Positron-annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy has fonnd limited application in study of the outdoor oxidative degradation of PP [46]. 

Reaction of a positron with an electron gives a metastable positronium (Ps) particle, which may have antiparallel spins (para-positronium, p-Ps) or parallel spins (ort/jo-positronium, o-Ps). Within a polymer, the longer lifetimes of o-Ps may be related to the size, concentration and distribution of free volume elements. There have been a number of studies of PIM-1 by positron annihilation lifetime spectroscopy (PALS) [33-36]. 

This work investigated PIM-1 membranes in the three states discussed above. Nickel-foil supported NaCl was used as a positron source and stacks of film samples, each about 1mm thick, were placed either side of the source. Annihilation lifetime decay curves were measured with an EG G Ortec fast-fast lifetime spectrometer. Measurements were made both in air and under an inert atmosphere (N2). However, o-Ps lifetimes in air were reduced due to quenching by oxygen, so only results obtained under N2 are discussed here. Results were analysed in terms of a four component lifetime distribution, which allowed obtaining better statistical fit. The two longest lifetimes, T3 and T4, for PIM-1 in 

This collaboration was made possible by support from INTAS (Project 05-l(XXXX)8-7862). The UK participants thank EPSRC for funding. 

Yuri YampolskiP, Ludmila Staramikova Nikolai Belov, Maria Gringoltf, Eugene Finkelshtein and Victor Shantarovich  

Membrane Gas Separation Edited by Yuri Yampolskii and Benny Freeman 2010 John Wiley Sons, Ltd 

The decrease in volume that accompanies the physical aging process can be associated to a change of the distribution of free volume holes. Since the size and concentration of free volume holes in amorphous polymers is closely linked to the 

One of the most useful tools for the detectionof free volume holes or voids, free volume, and free volume distribution, at an atomic scale, is positron annihilation lifetime spectroscopy (PALS). The technique exploits the fact that the positively charged positron (e ), the antiparticle to the electron, preferentially samples regions of low positive charge density. When injected in a polymer matrix, thermal-ized positrons can combine with an electron to form a bound state, known as positronium (Ps). This species can only exist in a void and it rapidly annihilates on contact with the electron cloud of a molecule. For polymer studies using PALS, it is ortho-positronium (oPs, a triplet state) that is of main interest. The oPs spin exchanges with electrons of opposite spin on the walls of the cavity and it is annihilated. Thus, the oPs lifetime, T3, gives a measure of the mean free volume cavity radius whereas the relative intensity of the oPs component, 1, can be related to the number of cavities. A semiempirical equation has been derived that correlates T3 with the cavity radius, r [81]  

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Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

In this work positron annihilation lifetime spectroscopy (PALS) was used to investigate structural diversity inside zeolite precursor matrix caused by the presence of alkali cations Na, K, Rb and Cs. PALS is an established and well-proven method for structural investigations of various materials, extensively used for metals and alloys, semiconductors and porous materials [3, 4]. In the investigations of zeolites PALS has been mostly used for their void structure and size study [5, 6, 7, 8], also in correlation to... 

Positron annihilation lifetime spectroscopy (PALS) provides a method for studying changes in free volume and defect concentration in polymers and other materials [1,2]. A positron can either annihilate as a free positron with an electron in the material or capture an electron from the material and form a bound state, called a positronium atom. Pnra-positroniums (p-Ps), in which the spins of the positron and the electron are anti-parallel, have a mean lifetime of 0.125 ns. Ortho-positroniums (o-Ps), in which the spins of the two particles are parallel, have a mean lifteime of 142 ns in vacuum. In polymers find other condensed matter, the lifetime of o-Ps is shortened to 1-5 ns because of pick-off of the positron by electrons of antiparallel spin in the surrounding medium. 

Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, Hall PJ, and Fernandez JJ. Characterization of activated carbon fibers by positron annihilation lifetime spectroscopy (PALS). In Unger KK, et al., eds. Studies in Surface Science and Catalysis, Characterisation of Porous Solids V, vol. 128, the Netherlands Elsevier Science. 2000 523-532. 

Independent of whether or not a well-defined crossover temperature can be observed in NS data above Tg, it has been well known for a considerable time that on heating a glass from low temperatures a strong decrease of the Debye-Waller factor, respectively Mossbauer-Lamb factor, is observed close to Tg [360,361], and more recent studies have confirmed this observation [147,148,233]. Thus, in addition to contributions from harmonic dynamics, an anomalously strong delocalization of the molecules sets in around Tg due to some very fast precursor of the a-process and increases the mean square displacement. Regarding the free volume as probed by positron annihilation lifetime spectroscopy (PALS), for example, qualitatively similar results were reported [362-364]. 

The main techniques used are positron annihilation lifetime spectroscopy (PALS) and the Doppler broadening (DB) or angular correlation (AC) techniques. The PALS parameters are the relative intensities (I j) and the... 

Suzuki, R., Ohdaira, T., Shioya, Y. and Ishimaru, T. (2001) Pore characteristics of low-dielectric-constant films grown by plasma-enhanced chemical vapor deposition studied by positron annihilation lifetime spectroscopy , Jpn. J. Appl. Phys. 40, L414. 

Petkov, M. P., Weber, M. H., Lynn K.G. and Rodbell, K. P. (2000) Probing capped and uncapped mesoporous low-dielectric constant films using positron annihilation lifetime spectroscopy , Appl. Phys. Lett. 77, 2470. 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

Forsyth, M., Meakin, P., MacFarlane, D.R., Hill, A.J. (1993) Positron annihilation lifetime spectroscopy as a probe of free volume in plasticized solid polymer electrolytes . Electrochimica Acta, 40(13), 2349. 

Bartos, J., Kristiakova, K., Sausa, O., Kristiak, J. (1996) Free volume microstructure of tetramethylpolycarbonate at low temperatures studied by positron annihilation lifetime spectroscopy a comparison with polycarbonate . Polymer, 37(15), 3397. 

Pore dimensions can be determined also by positron annihilation lifetime spectroscopy (PALS). Positron in a solid can create a bound structure with an electron, called positronium (Ps). Its triplet state (ortho-Ps) has an intrinsic lifetime in vacuum 142 ns, but when trapped in a free volume, like a pore, it lives shorter. The o-Ps lifetime is... 

Characterization of activated carbon fibers by Positron Annihilation Lifetime Spectroscopy (PALS). 

The use of Positron Annihilation Lifetime Spectroscopy (PALS) technique to characterize porous carbon materials has been analyzed. Positron annihilation lifetimes have been measured in two series of petroleum pitch-based activated carbon fibers (ACF) prepared by CO2 and steam activation. Two lifetime components were found a short-lived component, Ti from 375 to 393 ps and a long-lived component, 1 2 from 1247 to 1898 ps. The results have been compared to those obtained by Small Angle X-Ray Scattering (SAXS) and N2 and CO2 adsorption at 77K and 273K respectively The correlation found demonstrates the usefulness of PALS to get complementary information on the porous structure of microporous carbons. 

Positron annihilation lifetime spectroscopy (PALS) is a commonly used technique for the investigation of the electronic properties of condensed matter. The first application of positrons in condensed matter was in the study of electronic structure of metals and in the characterization of defects in solids [1]. 

The objectives of this research are basically two firstly, to analyze the use of positron annihilation lifetime spectroscopy to the study of carbon materials with high surface area and, secondly, to get a correlation between the parameters observed in PALS experiments and the results obtained in the characterization of porous materials by well-known methods like gas adsorption and Small Angle X-Ray Scattering (SAXS). 

In the present work, positron annihilation lifetime spectroscopy has been applied to characterize the porosity of activated carbons fibers. These materials are essentially microporous [16], with slit shaped pores and with a homogeneous pore size distribution. Because of that, they seem to be the most appropriate materials to analyze the application of PALS technique to the characterization of porous carbon materials. 

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