(430c) Effects Of Supercritical CO2 Conditioning On Uncrosslinked Polyimide Membranes For Natural Gas Separations

The effects of supercritical CO2 (SCCO2) conditioning on a 6FDA-based polyimide membrane are investigated through gas permeability, sorption, and secondary characterization methods. At pressures above the supercritical point, CO2 permeability unexpectedly declines suggesting a structural reorganization of the polymer matrix facilitated by the presence of a highly soluble species. High pressure permeability isotherms for He and N2 do not exhibit a similar decline. Upon depressurization, a typical hysteresis is not observed; rather, CO2 permeability remains at reduced levels compared to the original pressurization. CO2/CH4 mixed gas testing of the sample prior to and after SCCO2 conditioning shows an enhancement in membrane separating performance after conditioning with a corresponding reduction in the membrane productivity. Repressurization of the same sample in CO2 to the supercritical region continues to exhibit reduced CO2 permeability confirming that the polymer chain packing has been altered to a new state. Wide angle x-ray diffraction and density measurements reveal a reduction in chain spacing and an increase in bulk density. Dual-mode modeling of the SCCO2 conditioned polymer suggests a large decrease in CH', or the Langmuir sorption capacity parameter. This reduction in CH' is consistent with the hypothesis that the reorganization observed in the supercritical region is a result of the removal of excess free volume through enhanced chain mobility coupled with high hydrostatic pressure.