Linear viscoelasticity of an ultraphosphate glass

Linear viscoelastic stress relaxation and oscillatory shear experiments were performed on a mixed alkali ultraphosphate of composition 0.2Na2O+0.2Li2O+0.6P2O5 at temperatures in the vicinity of the glass transition temperature, Tg. The stress relaxation data were well described by a stretched exponential function G(t)=G0 exp [−(t/τ)]β. β decreased with increasing temperature. Oscillatory shear experiments were performed to determine the complex mechanical moduli, G*(ω)=G′(ω)+iG′′(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.