Thickness Effects on Morphology and Gas Permeability of Polystyrene-Grafted-Silica in a Polystyrene Matrix

Polymer-grafted-nanoparticles in a polymer matrix can self-assemble into anisotropic morphologies that can be controlled by varying system parameters, such as grafting density and film thickness. The relationships between film thickness, morphology, and gas permeability of these systems, however, are not well-understood. We present the film thickness effects on gas transport properties and the surface and bulk morphologies of polystyrene-grafted-silica (PS-g-SiO₂) in a polystyrene (PS) matrix. Our thinnest films (2.5 μm) of the polymer nanocomposite exhibited a 90% reduction in CO₂ permeability relative to that of PS, while those of bulk films (100 μm) increased by 200%. Morphology characterization suggests lightly grafted PS-g-SiO₂ components are conducive to diffusing to the surface; this effect is more pronounced in thinner films, where distance to the surface is much shorter than bulk films. Nanoparticles at the surface of thinner films would then rearrange into structures that disallow gas transport. Conversely, the enhanced relative permeability in bulk films is likely due to the decreased interfacial density between the nanoparticle and the polymer chains, allowing gas molecules to more easily flow through the film.