Nonlinear dielectric features of highly polar glass formers: Derivatives of propylene carbonate*

We have measured the nonlinear dielectric behavior of several highly polar propylene carbonate (PC) derivatives near T_g. Focus is on the effects of a large DC field on the frequency dependent permittivity and on the cubic susceptibility measured using a high AC field. Vinyl-PC shows both dielectric saturation and a field induced increase of dielectric relaxation times. The extent of the shift of the loss profile caused by the field correlates strongly with the peak magnitude of the cubic susceptibility, |χ₃|, showing a potential link between the |χ₃| 'hump' and electro-rheological behavior. Further support of this picture emerges from the observation that the most polar of these liquids, (S)-(-)-methoxy-PC ε_s ≈ 250, lacks both the electro-rheological effect in ε'(ω) as well as the 'hump' typically observed in |χ₃(ω)|. The absence of sensitivity of the dynamics to an electric field is contrary to the expectation that the electro-rheological effect correlates with the field induced entropy change. The results suggest that the dependence of the relaxation time on the electric field is not directly linked to the entropy change.