CAREER: Bottom-up construction of re-configurable entanglements toward polymer networks with switchable toughness

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。非技术概述：聚合物网络在我们的世界中无处不在，以粘合剂、橡胶和凝胶的形式表现和广泛应用。在许多应用中，这种材料的抗断裂能力对其性能和寿命至关重要。事实上，被废弃轮胎覆盖的田地的图像说明了机械损坏的聚合物网络的前景。提高聚合物网络的韧性取决于更深入地了解聚合物链在分子水平上的连接方式与宏观水平上的抗断裂能力之间的基本关系。具体来说，缠结——即聚合物链缠结在一个网络中——已被报道可以保护材料免受断裂；然而，这一现象的分子基础仍不清楚。该项目旨在开发精确的方法来安装、理解和控制聚合物网络中被困缠结的影响，最终提高聚合物网络的抗断裂能力，推进聚合物科学的基础。此外，通过结合本科生的课程研究经验，当地学校的示范，以及教师的远程研究经验，该项目将教育公众关于聚合物科学的基本知识，特别是聚合物网络的重要性。这些努力将培养未来几代学生对材料科学研究的欣赏，并将帮助他们考虑从事STEM领域的职业。技术概述：该项目的中心目标是开发分子精确控制和理解被困缠结及其对聚合物网络机械性能的影响。被困缠结——聚合物网络中聚合物链之间的拓扑交联——代表了聚合物科学的一个重要前沿：它们与抗断裂性的显著增强有关，这对软材料的应用、寿命和可持续性至关重要。然而，目前还缺乏详细的缠结拓扑-性质关系以及设计和操纵缠结的有效策略，这限制了人们定制聚合物网络抗断裂能力的能力。该项目通过三管齐下的方法解决了这些未满足的需求：(1)开发一种自下而上的策略，通过使用基于超分子金属配体配合物的模板来构建被困的纠缠；(2)通过改变模板拓扑来系统地研究纠缠-力学性质的关系；(3)将纠缠模板策略与刺激响应的动态共价化学结合起来，根据需要切换纠缠拓扑——从而改变材料的力学性质。该项目研究的主要力学性能包括模量、极限强度、韧性和阈值断裂能。该项目采用实验和理论相结合的工具来完成既定的目标：合成和表征具有模板缠结的新型凝胶材料，对这些凝胶进行机械测试，分子动力学模拟和理论推导。从这项工作中获得的对被困缠结的基本理解的进展将转化为提高聚合物网络材料的寿命，从而有助于减少它们在垃圾填埋场的积累。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。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).NON-TECHNICAL SUMMARY:Polymer networks are ubiquitous in our world, manifested and widely utilized in the form of adhesives, rubbers, and gels. The ability of such materials to resist fracture is critical to their performance and longevity in many applications. Indeed, images of fields covered with discarded tires illustrate the prospect for mechanically damaged polymer networks. Improvements in the toughness of polymer networks hinge on a deeper understanding of the fundamental relationship between how polymer chains are connected at the molecular level and the fracture resistance at the macroscopic level. Specifically, entanglements -- i.e., polymer chains tangled within a network -- have been reported to protect materials against fracture; however, molecular underpinnings of this phenomenon remain unclear. This project aims to develop precise methods to install, understand the effects of, and manipulate trapped entanglements in polymer networks to ultimately improve their fracture resistance and advance the fundamentals of polymer science.Additionally, through a combination of course-based research experiences for undergraduates, demonstrations for local schools, and remote research experiences for teachers, this project will educate the public on the fundamentals of polymer science in general, and the significance of polymer networks in particular. These efforts will nurture in future generations of students an appreciation for materials science research and will help them to consider careers in STEM. TECHNICAL SUMMARY:The central objective of this project is to develop molecularly-precise control and understanding of trapped entanglements and their effects on the mechanical properties of polymer networks. Trapped entanglements -- topological crosslinks among polymer chains in a polymer network -- represent a consequential frontier in polymer science: they have been correlated with dramatic enhancement of fracture resistance, critical for applications of soft materials, as well as their lifespan, and therefore with sustainability. Yet, detailed entanglement topology-property relationships and effective strategies to engineer and manipulate entanglements are lacking, which limits one’s ability to tailor the fracture resistance of polymer networks. This project addresses these unmet needs via a three-pronged approach: (1) development of a bottom-up strategy to construct trapped entanglements by using templates based on supramolecular metal-ligand complexes, (2) systematic investigation of entanglement-mechanical property relationships by varying the template topology, and (3) coupling the entanglement template strategy with stimulus-responsive dynamic covalent chemistry to switch the entanglement topology -- and therefore mechanical properties of materials -- on demand. Key mechanical properties investigated in this project include modulus, ultimate strength, toughness, and threshold fracture energy. The project employs a combination of experimental and theoretical tools to accomplish the stated goals: synthesis and characterization of novel gel materials with templated entanglements, mechanical testing of these gels, molecular dynamics simulations and theoretical derivations. Advances in the fundamental understanding of trapped entanglements derived from this work will translate to improved longevity of polymer network materials and thereby help to reduce their accumulation in landfills. .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.