Studies of Entangled Polymers Under Strong Flow Conditions

This project is aimed at understanding the behavior of polymeric solutions in strong flows beyond the linear viscoelastic limit, and for concentrations and molecular weights at which the polymers are sufficiently entangled that reptation-based theories are expected to be applicable. The project is based upon the use of transient shear and other homogeneous flows ranging from near-shear to a planar extensional flow, under conditions in which the conformation of polymer chains is far from the equilibrium state. This includes cases at sufficiently high shear rates that there can be significant chain stretch. This general class of problems is intresting because it provides a basis for testing the predictions of theoretical models, and also because an understanding of the relationship between flow and polymer configuration is an essential component in the dsign of processing systems that can lead to optimization of marcroscopic materal properties. The research proposed is dominantly experimental in nature, and is based on the use of novl flow cells that are coupled with transient optical measurement systems. These include both two-color and phase-modulated birefringence, as well as dynamic light scattering and Piv. This combination of experimental tools provides data on the transient configuration of polymer segments between entanglement points, as well as a complete characterization of the flow. Standard rheological tests (primarily linear viscoelasticity) are also used to characterize the fluids. The fluids to be studied are solutions with a bimodal moleclar weight distribution, and solutions of star polymers. These two studies represent a first step toward the consideration of polydispersity and polymer branching. Finally, recent extensions of the basic Doi-Edwards theory will be evaluated, and where possile improved, including particularly recent proposals for the incorporation of convective constraint release(CCR). %%% This research is aimed at achieving the fundamental understanding that is necessary for the design and development of processing strategies for the production of precision polymeric parts for structural and other applications, and for optimizatiton of the material properties by manipulation of molecular structure during processing. The ability to control design and optimize polymer processing is essential to the development of products with a high added value above the raw materials costs.