Abstract
The molecular theory of Rouse, Zimm, and Bueche correctly accounts for the viscoelastic properties of polymers in very dilute solution and, to a large extent, for those of polymers in bulk or in concentrated solution, as long as their mean molecular weight is below about 20 000. Above this MW limit, relaxation times appear which are longer than those provided for in this theory. The “viscoelastic plateau”, which then appears in the long relaxation time region of the dynamic spectrum, is ascribed to entanglements of molecular chains which behave like temporary crosslinks. An analogous phenomenon occurs in the same way in permanent polymer networks, such as rubber vulcanizates. In this case one finds abnormally slow relaxation or creep rates during the approach to equilibrium, as well as increased low-frequency mechanical energy losses under forced sinusoidal vibration. The presence of colloidal fillers, such as carbon blacks used to reinforce rubbers, also seems to increase this hysteresis within the polymer matrix, independent of thixotropic effects which result from the reversible rupture of filler particle aggregates under large-amplitude cyclic deformations. We propose to analyze here the results (obtained jointly at the Institut Français du Caoutchouc and at the laboratory of Professor J. D. Ferry, University of Wisconsin) of measurements over the entire rubbery spectrum of the dynamic properties and of stress relaxation on vulcanizates of natural rubber, cis-polybutadiene, and styrene-butadiene copolymer (SBR) in the absence of secondary crystallization or aging phenomena. Then we examine the interpretation of the behavior of these materials, both at low frequency and during the approach to equilibrium, by analogy with the theories of the “viscoelastic plateau” of linear polymers.
