An Oligomer-Based Grafting Approach to Overcome Interfacial Non-Ideality in MOF/PIM-1 Mixed-Matrix Membranes

Mixed matrix membranes (MMMs) offer a promising route to overcome the limitations of purely polymeric gas-separation membranes, yet their performance is often restricted by poor integration between porous inorganic fillers and organic polymer matrices. In particular, the pronounced mismatch in rigidity, surface chemistry, and processability between metal-organic frameworks (MOFs) and polymers of intrinsic microporosity (PIMs) can lead to fragile materials, interfacial defects, and inefficient filler utilization. Here, we introduce an interface-engineering strategy based on the synthesis of short PIM-1-based oligomers that act as molecular bridges between MOF particles and the polymer. Two hydroxyl-terminated oligomers with controlled molecular weight were synthesized from the monomeric building blocks of PIM-1 and subsequently grafted onto exposed Ce IV sites on the external surface of F4_MIL-140A(Ce) through post-synthetic modification. Comprehensive characterization confirmed successful functionalization while preserving the crystallinity, porosity, and adsorption properties of the parent MOF. Notably, direct incorporation of unmodified F4_MIL-140A(Ce) into PIM-1 produced highly brittle membranes that could not be reliably tested for gas separation, highlighting the critical importance of interfacial compatibility in these systems. In contrast, the grafted fillers enabled the fabrication of homogeneous, self-standing, defect-free MMMs with improved mechanical robustness. Gas permeation measurements show that the resulting membranes retain the high CO 2 permeability characteristic of PIM-1 (up to 7672 Barrer) while exhibiting enhanced CO 2 /N 2 ideal selectivity (up to 20). Gas transport properties indicate that the improved separation performance arises primarily from interfacial rigidification and suppression of non-selective transport pathways. Incorporation of the grafted fillers also significantly mitigates physical aging, resulting in lower permeability aging coefficients (β P ). This effect is particularly relevant for PIM-based membranes, whose progressive free-volume loss generally causes a substantial decline in permeability over time. The membrane containing the longer oligomer-grafted MOF exhibited exceptionally low β P values for He and H 2 transport, suggesting effective stabilization of the polymer microstructure.