Structures of a Semidilute Polymer Solution under Oscillatory Shear Flow

The structures of a semidilute polymer solution, comprised of ultrahigh molecular weight polystyrene as a solute and dioctyl phthalate as a solvent, under oscillatory shear flow were investigated by means of small-angle light scattering. The system exhibited a double-winged anisotropic scattering pattern called “butterfly”, characteristic of shear-induced phase separation known to occur under a continuous shear flow. Under oscillatory shear flow the phase separation strongly depended on both a strain amplitude γ0 and an angular frequency ω, being observed inside a quasi-parabolic line in the space of γ0 vs log ω. The system was brought to a dynamically stationary state in about 20 min after applying the shear flow, and the shear-induced structures changed with a strain phase φ of the oscillatory shear. At a given φ and γ0, the shear-induced structures strongly depended on ω: the characteristic wavelength of the structures decreased with increasing ω, and the mean square of the concentration fluctuations was a maximum at a certain ω.