Structural dependence of fast relaxation in glass-forming substances and correlation with the stretch exponent of the slow structural α-relaxation

The fast relaxation in glass-forming substances is often identified with the beta -relaxation of the idealized mode coupling theory (MCT), which involves a critical temperature, T-c, higher than the glass transition temperature, T-g. However, several recent experimental works indicate that the fast relaxation may not be so simply described. These findings include the breakdown of the predictions of the idealized MCT below T-c in 0.4Ca(NO3)(2)-0.6KNO(3) (CKN) and ortho-terphenyl (OTP), the existence of the fast relaxation also in crystalline OTP, and the flat dielectric susceptibility minimum (i.e., nearly frequency independent or constant loss) observed in CKN and propylene carbonate at temperatures below T-c. These experimental facts indicate that the fast relaxation observed below T-c is outside the predictions of the idealized MCT and a recent two-component schematic model of MCT, which includes hopping. It is unlikely that this non-MCT fast relaxation existing below T-c will abruptly vanish at temperatures above T-c. The need for an alternative model of the fast relaxation not only below but also above T-c is thus clear. We analyzed susceptibility spectra of many glass-formers with the assumption that the susceptibility minimum is caused by a temperature dependent near constant loss. Good fits to the susceptibility spectra were obtained and in the process the near constant loss was determined as a function of temperature. When compared in a plot against T-g-scaled temperature, the near constant losses of different glass-formers exhibited a pattern that correlates with the stretch exponent, beta (alpha)(T-g), of the Kohlrausch-Williams-Watts (KWW) function which describes the time dependence or the Structural alpha -relaxation at T-g. The correlation suggests that the fast relaxation and the slow structural alpha -relaxation are both determined by the same factor, which we attribute to the anharmonic intermolecular potential of interactions between the molecules. The fast relaxation is suggested to be the relaxation of a vibration by anharmonicity. The vibration is likely the one that attempts to relax the structure of the glass-former. (C) 2001 Elsevier Science B.V. All rights reserved.