The Yield Stress of a Soft and Water Swellable Microgel Aqueous Suspension in Semi-Dilute Regime

The rheological properties and the morphology of a water swellable microgel developed as a viscosity thickener for cosmetics were studied. The water swellable microgel was synthesized by a novel inversed microemulsion polymerization technique. The apparent yield stress appeared in semi-dilute regime, at around twice the overlapped concentration of the microgel and the value of the apparent yield stress increased with the concentration of the microgel. Although the deformation behavior of the microgel suspension at very small shear stress looked like elastic deformation from its flowcurve, it was revealed the suspension flowed in such case by creep measurement. It is implied that there would be a structure causing quite slow relaxation by creep and dynamic modulus measurements. The morphology of the microgel through the freeze-fracture TEM showed the microgel was swollen as a spherical shape and closely packed at a concentration at which the apparent yield stress appeared. One possible mechanism of the appearance of yield stress is the friction between the layers where the microgel particles arrange in a random closed packing state.