THE ENRICHMENT-DEPLETION DUALITY AS AN INDICATOR OF CRITICAL WETTING IN POLYMER BLENDS

We extend the mean field theory-based graphical technique of Cahn [J. Chem. Phys. 66, 3667 (1977)] and include a nearest-neighbor interactio n for the surface contribution, in order to identify particular binary mixture-wall combinations that are likely to exhibit critical wetting and duality. Our approach yields a molecular-based understanding of t he Schmidt and Binder wetting criteria [J. Phys. (Paris) 46, 1631 (198 5)], in terms of the effective wall composition and. We find that syst ems will exhibit critical wetting only if the wall is energetically eq uivalent to a ''frozen wall,'' a concept first introduced by Jerry and Nauman [Phys. Lett. A 167, 198 (1992)]. For a second-order transition , the solubility parameter for the wall must lie in between the solubi lity parameters of the mixture components. Our analysis reveals that s ystems having a critical wetting transition should also display an enr ichment-depletion duality. The verification of duality requires fewer experiments to be performed; thus experimentalists in search of system s exhibiting critical wetting are advised to seek this concomitant phe nomenon instead. We show that neither duality nor critical wetting sho uld occur at a free surface (a mixture-vacuum interface). (C) 1994 Aca demic Press, Inc.