Incorporating metal–organic frameworks (MOFs) into mixed matrix membranes (MMMs) has gained significant interest in gas separations due to the possible performance benefits of MOFs compared to pure polymers. However, the high loadings necessary for significant performance improvements are often inaccessible for a number of reasons, including poor interfacial compatibility between the MOF filler and polymer matrix as well as a reduction in film toughness leading to easy fracture. To address both of these challenges, a UiO-66-NH2 MOF was post-synthetically modified with oligomeric polyethylene glycol – diglycidyl ether (PEG-DE) to generate soft-shell MOF particles with dispersed and crosslinked PEG oligomers in the MOF pores. These functional MOFs were added to a 6FDA-Durene polymer matrix, improving interfacial compatibility with the matrix compared to unmodified UiO-66-NH2. Additionally, this process results in an immobilized PEG phase within the MOF crystals and chemical tethering between crystals, creating a mechanism for stress redistribution around the MOF nanoparticles. This approach leads to high performance in CO2 based separations, showing a permeability in the 40 wt% membrane of 750 barrer and a selectivity of 55 for CO2/N2 separations, exceeding the 2008 Robeson upper bound. Surprisingly, we observe a multi-order-of-magnitude increase in membrane toughness relative to the unfunctionalized MOF-based MMMs. This development allows for the use of MOFs at higher loadings in MMMs to boost CO2 separation performance while simultaneously improving the mechanical properties of the membrane.
