A propanediol monoester cross-linkable (PDMC) polyimide was synthesized for high-speed hollow fiber membrane spinning. The membranes were plasticization resistant and asymmetric, useful for natural gas purification. In fabricating the membrane material, it was found that significant chain scissioning could occur during the monoesterification reaction, which caused problems in subsequent membrane formation processes. Using NMR, GPC, and FTIR, it was observed that hydrolytic degradation occurs at the imide rings from the small amount of water produced during monoesterification. Workable synthesis conditions were found that enabled the cross-linkable polyimide to be synthesized with high conversions and without loss in the polymer molecular weight. The preservation of a high molecular weight (M w >100 000) was shown to be favorable for hollow fiber membrane formation.

A 6FDA-based, cross-linkable polyimide was characterized in the form of a defect-free asymmetric hollow-fiber membrane. The novel membrane was cross-linked at various temperatures and tested for natural gas purification in the presence of high CO2 partial pressures. The cross-linked membrane material shows high intrinsic separation performance for CO2 and CH4 (selectivity ∼49, CO2 permeability ∼161 barrer, with a feed at 65 psia, 35 °C, and 10% CO2). Cross-linked asymmetric hollow-fiber membranes made from the material show good resistance to CO2-induced plasticization. Carbon dioxide partial pressures as high as ∼400 psia were employed, and the membrane was shown to be promisingly stable under these aggressive conditions. The performance of the membrane was also analyzed using the dual-mode sorption/transport model.

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