The rheology of aqueous microgel dispersions

Aqueous dispersions of particles of polyN-isopropylacrylamide, cross-linked with bis NN' acrylamide have been prepared and characterised. Electron and optical microscopy as well as dynamic light scattering have been utilised to measure the temperature dependence of size. The polymer has an LCST of similar to 32 degrees C and so the particles were highly swollen at 15 degrees C and in a compact form at 45 degrees C. Capillary viscometry was used to study the effect of temperature and electrolyte on the viscosity of dilute solutions. In most cases the intrinsic viscosity was less than the value for hard spheres indicating that flow was occurring inside the particles. Oscillatory rheometry on concentrated dispersions indicated that the systems behaved as elastic solids and the high frequency elasticity was recorded. A model is suggested in which the compliance of the swollen gel particles reduces the electrostatic repulsion by allowing the particles to deform. The model was capable of giving a good fit to the experimental observations.