Degradation of polymers in high speed rotary homogenizers: A hydrodynamic interpretation

Dilute solutions of DNA in neutral phosphate buffer and of polystyrene in toluene are degraded to limiting molecular weights by high speed stirring in a Virtis homogenizer equipped with special thin, highly sharpened blades. Results are interpreted and justified in terms of the hydrodynamic theory of laminar boundary layers. Estimates of critical stress values for chain scission are obtained at various limiting molecular weights for the two polymer systems, and the treatment is used to rationalize the dependence of degradation upon homogenizer speed. The agreement between the hydrodynamic theory and experimental data implies that limiting molecular degradation occurs entirely in the boundary layer region extending a distance equal to approximately the molecular contour length from the leading edge of the blade tip and that laminar flow conditions prevail in this region. It therefore seems likely that polymer molecules subjected to a critical shearing stress are essentially completely extended in the streamlines of flow. The hydrodynamic theory is finally used to explain the peculiar kinetics of molecular degradation in high speed rotary homogenizers.