Rheological Scaling of Ionic-Liquid-Based Polyelectrolytes in Ionic Liquid Solutions

Polymerized ionic liquids (PILs) are a special class of polyelectrolytes with ionic liquid (IL) species being covalently attached to the repeating unit. The rheological properties of PIL in IL solutions are strongly influenced by the electrostatic screening between IL and PIL chains. However, the effect of IL electrostatic screening on the rheology of PIL in IL solutions remains elusive. To address this challenging yet important question, we conduct detailed rheological characterization of a model system containing a PIL [PC₄-TFSI: poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide)] in a mixture of a salt-free solvent (DMF: dimethylformamide) and an IL [Bmim-TFSI: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide] solution, with low to high IL concentrations, while spanning dilute and semidilute polymer regimes. We compare the specific viscosity _sp and the longest relaxation time of PILs measured at various Bmim-TFSI concentrations from 0 M (pure DMF) to 3.42 M (pure Bmim-TFSI) with the scaling predictions for ordinary polyelectrolyte solutions. We find good agreement at low IL concentrations but significant deviations at higher IL concentrations. We capture this discrepancy by proposing and validating a modified scaling law accounting for the modified screening length in concentrated solutions of ordinary salts. We propose that extended PIL chains initially shrink due to the charge screening effect at low IL concentrations but revert to expanded configuration at higher IL concentrations due to the charge underscreening effect when the screening length increases with increasing IL concentrations. Our results shed new insights on the conformation of PIL in IL solutions and, for the first time, provide a valid reference for the study of general polyelectrolyte solutions at high salt concentrations, where the Debye-Hückel theory is no longer valid.