M.F. Ferreira Marques

Free-volume studies in polyurethane membranes by positron annihilation spectroscopy

Abstract The free-volume parameters in various urethane/urea membranes obtained by varying the ratio of the structural constituents, polypropylene oxide and polybutadiene, were studied by positron lifetime and Doppler broadening measurements. On bi-soft segment membranes, a correlation was found between the composition of membranes, the normalized free volume, the radii of the holes and gas permeability. However, the correlation is not clear when PU data are also considered, indicating that other features must also play an important role in the permeation mechanism.

Positron lifetime studies in vinyl polymers of medical importance

Abstract Vinyl polymers used as artificial lens implants in ophthalmology were investigated by positron lifetime spectroscopy. The structure of these polymers with free volumes offers the possibility of charging them with anti-inflammatory drugs for sustained release. A correlation between the amount of normalised free volume and the ratio of the methyl methacrylate to ethyl-hexyl-acrylate, used as polymerisation monomers, was found.

Thymoquinone β-Cyclodextrin Nanoparticles System: A Preliminary Study

Thymoquinone is a natural product, the main constituent of Nigella sativa seeds, which exhibits anti-inflammatory and anticancer activities. Among several existing molecules capable of forming an inclusion compound structure, cyclodextrins are applied in the pharmaceutical industry either to increase solubility of hydrophobic molecules or to protect molecules from inactivation or degradation. β-Cyclodextrin is currently the most common cyclodextrin in pharmaceutical formulations and probably the best studied in humans. In order to study the properties of inclusion compounds based on cyclodextrins and thymoquinone Fourier Transform Infrared (FTIR), Ultraviolet-Visible, Positron Annihilation Lifetime (PAL) Spectroscopies and calorimetric studies by Differential Scanning Calorimetry (DSC) were used. The obtained results indicate the formation of a 1 : 1 inclusion compound between cyclodextrin and thymoquinone. PALS and DSC measurements also provided evidence of the inclusion compounds activity.

Gas Permeability and Temperature-Dependent Free-Volume Studies in Polyurethane Membranes by Positron Lifetime and Doppler Spectroscopies

Positron annihilation lifetime and Doppler broadening spectroscopies were used to study the free volume parameters in polypropylene oxide/polybutadiene bi-soft segment urea/urethane membranes (PU/PBDO), with PBDO content varying from 0 up to 75 wt % in the temperature range 298–324 K. The gas permeation features appear to be correlated with the free volume sizes determined by the lifetime measurements whereas phase separation of the various soft and hard segments in the membranes is mirrored by both the lifetime and Doppler results.

Analysis and modelling of positron lifetime spectroscopy data for the characterisation of water-in-oil microemulsion systems stabilised by Aerosol OT Effect of changing the counter-ion

In previous work using positron lifetime spectroscopy, a model was proposed leading to the satisfactory description of both positronium (Ps) formation and decay in sodium AOT–water–isooctane reverse micelles. The shorter lifetime found for the Ps triplet state in the aqueous core of the micelles, compared to that found in pure water, was attributed to Ps outdiffusion from these cores, enabling the quantification of this lifetime with two fitting parameters: r0, the aqueous core radius, and h, a factor which describes the ease of passage of Ps at the water/isooctane interface. To assess the validity of the model, the experiments have been extended to the study of the Ca(AOT)2 and 2:1 mole ratio Na(AOT):Ca(AOT)2–water–isooctane systems, to examine the effect of the counter-ion on r0 as a function of the water/AOT ratio, w0 and (for Ca2+) of temperature. Using, for all systems, the value of h previously found for the Na(AOT) microemulsions, the presence of the Ca2+ counter-ion appears to result in the formation of larger reverse micelle aggregates, compared to with Na(AOT), with radii in agreement with published data. With the Ps yields, the model implies a semi-empirical parameter, Atrap, related to the probability that the energetic positrons end up in the aqueous subphase before forming Ps. As expected and experimentally verified, Atrap is proportional to r02. Unexpectedly however, with the above value for h, the variation of Atrap with r02 is found to depend on the counter-ion. Therefore, the possible variation of h was considered. Using different values of h for each system, it is possible to obtain a unique variation of Atrap with r02, independent of the counter-ion. In this case, however, the radii for the microemulsions containing Ca2+ are significantly lower than those found in previous work.

EFFECTS OF CCL4 ON POSITRONIUM FORMATION IN PURE ISOOCTANE AND IN AOT/WATER/ISOOCTANE MICROEMULSIONS

Abstract Previous work, on positron lifetime spectroscopy (LS) measurements on AOT/water/isooctane microemulsions showed that most positronium (Ps) is formed in the aqueous subphase, and predicted that a water insoluble electron scavenger should inhibit much less Ps formation than if Ps was mainly formed in the organic solvent. To test this prediction, Ps inhibition by CCl 4 is examined in pure isooctane and in the micellar system. In the former case, from the combined analysis of LS and Doppler broadening results, CCl 4 is found to strongly inhibit Ps formation ( k = 36.6 M −1 ) by the successive capture of e − , to form CCl 4 − , and of e + , to form the PsCCl 4 bound-state. As expected, CCl 4 in the micellar system displays a much lower ability to inhibit Ps formation ( k ca. 9 M −1 ). However, a very good agreement with the data is only obtained by assuming that, besides Ps inhibition in the organic solvent, CCl 4 also promotes some inhibition of that fraction of Ps formed in the aqueous subphase. Quantitatively, the low inhibition constant found for this process (5.1 M −1 ) is quite consistent with what is expected when considering the fraction of the aqueous positron spurs that overlap with the organic subphase. Another hypothesis is discussed, by which CCl 4 would decrease the probability that the positrons become trapped in the aqueous subphase, due to an increase in the electron density of the organic solvent.

Preliminary Positron Lifetime Results on Free Volumes in Cyclodextrins

Positron annihilation lifetime spectroscopy was used to study the free-volume parameters in various pure -, - and -cyclodextrins samples and, in the case of β-cyclodextrin, with inclusion of S-carvone and thymoquinone. The results clearly indicate the presence of long lifetime components related to Ps-formation. The data show that the addition of S-carvone to β-cyclodextrin results in a decrease of o-Ps lifetime that we ascribe to a reduction of free volume holes from 81.8 to 63.7 Å3. The long lifetime component disappears when thymoquinone is added to -cyclodextrin, indicating this substance acts as an o-Ps quencher. For all samples studied, a decrease in the long lifetime component values was observed with increasing source in situ time, a result that might be attributed to the irradiation of the sample by the 22Na positron source.

Effect of water-soluble electron scavengers on positronium formation in AOT/water/isooctane microemulsions

Abstract In a previous work reporting on positron annihilation lifetime spectroscopy (LS) measurements in AOT/water/isooctane microemulsions, a model was established leading to the satisfactory description of both triplet positronium (o-Ps) formation and decay. The model proposes that a large proportion of Ps is formed in the aqueous pseudophase, a fraction of which can diffuse out to the organic phase. It thus predicts that an electron scavenger, soluble in the aqueous pseudophase only, should have an inhibiting action on both those species decaying in the aqueous (o-Psaq) and organic (o-Psorg) phases. To assess this prediction, measurements are performed with efficient inhibitors of Ps formation, incorporated into the reverse micelles: Te(OH)6, KNO3, and HgCl2. The last solute is also known as a Ps quencher in water. In all three cases, the lifetime results show that the radii of the water cores remain unaltered by the presence of the additives. As expected, the intensities of both o-Psaq and o-Psorg decrease with increasing solute concentration. However, the quenching power of HgCl2 is suppressed in the water aggregates; this is probably due to the association of the Hg2+ cations with the sulphonate groups of AOT. On this basis, the inhibition of HgCl2 is attributed to positron capture by the Cl− ions. The inhibition constant of Te(OH)6, KNO3 and Cl− are all found to be about 65% of what they are in pure water. Qualitatively, this is ascribable to the existence of mixed water–isooctane positron spurs.

EFFECT OF CCL4 ON POSITRONIUM FORMATION IN ISOOCTANE

The effect of CCl4 on positronium (Ps) formation is examined in isooctane through positron lifetime spectroscopy experiments, in the framework of a series of studies on AOT-based reverse micellar systems. This solute appears as a rather strong Ps formation inhibitor, as expected in view of its well-known electron scavenging properties. The reciprocal of the triplet Ps intensity (1/I3) varies linearly with CCl4 concentration (C), up to about 0.08M, allowing to derive a high inhibition constant,k=36.6M−1. Above 0.08M, however, the inhibition slackens out, which is attributed to an anti-inhibition effect. On the basis of previous reports on halocompounds, a general scheme of reactions is proposed to occur in the positron spur, including, besides the primary Ps formation through thee+/e− reaction and the recombination reactions, electron scavenging by CCl4 as the direct inhibiting reaction, followed by the reaction of the thus formed anion with the positron. The latter reaction may lead either to a [CCl4Ps] bound-state or, if the positron picks off the electron from the anion, to some additional Ps formation. By using a simple quantitative approach to this scheme, an equation descriptive of the variation ofI3 withC is derived which agrees well with the experimental data.

Positron Annihilation Study on Nanocrystalline Copper Thin Films Doped with Nitrogen

Pure and nitrogen doped copper thin films were produced by dc magnetron sputtering. The inclusion of nitrogen during the deposition process has revealed an important role in the reduction of the grain size and in the possibility to produce nanocrystalline copper thin films. Positron annihilation Doppler broadening measurements have been performed in order to identify lattice defects. Interfaces between grains and nanopores were identified as the main lattice defects and as candidates for nitrogen location in the films.