Layered double hydroxide polymer nanocomposites 9780081019047, 0081019041

Table of contents :
Front Cover......Page 1
Layered Double Hydroxide Polymer Nanocomposites......Page 4
Copyright Page......Page 5
Contents......Page 6
List of contributors......Page 18
Preface......Page 22
Acknowledgement......Page 26
1.1 Introduction......Page 28
1.2.1 Structure of layered double hydroxides......Page 29
1.2.3 Synthesis Routes of Layered Double Hydroxides......Page 31
1.2.3.2 Urea Hydrolysis......Page 32
1.2.3.8 Anion exchange......Page 33
1.3 Organic Modification of Layered Double Hydroxides......Page 34
1.3.1 One step Co-precipitation......Page 38
1.3.3 Memory Effect or Regeneration Method......Page 39
1.3.4 The delamination/restacking method......Page 40
1.4 Characterization of layered double hydroxides and modified layered double hydroxides......Page 41
1.5 Potential applications of layered double hydroxides, organically modified layered double hydroxides and layered double .........Page 48
1.5.1 Flame retardant applications......Page 50
1.5.2 Catalysis......Page 52
1.5.3 Water splitting......Page 53
1.5.4 Environmental remediation......Page 54
1.5.5 Electrode for super capacitor......Page 57
1.5.6 Biomedical applications......Page 59
1.7.1 Melt Compounding......Page 62
1.7.2 Solution Blending......Page 64
1.7.3.2 In-Situ LDH Synthesis......Page 65
1.7.4 Layer By Layer Assembly......Page 66
1.7.4.1 Preparation of LDH Nanosheets......Page 67
1.7.5 Two roll mill mixing......Page 68
1.7.7 High energy ball milling......Page 69
1.8.1 Flame Retardant Application......Page 70
1.8.2 Biomedical Application......Page 71
1.8.3 Gas sensing Applications......Page 76
1.8.4 Energy Applications......Page 77
1.8.5 Food Packaging Applications......Page 78
1.8.6 Water Purification......Page 79
1.8.7 Gas Barrier Materials......Page 80
1.8.8 Agricultural Applications......Page 81
1.8.9 Anti Corrosion Materials......Page 82
1.9 LDH based polymer hybrid nanocomposites......Page 83
1.10 Conclusion and perspectives......Page 90
1.11 Abbreviations......Page 93
References......Page 95
2.1 Introduction......Page 104
2.2.1.1 Mg–Al LDH–CO32−......Page 106
2.2.1.3 Mg–Al LDH–SO42−......Page 108
2.2.1.5 Mg–Al LDH–Cl−......Page 109
2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals......Page 110
2.2.3 FTIR spectra of layered double hydroxides containing three metals......Page 112
2.3 FTIR spectra of organo-modified layered double hydroxides......Page 114
2.4 Conclusion......Page 122
References......Page 123
3.1.1 Layered double hydroxides......Page 130
3.1.2 Modification of layered double hydroxides......Page 131
3.1.2.4 Mechanochemical approaches......Page 132
3.2.1 Introduction......Page 133
3.3.1 Preparation of carboxymethylcellulose/layered double hydroxide nanocomposites......Page 142
3.3.2 Preparation of pectin/layered double hydroxide nanocomposites......Page 143
3.3.3 Preparation of chitosan/layered double hydroxide nanocomposites......Page 144
3.3.4 Preparation of natural rubber/layered double hydroxide nanocomposites......Page 145
3.4 Preparation of synthetic polymer/layered double hydroxide nanocomposites......Page 147
3.4.1 Preparation of polyimide/layered double hydroxide nanocomposites......Page 149
3.4.2 Preparation of poly(methyl methacrylate)/layered double hydroxide nanocomposites......Page 150
3.4.4 Preparation of P(MMA-co-BA)/layered double hydroxide nanocomposites......Page 151
3.4.6 Preparation of poly(amide-imide)/layered double hydroxide nanocomposites......Page 154
3.4.7 Preparation of low-density polyethylene/layered double hydroxide nanocomposites......Page 158
3.4.8 Preparation of polyvinyl alcohol/layered double hydroxide nanocomposites......Page 161
3.4.9 Preparation of polyester/layered double hydroxide nanocomposites......Page 163
3.4.10 Preparation of polyvinyl chloride/layered double hydroxide nanocomposites......Page 164
3.4.11 Preparation of polypropylene-ethylene vinyl acetate/layered double hydroxide nanocomposites......Page 166
3.4.12 Preparation of silicone rubber/layered double hydroxide nanocomposites......Page 167
3.4.13 Preparation of epoxy resin/MoS2/layered double hydroxide nanocomposites......Page 168
3.4.14 Preparation of polyurethane/nitrile butadiene rubber Blend/layered double hydroxide nanocomposites......Page 170
3.4.15 Preparation of polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites......Page 171
3.4.16 Preparation of isotactic polypropylene/layered double hydroxide nanocomposites......Page 172
3.5 Conclusions and future perspectives......Page 175
References......Page 179
4.1 Introduction......Page 184
4.2 Microscopic characterization techniques for PNCs......Page 185
4.2.2 Scanning electron microscope......Page 186
4.2.3 Transmission electron microscope......Page 187
4.2.5 Scanning probe microscope......Page 189
4.2.6 Scanning tunneling microscope......Page 190
4.2.7 Atomic force microscope......Page 191
4.3 Microscopic characterization of polymer/LDH NCs......Page 192
4.3.1 Microscopic characterization of elastomer/LDH NCs......Page 193
4.3.2 Microscopic characterization of thermoplastic polymer/LDH NCs......Page 197
4.3.3 Microscopic characterization of thermosetting polymer/LDH NCs......Page 210
4.3.4 Microscopic characterization of polymer blend/LDH NCs......Page 219
References......Page 224
Further reading......Page 230
5.1 Introduction......Page 232
5.2 X-ray diffraction analysis......Page 233
5.3 X-ray diffraction analysis of layered double hydroxides and Modified Layered Double Hydroxides......Page 237
5.4 X-ray diffraction analysis of layered double hydroxide polymer nanocomposites......Page 243
5.5 Conclusion......Page 252
References......Page 253
6.1 Introduction......Page 258
6.2 Spectroscopy of polymer nanocomposites......Page 259
6.2.3 Energy-dispersive X-ray spectroscopy......Page 260
6.2.6 Nuclear magnetic resonance spectroscopy......Page 261
6.2.8 X-ray photoelectron spectroscopy......Page 262
6.3.1 Fourier transform infrared spectroscopy of layered double hydroxide polymer nanocomposites......Page 263
6.3.2 Raman spectroscopy of layered double hydroxide polymer nanocomposites......Page 266
6.3.3 Energy-dispersive X-ray spectroscopy of layered double hydroxide polymer nanocomposites......Page 270
6.3.4 Fluorescence spectroscopy of layered double hydroxide polymer nanocomposites......Page 274
6.3.5 Dielectric spectroscopy of layered double hydroxide polymer nanocomposites......Page 278
6.3.6 Nuclear magnetic resonance spectroscopy of layered double hydroxide polymer nanocomposites......Page 285
6.3.7 UV–vis spectroscopy of layered double hydroxide polymer nanocomposites......Page 289
6.3.8 X-ray photoelectron spectroscopy of layered double hydroxide polymer nanocomposites......Page 292
6.4 Spectroscopic characterization for the aging process......Page 297
6.5 Conclusions......Page 300
References......Page 301
Further reading......Page 306
7.1.1 The importance of rheological studies of polymer nanocomposites......Page 308
7.1.2 Rheology of polymer layered double hydroxide nanocomposites......Page 309
7.2 Rheology of thermoplastic polymer layered double hydroxide nanocomposites......Page 311
7.3 Rheology of thermosetting polymer layered double hydroxide nanocomposites......Page 324
7.4 Modeling of rheological properties......Page 328
References......Page 332
Further reading......Page 337
8.1 Introduction......Page 338
8.2.1 The techniques for determining thermal stability properties of polymers......Page 340
8.2.2 The techniques for determining the flame-retardant performance of polymers......Page 341
8.3.1 Thermal stabilizer introduction......Page 344
8.3.2.1 Effect of inorganic LDHs......Page 345
8.3.2.2 Effect of organic LDHs......Page 346
8.3.2.3 Effect of LDHs with other synergistic thermal stabilizers......Page 347
8.3.3 The mechanism of thermostability using LDHs......Page 349
8.4.1 Flame retardant introduction......Page 350
8.4.2.1 Effect of inorganic LDHs......Page 351
8.4.2.2 Effect of organic LDHs......Page 354
8.4.2.3 Effect of LDHs with other synergistic fire retardants......Page 357
8.4.3.1 Organic modification of LDHs......Page 364
8.4.4 The mechanism of flame retardancy using LDH......Page 365
References......Page 367
9.1 Introduction......Page 374
9.2 Preparative methods of LDH-elastomer and LDH-elastomeric blend nanocomposites......Page 376
9.4.1.1.1 XRD......Page 377
9.4.1.1.2 TEM......Page 381
9.4.1.2.1 XRD......Page 385
9.4.1.2.2 TEM......Page 386
9.4.1.3.1 XRD......Page 388
9.4.1.3.2 TEM......Page 389
9.4.1.4.1 XRD......Page 390
9.4.1.5.1 TEM......Page 391
9.4.1.6.1 XRD......Page 392
9.4.1.6.2 TEM......Page 393
9.4.2.1.1 XRD......Page 394
9.4.2.1.2 TEM......Page 395
9.4.2.2.1 XRD......Page 396
9.4.2.2.2 TEM......Page 397
9.5.1.1 PU-LDH nanocomposites......Page 399
9.5.1.2 EVA/LDH nanocomposites......Page 404
9.5.1.3 SR/LDH nanocomposites......Page 406
9.5.1.4 EPDM/LDH nanocomposites......Page 407
9.5.1.6 NBR/LDH and XNBR/LDH nanocomposites......Page 409
9.5.2.1 PU blend-LDH nanocomposites......Page 410
9.5.2.2 EVA Blend-LDH nanocomposites......Page 413
9.6.1.1 PU-LDH nanocomposites......Page 415
9.6.1.2 EVA-LDH nanocomposites......Page 417
9.6.1.3 SR/LDH nanocomposites......Page 421
9.6.1.4 EPDM-LDH nanocomposites......Page 422
9.6.1.5 NBR/LDH and XNBR/LDH nanocomposites......Page 424
9.6.2.1 PU blend/LDH nanocomposites......Page 425
9.6.2.2 EVA-EPDM/LDH nanocomposites......Page 428
9.7 Conclusion......Page 429
References......Page 430
10.1 A general introduction to LDH-carbon nanoform nanocomposites......Page 438
10.2 Graphene and graphene oxide/LDH nanocomposites......Page 442
10.2.1.1 Reassembly of graphene and LDHs......Page 444
10.2.1.2 Direct growth of LDH on graphene......Page 447
10.2.1.3 Graphene formation in LDH layers......Page 450
10.3 Carbon nanotubes/LDH nanocomposites......Page 452
10.3.1.1 Reassembly of CNTs and LDHs......Page 454
10.3.1.2 LDH formation on CNTs......Page 455
10.3.1.3 CNTs formation on LDHs......Page 456
10.4.3 Carbon spheres/LDH nanocomposites......Page 458
10.4.4 Carbon (nano)fibers/LDH nanocomposites......Page 459
10.5.1 Energy storage and conversion......Page 463
10.5.1.1 Batteries......Page 465
10.5.1.2 Supercapacitors......Page 466
10.5.1.3 Water splitting......Page 468
10.5.2 Catalysis......Page 470
10.5.3.1 Environment protection......Page 472
10.5.3.3 Materials science......Page 474
10.6 Conclusions......Page 475
10.7 Perspectives......Page 476
References......Page 477
Further reading......Page 487
11.1 Introduction......Page 488
11.2.1.1 Layer-by-layer assembly......Page 491
11.2.1.2 Physical blending......Page 493
11.2.2.1 Conventional emulsion polymerization......Page 495
11.2.2.2 Suspension polymerization......Page 498
11.2.2.3 Reversible deactivation radical polymerization (RDRP)......Page 499
11.2.3 Latex-templating approaches......Page 502
11.3.1 LDH-based nanocomposites......Page 508
11.3.1.1 Mechanical properties......Page 509
11.3.1.2 Flame retardancy......Page 510
11.3.2.1 Adsorption and extraction......Page 512
11.3.2.2 Catalysis and photocatalysis......Page 513
11.3.2.3 Electrochemical and magnetic properties......Page 514
References......Page 516
12.1.1.1 Conjugated polymers......Page 524
12.1.1.2 LDHs/conjugated polymer nanocomposites......Page 525
12.1.2 Fabrication and assembly of LDH/CP nanocomposites......Page 526
12.1.2.1 Layer-by-layer assembly method based on electrostatic interaction......Page 527
12.1.2.2 Layer-by-layer assembly method based on hydrogen bond interactions......Page 534
12.1.2.3 Layer-by-layer assembly method based on van der Waals forces......Page 538
12.1.2.4 Layer-by-layer assembly based on miscellaneous interaction......Page 539
12.2.1.1 Photostability of LDH/CP nanocomposites......Page 542
12.2.1.2 Luminescence properties of LDH/conjugated polymer nanocomposites and applications......Page 544
12.2.1.3 Fluorescence resonance energy transfer (FRET) of LDH/CP nanocomposites......Page 546
12.2.2.1 Photodetectors......Page 549
12.2.2.2 Photocatalysis......Page 550
12.3 Conclusions and outlook......Page 552
References......Page 553
13.1 Introduction......Page 558
13.2 Modification of LDHs with organic compounds......Page 559
13.3 Layered double hydroxide/Carbonaceous nanofiller hybrids......Page 562
13.4 Synthesis of LDH/Carbonaceous nanofiller hybrids......Page 563
13.5.1 Removal of pollution......Page 565
13.5.2 Supercapacitor......Page 570
13.5.3 Catalyst......Page 573
13.6 Polymer/LDH/Carbonaceous nanofiller hybrid nanocomposites......Page 575
13.6.1 Polymer/LDH/CNT hybrid nanocomposites......Page 577
13.6.2 Polymer/LDH/graphene hybrid nanocomposites......Page 581
13.6.3 Polymer/LDH/Other nanofiller hybrids......Page 583
13.7 Conclusions......Page 585
References......Page 586
14.1 Introduction......Page 592
14.2.1 Application of Layered Double Hydroxide Nanocomposites in Supercapacitors......Page 595
14.2.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Supercapacitors......Page 598
14.3 Batteries......Page 601
14.3.1 Application of Layered Double Hydroxide Nanocomposites in Batteries......Page 602
14.3.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Batteries......Page 604
14.4 Fuel Cells......Page 606
14.4.1 Application of Layered Double Hydroxide/Polymer Nanocomposites in Fuel Cells......Page 607
14.5 Other Electrical and Electronic Applications of Layered Double Hydroxide/Polymer Nanocomposites......Page 614
Acknowledgments......Page 618
References......Page 619
15.1 Introduction......Page 626
15.2.1.1 Cellulose/layered double hydroxide nanocomposites......Page 631
15.2.1.2 Starch/layered double hydroxide nanocomposites......Page 639
15.2.1.3 Chitosan/layered double hydroxide nanocomposites......Page 646
15.2.1.4 Alginate/layered double hydroxide nanocomposites......Page 651
15.2.1.5 Other polysaccharides......Page 654
15.2.2 Protein/layered double hydroxide nanocomposites......Page 659
15.2.3 PHA/layered double hydroxide nanocomposites......Page 660
15.2.4 PLA/layered double hydroxide nanocomposites......Page 670
15.2.5 PVA/layered double hydroxide nanocomposites......Page 680
References......Page 692
16.1 Introduction......Page 704
16.1.1 Layered double hydroxide nanocomposites......Page 705
16.1.2 Layered double hydroxide nanocomposites in the medical field......Page 707
16.1.2.1 Cellular uptake mechanism and biodistribution......Page 708
16.1.2.2 Tissue distribution of layered double hydroxide nanoparticles......Page 711
16.2.1 Layered double hydroxide nanocomposites in drug-delivery applications......Page 712
16.2.2 Layered double hydroxide nanocomposites in gene-delivery applications......Page 715
16.2.3 Bioimaging applications......Page 720
16.2.5 Layered double hydroxide nanocomposites for tissue engineering applications......Page 721
16.3.1 Alginate–layered double hydroxide nanocomposites......Page 722
16.3.1.1 Release of ibuprofen from alginate–zein bionanocomposite beads......Page 724
16.3.2 Chitosan–layered double hydroxide nanocomposites......Page 725
16.3.2.1 Drug-delivery applications......Page 726
16.3.2.2 Carboxymethyl chitosan–layered double hydroxide nanocomposite......Page 727
16.3.2.3 Bioimaging applications......Page 728
16.3.2.4 Ex vivo fluorescence image of rabbit ocular tissues......Page 729
16.3.2.5 Chitosan–layered double hydroxide nanocomposites in photodynamic therapy......Page 732
16.3.2.6 Chitosan–layered double hydroxide nanocomposites in tissue engineering applications......Page 734
16.3.3 Other polymer–layered double hydroxide nanocomposites......Page 736
References......Page 737
Further reading......Page 741
17.1 Introduction......Page 742
17.2 The history and evolution of chemical use in agriculture......Page 743
17.3 Principal agricultural problems to resolve with new technologies......Page 745
17.4 Layered double hydroxide applications in agriculture......Page 746
17.4.1 Layered double hydroxide matrices of slow-release fertilizers......Page 747
17.4.2 Layered double hydroxides for storage and gradual herbicide release......Page 753
17.4.3 Layered double hydroxides for storage and slow release of plant growth regulators......Page 754
17.4.4 Use of layered double hydroxides for pesticide removal......Page 757
17.5 Final considerations......Page 760
References......Page 761
Further reading......Page 768
18.1 Introduction......Page 770
18.1.1 Characterization and analytical techniques of polymer nanocomposites for food-packaging applications......Page 774
18.2 Layered double hydroxides as hosts of active molecules for potential in food-packaging applications......Page 775
18.3 Polymeric nanocomposites based on layered double hydroxide-active molecules......Page 781
18.3.1 Nanocomposites from oil-derived polymers......Page 783
18.3.2 Nanocomposites of bioplastics from fossil-based resources......Page 787
18.3.3 Nanocomposites of bioplastics from renewable sources......Page 791
18.4 Regulation issues......Page 793
18.5 Conclusions and future perspectives......Page 794
References......Page 795
19.1 Introduction......Page 808
19.2.2 Layered double hydroxide modification......Page 809
19.2.4 Mechanisms of adsorption......Page 810
19.3.1 Importance of using polymer/layered double hydroxide nanocomposites in water purification......Page 817
19.3.2 Polymer/layered double hydroxide-based adsorbents......Page 818
Acknowledgments......Page 825
References......Page 826
20.1 Introduction......Page 832
20.2 Applications of layered double hydroxides in catalysis......Page 834
20.3 Polymer/layered double hydroxide nanocomposites......Page 838
23.3.1.2 In situ polymerization......Page 839
20.3.1.4 Melt mixing......Page 840
20.4 Applications of polymer/layered double hydroxide nanocomposites in catalysis......Page 842
References......Page 856
Index......Page 862
Back Cover......Page 886

Citation preview

Layered Double Hydroxide Polymer Nanocomposites

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Woodhead Publishing Series in Composites Science and Engineering

Layered Double Hydroxide Polymer Nanocomposites Edited by

Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Saju Daniel International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India St. Xavier’s College Vaikom, Kottayam, Kerala, India

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2020 Elsevier Ltd. 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: 978-0-08-102261-0 (print) ISBN: 978-0-08-101904-7 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gwen Jones Editorial Project Manager: Andrea Gallego Ortiz Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

Contents

List of contributors Preface Acknowledgement 1.

Layered double hydroxides: fundamentals to applications Saju Daniel and Sabu Thomas 1.1 Introduction 1.2 Layered double hydroxides 1.2.1 Structure of layered double hydroxides 1.2.2 Differences between Ordinary Clays and Layered Double Hydroxides 1.2.3 Synthesis Routes of Layered Double Hydroxides 1.3 Organic Modification of Layered Double Hydroxides 1.3.1 One step Co-precipitation 1.3.2 Anion exchange reaction 1.3.3 Memory Effect or Regeneration Method 1.3.4 The delamination/restacking method 1.4 Characterization of layered double hydroxides and modified layered double hydroxides 1.5 Potential applications of layered double hydroxides, organically modified layered double hydroxides and layered double hydroxide nanocomposites 1.5.1 Flame retardant applications 1.5.2 Catalysis 1.5.3 Water splitting 1.5.4 Environmental remediation 1.5.5 Electrode for super capacitor 1.5.6 Biomedical applications 1.6 Introduction to Layered Double Hydroxide Polymer Nanocomposites 1.7 Different Fabrication Techniques of Layered Double Hydroxide polymer Nanocomposites 1.7.1 Melt Compounding 1.7.2 Solution Blending 1.7.3 In
Situ Methods 1.7.4 Layer By Layer Assembly 1.7.5 Two roll mill mixing

xvii xxi xxv 1 1 2 2 4 4 7 11 12 12 13 14

21 23 25 26 27 30 32 35 35 35 37 38 39 41

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Contents

1.7.6 Sonication 1.7.7 High energy ball milling 1.8 Applications of Layered Double Hydroxide Polymer Nanocomposites 1.8.1 Flame Retardant Application 1.8.2 Biomedical Application 1.8.3 Gas sensing Applications 1.8.4 Energy Applications 1.8.5 Food Packaging Applications 1.8.6 Water Purification 1.8.7 Gas Barrier Materials 1.8.8 Agricultural Applications 1.8.9 Anti Corrosion Materials 1.9 LDH based polymer hybrid nanocomposites 1.10 Conclusion and perspectives 1.11 Abbreviations References 2.

3.

FTIR characterization of layered double hydroxides and modified layered double hydroxides Meisam Shabanian, Mohsen Hajibeygi and Ahmad Raeisi 2.1 Introduction 2.2 Fourier transform infrared spectra of layered double hydroxides 2.2.1 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different anions 2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals 2.2.3 FTIR spectra of layered double hydroxides containing three metals 2.3 FTIR spectra of organo-modified layered double hydroxides 2.4 Conclusion References Fabrication technologies of layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Farbod Tabesh 3.1 Introduction 3.1.1 Layered double hydroxides 3.1.2 Modification of layered double hydroxides 3.2 Preparation of polymer/layered double hydroxide nanocomposites 3.2.1 Introduction 3.3 Preparation of Natural polymer/layered double hydroxide nanocomposites 3.3.1 Preparation of carboxymethylcellulose/layered double hydroxide nanocomposites

42 42 43 43 44 49 50 51 52 53 54 55 56 63 66 68

77 77 79 79 83 85 87 95 96

103 103 103 104 106 106 115 115

Contents

vii

3.3.2

3.4

3.5

Preparation of pectin/layered double hydroxide nanocomposites 3.3.3 Preparation of chitosan/layered double hydroxide nanocomposites 3.3.4 Preparation of natural rubber/layered double hydroxide nanocomposites 3.3.5 Other natural polymer/layered double hydroxide nanocomposites Preparation of synthetic polymer/layered double hydroxide nanocomposites 3.4.1 Preparation of polyimide/layered double hydroxide nanocomposites 3.4.2 Preparation of poly(methyl methacrylate)/layered double hydroxide nanocomposites 3.4.3 Preparation of polyvinyl acetate/layered double hydroxide nanocomposites 3.4.4 Preparation of P(MMA-co-BA)/layered double hydroxide nanocomposites 3.4.5 Preparation of wood flour/polypropylene/layered double hydroxide nanocomposites 3.4.6 Preparation of poly(amide-imide)/layered double hydroxide nanocomposites 3.4.7 Preparation of low-density polyethylene/layered double hydroxide nanocomposites 3.4.8 Preparation of polyvinyl alcohol/layered double hydroxide nanocomposites 3.4.9 Preparation of polyester/layered double hydroxide nanocomposites 3.4.10 Preparation of polyvinyl chloride/layered double hydroxide nanocomposites 3.4.11 Preparation of polypropylene-ethylene vinyl acetate/layered double hydroxide nanocomposites 3.4.12 Preparation of silicone rubber/layered double hydroxide nanocomposites 3.4.13 Preparation of epoxy resin/MoS2/layered double hydroxide nanocomposites 3.4.14 Preparation of polyurethane/nitrile butadiene rubber Blend/layered double hydroxide nanocomposites 3.4.15 Preparation of polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites 3.4.16 Preparation of isotactic polypropylene/layered double hydroxide nanocomposites Conclusions and future perspectives

116 117 118 120 120 122 123 124 124 127 127 131 134 136 137 139 140 141 143

144 145 148

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Contents

Acknowledgments References 4.

5.

6.

Microscopic characterization techniques for layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Shima Rashidimoghadam 4.1 Introduction 4.2 Microscopic characterization techniques for PNCs 4.2.1 Optical microscope 4.2.2 Scanning electron microscope 4.2.3 Transmission electron microscope 4.2.4 Field ion microscope 4.2.5 Scanning probe microscope 4.2.6 Scanning tunneling microscope 4.2.7 Atomic force microscope 4.2.8 X-Ray diffraction topography 4.3 Microscopic characterization of polymer/LDH NCs 4.3.1 Microscopic characterization of elastomer/LDH NCs 4.3.2 Microscopic characterization of thermoplastic polymer/LDH NCs 4.3.3 Microscopic characterization of thermosetting polymer/LDH NCs 4.3.4 Microscopic characterization of polymer blend/LDH NCs 4.4 Conclusions Acknowledgments References Further reading X-ray diffraction analysis of layered double hydroxide polymer nanocomposites Rodrigo Botan and Sabrina de Bona Sartor 5.1 Introduction 5.2 X-ray diffraction analysis 5.3 X-ray diffraction analysis of layered double hydroxides and Modified Layered Double Hydroxides 5.4 X-ray diffraction analysis of layered double hydroxide polymer nanocomposites 5.5 Conclusion References Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites Shadpour Mallakpour and Faezeh Azimi 6.1 Introduction

152 152

157 157 158 159 159 160 162 162 163 164 165 165 166 170 183 192 197 197 197 203

205 205 206 210 216 225 226

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Contents

6.2

Spectroscopy of polymer nanocomposites 6.2.1 Fourier transform infrared spectroscopy 6.2.2 Raman spectroscopy 6.2.3 Energy-dispersive X-ray spectroscopy 6.2.4 Fluorescence spectroscopy 6.2.5 Dielectric spectroscopy 6.2.6 Nuclear magnetic resonance spectroscopy 6.2.7 UV
vis spectrophotometry 6.2.8 X-ray photoelectron spectroscopy 6.3 Spectroscopic characterization of layered double hydroxide polymer nanocomposites 6.3.1 Fourier transform infrared spectroscopy of layered double hydroxide polymer nanocomposites 6.3.2 Raman spectroscopy of layered double hydroxide polymer nanocomposites 6.3.3 Energy-dispersive X-ray spectroscopy of layered double hydroxide polymer nanocomposites 6.3.4 Fluorescence spectroscopy of layered double hydroxide polymer nanocomposites 6.3.5 Dielectric spectroscopy of layered double hydroxide polymer nanocomposites 6.3.6 Nuclear magnetic resonance spectroscopy of layered double hydroxide polymer nanocomposites 6.3.7 UV
vis spectroscopy of layered double hydroxide polymer nanocomposites 6.3.8 X-ray photoelectron spectroscopy of layered double hydroxide polymer nanocomposites 6.4 Spectroscopic characterization for the aging process 6.5 Conclusions Acknowledgments References Further reading

7.

Melt rheological properties of layered double hydroxide polymer nanocomposites Appukuttan Saritha and Kuruvilla Joseph 7.1 Introduction 7.1.1 The importance of rheological studies of polymer nanocomposites 7.1.2 Rheology of polymer layered double hydroxide nanocomposites 7.2 Rheology of thermoplastic polymer layered double hydroxide nanocomposites 7.3 Rheology of thermosetting polymer layered double hydroxide nanocomposites

ix

232 233 233 233 234 234 234 235 235 236 236 239 243 247 251 258 262 265 270 273 274 274 279

281 281 281 282 284 297

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Contents

7.4 Modeling of rheological properties 7.5 Conclusions and future scope References Further reading 8.

9.

Thermal properties and flame-retardant characteristics of layered double hydroxide polymer nanocomposites Yanshan Gao, Lei Qiu, Dermot O’Hare and Qiang Wang 8.1 Introduction 8.2 The techniques for determining thermal stability properties and flame retardancy performance 8.2.1 The techniques for determining thermal stability properties of polymers 8.2.2 The techniques for determining the flame-retardant performance of polymers 8.3 LDH-based thermal stabilizer materials and their applications 8.3.1 Thermal stabilizer introduction 8.3.2 Thermal stability properties of LDH-based nanocomposites 8.3.3 The mechanism of thermostability using LDHs 8.4 LDH-based flame-retardant materials and their applications 8.4.1 Flame retardant introduction 8.4.2 Flame-retardant performance of LDH-based nanocomposites 8.4.3 Posttreatment of LDHs as flame retardants 8.4.4 The mechanism of flame retardancy using LDH 8.5 Conclusions and future development References Mechancial and dynamical mechanical properties of layered double hydroxide-filled elastomer and elastomeric blend nanocomposites Suneel Kumar Srivastava 9.1 Introduction 9.2 Preparative methods of LDH-elastomer and LDH-elastomeric blend nanocomposites 9.3 Different types of layered double hydroxide fillers used in the fabrication of elastomer and elastomeric blend nanocomposites 9.4 Morphology of elastomer-LDH and elastomeric blend-LDH nanocomposites 9.4.1 Morphology of elastomeric-LDH nanocomposites 9.4.2 Morphology of elastomeric blend-LDH nanocomposites 9.5 Mechanical properties of elastomer-LDH and elastomeric blend-LDH nanocomposites 9.5.1 Mechanical properties of elastomer-LDH nanocomposites 9.5.2 Mechanical properties of elastomeric blend-LDH nanocomposites

301 305 305 310

311 311 313 313 314 317 317 318 322 323 323 324 337 338 340 340

347 347 349 350 350 350 367 372 372 383

Contents

Dynamical mechanical properties of LDH-filled elastomer and elastomeric blend nanocomposites 9.6.1 Dynamical mechanical properties of elastomer-LDH nanocomposites 9.6.2 Dynamical mechanical properties of elastomer blend-LDH nanocomposites 9.7 Conclusion References

xi

9.6

10. Layered double hydroxide nanocomposites based on carbon nanoforms Gonzalo Abella´n, Jose A. Carrasco and Eugenio Coronado 10.1 A general introduction to LDH-carbon nanoform nanocomposites 10.2 Graphene and graphene oxide/LDH nanocomposites 10.2.1 Synthesis 10.3 Carbon nanotubes/LDH nanocomposites 10.3.1 Synthesis 10.4 Other CNF/LDH nanocomposites 10.4.1 Fullerene/LDH nanocomposites 10.4.2 Carbon quantum dot/LDH nanocomposites 10.4.3 Carbon spheres/LDH nanocomposites 10.4.4 Carbon (nano)fibers/LDH nanocomposites 10.4.5 Graphene/single-walled CNT/LDH nanocomposites 10.5 Applications of CNF/LDH nanocomposites 10.5.1 Energy storage and conversion 10.5.2 Catalysis 10.5.3 Miscellanea 10.6 Conclusions 10.7 Perspectives Acknowledgments References Further reading 11. Recent advances in layered double hydroxide/polymer latex nanocomposites: from assembly to in situ formation V. Prevot and E. Bourgeat-Lami 11.1 Introduction 11.2 Use of latex technology for the production of LDH-based composite materials and macroporous structures 11.2.1 Assembly of preformed LDH and latex particles 11.2.2 LDH-based nanocomposites by in situ emulsion and suspension polymerizations 11.2.3 Latex-templating approaches 11.3 Properties of LDH-based nanocomposites and LDH macroporous structures

388 388 398 402 403

411 411 415 417 425 427 431 431 431 431 432 436 436 436 443 445 448 449 450 450 460

461 461 464 464 468 475 481

xii

Contents

11.3.1 LDH-based nanocomposites 11.3.2 LDH-based macroporous structures 11.4 Concluding remarks and general trends References 12. Fabrication, assembly, and optoelectric properties of layered double hydroxide/conjugated polymer nanocomposites Yaping Huang, Harrone Muhammad Sohail and Jun Lu 12.1 Fabrication and assembly of LDHs/conjugated polymer nanocomposites 12.1.1 Introduction 12.1.2 Fabrication and assembly of LDH/CP nanocomposites 12.2 Optical and optoelectric properties of LDH/CP nanocomposites 12.2.1 Optical properties 12.2.2 Optoelectric properties 12.3 Conclusions and outlook References 13. Polymer layered double hydroxide hybrid nanocomposites Shadpour Mallakpour and Elham Khadem 13.1 Introduction 13.2 Modification of LDHs with organic compounds 13.3 Layered double hydroxide/Carbonaceous nanofiller hybrids 13.4 Synthesis of LDH/Carbonaceous nanofiller hybrids 13.5 Applications of LDH/Carbonaceous nanofiller hybrids 13.5.1 Removal of pollution 13.5.2 Supercapacitor 13.5.3 Catalyst 13.6 Polymer/LDH/Carbonaceous nanofiller hybrid nanocomposites 13.6.1 Polymer/LDH/CNT hybrid nanocomposites 13.6.2 Polymer/LDH/graphene hybrid nanocomposites 13.6.3 Polymer/LDH/Other nanofiller hybrids 13.7 Conclusions Acknowledgments References 14. Electrical and electronic applications of layered double-hydroxide polymer nanocomposites Shadpour Mallakpour and Forough Motirasoul 14.1 Introduction 14.2 Supercapacitors 14.2.1 Application of Layered Double Hydroxide Nanocomposites in Supercapacitors 14.2.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Supercapacitors

481 485 489 489

497

497 497 499 515 515 522 525 526 531 531 532 535 536 538 538 543 546 548 550 554 556 558 559 559

565 565 568 568 571

Contents

14.3

Batteries 14.3.1 Application of Layered Double Hydroxide Nanocomposites in Batteries 14.3.2 Application of Layered Double Hydroxide/Polymer Nanocomposites in Batteries 14.4 Fuel Cells 14.4.1 Application of Layered Double Hydroxide/Polymer Nanocomposites in Fuel Cells 14.5 Other Electrical and Electronic Applications of Layered Double Hydroxide/Polymer Nanocomposites 14.6 Conclusions Acknowledgments References 15. Applications of layered double hydroxide biopolymer nanocomposites Shadpour Mallakpour and Leila khodadadzadeh 15.1 Introduction 15.2 Biopolymer/layered double hydroxide nanocomposites 15.2.1 Polysaccharide/layered double hydroxide nanocomposites 15.2.2 Protein/layered double hydroxide nanocomposites 15.2.3 PHA/layered double hydroxide nanocomposites 15.2.4 PLA/layered double hydroxide nanocomposites 15.2.5 PVA/layered double hydroxide nanocomposites 15.3 Conclusions Acknowledgments References 16. Layered double hydroxide based nanocomposites for biomedical applications Raji Vijayamma, Nandakumar Kalarikkal and Sabu Thomas 16.1 Introduction 16.1.1 Layered double hydroxide nanocomposites 16.1.2 Layered double hydroxide nanocomposites in the medical field 16.2 Biomedical applications of layered double hydroxide nanocomposites 16.2.1 Layered double hydroxide nanocomposites in drug-delivery applications 16.2.2 Layered double hydroxide nanocomposites in gene-delivery applications 16.2.3 Bioimaging applications 16.2.4 Biosensor 16.2.5 Layered double hydroxide nanocomposites for tissue engineering applications

xiii

574 575 577 579 580 587 591 591 592

599 599 604 604 632 633 643 653 665 665 665

677 677 678 680 685 685 688 693 694 694

xiv

Contents

16.3

Layered double hydroxide polymer nanocomposites for biomedical applications 16.3.1 Alginate
layered double hydroxide nanocomposites 16.3.2 Chitosan
layered double hydroxide nanocomposites 16.3.3 Other polymer
layered double hydroxide nanocomposites 16.4 Summary References Further reading

17. Layered double hydroxide nanocomposites for agricultural applications Luı´z Paulo Figueredo Benı´cio, Frederico Garcia Pinto and Jairo Tronto 17.1 Introduction 17.2 The history and evolution of chemical use in agriculture 17.3 Principal agricultural problems to resolve with new technologies 17.4 Layered double hydroxide applications in agriculture 17.4.1 Layered double hydroxide matrices of slow-release fertilizers 17.4.2 Layered double hydroxides for storage and gradual herbicide release 17.4.3 Layered double hydroxides for storage and slow release of plant growth regulators 17.4.4 Use of layered double hydroxides for pesticide removal 17.5 Final considerations List of abbreviations References Further reading 18. Layered double hydroxide polymer nanocomposites for food-packaging applications Giuliana Gorrasi and Andrea Sorrentino 18.1 Introduction 18.1.1 Characterization and analytical techniques of polymer nanocomposites for food-packaging applications 18.2 Layered double hydroxides as hosts of active molecules for potential in food-packaging applications 18.3 Polymeric nanocomposites based on layered double hydroxide-active molecules 18.3.1 Nanocomposites from oil-derived polymers 18.3.2 Nanocomposites of bioplastics from fossil-based resources 18.3.3 Nanocomposites of bioplastics from renewable sources 18.4 Regulation issues 18.5 Conclusions and future perspectives

695 695 698 709 710 710 714

715 715 716 718 719 720 726 727 730 733 734 734 741

743 743 747 748 754 756 760 764 766 767

Contents

Acknowledgment References 19. Layered double hydroxide polymer nanocomposites for water purification Shadpour Mallakpour and Vajiheh Behranvand 19.1 Introduction 19.2 Pollutant elimination from water: why layered double hydroxides? 19.2.1 Structural properties of layered double hydroxides 19.2.2 Layered double hydroxide modification 19.2.3 Water pollutants 19.2.4 Mechanisms of adsorption 19.3 Pollutant elimination by polymer/layered double hydroxide nanocomposites 19.3.1 Importance of using polymer/layered double hydroxide nanocomposites in water purification 19.3.2 Polymer/layered double hydroxide-based adsorbents 19.4 Conclusions Acknowledgments References

xv

768 768

781 781 782 782 782 783 783 790 790 791 798 798 799

20. Layered double hydroxide polymer nanocomposites for catalysis Shadpour Mallakpour and Hashem Tabebordbar 20.1 Introduction 20.2 Applications of layered double hydroxides in catalysis 20.3 Polymer/layered double hydroxide nanocomposites 20.3.1 Preparation of polymer/layered double hydroxide nanocomposites 20.4 Applications of polymer/layered double hydroxide nanocomposites in catalysis 20.5 Conclusions Acknowledgments References

805

Index

835

805 807 811 812 815 829 829 829

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List of contributors

Gonzalo Abella´n Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain; Department of Chemistry and Pharmacy and Joint Institute of Advanced Materials and Processes (ZMP), University Erlangen-Nu¨rnberg, Fu¨rth, Germany Faezeh Azimi Chemistry Group, Pardis College, Technology, Isfahan, Islamic Republic of Iran

Isfahan

University

of

Vajiheh Behranvand Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Luı´z Paulo Figueredo Benı´cio Soil Department, Federal University of Vic¸osa, Brazil Rodrigo Botan Unifacvest University, Lages, SC, Brazil E. Bourgeat-Lami University of Lyon, Universite´ Claude Bernard Lyon 1, CPE Lyon, CNRS, UMR 5265, Chemistry, Catalysis, Polymers and Processes (C2P2), Villeurbanne, France Jose A. Carrasco Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain Eugenio Coronado Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain Saju Daniel International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India; St. Xavier’s College Vaikom, Kottayam, Kerala, India Sabrina de Bona Sartor Unifacvest University, Lages, SC, Brazil Yanshan Gao College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

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List of contributors

Giuliana Gorrasi Department of Industrial Engineering, University of Salerno-via Giovanni Paolo II 132, Fisciano, Italy Mohsen Hajibeygi Faculty of Chemistry, Kharazmi University, Tehran, Iran Yaping Huang Beijing University of Chemical Technology, Beijing, P.R. China Kuruvilla Joseph Department of Chemistry, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala, India Nandakumar Kalarikkal International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India Elham Khadem Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Leila Khodadadzadeh Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Jun Lu Beijing University of Chemical Technology, Beijing, P.R. China Shadpour Mallakpour Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Forough Motirasoul Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Dermot O’Hare University of Oxford, Oxford, United Kingdom Frederico Garcia Pinto Institute of Exact and Technological Sciences, Federal University of Vic¸osa - Rio Paranaı´ba Campus, Brazil V. Prevot CNRS, ICCF - Institut de Chimie de Clermont-Ferrand, Universite´ Clermont Auvergne, Clermont-Ferrand, France Lei Qiu College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

List of contributors

xix

Ahmad Raeisi Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak, Iran Shima Rashidimoghadam Department of Chemistry, Organic Polymer Chemistry Research Laboratory, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Appukuttan Saritha Department of Chemistry, School of Arts and Sciences, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India Meisam Shabanian Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, Iran Harrone Muhammad Sohail Beijing University of Chemical Technology, Beijing, P.R. China Andrea Sorrentino Institute for Polymers, Composites and Biomaterials (IPCB), National Research Council (CNR), Lecco, Italy Suneel Kumar Srivastava Department of Chemistry, Indian Institute of Technology, Khragpur, India Hashem Tabebordbar Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Farbod Tabesh Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India; School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India Jairo Tronto Institute of Exact and Technological Sciences, Federal University of Vic¸osa - Rio Paranaı´ba Campus, Brazil Raji Vijayamma International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India Qiang Wang College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China

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Preface

Recently the researchers have turned their attention toward layered double hydroxide (LDH) polymer nanocomposites because of the distinctive properties of LDHs such as wide range of chemical compositions, structural homogeneity, unique and excellent anion exchanging ability, easy synthesis, high bound water content, memory effect, nontoxicity and biocompatibility, and their wide range of applications. This book really showcases the researches of many famous scientists who have been working in the field of LDHs. The aspire of introducing this compendium is to replenish a solid understanding of the recent innovative advances in the fabrication, characterization, and applications of polymer LDH nanocomposites in various fields such as biomedical, aerospace, electrical and electronics, automotive industry, agricultural, water treatment, and catalysis. This book consists of 20 chapters and all the chapters cover various relevant topics and state of the art, new challenges, and opportunities in each field. Chapter 1 is actually a voyage from LDHs to LDH polymer nanocomposites so that one can acquire the knowledge of different strategies required to convert LDHs to exfoliated LDH polymer nanocomposites. Chapter 2 provides Fourier transform infrared spectroscopic characterization of various LDHs and organically modified LDHs with the help of which success of organic modification can be easily identified. Chapter 3 illustrates recent advances in the fabrication technologies of LDH polymer nanocomposites with suitable examples and it helps to find the novel and green procedure for the fabrication of LDH
based nanocomposites of each type of polymer. Chapters 4
6 highlight advanced morphological characterization techniques such as X-ray diffraction analysis, wide range of spectroscopic and microscopic characterization techniques for LDH, organically modified LDHs, and LDH polymer nanocomposites. In chapter 4 basal spacing of various organically modified LDHs are provided so that suitable organic modifiers required for the preparation of exfoliated polymer nanocomposites can be easily detected and the diffraction pattern of the polymer nanocomposite helps to predict the morphology of the composite—intercalated or exfoliated. Chapters 5 and 6 focus on spectroscopic characterization techniques such as nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, dielectric spectroscopy, fluorescence spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and microscopic characterization techniques such as optical microscopy, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy to help in characterizing the

xxii

Preface

nanoscale dispersions, phase segregation, and interface/interphases of polymer nanocomposites. Chapters 7
9 discuss specific properties of LDH polymer nanocomposites such as melt rheological properties, thermal and flame retardancy, and mechanical and dynamic mechanical properties. Chapter 7 reviews the rheology of nanocomposites of thermoplastics and thermosetting polymers filled with LDH and modified LDH with special emphasis on the modeling of rheological properties to understand the LDH particle dispersion and its influence on the melt flow behavior of the nanocomposites. Chapter 8 summarizes the thermal stability and flame retardancy of LDH polymer nanocomposites, their characterization techniques such as thermogravimetric analysis, cone calorimetry, limiting oxygen index and UL94 investigations, and the synergistic effect of LDH with other thermal stabilizers and fire retardants. Chapter 9 reviews recent advances on the mechanical and dynamical mechanical properties of LDH filled elastomer and elastomeric blend nanocomposites. Chapters 10
13 discuss fabrication and applications of carbon nanoform/LDH nanocomposites, LDH/polymer latexes nanocomposites, LDH/conjugated polymer nanocomposites, and LDH
based polymer hybrid nanocomposites. Chapter 10 deals with the fabrication of carbon nanoform/LDH nanocomposites and their applications in super capacitor, water splitting, catalysis, drug delivery, and environment protection. In Chapter 11 three main routes for the fabrication of LDH/polymer latexes nanocomposites such as electrostatic assembly, in situ polymerization, and latex-templating and their flame retardant applications are reviewed. Chapter 12 introduces various LDH/conjugated polymer nanocomposites fabricated by layerby-layer assembly through electrostatic interaction, hydrogen bonds, van der Waals force of attraction, and miscellaneous interactions for optoelectronic applications. Chapter 13 focuses on the state of the art in the preparation of polymer/LDH/carbonaceous nanomaterial hybrids and the influence of hybrid fillers on the properties such as thermal, mechanical, permeability, and drug delivery properties of polymers. Chapters 14
20 discuss wide range applications of LDH polymer nanocomposites, such as electrical and electronic applications such as super capacitors, batteries, and fuel cells, biomedical applications, such as controlled drug delivery, gene delivery, tissue engineering, and photodynamic therapy, and agricultural applications, such as controlled fertilizer, herbicide and plant growth regulators release and pesticide removal, food packaging, water purification, and catalysis. The main focus of Chapter 15 is on the fabrication of LDH
based polymer nanocomposites containing biopolymer matrix for the environmental protection and their important applications in the fields of water treatment, drug delivery, tissue engineering, packaging, and catalysis. This book is really an one stop reference book emphasizing recent advances in the technologies for the fabrication and characterization of LDHs, organically modified LDHs, LDH nanocomposites, LDH
based polymer nanocomposites, and LDH-based polymer hybrid nanocomposites and their wide range of applications thereby covering almost all the points of LDH chemistry. Therefore this book will

Preface

xxiii

lend a hand to academics, researchers, scientists, engineers, industrialists, and students in the field of polymer nanocomposites especially polymer LDH nanocomposites to discern solutions to their unreciprocated posers. Sabu Thomas and Saju Daniel

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Acknowledgement

We express our sincere thanks to all the authors who have taken so much effort and spent their valuable time for contributing chapters to this book. We are very much grateful to the peer reviewers for the valuable guidance. We would like to appreciate and acknowledge all the Elsevier team members of this project for their continuous and unforgettable support throughout the editing of this book.

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Layered double hydroxides: fundamentals to applications

1

Saju Daniel1,2 and Sabu Thomas1,3 1 International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India, 2St. Xavier’s College Vaikom, Kottayam, Kerala, India, 3School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

1.1

Introduction

In recent years, Layered double hydroxides have achieved a lot of attention both from academia and industries due to their easy and ecofriendly synthesis, low cost, non toxicity, structural and compositional tunability, high chemical and thermal stability, high biocompatibility and broad spectrum applications. By exploiting the three outstanding properties of layered double hydroxides, compositional flexibility in cations and anions, excellent anion exchange ability, memory effect, it is very easy to tailor layered double hydroxides into functional hybrid materials and nanocomposites for vast field applications. LDHs can be synthesized by simple methods which enable control over structure, composition and shape by properly varying the conditions. Recent developments in the synthesis methods have offered various morphologies such as microspheres, fibrous structures, nano sized belt, LDH films on substrates etc leads to tremendous range of possibilities for the fabrication of smart high-performance multifunctional materials. So the fabrication of layered double hydroxide based functional hybrid materials and nanocomposites have become one of the most fascinating topics of today’s research. One way of producing the LDH hybrid is the intercalation of smart inorganic or organic functional materials into the nanospace in the intergallery space by anion exchange to form inorganic/ inorganic or inorganic/organic hybrid system for controlled drug delivery, pesticide delivery, gene delivery, biosensor applications, bioimaging etc. Second way is to mix LDH with smart nanomaterials like graphene, carbon nanotube etc to form nanocomposites for energy storage and conversion like electrode for super capacitors, batteries, fuel cell, solar cell, photo catalysts and electro catalysts for water splitting, environmental remediation such as water treatment, removal of toxic gases from atmosphere. Third way is to introduce organically modified LDH or exfoliated LDH sheets or LDH/other nanofillers hybrid into the polymer matrix to develop polymer nanocomposites or LDH based polymer hybrid nanocomposites for energy, food packaging, agricultural, biomedical, flame retardant, gas barrier, anticorrosion, waste water treatment etc. Thin films and core-shell hierarchical hybrid nanostructures formed from LDH sheets and any other nanofillers or polymers attracted much Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00001-X © 2020 Elsevier Ltd. All rights reserved.

2

Layered Double Hydroxide Polymer Nanocomposites

attention because of their high performance in photoluminescence, drug delivery, electrode for super capacitors etc. The aim of this chapter is to make aware of the readers the importance of layered double hydroxides, layered double hydroxide nanocomposites and layered double hydroxide based polymer nanocomposites so as to motivate them to develop hierarchical hybrid nanostructures for the benefit of the society. For this purpose, from fundamentals to applications of layered double hydroxides, LDH hybrids and polymer nanocomposites are reviewed.

1.2

Layered double hydroxides

Layered double hydroxides are versatile and emergent class of two dimensional inorganic layered nanomaterials, natural or synthetic anionic clay minerals, of which the general formula is [M211-x Mx31 (OH) 2] x1 (An2) x/n. yH2O where MII is a divalent ion, MIII is a trivalent ion, An2 is an anion and charge density of LDH layers, x 5 MIII/MII 1 MIII whose value lies between 0.2 and 0.33 for pure LDH phase (Taviot-Gue´ho et al., 2018; Basu et al., 2014). This formula gives rise to a generic layer sequence [AcBZAcB]n for layered double hydroxides in which A and B represents layers of hydroxide anions, c represents layers of metal cations and Z represents layers of other anions such as carbonate ion, chloride ion, nitrate ion etc and neutral molecules like water molecules (https://en.wikipedia.org/wiki/ Layered_double_hydroxides). Hydrotalcite is one of the naturally occurring LDH clays and the parent member of the family layered double hydroxides with the chemical formula Mg6 Al2 (OH) 16CO3.4H2O and its name attributable to high water content (hydro) and talc like appearance (Selvam et al., 2014; Maheskumar et al., 2014; Mishra et al., 2018). Its existence was first declared by Hochstetter in 1842 and synthesized 100 years later by Feitknecht (Basu et al., 2014; Grosu et al., 2018). It is most common and its structure and properties were studied extensively and is considered as the representative of LDHs. So LDHs are also known as hydrotalcite-like compounds (Evans and Duan, 2006).

1.2.1 Structure of layered double hydroxides The structure of layered double hydroxides can be easily reviewed by analogizing its structure with that of brucite which has the formula Mg (OH) 2. Brucite has hexagonal close packing of hydroxide ions in which alternate octahedral sites are occupied by Mg21 ions so that hydroxide layers are neutral. The neutral hydroxide layers are stacked one upon the other and are held together by Vanderwaal’s force of attraction which results in a basal spacing of about 0.48 nm. Schematic representation of brucite structure is shown in Fig. 1.1 (Arizaga et al., 2007). It can be imagined that the substitution of some divalent ions in brucite structure by some trivalent ions isomorphously results in the formation of a positively charged mixed metal hydroxide layers [M1-x II M x III (OH) 2] x1 and the intercalation of anions in the inter layer regions counterbalance the residual positive charge on the

Layered double hydroxides: fundamentals to applications

3

Figure 1.1 Schematic representation of the brucite structure. (a) Side and (b) top view of the layer. Source: Adapted from Arizaga, G.G.C., Satyanarayana, K.G. and Wypych, F., 2007. Layered hydroxide salts: synthesis, properties and potential applications. Solid State Ionics, 178 (15
18), pp. 1143
1162. with kind permission of elsevier

Figure 1.2 Schematic representation of the structure of a generic LDH. (a) Side and (b) top view of the layer. Source: Adapted from Arizaga, G.G.C., Satyanarayana, K.G. and Wypych, F., 2007. Layered hydroxide salts: synthesis, properties and potential applications. Solid State Ionics, 178 (15
18), pp. 1143
1162. with kind permission of elsevier

metal hydroxide layers resembles layered double hydroxide structures. Water molecules in the interlayer region bind to the metal hydroxide layers and anions via extensive hydrogen bonding and help to stabilize the crystal structure of layered double hydroxides. Due to the intercalation of water molecules and anions in the inter lamellar region, the basal spacing has been increased from 0.48 nm in brucite to about 0.77 nm in hydrotalcite (Basu et al., 2014). Schematic representation of structure of layered double hydroxide is shown in Fig. 1.2 (Arizaga et al., 2007).

4

Layered Double Hydroxide Polymer Nanocomposites

1.2.1.1 Metal cations in the layers The metal cations both divalent and trivalent ions in the layers of LDHs are mainly from third and fourth periods of the periodic table. The divalent metal ions that are found commonly in the layers are Mg21, Ni21, Zn21, Co21, Fe21, Mn21, Cu21, Ti21, Cd21, Ca21 etc and the common trivalent metal ions that are found in the layers are Al31, Cr31, Fe31, Mn31, Ga31, V31, In31, Y31, La31, Ru31 etc.

1.2.1.2 Interlamellar anions The generally found interlamellar anions in LDHs are halides e.g. fluoride, chloride etc oxoanions such as carbonate, nitrate, sulphate, bromate etc, oxo and polyoxometallates like chromate, dichromate, (Mo7O24) 62, (V10O28) 62etc, anionic complexes such as ferro and ferricyanide, (PbCl4) 22 etc and organic anions like carboxylates, phosphonates, alkyl sulphates etc.

1.2.2 Differences between Ordinary Clays and Layered Double Hydroxides Even though LDHs have layered crystalline structures with substitutable ions in the interlamellar region similar to layered silicates such as montmorillonite, their chemical and structural features such as composition, geometries, and layer thickness are not identical. As already mentioned LDHs are anionic clays because of the presence of anions in the interlamellar gallery of positively charged layers whereas reversed structure can be observed in the case of layered silicates so the name cationic clays. Each crystal layer in LDHs consists of single octahedral metal hydroxide sheet whereas in layered silicates two or more sheets of metal oxides in the sandwiched structure are observed. In montmorillonite, single crystal layer is a sandwiched system of one octahedral sheet containing Fe, Al, and Mg between two silica tetrahedral sheets. Hence crystal layer thickness and rigidity of LDHs are lower than that of layered silicates (Basu et al., 2014; Das et al., 2008). Schematic diagram showing structural and chemical difference between LDH and MMT is given in Fig. 1.3 (Das et al., 2008).

1.2.3 Synthesis Routes of Layered Double Hydroxides A number of techniques can be applied for the synthesis of LDHs and the selection of the method highly depends on the type of cations in the hydroxide layers, the intercalated anions and the desired physicochemical properties such as phase purity, crystallinity, porosity, morphology, and electronic and optical characteristics of the final materials. The direct methods used for the synthesis of LDHs are co precipitation, urea hydrolysis, salt- oxide method, sol-gel synthesis, electrochemical synthesis, and in-situ film growth and the indirect method used for the synthesis of LDHs are anion exchange, reconstruction by memory effect, and delamination followed by restacking (Richetta et al., 2017).

Layered double hydroxides: fundamentals to applications

5

Figure 1.3 Schematic diagram showing structural and chemical difference between LDH and MMT. Source: Adapted from Das, A., Costa, F.R., Wagenknecht, U. and Heinrich, G., 2008. Nanocomposites based on chloroprene rubber: effect of chemical nature and organic modification of nanoclay on the vulcanizate properties. European Polymer Journal, 44 (11), pp. 3456
3465 with kind permission of elsevier

1.2.3.1 Co-Precipitation It is the most common and useful method for the preparation of large amount of layered double hydroxides and this method is also known as salt-base method. This method involves the slow addition of mixed aqueous solution of salts of both divalent and trivalent ions in proper proportion into water taken in a reactor and the co-precipitation of both the metal ions from the aqueous solution by the simultaneous addition of dilute solutions of sodium hydroxide and/or sodium bicarbonate, sodium carbonate, or ammonium hydroxide solution at controlled temperature and under vigorous shaking. Generally the pH of the reaction medium is kept constant in the range of 7
10, on the basis of nature of metal ions. Finally the suspension is subjected to hydrothermal treatment to obtain well crystallized sample (Basu et al., 2014; Richetta et al., 2017; Radulescu et al., 2008; Yan et al., 2016; Rives et al., 2013).

1.2.3.2 Urea Hydrolysis In this sophisticated method, an aqueous solution of the selected metal ions that required to be present in the LDH and urea in the stoichiometric molar ratio is heated in the temperature range from 100 C to reflux temperature for two to three days. At the end of the process, urea hydrolyses to ammonium carbonate that leads to the precipitation of LDH with carbonate ion as interlayer anion. This homogeneous precipitation method via urea hydrolysis offers LDH with high crystalline and narrow particle size distribution (Basu et al., 2014; Radulescu et al., 2008).

6

Layered Double Hydroxide Polymer Nanocomposites

1.2.3.3 Hydrothermal Crystallization In this method, an aqueous suspension of two oxides, one of trivalent metal ion, M2O3 and another of divalent metal ion, M2O are taken in a pressurized vessel and subjected to hydrothermal treatment at elevated temperature for a few days. During this process the precursor amorphous hydrated M2O3 crystallizes in the presence of reactive basic oxide M2O which acts as the crystallizing agent (Basu et al., 2014; Radulescu et al., 2008; Xu and Lu, 2005).

1.2.3.4 Sol-Gel Method In this method the sol- gel transition occurs during the strong acid hydrolysis of required metal precursors such as metal based alkoxides or acetyl acetonides in ethanolwater system into metal hydroxides using HCl or HNO3. The heating of the mixture to reflux with stirring should be continued until gel formation (Richetta et al., 2017).

1.2.3.5 Salt-oxide Method This is actually a solid-liquid reaction in which aqueous solution of the chloride salt of trivalent ion in excess is treated with an aqueous suspension of the metal (II) oxide (Richetta et al., 2017).

1.2.3.6 Electrochemical deposition In this method nitrate ions are reduced electrically to hydroxide ions on the working electrode due to which local pH value increases that induces the precipitation of LDH films. This is a good method for depositing LDH films of any desired thickness, morphology and film density on metal substrate with good adhesion (Richetta et al., 2017).

1.2.3.7 In-situ film growth In this method the substrate is immersed in water solution of other metal salt and a base is added to control the pH. Here the substrate shows dual role as a source of one of the reacting metal ions and act as the surface for film deposition (Richetta et al., 2017).

1.2.3.8 Anion exchange It is an indirect method widely used for preparing LDH with any desired anions in the inter lamellar region. In actual practice, an aqueous suspension of the LDH precursors or of the pre-synthesized LDH is stirred in the presence of a large excess of the salt of the anion to be intercalated. The anionic exchange occurring in the solution can be expressed as 

   M21 2 M31 2 A 1 B ! M21 2 M31 2 B 1 A

where A and B represent different anions

Layered double hydroxides: fundamentals to applications

7

The exchange of the anion depends on the electrostatic forces between positively charged LDH layers and the exchanging anions. The main criterion for anionic exchange is that the force of attraction between the host ion and the LDH sheet must be lower than that of guest ion and the sheet. The affinity between various anions and the positively charged LDH sheets is in the order NO32 , Br2 , Cl2 , F2 , OH2 , MoO422 , SO422 , CrO422 , HPO422 , CO322. This method can be used for the preparation of layered double hydroxides with any inorganic anions or organic anions. The entire process of anion-exchange must be carried out in an inert atmosphere (Basu et al., 2014; Selvam et al., 2014; Mishra et al., 2018; Richetta et al., 2017; Bullo Saifullah, 2015).

1.2.3.9 Reconstruction / rehydration method The noticeable property of LDHs is the regaining of original structure back after subjecting it to calcination between 400
500 C followed by rehydration. When LDH is heated between 400
500 C, it changes to mixed metal oxides. It is immersed in water or any other solution of anions to be intercalated. The overall process of rehydration must be carried out in an inert nitrogen atmosphere. This method can also be used for the synthesis of LDH with any inorganic or organic anions (Basu et al., 2014; Mishra et al., 2018; Richetta et al., 2017; Bullo Saifullah, 2015). Some examples for the synthesis of layered double hydroxides by various methods are given in Table 1.1.

1.3

Organic Modification of Layered Double Hydroxides

The prime objective of organic modification of layered double hydroxides is to enhance the interlayer spacing of LDH materials (Fig. 1.4) so as to make it easily accessible for the intercalation of large hydrophobic polymer chains. Some important organic modifiers used for the modifications of LDHs are given in Table 1.2 The fine tuning of the inter lamellar galleries of layered double hydroxides with suitable organic anions offer exfoliated layered double hydroxide polymer nanocomposites with multifaceted applications such as biomedical, energy, food packaging, flame retardant, gas barrier, agricultural, water purification etc. This is also an efficient way to tailor the nano space in the intergallery region with functional materials like pesticides, drugs, nucleic acids, enzymes etc for vast applications such as controlled pesticides release, drug delivery, gene delivery, biosensors etc. Organic modification can be carried out successfully by following any one of the important strategies such as anion exchange reaction, reconstruction, direct synthesis and restacking (Leroux and TaviotGue´ho, 2005). Sometimes bilayers, instead of mono layers of organic anions are formed in the inter gallery space as a result of which basal spacing becomes double that is a boon for producing polymer nanocomposites. This happens when the process is carried out in the presence of excess of organic anions for a long time with

Table 1.1 Some examples for the synthesis of layered double hydroxides by various methods (Yan et al., 2016; Prince et al., 2009; Wu et al., 2014; Xie et al., 2008; Baskaran et al., 2014; Yarger et al., 2008; Raynal et al., 2014; Liu et al., 2006a; Liu et al., 2006b). Method of Synthesis

Precursors

Reaction conditions

Type of LDH formed

References

Urea hydrolysis

Ni (NO3) 2  6H2O Fe (NO3) 3  9H2O Ni (NO3) 2  6H2O Fe (NO3) 3  9H2O Co (NO3) 2  6H2O, Al (NO3) 3  9H2O

NO32 /urea molar ratio of 0.25; hydrothermal treatment at 110 Cfor 24 h NO32/urea molar ratio of 3.0; hydrothermal treatment at 110 C for 24 h solution1of Co21, Al31salt; solution 2 of (NH4) 2CO3 and NH4OH; solution 2 was added dropwise to solution1 with constant stirring at 40 C for 1 h, with pH 8.5; washed, filtered, and dried at 80 C overnight NaOH solution was continuously dropped into the mixed salt solution until pH 7.8; stirred for 30 min; hydrothermally treated at 110 C for 3 h; filtered, washed, and then dried at 80 C for 12 h Solution 1of NaOH and Na2CO3, and solution2 of mixed salt were continuously dropped. into a beaker with constant pH 9.5; aged at 100 C for 13 h; filtered, washed, and dried at 100 C for 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 C for 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 Cfor 24 h HNO3 as hydrolysis acid, temperature of 0 C, ethanol as solvent, dried at 70 C for 24 h

NiFeCO3LDH

Wu et al. (2014)

NiFe
NO3LDH CoAl
CO3LDH

Wu et al. (2014)

MgCoAl
NO3 LDH

Xie et al. (2008)

MgAl
CO3 LDH

Zhao et al. (2014)

Mg Al LDH

Prince et al. (2009)

Ni Al LDH

Prince et al. (2009)

NiCoAl LDHs

Prince et al. (2009)

Co precipitation

Mg (NO3) 2  6H2O, Co (NO3) 2  6H2O, Al (NO3) 3  9H2O Mg (NO3) 2  6H2O, Al (NO3) 3  9H2O

Sol-Gel method

Mg (OEt) 2, Al (OsBu) 3 Ni (OAc) 2, Al (Os-Bu) 3 Co (OAc) 2, Ni (OAc) 2, Al (Os-Bu) 3

Baskaran et al. (2014)

Electro deposition

Zn (NO3) 2  6H2O Al (NO3) 3  9H2O

Working electrodes preparation by e-beam evaporation ˚ titanium and 600 A ˚ of platinum, followed by of 200 A ˚ gold on a cleaned glass substrate. After each 2000 A electro deposition wash the film with de ionized water and dry with gentle stream of N2 gas. Ag/AgCl electrode-reference electrode Pt wire counter electrode

Zn-Al NO3LDH

Yarger et al. (2008)

ZnSO4  7H2O CoSO4  7H2O

Zn21: Co21 5 2: 1 in molar ratioH2O2: Co21 55 5 1: 2 in molar ratio. A cleaned Ni foil (1 cm 3 1 cm in square) -working electrode, Ag/Ag Cl electrodereference electrode Pt wire counter electrode Al substrate dipped in Zn21 solution

ZnCo-LDH films

Li et al. (2014)

Zn Al LDH

Liu, Jinping, et al (2006a) Liu, Zhaoping, et al (2006b)

In-situ growth Anion exchange

CoCl2.6H2O AlCl3.6H2O

Co (NO3) 2.6H2O Al (NO3) 3. 9H2O

Co-Al-CO3 LDH was synthesized by urea method by mixing and refluxing 10, 5, and 35 mM solutions of CoCl2.6H2O, AlCl3.6H2O, and urea in 1 dm3 of deionized water for 2 days in the presence nitrogen with magnetic stirring. Treat Co-Al-CO3 LDH with salt-acid mixed solution (NaCl-HCl) in an inert atmosphere of nitrogen at ambient temperature. Disperse NaCl-HCl treated LDH sample into 500 cm3 of an aqueous solution containing 0.1 M sodium nitrate and sodium per chlorate respectively Solution 1 Co (NO3) 2.6H2O (1 M) and Al (NO3) 3. 9H2O (0.33 M) in 2.6 L deionized H2O. Solution 2 (NH4) 2CO3 (0.67 M) and NH4OH (3.27 M) in 2.225 L deionized H2O. Add solution 2 to solution1 with constant stirring at 40  C for 1 h. Silicate solutions of appropriate concentrations were added slowly to the HT gel at room temperature and stirred for 48 h. Exchange of CO322 by silicate anion

Co-Al-Cl LDH Co-Al-NO3LDH Co-Al-ClO4LDH

CoAl-HT-Si

Baskaran et al. (2014)

10

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.4 Schematic representation of ion-exchange technique. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186 with kind permission of Elsevier

Table 1.2 Some important organic modifiers used for the modifications of LDHs (Basu et al., 2014). Organic Modifiers

Structure

Symbol

Sodium dodecyl benzenesulfonate

SDBS

Sodium dodecyl sulfate

SDS

Stearic Acid

SA

Oleic Acid

OA

O OH

Lauric acid

LA

Bis (2-ethylhexyl) hydrogen phosphoric acid

BEHP

Layered double hydroxides: fundamentals to applications

11

Figure 1.5 Schematic showing the arrangement of MAPK anions in the OLDH interlayer region. (b) TEM images and size distribution of the OLDH. Source: Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

vigorous shaking. Xie et al. (2018) intercalated mitogen activated protein kinase (MAPK) into the interlamellar region of Zn Al LDH by solvent free high energy ball milling and the organically modified ZnAl-MAPK OLDH is incorporated into PBAT matrix to form nanocomposite for food packaging applications. By carefully analyzing the basal spacing for OLDH obtained from XRD analysis (4.07 nm), the alkyl chain length of mitogen activated protein kinase (MAPK) (1.86 nm) and the thickness of metal hydroxide layer (0.47 nm) in the LDH sheets, it can be obviously concluded that bilayer of MAPK anions are formed in the interlayer space (Fig. 1.5).

1.3.1 One step Co-precipitation In early days, organically modified layered double hydroxides are prepared via two step procedure. The first step is the preparation of LDH containing CO322, NO32 and Cl2 and the second step is the exchange of these anions by organic anions. Now- adays, in most of the reported works, researchers merge these two steps into one and called it as one step co- precipitation. One step co precipitation involves the slow addition of a solution of target anion into a solution containing divalent and trivalent ion. It is noted that thermal treatment is essential to improve the crystallinity and the addition of alkali maintains the pH at the required level to achieve the co precipitation of the two metallic salts (Mishra et al., 2018). Wang et al. (2015) extracted bio-based modifier (cardanol-BS) from renewable resource cardanol through the ring-opening of 1, 4-butane sultone (BS). Cardanol-BS modified layered double hydroxide (m-LDH) was prepared by one-step co- precipitation method and fabricated EP/ m-LDH nanocomposite for flame retardant application (Scheme 1.1).

12

Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.1 Diagrammatic Illustration of the Synthetic Route of Cardanol-BS Modified LDH. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanol-based surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290. with kind permission of ACS

1.3.2 Anion exchange reaction The procedure for the anion exchange reaction implicates the dispersal of pristine LDH into the aqueous solution of the anionic surfactant that has to be introduced into the inter lamellar galleries so as to replace the existing anion to produce the organically modified LDHs, with constant stirring at room temperature for several hours (Basu et al., 2014). Anion exchange plays a major role in the widespread applications of layered double hydroxides and their composites as this method is the principal step for introducing functional anions in the interlamellar region, pillaring and delamination.

1.3.3 Memory Effect or Regeneration Method It is the widely accepted method for the modification of layered double hydroxides in which heating of LDH in a Muffle furnace at 450 C for 3 hours leads to the formation of amorphous mixed metal oxide. The mixed oxide is dispersed into water containing anionic surfactant which is to be introduced into the inter lamellar galleries of LDH and kept for 24 hours with constant magnetic stirring. During this stirring carbonate anions already present in the inter lamellar galleries are replaced by the organic anions and the material memorize its original structure so the name

Layered double hydroxides: fundamentals to applications

13

3 hours @ 450°C Calcination in a muffle furnace

Unmodified LDH

Centrifuging and drying @ 60°C to a constant weight Modified LDH as solid residue

Addition of mixed oxide into a specific volume of a surfactant solution (0.1–0.2 M)

Mixed oxide

Stirring for 24 hours at ambient temperature Dispersion into aq. solution of the desired surfactant

Figure 1.6 A simplified schematic showing the synthesis route to obtain modified LDH from unmodified LDH. Source: Adapted from Basu, D., Das, A., Sto¨ckelhuber, K.W., Wagenknecht, U. and Heinrich, G., 2014. Advances in layered double hydroxide (LDH) -based elastomer composites. Progress in Polymer Science, 39 (3), pp. 594-626 with kind permission of elsevier

Figure 1.7 Mg/Al LDH loaded with 5-fluorouracil (5-FU) via memory effect. Source: Adapted from Peng, F., Wang, D., Cao, H. and Liu, X., 2018. Loading 5-Fluorouracil into calcined Mg/Al layered double hydroxide on AZ31 via memory effect. Materials Letters, 213, pp. 383-386 with kind permission of elsevier

memory effect (Basu et al., 2014) (Fig. 1.6). Peng et al. (2018a) successfully intercalated anticancer drug 5-Fluorouracil in the intergallery space of Mg/Al LDH via memory effect for drug delivery applications. Mg/Al LDH prepared in-situ by a hydrothermal process and the original layered structure is recovered by adding calcined LDH into a solution containing anticancer drug molecules 5-Fluorouracil (Fig. 1.7).

1.3.4 The delamination/restacking method In this method, the complete separation of LDHs into single sheets by using suitable solvent so that a stable colloidal suspension is obtained. The aqueous

14

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.8 Schematic illustration of the preparation process for metallo- porphyrin intercalated LDH nanocomposites via the exfoliation/ restacking route.

solution of organic anion is added to this colloidal solution in an inert atmosphere of nitrogen. Then the colloidal solution is subjected to drying, well-ordered LDHs intercalated with desired anions are obtained (Richetta et al., 2017). Ma et al. (2014) intercalated metalloporphyrin into the intergallery space of Ni-Al LDH and Mg-Al LDH by delamination /restacking method. The prepared LDH-CO3 is converted into LDH-NO3 by direct salt-acid method. It is delaminated by shaking with formamide and aqueous solution of metalloporphyrin is added to this colloidal solution in the presence of an inert atmosphere of nitrogen (Fig. 1.8) (Ma et al., 2014).

1.4

Characterization of layered double hydroxides and modified layered double hydroxides

The success of synthesis of LDHs and modification of LDHs can be evaluated by the characterization techniques such as SEM, TEM, AFM, XRD, FTIR, EDS, Raman spectroscopy and X-ray photoelectron spectroscopy and thermo gravimetric analysis. The sharp reflections corresponding to the (00n) planes in the powder X-ray diffraction patterns confirm the formation of LDH crystals and shifting of the typical and characteristic Bragg reflections in the diffraction pattern confirm the successful organic modification of the layered double hydroxides. The basal spacing of LDH and modified LDH can be calculated from the 2θ value corresponding to the first-order basal plane reflection by using Bragg’s equation. The shifting of the first-order basal plane reflection towards higher d-values or lower

Layered double hydroxides: fundamentals to applications

15

2θ values indicates the successful intercalation of organic anions into the inter lamellar gallery of layered double hydroxides. XRD analysis goes into the in-depth study of the crystal by providing the unit cell parameters, miller indices crystallite size or particle size (calculated with the help of Scherrer equation) etc. SEM and TEM give valuable information regarding the size and shape of the LDHs and modified LDHs. The chemical compositions of the LDHs and modified LDHs are determined by energy-dispersive X-ray spectra (EDS). The thickness of LDHs and modified LDHs can be detected from the AFM height images and the corresponding height profiles. The FTIR spectrum confirms the formation of LDH by providing peaks corresponding to interlayer anion, interlayer water molecules, O-H of metal hydroxide layer, M-O lattice etc (Nagendra et al., 2017a; Nagendra et al., 2015; Nagendra et al., 2017b). The increase in basal spacing obtained from XRD and the characteristic peak corresponding to the organic modifier from the FTIR spectrum and visualization of increase in basal spacing by taking the image by high resolution TEM etc indicates the successful modification of layered double hydroxides with the organic modifiers. Some of the characterization techniques are illustrated here with suitable examples, which are taken from some previously reported research work, for the better understanding of this topic. Nagendra et al (2017b) synthesized Co 2 Al, Zn 2 Al LDH and Co 2 Zn 2 Al LDH by co precipitation method. The size and shape of these LDHs were characterized by SEM and TEM analysis and visualized as circular platelets, hexagonal platelets with rounded edges, and hexagonal platelets with sharp edges morphologies respectively for Co 2 Al LDH, Zn 2 Al LDH, and Co 2 Zn 2 Al LDH (Fig. 1.9). This difference in shape is attributed to the difference in nucleation and growth mechanism of LDH in the presence of different metal salts. The crystallinity was confirmed by XRD and the prepared LDH crystals are found to be highly pure as there were no peaks other than typical LDH (Fig. 1.10a). Because of water molecules in the interlamellar region and hydroxyl groups in the LDH layers, a dominant oxygen peak was observed in EDS of different LDH. (Fig. 1.9) The FTIR bands at 1356 and1382 cm21 indicated the presence of carbonate and nitrate respectively, the bands at 3440 cm21 (ν (O-H) ) and 1632 cm21 (δ (H2O) ) indicated the presence of water molecules in the inter lamellar region (Fig. 1.10b). Xu et al. (2013b) synthesized LDH-CO3 by urea method and converted it to LDH-NO3 by an acid
salt direct exchange method. The reason for the conversion is that it is difficult to substitute interlayer CO322 anions by organic anions directly, because of high electrostatic attraction between CO322 and LDH platelets. After the first conversion, LDH NO3 was again converted to LDH-DS and LDH -DBS by anion exchange reaction. From the XRD data, it is clear that the basal space of LDH-NO3, LDH-DS, and LDH-DBS are 0.88 nm, 2.78 and 2.96 nm respectively which are found to be well agreement with the theoretical values (Fig. 1.11 and Fig. 1.12). The characteristic reflection peak of LDH-CO3 corresponding to 2θ 5 11.8 is absent in XRD graphs of all the other LDHs mentioned above which indicates complete conversion of CO322. XRD data of unmodified and some organically modified LDHs are given in Table 1.3. In addition to that, the characteristic absorption band of the carbonate at around 1352 cm21 was not observed in the

Figure 1.9 SEM, TEM, and EDS analysis of the as-synthesized LDH: (a) Co 2 Al LDH, (b) Zn 2 Al LDH, and (c) Co 2 Zn 2 Al LDH. Source: Adapted from Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

Figure 1.10 (a) Powder XRD patterns and (b) FTIR spectra of as-prepared Co 2 Al LDH, Zn 2 Al LDH, and Co 2 Zn 2 Al LDH. Source: Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

Layered double hydroxides: fundamentals to applications

Figure 1.11 Schematic diagrams showing the theoretical calculation results of the anion alignments in the interlayer of LDH-DS (a) and LDH-DBS (b). Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119 with kind permission of Elsevier

Figure 1.12 XRD patterns of the LDH-NO3, LDH-DS and LDH-DBS samples. Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119. with kind permission of elsevier

17

18

Layered Double Hydroxide Polymer Nanocomposites

Table 1.3 XRD data of unmodified and some organically modified LDHs (Basu et al., 2014; Xu et al., 2013b; Costa et al., 2008; Liu et al., 2008; Manzi-Nshuti et al., 2009). Type of LDH

Basal spacing (nm)

LDH NO3 LDH-DS LDH- DBS LDH-laurate LDH-oleate LDH-stearate LDH-BEHP (Bis (2-ethylhexyl) hydrogen 3phosphate-modified LDH)

0.89 2.78 2.96 2.45 3.4 3.37 1.52

FTIR spectrum of LDH-NO3, LDH-DS, and LDH-DBS. At the same time, some new peaks were observed at 1384 cm21, 2957, 2920 and 2845 cm21. The first peak arises due to the stretching mode of NO32 and the three other peaks arises as a result of asymmetric and symmetric stretching vibrations of CH3/CH2 group of long alkyl chains of the DS and DBS anions. Some additional bands were observed at 1220 cm21, 1200 and 1042 cm21, 1133 and 1011 cm21, first band results from the DS anions, a couple of bands (second and third) characteristic of the DBS absorption and the next pair of bands (fourth and fifth) characteristic of the C-H aromatic in-plane bendings of the LDH-DBS. In addition to all these bands, a broad absorption band appeared between 3700 and 3000 cm21 can be assigned to the O-H group stretching (Fig. 1.13). FTIR bands of some organically modified LDHs are given in Table 1.4. The information obtained from thermo gravimetric analysis of various types of LDHs reported in literature is that thermal stability of layered double hydroxides is only up to 200 C and above that temperature it decomposes to amorphous mixed metal oxides. Yang et al. (2002) conducted several in situ techniques such as DRIFTS, TG/DTA, MS, and HTXRD to detect the thermal evolution of the structure of an Mg
Al
CO3 layered double hydroxide (LDH) under an inert atmosphere (Fig. 1.14). The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) helped to determine the changes in the functional groups. TG/DTA investigate the changes in mass and energy changes (Fig. 1.15), MS identifies the products liberated during each stage of heating and HTXRD monitors the structure evolution. A model has been proposed on the basis of the study to describe the structural evolution of the Mg
Al
CO3 LDH. In this model, the conversion of layered double hydroxides into mixed metal oxides is divided into four stages. In the first stage, between 70
190 C, removal of interlayer water takes place and a change from Phase I with a basal spacing ranging from 7.5 to 7.3 A to Phase II with basal spacing of 6.6 A which indicates that no change in the LDH tactoid structure. In stage 2, between 190
280 C, the OH2 group bonded to Al31 disappears, in the third stage, between190
280 C, the OH 2 group linked to Mg21disappears and in the final stage, between 405
580 C, loss of CO322 takes

Layered double hydroxides: fundamentals to applications

19

Figure 1.13 FTIR spectra of the LDH-NO3, LDH-DS and LDH-DBS samples. Source: Adapted from Xu, K., Chen, G. and Shen, J., 2013. Exfoliation and dispersion of micrometer-sized LDH particles in poly (ethylene terephthalate) and their nanocomposite thermal stability. Applied Clay Science, 75, pp. 114-119. with kind permission of elsevier

place. At this temperature range, the material completely changes to an amorphous meta stable mixed solid oxide solution (Selvam et al., 2014; Yang et al., 2002). It is to be noted that thermal behaviour of modified LDH is different from that of unmodified LDH because of the presence of organic anions in the interlamellar region. The thermal behaviour of modified layered double hydroxides was greatly influenced by the nature of the anionic surfactant which can be easily understood from the Fig. 1.16. Costa et al. (2008) prepared Mg-Al-LDH by urea hydrolysis and modified it with DS, DBS, laurate and BEHP. In the Fig. 1.16 the loss of interlayer water below 225 C in all the modified form LDH-DS, LDH-DBS and LDH-laurate except LDH-BEHP are in a similar manner as that of unmodified LDH. The difference in LDH-BEHP is due to the less amount of water in the interlayer region due to the branching of alkyl chain. The shifting of first decomposition stage to lower temperatures in the modified samples can be attributed to the decrease in force of attraction of interlayer water to the sheet due to the increase in basal spacing during modification. There is marked variation in the second decomposition temperature in all the modified forms. In the case of SDBS-LDH, two step decomposition was observed between 240
600 C and the two peaks are found to be greater than that of unmodified LDH. It is due to the decomposition of SDBS in this region and the decomposition of aromatic ring and alkyl chain in the absence of oxygen delay

20

Layered Double Hydroxide Polymer Nanocomposites

Table 1.4 FTIR bands of some organically modified LDHs (Basu et al., 2014; Xu et al., 2013b; Costa et al., 2008; Liu et al., 2008; Manzi-Nshuti et al., 2009). Modified LDH

Band Region

Types of vibrations

LDH
SDS

2850
2965 1229 1065 630 671, 820, 1379 and 1468 426 2850
2965 1186 1038 615 1602, 1496, 1409 and 1450 674, 833, 1379 and 1467 426 2850
2965 1563 1412 680, 870, 1378 and 146 425 3500 3012 2800-3000 1400-1600 3500

ν -CH2ν S 5 O symmetric ν S 5 O asymmetric ν C-S Different vibration modes of CO322 M-O lattice vibration ν -CH2ν S 5 O symmetric ν S 5 O asymmetric ν C-S ν C-H aromatic in plane bending

LDH
DBS

LDH-laurate

LDH-oleate

LDH- stearate

LDH-BEHP (Bis (2-ethylhexyl) hydrogen phosphate-modified LDH)

2800-3000 1542 2850
2965 1037 and 1136 1220 671,880,1380 and 1465 443

Different vibration modes of CO322 M-O lattice vibration ν -CH2ν COO- (asymmetric) ν COO- (symmetric) Different vibration modes of CO322 M-O lattice vibration ν-OH layer hydroxide νCH attached to double bond ν-CH2-and CH3ν-COO2symmetric and asymmetric ν-OH layer hydroxide and interlayer water ν-CH2- and CH3ν COO- (asymmetric) ν -CH2ν P-O-C symmetric ν P 5 O anti symmetric Different vibration modes of CO322 M-O lattice vibration

the decomposition of the host material. A greater loss below 250 C was observed only for LDH-SDS because of the decomposition of SDS between 210250 C. In the case of LDH-laurate and LDH
BEHP large weight loss occurs between 250 C-350 C and is due to the decomposition of the interlayer surfactant anions.

Layered double hydroxides: fundamentals to applications

21

Figure 1.14 The thermal evolution of Mg
Al
CO3 LDH as a function of temperature. Source: Adapted fromYang, W., Kim, Y., Liu, P.K., Sahimi, M. and Tsotsis, T.T., 2002. A study by in situ techniques of the thermal evolution of the structure of a Mg
Al
CO3 layered double hydroxide. Chemical Engineering Science, 57 (15), pp. 2945-2953 with kind permission of elsevier

1.5

Potential applications of layered double hydroxides, organically modified layered double hydroxides and layered double hydroxide nanocomposites

Layered double hydroxides can be regarded as promising layered nano materials due to the remarkable properties such as uniqueness in structure, distribution of two or three different types of metal cations in the LDH layer, hydroxyl groups in the surface, simple synthesis methods, non toxicity, flexible tunability in both anions and cations, excellent anion exchangeability, memory effect, appreciable chemical and thermal stability, high power to deliver the intercalated anion in a sustained manner, biocompatibility, electrochemical activity, high surface to volume ratio, high adsorbing power, endothermic decomposition etc. Because of these improvising properties, layered double hydroxides itself, LDH intercalated with functional materials and LDH based nanocomposites with superior nano materials especially carbon nano materials are good precursors for world wide applications to meet the requirements of the society. Some important applications of layered double hydroxides and hybrid layered double hydroxides-intercalated and nanocomposites such as flame retardancy, catalysis, energy storage and conversion such as electrode for super capacitor,

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.15 In situTG/DTA of Mg
Al
CO3 LDH as a function of temperature. Source: Adapted fromYang, W., Kim, Y., Liu, P.K., Sahimi, M. and Tsotsis, T.T., 2002. A study by in situ techniques of the thermal evolution of the structure of a Mg
Al
CO3 layered double hydroxide. Chemical Engineering Science, 57 (15), pp. 2945-2953 with kind permission of elsevier

Figure 1.16 TGA plots of LDH and its modified forms. Source: Adapted from Costa, F.R., Leuteritz, A., Wagenknecht, U., Jehnichen, D., Haeussler, L. and Heinrich, G., 2008. Intercalation of Mg
Al layered double hydroxide by anionic surfactants: preparation and characterization. Applied Clay Science, 38 (3-4), pp. 153-164. with kind permission of elsevier.

Layered double hydroxides: fundamentals to applications

23

splitting of water etc, environment remediation such as waste water treatment and preventing air pollution, controlled drug delivery and pesticide release and all these applications are illustrated here with most relevant example for each application.

1.5.1 Flame retardant applications Layered double hydroxides are promising green flame retardant materials and can retard the growth of flame through three distinct means. (1) It performs as heat sink due to the endothermic decomposition (2) It decomposes to form mixed metal oxides which act as an insulating film on the surface (3) It releases bound water and carbon dioxide thereby diluting the flammable gases (Radulescu et al., 2008). The flame retardancy of LDHs can be enhanced by intercalating suitable anions such as borate, phosphate etc into the inter lamellar region of LDH so that a single substance such as LDH containing zinc cations and borate anion can achieve the synergistic effect of three commercial flame retardants magnesium hydroxide, aluminium hydroxide and zinc borate. The main advantage of usage of LDH over the commercial flame retardants is the requirement of only very low concentration, non toxicity and high efficiency (Gao et al., 2014b). Guo et al. (2017) applied a coating of Mg 2 Al layered double-hydroxide (LDH) on wood surface for flame retardant applications by a two step synthetic method and the formation of coating on the surface of wood was confirmed by SEM analysis (Fig. 1.17).

Figure 1.17 Schematic diagram and SEM images show the formation of the Mg 2 Al LDH coating via one-step co precipitation (a, e) and two-step process with (b, f) 8, (c, g) 10, and (d, h) 12 h hydrothermal treatment, respectively. Source: Adapted from Guo, B., Liu, Y., Zhang, Q., Wang, F., Wang, Q., Liu, Y., Li, J. and Yu, H., 2017. Efficient flame-retardant and smoke-suppression properties of Mg
Al-layered double-hydroxide nanostructures on wood substrate. ACS applied materials & interfaces, 9 (27), pp. 23039-23047 with kind permission of ACS.

24

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.18 CONE combustion parameters of untreated wood and the Mg 2 Al LDH-coated wood: (a) Total smoke production, (b) smoke production ratio, (c) heat release rate, (d) specific extinction area, (e) effective heat of combustion, and (f) total heat release. Source: Adapted from Guo, B., Liu, Y., Zhang, Q., Wang, F., Wang, Q., Liu, Y., Li, J. and Yu, H., 2017. Efficient flame-retardant and smoke-suppression properties of Mg
Al-layered double-hydroxide nanostructures on wood substrate. ACS applied materials & interfaces, 9 (27), pp. 23039-23047 with kind permission of ACS

The limiting oxygen index enhanced from18.9% to 39.1% and heat release index and smoke emission decreased by 40% and 58% respectively with respect to untreated wood (Fig. 1.18). The enhancement in flame retardancy can be attributed to the endothermic decomposition of LDH into mixed metal oxide which resulted in char formation. The hydrophilicity of LDH is changed to hydrophobic by surface modification by fluoro alkyl silane which is evidenced from contact angle measurement and the value is 152 . The increment in storage modulus by32% indicates that there is also increment in mechanical strength.

Layered double hydroxides: fundamentals to applications

25

1.5.2 Catalysis LDHs both as directly prepared and after thermal treatment and/or reduction have been widely used as stable and recyclable heterogeneous catalysts or catalyst supports for a variety of reactions benefitting the flexible tunability and uniform distribution of metal cations in the LDH layers and the facile exchangeability of intercalated anions. Layered double hydroxides are promising heterogeneous solid base catalysts for a number of organic transformations due to the abundance of hydroxyl groups. On heating between 450-500 C, LDHs have been changed to mixed metal oxides having a number of Lewis base sites and enhanced surface area which results in high catalytical efficiency. The rehydration of mixed metal oxide in the absence of CO2 can reproduce LDH with OH intercalated anion and this activated LDH possesses abundant Bronsted type basic sites. The number and the strength of OH sites can be controlled by varying the conditions of calcination. It is reported that the activated MgAl
LDH nano crystallites supported on carbon nanofibers (CNFs) is a very good catalyst for the trans esterification of glycerol with diethyl carbonate to form glycerol carbonate. This improvement in catalytical activity was attributed to the small size of the MgAl
LDH crystallites and number of accessible active OH sites in catalysts due to the accessible pores of the CNFs. The catalytic efficiency of binary LDH can be enhanced by incorporating third cation to form a ternary LDH. Polyoxometalate-intercalated catalysts can be widely used as heterogeneous catalysts for the acid-catalyzed esterification, oxidation of alcohols, alkenes etc (Fan et al., 2014). Wu et al. (2014) synthesized NiFe
NO3-LDH by urea hydrolysis for applying it as a catalyst for the one-pot synthesis of benzoin ethyl ether from benzaldehyde and ethanol. It was observed that the maximum percentage of conversion of benzaldehyde is 51.5% with a selectivity of 100% for benzoin ethyl ether. The catalytical activity of LDH in the above reaction is mainly attributed to the porous structure and L acid site of LDH. Baskaran et al. (2014) synthesized cobaltaluminium hydrotalcite (CoAl-HT-Si) with silicate ion as intercalated anion and proved that this LDH is a promising catalyst for variety of alcohol oxidation. The layer by layer assembly of LDH with oppositely charged species through electrostatic forces or/and hydrogen bonding will result in the formation of nanocomposites with excellent catalytic performances. Because of high specific surface areas, high aspect ratio, mechanical strength, thermal stability and electronic conductivity, carbon based nano materials such as carbon nanotube, graphene, carbon fibres etc are found to be excellent LDH catalyst supports in various heterogeneous reactions by enhancing their dispersion, heat, and mass transfer during the reaction, and provides mechanical strength for the whole composites. NiAl
LDH/CNTs nanocomposite modified electrode shows more electro catalytical activity for glucose electro oxidation than NiAl
LDH modified electrode or CNTs modified electrode. This can be attributed to the contribution of CNT in transporting more charge between Ni centers and electrode and promoting the diffusion of the reactants by providing a porous network like structure. A big challenge in this field is to improve simultaneously the activity, selectivity and stability of these LDH-based materials for catalytical purpose by finding new strategies to tailor the electronic structure of the catalysts and supports.

26

Layered Double Hydroxide Polymer Nanocomposites

1.5.3 Water splitting Layered double hydroxide nanocomposites can act as excellent photo catalysts and electro catalyst for water splitting process there by providing hydrogen fuel for energy production which is essential for solving the energy crisis in the coming years as its production via these methods is low cost, high efficiency, environmentally benign. Because of the unique properties of layered double hydroxide such as cation-exchange ability, anion-exchange ability, adsorption capacity etc, LDHs offer enhancement in light absorption, charge separation, electron transfer, electrode reaction kinetics and durability. For the last two decades, there has been rapid progress in the design of LDH based nanocomposites for water splitting purpose in which the above phenomena were increased by properly tuning the structure and size of LDH and selecting novel materials such as graphdiyne (Shi et al., 2018), graphene oxide (Ma et al., 2015), graphitic carbon nitrides (Bhowmik et al., 2018), carbon quantum dot (Tang et al., 2014), etc which are capable of promoting the photo and electro catalytic ability of layered double hydroxides. LDH materials containing transition metal cations, especially Ni and Co have been widely applied in electro catalytic reactions. The intercalation of simple metal oxy-anions and POMs in LDH hosts can lead to photo catalytic capability. Yan et al. (2016) have published a review article on recent advances in the synthesis of layered double hydroxide based materials and their applications in hydrogen and oxygen evolution. Shi et al. (2018) synthesized GDY@NiFe LDH composite by coupling reaction followed by electro deposition method (Fig. 1.19). The enhancement in the OER activity of the composite can be attributed to the increase in electrical conductivity as well as surface active areas due to the synergistic effect of LDH and GDY. The high electrical conductivity of GDY due to unique electronic structure helped the

Figure 1.19 (a) Schematic illustration for the formation of GDY@NiFe architectures. SEM images of (b) pure Cu foil, (c) GDY, (d) NiFe LDH, and (e) the GDY@NiFe composite. Source: Adapted from Shi, G., Yu, C., Fan, Z., Li, J. and Yuan, M., 2018. Graphdiynesupported NiFe layered double hydroxide nanosheets as functional electrocatalysts for oxygen evolution. ACS applied materials & interfaces, 11 (3), pp. 2662-2669. with kind permission of ACS

Layered double hydroxides: fundamentals to applications

27

rapid flow of electrons and LDH provide abundant active sites for oxygen evolution (Fig. 1.20). GDY@NiFe LDH composite shows good catalytic activity in an alkaline electrolyte for a small over potential of 260 mV to acquire the current density of 10 mA cm22 Scheme 1.2.

1.5.4 Environmental remediation Layered double hydroxides are capable of adsorbing toxic metals and ions and organic dyes from waste water and toxic gases liberated into the atmosphere from automobiles and industries owing to its large specific area, porosity, high anion exchangeability and non toxicity and are promising candidates for environment remediation technologies (Mishra et al., 2018). Mahjoubi et al. (2017) synthesized four layered double hydroxides with different anions Zn-Al-SO4, Zn-Al-Cl, Zn-AlNO3 and Zn-Al-CO3 by co precipitation method and dye adsorption experiments were conducted at various conditions. The precursors showed exceptional Langmuir maximum adsorption capacities of 2758, 2455, 2270 and 1684 mg/g for Zn-Al-SO4, Zn-Al-Cl, Zn-Al-NO3 and Zn-Al-CO3, respectively indicating that these materials are potential broad-spectrum adsorbent for dye removal from wastewater. Li et al. (2016) synthesized MgAl-CO322LDH by an ethanol-water mediated solvothermal method for adsorbing congo red dye from waste water and determined that maximum adsorption efficiency of the LDH and calcined LDH are 129.9 and 143.27 mg g21, respectively. The adsorption mechanism of LDH is anion exchange where as that of calcined LDH is reconstruction. LDH hybrids obtained by coupling it with different anions like humate, EDTA, glutamate, tartrate, MoS422, polysulfide, carbon-based materials like CNT, graphene, iron ferrite nanoparticles and with some other compounds like TiO2, MnO2 and silica are found to be more powerful adsorber of toxic pollutants than LDH. This is due to the enhancement in surface area, anion exchange power, stability, chelating and binding sites, excellent selectivity for different metal ions, low toxicity and easy way of separation and reusability when it is integrated with magnetic particles. Koilraj, P et al (2018) fabricated Mg Al-LDH/graphene oxide (GO), 2D/2D multifunctional nanocomposite by dispersion-coagulation method for the adsorption of Sr21 and SeO422 from waste water. The SeO422 removal happened due to the anion exchange ability of LDH and it exchanges NO32 with SeO422. The Sr21 removal takes place as a result of its coordination with
COO- or
CO- group in GO produced by the ring opening of epoxides. (Scheme 1.3) The observed sorption efficiency of SeO422 on the Mg Al-LDH/GO (5%) composite was found to be 0.835 mmol/g (65.9 mg/g) and that of Sr21 on the Mg Al-LDH/GO (5%) composite was 213.35 mg/g (2.435 mmol/g) of GO and the values remained constant for pH range 4-10 (Fig. 1.20). The Anionic/LDH obtained by incorporating anions such as humate, EDTA, tartrate etc into the interlamellar region of LDH by anion exchange method have cumulative characteristics of both the LDH and the anion agents, which consequently increased the adsorptive power towards toxic pollutants (Zubair et al., 2017). Garcia-Gallastegui et al. (2012) designed and fabricated Mg-Al-LDH /GO hybrid by the self mode layer by layer assembly of positively charged LDH sheet and

Figure 1.20 Sorption isotherms of (A) Sr21 cation and (B) SeO422 anion on MgAl-LDH/GO (5%) and MgAl-LDH/GO (20%) composites in the single electrolytic solution. Source: Adapted from Koilraj, P., Kamura, Y. and Sasaki, K., 2018. Cosorption Characteristics of SeO42
and Sr2 1 Radioactive Surrogates Using 2D/2D Graphene Oxide-Layered Double Hydroxide Nanocomposites. ACS Sustainable Chemistry & Engineering, 6 (11), pp. 13854-13866 with kind permission of ACS

Layered double hydroxides: fundamentals to applications

29

Scheme 1.2 Schematic Diagram of the Proposed Process of OER on the GDY@NiFe Sample. Source: Adapted from Shi, G., Yu, C., Fan, Z., Li, J. and Yuan, M., 2018. Graphdiynesupported NiFe layered double hydroxide nanosheets as functional electrocatalysts for oxygen evolution. ACS applied materials & interfaces, 11 (3), pp. 2662-2669. with kind permission of ACS

Scheme 1.3 Mechanism of Sr21 sorption onto the alkoxide moiety present in the MgAl-LDH/GO composite and their nano aggregate formation. Source: Adapted from Koilraj, P., Kamura, Y. and Sasaki, K., 2018. Cosorption Characteristics of SeO42
and Sr2 1 Radioactive Surrogates Using 2D/2D Graphene Oxide-Layered Double Hydroxide Nanocomposites. ACS Sustainable Chemistry & Engineering, 6 (11), pp. 13854-13866 with kind permission of ACS

negatively charged graphene oxide (Scheme 1.4). Here graphene supports layered double hydroxide for adsorbing carbon dioxide thereby increasing the efficiency. The enhancement in adsorption capacity and multicycle stability can be attributed

30

Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.4 Schematic Representation of the LDH and GO at Different Mass Ratios Highlighting the Degree of Surface Coverage Expected. Source: Adapted from Garcia-Gallastegui, A., Iruretagoyena, D., Gouvea, V., Mokhtar, M., Asiri, A.M., Basahel, S.N., Al-Thabaiti, S.A., Alyoubi, A.O., Chadwick, D. and Shaffer, M.S., 2012. Graphene oxide as support for layered double hydroxides: enhancing the CO2 adsorption capacity. Chemistry of Materials, 24 (23), pp. 4531-4539 with kind permission of ACS

to the increased particle distribution. The adsorption capacity of LDH has been increased by more than 60 % for only 7 wt% GO concentration (Fig. 1.21).

1.5.5 Electrode for super capacitor Layered double hydroxides are superior material for making pseudo-capacitor electrodes because of the presence of electrochemically active surfaces, environment friendly nature and low cost. On the basis of Faradic electrochemical reaction, LDHs containing transition metals show larger specific capacity in aqueous alkaline electrolytes than LDH with other metal ions. LDHs have some limitations for acting as super capacitor electrode due to low conductivity and strong stacking tendency. In order to avoid this, it is coupled with nano materials with high conductivity and specific area like graphene, multiwalled carbon nanotube etc and the nanocomposite thus produced have large redox activity. Lin, Yan, et al (2013) engineered three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite (3D-ARGON/NiAl-LDH) by the hydrothermal synthesis via in situ growth of ultrathin NiAl-LDH nanoflakes on the 3D-ARGON in an ethanol medium. The procedure for the preparation of 3D-ARGON/NiAl-LDH nanocomposite is shown in Fig. 1.22. The 3D nanocomposite thus produced have a macropore on the rim of a cup and large mesoporous structure on the wall of a cup which help the electron

Layered double hydroxides: fundamentals to applications

31

Figure 1.21 Average CO2 sorption capacities per mass of total adsorbent of pure LDH and carbon hybrids at 573 K and P (CO2) 5 0.2 bar, based on measured GO content, shown with standard errors. Source: Adapted from Garcia-Gallastegui, A., Iruretagoyena, D., Gouvea, V., Mokhtar, M., Asiri, A.M., Basahel, S.N., Al-Thabaiti, S.A., Alyoubi, A.O., Chadwick, D. and Shaffer, M.S., 2012. Graphene oxide as support for layered double hydroxides: enhancing the CO2 adsorption capacity. Chemistry of Materials, 24 (23), pp. 4531-4539 with kind permission of ACS

Figure 1.22 Procedure for the preparation of 3D-ARGON/NiAl-LDH. Source: Adapted from Lin, Y., Ruiyi, L., Zaijun, L., Junkang, L., Yinjun, F., Guangli, W. and Zhiguo, G., 2013. Three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite with super high electrochemical and capacitance performances. Electrochimica Acta, 95, pp. 146-154. with the kind permission of elsevier

transfer and mass transport during the faradaic redox reaction taking place in the supercapacitor. The maximum specific capacitance reported was 2712.7 F g21 at the current density of 1 A g21, which is more than 7 times that of pure NiAl-LDH, 3 times that of common reduced graphene oxide/NiAl-LDH and 1.8-fold that of twodimensional activated reduced graphene oxide/NiAl-LDH (Fig. 1.23). Some examples of LDH nanocomposites for energy applications are given in Table 1.5.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.23 Specific capacitances of the 3D-ARGON/NiAl-LDH (a) and pure NiAl-LDH (b) capacitor cells in different discharge current density. Source: Adapted from Lin, Y., Ruiyi, L., Zaijun, L., Junkang, L., Yinjun, F., Guangli, W. and Zhiguo, G., 2013. Three-dimensional activated reduced graphene oxide nanocup/nickel aluminum layered double hydroxides composite with super high electrochemical and capacitance performances. Electrochimica Acta, 95, pp. 146-154. with kind permission of elsevier

1.5.6 Biomedical applications LDHs have got a lot of attention from researchers in biomedical field for the innovative applications such as drug delivery, gene delivery, bio sensing and bio imaging area because of its non toxicity and in vivo and in vitro bio compatibilities. The great capabilities of LDHs in exchanging anions with drugs, nucleic acids (DNA, RNA), enzymes etc and the efficiency of layered double hydroxides in releasing the drug to the target location in a controlled and sustained manner at a particular pH give them an important position in the drug delivery applications. From the literature survey it is clear that many drugs such as non steroidal inflammatory, anti diabetic, cardiovascular, antibiotics, anti cancer, antioxidant, antimicrobial etc can be intercalated into the interlamellar region either by anion exchange reaction or by memory effect or by one step co precipitation. At the time of drug release, the drug delivery system breaks slowly releasing the drug at the required site. The slow releasing of drug from the interlayer sheet of LDH with respect to time is also a big factor in suppressing the toxicity of the drug. A dissolution test must be conducted to detect the drug release ability of the pillared LDH materials in a simulated intestinal fluid (buffer at pH 7.8). Researchers have been attempting to deliver RNA and

Layered double hydroxides: fundamentals to applications

33

Table 1.5 Some examples of LDH nanocomposites for energy applications. LDH nanocomposite

Electrode for super capacitor /battery

Specific capacitance/ Voltage

Reference

NiMn-LDH/CNT nanocomposite CoAl-LDH/MnO2

Electrode for super capacitor Electrode for super capacitor Electrode for super capacitor

2960 F g 21 at 1.5 A g 21 1088 F g 21 at 1A g 21 781.5 F/g at 5 mV. s21

(Zhao et al., 2014)

Electrode for super capacitor

1740 mF cm22 at 1 mA cm22

(Wan et al., 2015)

Electrode for super capacitor

1133.3 mF cm22 at 1 mA cm22

(Sekhar et al., 2017)

Electrode for super capacitor Electrode for super capacitor Cathode active material for Ni- metalhydride secondary battery

2130 F g21 at 2 A g21 2682 F g 21 at 3 A g 21 147 mV after 300 cycles 157 mV after 869 cycles

(Le et al., 2019)

Graphene Nanosheet/ Ni21/Al31 Layered Double-Hydroxide NiCo2S4 nanotube@ NiMn layered double hydroxide arrays/three dimensional graphene sponge Conductive silver nano wires-fenced carbon cloth fibers-supported layered double hydroxide nanosheets NiCo-LDH/rGO composites Ni Co-LDH@Ni Ni-AlLDH/C composites

(Diao et al., 2014) (Gao et al., 2011)

(Chen et al., 2014) (Be´le´ke´ et al., 2014)

DNA to mammalian cells in vivo by incorporating them with LDH alone or LDH and drug with the purpose of treating diseases (Mishra et al., 2018; Bullo Saifullah, 2015; Kuthati et al., 2015; Nakayama et al., 2010). Li, et al. (2014) designed CDsiRNA-5-FU/LDH nano complexes and delivered into the cancer cells and proved that this nano drug design is efficient in suppressing cancer cell growth. Schematic representation of immobilization of DNA in the inter gallery space of LDH and cellular uptake of DNA@LDH system followed by its action on the cell are demonstrated in Figs. 1.24 and 1.25 respectively (Mishra et al., 2018). Li et al. (2011) used layered double hydroxides as DNA vaccine delivery vector for enhancing antimelanoma immune response. The enzyme immobilized LDHs have been widely accepted as an ideal material for the construction of biosensor due to the high thermal stability, biocompatibility and the ability of LDH to protect the immobilized enzymes. The enzyme urease

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.24 Schematic image of DNA intercalated LDH. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186. with kind permission of elsevier

Figure 1.25 Schematic image of cellular uptake of DNA intercalated LDH nano hybrid and its action on cells. Source: Adapted from Mishra, G., Dash, B. and Pandey, S., 2018. Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153, pp. 172-186. with kind permission of elsevier

Layered double hydroxides: fundamentals to applications

35

was immobilized into the inter lamellar region of Zn/Al LDH and used the enzyme/ LDH system for the construction of urea biosensor. The rate at which urease dissociates urea is converted into an electric pulse from which the information regarding the amount of urea surrounding it can be obtained. Some examples for the applications of layered double hydroxides, layered double hydroxide nanocomposites and organically modified layered double hydroxides are given in Table 1.6.

1.6

Introduction to Layered Double Hydroxide Polymer Nanocomposites

It is very important to mention that layered double hydroxide polymer nanocomposites have a vital role in the material chemistry field because of their wide range of applications such as flame retardancy, thermal stability, water purification, catalysis, drug delivery, photo luminescence, agricultural applications, food packaging applications, energy applications etc. The high efficiency of layered double hydroxide polymer nanocomposite in these application fields can be achieved mainly by increasing the extent of exfoliation of LDH nanosheets in the polymer matrices. The remarkable exfoliation of LDH nano sheets in the polymer matrices is a big challenge due to the high interlayer interaction which arises from the high charge density of the layers. In order to solve this problem, there has been a prompt advancement in the fabrication of LDH
polymer nanocomposites to meet the promising applications in the recent years and so many new strategies have been developed and are categorized into three modes of preparation. First one is the intercalation of monomers into LDH or organically modified LDH followed by polymerization, which help to exfoliate and distribute LDH nanosheets uniformly throughout the polymer matrix and the second one is the direct intercalation of polymer into organically modified LDH and the intercalation of this large molecule leads to exfoliation of LDH nanosheets and the third one is to delaminate the sheets by using suitable solvents followed by mixing with the polymer (Fig. 1.26) (Wang and O’Hare, 2012). The following are the general techniques used for the fabrication of layered double hydroxide polymer nanocomposites through any one of the three modes of preparation which are mentioned above.

1.7

Different Fabrication Techniques of Layered Double Hydroxide polymer Nanocomposites

1.7.1 Melt Compounding Melt Compounding is the mixing of polymer and organically modified LDH or preexfoliated LDH by applying high local shear stresses in a melt mixer at high temperature, which should be above the softening point of the polymer (Radulescu et al., 2008; Ardanuy and Velasco, 2011; Unalan et al., 2014; Xiong, 2018).

Table 1.6 Some examples for the applications of layered double hydroxides, layered double hydroxide nanocomposites and organically modified layered double hydroxides. LDH/OLDH

Synthesis method

Mg-Fe/LDH and Ni-Fe/LDH

co-precipitation method Hydrothermal method

Mg-Al LDH Cu-Al layered double hydroxides

Hydrotalcite (Mg-Al) and Hydrotalcite-like compounds (MgFe, Zn-Al, and Zn-Fe) Mg
Al layered double hydroxide LDH (Cl) ) and LDH (CO3)

co-precipitation method

[NiFe]- (NO32) -LDH

pulsed laser ablation in liquids

anthraquinone-2-sulfonic acid sodium salt monohydrate (AQS) Ni-Fe LDH Homogeneous precipitation.

Graphene oxide, GOmodified Hummers method. Reduction of the Graphene Oxide Nanosheet. co-precipitation method

Mg
Al
CO322LDHs

Intercalating anion/other nanomaterial

5-Fluorouracil Memory effect Cu-Al/carbon fiber- LDH (CuAl/CF-LDH) Deoxyribonucleic acid (DNA) anion- exchange Intercalation of DNA into LDH ion-exchange and reconstruction BF4 2, Cl2, ClO4 2, CO322 C2O422, F2, I2, PO432 SO422 Anion exchange Anion exchange with sodium dodecyl sulfate exfoliation in water/ethanol LDH/rGO nanocomposite

GO from natural graphite powders by Hummer’s method LDH-assembled GO hydrogelsself-assembly approach

Applications

References

Arsenic Removal

(Nakahira et al., 2007) (Peng et al., 2018a) (Peng et al., 2018b)

corrosion resistance, Anti cancer agent catalysts for the degradation of ammonia and adsorption of azo dye Nonviral Gene Delivery Vehicles

(Balcomb et al., 2015)

DNA carrier

(Nakayama et al., 2010)

water oxidation catalysis

(Hunter et al., 2016)

oxygen evolution reaction (OER) efficiency

(Ma et al., 2015)

water purification

(Fang and Chen, 2014)

Layered double hydroxides: fundamentals to applications

37

Figure 1.26 Schematic illustration of the polymer nanocomposite preparation based on isotactic polypropylene and Mg 2 Al LDH layered double hydroxide. Source: Adapted from Nagendra, B., Mohan, K. and Gowd, E.B., 2015. Polypropylene/ layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS applied materials & interfaces, 7 (23), pp. 12399-12410 with kind permission of ACS

The shearing helps the delamination of LDH tactoids and high residence time allow the polymer chain to intercalate into the interlayer gallery space to obtain exfoliated polymer nanocomposites. From the literature survey, it has been found that this approach was successfully applied for the fabrication of polypropylene (Ardanuy and Velasco, 2011; Purohit et al., 2011; Purohit et al., 2014), poly (L-lactide) (Tang et al., 2016), poly (methyl methacrylate) (Nyambo et al., 2008), ethyl vinyl acetate (Wang et al., 2011), poly ethylene (Costa et al., 2011) Carboxylated nitrile rubber (Laskowska et al., 2014) layered double hydroxide nanocomposites.

1.7.2 Solution Blending In this method polymer LDH nanocomposites are prepared by dispersing organically modified LDH or exfoliated LDH into a polymer solution by heating at high temperature or mechanical stirring or ultra sonic assisted stirring followed by the evaporation of the solvent (Radulescu et al., 2008; Unalan et al., 2014; Xiong, 2018). Usually a homogeneous dispersion is produced by mixing the polymer which is dissolved in a suitable solvent and nanoparticles in the same or different solvent. The advantage of this method is that it is easy to prepare exfoliated polymer nanocomposites by delaminating LDHs using suitable solvent followed by the addition of polymer solution into it so that the interaction between the polymer and individual LDH nanosheet is high (Fig. 1.26). This method has been utilized for the successful fabrication of poly propylene (Nagendra et al., 2017a; Nagendra et al., 2015; Yang et al., 2015), poly acryl amide (Fu et al., 2010), silicone rubber (Pradhan and Srivastava, 2014), poly styrene (Suresh et al., 2016; Edenharter et al., 2016), poly amide (Shabanian et al., 2014), high density polyethylene (Gao et al., 2014a) layered double hydroxide nanocomposites.

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Layered Double Hydroxide Polymer Nanocomposites

1.7.3 In-Situ Methods 1.7.3.1 In-Situ Polymerization The preparation of monomer intercalated LDH followed by polymerization with the addition of initiators and/or excitation by heat or radiation is the basic principle of this method (Radulescu et al., 2008; Unalan et al., 2014; Xiong, 2018). (Fig. 1.27) Polyamide 6 (Peng et al., 2009), poly aniline (Hu et al., 2014), poly styrene (Nyambo et al., 2008), polyurethane (Guo et al., 2011; Kotal and Srivastava, 2011), polyimide (Dinari et al., 2015), PBMA (Kovanda et al., 2010) layered double hydroxide nanocomposites have been fabricated successfully by this method.

1.7.3.2 In-Situ LDH Synthesis In this method, synthesis of LDH from two constituent metal salts is carried out by co-precipitation in the presence of a polymer solution so that polymer chains can easily intercalate into the interlamellar galleries of LDHs (Radulescu et al., 2008; Xiong, 2018). Li et al. (2018) prepared beads of chitosan /layered double hydroxide nanocomposite for the adsorption of selenium oxoanions via in situ LDH synthesis. The precursors required for LDH synthesis were added into a chitosan solution and the gel thus formed was allowed to fall into sodium hydroxide solution, both of them co precipitated to form nanocomposite beads. The bead formations have taken place effectively only when the precursor concentration becomes 60%.

Figure 1.27 Schema of LDH/PBMA nanocomposite formation during in situ polymerization. Source:Adapted from Kovanda, F., Jindova´, E., Lang, K., Kuba´t, P. and Sedla´kova´, Z., 2010. Preparation of layered double hydroxides intercalated with organic anions and their application in LDH/poly (butyl methacrylate) nanocomposites. Applied Clay Science, 48 (1-2), pp. 260-270 with kind permission of elsevier

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1.7.3.3 Double in-situ method This method includes both in situ LDH synthesis as well as in situ polymerization. In this method, precursors for the preparation of LDH and monomers and initiators for the preparation of polymers are mixed together. The LDH is formed by the precipitation of metal ions and the monomer or polymer intercalate into the interlayer gallery of the LDH and the initiator continuously helps the conversion of monomers into polymers to form exfoliated layered double hydroxide polymer nanocomposites (Xiong, 2018).

1.7.4 Layer By Layer Assembly The general strategy involved in this method is the layer by layer assembly of the positively charged nanosheets produced by the delamination of LDHs alternatively with anionic polymers via electrostatic attraction. Yan, Dong peng, et al fabricated a multilayer (APPP/LDH) n thin film by alternate LBL assembly of sulfonated poly (p-phenylene) anionic derivate, APPP and exfoliated Mg-Al-LDH. The fluorescence properties of this UTF was found to be greater than APPP due to the suppression of π-π stacking of polymer chain with the introduction of exfoliated LDH nano sheets (Yan et al., 2009). In addition to this poly anions, neutral polymer which contains NH2 or -OH group can form hydrogen bond as driving force for LBL assembly and some other neutral conjugated polymer molecules can form Vander Waal’s forces as driving force for LBL assembly by utilizing delocalized π electrons on the conjugated polymer. The growth of the film can be monitored by uv-vis absorption spectroscopy Many research works reported on the development of LDH/polymer multilayer films by LBL assembly demonstrated that these nanocomposites are new platform materials for optical, electrical and magnetic applications. Xu et al. (2013a) developed (ZnAlLDH/PANI) n multilayer films by LBL technique in which UTF has been prepared by dipping the substrate into ZnAl-LDH colloidal solution first then into PANI solution and the procedure was repeated n times. The washing with pure water and drying in nitrogen was carried out after each deposition (Scheme 1.5).

Scheme 1.5 LBL assembly process for (ZnAl-LDH/PANI) n films. Source: Adapted from Xu, D.M., Guan, M.Y., Xu, Q.H. and Guo, Y., 2013. Multilayer films of layered double hydroxide/polyaniline and their ammonia sensing behavior. Journal of hazardous materials, 262, pp. 64-70. With kind permission of elsevier

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Layered Double Hydroxide Polymer Nanocomposites

The preparation of core-shell assembly of hybrid nanofillers followed by the incorporation of it into suitable polymer system leads to the formation of LDH/ polymer hybrid nanocomposites with wide range of novel applications such as flame retardancy, (Zhang et al., 2017) drug delivery, electrode for super capacitors etc. The core- shell structured materials can also be prepared from LDH and polymer by using layer by layer method which can also be used for wide range of promising applications.

1.7.4.1 Preparation of LDH Nanosheets The LDH nanosheets have very important role in the fabrication of multifunctional multilayer (LDH/polymer) n thin films and core-shell structured materials by LBL assembly. LDH nanosheets can be produced by dispersing either unmodified LDH or organically modified layered double hydroxides in suitable solvents such as butanol, formamide, a mixture of dimethyl formamide and ethanol, xylene (Nagendra et al., 2017b), water (Zhang et al., 2017), etc with the help of mechanical shaking, stirring, ultra sonication etc (Wang and O’Hare, 2012; Mao et al., 2017). Liu et al. (2006b) delaminated Co-Al-NO3LDH by agitating the dispersion in formamide with the help of mechanical shaker (Fig. 1.28). AFM analysis is an effective characterization technique for predicting the delamination of LDH into single sheets by providing the thickness of LDH and that of delaminated sheet from the AFM height images and the corresponding height profiles of the layered double hydroxide before and after exfoliation. Nagendra et al (2017b) synthesized Co 2 Zn 2 Al LDH by co precipitation and delaminated the LDH sheets by shaking it with xylene solvent and measured the thickness of the synthesized LDH and delaminated sheets by conducting AFM analysis. From the AFM height images and the corresponding height profiles, it is proved that the thickness of LDH before exfoliation is around a few hundreds of

Figure 1.28 Schematic illustration of the possible delamination mechanism for LDHs in formamide. Source::Adapted from Liu, Z., Ma, R., Osada, M., Iyi, N., Ebina, Y., Takada, K. and Sasaki, T., 2006. Synthesis, anion exchange, and delamination of Co 2 Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies. Journal of the American Chemical Society, 128 (14), pp. 4872-4880 with the kind permission of ACS

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Figure 1.29 AFM images and height profiles of (a) as-synthesized and (b) exfoliated singlelayer Co 2 Zn 2 Al LDH nanosheets. Source: Adapted from Nagendra, B., Rosely, C.S., Leuteritz, A., Reuter, U. and Gowd, E.B., 2017. Polypropylene/layered double hydroxide nanocomposites: Influence of LDH intralayer metal constituents on the properties of polypropylene. ACS Omega, 2 (1), pp. 20-31 with kind permission of ACS

nanometers whereas the thickness of exfoliated nanosheets is less than 1.0 nm which is in good agreement with the reported values of single sheet of LDH (Fig. 1.29).

1.7.5 Two roll mill mixing This method has been widely applied for the compounding of rubber and the first step in this method is the mastication of rubber for about 2 minutes before adding all the ingrediants. The ingrediants are added into the rubber in the sequential order, activators, accelerators, nanofillers and cross linking agents. The friction ratio between the two rolls must be kept constant throughout the cycle. The mixing is completed within 15 minutes and kept for maturation for about 24 hours. The compounded rubber has been cured in a compression molding press at 150  C for pre determined cure time which is detected with the help of moving die rheometer. Das et al. (2011) fabricated zinc oxide free rubber/St-LDH nanocomposites by two roll mill mixing and proved that St-LDH is a good candidate for the vulcanization of rubber as activator and it substitute the role of zinc oxide which make the method

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.30 Schematic presentation for the preparation procedure of LDH (LDH-St) and rubber/LDH-St composites.

more ecofriendly. Schematic presentation for the preparation procedure of LDH (LDH-St) and rubber/LDH-St composites is shown in Fig. 1.30. Recently, sonication and high energy ball milling techniques have been applied for the preparation of polymer nanocomposites.

1.7.6 Sonication In this method, the formation, growth, and collapse of bubbles in a liquid called acoustic cavitation produced by high intensity ultrasound waves leads to intense shockwaves that promote collisions between the particles, reduction in the size of LDH tactoids and, ultimately, the exfoliation of the layered filler (Unalan et al., 2014).

1.7.7 High energy ball milling Now- a-days, the fabrication of polymer nanocomposites has been carried out by implementing the high energy ball milling as this method requires no solvent and heating. It is mixing of components in the solid form at room temperature which makes the fabrication of composite more effective, convenient and green. It is a high-shear mixing technique in which the shear among balls of different diameters is capable of separating the sheets by overcoming the van der Waals force of attraction between the layers. The clay dispersion in the polymer is enhanced by the transfer of energy between the balls and polymer/clay mixture (Unalan et al., 2014).

Layered double hydroxides: fundamentals to applications

1.8

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Applications of Layered Double Hydroxide Polymer Nanocomposites

1.8.1 Flame Retardant Application Most of the commercially available flame retardants release toxic gases during combustion and introduction of some of them into the polymer matrix deteriorates the mechanical properties of polymers (Gao et al., 2014b). For the reason that, in the past two decades, polymer layered double hydroxide nanocomposites have achieved great attention due to the excellent and environment friendly flame retardant action of the green nanofiller, LDH. It retards the propagation of flame by acting as heat sink due to the endothermic decomposition of metal hydroxides into metal oxides which forms an insulating char on the surface that impedes the combustion process by downgrading the oxygen supply to the matrix. It liberates carbon dioxide and bound water during combustion thereby diluting the flammable gases (Basu et al., 2014; Matusinovic and Wilkie, 2012; Costa et al., 2007; Elbasuney, 2015). The factors which influence the flame retardancy of polymer layered double hydroxide nanocomposites are nature of cations and interlamellar anions of LDH, size and shape of LDH particles, type of polymer used and extent of exfoliation of LDH sheets into the polymer matrix (Gao et al., 2014b; Matusinovic and Wilkie, 2012). Matusinovic and Wilkie (2012) published a review article of flame retardancy and morphology of polymer layered double hydroxide nanocomposites which are mainly focused on the correlation between dispersion of layered double hydroxides in the polymer matrix and reduction in PHRR. Gao et al. (2014a,b) well studied about flame retardant polymer layered double hydroxide nanocomposites and published a review article on this topic in which PHRR reduction values of polymer nanocomposites with different types of polymers and LDHs are itemized. The various polymers used for this purpose are polypropylene, polystyrene, polyethylene, ethylene vinyl acetate, polyvinyl chloride, acrylonitrile
butadiene
styrene, unsaturated polyesters, epoxy, poly (lactic acid) ethylene
propylene
diene terpolymer etc. The generally used organic modifiers of LDH for flame retardant application are lauryl alcohol phosphoric acid ester potassium, benzoic acid, benzene sulfonic,4-amino benzoic, benzene phosphonic, and N- (2- (5,5-dimethyl-1,3,2dioxaphosphinyl-2-ylamino) -hexyl) acetamide-2-propylacid, N- (2- (5,5-dimethyl1,3,2-dioxaphosphinyl-2ylamino) -N-hexyl) formamide-2-propenyl acid, dodecyl sulfate, stearate anion,2-aminotoluene-5-sulfonic acid, dodecyl benzene sulfonate, 2-methyl-2-propene-1-sulfonate, 2-ethylhexyl sulfate, bis (2-ethylhexyl) phosphate etc. The flame retardant additives commonly added to LDH for synergestic effect are magnesium hydroxide, ammonium polyphosphate, micro-encapsulated red phosphorus, expandable graphite, intumescent flame retardant, triphenol phosphate, resorcinol diphosphate, decabromophenyl oxide/antimony oxide, silica, melamine, zinc borate etc. The efficient flame retardant LDH/Polymer nanocomposite can be fabricated by the proper selection of the polymer, the anions required for the modification of the LDH, cations of the LDH and flame retardant additives (Gao et al., 2014b; Matusinovic and Wilkie, 2012).

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Figure 1.31 Schematic illustration of the flame retardant mechanism of cardanol-BS modified LDH in EP composites. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290 with kind permission of ACS

Wang et al. (2015) fabricated EP/ Cardanol-BS modified layered double hydroxide nanocomposite by a combined effect of three roll mill and ultra sonication. Schematic illustration of the flame retardant mechanism of cardanol-BS modified LDH in EP composites is shown in Fig. 1.31. The LOI value of EP/m-LDH (6%) nanocomposite was found to be 29.2% with UL-94 V0 rating. The peak heat release rate, total heat release and total smoke production values of EP/m-LDH-6% were reduced by 62%, 19%, and 45%, respectively, compared to that of pure EP (Fig. 1.32). The flame retardant action depends on the dispersion of nanofillers in the polymer matrix which can improve the quality of char residue. The compact and continuous char residue restrict the escape of flammable gases liberating from the interior as well as acting as an insulating shield for the rest of the material.

1.8.2 Biomedical Application Layered double hydroxide polymer nanocomposites have vital role in biomedical sectors specifically in tissue engineering, drug delivery, gene therapy, photo dynamic therapy and stem cell therapy. In tissue-engineering the important role of fibrous scaffolds is to give biological and structural support to cell adhesion, proliferation, and differentiation there by helping the regeneration of tissues and organs. Fibrous scaffolds apt for such purpose can be prepared by introducing appropriate nanoparticles into a suitable polymer matrix. Shafiei et al. (2016) successfully prepared poly (ε-caprolactone) (PCL) / layered double hydroxide nanocomposite by electro spinning fabrication technique and observed that the addition of LDH in

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Figure 1.32 (a) Heat release rate and (b) total heat release versus time curves of EP and its flame retardant composites. Source: Adapted from Wang, X., Kalali, E.N. and Wang, D.Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustainable Chemistry & Engineering, 3 (12), pp. 3281-3290 with kind permission of ACS

PCL-LDH scaffold influenced the cell adhesion and proliferation remarkably and increased the adipogenic differentiation of mouse adipose derived stem cells (mADSCs) significantly. Lee et al. (2017) fabricated layered double hydroxide poly peptide thermo gel nanocomposite system for chondrogenic differentiation of stem cells which is necessary for cartilage repair. (Scheme 1.6) This study led to the conclusion that LDH/thermogel system can overcome the disadvantages of hydrogel system and future perspective of this study is that it can act as an injectable system in stem cell therapy. An immune fluorescence study proved that there is an increased chondrogenic differentiation in the nanocomposite systems. The immunofluorescence

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Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.6 Schematic Presentation of the Research Stem cells (green circles with a blue core), KGN (red dots), and RGD-coated LDHs (yellow hexagons) are incorporated in a hydrogel during the thermal-energy-driven gelation of the polymer aqueous solution. Stem cells adhere to the LDH surface by RGD, and KGN is continuously released in the system and induces chondrogenic differentiation of TMSCs. Source: Adapted from Lee, S.S., Choi, G.E., Lee, H.J., Kim, Y., Choy, J.H. and Jeong, B., 2017. Layered double hydroxide and polypeptide thermogel nanocomposite system for chondrogenic differentiation of stem cells. ACS applied materials & interfaces, 9 (49), pp. 42668-42675. with kind permission of ACS

images of the proteins secreted by the cells, COL II and SOX 9 are given in Fig. 1.33. Kapusetti et al. (2013) prepared bone cement / layered double hydroxide nanocomposite for the application of joint arthroplasty. It has been proved that this nanocomposite achieved superiority in properties such as thermal stability, fatigue resistance behaviour over pure bone cement due to strong interaction between the polymer and LDH. The high biocompatibility and bioactivity of these nanocomposites has been verified by MTT assay and in vivo studies on rabbits clearly revealed that it is better healing agent than pure bone cement. Photodynamic therapy (PDT) is a treatment in which a drug called photo sensitizer (drug) is administered and the tumor is illuminated with a suitable light in order to activate the drug. Wei et al. (2015) developed LDH
indocyanine green (ICG)
chitosan nanocomposites for the application in photodynamic therapy. In this study the photo instability and biodegradation of ICG has been protected by intercalating it in the interlayer gallery of amine modified LDH then coated with chitosan, a natural polymer. The coating of polymer on the LDH-ICG nanocomposites was obtained by the cross linking formed between amine groups on the surface of LDH as well as that of chitosan using glutaraldehyde, the cross- linking agent.

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Figure 1.33 Immunofluorescence images of COL II (a) and SOX 9 (b). (c) GAG content. The images were taken for each system 21 days after 3D culture of TMSCs. Source: Adapted from Lee, S.S., Choi, G.E., Lee, H.J., Kim, Y., Choy, J.H. and Jeong, B., 2017. Layered double hydroxide and polypeptide thermogel nanocomposite system for chondrogenic differentiation of stem cells. ACS applied materials & interfaces, 9 (49), pp. 42668-42675. with kind permission of ACS

ICG has low toxicity and high absorbing power in the wavelength region 800
805 nm which is transparent to the tissues and exposing the dye to NIR light enhances the penetration depths in tissues. Lv et al. (2015) fabricated DAS- (PAA/PAH) 10-LDH magnetic assembly by LBL technique for targeted drug delivery (Fig. 1.34). They have sufficient saturated magnetic strength and are sensitive towards external magnetic field which is a criterion for targeting drug delivery. It was observed that the drug loading and release ability of the multilayer assembly can be controlled by treating with suitable acidic solution and verified by loading and release of a simulated drug, methylene blue (Fig. 1.35). Hu, H., et al. (2013) fabricated P(DMAEMA)-grafted LDH nano hybrids for advanced gene delivery application. The introduction of ATRP initiation sites containing disulfide bonds onto LDH surfaces by using three step method followed by atom transfer radical polymerization (ATRP) of 2- (dimethylamino) -ethyl methacrylate (DMAEMA) develop novel gene delivery vectors (Scheme 1.7). The resultant LDH-PDs showed better ability to condense plasmid DNA (pDNA) and much higher efficiency to delivery genes in different cell lines including COS7and HepG2 cell lines (Fig. 1.36).

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Figure 1.34 Illustration for the fabrication of stabilized ZnAl-LDH via LbL process. Source: Adapted from Lv, F., Xu, L., Zhang, Y. and Meng, Z., 2015. Layered double hydroxide assemblies with controllable drug loading capacity and release behavior as well as stabilized layer-by-layer polymer multilayers. ACS applied materials & interfaces, 7 (34), pp. 19104-19111 with kind permission of ACS.

Figure 1.35 Release behavior of (a) (PAA/PAH) 10-LDH, (b) DAS- (PAA/PAH) 10-LDH, (c) B-DAS- (PAA/PAH) 10-LDH and (d) A-DAS- (PAA/PAH) 10
LDH toward MB. Source: Adapted from Lv, F., Xu, L., Zhang, Y. and Meng, Z., 2015. Layered double hydroxide assemblies with controllable drug loading capacity and release behavior as well as stabilized layer-by-layer polymer multilayers. ACS applied materials & interfaces, 7 (34), pp. 19104-19111 with kind permission of ACS.

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Scheme 1.7 Schematic Diagram Illustrating the Preparation Processes of P(DMAEMA) -Graft-LDH Hybrids via ATRP. Source: Adapted from Hu, H., Xiu, K.M., Xu, S.L., Yang, W.T. and Xu, F.J., 2013. Functionalized layered double hydroxide nanoparticles conjugated with disulfide-linked polycation brushes for advanced gene delivery. Bioconjugate chemistry, 24 (6), pp. 968-978. with kind permission of ACS

1.8.3 Gas sensing Applications Xu et al. (2013a) developed (ZnAl-LDH/PANI) n multilayer films by LBL technique for sensing ammonia gas. In this work it is proved that this UTF is an efficient sensor for ammonia gas. The change in resistance associated with the exposure of thin film to ammonia gas measures the ammonia sensing behaviour of UTF. The response of (ZnAl-LDH/PANI) n multilayer films towards ammonia is reversible at room temperature and it increases with increase in concentration of ammonia. When ammonia

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.36 Representative images of EGFP expression mediated by LDH (at the optimal weight ratio of 10), P(DMAEMA) (at the optimal N/P ratios of 15), and LDH-PD2 (at the N/P ratios of 15) in HepG2 cells. Source: Adapted from Hu, H., Xiu, K.M., Xu, S.L., Yang, W.T. and Xu, F.J., 2013. Functionalized layered double hydroxide nanoparticles conjugated with disulfide-linked polycation brushes for advanced gene delivery. Bioconjugate chemistry, 24 (6), pp. 968-978. with kind permission of ACS

comes in contact with poly aniline protonation
deprotonation of polyaniline will modulate the resistance change. It is mentioned that the introduction of LDH may enhance the voids for reaction with ammonia which facilitated the gas sensing response property of aniline.

1.8.4 Energy Applications Layered double hydroxide polymer nanocomposites can be used as proton exchange membrane in polymer electrolyte membrane fuel cells (PEMFCs) which are the most promising fuel cells due to their modularity and wide spectrum applications. Sulfonated polysulfone (SPSU), sulfonated poly (ether ketone) (SPEK), sulfonated poly (ether ether ketone) (SPEEK) are the polymers that are generally used for this application. Herrero, M., et al (2014) fabricated sulfonated polysulfone /layered

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double hydroxide nanocomposite membrane by solvent casting technique using the solvent dimethylacetamide for fuel cell application due to the easy availability, low cost and high thermal stability of polysulfone. In addition to these, polysulfone can be easily sulfonated very easily with a variety of sulfonating agents providing wonderful proton conducting membranes. The introduction of LDH enhances the mechanical properties, diminish the fuel and water permeability and keep humidity inside the membrane because of their hygroscopic properties and also affect the electrical and transport properties. Layered double hydroxide/conductive polymer (core/shell) nano platelet array can be applied as an electrode for high performance super capacitors as the LDH nanoplatelet core provide high energy-storage capability through a fast and reversible redox reaction where as the highly conducting polymer shell facilitates transport of electron during the charge
discharge process. Han, Jingbin, et al. (2013) developed LDH@PEDOT NPA electrode for supercapacitor with excellent electrochemical behavior-high specific capacitance, remarkable cycling performance and high specific energy and power. The largely enhanced pseudocapacitor behavior of the LDH@PEDOT NPA electrode can be correlated to the combined effect of LDH nanolatelet core and highly conductive PEDOT shell, the LDH nanoplatelet core offers abundant energy- storage capacity, whereas the highly conductive PEDOT shell and porous structural design smooth the progress of the electron/mass transport in the redox reaction. Dye-sensitized solar cells (DSSC) are promising clean energy device and have achieved a great deal of research attention because of low cost and easy processability and satisfactory power conversion efficiency. Layered Double Hydroxides are efficacious additive in polymer gelled electrolyte for the application in dye-sensitized solar cells. Ho et al. (2014) prepared LDH/poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) gelled electrolyte for DSSCs and have made the observation that there is hike in open circuit voltage. This is owing to the positive shifts in redox potential with increase in the amount of LDH which may be due to the high ion exchange efficiency of the LDH.

1.8.5 Food Packaging Applications Xie et al. (2018) fabricated excellent biodegradable food packing thin film of poly (butylene adipate-co-terephthalate) (PBAT) /OLDH nanocomposite by melt blending and blowing method (Scheme 1.8). A PBAT/OLDH film (1 wt % OLDH, O is alkyl phosphonate, C12H25PO422) showed excellent mechanical, thermal, optical, and water vapor barrier properties than pure PBAT film with a reduction of 37% in haze and 41.9% enhancement in nominal tensile strain at break. The water vapor barrier properties has been enhanced due to the hindrance to the flow of water molecules due to the uniform distribution of LDH sheets and their by increasing the path of water molecules. The food packaging experiment proved that the packaging efficiency of the PBAT/OLDH nanocomposite film containing 1% OLDH is greater than that of pure PBAT film and commercially available polyethene packing material (Fig. 1.37). Tammaro et al. (2014) fabricated Poly (ethylene terephthalate)

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Layered Double Hydroxide Polymer Nanocomposites

Scheme 1.8 Schematic Illustration for the Manufacture of the Biodegradable PBAT/OLDH Nanocomposite Films. Source:Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

(PET) nanocomposites by using seven different modified layered double hydroxides with the help of high energy ball milling. LDH helped the composite to prevent the unwanted oxidation of food by providing lower oxygen diffusion and permeability coefficients to the composites due to its high exfoliation into the polymer matrix. So these composites are promising candidates for food packaging applications.

1.8.6 Water Purification Mohamed et al. (2018) prepared polypyrrole nanofiber /Zn-Fe layered double hydroxide nanocomposite and applied it for removal of safranin dye from waste water because of their excellent adsorption power and photo catalytic properties (Fig. 1.38). It was observed that 5mg/L of safranin dye was completely removed by utilizing 0.05 gm of the composite after 120 minutes illuminating time in alkaline or neutral medium (Fig. 1.39). The enhancement in the photo catalytic removal of dye is attributed to the large specific surface area and reduction in the band gap due to composite formation. The mechanisms involved in the photo catalytic removal

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Figure 1.37 Photographs of banana samples with following treatment: (a) exposed to air, (b) packaged with pure PBAT film, and (c) packaged with OLDH-1 film. Source:Adapted from Xie, J., Wang, Z., Zhao, Q., Yang, Y., Xu, J., Waterhouse, G.I., Zhang, K., Li, S., Jin, P. and Jin, G., 2018. Scale-up fabrication of biodegradable poly (butylene adipate-co-terephthalate) /organophilic
clay nanocomposite films for potential packaging applications. ACS Omega, 3 (1), pp. 1187-1196 with kind permission of ACS

Figure 1.38 Photo catalytic degradation mechanism of safranin dye using PpyNF/Zn-Fe LDH nanocomposite. Source: Adapted from Mohamed, F., Abukhadra, M.R. and Shaban, M., 2018. Removal of safranin dye from water using polypyrrole nanofiber/Zn-Fe layered double hydroxide nanocomposite (Ppy NF/Zn-Fe LDH) of enhanced adsorption and photocatalytic properties. Science of the Total Environment, 640, pp. 352-363 wit kind permission of elsevier

are the adsorption of dye, charge transfer and production of oxidizing radicals due to the absorption of photon (Fig. 1.38).

1.8.7 Gas Barrier Materials The two dimensional layered double hydroxide nano sheets in the polymer nanocomposites are capable of hindering the gas penetration through the composite by providing an extensive diffusion path for the gas molecules due to low free space owing to the large aspect ratio of LDH and high interaction between the filler and

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.39 Effect of illumination time in degradation of several concentrations of safranin dye, (B) effect of catalyst dose in the removal of the dye with different time intervals, (C) effect of solution pH on the removal of safranin, and (D) stability of the catalyst for several runs of dye removal. Source: Adapted from Mohamed, F., Abukhadra, M.R. and Shaban, M., 2018. Removal of safranin dye from water using polypyrrole nanofiber/Zn-Fe layered double hydroxide nanocomposite (Ppy NF/Zn-Fe LDH) of enhanced adsorption and photocatalytic properties. Science of the Total Environment, 640, pp. 352-363 with kind permission of elsevier

polymer. Hence LDH/polymer nanocomposites are very good gas barrier material in tire and device packing industry. Wang et al. (2017) fabricated (U-mLDH/NBR) n multilayer films by LBL assembly by repeating the spin coating of U-mLDH suspension and NBR solution alternatively onto the substrate n times (Scheme 1.9). There was reduction in the oxygen transmission rate of the (U-mLDH/NBR) 30 nanocomposite by 92.2% with respect to NBR film and the value is found to be 0.626 cm3 m22 day21 atm21 (Fig. 1.40). The relative permeability of the nanocomposite is found to be lower than other rubber composites reported in literature and possesses high thermal stability, satisfactory mechanical strength so that it can act as a good gas barrier material.

1.8.8 Agricultural Applications Low density polyethylene/organically modified LDH nanocomposites are widely used as agricultural plastic films in greenhouses due to its infrared absorbing

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Scheme 1.9 Schematic illustration for the assembly of (U-mLDH/NBR) n film on PET substrate by layer-by-layer assembly technique. Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L. and Wei, M., 2017. Layer-by-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Composites Part A: Applied Science and Manufacturing, 102, pp. 314321 with kind permission of elsevier

ability. Wang et al.(2010) fabricated low density polyethylene (LDPE) / N, N-bis (phosphonomethyl) glycine modified LDH nanocomposite by master batch method. The IR absorption power of MgAlGLYPLDH/LDPE in both the broader 1428400cm21and narrower 1428-714 cm21ranges, and principally in the 1111-909 cm21range are higher than that of MgAl-CO3-LDH/LDPE film and it has selective and appropriate IR absorption to be applied in agricultural films.

1.8.9 Anti Corrosion Materials Hu et al. (2014) fabricated PANI/AD-LDHs nanocomposites through the grafting of PANI chemically onto LDH. The first step is the intercalation of anticorrosive decavanadate anion into Zn
Al
NO3 LDH through anion exchange reaction. After the intercalation, decavanadate- intercalated LDH (D-LDH) was treated with APTS in order to graft PANI onto the D-LDH (Scheme 1.10). The corrosion protection power of coated PANI/AD-LDHs nanocomposites on steel was studied by measuring the OCP versus time of exposure, Tafel and EIS after exposing the coating to 3.5 wt.% NaCl solution. The deviations in the OCP values, corrosion current and EIS spectra with time (Fig. 1.41) indicating that the coating of PANI/AD-LDH possesses higher corrosion resistant property than that of D-LDH and PANI. This can be attributed to the increase in barrier to diffusion, redox properties of PANI as well as the power of D-LDH to release inhibiting anions.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.40 (A) O2TR values for pristine PET, (U-LDH/NBR) n and (U-mLDH/NBR) n films (n 5 10, 20 and 30); (B) O2TR values for (U-mLDH/NBR) n films as a function of relative humidity (0%, 30% and 60%); (C) O2TR values of PET substrates coated with (N-mLDH/NBR) n multilayer films as a function of bilayer number n; (D) O2TR values for pure NBR, (N-mLDH/NBR) 15 and (U-mLDH/NBR) 30 films with nearly the same thickness (1 mm), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L. and Wei, M., 2017. Layer-by-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Composites Part A: Applied Science and Manufacturing, 102, pp. 314-321 with kind permission of elsevier

1.9

LDH based polymer hybrid nanocomposites

The introduction of hybrid nanoparticles system which consists of LDH and any other functionalized nanoparticles with high magnetic property, electrically conductivity, mechanical strength, flame retardancy etc, into any polymer matrix, there will be synergistic effect of properties of all the fillers of the hybrid system resulting in the fabrication of high performance polymer hybrid nanocomposites. The compatibility between the fillers and the polymer can be achieved by properly selecting the fillers in the hybrid system and the polymer. Kalali et al. (2016) fabricated Fe3O4@Ph-CDBS-LDH / epoxy hybrid nanocomposite by modifying LDH with green flame retardants followed by decoration of modified LDH with Fe3O4 to produce Fe3O4@Ph-CDBS-LDH hybrid and finally introduced this hybrid system into epoxy resin (Scheme 1.11). Schematic Illustration of the flame-retardant

Scheme 1.10 Schematic illustration of the preparation of AD-LDH/PANI nanocomposites. Source: Adapted from Hu, J., Gan, M., Ma, L., Li, Z., Yan, J. and Zhang, J., 2014. Synthesis and anticorrosive properties of polymer
clay nanocomposites via chemical grafting of polyaniline onto Zn-Al layered double hydroxides. Surface and Coatings Technology, 240, pp. 55-62. with kind permission of elsevier.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 1.41 Electrochemical impedance diagrams obtained after different exposure times to 3.5% NaCl solution for the steel covered by AD-LDH/PANI coating. Source: Adapted from Hu, J., Gan, M., Ma, L., Li, Z., Yan, J. and Zhang, J., 2014. Synthesis and anticorrosive properties of polymer
clay nanocomposites via chemical grafting of polyaniline onto Zn-Al layered double hydroxides. Surface and Coatings Technology, 240, pp. 55-62. with kind permission of elsevier.

Scheme 1.11 Preparation Process of a Fe3O4@Ph-CDBS-LDH Hybrid. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

Layered double hydroxides: fundamentals to applications

59

Scheme 1.12 Schematic Illustration of the Flame-Retardant Mechanism of (a) LDH/EP and (b) Fe3O4@Ph-CDBS-LDH/EP with EDX Mapping. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

mechanism of LDH/EP and Fe3O4@Ph-CDBS-LDH/EP with EDX Mapping are shown in Scheme 1.12. The introduction of 8 wt % this hybrid system into the EP matrix increased LOI value by 26.8 % and reduced the total smoke production and the peak heat release rate of the EP composite by 34% and 55% respectively with respect to EP, and achieved the UL-94V0 rating in the vertical burn test (Fig. 1.42). In addition to flame retardancy, this composite achieved high mechanical strength and thermal stability. Jiang et al. (2014) fabricated EP/ silica@Co 2 Al layered double hydroxide spheres hybrid nanocomposites for flame retardant applications. In this study mesoporous silica@Co 2 Al layered double hydroxide (m-SiO2@Co 2 Al LDH) spheres was prepared by LBL assembly (Scheme 1.13) and its formation confirmed by TEM and uv- visible spectroscopic analysis (Fig. 1.43). The presence of m-SiO2@Co 2 Al LDH in EP enhanced the flame retardant property by decreasing the PHRR by 39.3% and TSR by 23.8% (Fig. 1.44). This can be attributed to the synergestic effect of both the fillers that is LDH catalyzed formation of char and labyrinth effect of m-SiO2. Pradhan and Srivastava (2014) prepared three LDH/MWCNT hybrids using three different types of LDH by dry grinding and the resultant LDH/MWCNT hybrids

60

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.42 HRR versus time curves of epoxy and LDH-based epoxy composites obtained from cone calorimetry tests. Source: Adapted from Kalali, E.N., Wang, X. and Wang, D.Y., 2016. Synthesis of a Fe3O4 nanosphere@ Mg
Al layered-double-hydroxide hybrid and application in the fabrication of multifunctional epoxy nanocomposites. Industrial & Engineering Chemistry Research, 55 (23), pp. 6634-6642. with kind permission of ACS

Scheme 1.13 Synthetic Route of m-SiO2@Co 2 Al LDH. Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

were incorporated into silicon rubber by solution intercalation method. The tensile strength of SR/ Mg
Al-LDH/MWCNT, SR/Co
Al-LDH/MWCNT and SR/Li
AlLDH/MWCNT hybrid nanocomposites for 1% hybrid filler system were enhanced by 134%, 125% and 100% respectively with respect to neat SR. The solvent resistance property and cross link density were also found to be highest for SR/Mg
AlLDH/MWCNT. The maximum synergistic effect was observed for Mg
Al-LDH/

Layered double hydroxides: fundamentals to applications

61

Figure 1.43 TEM images, digital photos (A, B, C) and UV 2 vis absorption spectra (D) of m-SiO2, m-SiO2@Co 2 Al LDH (10 layers) and m-SiO2@Co 2 Al LDH (20 layers). Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

Figure 1.44 HRR curves of EP and its nanocomposites. Source;Adapted from Jiang, S.D., Bai, Z.M., Tang, G., Song, L., Stec, A.A., Hull, T.R., Hu, Y. and Hu, W.Z., 2014. Synthesis of mesoporous silica@ Co
Al layered double hydroxide spheres: layer-by-layer method and their effects on the flame retardancy of epoxy resins. ACS applied materials & interfaces, 6 (16), pp. 14076-14086. with kind permission of ACS.

MWCNT hybrid system owing to the highest surface area which led to maximum interaction between the SR matrix and Mg
Al-LDH/MWCNT. Huang et al. (2010) fabricated polyamide-6/CNT/LDH hybrid nanocomposite by preparing CNT/LDH hybrid filler from negatively charged CNT produced by the oxidation of CNT with nitric acid and nitrate intercalated Co-Al LDH produced by

62

Layered Double Hydroxide Polymer Nanocomposites

Figure 1.45 Schematic description of assembling exfoliated LDH/CNT hybrids: (a) CNT formamide suspension, (b) LDH formamide suspension, and (c) mixture of CNT and LDH suspensions. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T. and Liu, T., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. The Journal of Physical Chemistry B, 114 (50), pp. 16766-16772 with kind permission of ACS

the anion exchange of carbonate intercalated LDH and the hybrid filler was incorporated into polyamide-6 by converting ε-caprolactam into polyamide-6 in the presence of hybrid filler and 6-aminocaproic acid. The positively charged LDH nanosheet adsorb negatively charged CNTs on their surface through electrostatic attraction and this introduced a new path for producing exfoliated LDH nanosheet/ CNT hybrids. (Fig. 1.45). The synergistic effect of both the nano fillers enhanced the mechanical strength of the PA-6 nanocomposites (Fig. 1.46) due to uniform distribution of nanofillers and the strong interaction between the fillers and the polymer. This hybrid nanofillers preparation is necessary to design polymer hybrid nanocomposites for energy storage and generation, sensing applications and for polymer reinforcement. Chen et al. (2010) developed electrically conductive poly (vinyl alcohol) /graphene /layered double hydroxide hybrid film by layer-by-layer self-assembly. The various steps involved are dipping the substrate sequentially into exfoliated LDH suspension, aqueous PVA solution, exfoliated GO suspension and poly vinyl alcohol. The procedure also includes washing with water and drying in nitrogen after every dipping process. The dipping operations were repeated n times to develop multilayer films of (LDH/PVA/GO/PVA) n (Scheme 1.14). Here hydrogen bonding act as the driving force for LBL assembly which is formed through the hydroxyl group of PVA, carboxylate and epoxide group on the surface of GO and hydroxyl group of LDH nanosheet. Different fabrication routes to polymer nanocomposites and LDH based polymer hybrid nanocomposites and their applications are shown in Table 1.7.

Layered double hydroxides: fundamentals to applications

63

Figure 1.46 Typical stress-strain curves of neat PA6 and its nanocomposites. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T. and Liu, T., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. The Journal of Physical Chemistry B, 114 (50), pp. 16766-16772 with kind permission of ACS

Scheme 1.14 Schematic of Layer-by-Layer Assembly Procedure. Source: Adapted from Chen, D., Wang, X., Liu, T., Wang, X. and Li, J., 2010. Electrically conductive poly (vinyl alcohol) hybrid films containing graphene and layered double hydroxide fabricated via layer-by-layer self-assembly. ACS Applied Materials & Interfaces, 2 (7), pp. 2005-2011.

1.10

Conclusion and perspectives

This chapter is really an overview of current research on hybrid LDHs- intercalated, surface immobilized systems, LDH nanocomposites, LDH based polymer nanocomposites and LDH based hybrid polymer nanocomposites which are rapidly growing in the field of material chemistry. LDHs are good nano reservoirs for controlled

Table 1.7 Different fabrication routes to polymer nanocomposites and their applications. Type of LDH

Method of synthesis of LDH

Zn2Al 2 X LDH (X 5 CO322, Cl2, NO32, and SO422) Mg 2 Al LDH

Co precipitation

Exfoliation/organicmodification/ nanocomposites of LDH

Coprecipitation

Delamination in xylene

Zn-Fe LDH

Coprecipitation

Modified Zn-Al LDH

One step Coprecipitation

Mixing of pyrrole and Zn-Fe LDH. Addition of oxidant to pyrrole /Zn-Fe LDH mixture Salicylate, Parahydroxybenzoate, aleuriticcarboxylate, citrate, glycolate serine carboxylate, 2.2-bishydroxymethyl-propionate antioxidants

Sodiumdodecyl benzenesulfonateZnAl-LDH Mg/Al
Cl-LDH

One step Coprecipitation

NiFe, NiAl, and NiCr LDH-SDS

Coprecipitation

Coprecipitation

Flame-retardant Cyclophosphazene Compound

LDH based polymer nanocomposites, Method of fabrication, Application

Reference

HDPE/LDH Nanocomposites Solvent Mixing Method Flame Retardant Polypropylene/LDH nanocomposites Solution blending LDH improves the thermal stability, nucleation ability, and crystallization rate of iPP PpyNF/Zn-FeLDH nanocomposite Interfacial polymerization Water purification Highenergy ballmilling method Poly (ethylene terephtalate) (PET) Food packaging applications

(Gao et al., 2014a)

Polypropylene/DBS LDH nanocomposite melt blending Dielectric PCL-LDH fibrous scaffolds Electrospinning adipogenic differentiation of mesenchymal stem cells. PLA/HPCP/LDH-SDS composites thermal stability and flame retardancy

(Nagendra et al., 2015)

(Mohamed et al., 2018) (Tammaro et al., 2014) (Purohit et al., 2011) (Shafiei et al., 2016) (Shan et al., 2012)

Mg/Al-CO3-LDH

Mg 2 Al 2 CO3 LDH

Urea hydrolysis

Graphene Oxide (GO) - modified Hummers method surface modifications of LDH and GO using 3,4dihydroxybenzophenone 3,4dihydroxybenzophenone (DBP) and 1-dodecylamine (DDA), respectively

PS/LDH/GO hybrid nanocomposite solution blending from tetrahydrofuran utilizing a threeroll mill Flame retardant

(Edenharter et al., 2016)

Layered Double Hydroxide/Chitosan Nanocomposite Beads Sorbents for Selenium Oxoanions In situ LDH synthesis and Direct Mixing

(Li et al., 2018)

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Layered Double Hydroxide Polymer Nanocomposites

drug delivery, gene delivery, pesticide release etc due to their excellent anion exchange efficiency and intercalation properties. Biomolecule immobilization play a great role in the development of LDH chemistry as it contributes several innovative advanced materials for biotechnological applications such as biosensing, bioimaging etc. LDH based nanocomposites with carbon nanotubes; graphene, carbon nanofibers, graphydine etc. have remarkable importance in water splitting, environmental remediation like water and air purification, energy storage like super capacitor electrodes applications etc due to the reason that these hybrids are inherited by the combined properties of both the parental materials. For the usage of super capacitor electrode, LDHs containing transition metals are essential to achieve good results because of their variable valencies which are responsible for high redox activity. The hybrid LDHs are outstanding materials for the production of stimuli responsive systems on the basis of change in some properties by stimulating with heat, pH, light, pressure, magnetic field, and voltage for promising applications such as chemical switches, memory devices, or molecular sensor. The study in this field is found to be less so application of hybrid LDHs in this field has to be improved by properly selecting the candidates that having excellent stimuli response property. The exfoliation of LDH sheets into the polymer matrices by dispersing small amount of organically modified LDHs enhance properties like flame retardancy, mechanical strength, thermal stability etc. It is really a big challenge even though new strategies like pre-exfoliation have already been developed to overcome this problem which is not ecofriendly and economically viable because of the usage of highly expensive and toxic solvents. By properly designing the layered double hydroxides nanocomposites and its uniform distribution into the polymer system through suitable fabrication methods like melt compounding, solution blending etc enhance the dielectric, electric properties etc. thereby extending the applications to electrical, electronic, biomedical engineering fields. Because of these advantages, the fabrication of LDH-based polymer nanocomposites is a rapidly growing area in the research field. New technologies like layer by layer assembly has been extensively utilized for the fabrication of polymer/LDH thin films, core-shell structure with photoluminescence, magnetic, electrical and electronic properties which can be applied in light emitting diodes, targeting drug delivery, super capacitors etc, which is one of the reason for LDH based research more fascinating.

1.11 LDH XRD FTIR SEM TEM AFM MMT

Abbreviations Layered double hydroxide X-ray diffraction Fourier Transform Infrared Spectroscopy Scanning electron microscopy Transmission electron microscopy Atomic force microscopy Montmorillonite

Layered double hydroxides: fundamentals to applications

MAPK OLDH PBAT EP BS EDS DS DBS DRIFTS MS TG/DTA HTXRD BEPH SDS SDBS CNFs CNTs HT GO EDTA 3D-ARGON CD 5-FU PBMA LBL APPP PANI UTF St-LDH PHRR LOI PCL MADSCs PDT ICG KGN TMSCs COL II SOX RGD MTT NIR PAH PAA DAS ARPT P(DMAEMA) PEMFCs SPSU

Mitogen activated protein kinase Organically modified layered double hydroxide Poly (butylene adipate-co- terephthalate) Epoxy Resin 1, 4-butane sultone Energy-dispersive X-ray spectra Dodecyl sulfate anion Dodecyl benzene sulponate anion Diffuse reflectance infrared Fourier transform spectroscopy Mass Spectroscopy Thermogravimetry / Differential Thermal Analysis High temperature X-ray diffraction Bis (2-ethylhexyl) hydrogen phosphate Sodium dodecyl sulfate Sodium dodecyl benzene sulfonate Carbon nanofibers Carbon Nanotubes Hydrotalcite Graphene oxide Ethylene diamine tetra acetate ion Three-dimensional activated reduced graphene oxide nano cup Cell death 5-fluorouracil Poly butyl methacrylate Layer by layer Poly (p-phenylene) anionic derivate Poly aniline Ultra thin film Stearic acid modified LDH Peak of heat release rate Limiting oxygen index Poly (ε-caprolactone) Mouse adipose derived stem cells Photodynamic therapy Indocyanine green Kartogenin Tonsil-derived mesenchymal stem cells Type II A1 collagen Transcription factor Arginyl glycyl aspartic acid (3- (4, 5-dimethylthiazol-2-yl) 22, 5-diphenyltetrazolium bromide) Near infrared Poly- (allylamine hydrochloride) Poly (acrylic acid) 4, 4’diazostilbene-2, 2’-disulfonic acid disodium salt Atom transfer radical polymerization Poly ( (2-dimethylamino) ethyl methacrylate) Polymer electrolyte membrane fuel cells Sulfonated polysulfone

67

68

SPEK SPEEK PEDOT APTS NPA NBR LDPE GLYP D-LDH AD LDH EIS OCP Ph-CDBS EDX HRR TSR SR MWCNT CNT PA-6 PVA GO rGO PpyNF CF PET HPCP PLA PS

Layered Double Hydroxide Polymer Nanocomposites

Sulfonated poly (ether ketone) Sulfonated poly (ether ether ketone) Poly (3, 4-ethylenedioxythiophene) γ-aminopropyltriethoxysilane Nanoplatelet array Nitrile butadiene rubber Low density polyethylene N, N-bis (phosphonomethyl) glycine Decavanadate- intercalated LDH Decavanadate- intercalated and γ-aminopropyltriethoxysilane (APTS) -modified Zn
Al layered double hydroxide Electrochemical impedance spectroscopy Open circuit potential Phytic acid (Ph) and (hydroxypropyl) - sulfobutyl-β-cyclodextrin sodium Energy-dispersive X-ray spectroscopy Heat release rate Total heat release Silicone rubber Multiwalled carbon nanotube Carbon nanotube Polyamide-6 Poly vinyl alcohol Graphene Oxide Reduced Graphene Oxide Polypyrrole nanofiber Carbon fiber Poly (ethylene terephtalate) Hexaphenoxycyclotriphosphazene Poly (lactic acid) Poly styrene

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292.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

2

Meisam Shabanian1, Mohsen Hajibeygi2 and Ahmad Raeisi3 1 Faculty of Chemistry and Petrochemical Engineering, Standard Research Institute (SRI), Karaj, Iran, 2Faculty of Chemistry, Kharazmi University, Tehran, Iran, 3Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Arak University, Arak, Iran

2.1

Introduction

Infrared (IR) spectroscopy especially Fourier transform infrared (FTIR) spectroscopy is a technique that has been used over the years in both academia and industry for the structural and compositional analysis of organic, organometallic, polymeric, and inorganic materials, in addition to quality control of raw materials and commercial products. FTIR spectroscopy is a useful tool for functional group identification and quantification. Certain functional groups of an organic or inorganic structure can be identified easily using the FTIR technique. Also, FTIR spectroscopy can be used to confirm a pure compound or to detect the presence of specific impurities. The term “infrared” generally refers to any electromagnetic radiation falling in the region from 0.7 to 1000 μm. However, the region between 2.5 and 25 μm (4000
400 cm21) is the most attractive for chemical analysis. The relationship of the infrared region to other electromagnetic radiations is represented in Fig. 2.1. The “mid-IR” region includes the frequencies corresponding to the fundamental vibrations of virtually all of the functional groups and different bonds of metals in organic and inorganic compounds (Rives, 2001). The absorption bands in FTIR spectra are typically narrow and distinguished, making it possible to identify and monitor an absorption band related to the specific structural feature that is to be modified with a reaction. When a sample is exposed to an infrared beam, various wavelengths of radiation corresponding to the energies of the possible vibrational transitions in the molecule or crystal will be absorbed by the bonds of the sample. The remaining signals are recorded as an absorption band in the spectrum. In this process, those frequencies related to the infrared beam that match the natural vibrational frequencies of the bonds in the molecule are absorbed, and the energy absorbed serves to increase the amplitude of the vibrational motions related to the bonds. Note that a molecule can absorb only selected energies (frequencies) of infrared radiation, and not all bonds in a molecule are capable of absorbing infrared energy, even if the frequency of the Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00002-1 © 2020 Elsevier Ltd. All rights reserved.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.1 Electromagnetic spectrum and relationship of vibrational infrared to other radiations.

radiation exactly matches that of the bond motion (Pavia et al., 2008). Only the dipole bonds are capable of absorbing infrared radiation. Some molecules such as H2 or Cl2 are symmetric bonds and cannot absorb infrared radiation (Pavia et al., 2008). An electrical dipole must be present in an asymmetric bond in a molecule that is changing at the same frequency as the incoming energy of radiation to be transferred. Then the changing electrical dipole from the dipole bond can couple with the sinusoidally changing electromagnetic field of the incoming radiation. Thus a nonpolar bond (symmetric bond) that has identical or nearly identical groups on each end will not absorb in the infrared (Pavia et al., 2008). Near-infrared (NIR) spectroscopy is also known as “proton” infrared spectroscopy, as it covers the spectral region in which all the overtone and combination bands of vibrations involving hydrogen appear. The NIR spectral region has been defined by Kaye (1954, 1955) to extend from 700 to 3500 nm (14,285
2860 cm21). The only fundamental vibrations in the NIR region between 4000 and 10,000 cm21 are those associated with hydrogen atoms existing in hydroxyl groups or water in the case of minerals and inorganic compounds like layered double hydroxides (LDH). Whittet et al. (1997) reported average band positions for hydroxyl group and water in the NIR region around 4200 cm21 due to M
OH motions, 5200 cm21 as the H2O combination mode (bending 1 stretching), and around 7100 cm21 as the first OH stretching overtone. It is clear that the main structure can be obtained by mid-IR but NIR spectroscopy could be a suitable technique to study many compounds such as LDH, which contain both water and OH groups in their structure, to obtain more information about the local environments involved. By using the FTIR method, the structures and relative quantities of modifier molecules in the LDH surfaces can be analyzed. However, in some cases the low concentrations and aqueous environment for synthesized LDH can complicate the interpretation of measurement results. The absorbance of trace impurities or background noise can influence the FTIR absorbance result at low

FTIR characterization of layered double hydroxides and modified layered double hydroxides

79

concentrations of LDH. Since water strongly absorbs infrared light, the removal of water, as well as contamination, in the LDH and the nanocomposite films is necessary.

2.2

Fourier transform infrared spectra of layered double hydroxides

LDH or hydrotalcite-like compounds belong to the anionic clay family. The structures of these materials are made on the layers with a brucite-like structure carrying a net positive charge that is balanced by the anions intercalated between the positively charged layers. Positive charge on the electrostatically neutral brucite was created through the substitution of octahedral M21 by M31 cations (Qu et al., 2016a,b; Takehira, 2017; Chubar et al., 2017). One of the ways to identify the structures of LDH and intercalated anions between LDH layers is the FTIR technique. Mumpton et al. (1965) represented for the first time the FTIR spectrum of hydrotalcite like Mg
Al LDH. Also, Ross and Kodama exhibited the characteristic absorption bands of Mg
Al LDH (Ross and Kodama, 1967). In all FTIR spectra of prepared LDH, a broad absorption band was observed around 3480 cm21 with a shoulder band around 3000 cm21, which was related to OH stretching vibration. In general, the structure of LDH can be identified by different characteristic absorption bands in a typical FTIR spectrum. These bands were assigned to four series, as listed here. (1) The OH stretching vibration related to water molecules in the interlayer LDH and metal hydroxide layers, which usually appeared around 3300
3600 cm21. (2) The absorption band around 1620 cm21 related to the OH bending vibration. (3) The characteristic absorption bands in the region of 400
1100 cm21 can be attributed to the metal
oxygen and oxygen
metal
oxygen bands (Shabanian et al., 2014, 2016a,b). (4) The characteristic absorption related to anions in the interlayer LDH usually appeared in the range of 800
1700 cm21.

2.2.1 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different anions 2.2.1.1 Mg
Al LDH
CO322 The OH stretching frequency of the Mg
Al LDH appeared in a broad absorption band in the range of 3300
3500 cm21. The absorption band at about 1600
1650 cm21 was attributed to the bending motion of interlayer water. Generally, the absorption bands related to carbonate anion for asymmetric and symmetric stretching vibration appeared at about 1450 and 880 cm21, respectively. Three different absorption bands at 590, 637, and 667 cm21 were attributed to the M
O and O
M
O (M: Mg or Al) stretching vibration. In the Mg
Al LDH

80

Layered Double Hydroxide Polymer Nanocomposites

spectrum a medium absorption band appears at 450 cm21 due to the Al
O bond related to the [AlO6]32 structure (Valcheva-Traykova et al., 1993). Also some weak absorption bands around 3000 cm21 can be related to the OH hydrogen bonded stretching of water molecules intercalated in the LDH layer (Acharya et al., 2007). The main region in FTIR spectrum of Mg
Al LDH
CO322 as a typical LDH is illustrated in Fig. 2.2. In Fig. 2.2, for O
H stretching vibration a broad absorption band as well as a recognizable shoulder can be seen around 3300
3600 cm21. This

Figure 2.2 The approximate region of absorption bands of Mg
Al LDH
CO322.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

81

strong band is broad due to the overlapping of two and/or three possible absorption vibrations of the interlayer water. Due to a combination of lattice or vibrational modes, the brucite-type hydroxides can appear in this area as a strong and broad absorption band (Duan and Evans, 2006). Also due to O
H and CO322 interaction in the interlayer of LDH, the broad absorption band of hydroxyl group can appear as a broad shoulder band. In some FTIR spectra of Mg
Al LDH, a weak band appeared around 3040
3060 cm21, which could be related to water molecules that were solved in the microporosity area of the LDH structure (Chˆatelet et al., 1996). The O
H bond in LDH is shorter than in brucite, and its effect can shift the IR absorption band of the main O
H stretching vibration around 3550
3570 cm21 for brucite to around 3470 cm21 for Mg
Al LDH
CO322 (Kagunya et al., 1998).

2.2.1.2 Mg
Al LDH
NO32 Mg
Al LDH with nitrate anions has been reported many times (Shabanian et al., 2016a,b; Lennerova´ et al., 2015; Xu and Zeng, 2001; Zhao et al., 2012; Zhang et al., 2014; Wang et al., 2011; Nyambo et al., 2008). The characteristic absorption band around 1380 cm21 observed in the FTIR spectrum of all Mg
Al LDH
NO32 samples was attributed to the NO32 group. This intensive sharp absorption band was related to v3 vibrational mode with D3h symmetry in NO32 structure. In many FTIR spectra of Mg
Al LDH
NO32 a sharp and strong characteristic band appeared around 450 cm21, which was related to the metal
oxygen bond in the brucite-like lattice. The FTIR spectrum of Mg
Al LDH
NO32 prepared by the coprecipitation method from aluminum and magnesium nitrate is presented in Fig. 2.3. The characteristic absorption band centered at 3441 cm21 was attributed to the O
H stretching of the  metal
hydroxide layer and interlayer water molecules. The bending vibration of the water interlayer was reflected at 1621 cm21. Also, the NO32 stretching vibration appeared at 1383 cm21. The appeared absorption bands in the range of 580
870 cm21 were related to Al
O and Mg
O stretching modes. The shoulder bands at 2917 and 2856 cm21 were attributed to H2O
NO3 bridging vibration (Hajibeygi et al., 2017).

2.2.1.3 Mg
Al LDH
SO422 The Mg
Al LDH containing SO422 in its interlayer was prepared and the characteristic absorption band related to SO422 was reported. The characteristic absorption band related to sulfate ions in the interlayer LDH appeared at 1000
1300 cm21 (Acharya et al., 2007; Fahami and Beall, 2016). Also, OH as a weak absorption band appeared as two shoulder bands at 2920 and 2852 cm21 due to OH hydrogen bond stretching vibration of intercalated water molecules. In addition, the bands in the range of 500
1000 cm21 are related to M
O, O
M
O, and M
O
M lattice vibrations (M 5 Mg and Al) (Acharya et al., 2007; Obadiah et al., 2012).

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Layered Double Hydroxide Polymer Nanocomposites

Figure 2.3 FTIR spectrum of Mg
Al LDH
NO32.

2.2.1.4 Mg
Al LDH
PO432 and Mg
Al LDH
HPO422 The Mg
Al LDH with phosphate in its interlayer as well as its corresponding FTIR absorption bands was also reported (Shimamura et al., 2012). The absorption bands related to HPO422 appeared at 1085, 995, and 860 cm21 which can be related to antisymmetric stretching of P
O, symmetric stretching of P
O, and antisymmetric stretching of P
OH, respectively (Dartiguelongue et al., 2016). Three absorption bands related to HPO422 are transformed into a single broad absorption band at 1056 cm21. Also, a shoulder absorption band was obvious at 870 cm21 close to the antisymmetric stretching of P
OH. The characteristic absorption band related to phosphate in the interlayer of Zn
Al LDH was reported by other authors at 1056 cm21 (Costantino et al., 1997; He et al., 2010; Cheng et al., 2010).

2.2.1.5 Mg
Al LDH
Cl2 The preparation of Mg
Al LDH
Cl2 was reported via a coprecipitation method (Yue et al., 2017). The broad bands at 3466 and 1636 cm21 are associated with the stretching and bending vibrations of the 2 OH group of LDH layers and interlayer water molecules. The sharp band observed at 1372 cm21 is due to antisymmetric stretching of the CO322 ion, which may be introduced into the interlayer of Mg
Al LDH by absorption of CO2 during the preparation procedure (Li et al., 2009, 2014). The bands observed below 1000 cm21 (400
850 cm21) correspond to the characteristic lattice vibrations of Mg
O and Al2O3. Chloride did not have a significant and clear absorption band in the FTIR spectrum of LDH.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

83

Figure 2.4 The approximate region of absorption bands of typical interlayer anions.

By comparison of FTIR of LDH with different anions, the approximate region of absorption bands for some common anions are obtained and represented in Fig. 2.4. The absorption band related to the anions such as carbonate, nitrate, phosphate, and sulfate can be found in two regions (Fig. 2.4). Due to the presence of O
H bonds in the hydrogen phosphate anion, its absorption bands appeared in four or more regions.

2.2.2 Fourier transform infrared characteristic absorption bands of layered double hydroxides with different metals The LDH metals could be replaced with different metals (Hajibeygi et al., 2015; Rastin et al., 2017). The FTIR spectra in the range of 400
2000 cm21 related to some LDH with different M21 and Al31 are illustrated in Fig. 2.5. The Ca
Al and Cu
Al LDH were prepared from nitrate salts of pristine metals and for preparation of Ni
Al LDH, nickel chloride and aluminum nitrate solution salts were used. By comparison of these three spectra, it is clear that with changing metals in the LDH structure, their absorption bands were also changed. Some absorption bands in all three spectra are held in common such as the absorption band around 1375
1450 cm21, which is related to the presence of the nitrate ion and the CO322 group because of possible adsorption of CO2 during aging processes (Qu et al., 2016a,b). Also, a broad and weak band was observed at 1644 cm21 in all FTIR spectra related to the bending vibration of water (Zhong et al., 2017). The main differences between these three FTIR spectra are observed in the range of 400
900 cm21 related to typical stretching vibrations of metal oxides and metal hydroxide as well as O
M
O bonds in the LDH structure. In the FTIR spectrum of Ca
Al LDH, three shoulder and individual broad absorption bands appeared around 525, 765
870, and 1023 cm21. All of these bands were related to the typical stretching vibrations of M
O and M
OH (M 5 Ca and Al) in the LDH (Plank et al., 2006; Chen et al., 2015; Perioli et al., 2006).

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Layered Double Hydroxide Polymer Nanocomposites

In FTIR spectrum of Cu
Al the mentioned three absorption bands related to metal oxides and metal hydroxide can be seen, although the bands around 780 cm21 appeared as a broad band. The absorption bands which appeared in the range of 400
1040 cm21 were due to the pulsation of metal
oxygen and oxygen
metal
oxygen as well as metal hydroxide bonds in the brucite type (Chakraborty et al., 2015). Also, the absorption band at 1384 cm21 can be related to the nitrate ion vibration bands (Fig. 2.5) (Sahu and Pugazhenthi, 2011; Chakraborty et al., 2014). In the FTIR spectrum of Ni
Al LDH, a clear difference can be seen at 429 cm21. It can be related to presence of Ni
O bonds in the LDH structure (Chakraborty et al., 2014). Other absorption bands appeared as a broad and shoulder band in the region 500
1050 cm21 attributed to metal
oxide (Ni and Al) in the LDH structure that is typical of this kind of layered solids (Fig. 2.5) (Chakraborty et al., 2014). The absorption band of NO32 was observed as broad and sharp in Ni
Al as compared to Cu
Al and Ca
Al LDH. The bending vibration of interlayer H2O molecules (H\OH) appeared at 1637 cm21 confirming the presence of water molecules as bending modes. Also, other absorption bands in the region of 500
1000 cm21 as broad overlapped bands can be related to metal
oxide vibration modes (Costa et al., 2008). The FTIR spectra of Cu
Fe, Ni
Fe, and Ca
Fe LDH are presented in Fig. 2.6. All of the metals were used as their nitrate salts except for Ni which was used as a chloride salt. The absorption bands related to water bending vibration and nitrate anions are obvious around 1623 and 1355 cm21, respectively (Li et al., 2004).

Figure 2.5 FTIR spectra of Ca, Cu, and Ni
Al LDH.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

85

Figure 2.6 FTIR spectra of Cu
Fe, Fe
Ca, and Ni
Fe LDH.

In FTIR spectra of Cu
Fe and Ni
Fe LDH, the absorption band around 500 cm21 appeared in both spectra and can be related to M
O and M
OH lattice mode vibration (Nejati and Rezvani, 2012; Nejati et al., 2013; Zhang et al., 2010; Li et al., 2010). Due to the presence of chloride and nitrate anions, the absorption band around 1480 cm21 related to carbonate was decayed (Iwasaki et al., 2012). In FTIR spectrum of Ca
Fe, the absorption bands at 467, 588, and 853 cm21 were related to stretching vibration of M
O (M: Ca or Fe) and Ca
Fe
O in the LDH structure (Fig. 2.6) (Ferraro, 2012; Frost et al., 2009; Wu et al., 2012). The absorption band at 588 cm21 can be related to the Fe
O bond (Shabanian et al., 2015, 2016a,b).

2.2.3 FTIR spectra of layered double hydroxides containing three metals In recent years the preparation of ternary metal LDH has been investigated due to their unique application in electronic, magnetic, and optical areas. Ternary metal LDHs in general have better properties as compared to two-metal LDHs. These materials have better crystallinity and a well-defined hexagonal shape (Ma et al., 2010; Han et al., 2008, 2009; Yang et al., 2013). Some ternary LDH derivatives have been prepared by reaction of different M21 cations with M31 cations with different concentrations. FTIR characterizations of these ternary LDH derivatives are reviewed in the following sections.

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Layered Double Hydroxide Polymer Nanocomposites

It should be mentioned that all infrared spectra of LDH typically showed similar absorption bands, especially at high wavenumber regions such as the O
H stretching mode of the basal layer, the interlayer water, and stretching vibration of the anion (e.g., CO322). Moving to the wavenumber region below 1000 cm21, which showed the information on the absorption bands of the lattice, dual LDH materials showed bands of HO
M
OH and M
O, but in the ternary system usually the absorption bands shifted to higher wavenumbers with increasing concentration of the third ion. It should be noted that these shifts depended on the nature of the third ion. For example a broad absorption band of Cu substitution was found to be comparable to Co substitution. The possible explanation may be due to a Jahn
Teller distortion in Cu21 octahedral compounds that causes c-axis elongation (Fahel et al., 2016). The absorption bands of third cations are usually observed at lower wavenumbers as compared to the corresponding free hydroxide anions. The absorption bands related to different metals in LDHs may appear at different frequencies in the FTIR spectrum. Some ternary LDHs were prepared by different metals and their FTIR spectra were reported (Pe´rez et al., 2012). The characteristic absorption bands related to metal
oxide and O
M
O (M: Zn, Al, and Cr) appeared at around 428, 553, 608, 780, and 938 cm21. Also, for LDH containing Cd, Al, and Cr, the absorption bands appeared at around 419, 489, and 531 cm21. The preparation of ternary metal LDH containing Co, Ni, and Fe with carbonate anion has been reported (Ehlsissen et al., 1993). The FTIR of the above-mentioned LDH had some characteristic absorption bands that confirmed its structure. The OH related to water molecules in the interlayer of LDH appeared at around 3503 cm21 as a broad band due to hydrogen bonding (Ehlsissen et al., 1993). The antisymmetry vibration of carbonate anions in the interlayer was observed at around 1366 cm21 (Hernandez-Moreno et al., 1985). Also, some absorption bands appeared at 1385 and 1525 cm21, which can be attributed to the vibration mode of carbonate. The absorption band in the region 480
800 cm21 was related to metal
oxide vibration and the band at 646 cm21 was related to symmetric bending of carbonate that overlapped with absorption of metal
oxide and shifted to low frequency (Zhang et al., 2008). The FTIR spectrum of Mg
Zn
Al LDH that was prepared by the coprecipitation method has been reported (Eshaq and ElMetwally, 2016). The broad band in the range of 3445
3500 cm21 is ascribed to the O
H stretching vibration of the water molecule and metal hydroxide in the brucite-like layers (Yang et al., 2002). The band at 1640 cm21 is attributed to the O
H bending mode in the water molecule. The weak shoulder band appeared at approximately 3000 cm21 owing to the OH stretching mode of interlayer water molecules hydrogen bonded to interlayer anions. The characteristic bands in the low-frequency region (400
1000 cm21) are related to the metal
oxygen stretching vibrations (Zn
O, Mg
O, and Al
O). In addition, a strong band at 1370 cm21 indicates the presence of CO322 anions in the interlayer region. The FTIR spectra of Ni
Mg
Al LDH and Co
Mg
Al LDH have also been considered. The bands at 3478 and 1663 cm21 were ascribed to the stretching vibrations of the OH group of LDH layers and bending vibration of water molecules in

FTIR characterization of layered double hydroxides and modified layered double hydroxides

87

the interlayer (Parida et al., 2012). The sharp band appeared at 1388 cm21 due to antisymmetric stretching of the CO322 ion and the broad bands between 500 and 800 cm21 are attributed to the characteristic M
O vibrations (Bharali et al., 2015).

2.3

FTIR spectra of organo-modified layered double hydroxides

The use of LDH as a nanofiller to improve thermal, mechanical, and flameretardancy properties of a polymer matrix is one of the recent applications being investigated in both academia and industry (Costa et al., 2007; Manzi-Nshuti et al., 2008; Raeisi et al., 2017). However, the modification of LDH is necessary for preparation of uniform polymer LDH nanocomposites. Different methods have been used in the modification of LDH such as anion exchange (Choy et al., 2000) and regeneration in situ synthesis (Desigaux et al., 2006), etc. These methods have some drawbacks and so can be replaced by a onestep synthesized method (Wang et al., 2009). Accordingly, a one-step method as the correct approach to synthesize organo-modified LDH was reported from solution of metal salts and the anionic surfactant in a reactor (Wang et al., 2009). Many organic compounds that include an anionic segment in their structures have been used for the modification of LDH. In FTIR spectra of organo-modified LDH the presence of characteristic absorption bands related to functional groups of organic modifier can be helpful in investigating their structures. For the FTIR spectra of the organo-modified LDH two types of bands would be expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. The approximate region of absorption bands of some functional groups that can be presented in organo-modifiers are represented in Fig. 2.7. It is

Figure 2.7 The approximate region of absorption bands in organo LDH modifiers.

88

Layered Double Hydroxide Polymer Nanocomposites

noticeable that the FTIR absorption bands can slightly shift due to different factors such as hydrogen bonding, intramolecular interaction, and chemical structure. Characteristic P
O
C stretching vibration bands are usually observed about 1140 cm21 (symmetric) and 1037 cm-1 (antisymmetric). The P 5 O stretching vibrations are indicated by a strong band about 1225 cm21. The antisymmetric and symmetric stretching vibrations of COO2 (carboxylate anion) have usually been observed about 1570 and 1440 cm21, respectively. The corresponding FTIR bands of organic modifiers are expected to shift toward a lower wavenumber in comparison to their free-state absorption bands, as more energy is required for executing such vibrations due to the presence of restriction between the layers. The bands in the range 1000
1800 cm21 are mostly due to the functionalities of the modifier and also due to interlayer water molecules. The appearance of characteristic bands for CO32
(γ) means that some CO32
still exists in the interlayer region of the modified LDH. This is perhaps caused by partial free movement of the CO32
ions due to enlargement of the interlayer region after organic modification. Most of the modified LDH materials have exhibited strong absorption bands in the range 2850
3100 cm21, corresponding to the
CH
stretching vibration arising from the hydrocarbon tail present in each modifier. In FTIR spectra of the modified LDH, the presence of interlayer water is not clear. In this regard, the only difference observed is the disappearance of a weak band (in the form of a shoulder) in the region 3000
3100 cm21, which originates from the interaction between O
H groups and CO32
ions. One of the common organic compounds used as an organo-modifier of LDH is sodium dodecylbenzene sulfonate (SDBS). This organic sodium salt can change with the anions in the interlayer of LDH via an ion exchange reaction (Wang and O’Hare, 2012). The organo-modified LDH can be prepared by a one-step reaction from metal salt in the presence of organic modifier salt in sodium hydroxide solution with pH 5 10 (Wang et al., 2009). The structure of SDBS is represented in Fig. 2.8. In LDH
SDBS, the characteristic OQSQO stretching vibration bands have appeared about 1040 cm21 (antisymmetric) and 1070 cm21 (symmetric), whereas the corresponding bands in some modifiers appear at 1230 and 1186 cm21, respectively. The C
S stretching vibration band is also observed in the range of 610
630 cm21. SDBS additionally has shown multiple bands corresponding to the aromatic ring C
C vibrations in the range 1450
1610 cm21. In general, the presence of absorption bands of SDBS in FTIR spectrum of the modified LDH indicates a good intercalation anionic structure in the interlayers of

Figure 2.8 Molecular structure of SDBS.

FTIR characterization of layered double hydroxides and modified layered double hydroxides

89

LDH. The important bonds of SDBS are aromatic C
H and aliphatic C
H, double bond carbon
carbon groups, which showed the bands around 3020
3100, 2930
2990, and 1550
1650 cm21, respectively. The bands related to SO32 appeared at 1037 cm21 as well as 1182 cm21 (symmetric and antisymmetric). The mentioned region absorption bands for SDBS in the FTIR spectrum of a typical LDH
SDBS are illustrated in Fig. 2.9. The FTIR spectrum of a typical SDBS-modified Zn
Al LDH is illustrated in Fig. 2.10. The characteristic absorption bands can be observed at 2858, 2927, and 2962 cm21 which are related to the antisymmetric and symmetric CH3 and CH2 group vibration modes resulting from the long alkyl chains of the SDBS anion. The absorption bands at 3063, 1600, and 1131 cm21 are attributed to the aromatic CH

Figure 2.9 The approximate region of absorption bands of SDBS in a typical LDH
SDBS.

90

Layered Double Hydroxide Polymer Nanocomposites

Figure 2.10 FTIR spectrum of SDBS-modified Zn
Al LDH.

stretching, double bond aromatic carbon
carbon and C
H aromatic in-plane bending of SDBS, respectively. The two strong absorption bands at 1036 and 1172 cm21 are attributed to symmetric and antisymmetric stretching vibration of SO32 bands in the SDBS structure. Also, the absorption band around 1398 cm21 is related to NO32 bands which are maintained from metal salt in the preparation processes of LDH (Xu et al., 2013; Pavel et al., 2012; Hajibeygi et al., 2015). Other absorption bands in FTIR are related to the LDH structure. One of the other common organic modifiers for LDH is sodium dodecyl sulfate (SDS). The FTIR spectrum of SDS is similar to that of SDBS with only slightly modification. In SDS molecular structure there is no aromatic ring, therefore there are no absorption bands related to aromatic C
H and double bond carbon
carbon of the aromatic ring (Xu et al., 2013). The structure of SDS is presented in Fig. 2.11. The aspartic acid-modified Li
Al LDH was prepared and used for preparation of poly(ethylene terephthalate) LDH nanocomposites (Bunekar et al., 2016). In this work the organo-modified LDH was prepared using a two-step reaction. At first neat Li
Al LDH was synthesized from LiNO3  3H2O and Al(NO3)3  9H2O, and then organo-modified LDH was prepared via an ion exchange reaction with aspartic acid solution salt. The structure of aspartic acid is presented in Fig. 2.12. The FTIR spectra of LDH and aspartic acid-modified LDH were reported. The aspartic acid-modified Li
Al LDH spectrum exhibited some bands, included an absorption band related to the neat LDH as well as absorption bands related to functional groups of aspartic acid as modifier. A broad absorption band around

FTIR characterization of layered double hydroxides and modified layered double hydroxides

91

Figure 2.11 Molecular structure of SDS.

Figure 2.12 Molecular structure of aspartic acid.

Figure 2.13 Molecular structure of lauric acid.

3200
3500 cm21 appeared which can be ascribed to the hydrogen-bonded hydroxyl groups from both the hydroxide layers and interlayer water molecules. The antisymmetric vibration bands of NO32 or CO322 appeared at 1354 cm21. The absorption bands at 530 and 740 cm21 can be attributed to the metal
oxide stretching modes. Characteristic absorption bands related to alkyl C
H stretching vibration were observed in the region 2800
3000 cm21. The antisymmetric and symmetric stretching modes of the carboxylate group appeared at 1527 and 1404 cm21, respectively. The lauric acid-modified Mg
Al LDH was prepared by Katiyar et al. (2010). In this research work, laurate-modified LDH was prepared and used in the preparation of nanocomposite based on polylactic acid. The structure of lauric acid is illustrated in Fig. 2.13. The FTIR spectra of LDH, organo-modified LDH, and lauric acid are considered. In the FTIR spectrum of the neat LDH, the broad absorption band in the range of 3200
3700 cm21 related to OH stretching vibration as well as a shoulder band at 3000 cm21 indicating that the hydrogen bonding between water molecules and carbonate ions was observed (Komarewsky et al., 1953). The absorption bandrelated to bending vibration of water molecules appeared at 1638 cm21. The characteristic absorption bands attributed to the carbonate ion in the interlayer also appeared. Two absorption bands of the remaining nitrate ions were also observed at 1384 and 830 cm21 (Hansen et al., 1994) and the absorption band of lattice vibration of metal
oxide was observed at 411 cm21. In the FTIR spectrum of laurate-modified LDH some new absorption bands appeared and some absorption bands disappeared. The disappearance of the

92

Layered Double Hydroxide Polymer Nanocomposites

absorption band related to carbonate ion at 1373 cm21 and the appearance of absorption bands related to carboxylate groups indicated the lauric acid salt presence in the interlayer of LDH. The associated absorption bands of the carbonyl group stretching vibration of carboxylate appeared at 1561 and 1411 cm21 related to antisymmetric and symmetric stretching vibration of laurate COO2 in the anionic form (Borja and Dutta, 1992; Venkataraman and Vasudevan, 2000). Also, the absorption band for stretching vibrations related to antisymmetric and symmetric stretching of CH2 groups that appeared in the range of 2953
2858 cm21, while the CH2 bending vibration and CH2 rocking vibration appeared as single bands at 1467 and 721 cm21, respectively. Lauryl alcohol phosphoric acid ester potassium also was used as an organic modifier of LDH (Xie et al., 2016). The FTIR spectrum of the mentioned organomodified LDH revealed absorption bands at 438 and 3434 cm21 due to the presence of metal
oxide bonds and water molecules in the interlayer of LDH. Also, the absorption band at 2952 cm21 was related to symmetric and antisymmetric vibration of aliphatic C
H groups in the organic modifier structure. Manzi-Nshuti et al. (2009) reported the preparation of oleate-modified Zn
Al LDH. The oleate-modified LDH was prepared by coprecipitation method (Wang et al., 2005). The molecular structure of oleic acid is presented in Fig. 2.14. The FTIR spectrum of oleate-modified Zn
Al LDH revealed the absorption bands related to LDH as well as absorption bands related to oleate carboxylate salt as an organic modifier (Xu et al., 2004; Hibino, 2004). The symmetric and antisymmetric mode attributed to C
H aliphatic groups appeared around 2800
3000 cm21. Two strong bands appeared around 1400
1600 cm21, which was attributed to symmetric and antisymmetric carboxylate bands in oleate salt. An absorption band that appeared at 3006 cm21 as a weak band is related to C
H attached to a carbon
carbon double bond (Simons, 1978). The molecular structure of taurine (2-aminoethanesulfonic acid) is presented in Fig. 2.15. This organic compound was used in the preparation of organo-modified Mg
Al LDH (Lennerova´ et al., 2015).

O OH

Figure 2.14 Molecular structure of oleic acid. O

HO S O

Figure 2.15 Molecular structure of taurine.

NH2

FTIR characterization of layered double hydroxides and modified layered double hydroxides

93

In this work the FTIR spectra of neat LDH, modified LDH, as well as taurine as modifier, were compared. In the FTIR spectrum of modified LDH, the vibrational absorption bands related to taurine can be found. Three characteristic absorption bands appeared at 1241, 1184, and 1046 cm21, which were related to the SO32 group in the modified LDH structure. The stretching vibration of C
N appeared at 1046 cm21. Also, the absorption bands at 1512, 1305, and 1114 cm21 were attributed to different vibrations of the NH2 group. In some cases the bands attributed to 2 NH31 that were observed in the pure powders decreased significantly or vanished in the corresponding modified LDH. This can be attributed to the interaction between the N atom and the metal ions in the layers of LDH. The stretching vibration of CH2 groups in the taurine structure appeared at 1305, 963, 894, and 741 cm21 (Ohno et al., 1992). A sharp absorption band was observed in the FTIR spectrum of the neat LDH at 1383 cm21, which was related to nitrate anions in the interlayer of LDH. This absorption band appeared as a weak band in the FTIR spectrum of modified LDH. It can be related to a trace of nitrate anions remaining in the rehydrated LDH. Two organic compounds included 2-naphthalene sulfonate and 2,6-naphthalene disulfonate, which contain a naphthalene ring with one and two sulfonate groups being used for modification of LDH (Kameda et al., 2006). The structures of two aromatic modifiers are presented in Fig. 2.16. The FTIR spectrum of modified LDH indicated the naphthalene containing organic modifier anions in the interlayer region of LDH. In FTIR spectra of 2naphthalene sulfonate- and 2,6-naphthalene disulfonate-modified LDH, the absorption bands related to the organic modifier appeared as well as absorption bands related to the LDH structure. The absorption bands related to metal
oxide bonds appeared in the region of 500
1000 cm21. Also, the OH hydroxyl group in water molecules appeared as a broad band centered at 3500 cm21. The aromatic CH stretching vibration related to the naphthalene ring appeared around 3000
3100 cm21 and the stretching vibrations of the SO32 group have been observed at around 1120
1150 cm21 (Kameda et al., 2008). For the preparation of chiral organo-modified LDH, N,N0 -(pyromellitoyl)-bis-Lisoleucine diacid was used for modification (Mallakpour and Dinari, 2013). The molecular structure of N,N0 -(pyromellitoyl)-bis-L-isoleucine diacid as an organic modifier is shown in Fig. 2.17. The FTIR spectrum of organo-modified LDH showed two types of absorption bands: one related to the anionic organic modifier intercalated between LDH layers and the other attributed to the LDH structure.

SO3H

SO3H

HO3S

Figure 2.16 Molecular structures of naphthalene derivative modifiers.

94

Layered Double Hydroxide Polymer Nanocomposites

Figure 2.17 Molecular structure of a chiral organic modifier.

Figure 2.18 Molecular structure of diacid-diimide as an organic modifier of LDH.

An absorption broad and shoulder band was observed in modified LDH in the range 1600
1640 cm21, indicating the presence of H2O molecules as a bending vibration appeared in this region (Kloprogge et al., 2004). The characteristic absorption bands at 2930
3100 cm21 related to the aliphatic and aromatic C
H modes in the dicarboxylate salt have also been observed. FTIR spectra of Mg
Al LDH and diacid-diimide-modified Mg
Al LDH were reported in a research work (Hajibeygi et al., 2017). A diacid-diimide organic compound containing imide heterocyclic ring and aliphatic long chains was synthesized and used for modification of LDH. The molecular structure of diacid-diimide is shown in Fig. 2.18. The neat Mg
Al LDH was prepared via the coprecipitation method from magnesium and aluminum nitrate. The modified LDH was prepared by an ion exchange reaction between the neat LDH and carboxylate dianion salt of organic modifier. The FTIR spectra of neat LDH (A) and modified LDH (B) are shown in Fig. 2.19. In the FTIR spectrum of neat LDH (LDH
NO32), a broad and strong absorption band related to OH stretching vibration due to the presence of interlayer water molecules and metal hydroxide layers appeared at 3466 cm21. The bending vibration of the interlayer water molecules was reflected as a broad band centered at 1618 cm21. Also, the characteristic absorption band was observed at 1378 cm21 related to nitrate interlayer anions stretching vibrations. The Al
O and/or Mg
O as well as M
O
M (M: Al and/or Mg) stretching modes appeared as broad and shoulder absorption bands around 590
890 cm21. The FTIR spectrum of modified LDH revealed the absorption bands related to neat LDH as well as absorption bands attributed to an organic modifier. The shoulder absorption bands, which appeared around 2850
2930 cm21, were related to stretching vibration of CH aliphatic groups in the organic modifier structure. Two clear absorption bands at 1774 and 1711 cm21 were related to antisymmetric and symmetric stretching vibration of carbonyl in an imide heterocyclic ring in the

FTIR characterization of layered double hydroxides and modified layered double hydroxides

95

Figure 2.19 FTIR spectra of (A) neat LDH and (B) modified LDH.

organic modifier. The absorption band at 1383 cm21 was related to C
N vibration mode of the imide group. It is clear that to understand FTIR spectra of organo-modified LDH, comparison of FTIR spectra of neat LDH, neat modifier, and organo-modified LDH would be a promising method. However, some shift would be expected in the FTIR bands of organo-modified LDH as compared to the neat structures. These can be attributed to the new van der Waals forces, including repulsion and attraction between the atoms.

2.4

Conclusion

The infrared spectroscopic method is an excellent technique to study the structure of LDH and modified LDH. The infrared absorption bands identify molecular components and structures. This technique measures the absorption of infrared radiation by the sample material versus the wavelength. The presence of different polar bonds, LDH sheet structures, and anions in the interlayers (inorganic and organic compounds) can easily be detected using FTIR spectroscopy. To achieve good results it is necessary to compare FTIR spectra of the neat LDH, modifier, and modified LDH. The main functional groups and sharp absorption bands can be helpful in investigating the structures. All LDH showed some similar bands in FTIR spectra, such as a broad absorption band in the range 3300
3500 cm21

96

Layered Double Hydroxide Polymer Nanocomposites

attributed to O
H stretching mode of the basal layer and the interlayer water, the band at 1620
1650 cm21, assigned to the bending mode of the interlayer water, the anion bands, and the band related to M
O and M
OH between 400 and 800 cm21. For modified LDH two types of bonds are expected, one corresponding to the intercalated anionic modifier and the other corresponding to the host LDH material. Both the anionic modifier and LDH bands showed some shift in the FTIR bands of organo-modified LDH which can be attributed to the formation of new van der Waals forces in organo-modified LDH.

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Fabrication technologies of layered double hydroxide polymer nanocomposites

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Shadpour Mallakpour1,2,3 and Farbod Tabesh3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

3.1

Introduction

3.1.1 Layered double hydroxides Layered double hydroxides (LDHs) are one of the layered material types with metal oxide (positively charged)/hydroxide sheets with accommodated water molecules and exchangeable anions in the interlayer space. These materials are also known as hydrotalcite-like anionic clay, due to their resemblance to the natural structure of clays (e.g., montmorillonite), which   have been infixed with cations and water x1 p2 31 M21 M ð OH Þ ½A :mH OÞ and have a typical thickness of 0.5 nm 2 2 12x x x=p (Brnardi´c et al., 2013; Velasco et al., 2012; Yu et al., 2015, 2016). Octahedral M (OH)6 units form layers of LDH, where the metal (M) is coordinated by six hydroxyl groups (OH), therefore forming M(OH)2 brucite-like sheets. As shown in Fig. 3.1 (Richetta et al., 2016), divalent and trivalent metal cations and hydroxyl anions will occupy centers of the octahedral units and vertices, respectively (Velasco et al., 2012). Thus, various compositions can be prepared from the most prevalent Mg-Al LDHs to rare CaAl-LDHs. Also, monovalent cations (such as LiAl-LDHs) can be replaced by divalent cations and another type of LDH can be prepared where the trivalent cation is altered (most commonly to Fe31) (Brnardi´c et al., 2013). This possibility to prepare a wide range of LDHs created superiority over natural clay and the impact of the cation type on the specifications of LDHs and polymer/ LDH nanocomposites (NCs) is the aim of the research. Two general methods exist for the preparation of LDH nanosheets, which have been schematically illustrated in Fig. 3.2, top-down (delamination) and bottom-up (controlled nucleation), of which top-down is the most common (Wang and O’Hare, 2012).

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00003-3 © 2020 Elsevier Ltd. All rights reserved.

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Figure 3.1 Schematic view of the general structure of (Zn, Al) LDH, with Cl2 and NO32 anions intercalated in the brucite-like structure. Other possible chemical species eventually presented in the interlamellar space are shown. The basal spacing, d, is also indicated. Source: Adapted from Richetta, M., Digiamberardino, L., Mattoccia, A., Medaglia, P., Montanari, R., Pizzoferrato, R., et al., 2016. Surface spectroscopy and structural analysis of nanostructured multifunctional (Zn, Al) layered double hydroxides. Surf. Interf. Anal. 48, 514
518. With kind permission of John Wiley and Sons.

Figure 3.2 Schematic synthesis of top-down and bottom-up methods for LDH single layers. Source: Adapted from Wang, Q., O’Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124
4155. With kind permission of American Chemical Society.

The top-down method entails the modification of the interlayer environment of LDHs and, subsequently, the selection of a suitable solvent system, while, in bottom-up synthesis, an aqueous coprecipitation system is required (Omwoma et al., 2014).

3.1.2 Modification of layered double hydroxides Neighboring sheets in the LDH structure forcefully imbibe each other by hydrogen bonding and electrostatic attraction. The basal spacing (usually under 1 nm)

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between LDH layers prohibits polymer or monomer penetrating into the layers. Another barrier to homogeneous dispersal of LDH layers in the polymeric matrix is incompatibility of the LDH layers with polymer molecules which have hydrophilic and hydrophobic natures, respectively. The modification of LDHs to boost the interlayer spacing (Mallakpour and Khadem, 2017b) and diminish the hydrophilicity of the surface of the layer is an approach to overcome the barrier (Brnardi´c et al., 2013). There are four general methods to insert anions into the LDHs: direct synthesis, anionic exchange, reconstitution (rehydration) (Velasco et al., 2012), and mechanochemical approaches (Qu et al., 2016)

3.1.2.1 Direct synthesis The direct synthesis is conducted by coprecipitation and is the most common pathway to synthesize LDHs with inorganic anions. In this method, the desired anion is added to the solution of metal salts (usually chloride or nitrate salts) that will form the layers. To prevent coprecipitation of other phases such as oxide contamination from the metals, control of the pH is necessary. The tendency of the organic anions to hydroxide layers avoids the incorporation of metal salts anions used in the synthesis. The ability to control the charge density of the layers (M21 to M31 ratio) and producing high pure LDHs are the main benefits of this method.

3.1.2.2 Anionic exchange The anionic exchange route is very convenient and includes the dispersion of LDH precursor in a solution containing an excess of anions (e.g., PO432 can be incorporated) (Alibakhshi et al., 2016). Before picking out the precursor, it is crucial to be sure about the affinity of the anions with the LDH structure. Therefore, an enormous difficulty is carbonate anions, owing to the high tendency of the layers for small divalent anions. On the other side, among the most common monovalent anions presented in LDHs, hydroxide anions face the difficulty of the interchange.

3.1.2.3 Reconstitution (rehydration) In the reconstitution of LDHs, after being converted to oxides through calcination at temperatures between 500 C and 800 C, they can rehydrate to the original form in the presence of anions and water, which is called memory effect. A wide range of LDHs such as carbonates, organic-like naphthalene carboxylates, or carboxylates can be prepared with inorganic anions by this method. Despite the mentioned methods which are widely used, many issues should be considered, for example, treatment of aqueous waste, complex process, high energy expenditure, etc.— mechanochemical methods can effectively transcend these difficulties.

3.1.2.4 Mechanochemical approaches The mechanochemical methods are divided into two types: (1) a mechanohydrothermal process, in which at first precursors are ground and then hydrothermal

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treatment is done with a proper material (Zhang and Hou, 2018), and (2) direct mechanochemical synthesis, which has single-step grinding (including dry and wet grinding) and two-step grinding (dry and wet grinding) methods. Table 3.1 provides a comparison between these two methods. Irrespective of the method, the main obstacle in the preparation of high-purity hybrids is their affinity to carbonates, in which specific conditions should be conducted to avoid the presence of this anion. Thus, the alternative is the use of water-free carbonates (bidistilled water is normally used) and an inert atmosphere (nitrogen or other inert gases). Inorganic anions such as carbonates, chlorides, nitrates, and sulfates are the most common interlayer anions. The most used LDH is hydrotalcite, where magnesium and aluminum are the constituents, and carbonate, are the
main interlayer anions. Their typical composition formula is 31 22 Mg21 12x Alx ðOHÞ2 ðCO3x=n  mH2 OÞ (0.2 # x # 0.33) (Velasco et al., 2012).

3.2

Preparation of polymer/layered double hydroxide nanocomposites

3.2.1 Introduction Pure polymers suffer from some weaknesses such as poor mechanical strength, thermal stability, gas permeability, fire retardancy, adsorption capacity, etc. Therefore, some fillers such as SiO2 nanoparticles (NPs), CaCO3 NPs, carbon nanotubes, clay, graphene, cellulose, carbon dots, biochar, metal oxides, etc. can be used to cover those weaknesses. Due to the new mechanical, thermal, and optical properties, which seldom exist in the virgin macromolecule, the polymer/LDH NCs have grabbed significant attention in the material chemistry field. LDH would improve these limitations of polymers, for example, layer structure of LDHs can reduce the gas permeation through the polymeric matrix, existence of metal in the LDH Table 3.1 Advantages and disadvantages of the mechanochemical approaches (Qu et al., 2016) Approaches

Advantages

Disadvantages

Mechanohydrothermal process

Regular hexagonal shape particles Highly dispersed high crystallinity at low pressure Shorter time reaction The regular hexagonal shape of particles Salts, oxides, and hydroxides all possible raw materials Ease of operation Particle agglomeration

Solvent involvement Raw materials limited to soluble salts Energy waste Solvent involvement

Direct mechanochemical syntheses

Limited to Fe-based LDH Low crystallinity

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structure increases the char yield, thermal stability, and fire retardancy; the large surface area of this material can drastically increase the adsorption capacity of the polymer. The level of distribution (intercalation or exfoliation) of LDH layers into the polymeric matrix will determine the performance of the NCs, and NCs with exfoliated LDHs have favorable properties in comparison with NCs with intercalated LDHs, due to further dispersal into the polymeric matrix. On the other hand, the exfoliation of the LDHs into the polymer becomes more unfavorable due to powerful electrostatic interactions of the interlayer, which is the result of the great charge density (B300 mequiv/100 g) of LDH layers. As mentioned earlier, giant polymer chains cannot penetrate into the basal spacing of pristine LDH unless this distance is significantly increased. The three routes used for the fabrication of layered double hydroxide polymer nanocomposites shown in Fig 3.3 are: (1) intercalation of the monomers in the interlayer region of LDH or modified LDH and in situ polymerization resulting in intercalation, exfoliation, or both morphologies to the polymer nanocomposites; (2) direct intercalation of polymers into the interlayer region of LDH or modified LDH resulting in intercalated or exfoliated or both morphologies to the nanocomposite. The several methods coming under this category are: melt mixing, solvent blending, two-roll mill mixing, melt-spinning, freeze-drying, and melt extrusion; (3) preexfoliation of modified LDH with suitable solvents and restacking of the exfoliated layers over the polymer either by using polymers directly or monomer followed by in situ polymerization, which results in exfoliation, intercalation, or both morphologies to the nanocomposite. The several methods under this category are layer-by-layer (LbL) assembly, solvent blending, spin-spray LbL, and mechanical agitation. Intercalation of the monomers (in situ polymerization), in which the monomers are dissolved in LDH suspension and intercalated into LDH interlayer spaces. Then, the polymerization (radical, cationic, anionic, condensation, ring opening, etc.) will occur using a suitable initiator (depending on the type of polymerization), accelerator, and crosslinker (if needed) followed by exfoliation of LDH nanosheets within the polymer matrices. In some cases, an emulsifier agent is needed for better dispersion of hydrophobic substances (Fig. 3.4). Direct intercalation of expanded macromolecule chains, in fact, the polymer instead of monomer will be intercalated and lead to exfoliation of LDH nanosheets. In this method, modification of LDH with organic anions to increase the gallery spaces is required. This method contains several sub-categories, as described below. In melt mixing, the materials are melted in a mixer at desired conditions (specific temperature, pressure, roller speed, and time) and then mixed to produce a polymer/LDH NC. Predrying should be done to prevent moisture interference (Kong et al., 2018b). This method leads to less dispersion of LDH in the polymeric matrix. A wide range of polymer/LDH NCs can be produced with intercalated or partially exfoliated structures. The intercalation of polymer into the interlayer space can determine the degree of intercalation or exfoliation of the obtained polymer/ LDH NCs. One of the most important advantages of this method is independency on the solvent. Another advantage of this method is compatibility with common plastic processing technologies (Fig. 3.5).

Figure 3.3 Pathway of NC preparation by (A) monomer exchange and in situ polymerization, (B) direct polymer exchange, and (C) restacking of the exfoliated layers over the polymer. Source: Adapted from Wang, Q., O’Hare, D., 2012. Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets. Chem. Rev. 112, 4124
4155. With kind permission of the American Chemical Society.

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Figure 3.4 In situ polymerization. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

Figure 3.5 Melt mixing. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

Solvent blending requires dissolution of polymer and dispersion of LDH into a proper solvent. If the solvent is not water (like xylene), predrying for removal of moisture is essential (Suresh et al., 2018). The solvent is evaporated from the polymer/LDH NCs through several pathways, such as casting of the NCs to prepare NC films. In this method, the polymer can penetrate into LDH layers easier, and hence, this method causes better dispersion of LDH due to freedom of mobility of LDH in the solvent. Using organic solvents (in some cases) makes this method quite expensive (Fig. 3.6). Two-roll mill mixing is mainly used to fabricate rubbers. In this method, firstly, the polymer is added to a mixer, after a specific time, the LDH is included in this mixer. The mixing procedure breaks down the powder LDH particles, subsequently, diffusion of the particles and exfoliation of the layers occur. Depending on the conditions and procedure, after all these processes, the mixture is kneaded on to a

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Figure 3.6 Solution blending. Source: Adapted from Velasco, J.I., Ardanuy, M., Antunes, M., 2012. Layered double hydroxides (LDHs) as functional fillers in polymer nanocomposites. In: Gao, F. (Ed.), Advances in Polymer Nanocomposites. Woodhead Publishing Cambridge, United Kingdom, pp. 91
130. With kind permission of Elsevier.

two-roll mill and the required additives, like accelerators and curing agents, are added (Kong et al., 2018a). Melt-spinning is suitable for fabrication of synthetic fibers (Aranishi and Nishio, 2017). This method entails mixing of the polymer and LDH powder in an extruder (sometimes a compatibilizer is needed to improve the miscibility) and melting of a polymer along with LDH in a cylinder and the mixed materials are forwarded through a die at a specific flow rate (Kutlu et al., 2013) (Fig. 3.7). Freeze-drying is suitable for preparing polymer/LDH aerogels. Thus, first, a suspension or gel of polymer/LDH using water-soluble/dispersive polymer and LDH is prepared, then, this suspension or gel is frozen at low temperatures (using carbon dioxide/solvent bath, liquid nitrogen, etc.), and finally, the ice is sublimed with a high vacuum to gain aerogels (Chen and Schiraldi, 2018). During the freezing part, the microstructure of the aerogels is formed. The lower temperature in the freezing section leads to a larger number of ice nuclei, smaller crystals, and a denser structure. Also, the lower viscosity of the nanocomposite leads to better growth of the ice in a large lamellar ice layer, leading to obtaining a good NC aerogel (Fig. 3.8). Melt extrusion, in this route dry polymer and LDH (and in some cases a compatibilizer) or any ingredient [like pharmaceuticals (Patil et al., 2016)] are placed in an extruder with specific conditions including the desired temperature, rotation speed, time, and shear force (Bunekar et al., 2018). This method is a solvent-free method, therefore, dispersion of LDHs into the polymeric matrix occurs with the aid of heating, but the dispersion state in this manner is not as good as with the solvent blending method (Fig. 3.9). Preexfoliation is followed by mixing with the macromolecule. In this method, first, the LDH in a colloidal solution is laminated into nanolayers using a proper solvent or devices such as ultrasonics, then mixing with the polymer will happen (Nagendra et al., 2015; Wang and O’Hare, 2012). This method is divided into its subgroups as described below.

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Figure 3.7 Illustration of a typical melt-spinning process. Source: Adapted from Aranishi, Y., Nishio, Y., 2017. Cellulosic fiber produced by melt spinning. In: Blends and Graft Copolymers of Cellulosics. Springer, pp. 109
125. With kind permission of Springer.

Figure 3.8 The formation of microstructures of polymer/clay aerogel composites. Source: Adapted from Chen, H.-B., Schiraldi, D.A., 2018. Flammability of polymer/clay aerogel composites: an overview. Polym. Rev. 1
24. With kind permission of Taylor and Francis.

Layer-by-layer (LbL) assembly involves a core or sheet (polymer or other substrate) covered with a layer of a negatively charged polymer [like poly(ethylene terephthalate) (Wang et al., 2017), poly(sodium 4-styrene sulfonate) (PSS) or other polyelectrolytes (Li et al., 2005)] and then the LDH sheets are deposited on the core or sheet by dipping the substrate in the LDH suspension. This process can be repeated several times. The growth of the film can be controlled by, for example,

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Layered Double Hydroxide Polymer Nanocomposites

Matrix API Feeder Feed rate output Feed rate input Screw speed input

Barrel temperatures output

Material Spectrum pressure

Motor

Die Motor

Torque Screw speed output

Barrel temperatures input

Material temperature

Figure 3.9 Schematic of a typical extruder system. Source: Adapted from Patil, H., Tiwari, R.V., Repka, M.A., 2016. Hot-melt extrusion: from theory to application in pharmaceutical formulation. AAPS PharmSciTech. 17, 20
42. With kind permission of Springer.

Figure 3.10 Schematic illustration for the assembly of (U-mLDH/NBR)n film on PET substrate by the layer-by-layer assembly technique. Source: Adapted from Wang, L., Dou, Y., Wang, J., Han, J., Liu, L., Wei, M., 2017. Layerby-layer assembly of layered double hydroxide/rubber multilayer films with excellent gas barrier property. Comp. Part A: Appl. Sci. Manuf. 102, 314
321. With kind permission of Elsevier.

microgravimetry, which changes in frequency, showing an increase in the mass of layers (Katagiri et al., 2018) (Fig. 3.10). In solvent blending, a polymer or copolymer is dissolved in a suitable solvent (whether water or other solvents) along with dispersion of the desired amount of the

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modified LDH (preexfoliated) and then the mixing occurs in one pot with a better dispersion state of LDH in a polymeric matrix. The obtained mixture in most cases is cast to evaporate the solvent from the NC (Chakraborty et al., 2016). In this method, the polymer can penetrate into LDH layers not only due to the existence of the solvent which increases the mobility but also owing to the presence of a modifier in the LDH layers, which increases the gallery space of the LDH (Fig. 3.11). In the spin-spray LbL method, polymer or polyelectrolytes are dissolved in water as a solvent and the modified LDH is dispersed in water. The mixture is sprayed in a rotating substrate by some nozzles (depending on the number of materials) which are fed by pumps and nitrogen gas. Control of the nozzles, spraying time, and drying time are monitored by a microcontrolled circuit. The spraying and drying time can vary based on the materials. Also, the rinse stage can be run between each deposition (Larocca et al., 2018) (Fig. 3.12). In the mechanical agitation method, polymeric matrix and desired modified LDH are blended through mechanical agitation at a specific temperature to obtain a homogeneous suspension. Then the mixture can be moved into the preheated mold to gain a specific shape (Zhu et al., 2018). By this method, modified LDH is dispersed in the polymers, an advantage of this method is that solvent may not be used (in the case of resins) (Fig. 3.13). This chapter has presented researches about the preparation of polymer/LDH NCs based on natural and synthetic polymers and their characterization and applications.

Figure 3.11 Schematic diagram for the preparation of PMMA/ONi-Al LDH nanocomposites. Source: Adapted from Chakraborty, S., Kumar, M., Suresh, K., Pugazhenthi, G., 2016. Investigation of structural, rheological and thermal properties of PMMA/ONi-Al LDH nanocomposites synthesized via solvent blending method: effect of LDH loading. Chin. J. Polym. Sci. 34, 739
754. With kind permission of Springer.

Figure 3.12 Schematic of spin-spray layer-by-layer apparatus. For the sake of clarity, the gas and liquid connections are shown for only one nozzle. Source: Adapted from Larocca, N.M., Bernardes Filho, R., Pessan, L.A., 2018. Influence of layer-by-layer deposition techniques and incorporation of layered double hydroxides (LDH) on the morphology and gas barrier properties of polyelectrolytes multilayer thin films. Surf. Coat. Technol. 349, 1
12. With kind permission of Elsevier.

Figure 3.13 Reaction scheme of MCLDH microcapsules. Source: Adapted from Zhu, P., Gu, Z., Hong, S., Lian, H., 2018. Preparation and characterization of microencapsulated LDHs with melamine-formaldehyde resin and its flame retardant application in epoxy resin. Polym. Adv. Technol. With kind permission of John Wiley and Sons.

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Preparation of Natural polymer/layered double hydroxide nanocomposites

Natural polymers, like polysaccharides, are good candidates for NC matrices due to their unique properties such as biocompatibility, biodegradability, nonhazardous nature, etc. Even these biosafe macromolecules can be used as stabilizers for LDH for better dispersion, for example, utilization of carboxymethylcellulose (CMC) as a compatibilizer of ZnAl-LDH in potato starch (Wu et al., 2011).

3.3.1 Preparation of carboxymethylcellulose/layered double hydroxide nanocomposites In a study, CMC was used for a matrix for bio-NC
containing MgAl-LDH (Yadollahi et al., 2014). In this study, the LDH was obtained by a coprecipitation method and CMC/LDH NCs were successfully made through the solvent blending method using bidistilled water as a solvent. Possible interactions in the NCs are shown in Fig. 3.14. In this work, the effect of the amount of LDH on the polymer and subsequently on the NC properties has been studied.

Figure 3.14 Schematic illustration of the possible interaction of LDH particles with CMC chains. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym. 108, 83
90. With kind permission of Elsevier.

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As the content of the LDH increased, the water vapor permeability (WVP) decreased. The WVP of NCs was decreased to 37%, with the maximum amount of the LDH (8 wt.% with respect to the polymer weight). Also, at the low amount of the LDH exfoliation was observed while with increasing the content of the LDH intercalation of the LDH were observed. These observations were obtained using transmission electron microscopy (TEM) images (Fig. 3.15). As mentioned earlier, the type of cation can affect the property of the resulting LDH and subsequently the NCs. In a study, MgAl-LDH and NiAl-LDH were used to prepare CMC/LDH NCs (Yadollahi and Namazi, 2013). As can be seen in the thermogravimetry analysis (TGA) curves (Fig. 3.16), the thermal stability of pure CMC was increased using LDHs. Also, the NC-containing MgAl-LDH is more thermally stable than NiAl-LDH and it shows that the LDH with Mg is more stable than Ni.

3.3.2 Preparation of pectin/layered double hydroxide nanocomposites In addition to WVP, the antimicrobial activity of polymer/LDH is one of the aims of using LDHs and biopolymers. For example, pectin/LDH NCs were prepared using LDH containing antimicrobial active molecules such as benzoate (Bz), 2,4-dichlorobenzoate (DCBz), para-hydroxybenzoate (p-OHBz), and ortho-hydroxybenzoate (o-OHBz) (Gorrasi et al., 2012). The solvent blending was selected to prepare the

Figure 3.15 TEM images of the CMC
LDH NC films with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym. 108, 83
90. With kind permission of Elsevier.

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Figure 3.16 TGA curves obtained for (a) Na
CMC, (b) Ni-Al
CMC
LDH, and (c) MgAl
CMC
LDH. Source: Adapted from Yadollahi, M., Namazi, H., 2013. Synthesis and characterization of carboxymethyl cellulose/layered double hydroxide nanocomposites. J. Nanopart. Res. 15, 1563. With kind permission of Springer.

pectin/LDH NCs in distilled water. The complete delamination of LDHs into apple pectin was confirmed using wide-angle mode X-ray diffraction patterns (WAX) and absence of any other diffraction peak (Fig. 3.17). The antimicrobial activity test took 12 months and, after that, no mold formation for NCs with LDH containing antimicrobial active molecules was observed, meanwhile, in the case of pure pectin, mold formation occurred after 2 weeks (Fig. 3.18). This result proved that such NCs have promise for use in the packaging industry.

3.3.3 Preparation of chitosan/layered double hydroxide nanocomposites As discussed earlier, one of the methods used to prepare polymer/LDH is preexfoliation of the LDH. Beside the anions, some drugs can be used for this. A drugdelivery system has been made up of chitosan/LDH biohybrid beads coated with pectin through the LbL method (Ribeiro et al., 2014). Mg2Al-LDH was synthesized through the coprecipitation method and a nonsteroidal antiinflammatory drug [5-aminosalicylic acid (5ASA)] was used to modify the LDH. Fig. 3.19 shows a schematic of this system. The bio-NCs were prepared by adding LDH-5ASA into chitosan solution (from crab shells) and then dropping this mixture into the NaOH solution. The beads were coated with pectin (from citrus fruits). This system was prepared to achieve three goals: (1) pectin on the surface of the beads resists low pH (in the stomach); (2) the mucoadhesive property would be presented by the

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Figure 3.17 X-ray diffraction patterns of (A): (a) LDH-Bz, (b) LDH-DCBz, (c) LDH-oOHBz, (d) LDH-p-OH-Bz; and (B): (a) pectin, (b) pectin/LDH-Bz, (c) pectin/LDH-DCBz, (d) pectin/LDH-o-OHBz, (e) pectin/LDH-p-OHBz. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: preparation, characterization and physical properties. Carbohyd. Polym. 89, 132
137. With kind permission of Elsevier.

chitosan; and (3) controlling the kinetics of drug release would be achieved by incorporating the drug into the LDH. The in vitro results proved that this system is more efficient than only biopolymers or the immobilization of drug without LDH host.

3.3.4 Preparation of natural rubber/layered double hydroxide nanocomposites Also, natural rubbers can be reinforced with LDHs. For example, a mixture of metal LDHs (Zn/Mg-Al LDH) was prepared via an in situ pathway in the presence

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Figure 3.18 Pictures from a cast film of pectin and nanocomposites with nanohybrids after storage for 12 months at ambient temperature. Source: Adapted from Gorrasi, G., Bugatti, V., Vittoria, V., 2012. Pectins filled with LDHantimicrobial molecules: preparation, characterization and physical properties. Carbohyd. Polym. 89, 132
137. With kind permission of Elsevier.

of stearic acid (as an organic modifier) (Das et al., 2011). At first, stearic acidmodified LDH with zinc then accelerators and sulfur were added. Then, the nanocomposite was prepared using two-roll milling for 15 min. The obtained NCs were characterized using different analyses such as TEM, wide-angle X-ray scattering (WAXS), etc. Fig. 3.20 shows the WAXS and TEM image of the obtained NR/LDH NCs. The WAXS patterns of NR/LDH NCs reveal weak first basal reflection peaks in which the interlayer space obtained was 4 nm. This finding proves that the LDH in the rubber maintained its oriented structure. In addition, the moiety of intercalated/ partially exfoliated LDH reinforces the rubber matrix. Also, the TEM image of NR/ LDH NCs shows the exfoliated LDH in the rubber matrix. It is expected that the resultant NR/LDH NCs would be useful in heavy equipment industries, such as for gaskets, seals, etc.

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Figure 3.19 Pectin-coated chitosan bead incorporating 5-aminosalicylic acid (5ASA) intercalated in Mg2Al-LDH as a new drug-delivery system. Source: Adapted from Ribeiro, L.N., Alcˆantara, A.C., Darder, M., Aranda, P., Arau´joMoreira, F.M., Ruiz-Hitzky, E., 2014. Pectin-coated chitosan
LDH bionanocomposite beads as potential systems for colon-targeted drug delivery. Intern. J. Pharm. 463, 1
9. With kind permission of Elsevier.

3.3.5 Other natural polymer/layered double hydroxide nanocomposites Polymer/LDHs NCs can be used as a biocatalyst for C
C bond formation. For this, various polysaccharide/LDH aldolase biohybrid beads were prepared (Mahdi et al., 2015). In this work CMC, poly-galacturonic acid, sodium alginate, curdlan, oxidized cellulose, and carrageenan as biopolymers, fructose-6-phosphate aldolase (FSAwt) [purified from Escherichia coli (E. coli)] as an enzyme, and Mg2Al-LDH were used for the preparation of these biocatalysts. The FSAwt@Mg2Al-LDH and the beads were synthesized through coprecipitation and solvent-blending methods (in water), respectively. Oxidized cellulose, alginate, and carrageenan were the most compatible with the enzyme and its activity. As shown in Fig. 3.21 the aldol reaction of hydroxyacetone and formaldehyde was done to prove the catalytic efficiency of the biocatalytic and a yield of 80% was obtained after 3 h.

3.4

Preparation of synthetic polymer/layered double hydroxide nanocomposites

Some disadvantages of natural polymers encourage us to use synthetic polymers rather than natural polymers, such as designing polymers for specific uses,

Fabrication technologies of layered double hydroxide polymer nanocomposites

Mixed metal hydroxides layer Anionic layer

4.2 nm 3.1 nm

10

4

4.5-5.5 nm

Intensity (a.u.)

4.3 nm

LDH (pure) LDH-St (modified) NBR/LDH-St NR/LDH-St XNBR/LDH-St

121

Interlayer space

WAXS

103

0.9 nm

102

101

100 nm 2

4

6 8 2θ (deg)

10

12

14

Figure 3.20 WAXS patterns and TEM image of rubber/LDH nanocomposites. Source: Adapted from Das, A., Wang, D.-Y., Leuteritz, A., Subramaniam, K., Greenwell, H. C., Wagenknecht, U., et al., 2011. Preparation of zinc oxide free, transparent rubber nanocomposites using a layered double hydroxide filler. J. Mater. Chem. 21, 7194
7200. With kind permission of the Royal Society of Chemistry.

Figure 3.21 FSAwt@Mg2Al-LDH/carr beads mediated synthesis of L-1-deoxyerythrulose. Source: Adapted from Mahdi, R., Gue´rard-He´laine, C., Laroche, C., Michaud, P., Pre´vot, V., Forano, C., et al., 2015. Polysaccharide-layered double hydroxide
aldolase biohybrid beads for biocatalysed CC bond formation. J. Mol. Catal. B: Enzym. 122, 204
211. With kind permission of Elsevier.

withmore durability, more strength, more stability, lower processing cost, and so on. Thus, they can be used in many fields such as biomedical implants and devices, drug delivery, water treatment, wound dressing, etc.; on the other hand, modification of functional groups of synthetic polymers is easier than natural polymers. To overcome some weaknesses of synthetic polymers, blending two or more polymers or chemically linking (copolymers) makes an intermediate favorable property (Bhatia, 2016). In this section, the fabrication of important synthetic polymer/LDH using various methods is discussed.

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3.4.1 Preparation of polyimide/layered double hydroxide nanocomposites An organosoluble polyimide (PI) containing ZnCr-LDH has been synthesized (Dinari and Rajabi, 2016). The LDH was synthesized through the coprecipitation pathway, where Zn(NO3)2.6H2O and Cr(NO3)3.9H2O (Zn:Cr in the molar ratio of 2.0:1.0) were used. The NCs based on PI were prepared by in situ polymerization of 5-methyl-N,N-bis(4-nitrophenyl)naphthalene-1-amine with pyromellitic dianhydride (PMDA) using a different amount of LDHs. The resulting poly(amic acid) (PAA) as the precursor was further thermally imidized to the desired PI. Several techniques were applied to characterize the NCs. The morphology of LDH and PI/ LDH was studied via field emission scanning electron microscopy (FE-SEM) and the plate-like morphology and hexagonal crystallite for the LDH structure were given (Fig. 3.22). The incorporation of LDH into PI made it self-extinguishing, with the limiting oxygen index (LOI) for NCs in the range of 45.5
47.1%, which is good for these materials. It can be explained that the heat is distributed among layers. On the other hand, there are numerous
OH groups on the layers and they

Figure 3.22 FE-SEM images of NCs with (A and B) 2% and (C and D) 4% of LDH. Source: Adapted from Dinari, M., Rajabi, A.R., 2016. Structural, thermal and mechanical properties of polymer nanocomposites based on organosoluble polyimide with naphthyl pendent group and layered double hydroxide. High Perform. Polym., 1
9. doi:10.1177/ 0954008316665678. With kind permission of SAGE Journals.

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are H-bonded with polymer chains. Therefore, a large amount of energy should be consumed to break them. Thus, the heat transfer to the polymer will be postponed.

3.4.2 Preparation of poly(methyl methacrylate)/layered double hydroxide nanocomposites In another project (Tsai et al., 2016), LiAl- and MgAl-LDHs were organo-modified with two anionic modifiers, cocoamphodipropionate (K2) and sodium dodecyl sulfate (SDS). The multiorgano-modified LDHs were used as fillers in poly(methyl methacrylate) (PMMA) by in situ polymerization of methyl methacrylate (MMA) in the toluene and the properties of the cast NCs, for example, optical clarity and antiscratch, were investigated. From the wide-angle X-ray diffraction (WXRD) pattern of MgAl-LDH and MgAl-LDH-K2 1 SDS (Fig. 3.23), increasing in the basal space (d003) provides the evidence that modifiers were intercalated into the galleries of LDH. This change in basal spacing also occurred for the LiAl-LDH and LiAl-LDH-K2 1 SDS, as can be seen in Fig. 3.24. After adding LDHs into PMMA, the gas permeability was reduced from 0.9429 for pure PMMA to 0.1599 PMMA/LiAl-LDH-K2 1 SDS-3%. Also, the transparency of NCs was measured by ultraviolet-visible (UV-vis) spectroscopy and results indicated that LDHs did not affect the transparency of PMMA and show that LDHs were dispersed well in the matrix. Therefore, these three NCs are applicable as a glass substitute in many fields.

Relative intensity

(003)

(006) (b) (003) (006) (009)

0

20

40 2-Theta

(110) (113)

60

(a) 80

Figure 3.23 The WXRD diffraction patterns of (a) pristine MgAl-LDH and (b) MgAl-LDHK2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of Multiorganomodified LiAl- or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. With kind permission of John Wiley and Sons.

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Relative intensity

(002)

(004)

(006) (b) (002)

(004)

(101) 0

20

(006) (200) (115) 40 2-Theta

(303) (10,10) (a) 60

80

Figure 3.24 The WXRD diffraction patterns of (a) pristine LiAl-LDH and (b) LiAl-LDHK2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. With kind permission of John Wiley and Sons.

3.4.3 Preparation of polyvinyl acetate/layered double hydroxide nanocomposites Based on the literature, the delaminated or modified LDHs have better dispersion into the polymeric matrix. Another example for this claim is the work by Chen et al. (2015), where the MgAl-LDHs were delaminated in a microemulsion composed of octane, N-lauroyl-glutamate (LGA), and butanol as a solvent, surfactant, and cosurfactant, respectively. These delaminated LDHs, with different percentages, were dispersed in the vinyl acetate and the chain-growth polymerization occurred using benzoyl peroxide to give polyvinyl acetate (PVAc). As a comparison, virgin LDH-PVAc was prepared. The TEM micrographs of the samples proved that delaminated LDH has a better dispersion state than that of the LDH-PVAc. The dispersed delaminated sheets in the polymer have a fiber-like shape at the nanoscale size (Fig. 3.25). The obtained NCs containing LDH are more thermally stable than that of the polymer without LDH.

3.4.4 Preparation of P(MMA-co-BA)/layered double hydroxide nanocomposites As shown in Fig. 3.26, the NC consists of MgAl-LDH as nanofiller and a copolymer of MMA and butyl acrylate (BA) as a matrix. (P(MMA-co-BA)/LDH) has

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Figure 3.25 TEM micrographs of different LDHs in PVAc. (A) Delaminated LDHs (prepared in the microemulsion composed of octane, LGA, butanol, and water and the ratio of octane to LGA is 10); (B) NO3-LDHs. Source: Adapted from Chen, J., He, M., Wang, Y., Hu, W., Lv, L., 2015. Nanoscale dispersion of delaminated sheets of layered double hydroxides in polyvinyl acetate. Micro Nano Lett. 10, 465
468. With kind permission of IET Digital Library.

Figure 3.26 Schematic illustration of the LDH/P(MMA-co-BA) nanocomposite film formation. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

been prepared and the interaction between LDH layers and polymer chains has been reported as being electrostatic (Veschambres et al., 2016). This nanocomposite is suitable to be used in the coating industries. A coprecipitation approach was used to prepare the LDH and the latex was made through surfactant-free radical polymerization of MMA/BA (1:1) initiated by potassium persulfate (KPS) and the combination was transmitted into a Teflon cast. As shown in Fig. 3.27, the average particle size was obtained as 77 nm by dynamic light scattering (DLS), which fits well within the range of 30
140 nm from TEM images. TEM images of latex/LDH5 and latex/LDH10 samples are illustrated in Fig. 3.28. In the low-magnification image, the creation of a connected LDH lattice can be explicitly seen for either of the LDH polymer NCs. Since the network mesh

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Figure 3.27 DLS measurement (A) and TEM image (B) of MgAl-LDH. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

Figure 3.28 Low- and high-magnification TEM images of microsections from latex/LDH5 and latex/LDH10 samples. Source: Adapted from Veschambres, C., Halma, M., Bourgeat-Lami, E., Chazeau, L., Dalmas, F., Prevot, V., 2016. Layered double hydroxides: efficient fillers for waterborne nanocomposite films. Appl. Clay Sci. 130, 55
61. With kind permission of Elsevier.

size is in the range of the latex bead diameter (ca. 700 nm), the latex particles handle the morphology of this network. In the TEM images at higher magnification, since the LDH content was increased, there was more LDH layer stacking in the network wall, and, as expected, the thickness of the network wall was related to the LDH content.

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3.4.5 Preparation of wood flour/polypropylene/layered double hydroxide nanocomposites The NC based on Mg3Al-LDH and wood flour/polypropylene (WF-PP) has been reported (Peng et al., 2017). The pristine LDH, poplar WF, and PP were mixed using a high-speed blender and, after that, the mixture was melt-blended in an extruder. Of course, the main idea in this work was to study the effect of modified LDH on the NCs; therefore, the modification of Mg3Al-LDH with 5-sulfosalicylic acid (SA) anions was done via an ion exchange reaction and the resulting NCs were examined against the WF-PP/LDH NCs. The effect of the LDH on the photostability of WF-PP has been investigated through accelerated ultraviolet (UV) weathering. The UV resistance mechanism of SA-LDH is shown in Fig. 3.29. Addition of LDH into WF/PP brought some features, such as less color change, better thermal stability, fewer surface cracks, and better mechanical properties than WF/PP and, also, it reduced the photo-oxidation of WF/PP. Another important effect of modified LDH in WF/PP is the shield effect of the modified LDH against the UV light through the layers, as well as the presence of an interlayer anion (SA), which helps in chemical absorbance of UV light.

3.4.6 Preparation of poly(amide-imide)/layered double hydroxide nanocomposites In one study, N-tetrabromophthaloyl-glutamic was used for the modification of LDH and, subsequently, reinforcing of a poly(amide-imide) (PAI) by Mallakpour et al. (2016). The modification was done through the coprecipitation method and the solvent-blending route was used to prepare the NCs. The schematic mechanism of the modification and preparation of NCs are shown in Figs. 3.30 and 3.31, respectively.

Figure 3.29 UV resistance mechanism of LDH and LDH-SA. Source: Adapted from Peng, Y., Wang, W., Cao, J., Huang, Y., 2017. Synthesis of 5-sulfosalicylic acid-intercalated layered double hydroxide and its effects on wood flour/ polypropylene composites during accelerated UV weathering. J. Appl. Polym. Sci. 134. With kind permission of John Wiley and Sons.

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Figure 3.30 Preparation of the M-LDH. Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

Figure 3.31 Preparation of the PAI/M-LDH NCs. Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

Fourier transform infrared spectroscopy (FT-IR), TGA, FE-SEM, TEM, and XRD were applied to study the properties of the samples. Thus, FE-SEM images were employed to observe the morphology of the neat LDH, modified-LDH (MLDH) nanolayers, and PAI/M-LDH NCs. The FE-SEM photographs exhibited the plate-like morphology for neat LDH layers, which were stacked on top of each other (Fig. 3.32A,B). Evidence of exfoliation is provided by the existence of single layers. In the case of M-LDH, FE-SEM results exhibited that the M-LDH layers were exfoliated and lost their ordered stacking structure (Fig. 3.32C,D). FE-SEM images of PAI/LDH NCs show the good and uniform dispersion of MLDH into PAI, although by increasing the amount of M-LDH to 8 wt.%, some agglomeration is revealed (Fig. 3.33). The existence of an amino acid in the polymer NC made it a biodegradable NC and suitable for industrial applications. In

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129

Figure 3.32 FE-SEM images of LDH-CO322 (A, B) and M-LDH (C, D). Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

addition, PAI/LDH NCs due to the presence of LDH as an acid scavenger and PAI with its chemical resistance is useful in the coating industry. A chiral NC based on PAI/organo modified LDH has been synthesized (Mallakpour et al., 2014). In this project, coprecipitation was carried out for the preparation of the MgAl-LDH under ultrasonic irradiation, then the LDH was modified with a diacid. A schematic illustration of the procedure is shown in Figs. 3.34 and 3.35. For the synthesis of PAI, a polycondensation reaction of N,N0 -(pyromellitoyl)-bis0 L-phenylalanine diacid and 4,4 -diaminodiphenyl sulfone was carried out and the chirality of the polymer was confirmed by specific rotation {[α]25D 5
134 degrees (at a concentration of 0.5 g/dL in DMF at 25 C)}. A solvent-blending method was applied to prepare NCs in ethanol. TEM, FT-IR, TGA, XRD, and FE-SEM were used to characterize the NCs. Fig. 3.36 shows FE-SEM images of the LDH and modified LDH. Based on these images, the nature of LDH particles roughly included plate-like shapes with side-long dimensions and thicknesses ranging over a few nanometers were retained. As commonly observed for typical organic anionic intercalated LDH compounds, the formation of plate-like NPs with regular shape has been observed from the images of M-LDH. Over a few hundred and several tens of nanometers have been calculated for the diameters and thicknesses of NPs, respectively. As shown in Fig. 3.37, a carboxylated CNT was used to prepare a hybrid of flower-like MgAl-LDH/CNT for reinforcing of chiral PAI (Mallakpour and Dinari, 2015). N,N0 -(pyromellitoyl)-bis-L-phenylalanine as diacid and 2-(3,5-diaminophenyl)-benzimidazole as diamine were utilized as monomers in a step-growth

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Figure 3.33 FE-SEM images of PAI (A, B), PAI/NCs 2 wt.% (C, D), PAI/NCs 4 wt% (E, F) and PAI/NCs 8 wt% (G, H). Source: Adapted from Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
1329. With kind permission of John Wiley and Sons.

polymerization in the presence of tetra-n-butylammonium bromide (TBAB). Preparation of NCs was carried out using absolute ethanol as solvent and ultrasonic irradiation as a dispersant. At first, the PAI was dispersed in ethanol and sonicated at room temperature. Then a different amount of LDH (2, 4, and 8 wt.%) was added to the PAI suspension and sonicated. After removal of ethanol and drying in a vacuum oven, the PAI/LDH NCs obtained. The hybrid can be easily observed via TEM images. Synthesized LDHs show smooth, well-formed hexagonal forms and overlapping crystals under ultrasonic irradiation (Fig. 3.38A,B). Fig. 3.38C,D clearly shows the stiff connection between CNTs and LDH platelets. Random dispersion of the individual CNTs inside the microspheres and on the surface of LDH platelets can be clearly seen in the image. Accumulation of LDHs has been prevented by good interaction between CNTs and LDHs, which is indicated by the results. Also, in the TEM images of PAI/ LDH
CNT NC2%, LDH
CNT is well seen in the PAI matrix as well as good dispersion state of the LDH
CNT (Fig. 3.39). These NCs have potential to be used as water pollutant adsorbents, for example, for heavy metals, dyes, etc.

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Figure 3.34 Intercalation of diacid in the interlayer of LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

Figure 3.35 Synthesis of NCs of PAI and M-LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

3.4.7 Preparation of low-density polyethylene/layered double hydroxide nanocomposites The NC composed of low-density polyethylene (LDPE)/organo-modified LDH has been synthesized (Gorrasi and Bugatti, 2016). Aleurate, citrate, glycolate, parahydroxybenzoate, salicylate, and serine carboxylate were the organic anions intercalated into the ZnAl-LDH. Coprecipitation and ball-milling procedures were used to prepare the LDH and LDPE/LDH NCs, respectively. In this method, milling of modified LDH and LDPE powder was done in a centrifugal ball mill using five

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.36 FE-SEM images of (A and B) pristine LDH and (C and D) modified LDH. Source: Adapted from Mallakpour, S., Dinari, M., Hatami, M., 2014. Modification of Mg/Allayered double hydroxide with L-aspartic acid containing dicarboxylic acid and its application in the enhancement of the thermal stability of chiral poly (amide-imide). RSC Adv. 4, 42114
42121. With kind permission of the Royal Society of Chemistry.

Figure 3.37 Preparation of LDH
CNT hybrids. Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

steel balls, at a speed of 580 rpm, for 60 min, and at room temperature. Then, the mixture was molded at 130 C and quenched in an ice-water bath. The XRD patterns of the samples are given in Fig. 3.40. Peaks at 2θ 5 21.6 degrees and 2θ 5 23.8 degrees show the typical orthorhombic cell which proved the maintenance of LDPE in the nanocomposites. Also, the basal peak of modified LDHs has been observed at the same 2θ. The basal peak of salicylate and para-hydroxybenzoate-modified LDHs occurred at 2θ 5 5.2 degrees, related to the higher basal distance, which gives evidence for successful intercalation

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Figure 3.38 TEM micrographs of (A and B) the LDH and (C and D) LDH
CNT. Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

Figure 3.39 TEM micrographs of the PAI/LDH
CNT NC2% at different magnifications (a
d). Source: Adapted from Mallakpour, S., Dinari, M., 2015. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim. 119, 1905
1912. With kind permission of Springer.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.40 XRD diagrams of the nanocomposite LDPE/LDHs modified with the six organic anions: (a) aleuritic carboxylate; (b) citrate; (c) glycolate; (d) para-hydroxybenzoate; (e) salicylate; (f) serine carboxylate. Source: Adapted from Gorrasi, G., Bugatti, V., 2016. Mechanical dispersion of layered double hydroxides hosting active molecules in polyethylene: analysis of structure and physical properties. Appl. Clay Sci. 132
133, 2
6. With kind permission of Elsevier.

between LDPE and LDHs. Oxygen barrier properties of pure LDPE and its NCs have been investigated at atmospheric pressure and 25 C. The results indicated that organo-modified LDPE/LDH NCs are a good candidate for food packaging. Also, an overall migration test was conducted in acetic acid 3% (at nonswelling conditions) and ethanol 10% (at more swelling conditions) for LDPE and LDPE/LDH-salicylate NCs. The overall migrations were below 10 mg/dm2 as standard by the European Union legislation for food contact plastics.

3.4.8 Preparation of polyvinyl alcohol/layered double hydroxide nanocomposites A bio-NC (BNC) consisting of poly(vinyl alcohol) (PVA) and phenylalaninemodified LDH (M-LDH) has been prepared via the solvent-blending method in water using sonication as a green energy source (Mallakpour and Dinari, 2014). N,N0 -(pyromellitoyl)-bis-L-phenylalanine diacid was used as a modifier to modify MgAl-LDH through direct synthesis. In addition FT-IR, TGA, XRD, TEM, and FE-SEM observations were applied for the further validation of the exfoliation behavior and to conceive the structure of the M-LDH nanolayers and PVA/M-LDH BNCs. The FE-SEM images of neat LDH, M-LDH, and PVA/M-LDH BNC containing 8 wt.% of M-LDH are displayed in Fig. 3.41. It has been illustrated by the FE-SEM image of neat LDH that the nature

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135

Figure 3.41 FE-SEM photographs of LDH (A
C), M-LDH (D
F), and PVA hybrid with 8 wt.% M-LDH (G
I). Source: Adapted from Mallakpour, S., Dinari, M., 2014. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat. 77, 583
589. With kind permission of Elsevier.

of LDH particles roughly consists of a plate-like form, stacked on the top of each other with side-long dimensions ranging from a few micrometers and thickness of over a few hundred nanometers. The morphological features of the M-LDH were similar to that of the unmodified LDH, however it appeared floppier than the unmodified LDH. The FE-SEM images of PVA/MLDH BNC 8% showed that the morphology was changed. This morphological variation could be attributed to the reconstruction of the PVA matrix in the presence of the M-LDH, producing a packed lattice. It shows the uniform dispersion of particles in the polymeric matrix. The NCs of PVA/MLDH are a good candidate foruse in drug-delivery systems. Also, PVA was used as a matrix of a chiral LDH containing N-trimellitylimidoL-isoleucine and MgAl-LDH (Mallakpour and Dinari, 2016). The modification of LDH was conducted via direct synthesis assisted by ultrasonication. Through solvent blending, the NCs were prepared by mixing PVA and chiral LDH in distilled water and ultrasonic irradiation was used to make a well-dispersed LDH suspension. XRD, FT-IR, TGA, FE-SEM, and TEM were effective techniques to study the

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Figure 3.42 TEM micrographs of (A, B) LDH-CO322, (C, D) CLDH, and (E, F) NC4%. Source: Adapted from Mallakpour, S., Dinari, M., 2016. Bionanocomposite materials from layered double hydroxide/N-trimellitylimido-l-isoleucine hybrid and poly (vinyl alcohol) Structural and morphological study. J. Thermoplast. Comp. Mater. 29, 623
637. With kind permission of SAGE Journals.

properties of NCs. TEM photographs display smooth, overlapping crystals, wellformed, and in hexagonal form. The homogeneous contrast, uniform thickness, and reflection of the LDH ultrathin nature have been observed in the TEM images (Fig. 3.42). In the case of modified LDH, a hexagonal shape with rounded corners for the platelets and in the case of NCs of PVA and 4 wt.% of chiral diacid intercalated LDH (CLDH) a coexistence of LDH layers in the intercalated and partially exfoliated states have been observed.

3.4.9 Preparation of polyester/layered double hydroxide nanocomposites The manufacturing of fire-retardant LDH has been reported (Cai et al., 2016). Modification of MgAl-LDH (N-LDH) with spirocyclic pentaerythritol bisphosphorate

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Figure 3.43 Preparation of SPDP-LDH. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

diphosphoryl sodium (SPDP) anion was carried out by in situ synthesis (Fig. 3.43). An unsaturated polyester resin (UP) was applied as a matrix for NC preparation. At first, LDH was dispersed in the UP (with 1, 3, 5, and 10 wt.% of the UP weight) for 10 min at room temperature. Afterward, the mixture was moved to a mold and cured at room temperature for 1 h and in the next step 80 C curing for 3 h was done. The AFM was also used to study the particle size distribution and particle size of LDH and SPDP-LDH. Three-dimensional AFM images of LDH and SPDP-LDH are shown in Fig. 3.44. The lateral dimensions are in the range of 200
300 and 100
200 nm for the LDH and SPDP-LDH, respectively. Sizes of 40 and 30 nm were obtained for the height profiles of the images of LDH and SPDP-LDH, respectively. According to the data, to reduce the aggregation of LDH particles, the flame-retardant modification is very beneficial. According to the authors, the stacked layers of N-LDH are still stacked (Fig. 3.45A) and after modification with SPDP, some partial exfoliation in the polymeric matrix is observed (Fig. 3.45B). The fire retardancy of NCs was studied and the results proved the fire retardancy of NCs, and this property increased along with an increasing amount of LDHs. (LOI value of 21.74 for 10 wt.% of LDH in UP. The PHRR value for UP obtained was 736 kW/m2, while in the case of UP/modified LDH with 10 wt.% this value was 412 kW/m2.) This could be explained by the lower initial decomposition temperature of interlayer space and catalytic degradation action of the metal ion in LDH, causing fire retardancy of NCs.

3.4.10 Preparation of polyvinyl chloride/layered double hydroxide nanocomposites A reconstruction method was used for exfoliation of MgAl-LDH with laurylether phosphate through a urea hydrolysis method (Huang and Wang, 2009). The modified LDH was inserted into a polyvinyl chloride (PVC) matrix through solvent

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.44 AFM images of (A) N-LDH and (B) SPDP-LDH. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

Figure 3.45 TEM micrographs of (A) 5 wt.% N-LDH/UP and (B) 5 wt.% SPDP-LDH/UP nanocomposites. Source: Adapted from Cai, J., Heng, H.-M., Hu, X.-P., Xu, Q.-K., Miao, F., 2016. A facile method for the preparation of novel fire-retardant layered double hydroxide and its application as nanofiller in UP. Polym. Degrad. Stabil. 126, 47
57. With kind permission of Elsevier.

blending in the tetrahydrofuran. The morphology and diameter of particles are disk-like, with sizes of 30
60 nm, respectively, obtained by atomic force microscopy (AFM). The low-magnification TEM image showed the LDHs well dispersed throughout the polymer. Higher magnification displayed that the LDH nanolayers are exfoliated in the PVC matrix, but intercalated tactoids were also present (Fig. 3.46). Also, the presence of modified LDH increased the activation energy of degradation of PVC for 10
26 kJ/mol, according to the Flynn
Wall
Ozawa method, as well as improving the stability of polymer chains against dehydrochlorination.

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Figure 3.46 TEM micrographs of PVC/LDH nanocomposites with 20% loading of LDH at different magnifications (a
c). Source: Adapted from Huang, N., Wang, J., 2009. A new route to prepare nanocomposites based on polyvinyl chloride and MgAl layered double hydroxide intercalated with lauryl ether phosphate. Express Polym. Lett. 3, 595
604. With kind permission of Budapest University of Technology and Economics, Dept. of Polymer Engineering.

The application of LDH in PVC led to stabilization of the chlorine atom on the PVC chain, therefore, this nanocomposite could be useful in the coating, medical fields etc.

3.4.11 Preparation of polypropylene-ethylene vinyl acetate/ layered double hydroxide nanocomposites LDHs can affect the storage modulus of polymers or blends. A blend copolymer consisting of polypropylene (PP) and ethylene vinyl acetate (EVA) has been prepared using organo-modified Mg2Al-LDH by the melt-mixing method (Rafiee et al., 2016). In this research, sodium dodecyl benzene sulfonate (SDBS) was used as a modifier agent of the LDH. Modified LDH was synthesized using a proper precursor in one step. Dynamic mechanical thermal analysis (DMTA) revealed that the presence of LDH into PP matrix (without EVA) increased storage modulus (SM), while the addition of EVA into the PP matrix resulted in a decrease in the SM. These observations can be explained as an effect of the stiffness of LDH layers increasing, and of the free volume of EVA and high entropy of this which annihilate the crystalline region of PP, decreasing of SM. The sample containing 95 wt.% of PP and 5 wt.% of LDH has the most SM at room temperature. As LDH can increase the SM, it also can increase the loss modulus (LM) due to the decrease in crystalline phase, as well as

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the addition of EVA with a rubbery-like nature. Therefore, the LDH and EVA have the same effect on SM, while they oppose each other in the case of LM. Fig. 3.47 shows a TEM image of PP-EVA/MgAl-LDH NCs.

3.4.12 Preparation of silicone rubber/layered double hydroxide nanocomposites The hydrophilic LDH has been changed into an organophilic LDH by modification of MgAl-LDH with dodecyl sulfate (DS) by Pradhan et al. (2011). The coprecipitation and ion exchange pathways were proper methods to prepare LDH and modified LDH, respectively. This organophilic LDH was used to improve the properties of crosslinked silicone rubber (SR), at first, modified LDH was dispersed in CCl4 at 80 C and lasted for 6 h, then, the suspension was added to the solution of SR (in CCl4) and was then crosslinked with V430. Figs. 3.48 and 3.49 show the schematics of this process. A larger smooth area for neat SR has been observed from SEM micrographs of tensile fractured surfaces of SR (Fig. 3.50A). Also, an identical setback was designated for the matrix with no weaker region for crack starting.

Figure 3.47 TEM images of P75E25L5 samples with different magnifications (a
d). Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides. Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

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141

+ 0.48

SO4– 2.77nm

4

SO –

SO –

4

SO4–

SO4–

0.78

SO4–

SO4– SO4– SO4– SO4– SO4– SO –

4

SO4–

SO4– SO4–

Dodecyl sulfate ion

LDH

SO4–

DS-LDH

Figure 3.48 Schematic diagram of pure LDH and DS
LDH. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons. SO4–

SO4–

CH3 H2C 2.77nm

SO4– SO4–

SO4– SO4–

+

HC

DS-LDH

SO4–

Si

CH

CH2

CH3 CH3 Silicone Rubber + H H Si

SO4–

CH3

Si

Si

Si

H Crosslinker SR/DS-LDH nanocomposite

Figure 3.49 Scheme of the intercalation process between DS
LDH and the SR matrix. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons.

Nevertheless, many microvoids (designated by the arrows in Fig. 3.50B) around the dispersed nanoparticles roughly fractured the surface when 5 wt.% DS
LDH was added to SR. This can be described as particle clusters trapping a significant quantity of polymeric matrix and subsequently, during distortion, some of the energy will be absorbed and mechanical properties of the polymer/LDH NCs will be maximized. It has been observed that at the organic
inorganic interface, there are secondary cracks and break stages before unity with the spreading out early of cracks based on the results. As well as many other organic polymer NCs, the superior mechanical properties of the NC would be described by the rough surface of the SR polymer NCs.

3.4.13 Preparation of epoxy resin/MoS2/layered double hydroxide nanocomposites LDHs can help to reduce the fire retardancy of epoxy resin, as epoxy resins are highly flammable and toxic smoke is produced by their combustion. Hence, in one

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.50 SEM images of the tensile fracture surface morphology of (A) neat SR and (B) the SR/DS
LDH (5 wt.%) nanocomposite. Source: Adapted from Pradhan, B., Srivastava, S.K., Ananthakrishnan, R., Saxena, A., 2011. Preparation and characterization of exfoliated layered double hydroxide/silicone rubber nanocomposites. J. Appl. Polym. Sci. 119, 343
351. With kind permission of John Wiley and Sons.

Coprecipitation

Sonicaton

Fe3+ OH–

Ni2+ Co2+

LDH suspension Electrostatic attraction Self-assembly

n-BuLi

Hydrolysis Sonicaton

Solvothermal

MoS2-LDH hybrids

Bulk MoS2 Li+

Exfoliated MoS2 nanosheets

Figure 3.51 Illustration for the preparation of MoS2-LDH nanohybrids by the self-assembly method. Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

study (Zhou et al., 2017), a hybrid of MoS2 and LDHs was used as a fire-retardant agent in the epoxy resin (EP). As cobalt, iron, and nickel are capable of increasing char yield of polymers, NiFe- and CoFe-LDH were chosen for this aim, and prepared through a coprecipitation method. The MoS2 and LDH stack together by electrostatic force, as MoS2 is negatively charged and LDH has a positive charge. Fig. 3.51 shows the exfoliation of MoS2, synthesis of LDHs, and preparation of the hybrids. The nanocomposite was synthesized through a solution-blending method. In this manner, the hybrid of MoS2-LDH was dispersed and sonicated into acetone (as a solvent). Then, the melted EP (at 95 C) was poured into the MoS2-LDH suspension

Fabrication technologies of layered double hydroxide polymer nanocomposites

Pyrolysis products

EP/LDH/MoS2 composites

LDH/MoS2 hybrids MoS2 nanosheets LDH

Small organic molecules

Metal oxides

143

Char

Water molecules Flammable gaseous products

Figure 3.52 Schematic illustration of the flame-retardant mode of action of EP nanocomposites with LDH/MoS2 hybrids. Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

and the mixture was stirred with a mechanical stirrer for 6 h. The NC was then cured at 100 C and 150 C. The proposed mechanism of fire retardancy is as follows: 1. A maze or “tortuous path” was created by MoS2 to prevent the permeation of O2 and heat. 2. The degradation of LDHs generates a resistant oxide which protects the polymer from combustion and absorbs produced gases. 3. The release of volatile products is delayed by the catalytic carbonization effect of the LDH/MoS2 and it advances stable carbonaceous char as well as stopping the propagation of flammable gas, and oxygen and heat into the flame zone and polymer, respectively.

Also, water molecules from degradation of LDH play a role as cooling agents. In addition, the released heat is consumed for the degradation of LDH. These factors reduce the fire hazard risks of EP-LDH/MoS2. This mechanism is shown in Fig. 3.52. TEM images of different LDHs are presented in Fig. 3.53

3.4.14 Preparation of polyurethane/nitrile butadiene rubber Blend/layered double hydroxide nanocomposites In one study, MgAl-LDH was synthesized through a coprecipitation method and modification was done with SDS with a reconstruction (rehydration) method (Kotal

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.53 TEM images of exfoliated MoS2 (A), CoFe-LDH (B), CoFe-LDH/MoS2 (C), NiFe-LDH (D), and NiFe-LDH/MoS2 hybrids (E, F). Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS 2) nanosheets and layered double hydroxide (LDH): towards reducing fire hazards of epoxy. J. Hazardous Mater. 338, 343
355. With kind permission of Elsevier.

et al., 2010). These LDHs were used as filler in a blend of polyurethane (PU) and nitrile butadiene rubber (NBR) with amounts of 1, 3, 5, and 8 wt.% through the solution-blending method in tetrahydrofuran (THF). The LDH was added to the predissolved solution of PU/NBR (1:1) in THF. These NCs were crosslinked by sulfur. As the TEM images (Fig. 3.54) show, the LDH was well dispersed in NC 1 wt.% (Fig. 3.54A), which indicated the exfoliation of LDH and with increasing the amount of LDH (3 wt.%, Fig. 3.54B) both intercalation and exfoliation are seen. When increasing the amount of LDH continues to 8 wt.%, aggregation occurs (Fig. 3.54C). These NCs can be used in many applications such as tubing pipes, gaskets, protective covers, co-extrusion automotive gaskets, ballpen grips, automotive grips, etc.

3.4.15 Preparation of polyethyleneimine/poly(sodium 4-styrene sulfonate) hybrid/layered double hydroxide nanocomposites Polyethyleneimine (PEI) and poly(sodium styrene 4-sulfonate) (PSS) were used as a supporting polymer for LbL deposition of MgAl-LDH (Li et al., 2005). A coprecipitation method was utilized to prepare the LDH and then, formamide was used for exfoliation of the synthesized LDH. The procedure for preparation of the PSS/LDH NCs is as described here. First, a Si wafer and a quartz glass slide were cleaned with the aid of methanol/ HCl and subsequently H2SO4. This substrate was soaked first with PEI and then with PSS, followed by washing with water. This substrate was immersed in the

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145

Figure 3.54 TEM images of (A) PU/NBR/DS
LDH (1 wt.%) nanocomposite, (B) PU/NBR/ DS
LDH (3 wt.%) nanocomposite, and (C) PU/NBR/DS
LDH (8 wt.%). Source: Adapted from Kotal, M., Srivastava, S.K., Bhowmick, A.K., 2010. Thermoplastic polyurethane and nitrile butadiene rubber blends with layered double hydroxide nanocomposites by solution blending. Polym. Intern. 59, 2
10. With kind permission of John Wiley and Sons.

LDH suspension and then washed with water. This cycle was done several times to prepare a (PSS/LDH)n film. Fig. 3.55 shows an SEM image of LDH after ion exchange treatment and a TEM image of an LDH nanosheet. Uniform hexagonal crystals with a large lateral dimension can be found in the SEM image. Also, the TEM image clearly shows the ultrathin crystallite LDHs and implies a single LDH sheet. An AFM image of the PSS/LDH (Fig. 3.56) shows that the surface is tiled with LDHs with a lateral size of hundreds of nm to several microns. Although some overlaps can be seen, monolayer regions were predominant.

3.4.16 Preparation of isotactic polypropylene/layered double hydroxide nanocomposites The effect of LDH crystal size on the properties of isotactic polypropylene (iPP) has been investigated (Nagendra et al., 2015). In this study, two different

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.55 SEM image of LDH crystals after ion exchange treatment (left). Some etched pits in the crystals may have formed during the ion exchange process using the acidic solution. TEM image of the LDH nanosheet (right). Source: Adapted from Li, L., Ma, R., Ebina, Y., Iyi, N., Sasaki, T., 2005. Positively charged nanosheets derived via total delamination of layered double hydroxides. Chem. Mater. 17, 4386
4391. With kind permission of the American Chemical Society.

Figure 3.56 AFM image of the first LDH nanosheet layer on an Si wafer precoated with PEI and PSS; nanosheet concentration, 0.5 g/dm. Source: Adapted from Li, L., Ma, R., Ebina, Y., Iyi, N., Sasaki, T., 2005. Positively charged nanosheets derived via total delamination of layered double hydroxides. Chem. Mater. 17, 4386
4391. With kind permission of the American Chemical Society.

suspensions of MgAl-LDH were prepared using gel preparation of LDH and sonication of the LDH. The MgAl-LDH was synthesized through the coprecipitation method. LDH gel was obtained by dispersion and washing of LDH in acetone followed by dispersion in xylene. On the other hand, MgAl-LDH was dispersed in xylene using an ultrasonic bath. The NCs were prepared via solvent blending in xylene and 2.5% and 5% for LDH gel and 1%, 2.5%, 5%, and 10% for sonicated LDH. Fig. 3.57 shows a schematic illustration of iPP/LDH NCs. XRD patterns of the synthesized LDH, LDH gel, and sonicated LDH show that sonication did not affect the crystallinity of the prepared LDH, while the LDH gel has been changed (Fig. 3.58).

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147

Figure 3.57 Schematic illustration of the polymer nanocomposite preparation based on isotactic polypropylene and MgAl-LDH layered double hydroxide. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

Figure 3.58 Powder X-ray diffraction patterns of (a) as-prepared MgAl-LDH, (b) sonicated LDH, and (c) MgAl-LDH gel. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

The particle sizes for LDH gel and sonicated LDH were reported as 3
4 microns and 50
20 nm, respectively. A TEM image of LDH gel shows delaminated LDH platelets into single-layer nanosheets, in contrast, SEM (Fig. 3.59) and TEM (Fig. 3.60) images of sonicated LDH show broken LDH layers. The pale

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Layered Double Hydroxide Polymer Nanocomposites

Figure 3.59 Scanning electron microscope images of (A, B) MgAl-LDH dispersed in water and (C) sonicated LDH. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

contrast of both gel and sonicated LDH shows their single-layer nature in comparison with the bold contrast of the prepared LDH. As can be inferred from the XRD patterns of the iPP/LDH NCs, no changes to the crystallinity of the obtained NCs were made. Therefore, the crystal size of LDHs had no effect on the iPP, but smaller crystal size of LDH (sonicated LDH) caused better nucleation ability of the iPP due to the higher surface area of the sonicated LDH compared with LDH gel. Furthermore, Table 3.2 provides a summary of the preparation of important polymer/LDH NCs and their preparation methods. Also, several valuable books and reviews based on polymer/LDH NCs have been published and the properties of these NCs have been studied in detail (Basu et al., 2014; Jlassi et al., 2017; Mallakpour and Hatami, 2017a).

3.5

Conclusions and future perspectives

LDHs are a new promising material, with exceptional features which can improve the weaknesses of neat polymers and will enhance properties of polymers such as mechanical properties, thermal stability, reduced gas permeability, flame retardancy, electrical properties, water vapor permeability, viscoelasticity, the activation energy of degradation, oxygen barrier, etc. These materials can be modified with an appropriate modifier to raise the basal distance, which results in ease of NC preparation and better dispersion and subsequently shows the best performance. Direct synthesis, anionic exchange, reconstitution (rehydration), and mechanochemistry are the approaches to modify LDHs. Even antimicrobial modifiers can make LDHs antimicrobial, thus it can be used as a drug carrier. LDHs are capable of being modified with a wide range of inorganic or organic anions, and even CNTs can be used for this. Based on the literature, of the three general methods, the most commonly used method for the preparation of polymer/LDH NCs is preexfoliation of the

Fabrication technologies of layered double hydroxide polymer nanocomposites

149

Figure 3.60 TEM images of (A, B) as-prepared MgAl-LDH and corresponding EDS spectrum, (C, D) delaminated MgAl-LDH and corresponding EDS spectrum (inset) selected area electron diffraction (SAED) pattern of delaminated MgAl-LDH, (E, F) sonicated LDH and corresponding EDS spectrum. Source: Adapted from Nagendra, B., Mohan, K., Gowd, E.B., 2015. Polypropylene/layered double hydroxide (LDH) nanocomposites: influence of LDH particle size on the crystallization behavior of polypropylene. ACS Appl. Mater. Interf. 7, 12399
12410. With kind permission of the American Chemical Society.

LDH. This is due to the good dispersion of LDHs into polymeric matrices, consequently giving better results in the obtained NCs. These NCs can be easily characterized using different techniques including FT-IR, FE-SEM, TEM, XRD, TGA, DSC, WAX, etc. As has been reviewed, polymer/LDH NCs are widely used, thus, it is expected that novel methods for the preparation of these NCs will be discovered in the future. LDHs can be combined with other nanoscale materials, such as

Table 3.2 A summary of preparation methods of polymer/LDH NCs and LDHs used Polymer/LDH NCs

LDH

LDH preparation method

CMC/LDH NCs

MgAl-LDH

Coprecipitation

Pectin/LDH NCs

LDH-NO3

Urea method

Chitosan/LDH NCs Natural rubber/LDH NCs CMC, polygalacturonic acid, sodium alginate, curdlan, oxidized cellulose, and carrageenan/LDH NCs PI/LDH NCs

Mg2Al-LDH Zn/Mg-Al LDH

Coprecipitation Coprecipitation

Mg2Al-LDH

Coprecipitation

ZnCr-LDH

Coprecipitation

PMMA/LDH NCs

LiAl- and MgAl-LDHs MgAl-LDH

Coprecipitation

MgAl-LDH

Coprecipitation

Octane, LGA, and butanol


Mg3Al-LDH

Coprecipitation

5-Sulfosalicylic acid

PVAc/LDH NCs P(MMA-co-BA)/ LDH NCs WF-PP/LDH NCs




Modifier or surfactant
Benzoate, 2,4dichlorobenzoate, para-hydroxybenzoate, and orthohydroxybenzoate 5-Aminosalicylic acid Stearic acid Fructose-6-phosphate aldolase


K2 and SDS

Modification method


Preparation method

References

Solvent blending

Ion exchange

Solvent blending

Yadollahi et al. (2014) Gorrasi et al. (2012)

Direct synthesis Direct synthesis

LbL assembly Two-roll mill mixing

Ribeiro et al. (2014) Das et al. (2011)

Direct synthesis

Solvent blending

Mahdi et al. (2015)

In situ polymerization In situ polymerization

Dinari and Rajabi (2016) Tsai et al. (2016)




In situ polymerization

Chen et al. (2015)




In situ polymerization

Veschambres et al. (2016) Peng et al. (2017)


Ion exchange

Ion exchange

Melt blending

PAI/LDH NCs

Mg2Al-LDH

Coprecipitation

Direct synthesis

Solvent blending

Coprecipitation

N-tetrabromophthaloylglutamic L -aspartic-based diacid

PAI/LDH NCs

MgAl-LDH

PAI/LDH NCs

Direct synthesis

Solvent blending

MgAl-LDH

Coprecipitation

CNTCOOH

Direct synthesis

In situ polymerization

LDPE/LDH NCs

ZnAl-LDH

Coprecipitation

Direct synthesis

Ball milling

PVA/LDH NCs

Mg2Al-LDH

Coprecipitation

Direct synthesis

Solvent blending

PVA/LDH NCs

MgAl-LDH

Coprecipitation

Direct synthesis

Solvent blending

Polyester/LDH NCs PVC/LDH NCs

MgAl-LDH MgAl-LDH

Coprecipitation Urea method

Aleurate, citrate, glycolate, parahydroxybenzoate, salicylate, and serine carboxylate N,N0 -(pyromellitoyl)-bisL-phenylalanine diacid N-trimellitylimido-Lisoleucine SPDP Laurylether phosphate

Ion exchange Ion exchange

Mechanical agitation Solvent blending

PP-EVA/LDH NCs SR/LDH NCs EP/MoS2/LDH NCs

Mg2Al-LDH MgAl-LDH NiFe- and CoFe-LDH MgAl-LDH

Coprecipitation Coprecipitation Coprecipitation

SDBS DS MoS2

Direct synthesis Ion exchange Direct synthesis

Melt mixing Solvent blending Solvent blending

Mallakpour and Dinari (2014) Mallakpour and Dinari (2016) Cai et al. (2016) Huang and Wang (2009) Rafiee et al. (2016) Pradhan et al. (2011) Zhou et al. (2017)

Coprecipitation

SDS

Reconstruction

Solvent blending

Kotal et al. (2010)

MgAl-LDH

Coprecipitation

formaldehyde

Direct synthesis

LbL assembly

Li et al. (2005)

MgAl-LDH

Coprecipitation

xylene

Direct synthesis

Solvent blending

Nagendra et al. (2015)

PU/NBR Blend/LDH NCs PEI/PSS hybrid/LDH NCs iPP/LDH NCs

Mallakpour et al. (2016) Mallakpour et al. (2014) Mallakpour and Dinari (2015) Gorrasi and Bugatti (2016)

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Layered Double Hydroxide Polymer Nanocomposites

CNTs, metal oxides, graphene, etc. to create new properties or to improve and optimize some of their specific features. Currently, it is essential to mention that, in order to save our planet from pollutants, almost all scientists including physicians, pharmacists, materials scientists, chemists, physicists, and engineers in various fields, in academia as well as in industries, must use environmentally friendly methods and materials to develop technologies which will be compatible with the long-term future of the Earth (air, soil, and water). In this regard, using LDHs with biosafe materials such as amino acids, carbohydrates, vitamins, proteins, DNA, and other vital molecules from plants which are plentiful and have an endless natural supply, and which are biocompatible and biodegradable polymers, would be excellent choices to manufacture environmentally benign polymer/LDH NCs and utilize them for a wide variety of future technologies.

Acknowledgments The authors acknowledge the Research Affairs Division, Isfahan University of Technology (IUT), Isfahan, Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT) is gratefully acknowledged.

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42121. Mallakpour, S., Dinari, M., Talebi, M., 2016. Novel nanocomposites obtained by dispersion of LDH modified with N-tetrabromophthaloyl-glutamic in poly (amide-imide) having N-trimellitylimido-l-leucine and 4, 40 -diaminodiphenylether units. Polym. Comp. 37, 1323
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Microscopic characterization techniques for layered double hydroxide polymer nanocomposites

4

Shadpour Mallakpour1,2 and Shima Rashidimoghadam1 1 Department of Chemistry, Organic Polymer Chemistry Research Laboratory, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Material Institute, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

4.1

Introduction

Layered double hydroxides (LDHs) are one type of anionic clay material, also recognized as hydrotalcite like compounds. The chemical composition of LDHs can be explained by the general formula [M1
x21 Mx31 (OH)2]x1(Am2)x/m  nH2O, while M21 is a divalent metal cation (e.g., Mg21, Fe21, Co21, Cu21, Ni21, or Zn21), M31 is a trivalent metal cation (e.g., Al31, Cr31, Ga31, In31, Mn31, or Fe31) and 2 Am2 is an anion with charge m, for example, Cl2, CO22 3 , NO3 , etc., which is placed in the hydrated interlayer galleries and can be relatively and easily replaced. The value of x is equal to the molar ratio M31/(M21 1 M31) and is normally between 0.2 and 0.33 (Fan et al., 2014; Wang and O’Hare, 2012; Elbasuney, 2015). LDHs can be prepared in the laboratory by a variety of synthetic methods, for example, coprecipitation, urea hydrolysis, hydrothermal crystallization, and ion exchange methods (Costa et al., 2008). The functionalization of LDH is an unavoidable process in the fabrication of polymer nanocomposites (PNCs) and is carried out by exchanging the interlayer anions with anionic surfactants such as fatty acid salts (Focke et al., 2010), sulfonates (Wang et al., 2005), and phosphates (Zhang et al., 2015) and so on. Nowadays, PNCs have attracted attention due to the considerable enhancements in material properties, for example, mechanical strength, gas, and solvent barrier, toughness, flame retardancy, etc., in comparison with traditional polymer composites. As a result of the small size of the filler particles, their uniform distribution in the polymer matrix and consequently good interaction between the polymer and nanofiller was achieved. These nano-scaled fillers have a specific surface area which leads to larger matrix/filler interface and so more mutual interactions. Thus a significant improvement in composite properties at very low filler volume fractions was achieved (Mittal, 2010; Mallakpour and Rashidimoghadam, 2017a). Various Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00004-5 © 2020 Elsevier Ltd. All rights reserved.

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nano-fillers, such as LDHs (Becker et al., 2011; Nogueira et al., 2011; Mallakpour and Dinari, 2014a, 2016a), carbon nanotubes (CNTs) (Mallakpour and Rashidimoghadam, 2017b, 2018), metal oxides (Mallakpour and Adnany Sadaty, 2016; Mallakpour and Motirasoul, 2016), clay (Mallakpour and Khani, 2015; Mallakpour and Shahangi, 2013), graphene (Behzadi and Mirzaei, 2016; Huang et al., 2016), carbon black (Zhang et al., 2018; Liu et al., 2018), and graphite (Sa´nchez-Sa´nchez et al., 2018; Yuan et al., 2018) were employed for the preparation of PNCs. Among them, LDHs have been arousing tremendous consideration and research interest in the scientific community owing to their wide range of available chemical compositions, ability of being modified by different types of organic anionic surfactants, and endothermic decomposition liberating water vapor and metal oxide residue (Mallakpour and Hatami, 2017; Mallakpour and Khadem, 2017; Reny Costa et al., 2007; Cao et al., 2016). Thus the preparation of polymer/ LDH NCs has attracted considerable technological and scientific interest (Nogueira et al., 2016) and several techniques for the synthesis of polymer/LDH NCs, such as in situ polymerization, meltmixing, and solution blending and various microscopic characterizations for giving evidence to intercalated or exfoliated morphologies of LDH in different PNCs have been reported.

4.2

Microscopic characterization techniques for PNCs

Characterization of the NCs is essential to obtain valuable information about these materials such as quality of distribution of filler in the polymer matrix, influence of filler surface modification on filler dispersion and composite properties, interactions of the filler modification with the polymer chains, changes in the process parameters on the resulting morphology and properties, and study a wide spectrum of properties to determine the application potential of the NCs (Mittal, 2012). Over the years, various techniques have been developed to explore the structure of PNCs (Bhattacharya et al., 2008), including scattering techniques (Cipelletti et al., 2016), microscopic techniques (Leng, 2010; Adhikari and Michler, 2009), spectroscopic techniques (Gurses, 2016; Ponnamma et al., 2016), chromatography (Bhattacharya et al., 2008), melt state rheometry, solid-state analysis, calorimetry, and others. Microscopic characterization techniques investigate and map the surface and subsurface structure of a material using photons, electrons, ions, or physical cantilever probes. Microscopy is a technique that, combined with other scientific techniques and chemical processes, allows the determination of both the composition and the structure of a material. Different microscopic techniques, such as optical microscopes, scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), field ion microscopes (FIMs), scanning tunneling microscopes (STMs), scanning probe microscopes (SPMs), atomic force microscopes (AFMs), and X-ray diffraction topography (XRT) play a vital role in the characterization of PNC morphology on different length scales (Leng, 2010; Adhikari and Michler, 2009).

Microscopic characterization techniques for layered double hydroxide polymer nanocomposites

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4.2.1 Optical microscope The optical microscope, often referred to as a light microscope, is a type of microscope which uses visible light and a system of lenses to magnify images of small samples or to magnify the fine details of a larger object based on the principles of transmission, absorption, diffraction, and refraction of light waves, in order to examine minute specimens that cannot be seen by the naked eye. The optical microscope is the oldest design of microscope in which all of its parts work together. In the microscope, objects are enlarged or magnified with a convex lens that bends light rays by refraction. Diverging rays from points within the object (object points) are made to converge behind the convex lens and cross over each other to form image points (i.e., a focused image). The distance of the object from the lens divided by the distance of the focused image from the lens determines the magnification. The visibility of the magnified object depends on contrast and resolution. In general, the contrast or differences in light intensity between an object and its background or surroundings render the object distinct. An optical microscope has some advantages: cheap to purchase and operate, small and portable, the natural color of the specimen can be observed, living as well as dead material can be viewed, preparation is relatively quick and simple, requiring little expertise, and it is unaffected by magnetic fields. However, there are some drawbacks that provide limitations to its use in practice: magnifies objects up to 1500 3 , preparation may distort the specimen, the depth of the field is restricted, and it has a resolving power for biological specimens of around 1 nm. The advantages outweigh the disadvantages, however, in some circumstances an electron microscope maybe a better choice (Keller and Goldman, 1989).

4.2.2 Scanning electron microscope A SEM is a type of electron microscope that uses a focused beam of high-energy electrons (which are produced by an electron gun) to produce a variety of signals at the surface of solid specimens. The signals that derive from electron
sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. In most applications, data are collected over a selected area of the surface of the sample, and a two-dimensional image is produced that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 μm in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20 3 to approximately 30,000 3 , spatial resolution of 50
100 nm). The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semiquantitatively determining chemical compositions, crystalline structure, and crystal orientations. The design and function of the SEM is very similar to the electron probe microanalyzer and considerable overlap in capabilities exists between the two instruments. Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by

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Layered Double Hydroxide Polymer Nanocomposites

electron
sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons, diffracted backscattered electrons (that are used to determine crystal structures and orientations of minerals), photons (characteristic Xrays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence
CL), and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e., for rapid-phase discrimination). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbitals (shells) of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength (i.e., related to the difference in energy levels of electrons in different shells for a given element). Thus, characteristic X-rays are produced for each element in a mineral that is “excited” by the electron beam. SEM analysis is considered to be “nondestructive”; that is, X-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly. The advantages of SEM include its wide array of applications, the detailed three-dimensional and topographical imaging and the versatile information garnered from different detectors. SEM is also easy to operate with the proper training and advances in computer technology and associated software make operation user-friendly. The technological advances in modern SEMs allow for the generation of data in digital form. Although all samples must be prepared before being placed in the vacuum chamber, most SEM samples require minimal preparation. The disadvantages of SEM start with the size and cost. SEMs are expensive, large, and must be housed in an area free of any possible electric, magnetic, or vibration interference. Maintenance involves keeping a steady voltage, currents to electromagnetic coils and circulation of cool water. Special training is required to operate SEM as well as prepare samples. The preparation of samples can result in artifacts. The negative impact can be minimized with knowledgeable experienced researchers being able to identify artifacts from actual data as well as preparation skill. There is no absolute way to eliminate or identify all potential artifacts. In addition, SEMs are limited to solid, inorganic samples small enough to fit inside the vacuum chamber that can handle moderate vacuum pressure. Finally, SEMs carry a small risk of radiation exposure associated with the electrons that scatter from beneath the sample surface. The sample chamber is designed to prevent any electrical and magnetic interference, which should eliminate the chance of radiation escaping from the chamber. Even though the risk is minimal, SEM operators and researchers are advised to observe safety precautions (de Assumpc¸a˜o and Ferri, 2017; Stokes, 2008).

4.2.3 Transmission electron microscope Transmission electron microscopy is a method in which a beam of electrons is transmitted through a very thin specimen and the interactions between the

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electrons and the atoms can be used to observe features such as structure and morphology. This technology can give information about the structure, crystallization, morphology, and stress of a substance, whereas SEM can only provide information about the morphology of a specimen. However, TEM requires very thin specimens that are semitransparent to electrons, which can mean sample preparation takes longer. The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope. In TEM, the source of illumination is a beam of electrons of very short wavelength, emitted from a tungsten filament at the top of a cylindrical column of about 2 m height. The whole optical system of the microscope is enclosed in vacuum. Air must be evacuated from the column to create a vacuum so that the collision of electrons with air molecules and hence the scattering of electrons are avoided. Along the column, at specific intervals magnetic coils are placed. Just as the light is focused by the glass lenses in a light microscope, these magnetic coils in the electron microscope focus the electron beam. The magnetic coils placed at specific intervals in the column acts as an electromagnetic condenser lens system. The specimen stained with an electron-dense material and is placed in the vacuum. The electron beam passes through the specimen and scattered by the internal structures. The heated filament emits electrons which are then accelerated by a voltage in the anode. A higher anode voltage will give the electrons a higher speed. Thus the electrons will have a smaller de Broglie wavelength according to the equation, λ 5 h/mv. The resolving power of a microscope is directly related to the wavelength of the irradiation, which is used to form an image. The faster the electrons travel, the shorter their wavelength. As the wavelength is reduced, the resolution is increased. Therefore, the resolution of the microscope is increased if the accelerating voltage of the electron beam is increased. This technique involves a highvoltage beam of electrons emitted by a cathode and formed by magnetic lenses. The beam of electrons that has been partially transmitted through the very thin specimen carries information about the structure of the specimen. The spatial variation in this information (the “image”) is then magnified by a series of magnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or light-sensitive sensor like a CCD (charge-coupled device) camera. The image detected by the CCD may be displayed in real time on a monitor or computer. The advantage of this technique is that it magnifies specimens to a much higher degree than an optical microscope. Magnification of 10,000 times or more is possible, which allows scientists to see extremely small structures. For biologists, the interior workings of cells, such as mitochondria and organelles, are clearly visible. However, this technique has some limitations. TEM requires that specimens be put inside a vacuum chamber. Because of this requirement, the microscope cannot be used to observe living specimens, such as protozoa. Some delicate samples may also be damaged by the electron beam and must first be stained or coated with a chemical to protect them. This treatment sometimes destroys the specimen, however (Tang and Yang, 2017; Brydson, 2011).

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4.2.4 Field ion microscope The FIM is the first imaging tool to directly observe individual atoms. FIM relies on the electric-field-induced ionization of inert gas atoms in the vicinity of a charged surface. When a very sharp metallic needle is subjected to a high voltage of a few kilovolts, an intense electric field is generated at the surface. This electric field is generated by the positive charges present at the surface. Indeed, the application of the high voltage induces the free electrons to be, on average, displaced inwards by a small amount to screen the electric field, leaving partly charged atoms at the very surface. For a nonflat surface, protruding atoms are subjected to a greater charge. Since the electric field at the surface is directly proportional to the charge density, it is higher around these local protrusions. In the case of an atomically smooth curved surface, these protrusions correspond to the edges of atomic terraces. By imaging the distribution of the field intensity at the surface, the FIM provides an atomically resolved image of the surface itself. This technique has been applied mainly to the study of metals and semiconductors, but a few biological images have been obtained (Lefebvre-Ulrikson et al., 2016; Gault, 2012).

4.2.5 Scanning probe microscope The SPM is a family of tools used to make images of nanoscale surfaces and structures, including atoms. The SPM has a probe tip mounted on the end of a cantilever. The tip can be as sharp as a single atom. It can be moved precisely and accurately back and forth across the surface, even atom by atom. When the tip is near the sample surface, the cantilever is deflected by a force. SPMs can measure deflections caused by many kinds of forces, including mechanical contact, electrostatic forces, magnetic forces, chemical bonding, van der Waals forces, and capillary forces. The distance of the deflection is measured by a laser that is reflected off the top of the cantilever and into an array of photodiodes (similar to the devices used in digital cameras). SPMs can detect differences in height that are a fraction of a nanometer, about the diameter of a single atom. The tip is moved across the sample many times. This is why these are called “scanning” microscopes. A computer combines the data to create an image. SPM work differently than optical microscopes because the operator does not have a direct view of the surface but an image that represents the structure of the surface. This method is widely utilized for exploration of the nanoscale structure of materials, as well as their electronic and mechanical properties with its related spectroscopic modes of operation. SPM has many advantages. It provides researchers with a larger variety of specimen observation environments using the same microscope and specimen, reducing the time required to prepare and study specimens. Specialized probes, improvements and modifications to scanning probe instruments continue to provide faster, more efficient, and revealing specimen images with minor effort and modification. Unfortunately, one of the downsides of SPM is that images are produced in black and white or grayscale which can in some circumstances exaggerate a specimen’s actual shape or size. Computers are used to compensate for these exaggerations and

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produce real-time color images that provide researchers with real-time information, including interactions within cellular structures, harmonic responses, and magnetic energy (Paul and Gru¨tter, 2015).

4.2.6 Scanning tunneling microscope The STM is an electron microscope with a resolution sufficient to resolve single atoms. The sharp tip in the STM is similar to that in the SEM, but the differences in the two instruments are profound. In the SEM, electrons are extracted from the tip with a series of positively charged plates placed a few centimeters downstream from the tip. The electrons at the apex of the tip are confined to the region within the metal by a potential barrier. The attractive force from the positive charge on the plates is sufficient to permit the electrons to overcome the barrier and enter the vacuum as free particles. The apertures in the downstream plates form an electron lens that converts the diverging beam from the tip into a beam converging to a focus on the surface of the sample. In the STM, the plates that form the lens in the SEM are removed, and the tip is positioned close to the sample. The electrons move through the barrier in a way that is similar to the motion of electrons in a metal. In metals, electrons appear to be freely moving particles, but this is illusory. In reality, the electrons move from atom to atom by tunneling through the potential barrier between two atomic sites. In a typical case, with the atoms spaced five angstroms apart, there is a finite probability that the electron will penetrate the barrier and move to the adjacent atom. The electrons are in motion around the nucleus, and they approach the barrier with a frequency of 1017 per second. For each approach to the barrier, the probability of tunneling is 1024, and the electrons cross the barrier at the rate of 1013 per second. This high rate of transfer means that the motion is essentially continuous and tunneling can be ignored in metals. Tunneling cannot be ignored in the STM; indeed, it is all-important. When the tip is moved close to the sample, the spacing between the tip and the surface is reduced to a value comparable to the spacing between neighboring atoms in the lattice. In this circumstance, the tunneling electron can move either to the adjacent atoms in the lattice or to the atom on the tip of the probe. The tunneling current to the tip measures the density of electrons at the surface of the sample, and this information is displayed in the image. STMs are helpful because they can give researchers a three-dimensional profile of the surface, which allows researchers to analyze a multitude of characteristics, including roughness, surface defects, and determining things about the molecule size and conformation. It is capable of capturing much more detail than other microscopes. This helps researchers better understand the subject of their research on a molecular level. STMs are also versatile. They can be used for ultra-high vacuum, air, water and other liquids, and gases. They will activate in temperatures as low as zero Kelvin up to a few hundred degrees Celsius. STM works faster than AFM. AFM maximum sample size is 150 3 150 μm. On the other hand, STM generates mm size length and width. Lastly, resolution of STM is much better than AFM. There are very few disadvantages to using an STM. STMs can however be difficult to use effectively. There is a very specific technique that requires a lot of

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skill and precision. STMs require very stable and smooth surfaces, excellent vibration control, and sharp tips. STMs use highly specialized equipment that is fragile and expensive. Although, STM analyzes only conductive materials, AFM is used for conductive and insulator materials. STM requires a vacuum atmosphere but AFM can work even in liquid. For that reason AFM can be used for biological materials (Kalinin and Gruverman, 2007; Chen, 2007).

4.2.7 Atomic force microscope An AFM is a very-high-resolution kind of SPM which provides images of atoms on or in surfaces. An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive force between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, such that the tip makes contact with it, an increasingly repulsive force takes over and causes the cantilever to deflect away from the surface. A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection (and the subsequent change in direction of the reflected beam) is recorded by the PSPD. An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface (thus maintaining constant laser position) the AFM can generate an accurate topographic map of the surface features. AFM has many advantages. It is a powerful tool that is invaluable if you want to measure incredibly small samples with a great degree of accuracy. Unlike rival technologies it does not require either a vacuum or the sample to undergo treatment that might damage it. At the limits of operation however, researchers have demonstrated atomic resolution in high vacuum and even liquid environments. One of the major drawbacks of AFM is the single-scan image size, which is of the order of 150 3 150 μm, compared with millimeters for an SEM. Another disadvantage is the relatively slow scan time, which can lead to thermal drift on the sample. As the technology matures, researchers are relying on there being progress instrumentally, requiring improved signal-to-noise ratio, decreased thermal drift, and better detection and control of tip-sample forces, including the use of sharp probes. Novel solutions are steadily improving these performance issues. To further your understanding, you are welcome to follow this straight forward visual tutorial available online (Gautier et al., 2015; Johnson et al., 2009, https://www.researchgate.net/publication/259889212_Basic_Principles_of_Atomic_Force_Microscopy https://www.researchgate.net/publication/271709095_Atomic_force_microscopybased_force_measurements_on_animal_cells_and_tissueshttps://www.nanoscience.com/ techniques/atomic-force-microscopyhttps://www.weizmann.ac.il/Chemical_Research_

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Support/surflab/peter/afmworkshttps://warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpagswarwick/ex5/techniques/structural/afm).

4.2.8 X-Ray diffraction topography XRT is an imaging technique based on Bragg diffraction. The basic working principle of XRT is as follows: an incident, spatially extended beam (mostly of X-rays or neutrons) impinges on a sample. The beam may be either monochromatic, that is, consist a single wavelength of X-rays or neutrons, or polychromatic, that is, be composed of a mixture of wavelengths (“white beam” topography). Furthermore, the incident beam may be either parallel, consisting only of “rays” propagating all along nearly the same direction, or divergent/convergent, containing several greatly different directions of propagation. When the beam hits the crystalline sample, Bragg diffraction occurs, that is, the incident wave is reflected by the atoms on certain lattice planes of the sample, on condition that it hits those planes at the right Bragg angle. Diffraction from the sample can take place either in reflection geometry (Bragg case), with the beam entering and leaving through the same surface, or in transmission geometry (Laue case). Diffraction gives rise to a diffracted beam, which will leave the sample and propagate along a direction differing from the incident direction by the scattering angle. The cross-section of the diffracted beam may or may not be identical to that of the incident beam. In the case of strongly asymmetric reflections, the beam size (in the diffraction plane) is considerably expanded or compressed, with expansion occurring if the incidence angle is much smaller than the exit angle, and vice versa. Independently of this beam expansion, the relationship of sample size to image size is given by the exit angle alone: the apparent lateral size of sample features parallel to the exit surface is downscaled in the image by the projection effect of the exit angle. A homogeneous sample (with a regular crystal lattice) would yield a homogeneous intensity distribution in the topograph (a “flat” image). This method is capable of providing information on the nature and distribution of structural defects such as dislocations, inclusions/precipitates, stacking faults, growth sector boundaries, twins, and low-angle grain boundaries in single-crystal materials (Raghothamachar et al., 2010).

4.3

Microscopic characterization of polymer/LDH NCs

LDHs have been extensively employed as nanofillers in PNCs as a result of their highly tunable properties. Thus several researchers have focused their attention on the preparation and characterization of polymer/LDH NCs (Kutlu et al., 2014; Donato et al., 2012; Wang et al., 2011; Hu et al., 2012; Lv et al., 2009; Peng et al., 2009; Kuila et al., 2008; Hajibeygi et al., 2015; Gorrasi et al., 2012; Wu et al., 2011; Chung and Lai, 2010; Matusinovic et al., 2013). This chapter has been written to present an updated overview of the recent advances in microscopic characterization techniques of various polymer/LDH NCs reported in the years 2013
17. The emphasis is placed on the recent advances in SEM, TEM, and AFM characterization techniques of these NCs.

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4.3.1 Microscopic characterization of elastomer/LDH NCs Laskowska et al. studied the effects of LDHs with different structures, specific surface areas, and Mg/Al ratios on the curing behavior, crosslink density, mechanodynamical properties, transparency, thermo-optical properties, barrier properties, and morphology of carboxylated acrylonitrile
butadiene rubber (XNBR) composites. As a result of the polar surface of LDHs, good compatibility with XNBR, which have polar nitrile and carboxyl groups, is expected. SEM images of XNBR containing 10 phr (parts per hundred rubber) of the Mg-ion rich LDH [which is characterized by the lowest specific surface area (13 m2/g) but has the highest platelet aspect ratio, with layer dimensions of approximately .500 nm in width and 10
50 nm in thickness] at various magnifications showed that the composite exhibited good dispersion of intercalated/exfoliated layers with a thickness of approximately 30 nm (Fig. 4.1). The mechanical, parallel orientation of anisotropic filler layers in the rubber matrix under the conditions of elevated temperature and high shearing force (internal mixer) is advantageous and produces elastomer composites with improved mechanical and gas barrier properties (Laskowska et al., 2014). Two organic and inorganic phosphorus-containing anions (HPO22 and 4 HDEHP2) were successfully intercalated into the Mg/Al LDH following the rehydration process, in basic media by Go´mez-Ferna´ndez et al. Afterward, unmodified LDH and different amounts (1, 3, and 5 pphp) of modified LDH were added into a flexible polyurethane foam (PUF) matrix having a castor-oil-based polyether polyol. SEM analysis showed that the hexagonal shape was not lost in modified LDH, but the surface appeared to be more irregular, especially in the case of LDH-HPO4, which also featured an aggregation of particles due to the high surface energy of the modified clay. SEM micrographs of LDH-HDEHP showed particles with softer edges than LDH-HPO4, which presented flake-like morphology probably due to the

Figure 4.1 SEM images of XNBR containing 10 phr of LDH70 (LDH70 represents MgO/ Al2O3 5 70:30) at various magnifications. Source: Adapted from Laskowska, A., Zaborski, M., Boiteux, G., Gain, O., Marzec, A., Maniukiewicz, W., 2014. Effects of unmodified layered double hydroxides MgAl-LDHs with various structures on the properties of filled carboxylated acrylonitrile
butadiene rubber XNBR. Eur. Polym. J. 60, 172
185. With kind permission of Elsevier.

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Figure 4.2 (A) SEM images of (a) LDH-CO3, (b) LDH-HPO4, and (c) LDH-HDEHP. Magnification 3 10,000 (left) and 3 100,000 (right). (B) SEM micrographs with different magnifications ( 3 25 on the left, 3 100 on the right) of the polyurethane foam nanocomposites, PUF-REF (a), PUF-LCO33 (b), PUF-LHPO43 (c), and PUF-LHDEHP3 (d). [Control sample was denoted as PUF-REF and NC foams were named PUF-LCO31, PUF-LCO33, and PUFLCO35 (with 1, 3, and 5 pphp of LDH-CO3); PUF-LHPO41, PUF-LHPO43, and PUF-LHPO45 (with 1, 3, and 5 pphp of LDH-HPO4) and finally PUFLHDEHP1, PUF-LHDEHP3, and PUF LHDEHP5 (with 1, 3, and 5 pphp of LDH-HDEHP)] (pphp 5 parts per hundred parts polyol). Source: Adapted from Go´mez-Ferna´ndez, S., Ugarte, L., Pen˜a, C., Zubitur, M., Angeles Corcuera, M., Eceiza, A., 2016. Flexible polyurethane foam nanocomposites with modified layered double hydroxides. Appl. Clay Sci. 123, 109
20. With kind permission of Elsevier.

higher size of the organic anion (Fig. 4.2a). The SEM images of the prepared NC foam surfaces showed that foam cells have a polyhedral shape, with most of them consisting of open cells. NC foams demonstrated an uneven cell size distribution but cell structure did not appear collapsed or damaged, probably due to an increase in the reactive mixture viscosity along with the hindering caused by the presence of LDH during the growth of the bubbles, thus favoring the formation of a heterogeneous structure. Additionally, some cells collapsed and thus, bigger cells were observed in PUF-LHPO43 (Fig. 4.2b) (Go´mez-Ferna´ndez et al., 2016). Xu et al. prepared MgAl-LDH-loaded graphene hybrid (RGO-LDH) through the coprecipitation technique. Then, the heptamolybdate (Mo7 O62 24 )-modified RGOLDH hybrid (RGO-LDH/ Mo) was synthesized via the ion exchange technique. They showed that the flame retardancy and smoke suppression properties of polyurethane elastomer (PUE) were improved by incorporation of RGO-LDH/Mo into the PUE. The structure and morphology of GO, RGO, RGO-LDH, and RGOLDH/ Mo were observed by TEM. Fig. 4.3A demonstrates that GO has a very thin twodimensional sheet structure. Compared with GO, the folded layer region of RGO is obviously increased and some areas are restacking in Fig. 4.3B, which is because

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Figure 4.3 TEM images of (A) GO, (B) RGO (C) RGO-LDH (D) and RGO-LDH/Mo. Source: Adapted from Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40. With kind permission of Elsevier.

graphene is easy to re-aggregate. From Fig. 4.3C, it can be seen that a lot of nanolamella-loaded graphene sheets are attributed to the unique LDH hexagonal structure, indicating that LDH has been successfully loaded on the surface of the graphene layer. After the Mo7 O62 24 was intercalated into the RGO-LDH interlayer, as shown in Fig. 4.3D, the contours of the MgAl-LDH sheets were not obvious, because, after modification, the lattice structure was damaged, resulting in the crystal’s incompleteness, which was consistent with the X-ray diffraction (XRD) results. The TEM images of PUE5 (PUE (98 wt%) 1 RGO-LDH (2 wt%)) and PUE9 (PUE (98 wt%) 1 RGO-LDH/Mo (2 wt%)) demonstrate that the RGO-LDH and RGOLDH/Mo have no obvious agglomeration and their basic size ranges from 200 to

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Figure 4.4 TEM images of PUE5 (A) and PUE9 (B) composites. Source: Adapted from Xu, W., Zhang, B., Xu, B., Li, A., 2016. The flame retardancy and smoke suppression effect of heptaheptamolybdate modified reduced graphene oxide/layered double hydroxide hybrids on polyurethane elastomer. Compos. Part A: Appl. Sci. Manuf. 91, 30
40. With kind permission of Elsevier.

400 nm. These results indicate that both the RGO-LDH and RGO-LDH/Mo are dispersed well in PUE (Fig. 4.4) (Xu et al., 2016). He et al. reported the preparation of LDH-NO3 and sodium p-styrenesulfonate hydrate (SSS) modified LDH complex (LDH-SSS) and the synthesis of two NBR/ LDH composites using sulfur as the curing agent. The aging behavior of NBR/LDH composites was studied. After being heated at 90 C for 96 h for aging, the morphology of NBR/LDH composites were evaluated as shown in Fig. 4.5. It was found that the morphology of LDH and NBR material falls into two different dispersed states in the two composites. Compared with the NBR/LDH-NO3 composites before aging (Fig. 4.5A), the smaller-sized and more uniformly dispersed LDH-SSS particles can be observed on the fractured surface of NBR/LDH-SSS (Fig. 4.5B). In addition, the microvoids appear around dispersed LDH-NO3 particles (Fig. 4.5A); this can barely be observed in the NBR/LDH-SSS composites before aging (Fig. 4.5B). This shows that there is a weaker LDH/polymer interfacial interaction between LDH-NO3 and NBR than that between LDH-SSS and NBR, which makes the local failure of the interface easier for the NBR/LDH-NO3 composite. Localized damage at the polymer/ LDH interface obviously induces interfacial microvoids and is very similar to profuse crazing. Therefore, such isolated microvoids around dispersed LDH are formed as a result of the energy-absorbing process during tensile deformation. This leads to higher tensile strength and elongation at failure for NBR/LDH-NO3 before aging, compared to the unfilled NBR/LDH and NBR/LDH-SSS. During the thermal-oxidative aging of NBR composites, however, large and dense microvoids around dispersed LDH-NO3 (Fig. 4.5C) are visible, while fewer and much smaller microvoids are around dispersed LDH-SSS (Fig. 4.5D). In addition, the obvious NBR/LDH interfacial phase separation

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Figure 4.5 SEM images of fractured surface of (A) NBR/LDH-NO3 before aging, (B) NBR/ LDH-SSS before aging, (C) NBR/LDH-NO3 after aging, and (D) NBR/LDH-SSS after aging. Source: Adapted from He, X., Li, T., Shi, Z., Wang, X., Xue, F., Wu, Z., et al., 2016. Thermal-oxidative aging behavior of nitrile-butadiene rubber/functional LDHs composites. Polym. Degrad. Stabil. 133, 219
226. With kind permission of Elsevier.

can be observed in Fig. 4.5C, but it is difficult to identify this in Fig. 4.5D. These all show that the LDH-SSS have stronger interfacial interactions and better thermaloxidative aging properties than LDH-NO3 in the aspect of modified NBR (He et al., 2016).

4.3.2 Microscopic characterization of thermoplastic polymer/LDH NCs Kutlu et al. utilized a homogeneous precipitation technique for preparation of LDHs with a 2:1 magnesium/aluminum ratio. The fabricated LDHs were then organically modified with camphorsulfonic acid (CSA) and ciprofloxacin. The CSA-modified LDH (LDH1) and ciprofloxacin-modified LDH (LDH2) were melt-compounded with

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high-density polyethylene (PE) and the obtained NCs were further melt-spun using a piston-type spinning device. The interlayer distance of LDH was increased after the modification with CSA and ciprofloxacin because of the intercalation of selected anionic molecules into LDH galleries. The TEM images of the NCs show that LDH1 produced much better distribution in the matrix in comparison with LDH2 (Fig. 4.6) and modification with ciprofloxacin was not fully successful. The layered structure of LDH was retained after melt-mixing in both cases and intercalation and exfoliation of the LDH layers were also partially observed (Kutlu et al., 2014). Xue et al. evaluated the Zn2Al-NO3 LDHs which were prepared by a coprecipitation method, for the adsorption of anionic dye acid red 97 (AC97). They showed

Figure 4.6 TEM images of (A) LDH1/PE nanocomposites in low magnitude, (B) LDH1/PE nanocomposites high magnitude, (C) LDH2/PE nanocomposites in low magnitude, and (D) LDH2/PE nanocomposites in high magnitude. Source: Adapted from Kutlu, B., Meinl, J., Leuteritz, A., Bru¨nig, H., Wießner, S., Heinrich, G., 2014. Up-scaling of melt-spun LDH/HDPE nanocomposites. Macromol. Mater. Eng. 299, 825
833. With kind permission of Elsevier.

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the possibility of resourcing the LDH adsorbent sludge by synthesizing polypropylene (PP)/Zn2Al-AC97 LDH NCs by incorporation of different concentrations (0.2, 0.4, 1, 2, 4, 10 wt%) of the obtained Zn2Al-AC97 LDH into PP using a newly developed solvent-mixing method. The good distribution of filler in the PP matrix was observed when its loading was low (0.2
1 wt%) (Fig. 4.7). However, with the enhancement of an LDH amount from 2 to 10 wt%, the smoothness of the NC surface declines and more LDH nanoparticles can be clearly seen. The rapid precipitation of the polymer composite in hexane leads to the formation of spherical particles in all NCs, as shown in Fig. 4.7 (Xue et al., 2014). Shabanian et al. showed that the incorporation of organo-modified LDH into long aliphatic chain polyamide resulted in a considerable increase in the thermal stability, char yields, and flame retardancy of the NCs as compared to pure polyamide. The NCs were synthesized using a solution intercalation method under ambient conditions in N,N-dimethylacetamide as a solvent. The polyamide was synthesized using direct polycondensation reaction from an oleic acid-based monomer and

Figure 4.7 SEM images of (A) 0.2 wt%, (B) 2 wt%, (C) 4 wt%, and (D) 10 wt% PP/Zn2AlAC97 nanocomposites. Source: Adapted from Xue, T., Gao, Y., Zhang, Z., Umar, A., Yan, X., Zhang, X., et al., 2014. Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene. With kind permission of Elsevier.

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Figure 4.8 TEM micrographs of PA/LDH 3 and PA/LDH 6 (PA 5 polyamide, PA/LDH 3 5 NC containing 3 wt.% SDBS-LDH, and PA/LDH 6 5 NC containing 6 wt.% SDBSLDH). Source: Adapted from Shabanian, M., Basaki, N., Ali Khonakdar, H., Jafari, S.H., Hedayati, K., Wagenknecht, U., 2014. Novel nanocomposites consisting of a semi-crystalline polyamide and Mg
Al LDH: Morphology, thermal properties and flame retardancy. With kind permission of Elsevier.

Mg
Al LDH was modified with sodium dodecyl benzenesulfonate (SDBS) by a one-step method. TEM images (Fig. 4.8) demonstrate the random dispersion of LDH sheets in the polymer matrix even at the high amount of filler, indicating direct evidence of crystal layer exfoliation along with small agglomerations representing a partially delaminated NC. It seems that the dimensions of these agglomerations are too small to be detected by XRD method (Shabanian et al., 2014). The effect of Ni/Al LDH concentration on the rheological behavior of poly (methyl methacrylate) (PMMA)/Ni/Al LDH blends, which were prepared by solvent blending method, was studied by Chakraborty et al. Firstly, Ni/Al LDH was synthesized by a coprecipitation method at constant pH using their nitrate salts and then modified with sodium dodecyl sulfate (SDS). Afterward, the NCs were fabricated by the introduction of different amount of LDHs (3 and 5 wt.%) into PMMA using a solvent blending technique in the presence of methylene chloride as a solvent. FE-SEM images (Fig. 4.9) showed that pristine and modified LDH are principally in the form of flakes and large agglomerates and the layered LDH structure can be seen in both cases. Modified LDHs showed large agglomeration. The good distribution of the particles needed extensive stirring or ultrasonication techniques to break the LDH into smaller dimensions (Chakraborty et al., 2014). Du et al. employed an effective technique for excellent distribution of various amounts of MgAl-LDH (0
5.0 wt%) in poly(vinyl alcohol) (PVA) aqueous dispersion, which can considerably improve the mechanical properties and water resistance of the PVA films while maintaining high film transparency. MgAl-LDH was

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Figure 4.9 FESEM images of (A) pristine Ni/Al LDH, and (B) modified Ni/Al LDH. Source: Adapted from Chakraborty, S., Kumar, M., Suresh, K., and Pugazhenthi, G. (2014). Influence of organically modified NiAl layered double hydroxide (LDH) loading on the rheological properties of poly (methyl methacrylate) (PMMA)/LDH blend solution. Powder Technol. 256, 196
203. With kind permission of Elsevier.

prepared via coprecipitation followed by hydrothermal treatment. The results of SEM and TEM analysis (Fig. 4.10) demonstrated the large LDH aggregates in NCs. Some PVA chains partially intercalated into the structure in the TEM images. At LDH contents 5 1.0 wt%, the LDH nanoplatelets uniformly disperse and show partial intercalating and partial exfoliating structures. At LDH content 5 5.0 wt%, partial intercalating and partial exfoliating structures can also be observed, however, some large LDH aggregates appeared synchronously (Du et al., 2014). Mallakpour et al. examined the effect of novel modified LDHs on thermal and structural properties of PVA. They employed ion exchange reaction of LDH and N, N0 -(pyromellitoyl)-bis-L-phenylalaninediacid in distilled water for fabrication of organically modified chiral LDH (MLDH). The PVA-based NCs were prepared with different amounts of LDH (0, 2, 4, 6, and 8 wt%) by solution-intercalation technique via the ultrasound-assisted technique. The results of FE-SEM and TEM analysis showed that the LDH platelets were well-distributed within the PVA matrix and oriented along the PVA axis in a disorderly fashion (Fig. 4.11). The thermal properties of NCs were enhanced due to the uniform distribution of modified LDH in a polymeric matrix and the strong hydrogen bonding between O
H groups of PVA and the hydroxyl groups of LDH layers or carbonyl group as well as other polar groups of intercalated chiral dicarboxylated anion (Mallakpour and Dinari, 2014b). Polymer electrolyte fuel cells (PEFCs) have attracted much interest recently because of their high power density, low operating temperature, low pollution level, quiet operation, lower corrosion, simplification of stack design, and relatively quick start-up and shut-down (de lasHeras et al., 2017). Chen et al. fabricated a series of novel thermo-responsive NCs by free-radical copolymerization of

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Figure 4.10 SEM (A, B, C) and TEM (A0 , B0 , C0 , Av, Bv, Cv) images of PVA/LDH nanocomposites with different LDH contents. (A, A0 , Av) ɸLDH 5 0.5 wt%; (B, B0 , Bv) ɸLDH 5 1.0 wt%; (C, C0 , Cv) ɸLDH 5 5.0 wt% (ɸLDH 5 LDH contents). Source: Adapted from Du, M., Ye, W., Lv, W., Fu, H., Zheng, Q., 2014. Fabrication of highperformance poly(vinyl alcohol)/MgAl-layered double hydroxide nanocomposites. Eur. Polym J. 61, 300
308. With kind permission of Elsevier.

N-isopropylacrylamide (NIPAm) and the silylanized Mg/Al LDH (SiLDHs) with different mass ratios (PNIPAm-co-SiLDHs). With the aim of protecting the layered structure of LDH in the copolymerization reaction, the silanization process was carried out by the hydrolysis of a silane coupling agent (c-methacryloxypropyltrimethoxysilane, MPTS) on the surface of the wet LDH plates. They also 22 investigated the effect of interlayer anions of LDH (NO2 3 and CO3 ) on the surface characteristics and the sorption properties of the NCs. The results of SEM analysis showed that compared with the NO3
Mg/Al LDH, the MPTS-modified NO3
Mg/ Al LDH has a much smoother surface, as a consequence of the modification of MPTS (Fig. 4.12). PNIPAm-co-Si LDH exhibits the porous structure with an average diameter of about 8 μm. The large porous structure supplies a gallery for water to come in and out. The NO2 3 intercalated LDHs composite showed the stronger sorption capacity for Orange-II than that of CO22 3 because of the larger exposed external surface of the NO2 intercalated LDH NCs (Chen et al., 2015). 3 Nicotera et al. incorporated LDHs with different Mg21/Al31 metal ratios (2:1 2 2 and 3:1) and various interlayer anions (CO22 3 ; ClO4 ; NO3 ) in Nafion matrix by

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Figure 4.11 (A) FE-SEM photographs of LDH (a
c), MLDH (d
f) and PVA hybrid with 8 wt% MLDH (g
i). (B) TEM micrographs of MLDH (a and b) and PVA hybrids containing 8 wt% of MLDH (c and d). Source: Adapted from Mallakpour, S., Dinari, M., 2014b. Novel bionanocomposites of poly (vinyl alcohol) and modified chiral layered double hydroxides: Synthesis, properties and a morphological study. Prog. Org. Coat. 77, 583
589. With kind permission of Elsevier.

solution intercalation technique, in order for preparation of high proton-conducting Nafion-based NCs. The results of SEM analysis of the Naf-LDH-2/1(Mg21/Al31) 2 membranes with ClO2 4 and NO3 countervailing anions, obtained both in surface and in cross-section showed that these films are quite uniform, compact, smooth, and homogeneous on the whole volume without pores or agglomerates, indicating that there is a very good dispersion of the nanoplatelets in the polymeric matrix. Also, the surface, although having some texture with a little deposit of LDH sheets, is compact and homogeneous, and we can affirm that the layers maintain submicrometric dimensions (Fig. 4.13) (Nicotera et al., 2015). Mallakpour et al. synthesized novel Ni-Al LDH intercalated with a bio-active amino acid containing dicarboxylate via coprecipitation reaction of Ni(NO3)2. 6H2O, Al(NO3)3. 9H2O, and N,N0 -(pyromellitoyl)-bis-L-phenylalanine under ultrasonic irradiation. They incorporated different amounts of LDH (2, 4, and 8%) into poly(amide-imide) (PAI) matrix. The presence of amide, imide, and phenol groups in the backbone of the polymer matrix causes hydrogen interactions with functional groups of the modified LDH. FE-SEM analysis (Fig. 4.14) showed the uniform distribution of LDH into the polymer matrix. TEM images of diacid modified NiAl LDH and NC of PAI with 4% diacid modified LDH is shown in Fig. 4.15. TEM images of NC 4% demonstrate a coexistence of LDH layers in the intercalated and the partially exfoliated states. TEM micrograph shows two-dimensional objects which are oriented largely parallel to the grid surface and thin sheet-like objects

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Figure 4.12 SEM images of prepared samples (A) NO3
Mg/Al LDH, (B) SiLDHN, (C) P/ SN-2, (D) P/SC-2, and (E) EDS of P/SN-2 [SiLDHN 5 MPTS modified NO3
Mg/Al LDH, P/SN-2 5 PNIPAm-co-SiLDHN with mass ratios of 3:1 (3 5 NIPAm and 1 5 SiLDHN), P/ SC-2 5 PNIPAm-co-SiLDHC with mass ratios of 3:1 (3 5 NIPAm and 1 5 SiLDHC)]. Source: Adapted from Chen, H., Qian, G., Ruan, X., Frost, R.L., 2015. Abatement of aqueous anionic contaminants by thermo-responsive nanocomposites: (Poly(Nisopropylacrylamide))-co-silylanized Magnesium/Aluminun layered double hydroxides. J. Colloid. Interface. Sci. 448, 65
72. With kind permission of Elsevier.

with similar lateral dimensions. Both modified LDH and PAI/modified LDH NC4% present disc-like images, which are actually the LDH platelets lying flat on the substrate. In addition, the figure also shows that some platelets are overlapping on the edge (Mallakpour et al., 2015).

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Figure 4.13 SEM images of Nafion nanocomposite membranes loaded with 3 wt.% of LDHMg21/Al31 (2/1) and countervailing anions NO32 (A) and ClO42 (B). Source: Adapted from Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H2/air-fed fuel cells. Solid State Ionics 276, 40
46. With kind permission of Elsevier.

Zhao et al. incorporated Zn
Al LDH into poly(vinylidene fluoride) (PVDF) membranes using a phase inversion method. Fig. 4.16 demonstrates the threedimensional top surface images of the pure and LDH-incorporated PVDF membranes obtained from AFM analysis. The results showed that incorporation of LDH resulted in a great change in the surface morphologies of the pristine membranes. The surface of LDH-embedded membrane appeared to be smoother with both average roughness (Ra) and square roughness (Rq) dropping remarkably. The deep ups and downs with a large area and small quantities were replaced with flatter ones with a small area and large quantities. The change can be attributed to: (1) the existence of well-distributed LDH causing the morphology change of membrane surface; and (2) the incorporation of LDH influenced the shape, number, and size of pores on the membrane surface, producing more uniform pores with smaller size, which was confirmed by the SEM images (Fig. 4.17) (Zhao et al., 2016a). Mallakpour et al. reported the synthesis of bionanocomposite (BNC) materials based on isoleucine containing PAI and modified MgAl-LDH via solution intercalation technique for the first time. They modified MgAl-LDH by an ion exchange reaction in a solution of N,N0 -(pyromellitoyl)-bis-L-isoleucine in distilled water. Afterward, novel optically active PAI/modified LDH BNCs containing 2%, 4%, and 8% of modified LDH were successfully prepared using modified LDH with chiral isoleucine containing dicarboxylate and PAI chains with the same group via a solution intercalation technique for the first time. The results of FE-SEM analysis

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Figure 4.14 FE-SEM photographs of (A, B) modified NiAl LDH, (C, D) PAI/modified LDH NC 2%, and (E, F) PAI/modified LDH NC 8%. Source: Adapted from Mallakpour, S., Khani, M. (2015). Composites of semiaromatic poly (amide-ester-imide) based on bioactive diacid and oragnomodifiednanoclay produced by solution intercalation method: thermal and morphological study. Polym. Plast. Technol. Eng. 54, 541
547. With kind permission of Springer.

showed that the modified LDH is more floppy in comparison with neat LDH, which consists of plate-like shapes stacked on top of each other with lateral dimensions ranging from a few micrometers and thickness of over a few hundred nanometers. Also, it was found that, in BNC4%, the LDH nanosheets have a better dispersion in comparison with BNC with a high content of LDH (8%) (Fig. 4.18). The results of

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Figure 4.15 TEM micrographs of (A, B) modified LDH and (C, D) PAI/modified LDH NC 4%. Source: Adapted from Mallakpour, S., Khani, M. (2015). Composites of semiaromatic poly (amide-ester-imide) based on bioactive diacid and oragnomodifiednanoclay produced by solution intercalation method: thermal and morphological study. Polym. Plast. Technol. Eng. 54, 541
547. With kind permission of Springer.

Figure 4.16 AFM images of M0 and M3 membranes (M0 5 pure PVDF membranes, M3 5 2 wt.% LDH incorporated PVDF membranes). Source: Adapted from Zhao, Y., Li, N., Xu, B., Dong, B., Xia, S., 2016a Preparation and characterization of a novel hydrophilic poly(vinylidene fluoride) filtration membrane incorporated with Zn
Al layered double hydroxides. J. Ind. Eng. Chem. 39, 37
47. With kind permission of Elsevier.

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TEM studies showed that, for modified LDH, the platelets have a hexagonal shape with rounded corners. There are no signs of aggregation visible in the micrographs. The TEM analysis of BNC4% demonstrated a coexistence of modified-LDH layers in the intercalated and partially exfoliated states (Fig. 4.19) (Mallakpour and Dinari, 2016b). Zhao et al. reported the preparation of Zn-Al LDH-modified polyamide (PA) nanofiltration (NF) membranes with LDH loads varying from 0 to 0.250% (w/v) using interfacial polymerization of 1,3-phenylene diamine (MPD) in the aqueous phase and 1,3,5-benzenetricarbonyltrichloride (TMC) in the organic phase. The membranes with various contents of LDH were defined as PA-0, PA-1, PA-2, and PA-3. The surface morphology of various PA membranes was studied by FE-SEM analysis (Fig. 4.20i). The membrane without LDH showed the characteristic “ridgeand-valley” morphology but PA/LDH membranes showed fewer ridges and valleys and more “leaf-like” folds. Fig. 4.20i also presents the cross-section morphologies of PA-0 and PA-2. The addition of LDH to the PA membrane offered a much more even, or smoother surface without the same degree of notable outcropping against the background. Moreover, the thickness of the PA layer decreased, and PA-2 had a denser surface compared to the pure membrane. This phenomenon can be attributed to the hydrophilicity of LDH. Fig. 4.20ii demonstrates the AFM images. The ridges have been flattened and the valleys also rose in PA-2 compared to PA-0, which is consistent with the thickness change of the PA layer in them. In addition, the single ridge in PA-2 has a larger planar area, corresponding to the leaf-like surfaces of modified membranes (Zhao et al., 2016b). Kredatusova et al. described the multifunctional influence of modified LDH on the progress of ring-opening polymerization of ε-caprolactone under microwave irradiation. The modification of LDH was carried out with phosphonium-based ionic liquids (IL) containing phosphinate, carboxylate, and phosphate anions for the ion-exchange reaction with Mg-Al LDH. Poly(ε-caprolactone) (PCL)/LDH NCs were prepared by in situ ring-opening polymerization of ε-caprolactone under microwave irradiation (MROP) in the presence of organically modified LDH. They found that MROP of CL performed in the presence of LDH does not require any additional catalysts or initiators. The results of TEM analysis showed that the NCs prepared by in situ MROP of CL in the presence of phosphinate-LDH and carboxylate-LDH have highly delaminated morphology with a uniform distribution of nanolayers. But, the NC containing phosphate-LDH demonstrated homogeneous dispersion of LDH stacks smaller than 500 nm in size. The stacks of intercalated LDH were also observed in the PCL/LDH composite when the nonmodified LDH was applied as the catalyst for MROP of CL. The NC prepared from nonmodified LDH contained smaller stacks (diameter 200
300 nm) compared to the PCL/phosphate-LDH composite (Fig. 4.21) (Kredatusova´ et al., 2016). Shabanian et al. prepared novel poly(methyl-ether-imide) (PMEI)/ LDH NCs using the solution intercalation method. The synthesis of new PMEI was carried out by preparation of a methyl-rich bisphenol as starting material to produce a new diamine with an ether linkage which was utilized to the fabrication of PMEI. The SDBS was used for modification of Mg
Al LDH in one step and PMEI NCs were

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prepared by solution intercalation technique with various quantities of SDBS modified Mg
Al LDH (4 and 8 mass %). TEM images NC 4% showed that LDH sheets were dispersed in the PMEI matrix, indicating direct evidence of crystal layer exfoliation along with small agglomerations representing a partially delaminated NC (Fig. 4.22) (Shabanian et al., 2016). Hu et al. fabricated the decavanadate-anion with anticorrosive activity intercalated LDH (D-LDH) via anion-exchange reaction. Afterward, the D-LDH was grafted with ethoxy groups of γ-aminopropyltriethoxysilane (APTS) (AD-LDH) and employed in the fabrication of polyaniline (PANI) NCs by in situ polymerization. The results of SEM analysis showed that adding APTS in the D-LDH affected the morphology of the obtained material and reduced the D-LDH aggregation. Also, it is observed that the partial exfoliation of grafted intercalated LDH in the PANI matrix is quite unconspicuous but when comparing with the SEM image of PANI, partially exfoliated AD-LDH can be seen from the SEM image of AD-LDH/PANI composites. Therefore, the AD-LDH is well-dispersed in the PANI matrix and demonstrates partially exfoliated structures (Fig. 4.23) (Hu et al., 2015).

4.3.3 Microscopic characterization of thermosetting polymer/ LDH NCs

L

Li et al. showed that introducing 4.5 wt% of LDHs into sisal fiber reinforced phenolic composites might be effective to improve flame retardancy. The unmodified Mg-Al LDH (NLDH) and SDBS intercalated Mg-Al LDH (SDBSLDH) was incorporated into normal phenolic (PF)/silicone-modified novolac type phenolic resins (SPF) by solvent mixing. Then, biobased silicone modifier (TDS) was used to improve the electrical resistance and water absorption behavior. The results demonstrated that the modified composites optimally show a 60% reduction in total heat release (20.2 MJ/m2) compared to the composites without LDH. The SEM results revealed that all SPF-based composites (SPF-SF-NLDH, SPF-SFSDBSLDH, and SPF-SF) exhibit fiber breakage, while little pull out and debonding is observed. It displays remarkable toughness fracture surface morphology (Fig. 4.24). The excellent interfacial interaction is beneficial for the energy transfer and dissipation, leading to the SPF composites’ high impact strength. In comparison, the PF-based composites (PF-SFNLDH, PF-SF-SDBSLDH, and PF-SF) exhibit a large amount of Figure 4.17 The top surface (A) of M0 (a) and M3 (b) membranes and cross-section (B) morphologies of M0, M1, M2, M3, and M4 (a
e) membranes (M0 5 pure PVDF membranes, M1 5 0.67 wt.% LDH-incorporated PVDF membranes, M2 5 1.33 wt.% LDHincorporated PVDF membranes, M3 5 2 wt.% LDH-incorporated PVDF membranes, and M4 5 2.67 wt.% LDH-incorporated PVDF membranes). Source: Adapted from Zhao, Y., Li, N., Xu, B., Dong, B., Xia, S., 2016a Preparation and characterization of a novel hydrophilic poly(vinylidene fluoride) filtration membrane incorporated with Zn
Al layered double hydroxides. J. Ind. Eng. Chem. 39, 37
47. With kind permission of Elsevier.

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Figure 4.18 FE-SEM photographs of neat LDH (A, B), modified LDH (C, D), BNC4% (E, F), and BNC8% (G, H). Source: Adapted from Mallakpour, S., Dinari, M., 2016b. Using Mg-Al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly(amide-imide). Polym. Compos. 37, 3288
3295. With kind permission of John Wiley and Sons.

Figure 4.19 TEM micrographs of modified LDH (A and B) and BNC4% (C and D). Source: Adapted from Mallakpour, S., Dinari, M., 2016b. Using Mg-Al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly(amide-imide). Polym. Compos. 37, 3288
3295. With kind permission of John Wiley and Sons.

Figure 4.20 (i) FE-SEM images of the surfaces (A) of: (a) PA-0, (b) PA-1, (c) PA-2, (d) PA-3 and the cross-sections (B) of: (a) PA-0, and (b) PA-2. (ii) AFM images of PA-0 and PA-2 membranes. Source: Adapted from Zhao, Y., Li, N., Xia, S., 2016b. Polyamide nanofiltration membranes modified with Zn
Al layered double hydroxides for natural organic matter removal. Compos. Sci. Technol. 132, 84
92. With kind permission of Elsevier.

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Figure 4.21 TEM micrographs of poly(ε-caprolactone) composites prepared under microwave irradiation using various nanofillers: (A) nonmodified LDH, (B) phosphinateLDH, (C) phosphate-LDH, and (D) carboxylate-LDH. Polymerization conditions: constant power: 30 W, polymerization time: 10 min, filler content: 6 wt%. Source: Adapted from Kredatusova´, J., Beneˇs, H., Livi, S., Pop-Georgievski, O., Ecorchard, P., Abbrent, S., et al., 2016. Influence of ionic liquid-modified LDH on microwave-assisted polymerization of ε-caprolactone. Polymer. (Guildf). 100, 86
94. With kind permission of Elsevier.

fiber pull-out and debonding, which implies poor interactions between matrix and fiber, further affecting the energy transfer and dissipation. These two group composites are different only in the matrix resin, SPF versus PF, leading to such huge differences in impact strength and fracture surface morphology. The siliconecontaining SPF promotes the interactions between matrix and sisal fibers to be stronger than the PF composites (Li et al., 2016). Wang et al. reported the modification of LDH with a biobased modifier (cardanolBS) through a one-step coprecipitation technique. Then, various amounts of

Figure 4.22 TEM image of PMEIN 4. Source: Adapted from Shabanian, M., Ardeshir, H., Haji-Ali, S., Moghanian, H., Hajibeygi, M., Faghihi, K., et al., 2016. Efficient poly(methyl-ether-imide)/LDH nanocomposite derived from a methyl rich bisphenol: from synthesis to properties. Appl. Clay Sci. 123, 285-291. With kind permission of Elsevier.

Figure 4.23 SEM images of (A) D-LDH, (B) AD-LDH, (C) PANI, (D) AD-LDH/PANI. Source: Adapted from Hu, J., Gan, M., Ma, L., Zhang, J., Xie, S., Xu, F., et al., 2015. Preparation and enhanced properties of polyaniline/grafted intercalated ZnAl-LDH nanocomposites. Appl. Surf. Sci. 328, 325
334. With kind permission of Elsevier.

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Figure 4.24 SEM images of (A) SPF-SF-NLDH, (B) SPF-SFSDBSLDH, (C) SPF-SF, (D) PF-SF-NLDH, (E) PF-SF-SDBSLDH, and (F) PF-SF. Source: Adapted from Li, C., Wan, J., Pan, Y.-T., Zhao, P.-C., Fan, H., Wang, D.-Y., 2016. Sustainable, biobased silicone with layered double hydroxide hybrid and their application in natural-fiber reinforced phenolic composites with enhanced performance. ACS Sustain. Chem. Eng. 4, 3113
3121. With kind permission of the American Chemical Society.

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Figure 4.25 TEM micrographs of EP/m-LDH-1% (A, E), EP/m-LDH-4% (B, F), EP/mLDH-6% (C, G), and EP/LDH-6% (D, H) at different magnifications. Source: Adapted from Wang, X., Kalali, E.N., Wang, D.-Y., 2015. Renewable cardanolbased surfactant modified layered double hydroxide as a flame retardant for epoxy resin. ACS Sustain. Chem. Eng. 3, 3281
3290. With kind permission of the American Chemical Society.

Cardanol-BS-modified LDH (m-LDH) were incorporated into epoxy resins (EPs) using a combined technique of three-roll mill and ultrasonication. Fig. 4.25 gives the TEM images of EP/m-LDH and EP/LDH composites at various magnifications. The information about the distribution state was obtained from low-magnification images, whereas the high-magnification ones can distinguish whether intercalation and/or exfoliation have been obtained when LDHs are incorporated into epoxy matrices. In the case of EP/m- LDH-1%, a low-magnification TEM image (Fig. 4.25A) reveals a typical feature for epoxy-based NCs, a simultaneous presence of intercalated LDH plates, and small tactoids and stacks. Under high magnification (Fig. 4.25E), it was found that a well-intercalated NC structure has been formed. Similar TEM features were observed for EP/m-LDH-4% (Fig. 4.25B,F) and EP/m-LDH-6% (Fig. 4.25C,G). All of the EP/m-LDH NCs appear to be well-intercalated structures, and the m-LDH nanoplatelets are randomly oriented in the epoxy matrix. In contrast, the lowmagnification image of EP/LDH-6% (Fig. 4.25D) demonstrates that the distribution of neat LDH in the epoxy matrix is poor. Under high magnification (Fig. 4.25H), the unmodified LDHs form large aggregates with thick stacking. These results demonstrate that the uniform distribution of LDH within the epoxy matrix was achieved by modification of LDH by cardanol BS (Wang et al., 2015). Wang et al. reported that the fire hazard characteristics of EP can be improved by incorporation of LDH-wrapped β-FeOOH hybrid material into EP matrix. The SEM images show that the distribution of β-FeOOH rods throughout the EP matrix perform much better than that of LDH in the matrix (Fig. 4.26A). As can be observed in Fig. 4.26B, LDH agglomerates obviously in the EP matrix because of

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Figure 4.26 (A) SEM images for EP0 (a), EP/LDH (b), EP/b-FeOOH (c), EP/LDH-bFeOOH (d) of the fractured surfaces cryogenically broken after immersion in liquid nitrogen. (B) TEM ultrathin observations of the EP/LDH composites (a), EP/b-FeOOH composites (b) and EP/LDH-b-FeOOH composites (c, d). Source: Adapted from Wang, W., Pan, H., Shi, Y., Pan, Y., Yang, W., Liew, K.M., et al., 2016. Fabrication of LDH nanosheets on β-FeOOH rods and applications for improving the fire safety of epoxy resin. Compos. Part A: Appl. Sci. Manuf. 80, 259
269. With kind permission of Elsevier.

the H-bonding force of the LDH. The distribution of β-FeOOH throughout the EP matrix performs relatively well, but the structure of the rods collapses obviously because of stirring, ultrasonication, and shearing during the fabrication process. It can be seen that β-FeOOH rods surrounded by LDH demonstrate uniform distribution throughout the EP matrix. The physical area increases significantly because of the combination of LDH nanosheets and β-FeOOH rods, which is beneficial to the improvement of flame retardancy (Wang et al., 2016). Zhou et al. incorporated the LDH/MoS2 hybrids which were synthesized via self-assembly of exfoliated MoS2 nanosheets and LDH via electrostatic force, into epoxy to reduce its fire hazard risk. The results of SEM analysis showed that the pure epoxy has a smooth fracture surface, indicating the typical brittle failure of a thermosetting polymer. The fractured surface of epoxy systems shows entirely different fractographic features. For example, the MoS2-filled epoxy system demonstrates a rough surface with obvious agglomeration structure. A few individual MoS2 nanosheets exposed from the matrix can be observed, indicating the relatively weak interfacial interactions between MoS2 nanosheets and epoxy matrix. For the epoxy systems filled with LDH/MoS2 hybrids, it was observed that the LDH/MoS2 hybrids are dispersed well and completely embedded in the epoxy matrix without agglomeration, which is probably ascribed to the inhibition effect of deposited LDH on the restacking of MoS2 nanosheets. The uniform dispersion of LDH/MoS2 hybrids in epoxy matrix leads to prominent improvement of thermal stability and fire safety (Fig. 4.27) (Zhou et al., 2017). Shafiei et al. prepared the uniform and bead-free fibers of PCL composited with various amounts of LDH (ranging from 0.1 to 10 wt%) via an electrospinning method. They showed that the incorporation of LDH into PCL scaffold increased

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Figure 4.27 SEM images of the fractured surfaces for neat EP (A), EP/2% MoS2 (B), EP/ 2% NiFe-LDH-MoS2 (C), and EP/2% CoFe-LDH-MoS2 (D) composites (EP 5 epoxy). Source: Adapted from Zhou, K., Gao, R., Qian, X., 2017. Self-assembly of exfoliated molybdenum disulfide (MoS2) nanosheets and layered double hydroxide (LDH): Towards reducing fire hazards of epoxy. J. Hazard. Mater. 338, 343
355. With kind permission of Elsevier.

the in vitro degradation rate when subjected to accelerated degradation solution. The SEM analysis of fibers showed that the neat PCL scaffold has smooth surface morphology with a uniform diameter. However, it should be noted that all fabricated fibers were almost bead-free and randomly oriented in directions (Fig. 4.28). In order to use the PCL-LDH scaffolds in tissue engineering applications, the effect of electrospun PCL-LDH on the proliferation of mouse adipose-derived stem cells (mADSCs) was studied using SEM analysis (Fig. 4.29). It was found that, in both the PCL and PCL-LDH NC scaffolds, cells were well spread and flattened onto the fibers in an irregular pattern and secreted extracellular matrix (ECM). The sheet-like cells exhibited filopodia-like structures (star-like structures) and were elongated along the fibers as seen in the SEM image. It was also observed that cells organized into groups making chains of cells and colony layers. The spindle-like shapes of cells indicated appropriate media for supporting the growth of the cell. Some cells migrated into the pores of the PCL-LDH scaffolds, spreading on different layers, while this phenomenon could not be seen in PCL scaffolds. This might be because of the larger pores and presence of inducing agents that facilitate cell attachments and spreading (Shafiei et al., 2016).

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Figure 4.28 SEM micrograph of electrospun scaffolds; (A) PCL, (B) PCL 1 0.1% LDH, (C) PCL 1 1% LDH, (D) PCL 1 10%LDH (scale bars represent 10 μm). Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci. 127, 52
63. With kind permission of Elsevier.

4.3.4 Microscopic characterization of polymer blend/LDH NCs Rafiee et al. reported the preparation of PP/ethylene vinyl acetate copolymer (EVA) NCs containing organo-modified LDH by a one-step melt mixing process in the presence of a maleated PP(PP-g-MA) as compatibilizer. The results of TEM analysis demonstrated that LDH nanoparticles in the exfoliated and/or intercalated states were mainly localized in the EVA phase. The average particle size of EVA domains was reduced in the presence of LDH and PP-g-MA (Fig. 4.30). It has been observed that,

Figure 4.29 SEM observation of mADSCs cultured on (A, B) PCL, (C, D) PCL 1 0.1% LDH, (E, F) PCL 1 1% LDH, (G, H) and PCL 1 10% LDH scaffolds. Scale bars represent 100 and 2 μm. Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci. 127, 52
63. With kind permission of Elsevier.

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Figure 4.30 TEM images of P75E25C5L5 samples with different magnifications. Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides.Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

EVA as a minor phase forms droplet-type morphology within the PP matrix. EVA has a dark color because its electron density is much higher than that of PP and black areas represent LDH layers. TEM micrographs with high magnification (Fig. 4.30A, B) showed that LDH layers are broadly localized in the EVA domains in a disordered manner based on their polarization properties. LDH is seen mainly in the intercalated state within EVA domains. As shown in high-magnification TEM micrographs of the same sample (Fig. 4.30C,D), LDH layers were attracted to the EVA phase and formation of intercalated nanostructures is evident. There also exist some aggregates or stacks in this sample. Fig. 4.30A,B revealed characteristic TEM images of P25E75C5L5 (PP 5 25 wt%, EVA 5 75 wt%, compatibilizer 5 5 wt%, LDH 5 5 wt%) sample at different magnifications. In this sample, EVA is a major phase which attracts LDH particles. Fig. 4.31A confirms that LDH layers were partially

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Figure 4.31 TEM images of P25E75C5L5 samples with different magnifications. Source: Adapted from Rafiee, F., Otadi, M., Goodarzi, V., Khonakdar, H.A., Jafari, S.H., Mardani, E., et al., 2016. Thermal and dynamic mechanical properties of PP/EVA nanocomposites containing organo-modified layered double hydroxides.Compos. Part B: Eng. 103, 122
130. With kind permission of Elsevier.

dispersed within the EVA phase. The TEM image of this sample in low magnification in Fig. 4.31B shows the presence of intercalated LDH structures implying that EVA chains could penetrate into galleries of LDH layers (Rafiee et al., 2016). Bercea et al. showed that the chitosan/PVA/Mg-Al LDH NCs are a good candidate for preparation of novel composite biomaterials having various functional properties of chitosan coupled with good mechanical properties of PVA, on one side, and highly tunable properties, increased permeability, and retention of LDH nanoparticles, on the other. The Mg-Al LDH was prepared via the coprecipitation technique and was incorporated into chitosan/PVA mixture. The influence of pH on rheological and structural properties of the prepared NCs was also investigated. SEM images for chitosan/PVA/LDH NCs at pH 5 3, pH $ 7, and pH 5 10, showed that the polymer/clay mixture is uniformly dispersed at pH 5 3, but the network structure is not formed (Fig. 4.32). It was found that the porous structure obtained for a neutral or weakly basic environment where the gelation is induced by the polymer
polymer interactions and by the clay presence. Fracture surface analysis demonstrated that the LDH is uniformly distributed into the chitosan/PVA matrix. An SEM image of LDH shows that the LDH sample is highly crystalline and its structure consists of interconnected hexagonal-shaped particles (Bercea et al., 2015). Pak et al. prepared poly-3-hydroxybutyrate/poly(butyleneadipate-co terephthalate)/ LDH (PHB/PBAT/LDH) NCs from a binary blend of PHB/PBAT and stearate-Zn3Al LDH via a solution-casting technique. The dispersion of stearate-Zn3Al LDH in the

Figure 4.32 SEM micrographs of (A) CS/PVA/LDH composite at pH 5 3; (B) CS/PVA/ LDH composite at pH $ 7, and (C) LDH at pH 5 10 (CS 5 chitosan). Source: Adapted from Bercea, M., Bibire, E.-L., Morariu, S., Teodorescu, M., Carja, G., 2015. pH influence on rheological and structural properties of chitosan/poly(vinyl alcohol)/ layered double hydroxide composites. Eur. Polym J. 70, 147
156. With kind permission of Elsevier.

Figure 4.33 TEM micrographs of PHB/PBAT/2.0 wt% stearate-Zn3Al LDH. Source: Adapted from Pak, Y.L., Bin Ahmad, M., Shameli, K., Yunus, W.M.Z.W., Ibrahim, N.A., Zainuddin, N., 2013. Mechanical and morphological properties of poly-3hydroxybutyrate/poly(butyleneadipate-co-terephthalate)/layered double hydroxide nanocomposites. J. Nanomater. 2013, 8. Open access journal.

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NCs was studied by TEM analysis. The results demonstrated that the intercalated layers that were uniformly dispersed can be observed in PHB/PBAT. It was found that the stack consists of several layers, which showed the formation of intercalated NCs. The TEM analysis demonstrated that the LDH layers are in an intercalated but not well-ordered structure form and are dominantly distributed in the PHB/PBAT matrix (Fig. 4.33) (Pak et al., 2013).

4.4

Conclusions

Characterization of the NCs is necessary to obtain valuable information about these materials such as quality of distribution of filler in the polymer matrix, influence of filler surface modification on filler dispersion and composite properties, interactions of the filler modification with the polymer chains, changes in the process parameters on the resulting morphology and properties, and study of a wide spectrum of properties to determine the potential applications of NCs. This chapter describes the recent study of polymer/LDH NCs using different microscopic techniques such as SEM, TEM, and AFM. The capabilities of each microscopy technique allow investigation of NCs from different aspects. SEM analysis provides information from electrons scattered on the surface of the sample, so it is beneficial for imaging thicker and bulkier samples. TEM imaging, in which a high-energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe ultrafine patterns on nanostructures. AFM can provide a topographical and mechanical view of the surface, with an easier sample preparation protocol. All of these techniques would be suitable and are vitally important and essential to understanding the better formation of polymer/ LDH NCs for a wide variety of applications in different technologies. We are sure that in the near future the advances in the above methods will provide more information in this field and a greater insight into polymer/NC structures.

Acknowledgments The authors wish to express their gratitude to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I. R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran is gratefully acknowledged.

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Further reading Liau, C.P., Bin Ahmad, M., Shameli, K., Yunus, W.M.Z.W., Ibrahim, N.A., Zainuddin, N., et al., 2014. Preparation and characterization of polyhydroxybutyrate/polycaprolactone nanocomposites. Sci. World J. 2014, 9.

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X-ray diffraction analysis of layered double hydroxide polymer nanocomposites

5

Rodrigo Botan and Sabrina de Bona Sartor Unifacvest University, Lages, SC, Brazil

5.1

Introduction

The polymers are macromolecules consisting of repeating monomeric units. At present there is a wide variety of polymers with properties and applications in several fields of engineering. However, traditional polymers have limits of application that force their improvement or even the development of new materials that meet these needs. One of the alternatives for polymer materials improvement was the development of the polymer composites, which have as a definition: “Composite materials are a macroscopic combination of two or more distinct materials, having a recognizable interface between them” (Reinhart, 1987). In this combination, the constituents retain their identities and function together, which makes the properties of the composite better than that of each constituent individually. However, through the continuous development of the technology, polymer composites and neat polymers are no longer meeting some needs for improved properties, such as tensile strength, fracture toughness, high temperature resistance, and gas permeation resistance. In search of perfecting or developing new materials, research centers and industries around the world are creating and studying polymer nanocomposites in order to meet these needs. Nanocomposites are hybrid materials in which at least one of the components has nanometric dimensions. As in traditional composites, one of the components is the matrix, in which the particles of the second material (filler) are dispersed (Esteves et al., 2004; Wing Mai and Zhen, 2006). Among the many existing polymer nanocomposite types, those synthesized with layered double hydroxides (LDHs) have been highlighted, mainly because of the great versatility that these LDHs present in their synthesis and the properties of the new polymer nanocomposites. The properties that show improvement are mainly the mechanical properties, thermal properties, flammability properties, and reduced gas permeability (Wing Mai and Zhen, 2006; Wang and O’Hare, 2012).

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00005-7 © 2020 Elsevier Ltd. All rights reserved.

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The synthesis and characterization of polymer nanocomposites is a recent and developing area, so new methods of synthesis and properties never before reported are constantly presented in the literature. Among the most common methods used to synthesize polymer nanocomposites, it is possible to identify four routes: Melt mixing, exfoliation/adsorption, in situ nanoparticle synthesis, and in situ polymerization (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002). These polymer nanocomposites in all cases must be characterized using several characterization techniques, with the main objective of proving their effective synthesis, as well as identifying their morphologies and properties. One of the most important techniques used in the characterization of polymer nanocomposites is X-ray diffraction (XRD) analysis. Through this analysis it is possible to obtain valuable information about polymer nanocomposites, especially information on morphology. Nevertheless, this technique is also used to characterize the fillers used in polymer nanocomposites, LDH. Thus this chapter discusses this important technique of analysis, XRD, used to characterize LDHs polymer nanocomposites.

5.2

X-ray diffraction analysis

In 1895, Wilhelm Conrad Ro¨entgen, while working with cathode rays produced by Crookes tubes, discovered the X-ray. After publishing photographic observations of his wife’s hand, where the bones could be observed, X-rays were very quickly used to generate medical radiographies and technical applications. However, Ro¨entgen could not evaluate interference, reflection, or refraction effects (Guinebretie`re, 2007; Epp, 2016). During the period after the publication of Ro¨entgen’s discovery until the beginning of the First World War, many researchers studied X-rays and their applications, such as Thomson, Stokes, Rutherford, and others (Guinebretie`re, 2007). The properties of X-rays were gradually discovered: they propagated in a straight line, they were able to penetrate materials of different thicknesses, especially the less dense ones, they produced fluorescence in some materials and that they did not undergo refraction and also they were not reflected. Thus, through these studies, it was possible to expand the study of physical rays, such as cathode rays, X-rays, alpha rays, beta rays, and gamma rays. In April of 1912, Max von Laue, a German physicist, and Walter Friedrich, Arnold Sommerfeld’s assistant, along with Paul Knipping, one of Ro¨entgen’s students, irradiated a copper sulfate crystal with a polychromatic X-ray beam and observed on the photographic plate, for the first time, that X-rays can be diffracted by crystals. This experiment was the starting point of a new field of experimental physics, crystallography. Furthermore, Laue published several articles showing that the diffraction spots are distributed along conic curves and established the fact that X-rays are electromagnetic in nature. For this work, Max von Laue won the

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Nobel Prize in Physics in 1914 (Guinebretie`re, 2007; Glazer, 2013; “Max von Laue
Biographical,” 2017). Shortly after Laue’s studies, from 1912 to 1914, William Henry Bragg and his son William Lawrence Bragg, conducted some experiments using X-ray with NaCl and ZnS crystals. From the study of XRD in these crystals, W.L. Bragg deduced a formula that made it possible to calculate the position of atoms within a crystal through the form in which an X-ray beam is diffracted by the crystal lattice, and, in 1913, used XRD patterns to deduce the NaCl structure (Helliwell, 2013). From the knowledge of the NaCl structure (the first crystalline structure to be determined), W.L. Bragg derived an absolute X-ray wavelength and his father, W.H. Bragg, developed the X-ray spectrometer, allowing different types of crystals to be analyzed (Helliwell, 2013). The Nobel Prize in Physics 1915 was awarded jointly to Sir William Henry Bragg and William Lawrence Bragg “for their services in the analysis of crystal structure by means of X-rays” (“William Bragg
Biographical,” 2017). Currently, XRD methods are the most effective methods for determining the crystal structure of materials from different origins. Fundamentally, the phenomenon of diffraction is observed when a wave passes through an orifice or a slit, whose dimensions are the same size of magnitude. Due to its reduced wavelength, in the range of 0.01
10 nm, X-rays can only be diffracted by structures with atomic dimensions (Guinebretie`re, 2007; Leng, 2008). In crystalline structures, the dispersion centers (atoms or groups of atoms) are spaced periodically at fixed distances and are considered as three-dimensional diffraction networks for X-rays. When the radiation strikes the crystal structure, it is reflected by each of the planes of parallel atoms. Scattering occurs without modification of the wavelength (coherent scattering or Thomson scattering) or incoherently, that is, with a change in wavelength (incoherent scattering or Compton scattering). Due to the fact that it is coherent, the X-rays from Thomson scattering are responsible for the diffraction (Guinebretie`re, 2007). Diffraction theory, according to W.L. Bragg, explains that for certain directions and wavelengths, very pronounced peaks of intensity of scattered radiation were observed. Bragg assumed that the incident waves were reflected specularly by parallel planes of atoms of the crystal, and that the rays reflected from successive planes would produce constructive interference under certain conditions (Guinebretie`re, 2007). In a crystal lattice with the space between its planes, the optical path difference between the rays reflected by adjacent planes is given by 2dsinθ which should be equal to the integral multiple (n) of wavelength (λ) of X-ray used, for constructive interference to take place (Guinebretie`re, 2007; Leng, 2008). 2d:sinθ 5 n:λ

(5.1)

Eq. (5.1) is Bragg’s law, where θ is the complement of the angle of incidence and n is known as the order of the diffraction. Although the reflection in each plane is to speculate, only for certain values of θ will the reflections from all the parallel

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planes be added. For diffraction to be possible, the wavelength should be at most equal to twice the interplanar distance (λ # 2d). Bragg’s law also describes conditions for diffraction occurring, through the planes of a crystal. For graphical expression and interpretation, the Ewald sphere may be used, whose can predict diffraction pattern in a polycrystalline material, considered as an aggregate of the crystals with all possible orientations in threedimensional space (Guinebretie`re, 2007; Leng, 2008). Thus, X-ray scattering and Bragg diffraction provide information for X-ray diffractometry, a method based on the ability of crystals to diffract X-rays, making possible the study of the crystalline structures. Furthermore, morphological and structural information, like crystal atomic structure (positions and symmetry of the atoms in the unit cell), size and shape of the domain, identification of the crystalline phases, and quantitative determination of their weight fractions can be provided (Giannini et al., 2016). ˚ or 100 nm), the When the size of the crystals is less than 0.1 μm (1.000 A Scherrer equation or Debye
Scherrer equation (Eq. 5.2) is used to determine its size. This equation relates the size of submicrometer particles, or crystallites, in a solid to the broadening of a peak in a diffraction pattern (Burton et al., 2009; Uvarov and Popov, 2013). L 5 ðK:λÞ=ðβ:cosθÞ

(5.2)

where, L is the crystallite size, λ is the X-ray wavelength (nm), β is the width of the peak [full width at half maximum (FWHM) or integral breadth] after correcting for instrumental peak’s broadening (in radians), θ is the Bragg angle (in degrees), and K is the Scherrer constant. K is a constant related to crystallite shape, with typical value of about 0.9, and K values can be varied with the actual shape of the crystallite (Giannini et al., 2016; Burton et al., 2009). The use of the Scherrer equation allowed the development of the microstructural analysis of nano-scale particles and materials. Since the beginning of research with XRD in crystals, this technique has been developed and is widely used to characterize molecules of different origins, especially in engineering and materials science. XRD methods can be classified into two types: spectroscopic and photographic. The spectroscopic technique, known as X-ray powder diffractometry, or simply X-ray diffractometry, is the most widely used diffraction method. Photographic techniques are not widely used as diffractometry in modern laboratories (Guinebretie`re, 2007; Leng, 2008). The XRD instrument is called an X-ray diffractometer. There are a wide variety of types, configurations, and shapes of X-ray diffractometers, depending on the type of experiment being performed. Essential components of a typical X-ray diffractometer include: a source of X-rays, filters, mirrors and monochromators, and detectors (Guinebretie`re, 2007). X-ray sources are usually a sealed X-ray tube (Crookes tubes, Coolidge tubes, high-intensity tubes), where high-speed electrons collide with a metal target, or a synchrotron radiation. The main advantage of synchrotron radiation sources lies in

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the very high intensity of the X-ray beams they produce. After the X-rays source, a sequence of filters, mirrors, and associated monochromators are used, allowing efficient use of the divergent X-rays leaving the X-ray tube. Therefore, it can be possible to associate a single diffraction peak with each family of crystal planes and select one peak among all of those emitted by the tube. Detectors generate a pulse of current when they absorb an X-ray and the most commonly used types are single-photon detectors, linear (position-sensitive) detectors, area detectors, and Xray film (Guinebretie`re, 2007). A diffractometer presents, as a basic function, the ability to detect the XRD of samples and to relate the intensity of the diffraction with the diffraction angle (2θ) (Leng, 2008). Originally, in XRD analysis, powder samples were used. However, samples of crystalline aggregates and even liquid samples can be analyzed, since samples should contain a large number of tiny crystals (or grains) which randomly orient in three-dimensional space because standard XRD data are obtained from powder samples of perfectly random orientation (Guinebretie`re, 2007; Leng, 2008). Data acquisition in an X-ray diffratometer covers a range of diffraction intensity, from low to high values of 2θ. By continuously changing the incident angle of the X-ray beam, a spectrum of diffraction intensity versus the angle between the incident and the diffraction beam is recorded (Leng, 2008). Processing diffraction spectra, and the intensity peaks, where each peak represents diffraction from a certain crystallographic plane, it is possible to extract three essential values from each peak: the angular position, the integrated intensity, and the intensity distribution (Guinebretie`re, 2007). The diffraction pattern is unique for each crystalline compound, providing a “fingerprint” that allows the identification and characterization of the phases in polycrystalline or multiphasic materials by their diffraction patterns. The identification of the crystalline structure is performed by comparing the data from the obtained spectrum with a database containing over 60,000 diffraction spectra of known crystalline substances (Guinebretie`re, 2007; Leng, 2008). The principal advantages of the XRD technique for characterization of phases are simplicity, quickness, and accuracy of the results obtained, the possibility of quantitative analysis of composed or a mixture of materials or phases. W.L. Bragg’s papers, especially “The diffraction of short electromagnetic waves by a crystal,” published in 1912 and “The structure of some crystals as indicated by their diffraction of X-rays,” published in 1914, jointly with subsequent development of the X-ray spectrometer apparatus by W.H. Bragg, conducted a revolution in the scientific understanding of crystals and their atomic arrangements and was the beginning of the field of X-ray crystallography (Helliwell, 2013). Traditionally, XRD had been used in the field of materials science and engineering, particular qualitative and quantitative phase analyses, investigations of crystallographic textures, and residual stress measurements (Epp, 2016). Nowadays, XRD has been used by modern genetics, medicine, and biochemistry fields, and has been an important tool to establish the complete structures of crystals, from very simple NaCl to the most complex structures, such as proteins,

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viruses, and DNA. Since the first publications, around 20 or so Nobel Prizes in Physics and Chemistry have been awarded for research that has used the ideas described in W.L. Bragg and W.H. Bragg’s works.

5.3

X-ray diffraction analysis of layered double hydroxides and Modified Layered Double Hydroxides

Layered materials are a class of materials whose basic characteristics are structured or formed by crystals. These crystals form layers that stack two-dimensionally and hold together through weak bonds. Therefore, through this definition LDHs can be classified as layered materials (Arizaga et al., 2007). LDHs were discovered around 1842 in Sweden, the material discovered at that time is today known as hydrotalcite. After this discovery, a large number of minerals with similar structure to the new discovered material were reported and they were called by the mineralogists of the time “sjo¨grenite-hydrotalcite.” However, the exact formula of hydrotalcite, [Mg6Al2(OH)16] CO3.4H2O, was only reported in 1915 by Manasse (Wang and O’Hare, 2012; Manasse, 1915). The increase in the interest in LDH structure and synthesis occurred after the publication of a series of articles by Feitknecht, in which materials produced as “doppelschichtstrukturen” (double-layer structure) were identified (Feitknecht, 1938; Feitknecht and Gerber, 1942; Feitknecht, 1942). These materials have been described as a structure in which there is a layer formed by a metal hydroxide intercalated by another layer of a second metal hydroxide. However, this theory for the structure of these materials was refuted by Allmann (1968) and Taylor (1969), who after XRD analysis in a single crystal, proved that the same layer coexisted with the two cations (Khan and O’Hare, 2002). The development of studies and research on LDH since its discovery has made it possible for these materials at present to have a well-known chemical composition and structure. The chemical composition of these materials is represented by the following general formula: 

 M21 12x M31 x ðOHÞ2 ½Am- x=mUnH2 O

M21 represents a divalent cation, M31 represents a trivalent cation, Am
represents an m valent anion, and n represents the number of water molecules. A simple way to understand the structure of LDHs is to compare them with the structure of brucite, a mineral composed of magnesium hydroxide, which has the chemical formula Mg(OH)2. Structurally, the brucite presents magnesium cations located in the center of slightly distorted octahedra, which have hydroxyl anions at their vertices. These octahedra share edges forming a structure of plain and neutral layers, which are held together due to intramolecular forces (Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Crepaldi and Valim, 1998; Marangoni, 2009).

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When, in this layer structure, the isomorphic substitution of bivalent cations by trivalent cations occurs, the layer has a positive residual charge, but with the structure similar to the layer of brucite. This new structure does not satisfy the electrostatic valency principle and for the system, or layer, to become electrically neutral, the presence of ions (anions) between these layers (interlayer) is required. These anions, along with the water molecules, will promote the stacking of the LDH layers through weak forces in unorganized interlayer domains (Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Arizaga et al., 2007; Crepaldi and Valim, 1998; Marangoni, 2009; Tronto, 2006). The interlayer domain in the LDHs corresponds to the region between the adjacent inorganic layers. Studies show that in this region there are basically water molecules and anions. The amount of water molecules is determined by factors, such as the nature of the interlayer anion, water vapor pressure, and temperature (Khan and O’Hare, 2002; Brindley and Kikkawa, 1979; Brindley and Kikkawa, 1980). Another important structural parameter observed in LDHs is the stacking sequence of their structural unit. Studies have shown that there are two main forms of lattice, rhombohedral and hexagonal (Khan and O’Hare, 2002). Thus, through the options in the variation of chemical composition and structure a vast number of natural and synthetic LDHs have been produced and studied. This variation occurs mainly through the types and proportions of the metal cations used, as well as the type of interlayer anion used in LDHs. This vastness of existing natural and synthetic LDHs must be characterized by some characterization techniques. One of the most important characterization techniques for the structural study of LDHs is the XRD. In the first section of this chapter, the importance of XRD analysis was discussed and it was shown that one of the most important uses of XRD analysis is for the characterization of crystalline materials. As LDHs are crystalline materials, this technique is fundamental for understanding the structure of these materials, making it possible to identify information as atomic arrangements in the unit cell, unit cell parameters, and defects in the structure (Rives, 2001). Most LDHs that are removed from nature or synthesized are in the form of a fine powder. Thus the vast majority of XRD analyses occurs in the material in the form of powder or powder XRD method. In general, the methodology for XRD analysis in LDHs is presented in a simple way. The equipment that performs the XRD analysis, as already presented above, is a diffractometer. In this equipment a sample in the form of LDH powder is placed in the equipment. The parameters of the equipment are set according to what is desired and the analysis is started. Basically, the analysis consists of an X-ray beam emission in the sample, varying the angle of incidence. After the analysis, the equipment generates an XRD pattern. An example of an XRD pattern for an LDH is shown in Fig. 5.1. Thus, through a simple analysis of Fig. 5.1, it is possible to understand that the interpretation of the LDH structure is based on the positions and intensity of the peaks presented in the XRD pattern.

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Figure 5.1 XRD pattern for LDH formed by ZnAl cations and intercalated with chloride anion.

Interpreting the XRD patterns and consequently their peaks makes it necessary to understand what each peak means in the LDH structure. The peaks obtained in the XRD pattern for the structure of LDHs are called basal reflections. The basal reflections that present (00l) are directly related to the layered stacking, basal spacing (d) (Marangoni, 2009; Rives, 2001). The basal reflections (0kl), when present, are related to the organization of a layer relative to another layer of the LDHs. When basal reflections (hk0) are found, these refer to the organization of the atoms inside the LDH layers (Marangoni, 2009; Rives, 2001). Thus, through the identification and study of the basal reflections found in the XRD pattern for LDHs, it is possible, as already described, to characterize in detail all the structures of these materials. Fig. 5.1 show an XRD pattern for a synthesized LDH formed by Zn and Al cations (using the 2:1 ratio of Zn/Al) and chloride as the anion (ZnAl/Cl). The observed XRD pattern of this LDH is identified as an organized crystalline structure, it being possible to identify several basal reflections referring to all its crystalline structure. It is important to describe that when LDHs are used to synthesize polymer nanocomposites, these LDHs are mostly synthetic. This fact occurs mainly due to the ease of LDH synthesis, as well as the composition control and properties that these synthesized LDHs present. Therefore, in this chapter we focus on the presentation of XRD patterns for synthetic LDHs. However, natural LDHs can also be used in the synthesis of polymer nanocomposites. These natural LDHs are found in nature, and as already described earlier,

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it was the discovery of these natural LDHs that triggered the study of this materials class. Natural LDHs as found in nature present their XRD pattern, characteristics, and properties presented in mineral databases. Hydrotalcite, for example, has all its characterization presented in the American Mineralogist Crystal Structure Database Record (Downs and Hall-Wallace, 2003) or RRUFF Project (Lafuentes et al., 2015). In these databases it is possible to observe the XRD patterns for hydrotalcite originating from different regions of the world. When the LDH structures are studied with the main purpose of using them as nanofiller in polymer nanocomposites, the information of the basal spacing (d) of these materials is highlighted. The basal spacing is highlighted because LDHs present higher tunable charge density on layers, which causes a stronger interaction among the hydroxide layers and hinders the adsorption of monomers or polymers between these layers and consequently the synthesis of polymer nanocomposites with better morphologies and properties (Wypych and Satyanarayana, 2005; Botan et al., 2016). Thus, when LDHs are used as nanofillers for polymer nanocomposite synthesis, these LDHs are usually modified with ions that allow less interaction between the layers and consequently make it easier to adsorb monomers or polymers in LDHs. The basal spacing for LDHs is calculated using the angle obtained in the XRD pattern for the basal reflections relative to layered stacking and the Bragg’s diffraction law (Eq. 5.1). Generally, to obtain more precise basal spacing for LDHs, a mean of the basal spacings obtained from the different basal reflections of the XRD pattern is performed (basal spacing (d)
(d003 1 2d006. . . 1 nd00(3n))/n) (Xu et al., 2004; Xu and Braterman, 2010). The result of the basal spacing obtained for the LDH shown in Fig. 5.1 is ˚ . Through the basal spacing it is also possible to define the interlayer spacing 7.72 A ˚ ). Thus the value of the LDH, only removing the layer value from the LDH (B4.8 A ˚. of the interlayer spacing of ZnAl/Cl is 2.92 A This basal spacing result found for LDH, which has chloride as an anion, is a small basal spacing value and all the most common anions found in LDH, such as ˚ (Wang and fluoride, carbonate, nitrate, and sulfate, have values around 7
9 A O’Hare, 2012; Khan and O’Hare, 2002; Rives, 2001). The basal spacing of the ˚. hydrotalcite calculated by Downs et al. (1993) is 7.75 A These small anions with a small basal spacing do not allow a decrease in the interaction of LDH layers and consequently the effective synthesis of polymer nanocomposites. Thus, as previously mentioned, when it is intended to use LDHs as nanofiller for polymer nanocomposites, it is necessary to modify or synthesize these materials with different anions. The anions that have been most used and studied in LDH synthesis as nanofillers for polymer nanocomposites are sodium dodecyl sulfate (SDS), carboxylates, dodecyl benzenosulfonate (SDBS), bis(2-ethylhexyl) phosphate, and others (Rives, 2001; Botan et al., 2016).

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Figure 5.2 XRD pattern for LDH formed by ZnAl cations and intercalated with dodecyl sulfate anion.

When LDHs are synthesized with these anions a change in their structure and consequently the XRD pattern occurs. Fig. 5.2 shows an example of an XRD pattern for an LDH synthesized with ZnAl cations and with a SDS anion (ZnAl/SDS). In general, when XRD patterns for modified LDHs with different anions are analyzed, the basal reflections characteristic of the layered structure (layered stacking) of these materials can be observed. Fig. 5.2 shows the basal reflections of ZnAl/ SDS. However, the basal reflections regarding the organization of a layer relative to another layer and the organization of the atoms inside the LDH layers on the LDH structure are not easily identified. Thus, when XRD patterns are compared to LDHs with common anions, or unmodified, with modified LDHs, important information on the structure of these materials can be identified. In order to compare, identify, and explain the differences in XRD patterns for an unmodified LDH and a modified LDH, the examples presented in Figs. 5.1 and 5.2 are used. The first important information obtained from the comparison of XRD patterns is that although both present clear differences, the two XRD patterns present basal reflections characteristic of LDHs, showing that an effective modification occurred in one of the LDHs. The second important piece of information is that the modified LDH (ZnAl/ SDS) presents a lower crystallinity, or organization, when compared to unmodified LDH (ZnAl/Cl). The identification of all basal reflections for the modified LDH is not clear and, in general, the intensity of the basal reflections is smaller. The third important piece of information obtained is that the basal reflections of ZnAl/SDS when compared to ZnAl/Cl are shifted to values lower of 2θ ( ). Thus, in summary, through the information obtained by comparison of XRD patterns, it is possible to identify that modified LDH exhibits a lower crystallinity than

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unmodified LDH. This lower crystallinity is directly related to LDH modification, thus demonstrating that LDH was effectively modified with the anions used. The effective modification can also be identified by calculating the basal spacing of the modified LDH and also explains the displacement of the basal reflections to lower values of 2θ ( ). Calculating the basal spacing of the ZnAl/SDS in the same way as calculated for ˚ . The value of the interlayer spacing was 20.46 A ˚. ZnAl/Cl the result was 25.26 A Values much higher than those found for ZnAl/Cl, were found when the basal spac˚ . Table 5.1 presents the basal spacing for LDHs with different anions ing was 7.72 A reported in the literature. Fig. 5.3 shows more examples of XRD patterns for modified LDHs. In this figure an XRD pattern is presented for an LDH formed by the Mg and Al cations and modified (intercalated) with the SDS anion and another LDH formed by the

Table 5.1 Basal spacing (d) for LDHs with variation of the intercalated anions (Xu et al., 2004; Xu and Braterman, 2010; Nogueira et al., 2011; Gonc¸alves, 2012, 2015; Gonc¸alves et al., 2014; Meyn et al., 1990; Wang et al., 2009) Anions

Cations

MgAl CaFe ZnAl ZnCr CaAl

Phthalate ˚) (A

Terephthalate ˚) (A

SDS ˚) (A

SDBS ˚) (A

Laurate ˚) (A

Stearate ˚) (A



14.90 15.00 14.70

2 2 14.10 14.00 13.20

26.12 27.23 25.26 2 27.52

29.00 2 30.20 30.00 28.50

23.38 34.48 24.50 29.34 33.69

49.04 2 45.97 2 2

Figure 5.3 XRD patterns for modified LDHs with different anions.

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cations Zn and Cr, and modified with the anion laurate. Calculating the basal spac˚ for MgAl and ing (d) for these LDHs, the following values are obtained: 26.12 A ˚ for ZnCr, as shown in Table 5.1. 29.34 A This large increase in the value of the basal spacing for ZnAl/SDS, and other modified LDHs as presented, is due to the ion that was placed in the interlayer domain, which is due to its larger size expanding this region in the LDH. One can try to relate the exact size of the intercalated ion with the found basal spacing, however there is still no clear relationship between the values found, mainly because the intercalation of the ions in the interlayer domain can occur in different modes as reported in numerous scientific articles (Khan and O’Hare, 2002; Arizaga et al., 2007; Rives, 2001; Botan et al., 2016; Xu et al., 2004; Xu and Braterman, 2010; Kuehn and Porllmann, 2010; Nhlapo et al., 2008). When LDHs are used as nanofillers for polymer nanocomposite synthesis, the expansion of the interlayer spacing is important, because through this expansion the higher tunable charge density on layers, which causes a stronger interaction among the hydroxide layers is decreased, and consequently the probability of the monomer or polymer adsorbing in this space increases, also generating an increase in the probability of polymer nanocomposite synthesis with better characteristics and properties (Nogueira et al., 2011; Botan et al., 2012; Nyambo et al., 2008; Matusinovic et al., 2013). Thus the XRD analysis presents fundamental importance for the structural characterization of LDHs. When focusing the study of LDH for use as nanofiller in polymer nanocomposites this technique also has significant importance. Through this technique as presented it is possible to characterize the structure of natural or synthesized LDHs. It also allows identifying and confirming modifications that occur in LDHs with the main objective of making them more compatible with the polymer materials, besides identifying the structural characteristics of the modified LDH, helping to understand the possible characteristics and properties that the polymer nanocomposites can present.

5.4

X-ray diffraction analysis of layered double hydroxide polymer nanocomposites

Nanocomposites can be defined as the combination of a continuous phase (matrix) and other material (filler) that have at least one dimension in the nanometer range (Esteves et al., 2004; Wing Mai and Zhen, 2006). Nanocomposites are recently developed materials. One of the initial milestone versions for the study and development of polymer nanocomposites occurred with Fujiwara and Sakamoto (Unitcha Ltd, 1976), when these researchers produced a nanocomposite of polyamide 6 and clay (montimorilonite). Later, Toyota researchers, along with Fujiwara and Sakamoto, optimized this new material (Unitcha Ltd, 1976; Toyota Motor Co, 1988; McAdam et al., 2008). This new nanocomposite produced an extraordinary improvement in its mechanical, thermal, and physical

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properties when compared to neat polymer (Usuki et al., 1993). Since then, the new nanocomposites developed have presented significant gains, mainly in the mechanical properties, thermal properties, and flammability properties, when compared to neat polymers or traditional composites, besides using a small amount of nanofiller (Botan, 2014). Nanocomposites can be formed by two or more materials. These materials may in principle be identified as two types: the matrix or base material, with the main functions of keeping the reinforcements together, transmitting to them the applied stress and affording the final product shape, and the reinforcement or filler, which supports the request transmitted by the matrix. When a polymer material is used as a matrix, the nanocomposite can be called a polymer matrix nanocomposite or polymer nanocomposite (Botan, 2014). The use of inorganic reinforcements in polymer nanocomposites has been shown to be very promising, as may lead to nanocomposites with higher mechanical strength, higher thermal stability (Botan et al., 2012, 2016; Nogueira et al., 2011) or with better optical (Fogg et al., 1997), magnetic (Du et al., 1998) and/or electrical properties (Morais et al., 2003) than neat polymers or traditional composites. This improvement in polymer nanocomposite properties is primarily due to reinforcing nanoparticles, which have a high surface area when dispersed in the polymer matrix. This large surface area modifies the properties of the matrix or polymer, mainly due to the specific interactions that arise between the reinforcement and matrix. These interactions may influence the molecular dynamics of the polymer, resulting in significant changes in its physical properties, mainly in its thermal and/or mechanical behavior (Botan, 2014; Klabunde, 2001). The use of inorganic reinforcements in new polymer nanocomposites is very interesting, however various types of inorganic reinforcements that differ, for example, in morphological properties or in properties such as thermal resistance or chemical reactivity, may be used. Among the most common reinforcements used in composites and nanocomposites of polymer matrix with inorganic reinforcement are carbonates, aluminosilicates, and clays. Most of these works have focused on natural clay cation exchangers, but other natural and synthetic layered materials are gaining prominence (Botan, 2014). A layered material that is gaining prominence in the synthesis of polymer nanocomposites is LDHs. LDHs, besides allowing the synthesis of new polymer nanocomposites with better characteristics and properties, are considered materials of great versatility, easy production, and low cost. However, there is a fact of extreme importance that must be considered in polymer nanocomposite synthesis with LDH: The compatibility of the polymer and the LDH. This compatibility is necessary because the LDH (hydrophilic nature) has a low chemical affinity with polymers or monomers, in general, being predominantly hydrophobic. The compatibility of the LDH with the polymer matrix can be improved by chemical modification of the components (Botan, 2014). An outstanding type of LDH modification can be performed by the anion intercalation as discussed in Section 5.3.

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The production of polymer nanocomposites is a recent and developing area, so there is not yet an unambiguous classification for the different types of hybrid materials and respective synthesis methods. The synthesis of these materials, as well as their morphologies, are in constant development, thus enabling new methods and structures never previously reported to be developed (Esteves et al., 2004; Botan, 2014). Using the most common and generally described techniques for the synthesis of polymer nanocomposites, it is possible to identify four main routes. Melt mixing, exfoliation/adsorption, in situ nanoparticle synthesis, and in situ polymerization (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Botan, 2014). A brief description of the most common methods, the synthesis of nanocomposites from the melt mixing, as the name itself describes, consists in mixing the materials, polymer and reinforcement, in the molten state of the polymer material. For crystalline polymers, this temperature should be above the melting temperature (Tm) for amorphous polymers, above its glass transition temperature (Tg). Generally this process occurs in single or twin screw extruders. This method, until now, has been the most used in the synthesis of nanocomposites in the industry, mainly for nanocomposites that are reinforced with layered materials, because these present greater flexibility when compared with three-dimensional structures (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Botan, 2014). In exfoliation/adsorption, the reinforcement is exfoliated in a solvent, where the polymer to be used should also be soluble. This polymer is mixed with solvent and exfoliated reinforcement. In this mixture, the polymer tends to adsorb on the surface of the reinforcing layer and when the mixture undergoes an evaporation or precipitation process the nanocomposite is synthesized (Wypych and Satyanarayana, 2005; Botan, 2014). In the synthesis of nanoparticles in situ, the synthesis of nanoparticles involves chemical methods of controlled preparation of inorganic solids. The materials prepared according to this strategy generally have chemical bonds between the components, which results in more homogeneous and more consistent hybrids. There are two main methods for preparing nanocomposites by synthesis of nanoparticles in situ, they are: sol
gel reaction and synthesis in the presence of structuring materials (Esteves et al., 2004; Botan, 2014). The polymerization of the matrix in situ, occurs through the mixing of reinforcement already previously produced in a monomer. This monomer will adsorb the reinforcement structure and will subsequently be polymerized. The synthesis of nanocomposites by in situ matrix polymerization occurs according to the most widely used polymer synthesis routes, such as solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization. This strategy makes it possible to obtain a better dispersion of the reinforcements used, resulting in more homogeneous nanocomposites and, consequently, better processing (Esteves et al., 2004; Wypych and Satyanarayana, 2005; Khan and O’Hare, 2002; Botan, 2014). Thus, when polymer nanocomposites are synthesized, a uniform distribution of the reinforcements or fillers in the polymer matrix and a good adhesion at the

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interface of the two components is always sought, so that a synergism between the matrix and reinforcement occurs. The distribution and the form of the reinforcement in the polymer matrix are the morphology or structure of the polymer nanocomposites. The morphology of polymer nanocomposites can be classified or divided into three large groups. These three groups present profound dependence on the synthesis and process conditions by which these materials were subjected. Remembering that the reinforcement or filler used in this work is LDH, a layered material as already described, the definitions of the polymer nanocomposites morphology will be based on the use of the layered materials as fillers. The first group or class is called a microcomposite. In this group the layered compound does not delaminate. Delamination is the separation of layers from the layered compound in the polymer matrix. Thus there are aggregates of several intact layers of the layered compound distributed in the matrix. This morphology occurs mainly due to low or lack of affinity (miscibility) between the matrix and filler (Botan, 2014; Paul and Robeson, 2008). In this case the polymer or monomer, depending on the route of synthesis, cannot adsorb in the interlayer space of the layered compound. This structure shows the lowest gain in properties when compared to the other two morphologies. This structure also appears as the threshold between a nanocomposite material and a composite material. The second group is those named as intercalated nanocomposite. In this situation, the polymer or monomer is able to adsorb between the layers of the layered compound. Even though there is polymer between the layers of the layered compound, it is able to maintain its layered structure, which is only expanded. This structure is still not considered ideal, but presents considerable gains in properties when compared to neat polymers, traditional composites, and a microcomposition structure (Wypych and Satyanarayana, 2005; Botan, 2014; Paul and Robeson, 2008). The third group is the exfoliated nanocomposites. In this structure complete delamination of the layered compound occurs, its layers will be distributed randomly within the polymer matrix. In this case, there are nanosized layer particles increasing the surface area of contact with the matrix, resulting in stronger and larger interactions, allowing a wide synergism between the matrix and nanofiller. This structure presents the best gains in properties when compared to the other two structures discussed (Wypych and Satyanarayana, 2005; Botan, 2014; Paul and Robeson, 2008). However, these three structures are known and reported mainly in books or reviews for a better understanding of the polymer nanocomposites, but when studies are analyzed and developed with these materials, in the literature it is very common that polymer nanocomposites present a mixture of these structures, such as nanocomposites with intercalated and exfoliated structures at the same time. Nevertheless it is more usual to find nanocomposites with a morphology mix than fully intercalated or exfoliated nanocomposites (Botan et al., 2016; Nogueira et al., 2011, 2012; Botan et al., 2012, 2015; Botan, 2014).

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Layered Double Hydroxide Polymer Nanocomposites

Thus the study and identification of polymer nanocomposite morphology is of great importance for its characterization. The XRD technique is of great importance and is widely used for the study and identification of the polymer nanocomposite morphology. XRD analysis for polymer nanocomposites is usually performed via powder XRD method. Thus, if the nanocomposites are not produced in the form of powder, these materials generally must be crushed. However, there are some cases in which the crushing of nanocomposites becomes improbable and therefore other techniques besides the crushing of the nanocomposite and the use of it in powder can be used. For example, there are nanocomposites that exhibit high impact strength or glass transition temperature below room temperature, so in these cases it is necessary to perform cryo grinding. There are also nanocomposites with a preferred orientation of crystallites and these orientations are lost when the nanocomposite is crushed, so for these cases it is common to use small films or sheets for nanocomposite analysis. The methodology of XRD analysis for polymer nanocomposites is very similar to that for LDHs and is described in Section 5.3. It consists of placing the polymer nanocomposites powder in the diffractometer, setting the analysis parameters, and initiating the analysis. As a final result of the analysis, an XRD pattern will be generated for the analyzed polymer nanocomposites. The use of XRD analysis for neat polymers occurs mainly when one intends to study the crystalline structure of crystalline polymers. However, because the polymers do not exhibit a crystallinity as characteristic as the layered materials, their XRD patterns show differences. Fig. 5.4 shows the XRD patterns for different neat polymers, polyamide 6 (PA6), polymethyl methacrylate (PMMA), and polystyrene (PS). Comparing the XRD patterns of neat polymers with those of LDHs, presented in Section 5.3, the difference is clear. The polymers do not have the same crystalline

Figure 5.4 XRD patterns for polyamide 6, polymethyl methacrylate, and polystyrene.

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structure as LDHs, thus causing their XRD patterns to not show such intense and characteristic peaks. When comparing the neat polymers, it is possible to observe a differentiation, mainly between PA6 and the other two, PMMA and PS. This occurs because the polymers exhibit variation in their crystallinity. PA6 shows higher crystallinity when compared to PMMA and PS. Analyzing the XRD pattern for PA6 it is possible to identify two peaks in the   region of 21 and 24 degrees (2θ), which are attributed to α(200) and α(002/202) crystal planes, respectively (Peng et al., 2009). The XRD pattern of PMMA due to its small crystallinity does not have a wellcharacterized diffraction peak, but small deviations of linearity or halos. Thus, for this material its XRD pattern has two halos, one more prominent and centered around 15 degrees and another with less intensity and centered around 30 degrees (2θ) (Nogueira et al., 2012). The PS also has low crystallinity, so its XRD pattern has two small characteristic halos and is centered approximately in 10 and 20 degrees (2θ) (Botan et al., 2015). Thus, when XRD patterns of polymer nanocomposites reinforced with LDHs are analyzed, it is important to know the XRD patterns for each material individually, so that they help in the identification and study of the XRD patterns that will be obtained for the nanocomposites. The polymer nanocomposites are mostly composed of the polymer matrix, in general, the majority of studies do not present the use of more than 10% of LDH, so the polymer matrix corresponds to 90% of the whole nanocomposite. Therefore, when analyzing the XRD patterns of polymer nanocomposites they will present mainly the same global form of the neat polymer that was used as matrix. However, although XRD patterns of polymer nanocomposites follow those of neat polymers, it is possible to identify small differences that will suggest the possible morphologies that nanocomposites will present. The XRD patterns can be divided into three types or classes, which will be directly related to the three most common morphologies that polymer nanocomposites can present. In the first group the XRD patterns of the polymer nanocomposites present the general form of the neat polymers, however in these XRD patterns it is also possible to identify several peaks referring to the LDH basal reflections used as filler. The identification of the LDH basal reflections in the XRD patterns of polymer nanocomposites suggests that the layered structure of LDH remains intact within the matrix. Therefore, no type of adsorption of the polymer occurred on the interlayer spacing of LDH, showing that the affinity between the polymer and LDH was not high. Through the low affinity and presence of LDHs with their structure intact within the polymer matrix, this type of XRD pattern suggests the morphology of a polymer microcomposition, which also has a poor overall distribution of LDH in the polymer matrix. In the second group, the XRD patterns of the polymer nanocomposites, as well as the first group, presents a general form very similar to that of the neat polymer

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Layered Double Hydroxide Polymer Nanocomposites

that is used as matrix, but unlike the first group in these XRD patterns, it is not possible to identify several peaks referring to the basal reflections of LDHs. In this group, small reflection peak formations are identified mainly in low values of angles (2θ) in the XRD patterns. These small peak formations indicate that there is still some sort of organization of the LDH structure in the polymer matrix, suggesting that despite this small organization the vast majority of the layered structure of LDH was destroyed, suffering from delamination. Therefore, as the polymer was able to adsorb in the LDH structure, delaminating and/or expanding the LDH structure, the suggested morphology for this group is the intercalated. In the XRD patterns of these polymer nanocomposites, if a basal reflection peak characteristic of LDH is identified, it is possible to calculate the basal spacing value and compare it with the LDH spacing value, thus identifying how much the polymer has been able to expand the structure of the LDH. Fig. 5.5 shows an example of an XRD pattern for a polymer nanocomposite reinforced with a modified LDH (composition of 2% LDH), which shows the formation of a small peak. This suggests that the polymer nanocomposite has an intercalated morphology. Fig. 5.5 shows the XRD pattern of the neat polymer used in the matrix of this nanocomposite, the PS. In the third group the XRD patterns of the polymer nanocomposites are very similar to those of the neat polymers used as a matrix. In this case, unlike the others, there is no basal reflection peak which refers to the layered structure of LDH. Therefore, with the XRD pattern that does not present any peak regarding the layered structure of LDH, it can be suggested that in this nanocomposite a complete

Figure 5.5 XRD patterns for neat polystyrene and polymer nanocomposites formed by polystyrene and modified layered double hydroxide with a possible intercalated morphology.

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Figure 5.6 XRD patterns for neat polystyrene and polymer nanocomposites formed by polystyrene and modified layered double hydroxide with a possible exfoliated morphology.

breakdown of the layered structure of LDH has occurred. The polymer adsorbed into the interlayer space of LDH and ruptured its structure, thus demonstrating a great affinity between the polymer and LDH. Thus this characteristic of the XRD pattern suggests the exfoliated morphology for the polymer nanocomposites. The exfoliated morphology represents the complete disruption of the LDH layered structure, originating layers that will be randomly distributed in the polymer matrix. This justifies the absence of peaks in the XRD pattern referring to the layered structure of LDHs (Botan et al., 2016). Fig. 5.6 shows an example of a polymer nanocomposite with a possible exfoliated morphology, formed by PS and modified LDH, in which 3% of modified LDH was used in the polymer nanocomposite composition. Further examples of XRD patterns for polymer nanocomposites are shown in Figs. 5.7 and 5.8. Fig. 5.7 shows an XRD pattern for a nanocomposite formed by PMMA and a modified LDH, this nanocomposite presents in its composition 2% of the modified LDH. In this figure the XRD pattern for neat PMMA is also presented. As can be seen in Fig. 5.7 and following the XRD pattern analyzes for polymer nanocomposites described above, it is possible to identify that the polymer nanocomposite pattern is quite similar to the neat PMMA pattern, as well as no peaks being found in the XRD pattern of the polymer nanocomposite regarding the modified LDH. Thus it is possible to suggest that this PMMA/LDH nanocomposite presents a possible exfoliated morphology. Fig. 5.8 shows the XRD patterns for neat PA6 and a polymer nanocomposite formed by PA6 and modified LDH, using a composition with 3% modified LDH. Analyzing the XRD patterns, it is possible to suggest that this polymer nanocomposite may have an exfoliated morphology.

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Layered Double Hydroxide Polymer Nanocomposites

Figure 5.7 XRD patterns for neat PMMA and polymer nanocomposites formed by PMMA and modified LDH with a possible exfoliated morphology.

Figure 5.8 XRD patterns for neat PA6 and polymer nanocomposites formed by PA6 and modified LDH with a possible exfoliated morphology.

An important point to be identified for the analysis of the XRD patterns of polymer nanocomposites is their composition because, in general, polymer nanocomposites are synthesized with small amounts of LDHs. Polymer nanocomposites with amounts less than 1% LDH by weight may exhibit some peculiarities in their XRD patterns. Due to the small amount of reinforcement and the detection limit of XRD equipment, the XRD patterns for these nanocomposites may not show any peak relative to the LDH layered structure, however this absence of peaks may not be due to a

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breakdown of the LDH layered structure but due to the limit of detection of the equipment or an optimal global distribution of LDH in the polymer matrix (Nyambo et al., 2008; Matusinovic et al., 2013). Therefore, for polymer nanocomposites with low concentrations of LDH in their compositions, this alternative for interpretation of XRD patterns should always be considered. Thus the XRD analysis presents the fundamental importance for the characterization of polymer nanocomposite morphology. Through this analysis it is possible to identify indications of which morphology the polymer nanocomposite presents and consequently to relate its possible properties with its morphology. This technique also makes it possible to confirm the effective synthesis of polymer nanocomposites. However, for a complete analysis of the polymer nanocomposite morphology, it is desirable that besides the XRD analysis another technique of analysis be performed to confirm and complement the information obtained through XRD analysis. Generally the analysis used to complement XRD analysis for polymer nanocomposites is microscopy, specifically using a transmission electron microscope, which makes it possible to visualize the LDH structure in the polymer matrix. However, the transmission electron microscopy technique presents negative points, such as not very clear images, besides representing a small part of the studied material, generally a slice of average thickness of 100 nm. Other negative points in this technique are the considerable time spent in sample preparation and analysis, in addition to its high cost. Thus, characterization alternatives have been searched for to complement XRD analysis, a technique with this potential is X-ray microtomography (Pakzad et al., 2011; Awaja et al., 2011).

5.5

Conclusion

XRD is an analysis technique of great importance for the study of material structure. Since the discovery of this technique, it has been possible to develop a deep understanding of material structures. This technique is so important for the development of science and technology that about 20 Nobel Prizes in Chemistry and Physics are related to the ideas presented by the Bragg’s. In the characterization of LDH polymer nanocomposites the XRD analysis has fundamental importance. This importance is demonstrated, because through this technique it is possible to characterize the entire structure of LDHs, as well as the effective modification of these LDHs with the main objective of using them as nanofillers in polymer nanocomposite synthesis. XRD analysis also makes it possible to identify the polymer nanocomposite morphology. The identification of the polymer nanocomposite morphology is of great importance, because it makes it possible to understand all the behaviors and properties of these materials.

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In addition to all the structural and morphological characterization of all the materials involved in LDH polymer nanocomposite synthesis, and LDH polymer nanocomposites themselves, this analysis, when compared to other analysis techniques, is considered an easy-to-prepare, fast, and low-cost analysis. Therefore, XRD analysis is an indispensable characterization technique when it is intended to study, develop and/or understand LDH polymer nanocomposites.

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Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

6

Shadpour Mallakpour1,2,3 and Faezeh Azimi 3 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Research Institute for Nanotechnology and Advanced Materials, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Chemistry Group, Pardis College, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

6.1

Introduction

Layered compounds are an interesting class of material, consisting of a twodimensional sheet-like structure with strong bindings within the individual layer and weak van der Waals forces between the layers (Nejati et al., 2015; Nagendra et al., 2017; Naik et al., 2011). Layered double hydroxides (LDHs) are inorganic compounds composed of positively charged brucite-like layers in which some of the divalent metal cations are substituted via trivalent metal cations, producing positively charged layers (Veschambres et al., 2016; Tammaro et al., 2014; Mallakpour et al., 2015). The general formula of LDHs is [M211
x M31x (OH)2]x1 [(Am2)x/m. nH2O]x2, where, in this formula, M21 and M31 are divalent and trivalent cations, respectively, and An2 is usually an exchangeable anion. The quantity of M21, M31, and, An2 together with the value of x are adjustable, and thus the structure and behaviors of LDHs can be easily tailored (Fig. 6.1) (Wang et al., 2015a; Kang et al., 2013; Mallakpour et al., 2016). Lately, there has been a rapid evolution in methodology growth for the fabrication of polymer nanocomposites (NCs) comprising LDH, because of synergistic effects of this filler on mechanical, thermal, electrical, magnetic, and fire-retardant behaviors of various polymer/LDHs NCs (Purohit et al., 2014; Gaume et al., 2013; Mallakpour et al., 2014b). Polymer NCs are an original group of materials that can be created by incorporation of inorganic/organic nanofillers within the polymeric matrix. They exhibit unique physicochemical properties that cannot be obtained with individual components acting alone (Abdolmaleki et al., 2017; Hu et al., 2014). LDH-type materials present the potential for wide applications in the highperformance hydrogel, supercapacitor, ion exchanger, improvement in thermal Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00006-9 © 2020 Elsevier Ltd. All rights reserved.

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Figure 6.1 Schematic illustration of LDH structure and chemical components. Source: Adapted from Gu, Z., Atherton, J.J., Xu, Z.P., 2015. Hierarchical layered double hydroxide nanocomposites: structure, synthesis and applications. Chem. Commun., 51(15), 3024
3036, with kind permission of Elsevier.

resistance and flame retardancy of polymer composites, treatment of wastewater, photoluminescence, and preparation of organic
inorganic NCs because of their structural characteristics and diverse compositions (Lv et al., 2016; Mallakpour et al., 2014a; Basu et al., 2014). Due to its layered structure, large aspect ratio, diameter in the nanometer range, heat stability, and flame retardancy at low concentration, LDH is a potential candidate as a nanofiller for the fabrication of polymer/ layered material NCs. The efficiency of reinforced NCs with LDH is limited because of powerful interlayer electrostatic interactions between the sheets and important hydrophilic behaviors which are obtained from the high charge density of the LDH layers and the high anionic species and water molecules (Dinari and Mallakpour, 2015; Wang and O’Hare, 2012; Andronescu et al., 2014; Mallakpour and Dinari, 2015a). Chemical or physical modifications, based on the covalent/noncovalent bonding of functional groups on the LDH layers, are methods to facilitate the dispersion stability of LDHs. A wide diversity of anionic coupling agents, such as fatty acids, sulfonates, phosphates, and amino acids has been reported as modifiers for surface treatment of LDHs (Mallakpour et al., 2013; Kumar et al., 2012; Costa et al., 2005). The interaction between electromagnetic radiation and substances, as a function of wavelength, can be identified by spectroscopy. Displayed data in the spectrum are a plot of the response or variation of the interaction as a function of wavelength or frequency (Siddiqui et al., 2013; Njuguna et al., 2015). Here we report eight useful and practical spectroscopic techniques for the analysis and characterization of modified LDHs and polymer/LDHs NCs.

6.2

Spectroscopy of polymer nanocomposites

Due to the growing interest in NCs, molecular characterization of these materials is essential for understanding their properties and for the development of new

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materials. Spectroscopic techniques that bring information at a molecular level are unavoidable when characterizing polymers, fillers, and composites. The results of different analyses, such as Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, energy-dispersive X-ray (EDX) spectroscopy, fluorescence spectroscopy, dielectric spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, ultraviolet
visible (UV
vis) spectroscopy, and X-ray photoelectron spectroscopy (XPS) illustrate the surface of fillers, the state of filler dispersion in the host matrix, the extent of interaction between the polymer and the filler particles, or the dynamics of polymer chains at the polymer
filler interface. The potential applications of all these techniques can be summarized to solid-state structural simulations, mechanical, optical, and electrical properties of engineering materials, and quantum-chemical calculations of the electronic structure (Silva et al., 2012; Bokobza, 2017).

6.2.1 Fourier transform infrared spectroscopy FTIR spectroscopy is a powerful optical spectroscopy based on vibration measurements of an excited molecule by IR radiation at a specific wavelength range, which identifies the vibration characteristics of chemical functional groups in a sample (Ricci et al., 2015; Reichenb¨acher and Popp, 2012). FTIR spectroscopy provides information on molecular structures, chemical environments, orientations, and conformations of polymer chains. FTIR spectroscopy is one of the best methods for identifying molecular structure, such as functional group and bond, qualitatively according to the characteristic frequencies. In addition, FTIR spectra can determine the component content of the sample quantificatively according to band intensity (Chang and Tanaka, 2002).

6.2.2 Raman spectroscopy In Raman spectroscopy, the inelastic scattering of light is used to analyze vibrational and rotational modes of molecules. The recent method of coherent anti-Stokes Raman scattering possesses high sensitivity and is used for in vivo spectroscopy and imaging (Bokobza, 2017). Raman spectroscopy has the advantage of saving time when compared with other conventional methods. It is used to identify the structure, interface interactions, and physical properties of nanofillers, their functionalization as well as orientation. In a polymer NC, the interaction between nanofillers and polymers is reflected by a peak shift or a change in the peak intensity or width in the Raman spectrum. In other word, the shifts of the Raman band reveal mechanical deformation of nanofillers, polymer
nanofiller interactions, phase transitions of the polymer, stress state, as well as the Young’s modulus of the nanofillers (Yang et al., 2009).

6.2.3 Energy-dispersive X-ray spectroscopy EDX is a powerful method which has been utilized for the elemental investigation or chemical specification of a particular specimen. As a type of spectroscopy, it

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relies on the investigation of a sample by interactions among electromagnetic radiation and substance. The EDX method identifies the X-ray spectrum emitted by a sample bombarded via an electron beam to characterize the chemical composition of micro- and nanomaterials and shows the special distribution of the components through X-ray mapping (Michael and Iniyan, 2015; Mishra et al., 2017).

6.2.4 Fluorescence spectroscopy Fluorescence is a two-step process that requires absorption of light at a specific wavelength (excitation) followed by emission of light, usually at a higher wavelength. The emission of light is termed fluorescence. Fluorescence spectroscopy measures the change in the energy of the photons when the sample is exposed to high-energy photons which results in the emission of lower energy photons via the sample. It is an established technology for confocal microscopy, fluorescence resonance energy transfer, and fluorescence lifetime imaging in biochemical and medical fields (Albrecht, 2008). Fluorescence spectroscopy has a wide reach among polymer chemists and nanotechnologists. Their measurements can provide a wide range of detailed information regarding the molecular processes, including the solvent combinations in NC, rotational diffusion of molecules, distances between the nanofillers, conformational changes, and interaction of nanofiller with the polymer matrix (Goesmann and Feldmann, 2010).

6.2.5 Dielectric spectroscopy The dynamic measurements of nanoscale properties are performed with dielectric spectroscopy, which concludes the electric strength, complex dielectric constant components, and dielectric loss factor of numerous electric field frequencies and at various temperatures. The dielectric features of NCs are highly dependent on the filler content at lower frequencies. Further, alternating current and direct current dielectric spectroscopies were employed to determine the phase separation process, interactions between the divided phase, the influence of filler permittivity, filler purity (and conductivity), volume fraction, surface treatment, and processing situations (Krishnamoorti et al., 1996; Fumagalli et al., 2009).

6.2.6 Nuclear magnetic resonance spectroscopy NMR is a large range of phenomena associated with the interaction of electromagnetic radiation with materials. NMR spectroscopy involves putting a compound into a magnetic field and measuring the absorption of radio waves through the 1H, 13C, 19 F, 31P, or other nuclei. This analysis allows the observation of the special quantum mechanical magnetic behaviors of the atomic nucleus, especially for the macromolecular composite systems (M¨antylahti, 2014; Kitayama and Hatada, 2013). NMR is an effective tool to study polymers, their end-groups, branching,

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functionalization, and various properties. The use of NMR in characterizing the NC does not depend on the nature of the polymer or filler. However, in most of the composite cases, such as elastomers, thermoplastics, and biopolymers, NMR explains the polymer
filler interfacial interactions, nature of dispersion, interspatial distances of the filler particles, bonding, presence of voids, etc., in similar ways (Ponnamma et al., 2013; Postma et al., 2006).

6.2.7 UV
vis spectrophotometry UV
vis spectroscopy is a characterization method to evaluate the absorbance of samples at a certain wavelength. This technique is based on the principle of electronic transition in molecules or atoms, which is caused by absorption of light in the visible area of the electromagnetic spectrum (400
800 nm) under excitation of an electron from the ground state into a higher orbital. In UV
vis spectroscopy, there is a linear relationship between absorbance and absorber concentration, which causes this characterization technique to be especially attractive for making quantitative measurements. The UV ray can be separated into three parts by wavelength, as UV-A (320
400 nm), UV-B (280
320 nm), and UV-C (200
280 nm). UV-C and most of UV-B are absorbed by the ozone layer in the upper atmosphere, UV rays in sunlight reaching the ground are mostly UV-A (90%
99%), and a smaller amount of UV-B (1%
10%). The optical features of NCs based on the LDHs were investigated using UV
vis spectroscopy. This method was carried out to study the state of transition metals incorporated within the layered lattice and also changes the absorption features by intercalating the anions in the interlayer area of the LDHs (Permyakov, 2012; Yu and Xie, 2012; Peng et al., 2016). UV
vis spectroscopy is one of the most important characterization techniques to study the optical properties of polymer NCs. It helps to understand the interaction between the matrix and the nanofiller and analyzes the role of nanofillers in enhancing the property of the NCs. In addition, the UV
vis technique demonstrates the transparency, dispersion regions, refractive index, and optical band gap (Sharma et al., 2011).

6.2.8 X-ray photoelectron spectroscopy XPS is a surface analysis technique and is created by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that escape from the top 1
10 nm of the material being studied. This analysis probes a limited depth of the sample and provides both elemental and chemical state information of the elements that exist into a material (Haasch, 2014). XPS investigates surface modification of natural and synthetic polymers, carbon nanofillers, elemental chemical composition of a surface, and the bonding states of those elements, and is applied in the analysis of tuned catalysts, engineered polymer coatings, and nanoelectronic heterostructures (Yang et al., 2010).

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6.3

Layered Double Hydroxide Polymer Nanocomposites

Spectroscopic characterization of layered double hydroxide polymer nanocomposites

6.3.1 Fourier transform infrared spectroscopy of layered double hydroxide polymer nanocomposites FTIR investigates the existence of functional groups and probable interactions between inorganic lamellae and intercalated anion in LDH structures. The FTIR spectrum of LDH containing CO322 anions usually shows a broad band in the 3100
3650 cm21, which is related to the O
H stretching vibration of the metal hydroxide layer and interlayer water molecules. A characteristic shoulder at 3000
3100 cm21 is caused by the interaction between the anions (such as CO322) and H2O molecules, present in the interlayer region. The peaks located at the 1350
1380 cm21 could be related to CO322 antisymmetric stretching modes in the interlayer of the LDHs. A group of bands ranging from 400 to 680 cm21 is attributed to lattice vibration of metal hydroxide and metal oxides (Theiss et al., 2013b). Tsai et al. (2016) employed cocoamphodipropionate (K2) and sodium dodecyl sulfate (SDS), for surface modification of various types of LDHs (LiAl-LDH and MgAl-LDH) and the preparation of NCs based on poly(methyl methacrylate) (PMMA). The higher aspect ratio of LDH displays great efficiency in thermal, antiscratch, optical, and barrier gas in comparison with neat PMMA. The FTIR analysis of the multimodified LDHs and pure LDHs was performed to prove the creation of modified LDHs as shown in Figs. 6.2 and 6.3. The spectra of both pristine LDH samples displayed a broad absorption band at about 3500 cm21 owing to aOH stretching vibration of hydroxyl groups of LDH. Incorporation of K2 1 SDS in the LDH layers of treated LDH samples was demonstrated via the presence of new strong absorption peaks. A broad band is observed around 3500 cm21, which is ascribed to the aOH and aNH stretching vibrations of linked groups to LDHs. The characteristic peaks at 460 and 540 cm21 could be related to stretching vibration of metal oxides such as Mg-O and Al-O, respectively. The alkane chain (
CH2) and
NH bending of modified LDHs revealed the absorption peaks at 2920 and 1649
1550 cm21, respectively. On the other hand, the peak of CQO stretching vibration was shown at 1649 cm21, which overlapped and appeared in lower frequency due to H-bonding. Also, the vibration bands at 1032
1160 and 1200 cm21 were ascribed to the SO322 and C
O
C, respectively. In another research work, to enhance the distribution characteristics of LDH into the hosting polymer and change surface behaviors of LDH from hydrophilic to hydrophobic, surface modification of the LDH was done by Elbasuney (2015). This author introduced dodecanedioic acid (DDA) as an organic ligand and poly(ethylene-co-acrylic acid) as a polymeric surfactant to prepare polymer NCs with excellent flame retardancy and thermal properties. FTIR spectroscopy verified the successful attachment of the surfactants to the LDH layers. As shown in Figs. 6.4 and 6.5, due to the same crystalline structure of both LDH and organic treated LDHs, the region between 500 and 1500 cm21 (the fingerprint region) displayed intense and identical absorption of poly(ethylene-co-acrylic acid)-LDH and

Relative intensity (%T)

-SO4–2

(B) -C=O

-NH,-OH

-C-O-C -CH2

-CH2

(A)

CO2 Al-O-Al -CO3–

Al+3-O Mg+2-O

-OH 4000

3500

3000

2500

2000

1500

1000

500

Wavelength (cm–1) Figure 6.2 FTIR spectra of (A) MgAl-LDH and (B) MgAl-LDH-K2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. doi:10.1002/adv.21639, with kind permission of Elsevier.

Relative intensity (%T)

(B)

SO4–2 CH2 C=O C-O-C -NH,-OH

-CH2

(A) Al-O-Al CO3–

-OH 4000

3500

3000

2500

2000

1500

Al-O 1000

500

Wavelength (cm–1) Figure 6.3 FTIR spectra of (A) LiAl-LDH and (B) LiAl-LDH-K2 1 SDS. Source: Adapted from Tsai, T.Y., Bunekar, N., Liang, S.W., 2016. Effect of multiorganomodified LiAl-or MgAl-layered double hydroxide on the PMMA nanocomposites. Adv. Polym. Technol. doi:10.1002/adv.21639, with kind permission of Elsevier.

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Figure 6.4 FTIR spectra of poly(ethylene-co-acrylic acid)-LDH and uncoated LDH. Source: Adapted from Elbasuney, S., 2015. Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy. Powder Technol., 277, 63
73, with kind permission of Elsevier.

Figure 6.5 FTIR spectra of DDA-LDH and uncoated LDH. Source: Adapted from Elbasuney, S., 2015. Surface engineering of layered double hydroxide (LDH) nanoparticles for polymer flame retardancy. Powder Technol., 277, 63
73, with kind permission of Elsevier.

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Figure 6.6 FTIR spectra of the neat PAI and different NC materials. Source: Adapted from Mallakpour, S., Dinari, M., 2015b. Hybrids of Mg
Al-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the l-phenylalaninebased polymer nanocomposites. J. Therm. Anal. Calorim., 119(3), 1905
1912, with kind permission of Elsevier.

DDA-LDH in comparison with uncoated LDH. It can be seen that there is a significant difference in the infrared spectra (from 4000 to 1500 cm21) between organic treated LDHs and uncoated LDH which was related to the functional group area. The differences in FTIR absorption are due to the CQO and O
H stretching vibrations of the linked carboxylic surfactants. Mallakpour and Dinari (2015b) utilized hybrid nanofiller based on carboxylated multiwalled carbon nanotubes (MWCNTs) and Mg-Al LDH as a modifying agent by a simple coprecipitation method via ultrasonic radiation. Then, various loading amounts of LDH-MWCNTs were intercalated into the poly(amide imide) (PAI) and the obtained samples were studied by diverse techniques. Compared with the pure PAI, the FTIR spectra of PAI NCs were changed by intercalation of the LDHMWCNTs into the PAI matrix. Due to the presence of the LDH-CNTs in the PAI matrix, a new broad absorption band around 400
800 cm21 was observed which confirmed the formation of PAI/LDH-MWCNT NCs (Fig. 6.6).

6.3.2 Raman spectroscopy of layered double hydroxide polymer nanocomposites The Raman spectrum for LDHs containing MgAl illustrates strong and broad bands at 400
480 cm21, which are attributed to oxygen bonds of brucite-like layer,

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M
O
M as well as M
OH2-coordinated water. The vibrational mode of CO322 interacting with the hydroxyl groups of the brucite-like layer shows a weak band at 800
830 cm21. A faintly stronger band is displayed in the 2400 cm21, which is related to weak vibrational modes of adsorbed CO2 interacting weakly with the interlayer area (Balcomb et al., 2015). Nam et al. (2016) described a synthetic strategy to fabricate biomimetic multifunctional NCs in which a thin film of polydopamine (PDA) was deposited on the LDHs via interlayer polymerization. They used Raman spectroscopy to confirm the LDH intercalation. In this study, PDA was employed as an organic modifier and anion exchange facilitated via CoAl-LDH-NO3 (CAN) and MgAl-LDH-NO3 (MAN) as reactive precursors for creation of PDA/LDH NCs. The prepared NCs were denoted as CAN-PD1 and MAN-PD1 (the reactions of 0.2 mL of dopamine solutions with CAN and MAN, respectively) and CAN-PD2 and MAN-PD2 (the reactions of 0.1 mL of dopamine solutions with CAN and MAN, respectively). Raman spectra of the samples are illustrated in Fig. 6.7 which confirmed the intercalation of PDA into LDH. The spectra of NCs displayed a strong band at 1065 cm21, which is attributed to the symmetry vibration of the NO32. The band broadening for NO32 was seen, which corresponded to the presence of PDA, confirming enhanced disorder of LDH molecules via the intercalation. On the other

Figure 6.7 Raman spectra of PDA/LDH NCs: (A) CAN, (B) CAN-PD1, (C) CAN-PD2, (D) MAN, (E) MAN-PD1, and (F) MAN-PD2 (absorption band of X: silicon, ▼: Al-OH, K: PDA, ’: NO32). Source: Adapted from Nam, H.J., Park, E.B., Jung, D.Y., 2016. Bioinspired polydopaminelayered double hydroxide nanocomposites: controlled synthesis and multifunctional performance. RSC Adv., 6(30), 24952
24958, with kind permission of Elsevier.

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hand, the broad peaks at 1390 and 1590 cm21, which are observed in Raman spectra of NCs, are attributed to the vibrational modes of the carbon atoms arranged in graphitic-like domains and the aromatic CQC and C
N stretching mode of the basic indole structure. Furthermore, the phenolic C
O stretching and N
H bending created the small shoulder bands at 1215 and 1510 cm21. Shao et al. (2015) reported the design and production of a sophisticated nanoarray containing an LDH shell and a conducting polymer such as a polypyrrole (PPy) core, which was created through a two-step electrosynthesis technique. This product exhibited its great feature in high-efficiency flexible electrochemical capacitors. Raman spectra of PPy-LDH presented two peaks at 1559 and 1330 cm21, which are attributed to the π-conjugated structure and ring stretching mode of PPy backbone, respectively. Also, the new peak at 523 cm21 resulted from the stretching of OH
O bonds between CO322 and H2O in the interlayer of CoNi-LDH (Fig. 6.8). In order to remove U(VI) from aqueous solution, Yu et al. (2017) synthesized graphene oxide and NiAl-LDH (GO-LDH) NCs through a one-pot hydrothermal process. The obtained Raman data of GO-LDH NCs demonstrated two significant peaks in Fig. 6.9. The G band (located around 1580 cm21) is related to the vibration of sp2 banded carbon atoms in a two-dimensional hexagonal lattice. Another band at about 1350 cm21, called the D band, depicted the vibration of sp3 carbon atoms of disorder and defects. In addition, the 2D band (around 2700 cm21) is a broad and weak peak, which is attributed to disorder an account of an out-of-plane vibration mode. The intensity ratios of the D and G bands (ID/IG) helps to estimate structural defects and content of functional groups. The ID/IG ratio of GO-LDH NCs (1.008) shifted to a higher value than that of GO(0.926), which suggested a size reduction of the sp2 domains after LDH assembly.

Figure 6.8 Raman spectra of the LDH, pristine PPy and PPy-LDH. Source: Adapted from Shao, M., Li, Z., Zhang, R., Ning, F., Wei, M., Evans, D.G., et al., 2015. Hierarchical conducting polymer@ clay core
shell arrays for flexible all-solid-state supercapacitor devices. Small, 11(29), 3530
3538, with kind permission of Elsevier.

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Figure 6.9 Raman spectra of the GO-LDH. Source: Adapted from Yu, S., Wang, J., Song, S., Sun, K., Li, J., Wang, X., et al., 2017. One-pot synthesis of graphene oxide and Ni-Al layered double hydroxides nanocomposites for the efficient removal of U (VI) from wastewater. Sci. China Chem., 60(3), 415
422, with kind permission of Elsevier.

In other study, Huang et al. (2010) successfully fabricated exfoliated LDH nanosheet/CNT hybrids through electrostatic force. For this purpose, first, the positively charged LDHs (Co-Al-CO3 LDH) were prepared by precipitation via urea hydrolysis. Afterwards, the negatively charged CNTs (CNT-COONa) were prepared by nitric acid oxidation followed by treating with sodium hydroxide. Finally, the exfoliated LDH/CNT hybrids were fabricated through mixing positively charged LDHs and negatively charged CNTs. Raman spectra investigated the assembling process and nanostructure of the sample (Fig. 6.10). In the case of Co-Al-CO3 LDHs, a strong peak is observed at around 1059 cm21, which is attributed to the symmetry vibration peak of NO32. Raman data of pristine CNTs exhibited the D band (1300
1400 cm21) and G band (1500
1600 cm21), which are assigned to the disorder graphitic structure of nanotubes and tangential C
C stretching vibrations both longitudinally and transversally on the nanotube axis. In contrast, CNT-COONa displays the D band (defect/disorder-induced mode) at 1323 cm21 and G band (in plane stretching tangential mode) at 1573 cm21. Additionally, after nitric acid treatment, the ID/IG ratio for CNT-COONa (0.18) was higher than that of pristine CNTs. The enhancement of peak intensity may be attributed to an increase in the disorder in the nanotube structure during the oxidation process. The Raman spectrum of LDH/CNT hybrids showed a red shift in the G band and D band with an ID/IG ratio of 0.71. The strong interaction between positive LDH nanosheets and negative CNTs led to an increase in the energy necessary for vibrations to occur, which is reflected in the higher frequency of the Raman peak. The upshift in ID/IG

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Figure 6.10 Raman spectra of (A) Co-Al-NO3 LDH, (B) pristine CNTs, (C) CNT-COONa, and (D) LDH/CNT hybrids. Source: Adapted from Huang, S., Peng, H., Tjiu, W.W., Yang, Z., Zhu, H., Tang, T., et al., 2010. Assembling exfoliated layered double hydroxide (LDH) nanosheet/carbon nanotube (CNT) hybrids via electrostatic force and fabricating nylon nanocomposites. J. Phys. Chem. B, 114(50), 16766
16772, with kind permission of Elsevier.

ratio may be related to the strong interaction between the LDH sheets with local defects and the CNTs. Moreover, there is no NO32 band at 1059 cm21, thus suggesting that the negative CNTs replaced the NO32 anions to keep the charge balance of the LDH.

6.3.3 Energy-dispersive X-ray spectroscopy of layered double hydroxide polymer nanocomposites EDX analysis of the synthesized Fe-Ni-Co LDH indicated that the Fe/Ni/Co ratios for FeNi2Co2 LDH, Fe2-Ni5-Co3 LDH, Fe-Ni3-Co LDH, and Fe2-Ni7-Co LDH were 100:25:12, 100:29:10, 100:33:6, and 100:35:4, respectively. The Fe content in the Fe-Ni-Co LDHs is the highest, while the Co content is the lowest, which showed a notable difference between the material input ratio and real element content in the samples. The Fe content in all samples was nearly the same, and the Co content increased with an added amount of CoCl2, which was identified as 3%, 4%, 7%, and 9%, respectively (Gao et al., 2017). Nicotera et al. (2015) successfully synthesized hybrid membranes based on layer nanoadditives of the anionic clay family by solution intercalation. This class of nanosized materials includes Mg12/Al13 LDHs (at two metal ratios, 2:1 and 3:1) with diverse anions in the interlayer region (CO322, ClO42, NO32) which was incorporated into the Nafion matrix for the advancement of innovative hybrid NCs exploitable in the high-temperature polymer electrolyte membrane fuel cells. In order to measure the influence of Mg12/Al13-LDHs on the final membrane,

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Figure 6.11 EDX mapping of (A) filler-free Nafion membrane and (B) NC membrane loaded with 3 wt.% of LDH-ClO42. Source: Adapted from Nicotera, I., Angjeli, K., Coppola, L., Enotiadis, A., Pedicini, R., Carbone, A., et al., 2015. Composite polymer electrolyte membranes based on Mg
Al layered double hydroxide (LDH) platelets for H 2/air-fed fuel cells. Solid State Ionics, 276, 40
46, with kind permission of Elsevier.

fabricated hybrid membranes were characterized through different techniques. There is no overlapping of emissions between fluorine (contained in Nafion) and magnesium (contained in LDH) and gold employed for the coating, therefore these elements were selected for composite membranes. Fig. 6.11 corresponded to EDX maps of the pristine Nafion and Naf-LDH-ClO42 composite, a detailed analysis of these maps obviously displays the dispersion of fluorine (yellow spots) that can be attributed to polymer matrix and fluorine and magnesium (violet spots) corresponding to composite membrane, indicating that the Mg is well dispersed in the crosssection of membrane. In another work, Wang et al. (2014) prepared CuMoO4/Zn-Al LDH hybrids through an ion exchange and precipitation route. Subsequently, polypropylene (PP) composites with different loadings of CuMoO4/Zn-Al LDH hybrids as flameretardant nanofillers were prepared via a masterbatch-based melt. The presence of CuMoO4/LDH hybrids as flame-retardant nanofillers in the polymer matrix illustrates an essential role in improving the thermal and flame retardancy behaviors of the created NCs. For smaller particles, diffraction techniques can be performed using transmission electron microscopy (TEM) for identification, TEM-EDX as an advanced technique was used to characterize the synthesized CuMoO4/LDH hybrids, as shown in Fig. 6.12. Fig. 6.12A illustrates the image of LDHs with highly transparent thin layers which were well dispersed in alcohol without showing any noticeable aggregation. It can be evidently observed from Fig. 6.12B and C that nanocrystalline CuMoO4 was dispersed well on the LDH nanosheets. The presence of CuMoO4 and Zn, Al, Mo, and Cu elements is further displayed from the EDX of CuMoO4/LDH hybrids which these peaks confirmed the formation of CuMoO4/LDH hybrids (Fig. 6.12D). A variety of inorganic anions was intercalated into the Mg3Al-LDH (LDHs) as nanofiller for fabrication of NCs by Gao et al. (2013). In this contribution, it was concluded that properties of PP/LDH NCs significantly depend on the mentioned

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Figure 6.12 TEM observation of LDH (A), CuMoO4/Zn-Al LDH hybrids (B), (C) and the EDX analysis of CuMoO4/Zn-Al LDH hybrids (D). Source: Adapted from Wang, B., Zhou, K., Wang, B., Gui, Z., Hu, Y., 2014. Synthesis and characterization of CuMoO4/Zn
Al layered double hydroxide hybrids and their application as a reinforcement in polypropylene. Ind. Eng. Chem. Res., 53(31), 12355
12362, with kind permission of Elsevier.

inorganic anions. The elemental analysis of PP matrix and the LDH nanoparticles by EDX is exhibited in Fig. 6.13. The spectra of Mg and Al were detected in Fig. 6.13A, showing that it is Mg3Al-CO3 LDH, while only C can be seen in Fig. 6.13B. A scanning electron microscope (SEM) image and the elemental mapping Al and Mg of PP/Mg3Al-CO3 LDH 9 wt.% depicted a good dispersion of Mg3Al-CO3 LDH in the PP matrix as displayed in Fig. 6.14. In 2015, the impact of unmodified Zn-Al LDH mixed with ethylene propylene diene monomer (EPDM) rubber composites was investigated by Basu et al. (2016). In the present study, employing unmodified Zn-Al LDH, the thermal stability, mechanical properties, dynamic mechanical behaviors, flame retardancy, and rheological properties of EPDM composite were considerably enhanced. While any metal cations were not observed in the SEM/EDX analysis of the pure EPDM, the existence of Zn and Al atoms in the surface of EPDM composite with a content of 4 wt.% of Zn-Al LDH was confirmed by this analysis. Also, great incorporation of the LDH particles into EPDM rubber was depicted by EDX elemental mapping

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Figure 6.13 SEM-EDX analysis of PP/Mg3Al-CO3 LDH NCs. (A) EDX elemental analysis of the point representing LDH particles and (B) EDX elemental analysis of the point representing PP. Source: Adapted from Gao, Y., Wu, J., Zhang, Z., Jin, R., Zhang, X., Yan, X., et al. (2013). Synthesis of polypropylene/Mg 3 Al
X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites using a solvent mixing method: thermal and melt rheological properties. J. Mater. Chem. A, 1(34), 9928
9934, with kind permission of Elsevier.

of Al, Mg, S, and Zn atoms (Fig. 6.15A). The morphological characterization of EPDM filled with different contents of Zn-Al LDH is shown in Fig. 6.15B. It can be seen that the Zn-Al LDH particles in the submicron size were well dispersed in the EPDM matrix and the enhancement of inorganic filler amount did not lead to significant agglomeration. Due to the importance of the fabrication technologies for tissue engineering scaffolds, poly(ε-caprolactone) (PCL) is a well-known biopolymer which is employed as a fibrous scaffold in biomedical applications. Shafiei et al. (2016) prepared PCL-LDH NC scaffolds with different amounts of Mg/Al-LDH using an electrospinning technique. In Fig. 6.16, the EDX spectra of PCL-LDH NCs with 10 wt.% of LDH scaffold displayed low contents of Mg and Al atoms. In addition, it was demonstrated that the basic elements of the PCL are carbon and oxygen peaks. The EDX elemental map revealed a homogeneous dispersion of Mg and Al atoms within the fibers.

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Figure 6.14 (A) SEM image, (B) Mg mapping, and (C) Al mapping of 9 wt.% PP/Mg3AlCO3 NCs. Source: Adapted from Gao, Y., Wu, J., Zhang, Z., Jin, R., Zhang, X., Yan, X., et al. (2013). Synthesis of polypropylene/Mg 3 Al
X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites using a solvent mixing method: thermal and melt rheological properties. J. Mater. Chem. A, 1(34), 9928
9934, with kind permission of Elsevier.

6.3.4 Fluorescence spectroscopy of layered double hydroxide polymer nanocomposites The fluorescence spectra of a fluorescent anion (CPBA) and CPBA intercalated LDH (ZnAl-CPBA LDH) were investigated. The excitation spectrum and emission spectrum of CPBA both show a red shift. After intercalation, the emission spectrum of ZnAl-CPBA LDH remains unchanged with the maximum emission wavelength

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Figure 6.15 (A) SEM-EDX analysis of the pure EPDM and EPDM-4 LDH composite and the corresponding Al, Mg, S, and Zn-mappings, (B) SEM images of EPDM filled with different amounts of LDH. Source: Adapted from Basu, D., Das, A., Wang, D.Y., George, J.J., Sto¨ckelhuber, K.W., Boldt, R., et al., 2016. Fire-safe and environmentally friendly nanocomposites based on layered double hydroxides and ethylene propylene diene elastomer. RSC Adv., 6(31), 26425
26436, with kind permission of Elsevier.

located at 430 nm, while the excitation spectrum shows a slight blue shift from 364 to 352 nm, which can be attributed to intermolecular interactions between the excited guest and the LDH layers. The fluorescence lifetime of ZnAl-CPBA LDH is 4.83 ns, significantly longer than the value of 1.26 ns for pristine CPBA. The improvement of the fluorescence lifetime can be assigned to the high degree of organization of the CPBA moieties in the interlayer galleries of the LDH layers, which suppresses the thermal vibration and rotation of the CPBA anions (Wang et al., 2015b). Yan et al. (2010b) successfully fabricated (2-hydroxy benzo[a]carbazole3-carboxylate) (BCZC) intercalated LDH with different layer charge densities

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Figure 6.16 (A) FE-SEM image of PCL 1 10% LDH scaffolds. (B) The elemental distribution mapping of Mg (green dots) and Al (red dots) within the LDH containing fibers (C) EDX analysis of PCL 1 10% LDH scaffolds. Source: Adapted from Shafiei, S.S., Shavandi, M., Ahangari, G., Shokrolahi, F., 2016. Electrospun layered double hydroxide/poly (ε-caprolactone) nanocomposite scaffolds for adipogenic differentiation of adipose-derived mesenchymal stem cells. Appl. Clay Sci., 127, 52
63, with kind permission of Elsevier.

(LCD). They investigated the photoluminescence property of pristine BCZC aqueous solution, solid BCZC, BCZC in the LDH matrix with high (Mg/Al ratio 5 1.801 defined as sample A) and low (Mg/Al ratio 5 3.132 defined as sample B) LCD. Displaying data in the fluorescence emission spectra is related to pristine BCZC aqueous solution (10 μM), solid BCZC, sample A, and sample B (Fig. 6.17). The symmetrical emission peaks located at 499 nm with the full width at half maximum (FWHM) of 90 nm and at 517 nm with FWHM of 78 nm can be observed for the BCZC aqueous solution and its solid state, respectively. The effect of the π
π or dipole
dipole interaction of the conjugated BCZC molecule can create a red shift for the BCZC solid sample. Also, photoemission behavior of BCZC in the LDH matrix with high and low LCD is similar to that of the BCZC solid and aqueous solution state, respectively, indicating which luminescent features can be adjusted and controlled by regulating two LCDs (Mg/Al 5 1.801 and 3.132) of LDH.

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Figure 6.17 Normalized photoemission spectra for (A) pristine BCZC aqueous solution (10 μM), (B) solid BCZC, (C) BCZC/Mg
Al-LDH (Sample A), and (D) BCZC/Mg
AlLDH (Sample B). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Evans, D.G., Duan, X., 2010b. Benzocarbazole anions intercalated layered double hydroxide and its tunable fluorescence. Phys. Chem. Chem. Phys., 12(45), 15085
15092, with kind permission of Elsevier.

Yan et al. (2009) synthesized a sulfonated poly(p-phenylene) anionic derivate (APPP) and various exfoliated MgAl-LDH monolayers which were alternately assembled into ordered ultrathin films (UTFs) by a layer-by-layer method. The (APPP/LDH)n UTF (n 5 3
30) showed well-defined blue fluorescence and longrange order. Fig. 6.18A shows sharp emission peaks at around 415 nm (2.99 eV) of (APPP/LDH)n UTF, which are increased with rising “n.” In order to prove the obtained result, thin films were irradiated by UV light (Fig. 6.18B). The images display uniform bright blue luminescence the intensity of which is enhanced with an increase in the bilayer number “n.” Moreover, no red or blue shift of fluorescence spectra was detected from the UTFs, which proves a uniform distribution of APPP throughout the whole assembly processing. Compared with APPP pristine solution, an obvious blue shift (approximately 5 nm) occurred in the fluorescence spectra of UTFs without any broadening. This shift may be attributed to this consequence which the rigid framework of the LDH restricts the vibrations of the polymer backbone owing to nonbonding interactions. Also, they (Yan et al., 2010a) reported preparation of UFTs based on alternative layer-by-layer assembly of the sulfonated phenylenevinylene polyanion derivate (APPV) and exfoliated MgAl-LDH monolayers. The fluorescence peak at 2.25 eV (547 nm) of the (APPV/LDH)n UTFs (n 5 4
32) displayed an orderly growth of the UFTs upon increasing the number of deposition cycles (Fig. 6.19A). In comparison with the APPV pristine solution, the as-prepared UTFs excitation shifted, which indicates a homogeneous dispersion of APPV during the assembly process. Fig. 6.19B and C display irradiated thin films with UV light which can be depicted by homogeneous increased yellow luminescence of films with increasing “n.” Furthermore, according to the fluorescence lifetime values of (APPV/LDH)n UTFs (0.66
0.81 ns) compared to the pristine APPV solution (0.60 ns), it was concluded

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Figure 6.18 (A) Fluorescence spectra of (APPP/LDH)n (n 5 3
30) UTF and (B) photographs under 365 nm UV irradiation at different values of n. Source: Adapted from Yang, K., Gu, M., Guo, Y., Pan, X., Mu, G., 2009. Effects of carbon nanotube functionalization on the mechanical and thermal properties of epoxy composites. Carbon. N. Y., 47(7), 1723
1737, with kind permission of Elsevier.

that the LDH monolayers provided a confined environment for isolation of polymer chains and thus reduced the interlayer π
π stacking interaction. Anionic bis(2-sulfonatostyryl)biphenyl (BSB) especially can be used as a fluorescent brightener in the chemical industry; therefore, Yan and his co-workers (Yan et al., 2011) employed positively charged LDH nanosheets to assemble with BSB to obtain supramolecular UFTs. As shown in Fig. 6.20A, the fluorescence spectra of (BSB/LDH)n UTFs (n 5 4
32) have a maximum emission peak which is located at 444 nm with no shift or broadening of the emission peak. The obtained data demonstrated that no obvious change in intermolecular interactions of the BSB happen during the assembly process. Moreover, the thin films under UV illumination exhibited visible blue luminescence with enhanced brightness upon increasing “n” (Fig. 6.20B and C).

6.3.5 Dielectric spectroscopy of layered double hydroxide polymer nanocomposites LDHs containing Al or Ga as the trivalent ions and Mg or Zn as bivalent ions were investigated by broad band dielectric spectroscopy in a wide temperature range. Besides conduction effects, a relaxation process was observed which was assigned

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Figure 6.19 (A) Fluorescence spectra of (APPV/LDH)n (n 5 4
32) UTFs, (B) and (C) photographs of UTFs with different bilayer numbers under daylight and UV light (365 nm). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Wang, X., Evans, D.G., et al., 2010a. Anionic poly (p-phenylenevinylene)/layered double hydroxide ordered ultrathin films with multiple quantum well structure: a combined experimental and theoretical study. Langmuir, 26(10), 7007
7014, with kind permission of Elsevier.

Figure 6.20 (A) Fluorescence spectra of (BSB/LDH)n (n 5 4
32) UTFs, (B) and (C) are photographs of UTFs with different n under daylight and UV light (365 nm). Source: Adapted from Yan, D., Lu, J., Ma, J., Wei, M., Evans, D.G., Duan, X., 2011. Reversibly thermochromic, fluorescent ultrathin films with a supramolecular architecture. Angew. Chem., 123(3), 746
749, with kind permission of Elsevier.

to the reorientational fluctuations of water molecules adsorbed on the oxide surface or in the interlayer voids. A nonmonotonous temperature dependence of the relaxation rates of this relaxation process has been found. A quantitative description of this dependence was possible based on a model assuming two competing processes: rotational fluctuation of water molecules and formation of additional defects (Frunza et al., 2015).

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Purohit et al. (2012) employed modified LDH (ZnAl-LDH) with sodium dodecylbenzene sulfonate (SDBS) as a surfactant to prepare NCs based on polyethylene (PE). Here, the structure/property relationships of NCs based on PE and LDH are investigated mainly by dielectric spectroscopy, which is discussed in more detail below. In comparison with amorphous polymers, polymers with a high degree of crystallinity like PE have a dielectric behavior which follows a different nomenclature of relaxation processes. The glassy dynamic of amorphous polymers and polymers with a high degree of crystallinity are called α-relaxation and β-relaxation, respectively. Sometimes this causes confusion but because this nomenclature is well established, dynamic glass transition will be called β-relaxation here. The dielectric behavior of the PE and prepared NCs (PE2
PE16) with the different contents of ZnAl-LDH (2
16 wt.%) in the temperature domain at a fixed frequency of 103 Hz is displayed in Fig. 6.21. The dielectric response of pure PE is very weak because the asymmetry in the repeating unit of PE leads to no intrinsic dipole moment. The dielectric losses are weak due to the preference of impurities and defect in PE. Thus the dielectric probe technique was employed to investigate the molecular dynamics of polyolefins in detail. The isochronal spectra of neat PE illustrate numerous weak relaxation processes. A weak process of so-called α-relaxation is seen at high temperature, which is attributed to the crystalline lamella. Presumably, this process is due to a rotational translation of chain segments helped by a chain twisting. For NCs, in the temperature range around 275K, a process related to segmental oscillation in the disordered areas of PE is observed which is

Figure 6.21 Dielectric loss, ε00 , versus temperature, T, at a frequency of 1 kHz for PE (squares) and different NCs: PE2 (circles), PE4 (triangles), PE6 (inverted triangles), PE8 (rhombus), PE12 (stars) and PE16 (pentagons). Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

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shifted to higher frequencies with increasing temperature. At higher temperature, a further process is also observed in which its position and intensity are dependent on the concentration. This leads to the conclusion which this process is attributed to the existence of nanofiller. At first sight, this peak is attributed to an interfacial polarization (Maxwell
Wagner
Sillars polarization) which is originated by blocking of charge carried at the inner dielectric boundary layers on a mesoscopic length scale. In the case of NCs, this process is associated with Maxwell
Wagner
Sillars polarization and the effect of water absorption by the ZnAl-LDH. As can be seen, in comparison with the pure PE, the intensity of β-relaxation increases with increasing concentration of LDH. The increase in the measured dielectric loss is attributed to the concentration of polar molecules in the system. The only polar component in the system which increases with the concentration of LDH is the bulky anionic surfactant SDBS. In the polymer segments the alkyl tails of the SDBS are desorbed from the LDH surface and mixed with PE segments. This means the polymer segments near to the layers will oscillate together with the CH2 groups of the alkyl chains of the surfactant. Consequently, a remarkable increase in dielectric loss is observed with increasing concentrations of LDH. Due to the strong charge of the SDBS, this molecule is adsorbed at the exfoliated LDH layers. Thus dielectric spectroscopy selectively probes the mobility of segments located in an interfacial region near to the LDH sheets because the dielectric loss of the PE is low and so the matrix of the NCs is dielectrically invisible. Fig. 6.22 gives the dielectric loss for NC PE12 versus frequency and temperature in a 3D representation. The β-relaxation has a similar behavior to the pure PE at a lower temperature and higher frequency. At higher temperature and lower frequency regions, the Maxwell
Wagner
Sillars polarization is detected, which is not a relaxation process. The Maxwell
Wagner
Sillars polarization is very weak and so it was difficult to investigate. 00 Fig. 6.23 shows the variation of the dielectric constant (ε Þ of the NC PE6 versus frequency at the temperature 227K. As can be seen, two additional relaxation processes were identified. These processes are assigned to the molecular mobility of polymer segments to the surface of the LDH nanofiller. Process I at low frequency is related to the PE segments in close proximity to the LDH layers. Process II occurs at high frequency and is assigned to the oscillation of PE segments at a distance farther from the LDH sheets. In another research work, Purohit et al. (2011) focused on the fabrication and determination of dielectric behavior of new PP NCs (PE2
PE16) containing various amounts of organically modified ZnAl-LDH with SDBS (2
16 wt.%). The 3D representation of pure PP at a particular frequency depended on the temperature variation presented in Fig. 6.24. The asymmetry in the repeating unit of PP causes a low dipole moment which leads to reduced dielectric response of pure PP. As can be seen, the dielectric spectrum of pure PP has the main relaxation process, which is called β-relaxation. This process is assigned to the dynamic glass transition related to segmental fluctuations and is shifted to higher frequencies as the temperature increases. Moreover, due to the localized fluctuations, a γ-relaxation can be observed at lower temperatures (higher frequencies) than β-relaxation.

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Figure 6.22 Dielectric behavior of the sample PE12 versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

Figure 6.23 Dielectric loss versus frequency for the sample PE6 at T 5 277K. The dasheddotted lines correspond to individual relaxation processes. Source: Adapted from Purohit, P.J., Wang, D.Y., Emmerling, F., Thu¨nemann, A.F., Heinrich, G., Scho¨nhals, A., 2012. Arrangement of layered double hydroxide in a polyethylene matrix studied by a combination of complementary methods. Polymer. (Guildf)., 53(11), 2245
2254, with kind permission of Elsevier.

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–2.0 β-relaxation

–3.0

400 350 300 250 T( K) 200

150

–1

0

1

2

5

4

3

log ε''

–2.5

6

z)]

f(H

[ log

00

Figure 6.24 Dielectric loss ε of pure PP versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

Fig. 6.25 exhibits the dielectric spectrum of the NC PP16 in a 3D representation. The observed β-relaxation is similar to pure PP with an increased intensity in comparison with pure PP. At higher temperature, a dielectrically active process is observed which is not a relaxation process and may appear to be due to the preparation on NCs. Fig. 6.26 displays the dielectric behavior of the NCs containing various concentrations of LDH versus temperature at a frequency of 1 kHz. Compared to pure PP, the intensity of the β-relaxation process is strongly increased with the content of LDH. Also, the increase in the measured dielectric loss with the concentration of LDH is due to the increased content of the quite polar SDBS surfactant molecules, which increases with increasing concentration of LDH. The polar surfactant molecules are fluctuating together with the weakly polar PP segments and monitor the molecular mobility of the latter ones. Therefore, an increasing dielectric loss is observed with increasing concentration of LDH. Compared to the pure PP, for NCs the position of the β-relaxation is shifted by 30 K to lower temperature. According to the obtained result, the molecular mobility in the interfacial region between the LDH layers and the matrix is higher than that in the bulk unfilled PP. Therefore the glass transition temperature in the interfacial region is decreased compared to pure PP. Fig. 6.27 displays the dielectric loss of the NC PP16 at T 5 273.2K versus frequency. At higher frequencies, a well-defined loss peak can be observed. A more careful inspection of this peak shows that it has a so-called low-frequency contribution which originates from a further relaxation process. The two observed processes for the NCs are assigned to different regions of the molecular mobility of PP

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Process related to charge transport and/or percolation of electric exitations?

β-relaxation 2

log ε''

1 0

–1 –2

log

–2 0 2 4

z)] [f(H

6 150

250 300 200 T (K)

350 400

Figure 6.25 Dielectric behavior of the sample PP16 versus frequency and temperature in a 3D representation. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

00

Figure 6.26 Dielectric loss ε versus temperature T at a frequency of 1 kHz for PP (squares) and different NCs: PP2 (circles), PP4 (triangles), PP6 (inverted triangles), PP8 (rhombus), PP12 (stars), and PP16 (pentagons). Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

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Figure 6.27 Dielectric loss versus frequency for the sample PP16 at T 5 273.2K. The dashed-dotted lines correspond to individual relaxation processes. Source: Adapted from Purohit, P.J., Huacuja-Sa´nchez, J.E., Wang, D.Y., Emmerling, F., Thu¨nemann, A., Heinrich, G., et al., 2011. Structure
property relationships of nanocomposites based on polypropylene and layered double hydroxides. Macromolecules, 44 (11), 4342
4354, with kind permission of Elsevier.

segments depending on the distance from the surface of the LDH sheets. Process I appears at lower frequencies and is assigned to the PP segments in close proximity of the LDH layers. Their mobility is hindered by the strong adsorption of the polar head group of the surfactants at the LDH layers. Process II at higher frequencies is related to the fluctuations of the PP segments at a distance farther from the LDH sheets.

6.3.6 Nuclear magnetic resonance spectroscopy of layered double hydroxide polymer nanocomposites In 1H NMR the resonances attributed to lactate anion in the exfoliated MgAl-lactate LDH were observed at 1.32 ppm and 1.30 ppm (CH3), 4.09 ppm and 4.07 ppm (CH), and 3.33 ppm (C
OH), and in 13C NMR at 20.10 ppm (C3) and 68.55 ppm (C2), thus verifying the presence of lactate anion. In addition, in exfoliated MgAllactate LDH, the 1H NMR peak due to (C
OH) of the lactate anion is observed at a higher field compared to that of the pure lactate anion and indicates that the lactate anion is hydrogen bonded to the hydroxyl groups of the brucite-like layers (Indrasekara and Kottegoda, 2011). 11 B MAS NMR spectra of Mg/Al and Zn/Al type LDHs synthesized with interlayer triborate anions consisted of a singlet and a complex second-order quadrupolar broadened pattern. Peaks at 19.6 and 2.9 ppm were assigned to trigonal and tetrahedral boron, respectively. The relative intensity of the two boron peaks was between 2 and 3, which was consistent with intercalation of the triborate, which contains two trigonal and one tetrahedral boron atom (Theiss et al., 2013a).

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Tsai et al. (2014) employed MgAl-LDHs with different particle sizes, which were modified with a sulfanilic acid salt (SAS) via the hydrothermal method for the preparation of polyethylene terephthalate (PET) NCs. PET/MgAl LDH-SAS NCs were synthesized by intercalation of modified MgAl-LDH in the bis-hydroxy ethylene terephthalate (BHET) via an in situ polymerization route using MgAl LDH-SAS as a catalyst. The aim of this work was an examination of the effect of MgAl LDH-SAS on the thermomechanical, gas barrier behavior, and crystallization property of PET-based NCs. The chemical structures of the NCs were characterized by 1H NMR and 13C NMR, as shown in Fig. 6.28. These results confirmed the structure of PET and the presence of additional chemical bonds in the NCs which can be influenced by the adhesion between LDH nanofiller and PET molecules and

Figure 6.28 NMR spectra of PET NCs: (A) 1H NMR; (B) 13C NMR. Source: Adapted from Tsai, T.Y., Naveen, B., Shiu, W.C., Lu, S.W., 2014. An advanced preparation and characterization of the PET/MgAl-LDH nanocomposites. RSC Adv., 4(49), 25683
25691, with kind permission of Elsevier.

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do not do harm to the PET structures. The structure of PET NCs was further confirmed by protons of the polymeric CH2CH2 and C6H4 groups which are appeared as singlet peaks at 4.82 and 8.16 ppm, respectively. The integration ratio of these chemical shifts was exactly 1:1. However, neat PET and its NCs displayed the same peaks without any broadening or chemical shifts and/or the appearance of new peaks in the spectra. Thus, it is expected that linkages between BHET and SAS in LDH layers can happen during the ester interchange reaction, however, 1H NMR spectroscopy could not definitely identify extra peaks corresponding to bonds between SAS and PET. It seems that due to the very low content of LDH-SAS (1.0 wt.%), additional bonds between SAS and PET did not appear in the spectra. Finally, due to the results obtained from 1H NMR, it was demonstrated that there is no new chemical bond during loading of LDH to the polymerization. Hsueh and Chen (2003) synthesized LDH-amino benzoate (LDH-AB) by incorporating organic anions into the LDH. Subsequently, LDH-AB/polyimide (PI) NCs with different loadings of LDH-AB were successfully fabricated through the incorporation of LDH-AB in the PI matrix. In other words, the surface grafting of amino benzoate on the MgAl-LDH caused positive effects on the distribution and compatibility between inorganic MgAl nanolayers and organic PI matrix. The exfoliation of the MgAl nanolayers in the PI matrix to form LDH-AB/PI NCs was investigated by magic angle spinning 27Al NMR (27Al MAS NMR) spectrum and is shown in Fig. 6.29. The 27Al MAS NMR spectrum of LDH-AB in the narrow range of 210 to 120 ppm was referenced to the resonance of octahedrally coordinated aluminum within the brucite-like layers of the LDHs. The 27Al MAS NMR spectrum consisted of a signal resonance at 110 ppm, corresponding to octahedral coordination of all the aluminum atoms in the LDH-AB. Therefore NMR analysis indicated which LDH-AB with brucite-like layers were successfully fabricated. In 2005, Chen and Qu (2005) reported a facile approach to prepare NCs based on poly(methyl acrylate) (PMA) by in situ polymerization of methyl acrylate (MA)

Figure 6.29 27Al MAS NMR of LDH-AB. Source: Adapted from Hsueh, H.B., Chen, C.Y., 2003. Preparation and properties of LDHs/ polyimide nanocomposites. Polymer. (Guildf)., 44(4), 1151
1161, with kind permission of Elsevier.

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Figure 6.30 1H NMR spectra of (A) pure PMA and (B) PMA/OZnAl-LDH NC. Source: Adapted from Chen, W., Qu, B., 2005. Enhanced thermal and mechanical properties of poly (methyl acrylate)/ZnAl layered double hydroxide nanocomposites formed by in situ polymerisation. Polym. Degrad. Stability, 90(1), 162
166, with kind permission of Elsevier.

in the presence of Zn3Al(OH)8(C12H25SO4) (called OZnAl-LDH). Finally, the chemical structure of the PMA/OZnAl-LDH NCs was investigated compared with pure PMA. Fig. 6.30 shows the 1H NMR spectra of pure PMA and PMA/OZnAlLDH NC. The spectrum of pure PMA (Fig. 6.30A) displays chemical shifts centered at 3.46, 2.65, and 2.17
1.58 ppm which are assigned to the methyl, methane, and methylene protons, respectively, of the MA unit. The presence of the endstanding methyl group reveals a peak at 0.86 ppm. In Fig. 6.30B, the chemical shift of the PMA sample extracted from the PMA/OZnAl-LDH NCs is approximately the same as pure PMA. However, the small shift of position peaks is probably caused by the very small amount of OZnAl-LDH dispersed in the extracted PMA sample or the system error of the equipment. Also, the disappearance of peaks caused by CQC in MA monomer indicates the absence of MA molecules. In other words, the monomers absorbed in galleries of OZnAl-LDH have been converted to macromolecules after in situ polymerization. In another research work, Wang et al. (2005) presented PMMA NCs reinforced with MgAl-LDH as filler by two-stage in situ polymerization, where LDH layers had been modified with 10-undecenoate (LDH-U) to be well dispersed in PMMA matrix. The 27Al and 13C MAS NMR determined the structural and compositional details of the LDH-U. The 27Al MAS NMR spectrum of LDH-U demonstrated a signal resonance at 110.8 ppm. This resonance peak reveals that the aluminum atoms have an octahedral coordination geometry in the LDH-U. Therefore, the successful synthesis of LDH-U with a brucite-like layer was confirmed. Fig. 6.31 displays the 13C MAN NMR spectrum of the LDH-U. The spectrum exhibited three principal resonance peaks at 140.4 and 116.0 ppm, which are related to the carbon atoms of the vinyl group, Cb and Ca, respectively. The resonance peak of the

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.31 13C MAS NMR spectra of LDH-U. Source: Adapted from Wang, G.A., Wang, C.C., Chen, C.Y., 2005. The disorderly exfoliated LDHs/PMMA nanocomposite synthesized by in situ bulk polymerization. Polymer. (Guildf)., 46(14), 5065
5074, with kind permission of Elsevier.

carboxylic group appears at 185.0 ppm. Moreover, the presence of methylene carbons in the 10-undecenoate anion framework is observed in the region of 27
40 ppm. According to the literature, it is well established that the resonance peak at 170.6 ppm, corresponding to the interlayer charge balancing carbonate anion of the LDH, was not observed in the spectrum and disappeared. Restated, the interlayer region of the LDH sheets contains significant amounts of the 10-undecenoate anion. On the other hand, the resonance peak of Ck is slightly deshielded (shift downfield value) compared with sodium 10-undecenoate. The incorporation of the 10-undecenoate anion into the LDH layers, followed by strong electrostatic interaction between the carboxylic group and the inner surface of the LDH, causes a significant shift in this resonance peak.

6.3.7 UV
vis spectroscopy of layered double hydroxide polymer nanocomposites According to the literature, the broad absorption band of Mg/Fe LDH is due to the metal ligand charge transfer band of O2p !Fe31 and the M
M-charge-transfer spectrum of Mg21
O
Fe31 (Mohapatra et al., 2016). The Mn-containing LDHs with the lowest Mn amount displayed an intense band about 250 nm that can be attributed to the O22!Mn21 charge transfer transition. Mg/Al-NO3 LDHs and Zn/Al-NO3 LDH have absorption spectra and the maximum absorption wavelength results at 297 and 305 nm, respectively, which correspond with the existence of NO32 in the LDH interlayer (Peng et al., 2016). In the UV area of Mn-Ru LDH, the charge transfer O!M (MQMn or Ru) of the inorganic layers is superimposed upon the ligand transitions. The absorption band at about 272 and 367 nm is based

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on the direct charge transfer transitions from O22 2p to Mn21 3d (Dhanasekaran et al., 2017). The UV absorption intensity of ZnTi-LDH was nearly 10 times greater than that of MgAl-LDH and ZnAl-LDH at between 280 and 400 nm, which corresponds with the existence of Ti in the layers. The data displayed that the UV absorption intensity increase along with the enhancement of the Ti41 amount, demonstrating that the Ti component facilitates the absorption of photons and the obtaining electron transition (Wang et al., 2017). Hajibeygi et al. (2017) studied the influence of diacid-diimide modified Mg-Al LDH (DLDH) and sodium dodecylbenzene sulfonate (SDBS) modified Mg-Al LDH (SLDH) loading on the physical interactions and properties of developed NCs based on the PAI. It is clear from these results that high van der Waals interactions and hydrogen bonding between the modified LDH with a suitable modifier and PAI matrix caused a considerable increase in the thermal behavior of the PAI chain. The relative intensity of UV
vis spectra of PAI/DLDH (PAIDN) and PAI/SLDH (PAISN) was reduced by raising the LDH value. Solid-state UV
vis absorption of pure PAI and PAIDN 2, 5, and 8 mass% are displayed in Fig. 6.32. The absorption spectrum of pure PAI showed n!π and π!π transitions at 295
312 nm. These maximum absorption bands are ascribed to the atoms, such as oxygen, nitrogen, and also naphthalene rings, in the PAI structure. For LDH nanolayers, the maximum absorption bands at 450 nm are attributed to the metal charge transfer, which is caused by the 2p orbitals of oxygen to the 3d orbitals of Mg21 and Al31 ions (O2p!Mg3d, O2p!Al3d). The comparison of UV
vis spectra of PAI and PAINC showed that the maximum absorption bands were approximately in the range of

Figure 6.32 UV
vis spectra of PAI and PAIDN in the solid state. Source: Adapted from Hajibeygi, M., Shabanian, M., Omidi-Ghallemohamadi, M., 2017. Development of new acid-imide modified Mg-Al/LDH reinforced semi-crystalline poly (amide-imide) containing naphthalene ring; study on thermal stability and optical properties. Appl. Clay Sci., 139, 9
19, with kind permission of Elsevier.

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Figure 6.33 UV
vis spectra of PAI and PAINC at solution state. Source: Adapted from Hajibeygi, M., Shabanian, M., Omidi-Ghallemohamadi, M., 2017. Development of new acid-imide modified Mg-Al/LDH reinforced semi-crystalline poly (amide-imide) containing naphthalene ring; study on thermal stability and optical properties. Appl. Clay Sci., 139, 9
19, with kind permission of Elsevier.

400
460 nm. This region resulted from the transition electrons in the LDH layers and indicated the homogeneous dispersion of LDH nanostructures in the PAI matrix. Fig. 6.33 showed the UV
vis absorption spectra of virgin PAI and PAINC in the DMF solution. According to the related figure, the maximum absorption band of PAI solution at around 297 nm, is similar to solid state. Also, there are no maximum absorption bands of the LDH nanostructure in the spectrum of the PAINC solution, which is due to high dilution of the PAINC solutions. The absorption intensities were decreased by enhancement of LDH content. As can be seen, the UV
vis spectra of PAISN 2 and 5 mass% presented the lowest intensities. The strong interactions between PAI and LDH nanolayers are an important factor in this reduction. In another study, Shi et al. (2015) employed two surfactants, graphite-like carbon nitride (g-C3N4) and borate-modified LDH (LDH-B), for the preparation of PPgrafted maleic anhydride (PP-g-MA)/g-C3N4 and PP-g-MA/LDH-B NCs with 4 wt.% loadings. Among the nanoadditives, two-dimensional nanomaterials such as g-C3N4 with a stacked 2D structure have potential for significant improvement properties of NCs, especially by increasing the UV absorption capacity. On the other hand, the presence of LDH-B as nanofiller into the PP-g-MA matrix leads to a decrease in the optical behavior of the NCs (Fig. 6.34). The solution casting method in the presence of LDH layers and carboxymethyl cellulose (CMC) was carried out by Yadollahi et al. (2014). CMC-LDH NC films were fabricated with weight percentages of LDH from 0 to 8 wt.%. To evaluate the influence of LDH on the transparency of NC films, visible-light transmittance of

Spectroscopic characterization techniques for layered double hydroxide polymer nanocomposites

265

Figure 6.34 UV
vis spectra of LDH-B, g-C3N4, PP-g-MA and its NCs. Source: Adapted from Shi, Y., Gui, Z., Yu, B., Yuen, R.K., Wang, B., Hu, Y., 2015. Graphite-like carbon nitride and functionalized layered double hydroxide filled polypropylene-grafted maleic anhydride nanocomposites: comparison in flame retardancy, and thermal, mechanical and UV-shielding properties. Comp. Part B: Eng., 79, 277
284, with kind permission of Elsevier.

neat CMC film and CMC-LDH NC films as displayed in Fig. 6.35. For example, the optical transmission properties were measured under the visible-light region, NC films with LDH content lower than 8 wt.% illustrated a transmittance of 70%, while the optical behavior of NC 8 wt.% was notably decreased with increasing LDH layers. The reduction in transparency of NC films could be related to the defective distribution of LDH or the occurrence of partial aggregation. The results were completed by micrographs of pure CMC and NCs with 3 and 8 wt.% of LDH, which gives information about the distribution state. Figs. 6.36 and 6.37 show the TEM and SEM of the samples. These images clearly illustrate that CMC films without LDH exhibited a smooth and homogeneous surface. In the following, the dispersion state of NC films is indicated only by a small region of intercalated and agglomerated LDH layers in the NCs with 3 wt.% of LDH, but when the LDH amount was enhanced to 8 wt.%, the intercalation and aggregation of LDH layers increased slightly, leading to enhanced light scattering.

6.3.8 X-ray photoelectron spectroscopy of layered double hydroxide polymer nanocomposites XPS was employed in order to better characterize the chemical structure of the Zn-Al-PO432 LDH nanoparticles. The XPS full-survey spectra of Zn-Al-PO432 LDH show the peaks related to aluminum, carbon, and oxygen elements at binding

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Figure 6.35 Visible-light transmittance of pure CMC film and CMC-LDH NC films. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

energies of 72.4, 285, and 531 eV, respectively. Two peaks at binding energies of 1023 and 1045 eV are detected for zinc. Further, a peak corresponding to phosphorus element (P2p) at a binding energy of 133.8 eV was characterized. The observation of this peak represents the existence of phosphorus in the LDH structure. The deconvoluted spectra of the oxygen for Zn-Al-PO432 LDH demonstrate the highresolution O1s spectra in the range of 526
538 eV. The O1s spectrum was deconvoluted in three peaks showing three different oxygen species. The peaks centered at 530.55, 531.77, and 532.59 eV were ascribed to hydroxyl oxygen (aOH), the nonbridging oxygen (P
O) or Zn
O, and the bridging oxygen (P
O
P), respectively. The original O1s spectrum was then deconvoluted according to the assignments. These reveal that phosphorus was intercalated in the galleries of Zn-Al LDH. Also, there is a weak peak around 400 eV attributed to N1s, indicating the presence of a small amount of nitrate in the phosphate-intercalated LDH. The XPS results confirm successful loading of phosphate anions in the LDH structure (Alibakhshi et al., 2016). Huang et al. (2012) produced NCs based on ethylene vinyl acetate copolymer (EVA) as a host matrix mixed with a phosphorus nitrogen-containing compound, N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)hexylic)-acetamide-2-propyl acid (PAHPA) a modifier and LDH layers as nanofiller and obtained EVA/PAHPALDH NCs. They then investigated the effect of grafted PAHPA on the distribution and flame retardancy of LDH and EVA NCs. The XPS spectra of LDH are referenced to the O1s at 529.3 eV, C1s at 282.1 eV, Al2p at 71.8 eV, and Mg2p at 47.4 eV. The appearance of P2s, P2p, and N1s in the spectrum of PAHPA-LDH is due to nitrogen and phosphorus groups in modified LDHs. XPS of LDH revealed traces of carbonate group in the LDH layers with a 2.24% content of carbon, while the carbon amount of the PAHPA-LDH sample was around 6.33%. All of these

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267

Figure 6.36 SEM images of (A) CMC film, (B) the CMC-LDH NC film with 3 wt.%, and (C) CMC-LDH NC film with 8 wt.% LDH. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

results displayed favorable intercalation of PAHPA into the LDH layers by ion exchange (Fig. 6.38). Hu et al. (2015) reported an approach to increase the anticorrosion and flame retardancy of polyaniline (PANI). In this work, first, OH groups of decavanadate
intercalated LDH (D-LDH) were linked to ethoxy groups of γ-aminopropyltriethoxysilane (APTS) via covalent bonding, and finally, employed

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Layered Double Hydroxide Polymer Nanocomposites

Figure 6.37 TEM images of the CMC-LDH NC films with 3 wt.% LDH (A and B) and 8 wt.% LDH (C and D) at low and high magnifications, respectively. Source: Adapted from Yadollahi, M., Namazi, H., Barkhordari, S., 2014. Preparation and properties of carboxymethyl cellulose/layered double hydroxide bionanocomposite films. Carbohyd. Polym., 108, 83
90, with kind permission of Elsevier.

Figure 6.38 XPS spectra of LDHs and PAHPA-LDHs. Source: Adapted from Huang, G., Fei, Z., Chen, X., Qiu, F., Wang, X., Gao, J., 2012. Functionalization of layered double hydroxides by intumescent flame retardant: preparation, characterization, and application in ethylene vinyl acetate copolymer. Appl. Surf. Sci., 258 (24), 10115
10122, with kind permission of Elsevier.

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Figure 6.39 SEM images of (A) D-LDH, (B) AD-LDH, (C) PANI, and (D) AD-LDH/PANI, and (E) XPS survey spectra of AD-LDH and AD-LDH/PANI. Source: Adapted from Hu, J., Gan, M., Ma, L., Zhang, J., Xie, S., Xu, F., et al., 2015. Preparation and enhanced properties of polyaniline/grafted intercalated ZnAl-LDH nanocomposites. Appl. Surf. Sci., 328, 325
334, with kind permission of Elsevier.

(known as AD-LDH) for the preparation of PANI-AD-LDH NCs. The SEM micrographs of D-LDH, AD-LDH, and AD-LDH/PANI NCs are presented in Fig. 6.39. The morphology of the D-LDH seems to be aggregates of the plate-like particles due to relatively tight structural coherence in small particles. The images of ADLDH were changed by surface modification. The grafting agent caused a positive effect on the dispersion and, finally, reduction of agglomeration of clay particles. The SEM images of AD-LDH/PANI NCs illustrated well the distribution of ADLDH in the PANI matrix with the exfoliated structures. The XPS data of AD-LDH and AD-LDH/PANI are shown and summarized in the related figure and Table 6.1, respectively. The strong peak at 102.0 eV is due to the Si2p of AD-LDH, which demonstrated the existence of the APTS on the D-LDH surface as a source of silicon. No XPS signals from the Zn and Al were observed by this analysis. This result was in accordance with the relatively low concentration of the LDH in the

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Layered Double Hydroxide Polymer Nanocomposites

Table 6.1 The chemical composition of AD-LDH and AD-LDH/PANI determined from the analysis of the XPS data Sample

AD-LDH AD-LDH/PANI

C1s

O1s

N1s

Si2p

S2p

BE (eV)

BE (eV)

BE (eV)

BE (eV)

BE (eV)

At. (%)

At. (%)

At. (%)

At. (%)

At. (%)

284.1 70.61 284.6 73.45

531.6 26.05 531.3 14.63

399.1 2.41 399.6 9.06

102.0 0.93 102.2 0.27

165.1 2.59

AD-LDH/PANI NCs. The bonding energies related to the nitrogen 1s XPS spectra of AD-LDH and AD-LDH/PANI indicated the creation of a chemical interaction between AD-LDH and PANI matrix by a shift to the higher bonding energy. Gore et al. (2016) fabricated a new LDH/PAN NC via a plan of interweaving LDH with nanofibers based on polyacrylonitrile (PAN) film as an efficient adsorbent for Cr(VI) elimination from aqueous solution in an original column experimental plan. The characteristic peaks at 398.1 eV assigned to the CRN groups from the PAN chains are in agreement with the bonding energy value for the LDH/PAN NCs at 399.1 eV (Fig. 6.40A). The C1s spectrum of graphite carbon exhibited a peak at 284.7 eV (Fig. 6.40B). Also, the located peak at 532 eV (Fig. 6.40C) is attributed to the carbonyl oxygen atoms in amides, and also the oxygen atoms in hydroxyl groups. As such, XPS analysis of LDH/PAN NCs indicated the presence of whole elements from the LDH and PAN (Fig. 6.40D).

6.4

Spectroscopic characterization for the aging process

Since LDHs are an ultraviolet-light (UV)-resistant material, they were used to modify bitumen by Liu and his co-workers. The aging resistance of LDH-modified bitumen was investigated using a UV-aging oven. The viscosities of the base and modified bitumen before and after UV aging were indicated, and the addition of LDHs increased the viscosity of base bitumen. After UV aging, the viscosity of each bitumen also increased. However, the viscosity aging index (VAI) value for LDH-modified bitumen was lower than for the base bitumen. Moreover, the greater the content of LDHs, the lower the VAI value. LDH-modified bitumen at 5 wt.% had the lowest VAI value, and exhibited the best of UV aging resistance performance. Fig. 6.41A shows the penetration of base bitumen and LDH-modified bitumen before and after UV aging. As indicated, the addition of LDHs decreased the

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271

Figure 6.40 XPS spectra of PAN/LDH NC showing the presence of (A) nitrogen, (B) carbon, (C) oxygen, and (D) survey spectra of all elements. Source: Adapted from Gore, C.T., Omwoma, S., Chen, W., Song, Y.F., 2016. Interweaved LDH/PAN nanocomposite films: application in the design of effective hexavalent chromium adsorption technology. Chem. Eng. J. 284, 794
801, with kind permission of Elsevier.

Figure 6.41 (A) Penetration of bitumen before and after UV aging; (B) PRR of bitumen after UV aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

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penetration value of base bitumen because the LDHs made the bitumen more viscous. However, this value for the base bitumen became lower than that for modified bitumen after UV aging. As shown in Fig. 6.41B, the base bitumen had the lowest penetration retention rate (PRR) of 18.1%, indicating poor UV-aging resistance. After the addition of 5 wt.% LDHs, this value increased to 44.8%. This showed that the addition of LDHs can improve the UV-aging resistance of base bitumen. Fig. 6.42A shows the softening points of base bitumen and LDH-modified bitumen before and after UV aging. As indicated, the addition of LDHs increased the softening point value of base bitumen because the LDHs made the bitumen more viscous. However, this value for the base bitumen became higher than that for modified bitumen after UV aging. As shown in Fig. 6.42B, the base bitumen had the highest softening point increment (SPI) of 28 C, showing poor UV-aging resistance. After mixing with 5 wt.% of LDHs, this value decreased to 9.0 C. This indicated that the addition of LDHs can improve the UV-aging resistance of base bitumen. This corresponded well with the results of viscosity and penetration. Fig. 6.43 shows the FTIR spectra of the base and LDH-modified bitumens before and after UV aging. The UV-aging process can accelerate the oxidation of bitumens, and increase the peak area of carbonyl at 1700 cm21 and sulfoxide at 1030 cm21. After UV aging, the carbonyl index (IC5O) of base bitumen increased by 0.0184, and the sulfoxide index (IS5O) by 0.0370. However, the IC5O values of 3% and 5% LDH-modified bitumen only increased by 0.0163 and 0.0123, and the IS5O by 0.0290 and 0.0200. Therefore the addition of LDHs can inhibit the oxidation of bitumen during UV aging.

Figure 6.42 (A) Softening point of bitumen before and after UV aging; (B) SPI of bitumen after UV aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

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Figure 6.43 FTIR spectra of base and LDH-modified bitumen (A) before and (B) after UV-aging. Source: Adapted from Liu, X., Wu, S., Liu, G., Li, L., 2015. Optical and UV-aging properties of LDH-modified bitumen. Materials, 8(7), 4022
4033, open access journal.

6.5

Conclusions

In the recent progress, polymer/LDH NCs were characterized as one of the most promising investigations in chemistry and material science due to their twodimensional structures, enormous variety in physicochemical behaviors, and potential practical applications. Characterization of these NC materials is necessary to understand/analyze different facets of polymer NCs. This chapter introduced the various techniques of spectroscopy, which are available, for example, FTIR, Raman, XPS, UV
vis, EDX, Fluorescence, Dielectric and NMR for the investigation of polymer/LDH NCs.

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Layered Double Hydroxide Polymer Nanocomposites

Information obtained by these spectroscopic techniques includes the following: The chemical structure of samples, the attachment of the surfactants to the nanoparticle surface, the intercalation of modifiers in the basal spacing of the LDHs, and the existence of an intermolecular interaction between nanofillers and polymer matrix were confirmed by FTIR analysis. Also, Raman spectroscopy has been mainly employed to study the structural accommodation of interlayer species and the hydrogen bond network in LDH, particularly for oxo-anions such as CO322, NO32, SO422, CrO422, and organic carboxylate anions. The amount and distribution of nanoparticles and the elemental composition of NCs were confirmed by spectrum and elemental mapping using an EDX spectrometer combined with an SEM system. To characterize the optical behavior of samples, UV
vis and fluorescence analysis are performed which display the peak wavelength. The spectral bandwidth of UV
vis gave information such as particle size, shape, the material composition, and the local environment of NCs and nanoparticles. The dielectric behaviors of NCs as a function of frequency are measured by dielectric spectroscopy. Finally, empirical formula, the elemental composition, and electronic state of the elements in samples are identified by XPS technique.

Acknowledgments We thankfully acknowledge the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran for partial financial support. Further financial support from Center of Excellence in Sensors and Green Chemistry Research, IUT, is gratefully acknowledged. We also thank Mr. M. Hatami, Dr. V. Behranvand, Dr. F. Tabesh, and Dr. S. Rashidimoghadam from the Department of Chemistry, IUT, for their great help.

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749. Yang, K., Gu, M., Guo, Y., Pan, X., Mu, G., 2009. Effects of carbon nanotube functionalization on the mechanical and thermal properties of epoxy composites. Carbon. N. Y. 47 (7), 1723
1737. Yang, X., Tu, Y., Li, L., Shang, S., Tao, X.M., 2010. Well-dispersed chitosan/graphene oxide nanocomposites. ACS Appl. Mater. Interfaces 2 (6), 1707
1713. Yu, W., Xie, H., 2012. A review on nanofluids: preparation, stability mechanisms, and applications. J. Nanomater. 2012, 1. Yu, S., Wang, J., Song, S., Sun, K., Li, J., Wang, X., et al., 2017. One-pot synthesis of graphene oxide and Ni-Al layered double hydroxides nanocomposites for the efficient removal of U (VI) from wastewater. Sci. China Chem. 60 (3), 415
422.

Further reading Agu´, U.A., Oliva, M.I., Marchetti, S.G., Heredia, A.C., Casuscelli, S.G., Crivello, M.E., 2014. Synthesis and characterization of a mixture of CoFe2 O4 and MgFe2 O4 from

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layered double hydroxides: band gap energy and magnetic responses. J. Magn. Magn. Mater. 369, 249
259. Huang, N.H., Wang, J.Q., 2009. A new route to prepare nanocomposites based on polyvinyl chloride and MgAl layered double hydroxide intercalated with lauryl ether phosphate. Express Polym. Lett. 3, 595
604. Iyi, N., Matsumoto, T., Kaneko, Y., Kitamura, K., 2004. Deintercalation of carbonate ions from a hydrotalcite-like compound: enhanced decarbonation using acid 2 salt mixed solution. Chem. Mater. 16 (15), 2926
2932. Newman, S.P., Jones, W., 1998. Synthesis, characterization and applications of layered double hydroxides containing organic guests. New J. Chem. 22 (2), 105
115. Parida, K.M., Mohapatra, L., 2012. Carbonate intercalated Zn/Fe layered double hydroxide: a novel photocatalyst for the enhanced photo degradation of azo dyes. Chem. Eng. J. 179, 131
139. Zhao, Y., Liang, J., Li, F., Duan, X., 2004. Selectivity of crystal growth direction in layered double hydroxides. Tsinghua Sci. Technol. 9 (6), 667
671.

Melt rheological properties of layered double hydroxide polymer nanocomposites

7

Appukuttan Saritha1 and Kuruvilla Joseph2 1 Department of Chemistry, School of Arts and Sciences, Amrita Vishwa Vidyapeetham, Amritapuri, Kollam, Kerala, India, 2Department of Chemistry, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala, India

7.1

Introduction

7.1.1 The importance of rheological studies of polymer nanocomposites The most important milestone achievement in polymer technology has been the recent progress in polymer/layered nanocomposites. Polymer nanocomposites are promising candidates which unearth diverse applications in the vast arena of science and technology. Hence the information on polymer solutions is crucial for the polymer processing industry. Analysis of the rheology of polymeric melts encompasses the analysis of the mechanical behavior of the melt upon the exertion of mechanical stress or strain. The rheological behavior of polymers is of key significance for various polymer processing methods like extrusion, blow molding, injection molding, fiber spinning, and calendaring. Because of the growing economic importance for manufacturing of polymers, it is essential to consider the equipment and operational parameters for optimizing the design processes. The melt flow behaviors of composites encompassing particles are thoroughly dependent on how the particles aggregate and the interparticle, as well as the interaction of particles with the polymer. The behavior exhibited in this case can be significantly diverse from that of the unfilled melts (Barnes, 1997; Leonov, 1990). In reality, the analysis of rheological property is an imperative tool to explore the state of dispersion of the filler in polymeric systems and their response under external force. This can be considered equivalent to the techniques for morphological analysis like X Ray Diffraction (XRD), scanning electron microscope, transmission electron microscope, etc. One of the major advantages of this analysis is that it replicates the bulk properties of the matrix and also offers flow performance of the melt that are considered crucial in the melt processing of composites. Furthermore, it provides an awareness of the microstructure in the molten state. The flow behaviors are connected, generally qualitatively or at least semiquantitatively, with the dispersion of particles in the matrix. Conventionally rheological analysis is carried out by investigating the Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00007-0 © 2020 Elsevier Ltd. All rights reserved.

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viscoelastic response of a material in the linear flow regime, which means under very low shear strain or strain rate. As is known, polymeric melts are viscoelastic and their reaction to shearing is mainly dependent on the ratio between the timescales of shear experiments and the distinctive relaxation time of microstructures prevailing in those systems. In a high-molecular-weight unfilled melt, the term microstructure relates to molecular entanglements, whereas in a particle-filled system it refers to the structural relationship between the particles in the filler. Primarily, such microstructures indicate those structural features which physically function as a barrier against the flow of the polymer chains under stress. The system displays preferably an elastic response with an elevated value of storage modulus when the experimental timescale is far below the characteristic relaxation time whereas, at a large experimental timescale (experienced at low frequencies or shear rates), the system shows a viscous response. However, sufficiently high shearing actions can change and even exterminate these microstructures, which leads to an absolutely diverse material response. It is highly essential to have investigations in both these regions since they highlight the diverse mechanism of the reaction of the material towards external stress. The melts of polymeric systems are pigeon-holed by an acute strain beneath which stress and applied strain bear a linear relationship and their ratio (known as the relaxation modulus) shows a constant value independent of strain. Thus, the rheological behavior of polymeric melts below this critical strain to be a linear viscoelastic one. Above the critical strain, the relaxation modulus declines with strain, owing to changes in the microstructures, while stress becomes a nonlinear function of strain resulting in a nonlinear viscoelastic material response (Macosko, 1993). Consequently, the principal undertaking prior to performing rigorous rheological study is to define the point of transition between the linear and nonlinear regime of viscoelasticity. In order to determine this critical strain or a range about it, the polymeric melt is subjected to dynamic oscillatory shear using sinusoidal strain at constant frequency and varying strain amplitude. This serves to analyze the flow behaviors of the polymer melts devoid of terminating the interparticle interactions or network that may arise amongst them. The reaction of the fluid exhibiting viscoelasticity to nonlinear shearing (also known as flow reversal experiment) is employed to elucidate the development of structure amid the particles (Walker et al., 1995; Solomon et al., 2001; Li et al., 2003). To perform such an analysis, polymer melt is initially acted upon by a steady shear trailed by a definite relaxation period (shearing is stopped) followed by repeating the process in the opposite direction. It has been witnessed that development of any network structure is revealed in the stress shoot up in the flow reversal step. The time required to accomplish the stressshoot-up peak and its extent are absolutely dependent on the period of relaxation, rate of shear, and even the nature of dispersion of the particle in the melt.

7.1.2 Rheology of polymer layered double hydroxide nanocomposites Enhanced rheological properties for a system might be due to both particle
particle interactions and a polymer-based network where these two phenomena

Melt rheological properties of layered double hydroxide polymer nanocomposites

283

simultaneously contribute towards the reinforcement of the composite. As a result of complex interfacial interactions (polymer and particles) the molecular dynamics relaxation processes get reduced, which in turn suppresses relaxation, thereby affecting the performance of the nanocomposites and the nano-dispersion. Also, particle fractals within the polymer can disseminate stress and influence the system dynamics strongly. Nevertheless, it seems that the role of particle
particle interactions in inducing solid-like behavior is the more prevailing effect comparative to constrained layers, particularly at elevated concentrations and most notably for particles with active surfaces. Though the polymer
particle interactions are powerful, at concentrations higher than the threshold concentration, it is the formation of a network between the filler particles that provides the pseudo-solid rheological behavior at low frequencies. The percolation threshold depends on an assortment of parameters together with the morphology of the nanoparticles, the state of dispersion, molecular weight distribution of the polymer, and processing methods. Consequently, various values have been reported for percolation thresholds in diverse systems (Sinha Ray and Okamoto, 2003; Hussain et al., 2006; Habibi et al., 2010). It is noteworthy that a particle network is not inevitably the outcome of direct physical contacts between the fillers. Therefore, the reported rheological percolation threshold can be slightly less than the electrical percolation threshold. From a rheological perspective, a direct result of integration of filler in molten polymers is a noteworthy alteration in the resulting viscosity. The existence (or not) of yield stress is an important issue in polymer microcomposites (Hornsby, 1999), and therefore plays a key role in nanocomposites as these have even higher surface area. Below this specific stress, materials show solid-like behavior (i.e., they deform elastically) and, at stresses higher than the yield stress, the material behaves like a liquid. Studies conducted reveal that, in an approach analogous to the dynamic modulus, yield phenomena can also be correlated with the degree of particle interaction or polymer
filler interaction. Additionally, the accurate mechanism for yield stress in viscosity is also complicated. For example, in the case of clay, the percolation threshold was reduced and the modulus and viscosity at yield increased with increasing dispersion quality; while for silica exactly the opposite trend was observed (Jancar et al., 2010). The complexity of the interactions points toward the fact that depending on the system considered, the mechanism for solid-like behavior changes accordingly. The concept of percolation threshold concentration is very important from a practical perspective as it forms a crucial part in an assortment of applications such as mechanical reinforcement, electrical conductivity, flame retardancy, and permeability. Since the relaxation patterns change very strongly at percolation, a variation can be observed in many viscoelastic properties. Conversely, the storage modulus at low concentrations shows characteristic terminal behavior and, above percolation, a plateau is observed which increases with concentration. The rheological properties of the composites can be described by simple power-law equations (Winter and Mours, 1997) around percolation. These two types of interactions are displayed to an immense extent in the case of nanocomposites where the filler size falls in the nano regime. Undoubtedly, decreasing filler size and increasing surface area increase the tendency of the particles to interconnect. In addition, increased surface area of nanofillers enhances the probability of interfacial chain interactions. Hence micromechanical

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models fail to predict the resulting properties and excitingly particles with smaller sizes generally create stronger nanocomposites. In this context, it is apparent that the rheological analysis of nanocomposites requires mammoth attention and hence is considered decisive in the study of nanocomposites. Because of the diversity of particle morphology, the best candidate to be addressed is the aspect ratio of the filler counterparts, since a specific morphology must be defined accordingly (Kagarise et al., 2008). Although the part played by nanoparticles is significant, the relative size of the particle to the polymer chains is also another critical parameter influencing the viscoelastic properties. The literature suggests that when the radius of gyration of the tracer polymer (Rg) exceeds the radius of the nanoparticle (Rp), the subsequent reinforcement is much higher (Mu and Winey, 2007; Nusser et al., 2010). When the length of a polymeric chain is large, it can easily interweave with the particle and thereby amplify the time of relaxation of the polymer chain comparative to a shorter chain for which no entanglement with the particle is possible (Du et al., 2004; Picu and Rakshit, 2007; Anderson and Zukoski, 2009, 2010). Interestingly, most of the earlier studies were focused on polymer nanocomposites based on montmorillonite type of layered silicate clays. Lately, LDHs have emerged as an effective nanofiller for the preparation of polymer nanocomposites due to their tuneable properties and higher chemical purity. Moreover, polymer nanocomposites containing exfoliated LDH possessed more exfoliated clay layers as compared to layered silicate-based polymer nanocomposites, because each layer of the LDHs is comprised of a single octahedral sheet of mixed metal hydroxide, while layered silicates are composed of multiple octahedral/tetrahedral sheets of metal oxide/hydroxide (Leary et al., 2002; Lv et al., 2012). LDHs, generally termed anionic clays, constitute a type of anionic layered material consisting of positive brucite-like layers with an interlamellar space encompassing anions and solvation molecules to comthe charge. They possess the molecular formula of pensate x1 II III m
m2 MII12x MIII ð OH Þ ð A Þ  nH O, wherein M , M , and A represent a diva2 2 x=m x lent (or monovalent) cation, a trivalent cation, and the interlayer anion, respectively (Manzi-Nshuti et al., 2008; Zhao et al., 2009). Owing to their layered assembly and extraordinary anion exchange capacity, they are employed in numerous applications, such as catalysis (Wang et al., 2010), CO2 adsorbents, thermal stabilizers, UV absorbers, hosts for nanoscale reactions, and so on (Wang et al., 2012a,b, 2013a,b,c).

7.2

Rheology of thermoplastic polymer layered double hydroxide nanocomposites

Many thermoplastic polymers have been selected as the matrix for the incorporation of LDH-based fillers and they are found to exhibit excellent properties which make them befitting for quite a large number of applications. Polystyrene (PS) nanocomposites containing modified Co-Al layered double hydroxide (LDH) were prepared through a solvent blending technique (Suresh et al., 2016). The complex viscosity and rheological moduli of nanocomposites were found to be higher than that of

Melt rheological properties of layered double hydroxide polymer nanocomposites

285

pristine PS when the frequency increased from 0.01 to 100 s21. The storage modulus also showed increasing trend with a hike in the amount of LDH. The increase of storage modulus at lower frequency is the characteristic of pseudo-solid-like behavior due to the formation of network percolating LDH lamellae as seen in Fig. 7.1A. The growth of filler content in the nanocomposites causes it to change from a liquid-like to a solid-like nature. This transition concentration is called the rheological percolation threshold. The presence of a rheological percolation threshold in the nanocomposite samples can be credited to the development of incessant network of LDH and the polymeric chain. The same phenomena have been reported for PE/ MgAl LDH nanocomposites (Costa et al., 2006a,b,c) and polymer/layered silicate nanocomposites (Lim and Park, 2001). The nanocomposites show ascending complex viscosity values (Fig. 7.1B) with increasing concentration of LDH in the lower-frequency region, which slowly reverses as the frequency increases. The primary cause for this trend is adhesion between the LDH and PS and the cohesive interactions in the LDH layers. This also explains that the addition of LDH influences more frictional interactions. A changeover from Newtonian character to a shear thinning nature is also observed with increasing frequency, which is clarified by the point that the polymer chains have less time to entangle and the direction of randomly dispersed nanofiller at higher frequency. Since the elastic nature of the nanocomposites increases with increasing LDH content, it is seen that the loss factor of the nanocomposites is inferior to pristine PS and reduces with increasing LDH concentration, as seen in Fig. 7.2. In comparison with the storage moduli of samples, it is apparent that the loss modulus as seen in Fig. 7.3 is always higher than the storage modulus at lower frequency, indicating the dominance of the viscous part. Polypropylene (PP)/organomodified LDH nanocomposites were synthesized and the rheological properties investigated by Lonkar et al. (2012). It was observed that there is a robust effect of LDH particles on the flow performance of the composite melt which caused an augmentation of the rheological properties of nanocomposites. Figs. 7.4
7.6 show the disparity in the viscoelastic reaction of neat PP and

1000

(B) PS PS 1 PS 3 PS 5 PS 7

Complex viscosity (Pa-s)

Storage modulus (Pa)

(A) 10,000

100

10

0.01

0.1 1 10 Angular frquency (1/s)

100

PS PS 1 PS 3 PS 5 PS 7

1000

0.01

0.1 1 10 Angular frquency (1/s)

100

Figure 7.1 (A) Storage modulus versus angular frequency; (B) complex viscosity versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

10

Loss factor

PS PS 1 PS 3 PS 5 PS 7

1

0.01

0.1 1 10 Angular frequency (1/s)

100

Figure 7.2 Loss factor versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

Loss modulus (Pa)

10,000

1000

PS PS 1 PS 3 PS 5 PS 7

100

10 0.01

0.1 1 10 Angular frquency (1/s)

100

Figure 7.3 Loss modulus versus angular frequency of pristine PS and its nanocomposites (Suresh et al., 2016).

Figure 7.4 Difference in the viscoelastic response of unfilled polypropylene and high LDHfilled nanocomposite in a dynamic oscillatory frequency sweep experiment (Lonkar et al., 2012).

Melt rheological properties of layered double hydroxide polymer nanocomposites

287

Storage modulus (Pa), G'

104

103 PP PPL 1 PPL 3 PPL 5 PPL 7 PPL 10

102

101

100 0.1

1 10 Frequency (rad/s), ω

100

Figure 7.5 Storage modulus (G0) versus frequency (x) plots for unfilled PP and PP/LDH nanocomposite melts (Lonkar et al., 2012).

Figure 7.6 Complex viscosity (g ) versus frequency (x) plots for unfilled PP and PP/LDH nanocomposite melts (Lonkar et al., 2012).

nanocomposites containing high loadings of filler in a dynamic oscillatory frequency sweep experiment. In the experimental range of frequency, the loss modulus of the virgin PP was found to be higher than its storage modulus, which confirms that a dominant factor affecting the flow behavior of the unfilled PP melt is the viscous component. However, on increasing the frequency like a typical thermoplastic melt, the storage modulus of PP rose more than the loss modulus and at a certain

288

Layered Double Hydroxide Polymer Nanocomposites

high frequency, G0 crosses Gv. In contrast, the situation in PP/LDH nanocomposites is such that the storage modulus constantly exceeds the loss modulus demonstrating overriding elastic character of the material in the frequency range where the experiment is conducted. In the current situation, a high concentration of LDH in both the nano- as well as the microregime generates a robust physical barrier against the movement of the polymer chains and demonstrates that the liquid-like melt was gradually transformed into solid-like. The addition of LDH filler caused an upward shift of the low-frequency G0 values. The complex viscosity of nanocomposites increased with LDH concentration as illustrated in Fig. 7.6, and was attributed to the superior compatibility between the polymer matrix and LDH on account of its nano dispersion. The occurrence of nanostructured LDH particles in the melt not only enhances the melt viscosity but also provokes shear thinning nature in the lowfrequency region. The complex viscosity versus frequency plots for the polyethylene/LDH nanocomposites as shown in Fig. 7.7 were carefully analyzed by Costa et al. (2006a,b,c) and the figure clearly illustrates the traditional viscoelastic performance characterized by evolution from Newtonian at low frequency to shear thinning behavior at high frequency. The melt illustrates a changeover to pseudo-solid-like state from the liquid state and the extent of this transition with growing filler loading was studied by an index called shear-thinning exponent, “n.” In the case of Newtonian liquids, n is equal to zero and the neat polyethylene matrix acts more or less in a similar way with small negative value of n, while the nanocomposites demonstrate noteworthy divergence from this behavior. When the LDH concentration is increased, the negative value of n increases progressively in par with the budding

Complex viscosity,|η*| (Pa.s)

105

Shear thinning behavior

PE PEPB PE1LDH PE2.5LDH PE5LDH PE10LDH PE15LDH

104

103

Materials –n in|η*|~ ω PE 0.13 PEPB 0.04 PE1LDH 0.19 PE2.5LDH 0.23 PE5LDH 0.41 PE10LDH 0.71 PE15LDH 0.82

0

Newtonian behavior

102 0.1

1

10

100

Frequency, ω (rad/s)

Figure 7.7 Complex viscosity versus frequency plots for polyethylene/LDH nanocomposites. (PEPB is the blend of polyethylene and PE-g-MAH corresponding to the nanocomposite containing 15 wt.% LDH.) The variation of shear thinning exponent “n” with LDH concentration is also depicted (Costa et al., 2006a,b,c).

Melt rheological properties of layered double hydroxide polymer nanocomposites

289

shear-thinning nature which is distinctive of nanocomposites, where anchoring of polymer chains onto the surface of the particle leads to a decrease in relaxation (Costa et al., 2006a,b,c). Chakraborty et al. (2016) evaluated the dependence of frequency of PMMA nanocomposites containing Ni-Al LDH on the storage and loss modulus as depicted in Fig. 7.8. It is seen that at 190 C PMMA chains are completely stress-free in a relaxed state and display atypical homopolymer-like terminal performance provided the frequency is low. With a hike in frequency, there arises a corresponding enhancement in storage with a boom in the shear rate. When the shear rate shoots up, it is noticeable that there arises a perturbation in the long-time relaxation for all the filler loadings and LDH does not have much effect. The complex viscosity of PMMA and its nanocomposites analyzed at 190 C as displayed in Fig. 7.9 depicts the terminal relaxation zone. The viscosity diminishes

Figure 7.8 (A) Storage modulus and (B) loss modulus of PMMA and its nanocomposites (Chakraborty et al., 2016).

Figure 7.9 (A) Complex viscosity and (B) damping factor of PMMA and its nanocomposites (Chakraborty et al., 2016).

290

Layered Double Hydroxide Polymer Nanocomposites

at higher frequency as the chains begin to orient in the direction of flow and unscramble from each another. It is clear that the reduction in viscosity arises at high frequencies due to partial disruption of the three-dimensional complex patterns. Costa et al. (2005, 2006a,b,c) first reported the synthesis of intercalated LDPE nanocomposites using a melt intercalation using dodecylbenzene sulfonate
LDH as filler. An intricate feature was witnessed in the morphology of the nanocomposites, with the filler particles mostly positioned in the form of lean platelets and agglomerates in the dispersed state in the LDPE matrix. These materials portrayed a substantial variation of the linear viscoelastic response when compared with the matrix in the low-frequency region, which is attributed to the development of a threedimensional structure. The nature of the polyethylene matrix, that is, unmodified or MAH grafted, is of great concern with regard to its rheological behavior. To study the collapse and restoration processes associated with dispersed LDH phase in the LDPE systems, the nanocomposite melts were initially subjected to constant shearing until steady state is reached, and then subjected to reshearing after a short time. It is estimated that MAH-grafted polyethylene sticks to the surface of the filler with better strength than pure hydrocarbon chains. Study of the kinetics of the development of this type of structural changes under dormant conditions and their consequent breakdown could pave the way toward determining the nature of dispersion of the filler particles during processing. Fig. 7.10 illustrates the effects of particle
particle interaction and its kinetics due to nonlinear shearing of the LDH-based nanocomposites. Oleate-modified LDH was used as a filler in poly(butylene succinate) (PBS) nanocomposites by Zhou et al. (2010) and the flow constraint of PBS melt due to the occurrence of filler particles was effectively explained as shown in Fig. 7.11. Furthermore, the occurrence of a percolation threshold at an unusually low filler loading of 3
5% w/w was described by the survival of exfoliated platelets. When Mg2Al/CO3 was used as filler, the enhancement of the rheological properties was not as great as that for the organo-modified entity (Fig. 7.12), thus emphasizing the imperative role of the modifier in causing the shear-thinning behavior. It is noteworthy that the hanging groups fastened at the surface of the LDH platelets may generate some abrasion and the lamellar plates and the chains. The PBS chains occupy the lamellar gap of Mg2Al/oleate leading to a pseudo-exfoliation state at LDH loadings less than 5% w/w which possibly explains the occurrence of percolation threshold at lower loadings. Wang et al. (2012a,b) studied the effect of LDH nanosheets, [Mg2Al(OH)6] (DDS)-2H2O (Mg
Al
DDS) prepared using a one-step reverse microemulsion method in PP matrix and found that the reduced G0 and Gv as seen in similar phenomenon reports (Costa et al., 2008; Zhu et al., 2010) were attributed to the enhanced flexibility (relaxation) of restricted polymer chains at the interface of the PP
LDH layer. Such a relaxation behavior is characteristic of nanofillers, which well illuminates the nanosize of the LDH particles in the composites (Huwe et al., 1999; Baschnagel et al., 2000). The variation of moduli with frequency is depicted in Fig. 7.13. The traditional viscoelastic behavior (Zhu et al., 2011; Ugaz et al.,

Melt rheological properties of layered double hydroxide polymer nanocomposites

291

Figure 7.10 Response to nonlinear shearing of the LDH-based nanocomposites showing the effects of particle
particle interaction and its kinetics (Costa et al., 2006a,b,c).

1997) was customary in the nanocomposites till 8.0 wt% of LDH. The viscosity curve turned linear within the whole frequency range when the filler loading increases to 16.0 wt%. It is clear that the nanocomposites have entered into a state of rheological percolation from the evolution in the complex viscosity (Shenoy, 1999) as illustrated in Fig. 7.14. PET nanocomposites containing LDH were thoroughly analyzed (Lee et al., 2006) and the rheological properties were carefully examined after organically modifying the filler and incorporating it into the PET matrix. LDH having carbonate anion was organo-modified by various anionic surfactants such as dodecylsulfate (DS), dodecylbenzene sulfonate (DBS), and octylsulfate (OS) by a rehydration process. The Cole
Cole plots were analyzed and it was found that PET nanocomposites with 2.0 wt% of each of the modified fillers exhibit similar curves with a slope of about 1.27 indicating the heterogeneity. The viscosity curves in Fig. 7.15 display the information that although the viscosity of PET with all the modified LDH fillers demonstrates a shear thinning behavior from a low-frequency range,

292

Layered Double Hydroxide Polymer Nanocomposites

Figure 7.11 Frequency dependence of: (A) the storage modulus G0(x), (B) the loss modulus G0 0(x), and (C) the damping coefficient tan d of PBS, PBS:LDH/CO3 and PBS:LDH/oleate nanocomposite derivatives with different LDH/oleate loadings(expressed in % w/w) (Zhou et al., 2010).

the nanocomposites with LDH modified with DBS and OS reveal incessant curves exhibiting shear thinning due to the collapse of the structural network and slip between PET matrix and filler. It is also interesting to see that dodecylsulfatemodified LDH nanocomposites preserve the viscosity akin to a homo PET owing to the augmented interactions between filler and the PET matrix at high frequencies. LDH pillared by dodecyl benzenesulfonate (DBS) and modified with PANI/ DBSA prepared by solution adsorption method was melt-mixed with maleic anhydride modified polyethylene (PE-g-MA) and the properties of these nanocomposites were analyzed thoroughly (Kutlu et al., 2013). LDH was homogeneously dispersed throughout the matrix and accomplished a shear thinning exponent (STE) value of 20.84. An increase in filler loading also enhances the STE in a limited range, denoting the development of a denser network. Supply of an additional 5% of inorganic filler content does not smooth the progress of dispersion any further, as shown in Fig. 7.16.

Figure 7.12 Frequency dependence of complex viscosity of PBS, PBS:LDH/CO3 and PBS: LDH/oleate nanocomposite derivatives with different LDH/oleate loadings (expressed in % w/w) (Zhou et al., 2010). (A)

Storage modulus G' (Pa)

105

104 0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

103

102

10–1 (B)

100 101 Frequency (Hz)

102

Loss modulus G'' (Pa)

105

104 0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

103

10–1

100 101 Frequency (Hz)

102

Figure 7.13 (A) Storage modulus (G0 ) and (B) loss modulus (Gv) as a function of frequency for PP and PP/LDH nanocomposites (Wang et al., 2012a,b).

Complex viscosity (Pa.s)

0 wt% 0.5 wt% 1 wt% 1.5 wt% 2 wt% 4 wt% 8 wt% 16 wt%

104

103

102 100 101 Frequency (Hz)

10–1

102

Figure 7.14 Variation in complex viscosity as a function of frequency for PP and PP/LDH nanocomposites (Wang et al., 2012a,b). (B) 103

(A)

G' (Pa)

10

3

102 Eta' (Pa–s)

104

PET PET/MGALCO3 PET/MGALDS PET/MGALDBS PET/MGALOS

102 101

101

10

slope:2

0

100 101

102

103 G'' (Pa)

104

10–1 10–2

PET PET/MGALCO3 PET/MGALDS PET/MGALDBS PET/MGALOS

10–1

100 101 Freq. (rad/s)

102

103

Figure 7.15 (A) Cole
Cole plots, and (B) melt viscosity for PET nanocomposites with LDH content of 2.0 wt% at 270 C (Lee et al., 2006).

1,000,000 d 100,000 η (Pa-s)

c 10,000

b

1000 a 100 0.1

1 10 ω (rad/s)

100

Figure 7.16 Flow curves of PE-g-MA (A) and organomodified LDH/PE-g-MA conductive nanocomposites having 2.5% (B), 5% (C), and 10% (D) inorganic content. (The shear thinning exponent values in the low-frequency range are 20.09, 20.84, 20.91, and 20.94, respectively.) (Kutlu et al., 2013).

Melt rheological properties of layered double hydroxide polymer nanocomposites

295

In another study, poly(ethylene oxide co-propylene oxide-co-ethylene oxide) (PEO-PPO-PEO), a nonionic block copolymer, was utilized in an aqueous suspension of LDH for coagulating NBR latex. For understanding the effect of surfactant on the deagglomeration of LDH particles in aqueous media, the rheological behavior of the suspensions was analyzed and the particle size distribution, sedimentation testing, etc. were carried out. Owing to sturdier filler
rubber interaction there was a slight increase in viscosity of the latex suspensions (Fig. 7.17) (Braga et al., 2014) The role of LDH as a plasticizer was streamlined as a function of the intercalation of the nanocomposite to check how the intercalated LDH platelets behave with polarity of the polymer matrix (Nambo et al., 2008). The fact that the rheological behavior of a polymer is powerfully subjective to its molecular weight and molecular weight distribution (MWD) is made clear from the Cole
Cole illustration as

(A)

0.007 Dispersion water water/LDH water/LDHs

η (Pa.s)

0.006 0.005 0.004 0.003 0.002 0

200

(B)

400 600 • –1 γ (s )

0.010

1000

Suspension Latex Latex/LDH Latex/LDHs

0.008

η (Pa.s)

800

0.006

0.004

0.002 0

200

400 600 • γ (s–1)

800

1000

Figure 7.17 Viscosity of LDH aqueous dispersions (A) and latex/LDH suspensions (B) (Braga et al., 2014).

296

Layered Double Hydroxide Polymer Nanocomposites

shown in Fig. 7.18 (Colby et al., 1987; Mead, 1994; Leroux et al., 2014). The semicircular-shaped graph which is convex-shaped and corresponding to zero at x intercept, tallies to the Newtonian zero-shear viscosity (Fox and Flory, 1951; Friedman and Porter, 1975; Nichetti and Manas-Zloczower, 1998). It is interesting to notice that the intercalated structure of the composite led to a decline of the molecular weight. Another interesting study was conducted (Fu et al., 2010) in which nanoscale LDH-nitrate particles and sheets produced a strong effect on the rheological behavior of polyacrylamide solution followed by an interesting property, like formation of a gel at low filler loading. There was also a robust relationship between frequency and viscosity in the lower-frequency regime. This behavior can be credited with the formation of a network structure of the filler, which confines the motion of the polymer chains. Due to the spatially linked network structure there will be a lack of free rotation, and hence the relaxation of the structure is dramatically prevented. LDPE/LDH composites synthesized via melt compounding with an organomodified LDH and polyethylene-grafted maleic anhydride as the compatibilizer demonstrated an important fact that the length of the surfactant alkyl chain is an

(B)

1.2 103

18

1 10 η´´ (Pa.s)

2 102

40

200

3

12

1.5 102

10

8 102

100

η´´ (Pa.s)

(A)

8 14

6 102 0 4 102

200

600

16 PP 1000

20

16 14 10 12 8

18

PBS

1 102 0 5 101

100

200

2 102 0 0 0

2

2

2

2

3

0

3

5 101

2 10 4 10 6 10 8 10 1 10 1.2 10 η´ (Pa.s)

(C) 2 104

6000

2 102

18

4000

4

1.5 10 η´´ (Pa.s)

1 102 1.5 102 η´ (Pa.s)

PDMS 8

16 10 1214

10000

20000

2000 1 104 0 5 103 PDMS

0 0

3

5 10

4

1 10 1.5 104 η´ (Pa.s)

2 104

Figure 7.18 Cole
Cole ηv
η0 (ω) for LDH/Cn polymer composites: (A) PP, (B) PBS, and (C) PDMS; the values of n are indicated (Leroux et al., 2014).

Melt rheological properties of layered double hydroxide polymer nanocomposites

297

essential component in determining the dispersion as well as the rheological properties of the nanocomposites (Muksing et al., 2011). The strong dependence of interlayer ions on the rheology of LDH
EVA nanocomposites was studied in detail and the results demonstrate the variation in the
2 22 storage and loss moduli following the order of SO22 4 . NO3 . CO3 . Cl . Furthermore, both storage and loss modulus versus frequency curves for the nanocomposites with relatively high LDH particle loadings (17 wt%) approach the plateau at low frequencies which might contribute to the formation of an interconnected structure of the fillers or a strong particle polymer interaction. It is interesting to note that the consequence of the addition of LDH on the rheological behavior is relatively weak at high frequencies, demonstrating less sensitivity of LDH to affect the short-range dynamics of the EVA chains (Gao et al., 2016). A complete study was accomplished on PP/ethylene vinyl acetate copolymer (EVA)/organo-modified LDH composites to explore the consequence of LDH loading on small-amplitude oscillatory shear (SAOS) rheological properties and to correlate the properties with microstructure. The occurrence of LDH causes an increase in the storage modulus and complex viscosity, particularly in the terminal region. Additionally, the elasticity augmentation in EVA-rich hybrids was greater than the PP-rich counterpart, which can be well explained by the greater affinity of LDH to EVA than to PP. It was also elucidated from the study that the LDH platelets in the case of PP-rich samples are localized at the interface or within the EVA dispersed particles. On the other hand, in the case of EVA-rich samples, they are primarily confined inside the matrix (Aghjeh et al., 2017)

7.3

Rheology of thermosetting polymer layered double hydroxide nanocomposites

Thermosetting polymers constitute a significant class of materials that have accomplished extensive application in many areas. The prime benefit in the use of thermosetting polymers lies in the advantage of starting with a low-viscosity liquid before curing. This permits the creation of expendable adhesives and coatings with custom-made rheological properties. The characteristic features of thermosetting polymers is determined by their curing mechanism, which integrates the chemical reactions that govern both the chemorheology preceding gelation and subsequent development of an intricate crosslinked network upon complete cure. An organoclay of composition Zn2Al/LS was employed as an additive in poly(lactic acid) (PLA), poly(butylene) succinate (PBS), and poly(butylene adipate-co-terephthalate) (PBAT) (Hennous et al., 2013). An intercalated nanocomposite structure is obtained in the case of PLA and PBS. A robust upsurge of the complex viscosity |η | is observed for both PLA and PBS nanocomposites due to a chain extender behavior of the intercalated Zn2Al/LS platelets towards polymer chains, while an exactly opposite behavior is witnessed for PBAT owing to a plasticizing effect of the organoclay filler as perceived in Figs. 7.19 and 7.20.

298

Layered Double Hydroxide Polymer Nanocomposites 40

2500 Tan δ

30

|η∗| (Pa.s)

2000

20

b

10

a

0 0.1

1500

1

10 ω (rad/s)

100

1000 a 500

b 0.1

1

10

100

ω (rad/s)

Figure 7.19 Complex viscosity |η | versus ω for (A) PBAT and (B) PBAT-Zn2Al/LS. Inset shows the variation of tan δ versus ω (Hennous et al., 2013). 220 Tan δ

a

200

b

|η∗| (Pa.s)

180 0.1

160

1

ω (rad/s)

10

100

140 120

b

100

a 0.1

10

1

100

ω (rad/s)

Figure 7.20 Complex viscosity |η | versus ω for (A) PBS and (B) PBS
Zn2Al/LS. Inset shows the variation of tan δ versus ω (Hennous et al., 2013).

Dimethicones comprising of dispersed hydrotalcite-type materials previously organo-modified with amino acids were dispersed in silicone and the resulting nanocomposites were analyzed (Naime Filho et al., 2012). An exfoliated structure as revealed from XRD is confirmed by the drastic change in the rheological behavior from a quasi-Newtonian to an extensive developed gel-like structure for the nanocomposite derivatives as depicted in Figs. 7.21 and 7.22. The percolation threshold was noticeable at a filler loading of 5 wt%, signifying the occurrence of a fundamentally advanced interface between the added filler and the matrix. The upsurge of more than one order of magnitude in viscosity was clarified by the rather strong attrition between the fastened amino acid anions and the silicone chains. The

Melt rheological properties of layered double hydroxide polymer nanocomposites

299

(A) 1,000,000

|η∗| (Pa.s)

100,000

10,000 12.5 1000

100 0.1

0 1

ω (rad/s)

10

100

(B) 1,000,000

|η∗| (Pa.s)

100,000

10,000 12.5 1000

100 0.1

1

ω (rad/s)

10

100

0

Figure 7.21 Complex viscosity versus ω for (A) Mg2Al/Gly x: PE-100 and (B) Mg2Al/Ala x: PE-100. The percentage hybrid LDH filler loading x is indicated (Naime Filho et al., 2012).

fact that the shear-thinning parameter, as well as viscosity hike, for the nanocomposite can be attributed to the sturdy interactions between the particles and the polymer chains (Sochi, 2010), where the silicate layers function as a barrier to the flow of the melt (Zhao et al., 2005). The transformation from pseudo liquid to gel-like assembly has a close relationship with the degree of exfoliation (Wagener and Reisinger, 2003). The shear-thinning exponent and the relaxation parameter as depicted in Fig. 7.22 take the shape of a sigmoid curve for evolution which is characteristic of the presence of a percolation threshold occurring between 2.5 and 5 wt.% fillers. This is associated with a drastic increase of more than one order of

300

Layered Double Hydroxide Polymer Nanocomposites

1

–1.2 –1 G′(Ala) G′′(Ala) G′(Gly) G′′(Gly) η∗(Ala) η∗(Gly)

0.6 0.4

–0.8 –0.6 –0.4

n in |η∗|→ ωn ω→0

n in G′, G′′ → ωn ω→0

0.8

0.2 –0.2 0 0

2

4 6 8 LDH filler (wt.%)

10

12

0

Figure 7.22 Variation of the relaxation parameters from the terminal zone using G0 versus ω, Gv versus ω and h versus ω curve (Naime Filho et al., 2012).

magnitude G0 and |η | versus ω values with no significant improvement being observed for higher percentage loading. Such low loading (2.5
5% w/w) can be explained by the presence of exfoliated platelets only, and the exfoliation degree is found to be correlated to the shear thinning exponent. While the pure silicone is a viscous liquid with no capacity for elastic deformation and low plastic deformation capacity, the nanocomposites are quite elastic and capable of moderate plastic deformation. Zwitter ionic imidazolium-based ionic liquid (ZIL) constituted by imidazolium cation and sulfonate anion was used to modify both cationic and anionic clay minerals, represented by montmorillonite (Na1-Mt) and calcined hydrotalcite (CHT) (Soares et al., 2017). Then, the unmodified and modified clays were mixed with epoxy prepolymer based on diglycidyl ether of bisphenol A (DGEBA). The modified clays resulted in epoxy networked materials with higher intercalation degree, and good transparency. Fig. 7.23 illustrates the dependence of complex viscosity, η , with frequency for ER dispersions prepared with 2.5 phr of different clays. The addition of the Na1-Mt resulted in an increase of the viscosity, whereas the addition of CHT did not significantly affect the viscosity of the ER prepolymer, suggesting better dispersion of the former clay. In both systems, the presence of the functionalized clay minerals gave rise to higher viscous ER dispersions as compared with those prepared with Na1-Mt and CHT. This behavior suggests a better dispersion of these modified clays inside the ER prepolymer. Probably, the sulfonate anions and the imidazolium cations in the interlayer space of ZIL-Mt and ZILCHT, respectively, contribute to a better intercalation of the epoxy prepolymer because of the affinity between these groups and the polar groups (epoxy and hydroxyl groups) present in the epoxy prepolymer, thus generating a better matrix
filler interaction and higher interfacial area. Such interactions may provoke an increase in viscosity of the dispersion.

Melt rheological properties of layered double hydroxide polymer nanocomposites

301

(A) Complex viscosity (Pa.s)

c 18 16

b

14 a 12

1

10 100 Angular frequency (1/s)

(B)

Complex viscosity (Pa.s)

18

e

16

14

d a

12

1

10 100 Angular frequency (1/s)

Figure 7.23 Dependence of complex viscosity η of the epoxy (DGEBA), with frequency for the clay/ER dispersions containing 2.5 phr of clay minerals: (a) ER prepolymer, (b) Na1Mt and (c) ZIL-Mt; (B) (a) ER prepolymer, (d) CHT and (e) ZIL-CHT (Soares et al., 2017).

7.4

Modeling of rheological properties

The melt-state viscoelastic properties, for instance, shear viscosity and shear modulus, are recognized to amplify in the filled polymer melts up to two orders of magnitude compared to their values for the pure polymer matrix. The agglomerated filler particles can be shattered upon the application of a momentary shear flow. In the dormant state, when the flow is switched off reaggregation of particles occur again (Osman and Atallah, 2006). Such thixotropic effects occur due to the deprivation of the filler superstructure, such as a decrease or disappearance of the shear overshoot or decline of complex shear viscosity in the succeeding shear cycles. The layered crystalline materials like LDH, when exfoliated, organize into highly anisometric particles that can be oriented upon application of shear flow (Lin-Gibson et al.,

302

Layered Double Hydroxide Polymer Nanocomposites

2004). It has been reported that there is noteworthy arrangement of the silicate layers in block copolymer nanocomposites when shear rates are above 10 s21 (Ren and Krishnamoorti, 2003). Monte Carlo simulations have revealed that incorporation of solid spherical nanoparticles boosts the small-strain module in proportion to the particle loading and the peak enhancement was estimated to be three times compared to the unfilled melt (Sharaf and Mark, 2004) and, accordingly, the improvement is found to be much less than that calculated for the melt-state nanocomposites and the theory does not consider the agglomeration of filler particles. Attractive interactions between the surface of the nanoparticle and the polymer segments were taken into account in molecular dynamic simulations; nevertheless, the viscosity and dynamic shear modulus have been calculated to amplify less than one order of magnitude, even at high loadings, in comparison with the pure melt (Smith et al., 2002). It is obvious that contemporary computer simulations cannot deal with such an exceedingly sluggish process like the evolution of the microscopic filler structure under the application of external mechanical forces. Hence the need of the hour is to develop appropriate rheological models for these complex systems. Conventional rheological models were satisfactory only in the case of pure polymer melts and hence take for granted only slight modification of the polymer structure even in the nonlinear region. Of late, a structure network model based on a modified upper convected Jeffreys model has been used to portray the shear reversal trial in PP
organoclay nanocomposites (Lertwimolnun et al., 2007). This model makes use of a supposed structure parameter whose evolution is specified by the kinetic equation that correlates the structure breakdown with the energy dissipation due to the flow process (Yziquel et al., 1999). In another interesting work in which lowdensity polyethylene (PE) melt is filled with Mn
Al-based LDH, an approach based on the well-known Wagner model has been employed to address a thixotropic response in the nonlinear shear regime is probed (Costa et al., 2005, 2006a,b,c). For the prediction of the storage modulus, two equations were formulated by Einstein (Einstein, 1956): Gc 5 Gm 1 1 1:25Vf G c 5 G m 1 1 Vf







(7.1) (7.2)

Guth modified the Einstein equations as (Guth, 1945):   Gc 5 Gm 1 1 1:25Vf 1 14:1 Vf2

(7.3)

By the rule of mixtures it can be shown that (Nielsen and Landel, 1993): tan δc 5 Vf tan δf 1 Vm tan δm

(7.4)

For rigid fillers, the former term in the above equation can be ignored: tan δc 5 Vm tan δm

(7.5)

Melt rheological properties of layered double hydroxide polymer nanocomposites

303

It is presumed that the matrix along with the fillers contributes a stiffness equivalent to the minimum elastic modulus of the composite; and hence the equation becomes (Tung and Dynes, 1987): tan δc 5 Vf ðGm =Gc Þtan δm

(7.6)

where G is the storage modulus of the material. The graphs of experimental and theoretical storage moduli values at 200 C for PMMA and its nanocomposites (Kumar et al., 2016) are shown in Fig. 7.24. (A)

4000

Storage modulus (MPa)

3500

3000

2500

Experimental value Einstein Eq. 1 Einstein Eq. 2 Guth Eq. 3

2000

1500 Pure PMMA (B)

4.5 4.0

PMMA-5

PMMA-5-G

Experimental value Theoretical Eq.5 Theoretical Eq.6

Loss factor

3.5 3.0 2.5 2.0 1.5 1.0 Pure PMMA

PMMA-5

PMMA-5-G

Figure 7.24 Plots of experimental and theoretical storage modulus (A) and loss factor (B) of pure PMMA, PMMA-5, and PMMA-5-G nanocomposites at 200 C (Kumar et al., 2016).

304

Layered Double Hydroxide Polymer Nanocomposites

Normally, polymer-based solutions are inherently non-Newtonian in nature, that is, they exhibit shear thinning as the viscosity is not linearly related with shear rate. It is familiar that at elevated shear rates, the viscosity decreases due to the alignment of polymer molecules which in turn permits effortlessness of flow of molecules. Diverse mathematical correlations are used to explain the shear stress versus the shear rate of non-Newtonian fluids and they pave the way toward the identification of the flow characteristics and proficiency of a fluid to execute specific functions. Power law and Bingham plastic models are the two parameter models in this regard. The power law model assumes the form: τ 5 Kγ n

(7.7)

η 5 Kγ n21

(7.8)

where n 5 1 for a Newtonian fluid; n , 1 for shear thinning; and when n . 1 it is a shear thickening fluid. The Bingham plastic model is stated as: τ 5 τ 0 1 γU μp

(7.9)

where μp is plastic viscosity, and τ 0 is yield stress. There are numerous models that include three or more parameters. The third parameter helps to distinguish the fluid flow in the lower and upper Newtonian regions as well as in the power law region. The plastic viscosity term is replaced in the Herschel
Bulkley model by adding the power law expression. τ 5 τ 0 1 K γn

(7.10)

The Sisko model (Sisko, 1958) is used to articulate the flow behavior in the upper Newtonian region and the power law region. It is a three-parameter model, which relates the apparent viscosity with shear rate. η 5 ηN 1 Kγ n21

(7.11)

The Casson model (Casson, 1965) for non-Newtonian behavior is stated as: τ 5 τ o1=2 1 ηαγ1=2

(7.12)

where ηα is viscosity at infinite shear stress. The models used to express polymer solutions, food, activated sludge, and blood flow characteristics are the Cross and Carreau models. Each model consists of four parameters. The Cross model is given as (Cross, 1965):   η 5 ηN 1 ðηo 2 ηNÞ=ð1 1 ðC  γ Þ mÞ

(7.13)

Melt rheological properties of layered double hydroxide polymer nanocomposites

305

where, m is dimensionless exponent and C is time constant. ηo and ηN denote the initial viscosity at zero shear rate and final viscosity at infinite shear rate, respectively. Whereas, the Carreau model is stated as (Carreau et al., 1968):   η 5 ηN 1 ðηo 2 ηNÞ  ð1 1 ðλ  γ Þ 2Þ ðn 2 1Þ=2

(7.14)

where λ and n correspond to relaxation time and gauge of shear thinning properties, respectively.

7.5

Conclusions and future scope

The most imperative parameters governing the rheological properties were found to be the interactions between polymer and particles, which are related to the alteration of chain dynamics at the surface of the particle, and interactions between particles, which are related to the tendency of the particles to interconnect and create local fractals leading to a continuous network. Alternative parameters such as state of dispersion, particle size, and morphology were found to be significant as well, but their contribution can be explored by considering their effect on the aforementioned interactions. Though stupendous work has been carried out in the area of LDH-filled polymer composites, still a systematic study of the rheology of the different systems is lacking. Thermoplastic polymers are mostly the chosen candidates for the study of rheology and hence there is an on-going prospect for research to be unveiled in the rheological analysis of thermosetting polymers. Hence it is worthwhile considering the gaps in the literature for future research. Organically modified LDH is found to exhibit excellent intercalation/exfoliation in polymers and hence pave the way for the exhibition of outstanding properties. Hence there is still a lot of work to be carried out toward the exploration of various types of organic modification in LDH, which leads to novel network formations in both thermoplastic as well as thermosetting polymeric nanocomposites.

References Aghjeh, M.R., Mardani, E., Rafiee, F., Otadi, M., Khonakdar, H.A., Jafari, S.H., et al., 2017. Analysis of dynamic oscillatory rheological properties of PP/EVA/organo-modified LDH ternary hybrids based on generalized Newtonian fluid and generalized linear viscoelastic approaches. Polym. Bull. 74, 465
482. Anderson, B.J., Zukoski, C.F., 2009. Rheology and microstructure of entangled polymer nanocomposite melts. Macromolecules 42, 8370
8384. Anderson, B.J., Zukoski, C.F., 2010. Rheology and microstructure of polymer nanocomposite melts: variation of polymer segment-surface interaction. Langmuir 26, 8709
8720. Barnes, H., 1997. Thixotropy
a review. J. Non-Newtonian Fluid Mech. 70, 1
33.

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420. Wang, Q., Yu, J., Liu, J., Guo, Z., Umar, A., Sun, L., 2013c. Na1 and K1-exchanged zirconium phosphate (ZrP) as high-temperature CO2 adsorbents. Sci. Adv. Mater. 5, 469
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4016. Zhou, Q., Verney, V., Commereuc, S., Chin, I.-J., 2010. Fabrice Leroux, Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly (butylene succinate). J. Colloid. Interface. Sci. 349, 127
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Further reading Costa, F.R., Wagenknecht, U., Heinrich, G., 2007. LDPE/MgeAl layered double hydroxideS nanocomposite: thermal and flammability properties. Polym. Degrad. Stab. 92, 1813
1823. Cui, Z., Chin, 2010. Synergistic effects of layered double hydroxide with phosphorusnitrogen intumescent flame retardant in PP/EPDM/IFR/LDH nanocomposites. J. Polym. Sci. 28, 563
569. Drzal, L.T., Rich, M.J., Lioyd, P.F., 1983. Adhesion of graphite fibers to epoxy matrices: I The role of fiber surface treatment. Adhesion 16, 133
140. Feng, Y., Li, D., Wang, Y., Evans, D.G., Duan, X., 2006. Synthesis and characterization of a UV absorbent-intercalated Zn-Al layered double hydroxide. Polym. Degrad. Stab. 91, 789
794. Illaik, A., Taviot-Gue´ho, C., Lavis, J., Commereuc, S., Verney, V., Leroux, F., 2008. Unusual Polystyrene nanocomposite structure using emulsifier-modified layered double hydroxide as nanofiller. Chem. Mater. 20, 4854
4860. Katiyar, V., Gerds, N., Koch, C.B., Risbo, J., Hansen, H.C.B., Plackett, D., 2011. Melt processing of poly(L-lactic acid) in the presence of organomodified anionic or cationic clays. J. Appl. Polym. Sci. 122, 112
125. Krishnamoorti, R., Ren, J., Silva, A.S., 2001. Shear response of layered silicate nanocomposites. J. Chem. Phys. 114, 4968
4973. Li, D., Tuo, Z., Evans, D.G., Duan, X., 2006. Preparation of 5-benzotriazolyl-4-hydroxy-3sec-butylbenzene sulfonate anion intercalated layered double hydroxide and its photostabilizing effect on polypropylene. J. Solid State Chem. 179, 3114
3120. Lin, Y., Li, D., Evans, D.G., Duan, X., 2005. Modulating effect of Mg
Al
CO3 layered double hydroxides on the thermal stability of PVC resin. Polym. Degrad. Stab. 88, 286
293. Ren, J., Silva, A.S., Krishnamoorti, R., 2000. Linear viscoelasticity of disordered polystyrene 2 polyisoprene block copolymer based layered-silicate nanocomposites. Macromolecules 33, 3739
3746. Xu, Z., Saha, S.K., Braterman, P.S., D’Souza, N., 2006. The effect of Zn, Al layered double hydroxide on thermal decomposition of poly(vinyl chloride). Polym. Degrad. Stab. 91, 3237
3244. Zhao, X., Zhang, F., Xu, S., Evans, D.G., Duan, X., 2010. From layered double hydroxides to ZnO-based mixed metal oxides by thermal decomposition: transformation mechanism and UV-blocking properties of the product. Chem. Mater. 22, 3933
3942.

Thermal properties and flameretardant characteristics of layered double hydroxide polymer nanocomposites

8

Yanshan Gao1, Lei Qiu1, Dermot O’Hare2 and Qiang Wang1 1 College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China, 2University of Oxford, Oxford, United Kingdom

8.1

Introduction

The PlasticsaEurope statistics suggest that the consumption of polymer-based materials has been increasing rapidly in recent years (Fig. 8.1). Production increased from 1.5 million tons in 1950 to 311 million tons in 2014. This growth is around 9% a year on average. Polymer-based materials are now recognized as key components in many important industries such as construction, automotive, electronics, and aerospace, due to their remarkable combination of properties, low weights, cost-effectiveness, and ease of processing. Most polymers, however, suffer from thermal degradation and are highly flammable, which increases their fire hazards when used in practical applications, significantly reducing service life and severely restricting their uses in many areas (Morgan and Wilkie, 2007; Gao et al., 2014a,b; Hilado, 1998; Feng et al., 2012). Consequently, improving polymer flame retardancy is a major challenge for extending polymer use to most applications. In order to improve the thermal stability properties and flame-retardant performance of polymer resins, effective nano-sized flame-retardant fillers have been added to polymer matrices. Halogenated flame retardants have been in use since the 1930s. They are the most widely produced and used flame retardants due to the advantages of low cost, ease of processing, high flame retardancy, and miscibility (Morgan and Gilman, 2013). However, it has been found that some halogen-containing flame retardants are toxic and will produce smoke and brominated dioxins when burning, which is a great threat to both the environment and people. As the awareness of health care and environmental protection increase, the use of halogen-based additives is diminishing in Europe and the United States. In 2004, two formulations of halogen flame retardants were banned in Europe and North America, while a third was banned in 2008 (Liu and Zhu, 2014). Organic phosphorus compounds can be vapor phase or

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00008-2 © 2020 Elsevier Ltd. All rights reserved.

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Figure 8.1 Polymer production in million tons from 2004 to 2014.

condensed phase flame retardants, meaning that they can be useful in low loading levels when combined with polymers that inherently char on their own. But the phosphates in polymers can cause plasticization, which leads to a decrease in the mechanical properties of the polymers. Also, they can generate more smoke and CO during fire conditions because they help inhibit polymer combustion (Morgan and Gilman, 2013). Intumescent flame retardants provide excellent fire protection but tend to be limited to lower-temperature materials and fire protection barriers due to their water absorption issues. Metal hydroxides such as Al(OH)3 and Mg(OH)2 are the largest commercially manufactured flame retardants and are perceived to be very environmentally friendly. They generate greatly lower smoke and reduce overall toxic gas emissions when burning. Further, these fillers are fairly inexpensive and can be easily coated with surfactants to make their use in polymer easier. The main disadvantage of Al(OH)3 and Mg(OH)2 is their high loading (50
70 wt%) and inherent poor compatibility with hydrocarbon-based polymers. In addition, the mineral fillers can delay ignition and slow initial flame growth, but cannot stop it completely if enough constant external heat is applied (Morgan and Gilman, 2013). Layered double hydroxides (LDHs) are a class of lamellar compounds made up of positively charged mixed metal hydroxide layers with an interlayer region containing charge-compensating anions and water molecules. They can be described by the general chemical formula [Mz11
xM31x(OH)2]q1(Xn
)q/n  yH2O, where Mz1 represents divalent cations such as Mg21, Zn21, Ca21, etc., while M31 is trivalent cations such as Fe31 or Al31, and Xn
is a charge-balancing interlayer anion. LDHs are emerging as a new generation of thermal stabilizer and flame-retardant materials due to their unique chemical composition and layered structure. They are potentially eco-friendly flame retardants for polymer applications. In addition, by properly intercalating certain anions, such as borate into LDHs, LDHs might combine the advantages of both magnesium hydroxide (MH), aluminum hydroxide (AH), and zinc borate (xZnO  yB2O3  zH2O; known in the trade as Firebrake) (Gao et al., 2014a,b). In this chapter, we discuss the techniques for evaluating the thermal stability properties and flame-retardant performances and summarize LDH-based thermal

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stabilizer materials and fire-retardant materials, their applications, and the corresponding mechanisms.

8.2

The techniques for determining thermal stability properties and flame retardancy performance

In order to measure the thermal stability and flame retardancy of polymer composites, different standards and indices need to be considered. To date, thermogravimetric analysis (TGA) is usually used to measure the thermal stability of polymers and four approaches are commonly used to evaluate the fire properties of polymer/LDH nanocomposites, they are: microscale combustion calorimeter (MCC), limiting oxygen index (LOI), cone calorimeter (CONE), and Underwriters Laboratories (UL-94).

8.2.1 The techniques for determining thermal stability properties of polymers TGA is a kind of thermal analysis technology (Doyle, 1961; Wu et al., 2002), it is used to test the thermal stability and composition of polymer materials (Jain et al., 2016). It can offer the relationship between the sample weight and the heating temperature under the control of predefined program. Meanwhile it can be used with other analytical methods in the actual analysis of materials for carrying out comprehensive thermal analysis results (Qiu et al., 2015; Byrn et al., 1995; Zhao et al., 1997; Jeske et al., 2012). Briefly, TGA analysis was carried out with an established heating rate or a corresponding air (or CO2, N2, etc.) flow rate. When the substance which is being measured begins to sublimate, vaporize, decompose gas, or loss the crystal water in the heating process, the weight of measured materials will change. Then the thermogravimetric curve of the sample will not remain straight and instead drop little by little. By analyzing the curves of a material, we can discover the temperatures at which the sample starts to change, including the lost weight and its corresponding percentage. For example, in the TGA curves of ethylene-vinyl acetate copolymer (EVA)/LDH composites, all samples mainly underwent three stages of decomposition (Fig. 8.2) (Wang et al., 2011a,b,c,d). The first stage corresponds to the loss of physically absorbed water and interlayer water in lower temperatures (below 225 C) with a decomposition maximum at about 180 C in the DTG curve. The second step at higher temperatures (in the range of 225
500 C) is associated with the dehydroxylation of the metal hydroxide layers and the degradation of interlayer carbonates (Benito et al., 2010). Finally, the third step occurred at over 500 C, and is attributed to the removal of residual carbonate anions in TGA. Through TGA analysis, changes to crystal properties can be studied, such as the physical phenomena of melting, evaporation, sublimation, adsorption, and other experimental samples. At the same time, it is helpful to study the chemical phenomena of the materials including dehydration, dissociation, oxidation, reduction, and so on.

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100

80

100

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501 493

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Figure 8.2 TGA and DTG curves of pure EVA and its composites with 20 wt% LDHs.

8.2.2 The techniques for determining the flame-retardant performance of polymers MCC, LOI, CONE, and UL-94 are usually used to detect the flame-retardant properties of polymer composites. MCC is also known as pyrolysis combustion flow calorimeter (PCFC), which is a convenient and fast method for laboratory evaluation of flame-retardant properties. It is based on a TGA-like degradation of the polymer in nitrogen, followed by combustion of the gases produced in air (Gao et al., 2014a,b). MCC can quickly and easily measure the key fire parameters of plastics, wood and textiles, and composites. For the MCC test, just a few milligrams (c. 5 mg) of sample is heated to the setting temperature with a heating rate which

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was set up before, a wealth of information on material combustibility and fire hazard can be obtained in minutes, including the heat release rate (HRR), heat release capacity (HRC), total heat release (THR), and temperature of degradation. The LOI of a material is defined as the minimum oxygen concentration (expressed in volume percentage) required for the flame burning of the material to be carried out in a mixture gas system of oxygen and nitrogen. It is measured by passing a mixture of oxygen and nitrogen over a burning sample, and reducing the oxygen level until a critical level is reached. It is widely used to evaluate the flameretardant properties of materials (Weil et al., 1992). LOI values for different plastics are determined by standardized tests, such as the ISO 4589 and ASTM D2863. When doing the LOI test, the samples should be pressed into 120 3 60 3 3 mm sheets according to ASTM D2863. The values obtained by the test should be the averages of several tests for each sample. For LOI results, the higher the value, the more difficult it is for combustion to occur. Generally, we consider that the materials can burn easily when the LOI values are ,22, flammable when the value is between 22 and 27, and it is incombustible when the value is .27. LOI is a method to evaluate the relative combustibility of polymer materials, but it cannot give any useful information about the burning behavior (Alongi et al., 2011). Compared to MCC and LOI tests, CONE can provide useful information about the combustion of polymers and it is the most effective method for the laboratory evaluation of the flame-retardant properties of polymers. Approximately 30 g of composite samples was compression molded into 10 3 10 cm square plaques of uniform thickness (B3 mm) before the tests were performed. A cone-shaped heater with incident flux of 35 kW/m2 was used, and the spark was continuous until the sample ignited. The results obtained from CONE can be used to evaluate material-specific properties, setting it apart from many of the established fire tests which are designed to monitor the fire response of a certain specimen (Schartel and Hull, 2007). The CONE test can gather data regarding the ignition time (tig), average mass loss rate (AMLR), combustion products, HRR, THR, and other parameters associated with burning properties. HRR is generally considered to be the most important parameter for evaluating the flame-retardant performance of polymer composites. For example, pure acrylonitrile-butadiene-styrene (ABS) was observed to burn out within 830 s after ignition, and a very sharp HRR peak appears at the range of 150
700 s, with a peak heat release rate (PHRR) value of 489 kW/m2. While the ABS/MgAl LDH and ABS/ZnMgAl LDH composites showed a PHRR value of 196 and 214 kW/m2 with 60% LDH loading, respectively. The addition of LDHs prolonged the combustion times (Fig. 8.3) (Xu et al., 2012). UL-94 is a plastics flammability standard released by Underwriters Laboratories of the United States. The standard determines the material’s tendency to either extinguish or spread the flame once the sample has been ignited. UL-94 test is carried out with two standards: the vertical burn test (UL-94V) and the horizontal burn test (UL-94 HB) (Gao et al., 2014a,b). UL-94V provides useful information aimed at the dripping behavior of polymer composites. The dripping of the burning melt determines the spread of flame through secondary flaming during real situations

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Figure 8.3 Dynamic curves of HRR versus time for the pristine ABS and two LDH/ABS composites. Surface burn

Vertical burn

Horizontal burn 45º

Doesn’t ignite Under hotter flame UL 94 5VA UL 94 5VB

Self extinguishing UL 94 V-0 (Best) UL 94 V-1 (Good) UL 94 V-2 (Drips)

Slow burn rating Takes more than 3 min to burn 4 inches

Figure 8.4 UL-94 flammability ratings summary.

(Costa et al., 2007). This standard involves five parts including HB, V-2, V-1, V-0, and 5V, as shown in Fig. 8.4. The vertical burning test is measured on sheets 127 3 12.7 3 3.3 mm3 according to the standard UL-94 test ASTM D635. A good flame-retardant material should reach the UL-94 V-0 rating and have no dripping during the test. This is primarily an evaluation that is used to qualify a product. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. Jiang et al. (2016) studied PP nanocomposites consisting of Zn2Al-DBS LDHs in combination with zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (ZrP) compounds. Fig. 8.5 shows that the flame for pure PP was very vigorous and spread rapidly. With an increasing load of LDHs in combination with ZrP, the combustion speed of the PP composites slowed significantly, at the same time, in the burning process, the flame of the PP composites was weaker than pure PP. When the loading of LDHs and ZrP was 5 and 15 wt%, respectively, the PP composites can reach UL-94 V-0 rating. UL-94 is the most widely used standard for the flammability of plastic materials which is used to evaluate the ability of materials to be extinguished after being

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317

Figure 8.5 Images of vertical flame test for: (A) PP, (B) PP
LDH20, (C) PP
ZrP20, (D) PP
LDH10
ZrP10, (E) PP
LDH6.7
ZrP13.3, and (F) PP
LDH5
ZrP15 composites at different time points.

ignited. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. All of the above measures can be used for determining the flame retardancy performance of polymers. Different methods provide different information regarding the burning behavior. A variety of studies have been conducted to show correlations between each of the flammability tests. Weil et al. (1992) reported that the LOI value might be leveled with UL-94 or CONE data to some degree in certain conditions, but it was hard to show close relations between them. Also, it does not mean that higher LOI gives better UL-94V ratings.

8.3

LDH-based thermal stabilizer materials and their applications

8.3.1 Thermal stabilizer introduction The thermal stabilizer is one of the important additives to materials, especially polymers which are sensitive to high temperature. Generally, ideal thermal stabilizers should have properties such as high thermostability, good compatibility with polymer materials, as well as low volatility and proper lubrication. In recent years LDHs have attracted considerable interest from both industry and academia due to their good thermostability (Lin et al., 2005; Xu et al., 2006).

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It was found that this kind of novel heat stabilizer could bring out great environmental protection and economic benefits. For instance, when combining LDHs with organotin, a synergistic stabilization effect was obtained for rigid poly(vinyl chloride) (PVC) and the cost was reduced (Hua et al., 2001). More and more researches on LDHs as thermal stabilizers for polymer materials have been carried out aiming to make sure that the polymers can be used for various aspects.

8.3.2 Thermal stability properties of LDH-based nanocomposites 8.3.2.1 Effect of inorganic LDHs Due to the unique structure of LDHs, all the members can act as a thermal stabilizer. But different interlayer anions have different results. LDHs composed of Mg, Zn, and Al are preferred as inorganic fillers within the polymer matrix since the usage of these metals preserves the original color of polymers (Yang et al., 2015). Qiu et al. (2015) carried out a series of experiments on polypropylene (PP)/ Mg3Al-CO3 LDH nanocomposites systematically. The morphology-dependent performance of Mg3Al-CO3 LDHs (plate-like, spherical, and flower-like) as nanofillers within PP matrix has been studied. The results showed that the thermal stability of PP/LDH nanocomposites was significantly improved after incorporating Mg3AlCO3 LDHs with different kinds of morphologies. Specifically, the temperature at 50% weight loss (T0.5) of the PP/plate-like LDH nanocomposites (with a LDHs loading of 13.0 wt%) was increased by 61 C compared to that of pure PP. The results also obtained that the influence of LDH morphologies on thermal stability follows the order of plate-like . spherical . flower-like (Fig. 8.6). Gao et al. (2014a,b, 2016a,b) investigated the thermal stability properties of Zn2Al LDHs with different inorganic anions on high-density polyethylene (HDPE) and EVA. The results obviously suggest that different interlayer anions intercalated LDHs could lead to different performances on the same polymer. But the inorganic anions showed the same influence order for HDPE and EVA resin, which is: Zn2Al-Cl LDHs . Zn2Al-CO3 LDHs . Zn2Al-NO3 LDHs . Zn2Al-SO4 LDHs. Lin et al. (2006) investigated the thermal stabilization of PVC with Mg2Al-CO3 and Mg3Al2Zn-CO3 LDHs. The results showed that both LDHs improved the thermal stability of PVC resin, but compared with Mg2Al-CO3 LDHs, Mg3Zn2Al-CO3 LDHs enhanced the thermal stability of PVC in terms of both long-term stability and early coloring. In addition, Wang et al. (2017) added Ni cation into Mg3Al LDHs and studied the performance of Ni0.2Mg2.8Al-CO3 LDHs on EVA. The decomposition temperature of PP with 5 and 10 wt% Ni0.2Mg2.8Al LDHs composites is 34 C higher than pure PP. Moreover, inorganic LDHs had a good effect on enhancing the thermal stability by influencing the reaction process of different kinds of polymers including PVC (Zhao et al., 2008; Zhang et al., 2007), PP(Cui, 2010; Nyambo et al., 2008a,b), etc., either by changing the valence state of the metal cations, interlayer anions, or the species of the metal cations, and even the ratios between the different layers of the metals.

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(A) 100

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Figure 8.6 TGA analysis of: (A) PP/spherical LDH, (B) PP/plate-like LDH, (C) PP/flowerlike LDH nanocomposites, and (D) graph of T0.5 versus LDH loading.

8.3.2.2 Effect of organic LDHs The above researches have demonstrated that LDHs with inorganic interlayer anions can improve the thermal stability of polymers effectively. However, pristine LDHs with hydrophilic surface properties are not compatible with hydrophobic polymers such as PP or polystyrene (PS), which will affect the dispersity of LDHs in the polymer matrix (Yang et al., 2015). In view of this problem, organic anionmodified LDHs were reported to improve the dispersion of LDHs in the polymer matrix (Wang et al., 2012a,b,c,d; Manzi-Nshuti et al., 2009,b,c). Yang et al. (2015) modified LDHs with various anionic surfactants, such as lauric acid (LA), palmitic acid (PA), stearate (SA), lauryl phosphate (LP), or dodecyl sulfate (DS), then systematically discussed the thermal stability of PP nanocomposites containing the appropriate hydrophobically modified LDHs. The T0.5 of the PP/organo-LDH nanocomposites was significantly improved by 37
61 C, respectively, depending on the type and loading content of organo-LDHs compared to that of pure PP (Fig. 8.7). The surfactant-dependent (DS and stearic) performance of Mg3Al LDHs as nanofillers for PP matrix was evaluated by Qiu et al. (2018). The results showed that the thermal stability of the PP/LDH nanocomposites was greatly improved in terms of the value of T0.5, especially for the PP/stearic-LDH nanocomposites with a LDH loading of 20 wt%, the T0.5 was increased by 80 C compared to that of pure PP (Fig. 8.8). Wang et al. (2013a,b) investigated the thermal stability of PP/

320

Layered Double Hydroxide Polymer Nanocomposites

(A) 100

(B) 100 3 phr organoLDH/PP 90

90 80

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20

20

10

10

0 200

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300 350 400 Temperature (ºC)

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Figure 8.7 TG curves of PP and PP/organic LDH nanocomposites depending on: (A) the kind of organic LDHs and (B) the loading content of lauric acid (LA) LDHs.

tartrazine intercalated LDH nanocomposites, and the thermal stability of Mg3Altartrazine LDHloaded PP nanocomposites was significantly enhanced compared to pure PP. With only 0.4
0.8 wt% of LDHs, the T0.1 and T0.5 were increased by 26.2 C and 41.3 C, respectively. In addition, Zhang et al. (2014) synthesized 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP) intercalated Mg2Al LDHs (Mg2Al-BP LDHs) and investigated its thermal stabilization for PVC. Congo Red tests showed that the addition of the Mg2Al-BP LDHs can improve the static thermal stability time of PVC (Fig. 8.9). The dynamic thermal stability behavior of PVC was also enhanced after the addition of Mg2Al-BP LDHs. They also found that the stabilization mechanism may be attributed to the ability of Mg2Al-BP LDHs not only to scatter the incident light but also to absorb the released HCl, which improved the resistance of PVC to both accelerated weathering and thermal degradation. Besides, many studies have proved that other organic LDHs also can improve the thermostability of nanocomposites. For example, Nyambo et al. (2009a,b) prepared poly(methyl methacrylate) (PMMA)/Mg2Al-palmitate (C16) nanocomposites and the T0.1 was increased for all nanocomposites by 15 C. The T0.5 was increased compared to the pure PMMA by 27
35 C.

8.3.2.3 Effect of LDHs with other synergistic thermal stabilizers Some reports have conducted a series of researches on the synergistic effects of LDHs with other additives such as ammonium polyphosphate (APP), MH, and carbon-based materials, etc., as well. LDHs were used as synergistic agents of APP in poly(vinyl alcohol) (PVA) matrix by Zhao et al. (2008). The results showed that

Thermal properties and flame-retardant

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18

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Figure 8.8 TGA analysis of: (A) PP/DS
LDH, (B) PP/stearic
LDH nanocomposites, and (C) graph of T0.5 versus LDHs loading.

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Layered Double Hydroxide Polymer Nanocomposites

180 160 Stability time (min)

140 PVC/LDH

120 100

0 phr 0.5 phr 1 phr 3 phr 5 phr

PVC/LDH-BP

pure PVC

80 60

PVC/BP

40 20 0

Figure 8.9 Thermal stability time of PVC, PVC/Mg2Al-CO3 LDH, PVC/BP, and PVC/ Mg2Al-BP LDH composites measured by Congo Red test.

LDHs exhibit an obvious synergistic effect with APP. The Tinitalis (the initial decomposition temperature, temperature at 5% weight loss) further increased compared to LDHs alone, which may be due to the physical crosslinking effect among layered particles, APP molecules, and polymer chains. Meanwhile, all PVA/APP/LDH composites showed higher char residues than that of PVA/APP at 500 C, 600 C, or 700 C. Besides, Zhang et al. studied the synergistic effects of LDHs with hyperfine magnesium hydroxide (HFMH) in EVA (Zhang et al., 2007). TGA data demonstrated that the addition of LDHs can raise 5
18 C of EVA/HFMH/LDH nanocomposite samples with 5
15 phr (parts per hundred resins) LDHs compared with that of the EVA/HFMH sample when 50% weight loss is selected as a point of comparison. Gao et al. (2016a,b) found that for PP/Mg3Al LDH-oxidized carbon nanotube (OCNT) nanocomposites with 10 wt% LDHs and 0.5, 1, and 2 wt% OCNTs, T0.5 was increased by 41 C, 41 C, and 43 C, respectively. These increases are much higher than observed with PP/LDH nanocomposites without OCNTs. DTG analyses also clearly showed that compared to pure PP (340 C), the temperature of the maxima rose after adding LDHs, OCNTs, or a combination of LDHs and OCNTs. The temperatures of the maxima in DTG lie in the range of 350a390 C, suggesting that LDHs, together with OCNTs, act as a good thermal stabilizer for PP. Many research articles report the addition of LDHs together with other thermal stabilizers does produce a synergistic improvement in thermal stability of the polymeric host material. The physical explanation of this effect is not clear, perhaps the physicochemical properties of host material is changed by the formation of some intermediate products with high thermal stability. To determine the origin of these effects requires further analysis and research.

8.3.3 The mechanism of thermostability using LDHs Since LDHs could improve the thermostability of polymer materials, many experts have done a lot of work to explain the mechanism. During the heating process,

Thermal properties and flame-retardant

323

LDHs can release H2O and CO2 effectively, which can delay the scission of polymer chains, making the polymer composites more stable. However, if the loading of LDHs is too high, the T0.5 value will decrease when compared with low LDH loading. The reason is that high LDH loading results in a lot of metal and metal oxide, which in turn accelerates the catalytic degradation of polymer on heating (Jiang et al., 2014). In addition, when LDHs were modified with organic species, the thermal stability and the overall thermal decomposition behavior changed (Costa et al., 2009). The mechanism of chloride-containing polymers, such as PVC, is a little different from polymers without chloride such as PP. The HCl releases from PVC matrix can be absorbed by LDHs, preventing the further self-catalytic reaction of PVC. The addition of LDHs led to an enhanced and excellent thermostability of polymer materials, not only due to their barrier functions, but also concern with the changes of activation energy of thermal degradation, which play an important role in hindering the movement of small molecules during the polymer degradation process.

8.4

LDH-based flame-retardant materials and their applications

8.4.1 Flame retardant introduction As the American Chemistry Council has described, flame retardants are a key component in reducing the devastating impacts of fires on people, property, and the environment. They are added to different materials (e.g., textiles, plastics) to prevent fires from starting, limit the spread of fire, and minimize fire damage. Some flame retardants work effectively on their own, others act as “synergists” to increase the fire protective benefits of other flame retardants. A variety of flame retardants is necessary because materials that need to be made fireresistant are very different in their physical nature and chemical composition, so they behave differently during combustion. The elements in flame retardants also react differently with fire. As a result, flame retardants have to be matched appropriately to each type of material. Flame retardants work to stop or delay fire, but, depending on their chemical makeup, they interact at different stages of the fire cycle. When flame retardants are present in the material, they can act in three key ways to stop the burning process. They may work to: (1) Disrupt the combustion stage of a fire cycle, including avoiding or delaying “flashover,” or the burst of flames. (2) Limit the process of decomposition by physically insulating the available fuel sources from the material source with a fire-resisting “char” layer. (3) Dilute the flammable gases and oxygen concentrations in the flame formation zone by emitting water, nitrogen, or other inert gases. Therefore, the use of flame retardants is essential to stopping or slowing the spread of fire and LDHs have been increasingly used as fire retardants.

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Layered Double Hydroxide Polymer Nanocomposites

8.4.2 Flame-retardant performance of LDH-based nanocomposites 8.4.2.1 Effect of inorganic LDHs Due to the poor compatibility between inorganic LDHs and the polymer matrix, only a few inorganic anions intercalated in LDHs have been investigated as flame-retardant additives for polymers. Carbonate is the first and the most extensively investigated; carbonate-intercalated LDHs have been shown to be highly efficient in improving the thermal stability and flame-retardant performance of many polymers, such as EVA (Gao et al., 2016a,b; Shi et al., 2005a,b; Jiao et al., 2008; Shi et al., 2005a,b; Nyambo and Wilkie, 2009a,b), PVC (Xu et al., 2006; Zhang et al., 2004; Molefe et al., 2015), ABS (Xu et al., 2012; Zhang et al., 2004), HDPE (Gao et al., 2014a,b; Zhang et al., 2004), and so on. Zhang et al. (2004) studied the fire retardancy of MgAl-CO3 LDHs in various polymers. After PS, ABS, HDPE, and PVC were filled with the nano-LDHs with a loading of 60 wt%, their LOI values could be increased up to 28, 27, 26, and 33, respectively, and the polymers produced less smoke than the materials free of the nanoLDHs during burning. Shi et al. (2005a,b) incorporated 60 wt% MgAl-CO3 and ZnMgAl-CO3 LDHs into EVA-28, the LOI of EVA can be increased from 21 to 34 and 40, respectively. ZnO present in the mixed metal oxide formed by decomposition of ZnMgAl-CO3 LDHs can promote charring of the composite. As a result, incorporation of Zn21 into the layers of the LDHs was found to promote material charring and smoke suppression. In addition, MgAl-CO3 and ZnMgAlCO3 LDHs also showed a good flame-retardant performance to ABS resin. Both ABS/MgAl-LDH and ABS/ZnMgAl-LDH composites exhibit higher LOI, lower smoke density values, and a prolonged combustion time, compared to pristine ABS (Xu et al., 2012). Molefe et al. (2015) observed that MgAl-CO3 LDHs are a promising functional filler for plasticized PVC. Both the thermal stability and flame-retardant performance can be improved with 30 phr LDHs loading. In addition, a series of MgAlFe-CO3 LDHs have been added to EVA by Jiao et al. (2008). The results show that MgAlFe-CO3 LDHs are better than MgAl-CO3 LDHs in improving the flame retardation of EVA at the same additive loading level and reached the UL-94 V-0 rating when the LDH loading was 50 wt%. The addition of ZnAl-CO3 LDHs coated with oleate also can promote charring to retard the generation of flame for PVC (Xu et al., 2006). Gao et al. (2014a,b) synthesized HDPE/Zn2Al 2 X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites with different loadings from 10 to 40 wt% using a modified solvent-mixing method. The influence on flame-retardant properties followed the order of SO422 . NO32 . CO322 . Cl2. When adding 40 wt% LDHs, the PHRR was reduced by 54%, 48%, 41%, and 24%, respectively (Fig. 8.10). In 2005, Shi et al. (2005a,b) reported the borate intercalated MgAl LDHs as a flame-retardant filler for EVA for the first time. MgAl-LDHs showed a good flame-retardant performance, the LOI of EVA was increased from 21 to 29 after adding 60 wt% LDHs. EVA/MgAl-borate composites with 60 wt% LDH loading also showed a significantly better smoke suppression, which was 45% less than

Thermal properties and flame-retardant

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Figure 8.10 MCC analysis of: (A) HDPE/Zn2Al 2 Cl, (B) HDPE/Zn2Al 2 NO3, (C) HDPE/ Zn2Al 2 CO3, and (D) HDPE/Zn2Al 2 SO4 LDH nanocomposites.

that of pure EVA. Later, Nyambo and Wilkie (2009a,b) investigated the fire resistance performance of ZnAl- and MgAl-borate LDHs in EVA. At 40% loading, the reduction in PHHR observed in EVA composites containing LDHs reached to 74% and 77%, respectively. In addition, Wang et al. (2013a,b) synthesized PP/Zn2Al-borate and PP/Mg3Al-borate LDH nanocomposites using a modified solvent-mixing method. The results show that PP/Zn2Al-borate LDH nanocomposites exhibited superior performance to the equivalent PP/Mg3Alborate LDH nanocomposites. By considering both the thermal improvement and the flame-retardant performance, 15 wt% of the highly dispersed Zn2Al-borate LDHs in PP was found to be the optimal loading. The 15 wt% Zn2Al-borate LDHs in pristine (unmodified) PP resulted in a 64% reduction of the PHRR (Fig. 8.11). It is believed that borate promotes the formation of a ceramic-like MgO or Al2O3-based coating that forms over the char, which forms on the surface of a polymer during combustion and subsequently forms a vitreous phase, which acts as a binder to reinforce this ceramic coating, preventing further combustion (Shi et al., 2005a,b). In addition to borate, phosphate-intercalated LDHs were also studied by Ye and Qu (2008). They compared the flame-retardant properties of MgAl-CO3 and MgAlPO4 LDHs in the EVA blends. The LOI values of EVA/MgAl-PO4 samples with different loading levels are 2% higher than those of the corresponding MgAl-CO3 LDHs samples. Meanwhile, both EVA/MgAl-CO3 and EVA/MgAl-PO4 LDH

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100

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Figure 8.11 (A) TGA analysis of PP/Zn2Al-borate nanocomposites with various LDH loadings (0, 3, 6, 9, 15, and 30 wt%). (B) HRR of PP/Zn2Al-borate nanocomposites with various LDH loadings (0, 6, 15, and 30 wt%).

composites can pass the V-0 rating when the LDH loading reached 60 wt%. However, the composites with 55 wt% MgAl-PO4 LDHs can pass the V-1 rating while the composites with 55 wt% MgAl-CO3 LDHs cannot pass any rating in the UL-94 test. The flame-retardant mechanism of MgAl-PO4 LDHs can be ascribed to its catalysis degradation of the EVA resin, which promotes the formation of charred layers with the P-O-P and P-O-C complexes in the condensed phase and the compact charred layers formed from the EVA/MgAl-PO4 sample effectively protect the underlying polymer from burning. The SEM observation gives further evidence of this mechanism (Fig. 8.12). To sum up, LDHs intercalated with inorganic anions such as CO322, NO32, Cl2, SO422, borate, phosphate, etc. are potentially promising flame-retardant additives for polymers such as ABS, EVA, PP, and so on. But one problem with inorganic

Thermal properties and flame-retardant

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Figure 8.12 SEM images of charred residues of (A) EVA/MgAl-CO3 and (B) EVA/MgAlPO4 LDH composites with 60 wt% LDH loading.

LDHs is the high loading as nanofillers. In order to obtain a high-efficiency flame retardant with a low inorganic intercalated LDHs loading, more work needs to be done in the future.

8.4.2.2 Effect of organic LDHs Although inorganic anion-intercalated LDHs are potentially promising flameretardant nanofillers for some polymers, the hydrophilic surface of the inorganic LDHs is incompatible with hydrophobic polymers, which severely inhibits homogeneous dispersion of LDH layers within the polymer matrix (Manzi-Nshuti et al., 2009,b,c; Bao et al., 2008). Furthermore, the high charge density in the metal hydroxide layer leads to a strong electrostatic interaction between the hydroxide sheets, making separation of these sheets (exfoliation) very difficult. Therefore, it is important to modify LDHs with suitable organic anions to increase the gallery distance as well as reduce the hydrophilic character of the surface (Gao et al., 2014a, b). Till now, many organic anions have been intercalated into LDH interlayers including oleate, DS, SA, undecenoate, dodecyl benzene sulfonate (DBS), C16,N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexyl)formamide-2-propenyl acid (DPHPA), 2-carboxylethyl-phenyl-phosphinic acid (CEPPA), etc. Among the organic anions, oleate is most extensively studied. The long chain of oleate acts to compatibilize the LDHs with many polymers, such as PP, PE, EVA, PMMA, and poly(ethylene-co-butylacrylate) (PEBuA). Oleate exhibits an excellent combination of high thermal stability, good water solubility, and relatively low cost, and as a result it is usually preferred to other possible surfactants (ManziNshuti et al., 2009,b,c). It was found that ZnAl-oleate LDHs revealed a good flame-retardant performance for PE, which PHRR reduction was 58% with 10 wt% LDHs loading, followed by PMMA (28%) and PEBuA (2%) (Manzi-Nshuti et al., 2009,b,c). Wang et al. (2011a,b,c,d) investigated the EVA/ZnAl-oleate nanocomposites, the result showed that the PHRR reduction was 33% with 10 wt% LDH loading. In addition, MgAl-oleate LDHs showed a similar flame-retardant performance, in which PHRR reduction was 36%, with the same LDH loading in EVA resin. In addition, Manzi-Nshuti et al. (2009,b,c) synthesized PP nanocomposites with a

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series of oleate-intercalated ZnMg LDHs. It was found that Zn0.5Mg1.5Al-oleate LDHs showed the largest PHRR reduction, 38% with 4 wt% LDHs loading. For Zn2Al-oleate LDHs, the PHRR reductions are 25% and 5% with 2 and 4 wt% LDH loading. DS is an important anion applied to modify LDHs for flame-retardant applications. Ye and Wu (2012) investigated the flame-retardant properties of low-density polyethylene (LDPE)/LDH nanocomposites with DS-modified MgAl-LDHs. When the LDH loading was 5 phr, the PHRR values were reduced by 14.5% and 5%, respectively. In addition, Wang et al. (2012a,b,c,d) proved that DS-intercalated NiAl LDHs and EVA matrix had good compatibility and when the LDHs loading was 20 wt%, the PHRR was reduced by 74.9%. DBS is another important anion applied to modify LDHs as polymer additives. Costa et al. (2007) investigated the flammability properties of the nanocomposites based on LDPE and MgAl-DBS LDHs. The PHRR values were found to be reduced significantly with increasing LDH concentration. When the LDHs loading was 16.2% (PE-LDH6), the PHRR value of the nanocomposites was reduced by 68% (Fig. 8.13A). Tig, a parameter defined as the time at which the test samples catch fire, is also significantly increased with increasing LDH content. The pure LDPE has a Tig below 100 s and that increased to above 120 s with the addition of 16.2 wt % LDHs (Fig. 8.13B). Except oleate, DS, DBS anions, 1,4-butane sultone (BS) (Wang et al., 2015a,b), phenyl phosphate (PP) (Edenharter and Breu, 2015), C16 (Majoni, 2015; Nyambo et al., 2009a,b), 2-carboxy lethyl-phenyl-phosphinic acid (CEPPA) (Ding et al., 2015), [((1,1,3,3-tetramethyldisiloxane-1,3-diyl)bis(propane-3,1-diyl))bis(2-methoxy-4,1-phenylene)bis(phenylphosphonochloridate)(SIEPDP) (Li et al., 2015), N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-hexylacetamide-2-propyl acid (PAHPA) (Huang et al., 2012), N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexylformamide-2-propenyl acid (DPHPA) (Huang et al., 2011), and so on, were also intercalated into the LDH interlayer as flame-retardant nanofillers. For example, with only 6% cardanol BS modified MgAl LDHs (m-LDHs), the epoxy resin (EP) composite reached an LOI of 29.2% and UL-94 V-0 rating. The PHRR, THR, and total smoke production (TSP) values of EP/m-LDH-6% were decreased by 62%, 19%, and 45%, respectively, compared to those of pure EP (Wang et al., 2015a,b). Edenharter and Breu (2015) found that with a filler content of 5 wt%, MgAl-PP LDHs could be shown to significantly improve the flame-retardant properties of PS as compared to MgAl-CO3 LDHs, the PHRR values were 47% and 22%, respectively (Fig. 8.14A). Moreover, the heat release of the PP-LDH nanocomposites was spread over a wider range of time resulting in a higher burnout time (from 550 to 695 s), which indicates a slower transfer of mass and heat during the combustion of the polymer. The longer burning time at lower HRR may be related to the formation of a thin layer of char and residues of metal oxides that insulate the polymer from heat radiation (Fig. 8.14B). The addition of 5% and 10% of MgAl-C16 LDHs to PS also resulted in a substantial reduction in PHRR (47% and 61%, respectively) of the polymer (Majoni, 2015). In addition, Kang et al. (2013) investigated the effect of dye structure (acid yellow 36 and acid red 88)

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Heat release rate (HRR) (kW.m–2)

(A) 600

PE-LDH1 PE-LDH2

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400

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200 0

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Figure 8.13 Cone-calorimeter investigation results showing (A) variation of heat release rate (HRR) with time, (B) variation of time of ignition (Tig) and peak heat release rate (PHRR) with LDH content.

intercalated MgAl LDHs (d-LDHs) on the flammability of PP-g-MA composite. Compared with MgAl-NO3 LDHs, d-LDHs can significantly decrease the PHRR and THR of the composite, when the LDH loading was 5 wt%, the PHRR reduced by 22% and 33% for acid red 88 and acid yellow 36 intercalated LDHs, respectively, while it reduced by 11% for NO3-LDHs. In conclusion, organic intercalated LDHs show significantly enhanced flame retardancy compared with pure polymer matrix. Compared with the inorganic LDHs, the organic modified LDHs have much better compatibility with polymers, and much lower LDH loading is required to obtain a similar flame-retardant performance.

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(A)

(B)

HRR [kW/m2]

800 600 400 200 0 0 100 200 300 400 500 600 700 Time [s]

Figure 8.14 (A) Heat release rate for pristine polystyrene (blue) and nanocomposites filled with CO3-LDH (black) or PP-LDH (red) and (B) residue of the cone test for Polystyrene/PPLDH nanocomposite.

8.4.2.3 Effect of LDHs with other synergistic fire retardants A problem of inorganic LDH nanofillers for polymers is that a very high loading is required. Many other flame-retardant additives were combined with inorganic LDHs, such as red phosphorus (RP), APP, carbon nanotubes (CNTs), graphene oxide (GO), melamine (MA), mesoporous silica (m-SiO2), and TiO2, etc. APP is an important flame-retardant additive, and previous studies showed that using LDHs together with phosphorus-containing flame retardants can help to improve the dispersion of these additives within the polymer matrix (Nyambo et al., 2008a,b). Zhao et al. (2008) combined different cation (ZnAl-CO3, ZnFe-CO3, NiAl-CO3, and NiFe-CO3) LDHs as synergistic agents with APP to improve the flame retardancy of PVA matrix. When the content of the LDHs in PVA is 0.3 wt% and the content of APP is 14.7 wt%, the LOI of PVA increased from 20 to 31, 33, 34, and 34, respectively. Furthermore, the amount of residue increased in the order: PVA/APP , PVA/APP/ZnAl , PVA/APP/ZnFe , PVA/APP/NiFe , PVA/APP/ NiAl. Among the PVA/APP/LDH samples, PVA/APP/NiAl showed the best flameretardant performance, which may be attributed to the slightly different decomposition behavior around 450 C from other ternary composites. Furthermore, a study on the effect of MgAlZnFe-CO3 LDHs on the flame-retardant properties of APP and melamine (mass ratio 5:1) poly(butylene succinate) (PBS) composites was investigated by Liu et al. (2014). It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs and IFR was 1% and 19%, respectively, for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. The results showed that a suitable amount of MgAlZnFe-CO3 LDHs had a noticeable synergistic effect on IFR-PBS composites. A possible interaction between APP and LDH was also proposed byZhao et al. (2008). During burning, APP is first thermally decomposed to form poly(phosphoric acid), which may undergo a further dehydration in two traditional ways. The phosphate ester may react with the PVA chain or itself, which subsequently crosslinks with the formation of a three-dimensional network structure.

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After adding the LDHs as synergistic agents, the approach of char formation contributed more to flame-retardant PVA. In addition to the traditional dehydration methods, poly(phosphoric acid) may react with the LDH layers, releasing water molecules and producing bridges between APP chains. The formation of a small number of such bridges will induce a stabilization of APP and a decrease in the volatility of phosphorus, and thus more APP will be available for phosphorylation and char formation (Song et al., 2006; Chen et al., 2005). The crosslinks can increase the degree of polymerization of poly(phosphoric acid), which will increase the viscosity of the melt during pyrolysis and combustion, and therefore enhance the formation of compact and dense charred layer. CNTs are another kind of flame-retardant additive for polymers. It has been found that adding a small amount of CNTs to polymers can improve the flameretardant performance of the polymer composites significantly (Xie et al., 2016). Considering the fact that LDHs and CNTs possess different flame-retardant mechanisms, the potential synergistic effect between them in polymers was investigated by Gao et al. (2016a,b). They found that a system with 10 wt% of aqueous miscible organic (AMO)-LDH and 1 wt% OCNT showed a PHRR reduction of 40%, even greater than the PHRR reduction with PP/20 wt% AMO-LDH (31%) (Fig. 8.15A). The increased PHRR reduction after adding OCNTs is because the dense network of nonflammable OCNTs acts as a physical barrier to the diffusion of oxygen and also slows the escape of combustion products formed during decomposition, which can shield the polymer resin from external radiation and heat feedback from the flame (Kashiwagi et al., 2002; Song et al., 2008; Wang and Jiang, 2011a). They also found the degree of mixing between AMO-LDH and OCNT has a significant effect on the flame-retardant properties. Better mixing can lead to better flame-retardant performance (Fig. 8.15B). In addition, the incorporation of AMO-LDH-OCNT hybrids led to better thermal stability and mechanical properties. CNT and ZnAl-CO3 LDHs with good solubility in liquid media were also synthesized by Xie et al. (2016). It was established that CNT/ZnAl-CO3 LDHs could improve the thermal stability while reducing the PHRR and the total smoke release of polyurethane (PU) foams efficiently. Jiang et al. (2014) combined m-SiO2 and CoAl LDHs to improve their flame retardancy effectiveness in EP. The m-SiO2@CoAl LDHs were synthesized through a layer-by-layer assembly process. The strong Si and O signal across the sphere confirms the m-SiO2 core, while the Co and Al signals both detected in the surface region clearly suggest the adsorption of CoAl LDH particles (Fig. 8.16A). Incorporation of m-SiO2@CoAl LDHs into EP led to an increase of the char yield and a decrease of PHRR as well as THR values. Compared to pure EP, the addition of 2 wt% m-SiO2@CoAl LDHs brings about a 39.3% maximum decrease in PHRR (Fig. 8.16B), and a 36.2% maximum decrease in THR. In addition, the incorporation of m-SiO2@CoAl LDHs results in a significant improvement of the char yield (Fig. 8.16C). The results exhibit that the EP/m-SiO2@CoAl LDH nanocomposites present a good flame retardancy. They also proposed the mechanism for the improved fire-resistant property of EP/m-SiO2@CoAl LDH nanocomposites. During the combustion process, m-SiO2 with catalytic activity leads to the

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Figure 8.15 PHRR reduction of PP and the various nanocomposites prepared.

formation of pyrolysis products with lower carbon numbers, which can be easily catalyzed by carbonization in the presence of metal oxides. Meanwhile, CoAl LDHs can catalyze carbonization of degradation products. Moreover, m-SiO2 plays a role as a barrier that can absorb degraded products to extend the contact time with metal compound catalyst. Furthermore, the degraded products are dehydrogenated and catalytically converted into char by the combination of the mSiO2 labyrinth effect and the CoAl LDH catalysis effect. In addition, Jiao and Chen (2011) proved that TiO2 has a good flame-retardant synergistic effect with LDHs in the EVA/LDH/TiO2 blends. Only 2 phr TiO2 can make the EVA/LDH/TiO2 pass the UL-94 test. The PHRR values of the composite samples decreased with increasing loading of TiO2, and their burning was also prolonged to 600
650 s from 80
250 s. The mechanism of the reduction in PHRR is

Figure 8.16 (A) Dark-field STEM image and elemental mapping of m-SiO2@Co 2 Al LDH, (B) HRR curves of EP and its nanocomposites, and (C) digital photos of the residues from EP and its nanocomposites.

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mainly due to the physical processes in the condensed phase (Kong et al., 2006; Tang and Lewin, 2008). The accumulated TiO2 consequently formed a charred layer by collaborating with LDHs, which acts as a heat insulation barrier. This charred layer prevented heat transfer and transportation of degraded products between the melting polymer and surface, thus reducing the HRR and related parameters. Except APP, CNT, SiO2, and TiO2, microcapsulated red phosphorus (MRP) and intumescent flame retardant (IFR) also show good synergy with inorganic LDHs in polymers. The synergistic effect in the flame retardation between MgAlCO3 LDHs and MRP in EVA has been found by Jiao et al. (2006). The LOI value, the mechanical properties, and thermal stability are improved for the composites. When the loading of LDH and MRP is 70 and 10 phr, respectively, the LOI value increased from 32 to 39. Liu et al. (2014) studied the synergistic effect of IFR consisting of APP and melamine, and MgAlZnFe-CO3 LDHs. It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs was 1% (the total loading of flame retardant was 20%), for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. Although organically modified LDHs have much better flame-retardant performance than inorganic LDHs, organic anions are inherently combustible, thus the loading cannot be too high or both the thermal stability and flame-retardant properties will decrease. In addition, LDHs or organic LDHs alone, even at high concentrations, are not sufficient to obtain a high LOI value or V-0 rating in UL-94 testing. Thus, many synergistic flame-retardant additives are also combined with organic LDHs in order to achieve a desired result. MH is an example of a toxic-free, smoke-suppressing, halogen-free flameretardant additive. In order to improve the flame-retardant performance of polymer, a series of DS-intercalated LDHs, such as MgAl, ZnAl, and MgFe have been added to EVA/MH composites (Ding et al., 2011). The results show that the distribution of inorganics in EVA/MH/LDH composites is more homogeneous than the distribution in sample EVA/MH, which means that the addition of LDH can improve the distribution of MH in EVA. Composites containing LDH show good flame retardancy, when adding 45 wt% MH and 5 wt% LDHs, the PHRR reduction reached 88%, 68%, and 85% for MgAl LDHs, ZnAl LDHs, and MgFe LDHs, respectively. Especially for EVA/MH/MgAl LDH composite, it displays a remarkable reduction in PHRR of almost 60% relative to that of EVA/MH composite without LDHs. APP is a high-molecular-weight phosphate-based chain, it serves as both an acid source and a blowing agent in intumescent formulations to promote char formation during polymer decomposition. Phosphoric acids produced during pyrolysis promote charring, while the evolved NH3 improves swelling, hence slowing or preventing heat and mass transfer to and from the pyrolysis zone (Nyambo et al., 2008a,b). MgAl undecenoic acid LDH and APP were added to neat PS individually or in combinations at weight fractions no greater than 10% by Nyambo et al. (2008a,b). PS composites containing 5% and 10% LDH show reductions in PHRR of 17% and

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27%, respectively. When APP is added to PS at the same weight fractions, lower PHRR reductions were observed, 11% and 22%, respectively. Even though LDHs alone is more effective in reducing the PHRR than APP, their combination produced a better result than simply an additive effect. The observed reduction in PHRR for PS/LDH/APP was significantly higher, and was 42% with 5 wt% LDH and 5 wt% APP. Furthermore, APP/MgAl-DS LDH were added to poly(butyl acrylate
vinyl acetate) (P(BA-VAc)) (Zhao et al., 2011). Pure P(BA-VAc) is a readily flammable polymeric material with an LOI value of 20.0%, and it cannot pass the UL-94 test. The LOI value of flame-retardant P(BA-VAc) can reach up to 30.7 and UL-94 V-0 rating, particularly when the contents of organic LDH and APP in P (BA-VAc) are 0.5 and 14.5 wt%, respectively. IFR is an efficient flame-retardant system for polymer matrix. It is halogenfree and has low toxicity. A typical IFR system is APP and pentaerythritol (APP/ PER). Ding et al. (2010) proved that the addition of LDH nanofillers into the PP composites can obtain good flame-retardant synergistic effects with APP/PER additives at appropriate LDH loadings. Only small amounts of LDH fillers (lower than 5 wt%) can evidently increase the LOI values. For example, 5 wt% loading of LDH can increase the LOI value up to 35 of PP/IFR/ZnAl-DS LDH composites from 19 of pure PP. This indicates that a small amount of LDH fillers can give a good synergistic effect on flame-retardant properties with APP/PER additives. Wang et al. (2012a,b,c,d) synthesized maleic anhydride grafted ethylenepropylene-diene terpolymer (mEPDM)/IFR/MgAl-DBS composite; the results showed that the introduction of a small amount of LDH in the flame-retardant mEPDM led to a significant decrease in HRR. The PHRR value reduced by 55.2% when the IFR and LDH loading is 38 and 2 phr, respectively. In addition, when adding 30 phr IFR and 2 phr DBS-modified MgAl LDH into EPDM, the LOI value increased to 27% from 17.5% (pure EPDM), and the composite reached a V-0 rating in the UL-94 test (Shen et al., 2013). Furthermore, PMMA/IFR/ MgAl-DS LDH composites were investigated by Huang et al. (2014) when incorporating 5 wt% LDH and 10 wt% IFR, the PHRR reduction was 37.9%, the LOI value increased to 26.1 from 17.4 of pure PMMA. Wang et al. (2015a,b) investigated the effect of DBS-modified binary MgAl- and ternary MgZnAl-LDHs on flammability of flame-retardant PP composites in combination with IFR additives. The synergism between either binary or ternary LDH and IFR occurred during the combustion. The reduction of PHRR value was 79.2% and 77.7% for PP/18 wt% IFR/2 wt% MgAl-DBS and PP/18 wt% IFR/2 wt% MgZnAl-DBS LDH composites, respectively (Fig. 8.17A). In contrast to the MgAl LDHs, the MgZnAl LDHs showed superior char-formation ability and smoke suppression due to the presence of the element zinc. When IFR is partly substituted by LDH, most LOI values are slightly higher than that with IFR alone, except PP/16 wt% IFR/4 wt% MgAl-DBS LDH composites. An optimum is observed at 2.0 wt% of MgZnAl LDHs and 18.0 wt% of IFR, exhibiting the highest LOI of 32.5% and a UL-94 V0 rating in vertical burning test (Fig. 8.17B). The combustion process for PP/IFR/LDH composites can be divided into four stages based on the HRR curves: (1) predegradation of PP; (2) main burning

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Figure 8.17 (A) Heat release rate versus time curves of PP and its flame-retardant composites and (B) the LOI values and UL-94 results of PP and its flame-retardant composites.

process; (3) char formation; and (4) oxidation of char residues (Fig. 8.17A). The incorporation of both binary and ternary LDH has a significant influence on the fourth stage, which enhances the thermal oxidative resistance of the char layer, answering for the disappearance of the second peak of HRR curves for PP/IFR/ LDH composites. Except MH, APP and IFR, zinc borate (Wang et al., 2011a,b,c,d), triphenol phosphate (TPP) (Manzi-Nshuti et al., 2009,b,c), melamine (Manzi-Nshuti et al., 2008), and ZrP (Jiang et al., 2016) were also combined with organic LDHs as a synergistic flame-retardant additive. For example, as an effective synergistic flameretardant, the addition of the LDH/ZrP composites resulted in a significant decrease in the HRR compared with pure PP. When the loading of LDHs and ZrP was 5% and 15% (a ratio of 1:3), respectively, the PHRR was reduced by 28.2%. The improved flame-retardant performance may be because LDH and ZrP decomposed

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to produce noncombustible gases, which diluted the combustible gases, and a compact char layer that acted as a fire barrier (Jiang et al., 2016). Also, Huang et al. (2014) studied the flame-retardant effect of IFR/RGO/LDH on PMMA, when filled with 10 wt% IFRs, 1 wt% RGO, and 5 wt% LDHs; it can achieve an LOI value of 28.2% and UL-94 V-1 grade. Compared with neat PMMA, the PHRR of PMMA/ IFRs/RGO/LDHs is reduced by about 45%. Previous studies show that both the physical and chemical interactions between the LDHs and the halogen-free flame-retardant (HFFR) materials are responsible for the observed synergy in fire performance (Nyambo et al., 2008a,b). The LDHs are thought to impact char formation in the polymer/HFFR system. This char is very effective, making the polymeric substance less prone to combustion (Wang et al., 2010).

8.4.3 Posttreatment of LDHs as flame retardants Because of the intrinsic hydrophilic nature of LDHs, they generally do not exhibit native compatibility with nonpolar polyolefin hosts (e.g., PP), which may affect the flame-retardant characteristics of the materials. Therefore, to enhance the compatibility of the LDHs with the polymer matrices is very important. Till now, there are the following two methods to improve the dispersibility of LDH particles in polymers.

8.4.3.1 Organic modification of LDHs In order to apply LDHs as an inorganic filler in polymer nanocomposites, hydrophobic modification of LDH is necessary, and can be achieved by intercalating anionic surfactants with hydrophobic aliphatic carbon chains, such as fatty acids and dodecyl sulfate. Hydrophobic polymer chains can easily access the interlayer of LDH when the hydrophobic interlayer is swelled, resulting in polymer nanocomposites with highly dispersed LDH. Or, the LDH materials are treated by organic modification with anionic surfactants before they are incorporated into the polymers. The anionic surfactants include fatty acid, fatty acid metal salt, silane coupling agent, or titanate coupling agent, etc. (Feng et al., 2012; Tao et al., 2009). For example, pristine Mg2Al LDH was modified with three different organic acids (laurate, palmitate, and stearate) to increase its hydrophobicity. TEM analysis showed most of the layers of LDH modified with laurate and palmitate were sufficiently separated from each other, and randomly dispersed in the PP matrix, indicating that most of the LDH layers were exfoliated in the PP matrix (Fig. 8.18A,B). But in PP/ stearate-LDH nanocomposites, a swelled LDH structure with several layers (3
6 layers) and sufficiently separated layers was observed, indicating that both the PP intercalated LDH layers and the exfoliated LDH layers existed concurrently (Fig. 8.18C) (Yang et al., 2015). Treatment of the LDHs with 1
10 wt% of anionic surfactant, for example, sodium stearate or sodium oleate results in improvement of dispersibility and fluidity.

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Figure 8.18 Transmission electron microscopy images of (A) 3 phr laurate-LDH/PP, (B) 3 phr palmitate-LDH/PP, and (C) 3 phr stearate-LDH/PP, respectively.

8.4.3.2 Aqueous miscible organic solvent treatment In order to improve the compatibility between LDH and polymer, one solution is to intercalate surfactant anions into the LDH interlayers. The other method to improve the dispersibility of LDH particles in polymer matrix is using the solvent-mixing method. Currently, most polymer/LDH nanocomposites are prepared using the melt-mixing method, by which the polymer and dried LDH powders are mixed in an extruder at an elevated temperature. However, one problem is that the LDH nanoparticles aggregate severely when being dried, which can result in poor dispersion of the LDH nanoparticles in the polymer matrix. Therefore, in order to make highly dispersed polymer/LDH nanocomposites, solvent mixing is preferred. Recently, Wang et al. (2012a,b,c,d) reported a new method that makes it possible to disperse inorganic anion-intercalated LDHs in nonpolar solvents (e.g., xylene). Polymer/LDH nanocomposites can then be prepared by the solvent-mixing method. Due to the intrinsic hydrophilic nature of LDHs, they generally cannot be dispersed in nonpolar solvents and so they do not exhibit native compatibility with nonpolar polyolefin hosts. After aqueous miscible organic solvent treatment, these solvent-treated LDH nanoparticles can now be dispersed in xylene to give a stable suspension (see Fig. 8.19A). UV-visible data also indicate that the LDH suspension in xylene is optically transparent and stable (Fig. 8.19B) (Wang et al., 2012a,b,c,d). By using this method, polypropylene/Mg3Al-LDH nanocomposites were synthesized successfully by dissolving PP into a clear dispersion of Mg3AlLDH in xylene. Further, ZnAl and MgAl LDHs with different inorganic anions (such as Cl2, NO32, CO322, and SO422) were dispersed in the HDPE, EVA, and PP polymer matrix using the solvent-mixing method. The thermal stability and flame-retardant performance of the nanocomposites were significantly enhanced due to the good dispersion of LDHs (Gao et al., 2014a,b).

8.4.4 The mechanism of flame retardancy using LDH The origin of the excellent flame retardancy and smoke suppression properties of LDHs is derived from their unique chemical composition and layered structure.

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Figure 8.19 The effect of solvent treatment on the Mg3Al-LDH nanoparticles: (A) unmodified LDH cannot be dispersed in xylene, after the washing treatment the LDHs nanoparticles can be dispersed in xylene, (B) transmission UV-visible spectra of solventmodified Mg3Al-LDH dispersions in xylene.

Although no mechanism has been proposed for flame-retardant LDH nanocomposites (Nyambo et al., 2009a,b; Zhu et al., 2001; Chen et al., 2007), it is generally believed that the flame-retardant mechanism of LDHs is different from that of silicate clay-based nanocomposites (Nyambo et al., 2008a,b). During thermal decomposition, the LDH gradually loses its interlayer water when heated in the range from 50 C to 220 C. At higher temperature, it further loses its hydroxyl groups in the host sheet and carbonate anions in the interlayer spacings to produce water and carbon dioxide, and then it is converted into mixed metal oxides. The water vapor and carbon dioxide released from the LDHs can dilute flammable gases and prevent contact of the materials with oxygen and eventually stop the combustion when there is not enough fuel to propagate the reaction, and promote the formation of an expanded carbonaceous coating or char. Char formation protects the bulk polymer from exposure to air, thus reducing the heat release during the combustion and suppressing smoke production (Zammarano et al., 2005; Chen et al., 2002). The mechanism of action of the LDHs as the flame retardant can be described as the endothermic decomposition reducing the fire intensity, the shielding due to char formation, and stabilization of char and the dilution effect (Feng et al., 2012). Consequently, the mass loss rate will be significantly reduced due to the combination of the above-mentioned three functions (Gao et al., 2014a,b).

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Conclusions and future development

In this chapter, we have summarized the current research into flame-retardant polymer/LDH nanocomposites. As flame retardant nanofillers, the role of LDHs is summarized as having four functions: (1) heat absorption (endotherm), (2) gaseous dilution, (3) char formation, and (4) dispersion. A proper loading of LDH could also improve the thermostability of polymers because the released H2O and CO2 can delay the scission of polymer chains, so making the polymer composites more stable. In addition, the synergistic effect between LDHs and other HFFR additives was discussed. As a synergistic additive, LDH can not only enhance the flameretardant properties of polymer/LDH nanocomposites, but also reduce the loading of HFFR agents in polymer matrix so as to improve the thermal stability and mechanical properties of polymer nanocomposites. The synergistic function of LDHs is considered to impact the char formation of polymer/HFFR systems. But the detailed mechanism between LDHs and HFFR needs to be explored in future work.

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Mechancial and dynamical mechanical properties of layered double hydroxide-filled elastomer and elastomeric blend nanocomposites

9

Suneel Kumar Srivastava Department of Chemistry, Indian Institute of Technology, Khragpur, India

9.1

Introduction

Elastomers, according to the general IUPAC definition, are polymers exhibiting rubber-like elasticity (Evans and Slade, 2006). Their general characteristics involve viscoelastic behavior, low modulus of elasticity, high failure strain, along with very weak intermolecular forces. Their good heat resistance, ease of deformation at ambient temperatures, and exceptional elongation and flexibility before breaking make elastomers excellent and relatively cheap materials for extensive applications in a variety of commercial as well as domestic products (Choy et al., 2007; Bhowmick, 2008; Acharya et al., 2007b; Kura et al., 2014; Gu et al., 2015; Daud et al., 2006; Wang and O’Hare, 2012; Basu et al., 2013; Zvonimir and Wilkie, 2012; Velosco and Antunes, 2012; Kotal et al., 2009). The incorporation of nanofillers into elastomers further significantly improves their heat resistance, photo and UV stability, flammability, barrier properties, thermal, mechanical, and dynamic mechanical properties, etc., depending on the polymer matrices, nature of nanofillers, and their dispersion in elastomer matrices (Bhowmick, 2008). Although, a variety of inorganic fillers have been used in the fabrication of elastomeric nanococomposites, layered double hydroxides (LDHs) remain an important class of filler (Acharya et al., 2007b). LDHs exhibit a wide range of available chemical compositions and sufficient interlayer spacing. The general formula of LDHs can be represented as: [M z112xM31x(OH)2]q1(Am2)q/m  nH2O. Usually z 5 2, with M12 5 Ca12, Mg12, Zn12, Ni12, Mn12, Co12, or Fe12 and with M31 5 Al31, Cr31, Mn31, Fe31, Ga31, Co31, or Ni31 so that q 5 x. In one case z 5 1, with M1 5 Li1 and M31 5 Al31, giving q 5 2x
1. The values of x have been reported to fall between the range 0.1
0.5, however pure phases only exist for 0.2
0.33, that is, M12/M13 ratios are in the range of 2
4 and Am2 5 interlayer anions present in the LDH. The order of preference of exchangeable anions in the interlayer is: NO32 , Br2 , Cl2 , F2 , OH2 , SO422 , CO322. LDHs have a brucite-like Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00009-4 © 2020 Elsevier Ltd. All rights reserved.

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Figure 9.1 Structure of typical layered double hydroxide (Acharya et al., 2007b). Source: Reproduced with permission from Elsevier.

structure, where divalent octahedrally coordinated M(II) ions are partially substituted by trivalent M(III) ions. As a result, the positively charged metal oxide/ hydroxide layers are neutralized by other charge-balancing anions. Fig. 9.1 shows a schematic representation of a typical layered double hydroxide (Acharya et al., 2007b). A number of synthetic techniques have been successfully employed for the preparation of LDH, such as ion exchange process, coprecipitation, and reconstruction method. In addition, sol
gel synthesis, fast nucleation process followed by a separate aging step at elevated temperatures, and hydrothermal methods are also reported. However, pristine LDH prepared in this manner is not suitable in the development of elastomeric nanocomposites due to the smaller interlayer gallery space (B0.76 nm) (Velosco and Antunes, 2012). Therefore, pristine LDH is modified with anionic surfactants, for example, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). This replaces the counter anion and water molecules in the interlayer space. In addition, aromatic carboxylate anion, aliphatic carboxylate anion, phosphonates, and polymeric anions, etc., are also used for the organomodification of LDHs. These LDH-filled polymers typically show enhanced properties compared to neat polymer matrix. As a result, elastomer/LDH and elastomeric blend/LDH nanocomposites find their multifaceted applications in different fields (Acharya et al., 2007b; Kura et al., 2010a, 2013, 2014; Gu et al., 2015; Daud et al., 2006; Wang and O’Hare, 2012; Basu et al., 2013; Zvonimir and Wilkie, 2012; Velosco and Antunes, 2012; Kotal et al., 2009, 2010a,b, 2011, 2013; Kotal and Srivastava, 2011a,b; Kotal, 2012; Go´mez-Ferna´ndez et al., 2016; Srivastava and Kotal, 2013; Guo et al., 2011; Xiong et al., 2015; Yan et al., 2013; Zhang et al., 2007, 2008, 2013, 2016; Xu et al., 2016a,b; Roy et al., 2016a,b,c; Roy, 2017; Yu et al., 2013, 2014a,b; Yao et al., 2002; Wang and Pinnavaia, 1998; Kumar and Das, 2010; Kuila et al., 2007, 2008a,b, 2009a,b,c; Kuila, 2009; Ye et al., 2008; Costache et al., 2007; Wang et al., 2011, 2012a,b, 2013; Jiao et al., 2006, 2010; Pradhan, 2013; Pradhan et al., 2008, 2011, 2012; Pradhan and Srivastava, 2014; Nhlapo et al., 2008; Borja and Dutta, 1992; Meyn et al., 1990; Kanoh et al., 1999; Itoh et al., 2003; Shieh et al., 2010; Fornes et al., 2004; Fornes

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and Paul, 2003; Kong et al., 2006; Acharya et al., 2007a, 2008; Wu et al., 2011, 2004; Kim et al., 2001; Acharya, 2008; Chao et al., 2013; Das et al., 2011a,b,c, 2012; Basu et al., 2016; Zhao et al., 2014; Kuila et al., 2012; Cui and Qu, 2010; Kato, 2011; Acharya and Srivastava, 2017; Raman et al., 2013; Srivastava and Kuila, 2013; Srivastava and Himadari, 2010; Xiao et al., 2013, 2014; Wei-Na and Da-chen, 2012; Feng and Su, 2011; Sadhu and Bhowmick, 2004; Bharadwaj et al., 2013; He et al., 2016; Braga et al., 2014; Laskowska et al., 2013; Feng et al., 2013; Long-Chao et al., 2011; Fabiula et al., 2011; Eshwaran et al., 2015; Laskowska et al., 2014a; Costa et al., 2010; Srivastava and Mittal, 2017; Basu et al., 2014; Abdullah et al., 2010; Bottazzo et al., 2013; Srivastava, 2014). In view of this, the main aim of this review is to provide an overview of the mechanical and dynamical properties of LDH-filled elastomer and elastomeric blend nanocomposites. The elastomers addressed in this review consist of polyurethane (PU) (Kotal et al., 2009, 2010a,b, 2011, 2013; Kotal and Srivastava, 2011a,b; Kotal, 2012; Go´mezFerna´ndez et al., 2016; Srivastava and Kotal, 2013a; Guo et al., 2011; Xiong et al., 2015; Yan et al., 2013; Zhang et al., 2013, 2016; Xu et al., 2016a,b; Roy et al., 2016a,c; Roy, 2017; Yu et al., 2013), ethylene vinyl acetate copolymer (EVA) (Yao et al., 2002; Wang and Pinnavaia, 1998; Kumar and Das, 2010; Kuila et al., 2007, 2008a,b, 2009a,b,c; Kuila, 2009; Ye et al., 2008; Zhang et al., 2007, 2008; Costache et al., 2007; Wang et al., 2011, 2012, 2013; Jiao et al., 2010), silicone rubber (SR) (Pradhan, 2013; Jiao et al., 2006; Pradhan et al., 2011, 2012; Pradhan and Srivastava, 2014; Kong et al., 2006), ethylene propylene diene rubber (EPDM) (Acharya et al., 2007a; Wu et al., 2004; Kim et al., 2001; Acharya, 2008; Pradhan et al., 2008; Chao et al., 2013; Das et al., 2011a; Basu et al., 2016; Zhao et al., 2014; Kuila et al., 2012; Wang et al., 2012; Cui and Qu, 2010), styrene butadiene rubber (SBR) (Acharya and Srivastava, 2017; Acharya et al., 2008; Yu et al., 2014a,b; Raman et al., 2013; Xiao et al., 2013; Wei-Na and Da-chen, 2012; Das et al., 2011b, 2012; Feng and Su, 2011; Wu et al., 2011; Sadhu and Bhowmick, 2004), acrylonitrile butadiene rubber (NBR) (He et al., 2016; Braga et al., 2014; Xiao et al., 2014; Laskowska et al., 2013; Feng et al., 2013; LongChao et al., 2011; Fabiula et al., 2011; Eshwaran et al., 2015; Das et al., 2011a,c; Laskowska et al., 2014a; Costa et al., 2010; Srivastava and Mittal, 2017; Basu et al., 2014), natural rubber (NR) (Abdullah et al., 2010; Bottazzo et al., 2013; Srivastava, 2014), etc.

9.2

Preparative methods of LDH-elastomer and LDH-elastomeric blend nanocomposites

LDH/elastomers and LDH/elastometic blend nanocomposites have been prepared by in situ polymerization, solution blending, and melt blending. Depending on the filler distribution, polymer/LDH nanocomposites can be defined as intercalated nanocomposite, exfoliated nanocomposite, and partially exfoliated nanocomposite, depending on whether the LDH layers are arranged regularly and stacked parallel to

350

Layered Double Hydroxide Polymer Nanocomposites

each other, randomly distributed, or stacked parallel to each other, in addition to some randomly dispersed layers throughout the polymer matrix. The preparation of nanocomposites based on the solution blending method is considered not ecofriendly and cost-effective due to the use of excess amounts of organic solvents. In contrast, melt mixing of polymer with nanofiller is the most effective way to obtain nanocomposites for commercial application. The nanofillers are added to the molten polymer under a shearing force to obtain homogeneously dispersed inorganic filler
elastomer nanocomposites. The method is totally environmentally friendly and does not require solvent. Rubber naocomposites have also been prepared using two-roll mill mixing by introducing ingredients and controlling the gap distance between the rolls, setting the desired temperature and speed.

9.3

Different types of layered double hydroxide fillers used in the fabrication of elastomer and elastomeric blend nanocomposites

The incorporation of layered double hydroxide into elastomers improves significantly their mechanical, thermal, dynamic mechanical, barrier properties, flame retardancy, etc. Table 9.1 summarizes detailed information on the different types of LDH fillers used in fabrication of PU (and TPU), EVA, EPDM, SR, SBR, NBR, XNBR, and NR nanocomposites.

9.4

Morphology of elastomer-LDH and elastomeric blend-LDH nanocomposites

Polymer nanocomposites exhibit enhanced physicochemical properties compared to bulk polymer or conventional composites. This is ascribed to the nanolevel dispersion of the inorganic filler in the polymer matrix. Therefore, it is mandatory to investigate the morphology of polymer nanocomposites by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

9.4.1 Morphology of elastomeric-LDH nanocomposites 9.4.1.1 PU-LDH nanocomposites 9.4.1.1.1 XRD Kotal et al. (2009) studied XRD of dodecyl sulfate intercalated Mg-Al-LDH (referred to as DS-LDH)-filled TPU nanocomposites. These investigations revealed the absence of any diffraction peaks corresponding to neat PU in the range of 2θ  2
10 degrees. XRD of TPU/DS-LDH (1, 3, and 5 wt.%) showed the absence of diffraction peaks of DS-LDH corresponding to 2θ  3.41 (d003) and 6.89 degrees (d006), though a small hump appeared at around 2θ 5 2.5 degrees in the case of

Table 9.1 Different LDH used as fillers in preparation of elastomer and elastomeric blend nanocomposites Elastomer

Filler

Preparation method

References

TPU TPU PU prepolymer (isocyanate terminated) PU prepolymer

Solution blending Solution intercalation In situ polymerization

Kotal et al., 2009 Kotal et al., 2011 Kotal and Srivastava, 2011b Kotal et al., 2011a

PU prepolymer PU prepolymer Waterborne PU WPU Prepolymer PU PU prepolymer TPU/NBR (1:1 wt%) TPU/NBR TPU:NBR PU

Dodecyl sulfate intercalated LDH (1, 3, 5 and 8 wt.%) Stearate-intercalated LDH (1, 3, 5, and 8 wt%) DS- and St-LDH-modified LDHs, isocyanate grafted DS- and St-LDH Dodecyl sulfate intercalated Mg-Al layered double hydroxide (1, 3, 5, 8 wt.%) Stearate intercalated-LDH Dodecyl sulfate-modified Co-Al-LDH NiAl-LDH/ZnO ZnAl-LDH/ZnO Reduced graphite oxide-LDH DS-intercalated MgAl-LDH DS-Mg
Al-LDH CNF-LDH LDH:MWCNT Heptamolybdate intercalated MgAl LDHs

EVA-18

DS-Mg-Al-LDH

EVA-28

DS-LDH

EVA-40

DS-LDH

EVA-45)/EPDM(1:1 wt. ratio) EVA-60

DS-Mg-Al-LDH DS-Mg-Al-LDH

In situ polymerization In situ polymerization In situ polymerization In situ polymerization In situ polymerization Blending method In situ polymerization Solution blending, Solution intercalation Solution intercalation Prepolymerization method Solution blending method Solution blending method Solution blending method Solution blending method Solution blending method

Kotal et al., 2013 Guo et al., 2011 Xiong et al., 2015 Zhang et al., 2013 Xu et al., 2016a Zhang et al., 2016 Roy et al., 2016b Roy et al., 2016b Roy et al., 2016c Xu et al., 2016b Kuila et al., 2008b Kuila et al., 2007 Kuila et al., 2009a Kuila et al., 2009b Kuila et al., 2009c (Continued)

Table 9.1 (Continued) Elastomer

Filler

Preparation method

References

EVA-28/LDPE

DS-Mg-Al-LDH

Kuila et al., 2008a

EVA-18

DS-Zn-Al-LDH

LDPE/EVA-18 EVA-28 EVA-19 EVA-14 EVA-28 EVA-14 SR SR SR

Hyperfine magnesium hydroxide (HFMH)/DS-MgAl- LDH HFMH/organomodified Mg-Al-LDH LDH-CO3 modified with 2-aminotoluene-5-sulfonic acid Al2O3 with LDH Stearate-modified Mg-Al-LDH with different Ni content Nano hydrotalcite St- Mg
Al-LDH DS-Mg-Al-LDH MWCNT-Li
Al-LDH, MWCNT-Mg
Al-LDH, MWCNTCo
Al-LDH DS-Mg-Al-LDH DS-Mg-Al-LDH Sodium-1-decanesulfonate-modified LDH DS- Mg-Al-LDH, undecylenic acid-Mg-Al-LDH,(UA-LDH), oleic acid-Mg-Al-LDH(LDH-OA) Zn-Mg-Al-LDH Zn-Al-LDH Mg
Al, Ni
Al, and Cu
Al LDHs Sodium dodecyl benzene sulfonate-Mg-Al-LDH Zn-Al-LDH

Solution blending method Melt and solution intercalation Melt compounding Melt intercalation Melt blending Melt blending Melt blending Melt blending Solution blending Solution blending Solution blending

EPDM EPDM EPDM, XNBR EPDM EPDM EPDM EPDM EPDM Solution styrene butadiene copolymer (SSB) NBR NBR Latex

Sodium p-styrene sulfonate modified Mg-Al LDH Mg-Al-LDH

Solution blending Solution blending Solution intercalation Melt mixing Two-roll mill mixing Two-roll mill mixing Twin-roll mill Two-roll mill mixing Open two-roll mill mixing Roll mixing mill Mixing method

Zhang et al., 2008 Ye et al., 2008 Zhang et al., 2007 Costache et al., 2007 Jiao et al., 2010 Wang et al., 2013 Jiao et al., 2006 Pradhan et al., 2012 Pradhan et al., 2011 Pradhan and Srivastava, 2014 Acharya et al., 2007a Acharya, 2008 Pradhan et al., 2008 Chao et al., 2013 Das et al., 2011a Basu et al., 2016 Zhao et al., 2014 Wang et al., 2012b Das et al., 2012 He et al., 2016 Braga et al., 2014

NBR NBR

Sodum lignosulfonate-modified Mg-Al-LDH Mg-Al-LDH

Melt compounding Roll mixing mill

NBR NBR XNBR

LDH modified by sodium styrene sulfonate and sodium dodecyl benzene sulfonate Unmodified LDH and stearate-modified LDH MgAl-LDH MgAl-LDHs

X-NBR

Mg-Al-LDH

Open two-roll mill mixing Two-roll mill mixing Brabender Measuring Mixer Two-roll mill mixing

X-NBR

Organically modified Mg-Al-LDH

X-NBR NR NR/BR

Zn-Al-LDH DS-Zn-Al-LDH LDH (modified and unmodified)

Two-step melt compounding method Two-roll mill mixing Mixing method Two-roll mill mixing

Xiao et al., 2014 Laskowska et al., 2013 Feng et al., 2013 Eshwaran et al., 2015 Laskowska et al., 2014a Laskowska et al., 2014b Costa et al., 2010

Basu et al., 2014 Abdullah et al., 2010 Bottazzo et al., 2013

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Layered Double Hydroxide Polymer Nanocomposites

TPU/8 wt.% DS-LDH nanocomposite. All these observations clearly suggest that DS-LDH layers are partially or completely exfoliated at lower DS-LDH loadings, followed by intercalation at higher filler loadings. XRD patterns of TPU nanocomposites containing 1, 3, and 5 wt.% stearate intercalated Mg-Al-LDH (referred to as St-LDH) also exhibited similar behavior (Kotal et al., 2011). However, a comparatively broad peak appeared at 2θ  2.07 degrees in TPU/St-LDH (8 wt.%) corresponding to the interlayer spacing of 4.26 nm due to the intercalation of TPU chains within the interlayer spaces of St-LDH (Kotal et al., 2013). Kotal and Srivastava (2011b) also studied XRD patterns of PU/MDI- and PU/IPDI-grafted LDHs (pristine and organomodified) nanocomposites. These findings clearly demonstrated the intercalation of PU chains in the interlayer space of the MDI- or IPDIgrafted pristine LDHs. In contrast, (003) basal diffraction peak as well as the weak (110) of MDI-g-pristine LDH and IPDI-g-pristine LDH, were completely absent in 3 wt.% filled MDI- and IPDI-grafted DS and St-LDHs/PU nanocomposites. This suggested the formation of uniform dispersion and exfoliation of MDI- or IPDI-grafted DS-LDH or St-LDH layers in PU matrix. Kotal et al. (2010a) also made XRD studies on DS-LDH, neat PU and 1, 3, 5, and 8 wt.% DS-LDH-incorporated PU nanocomposites prepared by solution blending method. A diffraction peak (003) appeared at 2θ B 3.2 degrees corresponding to 2.75 nm basal spacing of DS-LDH layers. XRD of neat PU showed an absence of diffraction peak (003) in 2θ range, corresponding to the DS-LDH. This peak was also found to be absent in PU filled with 1 wt.% DS-LDH content due to the delamination or exfoliation of DS-LDH in the PU matrix (Xu et al., 2016a; Roy et al., 2016b) However, a slight hump appeared below 2θ B 3.2 degrees in the diffractograms of PU filled with 3, 5, and 8 wt.% DS-LDH, indicating partial exfoliation or intercalation of DS-LDH in PU. Guo et al. (2011) also studied XRD of neat PU and its nanocomposite consisting different loadings of CoAl-DS-LDH. The (003) basal peak of CoAl-DS-LDH is found to be absent in 0.5
2.0 wt.% filled PU indicating exfoliation of CoAl-DSLDH layers in PU matrix. XRD of PU/APS-DS-LDH nanocomposites showed absence of (003) basal diffraction peak of the nanocomposites (in the range from 1 to 3.0 wt.% LDH loading) due to the highly dispersed and/or exfoliated layers of APS-DS-LDH (Zhang et al., 2016). At 5.0 wt.% APS-DS-LDH loading in PU, (003) reflection slightly appears due to the aggregation of APS-DS-LDH. Kumar and Das (2010) observed delamination/complete exfoliation of organically modified Mg-Al-LDH layers in the polyurethane matrix.

9.4.1.1.2 TEM The morphology of 1, 5, and 8 wt.% filled DS-LDH and St-LDH nanocomposites of TPU was studied and corresponding TEM images are shown in Fig. 9.2 (Kotal et al., 2009) and Fig. 9.3 (Kotal et al., 2011), respectively. It is noted that a fraction of the DS-LDH layers is uniformly exfoliated in TPU/DS-LDH (1 wt.%) and TPU/ DS-LDH (5 wt.%) nanocomposites. Interestingly, in either case, intercalated tactoids of LDH crystallites, mostly in the form of thin platelets, are also present along with the exfoliated LDH layers, suggesting the formation of partially exfoliated

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Figure 9.2 TEM images of (A) PU/DS-LDH (3 wt.%) nanocomposite and (B) PU/DS-LDH (8 wt.%) nanocomposite (Kotal et al., 2009). Source: Reproduced with permission from Wiley.

morphology. However, an intercalated structure consisting of parallel DS-LDH layers is developed at 8 wt.% filler loading in TPU. The TEM of TPU/St-LDH (1 wt.%) and TPU/St-LDH (5 wt.%) nanocomposites suggested exfoliation of StLDH layers throughout the TPU matrix involving both exfoliated layers and intercalated tactoids of LDH crystallites in the form of thin platelets, respectively. However, exfoliated layers/intercalated tactoids of LDH crystallites and the aggregation of St-LDH are observed at 8 wt.% loading throughout the TPU matrix. All these findings clearly suggested St-LDH to be more compatible than DS-LDH in the formation of more exfoliated layers at lower loading of St-LDH. TEM images of modified and unmodified LDH (3 wt.%)-filled polyurethane nanocomposites showed delamination of filler in the polyurethane matrix (Kumar and Das, 2010). TEM images of PU/MDI-g-pristine LDH, PU/IPDI-g-pristine LDH, PU/MDI-g-DSLDH, PU/IPDI-g-DS-LDH, PU/MDI-g-St-LDH, and PU/IPDI-g-St-LDH nanocomposites at 3 wt.% filler loading are displayed in Fig. 9.4 (Kotal and Srivastava, 2011b).

356

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.3 TEM images at higher magnification of PU nanocomposites containing (A) 1 wt. % (B) 5 wt.%, and (C) 8 wt.% LDH
stearate (Kotal et al., 2011). Source: Reproduced with permission from Wiley.

Figure 9.4 TEM images of (A) PU/MDI-g-pristine LDH, (B) PU/IPDI-g-pristine LDH, (C) PU/MDI-g-DS-LDH, (D) PU/IPDI-g-DS-LDH, (E) PU/MDI-g-St-LDH, and (F) PU/IPDI-gSt-LDH nanocomposites showing the nature of dispersion of MDI- and IPDI-grafted pristine, DS, and St-LDH layers in PU matrix (Kotal and Srivastava, 2011b). Source: Reproduced with permission from Royal Society of Chemistry.

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It clearly indicated aggregation of MDI- and IPDI-grafted pristine LDH in PU (cf. Fig. 9.4A and B), while such aggregates are found to be absent in MDI- and IPDI-grafted DS-LDH/PU nanocomposites (Fig. 9.4C and D), respectively. However, some tactoids composed of several metal hydroxide sheets (indicated by the arrows) with thicknesses 10
30 nm are also present, along with exfoliated MDI-g-DS-LDH and IPDI-g-DS-LDH layers in PU matrix, resulting in the formation of partially exfoliated nanocomposites. Fig. 9.4 also provides an idea about the dispersion of MDI-g-St-LDH and IPDI-g-St-LDH in PU. MDI-g-St-LDH/PU exhibited relatively better exfoliation and homogeneous dispersion of LDH layers in the PU matrix compared to the IPDI-g-St-LDH. Morphology of PU/dodecyl sulfate-modified Mg-Al-LDH (DS-LDH) nanocomposites by in situ synthesis of PU from prepolymer and polyol TG was also studied by TEM analysis (Kotal et al., 2010a). The lower-magnification image showed inhomogeneous distribution of DS-LDH layers in PU matrix. However, the corresponding higher-magnification image confirmed the partial delamination of DS-LDH layers. This was ascribed to the polar interaction between DS-LDH layers and PU prepolymer/hydroxylfunctionalized polyol-TG (mixtures of glycerol and trimethylolpropane). TEM images of PU/DS-LDH (5 wt.%) nanocomposite suggested intercalation of DSLDH layers in PU matrix. However, agglomeration is observed at 8 wt.% loading of DS-LDH in PU matrix. Scheme 9.1 represents the formation of partially exfoliated and intercalated PU nanocomposites from pristine Mg
Al LDH and PU.

Scheme 9.1 Schematic diagram for the formation of partially exfoliated and intercalated PU nanocomposites from pristine Mg
Al LDH and PU (Kotal et al., 2010a). Reproduced with permission from American Scientific Publishers.

358

Layered Double Hydroxide Polymer Nanocomposites

TEM study of PU/APS-DS-LDH (3 wt.%) nanocomposites suggested the formation of partially exfoliated states for nanocomposites (Yu et al., 2013). However, stacking of APS-DS-LDH layers in the PU matrix was clearly visible in PU/APSDS-LDH (5 wt.%). Fractured surface morphology analysis of WPU/NiAl-LDHs/ ZnO composites showed homogeneous distribution of NiAl-LDH/ZnO (0.5, 1.0, 1.5 wt.%) (Xiong et al., 2015). Such uniform distribution of the filler in the WPU matrix plays an important role in improving the mechanical performance of the prepared composites. The increasing content of NiAl-LDHs/ZnO (2.0 wt.%) led to microphase separation due to closer contact of fillers in WPU in the composites. In another work, TEM image of LDH in PU nanocomposite containing 2 wt.% CoAlDS-LDH also confirmed exfoliation of CoAl-DS-LDH layers (Guo et al., 2011). TEM of waterborne hyperbranched polyurethane acrylate (WHPUA)/dodecyl sulfate-modified LDH nanocomposites based on hyperbranched aliphatic polyester boltorn and MgAl-LDH showed formation of intercalated and exfoliated structures (Wang and Pinnavaia, 1998).

9.4.1.2 EVA-LDH nanocomposites 9.4.1.2.1 XRD Srivastava and his group reported XRD studies of EVA-18/DS-LDH (Kuila et al., 2008b), EVA-28/DS-LDH (Kuila et al., 2007), EVA-40/DS-LDH (Kuila et al., 2009a), and EVA-60/DS-LDH (Kuila et al., 2009c). A sharp basal peak (003) appeared in XRD of DS-LDH (modified) and LDH (unmodified) corresponding to an interlayer distance of B2.76 and 0.78 nm, respectively. The observed increase of 1.98 nm in basal spacing of DS-LDH is due to the intercalation of dodecyl sulfate anion. XRD patterns of pure EVA-18, EVA-28, EVA-45, and EVA-60 exhibit no peak in the 2θ range of 2
10 degrees. But original (001) peak of DS-LDH is shifted to a lower angle in EVA-18/DS-LDH nanocomposites due to the intercalation of polymer chains within the interlayer spaces of DS-LDH. However, EVA containing 8 wt.% of DS-LDH clearly showed the presence of a peak at 2θ  2.5 degrees in all probability due to the aggregation of DS-LDH particles in the EVA18 polymer matrix. It is also noted that the original (003) peak of DS-LDH disappeared in 1 and 3 wt.% DS-LDH-loaded EVA-28, EVA-45, and EVA-60 composites. This indicates that the DS-LDH platelets are either randomly oriented or separated far enough to produce insignificant diffraction peaks. At higher DS-LDH loadings (5 and 8 wt.%), corresponding EVA-28, 45, and 60 nanocomposites showed the appearance of a broad diffraction peak (2θ  2.5 degrees), possibly due to the formation of partially exfoliated or intercalated structure. XRD data established formation of the exfoliated EVA/LDH nanocomposites by controlling the LDH loading of about 10% for the melt intercalation and 5% for the solution intercalation (Zhang et al., 2008). In another study, XRD of organic-modified LDH with hyperfine magnesium hydroxide (HFMH) in the halogen-free flame-retardant EVA (28% vinyl acetate content)/HFMH/LDH nanocomposites suggested exfoliation of LDH depending on its LDH loading in EVA (Zhang et al., 2007).

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359

9.4.1.2.2 TEM The dispersion of LDH layers in the EVA (vinyl acetate contents: 18, 28, 40, 45, 60) has been adequately investigated by TEM as displayed in Figs. 9.5
9.8 (Kuila et al., 2007, 2008b, 2009a,c; Kuila, 2009). TEM micrographs of EVA-18/DS-LDH (1 wt.%) composites revealed the very complex nature of their morphology. This indicated wide variation in the shapes/sizes of partially exfoliated and homogeneously dispersed LDH layers. The average thickness and lateral dimension of the DS-LDH layers in EVA-28/DS-LDH (1 wt.%) varied in the range of 6
8 and 40
60 nm, respectively. It is noted that the DS-LDH layers are homogeneously dispersed throughout the EVA matrix. EVA-45 loaded with 1 wt.% DS-LDH (thickness: about 1 nm; lateral size: 40
60 nm) showed random distribution of the stacked LDH layers and were consistent with XRD analysis. A completely delaminated morphology is observed in EVA-60 at 1 wt.% DS-LDH (thickness: 6
8 nm; lateral dimension: 30
40 nm). The morphology of EVA-18, 28, 45, and 60 loaded

Figure 9.5 TEM images of (A) EVA-18/DS-LDH (1 wt.%) and (B) EVA-45/DS-LDH (5 wt.%) nanocomposites (Kuila et al., 2008b). Source: Reproduced with permission from Wiley.

Figure 9.6 TEM images of EVA-28/DS-LDH (3 wt.%) nanocomposite (A) at low magnification, (B) at high magnification (Kuila et al., 2007). Source: Reproduced with permission from Wiley.

360

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.7 TEM images of (A) EVA-40/DS-LDH (1 wt.%) nanocomposite (B) EVA-40/DSLDH (3 wt.%) nanocomposite, and (C) EVA-40/DS-LDH (5 wt.%) nanocomposite (Kuila et al., 2009a). Source: Reproduced with permission from Wiley Interscience.

with 8 wt.% DS-LDH shows DS-LDH nanolayers not only intercalated, but also aggregated in the EVA matrix. These findings also established that the tendency toward exfoliation increases with increasing vinyl acetate (VA) content in EVA/ LDH nanocomposites, a fact that has also been inferred based on XRD analysis (Kuila, 2009). It is also noted that the degree of exfoliation predominates at low filler (DS-LDH) loading. However, equilibrium shifts towards intercalation at higher filler loadings in EVA. Zhang et al. (2008) observed homogeneous nano-dispersed layers of ZnAl-LDH (5 wt.%) in EVA through TEM in solution as well as melt intercalation methods. In another work, the effects of Ni cations on Mg-Al LDH prepared by conventional hydrothermal treatment (CHT) and microwave hydrothermal treatment (MHT) were studied on the morphology of corresponding EVA nanocomposites through TEM (Wang et al., 2011). More agglomerations of MgAl-CHT and NiMgAl-CHT compared to those of MgAl-MHT and NiMgAl-MHT in the EVA matrix were noted in EVA/LDHs (20 wt.%). However, NiMgAl-MHT shows the presence of more homogeneous nano-dispersed layers in NiMgAl-MHT-EVA composite, indicating better compatibility of NiMgAl-MHT with EVA matrix.

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Figure 9.8 TEM images of (A) fully exfoliated EVA-60/DS-LDH (1 wt.%) nanocomposite, (B) partially exfoliated EVA-60/DS-LDH (3 wt.%) nanocomposite, (C) partially exfoliated EVA-60/DS-LDH (5 wt.%) nanocomposites, and (D) aggregated morphology of EVA-60/ DS-LDH (8 wt.%) nanocomposite (Kuila et al., 2009c). Source: Reproduced with permission from Wiley.

9.4.1.3 SR-LDH nanocomposites 9.4.1.3.1 XRD Pradhan et al. (2012) studied XRD patterns of St-LDH, neat SR, and its St-LDH-filled nanocomposites. The (001) diffraction peak (2θB1.7 degrees) in the St-LDH pattern corresponds to basal spacing almost eight times greater (5.3 nm) than that of pristine LDH (Nhlapo et al., 2008; Borja and Dutta, 1992). The expanded basal spacing of StLDH clearly indicated intercalation of stearate ions (C18) in the interlayers of LDH (Meyn et al., 1990; Kanoh et al., 1999; Itoh et al., 2003). As a result of such an anion exchange reaction, formation of a bilayer structure between the LDH layers takes place as illustrated in Scheme 9.2 (Pradhan et al., 2012). It is noted that the characteristic crystalline basal peak of St-LDH (001) is completely absent in the patterns of 1, 3, 5, and 8 wt.% filled LDH in SR. This clearly suggested exfoliation of St-LDH layers in the polymer matrix. XRD of SR/DS
Mg-Al-LDH (1, 3, 5, 8 wt.%)

362

Layered Double Hydroxide Polymer Nanocomposites

Scheme 9.2 Effect of chain length of carboxylate anion on the d-spacing of LDH layers (Pradhan et al., 2012). Reproduced with permission from Wiley.

Figure 9.9 TEM images of (A) SS3 nanocomposite and (B) SS8 nanocomposite (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

nanocomposites also indicated the formation of exfoliated hydroxide layers dispersed in the SR matrix on a nanometeric scale (Pradhan et al., 2011). Pradhan and Srivastava (2014) noted disappearance of the diffraction peaks corresponding to Li-AlLDH/SR, MWCNT/SR, and Li-Al-LDH/MWCNT hybrid fillers in the corresponding LDH/MWCNT/SR nanocomposites (Pradhan and Srivastava, 2014).

9.4.1.3.2 TEM Fig. 9.9 displays TEM images of 3% and 8% loaded St-LDH-filled SR nanocomposites (Pradhan et al., 2012). It is clearly evident that St-LDH layers (thickness:

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Figure 9.10 TEM images of (A) SR/DS-LDH (5 wt.%) and (B) SR/DS-LDH (8 wt.%) nanocomposites (Pradhan et al., 2011). Source: Reproduced with permission from Wiley.

1
2 nm; lateral size: 75
100 nm) are exfoliated and homogeneously dispersed in the 3 wt.% filled SR matrix. In contrast, SR nanocomposite with 8 wt.% filler showed aggregation, although the corresponding peak(s) are found to be absent in the XRD pattern. TEM images of SR/DS-LDH (5 wt.%) and SR/DS-LDH (8 wt.%) nanocomposites are displayed in Fig. 9.10 (Pradhan et al., 2011). It confirmed that DS-LDH layers are well dispersed in the SR matrix to form exfoliated nanocomposites. The thickness and length of the individual exfoliated DS-LDH layers were found to be B1.5
2.5 and 30
80 nm, respectively. This TEM image provided positive evidence for the nanoscale dispersion of DS-LDH layers in the SR matrix. However, 8 wt.% filler loaded SR showed aggregation to some extent, resulting in the partial exfoliation of the SR chains in the gallery of DS-LDH. Pradhan and Srivastava, 2014) studied morphology of 1 wt.% loaded Li-Al-LDH/MWCNTs, Mg-Al-LDH/MWCNTs, and Co-Al-LDH/MWCNTs (1%)/SR through TEM as displayed in Fig. 9.11. It is inferred that MWCNTs are mainly aggregated in the LiAl-LDH/MWCNT/SR composite as compact bundles. In contrast, in Co-Al-LDH/ MWCNT/SR composite, MWCNTs are individually distributed in SR matrix along with a few bundled nanotubes. Interestingly, MWCNTs are homogeneously distributed in the SR matrix in Mg-Al-LDH/MWCNT/SR composite.

9.4.1.4 EPDM/LDH nanocomposites 9.4.1.4.1 XRD XRD of EPDM/LDH composites with varying DS-LDH loading showed disappearance of the basal (00 l) diffraction peaks of DS-LDH, indicating the possible formation of exfoliated nanocomposites (Acharya et al., 2007a). Pradhan and others (2008) used wide-angle X-ray scattering (WAXS) to understand the extent of dispersion of a varying amount of LDH particles in EPDM. It was noted that the position of the reflections corresponding to LDH-C10 (1-decanesulfonate-modified Mg
Al LDH) remained almost unaltered in EPDM/LDH nanocomposites.

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Figure 9.11 TEM images of Li
Al-LDH/MWCNT (1%)/SR (A), Mg
Al-LDH/MWCNT (1%)/SR (B), and Co
Al-LDH/MWCNT (1%)/SR (C) (Pradhan and Srivastava, 2014). Source: Reproduced with permission from Elsevier.

9.4.1.4.2 TEM The higher-magnification TEM images of EPDM/LDH composite with 3 wt.% DSLDH revealed a partially exfoliated nanostructure (Acharya et al., 2007a). TEM micrographs of EPDM containing 7.5 phr of MgAl-LDH-C10 are shown in Fig. 9.12 (Pradhan et al., 2008). It clearly shows that filler particles are dispersed as exfoliated fragments (lateral dimension ,100 nm) as primary particles (lateral dimension of a few hundred nm to 1
2 μm) and as soft clusters of the primary particles (lateral dimensions over a few μm). The proportions of the last two forms are much higher than the exfoliated fragments. Furthermore, the higher-magnification image of EPDM/Mg-Al LDH revealed highly disordered geometry of the MgAlLDH particles. EPDM/LDH (2 phr)/flame retardant comprised of pentaerythritol, ammonium polyphosphate, and methyl cyanoacetate (38 phr) confirmed formation of an exfoliated structure (Wang et al., 2012b).

9.4.1.5 SBR/LDH nanocomposites 9.4.1.5.1 TEM The low- and high-magnification TEM images of SBR/LDH-lignin composites in Fig. 9.13 showed improved dispersion of LDH in the presence of lignin

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Figure 9.12 TEM micrographs of EPDM/LDH nanocomposites with LDH-C10 content of 5 phr (magnification bar 1 μm) (Pradhan et al., 2008). Source: Reproduced with permission from Elsevier.

(Xiao et al., 2013). TEM studies also showed a decrease in the size of LDH particles with increasing lignin loading in SBR. This is clear evidence of lignin playing an important role in the dispersion of LDH in SBR.

9.4.1.6 NBR-LDH and XNBR-LDH nanocomposites 9.4.1.6.1 XRD Feng and Su (2011) studied XRDs of NBR and NO3-LDH, sodium styrene sulfonate (SSS)-LDH, and SDBS-LDH-filled NBR composites. It is noted that the basal spacing (d003) of NO3-LDH (0.83 nm) remained more or less unaltered, even after mixing with NBR matrix. The basal spacing (d003) of SSS (sodium styrene sulfonate)-LDH in NBR/SSS-LDH composite increased to 2.69 nm compared to 1.87 nm in SSS-LDH due to partial intercalation of polymer chains. In contrast, basal spacing (d003) of sodium dodecyl benzene sulfonate (SDBS)-LDH decreased from 2.96 to 2.69 nm in NBR/SDBS-LDH composite. Long-Chao et al. (2011), based on XRD analysis, reported formation of the exfoliated or partly intercalated NBR/organomodifed LDH (OLDH)/organomodifed MMT (OMMT)/hyperfine magnesium hydroxide (HFMH) nanocomposites. X-ray diffraction analysis of XNBR/organically modified MgAl-LDH nanocomposites shows dispersion of LDH particles as primary particles, as exfoliated layers, and as soft clusters (Pradhan et al., 2008). Das et al. (2011a) observed formation of the ordered structure by introducing ZnMg-Al LDH in XNBR rubber matrix. WAXS analysis of vulcanized XNBR/LDH-C10 (C10H21SO3Na) and XNBR/LDH-C16

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Layered Double Hydroxide Polymer Nanocomposites

Figure 9.13 TEM micrographs of LDH/SBR and lignin-LDH/SBR composites (high magnification): (A) 30LDH/SBR; (B) 30LDH-3lignin/SBR; (C) 30LDH-9lignin/SBR; (D) 30LDH15lignin/SBR. Xiao et al. (2013). Source: Reproduced with permission from Wiley.

(C16H33SO3Na) nanocomposite clearly indicated a different state of dispersions (Costa et al., 2010).

9.4.1.6.2 TEM According to Xiao et al. (2014), TEM study indicated improvement in the dispersion of LDH particles in the presence of sodium lignosulfonate (SLS) in NBR matrix. Long-Chao et al. (2011) observed homogeneous dispersion of organomodified LDH/organomodified MMT/HFMH in the NBR matrix. The morphology investigations through TEM of NBR-LDH-NO3 (L), NBR-SSS-LDH (SL), and NBR-SDBS-LDH (SDL) nanocomposites clearly revealed differences due to the different organic modifications of LDHs (Feng et al., 2013). LDH particles are mostly clusters in NBR/LDH nanocomposites. In the case of NBR/SDBS-LDH (SDL) nanocomposites, the proportion of clusters decreased and intercalated fragments predominated. The best dispersion was visualized in NBR/SSS-LDH (SL) nanocomposite with no clusters evident and LDH particle fragments homogeneously dispersed. The higher-magnification TEM image showed predominance

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of the extent of the exfoliated fragments with less or no cluster formation in XNBR/LDH (Das et al., 2011c). The appearance of some agglomerates in XNBR/ MgAl-LDH (30 phr) through SEM indicated not good dispersion of filler in polymer matrix (Laskowska et al., 2014a). TEM images of vulcanized XNBR/LDH-C10 showed the presence of a higher extent of “soft clusters” compared to vulcanized XNBR/LDH-C16 (Costa et al., 2010).

9.4.1.7 NR/LDH nanocomposites 9.4.1.7.1 XRD DS-modified Zn-Al-LDH-filled NR showed increased basal spacing corresponding to 1 wt.% (3.90 nm) and 5 wt.% (3.66 nm) compared to ZnAl-LDH-DS (interlayer spacing: 2.53 nm) (Abdullah et al., 2010). Bottazzo et al. (2013) also reported an XRD study of NR/BR (cis-1,4-polybutadiene)/unmodified LDH and NRBR/modified LDH composites.

9.4.1.7.2 TEM Abdullah et al. (2010) recorded a TEM micrograph for NR/ZnAl LDH-NO3 (7 phr) and compared it with NR/ZnAl LDH DS (7 phr). They noted exfoliation in addition to the intercalation of nanolayers of Zn Al-LDH-DS in NR/Zn Al-LDH-DS composite.

9.4.2 Morphology of elastomeric blend-LDH nanocomposites 9.4.2.1 PU blend-LDH nanocomposites 9.4.2.1.1 XRD Kotal et al. (2010b) studied 1, 3, 5, 8 wt.% DS-LDH-filled thermoplastic polyurethane/nitrile butadiene (TPU/NBR referred to as TN) rubber blends. The corresponding XRD of blend composites showed shifting of the characteristic basal reflection peak (003) of DS-LDH (2θ B 3.4 degrees ) to 2θ B 2.2, 2.4, 2.5, and 2.6 degrees, respectively. This was attributed to the intercalation of polymer chains within the Mg
Al layers of DS-LDH. Furthermore, XRD of PU/NBR/DS-LDH nanocomposites showed almost complete absence of the slight hump initially present in DS-LDH at 2θ B 6.79 degrees. WAXD patterns of TN blend and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% in SFCNT-MgAl-LDH (Roy et al., 2016c; Roy, 2017) and SFCNTZnAl-LDH (Roy et al., 2016a; Roy, 2017) hybrids have also been investigated. The appearance of a broad peak at 2θ B 20 degrees in the XRD pattern of TN matrix is attributed to the amorphous phase of TPU along with short-range regular ordered structure of the hard and soft segment and the characteristic amorphous state of NBR. The absence of (003) and (110) planes of SFCNT-LDH hybrid in blend nanocomposites suggested the possibility of partial exfoliation of hybrid filler in the TN matrix. An XRD study also showed that the position of the TN peak at 2θ B 20 degrees remains almost unaltered in SFCNT-LDH hybrid filled nanocomposites. The calculations of full width at half-maximum values of neat TN and its

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Layered Double Hydroxide Polymer Nanocomposites

nanocomposites suggested more ordered structure in SFCNT-LDH hybrid filled TN nanocomposites compared to neat TN. The WAXD pattern were recorded for TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% of both SFCNFMgAl-LDH (Roy et al., 2016b; Roy, 2017) and SFCNF-ZnAl-LDH (Roy et al., 2016a; Roy, 2017) hybrids. It is noted that a broad peak appeared in the hybridfilled TN nanocomposites with its position more or less identical with respect to neat TN. However, the absence of (003), (006), and (002) reflections of SFCNFLDH hybrid in TN nanocomposites indicated the formation of partially exfoliated TN nanocomposites.

9.4.2.1.2 TEM TEM images of PU/NBR blend nanocomposites containing 1, 3, and 8 wt.% DSLDH are displayed in Fig. 9.14A
C (Kotal et al., 2010b). It is inevitable that the DS-LDH (1 wt.%) layers are partially dispersed throughout the polymer matrix, suggesting the formation of partially exfoliated nanocomposites. A TEM image of PU/NBR containing 3 wt.% DS-LDH loading depicted the presence of both

Figure 9.14 TEM images of (A) PU/NBR/DS-LDH (1 wt.%) nanocomposite, (B) PU/NBR/ DS-LDH (3 wt.%), nanocomposite and (C) PU/NBR/DS-LDH (8 wt.%) nanocomposite (Kotal et al., 2010b). Source: Reproduced with permission from Wiley.

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Figure 9.15 TEM images of TPU/NBR nanocomposites containing (a) 0.50 and (b) 1 wt.% SFCNT-LDH hybrid (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

exfoliated layers and intercalated tactoids of LDH crystallites. However, 8 wt.% DS-LDH loaded in PU/NBR exhibited aggregation of DS-LDH layers in the blend matrix. Fig. 9.15 shows an HRTEM image of TPU/NBR nanocomposites containing 0.50 and 1 wt.% of the SFCNT-Mg-Al LDH (Roy et al., 2016c). The presence of an interconnected SFCNT-LDH network is clearly demonstrated throughout the TPU/NBR matrix. The fine dispersion of 0.50 wt.% SFCNT-LDH hybrid filler in the TPU/NBR matrix is also visible. However, aggregation of blend nanocomposite is observed in TPU/NBR/SFCNT-LDH (1.0 wt.%) nanocomposite. HRTEM images of TPU/NBR nanocomposites containing 0.50 and 1 wt.% of both the SFCNTZnAl-LDH and SFCNF-ZnAl-LDH hybrid fillers are also displayed in Fig. 9.16 (Roy et al., 2016a). It is evident that 0.50 wt.% SFCNT-ZnAl-LDH and SFCNFZnAl-LDH hybrids form an interconnected network uniformly spread throughout the TN matrix. In contrast, 1.0 wt.% hybrid filler loaded in TN is found to undergo aggregation. All these findings clearly suggest that enhanced properties could be achieved by incorporating 0.50 wt.% hybrid filler in TN matrix.

9.4.2.2 EVA blend-LDH and EPDM blend-LDH nanocomposites 9.4.2.2.1 XRD Kuila et al. (2009b) carried out an XRD study of EVA/EPDM/DS-LDH prepared by solution blending. The absence of a basal peak of 1 and 3 wt.% DS-LDH-filled in EVA/EPDM blend suggested the formation of a completely exfoliated or partially exfoliated structure. However, a very weak and broad peak appeared at 2θ , 3 degrees at higher DS-LDH filler loadings (5 an 8 wt %) in EVA/EPDM. This could be attributed to the formation of partially exfoliated nanocomposites. Kotal (2012; Kuila et al., 2008a) made an XRD study of EVA/LDPE/DS-LDH (EVA/LDPE ratio:30/70, 50/50, 70/30) consisting of 1, 3, 5, and 8 wt.% of DSLDH. The close observation of the XRD patterns of the composites suggests the maximum degree of exfoliation in the EVA/LDPE (50/50) blend. This is possibly

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Figure 9.16 HRTEM images of TN nanocomposites containing (a) 0.50, (b) 1 wt. of SFCNT-LDH hybrid, and (c) 0.50, (d) 1 wt.% of SFCNF-LDH hybrid (Roy et al., 2016a). Source: Reproduced with permission from Springer.

due to its better compatibility compared to 30:70 and 70:30 EVA/LDPE blends. EVA/LDPE/DS-LDH nanocomposites with EVA/LDPE ratios of 30:70 and 70:30 with 1 and 3 wt.% of DS-LDH content show no diffraction peaks. However, a small and smooth hump appeared at 2θ , 2.5 degrees in the composites consisting of 5 and 8 wt.% of DS-LDH in the EVA/LDPE blend. This was ascribed to the intercalation of polymer chains inside the interlayer gallery of DS-LDH. Therefore, it was concluded that exfoliation of DS-LDH layers occurs at lower filler loadings (1 and 3 wt.%) and polymer chains intercalated or partially exfoliated at high loadings (5 and 8 wt.%). These observations clearly suggested that the tendency toward intercalation increases with increasing DS-LDH content in the EVA/LDPE blend. However, the possibility of aggregation of DS-LDH layers at higher loadings cannot be ruled out. XRD of PP/EPDM/intumescent flame retardancy (IFR) of phosphorus nitrogen (NP) compound/organically modified Mg-Al LDH suggested formation of intercalated and/or exfoliated nanocomposites (Cui and Qu, 2010).

9.4.2.2.2 TEM A TEM image of EVA/EPDM/DS-LDH (3 wt%) showed that DS-LDH layers are dispersed homogeneously in a disordered fashion in the EPDM matrix

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Figure 9.17 TEM images of EVA/LDPE (50:50) nanocomposites with (A) 3 wt.% DS-LDH content at lower magnification, (B) at higher magnification, and (C) 8 wt.% DS-LDH content (Kuila et al., 2008a). Source: Reproduced with permission from Elsevier.

(Kuila et al., 2009b). The thickness and lateral size of the exfoliated LDH layers correspond to 4
6 and 30
50 nm, respectively. These observations are found to be in good agreement with XRD analysis demonstrating the formation of partially exfoliated EVA/EPDM/DS-LDH nanocomposites. Ye et al. (2008) observed LDH layers undergoing exfoliation in LDPE/EVA/HFMH/LDH. They also suggested these layers act as synergistic compatibilizer and dispersant to make the HFMH particles homogeneously dispersed in the LDPE matrix. Kuila et al. (2008a) also studied the morphology of EVA/LDPE (50:50)/DS-LDH composites consisting 3 and 8 wt.% of DS-LDH and corresponding typical TEM images are displayed in Fig. 9.17. The grayish-white areas represent the EVA/LDPE matrix and the black area represents the DS-LDH layers. The TEM image showed more or less homogeneous dispersion of DS-LDH layers throughout the polymer blend matrix. However, a higher-magnification image confirmed the presence of disorderly oriented LDH layers in blend. This provided ample evidence for the formation of delaminated/exfoliated nanocomposites. The thickness and lateral sizes of the exfoliated LDH layers in EVA/LDPE/DS-LDH (3 wt.%) and EVA/LDPE/DS-LDH

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Layered Double Hydroxide Polymer Nanocomposites

(8 wt.%) nanocomposites correspond to 8
10 and 30
40 nm, respectively. In addition, there also exist some aggregates or stacking of DS-LDH layers in these nanocomposites (as indicated by arrows). This may be explained similar to that observed on LDPE/Mg-Al layered double hydroxide nanocomposites as reported by Costa et al. (2007). According to this, more and more polymer chains enter the interlayer region with time forcing the delamination of the surface layers one by one from the surface of large organomodified LDH particles. TEM images of the EVA/LDPE/DS-LDH (8 wt.%) confirmed not only aggregated but also intercalated/ partially exfoliated DS-LDH layers present in the EVA/LDPE matrix. TEM of PP/ EPDM/IFR/LDH confirmed findings of XRD analysis on the formation of the intercalated and/or exfoliated nanocomposites (Cui and Qu, 2010).

9.5

Mechanical properties of elastomer-LDH and elastomeric blend-LDH nanocomposites

The conventional inorganic fillers have already been used extensively as conventional fillers in enhancing the mechanical properties of polymers. However, the introduction of inorganic nanofillers in polymer and polymer blends marks a significant improvement in their mechanical properties. Therefore, the search for novel materials acting as reinforcing fillers in polymer
matrix composites exhibiting superior mechanical properties compared to neat polymers remains a big challenge today. Considering this, enhancement in mechanical properties, such as tensile strength (TS), elongation at break (EB), Young’s modulus, toughness, ductility, etc. of important elastomer/LDH and elastomer blend/LDH nanocomposites is reviewed below.

9.5.1 Mechanical properties of elastomer-LDH nanocomposites 9.5.1.1 PU-LDH nanocomposites Figs. 9.18(A, B) show the variation of TS and EB of PU filled with varying concentrations of dodecyl sulfate (DS)-LDH (Kotal et al., 2009) and stearate (St)LDH (LDH:MgAl-LDH) (Kotal et al., 2011), respectively. It is noted that the TS and EB of all the TPU nanocomposites is in general always greater with respect to neat TPU. TS of 3 wt.% of DS-LDH and 1 wt.% of St-LDH loaded neat TPU results in maximum improvements by 67% and 45%, respectively. This is due to the strong interfacial interaction between polar hydroxyl groups of St-LDH (and DS-LDH) and polar urethane groups of TPU as displayed in Scheme 9.3. The EB of TPU/DS-LDH (3 wt%) and TPU/St-LDH (3 wt%) showed maximum improvement by 27% and 53%, respectively. This is in all probability due to the entanglement of the TPU chains, chain slippage, and platelet orientation of DS-LDH and St-LDH layers. However, relatively higher values of EB at all St-LDH loadings in TPU are possibly due to the plasticization effect of the long alkyl chain of St-LDH.

(A) 65 800

55

750

50 700

45 40

650 Elongation at break (%) Tensile strength (MPa)

35 30

0

2 4 6 DS-LDH content (wt %)

8

600

1000

60

950

56

900 52 850 48

800 750

44 Tensile strength

40

700

Elongation at break

Elongation at break (%)

Tensile strength (MPa)

(B)

Elongation at break (%)

Tensile strength (MPa)

60

650

36 0

2 4 6 LDH-stearate content (wt %)

8

600

Figure 9.18 (A) Variation of tensile strength (TS) and elongation at break (EB) of PU/DSnanocomposites with DS-LDH content (Kotal et al., 2009). (B) Variation of TS and EB of PU/LDH
stearate nanocomposites with LDH-stearate content (Kotal et al., 2011). Source: Reproduced with permission from Wiley.

Scheme 9.3 Formation of hydrogen bonding between LDH-stearate and PU chains (Kotal et al., 2011). Reproduced with permission from Wiley.

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Layered Double Hydroxide Polymer Nanocomposites

The improvements in mechanical properties have also been correlated on the basis of fracture surface behavior of TPU/DS-LDH and TPU/St-LDH nanocomposites using SEM. TPU/Mg-Al-DS-LDH nanocomposites showed maximum improvements in TS (407%) and EB (159%) at 3 and 8 wt.% DS-LDH loading, respectively (Kotal et al., 2010a). The increase in TS may be ascribed to the strong interfacial interaction between the hydroxyl group of DS-LDH and polar urethane groups of polyurethane resulting in good compatibility between the two components. In other words, the high surface area of LDH and formation of partially exfoliated structure enhances the interfacial interaction through bridge, loop, and tail linkages of the polymer chains with the DS-LDH layers via hydrogen bonding and polar
polar interactions (Scheme 9.4). The mechanical property investigations also showed an increase in toughness with increasing DS-LDH loading up to 3 wt.% and then decreases in TPU/Mg-Al-DS-LDH nanocomposites

Scheme 9.4 Proposed mechanism for interactions between DS-LDH and PU chains (Kotal et al., 2010a). Reproduced with permission from American Scientific Publishers.

Kotal and Srivastava (2011b) investigated the effect of interlamellar grafting of pristine and organomodified LDHs in the corresponding PU nanocomposites. The corresponding tensile strength, elongation at break, and modulus at different percentages of strain are presented in Table 9.2. It is observed that isocyanate-grafted organo-LDHs impart an improvement in tensile strength as well as elongation at break for PU nanocomposites. Table 9.2 also shows the maximum improvement in

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Table 9.2 Mechanical data of polyurethane and its nanocomposites with 3 wt.% MDI- and IPDI-grafted LDH, DS-LDH, and St-LDH Modulus (MPa)

PU PU/IPDI-g-LDH PU/MDI-g-LDH PU/IPDI-g-DSLDH PU/MDI-g-DSLDH PU/IPDI-g-StLDH PU/MDI-g-StLDH PU/DS-LDH PU/St-LDH

TS/MPa

EB (%)

100%

200%

300%

0.28 6 0.01 0.32 6 0.01 0.35 6 0.01 0.40 6 0.02

0.41 6 0.01 0.43 6 0.03 0.4 6 50.03 0.53 6 0.03

0.54 6 0.01 0.56 6 0.02 0.58 6 0.02 0.70 6 0.02

1.10 6 0.6 1.76 6 0.3 2.48 6 0.7 3.75 6 0.8

626 6 27 850 6 32 800 6 15 900 6 40

0.43 6 0.02

0.60 6 0.03

0.80 6 0.02

4.07 6 0.4

872 6 20

0.48 6 0.03

0.65 6 0.02

0.84 6 0.03

4.60 6 0.6

906 6 22

0.62 6 0.02

0.88 6 0.03

1.14 6 0.02

5.40 6 0.5

822 6 29

0.38 6 0.01 0.35 6 0.02

0.49 6 0.02 0.52 6 0.03

0.64 6 0.03 0.67 6 0.03

3.20 6 0.7 2.60 6 0.4

866 6 24 895 6 35

Source: Modified from Kotal, M., Srivastava, S.K., 2011b. Synergistic effect of organomodification and isocyanate grafting of layered double hydroxide in reinforcing properties of polyurethane nanocomposites. J. Mater. Chem. 21, 18540
18551, reproduced with permission from the Royal Society of Chemistry.

tensile strength achieved in PU/3 wt.% of MDI-grafted St-LDH (391%) and PU/ 3 wt.% of IPDI-grafted St-LDH (318%). Such improvements in the tensile strength may also be attributed to the homogeneous dispersion of IPDI-grafted St-LDH and MDI-grafted St-LDH layers in PU matrix. It is also noted that the tensile strength of MDI-g-DS-LDH/PU and IPDI-g-DS-LDH/PU nanocomposites is improved by 270% and 241%, respectively. These mechanical studies also established a greater improvement in tensile strength in PU/isocyanate-grafted organomodified LDHs than the individual grafted or organomodified LDH/PU nanocomposites. Table 9.2 also shows that MDI-grafted LDHs/PU nanocomposites in comparison with their corresponding IPDI-grafted LDHs/PU nanocomposites exhibit relatively lower elongation at break. However, MDI-g-St-LDH/PU nanocomposite showed maximum decrease in the percent of elongation (31.3%). This is possibly due to the fact that the PU chains in the nanocomposites are more restricted by the MDI-grafted StLDH layers owing to their increased stiffness as well as crosslink density. PU/ CoAl-DS-LDH nanocomposites always showed superior Young’s modulus and tensile strength compared to neat PU (Guo et al., 2011). The maximum enhancement in Young’s modulus and tensile strength is observed in PU/CoAl-DS-LDH (2.0 wt.%) corresponding to B149% and 89%, respectively. This is ascribed to the exfoliation of LDH nanolayers in the PU matrix and strong interfacial interactions between hydroxyl groups of CoAl-DS-LDH and polar group of PU chains. The improvement in elongation at break in PU/LDH (0.5 wt.%) nanocomposite may be ascribed to the synergistic effect of platelet orientation and chain slippage. However, reduced mechanical properties of PU/CoAl-DS-LDH (5.0 wt.%) could be due to the aggregation of CoAl-DS-LDH nanolayers. According to Yan et al. (2013),

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Layered Double Hydroxide Polymer Nanocomposites

1.05

800 Tensile strength (MPa) Elongation at break (%)

Tensile strength (MPa)

600 0.95

500 400

0.90 300 0.85

Elongation at break (%)

700 1.00

200 100

0.80 0

1

2 3 LDH-content (wt%)

4

5

Figure 9.19 Variation of tensile strength (MPa) and elongation at break (%) of PU/APS-DSLDH nanocomposites with different contents of APS-DS-LDH (Zhang et al., 2016). Source: Reproduced with permission from Royal Society of Chemistry.

increasing DS-LDH content in WPU increased the hardness and tensile strength of WPU/DS-LDH nanocomposite. In another study, mechanical properties showed improvements in polyurethane/NiAl-LDH/ZnO composites (Xiong et al., 2015). Zhang et al. (2013), incorporated ZnAl-LDH/ZnO in WPU and observed profound effect on the mechanical strength of WPU/ZnAl-LDHs/ZnO nanocomposite. This was ascribed to the good dispersion of ZnAl-LDH/ZnO in the WPU matrix. Zhang et al. (2016) studied the mechanical properties of PU/LDH nanohybrids. Fig. 9.19 shows the variation of tensile strength and elongation at break (%) of PU/APS-DSLDH nanocomposites. It is noted that EB is maximally increased by 430% for PU nanocomposites by incorporating 3 wt.% APS-DS-LDH loading. This has been attributed to interfacial bonding between APS-DS-LDH and PU matrix. This makes the PU/APS-DS-LDH hybrid less susceptible to breaking during extension. Alternatively, this may also be due to the plasticization effect of the long alkyl-chain of the intercalated DS-LDH, resulting in a flexible matrix. TS of PU/LDH nanocomposites was also maximally enhanced at 3 wt.% filler loading. This is ascribed to a stronger interfacial interaction between the
OH and
NH2 groups of APS-LDH and the
NHCOO group of PU. They also observed a slight decrease in modulus of PU/APS-LDH nanocomposites compared to neat PU. It was suggested that long hydrocarbon segments of APS and DS play a key role in the improvements of the matrix flexibility over the interfacial interaction between LDH and PU. Kotal and Srivastava (2011b) reported that the modulus of PU/MDI-g-St-LDH increased significantly compared to MDI-g-DS-LDH/PU nanocomposites. This is probably due to the nano-reinforcement effect/development of shear zones in nanocomposites under stress and strain conditions or high aspect ratio of LDH (Kotal et al., 2009).

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9.5.1.2 EVA/LDH nanocomposites The mechanical properties of LDH nanocomposites of EVA, consisting of different amounts of vinyl acetate content, have been studied (Kuila et al., 2007, 2008b, 2009c). The corresponding variation in tensile strength and elongation at breakage of EVA with varying DS-LDH is displayed in Fig. 9.20. It is noted that the tensile strength of 1 wt.% DS-LDH-filled EVA-18 and EVA-28 are maximally enhanced compared to neat EVA-18 and EVA-28, respectively. Such an improvement in tensile strength is likely due to the strong interfacial interaction between the polar hydroxyl groups present in the DS-LDH and the polar acetate groups of EVA. However, further addition of DS-LDH in EVA caused a reduction in TS due to aggregation of filler. It is also noted that elongation at break first decreases with the addition of DS-LDH in EVA-18 and EVA-28 followed by an increase at higher filler level. Such an improvement in elongation at break may be due to the platelet orientation of LDH nanolayers. In contrast, tensile strength as well as elongation at break increased with increased DS-LDH in EVA-45/DS-LDH and EVA-60/DSLDH nanocomposites due to the strong interfacial interaction between the polymer chains and DS-LDH. The tensile strength as well as elongation at break increased up to 5 wt.% DS-LDH content in EVA-45/DS-LDH nanocomposites. Beyond 5 wt. % of DS-LDH loadings, TS and EB of EVA-45 showed a decreasing trend. The tensile strength of EVA-60/DS-LDH nanocomposites also showed an increasing trend up to 3 wt.% of DS-LDH content. At higher filler loading (5 and 8 wt.%), a gradual decrease in TS is observed, though in either case the TS values are higher compared to neat EVA. Such a reduction in TS is associated with the extended aggregation of DS-LDH layers and augmented with increasing DS-LDH content in EVA. It is also evident from Fig. 9.20 that the addition of DS-LDH in EVA-60 results in an increase in the elongation at break and is at a maximum for 8 wt.% DS-LDH in EVA-60. Such improvements in EB may be due to the extensive entanglement of the crosslinkable polymer chains and synergistic effect of platelet orientation and chain slippage. The improvements in mechanical properties of EVA/DS-LDH nanocomposites have been correlated in terms of fracture behavior of the nanocomposites using SEM. The fracture surface morphology of neat EVA-18, EVA-45, and their corresponding nanocomposites consisting of 5 wt.% DS-LDH are shown in Fig. 9.21 (Kuila et al., 2008b). The fractured surface image of neat EVA-18 clearly shows the presence of some cracks, while their nanocomposites with 5 wt.% DS-LDH content do not show any prominent cracks. This is possibly due to the formation of some shear zones which reduce the formation of cracks and ultimately toughen the nanocomposite materials. A completely different fracture surface morphology is observed in EVA-45/DS-LDH (5 wt.%) compared to neat EVA-45. Considering the tensile mechanical data, it seems that the rougher the fracture surface, the better the mechanical properties of the related nanocomposites. Such a surface morphology variation is possibly due to the deviation of tear. It is also inferred from the SEM images that DS-LDH is compatible with EVA-45 and possibly its platelet orientation is likely to account for the improved mechanical properties in the corresponding nanocomposites.

378

Layered Double Hydroxide Polymer Nanocomposites

1000

(A) 32

950 900

28

850

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Tensile strength Elongation at break

22 20

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Elongation at break (%)

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Elongation break (%)

Tensile strength Elongation at break

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DS-LDH content (wt %)

Figure 9.20 (A) Variation of TS and EB (%) of EVA-18/DS-LDH nanocomposites (Kuila et al., 2008b). (B) TS and EB of EVA-28/DS-LDH nanocomposites (Kuila et al., 2007). (C) Variation of tensile strength and elongation at break with DS-LDH contents for EVA-40/DSLDH nanocomposites (Kuila et al., 2009a). (D) Variation of tensile strength and elongation at break with 0, 1, 3, 5, and 8 wt.% DS-LDH contents in EVA 60 matrix (Kuila et al., 2009c). Source: Reproduced with permission from Wiley.

According to Wang et al. (2012a), the tensile strength of EVA composites filled with 2 wt.% of rare earth-doped Ni-containing LDHs (S-Ni0.1MgAl-La, S-Ni0.1MgAlCe, S-Ni0.1MgAl-Nd) and S-Ni0.1MgAl (surface-modified Ni-containing LDHs) was significantly increased compared to pristine EVA. The elongation at break values increased only for the EVA/rare earth-doped Ni-containing LDHs (2 wt.%) with respect to pristine EVA. In other investigations, although LDHs reduced the elongation at break and the tensile strength of the composites, the addition of nickel

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(C)

1000

14

900 10

850 800

8 Tensile strength Elongation at break

Elongation at break (%)

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(D)

6

2000 1800

5 1600 4

1400

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Tensile strength (MPa)

Tensile strength Elongation at break

1000 2 1

3 5 DS-LDH content (wt %)

8

Figure 9.20 (Continued).

ions enhanced the ductility and strength of the composites in comparison with S-Mg3.0Al/EVA (Wang et al., 2013). EVA-NMH (nano-magnesium hydroxide) exhibited better mechanical properties than EVA-NLDH (nanohydrotalcite) (Jiao et al., 2006).

9.5.1.3 SR/LDH nanocomposites Pradhan et al. (2012) studied the variation of TS and EB of SR nanocomposites with varying St-LDH concentration. It is noted that SR/St-LDH (3 wt%) nanocomposite attained maximum improvement of tensile strength (97 %) and elongation at break (43%) compared to neat SR. Such improvements in the mechanical properties

380

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.21 Tensile fracture surface morphology of (a) EVA-18, (b) EVA-18/DS-LDH (5 wt.%), (c) EVA-45, (d) EVA-45/DS-LDH (5 wt.%) (Kuila et al., 2008b). Source: Reproduced with permission from Wiley.

are possibly due to the larger aspect ratio of the LDH layers and interaction between-OH functionality of St-LDH layers and Si
O polar groups of SR matrix, homogeneity of the SR nanocomposites, and easier exfoliation (Shieh et al., 2010; Fornes et al., 2004). The improvement of elongation at break of St-LDH/SR nanocomposite may be attributed to platelet orientation of St-LDH or chain slippage, or both, in the SR matrix. Young’s modulus of St
LDH (3 wt.%)/SR nanocomposite was found to be 55% higher than that for the pure SR. They also correlated improvements in mechanical properties in terms of fracture behavior of the St-LDH/SR nanocomposites. Fig. 9.22(A
C) represents the stress
strain plots of SR filled with different loadings (0, 0.5, 0.75, 1.0, and 1.5 wt.%) of Li-Al-LDH/MWCNT, Mg-Al-LDH/ MWCNT, and Co-Al-LDH/MWCNT, respectively (Pradhan and Srivastava, 2014). It is noted that tensile strength and elongation at break of the hybrid-filled/SR composites are always higher compared to neat SR, the maximum improvements are observed for 1 wt.% of Mg-Al-LDH/MWCNT, Li-Al-LDH/MWCNT, and Co-AlLDH/MWCNT-loaded SR. Such improvements in the mechanical properties of SR are ascribed to the synergistic effect of 1D MWCNTs and 2D LDH fillers.

9.5.1.4 EPDM/LDH nanocomposites Acharya et al. (2007a) achieved superior tensile strength of EPDM/LDH nanocomposites compared to that of neat EPDM. Such an enhancement in the tensile

Mechancial and dynamical mechanical properties

381

Stress (MPa)

(A)

0.4 a 0.2 0.0

0

50

Stress (MPa)

(B)

100 150 Strain (%)

200 d

e c b

0.6 0.4

a 0.2 0.0

0

50

(C)

100 150 Strain (%)

200

0.8 Stress (MPa)

d

e c b

0.6

d c

e

0.6

b

0.4

a

0.2 0.0

0

50

100 150 200 Strain (%)

Figure 9.22 (A) Stress
strain plots (a) neat SR, (b) Li-Al-LDH/MWCNT (0.5 wt.%)/SR, (c) Li-Al-LDH/MWCNT (0.75 wt.%)/SR, (d) Li-Al-LDH/MWCNT (1.0 wt.%)/SR, and (e) LiAl-LDH/MWCNT (1.5 wt %)/SR (Pradhan and Srivastava, 2014). (B) Stress
strain plots (a) neat SR, (b) Mg-Al-LDH/MWCNT (0.5 wt.%)/SR, (c) Mg-Al-LDH/MWCNT (0.75 wt. %)/SR, (d) Mg-Al-LDH/MWCNT (1.0 wt.%)/SR, and (e) Mg-Al-LDH/MWCNT (1.5 wt. %)/SR (Pradhan and Srivastava, 2014). (C) Stress
strain plots (a) neat SR, (b) Co-Al-LDH/ MWCNT (0.5 wt.%)/SR, (c) Co-Al-LDH/MWCNT (0.75 wt.%)/SR, (d) Co-Al-LDH/ MWCNT (1.0 wt.%)/SR, and (e) Co-Al-LDH/MWCNT (1.5 wt.%)/SR (Pradhan and Srivastava, 2014). Source: Reproduced with permission from Elsevier.

strength value with the addition of LDH may be ascribed to the strong interfacial interaction between EPDM and DS-LDH. Furthermore, partially exfoliated rigid DS-LDH layers efficiently transfer stress from the polymer and directly enhance the stiffness in the corresponding nanocomposites. Interestingly, EB also increases with the DS-LDH content in EPDM. Such an increase in EB could be attributed to the platelet orientation, chain slippage, or plasticization. However, TS and EB both increase slowly at higher DS-LDH loadings in EPDM, owing to the aggregation of

382

Layered Double Hydroxide Polymer Nanocomposites

the filler. It is also noted that the modulus increases with increasing LDH content in all EPDM nanocomposites. Such an increase in modulus could be attributed to the resistance exerted by the sterically hindered LDH surface itself, strong polymer
filler interaction, or due to the formation of microvoids around DS-LDH nanolayers. Pradhan et al. (2008) observed significant improvement in the mechanical properties of EPDM/MgAl-LDH nanocomposites. Organomodified (sodium dodecyl sulfate, undecylenic acid, oleic acid) layered double hydroxide-filled EPDM composites showed more improved tensile strength and elongation modulus (Chao et al., 2013). Das et al. (2011a) reported tensile strength of NBR vulcanizates cured by LDH to be comparable with the vulcanizates cured by conventional approach. Zhao et al. (2014) reported that 10 phr of Cu-Al-LDH-filled EPDM composite resulted in no obvious changes to tensile strength compared to that of pure EPDM matrix.

9.5.1.5 SBR/LDH nanocomposites The tensile strength and modulus of lignin-LDH/SBR nanocomposites increased from 4.0 to 10.9 MPa following the lignin content in the range 0
9 phr (Xiao et al., 2013). It was suggested that better dispersion of LDH particles in the presence of lignin in SBR accounts for such enhanced mechanical properties of lignin-LDH/ SBR nanocomposites. Das et al. (2012) reported that tensile strength, elongation at break, and 100% modulus of SBR increased with increasing filler loading up to 80 phr.

9.5.1.6 NBR/LDH and XNBR/LDH nanocomposites He et al. (2016) studied the stress
strain behavior of different NBR composites (unmodified NBR, NBR with 7 phr LDH-NO3, and NBR with 7 phr LDH-SSS) before and after aging. It was noted that NBR/LDH-NO3 shows a better tensile strength and elongation at failure before aging than unmodified NBR and NBR/ LDH-SSS, with a decrease in value with increasing LDH-NO3 content. Nevertheless, tensile strength and elongation at failure of NBR/LDH-SSS after aging are higher than those of NBR/LDH-NO3 and unmodified NBR. NBR/organomodified LDH (OLDH)/organomodified MMT (OMMT)/hyperfine magnesium hydroxide (HFMH) nanocomposites showed improvement in tensile strength and stress at 100% elongation (Long-Chao et al., 2011). Xiao et al. (2014) observed improved mechanical properties in LDH/NBR and sodium lignosulfonate (SLS)LDH/NBR composites compared to pristine NBR. In another work, NBR/sodium styrene sulfonate (SSS)-modified LDH, which exhibited two times higher tensile strength than cured pure rubber without significant loss of elongation, was obtained (Feng et al., 2013). Eshwaran et al. (2015) studied the stress
strain behavior of NBR filled with unmodified commercial LDH, modified commercial LDH, and modified synthetic LDH. It was noticed that stretched nanocomposites follow two different patterns. The first region (below 300% strain) of the plot corresponds to the broken local filler network, disturbance of the chain conformation, Mullins

Mechancial and dynamical mechanical properties

383

effect, and other minor effects. The second region (above 300% strain) of the plot corresponds to the unstrained undisturbed regions, respectively. Tensile strength, modulus at 200% elongation, and elongation at break of XNBR/LDH-C10 were found to be always superior compared to neat XNBR (Pradhan et al., 2008). The tensile strength of the XNBR also improved on loadings with MgAl-LDH (Laskowska et al., 2014a). This was ascribed to the existence of an ionic/polar interaction between MgAl-LDH and XNBR. Furthermore, elongation at break of the XNBR MgAl-LDH composites is remarkably reduced compared to the unfilled neat gum. This is in all probability due to the reduced free volume as a result of the interaction between the filler and the matrix, as well as the higher crosslink density. Costa et al. (2010) observed that sodium 1-decanesulfonate- and 1-hexadecanesulfonate-modified Mg-Al LDH acted as a very efficient reinforcing agent for XNBR matrix. Basu et al. (2014) investigated the mechanical properties of the XNBR/Zn-Al LDH composites. They observed much improved tensile strength at 40 phr filler loading compared to XNBR gum. The 100% modulus also increases significantly with increasing LDH content and reaches its highest value at 100 phr loading of LDH.

9.5.1.7 NR/LDH nanocomposites NR/Zn-Al LDH-DS (nanocomposites) exhibit higher tensile strength than NR/ZnAl LDH-NO3 microcomposites (Abdullah et al., 2010). NR attained a 21.57% increment in tensile strength at 7 phr loading of Zn-Al LDH-DS. This is ascribed to the intercalation of NR into the galleries of Zn-Al LDH-DS. The ultimate tensile strength decreased at higher loadings due to poor dispersion or agglomeration.

9.5.2 Mechanical properties of elastomeric blend-LDH nanocomposites 9.5.2.1 PU blend-LDH nanocomposites Kotal et al. (2010b) investigated the effect of DS-LDH filler on the mechanical properties of neat PU/NBR blend and the corresponding findings are displayed in Fig. 9.23. It is noted that the tensile strength and elongation at break of PU/NBR/ DS-LDH (1 wt%) nanocomposite compared to neat blend are enhanced by 156% and 21%, respectively. Beyond this, the PU/NBR/DS-LDH nanocomposites show a gradual decrease in tensile strength and elongation at break due to agglomeration of DS-LDH layers in the PU/NBR blend. However, the possibility of the formation of crack growths through filler agglomerates in the PU/NBR nanocomposites cannot be ruled out. The variations of tensile strength and elongation at break of SFCNF-Mg-AlLDH-filled TN blends are displayed in Fig. 9.24 (Roy et al., 2016b). The enhancement in mechanical properties clearly indicates the reinforcing effect of SFCNF-MgAl-LDH in the TN matrix. It is also noted that 0.50 wt.% SFCNFMgAl-LDH filler-loaded TN nanocomposite exhibits maximum improvement in

Layered Double Hydroxide Polymer Nanocomposites

Tensile strength (MPa) Elongation at break (%)

Tensile strength (MPa)

14

630 600

12

570

10

540 8 510 6

Elongation at break (%)

384

480 4 0

2 4 6 DS-LDH content (wt %)

8

450

Figure 9.23 Variation of tensile strength and elongation at break of PU/NBR/DS-LDH nanocomposites with DS-LDH content (Kotal et al., 2010b). Source: Reproduced with permission from Wiley.

Figure 9.24 Variation of the tensile strength and elongation at break of the TN nanocomposites versus the SFCNF-MgAl-LDH hybrid content (Roy et al., 2016b). Source: Reproduced with permission from Wiley.

tensile strength of 167% and EB of 1.51 times compared to neat TN. Such improvements in mechanical properties suggest that the SFCNF-MgAl-LDH hybrid acts as a better reinforcing filler in improving the mechanical properties. This could be attributed to the highest specific surface area of SFCNF-MgAl-LDH and its homogeneous dispersion leading to strong polymer
filler interfacial interaction. At higher loadings, tensile strength and EB gradually decreased due to the aggregation of the filler into the matrix. The mechanical properties of the TN blend in the presence of SFCNF-MgAl-LDH hybrid filler has been explained as proposed in Scheme 9.5.

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385

Scheme 9.5 TN blend without and with filler in order to explain the mechanical properties (Roy et al., 2016b). Reproduced with permission from Wiley.

The synergistic effect of SFCNF and LDHs has also been established on mechanical properties of TN nanocomposites. Roy et al. (2016a) studied the effect of SFCNT-ZnAl-LDH and SFCNF-ZnAl LDH loadings on mechanical properties of the TN nanocomposites and the findings are displayed in Fig. 9.25. It is noted that 0.50 wt.% SFCNT-ZnAl-LDH (and SFCNF-ZnAl-LDH) loaded TN nanocomposites achieved maximum improvement in tensile strength of 171% (126%) and EB of 1.8 times (1.50 times) compared to neat TN. Such improvements in mechanical properties are attributed to the optimum dispersion of SFCNT-LDH hybrid fillers leading to an enhanced interaction between matrix and nanofiller. Furthermore, tensile strength and elongation at break slightly decrease at higher filler loadings due to the tendency of the filler to agglomerate. Their findings also clearly confirmed the synergistic effect of SFCNT and LDH in reinforcing of TN nanocomposites. The variations of tensile strength and elongation at break of TPU/NBR/SFCNTMg-Al-LDH nanocomposites have also been studied and corresponding findings are displayed in Fig. 9.26 (Roy et al., 2016c). It is observed that 0.50 wt.% SFCNTLDH hybrid-loaded TPU/NBR exhibits about 171% and 1.8 times enhancement in tensile strength and elongation at break compared to pure TPU/NBR. The variation of stress–strain of 0.25 wt.% SFCNTs/TPU/NBR, 0.25 wt.% LDH/TPU/NBR, and 0.50 wt.% SFCNT-LDH/TPU/NBR established role of synergistic effect of individual fillers on the mechanical properties of hybrid-filled TPU/NBR nanocomposites. Alternatively, this could also be ascribed to the optimum dispersion of SFCNTLDH hybrid filler that causes an enhanced interaction between matrix and nanofiller.

386

Layered Double Hydroxide Polymer Nanocomposites

Figure 9.25 (A) Variation of tensile strength and elongation at break of TN nanocomposites with SFCNT-ZnAl-LDH hybrid content. (B) Variation of tensile strength and elongation at break of TN nanocomposites with SFCNF-ZnAl-LDH hybrid content (Roy et al., 2016a). Source: Reproduced with permission from Springer.

9.5.2.2 EVA Blend-LDH nanocomposites Kuila et al. (2009b) investigated variation of tensile strength and elongation at break with DS-LDH loading in an EVA/EPDM blend. It is noted that tensile strength as well as elongation at break increases up to 3 wt.% DS-LDH content in EVA/EPDM matrix. Such an increase in TS and EB suggests that the EVA/ EPDM blend is strengthened and toughened simultaneously by increasing the concentration of DS-LDH. Furthermore, the interfacial interaction between the hydroxyl functionality of DS-LDH and the polar acetate group of EVA provided

Mechancial and dynamical mechanical properties

387

600 14 12

500

10

450 400

8 350 Tensile strength Elongation at break

6

Elongation at break (%)

Tensile strength (MPa)

550

300 250

4 0.0

0.2

0.4

0.6

0.8

1.0

MWCNT-LDH hybrid content (wt%)

Figure 9.26 Variation of tensile strength and elongation at break of TPU/NBR nanocomposites with SFCNT-Mg-Al-LDH hybrid content (Modified) (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

good adhesion between these two phases, promoting their compatibility and reducing the extent of phase separation. The increase in EB may be attributed to the plasticization or chain slippage of the EVA/EPDM blend within the platelet orientation of DS-LDH nanolayers (Roy et al., 2016c). At higher DS-LDH loadings (5 and 8 wt.%), TS and EB of EVA/EPDM tend to decrease due to the aggregation of filler. The effect of DS-LDH contents (1, 3, 5, and 8 wt.%) on the tensile properties (tensile strength and elongation at break) has been investigated for EVA/LDPE (30:70, 50:50, and 70:30) blends (Kuila et al., 2008a; Kuila, 2009). The degree of improvement in mechanical properties of the nanocomposites is found to be different for all the blend systems. This is mainly attributed to the varying degree of dispersion of DS-LDH layers in the EVA/LDPE blends. It is also noted that EVA/ LDPE (30:70) blend with 3 wt.% of DS-LDH loadings led to the maximum improvements in tensile strength (103 %) and elongation at break (92%) with respect to the neat blend. Fig. 9.27 show that nanocomposites with EVA/LDPE (50:50) and 3 wt.% DS-LDH exhibited improvement in TS is 55%, whereas the EB of the nanocomposites decreases gradually with increasing concentration of DSLDH in the blend (Kuila et al., 2008a). Kuila (2009) also observed higher tensile strength (70%) and elongation at break (30%) compared to the pure blend in EVA/ LDPE (70:30)/DS-LDH (5 wt.%) blend nanocomposites. This was ascribed to the fine dispersion of DS-LDH particles offering the whole surface of the LDH layers available for the interaction with the polar group of the EVA in the EVA/LDPE blend. However, EVA/LDPE (30:70, 50:50) blends showed a decrease in tensile strength and elongation at break beyond 3 wt.% DS-LDH loading due to the aggregation of filler.

28

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T.S. (MPa) E.B. (%)

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Layered Double Hydroxide Polymer Nanocomposites

Tensile strength (MPa)

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300

DS-LDH content (wt%)

Figure 9.27 Variation of tensile strength (TS) and elongation at break (EB) with 1, 3, 5, and 8 wt.% DS-LDH contents in EVA/LDPE matrix. Kuila et al. (2008a). Source: Reproduced with permission from Elsevier.

9.6

Dynamical mechanical properties of LDH-filled elastomer and elastomeric blend nanocomposites

Dynamic mechanical results are generally given in terms of complex moduli or compliance. The complex moduli are defined by E 5 E/ 1 iE//, where, E is the complex shear modulus; E/, the real part of the modulus; E//, the imaginary part of the modulus; and i 5 O
1. E/and E// are also called the storage modulus and loss modulus respectively. The angle that reflects the time lag between the applied stress and strain is δ, and is defined by a ratio called the loss tangent or dissipation factor: tan δ 5 E///E/. DMA is also by far the most sensitive technique. The glass transition temperature (Tg) of polymers can also be measured by DMA. In view of this, elastic modulus, loss modulus, glass transition temperature of elastomer, and elastomeric blend nanocomposites are reviewed below.

9.6.1 Dynamical mechanical properties of elastomer-LDH nanocomposites 9.6.1.1 PU-LDH nanocomposites Kotal et al. (2011) studied temperature dependence of logarithm of storage modulus, loss modulus and tan δ of TPU, and its St-LDH-filled nanocomposites as displayed in Fig. 9.28 and the findings are summarized in Table 9.3. It is noted that the storage moduli increase with St-LDH contents in TPU over a wide range of temperature (
80 C to 180 C). Such improvements in storage moduli are possibly due to the greater surface area available due to exfoliated St-LDH layers in the TPU matrix. This is likely to lead to the strong interaction (hydrogen bonding)

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Figure 9.28 Temperature dependence of logarithm of storage modulus, loss modulus, and tan δ of TPU, and its St-LDH-filled nanocomposites (a) neat PU and its nanocomposites containing, (b) 1 wt.%, (c) 3 wt.%, (d) 5 wt.%, and (e) 8 wt.% LDH
stearate (Kotal et al., 2011) Source: Reproduced with permission from Wiley. Table 9.3 Glass transition temperature (Tg), tan δmax, storage modulus (E0 ), and loss modulus (Ev) of neat PU and its LDH
stearate nanocomposites (Kotal et al., 2011) Sample

Neat PU PU/LDH
stearate (1 wt%) PU/LDH
stearate (3 wt%) PU/LDH
stearate (5 wt%) PU/LDH
stearate (8 wt%)

Tg ( C) Tan δmax

2 46 2 44 2 41 2 40 2 31

0.49 0.45 0.44 0.43 0.41

log E0 (MPa)

log Ev (MPa)

225 C

25 C

225 C

25 C

7.12 7.61 7.73 7.82 8.27

6.68 7.15 7.47 7.52 8.0

6.41 6.71 6.97 7.02 8.0

5.44 5.96 6.16 6.22 6.51

Source: Reproduced with permission from Wiley.

between polar urethane groups of TPU and hydroxyl group of St-LDHs. As a result, the mobility of TPU chains is restricted, influencing the increase in stiffness with agradual introduction of St-LDH layers in the TPU matrix. It is also evident from Table 9.3 that the improvement in log E0 is relatively much higher in TPU/St-LDH

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nanocomposites, possibly due to the increased interaction of
OH and
NHCOO groups present in St-LDH and TPU, respectively. The Tg of the soft segment of TPU is shifted with increasing loading of St-LDH to the higher-temperature regions and is found to be maximum (15 C) at 8 wt.% in TPU nanocomposites. It is also evident from Table 9.3 that increasing the filler loading in TPU is accompanied by a gradual reduction in the tan δmax values. This is in all probability due to the restricted molecular chains of TPU as a result of the increasing interaction between TPU and filler. DMA studies of dodecyl sulfate intercalated LDH-filled TPU nanocomposites also exhibited more or less similar behavior (Kotal et al., 2009). Kotal and Srivastava (2011a) also reported DMA analysis of PU/DS-LDH nanocomposites. Fig. 9.29(A, B) and (C) show the temperature dependence of storage moduli (E0 ) and loss factor (tan δ) of the relevant nanocomposites respectively. E0 is found to be maximum (176%) at 3 wt.% loading and considerably increased over a wide temperature range. Furthermore, it is noted that Tg is maximum for 3 wt.% DSLDH loading in PU, where it increases from 213 C (Tg) for neat PU to 26 C due to the increased interaction between DS-LDH and the PU matrix, which results in the restricted mobility of PU chains leading to relaxation of PU/DS-LDH systems at higher temperature. Usually, the temperature range with tan δ . 0.3 is taken as a standard to evaluate damping materials (Kuila et al., 2008b). In light of this, Zhang et al. (2016) studied the damping properties of the PU/APS-DS-LDH nanocomposites at 1 Hz. The temperature range with tan δ . 0.3 of neat PU was about 44.6 C and the glass transition temperature and tan δ (max) were found to be
34 C and 0.95, respectively. They also noted that PU/APS-DS-LDH (3 wt.%) exhibits temperature ranges B52 C and tan δ reached a maximum value of 1.04. This clearly suggested an improvement in tan δ of PU/APS-DS-LDH (3 wt.%) compared with neat PU and accounted for its enhanced damping properties.

9.6.1.2 EVA-LDH nanocomposites The temperature variation of dynamic storage modulus (E/) of neat EVA-18, EVA28, EVA-45, EVA-60, and corresponding DS-LDH-filled nanocomposites are reported in Fig. 9.30 (Kuila, 2009). It is noted that the storage modulus of EVA-18 nanocomposites with 1 wt.% DS-LDH content is higher compared to neat EVA. However, the storage modulus decreases with a higher amount of DS-LDH content. The storage moduli of EVA-28/DS-LDH nanocomposites are enhanced at Tg and at higher temperatures (50 C
80 C). However, in the ambient temperature (10 C
30 C) region, the storage modulus of the nanocomposites decreases compared to neat EVA-28. The storage moduli of EVA-45/DS-LDH nanocomposites are significantly higher compared to neat EVA-45 and the storage modulus values increase gradually with increasing concentration of DS-LDH in EVA. In contrast, the storage modulus of EVA-60/DS-LDH (3 wt.%) nanocomposites is higher compared to the neat counterpart or EVA-60/DS-LDH (1 wt.%) and EVA-60/DS-LDH (5 wt.%) nanocomposites. These findings clearly showed that the storage modulus values of different nanocomposites are improved with respect to neat EVA. In all

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391

(A) 4.0 3.5 PU

log E′ (MPa)

3.0

1 wt% DS-LDH

2.5

3 wt% DS-LDH

2.0

5 wt% DS-LDH

1.5

8 wt% DS-LDH

1.0 0.5 0.0

–0.5 –1.0 –60 (B)

–40

–20

40

60

80

0.9 PU

0.8

1 wt% DS-LDH

0.7

tan δ

0 20 Temperature (ºC)

3 wt% DS-LDH

0.6

5 wt% DS-LDH

0.5

8 wt% DS-LDH

0.4 0.3 0.2 0.1 0.0 –60

–40

–20

0 20 Temperature (ºC)

40

60

80

Figure 9.29 Temperature dependence of (A) storage modulus and (B) tan δ for PU nanocomposites with DS-LDH content (Kotal and Srivastava, 2011a). Source: Reproduced with permission from Springer.

probability, incorporation of DS-LDH in the polymer matrix restricts the mobility of the polymer chains through chemical bonding between the polar acetate group of EVA and hydroxyl group of DS-LDH, enhancing thereby the stiffness of the nanocomposites. As a result, the storage modulus of the nanocomposites exceeds that of neat EVA. However, the degree of improvement of the storage modulus for

392

Layered Double Hydroxide Polymer Nanocomposites

(A)

(B) c

1E9

b a

d 1E8

log E⬘ (Pa)

log E⬘ (Pa)

1E9

e

1E8 a

c

b

d

1E7 1E7 –40

–20

0 20 40 Temperature (°C)

60

(C)

0 20 40 60 Temperature (°C)

80

d c log E⬘ (Pa)

log E⬘ (Pa)

–20

(D) 1E9

1E9 1E8 1E7

–40

b

1E7

c a

a

1E6

1E6 –40

1E8

–20 0 20 40 Temperature (°C)

60

80

–40

d

b –20

0 20 40 Temperature (°C)

60

80

Figure 9.30 Storage modulus vs. temperature curve of (A) EVA-18/DS-LDH, (B) EVA-28/ DS-LDH, (C) EVA-45/DS-LDH, and (D) EVA-60/DS-LDH nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH contents (Kuila, 2009).

different EVA/DS-LDH systems is not equivalent. This is probably due to the different degree of dispersion of DS-LDH layers in the nanocomposites. Fig. 9.31 displays variation of tan δ versus temperature for EVA-18/DS-LDH, EVA-28/DS-LDH, EVA-45/DS-LDH, and EVA-60/DS-LDH nanocomposites with 0, 1, 3, 5, and 8 wt.% DS-LDH contents (Kuila, 2009). It is noted that the tan δ peak position of EVA-18 nanocomposites shifted toward the negative temperature region. This indicates that the Tg of the nanocomposites decreases due to the plasticization effect of DS-LDH layers in EVA. The low concentration of vinyl acetate (VA) in EVA-18 is not sufficient for chemical interaction between the polar acetate group of EVA and the hydroxyl group of DS-LDH. The intensity of tan δ peak of the nanocomposites of EVA-28 and EVA-45 with DS-LDH filler decreases with increasing concentration of DS-LDH in EVA. The change in Tg is observed in EVA-60/DS-LDH nanocomposites. Hoewver, the heights of tan δ peak are similar to those of EVA-28 and EVA-45. The intensity of the tan δ peak decreased with increasing LDH content due to the stiffness of these materials. It is anticipated that incorporation of DS-LDH nanolayers restricts the mobility of the polymer molecules by the strong chemical bonding resulting in the reinforcement of the polymer

Mechancial and dynamical mechanical properties

393

(A)

(B)

0.25

ac

0.35

b d

0.20

a

b c

0.30

e

tan δ

tan δ

0.25 0.15 0.10

0.20

d

0.15 0.10

0.05

0.05 0.00 –40

–20

0

20

40

(C) 0.8

a

0.7 0.6

–40

60

Temperature (°C)

1.8 1.6

c

1.4

0

20

40

60

40

60

Temperature (°C)

(D)

d

b

–20

a

b c d

1.2

tan δ

tan δ

0.5 0.4 0.3

1.0 0.8 0.6

0.2

0.4 0.2

0.1 0.0 –40

–20

0

20

Temperature (°C)

40

60

0.0 –40

–20

0

20

Temperature (°C)

Figure 9.31 Tan δ vs. temperature curve of (A) EVA-18/DS-LDH, (B) EVA-28/DS-LDH, (C) EVA-45/DS-LDH, and (D) EVA-60/DS-LDH nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH contents (Kuila, 2009).

matrix. Therefore, DS-LDH layers exhibit higher stiffness compared to the pristine EVA and account for the reduction in the magnitude of the tan δ peak. The broadness of the tan δ peak is determined at half of its height, and is associated with the mobility of the polymer chain in the thermal relaxation process. For compatible polymer/inorganic nanocomposites, local polymer chains that are located close to the filler surface require higher temperature to induce the motion of the polymer chains. However, other polymer chains, far from filler surfaces, which are unaffected by the fillers, maintain the same properties as those of the pristine polymer. Such improvements in mechanical properties have been correlated in terms of fracture behavior of the nanocomposites using SEM. Zhang et al. (2007) studied the effects of LDH loadings on the Tg of EVA/ HFMH/LDH. It is observed that Tg values decrease with increasing LDH contents toward the Tg value of pure EVA (Tg:
12 C). Furthermore, it is noted that Tg values of EVA/HFMH (Tg 5 22 C) filled with 2, 5, 10, and 15 phr of organomodified LDH (OM-LDH) correspond to
3,
5,
7, and
10 C, respectively. It seems that the addition of OM-LDH enhanced the mobility of the polymeric chain

394

Layered Double Hydroxide Polymer Nanocomposites

4.5 4.0 3.5 c Log E⬘ (MPa)

3.0 2.5 2.0

b

e a

d

1.5 1.0 0.5 0.0 –0.5 –100

–50

0

50

Temperature (°C)

Figure 9.32 Storage modulus versus temperature of the St
LDH/SR nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% St
LDH (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

segments compared to EVA (100 phr)/HFMH (100 phr). Temperature variation of storage modulus of EVA/HFMH (100:100 phr), EVA/HFMH/LDH (100:2:98 phr), EVA/HFMH/LDH (100:5:95 phr), and EVA/HFMH/LDH (100:15:85 phr) have also been made at a constant frequency of 5 Hz. Their findings demonstrated much larger storage modulus value of EVA (without LDH) than that of the pure EVA sample. However, the storage modulus of the EVA/HFMH/LDH samples decreased with increasing loading of OM-LDH. This is attributed to the OM-LDH helping the dispersion of the HFMH evenly in the EVA matrix.

9.6.1.3 SR/LDH nanocomposites Figs. 9.32 and 9.33 display variation of the storage modulus (E) and tan δ for neat SR and the St-LDH/SR nanocomposites, respectively (Pradhan et al., 2012). These findings indicated the storage modulus of the St-LDH/SR nanocomposites at
75 C are greater compared to SR. It is also noted that the modulus is maximally improved by 714% higher for the St-LDH (3 wt.%)/SR nanocomposite. Such an enhancement in storage modulus could be attributed to the higher aspect ratio of the dispersed LDH layers. Alternatively, possibility of the interaction between the polymer chains and LDH restricting the flexibility of SR chains as well as the increase the stiffness of the composite also cannot be ruled out. The reduction in the storage modulus at higher filler loading in the SR matrix is likely to be due to the aggregation of St-LDH in SR (Kong et al., 2006). The tan δ versus temperature curves indicate that the incorporation of St-LDH lowers Tg of the corresponding SR

Mechancial and dynamical mechanical properties

395

0.08 a

0.06 0.04 Tan delta

0.02

b c d e

0.00 –0.02 –0.04 –0.06 –0.08 –100

–50

0

50

Temperature (°C)

Figure 9.33 Temperature dependence of tan δ of the St
LDH/SR nanocomposites with (a) 0, (b) 1, (c) 3, (d) 5, and (e) 8 wt.% St
LDH (Pradhan et al., 2012). Source: Reproduced with permission from Wiley.

nanocomposites approximately by 3 C and could be attributed to the plasticization effect of the St-LDH particles in the SR matrix.

9.6.1.4 EPDM-LDH nanocomposites The variation of tan δ and storage modulus (E0 ) versus temperature plots of the EPDM-LDH composites are displayed in Fig. 9.34 (Basu et al., 2016). It is noted that Tg shifted towards higher temperatures (by a small margin) on the addition of LDH. At a 4 phr loading the ZnO-containing sample shows a positive shift of B4 C as compared with the 4 phr LDH sample. However, on increasing the loading of LDH from 4 to 100 phr indicated an increasing tendency of Tg. The peak heights of the tan δ versus temperature plot decreased at a higher loading of LDH in the EPDM. The storage modulus also significantly improved in the region after the Tg and showed a steady rubbery plateau at a higher loading of LDH with EPDM. Acharya (2008) studied the variation of dynamic storage modulus (E0 ) and damping (tan δ) of EPDM/DS-LDH nanocomposites as a function of temperature, and the corresponding data are displayed in Table 9.4. It can be seen that the storage moduli of EPDM filled with 2-wt.% layered double hydroxide increased abruptly below Tg, after which, it decreases with increasing DS-LDH contents. Such a decrease is possibly due to the presence of amorphous regions in the corresponding nanocomposites. However, the storage modului of EPDM/LDH nanocomposites increase regularly with an increase in filler content above Tg. Such an improvement in the storage modulus of EPDM/DS-LDH nanocomposites is possibly due to the presence of stiff LDH filler in addition to the combined effect of

396

Layered Double Hydroxide Polymer Nanocomposites

(A) 2.0

–42.4°C

EPDM-4 ZnO EPDM-4 LDH EPDM-10 LDH EPDM-40 LDH EPDM-100 LDH

–38.5°C –37.5°C

1.5

–37.2°C

1.0

tan δ

–36.8°C

0.5

0.0 –80

–60

0

–40 –20

20

40

60

Temperature (°C) (B) EPDM-ZnO EPDM-4 LDH EPDM-10 LDH EPDM-40 LDH EPDM-100 LDH

E⬘ (MPa)

103

102

101

–80

–60 –40

–20

0

20

40

60

Temperature (°C) 4

(C)

log E⬘ (MPa)

3

EPDM-100 LDH EPDM-40 LDH EPDM-10 LDH EPDM-4 ZnO EPDM-4 LDH ~Ea/R

2

5.18 kJ mol

–1

2.78 kJ mol–1

1

1.54 kJ mol–1 1.12 kJ mol–1 1.34 kJ mol

0 0.0028

0.0035

–1

0.0042

0.0049

1/T (K–1)

Figure 9.34 (A) Loss tangent (tan δ) and (B) storage modulus (E’) vs. temperature and (C) log E’ vs 1/T plots of the EPDM
LDH composites (Basu et al., 2016). Source: Reproduced with permission from Royal Society of Chemistry.

Mechancial and dynamical mechanical properties

397

Table 9.4 Dynamic mechanical data of neat EPDM and EPDM/DSLDH nanocomposites (Acharya, 2008) Sample

DS-LDH contents (wt.%)

Tg

E0 (Pa) 3 107 at Tg

E0 (Pa) 3 106 at 25 C

Tan δ

EL0 EL2 EL3 EL4

0 2 3 4


61.2
48.5
47.4
47.2

3.8 4.7 3.7 3.5

2.5 4.2 4.5 6.6

1.18 0.76 1.05 0.92

aspect ratio and degree of dispersion of LDH particles. It is also noteworthy to mention that the glass transition temperature, Tg (β-relaxation), increases by  13.8 C for 3-wt.% of LDH content and remains more or less the same, with higher wt.% of LDH loading. Furthermore, it is noted that the tan δ value decreased and the peak area broadened in the nanocomposites. According to Das et al. (2012), storage modulus decreases with temperature, and loss modulus reaches a maximum in SBR/Zn-Al-LDH nanocomposites. The peak height of tan δ is reduced with the increase in the filler loading.

9.6.1.5 NBR/LDH and XNBR/LDH nanocomposites It is well known that ZnO along with stearic acid during vulcanization condition reacts with sulfur and organic accelerators. Eshwaran et al. (2015) reported that ZnAl LDH can be used in place of ZnO and stearic acid for vulcanization as well as in reinforcement of the rubber matrix. Fig. 9.35 shows dynamical mechanical thermal analysis of NBR nanocomposites consisting of unmodified LDH (uLDHc), modified commercial LDH (mcLDHc), modified synthesized LDH (mLDHs), and ZnO equivalent are presented in Fig. 9.35 (Eshwaran et al., 2015). A nominal negative shift and noticeable reduction in tan δ peak for mLDHc is also observed. NBR/ mLDHc exhibited B4 C lower Tg values as compared to uLDHc and mLDHs composites. Furthermore, it is noted that there is hardly any change in storage modulus (E0 ) with considerable change. Therefore, it can be concluded that mLDHc-based nanocomposites show a better filler
polymer interaction than uLDHc and mLDHs composites due to the reduction in the tan δ peak height. The variation of amplitude sweep with the amount of incorporated filler showed that mLDHs possesses a higher filler
filler interaction compared to the other nanocomposites. Pradhan et al. (2008) recorded that the tan δ maximum value steadily decreases with increasing filler concentration in XNBR/MgAl-LDH. Laskowska et al. (2014a) studied variation of tan δ, storage modulus (E0 ), and loss modulus (Ev) with temperature of XNBR and its MgAl-LDH nanocomposites at 10 Hz. They noted that the storage modulus (Ev) increases with increasing MgAl-LDH concentration, and the value is found to be highest at 30 phr loading in XNBR. This is probably due to the strong interfacial interactions between MgAl-LDH and XNBR. It is also evident from tan δ versus temperature plot that Tg of neat XNBR (
9.9 C) is increased

398

Layered Double Hydroxide Polymer Nanocomposites

(A)

1.5 Unmodified commercial LDH (uLDHc) Modified commercial LDH (mLDHc) Modified synthesized LDH (mLDHs)

3

10

10

tan δ

E⬘ (MPa)

1.0 102

0.5

1

100 –60

–40

–20

0

20

40

0.0 60

Temperature (°C)

(B) 4.0

Unmodified commercial LDH (uLDHc) Modified commercial LDH (mLDHc) Modified synthesized LDH (mLDHs)

E⬘ (MPa)

3.8

3.6

3.4

3.2

3.0 0.1

1

10

Dynamic strain (%)

Figure 9.35 (A) Temperature sweep curves and (B) amplitude sweep curves for NBR 1846 with different LDH (Eshwaran et al., 2015).

to
6.8 C in MgAl-LDH reinforced samples. Furthermore, a plot of loss modulus Ev versus temperature showed that Tg of XNBR (
22.2 C) increased in XNBR/ MgAl-LDH (
18.0 C) nanocomposites.

9.6.2 Dynamical mechanical properties of elastomer blend-LDH nanocomposites 9.6.2.1 PU blend/LDH nanocomposites The temperature dependence of storage modulus (E0 ), loss modulus (Ev), and tan δ of neat TN and its SFCNF-LDH hybrid-filled TN nanocomposites has been investigated and the findings are displayed in Fig. 9.36 (Roy et al., 2016b). It is noted that 0.50 wt.% SFCNF-LDH hybrid-filled TN achieved maximum E0 values up to 276% (at
60 C) and 261% (at 25 C). Ev values also increased in TN/SFCNF-LDH nanocomposites compared to the neat sample. At
30 C, the Ev values of the 0.50 wt.% hybrid-filled TN nanocomposite is maximally improved (254%) compared to the pure TN. The enhanced E0 and Ev values in TN nanocomposites suggest a strong influence of the hybrid filler on the elastic properties and their interaction with the

Mechancial and dynamical mechanical properties

399

(B)

(A) Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

1000

100

Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

100

E⬘⬘ (MPa)

E⬘ (MPa)

1000

10

10

1

0.1

1 –80

–60

–40

–20

0

20

40

60

–80

–60

–40

Temperature (°C)

–20

0

20

40

60

Temperature (°C)

(C) Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)

0.8

Tan δ

0.6

0.4

0.2

0.0 –80

–60

–40

–20

0

20

40

60

Temperature (°C)

Figure 9.36 (A) Temperature dependence of E0 values of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid. (B) E” values of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid and (C) tan δ of the neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNF-LDH hybrid (Roy et al., 2016b). Source: Reproduced with permission from Wiley.

polymer chains. This is likely to increase the friction between the filler and polymer because of the combined effect of the dispersion of filler and the filler
polymer interaction. The 0.50 wt.% SFCNF-LDH hybrid-filled TN nanocomposites showed a decrease in tan δ height (0.75) compared to that in the neat TN (0.79). Such findings were attributed to the internal friction among the nanofiller
nanofiller, nanofiller
polymer matrix, and polymer matrix
matrix under some external stresses. TN nanocomposites incorporated with 0.50 wt.% of filler exhibited the maximum positive shift in Tg (B3 C) compared to the neat sample because of strong polymer
filler interactions. Tg also shifted to higher values in the case of the 0.75 wt.% SFCNF-LDH filler loading followed by a reduction in the 1 wt.% filler-loaded TN blends in all probability due to the aggregation of the hybrid filler. DMA findings of neat TN and its SFCNT-LDH and SFCNF-LDH hybrid-filled nanocomposites are recorded in Table 9.5 (Roy et al., 2016a). The storage modulus and loss modulus of hybrid-filled TN nanocomposites are always higher compared to neat TN. TN nanocomposites filled with 0.50 wt.% hybrid showed maximum

400

Layered Double Hydroxide Polymer Nanocomposites

Table 9.5 Storage modulus at different temperatures, glass transition temperature (Tg), and height of tan δ value of pure TN and its composites (Roy et al., 2016a) Sample

E (MPa) at 260 C

% E (MPa) Improvement at 25 C

% Tg ( C) Improvement from Ev

tan δ height

Pure TN TN/SFCNT-LDH (0.25 wt.%) TN/SFCNT-LDH (0.50 wt.%) TN/SFCNT-LDH (0.75 wt.%) TN/SFCNT-LDH (1.0 wt.%) TN/SFCNF-LDH (0.25 wt.%) TN/SFCNF-LDH (0.50 wt.%) TN/SFCNF-LDH (0.75 wt.%)

571 1920


236

2.70 7.78


188

2 36 2 33

0.79 0.75

2402

321

10.30

281

2 30

0.66

1950

241

9.92

267

2 34

0.74

1840

222

7.77

187

2 35

0.76

1702

198

5.50

104

2 36

0.76

2158

278

10.15

276

2 32

0.71

1960

243

9.13

238

2 34

0.72

Source: Reproduced with permission from Springer (original Table 1, Roy, S., Srivastava, S.K., Mittal, V., 2016. Facile noncovalent assembly of MWCNT-LDH and CNF-LDH as reinforcing hybrid fillers in thermoplastic polyurethane/nitrile butadiene rubber blends. J. Polym. Res. 23: 36/1
36/11.

improvements in E0 . This could be ascribed to the excellent dispersion of filler into the matrix causing a strong interaction of filler with the polymer chains and resulting in efficient load transfer by the hybrid filler to the polymer. The loss modulus (E) value (at 230 C) of pure TN (47.22 MPa) has been improved by 237, 339, 219, and 200%, with 0.25, 0.50, 0.75, and 1 wt.% SFCNT-LDH hybrid loading, respectively. The corresponding improvements in loss modulus of SFCNF-LDHloaded TN blends are found to be 165, 262, 209, and 202%. These findings suggested that hybrid fillers not only influence the elastic properties strongly but also increase the friction between filler and polymer. The variation of tan δ versus temperature showed higher Tg of all the hybrid-filled nanocomposites compared to neat TN. It is noted that all the TN nanocomposites exhibit a slight positive shifting in Tg compared to neat TN. Furthermore, the SFCNT-LDH hybrid shows a relative increment in Tg compared to the SFCNF-LDH hybrid in TN matrix, which signifies that the SFCNT-LDH hybrid exerts comparatively better restriction than the other hybrid filler. Roy et al. (2016c) studied dynamic mechanical analysis of SFCNT-LDH hybridfilled TPU/NBR blend and the findings are displayed in Fig. 9.37. It is noted that the modulus of the 0.50 wt.% hybrid-filled TPU/NBR blend is significantly enhanced in both the glassy region (by 243% at 260 C) and rubbery state (by 241% at 25 C) compared to pure TPU/NBR. Such an enhancement in storage modulus is attributed to the homogeneous dispersion of the SFCNT-LDH nanofiller within the matrix, along with a strong interaction between the SFCNT-LDH hybrid filler and polymer matrix. The loss modulus (at 230 C) of TPU/NBR

Mechancial and dynamical mechanical properties

(B)

2500 Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

2000

1500

1000

200

100

500

50

0

0

–80

–60

–40

–20

0

20

40

60

Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

150

Loss modulus

Storage modulus (MPa)

(A)

401

80

–80

–60

–40

Temperature (°C)

–20

0

20

40

60

80

Temperature (°C)

(C) Neat blend Blend/MWCNT-LDH (0.25 wt%) Blend/MWCNT-LDH (0.50 wt%) Blend/MWCNT-LDH (0.75 wt%) Blend/MWCNT-LDH (1.0 wt%)

0.8

Tan δ

0.6

0.4

0.2

0.0 –80

–60

–40

–20

0

20

40

60

80

Temperature (°C)

Figure 9.37 Temperature dependence of (A) storage modulus, (B) loss modulus, and (C) tan δ of neat TPU/NBR and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt.% SFCNTMgAl-LDH hybrid (Roy et al., 2016c). Source: Reproduced with permission from Wiley.

nanocomposites filled with 0.25, 0.50, 0.75, and 1 wt.% hybrid is improved by 99, 254, 224, and 193%, respectively, compared to pure TPU/NBR. The temperature variation of the dissipation factor (tan δ) of neat TPU/NBR and its SFCNT-LDH hybrid-filled nanocomposites show decreases in the height of the tan δ curve in neat TPU/NBR filled with SFCNT-LDH hybrid. The maximum decrease in tan δ height (0.62) compared to neat TPU/NBR (0.79) is recorded in 0.50 wt.% SFCNTLDH hybrid-loaded TPU/NBR nanocomposite. Furthermore, it is also noted that a slight increase in Tg is observed in TPU/NBR/SFCNT-LDH nanocomposites compared to the TPU/NBR blend.

9.6.2.2 EVA-EPDM/LDH nanocomposites Kuila et al. (2009b) investigated the variation of dynamic storage modulus (E0 ), loss modulus (Ev), and damping (tan δ) of neat EVA/EPDM/DS-LDH nanocomposites as a function of temperature, which are displayed in Fig. 9.38. It is inferred that the storage modulus of EVA/EPDM/DS-LDH nanocomposites exceeds that of the neat EVA/EPDM blend. The absolute values of Ev at higher loss peak are

402

Layered Double Hydroxide Polymer Nanocomposites (B)

(A)

8

10 8

log E⬘ (Pa)

log E⬘ (Pa)

10

d c b

7

10

7

10

d c

a b a

6

10 6

10

–40

–20

0

20

40

60

80

–60

Temperature (°C)

–40

–20

0

20

40

60

80

Temperature (°C)

(C) 0.6 0.5

b a d c

Tan δ

0.4 0.3 0.2 0.1 0.0 –40

–30

–20

–10

0

10

20

30

40

Temperature (°C)

Figure 9.38 Temperature dependence of (A) Storage modulus (E0 ). (B) Loss modulus and (C) loss tangent tan δ of (a) pure EVA/EPDM blend, and their nanocomposites with (b) 3, (c) 5, and (d) 8 wt.% DS-LDH content (Kuila et al., 2009b). Source: Reproduced with permission from Wiley Interscience.

greater than the neat EVA/EPDM blend. The loss modulus (Ev) for the nanocomposites is found to be higher over the experimental temperature region. In EVA/ EPDM blends, Tg’s broaden and remain separated but shifted toward each other. It is also noted that both Tg’s shows little or no change with the addition of DS-LDH in EVA/EPDM. This may be attributed to the lack of surrounding entanglement of the polymer chains. The temperature dependence of tan δ of EVA/EPDM/DS-LDH nanocomposites shows that the height of tan δ peak decreases with increasing DSLDH concentration. This may be ascribed to the presence of rigid DS-LDH nanolayers accounting for increasing stiffness in the nanocomposites. Furthermore, it is noted that tan δ peaks of the nanocomposites are extended to a wider temperature range compared to that of the neat EVA/EPDM blend.

9.7

Conclusion

Several methods are reported for the synthesis of EVA, SR, EPDM, SBR, NBR, NR, and their blend nanocomposites using different types of individual/modified

Mechancial and dynamical mechanical properties

403

LDH and hybrid fillers of LDH. Following this, these nanocomposites have been investigated for the establishment of nanostructure and dispersion in polymer matrix through XRD, TEM, and SEM studies. Furthermore, mechanical and dynamical properties of most of these elastomer and elastomeric blend nanocomposites could be further enhanced compared to their neat counterparts under static and dynamic applications. The low mechanical properties of silicone rubber have also been successfully enhanced in LDHs and LDH-based hybrid-filled elastomer nanocomposites. However, more work in the future should be focused on investigation into the mechanical and dynamical properties of nanocomposites of LDH-filled SBR, SR, NR, NBR and XNBR elastomers and elastomeric blends.

References Abdullah, M.A.A., Ahmad, M.H., Yunus, W.M.Z.W., Rahman, M.Z.A., Hussein, M.Z., Dahlan, K.Z.H.M., 2010. Preparation and properties of natural rubber/layered double hydroxide nanocomposites. J. Sustain. Sci. Manage. 5, 58
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Layered double hydroxide nanocomposites based on carbon nanoforms

10

Gonzalo Abella´n1,2, Jose A. Carrasco1 and Eugenio Coronado1 1 Institute of Molecular Science (ICMol), University of Valencia, Valencia, Spain, 2 Department of Chemistry and Pharmacy and Joint Institute of Advanced Materials and Processes (ZMP), University Erlangen-Nu¨rnberg, Fu¨rth, Germany

10.1

A general introduction to LDH-carbon nanoform nanocomposites

Layered double hydroxides (LDHs) are anionic clays built from the stacking of positively charged brucite-type inorganic layers interleaved with anions. They can be formulated as [Mz11
xMy1x(OH)2]x1[An
]x/n  mH2O (Mz1, z 5 1 or 2; My1, y 5 3 or 4; An
5 organic or inorganic anions). Synthetic LDHs are quite promising based on their low cost, chemical versatility, and anion-exchange capabilities, which allows for tuning of their composition and properties in a wide range (Rives, 2001; Duan and Evans, 2006). Thanks to their layered nature, they can also be exfoliated in polar organic solvents by intercalation of suitable anions that minimize the interaction between layers to produce stable colloids containing single and fewlayer LDHs, which possess ultrahigh surface areas of c.1000 cm2/g in theory (Chen et al., 2015). These positively charged nanosheets can then be used as inorganic building blocks in the design of hybrid nanocomposites when mixed with other organic or molecular materials. In this context, an interesting ingredient of these nanocomposites is provided by the so-called carbon nanoforms (CNFs). These carbon-based materials include a wide variety of sizes and shapes ranging from C60 to metallofullerenes, carbon fibers, nanotubes, nano-onions, nanohorns, nanobuds, spheres, or graphene, among others (Delgado et al., 2008). Owing to their lowdimensional nature and sp2 hybridization of carbon atoms, and because of the sheer multitude of their esthetically pleasing structures, 0D fullerenes, 1D carbon nanotubes (CNTs), and 2D graphene, exhibit excellent physical and chemical properties, and stand as one of the most appealing building blocks in the development of nanocomposites. For instance, graphene (Geim and Novoselov, 2007; Geim, 2009), a one-atom-thick layer of graphite, is a semimetal, gapless two-dimensional (2D) semiconductor with unprecedented properties including ultrahigh room-temperature carrier mobility, or specific surface area (2630 m2/g, in theory). Moreover, graphene is an elastic film, with a high Young’s modulus (c.1.0 GPa) (Lee et al., 2008), it is Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00010-0 © 2020 Elsevier Ltd. All rights reserved.

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transparent like (or better than) plastic (Geim and Novoselov, 2007), and possesses outstanding electrical (c.106 S/cm) (Chen et al., 2008) and thermal (3000
5000 W/ m/K) (Seol et al., 2010) conductivities, better than any metal. Additionally, it behaves as an impermeable membrane (Abraham et al., 2017), and can be considered as both a solid and a macromolecule with molecular weights of more than 106
107 g/mol (Eigler and Hirsch, 2014). Graphene also exhibits a rich chemical phenomenology that ranges from supramolecular to covalent functionalization, endowing them with improved processability as well as allowing to controllably engineer the bandgap structure, create novel architectures, and manipulate the interfacial characteristics of this monolayer material (Eigler and Hirsch, 2014; Hirsch et al., 2013; Paulus et al., 2013; Criado et al., 2015; Abella´n et al., 2017). When it comes to nanocomposite formation, the limited solubility of pristine graphene is one of the main drawbacks. Along this front, negatively charged graphene oxide (GO) is usually employed as a very effective alternative. GO is a single layer of graphite oxide. During the formation of graphite oxide, the graphene layers in graphite become intercalated by an acid to form a stage 1 intercalation compound, with all layers being intercalated (Eigler and Hirsch, 2014). Subsequent oxygenation of such stage 1 intercalation compounds occurs on both sides of the basal plane and in this way graphite oxide is formed. Delamination of single layers of graphite oxide leads to GO. The exact nature of the functional groups in GO strongly depends on the reaction conditions, such as preparation time and temperature, as well as on the work-up procedure. Typically, GO consists of about 45 mass % carbon. Although several structure models have been proposed, GO is a rather polydisperse material, whose exact structure is very difficult to precisely define. The mobility of charge carriers depends on the density of defects and, therefore, mobility values range between 0.1 and 1000 cm2/V  s, very limited values compared to that of pristine graphene, which usually exceeds 15,000 cm2/V  s. In any case, its excellent processability has allowed the development of the first examples of graphene/LDH hybrid nanocomposites. With respect to 1D CNTs—which can exist with various flavors (i.e., a broad variation of helicities, single-walled, multiwalled)—they possess a low bandgap of 0
1.9 eV, high electrical conductivity of 0.17
2 3 105 S/cm (Baughman, 2002), and high thermal conductivity of 3000
6600 W/m  K (Kim et al., 2001). In addition, their excellent mechanical properties with Young’s moduli of 0.27
1.25 GPa and a tensile strength of 11
63 GPa (Yu, 2000), makes them excellent candidates for functional nanocomposite reinforcement. In stark contrast, LDHs exhibit an insulating behavior, low-to-moderate specific surfaces areas ranging from 20 to 365 m2/g, but a rich redox chemical behavior (Chen et al., 2015). The incorporation of transition metals in synthetic LDHs endowed them with excellent redox properties, which is extremely interesting in energy conversion and storage. Moreover, they can accommodate different metals in a well-defined atomic arrangement within the layers, a property that has been extensively used in heterogeneous catalysis. But LDHs are not only interesting as active materials per se, but also as functional precursors of layered mixed metal oxides or spinels, of utmost importance in catalysis or magnetism (Abella´n et al., 2014a, 2015b).

Layered double hydroxide nanocomposites based on carbon nanoforms

413

Probably, the most interesting property of LDHs is their anion-exchange ability (Rives, 2001). LDHs are capable of accommodating a multitude of inorganic and organic anions, including complex macrocycles, polymers, or even DNA strands (Duan and Evans, 2006). This allows the synthesis of hybrid functional materials with the coexistence, and in some cases, even interaction of different properties (Abella´n et al., 2014c, 2015a, 2015b; Clemente-Leo´n et al., 2011). Additionally, LDHs submitted to calcination at moderate temperatures exhibit the so-called “memory effect,” enabling the reconstruction/recrystallization of the initial solids in the presence of anions. This effect has been extensively used for environmental remediation and controlled delivery of anions (Goh et al., 2008). The intrinsic limitations of LDHs, mainly due to their poor conductivity, can be overcome by their combination with highly conducting CNFs, in the same way as the rich chemical reactivity of LDH perfectly complements the limited redox behavior of CNFs. In this sense, the design and synthesis of hierarchical nanocomposites is one of the most interesting approaches for combining their distinguishing properties, creating multifunctional hybrid materials endowed with high mechanical strength and hierarchical porosities, as well as improved heat and mass transfer properties (Abella´n et al., 2010; Coronado et al., 2010). The combination of LDHs with different CNFs has attracted an increasing level of attention during recent years due to their outstanding properties in different fields ranging from polymer reinforcement and environmental remediation, to catalysis or energy storage and conversion. Table 10.1 summarizes some properties of CNFs and LDHs, and Fig. 10.1 depicts a schematic illustration of an LDH material combined with the most typical CNFs. Along this line, Table 10.2 depicts the most typical LDH combination of metals and the capability of these inorganic layers to be mixed with CNFs giving rise to nanocomposites. There are different synthetic approaches for the preparation of CNF/LDH hybrids (Zhao et al., 2012). We can divide them into three main families, graphene/ LDH; CNT/LDH; and other nanocomposites combining LDHs with CNFs like fullerenes, carbon spheres, or mesoporus carbons. Fig. 10.1 summarizes the most common synthetic approaches pursued in the literature to prepare these systems. Firstly, based on the strong electrostatic interactions between exfoliated positively charged LDH nanosheets (Li et al., 2005b) and negatively charged CNFs, the reassembly of nanocarbons and LDHs is one of the most extensively used approaches for the synthesis of LDH/carbon hybrids. Secondly, owing to the strong adsorption ability of negatively charged CNFs for metal cations, their surface is perfectly suited for the direct seed nucleation and growth of LDH crystallites under basic pHs by means of coprecipitation or hydrothermal methods (Abella´n et al., 2014a; Du et al., 2014; Okamoto et al., 2007). Finally, LDHs can act as either as a catalytic support for the in situ formation of CNFs [employing chemical vapor deposition (CVD) using a carbon source like ethylene], or even as multilayer nanoreactors, where carbon-rich molecules are intercalated within LDH; the calcination of these systems leads to the formation of hybrid nanocomposites (Abella´n et al., 2012a, 2014a; Wang et al., 2014; Xu et al., 2001).

Table 10.1 Properties of different carbon nanoform materials and LDHs Properties

Fullerenes (Shi et al., 1992; Haddon et al., 1995; Yu et al., 1992)

CNTs (Yu, 2000; Krishnan et al., 1998; Yu et al., 2000; Wilder et al., 1998; Ebbesen et al., 1996; Berber et al., 2000; Peigney et al., 2001)

Graphene (Geim and Novoselov, 2007; Lee et al., 2008; Chen et al., 2008; Seol et al., 2010)

Graphene oxide (Montes-Navajas et al., 2013; Compton and Nguyen, 2010; Dreyer et al., 2010; Liu et al., 2012a; Suk et al., 2010; Go´mezNavarro et al., 2007)

LDHs (Chen et al., 2015)

Youngs modulus (GPa) Tensile strength/a Bandgap (eV) Electrical conductivity (s/cm)

15.9

0.27
1.25

c.1.0

380
470





1.5
2.0 10214
6  1028

11
63 0
1.9 0.17
2  105

130 0 c.106

130 0.1
4 1  1023
2

Thermal conductivity (W/m  K) Specific surface area (m2/g) Chemical reactivity

0.4

3000
6600

3000
5000

c.5000



Insulating (B62 S/cm for calcined CoNi-LDH)





150
1315

2630

736.6

20
365

Active

Active

Active

Active (redox)

Active (redox)

Source: Adapted from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Wei, F. 2012. Adv. Funct. Mater. 22 (4), 675 and extended with new data for graphene oxide.

Layered double hydroxide nanocomposites based on carbon nanoforms

415

Figure 10.1 Schematic illustration with some of the most typical nanocomposites between LDH and CNFs. (A) Nanocomposites with graphene: (A1) in situ growth of LDH flakes parallel or (A2) vertical to the graphene layer, (A3) intercalation of graphene layers into the interlayer space of the LDH and (A4) graphene growth on the surface of the LDH crystallite. (B) Nanocomposites with carbon nanotubes: (B1) in situ growth of LDH flakes in the surface of CNTs, (B2) in situ growth of CNTs vertical, or (B3) parallel aligned to the LDH surface. (C) Nanocomposites with, from left to right, fullerene, carbon spheres, and carbon fibers: (C1) direct assembly between LDH flakes and fullerenes located into the interlayer space, (C2) in situ growth of LDH crystallites on the surface of carbon spheres, and (C3) in situ growth of carbon fibers vertically aligned to the surface of the LDH flakes.

In the first part of this review we will discuss in detail the synthetic routes pursued to prepare LDH/carbon nanocomposites focusing first on 2D graphene, then on 1D CNTs, and finally on other CNFs. In the second part we will give an overview of the main applications of these nanocomposites in areas like energy storage and conversion and catalysis

10.2

Graphene and graphene oxide/LDH nanocomposites

As previously mentioned, all the examples of graphene/LDH nanocomposites developed in the literature using the reassembly of 2D building blocks have employed GO instead of pristine graphene, owing to its good processability and negatively charged surface. The reduction of GO can be performed by thermal or chemical treatment with reducing reagents like hydrazine, leading to rGO. The exfoliation of LDH leads to positively charged nanosheets with a unilamellar thickness (,1 nm) and ultimate 2D anisotropy (aspect ratio .100). The ideal perfect assembly should consist of electrostatic face-to-face stacking of positively charged LDH nanosheets

416

Layered Double Hydroxide Polymer Nanocomposites

Table 10.2 Most representative LDH compositions and their capability for synthesizing the main CNFs are labeled as follows: G (graphene), T (carbon nanotubes), F (fullerene), QD (carbon quantum dots), CS (carbon spheres), and F (carbon fibers) M3+,M4+/M2+,M+

Fe2+

Co2+

Ni2+

Cu2+

Zn2+

Ca2+

Mg2+

Mn2+

Li+

G, T, Fe3+

G, T, F

QD, CS

Co3+

G, T, F

Ni3+

G

F

G, T,

Al3+

CS, F

G, T, QD,

G, T

G

Mn3+ Ga

G

CS

CS Cr3+

G, T, F,

T, F

G, T

3+

In3+ Ti4+

G

Synthesized catalytic LDHs

LDHs without catalytic activity for nanocarbon growth

Not synthesized

and negatively charged GO in an alternating sequence at a molecular scale. However, due to the large disparity of particle sizes and intrinsic polydisperse nature of GO, a perfect sandwich-like hybrid remains a big challenge (Gao et al., 2011; Li et al., 2010). The first self-assembled graphene/LDH nanocomposite ever described was developed in 2010 (Chen et al., 2010). In that work, Li et al. synthesized a film of graphene and CoAl-LDH. Both halves were previously exfoliated and delaminated and then reassembled into the final nanocomposite. Graphene/LDH nanocomposites with different compositions such as NiAl (Gao et al., 2011), NiCo (Chen et al., 2014), or CoAl (Wang et al., 2011) have been described and applied as feasible supercapacitors, giving rise to excellent performance in terms of energy storage behavior, higher specific capacitance, and longer life cycles than expected for the separate materials.

Layered double hydroxide nanocomposites based on carbon nanoforms

417

10.2.1 Synthesis As mentioned above, we can classify the synthetic methods in three main branches, namely, reassembly of LDHs and graphene, direct growth of LDH on graphene, and formation of graphene in LDH layers.

10.2.1.1 Reassembly of graphene and LDHs This synthetic methodology lies in the self-assembly of preexfoliated 2D building blocks in solution, leading to the final restacked material, or in a layer-by-layer assembly on selected substrates (Daud et al., 2016; Cao et al., 2016) (Fig. 10.2). The exfoliation-restacking synthesis is based on the delamination of LDH previously intercalated with labile anions such as chlorides, nitrates, or surfactants prior to the combination with GO through electrostatic interactions (Wimalasiri et al., 2014). The main problem associated with the delamination of LDHs is the great facility in which the exfoliated nanosheets tend to reassemble, due to the electrostatic interactions between the cationic hydroxide layers and the interlamellar anions (Wang and O’Hare, 2012). So far, no exhaustive studies have been carried out on the full optimization of the liquid-phase exfoliation (LPE) (Latorre-Sanchez et al., 2012; Abella´n et al., 2012b) of LDHs, in contrast with the exfoliation of other 2D materials like graphene, transition metal dichalcogenides, or simple metal

Figure 10.2 (A) Scheme with the formation of a CoAl-LDH and graphene oxide (GO) nanocomposite. (B) Digital photographs of an aqueous dispersion of CoAl-LDH (left), of GO (middle), and the GO/CoAl-LDH nanocomposite (right). Source: Reproduced from Wang, L., Wang, D., Dong, X.Y., Zhang, Z.J., Pei, X.F., Chen, X. J., et al., 2011. Chem. Commun., 47 (12), 3556 with permission from the Royal Society of Chemistry.

418

Layered Double Hydroxide Polymer Nanocomposites

hydroxides which have been fully addressed (Backes et al., 2017). To avoid the inherent problems LDHs have in terms of delamination, polar aprotic solvents like formamide (Huang et al., 2015) or bulky amphiphilic interlamellar anions such as surfactant molecules (specially dodecyl sulfate) (Ma et al., 2015) are commonly used in order to enable the delamination of cationic nanosheets to be thermodynamically favorable, preventing unwanted aggregation. At this point, it is worth remarking that the conventional aqueous synthetic routes for LDHs usually lead to carbonate-intercalated LDH that requires further anion-exchange processes to weaken the interlamellar interactions, and allow proper exfoliation in solvents such as formamide or dimethylformamide. When it comes to nonaqueous routes, Gardner et al. described a synthetic procedure to obtain nanometric alkoxideintercalated Al-containing LDHs using alcohols as solvents (Gardner et al., 2001; Gursky et al., 2006). Recently, this methodology has been successfully extended to synthesize pure phases like: NiMn-, CoFe-, NiCo-, and NiFe-LDH, which can be exfoliated in water due to the hydrolysis of the interlamellar alkoxide groups (Fig. 10.3). (Chen et al., 2014; Latorre-Sanchez et al., 2012; Abella´n et al., 2012b, 2014b; Carrasco et al., 2016b; Xu et al., 2013). These chemical compositions are of utmost importance in magnetism, Li-ion batteries or water splitting. Along this line, Latorre-Sanchez et al. reported the first synthesis in water of a GO/NiMn-LDH with superparamagnetic behavior and high capacity values when used as an anode in Li-ion batteries (Latorre-Sanchez et al., 2012). The negative nature of GO reveals it to be crucial for combining with the positive LDH layers in the final nanocomposite. Controlling the GO synthetic process can modify the amount of negative charges. On the other hand, by differing the atomic ratio it is possible to modulate the positive charge in LDH to a certain extent. Fig. 10.4 (extracted from Wimalasiri et al., 2014) exhibits the schematic process for mixing GO (reduced with hydrazine) with LDH in a unique material, as well as electronic microscopy images that further corroborate this assembly.

Figure 10.3 (A) Idealized illustration of the preparation and structure of the hybrid GO/ NiMn-LDH. The scheme illustrates the size of the sheets and the large particle surface of GO on which the smaller NiMn-LDH nanosheets are supported. (B) Digital photographs of the aqueous dispersions of GO (left), NiMn-LDH (middle), and the hybrid GO/NiMn-LDH formed immediately after mixing the previous components. Source: Reproduced from Latorre-Sanchez, M., Atienzar, P., Abella´n, G., Puche, M., Forne´s, V., Ribera, A., et al., 2012. Carbon, 50 (2), 518 with permission from Elsevier.

Layered double hydroxide nanocomposites based on carbon nanoforms

419

Figure 10.4 (A) Schematic procedure for the synthesis of a graphene/NiAl-LDH nanocomposite and (B) the corresponding digital photographs of the different steps. Scanning electron microscopy (SEM) images of (C) NiAl-LDH platelets, (D) exfoliated NiAl-LDH nanosheets, (E) graphene nanosheets, and (F) graphene/NiAl
LDH nanocomposite. Source: Reproduced from Wimalasiri, Y., Fan, R., Zhao, X.S., Zou, L., 2014. Electrochimica Acta 134, 127 with permission from Elsevier.

Recently Ma and co-workers have revisited the synthesis of rGO/LDH hybrids by means of a fine control of the graphene:LDH mass ratios and adjusting the charge density of GO/rGO nanosheets, improving the stacking of the layers, which results in superlattice structures of up to 20 layers in the best case. Exploiting this strategy, the preparation of rGO/CoAl, rGO/MgAl, rGO/NiFe, and rGO/NiMn-LDH hybrids has been successfully developed, exhibiting great potential in supercapacitors, water splitting, and membranes (Ma et al., 2014, 2015, 2016; Sun et al., 2016) (Fig. 10.5). On the other hand, the hydrogen-bonding layer-by-layer (LbL) technique has been successfully applied in the assembly between positive hydroxide and negative GO nanosheets. The main difference with the exfoliation and restacking approach

420

Layered Double Hydroxide Polymer Nanocomposites

Figure 10.5 (A) XRD patterns of LDH nanosheets flocculated with GO (black trace) and rGO (red trace) nanosheets, respectively. Indices 001 are basal series of superlattice lamellar composites, whereas L100 and L110 are in-plane diffraction peaks from LDH nanosheets. (Inset) Schematic illustration of sandwiched LDH nanosheets and graphene. (B) TEM observations of the lamellar nanocomposites. (C) High-resolution image showing lamellar lattice fringes with different contrast. (D) Electron diffraction indexed to be in-plane diffraction rings of LDH (L100 and L110) and graphene (G100 and G110), respectively. Source: Reproduced from Ma, R., Liu, X., Liang, J., Bando, Y., Sasaki, T., 2014. Adv. Mater. 26 (24), 4173 with permission from Wiley.

relies on the usage of substrates and a polymeric solution to assist the layer-bylayer assembly. In the work of Liu et al. (Chen et al., 2010) the authors prepare the GO nanosheets by a slightly modification of the Hummers method, and the CoAlNO3 LDH is delaminated in formamide. For the LbL assembly, polyvinyl alcohol (PVA) is dissolved in deionized water in order to obtain an aqueous solution of PVA 1% in weight. After preparing the different components, cleaned quartz glass slides (using H2SO4/H2O2, which introduces some negative charges in the SiO2 surface) are submerged first in a suspension of LDH nanosheets, followed by dipping the substrates in the PVA solution, then in the exfoliated GO, and finally again in the PVA solution. Between each step, a thorough rinsing with deionized water is carried out. The complete cycle is repeated several times to obtain the final nanocomposite film. The reduction of GO to rGO has been performed by immersing the final film in hydrazine/N,N-dimethylformamide (0.5 mL 50% hydrazine/30 mL DMF) solution. (Fig. 10.6). Similar assemblies have been obtained between CoAlLDH exfoliated in formamide and aqueous GO suspensions using PDDA-coated ITO [where PDDA 5 poly(diallyldimethylammonium chloride) and ITO 5 indium tin oxide] and flexible PET substrates. In this case an iterative self-assembly process has been developed without using a polymer between the different building blocks. The reduction is obtained by heating at 200 C for 2 h under H2 atmosphere (Fig. 10.6). LBL is a promising technique as other driving forces than electrostatic interactions can be used for directing the self-assembly of LDH and CNFs, such as hydrophobic interactions or hydrogen bonding.

10.2.1.2 Direct growth of LDH on graphene Two main approaches have been used for the direct growth of LDH on graphene. On the one hand, the coprecipitation of LDH starting from the selected precursor

Layered double hydroxide nanocomposites based on carbon nanoforms

421

Figure 10.6 (A) Schematic procedure of a layer-by-layer assembly of LDH and GO nanosheets mediated through PVA chains. (B) Scheme of LbL assembly for the synthesis of multilayer films of GO/CoAl-LDH nanocomposite. Source: (A) Reproduced with permission from Chen, D., Wang, X., Liu, T., Wang, X., Li, J., 2010. ACS Appl. Mater. Interfaces 2 (7), 2005. Copyright 2010 American Chemical Society. (B) Reproduced with permission from Dong, X., Wang, L., Wang, D., Li, C., Jin, J., 2012. Langmuir 28 (1), 293. Copyright 2012 American Chemical Society.

salts (M21/M31) in the presence of ultrasonicated GO under controlled pH conditions. On the other hand, the hydrothermal approach used stainless steel Teflonlined autoclaves. Reducing agents like urea, hydrazine, glucose or sodium sulfide are commonly used to adjust the pH and reduce the GO to rGO (Liu et al., 2006). In this approach, GO acts as a template for the precipitation of the LDH, due to the adsorption capacity of the cations added during the precipitation process, thus limiting the final LDH particle size. Heating at moderate temperatures triggers the hydrolysis of urea that progressively increases the pH up to a final value close to 8. This enables the in situ reduction of GO layers into rGO, whilst assisting the formation of the LDH phase. Fig. 10.7 shows the first report on the synthesis of LDHrGO hybrids (Li et al., 2010). By following this synthesis, graphene/LDH nanocomposites of different compositions have been described, from the conventional CoAl-LDH (Huang et al., 2012) and MgAl-LDH (Yuan et al., 2013) to more sophisticated ZnCr-LDH (Lan et al., 2014), NiFe-LDH (Youn et al., 2015), or even ternary LDHs like NiCoAl-LDH (He et al., 2015). The work of Liu et al. (Gao et al., 2011) depicts a modified synthetic

422

Layered Double Hydroxide Polymer Nanocomposites

(A)

GO

RGO-Ni-Fe LDH

Exfoliated GO TMAOH

Ni2+, Fe3+ urea and TSC

Exfoliation

Adsorption

150ºC, 48 h

Fe3+ d = 0.82 nm

GO nanosheet

(B)

Hydrothermal reduction Graphene nanosheets

Ni2+

d = 0.78 nm

Ni-Fe LDH nanosheets

5000 cps

RGO-Ni-Fe LDH Intensity

500 cps Exfoliated GO

d = 0.82 nm

1000 cps

GO 10

20

30 40 2 Theta/degrees

50

60

70

Figure 10.7 (A) Schematic representation of the synthetic procedure of the hybrid rGO/ NiFe-LDH hybrid. (B) XRPD patterns of the precursor GO, the delaminated form, and the nanocomposite. The precursor GO exhibits a basal space of 0.82 nm, implying a complete oxidation of graphite into GO. The exfoliated sheets display a broad peak in the range 20
40 degrees, related with scattering effects between the exfoliated sheets and water as solvent. Lastly, the pattern of the hybrid material matches with an NiFe-LDH phase and no peaks of graphite are observed, therefore the restacking of the as-reduced graphene sheets is prevented. Source: Reproduced from Li, H., Zhu, G., Liu, Z.-H., Yang, Z., Wang, Z., 2010. Carbon 48 (15), 4391 with permission from Elsevier.

procedure in the formation of a graphene/NiAl-LDH nanocomposite. In this case, graphite oxide is exfoliated by ultrasonication and glucose is used as a reducing agent in order to obtain a graphene nanosheet suspension prior to the in situ growth of the LDH, avoiding the use of hydrazine due to its high toxicity (Zhu et al., 2010). After that, the suspension of graphene nanosheets is introduced in an autoclave where Ni and Al salts are present, as well as urea as a pH-controlling agent (Huang et al., 2013). The final nanocomposite has been successfully prepared after 24 h at 95 C (Gao et al., 2011) (Fig. 10.8). The annealing temperature is another important parameter that has to be optimized in order to avoid metal reduction at the expense of graphene that can act as a sacrificial reducing agent being converted into CO and CO2 (Fig. 10.9) (Abella´n et al., 2012b). In general, the direct growth of LDH on GO leads to strong interactions at the interface (Wang and Dai, 2013). Indeed, XANES measurement have been employed by Dai and co-workers to study the nature of these interactions. They observe for a related NiFe-LDH/CNT hybrid the formation of M 2 O 2 C (MQNi, Fe) bonding via the carboxyl group, leading to large perturbations to the carbon atoms in the carbonyl groups. The same is expected in the case of GO (Gong et al., 2013).

Layered double hydroxide nanocomposites based on carbon nanoforms

423

GO

GNS Glucose 95ºC 1 h GO nanosheet Adsorption

95ºC 24 h Ni/Al LDH nanosheets

Hydrothermal

Ni2+

Ni2+, Al3+ and urea

Graphene nanosheets

Al3+

Figure 10.8 Scheme with the formation of a graphene/LDH nanocomposite. Source: Reproduced with permission from Gao, Z., Wang, J., Li, Z., Yang, W., Wang, B., Hou, M., et al., 2011. Chem. Mater. 23 (15), 3509. Copyright 2011 American Chemical Society.

Figure 10.9 Idealized structure of the GO/NiMn-LDH hybrid. The remarkable difference in size between NiMn-LDH (c.100 nm) and GO (c.3 μm) is illustrated, as well as how the thermal treatment affects the segregation of Ni NP and Mn oxides. The right panel shows an FESEM image of an Ni NP of c.20 μm formed in the nanometallurgic process. Source: Reproduced from Abella´n, G., Latorre-Sa´nchez, M., Forne´s, V., Ribera, A., Garcı´a, H., 2012b. Chem. Commun. 48 (93), 11416 with permission from the Royal Society of Chemistry.

The main limitation of the direct nucleation and growth of the LDH phases on GO is the restricted accessibility of electrolyte to the active metal centers due to the formation of thicker flakes compared to the exfoliation and restacking approach. On the other hand, the strong interaction between both phases leads to a dramatic improvement in the electrical properties of LDHs.

10.2.1.3 Graphene formation in LDH layers Owing to its unusual anion intercalation property, noncatalytic systems like MgAlLDH can act as “nanoreactors” for the formation of CNFs within their interlamellar space. Indeed, Yang et al. (Sun et al., 2012) described a way to synthesize graphene

424

Layered Double Hydroxide Polymer Nanocomposites

Step I

Step II

Step III

(A)

Acid

Calcination

(B)

Monolayer graphene

Calcined LDH-1

LDH-1

Etching LDH-2

Calcined LDH-2

Bi-trilayer graphene

(C) Graphene LDH-3 DSO

Calcined LDH-3

Multilayer graphene

MMA

Figure 10.10 Scheme with the formation of MMA-derived graphene nanosheets with a different number of layers in the interlamelar space of an LDH. (A) Monolayer, (B) bi- and trilayer, and (C) multilayer graphene nanosheets after acid treatment of LDH-1, 2, and 3, respectively. Source: Reproduced from Sun, J., Liu, H., Chen, X., Evans, D.G., Yang, W., Duan, X., 2012. Chem. Commun. 48 (65), 8126 with permission from the Royal Society of Chemistry.

nanosheets with a certain control over their thickness after the calcination of MgAlLDH intercalated with dodecyl sulfonate (DSO) and methyl methacrylate (MMA) anions [see Fig. 10.10 extracted from Sun et al. (2012)]. These small and slightly oxidized graphene flakes can be isolated and characterized after acid etching of the nanocomposite. On the other hand, by template-directed CVD it is possible to use MgAl-LDH as precursors for the synthesis of mesoporous MgAl-layered double oxide (LDO) nanosheets consisting of MgO and MgAl2O4. The mesoporus structure arises from the Kirkendall diffusion of Mg21 during the transformation of octahedral Mg
O into tetrahedral Mg
O via dehydration of the interfacial
OH groups. These layered oxides have been proven excellent for actively decomposing CH4 (or related carbon sources) and depositing single-layer graphene due to the exposed oxygen atoms on their surface. In contrast to monometallic hydroxides like Mg(OH)2— which after calcination lead to the formation of MgO layers—the use of MgAlLDH gives rise to an uninterrupted and uniform unstacked graphene layer with protuberances rather than an aggregate of graphene nanocages (Xie et al., 2012). This is mainly due to the presence of mesopores (Fig. 10.11) (Zhao et al., 2014c). Aside from catalytically “innocent” metals like Mg, Zn, or Al, LDHs can be excellent catalyst precursors. Indeed, a wide variety of CNFs have been synthesized using the metal oxides or the metal nanoparticles obtained after calcination and/or reduction of LDH having Fe, Ni, or Co atoms (Abella´n et al., 2012a; Evans and Duan, 2006). A thorough revision can be encountered in the work of Wei et al., indeed using this methodology several graphene-containing hierarchical systems

Layered double hydroxide nanocomposites based on carbon nanoforms

425

Figure 10.11 Scheme for the synthesis of unstacked double-layer template graphene. A mesoporous oxide derived from the calcination of a LDH is used as a template, followed by its removal by acid etching. Source: Reproduced from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Tian, G.-L., Nie, J.-Q., Peng, H.-J., et al., 2014c. Nat. Commun. 5 with permission from Nature publishing group.

have been synthesized, including SWCNT/graphene hybrids (see below) (Zhao et al., 2012). Authors like Zhang et al. have used methane as a carbon source combined with different LDHs to obtain a mixture of graphene and CNT hybrids with LDHs (Zhao et al., 2013b, 2014b). Fig. 10.12 depicts part of the characterization of the nanocomposite graphene/SWCNT/CoAlMg-LDO extracted from Zhao et al. (2013b). Despite the aforementioned efforts, the directly controlled synthesis of highquality graphene within the interlamellar space of LDH remains an open challenge. In any case, the rich chemical versatility of LDH assures a promising future in the design of new graphene synthetic concepts.

10.3

Carbon nanotubes/LDH nanocomposites

As introduced in the previous section, the first works reported in the literature were related to the use of LDH phases as catalysts for the formation of CNF, and the subsequent elimination of the LDH phase by acid leaching. Indeed, since 1997 several works have explored the influence of composition, size, surface reactivity, and calcination temperatures in the CVD growth of CNF using LDH as catalytic precursors. As a matter of fact, Wei et al. reported the use of different LDHs as catalysts for the large-scale synthesis of high-quality single-walled CNTs by means of a fluidized-bed catalytic CVD procedure (Zhao et al., 2010a). Other examples where LDHs have been used as catalysts can be found elsewhere (Li et al., 2005a; Xiang et al., 2009; Cao et al., 2009; Zhao et al., 2007; Xue et al., 2010). In 2008 the first CNT/LDH nanocomposite was described by Zhang et al. (Su et al., 2008). In that work, a MWCNT/CoAl-LDH nanocomposite was synthesized

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Figure 10.12 (A) SEM image of CoMgAl-LDH platelets, (B) XRPD patterns for the CoMgAl-LDH and LDO, (C) SEM, (D) TEM, and (E, F) HRTEM images for the graphene/ single-walled CNT/CoMgAl-LDO nanocomposite. Source: Reproduced from Zhao, M.-Q., Zhang, Q., Huang, J.-Q., Tian, G.-L., Chen, T.-C., Qian, W.-Z., et al., 2013b. Carbon 54, 403 with permission from Elsevier.

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by means of homogeneous precipitation, and used as electrode material for supercapacitor purposes at high currents, leading to improved performances. Similarly, in 2010, a CNT/NiAl-LDH nanocomposite with controllable LDH mass ratio was described by Li and co-workers, showing LDH platelets with a lateral size of 10
15 nm (Wang et al., 2010; Du and Fang, 2010). At that time the important influence of the functionalization degree of CNT was pointed out, paving the way for future works (Du and Fang, 2010).

10.3.1 Synthesis The two main approaches for the synthesis of these nanocomposites are LDH formation directly on CNTs and the in situ growth of CNTs on the LDH structure. The reassembly of both halves is not usually chosen as a synthetic method but some examples can be found in the literature.

10.3.1.1 Reassembly of CNTs and LDHs Reassembly of CNTs and LDHs has been carried out by authors such as Qiao et al. (2013) and Wang et al. (2016). In both cases, the LDH material was delaminated in formamide, followed by the addition of an NaOH (Qiao et al., 2013) or aqueous solution (Wang et al., 2016) with functionalized CNTs. Electrostatic interactions between opposing charges (cationic sheets of LDH and anionic groups in the CNT) are the driving force for the self-assembly process. Fig. 10.13, extracted from Qiao et al. (2013), represents a schematic description of the procedure.

Figure 10.13 Scheme of the formation of multiwalled CNT/LDH nanosheet nanocomposites. Source: Reproduced from Qiao, Z., Gao, C., Sun, B., Ai, S., 2013. J. Inorg. Organomet. Polym. Mater. 23 (4), 871 with permission from Springer.

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10.3.1.2 LDH formation on CNTs LDH formation on CNTs is the most typical method of synthesis for CNT/LDH nanocomposites, where the LDH phase has been usually synthesized by means of a coprecipitation or solvothermal method. In 2013, Dai and co-workers described the synthesis of a NiFe-LDH/CNT hybrid with an excellent electrocatalytic activity for the oxygen evolution reaction (OER), which boosted the research on NiFe applications in water splitting (Gong et al., 2013). They used the nucleation and growth of NiFe-LDH on the surface of dispersed oxidized CNTs at moderate temperatures (85 C), and then the solvothermal growth at 160 C obtaining crystalline NiFe-LDH platelets strongly interacting with partially reduced CNTs. However, this synthetic approach leads to the formation of spinel impurities, a typical problem associated with the aqueous synthesis of non-Al-containing LDH, like NiFe or CoFe, for example (Abella´n et al., 2013a, 2013c). A similar work was developed by. Yang et al. (2013b), this time combining a ternary NiCoAl-LDH with activated multiwalled CNTs via a homogeneous urea precipitation method. The resulting nanocomposite exhibited high specific capacitances and good results in rate capability or cyclic stability. Zhao et al. (2014a) explored the formation of a nanocomposite between NiMn-LDH or CoMn-LDH and CNTs previously treated with HNO3. The formation of the hybrid material was achieved by mixing the corresponding metal salts and the functionalized CNTs by an in situ coprecipitation method [Fig. 10.14, extracted from the work of Zhao et al. (2014a) depicts a scheme of the procedure]. The resulting nanocomposite was tested as an electrode material in supercapacitors, giving rise to excellent cyclability and high specific capacitance.

Figure 10.14 Scheme with the formation of CNT/NiMn-LDH nanocomposite. Step (I): surface modification of the CNT by functional groups such as
OH
,
CO or
COO
. Step (II): grafting of the NiMn-LDH nanosheets to the CNT structure by an in situ growth method. Source: Reproduced from Zhao, J., Chen, J., Xu, S., Shao, M., Zhang, Q., Wei, F., et al., 2014a. Adv. Funct. Mater. 24 (20), 2938 with permission from Wiley.

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10.3.1.3 CNTs formation on LDHs This approach relies on using CVD procedures, which usually give rise to a mixture of carbon species like graphene, carbon spheres, or CNTs (Abella´n et al., 2014a; Hima et al., 2008; Pacuła et al., 2015). The work of Hima et al. (2008) clearly defines how the CVD conditions directly affect the final CNFs (see Fig. 10.15 extracted from that work). The authors stated that the different CNFs may arise from an overgrowth of the initially formed 1D tubular structures, which gain volume as long as the CVD time is increased. This fact also affects the wettability properties of the nanocomposite, leading to superhydrophobic films in the case of the longest growing times. The selective formation of a specific CNF is governed by several factors, being the size and surface density (NP/m) of the catalytic nanoparticles (e.g., Fe NPs) generated in situ, one of the most important parameters. The fine-tuning of the size by using FeMgAl-LDH intercalated with MoO22 4 , which act as a pinning center for the Fe NPs generated after calcination, can lead to the selective formation of SWCNTs and more complex helical structures (Zhao et al., 2010b). Another factor that was revealed as very important in the specific growth of CNTs was the final

Interwoven CSs

Nanometric Co particles

Sillicon wafer

Reduction

Sillicon wafer D CV in m 60 CVD

Caterpillar-like CFs

40 min C 20 VD mi n

Sillicon wafer CNTs

Calcined CoAl-LDH Sillicon wafer Sillicon wafer

CoAl-LDH Platelet-like particles

Calcination 700ºC/2 h

Sillicon wafer

Figure 10.15 Schematic formation of different nanocomposites of carbon structures after using CoAl-LDH particles as catalysts. Interwoven carbon spheres, caterpillar-like carbon fibers, and carbon nanotubes are reported. Source: Reproduced from Hima, H.I., Xiang, X., Zhang, L., Li, F., 2008. J. Mater. Chem. 18 (11), 1245 with permission from the Royal Society of Chemistry.

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disposition of the 2D crystallites. In this sense, magnetic NiFe-LDHs are a special class of hydroxides that exhibit the highest catalytic metal concentration allowed by these layered entities. However, it was not until 2014 that their application in direct nanocarbon formation was first studied (Abella´n et al., 2014a). Interestingly, bulk NiFe-LDH led to a wide variety of CNFs such as few-layer graphene, carbon fibers, and MWCNTs, whereas ultrathin films c.150 nm thick with parallel orientation of the crystallites conduce to a hierarchical growth of bamboo-like CNTs with a thickness of c.250 nm at relatively low temperatures (550 C) (Abella´n et al., 2014a) (Fig. 10.16).

Figure 10.16 FESEM images of bamboo-like CNT growth over NiFe-LDH platelets after (A, C, E) a chemical vapor deposition (CVD) and (B, D, F) a catalytic CVD (in absence of H2) procedures. Source: Reproduced from Abella´n, G., Carrasco, J.A., Coronado, E., Prieto-Ruiz, J.P., PrimaGarcı´a, H., 2014a. Adv. Mater. Interfaces 1 (6), 1400184 with permission from Wiley.

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10.4

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Other CNF/LDH nanocomposites

Despite the fact that graphene/LDH and CNT/LDH nanocomposites are the most studied families of hybrid materials consisting of LDHs and carbon species, nanocomposites based on LDHs and other CNFs like fullerene, carbon spheres, nanofibers, carbon dots, or carbon rings have been reported.

10.4.1 Fullerene/LDH nanocomposites This type of material has been described by authors like Liu et al. (2012b), Liu (2013), and Fortner et al. (2012). The first example of C60/LDH nanocomposite dates from 1996, and was synthesized by simply mixing powders of dodecyl sulfate-intercalated MgAl-LDH with C60 solutions in toluene or hexane. The hydrophobic character of the interlamellar space of the alkyl-functionalized LDHs allowed the diffusion of the C60 molecules within the layers, and the ulterior calcination of the system led to the decomposition of dodecyl molecules forming the final C60/LDH nanocomposite (Tseng et al., 1996). A few recent examples were reported using the anion-exchange capacity of the LDHs, for example, by preparing benzoic acid-intercalated LDHs from NO3-LDHs and then adding the fullerene in the interlamellar space by means of ultrasonic treatment (Liu, 2013). It is worth remarking that no thorough characterization studies have been reported for this family of hybrids, as its physical and chemical properties are still unknown. Figure 10.1C1 exhibits a schematic representation of the C60/LDH nanocomposite.

10.4.2 Carbon quantum dot/LDH nanocomposites These nanocomposites were firstly reported in 2014 by Kang et al. They decorated NiFe-LDH with 0D carbon quantum dots (CQDs) of c.5 nm in a three-step synthetic process consisting of mixing first the metal salts and CQD solutions at 85 C for 4 h. Then, they transferred the mixture to a solvothermal reaction at 120 C for 12 h, followed by a final solvothermal process at 160 C for 2 h. A similar approach was developed by both Liu et al. and Zhang et al. (Liu et al., 2016a; Wei et al., 2016) for the synthesis of hybrids with MgAl and NiAl-LDH, respectively. Fig. 10.17, extracted from Tang et al. (2014), exhibits some parts of the characterization of the CQD/NiFe-LDH nanocomposite. The hybrids were tested as electrocatalysts for water oxidation as well as supercapacitors, with good results due to the synergy between both parts of the material.

10.4.3 Carbon spheres/LDH nanocomposites These nanocomposites were synthesized by authors like Zhang (Gong et al., 2011) and He (Xu et al., 2014) with the aim of exploring their electrochemical applications since in this field carbon spheres have been demonstrated to be of interest (Chen et al., 2012; Wickramaratne et al., 2014). Regarding the synthetic

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Figure 10.17 (A) SEM, (B) TEM, and (C) HRTEM images of a carbon quantum dot/NiFeLDH nanocomposite. (D) XRPD spectra of the nanocomposite and the pure LDH phase. Source: Reproduced with permission from Tang, D., Liu, J., Wu, X., Liu, R., Han, X., Han, Y., et al., 2014. ACS Appl. Mater. Interfaces 6 (10), 7918. Copyright 2014 American Chemical Society.

methodology, the direct reassembly of the spheres and LDHs has been explored by means of ultrasonication procedures, preparing both building blocks separately and then mixing them at the same time (Gong et al., 2011), whereas the most typical approximation of synthesizing these hybrids is by the CVD process, using the LDH material as a catalyst and a carbon source like ethylene (Carrasco et al., 2016a) for the formation of the spheres at high temperatures (e.g., 900 C) (Jin et al., 2005). Fig. 10.18 depicts the schematic formation by direct assembly of a carbon sphere/ LDH nanocomposite (Gong et al., 2011).

10.4.4 Carbon (nano)fibers/LDH nanocomposites The main approach pursued to obtain this kind of nanocomposite is the in situ LDH formation on carbon fibers (Yu et al., 2016; Zhao et al., 2013a; He et al., 2014; Warsi et al., 2014). In a typical procedure, the LDH is synthesized by means of a hydrothermal method, adding the carbon fibers to the mixture. He et al. (2014) prepared a mixture of bohemite AlOOH/carbon nanofibers and then an Ni salt plus urea were added to the aqueous solution. After some time, the final solution was transferred to an autoclave and the reaction was set at 100 C for 48 h. The resulting

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Figure 10.18 Schematic procedure for the synthesis of a carbon sphere/LDH nanocomposite through a direct assembly approach. Source: Reproduced with permission from Gong, J., Liu, T., Wang, X., Hu, X., Zhang, L., 2011. Environ. Sci. Technol. 45 (14), 6181. Copyright 2011 American Chemical Society.

nanocomposite was collected after a centrifugation process. Authors like Zhao et al. (2013a) also used MnO2 nanowires to enhance the supercapacitive properties of the nanocomposite [see Fig. 10.19 extracted from Zhao et al. (2013a)]. The main driving force in the development of these hybrids was their applications related to electrochemistry, especially for supercapacitors in this case (He et al., 2014). In addition to the previously described LDH nanocomposites, there are also some punctual examples of hybrids with other CNFs. For example, carbon nanorings or nano-onions/LDH have been synthesized by the groups of Zeng (Xu et al., 2001), Coronado (Abella´n et al., 2012a) (Fig. 10.20), and Yang (Sun et al., 2013) using Fe, Co, or Fe-containing LDH as catalytic nanoreactors. Indeed the pioneering works of Zeng and co-workers revealed that the in situ generated low-valencestate cobalt oxides during the calcination of terephthalate anion (TA2
)-intercalated CoMgAl-LDH led to the formation of MWCNTs with a diameter of 10
35 nm and a length of 20
200 nm (Xu and Zeng, 2000). The intercalation of poly(vinyl sulfonate)-intercalated CoAl-LDH led to the formation of sulfate-doped CNF (Xu et al., 2001). It is worth mentioning that since 1995 several efforts have been developed for the synthesis of meso/microporous 3D carbons using LDH as containers (Putyera et al., 1995, 1996; Hibino, 1996; Leroux and Dubois, 2006; Leroux et al., 2006; Pre´vot et al., 2011). More recently, the thermal decomposition of an NiFeLDH intercalated with sebacate molecules, acting as a carbon source, enables the low-temperature formation of carbon nano-onions (CNOs) and multiwalled nanotubes (MWNTs) through a simple reaction (Abella´n et al., 2012a). This process benefits from the catalytic activity of the FeNi3 nanoparticles formed in situ, which facilitate the decomposition of the confined sebacate molecules to produce CNFs

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Figure 10.19 (A, B, C) SEM images of the MnO2/LDH/carbon fiber nanocomposites. The inset in (B) corresponds to the EDS results. (D) EDS mapping of a single nanostructure. (E) TEM image of an individual MnO2/LDH composite (the inset corresponds to the FFT pattern). (F) Raman spectra of MnO2, carbon fiber/LDH and the MnO2/LDH/carbon fibers nanocomposite. Source: Reproduced from Zhao, J., Lu, Z., Shao, M., Yan, D., Wei, M., Evans, D.G., et al., 2013a. RSC Adv. 3 (4), 1045 with permission from the Royal Society of Chemistry.

embedded into a graphitic shell via a bottom-up approach. Remarkably, this was the first example of low-temperature (400 C) formation of CNFs and metal alloy nanoparticles, instead of LDO using LDH hybrids as precursors (Abella´n et al., 2012a, 2013b; Carrasco et al., 2016a). By following similar approaches carbon nanorings have been reported by Sun et al. (2013).

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Figure 10.20 (A) Schem