When polymeric fluids flow, individual polymer chains may become deformed by hydrodynamic forces. Understanding this deformation process is a central research goal of polymer rheology, as the conformational dynamics of individual chains ultimately create non-Newtonian flow properties. Despite this obvious importance, few attempts have been made to study directly polymer chain dynamics in flow. In this project, low-angle light scattering is used to probe the stretching and orientation of polymers in steady uniaxial elongational flow of dilute solutions. Results from the previous funding period indicate that the level of chain stretching in a model flow, created between opposed jets, may not be as great as previously suggested by indirect optical methods (birefringence) or current molecular theories. The physical mechanisms responsible for controlling the rate of deformation and the ultimate deformation level of flexible polymers are now being examined by varying molecular weight, solvent type and quality, elongation rate, and backbone stiffness. Experiments are being conducted in a recently completed light-scattering apparatus capable of low-angle measurements off an extremely small scattering volume located at a stagnation point. These measurements will be complemented by birefringence studies and flow experiments in more transient stress environments. This research aims to understand macromolecular behavior in flow, which is of fundamental scientific importance as well as in technology, such as fiber spinning, turbulent-drag reduction, and enhanced oil recovery.
