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

Abstract

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.

Effect of steric constraints on the physico‐electrochemical properties of sulfonated polyaromatic copolymers

“…Co-monomers TEAsBTC, o-BPL, m-BPL and p-BPL All reported compounds were synthesized according to previously published methodologies. [37][38][39]51,56 The synthetic pathways utilized in the preparation of monomers TEAsBTC, o-BPL, m-BPL and p-BPL are outlined in Scheme 1. Additional information and the detailed synthetic methodology for each reported compound are outlined in Appendix S1.…”

“…Polymers sPPBpoH + -x% and sPPBpmH + -x% Acid-functionalized polymers were prepared using a multi-step approach as previously described, 37,38,51 involving (i) thermally mediated Diels-Alder polycondensation reactions between diene monomer TEAsBTC and various combinations of dienophile comonomers o-BPL, m-BPL and p-BPL, in a cumulative stoichiometric ratio of 1.015 eq. dienophile to 1.000 eq.…”

“…38 We recently reported an investigation on the structureproperty relationship of sulfonated phenylated poly(phenylene)s containing either linear (para) or angled (ortho or meta) biphenyl moieties. 51 This was achieved through modifications to the dienophile monomer, which directly translated to modifications to the homo-polymer backbone without varying the ionic content, hydrophobicity, or composition of the polymer. It was believed that, by introducing angled moieties into the polymer backbone, the resulting macroscopic structure may adopt flexible coils, 52 as opposed to rigid rods, in order to promote polymer chain entanglements and hence a reduction in water sorption, swelling, and elastic deformation.…”

“…40,[53][54][55] Contrary to this assumption, we found that incorporating nonlinear ortho and meta biphenyl moieties into the highly phenylated sPPB-H + backbone resulted in sterically encumbered, rotationally restricted polymers. 51 This in turn yielded membranes that possessed poor physical and electrochemical properties, probably due to reductions in macroscopic chain entanglement. 51 Herein we report on a series of copolymers with progressively increased nonlinear biphenyl content, from 0% to 10%.…”

“…51 This in turn yielded membranes that possessed poor physical and electrochemical properties, probably due to reductions in macroscopic chain entanglement. 51 Herein we report on a series of copolymers with progressively increased nonlinear biphenyl content, from 0% to 10%. In this way, the impact of inserting a small number of inflexible 'kinks' into an otherwise flexible polymer backbone is investigated, and a structure-property relationship is established.…”