Control of surface morphology in blockcopolymerhomopolymer thin films

Abstract

The structure of the thin films of symmetric diblock copolymer is governed by boundary conditions, such as commensurability and interfacial interactions. For thin films having lamellar morphology, film thickness is quantized to nL0 (where n is an integer and L0 is the bulk equilibrium lamellar period) in case of symmetric wetting or (n+1/2)L0 in case of asymmetric wetting. If the initial film thickness is not commensurate with this constraint, either holes or islands of height L0 are formed. In our work, unusual hierarchical structures of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) thin films were induced by residual solvent after annealing above Tg. In order to investigate how to reduce lateral strains imposed on the hole layer, PS-b-PMMA/PPO thin films were prepared. It is well known that (poly(2,6-dimethyl 1,4-phenylene oxide)) (PPO) is very miscible with PS block of PS-b-PMMA block copolymer (χPS-PPO<0), while it is immiscible with PMMA block (χPMMA-PPO>0). By adding PPO to block copolymer, L0 in bulk is increased after temporary decreasing and the hierarchical structures of the thin films were effectively suppressed. Surface morphology was investigated using optical microscopy (OM) and atomic force microscopy (AFM) and equilibrium lamellar domain pacing in bulk sample was measured by small angle x-ray scattering (SAXS). Keywords: blockcopolymer/homopolymer blend, thin films, islands and holes, hole-in-hole, optical microscopy, atomic force microscopy, small angle X-ray scattering.