Polymer conformations next to the surfaces of compressible and incompressible binary blends

The confinement of a polymer blend generally results in the enrichment of one component at the surface, due to a complex interplay of enthalpic and entropic factors. To study this phenomenon, we consider a lattice model of an interacting blend of two polymer species A and B with a third monomeric species C (representing free volume) between two interacting, planar surfaces of variable separation. B is a linear chain, and A may be either a linear chain or a star with a specified number of arms. The molecular weights of the two species are fixed for the case of a star-linear blend and may be either fixed or have temperature-dependent distributions for a linear-linear blend. Compressibility is determined by the bulk concentration of C. The interactions between A and B (determining the miscibility of the components), and of species A and C with the surface, may both be attractive, repulsive, or neutral. Composition profiles are numerically calculated as a function of distance from the surface for various blends, using the ultrafast recursive lattice method of Gujrati and Chhajer. This method is also used to calculate the entropy of the blend film as a function of the interaction between A and two identical surfaces. These calculations show the effects of polymer architecture, molecular weight disparity, polydispersity, film thickness, miscibility, and compressibility on surface segregation. It is found, in agreement with previous results of others, that the shorter chain in a linear-linear blend is enriched at a neutral surface, to a degree that increases with both molecular weight disparity and with compressibility. It is also found that the enrichment of stars at a neutral surface increases with the number of arms only up to a critical arm number, which, in turn, increases with compressibility.