TABLE OF CONTENTS Various variants of the memory function in the appropriate Boltzmann-Volterra equations have been suggested to date to describe processes of stress relaxation and creep. Combined description of these memory functions and their resolvent is present in monograph [112]. The memory function contains three or four parameters, moreover they usually possess a fractional exponent of time, because only in this case experimental data may be described with a good approximation by stress relaxation and creep.
Analysis of proposed memory functions has indicated that at proper selection of parameters they give the course of relaxation processes with acceptable accuracy. However, the physical meaning of these parameters is not always clear, although attempts to disclose the physical meaning of some memory functions have been made [74, 104]. Reference [7] suggested an approach to production of relaxation memory functions based on consideration of thermodynamic functions and their changes during relaxation.
Suppose that the stress relaxation proceeds as a result of interaction and diffusion of kinetic units - relaxants. Relaxants may be various groups of atoms, repeat units, larger fragments and the whole fragments of macromolecules. Among relaxants are also separate elements of the free (empty) volume, i.e. microcavities, stress concentrators, etc. Interacting with each other, these microcavities may unite, be rearranged and diffuse in a polymeric material during relaxation, forming a structure which promotes a decrease of relaxing stress. The polymeric material may then be considered as the one consisting of relaxants and non-relaxants, the most part of the sample material...
