Takeji Hashimoto

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(γ) given by ΔT(0) × [1 − (γγc)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.

Shear-induced homogenization of semidilute solution of polymer mixture. II: Composition dependence

Shear‐induced homogenization of semidilute solutions of polybutadiene (PB) and polystyrene (PS) with dioctylphthalate as a solvent was studied as a function of temperature T, polymer compositions at a fixed total polymer concentration of 3.3 wt%. The drop of homogenization temperature ΔTc(S) as measured in the increasing and decreasing cycles of shear rate S (s−1) was found to be given by ΔTc(S)/Tc(0)=(1.3±0.1)×10−3 S0.5±0.02 for all the mixtures with PS/PB compositions 30/70, 40/60, 50/50, 60/40, and 70/30 where ΔTc(S)=Tc(0)−Tc(S), Tc(S) is the homogenization temperature at a given S, and Tc(0) is the cloud point at S=0. A remarkable hysteresis effect was discovered on the shear‐induced homogenization for off‐critical mixtures with 20/80 and 80/20 compositions: the critical shear rates Sc,i and Sc,d for the homogenizations observed, respectively, in the increasing and decreasing cycles of S satisfy Sc,i≥Sc,d. The difference δSc≡Sc,i−Sc,d is greater at a lower ΔT(0)≡Tc(0)−T for a given mixture and at more...