Viscoelastic Behavior of the Polymerized Ionic Liquid Poly(1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonylimide))

Dynamic melt viscoelastic behavior for polymerized ionic liquids, poly(1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonylimide)) (PC₂VITFSI), with various molecular weights was investigated in the frequency/temperature range from the glass to the terminal region. Viscoelastic spectra at given temperatures for PC₂VITFSI with lower molecular weight were able to be well superimposed on each other to make a master curve. PC₂VITFSI with a molecular weight larger than 1.4 × 10⁵ were entangled with each other, showing a rubbery region in the master curves. However, viscoelastic spectra for PC₂VITFSI with higher molecular weight (larger than 2.2 × 10⁵) were not superimposed in the rubbery and terminal regions due to the presence of two rubbery elastic origins, i.e., chain entanglement and ionic aggregates. The presence of huge ionic aggregates was unable to be confirmed by SAXS measurements, but the formation of ion pairs between a cation and an anion was supported by the WAXS profiles. Master curves of PC₂VITFSI exhibited extended shoulders in the glass–rubber transition region; these arose from the relaxation mode due to segmental motions of positively charged polymer chains affected by neighboring TFSI^– anions. The entanglement and critical molar mass of PC₂VITFSI were determined to be 4.5 × 10⁴ and 2.2 × 10⁴, respectively.