Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 31

In view of the continuous pathway between the liquid and glass states, glass‐forming liquids cannot be purely Newtonian when they approach the glass transition. In fact, they are viscoelastic, with an elastic component that becomes increasingly important near T_g. More precisely, application of a shear stress first causes an elastic strain, which would be recovered if the stress were released, and then a viscous deformation. The response of a viscous melt subjected to stress thus is made up of an instantaneous, elastic response along with a delayed response. By combining the simplest representations of elasticity and viscous flow, Maxwell model has as a mechanical analogue a spring and a dash pot placed in series [37]. Its important result is that, if stresses are applied at low frequencies, as usually the case in viscometry, then a simple relationship holds between the viscosity, relaxation time, and shear modulus at infinite frequency (G_∞),

(7)

The fact that the glass transition is observed at values close to 10¹² Pa.s for widely different kinds of liquids thus indicate that G_∞ also weakly depends on composition, with a mean value of about 10 GPa, which varies by less than a factor of 10 with either temperature or composition at least for oxide glass‐forming liquids [38, 39]. Compared with the tremendous variations of viscosity with temperature and composition, G_∞ thus is almost constant. If the viscosity is known, structural relaxation times can be readily estimated from Eq. (7).