Interpretation of positron lifetime spectra arising from micellar solutions: Determination of the ortho-positronium diffusion coefficient in the solvent

Positron lifetime spectra arising from micellar solutions of sodium dodecylsulphate (SDS) are interpreted in terms of a classical positronium diffusion model published earlier. Unlike the generally accepted assumptions, this model results in a non-exponential ortho-positronium (o-Ps) lifetime density function. A new method is presented for the simultaneous fitting of this lifetime density function to independent lifetime spectra recorded under the same experimental conditions. Among the fit parameters D_p, the diffusion coefficient of o-Ps in the solvent (i.e. heavy water) phase is studied in detail; a detailed error analysis for D_p is also given. Provided that the mean aggregation number of SDS micelles is about 60, the published D_p values show that the diffusion coefficient of o-Ps in (heavy) water at room temperature is lower than that of small ions and molecules and the Arrhenius plot indicates a strong o-Ps localization in the solvent. The hydrodynamic radius of o-Ps is calculated from the o-Ps and micellar diffusion coefficients and from the micellar radius; it is greater than that of small ions and molecules and this can be considered as an independent indirect proof for the existence of o-Ps bubble in the (heavy) water.     

A positron annihilation lifetime study of isotactic polypropylene

Positron lifetime studies were performed on well-characterized annealed and quenched samples of isotactic polypropylene. The positron experiments were conducted from ?20 to 110°C as a function of both heating and cooling. Of the three decaying exponential components resolved from the lifetime spectra, only the long-lifetime ortho-positronium (o-Ps) pickoff component was affected by the changes in temperature. The behavior of both the lifetime and intensity of the o-Ps component was interpreted with the aid of x-ray diffraction, densitometry, and optical microscopy examinations and results from previously reported investigations of the thermal transition behavior of polypropylene. The present experiments demonstrate that o-Ps lifetimes were similar for both the annealed and quenched samples, independent of thermal cycling, while the o-Ps component intensity was significantly larger for the quenched material during heating, with both sample types exhibiting a significant hysteresis upon cooling. These results suggest that the mean free-volume cavity size is independent of prior thermal treatment, while the density of free-volume sites is a sensitive function of structure and prior thermal history. The variations of lifetime and of intensity with temperature have provided insight into polypropylene's glass transition phenomena.     

Diffusion and localization of o-Ps in D2O determined from positron annihilation in SDS micellar solutions

A microscopic diffusion model is presented for the determination of orthopositronium (o-Ps) lifetime in micellar solutions. Among other parameters, the lifetime density function depends on the o-Ps diffusion coefficient in the water phase. Orthopositronium diffusion coefficients are determined by fitting this lifetime density function to positron annihilation spectra obtained from 1 mol/dm³ solution of sodium dodecylsulphate (SDS) in D₂O at different temperatures. The activation energy of the o-Ps diffusion in D₂O obtained from an Arrhenius plot as E_a = 0.9₂₂ ± 0.1₀₃ eV indicates strong localization.     

Free volumes in solids under high pressure

A review of experimental work on ortho-positronium (o-Ps) lifetime in solids under the pressures up to 1 GPa is presented. Among the effects observed at high pressure one can mention: the disappearance of the energy level for Ps at the reduction of free volume size; pressure induced phase transitions; variation of Ps formation intensity with time; increase of o-Ps lifetime after intercalation of high pressure gas to the paraffin samples.     

Influence of vinylidenefluoride on the physical ageing of poly(chlorotrifluoroethylene)

The influence of vinylidenefluoride (3% composition) on the physical ageing of poly(chlorotrifluoroethylene) at 70 °C has been investigated using the positron annihilation lifetime technique. Both the ortho-positronium (o-Ps) pick-off lifetime and its intensity show a systematic variation with ageing time. The o-Ps intensity exhibits non-exponential character which can be fitted with two additive exponential decay curves and the free volume is found to exhibit the Doolittle type of free volume relaxation. The relaxation times were evaluated from the structural relaxation function and the non-exponentiality parameter was estimated using the Kohlrausch-Williams-Watts (KWW) function, which indicates the deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seems to label the different kinetic units/irregularity in the chain backbone of PCTFE structure participating in the relaxation process. Physical ageing seems to yield close packing of polymer chains.     

Positronium interactions in organic solvents

In the present work, the chemistry of the positronium (Ps) species has been investigated in pure benzene, pyridine and their mixtures with pyridine concentrations at 4.12, 6.18 and 8.24 M, respectively, using the Doppler-broadened line-shape analysis technique. It is seen that the intensities of the para-(p-Ps) and ortho-Ps (o-Ps) in benzene and that of p-Ps in pyridine follow the Ore-model predictions while the intensity of o-Ps in pyridine is much lower than expected from this model. On the basis of these observations and of decrease in the o-Ps lifetime with increasing pyridine concentration in various organic solvents as reported in literature, it is concluded that pick-off is not the only quenching mechanism for Ps in organic solvents and pyridine is a quencher of Ps-species rather than an inhibitor. Calculations carried out considering a diffusion-controlled mechanism of Ps-quenching in pyridine via unstable (dissociative) complex/adduct formation and the bubble model show that the quenching rate is diffusion controlled and the pick-off rate is in accordance with the free-volume model. These conclusions were confirmed in the mixtures of benzene and pyridine.     

Comments on the relation: positronium lifetime – free volume size parameters of the Tao–Eldrup model

The author is discussing the parameters appearing in the Tao–Eldrup (TE) model describing the ortho-positronium (o-Ps) lifetime dependence on the size of free volume in which Ps is trapped. Parameter values are not universal, applicable to all media. The Ps penetration to the bulk should depend on Ps work function; the o-Ps decay rate is strongly influenced by the contact density factor.     

The use of positron lifetime measurements in a study of electron scavenging by a range of radiosensitizer compounds

Positron lifetime measurements were made in aqueous solutions of a number of radiosensitizer compounds in order to measure the o-Ps inhibition and quenching constants. By making measurements in the presence and in the absence of perchloric acid, the inhibiting effects on the o-Ps yield due to epithermal and trapped electrons could be determined on the basis of the spur model. Inhibition constants relating to the scavenging by the sensitizer molecule of epithermal electrons. (K₁) and traped electrons, (K₂) were found for eight basic nitroimidazoles. The results are also discussed in terms of electron trap depth which is here taken to be equivalent to the one-electron redox potential, E¹₇. It is concluded that o-Ps lifetime measurements alone are not capable of determining the finer details of these spur reactions. Estimates of the respective yields of RS^- and OH^. species in oxygen and R—NO₂ compounds show that these are not able to explain the large differences in sensitizing efficiency.     

Magnetic quenching of positronium in organic solutions

The magnetic quenching of ortho-positronium (o-Ps) in some pure nonpolar liquids (n-hexane, cyclohexane and benzene) and solutions (nitrobenzene in n-hexane, cyclohexane and benzene; and carbon tetrachloride and biphenyl in n-hexane) is examined for steady magnetic fields up to 14 kG by the positron annihilation lifetime technique. The long lifetime of o-Ps is very sensitive to the strength of the external magnetic field, decreasing as the field strength increases. This effect follows from a well-known principle of atomic physics, the quadratic Zeeman effect. With one exception, all the liquids studied here appear to be normal in this regard. The exception is nitrobenzene in n-hexane, which shows a very marked enhancement of quenching beyond the Zeeman effect at low fields.     

Delayed formation and localisation of positronium in polymers at low temperatures

A theoretical model of the positron annihilation lifetime spectrum including the mechanisms of slow positronium (Ps) localisation and delayed Ps formation from a positron and a trapped electron was developed. The model was applied to two series of spectra for low-density polyethylene and high-density polyethylene (HDPE) collected at constant temperature (much below the glass temperature) as a function of measurement time. The Ps internal relaxation time and time of localisation of Ps in a free volume centre were determined. The results show that after long irradiation of the polymer a dominant fraction of positrons (unbound in Ps) annihilate from the trapped states. On the basis of parameters determined from the HDPE lifetime spectra, two S(t) curves (for sample in darkness and in light) were calculated. The predicted shapes of S(t) well agree with literature data obtained with the age–momentum correlation (AMOC) experiment. According to the new model the shapes of the para-Ps and the ortho-Ps (p-Ps) components are non-exponential. In spite of this, the multi-exponential decomposition of a polymer spectrum enables to determine correctly the value of the o-Ps lifetime, however the other parameters determined from the spectrum have no simple physical meaning.     

Studies of crystallization of SiO2-glass by positron annihilation

The crystallization of silica glass has been studied by positron lifetime spectroscopy and accompanying investigations using X-ray diffraction and nuclear magnetic resonance (NMR). To this end isothermal and isochronal heat treatments were performed in the range from 700 °C up to 1600 °C. The lifetime spectra are analyzed by two lifetime components. The decrease of the short lifetime (200 ps) is attributed to the increasing volume fraction of the crystalline phase. The long lifetime (1000 ps) is related with the pick-off annihilation of theo-Ps states in cavities.With starting formation of crystallization nuclei the long lifetime increases which is explained by expanding cavities at the interface between crystalline phase and amorphous matrix.     

Positron annihilation in hypercrosslinked polystyrenes

Positron annihilation in hypercrosslinked polystyrene, called CPS(0.3)150E, has been studied as a function of temperature in the range from 35 to 370 K. The positron lifetime spectra were resolved into five components using the PATFIT and MELT programs. The annihilation rate constant of the longest-lived component was found to increase linearly with increasing temperature. Remarkably, the distribution of o-Ps lifetimes resolved by MELT was shown to broaden with decreasing temperature. Moreover, an effect of the oxygen pressure on the positronium characteristics was also investigated in another hypercrosslinked polystyrene, CPS(0.3)100E. The oxygen pressure varied from vacuum to 6 atm. The annihilation rate constants of the long-lived components were observed to increase linearly as a function of the oxygen pressure, which demonstrates a dependence of the kinetic of o-Ps quenching on the pore size.     

comments on the paper “contribution of quantum effects to the mechanism of positronium reactions”

Some results recently published on the o-Ps quenching reactions promoted by FeCl₂ and K₂Cr₂O₇ in glycerol aqueous solutions were interpreted by supposing that a significant fraction of o-Ps atoms diffuses by a quantum-mechanical effect or in quasi-free states even at room temperature. It is shown that the results can be interpreted in a different manner, so they cannot be invoked for supporting the hypotheses mentioned above about the mechanism of o-Ps diffusion at room temperature.     

Microstructural studies of electron beam-irradiated cellulose pulp

The effect of electron beam irradiation on the microstructure of cellulose has been investigated using positron annihilation lifetime spectroscopy (PALS) and electron paramagnetic resonance (EPR) Spectroscopy. PALS studies of irradiated cellulose samples showed that ortho-positronium (o-Ps) lifetime increases with an increase in dose up to 80 kGy and decreases at higher doses. The EPR signal of the irradiated cellulose matrix showed the presence of multiple radical sites. These results are discussed on the basis of chemical and physical changes occurring at the microscopic level in the cellulose due to irradiation.     

Free volumes in liquid-crystalline main-chain polymer probed by positron annihilation

Free volumes in a copolymer consisting of p-hydroxybenzoic acid (HBA) and poly(ethylene terephthalate) (PET) were probed by positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 30 and 230°C. Above the glass transition temperature (ca. 60°C), the lifetime of ortho-positronium (o-Ps) annihilated in the free volumes and its annihilation probability were found to increase with increasing specimen temperature. These facts were attributed to the increase both in the size of the free volumes and in the concentration of such regions due to rearrangements of molecules. From the observed lifetime of o-Ps, it was found that the size of the free volumes increases from 0.05 nm³ to 0.1 nm³ in the temperature range between 30 and 230°C. At solid-mesophase transition temperature (150–220°C), the size of the free volumes monotonously increases, while the increase in the concentration of such regions saturates at 174°C. The clear relationship between the data obtained by the positron annihilation and those obtained by differential scanning calorimetry was confirmed. ©1995 John Wiley & Sons, Inc.     

Detection of free volumes in polyaniline complexes with various acids by using positron lifetime spectroscopy

The positron lifetime spectroscopy (PLS), a non-destructive characterization method, utilizes positronium (Ps; an electron–positron bound state) as a probe and measures its lifetime in polymer free volumes. For the first time the free volumes have been estimated by PLS in polyaniline (PANI) complexes with various inorganic and organic acids. It was found that the o-Ps lifetime increases and the intensity decreases with increasing ionic radius of the counter-ions in PANI complexes. Obviously, larger counter anions result in enhanced mean size of the voids corresponding to the free volume in the bulk polymer.Electrical conductivity has been measured by conventional four-probe technique. The glass transition temperature and temperature of removal of the absorbed water have been determined by using differential scanning calorimetry. It was established fairly well correlation of the mentioned polymer parameters with the o-Ps lifetime and the free volume of PANI complexes, respectively. The greater free-volume results in a decrease of conductivity, glass transition temperature and temperature of removal of the absorbed water.     

Free volume of cyanate resins studied by positron annihilation

The polymerization process of bisphenol-A dicyanate (BADCy) has been studied using a positron-annihilation lifetime technique (PAL). The polymerization was conducted at 150°C, and the process was followed by PAL. Seven kinds of samples with different curing times were also formed at 150°C, and the relation between the period of the curing time and the degree of polymerization was studied. It has been shown that theo-Ps lifetime increases in samples with a higher polymerization than 85%, which is consistent with measurements of the specific volume of BADCy.     

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.     

Temperature dependence of the free volume in polytetrafluoroethylene studied by positron annihilation spectroscopy

The temperature dependence of the free volume holes in pure polytetrafluoroethylene (PTFE) and doped with 25% glass have been studied in the temperature range (30–250 °C) using positron annihilation lifetime spectroscopy. The data clearly revealed the glass transition temperatures for pure and doped PTFE are 130 and 110 °C, respectively. As the temperature increases, the free volume distribution becomes positioned at larger free volume hole size. A good correlation between the electrical conductivity and the o-Ps parameters was achieved.     

Study of molecular motions in alkyl cyanobiphenyls using positron lifetime spectroscopy

Positron lifetime measurements have been carried out in two liquid-crystal-forming homologues of alkyl cyanobiphenyls, 7CB and 10CB. In each of these two compounds measurements were performed during the heating cycle of samples prepared either by quenching or by slow cooling from the respective liquid crystalline phase. In both compounds, the behavior of quenched and slow-cooled samples is found to be different. Unlike the slow-cooled sample, the material in the quenched sample seems to have transformed into a glassy solid. Theo-Ps pick-off lifetime in the quenched sample exhibits a strong temperature dependence. In each case, its value exhibits six broad peaks at various characteristic temperatures which have been ascribed to various motions associated with the molecules of these compounds. The quenched samples of 7CB and 10CB exhibit glass transitions at 268 and 266 K, respectively. The present work demonstrates an interesting application of positron lifetime spectroscopy (PLS).