Densification-induced hollow fiber membranes using crosslinked thermally rearranged (XTR) polymer for CO2 capture

Since thermally rearranged (TR) polymers were known as high gas permeable and processable materials, fabricating high performance hollow fiber (HF) membranes have been tried using them. However, an unexpected drawback emerged which is the gas productivity loss by thermal densification of skin layers during thermal treatment above their glass transition temperature (T-g). In this work, we used a recently reported crosslinked-TR (XTR) polybenzoxazole to develop a new class of high-flux TR hollow fibers by directly exploiting the thermal densification phenomenon. The TR temperature range (320-460 degrees C) and T-g (394 degrees C) were determined by thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The chain rigidity of the XTR polymer increased during an isotherm treatment at its T-g, suggesting a restricted densification. Surprisingly, the undesired pinhole-defects (pore diameter < 5 nm) on precursor fibers were perfectly healed after thermal treatment (> 400 degrees C), forming an ultrathin defect-free skin layer on thermally-densified XTR hollow fiber membranes. The pore-healed XTR hollow fibers exhibited an outstanding CO2 permeance of similar to 2300 GPU and a CO2/N-2 selectivity of 17.4 with a skin thickness of 103 nm.