Extensional viscosity and birefringence measurements of viscoelastic polymer solutions in filament stretching flows
- Harvard Univ., Cambridge, MA (United States)
The evolution of the transient extensional stresses in dilute and semi-dilute viscoelastic polymer solutions are measured with a filament stretching rheometer of a design similar to that first introduced by Sridhar et al. (1991). The solutions are polystyrene-based (PS) Boger fluids that are stretched at constant strain rates ranging from 0.6 {le} {var_epsilon}{sub 0} {le} 4 s{sup -1} and to Hencky strains of {var_epsilon} > 4. The test fluids all strain-hardening and Trouton ratios exceeding 1000 are obtained at high strains. Experimental data exhibit strain-hardening at lower strain levels than predicted by bead-spring and FENE dumbbell models. We also monitor the decay of the tensile viscoelastic stress difference in the fluid filament following cessation of uniaxial elongation as a function of the total imposed Hencky strain and the strain rate. The extensional stresses initially decay very rapidly upon cessation of uniaxial elongation, and this drop in stress is followed by a slower viscoelastic relaxation. The measurements at moderate strains deviate significantly from the predictions of nonlinear dumbbell models. Flow induced birefringence measurements performed during elongation yield insight into the deformation-dependent conformation of the polymer chains. Tests are conducted in a neutral density fluid to prevent gravitational sagging that would be prevalent for the large sample diameters used in this study. The results demonstrate a monotonic rise in birefringence from {delta}n{prime} = 10{sup -8} to 10{sup -4} for samples stretched to Hencky strains of {var_epsilon} = 5. Newtonian samples stretched to the same levels demonstrate no rise in birefringence. The stress-optical law appears to fail at strains {var_epsilon} > 3 and the principal tensile stress difference compute from the relationship {delta}{tau} = {delta}n{prime}/C is significantly lower than the directly-measured stress difference.
- OSTI ID:
- 602817
- Report Number(s):
- CONF-961108--
- Country of Publication:
- United States
- Language:
- English
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