Glass Transition at Interfaces
Seminar by Thomas Salez CNRS // University of Bordeaux // Hokkaido University
The simple geometry of a polymer film on a substrate with a step at the free surface– a stepped film – is unfavourable due to the excess interface induced by the step, thus
allowing for a fine rheological nanoprobe. After describing the experimental technique and the theoretical tools , we demonstrate how they enable to directly probe and understand the surface evolution of thin polymer films below the glass-transition temperature T g . While above T g the entire volume between the substrate and the free surface participates to the flow, below T g only a near-surface region responds to the excess interfacial energy. In the latter case, the developed thin-film theory for flow limited to the free-surface region is in excellent agreement with experimental data. Furthermore, a microscopic theory based on random cooperative strings of molecular rearrangements provides support to those observations and allows to explain the T g reductions observed in thinner films . Strikingly, the system transitions from whole-film flow to surface-localised flow over a narrow temperature region near the bulk glass-transition temperature.
 Physical Review Letters 109 128303 (2012)
 Science 343 994 (2014)
 Proceedings of the National Academy of Sciences of the USA 112 8227 (2015)