Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties
Introduction
The properties and applications of carbon nanotubes (CNTs) and related materials have been very active research fields over the last decade [1], [2], [3]. CNTs possess high flexibility, low mass density, and large aspect ratio (typically >1000), whereas predicted and some experimental data indicate extremely high tensile moduli and strengths for these materials. Individual single-walled carbon nanotubes (SWCNTs) can be metallic or semiconducting. The latter can transport electrons over long lengths without significant interruption which makes them more conductive than copper [4], [5]. It is indeed this combination of mechanical and electrical properties of individual nanotubes that makes them the ideal reinforcing agents in a number of applications. The first ever polymer nanocomposites using CNTs as fillers were reported in 1994 by Ajayan et al. [6]. Since then, there have been many papers dedicated to processing and resulting mechanical and/or electrical properties of fabricated polymer nanocomposites. However, as-grown CNTs are normally mixtures of various chiralities, diameters, and lengths, not to mention the presence of impurities and other defects. Furthermore, CNT aggregation has been found to dramatically hamper the mechanical properties of fabricated nanocomposites. Finally, due to their small size, CNTs are normally curled and twisted, and therefore individual CNTs embedded in a polymer only exhibit a fraction of their potential. Thus, the superb properties of CNTs cannot as yet be fully translated into high strength and stiffness finished products.
In view of the preceding, there has been an immense effort to establish the most suitable conditions for the transfer of either mechanical load or electrical charge to individual nanotubes in a polymer composite component. A prerequisite for such an endeavour is the efficient dispersion of individual nanotubes and the establishment of a strong chemical affinity (covalent or non-covalent) with the surrounding polymer matrix. Various methods of CNT chemical modification have been proved quite successful in introducing functional moieties which contribute to better nanotube dispersion, and eventually to efficient thermodynamic wetting of nanotubes with polymer matrices [7]. Another area of intense research is the grafting of macromolecules onto the nanotube surface. Indeed, the addition of a whole polymer chain is expected to have greater influence on the nanotube properties and their affinity to polymer matrices as compared to the addition of low molecular weight functionalities. The modification of CNTs by polymers is separated into two main categories, based on whether the bonding to the nanotube surface is covalent or not. The covalent modification itself involves either “grafting to” or “grafting from” strategies [8], [9], [10].
Apart from improving the chemical affinity of CNTs to polymer matrices, the various modification strategies also assist in effective processing to form CNT/polymer components with enhanced mechanical or electrical properties. As is well known, any aggregation of CNTs in polymer composites results in inferior properties, as it prevents efficient stress transfer to individual nanotubes [11]. So far, the majority of the processing methods lead to materials that contain low volume fractions of CNTs that, at least in absolute mechanical property values, cannot seriously compete with commercial polymer composites. For electrical applications, on the other hand, the percolation threshold is so low that large quantities of CNTs are not required and cost-effective composites can be fabricated [12]. Indeed, a large number of processing techniques have already been attempted, and useful conclusions may be drawn from a systematic review of the current situation.
In terms of tensile modulus, it has been established by numerous studies [13] that chemically modified nanotubes exhibit a significant increase in modulus as compared to the matrix resin. As mentioned earlier, this is mainly due to the fact that functionalization improves both dispersion and stress transfer. As yet however the values of strength improvement are disappointing, being orders of magnitude lower than the tensile strengths of CNTs, which range from 60 to 150 GPa [14]. However, in some cases certain improvements are observed as a function of CNT functionalization and, most importantly, CNT volume fraction. All these results are fully reviewed in the subsequent sections.
With reference to electrical properties [12], [15], [16], the present review compares the results obtained from a great number of un-reinforced and CNT reinforced polymers. The results are indeed quite revealing; in most cases, an enhancement of the electrical conductivity by several orders of magnitude is obtained by the addition of CNTs. Although a very broad range of both thermosetting and thermoplastic matrices have been employed and systematic trends are difficult to discern, it is evident that only small quantities of single or multi-wall carbon nanotubes are required to achieve relatively high values of electrical conductivity. Needless to say, that this result alone can guarantee the future commercial viability of CNT materials, provided of course that cost-effective dispersion methods are employed. Finally, the replacement of carbon black, the most commonly industrially used filler material, with CNTs for the preparation of electrically conducting polymer composites is expected to have a great impact on a wide range of industrial applications.
Obviously, it is impossible to make a comprehensive overview of all aspects of this large subject in the framework of one article. Therefore, to keep our task manageable, we confine ourselves to discussing the most characteristic and important recent examples, where the homogeneous dispersion of CNTs within polymer matrices plays a crucial role in the fabrication of multifunctional composites. More detailed information is available in topical reviews devoted to particular issues.
Section snippets
Modification of carbon nanotubes with polymers
As mentioned above, the modification of CNTs by polymers may be divided into two categories, involving either non-covalent or covalent bonding between CNT and polymer. Non-covalent CNT modification concerns the physical adsorption and/or wrapping of polymers to the surface of the CNTs. The graphitic sidewalls of CNTs provide the possibility for π-stacking interactions with conjugated polymers, as well as organic polymers containing heteroatoms with free electron pair. The advantage of
Composite processing
To maximize the advantage of CNTs as effective reinforcement for high strength polymer composites, the CNTs should not form aggregates, and must be well dispersed to enhance the interfacial interaction with the matrix. Several processing methods available for fabricating CNT/polymer composites based on either thermoplastic or thermosetting matrices have been described in past review articles [13], [203], [204]. They mainly include solution mixing, in situ polymerization, melt blending and
Mechanical properties of carbon nanotube/polymer composites
The one-dimensional structure of CNTs, their low density, their high aspect ratio, and extraordinary mechanical properties make them particularly attractive as reinforcements in composite materials. By now, hundreds of publications have reported certain aspects of the mechanical enhancement of different polymer systems by CNTs. These studies have been discussed in some excellent reviews [13], [204], [247], [248], [249]. The variation of many parameters, such as CNT type, growth method, chemical
Electrical properties of carbon nanotube/polymer composites
CNTs have clearly demonstrated their capability as fillers in diverse multifunctional nanocomposites. The observation of an enhancement of electrical conductivity by several orders of magnitude at very low percolation thresholds (<0.1 wt%) of CNTs in polymer matrices without compromising other performance aspects of the polymers such as their low weight, optical clarity, low melt viscosities, etc., has triggered an enormous activity world-wide in this scientific area. Nanotube-filled polymers
Acknowledgements
The authors acknowledge the financial support of the European Marie-Curie grant EU-TOK-FP7 “High Volume Fraction Nanocomposites Incorporating Modified CNT Reinforcement” [Contract No.: MTKD-CT-2005-029876]. The authors wish also to thank Dr. J. Mosnacek for fruitful discussion concerning the reaction mechanisms.
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Present address: Polymer Institute, Slovak Academy of Sciences, Dubravska Cesta 4, 842 36 Bratislava, Slovak Republic.