The effect of shear flow on the miscibility behaviour of the model pol
ymer blend polystyrene/poly(vinyl methyl ether) has been investigated
in real time by two-dimensional light scattering and optical microscop
y. Under certain shear and dynamic temperature conditions large 'waves
' have been observed which are aligned either in the direction paralle
l to the applied shear flow or just off it. Two-dimensional light scat
tering patterns from samples under the same conditions show sharp stre
aks aligned in the direction perpendicular to the flow. The evidence p
resented in this work indicates that these 'waves' are unlikely to be
elongated phase-separated domains; instead, they could be caused by fl
uid instabilities.