Glass Transition Temperature of Polymer Films
Ophelia K. C. Tsui
Department of Physics, Boston University, Boston, MA
Abstract
Unlike simple liquids, polymers typically transform into a solid-like glassy state by the glass transition – a process in which the liquid viscosity increases rapidly upon cooling past the glass transition temperature (Tg). There has long been a debate on the cause of reduction in Tg found in a variety of polymer films with decreasing film thickness. In this study, we measure the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures, and find that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in Tg observed before. More importantly, the data can be explained by a straightforward application of the hydrodynamic equation to the films presumed to possess a high-mobility layer at the free surface atop a bulk-like inner layer. This result suggests that the surface mobile layer promotes the dynamics of the films by providing an additional, high-mobility flow channel to the films.