INTERFACIAL BEHAVIOR OF COMPRESSIBLE POLYMER BLENDS

We develop a new density functional treatment for the liquid-liquid in terface of phase separated binary polymer blends. In contrast to previ ous density functional theory studies of these interfaces, we incorpor ate the compressibility (i.e., ''equation of state effects'') of the s ystem through use of a compressible version of Flory-Huggins theory. T he introduction of compressibility is demonstrated to severely complic ate the mathematical description. The coupled, nonlinear Euler-Lagrang e equations for the interfacial profile and tension are shown to becom e numerically unstable and permit analytic solution in the asymptotic profile wings. Our computations are limited to symmetric binary blends because this restriction enables us to compute numerically the centra l portion of the interfacial profile and thereby test more general var iational methods (applicable also to unsymmetric blends) that we devel op for computing the full interfacial profile and the interfacial tens ion. The total density of the symmetric blend is found to be reduced i n the profile center relative to the bulk density by 0.4%-1.5%. While this total density fluctuation costs free energy, it reduces unfavorab le polymer-polymer contacts, thereby diminishing the total interfacial tension. The applicability of our approximate scheme is tested.