Anomalous viscosity jump during the volume phase transition of poly( N-isopropylacrylamide) particles

Abstract

Dilute dispersions of cross-linked poly( N-isopropylacrylamide) (PNIPAM) microgels are studied by viscosity at low stress, dynamic light scattering and microelectrophoresis. The rheological measurements at low stress demonstrate that upon heating through the volume phase transition temperature (VPTT) there is a stress-dependent increase in viscosity that passes through a maximum before falling close to the value of water. On cooling only a much smaller viscosity rise is seen in the region of the VPTT. The particle size, measured in the absence of shear, decreases smoothly on increase in temperature as the microgel transitions through the VPTT from a swollen coil to a collapsed, globular latex. The magnitude of electrophoretic mobility of the swollen coil is low but of the collapsed globule is high, consistent with a charge-stabilised polymer latex. The collapse in particle size occurs at a lower temperature than the increase in magnitude of electrophoretic mobility. The viscosity rise on heating occurs in the range of temperature over which the hydrodynamic particle size falls, then the viscosity falls as the magnitude of the electrophoretic mobility increases. We propose that the viscosity rise is a shear-induced association of microgel particles via inter-chain association and the findings are consistent with a two-stage collapse during the coil–globule transition.