Microwave–Assisted Synthesis of Quaternized Carboxymethyl Chitosan in Aqueous Solution and its Thermal Behavior

Abstract

A novel promising polyampholyte, being water–soluble quaternized carboxymethyl chitosan (QCM–chitosan), can be prepared by grafting carboxymethyl groups and quaternary ammonium groups on chitosan. Traditionally, QCM–chitosan was obtained by a conventional heating method in organic solvent for a lengthy time. The present study was to prepare QCM–chitosan rapidly under microwave irradiation for 70 min; the whole preparation proceeded in water without any organic solvent. Firstly, chitosan was carboxymethylated, and sequentially carboxymethyl chitosan (CM-chitosan) was quaternized to obtain QCM–chitosan. QCM–chitosan's structure was characterized. GPC-LLS were used to analyze its weight – average molecular weight (Mw ), and the degree of substitution (DS) was determined by titration. The thermal stability of QCM–chitosan was measured by TGA. The results proved that QCM–chitosan was synthesized. The maximum degree of substitution (DS) of carboxymethyl groups and quaternary ammonium groups under the optimum reaction condition was 0.82 and 0.48, respectively. Compared to chitosan, the thermal stability of QCM–chitosan decreased because of disintegration of intermolecular hydrogen bonding and partial breaking of the molecular structure, moreover, as DS of quaternary ammonium and carboxymethyl groups increased, QCM-chitosan showed lower thermal stability. In summary, the microwave irradiation method is a rapid and environmentally friendly process to synthesize soluble chitosan derivatives.