THE RHEOLOGY OF DEFORMABLE AND THERMORESPONSIVE MICROGEL PARTICLES

This paper describes the preparation and characterization of two monodisperse, hydrophilic polymer latices, with different particle diameters, of poly-N-isopropylacrylamide crosslinked with N,N'-methylenebisacrylamide. The uncrosslinked polymer is water soluble and non-ionic and exhibits a lower consolute solution temperature of about 31-32 degrees C. The latex was prepared via a surfactant-free dispersion polymerization route using water as a solvent at a polymerization temperature of 70 degrees C. The physical properties of the resulting latex were strongly dependent on temperature. At high temperatures (65 degrees C), the particle diameters were 23 and 50 nm, whilst at 25 degrees C they were 130 and 470 nm, This five- to ten-fold increase in the particle diameter corresponds to a 100- to 1000-fold increase in volume. Consequently, the theological properties, both continuous and oscillatory shear, of these systems are strongly dependent on temperature. Volumetric and particle size changes observed by photon correlation spectroscopy and differential scanning calorimetry thermal analysis were in agreement, confirming that the volume phase transition is controlled and influenced by the same factors observed in the swelling/deswelling behaviour of microgels. The viscoelastic properties of these systems and the elasticity of the dispersion decreased as the temperature was increased and the fluid changed from a predominantly elastic to a viscous material. In addition, at lower temperatures, all the studies showed an increase in the storage modulus of the dispersion with increasing particle concentrations. The rheology of the dispersions as a function of the volume fraction of the particles was monitored and the viscosity and shear-thinning increased as volume fraction increased.