Rheological scaling of ionic-liquid-based polyelectrolytes in ionic liquid solutions: the effect of the ion diameter of ionic liquids.

We investigate the effect of the ion diameter a of ionic liquids (ILs) on the shear viscosity of polymerized ionic liquids (PILs) in IL solutions. When both the PIL and IL contain large PFSI anions ( a ≈ 0.57 nm), the specific viscosity η sp first decreases with increasing IL concentration c IL in the low c IL regime, reaches a minimum and then increases with increasing c IL in the high c IL regime. By comparing the measured η sp with the modified charge screening model proposed in our previous study [Matsumoto et al. , Macromolecules , 2021, 54 , 5648-5661], we attribute the observed non-monotonic trend of η sp against c IL to the charge underscreening phenomenon, i.e. , an increase of the screening length at high c IL leads to the upturn of η sp . On the other hand, when the PIL and IL contain small BF 4 anions ( a ≈ 0.34 nm), the η sp decreases asymptotically with increasing c IL , because the charge on the PIL chain is likely screened fully in the entire c IL regime. Our results demonstrate that the ion diameter of ILs plays an important role in governing the charge screening mechanism of PILs in IL solutions, and thus influencing the viscoelastic properties of PIL solutions.