New experimental features and revisiting the alpha and beta mechanical relaxation in glasses and glass-forming liquids

The dynamic mechanical moduli of various amorphous polymers, and more speci ally PMMA, in the glassy state and through their glass transition temperatu res are considered. Beyond the usual properties of the beta and alpha relax ations, new features are observed: departure from thermorheological simplic ity and the effect of physical ageing on the beta relaxation. A theory prev iously proposed in order to describe those relaxation phenomena on a physic al basis is extended so that all the experimental data can be explained in a self-consistent way and equations able to reproduce those data are given. The main new aspects of the theory is the temperature (T > T-g) and struct ure dependence of: (i) the correlation parameter determining the molecular mobility described in terms of hierarchical constraint; (ii) the elastic an d anelastic moduli; and (iii) the strength of the beta process. (C) 1999 El sevier Science B.V, All rights reserved.