Physical aging of 6FDA-based polyimide membranes monitored by gas permeability

The aging behaviors, as judged by gas permeability, of two glassy 6FDA-based polyimides, 6FDA-DAM and 6FDA-mPDA, in thick and thin film forms are reported. Their O2 and N2 gas permeabilities were monitored over several thousands of hours. In general, the properties of these thin films deviate dramatically from their bulk behavior, by showing much more rapid decrease in gas permeability and increase in selectivity. Owing to the high free volume, the 6FDA-DAM thin films have very high permeability at the very early aging time, followed by an order of magnitude decrease in permeability relative to the initial value over the course of 1000 h of aging. On the other hand, a thick 6FDA-DAM polymer film maintained a high permeability over thousands of hours of aging. 6FDA-mPDA thin films have moderate aging rates comparable to thin films made from other polyimides such as Matrimid®. Effect of PDMS coating on gas permeability and aging was examined using a few 6FDA-mPDA thin film membranes and was found to be insignificant.