(253a) Analysis of the Solubility of Gases in Mixed Matrix Membranes through the Nelf Model

Mixed matrix membranes have attracted significant attention due to the possibility of combining the benefits of both polymer and inorganic phases. Major attention has been devoted to the study and modelling of the permeability of gases, with rather little attention to the separate contributions of solubility and diffusivity. In this work, we consider the use the Non Equilibrium Thermodynamics for Glassy Polymers, and in particular the NELF model, to study the solubility of different penetrants in inorganic/polymeric mixed matrices. In particular attention will be focused on the evaluation of effect of the inorganic rigid phase on i) the polymer density and free volume, and ii) the polymer swelling coefficient, which are relevant in the case of swelling penetrants. We have considered the systems formed by Teflon AF® 2400 and hydrophobic fumed silica (FS), for which specific sorption data are available. The composite system does not follow closely an additive sorption behavior, and the observed gas solubility is higher than that predicted by the additive model, reasonably due to additional free volume created by insertion of the filler. Solubility data relative to one gas can be used by the NELF model to estimate the variation of the polymer fractional free volume and swelling capacity, that is the slope of the dilation isotherm. Consistently with what one expects, increasing the filler content yield an increase of the polymer fractional free volume and a decrease of its swelling coefficient. The new parameters can then be used to predict the solubility of other gases and pairs of gases in the mixed matrix membrane.