The viscosity of dilute poly(N-isopropylacrylamide) dispersions

The viscosity of dilute aqueous dispersions of poly(N-isopropylacrylamide) microgel particles is measured by capillary viscometry. The viscosity increases with particle mass fraction and on reducing temperature, particularly below the volume phase transition temperature (VPTT) of 32 degrees C. Converting the particle loading to volume fraction via the change in hydrodynamic size, the slope of the viscosity-volume fraction graph exhibits an increasing value beyond that for the equivalent effective hard-sphere size as the particles swell. This increase is due to the porosity of the particles. Two microgel samples of different collapsed size (124 and 59 nm at 50 degrees C) are investigated and the deviation from hard-sphere behavior is greater for the smaller particles. (C) 2011 Elsevier Inc. All rights reserved.