To investigate the polyamide thin-film composite membranes structure in regard to the variation in the free volume in thin-film composite membrane polyamide active layers, positron annihilation spectroscopy (PAS) experiments using a slow positron beam were performed. The correlation between the pervaporation performance and polyamide active layer free volume explored using the slow positron beam technique was also investigated. The composite membranes were prepared via the interfacial polymerization reaction between ethylenediamine (EDA) and trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane (EDA−TMC/mPAN). The variations in the S parameter at positron incident energies of 0.5−2.5 keV indicate a decrease in the S parameter when the concentration of EDA in aqueous solution increased from 0.5 to 10 wt %. The positron annihilation took place at the surface of the polyamide active layer for the variations at low S parameters and high W parameters (positron incident energy <0.5 keV). The positron annihilation occurred at the free volume in the polyamide active layer for the variations at high S parameters and low W parameters (positron incident energy: 0.5−2.5 keV). From the o-Ps lifetime results, the density of EDA−TMC polyamide active layer decreases along the thin-film growth direction from the aqueous phase toward the organic phase in the range of 1−2 keV positron incident energy. From the positron annihilation lifetime spectroscopy, the variation in the free volume in the EDA−TMC active layer showed a good correlation with the pervaporation performance.

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