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The shear-induced concentration fluctuations in semidilute solutions comprised of ultrahigh molecular weight polyethylene (UHMWPE) as a solute and paraffin as a solvent were studied by shearsmall-angle light scattering and shear-microscopy. In steady-state shear flow, a unique anisotropic scattering pattern was found to appear when the shear rate y is increased above a critical shear rate yc. This anisotropic scattering pattern was found to be essentially identical to the "butterfly pattern" observed in high molecular weight polystyrene solutions in dioctyl phthalate. This butterfly pattern, found at temperatures higher than the equilibrium melting temperature of UHMWPE, suggests that the shearinduced concentration fluctuations or phase separation occurs in the homologous mixtures of UHMWPE and paraffin having characteristics of athermal solutions.

The shear‐induced concentration fluctuations or phase separation of a semidilute solution comprised of polystyrene (PS) as a solute and dioctylphthalate (DOP) as a solvent (PS/DOP) was investigated by using real‐time and in‐situ shear‐small‐angle light scattering and shear‐phase‐contrast optical microscopy. When a transient shear flow with a fixed shear rate γ greater than a critical value γC was imposed on the solution, a unique anisotropic scattering pattern was observed some time after onset of shear. This pattern was found to be identical to the “butterfly pattern” previously reported for the same solutions under steady shear flow with γC. When the shear flow was ceased before the scattered intensity reached a steady state, the scattered intensity rapidly increased toward a maximum intensity, and then decreased toward the intensity of the quiescent solution with time. From the phase‐contrast microscopy, this immediate increment of the scattered intensity after the shear cessation was found to arise from the increment in amplitude of the concentration fluctuations along flow direction. The characteristic length scale of the fluctuations was about 2.5 μm in this experiment, almost independent of the shear rate imposed on the solution.

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