Positronium lifetime in an ellipsoidal free-volume hole of polymers

A discussion of the molecular mechanisms of moisture transport in epoxy resins

“…The issue has been scrutinized in the past with both experiment and theory. First, Jean and Shi 30 looked at positronium annihilation in elliptical nanopores. They found that the lifetime was dominated by the shorter, or minor, dimension of the ellipse.…”

Role of the density in the crossover region ofo‐terphenyl and poly(vinyl acetate)

“…Positron annihilation lifetime spectroscopy (PALS) is a nanoprobe of the free volume which is parametrized in terms of the orthopositronium ͑o-PS͒ lifetime o−PS [11,[41][42][43]. In condensed phases o-PS tends to be confined in the open space of the structure where the positron is annihilated by electrons localized on the surface of the nanocavities (the penetration depth of the positron into the electron cloud is ⌬R Ӎ 1.66 Å [40,41]). o−PS increases with the size of the holes and depends on their geometry.…”

“…This proves that PALS and ESR are probing the same length and time scales. To investigate the degree of correlation between the unoccupied volume and the rotational relaxation of TEMPO, we calculated the average size of the unoccupied volume r from o−PS assuming that the holes are spheres, cuboids, or ellipsoids [40,41]. The aspect ratio of cuboids-i.e., square parallellepipeds-and ellipsoids was taken to be equal to the one of TEMPO.…”

Free-volume hole relaxation in molecularly oriented glassy polymers

“…However, with the development of positron annihilation lifetime spectroscopy (PALS), direct detection of free-volume holes in polymers has become possible and has attracted much attention in modern polymer physics [4][5][6][7][8][9]. In the past, much work has been performed on the free-volume hole information in isotropic polymers [10][11][12][13][14][15] with only limited reports on oriented polymers (mostly oriented thin films) [16,17]. Due to the growing interest in oriented bulk polymers for structural and ballistic applications [18,19], the understanding of freevolume hole is particularly useful in addressing microcrack initiation and propagation in polymers under stress.…”