PHASE-SEPARATION VERSUS WETTING - A MEAN-FIELD THEORY FOR SYMMETRICALPOLYMER MIXTURES CONFINED BETWEEN SELECTIVELY ATTRACTIVE WALLS

Partially compatible symmetrical (N-A = N-B = N) binary mixtures of li near flexible polymers (A, B) are considered in the presence of two eq uivalent walls a distance D apart, assuming that both walls preferenti ally adsorb the same component. Using a Flory-Huggins type mean field approach analogous to previous work studying wetting phenomena in the semi-infinite version of this model, where D --> infinity, it is shown that a single phase transition occurs in this thin film geometry, nam ely a phase separation between an A-rich and a B-rich phase (both phas es include the ''bulk'' of the film). The coexistence curve is shifted to smaller values of the inverse Flory-Huggins parameter chi(-1) with decreasing D, indicating enhanced compatibility the thinner the film. In addition, due to the surface enrichment of the preferred species ( B), the critical volume fraction of A monomers is shifted away from ph i(crit) = 0.5 (where it occurs for D --> infinity due to the symmetry of the model) to the B-rich side. This behavior is fully analogous to the results established previously for the Ginzburg-Landau model of sm all molecule mixtures and to Monte Carlo simulations of corresponding lattice gas models. We argue that for symmetric walls the stable solut ions always are described by volume fraction profiles phi(z) that are symmetric as function of the distance z across the film around its cen ter, but sometimes the system is inhomogeneous the lateral direction p arallel to the film, due to phase coexistence between A-rich and B-ric h phases. Antisymmetric profiles obtained by other authors for symmetr ic boundary conditions are only metastable solutions of the mean field equations. The surface excess of B, whose logarithmic divergence as I n \phi - phi(coex)\ signals complete wetting for D --> infinity, stays finite (and, in fact, rather small) for finite D: hence studies of we tting phenomena in thin film geometry need to be analyzed with great c are.