Analytic RISM Theory of Polymer Alloys: Molecular Closure Predictions for Structurally Symmetric Blends

Polymer reference interaction site model (PRISM) theory with new molecular closure approximations is applied to analytically calculate the phase behavior and effective SANS X parameter of binary blends composed of long threadlike chains. A general analysis of the validity of incompressible approaches to computing thermodynamics and scattering patterns is presented. The special case of structurally symmetric mixtures is then considered in depth, anb the influence of density and coupled density/concentration fluctuations, or `compressibility effects', is investigated. Significant corrections to mean field Flory-Huggins theory are found for short and intermediate length polymers. The magnitude of these correlation corrections are strongly dependent on the precise nature of the interchain attractive potentials, blend composition, and molecular weight. Applications of the general formulas to isotopic blends are presented and qualitatively compared with experiments. The effect of strong `specific interactions' is also studied within the analytic PRISM framework. Under some circumstances such interactions merely quantitatively modify the upper critical solution temperature (UCST) process. However, depending on system-specific factors, different types of phase behavior are predicted to occur including lower critical solution temperature (LCST) phase separation. Within the present model the LCST phase transition is driven by thermally-induced local packing changes in the fluid.