ALPHA-MECHANICAL AND BETA-MECHANICAL RELAXATIONS IN AMORPHOUS POLYMERS - RUBBER GLASS-TRANSITION AND PHYSICAL AGING

The occurrence of molecular motion below T(g) of a glass (beta-relaxation) is considered as a precursor of the large-scale molecular motion (alpha-relaxation). In the case of amorphous polymers, the former is attributed to faster, uncorrelated, rotational motions of the smallest length of the main chain. These motions involve a distribution of both potential energy barriers and activation entropy resulting in the time tau(beta). The latter corresponds to the non-elastic deformation associated to the much slower hierarchically constrained motions of the surrounding sub-chain and which leads to the growth of sheared microdomains (smd) nucleated in the most disordered sites or quasi-point defects, i.e. randomly distributed sites with frozen-in density fluctuations. When the duration for the applied stress is long, or at high temperature, the smd grow and merge into the others which were nucleated at other sites, thus leading to an irrecoverable macroscopic deformation or viscous flow. This results in the main- or alpha-relaxation. The characteristic time is comparable to the average time for the molecular mobility around T(g); moreover, the rubbery effect is accounted for by introducing a parameter which is very sensitive to the length of chain segments between entanglements and physical or chemical crosslinking nodes. The model has allowed us to describe correctly the frequency and the temperature dependence of the dynamic modulus, and other features as well, in the case of a great number of amorphous polymers. In this work, our attention is focused on the low temperature side of the alpha-relaxation (shorter times) which appears to merge in the beta-relaxation and the consequences about physical ageing are emphasized.