Physical Aging in Ultrathin Polystyrene Films: Evidence of a Gradient in Dynamics at the Free Surface and Its Connection to the Glass Transition Temperature Reductions

We have measured the temperature dependence of the physical aging rate beta of thick (2430 rim) and thin (29 nm) polystyrene (PS) films supported on silicon using a new streamlined ellipsometry procedure that we have recently developed. The physical aging rates beta(T) for the similar to 30 nm thick films are found to be reduced at all temperatures, which is not consistent with a simple shift in beta corresponding to the average reduced glass transition temperature (T(g)) of these films. Instead, the beta(T) results correspond well with there being a gradient in dynamics near the free surface. Our beta(T) results can be well fit by both a two-layer model and a gradient model. The temperature-dependent length scale (of order 10 nm) that characterizes the depth to which the enhanced dynamics near the free surface propagate into the film is similar to that found previously by Forrest and Mattsson [Phys. Res. E 2000, 61, R53-R56] for the molecular weight (MW)-independent T(g) reductions of low-MW free-standing PS films, strongly suggesting that the same mechanism is responsible for both effects. This length scale grows with decreasing temperature, suggesting that the mechanism is cooperative in nature.