Structure-Property Relationships in Sterically Congested Proton-Conducting Poly(phenylene)s: the Impact of Biphenyl Linearity

摘要

The structure-property relationship of sulfonated phenylated poly(phenylene)s possessing either angled or linear backbone moieties was investigated. 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 polymer, the angled ortho biphenyl moiety was found to prevent membrane formation. The angled meta biphenyl-containing polymer, while forming a membrane, exhibited a 74% increase in volumetric expansion, 31% reduction in tensile strength, and 72% reduction in 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. Collectively, this study suggests that incorporating angled biphenyl linkages into sulfonated phenylated poly(phenylene)s leads to highly rigid, inflexible backbones that prevent chain entanglement and the formation of free-standing membranes.