On the Constitutive Equation for Polymeric Materials

Abstract

The validities of the linear and nonlinear constitutive equations for polymeric materials are discussed from non-steady experiments with a Weissenberg rheogoniometer model R-17 of Sangamo Controls. Lodge's linear constitutive equation is well supported by the measurements of normal and shear stresses under oscillatory shear flow. On the other hand, the so-called “rate type” nonlinear constitutive equation could not explain the experimental results of the stress growth and relaxation after the onset and the cessation of steady simple shear flow. A discussion on the failure of the theories for stress-overshoot phenomena is given based on the idea that a transition of the internal structure due to external force does not occur at the moment when the external force is given to the material but occurs after a certain amount of strain is accumulated in the material. Moreover, a method to estimate the energy for entanglement destruction in polymeric materials is proposed.