John H. Dillon Medal (2020): Thermorheological Complexity at Polymer Surfaces

A polymer glass surface is not so glassy, as there exists a layer with enhanced molecular mobility. Here, we present our results from combined experiments and simulations of creep at a polymer free surface. We reveal a distinct mode of molecular dynamics at a polymer free surface. This unique mode of surface dynamics causes chains near the free surface that are unentangled in the bulk to exhibit a mechanical response characteristic of bulk entangled polymers. In chains that are already entangled in bulk, this effect leads to prolonging of the entangled response. Moreover, in both cases, the breadth of the entangled response grows on cooling, leading to a breakdown in time-temperature-superposition at the free surface.