Free-volume Voids Research Articles

Simple and complex sorption−solution isotherms for membrane polymers: A statistical thermodynamic fluctuation theory

Simple, site-specific adsorption isotherm models, such as GAB and other generalizations of BET, fit strikingly well to experimental isotherms of simple shapes for glassy and rubbery polymers. However, the structures of dense polymers do not necessarily resemble the unrestricted layer-by-layer adsorption mechanism on one type of sorption site assumed by these models. Since polymers contain micropores, sorptive likely (ad)sorbs on the internal surfaces of the free (accessible) volume voids, and dissolves in the polymer phase. While the empirical dual mode sorption model and its variants address this duality, other common models assume chiefly only one of the two mechanisms. Moreover, the simplified models do not fit complex isotherms, such as those for alcohols and poly[(trimethylsilyl)propyne] (PTMSP). The statistical thermodynamic fluctuation theory is adopted here to capture the sorption-solution duality consistently even for the complex isotherms. The statistical thermodynamic ABC isotherm derived from this theory involves the sorbate-sorbent and mono-, di-, and tri-sorbate interactions expressed by sorbate number correlations. This work shows that BET, GAB, dual-mode sorption model, Flory-Huggins, and ENSIC models are special cases of the ABC isotherm. The isotherm multiplicativity principle has been derived from the number fluctuation relationship to model complex isotherms.

Influence of water quality factors on cake layer 3D structures and water channels during ultrafiltration process

The three-dimensional (3D) structure of the cake layer, which could be influenced by water quality factors, plays a significant role in the ultrafiltration (UF) efficiency of water purification. However, it remains challenging to precisely reveal the variation of cake layer 3D structures and water channel characteristics. Herein, we systematically report the variation in the cake layer 3D structure at the nanoscale induced by key water quality factors and reveal its influence on water transport, in particular the abundance of water channels within the cake layer. In comparison with pH and Na+, Ca2+ played more significant role in determining cake layer structures. The sandwich-like cake layer, which was induced by the asynchronous deposition of humic acids and sodium alginate (SA), shifted to an isotropic structure when Ca2+ was present due to the Ca2+ bridging. In comparison with the sandwich-like structure, the isotropic cake layer has higher fractions of free volume (voids) and more water channels, leading to a 147% improvement in the water transport coefficient, 60% reduction in the cake layer resistance, and 21% increase in the final membrane specific flux. Our work elucidates a structure-property relationship where improving the isotropy of the cake layer 3D structure is conducive to the optimization of water channels and water transport within cake layers. This could inspire tailored regulation strategies for cake layers to enhance the UF efficiency of water purification.

Enhanced gas separation performance of PIM-1 blend membranes incorporating ionic liquid (3-(trimethoxysilyl) propan-1-aminium acetate ([APTMS][Ac])) as filler: Investigation of morphology, compatibility and transport properties

Blend membranes were fabricated by incorporating an ionic liquid, 3-(trimethoxysilyl) propan-1-aminium acetate ([APTMS][Ac]), as a filler into the polymer of intrinsic microporosity PIM-1. The analyses using FTIR spectroscopy, AFM, and SEM confirmed a uniform dispersion and excellent polymer-filler compatibility, even at higher loadings. Gas transport properties were assessed with filler loadings up to 10 wt% of IL, revealing a slight reduction in permeability but an enhancement in selectivity. CO2 permeability of the blend membrane decreased by 37% at 10% filler loading, while the ideal selectivity increased from 19.6 to 29.0 for CO2/CH4 and from 20.9 to 31.7 for CO2/N2 upon increasing [APTMS][Ac] content (0 wt% − 10 wt%). The CO2/CH4 gas mixture selectivity rose by 58.62% and the CO2 permeability dropped by 40.44% compared to the pristine PIM-1 membrane. For the CO2/N2 gas mixture, the selectivity and CO2 permeability were enhanced by 40.11% and reduced by 39.07%, respectively. The permeability reduction can be ascribed to the filling of free volume voids in the PIM-1 polymer matrix by the IL [APTMS][Ac], while the selectivity improvement can be attributed to the strong interaction and high solubility of CO2 in the ionic liquid. The CO2 solubility coefficient increased with the rising IL concentration, indicating the high solubility of the IL in the polymer matrix.

Aggregation-Induced Emission by Molecular Design: A Route to High-Performance Light-Emitting Electrochemical Cells.

The emission efficiency of organic semiconductors (OSCs) often suffers from aggregation caused quenching (ACQ). An elegant solution is aggregation-induced emission (AIE), which constitutes the design of the OSC so that its morphology inhibits quenching π-π interactions and non-radiative motional deactivation. The light-emitting electrochemical cell (LEC) can be sustainably fabricated, but its function depends on motion of bulky ions in proximity of the OSC. It is therefore questionable whether the AIE morphology can be retained during LEC operation. Here, we synthesize two structurally similar OSCs, which are distinguished by that 1 features ACQ while 2 delivers AIE. Interestingly, we find that the AIE-LEC significantly outperforms the ACQ-LEC. We rationalize our finding by showing that the AIE morphology remains intact during LEC operation, and that it can feature appropriately sized free-volume voids for facile ion transport and suppressed non-radiative excitonic deactivation.

Photopolymerization shrinkage in dimethacrylate-based dental restorative composites probed by means of positron annihilation lifetime spectroscopy

Peculiarities of microstructure changes in void subsystem under light-curing polymerization in loosely- and densely-packed polymer/filler nanocomposites are studied using positron annihilation lifetime (PAL) spectroscopy, at the example of commercially available dimethacrylate-based dental restorative composites Dipol® and ЭСТА-3®. Main process governing volumetric shrinkage in these polymer/filler nanocomposites is identified as positron-to-Ps trapping conversion, initiated by fragmentation of Ps-decay holes due to crosslinking of monomer chains. Meaningful description of photopolymerization changes is developed within x3-x2-CDA (coupling decomposition algorithm).In loosely-packed nanocomposites (such as Dipol®), the Ps decay takes place in intrinsic holes of polymer matrix, and free positron trapping occurs in filler sub-system, specifically in the interfacial voids between filler particles themselves and filler particles contacting with surrounding polymer. Under photopolymerization, fragmentation of intrinsic holes in polymer matrix dominates over enhanced trapping of positrons in filler sub-system since filler-particles assemblies get more tightly covered by polymer owing to disappearing of interfacial free-volume voids at their surface.In densely-packed nanocomposites (such as ЭСТА-3®), where positron trapping in interfacial voids at the surface of agglomerated filler particles takes minor role, the polymerization-driven fragmentation of Ps-decay holes causes stress inside monolith filler-particles assemblies, resulting in disappearing of some interparticulate traps (positron-trapping voids between filler particles).

Light-curing effects in acrylic-type dental nanocomposites probed by annihilating positrons: the case of loosely monolith Dipol\xae restoratives

The possibility of application of positron annihilation lifetime (PAL) spectroscopy to commercially available dimethacrylate-based dental restorative composites Dipol® (Oksomat-AN Ltd, Ukraine) subjected to photopolymerization due to light curing is analyzed. The governing annihilation process in these composites is identified as mixed positron (e+)-positronium (Ps) trapping, where Ps decaying is caused entirely by input from free-volume holes in polymer matrix superimposed on free e+-trapping contribution from interfacial free-volume holes between filler nanoparticles and surrounded polymer matrix. Photopolymerization shrinkage is revealed through decrease in the average lifetime of annihilating positrons due to opposite changes in Ps-decaying and e+-trapping channels. The growing light-activated polymerization is characteristic of both intensities related to the second and third components in the unconstrained ×3-term decomposed PAL spectra, accompanied by decrease in the corresponding lifetimes. This process resulted in enhanced trapping rate in the defects and depressed fractional free-volume saturation with light curing. Light exposure causes smaller voids in composites owing to free-volume fragmentation in Ps- and e+-trapping sites. The microstructure scenario for these transformations includes photo-induced cross-linking of structural chains in the polymer matrix, followed by conversion of o-Ps traps in interfacial free-volume voids near agglomerated filler nanoparticles. A meaningful description of this process is developed on the basis of the semi-empirical model exploring the ×3– ×2-coupling decomposition algorithm.

Investigation of organophilic montmorillonites supported palladium catalytic composites by combined positron annihilation lifetime spectroscopy and X-ray diffraction

A characterization of organophilic montmorillonite (OMMT) supports and organophilic montmorillonite supported palladium (Pd@OMMT) catalytic composites was carried out using positron annihilation lifetime spectroscopy (PALS) and X-ray diffraction (XRD). XRD method supplies the fundamental information on the interlayer spacing of OMMT. And PALS method supplies the complementary information on the free volume voids variations inside the interlayer space of OMMT, which is occupied with organic cations and Pd species. In PALS analyzing, the size of the channel-like free volume voids inside the interlayer space of OMMT are estimated by both cuboidal and cylindrical free volume voids models from o-Ps lifetime. The OMMT and Pd@OMMT composites show bigger interlayer spacing values (by XRD) but smaller free volume void size values (by PALS) than MMT. It should be due to the fact that the interlayer space becomes more crowded after organic modifier intercalation and Pd immobilization inside the interlayer space of MMT. The prepared Pd@OMMT catalytic composites show high activity and good recyclability in typical Heck coupling reactions. The correlations between the microstructure and catalytic performances of the organophilic montmorillonites supported palladium catalytic composites were discussed.

Comparative study of free-volume nanostructurization in glassy and crystalline As2S3 re-examined with annihilating positrons

Evolution of free-volume atomic scale structure under crystal-to-glass transition was re-examined in arsenic trisulfide As2S3 in glassy and crystalline state, employing mutually correlated positron annihilation lifetime studies treated in a framework of two-state simple positron trapping model. In crystalline state represented by mineral orpiment As2S3 supplied from Wanshan mine (China), the trapping rate is restricted to ~1.14 ns−1, associated with 0.240 ns defect-free bulk and 0.402 ns defect-specific lifetimes due to free volumes stabilized preferentially within inter-layer spaces. The positron trapping rate is notably enhanced to ~1.51 ns−1 in glassy state presented by melt-quenched As2S3, the positron traps being smaller with decreased bulk (0.278 ns) and defect (0.380 ns) lifetimes due to nanostructurized free volumes stabilized within corrugated quasi-layers. In general, vitrification in As2S3 produces depressing and time-enhancing trend in the recorded PAL spectra peaks. The ratio of bulk positron lifetimes for glassy and crystalline states serving as criterion of vitrification completeness reaches 1.16, meaning that some additional volume is available for annihilation in a glass. The ratio of mean positron lifetimes approaching 1.12 reflects a degree of realized crystal-to-glass transformation. Vitrification-related changes in As2S3 can be imagined as fragmentation of existing free-volume voids due to crumpling of intrinsic inter-layer spaces.

The effect of bioadhesive on the interfacial compatibility and pervaporation performance of composite membranes by MD and GCMC simulation

Combing molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulation, the effect of bioadhesive transition layer on the interfacial compatibility of the pervaporation composite membranes, and the pervaporation performance toward penetrant molecules were investigated. In our previous experimental study, the structural stability and permeability selectivity of the composite membranes were considerably enhanced by the introduction of bioadhesive carbopol (CP). In the present study, the interfacial compatibility and the interfacial energies between the chitosan (CS) separation layer, CP transition layer and the support layer were investigated, respectively. The mobility of polymer chains, free volume in bulk and interface regions were evaluated by the mean-square displacement (MSD) and free volume voids (FFV) analysis. The diffusion and sorption behavior of water/ethanol molecules in bulk and interface regions were characterized. The simulation results of membrane structure have good consistency, indicating that the introduction of CP transition layer improved the interfacial compatibility and interaction between the separation layer and the support layer. Comparing the bulk region of the separation layer, the mobility and free volume of the polymer chain in the interface region decreased and thus reduced the swelling of CS active layer, revealing the increased diffusion selectivity toward the permeated water and ethanol molecules. The strong hydrogen bonds interaction between the COOH of the CP transition layer and water molecules increased the adsorption of water molecules in the interface region. The simulation results were quite consistent with the experimental results.

Free Volume Structure of Acrylic-Type Dental Nanocomposites Tested with Annihilating Positrons

Positron annihilation spectroscopy in lifetime measuring mode exploring conventional fast-fast coincidence ORTEC system is employed to characterize free volume structure of commercially available acrylic-type dental restorative composite Charisma® (Heraeus Kulzer GmbH, Germany). The measured lifetime spectra for uncured and light-cured composites are reconstructed from unconstrained x3-term fitting and semi-empirical model exploring x3-x2-coupling decomposition algorithm. The governing channel of positron annihilation in the composites studied is ascribed to mixed positron-Ps trapping, where Ps decaying in the third component is caused entirely by input from free-volume holes in polymer matrix, while the second component is defined by free positron trapping in interfacial free-volume holes between filler nanoparticles and surrounded polymer matrix. Microstructure scenario of the photopolymerization shrinkage includes cross-linking of structural chains in polymer matrix followed by conversion of bound positron-electron (positronium) traps in positron-trapping interfacial free-volume voids in a vicinity of agglomerated filler nanoparticles.

Nanostructurization effects in PVP-stabilized tetra-arsenic tetra-sulfide As4S4 nanocomposites

Nanostructurization in three types of tetra-arsenic tetra-sulfide As4S4 polymorphs composed by (1) preferential β-As4S4, (2) realgar α-As4S4 and (3) admixture of As4S4 in the form of β-realgar, pararealgar and intermediate χ-phase subjected to mechanochemical ball milling in a water solution of polyvinylpyrrolidone (PVP) are characterized with multiexperimental structure-sensitive probes. Void structure of pelletized As4S4-PVP nanocomposites is probed by positron annihilation lifetime spectroscopy complemented with atomic-sensitive techniques such as X-ray powder diffraction, Raman scattering and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Appearance of arsenolite As2O3 crystallites is character for all As4S4 polymorphs affected by wet milling. Positron annihilation lifetime data considered in terms of substitution positron-positronium trapping confirm complicated nature of nanocomposites. Modified x3-x2-coupling decomposition algorithm developed in addition to unconstrained x3-term fitting procedure is applied to parameterize annihilation channels in these nanocomposites. Interfacial free-volume voids between neighboring nanoparticles in PVP environment are defined as most favorable trapping sites, they being rather loosely composed by As4S4 crystallites in full respect to variety of crystallographic polymorphs used for milling. Detected annihilation channels are ascribed preferentially to intrinsic free-volume voids insensitive to trapping states at the surface of oxidized nanocomposites.

Cholesterol–Ceramide Interactions in Phospholipid and Sphingolipid Bilayers As Observed by Positron Annihilation Lifetime Spectroscopy and Molecular Dynamics Simulations

Free volume voids in lipid bilayers can be measured by positron annihilation lifetime spectroscopy (PALS). This technique has been applied, together with differential scanning calorimetry and molecular dynamics (MD) simulations, to study the effects of cholesterol (Chol) and ceramide (Cer) on free volume voids in sphingomyelin (SM) or dipalmitoylphosphatidylcholine (DPPC) bilayers. Binary lipid samples with Chol were studied (DPPC:Chol 60:40, SM:Chol 60:40 mol ratio), and no phase transition was detected in the 20-60 °C range, in agreement with calorimetric data. Chol-driven liquid-ordered phase showed an intermediate free volume void size as compared to gel and fluid phases. For SM and SM:Cer (85:15 mol:mol) model membranes measured in the 20-60 °C range the gel-to-fluid phase transition could be observed with a related increase in free volume, which was more pronounced for the SM:Cer sample. MD simulations suggest a hitherto unsuspected lipid tilting in SM:Cer bilayers but not in pure SM. Ternary samples of DPPC:Cer:Chol (54:23:23) and SM:Cer:Chol (54:23:23) were measured, and a clear pattern of free volume increase was observed in the 20-60 °C because of the gel-to-fluid transition. Interestingly, MD simulations showed a tendency of Cer to change its distribution along the membrane to make room for Chol in ternary mixtures. The results suggest that the gel phase formed in these ternary mixtures is stabilized by Chol-Cer interactions.

Positron trapping defects in free-volume investigation of Ge–Ga–S–CsCl glasses

Evolution of free-volume positron trapping defects caused by crystallization process in (80GeS2–20Ga2S3)100−х(СsCl)x, 0 ≤ x ≤ 15 chalcogenide-chalcohalide glasses was studied by positron annihilation lifetime technique. It is established that CsCl additives in Ge–Ga–S glassy matrix transform defect-related component spectra, indicating that the agglomeration of free-volume voids occurs in initial and crystallized (80GeS2–20Ga2S3)100−х(СsCl)x, 0 ≤ x ≤ 10 glasses. Void fragmentation in (80GeS2–20Ga2S3)85(СsCl)15 glass can be associated with loosing of their inner structure. Full crystallization in each of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is shown, that CsCl additives result in white shift in the visible regions in transmission spectra. The γ-irradiation of 80GeS2–20Ga2S3 base glass leads to slight long-wavelength shift of the fundamental optical absorption edge and decreasing of transmission speaks in favor of possible formation of additional defects in glasses and their darkening.

Free-Volume Nanostructurization in Ga-Modified As2Se3 Glass.

Different stages of intrinsic nanostructurization related to evolution of free-volume voids, including phase separation, crystalline nuclei precipitation, and growth, were studied in glassy As2Se3 doped with Ga up to 5 at. %, using complementary techniques of positron annihilation lifetime spectroscopy, X-ray powder diffraction, and scanning electron microscopy with energy-dispersive X-ray analysis. Positron lifetime spectra reconstructed in terms of a two-state trapping model testified in favor of a native void structure of g-As2Se3 modified by Ga additions. Under small Ga content (below 3 at. %), the positron trapping in glassy alloys was dominated by voids associated with bond-free solid angles of bridging As2Se4/2 units. This void agglomeration trend was changed on fragmentation with further Ga doping due to crystalline Ga2Se3 nuclei precipitation and growth, these changes being activated by employing free volume from just attached As-rich glassy matrix with higher content of As2Se4/2 clusters. Respectively, the positron trapping on free-volume voids related to pyramidal AsSe3/2 units (like in parent As2Se3 glass) was in obvious preference in such glassy crystalline alloys.

Structural-relaxation phenomena in As–S glasses as probed by combined PAL/DBAR technique

Experimental techniques exploring phenomena of positron–electron interaction, namely the positron annihilation lifetime spectroscopy and Doppler broadening of annihilation radiation, are shown to be very informative tools to study radiation- and thermally-induced phenomena in chalcogenide glasses of binary As–S system. Time-dependent processes of free-volume voids agglomeration (expansion), fragmentation (refining) and disappearing (contraction) are identified as main stages of physical aging in S-rich glasses, while a competitive channel of coordination topological defects formation associated with void charging becomes significant in a vicinity of near-stoichiometric glass compositions under γ-irradiation. The data of combined positron lifetime and Doppler broadening of annihilation radiation measurements are correlated with radiation-induced shift of fundamental optical absorption edge of the studied glasses. The meaningful model for γ-induced and relaxation-driven evolution in free-volume void structure of As–S glasses giving a unified insight on their structural-chemical nature is proposed.

Free-volume Study in GeS2-Ga2S3-CsCl Chalcohalide Glasses Using Positron Annihilation Technique

Positron annihilation lifetime spectroscopy combined with Doppler broadening of annihilation radiation was applied to study free-volume entities in Ge-Ga-S glasses having different amount of CsCl additives. It is shown that the structural changes caused by CsCl additives can be adequately described by positron trapping modes determined within two-state model. The results testify in a favor of rather unchanged nature of corresponding free-volume voids responsible for positron trapping in the studied glasses, when mainly concentration of these traps is a subject to most significant changes with composition.

Interphase of a Polymer at a Solid Interface

Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymer–solid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film.

Prediction of free-volume-type correlations in glassy chalcogenides from positron annihilation lifetime measurements

A newly modified correlation equation between defect-related positron lifetime determined within two-state trapping model and radius of corresponding free-volume-type defects was proposed to describe compositional variations in atomic-deficient structure of covalent-bonded chalcogenides like binary As–S/Se glasses. Specific chemical environment of free-volume voids around neighboring network-forming polyhedrons was shown to play a decisive role in this correlation, leading to systematically enhanced volumes in comparison with typical molecular substrates, such as polymers.

Simulation study of the impact of interface roughness and void inclusions on Cu(In,Ga)(Se,S)2 solar cells

Typical features of sequentially processed Cu(In1−xGax)(Sey−1Sy)2 (CIGSSe) thin film solar cells such as roughness at particular interfaces and free absorber volume (voids) might influence the device characteristics significantly. We investigated the influence of both, roughness and voids, on the solar cell performance parameters as well as on the device capacitance by means of two-dimensional (2D) device simulations. Increased roughness was found to enhance light absorption and carrier collection whereas the increased interface recombination at the textured heterojunction is of minor relevance compared to the performance gain. However, the enlarged interface area must be taken into account when considering measurements of the device capacitance. Void inclusions in the rear section of the absorber support infrared absorption because of internal reflection. This leads to slight improvement of the device performance unless the back contact coverage does not come below 30%. Additionally, voids appear to be less critical in terms of surface recombination at the void/CIGSSe interface given that increased gallium concentration is present.

Positron annihilation probing of crystallization effects in TAS-235 glass affected by Ga additions

Crystallization effects in Te20As30Se50 glass known also as TAS-235 affected by Ga additions to Ga2Te20As28Se50 and Ga5Te20As25Se50 compositions are probed with positron annihilation spectroscopy in the measuring modes exploring positron lifetimes and Doppler broadening of annihilation line. Occurring of cubic-phase Ga2Se3 droplets with character nanoscale sizes in partially-crystallized Ga2Te20As28Se50 alloy is shown to be associated with agglomeration of intrinsic free-volume voids, this process being enhanced over microcrystalline scale in Ga5Te20As25Se50 alloy. Crystallization changes in the void structure of TAS-235 glass are considered in terms of free-volume evolution under the same principal chemical environment responsible for positron trapping in amorphous and partially crystallized substances.