Exploring ultimate mechanical characteristics of polymers, from molecular to fracture mechanics

非技术总结所有形式的聚合物(包括常见的塑料)因断裂而发生的机械故障限制了聚合物产品的应用范围并增加了成本。该项目将试图澄清聚合物压裂的性质，以确定提高失效门槛的方法。这可能会导致石油基塑料的年消费量(数千亿磅)减少，从而通过降低垃圾填埋场的压力来建立更可持续的经济。该项目将探索并展示阐明抗折性与材料强度之间的联系的好处，这种联系可以通过化学和物理手段来改善。具体地说，将以一种不同于传统方式的方式检查聚合物材料的断裂行为，以更好地说明断裂的原因。现有知识和新观点之间的对比将被纳入一本关于聚合物力学所有方面的教科书，国际和平研究所计划进行这方面的工作。该项目还将通过培训新一代科学家和工程师，帮助填补这一领域的知识空白，他们能够为聚合物材料的性能和应用面临的现有和未来挑战提供新的解决方案。技术总结计划中的项目将通过建立基于双折射测量的应力标准来研究聚合物材料的断裂行为。总体目标是寻找更有效的方法来增加聚合物的韧性。如果没有适当的解释为什么韧性是一个材料参数，以及是什么决定了它的大小，就不可能实现这一点。PI计划探索材料机械强度和韧性之间的密切联系。对于所有形式的聚合物，即塑料、弹性体和热固性聚合物，将从裂纹尖端的应力状态的角度来研究它们的脆性断裂行为。调查将考虑以下影响。通过预熔拉伸增强链状网络可提高玻璃态无定形聚合物的延展性。在结晶过程中保留链状网络可避免玻璃态半结晶聚合物(如结晶聚对苯二甲酸乙二酯)的脆性断裂。增加单位面积承载股数的结构变化会产生更坚固的弹性体。这项拟议的研究将收集更多与裂纹尖端的应力状态明确决定聚合物断裂而不是格里菲斯-欧文类型的能量平衡论点相关的证据。整个项目将试图回答为什么韧性是一个材料常数，以及什么因素决定了各种形式的聚合物的韧性大小。为了改善聚合物的延展性和增韧，将探索一种有效的策略，通过增强承载链网的结构来增加固有的机械强度。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。

NON-TECHNICAL SUMMARYMechanical failure of polymers in all forms (including the familiar plastics) due to fracture limits the range of applications and increases the costs of polymeric products. This project will attempt to clarify the nature of polymer fracture in order to identify means to increase the threshold for failure. This could lead to reduction of the annual consumption (several hundred billion pounds) of petroleum-based plastics toward the goal of building a more sustainable economy by lowering the pressure for landfills. The project will explore and show the benefits of elucidating a connection between the resistance to fracture and the material strength that can be improved through chemical and physical means. Specifically, fracture behavior of polymeric materials will be examined in a different than conventional way to better illustrate the cause for fracture. The contrast between existing knowledge and new perspectives would be incorporated in a textbook on all aspects of polymer mechanics that the PI plans to work on. The project will also help to fill the knowledge gap in this area by training a new generation of scientists and engineers who can generate new solutions to existing and future challenges facing the properties and applications of polymeric materials. TECHNICAL SUMMARYThe planned project will investigate the fracture behavior of polymeric materials through establishment of a stress criterion based on birefringence measurements. The overall objective is to identify more effective means to increase the toughness of polymers. This cannot be achieved without proper explanation of why toughness is a material parameter and what dictates its magnitude. The PI plans to explore an intimate connection between material mechanical strength and toughness. For polymers in all forms, i.e., plastic, elastomeric and thermosetting, their brittle fracture behavior will be investigated from an angle that focuses on the stress state at the crack tip. The investigations will take into account the following effects. Enhancement of chain network through pre-melt stretching increases ductility of glassy amorphous polymers. Preservation of chain network during crystallization avoids brittle fracture in glassy semicrystalline polymers (e.g., crystalline polyethylene terephthalate). Structural changes to increase load-bearing strands per unit area result in stronger elastomers. The proposed research will gather more evidence relevant to a scenario where stress state at the crack tip explicitly dictates polymer fracture instead of Griffith-Irwin type energy balance argument. The entire project will attempt to answer why toughness is a material constant and what factors determine its magnitude for polymers in their various forms. For improvement of ductility and increased toughness of polymers an effective strategy will be explored through enhancement of structure of load-bearing chain network to increase the intrinsic mechanical strength.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.