A new apparatus which makes it possible to measure flow small-angle light scattering ("flow-SALS") and to observe the development of the supramolecular structure by an optical microscope ("shear microscopy") has been developed in our laboratory. It simultaneously provides information on reciprocal space from SALS and on real space from optical microscopy. Therefore, the development of the supramolecular structure in systems such as polymer solutions and polymer mixtures as well as liquid crystals and colloidal systems under shear can be investigated in depth. Shear-induced phase separation and shear-induced homogenization of some polymer solution systems were investigated using this apparatus.

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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, ␥ , above a critical shear rate, ␥ c,i . The solution was then brought back into a two-phase state by lowering ␥ below a critical shear rate, ␥ c,d . We previously reported a large hysteresis effect in solutions with off-critical compositions; ␥ c,i is always higher than ␥ 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 ␥ c,i . Thus the hysteresis effect was found to be due to a surprisingly slow ordering process at ␥ close to ␥ 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, ⌬T c (␥ ), given by ⌬T c (␥ )ϰ␥ 1.0Ϯ0.1 for the off-critical mixtures, rather than ⌬T c (␥ )ϰ␥ 0.5Ϯ0.1 found previously for the near critical mixtures. Finally, the incubation time was found to initially increase with ␥ and then decrease with a further increase of ␥ , suggesting that the ordering mechanisms are different in the low and high shear-rate regimes.