Doctoral Dissertations
Date of Award
8-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy
Major
Energy Science and Engineering
Major Professor
Tomonori Saito
Committee Members
Alexei Sokolov, Brian Long, Matt Korey, Sheng Dai
Abstract
The increasing demand for sustainable materials has driven the need for effective recycling methods for step-growth polymers such as poly(ethylene terephthalate) (PET), polycarbonates (PC), polyesters, and polyamides. This dissertation focuses on developing tailored deconstruction strategies to break down these polymers into monomers or oligomers that can be reprocessed into new materials. A key aspect of this work is catalyst design, aiming to identify efficient and cost-effective catalysts. A Hammett study using benzoic acid derivatives was conducted to assess the impact of substituent groups on catalyst reactivity. The most successful catalyst identified is the TBD:para-aminobenzoic acid system, which exhibits high thermal stability, cost-effectiveness, rapid synthesis, and scalability.
Tailored deconstruction enables selective recovery of oligomers from polymer breakdown. This study employs glycolysis, catalyzed by TBD:TFA, as the primary chemical deconstruction method. The kinetics of glycolysis are controlled by the ratio of ethylene glycol (EG) to nylon repeat units, where higher EG ratios accelerate deconstruction and extended reaction times yield lower molecular weight oligomers. Molecular weight control is primarily influenced by EG ratio, offering a tunable approach to achieving desired oligomer properties. The resulting nylon oligomers are repolymerized via mechanochemistry, specifically speedmixing, which circumvents the solubility challenges of nylon 6 oligomers. Copolymerization of these oligomers with poly(bisphenol A-co-epichlorohydrin) produces a material with enhanced processability and superior adhesive properties. This copolymer exhibits nearly double the lap shear stress of virgin nylon in steel-to-steel bonding and outperforms commercial epoxy-based adhesives, such as JB Weld, for steel-to-composite adhesion, achieving nearly three times higher bond strength.
Beyond pure polymer recycling, chemical deconstruction using TBD:TFA also addresses challenges in processing composite materials that are traditionally difficult to recycle due to their form factor. Nylon 6 composites with glass or carbon fiber are challenging to mechanically separate, but chemical recycling effectively reduces the molecular weight of the polymer matrix, enabling common organic solvents like tetrahydrofuran to extract residual oligomers. Scanning electron microscopy and single-fiber tensile testing confirm that the recovered fibers remain intact and free of residual polymer. This research demonstrates a scalable approach to recycling step-growth polymers and composites, providing pathways for circular material use.
Recommended Citation
Zheng, Jackie, "FROM WASTE TO A NEW PLACE: DEVELOPING UPCYCLING STRATEGIES TO TURN WASTE PLASTICS INTO INDUSTRIALLY RELEVANT UPCYCLED POLYMERS. " PhD diss., University of Tennessee, 2025.
https://trace.tennessee.edu/utk_graddiss/12670