Block copolymer ordering on elastomeric substrates of tunable surface energy

Abstract

We report a range of surface characteristics from strong and partial dewetting to complete wetting for thin films of cylinder and lamellae forming block copolymers (c-BCP and l-BCP, respectively) of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) on flexible substrates. BCP of various molecular weights (Mw) was applied on elastomeric polydimethylsiloxane (PDMS) substrates of distinct cross-link densities and various surface energy (SE) obtained by exposure to UV-ozone. We demonstrate that lower Mw c-BCP films dewet faster than l-BCP films of higher Mw applied on PDMS of lowest cross-link density and SE < 25 mJ/m2. Partial dewetting was observed for c-BCP films applied on the lowest cross-link density substrates with 25 mJ/m2 < SE < 32 mJ/m2, with a sharper transition from dewetting to wetting for l-BCP films. Finally, complete wetting (stable films) occurred for both c-BCP and l-BCP films on PDMS substrates with lowest cross-link density and SE > 32 mJ/m2, as well as on PDMS of the highest cross-link density with SE > 25 mJ/m2. In this extended SE wetting regime, c-BCP show vertical orientation when applied on both types of cross-linked PDMS in a narrow neutral SE range (39–40) mJ/m2, while being oriented parallel to the substrate when SE > 40 mJ/m2. The ratio of vertically oriented l-BCP however decreases gradually when applied on substrates with a higher SE range (40–65 mJ/m2) in comparison to the sharply transitioning c-BCP orientation, reflecting the intrinsic stability of vertical lamellae over a wider range of substrate SE, consistent with theoretical estimates. These results have important ramifications for design of next-generation flexible electronics utilizing BCP thin films.