Most of the chemicals used for a simple and efficient synthesis of benzoxazine monomers are based on petroleum resources. Because of environmental and cost concerns, increasing attention has been paid to renewable alternative raw materials that could replace petrochemicals. Formaldehyde is produced industrially by the catalytic oxidation of biomethanol, therefore the synthesis encourages the use of plant-derived phenolics instead of petroleum-based ones. Bio-based amines receive less attention due to their smaller natural stock. Nonetheless, extensive studies must be done, as there are still many unexplored natural components and obscurities in the relationship between the structure and properties of bio-based benzoxazines.
In experimental work two benzoxazine monomers based on vanillin and furfurylamine and based on guaiacol and furfurylamine have been successfully synthesized. Their structures have been confirmed by infrared spectroscopy and nuclear magnetic resonance spectroscopy. In an efficient and low-cost manner, I have developed a fully bio-based copolymer of benzoxazine monomer and chitosan. Thermal and mechanical properties of copolymers have been investigated by differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical analysis. The influence of chitosan on the polymerization of benzoxazine is extremely intricate. The condensation reaction of amine groups of chitosan with aldehyde group of vanillin leads to Schiff base formation. The formation of Schiff base, an extensive inter- and intramolecular hydrogen-bonding network along with covalent reactions resulted in remarkable cross-lining density and thermomechanical properties. In addition, the catalytic effect of the amine groups of chitosan effectively lowered the ring opening polymerization temperature. Furthermore, I also compared the properties of copolymers of chitosan and guaiacol based benzoxazine or vanillin based benzoxazine. The results suggest that the aldehyde group of vanillin, besides inherent reactive functional chitosan groups, is playing a crucial role to form extensive bonding network. Thermomechanical properties of vanillin based benzoxazine and chitosan copolymers were significantly enhanced.
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