Reconciling computational and experimental trends in the temperature dependence of the interfacial mobility of polymer films

摘要

Many measurements have indicated that thin polymer films in their glass state exhibit a mobile interfacial layer that grows in thickness upon heating, while some measurements indicate the opposite trend. Moreover, simulations and limited measurements on glass-forming liquids at temperatures above the glass transition temperature T-g exhibit a growing interfacial mobility scale xi upon cooling. To better understand these seemingly contradictory trends, we perform molecular dynamics simulations over a temperature regime for which our simulated polymer film enters a non-equilibrium glassy state and find that the relaxation time tau(alpha) within the film interior, relative to the polymer-air interfacial layer, exhibits a maximum near the computational T-g. Correspondingly, we also observe that the interfacial mobility length scale exhibits a maximum near T-g, explaining the apparent reversal in the temperature dependence of this scale between the glass and liquid states. We show that the non-monotonic variation of xi and the relative interfacial mobility to the film interior arise qualitatively from a non-monotonic variation of the gradient of the effective activation free energy of the film; we then obtain a quantitative description of this phenomenon by introducing a phenomenological model that describes the relaxation time layer-by-layer in the film for a temperature range both above and below T-g of the film as a whole. This analysis reveals that the non-monotonic trend in the relative interfacial mobility and xi both arise primarily from the distinctive temperature dependence of relaxation in the interfacial layer, which apparently remains in local equilibrium over the whole temperature range investigated.