Shear effect on the phase behaviour and morphology in oligomer blend of polystyrene/poly(methyl methacrylate)

The phase behaviour of an oligomer mixture of polystyrene/poly(methyl methacrylate) (PS/PMMA), which shows an upper critical solution temperature (UCST) type phase diagram, has been investigated under simple shear flow. The cloud points were strongly affected by the values of the applied shear rate. The phase diagrams under different values of shear rate, indicated shear-induced mixing for all measured compositions, and the magnitudes of the depression of the cloud points under the effect of shear rate were found to be composition dependent. The dramatic decrease of the homogenization temperature was investigated as a function of shear rate (γ̇), and the normalised shift in the cloud point |ΔT(γ̇)/T(0)| versus γ̇ was also studied and compared with that of simple liquid mixtures and polymer solutions, as well as high molecular weight polymer blends. The cloud points of this oligomer mixture showed high sensitivity to change under the effect of shear rate; a much higher sensitivity than both simple liquid mixtures and polymer solutions, on the other hand, slightly less sensitivity than the high molecular weight polymer blend (PMMA/poly(styrene-co-acrylonitrile)). The morphology of PS/PMMA=30/70 blend was also studied under different values of shear rate at 35°C below its quiescent cloud point, using the transmission electron microscopy (TEM). The results showed that the shear-induced mixing occurred at a high critical shear rate value (34 s−1), below which the dispersed PS-rich domains were slightly elongated and broken-up into very small domain sizes with very good particle distribution.