As discussed previously, conventional reinforced fibers such as glass, carbon fiber have been used to make PMCs. Superior properties of carbon nanotubes (CNTs) such as low-weight, very high aspect ratio, high electrical conductivity, elastic moduli in the TPa range, and much higher fracture strain make them an attractive candidate over conventional reinforced fibers and CNTs, and they are being used to replace conventional reinforced fibers to achieve advanced functional composites which provide excellent properties in terms of strength, aspect-ratio, and thermal and electrical conductivity (Breuer and Sundararaj 2004; Spitalsky et al. 2010; Seymour 1990; Mylvaganam and Zhang 2007).
Although CNTs came to light more than a decade ago, preparation of satisfactory polymer composites by CNTs has faced difficulties due to several challenges such as purification, dispersion, alignment, and adhesion of CNTs. In addition the limitations in interfacial load transfer must be overcome. CNTs are a bit expensive compared to conventional reinforced fiber, but it is worth it to get PMCs which are armed with superior properties, and it is only a matter of time to produce low cost CNTs (Liu et al. 2012; Sahoo et al. 2010).
This chapter will discuss the strategies taken by researchers to counter the above challenges, giving particular attention to the CNT-polymer composites (CNT-PMCs).
Purification and yield of CNTs are fundamental challenges in terms of cost and time. Generally soot produced through sublimation and recombination for CNTs preparation, contains inherent contaminants and needs a purification process. Most of the adopted processes, to get rid of inherent contaminants, were time consuming and produced small quantities. Latter, an efficient purification method was developed using coiled polymer to extract CNTs from their accompanying material with a high yield. In this method, a toluene containing poly(m-phenylene-co-2,5-dioctoxy-p-phenylenevinylene) solution was used to purify nanotube soot by a low power ultrasonic sonicated bath. It allowed inherent contaminants (solid material amorphous carbon) to settle to the bottom of the solution and nanotube composite suspension was then decanted.
Dispersion of nanotubes plays a crucial role in controlling the final properties of CNT-PMCs. The effective use of CNTs depends on the uniform dispersion of CNTs into matrix without reducing their aspect ratio. However, CNTs tend to remain in form stabilized bundles due to van der Waals attraction and very low solubility in solvents.
To overcome the dispersion problem, many mechanical/physical methods were adopted such as high shear mixing, solution mixing, melt mixing, electrospinning, ultrasonication, anionic, cationic, and nonionic surfactants surfactant addition (Pang et al. 2010). In addition in-situ polymerization and chemical modification to functionalize CNTs has been performed. These methods have drawn much attention to the preparation of high performance CNT-polymer composites.
In particular, functionalizing nanotube ends with carboxylic groups were reported quite some time ago. Recently, various functionalized CNTs were used to graft polymerization through anionic, ATRP, radical polymerization, click chemistry, and the preparation of CNT composites employing hyperbranched polymers to achieve good CNT dispersion to get polymer grafted CNT materials with improved dispersion ability (Sahoo et al. 2010).
Some of the scheme for chemical modification for CNT is given in Fig. 22.

Fig. 22 Scheme for covalent functionalization of CNTs

In mechanically reinforced composites, one of the most important issues is the interfacial stress transfer as discussed previously. It is responsible for interface failures in shear stress condition. CNTs have an inherent smooth non-reactive surface which limits the interfacial bonding between the CNT and the polymer chains that limits stress transfer. One of the approaches to overcome the above problem is chemical modification and functionalization of CNTs as stated previously, which can give better bonding sites to the polymer matrix, supported by computational calculation.
A simple method was followed for integrating CNTs into epoxy polymer via chemical functionalization of CNTs. First SWNTs were treated with oxidizing agent, e.g., concentrated H2SO4/HNO3 mixture, which generated –COOH and –OH group on CNTs surface (Sahoo et al. 2010). These functionalized CNTs were then dispersed in solvents like N, N-dimethylformamide/tetrahydrofuran; epoxy resin/polymer (Forney and Poler 2011). If needed a curing agent was added. This leads either to formation of covalent bonds between CNTs and polymer or better dispersion of CNT in the polymer of CNT-PMCs (Geng et al. 2008). It is reported that composite with 1 wt% functionalized CNTs showed an increase of 18 % and 24 % in tensile strength and modulus respectively over the polymer composites with unfunctionalized CNTs and a 30 % increase in tensile modulus over pure polymer (epoxy resin) (Mylvaganam and Zhang 2007; Geng et al. 2008).
In addition, the pi bond present in CNTs structure interaction can be used to make pi-pi compatible CNT-PMCs by choosing a suitable polymer as a matrix. An example of such a CNT-polymer composite is SWNT-polypropylene composite which is made by combining the uniformly dispersed nanotube/solvent mixture with polypropylene matrix/solvent mixture to form nanotube/solvent/matrix mixture. The final composite, with 1 wt% of CNTs, showed more than 50 % increase in tensile strength. This substantial increase in strength was believed to be due to an effect of pi-pi interaction which leads to uniform dispersion of CNTs in the matrix material (Mylvaganam and Zhang 2007).
The final properties of the CNT-PMCs can be controlled by using a different polymer and tuning the conditions used in making the composite.
Various CNT-polymer composites have been reported to tune electrical properties of composites depending on application as different applications need specific levels of conductivity, such as for electronic goods, semiconductor components, and circuit boards (Heeder et al. 2011).
CNTs are excellent candidates for the fabrication of electrically conducting composites due to their high aspect ratio and high electrical conductivity. The electrical conductivity of individual CNTs has been measured and found to be on the order of 106 S/m. The maximum electrical conductivity of SWCNT films has been reported to be in the range of 104–105 S/m due to the contact resistance between the individual nanotubes. Therefore, the range of electrical conductivity of CNT/polymer composites has tremendous potential, and can be tuned to the electrical conductivity of CNT/polymer composites by varying the amount and dispersion of CNTs in the composites considering other factors too. The CNT/polymer composites can be used for a variety of applications including electrostatic dissipation (<10-4 S/m), electrostatic painting (10-4 - 101 S/m), electromagnetic interference (EMI) shielding (>101 S/m), printable circuit wiring, and transparent conductive coatings. CNTs are being used as fillers for electrically conductive adhesives because of their high aspect ratio, high electrical conductivity, and high oxidation resistance. CNT/composites are widely used in photovoltaic devices and light-emitting diodes. CNT-conducting polymer composites have a potential application in supercapacitors. The PANI/MWNTs composites electrodes showed much higher specific capacitance (328 F g-1) than pure PANI electrodes (Sahoo et al. 2010).
Again, the electrical conductivity of CNT/polymer composites is widely defined by the percolation theory. The common factors affecting the percolation threshold of electrical conductivity are similar to mechanical properties such as dispersion, alignment, aspect ratio, degree of surface modification of CNTs, types and molecular weights of the matrix polymer, and composite processing methods.
Due to the superior mechanical and thermal properties of CNT/polymer composites, they have drawn great attention to the applications in high end areas such as aerospace and defense industries. The most possible application comes from substituting the metal composite with the significantly lighter weight CNT/polymer composites in the design of airframes which requires materials with low density, high strength, and modulus. O’Donnell et al. reported that CNT reinforced polymer composites can show a profitable effect on the commercial aircraft business due to lighter weight (less fuel consumption). CNTs can be used as additional filler to the carbon fiber-reinforced polymer (CFRP) composite to enhance its interlaminar fracture characteristics (Sahoo et al. 2012).
Potential application of PMCs composite can be found in sensing important materials that are critical to the environment, space missions, industrial, agricultural, and medical applications. Detection of NO2 and CO is important to monitor environmental pollution whereas sensing of NH3 is required in industrial and medical environments. Sensors based on individual SWNTs/polymer were demonstrated for sensing NO2 or NH3. During sensing, it is found that the electrical resistance profile of a semi-conducting SWNT changed significantly upon exposure of NO2 or NH3 gas (Penza et al. 2009). The existing electrical sensor materials including carbon black polymer composites operate at high temperatures for substantial sensitivity whereas the sensors based on SWNT exhibited a fast response and higher sensitivity at room temperature. A nanotube-poly (dimethylsiloxane) polymer composite film that can be used to form nanosensor, contains at least one conductive channel comprising an array of substantially aligned carbon nanotubes embedded in a polymer matrix, can be used to determine a real time physical condition of a material, and can be exploited in monitoring the physical condition of wing or chassis of a flying airplane or space shuttle (Mylvaganam and Zhang 2007).
Composites of conjugated polymers are becoming increasingly used for solar cells because of their great expectations for cheap energy conversion. In addition, low weight, flexibility, and inexpensive preparation procedures of polymer composites for solar cell application make them more attractive than crystalline inorganic semiconductors for future applications. There are many reports on the performance of polythiophene/fullerene solar cells with a hole-collecting buffer layer that was made using composite films of functionalized multi-walled carbon nanotube (f-MWCNT), poly(3,4-ethylenedioxythiphene):poly(styrenesulfonate) (PEDOT:PSS), and ZnO nano particle. P3HT:PCBM bases solar cell reported efficiency as high as 5 %.
Among various polymer electrolyte membranes, Nafion is the most suitable candidate for the fabrication of fuel cell membranes owing to its remarkable ionic conductivity and chemical Nafion-based membranes have a high production cost, low conductivity at low humidity and/or high temperature, loss of mechanical stability at high temperature, elevated methanol permeability, and restricted operation temperature. The higher methanol permeability not only decreases the fuel cell efficiency, but also the cathode performance. These problems can be overcome by the incorporation of CNTs into the Nafion membrane to improve the mechanical stability, the proton conductivity and to decrease methanol permeation of the Nafion membrane. Choi’s research group prepared functionalized MWCNTs (oxidized and sulfonated MWCNTs) with reinforced Nafion nanocomposite membranes for PEM fuel cell (Lee et al. 2011; Liu et al. 2012).
Fullerenes, a family of carbon allotropes, were discovered in 1985 by Robert Curl, Harold Kroto, and Richard Smalley. Spherical fullerenes are also called buckyballs. The structures of fullerene (C60) and methanofullerene phenyl-C61- butyric-acid-methyl-ester (PCBM) are shown in Fig. 23.

Fullerene (C60) has attracted continuous attention since its discovery due to its exceptional physical and chemical properties. Fullerene-containing materials have shown wide and promising applications in the field of superconductors, ferromagnets, lubrications, photoconductors, and catalysts (Prato 1997; Wudl 1992, 2002).
Organization of fullerene (C60) and its derivatives into nanostructures within polymer systems has potential applications in solar cells and biomedicine (Chen et al. 2009; Po et al. 2010; Sariciftci et al. 1993; Orfanopoulos and Kambourakis 1995). For example, interesting results on polymer solar cells were reported using a blend of fullerene derivative and a block copolymer poly(4-vinyl pyridine) of poly (3-hexyl thiophene) (P3HT-b-P4VP) (Sary et al. 2010). It was also found that fullerene in aqueous solution can generate singlet oxygen under photo-irradiation which has implications in the studies of biomedical and environmental science (Orfanopoulos and Kambourakis 1995; Anderson et al. 1994). One of the biologically most relevant features of C60 is the ability to function as a “free radical sponge” and quench various free radicals more efficiently than conventional antioxidants (Krusic et al. 1991). Fullerene has widespread applications ranging from drug-delivery and tissue-scaffolding systems to consumer products (Markovic and Trajkovic 2008), and it has been explored in the area of biological chemistry, such as enzyme inhibition, antiviral activity, DNA cleavage, and photodynamic therapy (Boutorine et al. 1994). However, many of C60 potential applications have been seriously hampered (Zhu et al. 1997; Ravi et al. 2005) by its extremely low solubility in water. Derivatization of the fullerene molecule with various functional groups and other solubilization procedures such as surfactants or long chain polymers (Ford et al. 2000; Mehrotra et al. 1997) is done through covalent interactions. Alok et al. fabricated functionally unmodified C60-containing nanostructures via a combination of an amphiphilic block copolymer P4VP8-b-PEO105-b-P4VP8 selfassembly and charge-transfer complexation between fullerenes and P4VP segments in organic solvent (Alok et al. 2011).
Recently, the development of graphene-based polymer nanocomposites has become a new direction of research in the area of polymer nanocomposites. Graphene is an allotrope of carbon with a two-dimensional structure in which sp2 bonded carbon atoms are densely packed in a honeycomb crystal lattice into a one-atom-thick planar sheet. Graphene possesses high thermal conductivity, superior mechanical strength, and excellent electronic conductivity. As compared with CNTs, graphene has become a relatively cheap nanomaterial because its synthesis procedure is much simpler than those methods used for synthesis of carbon nanotubes. It has been reported that the improvement in mechanical properties of polymers by adding graphene is much more efficient than that by nanoclay or other nanofillers. Therefore, graphene is considered as a good choice of nanofillers for making advanced polymeric nanocomposites (Sahoo et al. 2012).
The unique structure and high surface area of graphene sheets allow them to be used as composite fillers in fuel-cell and solar cell applications. Among various polymer electrolyte membranes, Nafion (Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer) has received significant attention due to its remarkable ionic conductivity, and chemical and mechanical stability. However, the disadvantages of Nafion-based membranes are high production cost, low conductivity at low humidity and/or high temperature (over 100o C), loss of mechanical stability at high temperature (around 100o C), elevated methanol permeability, and restricted operation temperature. PEMfuel-cell performance can be improved by increasing the proton conductivity of the membrane by incorporating 10 wt% of sulfonic acidfunctionalized graphene oxide (GO) into the Nafion matrix. These experimental results suggest that the functionalized GO/Nafion nanocomposites offer significant promise as electrolyte membranes for PEMFC applications (Sahoo et al. 2012).
When Nafion membrane is replace by poly(ethylene oxide) (PEO), which leads to a GO/PEO membrane, it shows proton conductivity of 0.09 S cm-1, at 60o C, and a power density of 53 mW cm-2 in a hydrogen PEMFC. It is due to partially existing –COOH groups on the GO in the form of –COO- and H+ at room temperature. This provides better ionic/protonic conductivity in the PEO/GO composite membrane (Sahoo et al. 2012).
Organic photovoltaic cells (OPV) are of great interest as a potential source of renewable energy and as a promising alternative to traditional inorganic solar cells due to their light weight, ease of manufacturing, compatibility with flexible substrates, and low cost. Currently, the most successful OPV cells are fabricated with a BHJ architecture based on poly(3-octylthiophene) (P3OT) as the donor and the fullerene derivative 6,6-phenyl C60 butyric acid methyl ester (PCBM) as the acceptor. Graphene based PMC has great potential to be used as an acceptor for photovoltaic devices due to its excellent electron transport properties and extremely high carrier-mobility. Most research focuses on replacing or cooperating with PCBM of polymer-based OPVs because its electron mobility is high and its energy level can be tuned easily through controlling its size, layers, and functionalization. However, the power conversion efficiency (PCE) values of the OPVs reported in the literature are only slightly higher than 1 %, indicating that graphene is still far from being qualified to act as this kind of material (Sahoo et al. 2012).
Numerous functional filler, reinforcements, function polymer, and fabrication techniques are being exploited by researchers to achieve various kinds of PMCs armed with various properties.