|Körber, T.; Krohn, F.; Neuber, C.; Schmidt, H.W.; Rössler, E.A.: Reorientational dynamics of highly asymmetric binary non-polymeric mixtures - a dielectric spectroscopy study, Physical Chemistry Chemical Physics, 23(12), 7200-7212 (2021) -- DOI: 10.1039/d0cp06652d|
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 Tg (ΔTg = 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-Tg component (α1) and the low-Tg 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-Tg molecules. From the extracted time constants, two glass transition temperatures Tg1 and Tg2 can be derived, showing a non-trivial concentration dependence for Tg2. 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 Δεαi as representation of molecular populations.