Rheological Scaling of Imidazolium-Based Polyelectrolyte in Ionic Liquid Semidilute Solutions

Polymerized ionic liquids (PILs) are a special type of polyelectrolytes with ionic liquid moieties covalently attached to a polymer backbone. Existing studies show that electrostatic interactions play a dominant role in determining the viscoelastic properties of ordinary polyelecrolytes. Recently, we found that the charge underscreening effect, in which the screening length increases with increasing the ion concentration, resulted in expanding PIL chains at high ion concentrations in the dilute regime. In this study, we investigate the effect of charge underscreening on the rheological properties of PILs in the semidilute unentangled regime by using a model system consisting of a PIL (PC₄–TFSI) in a mixture of an ionic liquid (BmimTFSI) and a non-ionic solvent (DMF). We observe: i) both specific viscosity η_sp and relaxation time λ are initially constants at low ion concentrations c_s and then decrease with increasing c_s at an intermediate c_s. ii) both η_sp and λ increase with increasing c_s. This result indicates that the charge underscreening effect is still dominant in the semidilute unentangled regime. We capture the observed trend by proposing and validating a modified scaling law accounting for the dependence of correlation length on c_s.