Some new operational modes and parameters of stress relaxation for the viscoelastic characterization of solid polymers. II. The 'vertical-shift' mode and intrinsic 'time-strain clock'

摘要

As in part I of this study, in the same manner in the present part II as well, by the same modus operandi way, an attempt was made to introduce, by means of a given operational mode, some further practical parameters for a "by-eye" but well-proven experimental viscoelastic characterization of a polymeric solid. Thus, through consideration of the peculiar vertical shift behavior of the apparent modulus ((E) over tilde) of isotactic polypropylene (iPP) and based on the KWW model, it is shown that, in an empirical and a formalistic sense, a relevant effective or equivalent (single) characteristic relaxation time can be introduced which can give some new interpretations for the linear and nonlinear viscoelastic behavior of a polymeric material, as that of a "time-strain clock," which, as an intrinsic function, is responsable for a functional time-strain shift of the relaxation time and, at the same time, for a shift toward to a more linear or to a more nonlinear behavior. In the above context of attempts, another functional relationship was shown, the so-called spectral shift function and its corresponding parameter of nonlinearity strength, through which some further interpretations and characterizations connected with the existence of the permanent internal stress could be made. Finally, the introduction of the so-called spectral isostrains and their corresponding "spectral inversion point" complete the "set" of the operative parameters proposed for the above-mentioned purpose. (C) 2002 Wiley Periodicals, Inc. [References: 12]