Self-consistent integral equation theory of polyolefins and comparison to X-ray scattering experiments

Self-consistent, polymer reference interaction site model (PRISM) calculations were performed on realistic models of three polyolefin melts: polyisobutylene (PIB), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP). In these calculations, both the intramolecular and intermolecular structure of the polymer liquid are determined in a self-consistent manner. The multiple-chain problem is mapped to an equivalent single-chain Monte Carlo simulation by representing the effect; of the other chains through a "medium-induced" pairwise-additive potential calculated from PRISM theory. The intramolecular structure factor, intermolecular radial distribution Functions, and medium-induced potentials are obtained numerically from a series of Monte Carlo simulations and PRISM calculations performed iteratively until a self-consistent solution is obtained. The resulting melt structure factors are in close agreement with X-ray scattering experiments on PIE at; 25 degrees C and IPP and sPP liquids at 180 degrees C. The individual radial distribution functions between pairs of methyl, methylene, and methyne (or C atom in the case of PIE) on different macromolecules show universal behavior on long length scales; however, significant differences in local packing are found for distances less than about; 12 Angstrom. The resulting intramolecular structure functions can be employed as input to PRISM theory to deduce the packing and miscibility characteristics in polyolefin mixtures.