Studies of Sustainable Polymers: Novel Lignins to Reprocessable Polymers

This dissertation includes two research topics. This first topic focuses on fundamental studies of monolignols and lignin, including polymerization and degradation. The second part reports a polymeric material that was crosslinked but can be reprocessed. In order to understand lignin from a molecular level and promote biopolymer conversion, we investigated the dehydrogenative copolymerization and degradation of two monolignols: caffeyl (C) alcohol and p-coumaryl (H) alcohol. The copolymerization and degradation were monitored by a quartz crystal microbalance with dissipation (QCM-D). Atomic force microscopy (AFM) was applied to investigate the topologies of the copolymer and degraded films. Horseradish peroxidase (HRP) was used as the enzyme for the dehydrogenative polymerization of monolignols and chelator-mediated Fenton chemistry was used to degrade the lignin. With constant monolignol concentration, we found that as the fraction of H in the polymerization feed increased, the amount of lignin formed increased, and the films became more rigid. For the degradation process of the resultant lignins, the presence of more C-monolignol during polymerization facilitated greater degradation. This work demonstrated the chemical factors that influenced the physical properties of lignin and lignin degradation, which could impact biofuel production. We further investigated the surface-initiated dehydrogenative polymerization of a new monolignol 5-hydroxyconiferyl (5H) alcohol using a QCM-D. HRP was immobilized on gold sensors. Various experimental conditions were studied. The dehydrogenative polymerization of 5H-monolignol was influenced by the concentration of monolignols and temperature, but was not affected by the hydrogen peroxide concentration, which was different from other monolignols. We also compared the polymerization kinetics of 5H-monolignol and the topology of the resulting lignin thin films with other monolignols. Furthermore, we utilized enzymatic and chemical degradation methods to treat the 5H-lignin. The 5H-lignin film was degraded thoroughly via a chelator-mediated Fenton reaction. This study provided a comprehensive understanding of 5H-monolignol polymerization and degradation and could be used as a reference for the exploration of the applications of the 5H-monolignol. In this dissertation, a separate study involved a vitrimer. It was a crosslinked polymer, but could be reprocessed and reshaped. The new vitrimer was based on poly (methyl methacrylate-co-hydroxymethyl methacrylate). Aromatic disulfides that underwent a dynamic exchange reaction were incorporated as crosslinkers. The structure of the material was identified by proton nuclear magnetic resonance spectroscopy (1H NMR) and Fourier transform infrared spectroscopy (FTIR). Thermal properties and mechanical properties were studied through thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and Instron tests. Furthermore, the chemical resistance was explored. Notably, that new material exhibited comparable mechanical performance for three cycles when reprocessed via a hot press to reprocess.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Chemistry

Department dc:contributor.department

Chemistry

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2022

Author dc:creator

• Liu, Tianyi

Chair dc:contributor.committeechair

• Esker, Alan R.

Committee members dc:contributor.committeemember

• Roman, Maren
• Schulz, Michael
• Iovan, Diana Alexandra

• Lignin
• Monolignol
• Degradation
• Quartz Crystal Microbalance with Dissipation Monitoring
• Vitrimer

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:34616

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/110416