STABILIZED POLYMER MICROPARTICLES BY PRECIPITATION WITH A COMPRESSED FLUID ANTISOLVENT .2. POLY(PROPYLENE OXIDE)-BASED AND POLY(BUTYLENE OXIDE)-BASED COPOLYMERS

Block copolymers containing either poly(propylene oxide) (PPO) or poly (butylene oxide) (PBO) stabilizer group(s) and a poly(ethylene oxide) (PEO) anchor group prevent flocculation of amorphous poly(methyl metha crylate) (PMMA) microparticles formed by spraying PMMA solutions into flowing liquid CO2 at 23 degrees C. When dissolved PPO-PEO-PPO tribloc k and PBO-PEO diblock copolymers are introduced with the CO2 feed stre am, 0.1-0.5 mu m primary PMMA particles are produced. However, larger, and in some cases more spherical microparticles (0.5-2.0 mu m) are fo rmed when these stabilizers are fed via the polymer solution phase, fo r the same overall quantity of stabilizer. The effectiveness of the st abilizer is described in terms of its concentration and how it partiti ons between the dispersed phase, the interface, and the CO2 phase. In many cases stabilizers with only moderate solubilities in CO2 are more effective than those with higher or lower solubilities. When the stab ilizer is introduced with the solution phase, it does not have to be s oluble in CO2 to prevent flocculation. The latex particle size, stabil ity, critical flocculation density, and reversibility of flocculation have been measured in-situ by turbidimetry to understand the mechanism of steric stabilization in supercritical fluids. The size of the prim ary particles in the product determined by scanning electron microscop y is consistent with in-situ measurements of particle size by turbidim etry.