Zusammenfassung
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.
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