Dynamics near Free Surfaces and the Glass Transition in Thin Polymer Films: A View to the Future

The past 20 years have seen a substantial effort to understand dynamics and the glass transition in thin polymer films. In this Perspective, we consider developments in this field and offer a consistent interpretation of some major findings. We discuss recent experiments that directly measure mobility at or near the surface of glassy polymers. These experiments indicate that enhanced mobility near the free surface can exceed bulk mobility by several orders of magnitude and extend for several nanometers into the bulk polymer. Enhanced mobility near the free surface allows a qualitative understanding of many of the observations of a reduced glass transition temperature T g in thin films. For thin films, knowledge of T g by itself is less useful than for bulk materials. Because of this, new experimental methods that directly measure important material properties are being developed.