Effect of structural isomerism on physical and gas transport properties of Tröger's Base-based polyimides

The physical and gas transport properties of Tröger's Base (TB)-based polyimides (PI-TBs) synthesized by isomeric monomers were rarely reported. Here, a series of PI-TBs was synthesized from two iso -ether dianhydrides [4,4′-oxydiphthalic anhydride (4,4′-ODPA)/3,4′-oxydiphthalic anhydride (3,4′-ODPA)] and two iso -biphenyl dianhydrides [3,3′,4,4′-biphenyltetracarboxylic dianhydride (4,4′-BPDA)/3,4′-biphthalic anhydride (3,4′-BPDA)], with two diamines, 2,5-dimethyl-1,4-phenylenediamine (DPD) or 2,6-diaminotoluene (DAT). The solubility, microporosity, chain packing, mechanical performance and thermal stability, and gas transport properties were investigated for the PI-TBs derived from the isomeric dianhydrides. The polyimides derived from DPD and appropriate dianhydrides had better solubility and film-forming capability, mechanical properties , and higher gas permeability than those from DAT analogues. Furthermore, the polyimides based on different isomeric dianhydrides showed good solubility, mechanical properties, and high thermal stability and glass transition temperatures ( T g s), up to 400 °C. The chemical structures of isomeric dianhydrides have a noticeable influence on the chain packing and gas separation performance of the PI-TBs. The PI-TBs with rigid 3,4’-/4,4′-BPDA residues showed microporosity and their BET values in the range of 235.2–324.9 m 2 /g. The PI-TB membranes from the 3,4′-BPDA showed both higher gas permeability and selectivity (H 2 /CO 2 , H 2 /CH 4 , CO 2 /CH 4 ) compared to those from the 4,4′-BPDA. On the contrary, the PI-TB membranes from the 3,4′-ODPA had lower gas permeability with improved gas selectivity compared to those from the corresponding 4,4′-ODPA.