“Network Topology and Associative Bond Exchange Control the Viscoelastic Response of Elastomeric and Glassy Vitrimers”

Dept: Chemical & Biomedical Engineering

Chair: Ralm Ricarte, Ph.D.

Abstract

Permanent polymer networks often suffer from inherent structural inhomogeneities and poor recyclability. While covalent dynamic associative networks, known as vitrimers, offer a promising solution to these limitations, the fundamental relationships between their network architecture, cross-link chemistry, and macroscopic relaxation remain incompletely understood. This dissertation systematically investigates how network topology and associative bond exchange govern the structural and viscoelastic properties of elastomeric and glassy vitrimers. By utilizing polybutadiene (PB) and polystyrene (PS) as model backbones, this work bridges the gap between microscopic bond dynamics and macroscopic material properties through three central investigations. First, the impact of cross-link density on elastomeric network topology is examined. Using PB networks containing dynamic dioxaborolane cross-links, associative exchange is shown to dynamically resolve topological defects. Compared to analogous permanently cross-linked networks, these elastomeric vitrimers possess fewer network defects and exhibit higher effective cross-link densities. Second, the linear viscoelasticity of unentangled glassy PS vitrimers bearing dynamic imine cross-links is investigated. Time-temperature superposition studies reveal that the terminal relaxation of these vitrimers is governed by a Slow Arrhenius Process (SAP). The activation energy associated with the SAP is significantly weaker than predictions from standard sticky Rouse models, suggesting that macroscopic flow is modulated by local elasticity fluctuations, matrix effects, and cross-linker diffusion rather than solely by intrinsic bond exchange kinetics. Finally, the pre-gelation evolution of PS solutions cross-linked by dynamic imine bonds is mapped using dynamic light scattering and diffusion-ordered NMR spectroscopy. This analysis quantitatively captures the microscopic structural transition from freely diffusing polymer chains to a fully percolated, heterogeneous network. Although classical Flory-Stockmayer theory fails to accurately predict the macroscopic gel point due to the reversible chemistry and potential intramolecular cross-links, it successfully describes the pre-gelation growth in cluster size and dispersity. Ultimately, this research establishes a comprehensive framework connecting microscopic diffusion and bond exchange dynamics directly to the macroscopic material performance of dynamic polymer networks.