An Integral Equation Theory of Polymer Blends: Athermal Mixtures

In order to understand the effects of molecular structure on the miscibility of polymer blends, we applied our polymer RISM theory to various athermal mixtures. For the case of athermal blends involving differences in local structure in the components, we obtained an analytical approximation for the generalized Flory-Huggins χ parameter in terms of composition, density, and the molecular parameters monomer size, stiffness, and aspect ratio. Numerical calculations were performed for two 'topological blends' involving differences in molecular structure on global length scales: the bimodal mixture and the chain/ring mixture. We find that the noncombinatorial entropy of mixing, which stabilizes the miscible mixture, increases when the structural asymmetry is increased. This stabilization is caused by spatially nonrandom packing, which is enhanced by structural differences. Our theory was also used to calculate the total and partial structure factors for athermal mixtures, and comparisons are made with the RPA theory.