Relative Fractional Free Volume Research Articles

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.

Structural, morphological, thermal, mechanical, positron annihilation, and γ-ray shielding studies of nano-tungsten oxide/polyvinyl alcohol composites

Tungsten oxide nanoparticles were synthesized. Metal/polymer composites of nano-sized WO3 and poly vinyl alcohol were blended by the casting method with concentrations of WO3. The structural, morphological, thermal, and mechanical characteristics of the prepared samples were investigated. The γ-ray attenuation properties of the synthesized composites were examined using γ-ray spectrometers, and the γ-ray mass absorption coefficients were measured using 662 keV γ-ray of 137Cs radioactive source. The defects and microstructure changes of the composites were probed using positron annihilation lifetime and Doppler broadening techniques. The shielding performance and thermal stability improved with increasing the concentration of WO3, and some of the mechanical properties, in particular the elongation at break, deteriorate. The contribution of WO3 has a strong effect on reducing the relative fractional free volume, which in turn helps to increase the γ-ray absorption of the composites. The use of a 50–70 % WO3/PVA composite is recommended.

Minute-Scale Evolution of Free-Volume Holes in Polyethylenes during the Continuous Stretching Process Observed by In Situ Positron Annihilation Lifetime Experiments

Using the newly developed positron annihilation lifetime spectroscopy (PALS) facility with a high count rate up to 3000 cps, in situ PALS experiments were performed for the first time on the continuous stretching process of polymers to quantitatively analyze the minute-scale evolution of free-volume holes. According to the stress–strain relationship and PALS results of four types of polyethylenes with different crystallinities, the tensile process could be divided into four distinct stages: elastic, initial nonlinear (until yield point), postyield, and strain hardening stages. The increase of o-Ps (orthopositronium) lifetime in the first three stages exhibits an enlargement of free-volume hole size with increasing strain. The decrease of the o-Ps lifetime in the last stage is most probably due to the increasing anisotropy of free-volume holes. The relative fractional free volume FFVr (derived from hole radius R (calculated from the Tao–Eldrup model) and o-Ps intensity) generally increases in the first two stages but remains nearly unchanged in the other two stages. This work demonstrates a new feasibility to disclose minute-scale evolution of microstructure of materials through in situ PALS experiments in the future.

Highly Cross-linked Epoxy Coating for Barring Organophosphate Chemical Warfare Agent Permeation.

Chemical warfare agents (CWAs) can be absorbed in polymeric coatings through absorption and permeation, thus presenting a lethal touch and vapor hazards to people. Developing a highly impermeable polymer coating against CWAs, especially against organophosphate CWAs (OPs), is challenging and desirable. Herein, fluorinated epoxy (F-EP) and epoxy (EP) coatings with different cross-link densities were prepared to resist OPs. The effects of the polymer coating structure, including cross-link density, chemical composition and free volume, on the chemical resistance to dimethyl methylphosphonate (DMMP, Soman simulant) were investigated in detail. Meanwhile, the chemical resistance to Soman and VX was examined. The results reveal that the cross-link density is a critical factor in determining the chemical resistance of the coatings. Highly cross-linked EP and F-EP coatings with dense and solid cross-linked networks can fully bar DMMP and OPs permeation during the test time. At low or medium cross-link densities, the EP coating with a lower retention of DMMP exhibited a higher resistance than the F-EP coating due to the lower interaction with DMMP and smaller free-volume holes and lower relative fractional free volume. These results suggest that increasing the cross-link density is a reasonable approach to control the chemical resistance of polymer networks against OPs.

Thermal barrier coatings for cellulosic substrates: A statistically designed molecular dynamics study of the coating formulation effects on thermal conductivity

A central composite design (CCD) was employed to investigate the effects of cellulose nanocrystal (CNC), cellulose nanofiber (CNF), and relative free volume on the thermal barrier properties of a pigment-based coating for cellulosic substrates, composed of calcium carbonate and poly(styrene-co-methacrylic acid) binder. Average room-temperature thermal conductivity based on three replicates was selected as response and calculated for the different coating formulations using reverse non-equilibrium molecular dynamics (RNEMD) simulation with the Müller-Plathe algorithm. The effects of CNC and relative free volume fractions on the thermal conductivity of the coating were found to be significant, while that of the CNF volume fraction was insignificant. Overall, relative free volume fraction (porosity) had much larger impact on thermal conductivity than CNC volume fraction. Moreover, a weak interaction was observed between these two significant factors. A pore size distribution analysis and average pore size calculation for the coatings (∼5.25 Å for the low and ∼6.50 Å for the high relative free volume fraction) did not reveal any significant effect of CNC on these properties at either the low or high relative free volume fraction. It is speculated that larger CaCO3-CNC interfacial phonon scattering at the low relative free volume fraction leads to the above observations.

Relationship between the Microstructure and Performance of Graphene/Polyethylene Composites Investigated by Positron Annihilation Lifetime Spectroscopy.

The quantitative characterization of microstructure is most desirable for the establishment of structure-property relationships in polymer nanocomposites. In this work, the effects of graphene on the microstructure, mechanical, electrical, and thermal properties of the obtained graphene/polyethylene (PE) composites were investigated. In order to reveal the structure-performance relationship of graphene/PE composites, especially for the effects of the relative free volume fraction (fr) and interfacial interaction intensity (β), positron annihilation lifetime spectroscopy (PALS) was employed for its quantitative description. The relative free volume fraction fr gives a good explanation of the variation for surface resistivity, melting temperature, and thermal stability, and the variation of tensile strength and thermal conductivity agree well with the results of interfacial interaction intensity β. The results showed that fr and β have a significant effect on the properties of the obtained graphene/PE composites, and the effect on the properties was revealed.

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.

Effect of Free-Volume Holes on Dynamic Mechanical Properties of Epoxy Resins for Carbon-Fiber-Reinforced Polymers

Six types of matrices of carbon-fiber-reinforced polymers were prepared from different epoxies, amines, and thermoplastics at different curing conditions. Dynamic mechanical analysis measurements were performed to investigate the dynamic mechanical properties of storage modulus E′, loss modulus E″, damping factor tan δ, and complex viscosity |η*|. Positron annihilation lifetime (PAL) spectroscopy was carried out to evaluate each size and fraction of free-volume holes in the sample. The correlations between the dynamic mechanical properties and relative free-volume fraction were studied by using the Williams–Landel–Ferry equation. With increasing relative free-volume fraction, regular changes of dynamic mechanical properties are revealed: log[E′(T)] and log[|η*|(T)] decrease linearly in the temperature range of Tg(PAL) < T < Trub(E′) (Tg(PAL) is the glass transition temperature determined by PAL measurements; Trub(E′) is the lowest temperature where E′(T) curve coincides with its fitting line in the rubber...

Dielectric and nano-scale free volume properties of polyaniline/polyvinyl alcohol nanocomposites

Polyaniline (PANI)/polyvinyl alcohol (PVA) nanocomposites has been prepared . Scanning electron microscopy, SEM of pure PVA and PANI/PVA nanocomposite films to monitor the morphology and dispersion of PANI on the surface of the PVA films. Positron annihilation lifetime measurements were carried out, in the air, at room temperature with a conventional fast–fast coincidence spectrometer. Dielectric permittivity \(\varepsilon^{{\prime }}\), dielectric loss \(\varepsilon^{{\prime \prime }}\) and AC conductivity \(\sigma_{ac}\) of PANI/PVA nanocomposites has been achieved in the range of frequencies 40 Hz–5 MHz and temperature range of 293–353 K. It was found that; O-ps lifetime, τ3, its intensity, I3 and relative fractional free volume, Fr increase as the PANI content increases. The dielectric permitivity \(\varepsilon^{{\prime }}\) and loss \(\varepsilon^{{\prime \prime }}\) increase with increasing the PANI content and decreases as the frequency increases. The values of the frequency exponent for the correlated barrier hopping \(s\) and the binding energy of the charge carriers \(W_{m}\) for the investigated nanocomposites decrease with increasing PANI content. Finally, the dielectric permittivity \(\varepsilon^{{\prime }}\) was found to be directly proportional to the relative fractional free volume, Fr.

Correlation of electrical and swelling properties with nano free-volume structure of conductive silicone rubber composites

The present study focuses on finding a correlation between the positron annihilation parameters of silicone rubber polydimethylsiloxane (PDMS) composites loaded with different conductive fillers and their swelling and electrical properties. Four types of conductive fillers have been used in this study, i.e., carbon black (CB), graphite (G), copper, and nickel powders. The investigated swelling parameters for PDMS composites such as the maximum degree of swelling Qm%, the penetration rate, P, and consequently the diffusion coefficient, D, decreased with increasing the filler content due to the reduction of the size of free-volume, which could be observed through a decrease of the probability of ortho-positronium (o-Ps) formation I3, that has been measured by the positron annihilation spectroscopy (PALS). Accordingly, a positive correlation has been found of 76.78% and 61.1% between Qm% and the o-Ps lifetime s3, representing the size of free-volume for the CB and G filled composites, respectively. It is worthwhile, mentioning that the CB filled composites exhibit relatively low values of P, D, and Qm% as compared to the G filled composites due to the difference in the physical properties of the filler, particle size, surface area as well as the tendency of the filler particles to make aggregates. On the other hand, the variation of the diffusion coefficient with the relative fractional free-volume Fr is found in good agreement with the Fujita’s free-volume theory. On the other hand, the electrical conductivity increases with increasing the conductive filler content. Positive temperature coefficient of conductivity (PTCC) behavior is detected, except for the composite containing 20 or 25 phr CB, which showed a metallic behavior. Besides, CB filled PDMS composites exhibit higher electrical conductivity as compared to the composites filled with the other three fillers. The electrical conductivity ln (r) is positively correlated with the probability of free annihilation of positrons at interfaces I2, thus suggesting an increase in the electron density with the filler content. The activation energy of conduction, Ea, decreases with the increase in the loading of conductive filler. Moreover, it was noticed that Ea varies with the filler type, the values of Ea increase as the particle size of the filler increases. Finally, a correlation between the free-volume Vf, determined by PALS with the DC electrical conductivity ln(r) is found to be in accordance with Miyamoto and Shibayma model. POLYM. COMPOS., 00:000–000, 2013. V C 2013 Society of Plastics Engineers

Positron annihilation lifetime spectroscopy for miscibility investigations of styrene–butadiene–styrene copolymer/polystyrene blends

The free volume parameters of styrene–butadiene–styrene copolymer/polystyrene (SBS/PS) blends were investigated with positron annihilation lifetime spectroscopy (PALS) in this study. The behaviors of free volume distribution, average free volume, and relative fractional free volume revealed the difference of interfacial miscibility. Based on different models, inter‐chain interaction parameter β, geometric factor γ, and hydrodynamic interaction parameter α obtained from free volume data were employed to further determine the effect of molecular architecture and styrene content on the miscibility. The results suggest the better miscibility in star‐shaped SBS/PS blends than that of corresponding linear SBS/PS systems, even than that of systems containing more styrene unit. In addition, differential scanning calorimetry, dynamic mechanical analysis, and scanning electron microscopy, which are sensitive to heterogeneities in larger domain size, give different results of miscibility from free volume data. It should be attributed the difference of characterization scale. The mechanical property corroborates the results of miscibility. POLYM. ENG. SCI., 54:785–793, 2014. © 2013 Society of Plastics Engineers

Investigations of microstructure and ionic conductivity for (PEO)8-ZnO-LiClO4 polymer nanocomposite electrolytes

The effects of nanosized ZnO on the microstructure, the free volume and the ionic conductivity of poly ethylene oxide (PEO) nanocomposite electrolytes (PEO)8-ZnO-LiClO4 are systematically studied by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and positron annihilation lifetime spectroscopy (PALS), respectively. The experimental results show that the presence of the nanosized ZnO brings about a reduction in the crystallinity of the PEO phase and a very marked increase in ionic conductivity. PALS discrete analysis shows that the free volume size, the free volume concentration and the relative free volume fraction significantly increase with the increase of nano-ZnO. Specially, it is the first time to observe the occurrence of peaks of the free volume distribution split after nano-ZnO has been filled, which indicates that the ZnO added into (PEO)8-LiClO4 has an important effect on the microstructure for nanocomposites due to the interaction between the nano-ZnO and matrix. A direct correlation between the free volume fraction and the ionic conductivity is observed, and the ionic conductivity mechanism is discussed.

The hydrogen bond and free volume property of poly(ether‐urethane) irradiated by neutron

AbstractPoly(ether‐urethane) (PEU) was irradiated by neutron in different atmospheres. The hydrogen bonding interaction was analyzed by Fourier transform infrared (FTIR), and the microstructure of PEU had been investigated by positron annihilation lifetime spectroscopy (PALS). The gas products were detected by gas chromatography after irradiation. The results demonstrated that the irradiation led to more hydrogen bonded carbonyl in PEU, smaller relative free volume fraction, and narrower free volume distribution. It suggested that increasing hydrogen bonds would result in the collapse of free volume. The irradiation induced micro‐phase merging together and the presence of oxygen would accelerate this tendency, which was revealed by PALS. All the results indicated that the chain relaxation led to more hydrogen bonds, and the hydrogen bonding interaction suppressed the free volume. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 381–388, 2010

Relationship between free volume properties and structure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) membranes via various crystallization conditions

The relationship between the crystalline structure and the free volume properties within the amorphous phase of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) membranes, which were prepared via isothermal crystallization processes at various crystallization temperatures, was investigated. From the data of positron annihilation lifetime (PAL) spectroscopy over a temperature range of 25–90°C, the temperature dependence of free volume size, amount, distribution, and relative fractional free volume and thermal expansion behaviors of free volume were discussed. The existence of a rigid-amorphous fraction (RAF) which constrained long-range motions in the semi-crystalline polymer was demonstrated by DSC trace and considered to interpret the temperature dependence of those free volume properties. Based on the crystallization conditions, the effect of the crystallization rate of PHBV polymer was proposed to explain the thermal expansion coefficients of free volume size. The membranes crystallized at faster crystallization rate would accompany higher thermal expansion coefficients of free volume size and the larger free volume size at higher temperatures. Larger distribution of the free volume size and higher thermal expansivity of o-Ps lifetime dispersion (dσ/dt) of isothermally melt-crystallized membranes were observed as a result of the bimodal distribution of the lamellar periodicity, which was obtained by small-angle X-ray scattering (SAXS), and less amount of RAF than that of a cold-crystallized membrane. Conclusively, free volume size and thermal expansion of the free volume in PHBV membranes were affected by the kinetics of crystallization; comparatively, the total amount of free volume and fractional free volume were determined by the final crystallinity. The size distribution of free volume was associated with the crystalline lamellar structure which was dominated by the crystallization conditions.

Free‐volume and crystallinity in low molecular weight poly(ethylene oxide)

AbstractPositron annihilation lifetime spectroscopy (PALS), differential scanning calorimetry, X‐ray diffraction, and polarized light optical microscopy were used to study six low molar mass poly(ethylene oxide) samples with average molar masses ranging from 1 × 103 to 10 × 103 g mol−1. Dynamic light scattering was used to determine molar mass and polydispersity rigorously. Polymer samples with 70–95% crystallinity, which is an unusual range in PALS studies, were prepared by molten material quenching. The ortho‐positronium pick‐off lifetime (τ3) and relative fractional free volume (fv), determined by the free volume model, correlated well with the average molar mass and crystallinity of the polymers. X‐ray diffraction and polarized light optical data support the interpretation of positron annihilation results. PALS parameter, I3, which is associated with high cavity content, remained approximately constant at 20–22% for all samples. The cavities are present as crystallite defects in the spherulitic open texture and the amorphous phase for the low crystallinity sample (e.g., for Mw = 1390) and at the interfaces and in interlamellar spherulite regions of the more crystalline materials. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2400–2409, 2007

The characterization of PP/NBR blends by positron annihilation lifetime spectroscopy (PALS): The effect of composition and dynamic vulcanization

The influence of dynamic vulcanization and blend composition on the free volume properties of polypropylene (PP)/nitrile butadiene rubber (NBR) blends has been studied by using a novel technique based on positron annihilation lifetime spectroscopy (PALS). The non-vulcanized blends presented an increase of average free volume size holes and relative fractional free volume, which is an indication of incompatibility between the blend components. The dynamic vulcanization decreased the PALS parameters because of the presence of the crosslinking both inside the rubber phase and, probably, at the interface. In spite of the significant difference in PALS parameters observed as a function of the vulcanization, the mechanical properties were similar for non-vulcanized and vulcanized blends of similar composition.

Differential scanning calorimetric and free volume study of reactive compatibilization by EPM‐g‐MA of poly(trimethylene terephthalate)/EPDM blends

AbstractDifferential scanning calorimetry (DSC) and positron annihilation lifetime measurements have been carried out to study the effect of the compatibilizer maleic anhydride grafted ethylene propylene copolymer (EPM‐g‐MA) in poly trimethylene terephthalate and ethylene propylene diene monomer (PTT/EPDM) immiscible blends. The DSC results for the blends of 50/50 and 30/70 compositions show two clear glass transition temperatures, indicating that the blends are two‐phase systems. With the addition of compatibilizer, the separation between the two glass transitions decreased, suggesting an increased interaction between the blend components with compatibilizer. At 5 wt % of compatibilizer, the separation between the Tgs reduced in both 50/50 and 30/70 blends. The positron results for the blends without compatibilizer showed an increase in relative fractional free volume, as the EPDM content in the blend is increased. This suggests the coalescence of free volume of EPDM with the free volumes of PTT due to phase separation. However, the effect of compatibilizer in the blends was clearly seen with the observed minimum in free volume parameters at 5% of the compatibilizer, further suggesting that this percent of compatibilizer seems to be the optimum value for these blends. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 740–747, 2006

Microstructure and thermal properties of ethylene‐(vinyl acetate) copolymer/rectorite nanocomposites

AbstractEthylene‐(vinyl acetate) copolymer (EVA)/rectorite nanocomposites were prepared by direct melt extrusion of EVA and organo‐rectorite. The microstructures and thermal properties of EVA nanocomposites were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), solid‐state nuclear magnetic spectroscopy, positron annihilation spectroscopy, thermal gravimetric analysis (TGA) and dynamic mechanical analysis techniques. XRD pattern and SEM images show that the intercalated structure is formed and rectorite is finely dispersed in EVA matrix. When organoclay content of the hybrid increases to 7.5 wt%, or pristine rectorite was used instead of organoclay, the crystallization behavior of EVA nanocomposite changes greatly and the ratio of the monoclinic to orthorhombic crystal increases significantly. The relative fractional free volume of the nanocomposite decreases with the increasing organo‐rectorite content, and the values of damping factor (tan δ) for all nanocomposites are lower than that of pure EVA. These facts illuminate that intercalated structure restricts the segment motion and mobilization of polymer chain. TGA results of EVA nanocomposites in air indicate that deacylation of EVA is accelerated because of the catalytic effect and the thermal degradation of the main chain is delayed owing to the barrier effect of silicate layers. Copyright © 2005 Society of Chemical Industry

Influence of free volume on the mechanical properties of Epoxy/poly (methylmethacrylate) blends

Positron lifetime measurements have been performed to investigate the free volume dependence of the mechanical properties of Epoxy/poly methyl methacrylate (PMMA) blends of varying composition of PMMA (2.5, 5, 7.5, 10, 12.5, 15, and 17.5 wt%). The mechanical properties of the blends have been evaluated according to the (ASTM) standard. The positron results indicate that the mechanical properties like tensile strength and surface hardness have a significant dependence on the free volume of the blends. It is also found that the blends of the present study show positive deviation from the familiar linear additivity rule suggesting the immiscible nature of the blends. Further, up to 5-wt% of PMMA in the blend, an increase in relative fractional free volume correlates well with a decrease of tensile strength.

Positron Annihilation in Metallocene Ethylene/1-Hexene Copolymers Related to Their Structure and Mechanical Properties

Positron lifetime measurements are reported for a linear polyethylene and several ethylene−1-hexene copolymers synthesized with metallocene catalysts. The lifetimes, their intensity, the relaxation time, and the relative free volume fraction obtained are correlated with comonomer content, crystallinity, location of β relaxation, and some mechanical parameters, such as elastic modulus and microhardness. A structural discontinuity is found with comonomer molar fraction that evidences the existence of two different regimes: the first one including the homopolymer and copolymers with contents up to around 5−7 mol % and the second regime for higher compositions. Possible reasons for this behavior are discussed. In addition, a linear relationship is found between the location of the β relaxation, which is associated with large-scale cooperative motions in the amorphous environments, and the free volume fraction: the higher the relaxation temperature is, the lower free volume value is obtained.