Deformation and orientation of dissolved polymer chains in an elongational flow

The addition of a low concentration of dissolved high molecular weight polymer can greatly modify the rheological properties of a simple. Many of the modification can be attributed to flow-induced departures of the average chain conformation from its isotropic value at quiescence. The statistical deformation and orientation of polymer chains in flow has been predicted by various molecular models, but these predictions have not been adequately tested. This research provides an important molecular-level understanding of chain conformation in dilute solutions undergoing elongational flow; the work applies light scattering and birefringence techniques to probe chain conformation in situ. We have investigated the influences of chain stiffness and solvent quality on the chain conformations produced in and around the stagnation point of opposed jet flow. By light scattering, the average radius of gyration of the examined polymers has been probed both parallel and perpendicular to the stretching axis for flows of various strength. Flexible polymers are not deformed affinely under any circumstances, with statistical coil deformation falling much below this limit. Although solvent quality has little impact, slightly more coil deformation is observed in a theta solvent than in a good one. For a relatively stiff polymer of approximately 15 persistence lengths, the behavior of chain deformation/orientation in opposed jet flow is different; Because less strain is required to orient a stiff polymer than to deform an analogous flexible polymer, conformation changes are less localized, extending outside the region between the jets. Nevertheless, the overall conformational changes remain less than that predicted by a rod model