Applying Zeolitic imidazolate frameworks in thin film architectures

Abstract

Zeolitic Imidazolate Frameworks (ZIFs) are of interest for highly specialised applications in fluid separation, chemical sensing and heterogeneous catalysis and the development of appropriate thin film structures that maximise the utilisation of these materials is of great importance. In this Dissertation, new techniques for the incorporation of ZIFs into various polymeric thin film architectures were explored to exploit their interesting structural and chemical properties. To this end, the chemical compatibility between ZIFs and polymers and the ability to exert control over the positioning of ZIFs on polymeric surfaces were explored in depth and new techniques developed to address these issues. A novel top down patterning process was successfully demonstrated and was the first top down technique described for ZIFs. A pre formed dense layer of ZIF 9 particles, over a layer of a photoresist material such as phenyltriethoxysilane or poly(o hydroxyl imide) on a silicon substrate, was exposed to intense X ray irradiation such that the photoresist underwent chemical hardening. The unexposed regions were then rinsed away to leave behind a patterned array of ZIF regions on the substrate, with no damage occurring to the ZIF material. A bottom up methodology was developed using radio frequency glow discharge plasma polymerisation to apply functional coatings over polytetrafluoroethylene substrates in a patterned manner using a physical masking process. It was found that plasma polymer coatings containing high quantities of oxygenated functionalities, such as carboxylic acid or hydroxyl groups, promoted the in situ growth of ZIFs on surfaces, while amine based plasma polymers instead inhibited growth. These coatings were also found to promote the growth of other porous framework materials. Subsequent functionalisation of the plasma polymer coatings in a second step was performed, using either 3 aminopropyltriethoxysilane or poly(styrene co maleic anhydride), to bind specifically to the plasma polymer treated regions of the patterned surfaces. The introduction of higher quantities of the favourable oxygenated functionalities further enhanced ZIF growth in a patterned manner and led to the formation of denser coatings of smaller crystalline particles over the surfaces. The improvement of the performance of mixed matrix membranes for gas separations was also explored via the inclusion of a room temperature ionic liquid as a third component in the structure. This component was added to fill interfacial voids between ZIF nanoparticles and a gas permeable polybenzimidazole matrix, caused by poor interfacial chemical compatibility, and thus improve the utilisation of the ZIF nanoparticles and the selectivity of the membrane for H₂ gas over CO₂ and N₂. A synergistic maximum loading of the ZIF and the ionic liquid loading within the membrane was observed such that the performance of the membrane was maximised for both its H₂ permeability and its selectivity of H₂ over CO₂ and N₂. With higher loadings beyond this synergistic level, the performance of the membranes decreased significantly as the polybenzimidazole matrix became excessively strained.