Structure and Properties of Carboxymethyl Chitin in the Salt, Acid, and Mixed Forms

Abstract

Different forms of 6-О-carboxymethyl chitin were prepared, namely, the salt form containing, the carboxy group in the –COONa form, the acid form containing the carboxy group in the –COOН form, and the mixed form containing the units in the acid and salt forms in the equimolar ratio. The composition and structure of the synthesized carboxymethyl chitin and films based on them were confirmed by Fourier IR spectroscopy, solid-state 13C NMR spectroscopy, thermal gravimetric analysis, and differential scanning calorimetry. The mechanical properties of the films prepared from 6-О-carboxymethyl chitin in the salt and acid forms were evaluated by comparing their deformation characteristics. The Fourier IR data confirm formation of hydrogen bonds and ion pairs in 6-О-carboxymethyl chitin in the acid form as a result of intermolecular interaction of functional groups of the polymer. The solid-state 13С NMR data show that the substitution of the –СН2СООNa group for hydroxyl proton in the course of the synthesis of 6-О-carboxymethyl chitin from chitin occurs exclusively at the С-6 atom of the glucopyranose ring. Comparison of the data of thermal gravimetric analysis and differential scanning calorimetry shows that the amount of water absorbed by the polymer depends on the form in which 6-О-carboxymethyl chitin occurs, with the salt form being the most hygroscopic. The deformation characteristics of films of 6-О-carboxymethyl chitin in the salt and acid forms were studied. The elastic modulus of 6-О-carboxymethyl chitin in the acid form significantly differs from that of 6-О-carboxymethyl chitin in the salt form. This difference may be due to higher content of water absorbed by the polymer in the films obtained from the salt form of 6-О-carboxymethyl chitin (20%) compared to the films obtained from the acid form of 6-О-carboxymethyl chitin (9%).