THE COMPLEX MECHANICAL MODULUS AS A STRUCTURAL PROBE - THE CASE OF ALKALI BORATE LIQUIDS AND GLASSES

Brillouin light scattering has been used to determine the high-frequen cy complex mechanical modulus of alkali berate liquids and glasses, as a function of the temperature. The temperature dependence of the comp lex modulus can be described by an enhanced Maxwell model for linear v iscoelastic systems. Accordingly, the module comprises relaxational co mponents and a temperature dependent static modulus, which is determin ed by the equilibrium volume fraction of kinetically arrested domains. Application of this model to the Brillouin data indicates that the st ructural relaxations in undercooled glass forming liquids occur via re latively distinct mechanisms, each one becoming thermally activated wi thin a different temperature range. The rate of degradation of the net work structure increases with increasing alkali content, and is commen surate of the fragility of the liquid. The structural features which a re subject to a change in the context of a particular degradation mech anism are released sequentially, i.e., relaxation, facilitated by the rupture of a given network link, is required before other links of the same type become affected by thermal motion. Mechanisms that are acti vated at high temperatures involve the diffusional displacements of va rious atomic species. Immediately above T-g, however, structural relax ations are characterized by the dominance of the bulk viscosity over t he shear viscosity, and by positive values of the imaginary part of th e complex poison ratio. This indicates that, to a significant degree, compressive deformations and head-on collisions between structural moi eties are involved in the structural relaxations at these low temperat ures. It is surmised that the deformation of boroxol rings, where a bo ron moves out of the BO3 plane to exchange one of its oxygen neighbors , is underlying to this relaxation mechanism, which results in an incr ease of the average network ring size. (C) 1995 American Institute of Physics.