The authors of the present study have recently presented evidence that the tumbling-snake model for polymeric systems has the necessary capacity to predict the appearance of pronounced undershoots in the time-dependent shear viscosity as well as an absence of equally pronounced undershoots in the transient two normal stress coefficients. The undershoots were found to appear due to the tumbling behavior of the director $\mathbf{u}$ when a rotational Brownian diffusion term is considered within the equation of motion of polymer segments, and a theoretical basis concerning the use of a link tension coefficient given through the nematic order parameter had been provided. The current work elaborates on the quantitative predictions of the tumbling-snake model to demonstrate its capacity to predict undershoots in the time-dependent shear viscosity. These predictions are shown to compare favorably with experimental rheological data for both polymer melts and solutions, help us to clarify the microscopic origin of the observed phenomena, and demonstrate in detail why a constant link tension coefficient has to be abandoned.

中文翻译：

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简单剪切作用下翻滚蛇形模型中的非恒定连杆张紧系数

本研究的作者最近提出了证据，表明聚合物系统的滚转-蛇形模型具有必要的能力，可以预测随时间变化的剪切粘度中明显下冲的出现，以及在瞬态两种情况下均不存在同样明显的下冲。法向应力系数。发现下冲是由于导演的翻滚行为而出现的$\mathbf{ü}$当在聚合物链段的运动方程中考虑旋转布朗扩散项时，已经提供了关于使用通过向列顺序参数给出的链节张力系数的理论基础。当前的工作详细说明了翻滚蛇形模型的定量预测，以证明其预测随时间变化的剪切粘度下冲的能力。这些预测结果与聚合物熔体和溶液的流变学实验数据相吻合，有助于我们阐明观察到的现象的微观起源，并详细说明了为什么必须放弃恒定的链节张力系数。