lmprovement of CO_2/CH_4 separation characteristics of polyimides by chemical crosslinking

摘要

Uncrosslinked and crosslinked polyimides and copolyimides have been synthesized in order to increase selectivity without an unacceptable loss in perrneabiIity. The goal was to reduce undesirable effects caused by CO_2 induced swelling in CO_2/CH_4 separation processes by stabilizing the polyin1ide structure with crosslinks. In the polymerization reaction 6fp.DA (4,4'- (hexafluoroisopropylidene)diphth anhydride) was used as dianhydride monomer and mPD (m-phenylene diamine) and DABA (diamino benzoic acid) were used as diamine monomers. With copolyimides containing strong polar carboxylic acid groups (i.e. 6 FDA-mPD/DABA 9:l ) reduced plasticization was seen up to a pure CO_2 feed pressure of l4 atm, presumably due to hydrogen bonding between the carboxylic acid groups. By chemical crosslinking of the free carboxylic acid groups of the 6fp.DA-mPD/DABA 9:l with ethylene glycol. the swelling effects due to CO_2 can be reduced at least up to a pure CO_2 feed pressure of 35 atm. With increasing degree of crosslinking, increasing CO_2/CH_4 selectivity was found because of reduced swelling and polyrner chain mobility. By using ethylene glycol as a cross]inking agent, CO_2 permeability was not significantly lowered because the reduced chain mobility was compensated by the additional free volume caused by the crosslinks.