On the control of surface enrichment in polymer blends and copolymers

We discuss a model for the equilibrium behavior of flexible polymer blends and copolymers near surfaces and focus on how the bulk properties of such melts affect their surface properties. Using a self-consistent 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. For an athermal structurally asymmetric blend, we predict no segregation-as is consistent with recent simulations. However, for a blend of typical polyolefins,,we predict that the segregation is controlled by differences in the pure-component ''beta'' parameters, beta = R(g)(rho(mol))(1/2), where R(g) is the radius of gyration and rho(mol) is the number density of chains. This result is consistent with both previous theoretical work and with experiments on polyolefin diblock copolymers. Lastly, we show how nearly all of these results can be understood from the perspective of a simple pure-component scaling theory.