S. Mawson et al., STABILIZED POLYMER MICROPARTICLES BY PRECIPITATION WITH A COMPRESSED FLUID ANTISOLVENT .1. POLY(FLUORO ACRYLATES), Macromolecules, 30(1), 1997, pp. 71-77
Poly(1,1-dihydroperfluorooctyl acrylate) (poly(FOA)) based stabilizers
greatly reduce, and in some cases eliminate, flocculation of amorphou
s poly(methyl methacrylate) (PMMA) and polystyrene (PS) microparticles
formed by precipitation into liquid CO2 at 23 degrees C. The micropar
ticle stabilization mechanism is explained in terms of the stabilizer-
CO2 phase behavior, the spray characteristics, and the interfacial act
ivity of the stabilizer. Compared with the homopolymer poly(FOA), the
diblock copolymer PS-b-poly(FOA) produces smaller and more spherical p
rimary particles (0.1-0.3 mu m) and also prevents flocculation at lowe
r stabilizer concentrations. These differences are due to the greater
interfacial activity of PS-b-poly(FOA). Steric stabilization commences
in the jet on the order of several tenths of milliseconds and continu
es for seconds throughout the precipitator. With the use of a coaxial
nozzle, precipitation is delayed and the stabilizers become even more
effective at preventing flocculation.