Dissolution of Chitosan in Ionic Liquids for Acylation Chemistry and Formation of Cellulose-Chitosan Blends

Abstract

Dissolution of chitosan in ionic liquids was accomplished. It has been shown that 1-butyl-3-methylimidazolium acetate (BMIMAc) is a better solvent than 1-butyl-3-methylimidazolium chloride (BMIMCl). Dissolution of chitosan in BMIMCl required a prior regeneration of chitosan from 1% acetic acid solution. In the case of BMIMAc, both dried chitosan and regenerated chitosan from acetic acid solution have been dissolved in a relatively short amount of time. While concentrations up to 2 wt% of chitosan in BMIMCl could be obtained, concentrations of 10 wt % of chitosan in BMIMAc were realized. Homogeneous phthalation and benzoylation of chitosan were achieved in these ionic liquids. According to FT-IR data both -OH and -NH2 groups of chitosan reacted with benzoyl chloride and phthalic anhydride, respectively. The reaction of chitosan with phthalic anhydride in the presence of a base or N-bromosuccinimide as catalyst and using ionic liquids as a solvent media was also studied. The presence of a base into system leads to an increase of the degree of substitution (DS = 0.41) of the functional groups of chitosan comparing with the reactions performed in the absence of a base (DS = 0.24), while the presence of a catalyst into system resulted in even higher increase of DS (0.85). The FT-IR data indicated that the hydroxyl groups of chitosan are being catalyzed to a greater extent than the amino groups. All the reactions products obtained in the presence of a catalyst were soluble in dimethyl sulfoxide and dimethylformamide. Chitosan-cellulose blends were prepared using BMIMAc as common solvent. Rheological measurements of polymeric solutions indicated the formation of a complex between chitosan and cellulose molecules. Films prepared from polymeric solutions were investigated by means of FT-IR, TGA, XRD and SEM measurements. The shifting of the band corresponding to –NH groups of chitosan from 1597 to 1565 cm-1 (FT-IR), the absence of diffraction peaks at 2è = 10.7 and 14.9o (XRD), the increased Ea for all polymeric blends (MTGA), and the presence of a homogeneous structure with no phase separation of the two polymers (SEM) serve as good evidence for the miscibility between chitosan and cellulose in the solid state.