Decoupling Role of Film Thickness and Interfacial Effect on Polymer Thin Film Dynamics

The film thickness and substrate interface are the two most common parameters to tune the dynamics of supported thin films. Here, we investigated the glass transition temperature (Tg) and thermal expansion of thin poly(methyl methacrylate) (PMMA) films with various thicknesses and different interfacial effects. We showed that, although the Tg of the thin films can be modulated equivalently by the two factors, their ability to change the expansivity (β) is quite different; that is, β increases notably with a reduction in the thickness, while it is insensitive to perturbations at the interface. We attribute the deviation in modulating β by the thickness and the interfacial effect to the disparate abilities to change the free volume content in the film by a free surface and substrate interface. This leads to a situation where thin films with dissimilar thicknesses and interfacial properties can have the same Tg but very different β values, suggesting that Tg alone cannot unequivocally quantify thin film dynamics.