Dimensionally-controlled densification in crosslinked thermally rearranged (XTR) hollow fiber membranes for CO2 capture

Abstract

Thermal densification in asymmetric hollow fibers fabricated using thermally rearranged (TR) polymers has been regarded as a challenging issue due to productivity reduction by severe permeance loss. However, it has recently been reported from our group that the densification phenomenon could be exploited to induce ultrathin skin layer from highly porous precursor fibers. We successfully prepared densification-induced crosslinked thermally rearranged (diXTR) fibers from porous XHPI precursor fibers. The proposed hollow fiber fabrication method using the XTR material effectively enhanced CO2 permeance by 2-fold, without any loss of CO2/N-2 selectivity, compared to a traditional method. Extending from our previous work, in this study it was found that the densification during TR process can be dimensionally restricted in order to maximize the gas permeance. Generally, the thermal densification induces omnidirectional shrinkage of heat treated fibers above Tg. The longitudinal shrinkage can be prevented by physically holding both ends of hollow fibers during thermal treatment. This approach was applied to diXTR hollow fibers, which allowed a remarkable CO2 permeance exhibiting around 4,600 GPU with 18 CO2/N-2 selectivity. It was discovered that an effective suppression of the thermal densification occurred at the vicinity of Tg. A skin layer thickness of 52 nm was achieved (calculated using O-2 permeance). Evaluation of mechanical properties resulted in no evidence of mechanical weakness in dimensionally-controlled diXTR (2D-diXTR) fibers. Additionally, a direct methanol treatment was adopted to restore CO2 permeance of 30 day elapsed 2D-diXTR fiber modules. The method effectively recovered CO2 permeance up to 92% for an original permeance.