Substrate vs. free surface: Competing effects on the glass transition of polymer thin films

The glass-transition temperature (Tg) of polymer thin films can be strongly influenced by the combined effects of the supporting solid substrate and the free surface. The relative importance of these two effects, which often compete with each other, depends on the strength of the substrate-film interactions. Utilizing an atomistically informed coarse-grained model for poly(methyl methacrylate) (PMMA), here we uncover the relationship between the substrate-film interfacial energy and the spatial distribution of Tg across thin films. We find that above a critical interfacial energy, the linear dependence of film Tg on the interfacial energy breaks down and film Tg attains an asymptotic value. Analyses on the spatial variation of Tg across the thin film reveal that the short-range interface near the cohesive surface generates a long-range interphase that leads a spatially uniform appreciation of Tg throughout the film, unlike weakly cohesive surfaces that show sharp gradients along the depth of film. These findings explain recent experiments and reveal a versatile approach for tuning film Tg via engineered substrate-film interactions.