Directed self-assembly of nanoparticles at the polymer surface by highly compressible supercritical carbon dioxide

We report a versatile route for self-assembly of polymer-soluble nanoparticles at the polymer surface using highly compressible supercritical carbon dioxide (scCO ₂). Polystyrene and poly(methyl methacrylate)-based nanocomposite thin films with functionalized polyhedral oligomeric silsesquioxane and phenyl C ₆₁ butyric acid methyl ester nanoparticles were prepared on Si substrates and exposed to scCO ₂ at different pressures under the isothermal condition of 36 °C. The resultant structures could be then preserved by the vitrification process of the glassy polymers via quick pressure quench to atmospheric pressure and subsequently characterized by using various surface sensitive experimental techniques in air. We found that the surface segregation of these nanoparticles is induced in the close vicinity of P = 8.2 MPa where the excess absorption of the fluid into the polymers maximizes. However, when the film thickness becomes less than about 4R _g thick (where R _g is the radius of polymer gyration), the uniform dispersion of the nanoparticles is favorable instead even at the same CO ₂ conditions. We clarify that the phase transition is correlated with the emergence of a concentration gradient of the fluid at the polymer/CO ₂ interface and is a general phenomenon for different polymer-nanoparticle interactions.