Phase equilibria in thin polymer films

Within self-consistent field theory and Monte Carlo simulations the phase b ehavior of a symmetrical binary AB polymer blend confined into a thin film is studied. The film surfaces interact with the monomers via short ranged p otentials. One surface attracts the A component and the corresponding semi- infinite system exhibits a first order wetting transition. The surface inte raction of the opposite surface is varied as to study the crossover from ca pillary condensation for symmetric surface fields to interface localization /delocalization transition for antisymmetric surface fields. In the former case the phase diagram has a single critical point close to the bulk critic al point. In the latter case the phase diagram exhibits two critical points which correspond to the prewetting critical points of the semi-infinite sy stem. Only below a triple point there is a single two-phase coexistence reg ion. The crossover between these qualitatively different limiting behaviors occurs gradually, however, the critical temperature and the critical compo sition exhibit a non-monotonic dependence on the surface field. The depende nce of the phase behavior for antisymmetric boundaries is studied as a func tion of the film thickness and the strength of the surface interactions. Up on reducing the film thickness or decreasing the strength of the surface in teractions we can change the order of the interface localization/delocaliza tion transition from first to second. The role of fluctuations is explored via Monte Carlo simulations of a coarse grained lattice model. Close to the (prewetting) critical points we observe 2D Ising critical behavior. Also, there is a rich crossover behavior between Ising critical, tricritical and mean field behavior. At lower temperatures capillary waves of the AB interf ace lead to a pronounced dependence of the effective interface potential on the lateral system size.