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

各种形式的聚合物（包括我们熟悉的塑料）由于断裂导致的机械失效限制了聚合物产品的应用范围，增加了聚合物产品的成本。该项目将试图澄清聚合物裂缝的性质，以确定提高失效阈值的方法。这可能会减少石油基塑料的年消耗量（数千亿磅），从而通过降低垃圾填埋场的压力来实现建立更可持续经济的目标。该项目将探索并展示通过化学和物理手段提高材料抗断裂性和强度之间的联系所带来的好处。具体来说，聚合物材料的断裂行为将以不同于传统的方式进行检测，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PI计划研究的聚合物力学各方面的教科书中。该项目还将通过培训新一代科学家和工程师来帮助填补这一领域的知识空白，这些科学家和工程师可以为聚合物材料的性能和应用所面临的现有和未来挑战提供新的解决方案。技术概述计划中的项目将通过建立基于双折射测量的应力准则来研究聚合物材料的断裂行为。总体目标是确定更有效的方法来增加聚合物的韧性。如果不适当地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，这是不可能实现的。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，它们的脆性断裂行为将从侧重于裂纹尖端应力状态的角度进行研究。调查将考虑到以下影响。通过预熔拉伸增强链网，增加了非晶态玻璃聚合物的延展性。在结晶过程中，链网的保存避免了玻璃状半结晶聚合物（例如，结晶聚对苯二甲酸乙二醇酯）的脆性断裂。结构变化，以增加每单位面积的承重股导致更强的弹性体。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的情况有关的证据，而不是Griffith-Irwin型能量平衡论点。整个项目将试图回答为什么韧性是一个材料常数，以及哪些因素决定了不同形式聚合物的韧性大小。为了提高聚合物的延性和韧性，将探索通过增强承载链网结构来提高其内在机械强度的有效策略。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.