Dilute polyacrylamide solutions under uniaxial extensional flow

The shear and extensional viscosities, dynamic viscosities, and normal forces of a series of polyacrylamide solutions of varying molar mass have been determined in a Boger fluid at varying concentrations. The critical strain rate under extensional flow that can be defined from the departure from Newtonian behavior, epsilon(c), was found to be independent of the concentration and varied in the same way as the critical coil-stretch strain rate with the molar mass. A quantitative comparison indicated that, is a decade lower than the strain rate predicted to characterize the coil-stretch transition. This difference indicates that c reflects the average deformation of the chains over the range of strain rates.