Liquid crystallinity in flexible and rigid rod polymers

We apply an anisotropic version of the polymer reference interaction site m odel (PRISM) integral equation description of flexible polymers to analyze athermal liquid crystallinity. The polymers are characterized by a statisti cal segment length, sigma(o), and by a physical hard-core thickness, d, tha t prevents the overlap of monomers on different chains. At small segment de nsities, rho, the microscopic length scale d is irrelevant (as it must be i n the universal semidilute regime), but becomes important in concentrated s olutions and melts. Under the influence of the excluded volume interactions alone, the chains undergo a lyotropic, first-order isotropic-nematic trans ition at a concentration dependent upon the dimensionless "aspect ratio," s igma(o)/d. The transition becomes weaker as d --> 0, becoming second order, as has been previously shown. We extend the theory to describe the transit ion of rigid, thin rods, and discuss the evolution of the anisotropic liqui d structure in the ordered phase. (C) 2000 American Institute of Physics. [ S0021-9606(00)51510-4].