EAGER: Exploring the Molecular Foundation for the Mechanics of Polymer Glasses

非技术总结：该项目旨在解决有关普通塑料（聚合物）机械性能的基本问题，这些塑料（聚合物）已成为现代社会中最重要的一类材料。每年用聚苯乙烯、聚甲基丙烯酸甲酯和聚碳酸酯等玻璃状聚合物制造出数十亿磅的商业产品。然而，这些玻璃塑料材料的机械强度还没有从分子参数（即通过其单个聚合物分子的行为）的角度来理解。例如，对于聚苯乙烯和聚（甲基丙烯酸甲酯）等材料为何如此脆弱，而一些聚碳酸酯却不脆弱，而且可以高度变形而不会断裂，人们显然缺乏了解。本研究旨在从分子水平上理解脆性断裂与韧性变形等重要力学行为。通过将实验、理论和分子动力学模拟相结合的综合方法，将对分子模型进行测试，以确定它如何能够或不能够提供框架，从而为玻璃聚合物的力学发展提供连贯和全面的知识基础。如果成功，它将为改善现有聚合物材料的机械性能，指导下一代聚合物材料的分子设计奠定基础，从而为社会带来巨大的经济效益。该项目还将有助于研究生的跨学科教育和培训，并有可能被纳入本科生和研究生的课程和教科书。技术概述：本研究将探索聚合物材料力学行为的重要问题的答案，例如为什么聚合物玻璃具有延展性以及什么决定了非晶聚合物固体的最终机械强度。该研究计划建立并扩展了一个新提出的分子模型，以描述聚合物玻璃的大变形行为，包括屈服和断裂。提出的工作结合了实验和计算机模拟，通过识别理论描述中的基本成分来测试分子模型。具体来说，将进行以下子项目，以提高当前对玻璃聚合物力学的认识和理解：(A)低分子量（MW）聚合物玻璃和含有低分子量组分的玻璃聚合物混合物将受到单轴压缩，以确定延性行为和大规模塑性变形的可能性。(B)将在室温下进行压缩试验，以查明压缩过程中是否也存在脆性-延性转变（BDT），并解决为什么玻璃状聚苯乙烯在室温下压缩时具有延性，但在拉伸拉伸时脆性的问题。(C) BDT将作为拉伸速率的函数来研究，以探索屈服如何可能是一个随时间变化的激活过程。(D)将进行分子动力学模拟，以探索在外部变形过程中链网络中承重链（LBS）的存在，并描述围绕LBS的活化相的预测出现，以便与分子模型中包含的物理图像进行比较。

NON-TECHNICAL SUMMARY:The project aims to address fundamental questions concerning the mechanical properties of common plastics (polymers) that have become a most important class of materials in modern society. Many billions of pounds of commercial products are manufactured annually from glassy polymers such as polystyrene, poly(methyl methacrylate), and polycarbonates. However, the mechanical strength of these glassy plastic materials has yet to be understood in terms of molecular parameters (i.e., through the behavior of their individual polymer molecules). For example, there is a distinct lack of knowledge about why materials such as polystyrene and poly(methyl methacrylate) are so brittle while some polycarbonates are not and can be highly deformed without breaking. This research aims to develop a molecular level understanding of such important mechanical behaviors as brittle fracture versus ductile deformation. Through an integrated approach combining experiments, theory, and molecular dynamics simulations, a molecular model will be subjected to tests to determine how it may or may not provide the framework on which to develop a coherent and comprehensive knowledge base for the mechanics of glassy polymers. If it is successful, it will provide a foundation that will not only enable improved mechanical performance of existing polymeric materials but also guide molecular design of next-generation polymeric materials, thus potentially bringing about significant economic benefits to society. This project will also contribute to the interdisciplinary education and training of graduate students and potentially be incorporated in courses and textbooks for undergraduate and graduate students as well. TECHNICAL SUMMARY:This research will explore answers to important questions on the mechanical behavior of polymeric materials such as why polymer glasses can be ductile and what determines the ultimate mechanical strength of amorphous polymer solids. The research program builds on and extends a newly proposed molecular model to describe large deformation behavior of polymeric glasses including yielding and fracture. The proposed work integrates both experiment and computer simulation to test the molecular model by identifying the essential ingredients in the theoretical description. Specifically, the following sub-projects will be pursued to improve current knowledge and understanding of glassy polymer mechanics: (A) Polymer glasses of low molecular weight (MW) and glassy polymer mixtures containing a low-MW component will be subjected to uniaxial compression to determine the possibility for ductile behavior and large scale plastic deformation. (B) Compression tests will be carried out below room temperature to find out whether there is also a brittle-ductile transition (BDT) in compression and to address the questions of why glassy polystyrene is ductile in compression at room temperature but brittle in tensile extension. (C) The BDT will be studied as a function of the extensional rate to explore how yielding may be a time-dependent activation process. (D) Molecular dynamics simulations will be conducted to explore the existence of load-bearing strands (LBS) in a chain network during external deformation and characterize the predicted emergence of activated phases surrounding the LBS for comparison with the physical picture contained in the molecular model.