Gradient overlap effects in the thin films

The dynamics of polymer and other glass-forming liquids can exhibit massive gradients in the nanoscale vicinity of interfaces – an effect that is not locally correlated with microscopic changes in structure. A major outstanding question is how these gradients behave in extremely thin films, where gradients emanating from distinct interfaces can interact. Here we report on the results of ultra-thin film simulations probing dynamics and glass formation locally and globally in this gradient-overlap regime. Results point to three general regimes of thin film behavior: one when the film thickness is greater than twice the gradient range; one in which the gradients overlap but do not individually span the film; and an ultra-thin-film limit in which each gradient span fully to the other interface. We report on distinct behaviors in these regimes in terms of the form of the gradients, the presence or absence of a bulk-like domain, and the breadth of the overall film glass transition. These findings have implications for the interpretation of dynamical data in ultra-thin films and for the underlying origin of alterations in dynamics in the nanoscale vicinity of interfaces.