Date of Award

2025

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical and Aerospace Engineering

Advisor 1

Trisha Sain

Committee Member 1

Parisa Abadi

Committee Member 2

Franck Vernerey

Committee Member 3

Rebecca Ong

Abstract

The long-term performance of polymeric materials in extreme environments is governed by complex chemical and physical aging processes that alter their molecular structure and mechanical integrity. Conventional thermoset epoxies, while widely used in structural applications, are particularly susceptible to oxidative degradation at elevated temperatures, leading to embrittlement and loss of structural integrity. In the first part of this work, diffusion-limited oxidation (DLO) in a bulk structural epoxy was systematically investigated to establish the link between heterogeneous microstructural evolution and macroscopic mechanical degradation. High-temperature oxidation experiments were conducted in a controlled environmental chamber, and the resulting chemical, thermal, and mechanical changes were characterized using Fourier transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), nanoindentation, and uniaxial tensile testing. The results revealed the formation of a distinct oxidized surface layer with elevated stiffness and increased carbonyl concentration, indicative of oxidative crosslinking. These localized chemical changes led to reduced viscoplastic deformation and an overall embrittlement of the bulk epoxy. Building on these findings, the second part of this work examined a recyclable epoxy vitrimer system to assess its environmental durability under oxidative and hydrolytic aging conditions. The virgin vitrimer, synthesized from DGEBA, glutaric anhydride, and zinc acetylacetonate, exhibited dynamic covalent adaptability while maintaining structural rigidity of a conventional thermoset. Accelerated aging experiments revealed distinct degradation mechanisms in oxidative versus hydrolytic environments. Hydrolysis involved an initial period of water uptake followed by reaction-driven mass loss and bulk erosion, while oxidation produced immediate degradation characterized by localized micro-porosity near the surface. Despite these differing mechanisms, both environments led to significant embrittlement and loss of mechanical integrity over time. Collectively, this work establishes fundamental structure–property relationships linking molecular-level aging phenomena to macroscopic mechanical degradation in both conventional and recyclable epoxy systems. The insights gained provide critical guidance for the design of more durable, recyclable polymer networks capable of sustaining structural performance in extreme environments.

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