Temperature and Shear Rate Dependence of Small Angle Neutron Scattering from Semidilute Polymer Solutions

We use small angle neutron scattering to study the effect of shear on a polymer solution (polystyrene, PS, in dioctyl phthalate, DOP) being in the entangled part of the semidilute regime, in a large range of temperatures and shear rates. The scattering of the induced structure, in the first stages of the flow effect, gives butterfly patterns, i.e., contours constituted of two lobes along the v direction. In later stages, as the shear is increased at constant temperature or the temperature is decreased at constant shear rate, the scattering becomes stronger, having greater dependence on q, and contours tend finally to have an elliptical shape. This means that the objects created by flow are larger. The temperature effect is not only due to the slowing down of the dynamics of the system at lower T, since after correction of the latter, using a time−temperature superposition (determined using oscillatory low strain), the influence of the vicinity of the cloud point on the flow-induced structure remains important. Conversely, steady flow viscosity measured under the same conditions can be corrected using such time−temperature superposition. Some of our results indicate that entanglements are not mandatory to obtain the butterfly effect and that the shear effect can lead to irreversible demixing for solutions closer to the dilute regime.