Free-Volume Depth Profile of Polymeric Membranes Studied by Positron Annihilation Spectroscopy:  Layer Structure from Interfacial Polymerization

Abstract

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.

Designing asymmetric multilayered membrane filters with improved performance

“…Currently, experimentally it is difficult to observe the internal particle trapping within a filter during the filtration process directly, with limited techniques only now beginning to emerge, such as positron annihilation spectroscopy [10,11]. Instead, deductions are made only after dissecting the porous medium once filtration has ceased.…”

Teflon AF2400/Ultem composite hollow fiber membranes for alcohol dehydration by high‐temperature vapor permeation

“…Figure 7 shows S parameter curves of HFs as a function of positron incident energy, which determines the penetration depth of positrons into the membrane surface. 38,40,48 For all the HF membranes, the S parameter near the surface is observed to decrease rapidly with increasing positron incident energy, which is typical due to the back diffusion and scattering of positronium. With an increase in positron incident energy, the S parameter quickly reaches the minimum value, which can be interpreted as the dense selective layer of the HF membranes.…”

Interfacially polymerized thin-film composite polyamide membrane: positron annihilation spectroscopic study, characterization and pervaporation performance

“…The S parameter value increases by increasing the relative contribution of low momentum electrons to the positron annihilation in the open volume voids. [18][19][24][25] DBES can, therefore, be applied to measure the variation in free volume in a composite membrane. The multilayer structure of a composite membrane can be analyzed from the S parameter variation (free volume variation).…”

“…[20][21][22][23] However, there are few studies on polymeric TFC pervaporation membranes investigated with PAS. [18][19][24][25] In this regard, to probe the variation in the free volume in the active polyamide layer, the TFC polyamide membrane was examined by conducting PAS experiments, in which a variable monoenergy slow-positron beam was used.…”