Reorientational dynamics of highly asymmetric binary non-polymeric mixtures - a dielectric spectroscopy study

We present an analysis of dielectric spectra measured for a specially designed non-polymeric asymmetric binary glass former characterized by a large difference of the component's T_g (ΔT_g = 216 K). We cover the whole additive concentration range from 4% up to 90% (by mass). Two main relaxations α₁ and α₂ are identified, which are characterized by well separated time scales and are attributed to the dynamics associated with the high-T_g component (α₁) and the low-T_g component (α₂). Frequency-temperature superposition does not apply. To cope with the extraordinary spectral broadening, we introduce a model consisting of a generalized Cole-Davidson (α₁) and a Havriliak-Negami function with a low frequency truncation (α₂). Whereas the α₁-relaxation reflects essentially homogeneous dynamics and its spectra mainly broaden on the high-frequency flank of the relaxation peak, the α₂-relaxation becomes broader on the low-frequency side reflecting pronounced dynamic heterogeneity in a more or less arrested matrix of high-T_g molecules. From the extracted time constants, two glass transition temperatures T_g and T_g can be derived, showing a non-trivial concentration dependence for T_g. Supplementary, we find a β-relaxation. The total relaxation strength Δε strongly deviates from ideal mixing, and therefore care has to be taken interpreting the corresponding Δε_α as representation of molecular populations.