Particle formation in precipitation polymerization: Continuous precipitation polymerization of acrylic acid in supercritical carbon dioxide

The morphology of the polymer produced during continuous precipitation polymerization of acrylic acid in supercritical carbon dioxide (scCO(2)) varied significantly with reaction conditions. Three different morphologies were observed: a coagulum of primary particles with diameters of 100-200 nm, irregular particles with diameters of 5-20 mu m, and spheres with diameters of 10-100 mu m. To explore the variables that control particle morphology, the glass transition temperature (T-g) of poly(acrylic acid) (PAA) was measured at several CO2 pressures using high-pressure differential scanning calorimetry. Sorption of scCO(2) into PAA also was measured at various temperatures and pressures with a quartz crystal microbalance. Chow's equation described the T-g reduction by CO2 quite accurately. Formation of large spherical particles of PAA was favored when the polymer molecular weight was relatively low and when the polymerization temperature was well above T-g.