Physics (Internal)

Anomalous Structural Relaxation in Thin Polymer Films

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract:
The glass transition and structural relaxation of the glassy state (commonly referred to as physical aging) in nanoconfined polymer films have been heavily studied. Physical aging studies in the research literature over the past 15 years have observed large increases in physical aging rate with decreasing film thickness. Surprisingly these effects are observed for micron thick films, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. Using a newly developed streamlined ellipsometry technique, we have done extensive measurements of the physical aging rate for polystyrene (PS) films of varying film thickness, aging temperature, and quench method. We find two sources of anomalous physical aging behavior in these films. By thermally quenching the films in different geometries, we find that the physical aging rate of polymer films are strongly affected by the way in which the films are supported during the thermal quench. These results suggest that the increased physical aging rate with decreasing film thickness observed in the research literature for micron thick films may be attributable to stress differences imparted in these films. A comparison of the temperature dependence of the physical aging rate for both bulk (2430 nm) and thin (29 nm) PS films supported on silicon demonstrates that the thinner films have reduced physical aging rates at all temperatures that are inconsistent with a simple shift in the temperature dependence of the aging rate corresponding to the shift in Tg observed in these films. The reduced aging rate values measured at all physical aging temperatures are consistent with a gradient in dynamics originating from the free surface of the film. Our data is well fit by a simple two-layer model that has been previously employed to explain the Tg reductions in ultrathin PS films, suggesting that the enhanced dynamics present at the free surface are responsible for both effects.