When Does a Glass Transition Temperature Not Signify a Glass Transition?

We present a simple model that links enhanced mobility at the free surface to the dilatometric glass transition temperature, T_g in thin films. The model shows that what is typically measured as a dilatometric T_g, characterized by the hallmark "kink" in the plot of film thickness versus temperature, only represents the dynamics of an infinitesimally thin layer of the sample. In other words, the measured dilatometric T_g value in thin films is no …