Phase transition of polymer mixtures under simple shear flow

Self-assembled structure in semidilute solutions of polystyrene and polybutadiene in dioctylphthalate just above (Regime V) and below (Regime IV) the critical shear rate c for shear-induced homogenization was investigated by a light scattering method. The scaled structure factor was distinctly different in Regimes IV and V, being squared Lorentzian in the former and simple Lorentzian in the latter. This difference implies that concentration fluctuations occur in phase-separated domains in Regime IV and thermal concentration fluctuations in the single-phase state in Regime V. The shear-rate γ dependence of the steady-state structure factors in Regimes IV and V as well as that of the time-evolution of structure due to spinodal decomposition after sudden change in shear-rate from Regime V to Regime IV were found to scale with the effective quench depth ΔT(γ̇) given by ΔT(0) × [1 − (γ̇γ̇c)12]. Thus, shearing affects structure self-assembly as if it pushes down the phase diagram and thus changes time- and spatial scales for the structure formation. Moreover, there was a close similarity between the γ dependence of the characteristic length (ξ⊥)d for the domains in Regime IV and the time dependence of the characteristic wavenumber for the coarsened structure after the cessation of shear flow across γc. In quantitative terms, increasing γ has the effect equivalent to shortening the linear time scale t, i.e., t∼γ−1.