Is there Universality in the Dielectric Response of Polar Glass Formers?

The frequency dispersion of structural α-relaxation obtained from broadband dielectric spectroscopy measurements is relatively narrow in many polar glass-formers. On the other hand, it becomes much broader when probed by other techniques, including photon correlation spectroscopy (PCS), nuclear magnetic resonance (NMR), and mechanical shear modulus. Therefore, the dynamics of glass-formers observed by dielectric permittivity spectroscopy (DS) is called into question. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari-Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same glass-former, there is a reduction of relaxation strength of α-relaxation relative to that of the JG β-relaxation. Additionally, the separation between the α and the JG β relaxations in dielectric permittivity) decreases when probed by mechanical shear modulus. These changes in relation of α- to JG β-relaxation, when examined by the other techniques, engender the non-negligible contribution of the latter to the former. Hence the apparent α-relaxation is broader than observed by the dielectric permittivity. The broadening is artificial because it is due to a confluence of the α and JG β relaxations with a disparity in their relaxation strengths much less when the other techniques than by dielectric permittivity are used. This explanation is supported by showing the α-relaxation of polar glass-formers becomes broader when the dielectric data are represented in terms of the electric modulus instead of permittivity. The broadening, in this case, is again due to a reduction of the relaxation strength of the α-relaxation relative to that of the JG β-relaxation in the electric modulus representation. A corollary of the explanation applicable to weakly polar glass-formers having JG β-relaxation widely separated from the α-relaxation is the prediction that the frequency dispersion of dielectric α-relaxation is nearly the same as that of the electric modulus, and there is no significant additional broadening when probed by the other techniques. A host of experimental data from the literature and our new measurements are given to support the explanation for polar glass-formers and the ancillary prediction for weakly polar glass-formers. Thus the narrow frequency dispersion of the intense relaxation in polar glass-formers observed by dielectric permittivity is real and genuinely represents the dynamically heterogeneous and cooperative dynamics of α-relaxation. By contrast, the broad dispersion found by the other techniques is artificial and misleading.