Bottom up approach to study the gas separation properties of PIM-PIs and its derived CMSMs by isomer monomers

Resolving the relationships between the molecular structure and gas separation properties of microporous polymers and their derived carbon molecular sieve membranes (CMSMs) are extremely important in obtaining highly efficient gas separation polymer membranes and CMSMs. To elucidate the correlations, we designed two microporous polyimides (CTPI and MTPI) derived from two triptycene-based diamine isomer monomers, that is, 2,6-triptycene diamine (C2 symmetry, CTA) and 2,7-triptycene diamine (Cs symmetry, MTA). The MTA shows a higher dipole moment than CTA (2.47 vs 1.83 Debye). As a result, the MTPI derived from MTA exhibits a slightly higher density (1.25 vs 1.23 cm3 g−1), smaller fractional free volume (0.250 vs 0.262), lower surface area (148 vs 193 m2 g−1) and smaller micropore size (7.5 vs 9.0 Å), higher glass transition temperature (345.8 vs 329.3 °C) and lower gas permeability but a slightly higher gas pair selectivity than CTPI. The MTPI also demonstrates a higher maximum derivative of weight loss temperature (547.6 vs 546.4 °C) than CTPI, and the 550 °C pyrolyzed MTPI (MTPI-550) shows a higher surface area (772 vs 703 m2 g−1) and larger micropore (6.4 vs 5.4 Å) than the corresponding CTPI-550. Consequently, the MTPI-550 shows higher gas permeability than CTPI-550. The CO2 permeability of MTPI-550 is 9878 Barrer combined with CO2/CH4 ideal selectivity of 44.5, which is one of the best performances among reported CMSMs. The barometric sorption results indicate that the higher permeability of CMSMs than the PIM-PI precursors is due to the larger diffusion and solubility coefficients because of huge enhanced microporosity, and the higher selectivity is attributed to their diffusion selectivity by the smaller pore size of CMSMs. The above findings clearly show that the larger dipole moment isomer leads to a compacter polymer chain packing of the corresponding PIM-PI, and a more open structure in the resulting CMSM due to the larger dipole-dipole interaction delayed its decomposition speed during the CMSM formation.