Tuning the Solution Viscosity of Ionic-Liquid-Based Polyelectrolytes with Solvent Dielectric Constants via the Counterion Condensation
Atsushi Matsumoto, Ryosuke Ukai, Hiroto Osada, Shinji Sugihara, Yasushi Maeda
Abstract
Ionic-liquid-based polyelectrolytes, or polymerized ionic liquids (PILs), are a special class of polyelectrolytes having advantageous properties of ionic liquids. Previous studies have found that the electrostatic interaction and therefore the number of dissociated counterions govern the conformation and dynamics of PIL chains in solutions. Herein, we investigate the effect of the solvent dielectric constant εr on the effective charge fraction feff for a model PIL, poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide) (PC4-TFSI), by measuring the shear viscosity of the PC4-TFSI in a series of non-ionic good solvents with dielectric constants ranging from 7.9 for tetrahydrofuran to 178 for N-methylformamide. For a given molar concentration cp of PC4-TFSI monomers, the specific viscosity ηsp decreases monotonically with decreasing εr. In the semidilute unentangled regime, the dependence of ηsp on cp in solvents at high εr follows the scaling law of ηsp (∝ cp0.5) for salt-free polyelectrolytes in good solvents, but its scaling relationship gradually shifts toward the scaling law of ηsp (∝ cp1.3) for charge neutral polymers in good solvents as εr is decreased. Moreover, the cp dependence of ηsp in solvents having different εr falls onto a single curve by adding an ionic liquid as salt ions to screen the charges on PC4-TFSI chains. These results indicate that the effective charge fraction on PC4-TFSI chains decreases with decreasing εr, in good agreement with the predicted trend by the Manning counterion condensation model. As a result, we demonstrate that the counterion condensation takes place in PIL solutions, indicating an ability to control the properties of PIL solutions simply by changing the solvent dielectric constant. Because PILs have recently attracted significant attention with a wide range of applications for batteries, molecular separations, and antimicrobial, our results can provide useful information about the rheological properties of PIL solutions, optimizing processing operations for PILs and thus PIL-based material design with improved system performance.