Free Volume Hole Size Research Articles

Effect of electron beam irradiation on hole structure of cellulose acetate membrane

Cellulose acetate (CA) is one of the most popular biomaterials used for filtration membranes in water treatment.In this study, the CA membranes immersing in water were irradiated by electron beam, and the hole structure of CA membranes at different irradiation doses was investigated. The SEM pictures show the combination of micro-meter sized pores in CA membrane during the irradiation process. The electrochemical impedance spectroscopy(EIS) results reveal the membrane resistance decreases with the irradiation dose, which implies easier ion permeation. The positron annihilation lifetime measurement (PAL) results indicate the mean free-volume hole size becomes smaller with increasing irradiation dose. The above results demonstrate that electron beam irradiation would lead to chain scission of CA, and then the formation of new pores and pores combination, which provides more channels for ion permeation. FTIR results suggest stronger hydrogen bonding in CA after the irradiation in water. The differential scanning calorimetry (DSC)results indicate that the crystallinity has been enhanced with the irradiation dose up to 10 kGy and reduces in the range from 10 kGy to 80 kGy. The results could provide the clues explaining the performance deterioration of CA membrane during treating radioactive waste water.

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Exposure: Observations with Positron Annihilation Lifetime Spectroscopy.

Atomic oxygen (AO) is one of the dominant components of the residual atmosphere in low Earth orbit. AO collides with spacecraft with a translational energy of 5 eV, forming nanoscale protrusions on polymeric materials. To clarify the influences of a polymer's chemical structure on the formation of AO-induced microstructures, this study investigated the size of free-volume holes and the layer thickness that interacted with AO for polyethylene (PE), polypropylene (PP), and polystyrene (PS) by positron annihilation lifetime spectroscopy. The injection energies of positrons varied from 1.3 to 10 keV to adjust the injection depth (range) into the polymers (40 nm-1.6 μm). For the pristine films, the lifetime of ortho-positronium (o-Ps, τ3) was longer in the order of PS, PP, and PE regardless of the injection energy of positrons, showing the different sizes of free-volume holes with radii of 0.29, 0.31, and 0.32 nm, respectively. The fraction of the decay component corresponding to o-Ps in all decay components (relative intensity of o-Ps, I3) was used to investigate the chemical change induced by AO exposure. The I3 values for the three polymers were decreased by AO exposure of (2-5) × 1018 atoms/cm2 or more at a depth of 40-48 nm, obtained by 1.3 keV positrons. This indicates that AO formed polar groups (i.e., an oxidized layer) on the polymer surfaces. The maximum depths of such chemical change for PE and PP were deeper than that for PS. The different sizes of free-volume holes would affect the diffusion or ballistic penetration of AO, resulting in the difference in the oxidized layers' thicknesses and surface morphologies.

Free volume can control biodegradation rate of polylactic acid

We evaluated whether the enzymatic degradation rate of biodegradable plastics is related to the amorphous structure of the polymer. The “free-volume,” meaning the cavity between the polymer chains in the amorphous part, is known to influence various physical properties. In this study, enzymatic degradation tests were performed on polylactic acid (PLA) films with free-volume hole sizes estimated by positron annihilation lifetime spectroscopy (PALS). Films with different free-volume hole sizes near the surface region degraded in different modes, and a positive correlation, with a correlation coefficient of 0.713, was observed between the degradation rate in the initial 30 min and the free volume size near the surface. These results suggest the possibility of controlling the degradation rate of the amorphous region by controlling the free volume size, which is important for future applications of biodegradable plastics in a wide range of fields.

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.

Molecular dynamics study of the environmental stress cracking agent assisted cavitation in linear and branched polyethylene

We used molecular dynamics (MD) simulation to investigate whether an environmental stress cracking (ESC) agent would help the cavitation process in linear polyethylene (LPE) and branched polyethylene (BPE) by examining the free volume hole size and spatial distributions at the [PE]-[ESC agent] interfaces. The BPE models used contained 10 and 82 butyl/hexyl branches/1000 backbone carbons and the ESC agent used was nonyl ethoxylate (NE) at concentrations 0.001–1.4 wt%. The results show that there exist no significant changes in the packings of LPE and BPE chains, as characterized by their radial distribution functions (RDFs), in the presence of NE regardless its concentration. However, molecular packings at the [PE]-[NE] interfaces vary with NE concentration. In particular, ethylene (E) segments of NE, compared to the ethylene oxide (EO) segments, exhibit stronger association with LPE and BPE chains. We used the Voronoi tessellation method to determine the size and spatial distributions of free volume holes at the [PE]-[NE] interfaces and found that more and larger free volume holes formed around the E-segments than the EO-segments. And such effect was found to be more pronounced in the case of LPE chains than the BPE chains. We further investigated the free volume holes coalescence dynamics using the Fourier mode analysis. The number of frequency modes and the corresponding intensities of the E-segments were significantly higher than those of EO-segments, suggesting that E-segments exhibited a higher tendency to form more and larger free volume holes. This suggests that [PE]-[E-segments] interfaces have a higher chance to form larger voids (i.e., cavitation, a feature of brittle failure that is experimentally observed in ESC). The tendency intensified with increasing NE concentration but was reduced by the presence of short chain branches. The high mobility of terminal carbons in the short chain branches is believed to suppress the formation of large free volume holes at the [PE]-[E-segments] interfaces by absorbing the newly formed free volume by that of the terminal carbons.

Positron annihilation lifetime spectroscopic analysis of Nafion and graft‐type polymer electrolyte membranes for fuel cell application

AbstractThe subnano free‐volume hole features of Nafion‐212 and poly(styrene sulfonic acid) grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes are investigated by using the positron annihilation lifetime spectroscopic analyses with three‐ and four‐component models (i.e., one‐ and two‐ortho‐positronium [o‐Ps] components). The four‐component model provides a more adequate description of free‐volume hole features for both membranes, in which the longer o‐Ps lifetime is assigned to the larger free volume hole sizes in the mobile side chains, while the shorter o‐Ps lifetime is associated with the smaller free volume hole sizes within the backbones (the rigid amorphous fractions and the crystalline‐amorphous interfaces) and the interfaces between the main chains and the side chains. The o‐Ps annihilation is found to occur primarily in the side chains. The subnano volume hole features revealed by the positron annihilation lifetime (PAL) spectra suggest the primary water uptake and conductance in the side chains and the possible presence of water molecules in the rigid amorphous fractions and the interfaces. Note that the three‐component model as usually reported in the literature may underestimate the lifetime and intensity of ortho‐positronium annihilation.

Influence of Crosslinking Extent on Free Volumes of Silicone Rubber and Water Diffusion after Corona Discharge.

Silicone rubber is widely used as an insulating material. In this article, silicone rubber samples were prepared by varying the content of crosslinker (2,5-bis(tert-butyl-peroxy)-2,5-dimethylhexane, DBPMH), and the free volume holes in the samples were investigated by means of positron annihilation lifetime spectroscopy (PALS) measurement. The surface chemical structure, surface micromorphology and water diffusion of the samples after corona discharge treatment were studied by FTIR, SEM and EIS measurements, respectively. As the crosslinker weight ratio increased from 0.2 wt.% to 1.5 wt.%, the mean free volume hole size first decreased and then remained unchanged. However, the concentration of free volume holes did not vary as the crosslinker weight ratio increased. SEM morphologies show that surface cracks were produced on samples having high crosslinking levels after corona treatment. The water diffusion coefficient of samples after corona treatment increased from 3.13 × 10−10 cm2 s−1 to 17.68 × 10−10 cm2 s−1 in the initial immersion period, as the crosslinker weight ratio increased from 0.2 wt.% to 3.0 wt.%. The results indicated that deterioration of samples with high crosslinking levels were more serious and water repellency more easily lost. The corona resistance ability of low crosslinking level silicone rubber stems from internal low molecular weight molecules.

Effect of conversion on epoxy resin properties: Combined molecular dynamics simulation and experimental study

We investigated epoxy resin consisting of diglycidyl ether of bisphenol A (DGEBA) and bis-( p -aminocyclohexyl)methane (PACM) and found that the density increased and decreased in the low- and high-conversion regions, respectively, by using experiments and all-atom (AA) molecular dynamics (MD) simulations. To understand this feature qualitatively, we conducted course-grained (CG) MD simulations. For the flexible and rigid CG models, the calculated density increased and decreased monotonically, respectively, in contrast to the experimental density. To develop a more realistic CG model, which is denoted as CG-EP, we derived angular parameters based on AA-MD simulations. It was found that the CG-EP successfully reproduced the trend of the experimental density, suggesting the importance of molecular flexibility. In addition, the progress of the conversion monotonically increased the free volume hole size, which is consistent with the result of positron annihilation lifetime spectroscopy. Furthermore, we experimentally observed that the Young's modulus suddenly decreased at 50%, as the conversion progressed. The CG analysis indicated that this trend was also attributed to the molecular flexibility. • Density of the present epoxy resin is maximum near the 50% conversion. • A developed coarse-grained model can reproduce the experimental density. • Molecular flexibility of epoxy resin significantly affects the density. • Molecular flexibility of epoxy resin significantly affects the Young's modulus.

Evaluation of transport mechanism and nanostructure of nonperfluorinated PVA/sPTA proton exchange membrane for fuel cell application

AbstractPolyvinyl alcohol (PVA) with sulfophthalic acid (sPTA) as a proton exchange membrane (PEM) for fuel cell applications was prepared by the solvent casting method. The fabricated membranes were obtained by varying the concentration of sPTA from 5 to 30 wt.% of PVA and thermally crosslinked at a temperature of 100°C for 1 h. Crosslinked PVA/sPTA membranes were characterized by FTIR spectroscopy, scanning electron microscopy (SEM), and wide‐angle X‐ray diffraction (XRD). The transport properties were investigated by determining ion exchange capacity (IEC), water uptake, methanol permeability, and proton conductivity. Free volume hole size and its distribution of crosslinked PVA membrane were measured by positron annihilation lifetime spectroscopy (PALS). With an increase in sPTA concentration in the PVA membrane, both IEC and proton conductivity increased, but water uptake and methanol permeability decreased. At room temperature, IEC values were increased from 0.3 to 1.6 meq/g, and proton conductivity varied from 0.2 to 3.5 S/cm. According to PALS data, increasing the sPTA content causes the free volume content of crosslinked PVA membranes to decrease. The results were analyzed and presented in the context of free volume theory, in which the concentrations of nanostructure free volume defects are related to the membrane's transport capabilities.

Modification of polyamide reverse osmosis membranes for the separation of urea

Reverse osmosis (RO) membranes are the gold standard for water desalination and have been in use for over three decades. Even though RO membranes exhibit excellent performance rejecting monovalent and divalent salt ions, they do not reject small, neutral, and uncharged molecules, such as urea, to a level to produce potable water, especially at near-neutral pH. Due to the fast, uncontrolled nature of the interfacial polymerization reaction, the polyamide layer contains both network and aggregate free volume holes (pores). Because urea rejection is dominated by the size exclusion mechanism, reducing the free volume to reduce the passage of the urea through the membrane is needed. In this regard, the modification of RO membranes to increase the degree of cross-linking and/or decrease the free volume hole size is an ideal approach. We hypothesize that if the polyamide layer can be modified with a diamine, then the urea rejection will be increased. In this work, we modified polyamide RO membrane separation layers of commercial membranes (Dupont XLE and BW30XFR) using the carbodiimide chemistry followed by the application of m-phenylenediamine (MPD) and heat treatment in the post-modification stage. The modified membranes were characterized using ATR-FTIR, XPS, SEM, contact angle goniometry, and electrokinetic analyzer. Membranes were performance tested for water permeance, NaCl rejection, and urea rejection using a dead-end stirred cell. Compared to the control membranes, the modified XLE membranes and modified BW30XFR membranes improved the urea rejection from 16.8% to 54.9% and from 48.4% to 64.6%, but a reduction in water permeance by a factor of up to 4.7 and 2.7 respectively. The results show that combining the application of MPD and heat treatment can enhance the urea rejection of the membranes significantly.

Relative effects of polymer composition and sample preparation on glass dynamics

Modern design of common adhesives, composites and polymeric parts makes use of polymer glasses that are stiff enough to maintain their shape under a high stress while still having a ductile behavior after the yield point. Typically, material compositions are tuned with co-monomers, polymer blends, plasticizers, or other additives to arrive at a tradeoff between the elastic modulus and toughness. In contrast, strong changes to the mechanics of a glass are possible by changing only the molecular packing during vitrification or even deep in the glassy state. For example, physical aging or processing techniques such as physical vapor deposition increase the density, embrittle the material, and increase elastic modulus. Here, we use molecular simulations, validated by positron annihilation lifetime spectroscopy (PALS) and quasi-elastic neutron scattering, to understand the free volume distribution and the resulting dynamics of glassy co-polymers where the composition is systemically varied between polar 5-norbornene-2-methanol (NBOH) and non-polar ethylidene norbornene (ENB) monomers. In these polymer glasses, we analyze the structural features of the unoccupied volume using clustering analysis, where the clustering is parameterized to reproduce experimental measurements of the same features from PALS. Further, we analyze the dynamics, quantified by the Debye-Waller factor, and compare the results with softer, lower density states. Our findings indicate that faster structural relaxations and potentially improved ductility are possible through changes to the geometric structure and fraction of the free volume, and that the resulting changes to the glass dynamics are comparable to large changes in the monomer composition.

Synthesis and characterization of PVA/sPTA proton exchange membranes for fuel cell applications

Using the casting method, proton exchange membranes PEMs for fuel cell were prepared by thermal cross-linking of polyvinyl alcohol (PVA) with sulfophthalic acid (sPTA). The membranes were prepared with different cross-linking temperatures with thickness ranged between 100 to 150 μm. The prepared membranes were characterized by different techniques such as ion exchange capacity, thermal analyses, water uptake and proton conductivity. The prepared membranes showed proton conductivity between 8.46-32.7 mS/cm. Increasing the cross-linking temperature increased the cross-linking density, resulting in an enhancement in the tensile strength and thermal stability of the prepared membranes. The membrane hole sizes were obtained using positron annihilation lifetime spectroscopy (PALS) measurements. Cross-linked PVA/sPTA membranes showed a strong correlation between the free volume hole size with the mechanical stability and methanol permeability of the membranes.

Characterization, nanostructure, and transport properties of styrene grafted PVA/SiO2 hybrid nanocomposite membranes: Positron lifetime study

AbstractGamma radiation‐induced graft copolymerization of styrene monomer on a polyvinyl alcohol film has successfully prepared a proton exchange membrane based on polyvinyl alcohol/silicon nanoparticles (PVA/SiO2), for use in fuel cells. The physical and chemical properties of the prepared hybrid nanocomposite membranes were examined by the Fourier transformer infrared spectrometer (FTIR) and the hardness test. The electrochemical properties were studied as a function of the degree of grafting at different doses of gamma irradiation. Ion exchange capacity (IEC) was found to improve with rising in the degree of grafting and then slightly decrease because of styrene homo‐polymerizes as the dose of gamma irradiation intensifies. As the graft grade increases, the proton's conductivity rises to 30% of the degree of grafting and then begins to stabilize. The free volume hole size obtained from positron annihilation lifetime (PAL) experiments was found to increase by the further accumulation of the styrene graft content. The results of PAL support electrochemical results. Also, a strong link between the results of nanoscopic properties from PAL, and the results of chemical and physical properties (macroscopic results) has been successfully established. The prepared PVA/SiO2‐grafted sulfonated styrene seems to be a potential alternative to Nafion for fuel cell applications.

Synthesis, structural, thermal, mechanical, and nano-scale free volume properties of novel PbO/PVC/PMMA nanocomposites

Novel nanocomposite films (PbO/PVC/PMMA) were prepared by solution casting. The influence of PbO loading, from 0.5 to 4 wt%, on the structural, thermal, mechanical, and free volume properties of the PVC/PMMA blend was reported. The structural results showed that the average crystallite size of PbO is 47.8 nm and it had nanoparticles morphology with average size, 46.3 nm as confirmed by HR-TEM. Loading PbO appeared new crystalline peaks attributable to α-PbO and increased the blend amorphicity. The blend had a smooth surface with fingerprint shape, which confirmed the miscibility between PVC & PMMA while PbO loading increased the surface roughness. Good distribution was reported to 2 wt% PbO loading. Thermogravimetric analysis (TGA) revealed an increase in the thermal stability of PNCs films. Higher storage moduli of PNCs films were reported as revealed from Dynamic Mechanical Analysis (DMA) results besides the increase in Tg with loading which confirmed the interaction between PbO & blend and enable PNCs to have better mechanical properties. Positron annihilation lifetime spectroscopy (PALS) measurements manifested a decrease in the ortho-positronium pick off lifetime (τ3) and the free volume (VF) with loading. This is due to the reduction in the size of free volume holes in regions near PbO, the polar groups in the blend matrix, and the hole-filling mechanism. τ3 & VF increased with heating due to the thermal expansion of the free volume holes within the PVC/PMMA matrix. As the main result of this work, PbO formed small free-volume structures within the PVC/PMMA matrix, which lead to the superior thermal and mechanical properties of nanocomposite films that could widen the blend applications.

Investigation and modelling of the water transport properties in unfilled EPDM elastomers

The water sorption properties (i.e. diffusivity and solubility) of three unfilled vulcanized EPDMs were investigated in wet atmosphere (typically between 0 and 95% RH) and in pure distilled water (denoted as 100% RH) at 70 °C with an IGAsorp dynamic sorption analyzer. In the 0–0.5 activity range, all EPDMs obey to the usual Fick and Henry's laws. In contrast, at activities higher than 0.5, the water sorption behavior of all EPDMs deviates from these two laws because of the formation of water clusters. Park's equation was used for simulating accurately the sorption isotherms and thus, accessing Henry's constant and the average size of water clusters. In wet atmospheres, clusters would be composed of about 8 ± 4 molecules corresponding approximately to the size of free volume holes. In contrast, in pure distilled water, clusters consist in a much larger number of water molecules (between 55 and 71) presumably because of an exacerbated swelling of EPDM samples. Thus, immersion in water solution is more drastic than exposure to fully saturated wet atmosphere. The impact of the EPDM formulation (nature and content of crosslinking agent) on these experimental results is clearly of the second order of magnitude.

Characterization and evaluation of Nafion HP JP as proton exchange membrane: transport properties, nanostructure, morphology, and cell performance

In this study, a novel membrane (Nafion HP JP) was studied and investigated intensively for use as a proton exchange membrane for fuel cell. A standard membrane, Nafion NRE212, was also studied under the same conditions for comparison. Fourier transform infrared spectroscopy, thermogravimetric analysis, wide-angle x-ray diffraction, atomic force microscope (AFM), proton conductivity, and mechanical test were used to study the structure and properties of the membranes. In addition, methanol permeability was measured as a function of methanol concentration. Nafion HP JP membrane exhibited a lower methanol permeability over the concentration range studied. Furthermore, Nafion HP JP has Young’s modulus of 88.3 MPa and tensile strength of 13.2 MPa which are higher than those of Nafion NRE212 (39.85 and 9.5 MPa, respectively). AFM data confirmed the higher value of water uptake for Nafion HP JP membrane. Free volume distribution data measured using positron annihilation lifetime (PAL) showed that the novel membrane has a smaller free volume hole size than that of Nafion NRE212. Moreover, the cell performance of the single cell test using the investigated membrane, hydrogen as fuel and oxygen as oxidant, was investigated at different temperatures and humidities. The power density delivered using Nafion HP JP is higher than that from Nafion NRE212 especially at 50 °C and 30% relative humidity. In addition, the membrane durability of both samples was performed for about 220 h and Nafion HP JP has an open circuit voltage reduction rate of 0.136 mV/h that nearly half that for Nafion NRE212 membrane. The optimal concentration for direct methanol fuel cell DMFC operation using both membranes was found to be 2 M which totally agrees with methanol permeability and PAL data. The obtained results reflect that Nafion HP JP shows great potential for fuel cell applications.

Correlation between the ion permeation and free volume property in ethyl cellulose film during the acid treatment

A correlation between the ion permeation and free-volume hole size was found by theoretical deduction and confirmed by the corresponding experiment.

Degradation mechanism of carbon fiber-reinforced thermoplastics exposed to hot steam studied by chemical and structural analyses of nylon 6 matrix

Degradation testing of nylon 6-based carbon fiber-reinforced thermoplastic (CFRTP) was performed by exposure to hot steam in a closed vessel. The degraded structure was investigated to understand the degradation mechanism. Aging-induced micro-cracking in the nylon 6 matrix of CFRTP indicated that internal stress was generated by changes in its local density, along with a decrease in the molecular weight due to hydrolysis. Aging process of nylon 6 increased the free volume hole size in the amorphous part and also induced crystallization, suggesting that the density of the amorphous part decreases with increasing crystallinity in the matrix. Therefore, the degradation mechanism of the CFRTP was proposed to begin with a change in the local density of nylon 6 owing to crystallization, followed by water infiltration and hydrolysis of the matrix. Stress due to density changes combined with reduced molecular weight forms micro-cracks, decreasing the mechanical strength of CFRTP.

Effect of TiO2 Nanofiller on Electrical Conductivity of ABS/TiO2 Polymer Nanocomposites: A Free Volume Study

Acrylonitrile Butadiene Styrene (ABS) / Titanium dioxide (TiO2) Polymer nanocomposites with different TiO2 wt % were prepared by solution casting method. Microstructural characterization of ABS/TiO2 polymer nanocomposites was performed by Positron Lifetime Spectroscopy. The increased positron lifetime parameters viz., o-Ps lifetime (τ3) and free volume size (Vf) from 0.2 to 1.0 wt% of TiO2 loading suggests the formation of interfaces due to the increased agglomeration of TiO2 nanoparticles. The increase of two orders of magnitude change in electrical conductivity (σ) of ABS/TiO2 polymer nanocomposites with the increasing free volume hole size (Vf) as a function of TiO2 nanoparticles loading . The increased electrical conductivity (σ) with the increased free volume hole sizes (Vf) indicates the increased interfacial space for the movements of charge carriers and TiO2 ions. Miyamoto and Shibayma model for ion conduction has been used to explore the effect of free volume hole size (Vf) on the electrical conductivity (σ) in ABS/TiO2 polymer nanocomposites. Fourier Transform Infrared Spectroscopy (FTIR) suggests the improved chemical and physical interaction between the functional groups of TiO2 and ABS polymer side chain

Natural weathering mechanism of isotatic polypropylene under different outdoor climates in China

In order to explore the natural weathering mechanism of isotatic polypropylene (iPP) served in different outdoor climates, iPP samples were exposed to four typical climate conditions of China for four years. The microstructural changes of iPP were investigated by positron annihilation lifetime spectroscopy (PALS), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC) and wide-angle X-ray diffraction (WAXD). As the degradation processed, the molecular defects such as carbonyl and hydroxyl groups increased. Because of the formation of the new imperfect crystal caused by chemi-crystallization, the crystallinity increased initially and then reached a plateau. Due to the increase of crystallinity, the amounts of defects in crystalline-amorphous interfacial phase increased and the concentration of free volume holes decreased with exposure time. In addition, the interaction between functional groups constrained the mobility of molecular segments, which persistently decreased the size of free volume holes. The results showed that the freed molecule segments released by chain scission formed new crystals in the amorphous region during the degradation process. In addition, the similar microstructural changes of iPP weathered in different stations showed the similar degradation mechanism. Furthermore, the influence of the temperature on the degradation rate of iPP was more significant than other environmental factors.