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. / Doctor of Philosophy / Lignin is a complex phenylpropanoid polymer and is one of the most abundant biopolymers in nature. Conversion of lignin into biofuels or other fine chemicals has drawn significant attention in recent years. Understanding molecular details of lignin formation and degradation is of fundamental importance for the biorefinery. Although a number of studies have improved our knowledge about lignin, many important aspects remain unknown.
Lignin arises from dehydrogenative polymerization of three types of monolignols, named p-coumaryl (H), coniferyl (G), and sinapyl (S) alcohols. Recently, a new monolignol, caffeyl (C) alcohol, has been found. In this work, the surface-initiated copolymerization of C-monolignol and H-monolignol was conducted through an in vitro synthesis. Furthermore, chelator-mediated Fenton reactions were applied to degrade the resulting lignin. The effect of C-lignin incorporation on degradation was studied. It was found that, when more C-lignin was incorporated, the percentage of degradation was larger. These findings are likely to guide the conversion of lignocellulosic biomass into value-added products.
A new monolignol, 5-hydroxyconiferyl (5H) alcohol, was investigated in this dissertation. The surface-initiated dehydrogenative polymerization of 5H was conducted under various experimental conditions, including different temperature, monomer concentration, and hydrogen peroxide concentration. Furthermore, degradation by enzymatic and non-enzymatic methods were studied. It was found that the 5H-lignin was recalcitrant to enzyme, but can be degraded by a non-enzymatic procedure. The synthesis and degradation were monitored by a quartz crystal microbalance with dissipation (QCM-D), which is a label-free method and can provide real-time data.
Thermosets are the materials that are chosen for many applications due to their structural stability and mechanical properties. However, due to their permanent crosslinkages, they cannot be reprocessed or recycled. In this dissertation, a new crosslinked polymer material called a vitrimer was reported. The material was developed based upon poly (methyl methacrylate) (PMMA) and aromatic disulfide linkages, which are exchangeable chemical bonds. The exchange reaction occurs very quickly at elevated temperature. As a result, the material can be easily reprocessed and also exhibited chemical stability and mechanical properties similar to conventional thermosets.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110416 |
| Date | 02 June 2022 |
| Creators | Liu, Tianyi |
| Contributors | Chemistry, Esker, Alan R., Roman, Maren, Schulz, Michael, Iovan, Diana Alexandra |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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