THE GLASS-TRANSITION - GENERAL SCENARIO AND CROSSOVER TEMPERATURE

Analysis of light and neutron scattering spectra of both fragile and n on-fragile glass forming liquids shows that the glass transition is no t a monotonic process, qualitative changes in the dynamics occur at a crossover temperature, T-x. The crossover temperature shows up in many processes and seems to occur for different liquids at approximately t he same viscosity (or structural relaxation time). A general scenario for the dynamics of strong and fragile glass forming liquids is discus sed. It is shown that at T > T-x it follows, the qualitative scenario suggested by mode-coupling theory (MCT) for the higher temperature reg ion. However, below T-x it does not agree with the asymptotic MCT pred ictions, not even qualitatively. The importance of a potential energy landscape for the dynamics in this low temperature range (T < T-x) is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.