Why We Need to Look Beyond the Glass Transition Temperature to Characterize the Dynamics of Thin Supported Polymer Films

Published

May 14, 2018

Author(s)

Jack F. Douglas, Wengang Zhang, Francis W. Starr

Abstract

There is significant variation in the reported magnitude, and even the sign of Tg shifts in thin polymer films with the same chemistry, film thickness and supporting substrate. The implicit assumption of these studies is that methods used to estimate Tg in homogeneous bulk materials are relevant for inferring dynamical changes in these films. To test the validity of this assumption, we perform molecular dynamics simulations of a coarse-grained polymer melt supported on an attractive solid substrate. As observed in many experiments, we find that Tg based on quasi-thermodynamic criteria (temperature dependence of film height or energy) decreases with decreasing film thickness, regardless of the strength of polymer-substrate interactions. In contrast, we find that Tg estimates based on a dynamical criteria (the relaxation of a dynamical correlation function) show a decreased Tg for weak substrate interactions with decreasing thickness (like the thermodynamic Tg definitions), but an increased Tg when substrate interactions exceed the polymer-polymer interactions. We show that these different trends in Tg reflect differing sensitivities to the mobility gradient across the film. The slow moving polymer segments near the substrate layer contribute to the shift of Tg in the dynamic measurement, while these chain segments contribute little to the thermodynamic estimate of Tg. Our results emphasize the limitation of using a single estimate of Tg to infer changes in the dynamics of thin supported polymer films, owing to the substantial mobility gradient. On the other hand, we show how different techniques to estimate of Tg can be combined to distinguish changes in mobility at the substrate from the remaining film. Ultimately, it is knowledege of this relaxation gradient that is needed for applications.

Citation

Proceedings of the National Academy of Sciences

Pub Type

Journals

Keywords

supported polymer film, glass transition, mobility gradient, relaxation

Citation

Douglas, J. , Zhang, W. and Starr, F. (2018), Why We Need to Look Beyond the Glass Transition Temperature to Characterize the Dynamics of Thin Supported Polymer Films, Proceedings of the National Academy of Sciences (Accessed April 20, 2024)