Variable Monoenergy Slow Positron Beam Research Articles

Fabrication and characterization of nitrogen selective thin-film metallic glass/polyacrylonitrile composite membrane for gas separation

Zr-based (Zr55Cu29Al11Ni5.0) thin-film metallic glass (TFMG) is a new material for gas separation membrane. In this study, we fabricated Zr-based TFMG/polyacrylonitrile (PAN) composite membranes via RF sputtering deposition. The surface morphology, crystallinity, and thermal stability of uncoated and TFMG-coated membranes were then characterized. Variable monoenergy slow positron beam analysis revealed a strong correlation between the pore size and thickness of the TFMG active layers. The resulting membranes were more permeable to N2 than to CO2 or O2 due to weaker interactions with the pore walls and faster diffusion. The best N2 performance was 4288 GPU with ideal selectivity of 1.81 for N2/O2 and 1.95 for N2/CO2. Overall, we demonstrated the considerable potential of TFMG/PAN composite as a membrane for nitrogen selective applications.

Effect of free volume and formation mechanisms of polyamide layers on nanofiltration membrane

Abstract Amine monomers of different chemical structures and multiple reactive functional groups, namely tris(2-aminoethyl)amine (TAEA), triethylenetetramine (TETA), and 1,3-diaminopropane (DAP) was reacted with acyl chloride monomers such as 1,3,5-benzenetricarbonyl chloride (TMC) and suberoyl chloride (SC) to synthesize polyamide thin film composite membranes through interfacial polymerization. The effect of membrane formation mechanisms on nanofiltration was investigated. A variable monoenergy slow positron beam was used to explore the differences in the free volume diameters of polyamide layers composed of different chemical structure. The results revealed that the polymerization degree, free volume diameter, and selective layer thickness of the polyamide layers were strongly correlated with each other. Specifically, a high polymerization degree induced a small free volume diameter and a thin selective layer. The cross-linked TAEA-TMC polyamide layer exhibited a free volume diameter of 3.9 A and was the thinnest (168 nm) among the synthesized membranes, whereas the linear DAP-SC polyamide layer was the thickest (873 nm) with a free volume diameter of 4.7 A. Finally, a stability test on the TAEA-TMC/PAN showed a slight fluctuation during the 36 h filtration, salt rejection maintained stability at around 97% and water flux of 22.3 L m −2 h −1 .

Non-destructive means of probing a composite polyamide membrane for characteristic free volume, void, and chemical composition

Positron annihilation spectroscopy measures free volume in membranes at the sub-nanometer scale (0.1–1 nm).

Surface modification of poly(dimethylsiloxane) by atmospheric pressure high temperature plasma torch to prepare high-performance gas separation membranes

In this study, an atmospheric pressure high temperature plasma torch (APHTPT) was used to modify poly(dimethylsiloxane) (PDMS) membranes in order to form a thin SiOx layer on the membrane surface. The chemical properties of the PDMS and APHTPT-treated membranes, as well as the conversion of polysiloxane, were determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) microanalysis. The microstructure of the membranes as a function of depth was obtained with a variable monoenergy slow positron beam (VMSPB) spectroscopy. Results of XPS and EDX indicated that the conversion of polysiloxane to silica was favored with increasing plasma power. VMSPB data demonstrated that the treated membrane exhibited an asymmetric organic–inorganic hybrid structure. Based on the analysis of the treated membrane, the free-volume size increased with the depth and showed a bi-modal distribution. The gas permeation properties of O2, N2, and CO2 were tested. The change in the applied plasma power was found to have a great effect on the membrane gas separation performance. The membrane selectivity for O2/N2 and CO2/N2 increased from 2.11 to 4.93 and from 9.54 to 17.19, respectively, after treatment at 10kW. The APHTPT-treated PDMS membrane was found to have the advantages of both organic and inorganic membranes, leading to outstanding gas separation performance.

Development of the Asymmetric Microstructure of Carbon Molecular Sieve Membranes as Probed by Positron Annihilation Spectroscopy

The development of the microstructure of carbon molecular sieve membranes (CMSMs) was examined using a variable monoenergy slow positron beam (VMSPB). To vary the structure of the CMSMs fabricated by pyrolyzing a Kapton precursor, different pyrolysis temperatures at a fixed heating rate under vacuum conditions and various periods of holding time at a given pyrolysis temperature were applied. The VMSPB was integrated with the platinum (Pt) capping technique, in which a layer of Pt was sputtered on the membrane surface to eliminate the back diffusion effect of positrons. On the basis of the depth profile obtained, the membranes carbonized at 800 and 900 °C were demonstrated to exhibit asymmetric microstructures at the top layer. The analysis of positron annihilation spectroscopic data of the membranes using a VEPFIT program revealed three structural layers at the most: a dense top layer, a transition layer, and an underlying layer. At prolonged holding time, it was found that the decrease in the gas permeat...

Correlating the microstructure of novel polyamide thin-film composite membranes with ethanol dehydration performances

Polyamide thin-film composite (TFC) membranes were prepared by interfacial polymerization. The purpose is to improve the pervaporation performance of polyamide membranes in separating an aqueous ethanol solution. A novel amine monomer 2,2′-dimethylbenzidine hydrochloride (m-tolidine-H) was reacted with an acyl chloride monomer trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane. The effects of the following interfacial polymerization conditions on the TFC membrane pervaporation performance were investigated: monomer concentration, immersion time of mPAN in aqueous m-tolidine-H solution, and interfacial polymerization (IP) time for reacting m-tolidine-H with TMC. Attenuated total reflectance Fourier transform infrared spectroscopy and scanning electron microscopy were used to characterize the chemical structure and the morphology of the membranes, respectively. To probe the variation in the fine-structure of the polyamide active layer and the free volume depth profile in the TFC membranes, positron annihilation lifetime spectroscopy (PALS) experiments coupled to a variable monoenergy slow positron beam were conducted. The densest portion of the polyamide layer was at the interface of the two immiscible monomer solutions, as detected based on the smallest o-Ps annihilation lifetime at positron incident energies of 1 and 2 keV with 0.05 and 0.5 wt% TMC, respectively. A high pervaporation performance of 2191 g/m2h permeation flux and 99 wt% water content of permeate was delivered by the polyamide TFC membrane that was prepared by immersing the mPAN membrane in a 1.5 wt% aqueous m-tolidine-H solution for 10 s, followed by contacting it with a 0.05 wt% organic TMC solution for 10 s.

Multilayered Poly(vinylidene fluoride) Composite Membranes with Improved Interfacial Compatibility: Correlating Pervaporation Performance with Free Volume Properties

A spin-coating process integrated with an ozone-induced graft polymerization technique was applied in this study. The purpose was to improve the poor interfacial compatibility between a selective layer of poly(2-hydroxyethyl methacrylate) (PHEMA) and the surface of a poly(vinylidene fluoride) (PVDF) substrate. The composite membranes thus fabricated were tested for their pervaporation performance in dehydrating an ethyl acetate/water mixture. Furthermore, the composite membranes were characterized by field emission scanning electron microscopy (FE-SEM) for morphological change observation and by Fourier transform infrared spectroscopy equipped with attenuated total reflectance (ATR-FTIR) for surface chemical composition analysis. Effects of grafting density and spin-coating speed on pervaporation performance were examined. The composite membrane pervaporation performance was elucidated by means of free volume and depth profile data obtained with the use of a variable monoenergy slow positron beam (VMSPB). Results indicated that a smaller free volume was correlated with a higher pervaporation performance of a composite membrane consisting of a selective layer of spin-coated PHEMA on a PHEMA-grafted PVDF substrate (S-PHEMA/PHEMA-g-PVDF). The composite membrane depth profile illustrated that an S-PHEMA layer spin-coated at a higher revolutions per minute (rpm) was thinner and denser than that at a lower rpm.

Solving the Charging Effect in Insulating Materials Probed by a Variable Monoenergy Slow Positron Beam

A variable monoenergy slow positron beam (VMSPB) operating at a high vacuum on insulating materials encounters a problem of significant surface charging effect with time. As a result, positronium formation is inhibited, and the positron annihilation radiation counting rate is reduced; these consequently distorted the experimental positron annihilation and results. To solve such problems, a technique of depositing an ultrathin layer of sputtering noble metals on insulators is developed. We report a successful method of sputtering a few atomic layers of platinum (∼1 nm) on a polyamide membrane to completely remove the charging effect for VMSPB applications in insulators.

Characterizing free volumes and layer structures in polymeric membranes using slow positron annihilation spectroscopy

Positron annihilation spectroscopy coupled with a newly built slow positron beam at National University of Singapore has been used to study the free volume, pore, and depth profile (0 – 10 μm) in cellulose acetate polymeric membrane at the bottom and top sides of membranes for ionic separation in water purification applications. The S and R parameters from Doppler broadening energy of annihilation radiation representing free volumes (0.1–1 nm size) and pores (>1 nm–μm) as a function of depth have been analyzed into multilayers, i.e. skin dense, transition, and porous layers, respectively. The top side of membrane has large free volumes and pores and the bottom side has a skin dense layer, which plays a key role in membrane performance. Positron annihilation lifetime results provide additional information about free-volume size and distribution at the atomic and molecular scale in polymeric membrane systems. Doppler broadening energy and lifetime spectroscopies coupled with a variable mono-energy slow positron beam are sensitive and novel techniques for characterization of polymeric membrane in separation applications.

Characterizing Free Volumes and Layer Structures in Asymmetric Thin-Film Polymeric Membranes in the Wet Condition Using the Variable Monoenergy Slow Positron Beam

The free volume sizes and distributions and multilayer structures in asymmetric thin layer polyamide membranes in the wet state have been first time measured using the variable monoenergy slow positron beam (VMSPB). This paper presents a newly developed method which combines the technique of plasma-enhanced chemical vapor deposition to form a barrier layer for high-vacuum condition to seal the composite polyamide and of interfacial polymerization between 2-aminoethanethiol (AETH) and trimesoyl chloride (TMC) on the surface of modified asymmetric polyacrylonitrile (mPAN) membrane (AETH-TMC/mPAN), under the wet condition encountered during the pervaporation process in which they are in direct contact with the liquid environment provided by the feed solution. During the VMSPB experiments under a high vacuum, our new method maintained the thin-film composite (TFC) membrane in the wet conditions typically experienced in VMSPB experiments wherein it directly contacts the liquid feed environment, which is essent...

Influence of the aminothiol structure on pervaporation dehydration of poly(thiol ester amide) composite membranes

In this study, poly(thiol ester amide) composite membranes were prepared via interfacial polymerization by reacting trimesoyl chloride (TMC) with 4-aminobenzenethiol (ABTH) or 2-aminoethanethiol (AETH) on the surface of a modified polyacrylonitrile (mPAN) membrane (ABTH–TMC/mPAN and AETH–TMC/mPAN). Pervaporation separation of an aqueous ethanol solution with poly(thiol ester amide) composite membranes was investigated as a function of aminothiol structure. To correlate variations in the free volume of poly(thiol ester amide) active layers with pervaporation performance, positron annihilation spectroscopy (PAS) experiments were performed with a variable mono-energy slow positron beam. The affinity of the poly(thiol ester amide) active layer for an aqueous ethanol solution increased upon introduction of a phenyl ring onto the poly(thiol ester amide) backbone. The active layer of AETH–TMC/mPAN composite membranes was thicker and possessed smaller pores ( τ 3) and a lower concentration ( I 3) of free volume than the active layer of ABTH–TMC/mPAN composite membranes did. Solution effects dominated the separation behavior of hydrated ethanol in all poly(thiol ester amide) composite membranes. Furthermore, ABTH–TMC/mPAN composite membranes possessed a higher permeation rate than AETH–TMC/mPAN membranes.

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

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 °C. 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. The variations in the free volume and the thickness of the active polyamide layers of the composite membranes were obtained by positron annihilation spectroscopy (PAS) experiments, in which a variable monoenergy slow-positron beam was used. The data on Doppler broadening Sparameters vs annihilation radiation energy spectra showed that the active MPDASA-TMC layer of the composite membrane had a lower free volume and was thinner than the active MPDA-TMC layer. This result corresponded well with that from the pervaporation performance of TFC polyamide membranes.

Plasma deposition of tetraethoxysilane on polycarbonate membrane for pervaporation of tetrafluoropropanol aqueous solution

An SiO x C y H z /polycarbonate (PC) composite membrane was prepared via plasma deposition of tetraethoxysilane (TEOS) monomer on the surface of the PC substrate for the purpose of applying it in the pervaporation of an 80 wt.% aqueous tetrafluoropropanol (TFP) solution. The effects of plasma power and deposition time on the chemical and physical characteristics of the composite membrane were investigated with Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). To correlate the free volume variation in the plasma deposition layer of the SiO x C y H z /PC composite membrane at different plasma treatment conditions, positron annihilation spectroscopy (PAS) experiments coupled to a variable monoenergy slow positron beam was performed. Under a condition of high-applied power, an SiO x C y H z layer with low porosity could be produced, and it showed good water selectivity in the pervaporation of an 80 wt.% aqueous tetrafluoropropanol (TFP) solution. The best pervaporation performance was obtained with the SiO x C y H z /PC composite membrane at an applied power of 150 W and a deposition time of 30 min as the plasma treatment conditions, giving a permeation rate of 346 g/(m 2 h) and a water concentration in the permeate of 99.3 wt.%.

Characterization, transport and sorption properties of poly(thiol ester amide) thin-film composite pervaporation membranes

The effect of acyl chloride chemical structure on the ethanol aqueous solution dehydration through the poly(thiol ester amide) thin-film composite membrane prepared by reacting 2-aminoethanethiol (AETH) with trimesoyl chloride (TMC) or succinyl chloride (SCC) on the surface of the modified asymmetric polyacrylonitrile (mPAN) membrane was investigated. SEM/EDX, ATR-FTIR and water contact angle were applied to analyze the S element, chemical structure, and hydrophilicity of the poly(thiol ester amide) active layer of the composite membrane. In order to estimate the variation in the free volume of the poly(thiol ester amide) active layer and correlate that with the pervaporation performance, positron annihilation spectroscopy (PAS) experiments were conducted, in which a variable monoenergy slow positron beam was used. Doppler broadening S parameters of annihilation radiation energy spectra showed a significant variation with the acyl chloride chemical structures of the poly(thiol ester amide) active layers. The S parameters of the AETH–TMC/mPAN thin-film composite membrane were found to be lower than those of the AETH–SCC/mPAN thin-film composite membrane. In the ethanol aqueous solution dehydration, the AETH–TMC/mPAN thin-film composite membrane exhibited a lower permeation rate and a higher water concentration in the permeate than the AETH–SCC/mPAN. This is in good agreement with the analysis by positron annihilation spectroscopy. The solution effect dominated the pervaporation separation behavior of the poly(thiol ester amide) thin-film composite membrane with TMC substituting for SCC in the poly(thiol ester amide) active layer. The AETH–TMC/mPAN membrane was found to exhibit superior performance compared with some membranes discussed in the literature.

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 suppo...