(410d) Influence of Hydration on Ion Transport Mechanisms in Ion-Containing Polymers

Ion transport in polymer electrolytes is of significant interest in various applications (e.g., water purification and batteries). A detailed understanding of the physics that govern ion transport in such polymers would enable rational design of new materials for these processes. Hydration is thought to strongly affect these physics, yet fundamental relationships between molecular level interactions (i.e., ion-water-polymer interactions) and macroscopic transport properties in ion-containing polymers over a broad range of hydration are unavailable. Furthermore, the various communities studying polymer electrolytes have explored ion transport under distinct physical regimes (e.g., rigorously dry polymers for batteries vs. highly swollen polymers for water purification). To connect these communities together and improve our fundamental understanding of ion transport, we systematically probed ion transport in LiTFSI doped polyethers from dry to wet conditions. Simultaneous measurements of water sorption and ionic conductivity in polyethers exposed to controlled humidity demonstrated that conductivity is relatively unaffected by the presence of water at low water activity (i.e., < 0.30), but increases by a factor of four to ten at higher water activities (i.e., > 0.60). Pulsed field gradient NMR and DSC experiments were conducted to determine ion dissociation and polymer dynamics as a function of hydration. These data showed that dissociation strongly increases with hydration due to strong solvation of ions with water imbibed within the polymer, while polymer dynamics changed marginally with hydration in the regime explored (i.e., rubbery polymer electrolytes sorbing < 10 vol% water). Complementary simulations provided additional insights, suggesting that ion transport is governed by ion pairing and polymer dynamics at low water content (i.e., < 10 vol%) and by water dynamics and percolation at high water content (i.e., > 20 vol%). The implications of these results on transport mechanisms and design strategies for ion specific selectivity will be discussed.