RHEOLOGICAL AND SMALL-ANGLE NEUTRON-SCATTERING INVESTIGATION OF SHEAR-INDUCED PARTICLE STRUCTURES OF CONCENTRATED POLYMER DISPERSIONS SUBMITTED TO PLANE POISEUILLE AND COUETTE-FLOW

Shear-induced particle structures of rheologically well-characterized concentrated polymer dispersions were investigated by small angle neutron scattering (SANS) in a wide range of shear rates. The dispersions consist of electrostatically stabilized styrene-ethylacrylate-copolymer spheres in glycol or water. Their viscosity functions show pronounced shear thinning and strong shear thickening versus shear rate as measured by various rotational rheometers and by capillary rheometry. A quartz slit die, which could be tilted with regard to the neutron beam, enabled us to achieve wall shear rates as low as 10(-5) s-1 and wall shear stresses up to 10(4) Pa. Part of the measurements were repeated using a Couette shear cell. Spheres of 320 nm mean diameter at a volume concentration of 58.7% in glycol show an amorphous structure at rest. In the range of strong shear thinning the halo intensity becomes anisotropic, the intensity in flow direction being increased, but no long range particle structure shows up. This result shows that drastic viscosity transitions may occur, whereas the structural changes detectable by SANS may remain weak. Nearly monosized 165 nm particles at 52.3% solid in glycol exhibit in the shear thinning regime distinct hexagonal maxima superimposed on the halo and also in a second and third ring indicating the formation of a long range particle structure. With increasing shear rate these maxima disappear and an anisotropy of the halo intensity shows up. An intensity increase in flow direction at small angles is observed in the shear thickening regime. The particle structures observed after stop of flow depend on the preceding shear rate. Similar structural changes are found at 43.4% solid for the 165 nm particles. The results obtained in Couette flow are in qualitative agreement with the slit data but show much sharper intensity maxima. The underlying type of superstructure is compared with nonequilibrium molecular dynamics (NEMD) simulations for a soft sphere model fluid. The intensity patterns extracted from the simulation bear a remarkable similarity to the directly measured SANS data.