Integrated Molecular Approach to Study Mechanical Behavior of Polymeric Materials

非技术总结：随着塑料材料越来越多地取代许多成熟的传统材料，聚合物玻璃的机械性能的研究变得更加重要。 改善这类重要材料的机械性能需要更深入地了解影响极限强度的因素和过程。 该项目旨在建立分子水平上的聚合物物理学，可用于提供如何制造更强的块状聚合物玻璃的预测设计原则。 具体来说，将进行理论上的实验和计算机模拟，以阐明由于链连接性而产生的独特机械特性，并解决为什么聚合物玻璃可以具有延展性以及如何使它们更能抵抗脆性破坏的问题。 研究计划的成功将使应用进一步扩大，以提高这类现代工程材料的经济价值。 由于新的模型和概念有望从研究活动中产生，这项工作将为有关玻璃态塑料的聚合物物理学常识提供实质性的进展，并加强聚合物科学和工程研究生教育的课程。该项目将各种实验与相关的分子动力学模拟相结合，以开发聚合物玻璃分子力学的概念框架，并寻找有关应力起源的物理原理，应力松弛、屈服以及大变形时脆韧转变的本质。 本研究的目的有四：（1）用实验和分子动力学（MD）模拟的方法研究压缩过程中的脆韧转变（BDT），（2）用实验和MD模拟的方法研究压缩过程中的应力来源，包括链内应力的贡献，（B）用实验和MD模拟的方法研究压缩过程中的应力来源。C）描述了链网络如何能够或不能通过玻璃体片段的活化和移动将聚合物玻璃驱动成塑性状态，其中实验和模拟将探索通过掺入低分子量组分来稀释链网络以制造不同组成的混合物的效果; D）研究在不同速率和温度下的变形如何增强分子流动性--变形是否总是导致增强的链段流动性，正如艾林活化思想所暗示的那样？(e.g.,应力是否总是出现以降低分子迁移率的Eyring公式中所描述的活化势垒？） 与连续介质力学中的大多数本构建模研究不同，该项目采用现象学和分子方法来确定将Eyring思想（对于单个粒子）应用于聚合物玻璃等复杂多体系统的先决条件。 解释，研究将检查的因果关系屈服和延展性的聚合物玻璃链网络的位移和变形的节段活化和最终的宏观屈服的原因。

NON-TECHNICAL SUMMARY:As plastic materials displace in increasing amount many of the well-established traditional materials the study of mechanical performance of polymeric glasses has become more important. Improving mechanical properties of this important class of materials requires a deeper molecular-level understanding of what factors and processes affect the ultimate strength. This project aims to establish the polymer physics on a molecular level that can be applied to provide predictive design principles on how to make stronger bulk polymeric glasses. Specifically, theoretically motivated experiments and computer simulations will be carried out to elucidate the unique mechanical characteristics due to chain connectivity and address the questions of why polymer glasses can be ductile and how they can be made even more resistant to brittle failure. The success of the research program will allow applications to be further broadened to increase the economical values of this class of modern engineering materials. Since new models and concepts are expected to emerge from the research activities, the work should offer substantial advances to the general knowledge of polymer physics concerning plastics in the glassy state and enhance the curriculum of graduate education in polymer science and engineering.TECHNICAL SUMMARY:The project integrates various experiments with pertinent molecular-dynamics simulations to develop a conceptual framework for molecular mechanics of polymeric glasses and to search for physical principles concerning the origin of stress, the nature of stress relaxation, yielding, as well as brittle-to-ductile transition during large deformation. The research has four objectives: A) characterize brittle-ductile transition (BDT) in compression using both experiment and molecular-dynamics (MD) simulation; B) elucidate the origin of stress with both experiment and MD simulation including intrachain contributions; C) depict how the chain network is able or unable to drive a polymer glass into a plastic state through activation and mobilization of vitreous segments, where experiment and simulation will explore the effect of diluting the chain network by incorporating a low molecular-weight component to make mixtures of different compositions; D) examine how deformation at different rates and temperatures enhances molecular mobility -- can deformation always lead to enhanced segmental mobility as implied by the Eyring idea of activation? (e.g., does stress always emerge to lower the activation barrier as depicted in the Eyring formula for the molecular mobility?) Unlike most constitutive modeling studies in continuum mechanics, this project takes a phenomenological and molecular approach to determine the prerequisites for the application of the Eyring idea (for a single particle) to such a complex many-body system as polymer glasses. Explicitly, the research will examine the causality for yielding and ductility in polymer glasses in terms of the displacement and deformation of the chain network as causes for segmental activation and eventual macroscopic yielding.