Osmotic de-swelling of polystyrene microgel particles

The effects of added free polymer on the swelling and stability of polystyrene microgel particles dispersed in ethylbenzene have been investigated. The microgel particles were characterized by photon correlation spectroscopy (PCS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) measurements. TEM measurements showed that particles deposited on grids from ethylbenzene (a good solvent for polystyrene) were invariably larger than the same particles deposited from water. SEM data revealed that removal of ethylbenzene favoured the collapse of the particles to a deformed oblate spheroid shape. The stability of the microgel particles in the presence of free (i.e. non-adsorbing) polystyrene was investigated by turbidity - wavelength measurements, and the stability thresholds established. Osmotic de-swelling of the swollen particles was observed in the presence of free polystyrene using PCS. The extent of de-swelling increased with an increase of the molecular weight and concentration of the free polystyrene. A thermodynamic model: based on Flory's theory of network swelling, is presented which, in the presence of excluded free polystyrene, provides a qualitative description of the deswelling of the microgel particles. The results are also compared to the predictions of scaling theory.