Molecular Dynamics Simulations of Polymerized Ionic Liquids: Mechanism of Ion Transport with Different Anions

All-atom molecular dynamics simulations were undertaken to study the ion-transport mechanism in poly(ethyl vinyl imidazolium) with different anions: Br-, BF4-, PF6-, and Tf2N-. Specifically, ion association was analyzed along with the statistics of hopping events at a relatively short time scale, which was defined from the instant the breaking and forming occurred of the ion association. Of some significance was the observation that BF4- and PF6- exhibited more hopping events than either Tf2N- or Br-. Higher temperature promoted more interchain hopping than intrachain hopping for all the polyatomic anions. At a longer time scale, the diffusivity, dynamic heterogeneity, and stringlike cooperative motion were examined. The dynamic heterogeneity decreased with the size of the counterion. Tf2N had a unique curve shape of the self-part of the van Hove function and a shorter length of stringlike motion. It was confirmed that only a small fraction of the counterions were mobile, and most of the hopping events did not lead to the effective transport. The diffusivity had universal correlation to both the structural relaxation time and the average lifetime of ion association for all the simulated chemistries. For each anion, the linear correlation was more possible between the diffusivity and the inverse of the structural relaxation time.