缠结聚合物剪切启动的原始链网络模拟中的壁滑移及其对剪切应力下冲的影响
Wall slip in primitive chain network simulations of shear startup of entangled polymers and its effect on the shear stress undershoot
AI总结：
针对缠结聚合物快速剪切启动中剪切应力下冲现象，研究通过扩展纳入壁面滑移的原始链网络模型开展模拟，发现壁滑移会削弱应力过冲与下冲幅度，接枝链与本体链的解缠结不贡献应力下冲，为该现象提供了新的机理解释。
摘要：
在近期针对缠结聚合物的快速剪切流启动过程中应力增长的部分实验里，研究人员观察到剪切应力在出现过冲之后、趋近稳态之前会发生下冲。尽管已有研究提出缠结链的翻转是该现象的根源，但本文探究了应力下冲的另一种可能成因，即聚合物与固体壁面之间的界面滑移。为此，我们对原始链网络（primitive chain network）模型进行了扩展，纳入了缠结聚合物流体与接枝有聚合物的固体壁面之间的界面滑移效应。我们通过设定本体聚合物中的剪切应力等于壁面接枝聚合物产生的剪切应力，确定了壁面处的滑移速度以及本体中的剪切速率。在确认稳态预测结果合理后，我们考察了瞬态行为。模拟结果证实，如先前所报道的，滑移会降低应力过冲的幅度。由于分子翻转的一致性降低，应力下冲的幅度也会减弱，甚至完全消失。换言之，在整个应力过冲区域内发生的接枝链与本体链之间的解缠结，并不会对应力下冲产生贡献。
英文摘要：
In some recent experiments on entangled polymers of stress growth in startup of fast shear flows an undershoot in the shear stress is observed following the overshoot, i.e., before approaching the steady state. Whereas tumbling of the entangled chain was proposed to be at its origin, here we investigate another possible cause for the stress undershoot, i.e., slippage at the interface between polymer and solid wall. To this end, we extend the primitive chain network model to include slip at the interface between entangled polymeric liquids and solid walls with grafted polymers. We determine the slip velocity at the wall, and the shear rate in the bulk, by imposing that the shear stress in the bulk polymers is equal to that resulting from the polymers grafted at the wall. After confirming that the predicted results for the steady state are reasonable, we examine the transient behavior. The simulations confirm that slippage weakens the magnitude of the stress overshoot, as reported earlier. The undershoot is also weakened, or even disappears, because of a reduced coherence in molecular tumbling. In other words, the disentanglement between grafted and bulk chains, occurring throughout the stress overshoot region, does not contribute to the stress undershoot.
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