Thomas Salez – 08 Novembre

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 [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 and allows to explain the Tg reductions observed in thinner films [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.

Figure :  The free interface of a glassy film truncates a string-like cooperative motion (red), with respect to a bulk motion (blue).
Artwork by Etienne Raphael.

Références :
[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)