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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To improve the pervaporation performance of polyamide membrane, thin-film composite (TFC) polyamide membranes were prepared through the interfacial polymerization between m-phenylenediamine (MPDA) or 1,3-phenylenediamine-4-sulfonic acid (MPDASA) and trimesoyl chloride (TMC) on the surface of the modified asymmetric polyacrylonitrile (mPAN) membrane and applied in the pervaporation separation of 70 weight % aqueous isopropanol solutions at 25 1C. The variations in the free volume and the thickness of the active polyamide layer of composite membrane were obtained by positron annihilation spectroscopy (PAS) experiments, in which a variable monoenergy slow-positron beam was used. FTIR-ATR spectroscopy, XPS, scanning electron microscopy, AFM and water contact angle measurements were applied to analyze chemical structures, surface elemental compositions, morphologies, surface roughness and hydrophilicity of the active polyamide layer of composite membrane. From the result of PAS experiments, the S parameter (corresponding to the free volume size and amount) and the thickness of the active polyamide MPDASA-TMC/mPAN layer were found to be lower than those of the active MPDA-TMC/mPAN layer. In the aqueous isopropanol solution dehydration, the MPDASA-TMC/mPAN membrane exhibited a higher permeation rate than but maintained the same water concentration in the permeate as did the MPDA-TMC/mPAN membrane. This is in good agreement with the analysis by PAS. Polymer Journal (2010) 42, 242-248; doi:10.1038/pj.2009.334; published online 13 January 2010Keywords: interfacial polymerization; PAS; pervaporation; polyamide; TFC membrane INTRODUCTION Polyamides have been studied as suitable membrane materials because of their high thermal stability, excellent mechanical strength and high resistance to organic solvents. Alcohol dehydration is one of the important areas in pervaporation separation processes. In pervaporation separation processes, polyamides show high selectivity in alcohol dehydration for a wide range of water concentrations. 1-5 Their high selectivity stems from the concept of high diffusion selectivity. This is because they have low free volume/low mobility and the size of water molecules is smaller than that of alcohol. However, they show low permeation rate because of their very low free volume and low water solubility. To increase the permeation rate of polyamide membranes without sacrificing selectivity, the membrane morphology must be converted from a dense thick film into an asymmetric or composite

In this study, a series of polycarbonate/polyacrylonitrile (PC/PAN) composite membrane has been successfully fabricated by vapor-induced phase separation and in-situ casting process. The effect of the casting solution concentration on the membrane morphology was investigated. The morphology of the PC/PAN composite membranes was characterized by SEM. The effect of the skin layer thickness on the water flux was measured at a fixed pressure. Compared with the traditional asymmetric PAN membrane with a dense skin layer and porous support layer, the PC/PAN composite membranes prepared by in-situ casting process effectively prevent the skin layer formation, resulting in the water flux increases.

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