Polymer positron annihilation lifetime

The free volume (FV) in polymer systems is of great interest because the size and concentration of its elements (holes) affect numerous transport and other physicochemical properties of polymers. Positron annihilation lifetime (PAL) spectroscopy is now one of the most efficient approaches for investigations of FV. The foundations of this method for probing polymers were based in particular on Walker-Brandt-Berko s free volume model (7). According to this model, Positronium, Ps, (a bound atomic system, which consists of an electron and the positron) tends to be localized or trapped before its annihilation in FV or, in other words, in areas with reduced electron density. Accordingly, annihilation characteristics (lifetimes and intensities of longer lifetime components of annihilation radiation) provide information regarding the concentration and sizes of FV elements. (2-5)... 

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. 

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. 

The analysis of the positron annihilation lifetime spectra is a very important aspect of using the PAL techniques to analyze polymers. Without proper data analysis interpretation of data might be misleading and important scientific information will be lost. In PAL studies of polymers the PAL spectrum can be analyzed in two ways (1) a finite lifetime analysis or (2) continuous lifetime analysis. In the finite lifetime analysis the PAL spectra is resolved into a finite number of negative exponentials decays. The experimental data y(t) is expressed as a convoluted expression (by a symbol ) of the instalment resolution function R(t) and a finite number (n) of negative exponentials ... 

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. 

Haidar, B., Singru, R.M., Maurya, K.K., Chandra, S. (1996) Temperature dependence of positron-annihilation lifetime, free volume, conductivity, ionic mobility, and number of charge carries in a polymer electrolyte polyethylene oxide complexed with NH4CIO4 . Phys. Rev. B. 54, 7143. 

Tanaka, K., Kawai, T., Kita, H., Okamoto, K., Ito, Y. (2000) Correlation between gas diffusion coefficient and positron annihilation lifetime in polymers with rigid polymer chains . Macromolecules, 33, 5513. 

Mazzroua, A., Mostafa, N., Gomaa, E., Mohsen, M. (2001) The use of positron annihilation lifetime technique to study the effect of doping metal salts on polyhydroxamic acid polymers . J. Appl. Poly. Sci. 81, 2095. 

Mohamed, H.F.M. (2001) Study on polystyrene via positron annihilation lifetime and Doppler broadened techniques . Polymer, 42, 8013. 

As for the volumes of the atoms, the thermal expansion and compressibility is composed of two main terms, the cavity and the hydration. An estimate of the contribution of each factor relies on assumptions that are not easy to check. An estimate of the expansion or compression of the cavities should be possible with positron annihilation lifetime spectroscopy. This approach has proven to be a useful tool for determining the size of cavities and pores in polymers and materials. The lifetime is sensitive to the size of the cavity in which it is localized. A number of empirical relations correlate the distribution of the lifetime and the free volume [33]. Data on the pressure effect on the lifetime are only available for polymers. The results suggest that there may be a considerable contribution of the reduction in cavity size to the compressibility of a protein. 

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-... 

MORPHOLOGY OF FREE-VOLUME HOLES IN AMORPHOUS POLYMERS BY MEANS OF POSITRON ANNIHILATION LIFETIME SPECTROSCOPY... 

Consolati, G., Quasso, R, Simha, R., and Olson, G. B., On the relation betwen positron annihilation lifetime spectroscopy and lattice-hole-theory free volume, J. Polym. Sci. B, 43, 2225-2229 (2005). 

Olson, B. G., Positron annihilation lifetime studies o polymers, Ph.D. dissertation. Case Western Reserve University, Cleveland, 2003. 

Srithawatpong, R., Peng, Z. L., Olson, B. G., Jamieson, A. M., Simha, R., McGervey, J. D., Maier, T. R., Halasa, A. R, and Ishida, H., Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene, high-vinyl polyhutadiene, and their miscihle hlends, J. Polym. Sci. B, 37, 2754-2770 (1999). 

Dlubek, G., Stejny, J., and Alam, M. A., Effect of cross-linking on the free-volume properties of diethylene glycol bis(allyl carbonate) polymer networks a positron annihilation lifetime study. Macromolecules, 31,4574-4580 (1998b). 

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). 

Jean, Y. C., Comments on the paper Can positron annihilation lifetime spectroscopy measure the free-volume hole size distribution in amorphous polymers Macromolecules, 29,5756-5757 (1996). 

POSITRON ANNIHILATION LIFETIME STUDIES OF FREE VOLUME IN HETEROGENEOUS POLYMER SYSTEMS... 

Wmberg, R, Eldrup, M., and Maurer, R H. J., Nanoscopic properties of silica-filled poly-dimethylsiloxane by means of positron annihilation lifetime spectroscopy. Polymer, 45, 8253-8264 (2004). 

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