Glass transition of a polystyrene surface as detected via two-dimensional diffusion of Au atoms during physical vapor deposition

Abstract

We investigated two-dimensional diffusion of Au atoms across the surface of a polystyrene (PS) film during physical vapor deposition to determine the surface glass transition temperature (Tg). Deposited Au atoms underwent two-dimensional diffusion on the PS surface until they coalesce with other atoms or aggregated particles. The diffusivity of Au on the PS surface was evaluated from the size and the number density of yielded Au nanoparticles. The diffusion coefficient D of Au was found to exhibit a drastic change near 356 K, which was assigned to the surface Tg of the PS substrate. A stronger temperature dependence of D observed near Tg suggests that Au diffusion is coupled with the segmental dynamics of PS. The change in the diffusion coefficient suggests an essential difference in the diffusion mechanism below and above the Tg of the PS surface. We assume that diffusion on a glassy PS surface reflects a more heterogeneous feature of the PS surface than that above Tg. This was qualitatively supported by the results from Monte Carlo simulations with varying activation energy distributions.