Unravelling Chemical Bond Exchange Pathways in Polyester-based Vitrimers and its Composites

Polymers and their composites are ubiquitous materials with applications across a broad range of sectors, including automotive, aerospace, biomedical, and construction industries. Despite their technological significance, conventional polymers face persistent challenges related to sustainability, recyclability, and multifunctionality. To address these limitations, vitrimers, a new class of polymeric materials possessing covalently adaptive networks (CANs) have emerged as promising candidates that bridge the gap between thermosets and thermoplastics. Vitrimers combine the mechanical robustness and thermal stability of thermosets with the reprocessability, repairability, and recyclability typical of thermoplastics, offering a transformative pathway toward circular materials design. However, their industrial adoption remains limited due to an incomplete understanding of the molecular parameters governing bond-exchange dynamics and network relaxation behavior.

This dissertation aims to elucidate the molecular-level mechanisms controlling relaxation dynamics in polyester-based vitrimer systems engineered for scalable manufacturing. A fast-relaxing vitrimer matrix was designed by incorporating dynamic ester linkages between rigid aromatic precursors (hard segments) and soft, rubbery components (soft segments), forming a dual-phase architecture that exhibits high mechanical toughness and robustness due to the rigid domains, while enabling rapid reprocessability and recyclability upon thermal activation via the flexible segments. The dynamic ester-based functionalities of this vitrimer matrix were further exploited to fabricate hierarchically structured fiber-reinforced composites (FRCs) that demonstrate exceptional mechanical strength and closed-loop recyclability, driven by reversible covalent interactions between hydroxyl-rich fibers and the vitrimeric ester matrix. To develop a comprehensive understanding of relaxation dynamics beyond chemical exchange phenomena, model systems were synthesized to isolate and analyze key parameters such as crosslink density, spatial bond orientation, and proximity of exchangeable sites. Furthermore, novel non-invasive spectroscopic methodologies, including low-field NMR and AFM-based nanoIR, were developed to accurately determine the topology freezing temperature (T_v) in vitrimers, effectively decoupling chemical bond exchange dynamics from local segmental motion.

The findings presented herein aim to advance fundamental understanding of molecular determinants governing bond-exchange and relaxation dynamics in ester-based vitrimer systems. These insights not only inform the rational design of vitrimer networks with tunable processability and performance but also provide guiding principles for scalable manufacturing of next-generation recyclable and repairable polymer composites.