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

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, y over dot, above a critical shear rate, y over dot(c,i). The solution was then brought back into a two-phase state by lowering y over dot below a critical shear rate, y over dot(c,d). We previously reported a large hysteresis effect in solutions with off-critical compositions; y over dot(c,i) is always higher than y over dot(c,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 y over dot(c,i). Thus the hysteresis effect was found to be due to a surprisingly slow ordering process at y over dot close to y over dot(c,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, Delta T-c(y over dot), given by Delta T-c(y over dot)proportional to y over dot(1.0+/-0.1) for the off-critical mixtures, rather than Delta T-c(y over dot)proportional to(0.5+/-0.1) found previously for the near critical mixtures. Finally, the incubation time was found to initially increase with y over dot and then decrease with a further increase of y over dot, suggesting that the ordering mechanisms are different in the low and high shear-rate regimes. (C) 1996 American Institute of Physics.