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<INF>g</INF> (Delta T<INF>g</INF> = 216 K). We cover the whole additive concentration range from 4% up to 90% (by mass). Two main relaxations alpha<INF>1</INF> and alpha<INF>2</INF> are identified, which are characterized by well separated time scales and are attributed to the dynamics associated with the high-T<INF>g</INF> component (alpha<INF>1</INF>) and the low-T<INF>g</INF> component (alpha<INF>2</INF>). Frequency-temperature superposition does not apply. To cope with the extraordinary spectral broadening, we introduce a model consisting of a generalized Cole-Davidson (alpha<INF>1</INF>) and a Havriliak-Negami function with a low frequency truncation (alpha<INF>2</INF>). Whereas the alpha<INF>1</INF>-relaxation reflects essentially homogeneous dynamics and its spectra mainly broaden on the high-frequency flank of the relaxation peak, the alpha<INF>2</INF>-relaxation becomes broader on the low-frequency side reflecting pronounced dynamic heterogeneity in a more or less arrested matrix of high-T<INF>g</INF> molecules. From the extracted time constants, two glass transition temperatures T<INF>g<INF>1</INF></INF> and T<INF>g<INF>2</INF></INF> can be derived, showing a non-trivial concentration dependence for T<INF>g<INF>2</INF></INF>. Supplementary, we find a beta-relaxation. The total relaxation strength Delta epsilon strongly deviates from ideal mixing, and therefore care has to be taken interpreting the corresponding Delta epsilon<INF>alpha<INF>i</INF></INF> as representation of molecular populations.