Relaxation dynamics of glass-forming liquids studied by ultrasonic spectroscopy: Stretched response of supercooled ethylbenzene

Glass forming liquids exhibit their strong or fragile behaviour as a function of temperature, featuring a smaller or greater deviation from a simple Arrhenius law. The size of such deviation on a typical time scale of 10(-6) s characterizes the 'kinetic fragility' F-1/2 of a given material. Ultrasonic experiments in the MHz region are then suitable to show how the relaxational properties are influenced by the 'fragile' character of the liquid investigated. On this basis, measurements of acoustic attenuation at fixed frequency (15 MHz) have been performed on glass forming liquid ethylbenzene in the temperature range 100 K-300 K and compared with previous results on simple supercooled liquids derived from benzene. To prevent crystallization each temperature point below 190 K was reached by cooling the sample directly from 300 K and using also different cooling rates. Measurements have given evidence of a mechanical relaxation process: below the melting temperature the acoustic attenuation exhibits a peak and correspondingly the sound velocity increases from liquid-like to solid-like values. A stretched response function is required to reproduce the observed behaviour. The values of the Kohlrausch-Williams-Watts stretching parameter beta(KWW) which describe the experimental data are very low, if compared to those obtained for other glass-forming liquids.