Order-to-disorder transition of block copolymer thin films and its dependence on thickness and preferential interaction

Abstract

Unlike the phase transitions in the bulk block copolymers (BCPs), the order-to-disorder transition (ODT) in film geometry is influenced by interfacial interactions. An intuitive prediction is that a preferentially selective interaction toward one block promotes microphase separation. Such a prediction is supported by a few theoretical and experimental reports, but a few studies including thermally generated defects or field fluctuation effects suggested an increase in (χN)ODT value with decreasing film thickness. In this study, we focus on the phase transition behavior of cylinder- and lamella-forming block copolymer films subject to an asymmetric wetting condition. Our self-consistent field theory calculation using a discrete bead-spring model with finite-range interactions exhibits a decrease in (χN)ODT with decreasing film thickness for both lamella- and cylinder-forming BCP films. This result is consistent with our expeirments using PS-b-P2VP BCP films with an asymmetric wetting condition that confines the films with selective interactions of the PS/air and P2VP/substrate. For both theory and experiment, the onset of the ODT shift was much thicker for the lamellar case (~20 layers) than for the cylinder case (~10 layers)