Polymer Nanotube Nanocomposites · 1.2 Properties 4 1.3 Synthesis 8 References 12 . 2. Overview of Polymer Nanotube Nanocomposites 15 . V. Mittal . 2.1 Introduction 15 2.2 Methods

Polymer Nanotube Nanocomposites

Synthesis, Properties, and Applications

Vikas Mittal BASF SE, Polymer Research, Germany

Scrivener

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Synthesis, Properties, and Applications

Vikas Mittal BASF SE, Polymer Research, Germany

Scrivener

©WILEY

Copyright © 2010 by Scrivener Publishing LLC. All rights reserved.

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Contents

Preface xiii

1. Carbon Nanotubes: An Introduction 1 V. Mittal 1.1 Introduction 1 1.2 Properties 4 1.3 Synthesis 8

References 12

2. Overview of Polymer Nanotube Nanocomposites 15 V. Mittal 2.1 Introduction 15 2.2 Methods of Nanotube Nanocomposites Synthesis 18

2.2.1 Solution Mixing 18 2.2.2 In-Situ Polymerization 20 2.2.3 Melt Mixing 22

2.3 Properties of Polymer Nanotube Nanocomposites 25 2.3.1 Mechanical Properties 25 2.3.2 Thermal and Electrical Properties 34 References 42

3. New Microscopy Techniques for a Better Understanding of the Polymer/Nanotube Composite Properties 45 K. Masenelli-Varlot, A. Bogner, C. Gauthier, L. Chazeau and J.Y. Cavaillé 3.1 Introduction 46 3.2 Near Field Microscopy 47

3.2.1 Principles of STM and AFM 48 3.2.2 Near Field Microscopy for Nanotubes 50 3.2.3 AFM and CNT Composites 50

vi CONTENTS

3.3 Transmission Electron Microscopy 52 3.3.1 Principles 52 3.3.2 Characterisation of Carbon Nanotubes 56 3.3.3 Characterisation of Polymer/

Nanotube Composites 58 3.4 Scanning Electron Microscopy 67

3.4.1 Overview of the Technique (SEI, BEI, CCI) 67 3.4.2 Application to the Study of Nanotubes 68 3.4.3 For Polymer CNT/Nanocomposites 69 3.4.4 Development of New Imaging Modes 72

3.5 Conclusions 75 References 77

4. Polymer Nanocomposites with Clay and Carbon Nanotubes 83 Qiang Fu, Changyu Tang, Hua Oeng and Qin Zhang 4.1 Introduction 83 4.2 Electrical Properties of Polymer

Composites with Clay and CNTs 86 4.3 Mechanical Properties of Polymer

Composites with Clay and CNTs 89 4.4 Thermal and Flame Properties of Polymer

Composites with Clay and CNTs 100 4.5 Conclusion and Future Outlook 106

Acknowledgement 108 References 109

5. Polyethylene Nanotube Nanocomposites 113 S. Kanagaraj 5.1 Introduction 113 5.2 Surface Modification of Carbon Nanotubes 115 5.3 Dispersion of Nanotubes in Polyethylene Matrix 117 5.4 Method of Preparation of CNT-PE Composites 119 5.5 Interfacial Bonding and Load Transfer 123 5.6 Material Characterization 126 5.7 Conclusions 135

Acknowledgement 136 References 136

CONTENTS vii

Properties of Polyurethane/Carbon Nanotube Nanocomposites Tianxi Liu and Shuzhong Guo 6.1 Introduction 6.2 Preparation of CNT-Based

Polyurethane Nanocomposites 6.2.1 Melt-Mixing 6.2.2 Solution Casting 6.2.3 In-Situ Polymerization 6.2.4 Sol-Gel Approach

6.3 Functionalization, Dispersion Morphology and Micro-/Nano-structures

6.4 Physical Properties 6.4.1 Mechanical Properties 6.4.2 Thermal Conductivity 6.4.3 Thermal Stability and Degradation 6.4.4 Fire Retardancy 6.4.5 Rheological Properties 6.4.6 Electrical Conductivity 6.4.7 Water Vapor Transport Properties 6.4.8 Shape Memory 6.4.9 Special Properties Related to Bio-Applications 6.4.10 Microwave Absorption 6.4.11 UV-Protection

6.5 Applications 6.6 Conclusions

Acknowledgements References

141

141

144 144 145 146 147

148 151 151 154 155 156 157 158 161 162 165 166 167 168 170 171 171

7. Properties of PMMA/Carbon Nanotubes Nanocomposites 177 R.B. Mathur, Shailaja Pande and B.P. Singh 7.1 Introduction 177 7.2 Fabrication/Processing of CNT-PMM A Composites 179

7.2.1 Solution Processing 181 7.2.2 Melt-Processing 183 7.2.3 In-Situ Polymerization Processing 185

viii CONTENTS

7.2.4 Coagulation Method 186 7.2.5 Surfactant, Compatibilizers and Co-Solvent

Assisted CNT-PMMA Composites 188 7.2.6 Chemical Modification of CNTs for Processing

of Nanocomposites 189 7.3 Mechanical Properties of CNT-PMMA Composites 190 7.4 Electrical Properties of CNT-PMMA Composites 198

7.4.1 Electrical Conductivity 198 7.4.2 Electromagnetic Interference (EMI) Shielding 200

7.5 Thermal Properties 204 7.6 Conclusion 216

References 216

8. Synthesis of Vinyl Polymer/Carbon Nanotube Nanocomposites Prepared by Suspension Polymerization and Their Properties 221 P. Slobodian 8.1 Introduction 221 8.2 Free Radical Polymerization 223 8.3 Suspension and Bulk Polymerization Techniques 225 8.4 In-situ Radical Polymerization in Presence of CNT 228

8.4.1 Polymer Matrix 228 8.4.2 Addition of Radicals onto CNT 229 8.4.3 CNT Degradation 232 8.4.4 Additional Analyses 233

8.5 Polymer/CNT Composite Microspheres 235 8.5.1 CNT Material Adsorbed onto

Polymer Microspheres 239 8.6 Electrorheology of Polymer/CNT Nanocomposites

Prepared by in-situ Suspension Polymerization 243 References 246

9. Polylactide-Based Carbon Nanotube Nanocomposites 249 Srikanth Pilla, Shaoqin Gong and Lih-Sheng Turng 9.1 Introduction 249 9.2 Synthesis of PLA 252 9.3 Carbon Nanotubes 254

CONTENTS ix

9.4 Preparation of PLA-CNT Nanocomposites 255 9.5 Viscoelastic Properties 264 9.6 Thermal Properties 268 9.7 Mechanical Properties 269 9.8 Thermal Degradation Properties 272 9.9 Electrical Conductivity Properties 273 9.10 Biodegradability 274 9.11 Applications 275 9.12 Conclusions 275

Acknowledgements 276 Note 276 References 276

10. Synthesis and Properties of PEEK/Carbon Nanotube Nanocomposites 281 A.M. Diez-Pascual, J.M. González-Domínguez, Y. Marttnez-Rubi, M. Naffakh, A. Ansón, M.T. Martínez, B. Simará ana M.A. Gómez 10.1 Introduction 281 10.2 Poly(ether ether ketone)s: Structure, Synthesis

and Properties 283 10.3 Synthesis, Purification and Characterization

of the SWCNTs 285 10.3.1 Synthesis of Laser-Grown SWCNTs 285 10.3.2 Synthesis and Purification

of Arc-Grown SWCNTs 285 10.3.3 Characterization of the Single-Walled

Carbon Nanotubes 287 10.4 Integration of the Carbon Nanotubes

in the PEEK Matrix 290 10.4.1 Covalent Grafting in Carbon Nanotube/PEEK

Nanocomposites 291 10.4.2 Wrapping of the SWCNTs

in Compatibilizing Agents 292 10.4.3 Pre-Mixing Stage and Melt Blending Approach 294

10.5 Characterization of PEEK/Carbon Nanotube Nanocomposites 295

x CONTENTS

10.5.1 Morphology 295 10.5.2 Thermogravimetric Study 297 10.5.3 Differential Scanning Calorimetry 298 10.5.4 X-ray Diffraction Analysis 303 10.5.5 Mechanical Properties 304 10.5.6 Electrical and Thermal Conductivity 307

10.6 Concluding Remarks 309 Acknowledgements 310 Glossary of Abbreviations 310 References 311

11. Synthesis and Properties of PVA/Carbon Nanotube Nanocomposites 315 C. Mercader, P. Poulin and C. Zakri 11.1 Introduction 315

11.1.1 Mechanical and Electrical Properties of Carbon Nanotubes 316

11.1.2 Why Use Nanotubes in Nanocomposites? 317 11.1.3 Poly(vinyl) Alcohol 319

11.2 Synthesis Methods and Structural Properties of Nanotubes / PVA Composites 320 11.2.1 Films 320 11.2.2 Fibers 324

11.3 Mechanical Properties of the Composites 327 11.3.1 Reinforcement 328 11.3.2 Stress Transfer Efficiency 331

11.4 Electrical Properties 333 11.5 Other Original Properties of PVA/

Nanotube Composites 335 11.5.1 Energy Absorption 335 11.5.2 Shape Memory Effect 337

11.6 Conclusion 339 References 340

12. Elastomers Filled with Carbon Nanotubes 345 Liliane Bokobza 12.1 Introduction 345 12.2 Composite Processing 347 12.3 Electrical Properties 350

CONTENTS xi

12.4 Mechanical Properties 355 12.4.1 Tensile and Swelling Behaviors 355 12.4.2 Dynamic Mechanical Analysis (DMA) and

Differential Scanning Calorimetry (DSC) 361 12.5 Spectroscopic Characterization 364 12.6 Thermal Stability 367 12.7 Conclusions 369

References 369

13. Specific Interactions Induced Controlled Dispersion of Multiwall Carbon Nanotubes in Co-Continuous Polymer Blends 373 Suryasarathi Bose, Arup R. Bhattacharyya, Rupesh A. Khare and Ajit R. Kulkarni 13.1 Introduction 373 13.2 Experimental 376

13.2.1 Materials 376 13.2.2 Preparation of the Modifiers 376 13.2.3 Melt Blending 377 13.2.4 Characterization 378

13.3 Results and Discussion 379 13.3.1 Specific Interactions: Spectroscopic and

Microscopic Evidences 379 13.3.2 AC Electrical Conductivity Measurements:

Assessing the State of Dispersion of MWNTs 381 13.3.3 Phase Morphology and Selective Localization

of MWNTs in the Blends 383 13.3.4 Melt-Interfacial Interactions 386

13.4 Summary 387 Acknowledgements 388 References 388

14. Effect of Structure and Morphology on the Tensile Properties of Polymer/Carbon Nanotube Nanocomposites 391 Jtngjing Qiu and Shiren Wang 14.1 Background 391 14.2 Structure and Morphology Characterization 392

14.2.1 Structure and Mechanical Properties of CNTs 392

xii CONTENTS

14.2.2 Characterization of CNTs/Polymer Composites 395 14.2.3 Structure and Tensile Properties of CNTs/

Polymer Composites 396 14.3 Concluding Remarks 417

References 418

15. Polymer Nanotube Composites: Promises and Current Challenges 423 Atnal M.K. Esawi and Mahmoud M. Farag 15.1 Carbon Nanotubes 424

15.1.1 Background 424 15.1.2 Synthesis of CNTs 425 15.1.3 Fabrication of CNT Polymer Composites 426 15.1.4 Electrical Properties of CNT Polymer

Composites 429 15.1.5 Mechanical Properties of CNT

Polymer Composites 432 15.2 Case Studies 435

15.2.1 Case Study: CNT-Based Strain Sensor 435 15.2.2 Case Study: Technical and Economic

Feasibility of Using CNT-Based Composites in Aerospace Applications 440

15.3 Conclusions 445 References 446

Index 449

Preface

Nanotube materials are the one of the best examples of novel nanostructures derived by bottom-up chemical synthesis processes. The chemical composition and atomic bonding configuration pres-ent in nanotubes is simple; however, these materials represent diverse structure-property relations among the nano-materials. Since the early discoveries of the nanotube materials, a large number of research studies have been devoted to demonstrate the potential of the nanotubes in improving the mechanical, electrical and thermal properties of materials. Polymer nanotube nanocompos-ites represent an important class of such hybrid materials where the nanotubes are embedded in the polymer matrices by employing various synthesis methodologies including solution casting, melt blending or in-situ polymerization in the presence of nanotubes. Uniform dispersion of nanotubes in the polymer matrix is of utmost importance in order to obtain an optimal improvement in the properties of the polymer at low fractions of nanotubes. Thus it is required to optimize the compatibility between the organic and inorganic phases as well as processing methodologies. The current book intends to assimilate the various polymer nanotube systems reported in the literature to underline the high potential of nanotubes as fillers and to provide pathways for the large scale commercial application of the nanotube nanocomposites.

Chapter 1 describes the properties and synthesis of nanotubes. It is clear from the properties of the nanotubes that tremendous gains in the properties of the composites can be achieved if the nanotubes and polymer phases are optimally mixed. Chapter 2 reviews the numerous polymer nanotube composite systems reporting superior composite properties and thus justifying the ever-increasing use of nanotubes as fillers for composite materials. Chapter 3 deals with the use of electron microscopy methods along with new

xm

xiv PREFACE

advancements in understanding the microstructure of polymer nanotube nanocomposites. Chapter 4 describes the combined use of clay as well as nanotubes as reinforcements in the composite syn-thesis and the synergistic effects of the two fillers in the property enhancements. Chapter 5 details the non-polar nanocomposites with polyethylene as the polymer. Polar polyurethane nanocom-posites with nanotubes is dealt with in Chapter 6. Commercially important PMMA nanotube nanocomposites are described in Chapter 7. Chapter 8 describes the suspension polymerization for the synthesis of vinyl polymer based nanotube nanocomposites. Biodegradable polymer system of polylactide has also been used for the synthesis of nanocomposites and properties of such com-posites are reported in Chapter 9. Engineering plastics like PEEK have also been reinforced with nanotubes and the microstructure and properties of these composites can be found in Chapter 10. Chapter 11 details the synthesis and resulting properties of poly vinyl alcohol nanotube nanocomposites, whereas Chapter 12 focuses on the reinforcement of elastomers with nanotubes. Controlled disper-sion of nanotubes in co-continuous polymer blends are detailed in Chapter 13. Effect of structure and morphology on the tensile prop-erties of polymer nanotube nanocomposites is the focus of Chapter 14. Chapter 15 with the help of case studies sums up the promises and current challenges present in front of the polymer nanotube nanocomposites technology.

At this juncture, I would like to express my gratitude to Scrivener Publishers for their kind support in the project. I dedicate this book to my family, especially to my mother, for being constant source of inspiration without which I would not have achieved this or any goal. Moreover, heartfelt thanks to my wife Preeti for her continuous help in the project from inception till completion.

Vikas Mittal Ludwigshafen, March 2010.

1

Carbon Nanotubes: An Introduction* V. Mittal

BASF SE, Polymer Research, 67056 Ludwigshafen, Germany

Abstract Carbon nanotubes represent high potential fillers owing to their remarkably attractive mechanical, thermal and electrical properties. The incorporation of nanotubes in the polymer matrices can thus lead to synergistic enhance-ments in the composite properties even at very low volume fractions. This chapter provides a brief overview of the properties and synthesis methods of nanotubes for the generation of polymer nanocomposites.

Keywords: nanotubes, nanocomposites, electrical, mechanical, vapor deposition, laser, arc.

1.1 Introduction

Carbon nanotubes, which are allotropes of carbon, are regarded as the ultimate carbon fibers (1,2). Their mechanical properties (strength, stiffness) are expected to approach that of an ideal carbon fiber, which has the perfect orientation of defect-free graphene (single layer of graphite) layers along the fiber axis. These graphene layers are also represented to be as thinnest possible layers to form the nano-tubes. Nanotube materials are the one of the best examples of novel nanostructures derived by bottom-up chemical synthesis processes (3). The chemical composition and atomic bonding configuration present in nanotubes is simple, however, these materials represent diverse structure-property relations among the nano-materials (2). A single-walled nanotube (SWNT) can be formed by rolling a sheet of graphene into a cylinder along an {m,n) lattice vector in the graphene

This review work was carried out at Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland. V. Mittal (ed.) Polymer Nanotube Nanocomposites, (1-14) ©Scrivener Publishing LLC

1

2 POLYMER NANOTUBE NANOCOMPOSITES

plane as shown in Figure 1.1 (3). The (m,n) indices determine the diameter and chirality, which are key parameters of a nanotube. Depending on the chirality (the chiral angle between hexagons and the tube axis), SWNTs can be either metals or semiconductors. As grown, the nanotubes are closed at both ends by a hemispherical cap which is formed by the replacement of hexagons with pentagons in the graphite sheet leading to curvature in the structure.

The credit for realizing the nanotubes in an arc discharge appara-tus is given generally to Iijima who could successfully prove the exis-tence of first multi walled carbon nanotubes (MWCNT) mixed with other forms of carbon (4). He observed a graphitic tubular structure

Figure 1.1. (a) Schematic honeycomb structure of a graphene sheet. Single-walled carbon nanotubes can be formed by folding the sheet along lattice vectors. The two basis vectors a, and a2 are shown. Folding of the (8,8), (8,0), and (10,-2) vectors leads to armchair (b), zigzag (c), and chiral (d) tubes, respectively. Reproduced from reference 3 with permission from American Chemical Society.

CARBON NANOTUBES: A N INTRODUCTION 3

in an arc discharge apparatus used for the production of fullerenes. Electron diffraction taken from individual nanotubes indicated that the orientation of the top and bottom portions of the individual nan-otube cylinders can be different indicating helicity. The identification of these materials as strong structural component for the enhance-ment of many polymer matrix properties caused an exponential rise in the number of studies dealing with carbon nanotube nanocom-posites. Though the existence of these materials was realized earlier also e.g. by Endo in 1976, however, no attention was paid to them (5). MWCNT were classified as either graphite sheets arranged in concentric seamless cylinders or a single sheet of graphite rolled in around itself. The interlayer spacing in the nanotubes is slightly larger than the single crystal graphite because of the existence of the geometrical strain by the formation of concentric seamless cylinders. Outer diameters from 4-50 nm were reported and length of these tubes was also up to few microns, whereas inner tube diameters as small as 2.2 nm were reported. A tremendous activity also in the field of nanotube synthesis followed the initial discoveries and nanotubes with many different methods were generated and characterized (6,7). Subsequently, single wall carbon nanotubes (SWCNT) were discov-ered in which as mentioned above, one-atom thick sheet of graphite is rolled up into a seamless cylinder. Diameters of the order of a nano-meter and length of the order of 1000 nm were achieved depending on the preparation and purification methods (8). Figure 1.2 shows the

Figure 1.2. High-resolution transmission electron microscopy images of (a) SWNT and (b) MWNT. Closed nanotube tips are also shown in Figure 2C (MWNT tips) and Figure 2D (SWNT tip, shown by arrows). Reproduced from reference 9 with permission from American Chemical Society.


typical images of the single wall and multiple wall nanotubes (9). One important point to note is that the conventional carbon fibers cannot match the structural perfection achieved in nanotubes and the carbon fibers synthesized by the traditional methods vary signifi-cantly in morphology and structure.

1.2 Properties

Carbon nanotubes have unique mechanical, electrical, magnetic, optical and thermal properties as demonstrated in Table 1.1 (10). In some special applications, such as space explorations, high-performance lightweight structural materials are required, and they can be developed by adding CNTs to polymers or other matrix mate-rials to generate nanocomposites. The organic-inorganic nanocom-posites can conventionally contain inorganic fillers falling into three

Table 1.1. Theoretical and experimental properties of carbon nanotubes. Property Specific gravity

Elastic modulus Strength

Resistivity Thermal conductivity Magnetic susceptibility

Thermal expansion Thermal stability Specific surface area

CNTs 0.8g/cm3forSWCNT; 1.8 g/cm3 for MWCNT (theoretical) ~1 TPa for SWCNT; -0.3-1 TPa for MWCNT 50-500 GPa for SWCNT; 10-60 GPa for MWCNT 5-50 μΩ cm 3000 W m"1 K-1 (theoretical)

22x l0 6 EMU/g (perpendicular with plane), 0.5 x 106 EMU/g (parallel with plane) Negligible (theoretical

> 700 °C (in air); 2800 °C (in vacuum) 10-20 m 2 /g

Graphite 2.26 g/cm3

1 TPa (in-plane)

50 μΩ cm (in-plane) 3000 W nr1 K1 (in-plane), 6 W m-1 K-1 (c-axis)

-1 xlO^K"1 (in-plane), 29 xlO-6K-' (c-axis) 450-650 °C (in air)

Reproduced from reference 10 with permission from Elsevier.


different categories by the virtue of their primary particle dimensions (11). When all the three dimension of the particles are in the nanome-ter scale, the inorganic fillers have the form of spherical particles like silica particles (12,13). Fillers with two dimensions in the nanome-ter scale whereas the third one in the range of micrometers include carbon nanotubes or whiskers (14,15). When the filler has two finite dimensions in the range of micrometers whereas one dimension is in nanometer scale, the fillers include layered silicate (or alumino-silicate) materials (16). Composites with all the three possible filler geometries have been synthesized, however, nanotube composites generate much high end application potential in the nanocomposites as compared to the nanofillers of other geometries. Thus, single and multi walled nanotubes have attracted the interest of scientists and a significant research effort is devoted to the synthesis and characterization of the nanotubes as well as to tune their surface functionalities to compatibilize them with the matrix materials.

Many experimental and theoretical studies have reported the modulus of the nanotubes to be in the same range as graphite fibers and the strength even at least an order of magnitude higher than the graphite fibers (17-21). The single-walled nanotube is supposed to be formed by rolling a graphene sheet and a Young's modulus of about 1 TPa was reported (22) as compared to 300-800 GPa for graphite fibers (17). Another literature study also reported the Young's modulus of 4.7 TPa, which is much higher than any of the filler material known (23). However, some computational studies found that the true mod-uli of the nanotubes were below the estimated values obtained from the graphene sheet (24). Several kinds of local defects, such as Stone Waals defect and dislocation of carbon atoms may influence the prop-erties of the nanotubes, which have been discussed in some reported computational works (25,26). The big range in the measured and esti-mated mechanical properties is generally due to the different sizes, lengths and numbers of wall layers. Also, it may be hard to produce identical nanotubes even in the same experiment (8). A typical ten-sile test for multi-walled nanotubes which was reported by Ruoff and Lorents is shown (27) in Figure 1.3. It has also been observed (24) that the inner walls of a multi walled carbon nanotubes are not effective in taking tensile loads applied at the both ends, and it is only the outmost layer of the nanotubes which takes the entire load. Therefore, failure in such a case starts at the outer wall of the multiwalled carbon nanotube by breaking the bonds among carbon atoms. The simulated version of such a process has been demonstrated in Figure 1.4. The strength


Figure 1.3. Tensile test for the determination of mechanical performance of single nanotube. Reproduced from reference 24 with permission from Elsevier.

Figure 1.4. Stretching process of a triple walled carbon nanotube. Reproduced from reference 24 with permission from Elsevier.

of nanotubes also depends on the distribution of defects, interlayer interactions in multi walled carbon nanotubes as well as bundles of single walled carbon nanotubes. The defect density is dependant on the growth process. The hollow morphology of the nanotubes has been observed to produce distinct mechanical response thus differing from conventional fibers or other reinforcing materials. Nanotubes can sustain even 40% strain without showing any brittle behavior.

Apart from mechanical properties, electronic properties of nano-tubes are equally important and useful. As observed earlier, both metallic and semiconducting nanotubes are generated depending

CARBON NANOTUBES: AN INTRODUCTION 7

on the helicity The measurement of electronic properties directly on a single nanotube is, however, a challenge. Four or more probe measurements have been developed to generate the information on transport properties of individual nanotubes. Multiwalled nanotubes represent more complex transport properties because of the presence of cylinders of different helicity. On the other hand, the transport properties in single walled nanotubes are more uniform in nature. Reports have also suggested that the conductivity in nanotubes can also be altered by suitably doping them which alters their original lattice structure. The conductivity behavior of the nanotubes has also been observed to have similar nature as conducting polymers.

Also, the presence of defects can generate reactive sites on the surface of nanotubes thus making them more reactive compared to graphite counterparts. Figure 1.5 (9) is the simulated representation of such defects which act as electrophilic reaction sites.

Figure 1.5. Top: ball-and-stick model for a single-walled nanotube with a bond rotation defect (gray atoms). Bottom: corresponding simulated 3D-local density of states image. Reproduced from reference 9 with permission from American Chemical Society.


1.3 Synthesis

The synthesis processes for the nanotubes have been continuously refined in the recent years and today, a number of methods are available to synthesize both single and multiwalled carbon nan-otubes. These methods include high temperature evaporation using arc-discharge (28-30), laser ablation (31), chemical vapor deposition etc. (32-34).

In the electric arc method, a DC arc plasma between the two car-bon electrodes in an inert atmosphere is generated. The inert atmo-sphere used is generally helium. A soft, dark black, fibrous deposit forms on the cathode as the anode is consumed. The deposit gen-erally consists of 50 vol% multiwalled carbon nanotubes. It is also possible to grow single walled nanotubes by the arc discharge method using hydrogen.

The laser ablation process uses a graphite target contain-ing small amounts of a metal catalyst. The target is placed in a furnace at roughly 1200°C in an inert atmosphere followed by evaporation using a high power laser. The nanotubes develop on the cooler surface of the reactor, as the vaporized carbon condenses. The yield of nanotube synthesis by this process is roughly 70%. The nanotubes generated by the above mentioned methods are relatively impure, with presence of unwanted car-bonaceous impurities. The resulting products are swept from the high temperature zone by using an inert gas and conical cop-per head (cooled with water) is used to deposit the nanotubes. These methods are also not operated at higher scale, therefore, the overall production costs are high. These methods are, thus, generally not suitable for the generation of large scale cheap production of nanotubes required for the production of polymer nanocomposites.

Chemical vapor deposition method is used as an alternative to circumvent the limitations of the other synthetic methods. In chemical vapor deposition method, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination of these materials. The diameter of the grown nanotubes is related to the size of the metal par-ticles. The substrate is heated to approximately 700°C. To initi-ate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen, hydrogen, etc.) and


a carbon-containing gas (such as acetylene, ethylene, ethanol, methane, etc.). The carbon vapor deposition method generates entangled nanotubes in general; however, more aligned tubes can be generated by using the synthetic conditions which lead to rapid and dense nucleation on the substrates (35-36). The nanotubes in the length range of millimeters have also been syn-thesized by extending the growth time. A number of commer-cial CVD routes to SWNT synthesis have been developed, but the so-called 'HiPco' method has been widely used. High pres-sure carbon monoxide is used as a carbon source to generate the gas phase growth of SWNT. Table 1.2 also provides a review of the different methods for the synthesis of carbon nanotubes (37). The quality and yield of the generated nanotubes is significantly affected by the used technique and the synthesis conditions used.

Table 1.2. A summary of the methods for the synthesis of carbon nanotubes.

Short name

Laser ablation (PLV)

dc arc discharge

Gas phase decomposition

CCVD

Technology of preparation [reference] Ablation from graphite doped with (Fe, Co, Ni,...) catalyst

First reported production. Modified Kratschmer reactor

Decomposition in an oxygen-free environment Typical: HiPco® (high pressure CO decomposition) Catalytic chemical vapor deposition. Supported metal catalysts are used

Typical mean diameter (nm) 1.4(1-1.8)

1.5(0.9-3.1)

1 (0.9-1.3)

1.5(1.3-2)

Product description High quality, good diameter control, bundled tubes; commercial Lesser quality, carbonaceous impurities abundant. CNTs grow bundled Easy purifica-tion, commercial, good quality

Cheapest, commercial, up-scalable. Most feasible from the application point of view

(continued)


Table 1.2. (continued) Short name

Flame pyrolysis

Solar furnace

Inner tubes of DWCNTs

Zeolite grown

Technology of preparation [reference] Carbon source + metallocene catalyst, conventional low pressure pyrolysis reactor

Solar rays focused on a metal doped graphite target. Growth dynamics similar to PLV

Catalyst free growth from peapods by coalescence of C^ molecules

CNTs grow by thermal decomposition of template molecules within zeolite channels

Typical mean diameter (nm)

2-3

1.4

0.7 (0.55-1)

0.45

Product description

Low yield, bad quality. Still under devel-opment. Plant technology available, large commercializa-tion potential

Good qual-ity CNTs, little amorphous car-bon. Spreading not expected in the near furture Well shielded, best qual-ity CNTs. Separation from outer tubes is very challenging Monodisperse diameter distri-bution, oriented tubes. CNTs metastable out-side the channels

Reproduced from reference 37 with permission from Elsevier.

Figure 1.6 (3) shows the ordered nanotube arrays synthesized by chemical vapor deposition methodology.

The organic-inorganic nanocomposites generated by the incor-poration of the nanotubes in organic materials require optimum dispersion of the nanotubes in order to translate the nanotube and polymer properties synergistically into composite properties. The nanoscale dispersion of nanotubes in the organic materials leads to the generation of large interfacial contacts between the organic and inorganic phases thus generating an altogether different mor-phology. The nanotubes, however, owing to their inert nature tend to form bundles with each other owing to the van der Waals forces


Figure 1.6. Carbon nanotube structures obtained by chemical vapor deposition synthesis, (a) SEM image of self-oriented MWNT arrays. Each tower-like structure is formed by many closely packed multiwalled nanotubes, (b) SEM top view of a hexagonal network of SWNTs (line-like structures) suspended on top of silicon posts (bright dots), (c) SEM top view of a square network of suspended SWNTs, (d) Side view of a suspended SWNT power line on silicon posts (bright) and (e) SWNTs suspended by silicon structures (bright regions). Reproduced from reference 3 with permission from American Chemical Society.

between them and thus do not disperse well in the organic matrices in their pristine state. This hindrance in the optimum dispersion of the nanotubes in the organic matrices is overcome by suitably enhancing the surface of the nanotubes. The surface functionaliza-tion makes these nanotubes more organophilic thus facilitating the nanoscale dispersion of the nanotubes in the organic materials. This behavior is similar to the clay based nanocomposites in which the suitable surface modifications of clay surface are required in order to disperse the clay in various polymer systems.

A number of nanotube surface modification approaches have been reported in the recent years. Non-covalent surface modifications aim to physically wrap polymer chains around the nanotubes or adsorb various surfactant molecules on the surface of nanotubes. Thus,


these modification methods retain the structural characteristics of the nanotubes. However, the modifications present on the surface are only physically bound, therefore, they are prone to be removed from the surface or may not be suitable enough for load transfer etc. Covalent means of surface modification of the nanotubes have also been developed. In such methods, polymer chains can either be grafted to the surface or grafted from the surface of the nanotubes.

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Polymer Nanotube Nanocomposites · 1.2 Properties 4 1.3 Synthesis 8 References 12 . 2. Overview of Polymer Nanotube Nanocomposites 15 . V. Mittal . 2.1 Introduction 15 2.2 Methods