SYNOPSISA constitutive equation to describe the yield behavior of poly(methy1 methacrylate) (PMMA) is useful not only from the technological point of view, but also for the comprehension of the nonlinear mechanisms acting in the material. In both compression and tension, the yield stress is usually represented as a function of the strain rate at different temperatures. In PMMA and other glassy polymers these curves are related by scaling, that is, they can be matched to form a master curve. Particularly in PMMA the temperature and strain rate dependence of the master curve has been characterized by two different models. The first involves two thermally activated rate processes, one acting only at high strain rates. The second model interprets the yield process as a cooperative movement of several independent structural units, all with the same activation energy. In this article it is demonstrated that only the second phenomenological model is correct because it provides a good fit to the master curve of PMMA both in compression and tension, and verifies the properties of a set of curves related by scaling. Moreover, it is pointed out that the first model leads to severe inconsistencies because it does not consider the nonlinear behavior of PMMA. Finally, the physical parameters obtained (internal stress, activation volume, and enthalpy) are compared with those given in the literature. 0 1995 John Wiley & Sons, Inc. I NTRO DUCT10 NThe dependence of the yield behavior on strain rate and temperature is useful not only to predict simple failure criteria, but to describe the mechanisms acting in a nonlinear isotropic viscoelastic material as well. Particularly, the yield process of glassy polymers has been examined from two different aspects: the molecular and structural theories, and the phenomenological models.The molecular theories assume that the nonrecoverable deformation is due to permanent changes in the polymeric chains. For instance, Robertson' considered that the intramolecular forces were the primary factor for deformation, its rate being a function of temperature. Moreover, Duckett et a1.' modified this theory by including the effect of the hydrostatic pressure. This modified model satisfactorily accounts for the plastic resistance near the CCC 002 1 -8995/95/010055-14 glass transition temperature, but fails at low temperatures over a wide strain-rate range.Some years later, Argon3 developed a molecular theory by assuming that the deformation was a consequence of the formation of a pair of molecular kinks. The energy required to create the kink pair arose from the elastic interactions between molecules ( intermolecular forces ). Argon also suggested that the yield process could not merely evolve through the formation of double kinks, but should require the cooperative change of several adjacent molecular segments. Although this theory turned out to be more relevant at very low temperatures, there are no general physical models to describe the yield behavior over a wide range of both temperature and strain ...

SYNOPSISData on the tensile yield behavior of poly(viny1 chloride) (PVC), reported in the literature, are interpreted in terms of a model involving a cooperative movement of several independent structural units, all with the same activation enthalpy. This analysis leads to physical parameters such as the internal stress, activation volume, and enthalpy, etc. These values are discussed and compared with those determined from thermodynamical considerations using stress relaxation tests and tensile curves at a constant strain rate. 0 1996 John Wiley & Sons, Inc.

Stress relaxation of commercial poly(vinyl chloride) (PVC) is measured at strains below 3% and at different temperatures below the glass transition temperature. First it is shown that below the yield point the material follows a linear viscoelastic behavior. Then the data at a fixed deformation level (0.03) are fitted by considering a lognormal distribution function of relaxation times. Furthermore, from the measured stress‐strain curves, the temperature dependence of the elastic tensile modulus is determined. The temperature dependence of the elastic modulus, the relaxation strength, and the parameters of the distribution: mean relaxation time, τm, and half‐width, β, are given. Moreover, the distribution function and the temperature dependence of its characteristic parameters are discussed in terms of a cooperative model of the mechanisms involved in the mechanical relaxation of glassy polymers. Finally, the relationship proposed between the tensile modulus and the free volume helps explain the temperature dependence of the relaxation strength. © 1996 John Wiley & Sons, Inc.