(123c) Anomalous Dynamics of Ionic Liquids near to the Glass Transition Temperature

Here we discuss a systematic study of the dynamic moduli of 1-butyl 3-methylimidazolium tetrafluoroborate [Bmim][BF₄], 1-butyl 3- methylimidazolium hexafluorophosphate [Bmim][PF₆], and 1-ethyl 3-methylimidazolium ethylsulfate ([Emim][EtSO₄]) in the vicinity of their respective glass transition temperatures. The results show an anomalous aging in that the dynamic and the low shear rate viscosities decrease with time at temperatures near to, but above, the glass transition temperature and this is described in detail in the conference. The samples that are aged into equilibrium obey the time temperature superposition principle and the shift factors and the viscosities follow classic super-Arrhenius behaviors with intermediate fragility values as the glass transition is approached. The data also show that these ionic liquids do not follow the Cox-Merz relationship between the steady shear viscosity and the dynamic viscosity. Finally, we note that the use of the van Gurp analysis provides an excellent representation of the time-temperature superposition of the systems and provides a ready means to illustrate the differences in glassy moduli of the three ionic liquids studied.