Gradient overlap effects in ultra-thin films*

The dynamics of polymers and other glass-forming liquids can exhibit largegradients in the nanoscale vicinity of interfaces. Progress has been made over the last 10 years in understanding the phenomenology of these gradients. An 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 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.