(543d) Dynamic Phase Transitions in Confined Polymer Glasses

Despite more than two decades of effort, several qualitative features regarding the segmental dynamics of thin films of glass-forming materials remainly poorly understood. This challenge is of paramount importance in thin film membranes, organic electronic devices, and critical steps of semiconductor manufacturing. In this talk, I will present the results of recent simulations and experiments that suggest the dynamics near surfaces are qualitatively different from those of a bulk supercooled liquid, suggesting that the dynamics near a free surface lose their glassy character. To explore this hypothesis farther, we have used molecular simulations to investigate a non-equilibrium phase transition associated with glass-forming materials. This transition is associated with a supercooled liquid that undergoes a sharp transition from a mobile dynamic phase to a phase with reduced mobility, and it provides a clear signal in the dynamics of the glass-forming polymer under confinement. I will describe our characterization of this transition in free-standing thin films of supercooled liquids, where we find that only the bulk of the film is able to undergo this transition. This suggests that the dynamics at the free surface are not glassy in nature, at least not at the temperatures accessible to our simulations.