Imidazolium-based poly(ionic liquid)/ionic liquid solutions: Rheology, structuration and ionic transport properties

RAFT-made poly(ionic liquid) chains (PIL), poly(1-[2-acryloylethyl]-3-methylimidazolium bromide) (Poly [AEmim][Br]), have been mixed with a ionic liquid (IL), namely butylmethylimidazolium chloride ([C4mim][Cl].) The similarities in terms of structure and polarity of these molecular and macromolecular imidazolium derivatives led to homogeneous and transparent solutions. The influence of the chain molar mass and the concentration was examined through rheology investigation. It was established that the steady-shear viscosity of the solutions, and the viscoelastic properties can be finely tuned by these two parameters and by temperature. The thermal-dependence of the rheological features allowed for determining high activation energies (E_a) (around 80 kJ/mol), compared to analogous PIL aqueous solutions. Such E_a values reflect high cohesion in PIL/IL solutions, related to the formation of multiple strong interactions, such as hydrogen bonds, van der Waals forces and Coulomb interactions between ionic species. Wide-angle and small-angle X-ray scattering experiments as well as scanning electron microscopy analysis evidenced a multi-scale local homogeneity of the PIL/IL solutions, which additionally exhibit high ionic conductivity (ca. 10⁻³ S/cm at 30 °C), as measured by broadband dielectric spectroscopy.