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

Abstract

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, O2, N2, CH4, CO2) and mixed gases (CO2/CH4). The selectivity of the cross-linked membrane can be maintained even under very aggressive CO2 operating conditions that are not possible without cross-linking. Moreover, the plasticization resistance was demonstrated up to 700 psia for pure CO2 gas or 1000 psia for 50% CO2 mixed gas feeds.

Physical aging in carbon molecular sieve membranes

“…Clearly, properties of polymeric membranes may be affected by contaminants or penetrant induced plasticization [37][38][39][40], etc. This fact notwithstanding, ''aging'' in polymeric membranes generally refers to physical aging in non-equilibrium glassy polymers undergoing physical rearrangements to approach an equilibrium state.…”

“…3. 6FDA-DAM and 6FDA/ BPDA-DAM were synthesized by a two-step reaction in which the first step produced a high molecular weight polyamic acid at low temperature ($5°C) followed by a second, imidization step and a final drying step at 210°C under vacuum to produce the polyimides [40].…”

Pure- and mixed-gas permeation properties of highly selective and plasticization resistant hydroxyl-diamine-based 6FDA polyimides for CO2/CH4 separation

“…However, many polyimides have similar limitations in their practical use as cellulose acetate because of CO 2 -induced plasticization, which often results in unacceptable loss in CO 2 /CH 4 selectivity [16][17][18][19]. However, recent studies demonstrated that plasticization in polyimides can be mitigated to a large extend by various methods, such as: (i) Thermal annealing [20][21][22][23], (ii) chemical cross-linking [22,[24][25][26][27][28][29][30] and (iii) formation of charge transfer complexes [31].…”

Carbon molecular sieve membrane structure–property relationships for four novel 6FDA based polyimide precursors

“…The inhibited chain packing introduced by the bulky 6F group provides high free volume and promotes increased solubility to enable facile processing and high permeability in the precursor polymers and resultant CMS materials[27]. Extensive earlier work on 6FDA-DAM, 6FDA:BPDA/DAM, and 6FDA/DAM:DABA CMS films[12,[28][29][30][31] showed DAM to be useful in forming high-performance CMS films; however DETDA, resulting from replacement of two methyl groups in DAM by two ethyl groups, has not been studied previously ODA unit with the packing disruptive 1-5ND unit, seemed like a useful strategy to potentially provide both high selectivity and high permeability.The synthesis of all polyimides involves polycondensation between relevant dianhydrides and diamines in two steps, asFigure 2illustrates for the 6FDA/DETDA:DABA(3:2) case. In the…”