Surface and Bulk Relaxation of Vapour-Deposited Polystyrene Glasses

We report an experimental and theoretical study of the surface and bulk relaxation of glassy polystyrene produced by physical vapour deposition. The as-deposited samples are shown to be stable glass analogous to liquid-cooled glass that would have been aged for ~ 300 years. Under the perturbation of gold nanoparticles, the localised surface flow of material has been measured from the build-up of polymer material around the nanoparticle as a function of time and temperature for both stable glasses and rejuvenated glasses. In all cases, the surface evolution of the as-deposited films and the rejuvenated films are enhanced compared to bulk and are not distinguishable from each other. Strong similarities with previous studies of surface flow behaviour of high-M_w PS films prepared by spin-coating are also observed. Comparisons to numerical solutions of the glassy thin film equation provide quantitative estimates of the surface mobility. For temperatures sufficiently close to the glass-transition temperature, particle embedding is also measured and used as a probe of bulk dynamics, and in particular bulk viscosity.