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 (Tg), 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 (Tg), 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 OEx 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 Tg and ϵC.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Inorganic Chemistry
- Materials Chemistry