Quantifying Metal-Organic Framework (MOF)-Polymer Adsorption Using Single-Particle Langmuir Isotherm Testing (SPLIT)

Interactions between polymers and metal-organic frameworks (MOFs) are a critical determinant of composite performance, yet methods to probe these interactions are generally indirect and ensemble averaging. Here, we present single-particle Langmuir isotherm testing (SPLIT), an atomic force microscope (AFM)-based approach that quantifies polymer adsorption onto the surface of a single MOF particle on an AFM probe. After immersing the MOF-tipped probe in a polymer solution, the adhesion force between the polymer-coated MOF and a reference surface is measured under ambient conditions and found to reversibly vary with polymer concentration analogously to a Langmuir isotherm. This process allows us to compute thermodynamic properties including MOF-polymer binding energies. Repeating SPLIT with different polymer molecular weights and in different solvents shows that this method also provides insight into Flory-Huggins interaction parameters and polymer self-entanglement. Finally, we find that SPLIT measurements can guide the design of mixed-matrix membranes (MMM) as they provide an estimate of polymer concentrations that lead to MOF aggregation. Collectively, this work shows that SPLIT can provide unique information about MOF-polymer interactions using only a single MOF particle with direct relevance to composite design.