Physico-chemical characterization of chitosans varying in degree of acetylation

Three commercial chitosans varying in average degree of acetylation (DA) were characterized in terms of average molar masses M-n, M-w, the second virial coefficient B and radius of gyration R-g,R-z, and intrinsic viscosities by membrane osmometry, static light scattering, and capillary viscometry in acetate buffer of pH 4.5. To obtain satisfactory light scattering data, the method used for clarification was of great importance. Combined ultracentrifugation and membrane filtration was found to be the method of choice. Using the model for wormlike chains with excluded volume and a logarithmic molar mass distribution, the persistence length L-p was determined as L-p = 6 nm. No concentration-dependent association was observed for the low concentrations studied. Samples were also fractionated on Sepharose CL-2B for subsequent light scattering and viscosity measurements to establish relationships between M-w and [eta] and R-g, respectively. These relationships were found to be independent of the DA for the bulk of molecularly dispersed chitosan. Model calculations aimed at checking the consistency of data led, as to be expected, to an increasing excluded volume effect with increasing molar mass and steadily increasing polydispersities of the fractions with increasing elution volume. The levelling off of the [eta]-M-w and R-g-M-w relations for fractions of increasingly high molar mass was assigned to chemical heterogeneity of samples in terms of DA distributions. (C) 1998 Elsevier Science Ltd. All rights reserved.