Rheology and molecular weight changes during enzymatic degradation of a water-soluble polymer

The rheological behavior and molecular weight characteristics of a natural polymer undergoing enzymatic hydrolysis were examined for aqueous guar solu tions. Changes in weight-average molecular weight (M-w), deduced from gel p ermeation chromatography (GPC), were used to construct a kinetic model for the process, such that 1/M-w proportional to kt, with the rate constant, k, varying inversely with polymer concentration. This relationship suggests t hat enzymatic degradation was zeroth-order in guar concentration. These fin dings contrast with previous studies of natural polymer degradation which u sually have interpreted the linear relationship between 1/M and time as fir st-order processes. Our analysis reveals that this linear relationship is e xpected regardless of the reaction order and that the true order can be det ermined only from the dependence of the degradation rate on initial polymer concentration. Rheological properties were sensitive to extent of degradat ion; several orders of magnitude change in zero shear viscosity were observ ed over the course of polymer chain scission. Moreover, the viscosity-time profiles for different enzyme concentrations could be collapsed onto a sing le curve by temporal scaling. This could be used to predict, a priori, guar solution viscosity as a function of degradation lime and enzyme concentrat ion. This "concentration-degradation time" superposition was based on a uni que relationship between zero shear viscosity, eta(0), and the product of e nzyme concentration and degradation time.