Electron irradiated low-density polyethylene studied by positron annihilation lifetime spectroscopy

A study of the degree and rate of cross-linking of low-density polyethylene (LDPE) as a result of irradiation by high-energy electron beam has been performed using positron annihilation lifetime (PAL) technique. The PAL measurements were carried out at room temperature with a conventional fast-fast coincidence system. The lifetime spectra were analysed into four components using the PATFIT program to extract the positron parameters such as lifetime, mean lifetime, intensities, free volume radius, and fractional free volume. Almost all parameters exhibited differences between the unirradiated and irradiated LDPE. It can be concluded that the effect of irradiation on polyethylene results in intensive network formation, which is intensified as the electron energy is increased. The results manifest another proof of the usefulness of positron techniques in the study of the microstructure of polymers. © 2003 Elsevier Ltd. All rights reserved

Re-emission of slow positrons from tungsten at elevated temperatures

A series of measurements have been made on the dependence of the intensity and energy spectrum of slow positrons re-emitted from polycrystalline tungsten foil held at temperatures of up to 500 °C. Irrespective of target history, no sharpening of the energy spectrum of re-emitted positrons was observed as the remoderator temperature was increased. Further, a steady decrease in the apparent re-emitted positron intensity was observed, attributed primarily to the increase in positronium formation, as the temperature of the foil was increased from 20 to 500 °C. It is concluded that maintaining a moderator or remoderator at elevated temperatures in standard positron beam systems has no beneficial consequences for the quality or intensity of the beam produced. Reasons for this are discussed. © 2002 Elsevier Science B.V. All rights reserved

Positron annihilation lifetime study of helium ions implanted polyethylene blends

The structural defects of linear low density polyethylene (LLDPE) and high density polyethylene (HOPE) blends implanted with helium ions were investigated using position annihilation lifetime (PAL) technique. The positron annihilation lifetime measurements were carried out at room temperature with a conventional fast-fast coincidence system. The lifetime spectra were analyzed into four components using the PATFIT program to extract the positron parameters such as lifetime components, and their corresponding intensities. Almost all parameters exhibited a correlation with microstructure changes resulting from implantation. The results were further discussed by comparison with modifications in the morphology of implanted samples using Scanning Electron Microscope (SEM) and Differential Scanning Calorimeter (DSC). Virgin Samples of Polyethylene blends are shown to be miscible by singlet DSC melting temperatures. After ion implantation the thermal properties of blends exhibit different behaviors depending on ion fluence and blending ratio i.e. polymer structure

Positron annihilation lifetime study of pure and treated polyvinyl chloride

Positron annihilation lifetime technique (PAL) has been applied to investigate the behavior of the free volume in pure and doped polyvinyl chloride (PVC) as a function of Pb concentration and temperature. It is noticed that the glass transition temperature T8 has been shifted to lower temperatures. This shift might explain the increase in the electrical conductivity. The dependence of the size and fractional free volume in PVC on temperature has been discussed on the basis of the free volume model

Self-implantation of Cz-Si: Clustering and annealing of defects

Observations of vacancy clusters formed in Czochralski (Cz) Si after high energy ion implantation are reported. Vacancy clusters were created by 2 MeV Si ion implantation of 1 × 10 15 ions/cm 2 and after annealing between 600 and 650 °C. Doppler broadening measurements using a slow positron beam have been performed on the self-implanted Si samples, both as-implanted and after annealing between 200 and 700 °C for time intervals ranging from 15 to 120 min. No change in the S parameter was noted after the thermal treatment up to 500 °C. However, the divacancies (V 2 ) created as a consequence of the implantation were found to start agglomerating at 600 °C, forming vacancy clusters in two distinct layers below the surface; the first layer is up to 0.5 μm and the second layer is up to 2 μm. The S-W plots of the data suggest that clusters of the size of hexavacancies (V 6 ) could be formed in both layers after annealing for up to an hour at 600 °C or half an hour at 650 °C. After annealing for longer times, it is expected that vacancies are a mixture of V 6 and V 2 , with V 6 most probably dominating in the first layer. Further annealing for longer times or higher temperatures breaks up the vacancy clusters or anneals them away. © 2005 Elsevier B.V. All rights reserved

The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene

This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques. © 2005 Elsevier B.V. All rights reserved

Gamma irradiation effects on polymethyl methacrylate

The effect of gamma-irradiation on the structure of polymethyl methacrylate (PMMA) has been investigated. In this study, the PMMA specimens were gamma-irradiated at doses in the range of 0-1000 kGy and measured by means of positron annihilation lifetime (PAL) spectroscopy. Irradiation dose dependence of the longest lifetime component (?3), attributed to ortho-positronium (o-Ps) decay, and the intensity of this component (I3) was studied by positron annihilation lifetime spectroscopy (PAL). The two physical quantities were correlated with the size and density of free volume sites in the examined specimens and were compared with the thermal transitions in the material obtained using the DSC technique. It was found that the size and amount of free volume decreases with increasing dose due to irradiation-enhanced crosslinking.Wiley Online Librar