Main and secondary relaxations of non-polymeric high-Tg glass formers as revealed by dielectric spectroscopy

A series of high- glass formers with values varying between 347 and 390 K and molar masses in the range of 341 and 504 g/mol are investigated by dielectric spectroscopy. They are compared to those of paradigmatic reference systems. Differently polar side groups are attached to a stiff non-polar core unit at different positions. Thereby, the dielectric relaxation strength varies over more than two decades. All the relaxation features typical of molecular glass formers are rediscovered, i.e. stretching of the main (α-) relaxation, a more or less pronounced secondary (β-) process, and a fragility index quite similar to that of other molecular systems. The position of the polar nitrile side group influences the manifestation of the β-relaxation. The α-relaxation stretching displays the trend to become less with higher relaxation strength Δ, confirming recent reports. Typical for a generic β-process is the increase of its amplitude above which is found to follow a power-low behaviour as a function of the ratio / with a universal exponent; yet, its relative amplitude to that of the α-relaxation varies as does the temporal separation of both processes. The mean activation energy of the β-process as well as the width of the energy distribution () increases more or less systematically with . The latter is determined from the dielectric spectra subjected to a scaling procedure assuming a thermally activated process. Plotting () as a function of the reduced energy scale / the distributions are centred between 20 – 30 and their widths differ by a factor 2 - 3.