Polymer Matrix and Filler Loading Effects in Polyimide Mixed-Matrix Membranes with Triptycene-Based Microporous Organic Polymers for CO2/CH4 Separation

Polyimide-based mixed-matrix membranes (MMMs) offer a promising route to overcoming the permeability–selectivity trade-off in gas separation. In this work, a triptycene-based porous organic polymer, TFAP-Trp, was systematically investigated as a filler in a wide range of polyimide matrices for CO2/CH4 separation performance. MMMs containing 10, 20, and 30 wt % TFAP-Trp were fabricated using two commercial polyimides and five structurally distinct 6FDA-based polyimides, to evaluate the combined effects of polymer chemistry and filler loading on gas transport behavior. The incorporation of TFAP-Trp did not compromise membrane thermal stability, with degradation temperatures remaining approximately 500 °C or higher. Wide-angle X-ray scattering revealed an increase in intersegmental distance with increasing filler content, consistent with changes in polymer chain packing. Gas transport measurements showed significant permeability enhancements for all matrices, while CO2/CH4 selectivity was largely preserved at 10 wt %. For Pi-DAPOH, CO2 permeability increased from 5.2 Barrer for the pristine polymer to 11.5 and 21.7 Barrer at 10 and 20 wt % TFAP-Trp, respectively, while CO2/CH4 ideal selectivity evolved from 74.3 to 76.7 and 60.2. Higher filler content led to diminishing permeability gains and increased nonideal transport behavior. Effective medium modeling showed improved SEMA permeability predictions at intermediate and high filler loadings compared to Maxwell–Garnett theory. Overall, a TFAP-Trp loading of 10 wt % provided the most consistent balance between permeability enhancement and selectivity preservation across diverse polyimide matrices. These results establish TFAP-Trp as a highly compatible and effective porous organic filler for next-generation polyimide MMMs targeting energy-efficient CO2/CH4 separations.