Particle encapsulation with polymers via in situ polymerization in supercritical CO ₂

Fine Dechlorane (DCR) particles with an average size of 12 μm were successfully encapsulated with poly(methyl methacrylate) (PMMA) and poly(1-vinyl-2-pyrrolidone) (PVP) polymers via in situ dispersion polymerization in supercritical carbon dioxide (scCO ₂). Adjusting the process parameters can control the coating thickness, surface morphology, and degree of particle agglomeration. A uniform thin-film encapsulation of the host particles has been achieved under appropriate operating conditions, which is very difficult to obtain using other methods of supercritical fluid processing. SEM micrographs from our experiments show a variety of coating morphologies. It is found that dispersed polymer particles can deposit and aggregate on a thin layer of polymer coated on DCR and undergo plasticization, coagulation, and fusion into the encapsulating polymer. When the pressure is low or the concentration of stabilizer is high, a coexistence of smooth thin-film coating in contact with uncoagulated polymer particles was found. These observations suggest that polymerization occurs simultaneously through two parallel routes: reaction in dispersed polymer particles and reaction in the polymer domains nucleated on the surface of the host particles, which later develop into a uniform polymer layer. The latter resembles a precipitation polymerization as compared to conventional dispersion polymerization. It is also found that the stabilizer plays an important role in polymer growth and particle coarsening on the surface of the host particles. Without the stabilizer, PMMA could not be coated on the host particles and with a large amount of stabilizer, coagulated rough surface morphologies were observed due to steric repulsion. The effects of various process parameters, such as concentrations of monomer, surfactant stabilizer and initiator, and reaction pressure, are also discussed.