Flow-induced anisotropy in mixtures of associative polymers and latex particles

The effect of associative polymers on the structure and rheological behavio r of colloidal suspensions is discussed. Adding associative polymer is know n to increase the viscosity of the suspensions. At high shear rates the inc rease is close to what could be expected on the basis of the hydrodynamic e ffects of the added polymer. At low shear rates the viscosity increases muc h more. Small-angle light scattering (SALS) during flow is used here to inv estigate the underlying structural mechanisms. The SALS patterns indicate t hat the associative polymer changes the particulate structure: characterist ic butterfly patterns appear even at relatively low particle volume fractio ns. They are not present in the suspensions without associative polymer. Th e patterns indicate that fluctuations in particle concentration are more pr onounced in the flow direction than in the vorticity direction and that ani sotropic particulate structures with an orientation along the vorticity dir ection develop. The evolution of their characteristic length scale during h ow has been followed over time. Changing the hydrophilic part of the polyme r from polyacrylamide to polyacrylic acid induces stronger associative inte ractions. In the suspensions this results in a reduction of the relative vi scosity rather than an increase, The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspens ions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competi tion between adsorption of the associative polymer on the particles with th e intermolecular associations between the polymer chains seems to be respon sible for the observed differences. (C) 2000 Academic Press.