ON THE CONTROL OF SURFACE ENRICHMENT IN POLYMER BLENDS AND COPOLYMERS

We discuss a model for the equilibrium behavior of flexible polymer bl ends and copolymers near surfaces and focus on how the bulk properties of such melts affect their surface properties. Using a self-consisten t field theory, derived previously from density functional theory, we obtain a general expression for the segregation in a structurally and interaction potential asymmetric blend near a flat, neutral surface. F or an athermal structurally asymmetric blend, we predict no segregatio n-as is consistent with recent simulations. However, for a blend of ty pical polyolefins,,we predict that the segregation is controlled by di fferences in the pure-component ''beta'' parameters, beta = R(g)(rho(m ol))(1/2), where R(g) is the radius of gyration and rho(mol) is the nu mber density of chains. This result is consistent with both previous t heoretical work and with experiments on polyolefin diblock copolymers. Lastly, we show how nearly all of these results can be understood fro m the perspective of a simple pure-component scaling theory.