Microscopic Solubility-Parameter Theory of Polymer Blends: General Predictions

A microscopic solubility parameter theory for polymer blends is developed based on liquid-state polymer reference interaction site model (PRISM) integral equation methods. The well-defined approximations to obtain such a description are identified, and tractable schemes to go beyond them are discussed. Analytical predictions for the purely enthalpic χ-parameter are derived using the Gaussian thread model. Many novel, non-Flory-Huggins effects are predicted including the failure of mean-field theory for random copolymer alloys, strong deuteration swap effects, nonadditivity of chemical and conformational asymmetry contributions to the χ-parameter, and unusual and subtle temperature dependences of χ due to thermally-induced density and chain dimension changes. Both general model calculations and applications to specific polyolefins are presented. The simple enthalpy-based theory accounts for the absolute magnitude of olefinic χ-parameters and a wide range of "anomalous" non-Flory-Huggins phenomena observed in recent small-angle neutron scattering and cloud-point experiments. New, experimentally testable predictions are also made. The fundamental origin of the non-mean-field behavior is the dependence of local intermolecular packing, and hence cohesive energy density, on the chain aspect ratio or effective stiffness. Numerical studies using more realistic semiflexible chain models are also presented and are in qualitative agreement with the analytic predictions. The influence of nonlocal, excess entropy of mixing processes is investigated and found to be a relatively small effect for chain aspect ratios characteristic of flexible polymers. Moreover, for blends which can be studied in the miscible phase at experimentally relevant temperatures, the enthalpic contribution to χ arising from conformational asymmetry is generally orders of magnitude larger than the excess entropic contribution. These conclusions provide support for a fundamental assumption of regular solution theory that spatially local enthalpic effects make the dominant contribution to the excess mixture free energy.