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. Recent simulations and experiments have provided compelling evidence that the dynamics near surfaces are qualitatively different from those of a bulk supercooled liquid, and in this talk I will describe our efforts at characterizing those dynamics by investigating 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.