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Designing ion-containing polymers for facile ion transport

Ralph H. Colby (Material Science and Engineering Penn State University)

Advanced batteries and fuel cells require membranes that transport cations. The utility of ionomers (polymers with covalently bonded anionic groups) is considered here, as these materials are single-ion conductors, with a cation transference number of unity. Our experimental results thus far are for ionomers made by reacting poly(ethylene glycol) (PEG) oligomers with the sodium salt of dimethyl 5-sulfoisophthalate and neutral dimethyl isophthalate ; both polyesters and polyurethanes (using isophorone diisocyanate) have been studied. The charge spacing on these chains is directly controlled by the molar mass of the PEG oligomers (we use M = 400, 600 and 900) and the ionic fraction of phthalate monomers used. SAXS and rheology suggest that these ionomers are quite unusual, in that their ions do not microphase separate, likely due to a combination of the regular placement of ionic groups along the chains and the stabilization of ions and ion pairs by the PEG ether-oxygens. Dielectric spectroscopy is used in the conventional way to measure conductivity and polymer segmental relaxations. Using the onset of electrode polarization (usually considered a nuisance in dielectric spectroscopy) the free ion concentration and mobility are quantitatively estimated, based on the 1953 MacDonald model and later work of Coelho. An Arrhenius temperature dependence of the free ion concentration is found, with activation energy that decreases in going from Li+ to Na+ to Cs+ , consistent with larger cations being less strongly bound to the sulfonate groups. In turn, cation mobility has a strongly non-Arrhenius temperature dependence, controlled by segmental motion of the PEG. However, the most stunning result is that, despite studying ionomers with very low charge densities, only one cation in roughly 10000 are free from their sulfonate group and contributing to conductivity ! This immediately suggests an opportunity to boost the conductivity by making more ions free. We close with a discussion of ab initio calculations and new polymer synthesis targets aimed at exploiting this opportunity.