Deformation kinetics of ageing materials

A constitutive equation of time-dependent, chemically stable materials, which stems from the basic ideas of the irreversible thermodynamics of an internal variable and Eyring's absolute reaction rate theory, has been extended to chemically unstable materials. This formulation is quite general and, in principle, can be applied to many types of materials. In this paper, the ageing behaviour of time-dependent network polymers undergoing chain scission is considered. In the network scission process, we postulate that the energy barrier is affected by a changing of the chemical crosslink density. An explicit equation to account for the energy barrier change, which influences the relaxation process, is formulated. For the purpose of illustration, the effect of different chemical crosslink density, ν, on the relaxation rate has been considered, from which the following theoretical expression of relaxation modulus ΔE(t) is obtained:

ΔE = ΔEt exp (γv/kT)]

It can be seen that a change in v leads to an effective change in the time scale, usually denoted by a_x. Here the analytical expression a_x = exp(γν/kT) correlates quite well with the experimental data.