An immiscible polymer blend containing 1 % polyisobutylene (PIB) in a polydimethylsiloxane (PDMS) matrix was subjected to shear in a combined rheo-SALS setup. The immiscible blend consists either of PIB droplets or PIB domains in PDMS. The PIB is an elastomer, produced by cationic vinyl polymerization of isobutylene and is often used in glues and sealing compounds.
The flow behavior of the polymer blend, subjected to logarithmic shear ramp from 1 s-1 to 200 s-1, is characteristic for unlinked polymers. For shear rates up to 20 s-1, the sample shows zero-shear viscosity. The shear force leads to orientation and deformation of the PIB domains but is counteracted by interfacial tension forces, which try to keep the droplets spherical, i.e the viscosity doesn’t change.
The scattering patterns obtained at rest and at low shear rates (images a and b in the figure below) show an isotropic angular distribution of the scattered intensity indicating a spherical shape of the PIB droplets in the PDMS matrix.
Increasing the shear rate, the shear forces across the droplets dominate the interfacial tension forces; the droplets start to deform and the viscosity decreases. Following the orientation and deformation of the PIB domains, the scattering patterns begin to change from a circular to an elliptical shape.
The higher the shear rate (images d and e), the more oriented and deformed the PIB domains become, leading to higher anisotropy of the scattering patterns.
SALS is an "inverse space" method: Large structures scatter light at small angles, whereas light scattering on small dimensions causes large scattering angles. Therefore, the deformation of the PIB domains in a shear flow direction results in light scattered at smaller angles in the shear direction, and at larger angles perpendicular to the shear direction. The light-scattering patterns change from circular to ellipsoid, and the ellipse is perpendicular to the shear direction.