Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties

https://doi.org/10.1016/j.progpolymsci.2009.09.003Get rights and content

Abstract

Carbon nanotubes have long been recognized as the stiffest and strongest man-made material known to date. In addition, their high electrical conductivity has roused interest in the area of electrical appliances and communication related applications. However, due to their miniscule size, the excellent properties of these nanostructures can only be exploited if they are homogeneously embedded into light-weight matrices as those offered by a whole series of engineering polymers. We review the present state of polymer nanocomposites research in which the fillers are carbon nanotubes. In order to enhance their chemical affinity to engineering polymer matrices, chemical modification of the graphitic sidewalls and tips is necessary. In this review, an extended account of the various chemical strategies for grafting polymers onto carbon nanotubes and the manufacturing of carbon nanotube/polymer nanocomposites is given. The mechanical and electrical properties to date of a whole range of nanocomposites of various carbon nanotube contents are also reviewed in an attempt to facilitate progress in this emerging area.

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.

References (488)

  • C.S. Wu et al.

    Study on the preparation and characterization of biodegradable polylactide/multi-walled carbon nanotubes nanocomposites

    Polymer

    (2007)
  • Y.L. Zeng et al.

    Functionalization of multi-walled carbon nanotubes with poly(amidoamine) dendrimer for mediator-free glucose biosensor

    Electrochem Commun

    (2007)
  • L. Sun et al.

    Mechanical properties of surface-functionalized SWCNT/epoxy composites

    Carbon

    (2008)
  • H.S. Kim et al.

    Nylon 610/functionalized multiwalled carbon nanotubes composites by in situ interfacial polymerization

    Mater Lett

    (2007)
  • H. Kitano et al.

    Functionalization of single-walled carbon nanotube by the covalent modification with polymer chains

    J Colloid Interface Sci

    (2007)
  • C. Zhou et al.

    In situ preparation and continuous fiber spinning of poly(p-phenylene benzobisoxazole) composites with oligo-hydroxyamide-functionalized multi-walled carbon nanotubes

    Polymer

    (2008)
  • X. Lou et al.

    Grafting of alkoxyamine end-capped (co)polymers onto multi-walled carbon nanotubes

    Polymer

    (2004)
  • H.C. Wang et al.

    Sensors for organic vapor detection based on composites of carbon nonotubes functionalized with polymers

    Sens Actuators B

    (2007)
  • H. Li et al.

    Water-soluble SWCNTs from sulfonation of nanotube-bound polystyrene

    Carbon

    (2007)
  • C.F. Kuan et al.

    The preparation of carbon nanotube/linear low density polyethylene composites by a water-crosslinking reaction

    Mater Lett

    (2007)
  • S.M. Yuen et al.

    Silane-modified MWCNT/PMMA composites—preparation, electrical resistivity, thermal conductivity and thermal stability

    Composites A

    (2007)
  • H.C. Kuan et al.

    Synthesis, thermal, mechanical and rheological properties of multiwall carbon nanotube/waterborne polyurethane nanocomposite

    Compos Sci Technol

    (2005)
  • D.M. Guldi et al.

    Carbon nanotubes in electron donor–acceptor nanocomposites

    Acc Chem Res

    (2005)
  • V. Sgobba et al.

    Covalent and noncovalent approaches towards multifunctional carbon nanotube materials

  • P.R. Bandaru

    Electrical properties and applications of carbon nanotube structures

    J Nanosci Nanotechnol

    (2007)
  • B.Q. Wei et al.

    Reliability and current carrying capacity of carbon nanotubes

    Appl Phys Lett

    (2001)
  • T. Durkop et al.

    Properties and applications of high-mobility semiconducting nanotubes

    J Phys Condens Matter

    (2004)
  • P.M. Ajayan et al.

    Aligned carbon nanotube arrays formed by cutting a polymer resin–nanotube composite

    Science

    (1994)
  • D. Tasis et al.

    Chemistry of carbon nanotubes

    Chem Rev

    (2006)
  • C.M. Homenick et al.

    Polymer grafting of carbon nanotubes using living free radical polymerization

    Polym Rev

    (2007)
  • N. Grossiord et al.

    Toolbox for dispersing carbon nanotubes into polymers to get conductive nanocomposites

    Chem Mater

    (2006)
  • M. Yu et al.

    Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load

    Science

    (2000)
  • D.E. Hill et al.

    Functionalization of carbon nanotubes with polystyrene

    Macromolecules

    (2002)
  • W. Guojian et al.

    Study of SMA graft modified MWNT/PVC composite materials

    Mater Sci Eng A

    (2008)
  • Y. Lin et al.

    Polymeric carbon nanocomposites from carbon nanotubes functionalized with matrix polymer

    Macromolecules

    (2003)
  • J.E. Riggs et al.

    Strong luminescence of solubilized carbon nanotubes

    J Am Chem Soc

    (2000)
  • K.A.S. Fernando et al.

    Poly(ethylene-co-vinyl alcohol) functionalized single-walled carbon nanotubes and related nanocomposites

    J Nanosci Nanotechnol

    (2005)
  • N. Zhang et al.

    Chemical bonding of multiwalled carbon nanotubes to polydimethylsiloxanes and modification of the photoinitiator system for microstereolithography processing

    Smart Mater Struct

    (2004)
  • A.B. Bourlinos et al.

    Silicone-functionalized carbon nanotubes for the production of new carbon-based fluids

    Carbon

    (2007)
  • B. Zhao et al.

    Synthesis and characterization of water soluble single-walled carbon nanotube graft copolymers

    J Am Chem Soc

    (2005)
  • Y. Ni et al.

    Chemically functionalized water soluble single-walled carbon nanotubes modulate neurite outgrowth

    J Nanosci Nanotechnol

    (2005)
  • D. Hill et al.

    Functionalization of carbon nanotubes with derivatized polyimide

    Macromolecules

    (2005)
  • T.Z. Wang et al.

    Polymeric carbon nanocomposites from multiwalled carbon nanotubes functionalized with segmented polyurethane

    J Appl Polym Sci

    (2007)
  • G.X. Chen et al.

    Controlled functionalization of multiwalled carbon nanotubes with various molecular-weight poly(l-lactic acid)

    J Phys Chem B

    (2005)
  • D. Baskaran et al.

    Grafting efficiency of hydroxy-terminated poly(methyl methacrylate) with multiwalled carbon nanotubes

    Macromol Rapid Commun

    (2005)
  • B.X. Yang et al.

    Enhancement of stiffness, strength, ductility and toughness of poly(ethylene oxide) using phenoxy-grafted multiwalled carbon nanotubes

    Nanotechnology

    (2007)
  • X. Wang et al.

    Polymer-functionalized multiwalled carbon nanotubes as lithium intercalation hosts

    J Phys Chem B

    (2006)
  • J.E. Riggs et al.

    Optical Limiting properties of suspended and solubilized carbon nanotubes

    J Phys Chem B

    (2000)
  • W. Huang et al.

    Sonication-assisted functionalization and solubilization of carbon nanotubes

    Nano Lett

    (2002)
  • Y. Lin et al.

    Functionalizing multiple-walled carbon nanotubes with aminopolymers

    J Phys Chem B

    (2002)
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    Present address: Polymer Institute, Slovak Academy of Sciences, Dubravska Cesta 4, 842 36 Bratislava, Slovak Republic.

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