Effect of solvent strength and operating pressure on the formation of submicrometer polymer particles in supercritical microjets

A supercritical antisolvent (SAS) process is described which utilizes a mixture of thermodynamically good and poor polymer solvents and micronozzles. Experiments were conducted on polyvinylpyrrolidone (PVP) solutions in a mixture of dichloromethane (DCM) (a good solvent) and acetone (a poor solvent). Decreasing the nozzle diameter and the fluid velocity were shown to favor the disintegration of supercritical jets into drops. Mass transport of CO2 into, and solvents out of, the falling supercritical drops, rather than mass transport during jet breakup, are found to control the particle formation. Varying the acetone content of the solvent, the nozzle diameter and the jet velocity are demonstrated to provide an efficient method to decrease the particle diameter to several tens of nanometers and smooth their surface irregularities. The proposed method is expected to be applicable to a wide variety of polymers.