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

Abstract

We have measured the phase separation morphology of polystyrene--poly (methyl methacrylate) (PS-PMMA) blend films of thickness $h$ on a silicon oxide $({\mathrm{SiO}}_{x})$ substrate with a ${\mathrm{SiO}}_{x}$ 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 ${\mathrm{SiO}}_{x}$ 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 ${L}_{c}.$