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, ε 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 ε c is a decade lower than the strain rate predicted to characterize the coil−stretch transition. This difference indicates that ε c reflects the average deformation of the chains over the range of strain rates.