Wetting reversal transition in phase-separated polymer mixtures

We investigate a wetting reversal transition in thin films of two-phase mix tures of poly(ethylene-propylene) (PEP) and its deuterated analogue (dPEP) on a substrate covered by a self-assembled monolayer (SAM) whose surface en ergy, gamma(SAM), is tuned by varying the SAM composition. As gamma(SAM) in creases from 21 to 24 mJ/m(2), a transition from a three-layer (air/dPEP/PE P/dPEP/SAM) to a two-layer (air/dPEP/PEP/SAM) structure occurs at increasin g T-c - T, where T-c and T are the critical and transition temperatures, re spectively. As the system structure changes from three-layer to two-layer, the thicknesses of the dPEP-rich wetting layers at the air/mixture and mixt ure/SAM interfaces are found to smoothly increase and decrease, respectivel y, while the thickness of the PEP-rich layer (ca. one half of the total fil m thickness) does not change. The dependence of the transition temperature on gamma(SAM) is predicted by a simple model using the experimental data on the surface energies of PEP/dPEP and estimates of the interfacial energy b etween PEP and dPEP.