Introduction to polymer–carbon nanotube composites
It is now well accepted that tubular structures of carbon, or carbon nanotubes (CNTs) existed for some time prior to their clear identification using microscopic techniques by Iijima in 1991 (Iijima, 1991) and to the subsequent reports on the predicted electronic structure of a fullerene tubule and large-scale synthesis of multi-walled CNTs (MWCNTs) in 1992 (Ebbesen and Ajayan, 1992; Mintmire et al., 1992), and the synthesis of single-walled CNTs (SWCNTs) by two independent groups the following year (Bethune et al., 1993; Iijima and Ichihashi, 1993). Raudushkevish and Lukyanovich in 1952 referred to ‘carbon tubes’ and Oberlin, Endo and Koyama in 1976 to ‘hollow carbon fibres’ and presented the first microscopic images at low magnification in which the hollow but not tubular structure of MWCNTs could be seen (Raudushkevish and Lukyanovich, 1952; Oberlin et al., 1976). In 1978, the term ‘nanotubes’ was used for the first time in the context of carbon nanotubes (Wiles and Abrahamson, 1978). In 1987, Hyperion Catalysis International (of Cambridge, MA, USA) started to publish a series of patents with regard to the production of ‘nanofibrils’ and their use in masterbatches with polymers (Tennent, 1987). Moreover, CNTs were found in 2006 in ancient Damascus sabres used during the Crusades more than 400 years ago, most possibly formed as a consequence of the sophisticated thermo-mechanical treatment of forging and annealing applied by craftsmen to refine the steel to its exceptional quality (Reibold et al., 2006). It is also most likely that Thomas Edison, when developing the light bulb circa 1880, using carbonised bamboo, produced similar carbon structures (Edison, 1879). The unique electrical, mechanical and thermal properties of CNTs have now been widely documented and, as was the case with other nanoparticles, the first large-scale commercial exploitation of carbon nanotubes will be as a functional filler for polymeric materials. The potential of CNTs to achieve electrically dissipative or conductive composites was first demonstrated by Hyperion Catalysis International in their masterbatches containing MWCNTs. The first reference to the potential of CNTs as mechanically reinforcing fillers for polymers was made indirectly by Ajayan et al. in 1994 (Ajayan et al., 1994) and directly in 1998 by Wagner et al. working on a composite of MWCNTs with an UV cured in-situ polymerised urethane– diacrylate oligomer and again in 1998 by the same group (Lourie et al., 1998) and Schadler et al. (Schadler et al., 1998), both studying epoxy–MWCNT composites. Since then, there has been exponential growth in the study of composites of CNTs with both thermosetting and thermoplastic polymers. A limited number of review articles have been published on this topic: by way of example, see (Breuer and Sundararaj, 2004; Moniruzzaman and Winey, 2006; Du et al., 2007; Ahir et al., 2008; Li et al., 2008; McClory et al., 2009; Byrne and Gun’ko, 2010; Spitalskya et al., 2010). Most have focused on specific aspects of polymer–CNT composites, such as on elastomer–CNT composites (Bokobza, 2007) and mechanical properties of polymer–CNT composites (Coleman et al., 2006).
However, at the time of compiling this text, no book had been published to date which describes comprehensively the preparation, properties, characterisation and applications of polymer–CNT composites. Therefore, we have attempted to provide a detailed account of the relevant topics and associated challenges in the field of polymer–CNT composites, with a particular emphasis on the state of the art at this time, through contributions from many of the leading researchers in this field. To this end, the book has been sub-divided into three parts.
Part I (Chapters 1–8) focuses on the preparation and processing of polymer–CNT composites. Specifically, in Chapters 2–4, the synthesis of polymer–CNT composites using non-melt mixing methods is described. In Chapter 1, Kaminsky reports on the in situ polymerisation techniques which can be readily employed to prepare polyolefin–CNT composites with MWCNT loading up to 25 wt%. The author describes how MAO catalyst can be anchored covalently to the surface of CNTs and ethylene or propylene polymerised from the surface. This approach yields good interfacial adhesion between the polymer and CNTs and the resultant composites have enhanced mechanical properties and thermal stability relative to the neat polymer. In Chapter 2, Dubois and co-workers describe the use of plasma technology to surface-treat CNTs prior to incorporation into a polymer matrix. Most interestingly, the authors report a plasma post-discharge treatment which avoids CNT degradation. Using this technique, they were able to graft amine groups onto the surface of CNTs which in turn were used as initiating sites for the ring opening polymerisation of ε-caprolactone. In Chapter 3, Mai and co-workers summarise the various methodologies available for the non-covalent and covalent functionalisation of CNTs for polymer nanocomposites. In particular, the authors emphasise the role of CNT functionalisation in dispersion of CNTs in polymer matrices and interfacial interaction between polymer and CNT, both important factors in obtaining efficient stress transfer from the polymer to the CNTs and achieving a reinforcing effect. Chapters 4 and 5 focus on the melt mixing/extrusion of polymer–CNT composites. In Chapter 4, Pötschke and co-workers describe how material and processing parameters influence CNT dispersion in polymer melts. The authors discuss both batch mixing using small-scale mixers and continuous melt mixing using twin-screw extrusion of different polymer–CNT systems. They report how polymer type, melt viscosity, polymer molecular weight, screw speed, processing temperature and residence time affect CNT dispersion. In Chapter 5, Li and Shimizu describe the application of very high shear (> 1000 sec−1) melt processing of polymer–CNT composites. The authors demonstrated the effectiveness of high shear melt mixing in achieving CNT dispersion in both a homopolymer and a polymer–polymer blend. Secondary melt processing and shaping of polymer–CNT composites, such as injection moulding, have hitherto not been studied in detail. In Chapter 6, Lew and Claes report on the relationship between polymer–CNT composite properties, including electrical conductivity and surface finish as a function of varying injection moulding parameters, such as injection melt temperature and injection speed. Chapters 1–6 focus on the preparation and processing of thermoplastic CNT composites, however the remaining two chapters in Part I, Chapters 7 and 8, describe the preparation and properties of elastomer– and thermoset–CNT composites, respectively. In Chapter 7, Klüppel and co-workers describe the preparation of elastomer–CNT and elastomer hybrid filler composites, highlighting the importance of adding a pre-dispersing agent to aid CNT dispersion. The authors also report the dielectric, electrical and thermal conductivity, mechanical and fracture properties of the resultant composites. In Chapter 8, Kenny and co-workers present a chemo-rheological approach and analysis of a number of epoxy–CNT systems where the effect of CNT type (SWCNT, DWCNT and MWCNT) and functionality on composite properties, including cure kinetics, was examined.
In Part II (Chapters 9–20) the properties of a number of different polymer–CNT systems (Chapters 9, 10, 11, 12, 16) and two of the principal techniques employed to characterise polymer–CNT composites (Chapters 14 and 15) are described. The remaining chapters focus on multi-scale modelling of polymer–CNT composites (Chapter 13), detailed studies of specific polymer–CNT composite systems (Chapters 17, 18 and 19) and, most timely, on the toxicity of CNTs (Chapter 20). As is widely accepted, the key challenge to fully exploiting polymer–CNT composites is achieving highly dispersed and distributed CNTs in the polymer matrix, the extent of which is in many instances the key factor to optimising composite properties. In Chapter 9, Pötschke and co-workers describe the quantification of CNT dispersion and distribution using extensive microscopic techniques. Furthermore, the authors provide a methodology to determine the spatial distribution and orientation of CNTs in a polymer matrix. Alig and co-workers in Chapter 10 discuss the inter-relationship between thermo-rheological history, CNT network structure and composite properties. Using well-defined shear-flow conditions and in-line measurements, the authors describe the influence of processing history on CNT network formation and destruction, electrical conductivity, and viscoelastic properties of CNT-filled polymer melts. In Chapter 11, Nanni and Valentini describe the underlying theory and electromagnetic properties of polymer–CNT composites. Goh, in Chapter 12, provides an introduction to the ‘grafting-to’ and ‘grafting-from’ methods for grafting polymers onto the surface of CNTs and a detailed review of the mechanical properties of a range of polymer–CNT composite systems reinforced with polymer-grafted CNTs. In Chapter 13, Odegard describes a general framework for multi-scale modelling of CNT composites, a topic hitherto poorly described in the literature due to multi-scale modelling approaches that incorporate both molecular- and continuum-level simulations being relatively new, limited experimental validation of the models available and the limitations of molecular dynamics simulations of polymers over long time scales. The author discusses the fundamental aspects of efficient and accurate modelling techniques and provides a review of current state-of-the-art modelling approaches and applies one such model to predict the modulus of CNT-filled polyethylene. Two key characterisation tools, Raman spectroscopy and melt rheology, have been used to study the structure–property relationships of polymer–CNT composites. In Chapter 14, Wagner describes the application of Raman spectroscopy to the study of polymer–CNT interactions and demonstrates the usefulness of the Raman signature of CNTs as a detector device for the presence of bulk matrix defects, the occurrence of polymer phase transitions, and the change in CNT orientation with respect to an applied stress. Nobile, in Chapter 15, demonstrates the effectiveness of rheology measurements in understanding the linear and non-linear viscoelastic behaviour of polymer–CNT composites. The effect of CNT dispersion, aspect ratio, and alignment on the rheology of polymer–CNT composites is discussed. The rheological behaviour of polymer–CNT composite melts in shear and uniaxial elongational flows and the role of MWCNTs in flow-induced crystallisation of polymer–CNT composites are reported. The thermal degradation/stability of polymer–CNT composites plays a crucial role in their melt processing and ultimately application. In Chapter 16, Su and co-workers review the mechanisms of thermal degradation improvement of polymers afforded by CNT addition and discuss the thermal degradation of a range of polymer–CNT composites. Morcom and Simon in Chapter 17 provide a detailed review of polyolefin–CNT composites. The authors discuss a wide range of processing methods used to prepare polyolefin–CNT composites and review the effect of CNT addition on polyolefin crystallinity as well as the mechanical, rheological, electrical, thermal, and wear properties of this family of composites. In Chapter 18, McNally and co-workers describe the preparation of composites of poly(ethylene terephthalate) (PET), an important engineering polymer, with MWCNTs. The authors report the electrical and rheological percolation of these composites as a function of CNT loading. They also provide the results of a detailed study of the crystallisation behaviour of PET on CNT addition, using a combination of differential scanning calorimetry and Fourier transfer infrared and Raman spectroscopy. Göldel and Pötschke, in Chapter 19, describe the localisation of CNTs in polymer blends during melt mixing. The authors discuss tailoring the localisation of CNTs and highlight the factors which influence transfer and localisation of CNTs and other nano-scale fillers. The impact of selective localisation of CNTs on polymer blend rheology and morphology is also described. The potential toxicity of CNTs and CNT composites continues to be a controversial topic with several conflicting reports published to date. As there may be many possible bioengineering and biomedical applications of polymer–CNT composites, the toxicity of CNTs is currently of immense interest. In Chapter 20, Martin and co-workers describe the toxicology of CNTs compared with other particulate materials and compare CNTs and asbestos, from respiratory exposure studies. The authors discuss the toxicity of CNTs and the parameters that influence their toxicity, including aspect ratio, level of CNT aggregation, surface chemistry and defects, and CNT impurity.
Finally, in Part III (Chapters 21–25), some of the more technologically interesting applications of polymer–CNT composites are discussed. In Chapter 21, Peijs and co-workers describe the production of polymer–CNT composite fibres and the orientation of CNTs and polymer during fibre production. The authors also report the electrical, mechanical and sensing properties of polymer–CNT fibres. Dunne and Mitchell in Chapter 22 provide a comprehensive review of the biomedical and bioengineering applications of polymer–CNT composites, with a particular emphasis on joint replacement surgery and dentistry. Initially, the use of CNTs in orthopaedic implants and dentistry, including in dental restorative materials, periodontal dentistry and denture base resins, is discussed. The authors then discuss the use of CNTs in regenerative medicine and tissue engineering, including targeted drug delivery systems, monitoring biological systems and biosensors. In Chapter 23, Dewaghe and co-workers describe the fire-retardant applications of polymer–CNT composites. The authors discuss fire testing, fire protection mechanisms, and the role CNTs can play in fire retardation, including the impact of CNTs on heat release rate and the influence of CNT dispersion. They also report, using examples, on the synergism between CNTs and ammonium phosphate in intumescent systems, and the use of CNTs in flame-resistant coatings. Beyer, in Chapter 24, describes the use of polymer–CNT composites in cable applications, with particular focus on the flammability of composites of LLDPE with MWCNTs, and EVA with MWCNTs and with a hybrid filler system based on MWCNTs and a nanoclay. The crack density and surface characteristics of charred MWCNT compounds are also assessed. In Chapter 25, Feller and co-workers discuss the concept of sensing with conductive polymer–CNT composites. The authors describe the synthesis, fabrication, characterisation and structure-sensing properties of polymer–CNT composites. Using examples, the authors also demonstrate the use of these composite materials in temperature, stress and chemical sensing applications.
It was the intention of the editors when compiling this book to address as many of the issues and topics associated with polymer–CNT composites as possible. Specifically, each contributor, a leading researcher in the field, was invited to include a review of the current-state-of-the-art of the relevant topic covered in their chapter and, where applicable, include new results. To the best of our knowledge, this is the first comprehensive text to be published which solely focuses not only on specific polymer–CNT composite systems but also on general relationships, characterisation and the properties of polymer–CNT composites. Many key challenges remain, all highlighted throughout the book, which must be addressed if polymer–CNT composites are to find widespread commercial application.