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Polymer Nanotube Nanocomposites
Scrivener Publishing100 Cummings Center, Suite 541J
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Publishers at Scrivener
Martin Scrivener([email protected])Phillip Carmical ([email protected])
Polymer Nanotube Nanocomposites
Edited by
Vikas Mittal
2nd Edition
Synthesis, Properties, and Applications
Copyright © 2014 by Scrivener Publishing LLC. All rights reserved.
Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts.Published simultaneously in Canada.
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Library of Congr ess Cataloging-in-Publication Data:
ISBN 978-1-118-94592-6
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
Contents
Preface xiii
1 Polymer Nanotube Nanocomposites: A Review of Synthesis Methods, Properties and Applications 1Joel Fawaz and Vikas Mittal1.1 Introduction 2
1.2 Methods of Nanotube Nanocomposites Synthesis 41.2.1 Direct Mixing 41.2.2 Solution Mixing 41.2.3 In-Situ Polymerization 71.2.4 Melt Mixing 12
1.3 Properties of Polymer Nanotube Nanocomposites 181.3.1 Mechanical Properties 181.3.2 Thermal Properties 261.3.3 Electrical Properties 331.3.4 Other Properties 37
1.4 Applications 38References 40
2 Functionalization Strategies for Single-Walled Carbon Nanotubes Integration into Epoxy Matrices 45J. M. González-Domínguez, A. M. Díez-Pascual, A. Ansón-Casaos,
M. A. Gómez-Fatou and M. T. Martínez.2.1 Introduction 46
2.1.1 SWCNTs in Composite Materials: The Case of Epoxy 49
2.1.2 The Processing and Functionalization of SWCNTs 50
2.2 Covalent Strategies for the Production of SWCNT/Epoxy Composites 512.2.1 Oxidation of SWCNTs 51
vi Contents
2.2.2 Functionalization with Terminal Amines 542.2.3 Functionalization with Terminal Oxirane Rings 592.2.4 Other Functional Groups 61
2.3 Non-covalent Strategies for the Production of SWCNT/Epoxy Composites 622.3.1 Adsorption of Reactive Species 622.3.2 Adsorption of Non-reactive Species 672.3.3 Dual-Affinity Adsorbed Species: The Use of Block
Copolymers in SWCNT/Epoxy Composites 722.4 Effect of Functionalization on the Epoxy
Physical Properties 762.4.1 Static and Dynamic Mechanical Properties 762.4.2 Thermal Behaviour and Stability 832.4.3 Electrical Conductivity and Percolation
Phenomena 962.4.4 Combined Properties: Electromechanical Effects 1012.4.5 Other Physical Properties 103
2.5 Applications of Functionalized SWCNTs in Epoxy Composites 104
2.6 Concluding Remarks and Future Outlook 106Acknowledgements 108References 109
3 Multiscale Modeling of Polymer–Nanotube Nanocomposites 117Maenghyo Cho and Seunghwa Yang3.1 Introduction 1173.2 Molecular Modeling and Simulation of CNT-Polymer
Nanocomposites 1213.2.1 Molecular Dynamics and Molecular Mechanics 1213.2.2 Force Fields for CNTs and Engineering Polymers 1243.2.3 Molecular Modeling and Simulation Procedures
for CNT/Polymer Composites 1253.3 Micromechanics Modeling and Simulation of
CNT-Polymer Nanocomposites 1323.3.1 Equivalent Inclusion Model 1323.3.2 Mathematical Homogenization Model 1373.3.3 Description of the Interphase Zone 1393.3.4 Weakened Interface between CNT and Matrix 1403.3.5 Effect of CNT Waviness 1423.3.6 CNT Agglomeration 143
Contents vii
3.4 Fully Integrated Multiscale Model for Elastoplastic Behavior with Imperfect Interface 1453.4.1 Hierarchical Integration of the Molecular Dynamics
and Continuum Model 1453.4.2 Two-Step Multiscale Model for the Elastoplastic
Behavior of CNT-Polymer Composites 1483.5 Conclusion and Perspective on Future Trends 158References 160
4 SEM and TEM Characterization of Polymer CNT Nanocomposites 167Francisco Solá4.1 Introduction 1674.2 Imaging CNTs in Polymer Matrices by SEM 1684.3 Mechanical Properties of CNT/Polymer Nanocomposites
by In-Situ SEM 1724.4 Imaging CNT in Polymer Matrices by TEM 1764.5 Mechanical Properties of CNT/Polymer Nanocomposites
by In-Situ TEM 1804.6 Conclusions and Future Outlook 181Acknowledgement 182References 183
5 Polymer-Nanotube Nanocomposites for Transfemoral Sockets 187S. Arun and S. Kanagaraj5.1 Introduction 188
5.1.1 Major Components in Transfemoral and Transtibial Amputee 188
5.1.2 Evolution of the Socket System 1885.1.3 Drawbacks of the Socket System 190
5.2 Materials Used for the Socket System 1905.2.1 Fiber-Reinforced Composites for the
Socket System 1915.2.2 Epoxy Nanocomposites 1925.2.3 FRP/CNTs Nanocomposites 2005.2.4 Aligned CNT Nanocomposites 203
5.3 Summary 204Acknowledgements 204References 204
viii Contents
6 Micro-Patterning of Polymer Nanotube Nanocomposites 211Naga S. Korivi6.1 Introduction 2116.2 Micro-Patterning Methods 213
6.2.1 Micromolding 2146.2.2 Selective Surface Modification 2226.2.3 Light-Based Methods 2256.2.4 Inkjet Printing 2286.2.5 Other Methods 230
6.3 Conclusions 230Acknowledgments 231References 231
7 Carbon Nanotube-Based Hybrid Materials and Their Polymer Composites 239Tianxi Liu, Wei Fan and Chao Zhang7.1 Introduction 2407.2 Structures and Properties of Carbon Nanomaterials 242
7.2.1 Fullerene 2447.2.2 Carbon Nanotubes 2457.2.3 Graphene Nanosheets 2457.2.4 Graphene Nanoribbons 246
7.3 Strategies for the Hybridization of CNTs with Carbon Nanomaterials 2477.3.1 CNT-Fullerene Hybrids 2477.3.2 CNT-GNS Hybrids 2507.3.3 CNT-GNR Hybrids 254
7.4 Preparation of CNT-Based Hybrid Reinforced Polymer Nanocomposites 2577.4.1 Solution Casting 2597.4.2 Melt Mixing 2597.4.3 In-Situ Polymerization 2607.4.4 Other Methods 260
7.5 Physical Properties of CNT-Based Hybrid Reinforced Polymer Nanocomposites 2627.5.1 Mechanical Properties 2627.5.2 Electrical Conductivity 2667.5.3 Thermal Conductivity 269
7.6 Summary 272Acknowledgements 273References 273
Contents ix
8 Polymer-Carbon Nanotube Nanocomposite Foams 279Marcelo Antunes and José Ignacio Velasco8.1 Introduction 2798.2 Basic Concepts of Polymer Nanocomposite Foams 2818.3 Main Polymer Nanocomposite Foaming Technologies 282
8.3.1 Continuous/Semi-continuous Foaming Processes 2838.3.2 Batch Foaming Processes 285
8.4 Polymer-Carbon Nanotube Nanocomposite Foams 2878.4.1 Types of Carbon Nanotubes and Production
Methods 2888.4.2 Properties of Polymer-Carbon Nanotube
Nanocomposite Foams 2918.5 Recent Developments and New Applications of Polymer-
Carbon Nanotube Nanocomposite Foams 3128.6 Conclusions 320Acknowledgements 322References 323
9 Processing and Properties of Carbon Nanotube/ Polycarbonate Composites 333Shailaja Pande, Bhanu Pratap Singh and Rakesh Behari Mathur9.1 Introduction 3339.2 Fabrication/ Processing of CNT/PC Composites 335
9.2.1 Melt-Processing 3379.2.2 Solution Processing Methods 340
9.3 Mechanical Properties of CNT/PC Composites 3449.4 Electrical Properties of CNT/PC Composites 355
9.4.1 Electrical Conductivity 3559.4.2 Electromagnetic Interference Shielding
Properties 3599.5 Conclusions 359References 361
10 Advanced Microscopy Techniques for a Better Understanding of the Polymer/Nanotube Composite Properties 365K. Masenelli-Varlot, C. Gauthier, L. Chazeau, F. Dalmas,
T. Epicier and J.Y. Cavaillé10.1 Introduction 36510.2 Near-Field Microscopies 367
10.2.1 Principles of STM and AFM 367
x Contents
10.2.2 Near-Field Microscopy for Nanotubes 370 10.2.3 AFM and CNT Composites 370
10.3 Transmission Electron Microscopy 372 10.3.1 Principles 372 10.3.2 Characterization of Carbon Nanotubes 376 10.3.3 Characterization of Polymer/Nanotube
Composites 37910.4 Scanning Electron Microscopy 387
10.4.1 Overview of the Technique (SEI, BEI, CCI) 387
10.4.2 Application to the Study of Nanotubes 388
10.4.3 For Polymer CNT/Nanocomposites 38910.5 Focused Ion Beam Microscopy 39510.6 Conclusions 396Acknowledgements 398References 398
11 Visualization of CNTs in Polymer Composites 405Wenjing Li and Wolfgang Bauhofer11.1 Introduction 40511.2 Experimental 40811.3 Visualization of CNTs at High Accelerating
Voltage (5-15 kV) 408 11.3.1 CNT Visualization in Stirred-Composites and
Calendered-Composites 408 11.3.2 Imaging Mechanism 409 11.3.3 Determination of Imaging Depth 412 11.3.4 CNT Visibility 414
11.4 Visualization of CNTs at Low Accelerating Voltage (0.3-5 kV) 417
11.4.1 CNT Visualization at Different Voltages 417 11.4.2 Imaging Mechanism 418
11.5 Essential Requirements and Tips for CNT Visualization 42311.6 Conclusion 424Acknowledgement 425References (with DOI) 425Reference List 426
Contents xi
12 Polymer Nanotube Composites: Latest Challenges and Applications 429Amal M. K. Esawi and Mahmoud M. Farag12.1 Carbon Nanotubes 430
12.1.1 Background 430 12.1.2 Synthesis of CNTs 431 12.1.3 Fabrication of CNT Polymer Composites 433 12.1.4 Electrical properties of CNT polymer
composites 435 12.1.5 Mechanical Properties of CNT Polymer
Composites 43712.2 Case Studies 440
12.2.1 Case Study: CNT-Based Strain Sensor 440 12.2.2 Case Study: Technical and Economic Feasibility
of Using CNT-Based Composites in Aerospace Applications 445
12.2.3 Case Study: CNT Composites for Wind Turbine Blades 451
12.2.4 Case Study: CNTs in Flexible Body Armor 45512.3 Conclusions 459References 460
Index 465
xiii
Preface
It is a pleasure to write this preface for the 2nd edition of Polymer Nanotubes Nanocomposites. Since the release of the 1st edition in 2010, a large number of advancements have been made in the science of polymer nanotube nano-composites in terms of synthesis and filler surface modification, as well as properties. Furthermore, a number of commercial applications have been realized. Thus, the aim of this second volume is to update the information presented in the first volume, as well as to incorporate recent findings.
Chapter 1 reviews various synthesis techniques and properties, as well as applications, of the polymer nanocomposite systems. Chapter 2 focuses on the functionalization strategies for single-walled nanotubes in order to achieve their nanoscale dispersion in epoxy matrices. Chapter 3 provides insights into the multiscale modeling of the properties of the polymer nanotube nanocomposites. Chapter 4 provides perspectives on the electron microscopy characterization of the polymer nanotube nanocomposites. In Chapter 5, the use of polymer nanotube nanocomposites for transfemoral sockets is described. Chapter 6 presents an overview of the different meth-odologies to achieve micro-patterning of polymer nanotube nanocom-posites. An overview of recent progress on hybridization modifications of CNTs with carbon nanomaterials and their further applications in polymer nanocomposites is given in Chapter 7. Chapter 8 provides details on the foams generated with polymer nanotube nanocomposites and concludes that hybrid materials based on metallic honeycombs filled with polymer-carbon nanotube foams and sheets built from different layers of polymer foams display excellent electromagnetic absorption, confirming their high potential for EMI shielding. Chapter 9 provides information on the syn-thesis and properties of polycarbonate nanocomposites. Chapters 10 and
xiv Preface
11 focus on the advanced microscopy techniques used for understanding polymer/nanotube composite interfaces and properties. Chapter 12 con-cludes the volume by summarizing the latest challenges as well as perspec-tives for the future of polymer nanotube nanocomposite materials.
Dr. Vikas Mittal Abu Dhabi
May 2014
Vikas Mittal (ed.) Polymer Nanotube Nanocomposites, (1–44) 2014 © Scrivener Publishing LLC
1
1Polymer Nanotube Nanocomposites:
A Review of Synthesis Methods, Properties and Applications
Joel Fawaz and Vikas Mittal*
Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, UAE
AbstractOwing to their high mechanical and electrical properties, nanotubes are ideal fillers for the generation of composites. Polymer nanotube nanocomposites are synthesized after achieving suitable surface modifications of the nanotubes using different synthesis methods like melt mixing, in-situ polymerization and solution mixing. All these methods have their own advantages and limitations and vary-ing degrees of success in achieving nanoscale dispersion of nanotubes in addi-tion to achieving significant property enhancement in the composite properties. The tensile modulus is generally reported to be significantly enhanced on the incorporation of even small amounts of nanotubes. Though the tensile strength and elongation at break in many cases are reported to improve, they are more likely dependent on the morphology of the nanocomposites. The glass transition temperature as well as degradation temperature are also observed to significantly increase mostly owing to the reinforcing effect of nanotubes. Many other proper-ties like electrical conductivity, heat deflection temperature, etc., also increase on the addition of nanotubes to the polymer.
Keywords: Nanocomposites, dispersion, aspect ratio, in situ, melt, morphology, tensile properties, glass transition temperature, degradation, functionalization, electrical conductivity, resistivity
*Corresponding author: [email protected]
2 Polymer Nanotube Nanocomposites
1.1 Introduction
Many experimental and theoretical studies have reported the modulus of the nanotubes to be in the same range as graphite fibers and even the strength at least an order of magnitude higher than the graphite fibers [1–11]. In any case, even if the real mechanical properties of nanotubes are actually somewhat lower than the estimated values, nanotubes still repre-sent high potential filler materials for the synthesis of polymer nanocom-posites. The surface area per unit volume of nanotubes is also much larger than the other filler fibers, leading to much larger nanotube/matrix inter-facial area in the nanotube-reinforced composites than in traditional fiber-reinforced composites. Figure 1.1 represents such an interface polymer fraction in nanotube-reinforced polymers where the ratio of the thickness t of the interphase versus the inclusion radius rf is plotted with respect to the volume fraction of the inclusion [1]. Owing to the interfacial contacts with the nanotubes, the interfacial polymer has much different properties than the bulk polymer. The conversion of a large amount of polymer into interface polymer fraction due to the nanoscale dispersion and high sur-face area of nanotubes generates altogether different morphology in the nanotube nanocomposites, which results in the synergistic improvement in the nanocomposite properties. In order to achieve optimized interfacial interactions between the polymer and nanotubes, nanoscale dispersion
1.0
0.050.250.501.0
Nanotube
Carbon fiber
Interphase
polymer
Nanotube/fiber Balk
polymer
t/rf
0.8
Fra
ctio
n o
f in
terp
ha
se p
oly
me
r
0.6
0.4
0.2
0.00.0 0.1 0.2 0.3 0.4
Fiber volume fraction
0.5 0.6
r1
t
Figure 1.1 Fraction of interphase polymer as a function of volume fraction of fiber inclusion, where t is the interphase thickness and rf is the radius of the nanotube/fiber inclusion. Reproduced from [1] with permission from Elsevier.
Polymer Nanotube Nanocomposites: A Review 3
of the filler is required, which necessitates compatibilization of the poly-mer and inorganic phases. Therefore, the nanotubes need to be surface modified before their incorporation into the polymer matrix. Therefore, as CNTs agglomerate, bundle together and entangle, it may lead to defect sites in the composites, subsequently limiting the impact of CNTs on nanocom-posite properties. Salvetat et al. [12] studied the effect of CNTs dispersion on the mechanical properties of nanotube-reinforced nanocomposites, and it was observed that poor dispersion and rope-like entanglement of CNTs caused significant weakening of the composites. Thus, alignment of CNTs is also equally important to enhance the properties of polymer/CNT composites [13,14]. Stress transfer property of the nanotubes in the com-posites is another parameter which controls the mechanical performance of the composite materials. Many studies using tensile tests on nanotube/polymer nanocomposites have reported the bonding behavior between the nanotubes and the matrix [15,16], in which there was an interfacial shear strength ranging from 35 to 376 MPa. The range of values was due to the different diameters of the nanotubes and the number of wall lay-ers. However, other behaviors have also been reported based on interfacial compatibility. In their study, Lau and Hui [17] observed that most of the nanotubes were pulled out during the tensile testing owing to no interac-tion at the interface.
It has also been reported that in the case of multiwalled nanotubes, the inner layers of nanotubes cannot effectively take any tensile loads applied at both ends owing to the weak stress transferability between the layers of the nanotubes [8,18]. This results in the outmost layer of the nanotubes taking the entire load. As a result, the failure of the multiwalled nanotubes could start at the outermost layer by breaking the bonds among carbon atoms.
Nanotube nanocomposites with a large number of polymer matrices have been reported in recent years. The composites were synthesized in order to enhance mechanical, thermal and electrical properties of the conventional polymers so as to expand their spectrum of applications. Different synthesis routes have also been developed in order to achieve nanocomposites. The generated morphology in the composites and the resulting composite properties were reported to be affected by the nature of the polymer, nature of the nanotube modification, synthesis process, amount of the inorganic filler, etc. This chapter reviews nanocompos-ite structures and properties reported in a few of these reports and also stresses the future potential of nanotube nanocomposites by mentioning some of their reported applications. Recent reviews were published and can be found in [19–21].
4 Polymer Nanotube Nanocomposites
1.2 Methods of Nanotube Nanocomposites Synthesis
1.2.1 Direct Mixing This method, unlike the others, is used only for thermoset polymers. The carbon nanotubes are dispersed into a low viscosity thermosetting resin, usually epoxy, by mechanical mixing or sonication [22]. Afterwards, the mixture is cured to produce the nanocomposite. Another direct mixing technique involves the use of solvent to lower the viscosity of the epoxy resin [23]. The CNTs are first exfoliated in ethanol under sonication before mixing them with the epoxy resin. Once dispersion is obtained, the sol-vent is evaporated and hardener is added to trap the CNTs in the polymer matrix.
1.2.2 Solution MixingSolution mixing method has the advantage that the viscosity of the system can be controlled to be low so as to achieve higher extents of nanotube dis-persion in the polymer systems. Both thermoset and thermoplastic poly-mers can be employed using this approach to achieve nanocomposites. The disadvantage associated with this method is, however, the requirement of a large amount of solvent for the nanocomposite synthesis, which for industrial applications may not be environmentally friendly or cost effec-tive. For thermoset nanocomposites, one can also use the prepolymer to disperse the nanotubes and the prepolymer can then be crosslinked dur-ing the evaporation of solvent. Suhr et al. [19] reported the solution mix-ing approach as shown in Figure 1.2 for the synthesis of polycarbonate nanocomposites. The nanotubes were first oxidized in nitric acid before dispersion as the acidic groups on the sidewalls of the nanotubes can inter-act with the carbonate groups in the polycarbonate chains. To achieve nanocomposites, the oxidized nanotubes were dispersed in THF and were added to a separate solution of polycarbonate in THF. The suspension was then precipitated in methanol and the precipitated nanocomposite mate-rial was recovered by filtration. From the scanning electron microscopy investigation of the fracture surface of nanotubes, the authors observed a uniform distribution of the nanotubes in the polycarbonate matrix [24].
Similarly, Biercuk et al. [25] reported the use of a solution mixing approach for the synthesis of epoxy nanocomposites. Epoxy prepolymer was dissolved in solvent in which the CNTs were also uniformly dispersed. The solvent was subsequently evaporated, and the epoxy prepolymer was crosslinked. The resulting nanocomposite was reported to have a good
Polymer Nanotube Nanocomposites: A Review 5
dispersion of nanotubes. In other studies, multiwalled nanotubes were mixed in toluene in which polystyrene polymer was dissolved [26, 27] to generate polystyrene nanocomposites. The nanocomposites were gener-ated both by film-casting and spin-casting processes. The solution mix-ing method has also been used to attain alignment of the nanotubes in the composites [28,29]. Aspect ratio and rigidity of the nanotubes were reported to be the two factors which affect the alignment of the nanotubes. If the nanotubes were longer and more flexible, the alignment of the nano-tubes in the composites was observed to deteriorate [30,31]. Stretching the cast film of the nanocomposite synthesized by the solution-mixing method resulted in the improvement of the nanotube alignment [30].
Liu and Choi [32] reported high quality dispersion of MWNTs at con-centrations up to 9 wt% in poly(dimethylsiloxane) (PDMS) matrix using solution mixing. For better dispersion, a systemic study was conducted to determine the optimal solvent for both CNTs and PDMS. Chloroform was selected over the other common solvents, such as THF and DMF, due to its
As-received SWNTs in
Nitric Acid (oxidation)Oxidized SWNTs in THF Polycarbonate in THF
Mix-up Together
Filtration
Precipitation in Methanol
Compressive Mold Nanotube-Composite
Figure 1.2 Schematic of synthesis of CNT polycarbonate nanocomposites by solution mixing approach. Reproduced from [24] with permission from American Chemical Society.
6 Polymer Nanotube Nanocomposites
high solubility of the components and stability of the mixture. Moreover, functionalization of the CNTs by carboxyl groups further enhanced dis-persion. The nanocomposite synthesis entailed the initial dispersion of fMWNTs in chloroform which was then sonicated for 1 hour. Meanwhile, PDMS base resin was dissolved in chloroform and magnetically stirred for 15 minutes. The separate mixtures were mixed together and sonicated for 1–2 hours. Solvent evaporation was efficiently performed by apply-ing vacuum at elevated controlled temperatures. This process enabled the retainment of the initial dispersion as can be seen in the SEM images in Figure 1.3.
Polyvinyl alcohol (PVA)/MWCNT nanocomposite membranes were reported by Shirazi et al. [33] as a means of dehydrating isopropanol. The nanocomposites were prepared by solution mixing in which PVA was dis-solved and stirred in deionized water at 90°C followed by filtration and removal of bubbles by vacuum. CNTs are then added to the solution and ultrasonicated for 4 hours followed by the use of crosslinker and a cata-lyst. Figure 1.4 illustrates the procedure followed to prepare the membrane. Good dispersion of MWCNTs was achieved up to 2 wt% loading; whereas, increasing the loading above 2 wt% tended to cause agglomeration.
Figure 1.3 SEM micrographs of fracture surfaces of PDMS nanotube composities containing 7 wt% filler at (a) 160x, (b) 1000x, (c) 3000x and (d) 10,000x. Reproduced from [32] with permission from Multidisciplinary Digital Publishing Institute.
Polymer Nanotube Nanocomposites: A Review 7
Moreover, when measuring the outer diameter of the nanotubes in the 2 wt% loading nanocomposite, it was found to be similar to that of the neat CNT. On the other hand, in the 4 wt% loading nanocomposites, the diam-eter was measured to be higher, signaling the formation of CNT bundles.
Martone et al. [34] compared solution and direct mixing in terms of dispersion. Different solvents (ethanol, acetone and sodium dodecyl sul-fate aqueous surfactant) and dispersion techniques (magnetic, mechanical and sonication) were used to disperse the CNTs in an epoxy matrix. It was observed that direct mixing using sonication yielded submicron and more uniform texture compared to other methods, as seen in Figure 1.5.
1.2.3 In-Situ PolymerizationThis mode of nanocomposite synthesis is beneficial owing to the fact that the nanotube dispersion can be achieved in a solvent in which monomer is also dissolved or suspended. The low viscosities encountered during this process lead to better dispersion of nanotubes. The subsequent polymer-ization of monomer then leads to the uniform intercalation of polymer around the nanotubes. In many instances, the generated polymer can also be chemically grafted to the nanotube surfaces either by the acidic func-tionalities generated on the surface by chemical treatment or by direct grafting of polymer chains from the surface of the nanotubes by using sur-face immobilized initiators. Barazza et al. used miniemulsion approach to achieve polystyrene nanocomposites [35]. Cetyltrimethylammonium bro-mide (CTAB) and sodium dodecyl sulphate (SDS) were used to function-alize the nanotubes. Hexadecane was used as a costabilizer and oil-soluble
Figure 1.4 Flow diagram describing the procedure of PVA nanocomposite membranes. Reproduced from [33] with permission from Elsevier.
as-grown CNTs PVA PVA membrane
heat treatment
drying in ambient
temperature
castingcrosslinker
and catalyst
sonication
sonication
PVA Solutionoxidation with air
acid treatment
8 Polymer Nanotube Nanocomposites
initiator AIBN was used for the polymerization. After the polymerization, the whole reaction contents were poured into a large volume of pure iso-propyl alcohol to recover the nanocomposite. The incorporation of nano-tubes in the polymer matrix was successfully achieved as demonstrated in Figure 1.6. The incorporation of nanotubes resulted in the black col-oration of the nanocomposites materials as well as significant reduction in the electrical resistivity of the composite material. Raman spectra for the composite material also indicated a reduced vibrational freedom of the polymer chains as a consequence of the nanotube incorporation. An adsorbed polymer layer on the nanotube bundles was achieved as shown in Figure 1.6 which was observed to contribute to a better dispersion of the nanotubes.
Velasco-Santos et al. [36] also reported the in-situ polymerization of methyl methacrylate with both the treated and untreated nanotubes to generate polymer nanocomposites. The amount of initiator AIBN, reac-tion time and temperature were controlled to tune the molecular weight of polymer in the composites. The treated nanotubes had COOH and COO- functionalities on the sidewalls as well as tips and resulted in bet-ter property enhancement of the composites as compared to the untreated
Figure 1.5 Optical micrographs for epoxy nanocomposites prepared via solution mixing in (a) acetone, (b) surfactant, (c) ethanol and (d) via direct mixing using sonication. Reproduced from [34] with permission from BMP-PT.
Polymer Nanotube Nanocomposites: A Review 9
nanotubes. The authors suggested that the use of in-situ polymerization as well as functionalization of the nanotubes lead to the synergistic reinforce-ment of the organic and inorganic components of the composite.
Some studies on the grafting of the polymer chains from the surface of the nanotubes have also been reported. Qin et al. reported the polym-erization of n-butyl methacrylate from the surface of nanotubes by using controlled living polymerization method [37]. Gao et al. also followed the similar grafting from the surface approach and polymerized methyl meth-acrylate on the surface by using atom transfer radical polymerization [38]. Figure 1.7 shows the schematic of the process in which an atom transfer radical polymerization initiator was covalently immobilized on the surface of the nanotubes which was subsequently used to graft polymer brushes
Figure 1.6 TEM micrographs showing nanotube bundles with an adsorbed polystyrene layer in a 8.5% weight SWNT-PS composite. Reproduced from [35] with permission from American Chemical Society.
Figure 1.7 Schematic of grafting of PMMA chains from the surface of nanotubes using atom transfer radical polymerization. Reproduced from [38] with permission from American Chemical Society.
Adsorbed polymer layer
7 nm
MWNT MWNT-Br MWNT-PMMA
O
Br3)
O
Br
O O
O
O
1) HNO3, SOCl2,
2) HOCH2CH2OH4)
CuBr/PMDETA
MMA, 60oC
O O
O
Br
Br
O
O O
O
Br
O
O O
O
Br
O
n
OCH3O
O O
O
Br
O
n
OCH3O
O
O O
O
Br
O
n
OCH3O
10 Polymer Nanotube Nanocomposites
from the surface. The use of controlled polymerization methods allow the benefits to control the molecular characteristics of the polymer grafts thus allowing to tune the properties of the hybrids. A variety of polymer archi-tectures like block copolymers, multi-arm brushes, etc., can also be grafted by using the controlled polymerization mode.
Bai et al. [39] reported the synthesis of grafted poly(3,4-ethylenedioxy-thiophene) (PEDOT)/MWNT composite using in-situ oxidative polymer-ization. This polymerization was conducted in a ternary phase system. The MWCNTs were sonicated with AOT surfactant dissolved in p-xylene, fol-lowed by the addition of FeCl3 solution. Eventually, the monomer EDOT was added drop-wise to the suspension with a reaction time of 24 hours. Lastly, the product was obtained through thorough washing and vacuum drying. The use of AOT surfactant allowed the uniform dispersion and sta-bility of the nanotubes. The composition of the nanocomposite was found to have 8 wt% MWCNTs. Moreover, it was determined that the PEDOT grafted itself on the walls of MWCNTs creating a 3-dimensional network that gives rise to excellent capacitor properties.
Mansourpanah et al. [40] synthesized polycaprolactone-modified MWCNTs (PCL-MWCNTs) followed by the fabrication of polyethersul-fone (PES)/PCL-MWCNTs. This was carried out using a variation of in situ polymerization and solution mixing. PCL-MWCNTs were first prepared by activating the CNTs in acid medium of H2S and HNO3 under reflux for 10 hours and later cooled, cleaned and dried. ε-caprolactone and modi-fied CNTs were added together and sonicated at controlled elevated tem-peratures. PCL-MWCNTs were extracted by precipitation. On the other hand, PES was dissolved in DMA and PVP; whereas different concentra-tions of PCL-MWCNTs were dissolved in choloform. The two mixtures were mixed and stirred at 50°C and 200 rpm for 5 hours . However, using a film applicator to prepare PES/PCL-MWCNT membranes, the evapora-tion step was cancelled and instead immersed in a water bath to remove the solvent and other water-soluble polymer. This procedure allowed for good dispersion as well as enhanced porosity.
Dash et al. [41] reported the synthesis of Poly(anthranilic acid) (PAA)/MWCNT composites via in situ chemical oxidative polymerization. The CNTs were first functionalized using H2SO4 and HNO3 to provide carboxylic acid groups at the surface. Then, the functionalized MWCNTs were soni-cated in a 1.2 M HCL solution for 2 hours before adding aniline and anthra-nilic acid to the suspension. APS reagent in HCL solution was added to the mixture and mechanically stirred. The co-polymer products obtained were filtered, washed and vacuum dried. SEM analysis showed that the diameter of the nanocomposite increased with increasing MWNT loading as PAA
Polymer Nanotube Nanocomposites: A Review 11
coats itself on the outer surface of the nanotubes, as shown in Figure 1.8. Dash et al. [41] stated that the coating arises from the strong interactions between the comonomer (i.e., aniline) and the functionalized MWNTs.
Wu and Liu [42] prepared PS/MWCNTs via solution free radical in situ polymerization. Without any pretreatment of MWCNTs, they were com-bined with styrene monomers, toluene and AIBN initiators and the mix-ture was heated at 90°C for 11 hours. The product was precipitated and vacuum dried. FTIR analysis concluded the successful grafting of PS onto the walls of CNTs. Moreover, qualitative relationships between initiator and temperature with monomer conversion and polymer grafting were established by the authors. It was noted that with increase of AIBN initia-tor, monomer conversion increases. However, the highest grafting% was achieved with 0.05 g AIBN. Increasing the polymerization temperature increases both grafting and conversion, as shown in Table 1.1.
Li and Kim [43] reported the synthesis of polyaniline (PANI)/MWCNT composites for sensor application. The synthesis was conducted via in situ oxidation polymerization in which the aniline monomers and MWCNTs were added to 1 M HCL followed by the addition of the ammonium per-sulfate (APS) reagent solution. The mixture was stirred for 2 hours at room temperature then the product obtained was filtered and washed. Core and shell structures were visible in SEM images signaling the typical struc-ture of polymer grafted nanocomposites and the diameter increased with increasing MWCNT.
Figure 1.8 SEM micrographs of (a) MWCNT, (b) c-MWCNT, (c) neat anthranilic acid, (d) PAA/c-MWCNT at 2 wt%, (e) PAA/c-MWCNT at 5 wt% and (f) PAA/c-MWCNT at 10 wt% filler content. Reproduced from [41] with permission from Springer.
12 Polymer Nanotube Nanocomposites
1.2.4 Melt MixingMelt mixing of polymer with the inorganic filler is a very attractive tech-nique to synthesize nanocomposites using a large variety of polymers. This technique has also been exploited in great details for the polymer clay systems and the generated knowledge and experience is applicable also to polymer nanotube nanocomposites in many ways. The advantage of this technique is the direct mixing of the polymer at high temperature with the filler thus requiring no solvent which makes this process more industri-ally attractive as well as environmentally friendly. The nanotubes have also been reported to have a lesser extent of fiber breakage during compound-ing in melt [44, 45]. Alig et al. [46] discusses in depth the relation between processing conditions, CNT dispersion and filler network morphology with the properties generated. It is stated that dispersion of CNTs involves several steps:
1. Wetting of initial agglomerates by the polymer. 2. Infiltration of polymer chains into the initial agglomerates
to weaken them.3. Dispersion of agglomerates by rupture and erosion.4. Distribution of individualized nanotubes into the matrix.
Table 1.1 Effect of polymerizing conditions on monomer conversion and polymer grafting percentages for PS nanocomposites. Reproduced from [42] with permission from Taylor & Francis.
MWCNTs-PS Polymerizing temperature (°C)
AIBN added (g)
C% of St PG%
1 90 0.01 9.9 2.92 90 0.02 30.5 4.93 90 0.05 39.0 15.64 90 0.10 55.2 4.25 90 0.15 58.3 0.86 90 0.20 59.7 0.87 80 0.05 34.1 2.28 70 0.05 19.0 1.59 60 0.05 13.1 0.910 50 0.05 9.0 0.6
Polymer Nanotube Nanocomposites: A Review 13
The dispersion is generally improved because of the presence of high extents of shear in melt compounding equipments. Longer processing times also lead to better mixing of organic and inorganic phases and align-ment of the nanotubes in the composites can also be improved when elon-gational flow is additionally applied. However, melt mixing may also lead to serious degradation of the polymer if the compounding temperature is too high or very long processing times are used. The organic surface modifications immobilized on the sidewalls of the nanotubes are also prone to thermal damage during the compounding thus requiring an opti-mal mixing temperature and mixing time which do not cause the thermal damage but are also high and long enough to ensure homogenous mixing. Increasing the mixing temperature would lower the viscosity of the poly-mer matrix and this in turn makes the dispersion worse [46]. It is impor-tant to note that this depends on the screw speed as well as the polymer grade. Moreover, high loadings of nanotubes limit wetting and infiltration and result in bigger agglomerates. Figures 1.9 and 1.10 illustrate the effects of mixing speed and mixing temperature on area ratio, degree of dispersion and distribution coefficient; respectively for MWNT/PC nanocomposites [46]. The higher distribution coefficient reflects an increase in agglomera-tion tendency.
Pötschke et al. [47] reported the polycarbonate nanocomposites by melt mixing method using twin-screw co-rotating intermeshing extruder.
14
12
10
8
6
4
2
00 50 100
Mixing speed (rpm)
Are
a r
ati
o, A
A (
%)
150 200 250
Low viscosityMedium viscosityHigh viscosity
300
Figure 1.9 Area ratio vs mixing speed for PC nanocomposites with different viscosities and at 1 wt% filler. Reproduced from [46] with permission from Elsevier.
14 Polymer Nanotube Nanocomposites
Compounding temperature of 240°C, screw speed of 280 rpm and a feed rate of 980 g/h were used for the composite generation. The SEM investiga-tions of the polymer nanotube masterbatches revealed random orientation of nanotubes and formation of interconnecting structures. The authors also reported that it was not possible to estimate fiber length from the micro-graphs owing to the complex nanotube network. The diameters of the nano-tubes in the composites were observed to in the range of 10 to 50 nm which is higher than the other studies reporting the diameter in the range of 10 to 15 nm. It was suggested that a thick polycarbonate layer existed on the surface of nanotubes thus increasing the diameter as well as indicating some extents of interphase mixing or phase adhesion. Maiti et al. [48] reported the synthe-sis of PC/PCL-MWCNT nanocomposites using melt mixing. A masterbatch of PCL-MWCNT was first prepared via melt mixing using internal mixer at 65°C and 60 rpm for 10 min. Then, the masterbatch was melt blended with pure PC at 280°C and 60 rpm for 10 min. This procedure yielded a homoge-neous dispersion of CNTs at low loadings as analyzed in SEM.
CNT/LLDPE nanocomposite fibers were synthesized using melt extruder as reported by Mezghani et al. [49]. The melted LLDPE pellets were mixed with aligned MWNTs using 24 mm diameter Thermo Haake twin screw extruder of length 40D. The temperature was maintained 160°C whereas the last zone of the extruder was maintained at 180°C. The spinneret die was used to produce the fibers with average extruded fiber diameter of 620 m which were air-cooled and drawn (6x) at room temperature. Good distribution of the CNTs with no agglomeration in the LLDPE matrix was noted due to its passing through 3 mixing stages. Moreover, alignment of CNTs was present due to drawing of the fibers.
1.0
0.8
0.48 0.23 0.1 0.05
s / MPas / MPa
0.02 0.48 0.23 0.1 0.05 0.02
0.6
0.4
0.2
0.0 0.0
1.0
1.5
2.0
(a) (b)
QP /
-
DT
EM
/-
0 220 240 260
TM/ oC
280 300 0 220 240 260
TM/ oC
280 300
Figure 1.10 Mixing temperature (TM) vs (a) degree of dispersion (b) distribution coefficient for PC nanocomposites at 1 wt% prepared via melt mixing at 50 rpm and 5 min. Reproduced from [46] with permission from Elsevier.
