Plasticization as a tool for tuning the dynamics of polymerized ionic liquids: Insights from molecular dynamics simulations

Abstract

Using molecular dynamics simulations, we show that the dynamical properties of a polymerized ionic liquid (PIL), which features backbone-embedded imidazolium rings, can be tuned by plasticization with a simple ionic liquid (SIL). Our structural analysis reveals a basically linear dependence of the local ionic environments on the PIL:SIL ratio. Moreover, we observe that the self diffusion coefficients D and structural relaxation times τ vary continuously between the limiting cases of the pure PIL and pure SIL when changing the mixing ratio. Thereat, the concentration dependence is well described by the Gordon-Taylor equation. Upon cooling, D and τ exhibit non-Arrhenius temperature dependence, while the Stokes-Einstein prediction D ∝ τ -1 is fulfilled to a high degree for all compositions. PIL-SIL mixing does not result in enhanced dynamical heterogeneity and leads to similar changes in the motions of the non-polymerized cations and anions, suggesting strong dynamical couplings between these constituents. Finally, Nernst-Einstein estimates of the room-temperature dc conductivity are reasonably high and amount to ∼10 -3 -10 -2 S/cm even at ~25-50% PIL fractions. We conclude that electrolytes with favorable and tunable transport properties can be obtained from SIL plasticization of PILs, in particular, when the polymeric component has backbone-embedded charges.