Han Zehua, Jiao Guisheng, Ma Changli, Zuo Taisen, Han Charles C, Cheng He
Spallation Neutron Source Science Center, Dongguan 523808, China.
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
Polymers (Basel). 2021 Sep 9;13(18):3042. doi: 10.3390/polym13183042.
Although many theories have been proposed to describe the nature of glass formation, its microscopic picture is still missing. Here, by a combination of neutron scattering and molecular dynamics simulation, we present the temperature-dependent atomic structure variation of polystyrene at the glass formation, free volume and cooperative rearrangement. When it is close to glass formation, the polymer is confined in tubes, whose diameter is the main chain-main chain distance, in a "static cage" from its neighbors. This definition can not only account for the kinetic pathway dependence of Williams-Landel-Ferry (WLF) free volume, but also be testified in a set of six polymers. However, the free volume which allows a monomer to move cannot be found in any frame of its real-space image. Monomers, thus, have to move cooperatively to be out of the cage. During glass formation, dynamic heterogeneity develops, and string-like cooperative rearrangement region (CRR) grows over a long range of time and length scales. All of these CRRs tend to walk through loose "static cages". Our observation unifies the concepts of free volume and cooperative rearrangement. The former is a statistical average leading to a polydisperse "static cage" formation; while a loose "static cage" provides the way that CRRs move.
尽管已经提出了许多理论来描述玻璃形成的本质，但其微观图像仍然缺失。在这里，通过中子散射和分子动力学模拟相结合的方法，我们展示了聚苯乙烯在玻璃形成、自由体积和协同重排过程中随温度变化的原子结构变化。当接近玻璃形成时，聚合物被限制在直径为主链-主链间距的“静态笼”中的管内，周围是其相邻分子。这一定义不仅可以解释威廉姆斯-兰德尔-费里（WLF）自由体积的动力学路径依赖性，还可以在一组六种聚合物中得到验证。然而，在其真实空间图像的任何框架中都找不到允许单体移动的自由体积。因此，单体必须协同移动才能离开笼子。在玻璃形成过程中，动态不均匀性发展，线状协同重排区域（CRR）在很长的时间和长度尺度上生长。所有这些CRR倾向于穿过松散的“静态笼”。我们的观察统一了自由体积和协同重排的概念。前者是导致多分散“静态笼”形成的统计平均值；而松散的“静态笼”为CRR移动提供了途径。
