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

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 BF₄ 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.