Impact of Ion Pairs on the Properties of Dynamic Ion Gels Formed by Complex Coacervation of Oppositely Charged Poly(Ionic Liquids)

Abstract

Dynamic ion gels (DIGs) obtained via complex coacervation of oppositely charged poly(ionic liquid)s (PILs) address the inherent trade-off between ionic conductivity (σ_DC) and mechanical strength (G′) of PILs by providing both enhanced ion transport and robust viscoelastic properties. In order to tune the strength of ionic cross-links through charge delocalization of ion pairs, we study a series of four DIGs obtained from the combination of a cationic PIL (PIL⁺) containing pendant imidazolium groups and free bis(trifluoromethylsulfonyl)imide (TFSI) counteranions with four anionic PILs (PIL^–) bearing pendant sulfonate anions and various free counter cations (i.e., 1-methyl-3-butylimidazolium, trimethylpropylammonium, tetrabutylammonium, and tetrabutylphosphonium). These new DIGs are produced via the formation of ionic cross-links with ion pairs having more localized charges (i.e., cations paired with sulfonate instead of TFSI). This results in stronger electrostatic interactions with counterions, reducing their mobility and significantly increasing the enthalpic driving force for ion exchange-induced coacervation. As a consequence, the four resulting DIGs release different free ionic liquids (ILs) consisting of TFSI anions associated with imidazolium, ammonium or phosphonium cations. The physical, ion-conducting, and viscoelastic properties of the resulting DIGs are systematically investigated by differential scanning calorimetry, broadband dielectric spectroscopy and rheology. The DIG having sulfonate-imidazolium ionic cross-links and releasing EMIM-TFSI ILs exhibits the best compromise between G′ = 62 kPa (at 25 °C, 1 rad s^–1) and σ_DC = 6.5 × 10^–6 S cm^–1 (at 25 °C), significantly outperforming the parent PILs. These results highlight DIGs as a highly promising class of materials with enhanced processability and mechanical integrity, making them ideal candidates for electrochemical applications such as supercapacitors, soft robotics, electrochromic devices, sensors, and solar cells.