INSP - 4 place Jussieu - 75252 PARIS Cedex 05 - Barre 22-23 - 3e étage, salle 317
Thomas Salez - Laboratoire de Physico‐Chimie Théorique (PCT), UMR CNRS 7083 GULLIVER, ESPCI ParisTech
Abstract
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 new rheological nanoprobe. After describing the experimental technique and theoretical tools [1], we demonstrate how they enable to directly probe and understand the surface evolution of thin polymer films below the glass transition temperature Tg [2]. While above Tg the entire volume between the substrate and the free surface participates to the flow, below Tg 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 [3]. Strikingly, the system transitions from whole film flow to surface localised flow over a narrow temperature region near the bulk glass transition temperature. The measurements and models presented provide a quantitative measure of surface mobility. Therefore, this study may contribute to feed further the ongoing debate around glass transition, at interfaces, and in confinement.
[1] Physical Review Letters 109 128303 (2012)
[2] Science 343 994 (2014)
[3] Proceedings of the National Academy of Sciences of the USA 112 8227 (2015)