AI 中文总结
该研究通过四步变形实验挑战传统玻璃态聚合物力学模型的假设，结合光漂白光学实验排除部分卸载时分子迁移率降低的可能，开发了以高效堆积材料分数为内部变量的新模型，可描述相关实验。
AI 中文摘要
传统玻璃态聚合物的应力-应变行为模型基于以下假设：应力-应变响应的关键特征可通过分子迁移率的变化来解释。由（i）初始恒定应变率加载、（ii）卸载至指定应力、（iii）在该应力下蠕变、（iv）第二次恒定应变率加载组成的四步变形实验对这一假设提出了挑战。具体而言，现有模型无法预测轻微卸载时实验观察到的较大第二次应力过冲。此前，人们仍存在一种可能性：在部分卸载而非完全卸载的情况下，分子迁移率实际更低，这若不考虑这些本构模型的特定细节，仍可保留其主要假设。通过在进行机械四步实验的同时，使用光漂白技术开展直接光学实验，结果表明部分卸载时不会出现更低的分子迁移率。由于传统模型无法解释这些实验结果，因此开发了一种新模型，其中分子结构的变化并非体现在弛豫时间上，而是体现在剪切模量上，剪切模量是内部变量（高效堆积材料的分数）的函数。该分数服从种群平衡方程，其中稳态分数由施加的应力控制。在无变形时，高效堆积分数会增加，这解释了低于玻璃化转变温度Tg时物理老化过程中模量的增加。该模型定性描述了四步实验以及单步加载实验。
英文摘要
Traditional models for stress-strain behavior of glassy polymers are based on the assumption that the critical features of the stress-strain response can be explained by changes in the molecular mobility. The four-step deformation experiments consisting of (i) an initial constant strain rate loading, (ii) unloading to specified stress, (iii) creep under that stress and (iv) second constant strain rate loading, challenges that assumption. Specifically, existing models fail to predict the experimentally observed large second stress overshoot in case of a slight unloading. Until now there has remained a possibility that the mobility was actually lower in case of a partial rather than complete unloading, which would preserve the main assumption, if not particular details, of these specific constitutive models. By performing direct optical experiments using the photobleaching technique simultaneously with the mechanical four-step experiments it is shown that a lower molecular mobility upon partial unloading does not take place. As traditional models cannot account for these experimental results, a new model has been developed where the changes of molecular structure manifest not in the relaxation time, but in the shear modulus, which is function of an internal variable that is the fraction of the efficiently packed material. This fraction obeys a population balance equation, where the steady-state fraction is controlled by the applied stress. In the absence of deformation, the efficiently packed fraction increases, which explains the increase in the modulus in the course of physical aging below Tg. The model qualitatively describes the four-step experiment as well as single step loading experiments.
Comments23 pages, 6 figures, 56 references, Supporting Information with 6 figures
Journal refMacromolecules (2022) 55 (15): 6351-6363
DOI：10.1021/acs.macromol.2c00711
