Melt rheology of zinc alkali phosphate glasses

The melt rheology of two zinc alkali phosphate glasses was studied with oscillatory flow experiments to accelerate efforts to melt process them with organic polymers. The glasses differed markedly in chemical durability and melt viscosity. Melt viscosities were time independent at temperatures near their glass transition temperature. The omega dependence of the complex viscosity eta* conformed excellently to predictions, eta*(omega) = eta(0)(1+iw tau), for Hookean dumbbells. Fit parameters eta(0) and tau were used to estimate a melt viscosity-average molecular weight M using a model reported by Ferry and co-workers [J. Applied Phys. 26, 359 (1955)]. Arrhenius activation energies Delta H double dagger for viscous flow were consistent with those traditionally found for inorganic glasses. At higher temperatures, the melt viscosity of both glasses increased monotonically with time. The viscosity increase was exponential at long times, and the transition time to the exponential dependence was found to be strongly dependent on the shear strain. The steep viscosity rise is expected to cause significant processing challenges. Sample crystallization is thought to be responsible for this behavior. (C) 1996 Society of Rheology.