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

摘要

A series of high-T-g glass formers with T-g values varying between 347 and 390 K and molar masses in the range of 341 and 504 g mol(-1) are investigated by dielectric spectroscopy. They are compared to paradigmatic reference systems. Differently polar side groups are attached to a rigid 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 (alpha-) relaxation, a more or less pronounced secondary (beta-) 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 beta-relaxation. The alpha-relaxation stretching displays the trend to become less with higher relaxation strength Delta epsilon(alpha), confirming recent reports. Typical for a generic beta-process is the increase of its amplitude above T-g, which is found to follow a power-law behaviour as a function of the ratio tau(alpha)/tau(beta) with a universal exponent; yet, its relative amplitude to that of the alpha-relaxation varies as does the temporal separation of both processes. The mean activation energy of the beta-process as well as the width of the energy distribution g(beta)(E) increases more or less systematically with T-g. The latter is determined from the dielectric spectra subjected to a scaling procedure assuming a thermally activated process. Plotting g(beta)(E) as a function of the reduced energy scale E/T-g, the distributions are centred between 19-35 and their widths differ by a factor 2-3.