Relaxation Dynamics of Glass-Forming Liquids Studied by Ultrasonic Spectroscopy: Stretched Response of Supercooled Ethyl
- PDF / 205,733 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 106 Downloads / 238 Views
CC5.4.1
Relaxation Dynamics of Glass-Forming Liquids Studied by Ultrasonic Spectroscopy: Stretched Response of Supercooled Ethylbenzene M. Cutroni and A. Mandanici Dipartimento di Fisica and INFM, Università di Messina 98100 Messina, ITALY ABSTRACT 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’ F1/2 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 βkww which describe the experimental data are very low, if compared to those obtained for other glass-forming liquids. INTRODUCTION A fast cooling rate usually allows to bring liquids in a supercooled state below their melting temperature Tm, avoiding crystallization [1-3]. In this supercooled temperature region the variation of dynamic variables like viscosity or relaxation times in all the so-called ‘strong’ liquids obeys the Arrhenius law τ = τ 0 ⋅ exp[E (k B T )]
(1) .
Instead in supercooled ‘fragile’ liquids those variables increase much faster than predicted by the Arrhenius law, when the temperature decreases [2,4]. The apparent activation energy, related to the slope of the η(T ) or τ(T ) curve in an Arrhenius-like plot, increases progressively. Molecular rearrangements become more and more unlikely until structural dynamic processes are arrested on the experimental observation time-scale at the glass transition temperature Tg, and a solid with the frozen-in disorder typical of liquids is obtained. The fragility concept allows for a classification of all materials with respect to their temperature dependent relaxational features [5]. The full understanding of the microscopic mechanism giving rise to the ‘fragile’ behaviour of supercooled liquids, and to the glass transition is still lacking although this has important implications in several fields like for
CC5.4.2
instance materials science, cryobiology and the food industry. In order to gain more information on the dynamics in the supercooled regime, some simple disubstituted benzene
Data Loading...
