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 rang e of 0.1-30%, 0.1-40% and 0.05-65% w/v, respectively. Static and dynamic li ght scattering, viscometry, and rheological techniques were applied. The fo rward scattering (at scattering angle theta = 0) could be separated in cont ributions resulting from repulsive interactions and the true molar mass M-w (c) at concentration c. A similar procedure was applied to the apparent rad ius 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 obta ined are discussed. Both molecular parameters remained unchanged up to thre e times the overlap concentration. At higher concentrations a pronounced in crease in M-w(c) and R-g(c) indicated association. The high M-w dextran dev eloped a reversible gel point and critical behavior of percolation theory. The time correlation function of dynamic light scattering displayed fast an d slow motions where the slow motion was assigned to clusters. Separating t he effect of thermodynamic interactions from the mutual diffusion coefficie nt 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 incre ase at higher concentrations.