Symmetric polymer blend confined into a film with antisymmetric surfaces: Interplay between wetting behavior and the phase diagram

We study the phase behavior of a symmetric binary polymer blend that is con fined in a thin film. The film surfaces interact with the monomers via shor t-range potentials. We calculate the phase behavior within the self-consist ent field theory of Gaussian drains. Over a wide range of parameters we fin d strong first-order wetting transitions for the st mi-infinite system, and the interplay between the wetting/prewetting behavior and the phase diagra m in confined geometry is investigated. Antisymmetric boundaries, where one surface attracts the A component with the same strength as the opposite su rface attracts the B component, are applied. The phase transition does not occur close to the bulk critical temperature but in the vicinity of the wet ting transition. For very thin films or weak surface fields one finds a sin gle critical point at phi (c)=1/2. For thicker films or stronger surface fi elds the phase diagram exhibits two critical points and two concomitant coe xistence regions. Only below a triple point there is a single two-phase coe xistence region. When we increase the film thickness the two coexistence re gions become the prewetting lines of the semi-infinite system, while the tr iple temperature converges toward the wetting transition temperature from a bove. The behavior close to the tricritical point, which separates phase di agrams with one and two critical points, is studied in the framework of a G inzburg-Landau ansatz. Two-dimensional profiles of the interface between th e laterally coexisting phases an calculated, and the interfacial and line t ensions analyzed. The effect of fluctuations and corrections to the self-co nsistent held theory are discussed.