Free-volume depth profile of polymeric membranes studied by positron annihilation spectroscopy: Layer structure from interfacial polymerization

The free-volume depth profile of asymmetric polymeric membrane systems prepared by interfacial polymerization is studied using positron annihilation spectroscopy coupled with a variable monoenergy slow positron beam. Significant variations of S, W, and R parameters from the Doppler broadened energy spectra vs positron incident energy up to 30 keV and orthopositronium lifetime and intensity are observed at different doping times of triethylenetetraamine (TETA) reacting with trimesoyl chloride (TMC) in an interfacial polymerization on modified porous polyacrylonitrile (PAN) asymmetric membrane. The positron annihilation data are analyzed in terms of free-volume parameters as a function of depth from the surface to nano- and micrometer regions of asymmetric membranes. A multilayer structure is obtained in polymerized polyamide (PA) on modified PAN membranes (m-PAN): a nanometer scale skin polyamide layer, a nanometer to micrometer scale transition layer from dense to porous m-PAN, and the porous m-PAN support. The results of free-volume parameters and obtained layer thicknesses are compared with the flux (permeability) and water concentration in permeate (selectivity) through the pervaporation separation of 70 wt % 2-propanol aqueous solution. It is found that the water concentration in permeate is mainly controlled by the free-volume properties of skin polyamide and weakly related to the transition layer from the skin to porous m-PAN. The obtained layer structures of asymmetric polymeric membranes are supported by the data obtained by AFM, SEM, and ATR-FTIR.