Shear-induced homogenization of semidilute solution of polymer mixture and unmixing after cessation of the shear

Abstract

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.