Probing Slow Dynamics in Supported Thin Polymer Films

We have used variable cooling rate ellipsometric measurements to probe the slow dynamics in thin supported polystyrene films. For the slowest cooling rates ($\sim 1\mathrm{K}/\min$) the measured $T_g$ values are reduced below the bulk value with the measured $T_g$ of 341 K for a 6 nm film. As the cooling rate is increased the $T_g$ reductions become smaller until at cooling rates $>90\mathrm{K}/\min$ there is only slight evidence for a film-thickness-dependent $T_g$ value. By relating the cooling rate to a relaxation time, we show that the relaxation dynamics of the thin films appears to become Arrhenius with an activation energy that decreases with decreasing film thickness. We discuss this in terms of a possible connection to a length scale for cooperative motion. Finally, the results can be used to resolve a number of outstanding contradictory reports in the literature.