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This is a reprint of articles from the Special Issue published online in the open access journal Molecules (ISSN 1420-30
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The addition of nanoparticles to polymer composites has led to a new generation of composite materials with enhanced and
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Sustainable development has been gaining momentum in the modern world, and the use of nanomaterials in various applicati
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Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing discusses the state of the art, emerging challenges, prope
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Table of contents :
Content: Preface xiii 1 Hybrid Nanostructured Materials for Advanced Lithium Batteries 1Soumyadip Choudhury and Manfred Stamm 1.1 Introduction 1 1.2 Battery Requirements 4 1.3 Survey of Rechargeable Batteries 7 1.4 Advanced Materials for Electrodes 9 1.5 Future Battery Strategies 38 1.6 Limitations of Existing Strategies 59 1.7 Conclusions 62 Acknowledgments 63 References 63 2 High Performing Hybrid Nanomaterials for Supercapacitor Applications 79Sanjit Saha, Milan Jana and Tapas Kuila 2.1 Introduction 80 2.2 Scope of the Chapter 82 2.3 Characterization of Hybrid Nanomaterials 82 2.4 Hybrid Nanomaterials as Electrodes for Supercapacitor 91 2.5 Applications of Supercapacitor 130 2.6 Conclusions 134 References 135 3 Nanohybrid Materials in the Development of Solar Energy Applications 147Poulomi Roy 3.1 Introduction 147 3.2 Significance of Nanohybrid Materials 148 3.3 Synthetic Strategies 162 3.4 Application in Solar Energy Conversion 167 3.5 Summary 175 References 176 4 Hybrid Nanoadsorbents for Drinking Water Treatment: A Critical Review 199Ashok K. Gupta, Partha S. Ghosal and Brajesh K. Dubey 4.1 Introduction 199 4.2 Status and Health Effects of Different Pollutants 201 4.3 Removal Technologies 203 4.4 Hybrid Nanoadsorbent 208 4.5 Issues and Challenges 217 4.6 Conclusions 224 References 225 5 Advanced Nanostructured Materials in Electromagnetic Interference Shielding 241Suneel Kumar Srivastava and Vikas Mittal 5.1 Introduction 241 5.2 Theoretical Aspect of EMI Shielding 243 5.3 Experimental Methods in Measuring Shielding Effectiveness 247 5.4 Carbon Allotrope-Based Polymer Nanocomposites 248 Fillers-Based Polymer Nanocomposites 265 5.5 Intrinsically Conducting Polymer (ICP) Derived Nanocomposites 276 5.6 Summary 300 6 Preparation, Properties and the Application of Hybrid Nanomaterials in Sensing Environmental Pollutants 321R. Ajay Rakkesh, D. Durgalakshmi and S. Balakumar 6.1 Introduction 321 6.2 Hybrid Nanomaterials: Smart Material for Sensing Environmental Pollutants 323 6.3 Synthesis Methods of Hybrid Nanomaterials 326 6.4 Basic Mechanism of Gas Sensors Using Hybrid Nanomaterials 330 6.5 Hybrid Nanomaterials-Based Conductometric Gas Sensors for Environmental Monitoring 331 6.6 Conclusion 342 References 342 7 Development of Hybrid Fillers/Polymer Nanocomposites for Electronic Applications 349Mariatti Jaafar 7.1 Introduction 350 7.2 Factors Influencing the Properties of Filler/Polymer Composite 353 7.3 Hybridization of Fillers in Polymer Composites 355 7.4 Hybrid Fillers in Polymer Nanocomposites 358 7.5 Fabrication Methods of Hybrid Fillers/Polymer Composites 362 7.6 Applications of Hybrid Fillers/Polymer Composites 365 References 366 8 High Performance Hybrid Filler Reinforced Epoxy Nanocomposites 371Suman Chhetri, Tapas Kuila and Suneel Kumar Srivastava 8.1 Introduction 372 8.2 Reinforcing Fillers 373 8.3 Necessity of Hybrid Filler Systems 376 8.4 Epoxy Resin 379 8.5 Preparation of Hybrid Filler/Epoxy Nanocomposites 380 8.6 Characterization of Hybrid Filler/Epoxy Polymer Composites 381 8.7 Properties of the Hybrid Filler/Epoxy Nanocomposites 383 8.8 Summary and Future Prospect 408 References 413 9 Recent Developments in Elastomer/Hybrid Filler Nanocomposites 423Suneel Kumar Srivastava and Vikas Mittal 9.1 Introduction 423 9.2 Preparation Methods of Elastomer Nanocomposites 426 9.3 Hybrid Fillers in Elastomer Nanocomposites 427 9.4 Mechanical Properties of Hybrid Filler Incorporated Elastomer Nanocomposites 440 9.5 Dynamical Mechanical Thermal Analysis (DMA) of Elastomer Nanocomposites 452 9.6 Thermogravimetric Analysis (TGA) of Hybrid Filler Incorporated Elastomer Nanocomposites 464 9.7 Differential Scanning Calorimetric (DSC) Analysis of Hybrid Filler Incorporated Elastomer Nanocomposites 468 9.8 Electrical Conductivity of Hybrid Filler Incorporated Elastomer Nanocomposites 476 9.9 Thermal Conductivity of Hybrid Filler Incorporated Elastomer Nanocomposites 477 9.10 Dielectric Properties of Hybrid Filler Incorporated Elastomer Nanocomposits 477 9.11 Shape Memory Property of Hybrid Filler Incorporated Elastomer Nanocomposites 478 9.12 Summary 478 Acknowledgment 479 References 479
Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])
Hybrid Nanomaterials Advances in Energy, Environment and Polymer Nanocomposites
Suneel Kumar Srivastava and Vikas Mittal
This edition first published 2017 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2017 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
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Cover images: Pixabay.Com Cover design by: Russell Richardson Set in size of 11pt and Minion Pro by Exeter Premedia Services Private Ltd., Chennai, India
Printed in 10 9 8 7 6 5 4 3 2 1
Contents Preface 1
Hybrid Nanostructured Materials for Advanced Lithium Batteries Soumyadip Choudhury and Manfred Stamm 1.1 Introduction 1.2 Battery Requirements 1.2.1 Primary and Secondary Batteries 1.2.2 Battery Market 1.3 Survey of Rechargeable Batteries 1.4 Advanced Materials for Electrodes 1.4.1 Benefits and Limitations of Nanostructured Battery Materials 1.4.2 Hybrid Materials as Anodes 1.4.3 Hybrid Materials as Cathodes 1.5 Future Battery Strategies 1.5.1 Post Lithium-Ion Batteries 1.5.2 Lithium-Sulfur Batteries 1.5.3 Lithium-Air Batteries 188.8.131.52 Non-Aqueous Li-Air Battery 184.108.40.206 Aqueous Li-Air Battery 1.6 Limitations of Existing Strategies 1.7 Conclusions Acknowledgments References High Performing Hybrid Nanomaterials for Supercapacitor Applications Sanjit Saha, Milan Jana and Tapas Kuila 2.1 Introduction 2.2 Scope of the Chapter
xiii 1 1 4 5 6 7 9 9 10 31 37 37 38 53 53 58 59 62 63 63 79 80 82
vi Contents 2.3
Characterization of Hybrid Nanomaterials 2.3.1 Morphological Characterization 2.3.2 Structural Characterization 2.3.3 Electrical and Electrochemical Properties 2.4 Hybrid Nanomaterials as Electrodes for Supercapacitor 2.4.1 Graphene Hybrid 2.4.2 Nanostructured Metal Oxide-Sulphide Hybrids 2.4.3 Conducting Polymer Hybrid 2.4.4 Carbon Balck and Carbon Fiber Hybrid 2.4.5 Carbon Nanotube and Fullerene Hybrid 2.5 A Applications of Supercapacitor 2.5.1 Energy Storage Smart Grid 2.5.2 Cold Start and Transportation 2.5.3 Emergency Power 220.127.116.11 Windmills 18.104.22.168 Emergency Door 22.214.171.124 Digital Cameras 126.96.36.199 Wireless Systems and Burst-Mode Communications 188.8.131.52 Toys 2.5.4 Strategic Sector 2.5.5 UPS and Inverter 2.5.6 Others 2.6 Conclusions References 3
Nanohybrid Materials in the Development of Solar Energy Applications Poulomi Roy 3.1 Introduction 3.2 Significance of Nanohybrid Materials 3.2.1 Use of Nanostructured Materials 3.2.2 Materials and Band Gap Engineering 184.108.40.206 Binary Metal Chalcogenides 220.127.116.11 Binary Metal Oxides 3.2.3 T Types of Hybrid Materials 18.104.22.168 Core-Shell Nanoheterostructures 22.214.171.124 Carbon-Based Hybrid Nanostructure 126.96.36.199 Polymer-Based Hybrid Nanostructure 3.3 Synthetic Strategies 3.3.1 Hot-Injection Method
82 83 84 89 91 91 98 115 124 126 130 130 131 132 132 132 132 132 133 133 133 134 134 135 147 147 148 149 151 151 155 157 157 159 160 162 162
Contents vii 3.3.2 Hydrothermal/Solvothermal Method 3.3.3 Electrochemical Anodization 3.3.4 Chemical Vapor Deposition 3.4 A Application in Solar Energy Conversion 3.4.1 Photocatalysis 3.4.2 Photoelectrochemical Water Splitting 3.4.3 Photovoltaic Devices 188.8.131.52 Dye-Sensitized Solar Cells 184.108.40.206 Quantum Dot-Sensitized Solar Cells 220.127.116.11 Si-Based Solar Cells 3.5 Summary References 4
Hybrid Nanoadsorbents for Drinking Water Treatment: A Critical Review Ashok K. Gupta, Partha S. Ghosal and Brajesh K. Dubey 4.1 Introduction 4.2 Status and Health Effects of Different Pollutants 4.3 Removal Technologies 4.4 Hybrid Nanoadsorbent 4.4.1 Synthesis of Material 4.4.2 A Application of Hybrid Nanoadsorbents 18.104.22.168 Arsenic 22.214.171.124 Fluoride 126.96.36.199 Heavy Metals 4.5 Issues and Challenges 4.6 Conclusions References Advanced Nanostructured Materials in Electromagnetic Interference Shielding Suneel Kumar Srivastava and Vikas Mittal 5.1 Introduction 5.2 Theoretical Aspect of EMI Shielding 5.3 Experimental Methods in Measuring Shielding Effectiveness 5.4 Carbon Allotrope-Based Polymer Nanocomposites 5.4.1 Carbon Fiber-Filled Polymer Nanocomposites 5.4.2 CNT-Filled Polymer Nanocomposites 5.4.3 Graphene and Graphene Oxide Fillers-Based Polymer Nanocomposites
164 165 166 167 167 169 171 171 172 173 175 176 199 199 201 203 208 208 208 208 216 214 223 224 225 241 241 243 247 248 249 252 265
Intrinsically Conducting Polymer (ICP) Derived Nanocomposites 5.5.1 P PANI in EMI Shielding Applications 5.5.2 PPy in EMI Shielding Applications 5.5.3 Core-Shell Morphology in EMI Shielding 5.6 Summary Acknowledgement References 6
Preparation, Properties and the Application of Hybrid Nanomaterials in Sensing Environmental Pollutants R. Ajay Rakkesh, D. Durgalakshmi and S. Balakumar 6.1 Introduction 6.2 Hybrid Nanomaterials: Smart Material for Sensing Environmental Pollutants 6.3 Synthesis Methods of Hybrid Nanomaterials 6.3.1 Sol-Gel Method 6.3.2 Hydrothermal Methods 6.3.3 Layer-by-Layer Deposition Method 6.3.4 T Template-Assisted Synthesis of Hybrid Materials 6.3.5 Physical Vapor Deposition 6.3.6 Gas-Sensing Principle of Hybrid Nanomaterials 6.4 Basic Mechanism of Gas Sensors Using Hybrid Nanomaterials 6.5 Hybrid Nanomaterials-Based Conductometric Gas Sensors for Environmental Monitoring 6.5.1 Hybrid Nanomaterials for Volatile Organic Components 6.5.2 Hybrid Nanomaterials for Ammonia Detection 6.5.3 Hybrid Nanomaterials for Hydrogen Detection 6.5.4 Hybrid Nanomaterials for Nitrous Oxide Detection 6.6 Conclusion References Development of Hybrid Fillers/Polymer Nanocomposites for Electronic Applications Mariatti Jaafar 7.1 Introduction 7.2 Factors Influencing the Properties of Filler/Polymer Composite 7.3 Hybridization of Fillers in Polymer Composites
276 281 287 289 300 300 300 321 321 323 326 326 327 327 328 329 330 330 331 332 334 337 339 342 342 349 350 353 355
Contents ix 7.4 7.5
Hybrid Fillers in Polymer Nanocomposites Fabrication Methods of Hybrid Fillers/Polymer Composites 7.6 A Applications of Hybrid Fillers/Polymer Composites References 8
High Performance Hybrid Filler Reinforced Epoxy Nanocomposites Suman Chhetri, Tapas Kuila and Suneel Kumar Srivastava 8.1 Introduction 8.2 Reinforcing Fillers 8.3 Necessity of Hybrid Filler Systems 8.4 Epoxy Resin 8.5 Preparation of Hybrid Filler/Epoxy Nanocomposites 8.6 Characterization of Hybrid Filler/Epoxy Polymer Composites 8.7 Properties of the Hybrid Filler/Epoxy Nanocomposites 8.7.1 Hybrid Fillers Based on CNT, GNP and GO 8.7.2 Hybrid Fillers Based on CB, CF, CNT and Graphene 8.7.3 Hybrid Fillers Based on Clay, CB, CNT and Glass Fibers 8.7.4 Hybrid Fillers Based on Ceramic Powder, CNT and GNP 8.7.5 Hybrid Fillers Based on Silica Particle Modified Graphene and CNTs 8.7.6 Hybrid Fillers Based on LDHs, Organohydroxide, MoS2, and Graphene 8.7.7 Hybrid Fillers Based on Silicate and Liquid Rubber 8.8 Summary and Future Prospect References Recent Developments in Elastomer/Hybrid Filler Nanocomposites Suneel Kumar Srivastava and Vikas Mittal 9.1 Introduction 9.2 Preparation Methods of Elastomer Nanocomposites 9.2.1 In-situ Polymerization 9.2.2 Solution Mixing 9.2.3 Melt Intercalation Method
358 362 365 366 371 372 373 376 379 380 381 383 383 386 390 392 397 405 406 408 413 423 423 426 426 427 427
x Contents 9.3
Hybrid Fillers in Elastomer Nanocomposites 9.3.1 Dispersion of Hybrid Fillers in Elastomer Nanocomposites 9.3.2 Dispersion of Hybrid Fillers in PU Nanocomposites 9.3.3 Dispersion of Hybrid Fillers in SR Nanocomposites 9.3.4 Dispersion of Hybrid Fillers in NR Nanocomposites 9.3.5 Dispersion of Hybrid Fillers in SBR, NBR, EPDM and EVA Nanocomposites Mechanical Properties of Hybrid Filler Incorporated Elastomer Nanocomposites 9.4.1 Mechanical Properties of Hybrid Filler Incorporated PU Nanocomposites 9.4.2 Mechanical Properties of Hybrid Filler Incorporated SR Nanocomposites 9.4.3 Mechanical Properties of Hybrid Filler Incorporated NR Nanocomposites 9.4.4 Mechanical Properties of Hybrid Filler Incorporated SBR, NBR, EPDM and EVA Nanocomposites Dynamical Mechanical Analysis (DMA) of Elastomer Nanocomposites 9.5.1 DMA of Hybrid Filler Incorporated PU Nanocomposites 9.5.2 DMA of Hybrid Filler Incorporated SR Nanocomposites 9.5.3 DMA of Hybrid Filler Incorporated NR Nanocomposites 9.5.4 DMA of Hybrid Filler Incorporated SBR, NBR, EPDM Nanocomposites Thermogravimetric Analysis (TGA) of Hybrid Filler Incorporated Elastomer Nanocomposites 9.6.1 TGA of Hybrid Filler Incorporated PU Nanocomposites 9.6.2 TGA of Hybrid Filler Incorporated SR Nanocomposites 9.6.3 TGA of Hybrid Filler Incorporated NR Nanocomposites
427 427 427 434 435 437 440 440 446 450
450 452 452 458 463 463 464 464 466 467
Contents xi 9.6.4
TGA of Hybrid Filler Incorporated SBR, NBR, EPDM and EVA Nanocomposites 9.7 Differential Scanning Calorimetric (DSC) Analysis of Hybrid Filler Incorporated Elastomer Nanocomposites 9.7.1 DSC of Hybrid Filler Incorporated PU Nanocomposites 9.7.2 DSC of Hybrid Filler Incorporated SR Nanocomposites 9.7.3 DSC of Hybrid Filler Incorporated NR Nanocomposites 9.7.4 DSC of Hybrid Filler Incorporated SBR and NBR Nanocomposites 9.8 Electrical Conductivity of Hybrid Filler Incorporated Elastomer Nanocomposites 9.9 Thermal Conductivity of Hybrid Filler Incorporated Elastomer Nanocomposites 9.10 Dielectric Properties of Hybrid Filler Incorporated Elastomer Nanocomposits 9.11 Shape Memory Property of Hybrid Filler Incorporated Elastomer Nanocomposites 9.12 Summary Acknowledgments References Index
468 468 468 472 474 475 476 477 477 478 478 479 479 491
Preface A hybrid material is defined as a material composed of an intimate mixture of inorganic components, organic components, or both types of components. In this regard, 3D hybrid materials have been receiving continuous attention. They can be prepared by hybridizing 1D (MWCNTs, CNF, etc.) and 2D (molybdenum disulfide, titanium disulfide, tungsten disulfide, Na-montmorillonite, layered double hydroxide, graphene, etc.) materials. In addition, formation of hybrid materials has also been reported considering other combinations. These different types of hybrid materials have currently been garnering tremendous attention for their possible use in developing materials for efficient energy harvesting. Nanostructured hybrid materials have also seen many significant advances in providing pollutant-free drinking water, sensing of environmental pollutants, energy storage and conversion. In addition, they have also been used in shielding material to interfere with electromagnetic waves originating from different electronic instruments and appliances, which deteriorate their performance and adversely affect human health. Ever since it was first reported that the work done by a group of researchers at Toyota dramatically improved the properties of polyamide 6 by incorporating modified low content of montmorillonite, immense interest has been generated in developing such high performing polymer nanocomposites for applications in the automotive, aerospace and construction sectors, among others. However, the aggregation of many types of fillers, such as clay, LDH, CNT, graphene, etc., remains a major barrier to their development. Recently, this problem has been overcome by the fabrication and application of 3D hybrid nanomaterials as nanofillers in a variety of polymers. More importantly, these 3D hybrid-filled polymer nanocomposites exhibit synergistic properties, unlike individual phases or their microcomposites alone. Therefore, the development of simple, convenient and efficient methods for the fabrication of hybrid nanomaterials and the realization of their applications in energy, environment and polymer nanocomposites remain a challenging task. xiii
xiv Preface In view of this, the chapters of this book entitled Hybrid Nanomaterials: Advances in Energy, Environment and Polymer Nanocomposites, introduce readers to the following emerging research topics: Chapter 1: Hybrid nanostructured materials for development of advanced lithium batteries Chapter 2: High performing hybrid nanomaterials for supercapacitor applications Chapter 3: Nanohybrid materials in the development of solar energy applications Chapter 4: Application of hybrid nanomaterials in water purification Chapter 5: Advanced nanostructured materials in electromagnetic interference shielding Chapter 6: Preparation, properties and application of hybrid nanomaterials in sensing of environmental pollutants Chapter 7: Development of hybrid fillers/polymer nanocomposites for electronic applications Chapter 8: High performance hybrid filler reinforced epoxy nanocomposites Chapter 9: Recent developments in elastomer/hybrid filler nanocomposites It is expected that these simple, attractive, versatile, technological developments in hybrid materials and their applications will provide a better understanding of the currently ongoing research in related fields. Finally, support from Mr. Martin Scrivener, publisher and our family members are gratefully acknowledged. Suneel Kumar Srivastava and Vikas Mittal March 2017
1 Hybrid Nanostructured Materials for Advanced Lithium Batteries Soumyadip Choudhury* and Manfred Stamm* Leibniz Institute of Polymer Research, Dresden, Dresden, Germany
Abstract Efficient energy storage devices are progressively gaining importance due to the limited reserve of fossil fuels and advancement of alternative energy sources. Lithium-based battery systems have acquired a leading position in electrochemical energy storage and have become an important element in the replacement of conventional gasoline-driven vehicles with electrically driven ones. State-of-the-art lithium-ion batteries still cannot fulfill capacity requirements, and lithium-sulfur and lithium-air batteries might be promising for the high-energy-density batteries of the future. In this chapter, a brief overview of common lithium-ion batteries as well as of advanced battery systems is provided, including principles of operation, methods of fabrication utilizing nanohybrids for improved performance, and some aspects for further improvements. Keywords: Nanostructured materials, hybrid materials, lithium-ion batteries, lithium-sulfur batteries, lithium-air batteries
In our society, the worldwide demand for electric energy consumption is progressively increasing day by day, and energy is being exploited in everything from mobile electronics to portable electronic gadgets and, ultimately, electrically driven vehicles. This increasing demand has caused a rapid rise of both primary and secondary batteries. In the 21st century, the steep growth of energy demand and environmental concerns associated *Corresponding authors: [email protected]; [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (1–78) © 2017 Scrivener Publishing LLC
2 Hybrid Nanomaterials with global warming, and a limited reserve of fossil fuels, has brought a serious note to the work of politicians and researchers in finding alternatives to the sole dependency on fossil fuels. Energy resources, such as hydroelectric, nuclear, and renewable resources like sun, wind, biological and tidal powers, are competing candidates as alternatives to fossil fuels. Hydroelectric power is a clean source of energy that requires storage of the potential energy of water in dams in suitable regions which are not available everywhere. Nuclear power, although used in different countries at large scale, causes radioactive hazards associated with long-term storage of radioactive wastes, and safety aspects are primary hindrances to be taken care of, especially in the wake of the Fukushima disaster. Although renewable sources offer clean energy, the intermittent nature of sources like the sun, wind or tidal waves practically restricts the continuous production of energy from these sources . In that case, the renewable energies have to be stored when they are available and supplied on demand. These systems can only be operated reasonably with powerful energy storage units, like thermal or chemical storage units including high-energy batteries, to strategically balance source variability and power requirement. The accumulators (or secondary or rechargeable batteries) can be exploited as a component of energy storage system for giant electric grids, but mostly for local energy storage for smart grids in localized communities; in addition, they are used in consumer electronics to a large extent and are essential for the progress of e-mobility. Nowadays, the rechargeable batteries find applications in laptops, cell phones, medical implants, power tools, toys and many different portable electronic gadgets. In recent years, there has been a strong drive towards research and development to replace gasoline-driven cars with e-cars with rechargeable batteries. So, secondary batteries are now being exploited in high-end applications; for example, in transportation sectors, defense, or aerospace applications as well. State-ofthe-art lithium-ion battery technology suffices for batteries for electronic gadgets, but to broaden the prospects of batteries in transportation sectors, a dramatic boost in the current battery technology has to be executed . In particular, to bring the global electrified transportation venture to reality, development of cheap, environmentally friendly, safe, and highenergy-density batteries is the challenge for the near future. However, the state-of-the-art Li-ion batteries presently existing in the market are limited to the energy density of 150 Wh/kg which is, taking weight limitations into account, below the performance of the gasoline-driven vehicles (Figure 1.1). Most advanced e-cars like Tesla Model S have now extended the range with a big battery pack to 500 km. Significant uplift of energy densities by a factor of 2–5 are required to reach the desired performance of
Specific energy (Wh kg–1)
Hybrid Nanostructured Materials
800 >400 km
>225 km >200 km
50 km 80 km 100 km Pb–acid Ni–Cd Ni–MH Li–ion Future Zn–air Li–S Li–ion
Price (US$ kW h–1) 200
100 μm for multicrystalline silicon solar cell . However, experimental study showed much deviation due to the electron-hole recombinations based upon their synthesis procedures and crystallinity . In DSSC, experimentally sintered TiO2 nanoparticle layer of 10 μm thickness or TiO2 nanotube layers with 20 μm thickness were found to be the optimum thickness for achieving high solar cell efficiency [4, 20, 21]. A comparative study between nanoparticulate layer and 1D nanostructures of TiO2 in DSSC showed that the diffusion coefficient (De) of electrons increases while the electron recombination lifetime ( r) decreases with an increase in the particle size up to 32 nm . It is believed that the r value usually is influenced by the surface area. The investigations suggest that the nanotube and nanoparticle films showed similar transport times, whereas recombination rate was 10 times slower in the nanotube films, and accounts for significantly enhanced charge collection efficiency in the case of nanotube photoelectrodes . Further, light scattering effect in nanostructured layer also plays a very important role. The light absorption ability is enhanced due to the increased optical length and optical confinement . In the absence of the scattering effect, the light transmits through the nanostructured film and the absorbance is considered as abs × d, where abs and d are the absorption coefficient and film thickness, respectively. While considering scattering effect, the photon travels longer than the film thickness d showing the absorbance greater than abs × d. It is also reported that the self-organized 1D nanostructures on the conductive substrate exhibit higher scattering effect in comparison to 0D nanostructure and thereby lead to better performance as photoanode [29, 30].
Nanohybrid Materials and Solar Energy Applications 151
Materials and Band Gap Engineering
The choice of material to harvest maximum possible light lies in the optical property of the materials. The band gap value of semiconductors determines absorption ability of a material with either narrow or wide band gap values being extensively used for various solar applications. The absorption ability of material can further be modified with proper band gap engineering by doping or fabricating heterostructured nanomaterials combining more than one semiconductor. The present section describes some important solar effective inorganic materials used in various applications.
188.8.131.52 Binary Metal Chalcogenides 184.108.40.206.1 Iron Sulfides Iron sulphide exists in different forms, e.g., FeS2, Fe2S3, Fe3S4, Fe7S8, Fe1-xS, Fe1+xS and FeS . Among them, FeS2 with a band gap value of 0.9 eV is considered as a potential material for various solar applications due to its abundant and nontoxic nature. Furthermore, the material exhibits an exceptionally large optical absorption coefficient in the visible region, which allows almost 90% light absorption at only 40 nm thickness as potential light absorber [31–33]. Considering the economy as well as performance in solar cells, about 4% efficiency with pyrite-based solar cells can be comparable with Si-solar cell with 20% efficiency . However, till now only 2.8–3% efficiency has been achieved in a photoelectrochemical cell . The limitation lies in the production of high dark current leading to very small open circuit potential [33–35]. The presence of phase impurities and large number of surface defect states due to the sulphur segregation at the surfaces are believed to be responsible for such behavior . However, Keszler and his group  ruled out the effects of phase impurities and stated that surface S-vacancies were the responsible factor. In order to overcome these problems several chemical approaches were attempted to prepare nanostructured iron sulphides. Bi et al.  synthesized highly stable, photosensitive and phase pure iron pyrite nanocrystals using surfactant-assisted hot-injection method. The nanocrystals show high carrier mobility leading to strong photoconductivity at room temperature. According to their investigations, trioctylphosphine oxide (TOPO) as surfactant stabilizes nanocrystral surfaces by passivating both surface Fe and S sites. Further, phase pure, single crystal iron pyrite nanocrystal ink was prepared by hot-injection method followed by sintering process in sulphur vapor as reported by Puthussery et al. . The suitable sintering process under sulphur vapor not only increases the average
152 Hybrid Nanomaterials grain size, increasing the carrier diffusion length, but also reduces possible sulphur deficiency at the surface, resulting in promising photovoltaic performance. Macpherson and Stoldt  synthesized phase pure iron pyrite nanocubes with controlled size and shape and (100) exposed facets by hot-injection method. They observed considerable contrast in the absorption spectroscopies of irregular nanocrystals and nanocubes with reduced size of 98%
Pb(II) – 90% Cr(VI) – 75%
218 Hybrid Nanomaterials
Micro-channel-embedded metal– carbon–polymer nanocomposite
Sepiolite-supported nanoscale zero-valent iron
Catecholamine-coated maghemite nanoparticles
Poly(itaconic acid/methacrylic acid)-grafted-nanocellulose/ nano-bentonite composite
2-hydro-xyethylammonium sulfonate immobilized on g-Fe2O3 nanoparticles
Schiff base ligand-based nanocomposite adsorbent
Carboxymethyl-β-cyclodextrin conjugated magnetic nanoparticles
Cu(II) – 162.48 mg/g Ni(II) – 94.43 mg/g
Cr(VI) – 43.86mg/g Pb(II) – 44.05mg/g
Hybrid Nanoadsorbents for Drinking Water Treatment 219
Type of nanoparticle
crosslinking chitosan/rectorite nanohybrid composite
Iron oxide nanoparticles-immobilized-sand
Xanthated nanobanana cellulose
Poly(acrylamide-co-sodium acrylate) incorporated nanohydrous manganese oxide
Amino-functionalized Fe3O4@SiO2 magnetic nanoadsorbent
Table 4.7 Cont.
Cu(II) Cd(II) Pb(II)
Cu(II) Cd(II) Pb(II)
Cu(II) Cd(II) Ni(II)
Cu(II) – 6.3 mmol/g Cd(II) – 4.0 mmol/g Pb(II) – 2.1 mmol/g
Cd(II) – 49.0 mg/g Ni(II) – 13.1 mg/g
Cu(II) – 1.26 mg/g Cd(II) – 0.53 mg/g Pb(II) – 2.09 mg/g
Cu(II) – 20.49 mg/g Cd(II) – 16.53 mg/g Ni(II) – 13.32 mg/g
220 Hybrid Nanomaterials
Nanoscaled zero valent iron/graphene composite
Multi-cyanoguanidine modified magnetic chitosan
Grapheme oxide modified with 2-pyridine-carboxaldehydethiosemicarbazone
Mercapto-functionalized nanomagnetic Fe3O4 polymers
poly(1-vinylimidazole)-grafted Fe3O4@SiO2 magnetic nanoparticles
polyaniline/reduced graphene oxide nanocomposite
Nano-TiO2 immobilized on diatomite
Hg(II) – 1433.3–1633.3 μmol/g Cd(II) – 433.3–833.3 μmol/g Pb(II) – 166.7–266.7 μmol/g
Hg(II) Cd(II) Pb(II)
Hybrid Nanoadsorbents for Drinking Water Treatment 221
Type of nanoparticle
Strontium hydroxyapatite embedding ferroferric oxide nanocomposite
Thiol-functionalized mesoporous silica-coated magnetite nanoparticles
CdS/graphene and ZnS/graphene nanocomposites
Table 4.7 Cont.
Cd(II) – 3.62 mg/g Pb(II) – 3.10 mg/g
Cd(II) – 97% Pb(II) – 99%
222 Hybrid Nanomaterials
Hybrid Nanoadsorbents for Drinking Water Treatment 223
Issues and Challenges
The inclination of using nanoparticles in different forms for the removal of water pollutants lies in the pronounced change in material property at nanoscale. The same material exhibits different properties at nanolevel as compared to its bulk counterpart such as physical, chemical, mechanical, magnetic, electrical properties, etc. In the case of adsorption, the nanoscale materials drastically increase catalytic properties and reaction rates . Moreover, the superparamagnetic behavior was observed below some critical crystalline sizes. The high surface reactivity and high surface area to mass ratio are the most favorable properties of a nanomaterial, leading to a higher adsorption potential compared to macro-sized material . However, some important issues are associated with nanoadsorbents such as separation of nanomaterial, regeneration, and subsequent reuse of adsorbent. The common methods of separation are application of external magnetic field for magnetic nanomaterial, filtration (membrane filters), centrifugation, etc. [8, 94, 165, 200, 201]. These additional methods of separation of adsorbent reduce the technological feasibility of the pollutant removal by adsorption process and increase the overall cost of the system. Consequently, this gave birth to the concept of developing hybrid nanomaterial. Magnetic nanohybrid material is often chosen to develop the magnetic properties in any adsorbent . Apart from that, immobilization of nanomaterial on a suitable support is of utter importance for ease of separation . In addition, the nanomaterials in fine powder form have very low hydraulic conductivity when applied to fixed-bed column studies . In this context, the composite materials in a stable form, such as beads, resin, etc., can be used by introducing larger hydraulic conductivity. Some cheap, porous, and easily available materials like sand, zeolite, activated carbon, activated alumina, ceramic materials, and organic polymers are preferred by researchers for this purpose [134, 135, 156, 161, 164]. However, the major challenge associated with the hybrid nanomaterial is the stability of the material in its nano form. The bonding with support material or stability of the material itself must be ascertained. Furthermore, the health issues related to nanomaterial are still unresolved. A number of studies have highlighted the toxic effect of nanomaterial [202–206]. Hence, the presence of nanoadsorbent in the treated water will create secondary pollution, which in turn significantly reduces the applicability of the adsorption process. In most of the literature, the adsorption studies are conducted at batch scale. There are still comparatively fewer fixed-bed studies, which hinders
224 Hybrid Nanomaterials the acceptability of material in real-life application. Moreover, the synthetic water in the laboratory does not produce the flavor of real-time water, as the characteristic of real-time water may possess a different and more complex scenario. The research should be performed with real-time water as a field-scale column-based study to confirm the applicability of material. There has been much less work reported in this area, which should be taken up in upcoming research projects. Many complex nanomaterials exhibited noticeable results in lab-scale studies. However, the preparation processes adopted in those studies are complicated. The practicability of the production processes of those materials and their subsequent use for pollutant removal in large-scale is uncertain and should be analyzed. The assessment of technical and financial aspects of the production of hybrid nanoadsorbent is a prominent gap area in this field. Overall, the promising future in this direction will depend on the future research to address the challenges identified in this chapter.
In this chapter, the performance of hybrid nanomaterial on the removal of various water pollutants based on different research papers is summarized. This review revealed that the performance of adsorbent in nanoscale is better than that on the macro level. This is mainly attributed to the fact that the drastic change in material property at nanoscale enhanced the adsorption potential of nanoadsorbent. However, several shortcomings of nanoadsorbents, such as separation of material after sorption, application in fixed-bed study, etc., are experienced in research. The recent trend toward the use of hybrid nanomaterial stems from this gap area. The hybrid nanoadsorbent, prepared with two or more metal ions or in the form of a composite material, enhances the hydraulic conductivity and ease of treatment process. Hybrid nanoadsorbent exhibited promising scavenging potential for arsenic, fluoride, and also for several heavy metals from drinking water. Iron- and aluminum-based adsorbents are preferred in many studies. However, the issues related to stability of material, regeneration capacity and toxicity associated with nanomaterial are still unresolved. The research should also be streamlined towards field-scale applicability to ensure technical feasibility and economic viability. This chapter critically analyzed the application of hybrid nanoadsorbent in water treatment with its advantages and disadvantages. The information highlighted in this review will be instrumental in motivating more research on the required
Hybrid Nanoadsorbents for Drinking Water Treatment 225 technical advancements in this area towards the future of hybrid nanoadsorbent in water treatment.
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5 Advanced Nanostructured Materials in Electromagnetic Interference Shielding Suneel Kumar Srivastava1* and Vikas Mittal2 1
Department of Chemistry, Indian Institute of Technology, Kharagpur, India Department of Chemical Engineering, Petroleum Institute, Abu Dhabi, United Arab Emirates
Abstract Conventionally, metal as shielding materials was an early choice in reflecting electromagnetic waves. However, their applications are limited by heavier weight, cost, processing difficulties, reproducibility and their aerial oxidation/corrosion. Therefore, a considerable amount of research has been focused on developing suitable EMI shielding materials either by controlling reflection or absorption. In view of this, developments on carbon nanostructure-based composite materials have been receiving more attention for electromagnetic interference (EMI) shielding. The fabrication of nanocomposites is especially found to be very promising as an effective EMI shielding material owing to their high shielding effectiveness, light weight, low cost, and easy processability. In view of this, this article attempts to review carbon-based materials, such as carbon black, graphite, carbon nanotube (CNT), carbon nanofiber (CNF) and graphene, either in pure form as well as nanocomposites fabricated by incorporating an individual or hybrid filler into polymer matrices and intrinsic conducting polymers. Keywords: Electromagnetic shielding, carbon nanotubes, graphene, conducting polymers, polymer nanocomposites
The interference of electromagnetic waves (EM) is an undesirable manifestation of radiation of deferent frequencies emitted due to extensive use of *Corresponding author: [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (241–320) © 2017 Scrivener Publishing LLC
242 Hybrid Nanomaterials electronic and wireless electronic devices such as mobile phones, computers, TVs, radios, etc. [1–8]. The interference of these radiations with different electronic instruments and appliances deteriorates their performance in terms of speed and secrecy, resulting in the loss of data storage, revenue, time and effort. In addition, it has an adverse effect on human health under certain circumstances leading to many diseases, such as leukemia, miscarriages and brain cancer, and needs an immediate solution. Therefore, it is desirable to isolate these devices/instruments and prevent electromagnetic interference (EMI). In view of this, blocking of the radiation by suitable means could be one of the best approaches to successfully carry out the signal processing and transportation without any interference. Therefore, extensive research has been in progress in developing suitable EMI shielding materials to overcome this either by controlling reflection or absorption. Metals (Al, Cu, Sn, Zn, Fe, Ni, Ag, etc.) are suitable electromagnetic shielding material for reflecting EM waves due to their free electrons, accounting for their high electrical conductivity and shallow skin depth [9–11]. In addition, some other options exist like: sprayed, painted or electrolessly applied conducting coating on a supporting material (plastic) or incorporation of metal (stainless steel) powder or fibers as conducting filler in a plastic matrix for EMI shielding . This prevents interference from affecting coated electronic equipment by absorbing or deflecting incoming EMI. However, heavier weight, cost, processing difficulties, reproducibility and susceptibility of metals towards aerial oxidation/corrosion often leads their inferior mechanical/electrical properties. Therefore, a large number of carbon-based materials, such as carbon black, graphite, carbon nanotube (CNT), carbon nanofiber (CNF), graphene and intrinsic conducting polymers (ICP), have been employed as filler in polymer in order to explore its EMI shielding [1–3]. However, poor dispersibility and high percolation threshold of carbon black and graphite account for their inferior EMI shielding performance [12–14]. On the contrary, CNT, CNF and graphene could be dispersed in various media [15–18], but their purification, functionalization and cost are not desirable as EMI shielding materials [16–19]. Figure 5.1 shows a pie chart of five carbon materials, carbon black (CB), carbon fibers (CFs), carbon nanotubes (CNTs), graphite and graphene, from 2001 to 2014 . It is evident that carbon nanotubes and graphene possess great promise in EMI shielding and EM absorption. Alternatively, conducting polymers could carry out reflection and absorption of the EM wave simultaneously, providing a significant advantage over metals. For that reason, intrinsically conducting polymers, especially polyaniline, polypyrrole and polythiophene, etc., have been investigated as suitable alternatives in EMI shielding application. This is ascribed to their simple
Advanced Nanostructured Materials in EMI Shielding 243 50
CB CF CNT Graphite Graphene
Figure 5.1 Publication numbers of five carbon materials: carbon black (CB), carbon fibers (CFs), carbon nanotubes (CNTs), graphite and graphene, from 2001 to 2014. (Reprinted from  with permission from The Royal Society of Chemistry).
processability, light weight, good environmental stability, tunable conductivity, and permittivity, etc. [9, 20, 22]. They exhibit mechanical properties of polymer and electronic/optical characteristics of a metal simultaneously. In view of this, this chapter review deals with various theoretical aspects and experimental methods of EMI shielding followed by the response of conductive fillers in corporate polymers in electromagnetic interference shielding. In view of all this, this chapter reviews the EMI shielding performance of carbon-based materials either in pure form or as nanocomposites fabricated by incorporating an individual or hybrid filler into polymer matrices and intrinsic conducting polymers
Theoretical Aspect of EMI Shielding
During its propagation through a medium, electromagnetic wave is hindered by the bulk materials. So, proper measurement technique of the shielding of EM wave is also necessary to make an assessment of the shielding material to be prescribed in a specific purpose of use [1–6]. Generally, it is measured by calculating the magnitude of reduced power or field strength of incident EM wave caused by the shielding material. The nature of the shielding material, thickness, distance from the source of EM wave (r) and frequency of the EM wave are some of the most important
244 Hybrid Nanomaterials parameters accounting for effective shielding. The distance (r) between the source of the EM wave and the shielding material (observation point) could be divided into near field, far field and transition regions. When electromagnetic radiations are incident on a material, their barrier towards the propagation could be attributed to the contribution originating from reflection, absorption and multiple internal reflections. The shielding mechanism of EM wave depending on wave nature and characteristics of materials is schematically depicted in Figure 5.2. Reflection generally occurs due to the difference in impedance of the corresponding layers (air and interacting material). It takes place in highly conducting materials (metals, carbon materials) due to the presence of mobile charge carriers, i.e., electrons and holes. EM waves also undergo multiple reflections (foamed materials and composite) between the opposite layers (internal) of a shielding material. On the other hand, absorption involves interaction between the electric/magnetic dipoles of the shielding materials with the electric/magnetic vector of the electromagnetic radiation incident on the materials. Generally, high dielectric constant materials (ZnO, SiO2, TiO2, BaTiO3, etc.) and high permeability magnetic material (Ni, Co, Fe and their carbonyl compounds, etc.) account for absorption of EM waves due to the presence of electric and magnetic dipoles respectively . The interaction of electric dipole (shielding material) and electric field (EM wave) results in generation of heat. Further, EM (thickness higher than skin depth) passing through a shielding nonconductive material (polyester, polypropylene glass) exhibits high transmission. Multifaceted aspects of the electrical Inc id wa ent ve
d ecte Refl ve wa
Tra nsm wa itted ve
rnal Inte tion c e refl
Figure 5.2 Mechanism of EMI Shielding: A schematic representation (Modified). (Adapted with permission from ; Copyright © Elsevier Ltd.).
Advanced Nanostructured Materials in EMI Shielding 245 properties and electromagnetic shielding response of electrically conducting materials based on an understanding of the shielding theory governing theoretical equations and relevant measurement techniques prerequisites are described below. Electromagnetic radiation is composed of an electric field and a magnetic field. Electromagnetic radiation is composed of two components: magnetic field (H-field) and electric field (E-field) oriented at right angles to each other. In EMI shielding, two regions, namely: near fielding and far shielding regions, are involved. When the distance between source of radia/ ( wavelength of source), it is considtion and shield is larger than /2 ered as far-field shielding. When the distance between source of radiation / ( wavelength of source), it is called near-field and shield is less than /2 shielding and theory based on contribution of electric and magnetic dipoles is used in EMI shielding . Shielding efficiency (SE) of a material acting as a shield corresponds to the ratio of the magnitude of the incident electric or magnetic field without shielding to electric or magnetic field with shielding.The electromagnetic interference shielding effectiveness (SET) of conducting material is strongly dependent on electrical conductivity, dielectric constant, thickness of materials, shielding efficiency, frequency, affinity for conducting fillers in the case of composites, etc. . Total SE can be obtained by summing up the individual contributions of reflection (R), absorption (A) and multiple reflections (M) in dB . EMI shielding effectiveness (SET) can be expressed in decibels (dB) as follow :
SET (dB) SER
SE A SE M
where SER, SE EA and SE EM correspond to contributions due to reflection, absorption and multiple internal reflections respectively. SET can also be expressed mathematically as [19, 20, 25]:
where PI, EI, HI are incident power, electric field and magnetic field intensities respectively; whereas PT, ET, HT are transmitted power, electric field and magnetic field intensities respectively. When SE EA > 10 dB, the magnitude of SEM can be ignored and total EMI shielding effectiveness can be expressed as:
SET (dB) SER
246 Hybrid Nanomaterials EA can be written as : SER and SE
SER (dB) 10 log10 (1 R)
SE A (dB) 10 log10 (1 Aeff ) 10 log10
The impedance mismatch between the free space and shielding material accounts for reflection loss (SER). Its magnitude under plane wave (far-field conditions) can be expressed as:
where s and refer to total conductivity, angular frequency, relative permittivity and relative permeability referred to as free space respectively. This suggests that high SER could be achieved for a material exhibiting high s and low or . Shielding due to absorption (SE EA) in dB is related to thickness (t) and skin depth ( ) of shielding material as :
SE A (dB)
20 log10 e
where (defined as the distance required by wave to be attenuated to 1/e of EA is directly related to the square its original strength) = (2/ T ) / Thus, SE root of T and r of the material and frequency of EM wave. Therefore, suffficient thicknesses, high value of electrical conductivity as well as permeability are prerequisite for a good EM wave-absorbing material. However, high conductivity of shielding materials should exhibit good connectivity in its conduction path (or percolation in the case of composite materials). Multiple internal reflections generally occur in very thin nonmagnetic samples, causing poor absorption of EM wave. In this, EM wave after penetrating the interacting first boundary gets reflected by second boundary and rereflected from the first boundary to be reflected again and again. The shielding contribution due to multiple reflections (SET) can be expressed  as:
Advanced Nanostructured Materials in EMI Shielding 247
Experimental Methods in Measuring Shielding Effectiveness
Generally, shielding effectiveness is measured by SNA (scalar network analyzer) and VNA (vector network analyzer) instruments. In SNA the amplitude of the signals can only be measured whereas in VNA both magnitude as well as phase measurement of the signal is possible. So for measurement of complex signals, i.e., complex permittivity and permeability, VNA is widely used instead of SAN. In two-port VNA the incident and transmitted wave signals are duly analyzed with respect to the inherent shielding performance of the shielding material.
The resultant complex scattering parameters (S) are mathematically expressed according to their direction of propagation to co-relate with the reflectance (R) and transmittance (T) T as below. According to the theory of wave propagation, the electromagnetic energy of a wave reflected and transmitted through a sample can be related in terms of reflection coefficient (R), transmission coefficient (T) T and absorption coefficient (A) as follows:
where R = |S11|2 and T T = |S12|2. In this, S11 and S21 parameters are generally used to describe the scattering through two interconnected ports, where the wave propagation is related by the port’s reflection and transmission behavior. The measurement of the S parameters in VNA includes transmission/ reflection method, open-ended coaxial probe technique, free-space technique, resonant cavity method and parallel plate technique. Among these, reflection method is widely used for S parameters including the respective complex permittivity and permeability. During analysis of the shielding effectiveness from S parameters, several conversion techniques have been followed. These include, for example, NIST iterative, new non-iterative, short-circuit line (SCL) and Nicholson-Ross-Weir (NRW). Among these,
248 Hybrid Nanomaterials NRW technique is the most widely used through the regression/iteration of the input S-parameters. This is because it can provide both the value of permittivity and permeability data.
Carbon Allotrope-Based Polymer Nanocomposites
The problem faced by metals could be overcome by incorporating small volume fractions of electrically conducting fillers in polymer matrices exhibiting poor electrical, magnetic and dielectric magnetic properties. In this regard, nanocomposites represent a special case of composites consisting of dimension of filler in nanometer range in a matrix. Generally, polymer remains the most desirable matrix in these nanocomposites due to its simple processing conditions, corrosion resistance, mechanical flexibility and low density and transparency towards electromagnetic radiations. Due to the skin effect, a composite material having a conductive filler with a small unit size of the filler is more effective than one having a conductive filler with a large unit size of the filler [1, 2]. When the concentration of electrically conducting particles in a composite exceeds a certain level (percolation threshold), the particles come into contact with each other and form a continuous path for electrons to travel, making it electrically conducting. The percolation limit depends on the shape of the conducting particles. For example, carbon nanotubes with high aspect ratio can form a conducting network at much lower volume fractions and potentially lower costs than cheaper, traditional fillers such as carbon fiber and carbon black. In the past, a large number of conducting materials, such as graphite, carbon black, CNT, graphene and ICP, have been used as fillers in polymer matrix and investigated for EMI shielding [1–4]. Carbon black has also been used to generate the conductivity in a large number of plastic materials. Therefore, several investigations have been conducted on developing these materials in view of their EMI shielding applications. Ramadin et al.  prepared electrical properties of laminated epoxy/carbon fiber composites and studied electromagnetic losses as a function of frequency and specimen thickness. Their study showed optimum SE of about 62 dB corresponding to the thickness of 30 mm at the frequency of 9 GHz. The effect of processing and operating variables on EMISE of excess conductive carbon black reinforced solid and microcellular EPDM vulcanizates has been investigated . This study showed that EMI shielding effectiveness of conductive carbon black reinforced microcellular EPDM rubber vulcanizates increased monotonically with
Advanced Nanostructured Materials in EMI Shielding 249 filler loading and showed a maximum of around 75 dB (10 GHz) of 60 phr loading.
Carbon Fiber-Filled Polymer Nanocomposites
Carbon fiber has also been widely used as conducting filler in EM shielding due to its relatively better electrical conductivity and tensile strength. Khastgir’s group  reported that nitrile rubber filled with short carbon fiber, conductive carbon black, and their filler blend with loading >30 phr can be used as EMI shielding materials in the frequency ranges of 200–1000 MHz and 8–12 GHz. Ni-coated carbon fiber paper (NCFP) and carbon fiber paper (CFP) at 8 wt% loading in epoxy composites of (thickness: 0.5 mm) exhibited EMI SE, 35 dB and 30 dB, respectively, with reflection being dominant shielding mechanism in 3.22–4.9 GHz frequency range . Jana et al.  investigated the role of aspect ratio of carbon fiber for their shielding efficiency in the frequency range of 8–12 GHz. It was noted that composites with higher aspect ratio (100) form better conducting network and account for the improvement in EMI shielding. In addition, composites associated with higher concentration and sample thickness of fiber showed good absorption loss and maximum SE. The higher crosslinking density in the short carbon fiber-filled polychloroprene rubber composites vulcanized by the conventional method gives rise to higher electrical conductivity . Further, 30 to 40 phr (parts per hundred parts of rubber) loading of fiber made the rubber composite a potential EMI shielding material. Das and coworkers [33–35] carried out significant work on EMI shielding measurements on composites of different polymers and blends filled with conductive fillers for EMI shielding application over the microwave frequency ranges of 200–2000 MHz and 8–12 GHz. In one of the works, they prepared flexible composites of CB and short carbon fiber (SCF)-filled EVA and NR and found that higher shielding in SCF/ NR (34 dB) compared to SCF/NR (30 dB). It was also observed that carbon fiber/EVA showed higher shielding (34.1 dB) than CB/NR composites (8.4 dB). Das established that 50/50 EVA/EPDM blend systems exhibit higher shielding effectiveness than pure EVA and EPDM SCF-filled composites . The SE of the composites was found to be frequency dependent and it increased with increasing frequency (100–2000 MHz and 8–12 GHz). However, the SE of the composites is enhanced with increasing amount of filler loading in EVA. These studies also indicated that electrically conductive SCF-filled composites are more effective for EMI shielding than conductive carbon black-filled ones. In another work, carbon-carbon
250 Hybrid Nanomaterials fiber/epoxy composites also exhibited better shielding . Chiang and Chiang  prepared composite consisting of Ni-coated fiber (NCF) treated with neophentyl(diallyl)oxy, tri(dioctyl) pyrophosites used as coupling agent by melt blending and examined it for EMI SE. It was noted that this composite could achieve 50 dB at 1 GHz. Katsumata et al.  prepared highly conductive plastic composite EVA with vapor-grown carbon fiber composite and observed EMI SE of 70 dB. In another work , high mechanical properties and improved electromagnetic performance were observed in Ni-coated carbon fiber-reinforced polypropylene. Huang and Pai [40, 41] fabricated electroless nickel carbon fiber (ENCF) blend with ABS and observed the best SE of 44 dB at 1 GHz. In another study, NCF and carbon fiber (CF)-filled ABS showed EMI SE of 47 dB at 1 GHz . Melt blending technique was also applied to prepare polycarbonate/ABS/ nickel-coated-carbon-fiber conductive composites and exhibited EMI SE of 47 dB at 1 GHz . Though, the materials were observed to be effective in EMI shielding application, phase separation occurs at high temperature between PC and ABS, causing the diffusion of oxygen in PC/ABS matrix leading to oxidation of Ni. Consequently, the specific resistance of the Ni increases, resulting in the decrease of EMI SE. Polymer composites filled with nickel-coated carbon fibers were prepared by a dry grinding method . It was observed that composites consisting of 15 wt% nickel-coated carbon fiber exhibited EMI SE of 80 dB at 30 MHz, and 100 dB at 1000 MHz. Injection molding technique was also applied to prepare nickel-coated carbon fiber-filled thermoplastic carbon/epoxy composites . Jou et al.  studied the influence of conductive carbon fiber orientation and its weight percentage in liquid crystal polymer composites on electromagnetic shielding effectiveness. The maximum EMI SE of 60 to 70 dB (130 to 200 MHz) was achieved in carbon/epoxy composites . Injection molding technique was also applied to prepare nickel-coated carbon fiber-filled thermoplastic resin. In this procedure, the EMI SE of the material was found to be > 70 dB at 25 MHz and > 95 dB at 1000 MHz . EMISE properties of 10 vol% CB-filled (100/0)–(10/90) PP/PS blend are found to be adequate for computer shielding applications . A model has also been developed for shielding effectiveness based on first principles by performing compounding runs following injection molding and shielding effectiveness testing of short carbon fiber/nylon 6,6 resins . This is reasonably accurate below the percolation threshold and can be used to estimate the amount of filler needed to provide a particular shielding effectiveness. Graphite nanoplatelet/poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) composites were prepared as EMI shielding material having
Advanced Nanostructured Materials in EMI Shielding 251 thermal conductivity. It is observed that 0.5, 10 and 25 wt% graphite nanoplatelet loaded samples show SE value of ~30, ~47 and 70 dB respectively . Enhancement of electromagnetic interference shielding efficiency of polyaniline through mixture and chemical doping is also reported . Fukushima et al.  reported the fabrication of acrylamide-grafted nanographite by the exfoliation of graphite to get low resistance (10 1.5Ω cm) material. Study of impedance showed that the polymer composites could be useful in EMI shielding application. In addition, water-based colloid of submicron graphite particles was also used to reduce the resistivity of cement paste. It was observed that while the coating made of graphite colloid and cement paste showed 11 dB SE, their admixture give 22 dB SE (at 1 GHz) . Al-Ghamdi and El-Tantawy  fabricated functional nanoconducting composite from polyvinyl chloride reinforced graphite-copper nanoparticles (PVC/GCu) for EMI shielding applications. The electrical conductivity increases with increasing GCu content within composites. The nanocomposites showed a high dielectric constant and a high dissipation factor in the frequency range of 1–20 GHz. The variation of EMI SE versus frequency (1–20 GHz) in Figure 5.3 shows that SE increases with increasing GCu content in PVC/GCu nanocomposites. It is also evident that increased GCu content in PVC enhanced EMI SE from 22 to 70 dB over a frequency range of 1–20 GHz and was attributed mainly to the conductive network. Further, GCu nanoparticles interact with the incident EM radiation and account for the high SE. The variation of reflection and absorption losses as a function of frequency of PVC/Gcu nanocomposites has also been investigated. It is noted that the reflection loss and absorption loss both increase with increasing frequency as well as increasing GCu content in the composites. All these features make the PVC/GCu nanocomposites ideal absorbing materials in electromagnetic wave shielding at microwave frequency. Cu-Ni alloy decorated graphite layers were prepared by decorating natural graphite with copper-nickel alloy nanoparticle for EMI suppression by simple low temperature reduction technique at (50–90 °C) . It was observed that the material showed 10.27 to 25.10 dB absorption loss (SEabs) and 5.62 to 18.69 dB reflection loss. EMI shielding studies have also been reported on composites of polyaniline/colloidal graphite , graphite/acrylonitrile butadiene styrene , styrene-acrylonitrile copolymer/graphite sheets , polyphenylene sulfide/graphite , graphite/ epoxy [60–62], polyethylene/graphite [63, 64], polypropylene/graphite , graphite metal oxide/acrylic sheet  and polyurethane matrix filled with Ni-coated graphite fibers .
252 Hybrid Nanomaterials 80 Experimental
GCu8 GCu12 GCu16
Figure 5.3 Variation of EMI SE against the measured frequency range from 1 to 20 GHz for the PVC/GCu nanocomposites. (Reproduced from  with permission from Elsevier Ltd.).
CNT-Filled Polymer Nanocomposites
It is well established that shielding of EM wave could be prevented by conductive method . However, highly conductive dispersed metal particles in water-based system are very prone to undergo fast oxidation. Therefore, developments of chemically and thermally stable lightweight conductive fillers are the most preferred choice today [69–74]. Li et al.  used homogeneously dispersed MWCNT in pure acrylic emulsion to prepare MWCNT/polyacrylate composites and applied it on building interior walls for electromagnetic interference shielding applications. Figure 5.4 shows the variation of room-temperature dc conductivity ( dc) of WMCNT/polyacrylate composites as a function of the WMCNT mass concentrations ( ) in log-log scale (Inset: the plot of dc as a function of in linear scale). It is observed that a low weight fraction of MWCNTs could achieve a high level of conductivity (room temperature) and a low percolation threshold for MWCNTs ( c ~ 0.58 wt%). EMI SE measurements
1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 1E-12 1E-13 1E-14
T = 2.29
T = 10.29
dc conductivity (S/cm)
dc conductivity (S/cm)
Advanced Nanostructured Materials in EMI Shielding 253
10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 1E-11 0.00 0.02
0.04 0.06 p
Figure 5.4 DC conductivity at room temperature (σσdc) of WMCNT/polyacrylate composites as a function of the WMCNT mass concentrations (p) in log–log scale. Inset: plot σdc as a function of p in linear scale. (Reprinted with permission from ; Copyright © 2008 Elsevier B.V.).
in radio frequency and X-band frequency range show high EMI SE of the MWCNT/polyacrylate composite films at a low mass concentration of ). The EMI SE finding of composite films MWCNTs (SEX-band > SERadio frequency q y consisting of 10 wt% MWCNTs agreed well with that of theoretical prediction in far field. The variation of EMI SE of MWCNT/polyacrylate composites with MWCNT contents at 450 MHz and 10 GHz shows that shielding effectiveness increases progressively with the increase of filler loading. The high performance of SE is attributed to conductive network formed due to the dispersion of MWCNTs . Huang et al.  applied multiwalled carbon nanotubes (MWCNTs) to electroless nickel-phosphorus (Ni-P) deposition, followed by electroless Ag plating (SCCNT). The asprepared SCCNT/polyacrylate composite coatings exhibited a low resistivity of 1.1 m ohm and a high EMI shielding effectiveness (SE) of more than 46.5 dB in the frequency range from 3.95 to 5.85 GHz with the maximum value of 53.25 dB at 4.05 GHz. In another study, thin MWCNT-PMMA films containing 0.1–10 vol% CNTs were fabricated using solvent casting method . Conductivity of these films increased with CNT content with a percolation threshold of about 0.5%. EMI shielding effectiveness of about 18 dB was achieved, suggesting the use of these composites as effective EMI shielding materials. Electromagnetic interference shielding effectiveness measurements on a 10 vol% CNT-loaded PMMA showed value of ~18 dB in the frequency
254 Hybrid Nanomaterials range 8.0–12 GHz (X-band). Pande et al.  also applied multiwalled carbon nanotubes–polymethyl methacrylate (MWCNT–PMMA) composites in similar methods and found them to be structurally strong EMI shielding material. SE up to 40 dB in the frequency range 8.2–12.4 GHz (X-band) was achieved by stacking seven layers of 0.3-mm thick MWCNT–PMMA composite films compared with 30 dB achieved by stacking two layers of 1.1 mm thick MWCNT–PMMA bulk composite. The composites with maleic anhydride modified MWCNTs (Mah-g-MWCNTs) and poly(methyl methacrylate) were prepared by the in-situ and ex-situ solution polymerization system . The EMISE of Mah-g-MWCNTs/PMMA composites increased with the increasing of the Mah-g-MWCNTs content and in-situ system showed higher EMI SE value. Epoxy resin is widely used in thermosetting polymer matrices for the development of advanced composites [80, 81]. When CNTs are incorporated into the epoxy matrix, they provide an excellent option in developing high-performance EMI shielding conductive composites at low filler contents [82–98]. Phan and coworkers  observed improvement in dispersion and electrical conductivity in epoxy-filled functionalized MWCNTs (functionalization reaction time: 2 h) compared to pristine MWCNTs. According to the available literature, SWNTs can be dispersed into epoxy with a very low percolation threshold [91– 93]. Huang et al.  reported EMI–SE of 18 dB with 15 wt% small SWCNTs and 23–28 dB with 15 wt% long SWCNTs. Li et al.  fabricated SWNT/epoxy composites as lightweight EMI shielding materials. The variation of EMI shielding effectiveness over the frequency range of 10 MHz–1.5 GHz for various SWNT loadings shows that SE increases with increasing wt% of SWNTs at fixed frequency. It was also noted that 15 wt% SWNT loaded epoxy showed highest EMI SE (49 dB) at 10 MHz. At higher frequencies, the 10 and 15 wt% SWNT loaded epoxy composites exhibited SE of ~ 20 dB at ~ 1 GHz. They also established strong relevance between the EMI SE and the dc conductivity in accordance with the EMI shielding theory. Figure 5.5 shows EMI SE data versus frequency (10 MHz–1.5 GHz) plots for three composites containing 10 wt% of SWNTs (SWNTs-long with aspect ratio ~ 240; SWNTs-short with aspect ratio ~ 139; and SWNTs-annealed). It is observed that at same wt% loading, shielding effectiveness of SWNTs-short composite is much lower than that of SWNTs-long composite, and agreed well with respective dc conductivity and EMI shielding theory. The significant increase in EMISE was recorded in SWNT annealed composites (based on annealed SWNTs-short material) compared to the same SWNT wt% composites made from SWNTs-short.
Advanced Nanostructured Materials in EMI Shielding 255 45 EMI shielding effectiveness (dB)
A - SWNTs-long B - SWNTs-annealed C - SWNTs-short
15 10 0 10
100 Frequency (MHz)
Figure 5.5 Impact of wall integrity and aspect ratio on the EMI shielding effectiveness of the composites containing 10 wt% SWNTs. (Reprinted with permission from ; Copyright © 2006 American Chemical Society).
Jou and coworkers  studied the influences of CNTs (denoted as CNTs–KUAS) of varying aspect ratios (not purified) of aspect ratio 200–500 and purified CNTs (denoted as CNTs–Desunnano) of aspect ratio 10,000 in liquid crystal polymers (LCPs) and melamine formaldehydes (MF). Their findings demonstrated that formation of conductive network in composites is assisted by the higher aspect ratio of raw material, which is a key factor for conductivity and SE. It is noted that the higher the aspect ratio, the higher is the SE of nanocomposites. They showed that SE of both CNT composites is higher than 40 dB wt% conductive carbon nanomaterials-filled LCPs and MF nanocomposites, and that SE of CNTs–Desunnano is the highest and almost 15 dB higher than that of CNTs–KUAS/LCPs composites at all frequencies. The effect of length of MWCNT on electrical and EMI shielding properties of MWCNT-epoxy composites up to 0.5 wt% loading has been investigated . The electrical conductivity was lower for short length MWCNT-epoxy compared to long MWCNT-epoxy composite at the same loading, and hence the percolation threshold in the short length MWCNTepoxy composites is significantly higher than that in MWCNT-epoxy composites. This is attributed to the short length of MWCNTs in all probability due to the reduction of their aspect ratio and reduction. It was also noted that absorption dominated EMI shielding effectiveness (−16 dB) for l-MWCNT compared to s-MWCNT (−11.5 dB) with 0.5 wt% loading
256 Hybrid Nanomaterials in Ku-band (12.4–18 GHz), due to very low percolation threshold and enhanced electrical conductivity. CNT/Fe nanocomposites show excellent microwave absorption characteristics . This is ascribed to confinement of crystalline Fe in carbon nanoshells. Fe-doped carbon nanotubes/epoxy composites showed highest EMI SE of −40.7 dB at 39.5 GHz for the composite with 8 wt% loading of Fe–MWCNTs . The method followed by nanocomposites derived by filling of epoxy by chemical coating of Ni on MWCNT also showed the electrical performance of EMI SE . Most of the earlier investigations on the EMI shielding properties of CPCs were focused on compression molded samples consisting of randomly distributed conductive filler rather than aligned filler existing in injection molded samples [84, 94, 99]. Motivated by this, effects of CNT alignment in injection molded sample of CNT/PS on EMI shielding properties were studied [100–102]. Arjmand and coworkers  observed that alignment of MWCNT induces an adverse effect on EMI shielding properties in MWCNT in MWCNT/PS composites. The nanocomposites prepared by injection molding process showed EMI SE (~ 47 dB) independent of the frequency in the range of 8.2 to 12.4 GHz. The corresponding compression molded samples at 20.0 wt% of MWCNT showed an EMI SE of 57.4 dB at 8.2 GHz, which increased to 66.4 dB at 12.4 GHz. The relatively inferior EMI of injection molded sample was related to lower electrical conductivity, real permittivity (polarization loss) and imaginary permittivity (Ohmic loss) leading to lower electromagnetic wave energy dissipation. The EMI SE of AgNW/PS and MWCNT/PS nanocomposites as a function of nanofiller loading has been studied . Sachdev et al.  carried out electrical and EMI shielding characterization of multiwalled carbon nanotube/polystyrene composites. Khatua and his group  reported preparation of CNT/PS nanocomposites following tumble mixing and compression molding at elevated temperature. Conductivity measurements indicated that an extremely small mass fraction of 0.05 wt% MWCNTs in PS is good enough for percolation threshold. The GNP sheets form strong physical bonds with the PS by π-π π interaction in between the phenyl ring of PS, sheets of GNP and MWCNT in melt-blended nanocomposites and form a GNP−CNT−GNP interconnected conductive network structure throughout the matrix, as shown in Figure 5.6. This could account for the reduced percolation threshold of the nanocomposites. Figure 5.7 shows the variation of the EMI shielding effectiveness of these nanocomposites of MWCNT and graphene nanoplatelets (GNP) loadings in the frequency range of 8.2–12.4 GHz. It is evident that PS/MWCNT/ GNP nanocomposites filled with 2 wt% MWCNTs and 1.5 wt% GNP
Advanced Nanostructured Materials in EMI Shielding 257
Interaction between phenyl ring of PS and MWCNT MWCNT Interaction between MWCNT and GNP Interaction between phenyl ring of PS and GNP GNP Phenyl ring of PS
Figure 5.6 Schematic representation of π–π π interactions between GNP, MWCNT, and PS in the PS/MWCNT/GNP nanocomposites. (Reprinted with permission from ; Copyright © 2013 American Chemical Society).
loading exhibited the EMI SE value of ~20.2 dB in the X-band region. This is attributed mainly to the formation of conducting interconnected network structure of GNP/MWCNT/GNP in the insulating PS matrix. They also extended their work on preparation of CNH/GNP/PS nanocomposites by incorporation of suspension-polymerized GNP/PS bead in the in-situ polymerized CNH/PS matrix during the polymerization reaction . It is proposed that addition of the beads not only acted as excluded volume but also facilitated the formation of GNP–CNH–GNP or CNH– GNP–CNH network structure. The percolation threshold was found to be significantly reduced to 0.07 wt% of CNH. The nanocomposites consisting of 1 wt% CNH and 0.15 wt% GNP showed a considerably high value of EMI SE (−24.83 dB). Polystyrene foam filled with carbon nanotubes has also been examined as a very effective, lightweight EMI SE material . Singh et al.  prepared MWCNT reinforced low density polyethylene nanocomposites by solvent casting followed by compression molding technique. They observed a sharp increase in the conductivity up to 7 wt% of CNT due to the formation of conducting links and increase in the number of charge transport paths. The conductivity of 0.63 S/cm was achieved at 10 wt% CNT loading. The variation of total EMI shielding effectiveness as a function of frequency in the 12.4–18 GHz range of MWCNT–LDPE nanocomposites show that the average value of EMI-SE reaches 22.4 dB for 10 wt% MWCNT-LDPE composites. This is attributed to significant improvement in the electrical conductivity of the composites. Melt mixing has also been applied to prepare electrically conducting nanocomposites
258 Hybrid Nanomaterials 22 20 18 EMI shielding (dB)
16 14 12 0.1 wt% MWCNT (50 wt% bead with 0.21 wt% GNP) 0.2 wt% MWCNT (50 wt% bead with 0.21 wt% GNP) 0.3 wt% MWCNT (50 wt% bead with 0.21 wt% GNP) 1.0 wt% MWCNT (60 wt% bead with 0.65 wt% GNP) 1.5 wt% MWCNT (70 wt% bead with 0.80 wt% GNP) 2.0 wt% MWCNT (70 wt% bead with 0.80 wt% GNP) 2.0 wt% MWCNT (70 wt% bead with 1.50 wt% GNP)
10 8 6 4 2 0 8.0 109
1.1 1010 1.0 1010 Frequency (Hz)
EMI shielding (dB)
3.8 3.7 3.6 3.5 3.4
50 wt% bead with 0.21 wt% GNP (0.3 wt% MWCNT) 60 wt% bead with 0.25 wt% GNP (0.3 wt% MWCNT) 70 wt% bead with 0.29 wt% GNP (0.3 wt% MWCNT)
3.3 3.2 8.0 109 (b)
1.1 1010 1.0 1010 Frequency (Hz)
Figure 5.7 EMI shielding of the PS/MWCNT/GNP nanocomposites vs. frequency at (a) different MWCNT loadings with different PS-GNP bead content and (b) various weight percents of PS-GNP beads at constant MWCNT loading (0.3 wt%). (Reprinted with permission from ; Copyright © 2013 American Chemical Society).
of high-density polyethylene (HDPE) filled with 20 wt% of carbon black (CB) and 0–1 wt% of MWNTs . The variation of EMI SE over the frequency range of 8.2–12.4 GHz at different loadings of MWNT in HDPECB-MWNT nanocomposites is found to be frequency dependent. The SE
Advanced Nanostructured Materials in EMI Shielding 259 20 a b c d e
Shielding effectiveness (dB)
18 16 14 12 10 8 6 4 2 0
11G 10G Frequency (Hz)
Figure 5.8 Effect of frequency on EMI SE of the nanocomposites [(a) HDPE + 20 wt% CB, (b) HDPE +20 wt% CB + 0.25 wt% MWCNT, (c) HDPE + 20 wt% CB + 0.5 wt% MWCNT, (d) HDPE + 20 wt% CB + 0.75 wt% MWCNT, (e) HDPE + 20 wt% CB + 1.0 wt% MWCNT].. (Reprinted with permission from ; Copyright © 2012 Taylor & Francis).
increases from 9.5 to 16 dB at 8.2 GHz. The highest EMI SE of the composites containing 1 wt% of MWNTs was measured at 16 dB in the X-band. The effect of filler loading on the EMI SE of the nanocomposites for the sample of thickness 0.35 mm at various frequencies is shown in Figure 5.8. It was observed that over the entire frequency range, SE increases with an increase in loading of MWNT. This is mainly attributed to the formation of conducting networks of CB and MWNT in the insulating HDPE matrix. Tran and others  fabricated highly expanded nanocomposite foams of polypropylene and carbon nanotubes (PP/CNT) as efficient EMI absorbers by using supercritical carbon dioxide technology. Khatua et al. developed PC–MWCNT conducting nanocomposites through solution blending  and melt blending [112, 113] in order to develop efficient electromagnetic interference shielding materials. The MWCNT–PS composites prepared by solution blending method exhibited high electromagnetic interference shielding and electrical conductivity at very low (~0.021 wt%) percolation threshold . It was also noted that sDC of the composites increases with increasing the PC bead content in the composites. It was proposed that the PC bead (insulating) acts as excluded volume in which MWCNT is not able to penetrate. This increases the effective concentration of MWCNT in the continuous PC region, which accounts for increases in the net sDC of the composites with the addition of PC bead at constant MWCNT loading. Figure 5.9 shows the variation of the EMI SE with frequency (8.2–12.4 GHz) for the MWCNT/PC composites with
260 Hybrid Nanomaterials 25
EMI shielding (dB)
10 0.1 wt% CNT with 70 wt% PC bead 1.0 wt% CNT with 70 wt% PC bead 1.5 wt% CNT with 70 wt% PC bead 2.0 wt% CNT with 70 wt% PC bead
–5 8.0 109
Figure 5.9 EMI Shielding vs. frequency of the MWCNT–PC composites containing different MWCNT loadings at 70 wt% PC beads content. (Reproduced from  with permission from Royal Society of Chemistry).
different MWCNT and 70 wt% bead loadings. It was noted that MWCNT– PC composites exhibited higher EMI SE value of ~23.1 dB (8.2–12.4 GHz) at 2 wt% of MWCNT loading and 70 wt% PC bead loadings, which was ascribed to the formation of conducting interconnected continuous network of CNT–CNT in insulating PC matrix. The electrical properties of uniformly dispersed MWCNT/PC composite system fabricated by solvent casting have been investigated in electromagnetic interference shielding applications . The as-synthesized MWCNT/PC composite films at 20 wt% loading exhibited EMI Se value of ~43 dB (8.2–12.4 GHz) following absorption as primary mechanism of shielding. Monnereau and coworkers  also reported gradient foaming of polycarbonate/carbon nanotube-based nanocomposites with supercritical carbon dioxide and their EMI shielding performances. The MWCNT (10 wt%)/polycarbonate composites achieved high EMI SE of ~ 27.2 dB in Ku band . The GNP – and MWCNT-embedded polycarbonate hybrid composites prepared by simple melt mixing exhibited high electromagnetic interference shielding with low percolation threshold . FESEM micrographs of the PC/GNP/MWCNT hybrid composites containing 0.5 wt% of (GNP/ MWCNT) (0.3 wt% GNP and 0.2 wt% MWCNT) in Figure 5.10a show that GNP and MWCNT are homogeneously and randomly distributed and form
Advanced Nanostructured Materials in EMI Shielding 261
300 nm 200 nm (a)
Figure 5.10 FESEM micrograph and (b) HRTEM micrograph of PC/GNP/MWCNT (0.5 wt% GNP/MWCNT loading) composites . Reproduced with permission from Wiley.
conductive network. The HRTEM (high resolution transmission electron microscopy) images in Figure 5.10b also supported a random and regular dispersion of the tube-like MWCNT and platelet-like GNP throughout the PC. EMI shielding versus frequency of the PC/GNP/MWCNT composites containing different (GNP/MWCNT) loadings in this frequency region of 8.2–12.4 GHz is displayed in Figure 5.11. It is noted that 4 wt% GNP/ MWCNT-filled PC/composites exhibit maximum EME SE value of 21.6 dB due to the continuous conducting network structure of CNT–GNP–CNT or GNP–CNT–GNP in insulating PC matrix. Successful investigations have been reported on application of MWCNT poly(ether sulfone) and poly(ether imide) in EMI shielding . Eswaraiah et al.  synthesized EMI shielding material consisting of MnO2 nanotubes (MNTs), functionalized multiwalled carbon nanotubes (f-MWCNTs) and polyvinylidene fluoride (PVDF). The electrical conduc(f tivity of the composites confirmed that the addition of MNTs in PVDF increases electrical conductivity and follows percolation behavior. Such increase in electrical conductivity can be attributed to the high aspect ratio and efficient dispersion of the MNTs in the PVDF matrix. Figure 5.12 shows that addition of 5 wt% MNTs-1 wt% of ff-MWCNTs to PVDF resulted in EMI SE of approximately 20 dB h in comparison with EMI SE of approximately 18 dB for 7 wt% off f-MWCNTs. The observed increase in EMI shielding effectiveness with the addition of nanofillers is attributed to the enhanced electrical conductivity of the composite due to the addition off f-MWCNTs and good homogeneity of the nanofillers in the polymer. Poly(trimethylene terephthalate) (PTT)/MWCNT (4.76 vol%) composites showed the highest EMI SE of PTT/MWCNT composites which was
262 Hybrid Nanomaterials 35
0.8 wt% (GNP/MWCNT) loading 2.0 wt% (GNP/MWCNT) loading 2.5 wt% (GNP/MWCNT) loading 3.0 wt% (GNP/MWCNT) loading 4.0 wt% (GNP/MWCNT) loading
EMI shielding (dB)
25 20 15 10 5 0 –5 8.0 109
0 wt% 1 wt% 2 wt%
3 wt% 4 wt% 5 wt%
10 11 Frequency (GHz) EMI shielding effectiveness (dB)
EMI shielding effectiveness (dB)
EMI shielding effectiveness (dB)
Figure 5.11 EMI Shielding versus frequency of the PC/GNP/MWCNT composites containing different (GNP/MWCNT) loadings. (Reproduced from  with permission from Wiley). 25 20 15 10
PVDF 5 wt% MNT s/1 wt% f-MWCNT s/P VDF
5 0 8
10 11 Frequency (GHz)
20 16 12
0 wt% 3 wt%
5 wt% 7 wt%
10 11 Frequency (GHz)
Figure 5.12 EMI shielding effectiveness of MNTs/PVDF, MNTs/f-MWCNTs/PVDF and f-MWCNTs/PVDF composites. (Reprinted from  with permission from Springer).
Advanced Nanostructured Materials in EMI Shielding 263
EMI shielding effectiveness (dB)
~23 dB at MWCNT loading . MWCNT (4.0 wt%)/PEG grafted nanocomposite film exhibited the maximum EMI SE of ~22.9 dB (4.14 GHz) . Shielding efficiency as high as 60 to 80 dB together with a low reflectivity was achieved in 0.25 vol% MWNTs-filled polycaprolactone nanocomposites . Rubber is an insulating and flexible material and its composites find many technological applications in EMI shielding [123–143]. The electrical conductivity and dielectric properties of insulating rubber medium can be modified by dispersing electrically conductive particles in the medium similar to thermosetting polymers. Conductive particles dispersed in the medium contribute to the dielectric properties and EMI shielding effectiveness of the composites . Though, fabrication of rubber reinforced by nanofillers is not new in today’s world, a very limited amount of work has been reported on investigations related to their applications in EM shielding. Jiang et al.  reported that acrylonitrile butadiene rubber reinforced by MWCNT exhibits excellent electrical properties. It was noted that conductivity below 3.0 phr dramatically increased by 10 orders of magnitude, indicating the formation of conductive network. Further, dielectric permittivity and dielectric loss of NBR/MWCNT composites increased with increasing the MWCNT content. Figure 5.13 shows the variation of 30 28 26 24 22
20 18 16 14
12 10 8 6 4 2 0
3 phr 1.5 phr 0.5 phr 0 phr 0
Figure 5.13 EMI shielding effectiveness as a function of frequency for NBR/MWCNT composites with various MWCNT loadings. (Reprinted with permission from ; Copyright © 2012 Society of Plastics Engineers).
264 Hybrid Nanomaterials EMI shielding effectiveness as a function of frequency (0.5–18 GHz) for NBR/MWCNT composites at different MWCNT loadings. These studies established that NBR/20 phr MWCNT composite (thickness: 1 mm) achieved EMI shielding effectiveness of 26 dB (16.7 GHz) due to enhanced conductivity and permittivity. It was also observed that the shielding effectiveness of the composites increased with increasing the frequency for the same MWCNT loading. Joseph et al.  prepared butyl rubber (BR)/ SWCNT composites and observed exhibited EMI shielding effectiveness in the range of 9–13 dB and frequency range of 8.2–18 GHz corresponding to 8 phr loaded SWCNT in BR. It was concluded that an increase in dielectric properties, conductivity and skin depth of BR-SWCNT composite with SWCNT loading significantly influence EMI shielding properties of the composites. Al-Hartomy et al.  investigated the effect of different loadings of CNT (2–10 phr) in NR on electromagnetic interference shielding effectiveness in the 1–12 GHz frequency range. The high conductivity, small diameter, high aspect ratio, and high electrical and thermal conductivity of these nanocomposites make them an excellent option for highperformance EMI shielding materials in the frequency range of 1–7 GHz at low filling concentration. Al-Saleh and Sundararaj  prepared CNT/ ABS and VGCNF/ABS nanocomposites by melt mixing method. Their study shows that electrical conductivity of the composites having 0, 0.50, 0.75 and 1.0 wt% of CNT correspond to 10 14, 10−12, 4 × 10−5 and 100 S/m (electrical percolation threshold: ~ 0.75 wt%). In addition, they also studied EMI SE of CNT/ABS nanocomposite (1.1-mm thick plate) as a function of frequency and CNT content in the EM frequency range of 100–1500 MHz. It is noticed that EME SE is independent of the frequency. Interestingly, EME SE increased remarkably with an increase in CNT loading and attains its value of ~ 40 dB (1 GHz) for15 wt% CNT-filled ABS nanocomposite. These findings also established considerably higher EMI SE in ABS/CNT compared to VGCNF-based nanocomposites at 1 GHz. This remarkable difference in EMI SE between VGCNF and CNT nanocomposites could be attributed to the difference in the nanocomposites’ microstructure and to the difference in nanofiller intrinsic conductivity (dominating factor). Homogeneously dispersed MWCNT-filled ABS (ABS/MWCNT) composites were prepared via solution-blending combined with hot-pressing . The volume resistivity measurements of the composites showed that the percolation threshold was low at about 1–3 wt% filler loadings. The variation of EMI SE of ABS/MWCNT composites with MWCNT loadings (1–15 wt%) over the frequency range of 100–1500 MHz shows the highest EMI SE of 36 dB (760 MHz) in ABS/MWCNT composite containing 15 wt% of MWCNTs. The electromagnetic wave absorbing composite films
Advanced Nanostructured Materials in EMI Shielding 265 were prepared by a dip-coating method using a uniform mixture of rare earth lanthanum nitrate-doped amorphous carbon nanotubes (ACNT) and polyvinyl chloride (PVC) . These findings indicated reflection loss (R) value of a lanthanum nitrate-doped ACNT/PVC composite was −25.02 dB at 14.44 GHz, and the frequency bandwidth corresponding to the reflector loss at −10 dB was up to 5.8 GHz within the frequency range of 2–18 GHz in 6 wt% lanthanum nitrate-doped composite. In addition, the formation and application of many other composites of CNTs have also been reported in the literature [129–144]. Table 5.1 records the preparative methods used, conductivity, percolation threshold, thickness for polymer nanocomposite of CNT films and corresponding EMI shielding data.
Graphene and Graphene Oxide Fillers-Based Polymer Nanocomposites
In comparision to CNTs or carbon nanofibers (CNFs), graphene, when reinforced in polymer, exhibits outstanding structural, electrical and mechanical properties [14, 145–152]. Interestingly, the low price and availability of pristine graphite followed by large quantities coupled with a relatively simple methodology applied in the formation of graphene, makes it one of the potential conductive fillers in the preparation of graphene-based composites. It is also an excellent choice in EMI shielding due to its low cost, light weight, high aspect ratio, saturation velocity, flexibility and mechanical strength . In addition, graphene-filled polymer nanocomposites showed promising EMI shielding response owing to their high aspect ratio, electrical conductivity and dispersion . Dispersion of these graphene sheets within the matrices of polymer leads to the onset of electrical conductivity. It is reported that at the onset of electrical conductivity, graphene-filled polymer nanocomposites at percolation threshold appeared at much lower loading than carbon-derived fillers . However, very limited work has been reported on graphene-based polymer nanocomposites compared to CNTs even now. For the first time, monolayer graphene was prepared indirectly from graphene by an inductively coupled plasma (ICP) chemical vapor deposition (CVD) process . The average value was found to be 2.27 dB corresponding to ~ 40% shielding of incident waves, which was more than seven times (in terms of dB) greater SE than gold film. The absorption rather than reflection is found to be the dominant mechanism in this process. Zhang and others  synthesized free-standing graphene paper prepared by CVD synthesis of 3D graphene pellets, extracting the
0.062 wt % (fitting value)
0.062 wt% (fitting value)
Epoxy/Long SWCNT 
By dispersion of SWCNTs’’ and epoxy resin in acetone In-situ
PMMA/MWCNT  In situ
10 vol % CNT
Filler in film 15 wt.%
10 vol %
Percolation thershold 3.5 wt%
PMMA/MWCNT  Solvent casting
PMMA/MWCNT  Solvent casting
PA/Ag coated CNT 
PA/ MWCNT 
Nanocomposites EVA/ SWCNT 
Preparation method Prepared in Brabander Plasticoder MWCNT dispersed in de-ionized water with SDS and acrylic emulsion Solution method
Thickness of film 1.5 mm
~ 0.14 S/cm (10 wt%)
10 MHz to 1.5 GHz
3.95 to 5.85 GHz
49 dB (10 MHz); 20 (1 GHz), Reflection
40 dB, absorption 8 dB (4 GHz), 11 dB (14 GHz) 25 dB
53.25 (4.05 GHz)
EMI SE (dB), Frequency dominant conductivity range, GHz mechanism 10–8 S cm–1 200–2,000 22–23 dB (3.5 wt%) MHz and (8–12 GHz ) 8–12 GHz Increase in 10 order 100–1000 25.1–25.8 dB magnitude MHz (8–12 GHz ) 8.2–12.4 GHz
Table 5.1 Preparative method, percolation threshold (wt%), wt/vol % filler in polymer film, its thickness, conductivity, EMI SE (Dominant shielding mechanism) and frequency range for polymer nanocomposites of SWCNT and MWCNT
266 Hybrid Nanomaterials
Compression molding Ultrasonic mixing of pretreated MWCNT, CTAB and epoxy
Fe@CNT in epoxy resin 
MWCNTs), Fe3O4 and Fe In epoxy resin 
10 wt% MWCNT 3 mm
5 and 10 wt%
134 wt% 1 mm (Ni content in MWCNT)
Fe-MWCNT/Epoxy  Epoxy/Ni@MWCNT 
0.02 wt% (l-MWCNT); 0.11 (s-MWCNT)
Dispersion with homogenizer
4.2 × 10−7 S/cm
3.22 GHz and 40 GHz.
7–12 GHz 5.7 × 10–3 S/cm (F-MWCNT-2h)
1.37 × 10–3/S cm (l-MWCNT), 0.95 × 10–3 S/cm (s-MWCNT) 3.2 × 10–1 S/m
66.4 dB (12.4 GHz), 56.92 dB (39.405 GHz)
RL*: below −10 dB (11.8–14.7 GHz) and minimum value: −31.71 (13.2 GHz), dielectric and magnetic losses
6.6 dB (at 7 GHz)
–16 dB (l-MWCNT); –11.5dB (s-MWCNT) –40.7 dB (39.5 GHz) 6.5 dB (1.0 GHz)
Advanced Nanostructured Materials in EMI Shielding 267
PS/ CNH/GNP 
0.07 wt % of CNH (fitted value)
2 wt% (MWCNT) with 70 wt% (PC beads). 1 wt% CNH and 0.15 wt% GNP
PC/ MWCNT 
40 MHz to 40 GHz. 8.2–12.4 GHz
3.69 S/m (31.4 GHz) ~ 4.57 × 10–3 S/cm (0.05 wt% MWCNT) 6.24 × 10–2 S/cm
1.02 × 10–4 S/cm (30 MHz)
Supercritical CO2 technology Solution blending
6.24 × 10–2 S/cm
conductivity 7.98 × 10–3 S/cm
~ –24.83 dB
~ 23.1 dB
16 dB (8.2 GHz), Reflection
18.56 dB, reflection
~ –24.83 dB
EMI SE (dB), Frequency dominant range, GHz mechanism 8.5–12 GHz 23.5 dB
Thickness of film
1 wt % CNH and 4 mm 0.15 wt % GNP 7 wt %
Filler in film 5 wt%
20 wt.% CB + 1 wt.% MWNTs 0.184 vol% CNT
Percolation thershold 0.05 wt%
HDPE/MWCNT[109 } Solution melt mixing process
CNT dispersed in PS/toluene through ultrasonication LDPE/MWCNT  Solution method
Nanocomposites PS/ MWCNT 
PS foam /CNT 
Preparation method Dry tumble mixing
Table 5.1 Cont.
268 Hybrid Nanomaterials
PC/GNP/MWCN T  Poly(ether sulfone),poly(ether imide)/ MWCNT  PVDF/ f-MWCNTs 
Grafted coupling between MWCNT and IPDI–PEG
PC a-MWCNT 
0.072 wt% (fitting value) 0.5 wt%
PC composites initially prepared by solution blending and subsequent dilution with SAN in next step during melt mixing. Solvent casting 2–3 wt%
PC/SAN/MWNT/ Fe3O4 
4.0 wt% MWCNTs
MWNT (3 wt%) and Fe3O4 (3 vol%)
3.2 × 10–1 S/m (2 wt% f-MWCNT) 4x 10–2 S/cm
~6.84 × 10–5 S/cm (3 wt%) 0.003 S/cm (2 wt% MWCNT
3 S/m (10 wt% a-MWCNT) 1.27 × 10–2 S/cm
8 fold increase in conductivity
8 to 12 GHz
–22.9 dB (4.14 GHz)
18 dB, absorption
Between 42–45 dB at 8 GHz.
43 dB, absorption -27.2 dB (12.4– 18 GHz), Absorption loss ~ 21.6 dB
8.2–12 GHz –32.5 dB and (18 GHz) 12–18 GHz
Advanced Nanostructured Materials in EMI Shielding 269
Ultrasonic dispersion in DMF. Direct growth of MWCNT on CF
Nanocomposites NBR/MWCNT 
ABS /MWCNT 
PVC/Lanthanum nitrate doped amorphous CNT 
SWNTs/soluble crosslinked PU 
MWCNT/Carbon foam (CF) 
Preparation method Latex technology
Table 5.1 Cont.
HPPy/Ag-2 (3.3 × 10–3) > HPPy (8.2 × 10–4). The observed increase in conductivity could be attributed to the progressive development of electronic path in HPPy/Ag nanocomposites. The abrupt increase in the conductivity in HPPy/Ag-10 could be assigned to the presence of interconnecting network generated by Ag nanoparticles itself. These observations have also been reflected in our subsequent EMI shielding studies of HPPy and HPPy/ Ag nanocomposites shown in Figure 5.22a. EMI SE of PPy, HPPy, HPPy/ Ag-2, HPPy/Ag-5, HPPy/Ag-10 in the frequency range of 0.5–8 GHz are found to be ~20 to 5, ~34.5 to 6, ~36.5 to 11.5, ~55.78 to 20, ~59 to 23 dB respectively. It was noted that wave from the inner shell wall of HPPy/ Ag-2 is dominating. In contrast, HPPy/Ag-5 and HPPy/Ag-10 undergo both internal as well as external reflection simultaneously, which accounts for their higher EMI (SE) compared to PPy. An anticipated scheme about trapping mechanism of EM wave through enhanced internal reflection in HPPy/Ag is also displayed in Figure 5.22b. Panigrahi and Srivastava [280, 282] reported an approach for the fabrication of mechanically and thermally enhanced conducting rubber blends (EPDM, NBR and NR) with polystyrene (PS)-polyaniline (PANI) coreshell (PS@PANI) which acts as an excellent tracker of EM wave. They compared the electromagnetic interference shielding performance with the similar blends prepared by simple individual mixing of PS, PANI and rubber under identical conditions. Their findings indicated the high shielding efficiency of rubber/PS@PANI blends (~30 dB: 1–8 GHz) compared to the blends prepared individually from PS, PANI and respective rubber (~18 dB: 1–8 GHz). Such high performance of rubber blends is attributed to the trapping of EM wave through enhanced internal reflection due to typical core-shell morphology of PS@PANI. It is anticipated that such lowcost, lightweight, corrosion-resistant and environmentally stable sheets of PS@PANI rubber blends could provide an effective alternative as EMI shielding material for commercial application purposes. Table 5.3 records the corresponding preparative methods used, percolation threshold, wt% of filler in composite, its thickness, conductive
300 Hybrid Nanomaterials frequency range and EMI shielding data of PANI, PPY and their graphene and graphite oxide composites.
With the advancement of more and more electronic instruments operating at different frequencies, there is a an urgent need for shielding due to their mutual interference. By conventional, metal-based shielding materials have been used. However, taking light weight and corrosion resistance into consideration, current developments on the carbon and intrinsic polymerbased conducting polymer nanocomposites has been receiving more attention in electromagnetic shielding. In view of this, the focus of this article has been on composites derived from carbon, graphene, graphene oxide, carbon nanotubes and conducting polymers. In view of this, a considerable amount of work has been reported on different types of nanocomposites for EMI shielding. But the imporant issue in the development of composites of carbon nanostructures remains dispersion. In addition, it is mandatory to improve their application over a wide range of frequencies and to understand shielding mechanism.
Acknowledgement S.K. Srivastava is thankful to his research scholars Dr. Ritwik Panigrahi and Kunal Manna for their help in literature survey and proof reading respectively.
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318 Hybrid Nanomaterials
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6 Preparation, Properties and the Application of Hybrid Nanomaterials in Sensing Environmental Pollutants R. Ajay Rakkesh, D. Durgalakshmi and S. Balakumar* National Centre for Nanoscience and Nanotechnology, Guindy Campus, Chennai, India
Abstract Environmental pollutants pose a major world problem which nanotechnologists are currently facing. This concern has led to efforts in developing new and ecofriendly techniques for the detection of gas molecules hazardous to the environment and human health. Conductometric gas sensors have been widely used in the fields ranging from health and safety to emission control in combustion process. Hybrid nanomaterials have emerged as promising materials for next generation gas sensors due to their unique physiochemical properties. This chapter provides a brief summary of recent research progress in the preparation, properties and application of hybrid nanomaterials in the sensing of environment pollutants. Keywords: Hybrid nanomaterials, conductometric gas sensors, environmental pollutants, hazardous gas molecules, graphene nanosheets
Drastically increasing environmental pollution has been accepted as a major concern in the universe, and the detection of such pollutants has become a high priority issue for human health [1–5]. This concern has led to efforts to develop new and eco-friendly techniques for the detection of gas molecules hazardous to the environment . Conductometric gas
*Corresponding author: [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (321–348) © 2017 Scrivener Publishing LLC
322 Hybrid Nanomaterials sensors have grown to be an emerging technology for monitoring flammable, explosive and toxic gases in the environment. These most recent sensors are admired because of their rapid response, selectivity, sensitivity, low cost and efficient detection property [7–11]. The marketing of gas sensor devices for the monitoring of environmental pollutants has been growing rapidly. The development of low-cost, highly reliable, miniaturized and portable sensing devices for the detection of environmental pollutants are urgently needed. The recent trends in the development of gas sensors have been examined in reviews by Miller et al. , Perreault et al. , and Fine et al. . A gas sensor is a device that consists of three basic components: (i) hybrid sensing material where the gas molecule recognition system or chemical reactions takes place, (ii) transducer which acts as an interface between the sensor and the gaseous environment (the developed physiochemical reaction must be transformed into an electrical output voltage, current or resistance), and (iii) signal processing system to measure the physical parameter and feed into a readable output [15–20]. The present chapter discusses the application of hybrid nanomaterials in sensing of environmental pollutants. Among typical gas sensors, special focus will be given to resistive-type sensors, which change their electrical conductivity in contact with some gases. They are the most suitable in terms of quick response to the occurrence of the chemical analyte of interest, the most facile and easy to fabricate, and have the potential to be selective to the gas of interest in the presence of interfering compounds [21, 22]. Metal oxides represent an important class of materials whose properties cover the entire range from insulators, semiconductors to metals. In addition, gas sensors based on inorganic metal oxide nanomaterials, such as molybdenum oxide (MoO3), titanium oxide (TiO2), tin oxide (SnO2), zinc oxide (ZnO), tungsten oxide (WO3), silicon oxide (SiO2), etc., show better sensing activity due to changing surface charge and oxygen stoichiometry in the sensing material [23–27]. However, these gas sensors require very high temperatures (~200−400 °C) to detect specific gaseous species. In order to overcome these issues, gas sensors based on organic conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and poly(3,4ethylenedioxythiophene) (PEDOT) are preferred as sensing materials because of their high conductivity and desired functionality which enhance the gas-sensing activity [28, 29]. However, they are occasionally unstable and show poor sensitivity due to the presence of environmental moisture in the material by high affinity of conducting polymers. Because of the abovementioned limitations, the use of conductometric gas sensors for practical applications is very limited.
Hybrid Nanomaterials in Sensing Environmental Pollutants 323
Number of reports
Organic-Inorganic sensors Organic-Inorganic patents Organic-Inorganic hybrids
1400 1200 1000 800 600 400 200 0 1995
Scheme 6.1 List of Publications and patents in the field of hybrid nanomaterials. (Reproduced with permission from ; Copyright © 2015 American Chemical Society).
Problems related to high operating temperature in inorganic materials and poor stability and low conductivity of organic materials, limits their use in gas sensor fabrication. The incorporation of hybrid nanomaterials of two different classes in a gas sensor may result in enhanced and excellent gas-sensing activities [30, 31]. It has been forecasted that two different energy bands combined with the use of organic and inorganic materials in hybrid forms may assist to get rid of their practical limitations due to synergetic effects, thus leading to the fabrication of efficient gas-sensing devices [32, 33]. These hybrid nanomaterials have received great interest in the field of gas-sensing applications. This chapter focuses on the preparation, properties and application of hybrid nanomaterials in sensing of environmental pollutants.
Hybrid Nanomaterials: Smart Material for Sensing Environmental Pollutants
Hybrid materials are composites consisting of two materials at nanoscale level. Commonly one of the materials is organic and another is inorganic
324 Hybrid Nanomaterials R H3C Si CH3 OH Si
Si O O O O O Si O Si
Strength of interaction
N+ R O–
M O O O
N+ R O–
N+ R O–
Si Si O O O O Si O Si Ionic
H OH Si O
Si Si O O O O Si O Si
H-bonding Si O
Si Si O O O O Si O Si
Figure 6.1 Possible strength of interaction in hybrid nanomaterials. (Reproduced with permission from ; Copyright © 2007 Wiley-VCH Verlag GmbH & Co.).
in nature. Figure 6.1 schematically defines the possible interactions connecting the organic and inorganic species on which hybrid materials can be classified. Class I hybrid materials are made up of weak interactions (hydrogen bonding, van der Waals or weak electrostatic interactions) between the two components. Class II hybrid materials are made up of strong chemical interactions between the components . The field of hybrid nanomaterials is a rapidly growing research area in advanced nanomaterials science. These materials can combine dissimilar properties of organic and inorganic components in one system. Due to many possible combinations of elements, the field of hybrid nanomaterials is very attractive as it provides the opportunity to discover a limitless set of new materials with a broad spectrum of known and unknown
Hybrid Nanomaterials in Sensing Environmental Pollutants 325 properties. Moreover, it has the possibility of developing multifunctional materials. For example, impregnation of inorganic nanomaterials with specific (optical, electrical, electronic or magnetic) properties in the organic networks that give desired functional properties. The most fascinating property of hybrid nanomaterials for many applications is their material processing temperature. Compared to pure inorganic components which require a high temperature treatment for their materials processing, hybrid nanomaterials require low temperature for materials processing due to the arrangement of crosslinked inorganic networks from the smaller molecular precursors [36–38]. Hence, these materials can be tuned in either bulk or thin film form. Although, from an economical point of view, hybrid thin film materials could be used in the biomaterials sector for anticorrosion or scratch-resistant coatings [39, 40]. The transition from the bulk to nanoscale level of the materials leads to changes in their own physical properties, the so-called quantum size effect. If the two components have opposite polarities, the system would thermodynamically phase separate. Thus, it could be another interesting property of hybrid nanomaterials that could react with environmental changes because of their compositional variations at the nanoscale level resulting in a fine tuning of the material properties [41–43]. The properties of hybrid nanomaterials depend not only on that of their individual components, but also on their morphology and interfacial interactions. The nature of interaction between the organic and inorganic components also has an impact on the properties of the materials. For example, if the surface atoms strongly interact with molecules by chemical bonding, resulting in electron transfer process, that opens up routes to novel technologies in forthcoming fields like electroactive materials, electrochromic materials, sensors and biohybrid materials, etc. [44, 45]. The most trusted and demanding field of research relates to the utilization of hybrid nanomaterials as sensing materials for monitoring environmental hazardous pollutants. Many studies have reported on the adsorption of gas molecules onto organic−inorganic nanocomposite materials. The role of physiochemical parameters, such as surface area, interfacial interaction of species, pore volume, and surface active sites, on the properties of hybrid materials to attain highly selective and sensitive detection of environmental pollutants, are still a matter of significant interest in the field of hybrid gas sensors [46–48]. Apart from interfacial interactions, morphology-dependent properties are an important characteristic that influences the hybrid nanomaterials performance. Hence, in the attempt to develop hybrid nanomaterials, a long-standing objective is to develop facile synthetic strategies to control the morphology, size, shape and
326 Hybrid Nanomaterials composition. This control allows the tuning of properties of hybrid nanomaterials. The versatile methods to synthesize hybrid nanomaterials of desired properties for specific application in sensing environmental gaseous pollutants are discussed below.
Synthesis Methods of Hybrid Nanomaterials
Many preparation methods have been used to synthesize hybrid nanomaterials for conductometric gas sensors. Some factors need to be examined when selecting a synthesis method which is sufficient to yield high-quality hybrid nanomaterial films. The hybrid nanomaterial thin films contain metal or semiconductors on the surface that show nanoporouslike structures. This could absorb the harmful gaseous molecules from the environment by allowing them to penetrate into the hybrid nanostructure matrix, resulting in a strong sensing behavior. Common techniques for making hybrid nanomaterial films for gas sensors are mainly the sol-gel, hydrothermal/solvothermal, layer-by-layer deposition, template-assisted deposition and physical vapor deposition (PVD) methods.
Sol-gel technique is a wet chemistry-based synthesis route (Figure 6.2) for hybrid materials which uses hydrolysis and polycondensation of RO RO RO
RO RO RO
HO RO RO
RO RO RO
n OR RO Si Si RO OR O Rings OR RO
HO RO HO
RO OR Si O O
OR Chains Si O RO O Si
Figure 6.2 Fundamental reactions involved in the sol-gel method. (Reproduced with permission from ; Copyright © 2007 Wiley-VCH Verlag GmbH & Co.).
Hybrid Nanomaterials in Sensing Environmental Pollutants 327 alkoxide-based metal precursors such as R4–nSiXn compounds (n = 1–4, X = OR’) . In this synthesis route, “sol” is prepared by dissolving an alkoxide precursor in a suitable solvent (e.g., various alcohols) and a “gel” is obtained by controlled addition of gelating agent (water) under acidic or basic conditions to begin the condensation reactions leading to the formation of a three-dimensional oligometric network by chemical reactions [49, 50]. The condensation process, solvent evaporation and syneresis of a species result in the formation of a gel. The obtained gel can be transformed into films on the desired sensor substrate via dipping, spin-coating or spraying techniques. Then, the deposited film is treated with a low temperature annealing process to adhere on the substrate and further heat treatment is allowed to remove all the organic residues and water molecules through evaporation, and also to provide densification. For the deposition of a hybrid nanomaterial film, extra care must be taken to ensure treatment of the film below the decomposition points of its components [51, 52].
The hydrothermal technique is a versatile route for the synthesis of hybrid nanophase materials, generally performed in polar solvents. Synthesis reactions can be carried out in a temperature range of 100° to 200 °C and at a pressure range of ~1 atmosphere; in most hydrothermal techniques experimentation takes place below the supercritical temperature of water, i.e., 374 °C [53, 54]. This method is greatly suited for fabrication of specific morphology and controlled size of the hybrid nanostructures. Moreover, it is an environmentally friendly synthesis route because reactions are carried out in closed setup at low temperatures, mostly with Teflon-lined stainless steel autoclave being used for the stimulated reaction process. It is economically beneficial because it consumes very little energy. The reactions can be carried out in double-distilled water or in any other polar solvent. The main advantages of the hydrothermal route are that (a) chemical reaction kinetics are greatly increased with a small change in temperature, (b) new metastable materials can be created, (c) the final products of the materials are of high purity even from impure feedstocks, (d) it is cost-effective with no need for precipitating agent, (e) it is eco-friendly, and (f) hybrid hydroxylated clays and zeolite cannot be prepared by any other synthesis method [55–57].
Layer-by-Layer Deposition Method
The synthesis technique for the controlled fabrication of hybrid nanomaterials relates to surface potential, and their interface with counterions is called
328 Hybrid Nanomaterials Negatively charged surface
Layer-by-layer assembly Positively charged polyelectrolytes
Negatively charged clusters
Figure 6.3 Schematic representation of layer-by-layer deposition method.
layer-by-layer deposition method. It allows for the fabrication of organicinorganic hybrid materials using different surface potentials of the organic and inorganic electrolytes. For example, the substrate surface with anionic charges can be used to deposit cationic charged polyelectrolytes over it (Figure 6.3) . After deposition of the organic species, the original charge on the surface is overcompensated and clusters or particles can be deposited again as a layer of negatively charged inorganic building blocks. Subsequently, a layer of the organic species is deposited over it and so on. This technique provides a procedure for the chronological deposition of oppositely charged building blocks . Because of its step-by-step characteristics, heterostructured multilayer hybrid materials are readily fabricated by this technique with controlled layer thickness, composition and function. This method is also used for the surface functionalization of nanoparticles.
Template-Assisted Synthesis of Hybrid Materials T
Template-assisted method can be used to develop hybrid materials for various purposes, like loading the space and/or directing the formation of specific nanostructures , schematically represented in Figure 6.4. Templates for the fabrication of hybrid nanomaterials can be preformed for different dimensions such as nanoparticles with 2D or 3D structures . In addition, the self-assembly of single molecules into layered 2D and 3D nanostructures can also be fabricated as a template. The most complicated 3D structures include hexagonal rod-like structure, lamellar structure or honeycomb interpenetrating networks [60, 61]. Some of the parameters, such as the concentration of the surfactants, the temperature and the pH, are desired for the fabrication of hybrid nanostructures. Once the hybrid nanomaterials are formed, they can be easily removed from the template. After removal of the template, which usually occurs by heat treatment at temperatures above 450 °C, the materials are highly inorganic in nature.
Hybrid Nanomaterials in Sensing Environmental Pollutants 329 Increasing surfactant concentration
Figure 6.4 Schematic illustration of template-assisted synthesis of hybrid materials. (Reproduced with permission from ; Copyright © 2007 Wiley-VCH Verlag GmbH & Co.).
Power supply Target
Target atoms Plasma
Hybrid material coated layer
Figure 6.5 Schematic illustration of PVD technique for the fabrication of hybrid nanostructures.
Physical Vapor Deposition
In PVD techniques, the synthesized hybrid nanomaterial to be deposited is put into the gas phase by sputtering (bombardment of the material by ions) (Figure 6.5). In a low pressure gas high energy field is used to drive ionization, creating a large number of ions and free electrons. Ions from plasma are attracted towards a target made of the hybrid composite material to be deposited. The ions strike the target, physically knocking target atoms loose, and the target atoms are then coated on the substrate. The substrate containing hybrid films was shown to have a level of conductivity and upon exposure to hydrogen, the conductivity was shown to increase [62, 63]. The
330 Hybrid Nanomaterials PVD is performed under vacuum; this could be an expensive technique for large-scale production. Growth rates of around 10 nm an hour make this technique less suitable for high throughput for industrial applications.
Gas-Sensing Principle of Hybrid Nanomaterials
The basic principle behind the gas-sensing behavior of hybrid nanomaterials is the change in their electrical resistance upon exposure to a gas due to electronic exchange between the materials and the gaseous molecules . It has been confirmed that introduction of oxidizing/reducing gas molecules onto hybrid nanomaterial film results in a change of electrical resistance. If we stop the flow of gas and purge the fresh air from the analyzing chamber, the original resistance value of the film is restored. The experimental change in the resistance value is due to the adsorption of oxidizing/reducing gaseous molecules onto the surface of the hybrid nanomaterial film. The interaction between the gaseous molecules with nanomaterial film that has been embedded by metal/metal oxide nanomaterials results in the acceptance/donation of electrons determined by the nature of gaseous molecules, thus decreasing/increasing the electrical resistance [64, 65]. Some of the factors that influence the gas-sensing behavior are particle size effect, depletion layer depth, quantum confinement effect and surface chemical bonding processes. In order to understand these better, it is important to attain background knowledge on the gas-sensing mechanism of metal oxide hybrid materials. It can be explained as having metal atoms (it could be one or more similar/dissimilar metals) bound to oxygen atoms. The active surface sites of the hybrid metal oxides are most important for chemical gas-sensing field.
Basic Mechanism of Gas Sensors Using Hybrid Nanomaterials
To understand their working mechanism, the responses of conductometric gas sensors have been widely investigated with respect to adsorption on the surface active sites, chemical processes, and resulting conductivity changes. Several models have been quoted to explain the sensing mechanisms on the basis of different sensing materials. Assuming an n-type semiconducting metal oxide-based conductometric sensor (e.g., ZnO, MoO3, SnO2), the sensing response of a target gas presented in the analyzing chamber relies on the surface reactions which occur between adsorbed oxygen species and the target gas . In air, oxygen molecules adsorbed on the surface dissociate
Hybrid Nanomaterials in Sensing Environmental Pollutants 331 o– e–
e– e– o–
Conduction band electrons (a)
e– Adsorbed oxygen
e– e– o–
Figure 6.6 Generalized mechanism of a gas sensor using n-type semiconducting metal oxides.
and trap the free electrons because of their high electron affinity, forming a potential barrier which determines the electrical resistance value. When a sensing device is placed in the presence of a reducing gaseous system like the environment, e.g., H2, CO, hydrocarbons, methanol, etc., the gas molecules adsorb on the sensor surface and react with the active oxygen species. Depending upon the temperature and reactivity of the sensing materials, the physiochemical reaction of surface oxygen species will vary [67, 68]. The interaction of the hybrid nanomaterials with the target gas decreases the potential barrier and allows electron to flow easily; in this manner the electrical resistance is reduced as a function of concentration of the target gas. On the basis of the scheme shown in Figure 6.6, the gas-sensing behavior of hybrid nanomaterials can be observed . The next section focuses on the application of hybrid nanomaterials-based conductometric gas sensors in detecting harmful gaseous pollutants for environmental monitoring.
Hybrid Nanomaterials-Based Conductometric Gas Sensors for Environmental Monitoring
The gas-sensing behavior of hybrid nanomaterial-based conductometric gas sensors is discussed in this section. Development in the area of sensing
332 Hybrid Nanomaterials materials and improvement in gas response parameters, such as stability, repeatability, detection limit, sensing range, and response/recovery time, are briefly discussed herein. Sensor device fabrication using hybrid nanomaterials for commercial forecasting and also to develop environmental monitoring of harmful pollutants is highlighted.
Hybrid Nanomaterials for Volatile Organic Components
The assessment of indoor air purity is currently an extensive environmental concern. Indoor pollutants consisting of volatile organic components (VOCs) like chloroform, ethanol, acetone, etc., may cause environmental pollution [69, 70]. Most people can smell very high levels of some VOCs but others are odorless. Odor does not specify the level of hazard during inhalation. There are different VOCs produced from building materials as well as home products and they can easily evaporate at room temperature. However, the detection of very low concentrations of VOCs present in our surroundings is a challenging task. Thus, there is great demand for the improvement of a standard technique to monitor indoor VOCs. Recently, hybrid nanomaterials were fabricated using sol-gel-derived spin-coating technique that has enabled the selective sensing of VOCs . In these hybrid nanomaterials, the selectivity is obtained by constructing different metal/metal oxides decorated onto the graphene nanosheets. This throws out the need for polymers in conductometric gas sensor devices for each specific analyte. The use of hybrid nanomaterial-based VOC sensors is briefly explained in this section. Wang et al.  used a facile, surfactant-free hydrothermal route to synthesize NiO and Au-loaded NiO hybrid nanostructures for sensing acetone vapors. The fabricated Au-loaded NiO hybrid sensor showed excellent sensing performance compared with NiO nanomaterial. Based on the practical application, response and recovery time are significant factors of conductometric gas sensors; rapid response and recovery typically permit only a short detection time. Figure 6.7 represents the electron micrographs of Au-loaded NiO hybrid nanostructures and dynamic sensing response–recovery curves with various acetone concentrations. The acetone vapor responses to 5, 10, 20, 50 and 100 ppm concentrations were 4.1, 5.6, 7.8, 10.2 and 15.3, respectively. The authors of the study  explained that the Au-loaded gas sensor showed about four-fold enhancement in sensitivity compared to pure NiO nanomaterial, which consecutively verifies the endorsement effect of the unique nanostructure and Au nanoparticles . Furthermore, the conductometric sensors showed ultrafast response, as exhibited by the drastic changes in resistance value when exposed to the target gas.
Hybrid Nanomaterials in Sensing Environmental Pollutants 333 0.9 0.8
Resistance (M )
Au-loaded NiO NiO
0.2 0.1 0.0
200 300 Times (s)
6.7 Electron micrographs with different magnification of Au-loaded NiO hybrid nanostructures and dynamic sensing response–recovery curves with various acetone concentrations. (Reproduced with permission from ; Copyright © 2012 Elsevier). 80
Methanol Ethanol Methyl acetate Acetone Water
Methanol Ethanol Methyl acetate Acetone Water
60 R/Ro (%)
Sensitivity ( R/Ro (%))
1200 1500 1800
O OH O
Figure 6.8 Gas-sensing response of (a) Ag-RGO/PIL and (b) pure RGO-based sensors upon periodic exposure to different polar VOCs. Schematic representation of chemical vapor absorbed onto the Ag microspheres decorated RGO sheets (c) and pure RGO sheets (d). (Reproduced with permission from ; Copyright © 2014 Springer).
Tung et al.  synthesized silver nanoparticle-decorated reduced graphene oxide (Ag-RGO) by a layer-by-layer deposition technique, in which poly(ionic liquid) (PIL) was used as a capping agent to develop VOC sensors. The enhanced sensing performance of the fabricated Ag-RGO/PIL hybrid sensors was compared with as-prepared RGO. Figure 6.8a shows the Ag-RGO/PIL sensing responses when exposed to VOC. The authors
334 Hybrid Nanomaterials explained  that upon purging the VOC from the sensor device, there was a large increase in resistance value with a response of ΔR/R0, followed by recovery over 5 min with nitrogen purging. The sensing response and recovery times are short (4–5 s to respond, 10 s to reach saturation and 100 s to recover initial conditions). In addition, the pure RGO sensor (Figure 6.8b) shows a lesser sensing performance with weak selectivity under different solvents. The author proposed that the enhanced sensing performance of Ag-RGO/PIL sensors arose from the synergistic effect of hybrid nanomaterials . A huge amount of organic pollutants can be adsorbed onto the RGO surfaces due to the large surface area and chemically active sites present in the hybrid material, as illustrated in Figure 6.8c, and thus efficiently transfer electrons between sensing material and target pollutant molecules for detection . For comparison, pure RGO and pure Ag-PIL-based sensors were also tested by exposing them to VOC for reference (Figure 6.8d). Even though Ag-PIL itself shows some sensing response to the target molecules, the sensing response is not well defined .
Hybrid Nanomaterials for Ammonia Detection
Ammonia is a colorless toxic gas which occurs naturally in our ecosystem through human wastes and industrial effluents. It has a characteristic sharp and pungent odor but we can sense it only at higher concentrations (more than 50 ppm) . So, it is common to be exposed to ammonia at lower levels in our day-to-day life. Ammonia is a corrosive substance and its main toxic effects on the human body are irritation to the skin, eyes, throat and respiratory systems . Hence, it is highly necessary to fabricate conductometric gas sensors with enhanced sensitivity and selectivity that can monitor and control low concentrations of ammonia. Enhanced selectivity, high sensitivity and greater repeatability towards the conductometric gas-sensing of ammonia at room temperature were obtained by Q. Feng et al. using reduced graphene-encapsulated Co3O4 nanofibers (Figure 6.9) . It was demonstrated that the rGO–Co3O4 hybrid nanofiber-based conductometric gas sensors showed good sensitivity towards the ammonia gas with different concentrations starting from 5 ppm to 100 ppm and greater selectivity at room temperature. This kind of hybrid sensor provides an ultimate gas response of only 4 s to attain 90% of the saturated value. The author claimed that the possible reason for such desirable response features may be attributable to the unique interaction of ammonia with the outer layer of rGO and the polarized C-Co3+ covalent centers within the porous nanofibers.
Hybrid Nanomaterials in Sensing Environmental Pollutants 335 )
90% of steady rate
100 105 Time (min)
1 wt % rGO/Co3O4
90% of maximum sensitivity
1.4 1.2 90
Resistance ( 10
NH3 T = 20 C unit: ppm
Resistance ( 10
y = –59.9958*exp(–x/40.98896) + 72.09816
Figure 6.9 Response–recovery curves of the rGO–Co3O4 composite nanofibers for ammonia gas sensing at room temperature. (Reproduced with permission from ; Copyright © 2016 Elsevier).
HOOC RGO-SnO2 sensing layer
400 1200 2000 2800
0.7 Gas off 0.6
1500 3000 Time (sec)
Recovery time (sec)
Hydrothermal (120 C)
Gas 0 on 400 1200 2000 2800 0 1000 2000 3000 Time (sec)
6 5 4 3 2 1 0
SnO2 HOOC nanoparticles
900 750 600
10:4 10:5 10:8 1:1 SnO2 10:3 Hydrothermal
450 300 150 0
Figure 6.10 Dynamic response of (a) 10:3 (RGO: SnO2) and (b) 10:4 (RGO: SnO2) towards four different concentrations of ammonia (400–1200 ppm). (c) Comparative response of intrinsic RGO, SnO2 and RGO–SnO2 hybrid sensors prepared hydrothermally and by varying the wt% of GO and SnO2 nanoparticles towards 1200 ppm ammonia, and (d) comparative plot of recovery times of intrinsic RGO, SnO2 and hybrid RGO–SnO2 sensors. (Reproduced with permission from ; Copyright © 2015 Royal Society of Chemistry).
Ghosh et al. have developed RGO−SnO2-based hybrid sensors for ammonia sensing (Figure 6.10) . It is demonstrated that RGO, intrinsically a p-type material, was prepared by thermally reducing the graphene oxide (GO). The n-type SnO2 nanomaterials were prepared using the hydrothermal method. The RGO−SnO2 hybrid nanostructure was prepared by two-step wet chemical route. The sensors show very large
336 Hybrid Nanomaterials response (larger than RGO or SnO2 response towards ammonia) and fast response, good selectivity and recovery even at room temperature. The enhanced sensing behavior of the hybrid nanomaterial film is due to the combination of p-type and n-type sensing material. The SnO2 donates electrons to RGO which recombine with the holes of RGO thereby shifting the Fermi level, and thus a depletion region forms. This depletion region is an additional site for the target gases and attracts electrons from the donor molecules (ammonia) and results in increase in conductivity. The response of the sensor was carried out in the presence of ammonia (25−2800 ppm) and different VOCs. Unique gas-sensing properties of hybrid nanomaterials are mainly associated with the heterogeneous interfaces that have been reported by Xu et al. . They demonstrated with SnO2−SnS2 hybrid nanomaterials prepared by the oxidation of SnS2 at 300 °C at different time intervals and reported high response to NH3 at room temperature (Figure 6.11). With
Gas in Gas flow rate control module
Test chamber 2.0
Data control module
SnS2 SnO2-SnS2-0.5 SnO2-SnS2-1 SnO2-SnS2-2 SnO2-SnS2-4 SnO2
2 1 0
300 400 Time (s)
U Gas sensor unit
300 Time (s)
SnO -SnS -1 2 2 SnO -SnS -0.52 2 2 SnO -SnS -2 2
SnO -SnS -4 2
100 200 300 400 500 100 ppm Concentration (ppm)
1.8 50 ppm 1.5
25 ppm 10 ppm
900 Time (s)
1500 Time (s)
Figure 6.11 (a) Schematic diagram illustrating the gas-sensing test platform. (b) Dynamic response-recovery curves. (c) Repetitive dynamic response-recovery curves. (d) Response-recovery curves of SnO2−SnS2–1 to NH3 in the concentration range from 10 to 500 ppm. The inset shows the response as a function of NH3 concentration. (Reproduced with permission from ; Copyright © 2015 American Chemical Society).
Hybrid Nanomaterials in Sensing Environmental Pollutants 337 the increasing oxidation time, the relative concentration of interfacial Sn bonds, O−Sn−S, among the total Sn species of the SnO2−SnS2 hybrids first increased and then decreased . Interestingly, it can be found that the response of SnO2−SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3 . They revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and also the importance of the interfacial bonds, which offer guidance for the material design to develop hybridbased sensors.
Hybrid Nanomaterials for Hydrogen Detection
Hydrogen has attracted significant attention among researchers as one of the best renewable and clean energy sources for society. Hydrogen is seen as an alternative energy source for fossil fuels because of its desirable properties like high burning velocity, high combustion heat, wide flammable range and low ignition energy . The final product of combusted hydrogen is water, which can be recycled to form hydrogen and oxygen. However, it has a low flammability point in air . Due to its colorless, odorless and tasteless nature, accurate detection and monitoring of hydrogen leakage is necessary for secure production, storage, and application. Phan and Chung  demonstrated a simple chemical method for hydrogen sensing using palladium nanocube-loaded graphene hybrid materials (Figure 6.12). The authors summarized the size effect of Pd cube on the sensing response value (S) of a H2 sensor using a Pd cube-graphene hybrid. The response value (S) of the H2 sensor increases with increasing Pd cube size. Moreover, the Pd cube-graphene hybrid-based H2 sensor worked well at room temperature. The basic sensing mechanism shows that Pd absorbs H2 molecules and changes them to PdHx, which possesses a lower work function than the pure Pd nanocube material, which encourages the free electron in Pd to transfer to the graphene to increase the resistance in Pd cube-graphene hybrids. The Pd cubes of 70–85 nm size were the best catalysts for H2 detection. They found that the effects of Pd cube size on the detection performance of H2 sensor based on Pd cubegraphene hybrid were explained by the spillover sensing mechanism and influence of Pd size on the hybrid state of two materials . Esfandiar et al.  fabricated novel Pd-WO3 nanostructures that were incorporated onto partially reduced graphene oxide (PRGO) sheets using a hydrothermal process (Figure 6.13). Gas-sensing properties of hierarchical
338 Hybrid Nanomaterials 20
16 Response (S) to 1% H2
Response (S) to 1% H2, @RT
14 12 10 8 6
12 8 4
Pd-Gr-25 Pd-Gr-40 Pd-Gr-70
40 55 70 Pd cube size (nm)
Tepmperature ( C)
Figure 6.12 (a) Effects of Pd cube size and (b) working temperature on H2 sensing properties of Pd cube-graphene hybrids and (c) spillover zone sensing mechanism. (Reproduced with permission from ; Copyright © 2014 Elsevier).
nanostructure thin films were studied for different hydrogen concentrations (from 20 ppm to 10,000 ppm) at room temperature. It was found that in the Pd-WO3 hybrid nanostructure, oxygen molecules adsorbed on the surface of WO3 nanomaterial to form a depletion layer with low electrical conductivity by capturing electrons from the conduction band of WO3. Hydrogen gas can react with oxygen-adsorbed groups and then remove the charged oxygen by H2O desorption. The electrons generated from these reactions can alter the space-charge layer in the grain boundaries and direct an injection of electrons to the depletion layer, thereby increasing the conductivity. Moreover, Pd nanoparticles increase the rate of these reactions by dissociation of the hydrogen molecules into active atoms. It reduces the height of Schottky potential barrier in Pd/WO3 interfaces. All of these results show the donation of electrons into the conduction band of WO3 and increases the electrical conductivity of the Pd-WO3 film, as observed from dynamic responses for sensors without graphene.
Hybrid Nanomaterials in Sensing Environmental Pollutants 339
200 160 Time (s)
H + H
Sensitivity ( G/G0)
1800 Pd-WO3 precursor
Response time Recovery time H2: 100 ppm
120 80 40
5000 ppm 0
25 25 C 100 C 150 C
50 75 100 125 150 Tepmperature ( C)
20 ppm 100 ppm
2500 ppm 500 ppm
Partially reduced graphene xoide
1000 1500 Time (s)
Sensitivity ( G/G0)
4.7 eV (
3.3–3.9 eV ( Wo3)
5.2 eV ( pd)
25 C 100 C 150 C
Sensitivity = kFC
2000 4000 6000 8000 10000 H2 concentration (ppm)
Sensitivity ( G/G0)
Pd SB (Air) EF
H2 500 ppm
CO CH4 500 ppm 500 ppm on off on off
KF: 2.6, 1/n:0.52 KF: 0.43, 1/n:0.89 KF: 0.42, 1/n:0.83
600 900 Time (s)
Figure 6.13 The procedure for hydrothermal synthesis of Pd-WO3 nanostructures on GO and PRGO are presented. The proposed mechanism for H2 sensing is also presented in the circle. (b) Changes in the energy band diagram of the Pd/WO3 on a graphene sheet due to hydrogen dissociations are schematically demonstrated before and after H2 exposure. (Reproduced with permission from ; Copyright © 2014 Elsevier).
However, Pd-WO3/PRGO in the graphene sheets can alter the potential barrier at the graphene-WO3 interfaces and give better pathways for carriers into the electrodes . Moreover, it is found from the above discussion that the graphene sheets can perform as a template for customized growth with low aggregation resulting in enhanced electrical conductivity between the Pd-WO3. Due to these factors, it is confirmed that the Pd-WO3/PRGO nanostructure could reveal an excellent gas sensor for monitoring both oxidizing/reducing gaseous molecules.
Hybrid Nanomaterials for Nitrous Oxide Detection
Nitrous oxide (NO2) is a toxic gaseous pollutant in the atmosphere and is hard to decompose at room temperature. It generates acid rain and spoils the ozone layer. The major risk of NO2 to human health is that it causes
340 Hybrid Nanomaterials lung-related problems. People affected with asthma are mostly sensitive to the gas, and lungs may become reddened, which leads to breathing problems [84, 85]. Air pollution caused by NO2 gas has turned out to be an important issue and the advancement of techniques for detecting NO2 gas is very urgent in the near future. Liu et al.  found that ZnO–reducing graphene oxide (RGO)-based conductometric gas sensors exhibit greater sensitivity, and much shorter response and recovery time than RGO-based sensors for the detection of NO2, representing that the gas-sensing performances had been improved by the addition of ZnO nanomaterials into RGO matrix . They dispersed ZnO–RGO hybrids in DMF solvent as sensing materials for fabrication of the NO2 sensors. The sensing element was obtained by dip-coating the ZnO–RGO dispersion onto the ceramic substrate prior to testing. The gas-sensing response and recovery of ZnO–RGO hybrid sensor and RGO sensor showed 5 ppm concentration of NO2 and the response and recovery curve of RGO sensor towards 25 ppm NO2, respectively, as shown in Figure 6.14. Deng and coworkers demonstrated gas sensors based on Cu2O nanowire decorated onto RGO sheets for the detection of NO2 gas . The nanowires had a highly anisotropic nature and possessed a different octahedral morphology. The sensing response of Cu2O/RGO hybrid material was 67.8% for 2 ppm concentration of NO2 gas, much higher than RGO (22.5%) and Cu2O nanowires (44.5%). The hybrid nanomaterial displayed a considerably better sensing performance at concentrations greater than 1.2 ppm. The nanowire architecture of the hybrid material can be kinetically inhibited by the variation of growth conditions, giving rise to high
14 499 s
10 8 6 4 2
1200 800 Time (s)
20 40 60 80 100 120 140 160 180 Time (s)
Figure 6.14 Response and recovery curve of sensor based on (a) ZnO–RGO hybrids towards 5 ppm NO2, and (b) RGO towards 25 ppm NO2. (Reproduced with permission from ; Copyright © 2014 Elsevier).
Hybrid Nanomaterials in Sensing Environmental Pollutants 341 specific surface area and improved conductivity. The RGO–Cu2O hybrid nanomaterials achieved an efficient sensitivity to NO2 at room temperature . The unique features of RGO–Cu2O hybrid nanostructures show them as a promising candidate for ultrasensitive gas sensors for monitoring hazardous gaseous molecules in the environment. Wei et al.  developed a novel SnO2 gas sensor doped with carbon nanotubes operating at room temperature by heat treatment of singlewalled carbon nanotubes (SWCNTs) followed by spin-coating techniques. The comparative gas-sensing studies revealed that SWCNTs/SnO2 hybrid nanosensor displays much higher sensitivity and recovery responses in monitoring NO2 gas at room temperature than the pure SnO2 sensor. It shows that doping with SWCNTs improves the sensitivity of SnO2 hybrid sensors. The authors schematically explain the gas-sensing mechanism of SWCNTs/SnO2 hybrid nanomaterials with the changes of the conduction and valence bands for hybrid SWCNTs/SnO2 sensors. Figure 6.15 shows two depletion layers, one is on the surface of the SnO2 material, and the other lies between the SWCNTs and SnO2 interfaces. Before absorption of the NO2 gases, the depletion widths of these two layers are given by d1 and d3, respectively. After adsorption, the widths are shown in d2 and d4, respectively. The potential barriers between the SWCNTs–SnO2 may change at the interfaces. If both these effects expand the depletion layers at the junction, then the resistances of the sensor increase with the gas concentrations . The gas molecules can easily be monitored at room
O NO2– – NO2– O O2 – O2
Depletion layer Ec
in oxidizing gas (NO2)
Detected gas in
1 cycle Rair = R1 V5–Vair Vair Vair
Detected gas off Rgas = R1
Ev Grain boundary
–500 200 250 300 350 400 450 500 550
Figure 6.15 Schematic representation of a potential barrier to electronic conduction at grain boundary for hybrid SWCNTs/SnO2 sensors and response curve of hybrid SWCNTs/SnO2 sensor to NO2 at room temperature. (Reproduced with permission from ; Copyright © 2004 Elsevier).
342 Hybrid Nanomaterials temperature due to the amplification effects of heterojunction structure. However, the gas-sensing activity greatly depends on the gas-adsorptive (SWCNTs) nanomaterial over the semiconducting SnO2 layer and also the morphology of the adsorptive layer.
This chapter reviewed recent research progress in the preparation, properties and application of hybrid nanomaterials in sensing of environment pollutants. These hybrid nanomaterials have been found to be an outstanding substitute for conventional gas-sensing materials due to their unusual morphology-dependent electrical properties that are beneficial for gas sensor improvement, which can easily be customized by variation of their preparation routes or precursors. Among the various hybrid nanomaterial systems, metal/metal oxide decorated graphene-based sensors were reviewed in detail due to their promising properties for monitoring environmental hazardous gaseous molecules. This fascinating class of hybrid nanomaterials shows interesting electrical and molecular properties that can be easily fabricated by cost-effective methods. The chapter clearly elucidates the sensing phenomena depending on both the composition of the hybrid materials and the geometry of the structures produced. This chapter also reviews the urgent need for making measurements at room temperature in the presence of pollutants and enabling the hybrid sensors to work in a real-world environment.
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344 Hybrid Nanomaterials 25. Capone, S., Siciliano, P., Quaranta, F., Rella, R., Epifani, M., and Vasanelli, L., Analysis of vapours and foods by means of an electronic nose based on a solgel metal oxide sensors array. Sens. Actuators B 69, 230, 2000. 26. Brailsford, A.D., Yussouff, M., and Logothetis, E.M., A first principles model of metal oxide gas sensors for measuring combustibles. Sens. Actuators B 49, 93, 1998. 27. Tsujita, W., Yoshino, A., Ishida, H., and Moriizumi, T., Gas sensor network for air-pollution monitoring. Sens. Actuators B 110, 304, 2005. 28. Nylabder, C., Armgrath, M., and Lundström, I., An ammonia detector based on a conducting polymer. Anal. Chem. Symp. Ser. 17, 203, 1983. 29. Nicolas-Debarnot, D., and Poncin-Epaillard, F., Polyaniline as a new sensitive layer for gas sensors. Anal. Chim. Acta 475, 1, 2003. 30. Barsan, N., Koziej, D., and Weimar, U., Metal oxide-based gas sensor research: How to?. Sens. Actuators B 121, 18, 2007. 31. Tan, O.K., Cao, W., Hu, Y., and Zhu, W., Nano-structured oxide semiconductor materials for gas-sensing applications. Ceram. Int. 30, 1127, 2004. 32. Kruk, M., and Jaroniec, M., Gas adsorption characterization of ordered organic−inorganic nanocomposite materials. Chem. Mater. 13, 3169, 2001. 33. Tran-Thi, T.H., Dagnelie, R., Crunaire, S., and Nicole, L., Optical chemical sensors based on hybrid organic-inorganic sol-gel nanoreactors. Chem. Soc. Rev. 40, 621, 2011. 34. Kaushik, A., Kumar, R., Arya, S.K., Nair, M., Malhotra, B.D., and Bhansali, S., Organic-inorganic hybrid nanocomposite-based gas sensors for environmental monitoring. Chem. Rev. 115, 4571, 2015. 35. Kickelbick, G. (Ed.), Hybrid Materials. Synthesis, Characterization, and Applications, pp. 1–48, Wiley-VCH Verlag GmbH & Co. KgaA: Weinheim, 2007. 36. Lobotka, P., Kunzo, P., Kovacova, E., Vavra, I., Krizanova, Z., Smatko, V., Stejskal, J., Konyushenko, E.N., Omastova, M., Spitalsky, Z., Micusik, M., and Krupa, I., Thin polyaniline and polyaniline/carbon nanocomposite films for gas sensing. Thin Solid Films 519, 4123, 2011. 37. Hong, L., Li, Y., and Yang, M., Fabrication and ammonia gas sensing of palladium/polypyrrole nanocomposite. Sens. Actuators B 145, 25, 2010. 38. Chang, Q., Zhao, K., Chen, X., Li, M., and Liu, J., Preparation of gold/polyaniline/multiwall carbon nanotube nanocomposites and application in ammonia gas detection. J. Mater. Sci. 43, 5861, 2008. 39. Sanchez, C., Julian, B., Belleville, P., and Popall, M., Applications of hybrid organic–inorganic nanocomposites. J. Mater. Chem.15, 3559, 2005. 40. Gómez-Romero, P., and Sanchez, C. (Eds.), Functional Hybrid Materials, Wiley-VCH: Weinheim, 2004. 41. Haas, K.H., Hybrid inorganic-organic polymers based on organically modified Si-alkoxides. Adv. Eng. Mater. 2, 571, 2000. 42. Sharp K.G., Inorganic/organic hybrid materials. Adv. Mater. 10, 1243, 1998. 43. Hüsing, N., and Schubert, U., Aerogels—Airy materials: Chemistry, structure, and properties. Angew. Chem. Int. Ed. 37, 22, 1998.
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7 Development of Hybrid Fillers/ Polymer Nanocomposites for Electronic Applications Mariatti Jaafar School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia
Abstract The development of polymer composites containing nanosized filler has been gaining in popularity due to several advantages. One of the main advantages of using nanosized filler is the increased internal interfacial area. This results in maximized polymer-nanoparticles interactions. Addition of nanosized filler has the potential to dramatically improve polymer performances, including thermal and electrical conductivities, mechanical strength and stiffness, thermal stability, as well as flame retardancy. All of these performance benefits are available without increasing the density of the base polymer. Factors such as type of nanofiller, size and shape of the nanofiller, volume of the nanofiller in the polymer, selection of polymer matrix and processing condition need to be fully understood because they might influence the properties of the polymer nanocomposites. Furthermore, hybrid nanofillers for polymer composites are gaining acceptance because they offer a range of properties that cannot be obtained with a single type of reinforcement. The development of this material has become a popular topic in materials science due to its ability to produce materials with diversified properties. With the correct selection of nanofiller and processing, hybrid nanocomposites with desired properties can be produced. Keywords: Hybrid fillers, nanofillers, polymer nanocomposites, hybrid nanocomposites
Corresponding author: [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (349–370) © 2017 Scrivener Publishing LLC
350 Hybrid Nanomaterials
The growth of modern plastics expanded during the first 50 years of the 20th century when at least 15 new types of polymers were introduced. The introduction of thermosetting plastics, thermoplastics, natural polymers, modified natural polymers and biodegradable plastic proved to be the success of the plastic materials industry. There are hundreds of plastics available commercially but only a few are recognized as commodity thermoplastics due to the high volume of consumption of these thermoplastics as well as their relatively low price . Figure 7.1 shows the world plastics demand according to the type of polymer. Polypropylene (PP), polyethylene (PE) with low density (LD), linear low density (LLD), and high density (HD), and poly(vinyl chloride) (PVC) represent the highest percent of plastics demand. This is followed by other types of plastics, namely poly(ethylene terephthalate) (PET) and polyurethane (PUR), polystyrene (PS), acrylonitrile butadiene styrene (ABS) and others. Among the most versatile polymer matrices, polyolefins, such as polypropylene (PP), are the most widely used thermoplastics in food packaging, automobile and other industrial sectors. Their well-balanced physical and mechanical properties, easy processability at a relatively low cost, high thermal stability and resistance to corrosion make them an excellent material [3, 4]. On the other hand, thermoset materials have commonly been used in semiconductors and integrated circuits (IC). Traditional thermoplastic
Others 6% PP 31%
PE (LD/LLD) 18%
Figure 7.1 World plastics demand by polymer type in the year 2015 .
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 351 materials have not been as resistant to soldering temperatures as epoxies and related thermosetting materials. Owing to their densely crosslinked structure, they exhibit a number of superior qualities such as high glass transition temperature, high modulus, high creep resistance, low shrinkage at elevated temperature and good resistance to chemicals. Thermoset polymers are typically insoluble and cannot be remelted or reformed into another shape after the curing process. The advantages of the low viscosity of thermoset polymers in the beginning of the process are that they can be molded into very complex shapes and are capable of wetting the reinforcement in the composites. Though thermoset plastics and thermoplastics sound similar, they have very different properties and applications. Both types of polymers are preferred for use in engineering applications due to their light weight and ease of processing. All of these plastic products are compounded with a material known as additives. Addition of additives transforms polymeric materials into more colorful, tougher, stiffer materials which are more resistant towards degradation. There are many types of additives such as filler, reinforcement, colorant, plasticizer, antioxidant and blowing agent. Each of these additives has its own function. The additives can be classified as a reinforcement, active filler and inactive filler depending on the aspect ratio (length/ diameter) and compatibility with the polymer matrix. Reinforcement is mostly referred to as long and continuous fibers. It is commonly used in thermoset polymer in uni-directional, 2-dimensional and 3-dimensional fiber arrangements. The long and continuous fiber can be used up to 60 to 70 vol%, which subsequently increases its mechanical properties. Active and inactive fillers are referred to as fillers that are used to improve certain properties of the mixed materials and fillers that are used to lower the compound cost, respectively. These fillers are commonly used in the form of short and discontinuous fiber, flake, and particulate geometry. Depending on the fabrication method, the fillers are arranged in random orientation and produce a product with isotropic properties. Generally 30 to 40 vol% of these fillers can be used to produce polymer composites. By the appropriate selection of these materials, not only the economics but also the other properties, such as processing and mechanical behavior, can be improved. Adding fillers to the polymer affects the viscosity as compared to the neat polymer. Generally, by adding fillers, the dimensional stability and stiffness of the products can be improved. Depending on the required properties, types of fillers are normally the first thing to consider when it comes to the materials selection for the fabrication of polymer composite. They can be based on natural and synthetic fillers, as shown in Figure 7.2. Natural fillers are obtained from sources such as
352 Hybrid Nanomaterials Fillers
Mineral + Organic
Mineral + Inorganic
Plant + Organic
Plant + Inorganic
Animal + Organic
Animal + Inorganic
Figure 7.2 Hybrid filler using different types of reinforcement and filler sources.
mineral, plant and animal. Synthetic filler can be further categorized into organic and inorganic fillers. Examples of synthetic inorganic fillers are synthetic diamond, carbon nanotubes (CNTs), graphene nanoplatelets, silicon carbide, aluminum nitride and boron nitride. Synthetic organic fillers are commonly based on polymer type of fillers. Addition of organic fillers may affect the flammability and chemical resistance of the mixture. Hybrid fillers can be obtained by combining different filler and reinforcement sources.
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 353 Addition of conductive fillers in polymer composites for electronic applications is mainly governed by price-performance relationships. Apart from reducing the price of the final material, conductive filler can also help to improve the thermal and electrical conductivities, shrinkage of the molding and stiffness, which are the principle limitations of bulk polymer . For example, in order to increase thermal conductivity of the composites, heat resistance at the filler-matrix interface needs to be minimized. This resistance results from phonon scattering process. Adding thermal conductive filler with a high aspect ratio, such as carbon nanotube, carbon nanofiber and nanowire, can easily form network structure and increase the thermal conductivity [6, 7].
Factors Influencing the Properties of Filler/Polymer Composite
Polymer composites are multifunctional materials in which their properties can be tailored to meet the requirement of the electronic packaging. It is generally known that polymer matrices are weak, low stiffness and viscoelastic materials. They have very poor mechanical properties, and low thermal and electrical conductivity. The properties of the polymer composites are contributed by the fillers or reinforcements that are incorporated into them. Their properties are strongly dependent on the types of fillers or reinforcements, volume fraction or loading, dispersion, as well as the microstructure of these fillers or reinforcements. Many research works have been done with the incorporation of suitable ceramic and metal fillers in advanced composite to make the properties, such as dielectric, electrical conductivity, thermal conductivity, coefficient of thermal expansion (CTE) and some mechanical properties, tailorable to the required value [8–10]. In the design of polymer composites, volume fraction or the loading of the filler is determined according to the application requirement. It is well known that there is always an improvement in polymer composite properties with the increase of filler loading. For example, to produce electrically conductive polymer composites, conductive filler such as copper or silver particles will be added into the polymeric materials. Suriati et al.  reported that with the increase of the conductive filler loading in the polymer composite, the electrical resistance decreases. This is due to the formation of conductive network throughout the polymer matrix. Average distance between the conductive fillers becomes smaller, which increases the number of conductive paths, leading to the decrease of the resistance in the composite . Increase in the filler loading also aids
354 Hybrid Nanomaterials in the enhancement of mechanical properties of the polymer composites such as the composite strength and modulus. However, it might reduce the toughness or elongation at break of the composite system. This is mainly due to the agglomeration of the filler or the poor bonding between filler and matrix [12, 13]. When a highly filled system is involved, the loading effect becomes a critical factor to be considered. There is always a maximum loading of filler that can be introduced before the properties of the polymer composite start to drop or be saturated. This is known as percolation loading. Suriati et al.  reported an increase in the resistivity in the polymer composite when the filler loading exceeded the percolation threshold, resulting in a poorly conductive polymer composite. This is due to the overloading of the conductive filler, which subsequently contributed to the contact resistance between the fillers, especially when the filler is nanosized. After the selection of the types of filler and filler loading according to the application requirement, the properties of the composite can be further enhanced with the proper selection of size and shape of the fillers. There are a few types of filler shapes known, which are spherical and irregularly shaped particulates and dimensionally oriented fillers such as flakes. For applications where surface roughness is concerned, such as coating or film, spherical-shaped filler gives a smoother surface finish to the composite product . In terms of conductivity, either electrical or thermal, dimensionally oriented fillers always have an advantage over the spherical and irregularly shaped particulates . In terms of the size of filler, Sanada et al.  investigated the thermal conductivity between the close-packed structure of hybrid fillers based on different sizes; micro- and nanofillers. It was found that the incorporation of nanosized multiwalled carbon nanotube significantly increased the thermal conductivity of the composites. When the size of the solid particles gets smaller, from micron- to nanosized, the tendancy of agglomeration is increased as a result of van der Waals forces. The most frequent nanofiller agglomeration issue in electronic packaging is the agglomeration of nanosilica in the underfill . This is due to the hydrophilic surface of the silica particles where they stick together through the hydrogen bonding. This agglomerated silica network in the polymer matrix causes the polymer to be trapped between the interparticle voids. This will affect the rheology of the composite system and increase the overall viscosity with the filler loading. Hence, the loading of the filler is limited due to the rise in viscosity. Besides, the incompatibility of the hydrophilic nanofiller and hydrophobic polymer matrix gives the poor interfacial interaction in the composite system. Hence, the optimum performance of the composite cannot be achieved at this low loading and poor interaction of filler.
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 355 In order to stabilize the dispersion of the nanofiller, surface modification of the nanofiller can be done physically and chemically. Poh et al.  proposed physical surface modification by using a common surfactant, Triton. The theory behind this surfactant is the hydrophilic and hydrophobic chain segments in the surfactant molecule. When the surfactant was added into the composite system during processing, adsorption occured between the hydrophilic nanofiller and the hydrophilic end of the surfactant molecule. This left the hydrophobic end of the surfactant molecule to orient itself towards the polymer matrix. Thus, the interfacial tension between the filler and the matrix is then lowered and the dispersion is improved. There are many factors affecting the surface modification of the nanofiller such as types of silane coupling agent, concentration and treatment time.
Hybridization of Fillers in Polymer Composites
Hybrid composites are gaining attention from the materials research community as they offer a wide range of possibilities to tailor their properties at various length scales. Hybrid composites are no stranger to many applications, including aircraft, sports equipments, etc. They continue to attract much attention because through the fabrications of hybrid composites, tailoring the properties needed for particular material is much easier compared to only single material. Composite materials are well defined in various literature as materials that have two or more types of fibers or fillers in a common matrix. With the existence of hybrid composites, it is possible to have material with specific characteristics that are suitable for the specified application of the end use . Hybrid composites are composed of two or more fillers, either same or different fillers, in a single matrix to achieve a balance between properties of single filler reinforced composites [20, 21]. With different types of available reinforcing materials in hybrid composite systems, the “hybrid effect” or the synergy effect of each material that becomes part of hybrid composites has been extensively studied. Researchers were inspired to produce hybrid composites that could fulfill the demand in various industrial applications . Based on Figure 7.2, hybrid composites can be produced with different types of reinforcement and fillers; for example, hybrid based on a combination of natural-based filler/reinforcement and synthetic filler/reinforcement. Hybridization could also be used as a means of increasing the cost effectiveness of this type of material. Hybrid composites have been considered for a variety of applications, and hybridization can be carried out on many levels, ranging from intimate blending fiber within plies, through
356 Hybrid Nanomaterials alternating ply to skin/core constructions or through mixing of various particulate fillers in a matrix or matrices. Figure 7.3 shows an illustration of hybrid composites. One example of the use of hybrid composites for mechanical performance reasons is in the construction of helicopter blades. Helicopter blades are often a hybrid construction of glass and carbon fiber, a combination necessary to provide the required levels of stiffff ness, fatigue resistance and damage tolerance . In addition, there are a wide range of applications using the hybrid concept, such as for structural, flame retardant and also dental purposes. Leong et al.  stated that there are generally three main aspects for evaluating the effects of hybrid composites. The most well-known one is the economic effect, where more expensive filler can be incorporated into cheaper material. The second effect of hybrid fillers is the ability to fabricate a broader range of properties, whether physical, thermal or mechanical, to specifically match desired characteristics. Finally, hybridization could bring about the advantage of improving functional and mechanical properties. Properties of each engineering material are guided by the mechanical, thermal, electrical and optical behavior that they inherit. But it is
Fiber A Fiber B
Fiber B (a)
Filler A Filler B
Figure 7.3 Hybridization of (a) intimate blending of fibers within plies, (b) alternating ply to skin/core construction and (c) various particulate fillers in a matrix.
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 357 known that one material is not able to satisfy the need of having all the desired properties, thus creating “holes” of property that can be filled by other types of material by producing hybrid materials that can occupy the “missing” properties . In Figure 7.4, materials A and B exhibit different properties, with A showing lower property (X) than B. Hybridization between material A and material B (Hybrid A and B) results in positive or negative hybrid effects or property that follows the prediction based on the “rule of mixtures.” In hybrid composites, the positive or negative hybrid effects refer to the deviation of a certain mechanical property from the prediction. Regions where the property of hybrid composite is above and below the prediction are referred to as positive and negative hybrid effects, respectively. Hybrid composites which follow the prediction will exhibit property that is located on or near the prediction line. Synergistic combination of hybrid composites will show an improvement in overall mechanical properties compared to individual ones. Mechanical properties of hybrid systems that have two single systems can be predicted using the rule of hybrid mixtures (RoHM); PH = PGVG + PCVC, where PH is the properties that need to be determined, PG and PC are the corresponding properties of the first and second system respectively,
Property that follows the rule of mixture (prediction)
Hybrid composites (A + B)
Figure 7.4 Properties of individual materials (A and B) and effect of hybridization between material A and material B.
358 Hybrid Nanomaterials while VG and VC are the volume fraction of both systems . In terms of mechanical properties, it is generally accepted that the elastic moduli can be predicted with the rule of mixtures behavior. Hybrid effects in glass/carbon reinforced in epoxy have been reported to improve strain and fracture compared to when the two fibers are used separately due to the stiffer fiber . Through hybridization, the balance between the two types of reinforcement in a common matrix can be achieved. As in the case of carbon and glass fiber, the properties of carbon fiber that are strong, stiff and have low density can be complemented with glass fiber that has higher fracture property but lower strength and stiffness .
Hybrid Fillers in Polymer Nanocomposites
Studies on hybrid composites has been growing in recent years and from the trend observed, hybrid composites proved to be one of the materials with many potentials that can be manipulated into diversified material properties. The importance of hybrid composites can be evaluated by three main aspects. Firstly, the demand for hybrid composites is mainly due to the economic advantage. Through hybridization, the main focus is on cost reduction by combining both expensive and cheap materials but with advantages from both materials. Specific characteristics can be obtained from combinations in hybrid materials which could offer wider options to develop materials for a specific design and purpose. Finally, through hybrid materials the improvement in mechanical and functional properties could be optimized when hybrid materials interact with each other as they are combined . With good selection of filler reinforcement and processing techniques, hybrid composites can be produced to suit various practical requirements . Generally, hybrid composite systems employing the addition of carbon nanotube into other fillers, such as boron nitride, synthetic diamond, carbon black, silica, aluminum and glass ceramic, result in an effective method to form conductive network or conductive bridge. The thermal conductivity properties of hybrid fillers show up to 50% enhancement from single composites . On the other hand, a new way of combining hybrid microsized silicon carbide with nanosized multiwalled carbon nanotubes (MWCNT) advances the improvement of the thermal conductivity of the composites . The hybrid between Al with silicon nitride shows increased thermal conductivity compared to Al with wollastonite-filled HDPE composites. The enhancement was induced by the higher thermal conductivity of silicon nitride shown in comparison with wollastonite .
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 359 Besides, thermal conductivity of hybrid boron nitride (BN) with carbon black (CB) and carbon fiber (CF) demonstrated different results. The BN and CB show an increase in thermal conductivity up to maximum loading; whereas, hybridization between BN and CF decreased the thermal conductivity as compared to single filler composites. This means that the addition of carbon fiber does not help in the formation of a filler network. This might be attributed to the low aspect ratio of CF after extrusion and the orientation effect induced by injection molding. Marcq et al.  stated that combination of micro- and nanofiller is one of the new ways to improve conductivity. A high surface area of double-wall carbon nanotubes (DWCNTs) in association with intermolecular van der Waals forces between DWCNTs leads to their arrangement in bundles and bundle agglomerates compared to multiwall carbon nanotubes (MWNTs), which are shorter, and thus well dispersed in MWNTs/microscale silverfilled epoxy composites. Hsien et al. , in their study on hybrid conductive fillers based on nano- and micronsized particles, reported that the mixture of 0.4 vol% of nanosilver filler loading and 1.5 vol% of micronsized silver gives significant electrical conductivity value. The nanosized silver contributes a conductive network and increases the electrical conductivity properties of the conductive adhesives. In hybrid silver conductive adhesive systems, silver nanoparticles occupy interstitial positions to improve particle-particle contact and increase electrical conductivities. A similar observation was reported by Khairul , where it was found that addition of nanosilver in micron silver/epoxy system results in 11% increase in the electrical conductivity values compared to single filler. However, addition of a high amount of silver nanoparticles (1.5 vol%) in micronsized silver conductive adhesives system does not improve the conductivity due to an increase in contact resistance. The concept of hybrid silver flake in microsized and silver nanoparticlesfilled epoxy composites was investigated by Suriati et al. . As shown in Figure 7.5, samples containing hybrid micron:nanofillers (i.e., 75:25, 50:50, and 25:75) show remarkable increments in storage modulus at 30 °C, with the highest value represented by 50:50. In this case, synergistic effects seem to occur as the storage modulus of hybrid filler was higher than the samples consisting of only one filler type (i.e., 100:0 and 0:100). The decrement of storage modulus at 25:75 and 0:100 can most probably be attributed to the increasing agglomeration of silver nanoparticles due to higher surface area governed by nanosized filler. Kong  used hybrid nanofillers based on three different sizes and shapes of synthetic nanodiamond (ND). Figure 7.6 shows thermal conductivity of hybrid composites consists of spherical shape ND (4–15 nm)
360 Hybrid Nanomaterials 4000
Tg ( C)
Storage modulus (MPa)
1000 Storage modulus Tg
75:25 50:50 25:75 Silver flakes: silver nanoparticles
Figure 7.5 CTE before and after Tg of hybrid silver-filled epoxy composites (Reprinted with permission from ; Copyright © 2012 Springer).
0.5 Thermal conductivity (W/mK)
ND1 ND2 0.4
Ratio (ND/ND1 and ND/ND2)
Figure 7.6 Thermal conductivity of 2 vol% of hybrid fillers-filled silicone rubber composites as a function of ND/ND1 and ND/ND2 ratio.
with irregular shape ND1 (100 nm) and ND2 (200 nm). It was concluded that hybrid filler composites are more effective than single filler composites in enhancing the thermal conductivity of silicone rubber composites. Formation of random networks from the particles with different sizes and shapes facilitates the phonon transfer and this leads to the increment of
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 361 ND
ND1 or ND2
Figure 7.7 Schematic illustrating the packing effect of different sizes and shapes of hybrid fillers in hybrid filler composite. (Figure adapted from Rothon and Hancock, 2001).
thermal conductivity of the composites (Figure 7.7). Results showed that better thermal conductivity is observed in a composite material with high ratio of larger particle size used in the hybrid composites. The phenomenon is attributed to the fact that higher numbers of large particles in a composite lead to less interfaces introduced along the conduction paths, resulting in enhancement of the thermal conductivity of the composites. An enhancement in impact strength and tensile strength were proposed by Mirmohseni and Zavareh  using hybridization between epoxy, poly(acrylonitrile-co-butadiene-co-styrene) (ABS), clay and TiO2. The obtained results indicated that the combination of materials would generate a synergistic effect on the impact and tensile strength of the epoxy polymer. In addition, although only a little agglomeration was observed in the SEM micrographs, it is reported that TiO2 and ABS still can act as crack stoppers. Lin et al.  reported that different sizes of hybrid filler between nanoparticles ZrO2 and microsized short carbon fiber loaded in polyetheretherketone (PEEK) resulted in good tensile properties and wear resistance. The tensile strength and tensile modulus showed improvement, indicating the presence of the synergetic effect of ZrO2 and short carbon fiber on the enhancement of mechanical properties. Another emerging type of carbon-based filler is exfoliated graphene nanoplatelets (GnP). Previous work has demonstrated that GnP, which has a graphitic structure, also has many of the superior thermal and electrical properties of CNTs when incorporated into a polymer composite. This might make GnP a promising and less expensive alternative filler in polymer composite systems when compared to CNTs. Li et al.  produced an epoxy-filled composite with hybrid GNP and CNT composites with a total mass fraction of 2 wt% and studied the effect of varying individual CNT/ GNP contents on the electrical, flexural, and fracture properties of epoxy
362 Hybrid Nanomaterials composites. Yang et al.  also combined fibrous 1D MWCNTs with planar 2D multi-graphene platelets to create a 3D graphene-based architecture that exhibited a synergistic effect on the mechanical and thermal conductivity of epoxy composites. The tensile modulus, tensile strength, and thermal conductivity of 0.9 wt% MGP and 0.l wt% MWCNT epoxy composites were improved by 23%, 15%, and 47%, respectively, compared to a pure epoxy system. Realizing the superior thermal and electrical properties of CNTs and GnP, Kong et al.  incorporated hybrid GnP and MWCNT fillers into silicon rubber. Results indicate that a significant change in their electrical and thermal conductivity behavior. Combining the GnP and MWCNT-OH as a hybrid filler system increases the electrical and thermal conductivity of the composite as compared to the single filler composite. For a total filler content of 4 wt%, the optimum ratio of GnP/MWCNT-OH is 3:1, which represents 75% GnP/25% MWCNT-OH (Figure 7.8). At this ratio, the thermal conductivity and electrical conductivity obtained is the highest among all composites produced, with a value of 0.392 W/m.K and 1.24 × 10–3 S/m, respectively. Based on morphology, it is observed that that there is geometrical synergy between nanoplatelets and nanotubes where the nanoplatelets and nanotubes form an interconnected hybrid network structure, which consists of nanotubes forming multiple junctions among them and exfoliated graphite nanoplatelets. The nanotubes form junctions among the nanoplatelets by touching two or more nanoplatelets which are separated far away.
Fabrication Methods of Hybrid Fillers/Polymer Composites
The fabrication method is the key to attaining a good composite design with a manufacturing process that can operate with minimum problems. The goals of the composite manufacturing process are the ability to achieve a consistent product, minimize voids and reduce the residual stress. During the fabrication method of hybrid particulate filler and matrix, mixing must occur in two fundamental mechanisms, which are dispersive mixing and distribution mixing. Dispersive mixing must overcome different viscosities, surface energies, chemical compatibilities, and melting temperatures. It is focused on short range blending of the compound. Distributive mixing depends on the types of equipment used . Preparation of hybrid
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 363
Thermal conductivity, (W/m.K)
0.36 0.34 0.32
0.30 0.28 0.26
0.24 0.22 0.20 0/4
Electrical conductivity, (S/m)
1.0E-06 0/4 (b)
Figure 7.8 Thermal and electrical conductivities of hybrid GnP/MWCNT composite at filler content of 4 wt%. (Reprinted with permission from ; Copyright © 2014 Elsevier).
364 Hybrid Nanomaterials nanofiller in thermoplastic polymer composites by melt-compounding technique is very popular because it implies the use of conventional polymer processing equipment and generates moderate production costs . A few pieces of equipment are used in melt compounding of the thermoplastic composites such as injection molder, extruder, compression mold, two roll mill and internal mixer . However, one of the most important drawbacks lies in the fact that nanoparticles such as CNT, carbon black and graphene show a great tendency to establish strong van der Waals forces which cause strong agglomeration phenomena. Hence, when mixed with polymer matrices, the fillers tend to segregate in tight bundles, precluding their effective distribution in composites. Homogeneous dispersion and distribution of hybrid nanofiller particles in polymer matrix is very important for improvement of mechanical properties of thermoplastic polymer composites. In other words, the potential problem in preparation of thermoplastic polymer composites is poor dispersion and distribution of nanoparticles in polymer matrix. Generally, agglomeration is highly dependent on dispersion of particles in a matrix, i.e., increase in the degree of particle dispersion results in decreasing particle agglomeration. On the other hand, distribution indicates how uniformly the primary particles or their agglomerates are distributed through the composites. Suitable strategies are strictly required to improve the thermoplastic polymer composites compatibility and dispersibility and to achieve the formation of homogeneous polymerbased composites with improved polymer-filler interfacial adhesion . The incorporation of hybrid nanofillers in thermoset polymer matrix leads to an exceptionally large quantity of particles and high surface area of fillers, resulting in difficulties in uniformly dispersing these particles. Dispersion of hybrid nanofillers in thermoset polymers is assisted by a few fabrication methods such as ultrasonication, ball milling, speed mixing, and extrusion. The most common techniques used usually apply high shear forces during a dispersion process in order to break up the agglomerates and distribute the individual filler homogenously in the thermoset matrix. However, it was believed that these techniques not only could separate the fillers from each other, but they can also fragment the filler that has high aspect ratio during processing. Moreover, the separated filler particles tend to re-agglomerate after some time due to the van der Waals attraction. This re-agglomeration phenomenon depends upon various factors such as matrix-filler interaction, filler-filler interaction, viscosity of the suspension and length of time before suspension was solidified. Moreover, ultrasonication method was reported to be effective in preventing the formation of large filler agglomerates and contributing to better dispersion of filler in
Hybrid Fillers/Polymer Nanocomposites for Electronic Apps 365 surrounding matrix . The propagation of ultrasonic waves in the liquid medium generates giant pressure, causing a massive stress that destroys the binding energy of the interparticles.
Applications of Hybrid Fillers/Polymer Composites
Research on the application of nanofillers in polymer composites has received increasing attention from many researchers in various fields. This is due to their extremely small sizes and large specific surface areas that exhibit novel properties in terms of optical, physical and chemical properties which differ from the bulk properties. In electronic applications, polymer composites with thermal conductivity properties were used in thermal management application. In electronic application, effective heat dissipation is crucial to enhance the performance and reliability of the electronic devices such as thermal conductance in circuit board, heat exchanger, heat sink, appliances and machinery. For most modern microelectronic devices, cooling is restricted by a low thermal conductivity of the polymeric packaging material (i.e., 0.10–0.25 W/m.K) as compared to commonly used metal or ceramic materials. Thermal management is required to remove the unwanted heat from sources, such as semiconductors, without negatively affecting the performance or reliability of adjacent component. The higher the temperature, the lower the reliability of the composites due to the degradation occurring at interfaces. Reliability is the performance against requirements over a period of time. It also can be said to be quality of product. A few applications of thermal management are heat sinks, enclosure fans, circuit boards, heat exchangers, appliances, and machinery. Heat sink is often used to cool microprocessor and air stream for a critical device. It is a good choice when an enclosure fan is impractical. As with enclosure fans, active heat sinks carry the drawbacks of reduced reliability, higher cost system and higher power system operation. Most materials used to fabricate active heat sink are thermoplastic, epoxy with copper (Cu) and aluminum fillers. While the enclosure fan is used to increase air flow, it significantly lowers the critical temperature of all components in the enclosure in personal computers. It also increases the cooling efficiency of heat sink. Testing has shown that more heat is dissipated when the fan blows cool outside air into a computer rather than when it sucks warm air from inside the chassis and moves it outside. Moreover, addition of carbon-based conductive fillers, such as carbon nanotube, synthetic diamond, alumina and hybrid thermal, in polymer improved the
366 Hybrid Nanomaterials thermal conductivity but maintained electrical insulation properties of plastics, which is desirable in some applications such as antistatic devices, capacitors, material for electromagnetic interference (EMI) shielding and sensors as well as thermal management . Besides, the addition of metal powders to thermoplastic and thermoset polymers represents an important group of engineering materials, with a great number of applications, such as conductive adhesives, discharging static electricity, heat conduction, electromagnetic interference shields, electrical heating, and converting mechanical signals to electrical signals. Micronsized silver flakes in epoxy have been widely used in electronic industries. However, application of metal nanoparticles and hybrid metal nanoparticles in polymer is believed to not only enable the ultrafine pitch capability, but also enhance the electrical properties such as reducing the percolation threshold and improving electrical conductivity. Metal-filled polymer composites are widely used for electromagnetic interference shielding. They have a lighter weight than metals and are less costly. Moreover, types of nanofillers, size and shape of the nanofiller, volume of the nanofillers in the polymer, selection of polymer matrix and processing conditions might influence the properties of the polymer nanocomposites.
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8 High Performance Hybrid Filler Reinforced Epoxy Nanocomposites Suman Chhetri1, Tapas Kuila1* and Suneel Kumar Srivastava2* 1
Surface Engineering and Tribology Division, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India 2 Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur, India
Abstract The intent of this chapter is to cover the latest progress and innovation in the field of hybrid filler-based epoxy nanocomposites. Hybrid filler, which is a combination of various particles having different shapes, sizes, and geometries, includes nanostructured carbon filler and other inorganic mineral particles. The motivation behind the fabrication of hybrid filler materials is to achieve expected improvement in the properties of the nanocomposites through synergistic effect. It is seen that the nanocomposites display properties that are not achievable with single filler while using hydride filler. Discussed in detail is the improvement in electrical, thermal, mechanical, flame retardant and other properties. The mechanism of improvement in the properties through the synergistic effect of the hybrid filler may be due to the formation of network structure and fine dispersion of the filler. The reinforcing competence of different carbon-based hybrid fillers in combination with ceramic materials is highlighted. Other than the network formation and dispersion, reinforcing ability of the hybrid fillers depends upon the optimum weight ratio of the filler materials, interactions between the fillers, their geometry and processing technique. Keywords: Hybrid fillers, polymers, nanocomposites, mechanical properties, thermal conductivity, electrical conductivity
*Corresponding authors: [email protected]; [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (371–422) © 2017 Scrivener Publishing LLC
372 Hybrid Nanomaterials
The incorporation of second phase or components called filler or reinforcing agent into the polymer matrix results in the formation of polymer composites with improved properties as compared to the neat polymer . Development of polymer composites by the incorporation of fillers of varied dimensions, different morphologies and geometrical forms is intended to achieve materials with improved mechanical, electrical, thermal and other properties. The other motive behind the fabrication of polymer composites materials has been to widen the prospect of existing materials and create novel resistive material with high strength but lightweight. Polymer composites materials are becoming indispensable to human life in modern society and are widely used in fields, such as aerospace, electronics, medical equipment, automotive, etc., due to their superior properties such as, low weight, superior mechanical, electrical, and thermal properties, corrosion resistance, high fatigue strength, etc. [2, 3–6]. Conventionally, microscale inclusions were in the spotlight in the field of polymer composites, which basically resulted in improvement in mechanical strength, stiffness and thermal stability . Fibrous filler such as carbon and glass fiber have been extensively used to develop rigid and cost-effective composites [8, 9]. Particulate fillers have been deployed to achieve some improvement in mechanical properties to enhance the fire resistance and electrical properties of the neat polymer. One of the limitations associated with the microscale inclusion is the requirement of heavy loading to achieve desirable properties, which is detrimental to the mechanical properties and opaque nature of the resulting composites [2, 10, 11]. Moreover, issues like incompatibility between the matrix and filler, poor interfacial interaction, and disparity between the coefficients of thermal expansion lead to a reduction in toughness, impact resistance, etc. In order to overcome these lacunas, nanoscale inclusion and its unique properties have led to the development of a new realm called nanocomposites. In contrast to microscale composites, nanocomposites exhibit improved mechanical, thermal, gas barrier, electrical properties, among others, even with small amount of nanofiller loadings, without compromising the optical clarity and density . Nanocomposites are basically defined as the mixture of two phases, polymer and filler, where, dimension of one phase is at least in the nanometer scale. Nanofillers possess large surface area for a given volume , and interfacial interactions with the host matrix can be tuned. Due to the large interfacial contact area of the nanofillers, efficient interaction between the nanofiller and polymer matrix takes place, which results in improved mechanical and other optoelectronic properties.
High Performance Hybrid Filler 373 The pioneering work of Toyota Central Research Laboratories in Japan in the early 1990s reported enhanced thermal and mechanical properties of Nylon-6/clay nanocomposite with small loadings of filler. This novel work opened a new avenue in the endeavour to find high-performance, low-cost polymer nanocomposite materials . The issues that were perplexing the use of conventional filler to reinforce properties of materials were improved with the introduction of nanofiller materials. Owing to the tuneable features of the nanofiller, the nanocomposite materials exhibit unique properties as compared to their conventional microcomposite counterparts.
The common fillers that are used in conventional polymer composites are glass fibers, carbon fiber, metal particles, wood sheets, etc. . To endow multifunctionality and other superior properties to the polymer matrix, currently carbon black (CB), silica, nanoclays, titanium dioxide (TiO2), layered silicates, carbon nanotubes (CNTs), carbon nanofibers (CNF), aluminum hydroxide, polyhedral oligomeric silsesquioxane (POSS), metal/ceramic nanoparticles, graphite intercalation compounds (GIC) and more recently graphene have been widely used [16–19]. The filler materials have been classified on the basis of their geometrical characteristics, namely shape, size and aspect ratio , with the possible classification of zero, 1D, 2D or 3D. Metallic filler particles may enhance both the electrical and thermal conductivity of the polymer; however, the loading that is necessary to obtain high values may increase the density of the composite materials, which is not desirable where light weight is preferred. Metal particles, such as aluminum, silver, copper and nickel, have been used as reinforcement for thermally conductive filler. Ceramic filler materials have been widely accepted in the field of electronic materials owing to their high thermal conductivity and electrical resistivity. Carbon fibers (CFs) have been widely used as reinforcement due to their high modulus and strength, good thermal and electrical conductivity, low linear coefficient of thermal expansion and good thermal stability, and their nanocomposites find applications in aerospace, engineering, transportation, etc. [20–22]. Though the intrinsic properties of CFs benefit the polymer matrix in improving the mechanical properties of the nanocomposites, a higher loading is usually required to achieve significant enhancement. In recent years, carbon nanofibers (CNFs) grown from the vapor phase with diameters on the order of 100 nm have been used to prepare polymer nanocomposites . CNFs have better reinforcing potential
374 Hybrid Nanomaterials than their microsize counterpart CFs. The surface area/volume ratio of CNFs is higher than CFs, which promotes effective interaction with the polymer matrix which leads to enhanced mechanical properties and electrical conductivity. The CNFs/polymer nanocomposites can be used in electromagnetic interference (EMI) shielding and electrostatic discharge materials. Though CNFs enjoy the benefit of being lower cost as compared to CNT, they have been studied less. The reason is that CNTs and multiwall CNTs (MWCNTs) have lower diameter and density and higher mechanical properties. The subsequent entry of nanographene reinforcement has made the field even more appealing by achieving high performance, novel functional materials at very low loadings . Thus, the discovery of carbonaceous nanofiller can be regarded as a major milestone in the area of polymer nanocomposites where small loading of outstanding properties bearing CNTs and graphene-like materials resulted in superior properties. Layered silicates, which belong to the family of 2:1 phyllosilicates, consist of an octahedral sheet of alumina or magnesia sandwiched between two silica tetrahedral sheets . The high aspect ratio of layered silicates facilitates intercalation of polymer chains or individually exfoliated layers in the presence of polymer matrix . The incorporation of nanolevel layered silicates exhibits significant improvement in various properties of the nanocomposites as compared to the neat polymer. Layered silicatefilled polymer composites have emerged as the major alternative to the conventional filler-based composite materials owing to their large surface area, swelling properties, high mechanical strength and modulus, chemical and thermal stability and low cost of the fillers. The composites find applications in the aerospace, automotive, and electronic fileds [26, 27]. Montmorillonite (MMT), layered double hydroxides (LDHs), laponite, and other clay minerals have been extensively used as nanofiller to improve the flame retardant properties of polymer nanocomposites. Both the naturally occurring or synthetic clay minerals form an insulating charred layer on the surface of the polymer nanocomposites during burning and disrupt the oxygen supply to the bulk. Moreover, the burning of these filler materials is an endothermic reaction and forms sufficient smoke and water vapor preventing the oxygen supply to the bulk of the nanocomposites. However, the low thermal and electrical properties of the clay minerals are major issues while preparing the polymer nanocomposites . In order to alleviate the issues associated with the clay minerals, carbon-based nanofillers, such as carbon black (CB), carbon nanofiber (CF), CNT and graphene, have been introduced as filler materials to fabricate polymer nanocomposites. Carbon black (CB) shows outstanding properties such as heat, chemical resistance, electrical conductivity, low thermal expansion and light weight,
High Performance Hybrid Filler 375 abundant source, low density and low cost. It has been extensively used as filler material to improve mechanical, thermal, electrical, and optical and damping properties of the nanocompoites [29, 30]. It has hugely beneficiated the field of rubber composites to enhance the tensile strength, improve tear and cut growth resistance, abrasion resistance, modulus, and hardness. Carbon black, the amorphous form of carbon is socalled “quasi-graphitic” and differs from the other two allotropes of carbon, diamond and graphite. The diameter of CB particles falls in the range of 10–400 nm and forms aggregates . The reaction between carbon atoms and aromatic radicals during the incomplete combustion of aromatic hydrocarbon, results in the formation of CB. The CB accumulates into agglomerates due to the van der Waals forces. It is extensively used to manufacture printing inks, colored plastics, and additive in batteries due to its good pigment properties . Graphite is widely accepted as an excellent conductive filler because of its good thermal conductivity, low cost and dispersability in polymer matrix [33, 34]. It has been widely exploited either in the raw form or expanded, which is an exfoliated form of graphite with a multilayer thickness of 100–400 nm. Expandable graphite (EG) is generally prepared with a sulphuric acid treatment of graphite accompanied by thermal expansion. It has also been used as filler material but its effectiveness as reinforcing filler is minimal due to its low specific surface area . Sonication can be used to further exfoliate EG to acquire graphite nanoplatelets (GNPs) of thickness down to 5 nm and a significantly increased surface area . Generally, GNPs include all types of graphitic structure from 100-nm thick platelets to monolayer graphene and can be prepared by ball milling as well as by the exfoliation of acid intercalated graphite via microwave radiation [37, 38]. GNPs show superior mechanical, electrical and thermal properties. They have been found to be more efficient than EG in improving nanocomposite properties. Generally, graphite-based exfoliated fillers are derived from the GICs, which are formed by the intercalation of alkali metals or mineral acids in the interlayer spacing of graphite. They can be further exfoliated to EG due to the weak van der Waals forces. The discovery of carbon nanotube (CNT) in 1991 by Iijima  and the subsequent report of the first CNT-reinforced polymer nanocomposites by the Ajayan group in 1994 , make it an ideal reinforcing filler due to its extraordinary mechanical and chemical properties and electrical and thermal conductivities. In addition, the high aspect ratio of CNTs has been successfully exploited to fabricate electrically and thermally conductive nanocomposites with very low percolation threshold [41, 42]. Single-walled nanotubes (SWNTs) can be considered as a cylinder with diameter on the order of 1 nm and length up to a few centimeters consisting of a rolled
376 Hybrid Nanomaterials single sheet of graphene , whereas MWNTs can be considered as a piling of such cylinders concentrically separated by an interspacing of 0.34 nm. In comparison to CNFs, CNTs have a small diameter in combination with high aspect ratio and low density, which makes them one of the reinforcing fillers with the most potential for polymeric materials. Depending upon the atomic arrangement, CNTs can be metallic or semiconducting. However, the large surface area of 1D CNTs inhibits their inherent properties from being translated into end nanocomposites, due to the tendency of restacking or aggregation, and hampers their mechanical properties. The potential of reinforcement and fabrication of polymer nanocomposites with high loading of CNTs is problematic due to the increased melt viscosity, small size of CNTs and large surface area. Issues like its dispersion into the polymer matrix and interfacial nature with the polymer matrix impede the realization of its complete properties in the host matrix. Thus, a full translation of the intrinsic potentials of CNTs as reinforcing fillers is yet to be realized completely. Furthermore, the high production cost of CNTs also limits its widespread use as effective filler. Covalent and noncovalent functionlization of CNTs has been considered an effective measure to counter the dispersion and interfacial issues but it introduces defective sites which hampers the end properties. The discovery of graphene in 2004 opened a plethora of avenues in the field of polymer nanocomposites . Its intriguing and exciting properties like high surface area, large aspect ratio, outstanding mechanical, electrical, thermal properties, flexibility, high transparency and optoelectronic properties make it one of the potential fillers. Graphene/ polymer nanocomposites show improved mechanical, thermal, barrier, and flame retardant properties . However, fabrication of high-performance, graphene-based nanocomposites is not an easy task, since the aggregation tendency of graphene due to π-π interaction impedes the degree of dispersion, which precludes the complete reinforcing potential of graphene.
Necessity of Hybrid Filler Systems
The quest for new smart materials is a never ending endeavour to ensure the development of materials for high performance and advanced applications. Therefore, the use of hybrid filler materials is slowly becoming ubiquitous in polymeric system to achieve multifunctional materials at low loadings and minimum cost. The word “hybrid” is a Greek-Latin word which signifies the combination of two or more different entities or fusion of two sometimes odd kinds or compositions. In the language of Ashby , the definition of hybrid materials goes like this: “a combination of two or more materials in a predetermined geometry and scale, optimally
High Performance Hybrid Filler 377 serving a specific engineering purpose.” So, hybrid materials are deliberate designing of new materials to have characteristics not offered by either one alone. The concept of hybridization can be considered an effective measure to create novel functionality through synergistic blending of different components which offers a solution to alleviate the shortcoming of functional properties that is unachievable by the use of any of the single components alone. Hybrid materials cannot be perceived only as materials of interest for academic research but their unusual properties can be exploited to develop components of innovative devices which could offer new technologies and business prospects. They offer unusual combinations of stiffness, strength and weight that are difficult to attain separately from the individual components. The performance of the nanocomposite can be improved by using hybrid fillers and has already been explored with different combinations of matrix and filler [46, 47]. Conducting nanocomposites of polymer occupy a significant domain of research activity as these materials are in high demand in the fields of electromagnetic interference (EMI) shielding, antistatic materials, conductive films, electronic equipment, etc. [48–50]. The common conducting fillers that have been used are metal particles, CB, CNFs, CNT and GNPs or graphene. Being lower in cost compared to other fillers, CB nanocomposites are overwhelmingly used in various applications. The intrinsic properties of fillers, their structure, loading level and extent of dispersion affect the conductivity of the polymer nanocomposites. Typically, significant improvement in electrical conductivity has been recorded for polymer nanocomposites using conventional microfiller at a loading level of greater than 10% . On the contrary, the percolation threshold has been achieved at a loading level of less than 1% of CNT and graphene . The percolation threshold, which is the minimum concentration of filler materials to transform insulating polymer matrix into conductor by forming threedimensional conduction network structures, depends upon the aspect ratio of the filler material among other factors. Increasing the concentration of the filler usually increases the electrical conductivity by forming continuous electrical networks, which is an expensive way to improve mechanical properties and density. For example, CB-filled polymer nanocomposites severely suffer from this drawback. Therefore, it is imperative to increase the electrical conductivity of polymer nanocomposites accompanied by moderate mechanical properties at very low level of loading. Lower volume fractions of fillers are desirable to improve the mechanical properties and ductility at minimum cost of processing . More recently, the practice of hybridization among carbon nanofillers of different geometries and intrinsic electrical properties has been widely pursued by the scientific community to lower the percolation threshold even more and to reduce the cost of
378 Hybrid Nanomaterials fabrication. The combination of two different geometry fillers, such as CB with GNP or CNT with GNP, leads to the formation of a co-supporting network structure for conduction at lower percolation threshold, which helps to render good mechanical properties. In addition, the electrically inert fillers are also used along with the conductive filler to reduce the percolation threshold . It is believed that these insulating fillers influence the distribution of conducting filler, leading to the formation of conductive 3D network structure for electron conduction. Hybrid filler has been found effective in improving the properties of the nanocomposites at lower loadings. However, the mechanism of synergetic effect is not yet fully understood. It is anticipated that the tunnelling of electrons takes place through the 3D network of the hybrid filler. Hybrid filler can be projected as the suitable alternative to achieve the enhanced properties at low level of loadings. The miniaturization of electronic devices has obligated the necessity of thermal interface materials that dissipate the generated heat quickly and keep the operating temperature of the device at minimum level. The conventional fillers, such as alumina (Al2O3), silver (Ag), SiC, BN, AlN and silica (SiO2), are mostly used to fabricate thermal interface materials and suffer from deteriorated mechanical properties due to the excess loading of fillers. So, to fabricate the materials with high thermal conductivity value accompanied by moderate improvement in mechanical properties at low loading is quite paradoxical. Although an increasing amount of filler renders efficient continuous network formation which enhances conductivity, the materials become brittle. Carbon nanofillers, such as MWCNTs, GNPs or graphene, outperform conventional fillers in an endeavour to achieve conductive nanocomposites at low level of loading. However, a complete realization of enhancement of thermal conductivity using MWCNTs has yet to be achieved due to the lower intrinsic tube conductivity, thermal interface resistance and scattering of heat-carrying phonons in the surrounding . Recently, doping of MWCNTs with metal particles has received attention to further enhance the thermal conductivity of the nanocomposites. Although fascinating properties of CNT or MWCNTs and graphene have been successfully exploited to fabricate thermally conductive materials, the use of co-filler has been even more promising and outperforms the contribution of each single component, resulting in better enhancement of thermal conductivity . There are plenty of reports where GNP-CNT hybrid filler has been successful in improving the thermal conductivity of the nanocomposites without hampering the mechanical properties. The combination of CNTs with GNPs helps to minimize the thermal interface resistance and also reduces the production cost by partially replacing the costlier CNTs. One of the critical factors is the agglomeration of
High Performance Hybrid Filler 379 nanoparticles in the polymer matrix, which subsequently hampers the expected properties of the nanocomposites. Design of suitable hybrid filler could be the solution to the above-mentioned problem, where one partner would help to inhibit the agglomeration of the other and at the same time reduce the amount of loading to achieve superior properties compared with the only single-component polymer binary system. The ceramic fillers have also been integrated with CNTs to enhance thermal conductivity. Introduction of insulating co-filler like boron nitride (BN) may not only improve thermal conductivity by increasing thermal conductive pathways but also diminish the electrical conductivity of the nanocomposites by disconnecting the conductive electric network [57, 58]. These types of nanocomposite materials can be used as encapsulation for dissipating the heat generated in the electronics devices, while protecting them from external shock. Therefore, the underlying purpose for designing hybrid filler for polymer nanocomposites is to achieve high thermally and electrically conductive materials accompanied by a moderate improvement in mechanical properties. In this chapter, the major developments in the area of hybrid/epoxy nanocomposites have been highlighted in detail. A wide range of hybrid fillers are reviewed, including GNPs-CNTs to CNTs-clay and CNT-CB to CNT-ceramic.
The term epoxy resin is inclusive of both the pre-polymer and the cured resin systems and it is characterized by a class of step-growth polymerization reaction between the two components: a pre-polymer containing reactive epoxide groups on both ends of the chains, and a hardener . Reportedly, the first synthesis history of what is now known as epoxy resin could date back to as early as 1891 [60, 61], but commercial epoxy resin appeared on the market only after the pioneering work conducted independently by Pierre Castan in Switzerland and Sylvan Greenlee in the United States in the 1940s. Epoxy, which was first commercialized by the Devoe-Raynolds Company in 1947, has become one of the most sought after materials in the area of research and industry . Bisphenol-A epoxy resin, which is the end product of bisphenol A and epichlorohydrin, was the earliest resin seen on the market and is still a widely followed method to prepare commercial epoxy resin. On the basis of the preparation techniques, epoxy resin can be categorized into two classes: glycidyl epoxy and nonglycidyl epoxy resins . Glycidyl epoxy resins are those prepared by the condensation reaction between
380 Hybrid Nanomaterials bis(4-hydroxy phenylene)-2,2 propane (called bisphenol A) and 1-chloroprene 2-oxide (called epichlorohydrin) and nonglycidyl epoxy resins are derived from the peroxidation reaction of olefinic double bonds [64, 65]. Epoxy resins contain a reactive three-membered ring called epoxide group which takes part in the reaction with hardener during the curing process. A large number of combinations are possible with the accessibility to a variety of epoxy resins and hardeners generating entirely new polymer structures. Epoxy resin becomes hard when it is crosslinked with hardener or curing agent. The curing process involves the crosslinked polymerization reaction between epoxy resin and hardener, which results in rigid three-dimensional structures. Choice of suitable hardener and controlling epoxy network structure during curing is necessary for successful application of epoxy resins as the chemical nature of curing agent largely influences the cure kinetics and glass transition temperature (Tg) . Apart from the nature of the curing agent, curing temperature also influences the properties of epoxy matrix. Some of the common hardeners are aliphatic amine, amido amine, aromatic amine, polyamine, polyamide, anhydrides, polyamidoamines, and imidazolines [62–65]. The curing temperature of aliphatic amines is lower compared to the aromatic amines and acid anhydrides. Epoxy is one of the most widely used thermoset polymers due to its good mechanical and adhesive properties, high specific strength, high chemical and heat resistance, easy processability and low density of ~1.3 gcm 2. In addition, the electrical insulation properties of epoxy are also used as encapsulate for circuit components due to their heat and chemical resistance . One of the drawbacks limiting its widespread use in various fields is its brittle nature. In order to improve the mechanical strength, thermal conductivity, electrical conductivity and toughness a wide range of fillers, from rubber to silicate, glass to polyaromatic nylon (Kevlar) fibers, EG to graphene, CNTs to ceramic particles, have been incorporated [67–71]. Epoxy/fiber networks, which combine fibers such as glass, carbon, and polyaromatic nylon (Kevlar) fibers with nanocomposites, have been widely used for structural applications in the aerospace, marine, and building construction industries due to their high strength-to-weight and stiffness-to-weight ratios [62, 63].
Preparation of Hybrid Filler/Epoxy Nanocomposites
The factors such as uniform dispersion, alignment of fillers and interfacial interaction between the filler and polymer matrix profoundly
High Performance Hybrid Filler 381 influence the final properties of the nanocomposites. The processing technique tries to minimize the issues that influence the end properties of the composites. The processing methods that are mainly used for fabricating hybrid/epoxy polymer nanocomposites are solution mixing and three-roll mill. In this method, first the hybrid filler is dispersed in a suitable solvent through shear mixing and sonication. The homogeneous dispersion of filler is then added into the epoxy resin followed by shear mixing for a few minutes. The solvent is evaporated at a certain temperature under high-speed stirring and the mixture is degassed in a vacuum chamber for the complete removal of solvent residue. The mixture is then allowed to cool to room temperature and curing agent is added by mechanical mixing for uniform dispersion. Finally, the gel-like material is poured into the silicon or stainless steel mold for further curing at various temperatures. The uniform distribution of filler materials in the epoxy matrix is the main advantage in solution mixing. However, the use of hazardous solvent and its complete removal are major issues with this processing technique. The inevitable problems of agglomeration preclude the effective capability of filler materials. Three-roll mill mixing technique has been extensively used in the fabrication of hybrid/epoxy composites. In three-roll mill machine processing, external shearing force is applied to disperse/ exfoliate the filler in the polymer matrix. Three-roll milling is a dispersion technique that utilizes both shear flow and extensional flow created by rotating rolls of different speeds to mix and disperse the filler particles. It consists of three cylindrical rollers, namely feeding, hopper and center rollers, which move at different speeds. The feeding and hopper rollers move in the same direction while the center roller moves in the opposite direction. The milling cycle can be repeated for fine dispersion. This processing technique is highly attractive and environmentally friendly since no solvent is required and the issue of evaporation does not exist.
Characterization of Hybrid Filler/ Epoxy Polymer Composites
The degree of dispersion, extent of interaction between the filler and matrix and state of aggregation are the parameters that profoundly influence the end properties of polymer nanocomposites. The state of dispersion and morphological features can be studied using microscopic methods such as
382 Hybrid Nanomaterials
Figure 8.1 SEM images showing the dispersion of CB/clay hybrid filler in epoxy matrix. (Reprinted with permission from ; Copyright © 2009 Elsevier Ltd.)
transmission electron microscopy (TEM), atomic force microscopy (AFM) or scanning electron microscopy (SEM) analysis. The SEM analysis is helpful to understand the morphology of the sample, dispersion state and crystalline nature of the nanocomposites. The SEM image of the fracture surface of the polymer nanocomposite samples reveals information about filler dispersion and interaction with the polymer matrix. The hybrid filler system throws a light on the influence of one component on the dispersion state of the other and, hence, the connectivity. Figure 8.1 shows the SEM micrographs of three epoxy nanocomposites containing 2.5 wt% CB and 0, 2.5 and 5 wt% of clay. Figure 8.1a shows that the CB particles are randomly dispersed and well connected with each other in the nanocomposites. These well-connected aggregates of CB impart the electrical conductivity to the nanocomposites. The SEM image in Figure 8.1b shows nanocomposites containing an equal amount of CB and clay. It shows fairly homogenous dispersion with relatively small aggregates of CB particles. It is very helpful to use TEM analysis to investigate the degree of dispersion and exfoliation of fillers in the polymer matrices. Figure 8.2 shows the dispersion of CNT-GNP hybrid filler system in the epoxy matrix. Figure 8.2b shows that the 2D graphene nanoplatelets are intercalated between the 1D nanotubes, which can help to generate the 3D filler network. Therefore, TEM image analysis is very helpful to understand the concept of the cosupporting network that formed between the two components. Frequently UV-vis spectroscopy is used to investigate the dispersion of fillers, particularly of CNTs and GNPs in the hybrid filler polymer nanocomposites. It is well know that the individual CNTs and graphene sheets are more active in the UV-vis region than bundled CNTs and stacked GNPs . Figure 8.3 shows that the CNTs and GNPs absorb light at around
High Performance Hybrid Filler 383
Figure 8.2 TEM images of dispersion of CNT0.8GNP0.2/epoxy composite. (Reprinted with permission from ; Copyright © 2014 Elsevier Ltd.) 0.6 1.6
Absorption at 500 nm
CNT CNT0.8GNP0.2 CNT0.6GNP0.4
0.4 0.2 (a)
CNT1-xGNPx 0.3 300
500 600 Wavelength (nm)
0.4 0.6 X, (wt%)
Figure 8.3 (a) UV-vis absorption spectra of the epoxy nanocomposite suspensions diluted in THF solvent and (b) absorption at 500 nm vs. GNP/CNT ratio. (Reprinted with permission from ; Copyright © 2014 Elsevier Ltd.)
260–270 nm and the absorption diminishes with the increase in CNT concentration, signifying CNTs dominating the absorption of hybrid filler in the UV region. For example, the absorption of CNT0.8GNP0.2 is at 500 nm, as shown in Figure 8.3b, which is higher than the CNTs or other hybrid composites and it reflects the fact that GNP prevents the re-aggregation of the CNT bundles. Thus, UV-vis spectra analysis suggests that the presence of co-filler alters the dispersion state of others.
Properties of the Hybrid Filler/Epoxy Nanocomposites Hybrid Fillers Based on CNT, GNP and GO
One of the factors driving the development of carbon nanofiller-based composite is their outstanding thermal and electrical conductivity. Reportedly,
384 Hybrid Nanomaterials MWCNTs showed thermal conductivity of ~3000 Wm 1K 1 and electrical conductivity in the range of 10–108 Sm 1 [73, 74]. However, transfer of intrinsic properties of CNTs to the polymers has not been realized yet. This may be due to factors like high thermal interfacial resistance and scattering of heat-carrying phonons. On the other hand, GNPs display the similar thermal conductivity of CNTs but are more efficient in enhancing the thermal conductivity due to their 2D structure and rigid nature as compared to the flexible CNTs. The GNP nanofillers are economically more viable for nanocomposite preparation as compared to the CNTs. The hybridization between 1D SWNTs and 2D GNP render additional channel for heat flow, in which 1D tortuous SWNTs make a bridge with 2D GNPs. Thus, CNT-GNP hybrid filler exhibits improved thermal conductivity without compromising the mechanical properties. In addition, partial replacement of expensive filler with cheaper filler results in the production of cost-effective materials. Yu et al. investigated the synergistic effect of the 1D-SWNTs/GNPs hybrid filler on the thermal conductivity of the epoxy nanocomposites . It is seen that the reinforcing efficiency of the hybrid filler at certain ratios is much better as compared to the individual fillers. The electrical conductivity depends upon the aspect ratio of the filler similar to the other filler materials. Large aspect ratio corresponds to the formation of bridge or conducting network between the fillers, facilitating the easy transfer of phonon or electrons. The increase in thermal conductivity can be accounted for by the formation of conductive network and diminished thermal interface. The thermal conductivity of the nanocomposites was found to be 1.75 W m 1 K 1 at a GNP-SWNT filler ratio of 3:1 (7.5 wt% GNPs and 2.5 Wwt% SWNTs in epoxy). However, the synergy was not observed in electrical conductivity. Huang et al. studied the synergetic effects of GNPs and CNT in epoxy nanocomposites at higher loading of hybrid filler from 10 to 50 vol% . Thermal conductivity of 6.31 Wm 1K 1 was recorded with 20 vol% CNT and 20 vol% GNP loadings. The thermal conductivity increased further in the nanocomposites containing 50 vol% GNPs or 50 vol% CNT loadings. The significant improvement in thermal conductivity has been realized in this study and is attributed to the synergy between the fillers at high loadings. Indeed, higher loading is required to achieve high thermal conductivity of the nanocomposites. However, the suitable method for dispersing high content of fillers in the polymers is very difficult. The bridging of CNT with GNPs generally forms 3D conductive network, which may enhance the thermal conductivity. Further, the electrical conductivity of CNT-GNP/epoxy is lower than that of the CNT/epoxy nanocomposites, possibly due to the low electrical conductivity of GNP. Development of nanocomposite materials having good heat dissipation ability and low thermal expansion coefficients is gaining much momentum
High Performance Hybrid Filler 385 with the growing demand for microelectronics. Yu et al. used CNTs grown on GNP support hybrid filler to fabricate hybrid/epoxy nanocomposites and observed ~300 and 50% improvement in through-plane thermal conductivity enhancement against individual GNPs and CNTs (~20 wt% hybrid filler loadings) . Further, this study recorded ~12% improvement in thermal conductivity in comparison to simply mixed CNTs and GNPs. Therefore, this encouraging result suggests that the CNT grown on GNP is more effective hybrid filler than simply mixed CNTs-GNPs hybrid. Such a type of hybrid filler can alleviate the problem of dispersion and separation of individual fillers associated with the hybrid filler. Li et al. fabricated epoxy nanocomposites using CNTs grown on the GNPs . The study recorded ~40 and 36% improvements in tensile modulus and tensile strength, respectively, at the loading of 0.5 wt% of CNT-GNP hybrid filler. The synthesized hybrid filler was found to be more effective as reinforcing filler than physically mixed CNTs-GNPs, as calculated from the modified Halpin-Tsai modeling. The improvement in mechanical properties can be owed to the unique architecture between the filler and its dispersion which facilitied the load transfer at the interface. Further, the authors claimed that the hybrid/epoxy nanocomposites can be used as sensor material to monitor the onset of irreversibly permanent deformation. Yue et al. prepared CNTs-GNPs/epoxy nanocomposites at different CNT-GNP ratios and studied the effects of hybrid filler content on the mechanical and electrical properties along with the state of filler dispersion . The state of dispersion of hybrid filler was studied by optical microscopy, rheological measurements, SEM, TEM, etc. The percolation threshold in electrical conductivity was reported at 0.62 wt% of CNT-GNP hybrid at 8:2 ratio. The attainment of such low percolation threshold in electrical conductivity can be attributed to the formation of 3D conduction structure. Furthermore, the rheological properties of the CNT-GNP hybrid/epoxy nanocomposites were investigated and it was found that the viscosity and storage modulus were higher as compared to the CNT/epoxy composite. Chatterjee et al. studied the effect of GNP size on the mechanical and thermal properties of CNT-GNP/epoxy nanocomposites . Fracture toughness and flexural modulus were found to increase by ~77 and 17% against pure epoxy with a CNT:GNP ratio of 9:1. Although the improvement in fracture toughness of the CNT-GNP (9:1) hybrid/epoxy nanocomposite was less than that of CNT/epoxy nanocomposites, improvement in flexural modulus was higher than that of the single filler system. The significant improvement in mechanical properties may be attributed to the efficient load transfer at the interface of matrix and CNT-GNP hybrid filler. Kim et al. investigated the effects of MWNT on the mechanical and rheological properties of GNP/epoxy nanocomposites . The improvements
386 Hybrid Nanomaterials in storage and loss modulus were recorded with the addition of MWNT. The impact strength was also found to increase by 35% at 1 wt% of MWNT content. The rheological behaviors of the nanocomposites showed more solid-like behavior with increasing the concentration of MWNTs. Li et al. studied the effects of hybrid filler composed of CNT and GNPs on the electrical and mechanical properties of epoxy nanocomposites . The hybrid filler content was fixed at 2 wt% and the electrical conductivity of the nanocomposite was found to be 0.47 Sm 1. The fracture toughness of the CNT-GNP hybrid/epoxy nanocomposite was increased by 21 and 57% as compared to the GNP/epoxy nanocomposite and neat epoxy, respectively. Yang et al. demonstrated the synergetic effects of the multi-graphene platelets (MGPs) and MWCNTs on the mechanical properties and thermal conductivity of epoxy nanocomposite . MWCNTs make a bridge with MGPs to avoid the aggregation of MGPs. The tensile strength of the hybrid/epoxy nanocomposites was found to be 35.4% higher against pure epoxy at the loading of 1 wt%. The study recorded ~146.9% improvement in thermal conductivity of the hybrid/epoxy nanocomposites at 1 wt% loading of the hybrid filler. Generally, the nanocomposite materials reveal enhancement in thermal conductivity at higher loading of filler, which adversely affects the mechanical properties. Hence, development of nanocomposite materials with higher thermal conductivity without sacrificing mechanical properties is very crucial to meet the increasing demand in a range of applications. In this direction, Im and Kim applied a new fabrication technique called wetting process to prepare thermally conductive GO/MWCNT/epoxy nanocomposite . The maximum enhancement in the thermal conductivity was recorded at ~0.36 wt% of MWCNTs and total amount of filler was fixed at 50 wt%. The improvement in thermal conductivity can be attributed to the conduction path that is formed due to the bridging of MWCNTs with GO. The study reported improved storage modulus in comparison to the solvent mixed nanocomposites. Thus, it can be deduced that the wetting process, which operates on the surface energy of GO and MWCNTs, is more effective than solvent mixing to fabricate mechanically robust hybrid nanocomposites.
Hybrid Fillers Based on CB, CF, CNT and Graphene
The design of hybrid filler containing different geometric characteristics and aspect ratios displays unique properties due to synergy which leads to enhanced electrical conductivity of the nanocomposites at lower loadings. Addition of a small amount of CNTs to the CB/polymer nanocomposites have resulted in improved properties. Further, the incorporation of
High Performance Hybrid Filler 387 low-cost co-fillers helps to fabricate cost-effective polymer nanocomposites with multifunctional properties. Ma et al. fabricated epoxy nanocomposites containing hybrid filler composed of CNT-CB and demonstrated the electrical and mechanical properties of the hybrid/epoxy nanocomposites . The percolation threshold in electrical conductivity was attained with 0.2 wt% CNT-CB (1:1) hybrid filler and ~6-fold increase in electrical conductivity (9.42 × 10–9 to 2.75 × 10 5 S m 1) was recorded for the nanocomposite. Further, it was recorded that the CB particles improved the ductility and fracture toughness of the nanocomposites due to the synergistic effect of the hybrid fillers. About 55% improvement in impact toughness confirmed the synergistic effect of hybrid fillers. Figure 8.4 shows the stress-strain curve of the nanocomposites containing single or hybrid filler. Ductility of the nanocomposites was found to be increased with the addition of CB. Sumfleth et al. studied the effect of MWCNT and nanosized CB on the electrical conductivity of epoxy nanocomposites by varying the weight fraction of the hybrid filler . The electrical behavior of the hybrid/ epoxy and MWCNT/epoxy nanocomposite systems were found to be alike. The synergistic effects in network formation and in charge transportation can be attributed to the preservation of electrical conductivity when half of the amount of MWCNT was replaced with CB. It is seen that the combination of conductive fillers not only preserved the electrical conductivity of the nanocomposites but also minimized the use of expensive fillers significantly. Figure 8.5 shows the variation of electrical conductivity of the binary and ternary system. 140
Flexural stress (MPa)
A: Neat epoxy B: 0.2% CNT only C: 0.2% CNT only D: 0.2% CB only E: 0.4% CB only F: 0.2% CNT + 0.2% CB
20 0 0
Flexural strain (%)
Figure 8.4 Stress-strain curves of the nanocomposites containing different fillers (Reprinted with permission from ; Copyright © 2009 American Chemical Society).
388 Hybrid Nanomaterials 10–1 Binary MWCNT composite Binary CB composite Ternary MWCNT+CB composite Computed sum conductivity of binary MWCNT and CB composites
10–3 10–4 10–5 10–6 10–7 10–8 10–9
f= 1khz 10–2
10–1 Filler content (wt. %)
Figure 8.5 Electrical conductivity for the nanocomposites exhibiting percolation behavior (Reprinted with permission from ; Copyright © 2009 Springer Science+Business Media, LLC).
Fan et al. fabricated GNPs-CB/epoxy nanocomposites by solution mixing method and studied the effect of CB on the electrical conductivity of GNPs/epoxy nanocomposites . The CB improved the dispersion of GNPs and favored the formation of robust conducting network. The percolation threshold in electrical conductivity was found to be 0.5 wt% for GNPs0.9-CB0.1/epoxy nanocomposites, which was lower than that of nanocomposites only filled with GNPs. The GNP-CB(9:1)/epoxy nanocomposites showed the electrical conductivity of 1.6 × 10 4 Sm 1 at 0.5 wt% for GNPs0.9-CB0.1 as compared to the epoxy nanocomposites filled with GNPs alone (4.9 × 10 7 Sm 1). Wei et al. used three different fillers of different geometries (GNPs, CB and CNTs) and studied the synergy on the electrical conductivity by incorporating GNP-CB and GNP-CB-CNT hybrid . The percolation threshold in electrical conductivity for GNP-CB/epoxy and GNP-CB-CNT/epoxy nanocomposites was achieved at 0.5 wt% of GNP-CB and 0.2 wt% of GNPCB-CNT loadings, respectively. The low percolation threshold values suggested that the electrical conductivity of the nanocomposites containing three different fillers is higher than that of the nanocomposites containing
High Performance Hybrid Filler 389 10–1
0.4 0.6 x (wt. %)
10–6 10–9 GNP0.7CB0.1CNT0.2/EP 10–12
10–15 10–180.0 (b)
1.0 1.5 2.0 2.5 Filler content (wt. %)
Figure 8.6 Variation of the electrical conductivity of the nanocomposite as a function of weight ratios of CB-GNP-CB and CNT-GNP-CB-CNT (total filler content is 1 wt%) (a); and the electrical conductivity of GNPs/epoxy, GNP0.9CB0.1/epoxy, and GNP0.7CB0.1CNT0.2/ epoxy as a function of the filler content (b). (Reprinted with permission from ; Copyright © 2010 Elsevier).
only two fillers. The electric conductivity of the GNP0.9CB0.1/epoxy (total filler loading is 1 wt%) nanocomposite was found to be 1.6 × 10−4 S m−1, which was three times larger than the base value (4.9 × 10−9) and the electrical conductivity value of 2.18 × 10−1Sm−1 recorded when 0.2 wt% CNTs was added into the GNP-CB/epoxy nanocomposite. The CB particles helped in the dispersion of GNPs and CNT particles contributed in the formation of conductive bridge to connect GNPs. Figure 8.6 presents the variation of electrical conductivity of the nanocomposite as a function of CB and CNT fraction. Ma et al. reported the preparation of MWCNTs-CF/epoxy nanocomposites by mixing-mold pressing method and showed the improvement in thermal conductivity and flexural and impact strength up to 2 wt% of MWCNTs loadings . The thermal conductivity value of 1.426 Wm 1K 1 was reported with 8 vol% treated MWCNTs and CF hybrid filler. The flexural and impact strength was found to increase and the optimum value was attained at 2 wt% of MWCNTs loadings. The increment was even more pronounced with surface treated MWCNTs-CF hybrid filler. The surface treatment of CF and MWCNTs not only improves the dispersion of the filler in the matrix but also helps in efficient interfacial formation which acts as bridge for load transfers. The deterioration of the mechanical properties at higher content of MWCNTs may be attributed to the introduction of more bubbles and stress concentration. Inam et al. fabricated multiscale micro-nanocomposite laminates using amino-modified
390 Hybrid Nanomaterials double-walled CNT (DWCNT-NH2:CF) hybrid/epoxy nanocomposite by a resin infusion technique . The incorporation of DWCNT-NH2-CF hybrid filler at the loading of 0.025, 0.05, and 0.1 wt% into epoxy resin resulted in the improvement of flexural modulus, flexural strength, and absorbed impact energy by 35, 5 and 6%, respectively. However, the mode I interlaminar toughness decreased by 23% as compared to the neat epoxy. Rahmanian et al. compared the mechanical properties of CNT/epoxy and CSCF (CNT grown short carbon fiber)/epoxy nanocomposites with the CNT-CSCF hybrid/epoxy nanocomposite . The enhancement in tensile strength and elastic modulus of the CNT(0.3%)-CSCF(1%) hybrid/ epoxy nanocomposite were ~37.3 and 38.8%, respectively, as compared to the neat epoxy. Further, the increase in CNT content was found to adversely affect the tensile properties, which can be due to the aggregation of CNTs in the epoxy matrix. The synergy enforcement in tensile properties can be attributed to the high stiffness of microfillers. The storage modulus was found to increase by 41% for CNT(0.3%):CSCF(1%) hybrid/epoxy nanocomposite and ~56% improvement in impact strength was recorded for the CNT(0.2%)-CSCF(3%) hybrid/epoxy nanocomposite. Shokrieh et al. studied the synergistic effects of graphene and CNF on the flexural fatigue life of epoxy polymer . The integration of GPL and CNF to form hybrid filler resulted in significant improvement in the fatigue life of epoxy resin against GPL alone and CNF-filled epoxy nanocomposites. About 37.3-fold improvement in flexural bending fatigue was recorded for 0.5 wt% of graphene-CNF hybrid/epoxy nanocomposites. The static bending strength of 123 Mpa was recorded for the nanocomposites filled with 0.25 wt% of graphene-CNF(1:1) hybrid filler. In contrast, the static bending strength of the GPL/epoxy and CNF/epoxy nanocomposite values was found to be 118 and 121 MPa, respectively. The improvement in fatigue life is attributed to the stiffening nature of graphene and the enhanced strength due to the pull-out mechanism of CNF during fracture. Further, it can be concluded that the hybrid filler materials greatly influenced the crack propagation mechanism of the polymer that can be traced to improvement in the fatigue life of the nanocomposites.
Hybrid Fillers Based on Clay, CB, CNT and Glass Fibers
Liu and Grunlan introduced clay minerals into the CNT/epoxy nanocomposites to enhance the dispersion of CNT, preserving the electrical conductivity and mechanical properties . It is known that clay is mechanically robust and displays good dispersion, therefore, efficiently transfers load across the interface in polymer nanocomposites. Thus, the concept of using
High Performance Hybrid Filler 391 clay along with CNT is to improve the electrical conductivity of the nanocomposites without compromising its mechanical properties. The addition of clay decreased the percolation threshold in electrical conductivity from 0.05 to 0.01 wt% SWNT content. The electrical conductivity was found to increase from 10–7 to 10–3 S m–1 with the addition of 0.2 and 0.05 wt% of clay and SWNT loadings, respectively. Further, the thermomechanical properties were studied and the storage modulus of the nanocomposites was found to increase with the increase in concentration of SWCNT. The storage modulus of the nanocomposites increased by 20.3 and 21.9% with the addition of 0.05 and 0.1 wt% of SWCNT in the 2 wt% clay-filled epoxy nanocomposites. The Tg of the nanocomposite was also found to increase by 4 °C. Etika et al. added clay particles to CB-filled epoxy nanocomposites in an attempt to enhance the electrical conductivity without harming the mechanical properties . The electrical conductivity of the CB/epoxy nanocomposites was found to be increased by an order of magnitude with the addition of 0.5 wt% clay but there was no significant improvement in storage modulus. The epoxy nanocomposites fabricated from equal concentration of clay and CB did not show any improvement in electrical conductivity but the storage modulus was increased by ~36 and 40% for 2.5 and 5 wt% CB filled into the epoxy. Further, both the electrical conductivity and storage modulus was found to increase at 1:2 clay-CB (wt/wt), which could be perceived as the optimum concentration of clay to improve the electrical and mechanical properties concurrently. The clay particles help to disperse and stabilize the CB particles, which ultimately accounts for the synergistic effects on the electrical and mechanical properties of the epoxy nanocomposites. Lin et al. used vacuum-assisted resin transfer molding (VARTM) process to fabricate layered silicate-glass fiber/epoxy hybrid nanocomposites and studied the synergistic effects of the filler on the mechanical properties of the nanocomposites . The glass fibers were placed in a parallel and perpendicular direction to the resin flow to examine the influence of fiber orientation on the clay dispersion. The onset of decomposition temperature corresponding to 1.2 wt% losses was increased by 29 °C for the 5 wt% clay-filled nanocomposites as compared to the neat epoxy. The significant improvement in thermal properties is attributed to the barrier properties of the layered clay minerals dispersed in the nanocomposites. The storage modulus in the rubbery region was found to increase with the clay loading while the glassy region remains unaffected. The flexural test was performed in longitudinal and transverse direction.The transverse and longitudinal flexural modulus was increased slightly and significantly with the clay
392 Hybrid Nanomaterials loadings. However, the longitudinal flexural strength and impact strength at direction perpendicular to the fiber was found to decrease with the clay loadings. Conventionally, carbon black and vapor grown carbon fiber (VGCF) were intensely used as conductive filler in polymer nanocomposites but usually higher loading is required to achieve the applicable electrical properties. The discovery of CNT has instantly substituted CB and VGCF as conductive filler for nanocomposite fabrication. Although the electrical conductivity value of CNT is much higher and it makes nanocomposites conductive with small amount of loading, the use of CB-VGCF hybrid would be much more economically viable. Kotaki et al. selected VGCF instead of CNTs to fabricate clay-VGCF/epoxy nanocomposites . Low volume resistivity was achieved with the addition of highly exfoliated clay and the addition of 5 wt% clay along with 1 wt% of VGCF resulted in low volume resistivity of 105 Ω m. The accomplishment of such low volume resistivity may be linked to the efficient dispersion of VGCF to form the conductive network which was further enhanced by the clay particles.
Hybrid Fillers Based on Ceramic Powder, CNT and GNP
Efficient heat dissipation or transfer in electronic components is very vital to maintain lower operating temperature, optimal circuit performance and longer working time. Primarily, materials having high thermal conductivity, low coefficient of thermal expansion, high electrical resistivity and low dielectric permittivity are desirable for packing the electronic components. The encapsulation of electronic circuit to shield the components against environmental damage, shocks, etc., using polymer composites having high thermal conductivity is highly desirable. The widely used smart carbon-based nanofillers having extraordinary thermal conductivity and low thermal contact resistance have an enormous prospect for thermal interface applications; however, electrical conductivity of so-formed nanocomposite materials is one of their demerits for microelectronic packaging applications . Thus, fabrication of hybrid filler employing diff ferent carbon-based nanofillers with inorganic particles can be perceived as a plausible solution for enhancing thermal conductivity and mechanical strength to improve thermal contact resistance. So, the fabrication of hybrid filler using conducting components, such as CNTs and graphene, along with ceramic-based fillers, such as SiC, AlN, BN, and Al2O3, has been carried out to accomplish lower electrical conductive materials with high thermal conductivity. Dispersion of such co-fillers along with the conductive fillers not only imparts insulating properties to the electrically
High Performance Hybrid Filler 393 conductive materials but also synergistically enhances the thermal conductivity accompanied by reduced thermal interface resistance, which are requisite for thermal interface materials (TIMs). The conductive fillers play a supportive role in heat transfer, which results in high thermal conductivity. Among the ceramic fillers, aluminum nitride has been extensively used as conductive filler owing to its high thermal conductivity (~320 W mK 1 at room temperature), lack of toxicity, stable crystal structure, and relatively low cost . Hexagonal boron nitride (hBN), also called “white graphite,” is an excellent electrical insulation, having high thermal conductivity, excellent dimension stability and desirably low dielectric loss and constant. Because of its high thermal conductivity, Micro-SiC has been extensively used in electronic industries. Zakaria et al. fabricated epoxy nanocomposites using CVD grown CNT-alumina (CNT-Al2O3) hybrid filler and physically mixed CNT-Al2O3 filler . The uniform dispersion of CNT-Al2O3 in the polymer matrix increased tensile strength and tensile modulus by 30 and 39%, respectively. Further, thermal conductivity and Tg was found to increase by 20 and 25%, which reflected the synergistic effect of the hybrid filler. Figure 8.7 shows the FE-SEM images of CNT-Al2O3 hybrid/epoxy nanocomposites. Li et al. prepared nano/micrometer hybrids by CVD grown CNTs on the SiC, Al2O3 and GNP followed by hybrid filler/epoxy nanocomposites . The reinforcement effect of CNT-GNP hybrid was more prominent than the rest. The improvement in tensile modulus and tensile strength was higher in the case of CNT-GNP hybrid/epoxy nanocomposites. About a 40% improvement in tensile modulus was recorded for the nanocomposites against neat epoxy. However, the improvement of 4, 9 and 18% was recorded in the case of CNT-SiC/epoxy, CNT-SiC-CNT–Al2O3/epoxy and CNT-Al2O3/epoxy nanocomposites. He and Tian grew CNTs on the surface of Al2O3 and fabricated CNT-Al2O3/epoxy nanocomposites by mechanical stirring . Homogeneous dispersion of CNTs into the epoxy matrix and enhanced interfacial interaction improved the mechanical properties of the nanocomposites. The tensile strength and Young’s modulus of the nanocomposites improved by 34 and 47%, respectively, due to the the synergistic effect of the hybrid reinforcement. Zha et al. fabricated alumina fibers (AFs)-GNPs/epoxy nanocomposites using a hot-pressing process and investigated the thermal conductivity as well as the dielectric properties of the nanocomposites . The nanocomposite containing 2 vol% GNPs and 50 vol% AFs showed a thermal conductivity of 1.62 W m 1 K 1. The decomposition temperature with 2 vol% of GNP and 50 vol% of AF loading was improved by ~100 °C. The improvement in thermal conductivity was ascribed to the thermal conduction path formed by high aspect
394 Hybrid Nanomaterials
Alumina Carbon nanotubes
1 m (a)
1 m (b) 16.48 nm
200 nm (c)
100 nm (d)
Figure 8.7 FE-SEM images of CNT-Al2O3 hybrid compound at different magnification (Reprinted with permission from ; Copyright © 2014 Elsevier Ltd.)
ratio GNP and the contribution of AFs in reducing the thermal interface resistance. The hybrid composites showed low dielectric permittivity at low concentration of GNPs and high insulating property with high concentration of AFs. Teng et al. used poly(glycidyl methacrylate) (PGMA) functionalized MWCNTs-zirconate coupling aluminum nitride (AlN) hybrid filler to fabricate epoxy nanocomposites and demonstrated its effect on thermal conductivity . Functionalization of MWCNT improved the dispersion and interfacial affinity, which was reflected in the enhanced thermal conductivity of 1.21 Wm 1K 1 at 25 vol% of modified AlN and 1 vol% of functionalized MWCNT loadings. It is noteworthy that ~50 vol% untreated AlN is required to achieve the similar thermal conductivity of the nanocomposites without the addition of MWCNTs. These results state that appropriate design of hybrid fillers improved the dispersion and could be more efficient than single component filler. Ma et al. reported a thermal conductivity of ~1.04 W m 1K for the MWCNTs-AlN/epoxy nanocomposites containing 29 wt% of hybrid filler (4 wt% MWCNTs + 25 wt%
High Performance Hybrid Filler 395 AlN) . The flexural and impact strength of the epoxy nanocomposites were found to increase with filler amount and were optimum with 1 wt% of MWCNTs and 5 wt% of AlN. The synergistic effect of the MWCNTs and AlN was also vividly reflected through the improvement in thermal decomposition temperature of the nanocomposites. Further, the surface treatment of MWCNTs and AlN with γ-glycidoxy propyl trimethoxysilane (KH-560) was performed and its nanocomposites with epoxy showed a positive effect on thermal conductivity. Choi et al. prepared nano-AlNdoped MWCNTs to improve the thermal conductivity of epoxy nanocomposites . The nano-AlN-doped MWCNTs contributed to the electrical and thermal conductivity of the nanocomposites through the formation of 3D networks within the matrix. Thermal conductivity of the nanocomposites containing 2 wt% nano-AlN-doped MWCNTs and 57.4 vol% micro-AlN was increased by 31.27 times against neat epoxy. This may be attributed to the interconnected link between the conducting filler and improved interfacial resistance. Shtein et al. reported the preparation of 3D network hybrid filler composed of nm-BN with μ-BN or GNP and fabricated epoxy nanocomposites to study their synergistic effect on the thermal conductivity and mechanical properties . The thermal conductivity was increased by 72 and 22% for the BN- and GNP-based hybrid filler, respectively. The flexural and compressive strength was found to increase for the nanocomposites as compared to the neat epoxy. The enhancement in thermal conductivity can be attributed to the contribution of dispersed nm-BN particles that formed the conducting pathways in the nanocomposites. Teng et al. fabricated thermally conductive epoxy nanocomposites with poly(glycidyl methacrylate) (PGMA) MWCNTs and zirconate coupling boron nitride (BN) . The covalent functionalization promoted the fine dispersion of the fillers in the epoxy matrix, which facilitated the generation of good interaction between the components and also helped to reduce the thermal interface. The thermal conductivity was found to increase by 743% (1.913 Wm 1K 1 as compared to 0.2267 Wm 1K 1 of neat epoxy) at a loading of 30 vol% modified BN and 1 vol% functionalized MWCNTs. The improvement in thermal conductivity can be attributed to the 3D network structure of hybrid filler which promoted heatflow. Ulus et al. investigated the mechanical properties of epoxy nanocomposites prepared by BN and MWCNT as hybrid filler . The amount of BN was kept constant at 0.5% and the loading of MWCNT was varied from 0.1 to 0.3 to 0.5 wt% to study the synergistic effect. It was found that the tensile properties were influenced by the addition of MWCNT. The maximum tensile strength was achieved at 0.5 wt% boron nitride nanoparticle (BNNP) and 0.3 wt% MWCNT loadings. The elastic modulus of the
396 Hybrid Nanomaterials nanocomposites was increased by ~38% against neat epoxy. Figure 8.8 shows the preparation procedure of epoxy nanocomposites. Ci and Bai used CVD grown aligned MWCNT on the micronsized SiC to develop hybrid filler and fabricated epoxy nanocomposites . The addition of SiC was not seen to improve the mechanical properties; rather, it was found that the fracture strain was reduced as compared to the neat epoxy. In contrast, the nanocomposites containing hybrid filler showed ~24 and 15% improvement in Young’s modulus and tensile strength, respectively. The improvement in mechanical properties can be attributed to the novel structure so formed, which rendered good interfacial affinity with the matrix. Zhou et al. observed 2.9 and 20.7 times higher thermal conductivity of MWCNT/epoxy and μSiC/epoxy nanocomposites than that of the pure epoxy . To further improve the thermal conductivity, the authors prepared the hybrid filler by partially replacing microfiller with nanofiller to fabricate epoxy/hybrid nanocomposites. The epoxy nanocomposites containing 5 wt% MWCNT and 55 wt% μSiC hybrid filler displayed 24.3 times higher thermal conductivity than that of pure epoxy. Yang and Gu prepared TETA-functionalized MWCNTs/epoxy and silane-modified nanosized SiC and studied the thermal conductivity of the single filler and hybrid filler nanocomposites . The nanocomposites containing 30% (volume fraction) of hybrid filler showed a thermal conductivity of 2 Wm 1 k 1 at room temperature. In contrast, the thermal conductivity of the nanocomposites containing 20% (volume fraction) MWCNT was found to be 1.6 Wm 1 k 1. Therefore, it is seen that the surface treatment of filler is very crucial to enhance dispersion and interaction with polymer matrix, which subsequently diminishes interfacial
Hardener Preparing the mold BNNP and CNT powder
Tensile test specimen Ultrasonic mixing
Figure 8.8 Preparation process of the epoxy composites. (Reprinted with permission from ; Copyright © 2013 Elsevier B.V.)
High Performance Hybrid Filler 397 thermal resistance. Wang et al. used direct iron-catalyzed heat-treatment procedure to grow SiC nanowires on the surface of graphene and obtained SiCNWs-GNPs hybrid nanostructure, which was subsequently used as hybrid filler to fabricate epoxy nanocomposites . The thermal conductivity was increased by 65.2% at 7 wt% of SiCNWs-GNPs loading in comparison to the neat epoxy and it was found that the insulation characteristic of the nanocomposites was preserved. The storage modulus of the nanocomposite was found to be 42.4% larger as compared to the neat epoxy. The degradation temperature at 5% weight loss (Td5%) and the maximum degradation temperature were improved while lower dielectric constant value was recorded. The enhancement in these properties could be linked to the bridging effect of SiC, which promoted fine dispersion of the hybrid filler in the matrix. Hong et al. incorporated AlN and BN into the epoxy matrix to investigate the effects of their particle size and the relative composition on the thermal conductivity of the resulting nanocomposites . It was predicated that the optimal thermal conductive path was strongly influenced by the packing efficiency and interfacial thermal resistance, which subsequently affected the heat conduction and dissipation. The thermal conductivity value of ~8.0 Wm 1K 1 was recorded in the 1:1 volume ratio of AlN-BN particles with similar particle sizes.
Hybrid Fillers Based on Silica Particle Modified Graphene and CNTs
Generally, silica particles are incorporated into the epoxy matrix for three purposes: (i) to reduce the co-efficients of thermal expansion (CTE), (ii) to lower the shrinkage upon curing, and (iii) as mechanical reinforcement [112–115]. The mechanical and thermal stability of materials becomes very vital when developing strong but lightweight structural components intended to operate in a wide range of temperatures, like those used in the aerospace industry for spacecraft. So, it is imperative and pragmatic to design such types of materials which display both mechanical and thermal stability. One way is to integrate different components having different features, which allows varying the properties of the composite materials in a wide range of application parameters. For example, the desired mechanical and thermal stability of the nanocomposite materials can be achieved by using CNF with large aspect ratio. In contrast, SiO2 possesses low CTE that is reflected in the thermomechanical stability of the nanocomposite in a wide temperature range. Therefore, the development of hybrid fillers possessing good mechanical, thermal and thermomechanical properties is in
398 Hybrid Nanomaterials (a)
(d) SiO2 GO
Figure 8.9 SEM images of the (a) raw SiO2 and (b) created SiO2-GO hybrid, scale bar 500 nm. (c) Transmission electron microscopy (TEM) image of a typical core-shell structured SiO2-GO hybrid. (d) High-resolution TEM (HR-TEM) images of grapheneencapsulating silica spheres. (Reprinted with permission from ; Copyright © 2012 American Chemical Society).
high demand to achieve high performance epoxy nanocomposites suitable for high temperature applications . Chen et al. prepared core-shell structured SiO2-GO hybrid filler and incorporated it into the epoxy matrix . The resulting nanocomposite showed improved modulus, strength and fracture toughness. The GO played a crucial role to bridge SiO2 with the polymer matrix, thereby improving interfacial interaction, which was reflected in the mechanical properties of the hybrid/epoxy nanocomposites. The epoxy nanocomposite containing 10 wt% of SiO2-GO hybrid filler showed 8.5, 22.1, and 45.3% improvement in Young’s modulus (EY), tensile strength (σs), and fracture toughness (G) as compared to the nanocomposites containing only SiO2-N2. Figure 8.9 shows the TEM and SEM images of SiO2 and GO-coated SiO2. The typical stress-strain curve is presented in Figure 8.10. Dynamic mechanical analysis (DMA) was conducted to further illustrate the interfacial interaction between the fillers and matrix, which showed ~41.7% improvement in storage modulus against pure epoxy at 10 wt% loadings. Further, the interaction between the hybrid filler and matrix was confirmed by the 10.7 °C rise in Tg. Figure 8.11 shows the storage modulus and tan δ curve against temperature.
High Performance Hybrid Filler 399 80
80 1) Neat epoxy 2) 0.1 wt% GO 3) 10 wt% SiO2 4) 10 wt% SiO2-NH2 5) 10 wt% (SiO2-NH2/GO) 6) 10 wt% SiO2-GO
2.5 Tensile strength (MPa)
5 1 3
70 2.0 60
0.02 0.04 0.06 Strain (mm/mm)
10 15 Content (wt%)
Figure 8.10 (a) Stress-strain curves for neat epoxy and epoxy composites; (b) effect of SiO2-GO content on the tensile strength and modulus of epoxy resins. (Reprinted with permission from ; Copyright © 2012 American Chemical Society).
Neat epoxy 0.1 wt% GO 10 wt% SiO2 10 wt% SiO2-NH2 10 wt% SiO2-GO
Storage modulus (MPa)
Neat epoxy 0.1 wt% GO 10 wt% SiO2 10 wt% SiO2-NH2 10 wt% SiO2-GO
0.0 150 200 Temperature ( C)
200 250 Temperature ( C)
Figure 8.11 Plots of dynamic mechanical curves for neat epoxy and epoxy composites: (a) storage modulus; (b) damping spectra (tan δ). (Reprinted with permission from ; Copyright © 2012 American Chemical Society).
Huang et al. used RGO-encapsulated SiO2 hybrids (SiO2@RGO), which were prepared by thermally reducing electrostatically assembled SiO2@ GO to fabricate SiO2@GO/epoxy nanocomposites and studied the thermal, dielectric and thermomechanical properties . The embedded SiO2 particles on thin RGO nanosheets forming core-shell nanostructure inhibited the agglomeration of RGO nanosheets, which efficiently promoted dispersion of RGO and network formation with the matrix, resulting in enhanced thermomechanical properties and thermal or electrical conductivity. The authors revealed a dielectric constant value of 77.23 at 1 kH at 20 wt% of SiO2@RGO, which was ~22 times higher than that of neat epoxy, which was further increased with increasing the content of the filler owing
400 Hybrid Nanomaterials to the formation of more effective conducting path. The electrical conductivity was found to increase significantly at 30 wt% of hybrid filler loadings. Further, the thermal conductivity increased to 0.452 Wm 1K 1 at 40 wt% of SiO2@RGO loadings. Hsiao et al. deployed sandwich structured hybrid nanosheets (NSs) consisting of thermally reduced GO (TRGO) and silica to fabricate thermally conductive and electrically insulating epoxy nanocomposite . The thermal conductivity was recorded as 0.322 Wm−1K−1 at 1 wt% of TRGOsilica loadings and was 61% larger than that of the neat epoxy. The layer of silica embedded in TRGO generated 3D phonon transport channel that can deteriorate the electrical conductivity. Silica plays a pivotal role in decreasing the thermal resistance between the matrix and TRGO. It might also mitigate the aggregation of TRGO and promote homogenous dispersion of hybrid filler in the matrix, enhancing the interfacial interactions. The storage modulus of the nanocomposites at 1 wt% TRGO-silica NS/epoxy nanocomposite was found to be 23.4% higher than that of the neat epoxy matrix. Wang et al. prepared graphene-nanosilica hybrid filler through a sol-gel and surface treatment process and incorporated it into the epoxy resin to prepare the nanocomposites . The concept behind the preparation of noble sandwich structure hybrid filler was to improve the thermal-oxidative decomposition of the layered structure under combustion. The unique structure not only improved the thermal stability and flame retardant properties but also improved thermal conductivity, mechanical and dielectric properties. The peaks of the heat release rate (HRR) and total heat release (THR) of the hybrid/epoxy nanocomposites were decreased by 39 and 10% against the neat epoxy at the loading of 1.5% m-SGO. The improvement in flame retardant properties could be accounted for by the high resistance to degradation and efficient interaction between the filler materials and epoxy matrix. The Tg value of the m-SGO1.5/EP nanocomposite was increase by 10 °C against neat epoxy. Tensile strength and thermal conductivity of the m-SGO1.5/epoxy nanocomposite were also found to be ca. 1.31 and 1.38 times higher as compared to the neat epoxy. Furthermore, high dielectric constant and low dielectric loss were recorded. Pu et al. used 3-aminopropyltriethoxysilane (APTES) to simultaneously functionalize and reduce the GO. Silica particles were formed in situ from the tetraethyl orthosilicate and deposited on the APTES functionalized graphene (A-graphene) sheets. The APTES modified silica-GO hybrid was incorporated into the epoxy matrix to investigate the thermal conductivity of the nanocomposite . The silica particles strongly acted as bridge to link the graphene and epoxy and are also believed to have played a pivotal role in improving the modulus mismatch between the fillers and the
High Performance Hybrid Filler 401 matrix. The thermal conductivity of silica-coated A-graphene (S-graphene) epoxy composites was enhanced by 72% against neat epoxy polymer, while preserving the electrical neutrality of the nanocomposites. Jiang et al. modified GO by APTES functionalized silica nanoparticles (ATGO) and incorporated it into the epoxy matrix . They studied the effect of filler loadings on the tensile strength and impact strength both at room temperature (RT) and cryogenic temperature (CT). It is seen that the tensile strength and tensile modulus of the ATGO (1 wt%)/epoxy nanocomposite were 29.2 and 22.0% higher than that of neat epoxy at CT. The impact strength at RT was much higher than that at CT. This may be attributed to the presence of multifunctional groups and, hence, good interaction between matrix and ATGO. Figure 8.12 shows the schematic for the preparation of GO, ATS, and ATGO. Jiang et al. also modified the surface of silica nanoparticles by APTES and then attached them with GO using isocyanate group-terminated flexible chains to synthesize modified filler (SATPGO) for the fabrication of epoxy nanocomposites . The impact strength of the SATPGO-filled (0.5 wt%) epoxy nanocomposite was found to be 154 and 92% higher than that of neat epoxy at RT and CT, respectively. Tang et al. prepared RGO-filled epoxy nanocomposites and investigated the influence of dispersion on the mechanical properties of the nanocomposites . They reported the improvement in KIC values by 24 and 52% at 0.2 wt% RGO loadings. This indicated that the state and extent of dispersion was greatly influenced by the end properties of the nanocomposite. OH
O O O
COOH O COOH O
Single layer of GO
Multiple layer of graphite oxide OH
Figure 8.12 Schematic for the preparation of GO, ATS, and ATGO (Reprinted with permission from ; Copyright © 2013 Elsevier Ltd.)
402 Hybrid Nanomaterials Practically, it is very difficult to achieve the theoretically predicated thermal conductivity of MWCNT/epoxy nanocomposites due to the thermal interfacial resistance [124, 125]. Therefore, surface modification of CNT has been widely exploited to diminish thermal interfacial resistance and to make the modulus compatible between the CNT and polymer matrix by promoting good interfacial interaction. The use of conductive filler enhanced the probability of network formation and facilitated heat transfer, which subsequently is reflected in the improved thermal conductivity. Cui et al. developed silica-coated CNT core/shell structured (MWCNT@ SiO2) by sol-gel method and employed as filler to fabricate epoxy nanocomposite . The layer of silica on CNT helped to diminish the modulus mismatch between the MWCNT and epoxy and improved the interaction between them. The increment in storage modulus confirmed good interaction between the hybrid filler and epoxy and 2.7 °C improvement in Tg was also recorded. The increment in Tg can be ascribed to the restricted mobility of the polymer segments, which was possible due to strong interaction with the filler. The thermal conductivities of the MWCNT@SiO2/ epoxy nanocomposite were found to increase by 51 and 67% at low filler loadings of 0.5 and 1 wt%, respectively. Hsieh et al. prepared MWCNTsSiO2 nanoparticle hybrid/epoxy nanocomposites and reported improved fracture toughness . The fracture toughness and fracture energy of the hybrid epoxy nanocomposites was found to be increased by ~33 and ~35% respectively with 0.18 wt% MWCNTs and 6.0 wt% silica nanoparticles. Young’s modulus values did not improve with the addition of MWCNT while only silica/epoxy nanocomposites showed little improvement. Lavorgna et al. studied the synergistic effects of 3-amminopropryltriethoxy silane (APTES) functionalization and silica-decorated CNT on the thermal and thermal-mechanical properties of hybrid/epoxy nanocomposites . The APTES-functionalized and silica-enriched MWCNT showed greater dispersion in the epoxy matrix as reflected in the 25 and 280% improvement in storage modulus in glassy and rubbery regions, respectively. Further, 20 °C increase in Tg and 4–7 °C improvement in onset decomposition temperatures (at 5 and 10% weight loss) was recorded for the nanocomposite. Figure 8.13 presents TGA curves of pristine and differently functionalized MWCNTs. Jang et al. used CNF and micronsized SiO2 particles to fabricate epoxy nanocomposites to investigate their mechanical damping and thermal stability by measuring the CTE . The hybrid filler/epoxy nanocomposites showed 18.6 and 15.8% improvement in storage modulus and damping loss factor at 3 wt% of each CNF and SiO2 loading. The CTE of the hybrid nanocomposite with 3 wt% loading was found to be decreased by 15%
High Performance Hybrid Filler 403 1.0
0.8 Loss weight, %
MWCNT-APTES-SiO2-x(OH)x 0.6 MWCNT-APTES 0.4
0.2 MWCNT 0.0 100
400 500 600 700 Tepmperature, C
Figure 8.13 TGA curves of pristine and differently functionalized MWCNTs. (Reprinted with permission from ; Copyright © 2012 Elsevier Ltd.)
against neat epoxy resin. The SiO2 was found to be more effective than hybrid filler in reducing the CTE but damping and stiffness of the nanocomposite got hampered. Likewise, CNF, due to its high aspect ratio, is more promising than SiO2 in improving damping and stiffness. Integration of the two components having different features to simultaneously attain multiple properties is very crucial to fabricate nanocomposite materials applicable for structural components in a wide temperature range. Tang et al. combined soft submicron-rubber and rigid nanosilica particles with MWCNTs to enhance the properties of the epoxy nanocomposites suitable for various engineering applications where both electrical conductivity and toughness are desirble . The electrical conductivity of MWCNT-silica/epoxy and MWCNT-liquid rubber/epoxy nanocomposites was reported as 1.63 × 10 1 and 4.82 × 10 2 S m 1, respectively, in comparison to the neat epoxy (2.33 × 10 9 18.6 and 15.8%). The high electrical conductivity of the MWCNT-silica/epoxy nanocomposites in comparison to MWCNT-LR/epoxy nanocomposites was attributed to the improved dispersion of MWCNT in the presence of silica compared to LR. The KIc and GIc values of MWCNT-LR/epoxy were found to be greater than MWCNT-silica/epoxy nanocomposites. The KIC and GIC values of 1.174 ± 0.046 and 454.68 ± 15.17 were reported for MWCNT-LR/epoxy nanocomposites against the 0.548 ± 0.056 and 89.33 ± 16.66 for pure epoxy composites, whereas the values of 0.882 ± 0.056 and 199.02 ± 14.33 were recorded for MWCNT-silica/epoxy nanocomposites. These results correspond to the
404 Hybrid Nanomaterials fact that rubber particle is a more effective toughener as compared to the rigid particles. However, MWCNT-silica/epoxy nanocomposites showed greater values in both stiffness and strength. The results signified that the combination of two fillers is very promising in improving the mechanical and electrical properties compared to the single filler particle. The high stiffness of CNT has encouraged studying its mechanical reinforcing ability when incorporated into the polymer matrix to fabricate nanocomposites. However, it is always desirable to achieve improvement in mechanical properties at lower CNT loading, but that is not always possible using simple processing techniques. In order to disperse CNTs efficiently and to realize enhancement at lower concentration of CNTs, employment of co-filler may be one way out. Sun et al. used exfoliated ZrP nanoplatelets to fabricate epoxy nanocomposites containing CNTs . The interaction between the inorganic ZrP nanoplatelets and CNTs promote the dispersion of CNTs in the epoxy matrix and improvement in mechanical properties was realized at very low loading of CNTs. The authors revealed ~41% improvement in Young’s modulus and ~55% in tensile strength. The uniform dispersion of filler in the host polymer matrix is one of the vital prerequisites for the transfer of properties of filler to the nanocomposite. Functionalization of filler materials, use of dispersing agent, and incorporation of a third phase have been identified as the probable measures to enhance dispersion of the filler. In order to improve dispersion, Sumfleth et al. modified MWNT with titania nanoparticles to achieve multiphase materials . The improvement in dispersion was evident from the increased shear storage modulus and decreased electrical conductivity of the MWNT-TiO2/epoxy nanocomposite. Du et al. fabricated magnesium oxide-coated graphene (MgO@GR)/ epoxy nanocomposites and reported improved thermal conductivity . It is believed that MgO not only helped to disperse GO in the matrix but also enhanced the interfacial interaction and thermal conductivity. The thermal conductivity was found to be increased by 76% at the loading of 7 wt% MgO@GR while the electrical resistivity was maintained at 8.66 × 1014 Ω m. Further, the dispersion of the hybrid material was confirmed by a 15 °C increase in Tg. Luan et al. fabricated 1D Ag nanowires (NWs), 2D chemically reduced GO (CRG)-CRG-AgNW hybrid/epoxy nanocomposites and reported enhanced mechanical properties, thermal resistance and higher electrical conductivity as compared to pure resin and composites with single filler alone . Due to the reduced tunnelling resistance between the nanowires, the percolation threshold was reduced from 30 to 10 wt% for hybrid epoxy nanocomposites. The break strength of the hybrid/epoxy nanocomposite was found to be increased by 50% due to the
High Performance Hybrid Filler 405 chemical effects between CRGs and hardener on crosslinking of the epoxy. Further, the improvement in glass transition temperature was also reported. Omrani et al. prepared epoxy nanocomposites containing hybrid nanosized TiO2-SiO2 (50–120 nm, 2.5–10% by weight) filler particles and studied viscoelastic as well as thermal properties . The addition of the hybrid filler resulted in the improvement of ~33 °C in Tg and storage modulus. The synergistic potential of the hybrid filler was reflected in the improvement temperature at 5% loss in mass (Td,5%) at 16 °C against neat epoxy. The increase in thermal stability was attributed to the capability of the hybrid filler to catalyze the curing and its role as thermal stabilizer.
Hybrid Fillers Based on LDHs, Organohydroxide, MoS2, and Graphene
Wang et al. prepared Ni-Fe LDH-graphene hybrids by a one-pot in-situ solvothermal method and fabricated epoxy nanocomposites to study the fire hazard effect . The Tonset value was found to increase by 25 °C at 2.0 wt% loading of Ni-Fe LDH-graphene hybrid filler. The peak HRR and THR values were decreased by 60 and 61%, respectively, as compared to the neat epoxy resin. The improvement in fire hazards can be owed to the synergistic effect of hybrid filler, where the filler acts as a barrier to reduce the release of combustible gas and also forms a charred layer that can prevent the polymer from further burning. Although aluminum hydroxide is favored in flame retardant materials because of its cost factor and environmental friendliness, typically a greater amount is required to accomplish appropriate properties and its compatibility with organic polymers is also not encouraging. So, many efforts have been undertaken to increase its effectiveness as retardant material. Jiajun et al. prepared aluminium-organophosphorus hybrid nanorods (AOPH-NR) using aluminum hydroxide (ATH) and dibenzylphosphinic acid (DBPA) to determine the limiting oxygen index (LOI) and fabricated epoxy nanocomposite to study the flame retardant and mechanical properties . The LOI was found to be decreased by 26.1 and 23% at the loading of 4.25 wt% AOPH-NR. The storage modulus was increased from 1260 to 2607 MPaat 4.25 wt% of AOPH-NR loadings without affecting the Tg significantly. The thermogravimetric measurements displayed the fact that AOPH-NR assisted the degradation, which is suggestive of the favorable release and migration ability of phosphorus species for the AOPH-NR/ epoxy nanocomposites. Wang et al. synthesized MoS2-GNS hybrids to prepare epoxy nanocomposite and studied its flame retardant properties . The modification of
406 Hybrid Nanomaterials 100
EP MoS2/EP GNS/EP MoS2/GNS/EP
EP MoS2/EP GNS/EP MoS2/GNS/EP
2 THR (MJ/m )
1000 800 600
EP MoS2/EP GNS/EP MoS2/GNS/EP
200 Time (s)
200 Time (s)
EP MoS2/EP GNS/EP MoS2/GNS/EP
2 SPR (m /s)
200 Time (s)
1000 0.1 500 0.0
200 Time (s)
Figure 8.14 (a) HRR, (b) THR, (c) TSR, and (d) smoke production rate versus time curves of epoxy and its composites obtained from cone calorimeter fire testing. (Reprinted with permission from ; Copyright © 2013 American Chemical Society).
GNS with MoS2 significantly improved the oxidative resistance of graphene. The PHRR, THR, and total smoke release (TSR) were found to be decreased by 45.8, 25.3 and 30.5%, respectively, for the nanocomposite containing 2 wt% of MoS2-GNS hybrid filler as compared to the neat epoxy. Furthermore, onset thermal degradation temperature was increased by (Tonset) 53 °C with the incorporation of 2 wt% of MoS2-GNS hybrid filler. The enhancement in flame retardant and smoke suppression properties was attributed to the insulating charred layer generated by MoS2-GNS, which guarded the inner polymer from being burned, and the formation of molybdenum oxide, which is a highly efficient smoke suppression agent. Figure 8.14 presents the HRR, THR, TSR, and smoke production rate versus time curves of epoxy and its nanocomposites obtained from cone calorimeter fire testing.
Hybrid Fillers Based on Silicate and Liquid Rubber
Many efforts have been expended towards toughening the epoxy matrix without sacrificing the strength, stiffness and thermal properties. The
High Performance Hybrid Filler 407 pioneering work of BF Goodrich researchers in the 1970s stimulated intensive work in the area to improve the toughness of thermosets using liquid rubber while balancing stiffness . One of the criteria that must be fulfilled in order to be a toughening agent is phase separation during curing, and to achieve phase separation, there must be compatibility between the epoxy matrix and rubber accompanied by a good degree of interfacial strength [139, 140]. In order to match the compatibility between the components, liquid rubbers having various types of reactive groups have been used such as butadi butadiene-acrylonitrile copolymers containing carboxyl (CTBN) or amine (ATBN). The end groups play a vital role in tuning the behavior and at the same time generate interfacial strength of the epoxy matrix. Although liquid rubber is promising for toughening the thermoset matrix, due to the mismatch of the modulus, the tensile modulus of the matrix gets hampered. The organophilic layered silicates, which are very promising reinforcements, may be coupled with liquid rubber to simultaneously enhance the modulus and toughness of the nanocomposites. Fröhlich et al. used stearate-functional liquid trihydroxy-terminated poly(propylene oxide-block-ethylene oxide) (PPO) to accomplish compatibility with organophilic fluorohectorite, modified by means of intergallery cation exchange with bis(2-hydroxyethyl)methyldodecylammonium and fabricated epoxy nanocomposites . The PPO not only established compatibility with nanofiller but also with epoxy matrix and induced phase separation. The stearate-functional PPO was produced by transesterification of PPO end groups with methyl stearate in order to vary the compatibility between liquid rubber and nanofiller as well as between liquid rubber and epoxy resin, which varied by transesterification of an average of 20% of PPO end groups with methyl stearate, thus producing a stearate-functional PPO. The authors studied the role of PPO compatibility with nanofiller to accomplish concurrent distribution of rubber particles and nanofiller phases. About 100% improvement in KIc was recorded for the nanocomposite containing unmodified PPO along with organohectorite. Further, the KIc value was improved by ~400% for the nanocomposite while using PPO-stearate in combination with high organohectorite hybrid filler. However, the toughness was found to be enhanced at the expense of stiffness, strength, and glass temperature. The results suggested that the higher content of organosilicate with stearatemodified PPO could be the ideal combination to achieve toughness/ stiffness balance. Figure 8.15 shows DMA results for the properties of the epoxy hybrid nanocomposites with additive loadings of 15 wt% in comparison to the neat epoxy resin.
408 Hybrid Nanomaterials 1010 1.2
–80 –60 –40 Temperature ( C)
Loss factor tan
neat epoxy resin ER-67PPO/15 ER-67PPO-stearate/15 ER-67PPO+adduct/15 Loss factor tan
Fluctural modulus (Pa)
Temperature ( C)
Figure 8.15 DMA diagrams of various epoxy hybrid nanocomposites with additive loadings of 15 wt% in comparison to the neat epoxy resin. The inset shows the loss factor between −100 and −20 °C in greater detail (Reprinted with permission from ; Copyright © 2003 American Chemical Society).
Summary and Future Prospect
The growing interest in hybrid fillers can be seen as an effort to reduce the content of expensive fillers in nanocomposites as much as possible, which benefits the development of low-cost products. The contribution of desirable properties from individual components may result in improved properties due to the synergistic effect. In some cases, imparting extra features to the materials through the synergy of more than one component is unachievable using a single filler. The field of polymer nanocomposites has become one of the most vibrant areas of research with the advent of carbon nanofillers such as CNTs, GNPs, graphene, etc. These nanofillers reinforce the physicochemical properties of the polymer matrix for application in diverse areas. The combinations of two or more nanometer-sized carbon fillers along with inorganic mineral fillers have been widely used to fabricate nanocomposites. These nanocomposites exhibit improved thermal, electrical and mechanical properties for use in automotive, aerospace, electronics, and other sectors. The composites of carbon nanofiller/ceramic hybrid could be potential materials for thermal management system. The mechanical properties of the best performing hybrid/epoxy nanocomposites are presented in Table 8.1 and the electrical and thermal conductivity values are shown in Table 8.2.
CNT (0.2%), CSCF (1%)
SWNT (0.05%), clay (2%)
4.0 wt %
Tensile strength (MPa)
Young’s modulus (GPa)
Flexural modulus (GPa)
Table 8.1 The mechanical properties of the hybrid filler epoxy nanocomposites.
Storage modulus (GPa)
High Performance Hybrid Filler 409
BN (0.5) MWCNT (0.3)
10 wt %
MWCNTs (0.18) silica (6.0)
MWNTs (0.4 %) ZrP (2.0%)
Table 8.1 Cont.
Tensile strength (MPa)
Young’s modulus (GPa) Flexural modulus (GPa)
Storage modulus (GPa)
410 Hybrid Nanomaterials
GNPs (7.5%) SWNTs (2.5%)
CNTs (20 vol%) GNPs (20 vol%)
SWNT (0.05%)/clay (2 wt%)
GNP (2 vol%) AF (50 vol%)
AlN (25 vol%) MWCNTs (1 vol%)
Alumina fibers -GNPs
1.6 × 10
Thermal conductivity in Wm–1K–1
2.75 × 10–5
Electrical conductivity in Sm–1
Table 8.2 The electrical and thermal conductivity of the hybrid filler epoxy nanocomposites.
High Performance Hybrid Filler 411
BN (30 vol%) MWCNTs (1 vol%)
8 wt %
Table 8.2 Cont.
1.63 × 10–1
Electrical conductivity in Sm–1
Thermal conductivity in Wm–1K–1
412 Hybrid Nanomaterials
High Performance Hybrid Filler 413 The final properties of the nanocomposites are the function of factors such as the intrinsic properties of filler and matrix, interfacial interaction/ adhesion between the two components and the area of the contact between the two. Selection of proper dispersion technique and the meticulous tuning of the surface of filler to improve the interaction is very crucial to achieve improvement in properties. Thus, physical and other functional properties of the nanocomposites strongly depend upon the microstructure of the nanocomposites, so utmost care should be taken to disperse the filler at molecular level and design of the interface. In order to improve the thermal interface resistance, more effort needs to be spent on engineering the chemical functionalities of the components. The rapid development of polymer nanocomposites in the processing technologies can be seen as a positive move towards meticulously controlling the morphology of the materials and engineering of the interface between the components, and it may offer a plethora of possibilities for practical application.
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9 Recent Developments in Elastomer/ Hybrid Filler Nanocomposites Suneel Kumar Srivastava1* and Vikas Mittal2 1
Department of Chemistry, Indian Institute of Technology, Kharagpur, India 2 Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab Emirates
Abstract In the last decades, a large number of investigations have been reported on the synthesis, properties and applications of various nanomaterials as fillers in polymer nanocomposites. In view of this, considerable amount of work has been reported in literature on development of inorganic materials filled elastomer nanocomposites. The work described in this chapter is mainly focused on the preparation of these hybrid-filled elastomer nanocomposites, along with their mechanical, thermal, electrical, dielectric and shape memory properties. Keywords: Elastomer, hybrid nanomaterials, blends, nanocomposites mechanical properties, TGA/DSC, conductivity, dielectric, shape memory
Polymer nanocomposites have been receiving considerable attention due to their enhanced mechanical, electrical, thermal, flame retardant and other functional properties compared to the neat polymer [1, 2]. Such improvements in properties strongly depend on the nature and properties of filler, dimension, aspect ratio, dispersion of the filler and interfacial interaction between matrix and the filler. These nanocomposites display new and improved mechanical, catalytic, electronic, magnetic, and optical properties not exhibited by the individual phases or by their *Corresponding author: [email protected] Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (423–490) © 2017 Scrivener Publishing LLC
424 Hybrid Nanomaterials macrocomposite and microcomposite counterparts. Since then, the majority of research has been focused on improving various properties of the polymer nanocomposites for their numerous potential applications as high performance materials in the automotive, aerospace, construction, and electronic industries fields and in many facets of day-to-day life. Elastomers are a very important class of polymer materials due to their extensive commercial as well as domestic applications [3–6]. Polyurethane (PU) is one of the most versatile block copolymers, exhibiting outstanding elastomeric properties controlled by the molecular chain structures of hard and soft segments . It finds multifaceted commercial applications as synthetic leathers, membranes, thermoplastic elastomers, rubbers, foams, adhesives and coatings, and in the biomedical field. PU exhibits high elasticity, good tear strength, biocompatibility, shock absorption, transparency and excellent abrasion resistance. Despite several advantages, poor thermal stability, barrier property and high combustibility of PU remain its limitations in various applications. Polysiloxanes, usually referred to as silicone, remain the other most important synthetic functional elastomers due to their unique properties, such as excellent weather resistance, good chemical stability, oxidation resistance, thermal stability, low-temperature toughness, electrical insulation, low surface energy, low toxicity and high optical transparency . These properties are attributed to the presence of inert backbone consisting of alternating silicone and oxygen atoms. Although, dimethylsiloxane unit is the basis of silicone polymers, the methyl groups in this unit could be substituted with other groups to achieve other desirable properties. The silicones, such as silicone oil, silicone grease, liquid silicone rubber, silicone gum and silicone resin, are widely used as lubricants, adhesives and sealants in the automotive and aerospace industry, cables for appliances and telecommunications, cooking, baking and food storage products, medical implants and in electrical insulation products, etc. However, despite having several advantages, the vulcanized neat silicone rubber (SR) usually has poor mechanical properties and low thermal/ electrical conductivity, restricting its use in many industrial applications. Natural rubber (NR) is also an important unsaturated elastomer and a polymer of isoprene. NR possesses many numerous commendable properties, including high strength, high tear resistance, low heat build-up, high resilience, retention of strength at elevated temperature, excellent dynamic properties and general fatigue resistance . Because of its elasticity, resilience and toughness, NR is the basic constituent of many products used in the transportation (e.g., tires), industrial (sealants), consumer (sports materials), hygienic and medical sectors. However, NR, unlike many other polymers, is highly susceptible to many forms of degradation: light, ozone,
Elastomer/Hybrid Filler Nanocomposites
radiation, humidity and heat, due to the presence of double bonds in the main chain. In addition, its inherent high flammability restricts its usage in many otherwise critical products. Styrene-butadiene rubber (SBR) exhibits better processability, heat aging and abrasion resistance but is inferior in terms of elongation, hot tear strength, hysteresis, resilience and tensile strength . Nitrile rubber (NBR) is widely used due to its great oil resistance, heat resistance in plasticizer, low gas permeability and high shear strength . Ethylene propylene diene monomer (M-class) rubber (EPDM) is a synthetic rubber with a wide range of applications in cold room doors for sealing purpose, industrial respirators, automotive paint spray environments, cable insulation, solar pool panels, and as a covering for water proofing, etc. . These elastomer nanocomposites exhibited improved properties for many multifaceted applications in same or different fields . According to the available literature, a large number of one-dimensional (1D) materials, such as carbon nanotubes (CNTs), carbon nanofiber, etc., and two-dimensional (2D) materials, such as natural layered silicate (e.g., montmorillonite), synthetic clay, e.g., layered double hydroxides, (LDHs), graphite (EG), graphite nanoplatelet (GNP), graphene, etc., have been used as reinforcing fillers in polymer [8–12]. However, these fillers need to be modified prior to the fabrication of nanocomposites due to their agglomeration in various solvents as well as in polymer matrix. Therefore, it remains a major challenge to find other ways to enhance the dispersion of these fillers in polymer matrix/solvent while preserving their intrinsic properties. In the case of CNT, CNF and graphene, they are subjected to covalent and noncovalent functionalizations [12–15], whereas montmorillonite (MMT) and layered double hydroxide (LDH) are organomodified [8, 9]. Such inorganic filler reinforced polymer nanocomposites exhibit enhanced mechanical, electrical, thermal, flame retardant and other functional properties compared to the neat polymer. The investigations revealed that these property improvements are strongly dependent on the dimension, aspect ratio and dispersion of the filler and its interfacial interaction with polymer matrix. A hybrid material has been defined as a “material composed of an intimate mixture of inorganic components, organic components, or both types of components” . The hybrid materials, especially those that are 3D, have been receiving continuous attention . These hybrids are prepared by hybridizing 1D (MWCNTs, CNF, etc.) and 2D (molybdenum disulfide, titanium disulfide, tungsten disulfide, Na-montmorillonite, layered double hydroxide, graphene, etc.) materials or by combining 0D/1D, 1D/3D and 2D/2D materials [17–91]. Zhang and Liu 
426 Hybrid Nanomaterials recently reviewed hybridization modification of graphene and its polymer nanocomposites. They concluded that the presence of interactions (such as van der Waals interaction, - stacking, etc.) between individual fillers is desirable to facilitate efficient load transfer in the polymer matrix. The hybrid materials inherit the merits of individual components manifesting from the synergistic effect. As a result, these hybrid nanomaterials have been receiving a considerable amount of attention in high performance supercapacitor nanocomposites [21–24, 46, 75, 85, 88], photodegradation , water purification [27, 55, 66, 87], catalyst/ electrocatalyst [31, 32, 58, 59, 67], transport conductor [37, 43], shape memory , antibacterial [54, 61], opto electronics , flame retardancy , Li-ion batteries [73, 76–79], sensors [74, 90, 91], photoanode , solar cells [89, 90], etc. Further, the literature has suggested that hybrid materials, especially those that are 3D, act as effective reinforcing fillers in the development of high performance polymer nanocomposites [46, 52–54, 61, 92–154]. These hybrid-filled polymer nanocomposites very often display enhanced properties not exhibited by the individual phases or by their macrocomposite and microcomposite counterparts. This is ascribed to the fact that hybrid filler overcomes the dispersion problem faced by individual components. As a result, these 3D hybridfilled polymers are likely to find better applications as high performance composite materials in the automotive, aerospace, construction and electronic industries and in many facet of day-to-day life. In view of this, the present chapter reviews the preparation of various hybrid fillers including 3D fillers and their application in development of elastomer and elastomeric nanocomposites, such as PU [46, 52–54, 61, 92–110], SR [111–123], NR [124–136], SBR [137–144], NBR [145–148], EPDM [148– 155], EVA , etc. Establishment of dispersion/nanostructures, mechanical and thermal properties of elastomer nanocomposites are also reviewed. Additionally, attempts have also been made to review these features for the elastomers filled with hybrids other than 3D fillers.
Preparation Methods of Elastomer Nanocomposites In-situ Polymerization
A calculated amount of filler is swollen within the liquid monomer or prepolymer in a suitable solvent and subjected to the polymerization in the presence of fillers.
Elastomer/Hybrid Filler Nanocomposites
In this method, nanofiller is dispersed in polymer (already dissolved in some suitable solvent) and subjected to thorough mixing by means of mechanical, magnetic stirrer or ultrasonication, followed by casting and solvent evaporation. It remains one of the preferred methods utilized more often in the laboratory for the synthesis of polymer nanocomposite due to precise control over homogeneous dispersion
Melt Intercalation Method
Melt blending method is an environmentally benign process compatible with current industrial processes. The nanofillers are mixed with molten PU in an extruder or injection molder in the absence of any organic solvent.
Hybrid Fillers in Elastomer Nanocomposites
The different types of hybrid fillers have been used in formation of various elastomers and their blend nanocomposites. Table 9.1 provides the details of individual components in 3D and other hybrids in fabrication of nanocomposites of PU, SR, NR, SBR, NBR, EPDM, etc. Further, dispersion of hybrid fillers in elastomer nanocomposites has been investigated by many workers as described below.
Dispersion of Hybrid Fillers in Elastomer Nanocomposites
Dispersion of the filler and its interfacial interactions with polymer accounts for the fabrication of high performance polymer nanocomposites. In this regard, TEM images provide direct information on the dispersion and establishment of nanostructure in hybrid-filled elastomer nanocomposites.
Dispersion of Hybrid Fillers in PU Nanocomposites
The TEM image of TPU/MWCNT-graphene nanocomposite revealed that graphene nanosheets were adhered to the MWCNT bundles, indicating interaction between MWCNTs and graphene sheets . Multiwalled carbon nanotubes (TWNT)/RGO hybrid exhibited good dispersion as well as interconnectivity between TWNT and RGO in HBPU/TWNT/RGO composites (7/3 TWNT/RGO composition with
428 Hybrid Nanomaterials Table 9.1 Details of 3D and other hybrid fillers used in preparation of elastomer nanocomposites. Elastomer
R-GNR/CNT hybrid 
Graphene/Nano SiO2 
Graphene NP/graphene oxide 
Besalt fiber-epoxy 
CNT/Ag nanoparticles 
TWNT/RGO composites 
Ag nanowires/silica nanoparticles 
CNT/carbon black 
Elastomer/Hybrid Filler Nanocomposites
Table 9.1 Cont. Elastomer
Precipitated silica/montmorillonite 
Expanded graphite/conductive carbon/polyaniline 
Si3N4/SiC whisker 
Kaolinite/carbon black 
Carbon black/nanoclay 
Two roll mill
Coal gangue/carbon black 
Poly(ethylene terephthalate)/halloysite nanotubes 
Roll mill mixing
Waste tire dust/carbon black 
Carbon black/nanoclay 
Roll mill mixing
Silica/carbon black 
Internal milling/roll milling
Carbon black/nanoclay 
Nanoclay-carbon black 
Compounding done in a mixer
Marble waste/rice husk derived silica 
Bagasse ash silica/precipitated silica 
Carbon black-graphene 
Roll mill mixing
Silica hybrid fillers 
CNT/thermally reduced graphene 
Tannic acid functionalized graphene/ tubular halloysite nanotubes 
Hybrid colloid co-coagulated with SBR emulsion (Continued)
430 Hybrid Nanomaterials Table 9.1 Cont. Elastomer
Silica hybrids 
Halloysite nanotubes- phthalocyanine 
Carbon black/graphene 
Roll mill mixing
Roll mill mixing
CNT-thermally Reduced Graphene 
Roll mixing method
Cloisite 30B/ nano-CaCO3 
Modified Clay–Carbon Black 
Clay/carbon black NBR 
Carbon Black/Silica 
Roll mill mixing
Silica/Rice husk 
Carbon black/graphite nanoplatelets 
Roll mill mixing
Carbon black/clay 
Roll mill mixing
Roll mill mixing
Expanded graphite/carbon black 
Roll mill mixing
Dodecylbenzenesulfonate-doped polyaniline/organoclay 
1 wt% nanocarbon content) as evident from TEM . Lee et al.  investigated the effect of MWCNT (3 wt%) distribution with the coaddition of 3 wt% of cloisite C30B and C25A on the morphology of melt-blended TPU (polyether and polyester based) nanocomposites and the corresponding TEM images are shown in Figures 9.1 and 9.2
Elastomer/Hybrid Filler Nanocomposites
1 m (a)
1 m (b)
C25A 1 m (c)
Figure 9.1 TEM images of ester-TPU nanocomposites containing 3 wt% MWCNT and 3 wt% of (a) none, (b) C30B, and (c) C25A. (Reproduced from  with permission from Wiley).
respectively. This study indicated that coaddition of C30B improved the dispersion of MWCNT in TPU. In contrast, some kind of network structure was observed to have been formed by MWCNT in the presence of C25A in TPU, indicating high affinity of MWCNT for C25A. Roy et al.  recorded TEM images of TPU nanocomposites containing 0.50 and 1 wt% of the MMT–MWCNT (1:1) filler loadings. It can be seen from Figure 9.3 that the MWCNT nanotubes and MMT nanoplatelets exist together and are homogeneously distributed in 0.50 wt% MMT–MWCNT (1:1) hybrid-filled nanocomposite due to the electrostatic attraction between MWCNT and MMT nanoplatelets. When MMT–MWCNT (1:1) loading is 1 wt% in TPU, the aggregation of filler is clearly evident from the corresponding TEM image. In another work on TPU/MWCNT/CRGO (chemically reduced graphite oxide) , the TEM images in Figure 9.4 clearly show the fine dispersion of 0.50 wt% of hybrid filler in TPU matrix. This is ascribed to the electrostatic attraction between MWCNT and CRGO. However, hybrid filler is aggregated at 1.0 wt% filler loading in TPU matrix, as evident from the corresponding
432 Hybrid Nanomaterials
1 m (c)
Figure 9.2 TEM images of ether-TPU nanocomposites containing 3 wt% MWCNT and 3 wt% of (a) none, (b) C30B, and (c) C25A. (Reproduced from  with permission from Wiley).
100 nm (a)
100 nm (b)
Figure 9.3 TEM images of TPU nanocomposites containing (a) 0.50 and (b) 1 wt% MMT-MWCNT (1:1) hybrid. (Reproduced from  with permission from Wiley).
TEM image. Also, these images clearly show the presence of interconnected MWCNT–CRGO (1:1) network throughout the TPU matrix, which could also be reflected in the properties of TPU. The TEM image of TPU/1 wt% sulfonated reduced graphene oxide (SRGO)/sulfonated
Elastomer/Hybrid Filler Nanocomposites
100 nm (a)
100 nm (b)
9.4 TEM images of TPU nanocomposites containing (a) 0.50 and (b) 1 wt% MWCNT-CRGO (1:1) hybrid. (Reproduced from  with permission from Wiley).
0.5 m (a)
0.5 m (b)
9.5 TEM images of TPU/NBR nanocomposites containing (a) 0.50 and (b) 1 wt% SFCNT-LDH hybrid. (Reproduced from  with permission from Wiley).
carbon nanotube (SCNT) suggested the formation of 3D interconnected network of SRGO/SCNT in TPU, where 1D nanotubes acted as a bridge and connected 2D nanosheets . Very limited morphological studies have also been done on blend nanocomposites of thermoplastic polyurethane [96–98]. An efficient approach has been applied to assemble MgAl layered double hydroxide onto pristine carbon nanotubes using sodium dodecylsulfate in developing TPU/ NBR nanocomposites . Figure 9.5 shows HRTEM images of TPU/NBR filled with 0.50 and 1 wt% of surfactant modified SFCNT (modified CNT)/ LDH MgAl-LDH which clearly showed the presence of an interconnected SFCNT–LDH network throughout the TPU/NBR matrix. Roy et al.  also observed homogeneous dispersion of CNF–LDH (MgAl-LDH and ZnAl-LDH) in TPU/NBR blends.
434 Hybrid Nanomaterials The establishment of nanosctructure and dispersion of hybrids other than 3D hybrids in PU has also been investigated by many researchers. Chen et al.  observed the homogeneous distribution of TiO2/RGO (1:1) hybrid filler from SEM images of the fracture surface of TPU/TiO2/RGO nanocomposites. Hybrids of NiAl LDH-ZnO (1.5 wt%)  and ZnAl LDH-ZnO  exhibited homogeneous dispersion. TEM images of PU/ Ag/TiO2/G (3.57 wt%) depicted the presence of some unevenly dispersed nanoparticles on the graphene nanosheets with a little aggregation spread on the graphene scaffold . The SEM analysis of HBPU/MWCNT–Fe3O4 showed that nanohybrids were dispersed in a HBPU matrix . Gaddam et al.  synthesized carbon–silica nanoparticle hybrid (CSH) hybrid and observed its uniform dispersion in PU matrix. In another work, the addition of poly(tetramethylene ether) glycol prevented the restacking of GNPs and facilitated the dispersion of the hybrid in PU matrix . Wei et al.  noticed that silver nanowires (AgNW) were debundled in the presence of nano-SiO2 in WPU/AgNW/nano-SiO2 nanocomposites. The fractured surface SEM images of WPU/Ag/halloysite (2 wt%) nanocomposites showed uniform dispersion of hybrid filler in polymer matrix, indicating strong interfacial interaction between WPU and hybrid filler . The TEM images of electrospun PU–MWCNT–AgNP nanofibers in Figure 9.6 indicated that Ag clusters tended to adhere to the surface of MWCNTs that combined the discrete Ag clusters together .
Dispersion of Hybrid Fillers in SR Nanocomposites
Although, formation of silicone rubber (SR) nanocomposites of hybrid fillers have been reported [111–123], very limited investigations have dealt with the study of their dispersion and morphology. In an early work, Hu et al.  reported a novel, scalable and inexpensive approach to fully disperse carbon nanotubes in silicone rubber. The corresponding SEM and TEM images in Figure 9.7a–d show the presence of well-dispersed graphene and MWNTs that form conductive network in the silicone rubber matrix. Recently, Pradhan et al. [113–116] used 3D hybrids in a 1:1 wt ratio and successfully dispersed them in a SR matrix in the presence of THF as solvent. The TEM image of VMQ nanocomposite filled with 0.375 and 0.75 wt% MWCNT–graphene in Figure 9.8 clearly shows better dispersion and homogeneity of MWCNT–graphene in the VMQ matrix. . Figure 9.9 shows MWCNTs to be dispersed as individual nanotubes at 1.0 wt% loading of MWCNT/MMT hybrid in SR . The possibility of enhanced dispersion of the hybrid filler in SR could be ascribed to
Elastomer/Hybrid Filler Nanocomposites
Cu 400 nm
MWCNT MWCNT 200 nm
Figure 9.6 TEM images of (a) PU–AgNP, (b) PU–MWCNT, and (c,d) PU–MWCNT– AgNP hybrid nanofibers. The inset in (a) is the EDX spectrum of PU–AgNP nanofibers, and the inset in (d) is the magnified image of the selected area. (Reproduced from  with permission from Royal Society of Chemistry).
the hydrogen bonding between -OH group of MMT (-OH) and SR (-OH). They also observed homogeneous distribution of 1 wt% loaded Mg– Al-LDH/MWCNT SR matrix . Witt et al.  also observed good dispersion of 2.5 phr carbon black (CB)/1phr CNT hybrid in SR due to the formation of filler network of the hybrid filler. SR/MMT/precipitated silica/montmorillonite hybrid was not found to be exfoliated .
Dispersion of Hybrid Fillers in NR Nanocomposites
Several studies have established the extent of dispersion of hybrid fillers in NR. Li et al.  developed a modified latex mixing method to disperse graphene (GE)/CNT hybrid in natural rubber (NR). The TEM images in Figure 9.10A show homogeneous interdispersion of graphene and CNTs in NR matrix. They concluded that GE nanosheets and CNTs interpenetrate and make contact with each other and result in a more compact hybrid filler network, which distinguishes the nanocomposite with the hybrid
436 Hybrid Nanomaterials
1 m (a)
50 nm (d)
MWNT TrG MVQ MVQTrG(1)/MWNT(2.5) MVQ/MWNT(5) 10
2 theta(˚) (e)
Figure 9.7 Silicone rubber/graphene/MWNTs ¼ 100/1/2.5: (a,b) SEM images, (c) TEM image, the insert is the enlargement of carbon nanotube in the white rectangle, (d) TEM image, (e) TEM image of silicone rubber/graphene/MWNTs ¼ 100/1/3 and (f) XRD profiles of MWNT, TrG, MVQ and MVQ composites. (The arrows point to the MWNTs and graphene is in white circles). (Reproduced from  with permission from Elsevier).
filler from the nanocomposites with the single-component fillers. Modified kaolinite (10 phr)/CB (40 phr) hybrid filler produced a disorderedly dispersion of kaolinite sheets covered by fine carbon black particles in rubber matrix as evident from Figure 9.10B . NR filled with 21 phr carbon black (N330) and 4 phr of Cloisite (C15A) showed CB aggregates that
Elastomer/Hybrid Filler Nanocomposites
100 nm (d)
100 nm (c)
100 nm (e)
Figure 9.8 TEM images of (a) MWCNT(0.375 wt%)/VMQ, (b) G(0.375 wt%)/ VMQ, (c) MWCNT-G (0.375 wt%)/VMQ, (d) MWCNT-G(0.75 wt%)/VMQ and (e) MWCNT-G(1.5 wt%)/VMQ. (Reproduced from  with permission from Wiley).
were well dispersed with exfoliated clay layers (distributed in the unoccupied space of CB) . In many cases, SEM has been used to identify the dispersion of hybrid fillers in NR. Chen et al.  found that coal gangue/ carbon black exhibited good dispersion in NR. Morphological study of the tensile fracture surfaces of the hybrid-filled composites showed that HNTs have better adhesion and are well-dispersed in NR matrix as compared to recycled poly(ethylene terephthalate) (R-PET) particles . In another work, waste tire dust (WTD)/CB  and CB/organomodified Na-MMT (OMMT) and CB/organomodified laponite LRD (ORD)  hybrid fillers exhibited improved filler dispersion in natural rubber matrix. The SEM photomicrographs of fracture surface specimens filled with silica/CB (20/30 and 30/20 phr) showed a similar extent of dispersion in NR vulcanizates and greater than NR vulcanizates containing silica/CB in the ratio of 0/50, 10/40, 40/10 and 50/0 .
Dispersion of Hybrid Fillers in SBR, NBR, EPDM and EVA Nanocomposites
The TEM images of SBR nanocomposites in Figure 9.11 show greatly improved dispersion of HNTs in the presence of tannic acid functionalized
438 Hybrid Nanomaterials
100 nm (a)
100 nm (c)
100 nm (b)
100 nm (d)
Figure 9.9 TEM image of (a) SR/0.5 wt% MMT, (b) SR/0.5 wt% MWCNT, (c) SR/1 wt% MMT/MWCNT (1:1), and (d) SR/2 wt% MMT/MWCNT (1:1) nanocomposites. (Reproduced from  with permission from Wiley).
graphene (TAG) . In addition, TAG sheets were uniformly dispersed around HNTs to form a co-supporting and continuous hybrid network where the HNTs are separated by the silk-like TAG. Rybiński and coworkers  recorded AFM images of NBR and SBR vulcanizates containing halloysite-zinc phthalocyanine (HZ) or halloysite-chloroaluminophthalocyanine (HC) hybrid fillers, as displayed in Figure 9.11. These are characterized by a better dispersion of components, and the dispersed particles of hybrid fillers are decisively smaller than those in the vulcanizates containing halloysite or the physical mixture of halloysite and the pigment . Carbon black-reduced graphene (CB-RG) in SBR matrix observed by TEM indicated homogeneous dispersion of graphene sheets . The TEM image of SBR composite filled with 40 phr MK + 10 phr PS showed kaolinite sheets covered by the fine silica particles . The NBR composites prepared by taking different organoclay and nano-CaCO3 content confirmed homogeneity by studying the dispersion
Elastomer/Hybrid Filler Nanocomposites
200 nm (A)
500 nm (a)
500 nm (b)
Figure 9.10 TEM image of (A) NR/0.5GE/5CNTs (Modified image). (Reproduced from  with permission from Royal Society of Chemistry); and (B) NR composite filled with fillers: (a) NR-2, 50 phr MK and (b) NR-3, 10 phr MK + 40 phr CB. (Reproduced from  with permission from Elsevier).
of nanoparticles by FESEM . The TEM measurements revealed that the use of calcium stearate facilitated dispersion of nanoclay in the NBR . The morphological observations revealed that the replacement of CB with GNPs reduces the CB aggregation in EPDM matrix . SEM study showed that halloysite nanotubes (HNTs) have better adhesion to EPDM matrix than palm ash . The dispersion of the EPDM-based nanocomposites was investigated by high-resolution transmission electron microscopic analyses . Morphology studies by scanning electron microscopy of cryofractured surfaces indicated that the conducting nanocomposites produced heterogeneously distributed aggregates in the continuous elastomeric matrix . The graphene/boehmite/EVA nanocomposite showed a more homogeneous dispersion than those of composites containing only a single nanomaterial .
440 Hybrid Nanomaterials
(b) SBR-H composites
SBR-HG composites HNTs TAG SBR chain
200 nm (c)
Figure 9.11 TEM images of (a) SBR-H40, (b) SBR-G4, (c) SBR-HG44 and (d) a schematic for the dispersion status of fillers in the composites. (Reproduced from  with permission from Royal Society of Chemistry).
Mechanical Properties of Hybrid Filler Incorporated Elastomer Nanocomposites Mechanical Properties of Hybrid Filler Incorporated PU Nanocomposites
The mechanical properties of PU nanocomposites completely depend on the nature of the filler, dispersion of filler into the matrix, molecular weight of the matrix and the interaction between them . In view of this, considerable work has been reported in recent years on mechanical property improvements in hybrid filler reinforced PU, electrospun PU, PU fiber, TPU HBPU, and WPU. The variation of mechanical properties of TPU composites as a function of reduced graphene nanoribbon (R-GNR)/CNT hybrid are shown in Table 9.2 . It is evident that the incorporation of 1 wt% of R-GNR/CNT hybrid in TPU matrix improved tensile strength (184%) and Young’s modulus (81%) of neat TPU. Such improvements in mechanical properties were attributed to the fine dispersion of the filler in the matrix along with good filler-polymer interaction. The elongation at
Elastomer/Hybrid Filler Nanocomposites
Table 9.2 Summary of mechanical properties of neat TPU and its composites. (Reproduced from  with permission from Elsevier). Samples
Tensile strength (MPa)
Elongation at break (%)
TPU/(R-GNR/CNT) (0.2 wt%)
TPU/(R-GNR/CNT) (0.5 wt%)
TPU/(R-GNR/CNT) (1.0 wt%)
TPU/(R-GNR/CNT) (1.5 wt%)
TPU/(R-GNR/CNT) (2.0 wt%)
break (EB) of neat TPU (545%) achieved a maximum (795%) in 0.2 wt% hybrid-loaded TPU due to strong interaction between hybrid and TPU. At higher loadings, TPU nanocomposites exhibited inferior mechanical properties. Toughness of TPU (68 Jg–1) also increased to 247 Jg–1 at 1 wt% R-GNR/CNT loading. Such enhancement in toughness was ascribed to the crosslinked structure formed by the interfacial interaction between R-GNR/CNT and TPU. Such enhancement in toughening may be related to the unique 3D structure of the assembled R-GNR/CNT. Lee et al.  investigated the effect of 3 or 6 wt% of C30B (and C25A) on the mechanical properties of TPU/MWCNT nanocomposites. They observed that tensile modulus of TPU/MWCNT nanocomposites increased significantly in the presence of C30B rather than C25A. This was attributed to the good dispersion and interaction of hybrid filler (MWCNT in the presence of C30 B) with TPU matrix. The mechanical property of HBPU has been investigated in the presence of TWNT/RGO hybrid filler . All the composites showed increased modulus in composites compared to the 70.2 MPa of pure HBPU film. Further, they noted modulus being dependent on the composition ratio of TWNT/RGO in the HBPU composites. TWNT/RGO (3/7 ratio) and TWNT/RGO (7/3 ratio) hybrids in HBPU attained moduli of 95.7 and 104.9 MPa respectively (at 1 wt% nanocarbon content). Such observations were attributed to the optimum nanocarbon composition ratio required to achieve the best mechanical results of the composites. Roy et al.  studied the effect of MMT-MWCNT (1:1) loadings on the tensile strength, elongation at break (EB) and Young’s modulus of TPU. Figure 9.12a shows that tensile strength of TPU is improved by 44, 57, 38,
442 Hybrid Nanomaterials and 36% at 0.25, 0.50, 0.75, and 1 wt% MMT-MWCNT loading respectively. This is ascribed to the homogeneous effect of chain slippage and the platelet orientation of MMT and the flexibility of MWCNT. It is also evident from Figure 9.12a that maximum improvement of EB (780%) is achieved at the lowest loading of 0.25 wt% MMT–MWCNT compared with 633% for neat TPU. This is likely due to the resistance exerted by the sterically hindered MMT-MWCNT (1:1) hybrid surface itself and strong polymer filler interaction. Figure 9.12b shows that the Young’s modulus in 0.50 wt%
Elongation at break (%)
Tensile strength (MPa)
Tensile strength Elongation at break
32 0.0 0.2 0.4 0.6 0.8 1.0 MMT-MWCNT hybrid (1:1) content (wt%) (a)
Young’s modulus (MPa)
24 20 16
12 0.0 0.2 0.4 0.6 0.8 1.0 MMT-MWCNT hybrid (1:1) content (wt%) (b)
Figure 9.12 Variation of (a) tensile strength, elongation at break and (b) Young’s modulus of TPU nanocomposites depending on MMT-MWCNT (1:1) hybrid content. (Reproduced from  with permission from Wiley).
Elastomer/Hybrid Filler Nanocomposites
MMT-MWCNT hybrid-filled TPU nanocomposite is maximum improvement (87.5%) compared to neat TPU. Roy and coworkers  also extended their work on the mechanical properties of MWCNT-CRGO hybrid reinforced TPU nanocomposites, as shown in Figure 9.13. The TS values of TPU filled with 0, 0.25, 0.50, 0.75, and 1 wt% MWCNT-CRGO (chemically reduced graohite oxide) loading are 38.3, 51.5, 60.8, 57.1, and 45.3 MPa, respectively. The corresponding values of EB are found to be 633, 786, 923, 773, and 720%. The Young’s modulus of TPU/MWCNT-CRGO (1:1)
Tensile strength Elongation at break
Tensile strength (MPa)
Elongation at break (%)
0.2 0.4 0.6 0.8 1.0 MWCNT-CRGO hybrid content (wt%)
Young’s modulus (MPa)
40 35 30 25 20 15 0.0
MWCNT-CRGO hybrid content (wt%) (b)
Figure 9.13 Variation of (a) tensile strength, elongation at break and (b) Young’s modulus of TPU nanocomposites with MWCNT-CRGO (1:1) hybrid content. (Reproduced from  with permission from Wiley).
444 Hybrid Nanomaterials nanocomposites is also found to be superior compared to neat TPU. All these findings clearly indicated the superior mechanical properties of TPU nanocomposites. This is ascribed to the homogeneous distribution of hybrid nanofiller throughout the TPU matrix and strong interfacial interaction between the nanofiller and TPU matrix. The role of the synergistic effect of 1D MWCNT and 2D CRGO on the mechanical properties of TPU has also been established. Their studies also confirmed the synergistic effect of CNT/ MMT  and CRGO/MWCNT  hybrids in reinforcing the mechanical properties of TPU nanocomposites. Blending of TPU with polymer, especially NBR, has received considerable attention [148–150]. The TPU component in the blends accounts for the improvement in tensile strength and weather, oxygen, ozone and fuel/oil resistance, while NBR enhances the thermal stability and solvent resistance of the TPU in the blend. It was noticed that TPU/NBR blend forms co-continuous morphology in a 50:50 (wt%) TPU/NBR blend. These blends find applications in tubing pipes, gaskets, protective covers, co-extrusion automotive gaskets and grips, ball pen grips, etc. Earlier work has been reported on the formation of carbon black-  and LDH-filled  TPU/NBR nanocomposites. Therefore, nanocomposites incorporating hybrid filler have been reported for their superior properties and applications of these blends in the above fields, including new ones [96–99]. In one such method, the effect of surfactant modified CNT (SFCNT)–Mg-Al LDH (LDH) loadings on tensile strength and elongation at break of TPU/ NBR were studied . The tensile strength of TPU/NBR filled with 0, 0.25, 0.50, 0.75 and 1 wt% SFCNT–LDH hybrid was found to be 5.09, 11.7, 13.8, 10.6 and 9.78 MPa, respectively. The corresponding values of elongation at break are 293, 530, 513, 436 and 413%. These findings suggest that the maximum enhancement in tensile strength (171%) and elongation at break (1.8 times) in TPU/NBR compared to pure TPU/NBR is due to the optimum dispersion of hybrid filler and its enhanced interaction with matrix. At higher filler loadings, the tensile strength and elongation at break slightly decrease due to the agglomeration of filler. The improvement in tensile strength of TPU/NBR was even superior to that reported earlier in TPU/NBR blend reinforced by CB  and LDH . Roy et al.  also studied the mechanical properties of ZnAl-LDH (LDH)–SFCNT and Mg-Al-LDH (LDH)–SFCNF reinforced TPU/NBR blend. It is noted that TPU/NBR/SFCNT–LDH (0.50 wt%) and TPU/ NBR/SFCNF–LDH (0.50 wt%) blend nanocmposites exhibited 126 and 122% improvement in tensile strength (and 1.50 and 1.43 times improvement in EB) compared to neat TNP/NBR respectively. Most likely, optimum homogeneous dispersion of hybrid fillers in the TNP/NBR matrix lead to strong interfacial interaction between hybrid filler and polymer
Elastomer/Hybrid Filler Nanocomposites
matrix. They also extended their study on 3D fillers prepared by hybridizing SCNF and Mg-Al-LDH (LDH) by noncovalent assembly with sodium dodecyl sulfate as a linker between them . The variation of the tensile strength and elongation at break of TNP/NBR with respect to SFCNF– LDH hybrid loadings clearly established its reinforcing effect. It was also noted that the 0.50 wt% filler-loaded TPU/NBR nanocomposite exhibited maximum improvements in the tensile strength (167%) and elongation at break (1.51 times) compared to the neat TPU/NBR. Such improvements in the mechanical properties were attributed to the homogeneous dispersion of SFCNF–LDH filler in the TN matrix and the strong interfacial interaction between the hybrid filler and polymer matrix. At higher loadings, the tensile strength and elongation at break gradually decreased due to the aggregation of the filler in the matrix. Their findings also confirmed the synergistic effect of hybrid fillers in the improvement of the mechanical property of TPU/NBR nanocomposites. In addition to 3D hybrid fillers, investigations on other hybrid-filled PU nanocomposites also showed improvements in mechanical properties. According to Kang et al. , PU/MWCNT-graphite showed higher tensile strength and strain at break compared to corresponding PU/MWCNT and PU/graphite nanocomposites at the same filler loading. They suggested that such increments in mechanical properties could be ascribed to good dispersion and interconnectivity of MWCNT and graphite in PU. In another work, Das et al.  studied the mechanical properties of HBPU nanocomposites based on carboxyl-functionalized MWCNT (MNC), Fe3O4 (FNC) and Fe3O4–MWCNT (NNC) fillers. The corresponding mechanical performance data is displayed in Table 9.3. It is inevitable that TS of HBPU increases from ~22 MPa to 52, 55 and 59 MPa in 1, 2 and 3 wt% hybrid-loaded HBPU respectively. This is ascribed to the better filler-matrix interaction and external tensile load transmission from the matrix to the nanohybrid due to the interfacial shear stress. The elongation at break followed the order: HBPU (~790%) < FNC (670%) < MNC (~700%) < NNC (650–550%). The elongation at break decreased for NNC with an increasing nanohybrid content due to the restricted molecular movement as inter- and intramolecular interactions increased. Khabashesku and coworkers  reported that the addition of 0.2 wt% hybrid filler (CNT-nanodiamond) in polyurea/polyurethane matrix exhibited improvement in tensile strength (64%) and elongation at break (1.24 times) compared to neat polymer. Fu et al.  noted that WPU/ Ag-HNT (3 wt%) exhibited enhanced tensile strength (294%) and Young’s modulus (1311%) due to strong interfacial filler and polymer interactions. In contrast, the decrease in EB was attributed to the increased crosslinking density of WPU nanocomposites and the rigid nanofiller. Limited
446 Hybrid Nanomaterials Table 9.3 Mechanical performance of HBPU and its NCs. (Reproduced from  with permission from Royal Society of Chemistry). Sample code
Tensile strength (MPa)
Elongation at break (%)
Scratch hardness (kg)
Impact resistance (cm)
22 ± 0.85
790 ± 13.0
4.8 ± 0.20
Li-AlLDH/MWCNT/SR (0.68 MPa) > SR (0.32 MPa). These findings clearly demonstrate that the tensile strength of SR is improved by about 134, 125 and 100% in Mg-Al-LDH/MWCNT/SR, Co-Al-LDH/MWCNT/SR and Li-Al-LDH/MWCNT/SR composites respectively. Such improvements in the mechanical properties of SR are ascribed to the synergistic effect of 1D MWCNT and 2D LDH fillers based on the stress-strain plots shown in Figure 9.15. It is also noted that EB of individually filled SR composites is considerably reduced with respect to neat SR. Interestingly, EB of SR is nearly recovered in the case of Li-Al-LDH/MWCNT/SR whereas it is improved by 14 and 11% in Mg-Al-LDH/MWCNT/SR and Co-Al-LDH/ MWCNT/SR respectively. In all probability, homogeneous dispersion leading to the strong interfacial interaction and efficient stress transfer between SR and Mg-Al-LDH/MWCNT fillers could account for the maximum improvement in the mechanical properties of Mg-Al-LDH/MWCNT/SR composites. SR/carbon black/carbon fiber , SR/expanded graphite/conductive carbon, SR/PS/MMT  and SR/conductive carbon composites  also exhibited improved mechanical properties.
Elastomer/Hybrid Filler Nanocomposites
Table 9.5 Summery of the mechanical properties of SR and its composites. (Reproduced from  with permission from Elsevier). Sample
SR MWCNT(0.5 wt%)/SR MWCNT(1.0 wt%)/SR Li-Al-LDH(0.5 wt%)/SR Li-Al-LDH(1.0 wt%)/SR Li-Al-LDH/MWCNT(0.5 wt%)/SR Li-Al-LDH/MWCNT(0.75 wt%)/SR Li-Al-LDH/MWCNT(1.0 wt%)/SR Li-Al-LDH/MWCNT(1.5 wt%)/SR
0.32 ± 0.02 0.48 ± 0.02 0.44 ± 0.01 0.40 ± 0.01 0.38 ± 0.02 0.52 ± 0.01 0.57 ± 0.02 0.68 ± 0.03 0.64 ± 0.02
192 ± 8 133 ± 6 108 ± 5 132 ± 6 114 ± 5 165 ± 4 169 ± 3 190 ± 4 175 ± 5
Mg-Al-LDH(0.5 wt%)/SR Mg-Al-LDH(1.0 wt%)/SR Mg-Al-LDH/MWCNT(0.5 wt%)/SR Mg-Al-LDH/MWCNT(0.75 wt%)/SR Mg-Al-LDH/MWCNT(1.0 wt%)/SR Mg-Al-LDH/MWCNT(1.5 wt%)/SR
0.42 ± 0.01 0.39 ± 0.01 0.61 ± 0.03 0.69 ± 0.02 0.75 ± 0.01 0.73 ± 0.01
162 ± 6 120 ± 4 202 ± 5 209 ± 7 219 ± 4 187 ± 4
Co-Al-LDH(0.5 wt%)/SR Co-Al-LDH(1.0 wt%)/SR Co-Al-LDH/MWCNT(0.5 wt%)/SR Co-Al-LDH/MWCNT(0.75 wt%)/SR Co-Al-LDH/MWCNT(1.0 wt%)/SR Co-Al-LDH/MWCNT(1.5 wt%)/SR
0.41 ± 0.01 0.41 ± 0.01 0.54 ± 0.01 0.66 ± 0.02 0.72 ± 0.01 0.62 ± 0.01
125 ± 4 111 ± 5 178 ± 7 201 ± 6 208 ± 3 163 ± 5
100 150 Strain (%)
0.2 0.0 0
0.2 0.0 0
50 100 150 200 250 Strain (%)
50 100 150 200 250 Strain (%)
Figure 9.15 (A) Stress-strain plots for neat SR (a), Li-Al-LDH(0.5 wt%)/SR (b), MWCNT(0.5 wt%)/SR (c), and Li-Al-LDH/MWCNT(1.0 wt%)/SR (d). (B) Stress-strain plots for neat SR (a), Mg-Al-LDH(0.5 wt%)/SR (b), MWCNT(0.5 wt%)/SR (c), and Mg-Al-LDH/MWCNT(1.0 wt%)/SR (d). (C) Stress-strain plots for neat SR (a), Co-AlLDH(0.5 wt%)/SR (b), MWCNT(0.5 wt%)/SR (c), and Co-Al-LDH/MWCNT (1.0 wt%)/ SR (d). (Reproduced from  with permission from Elsevier).
450 Hybrid Nanomaterials
Mechanical Properties of Hybrid Filler Incorporated NR Nanocomposites
The mechanical property of recycled poly(ethylene terephthalate (PET R-PET)/HNTs hybrid-filled natural rubber composites were shown by Nabil and Ismail . It is inferred that the tensile strength and elongation at break gradually increase with the weight ratio of HNTs. This was attributed to the dispersion of HNTs inside NR matrix and the intertubular interaction between HNTs and rubber matrix. The variation of stress (100% and 300% elongation) in R-PET/HNTs-filled natural rubber composites shows an increase in both stress values with weight ratio of HNTs due to the formation of more rigid and stiffer NR composites. Li et al.  reported stress-strain curves of the unfilled NR and its nanocomposites filled with graphene/CNT hybrid fillers. Figure 9.16 shows corresponding tensile strength and fracture toughness data. It is observed that hybridizing CNTs with graphene has a more significant reinforcement effect on the nanocomposites due to the synergistic effect. The mechanical properties of the carbon black and nanoclay hybridfilled natural rubber show a superior performance to rubber vulcanizates over carbon black at the same filler loading . Liu et al.  found that NR with carbon black and two poly(ethylene glycol) modified clay hybrid fillers exhibit superior mechanical properties over that with CB carbon black as single phase filler. In another study, NR/organoclay/ carbon black nanocomposites exhibited a reinforcing effect on mechanical properties . The fatigue crack propagation resistance and tensile strength of NR are found to be improved for dry mixed CNT silica hybrid systems .
Mechanical Properties of Hybrid Filler Incorporated SBR, NBR, EPDM and EVA Nanocomposites
According to Tang et al. , 3D network consisting of halloysite nanotubes (HNTs)–tannic acid functionalized graphene (TAG) hybrid filler reinforces SBR matrix, as demonstrated by the observed improvement of the modulus and strength of the composites. It was observed that hybrid filler network, together with the enhanced interfacial interaction account for the improvement of the modulus and strength of the composites synergistically. The tensile property of SBR/CB and SBR/CB-RG composites showed that SBR/CB-RG hybrid fillers exhibited significant improvements in tensile properties compared to the limited improvements noted in SBR/ CB blends . The moduli at 200% elongation and elongation at break of
Elastomer/Hybrid Filler Nanocomposites NR/0.5GE NR/0.5GE/0.1CNTs NR/0.5GE/0.5CNTs NR/0.5GE/ICNTs NR/0.5GE/3CNTs NR/ICNTs Neal NR
30 Stress (MPa)
7 Fracture toughness (GJ/m3)
5 4 3
N 1C R 0. N 5G 0 Ts 0. E/0 .5G 5G .1 E E/ CN 0. 0.5 Ts 5G CN 0. E/1 Ts 5G CN E/ Ts 3C NT s
Tensile strength (MPa)
400 Strain (%)
N 1C R N 5G 0 Ts 0. E/0 .5G 5G .1 E E/ CN 0. 0.5 Ts 5G CN 0. E/1 Ts 5G CN E/ Ts 3C NT s
Figure 9.16 Mechanical properties of the unfilled NR and the nanocomposites filled with either single-component fillers or the hybrid filler. (a) Representative stress-strain curves, (b) tensile strength, and (c) fracture toughness. (Reproduced from  with permission from Royal Society of Chemistry).
the SBR/CB-RG blends were higher than those of the SBR/CB. Tangudom et al.  used bagasse ash silica as co-reinforcing filler with precipitated silica in NR/SBR and observed increase in tensile strength and percent elongation at break. CB/silica hybrid-filled SBR/NBR blends with silane coupling agent exhibited higher tensile strength, tensile modulus and
452 Hybrid Nanomaterials elongation at break attributed to the better rubber-filler interaction than blends without it . The NBR nanocomposites reinforced with organoclay and nano-CaCO3 led to an improvement of 350% in tensile properties . In addition, hybrid filler system causes an increase in modulus and elongation at break of NBR. This is ascribed to the synergistic reinforcement of NBR by hybrid filler system consisting of organoclay and nano-CaCO3. The influence of the hybrid (HNTs-phthalocyanine) filler on the mechanical properties of butadiene-acrylonitrile (NBR) rubbers was also assessed . Malas and Das  found that the mechanical properties of the hybrid nanocomposites were highly influenced by the dispersion and exfoliation of the nanoclays in the EPDM matrix. Morphological studies of tensile fracture surfaces of PA/HNTs/EPDM hybrid composites indicated that HNTs have better adhesion to the EPDM matrix as compared to palm ash . Yuan et al.  observed that graphene/boehmite incorporated EVA increased the tensile strength by 37.1% due to the synergetic dispersion effect of graphene and AlOOH and a combination of the functions from the two nanofillers.
Dynamical Mechanical Analysis (DMA) of Elastomer Nanocomposites DMA of Hybrid Filler Incorporated PU Nanocomposites
Dynamical mechanical thermal analysis is used to study the viscoelastic properties of the polymer and measures the storage modulus (E ) corresponding to elastic deformation, loss modulus (E ) corresponding to / ), which help in determining the glass plastic deformation and tan δ (E /E transition temperature (Tg) of a polymer. Therefore, the viscoelastic properties of 3D and other hybrid-filled PU, TPU, WPU, HBPU and PU/NBR blend nanocomposites were studied. Chen et al.  noted that the storage modulus of TPU/TiO2-RGO (1:1) was relatively higher than that of pure TPU but lesser than TPU/RGO. Interestingly, the loss modulus and loss tangent of TPU composite containing 1 wt% TiO2-RGO (1:1) were much lesser compared to pure TPU and TPU/RGO (1 wt%). This was attributed to the weak interface between TPU matrix and RGO fillers due to their aggregation tendency, whereas TiO2-RGO (1:1) hybrid exhibited reduced loss modulus of the composites. Temperature variation of a) storage modulus and b) loss modulus of neat TPU and its nanocomposites containing 0.25, 0.50, 0.75 and 1 wt%
Elastomer/Hybrid Filler Nanocomposites
3500 neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
Storage modulus (E’)
3000 2500 2000 1500 1000 500 0 –60
neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
200 Loss modulus (E’’)
0 20 Tepmperature ( C)
0 –60 (b)
0 Tepmperature ( C)
Figure 9.17 Temperature variation of (a) storage modulus and (b) loss modulus of neat TPU and its nanocomposites containing 0.25, 0.50, 0.75 and 1 wt% MWCNT-CRGO (1:1) hybrid. (Reproduced from  with permission from Wiley).
MWCNT-CRGO (1:1) hybrid are depicted in Figure 9.17 . It is noted that TPU nanocomposites containing 0.50 wt% filler compared to neat TPU show that storage modulus is enhanced by 61 and 206% in both the glassy region (at −50 °C) and rubbery state (at 50 °C), respectively. The variation of loss modulus with temperature of TPU and its MWCNT-CRGO filled nanocomposites show that loss modulus (–30 °C) of TPU nanocomposites
454 Hybrid Nanomaterials filled with 0.25, 0.50, 0.75, and 1 wt% hybrid compared to neat TPU are improved by 100, 140, 119, and 69%, respectively. Maximum shifting in Tg (–37 °C) is observed in 0.50 wt% hybrid-filled TPU nanocomposite. The variation of dissipation factor (tan d) of neat TPU and its MWCNTCRGO (1:1) filled nanocomposites as a function of temperature is shown in Figure 9.18 . It is evident that TPU nanocomposite loaded with 0.50 wt% MWCNT–CRGO (1:1) filler shows maximum decrease in tan height (0.22) in comparison to neat TPU (0.37). Internal friction among the nanofiller–nanofiller, nanofiller–polymer matrix, and polymer matrix–matrix under some external stresses could be ascribed to such observation. Such improvements in thermal and mechanical properties have been attributed to homogeneous dispersion and strong interfacial interaction. Roy et al.  studied the variation of dynamic storage modulus (E ) versus temperature of TPU and its MWCNT-MMT (1:1) hybrid-filled nanocomposites. The storage modulus of TPU nanocomposite having 0.50 wt% MWCNT-MMT (1:1) hybrid is improved by 37% in the glassy region (−50 °C) compared to neat TPU. In the rubbery state (at 50 °C), the storage modulus is significantly improved (174%) in the 0.50 wt% MWCNT-MMT (1:1) loaded TPU nanocomposites compared to neat TPU. The variation of loss modulus (E ) as a function of temperature of TPU and its hybrid nanocomposites in Figure 9.19b show improved loss modulus (at −30 °C)
0.6 neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
0.4 0.3 0.2 0.1 0.0 –60
20 40 0 Temperature ( C)
9.18 Temperature-dependent curve of tan of neat TPU and its nanocomposites containing 0.25, 0.50, 0.75 and 1 wt% MWCNT-CRGO (1:1) hybrid. (Reproduced from  with permission from Wiley).
Elastomer/Hybrid Filler Nanocomposites
of TPU filled with, 0.25, 0.50, 0.75, and 1 wt% MWCNT-MMT (1:1) hybrids compared to neat TPU. It is also noted that peak of TPU at around −46 °C [Tgg(soft)] is shifted in all TPU nanocomposites to the higher temperatures. The temperature variation of dissipation factor (tan δ) of neat TPU and its MMT-MWCNT (1:1) hybrid-filled nanocomposites in Figure 9.19c show maximum decrease in tan δ peak height for TPU/MMT–MWCNT (1:1) hybrid nanocomposite (0.19) with 0.50 wt% when compared with neat TPU (0.37). The positive shift of the Tg in the presence of the hybrid filler signifies the restricted mobility of the soft domain at the close vicinity of the glass transition temperature. Srivastava and coworkers [96–98] made significant contributions in using a variety of 3D hybrid fillers in the development of TPU/NBR nanocomposites. DMA of CNT (modified)–LDH (Mg-Al LDH) hybridfilled TPU/NBR nanocomposites was carried out in the range of −80 to
neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
200 Loss modulus (E’’)
Storage modulus (E’)
neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
2000 1500 1000 500
150 100 50
0 20 Temperature ( C)
0 20 Temperature ( C)
neat TPU TPU/MWCNT-CRGO (0.25 wt%) TPU/MWCNT-CRGO (0.50 wt%) TPU/MWCNT-CRGO (0.75 wt%) TPU/MWCNT-CRGO (1.0 wt%)
0.4 0.3 0.2 0.1 0.0 –60
0 20 Temperature ( C)
Figure 9.19 (a) Storage modulus vs. temperature, (b) loss modulus vs. temperature, and (c) tan δ vs. temperature plots of neat TPU and its nanocomposites with MWCNT-MMT (1:1) hybrid. (Reproduced from  with permission from Wiley).
456 Hybrid Nanomaterials 80 °C . It is noted that storage modulus of the 0.50 wt% hybrid-filled TPU/NBR matrix is significantly increased in both the glassy region (by 243% at −60 °C) and rubbery state (by 241% at 25 °C) compared to pure TPU/NBR. Such enhancement in storage modulus is attributed to the homogeneous dispersion of the hybrid filler in TPU/NBR matrix and strong filler polymer interaction matrix. The observed improvements are also found to be much more impressive than those reported for LDHfilled TPU/NBR nanocomposites. The loss modulus (E ) of TPU/NBR nanocomposites filled with 0.50 wt% hybrid is improved by 254% compared to pure TPU/NBR. It is evident that SFCNT-LDH hybrid-filled TPU/NBR nanocomposites exhibit no significant shift in Tg compared to neat TPU/NBR (ca. −36 °C). They also observed maximum decrease in tan in TPU/NBR/0.50 wt% SFCNT-LDH (0.62) compared to neat TU/NBR (0.79). This is ascribed to the internal friction among the nanofiller–nanofiller, nanofiller–matrix and matrix–matrix under external stresses. Roy et al.  also investigated the dynamical mechanical properties of TPU/NBR (TN) blend filled with noncovalent assembly of SFCNT-LDH (Zn-Al-LDH) and SFCNF-LDH (Zn-Al-LDH) hybrid fillers, as shown in Figure 9.20. The temperature variation of storage modulus and loss modulus of TPU/NBR nanocomposites containing 0.25, 0.50, 0.75, 1 wt% SFCNTLDH and SFCNF-LDH are always higher compared to neat TPU/NBR. Such improvements in mechanical properties could be attributed to better homogeneous dispersion, stronger interfacial interaction and synergistic effect. These investigations showed that TPU/NBR blend containing 0.50 wt% MWCNT-LDH hybrid exhibit superior mechanical properties (storage modulus: 321% at −60 °C) compared to neat TPU/NBR. It is also noted that all the TN nanocomposites exhibit slight positive shifting in Tg compared to neat TN. This is in all probability due to restricted mobility of the polymer chains in the presence of hybrid filler. SFCNT-LDH hybrid shows relative increment in Tg compared to SFCNF-LDH hybrid in TPU/NBR matrix, which signifies that SFCNT-LDH hybrid exerts comparatively better restriction than the other hybrid fillers. Figure 9.21 depicts the effect of hybrid fillers on dissipation factor (tan δ) of neat TN as a function of temperature. The height reduction and shifting of tan δ peaks are evident in TPU/NBR nanocomposites compared to neat TPU/NBR, which could be attributed to the internal friction among the nanofiller–nanofiller, nanofiller–polymer matrix and polymer matrix–matrix under some external stresses. Fu et al.  studied the dynamic mechanical properties of WPU and WPU/Ag-HNT nanocomposites. Figure 9.22a shows that the storage
Elastomer/Hybrid Filler Nanocomposites Neat blend Blend/CNT-LDH (0.25 wt%) Blend/CNT-LDH (0.50 wt%) Blend/CNT-LDH (0.75 wt%) Blend/CNT-LDH (1.0 wt%)
1000 Storage modulus (MPa)
Temperature ( C)
Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)
Storage modulus (MPa)
1 –80 (b)
Temperature ( C)
Figure 9.20 Temperature dependence of (a) storage modulus of neat TPU/NBR and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt% SFCNT-LDH hybrid; and (b) storage modulus of neat TN and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt% SFCNF-LDH hybrid. (Reproduced from  with permission from Springer).
458 Hybrid Nanomaterials modulus and plateau value of the WPU nanocomposites increased gradually with increasing Ag-HNT nanofiller contents. This is ascribed to the strong interfacial interaction and effective load transfer from WPU matrix to the rigid filler. The variation of tan curves with temperature in Figure 9.22b shows the presence of a small peak at low temperature (Tg of soft segment) and another peak at high temperature (Tg of hard segment). They ascribed the improved Tg to the incorporated rigid nanoparticles, which restricted the motion of WPU molecular chains. Ouyang et al.  studied the temperature-dependent variation of tan δ and storage modulus of electrospun PU filled with MWCNT, AgNP and MWCNT/AgNP (Figure 9.23). They noticed positive shifting of Tg (soft segment) of PU/AgNP towards higher temperature. Such behavior was in contrast to the nearly unchanged Tg (soft segment) noted in PU/ MWCNT and PU/MWCNT/AgNP. Further, PU hybrid samples showed reduced storage modulus compared to pure PU nanofibers. Such behavior depicted that addition of MWCNTs and AgNPs was likely to promote the fusion of the soft segment and hard segment, thereby improving the compatibility of the two microphases. Gaddam et al.  observed 136% improvement in storage modulus in PU/carbon-silica (1.5 wt%) nanocomposite. These nanocomposites exhibited 25% enhancement in Tg compared to neat PU. Such remarkable change in PU nanocomposites was ascribed to strong interfacial interaction between the hybrid filler and PU matrix.
DMA of Hybrid Filler Incorporated SR Nanocomposites
Pradhan  also investigated the variation of storage modulus (E ) and tan δ versus temperature of neat SR, G(0.375 wt%)/SR, MWCNT(0.375 wt%)/SR, MWCNT/G(0.75 wt%)/SR, and MWCNT/ G(1.0 wt%)/SR, represented correspondingly in Figures 9.24A and 9.24B respectively. It is inferred that the storage modulus of the MWCNT/ G(0.75 wt%)/SR hybrid is significantly higher at −125 °C compared to either neat SR or its individually filled MWCNT or graphene composites. Interestingly, the storage modulus of G(0.375 wt%)/SR is lower than the neat SR. This suggests that G/SR composite could act as an effective damping material even if a small amount of graphene is present in SR. The reduction in the storage modulus of SR at 1.0 wt% MWCNT/G (1:1) hybrid loading is in all probability due to the aggregation of hybrid filler. It is also evident that Tg is decreased by (2 °C) in MWCNT/G(0.75 wt%)/SR compared to neat SR, possibly due to the plasticization effect of the hybrid
Elastomer/Hybrid Filler Nanocomposites
Neat blend Blend/CNT-LDH (0.25 wt%) Blend/CNT-LDH (0.50 wt%) Blend/CNT-LDH (0.75 wt%) Blend/CNT-LDH (1.0 wt%)
–20 0 Temperature ( C)
Neat blend Blend/CNF-LDH (0.25 wt%) Blend/CNF-LDH (0.50 wt%) Blend/CNF-LDH (0.75 wt%) Blend/CNF-LDH (1.0 wt%)
–20 0 Temperature ( C)
Figure 9.21 Temperature dependence of tan delta of (a) neat TPU/NBR (TN) and its nanocomposites containing 0.25, 0.50, 0.75 and 1 wt% SFCNT-LDH hybrid; and (b) neat TN and its nanocomposites containing 0.25, 0.50, 0.75 and 1 wt% SFCNF-LDH hybrid. (Reproduced from  with permission from Springer).
460 Hybrid Nanomaterials 6000
WPU/Ag-HNT-0 WPU/Ag-HNT-1 WPU/Ag-HNT-2 WPU/Ag-HNT-3
Storage modulus (MPa)
5000 4000 3000 2000 1000 0 –80
Temperature ( C)
(a) 0.35 0.30
WPU/Ag-HNT-0 WPU/Ag-HNT-1 WPU/Ag-HNT-2 WPU/Ag-HNT-3
0.20 0.15 0.10 0.05 0.00 –80 –60 –40
Temperature ( C)
Figure 9.22 Temperature variation of (a) storage modulus and (b) loss factor of WPU and WPU nanocomposite films. (Reproduced from  with permission from Elsevier).
filler in SR matrix. On the contrary, no significant change in Tg is noticed in SR/MWCNT or SR/G with respect to neat SR. Temperature variation of storage modulus (E ) and tan δ of neat SR, MMT(0.5 wt%)/SR, MWCNT(0.5 wt%)/SR and MMT/MWCNT(1.0 wt%)/ SR have also been studied and the corresponding findings are displayed in Figure 9.25 . It is observed that the storage modulus of the MMT/MWCNT/SR nanocomposite is higher compared to neat SR and
Elastomer/Hybrid Filler Nanocomposites 103 Storage modulus (MPa)
PU –20 (a)
Temperature ( C)
PU PU-MWCNT PU-AgNP PU-MWCNT-AgNP
100 –80 –60 –40 –20 0 20 40 Temperature ( C)
Figure 9.23 Temperature variation of (a) tan δ and (b) storage modulus for PU, PU-MWCNT, PU-AgNP and PU-MWCNT-AgNP nanofibers. (Reproduced from  with permission from Royal Society of Chemistry). 5
Log E’ (MPa)
4 3 2 1
0 –1 (a)
–50 0 Temperature ( C)
0.05 b 0.00
–0.05 d –0.10 (B)
–90 Temperature ( C)
9.24 (A) Storage modulus versus temperature. (B) Tan δ versus temperature curves of (a) neat SR, (b) G(0.375 wt%)/SR, (c) MWCNT(0.375 wt%)/SR, (d) MWCNT/G(0.75 wt%)/SR, and (e) MWCNT/G(1.0 wt%)/SR.
462 Hybrid Nanomaterials 4
a. neat SR b. SR 0.5 wt% MMT c. SR 0.5 wt% MWCNT d. SR 1.0 wt% MMT/MWCNT
Log E’ (MPa)
3 2 1 0 –1 –120 (a)
–60 –30 0 Temperature ( C)
a. neat SR b. SR 0.5 wt% MMT
c. SR 0.5 wt% MWCNT d. SR 1.0 wt% MMT/MWCNT
–100 –75 Temperature ( C)
Figure 9.25 Temperature variation of storage modulus (E ) and tan δ of neat SR, MMT(0.5 wt%)/SR, MWCNT(0.5 wt%)/SR and MMT/MWCNT(1.0 wt%)/SR.
individually filled SR composites. At −125 °C, the improvement in storage modulus is by 55% for 1 wt% MMT/MWCNT-loaded SR nanocomposites. The enhancement of the storage modulus is likely attributed to the exfoliation of MMT/MWCNT hybrid in SR matrix as well as confinement of SR on the homogeneously dispersed MMT/MWCNT hybrid. Figure 9.25 also shows that Tg of MMT/MWCNT/SR nanocomposites is lower by ~2 °C compared to neat SR. Pradhan and Srivastava  also noted that the height of the tan δ peak of MMT/MWCNT/SR is lowest compared to neat SR and individually filled SR composites. This was ascribed to the
Elastomer/Hybrid Filler Nanocomposites
restricted molecular motion of polymer chain that is imposed by reinforcement of well-dispersed MMT/MWCNT on SR matrix.
DMA of Hybrid Filler Incorporated NR Nanocomposites
Liu and others  performed dynamic rheological tests to elucidate the mechanism of reinforcement in explaining the superior performance of NR containing the organoclay and carbon black hybrid filler. It is noted that the storage modulus (G ) of uncured rubber composites exhibits a three order increase in magnitude for rubber filled with 15 parts per hundred parts of rubber carbon black and 10 parts per hundred parts of rubber organoclay as compared with gum rubber. The findings suggest that the reinforcement is due to a more developed filler network formation in hybrid filler system than that in single phase filler. Liu et al.  also reported the dynamic properties of NR nanocomposites based on carbon black and PEG modified clay hybrid filler. It was found that NR with hybrid filler exhibits superior mechanical properties over that with carbon black as single phase filler. The variation of storage modulus and tan curves of NR composites with m-coal gangue (CG), m-CG/CB, and CB has been studied in the temperature range from −120 to 100 °C . The CG-filled sample exhibited maximum storage modulus in the glassy region attributed to the good matrix-filler interaction. The strong interaction between the rubber and carbon black accounts for the decrease in tan delta and increases in Tg value. The storage modulus of CG in NR composites is higher than carbon black due to the reinforcement imparted by the CG leading to a strong and stiff interface between filler and NR matrix
DMA of Hybrid Filler Incorporated SBR, NBR, EPDM Nanocomposites
Salkhord and Ghari  studied the effect of simultaneous addition of the two nanofillers and also changes in concentration of nanoparticles on storage modulus behavior. It is noted that nano-CaCO3 in hybrid nanocomposites affects the storage modulus significantly. Furthermore, their investigation showed that the glassy region of hybrid nanocomposites exhibits higher storage modulus compared to single filler phase systems. Such a large increase in storage modulus has a more physical cause (fillerfiller interaction) than chemical cause (filler-polymer interaction).
464 Hybrid Nanomaterials The storage modulus of PAni-DBSA/organoclay/EPDM nanocomposite decreases as the temperature increases . However, storage modulus increases as a function of PAni-DBSA/organoclay nanocomposite content. This clearly indicates that there is a large mechanical reinforcement by the nanocomposite over the entire temperature range. Further, it is noted that the loss modulus of all PAni-DBSA/organoclayEPDM composites increased in comparison to pure EPDM.
Thermogravimetric Analysis (TGA) of Hybrid Filler Incorporated Elastomer Nanocomposites TGA of Hybrid Filler Incorporated PU Nanocomposites
Thermal stability behavior of PU depends on its molecular weight, composition, preparatory procedure and fillers used in development of its nanocomposites. According to the available literature, thermal degradation of PU occurs in three stages [93, 94]. First is the degradation step (~230–300 °C) corresponding to the rapid rapture of urethane linkages producing isocyanate and polyol. The second stage of degradation (~350–450 °C) is associated with the conversion of isocyanate to urea. Next, (above 500 °C) the earlier formed urea degrades to carbonaceous char. Investigations are in progress to improve the thermal stability of PU by introducing micro- and nano- (individual and hybrid) fillers into PU. In view of its poor thermal stability, several attempts have been made to enhance it by preparing its nanocomposites. Hybrid filler incorporated into PU, TPU, WPU and HBPU exhibited enhanced thermal properties compared to neat polymer. Liu et al.  achieved improvement in thermal stability by 13 °C (at 10% wt loss) in 1.5 wt% R-GNR/CNT-loaded TPU. The improved thermal stability of TPU was attributed to the presence of strong interfacial interaction between the TPU matrix and the hybrid nanofillers and interlocked nanostructures formed within TPU matrix. Figure 9.26 shows the TGA curves of TPU and its MMT-MWCNT (1:1) hybrid-loaded nanocomposites under nitrogen atmosphere . It is inferred that the decomposition of neat TPU as well as its nanocomposites is a two-step process. It is found to be maximum improved by 23 °C (at 10% weight loss) and 31 °C (at 50 wt% loss) in 0.25 wt% MMT-MWCNT (1:1) hybrid-filled TPU and better than some of the reported literature on individually filled MMT and MWCNT nanocomposites of TPU. It is also noticed that char residues of the nanocomposites are slightly higher compared with neat TPU. TGA of TPU nanocomposites containing 0.125 wt% MMT and 0.125 wt% MWCNT has been performed,
Elastomer/Hybrid Filler Nanocomposites
Weight loss (%)
Weight loss (%)
Temperature ( C)
neat TPU TPU/MMT-MWCNT (0.25 wt%) TPU/MMT-MWCNT (0.50 wt%) TPU/MMT-MWCNT (0.75 wt%) TPU/MMT-MWCNT (1 wt%)
Temperature ( C)
Figure 9.26 TGA curves of neat TPU and its nanocomposites containing 0.25, 0.50, 0.75, and 1 wt% MMT-MWCNT (1:1) hybrid. (Reproduced from  with permission from Wiley).
and the results were compared with the TGA data of neat TPU, and TPU nanocomposites containing 0.25 wt% MMT-MWCNT (1:1) hybrid confirmed the synergistic effect of individual fillers on the thermal stability of neat TPU. Roy et al.  also studied thermogravimetric analysis of neat TPU and its nanocomposites of MWCNT-CRGO (1:1) hybrid filler in nitrogen atmosphere. Their findings revealed that decomposition temperatures corresponding to 10 (and 50%) weight loss is maximum enhanced by 28 °C (and 40 °C) in 0.25 wt% MWCNT-CRGO (1:1) filled TPU nanocomposite. Thermal stability behavior of TPU/NBR blends has also been investigated in the presence of different 3D hybrid fillers [96–98]. It is observed that 0.50 wt% of LDH-MWCNT , MWCNT-LDH, CNF-LDH  and CNF-LDH  hybrids in TPU/NBR blend showed maximum improvement in thermal stability as determined at 50% weight loss corresponding to 20, 23 and 14 °C respectively. Carbon-silica hybrid (1.5 wt%) filled PU exhibited improved thermal stability (40 °C:Tonset) . In another work, the effect of GO/GNP hybrid was investigated on the thermal properties of PU . The addition of ZnAlLDHs/ZnO also improved the thermal stability of neat WPU . WPU nanocomposites of AgNWs/nano-SiO2 hybrids exhibited excellent thermal stability which was ascribed to the nanostructure of hybrid . They suggested that nano-SiO2 effectively prevented the oxidation of AgNWs, thereby improving thermal stability in WPU/AgNWs/nano-SiO2 nanocomposites.
466 Hybrid Nanomaterials The TGA of WPU/Ag/halloysite showed that thermal performance significantly improved with increment in filler content, as the hybrid filler acted as thermal insulator and mass transport barrier . The thermal stability of hard segments was improved more prominently than that of soft segments. Xiong et al.  studied thermal degradation of pure WPU and WPU/NiAlLDH/ ZnO composites. The TG curves of WPU exhibited slight weight loss (3 orders of magnitude) the electrical conductivity compared to C30B. In another study, the effect of the graphene oxide and graphene nanoplatelet fillers on the electrical conductivity of PU nanocomposites were investigated . According to Wei et al. , the electrical resistivity of WPU/AgNWs/nano-SiO2 nanocomposites in WPU in the presence of nano-SiO2 decreased ~5 × 103 times and the percolation threshold was reduced from 10.6 vol% to 3.6 vol%. It was observed that the electrical conductivity of the in-situ prepared PU nanocomposites based on the hybrid of GO and GNP were superior to the corresponding nanocomposite based on GNP alone at the same loading of the filler. Kang et al.  achieved electrical conductivity of 1.2 × 10–4 S/cm in PU/MWCNTs(1.25 wt%)/graphite(1.25 wt%). This was found to be higher by three orders of magnitude than PU nanocomposites containing 20.0 wt% graphite. Such a significantly enhanced value of electrical conductivity of PU/hybrid nanocomposite is ascribed to the interconnected network structure of MWCNT and graphite. Jee and coworkers  studied the electrical properties and heating performance of polyurethane hybrid nanocomposite films containing graphite and MWNT hybrid filler. The electrical resistivity of graphite/ MWCNT hybrid-filled PU nanocomposites lies in the range of 2.40 × 102 Ωcm (5.0%) to 2.79 × 101 Ωcm (9.0%). The impedance measurements have shown that PU/MWCNT and PU/AgNP nanocomposites are conductive at MWCNT and AgNP loadings greater than 1.5 and 5 wt% respectively . Interestingly, conductive behavior was observed in PU/MWCNTs (0.5 wt%)/AgNPs(3 wt%) nanocomposites. It was suggested that such a small amount of MWCNTs successfully bridges the discrete AgNP clusters in PU/MWCNT/AgNP owing to the high aspect ratio of MWCNTs and the orientation effe ff ct by electrospinning. Witt and coworkers  observed that incorporating hybrid fillers (CNT and CB)effectively enhances electrical properties of SR due to a synergistic effect of the particles. Silicone rubber composites with 20 phr of conductive carbon show a resistance of 26–30 kΩ . Further incorporation of 10 phr of EG increased electrical conductivity with the resistance
Elastomer/Hybrid Filler Nanocomposites
decreasing by two orders of magnitude, i.e., 90–100 Ω. The presence of clay/carbon black hybrid in the modified SR showed that increasing the filler resulted in a decrease in volume resistivity that was almost linear . The conductive composites based on dodecylbenzenesulfonatedoped polyaniline/organoclay nanocomposites and propylene-ethylidenenorbornene rubber have also been investigated . These composites exhibit high conductivities of up to 10−3 S cm−1 for 40 wt% of conducting nanocomposite.
Thermal Conductivity of Hybrid Filler Incorporated Elastomer Nanocomposites
Very limited work exists on investigations of the thermal conductivity of hybrid filler incorporated elastomers. Feng et al.  observed maximum thermal conductivity (1.47 W/mK) in TPU nanocomposites with SRGO/SCNT (3:1) compared to TPU nanocomposites with SRGO/SCNT (1:1) and SRGO/SCNT (1:3) hybrids. Such enhanced thermal conductivity was attributed to the synergistic effect between well-dispersed SGRO and SCNT nanofillers. The silicone rubber incorporating 50 vol% hybrid Si3N4 particles of different sizes at an optimal weight ratio exhibited the highest thermal conductivity of 1.48W/mK compared to using each single particle-sized filler alone . The studies have shown that sample consisting of EPDM/CB composite and an effective amount of GNPs dispersed in the matrix provide an increase of the thermal conductivity of the nanocomposites . EPDM/CB composite and an effective amount of GNPs dispersed in the matrix provide an increase of the thermal conductivity.
Dielectric Properties of Hybrid Filler Incorporated Elastomer Nanocomposits
A recent study showed that the dielectric properties of hybrid-filled TPU [55, 58, 76] are improved. Lee et al.  studied the room-temperature variation of relative dielectric coefficients ( r) with frequency change for TPU(ester and ether types)/MWCNT(3 wt%) nanocomposites as a function of C30B and C25A loadings. They observed that dielectric constant value was highest in the ester-TPU/MWCNT with 3 wt% C25A. In another report , dielectric constants of the GRN-CNT@PU films were higher
478 Hybrid Nanomaterials than the PU with GRN. TiO2-RGO nanocomposite of PU exhibited higher dielectric constant due to enhanced filler-matrix interface .
Shape Memory Property of Hybrid Filler Incorporated Elastomer Nanocomposites
Reports are also available on the shape recovery behavior of PU nanocomposites in the presence of hybrid fillers. Interconnected SRGO/SCNT hybrid nanofillers at a low weight loading of 1% dispersed in TPU showed good IR absorption and improved the crystallization of soft segments for a large-shape deformation . Yi et al.  observed that 7/3 TWNT/RGO composite showed 98% shape recovery and 91.6% shape fixity, whereas the pure TWNT and pure RGO composites showed shape recoveries of 95.6% and 90.5%, respectively, and shape fixities of 90.5% and 87.0%, respectively. Gaddam et al.  noted that shape recovery time in PU/carbonsilica nanocomposites was reduced with an increase in the carbon-silica content. Such behavior was ascribed to the filler/polymer interfacial interaction resulting in the storage of a large amount of elastic strain energy. Furthermore, this stored energy was released, causing speedy shape recovery when higher temperature was applied.
The elastomers constitute an important class of polymeric materials and their nanocomposites are fabricated by incorporating different types of nanofillers such as layered silicate clays, layered double hydroxides, carbon nanotubes, carbon nanofibers, calcium carbonate, metal oxides, silica nanoparticles, etc. As a result, these elastomer nanocomposites exhibit significant enhancement in their properties. Additionally, uniform dispersion of nanofillers in elastomer remains one of the most important parameters of achieving desired mechanical and physical characteristics. In this chapter, current developments in the field of elastomer nanocomposites reinforced with different hybrid fillers have been reviewed. Attention has been paid to summarizing dispersion, morphological, mechanical, thermal, electrical, dielectric and shape memory properties. The enhanced properties of these nanocomposites are invariably ascribed to the synergistic effect of the individual fillers. However, more research is still needed in the development of mechanically and thermally enhanced conducting hybrid filler incorporated PU, SR, NR, SBR, NBR, EPDM and EVA
Elastomer/Hybrid Filler Nanocomposites
elastomeric nanocomposites. It is anticipated that this research could help to visualize the utilization of these elastomeric nanocomposites in multifaceted applications.
Acknowledgment S.K. Srivastava acknowledges his research scholars, Dr. Himadari Acharya, Dr. Tapas Kuila, Dr. Bratati Pradhan , Dr. Saheli Roy, Mr. Bhagabat Bhuyan and Mr. Kunal Manna for all their help. S.K. Srivastava is also thankful to his daughter Mili Srivastava for providing all necessary facilities to complete this work in Toronto, Canada.
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Advanced Surface Engineering Materials Edited by Ashutosh Tiwari, Rui Wang, and Bingqing Wei Published 2016. ISBN 978-1-119-24244-4 Advanced Ceramic Materials Edited by Ashutosh Tiwari, Rosario A. Gerhardt and Magdalena Szutkowska Published 2016. ISBN 978-1-119-24244-4 Advanced Engineering Materials and Modeling Edited by Ashutosh Tiwari, N. Arul Murugan and Rajeev Ahuja Published 2016. ISBN 978-1-119-24246-8 Advanced 2D Materials Ashutosh Tiwari and Mikael Syväjärvi Published 2016. ISBN 978-1-119-24249-9 Advanced Materials Interfaces Edited by Ashutosh Tiwari, Hirak K. Patra and Xumei Wang Published 2016. ISBN 978-1-119-24245-1 Polyethylene-Based Blends, Composites and Nanocomposites Edited by Visakh P. M and María José Martínez Morlanes. Published 2015. ISBN: 978-1-118-83128-1 Advanced Bioelectronics Materials Edited by Ashutosh Tiwari, Hirak K. Patra and Anthony P.F. Turner Published 2015. ISBN 978-1-118-99830-4 Graphene An Introduction to the Fundamentals and Industrial Applications By Madhuri Sharon and Maheswar Sharon Published 2015. ISBN 978-1-118-84256-0 Advanced Theranostic Materials Edited by Ashutosh Tiwari, Hirak K. Patra and Jeong-Woo Choi Published 2015. ISBN: 978-1-118-99829-8 Advanced Functional Materials Edited by Ashutosh Tiwari and Lokman Uzun Published 2015. ISBN 978-1-118-99827-4
Advanced Catalytic Materials Edited by Ashutosh Tiwari and Salam Titinchi Published 2015. ISBN 978-1-118-99828-1 Graphene Materials Fundamentals and Emerging Applications Edited by Ashutosh Tiwari and Mikael Syväjärvi Published 2015. ISBN 978-1-118-99837-3 DNA Engineered Noble Metal Nanoparticles Fundamentals and State-of-the-art-of Nanobiotechnology By Ignác Capek Published 2015. ISBN 978-1-118-07214-1 Advanced Electrical and Electronics Materials Process and Applications By K.M. Gupta and Nishu Gupta Published 2015. ISBN: 978-1-118-99835-9 Polymer Nanotube Nanocomposites, 2nd edition Synthesis, Properties, and Applications Edited by Vikas Mittal Published 2014. ISBN 978-1-118-94592-6 Advanced Materials for Agriculture, Food and Environmental Safety Edited by Ashutosh Tiwari and Mikael Syväjärvi Published 2014. ISBN: 978-1-118-77343-7 Advanced Biomaterials and Biodevices Edited by Ashutosh Tiwari and Anis N. Nordin Published 2014. ISBN 978-1-118-77363-5 Biosensors Nanotechnology Edited by Ashutosh Tiwari and Anthony P. F. Turner Published 2014. ISBN 978-1-118-77351-2 Advanced Sensor and Detection Materials Edited by Ashutosh Tiwari and Mustafa M. Demir Published 2014. ISBN 978-1-118-77348-2
Advanced Healthcare Materials Edited by Ashutosh Tiwari Published 2014. ISBN 978-1-118-77359-8 Advanced Energy Materials Edited by Ashutosh Tiwari and Sergiy Valyukh Published 2014. ISBN 978-1-118-68629-4 Advanced Carbon Materials and Technology Edited by Ashutosh Tiwari and S.K. Shukla Published 2014. ISBN 978-1-118-68623-2 Nanomedicine for Drug Delivery and Therapeutics Edited by Ajay Kumar Mishra Published 2014. ISBN 978-1-118-41409-5 Responsive Materials and Methods State-of-the-Art Stimuli-Responsive Materials and Their Applications Edited by Ashutosh Tiwari and Hisatoshi Kobayashi Published 2013. ISBN 978-1-118-68622-5 Nanomaterials in Drug Delivery, Imaging, and Tissue Engineering Edited by Ashutosh Tiwari and Atul Tiwari Published 2013. ISBN 978-1-118-29032-3 Biomimetics Advancing Nanobiomaterials and Tissue Engineering Edited by Murugan Ramalingam, Xiumei Wang, Guoping Chen, Peter Ma, and Fu-Zhai Cui Published 2013. ISBN 978-1-118-46962-0 Polymers for Energy Storage and Conversion Edited by Vikas Mittal Published 2013. ISBN 978-1-118-34454-5 Encapsulation Nanotechnologies Edited by Vikas Mittal Published 2013. ISBN 978-1-118-34455-2
Biomedical Materials and Diagnostic Devices Devices Edited by Ashutosh Tiwari, Murugan Ramalingam, Hisatoshi Kobayashi and Anthony P.F. Turner Published 2012. ISBN 978-1-118-03014-1 Integrated Biomaterials for Biomedical Technology Edited by Murugan Ramalingam, Ashutosh Tiwari, Seeram Ramakrishna and Hisatoshi Kobayashi Published 2012. ISBN 978-1-118-42385-1 Integrated Biomaterials in Tissue Engineering Edited by Murugan Ramalingam, Ziyad Haidar, Seeram Ramakrishna, Hisatoshi Kobayashi, and Youssef Haikel Published 2012. ISBN 978-1-118-31198-1
Index Active fillers, 351 Agglomeration, 354, 359, 361, 364 Ammonia detection, 334 Asymmetric, 90, 91, 100, 101, 104–106, 119, 129, 134, 135 Boron nitride, 379, 393, 395 Carbon allotrope-based polymer nanocomposites, 248 carbon black, 249–252 carbon fiber, 248 carbon nanotubes, Carbon black, 374–375, 392 Carbon fiber-filled polymer nanocomposites, 249 aspect ratio, 250 carbon-carbon fiber/epoxy composites, 249 electrically conductive SCF-filled composites, 251–252 functional nanoconducting composite, 250–251 graphite nanoplatelet/poly(3,4ethylenedioxythiophene)poly(styrene sulfonate) composites, 249 higher crosslinking density, 250 Ni-coated fiber, 249 short carbon fiber (SCF)-filled EV, 205 Carbon fibers, CFs, 373
Carbon nanotube (CNT)-filled polymer nanocomposites, 256 AgNW/PS and MWCNT/PS nanocomposites, 255 aspect ratios, 256 compression molded samples, 254 epoxy resin, 261, 262 functionalized multiwalled carbon nanotubes, 260–261 GNP and MWCNT-embedded polycarbonate hybrid composites, 256–257 GNP/MWCNT/GNP, 257 low density polyethylene nanocomposites, 253–254 MWCNT/polyacrylate composites, 259–260 MWCNT-PMMA films, 252–253 PC–MWCNT conducting nanocomposites, 261 polyvinylidene fluoride, 261, 263 PTT/MWCNT composites, 263–265 rubber, 149 Carbon nanotubes, CNTs, 373 Carbon-based hybrid nanostructures, Charge collection efficiency, 166–167 Charge storage, 79, 80, 92, 99, 113, 115, 134 Chemical vapor deposition (CVD), 205 CNFs, 373–74, 376–77 CNT, 81, 83–85, 87, 89–100, 103, 104, 110, 114, 115, 119, 125, 126, 129
Suneel Kumar Srivastava and Vikas Mittal (eds.) Hybrid Nanomaterials, (491–496) © 2017 Scrivener Publishing LLC
492 Index CNT-filled polymer nanocomposites (see Carbon nanotube (CNT)-filled polymer nanocomposites), 276 CNH/GNP/PS nanocomposites, 265 CVD synthesis, 277–280 dominant shielding mechanism, 277–280 EPDM rubber, 275 frequency range, 274 paper-like material, 277–280 percolation threshold, 277–280 preparative method, 281 probing, 275–276 PU/D-graphene nanocomposites, 277–280 thickness and conductivity, 159–160 Coagulation-precipitation, 153–154 Cold start, 79, 131 Composites, 372–373 Conducting polymer, 79–81, 91, 115, 124–126 Conductive fillers, 353–354, 359, 365 Conductometric gas sensor, 321 Copper-based chalcogenides, 157–159 Core-shell nanoheterostructures, 213–215 Defluoridation, Dispersion, 376–77, 380–385, 388–397, 400–404, 407–408, 413 Doping, 81, 94, 120 Drinking water treatment systems, contaminant removal, geogenic pollutants, 199–200 human need, 201 nanotechnology, 200 synthetic adsorbents, 200 total available freshwater reserve, 171–172
EDLC, 80, 91, 94, 97, 99, 103, 117, 128, 131, 134 Elastomer nanocomposites, 426 hybrid filler (see Hybrid filler incorporated elastomer nanocomposites), 427 in-situ polymerization, 427 melt intercalation method, solution mixing, 165–166 Electrical conductivity, 82, 83, 87–89, 91–93, 95–98, 100, 103, 106, 107, 110, 112, 117, 120, 121, 124, 126, 134, 373–374, 377, 379–380, 382–392, 399–404, 411–412 Electrochemical anodization process, 205 Electrochemical process, Electrolyte, 80, 82, 83, 89, 91–99, 101–103, 106–108, 110–112, 117, 122, 126, 128, 129, 131, 135 Electromagnetic interference (EMI) shielding, 242 carbon fiber-filled polymer nanocomposites, 252–265 carbon-based materials, 249–252 CNT-filled polymer nanocomposites, 242–243 conducting polymers, 245 electromagnetic radiation, 265, 274–281 graphene and graphene oxide fillers-based polymer nanocomposites, 244 high dielectric constant materials ICP derived nanocomposites (see Intrinsically conducting polymer (ICP) derived nanocomposites), 244 mechanism, 242 metals, 244, 246 multiple internal reflections, 243–244
Index 493 parameters, 244 reflection and absorption, 246 reflection loss, 245, 247–248 shielding effectiveness, Emergency power, 79, 132 EMI, 374, 377 Energy density, 80, 90, 93, 96–103, 106, 107, 112, 114, 115, 119–121, 125, 133, 134, 135 Ethylene propylene diene monomer (M-class) rubber (EPDM)-based nanocomposites, 474 differential scanning calorimetric analysis, 463–464 dynamical mechanical analysis, 452 mechanical properties, 463–464, 468 thermogravimetric analysis, 439 transmission electron microscopic analyses, 149 Exciton Bohr radius, 205 Expandable graphite, see as EG, 375380 Faradic, 79, 80, 81, 82, 90, 104, 106, 107, 110, 111, 112, 115 Filtration, 261 Gas sensor, environmental monitoring, 331 mechanism, 330 principle, 330 volatile organic components (VOC), 332 Glass Fibers, 373, 390, 391 Graphene, 81–85, 88, 89, 91–95, 97, 98, 101–106, 108–110, 112, 114, 116, 118–122, 123–125, 129, 134 Graphene and graphene oxide fillers-based polymer nanocomposites, 276 Bouveault-Blanc approach, 275 CNH/GNP/PS nanocomposites, 265
CVD synthesis, 277–280 dominant shielding mechanism, 277–280 frequency range, 274–275 laminated magnetic graphene/ Fe3O4, 274 paper-like material, 277–280 percolation threshold, 277–280 preparative method, 281 probing, 275–276 PU/D-graphene nanocomposites, 277–280 thickness and conductivity, 250–251 Graphite nanoplatelet/poly(3,4ethylenedioxythiophene)poly(styrene sulfonate) composites, 299 Graphite nanoplatelets, 381, 383–386, 388–389, 392, 395, 397, 408–412 Hollow microspheres of pyrrole (HPPy), 162–163 Homogeneous, 364 Hybrid composites, 355–359, 361, 367–369 Hybrid filler incorporated elastomer nanocomposites, 477–478 dielectric properties, 476–477 electrical conductivity, EPDM-based nanocomposites, 474 hybrid fillers, 474–475 natural rubber nanocomposites, 463 NBR composites, 474 PU nanocomposites, 445 SBR nanocomposites, 474 shape memory property, silicone rubber nanocomposites, 472–474 thermal conductivity, Hybrid Nanoadsorbent, 206, 208 adsorption potential, 208–213 Application of Hybrid Nanoadsorbents, 208 arsenic removal, 213–215
494 Index defluoridation, 216–222 heavy metal removal, 223 issues and challenges, 208, 217 magnetic property, 208 material synthesis, 164 Hybrid nanomaterials, 323 Hybridization, 355–359, 361 Hydrogen detection, 337 Hydrothermal method, 327 Hydrothermal/solvothermal method, 173 Inactive fillers, 351 Incident photon-to-electron conversion efficiencies (IPCE), Internal combustion engine, 81 Intrinsically conducting polymer (ICP) derived nanocomposites, 290–292, 299–300 core-shell morphology, 293–298 dominant shielding mechanism, 293–298 frequency range, 293–298 percolation threshold, polyaniline, 286 polypyrrole, 289 preparative method, 281 structures, 293–298 thickness and conductivity, 205 Inverter, 133 Ion exchange, 151–152 Iron-based chalcogenides, 154–155 Layer-by-layer deposition method, 327 layered double hydroxides, 374, 405 Layered transition metal dichalcogenides, 150 Light scattering effect, 213 Liquid Rubber, 403, 406, 407 Magnetic nanosized adsorbent, 159 Mechanical properties, 372–374, 376–379, 384–387, 389–395, 397–398, 401, 404, 405, 408
Metal oxide, 79–85, 87, 89, 90, 91, 106, 110, 115, 126, 134 Metal oxides, 322 Metal-catalyzed electroless etching (MCEE), 205 Multiwalled carbon nanotubes (MWCNTs)-filled polymer nanocomposites. See Carbon nanotube (CNT)-filled polymer nanocomposites, 205 Nanoclay, 373 Nanofiller, 372–374, 377–384, 392, 396, 407–408 Nanofiltration, Nanohybrid materials, 159–160 1D nanostructure, 149 carbon-based hybrid nanostructures, 149 charge collection efficiency, 149 charge transfer kinetics, 157–159 core-shell nanoheterostructures, 149 electron transportation rate, 149 fabrication methods, 166–167 light scattering effect, 160–162 polymer-based hybrid nanostructure, 150 quantitative charge collection, 149 solar effective inorganic materials, 154 solar energy conversion, 171–172 TiO2 nanotube structures, 213 Nanohydroxyapatite/chitosan, 290 Nanostick-shaped polypyrrole composites, 285 Natural fillers, 351 Natural graphite flakes (NGF), Natural rubber nanocomposites, 474–475 differential scanning calorimetric analysis, 463 dynamical mechanical analysis, 436 kaolinite sheets, 450
Index 495 mechanical properties, 437 SEM images, 434, 439 TEM images, 467–468 thermogravimetric analysis, 152 Nickel-based chalcogenides, 250 Nickel-coated carbon fibers, 284 Nickel-coated fly ash (Ni-FAC)-doped polyaniline composite film, Nitrile rubber (NBR) composites, 474 differential scanning calorimetric analysis, 463–464 dynamical mechanical analysis, 452 mechanical properties, 438–439 preparation, 439 TEM measurement, 468 thermogravimetric analysis, 167–169 Nitrous oxide detection, 339 Photocatalysis, 169–171 Photoelectrochemical water splitting, 157–159 Physical vapour deposition (PVD), 329 Physicochemical properties, 98 Plasma-enhanced chemical vapor deposition (PECVD) technique, 285 Poly(trimethylene terephthalate) (PTT)/MWCNT composites, 299 Polyaniline-MWCNT nanocomposites, 160–162 Polymer-based hybrid nanostructure, 261, 263 Polystyrene (PS)-polyaniline (PANI) core-shell (PS@PANI), Polyurethane (PU) nanocomposites, 445 CNT-nanodiamond, 431 coaddition of C30B, 468–472 differential scanning calorimetric analysis, 452–458 dynamical mechanical analysis, 431
ester-TPU nanocomposites, 445, 446 HBPU, 441 mechanical properties of, 431, 441–442 MMT–MWCNT, 443–444 MWCNT-CRGO hybrid, 445 PU/MWCNT-graphite, 440–441 R-GNR/CNT hybrid, 434 silver nanowires, 432–433 SRGO/SCNT, 464–466 thermogravimetric analysis, 444–445 TPU/NBR nanocomposite, 433 TPU/NBR nanocomposites, 427 TWNT/RGO composition, 251–252 Polyvinyl chloride reinforced graphitecopper nanoparticles (PVC/ GCu), 149 Power density, 79, 80, 90, 96, 99, 100, 101, 103, 106, 107, 110–112, 114, 119–122, 124–126, 129, 130, 134 Quantum confinement effect, 172–173 Quantum dot-sensitized solar cells (QDSSC), 160 Quantum size effect, 325 Reduced graphene oxide (RGO), 205 Reinforcement, 349, 351–353, 355, 358, 368 Removal Technologies, 203 Resistance, 89, 91, 92, 95, 100, 107, 111, 126, 127, 131 Reverse osmosis, 247–248 Rheological properties, 385, 386 Scalar network analyzer (SNA), 160 Sensing materials, 325 Si-based Schottky junction solar cells, 173–175
496 Index Si-based solar cells, 434–435 Silicon carbide, see as SiC, 378, 392, 393, 396, 397 Silicone rubber (SR) nanocomposites, 472–474 differential scanning calorimetric analysis, 458–463 dynamical mechanical analysis, 434, 436 graphene/MWNTs, 446–449 mechanical properties, 434 MWCNT/MMT, 437, 438 TEM images, 466–467 thermogravimetric analysis, Sol-gel method, 326 Status and health effects of different pollutants, 201 Styrene-butadiene rubber (SBR) nanocomposites, 474 differential scanning calorimetric analysis, 463–464 dynamical mechanical analysis, 450–452 mechanical properties, 437–438, 440 TEM images, 468 thermogravimetric analysis, 432–433
Sulfonated reduced graphene oxide (SRGO)/sulfonated carbon nanotube (SRGO/SCNT), 155–156 Synergy effect, 355 Synthetic fillers, 351–352, 355 Template assisted synthesis, 328 Thermal conductivity, 373–375, 378–380, 384–386, 389, 392–397, 400–402, 408–412 Thermomechanical properties, 391, 397, 399, 402 Thermoset, 351, 364, 366 TiO2 nanostructure, 155–156 Titanium dioxides, 205 Ultrafiltration, 247–248 UPS, 133, 134 Vector network analyzer (VNA), 156–157 Windmills, 133 Wireless Systems, 133 Working potential, 100, 102