Solvent Effects on the Non-Newtonian Viscosity of Dilute Solutions of Flexible Linear Macromolecules

Abstract

The average density of segments within a macromolecular coil decreases when the molecule is deformed by shear. Consequently, the excluded volume effects, and therefore the intrinsic viscosity, diminish as the rate of shear increases. The magnitude of the effect has been calculated for flexible polymers introducing the following simplifications: (1) An approximate density distribution of segments is used. (2) The degree of expansion α of the hydrodynamically deformed macromolecular coil is calculated using Flory's method. (3) The expanded macromolecule behaves hydrodynamically as a Gaussian coil of the same size. Some interesting conclusions are derived from the theory and are supported by the experimental results: (1) The non-Newtonian viscosity of polymer solutions should be studied in θ solvents to eliminate excluded volume effects. (2) The influence of shear increases with solvent power. (3) At large rates of shear all solvents behave as θ solvents. (4) Stiff chains and polyelectrolytes show a much more pronounced influence of rate of shear at relatively low molecular weights, which cannot be explained by the present theory.