Dynamical gradients, barrier factorization and interface coupling in thick and thin films of glass-forming liquids

We have developed a microscopic theory for the spatially heterogeneous dynamics of glassy polymer liquids near a vapor interface. The key activated event involves cage scale hopping facilitated by a collective elastic distortion of the surrounding medium. Three coupled physical effects enter for thick films: reduction of neighbors and weakened caging constraints nucleated very near the surface, dynamical transfer of weakened constraints in a layer-by-layer manner into the bulk, and modification of the collective elastic barrier both near and far from the interface. Predictions include an exponential spatial variation of caging constraints and the local glass transition temperature, the near factorization of the temperature and spatial location dependences of the total activation barrier, a double exponential form of the alpha time gradient characterized by a nearly constant correlation length, and position-dependent power law decoupling of the relaxation time from its bulk analog. Generalization of the ideas to thin films predicts nonadditive dynamical gradient interference effects resulting in a further enhancement of relaxation and reduction of the film averaged effective barrier with decreasing film thickness. Comparisons to simulation and experiment will be presented.