Rheological Scaling of Ionic Liquid-Based Polyelectrolytes in the Semidilute Unentangled Regime from Low to High Salt Concentrations

Polymerized ionic liquids (PILs) are a special class of ion-containing polymers with ionic liquid structures. The viscoelastic properties of PILs in IL solutions are expected to be influenced by both polymer-polymer interaction and charge screening by IL ions, which becomes complex at high IL concentrations due to strong ionic correlations. In this work, we aim to understand the effect of the ionic correlations on the shear rheology of PIL in IL solutions in the semidilute unentangled (SU) polymer regime, with moderate polymer-polymer interactions. We conducted systematic rheological characterization of a PIL, poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide), in a mixture of an IL, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and a non-ionic solvent, dimethylformamide, to evaluate the specific viscosity ηsp and the longest relaxation time λ with varying IL concentrations cIL, at a fixed polymer concentration. We found that both ηsp and λ initially decreased with the increasing cIL, following the scaling theory of polyelectrolyte solutions in the low salt concentration regime. However, these values exhibited an upturn at high cIL. We explained the observed non-monotonic viscoelastic behavior by proposing a charge screening model for SU solutions, accounting for both modified screening length in concentrated IL solutions and complete charge screening. Our results demonstrate how the strong ionic correlation in concentrated IL solutions modifies the electrostatics of PILs and their corresponding viscoelastic properties in the presence of moderate polymer-polymer interactions.