The interplay between wetting and phase behaviour in binary polymer films and wedges: Monte Carlo simulations and mean field calculations

By confining a binary mixture, one can profoundly alter its miscibility behaviour. The qualitative features of miscibility in confined geometry are rather universal and are shared by polymer mixtures as well as small molecules, but the unmixing transition in the bulk and the wetting transition are typically well separated in polymer blends. We study the interplay between wetting and miscibility of a symmetric polymer mixture via large scale Monte Carlo simulations in the framework of the bond fluctuation model and via numerical self-consistent field calculations. The film surfaces interact with the monomers via short-ranged potentials, and the wetting transition of the semi-infinite system is of first order. It can be accurately located in the simulations by measuring the surface and interface tensions and using Young's equation. 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/delocalization occurs which gives rise to phase diagrams with two critical points in the vicinity of the pre-wetting critical point of the semi-infinite system. 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 width Δ for antisymmetric surface fields. Upon decreasing the film width the two critical points approach the symmetry axis of the phase diagram, and below a certain width, Δtri, there remains only a single critical point at symmetric composition. This corresponds to a second order interface localization/delocalization transition even though the wetting transition is of first order. At a specific film width, Δtri, tricritical behaviour is found. The behaviour of antisymmetric films is compared with the phase behaviour in an antisymmetric double wedge. While the former is the analogy of the wetting transition of a planar surface, the latter is the analogy of the filling behaviour of a single wedge. We present evidence for a second order interface localization/delocalization transition in an antisymmetric double wedge and relate its unconventional critical behaviour to the predictions of Parry et al (1999 Phys. Rev. Lett. 83 5535) for wedge filling. The critical behaviour differs from the Ising universality class and is characterized by strong anisotropic fluctuations. We present evidence that the transition in large double wedges can be of second order although there is a first order wetting transition on a planar substrate.