The viscoelastic behavior of polymerized ionic liquids made up of poly(1-vinylimidazole-ran-1-ethyl-3-vinylimidazolium bis(trifluoromethane)sulfonylimide) (P(VI/C
2TFSI)) with varying degrees of quarternization (χ
q) was investigated to understand the relationship between macro physical properties and the charge density of polymer chains. The time-temperature superposition principle was found to be valid for P(VI/C
2TFSI) irrespective of χ
q value and yielded a universal master curve. The terminal response times decreased with increasing χ
q due to the dilution of polymer chains caused by the increasing TFSI
- concentration. Master curves of P(VI/C
2TFSI) with high χ
q showed broad shoulders in the glass-to-rubber transition region, which reflected either rotational motion of TFSI
- or sub-Rouse motion of the polymer chains. The fragility index decreased with increasing χ
q, indicating that the polymerized ionic liquids were less fragile in comparison with the ionomers or electrically neutral polymers. By observing the change in the fragility index, the transition between ionomers and polymerized ionic liquids was determined to be χ
q = 0.10 ∼ 0.25.
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