Sonochemical and Hydrophobic Modification of Chitin and Chitosan

Chitin and chitosan are linear polysaccharides consisting of acetyl-glucosamine and glucosamine units with many potential applications in biomedicine, agriculture and food industry. In the food industry, chitosan can be used as antimicrobial agent and to create active packaging to improve food quality and extend shelf life. Commercial applications of chitosan are closely associated with its functional properties and biological activity, which are primarily governed by two structural properties: degree of acetylation (DA) and molecular weight (MW). The overall goal of this study was to develop methods for tailored modification of DA, MW and functionalities of chitosan. The research was conducted in four phases. In the first phase, an accurate and rapid method to determine DA for both chitin and chitosan was developed. By employing concentrated phosphoric acid as a solvent for highly acetylated samples and by optimizing the analytical parameters through investigation of underlying chemical reactions, it was the first time that the DA of both chitin and chitosan could be analyzed by a single method in an accurate, rapid and economical way. In the second phase, high intensity ultrasound (HIU) was investigated as a pretreatment method for the deacetylation process of chitin, and data indicated that HIU pretreatment of chitin flakes lasting up to 30 minutes was insufficient to change the DA of the resulting chitosan. Since the HIU treatment has been considered as an efficient way to modify the MW of polysaccharides in general, the third phase of this study focused on tailored modification of chitosan MW with HIU process. The factors that influence the ultrasonic degradation process of chitosan were investigated and the results showed that the MW, radius of gyration and polydispersity of chitosan were efficiently reduced, whereas the chain conformation and DA were unchanged after sonication. The degradation of chitosan by ultrasound was primarily driven by mechanical forces and degradation mechanism could be described by a random scission model. The degradation process was affected by ultrasound intensity, solution temperature, polymer concentration and ionic strength, while acid concentration had little effect. Additionally, the data indicated that the degradation rate coefficient was affected by the degree of deacetylation of chitosan and independent of the initial molecular weight. In the fourth phase, hydrophobic modification of chitosan (HM-chitosan) was performed to probe new way to improve the functionality of chitosan. The physicochemical properties and antimicrobial activity of HM-chitosan and HM-chitosan based films were investigated, and the data indicated that hydrophobic modification introduced thermal responsive and self assembly properties, while maintained the antimicrobial activity of chitosan. Furthermore, the water vapor permeability and mechanical properties of HM-chitosan based films decreased. Contributions of this research include developing a practical method for the DA analysis, elucidating the effects of HIU pretreatment on the deacetylation process and factors that affect the ultrasonic degradation process of chitosan, along with the effects of hydrophobic modification on the functionalities of chitosan and chitosan films