Why we need to look beyond the glass transition temperature to characterize the dynamics of thin supported polymer films

Abstract

There is significant variation in the reported magnitude and even the sign of Tg shifts in thin polymer films with nominally the same chemistry, film thickness, and supporting substrate. The implicit assumption is that methods used to estimate Tg in bulk materials are relevant for inferring dynamic changes in thin films. To test the validity of this assumption, we perform molecular simulations of a coarse-grained polymer melt supported on an attractive substrate. As observed in many experiments, we find that Tg based on thermodynamic criteria (temperature dependence of film height or enthalpy) decreases with decreasing film thickness, regardless of the polymer–substrate interaction strength ε. In contrast, we find that Tg based on a dynamic criterion (relaxation of the dynamic structure factor) also decreases with decreasing thickness when ε is relatively weak, but Tg increases when ε exceeds the polymer–polymer interaction strength. We show that these qualitatively different trends in Tg reflect differing sensitivities to the mobility gradient across the film. Apparently, the slowly relaxing polymer segments in the substrate region make the largest contribution to the shift of Tg in the dynamic measurement, but this part of the film contributes less to the thermodynamic estimate of Tg. Our results emphasize the limitations of using Tg to infer changes in the dynamics of polymer thin films. However, we show that the thermodynamic and dynamic estimates of Tg can be combined to predict local changes in Tg near the substrate, providing a simple method to infer information about the mobility gradient.