Mechanical confinement effects on the phase separation morphology of polymer blend thin films

We have measured the phase separation morphology of polystyrene-poly (methyl methacrylate) (PS-PMMA) blend films of thickness h on a silicon oxide (SiOx) substrate with a SiOx capping layer. We observe a novel phase separation morphology for small capping layer thicknesses L and a transition from lateral to lamellar morphology as L is increased. We present a simple model that explains the observed lateral morphology and the transition in morphology in terms of a balance between the free energy increase associated with forming the interfaces between PS-rich and PMMA-rich domains and the free energy increase associated with the elastic bending of the SiOx capping layer. The simple model reveals the dependence of the transition capping layer thickness L-c on the polymer blend film thickness h, and gives a reasonable quantitative prediction of L-c.