Polymer nanocomposites, which comprise of additives/fillers and polymer matrices, are considered to be an important group of relatively inexpensive materials for many engineering applications. The polymer matrices may refer to all type of polymers including thermoplastics, thermosets, elastomers, and even polymer blends. Two or more materials are usually combined to produce composites which possess properties that are unique and cannot be obtained by each material alone. For example, high-modulus carbon fibers or silica particles are added into a polymer to produce reinforced polymer composites that exhibit significantly enhanced mechanical properties including strength, modulus, and fracture toughness. Therefore, due to their unique and superior properties as well as ease of production at low cost, polymer-based composites are currently important engineering materials with many applications which include high-performance composites even used in aerospace application, filled elastomers for damping, electrical insulators, thermal conductors, and other special applications in which a particular superior property is needed.
Special materials with extraordinary properties are chosen to create composites with desired properties; for example, high-modulus but brittle carbon fibers are added to low-modulus polymers to create a stiff and lightweight composite with a reasonable degree of toughness. In recent years, although the highest level of optimization of composite properties with traditional micrometer-scaled fillers has been reached, a large bundle of opportunities has been opened to overcome the limitations of traditional polymer composites by using newly available nanometer-scaled fillers – polymer nanocomposites in which the filler is <100 nm in at least one dimension as shown in Fig. 14 (Schadler 2004).

Unlike traditional polymer composites containing microscale fillers, when small amounts of nanoscale fillers, such as carbon nanotube tubes (CNTs) or nanoclay flakes, are incorporated into a polymer system, the properties of such nanocomposites can be largely modified even at an extremely low content of filler due to a very short distance between the fillers. Although polymer nanocomposites can be considered as a new class of materials, some nanofillers, such as carbon black (Donnet et al. 1993) and fumed silica, (Sumita et al. 1984; Kuriakose et al. 1986) have been used for more than a century for the fabrication of polymer-based nanocomposites. However, the research and development of polymer nanocomposites have greatly increased only in the recent years for the following reasons.
First of all, some of the polymer nanocomposites showed unpredicted combinations of properties (LeBaron et al. 1999). For example, the incorporation of equiaxed nanoparticles in thermoplastics, and particularly in semicrystalline thermoplastics, increases the yield stress, the tensile strength, and Young’s modulus (Sumita et al. 1983) compared to pure polymer. A volume fraction of only 0.04 mica-type silicates (MTS) in epoxy increases the modulus below the glass transition temperature by 58 % and the modulus in the rubbery region by 450 % (Messersmith and Giannelis 1994). In addition, the permeability of water in poly(e-caprolactone) decreases by one order of magnitude with the addition of 4.8 % silicate by volume (Messersmith and Giannelis 1995). Yano et al. (1993) showed a 50 % decrease in the permeability of polyimides at a 2.0 wt% loading of MTS. Many of these nanocomposites are optically transparent and/or optically active.
The second reason for the large increase in research and development efforts was related to the discovery of carbon nanotubes by Iijima in the early 1990s (Iijima 1991). Although there is an argument that carbon nanotubes have been observed since the 1960s, (Colbert and Smalley 2002) it was only in the mid-1990s that they became popular to the researchers and the scientists around the world, due to the ability to produce them in the quantities required for property evaluation of composites. The unpredictably sophisticated properties of these carbon nanotubes, especially their superior mechanical and electrical properties over those of the traditional fillers, offer an exciting potential for new composite materials.
Finally, recent significant development in chemistry, such as click chemistry, (Kolb et al. 2001) has brought remarkable control over the chemical modification of carbon nanotubes and the in situ process for polymer nanocomposites. It has also created an almost unrestricted possibility to control the interfacial interaction between the polymer matrix and the filler.

Carbon Nanotube/Polymer Composites

One of the well-known nanofillers for polymer nanocomposites is the carbon nanotubes. Due to the outstanding properties of CNTs, they have become an attractive candidate for scientists to develop advanced polymer nanocomposites with multifunctional features. The first polymer nanocomposites using CNTs as fillers were reported by Ajayan et al. in 1994 (Ajayan et al. 1994). Since then, there has been a lot of research dealing with fabrication of CNT/polymer composites. On the basis of extraordinary physical properties as well as the large aspect ratio of CNTs, most of CNT/polymer composites show unexpected properties in many aspects such as mechanical, electrical, and thermal properties. In general, the homogeneous dispersion, alignment, and content of CNTs in polymer matrices are the key parameters to enhance the physical properties of CNT/polymer composites.
Like other nanofillers, the dispersion state of CNTs in their matrices seems to be an important factor that determines the physical properties of the resultant CNT/polymer composites. However, it is well known that CNTs tend to form stable aggregates or bundles in a polymer matrix due to the very strong van der Waals forces among them (Sahoo et al. 2010). Therefore, they are difficult to be separated into individual nanotubes and dispersed homogeneously in a polymer matrix, which hampers the mechanical and electrical properties of fabricated nanocomposites.
Many research efforts have been made in the production of CNT/polymer composites for functional and structural applications (Liu et al. 2008; Thostenson et al. 2001; Coleman et al. 2006). However, even after a number of decades of research, the potential for CNTs as reinforcing fillers has been brutally restricted due to the drawback associated with dispersion of entangled CNTs during a fabrication process and poor interfacial interaction between CNTs and a polymer matrix. As CNTs are characteristic of small diameter in nanometer scale with high aspect ratio (>1,000) and thus extremely large surface area, their nature of poor dispersion in a polymer matrix is rather different from that of other conventional fillers, such as spherical particles and carbon fibers. Therefore, the agglomeration of CNTs in a polymer matrix can be considered as the main reason for the reduced mechanical, thermal, and electrical properties of their nanocomposites as compared with theoretical predictions based on individual CNTs. The critical challenge is, therefore, how to incorporate individual CNTs, or at least relatively thin CNT bundles or disentangled CNTs, into a polymer matrix. In other words, dispersion of CNTs not only is a geometrical problem due to the length and size of the CNTs but also relates to a method for how to separate individual CNTs from CNT agglomerates and stabilize them in a polymer matrix to avoid secondary agglomeration (Ma et al. 2010). In addition, the most suitable processing conditions are required for the efficient transfer of either mechanical load or electrical charge among individual carbon nanotubes in a polymer matrix towards a successful fabrication of CNT/polymer composites (Sahoo et al. 2010).
As mentioned above, the agglomeration of CNTs in a polymer matrix and the poor interfacial interaction between the CNTs and the polymer molecules are the most critical issues in the fabrication of CNT/polymer composites. Fortunately, there are several possibilities to improve the dispersion of CNTs in polymer matrices such as solution mixing, melt blending, and in situ polymerization methods. Moreover, several methods are also available to enhance the interaction between the CNTs and the polymer molecules. Especially, the surface modification of CNTs is an effective way to prevent carbon nanotube aggregation by improving their chemical compatibility with the polymer matrixes, which helps CNTs to disperse better and stabilize within a polymer matrix. There are mainly two approaches for surface modification of CNTs, namely, physical modification (noncovalent functionalization) and chemical modification (covalent functionalization).
For example, CNTs are used in the development of the stiff and lightweight polymer nanocomposites. CNT/polymer composites show considerably improved mechanical properties even at a low CNT content. For example, with an addition of 0.5 wt% MWCNTs, the tensile strength and modulus for high-density polyethylene nanocomposite films remarkably increased by  ~30 % and ~20 %, respectively (Zhang et al. 2006b). CNTs can also be used as a nucleating agent for crystallization of polymers. Several groups have studied the crystallization of polypropylene in the presence of CNTs (Valentini et al. 2003; Manchado et al. 2005; Bhattacharyya et al. 2003). Assouline et al. (2003) studied the non-isothermal crystallization of MWCNT/isotactic polypropylene (iPP) composites. The crystallization behavior of MWCNT/iPP composite was significantly different from that of the neat iPP. With an addition of 1.0 wt% MWCNTs into iPP, the crystallization rate was increased with evidence of fibril crystal growth rather than spherulite growth. Many research groups have observed the improved thermal stability in CNT/polymer composites. For example, Kashiwagi et al. reported that the addition of MWCNTs into polypropylene enhanced the thermal stability of PP both in nitrogen and in air. Besides, the MWCNTs could significantly reduce the heat release rate of PP. Generally, the thermal stability of the CNT/polymer composites increases due to the higher thermal conductivity of MWCNTs that facilitates heat dissipation within the composites (Huxtable et al. 2003). The results show a great potential for the use of CNTs as a flame retardant for polymer materials.
Therefore, in the following sections, thermal and mechanical properties and applications of CNT/polymer composites will be covered.

Thermal and Rheological Properties of CNTs Base Polymer Composites

The glass transition temperature (Tg) is a measure of the thermal energy required to allow polymer motion involving 10–15 monomeric units and corresponds to the softening of a polymer. Park et al. (2002) reported that Tg did not change for their in situ polymerized SWCNT/polyimide composites. The SWCNT/PMMA composites produced by the coagulation method have the same Tg over a wide range of nanotube loadings (Du et al. 2004). Therefore, it can be concluded that the addition of CNTs does not significantly change the glass transition temperature in CNT/polymer composites, because in the absence of strong interfacial bonds and at low nanotube loadings, the majority of polymer molecules are locally constrained only by other polymer molecules but not by CNTs.
On the other hand, in larger districts, carbon nanotubes do obstruct the motion of polymer molecules as measured by rheology. Rheological (or dynamic mechanical) measurements at low frequencies probe the longest relaxation time of a polymer which corresponds to the time required for an entire polymer molecule to change its conformation. Du et al. (2004) found that, although it has little effect on polymer motion at the length scales comparable to or less than an entanglement length, the presence of CNTs has a substantial influence at large length scales corresponding to an entire polymer chain. The storage modulus, G’, at low frequencies becomes almost independent of frequency as CNT loading increases. This shows a transition from a liquid-like behavior (which has short relaxation times) to a solid-like behavior (in which the relaxation times will be infinite) with increasing CNT loading. By plotting G’ versus CNT loading and fitting with a power law function, they reported that the rheological threshold of these nanocomposites was ~0.12 wt%. This rheological threshold could be attributed to a hydrodynamic CNT network that impedes the large-scale motion of polymer molecules. A similar phenomenon has previously been observed in nanoclay/polymer composites by Krishnamoorti and Giannelis (1997). They reported that a network of nanoscale fillers restrains polymer relaxations, leading to a solid-like or nonterminal rheological behavior. Therefore, any factor that changes the morphology of the CNT network will influence the low-frequency rheological properties of their nanocomposites.
The factors influencing the polymer chain mobility are the aspect ratio of CNTs, dispersion and alignment of CNTs in the polymer matrix, and the molecular weight of the polymer matrix. Du et al. (2004) reported that higher aspect ratio, better dispersion and less alignment of the CNTs, and longer polymer chains would result in more restraint on the mobility of the polymer chains, i.e., the onset of a solid-like behavior occurs at lower nanotube contents. In addition to these factors, the content, size, and interfacial properties of CNTs are expected to influence rheological properties of CNT/polymer composites. For example, at a fixed loading, nanotubes with smaller nanotube diameters and larger aspect ratios will produce a network with smaller mesh size and larger surface area/volume, which might restrain polymer motion to a greater extent (Du et al. 2004). Experimental results support this hypothesis. Lozano et al. (2001) observed a rheological threshold of 10–20 wt% in carbon nanofiber/polypropylene composites in which the diameter of the carbon nanofiber is ~150 nm. The rheological threshold is ~1.5 wt% in MWCNT/polycarbonate composites and only 0.12 wt% for the SWCNT/PMMA system (Du et al. 2004). Even if these three systems have different polymer matrices as well as their states of dispersion are unclear, the diameters among carbon nanofibers, MWCNT, and SWCNT differ by orders of magnitude. It can be concluded that if the diameter of filler decreases, the filler loading required for a solid-like behavior increases significantly.
The constraints imposed by CNTs on polymer matrices in nanocomposites are also evident in the polymer crystallization behavior. Bhattacharyya et al. (2003) studied crystallization in 0.8 wt% SWCNT/PP composites using optical microscopy (with cross-polars) and differential scanning calorimetry (DSC). From Fig. 15, the spherulite size in PP is much larger than that in SWCNT/PP composites. The authors also reported that upon cooling, the SWCNT/PP composites began their crystallization at the temperature which was about 11 C higher than that for PP’s crystallization, suggesting that nanotubes acted as nucleating sites for PP crystallization. They also observed that both melting and crystallization peaks in the nanocomposite are narrower than those in neat PP. Therefore, they proposed that higher thermal conductivity of the CNT as compared to that of the polymer at least in part should be responsible for the sharper but narrower crystallization and melting peaks, as heat would be more evenly distributed in the nanocomposite samples containing CNTs.

Mechanical Properties CNTs Base Polymer Composites

CNTs exhibit excellent mechanical properties with Young’s modulus as high as 1.2 TPa and tensile strength of 50–200 GPa (Qian et al. 2002). The combination of these exceptional mechanical properties along with the low density, high aspect ratio, and high surface area makes CNTs an ideal candidate of reinforcing fillers for fabrication of stiff and lightweight nanocomposites. Both SWCNTs and MWCNTs have been utilized for reinforcing thermoplastic polymers, such as polyethylene, polypropylene, polystyrene, nylon, and polycarbonate, as well as thermosetting polymers, including epoxy, polyurethane, and phenol–formaldehyde resins. Generally the CNT-reinforced nanocomposites can be considered as particulate composites or short fiber composites with the filler dimensions on the nanometer scale and a high aspect ratio. Therefore, the mechanics of CNT/polymer composites is governed by that of particulate composites or short fiber composites. On the other hand, unlike the macroscopic particulate composites, mechanical properties of CNT/polymer composites mainly depend on the dispersion state of nanofillers, apart from the properties of filler and matrix themselves. In addition to dispersion, there are other important factors that determine an effective reinforcement of CNTs in nanocomposites: they include a high aspect ratio, alignment, and interfacial interactions between CNTs and polymer matrix. The aspect ratio must be sufficiently large to maximize the load transfer between CNTs and the matrix and, thus, to achieve enhanced mechanical properties. For example, polystyrene nanocomposites reinforced with well-dispersed 1.0 wt% CNTs of a high aspect ratio had more than 35%and 25%increases in elastic modulus and tensile strength, respectively (Qian et al. 2000). Similar promising results have also been reported, (Coleman et al. 2006; Jiang et al. 2007) but other reports demonstrated only modest improvements in modulus and strength. For example, the impact resistance and fracture toughness of the CNT/epoxy composites containing CNTs of a larger aspect ratio were improved much better than those of the CNT/epoxy composites containing CNTs of a smaller aspect ratio (Hernández-Pe´rez et al. 2008). However, the corresponding tensile modulus and strength showed very limited improvements of less than 5.0 %, probably due to weak bonds between the CNTs and the matrix molecules as well as agglomeration of CNTs. In reality, the dispersion is known as the foremost important issue in producing CNT/polymer composites. Many different techniques, including the functionalization of CNTs and processing of CNT/polymer composites, have been employed for CNT dispersion, as discussed in sections “Other Types of Nano-Biocomposites.” A good dispersion not only makes more filler surface area available for bonding with a polymer matrix but also prevents the aggregated filler from acting as a stress concentrator that is detrimental to mechanical performance of nanocomposites (Liu and Wagner 2005).
However, to obtain a uniform CNT dispersion in nanocomposites, some parameters, such as CNT content in nanocomposites, length and entanglement of CNTs, as well as viscosity of matrix, are still needed to optimize. There were many reports (Ma et al. 2007, 2008b, 2009) showing that there is a critical CNT content in the matrix below which the strengthening effect for CNT/polymer composites increases with increasing CNT content. Above this critical CNT content, however, the mechanical strengths of CNT/polymer composites decrease, and in some cases, they decrease below those of the neat matrix materials. These observations can be attributed to (i) the problems associated with uniform dispersion of CNTs at high CNT contents and (ii) lack of polymerization reactions that are adversely affected by the high CNT content for an in situ process. The latter effect becomes more pronounced when functionalized CNTs are employed to produce CNT/polymer composites. To a large extent, the technique employed for CNT dispersion can influence the mechanical properties of CNT/polymer composites.
It should be noted that the definition of a dispersion state of CNTs in a polymer matrix is totally dependent on the magnification or scale used for the analysis. According to the study by Li et al. (2007b) using the term uniform or good dispersion to evaluate the CNT dispersion without any distinctive description may simply be misleading or inaccurate. This is because, for the conventional composites, uniform or good dispersion generally refers even distribution of fillers in a matrix medium without aggregation. However, for CNT/polymer composites, dispersion has two major aspects: (i) disentanglement of CNT bundles or agglomerates, which is referred as the nanoscale dispersion, and (ii) uniform distribution of individual CNTs or their agglomerates throughout the nanocomposites, which is a micro- and macroscale dispersion. From geometric consideration, the difference between random orientation and alignment of CNTs can result in significant changes in various properties of nanocomposites.
The storage moduli of the polystyrene composite films containing random and oriented CNTs were 10 % and 49 % higher than the unreinforced bulk polymer, respectively (Thostenson and Chou 2002). The alignment can be regarded as a special case of CNT dispersion. A few techniques, including mechanical stretching (Jin et al. 1998), melt-spinning (Fornes et al. 2006), dielectrophoresis, and application of an electrical or magnetic field (Park et al. 2006; Steinert and Dean 2009), have been employed during the composite fabrication to align CNTs in a polymer matrix. The degree of CNT alignment in the composite can be governed by two factors: (i) aspect ratio of CNTs and (ii) CNT content. A smaller diameter of CNT can enhance the degree of CNT alignment due to the greater extensional flow, and a higher CNT content decreases their alignment because of the CNT agglomeration and restrictions in motion from neighboring CNTs (Desai and Haque 2005). While alignment is necessary to maximize the strength and modulus, it is not always beneficial because the aligned nanocomposites have very anisotropic mechanical properties, i.e., the mechanical strengths along the alignment direction can be enhanced, whereas these properties are sacrificed along the direction perpendicular to this orientation.
In addition, the interfacial properties between CNTs and matrix molecules play an essential role for mechanical properties of such nanocomposites. A strong interfacial adhesion corresponds to high mechanical properties of nanocomposites through enhanced load transfer from matrix to CNT. Chemical and physical functionalizations of CNTs have proven to enhance the interfacial adhesion. Table 4 summarizes the effects of CNT functionalization on the mechanical properties of CNT/polymer composites made from thermoplastic polymers. These results indicate clearly that functionalization of CNTs can greatly enhance the modulus, strength, as well as fracture resistance of CNT/polymer composites.