Light scattering and viscosity behavior of dextran in semidilute solution

The solution properties of three dextrans in water with molar masses of 334 000, 506 000 and 2 660 000 g/mol were investigated in a concentration range of 0.1-30%, 0.1-40% and 0.05-65% w/v, respectively. Static and dynamic light scattering, viscometry, and rheological techniques were applied. The forward scattering (at scattering angle theta = 0) could be separated in contributions resulting from repulsive interactions and the true molar mass M-w(c) at concentration c. A similar procedure was applied to the apparent radius of gyration to derive the true radius of gyration R-g(c). A mean field and a scaling approach were applied, and the difference in the results obtained are discussed. Both molecular parameters remained unchanged up to three times the overlap concentration. At higher concentrations a pronounced increase in M-w(c) and R-g(c) indicated association. The high M-w dextran developed a reversible gel point and critical behavior of percolation theory. The time correlation function of dynamic light scattering displayed fast and slow motions where the slow motion was assigned to clusters. Separating the effect of thermodynamic interactions from the mutual diffusion coefficient allowed the self-diffusion coefficient to be obtained, which is governed by hydrodynamic interactions. The range of this interaction was estimated and compared with the cluster size. The zero shear viscosity showed common behavior with a fairly weak increase in the dilute regime and a steep increase at higher concentrations.