The Effect of Oligo(oxyethylene) Moieties on Ion Conduction and Dielectric Properties of Norbornene-Based Imidazolium Tf2N Ionic Liquid Monomers

Ion transport and segmental dynamics were studied for one- and two-armed norbornene ionic liquid monomers (ILMs) including imidazolium (Im+)-bis­(trifluoromethanesulfonyl)­imide (Tf2N–) pairs, with different imidazolium pendant structures containing either alkylene [(CH2)2] or oxyethylene [(OCH2CH2) x =(OE) x , x = 1, 2, or 3] units as the linkers between the norbornene and the imidazolium cation, using dielectric relaxation spectroscopy and oscillatory shear. All ILMs exhibit three dipolar relaxations, assigned to the fastest frequency glassy β relaxation, attributed to local chain motions of pendant groups, the intermediate frequency segmental α relaxation, associated with the glass transition (T g), and the slowest frequency ionic α2 relaxation, attributed to ions rearranging; the former has an Arrhenius character, while the latter two have Vogel temperature dependences. The incorporation of the OE linkers lowers the glass transition temperature (T g), accelerating the α and α2 relaxations, and increases the static and Coulombic dielectric constants (εs after the α2 relaxation and εC between the α and α2 relaxations), compared to the monomer without the OE linker. From the analysis of εs using the Onsager theory, the one-armed norbornene ILM εs is well predicted, but the two-armed ILMs exhibit lower experimental εs values compared to the Onsager prediction. This suggests that there is more ionic aggregation in the two-armed ILMs due to strongly overlapping polarizability volumes, consistent with the Kirkwood g correlation factor less than unity and higher polarizability volume overlap parameter, indicating that Im+Tf2N– ion pairs are strongly interacting and preferring an antiparallel alignment, thereby lowering εs. Furthermore, εC increases with the OE x linker length, consistent with Landau and Lifshitz’s mixing rule, revealing that with higher OE content, the environment is more polar for the ionic dissociation of the Im+Tf2N– ion pairs. This is directly reflected in ionic conductivities (σDC), which are strongly correlated with both T g and εC.