Free Volume Radius Research Articles

Investigating the properties of ortho-positronium in liquid scintillators under oxygen quenching

Investigating the mechanism of positron annihilation in liquid-scintillator based neutrino experiments could be helpful for positron reconstruction algorithms and positron-electron discrimination analysis. Based on this, we utilize a novel positron annihilation lifetime spectrometer to characterize a series of liquid scintillator samples without direct contact with the positron source by applying the anti-coincidence method, which facilitates the measurement of liquids with high accuracy and low background. We obtain an ortho-positronium (o-Ps) lifetime value of 3.02 ns for liquid scintillators composed of linear alkylbenzene and two solutes, and we also measure liquid scintillator samples by bubbling different gases to study the interaction of oxygen dissolved with positronium. The discussion of the annihilation behavior of o-Ps in liquid scintillators further clarify the factors affecting the lifetime and intensity of o-Ps, and the calculation of annihilation rate and free volume radius within the samples has potential applications in characterizing gas solubility and free volume in liquids with o-Ps as probe.

Investigating the Free Volumes as Nanospaces in Human Stratum Corneum Lipid Bilayers Using Positron Annihilation Lifetime Spectroscopy (PALS).

This work is the first one that provides not only evidence for the existence of free volumes in the human stratum corneum but also focuses on comparing these experimental data, obtained through the unique positron annihilation lifetime spectroscopy (PALS) method, with theoretical values published in earlier works. The mean free volume of 0.269 nm was slightly lower than the theoretical value of 0.4 nm. The lifetime τ3 (1.83 ns with a coefficient of variation CV of 3.21%) is dependent on the size of open sites in the skin. This information was used to calculate the free volume radius R (0.269 nm with CV 2.14%), free volume size Vf (0.081 nm3 with CV 4.69%), and the intensity I3 (9.01% with CV 10.94%) to estimate the relative fractional free volume fv (1.32 a.u. with CV 13.68%) in human skin ex vivo. The relation between the lifetime of o-Ps (τ3) and the radius of free volume (R) was formulated using the Tao-Eldrup model, which assumes spherical voids and applies to sites with radii smaller than 1 nm. The results indicate that PALS is a powerful tool for confirming the existence of free volumes and determining their size. The studies also focused on describing the probable locations of these nanospaces in SC lipid bilayers. According to the theory, these play an essential role in dynamic processes in biological systems, including the diffusion of low-molecular-weight hydrophobic and moderately hydrophilic molecules. The mechanism of their formation has been determined by the molecular dynamics of the lipid chains.

Mechanism Insights in Anticorrosion Performance of Waterborne Epoxy Coatings Reinforced by PEI-Functionalized Boron Nitride Nanosheets.

Functionalized hexagonal boron nitride nanosheets (BNNSs) have arisen as compelling anticorrosive additives, yet the precise mechanism of their corrosion resistance enhancement in coatings remains unclear. Here, polyethylenimine functionalized BNNSs (PEI-BNNSs) with approximately 6-11 layers were prepared through a "one-step" method. Then, the PEI-BNNSs/Waterborne epoxy (WEP) composite coatings were incorporated via the waterborne latex blending method for the anticorrosion of the Q235 substrate. The impedance modulus (|Z|f = 0.01 Hz) of 0.5 wt % PEI-BNNSs/WEP composite coating soaked in 3.5 wt % NaCl solution for 35 days increased by 4 orders of magnitude compared to pure WEP coating, exhibiting exceptional long-term resistance against corrosion. The positron annihilation lifetime spectroscopy and corrosion product analysis demonstrated that the reinforced anticorrosion capabilities are not solely ascribed to the "tortuous path effect" arising from BNNSs impermeability. These mechanisms also encompass the reduction in free volume fraction and radius of the free volume cavities within the composite coating brought about by the PEI molecules. Additionally, the increase in coating adhesion, promoted by PEI, plays an important role in augmenting the barrier properties against corrosive agents. This study provided a full comprehension of the role played by functionalized BNNSs in fortifying the anticorrosion attributes of WEP coatings.

Polyurethane and nano-TiO2 modifiers mitigate aging of asphalt binders by inhibiting aggregation of polar molecules: A molecular dynamics study

The aim of this study was to employ molecular dynamics to investigate how the addition of polyurethane and nano-TiO2 modifiers affects the strengthening mechanism and anti-aging properties of asphalt binders. In this study, asphalt molecular model, nano-TiO2 cluster model, PU molecular model, and PU/nano-TiO2/asphalt molecular model were constructed. The radius of gyration, free volume, diffusion coefficient, and indicators of structural alterations were used to analyze the impacts of PU and nano-TiO2 modifiers on the molecular structure of asphalt. The results indicate that the aging process of asphalt introduces oxygen-containing active functional groups. These groups increase the interaction forces among polar molecules, resulting in the aggregation of polar molecules and the disruption of the original colloidal structure. The spatial barrier effect of polyurethane chains hinders the self-aggregation behavior of asphaltene molecules. Asphaltenes are adsorbed onto the nano-TiO2 clusters, thereby impeding the formation of large-sized polar molecular aggregates. The primary role of polyurethane and nano-TiO2 is to mitigate the aging-induced aggregation of asphaltene polar molecules, preserving asphalt performance. However, it's important to note that asphalt will inevitably undergo aging over time.

Effect of electron beam irradiation on pore structure of ethyl cellulose membranes

In this paper, ethyl cellulose (EC) membranes were successfully prepared by phase conversion method, and irradiated by electron beam. The as prepared and irradiated EC membranes were characterized by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), positron annihilation lifetime spectroscopy (PALS) and differential scanning calorimetry (DSC). The EIS test results show that the resistance of EC membrane increases obviously after irradiation, implying it is more difficult for ions permeation. SEM suggests that the irradiation might shrink the pores of EC membranes might be smaller after irradiation. PALS results indicate that the o-Ps lifetime decreases with the irradiation dose, while the o-Ps intensity increases. DSC results indicate the crystallinity of EC increases with the irradiation dose. All the above results imply the irradiation induces the chain scission and rearrangement of chain segment. Thus, the large pores were divided into more small pores, and the crystallinity increases with the increase of irradiation dose. Moreover, there is an inverse relationship between the membrane resistance obtained from EIS and the square of free volume radius calculated from PALS results, which suggests the free volume holes play an important role in ion permeation.

ELECTRON IRRADIATED EFFECT IN POLYURETHANES STUDIED BY POSITRON ANNIHILATION LIFETIME SPECTROSCOPY

The intensity variation of positronium as a result of electron irradiation has been studied on polyurethanes (PU) using positron annihilation lifetime spectroscopy (PALS). The PALS measurement was carried out at room temperature with a conventional fast-fast coincidence system. The lifetime spectra were analyzed into four components using melt program to extract the positron parameters such as lifetime and intensity. Due to irradiation a decrease of intensity about 2.7% was observed up to 10kGy. Free volume radius has been calculated for electron irradiated PU. PALS measurement has also been performed to study the effect of temperature variation in the range 30K to 310K. Analysis by melt program on the spectra shows that the fourth lifetime component increases significantly above glass transition temperature.

A new type of mixed vegetable insulating oil with better kinematic viscosity and oxidation stability

• The molecular dynamics model of mixed vegetable insulating oil with different proportion was established. • The effects of different mixing ratios on the kinematic viscosity of mixed oil were studied by molecular dynamics. • The difference of oxidation stability between palm fatty acid ester and rapeseed oil was studied by First-Principle. • A mixing ratio with good kinematic viscosity and oxidation stability was obtained. Natural ester is a new type of environment-friendly energy, and the research on its performance is crucial in order to be widely popularized and applied in electrical equipment. The mixing of different natural esters can ameliorate the performance handicap of natural esters. Therefore, two widely applied natural esters, rapeseed oil and palm fatty acid ester (PFAE) were selected in this paper. The kinematic viscosity and oxidation stability of mixed oil were analyzed on the basis of the establishment of the mixture of two oils with different mixing proportion by simulation. Results indicate that the kinematic viscosity of the mixed oil decreased with the addition of PFAE. The kinematic viscosity of the mixed oil decreased by 51.3% when the mixing ratio was 80% rapeseed oil and 20% PFAE. The addition of short chain oil molecules increases the mean square displacement, mean-square radius of gyration and free volume of the mixed oil. The oil molecules in the mixed system have fewer tangles and reduced internal friction between them. DFT calculation shows that numerous electron traps in PFAE provide a channel for the transition of valence electrons. The methyl group in PFAE has strong positive electrostatic potential and strong electron affinity, which makes the oxidation stability of PFAE lower than that of rapeseed oil. Therefore, 80% rapeseed oil and 20% PFAE are the best mixing ratio in this study. This ratio has the best improvement on the kinematic viscosity of the mixed oil and the least influence on the oxidation stability.

P.672 Serum nerve growth factor levels in children with specific learning disorder

To comprehensively investigate the correlation between free volume properties (size and distribution) in the interior of polyamide (PA) active layer and mass transport mechanism, six polyamide thin film composite (TFC) membranes were characterized using positron annihilation lifetime spectroscopy (PALS). In case of pressurized filtration conditions, the rejection rate of boron across all PA membranes was found to be inversely proportional to the free volume size. The more boron transport occurred at the membrane containing larger free volume. In addition, the transition of the neutral boron transport mechanism from convection to diffusion was found to occur at a membrane free volume radius around 0.275 nm within the applied pressure range (2 and 10 bar), as verified by PALS and the dimensionless Peclet number. We believe that mass transport mechanism transition from convection to diffusion is caused by the compression of polyamide active layer due to applied hydraulic pressure (10 bar), resulting in lowering the convective mass transport pathway inside polyamide active layer. These membrane free volume size criteria and experimental filtration results may subsequently be used as new design guidelines for the development of high boron rejection polyamide TFC membranes having a low energy consumption.

Electrical properties and conduction mechanisms of K, Ga co-substituted Na0.5Bi0.5TiO3 ferroelectrics

Ferroelectric Na0.5Bi0.5TiO3 has potential in exhibiting oxide-ion conductive characteristics at higher temperatures due to their high leakage current. In this study, K+ and Ga3+ co-doped Na0.5Bi0.5TiO3 ceramics are synthesized using the conventional solid-state reaction method, and their conduction mechanism and electrical performance are investigated. The compounds with dominant oxygen-ion conduction are identified and their stability in the presence of nitrogen is analyzed. The bulk conductivity of the ceramics is improved by decreasing the concentration of K+ associated with an enhanced Na/K–O vibration. Excess K+ dopants hinder oxygen vacancy migration owing to the attraction effect on oxygen vacancies VO•• and A-site acceptor KBi'', and narrow the oxygen vacancy migration channel. The oxygen vacancy concentration, tolerate factor, specific free volume, relaxation parameter, and critical radius are used to discuss the conductive properties. The results demonstrate that an appropriate amount of K+ and Ga3+ doping promotes the oxide-ionic conductivity, which is helpful for tailoring the electrical properties of Na0.5Bi0.5TiO3-based oxygen-ion conductors.

Free volume correlation with electrical conductivity of polycarbonate/reduced graphene oxide nanocomposites studied by positron annihilation lifetime spectroscopy

ABSTRACTThe reduced graphene oxide (RGO) was added to polycarbonate (PC) through solvent mixing. The influences of RGO on the free volumes and electrical conductivity of PC were systemically studied by positron annihilation lifetime spectroscopy (PALS), Fourier‐transform infrared spectroscopy, transmission electron microscope, and electrical conductivity measurements for PC/RGO nanocomposites. The nanosized effect, good conductivity of RGO, and strong interfacial interaction between RGO and PC result in the low conductive threshold of 0.36 wt %. Ten orders of magnitude increase in electrical conductivity was obtained. The results of PALS indicate the properties of free volumes fluctuate around the conductive percolation threshold. The conformational change of PC segments in the interfacial region induced by the strong interfacial interaction leads to an increase in free volume radius. The concentration of free volume decreases due to the depression of free volume holes as the introduction of RGO. Significantly, an exponential function is proposed to describe the effect of relative fractional free volume on the electrical conductivity, suggesting that free volume plays an important role in regulating electrical conductivity of PC/RGO. The physical quantity fd that refers required fractional free volume of polymer composites from insulating to conducting phase is found. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48207.

Morphology correlated free volume studies of multi-walled carbon nanotube plasticized poly (vinyl chloride) nanocomposites: Positronium probes and electrical properties

Herein we present a new approach to develop nanocomposites based on plasticized poly (vinyl chloride)/multi-walled carbon nanotubes. The free volume is found to increase with increase in MWCNT content up to 1% CNT, but exceptional decrease was observed at higher loading. The increase in free volume is as a result of intercalation of carbon nanotubes between polymer chains, there by introducing slight disorder to nanocomposites. The decrease in free volume is as a result of bridging of polymer chains by carbon nanotubes, resulting in the decrease in free volume at the vicinity of carbon nanotubes. Structural studies via Raman spectra and Doppler Broadening reveals disorder and interfacial interactions in nanocomposites. A frequency up shift of G band is due to π electron transfer from MWCNT to chlorine in PPVC. Using Tao-Eldrup model, radius of free volume (R) and volume of free volume (Vf) are calculated for nanocomposites. Morphological studies via FESEM and TEM reveal uniform dispersion of MWCNT throughout the polymer matrix. Hence, nanocomposite with maximum MWCNT loading showed electrical conductivity of 4.2E-10 S/cm. Thus this is a new composite material for antistatic application.

Performance evaluation of polyamide TFC membranes: Effects of free volume properties on boron transport

To comprehensively investigate the correlation between free volume properties (size and distribution) in the interior of polyamide (PA) active layer and mass transport mechanism, six polyamide thin film composite (TFC) membranes were characterized using positron annihilation lifetime spectroscopy (PALS). In case of pressurized filtration conditions, the rejection rate of boron across all PA membranes was found to be inversely proportional to the free volume size. The more boron transport occurred at the membrane containing larger free volume. In addition, the transition of the neutral boron transport mechanism from convection to diffusion was found to occur at a membrane free volume radius around 0.275nm within the applied pressure range (2 and 10bar), as verified by PALS and the dimensionless Peclet number. We believe that mass transport mechanism transition from convection to diffusion is caused by the compression of polyamide active layer due to applied hydraulic pressure (10bar), resulting in lowering the convective mass transport pathway inside polyamide active layer. These membrane free volume size criteria and experimental filtration results may subsequently be used as new design guidelines for the development of high boron rejection polyamide TFC membranes having a low energy consumption.

High-performance UiO-66/polyimide mixed matrix membranes for ethanol, isopropanol and n-butanol dehydration via pervaporation

We have designed novel mixed matrix membranes (MMMs) consisting of UiO-66 nanoparticles and 6FDA-HAB/DABA polyimide for the dehydration of ethanol, isopropanol and n-butanol via pervaporation. The UiO-66 nanoparticles have a particle size of around 100nm. They can be evenly dispersed in the 6FDA-HAB/DABA polyimide matrix without visible agglomeration even at the highest 30wt% loading. The incorporation of UiO-66 into the 6FDA-HAB/DABA polyimide not only significantly enhances both free-volume radius and fractional free volume as confirmed by positron annihilation lifetime spectroscopy but also remarkably improves the normalized flux of MMMs for the dehydration of ethanol/water, isopropanol/water and n-butanol/water systems. The MMMs show excellent separation efficiency for the dehydration of isopropanol and n-butanol. At the highest UiO-66 loading of 30wt%, the MMMs have the water permeability of 0.329 and 0.292mgm−1h−1kPa−1 and mole-based selectivity of 2209 and 14214 respectively for isopropanol/water and n-butanol/water systems, outperforming most literature data. These experimental results strongly suggest the newly developed UiO-66/polyimide MMMs have great potential for isopropanol and n-butanol dehydration via pervaporation.

Influence of UV-Irradiation on Latent Tracks in Polyethylene Terephthalate Films**Supported by the National Natural Science Foundation of China under Grant Nos 11335003 and 91323105.

Polyethylene terephthalate (PET) films in thickness of 12 μm are irradiated by Xe and Au ions at the energies of 9.5 and 11.4MeV/u and with the ion fluence from 5 × 109 cm−2 to 1 × 1011 cm−2. After irradiation, ultra-violet lights are used to illuminate the samples with latent tracks at the wavelength of 365 nm with flux density of 4.2 mW/cm−2. UV-irradiation effects on tracked PET are investigated by the UV-vis spectrum and positron annihilation lifetime spectroscopy (PALS). It is found that carbonaceous clusters in PET films are generated by ion irradiation and decomposed with UV illumination by calculating the optical energy band gap Eg in the UV-vis spectrum. The free volumes behave differently in track and bulk after UV illumination. In our experiment, the PALS results show an increase in radius and density of free volume in tracked PET films after UV treatment, which indicates an expansion in radius of latent tracks.

Physicochemical effects of hydrolyzed asymmetric polyacrylonitrile membrane microstructure on dehydrating butanol

Traditionally, polyacrylonitrile (PAN) is coated with a selective layer to tailor it for dehydrating butanol by pervaporation. But we hydrolyzed PAN with sodium hydroxide for such a purpose. Fourier transform infrared and X-ray photoelectron spectroscopy indicated that PAN׳s CN group was hydrolyzed to CONH2, which was further converted to COOH. As a result, the hydrolyzed PAN (HPAN) was made more hydrophilic, as evidenced from water contact angle data. CONH2 and COOH formed hydrogen bonds between them, causing molecular chains to rearrange (according to X-ray diffraction) and pores to shrink (shown by scanning electron microscopy). Positron annihilation lifetime spectroscopy revealed that hydrolysis led to free volume radius decrease; however, prolonged hydrolysis caused breakage of molecular chains. The HPAN produced after 4h hydrolysis exhibited narrow free volume distribution and delivered exceptional performance (water concentration in permeate=99.1wt%; permeation flux=2.03kg/m2h).

Effect of confinement on glass dynamics and free volume in immiscible polystyrene/high‐density polyethylene blends

AbstractThe effect of confinement on glass dynamics combined with the corresponding free volume changes of amorphous polystyrene (PS) in blends with semi‐crystalline high‐density polyethylene (HDPE) have been investigated using thermal analyses and positron annihilation lifetime spectroscopy (PALS). Two different glass transition temperatures (Tg) were observed in a PS/HDPE blend due to the dissimilarity in the chemical structure, consistent with an immiscible blend. However, Tg of PS in the incompatible PS/HDPE blend showed an upward trend with increasing PS content resulting from the confinement effect, while Tg of the semi‐crystalline HDPE component became lower than that of neat HDPE. Moreover, the elevation of Tg of PS was enhanced with a decrease of free volume radius by comparing annealed and unannealed PS/HDPE blends. Positron results showed that the free volume radius clearly decreased with annealing for all compositions, although the free volume hole size agreed well with linear additivity, indicating that there was only a weak interaction between the two components. Combining PALS with thermal analysis results, the confinement effect on the glass dynamics and free volume of PS phase in PS/HDPE blends could be attributed to the shrinkage of HDPE during crystallization when HDPE acted as the continuous phase. © 2015 Society of Chemical Industry

The size of smallest subnanometric voids estimated by positron annihilation method. Correction to the Tao-Eldrup model

The Tao-Eldrup model, commonly applied in estimation of free volume sizes in solids, uses the concept of infinitely deep potential well, which can lead to inaccuracies when the ortho-positronium (o-Ps) lifetime is very short (less than 1.5ns). A simple correction allowing to estimate properly the free volume radii at the o-Ps lifetime down to 0.8ns is proposed.

A study on the characteristics and pervaporation performance of polyamide thin-film composite membranes with modified polyacrylonitrile as substrate for bioethanol dehydration

To improve the pervaporation performance in separating an aqueous ethanol solution, polyamide thin-film composite (TFC) membranes (m-tolidine-H-TMC/mPAN) were prepared through the interfacial polymerization reaction between trimesoyl chloride (TMC) and 2,2'-dimethylbenzidine hydrochloride (m-tolidine-H) on the surface of a modified polyacrylonitrile (mPAN) membrane. The effects of the feed ethanol concentration on the pervaporation performance and the durability of m-tolidine-H-TMC/mPAN TFC membranes were investigated. To choose the optimal mPAN membrane as the TFC substrate, the effect of hydrolysis time on the chemical properties and separation performance of an mPAN substrate was also studied. An appropriate hydrolysis time of 15 min was chosen to obtain the mPAN substrate due to the corresponding high permeation flux. The m-tolidine-H-TMC/mPAN TFC membrane exhibited a high pervaporation performance for ethanol dehydration. A positron annihilation lifetime spectroscopy experiment was used to estimate the mean free-volume radius of the m-tolidine-H-TMC polyamide selective layer, which lay between the radii of the water and ethanol molecules. © 2013 Society of Chemical Industry

Free volume-related microstructural properties of lithium perchlorate/sodium alginate polymer composites

The effect of salt (LiClO4) on free volume-related microstructural, optical, and dielectric properties of lithium perchlorate (LiClO4)-doped sodium alginate (NaC6H7O6)n solid polymer electrolyte were studied using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), UV–Visible, dielectric measurements, and positron annihilation lifetime spectroscopy (PALS) techniques. The FTIR study reveal that the LiClO4 interact with the OH groups of sodium alginate (NaAlg) and forms charge transfer complex (CTC). These formed CTC within the composite film affect the optical property of the polymer composite, which was reflected from UV–Visible study. The free volume was probed by the PALS technique, which uses the o-Ps pickup lifetime τ3, as an indication of local electron density and mean free volume radius. The o-Ps pickup intensity I3, reflects the probability of o-Ps formation. The o-Ps components τ3 and I3 increases with salt concentration, which shows microstructural modifications due to the decrease in the crystallinity of the NaAlg as observed from XRD studies. Therefore, electrical studies on the doped NaAlg reveal that the ionic movement in the composites is mainly due to the segmental motion of the polymer backbone. So, the complex formation due to doping affects the free volume-related microstructure and hence the dielectric properties including the AC conductivity of the polymer composites. POLYM. COMPOS., 35:1267–1274, 2014. © 2013 Society of Plastics Engineers

Sorption, swelling, and free volume of polybenzimidazole (PBI) and PBI/zeolitic imidazolate framework (ZIF-8) nano-composite membranes for pervaporation

We have studied alcohol- and water-induced swelling and sorption of PBI and PBI/ZIF-8 nano-composite membranes and characterized both dry and wet membranes by positron annihilation lifetime spectroscopy (PALS). The degrees of membrane swelling after immersing in alcohols and water follow the order of ethanol > methanol > water > n-butanol. Addition of ZIF-8 particles into PBI membranes suppresses the ethanol-, methanol- and water-induced PBI swelling. The water-induced swelling is subdued most severely because of the hydrophobic nature and rigid structure of ZIF-8 particles, while the n-butanol-induced swelling is enhanced owing to a greater free volume in the PBI/ZIF-8 membrane. The PALS studies confirm that the high pervaporation permeability of PBI/ZIF-8 membranes is attributed to the high fractional free volume (FFV) created by large cavities of ZIF-8 particles. In addition, the alcohol saturated PBI/ZIF-8 membranes have significantly smaller free volume radii than alcohol saturated PBI membranes. A correlation among FFV, pervaporation permeability and sorption diffusion coefficient has been developed. This study also demonstrates the importance of using PALS to characterize membrane structure changes in solvent-saturated membranes at a molecular level and to correlate pervaporation performance with solvent-induced swelling.