Pretransitional thinning of a polymer wetting layer

Mixtures of two immiscible polymers A and B phase-separate into A-rich (alpha) and B-rich (beta) phases. Thin films of such mixtures exhibit a transition from an equilibrium 3-layer structure (substrate/alpha/beta/alpha/air) to a 2-layer structure (substrate/beta/alpha/air) as the surface energy of the substrate is increased. In this letter we show that as an (equilibrium) precursor to this transition, the thickness of the alpha layer in the 3-layer structure at the substrate/polymer interface decreases, while that of the alpha layer against air increases until the alpha layer against the substrate vanishes leaving the 2-layer structure. This pretransitional behavior originates from the long-range nature of the van der Waals interaction between layers and is predicted bg a simple model that considers the dependence of the free energies of the two alpha layers on their thicknesses.