Phase diagram of polymer blends in confined geometry

Within self-consistent field theory we study the phase behavior of a symmet rical binary AB polymer blend confined into a thin film. The film surfaces interact with the monomers via short range potentials. One surface attracts the A component and the corresponding semi-infinite system exhibits a firs t order wetting transition. The surface interaction of the opposite surface is varied as to study the crossover from capillary condensation for symmet ric surface fields to the interface localization/delocalization transition for antisymmetric surface fields. In the former case the phase diagram has a single critical point close to the bulk critical point. In the latter cas e the phase diagram exhibits two critical points which correspond to the pr ewetting critical points of the semi-infinite system. Only below a triple p oint there is a single two phase coexistence region. The crossover between these qualitatively different limiting behaviors occurs gradually, however, the critical temperature and the critical composition exhibit a non-monoto nic dependence on the surface field. The dependence of the phase behavior for antisymmetric boundaries is studie d as a function of the film thickness and the strength of the surface inter actions. Upon reducing the film thickness or decreasing the strength of the surface interactions we can change the order of the interface localization /delocalization transition from first to second. The role of fluctuations is explored via Monte Carlo simulations of a coars e grained lattice model. Close to the (prewetting) critical points we obser ve 2D Ising critical behavior. At lower temperatures capillary waves of the AB interface lead to a pronounced dependence of the effective interface po tential on the lateral system size. (C) 2001 Elsevier Science B.V. All righ ts reserved.