DYNAMICS OF ENTANGLEMNT STRUCTURE UNDER MULTI-STEP SHEAR FLOW

在高相对分子质量的稠密聚合物液体中，聚合物分子相互缠绕，形成准网络结构。非线性粘弹性范围内纠缠结构的变化可以通过测量橡胶平台中部频率上的大变形叠加小振荡形变的不同动模G^&lt；**&gt；(omega，Gamma；t)来观察。该装置允许我们在垂直于大变形剪切面的平面上给出小的振动。我们进行了两个实验，即剪切变形中的应力松弛和恒定剪切流动开始后的应力发展。从第一个模型出发，发现G‘(omega，Gamma；t)的应变依赖关系符合用Doi-Edwards理论很好地解释的松弛模数中的倾倒函数。从第二个模型开始，观察到在有限剪切速率下，G‘从平衡值向定常值的转变。这一转变对应于钢的应力超调曲线。当剪切速率减小到线性粘弹性时，G‘值与稳态值基本一致。结果表明，非线性粘弹性是由缠结结构的变化引起的，缠结结构的变化是不可逆的。

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.