Soluble Polyimides Containing Benzocyclobutene Moieties: Synthesis, Cross-Linking Behavior, and Physical and Gas Transport Properties

In this study, a novel diamine monomer (BTPA) containing a benzocyclobutene (BCB) cross-linking site was synthesized, and two polyimides (PIs), PI-BCB-1 and PI-BCB-2, were synthesized using BTPA with dianhydrides, 4,4′-(hexafluoroisopropyl) diphenyl anhydride (6FDA) and 4,4′-(4,4-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), respectively. The BCB-based PIs were soluble in common organic solvents and exhibited a desirable toughness. The cross-linking behavior of the BCB-based PIs was investigated using nonisothermal differential scanning calorimetry, and a mathematical model for the curing kinetics was established. Further, the effects of BCB cross-linking on the interchain packing, thermal performance, and gas transport properties of the PIs were thoroughly examined. The cross-linked PIs exhibited excellent heat resistance with glass-transition temperatures exceeding 400 °C. In addition, the CO2 and CH4 permeabilities for the cross-linked PI membranes increased 16- and 27-fold, respectively, compared with their counterparts before cross-linking. Remarkably, the CO2 permeability for PI-BCB-1 increased from 33 to 1559 Barrer after cross-linking, while maintaining a nearly unchanged CO2/CH4 selectivity of 35 (39 for its uncross-linked counterpart), surpassing the 2008 Robeson upper limit. In addition, the cross-linked PI membranes showed desirable aging resistance and excellent plasticization resistance. This study offers insights for designing PIs containing BCB side group-cross-linked units to enhance membrane-based gas separation performance.