Hybrid Polymer Composite Materials. Structure and Chemistry [1st Edition] 9780081007921, 9780081007914

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Hybrid Polymer Composite Materials. Structure and Chemistry [1st Edition]
 9780081007921, 9780081007914

Table of contents :
Content:
Front-matter,Copyright,List of ContributorsEntitled to full text1 - Structure and chemistry of polymer/nanodiamond composites, Pages 1-21, Ayesha Kausar
2 - Chitosan-based hybrid polymer composites: Structure and chemistry, Pages 23-30, Katelyn Musumeche, Melanie Sanders, Dilip Depan
Chapter 3 - Chemistry of hybrid multifunctional and multibranched composites, Pages 31-63, Sergio D. Garcia Schejtman, Verónica Brunetti, Marisa Martinelli, Miriam C. Strumia
4 - Green hybrid composites from cellulose nanocrystal, Pages 65-99, Shahab Kashani Rahimi, Joshua U. Otaigbe
5 - Tailoring the interfaces in conducting polymer composites by controlled polymerization, Pages 101-134, Gergely F. Samu, Csaba Janáky
6 - Biomedical polymer hybrid composites, Pages 135-162, Tavakoli Javad, Dong Yu, Tang Youhong
7 - A review of the structure–property research on hybrid-reinforced polymer composites, Pages 163-191, Shahad Ibraheem, Sri Bandyopadhyay
8 - Structure and chemistry of fiber metal laminates, Pages 193-234, Barbara Surowska, Patryk Jakubczak, Jarosław Bieniaś
9 - Opportunities and challenges in the use of layered double hydroxide to produce hybrid polymer composites, Pages 235-261, Shadpour Mallakpour, Elham Khadem
10 - Green hybrid nanocomposites from metal oxides, poly(vinyl alcohol) and poly(vinyl pyrrolidone): Structure and chemistry, Pages 263-289, Shadpour Mallakpour, Vajiheh Behranvand
11 - Hybrid polymers composite: Effect of hybridization on the some propers of the materials, Pages 291-312, Vicente de Oliveira Sousa Neto, Gilberto Dantas Saraiva, Raimundo Nonato Pereira Teixeira, Diego de Quadros Melo, Francisco Cláudio de Freitas Barros, Ronaldo Ferreira do Nascimento
12 - Effect of stacking patterns on morphological and mechanical properties of luffa/coir hybrid fiber-reinforced epoxy composite laminates, Pages 313-333, Sudhir K. Saw
Index, Pages 335-344

Citation preview

Hybrid Polymer Composite Materials

Related titles Tribology of Polymeric Nanocomposites, 2e, (ISBN: 978-0-444-59455-6) Tribology of Polymeric Nanocomposites, (ISBN: 978-0-444-53155-1) Polymer-Layered Silicate and Silica Nanocomposites, (ISBN: 978-0-444-51570-4)

Woodhead Publishing Series in Composites Science and Engineering

Hybrid Polymer Composite Materials Structure and Chemistry

Edited by

Vijay Kumar Thakur Manju Kumari Thakur Raju Kumar Gupta

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 © 2017 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: 978-0-08-100791-4 (print) ISBN: 978-0-08-100792-1 (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: Charlotte Rowley Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

List of Contributors

Sri Bandyopadhyay School of Materials Science and Engineering, UNSW, Sydney, NSW, Australia Vajiheh Behranvand Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Jarosław Bienia´s Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology, Lublin, Poland Vero´nica Brunetti Department of Physical Chemistry (INFIQC, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina Francisco Cla´udio de Freitas Barros Department of Analytical Chemistry and Physical Chemistry, Federal University of Ceara, Fortaleza, Brazil Vicente de Oliveira Sousa Neto Department of Chemistry, State University of Ceara´ (UECE-CECITEC), Quixada´, Brazil Diego de Quadros Melo Federal Institute of Serta˜o Pernambucano, Petrolina, Brazil Dilip Depan Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette, LA, United States Ronaldo Ferreira do Nascimento Department of Analytical Chemistry and Physical Chemistry, Federal University of Ceara, Fortaleza, Brazil Sergio D. Garcia Schejtman Department of Organic Chemistry (IPQA, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina Shahad Ibraheem School of Materials Science and Engineering, UNSW, Sydney, NSW, Australia Patryk Jakubczak Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology, Lublin, Poland

x

List of Contributors

Csaba Jana´ky Department of Physical Chemistry and Materials Science, University of Szeged, Szeged, Hungary; MTA-SZTE “Lendu¨let” Photoelectrochemistry Research Group, Szeged, Hungary Tavakoli Javad Medical Device Research Institute, School of Computer Science, Engineering and Mathematics, Flinders University, Adelaide, SA, Australia Shahab Kashani Rahimi School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, MS, United States Ayesha Kausar Nanosciences Division, National Center for Physics, Quaid-iAzam University, Islamabad, Pakistan Elham Khadem Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Shadpour Mallakpour Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan, Islamic Republic of Iran; Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran Marisa Martinelli Department of Organic Chemistry (IPQA, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina Katelyn Musumeche Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette, LA, United States Joshua U. Otaigbe School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, MS, United States Raimundo Nonato Pereira Teixeira Department of Chemistry, Regional University of Cariri (URCA), Crato, Brazil Gergely F. Samu Department of Physical Chemistry and Materials Science, University of Szeged, Szeged, Hungary; MTA-SZTE “Lendu¨let” Photoelectrochemistry Research Group, Szeged, Hungary Melanie Sanders Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette, LA, United States Gilberto Dantas Saraiva Department of Physic, State University of Ceara´ (UECEFECLESC), Quixada´, Brazil

List of Contributors

xi

Sudhir K. Saw Central Instrumentation Facility, Birla Institute of Technology (Deemed University), Ranchi, India Miriam C. Strumia Department of Organic Chemistry (IPQA, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina Barbara Surowska Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology Lublin, Poland Tang Youhong Centre for NanoScale Science and Technology, School of Computer Science, Engineering and Mathematics, Flinders University, Adelaide, SA, Australia Dong Yu Department of Mechanical Engineering, School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia

1

Structure and chemistry of polymer/nanodiamond composites Ayesha Kausar Nanosciences Division, National Center for Physics, Quaid-i-Azam University, Islamabad, Pakistan

Chapter Outline 1.1 1.2 1.3 1.4 1.5

Introduction 1 Introduction to nanodiamond: background and invention Synthesis strategies for nanodiamond 3 Structure and chemistry of NDs 4 An overview of polymer/nanodiamond composite 6

2

1.5.1 Mechanical features of nanodiamond reinforced matrix 7 1.5.2 Thermal properties of nanodiamond reinforced polymer 9 1.5.3 Conducting properties of nanodiamond reinforced hybrid 12

1.6 Significance of polymer/nanodiamond composite

12

1.6.1 High strength performance materials 14 1.6.2 Thermal resistance materials 15 1.6.3 Li-ion batteries and electronic devices 16

1.7 Conclusions and outlook References 18

1.1

18

Introduction

Polymeric materials have become one of most extensively utilized materials because of their distinctive features mainly ease of manufacture, light weight, and ductile nature. Moreover, in comparison to metals, ceramics polymeric systems have accompanied lower strength and modulus [1]. Carbon-based nanomaterials such as carbon nanotube (CNT), detonation ND, and fullerene have been employed as physical modifier in polymer science. Subsequently, carbon nanomaterial based nanocomposite and layered silicates have been utilized for the enhancement of electrical, thermal, mechanical, and gas barrier features of polymer. Nanofillers have been recognized to considerably enhance the material features, in comparison to that of microsized filler, polymer, and composites itself [2]. The use of CNT as strengthening constituents has been restricted because of their high affinity to Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00001-X Copyright © 2017 Elsevier Ltd. All rights reserved.

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Hybrid Polymer Composite Materials: Structure and Chemistry

undergo agglomeration in polymeric matrix. Therefore, discovery of nanodiamond (ND) with its remarkable features, and dispersion tendency in polymer matrices has led to the progress of new class of nanocomposite [3]. Employment of ND has developed distinctive field of carbon-based nanoconstituent. ND has comprised of nanosized tetrahedral linkage and has been characterized as zero-dimensional material. The most significant traid of carbon nonmaterial have grown to be the forefront of material research [4]. The nanoscale diamond particles were first accidentally produced in Union of Soviet Socialist Republics by detonation [5]. Although until the end of 1980s, they remained unfamiliar materials to the rest of world. Commercially, NDs have been synthesized by detonation technique, laser ablation, and high energy ball milling of diamond microcrystals at high temperature and high pressure. Nanosized diamond powder with average size of 4 5 nm has been prepared by detonating the mixture of negative oxygen balance for example, mixture of hexogen and trinitrotoluene (TNT) in inert atmosphere [6]. Detonation soot is made up of 25 30 wt.% ND particles from graphene shells and amorphous carbon. Acid-purified NDs comprises inert diamond core that was surrounded by various covalently bonded surface groups such as OH, COOH, C 5 O, CH2, and CH3 [7]. ND comprises outstanding features such as bulk modulus, high thermal conductivity, chemical inertness, wear resistance, and excellent electrical insulating features. Subsequently, the large and accessible surface (300 500 m2 g21) and small diameter (B5 nm) specified great potential of ND for the enhancement of thermal, electrical, mechanical, and gas barrier features of polymers [8]. Lithium-ion batteries have gained extraordinary significance as energy storage of portable devices mainly laptops, cell phones computers, and others. In this regard, polymer-based electrolytes and electrodes have gained increased research attention because of good thermal stability, fine ionic conductivity, and other significant features [9,10]

1.2

Introduction to nanodiamond: background and invention

Amongst the excellent features of polymer/nanomaterials, the large surface-tovolume ratios and mechanical features are of immense significance. These features present numerous remarkable fields of research. The hardness and ultrahigh stiffness comprised by nanomaterials render them significance in polymer matrix reinforcement. In the view of recent research, small amount of nanomaterial (B1 wt.%) may considerably enhance the mechanical features (up to 100%). Although the actual mechanism responsible for this dynamic improvement has not clearly understood [11], it is supposed that the nanomaterials and polymer matrices organize molecular interface which plays vital role in property improvement. Range of nanomaterials have been prepared and characterized, amongst them, nanocarbon is of great attention with diverse dimensionalities for example, nanotube,

Structure and chemistry of polymer/nanodiamond composites

3

grapheme, and ND with one, two, and zero dimensionalities, correspondingly [12]. The mechanical features of the polymer matrices were strengthened with nanocarbons. Nature of linkage of nanocarbon components with polymer matrices depend on the dimensionality of filler component. For example, the ND may integrate with polymers at any point, while CNT may integrate over the polymer chain length. However, mechanical features of polymer matrices have been recognized to enhance by different type of nanocarbons. The synergetic influence of characteristic features on final composites may be obtained by the integration of two types of nanocarbons, where each of them integrates contrarily with the matrix [13]. In the year 1963, the explosive, named mixture B, was utilized by army for the first time. An immense ND concentration was found in smoke after explosion in the military secrecy. It was until the 1990s that the mass manufacture of ND has been initiated [14]. Current applications of ND are polymer compositions, electrochemical coatings and polishing, antifriction coatings, lubricants, biosensors, imaging probes, implant coatings, and drug carriers. Moreover, the absence of definite functional groups on ND surface restricts its applications. At present, functionalization of ND has become a research emphasis. Thus, halogen, amines, alkyl, and carboxylic acids have been grafted to ND surface [15].

1.3

Synthesis strategies for nanodiamond

For the preparation of ND, three routes have been successfully commercialized so far. Most wide and broad scale synthesis route is De Pont route in which carbon precursors for example, graphite, coal, and carbon black are transformed into diamond in a capsule by circular shock wave (B140 GPa), outside of the capsule. In the inner tube, the precursor of carbon are placed which in turn is placed in outer tube enclosed with driving tube. In the tube, free space is filled with explosive. The circular shock waves are synthesized by combustion of explosive at one end of the apparatus, through which the driving tubes are compressed, thus the transformation of sp2 carbon materials into ND particles may occur. For prevention of graphitization of diamond, a mixture of metallic powder (Al, Ni, and Cu) and graphite (6% 10%) is employed. The diamond yield is 5% of initial loaded material in capsule or 60 mass% of carbon phase. The ND prepared by this route has a bimodal type of size compensation that is, the size of first particles was 1 4 nm and the size ranges from 10 to 150 nm. Moreover, there are numerous alteration proposed of this route [16]. The second route for bulk-scale manufacture of ND is based on explosion of carbon comprising constituents with explosives. In this route, synthesis of diamond take place both within carbon comprising materials as well as by carbon atoms condensation present in explosives. In air atmosphere, the landsdalite particle size is smaller than 8 nm, while in inert atmosphere, a diamond cubic phase not more than 20 nm is generated. The diamond yield is about 3.4% of mass of explosives and 17% of mass of initial carbon. For the

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Hybrid Polymer Composite Materials: Structure and Chemistry

Nanodiamond carbon

Nanodiamond carbon

Purification

COOH

C

Good grain boundaries

OH

O

Incombustible impurities SOOT

Incombustible impurities COMMERCIAL ND

Figure 1.1 Major structural components of commercial DND product.

synthesis of ND, the third route is explosion route. By this route diamond clusters are formed from explosives employed as precursor constituents [17]. At present, numerous large industries reactors are in operation for the synthesis of ND. A number of explosive constituents can be employed such as mixture of TNT and hexogen (comprised of C, N, O, and H with negative oxygen balance). So, carbon is present in the system in additional amount. The significant state for ultradispersed diamond (UDD) ND formation is negative oxygen balance in system. The explosive is placed in a detonation chamber in this system and by nonoxidizing cooling water medium (ice) or gas (N2, Ar, and CO2), the ignition is carried out. In this process, no additional carbon material is needed for the generation of diamond soot, thus the explosive itself did not deliver the carbon. By employment of 3:2 mixture of TNT and hexogen, maximum production of diamond was achieved [18]. 2,4,6-Trinitro-1,3,5-benzotriamine and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine are the other explosives employed to carry the process. The detonation soot comprises diamond a nanoparticle up to 80% in which yield of carbon is around 4% 10% [19] (Fig. 1.1). By employment of explosives, there are two main technical needs for UDD synthesis that is, (1) explosive composition must offer thermodynamic condition and (2) for prevention of transformation to graphite, process of quenching is needed. The production is effected by explosive shape for example, the spherical shape is excellent, but cylindrical shape is usually utilized for accessibility [20].

1.4

Structure and chemistry of NDs

The ground-state electronic configuration of elemental carbon is 1S2, 2S2, 2P2. In 1S orbital, there are two core electrons while the remaining four electrons

Structure and chemistry of polymer/nanodiamond composites

5

≤ nm Noncrystalline carbon envelope; mixture of sp3 and sp2 bonding

Distorted diamond layer Diamond core (sp3bonded) Oxygen terminated surface

5 μm

Figure 1.2 Model for diamond structure.

(2S2, 2P2) are valence electrons. To give two-dimensional planar hexagonal structures to graphite, carbon atom endures sp2 hybridization while by sp3 hybridization; tetrahedral structure of diamond is formed. In the structure of diamond, the distinctive stacking of layers of carbon atoms are formed and has a modified face centered cubic structure [21]. The deficiency of free electrons in diamond structure makes it hardest and inert solid, best thermal conductor, and transparent. Germanium, silicon, and gallium arsenide also have diamond like structure. The structure of ND particle is complicated comprising of three parts: (1) central part (diamond core) is made up of sp3 hybridized carbon; (2) shell (analogous to fullerene) of sp2 hybridized carbon by which the core is partially covered; and (3) functional groups are formed by carbon atoms on outer surface (Fig. 1.2) [22]. On the pristine ND surface, numerous groups have been recognized comprising hydrocarbon functional groups, CH3, CH2, and CH in varying configuration (C CH2, O 5 CH2, O H, and O CH) and also oxygen comprising groups such as lactones (O C 5 O), carboxylic ( COOH), ether ( C O C ), and carbonyl groups ( C 5 O) (Fig. 1.3). The current commercial diamond nanoparticles are commercialized into three groups of products that is, (1) nanocrystalline particles; (2) ultrananocrystalline particles; and (3) diamondiods. The sizes of ultrananocrystalline particles are of several nanometer, while the sizes of the nanocrystalline particles are in the range of tens of micrometers. The diamondoids, well-defined hydrogen terminated molecular forms, comprise of numerous tens of carbon atoms [23].

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Hybrid Polymer Composite Materials: Structure and Chemistry

O H2 C

O

NH2

O

O O H2 C H2N

H2N N H

CF O

O

CI

O

R O

O

O Ar

O O

O

COOH

Figure 1.3 Various groups present on nanodiamond surface.

1.5

An overview of polymer/nanodiamond composite

Historically, nanoparticle-based polymer strengthening was proposed in the 1990s. Initially, carbon-black-filled rubber was one of the chief materials that can be recognized as polymer nanocomposite. Rendering to modern definition, the material comprised of filler nanoparticle and polymer matrix is called nanocomposite. The nanofiller must has at least one dimension in nanoscale range (,100 nm). They can be inorganic or organic in the sense utilized in chemistry [24]. The nanofillers are promising because of their outstanding and excellent resistant combination of features such as electrical conductivity, mechanical strength, and electrical conductivity [25]. Amongst carbon-based nanomaterials, spherical nanoparticles have gained remarkable interest owing to their excellent surface-to-volume ratio. Subsequently, ND has been recognized as widespread strengthening material because of nanosize, spherical shape, and extraordinary mechanical and physiochemical features [26]. In this respect, π-conjugated or conducting polymers have been recognized as an important material due to tuneable physical, electronic, optical, and mechanical features. These polymers have been extensively employed in electrochemical devices, batteries, anticorrosive coating, photovoltaics, organic transistors, and light emitting diodes [27]. Polymers such as polythiophene and polypyrrole are among the most

Structure and chemistry of polymer/nanodiamond composites

7

commonly utilized conducting organic polymers. These conducting organic polymers in combination with ND display extraordinary thermal, mechanical, and electrical features [28].

1.5.1 Mechanical features of nanodiamond reinforced matrix ND is also recognized as UDD. It comprises outstanding features of diamond core mainly excellent hardness and thermal conductivity with altering surface. By detonation preparation, it can be obtained in large amount and comparatively cheap price. The effect of ND (B2 wt.%) addition on mechanical features of rubber was studied by Dolmatov [29]. The utilized particle size was about 44 nm. Kurkin et al. [30] investigated polyvinyl alcohol/7 wt.% ND fiber coating. They examined B200% improvement in stiffness and large improvement in breaking strength (Table 1.1 and Fig. 1.4) [4]. These composites had poor ductility, which could be because of large ND content. It was currently reported by Behler et al. [31] that by the integration of ND (B20 wt.%) in electro spun polyamide 11 fibers, an enhancement of B400% Table 1.1

Mechanical properties of PVA-ND composite [4]

Nanodiamond content (wt.%)

Crystallinity, X (%)

Elastic modulus, E (GPa)

Hardness, H (MPa)

0 0.2 0.4 0.6

42.0 52.6 55.0 56.6

0.67 6 0.01 0.87 6 0.01 0.96 6 0.02 1.33 6 0.02

38.3 6 0.02 43.7 6 0.03 52.8 6 0.12 68.4 6 0.3

60

Crylallinity, χ (%)

55

50

45

40

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

ND content (% wt)

Figure 1.4 Variation of degree of crystallinity, X, as a function of ND concentration [4].

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Hybrid Polymer Composite Materials: Structure and Chemistry

in Young modulus and B200% increase in hardness was achieved. Moreover, the agglomerate formation takes place by higher filler concentration. The improvement rate in mechanical features was highest when the concentration of nanofiller was at dilute limit. With less than 1 wt.% ND, there was found limited investigations on the efficiency of ND in improving the mechanical features [4]. The reduced recovery for 12 vol.% ND samples are shown in Fig. 1.5A. The decreased mobility of polymeric chains was restrained among ND particles in the network. For 18 and 25 vol.% ND samples, the reduced elastic recovery was observed [32]. For 25 vol.% ND composite, there was 350% enhancement in Creep

(A)

222 nm

91 nm 25 vol.% ND 18 vol.% ND

20

12 vol.% ND 4 vol.% ND 2 vol.% ND 0 vol.% ND

Load on sample (mN)

18 16 14 12 10 8 6 4 2

Reduced recovery

0 0

500

1000

1500

2000

2500

3000

Displacement (nm) (B) 0.35

25 vol.% ND

Strees (N mm–2)

0.30 0.25 0.20

18 vol.% ND

0.15 0 vol.% ND

0.10 0.05 0.00

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Strain

Figure 1.5 Load displacement curves of epoxy ND samples with varying ND concentrations from 0 to 25 vol.% (A). Besides a clear decrease in the maximum indentation depth, a reduction of creep from 222 to 91 nm is observed for the 25 vol.% ND composite. (B) Stress strain curves show an improved Young’s modulus [24].

Structure and chemistry of polymer/nanodiamond composites

9

20

Impact resistance (kJ/m2)

18 16

A - Neat Epoxy B - 0.1wt% MWCNTs C - 0.1wt% NDs D - 0.05wt% MWCNTs/0.05wt% NDs E - 0.1wt% MWCNTs/0.1wt% NDs F - 0.2wt% MWCNTs/0.2wt% NDs

14 12 10 8 6 4 A

B

C

D

E

F

Figure 1.6 Charpy impact resistance values of neat epoxy and nanocomposites [34].

Young modulus of neat epoxy (3.4 6 0.5 to 12 6 2 GPa). The experimental values of Young modulus were dependant on the measurements and data analysis techniques [33]. For raw nanoindentation data, the stress strain curves have been calculated as shown in Fig. 1.5B [24]. To investigate the enhancement in impact resistance of neat epoxy with ND and multiwalled CNT (MWCNT) addition, charpy impact test was applied on neat epoxy and composites specimens. Thus, it was observed that by addition of 0.1 wt.% MWCNT an enhancement in impact resistance from 6.4 6 0.9 to 12.8 6 1.2 kJ m22 (100% rise) was observed. By the addition of both reinforcements in individual sizes of 0.05, 0.1, and 0.2 wt.% a continuous enhancement in impact resistance from 11.9 6 1.2, 13.7 6 1.1, and 16.7 6 1.7 kJ m22 (86%, 114%, and 161%) was observed (Fig. 1.6). Remarkable enhancement in impact resistance was observed for epoxy matrix composites comprising halloysite nanotubes up to 2.3 wt.% [34]. Together with the improvement in tensile strength and young modulus, an enhancement in fracture toughness was also observed [35].

1.5.2 Thermal properties of nanodiamond reinforced polymer Generation of inexpensive heat sinks for electronic devices, mainly for computer processors, high-power microchips, semiconductor lasers, and electronics components, has attained recent research interest. Conventionally, aluminum and copper have been employed for heat sinks of such devices with thermal conductivities of 250 and 400 W (m K)21, respectively. The thermal expansion coefficient of metal varies greatly than those of constituents employed in semiconductor electronics (mainly silicon). Thus, metals with low electrical resistivity should be added [36]. In contrast, natural single-crystal diamond has been recognized to feature the greatest thermal conductivity of bulk constituents investigated. However, due to

10

Hybrid Polymer Composite Materials: Structure and Chemistry

high cost it cannot be applied in heat removal technology. At present, SiC- and AlN-based ceramics have initiated appreciating extensive employment as heat sink materials. Moreover, polymer/ND composites are inferior to that of metals by a factor of two thermal conductivities [37]. To investigate the thermal conductivity, DSC studied was carried out with ND-butyl/low density polyethylene (LDPE) nanocomposite. ND-butyl was selected amongst the ND-alkyl series because its chain length is roughly between alkyl groups employed. In Fig. 1.7, DSC data for ND-butyl/LDPE nanocomposite were shown. It was observed that by the addition of ND-butyl, the crystallization temperature (Tc) of low density polyethylene (LPDE) became higher initially but with the further addition at ND content it was decreased (Fig. 1.7A). For example, by addition of 7 wt.% ND content, an improvement in Tc of pure LPDE from 76.71 to 82.85  C was observed [38]. Furthermore, by the addition of 11 wt.% ND, a reduction to 74.71  C was observed (Fig. 1.7B). The melting (A)

Heat flow (W g–1) Endo

11 % 9% 7% 5% 3% 1% Blank

80

70

90

100

110

120

Temperature (°C) (B) 11 % Heat flow (W g–1) Endo

9% 7% 5% 3% 1% Blank

40

50

60

70

80

90

100

110

120

Temperature (°C)

Figure 1.7 Cooling (A) and heating (B) DSC thermograms of ND-COO(CH2)3CH3/LDPE nanocomposites: the cooling scan (A) and heating scan (B) [38].

Structure and chemistry of polymer/nanodiamond composites

11

temperature (Tm) was raised up to 5 wt.% and then decreased. In comparison to pure LPDE, the Tm at 11 wt.% was lowered [39]. The enthalpies of crystallization and melting were calculated from the thermograms and shown in Table 1.2. In Fig. 1.8, the influence of particle volume fraction on thermal conductivity of coated and uncoated composites sintered at 920  C is displayed. It was seen that the variation trend in thermal conductivity was analogous to that of relative density, but the distinction was more improved in comparison to uncoated composites. The coated composites displayed a higher thermal conductivity. As compared to uncoated composites the composite comprising 50 vol% of coated ND displayed thermal conductivity of 284 W m21 K21, which was related to enhancement of 47% [40].

Thermophysical properties of ND-butyl/LDPE nanocomposites at the various ND content [38]

Table 1.2

Add percentage

Tc ( C)

Tm ( C)

ΔHc (J g21)

ΔHm (J g21)

% Crystallinity

0 1 3 5 7 9 11

76.71 77.01 77.34 81.57 82.85 76.68 74.71

97.27 98.04 99.09 100.55 95.02 94.90 93.37

99.1 97.5 94.0 90.3 90.2 84.2 83.1

190.1 192.3 195.3 189.5 176.4 171.2 165.0

47.9 49.9 51.3 52.2 47.6 45.8 43.1

Thermal conductivity (W m–1 K–1)

300

250

200

150 Coated uncoated 100 40

45

50

55

60

65

Particle volume fraction/%

Figure 1.8 The thermal conductivity of Cu/diamond composites with coated and uncoated diamond particles sintered at 920  C as a function of particle volume fraction [40].

12

Hybrid Polymer Composite Materials: Structure and Chemistry

1.5.3 Conducting properties of nanodiamond reinforced hybrid The powder of ND is also known as UDD. It comprises exceptional features of diamond on nanoscale and is becoming one of most extensively investigated nanomaterials. The broad band gap of diamond (5 eV) imparts it highly absorptive to UV light, but it is transparent in IR and visible region. The synthesis of ND in large volume was carried out by detonation method. It is comparatively cheap carbon nanomaterial for wide range of prospective applications. As powder, ND can be used in coatings, fibers, or other shapes to bind its convenient features [41]. Kausar et al. [42] studied the electrical conductivity of heteroaromatic ND/polyazopyridine (PAP)/polyaniline (PANi)/polypyrrole (PPy) (NDs/PAP/PANi/PPy) composite with both non functionalized and functionalized ND. The multilayered pyridine, thiophene, and azo moieties were found to improve the nanocomposite conductivity. It was observed that the conductivity of NF-NDs/PAP/PANi/PPy was 3.8 S cm21, and by the addition of functionalized filler in F-NDs/PAP/PANi/ PPy the conductivity was raised to 5.4 S cm21. In the case of NF-NDs/PANi/PPy/ PTh and F-NDs/PANi/PPy/PTh nanocomposite, comparatively lower values of conductivity were observed (2.9 and 3.7 S cm21). In literature, NDs have also been found to improve the electrical features of final composite material [43]. It was investigated by structural and spectroscopic analysis that the polymeric constituents of composite fibers retained structurally and chemically compatible form [44]. Atomic-force microscopy (AFM) was performed on isolated polyaniline/ND (PANi/NDs) (Fig. 1.9). On one end of fiber network, Ag electrode was realized. Consequently, by applying a DC voltage between the tip and electrode, the sample was imaged in contact mode acquiring current flowing from tip to sample (Fig. 1.9A) [45]. At each point of scanned area, the current signal was recorded concurrently with morphological reconstruction of sample, so permitting current map creation. The AFM images and consistent current maps were achieved by employing DV1/42V between AFM tip and two isolated PANI-ND fibers (Fig. 1.9B and C). In correspondence of substrate, no current signal was composed above noise and the direction of electric current flow inversion was estimated by reversal of DV sign (Fig. 1.9D). Thus, the rough estimation of single PANi fiber conductivity gives values as high as few hundred S m21, which were well in range with pure PANi fiber. Thus, it was observed that PANi-ND fibers were conductive and ND particles insertion inside PANi matrix did not influenced the charge transport features of conducting polymer [46].

1.6

Significance of polymer/nanodiamond composite

ND is less recognized member of carbon nanomaterial family. ND holds excellent potential for ultimate polymer matrix strengthening because of outstanding mechanical features, large, and accessible surface area (300 500 m2 g21), small diameter (5 nm), and rich, tailorable surface chemistry. In comparison to large nanofiller

Structure and chemistry of polymer/nanodiamond composites

13

(A)

(B) μm

μm

12 1.5

10 8

1.0

6 4

1.5

2 0

0 0

5

10

15

20

μm

25

(C) μm

nA

12 15

10 8

10

6 4

5

2 0 0

5

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μm

25

(D) 20

ΔV= +2V ΔV= –2V

It–s (nA)

10

0

–10

–20 0

1

2

3

4

5

6

7

8

9

10

Figure 1.9 Conductivity imaging of individual PANI-DND fibers: (A) schematic diagram of the measuring method based on a conductive Pt tip of an AFM apparatus; (B) AFM image and (C) current maps of two isolated PANI-DND fibers; (D) current It s flowing from tip to PANI-DND fibers sample recorded at each point of the scanned area [44].

14

Hybrid Polymer Composite Materials: Structure and Chemistry

such as nanosilica, grafting density of functional groups present on ND is remarkˇ ´ et al. [47] reported reduction in storage modulus of ably greater as well. Spitalsky epoxy/ND composite. Thus, it was observed that because of ND addition, the young modulus and hardness values for different polymers were slightly enhanced. ND fascinated great interest of polymer nanocomposite with outstanding mechanical features (1,220 GPa Young modulus and hardness 10 on Mohs scale), excellent thermal conductivity, low friction coefficient, and biocompatibility. Thus, ND links an inert diamond core with large accessible surface area of 5 nm or less in diameter. For electrochemical and biomedical composites, it has been proved to be outstanding candidate. It has been displayed by numerous studies on polymer/ND nanocomposite that the ND can be employed to enhance mechanical features of polymer matrices [48]. To obtain these enhancements, it is of utmost significance for adjustment of ND surface chemistry to polymer. For example, hydrophobic ND achieved by functionalization employing acta-decylamine outperformed unmodified ND in hydrophobic biodegradation. Detonation ND has small particle size of about 5 nm, tremendously rich surface chemistry, and high young modulus. The greater surface area was responsible for chemical modification. These features make ND distinctive for composite material formation by strong covalent bonding with polymer matrix. On polymer ND composite, a limited number of investigations have been reported. Primarily, ND was added to lubricants and rubbers for enhancement of wear resistance and functionality. At present, ND composites have become an emergent field and related nanocomposite research is going. The collective outstanding features by both organic and inorganic components in single material on nanoscale level make nanocomposite fascinating for next generation of biocompatible materials. The composite constituents of detonation type polymer/ND comprise spatial organization of constituents with new structural and physical features. They also have complex function because of strong synergistic influence between polymer and nanoparticle [49]. To achieve composites of siliconbased polymers, the polymerization (PP) route was selected in which detonation generated integrated ND (DND) particles. The layers of composites were chemically resistant, homogenous, mechanically, and thermally stable. A large quantity of biological constituents was loaded on their surface and to be employed in tissue engineering, implants, regenerative medicine, biosensors, stents, and other biological and medical devices.

1.6.1 High strength performance materials ND is also recognized as UDD. It combines outstanding features of diamond core such as excellent thermal conductivity and hardness with large readily adaptable surface. It can be achieved in large amount by detonation synthesis and is comparatively cheap with extensive applicability. Numerous research efforts have concentrated on integration of ND powder into thermoplastic polymers. For the enhancement of mechanical features of poly(I-lactic acid) and poly(vinyl alcohol), small additions of ND were reported. Thus, 200% improvement in hardness and 400% enhancement in Young’s modulus were observed by the addition of 20 wt.%

Structure and chemistry of polymer/nanodiamond composites

15

ND in polyamide 11 [31,50]. The other significant class of carbon nonmaterial is defined as nanocrystalline carbon powders synthesized by explosion techniques. ND comprises distinctive surface features because of very small particle size (2 10 nm). Therefore, surface functionalization of ND grains may influence bulk features of this constituent more efficiently than those of micro- and macroscale diamonds. ND powered may form good coarse suspensions and pastes for high precise polishing. The polymer/ND composite have been employed for manufacturing aircrafts, ships, cars, as well as hard and wear resistant surface coatings. They are deliberated as potential medical agents because of high specific surface area, high absorption capacity, and chemical inertness [51]. Numerous studies reported the thermoplastic ND. Even thermoplastic can be strengthened with ND without surface functionalization and at low concentration. Thus, strengthening of polydimethylsiloxane (PDMS) with glass beads in combination with 0.1 wt.% as received-ND reduced the mechanical stress. It was illustrated by viscosity measurement of melt that PDMS molecules [52]. However, polyurethane-2-hydroxymethylmethacrylate (PU-PHEMA/ND) composites with enhanced features were reported. Consequently, an increase in Tg and Young’s modulus were observed with 0.25 wt.% of as received-ND. The improvement was explained by the reaction between isocyanate groups generated during PP of PU-PHEMA and COOH group of ND. The polymer polyethylene (PE), having CH2 backbone without side chain functionality, is extensively employed polymer. Alkyl chains of altered length were grafted for the improvement of its efficiency [53]. It was revealed by DSC studies of PE/ND composites that with the increase of ND content and length of alkyl chain, the crystallization, melting, and crystallinity of nanocomposite were increased. While the improvement in hardness and Young’s modulus was observed by a factor of 4.5 and 2.5 correspondingly. To reduce fraction between two surfaces in such polymer/ ND is important in tribological applications. Polytetraflouroethylene has been frequently utilized as low friction polymer with elevated chemical and thermal stability [38]. In addition, the mixture was strengthened with ND for enhancement in tribological features. In addition, reduction in coefficient of friction and enhancement in wear resistance were calculated. The relation among wear potential and ND aggregates size were also studied. The reduced wear of poly(methyl methacrylate) (PMMA)/ND, polyethylene (PE)/ND, and polyacrylamide (PAA/NDZ) was also reported. With ND addition PU/ND composites also displayed enhancement in tribological features [54].

1.6.2 Thermal resistance materials A great challenge that restricts the potential of a polymer is the low melting temperature and thermal stability. However, the strengthened ND in polymer matrix is known to influence its thermal features. The increased mechanical features, low crystallization temperature (Tc), and glass transition temperature (Tg) was displayed by differential scanning calorimetry (DSC) studies of polylactic acid (PLA)/ND nanocomposite. It was believed that the ND particles act as nucleation center for crystallization [55]. The increased Tg and higher degradation temperature

16

Hybrid Polymer Composite Materials: Structure and Chemistry

were observed by the incorporation of 1 wt.% ND in liquid crystal polyester. The enhancement in Tg (calculated by employment of capacitance bridge) was observed by incorporation of 2 wt.% onion like carbon (OLC). In another attempt, enhancement in thermal degradation resistance of PMMA was observed by the incorporation of 2 wt.% of purified ND (HNO3). Consequently, the thermal stability of polymer was efficiently enhanced by the employment of ND and OLC. Diamond has outstanding thermal conductivity [56]. Therefore, its addition in polymers may enhance the thermal conductivity. Enhancement in thermal conductivity of matrices was observed by the incorporation of ND. By transient state method, 15% enhancement in thermal conductivity was calculated for 2 wt.% ND strengthened PDMS. This improved thermal conduction displayed strong polymer matrix and nanofiller interface, so decreasing interfacial phonon scattering [57]. For carbon-fiber strengthened composites, epoxy is common thermosetting polymer extensively employed as matrix material in ship buildings, aerospace, and sports industries. For reinforcement of epoxy systems, numerous nanofillers have been studied. By comparing epoxy/ND and epoxy/CNT nanocomposite at analogous nanofiller loading, remarkable enhancement in Tg that is, 17  C for CNT and 37  C for ND was observed. In comparison to neat epoxy, the fracture surface of both composites displayed better resistance to crack propagation [58]. The most fascinating development in composite fabrication was performed by Schubert et al. through infiltrating copper into diamond [59]. The thermal conductivity of composite was reported to reach 700 and 640 W m21 K21. Aluminum was studied for the employment as matrix material. For such composites, the thermal conductivity was reached as high as 580 and 670 W m21 K21. It was considered to be an excellent achievement for aluminum-based composite. The Si, Ti, Cr, and W were employed in order to make matrix materials dependant on aluminum, copper, wet diamond, and carbide forming metals. The requirement for carbide formation on diamond surface has been efficiently demonstrated by Weber et al. [60]. They illustrated the effect of thermal conductivity and coefficient of thermal expansion of diamond copper/chromium composites on chromium concentration in copper. Thus, the transition of diamond/ matrix from weak to strong bonding was studied. This gave rise to thermal conductivity growth (above 600 W m21 K21) and coefficient of thermal expansion reduction (to less than 10 3 1026 K21) of nanocomposite. Thermal conductivity of carbides of elements and refractory metals stated lie frequently in range of 7 170 W m21 K21. Thus, the thickness of coating influenced the total thermal conductivity of composite. The carbide coating thickness, at which composite thermal conductivity reached maximum, was 100 200 nm [61].

1.6.3 Li-ion batteries and electronic devices In rechargeable battery, there is movement of lithium ions from negative to positive electrode during discharging and backs when charging occur. Majority of rechargeable battery consists of electrodes ensuing two ultimate designs that is, thin film battery electrode and composite electrode. Both traditional essential designs comprise of a cell which is dependent on cathode, anode and electrolyte. By solution or vapor phase deposition, thin film batteries have been synthesized

Structure and chemistry of polymer/nanodiamond composites

17

in solid state. While the composite electrodes comprise percolating electronic conductor, an integration of active electrode materials, and binder phase [62]. Composite electrodes and liquid electrolytes may come in direct contact for production of lithium-ion battery. Mixtures of composite electrode and polymer electrolytes have also been used to form polymer lithium-ion batteries. Cross-linked trimethylpropane trimethylacrylate-based gel polymer electrolyte (GPE) has been fabricated with outstanding electrochemical features. Continuous enhancement in ionic conductivity was observed with improvement in constituents of liquid electrolytes. The significant electrochemical features of GPE comprise elevated ionic conductivity, excellent reversible capability, extensive electrochemical window, outstanding cycling ability, and superior rate capability [63]. By reacting different proportions of PVC with boric acid, oxalic acid, and lithium carbonate, the synthesis of single-ion-conducting polymer electrolyte membrane based on poly(vinylalcohol) (LiPVAOB) was carried out. The synthesized materials were studied using variety of techniques including scanning electron microscopy (SEM), differential thermal analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy, linear sweep voltammetry, and electrochemical impedance spectroscopy. Because of stable potential window and outstanding ionic conductivity, synthesized materials have been fascinating for lithium-ion batteries application [60]. On the basis of ionic liquid, Hofmann et al. [64] have synthesized GPE. The enhanced cell potential was examined using graphite anode and LiCo1/3Mn1/3Ni1/3O2 cathode. The improved cell performance was observed with anode of graphite and cathode of LiCo1/3Mn1/3Ni1/3O2. The improved performance was also attributed to the addition of 4-vinylpyridine and vinylene carbonate. Angulakshmi et al. [65] articulated solid composite polymeric membrane comprising LiPF6 and MgAl2O4 by employment of hot process. DSC, TGA, SEM, XRD, tensile, compatibility, impedance spectroscopy, and transport number studies displayed information about the membrane. Consequently, by integration of MgAl2O4, an enhancement in mechanical integrity, compatibility, and ionic conductivity membrane were observed. The cell assembling was carried out by composite polymer electrolyte (CPE) sandwiching between LiFePO4/CPE/LiFePO4. A discharged capacity of 110 mAh g21 at 1 C-rate even after prolonged cycles (up to 100) was observed [66]. In comparison to conventional lithium-ion batteries with liquid electrolytes, lithium-ion batteries with polymer composite have revealed enhanced electrochemical potential. A new type of biosensor consisted of polyaniline (PANI) grafted on ND (gold particles in film and immobilized cytochrome c (PANI-G-ND/Au/cyt c) was fabricated by Gopalan et al. [67]. The uniform distribution of Au and presence of fibrous PANI embedded into the ND galleries were observed. It was interpreted that biosensor comprises a tremendous electrocatalysis toward recognition of nitrite ion. The effect of pressure (B7 GPa) and temperature (in range of 700 2000  C) on sintering behavior of DND was experiential. Consequently, polymeric ND nanocomposite are capable materials for the fabrication of structural constituents with predominant features such as coatings, membranes, photo-electronic and optical devices, hybrid fire proof materials, and biocompatible material for sensors.

18

1.7

Hybrid Polymer Composite Materials: Structure and Chemistry

Conclusions and outlook

This chapter initially reviews the synthesis routes usually employed for ND preparation. For the preparation of ND, three routes have been successfully commercialized, out of which, most widely used and large scale method is De Pont route in which carbon precursors for example, graphite, coal, and carbon black are transformed into diamond in a capsule by circular shock wave (B140 GPa). The second route for bulk-scale manufacture of ND is based on explosion of carbon comprising constituents with explosives. Third route for ND is explosion route through which diamond clusters are formed from explosives employed as precursor constituents. The chemistry and structure of ND have also been discussed. The reinforcement effect of ND filler on polymer properties and polymer/ND nanocomposite preparation techniques were also reviewed here. It was experiential that in comparison to CNT, significant enhancement in polymer properties has been achieved by ND addition. Consequently, thermal, mechanical, and electrical properties of nanocomposite have also been discussed. These polymer nanocomposite have potential utilization in various important technical fields such as electronics, EMI shielding, radar absorbing materials, batteries and electronic, aerospace and sporting goods, and so on.

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[31] Behler KD, Stravato A, Mochalin V, Korneva G, Yushin G, Gogotsi Y. Nanodiamondpolymer composite fibers and coatings. ACS Nano 2009;3:363 9. [32] Ullah M, Kausar A, Siddiq M, Subhan M, Abid Zia M. Reinforcing effects of modified nanodiamonds on the physical properties of polymer-based nanocomposites: a review. Polym-Plast Technol Eng 2015;54:861 79. [33] Kalidindi SR, Pathak S. Determination of the effective zero-point and the extraction of spherical nanoindentation stress strain curves. Acta Mater 2008;56:3523 32. [34] Subhani T, Latif M, Ahmad I, Rakha SA, Ali N, Khurram AA. Mechanical performance of epoxy matrix hybrid nanocomposites containing carbon nanotubes and nanodiamonds. Mater Des 2015;87:436 44. [35] Ye Y, Chen H, Wu J, Ye L. High impact strength epoxy nanocomposites with natural nanotubes. Polymer 2007;48:6426 33. [36] Kidalov SV, Shakhov FM. Thermal conductivity of diamond composites. Materials 2009;2:2467 95. [37] Slack GA. Nonmetallic crystals with high thermal conductivity. J Phys Chem Solids 1973;34:321 35. [38] Jee A-Y, Lee M. Thermal and mechanical properties of alkyl-functionalized nanodiamond composites. Curr Appl Phys 2011;11:1183 7. [39] Liu Y, Gu Z, Margrave JL, Khabashesku VN. Functionalization of nanoscale diamond powder: fluoro-, alkyl-, amino-, and amino acid-nanodiamond derivatives. Chem Mater 2004;16:3924 30. [40] Chu K, Liu Z, Jia C, Chen H, Liang X, Gao W, et al. Thermal conductivity of SPS consolidated Cu/diamond composites with Cr-coated diamond particles. J Alloys Compd 2010;490:453 8. [41] Shenderova O, McGuire G. Nanocrystalline diamond. In: Gogotsi Y, editor. Nanomaterials Handbook. Boca Raton, FL: CRC Press; 2006. p. 203 38. [42] Ashraf R, Kausar A, Siddiq M. High-performance polymer/nanodiamond composites: synthesis and properties. Iran Polym J 2014;23:531 45. [43] Mochalin VM, Shenderova O, Ho D, Gogotsi Y. The properties and applications of nanodiamonds. Nat Nanotech 2012;7:11 23. [44] Tamburri E, Guglielmotti V, Orlanducci S, Terranova ML, Sordi D, Passeri D, et al. Nanodiamond-mediated crystallization in fibers of PANI nanocomposites produced by template-free polymerization: conductive and thermal properties of the fibrillar networks. Polymer 2012;53:4045 53. [45] Cai CD, Zhou JZ, Qi L, Xi YY, Lan BB, Wu LL, et al. Conductance of a single conducting polyaniline nanowire. Acta Physico-Chim Sin 2005;21:343 6. [46] Xiong S, Wang Q, Chen Y. Study on electrical conductivity of single polyaniline microtube. Mater Lett 2007;61:2965 8. ˇ ˇ ´ Z, Kromka A, Matˇejka L, Cernoch [47] Spitalsky P, Kova´rˇova´ J, Kotek J, et al. Effect of nanodiamond particles on properties of epoxy composites. Adv Composites Lett 2008;17:29 34. [48] Beyler-C¸i˘gil A, C¸akmakc¸ı E, Kahraman MV. Thermal properties of phosphorylated nanodiamond reinforced polyimides. Polym Composites 2015; Apr 1, DOI 10.1002/pc. [49] Hikov T, Mitev D, Radeva E, Iglic A, Presker R, Daniel M, et al. Studying the influence of nanodiamonds over the elasticity of polymer/nanodiamond composites for biomedical application. J Phys: Conf Ser 2014;558(1):012060, IOP Publishing. [50] Das B, Prasad KE, Ramamurty U, Rao CNR. Nano-indentation studies on polymer matrix composites reinforced by few-layer graphene. Nanotechnology 2009;20:125705.

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Chitosan-based hybrid polymer composites: structure and chemistry

2

Katelyn Musumeche, Melanie Sanders and Dilip Depan Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette, LA, United States

Chapter Outline 2.1 Chitosan as biomaterial: structure property functional relationship 23 2.2 Chitosan as a fourth-generation biomaterial 24 2.3 Recent advances with chitosan as a biomaterial 25 References 29

2.1

Chitosan as biomaterial: structure property functional relationship

As discussed above, chitosan (CS) is a unique polysaccharide derived from partial deacetylation of chitin, which is, after cellulose, the most abundant natural polysaccharide [1]. CS has reactive amino ( NH2) and hydroxyl ( OH) groups that provide many possibilities for covalent and ionic modifications (Fig. 2.1). The advantages of CS are as follows: (1) CS intrinsically possesses strong biological activity; (2) it is biocompatible, biodegradable, bioresorbable, and has a hydrophilic surface, which facilitates cell adhesion, proliferation, and differentiation [2]; and (3) due to its cationic nature in physiological pH, CS mediates nonspecific binding interactions with various proteins. Since then, CS material has been widely investigated in a number of biomedical applications [3 8], from wound dressings, drug or gene delivery systems, and nerve regeneration to space filling implant. CS is currently sold in the USA as a dietary supplement to aid weight loss and lower cholesterol and is approved as food additive in Japan, Italy, and Finland. It has excellent potential for engineering numerous tissue systems, including bone tissue, by serving as a structural base material on which normal tissue architecture is organized. However, although pure CS has very attractive properties, it lacks bioactivity and is mechanically weak [9]. These drawbacks limit its biomedical applications. For these reasons, it is highly desirable to develop a hybrid material made of CS and appropriate filler, hoping

Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00002-1 Copyright © 2017 Elsevier Ltd. All rights reserved.

24

Hybrid Polymer Composite Materials: Structure and Chemistry

Primary amino function OH O HO

OH

NH2

O

NH2

O

O O

O HO

NH OH

O

O

O

NH2

Secondary hydroxyl function

OH Primary hydroxyl function

Figure 2.1 Structure of chitosan.

that it can combine the favorable properties of the materials, and further enhance tissue regenerative efficacy. Of particular relevance is that CS is ideally suited to complex with anionic form of cellulose (carboxymethyl cellulose) to enable a number of applications and can be conjugated with functional molecules, antibodies, biotin, and heparin [10 14]. The advantage of blending CS is not only to improve its biodegradability and its antibacterial activity, but also the hydrophilicity introduced by addition of the polar groups able to form secondary interactions ( OH and NH2 groups involved in H bonds with other polymers). The most promising developments at present are in pharmaceutical and tissue engineering areas. For the recent breakthroughs in tissue engineering applications of CS, an attempt is made in this chapter to consolidate some of the recent findings on the effect of incorporation of cellulose nanocrystals on the mechanical, structural, and biomedical applications and the development of nanocomposites of CS and cellulose nanocrystals.

2.2

Chitosan as a fourth-generation biomaterial

In order to better understand the evolution of biomaterials, a brief history of their origin is helpful. As previously mentioned, a biomaterial can be either benign or bioactive. It is believed that the first device which can be called a biomaterial was a wooden prosthetic of a toe in Thebes West tombs in Egypt, which was estimated to have been created around 1065 740 BC [15]. Materials which scientists now classify as first-generation biomaterials were developed during the period from the 1960s to the 1980s. These first-generation biomaterials are generally described as being “inert” or benign and were meant to minimize any immune system response from the patient [16]. The second-generation of biomaterials differ from the first in that they were no longer classified as “inert.” Termed bioactive, some biomaterials were constructed

Chitosan-based hybrid polymer composites: structure and chemistry

25

with the intention of prompting a reaction from the patient upon application or implantation. And still others, termed resorbable, were constructed with the intention of the material degrading at a rate as close to possible as the rate of regeneration of the patient’s tissue [17]. The knowledge derived from discoveries made during first and second generations of biomaterials gave rise to the third generation of biomaterials. While the first and second generations were classified as being inert, bioactive, or resorbable, the new third generation of biomaterials were created to produce a response on a molecular level [17]. One example of CS being used as a third generation biomaterial is in the form of injectable hydrogels combined with collagen. An application of this biomaterial is in bone tissue engineering. The presence of collagen in the extracellular matrix of bones combined with the natural characteristics of CS (discussed elsewhere in this chapter) make this particular biomaterial well suited for bone tissue engineering [18]. Fourth-generation biomaterials, also referred to as smart or biomimetic materials [19], increase even further the extent of interaction between the materials and the surrounding tissue in that they gather information from the surrounding tissues and react accordingly. It has been speculated that nanoparticles and implants will be developed in coming years that are able to process information received from both chemical and physical sensors in order to determine their next function. This biomaterial technology would also likely give rise to biological sensors that can detect infections or predict diseases [16]. CS has shown promise in the area of fourth-generation biomaterials. For example, the contents of printer cartridges used in a commercial printer were replaced with drug solutions and a polymer solution (containing CS, among other chemicals) to create a three-dimensional (3D) drug gradient within a film. The drug release kinetics of the mixture were studied [19]. This study, combined with the study done by Yan et al. in which they printed a 3D hydrogel scaffold that included hepatocytes using extrusion-based bioprinting [20], and the ability to incorporate conductive polymers [21], will likely give rise to a true fourth-generation biomaterial in the near future.

2.3

Recent advances with chitosan as a biomaterial

In a recent study, 3D porous scaffolds of CS collagen and hyaluronic acid (HA) were prepared by freeze drying method to improve the physical and biological properties of the composite scaffolds [22]. The authors indicated that the above outlined properties of the scaffolds were improved after adding HA. HA interacts with collagen and CS through the electrostatic and hydrogen bonds and modifies the properties of 3D scaffolds. Further, the addition of HA to CS collagen improves its elasticity and leads to the increase of porosity, and therefore may improve cell attachment to scaffold surface and support cells migration to its deeper layers.

26

Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 2.2 Scanning electron microscopy images of (A) CTS/Coll; (B) CTS/Coll/1HA; (C) CTS/Coll/2HA; and (D) CTS/Coll/5HA. Source: [Reproduced from A. Sionkowska et al., Int. J. Biol. Macromol. 89, 442, 2016. With Permission].

Biopolymer CS/montmorillonite nanocomposites loaded with silver sulfadiazine for wound healing purposes were prepared via intercalation solution technique [23]. Fig. 2.3 shows the detailed reaction for this process. Structure and morphology of loaded nanocomposites were studied and compared with pure components and unloaded nanocomposites. The authors found that the drug was effectively loaded in the 3D nanocomposite structures, in which CS chains were adsorbed in monolayers into the clay mineral interlayer spaces. The results further confirmed the formation of well-dispersed ordered intercalated assembled layers of montmorillonite in CS matrix. Solid state characterization of loaded and unloaded nanocomposites confirmed the effective interaction between the organic and inorganic components and the successful drug loading of clay/CS nanostructures. Another investigation reveals a novel amperometric glutamate biosensor based on covalent immobilization of glutamateoxidase (GluOx) onto carboxylated

Chitosan-based hybrid polymer composites: structure and chemistry

-COOH

O

EDC+NHS

27

O N

O

O

-NH2

O NH C

O O O

N O

Chitosan Electrodeposition -NH2

AuNPs Coelectrodeposition

O

O

O

NH C

O

N O

Au electrode

-NH2

Glutamate oxidase

O NH C

CO NH

O

O

O

NH C

O

N O

Figure 2.3 Schematic representation of chemical involved in the fabrication of GluOx/ cMWCNT/AuNP.CHIT/Au electrode. Source: [Reproduced from C. Aguzzi et al., Colloids Surf. B Biointerf. 113, 152, 2014. With Permission].

multiwalled carbon nanotubes (cMWCNT), goldnanoparticles, and CS composite film electrodeposited on the surface of Au electrode [24]. The biosensor was evaluated and employed for determination of glutamate in sera from apparently healthy subjects and persons suffering from epilepsy. Fig. 2.4a,b,c show the results of studies done on the electrodes obtained from this process. Silver nanoparticles can be easily loaded with CS. The development, processing, and characterization of silver CS-based nanocomposites are a novel and exciting areas of research in biomaterials science. In this regards, the effect of using glycerol as plasticizer on mechanical and physical properties of silver CS nanocomposite films have been studied [25]. Hybrid membranes were prepared via three steps consisting of silver CS colloidal nanocomposites preparation, adding of glycerol to colloids and silver CS nanocomposites films formation. Experimental investigations indicated elongation at break and tensile strength of the films were enhanced at high concentration of glycerol of 0.8 to 1.0 mL in 60 mL 1% CS solution or 160% to 200% (w/w) (glycerol/CS). Furthermore, it has also been observed that, the swelling capacity, WVP, and crystallinity of the films also increases by increasing the glycerol concentration. The investigation also reveals that the thermal resistance of the plasticized film is lower than that of unplasticized films due to the weak intermolecular forces among CS structures.

28

Hybrid Polymer Composite Materials: Structure and Chemistry

(A)

(B)

0.05 4 µM

–0.05

Current (µA)

Current (µA)

0.20

1 µM 2 µM

0.00

6 µM 8 µM

–0.10 100 µM

–0.15 –0.20

12 µM 22 µM 50 µM

0

50

0.15 0.10 0.05 l (µA) =0.01053 + 0.001320 c(µM) (R=0.9947)

0.00

100 150 200 250 300

0

50

100 150 200 250 300

H2O2 Concentration (µM)

Time (s) (C)

1/lss / µA–1

160 120 80 40 1/lss(µA–1) =5.3140 + 354.0827/c (µM–1XR=0.9978)

0 0.0

0.2

0.4

0.6

0.8

1.0

1/cH2O2 (µM–1)

Figure 2.4 (A) Amperometric responses of Hb-CNDs-chitosan/GC electrode upon successive addition of H2O2 in deoxygenated pH 7.4 PBS at 20.30 V, (B) The corresponding calibration plot of current against H2O2 concentration. Each data point represents the average value of five independent experiments with error bars indicated and the relative standard deviation is 3.4%, (C) The plot of 1/l/A against 1/c. Error bars represent the range calculated using three different electrodes. Source: [Reproduced from B. Batra et al., Biosensors Bioelectronics. 47, 496, 2013. With Permission].

In another study, a simple, sensitive, and label-free aptamer-based biosensor for the detection of human immunoglobulin E (IgE) is developed using the electrochemical transduction method [26]. In this study, special immobilization interface consisting of multiwalled carbon nanotubes/ionic-liquid/CS nanocomposite (MWCNTs/IL/Chit) is employed to improve the conductivity of the biosensor as well as to increase the loading amount of aptamer DNA sequence. The improved specificity of this sensing system for the detection of IgE is also demonstrated by using bovine serum albumin (BSA) and lysozyme. Fig.2.5 gives a graphical representation of the reaction steps for this process. From these results, it could be

Chitosan-based hybrid polymer composites: structure and chemistry

29

Figure 2.5 Schematic outline of the principle for label-free electrochemical IgE biosensing. Source: [Reproduced from S. Khezrian et al., Biosensors Bioelectronics. 43, 218, 2013. With Permission].

concluded that the proposed approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies. There are many studies concerning the improvements of the mechanical properties of CS-based porous scaffolds by incorporating another biopolymer. In this connection, collagen CS scaffolds of different compositions were developed using emulsion air-drying method [27]. The author’s investigations revealed that the scaffolds with 10 30 wt.% of CS to collagen improved the mechanical properties of the composite scaffold. Further, composition with 7:3 ratio (collagen: CS) was found to be a better composite having a tensile strength of 13.57 MPa with 9% elongation at break. The authors also performed water uptake studies and found to be ameliorated for the composite scaffolds compared to pure collagen and CS scaffold, respectively. Scanning electron microscopy revealed a porous structure with well inter connected pores ranging from 100 to 300 µm, and their uniform distribution. As expected, the scaffold decreased the bacterial counts and supported fibroblasts attachment and proliferation, thus demonstrating this composite to be a good substrate for biomedical application.

References [1] Khor E, Lim LY. Biomaterials 2003;24:2339. [2] Hejazi R, Amizi M. Polymeric biomaterials. NY: Marcel Dekker; 2001. [3] (a) Ravi Kumar MNV. Reac Funct Polym 2000;46:1. (b) Harish Prashanth KV, Tharantharan RN. Trends Food Sci Technol 2007;18:117. [4] Periodontal PR, Vandemark L, Kenney EB, Bernard GW. J Periodontal 1996;67:1170. [5] Suh JK, Matthew HW. Biomaterials 2000;24:2589. [6] Domard A, Domard M. Polymeric biomaterials. NY: Marcel Dekker; 2001. [7] Malik DK, Baboota S, Ahuja A, Hasan S, Ali J. Curr Drug Del 2007;4:141.

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[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

[19] [20] [21] [22]

[23]

[24]

[25] [26]

[27]

Hybrid Polymer Composite Materials: Structure and Chemistry

Tan W, Krishnaraj R, Desai TA. Tissue Eng 2001;7:203. Lahizi A, Sohrabi A, Hungerford DS, Frondoza CG. J Biomed Mater Res 2000;15:586. Zhang L, Jin Y, Liu H, Du Y. J Appl Polym Sci 2001;3:584. Rusmini F, Zhong Z, Feijen J. Biomacromolecules 2007;6:1775. Slutter B, Soema PC, Ding Z, Verheul R, Hennink W, Jiskoot W. J Controlled Release 2010;143:207. Fernandez-Megia E, Novoa-Carballal R, Quinoa E, Riguera R. Biomacromolecules 2007;3:833. Adriano WS, Mendonica DB, Rodrigues DS, Mammarealla EJ, Giordano RLC. Biomacromolecules 2008;8:2170. Huebsch N, Mooney DJ. Inspiration and application in the evolution of biomaterials. Nature 2009;462(7272):426. Hench LL, Polak JM. Third-generation biomedical materials. Science 2002;295 (5557):1014. Available from: http://dx.doi.org/10.1126/science.1067404. Ferreira AM, Gentile P, Chiono V, Ciardelli G. Collagen for bone tissue regeneration. Acta Biomater 2012;8(9):3191. Holzapfel BM, Reichert JC, Schantz JT, Gbureck U, Rackwitz L, Noth U, et al. How smart do biomaterials need to be? A translational science and clinical point of view. Adv Drug Deliv Rev 2013;65(4):581. Sears NA, Seshadri DR, Dhavalikar PS, Cosgriff-Hernandez E. A review of three-dimensional printing in tissue engineering. Tissue Eng: B 2016;22(4):298. Yan Y, Wang X, Pan Y, Liu H, Cheng J, Xiong Z, et al. Fabrication of viable tissue-engineered constructs with 3D cell-assembly technique. Biomaterials 2005;26(29):5864. Dawson E, Mapili G, Erickson K, Taqvi S, Roy K. Adv Drug Del Rev 2008;60:215. Sionkowska A, Kaczmarek B, Lewandowska K, Grabska S, Pokrywczynska M, Kloskowski T, et al. 3D composite based on the blends of chitosan and collagen with the addition of hyaluronic acid. Int J Biol Macromol 2016;89:442 8. Aguzzi C, Sandri G, Bonferoni C, Cerezo P, Rossi S, Ferrari F, et al. Solid state characterisation of silver sulfadiazine loaded on montmorillonite/chitosan nanocomposite for wound healing. Coll Surf B: Biointerf 2014;113:152 7. Batra B, Pundir CS. An amperometric glutamate biosensor based on immobilization of glutamate oxidase onto carboxylated multiwalled carbon nanotubes/gold nanoparticles/ chitosan composite film modified Au electrode. Biosens Bioelectr 2013;47:496 501. Susilowati, E.; Kartini, I.; Santosa, S.J.; Triyono, Effect of glycerol on mechanical and physical properties of silver chitosan nanocomposite films IOPscience. 2016. Khezrian S, Salimi A, Teymourian H, Hallaj R. Label-free electrochemical IgE aptasensor based on covalent attachment of aptamer onto multiwalled carbon nanotubes/ionic liquid/chitosan nanocomposite modified electrode. Biosens Bioelectr 2013;43:218 25. Kumar BS, Aigal S, Ramesh DV. Air-dried 3D-collagen-chitosan biocomposite scaffold for tissue engineering application. Polym Compos 2012;33(11):2029 35.

3

Chemistry of hybrid multifunctional and multibranched composites

Sergio D. Garcia Schejtman1, Vero´nica Brunetti2, Marisa Martinelli1 and Miriam C. Strumia1 1 Department of Organic Chemistry (IPQA, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina, 2Department of Physical Chemistry (INFIQC, CONICET-UNC), Faculty of Chemical Sciences, National University of Co´rdoba, Co´rdoba, Argentina

Chapter Outline 3.1 Introduction 31 3.2 Hyberbranched versus dendritic macromolecules 35 3.3 Hyperbranched and hyperfunctional hybrid organic 39 3.3.1 Flat surfaces 39 3.3.2 Curved surfaces in 3D materials 40

3.4 Hybrid inorganic-hyperbranched polymer composites

43

3.4.1 Flat solid surfaces 44 3.4.2 3D Structures 48

3.5 Conclusion 57 3.6 Acknowledgments References 58

3.1

58

Introduction

The demand for complex, highly specific materials in the field of technology has grown inexorably in recent years. In the design, synthesis, and engineering of such materials, chemists have found inspiration in nature itself. However, to achieve a similar degree of control over the properties of synthetic materials requires harnessing multiple concepts from various disciplines. The ability to predict structure/property relationships using different kinds of building blocks is a multidisciplinary, hierarchical approach to the manufacture of hybrid materials with novel functionalities. It is first of all important to define the scope of the concept of hybrid materials. A hybrid material is the result not only of a combination of organic and inorganic components, called composites, but also of the combination of materials of different classes based on their structural properties. Fig. 3.1 shows several kinds of hybrid Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00003-3 Copyright © 2017 Elsevier Ltd. All rights reserved.

32

Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 3.1 Different examples of hybrid materials.

materials formed by combining different fragments such as dendritic-linear polymers, nanostructured systems, which could include inorganic substrates, among others [13]. Several of the products that form part of our daily life today are based on hybrid structures developed more than a decade ago. Thus, hybrid materials represent a critical bond between present technologies and potential applications. The unique and complex purpose behind the design of hybrid materials is to achieve new or specific functional properties either by using available structures or by chemical modification of existing structures. Hence, the combination or adaptation of each component of a hybrid material in a desired arrangement becomes the key to understanding the final properties and applications. Some recently published methodologies have emerged from the advancement of nanoscience and provide powerful tools for the preparation of new hybrid materials, making it possible to envision strict control of size, shape, and precision in introducing the desired chemical functionality [4]. Innovation and collective knowledge of architecture and structure in macromolecular chemistry has been one of the most important advances in addressing potential new applications in different areas including biology [5], energy storage [6], sensors [7], self-healing coatings [8], and drug delivery [9].

Chemistry of hybrid multifunctional and multibranched composites

33

In recent years, global interest in the study of macromolecular architecture has been driven by advances in the controlled synthesis techniques of polymers. Controlled/living radical polymerization (CRP) provides a versatile route for synthesis of polymers with narrow molecular weight distribution, designed architecture, and useful end functionalities [10]. The various methods of CRP include nitroxide-mediated polymerization, atom transfer radical polymerization (ATRP), cobalt-mediated radical polymerization, and reversible additionfragmentation chain-transfer polymerization (RAFT) [11]. Most of these methodologies allow precise adaptation of the chemical composition chain and degree of polymerization. Undoubtedly, one of the most used CRP is ATRP. Previous studies presented by Matyjaszewski et al. have demonstrated the several advantages of this kind of CRP [12]. For example, the results of morphology, swelling, and degradation behavior of nanogels prepared by ATRP are superior to the corresponding results when prepared by conventional-free radical polymerization [13]. More recently, synthetic approaches using “click” chemistry concepts have facilitated the preparation of a vast diversity of functional materials [14]. The preparation of functional polymers, dendronized polymers, or even postmodification of linear polymers and linear-dendritic hybrids have been successfully reached via “click” chemistry strategies [15]. Of the many click reactions typically used, the coppercatalyzed azidealkyne click reaction is the most frequently reported owing to its high yield and efficiency, plus the fact that the conditions it requires are simple, and the subsequent purification steps are relatively straightforward [16]. Due to the inherent toxicity of copper, the copper catalysts are required to be removed if the synthesized materials are required for biological systems, leading to very low residual levels. Other interesting alternatives are thiol-ene (click) and thiol-Michael addition chemistries [17]; however, thiol-based chemistries present their own inherent disadvantages mainly derived from the manipulation of thiol functional compounds which are sensitive to oxidation and usually present a pungent aroma, making them commercially limited to a relatively small range of simple molecules [16]. Faced with growing demand for sustainable and green polymer chemistry, researchers were encouraged to develop efficient, simple synthetic methodologies with high atomic economy. Such chemical reactions include amidation, esterification, etherification, the Knoevenagel reaction, SuzukiHeckFriedelCraftsSonogashira coupling, among others. Particularly interesting are other click-chemistry reaction types proposed by Sharpless [18], such as cycloaddition (alkyne-azide and DielsAlder), nucleophilic/electrophilic (thiol-click-chemistry), and radical-initiated reactions. The alkyne-azide cycloaddition reactions can be catalyzed by copper or carried out metal-free [3]. Recent studies have shown how the presence of multibranches, through covalent or noncovalent interaction, has produced significant changes in the properties of the hybrid materials [1921]. Dendritic structures are an example of the results of recent studies to obtain multibranched and multifunctional controlled macromolecules. Dendrimers, dendrons and hyperbranched and dendrigraft polymers are highly branched structures of great interest as new materials in different areas of application [22]. A dendrimer, for example, is a wedged-shaped molecule with a well-defined molecular weight (low polydispersity) and is composed of dendrons

34

Hybrid Polymer Composite Materials: Structure and Chemistry

representing structural components of the parent dendrimer. Thus, dendrimers/dendrons have many key architectural features, such as an initiator core/focal point, interior branching units/branch cells and well-defined numbers of functional surface groups as a function of generation (G) level [23]. The combination of all these properties and more, known cumulatively as “dendritic effects” together with the ability to control the nanoscale size, shape, and chemical functionality of these dendritic structures, has led to many new unprecedented properties [24]. Considerable obstacles continue to exist for cost-effective applications of dendrimers, including their synthetic complexity and molecular weight/physical size limitations. Faced with such obstacles, the dendronization methodology constitutes an interesting route of synthesis to obtain dendritic effects. In this context, the most used synthetic strategies are coupling of suitably functionalized fragments; growth of dendritic molecule from the polymer chain; growth of polymer chain from the reactive groups of dendrimers; and polymerization of macromonomers containing dendritic and linear blocks (dendron with polymerizable group), Fig. 3.2. The synthesis of dendrons and their combination with other organic/inorganic building blocks facilitates and accelerates the obtention of materials with dendritic properties. The dendronization methodology is a good example of hybrid material synthesis ideally suited as a synthetic tool for achieving interesting dendritic structures. Utilizing dendrons as building blocks allows the synthesis to be accelerated, lowering the number of reaction steps and the need for tedious purification procedures, while at the same time diminishing the need for excessive levels of reagents. This could lead to their use in new applications and improved levels of commercialization. Dendrons have already been used as building blocks, spacer arms, or functionalizing agents [2528]. Regularly branched dendrons can be attached to a point, a line, or a surface, thus giving rise to dendrimers, dendronized polymers, and dendronized surfaces known as forests [29]. Certain aspects of the behavior of

Figure 3.2 Synthetic strategies for preparing dendronized polymers.

Chemistry of hybrid multifunctional and multibranched composites

35

dendrons are dominated by structural and geometric features, since the functional groups on the periphery (polar or nonpolar) and their architecture or size define their spatial disposition and final properties [1,2]. The aim of this chapter is to illustrate, through different examples, the importance of the design of hybrid materials and the properties obtained as a consequence of the presence of hyperbranching and adequate functionalization. We will show prominent examples of chemical modification on different substrates (flat surfaces or curved 3D structures). As substrates, we have selected inorganic or highly crosslinked organic polymers such as porous polymeric supports, inorganic nanoparticles, or electrodes, in order to obtain greater insight into the control and adjustability based on size, architecture, and functionality. We place special emphasis on structure/property relationships and provide interesting examples of potential applications and uses.

3.2

Hyberbranched versus dendritic macromolecules

The development of polymer science in the period between 1930 and 1970 was based on control of the chemical structure and its influence on the properties of the polymeric material, at first taking into account only two possibilities: linear and crosslinked structures [30]. In the period between 1960 and 1970, pioneering research into the branching reaction of long-chain polyolefins gave rise to the appearance of a new type of polymeric structure. This third class of polymers, comprising branched polymers, provided clear evidence of the influence of molecular structure on the properties of the final products. Each step in the historical evolution of polymer structure signified an increase in the level of branching control. The introduction of branching involved substantial changes in rheological properties as well as multiplication of the terminal functional groups. Branched polymers may have a very symmetrical organization, with low polydispersity of molecular weights, or they may be irregular with considerable polydispersity of molecular weights. This new class of polymers constituted the foundations for developing the next step: hyperbranched polymers, dendrimers, and dendritic polymers. Hyperbranched polymers and dendrimers represent a new class of molecules with properties beyond the mere linear and occupy a separate field in the area of polymer chemistry [31]. They are characterized by a large number of branch points with numerous terminal groups. Hyperbranched polymers have a random distribution in terms of molecule size and degree of branching, whereas dendrimer macromolecules are well-defined with perfect branching. This structural difference stems from the fact that hyperbranched polymers are prepared by polymerizing a monomer AB2 in one-step (one-pot), while dendrimers are prepared by laborious methods using conventional organic synthesis. Lederer et al. stated in their article published in Angewandte Communications: “compared to the perfectly constructed dendrimers, hyperbranched polymers appear like their ugly cousins” [32]. The main structural differences between hyperbranched and dendrimers and the typical properties of both are shown in Table 3.1.

36

Hybrid Polymer Composite Materials: Structure and Chemistry

Structural differences between hyperbranched dendrimers and dendronized polymers

Table 3.1

Polymer

Hyperbranched

Dendrimer

3D, irregular One-step, relatively simple Precipitation or classification Simple Discrepant .1.1 Low High At linear and terminal units High

3D, regular Multistep, laborious Chromatography Difficult Identical 1.0 (,1.05) Very low High On periphery (terminal units) High

Structure

Topology Synthesis Purification Scaling-up MW PDI Viscosity Solubility Functional group Reactivity

Adapted from Ref. [3] with permission of The Royal Society of Chemistry.

Since hyperbranched polymers are synthesized via one-pot solution polymerization, they have a poorly defined structure with broad molecular weight distribution. Recently, Shi et al. reported three approaches to reduce the polydispersity of hyperbranched polymers: slow addition of monomer into the multifunctional core; use of a core molecule with higher reactivity than monomers; and polymerization within a confined space [33]. Another property of multifunctional molecules is the previously mentioned dendritic effect derived from biological interaction. In the biological field, polyvalent (or multivalent) interactions are defined as simultaneous binding of multiple ligands on a molecule (or a surface) to multiple receptors on another. These interactions have many advantages that monovalent interactions do not, since the binding itself can be collectively much stronger than the corresponding sum of monovalent interactions; in other words, synergism is observed in multivalency [34]. In recent years, several authors have used this property for the development and application of ligands, inhibitors, and drugs based on multivalency [35]. A further aspect of multivalent effects is the availability of functionality in space in order to be able to perform the polyvalent interactions. This effect can differ in dendritic structures with the increasing generation of dendrimers, the so-called generation effect, which generally involves periphery functions or sometimes the core under the influence of the branches (see Fig. 3.3) [36]. The dendritic effect can be

Chemistry of hybrid multifunctional and multibranched composites

37

Figure 3.3 (A) Multivalent binding between cell and virus; (B) dendritic effect. Adapted from Ref. [36] with permission of The Royal Society of Chemistry.

observed in both dendrimers/dendrons and hyperbranched molecules, and for any type of application, though it has generally been tracked in catalysis and biology, and to a lesser extent in the field of functional materials. It is important to note that the dendritic effect can be positive or negative, depending upon whether the observed effect is amplified or attenuated, respectively. Examples of these effects will be provided in the next sections. Various synthetic methodologies have been proposed for the preparation of hybrid polymers in accordance with the type and amount of the different starting monomers and the architecture of each moiety (linear or branched), whether it is hyperbranched or a dendritic polymer. Such methodologies include polymerization

38

Hybrid Polymer Composite Materials: Structure and Chemistry

techniques such as the classic step-growth ABn polycondensation, self-condensing vinyl, self-condensing ring-opening, and proton-transfer, among others. The amount of branching units generated and the complexity of the subsequent structure will depend on the number of reactive groups of the starting monomers. Fre´chet et al. reported numerous synthetic methodologies, affording various interesting topologies of hyperbranched polymers [3739]. Hyperbranched polymers with abundant vinyl groups can be synthesized by a combination of free radical polymerization with controlled radical polymerization of divinyl monomers or oligomers. Commercial acrylate monomer derivatives were applied to prepare hyperbranched polymers with vinyl groups [40]. Thermally curable hyperbranched polymers were synthesized in a one-step reaction by free radical bulk and solution polymerizations of a divinyl monomer, ethylene glycol dimethacrylate, in the presence of methyl 2-(bromomethyl)acrylate as an additionfragmentation chain transfer agent. The resulting hyperbranched polymers showed good thermal stability with a larger number of methacryloyl groups [41]. Furthermore, various amphiphilic hyperbranched-linear polymers were successfully synthesized by a combination of self-condensing vinyl (co)polymerization and ATRP and evaluated as potential unimolecular micelles. Hyperbranched hydrophobic macroinitiators were first synthesized and then extended with poly(ethylene glycol) methacrylate [42]. The self-condensing ring-opening methodology allowed the synthesis of hybrids with a diverse combination of hyperbranched and linear moieties, such as linear-hyperbranched, hyperbranched-linear-hyperbranched and hyperbranchedhyperbranched blocks. Hydroxy-functional polymers were synthesized by ring-opening copolymerization of trioxane and 1,3-dioxolane with glycidol. First, formic acid as a chain transfer agent produced a linear poly(oxymethylene) polymer, providing the macroinitiador for the subsequent ring-opening polymerization and formation of branched arms [43]. Proton-transfer polymerization for thermoresponsive hyperbranched polymers by nucleophilic ring-opening reaction has been developed, showing a wide range of critical solution temperatures [44]. In particular, the couple-monomer or multicomponent methodologies are employed when the monomers have nonequal reactivity of their functional groups [45]. Hyperbranched and dendritic structures confer a number of advantages on nanocomposites over other architectural forms of polymers, including monodispersity, nanoscale size, hostguest potential, and high ability as molecular vehicles, facilitating their passage through biological barriers [21]. In all cases, the main benefit of using dendrons or hyperbranched polymers instead of simple monomers for the modification of surfaces is the number of anchoring points afforded by these molecules, which improves the stability of the interface. The methods used to reach this point will be explained in the following paragraphs and their use in electrochemistry (mainly as electrode modifier), purification/separation (mainly as chromatography support) and biopolymers design shown in the subsequent sections.

Chemistry of hybrid multifunctional and multibranched composites

3.3

39

Hyperbranched and hyperfunctional hybrid organic

Dendritic-based hybrid organic composites are an alternative to conventional organic polymers whilst at the same time maintaining the dendritic effect. The immense scope for variation in dendritic molecules (hyperbranching and type of scaffold, generation number, nanosized, solubility, hydrophobicity/hydrophilicity and rigidity/flexibility balance, niche/cavity formation, selectivity, chirality, etc.), and their functional versatility (the possibility of multivalent binding) permit the design of highly improved novel hybrid organic substrates [46] for which there are a number of applications [1,2]. The aim of this section is to show systems based on polymer and films/hydrogels based biopolymer and crosslinked polymer beads, and their surface modification with hyperbranched molecules, mainly for application in biomedical areas and as support materials in purification/separation methodologies.

3.3.1 Flat surfaces Films/hydrogels based biopolymer is widely studied because of properties such as biocompatibility and biodegradability that allow them to be used for example in the biomedical field as carriers, for controlled drug release systems and in tissue engineering [47]. To obtain hybrid material, Stancu designed a system based on aminoterminated polyamidoamine (PAMAM) dendrimers with 16 and 64 amino groups (G2 and G4 dendrimer, respectively) chemically immobilized on crosslinked gelatin-glutaraldehyde (GA-gelatin) hydrogel [48]. PAMAM dendrimer was immobilized on a GA-gelatin scaffold though Schiff-base formation between amines and free aldehyde from partially unreacted GA. This material has nanorough surfaces and a high density of amino groups, available for further functionalization with amino-reactive molecules, producing scaffolds with potential application in biomedicine and especially for hard-tissue engineering applications. Another strategy for obtaining modified biopolymer scaffolds with dendritic molecules could be through selective on-side chemically immobilized isocyanate triester Newkome-type dendrons on crosslinked gelatin or chitosan films, forming urea/urethane between isocyanate and free amines/alcohols of the biopolymers (see Fig. 3.4) [49,50]. These modified systems provide selective hydrophobic sides due to the t-butyl periphery of dendrons and could be used for biomedical application, in particular wound dressing.

Figure 3.4 Selective surface modification of polymer films.

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Hybrid Polymer Composite Materials: Structure and Chemistry

3.3.2 Curved surfaces in 3D materials There is currently a great deal of interest in the polymer science of support materials for different applications in the life sciences and engineering, such as separation/purification, catalysis in organic synthesis, sensors, among others. Each support material must have specific properties according to its application, but interaction between support and sample, solubility (polar and nonpolar properties) and size (particles size, pores size) is very important in support-type polymers. It should be taken into account that many of these systems comprise porous material allowing increased flow through the structure with reasonably low back pressure. However, Lei et al. showed that porous support is not always necessary: the authors describe a nonporous composite hydrogel of zirconia and urea-formaldehyde (ZrO2UF), modified using PAMAM via “graft from” approach, with the imido groups on the scaffold achieved through the Michael reaction with methyl acrylate and the amination reaction with ethylene diamine, finally obtaining G3 amineterminated polyamidoamine. Thus, a dendritic material for specific molecular recognition, using immobilized Br-substituted ribonucleic acid (RNA) as ligand, was achieved as a support for application for instance in affinity chromatography [51]. The activated stationary phase [RNA(PAMAM)(zirconiaUF resin)] showed more powerful separation ability for the protein-bonded nucleic acids and peptide nucleic acids. Macroporous bead polymers based on poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) and polyester dendron was obtained by Ling et al., and this material was used as a chiral stationary phase (CSP) for enantioselective separation [52]. The aim of the latter work was to first prepare a chiral selector from L-proline; then, to apply two general strategies (convergent and divergent approach) to obtain dendritic polyester linkers connecting with a selector to the porous polymer support. The separation of racemic N-(3,5-dinitrobenzoyl)-R-amino acid alkyl amides using these new CSPs under normal-phase high-performance liquid chromatography (HPLC) conditions showed better results when the spatial disposition of the chiral selector was within the dendritic structure, as compared with monovalent CSP. This multivalent effect can be explained by the intercalation of the analyte between two selectors located in a precise position. However, the separation factor decreases when generation goes from G4 to G1; thus, as mentioned in the previous section, synthesized dendritic CSPs have a negative generation effect. On the other hand, Martinelli et al. obtained functionalized organic supports with sugar dendritic ligand from chemical modification of the poly(hydroxylated polybutadienic-hydroxyethyl methacrylate) matrix, using G1 dendrons with an NH2 group as focal point and six carboxyl groups in periphery [53]. The dendritic poly(PB-HEMA) was then modified using glucose, as shown in Fig. 3.5, and these novel supports were used as sorbents to retain Concanavalin A. Reverse phase chromatography (RPC) is one of the most widely used support polymer systems (with aliphatic properties), as the stationary phase of HPLC to effectively retain nonpolar analytes. However, since there are certain limitations associated with this technique, such as the difficulty in retaining/resolving ions and

OH

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Figure 3.5 Dendronization of poly(PB-HEMA) matrix. Reprinted from Reactive & Functional Polymers, 67, M. Martinelli, M. Caldero´n, C.I. Alvarez I, M.C. Strumia, Functionalised supports with sugar dendritic ligand, 10181026, Copyright (2007), with permission from Elsevier.

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Figure 3.5 (Continued)

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Chemistry of hybrid multifunctional and multibranched composites

43

polar molecules using RPC, mixed-mode chromatography methods and hydrophilic interaction liquid chromatography (HILIC) can be used. Usually, materials using RPC as stationary phase are formed by alkyl and/or phenyl networks; however, any polar functionality present in the structure is able to form water-rich regions on the structure of the stationary phase, and for this reason, it can be used for HILIC application [54,55]. Thus, the HILIC mechanism of retention depends on the chemistry of the stationary phase (disposition of hydrophilic zones), analytes, and mobile phase. Furthermore, depending on the final properties of the single column, a mixed-mode stationary phase can achieve multiple separation capabilities, such as RPC, ion-exchanged chromatography, size-exclusion chromatography, and HILIC. The functionalization of different conventional supports with multibranched molecules provides a perfect alternative for obtaining dendritic-based hybrid composites with potential application as a mixed-mode stationary phase. Macroporous bead polymers modified with dendritic molecules have been mentioned, but it is known that monolith materials (consisting of one piece of a continuous and porous material) often have better properties than conventional beads in terms of packing, more homogeneous pore size distribution, larger pore size, among others [56]. Thus, in seeking the best properties for chromatography applications, the alternative to dendritic-based hybrid polymers is ideally a combination between macroporous polymer monoliths and hyperbranched molecules. These hybrid materials can be obtained using divergent or convergent approaches within a monolithic structure or using a dendritic macromonomer to form the dendronized homopolymer or copolymer monoliths. Whichever approach is chosen, cavities along the polymer and welldefined hydrophilic/hydrophobic zones reflecting multifunctional properties, will be obtained. Designed macroporous materials have a high specific surface area which as mentioned, facilitates good throughflow; they now also have a high density of functional groups which change both chemical environment and polar/nonpolar properties.

3.4

Hybrid inorganic-hyperbranched polymer composites

Surface immobilization of dendritic molecules is of particular interest, and extensive studies on coating an electrode with multifunctional and multibranched molecules have resulted in a more sensitive electrochemical sensor owing to the increased density of functional groups [57]. Nanoparticles can also be modified with dendritic molecules for better dispersion stability or enhanced capture of target biomolecules. Application of dendritic molecules onto a surface via covalent bonding provides improved chemical and thermal stability whilst remaining a simple procedure. Among the various methods for covalently linking dendrons onto surfaces, the most commonly used technique is to form a dendritic layer on a gold surface by covalent attachment via a thiol group at the focal point. Another widely used method is to hydroxylate the surface and use a coupling agent to covalently bond the dendron by forming ester bonds or to activate the surface with amine

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Hybrid Polymer Composite Materials: Structure and Chemistry

groups and link dendrons by using coupling agents [3]. Dendrons ending with SiCl can be modified via SiOSi bonds [58]. All of these methods provide strong binding of the dendritic molecules to the surface. A selection of applications of dendrons and hyperbranched polymers covalently bound to surfaces are mentioned in the following sections. In addition to covalent bonding, multibranched modification of a surface can also be achieved through noncovalent bonds such as ionic attraction, hydrophobic interactions, or ππ stacking. Linking molecules through bonds is quite simple as long as the dendritic molecules are appropriately designed and the corresponding surfaces are correctly selected. The noncovalent approach generally involves ionic interactions between oppositely charged entities. For example, hyperbranched structures can have multifunctional groups at the periphery such as carboxylate groups and thus multiple ionic bonds with a cationic surface are feasible. The noncovalent binding directed by the end groups of dendrons has been termed “fingertipguided” functionalization [59].

3.4.1 Flat solid surfaces Gold electrodes. One interesting example is the self-assembled monolayer formation of a series of thiophene dendron thiols with different generations and alkyl chain lengths onto gold surfaces [60]. The purpose of the cited study was to understand the chemisorption behavior of a series of hyperbranched thiols formed onto gold electrodes in terms of parameters such as size, alkyl chain length, and bulk concentration. The chemical structures of these dendron thiols are shown in Fig. 3.6A. A rearrangement adsorption/desorption kinetics to describe their behavior at the interface was elucidated by using surface plasmon resonance, electrochemistry, quartz crystal microbalance and water contact angle measurements [60]. An empirical three-step model explained the observed adsorption kinetics, which involved a two-step rearrangement process including a relatively faster short range and a much slower long range course, followed by an initial fast adsorption/desorption. For the lower generation dendron thiols, much faster adsorption rate constants were found compared to higher generation dendron thiols, due to the additional adherence of the thiophene sulfurs to gold surfaces. Dendrons with the longest alkyl chain showed the most tightly packed monolayer arrangement [60]. Another focally substituted organothiol dendron shown in Fig. 3.6B) also adlayers on gold by chemisorption of the thiol moiety onto this surface [61]. The covalent approach onto gold surfaces is also achieved with sulfides at the focal point as shown by Friggeri et al., who investigated the insertion process of individual dendron-sulfide molecules (Fig. 3.6C) into self-assembled monolayers of 11mercapto-1-undecanol by atomic force microscopy, wettability, and electrochemical measurements [62]. A mechanism consisting of rapid dissociation of surface thiols followed by slow dendron adsorption was proposed considering the insertion of the individual dendron molecules as the rate-determining step of the process. The immersion of alkanethiol self-assembled layers (SAM) in solutions of increasing concentrations of dendron-sulfide leads to an increase in the number of dendritic

Chemistry of hybrid multifunctional and multibranched composites

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Figure 3.6 Scheme of some examples of (A and B) dendron-thiol and (C) dendron-sulfide molecules. (A) Adapted from Ref. [60] Copyright (2010), with permission from Wiley; (B) Adapted with permission from Ref. [61] Copyright (1998) American Chemical Society; (C) Adapted with permission from Ref. [62] Copyright (2000) American Chemical Society.

molecules inserted into the thiol layer; on the contrary, the initial SAM quality (reached at different incubation times of alkanethiols) is not a determining factor for the dendron insertion process. Dendrons with a carboxylic acid at the focal point [3,5-bis (3,5-dinitrobenzoylamino) benzoic acid] and nitro groups at the periphery (G1-NO2) were immobilized onto gold surfaces in a noncovalent approach and studied by electrochemistry and scanning tunneling microscopy (STM) [63]. G1-NO2 adsorbs onto gold electrode surfaces spontaneously by dipping the metal surface in dendron solution and also via grafting of cystamine covalently attached to a gold electrode. The reduction of these layers exhibits a well-behaved redox response for the adsorbed nitroso/ hydroxylamine couple, useful for the electrocatalysis of reduced nicotine adenine dinucleotide (NADH), an important feature in amperometric sensors design [63]. Dong et al. reported the formation of special surface structures on gold arising from the precisely tailored structure of surface-bound thiol-dendrons with different main structures and peripheral substituents [64]. Among the factors controlling aggregation behavior, chemisorption is fairly similar in these compounds. Thus, intermolecular interaction greatly influences the configuration of dendrons on gold and different results were obtained from SAMs composed of symmetrical and asymmetric structures (Fig. 3.7). The different aggregation behavior observed was

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Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 3.7 Chemical structure of gold-bonded dendron-thiols and the corresponding STM images. Adapted from Reference [64], Copyright (2003), with kind permission of Wiley.

probably influenced by peripheral substituents and the coadsorption process, as reflected in the results of both STM and electrochemical measurements [64]. A library of compact and monodisperse dendritic scaffolds based on the nontoxic 2,2-bis(methylol)propionic acid (bis-MPA) was explored for binding SAM onto gold surfaces to exploit the design of hydrophilic dendritic structures bearing sulfur-containing core functionalities [65]. The size of the dendritic framework (G1G3), the nature of the sulfur (whether thiol or disulfide), the functional end group (mannose or hydroxyl), and the distance between the dendritic wedge and the sulfur were key structural elements affecting the packaging densities assembled on the substrates. Surface interactions between multivalently presented motifs and cells in a controlled surface setting were evaluated by the cell scavenging ability of these sensor surfaces for Escherichia coli Ms7fim 1 bacteria that revealed 2.5-fold enhanced recognition for G3-mannosylated surfaces compared to G3-hydroxylated SADM surfaces [65]. Dendritic polyglycerol (PG) derivatives with different numbers of amino groups have been attached onto gold substrates via thioctic acid linker and the selective interaction of complementary fluorescently labeled DNA proved the availability of such end groups for biomolecule attachment. These results demonstrate a new way to tailor hyperbranched surfaces by introducing amino moieties which can act as suitable anchoring sites for specific biomolecule interactions, while maintaining the resistant properties against non-specific protein adhesion. The protein-resistant

Chemistry of hybrid multifunctional and multibranched composites

47

Figure 3.8 Chemical structure of G1-NO2 and G2-NO2 dendrons and the corresponding AFM images of these dendrons attached to carbon surfaces.

properties of these PG-coated surfaces depend on the amino content, providing a combination of both important characteristics in bioelectronics or in the development of biosensing platforms with improved sensitivity [1,2]. Carbon surfaces. The functionalization of carbon surfaces with dendrons provides controllable properties for the electrode surface due to multifunctional groups of these molecules. For example, the cooperative effect of phenyl rings and the multifunctionality of G1-NO2 and G2-NO2 dendrons (Fig. 3.8) allow a direct, rapid and spontaneous physisorption of these dendrons onto carbon surfaces [66]. The AFM images show a network film with embedded aggregates that completely cover the carbon surfaces after only a few minutes, with average heights suggesting a tilted preferred adsorption in the early stages of the film formation, and highlighting a noticeable increment of this effect with increasing dendron generation. These molecules form a layer covering the whole surface, but do not block the electron transfer reaction of redox probes like Fe(CN)632/42 or Ru(NH3)631/21. This effect, together with the remarkable simplicity of obtaining nitroaryl-ended films, makes these modified electrodes promising for electrocatalysis and biosensing platforms [67]. Boltorn H30 molecules are the third generation of commercial polyhydroxylated hyperbranched polymers, are approximately spherical in shape, have an average diameter larger than 3 nm and inner cavities that can be used for small molecule or ion inclusion. The spontaneous adsorption of Boltorn H30 onto carbon substrates was carried out with the idea of generating a nanostructured layer capable of retaining copper cations (II) inside. The modified electrode was subsequently incubated for a certain length of time (between 1 and 3 h) in CuCl2 solutions, washed in water and taken to an electrochemical cell to reduce the cations and generate the metallic nanoparticles [68]. These platforms are valuable for the electrocatalysis of hydrogen peroxide.

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Silicon surfaces (Si wafers in air are covered with a thin SiO2 layer bearing OH groups as final functions). Monodisperse dendrons with urea/malonamide are used as linkages for organic thin-film transistor gate insulators [69]. These dendrons are hydrogen bond-rich and able to form stable self-assembled structures. Due to their strong intermolecular interactions, they have been successfully employed in nonlinear optics. In addition, dendrons with hydrogen bond-rich urea/malonamide linkages and peripheral long alkyl chains would certainly interact with tetracarboxylic diimide derivatives possessing perfluorinated alkyl chains on imide rings [69]. Indium tin oxide (ITO) electrodes (transparent conducting films). Recently, new hyperbranched p-conjugated macromolecules were obtained by the versatile electropolymerization method and exhibit interesting electronic properties that yield thin and stable layers with good electrical conductivity. Mangione et al. reported the synthesis and properties of peripherally carbazole (CBZ) functionalized starburst monomers, featuring the presence or absence of electroactive central core triphenylamine (TPA) moieties connected by conjugated or saturated branches. For this purpose, the dendron monomers were obtained by a convergent strategy and the CBZ residues allowed the formation of hyperbranched polymeric layers over conductive substrates by electrochemical polymerization. The radical cation coupling of oxidized CBZ leads to the growth of the dendritic structures. Thus, two different fully p-conjugated dendritic polymers are formed, with and without an electroactive central core connected to peripherals moieties (Fig. 3.9) [70].

3.4.2 3D Structures The exclusive use of dendrons as stabilizers of inorganic nanoparticles gives rise to a new class of hybrid dendritic material called nanoparticle-cored dendrimers (NCDs). Briefly, NCDs are coreshell materials that possess nanometer-sized

Figure 3.9 Chemical structure of p-conjugated dendrimers. Reprinted from Ref. [70]. Copyright (2016), with permission from Elsevier.

Chemistry of hybrid multifunctional and multibranched composites

49

inorganic clusters at the core surrounded by a shell of dendrons of different generations which are attached radially to the core, allowing their stabilization and impeding aggregation [71]. Other different dendritic-inorganic 3D nanostructures are dendrimer-encapsulated inorganic nanoparticles and dendrimer-stabilized nanoparticles as shown in Fig. 3.1. Gold nanoparticles. A well-known strategy consists of performing ligand exchange reactions between alkylthiolgold colloids of the Brust type and thioldendrons, keeping the size of the colloids constant during the ligand exchange reactions and retaining their low polydispersity. One good example of this approach is the synthesis of the silylferrocenyl- or amidoferrocenyl-ended dendrongold particle assemblies represented in Fig. 3.10 [72,73]. The viability of using dendrons for functionalization of gold nanoparticles lies in the ability to control the size and surface reactivity of the nanoparticles. One important aspect of this structural feature for size control is the dependence of the binding strength of dendrons to gold particles on the umbrella-like structure, which confers the ability to control assembly size. Another important aspect is the combination of weak interactions and voids between particles that facilitates surface reactivity by molecular linkers. One more successful example of this approach is the synthesis of gold NCDs for which size and surface reactivity of the particles are controlled by the molecular sizes of dendritic arenethiols as capping agents [74]. This strategy exploits two important attributes of dendritic-thiol molecules: first, involving the use of a single thiol as the anchorage handle and, the second, involving the expandable dendritic structure as a spacing-tunable limit. The ability to tailor the terminal groups of dendrons to functionalize nanoparticles opens the possibility of creating novel organic-inorganic hybrid materials with other enhanced properties. A family of Newkome-type dendritic molecules, bearing a disulfide anchor group and different peripheral groups, sizes or generations, was synthesized and used as a ligand for the synthesis of NCDs (Fig. 3.11). These hybrid materials were characterized in detail by microscopy and spectroscopic techniques showing that the capping molecules of the organic shell determine the solubility and stability of the different NCDs, as well as the characteristics of the inorganic core, through a dendritic control characteristic for Newkome-type ligands [75]. Gold surfaces can also be modified by the electrochemical reduction of aryl diazonium salts of the general formulae A2, 1N2C6H4R, where A2 is the anion and R stands for a variety of functional groups; thus, Au(III) spontaneously reduces to gold in the presence of dendron-N21 as described in Fig. 3.12, to create potential catalytic nanomaterial [76,77]. Silver nanoparticles. Silver nanoparticles are very interesting since they display activity against several micro-organisms, but only in their positively charged form. In addition, some dendrimers have recently attracted interest as antimicrobials but at varying levels of toxicity; thus, an additive effect is feasible by forming hybrid silver-dendritic nanocomposites. As an example of this approach, dendrimerentrapped silver NPs were prepared using amine-terminated poly(amidoamine) dendrimers of generation 5 as templates and subjected to acetylation in order to

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Figure 3.10 Synthesis of dendronized gold nanoparticles using the thiol-ligand substitution procedure. Adapted with permission from Ref. [72]. Copyright (2003) American Chemical Society.

Si O

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Chemistry of hybrid multifunctional and multibranched composites

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Figure 3.11 Scheme of gold-cored Newkome-type dendrimers.

O

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Figure 3.12 Schematic view of dendron-modified nanoparticles via electrochemical reduction of aryl diazonium salts.

neutralize the surface positive charges [78]. In addition, silver NCDs protected by first-to-third generation poly(amidoamine) dendrons with an anthracenyl-focal point were produced by photo-promotion of silver ions in aqueous dendron solutions; briefly, the light-promoted electron transfer from amine groups of dendron to silver ions through anthracenyl moiety leads to the generation of silver nanoparticles [79]. Two different nanoparticles were synthesized from this approach: spherical facecentered-cubic nanoparticles of around 1520 nm in size and polygonal hexagonalclose-packing single crystals of 2035 nm in size, indicating competition between the increasing number of amine groups resulting in the reduced efficiency of photopromoted electron transfer with generation.

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Hybrid Polymer Composite Materials: Structure and Chemistry

Single-walled (SWCNTs) or multiwalled (MWCNTs) carbon nanotubes. Functionalized MWCNTs can be used as nanowires to link solid substrates with organic molecules (or biomolecules). Aromatic dendrons can interact by means of ππ stacking interactions and successfully functionalize these surfaces through the noncovalent interaction between the peripheries of the dendrons and the sidewalls of MWCNTs. The main advantages of dendronized MWCNTS are an enhancement of their dispersibility in all solvent types due to the ability to tailor the terminal functional groups and the distinct spacing between the dendron-ended groups and the substrates. Park et al. reported a systematic approach for the construction of a series of modified MWCNTs by the cooperative self-assembly of cyclodextrins and dendrons that were designed to control structure and specific functions; thus, various functional groups and/or biological molecules such as biotin could be introduced onto the MWCNTs surface [80]. In addition, MWCNTmetal nanoparticle hybrids could also be constructed by using electrostatic interactions between the specific surface functionality added onto the MWCNTs and the precursors of metal nanoparticles. Self-assembled dendron-cyclodextrin MWCNTs with selected surface functionalities could be used as templates for the formation of complexes with other polymers. For example, the negatively charged surfaces of dendron-cyclodextrin MWCNTs were covered with a positively charged polyethylenimine (PEI) layer using electrostatic interactions, and the resulting dendronized-MWCNTs-PEI complex, having a positively charged surface, exhibited intracellular uptake capability for DNA complexation with reduced enzymatic degradation, higher transfection efficiency, and lower cytotoxicity than PEI, making them potentially useful as gene delivery vectors [81]. In contrast, through a covalent link (Fig. 3.13), PAMAM-like dendrons have been grown from surface-mounted SWCNTs and investigated by Bissett et al. in the context of solar cell applications [82]. TiO2 nanoparticles. The energy conversion of dye-sensitized solar cells (DSSCs) is based on the electron injection from a photoexcited state of the sensitizer (dye) attached to TiO2 nanoparticles into the conduction band of the TiO2. The oxidized sensitizers are reduced by a redox couple (usually I2/I32) present in the electrolyte and this redox couple is then renewed in the counter-electrode, producing photoelectrochemical cell regeneration. It is believed that insulating the molecular layer effectively shields the back electron transfer from the TiO2 conduction band to the redox moiety (I32); therefore, a well-defined structural material like a dendron could possibly determine the optimal coverage of the recombination center of TiO2 nanoparticles. Different generations of polyester hydroxyl acetylene bis(hydroxymethyl)propanoic acid dendrons having a carboxylic acid group at the focal point were consequently chosen as coadsorbents for TiO2 nanoparticles in order to understand the coverage effect on the overall conversion efficiency [83]. Shin et al. found that the addition of dendrons increased the short-circuit current density of the DSSCs and had no adverse effect on the open-circuit voltage, achieving an increment of up to 40% in the total power conversion efficiency by using the G5 dendron as the coadsorbent [83]. Magnetic nanoparticles (MNPs). Heuze´ et al. have synthesized dendrons functionalized by a fluorescent tag and grafted onto coreshell MNPs by a convergent approach [84]. Since the dendritic parts are synthesized and characterized before

NH2

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Figure 3.13 Functionalization scheme for modifying SWCNT. Step 1: attachment of p-phenylendiamine leading to generation 0.5. Step 2: attachment of methylacrylate leading to generation 1.0. Steps 1 and 2 are alternated for higher generations. Adapted from Ref. [82] with permission of the PCCP Owner Societies.

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Hybrid Polymer Composite Materials: Structure and Chemistry

their grafting onto the MNP surface, this methodology using well-defined dendrons leads to a high percentage of grafting, as demonstrated by the use of dendrons bearing fluorescent units. As expected, the resulting dendron-grafted MNPs show a larger number of fluorescent sites than those grafted with their linear analog maghemite [84]. As described in the previous paragraphs, dendritic moieties can be localized on nanoparticles by direct dendron binding to the surface, via ligand exchange of previously stabilized nanoaprticles, or via divergent dendritic growth from functionalized nanoparticles. The dendritic-hybrid nanostructures synthesized in diverse ways may vary in their structural parameters such as dendron loading, presence of other organic molecules within the dendritic film, ordering and packing density of the coating, and thus, their macroscopic and/or physicochemical properties can differ. A combination of organic functionalization consisting in silanization and dendronization was carried out on γ-Fe2O3 to synthesize three different redox-active hybrid nanoparticles (Fig. 3.14) for the purpose of analyzing the role played by each modifier and how they interact not only among themselves, but also with the dispersing media during dendronization [31]. While silanization was performed to introduce architectural differences in the organic layer, dendronization was carried out to introduce a high amount of electroactive nitro groups. The electrochemical and spectroscopical results show that chemical composition and the way in which the organic layer was organized were also a consequence of the order in which the chemical modification was realized [85]. Silica particles. Silica is a very common material used in a variety of methodologies in both its unmodified and modified form, in particles and even monoliths, owing to its significant advantages such as being well packed, having physical stability properties and a well-defined porous structure (totally porous, coreshell particles as well as monoliths), generating high flow through the matrix, good loadability, and high reproducibility [86]. However, limitations for some applications require that such materials be modified for specific methodologies. Hybrid materials from silica and hyperbranched molecules are good alternatives in these cases. Silane modification is a well-known method for silica functionalization [87]. Some authors synthesized dendritic structure in silica using molecules with silane groups; Buszewski et al. first prepared a silane-modified silica gel with (3-aminopropyl)triethoxysilane (APTES), obtaining silanized-silica to separate the complex mixture [88]. To improve functionalization, a novel stationary phase for chromatography based on silanized-silica gel with a dendritic structure from reaction with methylamine and 1,4-butanedioldiglycidyl ether (BDDE) was then obtained [89]. The presence of quaternary ammonium groups in modified silica allows for anion retention in chromatographic systems. Thus, this material has application as a support in anion exchange chromatography, allowing the separation of many inorganic anions (F2, Cl2, NO22, Br2, NO32, HPO422, SO422, and ClO42) with good resolution and even good stability in mobile phases with pH up to 9. On the other hand, Li et al. prepared a dendritic-silica stationary phase by a divergent synthesis from propylamine on silica by consecutive amine-epoxy reactions with 1,4-butanedioldiglycidyl ether and aniline, as shown in Fig. 3.15 [90].

Chemistry of hybrid multifunctional and multibranched composites

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Figure 3.14 Synthetic pathways followed to prepare different NCDs: (A) dendron is directly attached to the NPs, (B) attachment is achieved in two steps: silanization with APS followed by dendronization, placing APS between the rigid dendron and the NP surface, and (C) dendron is attached first, followed by the APS agent. Reaction conditions: (i) and (iii) G1-ClNO2, DMAc, 0  C—r.t., N2, 24 h; (ii) and (iv) APS, toluene, ultrasonic bath, N2, 30  C, 3 h. Reprinted from Ref. [85]. Copyright (2014), with kind permission from Wiley.

These systems generate hydrophobic, electrostatic, as well as hydrophilic interactive domains because of a combination of phenyl rings with a quaternary ammonium and tertiary amines at the branch point along with embedded polar functionalities in the branch; in consequence, dendritic silane-modified spherical silica gel have potential application as a mixed-mode stationary phase, as explained in the previous section. Indeed, reversed-phase capability achieved separation of polycyclic aromatic compounds, while ionic molecules were separated under mixed-mode reverse phase/ionic exchange chromatography, and small polar compounds (nucleobases and nucleosides) were also separated using HILIC mode.

Figure 3.15 Synthetic route of dendritic silane-modified spherical silica gel. Reprinted from Journal of Chromatography A, 1337, Y. Li, J. Yang, J. Jin, X. Sun, L. Wang, J. Chen, New reversed-phase/anion-exchange/hydrophilic interaction mixed-mode stationary phase based on dendritic polymer-modified porous silica, 133139, Copyright (2014) with permission from Elsevier.

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Figure 3.16 Illustration of the confinement of ferrocenyl dendrimers in the silica pores. Reproduced from Ref. [92] with permission of The Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association and the RSC.

Another strategy for modifying dendritic surfaces was reported by Dı´az et al., who obtained a novel type of redox-active material consisting in the confinement of ferrocenyl dendrimers in the silica mesoporous by a simple and efficient synthetic route involving the incorporation of poly(propyleneimine) dendrimers containing 4, 8, and 64 amidoferrocenyl moieties (13) into MCM-41 after silanization (Fig. 3.16). These composite materials contain a controlled number of ferrocenyl units, which are easily accessible to electrochemical oxidation [91].

3.5

Conclusion

One of the most exciting achievements in the field of organic and polymer chemistry in recent decades is the ability to create large synthetic macromolecules with a well-defined shape and atomic structure in a rational and efficient manner. The combination of fragments with different structures and functionalities provides a promising approach to obtaining materials with a wide range of potential applications; handled adequately, such combinations can lead to unexpected new behaviors, rather than just the sum of behaviors of the individual components. There have been notable developments in the dendrimer field since the beginning of this century and research relating to hyperbranched materials is following a similar trend. Though various studies have been carried out of hybrid materials based on organic and inorganic surfaces, the paucity of available data on

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structureproperty relationships hinders the comprehensive evaluation of their application potential. However, some reports already indicate their superior capabilities as nanocarriers, green chemistry nanoreactors, sensors and in theranostics, and others. The various examples presented in this chapter illustrate the crucial importance of design and architectural control in hyperbranched hybrid materials, showing how it is possible to tune the materials’ properties to specific applications such as controlling hydrophilic/hydrophobic balance through the presence of polar or apolar functional groups; developing more sensitive electrochemical sensors by increasing the density of functional groups on the surface after immobilization; enhancing capture of target biomolecules; and achieving improved dispersion stability of nanoparticle-dendritic molecule composites, among others. We believe that further research into hybrid composites will not only facilitate full exploitation of already observed properties but will also trigger new applications in which a rational approach could bring enormous benefits.

3.6

Acknowledgments

Financial support from ANPCyT (2011-0654), CONICET (112-20110101029), SECYT-UNC and CYTED (214RT0482) is gratefully acknowledged. Sergio David Garcı´a Schejtman thanks CONICET for the fellowship awarded.

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4

Green hybrid composites from cellulose nanocrystal Shahab Kashani Rahimi and Joshua U. Otaigbe School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, MS, United States

Chapter Outline 4.1 Introduction 65 4.2 Preparation of cellulose nanocrystals: sources and extraction methods

67

4.2.1 Cellulose nanocrystals from acid hydrolysis 68 4.2.2 Cellulose nanofibers from mechanical processes 71 4.2.3 Cellulose nanocrystals from ionic liquid process 71

4.3 Surface modification of cellulose nanocrystals: polymer/nanocellulose interfaces 73 4.3.1 4.3.2 4.3.3 4.3.4

TEMPO-mediated surface oxidation 73 Silylation and acetylation of cellulose nanocrystal surfaces 74 Polymer grafting by surface-initiated polymerization 75 Cellulose surface modification by electrostatic and physical adsorption 76

4.4 Processing and development of CNC-based hybrid polymer nanocomposites 4.4.1 4.4.2 4.4.3 4.4.4

4.5 Properties of polymer/cellulose nanocrystals nanocomposites 4.5.1 4.5.2 4.5.3 4.5.4

82

Mechanical properties 82 Thermal properties 85 Melt rheological properties 86 Gas barrier properties 89

4.6 Conclusion and future perspective Acknowledgments 90 References 91

4.1

76

Solvent casting 76 Melt processing 79 In-situ polymerization 80 Layer-by-layer assembly 81

90

Introduction

Over the past decade, tremendous research and development efforts have been focused on cellulose nanomaterials both as reinforcing and functional additives for polymer composite application as well as building block for development of novel functional materials. These efforts are motivated by a number of enhanced benefits of cellulosic fibers such as natural abundance and availability, inherent biorenewability and Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00004-5 Copyright © 2017 Elsevier Ltd. All rights reserved.

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sustainability, exceptional structural and mechanical properties, low cost, low density, and biodegradability. One of the main challenges in using natural fibers as reinforcing components of polymer composites is the variation of properties based on their original climatic conditions, species, age, and spatial distribution of properties within the lingocellulosic fibers. One approach to avoid the negative impact of this variation of properties is to eliminate the hierarchical structure inherent to cellulosic fibers by removing the fiber constituents to extract the highly rigid and crystalline core of the fibrillar assembly known as cellulose nanocrystals (CNCs) or cellulose whiskers. These nanoscale cellulosic moieties have attracted a great level of academic and industrial attention not only because of their superior structural and mechanical properties (as will be discussed in this review) but also due to their nanoscale dimensions, high surface area, and natural abundance of their source cellulosic material which makes them superior alternatives to conventional nanoadditives and fibers such as silicates, glass, and inorganic nanomaterials traditionally used in composites applications. As an example, taking the density of CNCs to be at approximately 1.5 g cm23 (cf. 2.63 g cm23 for inorganic clays), a significant weight reduction of the final nanocomposite material is expected to be obtained simply by replacing clays with CNCs in the nanocomposite [1]. In fact, CNCs or cellulose whiskers are the highly crystalline core fraction of cellulosic fibers where other components such as lignin, hemicellulose, proteins, extractives, and paracrystalline interfibrillar regions are removed in the extraction process, leaving behind the tightly packed rod-shape defect-free cellulose fraction that are held together by strong hydrogen bonding of cellulose macromolecules. The hierarchical structure of cellulose fiber is shown in Fig. 4.1 [2].

Figure 4.1 Hierarchical structure of cellulose derived from wood/plants. Reproduced from Ref. [2]: {2015} {John Rojas, Mauricio Bedoya and Yhors Ciro}. Originally published in Current Trends in the Production of Cellulose Nanoparticles and Nanocomposites for Biomedical Applications, Cellulose  Fundamental Aspects and Current Trends, Dr. Matheus Poletto (Ed.), InTech, under CC BY 3.0 license. Available from: DOI: 10.5772/61334.

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These nanocrystals possess exceptional mechanical properties with axial Young’s modulus reported to be as high as 160 GPa, a value surpassing that of Kevlar and steel [3]. Cellulose macromolecule is comprised of rigid linear chain of ringed glucose units that are formed by a covalent link of β 14 glucosidic bond between the anhydroglucose rings (C6H10O5) [4]. Strong intermolecular hydrogen bonding via hydroxyl units of the glucose and oxygen of neighboring ring unit stabilizes the links, resulting in linear chain configuration with chair conformation [5]. This strong hydrogen bonding is the basis of formation of this elementary fibrillar structure which further aggregates into microfibrillar arrangement. Depending on the nature of the natural cellulose fiber, the arrangement of the fibrils and the degree of polymerization of the cellulose (or its length) vary among different species and sources. The microfibrils are the basic building blocks of the wood/plant cell wall. These fibrils are composed of cellulose crystallites that are connected via amorphous regions which are further wrapped in a polyglucosan material and hemicellulose. These microfibrils are held together via a matrix of lignin, proteins, and extractives. Therefore, certain processes have been developed in order to isolate the highly crystalline cellulose in the core of the fibrils. In this chapter, a survey is provided that covers previous research efforts together with some recent developments in the area of polymer/CNC nanocomposites and properties of CNCs, processing, and properties of CNC-based nanocomposite materials.

4.2

Preparation of cellulose nanocrystals: sources and extraction methods

CNCs are usually prepared using a number of different methods such as acid hydrolysis, mechanical process, enzymatic synthesis, and a recent approach using ionic liquids (ILs) [5]. Normally, in a typical process, a two-stage procedure is followed which depends on the cellulose source material. The first step involves the removal of polyglucosan components (except the cellulose fibrils), and the second stage involves extraction of the nanocrystalline regions. Specifically, lingocellulosic fibers are first chemically treated to remove the lignin, hemicellulose, and extractives. A more recent approach that has been used in a number of studies is based on steam explosion technology [6,7] to remove the lignin and hemicellulosic portions of the biomass where typically, the lingocellulosic fibers are subjected to high-pressure steam at a pressure of around 15 bars for a certain amount of time (usually less than 20 min) at temperature range of 220270 C. The fibers are then immediately exposed to atmospheric pressure by opening the chamber which causes the lignin/hemicellulose fractions to explode. These fractions can then be removed by extraction leaving the highly crystalline cellulose fibrils for further processing of CNC extraction. In the case of CNCs derived from the tunicate [8], the mantle is isolated from the animal followed by removal of the encapsulating protein components surrounding the microfibrils. In the case of bacterial and enzymatic cellulose production, after the cellulose microfibrils are cultivated, the walls and other components are removed

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by washing with alkaline solutions [9,10]. After the removal of the matrix material in the cellulose source, the second stage of the production of CNCs usually involves a number of treatments that include acid hydrolysis, mechanical process, and a bacterial/enzymatic treatment that will be described later in detail in this section. One of the major factors that dictates the final morphology and structure of the CNCs is the source material used to isolate the CNCs. Various cellulosic materials have been used to prepare the CNCs including wood fibers such as bleached softwood [11] and sugar beet pulp [12], cotton fibers [13], plant fibers such as flax [14], sisal [15], ramie [16], and hemp fibers [17]. In addition, the major nonplant/ wood-based sources are various types of bacteria [10] as well as tunicate [8]. Typical CNCs are rod-like whiskers with lengths of 251000 nm and diameters of 450 nm. It has been shown that the CNCs obtained from tunicate and bacterial growth method have typically larger lengths due to the higher amount of crystalline fraction in the cellulosic part of these materials [18]. For example, De Souza Lima et al. [19] reported a length value of 1160 nm and a diameter of 16 nm for whiskers obtained from tunicate giving an aspect ratio of 72.5 while the whiskers obtained from cotton had a length of 255 nm and diameter of 15 nm with an aspect ratio of 17. Fig. 4.2 depicts Transmission Electron Microscope (TEM) images of typical morphology of the CNCs obtained from various sources. A brief review of the main approaches to fabricate CNCs is discussed in the following sections.

4.2.1 Cellulose nanocrystals from acid hydrolysis Sulfuric acid hydrolysis is the most common method for fabrication of CNCs [5,6]. In a typical process, the cellulose starting material (i.e., after first stage of removal of the matrix containing the crystalline regions in the fiber) is suspended in de-ionized (DI)

Figure 4.2 TEM images of cellulose nanocrystals extracted from (A) ramie fiber, (B) Tunicate, (C) Bacteria, (D) cotton, and (E) from sugar beet. Images reproduced from Refs. [8,12,13,20,21] respectively with permissions from Royal Society of Chemistry, Springer and American Chemical Society.

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water followed by addition of sulfuric acid at prescribed composition. Then, the reaction continues for a set amount of time at a fixed temperature. The final mixture is quenched using ice cubes, filtered or centrifuged, and dialyzed against water until a neutral pH is achieved [22]. The purpose of the acid hydrolysis process is to remove the amorphous or paracrystalline regions surrounding the highly crystalline cores of the cellulose fibrils to leave the CNCs that are more resistant toward acidic medium in the final mixture. Sulfuric acid hydrolysis results in surfacenegative charges (sulfonate groups) that are responsible for colloidal stability of CNCs in aqueous solution at the price of reducing the thermal stability. A number of extensive studies have been carried out to investigate the effect of acid hydrolysis parameters such as time, temperature, and concentration of acid. For example, Dong et al. [23] found that a concentration of 64% (w/v) with a liquor ratio of 1:8.75 with the reaction conditions of 1 h at 45 C and ultrasonic treatment time of 5 min result in a suspension with anisotropic behavior above 4.5% (w/v). In a comprehensive study by Bondeson et al. [24], the yield of the hydrolysis process as well as the particle polydispersity was examined and the obtained results correlated to hydrolysis conditions. Their results indicated that particles with an average length between 200 nm and 400 nm and diameter of less than 10 nm with a yield of 30% could be achieved with a reaction time of 2 h while longer reaction times decreased the CNC length (via depolymerization of cellulose) and increased surface-negative charge. In a systematic study by Hamad et al. [25], it was found that a temperature of 65 C with shorter reaction times of about 5 min resulted in the highest CNC yield of 38%. In another study [26], the effect of hydrolysis temperature on preparation of CNCs obtained from cotton fibers was investigated. Using a fixed reaction time of 30 min and sulfuric acid concentration of 65%, the authors showed that by incremental increase in temperature from 45 to 72 C, the length of the crystals reduced from 141 to 105 nm while the polydispersity of crystal size increased from 1.15 to 1.21 indicating a more nonhomogenous hydrolysis process at elevated temperatures. Interestingly, Beck-Candanedo et al. [11] in a study of the effect of sulfuric acid hydrolysis time and acid-to-pulp ratio on dimensions of CNCs obtained from black spruce wood pulp showed that acid hydrolysis at longer reaction times produced shorter nanocrystals with lower size distribution (or polydispersity). The effect of acidto-pulp ratio was found to be inversely related to the nanocrystal dimension, implying that higher acid-to-pulp weight ratio results in formation of smaller crystals. The effect of acid-to-pulp ratio was found to be dependent on the retain time in such a way that the effect of acid-to-pulp ratio is more pronounced at shorter reaction time. In an effort to reduce the polydispersity of the nanocrystals obtained by acid hydrolysis, differential centrifugation [27], and ultracentrifugation [28], methods have been adopted. Bai et al. [27] used a multistep centrifugation process with stepwise incremental increase in the centrifugal speed to separate various fractions of CNC from the suspension at different velocities. It was found that smaller nanocrystal fractions could be separated at higher centrifugal speeds while each fraction at each speed showed a narrow-size distribution.

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In addition to sulfuric acid, a number of other mineral acids have also been used to prepare CNCs by the hydrolysis process. Yu et al. [29] studied the effect of hydrochloric acid hydrolysis parameters in preparation of CNCs. The optimized conditions to achieve highest yield and smallest diameter were reported to be a reaction time of 3 h at 110 C with acid-to-pulp ratio of 60 ml g21. Under these conditions, the CNC production yield from cotton and wood pulp material was 85% and 81%, respectively. In addition, it was found in the same study that the HClextracted nanocrystals improved thermal stability compared with that of the samples prepared by sulfuric acid hydrolysis, as well as, produced relatively narrower size distribution of the CNCs. However, one of the major disadvantages of extraction of CNC with hydrochloric acid is the lack of surface charge after the treatment process that results in significant flocculation and aggregation of whiskers and poor dispensability [30]. This characteristic is opposite to that of the treatment with sulfuric acid hydrolysis as the sulfate anions on the surface of CNC provide colloidal stability in aqueous medium. As shown in AFM images of Fig. 4.3, it is clearly seen that a cast film of the dispersion of CNC prepared from sulfuric acid hydrolysis shows a much better dispersion of whiskers compared to an aggregated structure obtained from hydrochloric acid hydrolysis method. In a series of studies by Wang et al. [32,33], a mixture of sulfuric and hydrochloric acid was used under ultrasonic treatment which resulted in development of spherical CNCs where the high polydispersity of the spheres resulted in formation of a liquid crystalline phase. A number of other researchers have reported the use of phosphoric acid in the hydrolysis process [34,35]. Camarero Espinosa et al. [34] optimized the hydrolysis process with phosphoric acid at 100 C with an acid concentration of 10.7 M for 90 min. The whiskers obtained under these conditions had an average length of 316 nm and diameter of 31 nm. A conductometric titration study showed a 10 times less surface phosphate groups compared to the sulfate groups after hydrolysis with sulfuric acid, indicating of very low charge density on the CNC surface. The thermal stability of the CNCs was studied with thermogravimetric analysis and compared with the CNCs obtained from sulfuric and hydrochloric acid hydrolysis. The results revealed that the phosphoric acid hydrolyzed CNCs had higher thermal

Figure 4.3 AFM images of cellulose nanocrystals obtained from (A) sulfuric acid and (B) hydrochloric acid hydrolysis of cotton. Reproduced with permission from Ref. [31]. Copyright (2011) American Chemical Society.

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stability compared to those bearing sulfate groups, but with less thermal stability compared to those obtained by hydrochloric acid process. In another study [36], hydrobromic acid was used to prepare CNCs from cotton fibers. The optimization of the hydrolysis process was carried out by varying the reaction time, temperature, and acid concentration. It was found that the hydrolysis temperature of 100 C for 3 h of reaction with acid concentration of 2.5 M of HBr. Increasing the acid concentration from 1.5 to 2.5 M increased the yield. However, higher concentration of HBr was found to result in side reactions and produced darker nanocrystals. In addition, it was found that the application of ultrasonic energy especially at the intervals during the hydrolysis process enhanced the final yield of CNC extraction especially at lower reaction temperatures. This was attributed to the fact that the ultrasonic waves can break apart the microaggregates and provide higher surfaces for acid hydrolysis. In addition, at lower temperatures, the input from the ultrasound treatment can compensate for the lower thermal energy and significantly increase the overall yield.

4.2.2 Cellulose nanofibers from mechanical processes A number of mechanical processes such as high-speed grinders [37], crushing in cryo-conditions [38], and high-pressure homogenizers [39] have been used to fabricate cellulose nanofibers from various starting source material. For example, Stelte et al. [39] studied the fibrillation process on soft and hard wood pulp to extract cellulose nanofibers. It was shown that the hard wood pulp needed more refining and high-pressure homogenization treatments by pressure-explosion technique to obtain nanofibers with similar properties. The nanofibrillation process is based on application of high shear force applied on the longitudinal axis of the cellulose fibrils, resulting in extraction of micro/nanofibrillar structures. In fact, additional repeated steps of high shear defibrillation result in relatively more uniform and smaller cellulosic fibrillar domains but with a compromise of less mechanical properties due to more damage impacted on the cellulose structure at each step. It should, however, be noted that in order to extract the cellulose whiskers or nanocrystals, usually, a postchemical treatment is required to remove any remaining amorphous areas in the cellulose fibrils [40]. For example, Cherian et al. [41] developed cellulose whiskers from pineapple leaf fibers by a combination of highpressure defibrillation and acid hydrolysis. They reported that by using repeated steps of acidic treatment using oxalic acid in autoclave with a high pressure resulted in effective defibrillation of banana fibers into nanoscale fiber with a length of about 200250 nm and diameter of 45 nm.

4.2.3 Cellulose nanocrystals from ionic liquid process ILs are molten salts that typically have melting points below 100 C due to their weak coordination of ions; and a number of them are liquid at or below ambient (room) temperature [42]. ILs have attracted considerable research attention over the past years in academia because they possess a large number of interesting physiochemical

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properties like high electrical conductivity, chemical stability, nonflammability, nearzero vapor pressure, and tunable structure with a wide range of anions and cations [43]. Kilpela¨inen et al. [44] showed that imidazolium-based ILs such as 1-butyl-3methylimidazolium chloride are capable of dissolving cellulose under mild conditions and the regenerated cellulose from the IL solution shows various types of microstructure and morphologies. In one of the early studies, Li et al. [45] described a novel method based on using sulfuric acid hydrolysis process in 1-butyl-3methylimidazolium chloride that significantly accelerated the reaction rate without any pretreatment. It was observed that in the course of the reaction, both endoglycosidic and exoglycosidic scission occurred, with the former being the dominant product in the initial stage of the reaction. The mechanism proposed by the authors for the accelerated reaction rate was ascribed to the dissolution of the cellulose in IL making the β-glucosidic bonds more accessible to the H1 of the acid. In addition, the Cl2 from the dissociation reaction of the IL was thought to have a weakening effect on the glucosidic bonds thereby favoring the reaction under acidic conditions. In a study by Man et al. [46], a novel approach based on using the 1-butyl-3methylimidazolium hydrogen sulfate (bmim[HSO4]) was used to prepare CNCs from microcrystalline cellulose (MCC). This preparation method was performed at a temperature range of 7090 C for 1 h. The reported results indicated that the regenerated cellulose after treatment with IL maintained the same cellulose I structure with an increase in crystallinity index as the treatment temperature increased. It was also found that no cellulose derivative was formed upon treatment with IL and the length and diameter of the nanocrystals obtained from this method were, respectively, 50300 and 1422 nm and had less thermal stability compared to MCC. In a recent study, Tan et al. [47] used similar IL (bmim[HSO4]) both as a solvent and reactant to extract CNCs from MCCs. They also observed that increasing the reaction temperature from 70 to 90 C resulted in enhanced crystallinity of the final CNCs. The main advantage of the IL-based techniques was the recovery of the IL after reaction ( . 90%) with no harmful reaction byproducts. In another interesting research reported by Abushammala et al. [48], CNCs were directly derived from wood particles using an acetate IL (i.e., [EMIM][OAc]) at 60 C with 2 h of reaction. A 20% by weight of the original wood mass was recovered in the form of CNCs with over 70% crystallinity in the form of cellulose I. The obtained CNCs were found to be partially acetylated from the solvation and reaction in the IL. The main mechanisms behind this process were reported to be (1) dissolution of lignin directly in the IL solution and swelling of the cellulosic portion, (2) decreased intermolecular hydrogen bonding via partial acetylation, and (3) catalysis of cellulose hydrolysis to produce the CNCs. In fact, the acetate IL ([EMIM][OAc]) had been shown previously to be able to dissolve lignin [49]; therefore, it is of great advantage to be able to remove the delignification process as a separate step in preparation of CNCs directly from wood particles. It is worthy to note that use of ILs is a promising approach to extracting CNCs from cellulose sources and the future studies be aimed at application of proper anion/cation pair in the IL and optimized reaction conditions to increase the CNC recovery yield with relatively higher aspect ratio whiskers.

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Surface modification of cellulose nanocrystals: polymer/nanocellulose interfaces

Cellulose whiskers possess hydroxyl-rich surfaces with the potential of developing interconnected percolated network structure of hydrogen-bonded whiskers in polymer matrix with significant improvement of structural properties [50,51]. Note that this network structure may pose a challenge regarding both interfacial adhesion and efficient dispersion in relatively more hydrophobic polymer matrices [52,53]. This special surface property of the cellulose at nanoscale just mentioned, if controlled properly, can provide significant opportunities that can be exploited to engineer the surface in order to enhance the polymer/CNC interfacial adhesion and to improve CNC dispersion in the polymer to achieve optimal interfacial area with the host polymer. However, care should be taken to ensure that the morphology and structural properties of the CNCs are preserved during the surface modification reaction. The approaches that have been adopted so far to tailor the surface chemical functionality of CNCs can be categorized into three major routes: (1) generation of various chemical functionalities depending on the application using surface synthetic methods, (2) physical adsorption of surfactants/compatibilizers via physical forces such as electrostatic or hydrogen bonding, and (3) grafting of polymeric chains using both “graft onto” and “graft from” approaches. Some of the recent advances using these techniques are now described briefly as in the following sections.

4.3.1 TEMPO-mediated surface oxidation TEMPO [(2,2,6,6-tetramethylpiperidine-1-oxyl) nitroxy radical]-mediated surface oxidation is based on using in combination with strong oxidizing agents such as sodium hypochlorite (NaOCl) that selectively oxidizes the surface methylol groups (primary alcohol) into carboxylic acid units [54,55]. A study by De Nooy et al. [56] showed that methylol groups are the only hydroxyls that can undergo the oxidation process while the secondary hydroxyl groups remain intact as they are deeply embedded within the whisker highly crystalline structure and, therefore, not accessible for the oxidation reaction. The carboxyl functional groups on the surface of cellulose can promote aqueous dispersion by electrostatic repulsion and provide colloidal stability. While the TEMPO-mediated oxidation has been shown to be easily tunable to control the extent of surface carboxylation by controlling the initial oxidizing agent content, excessive treatment is known to result in a decrease in the nanocrystal size due to the “peeling” reaction that removes the outer layer of cellulose from the crystal surface [57]. Li and et al. [54] developed nanocomposites of polyphenol via enzymatic polymerization in presence of TEMPO-oxidized nanocellulose. Their reported result revealed that due to the strong interfacial interaction between the polyphenol matrix and the surface carboxyl groups of the CNC, the thermal stability of the nanocomposite increased while the observation of the nanocomposite fractured surface indicated improved fracture toughness. The surface carboxyl groups can also be

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utilized for further surface modification of the CNC to enhance the compatibility with the matrix. For example, Benkaddour et al. [58] developed a grafted polycaprolactone (PCL) on the cellulose whiskers that were previously oxidized by TEMPO. The authors demonstrated that the surface carboxyl groups were first esterified with 10-undecyn-1-ol, which was then subsequently utilized to undergo “click” chemistry with azide functionalized PCL that resulted in formation of surface PCL-g-CNC nanoparticles.

4.3.2 Silylation and acetylation of cellulose nanocrystal surfaces The idea of sylilation of CNCs originated from the modification of cellulosic fibers with organosilane coupling agents that is a widely used method of surface functionalization of cellulose and wood fibers with prescribed functionality especially in polymer composite applications [5961]. In this method, the surfaces of the CNCs are covered with crosslinked polysiloxane layers with desired functionality on the surface of the silane layer chosen to be compatible with the polymer matrix to enhance the interfacial adhesion. Typically, alkyl-dimethyl chlorosilanes are used and the surface chemistry involves evolution of HCl and formation of SiOC bond between the siloxane layer and CNC surface. Gousse´ et al. [62] studied the role of the alkyl chain length on the structure of partially silylated CNCs ranging from isopropyl to n-butyl, n-octyl, and n-dodecyl moieties. The surface of the CNCs was characterized based on degree of substitution (or DS) of the silane. It was found that a DS value of 0.61 preserved the whisker morphology of the CNCs. However, DS values higher than 1 resulted in deformation and loss of the original whisker morphology. In addition, the partially silylated whiskers were found to be readily dispersible in organic solvents such as THF. In order to reduce the surface hydrophilicity and make the CNCs compatible with cellulose butyrate acetate matrix, Grunert et al. [21] carried out trimehtylsilylation of the CNC surface in formamide. Yu et al. [63] used 3-isocyanatopropyltriethoxy silane via the reaction of isocyanate groups of the silane coupling agents with CNC surface hydroxyls catalyzed by Sn(Oct)2 in anhydrous Dimethyl Formamide (DMF) and incorporated the modified CNCs in silicon elastomer. These surface modifications gave better dispersion of CNCs in the respective polymer matrices and significant improvement of mechanical properties of the final nanocomposite elastomers. In an alternative method, Raquez et al. [64] used an aqueous solution of methacryloxypropyltrimethoxysilane in a suspension of cellulose whiskers followed by the hydrolysis of the silane. The observed modified CNCs were recovered through centrifugation and vacuum drying or freezedrying after the adsorption step. The condensation of silane on CNC surface was achieved by curing the silane-adsorbed-CNC particles in a vacuum oven at 110 C. In addition, the authors showed that application of an excessive amount of silane (more than 200 mM) was not attainable due to the formation of a biphasic solution. This method was called a “green and sustainable” approach because it eliminates the evolution of HCl as in the case of chlorosilane agents. Surface acetylation of CNCs has mostly been achieved through utilization of various anhydride-based compounds. Yu et al. [65] prepared acetylated CNCs by

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reacting succinic anhydride with CNCs under pyridine reflux and the unreacted anhydride was removed by successive washing with water, acetone, and ethanol. These carboxylated CNCs were used for metal ion adsorption applications. In another simple approach [66], alkenyl succinic anhydride aqueous emulsion was mixed with CNC suspension and freeze-dried followed by heating at 105 C to produce highly hydrophobic CNC particles that were dispersible in low polarity solvents such as 1,4-dioxane. Sassi et al. [67] studied the role of the reaction medium on structure of CNCs during acetylation reaction. Using a nonswelling reaction mechanism where only surface cellulose chains are considered, they found that when acetylation is carried out under homogenous reaction conditions, the acetylated layers of the whisker are immediately released in the reaction medium after obtaining sufficient solubility. However, under heterogeneous reaction conditions, only surface cellulose chains are acetylated, resulting in formation of a layer of nonsoluble cellulose acetate that surrounds the highly crystalline core of unreacted cellulose.

4.3.3 Polymer grafting by surface-initiated polymerization Ring-opening polymerization (ROP) has been one of the major routes to grafting polymer chains from the surface of cellulose substrates due to the presence of surface hydroxyl groups that can act as polymerization initiation sites [68]. In an early study, Hafre´n et al. [69] used surface-initiated ROP to graft PCL onto cotton and filter paper surface where the reaction was catalyzed by organic and amino acids. Habibi et al. [20] used the same concept to graft PCL through surfaceinitiated ROP catalyzed by Sn(Oct)2 from the CNC surface. Their results suggested that the structure and morphology of the CNCs remained intact after the grafting reaction. The obtained modified CNC particles showed significantly improved dispersion and compatibility with a PCL matrix in a nanocomposite material. Carlsson et al. [70] studied the effect of ROP reaction time on surface properties and grafting density of the PCL on the CNC surface characteristics. Their results showed that the surface graft density was constant at 3%7% and independent on the ROP reaction time. In order to enhance the efficiency of the reaction, Lin et al. [71] used microwave-assisted surface-initiated ROP to graft PCL on CNC surface. The obtained modified CNC was melt mixed with PLA matrix to give a nanocomposite that showed enhanced interfacial compatibility with the hydrophobic matrix. In another approach to prepare hydrophobically modified CNCs, Morandi et al. [72] grafted polystyrene chains onto CNC surface via surface-initiated atom transfer radical polymerization (ATRP). This approach used a 2-bromoisobutyryl bromide as the ATRP initiating site on the CNC surface followed by polymerization. The brush chain length and grafting density were easily controlled by adjusting the reaction conditions. Using a similar approach, Zeinali et al. [73] prepared thermoresponsive CNC whiskers by grafting poly(N-isopropyl acrylamide) and pol(acrylic acid) through surface-initiated reversible-addition fragmentation transfer polymerization by attaching 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid as chain transfer agent followed by polymerization of acrylic monomers.

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4.3.4 Cellulose surface modification by electrostatic and physical adsorption This approach toward modification of CNC surface is based on utilization of physical forces such as electrostatic attraction or hydrogen bonding to adsorb various functional molecules or polymers on the CNC surface especially when the CNC is prepared by sulfuric acid hydrolysis that leaves the surface negatively charged. Application of surfactants has been shown to be a promising method of interfacial compatibilization of cellulosic fibers with polymer matrix [74,75] where the hydrophilic head of the surfactant interacts with the hydrophilic surface of cellulose, and the hydrophobic tail interacts with the matrix. For example, Hu et al. [76] used surfactants such as didecyldimethylammonium bromide and cetyltrimethylammonium bromide to modify the surface of CNC through electrostatic attraction of positively charged surfactant and negatively charged CNC surface. They found that the morphology of the surface layer on CNC surface was concentration dependent where low concentrations resulted in brush-like morphology with hydrophobic tails of the surfactant pointing outward thereby rendering the CNC surface highly hydrophobic. On the other hand, higher concentrations resulted in aggregation of surfactant on the CNC surface with a decrease in hydrophobic character. Salajkova´ et al. [77] modified the surface of the TEMPO-oxidized nanocellulose with various functional groups such as epoxide, benzyl, and acrylate groups through the use of the corresponding ammonium salt adsorbed on the surface through electrostatic force. This was demonstrated to be a promising method for composite applications because various chemical functionalities can be introduced on the CNC surface that, in turn, could potentially interact/react with the host matrix polymer. The use of nonionic surfactants such as sorbitan menstruate has also been used to disperse CNCs in hydrophobic matrix [78,79]. Another approach for modification of CNC surface is based on layer-by-layer (LbL) adsorption as described in the next section. In summary, a schematic representation of various CNC surface modification techniques is shown in Fig. 4.4 for clarity and easy access.

4.4

Processing and development of CNC-based hybrid polymer nanocomposites

In this section, a brief overview is given of the most commonly used approaches in preparation of thermoplastic and thermoset polymer nanocomposites based on CNCs.

4.4.1 Solvent casting Solvent casting is the most widely used technique for preparing CNC-based nanocomposites [80]. In a typical process, the CNCs are dispersed in the dispersing medium which is mostly aqueous dispersions although other solvents have been used. Once fully dispersed, the polymer is added to the dispersion, and the final

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Figure 4.4 Common surface modification chemistries of cellulose nanocrystals.

nanocomposite hybrid material is developed by removing the solvent. Aqueous dispersion is a highly favorable dispersion medium because the CNCs can be readily dispersed in water at nanoscale without aggregation. However, one limitation of this technique is the fact that only water-soluble/dispersible polymers or latex materials can be formulated. Paralikar et al. [81] prepared nanocomposite membranes of CNC incorporated into polyvinyl alcohol (PVOH) through solvent casting in water. The membranes had CNC concentration of 020 wt%. The process involved mixing of two separate master batches of a solution of PVOH and dispersion of CNC in water followed by a sonication of 25 min for breaking up any agglomerates formed during mixing. Because of the high solubility of PVOH and dispersibility of CNC in water, a synergistic effect of the CNC/PVOH interaction was observed as evidenced by highly dispersed CNCs in the PVOH membrane that resulted in enhanced physical and mechanical properties. Azizi Samir et al. [82] used waterbased casting technique to prepare poly(oxyethylene) (POE) nanocomposite reinforced with CNCs. In their method, CNCs were dispersed in the POE solution and mixed for 24 h followed by drying at 40 C for a week and at 100 C for 72 h. In addition to water, a number of polar organic solvents have also been successfully used as dispersing medium for preparation of CNC-based nanocomposites such as DMF [83], dimethyl sulfoxide (DMSO), and N-methyl 2-pyrrolidone (NMP) [84].

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Figure 4.5 Dispersion of sulfuric acid hydrolyzed CNC through cross polarizers from left to right: as prepared in water, freeze-dried, and dispersed in water, DMF, DMSO, NMR, formic acid, and m-cresol. Reproduced with permission from Ref. [84]. Copyright (2007) American Chemical Society.

In a recent study by Van den Berg et al. [84], the dispersibility of the cellulose whiskers in various organic solvents based on the surface charge of the whiskers was studied. It was found that the presence of negative surface charges (sulfate groups) obtained from sulfuric acid hydrolysis as already described is necessary for successful dispersion of whiskers in polar solvents such as DMSO, DMF, and NMP as shown in Fig. 4.5. However, protic solvents such as the formic acid and m-cresol are able to disperse even the CNC whiskers with neutral surfaces due to their ability to disrupt the intraparticle hydrogen-bonded network structure. Marcovich et al. [85] used DMF as suspending agent to obtain a stable suspension of CNCs using ultrasonic treatment. This stable suspension was then added to a mixture of polyol-isocyanate to obtain CNC reinforced polyurethane films. Using a similar approach, Liu et al. [86] fabricated PMMA nanocomposite reinforced with up to 10 wt% CNCs. This facile preparation method involves mixing of a stable suspension of CNCs in DMF with a solution of hydrophobic PMMA in DMF and drying the resulting mixture to give cast solid composite films with enhanced benefits. Solvent exchange process is another method of transferring cellulose whisker dispersions to organic solvents from aqueous dispersion. The advantage of this technique is the fact that a percolating network structure of CNC whiskers can be obtained in the aqueous solutions that can be directly transferred to organic solvent in the form of “organo-gels.” This method is known as the template approach developed by Capadona et al. [87,88]. In this method, a dispersion of CNC in water is solvent exchanged with acetone over a period of a week through formation of aqueousorganic bilayer mixture. The solvent exchange results in development of an organo-gel of CNCs in acetone. Nanocomposites of ethylene-oxide/epichlorohydrin copolymer were prepared by the template approach by placing the CNC-inacetone organo-gel in the copolymer solution followed by compression molding and drying. The obtained nanocomposites developed by this novel approach had mechanical properties comparable with that of samples that were directly solutioncast in DMF. Wang et al. [89] used the same organo-gel template approach to prepare poly(propylene-carbonate) green nanocomposites reinforced with CNCs. The reported morphological observations showed a submicron scale dispersion of

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the CNCs within the matrix polymer which was further confirmed by enhancement of mechanical properties. The solvent exchange process has also been used to transfer CNCs from aqueous solutions to highly water-immiscible solvents such as toluene [90] in order to prepare atactic PP/CNC nanocomposites. By contrast, severe aggregation of CNCs was observed in toluene. Other approaches such as surface modification of CNCs with more hydrophobic functionalities [91] or long chain hydrophobic moieties [92] have also been used in order to disperse the CNCs in water-immiscible solvents. Note, however, that although a good dispersion of particles may be achieved, the interaction of particles through hydrogen bonding and formation of interconnected network structure would be severely limited.

4.4.2 Melt processing Although solvent casting process is effective in achieving fine dispersion of CNCs in polymer matrices that is a necessary requirement of effective property enhancement of the host polymer matrix, it is both lengthy and noneconomical approach from the practical application point of view because the plastic industry is more interested in solvent-free “green” processing methods with significantly shorter cycle times. In this context, melt extrusion is the most widely used polymer processing technique in industry for fabrication of composites and nanocomposites [93]. However, there is a challenge of obtaining a well-dispersed morphology of CNC in the polymer matrix during extrusion because CNC tends to severely aggregate when blended with hydrophobic thermoplastics. A significant number of studies have been reported in the literature for the preparation of polymer/CNC nanocomposites using extrusion process such as polyethylene [94,95], polypropylene [96,97], polystyrene [98], polylactic acid [99,100], polyvinyl chloride [101] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [102], PCL [103], and thermoplastic starch [104]. Typically, a surface modifier such as a graft polymer layer or surfactant or simply a compatibilizer is used in order to enhance the CNC dispersion and compatibility with the matrix. For example, Bondeson et al. [105] prepared a suspension of the cellulose whiskers with PVOH and introduced the mixture in the PLA matrix with extrusion process using dry and liquid feeding techniques. It was found that although liquid feeding produced a better dispersed morphology of the CNCs in the PLA, the majority of the particles were located inside a discontinuous phase of PVOH within the continuous PLA matrix phase. Direct pumping of an aqueous solution of CNCs into the PLA matrix in the extrusion process did not show any improvement in the structural properties of the nanocomposites due to poor dispersion of CNCs in the matrix [106]. Application of 5 wt% anionic surfactant [99] was found to improve the dispersion of CNCs in the PLA matrix during extrusion with improved mechanical properties. However, higher surfactant content was found to degrade the PLA matrix. In another approach, Goffin et al. [107] grafted PLA chains on the CNC surface via surfaceinitiated ROP and melt extruded the modified CNC particles with PLA matrix. The results showed enhanced compatibility between the obtained PLA-g-CNC and PLA matrix indicated by improvement of mechanical properties and promotion of crystallization nucleation in the matrix. A similar strategy [20] was used to graft PCL

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on the CNC surface by ROP and reported that gave significantly improved dispersion as a consequence of the PCL graft layer due to enhanced interfacial compatibility. In the case of polyethylene matrix, de Menezes et al. [94] grafted fatty acid chains on the CNC surface and melt extruded the modified particles with low-density polyethylene matrix. The results showed that increasing the fatty acid chain length enhanced the dispersion of the CNCs in the LDPE matrix. Ben Azouz et al. [95] prepared a dispersion of CNCs in high molecular weight polyethylene oxide (PEO) solution to wrap the CNC surface with PEO layers. Subsequently, they freeze-dried the mixture and used in the desired product in an extrusion process to incorporate the modified CNCs in the Polyethylene (PE) matrix. As shown in Fig. 4.6, the extrusion of PEO-modified CNC with PE resulted in no significant degradation of particles while direct melt mixing of CNC with PE resulted in severe discoloration. This is thought to be a promising approach in order to effectively disperse the CNCs in the hydrophobic matrix while preserving their thermal stability during melt processing. Interestingly, similar approach [102] used polyethylene glycol (PEG) instead as the surface wrapping agent for CNCs in PHBV matrix during the high shear extrusion process, and the obtained results showed that the PEG layer was completely removed from the CNC surface and mixed with the PHBV matrix, leaving the cellulose whiskers aggregated in the matrix. Consequently, therefore, the molecular weight of the wrapping agent was found to be the determining success factor in this method.

4.4.3 In-situ polymerization In-situ polymerization is a versatile approach in the preparation of nanocomposite materials with possibilities of obtaining highly desirable morphologies and tunable

Figure 4.6 Melt extruded PE nanocomposite reinforced with unmodified and PEO-modified CNC and the subsequent color change during processing. Reproduced with permission from Ref. [95]. Copyright (2011) American Chemical Society.

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functionalities of hybrid materials [108]. In this method, the nanoparticles are dispersed or mixed with the monomer (in liquid form) followed by polymerization of monomer in presence of nanoparticles through various polymerization mechanisms [108]. In the case of CNC nanocomposites, this approach has been utilized for both thermoset and thermoplastic matrix materials. Liu et al. [109] prepared phenolic thermoset polymers reinforced with cellulose whiskers. Phenol-formaldehyde (PF) resin was first mixed with an aqueous dispersion of CNC which as then solvent exchanged with DMF through three successive cycles. The solution was then treated with a stepwise curing profile to obtain fully cured PF/CNC nanocomposite films. Tang et al. [110] prepared epoxy-based nanocomposites reinforced with cellulose whiskers. The preparation involved mixing a dispersion of CNCs in DMF with DGEBA monomer and toluenediamine-based curing agent followed by casting and curing. No phase separation occurred during curing and the CNC whiskers were found to be evenly dispersed within the epoxy matrix. Polyurethane-based nanocomposites with CNCs have also been prepared with in-situ polymerization approach [14,111113]. Li et al. [111] prepared PU nanocomposite foams with CNCs using sucrose-based polyol, a polymeric diphenylmethane diisocyanate, and a glycerol-based polyol. Upon polymerization, development of extra hydrogen bonds and additional crosslinks between the PU and hydroxyl groups of the CNC surface was confirmed, resulting in significant enhancement of properties at low CNC volume fractions. Strong reinforcement effect of CNCs was observed in elastomeric PU nanocomposites [112] which was attributed to the increasing number of crosslink density due to the interfacial bond formation between the CNC surface and PU hard-microphase domains. Furfuryl alcohol (FA) has been in-situ polymerized in presence of cellulose whiskers [114,115] to fabricate fully bio-based nanocomposite materials where the FA acted both as the dispersant of the CNC as well as the polymerization precursor. It was found that the residual sulfonic acid groups on the surface of hydrolyzed cellulose whiskers catalyze the ROP of FA. In our previously reported study [116], we demonstrated the development of polyamide 6 nanocomposites reinforced with CNCs via in-situ ROP of ε-caprolactam. Nanocomposites containing up to 2 wt% of CNC in PA6 were prepared and found that the increasing the CNC content resulted in less monomer conversion due to anionic polymerization inhibition effect of the CNC particles. Analysis of the properties and morphology of the PA6/CNC nanocomposites revealed the formation of an interconnected CNC fibrillar structure that significantly changed the rheological behavior of the PA6 matrix and the creep resistance of the matrix was significantly increased.

4.4.4 Layer-by-layer assembly LbL assembly has been shown to be a promising method of fabrication of nanocomposite thin films especially in biomedical application where high loading of nanoparticles such as carbon nanotubes in thin films has been achieved [117119]. One of the early reports of preparation of composite thin films by LbL method was by Podsiadlo et al. [117] who successfully prepared a multilayered structure of

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negatively charged sulfated CNCs with a poly-(dimethyldiallylamonium chloride) (PDDA) polycation on a glass substrate through multistep dipping in CNC and PDDA solutions. Mesquita et al. [120] developed bio-based highly deacetylated chitosanCNC multilayered composite films through LbL assembly. The driving force for development of the layered film is reported to be the electrostatic attraction between the negative surface charge of CNC and positive charge of chitosan as well as the strong hydrogen bonding among layers. Morphological observations on the samples revealed a 7-nm thickness for each bilayer. In another study [121], high aspect ratio CNCs were assembled into “flattened matchstick pile” structures for antireflective coating applications. At optimum assembly conditions, the authors reported a 100% light transmittance through the film. Thin films of polyelectrolyte/CNC nanocomposites [122] were prepared using solution-dipping and spin-casting methods with improved optical properties. It was reported that the spin-coated films were much thicker than the films prepared by solution-dipping method. In addition, the former system showed thin film interference colors and optical properties that were easily tunable through processing method. In addition to the reported studies on development of bio-based thin film composites by LbL assembly approach with superior gas barrier properties [123,124], it is worthy to note that these novel thin films have great potential, when combined with biodegradable and biocompatible polymers such as collagen [125], to be applied as extracellular matrix with significant potential in biomedical applications.

4.5

Properties of polymer/cellulose nanocrystals nanocomposites

4.5.1 Mechanical properties CNCs are attractive potential reinforcing additives for polymer matrices due to their exceptionally high mechanical properties and relatively lower density compared to most conventional nanoreinforcing agents that translates into nanocomposites with relatively lighter weight [126]. This is evidenced by considering the longitudinal modulus of CNCs to be in the range of 100170 GPa with an average value of 130 GPa which is almost equivalent to that of aramid fibers [3]. Depending on the source of CNC extraction, various longitudinal modulus values have been reported (e.g., 105 GPa for the CNCs from cotton and 143 GPA for CNCs from tunicate) [127,128]. Transverse elastic moduli of cellulose whiskers were investigated by Lahiji et al. [129] at 30% and 0.1% relative humidity by Atomic Force Microscopy (AFM). They measured a transverse elastic modulus (ET) value of 1850 GPa for wood-derived CNC whiskers and the effective stiffness in higher relative humidity condition was found to be slightly higher. Wagner et al. [130] used AFM forcedisplacement measurements to estimate the transverse modulus of CNC whiskers derived from tunicate and found to range from 2 to 37 GPa. This large variation in measurement of ET by AFM was attributed to the uncertainties related with the sensitivity of AFM tip rather than the property variation.

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Cao et al. [131] reported a significant mechanical property enhancement by CNCs in thermoplastic starch with tensile strength increasing from 3.9 to 11.5 MPa and elastic modulus increasing from 31.9 to 823.9 MPa, respectively, with increasing CNC content up to 30 wt%. In a study [132] on electro-spun PLA mats, addition of up to 5 wt% of CNCs resulted in 5- and 22-fold increase in tensile strength and modulus compared to that of neat PLA fiber mats, respectively. Biodegradable poly (butylene succinate) (PBS) foams reinforced by CNCs was developed by Lin et al. [133] and incorporation of 5 wt% of CNC in the foam resulted in 50% and 62.9% improvement in flexural strength and modulus, respectively, compared to neat PBS foams. The reinforcing capability of the CNCs has been demonstrated in a number of other studies on thermoplastic polymers such as PVA [134], PLA [135], PMMA [136], poly(vinylidene-fluoride) [137], poly(3-hydroxybutyrate) [138]. Engineering thermoplastics such as polyamide 6 have also been reinforced with CNCs [139,140]. In addition, CNCs have been used to mechanically reinforce elastomeric polymers as well. Biocompatible waterborne polyurethane matrix was reinforced with small volume fraction of CNCs (1 wt%) that increased the tensile strength and Young’s modulus from 5.43 to 12.22 MPa and from 1.16 to 4.83 MPa, respectively [141]. The reinforcing efficiency of CNCs has also been demonstrated in natural rubber nanocomposites [142]. However, the addition of CNCs has been shown to decrease the elongation at break in the nanocomposite material compared to the host polymer matrix due to the stiffening effect of CNCs in the polymer [20,90]. Furthermore, the mechanical property enhancement has also been studied in thermoset polymers. Pan et al. [143] studied the reinforcing ability of cellulose nanocrystals in an epoxy-acrylate UV-curable transparent film. The DMA results indicated a significant enhancement of modulus above the glass transition temperature (i.e., rubbery state) with increasing CNC loading. The reinforcing effect of CNCs in a phenolic thermoset resin [109] was adequately described by HalpinKardos model suggesting the domination of matrixfiller interaction over fillerfiller interaction. The studies of the effects of CNC on mechanical properties of unsaturated polyester resin were studied [144] where it was found that surface modification of CNC with organosilane coupling agents resulted in improvement of strength and stiffness of the of the polyester resin whereas no significant changes were observed on the impact energy of the polyester nanocomposites after the surface treatment. There are a number of important characteristics of the cellulose whiskers that play a critical role in its mechanical reinforcement efficiency in polymer composites. Cellulose whiskers, owing to their nanoscale dimensions, have significantly large specific surface area which has been reported to be in the range of 100 m2 g21 to several hundreds m2 g21 [145,146]. This large available surface could in fact be exploited to enhance the interfacial interaction in CNC/polymer nanocomposites through favorable interfacial interactions such as hydrogen bonding [147]. Moreover, the available surface interfacial area between the CNC whiskers and the host polymer is governed by the state of dispersion of CNCs in the matrix as well as the effect aspect ratio of the whiskers. Good CNC dispersion in the polymer at the molecular level and its relatively large aspect ratio (i.e., ratio of length to diameter) will increase the interfacial area between the particles and the polymer.

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The increased interfacial area will facilitate the stress transfer under mechanical load from the neighboring polymer chains to relatively very stiff and strong whiskers [148,149]. On the other hand, the highly hydrophilic nature of CNCs may lead to severe aggregation of whiskers in the polymer matrix which, in turn, reduces the available surface area, the effective aspect ratio, and the reinforcing potential of the whiskers. Rusli et al. [31] studied the role of CNC surface charge and aspect ratio and the corresponding microstructural properties on stress transfer efficiency from matrix to whiskers in an epoxy matrix by Raman spectroscopy. Their results demonstrated that the tunicate-derived cellulose whiskers had significantly higher stress transfer capability due to their exceptionally large aspect ratios compared to that of the cotton-derived whiskers. In addition, the sulfuric acid hydrolyzed samples had better dispersion throughout the matrix while the hydrochloric acid hydrolyzed whiskers showed negligible stress transfer capacity due to their excessive aggregation as a result of surface neutrality that, in turn, reduced their effective aspect ratio in aggregated state. In addition, the effect of interfacial compatibility between the matrix and CNC particles on mechanical properties was demonstrated by Goffin et al. [103] who used unmodified and PLA-g-CNC particles (by surface-initiated ROP of lactide) to reinforce PLA matrix. The reinforcing efficiency of the interfacial compatibility was shown by enhanced stiffness of the matrix above the Tg in the case of grafted CNCs. The strong mechanical reinforcing ability of cellulose whiskers has spurred a number of researchers to use micromechanical models to theoretically describe the mechanical properties of polymer/CNC nanocomposites. However, it was found that the use of conventional short fiber composites models such as HalpinTsai equations failed in a number of polymer/CNC nanocomposites systems reported in the literature by underestimating the reinforcing ability of CNCs [150]. This was attributed to the formation of a percolating rigid network structure of CNC whiskers that are strongly bonded together through hydrogen bonding that is not accounted for in the theoretical equations. To better understand the mechanical reinforcing property of the CNC/ polymer nanocomposites with a percolated structure, the series-parallel model of Takayanagi et al. [151] (as modified by Ouali et al. [152]) can be applied to predict the elastic shear modulus of the composite according to the following equation:  0 0 1 2 2ψ 1 ψχr G r G s 1 ð1 2 χr ÞψG0r 2  Gc 5 χr 2 ψ G0 s 1 ð1 2 χr ÞG0 r 0

(4.1)

where the subscripts r and s refer to rigid (whisker) and soft (polymer) phases and ψ is related with the volume fraction of the percolating phase (in this case the whiskers). The ψ parameter can be obtained using the following equations: ψ 5 0 for χr , χc ψ 5 χr

  χr 2χc b for χr , χc 12χc

(4.2) (4.3)

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In these equations, b 5 0.4 for a three-dimensional percolating network, χc is the critical volume fraction at which the percolation begins. According to Favier et al. [50], this parameter can be estimated using the equation χc 5 0.7d/l, where d and l are the diameter and length of the whiskers. χr is the volume fraction of the rigid whiskers phase. At sufficiently high temperatures, the stiffness of the matrix can be assumed to be zero and the equation is then simplified to G0 c 5 ψG0 r. It has also been explained that the good agreement between the experimental data and the values predicted by the model just described that takes the percolating phase structure into account is ascribed to the formation of infinite agglomerations of cellulose whiskers. It is also worth noting that the percolation threshold and structure formation of CNC network structure in the matrix can be controlled by the interfacial interaction, compatibility with the matrix the dispersion quality, and the original aspect ratio of the fibers. The larger the effective in-situ aspect ratio of the CNCs in the matrix, the lower the volume fraction at which a percolating network structure of the CNCs is formed.

4.5.2 Thermal properties The thermal properties of CNC/polymer nanocomposites such as the thermal stability, glass transition temperature, melting, and crystallization behavior are important variables in development of the novel functional nanocomposites. Roman et al. [153] studied the effect of sulfate groups on the CNC surface on the thermal stability of sulfuric acid hydrolyzed bacterial cellulose whiskers through controlling the sulfuric acid hydrolysis conditions. They reported a significant decrease in thermal stability of the CNC as the sulfate groups increased on the surface. It was also reported that the sulfate groups increase the char fraction after thermal degradation, implying the additional role of the CNCs as flame retardants. Note that the presence of sulfate groups catalyzed the degradation processes as indicated by relatively lower thermal degradation activation energy in CNCs with high sulfate group’s concentrations. Similar observations were made with thermal stability of the CNCs prepared by enzymatic process which was found to be higher than those of sulfuric acid hydrolyzed [10]. Incorporation of bacterial grown CNCs into PVA matrix resulted in significant thermal stability improvement from the onset temperature of thermal degradation of 184 C in the case of sulfuric acid hydrolyzed CNC in PVA to 378 C in the case of the bacterial grown CNC in PVA matrix. The presence of CNCs in a PMMA matrix [86], however, showed only slight reduction in the onset temperature of thermal degradation with the addition of CNCs. It is clear that a strong interfacial interaction or chemical bond facilitates enhancement of thermal stability in CNC-based nanocomposite. In such cases, the presence of CNC in the matrix is advantageous for thermal stability of the host polymer matrix by increasing the energy required for the onset of polymer decomposition and associated reduction in thermal expansion [154,155]. Considering the effect of CNCs on the glass transition temperature of host polymers, a great number of studies have reported no obvious change in systems including but not limited to poly(styrene-co-butyl acrylate) [156], PVC [157], PP [90],

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POE [82], and natural rubber [158] nanocomposites. On other hand, polymers that form strong interfacial interaction with the CNC surface through hydrogen bonding have typically increased Tg with incorporation of CNCs. For example, in our previously reported study [116], we demonstrated that the Tg of the amorphous portion of polyamide 6 increased slightly with a small volume fraction of CNCs. In addition, a similar effect has been observed in strong hydrogen bond-forming matrices such as PVOH [10] and glycerol plasticized starch [159] where the strong interfacial interaction severely inhibits the molecular motion and viscous flow of the polymer chains, resulting in an increasing trend of glass transition temperature with addition of CNCs. The melting temperature of semicrystalline polymers has also been affected by CNCs in various ways to an extent that depends on the surface chemistry and interfacial interaction of polymer with CNC surface. For example, in a number of systems such as poly(ethylene oxide) [82], cellulose acetate butyrate [21], plasticized starch [160], and PCL-based nanocomposite [161], no significant change in melting behavior of the polymer was observed. However, surface-modified CNCs have been shown to promote the crystallization of some particular polymer systems. For example, in the cellulose acetate butyrate system, trimethylsilylation of CNC surface resulted in enhanced crystallization and increased melting temperature of the matrix that was attributed to the promotion of polymerCNC interfacial interaction by surface modification. In another study [113], a completely amorphous polyurethane elastomer was found to transform into a thermoplastic-like material with partial crystallinity when CNCs with a surface layer of grafted polyurethane prepolymer were incorporated in the matrix. Crystallization behavior of the CNC nanocomposites is also affected by the presence of the CNCs depending on their surface chemistry, dispersion quality, and microstructure development within the host polymer matrix. Pei et al. [162] studied the role of surface modification of CNCs with silane agent on crystallization behavior of PLA and found that the modified CNC particles were very well dispersed in the PLA matrix and enhanced the crystallization kinetics by acting as effective nucleating agents. However, no such effect was found for nonmodified CNCs because they formed highly aggregated structures in the matrix. Gray et al. [163] reported strong nucleating effect of CNC in PP matrix as evidenced by the development of a transcrystalline layer on the cellulose surface as shown in Fig. 4.7. Han et al. [164] studied the role of cellulose whiskers in polyurethane matrix during isothermal crystallization by using Avrami model and found that the CNCs act as nucleating agents during isothermal crystallization.

4.5.3 Melt rheological properties Rheology is a powerful tool to study the effect of nanoparticle additives and their interaction with matrix on viscoelastic and microstructural properties of the nanocomposites. In addition, rheological data provide helpful insights into the behavior of the nanocomposite and hybrid materials during the melt processing stage. As in the case of nanoparticulate-filled polymer composites, rheological properties can provide fundamental understanding of the structureproperty relationships in

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Figure 4.7 Development of transcrystalline layer iso-PP crystal on the CNC film surface. Reproduced from Ref. [163] with permission from Springer.

CNC-based nanocomposite materials. Typically, this is accomplished through observation of changes in viscoelastic variables such as the storage and loss modulus (G0 and Gv), complex viscosity (η ), and tan δ. In a series of studies on rheological behavior of CNC suspensions [165167], it was observed that the suspension develops strong elasticity at high concentrations where the behavior resembles that of an elastic gel. Temperature sweep experiments indicated a structural rearrangement between 30 C and 40 C, where G0 initially increases and then decreases at higher temperatures. The structure formation was also confirmed in a sonicated CNC suspension by observation of the failure of CoxMerz rule [168]. Rheological analysis of nanocomposites of PHBV reinforced with CNCs [169] revealed that the most rapid changes in the G0 and Gv occurred in the concentration range of 0.52 wt% of CNC while the viscoelastic transition crossover point occurred at 1.2 wt% of CNC above which the nanocomposite melt behaved like an elastic gel. In our previously reported study [116] on polyamide 6 nanocomposites, we demonstrated the formation of CNC network structure within the PA6 matrix. It was shown that the formed structure could be broken apart by application of 10 s21 shear rate and reformed upon removal of shear. In fact, the failure of the CoxMerz rule for the samples with the highest concentration of CNC studied (i.e., 2 wt%) confirmed the structure formation. Increasing the CNC concentration from 0.6 to 2 wt% was also associated with a decrease in the slope of the terminal zone of the plot of G0 and Gv versus frequency that was attributed to enhanced elasticity of the melt upon increasing the CNC concentration. In a study on polyurethane nanocomposites reinforced with CNCs [85], the onset of percolation was found to be 1 wt% of the CNC. Above this concentration, a network of H-bonded whiskers formed throughout the matrix. This network was easily destructible by shear in the nonlinear viscoelastic zone. Mahi et al. [170] studied

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the linear and nonlinear rheological properties of ethylene vinyl acetate copolymer nanocomposite reinforced with CNCs. Small amplitude oscillatory shear experiments showed a significant enhancement of melt elasticity as shown in Fig. 4.8A and B by development of a nonterminal behavior in storage and loss modulus versus frequency which is indicative of transition to pseudosolid like behavior. The onset of percolation and network formation significantly affects the long-range motion of polymer chains in the low-frequency region and prevents the polymer relaxation as in the case of the neat polymer melt. The transient shear experiments did not show an overshoot in low shear region while an overshoot developed at high shear rates. This observation was attributed to the nanonetwork break up and orientation in the flow direction as schematically depicted in Fig. 4.8C and D. A similar increase of storage and loss modulus was also observed in various CNC-based nanocomposite systems such as polyurethane [171], poly(ethylene-glycol) [172], nitrile rubber [173], POE [95], and poly(vinyl acetate) [174]. The complex viscosity (η ) of the nanocomposites is also affected by the presence of nanocrystals in the polymer melt. Normally, addition of stiff rigid nanoparticles with high surface area results in increase of the complex viscosity as a result of molecular motion restriction imposed by the nanoparticles [175]. This effect is dependent on the state of dispersion of particles in the matrix. For example, in a study on PLA nanocomposites reinforced with CNC, it was found that for up to 2.5 wt% of CNC, the complex viscosity increased. However, further addition of CNC resulted in decrease in complex viscosity. This effect was attributed to severe agglomeration of CNCs at higher concentrations that resulted in relatively lower molecular entanglement density which, in turn, reduced the viscosity of the matrix.

Figure 4.8 Variation of (A) storage and (B) loss modulus of EVA melt with CNC concentration. Shear-induced orientation of (C) polymer chains and (D) CNC nanoparticles. Reproduced from Ref. [170] with permission from Springer.

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The complex viscosity CNC/polymer nanocomposites show strong dependence on frequency and the nanocomposites exhibit shear-thinning behavior. While the neat polymer matrix shows no dependence of complex viscosity on angular frequency at low-frequency values as evidenced by and a plateau in the complex viscosity versus frequency plot, addition of CNC was shown to significantly promote non-Newtonian behavior and increase of viscosity at low-frequency ranges which is related with the nanostructure signature [176]. In addition, surface chemistry and interfacial compatibility with the matrix play a major role in rheological behavior of CNC nanocomposites. Application of polyaniline-g-CNC was found to develop non-Newtonian shear-thinning behavior in PU matrix [177] at low concentrations unlike the nonmodified CNC. It was suggested that the liquidsolid transition is significantly promoted through the compatibility of the surface PANI layer with the PU matrix. Goffin et al. [103] studied the role of PCL-g-CNC in a PCL matrix through rheological characterization. It was seen that above 8% of the modified CNC, a network-like structure formed within the matrix that enhanced the elasticity and solid-like behavior of the matrix. This finding is ascribed to the coentanglement of the surface-grafted PCL layer with the PCL matrix which promoted the stress transfer to particles and imposed significant molecular motion restriction on the matrix.

4.5.4 Gas barrier properties There has been an increasing demand in plastic packaging industry for ecofriendly alternatives of petroleum-based polymers for use in packaging materials. The major requirement in this application is high gas barrier property of the film. The application of CNCs in gas barrier membranes has been reported in a number of research papers in the literature to improve the gas barrier property of cellulose-based thin films. In one of the early studies on PVOH membranes reinforced with CNCs [81], it was observed that the CNC effectively reduced the gas permeation flux while it allowed the moisture to pass through. Surface modification of CNC by acetylation resulted in better dispersion in PVA matrix and significantly improved the barrier properties. Fortunati et al. [178] dispersed CNCs in a PLA matrix with the aid of surfactant and observed 34% reduction in water permeability with only 1% of surfactant-modified CNC. They also reported good oxygen barrier property for 1 and 5 wt% of unmodified and modified CNCs dispersed in the PLA matrix. Khan et al. [123] 27% decrease in water vapor permeability in a chitosan/CNC biodegradable films containing 5 wt% of CNC. Li et al. [124] developed chitosan/CNC multilayered thin films on PET substrate by LbL assembly technique and observed surprisingly low oxygen permeability value of 0.043 cm3 μm m22 24 h21 kPa21, which is close to that of EVOH copolymers, under dry conditions. In other similar studies, promising reduction in water vapor transmission rate for carboxymethyl cellulose [179] and plasticized starch films [180] by addition of CNCs were reported. In a study on PCL nanocomposites reinforced with CNCs [181], it was reported that the reduction in rate of water vapor permeation is due to the reduction in gas diffusion because of relatively longer tortuous travel path of the water molecules through the rod-shaped cellulose whiskers. It should however be mentioned that the development of the CNC network is likely to be a most important

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requirement for superior barrier property as the comparison study of microfibrillated cellulose (MFC) and cellulose whiskers for their barrier property [182] showed that the MFC had better barrier property due to the network-like structure and entanglement of cellulose filaments, while CNC had much higher porosity to allow the gas molecule passage.

4.6

Conclusion and future perspective

The plethora of research advancements and scientific discoveries over the past decade in the investigation of physics and chemistry of CNCs reveal a number of potential uses of green hybrid polymer/CNC nanocomposites with enhanced benefits. This perspective is strongly motivated by research efforts in the scientific and industrial communities to develop sustainable materials from biorenewable resources to address increasing environmental legislations and concerns with conventional petroleum-based products. One of the challenges in the development of polymer/CNC nanocomposites materials is the problem associated with large scale industrial production. This drawback is being addressed by recent major research and development initiatives such as the establishment of CNC production pilot plants in the US Forest Product Laboratory that is capable of producing large quantities of CNCs as a first step toward commercialization of CNC production facilities. From a scientific research perspective, there are two major future directions to be explored. One area is the application and utilization of CNCs as functional additives for polymer nanocomposites as composite reinforcements, rheological modifiers, and gas barrier additives. The second area is the development of reliable strategies to improve the dispersion of the CNCs in polymer matrices using scalable processing methods such as extrusion and injection molding to afford novel functional materials and devices using CNCs in the field of polymeric electronics like recyclable and reusable solar cells [183], green low-cost nanopaper flexible electronic substrates [184,185], and biomedical applications such as drug delivery [186], injectable scaffolds and tissue engineering [187], and other pharmaceutical applications [188]. It is hoped that this chapter will provide a basis for further development of functional polymer/CNC nanocomposites for a number of uses in existing and new applications.

Acknowledgments This work was supported by the US National Science Foundation Division of Civil, Mechanical, and Manufacturing Innovation through CMMI-1161292 grant award and Office of International and Integrative Activities through IIA-1346898. The research work of J.U.O’s former graduate students and postdocs and funding by the US Department of State, the French Ministry of Higher Education and Research, and the Franco-American Commission of his Fulbright-Tocqueville Distinguished Chair award in Engineering at the University of Lyon 1 are gratefully acknowledged. We are indebted to our collaborators, with whom we had the privilege of working on projects cited in this chapter.

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Tailoring the interfaces in conducting polymer composites by controlled polymerization

5

Gergely F. Samu1,2 and Csaba Jana´ky1,2 1 Department of Physical Chemistry and Materials Science, University of Szeged, Szeged, Hungary, 2MTA-SZTE “Lendu¨let” Photoelectrochemistry Research Group, Szeged, Hungary

Chapter Outline 5.1 General considerations 101 5.2 Classification of synthetic procedures 102 5.3 In-situ chemical methods 103 5.3.1 Chemical formation of CPs in the presence of inorganic nanoparticles 5.3.2 One-pot synthesis 108

5.4 Electrochemical approaches

103

109

5.4.1 Nanoparticle-based hybrids 111 5.4.2 Nanostructured systems 112

5.5 Photo-assisted methods 121 5.6 Concluding remarks, outlook 125 References 127

5.1

General considerations

Hybrid materials based on organic conducting polymers (CPs) and inorganic materials have been in the focus of interest for decades. The synthesis of these hybrids has been mostly driven by the exploitation of complementary properties of the components, but in special cases, hybridization led to the rise of new functionalities. Great efforts are devoted to developing new synthesis tools to prepare such complex systems, understanding their working principles, and on expanding the field of applications. These span through optoelectronic devices (e.g., photovoltaic cells [1,2]), sensors [3,4], charge storage devices [5,6], and thermoelectric cells [7,8]. CPs constitute a special class of macromolecules, which (in their oxidized state) possess high electrical conductivity, hence their name synthetic metals. CPs can form hybrids with a wealth of inorganic materials (e.g., metals, semiconducting Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00005-7 Copyright © 2017 Elsevier Ltd. All rights reserved.

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oxides and chalcogenides, carbon nanotubes (CNTs), and graphene). These inorganic materials often have entirely different chemical and physical properties, which greatly limits the applicable synthesis tools and conditions to realize the organic/inorganic hybrids. Accordingly, careful attention has to be devoted to the choice of synthesis procedures because the performance of the derived hybrid material strongly depends on the quality of the interfaces between its constituents. The specific requirements toward the materials properties are usually application-dependent and customized synthesis procedures are necessary to realize them. In terms of optimizing the organic/inorganic interface, both the wetting and adhesion properties of the components should be taken into consideration, together with their electrical and optical properties. This picture is further complicated at the nanoscale because several electronic phenomena are localized in this finite region (such as charge injection and charge carrier recombination). The efficiency of these processes can only be maximized by ensuring an intimate contact between the inorganic and organic components. Consequently, there is a strong need for advanced synthetic procedures, which form the basis of this chapter.

5.2

Classification of synthetic procedures

Several different preparation strategies can be employed to prepare organic/inorganic nanocomposites. Each of these techniques has its own merits and drawbacks [9]. In terms of the synthesis methodology, three major groups can be distinguished (Fig. 5.1). During the sequestered approach, both the inorganic and organic constituents are prepared separately and combined later in a separate step of the synthesis. The main benefit of this method is that each component is prepared without the

Figure 5.1 Updated overview of the synthesis techniques of organic/inorganic composites [9]. Adapted with permission from Elsevier. Copyright 2015.

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interference of the other one; thus, a multitude of well-established synthesis tools can be applied. The composite formation step can be performed by simple ex-situ physical methods, such as mechanical mixing of the components, solution blending, spin coating, layer by layer assembly, and polymer grafting. In these cases, however, the control over the interfacial properties is rather limited and often, the presence of binding agents is necessary for electrochemical applications to ensure mechanical stability. Further complications may arise in the case of nanostructured systems, where simple filtration techniques are incapable of completely filling the host inorganic matrix [10], unfavorably affecting the performance of the resulting composite material. In spite of these shortcomings, currently, these methods are favored for large-scale production because they involve steps which are easily and cost-effectively implementable into existing industrial technologies. In the case of sequential methods, one of the components (either the inorganic or the organic component) is in situ generated in the presence of the other one. In this fashion, hybrids based on different nanostructures (e.g., nanoparticles, nanorods, and nanotubes) or thin films can be realized. However, distinctly different synthesis protocols should be followed in the case of suspended nanoparticles (distinct and separated entities) and interconnected nanostructured surfaces. In the case of CPs, the possibilities are not limited to chemical polymerization. Due to the intrinsic conducting property of CPs, the formed polymer layer remains conductive during polymerization. This conductivity enables the use of various electrochemical methods as synthesis tools, where supreme control over several key properties (e.g., overall and phase composition, morphology, and thickness) of the formed composites can be achieved. Electrodeposition techniques can be further enhanced by different photo-assisted methods, where the semiconducting nature of these materials is exploited during the composite formation. In concurrent synthesis techniques, both components are generated in situ, in the presence of each other. This can be considered as the least controlled synthesis technique. For such composite formation techniques, both chemical (one-pot) and electrochemical (codeposition) routes are possible. The focus of this chapter is on sequential methods, where the interfacial properties of CP/inorganic nanohybrids can be broadly tailored. The main characteristics of each approach are critically evaluated in what follows.

5.3

In-situ chemical methods

5.3.1 Chemical formation of CPs in the presence of inorganic nanoparticles As mentioned in the previous section, composites prepared via ex-situ methods often suffer from the lack of intimate contact between their constituents. To overcome this problem, it is imperative to establish chemical connection between the inorganic and the organic components. This can be achieved through either electrostatic or covalent binding of the monomeric or polymeric units on the surface of the

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inorganic nanoparticles prior to polymerization. In this sense, the functional groups on the nanoparticles—either inherent or postattached—play an important role in determining the quality of the inorganic/organic interface. The most straightforward approach to obtain inorganic/organic hybrids is the oxidative polymerization of monomers on the surface of inorganic materials. In a typical synthesis, the nanoparticles are dispersed in the monomer-containing solution, where the oxidizer (usually FeCl3 or (NH4)2S2O8) is added in a subsequent step. As the first step of the polymerization, the oxidation of the monomers occurs. The as formed radical cation can react with either another radical cation (radicalradical, RR mechanism) or a monomer (radical-substrate, RS mechanism), thus forming a dimer and subsequently oligomers [11]. The nanoparticles can act as nucleation sites for the oligomers, which further grow and form the polymer. There are two main difficulties with this approach: (1) aggregation and sedimentation of the nanoparticle dispersions and (2) the uncontrolled nature of chemical polymerization. The former leads to the inhomogeneous dispersion of the nanoparticles inside the host matrix, while the latter results in inferior control over the properties of the formed polymer. In addition, to circumvent the instability issue of the nanoparticle dispersions, surface-active polyanionic molecules are usually added, which enhances the repulsive forces between the individual nanoparticles. However, the introduction of electrochemically inactive surfactants should be avoided because of their insulating behavior, which can detrimentally affect the charge transfer at the organic/inorganic interface. The inherent surface groups of nanoparticles can be exploited during composite formation as it was demonstrated for 3-thiophene acetic-acid/Fe3O4 [12]. The surface of magnetite (or generally metal oxide) nanoparticles in aqueous solutions is dominated by surface OH groups. Depending on the pH of the solution, these surface groups can undergo protonation and deprotonation. By choosing a monomeric unit with appropriate functional groups (in this case, carboxylic groups), the electrostatic (or chemical) interactions can effectively bind the monomers to the surface of the nanoparticles (as proved by infrared-spectroscopy measurements, Fig. 5.2) [13]. It was also demonstrated that the fraction of incorporated Fe3O4 can be tuned by the alteration of the slurry composition [12]. In the case of carbon-based nanomaterials, direct chemical polymerization can undergo in a more ordered fashion because of the ππ interaction between the constituents, which provides an additional arranging force during the synthesis. A multitude of such composites were realized and different prominent application avenues were demonstrated [6,14]. It was shown for polyaniline (PAni)/reduced graphene oxide (rGO) composites that the addition of rGO has an enhancing effect on the electrical properties of the formed material [15]. The electrical conductivity of the composites is greater than expected from the particle mixture rule, which was rationalized by the strong ππ interaction between the formed polymer and the rGO together with the templating effect of rGO sheets. This latter also manifests in an increased charge carrier lifetime, as the more ordered structure decreases the electron-hopping barrier height. Various CP-based composites, such as PAni, poly (3,4-ethylenedioxythiophene) (PEDOT), and polypyrrole (PPy), were prepared in a

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Figure 5.2 Interaction between the surface groups of Fe3O4 nanoparticles and 3-thiophene acetic-acid monomer [13]. Reprinted with permission from the American Chemical Society. Copyright 2009.

similar fashion [16]. The excellent electrical contact between the organic/inorganic components was evidenced by cyclic voltammetry (absence of distortion caused by resistive element between the interfaces). In a similar manner, simple chemical oxidative polymerization can be employed for the fabrication of CP/CNT hybrids. The main difficulty of chemical polymerization is that pristine CNTs are insoluble in aqueous electrolytes and the stability of such suspensions is low. Multiple strategies were employed to overcome this issue: (1) chemical functionalization of CNTs,

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(2) stabilization through polymeric molecules (surfactants, polyanions, and polymers), and (3) altering the polymerization media. We also note that chemical polymerization is not exclusive to random arrangements, but also ordered structures are used as templates [17,18]. To gain better control over the polymerization, the use of more sophisticated synthesis methods is necessary. These involve the premodification of the nanoparticle surface by the formation of specific self-assembled monolayers. The surface modification plays a twofold role: (1) it can enhance the stability of the dispersion and (2) provides a starting ground for the polymerization. Usually, these molecules are covalently bonded to the underlying surfaces; thus, the premodification of the nanoparticle surface plays a vital role [19]. The type of surface modifying agent is predestined by the used inorganic particles (metals, metal oxides, and carbon-based nanomaterials) and a brief summary is given in Table 5.1. The grafting of CPs on inorganic surfaces can be achieved through three different approaches (see the analogy with the copolymer formation nomenclature) as depicted in Fig. 5.3. The (1) grafting to approach involves the attachment of the Table 5.1

Summary of surface modifying agents

Inorganic nanomaterial

Surface modification agent

Metals

Thiols (RSH, RSSR)

Metal oxides

Silane derivatives (RSiCl3, RSi(OEt)3) Phosphonic acids (RH2PO3, RH2PO4)

Metal chalcogenides

Dicarboxilates (R(COOH)2) Phosphonic acids (RH2PO3, RH2PO4) Carbodithioic acids (RC(S)SH)

Carbon

Diazonium salts (RC6H4N21) O-phenylenediamine (C6H4(NH2)2)

Figure 5.3 General approaches of polymer grafting [20]. Adapted with permission from Elsevier. Copyright 2014.

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preformed polymer chains via the reaction through their end groups and the reactive groups of the surface (note that this approach actually falls into the sequestered category). These end groups can be either located at the end of the polymer backbone or at the ends of the side chains of the polymer. The density of polymer brushes obtained by this method is low because of mass transport limitations and the steric hindrance by the polymer chains already present on the surface. Thicker brushes can be synthetized via the (2) grafting from approach, where the reaction involves the monomers and a monolayer of initiator molecules on the surface (it is often also named as surface initiated polymerization). In this case, the chain growth starts from the surface and is only limited by the mass transport of the small monomer. The (3) grafting through method can be considered as the mixture of the previous two approaches. Here, the surface contains monomer groups and the polymerization itself is carried out in the solution phase from the monomers present in the solution. The as formed polymeric chains attach to the nanoparticle surface through the grafted monomeric unit. From here, the propagation of the chain growth proceeds through the monomers present in the solution phase (unlike in the (1) case). Note, however, that all of these grafting methods favor the formation of separated CPcoated individual nanoparticles; thus, no percolation pathways of inorganic nanoparticles are formed, which confines the application areas of such hybrids. After the above-described self-assembled monolayer is formed on the surface, the polymerization can be carried out in various ways (e.g., chemical oxidative polymerization, light-assisted polymerization, catalyst transfer polymerization). The available synthesis tools were summarized in a recent review [20]. As a model system for hybrid organic/inorganic photovoltaic cells, the properties of the interface between TiO2/poly(3-hexylthiophene) (P3HT) are of particular interest. Through the surface modification of TiO2 nanotubes, its hybrids with P3HT were realized [21,22]. Stationary photoluminescence quenching experiments showed that the charge transfer process between P3HT and TiO2 is more efficient when the polymer is grafted on the TiO2 [21]. This trend can be explained by the chemical interaction through the binding layer which facilitates electron transport. This phenomenon was also demonstrated by femtosecond fluorescence dynamic measurements on a P3HT/TiO2 nanorod system [22] where the initial fast decay was assigned to photoinduced charge transfer between the TiO2 and P3HT. Furthermore, these experiments revealed that the formed polymer adapted a coillike structure, which induced torsional defects resulting in a reduced effective conjugation length [22]. The role of the linker compound was further verified through the investigation of the distance dependence of the charge transfer rate, studying a homologous series of molecules consisting of terthiophene connected to TiO2 nanoparticles [23]. It was revealed that longer spacers lowered the charge transfer rate, but the attenuation per bond was less than expected and was not strictly exponential. These deviations were discussed in terms of constructive interference effects or back folding of the flexible spacer [23]. Similar dependence was also demonstrated for carbon-based composites [24]. Hybrids of multiwalled carbon nanotubes (MWCNTs) and P3HT were realized via the grafting from method and it was shown that the thickness of the polymer coating could be varied [24].

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The inverted approach to prepare inorganic/organic hybrids by chemical means is a less studied area. A unique approach for the deposition of metal nanoparticles is electroless deposition [25]. It is based on the intrinsic property of CPs that allows them to adopt different oxidation states. During this process, the metal nanoparticles are generated in situ on the surface of CPs, through redox reactions between metal ions in the solution and the reduced CP. This method works mainly for noble metals (e.g., Au, Ag, Pt, Pd, etc.) and is driven by the difference between the reduction potential of the metal precursor and the oxidation potential of the CP. The hybrid formation can be achieved either in the presence of polymer dispersions [26] or thin films [27]. Several important parameters affect the shape and size distribution of the obtained metal deposits [28]. From the CP side: the initial CP oxidation state [29], the doping anion [30,31], and the surface morphology [32] of the CP. As for the metal: the solution concentration [33] and any specific interaction with the polymer. These specific interactions can be harnessed to an extent, where the metal deposition is no longer confined to the surface of the polymer layer. For instance, in the case of poly(1,8-diaminocarbazole), the free standing amino functional groups of the polymer can effectively entrap AuCl42 ions into the deeper regions of the polymer layer [27]. The subsequent chemical or electrochemical reduction step resulted in the formation of Au nanoparticles inside the polymer layer. In addition, the size distribution of the metal deposits was fine-tuned by the metal ion concentration in the solution. At low concentrations, the kinetics of metal deposition was dominated by diffusion, which resulted in comparable nucleation and growth kinetics causing a broad size distribution. In the case of high metal ion concentration, a narrower size distribution was obtained because of the instantaneous metal deposition. On the other hand, there was no control over the spatial distribution of the nanoparticles, without using external electrical aid [27]. Electroless deposition can be effectively used to prepare different metal/poly(alkyl-thiophene) hybrids with enhanced thermoelectric properties [34,35]. In these cases, the formation of the metal nanoparticles mainly occurs on the microstrains of the polymer, thus, no percolation pathways were formed; however, the electrical conductivity of the polymer was enhanced, which was an indicator of the intimate electronic contact between the formed metal nanoparticles and the polymer network.

5.3.2 One-pot synthesis A unique approach to perform chemical syntheses is one-pot synthesis. This technique can be extended for the preparation of organic/inorganic hybrids. During such synthesis, the reactants are subjected to successive chemical reactions in a single reactor. Because the formation of the CP and the inorganic constituent occurs simultaneously and at one place, special interactions between the CP and the inorganic participant can arise. The one-pot synthesis of TiO2/PAni was realized through the simultaneous oxidative polymerization of aniline and hydrolysis of Tiisopropoxide [36]. The formed PAni nanofibers were decorated with 510 nm diameter TiO2 nanoparticles and the obtained hybrid showed high surface area,

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which was exploited in supercapacitor applications [36]. This synthesis strategy was extended to other semiconductor oxides and in succession, MnO2/PAni hybrids were realized [37], where KMnO4 acted as the precursor. In a similar manner, the uniform distribution of WO3 was achieved in a PAni matrix, by employing peroxotungstic acid precursor [38]. The scope of this synthesis strategy extends also to the preparation of metal/CP hybrids [39,40]. Ag/PAni and Au/PAni hybrids were obtained through a unique one-pot synthesis strategy, where γ-rays acted as the initiator for hybrid formation [40]. It was demonstrated that the shape and the size of the metal nanoparticles can be varied from nanometer-sized spheres to micron-sized dendrites just by increasing the metal-to-aniline ratio. Moreover, the electrical conductivity of the composites showed a 50-fold increase, compared to PAni [40].

5.4

Electrochemical approaches

Electrochemical oxidation of the monomers (and subsequently oligomers) can also be a driving force of the polymerization. During this type of synthesis, the CP is deposited as a thin film on an electrode surface. Because of the highly controlled nature of electrochemical polymerization, the resulting CP films possess welldefined physical and chemical properties, compared to their counterparts prepared via chemical routes. For the preparation of CP-based hybrids, similar electrochemical protocols can be employed, under either current or potential control (Fig. 5.4). A brief summary of the advantages and disadvantages of the different electrochemical deposition techniques is given in Table 5.2.

Figure 5.4 Electrochemical methods, typically used for polymerization.

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Table 5.2 Summary of the different electrochemical polymerization protocols Deposition method

Advantages

Disadvantages

Galvanostatic

By varying the applied current density, the polymerization rate can be tuned The passed polymerization charge is proportional to the deposited polymer amount

To keep the current constant, the potential can change drastically, which can lead to unintended side reactions The applied current should be chosen so that the polymerization is not limited by the transport of monomers to the electrode surface

Potentiostatic

The polymerization reaction can be selectively performed

Any phenomena that cause a potential drop in the system (e.g., resistive film formation) can result in a sudden current drop

Potential step

Short polymerization pulses can be used in conjunction with prolonged rest periods to ensure the replenishment of monomers near the electrode surface The deposited amount of CP can be easily estimated

Large and uncontrolled current spike may occur at the beginning of each step

Potentiodynamic cycling

The formed CP is immediately reduced after each cycle, thus it adopts the most stable structure (via periodic swelling and shrinking)

Not applicable for all monomers

In an electrochemical synthesis, two sets of parameters must be specified: (1) parameters related to the employed protocol (e.g., potential window, sweep rate, number of cycles, potential or current density, passed charge, and length of pulses) and (2) parameters related to the system (type of solvent and electrolyte, temperature, pH of the solution, and concentration). These parameters are highly interdependent and thus special care must be taken in choosing the appropriate synthesis conditions for the preparation of a selected hybrid material. Generally, two main types of hybrid architectures can be fabricated through electrochemical means: (1) thin CP films, with the inclusion of different nanoparticles (e.g., as dopants) and (2) inorganic nanostructures coated by the CP.

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5.4.1 Nanoparticle-based hybrids During the formation of CPs, the incorporation of anions from the solution is necessary to fulfill charge neutrality (CPs are formed in their oxidized state). The early studies on the incorporation of different metal oxide particles into CPs aimed to exploit this coulombic interaction. This method works remarkably well for metal oxides having negatively charged surface (low point of zero charge) in the polymerization medium. In this manner, the incorporation of WO3 into PPy films was realized [41]. By the addition of ClO42 to the polymerization solution, a competitive behavior was found between the inclusion of the metal oxide and the perchlorate anion. Furthermore, by adjusting their ratio, the composition and thus the electrooptical properties of the formed hybrid films was tuned. For metal oxides such as TiO2, where the surface is positively charged (high point of zero charge), however, no incorporation could be detected [42]. The surface charge of metal oxide particles is also affected by different ionic species adsorbed on their surface. Thus, the incorporation of TiO2 into PPy films is achievable only by using certain anions that can effectively adsorb on the surface of the nanoparticles (e.g., I2 and SO422). Note that the adjustment of the pH to a higher level (to ensure negatively charged TiO2 particles) is not a viable option here. This simple method is hampered by the fact that CPs can only be polymerized in the acidic or neutral pH region. By shifting the solution pH, the OH2 ions present in the solution would act as a radical scavenger and terminate the polymerization. Incorporation of Fe3O4 into different CPs was also achieved via a similar strategy [4345]. The use of potassium oxalate as the electrolyte during the electropolymerization resulted in the formation of Fe21 and Fe31 oxalate species on the surface of the nanoparticles (Fig. 5.5), which in turn provided the necessary negative surface charge for the particles. This was, however, accomplished through etching and subsequently the partial dissolution of the Fe3O4 particles, which necessitated the careful optimization of the electrolyte/nanoparticle ratio.

Figure 5.5 Surface modification of Fe3O4 particles, using potassium tetraoxalate supporting electrolyte [44]. Reprinted with permission from the American Chemical Society. Copyright 2010.

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5.4.2 Nanostructured systems Nanostructured electrodes offer several advantageous properties compared to their planar counterparts in various application scenarios. These nanostructures possess higher surface area and, as illustrated in Fig. 5.6, the charge transport in ordered nanostructured systems involves less grain boundaries, which can enhance the charge carrier lifetime. Furthermore, in hybrid materials of these architectures, all the constituents are in contact with the back electrode (unlike in the previous cases where segregated particles were formed in the CP films). The complete filling of nanostructured electrodes by physical infiltration techniques often poses a difficulty because of mass transport limitation, arising from the diffusion of the large-molecular weight polymers into the porous structures. Therefore, it is necessary to utilize more sophisticated methods, where CPs are in situ generated in the pores of the nanostructures from their monomeric building blocks. Generally, two approaches are available to fill such nanostructured systems. The bottom-up approach, where the CP is generated on the underlying planar electrode surface. During such processes, the gradual filling of the nanostructures results in inhomogeneous distribution of the polymer throughout the template. This type of growth mechanism can be observed in cases where the used inorganic material is poorly conductive and the CP shows little or no affinity toward the host matrix. The other strategy involves the growth of the CP directly on the nanostructures themselves; therefore, a much more homogenous CP distribution is ensured [46].

5.4.2.1 Carbon-based materials In constructing different flexible devices (such as wearable electronics), carbonbased hybrids are among the most promising candidates [6]. The preparation of the host nanocarbon electrode can be achieved via simple drop- or spin-coating of the carbon suspensions on an inert electrode surface. Even freestanding electrodes can be fabricated, by passing a CNT suspension through a filtration membrane [47]. The CNTs remain on the filtration membrane, forming a flexible freestanding film with entangled internal networks (Fig. 5.7BD). To deposit a CP (in this case, PPy) into such templates, a pulse technique was proposed, which involved multiple polymerization pulses at 1.05 V (vs. Ag/AgCl) and a long rest period at open circuit potential among the pulses. The rest period was built in the protocol to ensure the

Figure 5.6 Possible geometries of nanostructured electrodes.

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Figure 5.7 (A)(C) Continuous CP/CNT reticulate structure and (B)(D) randomly overlapped CP/CNT bundles. Blue and brown parts represent CNT skeleton and CP skin, respectively [48]. Reprinted with permission from the Royal Society of Chemistry. Copyright 2012.

refill of the monomer in the solution phase in contact with the CNTs before the next polymerization pulse. It was demonstrated that by increasing the length of the polymerization pulses, the PPy coating became less homogeneous throughout the membrane (was thicker at the exterior parts). This trend was rationalized by the diffusion limitation arising during long potentiostatic polymerization. Such inhomogeneity led to inferior charge storage performance of the hybrid prepared by longer polymerization pulses [47]. To improve the electrical contact among the individual coated nanotubes, an interconnected CNT network was proposed as a host of the CP (in this case PAni) [48]. The derived hybrid, termed as a “skeleton/skin” architecture (Fig. 5.7AC), was completely free from overlaying coated nanotubes, where through “skin-contact” the CPCP junctions could hinder the overall charge transfer properties of the hybrid. The possibility to fabricate vertically aligned CNT arrays opened up new avenues for the electrochemical synthesis of CP/CNT hybrids. In such a configuration, high electrical conductivity, adsorption properties, and large surface area of the carbon material are retained, while the CP component serves as the active material and stabilizes the nanocarbon framework. Several applications, where efficient charge transport is necessary (e.g., supercapacitors, thermoelectrics, Li-ion batteries), can benefit from CPs grown on such substrate. In this vein, various CPs, such as PAni [4951], PEDOT [50], PPy [51,52], and P3HT [50,53], were deposited on CNTbased nanoarchitectures. In the case of PEDOT/MWCNT hybrids, the relationship between the used anions/ solvent and the morphology of the CP coating was studied [54]. It was found that the morphology of the formed CP layers mainly depends on the type of the nucleation mechanism. Nucleation can occur via two different routes: (1) instantaneous or (2) progressive. Only a small number of nuclei are formed during instantaneous nucleation and with the progress of polymerization, only their growth is observed, finally resulting in a granular morphology. In contrast, new nuclei are formed continuously during

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the 3D progressive nucleation, which results in a smoother surface. In aqueous media, the evolution of a granular morphology was observed for KCl and Na-(polystyrene sulfonate) electrolyte, whereas the use of a surfactant, such as Na-(dodecylbenzenesulfonate), resulted in a homogeneous coating. Similar behavior was found for a polymerization in nonaqueous media (acetonitrile/Bu4NPF6 electrolyte) [54]. This difference was attributed to the different wettability of the hydrophobic CNT surface, which altered the CP nucleation mechanism [54]. It is worthwhile to note that polymerization on carbon substrates is initiated at lower potentials compared to noble metal electrodes (see, e.g., the case of P3HT/single-walled CNT hybrids [53]). This is a clear indication of the electronic interaction between the aromatic monomer and the conjugated electron system of the CNTs. The previously shown composite materials often suffer from the same drawback: the short length of the CNTs (i.e., small thickness of the hybrid films), which often hampers their practical application. To move toward application-oriented systems, the deposition of PAni, P3HT, and PEDOT was performed on aligned MWCNT arrays with macroscopic dimensions (A 5 1 cm2, with a height of up to 2 mm) [50]. To achieve high polymer loadings, an electrochemical conditioning step was employed before electrodeposition. This involved potential cycling in the monomer-containing solution within a carefully selected potential window to avoid polymer formation. This step served two purposes: (1) deaerate the freestanding CNT structure and (2) to ensure proper wetting of the MWCNT arrays. To monitor the deposition procedure, Raman spectroscopic measurements together with scanning electron microscopy (SEM) images were taken at different points of the arrays (Fig. 5.8B). The characteristic vibration modes for both P3HT and MWCNT were observed (Fig. 5.8A) in the whole depth of the array for both potentiostatically (Fig. 5.8C) and potentiodynamically (Fig. 5.8D) deposited samples. The striking contrast lied in the relative intensity of the peaks. Constant relative intensity found in the case of potentiostatic deposition (Fig. 5.8C) indicating a uniform coating, whereas the continuous change in the relative intensity from bottom to top for potentiodynamic deposition (Fig. 5.8D) indicated an uneven P3HT distribution. SEM images revealed a significant increase in the diameter of the nanotubes after polymerization, compared to the neat MWCNT arrays (Fig 5.9AC). Two overlapping processes could be distinguished: (1) the thickening of individual tubes (2) bundle formation by the fusion of coated CNTs. With the increase in the polymerization time, the initial voids were filled with the polymer (Fig. 5.9B and C). Furthermore, the absence of cauliflower like objects and the smooth coating indicates the strong interaction between the CNT and P3HT. The importance of a pretreatment step was further demonstrated in the preparation of PPy/MWCNT hybrids [52]. Before polymerization, the MWCNT arrays were subjected to a wide range potentiodynamic cycling procedure in a monomerfree medium. The potential window was chosen in order to oxidize the surface of the CNT arrays. This treatment induced defects and different oxygen-containing moieties on the CNT surface (as evidenced by X-ray photoelectron spectroscopy) and altered the wetting behavior of the MWCNT arrays [52]. PPy deposition was more pronounced on these surfaces and enhanced charge storage capability of the formed hybrids was demonstrated [52].

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Figure 5.8 Raman spectroscopic investigation of the effect of different electrochemical deposition techniques on the homogeneity of the formed P3HT/MWCNT composites [50]. Reprinted with permission from Wiley. Copyright 2015.

Figure 5.9 Side-view SEM image of the bare MWCNT array (A), a potentiostatically prepared (t 5 10,000 s) P3HT/MWCNT composite (B), and a potentiostatically prepared (t 5 50,000 s) P3HT/MWCNT composite (C) [50]. Reprinted with permission from Wiley. Copyright 2015.

5.4.2.2 Metal-oxide-based materials Several different metal oxide nanostructures (e.g., TiO2 nanotubes, WO3 nanoporous electrodes, and ZnO nanocolumns) were studied as the matrice for CP electrodeposition. The preparation of such nanoarchitectures is often performed by electrochemical anodization, when the oxidation of the respective metal foil is carried out in the presence of water and a complexing agent (usually F2 ions) [55].

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As the first step, a barrier oxide layer is formed, which is gradually etched out by the complexing agent. When certain conditions (e.g., applied voltage, water content, and electrolyte concentration) are met, the self-organization of the pores can occur. Consequently, the as prepared materials possess high surface area and good electrical contact with the underlying electrode. This technique offers the ability to shape the formed nanostructures (e.g., tunable pore diameter, interpore distance, wall thickness, and depth of the layer). Furthermore, the formation of a compact blocking layer is the inherent property of the synthesis which can be practically relevant for solar cell applications. The anodization of Ti and Nb foils results in the formation of highly ordered metal oxide nanotube arrays, whereas W and Ni adopt a more disordered porous morphology [55]. Hybrid materials based on oxide nanostructures and CPs received considerable attention because of their prospective use as supercapacitor electrode materials [5,56]. Such hybrid formation can effectively address the relatively poor cycling stability of CPs, and the complementary pseudocapacitive properties of the components can also be harnessed. To completely fill the pores of the nanostructures, either dynamic- or slow static electropolymerization methods are favored. This is mainly because of the slow replenishment of the monomer in the deeper areas of the nanostructures. If the employed deposition procedure is too fast and depletes the solution phase in the monomer, an inhomogeneous polymer coating forms. Even worse, the clogging of the pores can result in the formation of a bulk polymer layer on top of the oxide nanostructures. The diffusion limitation of such processes was demonstrated when PEDOT was electrodeposited on ZnO nanocolumns of different lengths [57]. If the same polymerization potential was used, the shorter nanocolumns were uniformly coated with the polymer. In the case of longer columns, however, a more pronounced polymer formation was observed on the top of the columns, and homogeneous coatings were only seen if pulsed methods were used. This trend is an indicator of the inadequate refill of the monomer in the lower regions of the nanostructures in the case of static procedures. However, several CPs, such as PAni [58], polythiophene [59], and PEDOT [60], were deposited on TiO2 nanotubes with potentiostatic methods, despite its above listed shortcomings. In the case of PAni, even a predipping step in a more concentrated monomer solution was performed, thus increasing the local monomer concentration. But still, the PAni formation was more pronounced on the top of the nanostructures. The morphology of this top layer was characteristic of free standing PAni films, which consists of long randomly intertwined fiber-like structures [58]. Different dynamic electropolymerization methods can effectively address the inhomogeneous coverage of the nanostructures. There is a wealth of studies on the electrodeposition of PPy on TiO2 nanotube arrays by pulsed methods [6163]. It was demonstrated by spectroscopic measurements that the formed PPy retained its characteristic vibrations, indicating the deposition of an intact PPy layer [61]. In a typical synthesis, a short anodic pulse initiates the polymerization and the subsequent doping of the formed layer [62,63]. This is followed by a short cathodic pulse to discharge the hybrid and a long rest period to ensure the refill of the monomer. In the case of nanotube arrays, there are three possible places for the polymer formation

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Figure 5.10 Electrodeposition of PEDOT on TiO2 nanotubes and the effect of applied potential on the obtained PEDOT structures [60]. Reprinted with permission from Wiley. Copyright 2012.

(Fig. 5.10A): (1) inside the tubes, (2) between the tubes, and (3) on top of the nanostructure. In the case of PPy, polymerization proceeds in all three places, regardless of the applied current density. To achieve preferential deposition, the template itself was altered [62], and by sufficiently decreasing the intertubular space, the polymer formation among the tubes could be completely eliminated [63]. The same strategy was extended to PEDOT-based hybrids [60,64]. In contrast to PPy, the alteration of the potential (or current) caused the preferential formation of PEDOT on the tube walls, even without the premodification of the substrate. It was proposed that the difference in the surface energies of polymerization sites, together with the change in the nucleation mechanism with the applied potential (current), contributed to the observed behavior. After selectively dissolving the TiO2 nanotube template, PPy and PEDOT nanopore arrays and nanowires were obtained (Fig. 5.10B), which underlines the utility of this approach to fabricate ordered CP nanostructures. Carefully controlled dynamic synthesis methods are also capable to vary the thickness of the CP layer on the TiO2 nanotubes. During potentiodynamic deposition, the onset potential of PEDOT formation on the TiO2 nanotubes was determined (Fig. 5.11). It showed an E 5 0.15 V increase compared to Pt electrodes, caused by the higher resistance of the TiO2 sample (note the difference to carbonbased electrodes).

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Figure 5.11 Potentiodynamic electrodeposition of PEDOT on TiO2 nanotubes [65]. Reprinted with permission from the American Chemical Society. Copyright 2010.

To ensure homogeneous polymer coating in such cases, a special deposition procedure was introduced. A short initial phase at a relatively high potential was performed, which initiated oligomerization in the solution phase. This was followed by the slow potentiostatic deposition of the polymer at a lower potential (to avoid overoxidation of the polymer). As the polymerization time increased, the simultaneous shrinking of the inner tube diameter and the thickening of the walls were observed (Fig. 5.12A and B) [65]. As also seen in the previous examples, in the case of semiconducting oxides, further complications may arise from the low conductivity of the electrode material. To overcome this difficulty, high polymerization potentials are to be used, which carries the risk of the overoxidation of the forming CP. To circumvent this issue, in the case of PAni/WO3 hybrids, their similar H1 conduction mechanism can be exploited [66,67]. The enhanced electroactivity of WO3 in acidic media is rooted in the formation of tungsten bronze (HxWO3), supported by H1 uptake/release (as charge compensation) during the W61/W51 redox process. By employing a potentiodynamic polymerization protocol, hybrids with different amount of PAni were synthetized. With the increase in the number of polymerization cycles, the growth of PAni became more pronounced (Fig. 5.13). In the first few cycles, the deeper pores of the nanoporous WO3 template were filled (1 cycle—B) and thickening of the outstretched branches of the nanostructure was observed (3 cycles—C). As the polymerization proceeded, the growth of PAni occurred mainly at the top of the sample (5 cycles—D) until complete coverage was reached (10 cycles—E). Importantly, the initial morphology of the underlying nanoporous WO3 template was conserved even after the voids are completely filled. The surface layer of PAni adapted a fractal-type morphology even at prolonged depositions, which is particularly favorable for supercapacitor applications.

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Figure 5.12 (A) SEM images of PEDOT/TiO2 hybrids at different stages of polymerization. (B) The change of inner tube diameter and wall thickness during polymerization [65]. Reprinted with permission from the American Chemical Society. Copyright 2010.

Figure 5.13 SEM images of pristine WO3 (A) and four PAni/WO3 hybrids prepared by 1 (B), 3 (C), 5 (D), and 10 (E) potentiodynamic polymerization cycles [66]. Reprinted with permission from Springer. Copyright 2015.

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It is apparent from Fig 5.14A that both the deposited PAni and the underlying nanoporous WO3 retained their electroactivity in the hybrid material. This fact was attributed to facile H1 transport throughout the hybrid material. Furthermore, the preservation of WO3-related redox peaks in the hybrid was an indicator that the underlying nanoporous WO3 is still accessible to H1 ions from the solution phase. The photoelectrochemical studies of PAni/WO3 hybrids revealed further insights on the nature of the contact between these constituents [67]. The photoelectrochemical behavior of the hybrid material showed features of both of its components (Fig. 5.14B): p-type behavior of PAni and n-type behavior of WO3. The careful examination of the shape of the photocurrent transients revealed that in the case of the hybrid material, efficient separation of the charge carriers was achieved before

Figure 5.14 (A) Comparison of the electrochemical behavior of WO3, PAni, and PAni/WO3 samples [66]. (B) Comparison of photocurrent transients at three different bias potentials [67]. Reprinted with permission from Springer and the American Chemical Society. Copyright 2015 and 2012.

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they undergo recombination. This observation could be ascribed to both the nanostructured nature of the semiconductor support and the homogeneous distribution of the deposited polymer.

5.5

Photo-assisted methods

Different strategies are available for the utilization of incident light for synthetic purposes (Fig. 5.15). Photocatalytic reaction schemes (Fig. 5.15A and B) and photoelectrochemical synthesis techniques (Fig. 5.15C and D) can be distinguished. The available arsenal of photoelectrochemical approaches and their specific features has been reviewed recently [9]; thus, the focus of this section will be centered on the very new advancements of this field. The use of inorganic semiconductors in photocatalytic reaction schemes is a prominent field in solar energy conversion and environmental remediation. During photoexcitation, the absorption of light promotes electrons to the conduction band (CB) (and leaves a positive hole in the valence band (VB)). If the band positions lie at the proper energy levels (CB higher then desired reaction—reduction or VB lower then desired reaction—oxidation), several reactions can be driven (e.g., mineralization of organic pollutants and immobilization of toxic metal ions). The use of light irradiation as a synthetic tool was explored for the deposition of different species (e.g., metals [68,69], polymers [70], and Cd-chalcogenides [71,72]) on the inorganic surfaces. It is surprising, however, that only a few studies focused on the photocatalytic deposition of CPs on inorganic semiconductors (Fig. 5.15A). The pioneering studies in this field were carried out in a TiO2 slurry, for the deposition of PPy [73,74]. The as formed PPy coatings were insensitive to the used anions and were smoother compared to their electrochemically synthetized counterparts [74]. We highlight that the CP layers deposited in this manner were ultrathin because of the optical shielding effect of the forming CP layer. The details of the polymerization mechanism were also investigated, where the active role of the photogenerated holes was confirmed [75]. This approach was extended to other

Figure 5.15 Schematic illustration of photocatalytic polymerization of CPs on semiconductor electrodes (A), photocatalytic deposition of inorganic materials on CPs (B), photoelectrochemical polymerization on bare (C), and sensitized (D) semiconductor electrodes [9]. Adapted with permission from Elsevier. Copyright 2015.

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polymers (e.g., polythiophene [76,77], PEDOT [78,79], and PAni [80]); and to other inorganic semiconductors (e.g., WO3 [76], SiC [80], CdS/CdTe [81], and CdSe(ZnS) [82]). Furthermore, the applicability of this procedure was also shown for nanoparticulate [79], nanostructured [83,84], and mesoporous [85] systems. It has been demonstrated (for the case of PEDOT) that it is useful to employ electronscavenging materials (such as H2O2 or O2) to prohibit electronhole recombination so that the reaction rate can be boosted [78]. This photocatalytic synthesis was further extended to the formation of copolymers recently. The preparation of a PPyPEDOT was demonstrated on the surface of TiO2 [86]. It was found that the composition of the copolymer strongly depends on the irradiation time. At the early stages of the polymerization, mainly PPy was formed and PEDOT only appeared at later stages [86]. Further investigations were carried out to reveal the role of the spectral composition of irradiation on the quality of the formed CP layer in the case of PAni/SiC hybrids. Remarkably, by filtering out hard UV light (below 300 nm), higher quality PAni coatings could be obtained (as evidenced by vibrational spectroscopic and electrochemical measurements) [80]. Interestingly, the inverted approach is also feasible; thus, the intrinsic semiconducting nature of CPs can also be harnessed for photocatalytic deposition of inorganic species (Fig. 5.15B) (e.g., CdS on P3HT nanofibers) [87]. This arrangement is considered as a benchmark system for organic/inorganic hybrid solar cells, and the formation of this hybrid was mainly achieved through sophisticated chemical grafting techniques so far. As depicted in Fig. 5.16A, the CB edge of P3HT lies at a proper position for the deposition of CdS (the reduction of Cd21). It was evidenced that upon visible light irradiation, the generated photoelectrons, reaching the solid/ electrolyte interface, were responsible for the reduction of the Cd21 ions, adsorbed on the P3HT surface (Fig. 5.16B). As the synthesis is carried out in the presence of dissolved sulfur, CdS is formed instead of metallic Cd. The use of a sacrificial agent (ascorbic acid) increased the rate of the photocatalytic reaction by scavenging the photogenerated holes. It was found that with the progress of deposition time, the size of the deposited CdS quantum dots increased, which resulted in the monotonous decrease in the bandgap energy of the hybrid material [87]. Photoelectrochemical evaluation of the hybrid revealed that both constituents contribute to the overall behavior (Fig. 5.17). Furthermore, the photocurrents observed for the hybrid surpassed the performance of the individual materials, which is an indicator of the efficient charge separation at the CdS/P3HT interface. Light irradiation of inorganic semiconductor electrodes can also be beneficial for the deposition of CPs on their surface (Fig. 5.15C). Conventional electrodeposition in these systems is often hampered by the low conductivity of the inorganic host matrix. In these systems to initiate polymerization, the use of high potentials is often necessary. In such conditions, however, CPs are often prone to irreversible oxidation [46,88]. To overcome this barrier, light irradiation of the inorganic semiconductor electrodes can be employed, which has two benefits: (1) it can initiate photocatalytic polymerization (as demonstrated before), and (2) it can increase the conductivity of the semiconductor electrode (through photoconductivity). During photoelectrochemical polymerization, an external bias is applied in conjunction

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Figure 5.16 (A) Comparison of reduction potentials of different metal cations and the band positions of TiO2, CdS, and P3HT. (B) Schematic illustration of the photocatalytic synthesis of CdS/P3HT [87]. Adapted with permission from the American Chemical Society. Copyright 2015.

with the light irradiation. The role of the external bias is to drain the photogenerated electrons into the external circuit suppressing the recombination of the photogenerated species, thus making the use e2 scavengers obsolete. Usually, the applied external bias is not high enough to initiate electropolymerization. It was found, however, that the contribution of electropolymerization cannot be ruled out entirely. The formed CP via photopolymerization can often act as a seed layer for further growth through electrochemical polymerization [89]. Several studies aimed to unveil the mechanism of photoelectrochemical polymerization. In this vein, detailed investigations were carried out on the deposition

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Figure 5.17 Linear sweep photovoltammograms of P3HT, CdS, and CdS/P3HT electrodes under visible light irradiation [87]. Reprinted with permission from the American Chemical Society. Copyright 2015.

of PAni and PPy on TiO2 and WO3 substrates [90]. The contribution of photoelectrochemical and electrochemical polymerization was successfully separated. It was evidenced that as the potential window was extended into the region of electrochemical polymerization, the morphology of the formed hybrids gradually changed. In a procedure where only photopolymerization occurred, the formed CP retained the structural features of the host template, whereas with increasing contribution from electrochemical deposition, globular structures were formed on top of the nanostructures [90]. This latter is often a signature mark of electrochemically polymerized CP hybrids. The effect of pH and wavelength of excitation were also explored [91]. It was found that these parameters greatly affect the electrochromic response time of the hybrids. Significantly, faster response times were recorded for the hybrids compared to flat PPy films, which indicated the efficient electronic contact between the constituents. The effect of the counterions during photoelectropolymerization was also investigated [88]. The use of smaller, more mobile ions (ClO42) facilitated the formation of a more electroactive CP layer [88]. Matrix-assisted laser desorption/ionization mass spectrometric analysis revealed that photoelectrochemically generated PEDOT, from acetonitrile, is composed of longer chains, compared to its counterpart prepared in aqueous micellar electrolyte [92]. Moreover, in aqueous solution, a considerable amount of PEDOT chains possessed a thiophenone moiety as one of the end-units. This by-product was formed via the nucleophilic attack of the oligomers by a water molecule. It was also demonstrated that the presence of such thiophenone terminal groups further decrease the conductivity of the obtained CP [92]. A new variant of this synthesis approach employs light pulses, under potentiostatic conditions, to achieve homogeneous CP coatings [89,93]. Here, the underlying principle is the same as in the case of potential pulses. Similarly to that scenario, the necessary time for the refill of the monomer close to the

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nanostructured surface must be ensured. The gradually decreasing photocurrent with subsequent light pulses indicated the thickening of the formed CP layer [89]. The picture can become even more complex when the inorganic semiconductor templates are already sensitized (with dye molecules or quantum dots) before the CP formation (Fig. 5.15D). Note that this is the typical arrangement of an actual solid state dye-sensitized solar cell, where the hole selective contact (and hole transporter) can be obtained in this manner. In such systems, the photoelectrochemical polymerization can be carried out by either exciting the dye and semiconductor simultaneously (white light irradiation) or selectively (illumination with selected wavelengths). The efficiency of the photoelectrochemical polymerization is closely related to the energy difference between the oxidation potential of the monomer and the valance band position of the excited SC. Other contributions, however, should also be considered, such as the light intensity, light absorption properties of the sensitizer, electron transfer rate of precursors toward the dye, and the precursor concentration [94,95]. The comparison of different organicmetal-free sensitizers revealed that if the thermodynamic driving force was greater, longer polymer chains were formed, which resulted in superior device performance [96]. It was further demonstrated that the used solvent affects the oxidation potential of the monomer (in this case, EDOT) which can shift its oxidation potential lower than the HOMO level of the sensitizer [96]. To circumvent this issue, dimers and trimers of EDOT or its derivatives with a lower oxidation potential can be employed as precursors for the CP generation [9799]. Selective photoelectrochemical polymerization via exciting the sensitizer was carried out almost exclusively for organicdye-sensitized semiconductor nanostructures so far [9499]. This strategy, however, can be further extended to inorganic quantum-dot-sensitized architectures. It was shown for metal-chalcogenide (CdS, CdSe)-decorated TiO2 nanotube arrays that efficient photoelectrochemical deposition of PEDOT can be achieved via both collective and selective excitation (Fig. 5.18A) [100]. Parallel optimization of the monomer, irradiation wavelength, and the electrochemical method was also carried out. SEM images unveiled the morphological aspects of the synthetized hybrids (Fig. 5.18B) and indicated that the sensitizer QDs act as seeds for the initial growth of PEDOT [100].

5.6

Concluding remarks, outlook

This chapter summarized the various strategies for the controlled synthesis of CP/ inorganic nanostructure interfaces. We aimed to give a brief overview on the wealth of possibilities, also highlighting the particular importance of each method. The presented approaches span through chemical, electrochemical, and photopolymerization of the CP, but certain examples for the in-situ formation of the inorganic component were also demonstrated. One common feature in all cases was the precise control over the properties of the organic/inorganic interface. Such control may include the manipulation of the composition, morphology, and optoelectronic properties of the hybrid assembly. It was shown that it is indeed possible to tailor the

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Figure 5.18 SEM images for TiO2, CdS/TiO2, and CdSe/TiO2 samples before (AC) and after PEDOT deposition (DF). Parameters of potentiodynamic deposition: 10 cycles, 100 mV s21, 0.01 M EDOT (D), or bis-EDOT (E and F) in acetonitrile [100]. Illustration of the selective photoexcitation and polymerization procedure. Reprinted with permission from Elsevier. Copyright 2015.

features of a hybrid architecture toward a specific application, where a certain set of properties is required. This also means that there is no optimal structure or preparation method, both of them have to be designed to fulfill the requirements of the targeted application. It is also important that various comparative studies evidenced the superior performance of such prepared hybrid materials compared to their

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counterparts obtained via simple mechanical mixing. We really hope that the materials science community will increasingly exploit these smart methods in the future to obtain nanohybrid materials with advanced properties.

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6

Biomedical polymer hybrid composites

Tavakoli Javad1, Dong Yu2 and Tang Youhong3 1 Medical Device Research Institute, School of Computer Science, Engineering and Mathematics, Flinders University, Adelaide, SA, Australia, 2Department of Mechanical Engineering, School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia, 3Centre for NanoScale Science and Technology, School of Computer Science, Engineering and Mathematics, Flinders University, Adelaide, SA, Australia

Chapter Outline 6.1 Introduction 135 6.2 Biomedical polymer hybrid composites classification 6.2.1 6.2.2 6.2.3 6.2.4

6.3 Biomedical application of polymer hybrid composites 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5 6.3.6

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Natural polymer polymer hybrid composites 138 Biopolymer polymer hybrid composites 141 Mineral polymer hybrid composites 143 Metal polymers hybrid composites 143 Controlled drug-delivery applications 146 Tissue engineering applications 147 Photodynamic therapy 149 Wound-dressing applications 151 Bioadhesion applications 151 Biomedical applications of composition-based hybrid composites

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152

6.4 Conclusions 155 References 155

6.1

Introduction

Although synergistic integration of organic organic compounds in one material to achieve enhanced properties is as old as mankind, recent opportunities for their combination in molecular scale makes them interesting for biological applications and biooriented research [1,2]. In the biomedical and biological fields, hybrid biomaterials represent one of the most rapidly growing novel material categories at the cutting edge of highly technological innovation with continuing interest in the development. The ongoing attention to the development of novel biomedical hybrids is due to the fact that diseases that are becoming more complex put humans Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00006-9 Copyright © 2017 Elsevier Ltd. All rights reserved.

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at risk of different unknown requirements or disabilities in the worst scenario. Combining synthetic molecules with biological entities enables material scientists to create novel hybrids with tailorable properties, whose biomedical and biological applications depend on components’ long-term stability, biofunctionality, and biocompatibility [3,4]. Therefore, a careful selection of synthetic materials is an important prerequisite so as not to alter biological properties while improving other material characteristics [5]. In contrast to the broad definition of hybrid materials by the International Union of Pure and Applied Chemistry, a narrow-size-based classification has been suggested to limit the disadvantage of covering a wide range of materials [6]. Nevertheless, for biological approach, it seems that all definitions must include the term “biocompatibility” [7]. Thus, in this chapter, a “biomedical hybrid material” is defined as “a biomedical hybrid composite consisting of at least two biocompatible molecularly dispersed components—organic or inorganic—with the ability to perform an appropriate host response in a specific situation.” A special case of biocomposites with biological properties, in the size range of 1 100 nm (for at least one component), is “biomedical hybrid composites.” It is worth noting that material properties are subject to change when mixed at a molecular scale. Fig 6.1 presents the classification and importance of biomedical hybrid composites.

Figure 6.1 Schematic diagram of the “materials 1 shape 1 scale” method for biomaterial property optimization during the composite fabrication. Hybridization offers a certain portfolio of properties based on a precise engineering design space.

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Combinations of functional organic inorganic components, biological substances, and biomaterials have made the properties of hybrid compounds more promising with the enhancement of their physicochemical, mechanical, biological, and thermal characteristics. The field of biomedical polymer hybrids has been among the fastest growing research areas in technology, science, and engineering in recent years, which is due to improved properties of hybrids revealing their relevance to new health-related disciplines. Moreover, the number of different technological and science-based routes to alter the structure and composition of biomedical polymer hybrids has increased, along with the potential to control their mutual relations and fabrication methods. These are some of the reasons for the increasing contribution of hybrid composites in biomaterial and biomedical research. In the field of organic inorganic hybrid materials, from 2010 to 2015, the number of publications show about a 25% increase in comparison to the same previous period. Fig. 6.2 demonstrates the increasing rate of scientific publications in a 5-year interval from 1985 to 2015. Polymer hybrid composites have come to shape an interdisciplinary area, bringing together nanotechnology, material science, and biology, significantly impacting biomedical science. These developments, along with the possibility of enhancement of material properties at the molecular level, have resulted in an opportunity for such materials to become established as a promising category with significant applications in the biomedical arena [8,9]. On the other hand, their extraordinary biomedical versatility is a consequence of the availability of organic components. Existing organic components mainly include, but are not limited to, biopolymers, natural polymers, minerals, and metal nanoparticles. Fig 6.3 presents the biomedical applications of polymer hybrid composites.

Figure 6.2 The rate of scientific publication for the development of hybrid composites with possible applications in the biomedical field (searched in SciFinder Scholar http://www.cas.org).

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Figure 6.3 Biomedical polymer hybrid composite classification and applications.

6.2

Biomedical polymer hybrid composites classification

6.2.1 Natural polymer polymer hybrid composites In the search for biocompatible and sustainable polymeric hybrids, natural polymers are an attractive alternative for biomedical applications, compared to other materials [10 12]. Chitosan [13,14], collagen [15,16], starch [17,18], gelatine [19,20], cellulose [21,22], alginate [23,24], and silk [25] are material examples that have currently attracted much attention. The remarkable advantages of natural polymers, including low density per unit volume, acceptable specific strength, degradability features, along with availability and low cost, make them suitable to use in biomedical polymer hybrids [23]. However, as the chemical composition of natural polymers is source dependent, the overall properties of hybrid composites are subjected to changes [11]. Surface modification of natural polymers can lead to effective interactions between the components of hybrid composites while deleteriously affecting their dielectric properties [26]. The properties that are less influenced by polymer type may be altered as a function of natural polymer types, ratios, and frequencies ranging from kHz to MHz in accordance with ASTM D-150-10 standard. The effects of process parameters, type, distribution, size, and fraction of natural polymers on physical and mechanical properties of natural polymer hybrid composites have been investigated [2,27,28]. Natural polymer hybrid composites have presented mechanical properties such as stiffness, flexibility, and modulus that are

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superior to the properties of most synthetic polymer hybrids [29,30]. Natural polymer content affects the swelling properties of polymer hybrid composites [31,32]. This can result in increased extensibility of hybrids (up to 10 times), along with their enhanced surface properties. On the other hand, increasing water absorption capability negatively affects mechanical properties of hybrids. Chitosan, a natural polysaccharide, is one of the most commonly used and distributed biomaterials after cellulose. The negative charges of chitosan make it more outstanding among other natural polysaccharides with hydroxyl groups alone [33]. Chitosan’s therapeutic effects, such as antibacterial effect, hemostasis ability, biocompatibility, binding ability to some organic compounds, enzymatic hydrolysis tendency, intrinsic biological activity, and pain alleviation property, have encouraged researchers to try to harness its medical and pharmaceutical applications in chitosan polymer hybrid composites [28,34 36]. The poor solubility of chitosan at physiological pH, which has been described as an inherent drawback, has been resolved by introducing its different derivatives [37]. Chitosan polymer hybrid composites have been considered in anionic drug-delivery systems for mucoadhesion and gene expression purposes, in a variety of shapes from tablet to in-situ gelation when injected [38]. Chitosan hybridization with hydrogels, mainly poly (ethylene glycol), as nanoparticles has been investigated for the detection, diagnosis, and treatment of cancer, where chitosan’s supplementary antitumor effects lead to much better efficacy [39,40]. In aerosol devices, the hybrid’s gel transition must be designed for better mucoadhesion property through nasal delivery [41]. On the other hand, it has been reported that in the form of films, chitosan polymer hybrids can be used as transcutaneous drug-delivery systems or wound dressing. It was found that chitosan-hybrid composites accelerated wound-healing process, ameliorated wound antiinflammatory responses, decreased healing time with minimum scar formation and displayed an aptitude to stimulate fibroblast cell proliferation. In this case, the flexibility of the dressing, its water uptake ability, oxygen permeability, control of water vapor transition rate, and transparency played important roles [14,42]. The importance of chitosan-hybrid composites as biosensors has been revealed by studies of its ion-exchange properties. The presence of hydroxyl amino groups in chitosan-based hybrid composites provides a suitable environment for the selective immobilization of different biomolecules in order to improve biosensing properties [43]. Many options for enzyme immobilization in hybrid composite systems including chitosan have been well established, not only in the biosensing field but also in various practical fields such as material property enhancement of artificial skin, immobilizing matrices, and contact lenses. Hybrid nanocomposites have thus provided an electrochemical platform for the design and fabrication of bioelectrochemical devices with high sensitivity and acceptable stability [44]. Depending on the type, surface-loading concentration, surface area, porosity, and morphology of immobilized enzymes, the biosensor’s detection limit may vary up to 1028 mol L21 with a linear range of 1028 1025 mol L21 within less than 5 s [45]. The reported extraordinary performance of chitosan hybrid composite biosensors is probably due to the permeability, pH sensitivity, and biocompatibility of chitosan, whose

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combination with graphene, ion liquids, and metallic nanoparticles make it highly electrochemically sensitive as a consequence of the increment in conductivity [46]. Using chitosan hybrid composites, various porous scaffolds have been fabricated for regenerative medicine and tissue engineering. On the basis of their desirable physicochemical and biological properties, nontoxic behavior, and biodegradability, these scaffolds have been assigned as potential biomaterials for cartilage, bone, liver, nerve, and musculoskeletal tissue regeneration. Due to the bioactivity of chitosan hybrids, cell differentiation and proliferation are potentially excited while the porous and interconnected structure of chitosan hybrids is strong enough for cell adhesion and mechanical support [30,47,48]. Collagen fibrils and their network play a dominant role in preserving the biological integrity of extracellular matrix, while a vivid remodeling procedure is undertaken for proper physiological functions. Therefore, the restoration of delicate collagen networks polymer hybrids, under which normal physiological regeneration occurs, has been highlighted recently [49]. In view of the diversity of roles played by collagen in different tissues, attempts have been made to focus on evolving novel biomaterial hybrids to mimic the intricate architecture of collagen-based tissues to function as cell scaffolds [16,50 53]. Although the development of artificial collagen-like materials is an important area of research, animal-derived collagens are still acknowledged as those among the most useful biomaterials available for hybrid composites, which are now widely used for tissue engineering, cosmetic surgery, and drug-delivery systems [20]. They are used either in their native fibrillar form or after denaturation in variously fabricated forms such as sponges, sheets, plugs, and pellets. Collagen-based hybrid films have wound-healing characteristics along with enhanced dermal cell proliferation. Therefore, hybrids made of collagen and glycosaminoglycan have been used as a soft scaffold containing human epidermal keratinocytes for skin substitution [54]. Depending on the design criteria and required biomaterial properties, the hybridization of collagen with many synthetic polymers has been suggested. However, some limitations of use need to be fully considered, such as degradation, instability, and cytotoxicity due to crosslinking processes and agents [16,52]. To move beyond these limitations, collagen crossfertilizing processes with natural polymers (starch) have been suggested and selective biomechanical properties of collagen hybrid films have been improved when produced by this method [55]. Starch, as a completely biodegradable, environment friendly, and cheap material, has been used recently to achieve biodegradable polymer hybrids for biomedical applications. The hybridization of starch with other biomedical synthetic polymers has been described as an acceptable method for ameliorating unwanted properties (mechanical or thermal) while preserving valuable properties of bio- and blood compatibility. The chemical modification of starch based on derivation or grafting overcomes poor miscibility, leading to the improvement in mechanical properties of hybrids [17,18]. Cellulose is a complex polysaccharide with crystalline morphology, which usually contributes to hybrid composites with fibers by virtue of its shape and nanodimensions (10 100 nm). Cellulose nanofibril extraction is performed by chemical

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and mechanical processes including alkaline pretreatment and treatment, acid hydrolysis, cryocrushing, and high-pressure defibrillation [11]. To improve mechanical properties of cellulose-based hybrid composites, surface modification of cellulose nanofibers (including bacterial treatment, silylation, and grafting) has been suggested [56]. It was reported that surface chemical modification of nanofibers [40,57]. On the other hand, the dispersion of nanofibers in nonaqueous ground matrices may be subjected to the change by surface modification [58]. It has been acknowledged that cellulose-fiber-based hybrid composites offer an alternative biomaterial to glass fiber-based counterparts due to their good mechanical properties and low specific mass. In addition, it has been reported that the length of nanofibers affects the strength and modulus of hybrid composites and remains constant after a specific value [59,60]. Alginate with anionic linear polysaccharide structure is obtained from brown seaweed and produces thermostable gel in cold water, with extensive applications in pharmaceutics [61]. Use of alginate hydrogel hybrids has been reported for cell encapsulation or semipermeable membrane fabrication in monolithic drug-delivery devices [35,62]. Hard-to-control gelation processes as well as swellability and porosity of alginate/Ca21 hydrogel hybrids have been known as parameters with a negative impact on their biomedical application. Therefore, the contribution of secondary molecules (polyols, chitosan, glucose, or polycationic polymers) as crosslinking agents has been implemented to enhance mechanical and physicochemical properties. Among those, chitosan may lead to a better and more stable structure with controllable and consistent porosity that improves the efficiency of drug-delivery systems [63]. In the case of regenerative medicine that combines the fundamentals of tissue engineering and drug-delivery systems, alginate/chitosan/ gelatine hybrid composites have been extensively used. The composites were found to be highly biocompatible, supporting cell attachment and extracellular matrix products and allowing cells to proliferate longer [64]. On the other hand, they have the potential for releasing many biological and pharmaceutical agents over a long period of time.

6.2.2 Biopolymer polymer hybrid composites Biopolymers are produced from renewable resources and offer important contributions to biomedical applications, by virtue of both their biodegradability and their high biocompatibility [65]. Natural polymers may be categorized as the group of biopolymers that are found naturally. However, food and agricultural wastes or any renewable resources can be used to produce biopolymers. For example, lactic acid monomer is a hydroxyl carboxylic acid that may be obtained via the bacterial fermentation from starch or sugars [66]. Table 6.1 presents important biopolymers with biomedical applications. Aliphatic polymers consist mainly of polyglycolic and lactic acid and caprolactone hydrolyzed by enzymes, whose backbone cleavage makes polymeric hybrid composites biodegradable [67,68]. Their initial good mechanical properties and deterioration into nontoxic products make them reasonable choices for use in many

142

Table 6.1

Hybrid Polymer Composite Materials: Structure and Chemistry

Biomedical biopolymer specifications

Biopolymer

Family

Production method

Refs.

Polylactic acid (PLA)

Aliphatic polyesters

Bacterial fermentation from corn or sugars

[66 70]

Polyhydroxyalkanoates (PHAs)

Polyesters

Bacterial fermentation from conventional hydrocarbonbased polymers

[65,71]

Polybutylene succinate (PBS)

Aliphatic polyesters

Condensation of succinic acid and 1,4-butandiol originally produced by petroleum-based monomers or bacterial fermentation methods

[66,72,73]

Biopolyethylene

Green polymers

(Bio)ethanol dehydration through microbial fermentation

[65]

biomedical applications such as biodegradable implants and internal fixations, tissue scaffolds, cardiovascular grafts, orthopedic prostheses, and drug-delivery systems [69,70]. It has been acknowledged that inadequate mechanical properties of pure polylactic acid (in both D and L derivatives) in high loadings could be compensated for by contributing to hybrid structures with desirable properties while they have the ability to copolymerize or blend easily with other polymers [68]. On the other hand, synthesizing self-reinforcement hybrid composites by integrating oriented nanofibers and ground matrices of the same material mimics collagenousbased soft tissues [72]. The poor physicochemical properties of polyglycolic acid, including high melting point and water solubility, confine its application in biomedical fields. However, its copolymerization with polylactic acid helps to control the rate of biodegradation of hybrid composites [73]. Polycaprolactone is another aliphatic polymer extensively used in biomedical applications [74]. The promising characteristic of polycaprolactone-based hybrid composites lies in their thermomechanical properties. The low melting point and glass transition temperature of caprolactone permit low molecular weight diffusion at body temperature in the rubbery state. For this reason, they are accepted as a good candidate for tissue engineering and drug delivery [20,49,71,75,76]. The improvement of selective properties of polycaprolactone, such as hydrophilicity and water uptake as well as cellular response, directly influences its biomedical applications and can be achievable through the hybridization with natural polymers. Significant enhancement of hydrophilic behavior resulting in hybrid biological activity has been reported recently in natural aliphatic polymer hybrid composites [19,77]. Polypeptides were introduced as hybrid composites with specific physical, chemical, and biological properties that are not available with synthetic

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biomaterials [78]. Their exceptional capability of adaptation to different hierarchical structures (coil, helix, and sheet) endows them with the desirable capacity for extensive and specific applications in the biomedical field. Chemically, polypeptides comprise fewer than 100 amino acids that are covalently linked by peptide bonds. It has been well described that the molecular weight of polypeptides influences the final structural conformation of hybrid composites. Nonetheless, some environmental factors such as ionic strength and temperature must be considered in fabrication methods [79]. Therefore, mechanical and thermal properties of polypeptide-based hybrid composites need to be improved by contributing other natural or synthetic polymers. In this case, all nanoconstituents must be folded intramolecularly under desirable solution conditions that create the possibility of introducing fibril structures into hybrid composites [80]. Like aliphatic-base hybrids, hybrid composites of polypeptides are biodegradable, and their rate could be controlled as a function of fibrils’ ultrastructure and functionality [81].

6.2.3 Mineral polymer hybrid composites Although most minerals are difficult to shape, inflexible, brittle, and do not disperse easily in composite materials, their incorporation in hybrid composites is a suggested method to overcome undesirable properties for biomedical applications. On the other hand, their excellent biocompatibility and osteoconductivity make them a preferred candidate for bone repair and implantation. Hydroxyapatite, nanoclays, bioglasses, and ceramic nanopowders are important biominerals that have been used in mineral polymer biomedical hybrid composites [82]. Synthesizing mineral hybrids with natural or biopolymers can not only produce highly flexible hybrid composites that form easily into any desired shape but also makes them osteoblastconductive while enhancing cell attachment, growth, and proliferation [63]. Therefore, the hybridization of polysaccharides or polypeptides (which are abundant in nature and involve low preparation cost) with biomineral nanopowders is a cost-effective method for turning them into bioactive biomaterials with potential applications in tissue engineering and drug delivery [38]. It has been claimed that, with the use of the precipitation of minerals in polymeric solutions, undesirable aggregation problems were solved and porous scaffolds were produced that had strong potential for the hard tissue substitution [82]. Table 6.2 shows the advantages of hybridization of different minerals with natural and biopolymers for biomedical applications.

6.2.4 Metal polymers hybrid composites Different types of metallic nanoscale particles, such as gold, silver, and metallic oxides (iron oxide, titania, zirconia, alumina), have been used to fabricate metal polymer biomedical hybrid composites due to their favorable physical properties (i.e., magnetic, antibacterial, and electrical conductive properties) [91]. These hybrid composites can be used as conductive scaffolds, smart drug-delivery systems, imaging agents, and biosensors [92]. The main problem of metal polymer

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Hybrid Polymer Composite Materials: Structure and Chemistry

Table 6.2 Applications of mineral constituents for improvement of the properties of mineral polymer hybrid composites Mineral

Polymer

Enhanced properties

Hydroxyapatite [82 84]

Polysaccharides

Strengthened weak and soft scaffolds while improving biological reactionA good candidate for hard tissue engineering

Polypeptides

Increased extracellular mineralization and improved intercellular communicationSignificant in treatment of bone dysfunctions and osteoporosis

Natural/ Synthetic

Increased osteoblast cells’ adhesion, proliferation, and expression. Good control of nutrient diffusion and wastesA good candidate for porous scaffolds

Calcium phosphate [85,86]

Natural/ Synthetic

Influenced bone cell signaling that led to good cell-surface interaction and prevented bone loss on an implant surfaceA good candidate for hard tissue engineering

Silica [15,87]

Natural/ Synthetic

Stimulated osteogenic differentiation in human stem cells and promoted collagen type I synthesisAn appropriate candidate for tissue scaffolds with bioactive characteristics

Nanoclay [67]

Natural/ Synthetic

Resulted in high surface interactions between hybrid constituents that enhanced elastomeric propertiesExcellent for joint connectors such as ligaments and tendons

Synthetic silicates [88]

Natural/ Synthetic

Induced osteogenic differentiation in absence of exogenous growth factorsA good candidate for injectable fillers while triggering specific cellular response

Bioglass [89,90]

Natural/ Synthetic

Produced bioactive hybrid composites while reinforcing polymeric networkSignificant in biodegradable implants while dissolution products can result in favorable biological responses

hybrid processing is a weak interaction between components that necessitate the surface functionalization of metallic nanoparticles. It has been reported that, although such improvement can be difficult, it also significantly influences physicochemical, mechanical, and biological properties of hybrid composites [93]. Depending on structural characteristics of polymeric matrices, the functionalization

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of nanometals with thiol incorporated into hybrid crosslinking results in the enhancement of mechanical properties. The enhancement of electrical conductivity for hybrid polymeric scaffolds was incorporated into the fabrication of smart hybrid composites with the facility of electric signal propagation in specific tissues [94]. Moreover, electroconductive hybrid composites have been suggested to be used as neural implants with nerve regeneration ability and cardiac markers. Covalently conjugated metallic nanoparticles with thermoresponsive polymers have been used for cancer therapy applications [92,95,96]. The heat generated during the incorporation of an external magnetic field into a metal polymer hybrid was shown to target infected cells and tumors while increasing the possibility of release of chemotherapeutics at different rates on specific sites. This targeted drug-delivery approach has been made achievable by the incorporation of metal polymer hybrid composites into cancer therapeutic methods. Osteoblast adhesion and proliferation have been seen as consequences of the incorporation of nanodimensional alumina and titania into polymeric matrices. In efforts to achieve desirable physical properties of metal polymer hybrid composites, functionalizing titania surfaces with amine groups was reported to facilitate covalent interactions between nanoparticles and polymeric chains [97]. Also the use of poly(ethylene glycol) as a spacer that conjugated to aliphatic polyesters could physically increase polymer crosslinking density while enhancing mechanical and bioadhesion properties of e hybrid composites [98]. It seems that the addition of spacers (as surface modifiers) such as polyethylene glycol, polyvinyl alcohol, and dopamine methacrylate to metal polymer hybrids enhances the hybrid’s energy dissipation, stiffness, elongation at break, and tensile strength properties by improving interfacial properties of its components. Such multicomponent hybrid composites can potentially be used to construct scaffolds for tissue engineering with good elastomeric behavior [99].

6.3

Biomedical application of polymer hybrid composites

Polymer hybrid composites with controlled morphology, physicochemical, and mechanical properties at the nanoscale comprise a sensational class of materials with unique properties that can be used for extensive biomedical applications [2]. These applications can be categorized into the five main groups of tissue engineering (hard tissue implants and soft tissue replacements), drug-delivery systems, imaging agents, medical devices, and therapeutic purposes. Their boundaries are merging, however, in relation to the living characteristics of body tissue, and next generation of hybrid composites should exhibit appropriate simultaneous combinations of these categories. Nowadays, tissue-engineered scaffolds need to demonstrate apposite combinations of mechanical and physicochemical support and morphological regulation for cell attachment, proliferation, and growth, while simultaneously serving substrate for therapeutic agents with sustained and targeted delivery. Thus, although here we have followed the classical categorization of

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polymer hybrid composites to better organize the presentation of their biomedical applications, the multidisciplinary application of polymeric hybrid composites should be kept in mind.

6.3.1 Controlled drug-delivery applications The development of controlled and sustained delivery of different pharmaceutical and biological drugs to patients has been seen over past decades, enhanced recently by new approaches in the fields of biotechnology, tissue engineering, biology, and material engineering and science [100]. Enhanced treatment effectiveness, reduction of side effects and complications, support for users’ convenience, as well as cost reduction of treatment are all advantages of delivering the proper amount of drugs at the right place and time. Polymer hybrid composites are currently being considered for drug-delivery applications with the promising ability to conjugate with biological macromolecules in various forms including membranes, films, particles (capsules or spheres), and even in scaffolding shapes in the macro size [38,101]. Innovative hybrid composites that can interact with subcellular structures to release pharmaceutical compounds directly into the cell and overcome the problem of biological barriers have attracted much attention [102]. Meanwhile, hydrogel hybrid composites have raised notable interest due to their specific characteristics and envirosensitivity. Generally, polymeric gels are either natural (chitin and chitosan, alginic acid, agarose, pectin, dextran, and different derivatives of these polymers) or synthetic [poly(ethylene glycol), poly(vinyl alcohol), poly(acrylic acid), polyacrylamide, and poly(methyl methacrylate)] [103]. With respect to the purpose of use, different organic and inorganic materials have been suggested for hybrid composite fabrication. Mineral polymer hybrid composites have been significantly used for drug delivery in bone treatment. In this case, the selection of mineral type must be on the basis of bioactivity, osteoconductivity or bioresorbability [104]. Bioglass as a bioactive material has been recommended when physical or chemical bonding to bone is required, whereas resorbed compounds like calcium sulfate are ideal for new bone formation in that bones will gradually be substituted for by hybrid composites. Calcium phosphate as an osteoconductive material is apposite for cell attachment, growth and proliferation and osteoinductive minerals (hydroxyl apatite) are beneficial for simulating stem cells. Therefore, bioglasses, osteoinductive, and osteoconductive materials are mainly used in fabrication of tissue engineering scaffolds with drug-delivery capacity. Nevertheless, resorbable materials are mainly preferred to use in the design and fabrication of biodegradable hybrid composites. Table 6.3 indicates applications of polymeric hybrid composites for drug delivery in bone. In-situ mucoadhesive hybrid composites are usually offered for vaginal, intestinal, respiratory, or nasal drug deliveries, in which their bioadhesion property is significant [36,88]. Thermosensitive in-situ-forming hybrids have received a great deal of attention because they are easily administered in a liquid form, whereas their gelation occurs in response to environmental changes [41,109 111].

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Mineral polymer hybrid composite applications in drug delivery

Table 6.3

Hybrid composites

Pharmaceutic agents

Calcium phosphate [14,101,105]

Polycaprolactone Polylactic glycolic acid) Chitosan Collagen

Antibiotics, growth factors, and hormones

Bioglass [106]

Polylactic glycolic acid Polymethyl methacrylate Polylactic acid

Antibiotics

Hydroxyl apatite [49,107]

Polycaprolactone Collagen Alginate Gelatine

Antibiotics, pain relief, and chemotherapy

Titanium zirconium [35,108]

Polycaprolactone Polyethylene

Antibiotics

Silica [63]

Polymethyl methacrylate

Pain relief

For their drug-carrier capabilities in acrylic-based hybrid polymer networks, functionalized natural polymers (chitosan, collagen, gelatine) have been suggested for ophthalmologic and gastrointestinal controlled release purposes [112]. Furthermore, the delivery of macromolecules (insulin, gene, DNA) by selfassembly of chemically modified natural polymers into hybrid composites has been considered [113]. Fractional conjugation via different linkages to soluble natural polymers yields the self-aggregation at different pH levels and enables hybrid composites to trap macromolecules within the structures [112,113]. Table 6.4 shows different polymers that have been used in hydrogel hybrid composites. Recently, applications of hybrid composites with the inclusions of carbon nanotubes (CNTs) and hydrogel have risen significantly in the fabrication of drugdelivery systems where miscibility at the molecular level might open new horizons to molecular chemotherapies. In addition, the possibility of finding stimulating hydrogels using either CNTs alone or nanotube graphene hybrid hydrogels should be mentioned as new research activities in this field [21,114].

6.3.2 Tissue engineering applications Although natural or synthetic polymers can be used alone for scaffold preparation, their hybrid composites with minerals are currently under development with the aim

148

Table 6.4

Hybrid Polymer Composite Materials: Structure and Chemistry

Hydrogel hybrid composites Natural hydrogels

Neutral

Cationic

Anionic

Amphipathic

Agarose, dextran

Lysine, chitosan and its derivatives

Hyaluronic acid, pectin, chondroitin sulfate, alginate

Chitin, fibrin, collagen, and gelatine

Synthetic hydrogels Polyethylene glycol, polylactic acid, polyglycolic acid, polyacrylic acid, polyacrylamide, polymethyl methacrylate, polyvinyl alcohol, polycaprolactone, polyacrylonitrile, polyhydroxyl methyl methacrylate

of increasing mechanical stability and biological interaction [115]. The use of scaffolds with drug-delivery capacity has been shown to be more effective than that of local release growth factors or antibiotics [35,116]. On the basis of research, it has been concluded that ceramic polymer hybrid composites with biodegradation properties seem to be a promising choice for bone tissue engineering [36]. The most frequently used polymers in this field are poly-α-hydroxyl esters mainly including polyglycolic acid, polylactic acid or their copolymers, where hydrolytic degradation by esterification makes them biodegradable and, more importantly, where nontoxic wastes from degradation are removed through natural pathways [70,78]. Polypropylene fumarate and polyhydroxyalkanoates aliphatic polyesters are other polymers that have been used as hybrid composites for scaffold preparation [117]. Because of the different mechanical properties of minerals and bone, most minerals cannot be used for load-bearing scaffolds unless introduced as hybrid composites. Table 6.5 gives the Young’s moduli for selected scaffold hybrid composites. Compared to other applications of hybrid composites, process and fabrication strategies in tissue engineering are important because mechanical and biological properties of scaffolds can vary as a function of the methods chosen [51,76,118,119]. It must also be kept in mind that the addition of bioactive phase to ground substances may alter the degradation property and kinetics of the hybrid as hydrophobicity and water uptake properties change. In Table 6.6, scaffold processing methods for various hybrid composites are compared. Although most minerals are bioactive agents, the incorporation of biomolecules has a significant scope in the application of scaffold surface modification. Immobilization of arg-gly-asp (RGD) proteins to a surface promotes cell-surface adhesion and interaction by increasing the wettability property [120]. In-vitro characterization of hybrid polymer composite scaffolds has been performed using cell culture from mouse fibroblasts (L929, 208F) marrow stromal cells, human lung carcinoma (A549), human osteosarcoma cell line (MG-63, SaOS-2), and human trabecular bone or rat osteoblasts [24,83,121,122].

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Table 6.5 Young’s moduli between selected hybrid scaffold composites and bone (dense and porous) Polymer

Mineral

Porosity

Modulus (GPa)

Hydroxyl apatite

Dense

6 14

Hybrid composite Polylactic acid (L) Polylactic acid (D, L)

1 3

Polylactic-co-glycolic acid

1000

Polylactic-co-glycolic acid

Hydroxyl apatite

Porous

Polylactic acid (L) Polylactic acid (L)

2 1500 9 12.9

Tricalcium phosphate

Dense

Polypropylene fumarate

6000 130 200

Polylactic-co-glycolic acid

Tricalcium phosphate

Porous

60 70

Polylactic acid (L)

Bioglass

Porous

130 250

Polylactic acid (D, L)

0.5 1.5

Polylactic-co-glycolic acid

50

Bone Cancellous bone

Porous

100 500

Cortical bone

Dense

12,000 18,000

6.3.3 Photodynamic therapy Photodynamic therapy acts by the localization of photosensitizer molecules in a tumor upon systematic administration. In comparison to chemotherapy or radiation therapy, this method of fighting cancer is more effective as it minimizes the damage to surrounding healthy tissues. The role of hybrid composites in photodynamic therapy becomes more obvious because most photosensitizer molecules are hydrophobic and cannot be simply injected intravenously. Moreover, an accurate predefined concentration of these molecules in tumors is required to reduce the risk of damage to healthy tissue [123 125]. To overcome these major challenges, the use of hybrid polymer composites in nanoscale has attracted more attention than that of the carrier of photosensitizer molecules. These special carriers have hydrophilic properties with a large surface area to size ratio, which makes them suitable for surface modification with functional groups to achieve deeper and better penetration into specified cells [126].

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Hybrid Polymer Composite Materials: Structure and Chemistry

Comparison of different scaffold fabrication methods for hybrid composites

Table 6.6

Controlling parameter

Highlight

Suggested applications

Processing method: thermally induced phase separation Macro and microstructure, pore morphology, mechanical properties, bioactivity, and degradation rates

Highly porous with extensive pore interconnectivity

Nerve, muscle, tendon, ligament, intestine, bone, and teeth

Processing method: solvent casting, aggregation, and particle leaching Porosity

Isotropic structure, organic solvent remaining, and uncontrollable pore interconnectivity

Bone

Processing method: coating Thickness

Weak adhesion, changing scaffold surface morphology, and composition

Implant surface modification

Processing method: nanoparticle sintering Controllable size of porosities and achievable complex shapes

Poor structural interconnectivity

Bone, teeth

Processing method: 3D bulk formation Highly controllable shape, properties, and structure by computer

Scaffold can be tailored to host biological tissue with good interface

Nerve, muscle, tendon, ligament, intestine, bone, and teeth (limitation for material selection)

Although all polymeric hybrid composites that have been used for drug-delivery systems can be selected for photodynamic therapy purposes, a ceramic-based hybrid composite seems to be more effective as size, shape, porosity, and monodispersity of the particles are easily controlled during the preparation. Also, no swelling or porosity changes occur as a function of environmental alteration (e.g., pH values and temperature) [127].

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6.3.4 Wound-dressing applications The healing of a wound is a complex physiological process involving cell growth, proliferation, and migration, which requires collaborative efforts of many different tissues and cell linkages. The criteria that affect the performance and quality of wound dressing include semipermeability to ensure sufficient nutrients and gas exchange, notable swellability to control the wound’s environmental moisture, prevention of infection, and provision of a favorable wound microenvironment for all cell activities. Recent research focusing on wound care has emphasized new therapeutic approaches and the development of technologies for acute and chronic wound management. Substantial effort has been devoted to exploring new materials to accelerate wound healing or improve wound-dressing properties by synthesizing and modifying materials that are ecofriendly and sustainable [128]. New approaches to wound care involve the use of polymeric hybrid composites via advanced technologies in combination with the provision of enhanced swelling capability and desirable mechanical and physiochemical properties [42]. Natural polymers consisting of a large number of glucose units joined by glycoside bonds have been reported to affect the intrinsic coagulation pathway by virtue of their physicochemical properties. Furthermore, they have the potential to act as natural adhesives by dehydrating mucosa, causing reversible shrinkage of epithelial cells and promoting repair of damaged tissue. In light of these merits, natural polymer hybrid composites in recent years have been considered for wound management. Scaffolds prepared from natural polymers satisfy the requirements of adequate thermal stability and minimal interference with flow properties essential for wound-dressing applications. However, natural polymer-based products have some disadvantages, such as relatively poor mechanical properties and highly hydrophilic character. These disadvantages limit their use in a variety of applications. The use of natural polymer hybrid composites is a strategy to improve these undesirable properties [11]. Gelatine is a good candidate for enhancing the properties of other natural polymers. Although soft and porous gelatine would have direct contact with tissues, it is not expected to cause any damage to wound areas [129]. In its biological properties, gelatine shows no antigenicity, exhibits the activation of macrophages with a high hemostatic effect. Research has focused on the inclusion of cell-growth-enhancing factors in gelatine and the use of gelatine-based dressing materials. Fabrication methods interactions between wound environment and dressing, wound-dressing properties, release of pharmaceutical agents, and infection control have also been reported in other research related to polymer hybrid composites.

6.3.5 Bioadhesion applications The uses of polymer hybrid composites for bioadhesion purposes can be divided into two major categories. The first is the improvement of surface characterization of implants by biofilm formation or coating adhesion [130,131]. The second category is the application of polymeric hybrid composites as bioadhesives for soft or

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Hybrid Polymer Composite Materials: Structure and Chemistry

Optimization methods for enhancing the bioadhesion properties of biomedical hybrid composites

Table 6.7

Optimization

Process definition

Chemical

Chemical attachment of specific functional groups to the hybrid composite surface

Mechanical

Surface alteration through texturing, changing the morphology and roughness of the surface

Physical

Modification including cleaning or chemical composition substitution subsequent to physical processing

hard tissue adhesion, clotting purposes or mucosal drug-delivery administrations [84,132,133]. In the latter case, bioadhesion refers to the series of different reactions (temporary or permanent) at the hybrid biological interface, helping the composites to adhere to the tissue, which are relevant in drug delivery, dental, surgical and wound-healing applications [88,134]. Bioadhesive hybrid composites usually consist of fibrinogen, plasma glutamines, thrombin, and lectins. The optimization of bioadhesion process is usually established via physical modifications, mechanical interactions, chemical processing, or their combinations. Table 6.7 provides more information about the processes of optimization. The adhesion of hybrid composites to biological surfaces occurs mainly through three different pathways: (1) binding to the specific cell’s surface receptor sites, (2) changing in nature when placed in water, and (3) attracting by the formation of a series of electrostatic interaction bonds. Understanding of bioadhesion mechanisms is therefore important to explore desired applications as well as to ensure the successful prevention of undesired adhesion to biomolecules, cells, or organisms [135]. From the practical point of view, bioadhesive hybrid composite systems have been used for many years in medical applications such as dentistry and orthopedics and are now entering new fields such as tissue sealing and directed drug-delivery systems.

6.3.6 Biomedical applications of composition-based hybrid composites As categorized already, the addition of natural polymers, minerals, biopolymers, or metals to the polymeric substrate in the molecular scale can create biomedical hybrid composites. This is an area of multidisciplinary research where scientist, engineers, and biologists come together. The categorization of these novel materials may be achieved by differences in structure or by types of materials used. The applications of hybrid composites are presented on the basis of composition below (Tables 6.8 6.11).

Biomedical applications of natural polymer hybrid composites

Table 6.8

Cellulose-based applications Fibrous cellulose and cellulose acetate [18,68,136]

Textile wound dressings, barrier films, packaging

Cellulose esters [18,60]

Separation membrane

Carboxylate methyl cellulose [136]

Drug-delivery systems, pharmaceutical agents, cosmetics

Cellulose particle [136]

Analytical biochemistry

Collagen-based applications Collagen [16,137,138]

Ophthalmological drug-delivery systems, skin replacement, bone scaffolds, artificial heart, and vascular implants

Gelatine-based applications Gelatine [61,139]

Pharmaceuticals and cosmetics, wound dressings, microencapsulation

Starch-based applications Starch [65,72]

Bone filler, blood coagulation agent, bone replacement, orthopedic implants and fixation, bone cement

Modified starch (derivatives) [65]

Food and nutrition, drug-delivery systems

Thermoplastic starch [65]

Bioadhesive

Alginate-based applications Alginate [35,63,140]

Table 6.9

Pharmaceutical and drug-delivery systems, dental materials

Biomedical applications of biopolymer hybrid composites

PBS hybrid composites [141]

Medical materials, orthopedic applications, and encapsulating agents

PHAs hybrid composites [83]

Tissue engineering: scaffolds for nerve regeneration, skin regeneration, cardiac implants, heart valve scaffolds, and artificial heart valves Pharmaceutics: drug-delivery systems, drug molecules, and cosmetics Medical treatment: gastrointestinal patches, sutures

PLA PGLA hybrid composites [68,142,143]

Biodegradable implants and sutures, orthopedic implants, fixation and metal coatings, bone screws, wires and plates, cardiovascular and neurological implants, cellular based scaffolds, gene therapy applications, and drug-delivery systems

Biopolyethylene [65]

Plastic part of orthopedic implant with the same physicochemical properties as petroleum-based polyethylene

154

Table 6.10

Hybrid Polymer Composite Materials: Structure and Chemistry

Biomedical applications of mineral hybrid composites Mineral hybrid composites

Modified

Unmodified

Organomineral

Mineral polymer hybrid

Mineral drug hybrid

Synthetic

Natural

Types of polymeric ground substance Chitosan, polyglycolic acid, polylactic acid, polyisopropyl acrylamide, gelatine, cellulose, polyethylene oxide, polyurethane, polyvinyl alcohol

Application Cosmetic [144]

Sunscreen, personal care products

Biosensors [145 147]

Electrochemically based sensors

Medical devices [148]

Medical plastics, isolators, patches, implants

Pharmaceutics [63,101]

Excipients, drug carriers, active ingredients

Biomaterial [148]

Tissue scaffolds, tissue engineering, drug-delivery systems, bone replacement, injectable gels, bone fillers

Table 6.11

Biomedical application of metal hybrid composites

Bioimaging [149,150]

Metal hybrid composites as contrast agents can enhance the signal from the imaging system. Utilizing nanomagnetic particle based hybrids to enhance the signal from the targeted living system has been attempted in various biotools

Biosensing [91,151]

Nanomagnetic particles as a part of polymeric hybrid composites with the ability to quench fluorescence have been used for immune-biosensing activities

Bioseparation [152]

Metal hybrid composites have been suggested for separating biomolecules in a highly viscous fermentation because particles in such solutions are not easily centrifuged, filtered, or separated by packed bed chromatography

Hyperthermia for cancer therapy [8,149,153]

Low-heat hyperthermia is a cancer treatment with minimal damage to the normal tissue while using metal polymer hybrids for hyperthermic therapy

Biomedical polymer hybrid composites

6.4

155

Conclusions

Combined with increased understanding of the function and interaction of implant tissue interfaces and accumulated knowledge of polymer hybrid composites including structure, fabrication, and characterization, it is now clear that the use of hybrid composites for biomedical applications is more compatible with host tissues. Polymer hybrid composite biomaterials are particularly attractive because of their tailorable manufacturing processes. Such materials have therefore been deemed as a promising class of polymer-based hybrid biomaterials with applications ranging from tissue engineering for hard tissue to soft tissue replacements.

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7

Shahad Ibraheem and Sri Bandyopadhyay School of Materials Science and Engineering, UNSW, Sydney, NSW, Australia

Chapter Outline 7.1 Introduction 163 7.1.1 7.1.2 7.1.3 7.1.4

Composite materials 164 Matrix material 164 Matrix material 167 Examples of recent fillers used as reinforcement materials (CNT, MCF, and FA) 169

7.2 Coal combustion products

172

7.2.1 Fly ash 172

7.3 Conclusions 182 References 186

7.1

Introduction

More and more attention is being paid to the problems encountered during conservation work because of the steel structure which are subject to corrosion and fatigue problems, so it need to be prepared, or when carrying out seismic upgrading to prevent or limit the effect of earthquakes on building stock [1 3]. This attention ranges from ordinary buildings of historical and artistic interest and has never before been the subject of so much interest, especially in seismic countries like the United States of America, Japan, and Italy. For strengthened applications, structural adhesives need to be used. The most common adhesive used in such applications is epoxy for its excellent adhesion properties to other materials, but epoxy is brittle; so, there is a need to get rid of the brittleness of the epoxy resin by toughening/strengthening it with other rigid fillers such as carbon nanotube (CNT), fly ash (FA), and milled carbon fiber (MCF) [3 9]. Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00007-0 Copyright © 2017 Elsevier Ltd. All rights reserved.

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CNTs recently have been used as filler to enhance strength and modulus values for the composite in order to enhance mechanical, physical, and morphological properties of the matrix [10]. FA is a waste by-product produced with large amounts in thermal power stations. FA is cheap and light in weight compared to other filler materials such as calcium carbonate (CaCO3) [11,12]. FA also is abundantly available locally as a waste material and is technically equivalent or superior to high-class ceramics [11]. MCF is derived from recycling of carbon-fiber-reinforced materials. The only solution to dispose such carbon-fiber-reinforced polymer (CFRP) is dumping it in landfilling or waste incineration as carbon fibers do not dispose naturally [3,13]. The efforts are being recently focused on using recycled MCF, which have good properties, low cost, and also to limit the dumping process.

7.1.1 Composite materials Composite material is defined as a combination of two or more materials—those materials are different in their properties—but once they mix together, they often bind with each other physically, chemically, physics-chemically, and/or mechanically. Binding does not mean solving, but it implies overlapping between the two phases [14,15]. Composite materials have many significant advantages to the industry like weight reduction (approximately 20% 50%), corrosion resistance, fatigue resistance, equivalent or better mechanical properties and fewer fasteners leading to lower assembly costs, and superior fracture-resistance properties [14,15]. The designing of a composite for structural and aerospace application from epoxy matrix-reinforced CNT, FA, and MCF can be shown as below [14]: 1. If the chosen reinforcements have high strength and stiffness with low density. 2. The chosen matrix has good shear properties with low density. 3. Then, the product will be a composite which will have high strength, stiffness, and shear properties with low density.

7.1.2 Matrix material Matrix material actually is a polymer usually less tough and also weak, but by adding one or more reinforcement materials, this addition would enhance the strength/ or elasticity of the polymer (matrix) [16 18]. Fig. 7.1 shows the polymer matrix composites (PMCs) and reinforcing materials. The advantages of polymer (matrix) used for PMCs are as follows: 1. 2. 3. 4.

Get light weight material [20] Reduce corrosion [21] Reduce fatigue [21] Good mechanical properties [21]: Matrix material works as a fiber alignment. Matrix material also works as load transformer among the fibers. G

G

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Figure 7.1 Polymer matrix composites [19].

Figure 7.2 Airplane [21].

G

G

The toughness of the composite increases when using PMC. Matrix material works as filler damage protection [22].

For these reasons the polymer composites has wide range of applications. Some applications for the polymer composites: 1. Airplane is a good example for polymer composites [21,22]. Fig. 7.2 shows an airplane and Fig. 7.3 shows polymer composites that have been used in the airplane. Polymer composites have been used in the airplane as follows: a. Aircraft frame b. Protection of the windows glass layer (because once the glass is scratched, it increases the risk of damage (break) and putting the passengers in big danger) c. Insulate the electric parts d. Aircraft internal accessories [22].

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Figure 7.3 Fiber-reinforced plastics used in the airplanes [22,23]. 2. Boat frame Now, 90% of modern boats are made of PMCs like polyester-resin reinforced by fiberglass. This combination of the composite is better in both chemical and mechanical properties from woods, as well as metals. Figs. 7.4 and 7.5 show modern boat and modern boat structure [24]. 3. Structural strengthening To reduce the danger of earthquakes on the structural applications, fiber-reinforced plastics (FRPs) have been used. The fibers used are either carbon or glass and used as rods, fabrics, plates, and as meshes [1]. Fig. 7.6 shows how FRPs have been used in structural applications. Fig. 7.7 shows polymer composite used as a filler in dental applications. 4. Dental composites (white filling)

Dental composite is a resin reinforced with filler such as silica, glasses, and glasses ceramics [25,26]. The reason behind using filler is to improve the wear and fracture resistance of the composite, translucent, and used as well for teeth cosmetics [25,27]. Bonding agent like silane is used to ensure that the filler and the resin are perfectly adhesive to each other. Then when the mixture is filled in to the tooth, the ultra violet light has to be used to cure the resin composite [28].

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Figure 7.4 The figure shows modern boat [24].

Figure 7.5 The figure shows boat frame structure; polymer fiber glass reinforcement [24].

7.1.3 Matrix material 7.1.3.1 Epoxy resins Epoxy resin, a thermoset polymer, is widely used in PMCs [9]. Epoxy resin belongs to the thermoset family which consist of dipoxy (the resin) where react together or get “cured” [29]. The curing process can be affected or controlled by temperature [29]. The final product obtained is not a linear polymer, instead, it is a crosslinked network. Thus, all diamines and all the epoxy molecules become one big molecule [30]. So, the result is a hard substance that is very strong and cannot be processed further.

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Figure 7.6 The figure shows different kind of structural applications for the fiber-reinforced polymer [1].

Figure 7.7 The figure is showing curing by UV light using in dental application [25].

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In general, thermosets are more strong than thermoplastics because of the threedimensional bonds [31]. Epoxy has 1. 2. 3. 4. 5.

excellent adhesion to other materials, chemical resistance, heat resistance, good mechanical properties, and electrical insulation [9,32]

Because of the above-mentioned properties, epoxy has many uses in fiber applications, optoelectronics, and dentistry [30], laminates, castings, fixtures, and molds [29,33]. Also, epoxy is used in electronics industry to make insulators, transformers, generators, and switch gear [29,33,34]. Epoxy resins also have excellent use in repairing (as adhesive), pottery, glass, wood, metal, and leather objects [29,32,33]. Epoxy can be used in jewelry making [33] and in paints and coatings [29,32].

7.1.4 Examples of recent fillers used as reinforcement materials (CNT, MCF, and FA) 7.1.4.1 Carbon nanotubes CNTs are the most stiff fiber in the industry; it has tensile strength value 50 100 GPa and also has the highest modulus value known 1.4 TPa. So, CNTs have been recorded as an attractive and a great interest recently as structural reinforcement [35,36]. Although CNTs came from same graphite sheet, they are different in 1. Length 2. Thickness of the spirals 3. Layers number [36 41].

Types of CNTs: Single-walled CNTs: Single-walled CNT (SWCNT) is one layer thick from graphite atoms (graphene), which is wrapped in to a “seamless cylinder” (see Fig. 7.8) [36,42]. SWCNTs have diameter about 1 nm with length of thousands of times larger. Multiple-walled CNTs: Multiple-walled CNTs (MWCNTs) are graphite sheets, which roll in layers shape. MWCNTs can be either concentric cylinders of graphite sheets (sheets of graphite) which rolled around it selves or a scroll of parchment of one graphite sheet which can be roll around itself making something like a rolled newspaper (Fig. 7.9) [36,43]. Double-walled CNTs: Double-walled CNTs (DWCNTs) are formed from only two layers of graphite, the simplest member of the family of MWCNTs, which are formed from two layers of graphite (coaxial layers) [36,44,45] as seen in Fig. 7.10A and B.

Figure 7.8 (A) SWCNTs and (B) TEM for SWCNTs [36,42].

Figure 7.9 (A) MWCNT and (B) TEM for MWCNTs [36].

Figure 7.10 (A) DWCNT and (B) SEM for DWCNTs [46,47].

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Figure 7.11 The figure shows (A) CSCNTs and (B) SEM for CSCNTs [36]. DWCNTs have better physical and mechanical behaviors in comparison with SWCNTs as well as MWCNTs [46,47]. Cup-stacked CNTs: Hollow tubular shape consists of cup-shaped carbon units. The diameter varies from 50 to 150 nm and length can be up to 200 µm (see Fig. 7.11) [36,46 50].

7.1.4.2 Milled Carbon Fiber MCF is recycling to the carbon fiber, with length ranging 1 mm or less; so, it is shorter than the chopped carbon fiber [3,13]. CFRP is usually used in structural and aerospace applications. CFRP have high tensile strength (tensile value about 3800 MPa and modulus of elasticity about 240 GPa). CFRP is also perfect to resist corrosion and fatigue [3,51,52]. Carbon fiber is expensive, about 8 10 times more compared to E-glass fiber; so, the need for a replacement filler material is always subject to further study. Not many studies have been done on the epoxy-filled low-cost MCFs [3,53]. In this paper, use of MCF will be highlighted with other fillers such as FA and CNT with epoxy resin as matrix—as new and exclusive research—and investigate how the matrix and the fillers will behave under different tests.

7.1.4.3 Fly ash Several by-products are produced when coal is burnt to generate electricity in thermal power stations. Those products of burnt coals are as follows: G

G

G

G

FA Bottom ash Boiler slag Gypsum from desulfurization of flue gas

These are known collectively as coal combustion products (CCPs) [11,54 57]. FA is major product from CCPs [11,58]. It comprises up to 60% 90% of the total CCPs produced.

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Coal combustion products

7.2.1 Fly ash FA generated in coal power stations [5,38 40,59]. It is a mixture of oxides, which can enhance their potential for use in different ceramic applications. The size of FA particles varies between 0.5 µm and 100 µm [45], and they are typically present as solid, irregularly shaped spheres—or as cenospheres (hollow spherical shapes) [12,60 62]. FA is typically gray-black in color, varying from light color to dark color depending on the amount of unburnt carbon in the FA and also other oxides impurities. If the FA has light color, it is due to having low residual carbon [63 66]. The physical properties (size, color) and chemical properties (composition) can be different due to the used coal type [63,64,67,68]. Fig. 7.12 can show two scanning electron microscopy (SEM) images for FA in two different magnifications. The amount of FA in CCP varies between 55% and 65% or even more, and it depends on quality and composition of coal and also the method used for combustion [69 71]. For example, in year 2006, FA produced in the USA was 58% of the total CCP produced; whereas in Europe, it was 66% [71] (see Table 7.1). Sources of the FA FA is produced in large amounts when burning carbon in combustion chamber to generate electricity in electric power stations [72]. Fig. 7.13 shows the steps of FA production process. FA advantages: 1. Light weight 2. Cheap, and 3. Wisely available locally material [11,12].

FA has good physical and chemical properties. FA consists of various oxides such as silica oxide—SiO2, alumina oxide—Al2O3, and iron oxide—Fe2O3 [74] and also has small amounts of magnesium oxide—MgO, calcium oxide—CaO, potassium oxide, sodium oxide, and titanium oxide [74 76].

Figure 7.12 SEM images of fly ash particles [11].

Table 7.1

Total CCPs generated with share of each CCP in the United States and Europe [71]

Country

Fly ash Bottom ash Boiler slag FGD and other Total

USA (million tons)

Europe (million tons)

2002

2004

2006

2008

2002

2004

2006

2008

76.5 (59%) 19.8 (14%) 19.1 (1.4%) 30.4 (23%) 128 (100%)

70.8 (57%) 17.2 (14%) 22.02 (1.7%) 31.39 (25.6%) 122.4

72.4 (58%) 18.6 (14%) 20.02 (1.6%) 30.18 (24.1%) 122.5

72.5 (53%) 18.4 (13%) 20.2 (1.4%) 25.4 (18%) 136

NA NA NA NA NA

43.4 (67%) 5.8 (9%) 1.9 (2.9%) 13.0 (20%) 64.1

40.4 (67%) 6.13 (10%) 1.7 (2.7%) 12.7 (20%) 60.9

NA NA NA NA NA

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Figure 7.13 Schematic of a typical coal-powered power station [73].

There are two types of FA depending on ASTM (The American Society for Testing and Materials) C618: 1. FA type F 2. FA type C [72] 1. FA type F [59,86]: The calcium ranging in the FA type F is between 1% and 12% [86]. The lime content is around 10% [69]. 2. FA type C [59,86] The calcium content is higher ranging between 20% and 40% [76]. The lime content is higher than 20% [76]. G

G

G

G

FA applications: FA has been used in structural field, cement, concrete [54,55,76,77], road base, paints [11], FA bricks/tiles, [11,56,57,76], roofing tiles [11], mining application, rod base/subbase, mineral fill, gypsum panel products [11,78], gypsum panel products [11], and filler in wood [11]. In the recent years, the effort has been made to utilize FA in polymer composites [79] such as FA polypropylene composite [80,81], FA polyester [82], and FA epoxy composites [83], whilst the epoxy thermoset has high strength. The properties of the composite produced depend on the size, shape, concentration, and the way the filler distributes in the matrix.

7.2.1.1 Researches reported on fillers reinforced plastic A. First Study: “Epoxy enhanced by recycled milled carbon fibres in adhesively bonded CFRP for structural strengthening” [3]: This study dealt with MCF to enhance the mechanic behavior and electric resistance for the epoxy. MCFs ratios used for this research were 1.5%, 3%, and 5% [3].

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Tensile valuation was done on ASTM specimens D638. The results of the tensile test showed that modulus of elasticity and tensile strength has improved for more than 1.5 wt%. SEM also has been done for previous weight percentages and SEM images for the fracture surface showed that the short MCFs have “pulled out” from the epoxy matrix, which enhanced the mechanical properties [3]. With addition of 5 wt% MCF, there was reduction in electrical resistivity by the magnitude by four orders versus pure epoxy because of the conductivity properties of the MCFs. Tension test was done on the “Steel/CFRP double strap joints (with either CFRP sheets or CFRP laminates)” which were mixed with epoxy—no apparent increase in stiffness and strength values was observed [3]. The fracture surface for both pure epoxy and reinforced epoxy was studied and evaluated by SEM [3]. Main concluding points of this study: G

G

G

Adding MCFs more than 1.5 wt% to the epoxy could improve E-modulus as well as tensile strength by 7.2% for 3 wt% MCF, also 15.2% for 5 wt% MCF. E-modulus was 30.1% higher when 3 wt% of MCF was added, and 50.5% higher with 5 wt% of MCF [3]. SEM images showed good dispersion for MCFs particles in the epoxy in a different “orientation’” in the fracture surface. Some pullout MCFs were found in the SEM images which give better strength and modulus values. Debonding was found in the composite due to the composite ductility [3]. No countable values for stiffness and strength were found for steel/CFRP for either sheets or laminate joints reinforced epoxy [3]

Figs. 7.14 7.16 and Tables 7.2 7.4 show mechanical properties and SEM images for all composites.

Figure 7.14 The figure shows the mechanical test for (A) EP specimens; (B) DJS specimens; and (C) DJL specimens [3].

Figure 7.15 Stress strain diagram for pure epoxy (EP0-3) and epoxy-reinforced MCFs (EP1.5-3, EP3-3, and EP5-3) [3].

Figure 7.16 SEM for (A) 0 wt% MCF; (B) 1.5 wt% MCF; (C) 3 wt% MCF; and (D) 5 WT% MCF [3].

Table 7.2 The table shows the mechanical test values for the MCFs reinforced epoxy [3] Scenarios

Materials

Specimens

Weight ratio of milled carbon fiber

Carbon fiber sheet or laminate layers

EP (epoxy)

Milled carbon fiber, epoxy

dog-bone coupons

0%, 1.5%, 3%, and 5%

NA

DJS (doublestrap joints with CFRP sheets)

Milled carbon fiber, epoxy, CFRP steel, steel plate

double strap joints

0%, 1.5%, 3%, and 5%

1 and 3

Milled carbon fiber, epoxy, CFRP laminate, steel plate

doublestrap joints

0% and 5%

1

Table 7.3 The table shows E-modulus, strength, and U-strain for epoxy reinforced (average of three specimens) [3] Specimen

Weight ratio (%)

E-modulus (MPa)

Strength (MPa)

EP0-y EP1.5-y EP3-y EP5-y

0 1.5 3 5

2.06 6 0.06 (100%) 2.20 6 0.10 (106.8%) 2.70 6 0.17 (131.1%) 3.10 6 0.73 (150.5%)

34.8 6 1.1 (100%) 32.8 6 1.4 (94.3%) 37.3 6 1.7 (107.2%) 40.1 6 3.3 (115.2%)

The table shows electrical resistivity for only two composites 0 wt% MCF-epoxy and 5 wt% MCF-epoxy [3] Table 7.4

Samples

Length a (mm)

Width b (mm)

Thickness t (mm)

Resistivity ρ (Ω m)

Epoxy with 0% MCF weight ratio

E1 E2 E3 E4

20.0 19.96 19.99 19.99

5.77 5.81 5.82 5.82

0.69 0.69 0.93 1.23

. 1.67 3 107 . 1.68 3 107 . 1.25 3 107 . 9.46 3 106

Epoxy with 5% MCF weight ratio

EM1 EM2 EM3 EM4 EM5 EM6 EM7 EM8

19.94 19.94 20.0 19.96 20.04 19.95 19.96 19.96

5.95 5.94 5.96 5.95 5.97 5.90 5.88 5.93

1.46 1.33 1.52 1.17 1.35 1.05 1.28 1.23

. 8.13 3 106 . 8.91 3 106 . 7.84 3 106 8.44 3 101 . 8.86 3 106 2.23 3 102 4.00 3 102 1.10 3 102

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Figure 7.17 20 wt% FA epoxy [84].

B. Second study [84,85]: It is combination of two studies for same materials done by the present authors with different test techniques a. “SEM study of debonding/pullout features & FTIR analysis of bonding in fly ash epoxy particulate composites” [84] b. “Use of Secondary Ion Mass Spectrometry (SIMS) to identify fly ash mineral spatial and particulate distribution in epoxy” [85]

In this study, the epoxy matrix was mixed in 100:50 parts (resin:hardener), then 0%, 10%, 20%, 30%, 40%, and 50% FA by weight were added to the matrix and mixed thoroughly [84]. As can see in Fig. 7.17; five tensile specimens were casted for 20 wt% FA epoxy prior to tensile testing.

7.2.1.2 SEM study [84] 5000 3 magnification: SEM figures (Fig. 7.18A F) for various weight percentages of FA epoxy composites for tensile fraction using high magnification 5000 3. SIMS for 10 wt% FA epoxy [85] A very advanced SIMS test was used for the 10 wt% and 50 wt% FA epoxy composites to verify the distribution of the elements as well as the contaminations. Fig. 7.19A G shows the 10 wt% FA epoxy [85]. SIMS for 50 wt% FA [85]: In Fig. 7.20A G, one can find various elements as detected by SIMS test for 50 wt% FA epoxy.

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Figure 7.18 (A) 0 wt% FA [84], (B) 10 wt% FA [84], (C) 20 wt% FA [84], (D) 30 wt% FA [84], (E) 40 wt% FA [84], and (F) 50 wt% FA [84].

Summary of the second study [84,85] Australian FA has been used to fabricate 0, 10, 20, 30, 40, and 50 wt% FA epoxy composites. DGEBA (Diglycidal Ether of Bisphenol A) with cycloaliphatic polyamine as a crosslinking agent have been used and cured under 120 C/2 h. G

G

SEM test has been done for the fractured specimens to evaluate the strength and the material boning, debonding, and pullout [84]. Good bonding has been observed between FA and the epoxy [84].

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Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 7.19 (A) SIMS for Si [85], (B) SIMS for Mg [85], (C) SIMS for Fe [85], (D) SIMS for S [85], (E) SIMS for Ca [85], (F) SIMS for K [85], and (G) SIMS for Al [85].

G

G

G

G

Evidence of debonding and pullout in the fractured surface which indicates good adhesion between FA filler and epoxy matrix [84]. SIMS of the two composites 10 wt% and 50 wt% FA-epoxy has been studied [85]. Little amounts of Si, Mg, Fe, S, Ca, K, Al, Mn, and O were detected in FA epoxy composites using SIMS [85]. SIMS is a useful test to detect elements distribution and contamination as well [85].

C. Third study: “Mechanical properties of epoxy resin fly ash composite” [9] The study deals with raw FA [9]: G

G

Epoxy resin is Araldite (LY-554). HY-951 as a hardener used to fabricate all samples.

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Figure 7.19 (Continued). G

G

G

Glass fiber used E-glass (E-300) Material. Mixing ratio was (100 parts) of Araldite and (10 parts) for the hardener. The fabrication was done at room temperature [9].

Their observations were detailed in Table 7.5 and Figs. 7.21 7.27. Overall, they found that the compressive strength increased whilst the impact strength decreased with FA addition. G

G

Compression test Fig. 7.21 shows compression strength versus different wt% FA; as can be seen from the figure, the compressive strength increases because some FA particles are hollow due to the hollowness of FA [9]. Impact testing Fig. 7.22 can show the impact strength for FA epoxy composites [9].

Summary of the third study: G

As a result, the hollow FA particles give strength to the composite.

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Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 7.20 (A) SIMS for Al [85], (B) SIMS for Mg [85], (C) SIMS for Ca [85], (D) SIMS for K [85], (E) SIMS for Fe [85], (F) SIMS for Si [85], and (G) SIMS for S [85]. G

G

Using fiber glass enhanced impact and compression strength because of energy absorption with the pullout fibers. SEM test shows that FA particles have been uniformly segregated (Figs. 7.25 7.27).

7.3

Conclusions

1. Recently, many attempts have been made to utilize nanofillers such as CNTs due to their excellent properties. 2. FA has lately been recognized as a potential reinforcement in epoxy resin [70], and it appears that FA can be attractive because of its easy availability, chemical reactivity, and bonding characteristics [77,86]. 3. Reducing the composite cost is another important subject to study. Low cost of FA and MCFs will help to reduce the composite cost [87] as well as give better properties.

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Figure 7.20 (Continued).

4. It is interesting to see that applications of both MCF and FA in recycling lead to help the environment and reduce pollution. 5. It has been found in the studies that using CNTs, FA, and MCFs as reinforcement materials in polymers resulted in improved mechanism [9,34,88]. 6. Epoxy is the most used polymer among PMCs [59,86]. 7. However, epoxies themselves can be modified as improved, strengthened, and toughened matrix materials. Various materials including hybrid reinforcements have been used for this purpose [76,89 92]. 8. Composites based on epoxy resins need strengthening, stiffening, and toughening; thus, particulate and fiber reinforcement (such as FA, MCF, CNT, glass, and carbon) are used for that purpose to make a better and improve matrix material [87,90,93]. 9. The failure mechanisms of the composites are governed by fracture, debonding, and pullout of the reinforcement—thereby absorbing crack tip energy and contributing to improved fracture properties as well as strength and stiffness [34,87 89,94,95].

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Table 7.5

Nomenclature of material fabricated [9]

Material designation

% Fly ash (by weight)

% Resin (by weight)

% Glass fiber (by wt)

C1 C2 C3 C4 C5

30 38 46 54 38

70 62 54 46 60

Nil Nil Nil Nil 2

110

Copresive strength

105 C1 100

C2

95 90 C3 C4

85 80 20

30

40 50 % fly ash (by weight)

60

70

Figure 7.21 Comparison strength versus wt% FA in various percentages [9].

1 C1

Impact strength

0.9 0.8 C2 0.7 0.6 C3 C4

0.5 0.4 20

30

40 % fly ash (by wieght)

Figure 7.22 Impact strength versus various wt% from FA [9].

50

60

A review of the structure property research on hybrid-reinforced polymer composites

Figure 7.23 Compressive strength values [9].

Figure 7.24 Charpy Impact Strength values [9].

Figure 7.25 The figure shows SEM image for of fibers and epoxy matrix.

185

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Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 7.26 The figure shows the fracture surface of fiber glass [9].

Figure 7.27 Fly ash in epoxy matrix [9].

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[52] Hollaway LC. A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties. Constr Build Mater 2010;24:2419 45. [53] Chand N, Nigrawal A. Investigations on D.C. conductivity behaviour of milled carbon fibre reinforced epoxy graded composites. Bull Mater Sci 2008;31:665 8. [54] Maharashtra fly ash information, http://www.fly-ash-informationcenter.org.in/index, Aug 6, 2010. [55] Making of better concrete with material from America’s coal ash leader, headwater resources fly ash for concrete. http://www.flyash.com/, 2013. [56] N. Bouzoubaa and S. Foo, (January 2005), “Use of fly ash and slag in concrete: a best practice guide, material technology laboratory”, MTL 2004-16 (TRR), http://www.docstoc.com/docs/27782091/Use-of-Fly-Ash-and-Slag-in-Concrete-A. [57] Fatih T, Atalay U. Utilization of fly ash in manufacturing of building bricks, international ash utilization symposium. Eastern Turkey: Centre for applied Energy Research, University of Kentucky; 2001. p. 13 8. [58] ADAA/Ash Development Associated of Australia, Copyright © 2011 Ash Development Association of Australia Copyright © 2011 Ash http://www.adaa.asn.au/ ?/products, 2017. [59] Zaeni A, Bandyopadhyay S, et al. Colour control in fly ash as combined function of particle size and chemical composition. Fuel Sci Technol Fuel Energy 2010;89:399 404 http://www.simpas.unsw.edu.au/publications/2010/Colour%20control %20in%20fly%20ash%20as%20a%20combined%20function%20of%20particle%20size %20and%20chemical%20composition,%20Fuel.pdf. [60] Bulletin No.2 chemical comparison of fly ash and Portland cement, Headwater resources, Rev 3/05, http://www.flyash.com/data/upimages/press/TB.2%20Chemical %20Comparison%20of%20Fly%20Ash%20and%20Portland%20Cement.pdf; 2015. [61] Coal ash fact sheet, American Coal council and American Coal Ash Association http:// www.uswag.org/ACCACAA_Ash_Fact_Sheet.pdf, 2008. [62] Mehta PK. Influence of fly ash characteristics on strength of Portland cement-fly ash mixture. Cem Concr Res 1985;15:669 74. [63] Raask E. Cenospheres in pulverized-fuel ash. J Inst Fuel 1968;41(332):339 44. [64] University of North Dakota, energy environmental research centres. http://www. undeerc.org/, 1992. [65] Headwater resources, bulletin no 6, fly ash decreases the permeability of concrete, Rev 3 2005, http://www.flyash.com/data/upimages/press/TB.6%20Fly%20Ash%20Decreases% 20the%20Permeability%20of%20Concrete.pdf; 2005. [66] “Standard methods of test for rapid determination of the chloride permeability of concrete”, American Association of State Highway and Transportation Officials, AASHTO, Washington 1989. [67] Headwater resources, bulletin number 3, strength of fly ash concrete, Rev. 3/05, http://www.flyash.com/data/upimages/press/TB.3%20Strength%20of%20Fly%20h %20Concrete.pdf; 2013. [68] Headwater resources, bulletin number 5, “Fly Ash Improves Workability”, 2015. [69] Seshadri B, Bolan NS, Naidu R, Brodie K. The role of coal combustion products in managing the bio-availability of nutrients and heavy metals in soil. J Soil Sci Plant Nutr 2010;10(3):378 98. [70] Fly ash Australia http://www.flyashaustralia.com.au/WhatIsFlyash.aspx. The American Coal Ash Association (ACAA) http://acaa.affiniscape.com/displaycommon.cfm? an 5 1&subarticlenbr 5 149Copyright 2008 ACAA, 2008.

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[71] S.T. Cholake, S. Bandyopadhyay: Chapter 1—“Coal fly ash: a valuable recycling treasure in construction and environmental application”. In: Daniels JA, editors. Advances in environmental research. Volume 23. Nova Science Publishers, USA, https://www. novapublishers.com/catalog/product_info.php?products_id 5 29692, 2012. [72] Headwaters Resources. About fly ash, , http://flyash.com/about-fly-ash/ . ; 2013. [73] Bulletin No.1 fly ash—types and benefits, Headwaters Resources, Rev 3/05 http:// www.flyash.com/data/upimages/press/TB.1%20Fly%20Ash%20%20Types%20&% 20Benefits.pdf, 2015. [74] Sherly R, Shantha Kumar S. Valuable products from fly ash. J Ind Pollut Control 2011;27(2):113 20. [75] Thomas M. Optimizing the use of fly ash in concrete. Portland Cement Organization, , http://www.cement.org/docs/default-source/fc_concrete_technology/is548-optimizing-the-use-of-fly-ash-concrete.pdf . ; 2007. [76] McKerall WC, Ledbetter WB, Teague DJ. Analysis of fly ashes produced in Texas. College Station, Texas: Texas Transportation Institute, Research report no. 240-1, Texas A&M, University; 1982. [77] K. Ali, G. Rodney, (2001). “Characterization of fly ash from the Kangal power plant, Eastern Turkey”, International ash utilization symposium, Centre for Applied Energy Research, University of Kentucky, Eastern Turkey, http://www.flyash.info/2001/chemin1/04karay.pdf. [78] Fly Ash Australia. Typical applications, , http://www.flyashaustralia.com.au/ TypicalApplications.aspx . ; 2017. [79] Kumar B, Garg R, Singh U. Utilization of fly ash as filler in HDPE/Fly Ash polymer composites: a Review. Int J Appl Eng Res 2012;7(11) ISSN 0973-4562. [80] Nath D, Banyopadhyay S, Yu A, White C. Novel observation on kinetic of nonisothermal crystallization in fly ash filled isotatic-polypropylene composites. J Appl Polym Sci 2010;115(3):1510 17. [81] Jarvela PA, Jarvela PK. Multicomponent of polypropylene. J Mater Sci 1969;31:3853. , http://dx.doi.org/10.1007/BF00352802 . . Huang X, Hwang JY, Gillis JM (2003) J Miner Mater Charact Eng 2(1):11. [82] Guhanathan S, Saroja Devi M. Studies on interface in polyester/fly-ash particulate composites. Compos Interface 2004;11(1):43 66. [83] Gupta N, Brar BS, Woldesenbet E. “Effect of filler addition on the compressive and impact properties of glass fibre reinforced epoxy”. Bull Mater Sci 2001;24(2):219 23. [84] Ibraheem S, Koshy P, Standard O, Deavasahayam S, Banyopadhyay S. “SEM study of debonding/pull-out features & FTIR analysis of bonding in fly ash epoxy particulate composites”. Int J Enahnced Res Sci Technol Eng, ISSN: 2319-7463 2016;5(3) India. [85] Ibraheem S, Devasahayam S, Standard O, Bandyopadhyay S. Use of secondary ion mass spectrometry (SIMS) to identify fly ash mineral spatial and particulate distribution in epoxy polymer. Int J Miner Process IJMP- ELSEVIER 10 September 2015;142:139 46 USA. [86] Li MG, Sun CJ, Gau SH, Chuang CJ. Effects of wet ball milling on lead stabilization and particle size variation in municipal solid waste incinerator fly ash. J Hazard Mater 2010 Feb 15;174(1 3):586 91. [87] Wikipedia. “Fly Ash”: chemical composition and classification, , http://en.wikipedia. org/wiki/Fly_ash#Chemical_composition_and_classification . ; 2015. [88] American Association for State Highway Transportation Officials (AASHTO). http:// www.transportation.org/, 2013.

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[89] ASTM C618—08 Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. ASTM International. http://www.astm.org/cgi-bin/ SoftCart.exe/DATABASE.CART/REDLINE_PAGES/C618.htm?L 1 mystore 1 lsft6707. Retrieved 2008-09-18, 2008. [90] Meier U. Strengthening of structures using carbon fibre/epoxy composites. Constr Build Mater 1995;9:341 351. Application of polymeric materials construction industry, , http://www.sciencedirect.com/science/article/pii/0950061895000712 . . [91] Sajwan KS, Twardowska I, Punshon T, Alva AK. Coal combustion byproducts and environmental issues. New york: Springer Science 1 Business Media Inc; 2006. [92] Cholake ST, Moran G, Joe B, Bai Y, Singh Raman RK, Zhao XL, et al. Improved Mode i fracture resistance of CFRP composites by reinforcing epoxy matrix with recycled short milled carbon fibre. Constr Build Mater 2016;111:399 407 http://dx. doi.org/10.1016/j.conbuildmat.2016.02.039. [93] Bandyopadhyay S, Cholake ST, Morgan G, Joe B, Bai Y, Singh RK, et al. Improved fracture toughened epoxy matrix system reinforced with recycled milled carbon fibre. Ann Mater Sci Eng 2015;2:1023 8. [94] Bandyopadhyay S. Review of the microscopic and macroscopic aspects of fracture behaviour of unmodified and modified epoxy resins. Mater Sci Eng 1990; A125:157 84. [95] Bandyopadhyay S. Correlation of macroscopic fracture behaviour with microscopic aspects of deformation in toughened epoxies. Polym Mater Sci Eng 1990;63:32 7.

Structure and chemistry of fiber metal laminates

8

Barbara Surowska, Patryk Jakubczak and Jarosław Bienias´ Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology, Lublin, Poland

Chapter Outline 8.1 Introduction 193 8.2 Macrostructure characterization 195 8.2.1 8.2.2 8.2.3 8.2.4

Type of metal sheets and surface treatment 195 Type of composite materials 206 Configuration of plies 210 Defects in FMLs structure 217

8.3 Microstructure characterization 220 8.3.1 Microstructure of polymer composite layers 222 8.3.2 Microstructure of metal surface layers 223 8.3.3 Microstructure of metalcomposite interface 223

8.4 Physical chemistry of interface Acknowledgment 229 References 229

8.1

224

Introduction

Fiber metal laminates (FMLs) are a modern group of hybrid materials designed at the end of the 20th century for the aircraft industry. Until the middle of the 20th century, primary structures in the aircraft industry were dominated by the use of metals. In the second half of the 20th century, metal thin-walled semimonocoque structures, which in safe load conditions may exhibit a local, aerodynamically adverse loss of stability, were replaced with sandwich structures. Such structures were characterized by cellular cores, the so-called honeycombs. They were made mostly from aluminum and Nomex sheets [poly(meta-phenylene isophthalamide)]. Constructions based on such a structure allow for a high level of integration, although their mass is sometimes comparable to the mass of metal thin-walled constructions. The core, which may be given any outline, is joined to faces with bonding. The load-carrying structures of airframes based on sandwich structures are Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00008-2 Copyright © 2017 Elsevier Ltd. All rights reserved.

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characterized by high stiffness, which allows for maintaining a desirable geometry across the entire range of safe loads of an aircraft. This solution is also commonly used in helicopter rotor blades. The development of polymer materials and polymermatrix fiber composites has allowed for further structure modifications. In the 21st century, metal foam and FMLs are used for skins and load-carrying structures, and piezoelectric fibers are used as components of sensors monitoring the condition of such constructions [16]. FMLs consist of thin layers of metal sheet and polymerglass/carbon/aramid fiber composite. Such laminates are characterized by excellent properties of both metal and polymer composite. This combination has given rise to a new generation of hybrid materials that can inhibit or block crack growth upon cyclic loading, and which have very good load characteristics and impact resistance, as well as low density. They facilitate the production and repair of structures. They can be adjusted to meet different needs through joining different fiber/resin systems, the use of different grades of metal alloys, sheets of different thickness, different sequences of stacking laminate layers, different fiber orientation, surface pretreatment, and many more. The first group of FMLs manufactured for the aircraft industry was ARALL (Aramid aluminum laminate) in which the fiber component is made from aramid fibers [1,2,7,8]. Two grades of these laminates were patented: ARALL 1 and ARALL 2. 7075 Aluminum alloy sheet was used in the ARALL 1 laminate and 2024 aluminum alloy sheet was used in the ARALL 2 laminate. These laminates were used for commercial purposes by the ALCOA company in 1982 for the cargo hatch in the C17 military aircraft. The ARALL laminates had a number of flaws resulting from the used components. First of all, aramid fiber laminates exhibited low fibermatrix interface strength. As a result, the fiber volume fraction could not exceed 50%. Moreover, the emergence of cracks in the fiber layers as a result of mechanical fatigue was observed, although the fatigue crack growth resistance of hybrid materials is decidedly better than that of aluminum alloy sheet. Anisotropic properties of ARALL with unidirectional fiber distribution prevented the use of this material for skins subjected to bidirectional stresses. Another flaw is a relatively low resistance to notch effect (compared to monolithic aluminum alloy). The cause of this unsatisfactory resistance is low aramid fiber elongation (2.4%). Therefore, research on laminates has been conducted in which aramid fibers were replaced with glass fibers, which are cheaper to manufacture and have similar strength characteristics. Materials from this group have been named GLARE (Glass reinforced), and their basic intended use was components of fuselage skins of aircraft. ARALL and GLARE have been trademarked by Structural Laminates Company. GLARE was patented in 1987 by the AKZO NOBEL company. Basic research on GLARE was conducted at the Delft University of Technology [7,9]. More comprehensive information about the technology and properties of these materials have been published since 2001, after GLARE was introduced into the production of Airbus A-380 [710]. Since the beginning of the 21st century, these laminates have been the subject of research conducted in many research facilities around the world.

Structure and chemistry of fiber metal laminates

195

Due to the development of the carbon fiber production technology and the expansion of the application of carbon fiber reinforced polymer (CFRP) composites, the CARALL (carbon reinforced aluminum laminate) have become the subject of research [7,11,12]. Since galvanic corrosion is an issue in this type of FMLs, studies are conducted aimed at replacing aluminum sheet with other metals, for instance, titanium [1315]. In the present chapter, the macro- and microstructures of FML are described with respect to the selection of the type of components and their distribution for the production of functional materials with forecast properties.

8.2

Macrostructure characterization

The introduction of laminated elements is associated with a new design philosophy based on the premise that the material and the construction are made at the same time and on the fact that the properties of a hybrid material are more difficult to estimate at the design stage than in the case of traditional construction materials. The basic requirements for FML components in the aircraft industry are quality that satisfies aviation standards with respect to primary and secondary structures, low density, and resistance to the environment in-service conditions. Joining the layers of metal and composite at the construction production stage makes it possible to design its physical, chemical, and mechanical properties. Adjustment to constructors’ requirements is achieved through the use of different grades of metal alloys, polymer materials and fibers, sheets and prepregs of different thickness, different number of layers, joining different fiber/polymer systems, different sequences of stacking laminate layers, different fiber orientation, and many more.

8.2.1 Type of metal sheets and surface treatment Due to their intended use, FML should have low density. Therefore, light metals and their alloys are used for the metal layers. The most widely used are aluminum alloys. FMLs with titanium, titanium alloys, and magnesium alloys are also studied and implemented.

8.2.1.1 Aluminum alloys 2xxx series alloys are a particularly advantageous group of aluminum alloys. They are widely used and relatively cheap. They have good physical, chemical, and mechanical properties, can be subjected to heat treatment to enhance their properties, and there exists a proven process of surface pretreatment, especially through anodizing. These alloys are characterized by the stability of properties with variations of temperature (there is no significant loss of impact resistance in low temperatures). Moreover, 2xxx series alloys have one of the highest damage tolerances due to their capacity to inhibit cracks. The basic grade of this series is the AlCu4Mg1 alloy after T3 treatment (AW 2024 T3) (Table 8.1) used in GLARE 2-6, ARALL, CARALL, and other

Table 8.1

Chemical composition of aluminum alloys used in FML Element (wt.%)

Alloy

Cr

Zn

Si max

Mg

Mn

Cu

Ti max

Fe max

Others Each

Total

Al

2024

Max. 0.1

Max. 0.25

0.5

1.21.8

0.30.9

3.84.9

0.15

0.5

0.05

0.15

Bal.

7075

0.180.28

5.16.1

0.4

2.12.9

Max. 0.3

1.22

0.2

0.5

0.05

0.15

Bal.

5052

0.150.35

Max. 0.1

0.25

2.22.8

Max. 0.1

Max. 0.1



0.4

0.05

0.15

Bal.

Source: Based on EN 573-3:2008 Aluminium and aluminium alloys. Chemical composition and form of wrought products [16].

Structure and chemistry of fiber metal laminates

197

experimental laminates. T3 treatment consists in solution heat treatment, cold work, and natural ageing. The solution-treating temperature for the 2024 T3 alloy is 495 C, with quenching in cold water, preferably under pressure or with water jets [17] (Table 8.2). The most common application of this alloy is aircraft and other aerospace and industrial applications that need the lightweight and high strength (HS) components. Sheet products are used extensively in commercial and military aircraft for fuselage skins, wing skins, and engine areas where elevated temperatures to 393K (120 C) are often encountered. The prevalent use of the 2024 aluminum alloy results mostly from its moldability by working techniques without the loss of its advantageous operational properties, which is particularly significant in the case of aircraft skins and construction components. The thickness of aluminum layers used in the FML does not exceed 0.5 mm, which allows for the configuration of complex structures while maintaining low end mass of a given element. 7xxx series aluminum alloys are the strongest of all the aluminum wrought alloys. Among these series, AlZn5.5MgCu (AW7075) alloy is extensively used in aircraft and armaments industries because of its superior strength. Usually, the mechanical properties of 7075 alloy are improved by reducing its iron and silicon contents and altering quenching and ageing conditions. It exhibits good damage tolerance and high resistance to fatigue crack propagation in T6-aged condition [19]. The T6 temper consists of solution heat treatment at 480 C/1 h, rapid water quenching, and age hardening at 121 C/ 24 h. This alloy is susceptible to machining. It is characterized by moderate corrosion resistance. Nowadays, the main use of this alloy is for aircraft application including circumferential frames, stringers, lower wing spars, and upper wing skins. It has been used in the GLARE 1 laminate in the T6 condition. The microstructure of these aluminum alloys is very complicated: Exhibiting multiphase particles, periphery phases around composite particles, and clustering [20]. The types of intermetallic phases found on 2024-T3 and 7075-T6 aluminum alloys that have been identified are presented in Table 8.3. These precipitations increase mechanical properties but reduce local corrosion resistance. A potential grade of aluminum alloy to use in FMLs is 5xxx series alloy AlMg2.5 in the H34 condition (AW5052 H34) (Tables 8.1 and 8.2). Hardening to half-hard (H34 treatment) ensures mechanical properties sufficient for the use in the aircraft and automotive industry for components formed by working. Therefore, this alloy is considered as a component of laminates containing thermoplastic FML (TFML - Thermoplastic Fibre Metal Laminates) composites [24]. Studies are being conducted on the use of modern aluminum alloys—lithium— due to their density, which is lower than the density of aluminum, and the expected improvement of stiffness and mechanical fatigue resistance, compared to GLARE [25,26].

8.2.1.2 Titanium and titanium alloys Titanium is the second, after aluminum, most widely used light metal in the aircraft industry. Due to a relatively low specific gravity, high mechanical properties, very good corrosion resistance, and proven production technologies of titanium sheets

Table 8.2

Physical properties of metals used in FML [18]

Physical properties

AW 2024 T3

AW 7075 T6

AW 5052 H34

Ti grade 2

AZ31B

Density (g cm23)

2.87

2.81

2.68

4.61

1.77

Electrical resistivity (Ω cm)

5.82 3 10

5.15 3 10

4.99 3 10

5.2 3 10

9.2 3 1026

CTE (Coefficient of Linear Thermal Expansion), linear 250 C (31026K21)

24.7

25.2

25.7

92

26

Specific heat capacity (J (g K)21)

0.875

0.96

0.88

0.523

1.0

Thermal conductivity (W (m K)21)

121

130

138

B18

96

T3

T6

H34



H24.H26

Solution heat treatment ( C)

495/cold water

466482/cold water

Annealing, 343

Annealing, 345

Plastic working

Cold working



Strain hardening 1/2 hard, 260510 C

Hot working 230425

Ageing ( C)

Naturally

Artificially, 121



Processing condition 

26

26

26

25





Structure and chemistry of fiber metal laminates

Table 8.3

199

Intermetallic phases in aluminum alloys

Alloys

Particles

References

2024 T3

Al2CuMg (S-phase)

[21,22]

Al2Cu (Θ-phase)

[21,22]

(Al,Cu)x(Fe,Mn)ySi

[23]

Al20(Cu,Mn,Fe)5 Si

[20]

Al23CuFe4

[19]

MgZn2 (η-phase)

[19]

AlMgCu

[19]

7075 T6

and alloys, studies are being conducted on FMLs and TFMLs with titanium layers used instead of aluminum alloys [1315, 2730]. Commercially pure (CP) titanium is unalloyed. At room temperature, it consists of hcp alpha phase. As a single-phase material, its properties are controlled by chemistry (iron and interstitial impurity elements) and grain size. CP Ti Grade 2 (Table 8.4) has a minimum yield strength of 275 MPa and relatively low levels of impurity elements. Grade 2 is widely used because it combines excellent formability and moderate strength with superior corrosion resistance. CP Titanium Grade 2 may be considered in any application where formability and corrosion resistance are important, and strength requirements are moderate. Some examples of aerospace applications have included airframe skins in “warm” areas, ductwork, and brackets. Apart from Ti Grade 2, attempts are made to use the Ti6Al4V alloy (Grade 5), which is very popular in engineering (Table 8.4). It is a two-phase (alpha 1 beta) alloy susceptible to hot, warm, and cold working. Therefore, it should be suitable for the forming of FML structures [3133].

8.2.1.3 Other alloys The group of light metal alloys used in the aircraft industry includes also magnesium alloys. Although they exhibit limited corrosion resistance, they are an attractive construction material due to their low density and sufficient mechanical properties. The AZ31B alloy (Al 2.5%3.5%, Zn 0.7%1.3%, Mn 0.2%1%, according to ASTM B90) is tested in FMLs. This alloy is weldable, relatively corrosion resistant [3436]. Increased strength is obtained by strain hardening with a subsequent partial annealing (H24 and H26 tempers, Table 8.2). If low density is not required, but high corrosion resistance or HS is needed, FMLs with stainless steel sheets [37,38] and two-phase, HS steel sheets [39] are tested. Modern, HS, and very light aluminumlithium alloys are also beginning to attract attention [40,41].

Table 8.4

Chemical composition of titanium and titanium alloy used in FML according to ASTM B265 Element (wt.%)

Alloy

C max.

Fe max.

H max.

N max.

O max.

Al

V

Others max. Each

Total

Ti

CP Ti Grade 2

0.08

0.3

0.015

0.03

0.25





0.1

0.4

Bal.

Ti6Al4V (Grade 5)

0.1

0.40

0.015

0.05

0.02

5.506.75

3.504.50

0.1

0.4

Bal.

Structure and chemistry of fiber metal laminates

201

8.2.1.4 Surface treatment All metal alloys used in FMLs belong to a group of materials whose corrosion resistance results from self-passivation. However, a spontaneously formed passive layer does not provide sufficient adhesion to the composite. Therefore, a surface treatment of sheets is necessary before the joining process. A proper metal surface treatment should result in a surface free from impurities, with a macro- or microscale roughness, covered with a fresh, stable oxide layer with a desirable chemical composition. Before an oxide layer is formed, the natural passive layer should be removed.

Anodizing aluminum A typical pretreatment of aluminum sheets before an anodizing process consists in degreasing, chemical pickling, water rinsing, brightening in HNO3, and final water rinsing. Solvent degreasing is important because it removes contaminant materials which inhibit the formation of the chemical bonds. The degreasing stage usually makes use of chlorinated solvents such as trichloroethylene, 1,1,1-trichloroethane, perchloroethylene, or dichloromethane, or alternatively, nonchlorinated solvents including methyl ethyl ketone, methanol, isobutyl alcohol, and toluene or acetone [42]. Chemical pickling is usually performed with the use of chromicsulfuric acid (CAE) [4244] or sulfoferric acid (P2) [45] or as alkaline etching [43,46] in NaOH or KOH (potassium hydroxide). CAE is prepared as a solution of CAE with potassium dichromate or ferric sulfate. P2 is a water solution of Fe2(SO4)3 and concentrated H2S. The oxide layer on aluminum and its alloys is formed in an anodic oxidation process. The formation of the oxide layer includes three stages: The movement of Al31 ions from metal to the oxide layer, diffusion of Al31 ions through the barrier layer, and coatingforming oxidation reaction. Regardless of the conditions of the aluminum anodizing process, the oxide layer is formed through the following reactions [47]: 2Al 1 3H2 O ! Al2 O3 1 3H2

(8.1)

Al ! Al31 1 3e2

(8.2)

2Al31 1 3H2 O ! Al2 O3 1 6H1

(8.3)

H2 O ! 2H1 1 O22

(8.4)

2Al 1 3O22 ! Al2 O3 1 6e2

(8.5)

Al2 O3 1 6H1 ! 2Al31 1 3H2 O

(8.6)

The formation of the oxide layer consists of three stages:

202

Hybrid Polymer Composite Materials: Structure and Chemistry

 Formation of a thin (0.010.1 μm) and compact barrier film. As a result of the migration of Al31 ions in the electric field and their reaction with O22 or OH2 ions, anhydrous Al2O3 is formed (with high electrical resistance). Reformation of the barrier film at the oxide-electrolyte phase boundary—formation of the porous layer. Due to the increased volume of the produced oxide in comparison to the volume of the converted metal, tensile stresses are generated on the film. This leads to an emergence of cracks in the barrier layer, which contributes to the formation of pores in which the electrolyte diffuses.  Increase of the thickness of the porous layer of column morphology. At the bottom of the pores, two competing processes take place: Formation of an oxide coating and its electrolytic dissolution. This results in the deepening of the pores and in the buildup of the layer deeper into metal. Depending on the intended use, the thickness of this layer may range from several to about 150 μm. G

The formation rate of the oxide layer results from the balance between the oxide layer formation process and the resolution phenomenon. These processes take place at a specific rate: G

G

The rate of oxide creation, without side reactions, can be captured with Faraday’s law, according to which the mass of oxide produced is directly proportional to the current intensity and anodizing time. The rate of the electrolytic dissolution of the already formed oxide film is a quantity that depends on the type of the electrolyte, its concentration, and temperature.

The selection of the type of electrolytic bath depends on the further intended use of the anodized element. The three electrochemical processes are mostly used in aircraft applications: Chromic acid anodizing (CAA), sulfuric acid anodizing (SAA), and phosphoric acid anodizing (PAA) [46,48,49]. CAA process is a very good pretreatment for FML bonding due to high adhesion properties of the films incorporating some Cr (VI) and Cr (III). Anodizing in chromic acid was widely used during World War II for military equipment. It was developed in Great Britain by Bengough and Stuart. The constant voltage method (the BengoughStuart method) uses from 2.5 to 9% of chromic acid and cyclic voltage ramping. The temperature of the electrolytic bath is maintained at 3540 C. The applied time of coating formation is usually about 40 min. The advantage of this method is the fact that chromic acid residues in the oxide pores are not very aggressive [49]. However, the use of Cr (VI) is unadvised from a health and environmental point of view since it is toxic and carcinogenic. Therefore, this process is gradually out of use, especially in EU, and forbidden in the USA. SAA is an alternative anodizing method, which has replaced CAA to a great extent [50]. It is usually conducted with 5%22% sulfuric acid concentration, using distilled water with the temperature of about 035 C and initial voltage of 530 V, gradually increased to 60 V. PAA process was developed in the USA by Boeing. In Europe, there is interest in boric acid anodizing (BAA). Besides PAA, a mixed sulfuric acid/boric acid process (BSAA), carried out at lower temperature, has been patented by Boeing too.

Structure and chemistry of fiber metal laminates

203

BSAA is replacement to CAA to meet environmental regulations [49]. BAA or BAA/SAA-treated adhesive joints have better corrosion resistance but lower adhesive property then CAA [51,52]. Apart from DC (Direct Current) anodizing, the AC (Alternating Current) anodizing in phosphoric and sulfuric acids is tested. A distinct difference from DC anodizing is hydrogen gas generation on the treated surface during the hot AC anodizing process [53]. After completing the anodizing process, the coating is often sealed with primers. The primers are epoxides with corrosion inhibitors [12,54,55]. Their purpose is to increase adhesion and secure the anodized layers. The sealing of pores in an absorbent oxide layer secures it against the effects of external physical and chemical factors. Moreover, a surface with primer has a higher surface-free energy (SFE), and hence, it increases the strength of the adhesion between aluminum and resin during FML production [11,56,57]. There are also other methods of aluminum sheet surface pretreatment proposed to increase the metalcomposite adhesion, for example: Mechanics—grit-blasting, ultrasonic machining, excimer laser texturing, plasma treatment; coupling/oxidation— silanization, solgel; ablation/oxidation—plasma sprayed coating, ion beam enhanced deposition, discussed in the review paper [11].

Titanium surface treatment To join titanium with a polymer material, the titanium surface and alloys are prepared by means of a mechanical and chemical or electrochemical treatment [5862]. In the simplest case, mechanical treatment consists in wire brushing. Since this method is primitive and does not ensure repeatability of the surface condition, especially in the case of large elements, blast cleaning (sandblasting) is used much more frequently. Corundum (Al2O3) and air jets under pressure of the order of 0.20.3 MPa are usually used in blast cleaning. The obtained roughness depends on the size of the powder grains. It has been observed that large grains with the diameter of the order of 250 μm increase adhesion, in comparison to small grains with the diameter of the order of 50 μm. This difference is accounted for by the possible unfavorable absorption of very small grains of aluminum oxide on the titanium surface. Sandblasting results in a surface of specific roughness, without oxide. After sandblasting, cleaning, and surface degreasing, for example, using isopropylene is necessary to remove corundum particles [58,59]. Chemical prepickling of titanium and its alloys is usually performed in an aqueous solution of the mixture of 10%30% nitric acid and 1%3% hydrofluoric acid. HF acid causes rapid reaction with TiO2 which produces soluble titanium fluoride and hydrogen. Since hydrogen can diffuse into the upper titanium layer, HF and HNO3 solution at a ratio of 1:10 is used, which significantly reduces the concentration of free hydrogen. Good effects of cleaning titanium surface are also obtained by means of pickling in an HCl solution, which satisfactorily dissolves titanium salts without damage to the metal surface. Acid pickling results in the presence of an oxide film on the titanium surface with a predominance of TiO2 with the thickness below 10 nm [59,60]. An alternative method is pickling in a hydroxide and hydrogen

204

Hybrid Polymer Composite Materials: Structure and Chemistry

peroxide solution at room or elevated temperature [58]. Pickling results in the change of the surface roughness to the extent dependent on the pickling solution and the process duration. Treatment in alkaline solutions (sodium hydroxide) produces a gel layer of sodium titanate with thickness of the order of 1 μm. Oxidation with hydrogen peroxide results in the formation of a two-layer structure—inner compact layer and outer porous layer, each with the thickness of B5 nm. Moreover, a strong silanization is used on the surface [6366]. Silanes (8.7) are organicinorganic hybrid compounds which initialize adhesion between different materials, for example, ceramics and a polymer. They contain an organic functional group capable of copolymerization. They also contain three alkoxy groups (e.g., methoxy O-CH3) capable of hydrolysis and reactions with surface hydroxyl groups of inorganic substrates (e.g., silica).

ð8:7Þ

The treatment involves blasting with Al2O3, surface modified with SiO2, and silanization with organosilicon compounds under the pressure of 0.25 MPa. The process is executed in three stages. The first stage includes an initial surface cleaning with Al2O3 powder. The second stage is ceramization, a partial coating of the surface with silicone dioxide, using a specially modified sandblasting material (Al2O3 grains, the size of 30 μm, coated with a SiO2 layer). During the impact, the aluminum oxide grain transmits its coating through a kinetic chemical reaction onto the sandblasted surface. In the third stage, an ethanol silane solution creating chemical bonds with SiO2 is used. Other often employed techniques, such as high-density argon plasma cleaning, may be treated as alternative methods for oxidized surface cleaning. Many studies have proven that plasma treatment leads to achieving a contamination-free surface with good wettability and high surface energy [13,58,59]. Electrochemical treatment is low-voltage anodic oxidation [60,61,67] and increasingly often used plasma electrolytic oxidation (PEO) [6871]. Anodizing is carried out in acid solutions, mainly chromic acid, hydroxide solutions, mainly sodium hydroxide, and, less often, in fluoride-based solutions [31,33,62,67]. The use of titanium, an element easily reacting with oxygen, capable of passivation by spontaneous oxidation to TiO2, requires careful supervision of the state in which the metal surface remains. The titanium oxide produced in the anodic reaction is characterized by high resistance toward the electrolyte and metallic parts of the electric circuit. The oxide layer builds up as long as the ionic flow through the oxide is possible. TiO2 with anatase, rutile, or amorphous structure (depending on the process parameters) has the highest stability among the formed oxides. Anatase and rutile are crystallographic tetragonal variants of titanium dioxide, differentiated by structure parameters and physicochemical properties (Table 8.5).

Structure and chemistry of fiber metal laminates

Table 8.5

205

Structure and some properties of crystalline TiO2 Rutile

Anatase

Crystal lattice

Tetragonal

Tetragonal

Lattice parameter (nm)

a 5 0.459

a 5 0.378

c 5 0.296

c 5 0.951

Density (mg m )

4.245

3.893

Dielectric constant

110

48

23

The use of PEO process parameters, mainly a relatively high voltage (above 140 V), additionally causes an electrolysis of the solution and an electric discharge at the surface of the metal (plasma oxidation). Low voltage of traditional oxidation causes the formation of an amorphous structure with good density, while an increase of voltage causes the formation of crystal structure of anatase, which transforms into rutile at the highest voltage levels or under heat treatment. Elements from the electrolyte, for example, sulfur from the sulfuric acid, phosphorus from the phosphoric acid, or calcium from organic solutions containing oxygen, build into the oxide layer. Simultaneously, the layer changes morphology, porosity increases, and sparkling occurs. The oxidation under spark discharge process produces a layer with considerable roughness and characteristic topography with craters [6972]. Such topography seems beneficial for obtaining an adhesive bond with a polymer, as the experience with GLARE laminates shows that roughness of the surface is an essential parameter. The anode oxidation under spark discharge method, requiring a stand as for aluminum anodizing, can be used in industrial conditions. The solgel method, currently used for creating oxide layers on biomaterials, is simple and inexpensive. Due to chemisorption, after submerging in an appropriately prepared sol, on the surface of the metal appears gel, which is then rinsed, hydrolyzed, and dried. The chemisorptionhydrolysisdrying cycle repeated several times leads to obtaining the assumed thickness of the layer of, for example, TiO2. A layer produced in such a way is amorphous, with micro- and nanopores of 550 nm. Heating the gel in temperatures below 300 C retains the amorphicity of the structure. Partial crystallization begins at 400 C. During heating in 500 C, a mixture of two crystalline phases—anatase and rutile—can be observed, porous but with good adhesion toward the substrate [7375]. Higher curing temperature of about 600650 C leads to obtaining chiefly rutile as the dominant oxide. A similar phase composition can be produced by heating gel obtained through chemical oxidation [76]. From the technical and economical viewpoint, this method will not be useful for the FMLs if the amorphous TiO2 structure cannot provide good adhesion. Laminate assembling and prepreg curing should occur simultaneously in the autoclave process, and the temperature for currently used thermosetting polymers does not exceed 200 C. For this reason, SiO2 layers, whose firing temperature cannot be lower than 400 C, will also be useless. What is more, the solgel method produces

206

Hybrid Polymer Composite Materials: Structure and Chemistry

partially diffusive layers, but only at the stage of gel heating. Relatively low temperature in autoclave, despite increased pressure, may not provide a sufficient diffusion bond. However, these methods may be used for bonding titanium with thermoplastic composites, whose molding temperature exceeds 400 C. The above-mentioned methods are used for various titanium products with modified surface. Producing aircraft structure from FMLs and TFMLs involves preparing a surface with good adhesion to the polymer. The literature on preparing the titanium and titanium alloys sheet surface in FMLs is particularly scarce. There have been studies [29,30] on adhesive bonds of the Ti Grade 2 titanium sheet with ceramic glue and with polymer nanocomposites reinforced with SiO2 particles, the surface prepared by mechanical cleaning with wire brush, degreasing and anode oxidation in a sodium hydroxide solution and, instead of anodizing, ion nitriding. In the works [13,77], titanium alloy Ti 15-3 bonds with glass fiber/thermosetting resin and glass fiber/thermoplastic resin composites, using degreasing and laser ablation as the initial treatment was created and examined. The results were compared to laminates whose titanium alloy surfaces were sandblasted, sandblasted and with plasma sprayed TiO2, anodized in NaOH, and anodized in CrO3 1 NH4HF2. The best result was achieved by anodizing with sodium hydroxide. After the laser ablation, a beneficial roughness increase was observed. In the paper [27], the durability of Ti6Al4V alloy laminates with a carbon fiber reinforced composite was tested. The surface was prepared in a manner typical for titanium: Sandblasting, cleaning, and pickling. In some works, thin adhesive polymer plies, applied like paint coating, were additionally used for FMLs [13,58,77] as well as TFMLs [78]. The authors of the studies [31,32] examined the influence of the Ti6Al4V alloy anodizing on bisphenol-F epoxy resin adhesion. The effect of PEO pretreatments on the adhesive bonding of titanium to epoxy was investigated using lap-shear tests by Ref. [70]. The research conducted by the authors at present indicates that mechanical preparation of the Grade 2 titanium surface and traditional anodizing in CAEs does not provide sufficient adhesion in CFRP (Carbon Fibre Reinforced Polymer) GFRP (Glass Fibre Reinforced Polymer) laminates. Better results can be obtained when anodizing with hydrofluoric acid is used.

8.2.2 Type of composite materials The manufacturing of FMLs involves prepregs composed of unidirectional ceramic or polymer fibers in a thermosetting (FMLs) or TFMLs, party polymerized, resin matrix. Commercially available prepregs dedicated for specific purposes are used, but the main group consists of thermosetting matrix prepregs for the aircraft industry. Thermoplastic matrix prepregs are tested in TFML and intended for aircraft and other means of transportation but have not yet been widely implemented. The type of the resin and fibers is selected on the basis of expectations regarding laminate properties.

Table 8.6

Selected physical and mechanical properties of polymer matrix [7983] Density (g cm23)

Young’s modulus E (GPa)

Flexural strength (MPa)

Flexural strain (%)

Tensile strength (MPa)

Longitudinal thermal expansion α ( 3 1026K21)

Glass transition, melting point ( C)

HexPly M12

1.24

5.1

n.d.

n.d.

64

72

135 (Tg dry) 120 (Tg wet)

HexPly M21

1.28

3.5

147

5.0

n.d.

n.d.

195 (Tg wet)

CYCOM 985

1.25

4.05

155

4.5

n.d.

n.d.

228 (Tg dry) 175 (Tg wet)

1.25

4.6

163

4.5

103

n.d.

300 (Tg dry) 200 (Tg wet)

TORELINA PPS

1.35

4

135

n.d.

80

50

90 (Tg) 278 (Tm)

Fortron PPS Celanese

1.35

3.45

n.d.

n.d.

n.d.

19

85 (Tg) 285 (Tm)

PES

1.37

2.6

n.d.

n.d.

129

55

230 (Tg)

PEI Flexile PEI

1.27

3.6

165

n.d.

110

62

217 (Tg)

PEEK (Cytec APC-2)

1.32

3.6

170

1.4

100

45

143 (Tg) 334 (Tm)

Flexile PEEK

1.30

3.7

175

n.d.

110

n.d.

343 (Tm)

APC-PEKK Cytec

1.31 (30% cryst.)

4.5

n.d.

n.d.

102

n.d.

159 (Tg dry) 135 (Tg wet) 337 (Tm)

Matrix

Epoxy

BMI CYCOM5250-4

Thermoplastics

208

Hybrid Polymer Composite Materials: Structure and Chemistry

The most popular thermosetting composites, used for example, in GLARE, ARALL, CARAL laminates, contain epoxy resin, due to the good mechanical properties. Phenolic resin is used when increased fire resistance is needed. Bismaleimide (BMI) and polyimide resins, on the other hand, ensure great resistance against high temperature. Basic properties of these resins, based mostly on the information provided by the producers, can be seen in Table 8.6 [7983]. A variety of prepregs with thermoplastic matrices which can possibly be used in TFMLs are now available. Beside PP (polypropylene) and PA 6 (polyamide 6) popular in construction composites, the most interesting matrices in terms of properties are PEEK [poly(ether ether ketone)], PEKK [poly(ether ketone ketone)], PPS [poly (phenylene sulfide)], PES (polyethersulphone), and PEI (polyetherimide) (Table 8.6). The reinforcement in thermosetting prepregs is G

G

G

glass continuous fibers, type E and S (GFRP) (Table 8.7) carbon continuous fibers—standard middle modulus (IM), high modulus (HMs), HS (CFRP), aramid continuous-fibers HMs, ultra high modulus (UHM), HS (Aramid Fibre Reinforced Polymer (AFRP)).

The basic properties of glass and aramid fibers can be seen in Table 8.8, while the properties of carbon fibers in Table 8.9. S glass fibers display higher mechanical properties and lower thermal expansion ratio (CTE) than type E fibers (Table 8.8). Aramid fibers, known as Kevlar, are manufactured by Du Pont [85]. Moreover, para-aramid fibers (poly-paraphyenylene terephthalamide) Twaron and Technora made by Teijin Ltd are used [86]. Aramid fibers are organic fibers with a fairly good tensile strength-to-weight ratio [87]. Unfortunately, other mechanical and physicochemical properties do not meet the reinforcement requirements in composites as FML components. Carbon fibers, mostly obtained through the PAN (polyacrylonitrile) technology, surpass glass and aramid fibers in strength properties, especially in stiffness. In addition, they have a small negative CTE. Lower density and smaller fiber dimension allow the manufacturing of thinner and lighter prepregs which at the same time have higher strength properties. One distinctive feature of carbon fiber is its conductivity, which is advantageous in health monitoring but can cause corrosion in FML. Due to the form of fiber arrangement, prepregs are used as unidirectional tape and fabric-based prepregs. Usually, unidirectional fiber prepregs are used in FMLs, as Table 8.7

Chemical composition of glass fiber Chemical composition, wt.%

Glass fiber

SiO2

Al2O3

CaO

MgO

TiO2

ZrO3

Na2O

Fe2O3

Type E

59

12.1

22.6

3.4

1.5



0.9

0.2

Type S

6065.5

2325

09

611



01

00.1

00.1

Source: Based on Wallenberger F, Watson J, Li H. Glass fibers, in ASM handbook Vol. 21 composites. ASM International, Ohio, USA; 2001 [84].

Table 8.8

Selected properties of glass and aramid fibers [8587]

Fiber

Density (g cm23)

Young’s modulus (GPa)

Tensile strength (MPa)

Elongation (%)

Thermal expansion coefficient (31026K21)

Fiber dimension (μm)

E glass

2.54

7679

31003800

4.8

5

520

S glass

2.48

8891

4400

5.7

2.9

510

Kevlar K 49 Aramid HM

1.44

131

36004100

2.8

24

12

Kevlar K 149 Aramid UHM

1.47

179

3450

1.5

24

12

Kevlar 49 Aramid HS

1.44

83

3600

4.0

24

12

210

Table 8.9

Hybrid Polymer Composite Materials: Structure and Chemistry

Selected properties of carbon fibers

Property

Standard low modulus fiber

Aircraft fibers Low modulus

Middle modulus

High modulus

Young’s modulus (GPa)

228

220241

290297

345448

Tensile strength (MPa)

380

34504830

34506200

34505520

Elongation (%)

1.6

1.52.2

1.32.0

0.71.0

Resistivity (μΩ cm)

1650

1650

1450

900

Thermal conductivity (W (m K)21)

20

20

20

5080

CTE ( 3 1026K (on-axis))

2 0.4

2 0.4

2 0.55

2 0.75

Density (g cm23)

1.8

1.8

1.8

1.9

Carbon concentration (%)

95

95

95

99

Fiber dimension (μm)

68

68

56

58

Source: Based on Walsh P. Carbon fibers, in ASM handbook Vol. 21 composites. ASM International, Ohio, USA; 2001 [88].

laminates exhibiting anisotropic properties can be molded. Examples of prepregs reinforced with glass and carbon fiber, suitable for FML structures, can be seen in Table 8.10. In Glare laminates, S2-glass fibers in a FM94 adhesive system were used [89]. Such a prepreg is currently unavailable in Cytec, while the FM 94 adhesive is available [80]. Thermoplastic composites with continuous-fiber reinforcement are used more and more often in aircraft, taking place of other plastics and composites. Unidirectional prepregs, fabric-based prepregs, pultruded sheets, or tapes are seen as plies in TFMLs. The polymer selected depends on the application and cost [90]. The advantages of thermoplastic composites in FMLs include easier profile forming, higher working temperatures, lower moisture absorption, no expiry date, and incombustibility. Nonetheless, prepregs with matrices made of PEEK, PEKK, and PPS high-temperature thermoplastics are relatively expensive and require considerably higher curing parameters (heat and pressure).

8.2.3 Configuration of plies FMLs (TFMLs) are built of metal and unidirectional continuous fiber composite plies; the external plies are always metal. The configuration is usually symmetrical, from the center out. The basic configuration is 2/1 (Fig. 8.1A and B), that is, two metal external plies and a composite internal ply. Further configurations, with increasing number of metal plies, are 3/2 (Fig. 8.1C) and 4/3.

Table 8.10

Selected prepregs for FMLs [79,80]

Type of prepreg/ matrix

Fiber configuration

Tensile strength (room temperature (RT) dry) (MPa)

Tensile modulus (RT dry) (GPa)

Compression strength, (RT dry) (MPa)

In-plane shear strength, (RT dry) (MPa)

R-glass UD 60 vol.%

1560 (0 )

56 (0 )

1300 (0 )

67

Advantages

GFRP HexPly M12/ epoxy

E-glass woven 50 vol.%

CYCOM 7714/ epoxy

G

G

55 (90 )

16 (90 )

214 (90 )

510 (0 )

29 (0 )

500 (0 )

G

80

G

G

415 (90 )

26 (90 )

460 (90 )

E-glass 8 HS woven 5055 vol. %

345415

2128

331379

Carbon twill woven 50 vol.%

550 (0 )

5562

G

G

G

Low energy cure Good hot/wet performances up to 100 C Low moisture absorption Improved fatigue life on E, R-Glass UD laminates Tough epoxy matrix for use in helicopter blades Self-extinguishing Self-adhesive Solvent resistant

CFRP HexPly M12/ epoxy

65 (0 )

650 (0 )

75

(Continued)

Table 8.10

(Continued)

Type of prepreg/ matrix

HexPly M21/ epoxy

Tensile strength (room temperature (RT) dry) (MPa)

Tensile modulus (RT dry) (GPa)

Compression strength, (RT dry) (MPa)

650 (90 )

57 (90 )

730 (90 )

Carbon AS7 (HS) UD, 58.9 vol.%

2350 (0

148

1560 (0 )

109

Carbon IMA, UD, 59,2 vol.%

3050 (0 )

178

1500 (0 )

94

Fiber configuration

In-plane shear strength, (RT dry) (MPa)

Advantages

G

G

G

G

G

HexPly M91/ epoxy

Carbon AS7 (HS), UD, 58,7 vol.%

2580 (0 )

142

1350



Carbon AS7 (HS), UD, 58.9 vol.%

3520 (0 )

176

1880



G

G

G

G

Excellent toughness, in particular at high energy impact. High residual compression strength after impact. Good hotwet properties up to 150 C Low exotherm behavior allowing simple cures of thick structures up to 40 mm Good tack life Excellent toughness with very high residual compression strength after impact. Good translation of HexTow fiber properties, both for intermediate modulus and high strength carbon fiber. Good tack life and out-life. Low exotherm behavior allowing simple cures of

G

CYCOM 985/ epoxy

Carbon, UD, 56 vol.%

1980 (0 )

128 (0 )

1241 (0 )



G

G

40 (90 )

8.3 (90 ) G

G

G

CYCOM 5250-4/BMI

Carbon IM7, UD,

2618 (0 ) 248 (90 )

162 (0 ) 9.7 (90 )

1620 (0 )

103

G

G

G

G

G

G

HexPlyM65/ BMI

Carbon IM7, UD, 59 vol.%

1579 (90 ,0 )2s

85.4 (90 ,0 )2s

1234 (90 ,0 )2s

129 (645 )2s

G

G

G

thick structures up to 40 mm Good hotwet properties up to 120 C Modified epoxy system Excellent hot/wet strength retention Good impact resistance Ease of processing Good tack and drape Superior hot/wet properties at 104 to 190 C Excellent toughness High-temperature-resistance low thermal conductivity Excellent compression strength properties after impact Fluid/solvent resistant Specifically formulated for use in primary aircraft structures High laminate mechanical strengths and strains Improved compression after impact properties Excellent electrical properties

214

Hybrid Polymer Composite Materials: Structure and Chemistry

Figure 8.1 FML: (A) scheme of 2/1 (0/90) configuration, (B) laminate 2/1, and (C) laminate 3/2.

Table 8.11

Examples of configuration in FML

Grade

Lay out

Metal sheet alloy

Prepreg

Prepreg orientation in each fiber layer

Al/GFRP/Al (Glare 2A) [91]

2/1

2024-T3

Glass epoxy

0/0

Al/GFRP/Al (Glare 5) [91]

2/1

2024-T3

Glass epoxy

0/90/90/0

Al/GFRP/CFRP/ GFRP/Al

2/1

Al alloy

Glass epoxy/ carbon epoxy

0/0/0

Ti/CFRP/Ti/CFRP/ Ti

3/2

Ti

Carbon epoxy

245/ 1 45

The composite plies are composed of several laminas with the same or different orientation, which helps achieve isotropic, quasi-isotropic, or anisotropic properties. Most purposes, particularly aircraft primary structures, demand anisotropic properties. For the FML structure manufacturing, usually unidirectional prepregs are used, arranged according to the planned orientation. Because many lay-ups are possible, a clear coding system is used to identify the lay-up. All rolled metal plies in typical laminates have the same orientation. The direction of prepreg fibers corresponding to the direction of rolling is defined as 0 ; the perpendicular direction is defined as 90 . Configurations with the orientation of 0/0, 90/90, and 0/90/0 are symmetrical, while those of 0/90 asymmetrical. The configuration of 145/ 2 45 results in quasiisotropic properties. The examples of FML configurations can be seen in Table 8.11, in configurations 2/1 and 3/2. Composite plies in a laminate are made of one prepreg type or a combination of different types, creating a hybrid composite. One such example is Al/GFRP/CFRP/ GFRP/Al laminate, where glass composite laminas decrease the possibility of galvanic corrosion between aluminum and carbon composite [7,11].

Structure and chemistry of fiber metal laminates

215

The effective metal layer thickness for laminates lies within the range of 0.20.5 mm, and the thickness of a single composite layer within 0.10.5 mm [2,7,8]. On the basis of the number and thickness of each component layer (metal and composite), it is possible to calculate the metal volume fraction (MVF) coefficient in a laminate. MVF is defined as the ratio of the sum of individual metal layers thickness to total laminate thickness, Eq. (8.8). MVF 5

Pp

1 tm

tlam

(8.8)

where tm is the thickness of a single metal layer, tlam is the total thickness of a laminate, and p is the number of metal layers. On the basis of the MVF value, using the rule of mixtures, it is possible to estimate the value of a given static property [92,93]. FMLs and their structures are manufactured in the autoclave process. The main advantage of the autoclave technology is obtaining a laminate with considerably high quality in terms of porosity and metalcomposite adhesion. Laminate consolidation parameters depend on the prepreg curing parameters. The preparation of the metal sheet surface as well as tension during the curing process determines the ultimate metalcomposite adhesion and shape. The FML profile manufacturing

Figure 8.2 FML structure manufacturing: (A) components preparation, (B) layer configuration, (C) vacuum bag preparation, and (D) load prepared for autoclave curing; Scholz autoclave in Department of Materials Engineering, Lublin University of Technology.

216

Hybrid Polymer Composite Materials: Structure and Chemistry

process is carried out similarly to composite profile manufacturing. The main stages are preparing metal and composite components (Fig. 8.2A), arranging the layer configuration (Fig. 8.2B), forming a vacuum bag, and loading the autoclave (Fig. 8.2C and D). Nonflat metal components are formed in cold working, such as bending. It is necessary to optimize the bending radius of individual metal layers, depending on the composite filling thickness. The radius of the lower layer needs to be adjusted to the mold radius and the mold radius—to geometrical requirements. The radii of further layers result from the thickness of the composite filling after curing [94]. The curing process in an autoclave is realized through a rapid temperature increase in adjustable pressure conditions, isothermal heating for the time required for the process to take place, and then cooling. Process parameters (vacuum, overpressure, temperature, and time) are set for a specific composite or laminate. As regards thin structures, the heating process can be carried out continuously from RT to the curing temperature (Fig. 8.3A). As regards thicker components and prepregs with higher curing temperature, indirect heating is used (Fig. 8.3B).

Figure 8.3 Sample schemes of FML profile curing process: (A) continuous heating process and (B) gradual heating process.

Structure and chemistry of fiber metal laminates

217

Pressure change is realized in two stages (Fig. 8.3). The first stage involves lowering the pressure until 20.1 MPa vacuum is reached (in the vacuum bag). The second stage involves raising the pressure in the autoclave to 0.40.7 MPa along with the temperature to the value recommended for a given prepreg type (below 180 C for thermosets) and heating for about 2 h. The final stage involves lowering the temperature to about 70 C or lower at 0.033K s21, lowering the pressure to normal level, closing the vacuum, and turning off the autoclave. Decreasing the pressure is the key operation in this process. Moreover, it is necessary to control the heating and cooling rate. If the temperature rises too quickly, thermal stress in the material increases. Rapid cooling reduces initial and thermal stress relaxation, which may cause waving during the forming process [9498]. For some prepregs, especially with BMI matrix, postcuring in oven is recommended [79,80]. Replacing thermosetting composites with thermoplastic ones enables the TMFL profiles to be shaped during the consolidation and curing processes in the Out-ofAutoclave technology (OOA). An advantage of the OOA processes is that there are no size limits of the structure. The benefits of the use of thermoplastic prepregs include the possibility to shape the profile out of a flat packet of metalcomposite layers, the possibility to correct the profile’s geometry, as well as their theoretically unlimited durability. Prepregs, however, are much higher priced, especially the high-temperature ones, such as PEEK, PEKK, or PPS. They also require a higher polymerization temperature (300400 C) and higher pressure (up to 1.4 MPa). In the OOA process, it is necessary to design heating of forms in which the element is shaped. An alternative to this solution is to use self-reinforced polypropylene composite, for which both the temperature and the pressure requirements are lower, but which consolidate with metal by the means of additional adhesive layers [99101].

8.2.4 Defects in FMLs structure FMLs are mainly intended for primary structures. These are required to be of particularly high quality. They are subject to thorough quality control at every stage of manufacture, as well as during operation. Defects in FMLs can be classified either as defects arising in the manufacturing process or in-service damage. Basic types of defects which arise in the structure production process and are characteristic of hybrid metalcomposite materials include G

G

In respect of composite layers: Pores in the polymer matrix, delaminations on the fibermatrix boundary, uneven reinforcement distribution. These sorts of defects are typical of structural composites for general use. For the manufacture of FML structures, highquality prepregs are employed. Provided that they have been cured properly, their porosity does not exceed 1%, but it is even lower in practice. The joint between the fiber, and the matrix in those prepegs is continuous. In respect of metal: Uneven anodic oxide film thickness, inclusions, and discontinuities in the oxide layer—these are the types of defects which may occur when anodizing process is performed improperly. They should be detected during the sheet quality control.

218

G

G

Hybrid Polymer Composite Materials: Structure and Chemistry

Anodized surface contamination, which reduces adhesion, is usually impossible to detect directly. It results from failure to provide cleanliness of the technology. In respect of the metalcomposite interface: Delaminations, direct contact between fiber and metal, pores—these defects arise from improper curing cycle; excessively low adhesion—this defect is due to improper metal surface preparation and is identifiable only indirectly. In respect of FML as a whole: Shape defects resulting from incorrect layer arrangement, poor mold quality, improper profile design, and many more, as well as from thermal stress.

During the operation, defects in technology can lead to the development of microscopic cracks and delaminations, which results in loss of structure continuity or reduced performance properties. Inappropriate operation, for example, impact of foreign matter, may cause internal damage, not visible in macroscopic observations, but possible to detect by the means of NDT (nondestructive testing) methods. Environmental factors, such as temperature fluctuations or aggressiveness of the environment, can contribute to the corrosion of the metal layers, if the protective film on metal and FML edges is damaged and given the electrical conductivity of carbon fiber. Corrosion centers also need to be classified as in-service defects since they affect the reliability of the structure. Nondestructive methods of defect and damage detection are an important element in assessing the quality of materials and structures, as well as in diagnosing in-service damage. Nondestructive examination methods of FMLs are currently being developed. The results obtained are successively published in the available literature [102106]. As a result of connecting materials of different physical properties, NDT methods that have been used in composites prove insufficient for

Figure 8.4 Identification of Al/CFRP laminate surface damage following an impact of 2.5 J; ultrasound method.

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effective diagnosis of such structures. Modern methods may be used for the quality control; yet, their results should be selectively confirmed by the means of destructive methods of structure condition evaluation. One of the most appropriate methods of diagnosing FMLs is phased array ultrasound. This method allows for assessing the condition of FMLs in C-scan view (top view expressed in a color scale), and subsequent signal analysis in B-scan and A-scan of potentially faulty areas (Fig. 8.4). Due to the multilayered nature of the laminate, as well as the alternately stacked layers of materials of different acoustic impedance (different value of elastic wave propagation), wave propagation speed in the material must be set at an average value level between the composite and the metal. Multilayered nature of the laminate comprising layers which are different in terms of physical characteristics results in secondary reflections from each layer visible in the ultrasound image. B-scan cross section imaging presents single layers. What is essential about studying such materials by the means of this method is to capture the reference area of the laminate and to compare the shape of the signals received in this area with areas

Figure 8.5 Location of Al/CFRP laminate surface damage following an impact; mapping using sensor method; left to right: 3 J, 6 J, and 9 J.

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Figure 8.6 Detection of Al/CFRP laminate surface damage using X-ray CT; tomogram of a sectional FML sample at the point of impact: (A) 3 J and (B) 6 J.

examined in other locations. C-scan imaging is conducive to comparative analysis since any signal changes are visible on the color scale. Points of a different color undergo a secondary signal analysis in both A-scan and B-scan modes [104106]. Other NDT methods adapted to FML examination include pulse thermography, particularly useful for the detection of internal damage caused by impact [103,107], as well as methods based on the piezoelectric sensors placed on the surface or inside the laminate structure, which allow for continuous monitoring of the structure condition (structural health monitoring, SHM) [108,109]. Capabilities of damage detection by the means of ultrasound and sensors are presented in Figs. 8.4 and 8.5. Tomography and X-ray microtomography have been known as nondestructive methods which allow for the analysis of objects limited as to their dimensions. At low absorption levels through the polymer matrix, the difference in the absorption of component X-ray radiation allows for precise quantitative determination of porosity and discontinuity degree in the laminates. An example of the FML 2D scan after impact is presented in Fig. 8.6.

8.3

Microstructure characterization

Microstructure studies constitute a basic material analysis. Both qualitative and quantitative microstructure observations are necessary to assess the correctness of technology, especially for interfaces between components of hybrid materials. They are conducted on specimen coupons. Microstructure characterization on materials following various tests for example, mechanical, fatigue, heat, or corrosive examinations, and many more allows for the identification of degradation mechanisms, as

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it expands the knowledge of the relationship between the component type and configuration and the properties obtained. Furthermore, the features of the actual structure are used to verify numeric simulations. Thus, it is possible to optimize the structure and characteristics of the functional materials designed, that is, FML and TFML, much more efficiently. Characteristic microstructure features of FML are presented below, on the example of GLARE and CARALL laminate types, produced for research purposes by the authors.

Figure 8.7 Microstructure of UD laminate: (A) Ti/GFRP 2/1 (0/0)2, (B) Al/GFRP 2/1 (90/0)2, SEM.

Figure 8.8 Microstructure of UD laminate Ti/CFRP 2/1 (0/90); Ti sheets—white bars up and down, LM.

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8.3.1 Microstructure of polymer composite layers Microstructure uniformity depends on such factors as prepreg quality, number of layers, and their configuration, as well as curing parameters, such as pressure in particular. Microstructure obtained through curing of laminates with unidirectional fabric is normally homogeneous, without a visible prepreg boundary for one configuration (Fig. 8.7A) or with a visible connection between layers of different arrangement (Fig. 8.7B). Curing of the multilayered laminates, especially of different orientation, may result in a slightly bigger amount of resin on the boundary of these layers, which can be seen on microscope imaging (Fig. 8.8). This is characteristic of this particular technology and is not considered as a defect. The thickness of an area with additional resin does not exceed several micrometers. These areas are not privileged sites for decohesion.

Figure 8.9 CAA Anodic layer on 2024 aluminum alloy T3: (A) surface image and (B) cross section (Al alloy-up), SEM.

Figure 8.10 Anodic layer on cpTi (grade 2): (A) surface microstructure, SEM and (B) surface topography.

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8.3.2 Microstructure of metal surface layers The surface of a metal sheet intended to be connected with a composite material is usually prepared by the means of anodization, as shown in Section 8.2.1.4. Examples of anodized layer microstructure are presented in Figs. 8.9 and 8.10. Anodized layer on aluminum alloys, with a thickness of 35 μm, is characterized by a microcolumnar structure and rough surface, attenuated depending on the layer growth parameters (Fig. 8.9). An anodic layer on titanium, with a thickness of less than 1 μm, is presented in Fig. 8.10. It exhibits a uniform microstructure and surface development, which facilitates composite adhesion.

8.3.3 Microstructure of metalcomposite interface Metalcomposite interface is the most important area of the FML, as it affects its durability and, to a large extent, its mechanical properties. Interface microstructure

Figure 8.11 Proper microstructure of metalcomposite interface: (A) cross section and (B) longitudinal section; Al/GFRP, SEM.

Figure 8.12 Inaccurate metalcomposite interface: (A) excessive resin in profile bend, LM and (B) unbound area, SEM.

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Figure 8.13 Al/GFRP laminate microstructure with bending load: (A) cohesion crack in composite, adhesion crack on the boundary with the anodic layer, transverse cracks in the anodic layer, cross section and (B) longitudinal cracks in the fabric, matrix and anodic layer, longitudinal section; SEM.

observation enables the assessment of the laminate quality in respect of consolidation. Such studies, however, do not imply the durability of the connection. Adhesive joint of anodizing layer composite, if need be with thin film of primer or adhesive interlayer, is effective interface in FML or TFML. Proper interface microstructure involves continuous adhesion of the composite matrix polymer to the outer metal surface (Fig. 8.11). It is detrimental to the link durability if the reinforcing fibers contact this layer. During the profile production, an excess of resin may occur (Fig. 8.12A). Inclusions, pores, and unbound areas (Fig. 8.12B) are prone to develop into delaminations when the laminate is in use. Static and dynamic loads, as well as environmental factors, may result in microstructural degradation during operation. Decohesion in the composite area may occur at the cracks in the matrix, which usually propagate along the fibermatrix boundary (Fig. 8.13A) and fiber cracks (Fig. 8.13B). Delaminations in the area of metalcomposite interface occur along the boundary between the anodic layer and the composite (Fig. 8.13A). This layer may also crack (Fig. 8.13A and B). Should the laminate lose its continuity, fracture in the composite area is described in the same way as the composite material in terms of microstructure.

8.4

Physical chemistry of interface

Proper microstructure in the interface area does not ensure sufficient durability of the metalcomposite interface in FMLs. It is rather determined by adhesion between the layers connected. The phenomenon of adhesion can be explained on the basis of the following theories: Adsorption, mechanical interlock, diffusion, chemisorption, electrostatic, and

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weak boundary layer [110]. In respect of interaction between the anodic layer on metal and the polymer resin of the composite, it seems that the adsorption theory and the mechanical interlock theory are the closest to the actual description of the phenomenon of adhesion. The mechanical interlock theory is based on the factor that, at the microscopic level, all surfaces are very rough consisting of crevices, cracks, and pores. The adhesive penetrates these features and hardens such that it keys into the surfaces and forms a strong surface bond. Penetration of the adhesive in the surface pores may lead to high stress in the interface. This may have a large impact on the interface durability, provided that the adhesion surface has been properly prepared. The adsorption theory is based on the assumption that the adhesive wets the surface of the adherent when the join is formed. The adhesive should have a lower surface tension than the adherent surfaces as epoxy resins which wet metal and result in a good bond. According to this theory, in the event of intimate contact between the adhesive and the adherent, the adhesive strength arises as a result of secondary intermolecular forces at the interface. These may include van der Waals forces (dipoledipole, dipole-induced dipole interactions, and hydrogen bonds). The value of those forces is contingent upon basic thermodynamic values, such as SFE of both surfaces. Good adhesiveness results from good wettability. It is a necessary prerequisite, but not a sufficient one [110,111]. The number of hydroxy and epoxy functional groups in the epoxy resin (as composite matrix, primer, or auxiliary adhesive layer) constitutes one of the parameters determining the interface creation. These groups are responsible for strong chemical reactions with polar metallic surfaces. Polymermetal adhesion usually results from intermolecular chemical bonds and mechanical forces. Moreover, some metals, such as aluminum and titanium, have a strong coherent oxide layer, which is suitable for bonding. Adhesiveness is measured by SFE value, determined by the impact of a contact liquid on a solid substrate. Expressed quantitatively, adhesiveness equals the work required to create a new surface with a unit value on the boundary of the liquid and the solid phase, balanced in a reversible isothermal process. SFE is one of thermodynamic functions which define the atom balance in the outer layer of materials. It is therefore a characteristic value of every physical. In order to designate the value of SFE of the liquid, multiple direct methods are employed, while in the case of solids, there are no such techniques. Various indirect methods are in use, including Fowkes, OwensWendt, Wu, Zisman, Neumann, and van OssChaudhuryGood [112]. OwensWendt method is commonly used to determine the SFE of polymer materials [112,113]. According to this technique, it is necessary to define the dispersion and polar SFE components on the basis of Berthelot’s hypothesis. It states that the interaction between molecules of two different objects located in their outer layers is equal to the geometric mean of intermolecular interaction of each object. The method assumes that SFE γ S comprises two components: dispersion γ Sd and polar γ Sp. It also implies a correspondence between the abovementioned values:

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γ S 5 γ dS 1 γ pS Wa 5 γ LV ð1 1 cos}Þ

(8.9) (8.10)

By comparing those equations, the SFE of the solid object and SFE components is determined as follows: Wa 5 2ðγ S d γ LV d Þ0:5 1 2ðγ S p γ LV p Þ0:5

(8.11)

where γ Sd is the dispersion component of the SFE of the materials studied, γ Sp is the polar component of the SFE of the materials studied, γ d is the SFE of diiodomethane, γ S is the SFE of the solid object, γ SL is the surface energy on the phase boundary between the solid and the liquid object, γ L is the SFE of the liquid object, and ʘ is the contact angle measured on the actual surface studied. In order to define the polar and dispersion SFE components, it is necessary to measure the contact angle on the surface of the materials studied by the means of two liquids. The most popular liquids used for the measurement are distilled water (polar liquid) and diiodomethane (apolar liquid) with known SFE properties. This method was used by a number of researchers, including the authors, to assess the methods of component surface preparation, especially in the case of metal, for the FML manufacture [5658,64,114,115]. The link between the contact angle, the surface energy and the durability of the connection between various thermoplastic polymers and steel have been presented by Ochoa et al. [114]. The higher the surface energy, the higher the durability of the connection with steel, measured in shear strength studies. Ochoa et al. have noted that the surface energy value is directly linked to the number of polar groups or reactive ions on the polymer surface. It has thus been proven that surface wettability indicates its adhesiveness. They have also shown that higher surface energy implies better adhesion to a surface with a similar polarization. According to several studies, contact angle and roughness are the most adequate values to be measured in order to assess the modification of metal surface in respect of connecting it to a polymer composite. Both the initial bond strength and its later durability are contingent primarily on the interaction between the resin and the adhered material. In his studies on adhesion, Botelho [115] utilized aluminum surfaces prepared for adhesion by the means of anodizing in CAE. Contact angles were measured in order to select the proper method of preparing metal surface before producing hybrid laminates. Unprocessed samples and samples anodized in sulfuric acid exhibited larger contact angles compared to aluminum samples anodized in chromic acid. Anodizing metal in chromic acid has been therefore proven to be a better method of preparing surfaces for FML production. Liu et al. [59] showed that adhesion properties are determined by the molecular architecture of the polymer, chemical properties, and surface roughness of the metal substrate. They conducted shearing studies, according to which interphase adhesion is largely influenced by the orientation of sheet rolling and chemical surface properties engendered

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Figure 8.14 Surface morphology of an anodized aluminum sheet: (A) CAA and (B) SAA after a shear attempt; Al/GFRP laminate (0/0), SEM [56].

by various changes in the environment humidity. Own research [57,116] demonstrated better surface wettability for 2024 sheets subjected to CAA than for sheets subjected to SAA. Both anodization methods lead to adhesion interface which endures shear stress exceeding 3 MPa, whereas the character of matrix cracking along the metalcomposite boundary differs depending on the anodization method employed. Interfaces with metal anodized in chromic acid exhibited larger displacement of elements before the continuity loss at lower failure stress. Interfaces anodized in sulfuric acid, however, transfer higher shear stress with a much lower displacement effect on the components. Both sample types undergo composite decohesion due to resin delamination along the metalcomposite boundary. A polymer layer which was bound by adhesion remains on the metal surface, but the matrix on surfaces anodized in chromic acid exhibits more intensive signs of microflow and rolling, which may be indicative of plastic deformation (Fig. 8.14A). On the surface anodized in sulfuric acid, polymer shows signs of cold cracking and plastic cracking, hence smaller displacement when the material is subjected to shear force (Fig. 8.14B). For laminates with (0/0) orientation, shearing occurs along with

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Figure 8.15 Surface morphology of an anodized aluminum sheet: (A) CAA and (B) SAA after a shear attempt; Al/GFRP laminate ( 6 45), SEM [56].

composite decohesion, but the fabric cracking is more intensive. The layer which is bound to the substrate by the means of adhesion is richer in fiber fragments, but it also shows signs of cold and plastic cracking in the matrix along the boundary with the layer anodized in sulfuric acid (Fig. 8.15). What is therefore essential for the laminate layer interface is the choice of resin and the method of preparing the sheet surface in a way which would provide a higher adhesive durability of the interface than the cohesive durability of the composite layer. It should be noted that the difference in coefficient of thermal expansion between the adhesive and adherent (Tables 8.2, 8.6, 8.8, and 8.9) can have an important effect on the joint design working over a significant temperature range. Adverse stress condition, which may lead to the structure deformation or loss of interface adhesion, develops especially in laminates with unidirectional carbon reinforcement [9498,117].

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Acknowledgment Presented authors’ own research was financially supported by the National Science Centre allocated on the basis of the decision number UMO-2014/15/B/ST8/03447.

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Opportunities and challenges in the use of layered double hydroxide to produce hybrid polymer composites

9

Shadpour Mallakpour1,2,3 and Elham Khadem1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

Chapter Outline 9.1 9.2 9.3 9.4

Introduction 235 Introduction to LDH, structure, and preparation 236 Modification 237 Preparation and characterization of LDH-based polymer NCs 240 9.4.1 In-situ polymerization method 241 9.4.2 Solution intercalation 242 9.4.3 Melt compounding method 244

9.5 Properties of polymer/LDH composites 9.5.1 9.5.2 9.5.3 9.5.4 9.5.5

245

Thermal properties 245 Mechanical properties 248 Rheological properties 250 Swelling properties 252 Other properties 253

9.6 Conclusions 255 Acknowledgments 255 References 256

9.1

Introduction

For many decades, layered double hydroxides (LDHs) have been an interesting class of hydrotalcite-like materials in both industry and academia, which can be designed to obtain wide ranges of properties for using in various large- and Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00009-4 Copyright © 2017 Elsevier Ltd. All rights reserved.

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small-scale applications, such as catalysis [1,2], electrochemistry [3], flame retardants [4], adsorbent [5], drug delivery [6], super capacitors, fuel cells [7,8], electrochemical sensors [9], biosensors [10], and many other fields. These applications give rise to the properties such as their low cost, two-dimensional structure, high surface area, positive charge on the surface, high anion-exchange capacities, tunable interior architecture, and resistant to changes upon heating which make LDHs useful materials [11]. LDHs, due to the similarities, share with cationic clays or hydrotalcite-like materials and sometimes introduce as anionic clays developed from the natural hydroxycarbonate of Mg and Al which was first reported by Hochstetter in 1842 and synthesized 100 years later by Feitknecht [12,13]. Unlike layered silicates which naturally form on the Earth, only a few anionic clays have been occurred in nature, and most can be readily and artificially fabricated in the laboratory [14]. So, the morphology and natural features of the prepared LDHs can be easily designed and controlled, while the type and quality of layered silicates like MMT are constrained by nature [15]. LDHs are suitable candidates for the synthesis of polymer nanocomposites (NCs) due to most fascinating properties such as large surface area, layered crystalline geometry and interlayer anions interchange ability with much larger organic anionic species. The layered structures in these materials induce interlayer spacing which aids the intercalation of the polymer chains and offer a high degree of interaction between filler and polymer. These characteristics enhance the mechanical performance and properties of the composites [16]. However, the pristine LDHs are not appropriate for the penetration of giant polymer chains or chain segments into their gallery space unless the original interlayer distance is considerably increased through organomodification treatment to improve the compatibility of LDHs with polymer matrices [17,18]. In this chapter, we collect the information published, roughly from 2010, on the structure and the synthesis and modification methods of LDHs, as well as the use of them as nanolayers for improvement of the polymeric matrixes properties.

9.2

Introduction to LDH, structure, and preparation

LDHs or hydrotalcite clays are an organized broad family of synthetic twodimensional lamellar nanostructures [19]. The majority chemical composition of LDH materials is represented by normalized formula 21 31 x1 n2 31 ½M21 M ðOHÞ  ðA ÞUmH O, where M and M usually stand for the divalent 2 2 12x x x=n (such as Mg21, Zn21, Cu21, or Co21) and trivalent (such as Al31, Fe31, or Cr31) 2 22 22 metal ions; An2 (e.g., Cl2 ; NO2 3 ; ClO4 ; CO3 ; SO4 ) is an anion in the interlamellar region, and m is the amount of water present in the same region, and x, the constant value x(5M21/(M21 1 M31)), usually corresponds to 0.20 , x , 0.33 in forming a pristine LDH phase [20,21]. Their structures are based on brucite structure, which consist of one divalent metal cation in the center and six hydroxyl groups at vertexes and can be presented by idealized surfaces of 2:1 and 3:1 (MII/MIII)-type LDHs (Fig. 9.1) [15,18]. When a fraction of the divalent metal is substituted with a trivalent

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Figure 9.1 Schematic representation of the LDH structure. Adapted from Ref. [18], with kind permission of Elsevier.

metal, the layers lead to a net positive charge which is neutralized by intercalation of anions together with the water molecules in the hydrated interlayer regions [22]. Several techniques are reported in literatures for synthesizing LDH materials that have been summarized in different books, reviews, and thematic journal issues [19,23]. Among various methods, the most common are coprecipitation, urea reduction, saltoxide method, hydrothermal, electrochemical, solgel, structure reconstruction, and the anionexchange method [2427]. It has been reported that the coprecipitation method is the most common and useful method for synthesizing large amounts of LDH, while electrochemical and solgel are the least used methods [12,16]. By adjusting the synthetic parameters during coprecipitation or during postprecipitation treatment such as concentration of reactants, temperature, and aging time, LDH materials can be synthesized with different particle sizes, morphologies, and crystallization degrees [19,28].

9.3

Modification

The modification of LDH materials is an inevitable process in obtaining a better dispersion of LDHs into the polymer matrix. The compounds used for

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modification universally have negatively charged functionalities due to the positively charged hydroxylated layers [29]. The main goal of organic modification of LDH materials is to enlarge the gallery space of LDH materials to make the intercalation of large species, like polymer chains and chain segments, easier than to the unmodified LDHs. The anionic organic surfactants containing at least one anionic end group and a long hydrophobic tail are the suitable compounds for serving the desired purpose [12,17]. The modification and intercalation into LDHs is commonly made by one of the following three routes [30]: (1) coprecipitation (direct synthesis), in this method, the layers will fabricate by addition of the desired anion to a solution containing metal salts of the ions. The control of the charge density of the layers (M21 to M31 ratio) and the high purity of the obtained LDHs are the major importance of this technique, (2) anion exchange consists of the transformation of the host material into a colloidal system containing an excess of the organic anions to be incorporated. The anion exchange of interlayer carbonate anions due to the high affinity of the layers for CO22 is 3 extremely difficult which limit the industrial applications of LDHs, and (3) reconstruction; the LDHs lamellar framework after heating at temperatures greater than 450 C for several hours, they are converted into an amorphous mixed oxide, which after dispersion in an aqueous solution, are able to rehydrate to their original form in the presence of anions and water. This phenomenon is a peculiarity of LDH system, known as a “memory effect” [15,31]. Zhan et al. [32] showed that the adsorption capacity protein (hemoglobin, Hb) on hydroxyl functionalized ionic liquid (HFIL)/ZnAlLDH through coprecipitation technique was higher than that of LDH because of the more adsorption sites endowed by HFIL. Compared to the LDHHb with the thinner and irregular flaky particles (Fig. 9.2A), HFILLDHHb displayed the thicker lamellar particles with more ill-defined shape (Fig. 9.2B), due to the stronger interaction between Hb and HFILLDH support. transmission electron microscopy (TEM) image of HFILLDHHb presented a better dispersibility with the smaller average particle size about 400 nm than that of LDHHb (Fig. 9.2C and D). Mallakpour et al. prepared the modified LDHs (m-LDHs) by the coprecipitation reaction of Al(NO3)3  9H2O, Mg(NO3)2  6H2O, and various chiral diacid such as N-trimellitylimido-L-valine [33], N-trimellitylimido-L-isoleucine [34], L-aspartic acid [35], and N,N0 -(pyromellitoyl)-bis-amino acids based on L-leucine, L-alanine, L-methionine, L-phenylalanine, and L-valine [36], under ultrasound irradiation (Fig. 9.3). In another recent report by the same group, N,N0 -(pyromellitoyl)-bis-L-a-amino acids (L-isoleucine, S-valine, L-leucine, and L-phenylalanine) were used as modifying agents for intercalation into MgAl LDH via ion exchange reaction in distilled water (Fig. 9.4) [37]. In one case, TEM image of LDH/N,N0 -(pyromellitoyl)-bis-Lphenylalanine shows the hexagonal platelets with rounded corners that do not observe any signs of aggregation visible in the micrographs (Fig. 9.5). They applied these treatment LDH in preparation of polymer NCs based on poly (amide-imide) (PAI), poly(vinyl alcohol) (PVA), and poly(vinyl pyrrolidone) (PVP) for improving thermal and mechanical properties.

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Figure 9.2 SEM (A and B) and TEM (C and D) images of LDHHbcop and HFILLDH Hbcop. Adapted from Ref. [32], with kind permission of Elsevier.

Figure 9.3 Preparation of chiral diacid, LDH-diacid. Adapted from Ref. [33], with kind permission of Springer.

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Figure 9.4 Proposed models for the LDH products with chiral dicarboxylates. Adapted from Ref. [37], with kind permission of Springer.

Figure 9.5 TEM images of modified LDH with N,N0 -(pyromellitoyl)-bis-L-phenylalanine diacid. Adapted from Ref. [37], with kind permission of Springer.

9.4

Preparation and characterization of LDH-based polymer NCs

The utilization of solid materials with layered crystalline structures as reinforcing agents in the polymer matrixes relies on the rupture of their original structure (exfoliation of layers) or intercalation of organic molecules between the platelets and also on their uniform dispersion in the polymer matrix. Compared to layered silicates, LDHs due to having the advantages including the structural homogeneity,

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tailor-made structures and chemical compositions, nontoxicity, high bound water content, and high reactivity toward organic anionic species can be used as potential candidates for a wide range of applications [38,39]. The incorporation of m-LDHs in the polymer matrix may proceed via different strategies such as in-situ methods, solution intercalation, and melt compounding method [40]. It has been obviously demonstrated that various parameters such as the chemical nature and concentration of the anions in the inorganic layers, the interlayer distance between the layers, as well as the nature of the interlayer anions, the particle concentration in the composites, the preparation method, and the nature of the polymer matrix can affect the exfoliation of the LDH particles in the matrix [41,42]. Until now, several analytical techniques, including X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), TEM, and thermogravimetric analysis (TGA), have been applied to characterize the organically m-LDHs and polymer/LDH NCs. XRD is one of the most commonly used methods to identify whether the organic compounds have been intercalated into LDH spaces successfully. A shift of the d0 0 3 value diffraction peak to lower 2θ angle will show an enlargement of interlayer galleries, due to a successful intercalation of organic compound [43]. Mallakpour et al. indicated the interlayer distance of LDH sheets increase after intercalation of dicarboxylated anions; consequently, the d0 0 3 value (c. 0.76 nm) has shifted to higher d value (c. 2.15 nm and 2 nm for N,N0 -(pyromellitoyl)-bis-L-phenylalanine and N,N0 -(pyromellitoyl)-bis-L-methionine, respectively) [44,45]. The state of dispersion and exfoliation of LDH platelets and qualitative understanding of the internal structure are examined by SEM or TEM images. Also, from FTIR, TGA analyses are used to study the chemical structures and the content of interlayer organic anions [46,47]. Numerous review papers are available to summarize the research results mostly with focuses on the characterization of m-LDH and their composites [15,42,48].

9.4.1 In-situ polymerization method In-situ polymerization, the m-LDH is first swollen by a liquid monomer or a monomer solution so that the polymer formation can occur between the intercalated sheets, the monomer by migrating monomers into the galleries of the layered silicate. Polymerization can be performed either by heat or radiation, by the diffusion of an appropriate initiator or by an organic initiator or catalyst fixed through cationic exchange inside the interlayer before the swelling step by the monomer [31,42]. Fig. 9.6 indicates the general principle for accomplishing in-situ polymerization within the layers of LDH crystals. The rupture of the lamellar structure and high exfoliation of the layered particles are the main advantages of this method which improved the homogeneous dispersion of layered in the polymer matrix [42]. However, it due to somewhat complex process is rather industrially unfeasible for commodity polymers. Numerous NC materials were prepared by the polymerization of monomers into the interlayer space of LDHs, which have been reported in the literatures. Youssef et al. prepared a free-emulsion polymerization of styrene monomer intercalated ZnAl LDH [49],

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Figure 9.6 In-situ polymerization method.

and Ref. [50] prepared PET [poly(ethylene terephthalate)]LDH composites by microwave heating routes. Further, Katiyar et al. prepared poly(L-lactide) (PLLA) LDH NCs by an in-situ intercalative ring-opening polymerization method [51]. TEM images shown in Fig. 9.7 illustrate the distribution of LDH platelets in the PLLA/LDHCO3 5% (A) and PLLA/laurate-m-LDH (LDH-C12) compounds containing 5% (B), 3% (C), and (D) 1% LDH loadings. Fig. 9.7A shows a situation in which LDH layers did not mix well with the PLLA matrix. They considered the effect of LDH-C12 loading in morphologies of PLLA and observed random dispersion of LDH without any pattern at lower concentrations and a tendency to LDH platelet orientation at higher loading. Further, this method has been successfully used to prepare LDH-NCs-based poly(butyl methacrylate) [52], PVA [53], PET [54], polyurethane/polymethyl methacrylate (PMMA) [55], polyamide 6 [25], polystyrene (PS) [56], and poly(acryloyl morpholine) [57].

9.4.2 Solution intercalation Following this technique, the preexpanded LDHs are dispersed in a polymer solvent to promote diffusion of the macromolecules having functionalities in the LDH interlayer spacing. Finally, removing the solvent, either by vaporization, usually under vacuum, or by precipitation, is resulted in the polymer incorporated between the inorganic composite platelets [17]. The mechanism of this method is based on the hydrogen intercalation of polymers with little or no polarity into layered structures, which produced the thin films with a high orientation of the inorganic particles (Fig. 9.8). This procedure is quite simple for the intercalation of polymers; nevertheless, it is difficult to apply in industry owing to problems associated with the use of the organic solvents which is usually environment unfriendly and economically prohibitive [40]. The structural and thermal properties of PS/CoAl LDH NCs are synthesized via simple solvent blending method studied by Ref. [58] and rheological properties of PMMA/Ni-Al LDH NCs investigated [59]. The results indicate that regression coefficient value of the HerschelBulkley is the best fit among all models followed by

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Figure 9.7 TEM bright field images of PLLA/LDH-CO3 (5%) (A), PLLA/LDH-C12 (5%) (B), PLLA/LDH-C12 (3%) (C), and PLLA/LDH-C12 (1%) (D). Adapted from Ref. [51], with kind permission of Elsevier.

Casson and Power law model. Mallakpour and Dinari [60,61] studied the thermal and mechanical properties of PVA composite films containing 2, 4, 6, 8 wt% of organically m-LDH, prepared by solution intercalation method under ultrasonic irradiation. Fig. 9.9 shows the TEM images of 10 wt% Poly(L-lactide) (PLLA)/γ-polyglutamatem-LDH fabricated using solution mixing process [62]. TEM micrographs show that the original stacked lamellar structure of LDH layers could be modified to form the disorderly dispersed morphology in the PLLA matrix, and the hydroxide layers could stay stacking together. Over the past decade, several polymers seek to develop polymer/LDH NCs with different properties, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [63], PLLA [64,65], PAI [64], poly(vinyl chloride) [66], copolymerization of bis[2(methacryloyloxy)ethyl] phosphate and methyl methacrylate [67], and PVA [68].

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Figure 9.8 Solution blending method.

Figure 9.9 Bright-field TEM micrographs of 10 wt% PLLA/γ-LDH NCs. The inset in (b) is the high magnification TEM images of 10 wt% PLLA/γ-LDH samples. Adapted from Ref. [62], with kind permission of Elsevier.

9.4.3 Melt compounding method This technique consists of blending the LDH particles with the polymer matrix by applying high local shear stresses and using rather high temperatures in a meltmixer dispositive. Under such conditions, the polymer chains can crawl into the interlayer gallery and produce either an intercalated or an exfoliated NC (Fig. 9.10) [42]. The synthesis of polymer/LDH NCs by the melt compounding method was first reported by Nichols et al. in 1999 [69]. Leng et al. [70] prepared NCs based on PLLA and organically modified MgAl LDH by melt mixing under a corotating twin-screw microextruder at 463 K with 200 rpm screw speed for 10 min. Kutlu et al. [71] showed that the tensile strength and the durability of m-LDH melt compounded with polypropylene (PP) increased than that of the neat PP. Wang et al. [72] have fabricated PP/Co-Al LDH NCs by melt intercalation of PP into the part of CoAl LDH interlayers and studied the thermal degradation of NCs using different kinetic models.

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Figure 9.10 Melt-mixing method.

An extensive number of other literatures over the past decade reported to develop polymer/LDH-based NCs using different types of matrixes such as poly (butylene succinate) (PBS) containing oleate, anions modified MgAlLDH [73], PP containing sodium dodecyl benzene sulfonateZnAlLDH [74], PLLA containing γ-polyglutamate-modified MgAlLDH [75], and poly(p dioxanone) containing modified MgAlLDH with dodecylbenzene sulfonate and 4-hydroxybenzene sulfonate [76].

9.5

Properties of polymer/LDH composites

NCs consisting of LDH (organically modified or not) and polymer matrix frequently reveal moderate enhancement in mechanical, thermal, and various other properties than that the neat polymer. The stronger interfacial interaction between the polymer matrix and LDH particles with nanoscale dimensions play an important role in improvement properties in NCs.

9.5.1 Thermal properties Generally, the dispersion of LDHs into polymer matrixes was found to increase thermal stability by acting as a superior insulator. The increase in the degradation temperature is generally regarded as a consequence of the low permeability of the NCs hindering the diffusion of oxygen and volatile products throughout the polymer. Also, improvement in thermal stability may be due to the formation of a char layer after thermal decomposition of the organic matrix. In addition, inorganic materials by providing a cooling effect can retard the combustion stage of the organic species (surfactant anion, polymer chain segments, etc.) constrained inside the interlayer space of the LDH particles [15]. Mallakpour and Dinari [77] reported a systematic improvement in the thermal stability of the PAI matrix by the incorporation of different amounts of LDH/carbon

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Figure 9.11 TEM micrographs of LDH-CO22 3 (A, B), LDH-diacid (C, D), and PVP containing 8 wt% of LDH-diacid (E and F). Adapted from Ref. [33], with kind permission of Springer.

nanotube (2, 4, and 8%). According to authors, the 5% and 10% mass loss temperatures of the NCs were higher than those of the neat PAI, and the degradation temperature of the NCs has been shifted to higher region as the concentration of LDHCNT is increased. In another work [33], they prepared PVP NCs with novel chiral diacid intercalated LDHs (28 wt%) as nanofillers via ultrasonic irradiation. Fig. 9.11 presents actual images of LDH nanoplatelets before and after loading in the polymer matrix. According to those, the sheets have uniform contrast, reflecting their ultrathin nature and homogeneous thickness. In NC, LDH similar to metal hydroxide takes off heat from the surrounding environment, and the liberated water vapor decreases the content of flammable volatiles in the vicinity of the polymer so that m-LDHs could retard the pyrolysis of PVP. The obtained results for limiting oxygen index values (3540) based on Van Krevelen and Hoftyzer equation confirmed that some polymer NCs such as PAI-containing LDH can be categorized as the self-extinguishing materials [77,78]. Zhang et al. [79] compared the thermal stability of PVC combined with 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP), LDH, LDHBP under nitrogen flow. According to the authors, static and dynamic thermal stability of PVC/LDHBP composite remarkably improves in comparison to PVC/BP and PVA/LDH. This result is related to the homogeneously dispersed LDHBP and the elimination of the alkali of LDH after intercalation. Also, LDHBP has strong absorption capacity to the liberated HCl during the dehydrochlorination process of PVC. Labuschagne et al. [80] used LDH derivatives as heat stabilizer for flexible PVC compound. Fig. 9.12 shows TGA traces of the PVC/LDH composites,

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Figure 9.12 TGA traces in air for the PVC-LDH composites. Temperature was scanned from 25 to 900 C at a scan rate of 10 C min21 with nitrogen flowing at a rate of 50 mL min21. Adapted from Ref. [80], with kind permission of Elsevier.

LDH derivatives: Apparent elemental composition calculated from ICPOES, TGA residue at 900 C and d spacing from XRD results Table

9.1

Sample

Apparent composition

Residue (%)

d Spacing (nm)

MgAlLDH MgCuAlLDH

[Mg2.092Al0.908(OH)6](CO3)0.454 [Mg1.570Cu0.597Al0.833(OH)6] (CO3)0.417 [Mg1.527Zn0.554Al0.918(OH)6] (CO3)0.459 [Mg2.062Fe0.198Al0.740(OH)6] (CO3)0.370 [Ca2.275Al0.725(OH)6] ((OH)2CO3)0.182

57.0 65.6

0.761 0.761

61.9

0.758

56.3

0.768

59.2

0.755

MgZnAlLDH MgFeAlLDH CaAlLDH

Adapted from Ref. [80], with kind permission of Elsevier.

compared with virgin PVC in a nitrogen atmosphere. Generally, the degradation of composites based on derivatives LDH displays a two-step mechanism. First stage of weight loss is associated with a combination of PVC degradation (mainly dehydrochlorination) events and volatilization of the plasticizer, and the second stage is related to pyrolysis processes that give rise to a carbonaceous char rest. The obtained results of elemental composition, TGA residue at 900 C, and d spacing from XRD analysis are summarized in Table 9.1.

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The TGA results of PVC composites in another work [81] showed that adding of 5 wt% LDHdodecyl-sulfate (DS)toluene-2,4-di-isocyanate (TDI), the Tonset, Tmax1, and Tmax2 of PVC enhances from 279, 298, and 479 to 291, 314, and 487 C, respectively, higher than that of pristine PVC and PVC/LDHNO3TDI composite. The increase may be attributed to add inorganic LDH content and form char by m-LDH during the degradation process. In other word, LDHs can react with HCl and delay of pyrolysis process of PVC [82]. The incorporation of flame retardant modified MgAlLDH results in dramatically enhanced thermal stability properties of unsaturated polyester (UP) resin [83]. The degradation patterns for neat UP and their NCs with m-LDH were different, indicating different mechanisms for the composite degradation. Under a nitrogen atmosphere, there are three steps of degradation. The first below 160 C is associated with the loss of surface adsorbed and interlayer water by about 7.5 wt%, the second range (250400 C) corresponds to the dehydroxylation of the brucite-like layers and decomposition of NO2 3 between the layers of LDH and leaves about 50 wt% solid residue, whereas the third step beyond 600 C was attributed to dehydroxylation of host layers, and after destroying, the metallic oxide was produced with a total residual of 52%. Chiang et al. synthesized biodegradable PLLA reinforced with MgAl LDH. To enhance the compatibility of PLLA and filler, the surface of LDH was modified with polylactide-COOH (PLA) (henceforth designated as p-LDH). They showed that the thermal stability of NCs unexpectedly worsened by increasing p-LDH in PLLA and obviously shifted the degradation starting temperature downward. [65]. The incorporation of oleate-LDH (o-LDH) in PP matrix improved the PP relative crystallinity, induced crystalline orientation, and declined the glass transition temperature. Furthermore, the PP containing 0.45 and 0.90 wt% of o-LDH showed improved initial resistance to thermal degradation and stiffness [84]. In a typical example, Ref. [85] studied the thermal stability of PLLA and NCs prepared using 1 and 2 wt% of LDH modified by a dodecyl sulfate anion (DDS) via in-situ intercalative bulk polymerization. The Tg of PLLA/CaAlDDS NCs was about 20 C higher than that of pure PS which can be attributed to uniformly dispersion of LDH particles in the matrix and the decrease of polymer chain mobility in vicinity particle.

9.5.2 Mechanical properties A main reason for incorporation nanofillers to polymer matrix is to enhance the modulus or stiffness via reinforcement mechanisms illustrated by theories for NCs. The large surface area of LDHs, in some cases reaching 80 m2 g21, causes a large interaction and adhesion with polymer molecules and leads to a large mechanical reinforcement effect. Properly dispersed and aligned layered particles have proven to be very effective for improving the stiffness and tensile properties, particularly the modulus and tensile strength, of polymers even when present in small concentrations. Higher LDH amounts enhance the probability of particle cluster formation, resulting in decline of the mechanical properties [41,86]. In general, rigid fillers are naturally resistant to straining, thus, when a relatively softer matrix is reinforced with such fillers, the mobility of polymer, particularly

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polymer chains adjacent to the filler particles, becomes highly restricted and creates ¨ zgu¨mu¨s¸ et al. [87] investigated the effect the high modulus in filler-loaded polymer. O of MgAlLDH on the mechanical properties of the polyampholyte NC hydrogels (NH-LDHs) prepared by an in-situ free radical addition polymerization method in aqueous media. Fig. 9.13 presents the relationship between MgAlLDH content and compressive strength and elongation of the NH-LDHs. As shown in Fig. 9.13, the elongation at break of the gels considerably enhanced from 68.8% to 93.5% with increasing MgAlCl LDH content in the range of 15 wt%, while in content above of 5 wt%, the elongation at break of the NC hydrogels decreased to approximately 48.6%. In addition, maximum compressive strength and modulus of the composite were observed for MgAlLDH content increased up to 1%. This event is most likely caused by the crosslink density of the NH-LDHs which produced the tough polymer network. Also, higher amount of MgAlCl LDH in the NH-LDHs structure produces the additional cocrosslinking points effect, which significantly reduced the molecular mobility of the polyampholyte hydrogel structure. In the case of PLLA, addition of 1.2 wt% PLAcarboxylic acid (COOH) m-LDH (p-LDH) showed significant enhancements in the storage modulus than that of neat PLLA [85]. Adding more PLDH (0.20.4 wt%) into the PLLA matrix induced a decrease in the storage modulus of PLLA/p-LDH NCs. This finding can be ascribed to the excessive concentration of PLACOOH moleculars with low mechanical properties. Li et al. [88] studied the effect of adding organic modified LDH (OLDH) to poly(propylene carbonate) (PPC) and PPC-based NCs containing 03 wt% OLDH. They found that the tensile strength of PPC composites containing 2% OLDH is 83% higher than that of pure PPC with only small loss of elongation at break. The effect of LDHs modified with organic diacid in various contents (2, 4, 8 wt%) on the mechanical properties of PVA was reported by Mallakpour et al. [89].

Figure 9.13 The effect of the MgAlCl LDH content on the compressive strength and elongation of the NH-LDHs. Adapted from Ref. [87], with kind permission of Elsevier.

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Figure 9.14 SEM images of (A) neat PVA hydrogel and PVA/LDH NC hydrogels with (B) 0.2 wt% and (C) 0.5 wt% LDH prepared through 11 freezing/thawing cycles. Adapted from Ref. [90], with kind permission of Springer.

Mechanical data showed the Young’s modulus, tensile strength of PVA NC hydrogels were greatly enhanced by adding of m-LDH even at low-loading level. The improvement of properties was attributed to strong interfacial adhesion and increase of the physical crosslinking density of m-LDH with PVA via hydrogen bonding. In other hand, an addition of m-LDH nanoclay results in decrease in the elongation at break for the NCs which is due to present of the rigid crystalline nanoclay in the PVA matrix. In another work, Huang et al. [90] showed that Young’s modulus, tensile strength, and elongation at break of PVA NC hydrogels containing of LDH (0.2 and 0.5 wt%) were significantly improved for different freezing/thawing cycles, after adding filler. Nevertheless, the increase of 300% of the fracture strength and 150% of the Young’s modulus of PVA/LDH (0.5 wt%) NC hydrogels prepared through 11 freezing/thawing cycles can be attributed to morphological changes mainly involving a growth of the crystallite size and an increase of the number of crystallites (Fig. 9.14). LDH nanoparticles can act as a heterogeneous nucleation sites in which polymer chains nucleate and crystallize, thus improving the crystallization of PVA hydrogels.

9.5.3 Rheological properties The study of the rheological properties of any organic macromolecule is crucial to gain fundamental understanding of the processibility of that material. In the case of filled polymeric systems, rheological behavior is provided important information about the degree of LDH dispersion in the polymer and possible particle/particle or particle/polymer interactions by measuring their effects on the flow behavior in comparison with the corresponding pure polymer melts. The rheological properties of filled polymeric systems are sensitive to the structure, particle size, shape, and surface characteristics of the dispersed phase, and as well as state of dispersion of particles in the polymer matrix [59]. Thus, rheology can be applied as a complementary analysis in the materials traditional characterization. Rheological studies on the PMMA showed that the viscosity and shear stress properties of PMMA/NiAlLDH are increased by increasing the LDH loading in the polymer solution [59].

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Bercea et al. [91] investigated viscoelastic properties of chitosan/PVA/LDH composites in various PH conditions. The viscosity of CS/PVA/LDH mixture is very sensitive to structural changes so that at low pH values, composites have displayed a liquid-like behavior, and the sample is preponderant viscous. In such condition, the elastic recovery is very small, and the degree of recovered strain is about 4% of the maximum value obtained by the strain in the creep test. In weakly acid and basic conditions, by applying a constant stress up to 120 Pa during creeprecovery tests, a high elasticity was detected for CA/PVA/LDH. In such condition, LDH acts as a crosslinker for the polymer mixture and a network structure is formed, which presented a solid-like behavior in oscillatory shear conditions. Hennous et al. [92] studied the rheological properties of three biorelated polyesters, PLA, PBS, and poly(butylene adipate-co-terephthalate) filled with Zn2Al/lignosulfonate (LS). The rheological parameters were increased by the addition of 5 wt% Zn2Al/LS to the PLA and PBS compared to the polyester itself. In such condition, the Zn2Al/LS platelet is acted as a chain extender when its associated structure is intercalated by the polymer chain. The rheological data in ColeCole representation, ηvη0 (ω), is shown as an increase of the real viscosity in the low-ω region. Fig. 9.15 shows data in ColeCole representation of PLA as an example. In other word, the increase in the chain dimension and transition from liquid (Newtonian polymer) to gel-like structure has caused some restricted segmental motions at the interface between filler and polymer chain [92]. From other effective factors in rheological behavior of polymer are molecular weight and molecular weight distribution that are modified upon addition of filler [93].

Figure 9.15 ColeCole representation ηv vs. η0 (ω) for (a) PLA and (b) PLA-Zn2Al/LS. An enlarged domain is displayed in inset to visualize η0 0. Adapted from Ref. [92], with kind permission of Elsevier.

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9.5.4 Swelling properties The swelling properties of LDH are an important characterization in NCs, depending on the application for which they are envisaged. These properties for preparation of materials with good water resistance to maintain their physical strength and as well as for superabsorbent materials for drug delivery and waste water treatment are significantly required [94]. The pH-sensitive swelling behavior of the carboxymethyl cellulose/LDH-cephalexin NC beads in the buffer solutions revealed which swelling behavior of all the hydrogels enhanced with the increase of the pH from 1.2, 6.8, and 7.4 [1]. Herrero et al. [7] studied the effect of temperature on the water uptake properties of LDH/sulfonated polysulfone (SPSU) membranes prepared by the casting method using dimethylacetamide as solvent. In Fig. 9.16, the amount of water absorbed of the different SPSU/LDH composite membranes is lower than SPSU membrane. However, the water uptake of composites, due to the formation of hydrogen bonding between sulfonic acid group and the hydrotalcite type compound, increases with the amount of LDH. In another work [87], swelling properties of polyampholyte NC hydrogels (NH1-LDHs) based on acrylic acid, 2-(diethylamino) ethyl methacrylate, N,N-methylenebisacrylamide, and LDH (MgAlCl LDH) synthesized using an in-situ free radical addition polymerization method in aqueous media, were investigated as a function of time at room temperature. According to Fig. 9.17 in all selected NC,

Figure 9.16 Water uptake measurements at different temperatures (25, 60, and 80 C) for SPSU and SPSU composites. Adapted from Ref. [7], with kind permission of Elsevier.

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Figure 9.17 Swelling ratio as a function of time for NH1-LDHs in deionized water at room temperature. Adapted from Ref. [87], with kind permission of Elsevier.

at first, the rate of water absorption sharply increases and then mostly becomes steady. The time required to achieve the equilibrium water absorbency for H1, NH1-1LDH, NH1-2LDH, and NH1-7LDH is about 48, 24, 24, and 7.5 h, respectively. Sulfonated poly(ether ether ketone) was prepared in present of sulfuric acid (95%) in under nitrogen atmosphere at 55 C for 4 and 6 h to prepare SP4 and SP6 [95]. The water uptake of SP4 and its composites (containing MgAl or NiTi) at different temperature showed that the water uptake values increase with increasing LDH content up to 3 wt%. The highest water uptake of 29.3 wt% at 80 C has been found for MgAl-based SP4, which is maybe owning to the better LDH dispersion at low LDH content in SP4. In contrast to the previous results, in SP6-based membranes, addition of LDH has led to reduced water uptake, because of the high degree of sulfonation in SP6, and the decrease in water uptake further escalates with increasing MA contents.

9.5.5 Other properties Incorporation of LDH (either pristine or organically modified) into the polymer matrix results in improvement of other properties such as electrical properties, barrier properties, flame retardant, etc. Herrero et al. [7] prepared SPSU membrane and indicated that the membrane electrical resistance has a clear dependence to sulfonation degree and the amount of the LDH added. In other word, the proton conductivity improves with the amount of LDH powder, so the lowest electrical

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resistivity the SPSU was observed with 5 wt% of LDH and can be used as protonexchange membranes. Water or gas barrier property is one of the most important properties in LDH-based NC and has noticeable role in deteriorative reactions and microbial growth in food packaging. It is believed that the addition of layered inorganic particles in the polymeric matrix would improve the long tortuous path for gas and water vapor diffusion due to the waterproof clay layers distributed in the polymer matrix consequently decelerating the process of penetration (Fig. 9.18). Demirkaya et al.’s study support this conclusion [96]. Also in other work, Li et al. [88] observed that the PPC composites films containing OLDH were more effective in reducing both oxygen permeability coefficient (OP) and water vapor permeability coefficient (WVP) than that of pure PPC. The OP and WVP values of composite film are reduced about 54% and 20%, respectively, just upon 2% OLDH incorporation. This phenomenon is mainly attributed to the high aspect ratio of OLDH platelets within PPC matrix, which present a long diffusion path of the oxygen and water vapor molecules. One of the significant impacts of nanotechnology on polymeric material’s flammability is the enhancement of the flame retardancy of polymer NC. Hydrated material such as LDH as a heat sink material has the potential to deliver a flame retardant action to the hosting polymer by absorbing heat, releasing water under firing condition, and forming a protective oxide layer which can retard the combustion process via its endothermic decomposition during polymer degradation [4,97]. differential scanning calorimetry (DSC) analysis demonstrated that LDH provoked an endothermic decomposition at 354 C with absorption of 478.6 J g21 (Fig. 9.19). So, LDH as nanofiller in polymer has the potential to cool down the burning polymer surface, with the formation of protective oxide layer.

Figure 9.18 Model for tortuous path of gas through PPC/OLDH composite. Adapted from Ref. [88], with kind permission of Springer.

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Figure 9.19 The heat sink action of LDH by DSC. Adapted from Ref. [4], with kind permission of Elsevier.

9.6

Conclusions

PolymerLDH NCs, although known for many years, have attracted recent attention due to the report of the research groups on the improved properties of polymer NCs. This chapter has provided the fundamental insights into the strategies for development of LDH-based compounds and introduced the most properties (period 201016) of these NCs that offer great potential in multidiscipline applications. As expected, given their low cost, ease of preparation, excellent thermal stability, high surface area, good adsorption, and anion-exchange abilities; LDHs have shown primary potential as filler in preparation of polymer NC. In-situ methods, solution intercalation, and melt compounding method have been introduced as feasible methods of preparation for various polymer/LDH NC. In most cases, LDHs first need to be modified with organic surfactants, in order to become miscible with polymeric matrices. It has been clearly demonstrated that different factors, such as type of polymer, LDH dispersion, polymer/LDH affinity, the preparation technique, and LDH content, can affect the structure and the properties of composites. On the basis of the current literature on polymer/MMT NCs, interesting characterizations can be introduced for these materials, which will broaden the scope of applications. From the main advantages of LDHs are the improvements in mechanical, thermal, and barrier properties, as well as flame retardant of polymer composites.

Acknowledgments Authors acknowledge the support of the Research Affairs Division of Isfahan University of Technology (IUT) and National Elite Foundation (NEF).

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[66] Liu J, Chen G, Yang J. Preparation and characterization of poly (vinyl chloride)/layered double hydroxide nanocomposites with enhanced thermal stability. Polymer (Guildf) 2008;49:39237. [67] Wang L, Su S, Chen D, Wilkie CA. Fire retardancy of bis [2-(methacryloyloxy) ethyl] phosphate modified poly (methyl methacrylate) nanocomposites containing layered double hydroxide and montmorillonite. Polym Degrad Stab 2009;94:111018. [68] Zhao C-X, Liu Y, Wang D-Y, Wang D-L, Wang Y-Z. Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly (vinyl alcohol). Polym Degrad Stab 2008;93:132331. [69] Nichols KL, Chou C-J. Polymer composite comprising a inorganic layered material and a polymer matrix and a method for its preparation. Washington, DC: U.S. Patent and Trademark Office, Patent No, 5,952,093, 1999. [70] Leng J, Purohit PJ, Kang N, Wang D-Y, Falkenhagen J, Emmerling F, et al. Structureproperty relationships of nanocomposites based on polylactide and MgAl layered double hydroxides. Eur Polym J 2015;68:33854. [71] Kutlu B, Leuteritz A, Ha¨ußler L, Oertel U, Heinrich G. Stabilization of polypropylene using dye modified layered double hydroxides. Polym Degrad Stab 2014;102:914. [72] Wang D-Y, Das A, Leuteritz A, Boldt R, Ha¨ußler L, Wagenknecht U, et al. Thermal degradation behaviors of a novel nanocomposite based on polypropylene and CoAl layered double hydroxide. Polym Degrad Stab 2011;96:28590. [73] Zhou Q, Verney V, Commereuc S, Chin I-J, Leroux F. Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly (butylene succinate). J Colloid Interface Sci 2010;349:12733. [74] Purohit PJ, Wang D-Y, Wurm A, Schick C, Scho¨nhals A. Comparison of thermal and dielectric spectroscopy for nanocomposites based on polypropylene and layered double hydroxideproof of interfaces. Eur Polym J 2014;55:4856. [75] Chiang M-F, Wu T-M. Preparation and characterization of melt processed poly (L-lactide)/layered double hydroxide nanocomposites. Composites, B: Eng 2012;43: 278994. [76] Zubitur M, Gomez M, Cortazar M. Structural characterization and thermal decomposition of layered double hydroxide/poly (p-dioxanone) nanocomposites. Polym Degrad Stab 2009;94:8049. [77] Mallakpour S, Dinari M. Hybrids of MgAl-layered double hydroxide and multiwalled carbon nanotube as a reinforcing filler in the L-phenylalanine-based polymer nanocomposites. J Therm Anal Calorim 2015;119:190512. [78] Mallakpour S, Dinari M. Using Mg-al-layered double hydroxide intercalated with chiral dicarboxylic acid for the reinforcement of isoleucine amino acid containing poly (amide-imide). Polym Compos 2016;37:328895. [79] Zhang X, Zhou L, Pi H, Guo S, Fu J. Performance of layered double hydroxides intercalated by a UV stabilizer in accelerated weathering and thermal stabilization of PVC. Polym Degrad Stab 2014;102:20411. [80] Labuschagne FJ, Molefe DM, Focke WW, Van der Westhuizen I, Wright HC, Royeppen MD. Heat stabilising flexible PVC with layered double hydroxide derivatives. Polym Degrad Stab 2015;113:4654. [81] Liu J, Chen G, Yang J, Ding L. Improved thermal stability of poly (vinyl chloride) by nanoscale layered double hydroxide particles grafted with toluene-2, 4-di-isocyanate. Mater Chem Phys 2009;118:4059.

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Green hybrid nanocomposites from metal oxides, poly(vinyl alcohol) and poly(vinyl pyrrolidone): structure and chemistry

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Shadpour Mallakpour1,2,3 and Vajiheh Behranvand1 1 Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 2Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan, Islamic Republic of Iran, 3Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan University of Technology, Isfahan, Islamic Republic of Iran

Chapter Outline 10.1 Introduction

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10.1.1 Metal oxides 264 10.1.2 Chemical structure and properties of PVA 264 10.1.3 Chemical structure and properties of PVP 265

10.2 Polymer/metal oxide nanocomposites 265 10.3 Properties and applications of PVA/metal-oxide nanocomposites

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10.3.1 PVA/metal-oxide nanocomposites 266 10.3.2 PVA/nonmetal-oxide nanocomposites 274 10.3.3 PVA blends/metal-oxide nanocomposites 276

10.4 Properties and applications of PVP/metal-oxide nanocomposites

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10.4.1 PVP/metal-oxide nanocomposites 278 10.4.2 PVP/nonmetal-oxide nanocomposites 280 10.4.3 PVP blends/metal-oxide nanocomposites 282

10.5 Conclusions 283 Acknowledgments 284 References 284

Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00010-0 Copyright © 2017 Elsevier Ltd. All rights reserved.

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Hybrid Polymer Composite Materials: Structure and Chemistry

Introduction

10.1.1 Metal oxides Nanomaterials have extensively attracted interests for their small and quantum-size effects. They can exhibit novel and significant mechanical, electronic, magnetic, and optical properties in comparison with their bulk counterparts. Metal-oxide nanoparticles (NPs) belong to a family of nanomaterials that have been manufactured on a large scale are of increasing interest for applications in the fields of physics, chemistry, biology, and medicine [1 3]. They have large surface area, unusual adsorptive properties, surface defects, and fast diffusivities [2]. In this chapter combined with our research experiences, the effect of various metal oxides such as titanium dioxide (TiO2), zinc oxide (ZnO), silicon dioxide (SiO2), Fe3O4, and so on, as well as nonmetal oxides, on the morphology, properties, and applications of poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) green polymers has been investigated. Different interactions are illustrated in this chapter to demonstrate the strong attractive interactions between the metal oxide and nonmetal oxide surface (with or without chemical modifications) and the PVA and PVP matrix in the preparation of nanocomposites. This is followed by studies of the influence of the mentioned materials on the thermal, morphological, electrical, mechanical, and other properties of PVA and PVP matrixes. Finally, in conclusion, the summary of the applications of whole work regarding the use of metal oxide and nonmetal oxide as nanofillers in the PVA and PVP matrix are presented.

10.1.2 Chemical structure and properties of PVA PVA is a hydrophilic synthetic polymer containing pendant hydroxyl groups and its aqueous solution can form transparent films. It is essentially made from partially or fully hydrolysis of poly(vinyl acetate) (Fig. 10.1). PVA has significant tensile strength, more flexibility, hardness and gas, and aroma barrier features [4,5]. It has good adhesive and mechanical properties. However, it is not soluble in cold water and must be heated at temperatures higher than 90 C. It is a nonionic surfactant, used in pharmaceutical manufacturing as a stabilizing agent, viscosity modifier, and lubricant or as an

Figure 10.1 Structure formula for PVA: (A) partially hydrolyzed; (B) fully hydrolyzed. Source: Adopted from Gaaz TS, Sulong AB, Akhtar MN, Kadhum AAH, Mohamad AB, Al-amiery AA. Properties and applications of polyvinyl alcohol, halloysite nanotubes and their nanocomposites. Molecules 2015;20:22833 22847.

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Figure 10.2 Structure formula for PVP.

excipient for tablet formation [6]. PVA is biocompatible polymer and the good mechanical properties of PVA hydrogels make them a good substitute for articular cartilage. In the paper industry, PVA also is used to reinforce mechanical properties. It is used in many areas of science and technology, including membrane separation, adhesives, drug delivery systems, artificial biomedical devices, binding of pigments and fibers, film packaging in food, protective strippable coatings, cement reinforcement, the manufacture of detergents and cleansing agents, adhesives, emulsion paints, and fuel cell membranes [7 10]. Besides, because of its biocompatibility and excellent chemical and physical characteristics, PVA is one of the promising representatives of polymeric material in the preparation of green nanocomposites [8,11].

10.1.3 Chemical structure and properties of PVP PVP (Povidone) is one of the most commonly used biomaterials. It is a class of organic compounds that has a well-defined structure with a N-vinylpyrrolidone monomer connected as a long chain [12,13]. In fact, it has long and soft polyvinyl backbone and an amide group in each monomer [14]. The polyvinyl backbone serves as a tail group (hydrophobic) whereas the pyrrolidone group serves as a head group (hydrophilic) (Fig. 10.2). The pyridyl group can bind strongly to metals or semiconductor NPs. So, it can act as a reducing agent as well as a surface capping agent for the synthesis of NPs [12,15]. It has been used as stabilizer for the synthesis of cadmium oxide NPs [16], ZnO NPs [17], MoS2 [18], copper nanoplates [19], Fe3O4 NPs [20] and so on. Because PVP is reactive, inexpensive, nontoxic, water-soluble, and biocompatible polymer, it provides numerous potential biomedical and packaging applications [21]. The addition of PVP to membrane casting solutions results in the increase of membrane porosity as well as increase in the number of pores [22].

10.2

Polymer/metal oxide nanocomposites

Design and synthesis of polymer/metal-oxide nanocomposites have attracted great interest over the last two decades because they could exhibit better thermal, optical, mechanical, and electronic properties compared with corresponding neat metal oxide or polymer [23]. On one hand, inorganic NPs usually suffer from high-production expense, and the shaping and further processing of these materials is often difficult

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and demanding or impossible. On the other hand, polymers are flexible lightweight materials and can be manufactured at a low cost. They have easy processability and can be shaped into thin films. Hence, the disadvantages of using inorganic nanostructured materials can be overcome by using a polymer matrix to embed a relatively small content of inorganic NPs [24]. Metal oxide NPs can be employed to enhance the stiffness, toughness, and possibly the life of polymeric materials [25].

10.3

Properties and applications of PVA/metal-oxide nanocomposites

10.3.1 PVA/metal-oxide nanocomposites PVA-based metal-oxide nanocomposites have held outstanding interest by the scientific researchers due to their ability to combine the properties of both PVA and dopants. They have many applications because of their electron transport, mechanical, and optical properties in medical and engineering technology [26]. Cryogels composed of PVA were prepared by repeated freeze thaw method followed by in situ precipitation of ZnO NPs within the cryogel networks [27]. Cryogels made from natural and synthetic polymers have emerged as favorable materials to produce new macro porous architectures which may find novel applications in biomedical and biotechnological fields. FT-IR spectra showed broadening and shifting of O H stretching from 3600 (ZnO unloaded) to 3550 cm21 (ZnO loaded) which these observation confirmed the interaction of hydroxyls of native PVA and ZnO NPs. The scanning electron microscopy (SEM) studies revealed that the PVA cryogel ZnO nanocomposite has porous morphology with pore size varying from 20 to 50 μm. The prepared nanocomposites were also examined for swelling and deswelling behaviors. The results revealed that both of them depend on the chemical composition of the nanocomposites, number of freeze-thaw cycles, pH, and temperature of the swelling medium. The nanocomposites exhibited excellent antibacterial activity against Gram-positive and Gram-negative bacteria which is suitable in medical application. Abbaspour-fard et al. [28] added ZnO NPs to PVA biopolymer to compare the microstructural, mechanical, antibacterial, and physical properties of bionanocomposite films reinforced with various loading contents (1, 3, and 5 wt%). Water vapor penetration (WVP) properties as one of the most important parameters for biodegradable films were studied for the obtained films. By adding the NPs and increasing the film thickness, the WVP of the film decreased. Moreover, PVA/ZnO nanocomposites showed a good antibacterial property, thus these films can be used as antibacterial food packaging materials. PVA/ ZnO quantum dots (QDs) nanocomposites were prepared by Tang et al. [29]. The photoluminescence spectra of the PVA/ZnO QDs nancomposites showed a blue shift, as compared with the ZnO QDs alone, and the thermal stability of the PVA was sharply improved after incorporation of the ZnO QDs. Crasta et al. [30] prepared polymer composites of ZnO and tungsten oxide (WO3) NPs doped PVA

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matrix under ultrasonic irradiations. The FT-IR study showed that the Zn and W ions of NPs interacted with the OH groups of PVA and forms a complex via intra/ intermolecular hydrogen bonding as was shown in Fig. 10.3. It was confirmed due to shifts in the corresponding bands with a change in intensities. Uniform distribution of nanosized ZnO and WO3 dopants in PVA caused increases in the tensile strength and Young’s modulus of the obtained films. The effect of ZnO cerium oxide (Ce2O3) NPs on the structural characteristics and optical properties of PVA matrix was investigated by Siddaramaiah et al. [31]. Ce2O3 is well known for its optical properties, and it can filter ultraviolet (UV) rays. They expressed that Ce2O3 ZnO nanocomposite in a solution phase synthetic system can provide an efficient way to improve the field emission performance of ZnO nanostructures, which could be extended to other potential applications, such as optoelectronic devices, chemical sensors, and photocatalysts. PVA is a very strong hydrophilic polymer, in which the reinforcement of Ce2O3 ZnO NPs has decreased the hydrophilicity of PVA. Thermogravimetric analysis (TGA) showed improved thermal stability for the composites as compared with the PVA matrix. The band gap energy measured based on UV vis analysis represented enhanced conductivity of PVA films. Their investigations will support the PVA nanocomposites for possible applications in optoelectronic devices. PVA films doped with different concentrations namely, 0.5, 1.0, and 2.0 wt% of the carbon-coated ZnO (C ZnO) NPs were

Figure 10.3 The interaction between ZnO and WO3 ions and PVA. Source: Adopted from Kumar NBR, Crasta V, Bhajantri RF, and Praveen BM. Microstructural and mechanical studies of PVA doped with ZnO and WO3 composites films. J Polym 2014;2014:1 7.

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Figure 10.4 TEM of PVA/TiO2 (20 wt%) nanocomposite films (A) low resolution and (B) high resolution. Source: Adopted from Liu X, Chen Q, Lv L, Feng X, Meng X. Preparation of transparent PVA/TiO2 nanocomposite films with enhanced visible-light photocatalytic activity. Catal Commun 2015;58:30 33, with kind permission of John Wiley and Sons.

prepared by solution casting method in order to investigate the effect of C ZnO addition on the electrical, optical, structural, and morphological properties of PVA [32]. Results showed as C ZnO content increases, the dielectric permittivity, dielectric loss, and conductivity of PVA host decreases. Meng et al. [33] prepared highly transparent PVA/TiO2 nanocomposite films via simple hydrothermal method. Fig. 10.4 shows the transmission electron microscopy (TEM) images of a PVA/TiO2 nanocomposite film with TiO2 content of 20 wt%. As can be observed, TiO2 NPs are uniformly dispersed in PVA matrix (Fig. 10.4A) with the average size of about 10 nm. The lattice fringes with an interplanar distance of 0.36 nm are well consistent with the (1 0 1) lattice plane of anatase TiO2 (Fig. 10.4B). The small TiO2 size may be due to the confined space of PVA macromolecular chains, which limit the growth of TiO2 NPs during the hydrothermal process. Visible-light photocatalytic activity of the films using methyl orange (MO) as model pollutant was investigated too. Fig. 10.5 shows the photocatalytic activity of all the as-prepared PVA/TiO2 nanocomposite films. As expected, neat PVA film didn’t show photocatalytic activity for MO degradation. While PVA/TiO2 nanocomposite films exhibited much higher photocatalytic activity than that of PVA/ P25 film that had been prepared with doping of PVA with commercial TiO2 (20 wt %). After 15 min irradiation, MO is almost photodegraded by PVA/TiO2 nanocomposite films. The small grain size with anatase phase of PVA/TiO2 nanocomposite films caused improvement in photocatalytic activity. Fig. 10.5B shows the photodegradation curves of phenol under visible light irradiation. As it is clear, phenol solution has a slower photodegradation rate than that of MO photodegradation. Besides, PVA/TiO2 nanocomposite films also exhibited stable photocatalytic activity during recycle degradation.

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Figure 10.5 Photocatalytic activities of PVA/TiO2 nanocomposite films on the degradation of MO (A) and phenol (B) under visible light irradiation (N 420 nm). Insert in (A): cycle runs of PVA/TiO2 (20 wt%) nanocomposite film for degradation of MO under visible-light irradiation. Source: Adopted from Liu X, Chen Q, Lv L, Feng X, Meng X. Preparation of transparent PVA/TiO2 nanocomposite films with enhanced visible-light photocatalytic activity. Catal Commun 2015;58:30 33, with kind permission of John Wiley and Sons.

PVA nanofibers doped with different amount of graphene oxide (GO) were fabricated via electrospinning method by Chen et al. [34]. Followed with interface sol gel reaction, the PVA/GO/TiO2 core shell fibers were obtained. Fig. 10.6A and B is the TEM images of TiO2/PVA fibers. They found that TiO2 NPs aggregate together and cannot uniformly disperse across the surface of PVA nanofibers, which is negative to attain uniform core shell structure. Though, when GO was doped in PVA fibers, TiO2 shell with thickness of tens of nanometers could uniformly grow on the surface of fibers, as shown in Fig. 10.6C and D. These results showed that GO could be used as hard template to assist the growth of TiO2, which is a key point to prepare composite fibers with core shell structure. GO-doping increased the degree of crystallinity and tensile strength of the PVA nanofibers and decreased the glass transition temperature, decomposition temperature, and elongation at break of them. The thermal properties of GO/PVA nanofibers were probably determined by two factors. On one hand, CO, CO2, and H2O released when GO undergoes reduction around 200 C that may accelerate the PVA decomposition. On the other hand, the synthesized nanofibers have the diameters of only 100 600 nm, while the GO curled inside the PVA matrix have a typical size of 1 10 μm. That means GO could act as air bubbles in the matrix and minor outgassing from inside might finally cause a rapid breakdown of the PVA polymer matrix. They proposed a lot of applications for these composites in the fields of photocatalytic, dye-sensitized solar cell, and electrochemistry. A PVA/TiO2 nanofiber adsorbent modified with mercapto groups was synthesized by electrospinning [35]. It was employed for uranium(VI) and thorium(IV) removal from aqueous solution. The results showed that the sorption capacities of both metal ions for the modified PVA/TiO2 nanofibers were remarkably greater than those of the unmodified nanofibers. The maximum sorption capacities of

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Figure 10.6 TEM images of composite nanofibers: TiO2/PVA (A and B) and TiO2/GO/PVA (C and D). Source: Adopted from Wang B, Chen Z, Zhang J, Cao J, Wang S, Tian Q, et al. Fabrication of PVA/graphene oxide/TiO2 composite nanofibers through electrospinning and interface sol gel reaction: effect of graphene oxide on PVA nanofibers and growth of TiO2. Colloids Surf, A: Physicochem Eng Aspects 2014;457:318 325, with kind permission of Elsevier.

U(VI) and Th(IV) by Langmuir Isotherm were estimated to be 196.1 and 238.1 mg g21 at 45 C with pH of 4.5 and 5.0, respectively. Fu et al. [36] introduced Fe3O4 decorated Ag-nanowires into the PVA matrix to prepare Fe3O4 coated Ag-nanowire/PVA nanocomposite films with intriguing electrical properties. Corresponding to the magnetic properties of Fe3O4 coated Ag-nanowires, it is supposed that the Fe3O4-coated Ag-nanowires in the PVA solution can be aligned along the magnetic field produced by the two magnets (Fig. 10.7A). Fig. 10.7B shows a typical digital photograph of a flexible oriented Fe3O4 coated Ag-nanowires/PVA composite film. The random Fe3O4 coated Ag-nanowires/PVA composite films were fabricated under no applied magnetic field for the purpose of comparison.

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Figure 10.7 (A) Schematic illustration of the magnetic field-assisted synthesis of oriented Fe3O4 coated Ag-nanowire/PVA composite films and (B) a digital photograph of an as prepared flexible free-standing PVA composite film. Source: Adopted from Li N, Huang GW, Xiao HM, Fu SY. Preparation of aligned Fe3O4@Ag-nanowire/poly(vinyl alcohol) nanocomposite films via a low magnetic field. Composites Part A 2015;77:87 95, with kind permission of Elsevier.

The composite film with the 20 wt% Ag-nanowire content presented conductivity up to B4500 S cm21 in the parallel direction, which is much higher than that of the polymers based on Ag-nanowire and even on conductive matrix such as graphene with higher Ag-nanowire contents. The high-electrical conductivity and the high-anisotropic conductivity were obtained for the nanocomposites which were mainly ascribed to the successful coating of Fe3O4 onto the surface of high aspect ratio Ag-nanowires and the preferential alignment of Fe3O4 coated Ag-nanowires along the applied magnetic field. PVA/α-MnO2 nanocomposites containing 1, 3, and 5 wt% of modified α-MnO2 nanorods with valine amino acid were prepared through an ultrasound-assisted technique by Mallakpour et al. [37]. The MnO2 has wide applications in catalysts, removal of heavy metal ions, and energy storage systems. TEM images of PVA/α-MnO2 L-valine NC 3 wt% demonstrated that the nanorods are well dispersed in PVA matrix due to the presence of many hydrogen

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bonding interactions between nanofiller and polymer (Fig. 10.8). Thermal stability and mechanical resistance of the NCs were enhanced due to high dispersion of nanofiller in the PVA matrix. NC 3 wt% as an adsorbent exhibited good adsorption for removal of Pb(II) and Cd(II) in aqueous solution. PVA has hydroxyl groups that can act as chelating sites. While PVA/α-MnO2 L-valine NC 3 wt%, in addition to the hydroxyl groups related to PVA, is containing hydroxyl due to the MnO2 (Mn OH) and amine group (L-valine) due to the existence of modifier which are involved the adsorption process that can serve as chelating sites. Mallakpour et al. fabricated a series PVA nanocomposites embedded with different NPs such as modified TiO2, SiO2, Al2O3, and CuO NPs with biocoupling agents via solution casting along with ultrasonic method [38 41]. Uniform dispersion of modified NPs was obtained in the PVA matrix. Mechanical properties such as tensile strength and E-modulus dramatically

Figure 10.8 TEM images of PVA/α-MnO2 L-valine NC 3 wt% with different magnifications. Source: Adopted from Mallakpour S, Motirasoul F. Covalent surface modification of α-MnO2 nanorods with l-valine amino acid by solvothermal strategy, preparation of PVA/a-MnO2-L-valine nanocomposite films and study of their morphology, thermal, mechanical, Pb(II) and Cd(II) adsorption properties. RSC Adv 2016;6:62602 62611.

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Figure 10.9 Expected chemical reactions during the formation of PVA/SiO2/GA membranes. Source: Adopted from Dodda JM, Bˇelsky´ P, Chmelaˇr J, Remiˇs T, Smolna´ K, Toma´sˇ M, et al. Comparative study of PVA/SiO2 and PVA/SiO2/glutaraldehyde (GA) nanocomposite membranes prepared by single-step solution casting method. J Mater Sci 2015;50:6477 6490, with kind permission of Springer.

increased. The obtained composites were more stable and resistance to thermal degradation compared to the pure PVA. Besides, due to increased UV absorption of PVA NCs, they may be suitable for food and medical packaging. Dodda et al. [42] designed a single-step solution casting method for the preparation of PVA/SiO2 and PVA/SiO2/glutaraldehyde (GA) nanocomposite membranes through direct incorporating of silica and GA directly into the PVA matrix which could be economically viable. Membranes with and without GA exhibited significantly different characteristic features. GA preferably reacted with tetraethyl orthosilicate in the case of PVA/SiO2/GA membranes, forming submicron particles ( . 25 nm) different from PVA/SiO2 membranes (B1 nm), which reduced the thermal stability of these membranes (Fig. 10.9). The water uptake of the PVA/SiO2 membranes increased with temperature, but the PVA/SiO2/GA membranes were completely dissolved above 50 C. They concluded that the addition of GA declined the properties of PVA/SiO2 membranes. This due to the fact that the molecular structure of the PVA/SiO2/GA membranes is not efficiently reinforced and that their thermal and mechanical stability is relatively poor.

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10.3.2 PVA/nonmetal-oxide nanocomposites Different kinds of nonmetal oxide fillers including layered double hydroxide (LDH) [43], exfoliated graphene [44,45], carbon nanotube (CNT) [46], carbon nanofiber, metal NPs [47], and others have been employed to develop high-performance composite materials for different commercial, technological, and household applications [45]. Koul et al. [47] synthesized a novel, elastic, nonadhesive, and antimicrobial hydrogel PVA scaffold (loaded with AgNPs) using freeze-thaw method. Obtained products based on silver have proven to be promising candidates for antimicrobial activity against antibiotic-resistant bacteria. Silver is biologically active in the soluble form (Ag1 or Ag0 clusters). A microporous, filamentous network for polymer micrograph of normal and AgNPs loaded PVA hydrogels was observed in SEM micrographs (Fig. 10.10). Surface morphology of both the hydrogels was observed like a porous web-like structure.

Figure 10.10 (A) PVA Hydrogel, (B) AgNPs loaded PVA hydrogel; SEM image at 5000 magnification of (C) PVA Hydrogel, (D) AgNPs loaded PVA hydrogel. Source: Adopted from Bhowmick S, Koul V. Assessment of PVA/silver nanocomposite hydrogel patch as antimicrobial dressing scaffold: synthesis, characterization and biological evaluation. Mater Sci Eng, C 2016;59:109 119, with kind permission of Elsevier.

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PVA hydrogel could release silver NPs in a controlled manner for at least 96 h that maintain a sterile environment. In fact, increased mechanical property, swelling property, easy peal ability, and high-water content (to absorb the exudates) of the obtained hydrogel caused its application as hydrogel-based wound dressing material. Tabassum et al. [48] designed mesoporous PVA/hydroxyapatite composites for better biomedical coating adhesion. Their results showed that amount of PVA in the prepared composites had an influence on surface properties such as surface area, pore size, and pore volume. To facilitate the intercalation of LDH within PVA matrix and to improve dispersion, Mg Al CO22 LDH was modified with 3 organic diacid by Mallakpour et al. [43]. The mechanical and thermal properties of nanocomposites were enhanced due to many hydrogen bonding and well dispersion of the modified LDH in the PVA matrix. George et al. [46] fabricated novel PVA nanocomposite membranes reinforced with acid functionalized multiwalled CNTs (a-MWCNT) (0.5, 1, 1.5, and 2 wt%). Their outcomes were as follows: the contact angle analysis showed increase in hydrophilicity of composites. When the concentration of MWCNT increased to 1.5 wt%, pure polymer undergoes transition from electrically insulated to semiconductive. At lower filler loading, especially PVA with 0.5 wt% a-MWCNT loaded membranes exhibited favorable performance for separation of water and ethanol. Polymer composites, based on graphitic nanostructures, have attracted increased attention due to their unique barrier, mechanical, electric, and optical properties [49,50]. When graphene/reduced GO (r-GO) is well distributed in PVA, it brings outstanding enhancements in various properties so surface modification of them is an essential step for obtaining a molecular level dispersion in a polymer matrix [51,52]. Kurihara et al. [44] reported the preparation of GO/PVA composite structures that may be candidates for usage as effective heat dissipation materials. In this work, the thermal conductivity increased as the GO content was increased. Lee et al. [45] prepared two types of PVA composites containing r-GO and a novel aryl diazonium salt functionalized graphene (ADS-G) as nanofiller by a simple solution casting. Fig. 10.11 shows the schematic for the preparation of PVA/ADS-G composites.

Figure 10.11 Schematic for the preparation of PVA/ADS-G composites. Source: Adopted from Yue DS, Kuila T, Kim NH, Lee JH. Enhanced properties of aryl diazonium salt-functionalized graphene/poly(vinyl alcohol) composites. Chem Eng J 2014;245:311 322, with kind permission of Elsevier.

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As can be seen, the ADS-G contains several polar functional groups such as OH, 2SO2 N 5 N . On the other hand, PVA is consisting plenty of OH groups 3 , and in its chain. Hence, functionalized graphene sheets could interact well with the PVA matrixes, resulted improvements in mechanical, thermal, and electrical properties of the composites at very low filler loadings.

10.3.3 PVA blends/metal-oxide nanocomposites Even though PVA is biodegradable, with good mechanical properties in dry state, but it is highly hydrophilic polymer which limits its scope of application mainly in humid environment. In addition, PVA does not have fixed charges. Thus, several organic groups like hydroxyl, amine, carboxylate, sulfonate, and quaternary ammonium can be introduced to impart hydrophilicity and/or ionic properties. Several approaches are recognized for cross linking/blending of compounds to attain 3D networks in PVA for different applications [53,54]. PVA/chitosan polymer has received increasing attention corresponding to its high hydrophilicity, good mechanical stability, and good biocompatibility. This blend has been applied as adsorbent for heavy metal ions from aqueous solution. On one hand, chitosan has 2 NH2 and 2 OH groups which can act as active sites for chelating metal ions and in another hand, PVA contains large number of OH [55]. Wang et al. [55] prepared PVA/chitosan magnetic composite for the aim of Co21 removal. The schematic structure of PVA/chitosan magnetic composite was shown in Fig. 10.12.

Figure 10.12 Schematic structure of PVA/chitosan magnetic composite. Source: Adopted from Zhu Y, Hu J, Wang J. Progress in nuclear energy removal of Co21 from radioactive wastewater by polyvinyl alcohol (PVA)/chitosan magnetic composite. Prog Nucl Energy 2014;71:172 178, with kind permission of Elsevier.

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The PVA/chitosan magnetic composite beads fabricated in this investigation showed a relatively high-adsorption capacity compared to other adsorbents such as ammonium molybdophosphate polyacrylonitrile and phosphate-modified montmorillonite. The Co21 contains empty orbital and as a Lewis acid can accept electron pairs from 2 NH2 and 2 OH Lewis bases which present in PVA and chitosan. In another research work, Abdelgawad et al. [56] prepared nanofiber mats from colloidal dispersions of chitosan-based AgNPs blended with PVA for antimicrobial applications by electrospinning method. The antibacterial experiment indicated that the electrospun mats of PVA/chitosan AgNPs blends exhibited better antibactericidal activity than PVA/chitosan blends. The presence of AgNPs in PVA/chitosan-blend solutions improved not only the electrospinning performance but also caused the antibacterial ability which supports a good wound dressing material. Maghemite (γ-Fe2O3) NPs were incorporated into the PVA and alginate matrix by Rahman et al. [57] to produce beads that could remove 96% of Hg(II) during the photocatalytic activity in aqueous solution. They expressed that γ-Fe2O3 as a photocatalyst has an excellent efficiency in changing dangerous metal ion to harmless metals and when inserted in PVA beads provide clean and easy separation after the photocatalytic process. Lvʼs research group [58] synthesized Fe0 Fe3O4 nanocomposites embedded PVA/sodium alginate (SA) (IPS) beads which exhibited an excellent physical properties and catalytic reactivity. The optimal proportion for beads molding was 1.5 wt% SA with 5.0 wt% PVA, and the followed acidification and reduction treatments were critical to ensure high mechanical strength and high Cr (VI) removal ability of beads. The image in the middle of Fig. 10.13 provides the

Figure 10.13 Changes during immobilization process: macroscopic photos for PS and IPS beads (middle); SEM images for PS (left) and IPS (right) beads in low magnification (top), and in high magnification (bottom). Source: Adopted from Lv X, Jiang G, Xue X, Wu D, Sheng T, Sun C, et al. Fe0 Fe3O4 nanocomposites embedded polyvinyl alcohol/sodium alginate beads for chromium (VI) removal. J Hazard Mater 2013;262:748 758, with kind permission of Elsevier.

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macroscopic picture of PS and IPS beads. As can be observed immobilization of Fe0 Fe3O4 made the IPS beads totally black. Uniform wrinkle and crumple spread over the surface of PS bead were observed in SEM images in Fig. 10.13A whereas after adding of Fe0 Fe3O4 nanocomposites, the surface became more folded and undulated, as shown in Fig. 10.13B. These differences are more obvious under higher magnification in Fig. 10.13C and D. Acidification and reduction processes made the iron beads much more elastic and compact with higher mechanical strength, as well as increased their Cr(VI) removal efficiency from 65.56% to 97.7%. Overall, Fe0 Fe3O4 nanocomposites immobilization within PVA/SA could mitigate fears about potential risk of nanomaterial while provided high-reaction efficiency. A series of superabsorbentsilver (as nonmetal oxide filler) nanocomposites based on SA and PVA was synthesized by Ghasemzadeh et al. [59]. The hydrogel nanocomposite samples were dispersed in distilled water and ethanol with an ultrasonic homogenizer. The presence of nearly spherical and well-separated AgNPs with diameters in the range of 4 10 nm was observed in TEM micrographs. The obtained hydrogel presented enhancement in the thermal stability with the existence of AgNPs. Also, it showed very good antibacterial activity on Gram-positive and Gram-negative microorganisms. The synthesized hydrogel silver nanocomposites could be appropriate in many fields such as in wound dressing, biological systems, catalysis, and water purification.

10.4

Properties and applications of PVP/metal-oxide nanocomposites

10.4.1 PVP/metal-oxide nanocomposites CeO2 are among the 10 most produced engineered NPs in the world which are photo-sensible and applied as catalysis and catalyst support because they are transparent to visible light but absorb UV radiation [60]. In 2016, Wang et al. [61] prepared the CeO2 NPs/PVP nanofibers (PC) by electrospinning of PVP solution with CeO2 NPs working as the sensitive layer as was shown in Fig. 10.14. Finally, CeO2/PVP nanofibers were coated onto the surface of surface acoustic wave (SAW) devices by electrospinning method. The SEM images of the samples with different ratios of CeO2 NPs and PVP were shown in Fig. 10.15. It was seen that PC0.1 and PC0.5 exhibited uniform nanofibers with different concentrations of CeO2 NPs encapsulated in the PVP. While in PC2 the diameters of fibers were up to micrometer range as shown in Fig. 10.15D. Because the interaction energy of hydrophilic PVP and water molecule is higher than other polymers such as poly(vinylidene fluoride) and poly(methyl methacrylate) (PMMA), so the SAW humidity sensor with hydrophilic PVP nanofibers has higher capability to absorb water molecules. PVP as a biocompatible polymer

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Figure 10.14 Schematics of the fabrication processes of PC nanofibers. CeO2 NPs were synthesized by hydrothermal method. Source: Adopted from Liu Y, Huang H, Wang L, Cai D, Liu B, Wang D, et al. Electrospun CeO2 nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor. Sens Actuators, B: Chem 2016;223:730 737, with kind permission of Elsevier.

Figure 10.15 SEM images of (A) pure PVP, (B) PC0.1, (C) PC0.5, and (D) PC2. Source: Adopted from Liu Y, Huang H, Wang L, Cai D, Liu B, Wang D, et al. Electrospun CeO2 nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor. Sens Actuators, B: Chem 2016;223:730 737, with kind permission of Elsevier.

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matrix, biosafe citric acid and vitamin C as modifier agents instead of nonsafe agents as well as ZrO2 NPs as biocompatible nanofillers were selected by Mallakpour et al. [62] for the preparation of NCs. Thermal stability of NC with 7 wt% ZrO2 loading dramatically increased. Also, this group incorporated modified SiO2 NPs with citric acid and vitamin C (3, 5, and 7 wt%) into the PVP matrix and obtained highest thermal stability with 7 wt% loading too [63]. In both cases (incorporation of ZrO2 or SiO2), NCs showed higher UV absorptions than pure PVP and this feature increased with increasing the nanofiller content. PVP/SiO2/3-aminopropyltriethoxysilane (PVP/SiO2/APTES) composite nanofiber was synthesized by electrospinning method, and its application for the adsorption of Cd(II), Pb(II), and Ni(II) ions from aqueous solution was examined by Keshtkar et al. [64]. The results indicated that the adsorption capacity of Cd(II), Pb(II), and Ni(II) metal ions increased with increasing APTES amounts up to 10%. This increase is because of more uniform surface, more regular pore structure, and also increases in amino groups present in the structure. The reduction in adsorption capacity of the three metal ions in APTES amounts higher than 10% may be related to shrink in surface area and pore volume limiting the active sites of PVP/SiO2/APTES nanofiber adsorbent for the adsorption process.

10.4.2 PVP/nonmetal-oxide nanocomposites Tripathi et al. [65] synthesized CdSe/PVP nanocomposite by using PVP as a polymer matrix for memory device application. Other CdSe/polymer nanocomposites such as CdSe/PMMA and CdSe/polystyrene [66,67] have been applied in memory devices, but these hybrids need to organic solvents for synthesis processes which not only use from toxic materials but also are complicated and expensive methods. Thus, they used from PVP as a water soluble, inert, and nontoxic polymer with good charge storage capacity. As it was mentioned PVP consists of both hydrophobic and hydrophilic parts and PVP plays two roles during the synthesis of CdSe/ PVP NC: (1) it acts as a stabilizing agent due to the hydrophobic part and causes steric hindrance which prevents the NP agglomeration, (2) it controls the growth of CdSe NPs by creating a coordination bond between the nitrogen atom of PVP and Cd21 ion, forming a passivating layer around the CdSe NPs. Fig. 10.16 shows the possible interaction of CdSe NPs with PVP. Cadmium sulfide (CdS) NPs filled PVP were prepared by in-situ wet chemical precipitation technique by Abdelghany et al. [68]. They tried to understand the complex interaction between PVP matrix and CdS. They used from density function theory (DFT) as a measure for agreement between theoretical and experimental results of complex interaction between PVP and CdS. Fig. 10.17A D shows four different probabilities of interaction between PVP and CdS. The CdS NPs must be stabilized by chemical interactions with the oxygen atoms of the poly(pyrrolidone) units of PVP, which can be confirmed by the FT-IR spectrum, also displayed that the nitrogen has a weak interaction with the metal surface. DFT results revealed that CdS NPs mainly stabilized by chemical interaction with the oxygen atom of the poly(pyrrolidone) unit of PVP.

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Figure 10.16 Interaction of CdSe NPs with PVP chains. Source: Adopted from Kaur R, Tripathi SK. Study of conductivity switching mechanism of CdSe/PVP nanocomposite for memory device application. Microelectron Eng J 2015;133:59 65, with kind permission of Elsevier.

Figure 10.17 Mechanism of interaction between NPs and PVP monomer (M 5 Cd1). Source: Adopted from Abdelghany AM, Abdelrazek EM, Rashad DS. Impact of in situ preparation of CdS filled PVP nano-composite. Spectrochim Acta, A: Mol Biomol Spectrosc 2014;130:302 308, with kind permission of Elsevier.

A novel biomimetic fluorine substituted synthetic fluor-hydroxyapatite (f-HAp) (f-Hap)/PVP/Ag nanocomposite was synthesized under fluorine based ionic liquid by Sundrarajan et al. [69]. f-HAp is the most important for its biological applications and AgNPs exhibit an excellent antibacterial activity against a broad range of pathogens and it can be incorporated into the f-HAp nanocomposites for long-term load bearing applications. This work showed sustained delivery of fluorine ions and silver from the f-HAp surface to the bacterial surface and presented significant bactericidal activity against different bacterial pathogens. PVP/GO and PVP/r-GO nanocomposites were synthesized by Chen et al. [70], and their structure and properties were characterized. Good dispersion of r-GO sheets in PVP matrix could be obtained if the sheets get coated with the polymer during the reduction step. On the other hand, GO has more oxygenated functionalities to counterbalance the van der

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Waals interactions and avoid its aggregation, which contributes to good exfoliation and dispersion in a PVP matrix. Under low filler content (0.05, 0.1, 0.3, and 0.5 wt %), the enhancement of onset degradation temperatures of PVP nanocomposites was not clearly observed, but the amounts of residual char of the PVP nanocomposites were obviously increased.

10.4.3 PVP blends/metal-oxide nanocomposites Polymer blending is a simple and practical method to generate novel materials. Modified physical and mechanical properties of the samples will obtain by blending of two or more polymers. Moreover, the blending of synthetic polymers may improve the cost-performance ratio of the resulting films [71]. To increase conductivity, mechanical strength as well as to reduce high-water solubility of PVP have been used from various NPs [72]. Ramesan et al. [73] prepared nanocomposites of PVA/PVP/Ag-doped ZnO ternary blends (with 3, 5, 7, 10, and 15 wt% Ag-doped ZnO). When PVA and PVP are mixed, the interaction of PVA and PVP takes place through hydrogen bonding between the hydroxyl group of PVA and the carbonyl group of PVP. Hence, blending of such polymers with polar inorganic salt would direct to the uniform dispersion of fillers in the polymers. Some changes in absorption frequencies could be detected in the FT-IR spectrum of PVA/PVP/metal oxide as compared to the blend spectrum. There was a higher intensity and broadness of the peak around 3200 3600 cm21, and the absorption band of the composite (OH group) was shifted to a higher frequency which was corresponding to the strong hydrogen bond interaction between the blend and metal oxide particles. Also, a new absorption band was seen at 1084 cm21 as a result of the interaction between the blend and the NPs. Better thermal resistance of nanocomposites than a pure blend was found from the TGA study and it increased with an increase in the concentration of NPs. The electrical properties of the composites were increased with an increase in content of NPs up to a certain concentration (5 wt%). PVA and PVP with weight ratios of 50/50 were chosen as the matrix for the preparation of novel nanocomposites films by Mallakpour et al. [74]. For the better dispersion of nanosilica in the polymer matrix, the nanosilica surface was modified with citric acid and vitamin C. UV vis spectroscopy showed more UV absorption for the nanocomposites and also thermal resistant of nanocomposites was improved. Actually, nanosilica can act as an insulator to the volatile products generated during thermal decomposition. Javanbakht et al. [75] used from BaZrO3 NPs as mixed metal oxides, with provision of strong acid sites and good hydrophilic nature for the preparation of PVA/PVP BaZrO3 (PPB) proton exchange membranes (PVA:PVP 76:23). PVA membrane has low proton conductivity and high swelling which limits its application in proton exchange membrane fuel cells. Blending of hydrophilic polymers such as PVP with PVA increases the proton conductivity of PVA based membrane. In addition, the OH groups on the surface of BaZrO3 NPs provide strong hydrogen bonding sites and increase the contents of the bound to free water ratio into the membrane matrix. PPB membranes showed higher water uptake and proton conductivity compared to those of the PVA-based membrane due to the strongly hydrophilic character

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of PVP and BaZrO3 NPs. The obtained membranes demonstrated high-proton conductivity and they established the potentiality of the prepared nanocomposite membranes as electrolytes in fuel cell devices. A mixture (1:1, v/v) of chitosan and PVP was prepared by Dutta et al. [76]. Then, 0.001, 0.010, and 0.100 mg of silver oxide NPs were added to the above mixture for the preparation of wound healing material. Chitosan and PVP are miscible polymers that the carbonyl groups in the pyrrolidone rings of the PVP can interact with amino and hydroxyl groups exist in chitosan via hydrogen bonding formation. Therefore, the blending of these two polymers may lead to the preparation of new biocompatible and homogeneous blend matrix [77]. Their results showed that aside from good swelling capability as one of the important factor for decreasing the risk of wound dehydration it also exhibited good antibacterial activity due to the presence of both chitosan as well as silver oxide. This group also prepared the blends of chitosan, PVP, and TiO2 with the aim of its using as wound-dressing material [78] and the outcome of the in-vivo evaluation showed that the reported nanocomposite film is effective for wound care.

10.5

Conclusions

The present publication has tried to highlight and describe some recent properties and applications of PVA, PVP, and their blends nanocomposites containing metal oxides as well as nonmetal oxides. PVA and PVP were chosen as basic polymers because of eco-friendly, nontoxic, water soluble, degradable nature, and the easiness of making thin films with metal oxide. PVA is an artificial polymer that has been employed in the medical and other fields for the last 30 years. The OH groups in PVA can be a source of hydrogen bonding and therefore contribute the formation of polymer composite. Recent applications of PVA/metal oxides or nonmetal-oxide nanocomposites which have been described in this chapter are listed as follows: G

G

G

G

G

G

G

Food and medical packaging, Transparent membranes, Wound-dressing materials, Water purifications, Optoelectronic devices, Photocatalytic, dye-sensitized solar cell, and electrochemistry, and Antibacterial food packaging.

PVP provides numerous potential various applications as it is reactive, low cost, nontoxic, hydrophilic and biocompatible. Recent applications of PVP/metal oxides nanocomposites which have been presented in this chapter are as follows: G

G

G

G

Water purifications, Memory devices, Fuel cell devices, and Wound-dressing materials.

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It may be concluded that the number of literatures which have selected PVP as polymer matrix for the preparation of polymer nanocomposites is fewer than that for PVA-based nanocomposites. Actually, PVP has more important and key role as a stabilizing agent during the synthesis of NPs; PVP molecules have pyrrolidone functional groups that can easily arrest crystals of NPs, they can help to form ultrasmall particles and stop their aggregation.

Acknowledgments The authors would like to thank from the financial support provided by the Research Affairs Division, Isfahan University of Technology (IUT), Isfahan. The authors are also grateful to the Iran Nanotechnology Initiative Council (INIC), National Elite Foundation (NEF), and Center of Excellency in Sensors and Green Chemistry (IUT).

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[42] Dodda JM, Bˇelsky´ P, Chmelaˇr J, Remiˇs T, Smolna´ K, Toma´sˇ M, et al. Comparative study of PVA/SiO2 and PVA/SiO2/glutaraldehyde (GA) nanocomposite membranes prepared by single-step solution casting method. J Mater Sci 2015;50:6477 90. [43] Mallakpour S, Dinari M, Hatami M. Novel nanocomposites of poly(vinyl alcohol) and Mg Al layered double hydroxide intercalated with diacid N-tetrabromophthaloylaspartic. J Therm Anal Calorimetry 2015;120:1293 302. [44] Kim S, Azuma Y, Kuwahara Y, Ogata T, Kurihara S. Preparation of graphene oxide/ polyvinyl alcohol microcomposites and their thermal conducting properties. Mater Lett 2015;139:224 7. [45] Yue DS, Kuila T, Kim NH, Lee JH. Enhanced properties of aryl diazonium saltfunctionalized graphene/poly(vinyl alcohol) composites. Chem Eng J 2014;245: 311 22. [46] Jose T, George SC, Mg M, Thomas S. Functionalized MWCNT and PVA nanocomposite membranes for dielectric and pervaporation applications chemical engineering and process technology. J Chem Eng Process Technol 2015;6:1 8. [47] Bhowmick S, Koul V. Assessment of PVA/silver nanocomposite hydrogel patch as antimicrobial dressing scaffold: synthesis, characterization and biological evaluation. Mater Sci Eng, C 2016;59:109 19. [48] Hussain R, Tabassum S, Gilani MA, Ahmed E, Sharif A, Manzoor F, et al. In situ synthesis of mesoporous polyvinyl alcohol/hydroxyapatite composites for better biomedical coating adhesion. Appl Surf Sci 2016;364:117 23. [49] Cao YC, Wei W, Liu J, You Q, Liu F, Lan Q, et al. The preparation of graphene reinforced poly(vinyl alcohol) antibacterial nanocomposite thin film. Int J Polym Sci 2015;2015:1 7. [50] Tantis I, Psarras GC, Tasis D. Functionalized graphene poly(vinyl alcohol) nanocomposites: physical and dielectric properties. Express Polym Lett 2012;6:283 92. [51] Bao C, Guo Y, Song L, Hu Y. Poly(vinyl alcohol) nanocomposites based on graphene and graphite oxide: a comparative investigation of property and mechanism. J Mater Chem 2011;21:13942 50. [52] Kuilla T, Bhadra S, Yao D, Kim NH, Bose S, Lee JH. Recent advances in graphene based polymer composites. Prog Polym Sci 2010;35:1350 75. [53] Sharma DK, Li F, Wu Y. Electrospinning of Nafion and polyvinyl alcohol into nanofiber membranes: a facile approach to fabricate functional adsorbent for heavy metals. Colloids Surf, A: Physicochem Eng Aspects 2014;457:236 43. [54] Tudorachi N, Lipsa R. Synthesis and characterization of poly(vinyl alcohol-co-aspartic acid) copolymers. Polimery 2010;55:562 7. [55] Zhu Y, Hu J, Wang J. Progress in nuclear energy removal of Co21 from radioactive wastewater by polyvinyl alcohol (PVA)/chitosan magnetic composite. Prog Nucl Energy 2014;71:172 8. [56] Abdelgawad AM, Hudson SM, Rojas OJ. Antimicrobial wound dressing nanofiber mats from multicomponent (chitosan/silver-NPs/polyvinyl alcohol) systems. Carbohydr Polym 2014;100:166 78. [57] Rahman AHA, Teo CL, Idris A, Misran E, Asma S, Leong N. Polyvinyl alcohol alginate ferrophoto gels for mercury (II) removal. J Ind Eng Chem 2016;33: 190 6. [58] Lv X, Jiang G, Xue X, Wu D, Sheng T, Sun C, et al. Fe0 Fe3O4 nanocomposites embedded polyvinyl alcohol/sodium alginate beads for chromium (VI) removal. J Hazard Mater 2013;262:748 58.

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[59] Ghasemzadeh H, Ghanaat F. Antimicrobial alginate/PVA silver nanocomposite hydrogel, synthesis and characterization. J Polym Res 2014;21:1 14. [60] Trujillo-Reyes J, Vilchis-Nestor AR, Majumdar S, Peralta-Videa JR, Gardea-Torresdey JL. Citric acid modifies surface properties of commercial CeO2 nanoparticles reducing their toxicity and cerium uptake in radish (Raphanus sativus) seedlings. J Hazard Mater 2013;263:677 84. [61] Liu Y, Huang H, Wang L, Cai D, Liu B, Wang D, et al. Electrospun CeO2 nanoparticles/PVP nanofibers based high-frequency surface acoustic wave humidity sensor. Sens Actuators, B: Chem 2016;223:730 7. [62] Mallakpour S, Nezamzadeh AE. Surface modification of ZrO2 nanoparticles with biosafe coupling agents, preparation of poly(vinyl pyrrolidone) nanocomposites: optical, thermal, and morphological studies. Adv Polym Tech 2016;1 10. Available from: http://dx.doi.org/10.1002/adv.21699. [63] Mallakpour S, Naghdi M. Fabrication and characterization of novel polyvinylpyrrolidone nanocomposites having SiO2 nanoparticles modified with citric acid and L(1)-ascorbic acid. Polymer (Guildf) 2016;90:295 301. [64] Keshtkar AR, Tabatabaeefar A, Vaneghi AS, Moosavian MA. Electrospun polyvinylpyrrolidone/silica/3-aminopropyltriethoxysilane composite nano fiber adsorbent: preparation, characterization and its application for heavy metal ions removal from aqueous solution. J Environ Chem Eng 2016;4:1248 58. [65] Kaur R, Tripathi SK. Study of conductivity switching mechanism of CdSe/PVP nanocomposite for memory device application. Microelectron Eng J 2015;133:59 65. [66] Kim JM, Lee DH, Jeun JH, Yoon TS, Lee HH, Lee JW, et al. Non-volatile organic memory based on CdSe nano-particle/PMMA blend as a tunneling layer. Synth Met 2011;161:1155 8. [67] Li F, Son DI, Cha HM, Seo SM, Kim BJ, Kim HJ, et al. Memory effect of CdSe/ZnS nanoparticles embedded in a conducting poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] polymer layer. Appl Phys Lett 2007;90:222109(1) 222109(3). [68] Abdelghany AM, Abdelrazek EM, Rashad DS. Impact of in situ preparation of CdS filled PVP nano-composite. Spectrochim Acta, A: Mol Biomol Spectrosc 2014;130:302 8. [69] Jegatheeswaran S, Selvam S, Ramkumar VS, Sundrarajan M. Novel strategy for f-HAp/PVP/Ag nanocomposite synthesis from fluorobased ionic liquid assistance: Systematic investigations on its antibacterial and cytotoxicity behaviors. Mater Sci Eng, C 2016;67:8 19. [70] Chen S, Cheng B, Ding C. Synthesis and characterization of poly(vinyl pyrrolidone)/ reduced graphene oxide nanocomposite synthesis and characterization of poly(vinyl pyrrolidone)/reduced graphene oxide nanocomposite. J Macromol Sci Part B Phys 2015;54:481 91. [71] Abdelghany AM, Abdelrazek EM, Badr SI, Morsi MA. Effect of gamma-irradiation on (PEO/PVP)/Au nanocomposite: materials for electrochemical and optical applications. Mater Des 2016;97:532 43. [72] Chaudhuri B, Mondal B, Ray SK, Sarkar SC. A novel biocompatible conducting polyvinyl alcohol (PVA)-polyvinylpyrrolidone (PVP)-hydroxyapatite (HAP) composite scaffolds for probable biological application. Colloids Surf, B: Biointerfaces 2016;143: 71 80. [73] Ramesan MT, Varghese M, Periyat P. Silver-doped zinc oxide as a nanofiller for development of poly(vinyl alcohol)/poly(vinyl pyrrolidone) blend nanocomposites. Adv Polym Tech 2016;21650:1 7.

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[74] Mallakpour S, Naghdi M. Evaluation of nanostructure, optical absorption, and thermal behavior of poly(vinyl alcohol)/poly(N-vinyl-2-pyrrolidone) based nanocomposite films containing coated SiO2 nanoparticles with citric acid and L(1)-ascorbic acid. Polym Compos 2016;1 8. Available from: http://dx.doi.org/10.1002/pc.24161. [75] Attaran AM, Javanbakht M, Hooshyari K, Enhessari M. New proton conducting nanocomposite membranes based on poly vinyl alcohol/poly vinyl pyrrolidone/BaZrO3 for proton exchange membrane fuel cells. Solid State Ionics 2015;269:98 105. [76] Archana D, Singh BK, Dutta J, Dutta PK. Chitosan-PVP-nano silver oxide wound dressing: in vitro and in vivo evaluation. Int J Biol Macromol 2015;73:49 57. [77] Achaby ME, Essamlali Y, Miri NE, Snik A, Abdelouahdi K, Fihri A, et al. Graphene oxide reinforced chitosan/polyvinylpyrrolidone polymer. J Appl Polym Sci 2014; 41042:1 11. [78] Archana D, Singh BK, Dutta J, Dutta PK. In vivo evaluation of chitosan—PVP—titanium dioxide nanocomposite as wound dressing material. Carbohydr Polym 2013;95:530 9.

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Vicente de Oliveira Sousa Neto1, Gilberto Dantas Saraiva2, Raimundo Nonato Pereira Teixeira3, Diego de Quadros Melo4, Francisco Cla´udio de Freitas Barros5 and Ronaldo Ferreira do Nascimento5 1 Department of Chemistry, State University of Ceara´ (UECE-CECITEC), Quixada´, Brazil, 2 Department of Physic, State University of Ceara´ (UECE-FECLESC), Quixada´, Brazil, 3 Department of Chemistry, Regional University of Cariri (URCA), Crato, Brazil, 4Federal Institute of Serta˜o Pernambucano, Petrolina, Brazil, 5Department of Analytical Chemistry and Physical Chemistry, Federal University of Ceara, Fortaleza, Brazil

Chapter Outline 11.1 Introduction 291 11.2 Hybrid materials 292 11.2.1 11.2.2 11.2.3 11.2.4

Well-defined organicinorganic hybrid polymers 292 Hybrid materials: PDMS-SiO2 293 Polymer hybrid nanocomposites with carbon nanomaterials Mesoporous hybrid materials 300

11.3 Materials applications

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305

11.3.1 Optical application 305 11.3.2 Antifouling performance 305 11.3.3 Photoactive hybrid materials 306

11.4 Final considerations References 308

11.1

307

Introduction

The design of polymer-based materials in which multiple constituents are organized in across multiple length scales to facilitate novel functionalities has become a versatile approach to resolve challenges for a wide range of material technologies, such as transparent conductors, energy generation, and drug delivery [1]. The unique property combinations as a consequence of the particular arrangement Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00011-2 Copyright © 2017 Elsevier Ltd. All rights reserved.

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and interactions between the constituents provide opportunities for novel disruptive material technologies based on polymer hybrid materials. In this connection, the polymer hybrid materials are materials, comprising synthetic polymer, as well as biological or inorganic derived constituents that represent one of the most rapid growing research areas in current polymer science. The central point to recent developments in polymer hybrid materials is the concept of structure-related “emergent” properties is the more established field of polymer nanocomposites that is more concerned with the engineering of the effective properties of materials by suitable blending of constituents. In this context, two requisites have been fundamental for the recent interest in polymer hybrid materials. First, recent advancements in the field of polymer chemistry now facilitate the precise coupling of synthetic polymer and biological- or inorganic-derived constituents into complex-structured supramolecular entities that can serve as building blocks for functional materials. Second, progress in understanding of the physics underpinning the evolution of structure and properties in multiphase materials for the design of multicomponent materials. These individual constituents autonomously can be organized into superstructures with tailor properties. The potential use of polymeric hybrid materials will require an interdisciplinary effort to carry out more detailed understanding of the interaction between the synthesis, processing, structure, and performance of these complex materials.

11.2

Hybrid materials

The words “hybrid materials” stand for a combination of (a minimum of) two materials of different nature into a new one having the combined properties of the starting materials together with some added values. The hybrid materials’ properties are often superior to the sum of the intrinsic properties of the components and often have a functionality that is not present in either of the individual materials [2]. The possibilities to create novel materials by synergetic combination of inorganic and organic components on the molecular scale make this materials class interesting for application-oriented research of chemists, physicists, and materials scientists [3].

11.2.1 Well-defined organicinorganic hybrid polymers The construction of well-defined organicinorganic hybrid polymers often requires polymers with precise structures, which could be achieved through living polymerization techniques [37] (Fig. 11.1). Lately, many kinds of well-defined hybrid polymers with a variety of architectures have been developed by controlled living radical polymerizations in the presence of inorganic compounds, or by the coupling reaction of functional polymers with inorganic nanoparticles [8,9]. These living polymerization techniques have involved atom transfer radical polymerization [10,11], reversible addition-fragmentation chain transfer polymerization, [4]

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Figure 11.1 Examples of well-defined organicinorganic hybrid polymers. Printed with permission from Z. Zhang, P. Zhang, Y. Wang, W. Zhang. Recent advances in organic-inorganic well defined hybrid polymers using controlled living radical polymerization techniques, Poly Chem, 7 (2016) 39503976.

and coupling reactions that provide the high flexibility to prepare hybrid materials with predesigned structures and properties [12]. The properties of organicinorganic well-defined hybrid nanomaterials are deeply influenced by their nanostructure using the radical polymerization techniques as shown in Fig. 11.1 [7].

11.2.2 Hybrid materials: PDMS-SiO2 The hybrid materials based on the binary system polydimethylsiloxane-silica (PDMS-SiO2) have been identified as promising for applications as diverse as coatings [13], biomaterials [14,15], and photonics [16]. This diversity of applications is related to the flexibility that this system presents in terms of microstructures ranging from macroporous solids to transparent films with microporosity. In Fig. 11.2, cited by Almeida et al. [17], are shown Si structures and symbols proposed by Glaser et al. [18] according to the type of synthesis and hybrid materials obtained by hydrolysis of tetraethyl orthosilicate (TEOS) and PDMS (or its precursors), and the subsequent cocondensation of SiOH and Si(CH3)2OH ending groups, lead to the formation of structural groups such as tetrafunctional Qn (n 5 3,4) structural units (SiO4) crosslinked to difunctional Dn (n 5 1,2) structural units [(CH3)2  SiO2], where n is the number of bridging oxygen atoms surrounding Si. It is known that in an aqueous solgel process under acidic conditions, TEOS hydrolysis and subsequent crosslinking with hydrolyzed difunctional units are very fast, preventing the development of long PDMS chains [19,20]. An increase in the content of acid and water results in an increase of gelation rate and a porous monolithic xerogel with enhanced surface area. Thus, different types of microstructures

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Figure 11.2 Notation of Si structures and symbols. Notation proposed by Glaser et al. Adapted and printed with permission from C. Almeida, A. Wacha, A. Bo´ta, L. Alma´sy, M.H., V. Fernandes, F.M.A. Margac¸a, et al., PDMS-SiO2 hybrid materials—a new insight into the role of Ti and Zr as additives, Polymer (Guildf), 72(2015) 4051.

can be obtained by varying experimental parameters such as water and acid content, PDMS molecular weight, solvent type, or processing temperature. Almeida et al. [17] produced two different groups of PDMS-SiO2 hybrid materials by solgel using different contents of water and acid catalyst. They observed that an increase of the wateracid contents resulted in an increase of the specific surface area of these materials, from transparent (low contents) to porous (high contents) monoliths.

11.2.2.1 PDMS-coated silica nanoparticles: preparation and application Many chemical sensors, playing a crucial role in environmental science and technology, are based on the detection of vapors, and these sensors are often easily contaminated by the introduction of aqueous liquids [21,22]. According to literature, a shielding layer in the aperture of the sensor can be used to protect it from contamination by aqueous liquid and allow selective permeation of gas molecules. Park et al. [23] developed a wet-chemical process for the preparation of such shield layers, thereby inhibiting permeation of aqueous liquids and allowing gas transmission. The method is based on simple and cost-effective wet-chemical dip coating and SiO2 nanoparticles coated by PDMS [2325].

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Thermal vapor deposition method According to authors in order to fabricate PDMS-coated silica nanoparticles, a thermal vapor deposition method was used. In this process, bare silica nanoparticles and fluidic PDMS were placed at a weight ratio of 1:1 in a stainless steel reactor. PDMS and bare silica nanoparticles were separated with a metal mesh (30 mesh) partition (Fig. 11.3A). Then, the reactor was sealed with polyimide (PI) tape and heated to 300 C for 15 h. The reactor was equipped with a power supply and temperature controller. Using this procedure, PDMS vapor was deposited on the surfaces of bare silica nanoparticles, forming a thin PDMS layer.

Dip-coating solution method Park et al. [23] developed a new method to prepare super hydrophobic films by dip-coating solution. For this, the authors used solution A and solution B, such as described: Solution A was prepared by dispersing PDMS-coated silica nanoparticles (0.2 g) in hexane (29 mL), and solution B was a mixture of solution A (29 mL) and an adhesive solution (1 mL), which was prepared by dissolving PDMS (1 mL) and curing agent (0.1 mL) in hexane (8 mL). The procedure was as described: Metal mesh-1 was used as a substrate for dip coating. The mesh surface was cleaned in ethanol for 10 min, wiped clean, and dried at room temperature. The

Figure 11.3 Schematic diagram of the experimental setup for (A) the preparation of hydrophobic coating on silica nanoparticles, and (B) the preparation of dip-coating solution and super hydrophobic films. Printed with permission from E.J. Park, B.R. Kim, D.K. Park, S.W. Han, D.H. Kim, W.S. Yun, et al., Fabrication of superhydrophobic thin films on various substrates using SiO2 nanoparticles coated with polydimethylsiloxane: towards the development of shielding layers for gas sensors, RSC Adv, 5 (2015) 4059540602.

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mesh was dipped into solution B for 10 s, placed vertically, and dried for 5 min under atmospheric conditions. This dip-coating process was repeated three times. After, the mesh-1 coated with solutions B and A are referred to as mesh-1B and mesh-1A, respectively. Other substrates, mesh-2, PTFE-1 and 2, fabric-1 and 2 and paper, were coated using the same method (Fig. 11.3B). In this work, they reported a simple and versatile method for the fabrication of super hydrophobic films with gas permeability using a dip-coating process. According to authors, the high-water repellent property of coated substrates was revealed by measuring the water contact angle on the film surface. Also, by adding PDMS and curing agent to the coating solution, adhesion of PDMS-coated silica nanoparticles to a substrate was enhanced. Tests with exposure to acidic and basic environments and UV-irradiation showed that the super hydrophobicity of the films was maintained. They also demonstrated the fabrication of a gas permeable membrane with highly water repellant properties using the proposed method. According to researchers, the super hydrophobic and gas permeable membrane can be used as a shielding layer in gas sensors, thus preventing contamination of the sensor with aqueous liquids.

11.2.3 Polymer hybrid nanocomposites with carbon nanomaterials Carbon nanotubes (CNT) are long cylinders of covalently bonded carbon atoms. There are two basic types of CNT: single-wall CNT (SWNT) and multiwall CNT (MWNT). SWNT can be considered as a single graphene sheet (graphene is a monolayer of sp2-bonded carbon atoms) rolled into a seamless cylinder. The carbon atoms in the cylinder have partial sp3 character that increases as the radius of curvature of the cylinder decreases. MWNT consist of nested graphene cylinders coaxially arranged around a central hollow core with interlayer separations of B0.34 nm, indicative of the interplane spacing of graphite [26]. A special case of MWNT is double-wall nanotubes (DWNT) that consist of two concentric graphene cylinders. DWNT are expected to exhibit higher flexural modulus than SWNT due to the two walls and higher toughness than regular MWNT due to their smaller size [27]. The nanotubes can be filled with foreign elements or compounds, for example, with C60 molecules, to produce hybrid nanomaterials with properties, such as transport properties [27]. These hybrid nanomaterials currently have limited availability, but as production increases this might be a new opportunity for polymer nanocomposites. Hybrid composites have lately attracted the attention of researchers with different mixtures being tried out, for example, multiwalled CNT (MWCNT) with carbon black [28,29] few layer graphene with single-walled CNT and nanodiamonds [29] and MWCNT with graphene platelets [30]. In Fig. 11.4 [31], it is possible to visualize cutting the graphite sheet along the dotted lines [31].

11.2.3.1 Nanotube/polymer composites: production and application Yang et al. [30] designed a strategy to improve the mechanical properties and thermal conductivity of epoxy multigraphene platelets (MGPs) filled composites by

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Figure 11.4 Diagram showing how a hexagonal sheet of graphite is “rolled” to form a carbon nanotube. Printed with permission from E.T. Thostenson, Z. Ren, T.W. Chou, Advances in the science and technology of carbon nanotubes and their composites: a review, Compos Sci Technol 61 (2001) 18991912.

combining one-dimensional multiwalled CNT (1D MWCNTs) and 2D MGPs. They showed that the long and tortuous MWCNTs can bridge adjacent MGPs and inhibit their aggregation, resulting in an increased contact surface area between MGP/ MWCNT polymer (Fig. 11.5) [30].

Production of hybrid polymers: nanotube/polymer composites The CNT/polymer composite have properties that depend on the following parameters: (1) CNT production process, (2) CNT purification process, (3) amount and type of impurities in the CNT, (4) aspect ratio (length:diameter) of the CNT in the composite, and (5) CNT orientation in the polymer matrix composite [32,33]. These parameters are difficult to quantify, and they vary widely in the literature. Thus, more complete reports are needed to reduce dissimilarities between published results of similar composites [33]. There are three production methods that result in good CNT dispersion and CNT alignment: (1) method of solution blending (Fig. 11.6A) [34], (2) melt blending (Fig. 11.6B), and (3) in situ polymerization (Fig 11.6C). These three techniques are widely applied to produce nanotube/polymer composites and will be summarized below [34]. 1. Solution blending: This is the most common method for fabricating polymer nanocomposites because it is both amenable to small sample sizes and effective. Solution blending involves three major steps: disperse nanotubes in a suitable solvent, mix with the polymer (at room temperature or elevated temperature), and recovery of the composite by precipitating or casting a film. It is difficult to disperse the pristine nanotubes, especially SWNT, in a solvent by simple stirring. Commonly, the CNT/polymer composites are prepared by mixing CNTs and polymer in a solvent that prevents the aggregation of the nanotubes while its dispersion occurs [33]. Both organic and aqueous medium have been used to produce CNT/polymer nanocomposites [30,35,36]. In this method the dispersion of

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Figure 11.5 The model of microstructural scheme in epoxy composites with various weight ratios of MWCNTs and MGPs. Printed with permission from S.-Y. Yang, W.-N. Lin, Y.-L. Huang, H.-W. Tien, J.-Y. Wang, C.-C.M. Ma, et al., Synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites, synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites, Carbon, 49 (2011) 793803. nanotube can be achieved by magnetic stirring, shear mixing, reflux, or most commonly, ultra sonication. Sonication can be provided in two forms, mild sonication in a bath or high-power sonication. The use of high-power ultrasonication for a long period of time can shorten the nanotube length which is detrimental to the composite properties [37]. One variation of the solution blending method uses surfactants to disperse higher loadings of nanotubes [38,39]. Islam et al. [38] dispersed SWNT (20 mg mL1) in water with the aid of the surfactant NaDDBS (NaDDBS:SWNT 5 1:10) using low-power, highfrequency (12 W, 55 kHz) sonication for 1624 h (Fig. 11.7) [38].

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Figure 11.6 Representation of different steps of polymer/CNTs composite processing: solution mixing (A); melt mixing (B); in situ polymerization (C). Printed with permission from E. Beyou, S. Akbar, P. Chaumont, P. Cassagnau, Polymer Nanocomposites Containing Functionalised Multiwalled Carbon NanoTubes: a Particular Attention to Polyolefin Based Materials In Nanotechnology and Nanomaterials in “Syntheses and Applications of Carbon Nanotubes and Their Composites”, book edited by Satoru Suzuki, ISBN 978-953-51-1125-2, Published: May 9, 2013 under CC BY 3.0 license.

Figure 11.7 Representation of how surfactants may adsorb onto the nanotube surface. Printed with permission from (a)M.F. Islam, E. Rojas, D.M. Bergey, A.T. Johnson, A.G. Yodh, High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett, 3 (2003) 269273;(b) N.G. Sahoo, S. Rana, J.W. Cho, L. Li, S.H. Chan, Polymer nanocomposites based on functionalized carbon nanotubes, Prog Polym Sci, 35 (2010) 837867. 2. Melt Blending: Solution processing is a valuable technique for both nanotube dispersion and nanocomposite formation (Fig. 11.6B), it is less suitable for industrial scale processes. Melt processing is an uncomplicated and low-cost method of production on a large scale, which is why it is preferred for industrial uses. In this method, molten polymer is used to disperse the CNTs after applying a high-shear force [33]. Advantages of this technique are its speed and simplicity and its compatibility with standard industrial techniques [40]. Any additives, such as CNT can be mixed into the melt by shear mixing. However, it is

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important that processing conditions are optimized for the whole range of polymernanotube combinations. High temperature and shear forces in the polymer fluid are able to break the CNT bundles, and CNTs can additionally affect melt properties such as viscosity, resulting in unexpected polymer degradation [41]. 3. Situ polymerization: The carbon filler is mixed with a monomer. A polymerization reaction (Fig. 11.6C) will produce polymer-grafted filler particles in which carbon filler form covalent bonds with polymer chains. The small size of the monomer leads to production of a composite with a high degree of homogeneity. In this sense, the method allows the preparation of composites with high-conducting filler weight fraction. This technique is particularly important for the preparation of insoluble and thermally unstable polymers, which cannot be processed by solution or melt blending.

Bonduel et al. [42] performed the method of in situ polymerization by mixing CNTs with olefin monomers using methylaluminoxane as cocatalyst of the polymerization process. The polymerization was the same with or without the presence of pristine MWCNTs [42]. Epoxy nanocomposites comprise the majority of reports using in situ polymerization methods [43] where the nanotubes are first dispersed in the resin followed by curing the resin with the hardener. Zhu et al. [44] prepared epoxy nanocomposites by this technique using end-cap carboxylated SWCNTs and an esterification reaction to produce a composite with improved tensile modulus.

Nanotube/polymer composites: application In situ polymerization has been used by Bonduel et al. [42] to produce a CNT/polymer composite (Fig. 11.8 [42]). Initially, the fixation of methylaluminoxane cocatalyst (MAO) was performed on the surface of CNT through a reaction in toluene at 40 C for 1 h (step 1). Then, the mixture was maintained at 150 C for 2 h to eliminate toluene by evaporation (step 2). The yield MAO immobilization reaction onto the surface of CNT was 98%. Then, the catalyst bis(pentamethyl-g5-cyclopentadienyl)zirconium (IV)dichloride (CP2 ZrCl2) and surface-activated CNTs were mixed in n-heptane (step 3). The CP2 ZrCl2 reacts with MAO anchored to form CP2 ZrMe1 species. These cations are immobilized on the surface of the CNTs via electrostatic interactions with MAO counter ions formed. Then, polyethylene is produced from ethylene near the CNTs, precipitating on its surface (step 4) and coating them (step 5) [42].

11.2.4 Mesoporous hybrid materials The chemical strategies for the production of mesostructured and mesoporous hybrid phases have been presented in detail in several reviews and themed issues, comprising general approaches and concepts, and specific routes toward mesoporous hybrid materials (MPHM), processed as powders, gels, or thin films [4547]. There are two ways to add organic components. In the first way, the organic components are added during the synthesis of the mesostructure. In the second way, the organic components are added in one ensuing functionalization process, which can be done via direct synthesis (one-pot path) or through a post-grafting route (twostep path). The one-pot path involves the cocondensation of the functional inorganic precursor (e.g., an organosilane) with a nonfunctional precursor in the presence of a

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Figure 11.8 Scheme of the polymerization-filling technique (PFT) applied to carbon nanotubes. Printed with permission from D. Bonduel, M. Mainil, M. Alexandre, F. Monteverde, P. Dubois, Supported coordination polymerization: a unique way to potent polyolefin carbon nanotube nanocomposites, Chem Commun, 6 (2005) 781783.

supramolecular aggregate [48]. The two-step path cosists in the postsynthesis treatment of the mesostructured materials by either solution impregnation or by exposure to volatile vapors for subsequent addition of organic functional groups. Periodic mesoporous organosilica (PMO) materials with very active area can be obtained with the use of bridged organosilica hybrid precursors. These PMO materials have organofunctions cross-linked by SiOSi bonds within their inner walls. Although the synthesis of PMO can be considered a “one-pot” path, the different nature of the precursor and the possibility of locating organic groups within the pore walls make it stand out from ordinary direct synthesis and will be treated separately. Soler-Illia and Azzaronic [48] illustrated the three main routes leading to ordered MPHM (Fig. 11.9) [48] focusing on the oxide-based MPHM, which is the most extensively studied field.

11.2.4.1 Postgrafting methods Postgrafting approach (Fig. 11.9, route A) [48]. Mesoporous matrices present large surface areas rich in silanols or other MOH groups; for example, the SiOH surface group density is of c. 12 nm2 in the case of mesoporous silica [49]. The existence of a highly accessible surface allows rapid functionalization of the pores by bifunctional molecules with various anchoring groups. On silica surfaces are used alkoxysilane or silazane groups. Carboxylates, phosphates, or acetylacetonates

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Figure 11.9 Scheme of the conventional routes toward mesoporous hybrid materials carrying organic functions. Printed with permission from G.J.A.A. Soler-Illia, O. Azzaronic, Multifunctional hybrids by combining ordered mesoporous materials and macromolecular building blocks, Chem Soc Rev, 40(2011) 11071150.

groups are used for metal oxides and thiols groups are used for metals [48]. In this method, functional groups extend into the pores due to surface reactions. The surface coverage and strong grafting are achieved from the control of three factors: (1) preventing the condensation located by adjusting the hydrophilicithy and reactivity

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of the surface with the organic functional group, (2) preventing the competition of the anchoring group with other species in solution (e.g., water and other nucleophiles in the case of silica mesoporous), and (3) preventing the formation of (R-SiO)x polymers that block the pore entrances due to self-condensation of the functional molecules [50]. Postgrafting proceeds typically in anhydrous conditions, in which clustering is minimized; in some cases, postgrafting can also take place by breaking SiOSi bonds, through nucleophilic displacement at the surface silicon atom by the entering alkylsiloxane [51]. Depending on the postgrafting conditions (solvent, function solubility, etc.), the framework can be partially dissolved or MOM bonds can be cleaved in the procedure.

11.2.4.2 One-pot methods One-pot synthesis (Fig. 11.9, route B) [48]. In this route, the organic precursor containing the functional group to be incorporated is included in the initial solution. The cocondensation of the inorganic precursor (e.g., tetramethyl orthosilicate or TEOS) and organofunctional precursor (e.g., organotrialkoxysilanes) will occur or in the initial solution or during the synthesis process or during the precipitation or the formation of liquid crystalline mesophase [48]. This is an attractive synthesis, as it constitutes the easiest way to grant incorporation of organic groups embedded within the metal-oxo skeleton. However, certain aspects should be taken into account in order to design a proper synthesis procedure 1. To prevent damage to organofunctional groups, removal of template should be done under bland conditions, in two steps: (i) heating the mesostructure (150200 C) to enhance condensation, and (ii) extracting the template using suitable solvent (e.g., toluene) [48]. 2. In order to obtain ordered mesostructures reproducibly, the organic content must be kept below 30%40% mole fractions. Often dangling organic functions produce changes in hydrophiliclipophilic balance in the reaction mixtures that degrade the relative stability of mesophases [48]. The presence of polar organofunctions can lead, however, to a good organization, and even to a change of mesostructure, as observed in nitrogen-containing mesoporous hybrid thin films derived from cocondensation [52]. 3. Clusters with different concentrations of organics lead to irregular chemical composition of the materials. In order to avoid the formation of these agglomerates, the hydrolysis and condensation rates of the functional alkoxides and the inorganic precursors must be tuned. For example, propylamino group in (EtO)3Si(CH2)3NH2 is an alkaline group which may accelerates the inorganic condensation creating less ordered mesostructures [48]. 4. As a consequence of the composition of the initial mixtures, an important fraction of the incorporated organic functions is partially buried in the inorganic walls. This brings consequences in the mechanical and chemical properties of the materials, due to the connectivity of the hybrid framework, which is different case of pure inorganic [47]. Recent work in amino-containing mesoporous thin films showed that c. 37% of amine groups are available for quantitative reactions with organic functions in postgrafted films, whereas only about 16% are reactive in the materials obtained from cocondensation. This result can be understood in terms of the different distribution of functionalities obtained in both reaction routes [53].

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One-pot synthesis with bis- or polysilylated precursors (Fig. 11.9, route C) [48]. This route is analogous to route B, but generally implies the condensation of a single-source bridged precursor, typically a bis-trialkoxysilylorganosilane molecule [i.e., (RO)3SiR0 Si(OR)3], although multisilylated precursors can also be used. Synthesis conditions involve hydrothermal treatment and extreme pH conditions, in which sometimes integrity of the SiC bonds might be damaged. An advantage of this route is that organic functions are integrated within the framework walls: organic functions are accessible but do not modify pore size. In some cases, and depending on the size and shape of R0 , assembly at the molecular scale also takes place. Inagaki and collaborators reported an ordered benzenesilica hybrid material presenting a hexagonal array of 5.2-nm diameter mesopore channels, and ˚ crystal-like pore walls exhibiting structural periodicity with a spacing of 7.6 A along the channel direction. This periodicity at the molecular scale is driven by the ππ stacking of benzene residues that provides a structure-directing driving force that enters in synergy with the interactions between the precursor molecules and the surfactants, determining the ordering at two different length scales [54]. Cocondensation of a bridging precursor and a terminal organotrialkoxysilane [i.e., (RO)3SiRv], or a second bis-silylated precursor can lead to a rich variety of bifunctional materials that permit a variety of function exposure at the pore surface or double functions within the walls, respectively [48]. An additional advantage of PMOs is the possibility to perform chemical reactions on the hybrid framework, and several examples with applications in catalysis have been recently reviewed [55]. In summary, cocondensation and postgrafting routes are complementary, and their application depends on the features desired for the mesoporous system. Postfunctionalization has several advantages: a higher fraction of active surface species is available, grafted on a well-defined, robust mesoporous framework. However, the reactivity of surface silanols is not always easy to control, due to the different reactivity of isolated, terminal, geminal, or hydrogen bonded surface species. The control of the extent of grafting reactions is an important drawback of postfunctionalization [48]. On the other hand, “one-pot” synthesis is somehow complicated by the presence of organically modified alkoxides with different reaction rates and hydrophilicity that can modify or even hinder the formation of highly ordered pore arrays upon coassembly of the framework building blocks with the template. Degradation of the organic groups during thermal treatment also has to be taken into account, and a large fraction of the functional groups remains buried [48]. However, direct methods bring out the possibility of obtaining homogeneous function distribution, and the use of bridged precursors opened the rich field of PMOs. Overall, both routes can be combined in order to obtain complex multifunctional pores. For example, the consecutive application of one-pot and postgrafting permits to produce bifunctional mesoporous films [56]. The ultimate goal of MPHM synthesis is to use all the possible synthetic tools described above in order to design controlled size pore systems with any desired functional groups attached to the surface or within the pore walls, and to control the interactions between those functional groups in the confined conditions imposed by the pore size and shape. In particular, the control of spacing between the organic

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functions, and their spatial location in the macroscale are central features, if advanced multifunctional systems are sought [48]. A vast catalog of construction rules are ready to be used for building up designed multifunctional MPHM, which includes total pore control, selective reactivity, tuning of surface charge, or hydrophilicity, formation of self-assembled monolayers, and positional control of functional groups [57,58]. The intelligent choice among these nanoscale-molecular scale tools permits to imagine complex systems with well-crafted chemical species located in spatially arranged functional domains.

11.3

Materials applications

11.3.1 Optical application Optical products are probably the most common examples among hybrid coating applications. Some of them are illustrated in Fig. 11.10 [59]. Thus, one interesting application is dealing with the development and commercial use of organo-silicated hybrid sealants for liquid-based optical lenses, diaphragms, or zooms. The Varioptic Company has replaced organic glue for such optical devices with a hybrid one in order to get a more versatile sealing agent suitable for various liquids, different substrates (glass, plastic, and metal), and lens designs. Use of hybrid-containing liquid-based optical devices for cameras, cell phones, or endoscope applications is then affordable.

11.3.2 Antifouling performance In the last few decades, it has been observed an increasing demand of smart materials able to combine different useful properties, for possible application in the most diverse industrial fields, following the needs of the recent technological breakthroughs. Regarding the field of coatings technology, the organicinorganic hybrid (OIH) coatings represent the multifunctional response for the industrial demand. These materials, in fact, are at the interface of organic and inorganic systems, and they find a compromise between the typical features of organic materials, for example, film forming ability, density, hydrophobia, and others, and the ones of inorganic materials, such as mechanical or thermal stability [60]. Oldani et al. [60] prepared organicinorganic hybrid coatings interspersing a silica network (obtained by the solgel synthesis) with a fluoropolymeric matrix, following the first class synthetic approach, that is, the hydrolysis and condensation of tetraethylorthosilicate (TEOS, the silica precursor) was made in presence of a α,ω-triethoxysilane perfluoropolyether derivative (Fig. 11.11) [60]. The organic and the inorganic phases composing these OIH coatings were carefully chosen in view of their specific application, that is, fouling mitigation. The main feature that we aimed to confer to the antifouling coating was the hydrophobicity [60,61]. According to authors, in fact, it has been demonstrated that lowenergy surfaces are able to support an air film against the water in which they are

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Figure 11.10 Coatings of hybrid materials with anticorrosion. Printed with permission from C. Sanchez, P. Belleville, M. Popall, L. Nicole, Applications of advanced hybrid organicinorganic nanomaterials: from laboratory to market, Chem Soc Rev, 40 (2011) 696753.

immersed; thanks to that, it is possible to reduce the water-wetted area and, consequently, also the probability that biological organism or organic/inorganic particles, present in water, encounter the solid surfaces [61,62].

11.3.3 Photoactive hybrid materials Direct addition of photoactive molecules onto the sidewall of SWNTs can provide photoactive moleculeSWNT composites with robust and well-defined structures in comparison with the functionalization with carboxylates introduced at the terminals and defect sites of the SWNT by the acid treatments. Therefore, such covalently sidewall-functionalized SWNTs by photoactive molecules are attractive for a

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Figure 11.11 The solgel synthesis of a hybrid coating composed by tetraethylorthosilicate and a perfluoropolyether. Printed with permission from V. Oldani, G. Sergi, C. Pirola, B. Sacchi, C.L. Bianchi, Solgel hybrid coatings containing silica and a perfluoropolyether derivative with high resistance and anti-fouling properties in liquid media, J Fluor Chem, 188 (2016), 4349.

better understanding of the interactions between the two components in the ground and excited states [63,64]. Ito et al. [63] also connected zinc porphyrins covalently to the sidewall of SWNTs (ZnP-P-SWNT, Fig. 11.12) [64] by a one-step reaction, that is, dipolar cycloaddition of the azomethineylide [64]. Chirality sorted semiconducting SWNTs, that is, SWNT and SWMT, were used in the study [6466]. Both ZnP-PSWNT and ZnP-PSWNT showed moderate fluorescence quenching (c. 40%) compared to the porphyrin reference compound [65,66]. The nanosecond transient absorption technique confirmed an electron transfer process, producing ZnPK1-PSWNTK2 charge separation species with lifetimes of 170210 ns [67]. The merit of charge-separated state stabilization, which was calculated from the rate of charge separation and rate of charge recombination, was slightly enhanced in ZnP-PSWNT in comparison with ZnP-PSWNT [6668]. Consistently, the photoelectrochemical device with ZnP-PSWNT showed a higher IPCE value (max. 2.2% at 430 nm) than that with ZnP-P-SWNT (max. 1.1%) [65,66]. These results suggest that the higher conduction band level of SWNT is more favorable for charge separation via 1ZnP relative to SWNT with the lower conduction band level.

11.4

Final considerations

Hybrid materials represent one of the most fascinating developments in materials chemistry in recent years. The materials represent one of the most growing new material classes at the edge of technological innovations. Unique possibilities to create novel material properties by synergetic combination of inorganic and organic components on the molecular scale makes these materials class interesting for application-oriented research of chemists, physicists, and materials scientists. The modular approach for combination of properties by the selection of the best suited components opens new options for the generation of materials that are able to solve

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Figure 11.12 Structures of ZnP-ESWNT, ZnP-PSWNT, and H2P-DSWNT. Reprinted with permission from T. Umeyama, H. Imahori, Photofunctional hybrid nanocarbon materials, J Phys Chem C, 117 (2013) 31953209.

many technological problems. This chapter showed in selected examples how science and technology-driven approaches can help to design better materials for future applications.

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Sudhir K. Saw Central Instrumentation Facility, Birla Institute of Technology (Deemed University), Ranchi, India

Chapter Outline 12.1 Introduction 313 12.2 Experimental and characterization 12.2.1 12.2.2 12.2.3 12.2.4 12.2.5 12.2.6 12.2.7

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Materials 316 Alkali treatment of luffa and coir fiber 317 Fabrication of composites 318 Optical microscope 319 Static mechanical analysis 319 Dynamic mechanical analysis 319 SEM studies 320

12.3 Results and discussion

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12.3.1 Mechanical properties 320 12.3.2 Dynamic mechanical properties 321 12.3.3 Morphological studies of fiber and tensile fractured hybrid composite surfaces 328

12.4 Conclusions and future perspectives Acknowledgment 330 References 330

12.1

330

Introduction

There is always an increasing demand for advanced materials with better properties to meet new requirements or to replace existing materials. In recent years, rapid growth has occurred in the consumption of natural-fiber-reinforced polymer composites, Hybrid Polymer Composite Materials: Structure and Chemistry. DOI: http://dx.doi.org/10.1016/B978-0-08-100791-4.00012-4 Copyright © 2017 Elsevier Ltd. All rights reserved.

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which yield a unique combination of high performance, great versatility, and processing advantages at memorable cost [13]. With growing environmental awareness and ecological concern, natural-fiber-reinforced polymer composites have received increasing attention during the recent decades. Natural fibers, environment-friendly materials, are potential user and have been proved to be good reinforcement in polymeric matrices [47]. Natural fiber-reinforced epoxy composites have been evaluated both for strength, performance, and cost and have proved to be competitors for glass fiber/epoxy composites [8,9]. In the past few years, the hybrid composite materials are replacing the conventional composite materials because of their superior properties. Hybridization of two different fibers has proved to be an effective method to design materials suited for various requirements [10]. A number of research groups proved that luffa and coir fibers (CFs) could be used as effective reinforcements in a polymer matrix [1113]. These fibers were hybridized with glass fibers to obtain better mechanical performance [1417]. Luffa cylindrica is commonly called as loofa, sponge gourd, vegetable sponge, bath, or kitchen sponge, is a member of cucurbitaceous family [18]. The luffa cylindrica is a subtropical plant abundantly available in China, Japan, India, and other countries in Asia as well as in Central and South America [19]. The fruit luffa cylindrica can be eaten as a vegetable when it is young. But mature fruits can’t be eaten because of its bitter taste due to development of purgative chemicals. Due to its purgative property, it is used as medicine for remedy of dropsy, nephritis, and chronic bronchitis and lung complaints [20]. The luffa fruit has a fibrous vascular system that forms a natural mat when dried. The natural luffa mat possesses remarkable strength, stiffness, and energy absorption capacity comparable to metallic cellular material in a similar density range [21]. As reported by researchers [22,23], this fiber contains cellulose 55%70%, hemicelluloses 8%22%, lignin 10%23%, extractives 3.2%, and ash 0.4%. Chemical composition of this fiber seems to be suitable for polymeric composites. Accordingly, researchers [24] studied the effect of fiber surface treatment on mechanical properties of luffa cylindrica polymer composites. They have the opinion that various chemical treatments of luffa fiber (LF) enhance the mechanical properties of the composites. They studied the effect of water ageing on the mechanical properties of luffa cylindrica polyester composites and observed that water ageing reduces the mechanical properties of the composites [22,25]. The coconut coir (Cocos nucifera) fiber is an important lingo-cellulose hard and stiff fiber obtained from coconut trees, which grow extensively in tropical countries such as India, Sri Lanka, Thailand, and others. Several scientists studied that have been carried out to understand the structure, properties, and the influence of chemical modification on CFs. Because of its hardwearing quality, durability, and other advantages, it is used for making a wide variety of floor furnishing materials, yarn, rope, and others Espert et al. [26]. However, these traditional coir products consume only a small part of the potential total world production of coconut husk. Hence, research and development efforts have been underway to find new applications area for CFs including utilization of CFs as reinforcement in polymer composites due to their higher mechanical and physical characteristics in relation to high-lignin content, microfibrillar angle, and strain value [27,28].

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The viscoelastic or dynamic mechanical properties of polymeric materials are of considerable practical significance for several reasons, particularly, if they are determined over wide ranges of frequency and temperature. They can yield insight into various aspects of material structure, provide a convenient measure of polymer transition temperatures, and may influence other important properties such as fatigue and impact resistance. The dynamic properties are also of direct relevance to a range of unique polymer applications concerned with the isolation of vibrations or dissipation of vibrational energy in engineering components. The dynamic properties are generally expressed in terms of storage modulus, loss modulus, and damping factor, which are dependent on time, frequency, and temperature. Generally, the introduction of a filler in a polymeric matrix leads to a reduction in mobility of the macromolecular chains in the vicinity of filler. This is evident from the increase in the temperature of the main relaxation associated with the glass transition. Therefore, the extent of fiber/matrix interaction and structure property relationship could be understood from dynamic mechanical analysis (DMA) [29]. Hybrid composites may be defined as systems in which one kind of reinforcing material is incorporated into a mixture of different matrices (blends) [30,31], or when two or more reinforcement/filler materials are present in a single matrix [32] or also if both approaches are combined [33]. Hybridization with more than one fiber type in the same matrix provides another dimension to the potential versatility of fiber-reinforced composite materials. Properties of the hybrid composites may not follow a direct consideration of the independent properties of the individual components [3436]. Valea et al. investigated the influence of cure conditions and the exposure to various chemicals on the dynamic mechanical properties of several vinyl ester and unsaturated polyester resins containing glass fiber. Exposure to aromatic solvents was found to modify the viscoelastic character of these materials [37]. Bledzki and Zhang investigated the dynamic mechanicalthermal behavior of jute fiber-reinforced epoxy foams. It was observed that temperature of the log decrement peak for the jute fiber-based composites was shifted by about 5 C, in comparison with that of pure epoxy resin [38]. Jawaid et al. studied the mechanical and thermal properties of palm/jute bilayer hybrid composites. From this study, the thermogravimetric analysis showed that thermal stability of oil palm composites increased with incorporation of jute fiber due to higher thermal stability of jute fiber [39]. Idicula et al. studied the dynamic mechanical performance of banana/ sisal hybrid fiber-reinforced polyester composites. She also studied kinetic parameter by using Arrhenius relationship to calculate activation energy of the glass transition of the composites [40,41]. A great deal of work has already been reported on single fiber reinforced polymer composites. However, very limited literature is available on thermomechanical properties of hybrid natural fiber reinforced composites. Keeping in view the easy availability, low cost, and ecofriendly advantages, luffa and CFs were selected to hybridize and reinforce with epoxy resin to develop cost-effective, highperformance composites. The inherent physicomechanical properties of luffa and CFs are given in Table 12.1, which were taken from the literature. Fibers having high-cellulose content and low microfibrillar angle possess high-tensile properties.

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The inherent physicomechanical properties of luffa and coir fiber

Table 12.1

Parameter Density (g cm3) Cellulose content (%) Hemicelluloses content (%) Lignin content (%) Moisture content (%) Microfibrillar angle ( ) Aspect ratio (L/D) Diameter (μm) Lumen width (μm) Ash (%) Tensile strength (MPa) Young’s modulus (MPa) Elongation at break (%)

Raw luffa fiber

Treated luffa

Raw coir fiber

Treated coir

1.2 6 0.1 60.42 6 1.8 20.88 6 1.4

  

1.1 6 0.1 33.28 6 1.21 12.67 6 1.44

  

11.69 6 1.2 761 11 6 2 350 6 10 150 6 20 562 0.4 6 0.1 202.3 4500 2.5 6 0.2

       238.5 5892.6 1.69

46.84 6 0.8 961 39 6 5 100 6 5 240 6 10 12 6 5 0.2 6 0.1 144.6 3101.2 32.3 6 0.2

       196.0 4871.8 19.7

The cellulose contents of both of these fibers are almost the same. However, LF has a microfibrillar angle of 11 , which is less compared to CF (39 ). Hence, the inherent tensile properties (Table 12.1) of LF are higher than CF. The CF is compressive in nature due to its high-lignin content and shows high failure strain value in composites. This supports compressive strength and extensibility in composites. Due to high value of spiral angle and larger in lumen size causes high impact strength in coir-reinforced composites [28]. Therefore, luffa and CFs can be selected to hybridize and reinforce epoxy resin and combined properties of both the fibers can be achieved in the hybrid composites. Short randomly oriented luffa/coir hybrid fiber-reinforced epoxy composites, having a weight ratio of luffa and coir of 1:1, were prepared at a total weight fraction of 0.40 and their mechanical performance under static and dynamic conditions was studied. The effect of stacking patterns such as bistacking (luffa/coir), tristacking (luffa/coir/luffa) and (coir/luffa/coir), and ultimate mix composites on storage modulus, loss modulus, and damping peaks (Tan delta or mechanical loss factor) was investigated. Fibermatrix interaction was analyzed micro structurally from the dynamic mechanical data.

12.2

Experimental and characterization

12.2.1 Materials The CFs were obtained from Central Coir Research Institute (CCRI, Coir Board), Kerala, India. LFs were locally collected from agricultural fields of Jharkhand (India)

Effect of stacking patterns on morphological and mechanical properties

317

Figure 12.1 A photograph of (A) luffa and (B) coir fiber.

The evaluation of physicomechanical properties of liquid epoxy resin Table 12.2

Parameter

Evaluation

Appearance Specific gravity at 25 C (g cm3) Viscosity at 25 C (cps) Epoxy equivalent number Gel point in minutes Solid content (%) Tensile strength (MPa) Tensile modulus (MPa) Flexural strength (MPa) Flexural modulus (MPa) Impact strength (kJ m2)

A milky white liquid 1.12 475 187 26 84 6.9 165 14.3 364 1.1

after 1-year matured fruits from 2015 crops. Details of luffa and CF from collection to preparation are shown in Fig. 12.1. The physicomechanical properties of luffa and CFs are given in Table 12.1. Epoxy resin (LY 556) is used as matrix material and its common name is bisphenol-A-diglycidyl-ether chemically belongs to epoxide group. Hardener (HY-951) (2-amino ethylene ethane-1,2-diamine) is used as curing agent. Both the resin and the corresponding hardener were supplied by Ciba Geigy Ltd, India. The evaluation of physicomechanical properties of liquid epoxy resin is given in Table 12.2, respectively.

12.2.2 Alkali treatment of luffa and coir fiber To enhance interfacial bonding with matrix material and to reduce moisture absorption, surface modification (alkali treatment) of the ligno-cellulose luffa and CFs was performed. Prior to alkali treatment, both the fibers were washed thoroughly with fresh water. The NaOH solution of 5% concentrations was prepared in separate

318

(A)

Hybrid Polymer Composite Materials: Structure and Chemistry

Luffa

Coir

Coir

Luffa

Luffa

Coir

Tri-stacking (L/C/L)

(B)

Tri-stacking (C/L/C)

(D)

Ultimate mix

Luffa

Coir

(C)

Bi-stacking (L/C)

Figure 12.2 A schematic representation of different stacking patterns of hybrid composites (A) tristacking (luffa/coir/luffa), (B) tristacking (coir/luffa/coir), (C) bistacking (luffa/coir), and (D) ultimate mix.

water bath. Then the washed luffa and CFs were immersed in 5% NaOH solutions for 2 h with subsequent stirring. Then the alkali solution was drained out and the fibers were washed with fresh water to remove any NaOH sticking to fiber surface. Finally, the fibers were washed with distilled water containing a little dilute acetic acid and pH of 7 was maintained. The fibers were then air dried for 48 h followed by air oven drying at 70 C for 6 h.

12.2.3 Fabrication of composites Prepeg route was followed for the preparation of composites. Wet hand lay-up technique was adopted for composite fabrication. Chopped luffa and CFs of about 25cm length were used to prepare the composites. The curing of resin was done by the incorporation of epoxy and hardener in volume ratio of 60:40 in 100 mL of acetone (act as thinner or diluents). Keeping the combined fiber weight fraction as 0.40 and the weight ratio of two fibers as 1:1; ultimately mixed, tristacking (luffa/coir/ luffa and coir/luffa/coir), and bistacking (luffa/coir) composites were prepared. A sketch of the different configuration of hybrid composite laminate is given in Fig. 12.2AD. Mats of chopped luffa and CFs of 25-cm length were impregnated in epoxy resin. The prepeg was kept on mold stage made by teflon sheet with hole

Effect of stacking patterns on morphological and mechanical properties

319

at one corner and sealed with plastic cover. It was kept under vacuum for 12 h applying 600 kg cm2 pressure by means of vacuum pump machine to remove air bubbles and minimize voids, nicks in the composites and then it was left for another 12 h in sealed condition to complete curing. The postcuring of composite was done at 80 C in air-dried oven for 5 h. Neat epoxy samples (Epoxy resin cured for 24 h at room temperature followed by postcuring for 5 h at 80 C, without incorporating fibers) were also prepared.

12.2.4 Optical microscope The diameter of fibers was measured using a Leica optical stereo microscope (Model-Leica FW4000) by taking photographs with the help of digital camera (Leica stereo zone X3.2). The diameter of LF is less than that of coir, which can be observed in Table 12.1.

12.2.5 Static mechanical analysis Tensile test was carried out with dumbbell shape specimens (width 5 10 mm and thickness 5 3 mm) using a universal tensile machine (UTM 3366, Instron, UK) according to ASTM D 638. A crosshead speed of 5 mm min1 was applied. All tests were conducted under ambient conditions. The data reported were averages of at least six measurements. The flexural properties were evaluated according to ASTM D 792 using same tensile machine. The test speed was maintained at 1.5 mm min1. For pure epoxy and its composites, six measurements were executed with each sample. The tensile properties of untreated and chemically treated, both of these luffa and CFs were determined using the same tensile machine at a strain rate of 1 mm min1 and a gripping length of 50 mm at 23 6 1 C and 58% relative humidity. The results of mechanical analysis are reported in Table 12.1. The impact strength was calculated using an Izod impact testing machine, model-IT 1.4 with the specification of energy range from 0 to 1.4 J, hammer weight of 4.580 kg, and height of 203.70 mm from Fuel Instruments, Maharashtra, India. Izod impact strength was determined using ASTM D 256. The unnotched samples for measurements were cut to 70 3 10 3 3 mm3 dimensions. Six specimens were tested, and average values were reported in Table 12.2.

12.2.6 Dynamic mechanical analysis Dynamic mechanical experiments for different configuration of hybrid composites having the dimensions of 60 3 10 3 3 mm3 were conducted according to ASTM D5026 standard. A dynamic mechanicalthermal analyzer of TA Instruments, USA (model—Q800) was used for the evaluation of dynamic moduli and mechanical damping (tanδ). Three point-bending modes were used. The temperature range over which properties were measured was 30200 C at a heating rate of 3 C min1. The tests were carried out at frequencies of 1, 10, and 20 Hz.

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Hybrid Polymer Composite Materials: Structure and Chemistry

12.2.7 SEM studies A scanning electron microscope (SEM) (Model: Jeol JSM 6390 LV) was used to characterize the morphology of tensile fractured surfaces of fiber samples and its various hybrid composites. The samples were mounted on the stub by carbon tape. A thin film of gold is vacuum evaporated on the surface of sample to avoid charging before the photomicrographs were taken.

12.3

Results and discussion

12.3.1 Mechanical properties The importance of mechanical properties is to quantify the reinforcing potential of the composite system. However, mechanical properties can also give indirect information about interfacial behavior in composites, because the interaction between the components has a great effect on the mechanical properties of the composites. The properties of composites were strongly influenced after pretreatments of fibers. Various mechanical properties of modified and unmodified, luffa, and CFs were reported in Table 12.1. It is observed that the tensile strength and modulus of LF as well as aspect ratio is higher than that of CF. The tensile properties of different stacking pattern of hybrid composites consisting of tristacking (such as luffa/coir/ luffa or coir/luffa/coir), bistacking, and ultimate mix composites are shown in Fig. 12.3A and B. As the relative weight fraction of the two fibers is the same (i.e., luffa:coir 5 1:1), there is not much difference in their properties. The tensile strength was observed to be higher when luffa was used as the skin material and coir as the core material. The tensile strength will be higher when the high-strength material is used as the skin, which is the main load bearing component in tensile measurements. The tensile strength of the ultimate mix composite is comparable with the composite having luffa as the skin material. Maximum stress transfer occurs in ultimately mixed composites. In coir/luffa/coir, the value is slightly lower because the low-strength CF is used as the skin material. In bistacking, the tensile strength is again lowered. The tensile modulus is found to be higher for the tristacking (luffa/coir/luffa) composite and the same in the other pattern [40,41]. The flexural properties of tristacking (such as luffa/coir/luffa or coir/luffa/coir), ultimate mix, and bistacking composites were also shown in Fig. 12.3C and D. Flexural strength is found to be the highest in the bistacking composite and lowest in ultimate mix composites. In tristacking composites, the values are in between the other patterns. Shear and tension are the main forces in flexural stress. Shear force will depend mainly on stacking patterns. The delamination mechanism will be different in tristacking and bistacking composite. In tristacking composites, two interlaminar planes are there, and the possibility of delamination is higher. But in bistacking composites, there is only one interlaminar plane, and the possibility of delamination is lesser than that of tristacking composites. Hence, higher flexural strength is observed in bistacking composites [40,41].

Effect of stacking patterns on morphological and mechanical properties

(A)

321

(B) 1000

35

Tensile modules (MPa)

Tensile strength (MPa)

30 25 20 15 10 5 0

600 400 200

0 LC

LCL

CLC

Ultimate

(C)

LC

LCL

CLC

Ultimate

LC

LCL

CLC

Ultimate

(D) 2000 Flexural modules (MPa)

80 Flexural strength (MPa)

800

70 60 50 40

1600

1200

800

400

30 LC

LCL

CLC

Ultimate

Figure 12.3 The mechanical properties of various configuration of luffa/coir hybrid composites.

12.3.2 Dynamic mechanical properties In this study, the viscoelastic properties of hybrid composites were investigated under sinusoidal force is applied to the composite specimens and the resulting deformation (strain) is measured. Dynamic mechanical properties of fiberreinforced composites depend on the nature of the matrix material and the distribution and orientation of the reinforcing fibers and the nature of the fibermatrix interfaces and of the interphase region. Even a small change in the physical and chemical nature of the fiber for a given matrix may result in remarkable changes in the overall dynamic mechanical properties of composites.

12.3.2.1 Effect of stacking pattern on storage modulus (E 0 ) The storage modulus attained from the DMA study represents the elastic modulus of the hybrid composites, which determines the recordable strain energy in the

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Hybrid Polymer Composite Materials: Structure and Chemistry

deformed samples. In simple terms, it is the measure of elastic behavior of samples when subjected to temperature changes. Storage modulus measures stored energy, which reflects the stiffness of hybrid composites [39]. Fillers/reinforcements have an active role in increasing the modulus of polymeric materials. Fig. 12.4 shows the effect of stacking pattern on the storage modulus values with temperature of the hybrid composites at a frequency of 10 Hz. The storage modulus of neat epoxy resin can also be seen in Fig. 12.4. In all cases, storage modulus was found to be decreased with increase of temperature. At low temperature, the E0 values of the matrix and the composites are very close, that is, at low temperature fibers do not contribute much to imparting stiffness to the material. In the case of the neat epoxy resin, there was a sharp fall in E0 on passing through the glass transition temperature (Tg), due to the increased molecular mobility of the polymer chains above Tg. The drop in the modulus on passing through the glass transition temperature was dramatically reduced for reinforced hybrid composites compared to the neat resins, which shows the greater reinforcing effect of luffa and CF on the storage modulus above Tg than below it. This can be attributed to the combination of the hydrodynamic effects of the fibers embedded in the viscoelastic medium and to the mechanical restraint introduced by the fibers at high concentrations, which reduce the mobility and deformability of the matrix. The dynamic modulus curves of the filled systems showed a higher E0 value than the unfilled sample above the Tg region in the rubbery plateau. L/C/L represents a tristacking composite having luffa as the skin and coir as the core material and C/L/C represents a tristacking composite having coir as the skin and luffa as the core material. Ultimate mixed composites and bistacking composites can also be seen. L/C/L and ultimate mixed composites had almost the same values of E0 at Tg. But above Tg,

5000

Log E' (MPa)

4000 3000 2000

Epoxy Bi-stacking C /L /C L /C /L Ultimate mix

1000 0 20

40

60

80 100 120 140 160 180 200 220 Temperature (°C)

Figure 12.4 Effect of stacking pattern with temperature on storage modulus of the hybrid composites at a frequency of 10 Hz.

Effect of stacking patterns on morphological and mechanical properties

323

Table 12.3 Tan δ peak height and peak width, coefficient ðCÞ, Tan δmax ðTgÞ, E00max ðTgÞ, and activation energy of different stacking patterns Composite

ðCÞ

Tan δ peak height (cm)

Tan δ peak width (cm)

Epoxy resin Bistacking L/C/L C/L/C Ultimate mix



9

2

0.232 0.168 0.174 0.164

6.5 3.1 4.7 2.2

4.7 5.4 5.6 5.6

Tan δmax ðTgÞ at 10 Hz ( C)

97.3 103.9 113.6 106.5 112.8

E00max ðTgÞ at 10 Hz ( C) 96.84 104.3 111.8 105.3 115.6

Activation energy (kJ mol1) 47 52 64 53 67

the E0 value was slightly greater in L/C/L. The effectiveness of fillers on the moduli of the composites can be represented by a coefficient, C [29] as  E0G =E0R composite  C5 E0G =E0R resin

(12.1)

where E0G and E0R are the storage modulus values in the glassy and rubbery region, respectively. The higher the value of the constant C, the lower the effectiveness of the filler. The measured E0 values at 65 and 140 C (for epoxy resin) were employed as E0G and E0R , respectively. The values of C obtained for different hybrid composites are given in Table 12.3. The value of C is minimum for L/C/L and ultimate mix composites. The value of C is maximum for the bistacking composite. The high stiffness of these composites (L/C/L and ultimate mixed) is in agreement with their tensile properties reported in Table 12.1. Above Tg, the E0 value of C/L/C was comparatively lower than L/C/L and ultimate mixed composites. The E0 value of the bistacking composite was very low compared to the others, showing more molecular motions in it and poor stress transfer between fiber and matrix. As coir has lower tensile properties compared to luffa (Table 12.1), the stiffness of the composites will be decreased when coir is used as the skin material. As the diameter of CF is greater than that of LF, the aspect ratio of the fiber per unit area of the composite is lower in the case of coir. Hence, stress transfer between the fiber and matrix is decreased [42].

12.3.2.2 Effect of stacking pattern on the mechanical loss factor (tan δ) with temperature The variation of tan δ values of the above composites with temperature at a frequency of 10 Hz is given in Fig. 12.5. It was observed that the tan δ peak obtained

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Hybrid Polymer Composite Materials: Structure and Chemistry

0.40 Epoxy Bi-stacking C /L /C L /C /L Ultimate mix

0.35

Tan delta

0.30 0.25 0.20 0.15 0.10 0.05 0.00 20

40

60

80

100

120

140 160

180

200

220

Temperature (°C)

Figure 12.5 Effect of stacking pattern with temperature on tan δ values of the hybrid composites at a frequency of 10 Hz.

at 109 C (Tg) of the unfilled sample was very high. It is associated with the glass transition of epoxy resin. With the incorporation of fibers, the tan δ peak was lowered as expected. This is due to the decrease in weight fraction of the matrix by the incorporation of fibers. The peak height and peak width values are given in Table 12.3. A wider and lower tan δ peak was obtained for ultimate mixed composites. When fibers are ultimately mixed with each other, better dispersion takes place and stress is transferred from fiber to matrix easily without the failure of the matrix [43]. In L/C/L, the peak height was slightly higher, but peak width was closer to the ultimately mixed composite. The peak height of the bi-stacking composite was found to be very high and its peak width was very small. This indicates poor fiber/matrix adhesion and is consistent with its E0 values. The positive shift in tan δ values shows the effectiveness of the fiber as a reinforcing agent. Elevation of Tg is taken as a measure of interfacial interaction. The Tg values are given in Table 12.3. L/C/L and ultimate mixed composites showed the highest values of Tg with C/L/C and bi-stacking composites. The shift of Tg of epoxy resin to higher temperatures is associated with the decreased mobility of the polymer chains due to their interaction with the fibers.

12.3.2.3 Effect of stacking pattern on loss modulus (E 0 ) with temperature The loss modulus (E0 ) is a measure of the viscous response of the material, which indicates the energy dissipated in the form of heat, showing the viscous region of the hybrid composites. Elsewhere, it is considered a measure of energy loss as heat/cycle under deformation [39]. The effect of temperature on the loss modulus of the neat epoxy resin as well as the composites at a frequency of 10 Hz can be observed in

Effect of stacking patterns on morphological and mechanical properties

325

300 Epoxy Bi-stacking L /C /L Ultimate mix C /L /C

250

Log E" (MPa)

200 150 100 50 0 20

40

60

80

100 120 140 160 180 200 220 Temperature (°C)

Figure 12.6 Effect of stacking pattern with temperature on loss modulus values of the hybrid composites at a frequency of 10 Hz.

Fig. 12.6. The maximum heat dissipation occurred at the temperature where E0 was maximum, indicating the Tg of the system [44]. Above Tg, the E0 value of L/C/L was the highest and that of the bi-stacking was the lowest. The effect of filler was promi00 nent above the glass transition temperature in this case also. The Emax ðTgÞ of the different composites can be seen in Table 12.3. The highest Tg is observed for the ultimately mixed composites followed by L/C/L, C/L/C, and bistacking composites.

12.3.2.4 Effect of variation of applied frequency The viscoelastic properties of a material are dependent on temperature, time, and frequency. The storage modulus, damping peak, and loss modulus are affected by frequency. Fig. 12.7 shows the effect of frequency on storage modulus of the bistacking composite as a function of temperature. The modulus values were found to be decreased from 50 and 150 C. It can be observed that there was an increase in storage modulus with increase in frequency, which was more prominent when the frequency was increased from 1 to 20 Hz; after that the modulus value remained unchanged. Modulus measurements performed over a short time (high frequency) result in higher values, whereas measurements performed over long times (low frequency) result in lower values. This is due to the fact that the material undergoes molecular rearrangement in an attempt to minimize the localized stresses [45]. The tan δ values were also affected by frequency. Fig. 12.8 represents the effect of frequency on tan δ of the tristacking (L/C/L) composite. The tan δ peak was shifted to a higher temperature with increase in frequency. The damping peak is associated with the partial loosening of the polymer structure so that groups and small chain segments can move. The tan δ peak, which is indicative of the glass transition temperature, is also indicative of the extent of crosslinking of the system.

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Hybrid Polymer Composite Materials: Structure and Chemistry

4500

1 Hz 10 Hz 20 Hz

Log E' (MPa)

4000 3500 3000 2500 2000 1500

20

40

60

80

100 120 140 160 180 200 220

Temperature (°C)

Figure 12.7 Effect of frequency on storage modulus with temperature of bistacking composite. 0.22 0.20

20 Hz 10 Hz 1 Hz

0.18

Tan delta

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 60

80

100

120

140

160

180

200

220

Temperature (°C)

Figure 12.8 Effect of frequency on tan δ values with temperature of L/C/L composite.

12.3.2.5 Energy of activation for glass transition temperature (ΔE) The activation energy, ΔE, for the glass transition of the composites can be calculated from the Arrhenius equation  f 5 fo exp

2 ΔE RT

 (12.2)

Effect of stacking patterns on morphological and mechanical properties

327

where f is the measuring frequency, fo is the frequency when T approaches infinity, and T is the temperature corresponding to the maximum of the tan δ curve. Activation energy of the epoxy sample and hybrid composites is given in Table 12.3. Activation energy of the neat epoxy resin is only 53 kJ mol1. Ultimately mixed and L/C/L showed the highest activation energies, followed by C/ L/C and bistacking. This result is also in agreement with the extent of reinforcement.

12.3.2.6 ColeCole plots The magnitude of polarization within a material is represented by the dielectric constant, which can be represented by the Debye and Onsagar equations [46]. The single relaxation peaks are inadequate to describe the viscoelastic response of polymers. ColeCole is a particular treatment of dielectric relaxation data obtained by plotting E00 against E0 , each point corresponding to one frequency. Structural changes taking place in crosslinked polymers after fiber addition to polymeric matrices can be studied using the ColeCole method. The dynamic mechanical properties when examined as a function of temperature and frequency are represented on the ColeCole complex plane. E 5 f ðE0 Þ

(12.3)

Fig. 12.9 shows the ColeCole plot, where the loss modulus, E00 , data are plotted as a function of storage modulus, E0 . The nature of the ColeCole plots is 400 360

Loss modulus, E" (MPa)

320 280

C /L /C Bi-stacking L /C /L Ultimate

240 200 160 120 80 40 2100

2400

2700

3000

3300

3600

3900

4200

Storage modulus, E (MPa)

Figure 12.9 ColeCole plots of the hybrid composites having different stacking pattern.

328

Hybrid Polymer Composite Materials: Structure and Chemistry

reported to be indicative of the homogeneity of the system: homogeneous polymeric systems are reported to show a semicircular diagram [47]. The ColeCole diagrams of different stacking patterns shown in Fig. 12.9 are imperfect semicircles. However, the shape of the curve points toward the relatively good fibermatrix interaction.

12.3.3 Morphological studies of fiber and tensile fractured hybrid composite surfaces SEM provides an excellent technique for examination of fiber surface morphology. The physical appearance of the fibers before and after surface modification gives an idea about wettability of fiber with resin. Fig. 12.10 (micrographs A through D, respectively) shows the SEM micrographs of untreated and alkali treated, luffa, and CF. The shape of fiber seems long strips and has not a flat surface. A large number of grooves or channels run more or less parallel along with the longitudinal direction of fibers. There are waxes, oils, and small particles providing a protective layer on fiber surface as observed in Fig. 12.10A. Fig. 12.10B shows a larger number of surface cracks and separation of fiber bundle, compared to Fig. 12.10A. Very significantly, the intercellular gaps are clearly distinguished and the unit cells are partially exposed which was not obvious in the untreated LF. These features might result from the partial removal of wax and oily substances and loss of cementing materials such as lignin and hemicelluloses during treatment with sodium hydroxide. SEM analysis also examined the surface morphology of treated and untreated CFs. The removal of surface impurities on plant fibers is advantageous for fibermatrix adhesion as it facilitates both mechanical interlocking and the bonding reaction. In Fig. 12.10C, there is a porous structure observed for untreated CFs. It can be seen from Fig. 12.10D that after alkali treatment, the surfaces of the fibers become rougher due to removal of the intercellular binding materials and the amorphous waxy cuticle layer. This surface roughness of the alkali-treated fiber would enhance the mechanical interlocking with resins. The mechanical properties of hybrid composites could be corroborated with the morphological evidences. The fibermatrix adhesion in the hybrid composites can be understood by examining the SEM micrographs of cryogenically fracture surfaces of tensile test specimens. Fig. 12.10E and F shows SEM micrographs of the freshly tensile fracture surfaces of luffa/coir/luffa and coir/luffa/coir hybrid composites based on epoxy matrices, respectively. A large number of fiber pullout, fiber agglomeration, fibermatrix incompatibility, and matrix cracking were noticed in the epoxy-coir/luffa/coir hybrid composites (Fig. 12.10F), compared with epoxy-luffa/coir/luffa hybrid composite (Fig. 12.10E). The observed fiber pullout phenomenon in the fracture surfaces of the composites is a kind of index of the adhesiveness between the fibers and the matrix resin. Fracture of fiber with little or no pullout of LF is observed in the SEM micrograph of epoxy-luffa/coir/luffa hybrid composite (Fig. 12.10E). This observation indicates lesser extensibility of LFs leading to little fiber pullout and matrix failure under tensile loading. It can

Effect of stacking patterns on morphological and mechanical properties

329

Figure 12.10 SEM micrographs of fiber and its hybrid composite surfaces.

also be noted for these composites that the fiber failed by tearing, but no complete interfacial failure was observed; indicating that adhesion between the hybrid fibers and epoxy matrix was quite good for reinforcing. There is substantial epoxy matrix adhering to the fiber surfaces, indicating that the interfacial bond strength is fairly high in configuration of luffa/coir/luffa composites than coir/luffa/coir due to difference in the interfacial characteristics between the fibers and the matrix.

330

12.4

Hybrid Polymer Composite Materials: Structure and Chemistry

Conclusions and future perspectives

In the present study, static and dynamic mechanical analysis of short randomly oriented luffa/coir hybrid fiber reinforced epoxy composites was investigated with special reference to different stacking patterns of the composites. The effect of temperature and frequency on storage modulus (E0 ), mechanical damping (tan δ), and loss modulus (E00 ) was studied by keeping the relative weight fraction of luffa and coir 1:1 and the total fiber loading to a 0.40 weight fraction. Above Tg, the storage moduli of the composites were very high compared to neat epoxy. The tristacking composite, in which luffa was as the skin and coir as core material, showed maximum stiffness. The storage modulus of the ultimate mix composite is comparable with the composite having luffa as the skin material. The bistacking composite showed maximum damping property. Activation energy for the glass transition of neat epoxy and composites was evaluated, and it was found that ultimately mixed and L/C/L had the higher activation energies. Finally, we add that, by hybridizing luffa and coir, we can prepare userfriendly and cost-effective composite materials possessing appropriate stiffness and damping behavior for tailor-made applications. The ColeCole plot was constructed and showed good fibermatrix adhesion. The morphological features of the composites were well corroborated with the mechanical properties. On the basis of above studies, it can be concluded that an optimal configuration of luffa-coir hybrid fibers could effectively reinforce the epoxy resin and enable to achieve satisfactory properties of the composites for various engineering applications. For future application of CF, the hybrid composites could be studied with other plant fibers that complement them. The thermal insulation performance should be detected further. Also, the optimization design of the CF hybrid composite must be performed. The present work can be further extended to study the aspects of such hybrid composites. In practice, these composites are designed to perform in different static and dynamic conditions. The dielectric and corrosion behaviors of hybrid composites are needed to be investigated for finding their potential applications as dielectric materials.

Acknowledgment I am thankful to Central Instrumentation Facility (CIF) of Birla Institute of Technology, Mesra, Ranchi, India to carry out experiments described in this chapter.

References [1] Singha AS, Thakur VK. Physical, chemical and mechanical properties of Hibiscus sabdariffa fiber/polymer composite. Int J Polym Mater 2009;58:21728. [2] Singha AS, Thakur VK. Morphological, thermal, and physicochemical characterization of surface modified pinus fibers. Int J Polym Anal Charact 2009;14:27189.

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Index Note: Page numbers followed by “f ” and “t” refer to figures and tables, respectively. A Acetone, 78 79, 201 Acetylacetonates, 301 303 Acetylation, of cellulose nanocrystal surfaces, 74 75 Acid-purified NDs, 1 2 Acid-to-pulp ratio effect, 69 Adhesion, 152, 224 225 fiber matrix adhesion, 328 330 Airplane, 165, 165f fiber-reinforced plastics used in, 166f AlCu4Mg1 alloy, 195 197 Aliphatic polymers, 141 142 Alkali treatment of luffa and coir fiber, 317 318 Alkyl-dimethyl chlorosilanes, 74 Alkyne-azide cycloaddition reactions, 33 Aluminum, 9 11, 15 16 anodizing, 201 203 Aluminum alloys, 194 197 intermetallic phases in, 199t microstructure of, 197 Aluminum lithium alloys, 199 AlZn5.5MgCu (AW7075) alloy, 197 Amperometric glutamate biosensor, 26 27 Amphiphilic hyperbranched-linear polymers, 38 Anodized aluminum sheet, surface morphology of, 227f, 228f Anodizing aluminum, 201 203 Aptamer-based biosensors, for human IgE detection, 28 29 ARALL (Aramid aluminum laminate), 194 Aramid fibers, 82, 194, 208, 209t Arrhenius equation, 326 327 Arrhenius relationship, 315 Aryl diazonium salt functionalized graphene (ADS-G), 275 276, 275f Aryl diazonium salts, electrochemical reduction of, 49, 51f

Atom transfer radical polymerization (ATRP), 33, 38, 75, 292 293 Atomic-force microscopy (AFM), 12, 47, 82 AZ31B alloy, 199 B Bioadhesion, 145, 151 152 applications, 151 152 Biocompatibility, 136 Bioglass, 144t, 146, 147t Bioimaging, 154t Biomaterials, 24 25, 140 advances with chitosan as, 25 29 Biomedical polymer hybrid composites, 135 applications, 145 154 bioadhesion applications, 151 152 composition-based hybrid composites, 152 154 controlled drug-delivery applications, 146 147 photodynamic therapy, 149 150 tissue engineering applications, 147 148 wound-dressing applications, 151 classification, 138 145, 138f biopolymer polymer hybrid composites, 141 143 metal polymers hybrid composites, 143 145 mineral polymer hybrid composites, 143 natural polymer polymer hybrid composites, 138 141 specifications, 142t Biopolyethylene, 142t, 153t Biopolymer CS/montmorillonite nanocomposites, 26 Biopolymer hybrid composites, biomedical applications of, 153t

336

Biopolymer polymer hybrid composites, 141 143 Biosensing, 139 140, 154t Bioseparation, 154t Biotin, 52 Bis- or polysilylated precursors, one-pot synthesis with, 304 305 Bismaleimide (BMI), 208 2,2-Bis(methylol)propionic acid (bis-MPA), 46 Bisphenol-A-diglycidyl-ether, 316 317 Boltorn H30 molecules, 47 Boric acid anodizing (BAA), 202 203 2-Bromoisobutyryl bromide, 75 Bulk-scale manufacture of ND, 3 4, 18 1-Butyl-3-methylimidazolium chloride, 71 72

C Cadmium sulfide (CdS), 122, 123f, 124f, 280 Calcium phosphate, 144t, 146, 147t CARALL (carbon reinforced aluminum laminate), 195, 220 221 Carbazole (CBZ), 48 Carbon fibers, 171, 208 selected properties of, 210t Carbon nanomaterials, polymer hybrid nanocomposites with, 296 300 nanotube/polymer composites, 296 300 application, 300 hybrid polymers, production of, 297 300 Carbon nanotubes (CNTs), 1 3, 81 82, 104 106, 112 113, 147, 148t, 164, 169 171, 296, 297f Carbon surfaces, 47 Carbon-based materials, 112 114 Carbon-based nanomaterials, 1 2, 6 7 Carbon-black-filled rubber, 6 7 Carbon-fiber strengthened composites, 15 16 Carbon-fiber-reinforced polymer (CFRP), 164, 171, 195 Carboxylated multiwalled carbon nanotubes (cMWCNT), 26 27 Carboxylates, 301 303, 306 307 Carboxymethyl cellulose, 24, 89 90

Index

Cellulose, 67, 140 141 Cellulose fiber, hierarchical structure of, 66f Cellulose nanocrystals (CNCs) green hybrid composites from, 65 preparation, 67 72 from acid hydrolysis, 68 71 from ionic liquid process, 71 72 from mechanical processes, 71 processing and development, 76 82 in-situ polymerization, 80 81 layer-by-layer assembly, 81 82 melt processing, 79 80 solvent casting, 76 79 properties gas barrier, 89 90 mechanical, 82 85 melt rheological, 86 89 thermal, 85 86 surface modification, 73 76 by electrostatic and physical adsorption, 76 polymer grafting by surface-initiated polymerization, 75 sylilation and acetylation, 74 75 TEMPO-mediated surface oxidation, 73 74 Cellulose whiskers. See Cellulose nanocrystals (CNCs) Central Coir Research Institute (CCRI), 316 317 Ceramics polymeric systems, 1 2 Cetyltrimethylammonium bromide, 76 Charpy impact resistance values of neat epoxy and nanocomposites, 9f Chemical pickling, 201 Chemisorption, 44 46 Chiral diacid, preparation of, 239f Chiral stationary phases (CSPs), 40 Chitosan (CS), 139, 282 283 advancements with, 25 29 as biomaterial, 23 24 as fourth-generation biomaterial, 24 25, 26f structure of, 24f Chitosan hybrid composites, 139 140 Chitosan polymer hybrid composites, 139 Chromic acid anodizing (CAA), 202 203, 226 228 Circular shock waves, 3 4, 18

Index

Coal combustion products (CCPs), 171 182, 173t Coconut coir (Cocos nucifera) fiber, 314 Cole Cole plots, 327 328, 327f Collagen-based hybrid films, 140 Commercially pure (CP) titanium, 199 Composite materials, 164, 206 210 Composite polymer electrolyte (CPE) sandwiching, 16 17 Composition-based hybrid composites biomedical applications of, 152 154 Concurrent synthesis techniques, of organic/ inorganic composites, 103 Conducting polymers (CPs), 101 102 chemical formation, in presence of inorganic nanoparticles, 103 108 photocatalytic polymerization on semiconductor electrodes, 121f polymer grafting approaches, 106f Controlled/living radical polymerization (CRP), 33 Copper, 9 11 Copper-catalyzed azide alkyne click reaction, 33 Corundum, 203 Cryogels, 266 267 Crystalline TiO2, structure and some properties of, 205t Cycloaddition reaction, 33

D De Pont route, 3 4, 18 Dendrimers, 33 35 Dendritic effects, 33 34, 36 37, 37f Dendritic polyglycerol derivatives, 46 47 Dendritic silane-modified spherical silica gel, 54 55, 56f Dendronized polymers, 33 35, 36t synthetic strategies for preparation of, 34f Dendrons, 33 35, 39, 45 Dendron-thiol and dendron-sulfide molecules, 44 45, 45f Density function theory (DFT), 280 Dental composite, 166, 168f Detonation generated integrated ND (DND) particles, 4f, 12 14 Detonation ND, 1 2, 12 14 Detonation soot, 1 4

337

Diamond graphitization of, 3 4 tetrahedral structure of, 4 5 Diamond nanoparticles, commercialization of, 5 Diamond structure, model for, 5f Diamondiods, 5 Dichloromethane, 201 Didecyldimethylammonium bromide, 76 Differential scanning calorimetry (DSC) studies, 15 16 Differential thermal analysis, 16 17 Dimethyl sulfoxide (DMSO), 77 78 Dip-coating solution method, 295 296 DJS (double-strap joints with CFRP sheets), 177t DMF, 77 81 2-(Dodecylthiocarbonothioylthio)-2methylpropionic acid, 75 Double-wall nanotubes (DWNT), 296 Double-walled CNTs (DWCNTs), 169 170, 170f Dye-sensitized solar cells (DSSCs), 52 Dynamic mechanical analysis (DMA), 315, 319 E Electrochemical approaches, 109 121 electrochemical deposition techniques, 110t nanoparticle-based hybrids, 111 nanostructured systems, 112 121 photo-assisted methods, 121 125 Raman spectroscopic investigation, 115f used for polymerization, 109f Electrochemical impedance spectroscopy, 16 17 Epoxy resin fly ash composite, mechanical properties of, 180 182 Epoxy resins, 167 169, 316 317 Ethylene-oxide/epichlorohydrin copolymer nanocomposites, 78 79 Explosion route, 3 4, 18 F Fabrication of composites, 318 319 Fe3O4 nanoparticles and 3-thiophene aceticacid monomer, interaction between, 104, 105f

338

Fe3O4 particles, surface modification of, 111, 111f Ferrocenyl dendrimers in silica mesoporous, confinement of, 57, 57f Fiber metal laminates (FMLs), 193 194, 206, 214f, 215 216, 224 defects in FMLs structure, 217 220 examples of configuration in, 214t macrostructure, 195 220 configuration of plies, 210 217 defects in FMLs structure, 217 220 type of composite materials, 206 210 type of metal sheets and surface treatment, 195 206 microstructure, 220 224 of metal composite interface, 223 224 of metal surface layers, 223 of polymer composite layers, 222 physical chemistry of interface, 224 228 physical properties of metals used in, 198t profile curing process, 216f selected prepregs for, 211t structure manufacturing, 215f Fiber-reinforced plastics (FRP), 166, 166f Fillers reinforced plastic, 174 178 “Fingertip guided” functionalization, 44 5xxx series aluminum alloy, 197 Fluorine substituted synthetic fluorhydroxyapatite (f-HAp), 281 282 Fluoropolymeric matrix, 305 Fly ash (FA), 164, 171 182 in epoxy matrix, 186f SEM study, 172f, 178 182 Formic acid, 38 Fourier transform infrared (FTIR) spectroscopy, 16 17, 240 241 Fourth-generation biomaterials, 25 Fullerene, 1 2 Furfuryl alcohol (FA), 81

G G1-NO2, 45 chemical structure of, 47f Gallium arsenide, 4 5 Galvanostatic deposition method, 110t Gas barrier properties, of cellulose nanocrystals, 89 90

Index

Gel polymer electrolyte (GPE), 16 17 Gelatine, 151 Gelatin-glutaraldehyde (GA-gelatin) hydrogel, 39 Generation effect, 36 37, 37f Germanium, 4 5 GLARE (Glass reinforced), 194, 205 Glass and aramid fibers, selected properties of, 209t Glass fiber, chemical composition of, 208t Glutamate oxidase (GluOx), 26 27 Glycerol, 27 Gold electrodes, 44 45 Gold nanoparticles, 26 27, 49 Gold-bonded dendron-thiols, chemical structure of, 46f Gold-cored Newkome-type dendrimers, scheme of, 51f Graphene oxide (GO) doping, 269 Graphene/reduced GO (r-GO), 275 “Green and sustainable” approach, 74 Green hybrid composites, from cellulose nanocrystal, 65 Green hybrid nanocomposites from metal oxides, 263 metal oxides, 264 poly(vinyl alcohol) (PVA), chemical structure and properties of, 264 265 poly(vinyl pyrrolidone) (PVP), chemical structure and properties of, 265 polymer/metal oxide nanocomposites, 265 266 PVA blends/metal-oxide nanocomposites, 276 278 PVA/metal-oxide nanocomposites, properties and applications of, 265 266 PVA/nonmetal-oxide nanocomposites, 274 276 PVP blends/metal-oxide nanocomposites, 282 283 PVP/metal-oxide nanocomposites, 278 280 PVP/nonmetal-oxide nanocomposites, 280 282 H Halpin Kardos model, 83 Hardener (HY-951) (2-amino ethylene ethane-1,2-diamine), 316 317

Index

Hexogen, 1 2 HFIL LDH Hb, 237 238 High-performance liquid chromatography (HPLC), 40 43 Honeycombs, 193 Hyaluronic acid (HA), 25 Hyberbranched versus dendritic macromolecules, 35 38 structural differences, 36t Hybrid inorganic-hyperbranched polymer composites, 43 57 3D structures, 48 57 flat solid surfaces, 44 48 Hybrid materials concept of, 31 32 examples of, 32f Hybrid polymers, production of, 297 300 melt blending, 299 300 situ polymerization, 300 solution blending, 297 298 Hybrid polymers composite, 291 final considerations, 307 308 materials applications, 305 307 antifouling performance, 305 306 optical application, 305 photoactive hybrid materials, 306 307 mesoporous hybrid materials, 300 305 one-pot methods, 303 305 postgrafting methods, 301 303 polydimethylsiloxane-silica (PDMS-SiO2), 293 296 dip-coating solution method, 295 296 thermal vapor deposition method, 295 polymer hybrid nanocomposites with carbon nanomaterials, 296 300 hybrid polymers, production of, 297 300 nanotube/polymer composites, 300 well-defined organic inorganic hybrid polymers, 292 293, 293f Hydrobromic acid, 71 Hydrochloric acid hydrolysis, 70 Hydrogel, 147 Hydrogel hybrid composites, 146 147, 148t Hydrophilic interaction liquid chromatography (HILIC), 40 43 Hydrophobic ND, 12 14 2-Hydroxy-4-methoxybenzophenone-5sulfonic acid, 246 247

339

Hydroxyapatite, 144t Hydroxy-functional polymers, 38 Hydroxyl apatite, 147t Hyperbranched and dendrigraft polymers, 33 34 Hyperbranched and hyperfunctional hybrid organic composites, 39 43 curved surfaces in 3D materials, 40 43 flat surfaces, 39 Hyperthermia for cancer therapy, 154t I Indium tin oxide (ITO) electrodes, 48 In-situ chemical methods conducting polymers, chemical formation, 103 108 one-pot synthesis, 108 109 In-situ mucoadhesive hybrid composites, 146 In-situ polymerization approach, in preparation of nanocomposite materials, 80 81 In-situ polymerization method, 240f, 241 242, 300 Ionic liquids (ILs), cellulose nanocrystals from, 71 72 Isobutyl alcohol, 201 Isocyanate, 39 L Layer-by-layer (LbL) assembly, 81 82 Layered double hydroxide (LDH), 235 derivatives, 247t LDH-based polymer NCs, preparation and characterization of, 240 245 in-situ polymerization method, 241 242 melt compounding method, 244 245 solution intercalation, 242 243 modification, 237 239 polymer/LDH composites, properties of, 245 254 mechanical properties, 248 250 rheological properties, 250 251 swelling properties, 252 253 thermal properties, 245 248 preparation of LDH-diacid, 239f structure and preparation, 236 237

340

Leica optical stereo microscope, 319 Li-ion batteries and electronic devices, 16 17 Linear sweep voltammetry, 16 17 Lingocellulosic fibers, 67 Lithium-ion batteries, 1 2, 16 17 Loss modulus with temperature, effect of stacking pattern on, 324 325 Low density polyethylene (LDPE) nanocomposite, 9 11 Luffa and coir fiber, 313 314 alkali treatment of, 317 318 inherent physicomechanical properties of, 316t Luffa cylindrica, 314

M Maghemite NPs, 277 278 Magnetic nanoparticles (MNPs), 52 54 “Materials 1 shape 1 scale” method, 136f Matrix material, 15 16, 164 169 Melt blending, 299 300 Melt compounding method, 244 245 Melt processing, for cellulose nanocrystals, 79 80 Melt rheological properties, of cellulose nanocrystals, 86 89 Memory effect, 237 238 Mesoporous hybrid materials (MPHM), 300 305 one-pot synthesis, 303 304 with bis- or polysilylated precursors, 304 305 postgrafting methods, 301 303 Metal hybrid composites, biomedical application of, 154t Metal oxides, 264 Metal surface layers, microstructure of, 223 Metal volume fraction (MVF) coefficient, 215 Metal composite interface inaccurate, 223f microstructure of, 223 224, 223f Metal-oxide nanoparticles (NPs), 264 Metal-oxide-based materials, 115 121 Metal polymers hybrid composites, 143 145 Methacryloxypropyltrimethoxysilane, 74

Index

Methanol, 201 Methyl ethyl ketone, 201 Microcrystalline cellulose (MCC), 72 Microfibrils, 67 68 Milled carbon fiber (MCF), 164, 171 SEM images of, 175 Mineral polymer hybrid composites, 143, 144t in drug delivery, 147t Modified LDHs (m-LDHs), 238, 240 241, 240f, 275 Multigraphene platelets (MGPs), 296 297 Multiple-walled CNTs (MWCNTs), 169 Multivalency, 36, 37f Multiwall CNT (MWNT), 296 Multiwalled carbon nanotubes (MWCNTs), 8 9, 52, 107, 113 114, 115f, 296 Multiwalled carbon nanotubes/ionic-liquid/ CS nanocomposite (MWCNTs/IL/ Chit), 28 29 N Na-(dodecylbenzenesulfonate), 113 114 Nanoclay, 144t Nanocrystalline particles, 5 Nanodiamond background and invention of, 2 3 ND-butyl, 9 11 structure and chemistry of, 4 5 synthesis strategies for, 3 4 Nanodiamond reinforced hybrid, conducting properties of, 12 Nanodiamond reinforced matrix, mechanical features of, 7 9 Nanodiamond reinforced polymer, thermal properties of, 9 11 Nanodiamond surface, various groups present on, 6f Nanofillers, 1 2, 6 7 Nanoparticle-based hybrids, 111 Nanoparticle-cored dendrimers (NCDs), 48 49, 55f Nanoscale diamond particles, 1 2 Nanosized diamond powder, 1 2 Nanostructured systems carbon-based materials, 112 114 conducting polymers and, 112 121 geometries, 112f metal-oxide-based materials, 115 121

Index

Nanotube/polymer composites, 296 300 application, 300 hybrid polymers, production of, 297 300 in situ polymerization, 300 melt blending, 299 300 solution blending, 297 298 Natural polymer hybrid composites, 138 139 biomedical applications of, 153t Natural polymer polymer hybrid composites, 138 141 Natural polymers, 151 Natural-fiber-reinforced polymer composites, 313 314 Nicotine adenine dinucleotide (NADH), 45 N-methyl pyrrolidine (NMP), 77 78 Nucleophilic/electrophilic and radicalinitiated reactions, 33 O One-dimensional multiwalled CNT, 296 297 One-pot synthesis, 108 109, 303 304 with bis- or polysilylated precursors, 304 305 Optical microscope, 319 Organic modified LDH (OLDH), 254 Organic/inorganic composites, synthesis techniques of, 102 103 Organic inorganic hybrid (OIH) coatings, 305 Organo-gels, 78 79 Osteoblast adhesion and proliferation, 145 Out-of-Autoclave technology (OOA), 217 Owens Wendt method, 225 226 Oxygen permeability coefficient (OP), 254 P P3HT/TiO2, 107, 113 114, 115f P-conjugated dendritic polymers, 48 P-conjugated/conducting polymers, 6 7 PEEK [poly(ether ether ketone)], 208 PEI (polyetherimide), 208 PEKK [poly(ether ketone ketone)], 208 Perchloroethylene, 201 Periodic mesoporous organosilica (PMO) materials, 300 301 PES (polyethersulphone), 208

341

PET [poly(ethylene terephthalate)] LDH composites, 241 242 Phenol-formaldehyde (PF) resin, 80 81 Phosphates, 301 303 Phosphoric acid, in hydrolysis process, 70 71 Phosphoric acid anodizing (PAA), 202 203 Photoactive hybrid materials, 306 307 Photo-assisted methods, 121 125 Photodynamic therapy, 149 150 Photoelectrochemical polymerization, 122 125 Plasma electrolytic oxidation (PEO), 204 Plies, configuration of, 210 217 PLLA, 242 243, 248 249 Poly(1,8-diaminocarbazole), 108 Poly(2-hydroxyethyl methacrylate-coethylene dimethacrylate), 40 Poly(3,4-ethylenedioxythiophene) (PEDOT), 113, 116 117, 117f, 118f, 119f Poly(butylene succinate) (PBS), 83, 245 Poly(L-lactide) (PLA), 241 242 Poly(methyl methacrylate) (PMMA), 14 15, 278 280 Poly(PB-HEMA) matrix, dendronization of, 42f Poly(vinyl alcohol) (PVA), 264 chemical structure and properties of, 264 265 PVA/ADS-G composites, preparation of, 275f PVA blends/metal-oxide nanocomposites, 276 278 PVA/chitosan magnetic composite, 276f, 277 PVA hydrogel, 249 250, 275 PVA/metal-oxide nanocomposites, 265 266 PVA/nonmetal-oxide nanocomposites, 274 276 PVA/PVP BaZrO3 (PPB) proton exchange membranes (PVA:PVP 76:23), 282 283 PVA/SiO2 membrane, 273 PVA/TiO2 nanocomposite films, 267 268 structure formula for, 264f

342

Poly(vinyl pyrrolidone) (PVP), 264 chemical structure and properties of, 265 PVP blends/metal-oxide nanocomposites, 282 283 PVP/metal-oxide nanocomposites, 278 280 PVP/nonmetal-oxide nanocomposites, 280 282 PVP/SiO2/3-aminopropyltriethoxysilane (PVP/SiO2/APTES) composite nanofiber, 278 280 structure formula for, 265f Poly(vinylidene fluoride), 278 280 Polyacrylamide (PAA/NDZ), 14 15 Polyamidoamine (PAMAM) dendrimers, 39 Polyaniline (PANi), 16 17, 108 109, 114, 116, 118 PANi-ND fibers, 12 PAni/reduced graphene oxide (rGO) composites, 104 106 Polybutylene succinate (PBS), 142t Polycaprolactone (PCL), 73 74, 142 Polydimethylsiloxane (PDMS), 14 15 Polydimethylsiloxane-silica (PDMS-SiO2), 293 296 PDMS-coated silica nanoparticles, 294 296 dip-coating solution method, 295 296 thermal vapor deposition method, 295 Polydispersity, 69 Polyelectrolyte/CNC nanocomposites, 82 Polyester dendron, 40 Polyethylene (PE), 14 15 Polyethylenimine (PEI) layer, 52 Polyhydroxyalkanoates (PHAs), 142t, 147 148 Polyimide resins, 208 Polylactic acid (PLA), 142t Polymer blending, 282 Polymer composite layers, microstructure of, 222 Polymer films, surface modification of, 39f Polymer hybrid composites, 137 biomedical application of, 145 154 Polymer matrix incorporation nanofillers to, 248 mechanical features of, 2 3 physical and mechanical properties of, 207t

Index

Polymer matrix composites (PMCs), 164 165, 166f Polymer/cellulose nanocrystals nanocomposites gas barrier properties, 89 90 mechanical properties, 82 85 melt rheological properties, 86 89 thermal properties, 85 86 Polymer/metal oxide nanocomposites, 265 266 Polymer/nanodiamond composites, 1 nanodiamond reinforced hybrid, conducting properties of, 12 nanodiamond reinforced matrix, mechanical features of, 7 9 nanodiamond reinforced polymer, thermal properties of, 9 11 significance of, 12 17 high strength performance materials, 14 15 Li-ion batteries and electronic devices, 16 17 thermal resistance materials, 15 16 Polymeric materials, 1 2, 35 Polymeric ND nanocomposite, 16 17 Polymerization-filling technique (PFT), 301f Polypeptides, 142 143 Polypyrrole (PPy) films, 6 7, 111, 114, 116 117, 123 124 Polytetraflouroethylene, 14 15 Polythiophene, 6 7 Polyurethane-2-hydroxymethylmethacrylate (PU-PHEMA/ND) composites, 14 15 Polyurethane-based nanocomposites with CNCs, 81 Polyvalent interactions, 36 Postgrafting methods, 301 303 Potential step deposition method, 110t Potentiodynamic cycling deposition method, 110t Potentiostatic deposition method, 110t Povidone. See Poly(vinyl pyrrolidone) (PVP) PPS [poly(phenylene sulfide)], 208 Proton-transfer polymerization, for thermoresponsive hyperbranched polymers, 38 PS/CoAl LDH NCs, structural and thermal properties of, 242 243 PVC, thermal stability of, 246 247

Index

R Radical polymerization, 38, 292 293 Resorbable materials, 146 Reverse phase chromatography (RPC), 40 43 Rheology, 86 88 Ring-opening polymerization (ROP), 75

S Scaffold fabrication methods, for hybrid composites, 150t Scanning electron microscopy (SEM), 16 17, 240 241, 320 Secondary Ion Mass Spectrometry (SIMS), 178, 183f Self-condensing ring-opening methodology, 38 Self-condensing vinyl (co)polymerization, 38 Semicrystalline polymers, 86 Sequential methods, of organic/inorganic composites, 103 Sequestered approach, of organic/inorganic composites, 102 103 7xxx series aluminum alloys, 197 Si structures and symbols, 293, 294f Silica, 54, 144t, 147t Silicon, 4 5, 48 Silver nanoparticles, 27, 49 51 Silylferrocenyl- or amidoferrocenyl-ended dendron, 49 Single-wall CNT (SWNT), 296, 306 307 Single-walled carbon nanotubes (SWCNTs), 52, 53f, 169, 170f Smart/biomimetic materials. See Fourthgeneration biomaterials Sodium hypochlorite (NaOCl), 73 Sol gel method, 205 206 Solution blending, 297 298 Solution intercalation, 242 243 Solvent casting, for preparing CNCbased nanocomposites, 76 79 Solvent degreasing, 201 Sorbitan menstruate, 76 Spherical nanoparticles, 6 7

343

Stacking patterns of luffa/coir hybrid fiberreinforced epoxy composite laminates, 313 dynamic mechanical properties, 321 328 applied frequency, effect of variation of, 325 Cole Cole plots, 327 328 glass transition temperature, energy of activation for, 326 327 loss modulus with temperature, effect of stacking pattern on, 324 325 mechanical loss factor with temperature, effect of stacking pattern on, 323 324 storage modulus, effect of stacking pattern on, 321 323 experimental and characterization, 316 320 alkali treatment of luffa and coir fiber, 317 318 dynamic mechanical analysis, 319 fabrication of composites, 318 319 materials, 316 317 optical microscope, 319 scanning electron microscope (SEM) studies, 320 static mechanical analysis, 319 future perspectives, 330 mechanical properties, 320 morphological studies of fiber and tensile fractured hybrid composite surfaces, 328 329 Starch, 140 Structure property research on hybridreinforced polymer composites, 163 coal combustion products, 172 182 fly ash, 172 182 composite materials, 164 matrix material, 164 166 epoxy resins, 167 169 recent fillers as reinforcement materials, 169 171 carbon nanotubes, 169 171 fly ash, 171 milled carbon fiber, 171 Sulfonated poly(ether ether ketone), 253 Sulfonated polysulfone (SPSU) membranes, 252 Sulfuric acid anodizing (SAA), 202

344

Sulfuric acid hydrolysis, 68 69 Super hydrophobic films, 295 296 Surface acetylation of CNCs, 74 75 Surface modifying agents, 106t Surface treatment, 201 206 anodizing aluminum, 201 203 titanium surface treatment, 203 206 Surface-free energy (SFE), 203, 225 Sylilation, of cellulose nanocrystal surfaces, 74 75 Synergism, 36 Synthetic silicates, 144t T Template approach, 78 79 TEMPO [(2,2,6,6-tetramethylpiperidine-1oxyl) nitroxy radical]-mediated surface oxidation, 73 Tetraethylorthosilicate (TEOS), 305, 307f Thermal vapor deposition method, 295 Thermogravimetric analysis (TGA), 16 17, 240 241 Thermoplastic FML (TFML) composites, 197, 206, 208, 224 TiO2 nanoparticles, 52 TiO2/poly(3-hexylthiophene) (P3HT), 107, 123f Titanium and titanium alloys, 197 199 chemical composition of, 200t Titanium surface treatment, 203 206 Titanium zirconium, 147t Toluene, 78 79, 201 Tomography, 220 1,1,1-Trichloroethane, 201 Trichloroethylene, 201 Trinitrotoluene (TNT), 1 2

Index

Triphenylamine (TPA) moieties, 48 Tristacking, flexural properties of, 320 2xxx series aluminum alloys, 195 U UD laminate, microstructure of, 221f Ultradispersed detonation ND formation, 3 4 Ultradispersed diamond, 12 Ultrananocrystalline particles, 5 10-Undecyn-1-ol, 73 74 Unsaturated polyester (UP) resin, 248 V Varioptic Company, 305 Viscoelastic/dynamic mechanical properties of polymeric materials, 315 W Well-defined organic inorganic hybrid polymers, 292 293, 293f X X-ray diffraction, 16 17 X-ray diffractometry, 240 241 X-ray microtomography, 220 Y Young’s modulus, 14 15, 83, 249 250 Z Zirconia and urea-formaldehyde (ZrO2 UF), 40 ZnO cerium oxide (Ce2O3) NPs, 267 268