Linear viscoelasticity of an ultraphosphate glass

Linear viscoelastic stress relaxation and oscillatory shear experiments wer e performed on a mixed alkali ultra-phosphate of composition 0.2Na(2)O + 0. 2Li(2)0 + 0.6P(2)O(5) at temperatures in the vicinity of the grass transiti on temperature, T-g. The stress relaxation data were well described by a st retched exponential function G(t)= G(0) exp[-(t/tau)](beta). beta decreased with increasing temperature. Oscillatory shear experiments were performed to determine the complex mechanical moduli, G'(omega) = G'(omega) + iG "(om ega), as a function of angular frequency, omega. G'(omega) is the elastic m odulus and G "(omega) is the loss modulus. The breadth of G "(omega), which is a direct measure of the distribution of relaxation times, increased wit h increasing temperature in the same temperature range. The temperature dep endence of beta and of G "(omega) indicates a failure of thermorheological simplicity in this grass. The shear viscosities of the glass determined fro m G(t) and the zero shear viscosities determined from G "(omega) were found to be in agreement. (C) 2000 Elsevier Science B.V. All rights reserved.