86,573 research outputs found
Shapes of the free volume holes in amorphous polymers as estimated by positron annihilation lifetime spectroscopy
No full textIntermolecular spaces in polymer chains form the free volume, a useful concept to understand mechanical and transport properties of polymers. Quantification of the free volume can be obtained theoretically, using appropriate lattice models, as well as experimentally, through suitable probes. Among these, positronium (Ps) has become widespread due to the nondestructive character of the technique and the correlation between the Ps lifetime and the size of the holes. In most of the investigations, the cavity is approximated to a sphere. However, this may bias the evaluation of the free volume fraction. We show that by coupling results from Ps lifetime and specific volume measurement for amorphous polymers at equilibrium and the predictions of the Simha–Somcynsky equation of state, it is possible to shed light on dimensions of the holes as well as on their morphology
Positronium Contact Density in Ferrocene
No full textIn a medium where positronium (Ps) is formed, a lowering of the electron density at the positron position (contact density) with respect to a vacuum is generally observed. The effect is usually attributed to the interaction with the medium, which induces Ps polarization. The contact density reduction, explainable in polar media due to Debye forces, can be expected also in non-polar media, where London forces are present. To investigate this subject we measured the Ps contact density in ferrocene, an organometallic compound whose molecule does not have an intrinsic dipole moment; therefore, only induced dipole-induced dipole interactions may be present. Contact density was obtained by means of two different experimental techniques. A lowering by 23% with respect to vacuum value was observed, which allowed us a simple estimation of the order of magnitude of the effective microscopic electric field mimicking the Ps-ferrocene interaction
Swelling of Positronium Confined in a Small Cavity
No full textThe electron density at the positron (contact density) in the ground state positronium (Ps) formed in condensed matter is generally found to be lower than in vacuum. This is usually attributed to microscopic electric fields which polarize Ps, by acting on the two particles of the atom. In this paper we quantitatively investigate an opposite effect. It is due to the confinement of Ps in small cavities existing in the host solid (e.g. free volume in polymers), which increases the contact density. Although this phenomenon is greater, the smaller is the size of the cavity, Ps polarization seems to play anyway a predominant role
Outdoor ageing of an ethylene-propylene copolymer: a positron annihilation study
No full textAn investigation on the environmental degradation of an ethylene-propylene copolymer was carried out through positron annihilation lifetime spectroscopy, density and differential scanning calorimetry measurements. Polymer sheets were exposed to sunlight up to 11000 hours. Density and melting enthalpy increase with ageing, while ortho-positronium intensity decreases; ortho-positronium lifetime does not show significant variations up to 4400 hours. At higher exposures it starts to decrease. Outdoor ageing produces an increased crystallinity of the copolymer; also a degradation occurs with formation of carbonyl groups. The corresponding Ps inhibition is the main reason for the decrease in ortho-positronium intensity, the other one being the reduced amount of the amorphous phase available. The lifetime decrease observed at the highest exposures can be interpreted in terms of a shrinking of the nanoholes trapping positronium
Polarization of Positronium in Amorphous Polar Polymers: a Case Study
No full textThe features of positronium in an amorphous copolymer (polyvinyl acetate-crotonic acid) in a range of temperatures including the glass transition were investigated by means of positron annihilation lifetime spectroscopy. In particular, para-positronium lifetime was found to be longer than in a vacuum and to decrease with the temperature. This was attributed to the electron density at the positron (contact density), which is lower than in vacuo due to the presence of polar groups in the copolymer. A three quantum yield experiment confirmed the lifetime results
Discrete and continuous analysis of positron lifetime spectra : a comparison
No full textPositron lifetime spectra in poly[1-(trimethylsilyl)]propine membranes were collected at different temperatures and various number of accumulated counts, in order to compare the results from analyses obtained by different decay models: discrete (that is, purely exponential) or continuous (with at least one distribution of lifetimes). Analysis into four discrete components is inadequate, unless a low number of counts (1 to 2 millions) is accumulated. Conversely, deconvolutions which allow the ortho-positronium lifetime to be distributed give satisfactory fits, even at the highest statistics (60 millions). An equivalent good fit in terms of χ test is obtained with a five discrete components analysis, from which it is possible to extract information on the first moments (centroid and standard deviation) of ortho -positronium lifetime distribution
Ortho-para conversion of positronium in some rare earth ion aqueous solutions
No full text We have extended a previous investigation by studying the ortho–para conversion of positronium in aqueous solutions of some lanthanide ions (Yb3+, Tm3+, Er3+, Dy3+, and Tb3+). The corresponding cross sections were found to be proportional to the number of unpaired electrons in the ion. Furthermore, the contact density of the electron–positron pair in the origin was obtained for a 1 M aqueous solution of ErCl3. </jats:p
Effects induced by gamma irradiation of different polyesters studied by viscometry, thermal analysis and positron annihilation spectroscopy
No full textAn experimental study on different polyesters (PET, PBT, PEN, PEA, PCT-co-ET) subjected to g-irradiation up to 1 MGy was
carried out through viscometry, thermal analysis and positron annihilation lifetime spectroscopy. Chain scission induced by
irradiation prevails for doses up to 100 kGy; it is influenced by the length of the aliphatic chain as well as by the presence of the
aromatic ring. Crosslinking dominates for doses between 100 and 300 kGy, while the oxidative degradation mechanism, present at
the highest doses, increases its importance with the amorphous fraction of the polymer. Free volume structure is not modified by
irradiation
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