标题： 未缠结聚合物熔体在单轴伸长流动下的机械降解 显示英文标题

标题： Mechanical Degradation of Unentangled Polymer Melts under Uniaxial Extensional Flow

摘要： 复杂的流场控制着聚合物在各种制造过程中的变形。然而，高流动速率可能触发原材料聚合物中的反应事件（即键断裂或机械力子的不良反应），导致制造结构的机械或功能退化。此外，由于时间和长度尺度的限制，在实验室中很难完全表征这种分子级别的流动。在这项研究中，我们进行了非平衡分子动力学（NEMD）模拟，以探索在单轴拉伸流动（UEF）下未缠结聚合物熔体的流变和机械降解，并允许链断裂。我们的模拟表明，随着UEF拉伸速率的增加，会出现剪切增稠-变稀-再增稠的阶段，这是由流诱导的链构象变化引起的。随着UEF拉伸速率的进一步增加，键断裂势能导致另一个流动变稀阶段。有趣的是，由于键断裂前需要高度拉伸的聚合物链，裂纹动力学最初是一级的。当键断裂在链被拉伸之前发生时，它就不再是一级的了。我们的计算工作为聚合物材料制造工艺的最佳设计提供了见解。 显示英文摘要

摘要： Complex flow fields govern the deformation of polymers in various manufacturing processes. However, high flow rates may trigger reaction events (i.e., bond breaking or undesirable reaction of mechanophores) in raw polymeric materials, leading to the mechanical or functional debasement of manufactured structures. Additionally, it is difficult to fully characterize such molecular-level flow in the laboratory due to time- and length-scale limits. In this study, we perform non-equilibrium molecular dynamics (NEMD) simulations to explore the rheological and mechanical degradation of unentangled polymer melts under uniaxial extensional flow (UEF), allowing for chain breaking. Our simulations demonstrate shear thickening-thinning-thickening stages with the increase of UEF extension rates, resulting from flow-induced changes of chain conformation. With further increasing UEF extension rates, a bond-breaking potential leads to another flow thinning stage. Interestingly, fracture kinetics is originally first-order owing to the need for highly stretched polymer chains before bond fracture. It is no longer first-order when bond fracture is instigated before chains are stretched. Our computational work provides insight into the optimal design of the manufacturing process for polymeric materials.