Sub-Tg Cross-Linking of a Polyimide Membrane for Enhanced CO2 Plasticization Resistance for Natural Gas Separation

Decarboxylation-induced thermal cross-linking occurs at elevated temperatures (∼15 °C above glass transition temperature) for 6FDA–DAM:DABA polyimides, which can stabilize membranes against swelling and plasticization in aggressive feed streams. Despite this advantage, such a high temperature might result in collapse of substructure and transition layers in the asymmetric structure of a hollow fibers based on such a material. In this work, the thermal cross-linking of the 6FDA–DAM:DABA at temperatures much below the glass transition temperature (∼387 °C by DSC) was demonstrated. This sub- T g cross-linking capability enables extension to asymmetric structures useful for large scale membranes. The resulting polymer membranes were characterized by swelling in known solvents for the un-cross-linked materials, TGA analysis, and permeation tests of aggressive gas feed stream at higher pressure. The annealing temperature and time clearly influence the degree of cross-linking of the membranes, and results in a slight difference in selectivity for membranes under various cross-linking conditions. Results indicate that the sub- T g thermal cross-linking of 6FDA–DAM:DABA dense film membrane can be carried out completely even at a temperature as low as 330 °C. Permeabilities were tested for the polyimide membranes using both pure gases (He, O 2, N 2, CH 4, CO 2 ) and mixed gases (CO 2 /CH 4 ). The selectivity of the cross-linked membrane can be maintained even under very aggressive CO 2 operating conditions that are not possible without cross-linking. Moreover, the plasticization resistance was demonstrated up to 700 psia for pure CO 2 gas or 1000 psia for 50% CO 2 mixed gas feeds.