Influence of polymer modification on intra-MOF self-diffusion in MOF-based mixed matrix membranes

Mixed-matrix membranes (MMMs) represent a promising membrane type for gas and liquid separations. Such membranes can be formed by dispersing crystals of metal-organic frameworks (MOFs) in polymers. For liquid separations, crosslinked polymers are desirable because crosslinking reduces polymer dilation and plasticization effects by liquid sorbates. However, polymer crosslinking processes can unfavorably change transport and related structural properties of MOF fillers. In this work, 13C pulsed field gradient (PFG) NMR was used to investigate possible changes in intra-MOF self-diffusion of p-xylene and o-xylene after MMM polymer crosslinking. The studied MMMs were formed by dispersing MOF crystals of the type ZIF-71 in Torlon (a poly(amide-imide)) or Matrimid (a polyimide) polymers. The reported PFG NMR data indicate that the polymer crosslinking process used does not influence the intra-ZIF diffusivities when all effects on these diffusivities that are related to diffusing molecules crossing over the crystal boundaries or reflected away from these boundaries are removed. In contrast, these data show that the crosslinking process resulted in a significant decrease of the effective size of ZIF-71 crystals inside the studied MMMs, especially in the ZIF-71/Matrimid MMM. This crystal size decrease is attributed to a partial degradation of the ZIF-71 crystals inside the MMMs due to the crosslinking process. This conclusion was found to be in agreement with the results of electron microscopy analysis. PFG NMR studies and data analysis similar to those presented here can be used for quantifying any types of MOF crystal degradation inside MOF-based MMMs.