Viscoelastic effects on dynamics of concentration fluctuations in semi-dilute polymer solution in the good solvent regime

Abstract

The relaxation processes of the concentration fluctuations in semi-dilute polystyrene (PS) dioctyl phthalate (DOP) solution are investigated by using dynamic light scattering (DLS) in the good solvent regime. Slow modes were found in the dynamic structure factors in the good solvent regime as found in its theta condition. They were almost independent of wavenumber, implying that the slow modes originate from the viscoelastic effects arising in dynamically asymmetric two component systems where the stress and diffusion are coupled. In order to clarify whether the viscoelastic effect indicates the slow modes, the dynamical structure factors were calculated by using Doi-Onuki (DO) theory, where the viscoelastic effects are considered in the dynamics of the concentration fluctuations of binary systems, and the calculated structure factors were compared with experimental ones. In contrast to the theta condition, the deviation was found between the calculated structure factors and experimental ones at longer time region. This deviation originates from the estimation of the osmotic compressibility from the small-angle X-ray scattering. On the other hand, interdiffusion coefficient and cooperative diffusion coefficient from calculated dynamical structure factors agree with those from experimental dynamical structure factors, indicating that both diffusion coefficients are insensitive to the relaxation process at longer time.