Typical Physicochemical Behaviors of Chitosan in Aqueous Solution

Physicochemical properties of a homogeneous series of chitosans with different degrees of acetylation and almost the same degree of polymerization were investigated in an ammonium acetate buffer. Techniques such as interferometry, static light scattering (in batch or coupled on line with a chromatographic system), and viscometry were processed. All of the results agree with a unique law of behavior only depending on the degree of acetylation of the polymer. Indeed, values of the refractive index increment, radius of gyration, second viral coefficient, and intrinsic viscosity are decreasing in the same way as DA is increasing. Three distinct domains of DA were defined and correlated to the different behaviors of chitosans: (i) a polyeletrolyte domain for DA below 20%; (ii) a transition domain between DA = 20% and 50% where chitosan loses its hydrophilicity; (iii) a hydrophobic domain for DAs over 50% where polymer associations can arise. Conformations of chitosan chains were studied by the calculations of the persistence lengths (L(p)). The average value was found to be close to 5 nm, in agreement with the wormlike chain model, but no significant variation of L(p) with the degree of acetylation was noticed.