(541b) Sorption and Diffusion of Organic Vapors in Mixed Matrices Based on Teflon AF2400 and Fumed Silica

The transport properties of two model organic vapors in mixed matrix membranes based on amorphous Teflon® AF2400 and nanoscale fumed silica as filler have been studied at 25°C. Solubility and diffusivity of normal butane and normal pentane have been collected on pure AF2400, pure fumed silica and filled matrices with weight fractions of filler 0.25 and 0.40. The experimental data show that there is no great effect of the filler content of the solubility, when the latter is evaluated over the total mass of the composite. The infinite dilution diffusivity, on the other hand, increases significantly with filler content, and the observed increase of diffusivity with penetrant concentration levels off with increasing filler content. The behavior of diffusivity has been already observed in previous works on the same materials and can be explained by the fact that the free volume increases after addition of filler. Such an increase in free volume may be estimated either directly with density measurements, or indirectly, considering other binary data, such as the solubility. To this aim, a semi-additive model was considered, in which the filler sorption capacity is assumed to remain unchanged, while the polymer sorption capacity increases after the addition of the filler, due to higher free volume that is also introduced. The solubility in the polymeric phase alone can then be obtained at different filler volume fractions. The variation of polymer density with filler content may be calculated by fitting the NELF model on the solubility isotherms, thus obtaining satisfactory results. Noteworthy, the corresponding fractional free volume which is obtained from the solubility data correlates well also the observed dependence of diffusivity versus filler content. The experimental data also allow to estimate permeability and selectivity, and to calculate the ideal solubility and diffusivity selectivity for the penetrants considered. The permeability increases monotonously with filler content, while the selectivity slightly decreases.