Tuning interchain cavity of fluorinated polyimide by DABA for improved gas separation performance

Fluorinated polyimides with biphenyl structure and –CF3 with rigid chain structure exhibit great potential in membrane-based gas separation. However, the comprehensive separation performance of this type polyimides is not competitive as a gas separation material. Structure regulation is of great significance to improve the separation performance of polymer membrane. Few studies systematically investigated structure-property relationships in fluorinated polyimide structure. In this study, carboxylic groups are introduced into a fluorinated polyimide(6FDA-TFMB) via copolymerization with DABA diamine monomer. The hydrogen bond between –CF3 group in TFMB and –COOH group in DABA is proposed to enhance chain segment rigidity and optimize polymer chain arrangement. WAXD results and FFV data indicate that inter-chain cavity of fluorinated polyimide could be finely tuned by adjusting molar ratio of TFMB and DABA in the copolyimide. The resulted copolyimide membranes thus demonstrate tunable gas permeation properties. Gas permeability of 6FDA-TFMB/DABA firstly increases and then decreases, and ideal gas selectivity continuously increases with increasing DABA molar ratio. Among the polyimides, 6FDA-TFMB/DABA(6:4) membrane demonstrated superior separation performance, e.g. CO2 permeability of 135.6 Barrer and CO2/CH4 ideal selectivity of 31.6, surpassing the commercial cellulose acetate and Matrimid membrane. Attributed to the optimized interchain interaction, the 6FDA-TFMB/DABA (6:4) membrane displays an optimized CO2 plasticization resistance and physical aging in comparison with 6FDA-TFMB membranes. This study provides a guideline for controlling and optimize the performance of biphenyl fluorinated polyimide gas separation membrane.