Elucidating Interfacial Interactions in Mixed Matrix Membranes Incorporating Monodispersed Covalent Organic Frameworks Combining Gas Transport and Mechanical Analysis ^†

Comprehensive Summary Understanding the interfacial interactions between covalent organic frameworks (COFs) and polymer matrices remains a critical challenge for the development of high‐performance mixed matrix membranes (MMMs) for gas separation and mechanical robustness. Here, we systematically study MMMs made of highly crystalline and monodispersed 2D PDA‐HTA COF and 3D‐Py‐COF with commonly used PIM‐1 and 6FDA‐DAM as matrix. A comprehensive gas permeation, electron microscopy and mechanical properties analysis revealed that the incorporation of these porous fillers universally decreased gas permeability, which is mainly due to polymer chain infiltration. The large pores of the 2D COF promote deep polymer penetration, leading to pore blockage and the formation of a rigidified, selective interfacial region. In contrast, the small pores of the 3D COF largely prevent infiltration, resulting in a more classic, weakly‐adhered filler. Crucially, this same infiltration mechanism dictates the composite's mechanical properties, inducing complex plasticization and reinforcement phenomena that are highly dependent on the specific COF‐polymer pairing. These findings offer mechanistic insights and design principles for optimizing the interface in MMMs, paving the way for advanced membranes with both excellent separation and mechanical performance.