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

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