Aqueous heterogeneous radical polymerization of styrene under compressed ethane

Abstract Pressurization of emulsions with compressed gases potentially offers a low-energy route for preparation of miniemulsions that can be polymerized to produce polymeric nanoparticles. Conventional approaches typically require high-energy mixing to generate the miniemulsion and this impedes their industrial application. Compressed ethane is employed in this study to generate oil-in-water (mini)emulsions based on styrene/water and the anionic surfactant Dowfax 8390. The initial, turbid emulsion becomes transparent under compressed ethane at 60 °C and 6 MPa. Aqueous heterogeneous radical polymerization of styrene at these conditions yields nanoparticles with diameters of around 60 nm. However, mechanistic investigations reveal that particle formation proceeds via a classical emulsion polymerization mechanism (aqueous micellar nucleation) rather than direct polymerization of monomer droplets (droplet nucleation). The transparency phenomenon is attributed to matching refractive indices in the continuous and dispersed phases. The Lorentz-Lorenz mixing rule is used to estimate both the refractive index and ethane solubility in the dispersed phase at the transparent condition.