Linear viscoelasticity of an ultraphosphate glass

Abstract

Linear viscoelastic stress relaxation and oscillatory shear experiments were performed on a mixed alkali ultraphosphate of composition 0.2Na 2O+0.2Li 2O+0.6P 2O 5 at temperatures in the vicinity of the glass transition temperature, T g. The stress relaxation data were well described by a stretched exponential function G(t)=G 0 exp [−(t/τ)] β. β decreased with increasing temperature. Oscillatory shear experiments were performed to determine the complex mechanical moduli, G *( ω)= G ′( ω)+i G ′′( ω), as a function of angular frequency, ω. G ′( ω) is the elastic modulus and G ′′( ω) is the loss modulus. The breadth of G ′′( ω), which is a direct measure of the distribution of relaxation times, increased with increasing temperature in the same temperature range. The temperature dependence of β and of G ′′( ω) indicates a failure of thermorheological simplicity in this glass. The shear viscosities of the glass determined from G( t) and the zero shear viscosities determined from G ′′( ω) were found to be in agreement.