PHYSICOCHEMICAL CHARACTERIZATION OF CHITOSANS VARYING IN DEGREE OF ACETYLATION

Three commercial chitosans varying in average degree of acetylation (D A) were characterized in terms of average molar masses M-n, M-w, the s econd virial coefficient B and radius of gyration R-g,R-z, and intrins ic viscosities by membrane osmometry, static light scattering, and cap illary 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 f ound to be the method of choice. Using the model for wormlike chains w ith excluded volume and a logarithmic molar mass distribution, the per sistence length L-p was determined as L-p = 6 nm. No concentration-dep endent association was observed for the low concentrations studied. Sa mples were also fractionated on Sepharose CL-2B for subsequent light s cattering and viscosity measurements to establish relationships betwee n M-w and [eta] and R-g, respectively. These relationships were found to be independent of the DA for the bulk of molecularly dispersed chit osan. Model calculations aimed at checking the consistency of data led , as to be expected, to an increasing excluded volume effect with incr easing molar mass and steadily increasing polydispersities of the frac tions 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 dist ributions. (C) 1998 Elsevier Science Ltd. All rights reserved.