Polymers in Ionic Solvents: Effects of Limited Hydration

The interactions of ions with polymers and polyelectrolytes lie at the heart of design of polymer membranes and thermal energy storage materials, in which conditions of limited hydration are often observed. This talk will start by discussing the impact of fully hydrated ions on polyelectrolytes in solution and will continue with contrasting these experiments performed in the abundance of water with the behavior of polymers in inorganic salt hydrate (ISH) solvents where water is scarce. Specifically, we show that water scarcity leads to polymer gelation of neutral polar polymers in liquid ISH which offers an efficient route to reversibly shape stabilize liquid ISHs during their use as phase change materials (PCMs) for thermal energy storage. In the extreme salinity environment of ISHs (8-18M salt concentrations), most polyelectrolytes precipitate due to interactions with incompletely hydrated inorganic salt ions, but neutral polar polymers can form UCST-type gels. The gelation temperature (T_gel) determined from temperature-sweep rheological and UV studies, as well as the gel mechanical properties are strongly dependent on the water content, with the largest changes occurring when the water content in the system is increased to saturate the first ionic hydration shells. Correlations of this behavior with the formation of ionic contact pairs in ISHs as determined by FTIR and simulations, and with the inclusion of polar polymer units within the first hydration shell of the ions will be discussed.

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