Physical aging of polystyrene films tracked by gas permeability

Most studies using gas permeation to characterize physical aging in thin polymer films have focused on polymers of interest as membrane materials, such as polysulfone (PSF) and Matrimid. Many other physical aging studies, using techniques other than gas permeation, focus on polystyrene (PS). In this work, physical aging in bulk PS films and PDMS-coated thin PS films was studied using well-established gas permeation techniques. The ～400 nm PS films aged slightly faster than bulk PS. However, the difference between rates of aging in thin and thick films was much less than that reported in PSF and Matrimid films of similar thicknesses. The ～800 nm films aged in a manner generally similar to bulk PS. Comparison of the normalized oxygen permeability of ～400 nm films of PS, PSF, and Matrimid revealed that a ～400 nm PS film experiences a slower decline in relative permeability than a PSF or Matrimid film does. Unlike what has been observed previously in studies of PSF and Matrimid films, PS films do not appear to show aging behavior that is strongly dependent on film thickness or highly accelerated relative to bulk. Because it would be difficult to use the results of PS aging studies to predict the aging behavior of typical gas separation polymers, we suggest that PS is not a good model for the aging behavior of commercially useful gas separation membrane materials.