Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O2, N2, and CH4, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm–62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer Tg to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below Tg for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14–15% increase in O2/N2 selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.