Morphology of Ultrathin Supported Diblock Copolymer Films: Theory and Experiment

Abstract

An analysis of the morphological behavior of substrate-supported diblock copolymer films for thicknesses t below the equilibrium period L0 of the copolymer is presented. Substrate-supported films generally exhibit dissimilar interactions between the copolymer block components and the free and substrate surfaces. Accordingly, in this study, self-consistent-field calculations that incorporate asymmetric surface energetics were used to assess equilibrium film morphologies. Phase diagrams were constructed as a function of film thickness, surface interaction energies, the segmental interaction, and the chain length. In conjunction, experiments were conducted on a series of polystyrene-b-poly(n-alkyl methacrylate) copolymer films supported by silicon substrates. These employed a novel atomic force microscopy technique that allowed for the precise tracking of morphology as a function of film thickness. Comparison of the experimental results and calculations revealed several common trends. In particular, hybrid morphologies, incorporating both surface-parallel and surface-perpendicular elements, were observed both experimentally and through the calculations for the thickness regime, t ∼ 0.5L0. The stability of such structures was found to be closely linked to the symmetry of the surface energetics.