DYNAMIC MECHANICAL STUDY OF 4 AMORPHOUS POLYMERS AROUND AND ABOVE THE GLASS-TRANSITION - BREAKDOWN OF THE TIME TEMPERATURE SUPERPOSITION PRINCIPLE IN THE FRAME OF THE COUPLING MODEL

The dynamic mechanical behavior of four structurally related amorphous polymers has been investigated around and above their glass transition. In this temperature range two dispersion processes associated with segmental motion and the terminal zone, respectively, have been well resolved. However, the time-temperature superposition principle cannot be applied simultaneously to these processes. Segmental motion and terminal zone processes have been described in terms of the Havriliak-Negami relaxation function. We have also found that the time domain Fourier transform of those H-N behaviors can be well described by means of stretched exponential functions, and therefore it is possible to characterize process by means of a single shape parameter, beta. The so-obtained beta-parameters for the two dispersion are related to the corresponding apparent activation energies in the way expected by the coupling model when a common microscopic friction coefficient is assumed to control both processes.