Modeling Diffusion in Foamed Polymer Nanocomposites

Two-way multicomponent diffusion processes in polymeric nanocomposite foams, where the condensed phase is nano-scopically reinforced with impermeable fillers, are investigated. The diffusion process involves simultaneous outward permea-tion of the components of the dispersed gas phase and inward diffusion of atmospheric air. The transient variation in thermal conductivity of foam is used as the macroscopic prop-erty to track the compositional variations of the dispersed gases due to the diffusion process. In the continuum approach adopted, the unsteady-state diffusion process is combined with tortuosity theory. The simulations conducted at ambient temperature reveal distinct regimes of diffusion processes in the nanocomposite foams owing to the reduction in the gas-transport rate induced by nanofillers. Simulations at a higher temperature are also conducted and the predictions are com-pared with experimentally determined thermal conductivities under accelerated diffusion conditions for polyurethane foams reinforced with clay nanoplatelets of varying individual lamellar dimensions. Intermittent measurements of foam thermal con-ductivity are performed while the accelerated diffusion pro-ceeded. The predictions under a celerated diffusion conditions show good agreement with experimentally measured thermal conductivities for nanocomposite foams reinforced with low and medium aspect-ratios fillers. The model shows higher devi-ations for foams with fillers that have a high aspect ratio.