THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday October 12, 2021

10:00 AM

via

Bluejeans Video Conferencing

https://bluejeans.com/942835183/0498

will be held the

DISSERTATION DEFENSE

for

Luc Le

“Development of a High-Performance Reversible Crosslink Epoxy Composite by using Diels Alder Chemistry”

Committee Members:

Prof. Karl Jacob, Advisor, MSE/ME

Prof. Kyriaki Kalaitzidou, Co-advisor, ME/MSE

Prof. Paul Russo, MSE/CHEM

Prof. Youjiang Wang, MSE

Prof. Anselm Griffin, MSE

Abstract:

Thermoset polymers, due to a unique combination of high mechanical strength, thermal stability, and chemical resistance, are widely used by multiple industries for both commercial/industrial and residential applications. Epoxy resins are the most commonly used thermosets as they display high heat resistance, good chemical and corrosion resistance, high tensile strength, high modulus, high adhesion, low percent shrinkage in the cure, and excellent adhesion to various substrates, etc. However, due to the permanent chemically cross-linked structure they are non-recyclable at the end of life and end up in landfills or are incinerated. This challenge can be overcome by creating a dynamic covalent adaptable network (CAN) via reversible chemical reactions to replace the permeant crosslinks.

The goal of this research is to synthesize epoxy resins able to be recycled at the end of life using renewable materials by utilizing the thermally reversible dynamic bonds of the Diels-Alder (DA) reaction between furan and maleimide. In the first objective, the semi bio-based linear furan grafted epoxy prepolymer was synthesized by step-growth polymerization in one pot reaction that enabled control of the prepolymer’s molecular weight using the mono-functional group as the chain stopper. Furthermore, the thermodynamics and kinetics of DA crosslink formation in the epoxy network were investigated by selecting two different electronic structures: aromatic versus aliphatic maleimide to react with furan on the epoxy prepolymer to tune the retro DA temperature. This provided understanding on how the process parameters including curing temperature, dictate the upper temperature for the polymer structure stability as well as the reprocess conditions. The results indicate that the aromatic maleimide has higher kinetics reactivity toward furan in prepolymer and lower retro DA temperature for the adducts than the alkyl maleimide.  The efficacy of the CAN polymer and its thermo-mechanical properties strongly depend on the architecture polymers such as prepolymer chain characteristics, crosslink density and distribution, etc. The second objective was to elucidate the process-structure-property relationship of the CAN system as a function of prepolymer chain length and the crosslinker prepolymer-crosslinker ratio. The higher percentage of furan grafting on the prepolymer backbone results to shorter chain prepolymer and higher DA crosslink density in the polymer network that helps intrinsic self-healing. On the other hand, the network loses its structural integrity at temperatures higher than the glass transition temperature and exhibits poor solvent resistance. A 1:1 stoichiometric ratio of maleimide crosslinker to the prepolymer’s furan from results in more homogeneous crosslink distribution leading to a network where the bonds reverse at higher temperature. The third objective was to investigate the effect of nanocellulose that was modified to have with diene or dienophile characteristic on the mechanical strength and thermal reversibility of DA adduct of the optimized epoxy network. It is concluded that nanocellulose enhanced the mechanical properties of the epoxy network, yet it interferes with the CAN system at the retro DA temperature inhibiting the thermal reversibility of the network. The research provides fundamental understanding on how the crosslinker type and prepolymer length for the CAN system can be selected to fine tune the mechanical, viscoelastic, and reversible response to a thermal trigger enabling greener and recyclable epoxy resins.