Multistep Deformation Experiment and Development of a Model for the Mechanical Behavior of Polymeric Glasses
Multistep Deformation Experiment and Development of a Model for the Mechanical Behavior of Polymeric Glasses
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DOI：
10.1021/acs.macromol.2c00711
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发表时间：
2022-07
期刊：
影响因子：
5.5
通讯作者：
G. Medvedev;Enran Xing;M. Ediger;J. Caruthers
中科院分区：
文献类型：
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作者：
G. Medvedev;Enran Xing;M. Ediger;J. Caruthers
玻璃态聚合物的应力-应变行为的传统模型是基于这样的假设，即应力-应变响应的关键特征可以通过分子迁移率的变化来解释。四步变形实验，包括（i）初始恒应变率加载，（ii）卸载到指定的应力，（iii）蠕变下的应力，和（iv）第二恒应变率加载，挑战这一假设。具体地说，现有的模型无法预测实验观察到的大的第二应力过冲的情况下，轻微卸载。到目前为止，仍然存在这样的可能性，即在部分卸载而不是完全卸载的情况下，流动性实际上更低，这将保留这些特定本构模型的主要假设，如果不是特定细节的话。通过进行直接的光学实验，同时使用的光漂白技术与机械四步实验，它表明，较低的分子流动性部分卸载时不会发生。由于传统的模型不能解释这些实验结果，已经开发了一种新的模型，其中分子结构的变化不表现在弛豫时间，但在剪切模量，这是一个内部变量的函数，即，有效填充材料的分数。这部分服从人口平衡方程，其中的稳态分数是由所施加的应力控制。在没有变形的情况下，有效填充的分数增加，这解释了在低于Tg的物理老化过程中模量的增加。该模型定性地描述了四步实验以及单步加载实验。
Traditional models for the 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 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, challenge this 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 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 in the molecular structure manifest not in the relaxation time but in the shear modulus, which is a 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 belowTg. The model qualitatively describes the four-step experiment as well as single-step loading experiments.