Microcellular microspheres and microballoons by precipitation with a vapour-liquid compressed fluid antisolvent

Abstract

A new type of precipitation with a compressed fluid antisolvent (PCA) is demonstrated for the formation of porous polymeric microspheres and microballoons (hollow microspheres). The antisolvent is composed of pure saturated vapour over saturated liquid CO 2. A polystyrene (PS) in toluene solution is sprayed through a capillary into CO 2 vapour to form droplets, which fall into liquid CO 2 where they are rapidly dried and vitrified. Both the thickness and porosity of the microcellular shells can be controlled by changing the initial solution composition. The thickness is inversely proportional to the initial PS concentration. As the concentration is increased there is a transition from porous microballoons to porous microspheres. The cell sizes and surface areas of the microspheres are approximately 1–20 μm and 3–40 m 2g −1, respectively. The mass transfer pathway may be altered by addition of CO 2 to the polymer solution before spraying, resulting in greater and more uniform porosity. Compared with methanol as an antisolvent, CO 2 produces more porous and spherical microspheres, with 7–14 times faster precipitation.