Hierarchical polymer patterns driven by capillary instabilities at mobile and corrugated polymer–polymer interfaces

Abstract

We investigate the spontaneous formation of hierarchical structures during the annealing of polystyrene (PS) films cast on topographically patterned poly(methylmethacrylate) (PMMA) substrates at temperatures higher than the glass transition temperatures for both polymers. The mobile and corrugated PS/PMMA interfaces give rise to two sequential events of capillary instability of PS stripes segregated on the PMMA mesas and PS threads confined in the PMMA trenches. The latter occurred through a simultaneous “out-of-phase” fashion between the neighboring confined PS threads. Both the size and the spatial distribution of the PS droplets formed are determined by the geometric parameters of the patterns and films, and thus can be controlled for different applications. In addition to both capillary instabilities, a larger scale “wrinkling” fluctuation was also observed, with a wave vector along the original patterned line direction. The morphological evolution and structure formations are unique to the current system, and are drastically different from the polymer film dewettings on a planar surface, a chemically patterned surface, or a topographically patterned rigid surface.