A direct quantitative measure of surface mobility in a glassy polymer

Thin polymer films are widely used in applications and have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature $T_{\textrm{g}}$. Above $T_{\textrm{g}}$ the entire film flows, while below $T_{\textrm{g}}$ only the near surface region responds to the excess interfacial energy. An analytical thin film model for flow limited to the free surface region is developed and shows excellent agreement with sub-$T_{\textrm{g}}$ data. The system transitions from whole film flow to surface localized flow over a narrow temperature region near the bulk $T_{\textrm{g}}$. The experiments and model provide a measure of surface mobility in a sample geometry where confinement and substrate effects are negligible. \\[4pt] This work has been done in collaboration with Y. Chai, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1; T. Salez, Laboratoire de Physico-Chimie Theorique, UMR CNRS Gulliver 7083, ESPCI, Paris, France; J.D. McGraw, Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada, L8S 4M1; M. Benzaquen, Laboratoire de Physico-Chimie Theorique, URM CNRS; K. Dalnoki-Veress, Department of Physics and Astronomy, McMaster University and Laboratoire de Physico-Chimie Theorique, UMR CNRS; and J.A. Forrest, Department of Physics and Astronomy and Guelph-Waterloo Physics Institute,University of Waterloo.