Rheology of a temperature sensitive core-shell latex

Rheological properties of concentrated colloidal dispersions of a core-shell latex with a poly(styrene) (PS) core and a cross-linked, temperature sensitive poly(N-isopropylacrylamide) (PNiPAM) shell were investigated as function of temperature and concentration. The core-shell particles which have been studied previously by small-angle X-ray scattering were colloidally stable above the volume transition temperature of PNiPAM. The hydrodynamic shell thickness decreased with temperature, which results in a strong dependence of shear viscosity, modulus, and yield stress on temperature. At temperatures below 30 degrees C, the zero-shear viscosity superposed a master curve when plotted versus the effective volume fraction and resembled hard sphere behavior. The plateau modulus displayed a power law concentration dependence similar to that for microgels. Thus by chemically fixing a thermosensitive PNiPAM shell onto the surface of a charge-stabilized PS core, a latex was obtained the rheological properties of which can be controlled by temperature and which is stable against flocculation even at elevated temperatures.