Shear degradation and deformation of polysaccharides in thin liquid film flow on a rotating disk

The shear rate in thin liquid film flow on a rotating disk is governed by t he flow rate of the polymer solution and the rotation rate of the disk. Des pite this fact we show that shear degradation is determined solely by the r otation rate, whereas a variation of the flow rate has no distinct effect. Also no significant influence of the polymer concentration in a range of 1- 10 mg/ml could be observed. Threshold values of the rotation rate that caus e shear degradation in different samples of dextran and purified polysaccha rides of Viscum album L. berries are reported and changes in the molecular weight distributions of these samples are shown. Some of the polymers are p resumably deformed in their geometrical shape under the influence of sheari ng. In gel permeation chromatography these polymers are eluted after polyme rs with lower masses, so that most probably the hydrodynamic radii of the d eformed polymer particles are smaller while their molar masses are larger. A mathematical analysis of the light scattering data gives evidence to this assumption. Degradation rates of the dextran samples are compared with res ults of a previous study. (C) 2000 Elsevier Science Ltd. All rights reserve d.