Microstress mechanism for the time dependence of the modulus of crystalline polymers following an imposed change in volume

Abstract

PARRY and Tabor¹ have observed a significant time dependence of the dynamic shear modulus (G′) of crystalline polymers at high hydrostatic pressure (10,000 pound inch⁻²). The modulus observed a few seconds after the increase in pressure is lower than the modulus observed at later times. There are two obvious explanations of this effect. The first is that it is due to a time-dependent bulk compliance (B). A pressure-induced reduction in volume increases G′. If B is time dependent, then the volume will also be time dependent and will continue to decrease after the increase in pressure, thus rendering G′ time dependent. The second explanation is that the time dependence of G′ is due to the pressure-induced generation of shear stresses in the heterogeneous solid. Of these explanations Parry and Tabor¹ favour the second. This view is supported by the following argument.