Doppler Broadening Energy Spectroscopy Research Articles

Ultrathin polyamide nanofilm with an asymmetrical structure: A novel strategy to boost the permeance of reverse osmosis membranes

Ultrathin polyamide (PA) nanofilm based separation membranes have attracted drastically increasing attention recently. Typically, PA nanofilms with the thickness of around tens of nanometers are supported by a PSF substrate membrane which provides mechanical support. However, the low surface porosity of the PSF substrate membrane has required the transverse diffusion (parallel to the membrane plane) of water molecules in the nanofilm, which causes much longer mean diffusion paths compared to the thickness of the nanofilm. In this study, we address this problem by introducing a much looser polypiperazinamide (PPA) interlayer in between the PA nanofilm and the PSF support membrane, with the PPA nanofilm serving as a low resistance region for water molecules. A dual interfacial polymerization strategy was applied to create an asymmetrical ultrathin polyamide selective layer comprised of a high permeability loose PPA sublayer and a high selectivity dense PA top layer. Quartz crystal microbalance with dissipation (QCMD) techniques and Doppler broadening energy spectroscopy (DBES) were applied to study the asymmetry structure of the ultrathin polyamide nanofilms. Compared with the home-made traditional ultrathin polyamide (uPA) membrane, the asymmetrical ultrathin polyamide (A-uPA) membrane has 2–2.5 folds higher permeability while maintaining higher salt rejection. Our study demonstrates that the asymmetrical structure can significantly enhance the flux for ultrathin polyamide membranes. Further, the impact of the structure of the top layer and the sublayer on the membrane separation performance was explored by tuning the recipe of the PA top layer and the PPA sublayer.

Thin film composite nanofiltration membrane prepared by the interfacial polymerization of 1,2,4,5-benzene tetracarbonyl chloride on the mixed amines cross-linked poly(ether imide) support

A novel thin film composite (TFC) nanofiltration (NF) membrane was prepared by the interfacial polymerization of 1,2,4,5-benzene tetracarbonyl chloride (BTC) on the cross-linked poly(ether imide) (PEI) support via the mixed amines of the linear ethylenediamine (EDA) and the hyperbranched poly(ethyleneimine) (HPEI). Characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, Raman mapping, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), etc.; it is indicated that the TFC NF membrane is synthesized through the interfacial polymerization between BTC and the terminal amine groups on the EDA-HPEI cross-linked PEI support. The optimal TFC NF membrane exhibited the methanol permeance of 14.2Lm−2h−1MPa−1 and a RB rejection of 100%. After the solvent activation of dimethylformamide (DMF) and dimethylsulfoxide (DMSO), the TFC NF membrane exhibited the methanol permeance of 30.4Lm−2h−1MPa−1 and 51.5Lm−2h−1MPa−1 with the RB rejection of 90.4% and 74.4%, respectively. Characterized by ATR-FTIR, SEM and Doppler broadening energy spectroscopy (DBES), the effects of solvent activation via DMF and DMSO respectively were investigated on the PEI-based TFC NF membranes.

Pore structure characterization of supported polycrystalline zeolite membranes by positron annihilation spectroscopy

It is important to characterize the pore structure of supported polycrystalline zeolite membranes but it has remained a major challenge in studying microporous inorganic membranes. This paper reports the use of positron annihilation spectroscopy including positron annihilation lifetime spectroscopy and Doppler broadening energy spectroscopy to non-destructively characterize the pore structure of four MFI zeolite membranes of different microstructures on alumina supports. Positron annihilation lifetime spectroscopy analysis reveals a bimodal pore structure consisting of intracrystalline zeolitic micropores of around 0.6nm in diameter and irregular intercrystalline micropores of 1.4−1.8nm in size for the four MFI zeolite membranes studied. Distributions of the micropores along the membrane thickness direction can be inferred from Doppler broadening energy spectroscopy results, illustrating development of intercrystalline gaps during the growth of the zeolite layer. The amount and size of the intercrystalline micropores of the zeolite membranes vary with the synthesis method, and are the smallest for the randomly oriented MFI zeolite membrane synthesized without template and the largest for the c-oriented MFI zeolite membrane synthesized with the template. The c-oriented membrane has an asymmetrical distribution of intercrystalline pores along the film growth direction as compared to the uniform distribution of the bimodal structure for the other three membranes. The pore structure data obtained by positron annihilation spectroscopy are consistent with the xylene isomer separation performance of these membranes.

Depth Profile of Chemical Composition and Free Volume of Polyurethane-Urea/Clay Nanocomposite

Depth profile of subsurface chemical composition and free volume in segmented polyurethane-urea/clay nanocomposites was studied by angle resolved X-ray photoelectron spectroscopy (ARXPS) and Doppler broadening energy spectroscopy (DBES) using slow positron beam. The ARXPS studies revealed increasing N/C atomic ratio (hard segment to soft segment ratio) at any given depth for the clay loaded samples compared to the neat polymer. DBES study revealed significant microstructure modification with clay loading. Self segregation of hard and soft segments in neat polymer and their interspersing with clay loading was observed from DBES measurements.

Applications of Positron Annihilation Spectroscopy to Life Science

Positron annihilation spectroscopy (PAS) is a novel method that can provide molecular-level information about complex biological and macromolecular structure in a manner which is different, but complementary, to conventional medical and biochemical research methodology. Positron annihilation lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES), coupled with a slow positron beam have been extensively applied to the life science research recently. These techniques provide new information about the atomic and molecular level free-volume and void sizes, and their distributions 0.1 nm to a few nm, molecular bonding, structures at depth-layers, and phase transitions. This paper is to review recent research on positron annihilation spectroscopy applied to the area of life science and also focus on current bioscience-related work in the positron group at the University of MissouriKansas City (UMKC).

Study of the Chemical Environment inside Free Volume Holes in Halogenated Styrene Polymers Using Positron Annihilation Spectroscopy

In this paper, a systematic study is provided of the chemical environment inside free volume holes in a series of halogenated polystyrenes (p-position), −[CH2CH C6H5X]n– (X = F, Cl, Br, I), by using positron annihilation spectroscopy. In such polymers it was determined that the chemical environment is the major effect on Doppler broadening of two 511 keV γ photons from positron–electron annihilation. Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime spectroscopy (PALS) were combined in a novel approach to study the chemical environment in a polymer system. A highly linear relationship between Doppler broadenings caused by electron kinetic energies of valence electrons and the ionization potentials of halogen elements was obtained, as well as a similar correlation with the results from ab initio molecular orbital (MO) calculations for monohalogenobenzenes, C6H5X (X = F, Cl, Br, I). The results demonstrate that the combination of DBES and PALS may provide an effective way to study the chemical environment inside free volume holes in polymers.

The chemical crosslinking of polyelectrolyte complex colloidal particles and the pervaporation performance of their membranes

Water dispersible polyelectrolyte complexes (PECs) of sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA) were crosslinked via glutaraldehyde (GA) to improve the water resistance and stability performance of their pervaporation membranes. Relative crosslinking degrees (RCDs) of PECs with different GA contents were obtained by Fourier Transform Infrared Spectroscopy (FT-IR). Free volume of crosslinked PECs membranes (CPECMs) was examined by Doppler Broadening Energy Spectroscopy (DBES). The surface hydrophilicity and morphology of CPECMs were investigated by dynamic contact angle (DCA) and atomic force microscope (AFM), respectively. CPECMs were subjected to pervaporation dehydration of aqueous isopropanol mixtures. It was found that CPECMs showed very high water resistance, super permeability and good selectivity. Crosslinked PECs membrane with 0.5 wt% GA (CPECM0.5) showed a permeate flux of 21.2 kg/(m 2 h) and water content in permeate of 98.1 wt% in dehydrating 50 wt% aqueous isopropanol mixture at 70 °C. Moreover, good long time stability of CPECMs for dehydration of aqueous isopropanol mixture was also observed. These excellent performance are attributed to the chemical crosslinking of polyelectrolyte complex colloidal particles.

Corrosion of pure aluminium and aluminium alloy: a comparative study using a slow positron beam

Corrosion-related defects in pure Al and AA 2037 Al alloy have been investigated by positron beam-based Doppler broadening energy spectroscopy. Defect profiles have been analyzed by measuring the S parameter as a function of incident positron energy up to 30 keV. When pure Al samples are immersed in 1M NaOH for various times, a significant increase in the S parameter near the surface is observed. This implies that the corrosion process involves the creation of defects and nanometer voids. In contrast, a significant decrease in the S parameter is observed after the corrosion of water-quenched Al alloy by the same method, which is interpreted as being a result of Cu enrichment near the metal-oxide interface layer.

Effects of Surface Properties of Different Substrates on Fine Structure of Plasma-Polymerized SiOCH Films Prepared from Hexamethyldisiloxane (HMDSO)

In this study, Doppler broadening energy spectroscopy (DBES) combined with slow positron beam was used to discuss the effect of substrate types on the fine structure of a plasma-polymerized SiOCH layer as a function of depth. From the SEM pictures, the SiOCH films formed on different substrates showed hemispherical macrostructures, and the deposition rate was dependent on the mean pore size. It appears that the morphology of the plasma-polymerized SiOCH films was associated with the porosity-related characteristics of the substrate such as the size/shape of pores. As deposited on the MCE-022 substrate (mixed cellulose esters membrane with a mean pore size of 0.22 μm) with a nodular structure, the SiOCH films had pillar-like structures and high gas permeabilities. DBES results showed that the SiOCH films deposited on different substrates were composed of three layers: the SiOCH bulk layer, the transition layer, and the substrate. It was observed that the microstructure of the SiOCH films was affected layer by layer; a higher surface pore size in the substrates induced thicker transition layers with higher microporosities and led to thinner bulk layers having higher S parameter values during the plasma polymerization. It was also observed that the change in O(2)/N(2) selectivity was consistent with the DBES analysis results. The gas separation performance and DBES analysis results agreed with each other.

Interfacially polymerized thin-film composite polyamide membranes: Effects of annealing processes on pervaporative dehydration of aqueous alcohol solutions

High-performance thin-film composite polyamide membranes for pervaporative dehydration processes were prepared by means of the interfacial polymerization of triethylenetetramine (TETA) and trimesoyl chloride (TMC) on the surface of a modified polyacrylonitrile (mPAN) membrane support. The effects of annealing processes applied during and after the composite membrane preparation on the pervaporation performance were investigated. Two annealing processes were applied. One was to the composite membrane formed after the interfacial polymerization by subjecting it to different annealing temperatures and the other to the aqueous amine (TETA) solution used during the interfacial polymerization process by varying its temperature. Positron annihilation spectroscopy experiments using a slow positron beam were carried out. One of the positron annihilation techniques, Doppler broadening energy spectroscopy (DBES), was used to study the effect of the annealing processes on the S parameter (corresponding to the free volume in the membrane). The first plateau (S1 = 0.47798 ± 1.86E−4, L1 = 182 ± 84 nm) in the curve obtained for the polyamide layer in the Type-B composite membrane was higher than that (S1 = 0.47659 ± 2.74E−4, L1 = 196 ± 52 nm) in the Type-A composite membrane. In other words, this corresponds to a higher S parameter and a thinner layer for the Type-B membrane than the Type-A membrane. This implies that the free-volume amount in the former membrane is higher than that in the latter. It was found that these two annealing processes greatly improved the pervaporation separation performances of the thin-film composite polyamide membranes prepared in this study in dehydrating aqueous alcohol solutions.

Investigation on the variation in the fine structure of plasma-polymerized composite membrane by positron annihilation spectroscopy

An SiO x C y H z /mixed cellulose ester (MCE) composite membrane was fabricated by plasma deposition of hexamethyldisiloxane (HMDSO) monomer on the surface of an MCE substrate. The purpose is to apply the membrane for the oxygen enrichment of an O 2/N 2 mixture. The variation in the fine structure of the plasma-polymerized layers (SiO x C y H z ) prepared at different plasma conditions was identified using slow-positron beams. Although the SiO x C y H z layers displayed similar FTIR spectra, they evidently showed differences based on the data from Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime spectroscopy (PALS). These data indicated distinct differences in the physical structure despite the similarities in the chemical structure of the films. The deposited layer (mixture of HMDSO species and MCE species) had more hydrocarbon group, resulting in a looser structure of the deposited layer at the initial period of plasma deposition. However, the MCE species content of the deposited layer decreased with the deposition time, and the deposited layer became denser. From PALS, the o-Ps lifetime ( τ 3, corresponding to the free-volume size) decreased along the plasma deposition layer growth direction. These results corresponded with those obtained from XPS very well. The plasma-deposited membrane was applied for O 2/N 2 separation. The O 2 permeability decreased and the concentration of O 2 in the permeate increased with the deposition time and as the plasma power increased.

Characterization of a chromate-inhibited primer by doppler broadening energy spectroscopy

A chromate-inhibited primer has been characterized by Doppler broadening energy spectroscopy (DBES), scanning electron microscopy (SEM), and Raman spectroscopy. The S-parameter obtained by DBES exhibited a three-layer depth profile, which is attributed to the presence of a “skin” layer in the polymer matrix of the primer and a concentration gradient in the inorganic phases distributed through the primer. The primer was also examined by DBES following immersion in solution. The change observed in the S-parameter upon immersion indicated water filling the molecular free volumes in the primer. The ability to monitor the depth of water uptake as a function of immersion time illustrates that the primer does not need to be fully saturated for chromate release to occur.

Salt weathering effect of polymer coatings studied by positron annihilation spectroscopy

Deterioration of polymer coatings due to UV light and salt effects as a function of exposure time was studied by using positron annihilation spectroscopy. Doppler broadening energy spectroscopy (DBES) of annihilation radiation and positron annihilation lifetime (PAL) were measured as a function of positron incident energy (0–30 keV). A significant decrease in the S-defect parameter from DBES and the intensity of o-positronium from PAL was observed as a result of the UV and salt exposures. Salt significantly increases the deterioration of polymeric coatings under UV irradiation by a factor of 2. Direct correlation between the decrease in gloss and the decrease of S parameter from positron data is observed.

Deterioration of a polyurethane coating studied by positron annihilation spectroscopy: Correlation with surface properties

AbstractDeterioration of a polyurethane coating by Florida natural environments as a function of time up to 16 weeks was studied by positron annihilation spectroscopy. Doppler broadening energy spectroscopy (DBES) of annihilation irradiation and positron annihilation lifetime (PAL) were measured as a function of incident positron energy (0–30 keV). A significant decrease in the S‐defect parameter from DBES and the intensity of orthopositronium from PAL was observed as a function of weathering time. This is interpreted as a loss of free volume and holes as a result of the weathering process. The gloss and surface morphology in the same system were measured by glossimetry and atomic force microscopy (AFM), respectively. The gloss decreased and surface roughness increased as a function of weathering time. The AFM images showed a new feature of a spherically coagulated microstructure on the surface after weathering. Direct correlations between the decrease in gloss and the increase in roughness as well as the decrease in the S‐defect parameter from the DBES data and in the free volume from the PAL data were observed. These results were used to discuss the weathering process in terms of chemical and physical changes as a result of photodegradation in protective polymeric systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2290–2301, 2001