Positron annihilation lifetime spectroscopy free volume polymers

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. 

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. 

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). 

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). 

Peng, Z. L., Olson, B. G., Srithawatpong, R., McGervey, J. D., Jamieson, A. M., Ishida, H., Maier, T. M., and Halasa, A. R, Study of free volume in high vinyl-polybutadiene/cis-polyisoprene blends by positron annihilation lifetime spectroscopy, J. Polym. Sci. Polym. Phys., 36, 861-871 (1998). 

Positron Annihilation Lifetime Spectroscopy. The details of PALS will not be discussed here since several other chapters in this book are specifically concerned wifii this experimental technique and provide more than adequate description. Suffice it to say here that or//ioPositronium (oPs), which is formed when a positron binds to an electron of parallel spin, tends to locate in free volume sites within a polymer, and the time required for it to annihilate via pickoff with an electron of anti-parallel spin residing in the surrounding material is related to the size of the free volume site. There are two parameters that are sensitive to polymer free volume the oPs pickoff component lifetime, xa, which is related to the mean radius of the free volume cavities, and the oPs pickoff component intensity, I3, which is related to the concentration or number of fr volume cavities. 

Positron Annihilation Lifetime Spectroscopy of HIQ-40 Films Positron annihilation lifetime spectroscopy has emerged as a sensitive technique to probe free volume in polymers (33, 34), PALS uses orthoPositronium [oPs] as a probe of free volume in the polymer matrix. oPs resides in regions of reduced electron density, such as free volume elements between and along chains and at chain ends (33), The lifetime of oPs in a polymer matrix, T3, reflects the mean size of free volume elements accessible to the oPs. The intensity of oPs annihilations in a polymer sample, la, reflects the concentration of free volume elements accessible to oPs. The oPs lifetime in a polymer sample is finite (on the order of several nanoseconds), so PALS probes the accessibility of free volume elements on nanosecond timescales (33),... 

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]. 

The generally recognized and the most reliable method for investigation of free volume in polymers is positron annihilation lifetime spectroscopy (PALS). It was applied for investigation of PTMSN and related polymers. This method is based on the measurement of lifetime spectra of positrons in polymers - lifetimes (ns) and corresponding intensities li (%). Longer lifetimes (or T3 and T4) (so-called o-orthopositronium lifetimes) can be related to the mean size of free volume R. 

Wang et al. [60] utUized positron annihilation lifetime spectroscopy to measure the polymer free volume in mont-morillonite-styrene-butadiene rubber nanocomposites. There was an apparent reduction of the free volume of the polymer in the nanocomposite. The authors speculated that the reduction was primarily at the clay surface. This information is consistent with the crosslink density results reported above. 

Positron annihilation lifetime spectroscopy (PALS) can be used to measure the free volume in various materials. Jean et al. discussed the application of positron annihilation spectroscopy (PAS) in the detailed study of polymers and polymers with fillers. The primary experimental PAS technique used in this research is PALS, one of the three techniques in the PAS family and a powerful tool for measuring the free volume in various materials. The free volume has a great role in polymer research and is widely used to explain the behaviour of physical properties such as glass transition temperature, viscosity and physical ageing. Free volume is affected by the blending of polymers, ageing and addition... 

Positron Annihilation Lifetime Spectroscopy (PALS) provides a measure of free volume holes or voids, free volume, and free volume distribution, at an atomic scale. 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, thermalized 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) which is of interest. The oPs spin exchanges with electrons of opposite spin on the walls of the cavity and it is annihilated. Thus, the oPs lifetime, 13, gives a measure of the mean free volume cavity radius, whereas the relative intensity of... 

Positron annihilation lifetime spectroscopy (PALS) is normally applied to determine the free volume properties of a cured thermoset. The theory and methodology of PALS [27, 28] is briefly described next. The positron, an antiparticle of an electron, is used to investigate the free volume between polymer chains. The birth of the positron can be detected by the release of a gamma ray of characteristic energy. This occurs approximately 3 ps after positron emission when the Na decays to Ne. Once inside the polymer material, the positron forms one of the two possible types of positroniums, an ort o-positronium or a p(3 ra-positronium, obtained by pairing with an electron abstracted from the polymer environment. The decay spectra are obtained by the death event of the positron, pi ra-positronium or ort o-positronium species. By appropriate curve fitting, the lifetimes of the various species and their intensity can be determined. The lifetime of an ort o-positronium (Xj) and intensity (I3) have been found to be indicative of the free volume in a polymer system because this is where the relevant species become localised. X3 is related to the size of the free volume sites and I3 to their number concentration. The free volume properties of difunctional and multifunctional epoxies are shown in Table 3.5. The data clearly... 

Positronium Annihilation Lifetime Spectroscopy. Positron annihilation lifetime spectroscopy (pals) is primarily viewed as techniqne to parameterize the imoccnpied volnme, or so-called free volume, of amorphous polymers. In vacuo, the ortho-positronium (o-Ps) has a well-defined lifetime T3 of 142 ns. This lifetime is cut short when o-Ps is embedded in condensed matter via the pick-oflT mechanism whereby o-Ps prematurely annihilates with one of the surroimding boimd electrons. The quantum mechanical probability of o-Ps pick-off annihilation depends on the electron density of the medium, or the size of the heterogeneity. Typically the heterogeneity is assiuned to be a spherical cavity (164,165) so that T3 can be easily related to an average radius R (Ro = R -i- AR) of the nanopore ... 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

Positron Annihilation Lifetime Spectroscopy The principal experiment utilized in examination of the free-volume hole size has been positron annihilation lifetime spectroscopy (PALS), first developed by Kobayashi and co-workers (90,91). Positrons from a Na source are allowed to penetrate the polymer, and the lifetime of single positrons is registered. The... 

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