Shear-induced structures of concentrated temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel suspensions have been studied employing small angle neutron scattering (rheo-SANS). The interaction potential of swollen PNiPAM microgels could be varied from repulsive at temperatures below the lower critical solution temperature to attractive at temperatures above the lower critical solution temperature. In contrast to the case for suspensions of rigid spheres, the effective volume fraction could be changed by means of temperature while the mass concentration and particle number density were kept constant. Thus, aqueous PNiPAM microgels are interesting model systems with unique colloidal properties. Complementary information about shear-induced changes of both the internal particle structure and the overall microstructural phenomena were obtained from rheo-SANS experiments with PNiPAM microgels with different particle sizes. The shear-induced particle arrangements strongly depended on the particle–particle interaction potential. When the interaction potential was repulsive at temperatures below the lower critical solution temperature, no significant deformation of the swollen PNiPAM particles was observed even at high shear rates. Shear-induced ordering was found at high shear rates resulting in the formation of two-dimensional hexagonal close packed layers that aligned along the flow direction giving rise to shear thinning. The formation of sliding hexagonal close packed layers under shear flow is therefore proposed to be a general property of colloidal dispersion independent of the internal structure of the particle. At temperatures near the lower critical solution temperature, when the particle interaction potential is not yet strongly attractive, shear flow induces the collapse of an individual particle in concentrated suspension at high shear rates. A so-called butterfly scattering pattern indicates the shear-induced enhancement of concentration fluctuations along the flow direction leading to solvent being squeezed out of the particles until phase separation occurs finally.
- angle neutron-scattering
- colloidal dispersions