Integrated Molecular Approach to Study Mechanical Behavior of Polymeric Materials

非技术综述：随着塑料材料越来越多地取代许多成熟的传统材料，聚合物玻璃的力学性能研究变得更加重要。要提高这类重要材料的力学性能，需要在分子水平上更深入地了解影响极限强度的因素和工艺。这个项目的目的是在分子水平上建立聚合物物理，可以应用于提供关于如何制造更坚固的大块聚合物玻璃的预测性设计原则。具体地说，将进行理论上有动机的实验和计算机模拟，以阐明由于链连接而产生的独特机械特性，并解决为什么聚合物玻璃可以延展性以及如何使其更耐脆性破坏的问题。该研究项目的成功将使应用范围进一步扩大，以增加这类现代工程材料的经济价值。由于研究活动有望产生新的模型和概念，这项工作将实质性地促进与玻璃态塑料有关的聚合物物理常识的发展，并加强聚合物科学与工程的研究生教育课程。技术摘要：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，并探索关于应力来源、应力松弛性质、屈服以及大变形过程中的脆韧性转变的物理原理。这项研究有四个目标：A)用实验和分子动力学(MD)模拟来表征压缩过程中的脆韧转变(BDT)；B)用实验和MD模拟来阐明应力的来源，包括链内的贡献；C)描述链网络如何能够或不能通过激活和动员玻璃片段来驱动聚合物玻璃进入塑性状态，其中实验和模拟将探索通过加入低分子量组分来制备不同组成的混合物来稀释链网络的效果；D)研究不同速率和温度下的形变如何增强分子的流动性--形变是否总是像艾林的激活思想所暗示的那样，导致节段流动性的增强？(例如，是否总会出现应力来降低分子迁移率的艾林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模型研究不同，该项目采用唯象和分子方法来确定将艾林思想(针对单个粒子)应用于聚合物玻璃等复杂多体系统的先决条件。明确地说，这项研究将根据链状网络的位移和变形来检验聚合物玻璃屈服和延展性的因果关系，链网络是分段激活和最终宏观屈服的原因。

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.