  We study the phase behavior of a symmetric binary polymer blend which is
confined into a thin film. The film surfaces interact with the monomers via
short range potentials. We calculate the phase behavior within the
self-consistent field theory of Gaussian chains. Over a wide range of
parameters we find strong first order wetting transitions for the semi-infinite
system, and the interplay between the wetting/prewetting behavior and the phase
diagram in confined geometry is investigated. Antisymmetric boundaries, where
one surface attracts the A component with the same strength than the opposite
surface attracts the B component, are applied. The phase transition does not
occur close to the bulk critical temperature but in the vicinity of the wetting
transition. For very thin films or weak surface fields one finds a single
critical point at $\phi_c=1/2$. For thicker films or stronger surface fields
the phase diagram exhibits two critical points and two concomitant coexistence
regions. Only below a triple point there is a single two phase coexistence
region. When we increase the film thickness the two coexistence regions become
the prewetting lines of the semi-infinite system, while the triple temperature
converges towards the wetting transition temperature from above. The behavior
close to the tricritical point, which separates phase diagrams with one and two
critical points, is studied in the framework of a Ginzburg-Landau ansatz.
Two-dimensional profiles of the interface between the laterally coexisting
phases are calculated, and the interfacial and line tensions analyzed. The
effect of fluctuations and corrections to the self-consistent field theory are
discussed.
