Sorption and diffusion of organic vapors in mixed matrices based on Teflon AF2400 and fumed silica

The transport properties of two organic vapors in mixed matrix membranes based on amorphous Teflon® AF2400 and nanoscale fumed silica have been studied at 25°C. In particular, solubility and diffusivity of normal-butane and normal-pentane have been collected in pure AF2400, pure fumed silica and mixed matrices with weight fractions of filler equal to 0.25 and 0.40. The experimental data show that there is no great effect of the filler content on the solubility, when the latter is evaluated over the total mass of the composite. On the other hand, by estimating the solubility in the polymeric fraction alone of the composite material, the solubility of the vapors increases monotonically with increasing filler content. The infinite dilution diffusivity evaluated from transient sorption data increases significantly with filler content, and the observed increase of diffusivity with penetrant concentration levels off with increasing filler content. The behavior of diffusivity is consistent with previous works and can be explained by the fact that the free volume increases after addition of filler. Such an increase in free volume is estimated by fitting the NELF model to the experimental solubility isotherms of composite matrices. The values of free volume estimated in this way increase with filler content, as expected, and correlate well also with the diffusivity data when a commonly used semiempirical law based on free volume theory is considered.