In this work, CO2 adsorption at 273.15 K and N2 adsorption at 77 K of mixed matrix membranes has been studied, as a method to directly determine their fractional free volume (FFV). These membranes consist of a continuous phase of copoly(o-hydroxyamide)s (HPA) or copoly(o-hydroxyamide-amide)s (PAA) and a relatively highly porous polymer network filler (PPN1). Both the pure copolymers and the mixed matrix membranes (MMMs) have been analyzed before and after a thermal rearrangement (TR) process. The CO2 adsorption results have allowed characterizing the pore size distribution of the studied membranes in the 3–15 Å range, by using the Non-Local Density Functional Theory (NLDFT). Whereas the N2 adsorption has allowed determining the pore size distributions in the range between 20 and 250 Å. The experimental determination of the pore volume and the density allows the direct calculation of the membranes’ free volume fractions, which were in good agreement with the most usual FFV evaluation methods. In addition, part of the pore volume detected by N2 adsorption was associated with defects and poor integration of the membrane components. This correction has allowed us to make a new evaluation of the density of these materials.
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