DIRECT CONNECTION BETWEEN ANISOTROPIC OPTICAL-PROPERTIES, POLARIZABILITY AND RHEOLOGICAL BEHAVIOR OF SINGLE-PHASE GLASS MELTS

Rheological properties of glass melts can be determined by the method of uniaxial deformation of cylindrical glass specimens in tensile and compressive experiments. The glass melts investigated show non-Newtonian flow behaviour at high deformation rates due to alteration of the isotropic structure to an anisotropic structure. If this anisotropic state is frozen-in by a controlled cooling process under constant load, the glass samples show a permanent frozen-in birefringence very similar to that of an optically uniaxial crystal which represents - at least partly - the anisotropic structure of the glass melt during the flow process. This permanent birefringence varies in a characteristic manner analogously to the load- and deformation-dependent rheological properties. The two properties are connected to each other very closely. Under non-Newtonian flow conditions, the specific birefringence or the Maxwell constant is much larger than under Newtonian flow conditions, indicating a principal structural change of the glass melts from a slightly isotropic structure with frozen-in strains to an essentially larger anisotropic one with oriented flow units or network fragments, respectively, depending on the degree of interconnection of the tetrahedra. Characteristic examples are given for float, metasilicate and metaphosphate glass melts.