Tröger's Base Polyimide Membranes with Enhanced Mechanical Robustness for Gas Separation

The rigid V-shaped Tröger's base (TB) unit has been proven efficacious in creating microporosity, making TB-based polyimides (PIs) exhibiting significant advantages in simultaneously increasing gas permeability and selectivity for the separation industry. However, TB-based PIs commonly display undesired mechanical performance due to the low molecular weight resulting from the evident steric hindrance and low reactivity of TB-containing diamines. Herein, a novel diamine-containing bisimide linkage (BIDA) has been synthesized and then polymerized with paraformaldehyde via a moderate "TB polymerization" strategy to furnish polymers simultaneously, including imide linkages and TB units in the polymer main chains, namely, TB-PIs. This TB polymerization strategy avoids the direct polymerization of dianhydride with low-reactivity TB diamine. After incorporating a meta-methyl substituent into BIDA diamine, the m-MBIDA diamine-derived m-MTBPI ultimately exhibits a high molecular weight, good tensile strength (90.4 MPa) and an outstanding fracture toughness (45.1 MJ/m³). And more importantly, the m-MTBPI membrane displays an evidently enhanced gas separation ability in comparison with BIDA-derived TBPI, with overall separation properties much closer to the 1991 Robeson upper bound. Moreover, no sign of plasticization appears for the m-MTBPI membrane when separating a high-pressure CO₂/CH₄ mixture (v/v = 1/1) up to 20 bar, with the CO₂/CH₄ mixed-gas separation performance approaching the 2018 upper bound.