We have measured the phase separation morphology of polystyrene–poly (methyl methacrylate) (PS-PMMA) blend films of thickness $h$ on a silicon oxide $\left({\mathrm{SiO}}_x\right)$ 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 $L_c.$

• Received 28 October 1997

DOI:https://doi.org/10.1103/PhysRevE.57.5811