CORRELATION-EFFECTS AND ENTROPY-DRIVEN PHASE-SEPARATION IN ATHERMAL POLYMER BLENDS

Polymer reference interaction site model (PRISM) theory with the Percus-Yevick closure approximation has been applied to investigate the intermolecular correlations, effective chi-parameters, and spinodal phase separation of athermal binary polymer blends. These model mixtures an composed of structurally asymmetric semiflexible chains interacting via purely hard core potentials. In strong contrast to PRISM predictions for the idealized Gaussian thread model, nonlocal entropy-driven phase separation is predicted under certain conditions. By examining the intermolecular pair correlation functions we identify the physical driving force as local packing frustration associated with the different backbone stiffnesses of the blend components, which is propagated to macromolecular scales by chain connectivity and persistence. These entropic packing effects display many nonuniversal features including a sensitive dependence on chain length, blend composition, monomer volume difference, and both the mean and relative aspect ratios of the polymers. The sensitivity of the athermal blend fluctuation phenomena to local chain rigidity and nonzero liquid compressibility is emphasized. For model parameters characteristic of most flexible polymers of experimental interest the athermal packing frustration effect is found to generate only a small amount of thermodynamic incompatibility. Perturbative estimates of the enthalpic chi-parameters associated with (local) structural asymmetries suggest they are much more important than the purely entropic contribution for hydrocarbon alloys such as the polyolefins. Recent incompressible field theories for athermal conformationally asymmetric blends are derived within the liquid state integral equation framework by identifying an alternative, mean-field-like closure approximation coupled with the imposition of a zero compressibility constraint. (C) 1995 American Institute of Physics.