Imidazole Polymers Derived from Ionic Liquid 4-Vinylimidazolium Monomers: Their Synthesis and Thermal and Dielectric Properties

The synthesis of 1-ethyl-3-methyl-4-vinylimidazolium triflate, its polymerization, and ion exchange to yield a family of 4-imidazolium polymers with a variety of anions are described. For comparative purposes, the synthesis, polymerization, and ion exchange of an analogous set of 1-vinylimidazolium polymers are also presented. The comparative thermal and dielectric characteristics of the 4-vinyl-and 1-vinylimidazolium salts were evaluated. The trends in the glass transition (T-g) characteristics of the various 4-vinylimidazolium and 1-vinylimidazolium polymers were similar; however, the glass transition temperatures of poly(4-vinylimidazolium) BF4- PF6-, AsF6-, and CF3SO3- salts were significantly higher than those of the corresponding poly(1-vinylimidazolium) salts. This difference and the increase in T-g in going from BF4- to AsF6- in the 4-vinylimidazolium series were attributed to enhanced intramolecular bridging between imidazolium moieties positioned 1,3 or 1,5 along the polymer chain. In the dielectric spectra of I-vinylimidazolium salts at temperatures in excess of 30 degrees C, one relaxation mode distinct from that for electrode polarization is observed. The single mode appears to correspond to the alpha-relaxation peak in poly(3-ethyl-1-vinylimidazolium salts) recently identified and attributed to ion-pair motion by Nakamura et al. In the 4-vinylimidazolium polymer spectra set, at temperatures in excess of 30 degrees C, two relaxation modes, distinct from that for electrode polarization, are apparent: the alpha peak also observed in the 1-vinylimidazolium polymer set and a new relaxation peak observed at lower frequency. The lower frequency relaxation peak is identified in this work as the alpha'-relaxation and is also associated with ion-pair motion. Assuming the relaxation processes to be Arrhenius in nature, the activation energy of the alpha-relaxation in poly(4-vinylimidazolium) BF4-, PF6-, CF3SO3-, TFSI-, and C2N3- salts ranged from 83 to 28 kJ/mol and appears to scale with the glass transition temperature.