ORIENTATION OF POLYMER COILS IN DILUTE-SOLUTIONS UNDERGOING SHEAR-FLOW - BIREFRINGENCE AND LIGHT-SCATTERING

Polymer orientation can be investigated on different length scales by means of various measurement techniques such as flow birefringence or light scattering. We start from equations that relate the orientation angle of polymers to the tensorial quantities measured by experimental means. A quantity called ''orientation resistance'' of the polymer coils is introduced for the mentioned techniques and for translational diffusion. We calculate the orientation resistance for dilute Theta solutions undergoing shear now in the framework of bead-spring models including hydrodynamic interaction by various approximation schemes. We show that the results of the Gaussian approximation, as well as the perturbation theory, differ considerably from the simple Rouse and Zimm results. In order to make a comparison to flow birefringence and light scattering experiments as meaningful as possible, we carefully study the polydispersity effect on the orientation angle measurements for Theta and good solvents. It turns out that the predictions for the orientation angle are very sensitive to polydispersity and that in the Limit of vanishing shear rate this effect can be handled in a simple way. A comparison to experiments from the literature suggests that orientation angle measurements could be used to determine the polydispersity of a polymer system in dilute solutions.