Abstract:

Relaxation and retardation spectra comprise both the statistical processes and the macroscopic behavior of a linear viscoelastic material. Effectively, they provide the distribution function of the mechanical processes that evolve in the polymeric system and their integrals give the difference between the limiting moduli or compliances. Consequently, the normal procedure used for the interconversion between the spectra does not allow to separate the statistical characteristics of the mechanical processes from the macroscopic mechanical response. Nevertheless, the normalized spectra, characterized only by geometrical parameters, are more suitable because they give the probability of evolution of the internal processes without including an explicit dependence on the mechanical property under consideration. Furthermore, in the case of linear viscoelastic solids the interconversion of normalized spectra let to separate the statistical description from the intensity of relaxation Δ, which is the only macroscopic factor involved. In order to analyze this separation of variables, the interconversion of well-known distribution functions is considered. It is found that, for high values of Δ generally met in polymers, the converted spectrum of any distribution function, bounded or not, can be described virtually by a single process. This ambiguous description of the same polymeric system, given by a complex distribution and by a single process, simultaneously, is originated from the determination of Δ from the master curves. Therefore, an alternative treatment of experimental data is suggested in this paper, considering the individual curves measured at different temperatures without making any extrapolation. © 1992 The Society of Polymer Science, Japan.