Dielectric and Viscoelastic Responses of Imidazolium-Based Ionomers with Different Counterions and Side Chain Lengths

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.