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Structure-property relationships in sterically-congested proton-conducting poly(phenylene)s: The impact of backbone linearity

Date created: 
Proton exchange membranes

Decarbonatization of the World’s primary energy supply is becoming increasingly more important due to a rapidly changing climate. A hydrogen-based economy offers a potential means of zero-carbon energy production through the use of fuel cells and water electrolyzers. The development of robust, thermochemically-stable hydrocarbon-based proton exchange membrane materials that resist swelling for use in these devices represent a significant hurdle in their commercial adoption. In this thesis, the structure-property relationship of hydrocarbon-based sulfonated phenylated poly(phenylene) proton exchange membranes possessing either angled or linear backbone moieties is discussed. Polymers were synthesized using either bent (ortho or meta), or linear (para) biphenyl linkages and evaluated for differences in physical and electrochemical properties. Model compounds, structurally-analogous to the polymers, were prepared and characterized using spectroscopic and computational methods to elucidate structural differences and potential impacts on the properties of the respective polymers. A highly angled ortho biphenyl linkage resulted in a sterically hindered, rotationally-restricted molecule. When incorporated into a homo-polymer, the angled ortho biphenyl moiety was found to prevent membrane formation. The angled meta biphenyl-containing homo-polymer, while forming a membrane, exhibited a 74% increase in volumetric expansion, 31% reduction in tensile strength, and 72% reduction in the elongation at break when compared to the linear para biphenyl-containing analogue. The differences observed are attributed to a rotationally-restricted backbone in the angled biphenyl systems. Co-polymers containing a small fraction (≤5%) of the ortho or meta biphenyl linkage in an otherwise para biphenyl containing system were found to have a significantly lower degree of swelling than those containing solely para biphenyl linkages. Collectively, the work presented in this thesis suggests that incorporating angled biphenyl linkages into sulfonated phenylated poly(phenylene)s leads to highly rigid, inflexible backbones that prevents chain entanglement and the formation of free-standing membranes.

Document type: 
This thesis may be printed or downloaded for non-commercial research and scholarly purposes. Copyright remains with the author.
Steven Holdcroft
Science: Department of Chemistry
Thesis type: 
(Thesis) M.Sc.