<br>聚合物薄膜颈缩：缠结和平面应力条件下的延展性,Macromolecules

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聚合物薄膜颈缩：缠结和平面应力条件下的延展性
Macromolecules ( IF 5.1 ) Pub Date : 2024-06-17 , DOI: 10.1021/acs.macromol.4c00656
Siteng Zhang ₁ , Zhiqiang Cao ₂ , Xiaodan Gu ₂ , Ting Ge ₁

Affiliation

聚合物薄膜的延展性对于有机电子和分离膜等许多应用至关重要。进行大规模分子模拟以重现实验观察到的颈缩，这是一种延性变形模式。模拟表明，颈缩薄膜的形态与脆性聚合物中的银纹原纤维有本质上的不同。通过超越实验能力的细节揭示了薄膜颈缩的微观力学。薄膜的自由边界促进平面应力条件并允许通过应变局部化产生颈部。下面的缠结网络通过防止链条拉出来稳定颈部。颈部区域缠结聚合物的应变硬化补偿了厚度的减少，并支持在恒定拉力下稳定的颈部扩展，而不发生键断裂。尽管缠结起着关键作用，但颈部的宽度远大于缠结间距。 Considère 结构可以很好地预测颈缩的开始，但不能预测颈缩聚合物的拉伸比，因为其中的空隙破坏了体积守恒。 Krupenkin 和 Fredrickson 基于纠缠网络链延伸的几何论证能够预测拉伸比，这一点已通过使用 Z1+ 软件包的拓扑分析得到验证。在模拟中始终观察到延性薄膜颈缩，其厚度大于未受扰动的聚合物链尺寸，温度低于玻璃化转变温度，变形率远高于有限的单体迁移率。

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Polymer Thin Film Necking: Ductility from Entanglements and Plane Stress Condition

The ductility of polymer thin films is critical to many applications such as organic electronics and separation membranes. Large-scale molecular simulations are performed to reproduce the experimentally observed necking, a ductile deformation mode. The simulations show that the morphology of a necked film differs qualitatively from craze fibrils in brittle polymers. The micromechanics of thin film necking are revealed with details transcending the capability of experiments. The free boundary of a thin film promotes the plane stress condition and allows the onset of a neck via strain localization. The underlying entanglement network stabilizes the neck by preventing chain pullout. The strain hardening of entangled polymers in the neck region compensates for the reduction in thickness and supports stable neck propagation under a constant tensile force with no bond breaking. Despite the critical role of entanglements, the width of the neck is much larger than the entanglement spacing. The Considère construction predicts well the onset of necking but not the draw ratio of necked polymers, where voids break down the conservation of volume. Krupenkin and Fredrickson’s geometric argument based on the extension of entanglement network strands is able to predict the draw ratio, as verified by the topological analysis using the Z1+ package. The ductile thin film necking is consistently observed in the simulations with thicknesses larger than the unperturbed polymer chain size, temperatures below the glass transition, and deformation rates much higher than the limited monomer mobility.

更新日期：2024-06-17

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