A molecular-level understanding of dynamics in imidazolium-based ionomers with different counterions and side chain lengths was investigated using X-ray scattering, oscillatory shear, and dielectric relaxation spectroscopy (DRS). Variations of the counterion size and side chain length lead to changes in glass transition temperature (T g ), extent of ionic aggregation, and dielectric constant, with consequences for ion transport. A physical model of electrode polarization is used to determine the number density of simultaneously conducting ions and their mobility. Imidazolium-based ionomers with larger counterion and longer side chain have lower T g , resulting in higher ionic conductivity and mobility. The ionic mobility is coupled to ion motions that are directly measured as a second segmental process in DRS, as these are observed to share the same Vogel temperature. Time−temperature superposition (tTS) was applied to create linear viscoelasticity master curves and to investigate the delay in chain motion related to ionic associations. tTS works well for these materials, and the terminal relaxation time increases with decreasing side chain length and smaller counterion size. X-ray scattering confirms the extent of ionic aggregation and helps to rationalize the observed dielectric constants. Larger counterions or longer side chains diminish ionic aggregation, and their ionomers have higher dielectric constants, which agree reasonably with the Onsager prediction at all temperatures studied. Smaller counterions or shorter side chains promote ionic aggregation, and their ionomers have lower dielectric constants, which are directly reflected in the lower content of simultaneously conducting ions.

We use X-ray scattering to investigate morphology and dielectric spectroscopy to study ionic conduction and dielectric response of imidazolium-based single-ion conductors with two different counterions [hexafluorophosphate (PF 6 − ) or bis(trifluoromethanesulfonyl)imide (F 3 CSO 2 NSO 2 CF 3 − = Tf 2 N − )] with different imidazolium pendant structures, particularly tail length (n-butyl vs n-dodecyl). A physical model of electrode polarization is used to separate ionic conductivity of the ionomers into number density of conducting ions and their mobility. Tf 2 N − counterions display higher ionic conductivity and mobility than PF 6 − counterions, as anticipated by ab initio calculations. Ion mobility is coupled to polymer segmental motion, as these are observed to share the same Vogel temperature. Ionomers with the n-butyl tail impart much larger static dielectric constant than those with the n-dodecyl tail. From the analysis of the static dielectric constant using Onsager theory, there is more ionic aggregation in ionomers with the n-dodecyl tails than in those with the nbutyl tails, consistent with X-ray scattering, which shows a much stronger ionic aggregate peak for the ionomers with dodecyl tails on their imidazolium side chains.

Dielectric spectroscopy was used to determine the static dielectric constants (ε s ) of imidazolium acrylates and methacrylates and their ionomers, with different imidazolium pendant structures containing a combination of alkylene [(CH 2 ) n , n = 5 or 10] and ethyleneoxy [(CH 2 CH 2 O) n , n = 4 or 7.3 (the average of a mixture of n = 1 to 20)] units as spacers between the backbone and the imidazolium cation. All monomers and polymers exhibited two dipolar relaxations, assigned to the usual segmental motion (α) associated with the glass transition and a lower frequency relaxation (α 2 ), attributed to ions rearranging. From the analysis of the static dielectric constants using the Kirkwood g correlation factor, the dipoles in conventional (smaller) ionic liquids prefer antiparallel alignment (g ≈ 0.1), lowering ε s values (≤30), because their polarizability volumes V p strongly overlap, whereas the dipoles in the larger ionic liquid monomers display g of order unity and 50 ≤ ε s ≤ 110. A longer spacer leads to higher static dielectric constant, owing to a significant increase of the relaxation strength of the α 2 process, which is directly reflected through an unanticipated increase of the static dielectric constant with ionic liquid molecular volume V m . The glass transition temperature of polymerized imidazolium ionic liquids with various counterions is also shown to simply be a monotonically decreasing function of V m . Furthermore, the ionomers consistently exhibit 1.5−2.3 times higher static dielectric constants (ε s up to ∼140 at room temperature) than the monomers from which they were synthesized, suggesting that polymerization encourages the observed synergistic dipole alignment (g > 1).

Polymerizable imidazolium acrylates and their polymers with pendant imidazolium cations were synthesized with hexafluorophosphate and bis(trifluoromethanesulfonyl)imide counterions and characterized using calorimetry and dielectric spectroscopy. The ionic polymers containing a diethyleneoxy unit as an N-substituent on the imidazolium cation display higher ionic conductivities than the analogous N-n-butyl polymers. Using a physical model of electrode polarization, we separate the conductivity of single-ion conductors into number density of conducting ions p and their mobility μ. The monomers invariably possess higher conducting ion number density than the polymers, owing to the cation being part of the polymer, but p is insensitive to the N-substituent. In contrast, the diethyleneoxy N-substituent imparts higher mobility than the n-butyl N-substituent, for both monomers and polymers, owing to a lower binding energy between the imidazolium and the counteranions, which is not directly reflected in glass transition temperatures.

Three borate monomers: lithium triphenylstyryl borate (B1), a variant with three ethylene oxides between the vinyl and the borate (B2) and a third with perfluorinated phenyl rings (B3) were synthesized and used to prepare polysiloxane ionomers based on cyclic carbonates via hydrosilylation. B1 ion content variations show maximum 25 °C conductivity at 8 mol %, reflecting a trade-off between carrier density and glass transition temperature (T g ) increase. Ethylene oxide spacers (B2) lower T g , and increase the dielectric constant, both raising conductivity. Perfluorinating the four phenyl rings (B3) lowers the ion association energy, as anticipated by ab initio estimations. This increases conductivity, a direct result of 3 times higher measured carrier density. The ∼9 kJ/mol activation energy of simultaneously conducting ions is less than half that of ionomers with either sulfonate or bis(trifluoromethanesulfonyl) imide anions, suggesting that ionomers with weak-binding borate anions may provide a pathway to useful single-ion Li + conductors, if their T g can be lowered.