Convenient Preparation of Graphene Oxide from Expandable Graphite and Its Characterization by Positron Annihilation Lifetime Spectroscopy

Graphene oxide (GO) is conveniently prepared from expandable graphite using a simplified Hummers’ method. The product is thoroughly characterized by usual techniques (UV-vis, Fourier-transform infrared (FTIR) and Raman spectroscopies, zeta potential, electron microscopy, X-ray diffraction, nitrogen adsorption) to confirm the success of synthesis. Positron annihilation lifetime spectroscopy (PALS) is then used to extract information on the microenvironment in between the layers of graphene oxide

Morphology of free volume holes in amorphous polymers by means of positron annihilation lifetime spectroscopy

Morphology of free volume holes in amorphous polymers by means of positorn annihilation lifetime spectroscop

Morphology of free volume holes in an amorphous polyether-polyester polyurethane of biomedical interest

Positron annihilation lifetime spectroscopy (PALS) is a valuable technique for assessing the typical sizes of free volume holes in polymers. By coupling the results obtained from the positron annihilation spectra with those supplied by dilatometry, more precise information can be extracted on the free volume fraction and novel insight can be acquired on the shapes of the holes, demonstrating the limitations of the spherical approximation. In the present paper, we apply the combined PALS-dilatometry approach to study an amorphous polyether-polyester polyurethane, a polymer of great technological interest. We find that flattened and elongated holes, rather than spherical ones, better account for the behavior of the free volume fraction as a function of the temperature

Temperature dependence of free volume in atactic polypropylene

Positron annihilation lifetime spectroscopy data are often used to get information on the typical sizes of sub nanometric holes forming the free volume in polymers. To this purpose, the cavities are modelled as spheres or, more generally, using geometries which assume an isotropic expansion with the temperature. However, this guess could be unrealistic owing to the irregular shape of holes and the constrained movements of the macromolecules. In this work positron and dilatometric data are used to estimate the free volume fraction in atactic polypropylene. Comparison with the prediction of the Simha-Somcynsky theory supplies information on the thermal dependence of the volume of holes, whose behaviour can be interpreted in terms of anisotropic expansion

Positronium Contact Density in Ferrocene

In 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

Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy

Seven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with At and N(2) adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite