Free Volume Hole Size Research Articles

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.

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...

In situ measurements of free volume during recovery process of a shape memory polymer

The physics and mechanics governing the shape memory effect in shape memory polymers is not fully understood at the nanoscale. A very recent publication highligths a possible link between the free volume of the polymer and its ability to recover the original shape in shape memory cycles. In this study, it is demonstrated that there is a reorganisation of the monomeric unit of polymer chains via molecular sized free volume that makes the shape recovery possible. For that purpose, the free volume properties are measured in situ during the shape recovery process of a crosslinked polycyclooctene shape memory polymer by employing positron annihilation lifetime spectroscopy. By this technique, the size and distribution of the dynamic subnanoscale free volume holes are obtained by measuring the lifetime of the positrons inside the polymer at different stages of the shape recovery.

Study on poly (ethylene-co-vinyl acetate), acrylonitrile butadiene copolymer, and their blend reinforced with carbon black using position annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) have been used to investigate the effect of filling each of poly (ethylene-co-vinyl acetate) (EVA), acrylonitrile butadiene rubber (NBR), and their blend (50/50wt-%) with different concentrations of high-abrasion furnace (HAF) (0–60 phr). PALS is characterised by the free-volume parameters (Vh and F %), which is a measure of the size and fraction of free-volume holes in the composites. The results show that the variation in Vh and F % of free-volume holes depends on the chemical structure and the degree of crystallinity of each polymer. The NBR has a smaller size (127.78 Å3) and lower fraction (4.18%) of free-volume holes than the corresponding free-volume holes (177.48 Å3 and 7.03%) in EVA. The size of the free-volume holes increases in EVA with increasing HAF content up to 20 phr in polymer matrix while it decreases in NBR. The fractions (F %) of free-volume holes decrease due to the reduction in positronium formation in polymer and positron trapping at polymer interface. The results also show an inverse correlation between free-volume parameters (Vh and F %) and tensile strength value, and a direct correlation with elongation at break as a function of HAF content in EVA and NBR. The electrical properties are negatively correlated with lifetime of free positron, τ2, and positively with its relative intensity (I2).

Electrically conductive epoxy-polyester-graphite nanocomposites modified with aromatic amines

Abstract Epoxy resin nanocomposites containing graphite nanosheets (GNS) and unsaturated polyester resin (UPR) were prepared by incorporating GNS in semi interpenetrating network (semi-IPN) of epoxy resin and unsaturated polyester resin (E-UPR). GNS are prepared by thermal shock treatment of sulphuric acid-graphite intercalation compound (GIC) followed by ultra-sonication. The E-UPR-GNS semi-IPNs were further modified with aromatic amines such as benzidine (Bz) and diphenyl amine (DPA). All the samples were prepared by mixing Triethylenetetramine (TETA) as the curing agent at two different temperatures viz. room temperature (RT) and low temperature (LT; 5 °C). A transition is observed in the conducting properties of the samples: from an electrical insulator to an electrical semiconductor in case of nanocomposites especially on addition of GNS to E-UPR semi-IPNs. A sharp improvement is observed in conductivity after modification with diphenyl amine (DPA) cured at LT. The nanocomposites obtained at LT exhibit homogeneity with minimum anisotropy in conductivity. The morphology, mechanical, thermal and electrical properties were evaluated using various analytical techniques. Positron annihilation spectroscopy was carried out to correlate the measured properties with the free volume and free volume hole size distribution. The results are consistent with the reported studies.

Degradation of encapsulants for photovoltaic modules made of ethylene vinyl acetate studied by positron annihilation lifetime spectroscopy

The structure of ethylene vinyl acetate (EVA) encapsulants of crystalline-Si photovoltaic modules after the damp heat (DH) test was evaluated by positron annihilation lifetime spectroscopy (PALS). A reduction in free-volume hole size, which indicates the progress of deacetylation, was observed after the DH test. The difference in lifetime (Δτ) between the initial and DH-tested samples clearly increased after the DH test for 3000 h. The increase in Δτ was correlated with the acetic acid concentration in the EVA estimated by ion chromatography. The depth profile analysis by slow positron beam PALS revealed that Δτ in the near-surface region of the Si-cell side was significantly larger than that of the cover-glass side. This result indicates that deacetylation near the Si cell/EVA interface is accelerated.

Subnanoscopic inhomogeneities in model end-linked PDMS networks probed by positron annihilation lifetime spectroscopy and their effects on thermomechanical properties

The subnanoscopic free volume hole properties of model poly(dimethylsiloxane) (PDMS) networks with unimodal and bimodal distribution of chain lengths between the cross-links were determined using positron annihilation life time spectroscopy (PALS). Wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) studies were carried out to further understand the structural inhomogeneities of the networks at different length scales. The networks were prepared by end-linking combinations of sufficiently longer (Mn = 18,952) and shorter (Mn = 4323) precursor PDMS chains relative to entanglement spacing (Me). The macroscopic properties of the networks were studied by tensile testing, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), as well as equilibrium swelling measurements. Through PALS studies it was possible to delineate the subtle variations in the elementary mesh sizes, a measure of the segmental spacing, of the unimodal short and long chain networks influenced by cross-links or entanglements. The intersegmental spacing of the unimodal PDMS networks at a length scale of few angstroms obtained from WAXD measurements were found to be almost in quantitative agreement with the free volume hole sizes obtained from PALS. SAXS measurements on solvent swollen PDMS networks from short and long chain precursors were used to obtain correlation length of the networks at a higher length scale of a few nanometers. Further, the studies revealed that the enhanced mechanical properties seen in certain bimodal elastomer networks are connected with topological features of the networks. A noteworthy finding from PALS studies was that the bimodal composition with 20 wt% short chain, which showed the best combination of mechanical properties with a feature of stress-upturn in tensile testing, exhibited the least free volume size and relatively lower dispersion in its size distribution among the networks. These fundamental studies are expected to provide new insights into the network structure of model elastomers at a molecular scale and its possible connection with macroscopic properties.

Whole-Pattern Fitting and Positron Annihilation Studies of Magnetic PVA/α-Fe2O3 Nanocomposites

A low-temperature solution combustion method was used to synthesize α-Fe2O3 nanoparticles. Magnetic polyvinyl alcohol (PVA)/α-Fe2O3/NaCl nanocomposites were prepared by solvent cast method. The Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) results are in confirmation with X-ray diffraction (XRD) results indicating the formation of nanocomposites. The microcrystalline parameters, crystallite size (<N>), lattice strain (g in %), stacking faults (α d ), and twin faults (β) of prepared polymer nanocomposites were evaluated by whole-pattern fitting technique. The refinement was carried out using the computed microstructural parameters in which the twin faults and stacking faults did not vary much and statistical deviation was less than 5 %. Positron annihilation lifetime spectroscopy (PALS) was used for microstructural characterization. PALS results show that the ortho-positronium (o-Ps) lifetime (τ3) increases gradually as a function of nanoparticle concentration and about 219 ps increase observed from1.50 to1.71 ns at 3 wt%. This indicates the increase of free volume hole size (V f ) from 54.47 to 72.18 Å3. The o-Ps intensities (I 3) decrease indicating the inhibition of o-Ps formation upon incorporation of nanoparticles into PVA. The increase in I 2 values suggests the increased annihilation at the interface region. Positron lifetime parameters, viz., o-Ps lifetime, and its intensities indicate the effect of quenching and inhibition upon incorporation of metal oxide nanoparticles and inorganic salt into PVA.

Addition of Monovalent Electrolytes to Improve Storage Stability of Freeze-Dried Protein Formulations

This study investigates the effect of low levels of electrolytes on storage stability in freeze-dried sucrose-based protein formulations. Both bovine serum albumin and recombinant human serum albumin were freeze dried with sucrose and alkali halides (LiCl, NaCl, KCl, RbCl, and CsCl) at selected low levels. All formulations were stored at 50°C and 65°C up to 2 months and then assayed for protein aggregation. The data demonstrate that low levels of LiCl and NaCl enhance stability. No obvious correlations with either protein secondary structure or global dynamics (structural relaxation time) were found. However, good correlations were found between stability and both free-volume hole size via positron annihilation lifetime spectroscopy (PALS) and fast dynamics by neutron scattering. Volume changes on mixing and the partial molal volume of salt were also studied in an effort to detect decreases in free volume. These data did not support the hypothesis that reduction in free volume was the primary mechanism for salt-induced stabilization. Finally, a positive effect of postlyophilization annealing on stability was demonstrated. In summary, we find that small amounts of LiCl and NaCl significantly stabilize these proteins, which is a result at variance with conventional formulation wisdom.

Molecular scale cure rate dependence of thermoset matrix polymers

This manuscript demonstrates the molecular scale cure rate dependence of di-functional epoxide based thermoset polymers cured with amines. A series of cure heating ramp rates were used to determine the influence of ramp rate on the glass transition temperature (Tg) and sub-Tg transitions and the average free volume hole size in these systems. The networks were comprised of 3,3′-diaminodiphenyl sulfone (33DDS) and diglycidyl ether of bisphenol F (DGEBF) and were cured at ramp rates ranging from 0.5 to 20°C/min. Differential scanning calorimetry (DSC) and NIR spectroscopy were used to explore the cure ramp rate dependence of the polymer network growth, whereas broadband dielectric spectroscopy (BDS) and free volume hole size measurements were used to interrogate networks’ molecular level structural variations upon curing at variable heating ramp rates. It was found that although the Tg of the polymer matrices was similar, the NIR and DSC measurements revealed a strong correlation for how these networks grow in relation to the cure heating ramp rate. The free volume analysis and BDS results for the cured samples suggest differences in the molecular architecture of the matrix polymers due to cure heating rate dependence.

Connecting free volume with shape memory properties in noncytotoxic gamma‐irradiated polycyclooctene

ABSTRACTThe free volume holes of a shape memory polymer have been analyzed considering that the empty space between molecules is necessary for the molecular motion, and the shape memory response is based on polymer segments acting as molecular switches through variable flexibility with temperature or other stimuli. Therefore, thermomechanical analysis (TMA) and positron annihilation lifetime spectroscopy (PALS) have been applied to analyze shape recovery and free volume hole sizes in gamma‐irradiated polycyclooctene (PCO) samples, as a noncytotoxic alternative to more conventional PCO crosslinked via peroxide for future applications in medicine. Thus, a first approach relating structure, free volume holes and shape memory properties in gamma‐irradiated PCO is presented. The results suggest that free volume holes caused by gamma irradiation in PCO samples facilitate the recovery process by improving movement of polymer chains and open possibilities for the design and control of the macroscopic response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1080–1088

Free volume and gas permeation in anthracene maleimide-based polymers of intrinsic microporosity.

High free-volume copolymers were prepared via polycondensation with 2,3,5,6,-tetrafluoroterephthalonitrile (TFTPN) in which a portion of the 3,3,3',3'-tetramethyl-1,1'-spirobisindane (TTSBI) of PIM-1 was replaced with dibutyl anthracene maleimide (4bIII). An investigation of free volume using positron annihilation lifetime spectroscopy (PALS), and gas permeation measurements was carried out for the thin film composite copolymer membranes and compared to PIM-1. The average free volume hole size and the gas permeance of the copolymer membranes increased with decreasing TTSBI content in the copolymer.

Microstructure studies of isotactic polypropylene under natural weathering by positron annihilation lifetime spectroscopy

The microstructure evolution of isotactic polypropylene (PP) exposed to subtropical humid climate of Guangzhou of China, were investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, differential scanning calorimeter, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. Positron data showed that the free volume of PP matrix decreased with involving a shrinking of the free volume hole sizes as the extent of weathering degradation of PP aggravated. The shrinkage of free volume hole sizes may be traced to the loss of mobility of molecules of PP matrix. The increase of the glass transition temperature substantiated undoubtedly the decrease of molecular mobility of PP chains. The increase in crystallinity might increase the amount of rigid amorphous fraction of PP matrix, which induced the loss of molecular mobility. Furthermore, the decrease of ortho-positronium formation ought to be correlated to the increase in crystallinity and the increasing amount of scavenchers which were in this work represented by the carbonyl groups.

Dispersion and interaction of graphene oxide in amorphous and semi-crystalline nano-composites: a PALS study

The influence of dispersion and interaction of Graphene Oxide (GO) in semicrystalline Polyhydroxy butyrate (PHB) and glassy amorphous Poly(tBP-oda) is explored by Positron Annihilation Lifetime Spectroscopy (PALS). The ortho-Positronium lifetimes which represent the main free volume hole size of both polymers are mainly affected by the large differences in internal stresses built up by the shrinkage of the polymers during their preparation, restricted by the platelet structure of GO. The ortho-Positronium intensities, which represent the ortho-Positronium formation probabilities, suggest a strong dependency of on the dispersion of the nano-particles and their aspect ratio.

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

Characterization of Some Hydrogels Used in Water Purification: Correlation of Swelling and Free-Volume Properties

In this study, the main objective is to develop a good chelation and ion exchange hydrogel. This hydrogel is obtained by polymerization of dimethyl amino ethyl methacrylate (DMAEMA) and acrylic acid (AAc) by gamma irradiation, for the purpose of separation of some heavy and toxic metals from water. UV spectroscopy is applied to determine the metal ion concentration before and after treatment. The FTIR spectral analysis has identified the bond structure of PAAc, DMAEMA and P (DMAEMA/AAc) hydrogels. Microstructure and nanostructure are investigated by means of SEM and positron annihilation lifetime spectroscopy (PALS) respectively. A maximum swelling percent is found for 80/20 DMAEMA/AAc at free-volume hole size and fraction of 97?3 and 3.4% respectively. The P (DMAEMA/AAc) and PAAc hydrogels have been applied for Cu+2, Co+2 and Ni+2 removals from aqueous solutions, and the factors affecting the adsorption capacity are determined. The adsorption capacity of P (DMAEMA/AAc) is found to be higher than the corresponding ones PAAc. Its experimental results showed that, the maximum adsorption of P (DMAEMA/AAc) after 24 h occurs at pH 7 with concentration of 250 ppm for Ni+2 ions and at pH 5 with concentration of 40 ppm for Cu+2 and Co+2 ions. The adsorption affinity of P (DMAEMA/AAc) hydrogel at different treatment contact times is in the following order Ni > Cu > Co. However, the order becomes Co > Ni > Cu by the variation of the pH of the metal ion solution; the variation of its concentration leads to a different order of Cu > Co > Ni.

Free-volume hole size evaluated by positron annihilation lifetime spectroscopy in the amorphous part of poly(ethylene terephthalate) degraded by a weathering test

The degradation of poly(ethylene terephthalate) (PET) was investigated through a weathering test by using a xenon weather meter. The degraded structure was evaluated by means of gel permeation chromatography (GPC), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and positron annihilation lifetime spectroscopy (PALS). The GPC results indicated a decrease of number average molecular weight and a broadening of polydispersity with the degradation of PET, probably due to chain scission and some networking reactions. The DSC results indicated that the crystallinity index was changed through the weathering test. However, the change of crystallinity was poorly correlated with the progress of degradation. In the Tan δ measurement by DMA, suppression of molecular mobility during the degradation was implied by the increase in Tg with increasing exposure time. The PALS results revealed that the free-volume hole size decreased with increasing exposure time. The decrease in the free-volume hole size can be attributed to the interaction between functional groups produced by hydrolysis or photochemical chain scission. The change of properties and structure in the amorphous part of PET was found to be correlated with the progress of the degradation in the weathering test.

Effects of thermo-fatigue loading, hydrostatic pressure, and heat aging on the free-volume and crystalline structure of polypropylene-co -ethylene random copolymer pipes

Plastic pipes composed of polypropylene-co-ethylene random copolymers were subjected to accelerated aging by thermo-fatigue loading (i.e., thermal cycling using hot and cold water alternately under pressure), application of hydrostatic pressure at elevated temperatures, and heat aging in an oven. The effects of these accelerated aging techniques on the molecular and crystalline structure of the material were investigated using positron annihilation lifetime spectroscopy, Fourier-transform infrared spectroscopy, and differential scanning calorimetry. It was found that all three types of applied aging techniques decreased the size of the defects in the crystalline–amorphous interphase of the material, increased the density of these defects, and had a negligible impact on the free-volume hole size and intensity of the amorphous phase. Thermo-fatigue loading resulted in decreased lamellae thickness and lamellae thickness distribution; in contrast, hydrostatic pressure loading resulted in increased lamellae thickness and lamellae thickness distribution. The effect of thermo-fatigue loading on the chemical degradation of polypropylene was more pronounced than the effects of hydrostatic pressure and heat aging. POLYM. ENG. SCI., 55:641–650, 2015. © 2014 Society of Plastics Engineers

Depth profiling of the free-volume holes in cellulose triacetate hollow-fiber membranes for reverse osmosis by means of variable-energy positron annihilation lifetime spectroscopy

Two types of commercial cellulose triacetate hollow-fiber membranes with different water fluxes, apposite for a reverse osmosis separation process, were investigated by means of positron annihilation lifetime spectroscopy with a 22Na-radioisotope-based energy-tunable positron beam, in order to elucidate a depth profile of the free-volume hole size in a subnanoscopic viewpoint. The lifetimes of ortho-positronium (o-Ps), the triplet positron-electron pair, for the two membranes were measured as a function of positron incident energy E. The E dependence of the hole size, quantified from the o-Ps lifetime, indicated that the separation-active dense layers with the smallest hole sizes are located at the near-surface region for both the membranes. The ratio of the dense-layer thicknesses for the two membranes was estimated to be more than 50% of the ratio of the respective effective thicknesses calculated from the water flux based on Darcy's law. The result signified that the dense layer with the smallest hole size at the near-surface region considerably contributes to the difference in the water flux.

Synthesis and gas transport properties of novel poly(aryl ether ketone)s with branched structure

Poly(aryl ether ketone)s (PAEKs) based on 2-(3′-trifluoromethylphenyl) hydroquinone and 4,4′-difluorobenzophenone were synthesized and characterized in the presence or absence of 2,4′,6-trifluorobenzophenone (BB′2 monomer). The influence of the incorporation of a branched structure (BB′2 monomer) on the gas transport properties of PAEKs was investigated. The results showed that PAEKs with a branched structure possess a higher permeability and selectivity than PAEKs without a branched structure. Moreover, improvements in the permeability and selectivity were enhanced with increasing content of BB′2 monomer. This synergistic effect on permeability and selectivity was mainly due to the higher fractional free volume and the unique size and distribution of free volume holes arising from the incorporation of the branched structure. © 2013 Society of Chemical Industry