The kinetics of volume relaxation and related property changes during physical aging of poly(ether ether ketone) (PEEK)

Structural relaxation is accompanied by a simultaneous change in all structure-sensitive physical mechanical properties. Naturally, a fundamental correlation between structural relaxation and the variation of the properties of the glass would be of great interest to scientists and engineers. Changes in mechanical and dielectric properties during the course of structural relaxation are of particular interest, but one must remember that their measurements involve the use of external stress (or strain), which, regardless of its magnitude, will affect the ongoing structural relaxation. The application of even the smallest stress or strain could shorten the relaxation time by one or more orders of magnitude and hence caution must be exerted in seeking correlations between volume and enthalpy relaxation on the one hand, and viscoelastic (mechanical or dielectric) measurements on the other. The two approaches can be viewed as manifestations of the same structural relaxation although one must bear in mind that they differ fundamentally in terms of the cause and the effect, and while the theoretical background of correlation between structural and viscoelastic relaxation has not been explored, in this paper an empirical correlation between the two will be reported.