Phase transition for blends of polystyrene (PS) (Mw=800, Mw/Mn=1.3)/polyisobutylene (PIB) (Mw=500) were studied in the quiescent state and under shear flow. The cloud points of blends were determined by turbidity measurements at a fixed cooling rate of 0.5 °C/min. The phase diagram of PS/PIB blends shows a UCST behavior with a bimodal miscibility gap at low temperature. The shear effects on the phase transition behavior of PS/PIB blend was investigated by a flow‐turbidity apparatus which was constructed by modifying a Rheometrics Mechanical Spectrometer (RMS). No shear‐induced phase separation was observed in the one‐phase region. For temperatures within the two‐phase region, the transmitted light intensity increased abruptly when the shear was applied. The intensities decreased and approached to the initial values when the shear was removed. Several possible explanations will be discussed to explain these results. The one that appears to be most valid is that of shear‐induced phase mixing.

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