Shear-flow effects on self-assembly of semidilute solutions of off-critical polymer mixtures: Shear-hysteresis effects

Abstract

We use flow light scattering to study the phase transitions in semidilute solutions of polystyrene and polybutadiene in dioctylphthalate under simple shear flow. The phase-separated solution was brought into a single-phase state by increasing the shear rate, γ̇, above a critical shear rate, γ̇c,i. The solution was then brought back into a two-phase state by lowering γ̇ below a critical shear rate, γ̇c,d. We previously reported a large hysteresis effect in solutions with off-critical compositions; γ̇c,i is always higher than γ̇c,d. Shear-drop experiments were conducted to illuminate the origin of this hysteresis effect. The experimental results showed that at temperatures close to the cloud point temperature the formation of phase separated structures did not occur until up to 22 h after lowering the shear rate below γ̇c,i. Thus the hysteresis effect was found to be due to a surprisingly slow ordering process at γ̇ close to γ̇c,i. The ordering induced by the shear drop needs a much longer time than the homogenization induced by increasing shear. If the time scale of observation is sufficiently long, the hysteresis effect disappears, yielding the drop of cloud point temperature with shear, ΔTc(γ̇), given by ΔTc(γ̇)∝γ̇1.0±0.1 for the off-critical mixtures, rather than ΔTc(γ̇)∝γ̇0.5±0.1 found previously for the near critical mixtures. Finally, the incubation time was found to initially increase with γ̇ and then decrease with a further increase of γ̇, suggesting that the ordering mechanisms are different in the low and high shear-rate regimes.