Thermal depolymerization of chitosan chloride

The thermal depolymerization of chitosan chloride in solid state has been examined. Depolymerization was followed by measuring the apparent viscosity and intrinsic viscosity. The initial rate constants were determined from the intrinsic viscosity data and were found to increase markedly with increasing degree of acetylation, FA, showing that the FA is an important parameter for the rate of thermal degradation. The activation energies of the three chitosan chlorides with degrees of acetylation, F-A = 0.02, F-A = 0.16 and F-A = 0.35 were determined to be 114 +/- 11 kJ/mol, 112 +/- 5 kJ/mol and 109 +/- 5 U/mol, respectively. The rate of degradation was found not to be dependent on the presence of oxygen. On the other hand, the initial rate constant for chitosan chloride prepared by freeze-drying of a solution at pH 4 was about 30 times greater than that of a sample freeze-dried at pH 6, showing that the pH of the chitosan is important for its ability to degrade. H-1 and C-13 NMR spectroscopy of the thermally degraded chitosan with FA = 0.35 was used to identify the specificity in the reaction. The rate of acid hydrolysis of the glycosidic bond in chitosan solutions was recently (Proceedings of the 1st International Conference on Chitin and Chitosan (1997),168) found to be in the order A-A approximate to A-D much greater than D-A approximate to D-D, which appeared also to be valid for thermal depolymerization of chitosan. The NMR spectra also indicated that hydrolysis of the N-acetyl bond (de-N-acetylation) at the new reducing ends occurs in addition to the cleavage of the glycosidic bond. The results reported herein show that acid hydrolysis is the primary mechanism involved in the thermal depolymerization of chitosan chlorides in the solid state and that cleavage of the A-A and A-D linkages is mainly responsible for the degradation in the range of acetyl content investigated here. (C) 2001 Elsevier Science Ltd. All rights reserved.