Shavit Amit, Riggleman Robert A
Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Phys Chem Chem Phys. 2014 Jun 14;16(22):10301-9. doi: 10.1039/c3cp55330b.
Although the origin of ductility in crystalline materials is well understood through the motion of dislocations and defects, a similar framework for understanding deformation in amorphous materials remains elusive. In particular, the difference in the mechanical response for small-molecule amorphous solids, such as organic glasses that are typically brittle, and polymer glasses, which are frequently very tough, has not been systematically explored. Here, we employ molecular dynamics simulations to investigate the mechanical response of model glassy polymers confined to a nanoscopic pillar under tensile deformation. We vary the chain length, cooling rate for forming the glass, and the deformation rate and investigate the changes in the mechanical response. We find that samples that are cooled at a slower rate and deformed at a slower rate are more prone to localization of the strain response, or shear banding. Interestingly, this effect is independent of chain length over the range of parameters we have investigated so far, and we believe this is the first direct observation of shear banding in deformed polymer glasses under cylindrical confinement. Finally, by using the isoconfigurational ensemble approach, we provide evidence that the location where the shear band forms is due to structural features that are frozen in place during sample preparation.
尽管通过位错和缺陷的运动，人们已经很好地理解了晶体材料的延展性起源，但用于理解非晶材料变形的类似框架仍然难以捉摸。特别是，对于小分子非晶固体（如通常易碎的有机玻璃）和聚合物玻璃（通常非常坚韧）的力学响应差异，尚未进行系统研究。在这里，我们采用分子动力学模拟来研究在拉伸变形下限制在纳米柱中的模型玻璃态聚合物的力学响应。我们改变链长、形成玻璃的冷却速率和变形速率，并研究力学响应的变化。我们发现，以较慢速率冷却并以较慢速率变形的样品更容易出现应变响应的局部化，即剪切带。有趣的是，在我们目前研究的参数范围内，这种效应与链长无关，我们认为这是首次在圆柱形限制下对变形聚合物玻璃中的剪切带进行直接观察。最后，通过使用等构型系综方法，我们提供了证据表明剪切带形成的位置是由于在样品制备过程中固定下来的结构特征。
