Accounting for the thickness dependence of the Tg in supported PS films via the volume holes diffusion model

The glass transition temperature (T-g) of polymer thin films has been a subject of controversy in the last two decades. Determinations of the thermal T-g, that is the temperature at which a discontinuity in thermodynamic coefficients is observed, generally suggest a significant depression, whereas the molecular mobility is found to be unchanged. These experimental evidences have been recently rationalized within the volume holes diffusion model. Such model was previously applied to the description of the out-of-equilibrium dynamics in different nanostructured systems, included polymer freestanding films. According to the model, T-g depression emerges from the ability of thinner films to maintain equilibrium, due to the shorter distance free volume holes have to diffuse to the polymer interface, the molecular motion determining the diffusion coefficient being thickness independent. The present study aims to extend the application of the model to supported films that is systems where adsorption at the polymer/substrate interface limits the amount of "free" interface available to free volume holes elimination. Our analysis demonstrates that the model is able to quantitatively capture the thickness as well as the cooling rate dependence of the T-g depression in a variety of sample configurations. (C) 2013 Elsevier B.V. All rights reserved.