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

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

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.