Positron annihilation lifetime spectroscopy volume holes

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). 

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). 

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. 

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... 

The dynamic heat capacity and Tg of PS/vinyl acetate-butyl acrylate copolymer latex films as a function of annealing time were examined with modulated-temperature DSC. The mean free volume and relative concentration of holes at room temperature as a function of time were obtained using positron annihilation lifetime spectroscopy. The effect of residual water in the structured latex films on thermal properties is discussed. 33 refs. 

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 results summarized in Table 9 show that intrinsic porosity of the polymer, i.e. the fraction of its total volume accessible to N2 molecules at 77 K, can exceed 20% for the sample conditioned at high propylene pressure and room temperature, decreasing down to ca. 6% upon annealing in vacuum at 373 K. These results allow assuming that the variations in propylene permeability through a membrane described above apparently stem from the changes in the free volume structure, i.e. accessibility of the intrinsic micropores for the gas molecules. Similar conclusion on the expansion of the free volume hole size and the increase in the number of holes upon sorption of CO2 in polycarbonate has been made in [47] on the basis of positron annihilation lifetime spectroscopy investigations. 

In recent years, positron annihilation lifetime (PAL) spectroscopy has been demonstrated to be a special sub-nanometer probe to determine the free-volume hole size, fraction and distribution in a variety of polymers (4-9). In this technique, measured lifetimes and relative intensities of the positron and positronium, Ps (a bound atom which consists of an electron and a positron), are related to the size and fraction of sub-nanometer holes in polymeric materials. Because of the positive-charge nature, the positron and Ps are repelled by the ion core of polymer molecules and trapped in open spaces, such as holes, free volumes, and voids. The observed... 

Free volume or hole volume is ostensibly measured experimentally by positronium-annihilation-lifetime spectroscopy (PALS). In organic glasses, including amorphous polymers, the ortho-positronium (o-Ps) bound state of a positron has a strong tendency to localize in heterogeneous regions of low electron density. In vacuo, an... 

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)... 

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

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