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⁻¹ 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.