Effect of polymer polarity on the positronium formation

Positron annihilation lifetime spectra have been measured on styrene–methyl methacrylate, styrene–acrylonitrile, and styrene–butyl methacrylate copolymers. The results show that the longest lifetime τ₃ remains constant during extended PALS measurement in all experiment copolymers, and relative intensity I₃ decreases to a certain extent with measured time in weak polar copolymers and remain almost unchanged in strong polar copolymers as well as ST–BMA copolymers. The observed decrease in I₃ have been found to be unrelated to the microstructural change of copolymers, and, instead, to be more likely a result of the buildup of an electric field inside the copolymers during the prolonged PALS measurement. The field effect can result in the decrease of I₃ due to the increase in the positrons and electrons diffusing out of the spur, but the influence of the electric field on I₃ decrease with increasing the polarity of the copolymers and the softness of side groups of macromolecular chains in the copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 435–442, 2000     

Effect of dielectric screening on positronium formation in acetone-acetic acid polar solvent blends

The influence of polarization of a medium in the vicinity of the positron-electron pair produced at the end of a fast-positron track on the formation of positronium (Ps) was considered. Weakening interaction between the positron and electron, polarization leads to a decrease in the Ps formation probability. The transience of Ps formation requires that the time delay of medium polarization should be taken into account. In turn, the magnitude of dielectric screening in acetone-acetic acid mixtures strongly depends on the ratio between the amounts of polar monomeric acetic acid and its nonpolar dimer. Sequentially taking account of these effects, it is possible to explain both the complex behavior of the Ps yield and the dependence of dielectric permittivity of the mixture on its composition.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 153–159.Original Russian Text Copyright © 2005 by Stepanov, Byakov, Stepanova.This revised version was published online in April 2005 with a corrected cover date.     

Radiation effect on positronium formation in low-temperature polyethylene

The irradiation effect of γ-rays on polyethylene (PE) has been studied by positron annihilation lifetime spectroscopy (PALS). In the case of non-irradiated PE samples, at a low temperature below the glass-transition temperature, the intensity of the long-lived component of positronium, I₃, increases due to an increase in the concentration of trapped electrons. However, the increase in I₃ obtained in a few MGy γ-irradiated samples became very small due to the effect of induced radicals. It has been observed that the trapped electrons were affected by the intensity of the positron sources used for a PALS experiment.     

Study of irradiation effect on positronium formation in polypropylene

⁶⁰Co γ-rays and positron irradiation effect on positronium (Ps) formation in the bulk and near surface region of polypropylene (PP) was studied. It is revealed that the reduction of Ps formation probability in PP bulk as a function of elapsed time may not be due to enhanced concentration of oxygen groups, and should not be ascribed to a simple accumulation of free radicals. Results indicate that Ps in PP is not formed through only one branch. That branch sensitive to the irradiation as well as the existence of sample surface seems related to relatively mobile positrons.     

Effects of aging and oxidation on positronium formation in polyethylenes

Positron annihilation lifetimes (PAL) have been measured for polythylenes (PEs) under differenr conditions of heat treatment. It is shown that the intensity, I₃, of the long-lived component of positronium (Ps) shows hystereses between the heating and cooling processes due to aging effect. It is also shown that about half of Ps observed at low temperatures below the glass transition is inhibited after the sample is heat-treated above the melting point in the atmosphere containing oxygen. It is suggested that carbonyl groups (C=0) formed by thermal oxidation can inhibit Ps formation through trapping the precursors of Ps, e⁺ and/or e^-.     

Low-temperature formation of positronium in nonpolymer organic systems

The phenomenon of the low-temperature formation of positronium (bound electron-positron system) observed during positron irradiation of a test substance is discussed. The effect is additional relative to the formation of positronium under normal conditions and, like the principal phenomenon, is radiation-chemical in nature. This effect has been discovered and studied mainly for polymers, in which the role of weakly bound electrons and positron localization (trapping) in the intensity of manifestation of this phenomenon was revealed. This review focuses on studies concerning barely investigated nonpolymer (polar and nonpolar) organic systems, in which a relatively high mobility of molecules as compared to polymers can open an opportunity for a more pictorial manifestation of positron and electron trapping by polar centers, both belonging to separate molecules and resulting from collective orientation of the molecules.     

CCl4 对左旋氧氟沙星超声降解的影响

研究了CCl4对超声降解喹诺酮类抗生素左旋氧氟沙星(Levofloxacin)的影响, 考察了CCl4添加量、 超声功率、 溶液初始pH值及左旋氧氟沙星初始浓度等影响因素, 并采用HPLC和LC-MS/MS对超声降解产物进行了初步分析. 结果表明, CCl4增强了左旋氧氟沙星的超声降解, 当反应液体积为50 mL, 超声35 min时, 随着CCl4体积分数的增大(0~0.06%), 左旋氧氟沙星的降解率由1.9%增至69.2%; 超声功率为100~200 W时, 降解率随着功率的升高而增大, 功率为200~400 W时降解率有所降低; pH值对左旋氧氟沙星的超声降解影响很大, pH =7.14时容易超声降解, pH过低或过高均导致降解率显著减小; CCl4的体积分数一定时, 左旋氧氟沙星的降解率随其初始浓度的增大而降低; 左旋氧氟沙星的降解率在33~49 ℃时最大. CCl4强化超声降解左旋氧氟沙星过程主要是由·OH和一系列氯自由基参与的反应. HPLC分析发现, 降解过程中同时生成了2个产物, 并通过LC-MS/MS对其进一步鉴定.     

Mechanism of enhanced positronium formation in low-temperature polymers

An enhanced positronium (Ps) formation in low-temperature polymers has been widely observed. The additional positronium formation is due to shallow trapped electrons in them. Positron annihilation lifetime spectroscopy was applied to investigate the Ps formation in a series of polymers, such as low-density polyethylene, ethylene-methyl methacrylate copolymers with various methyl methacrylate contents, and pure poly(methyl methacrylate) at low temperature. An analysis of the experimental data based on simple kinetic equations enables one to understand the Ps formation mechanism in polymers during low-temperature positron annihilation experiments as functions of the temperature and elapsed time. Good fittings of the experimental data were obtained. The fitting parameters seemed to show clear physical meanings.     

四氯化碳和吡啶对苯和环己烷中正子素生成的影响

研究了在含不同浓度电子清除剂CCL_4或正穴清除剂吡啶的苯溶液和环己烷溶液中o-Ps强度的变化。结果表明,环己烷中电子和正离子的迁移率是很高的,苯中的迁移率也较高,但比环己烷中的小些。由我们导出的有关正穴清除剂使o-Ps强度增加的关系式表明,o-Ps强度的增强或抑制是电子清除和正穴清除这两个过程竞争的结果。     

Temperature effects on positronium formation in solution of sodium acetate in methanol

The influence of the acetate ion (Ac^? in solutions of methanol on the Ps yield has been examined at 203, 248 and 294 K. Ac^? was found to be both a strong “limited” inhibitor through e⁺ scavenging and an enhancer through hole trapping. The temperature effects observed are interpreted by a change in the relative probabilities ofthe two processes.     

Electric field effect on positronium formation in gamma-irradiated polypropylene and polyethylene

The effect of an external electric field (up to 48 kV cm⁻¹) on positronium (Ps) formation in γ-irradiated polypropylene and polyethylene has been studied by means of positron lifetime spectroscopy. The application of an electric field to unirradiated polypropylene and polyethylene considerably decreased the Ps formation. For example, the intensity of the longest-lived component (I₃) for one of the polypropylene samples was reduced from 26% to 18% by a field of 48 kV cm⁻¹. It was found that the electric field effect for the irradiated samples becomes weaker when increasing the irradiation dose. This result is discussed on the basis of the spur reaction model of Ps formation.     

Effect of temperature on positronium annihilation in silica gel

A systematic temperature-dependent study of positronium annihilation rate within the void spaces (micro- and mesopores) of silica gel material has been performed through positronium annihilation spectroscopy. The results find their plausible interpretation through a novel theoretical explanation based on vibrational interaction of thermally energized atoms on the surface layer of the pores with positronium, which in fact justifies the observed increase in the annihilation rate of the latter, with the increase in temperature.     

Temperature effects in positronium formation: Solid aromatic hydrocarbons

The temperature dependence of the probability of positronium formation was investigated in a series of solid aromatic hydrocarbons. The results are dis     

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.     

Electron-impact ionization of CCl4 and CCl2F2

Absolute partial and total cross sections for electron-impact ionization of CCl4 and CCl2F2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for production of CCl3(+), CCl2(+), CCl+, C+, Cl2(+), and CCl3(2+) from CCl4; and for production of CCl(2)F+, CClF2(+), CClF(+), (CCl+ + CF2(+)), Cl+, CF+, F+, and C+ from CCl2F2. Data are also reported for formation of (CCl2(+),Cl+) and (CCl+, Cl+) ion pairs from CCl4. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in the absolute cross sections for most of the singly charged ions is +/- 5-7 %. The present partial cross sections for lighter fragment ions are found to be considerably greater than had been previously reported but the most recent total cross section measurements agree well with those reported here. Neither the binary-encounter-Bethe theory nor the Deutsch-Mark theory reproduces the experimental cross sections correctly for both targets.     

Effect of alkali additives on the kinetics of WO3+CCl4 reaction

The chlorination kinetics of alkali-added (K and Li) tungsten trioxide were studied by thermogravimetry, using gaseous CCl₄ as chlorinating agent. The reactivity of the modified samples was compared to the results on the chlorination of pure WO₃. Similar apparent activation energies were found for the pure and alkali-added samples. However, potassium additive resulted in a strong decrease of the initial reaction rate, while surface lithium has no influence on it.During the chlorination a continuous decrease of the linear reaction rate was observed for both samples, which was explained by retarding effects of surface by-products and alkali additives. For describing the isothermal TG curves an appropriate kinetic model, based on the monotonously increasing inhibition of these species was assumed. The curve calculated with this model fits well to the experimental results.

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Zusammenfassung Mittels Thermogravimetrie wurde die Chlorierungskinetik von alkaliversetztem (K und Li) Wolframtrioxid untersucht, wobei CCl₄ als Chlorierungsreagens fungierte. Die Reaktivität der modifizierten Proben wurden mit der von reinem WO₃ verglichen. Für die reinen und für die alkaliversetzten Proben wurden ähnliche scheinbare Aktivierungsenergien gefunden. Der Zusatz von Kalium verursacht jedoch eine starke Abnahme der ursprünglichen Reaktionsgeschwindigkeit, während oberflächiges Lithium keinen Effekt zeigt.Während der Chlorierung wurde eine ständige Senkung der linearen Reaktionsgeschwindigkeit für beide Proben festgestellt, was mit Rückhalteeffekten von oberflächigen Nebenprodukten und Alkalizusätzen erklärt wird. Zur Beschreibung der isothermen TG-Kurven wurde ein entsprechendes kinetisches Modell angenommen, welches auf einer monoton steigenden Inhibition dieser Proben basiert. Die auf der Grundlage dieses Modelles berechnete Kurve stimmt recht gut mit den experimentellen Ergebnissen überein.

     

Plasma-chemical reactions in weakly decomposed CCl4

For the case of weak feed gas decomposition, where the concentration of CCl₄ exceeds those of decomposition and built-up products, the emission of CCl* is shown to originate from dissociative excitation of CCl₄. With electron concentration measured independently, the kinetics of CCl₄ decomposition has been extracted from the time dependence of the CCl* intensity. Supported by EPR determinations of radical concentrations in rapidly flowing CCl₄ and CCl₄/O₂ afterglows, the primary decomposition reaction is shown to be the electron impact dissociation into CCl₃ and Cl. Its rate constant (k ₁=4×10^–8 cm³s^–1) indicates strongly that dissociative electron attachment is the main reaction channel at least at r.f. power densities just above the threshold of a self-maintaining discharge. At extremely low mean electron energies the emission of a continuum is observed, which is tentatively ascribed to the radiative CCl₃-Cl recombination.     

Comprehensive study of positronium formation in polymer blends between polyethylene and ethylene-vinylacetate copolymer

The intensity I₃ of ortho-positronium (o-Ps) in a polymer blend system consisting of polyethylene (PE) and ethylene-vinylacetate (EVA, random copolymer with a vinylacetate content of about 14%) was measured as functions of EVA weight content (Φ=0–100%), electric field (E=0–60 kV/cm ), positron irradiation time (t=0–200 h) and temperature (T=100–300 K). It was found that the addition of small amounts of EVA to PE significantly alters the electric field, positron irradiation time and temperature dependence of I₃. Positron trapping on polar EVA is suggested to be responsible for the sensitive effects of EVA.