DYNAMICS OF ENTANGLEMNT STRUCTURE UNDER MULTI-STEP SHEAR FLOW

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In dense polymer liquids of high molecular weight, polymer molecules are entangled with each other, forming a quasi-netwirk structiure. The change in entanglement structure in the range of nonlinear viscoelasticity can be observed by measuriong diufferential dynamic modulus G^<**> (omega, gamma ; t) by small oscillatory defomations superposed on a large deformation at frequency in the middle of the rubbery plateau.In this work, we have constructed new type of biaxial rheometer equipped with two driving and two detection systems. This apparatus allows us to give small oscillations in the plane perpendicular to the shearing plane of the large deformation.We have performed two experiments, stress relaxation in laege shearing deformation and stress development after onset of steady shear flow. From the first one, the strain dependence of G' (omega, gamma ; t) was found to coincide with the dumping function in relaxation modulus which is well explained by the Doi-Edwards theory. From the second one, transition of G'from the equilibrium value to a steady one at a finite shear rate was observed. The transition corresponded to the stear stress overshoot curve. When the shear rate decresed to the one in the linear viscoelasticity, the value of G'did not changed from the steady value. It was concluded that non-linear viscoelasticity results from a change in entabglement structure and the change in entanglement structure is irreversible.