Theory of spatially heterogeneous activated glassy relaxation in supported thin films

Glassy dynamics in supported thin polymer films is investigated using the Elastically Collective Nonlinear Langevin Equation theory under neutral confinement and sharp interface conditions where there is no change of equilibrium properties. In this theoretical framework, the alpha process is a mixed local-nonlocal event determined by large amplitude cage scale barrier hopping coupled with facilitating spontaneous elastic fluctuations of the surrounding particles. Both aspects are strongly modified under confinement in distinctive manners which depend on temperature, density, film thickness and the nature of the interfaces or substrates. Large enhancements of relaxation at the vapor interface and well into the film are predicted to coexist with substrate-stiffness-dependent suppression of motion as the solid surface is approached resulting in massive mobility gradients across the film. Bilayers with one interface have also been studied. Asymmetric mobility gradients and Tg profiles are predicted which depend on the absolute and relative stiffness of the two materials. Calculations of relaxation time and Tg gradients for model hard sphere and polymeric systems, and comparison with experimental measurements, will be presented.