Dilute polystyrene solutions in extensional flows: Birefringence and flow modification

Abstract

The coil—stretch transition in dilute polymer solutions has been studied using four- and two-roll mills to generate two-dimensional extensional flows. Flow birefringence measurements give the polymer conformation in the flow, while velocity gradient measurements using homodyne light scattering reveal the effects of the polymer on the flow field. When the polymers become sufficiently extended there is a distinct onset of measurable decreases in the velocity gradients. A necessary condition for this occur is a very high level of domain overlap between the extended molecules. If we estimate this by the volume concentration of circumscribed spheres, φ eff (i.e., the sphere diameter equals the extended length of the polymer), the onset of an effect of polymer on the flow does not occur until φ eff ≈ 4000–6000. Nevertheless, our measurements show that dilute polymer solutions do inhibit the development of large strain rates in regions of persistent extensional flow. The present flow birefringence measurements also extend previous studies to a wider range of flow types, thus providing new information that can be used in the evaluation of molecular models for polymer flow dynamics. In the present paper, we compare experimental data with predictions from the elastic dumbbell model with various combinations of a nonlinear spring, conformation-dependent anisotropic friction (including strain-inefficient rotation), and internal viscosity.