Ion Transport in Polymerized Ionic Liquids: Effects of Side Chain Flexibility and Specific Interactions

Atomistic molecular dynamics (MD) simulations were undertaken on polymerized ionic liquids (polyILs) consisting of a poly(methyl methacrylate) (PMMA)-based backbone with attached imidazolium cations through a series of linkers and mobile anions: TFSI^-, PF₆^-, BF₄^-, and Br^-. Ionic correlations and the mechanisms of ion transport in these polyILs were systematically investigated to understand the effects of side chain flexibility coupled with specific interactions. The simulations revealed that the linker length has only a minimal effect on cation-anion interactions particularly when compared to the change with the anion. An increase in the linker length enhances the anion diffusivity to a varying extent for the different anions arising from significant increases in diffusive motion but a lesser reduction in ion hopping. Specifically, the small anions exhibit large increases in the anion diffusivity when the linker length is increased since the rigid “cage” appears to be strongly softened by increasing side chain flexibility, suggesting a size-dependent effect. Dynamical heterogeneity in the anion mobility further confirms the softening effect of the long linkers. Moreover, only a small fraction of the anions was observed to travel farther than the characteristic distance determined by the self-part of the van Hove function within the characteristic time of the strongest heterogeneity. Fast anions exhibit dominant interchain hopping associated with more cations from more chains compared with the slow anions. The string-like cooperative motion was observed in these fast anions and the string length decreases as the linker length increases. More importantly, distinct anion-anion correlation suppresses the conductivity in a barycentric reference frame, while this correlation enhances the conductivity within a polycation-fixed reference frame. The cage escape seems to serve as the primary ion transport rate-controlling mechanism in these systems. These findings may lead to interesting implications for the design of polyILs with enhanced ionic conductivity for electrochemical applications.