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Functional polymers for anhydrous proton transport
Anhydrous proton conducting polymers are highly sought after for applications in high temperature polymer electrolyte membrane fuel cells (PEMFCs). N-heterocycles (eg. imidazole, triazole, and benzimidazole), owing to their amphoteric nature, have been widely studied to develop efficient anhydrous proton transporting polymers. The proton conductivity of N-heterocyclic polymers is influenced by several factors and the design and development of polymers with a delicate balance among various synergistic and competing factors to provide appreciable proton conductivities has been a challenging task. In this thesis, the proton transport (PT) characteristics of polymers functionalized with two diverse classes of functional groups— N-heterocycles and phenols have been investigated and efforts have been made to develop the molecular design criteria for the design and development of efficient proton transporting functional groups and polymers. The proton conduction pathway in 1H-1,2,3-triazole polymers is probed by employing structurally analogous N-heterocyclic (triazole, imidazole, and pyrazole) and benz-N-heterocyclic (benzotriazole, benzimidazole, and benzopyrazole) polymers. Imidazole-like pathway was found to dominate the proton conductivity of triazole and pyrazole-like pathway makes only a negligible contribution, if any. Polymers containing benz-N-heterocycles exhibited higher proton conductivity than those with the corresponding N-heterocycles. Pyrazole-like functional groups, i.e. the molecules with two nitrogen atoms adjacent to each other, were found not to be good candidates for PT applications. A new class of proton transporting functional groups, phenols, has been introduced for anhydrous PT. One of the highlighting features of phenols over N-heterocycles is that the hydrogen bond donor/acceptor reorientation can happen on a single -OH site, allowing for facile reorientational dynamics in Grotthuss PT and enhanced proton conductivities in phenolic polymers. Unlike the case of N-heterocycles, comparable conductivities were achieved between poly (3,4,5-trihydroxy) styrene and the corresponding small molecule, pyrogallol. This observation suggests that reorientation should be considered as a crucial design parameter for PT functional groups. The PT characteristics of phenol-based biaryl polymers are studied and compared with the analogous phenol-based linear styrenic polymers. The two-dimensional disposition of -OH moieties in biaryl polymers, although resulted in lower apparent activation energies (Ea), did not improve the net proton conductivity due to the accompanying increase in glass transition temperature (Tg). Thus, the ease of synthesis and lower Tg values of phenol-based styrene polymers make the styrenic polymer architecture preferable over the biaryl architecture. Finally, the synthesis of a series of poly(3,4-dihydroxy styrene)-b-polystyrene block copolymers has been demonstrated via anionic polymerization. These block copolymers will provide an opportunity to systematically investigate the effect of nanoscale morphology on proton transport.
Organic chemistry|Polymer chemistry|Materials science
Chikkannagari, Nagamani, "Functional polymers for anhydrous proton transport" (2012). Doctoral Dissertations Available from Proquest. AAI3498336.