Effect of supercritical carbon dioxide on nanoporous polyhexafluoropropylene

The effect of supercritical CO₂ (sc-CO₂) on the size distribution of free volume holes (nanopores) in a polyhexafluoropropylene (PHFP) polymer matrix has been studied. Residual (after CO₂ release) swelling improves gas transport properties of the material. Relaxation of these properties over time has been compared with changes in permeability and nanoporosity. For PHFP samples with different histories, the data on nanoporosity have been obtained using positron annihilation lifetime spectroscopy (micropores) and the lowtemperature gas sorption technique (mesopores and part of micropores). Matching of the data is intended to reveal the role of pores of different sizes in permeability of membrane materials to different gases and determine the specifics of application of the positron annihilation technique to studying sc-CO₂-modified objects.     

Positron annihilation lifetime studies of free volume in gelatin

The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.     

Influence of Chemical Reactions of Positronium on Annihilation Characteristics in Polymeric Sorbents

Size distribution of free volume holes for several types of polymeric sorbents based on crosslinked polystyrene was studied using the positron annihilation and Brunauer–Emmett–Teller low-temperature nitrogen sorption techniques. It turned out that the ranges of free volume size detectable by the positron and sorption techniques partially overlap, wherein the positron annihilation technique covers the region of nanopores and the sorption method covers submicronic pores and a part of nanopores. Thus, both methods well complement one another. Information obtained by means of the positron technique can be confused if the positronium atoms produced by positrons injected in a material of interest take part in chemical reactions. Cases when such reactions can be suppressed by lowering the temperature are discussed.     

Evaluation of nonuniformity of polymeric membrane materials by positron annihilation technique

Positron annihilation lifetime distribution was experimentally studied in some polymers, including polymeric membrane materials, in an air and a nitrogen atmosphere. Basic attention was paid to the long-lived distribution component, i.e., to annihilation of orthopositronium (the positron-electron bound system). It was found that ambient oxygen affects the annihilation characteristics of positronium. Newly developed ideas on the mechanisms of formation, localization, and annihilation of positronium in the polymers lead to the conclusion that the distribution of free volumes of different size in the polymeric matrix is not uniform. The number concentration and size of free-volume holes and the size of microirregularities containing these holes were determined     

Correlations between the free-volume properties and the miscibility of chitosan/polar polymers blend membranes

The miscibility of chitosan (CS)/polar polymer blend membranes has been studied by positron annihilation and other methods. The miscibility of these two blend systems (CS/polyvinyl pyrrolidone (PVP) and CS/polyethylene glycol (PEG)) is good in the solution state due to the hydrogen interaction between the functional groups of the studied polymers. However, the miscibility of these two blend systems in the solid state is better in the CS/PVP system than in the CS/PEG system. The differences in miscibility of such two blend systems in the solid state were powerfully demonstrated with positron annihilation lifetime spectroscopy (PALS) methods. The CS/PEG blend system had much larger free-volume size and lower free-volume concentration. For their poorer interaction and phase separation fact, the molecules in the interfacial zone of the CS/PEG blend are less compact than the CS matrix. Therefore, the free-volume size in the interfacial zone was much larger than it in the CS matrix.     

Positron lifetime spectroscopy as a probe of nanoporosity of cement-based materials

Hydrating cement pastes prepared with water-to-cement ratio equal to 0.5 in weight were investigated through positron annihilation lifetime spectroscopy over a period of 4 weeks. Two series of samples were prepared using different pre-treatment procedures, in order to assess the influence of sample manipulations on positron annihilation measurements. The technique is sensitive to changes in the pore structure with ageing time. Moreover, from the positron data an estimate of the first two moments of radii distribution of the nanopores was obtained.     

Nanoporosity of Polymer Membrane Materials and Sorbents According to Positron Annihilation and Low-Temperature Gas Sorption Data

Based on the experimental data obtained by the authors in a number of previous studies, the limits of applicability of positron annihilation lifetime spectroscopy (PALS) and low-temperature gas sorption (LTGS) to determination of nanoporosity (size distribution of nanopores in the range from a few fractions of a nanometer to 50 nm) in polymeric membrane materials and sorbents are discussed. It turns out that none of these methods is universal. The possibility of using each of them is determined by different factors, with the cases considered being finely divided polymer materials and the membranes per se cast from powders. It has been shown that the particle size factor is important for the applicability of LTGS. The possibility of using PALS depends on the concentration of nanopores of a given size.     

Probing the microstructure of Nafion-117 using positron annihilation spectroscopy

We report a new result on positron annihilation studies in acid- and cation-neutralized (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, UO₂²⁺, Ni²⁺) Nafion membranes using positron lifetime and Doppler-broadened annihilation radiation (DBAR) measurements. The free-volume structure is characterized using a simple quantum mechanical model of positronium (Ps) in a spherical well. Our studies indicate that formation and expansion of clusters is always associated with a change in free-volume structure resulting in smaller free-volume holes. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 771–776, 1997     

Correlations between gas permeation and free-volume hole properties probed by positron annihilation spectroscopy

Free-volume hole sizes, fractions, and distributions in a series of polycarbonate-based polymers are measured by using positron annihilation lifetime (PAL) spectroscopy. Correlations between the obtained free-volume hole properties and gas diffusion coefficients are observed. © 1995 John Wiley & Sons, Inc.     

Positron annihilation and Xe NMR studies of free volume in polymers

The existence and the average size of free volume in bisphenol-A polycarbonate (PC), low-density polyethylene (LDPE), poly (2,6-dimethyl-phenyleneoxide)(PPO), and polytetrafluoroethylene (PTFE) were studied by positron annihilation and ¹²⁹Xe NMR measurements. The ¹²⁹Xe NMR chemical shifts for xenon adsorbed in the polymers indicated that the average pore size of the free volume increased in the following order: PC, LDPE, PPO, and PTFE. This order of the pore size of the free volume agrees well with that estimated from the longest lifetime (τ₃) of ortho-positronium formed in the polymers. The unique correlation that δ⁻¹∝ r is established between the ¹²⁹Xe NMR chemical shift (δ) and the pore size (r), which is deduced from the positron annihilation measurements.     

Positron annihilation lifetime studies of free volume in ovalbumin gels

The dependence upon concentration of sodium chloride of free volume in ovalbumin gels, a main component of an egg white, is studied by the positron annihilation lifetime technique. The average free-volume radius in ovalbumin gels was about 0.27 nm at 298 K, smaller than those of organic polymers such as low-density polyethylene (0.34 nm at 300 K) and polystyrene (0.29 nm at 300 K). These differences suggest that the positronium annihilates in free volumes located close to hydrogen bonds, thus decreasing average free-volume radius. Free-volume content decreased with the increase of NaCl concentration up to 30 mM. At lower concentrations of NaCl, it seems that a correlation exists between microscopic free volume and macroscopic hardness of the ovalbumin.     

Crystallinity dependence of free-volume size and its distribution of ethylene-vinyl alcohol copolymer evaluated by positronium lifetime measurement

The size distribution of free-volume (<~0.1 nm³) of ethylene-vinyl alcohol copolymer with various crystalline degrees was estimated by positronium lifetime measurement. With increasing degree of crystallinity, the size distribution significantly narrowed and the intensity of positronium decreased. This indicates that the inhomogeneity reduces with the increase of the degree of crystallinity. It is found that the free-volume fraction estimated is reflected by the fraction of the amorphous region.     

Positron annihilation: a unique method for studying polymers

Positron annihilation is a unique technique for studying the local free volume of polymers. Employing the positron annihilation lifetime spectroscopy (PALS) the size and size distribution of subnanometer size holes which constitute the excess free volume may be studied. In combination with macroscopic volume data the fractional free volume and the number density of holes may be estimated. After presenting the principles of the method, some examples typically for the investigation of the free volume in polymers will be given. Moreover, the study of interdiffusion in demixed polymer blends and further applications are shortly reviewed     

Thermal variation of free-volumes size distribution in polypropylenes. Probed by positron annihilation lifetime technique

Positron annihilation lifetime measurement was applied to the study of free-volume properties in three kinds of polypropylene as a function of temperature in the range of 25–180°C at thermal equilibrium. Positron lifetime data for polypropylenes were analyzed with a Laplace inversion technique in order to obtain continuous positron annihilation lifetime (PAL) distributions. At each temperature, four distinct PAL distributions were recognized. The distribution of the longest lived component was associated with a pick-off annihilation of ortho-positronium (o-Ps) trapped in free-volume of amorphous region, which grew bigger as the temperature increased. The hole radius distributions of free-volumes were estimated from the results of o-Ps lifetime distributions. A detailed analysis showed a mean radius of free volumes was 0.34 nm at room temperature and that was 0.42 nm near the melting point for each specimen. The distributions of hole radii of free volumes were found to be broader after thermal treatments. The relaxation of free volumes was attributed to the thermal equilibrium and the evacuation of included molecules in free volumes. © 1995 John Wiley & Sons, Inc.     

Free‐volume change in volume phase transition of polyacrylamide gel as studied by positron annihilation: Salt dependence

Variation of free‐volume parameters—average radius size, number concentration, and size distribution—of a polyacrylamide (PAAm) gel containing 4 mol % carboxylate anions is studied during a volume phase transition (VPT) caused by a change of sodium chloride (NaCl) concentration. A positron annihilation lifetime technique is used for the determination of the free‐volume characteristics. The measurement is performed in an acetone–water 3 : 2 (v/v) [0.27 : 0.73 (mol/mol)] mixed solvent at 20°C, and the free‐volume parameters deduced from the analysis of a positron annihilation curve are utilized. An average free‐volume size of the swollen PAAm gel, ∼ 0.32 nm in radius, almost agrees with that of the mixed solvent for a corresponding salt concentration, while the size of the collapsed gel, which is ∼ 0.28 nm in radius, is smaller than that of the mixed solvent. The results for the collapsed gel indicate that the hydrogen bond plays a significant role in the nanoscopic environment. The radius of the free‐volume of the swollen PAAm gel seems to be influenced by the composition between acetone and water. An inhomogeneity of the nanoscopic structure inside the PAAm gels is discussed in terms of a dispersion of a size distribution of the free‐volume. It is concluded that a change of the nanoscopic environment of the PAAm gel during the VPT can be monitored through the free‐volume parameters obtained by the positron annihilation lifetime technique. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2634–2641, 1999     

Positronium chemistry in porous materials

Porous materials have fascinated positron and positronium chemists for over decades. In the early 1970s it was already known that ortho-positronium (o-Ps) exhibits characteristic long lifetimes in silica gels, porous glass and zeolites. Since then, our understanding of Ps formation, diffusion and annihilation has been drastically deepened. Ps is now well recognized as a powerful porosimetric and chemical probe to study the average pore size, pore size distribution, pore connectivity and surface properties of various porous materials including thin films. In this paper, developments of Ps chemistry in porous materials undertaken in the past some 40 yr are surveyed and problems to be addressed in future are briefly discussed.     

Testing of the structure of macromolecular polymer films containing solid active pharmaceutical ingredient (API) particles

The aim of the present study was to investigate the structure of free films of Eudragit^® L 30D-55 containing different concentrations (0%, 1% or 5%) of diclofenac sodium by positron annihilation spectroscopy. The data revealed that the size of the free-volume holes and the lifetimes of ortho-positronium atoms decreased with increase of the API concentration. Films containing 5% of the API exhibited a different behavior during storage (17 °C, 65% relative humidity (RH)) in consequence of the uptake of water from the air.     

Relation of gas-transport parameters of amorphous glassy polymers to their free volume: Positron annihilation study

Correlations of transport parameters (diffusion coefficients D and permeabilities P of gases) and thermodynamic parameters (solubility coefficients S and parameters C′ _H of sorption isotherms) with the sizes of free-volume elements, v _h, as estimated via positron annihilation lifetime spectroscopy are analyzed for the first time on the basis of the data array obtained for glassy polymers. Correlations of logD and logP with 1/v _h that agree with the free-volume model under the condition of a weak change in the concentration of free-volume elements in different polymers are ascertained. Certain deviations from linear correlations with 1/v _h for polymers with high free volumes are interpreted as evidence that the connectivity (openness) of pores increases with the sizes of free-volume elements. For solubility coefficients and Langmuir parameters of sorption capacity C′ _H , good linear correlations with the value of v _h are demonstrated.     

Positron annihilation and free volume studies in polymer glasses

In this article, the interconnections between the ortho-positronium (the bound state of the positron and electron having parallel orientation of spins, ^TPs) pick-off annihilation characteristics and concentration and effective size of elementary free volumes in polymer structures are discussed. Free volume parameters are responsible for many important properties of polymers such as permeability to gases, selectivity, ageing mechanical strength, etc. However, the ways of quantitative estimations of size distributions of the free volume elements on the bases of experimental data are sometime not obvious. Various approaches to this problem are analyzed in this review mostly on examples of glassy polymer membrane materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2485–2503, 2008     

Structural heterogeneity in elastic and glassy polymeric materials revealed by positron annihilation and other supplementary techniques

Elastic and glassy polymers are normally considered as homogeneous disordered materials with unimodal (statistical) distribution of elementary free volumes. In this work, we discuss the results of our positron annihilation lifetime (PAL) measurements for a number of elastic and glassy polymers, where observations of the two long-lived ortho-positronium components in the PAL spectra and irregular (nonlinear) variations of annihilation characteristics in the vicinity of glass transition temperature reveal structural heterogeneity of these systems. The conclusion is confirmed by measurements of thermo-stimulated luminescence, thermo-mechanics and mobility of penetrants.