Effects of CO2 exposure and physical aging on the gas permeability of thin 6FDA-based polyimide membranes: Part 1. Without crosslinking

The changes in permeability of He, O2, N2, CH4 and CO2 through thin films (∼350 nm) of 6FDA-based polyimides with and without DABA units in the chain were monitored as a function of aging time at 35 °C using different methodologies that varied the exposure of these films to CO2. A pulse of CO2 exposure after aging causes the permeability of each gas to increase; the change is greater the larger the probe gas molecule. Continuous exposure to CO2 over 100 h leads to a larger increase in permeability than periodic CO2 exposure over the same period, but the trends are basically similar for the two cases. After a CO2 pulse, the permeability relaxes and approaches the aging response established prior to CO2 exposure. The change in refractive index for CO2 exposed films is consistent with the change in gas permeability in most regards; however, the recovery of refractive index following CO2 exposure is somewhat less than observed by permeability. Periodic exposure to CO2 during aging seems to retard the decline in gas permeability with the effect being greater for larger probe molecules. An increase in CO2/CH4 and O2/N2 selectivities with aging was observed for thin films that were never exposed to CO2 while these selectivities decrease with aging when periodic exposure to CO2 was imposed.