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

Abstract

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.

Polarization of ionic liquid and polymer and its implications for polymerized ionic liquids: An overview towards a new theory and simulation

“…Moreover, Choi et al observed that the addition of oligo (oxyethylene) (OE) linkers to (Im)(Tf 2 N) ILs lowered T g , accelerating the segmental α relaxation and the α 2 relaxation due to ions rearranging, and increased the static and Coulombic dielectric constants that were measured after the α and α 2 relaxations, respectively. 26 Furthermore, the β relaxation due to local OE-dipole motions was observed. An analysis of the three relaxation modes suggested that ion-pairing and OE-dipole motions substantially affect dielectric response in this electrolyte, through which ionic conductivity is strongly correlated with T g and the Coulombic dielectric constant.…”

“…The analysis of different counterions and chain lengths suggested that these parameters substantially alter T g , the extent of ionic aggregation, and the dielectric constant, and accordingly affect ion transport. Moreover, Choi et al observed that the addition of oligo (oxyethylene) (OE) linkers to (Im)(Tf 2 N) ILs lowered T g , accelerating the segmental α relaxation and the α 2 relaxation due to ions rearranging, and increased the static and Coulombic dielectric constants that were measured after the α and α 2 relaxations, respectively 26 . Furthermore, the β relaxation due to local OE‐dipole motions was observed.…”

“…24,25 With these pros and cons considered, different research groups focused on modifying PILs, such as using various types of ions and altering the structures of the polymer backbone and side chains. 26 Accordingly, electrochemical and mechanical properties have been extensively improved, yet some fundamental mechanisms, such as the coupling between the segmental motion and ion diffusion and between the dipole distribution in PILs and ion-pairing, largely remain unclear or poorly understood. Nevertheless, Colby and his coworkers suggested the relation for ionic conductivity σ $ εω, where ε and ω designate the dielectric constant and the angular frequency of dielectric spectroscopy.…”

“…Yet, several experimental studies have shown that PILs demonstrated substantial decrease in the ionic conductivity (by two to three orders of magnitude) as compared to pristine ILs, and this reason may arise partially from the fact that the counterions are strongly bound to sluggish ions covalently bonded to the polymer backbones, thus causing limited ion mobilities 24,25 . With these pros and cons considered, different research groups focused on modifying PILs, such as using various types of ions and altering the structures of the polymer backbone and side chains 26 . Accordingly, electrochemical and mechanical properties have been extensively improved, yet some fundamental mechanisms, such as the coupling between the segmental motion and ion diffusion and between the dipole distribution in PILs and ion‐pairing, largely remain unclear or poorly understood.…”

A Review on Ionic Liquid Gas Separation Membranes

“…Chain growth polymerization (i) requires an IL monomer with an unsaturated or cyclic chemical structure that can be activated during the polymerization step ( Figure 5 ). Such a bond is often synthetically introduced into IL monomers (ILM) through the incorporation of methacrylic and methacrylamide [ 112 , 148 , 149 , 150 ], vinyl and styrene [ 116 , 151 , 152 , 153 , 154 , 155 ], or norbornene [ 156 , 157 , 158 ] or epoxy moieties. The sheer number of these reports demonstrates the applicability of this approach and underlines its advantages.…”