Nonlinear Viscoelastic Behavior of Glassy Polymers: Unification of the Mechanical and Enthalpic Response

玻璃聚合物是当今使用的一些最重要的工程材料。它们用于各种产品，从航空航天复合材料中的基质树脂，到航空航天、汽车和家居应用中的高冲击透明材料，再到众多消费品。尽管这些材料具有重要的商业价值，但对玻璃态的基本理解仍然是科学和工程中的悬而未决的问题之一。由玻璃聚合物制成的产品的设计需要对这些材料在变形时的反应有基本的了解。该奖项支持研究，以揭示控制玻璃聚合物变形行为的机制，然后将这一理解纳入描述变形和导致变形所需的力之间关系的量化模型中。这项研究是朝着建立描述材料对复杂变形的响应的本构模型迈出的重要一步。这些模型是用于设计由玻璃聚合物制成的产品的计算机代码的核心。由于使用这些材料制造的产品的经济重要性，这项研究最终将使美国经济受益，从而促进国家健康、繁荣和福利。该奖项还将通过监督荣誉论文来支持本科生的研究，并与印地卡赛车社区合作，将基于赛车的STEM推广到初中和高中。目前在模拟玻璃聚合物的变形行为方面的最先进水平并没有纳入聚合物物理学的关键分子概念之一。具体地说，尽管玻璃聚合物看起来在空间上是均匀的，但在纳米尺度上，玻璃材料表现出显著的动态异质性，其中邻近的空间区域在局部迁移率方面可能有数量级的差异。该奖项支持继续开发一类明确纳入动态异质性的新模型的研究。此外，当材料变形时，它们既表现出变形行为，又表现出热响应；然而，现有的模型忽略了热响应，热响应可以包含高达总变形能量的一半。这项研究拨款的一个主要目标是统一变形和热响应，以提供对玻璃聚合物中变形过程的更多物理描述。最后，获得该奖项支持的研究人员将参与一个主要的STEM项目，在该项目中，高中生设计、制造和测试电动卡丁车，然后将其作为Indy500庆祝活动的一部分进行比赛。材料特性是STEM项目课堂方面的关键组成部分，其中明确分析了聚合物的机械特性。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Glassy polymers are some of the most important engineering materials in use today. They are used in a wide variety of products ranging from the matrix resin in aerospace composites to high impact transparent materials used in aerospace, automotive and home applications to numerous consumer goods. Notwithstanding the commercial importance of these materials, a fundamental understanding of the glassy state remains one of the outstanding questions in science and engineering. The design of products made from glassy polymers requires a fundamental understanding of how these materials respond when deformed. This award supports research to uncover the mechanisms that control the deformation behavior of glassy polymers and then incorporate this understanding in quantitative models that describe the relationship between the deformation and the forces required to cause that deformation. This research is a major step towards developing the constitutive models that describe the material response to complex deformation ? models that are at the heart of the computer codes that are used to design products made from glassy polymers. Because of the economic importance of products made using these materials, this research will eventually benefit the U.S. economy, and thus, advance national health, prosperity, and welfare. The award will also support undergraduate research through supervision of honors thesis, and motorsports-based STEM outreach to middle and high schools in partnership with the IndyCar racing community.The current state-of-the-art in modeling the deformation behavior of glassy polymers does not incorporate one of the key molecular concepts of polymer physics. Specifically, although glassy polymers may appear to be spatially uniform, on the nanometer size scale glassy material exhibits significant dynamic heterogeneity, where neighboring spatial regions can have order-of-magnitude differences in local mobility. This award supports research that will continue development of a new class of models that explicitly incorporates dynamic heterogeneity. In addition, when materials are deformed they exhibit both deformation behavior and a thermal response; however, existing models ignore the thermal response which can contain up to half of the overall deformation energy. A major objective of this research grant will be to unify the deformation and thermal response to provide a more physical description of the deformation process in glassy polymers. Finally, researchers supported by this award will participate in a major STEM program, where high school students design, build and test electric go-karts that they then race as part of the Indy500 festivities. Material properties are a key component of the classroom aspects of this STEM program, where mechanical properties of polymers are explicitly analyzed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.