Single amorphous polymeric nanofibers exhibit tunable properties when their sizes drop down below a specific onset value, including increases in modulus and strength. Understanding the detailed mechanism and corresponding microstructure change at the molecular level is key for reaching the long-term goal of controlling their properties and targeting applicable designs. In particular, the experimental study of polymeric chains and covalent bonds in amorphous nanofibers has been proven extremely challenging due to the scale limitation. Here, we investigate the role of chain alignment and backbone bond length on the diameter dependence of individual amorphous polystyrene (PS) by molecular dynamics (MD) simulation. For the first time, the diameter of the modeled nanofibers in MD can be directly comparable to the experimental scale. We observed a linear increase of ultimate strength and an exponential increase of Young’s modulus when reducing the nanofiber diameter to a certain onset value. The increase of ultimate strength is found to be more related to the linearly increased chain alignment while the variation in Young’s modulus is directly attributed to the exponentially increased backbone bond length. The parameter “normalized diameter” is also introduced to evaluate the extent of chain confinement in nanofibers. Finally, MD simulation is validated by comparing with experimental results, and a model demonstrating the evolution of molecular structure with diameter is proposed.

中文翻译：

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聚苯乙烯纳米纤维力学性质的尺寸依赖性中链取向和键长的相互作用

当单个非晶态聚合物纳米纤维的尺寸下降到特定的起始值以下时，其表现出可调性，包括模量和强度的增加。在分子水平上了解详细的机理和相应的微结构变化是实现控制其性能和针对适用设计的长期目标的关键。特别地，由于规模限制，已证明对非晶态纳米纤维中聚合物链和共价键的实验研究极具挑战性。在这里，我们通过分子动力学（MD）模拟研究链对准和主链键长对单个无定形聚苯乙烯（PS）的直径依赖性的作用。在MD中，建模的纳米纤维的直径首次可以直接与实验规模相比。当将纳米纤维直径减小到一定的起始值时，我们观察到极限强度的线性增加和杨氏模量的指数增加。发现极限强度的增加与线性增加的链排列更相关，而杨氏模量的变化直接归因于指数增加的骨架键长。还引入参数“规格化直径”以评估纳米纤维中的链约束程度。最后，通过与实验结果的比较验证了MD模拟的有效性，并提出了一个模型来证明分子结构随直径的变化。发现极限强度的增加与线性增加的链排列更相关，而杨氏模量的变化直接归因于骨架键长的指数增加。还引入参数“规格化直径”以评估纳米纤维中的链约束程度。最后，通过与实验结果的比较验证了MD模拟的有效性，并提出了一个模型来证明分子结构随直径的变化。发现极限强度的增加与线性增加的链排列更相关，而杨氏模量的变化直接归因于骨架键长的指数增加。还引入参数“规格化直径”以评估纳米纤维中的链约束程度。最后，通过与实验结果的比较验证了MD模拟的有效性，并提出了一个模型来证明分子结构随直径的变化。