A novel cross-linking approach for a carboxylic acid-containing 6FDA-based copolyimide is presented and characterized. This new type of cross-linking renders the polyimide insoluble in typical solvents and greatly enhances the resistance to plasticization under high CO2 pressure. Charge transfer complexing, oligomer cross-linking, decomposition, and dianhydride formation are all ruled out as possible causes of these enhanced properties. Rather, high thermal annealing temperatures decarboxylate the pendant acid group which creates a phenyl radical capable of attacking other portions of the polyimide for cross-linking. CO2 permeation isotherms reveal the enhanced stability against plasticization, and IR and C NMR confirm the evolution of CO2 and loss of the carbonyl carbon from the polymer as a result of the cross-linking.

The effects of supercritical CO 2 (scCO 2 ) conditioning on high-performance cross-linked polyimide membranes is examined through gas permeation and sorption experiments. Under supercritical conditions, the cross-linked polymers do not exhibit a structural reorganization of the polymer matrix that was observed in the non-cross-linkable, free acid polymer. Pure gas permeation isotherms and mixed gas permeabilities and selectivities show the cross-linked polymers to be much more stable to scCO 2 conditioning than the free acid polymer. In fact, following scCO 2 conditioning, the mixed gas CO 2 permeabilities of the cross-linked polymers increased while the CO 2 /CH 4 separation factors remained relatively unchanged. This response highlights the stability and high performance of these cross-linked membranes in aggressive environments. In addition, this response reveals the potential for the preconditioning of cross-linked polymer membranes to enhance productivity without sacrificing efficiency in practical applications which, in effect, provides another tool to "tune" membrane properties for a given separation. Finally, the dual mode model accurately describes the sorption and dilation characteristics of the cross-linked polymers. The changes in the dual mode sorption model parameters before and after the scCO 2 exposure also provide insights into the alterations in the different glassy samples due to the cross-linking and scCO 2 exposure.

The effects of supercritical CO2 (scCO2) conditioning are examined through changes in gas permeability and solubility for 6FDA-based polyimide membranes. At pressures above the supercritical point, both CO2 permeability and sorption unexpectedly decline, suggesting a structural rearrangement of the polymer 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. Permeability isotherms of the conditioned polymer show a reduction in transport compared to unconditioned samples; however, sorption isotherms show an increase in penetrant solubility following scCO2 conditioning. Pre- and post-CO2/CH4 mixed gas permeation testing at 35 °C shows a reduction in membrane permeability with a corresponding increase in the separation factor after scCO2 conditioning.

No abstract

No abstract