Interplay between wetting and phase behavior in binary polymer films and wedges: Monte Carlo simulations and mean field calculations

Abstract

We study the interplay between wetting and miscibility of a symmetric polymer mixture via MC simulations and SCF calculations. The film surfaces interact with the monomers via short ranged potentials and the wetting transition is of first order. If both surfaces in a film attract the same component, capillary condensation occurs and the critical point is close to the critical point of the bulk. If surfaces attract different components, an interface localization occurs which gives rise to phase diagrams with two critical points in the vicinity of the pre-wetting critical point. The crossover between these two types of phase diagrams as a function of the surface field asymmetry is studied. We investigate the dependence of the phase diagram on the film thickness D for antisymmetric surface fields. Upon decreasing the film thickness the two critical points approach the symmetry axis of the phase diagram, and below a certain thickness D_tri there remains only a single critical point at symmetric composition. This corresponds to a second order interface localisation transition even though the wetting transition is of first order. At a specific film thickness, D_tri, tricritical behavior is found. The behavior of antisymmetric films is compared with the phase behavior in an antisymmetric double wedge. While the former is the analogon of the wetting transition of a planar surface, the latter is the analogon of the filling behavior of a single wedge. We present evidence for a second order interface localization transition in an antisymmetric double wedge and relate its unconventional critical behavior to the predictions of Parry et al. PRL 83 5535 (1999) for wedge filling. The critical behavior differs from the Ising universality class and is characterized by strong anisotropic fluctuations.