Free volume positron annihilation lifetime spectroscopy

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. 

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]. 

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... 

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). 

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., Bondarenko, V., Al-Qaradawi, I. Y., Kilburn, D., and Krause-Rehberg, R., Structure of free volume in SAN copolymers from positron lifetime and PVT experiments II. Local free volume from positron annihilation lifetime spectroscopy (PALS), Macromol. Chem. Phys., 205, 512-522 (2004c). 

Hong, X., Jean, J. Y., Yang, H., Jordan, S. S., and Koros, W. J., Free-volume hole properties of gas-exposed polycarhonate studied by positron annihilation lifetime spectroscopy. Macromolecules, 29, 7859-7864 (1996). 

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). 

Kanaya, T., Tsukushi, T., Kaji, K., Bartos, J., and Kristiak, J., Microscopic basis of free-volume concept as studied by quasielastic neutron scattering and positron annihilation lifetime spectroscopy, Phys. Rev. E, 60,1906-1912 (1999). 

Ma, W., Andersson, A., He, J., and Maurer, F. H. J., Free volume changes, crystallization, and crystal transition behavior of S5Uidiotactic polystyrene in supercritical CO2, revealed by positron annihilation lifetime spectroscopy. Macromolecules, 41, 5307-5312 (2008). 

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). 

Schmidt, M., and Maurer, R H. J., Isotropic pressure-densified atactic poly(methyl methacrylate) glasses free-volume properties from equation-of-state data and positron annihilation lifetime spectroscopy. Macromolecules, 33, 3879-3891 (2000a). 

The free-volume concept dates back to the Clausius [1880] equation of state. The need for postulating the presence of occupied and free space in a material has been imposed by the fluid behavior. Only recently has positron annihilation lifetime spectroscopy (PALS see Chapters 10 to 12) provided direct evidence of free-volume presence. Chapter 6 traces the evolution of equations of state up to derivation of the configurational hole-cell theory [Simha and Somcynsky, 1969 Somcynsky and Simha, 1971], in which the lattice hole fraction, h, a measure of the free-volume content, is given explicitly. Extracted from the pressure-volume-temperature PVT) data, the dependence, h = h T, P), has been used successfully for the interpretation of a plethora of physical phenomena under thermodynamic equilibria as well as in nonequilibrium dynamic systems. 

Cangialosi, D., Schut, H., van Veen, A., and Picken, S. J., Positron annihilation lifetime spectroscopy for measuring free volume during physical aging of polycarbonate. Macromolecules, 36, 142-147 (2003). 

In the work reported here, changes in molecular free volume and tensile properties of PET have been followed as a function of physical aging. Positron annihilation lifetime spectroscopy (PALS) was used to measure free volume. The effect of physical aging on the competition between yielding and crazing without exposure to corrosive chemical environments was also investigated. Finally, the effect of physical aging on craze initiation at low strains due to exposure to a variety of solvents was determined. 

The positron annihilation lifetime spectroscopy (PALS) iq>paratus used to measure free volume in this work consisted of an automated EG G Ortec fast-fast coincidence system. The 1.3 MBq NaCl source was a 2 mm t source sandwiched between two Ti foils (2.54 pm foils). The source gave a two-component best fit to 99.99% pure, aimealed, chemically polished aluminum (x = 169 2 ps, Ii = 99.2 0.4 %, X2 = 850 25 ps, I2 = 0.8 0.4%). No source correction was used in the analysis of the data with the PFPOSFIT program (9). Measurements were made in air at 50% relative humidity with temperature control of 0.7 C. 

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. 

---