Decarboxylation crosslinking of polyimides with high CO2/CH4 separation performance and plasticization resistance

Abstract Two carboxylic acid containing diamines, CADA1 and CADA2, with or without bulky CF 3 group were synthesized and polymerized with 6FDA, BTDA and DSDA to form four polyimides, denoted as 6FDA-CADA1, 6FDA-CADA2, BTDA-CADA1 and DSDA-CADA1. After the thermal induced decarboxylation crosslink reaction, polyimides with 100 times’ higher CO 2 permeabilities, similar CO 2 /CH 4 selectivities and much better plasticization resistant properties were obtained compared with the un-crosslinked polyimides. The polymer d-spacing increased with the heating temperature and those of the fully crosslinked polyimides, 6FDA-CADA1-450 and DSDA-CADA1-450, were 5.81 A and 5.51 A, respectively, which were very close to the estimated distance between the crosslinked polymer chains (5.57 A, calculated by Material Studio). The CO 2 -induced plasticization phenomenon was not observed at a CO 2 pressure up to 30 atm for polyimides after thermally treated at temperatures above 425 °C. Moreover, polyimides crosslinked at different temperatures had almost the same selectivities, but their permeabilities gradually increased with the crosslinking temperature. For CO 2 /CH 4 gas pair, the separation performances of 6FDA-CADA1-425 and BTDA-CADA1-450 membranes surpassed the “2008 Robeson Upper Bound”, and other membranes thermally treated at 400 and 425 °C were all positioned near the “1991 Robeson Upper Bound”.