Primary and secondary relaxation time dispersions in fragile supercooled liquids

Abstract

The relaxation time dispersions of the primary and secondary dielectric relaxations are studied for molecular glass-forming liquids regarding their dependence on structural relaxation time or lack thereof observed as time-temperature superposition TTS , their changes with fragility, and a possible correlation of the values for the and processes. Toward more fragile liquids, the width w of the peak at the glass transition temperature Tg is known to increase significantly. Additionally, we observe that TTS extends over a broader range of peak relaxation times in the case of higher fragility, with the approach to exponential Debye relaxation being delayed to faster dynamics. The width w of the relaxation at Tg is somewhat lower for more fragile liquids and appears correlated with that of the primary relaxation. The extrapolated coincidence of the dispersions of the and processes occurs at the fragility “limit” of m=170, implying that w w for all molecular supercooled liquids. Materials of high fragility are also those with no clear low-frequency cutoff in the relaxation time distribution—i.e., their susceptibilities require Havriliak-Negami instead of Cole-Davidson fits. For sorbitol, the value of w is seen to change steadily through the -merging region, whereas w displays a kink at the crossover temperature.