Thermal Cyclotrimerization Bridges Polymer‐Filler Interfaces for High‐Performance CO 2 Separation Membranes

ABSTRACT Mixed‐matrix membranes (MMMs) that combine polymers with porous fillers hold promise for scalable CO 2 capture, but their performance is often limited by inadequate polymer–filler compatibility and suboptimal micropore structure. Here we demonstrate a thermal cyclotrimerization strategy that simultaneously tunes the microstructure of PIM‐1 and improves continuity at the MOFpolymer interface. Using MAF‐stu‐1 as a model filler, the resulting thermally rearranged MMMs achieve CO 2 permeabilities above 10 000 barrer with a 2.5‐fold increase in CO 2 /N 2 selectivity (from 18 to 46). Sorption measurements, spectroscopy, surface energy analysis, and molecular simulations reveal that cyclotrimerization introduces triazine units that both restrict polymer chain packing and create favorable interfacial interactions with MAF‐stu‐1, leading to enhanced pore accessibility and molecular discrimination. Importantly, these material‐level improvements translate into stronger process performance, reducing the specific CO 2 capture cost by 43.9% compared to the untreated membrane. This work establishes thermal cyclotrimerization as a versatile strategy for engineering robust MMMs, linking interfacial chemistry to process‐level outcomes and advancing the development of practical membrane technologies for industrial carbon capture.