Physical aging of thin glassy polymer films

Abstract

This research work was designed to systematically investigate the physical aging of glassy polymer thin films in terms of the effects of chemical structure of the polymer, film thickness, aging temperature and molecular weight. This research is fundamental in nature but is of both scientific interest and practical importance, especially to gas separation industry where polymer thin films are essentially used as the selective layer in the asymmetric or composite membrane structures. Three glassy polymers relevant to gas separation industry, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide) were studied; these polymer films having thicknesses from ~ 400 nm to 62 µm were subjected to isothermal aging at three temperatures, ranging from 35 to 55ºC, for a period of up to ~ 400 days. Two major techniques were employed in probing the aging process including gas permeability measurement and refractive index measurement. Rigorous methodologies have been developed for studying the physical aging of free-standing thin polymer films to eliminate any other external effect that might impact the aging behavior. Ellipsometry has been employed to determine the thicknesses and refractive indices of these thin films. By using the methods developed, the reproducibility of gas permeability and refractive index change during physical aging was demonstrated as well as the thermoreversibility of physical aging. Ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial aging response via the permeability and refractive index changes, 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 Lorentz-Lorenz equation was used to relate changes in refractive index to densification, or volume relaxation, with aging time. 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, about 14 to 15% increase in O2/N2 selectivity and about 0.6 to 1.5% increase in density at an aging time of 1,000 hours.