Characterizing and Controlling the Interface of Metal-Organic Framework and Polymer Components in Mixed-Matrix Membranes
Metal-organic frameworks (MOFs) are a class of materials that combine the chemical tunability of organic molecular chemistry with the porosity of zeolites. The crystallinity, porosity, and chemical versatility of MOFs has led to their study for applications including storage and delivery of gases, catalysis, and separations. In order to utilize MOFs for industrial applications, however, they must be processed into other forms. Previous work showed that small amounts of flexible polymers can be combined with different MOFs to form ‘sheets’ of MOF, termed mixed-matrix membranes (MMMs), that display useful MOF porosity and chemical accessibility in a flexible, stable form factor.
To build on this previous work, MMMs using three polystyrene-based polymers that are similar in chemical structure but provide a range of flexibility are developed in Chapter 2. MOF accessibility and MMM cohesion at up to 90 wt% MOF in styrene/butadiene copolymers is demonstrated, and it is shown that the starting flexibility of the polymer affects the utility of the resulting MMM.
To assess the factors that influence MMM cohesion, computational experiments predicting the microscopic interfacial structures of MOF-polymer MMMs are performed in Chapter 3. This computational modeling is then correlated with experimental findings to elucidate the structure-compatibility relationship of the MOF-polymer pairs studied, and two examples of MOF-polymer MMMs are found to contain both a flexible MOF-polymer interface and strong MOF-polymer hydrogen bonding interactions.
Significant differences in MOF pore accessibility between these two MMMs prompted further study and comparison, and NMR experiments detailed in Chapter 4 suggest the polymer component infiltrates into MOF pores to differing extents depending on the nature of the polymer used. This differing polymer infiltration behavior directly affects the utility of the composite as assessed through other experiments.
Chapter 5 describes the fabrication of new MMMs from an inexpensive commodity polymer, poly(ethylene-co-vinyl acetate) (EVA). These MMMs are then used in liquid-phase applications including sorption and removal of harmful perfluoroalkane substances and catalytic breakdown of a chemical warfare agent simulant. When EVA-based MMMs are compared with MMMs prepared using different polymers discussed in previous chapters, polymer-dependent differences in MMM performance are observed.