Jin Tianyi, Coley Connor W, Alexander-Katz Alfredo
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Phys Rev E. 2022 Jul;106(1-1):014506. doi: 10.1103/PhysRevE.106.014506.
The glass transition temperature (T_{g}) is one of the most fundamental properties of polymers. T_{g} is predicted by some theories as a sudden change in a "macroscopic" quantity (e.g., compressibility). However, for systems with "soft" glass transitions where the change is gradual it becomes hard to pinpoint precisely the transition temperature as well as the set of molecular changes occurring during this transition. Here, we introduce two new molecular signatures for the glass transition of polymers that exhibit clear changes as one approaches T_{g}: (i) differential change of the probability distribution of dihedral angles as a function of temperature and (ii) the distribution of fractional of the time spent in the different torsional states. These new signatures provide insights into the glass transition in polymers by directly exhibiting the concept of spatial heterogeneity and dynamical ergodicity breaking in such systems, as well as provide a key step to quantitatively obtain the transition temperature from molecular characteristics of the polymeric systems.
玻璃化转变温度（(T_{g})）是聚合物最基本的性质之一。一些理论预测(T_{g})是“宏观”量（如压缩性）的突然变化。然而，对于具有“软”玻璃化转变的体系，其变化是渐进的，很难精确确定转变温度以及在此转变过程中发生的分子变化集。在此，我们引入了两种用于聚合物玻璃化转变的新分子特征，当接近(T_{g})时，它们会呈现出明显变化：（i）二面角概率分布随温度的微分变化，以及（ii）在不同扭转状态下所花费时间的分数分布。这些新特征通过直接展示此类体系中的空间异质性和动力学遍历性破坏的概念，为聚合物的玻璃化转变提供了见解，同时也为从聚合物体系的分子特征定量获得转变温度提供了关键步骤。
