Alkyl Chain Length Dependence of Backbone-to-Backbone Distance in Polymerized Ionic Liquids: An Atomistic Simulation Perspective on Scattering

The backbone-to-backbone correlation length in polymerized ionic liquids (polyILs) manifested as the low-q peak in scattering profiles increases with increasing alkyl chain length, concomitant with ever-growing nonpolar domain size. Understanding the dependence of the correlation length on the pendant alkyl chain length is crucial to effectively designing polyILs for electrochemical applications. Herein, extensive atomistic MD simulations for a complete homologous series of poly(n-alkylvinylimidazolium bis(trifluoromethylsulfonyl)imide), poly(CnVim Tf2N) (n = 2–8), have been carried out to investigate the liquid structure with emphasis on scattering. Neutron scattering with isotopic labeling affords more versatile ways to vet the intrinsic structure, but different isotopically labeled neutron and X-ray scattering data do not necessarily lead to the same position of the low-q peaks. Such an unintended consequence has a nontrivial implication on exploitation of scattering experiments.