Linear viscoelastic creep under arbitrary temperature variations

Abstract

We discuss various temperature jump methods for the accurate determination of the activation energy ΔH of creep for a linear viscoelastic material, and carry out a numerical computation in a discrete relaxation model to show that the effect arising from the temperature dependence of the compliances ( αΔT effect) quickly decays after the temperature jump. A general and compact expression is then derived for the creep rate of a viscoelastic solid under constant stress and subject to arbitrary temperature variations. Application of the expression to various experimental situations is discussed, and the double T-jump method is extended to cases in which a temperature pulse of arbitrary shape is imposed on the sample. A systematic analysis shows that the αΔT effect makes a significant contribution to the expression only during a short period immediately after a T-jump and that it is otherwise totally negligible for any reasonable distribution function in relaxation time. Measurements of ΔH for polypropylene at 40°C by single and double T-jump are in good agreement and support this theoretical prediction. The double T-jump technique is so precise that observations of variation of ΔH with retardation time is now possible.