Temperature-induced transition of the diffusion mechanism of n-hexane in ultra-thin polystyrene films, resolved by in-situ Spectroscopic Ellipsometry

In-situ Spectroscopic Ellipsometry is used to study diffusion of liquid n-hexane in silicon wafer supported 150 nm thick polystyrene films, in the temperature range 16–28 °C. In the higher part of this temperature range Case II diffusion is shown to be dominant. In this case the temporal evolution of the ellipsometric spectra is adequately described by an optical model that divides the polystyrene into a swollen layer and a non-swollen layer, separated by a sharp diffusion front, i.e. mimicking the essential characteristics of the Case II diffusion process. This description is found to be applicable for tracking of the penetrant wave progressing through films with a thickness between 100 nm–1000 nm, covering much of the technologically important thin polymer film range. For temperatures below 20 °C transport is shown to occur by a combination of Fickian diffusion and polymer relaxation (Fickian Relaxation). The temperature-induced transition from Case II to Fickian Relaxation finds validation in the Spectroscopic Ellipsometry modeling and is related to the occurrence of penetrant induced glass transition of the system at around 20 °C. For the kinetics of swelling below the glass transition of the n-hexane – polystyrene mixture Berens – Hopfenberg formalism is applied to extract the penetrant diffusion coefficient and characteristic chain relaxation time.