UC San Diego

Inorganic–organic hybrid materials combine the desirable properties of organic polymers such as flexibility and easy processibility, with the tunable crystallinity, thermal stability, and high conductivity of some inorganic materials. Polyoxometalates (POMs) are a specific class of inorganic materials has metal–oxo bonds which diffuse negative charge over their structure. Chapter 1 introduces the field of POM-based hybrid materials, which have shown great success in forming morphologies alternative to powders, such as gels and films, with potential applications in energy science, flexible electronics, sensors, photo-electrochromic devices, and biomedical/bioactive materials. Many of these POM-based hybrid gels and films are physical mixtures of polymers and POMs with morphologies that change as a results of polymer concentration. Recently, it was discovered that incorporation of polyethylene glycol (PEG) into polyoxometalate-based frameworks enables the formation of switchable morphologies, or form-factors, such as gels and films without modifications the concentration of POM or polymer. POM frameworks are synthesized from the bottom up and are atomically precise with pores that can host water-soluble polymers within the crystal lattice. In addition, polymer-infiltrated crystals have shown strong intermolecular interactions, improved stability, and higher proton conductivity compared to their non-polymer infiltrated predecessor.Chapter 2 presents the synthesis of PEG-infiltrated polyoxometalate frameworks comprised of the Preyssler anion, [NaP5W30O110]14−, bridged with a variety of transition metal cations. Crystals of these frameworks can be dissolved and recast into gels and flexible and rigid free-standing films while maintaining short-range order. Raman spectroscopy reveals that the hydrogen bonding in these frameworks is stronger than that in the both PEG-free frameworks as well as non-linked PEG-containing crystals. These observations indicate that both the transitional metal framework and polymer are important in forming these unique morphologies. The investigation of the interplay between macroscopic flexibility of form factors (gels and film) and molecular mobility are studied by Solid State NMR in Chapter 3. Specifically, the dehydrated form (rigid films) has the most hydrogen bonding, while the most hydrated form (gel) has the least hydrogen bonding. This trend follows that observed in “conventional” hydrogel materials, which can be swollen by soaking in water. The mobility of the cluster in the various form-factors was evaluated 31P solid-state NMR shown to correlate inversely with hydration. In addition as, water content increases from rigid film to flexible film to gel, there is decrease in domain size and a decrease in the 31P spin—lattice relaxation times. PEG-infiltrated polyoxometalate frameworks are dynamic and flexible by combining crystalline order with structural transformability. It is possible that continuing to tailor the building blocks of polymer-infiltrated polyoxometalate frameworks (flexible transition metal linker, polymer selection, cluster, and counterions) improvements can be made to water adsorption, mechanical strength, proton conductivity, for making rationally designed devices.