Pattern-directed to isotropic dewetting transition in polymer films on micropatterned surfaces with differential surface energy contrast.

Abstract

Surface chemical patterns can both cause and direct dewetting in overlying thin polymer films. In this paper we focus on a key factor in this phenomenon, the magnitude of the surface energy difference between surface pattern domains (Δ). To probe the influence of Δ on film dewetting, we utilize novel combinatorial test patterns exhibiting a gradient in Δ. Specifically, our test patterns consist of a series of micron-scale striped regions that continuously change in their surface energy () relative to background striped regions having a fixed and calibrated . Using polystyrene (PS) films as a demonstration case, we employ these test patterns to quantify the morphology and kinetics of dewetting as Δ diminishes. Our study indicates a transition from pattern-directed to isotropic PS dewetting at critical Δ values. For Δ > 14 mJ m, ordered droplet arrays are formed, while for Δ < 7 mJ m, the dewetting is isotropic. A competition between these limiting behaviors is found for a "crossover regime", 7 mJ m < Δ < 14 mJ m. These combinatorial test patterns provide a powerful approach for investigating the large number of parameters that govern the stability of ultrathin polymer films, and the physical factors that influence the dewetted film morphology.