Polymer nanocomposites have attracted great attention due to the unique properties introduced by nanofillers, which typically refer to carbon blacks, silicas, clays, or carbon nanotubes (CNT). The polymer matrix acts as a supporting medium and the improvement in the properties of the nanocomposites generally originates from the nature of these nanofillers. Compared to other nanofillers, the unique structures of CNT potentially provide superior mechanical, electrical, and thermal properties. There are two types of CNT with high structural perfection, single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT) (Figure 7.1). SWNT can be considered as a single graphite sheet seamlessly wrapped into a cylindrical tube. MWNT comprise an array of such nanotubes that are concentrically nested. Theoretical and experimental results1 on individual CNT have shown extremely high elastic modulus, greater than 1TPa, and their reported strength is many times higher than the strongest steel at a fraction of the weight. The fiber-like structure of CNT with low density makes them particularly attractive for reinforcement of composite materials. The nearly one dimensional (1D) electronic structure of CNT allows electrons to be transported along the nanotubes without scattering, enabling them to carry high current with essentially no heating. Phonons also propagate easily along the nanotubes, such that the measured room temperature thermal conductivity for an individual MWNT2 (>3000 W/m K) is greater than that of natural diamond (∼2000 W/m K). The thermal

conductivity of an individual SWNT is expected to be even higher.3 The high aspect ratio (length/diameter) of CNT suggests lower percolation thresholds for both electrical and thermal conductivity in their nanocomposites. Therefore, CNT have considerable potential in multifunctional polymer nanocomposites for structural, electrical, and thermal applications.