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, Tg in thin films. The model shows that what is typically measured as a dilatometric Tg, 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 Tg value in thin films is no longer a good reporter of the dynamics. Calculations based on the model are found to agree with a vast body of thin film Tg measurements. While mathematically simple, the model contains all the necessary physics of a near surface layer with enhanced dynamics and a length scale over which the surface dynamics monotonically varies from surface enhanced to bulk-like. The model demonstrates that the typical dilatometric measurement of the glass transition is not necessarily a real glass transition.