A.P. de Lima

Structural characteristics and gas permeation properties of polynorbornenes with retained bicyclic structure

Abstract Polynorbornenes of different molecular weights were synthesised by addition polymerisation, using a nickel based catalyst. This polymerisation route leads to amorphous polymers that were shown to display unique properties: high glass transition temperature, rigid random coil conformation, and dense packing in the amorphous state. Gas permeation membranes were prepared from these addition type non-substituted polynorbornenes. A study of their gas transport properties was performed and compared with both substituted addition polynorbornenes and ring opening metathesis polymerised polynorbornenes. The observed behaviour, in terms of permeability and selectivity, is in accordance with the low free volume and the dense packing confirmed by positron annihilation spectroscopy and WAXD in these polymer structures.

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.

Study of NbH phases using perturbed angular correlation techniques

Abstract Niobium hydride phases were investigated by the perturbed angular correlation technique. By this method the electric field gradient (EFG) at radioactive probe atoms inside the NbH lattice is measured. We find two distinct EFGs in the region of the ϵ phase of NbH whereas, in the temperature range of the ζ and β phases, only a single EFG with a spread around the mean value is observed. Only a very weak disturbance of the cubic symmetry is found in the α phase. The observed EFGs are discussed in the frame of the point charge model.

Influence of the a-Si:H structural defects studied by positron annihilation on the solar cells characteristics

The influence of the power density, P , on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.- D PECVD, is studied by the constant photocurrent method, (CPM), and by slow positron beam spectroscopy, respectively.Deep defect density, N , remains approximately constant at 10 cm , typical of device quality material, for P in the range 7-20 16 y3 DD D mWOcm , as calculated from CPM.Out of this range, N increases roughly one order of magnitude for both lower and higher y3 DD

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.

Investigation of defect structures in deuteron irradiated and deformed stainless steel 316 by positron annihilation

Abstract Deuteron-irradiated and deformed stainless steel specimens were investigated by positron lifetime and Doppler broadening measurements. The evolution of the defect structures was studied as a function of the isochronal annealing temperature and for various degrees of deformation. A different behaviour was observed for deformed and irradiated stainless steel specimens. Evidence for vacancy clusters was found in the deuteron-irradiated steel. These clusters disappear after annealing around 900 K.

Electric field gradients in different niobium hydride phases

Abstract Various phases of the NbH system were investigated by the perturbed angular correlation technique. The 181HfNb samples were charged with various hydrogen concentrations (atomic ratios of 0.20, 0.75 and 0.90). For each sample, measurements were performed at several temperatures. The observed interactions are discussed in terms of the phases involved in the NbH phase diagram. Asymmetry parameters are compared with point-charge model calculations.

Identification of {sup 162}Gd and a new type of identical bands

From γ-γ-γ coincidence measurements in spontaneous fission of 252Cf, level energies were established to spins 14+ to 20+ in many neutron-rich nuclei. New isotope 162Gd was identified. Yrast bands of 152,154,156Nd, 156,158,160Sm, and 160,162Gd exhibit near-identical transition energies and moments of inertia (MOI) shifted by the same constant amounts for every spin state from 2+ to 12+ or 14+ for nuclei differing by 2n, 4n, 2p, 4p, α, α+2p, and α+2n. These shifted identical bands (SIB) are a new phenomenon. Analysis of all known even-even proton- to neutron-rich nuclei from Ba(Z=56) to Os(Z=76) reveal no SIB for proton-rich nuclei and few cases of SIB for the most neutron-rich pairs around N=98–102 separated by 2n and 2p.