removal of cadmium ions and copper ions from aqueous solution. The authors found that the composite had a good affinity with the ions and that the contact time, pH, and initial concentration of the ions influenced the adsorption. The

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studied composite showed characteristics of a suitable adsorbent of heavy metal ions from the wastewater stream. Our research group also developed biocomposites of chitosan with SF fibers. Two types of fibroin fibers were used: (1) fibers extracted directly from the cocoons of Bombyx mori silkworm, here called “extracted fibers” and (2) fibers supplied by silk spinning company (Bratac, Brazil), type Gregia, which consists of the physical combination of at least seven fibroin filaments extracted from seven different cocoons, here called “processed fibers.” The biocomposites were prepared by adding SF fibers with lengths of 1.0 or 0.2 cm and mass fractions of 20% or 60% into chitosan solution 0.1 g/L using the solvent casting method, followed by cross-linking with NaOH 1 mol/L for 24 h. Both SF fibers were nontoxic to cells (Fig. 9.7A) showing that they could be used in medical and pharmaceutical devices. Also, extracted fibers showed higher hydrophilic character (swelling of 12.1 6 0.5 g of PBS/100 g fiber) than processed fibers (swelling of 6.4 6 0.3 g of PBS/ 100 g fiber), which resulted in nonuniform biocomposites, with fibers agglomeration in many regions (Fig. 9.7B). Regarding the mechanical properties, the strength at break of chitosan did not improve with the addition of SF fibers, regardless of the

FIGURE 9.7 Biocomposites of chitosan and silk fibroin (processed and extracted fibers) prepared by casting. (A) Results of cytotoxicity; (B) photographs of biocomposites containing 60% of fibroin fibers of 0.2 cm in length; and (C) tear propagation force of biocomposite reinforced with processed fibers.

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type of fiber, length, or mass fraction. On the other hand, the addition of 60% of processed fibers of 0.2 cm in the chitosan matrix increased the tear propagation force by 20 times when compared to the result found for chitosan matrix, from 0.08 6 0.005 to 1.57 6 0.63 N (Fig. 9.7C), showing a significant mechanical improvement, remarkable for application as high-performance wound dressings.

9.4 Conclusion Studies of chitosan biocomposites have significantly grown in the last 20 years with reinforcements of animal and, especially, vegetable fibers. The search for entirely natural materials has been highly valued in recent years; in this sense, studies with materials of plant and animal origin as fillers have a greater environmental appeal, mainly because some of them are linked to the use of agribusiness residues. In this chapter, we revised several recent publications that used animal or vegetable fibers in the reinforcement of chitosan. The improvement in mechanical properties still stands out; however, improvements in thermal and antibacterial properties, as well as in the capacity of binding pollutants (such as dyes and heavy metals) were also observed. Regarding the applications of the biocomposites, food packaging predominates, as well as wastewater treatment and wound dressings.

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Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A AAm. See Acrylamide (AAm) Abiotic factors, 198 199 Acetamide group, 248 Acetic acid, 251 252 Acetylation of fibers, 280 Acrylamide (AAm), 65 66 Acrylic acid, 249 Active packaging, 206 208 AFM. See Atomic force microscopy (AFM) Agricultural applications of chitin and chitosan, 211 213 AIT. See Allyl isothiocyanate (AIT) Al2O3/chitosan nanocomposites characterization of, 151 155 DSC, 155 FTIR, 151 152 SEM, 154 XRD, 153 154 kinetics of thermal degradation, 155 170 degradation model, 166 170 estimation of degradation mechanism, 158 theoretical background, 156 158 thermal degradation kinetics, 161 166 thermogravimetric analyses, 158 161 preparation, 151 Alcohols, 234 235, 234f Alginate, 68, 105, 106f Aliphatic isocyanates, 236 Alkali treatment, 279 280 1-Ally-3-methylimidazolium bromide (AMIMBr), 50, 50f Allyl isothiocyanate (AIT), 207 208 1-Allyl-3-methylimidazolium acetate (AMIMOAc), 48 49 α-chitin, 62 63 AMIMBr. See 1-Ally-3-methylimidazolium bromide (AMIMBr) AMIMOAc. See 1-Allyl-3methylimidazolium acetate (AMIMOAc)

Amines, 234, 234f Animal fibers, 278, 290 294. See also Plant fibers feathers, 290 291 silk fibroin, 292 294 wool, 291 292 Antheraea mylitta. See Nonmulberry silk fibroin (Antheraea mylitta) Antibacterial process, 30 31 Antimicrobial activities, CN and CS application in, 23 24 Antimicrobial properties of chitin and chitosan, 189 202 abiotic factors, 198 199 antimicrobial mechanisms, 201 202 binding of low molecular weight of chitosan with DNA and RNA, 202 chitosan derivatives, 199 200 DD, 196 gram-negative bacteria, 197 gram-positive bacteria, 197 metal chelation by amine groups in chitosan chain, 202 molecular weights, 200 parameters affecting, 196 200 sources, 200 stimulation of secondary cellular effects, 201 202 surface charge density of bacteria membrane, 197 198 of films, 264 267 Antioxidant properties of chitin and chitosan, 202 205 antioxidant assays, 204 205 deacetylation period, 203 mechanism, 204 molecular weight, 204 parameters affecting, 203 204 of films, 267 268 Arsenic removal in water treatment process, 29 Aspergillus fumigatus CCRC 30502, 196

299

300

Index

Aspergillus parasiticus CCRC 30117, 196 Atom transfer radical polymerization (ATRP), 56 Atomic force microscopy (AFM), 121 125 AuNPs. See Gold NPs (AuNPs) Avrami Erofeev model, 167 170

B Bac-Shield, 189 Bacillus cereus CCRC 10250, 196 Bacillus subtilis, 22 23 Bamboo, 288 289 Banana, 287 288 Barley, 285 Barley straw arabinoxylan (BSAX), 207 208 Barrier properties of chitin and chitosan, 179 189 development, 186 189 parameters affecting, 181 185, 181f CED, 182 chemical structures, 181 182 cross-linking, 185 crystallinity, 183 184 degree of dispersion of platelets, 185 free volume, 182 183 orientation, 184 permeability tests, 185 186 BB. See Bismarck brown Y (BB) BDO. See 1,4-Butanediol (BDO) Beauveria bassiana, 249 Bentonite/chitosan characterization of, 151 155 DSC, 155 FTIR, 151 152 SEM, 154 XRD, 153 154 kinetics of thermal degradation, 155 170 degradation model, 166 170 estimation of degradation mechanism, 158 theoretical background, 156 158 thermal degradation kinetics, 161 166 thermogravimetric analyses, 158 161 preparation of, 142 151 β-chitin, 62 63 β-glycerophosphate (β-GP), 69 Biobased polymers, 2 Biocomposites of chitin/chitosan with natural fibers chitin/chitosan biocomposites with natural fibers, 280 294 fundamentals on natural fibers, 277 280

number of publications about chitin/ chitosan biocomposites, 277f percentage of publications on chitin/ chitosan biocomposites, 275f Biodegradable film production methods, 248 250 biodegradable materials application to produce polymers, 250 biopackaging production methods, 250 combination of biodegradable materials with synthetic polymers, 248 250 Biodegradable polymers, 98 Biodiversity, 62 Biofilm production, 250 Biofragmentation, 249 Bionanocomposites, 285 286 Biopackaging production methods, 250 Biopolymer-based stimuli-responsive CS, 13 Biopolymers, 2, 97 98, 98f application combined base materials, 249 250 as filling materials, 249 Bismarck brown Y (BB), 125 126 Biurethane cross-links, 236 Bleeding control, CN and CS application in, 35 36 BLG-NCA. See γ-benzyl L-glutamate-NCA (BLG-NCA) BMIMCl. See 1-Butyl-3-methylimidazolium chloride (BMIMCl) Bombyx mori silkworm, 293 Box Behnken design, 10 BSAX. See Barley straw arabinoxylan (BSAX) 1,4-Butanediol (BDO), 237 240 1-Butyl-3-methylimidazolium chloride (BMIMCl), 48

C Cadmium ions, 285 Calcium carbonate, 189 Callicarpa nudiflora, 25 Camellia sinensis, 212 213 Candida albicans CCRC 20511, 196 Carbon nanofibers (CNs), 71 Carbon nanotubes (CNTs), 71 Carboxylic acids, 234 235, 235f Carboxymethyl cellulose (CMC), 9, 53 54, 186, 266 Carboxymethyl hexanoyl CS (CHC), 9 Carnauba, 284 285

Index

Carrageenan, 106, 107f Cationic dyes, 125 126 CED. See Cohesive energy density (CED) Cellulose, 229 232, 278 279 Cellulose nanofibers (CNF), 265 Cellulose nanowhiskers (CNW), 80 81 CEO. See Clove essential oil (CEO) CHC. See Carboxymethyl hexanoyl CS (CHC) Chemical methods of fiber treatment, 279 280 Chitin (CN), 2, 47 48, 48f, 62 63, 98 101, 99f, 140 141, 140f, 176, 229 231, 230f, 248, 274, 275f antimicrobial properties, 189 202 antioxidant properties, 202 205 applications, 16 36, 18t, 125 127, 205 214 in antimicrobial activities, 23 24 in bleeding control, 35 36 in drug release, 35 in food packing, 17 23 in water treatment process, 26 31 in wound-healing activities, 24 26 barrier properties, 179 189 blends characteristics, 107 110, 107f classification, 107f, 109f blends and composites processing, 4 16 coprecipitation method, 15 electrospinning, 11 12 lyophilization, 10 11 melt extrusion method, 14 15 phase inversion/separation process, 16 polymerization, 15 16 simple blending method, 4 10 solution/solvent casting method, 12 14 spray drying and freeze spray drying, 11 techniques for, 5t characterization, 151 155 DSC, 155 FTIR, 151 152 SEM, 154 XRD, 153 154 chitin-based gel properties, 104 107 chitin-based PUs, 229 230 characterization and applications, 240, 241t chitin and chitosan, 230 231 development methods for, 236 240

301

polyols in synthesis of, 239t PUs, 231 236 chitin/chitosan biocomposites with natural fibers, 280 294, 281f animal fibers, 290 294 plant fibers, 281 290 composites study, 110 111 dissolution and gelation of chitin with ionic liquids, 48 52 fabrication of chitin-based blend and composite materials, 52 56 fillers, 188 189 insect sources, 177, 178t kinetics of thermal degradation, 161 marine sources, 177 179, 180t preparation, 142 151 Chitin nanowhiskers (ChNW), 186 Chitoderm, 189 Chitosan (CS), 2, 62 63, 98 101, 98f, 140 141, 140f, 176, 178t, 229 231, 230f, 248, 274, 275f aerogel, 114 antimicrobial properties, 189 202 antioxidant properties, 202 205 applications, 16 36, 18t, 125 127, 205 214 in antimicrobial activities, 23 24 in bleeding control, 35 36 in drug release, 35 in food packing, 17 23 tissue engineering, 31 35 in water treatment process, 26 31 in wound-healing activities, 24 26 barrier properties, 179 189 blends and composites processing, 4 16 coprecipitation method, 15 electrospinning, 11 12 lyophilization, 10 11 melt extrusion method, 14 15 phase inversion/separation process, 16 polymerization, 15 16 simple blending method, 4 10 solution/solvent casting method, 12 14 spray drying and freeze spray drying, 11 techniques for, 5t blends characteristics, 107 110 characterization, 101f, 151 155 DSC, 155 FTIR, 151 152 SEM, 154

302

Index

Chitosan (CS) (Continued) XRD, 153 154 chitosan-acetylated jute film, 282 283 chitosan-based blends, 69 71 melt blending, 76 natural polymer blended chitosan, 70 71 processing techniques for, 75 83 solution blending, 75 76 synthetic polymer blended chitosan, 69 70 chitosan-based composites and nanocomposites, 71 73 natural filler/chitosan-based composites and nanocomposites, 72 73 synthetic filler/chitosan-based composites and nanocomposites, 71 72 chitosan-based gels, 66 69, 102 111 chitosan gels without external crosslinkers, 66 67 ionic chitosan macrogels, 67 68 ionic chitosan micro-and nanogels, 68 properties, 104 107 thermosensitive gels, 69 chitosan-based IPN, 63 66 IPN based on CS and natural matrices, 66 IPN based on CS and synthetic ionic matrices, 64 65 IPN based on CS and synthetic nonionic matrices, 65 66 chitosan-based nanocomposites, 71 73 electrospinning, 81 83 freeze-drying, 79 80 layer-by-layer techniques, 80 81 mechanical stirring, 77 processing techniques of, 76 83 solvent casting, 77 79 chitosan/nanoclay films, 258 chitosan/poly(acrylic acid)/rice husk ash composites, 284 chitosan polyvinyl alcohol films, 258 chitosan thymol nanoparticles, 259 composite scaffolds, 292 composites study, 110 111 CS-based PUs, 229 230 characterization and applications, 240, 241t chitin and chitosan, 230 231 development methods for, 236 240

polyols in synthesis of, 239t PUs, 231 236 derivatives, 199 200 insect sources, 177 kinetics of thermal degradation, 161 marine sources, 177 179 preparation, 142 151 processing techniques for CS-based IPNs and gels, 73 75 cross-linking, 74 photopolymerization, 73 74 physical interaction, 74 75 properties, 102f sources and extraction, 3f structure, 177f ChNW. See Chitin nanowhiskers (ChNW) Cinnamaldehyde (CNE), 267 268 CL. See ε-caprolactone (CL) Clove essential oil (CEO), 267 CMC. See Carboxymethyl cellulose (CMC) CN. See Chitin (CN) CNE. See Cinnamaldehyde (CNE) CNF. See Cellulose nanofibers (CNF) CNs. See Carbon nanofibers (CNs) CNTs. See Carbon nanotubes (CNTs) CNW. See Cellulose nanowhiskers (CNW) Coating technology, 208 209 Cohesive energy density (CED), 182 Coir pith activated carbon (CPAC), 28 Collagen (COL), 9 collagen-based biomaterials, 70 71 Composites, 273 274 Contact angle of water drop on film surface, 263 264 Coprecipitation method, 15 Cosmeceutical applications of chitin and chitosan, 213 214 Cotton, 286 287 CPAC. See Coir pith activated carbon (CPAC) CRABYON, 189 Cross-linking gel, 74 Cryo-SEM, 112 113 Crystal violet (CV), 125 126 Crystallinity, 183 184 CS. See Chitosan (CS) CT. See Chitin (CN) CV. See Crystal violet (CV)

D Date palm, 285 286 DBTDL. See Dibutyltin dilaurate (DBTDL)

Index

DD. See Degree of deacetylation (DD) Deep eutectic solvents (DESs), 48 49 Degradable COL-CS composite materials, 34 35 Degradable materials, 248 249 Degradation mechanism estimation, 158 Degradation model, 166 170 Degree of deacetylation (DD), 99, 158 159, 196, 251 252 Degree of dispersion of platelets, 185 Demineralization, 231 Deodorization, 231 Deproteinization, 231 DESs. See Deep eutectic solvents (DESs) Dextran sulphate, 106, 106f Di-hydroxyl stearic acid, 282 Dibutyltin dilaurate (DBTDL), 237 240 Dielectric spectroscopy, 276 277 Dietary fiber, 210 Dietary supplement applications of chitin and chitosan, 210 211 Differential scanning calorimetry (DSC), 141 142, 155, 259, 261 263, 263f Differential thermal analysis (DTG), 141 142 Differential thermogravimetric and thermal analyses (TGA/DTA), 156 Diisocyanates, 231 232 Dimethylol propionic acid (DMPA), 237 240 2,2-Diphenyl-1-picrylhydrazyl radical scavenging (DPPH scavenging), 204 205 Diphenylmethane 4, 4-diisocyanate (MDI), 237, 238t DMA. See Dynamic mechanical analysis (DMA) DMAc. See N,N-dimethylacetamide (DMAc) DMPA. See Dimethylol propionic acid (DMPA) DMTA. See Dynamic mechanical thermal analysis (DMTA) DPPH scavenging. See 2,2-Diphenyl-1picrylhydrazyl radical scavenging (DPPH scavenging) Drug delivery systems, 96 Dry method, biofilm production, 250 DSC. See Differential scanning calorimetry (DSC) DTG. See Differential thermal analysis (DTG)

303

Durian, 288 Dye removal, 28 29 Dynamic mechanical analysis (DMA), 141 142 Dynamic mechanical thermal analysis (DMTA), 259 263 DSC, 261 263

E EB. See Elongation at break (EB) Edible films, 10, 207 208 EG. See Ethylene glycol (EG) Elastic modulus (EM), 253 254 Elasticity, 292 Electron density, 234 Electrospinning, 11 12, 81 83, 209 210 Electrospun nanofibers, 11 12 Electrospun silk fibers, 292 Elongation at break (EB), 251 252 EM. See Elastic modulus (EM) EMIMOAc. See 1-Ethyl-3methylimidazolium acetate (EMIMOAc) ε-caprolactone (CL), 55 56 Escherichia coli, 17 22 E. coli ATCC 11775, 197 198 E. coli CCRD 10674, 196 Esterification method. See Acetylation of fibers 1-Ethyl-3-methylimidazolium acetate (EMIMOAc), 48 49 Ethylene glycol (EG), 252 Ethylene vinyl alcohol copolymers (EVOH copolymers), 207 EVOH copolymers. See Ethylene vinyl alcohol copolymers (EVOH copolymers) Exfoliation, 185 “Extracted fibers”, 293

F Feathers, 290 291 Ferric ion reducing antioxidant power (FRAP), 204 205 FESEM. See Field emission SEM (FESEM) FFV. See Fractional free volume (FFV) Fiber(s), 231, 276 fiber polymer matrix interfacial adhesion, 288 physical methods of fiber treatment, 279 Fick’s law, 180 181 Field emission SEM (FESEM), 119

304 Filling materials, biopolymers application as, 249 Films antimicrobial properties, 264 267 antioxidant property, 267 268 barrier properties, 255 259 permeability to gases, 255 256 WVP measuring method, 257 259 WVP of films, 257 functional properties, 251 268 mechanical properties, 251 255 effective factors on mechanical behavior of films, 251 253 measuring, 253 255 production method, 252 thermal properties, 259 263 contact angle of water drop on film surface, 263 264 DMTA, 260 263 methods of determination, 259 Flax, 281 282 Fluoride removal in water treatment process, 30 Food packing, CN and CS application in, 17 23 Formic acid, 251 252 Fourier-transform infrared spectroscopy (FTIR), 142, 151 152, 249 Fractional free volume (FFV), 182 183 FRAP. See Ferric ion reducing antioxidant power (FRAP) Free radicals, 203 Free volume, 182 183 Freeze spray drying, 11 Freeze-drying method, 79 80 FTIR. See Fourier-transform infrared spectroscopy (FTIR) Fusarium oxysporum CCRC 32121, 196 Fusarium wilt of tomato, 212 213

G

γ-benzyl L-glutamate-NCA (BLG-NCA), 55 56 γ-chitin, 62 63 Gas diffusion, 257 permeability, 255 256 effect of factors on gas permeability of films, 256 Gelatin (GL), 9 Gelatine nanoparticles (GNPs), 82 83 Gelatinized starch, 249

Index

Generally Recognized as Safe (GRAS) material, 2 3 Gentamicin, 209 GL. See Gelatin (GL) Glass transition temperature (Tg), 259 Glutaraldehyde, 290 Gluten glycerol/chitosan polyethylene glycol (Gly/Glut-CP), 283 284 GLY. See Glycerol (GLY) Gly/Glut-CP. See Gluten glycerol/ chitosan polyethylene glycol (Gly/ Glut-CP) Glycerol (GLY), 252, 282 GNPs. See Gelatine nanoparticles (GNPs) Gold NPs (AuNPs), 12 Gram-negative bacteria, 197 Gram-positive bacteria, 197 Graphene oxide, 189

H HA. See Hyaluronic acid (HA); Hydroxyapatite (HA) Halloysite nanotubes (HNT), 267 HAT. See Hydrogen atom transfer (HAT) HDA. See Hexa-1,6-diamine (HDA) HDI. See Hexane-1,6-diisocyanate (HDI) HEA. See 2-Hydroxyethyl acrylate (HEA) Heavy metal removal, 26 28 HEMA. See 2-Hydroxyethyl methacrylate (HEMA) Hemicellulose, 278 279 Hemolysis assays, 267 268 inhibition effect, 268 Hexa-1,6-diamine (HDA), 231 232 Hexane-1,6-diisocyanate (HDI), 231 232 HMMI. See N-hydroxymethylmaleimide (HMMI) HNT. See Halloysite nanotubes (HNT) Homologous polymer blend, 109 110 HTCC. See N-(2-hydroxyl) propyl-3trimethylammonium CS chloride (HTCC) Humic acid removal in water treatment process, 30 Hyaluronan, 68 Hyaluronic acid (HA), 105, 106f Hydrogen atom transfer (HAT), 204 205 Hydrogen peroxide (H2O2), 203 Hydroxyapatite (HA), 72 73, 77 79 2-Hydroxyethyl acrylate (HEA), 56

Index

2-Hydroxyethyl methacrylate (HEMA), 65 66, 73 Hydroxyl groups, 229 230, 236 Hydroxyl radical (•OH radical), 203 Hydroxypropyl chitosan nail lacquer, 214

I ICTAC. See International Confederation for Thermal Analysis and Calorimetry (ICTAC) IEP. See Isoelectric point (IEP) IL-8. See Interleukin-8 (IL-8) Inhibitory activity, 202 Insect sources of chitin and chitosan, 177, 178t Interleukin-8 (IL-8), 203 International Confederation for Thermal Analysis and Calorimetry (ICTAC), 164 166 Interpenetrating polymer network (IPN), 64, 102 chitin-based IPN, 102 104 chitosan-based IPN, 64 66, 102 104 IPN based on CS and synthetic ionic matrices, 64 65 and synthetic nonionic matrices, 65 66 Ionic chitosan macrogels, 67 68, 104 105 microgels, 68 nanogels, 68 Ionic liquids, 48 dissolution and gelation of chitin with, 48 52 imidazolium acetates and alkanoates, 49f fabrication of chitin-based blend and composite materials using, 52 56 IPN. See Interpenetrating polymer network (IPN) Isocyanates, 233 236, 234f reaction between carboxylic acid and, 235f reaction between urea and, 235f reaction between urethane and, 235f reaction between water and, 235f used in synthesis of chitin/chitosanbased PUs, 238t Isoelectric point (IEP), 64 65 Isomorphic polymer blends, 109 110

J Jute, 278, 282 283

305

K Kenaf, 282 Kinetics triplet, 156 Kyotocel, 189

L L-lactide (LA), 55 56 Lactobacillus bulgaricus IFO 3533, 197 198 Layer-by-layer techniques (LbL techniques), 80 81 LCST. See Lower critical solution temperature (LCST) LDL. See Low-density lipoprotein (LDL) Light-emitting devices (LEDs), 126 127 Lignin, 278 279 Lignocelluloses, 290 Lignocellulosic fibers. See Plant fibers Lipid autoxidation, 203 Lipid hydroperoxides (LOOH), 203 Listeria monocytogenes, 17 22 Listeria monocytogenes LM-LM, 196 LMWHA. See Low-molecular-weight hyaluronic acid (LMWHA) Long chain polysaccharide, 278 279 Low-density lipoprotein (LDL), 204 205 Low-molecular-weight hyaluronic acid (LMWHA), 9 Low-Mw chitosan, 204 Low-pressure superheated steam drying (LPSSD), 252 253 Lower critical solution temperature (LCST), 64 65 Lyophilization, 10 11

M Macramin, 199 Macromolecules, 96 Magnetic microsphere, 29 Mahua oil-based polyurethane composite films, 22 Maleimides (MI), 73 74 MAP. See Modified atmosphere packaging (MAP) Marine sources of chitin and chitosan, 177 179, 180t Mechanical stirring, 77 Medium density fiberboard (MDF), 289 MEK. See Methyl ethyl ketone (MEK) Melt blending, 76 Melt extrusion method, 14 15 Melting temperature, 276 277

306

Index

Mentha piperita L. essential oils (MPEO), 209 Mentha X villosa huds essential oils (MVEO), 209 Mercerization, 279 280 Mercury removal in water treatment process, 29 Metal chelation by amine groups in chitosan chain, 202 Methyl ethyl ketone (MEK), 237 240 MI. See Maleimides (MI) Micro X-ray computed tomography, 276 277 Microscopic study, 112 113 AFM, 122 125 applications of chitin and chitosan-based products, 125 127 biopolymers, 97 98 chitin and chitosan-based gels, IPN, blends, and composites, 102 111 chitosan and chitin, 98 101 future, 127 128 optical microscopy, 117 122 SEM, 113 116 TEM, 116 117 Mineral, 276 fibers, 278 Minimal lethal concentration (MLC), 196 MMT. See Montmorillonite (MMT) Modified atmosphere packaging (MAP), 255 Molecular weight (Mw), 179, 200 Monomers, 96 Montmorillonite (MMT), 72 73, 207 MPEO. See Mentha piperita L. essential oils (MPEO) MVEO. See Mentha X villosa huds essential oils (MVEO) Mw. See Molecular weight (Mw)

N N,N,N-trimethyl chitosan chloride salt (TMC), 199 N,N-dimethylacetamide (DMAc), 47 48, 62 63 N,O-carboxymethylated chitosan, 200, 288 N-(2-hydroxyl) propyl-3trimethylammonium CS chloride (HTCC), 13, 82 N-acetylation, 62 63 N-furfuryl-N,N-dimethyl chitosan, 199 200 N-hydroxymethylmaleimide (HMMI), 73

N-isopropylacrylamide (NIPAAm), 65 66 1-N-phenylnaph-thylamine (NPN), 201 202 N-poly-methylated chitosan, 199 N-succinyl CS (NSC), 4 9 N-vinylpyrrolidinone (NVP), 73 Nano ZnO, 189 Nanocellulose extraction process, 285 286 Nanochitin, 186 Nanoclay, 207 Nanocrystalline cellulose (NCC), 284 Nanofiltration, 30 31 Nanohydroxyapatite, 9 Nanoparticles (NPs), 3 4, 189, 276 morphology, 114 115 Natural antibiotics, 200 Natural fibers, 277 280 chemical methods of fiber treatment, 279 280 chitin/chitosan biocomposites with, 280 294 physical methods of fiber treatment, 279 Natural filler/chitosan-based composites and nanocomposites, 72 73 Natural matrices, IPN based on CS and, 66 Natural polymers, 96, 107 blended chitosan, 70 71 Natural rubber (NR), 54 55, 70 71 NCC. See Nanocrystalline cellulose (NCC) Nigella sativa, 207 208 NIPAAm. See N-isopropylacrylamide (NIPAAm) Nitrate removal in water treatment process, 30 NMR. See Nuclear magnetic resonance (NMR) N N-propyl-N,N-dimethyl chitosan, 199 200 Noncontact mode, 123 Nonmulberry silk fibroin (Antheraea mylitta), 34 35 NPN. See 1-N-phenylnaph-thylamine (NPN) NPs. See Nanoparticles (NPs) NR. See Natural rubber (NR) NSC. See N-succinyl CS (NSC) Nuclear magnetic resonance (NMR), 259 NVP. See N-vinylpyrrolidinone (NVP)

O Octamethylene diisocyanate, 231 232 Oil palm, 285

Index

OP. See Oxygen permeability (OP) Optical microscopy, 112, 117 122 ORAC. See Oxygen radical absorbance (ORAC) Orientation, 184 OTR. See Oxygen transfer rate (OTR) Oxygen permeability (OP), 256 Oxygen radical absorbance (ORAC), 204 205 Oxygen transfer rate (OTR), 189, 190t

P P. fluorescens ATCC 14028, 197 198 PAA. See Poly(acrylic acid) (PAA) PAAm. See Poly(acrylamide) (PAAm) Packaging applications of chitin and chitosan, 205 210 active packaging, 206 208 coating technology, 208 209 electrospinning technology, 209 210 smart packaging, 208 of films biodegradable film production methods, 248 250 functional properties of films, 251 268 PAN. See Polyacrylonitrile (PAN) Paper, 289 290 PBS. See Poly(butylene succinate) (PBS) PBSA. See Phenylbenzimidazole sulfonic acid (PBSA); Poly(butylene succinate adipate) (PBSA) PBTA. See Poly(butylene terephthalate adipate) (PBTA) PCL. See Polycaprolactone (PCL) PDMAEM. See Poly(dimethylaminoethyl methacrylate) (PDMAEM) PEG. See Polyethylene glycol (PEG) PEGME. See Polyethylene glycol methyl ether (PEGME) PEO. See Polyethylene oxide (PEO) Permeability tests, 185 186 Peroxyl radical hemolysis, 268 PET. See Polyethylene terephthalate (PET) PG. See Propylene glycol (PG) PGA. See Poly(glutamic acid) (PGA) pH, 251 252 Phase inversion/separation process, 16 Phenylbenzimidazole sulfonic acid (PBSA), 286 Phosphate removal in water treatment process, 30

307

6-Phosphogluconic trisodium salt (6-PGNa1), 67 68 Photoinitiator (PI), 73 Photopolymerization, 73 74 Physical methods of fiber treatment, 279 PI. See Photoinitiator (PI) Pickering emulsion polymerization, 53, 53f Pineapple, 286 PLA. See Polylactic acid (PLA) PLA-PCL. See Polylactide-co-PCL (PLAPCL) Plant fibers, 278. See also Animal fibers bamboo, 288 289 banana, 287 288 barley, 285 carnauba, 284 285 cotton, 286 287 date palm, 285 286 durian, 288 flax, 281 282 jute, 282 283 kenaf, 282 lignocelluloses, 290 oil palm, 285 paper, 289 290 pineapple, 286 rice, 283 284 sisal, 285 sugarcane, 289 sunflower, 283 wood, 289 Plasticizers, 282, 286 type and concentration of, 252 PLLA. See Poly(L-lactic acid) (PLLA) PMAA. See Poly(methacrylic acid) (PMAA) Poly(1,8-octanediol citrate) (POC), 12 13 Poly(acrylamide) (PAAm), 65 66 Poly(acrylic acid) (PAA), 64 65 Poly(butylene succinate adipate) (PBSA), 76 Poly(butylene succinate) (PBS), 76 Poly(butylene terephthalate adipate) (PBTA), 76 Poly(dimethylaminoethyl methacrylate) (PDMAEM), 64 65 Poly(glutamic acid) (PGA), 69 70 Poly(L-lactic acid) (PLLA), 52 Poly(methacrylic acid) (PMAA), 64 65 Poly(N-acryloylglycine), 64 65 Poly(propylene oxide) glycol, 236 Poly(propylene/ethylene) glycols copolymers, 236

308

Index

Poly(β-(1 4)-N-acetyl-D-glucosamine), 62 63 Poly(γ-glutamic acid) (γ-PGA), 80 81 Poly[β-(1 4)-N-acetyl-D-glucosamine], 251 Polyacrylonitrile (PAN), 65 66 Polycaprolactone (PCL), 4 9, 69 70, 237 Polyether polyols, 236, 237f Polyethylene, 249 polyethylene-graft-maleic anhydride, 249 250 Polyethylene films, 249 Polyethylene glycol (PEG), 10, 65 66, 252 Polyethylene glycol methyl ether (PEGME), 3 4 Polyethylene oxide (PEO), 11 12, 209 Polyethylene terephthalate (PET), 14 15, 259 Polylactic acid (PLA), 11, 76 Polylactide-co-PCL (PLA-PCL), 11 12 Polymeric blends, 69 Polymerization, 15 16, 96, 236 237 Polymers, 96 97, 233 biodegradable materials application to produce, 250 blending, 75 nanocomposites, 76 77 Polyols, 236 in synthesis of chitin/chitosan-based polyurethanes, 239t Polysaccharides, 68, 97 98, 100 101, 231 Polyurethane (PU), 231 237 conventional synthesis of, 233f isocyanates, 234 236 polyols, 236 reactions to, 233 234 synthesis reaction, 232 types of PUs and applications, 233t Polyvinyl alcohol (PVA), 4 9, 52 53, 65 66, 69 70, 249, 285, 289 Polyvinyl pyrrolidone (PVP), 65 66, 75 76 Polyvinylidene fluoride (PVDF), 16 “Processed fibers”, 293 Propylene glycol (PG), 252 Pseudomonas aeruginosa, 82 83, 249 ATCC 27853, 197 CCRC 10944, 196 PU. See Polyurethane (PU) PVA. See Polyvinyl alcohol (PVA) PVDF. See Polyvinylidene fluoride (PVDF) PVP. See Polyvinyl pyrrolidone (PVP)

R Ramie, 278 Reactive oxygen species (ROS), 203 Reduced graphene oxide (rGO), 77 79 Relative moisture (RH), 255 Retinol, 128 Retting process, 279 rGO. See Reduced graphene oxide (rGO) RH. See Relative moisture (RH) Rice, 283 284 ROS. See Reactive oxygen species (ROS)

S Salmonella typhimurium, 197 198 S. typhimuruim ATCC 14082 S. typhimuruim CCRC 10746, 196 Scanning electron microscope (SEM), 112 116, 142, 154 Scanning tunneling microscopy, 276 277 SDBS. See Sodium dodecyl benzene sulfonate (SDBS) Second-harmonic generation (SHG), 117 118 Self-assembled chitin nanofibers, 53 self-assembled chitin nanofiberreinforced cellulose films, 54 SEM. See Scanning electron microscope (SEM) Semi-IPN (semi-IPN), 66 Serratia marcescens, 249 SET. See Single electron transfer (SET) SF. See Silk fibroin (SF) SHG. See Second-harmonic generation (SHG) Shigella dysenteriae CCR 13983, 196 Silanization, 280 Silk fibroin (SF), 292 294, 293f Silver nanoparticles, 189 Single electron transfer (SET), 204 205 Sisal, 285 Smart packaging, 208 Sodium dodecyl benzene sulfonate (SDBS), 264 Sodium hydroxide, 279 280, 284 285 Solution/solvent casting method, 12 14 Solvent casting technique, 77 79 Spray drying method, 11, 121 122 Stalks, 278 Staphylococcus aureus, 82 83 S. aureus CCRC 12652, 196 Starch chitosan films, 250

Index

starch chitosan flax fabric composite, 281 Step-growth processes, 236 237 Storage modulus, 260, 261f Sugarcane, 289 Sulfur nanoparticles without capping agent, 264 Sunflower, 283 Superoxide anion radical (O2•2radical), 203 Surface charge density of bacteria membrane, 197 198 Surface oxidation activation, 279 Synthetic ionic matrices, IPN based on CS and, 64 65 Synthetic nonionic matrices, IPN based on CS and, 65 66 Synthetic polymers, 96 biodegradable materials combination with, 248 250 biopolymers application of as filling materials, 249 application of biopolymers as one of combined base materials, 249 250 blended chitosan, 69 70

T Tapping mode, 122 TEA. See Triethylamine (TEA) TEAC. See Trolox equivalence capacity (TEAC) TEM. See Transition electron microscopy (TEM) TEMPO. See 2,2,6,6-Tetramethylpiperidine1-oxyl radical (TEMPO) TEMPO-CNFs, 265 266 TEMPO-oxidized. See 2,2,6,6Tetramethylpiperidine-1-oxyloxidized (TEMPO-oxidized) Tensile strength (TS), 248, 253 254 Tensile testing machine, 253 254, 253f Tetramethylene, 231 232 2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO), 265 2,2,6,6-Tetramethylpiperidine-1-oxyloxidized (TEMPO-oxidized), 287 TGA. See Thermogravimetric analysis (TGA) TGA/DTA. See Differential thermogravimetric and thermal analyses (TGA/DTA) Thermal degradation kinetics, 161 166 chitin and chitosan, 161

309

nanocomposites of 5-bentonite/chitosan and 10-Al2O3/chitosan, 161 166 Thermogravimetric analysis (TGA), 141 142, 158 161 chitin and chitosan, 158 159 nanocomposites of 5-bentonite/chitosan and 10-Al2O3/chitosan, 160 161 Thermoplastic method, biofilm production, 250 Thermosensitive gels, 69 Tissue engineering, CN and CS application in, 31 35 Titanium dioxide (TiO2), 286 TMC. See N,N,N-trimethyl chitosan chloride salt (TMC) Total radical-trapping antioxidant parameter (TRAP), 204 205 TPF. See Two-photon fluorescence (TPF) TPP. See Tripolyphosphate (TPP) Transition electron microscopy (TEM), 112, 116 117 TRAP. See Total radical-trapping antioxidant parameter (TRAP) Triethylamine (TEA), 237 240 Tripolyphosphate (TPP), 67 68 Trolox equivalence capacity (TEAC), 204 205 TS. See Tensile strength (TS) Two-photon fluorescence (TPF), 117 118

U Ultimate TS (UTS), 253 254 Urethane, 235f, 236 reaction between isocyanate and, 235f structure of urethane formation, 233f units, 231 232, 232f US Food and Drug Administration (USFDA), 2 3 UV irradiation, 292

V Vibrio chlolerea CCRC 13860, 196 Vibrio parahaemolyticus ATCC 17802, 197 198 Vibrio parahaemolyticus CCRC 1806, 196 Vinyl acetate, 249 Vinyl alcohol, 249

W Water treatment process, CN and CS application in arsenic and mercury removal, 29

310 Water treatment process, CN and CS application in (Continued) dye removal, 28 29 fluoride removal, 30 heavy metals removal, 26 28 nanofiltration and antibacterial process, 30 31 phosphate, nitrate, and humic acid removal, 30 Water vapor diffusion, 257 Water vapor permeability (WVP), 257 of films, 257 measuring method, 257 259 Water vapor transfer rate (WVTR), 189, 190t, 257 Wet methods, biofilm production, 250 Wood, 289

Index

Wool, 291 292 Wound-healing activities, CN and CS application in, 24 26 WVP. See Water vapor permeability (WVP) WVTR. See Water vapor transfer rate (WVTR)

X X-ray diffraction (XRD), 142, 153 154, 276 277

Y Young modulus, 254

Z Zeolite (ZE), 72 73 Zinc oxide, 189