Metal-organic frameworks (MOFs) are microporous materials poised to revolutionize the fields of catalysis, separations, and storage. MOFs are generally micro- to nano-crystalline powders with regular, tunable porosity that make them competitive with traditional heterogeneous catalysts such as zeolites and potential replacements for separation processes such as air purification. However, their powdered, insoluble form has precluded incorporation of MOFs in many commercial applications that could one day be improved or replaced by a MOF. Due to this shortcoming, industry-ready form factors of MOFs are being explored. One such form factor is MOF-dominant mixed-matrix membranes (MMMs) that incorporate up to 90% MOF by weight into small amounts of binder polymer to create freestanding materials that perform like a MOF, in the form of a polymer membrane. Understanding the nature of the interface between MOF and polymer is key to the performance and mechanical integrity of these materials, yet tools to assess and quantify these interactions are insufficient. Herein, we directly observe MOF/polymer interactions in actual membranes for the first time by solid-state NMR. In a series of MMMs made from the MOF UiO-66 and polymer PEO (poly(ethylene oxide)), we observe complete polymer infiltration and pore-filling of the MOF at MOF loadings up to 70 wt%. In contrast, when the same MOF is incorporated into fluorinated polymer PVDF (poly(vinylidene fluoride)) at 70wt% UiO-66, NMR experiments demonstrate only surface-level interactions of PVDF and MOF particles that allow access to the useful porosity of the MOF. These fundamental interface studies validate a new tool to shed direct light onto MOF/polymer interactions for the first time. Our increased understanding of these interactions will guide innovation in the successful application of MMMs and other MOF-based composites.

MOF/polymer interactions in composite materials visualized <i>in situ</i> with solid-state NMR

2019 Fall National Meeting

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MOF/polymer interactions in composite materials visualized in situ with solid-state NMR