MOLECULAR-CONFORMATION DURING STEADY-STATE MEASUREMENTS OF EXTENSIONAL VISCOSITY

Recent measurements by Tirtaatmadja and Sridhar (1993) of extensional viscosity of polymer solutions show that a steady state was reached in some cases. Since their test fluids were dilute or nearly dilute solutions, and since the strains were as high as seven by the end of the uniaxial stretching, it is supposed that the polymer chains were fully extended. Two analyses are carried out to investigate whether this assumed conformation of the linear macromolecules is consistent with the conditions observed at steady state. The first analysis uses Batchelor’s formula for a suspension of elongated particles to estimate the steady-state values of extensional viscosity. Values calculated from the formula are about an order of magnitude higher than the experimental ones, which suggests that the chains were not fully extended at steady state. The second analysis predicts the strain necessary to completely unfold a randomly coiled chain. The analysis is based on a new lower-bound equation and on prior simulations and shows that strains of at least 6.5 are required. However, steady state was reached in the experiments at strains of 5, which is further evidence that the chains. were never fully extended. Various explanations are explored and it is thought that entanglements likely prevented the chains from completely uncoiling. © 1995, The Society of Rheology. All rights reserved.