STRESS-RELAXATION AND ELASTIC DECOHESION OF VISCOELASTIC POLYMER-SOLUTIONS IN EXTENSIONAL FLOW

The evolution of the transient extensional stresses in dilute and semi -dilute viscoelastic polymer solutions was measured with a filament st retching rheometer of a design similar to that first introduced by Sri dhar et al. The solutions were polystyrene-based Boger fluids which we re stretched at constant strain rates in the range 0.6 less than or eq ual to epsilon(0) less than or equal to 4 s(-1) and to Hencky strains of epsilon > 4. The test fluids all strain-hardened and Trouton ratios exceeding 1000 were obtained at high strains. In addition to measurin g the transient tensile stress growth, the decay of the tensile viscoe lastic stress in the fluid column following cessation of uniaxial elon gation was also monitored as a function of the total imposed Hencky st rain and the strain rate. The measured relaxation functions were found to be significantly different from those observed following cessation of steady shear flow. The extensional stresses initially decayed very rapidly upon cessation of uniaxial elongation followed by a slower vi scoelastic relaxation. For the most elastic fluids, partial decohesion of the fluid filament from the endplates of the rheometer was observe d in tests conducted at high strain rates. This elastic instability is initiated near the rigid endplate fixtures of the device and it resul ts in the progressive breakup of the fluid column into individual thre ads or 'fibrils' with a regular azimuthal spacing. These fibrils elong ate and bifurcate as the fluid sample is elongated further. In tests c onducted at the highest Deborah numbers, complete sample decohesion fr om the endplates and rapid elastic recoil were sometimes observed. The critical stress and strain at the onset of the instability were deter mined by monitoring the tensile force exerted by the filament, the sam ple radius, and were used to construct an approximate stability diagra m. Flow visualization experiments using a modified stretching device s howed that the instability develops as a consequence of an axisymmetry -breaking meniscus instability in the non-homogeneous region of highly deformed fluid near the rigid endplate.