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 solutions. Changes in weight-average molecular weight (M-w), deduced from gel permeation 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 that enzymatic degradation was zeroth-order in guar concentration. These findings contrast with previous studies of natural polymer degradation which usually have interpreted the linear relationship between 1/M and time as first-order processes. Our analysis reveals that this linear relationship is expected regardless of the reaction order and that the true order can be determined only from the dependence of the degradation rate on initial polymer concentration. Rheological properties were sensitive to extent of degradation; several orders of magnitude change in zero shear viscosity were observed over the course of polymer chain scission. Moreover, the viscosity-time profiles for different enzyme concentrations could be collapsed onto a single curve by temporal scaling. This could be used to predict, a priori, guar solution viscosity as a function of degradation lime and enzyme concentration. This "concentration-degradation time" superposition was based on a unique relationship between zero shear viscosity, eta(0), and the product of enzyme concentration and degradation time.