Measurement of the depth-dependent local dynamics in thin polymer films through rejuvenation of ultrastable glasses

Anomalous dynamics and glass transition temperatures in thin polymer films have been studied for 3 decades and continues to be an active research area. While it has long been noted that a measure of depth dependent dynamics is the necessary measurement to develop our understanding, such measurements have been elusive. We measure the isothermal rejuvenation of stable glass films of poly(styrene). We demonstrate that the propagation of the front responsible for the transformation to a supercooled-liquid state can serve as a highly localized probe of the local supercooled dynamics. We use this connection to probe the depth-dependent relaxation rate with nanometric precision for a series of polystyrene films over a range of temperatures near the bulk glass transition temperature. The analysis shows the spatial extent of enhanced surface mobility and reveals the existence of an unexpected large dynamical length scale in the system. The results are compared with the cooperative-string model for glassy dynamics. The data reveals that the film-thickness dependence of whole film properties arises only from the volume fraction of the near-surface region. While the dynamics at the middle of the samples shows the expected bulk-like temperature dependence, the near-surface region shows very little dependence on temperature.