Dynamics of strong and fragile glass formers and a scaling procedure for the temperature dependence of the viscosity

We review results from Raman scattering experiments on different types of glass formers which have been analysed in order to describe the fast relaxations occurring above T_g. Both fragile and strong glass formers exhibit similar dynamics which can be qualitatively reproduced by mode coupling theory (MCT). In particular, a crossover temperature T_c can be extracted where T_c>T_g. Quantitatively, deviations from the MCT predictions are larger, the stronger the glass former is. One reason for the deviations siems from lew-frequency vibrations which also contribute to the dynamic structure factor and which are small in fragile but large in strong liquids. A change of dynamics at T> T, is also indicated in simple liquids by the emergence of a slow secondary relaxation process and by the break-down of rotational-translational coupling. Furthermore, a viscosity analysis is performed by introducing a crossover temperature T_x which marks the boundary between two dynamical regimes. For all glass formers a master curve is obtained at T> T_x. T_x is found to be close to T_c. Applying this scaling procedure to alurainosilicate melts, systematic trends as a function of the composition are revealed.