Unlocking the potential of crosslinked polymers: from dynamic covalent bonds to recyclable thermosets
Date
2022
Authors
Clarke, Ryan William, author
Chen, Eugene Y.-X., advisor
Miyake, Garret M., committee member
Li, Yan V., committee member
Crans, Debbie C., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
This dissertation describes the development of dynamic covalent chemistries and bond exchange principles in the context of crosslinked polymers demonstrating high performance properties and sustainable end-of-life avenues. Traditional crosslinked polymers, or thermosets, present a significant challenge in meeting the goals of a circular materials economy caused by a strict orthogonality between performance and recyclability. Covalent adaptable networks (CANs) are a developing class of responsive, crosslinked materials that mimic the performance qualities of static thermosets but access flow-state melt processability by triggering crosslink-bond exchange. Several impactful and fundamental advances to this topic and broader sustainable chemistry are discussed herein: demonstration of new dynamic covalent bonds, establishment of intra- and inter-domain exchange principles, orthogonal working/healing exchange conditions for classic thermoset behavior, and upcycling compatibilization of mixed-feed commodity plastics via dynamic crosslinking. The multidisciplinary knowledge developed herein spans the themes of polymer synthesis, catalysis, block copolymers, self-assembly, structure/property relationships, mechanism elucidation, sustainability, materials chemistry, and polymer physics. Chapter 1 introduces dynamic covalent bonds, sustainable thermoset strategies, and standing challenges impeding wide-scale adoption. Background is also supplied on relevant themes to the following chapters, including pathways for plastics recycling, block copolymer technology, and mechanical reprocessing. Chapter 2 describes the Lewis pair polymerization (LPP) of renewable indenone to a high Tg (> 300 oC), optically transparent bio-polymer. Special emphasis is made for the unique upcycling of polyindenone, as well as the Lewis pair polymerization mechanism and methodology which is critical for complex polymer synthesis in Chapters 3 & 4. Chapter 3 discloses the merging of A-B-A triblock copolymer self-assembled thermoplastic elastomer (TPE) structure with CAN dynamic bond exchange function to establish tethered domain-restriction as an effective strategy to lock out creep deformation during working conditions. We also define the fundamental principles governing inter- and intra-domain exchange and thermomechanical activation of newly established dynamic covalent bonds. Chapter 4 describes our compounded-sequence-control LPP technology applied to produce architecturally and topologically sophisticated cyclic and linear A-B-A-B multiblock copolymers in timely (< 10 min), scalable (~40 g), one-pot two-step processes. Structure/property relationships are meticulously investigated in solution, bulk, and film phase for differentiation of isomeric products. Chapter 5 reports a highly collaborative, multidisciplinary (molecular, material, and computational) study on bis(diazirine) crosslinker molecules with imbedded dynamic covalent bond functionality as universal crosslinking agents for upcycling commodity thermoplastics (polyethylene, etc.) to high-performance, recyclable thermosets. Demonstration of successful crosslinking by C-H insertion, improvements to material performance (tensile pulling, creep-recovery, and thermomechanical durability), and reprocessability are thoroughly discussed. Most critical, we establish the compatibilization of (otherwise immiscible) mixed plastics in dynamically crosslinked polymer blends and examine the phenomenon by molecular modeling and structural characterization. Chapter 6 offers conclusions, remaining challenges, and future opportunities.
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Embargo Expires: 01/09/2025
Subject
Lewis pair polymerization
rheology
thermosets
material characterization
covalent adaptable networks
sustainable polymer science