Fabrication of sub-Tg cross-linked and thermally rearranged poly (benzoxazole-co-imide) hollow fiber membrane derived from phenolphthalein-based copolyimide for CO2/CH4 separation

The high temperature (above 400 °C typically) of thermal rearrangement reaction often caused pore collapse and skin layer densification of asymmetric hollow fiber, and leading to low gas permeance. To solve this problem, a novel hollow fiber membrane based on phenolphthalein was prepared and undergone a cross-linked thermally rearranged (XTR) treatment. The thermal labile lactone rings from the phenolphthalein moities were employed to cross-link the hollow fiber membrane at sub-Tg. Subsequently, the cross-linked hollow fiber was heated over 400 °C and turned into thermally rearranged membrane. The CO2 permeance increased by 70 % when the hollow fiber precursor was first cross-linked at sub-Tg for 2 h. This demonstrated the efficiency of sub-Tg cross-linking, which mitigated the sub-layer collapsing as shown in SEM images. Additionally, by optimizing the hollow fiber spinning condition and the thermal treatment procedure, the CO2 permeance was further increased to 436.22 GPU with a CO2/CH4 selectivity of 30.63, surpassing all TR hollow fiber membranes for CO2/CH4 separation. When used high pressure CO2, CO2/CH4, and CO2/CH4/N2/ethane/benzene mixed gases as feeds, no significant signs of plasticization were observed. Therefore, the anti-plasticized high performance XTR hollow fiber membrane had a great potential for natural gas purification. [Display omitted] •For the first time phenolphthalein-based XTR hollow fiber membrane was made.•Sub-Tg cross-linking alleviated sublayer collapsing and thickening of skin layer.•XTR hollow fiber had high gas separation property and anti-plasticization.