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 Lc on the polymer blend film thickness $h,$ and gives a reasonable quantitative prediction of Lc.