Viscosity of glass-forming melts

We suggest a model able to predict the changes of the viscosity eta with chemical composition. Viscosity is related to entropy of the system. Earlier, in the framework of the same model we have described successfully the dependence of viscosity on temperature and on pressure. Angell and co-workers compare the thermal properties of many glasses to their "fragility". The dimensionless fragility is a measure of the dependence of the activation energy for spatial rearrangement on the changes of structure. We propose to use the power (alpha) of our equation as a fragility parameter. The present treatment gives a simple interpretation of the glass transition temperature T-g: The glass transition takes places when disarray in the amorphous system decreases to an extent that the dispersity a is only 3.5% of maximal value of the activation energy E-max. At the glass transition temperature the dimensionless activation energy epsilon = E/RTg is about 30. (C) 1998 Published by Elsevier Science B.V. All rights reserved.