(604a) Calculation of Thermodynamic Stabilities of Polymer/Carbon Nanotube Composites

A method based on molecular mechanics for estimating the free energy of mixing of carbon nanotubes with polymers is formulated. The formation of the nanocomposite is analyzed in terms of a simple path in which the nanotubes are exfoliated from a bundle and dispersed in a distorted polymer with cylindrical cavities to accommodate the nanotubes. This method is applied to polystyrene/carbon nanotube composites and the factors that determine their thermodynamic stability are identified. To a first approximation, the free energy of mixing normalized by the surface area of the nanotubes is independent of the lengths, but dependent on the diameters of the component nanotubes. The calculations indicate that, in the case of polystyrene, a thermodynamically stable nanocomposite can be obtained by using carbon nanotubes with diameters greater than about 9 nm.