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

초록

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 beta relaxation, attributed to local chain motions of pendant groups, the intermediate frequency segmental alpha relaxation, associated with the glass transition (T-g), and the slowest frequency ionic alpha(2) 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 alpha and alpha(2) relaxations, and increases the static and Coulombic dielectric constants (epsilon(s) after the alpha(2) relaxation and epsilon(C) between the alpha and alpha(2) relaxations), compared to the monomer without the OE linker. From the analysis of epsilon(s) using the Onsager theory, the one-armed norbornene ILM epsilon(s) is well predicted, but the two-armed ILMs exhibit lower experimental epsilon(s) 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(+)Tf(2)N(-) ion pairs are strongly interacting and preferring an antiparallel alignment, thereby lowering epsilon(s). Furthermore, epsilon(C) 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(+)Tf(2)N(-) ion pairs. This is directly reflected in ionic conductivities(sigma(Dc)), which are strongly correlated with both T-g and epsilon(C).

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