Evaluation of Nanoparticle Effect on Bubble Nucleation in Polymer Foaming

We present a density functional approach to calculate the free-energy barriers, critical radii, and nucleation rates of bubble nucleation in polystyrene and poly(methyl methacrylate) nanocomposites. In particular, the effects of surface geometry and chemistry of nanoparticles on bubble morphology and cell density have been evaluated with consideration of the local supersaturation of dissolved CO2 molecules and the local subsaturation of polymer chains. It is shown that addition of SiO2 or fluorinated SiO2 particles can improve the nucleation rates up to 4 or 5 orders of magnitude, and the critical radii shrink down to approximately half of the homogeneous nuclei, which are very helpful to fabricate low density foaming materials. The theoretical approach has been tested by the available experimental data and is expected to provide a reasonable explanation for the mechanism of inhomogeneous polymer foaming at the molecular level.