PMMA玻璃在恒定应变速率变形期间迁移率的力学测量与分子测量对比
Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass
AI总结：
通过对Tg-19K下的PMMA玻璃开展实验，对比屈服前后力学与分子探针测得的迁移率差异，发现屈服前二者变化幅度差异大、屈服后相近，动态非均质性先降后稳，为聚合物玻璃变形理论建模带来挑战。
AI中文摘要：
我们在Tg - 19 K下对聚甲基丙烯酸甲酯（poly(methyl methacrylate)，PMMA）玻璃进行了恒定应变速率变形和应力松弛实验，采用三种应变速率，并在大范围应变值下启动应力松弛。根据前人的研究，我们将松弛实验中应力衰减的初始速率解释为该系统迁移率的纯力学测量值。在我们的实验中，通过这种方式获得的力学迁移率在屈服前的变化小于3倍。在这些力学实验期间，我们还基于分子探针的重新取向对链段迁移率进行了光学测量；我们观察到探针迁移率在屈服前可增加多达100倍。相比之下，在屈服后区域，通过力学方法和探针重新取向确定的迁移率非常相似，且显示出对应变速率的相似依赖性。研究发现，动态非均质性在恒定应变速率变形期间最初会降低，随后在屈服后区域保持恒定。这些关于力学迁移率、探针迁移率和动态非均质性的综合观测结果，对聚合物玻璃变形的理论建模提出了挑战。
英文摘要：
We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at Tg - 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based upon the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post-yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence upon the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post-yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation.
