Positron Annihilation Lifetime Spectroscopy PALS

The decay curve of positron lifetimes measured experimentally can be represented mathematically as  

Generally, three to four lifetime components are resolved in polymers, and their attribution is as follows. The shortest lifetime component ri with intensity h is attributed to contributions from free positron annihilation (inclusive of p-Ps lifetime). The intermediate lifetime component Z2 with intensity 12 is considered to be due to the annihilation of positrons trapped at defects present in the crystalline regions, or those trapped at the crystalline-amorphous interface boundaries. The longest-lived component T3 with intensity 1, is due to pick-off annihilation of the o-Ps in the free volume cavities present mainly in the amorphous regions of the polymer [42,43]. The simple model of a Ps atom in a spherical potential well of radius R leads to a correlation between o-Ps hfetime and R [70,128-130]  

1 Free Volume Distribution-Lifetime Analysis by Laplace Transform Method 

In many porous media such as polymers, composites and proteins, the heterogeneity of the local molecular environment from which positron annihilation occurs is expected to generate a distribution of lifetimes. In such cases it is necessary to replace the sum in Eq. (27.10) by an integral. 

The fraction of positrons annihilating with rates between A and A - - dInA is represented as Aa(A)dlnA. This is the annihilation rate PDF employed by CONTIN-PALS2. The corresponding annihilation lifetime PDF is expressed as A a(A). The fraction of positrons annihilating with lifetimes between r -h dr is represented as A a(A)dr. 

Below the glass transition temperature, 230 K, the lifetime is weakly temperature dependent. Above Tg, the lifetime increases to a point whereupon the rate of increase slows down. The intensity of the o-Ps component grows in an approximately linear fashion and does not perceptibly change at The observation of is an intrinsic feature of Tg. As we have seen from the dielectric relaxation studies, the Tg process is associated with reorientation of dipoles about the backbone of the chain. The o-Ps will reside in a cavity formed from the surrounding polymer chains for a time that is dictated by its exchange lifetime. If, however, during this lifetime the polymer chains which form the 

It is apparent from comparison of the results of the above observations that the glass transition process is associated with the collective motion of elements of the polymer backbone about the polymer axis. For this motion to be able to occur there must exist in the neighbourhood of the moving segment a lack of material, voids, or as it is usually termed, free volume. The free volume is therefore the amount of volume required for co-operative motion about the backbone to occur. 

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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... 

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. 

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]. 

Three blends have been studied by Chang et al. [1997] using stress relaxation measurements and positron annihilation lifetime spectroscopy (PALS). It was observed that for blends of PS with PPE and PS with PVME the stress relaxation rates were faster for the blends in comparison with PS alone, whereas the opposite was true for a PMMA/PEG blend when compared with neat PMMA. 

Positron annihilation lifetime spectroscopy (PALS) allows the quantitative investigation of the polymer free volume [1, 2]. Additionally, the PALS beam technique makes a direct depth resolution possible, by implanting the probe - the positron - within a definite sample depth interval depending on the positron kinetic energy [3]. It is one of the very few nondestructive techniques for investi-... 

During the past two decades, positron annihilation lifetime spectroscopy (PALS) has developed to be the most important experimental method for studying the free volume... 

Dlubek, G., Pionteck, J., Bondarenko, V., Pompe, G., Taesler, Ch., Fetters, K., and Krause-Rehberg, R., Positron annihilation lifetime spectroscopy (PALS) for interdiffusion studies in disperse blends of compatible polymers a quantiative analysis, Macromolecules, 35, 6313-6323 (2002c). 

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