Positron annihilation lifetime spectroscopy PALS , free volume

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]. 

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... 

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., 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). 

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. 

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. 

Further information about the sizes of free volume elements or micropores can be obtained from inverse gas chromatography and positron annihilation lifetime spectroscopy (PALS), as is discussed later. 

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. 

Positron annihilation lifetime spectroscopy (PALS) studies the lifetime spectrum of ortho-positrons after being injected into the sample [3,4]. This lifetime depends on the probability of the ortho-positronium (o-Ps) particle (a hydrogen-like bound state formed by a positron-electron pair) to be quenched and annihilate. This probabihty is higher in condensed matter than in vacuum. Of all the probe methods PALS is nowadays probably the most versatile one and the most widely used. The o-Ps particle is the smallest probe available and can thus detect the smallest free volume elements furthermore, the method furnishes information on the average free volume size and on the FV size distribution. 

It has been observed [24] that for PEG (200g/mol) modified Pebax membrane for CO2 separation the CO2 permeability increased by a factor of about 2 (from 73 to 151 Barrer) and the separation factor CO2/H2 also increased by PEG addition (50 wt.%). This enhancement was attributed to the appearance of additional ethylene oxide (EO) units and free volume increase. Higher content of EO units results in an increase in the solubility of CO2. Later, the total free volume increase and hence the increase of the permeability was demonstrated by measurements of density and by positron annihilation lifetime spectroscopy (PALS) analysis [75]. 

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... 

The effect of copolymer composition on free volume and gas permeability of PECT copolymers as well as PET and PCT homopolymers was studied by Hill et al. (97). The free volume was studied by positron annihilation lifetime spectroscopy (PALS) in order to determine the relative size and concentration of free volume cavities in the copolymers. The logarithm of the permeability to oxygen and carbon dioxide increased linearly with the %mol content of 1,4-CHDM units in the copolymer, which was in agreement with the free volume cavity size and relative concentration observed by PALS measurements. Light et al. (98) studied the effect of sub-T relaxations on the gas transport properties of PET, PCT and PECT polyesters. They observed that modification of PET with 1,4-CHDM increased the magnitude of the p-relaxation, as well as the diffusion and solubility coefficients for oxygen and CO. ... 

When dealing with the free volume or interstices in dense membranes, small angle X-ray spectroscopy (SAXS) is very helpful, along with positron annihilation lifetime spectroscopy (PALS) [30], or even ellipsometry [31, 32], which measures density locally giving a first insight on the space free volume distribution in depth. 

Positron annihilation lifetime spectroscopy (PALS) is a more recent tool used to probe free volume and free volume distribution in polymers (38, 59). 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 (38). The lifetime of oPs in a polymer matrix reflects the mean size of free volume elements accessible to oPs. The intensity of oPs annihilations in a polymer sample reflects the concentration of accessible free volume elements. The oPs lifetime in a polymer sample is finite (on the order of several nanoseconds), so PALS probes the availability of free volume elements on nanosecond timescales (40). The minimum free volume cavity diameter required by oPs for localization is 3.SA (41), which is equal to the kinetic diameter of methane (42). Thus, PALS probes the dynamic availability of free volume elements similar in size to those important for gas separations applications. Several recent studies demonstrate the strong correlation of PALS parameters and transport properties in polymers (34, 38, 43-45). The chapter by Yampol skii and Shantarovich in this book describes the use of PALS to characterize free volume distribution in membrane polymers. 

Positron annihilation lifetime spectroscopy (PALS) is an efficient tool for measuring free volume and sizes of free volume elements in polymeric materials. This is particularly inq)ortant for studies of membrane materials, since free volume determines the permeation rate of small molecules. Free volume was studied by means of PALS in polymers characterized by extremely high permeability poly(l-trimethylsilyl-l-propyne) and copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole and tetrafluoroethylene. The results obtained were compared with those observed for conventional glassy polymers. For the first time, the size distribution of free volume has been determined for these membrane materials. 

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