Why We Need to Look Beyond the Glass Transition Temperature to Characterize the Dynamics of Thin Supported Polymer Films

There is a significant variation in the reported magnitude, and even the sign of T_g shifts in thin polymer films. Many measurements assume that methods to estimate T_g of bulk materials can be applied to ultra-thin films. We validate this assumption by examining T_g from several methods using molecular simulations. As observed in many experiments, we find T_g from the variation of thermodynamics decrease for thinner films for all polymer-substrate interaction strengths. In contrast, T_g defined dynamically from the density-density correlation function similarly shows a T_g decrease with decreasing thickness for weak polymer-substrate interactions, but a T_g increase with strong polymer-substrate interactions. We show the sensitivity of these measures to the mobility gradient across the film profile is key to resolving contradictory T_g shifts. The slow-moving substrate layer of the film dominates the overall dynamic T_g, while the thermodynamic T_g is not sensitive to the substrate behavior. Our results emphasize the limitation of using a single T_g to characterize film dynamics. Instead, it is vital to consider the mobile gradient to understand changes in dynamics.