Shear-induced homogenization of semidilute solutions of polybutadiene and polystyrene mixtures with nearly critical composition was studied by the flow small-angle light scattering method. The dramatic drop of the homogenization temperature ΔTc(S)=Tc(0)−Tc(S) was observed, and studied as a function of shear rate S, where Tc(S) is the homogenization temperature at a given S and Tc(0) is the temperature under the quiescent state. The experimental results showed that ΔTc(S)/Tc(0)=KS0.50±0.02 where the prefactor K was found to be (2.6±0.6)×10−3 for the solutions containing 3.0 wt. % of total polymer weights. This prefactor is much larger than that reported for a case of simple-liquid mixture by 3 or 4 orders of magnitude. The large prefactor is proposed to be due to a long lifetime of the concentration fluctuations in the single-phase state and slow growth rate of the fluctuations in the two-phase state, compared with the corresponding quantities for the simple-liquid mixture. In the shear-induced homogenized state, the light scattered intensity I(qy) perpendicular to the flow direction was found to obey the Ornstein–Zernike equation. The correlation length ξ and the intensity at qy=0, I(0), were found to depend on S, i.e., ξ−2∼S1/2−S1/2c and I(0)−1∼S1/2−S1/2c, and diverge at the critical shear rate Sc. These experimental results on Tc(S), ξ−2 and I(0)−1, suggest the mean-field behavior of the solution of the polymer mixture under shear. The early-stage unmixing behavior after the cessation of the steady-state shear at S>Sc was also examined, the results of which indicate that the hydrodynamic interaction is suppressed, giving rise to the small mode–mode coupling contribution Dhydro/Dapp=0.2 to 0.45, but is not yet completely screened out. Hence the hydrodynamic interaction cannot be still ignored for this system.

The domain structure self-assembled under a steady Couette flow was investigated on a semidilute solution of polymer mixture (polymer A + polymer B + solvent) at a composition near the critical one by use of the in situ light scattering method. This method permits a quantitative analysis of the scattering profile I(qy) in the plane perpendicular to the shear flow as a function of shear rate S at a given quench depth ΔT(0)=Tc(0)−T. Here, qy is the component of the scattering vector q in the plane concerned, Tc(0) the critical temperature at S=0, and T[<Tc(0)] the temperature of the experiment. Effects of shear rate on the self-assembled structure were pronounced, and they were classified into five regimes A to E. At the lowest S (regime A), I(qy) was expressed by a linear combination of the Porod scattering and the Ornstein–Zernike (OZ) scattering, the former being due to the domain structure and the latter to critical composition fluctuations inside the domains. At higher S (regime B), I(qy) was complex. However, with increasing S further (regimes C and D), it was universally represented by the squared Lorentzian form {1+[qy(ξ⊥)d]2}−2, with a shear-rate-dependent length parameter (ξ⊥)d which depends on S−n with n=1/4 to 1/3. This fact indicates the self-assembling of a domain structure which we call oriented random two-phase structure. At the highest S (regime E), I(qy) followed the OZ scattering, thus suggesting the system to change to the shear-induced homogenized state.