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 (Tgs), 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 m2/g. The PI-TB membranes from the 3,4′-BPDA showed both higher gas permeability and selectivity (H2/CO2, H2/CH4, CO2/CH4) 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.