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。非技术性总结：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并广泛使用。这种材料抗断裂的能力对于它们在许多应用中的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。聚合物网络韧性的提高取决于对聚合物链在分子水平上如何连接与宏观水平上的抗断裂性之间的基本关系的更深入理解。具体来说，纠缠--即，聚合物链缠结在一个网络中--据报道可以保护材料不断裂;然而，这种现象的分子基础仍然不清楚。该项目旨在开发精确的方法来安装，理解和操纵聚合物网络中捕获的缠结的影响，以最终提高其抗断裂性并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，当地学校的演示和教师的远程研究经验，该项目将教育公众了解聚合物科学的基本原理，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STEM职业。 技术摘要：该项目的中心目标是发展分子精确控制和理解捕获的缠结及其对聚合物网络机械性能的影响。被困的缠结-聚合物网络中聚合物链之间的拓扑交联-代表了聚合物科学中的一个重要前沿：它们与抗断裂性的显着增强有关，这对软材料的应用至关重要，以及它们的寿命，因此与可持续性有关。然而，详细的缠结拓扑性质的关系和有效的策略，工程和操纵缠结缺乏，这限制了一个人的能力，定制的断裂阻力的聚合物网络。该项目通过三管齐下的方法解决这些未满足的需求：（1）通过使用基于超分子金属-配体络合物的模板来开发自下而上的策略以构建捕获的缠结，（2）通过改变模板拓扑结构来系统地研究缠结-机械性质关系，以及（3）将缠结模板策略与刺激响应动态共价化学耦合以切换缠结拓扑--以及材料的机械性能。在这个项目中研究的关键机械性能包括模量，极限强度，韧性和临界断裂能。该项目采用了实验和理论工具的组合来实现既定目标：合成和表征具有模板缠结的新型凝胶材料，这些凝胶的机械测试，分子动力学模拟和理论推导。从这项工作中获得的对被困纠缠的基本理解的进展将转化为聚合物网络材料的寿命提高，从而有助于减少它们在垃圾填埋场的积累。该奖项反映了NSF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.