The effect of chain rigidity and microporosity on the sub-ambient temperature gas separation properties of intrinsic microporous polyimides

The relationship between the rigidity, microporosity and the ideal gas separation properties of polymers of intrinsic microporosity (PIM) is very important but seldom studied in detail. Herein, we designed a novel bromine substituted intrinsic microporosity polyimide (PIM-DB-PI), and compared its gas separation properties with a more rigid and microporous PIM-PI-1 (680 vs 435 m2 g−1) from −30 to 30 °C for the first time. Both PIM-PIs showed improved ideal gas separation properties with their performance for H2/N2, O2/N2, CO2/N2 and CO2/CH4 changed from well below the 2008 trade-off lines to approach or even above their latest trade-off lines upon decreasing temperature. The PIM-PI-1 with higher microporosity and rigidity showed higher gas permeability, diffusion and solubility coefficients as well as larger activation energy of permeation (Ep) and diffusion (Ed) than PIM-DB-PI, whereas PIM-DB-PI exhibited higher gas pair selectivity derived from its larger diffusion selectivity (aD), which was attributed to its higher enthalpic selectivity. Besides, the PIM-DB-PI showed a much larger critical penetrate size ((f/c)1/2) than PIM-PI-1 due to its higher flexibility. The above finding indicates that both rigidity and microporosity are very important in fine-tuning the gas transport through the polymer membrane. The microporosity has dominant effect on the gas diffusion selectivity of the membrane and the rigidity has more effect on the critical diffusion penetrate size ((f/c)1/2) of PIM-PI membranes.