Spectroscopic Ellipsometry Investigation of the Swelling of Poly(Dimethylsiloxane) Thin Films with High Pressure Carbon Dioxide

The thickness and refractive index of poly(dimethylsiloxane) (PDMS) thin films on silicon, exposed to high-pressure carbon dioxide, have been measured in-situ by spectroscopic ellipsometry. The swelling of the film is determined from the thickness and the sorption of CO 2 from the refractive index. The sorption and swelling values for the thin films exceed those of the bulk films. These increases may be attributed to excess CO 2 at the polymer thin film−CO 2 and polymer−silica interfaces and the influence of the silica surface and CO 2 on the orientation of the polymer. Pressure-induced birefringence of the fused-silica windows was addressed in the model calculations and corrections were found to be successful for a thermal oxide reference wafer and the PDMS films. Large surface excesses of CO 2 on the reference wafer were observed in regions where CO 2 is highly compressible due to critical adsorption.