Reconciling Computational and Experimental Trends in the Temperature Dependence of Interfacial Mobility in Polymer Films

Many measurements have indicated that thin supported polymer films in their glass state exhibit an interfacial layer of enhanced mobility whose thickness grows upon heating, as found also in crystalline materials approaching their melting temperature,T_m, while simulations and limited measurements of such films above their glass transition tempearture T_g instead exhibit a region of enhanced mobility whose thickness ξ grows upon cooling. To better understand these contradictory trends, we performed MD simulations over a T range over which our simulated polymer films enters a glassy state, and found that the relaxation time τ_α within the film interior, relative to the polymer-air interfacial region, exhibits a maximum near the observed computational glass transition temperature, T_g,c, reconciling previous measurements and simulations of supported polymer films in their glass and liquid states. Correspondingly, we also observe that the interfacial mobility scale exhibits a maximum near T_g,c, but the scale of collective polymer segment exchange motion increases monotonically upon cooling below T_g,c so that the interfacial mobility scale is no longer linked to the scale of collective motion in the non-equilibrium glass state.