Effects of Thermal Treatment and Physical Aging on the Gas Transport Properties in Matrimid®

Carbon dioxide and methane transport in a commercial polyimide, Matrimid 5218®, has been characterized in order to evaluate the effect of membrane thermal treatment and physical aging on its potentialities for CO2/CH4 separation. In particular, CO2 and CH4 permeabilities and diffusion coefficients were measured at three different temperatures (35, 45 and 55°C) in films pretreated at 50, 100, 150 and 200°C, respectively. The performances of each sample were examined for a period of more than 3 000 hours. Permeability and diffusivity values for both penetrants showed a marked decrease with increasing the pretreatment temperature up to 150°C and remained, then, substantially unchanged for specimens pretreated at the highest temperature. Interestingly, the samples characterized by the higher flux after film formation also showed a faster aging phenomenon, leading to a 25% decrease of CO2 permeability in the period inspected. Conversely, the samples pretreated at temperatures of 150°C, or higher, displayed very stable transport properties for the entire duration of the monitoring campaign. At the end of the aging period considered, the differences among samples were definitely reduced, suggesting that the initial behaviors are related to different polymer structures induced by pretreatment, which slowly evolve in time towards more similar and more stable configurations. Such aging rearrangements affect both CO2 and CH4 permeability in a similar way, so that no significant changes were observed for selectivity, which showed only a slight increment by increasing the temperature of the thermal treatment or the duration of the aging period.