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.

Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and acceptor-type defects in −c GaN.

. Probing the internal structure of reverse osmosis membranes by positron annihilation spectroscopy: gaining more insight into the transport of water and small solutes. Journal of Membrane Science, Probing the internal structure of reverse osmosis membranes by positron annihilation spectroscopy: gaining more insight into the transport of water and small solutes AbstractReverse osmosis (RO) has been employed as a key separation process in many industrial applications. In recent years, the use of positron annihilation spectroscopy (PAS) including positron annihilation lifetime spectroscopy (PALS) and Doppler broadening of annihilation radiation to characterise the internal structure of the skin layer of thin film composite membranes has renewed research interest for further development and optimisation of the RO process. In this paper, we highlight the need for better understanding of the skin layer internal structure. We review relevant PAS techniques that could provide an unprecedented level of insight to our understanding of the internal structure of the active skin layer of RO membranes. PALS data reported in previous studies revealed that commercially available RO membranes have a mean free-volume hole-radius of 0.20-0.29 nm in the active skin layer. Data corroborated from the literature show a good correlation between the mean free-volume hole-radius of RO membranes and the rejection of boric acid which can be considered as a model small and neutral solute. The data also highlight the need for a comprehensive inter-laboratory study to standardise free-volume hole-radius measurement using PALS. In addition to free-volume holeradius, free-volume fraction and thickness of the active skin layer appear to be important membrane properties governing neutral solute rejection. A roadmap is suggested to enhance the understanding of the transport of small and neutral solutes in RO. This includes integrating PAS with other techniques (e.g. molecular dynamics simulation) to describe the internal structure of RO membranes. Reverse osmosis (RO) has been employed as a key separation process in many industrial 2 applications. In recent years, the use of positron annihilation spectroscopy (PAS) including 3 positron annihilation lifetime spectroscopy (PALS) and Doppler broadening of annihilation 4 radiation to characterise the internal structure of the skin layer of thin film composite 5 membranes has renewed research interest for further development and optimisation of the RO 6 process. In this article, we highlight the need for better understanding of the skin layer 7 internal structure. We review relevant PAS techniques that could provide an unprecedented 8 level of insight to our understanding of the internal structure of the active skin layer of RO 9 membranes. PALS data reported in previous studies revealed that commercially available RO 10 membranes have a mean free-volume hole-radius of 0.20-0.29 nm in the active skin layer. 11Data corroborated from the literature show a good correlation between the mean free-volume 1...

Wood cellulose nanofibril films with sodium carboxylate groups prepared from a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized pulp exhibited an extremely low oxygen permeability of 0.0008 mL μm m(-2) day(-1) kPa(-1) at 0% relative humidity (RH). Positron annihilation lifetime spectroscopy (PALS) was used to determine the pore sizes in wood and tunicate TEMPO-oxidized cellulose nanofibril (TOCN-COONa) films in a vacuum (i.e., at 0% RH). PALS analysis revealed that the pore size of the wood TOCN-COONa films remained nearly at 0.47 nm from the film surface to the interior of the film. This is probably the cause of this high oxygen-barrier properties at 0% RH. The crystalline structure of TOCN-COONa also contributes to the high oxygen-barrier properties of the wood TOCN-COONa films. However, the oxygen permeability of the wood TOCN-COONa films increased to 0.17 mL μm m(-2) day(-1) kPa(-1) at 50% RH, which is one of the shortcomings of hydrophilic TOCN-COONa films.