Molecular Association and Rheological Properties in Chitosan-Weak Acid Solutions

Abstract

Light scattering and rheological properties of the chitosan (degree of deacethylation=74.8%) solutions in 8.5% formic acid/0.5M sodiumformate (F/FNa) or 2% acetic acid (Ac) aqueous solutions were measured by means of a low angle laser light scattering photometer (Chromatix, KMX6) and a cone-and-plate type Weissenberg Rheogoniometer (Sangamo Controls, R18). Rayleigh ratio of the extremely dilute F/FNa system (~10-5g/ml) increases with increasing time and becomes constant within 20~30min. Weight average molecular weight Mw increases from 87.3×104 to 625×104. That is, even in the low concentration, chitosan molecules are likely to associate in the F/FNa system. On the other hand, molecular association can not be observed in the Ac system. The persistence length of the chitosan molecule which is obtained with SAXS is about 120Å and 147Å in the F/FNa and the Ac system, respectively. The zero shear viscosity of solution subtracted the solvent viscosity is proportional to c1 in c<0.1% and to c4·4 in c<0.5% in the F/FNa system and it is proportional to c1·8 and c5·4 in the Ac system below and above 1.29%, respectively. Here, c is the concentration of the chitosan. Although the apparent molecular weight of the chitosan in the F/FNa system is much higher than that in the Ac system, the viscosity of the former system is lower than that of the latter system in dilute region (c<0.7%). This suggests that the chitosan molecules associate relatively compact in the F/FNa system.