| The effect of simple flow fields on the detailed dynamics of concentration fluctuations in semidilute polymer solutions in near-theta and poor solvents has been investigated using light scattering, dichroism and birefringence. There are several conclusions that may be drawn. The concentration and velocity fields couple strongly, and this coupling increases in strength with decreasing osmotic pressure as the solution approaches the coexistence curve. The coupling with flow causes anisotropic concentration fluctuations to grow in such a way that the direction of the greatest concentration gradient is parallel to the direction of the principle axis of the stress of the applied flow. The resulting scattering patterns display various symmetries including two-lobed and four-lobed patterns, and these symmetries are in agreement with the predictions of the two-fluid and Helfand-Fredrickson models. Concentration fluctuations are also observed to relax by a mechanism that couples both cooperative diffusion and stress relaxation of entanglements of the transient polymer mesh. This time-dependent behavior is in agreement with the predictions of the Brochard-de Gennes model. |
