E.E. Abdel-Hady

Positron annihilation lifetime study of irradiated and deformed low density polyethylene

Positron annihilation lifetime spectroscopy has been used to study the free volume hole properties of unirradiated and irradiated low density polyethylene (LDPE) as a function of deformation in the range from 0 to 110%. Deconvolution of the lifetime spectra into three components reflected three different morphologies of the polymer structure. The variation of positron annihilation parameters with deformation shows three stages; the elastic range, strain-softening region, and plastic region. The free volume was probed using ortho-positronium (o-Ps) pick-off annihilation lifetime parameters as a measure of electron density and the mean free volume hole radius. For unirradiated sample, the average of the free volume hole increases with deformation in the elastic region, remains unchanged within the strain softening region, and finally tends to increase in the plastic region. A reduction in the size of the free volume holes and concentrations has been observed in the case of irradiated sample which is due to a structural change. The distribution of the free volume for unirradiated sample shifts from a small to large size as the deformation increases and has very similar Gaussian-like distribution. For irradiated one, the distribution becomes broader and has an asymmetric appearance. The results show that positron lifetime is a sensitive means to probe the microstructural change of polymers during deformation.

Microstructure changes of poly(vinyl chloride) investigated by positron annihilation techniques

Abstract The properties of the free volume hole in pure and doped poly(vinyl chloride) (PVC) have been investigated as a function of cadmium content and temperature using positron annihilation lifetime and Doppler broadening of annihilation radiation techniques. The observed spectra were decomposed into three components, with the longest component (lifetime, τ3 and intensity, I3) being associated with the pick-off annihilation of o-Ps trapped by free volume. The temperature coefficient of τ3 and I3 showed a change at the glass transition temperature (Tg) to lower temperature. Further, the behavior of the annihilation parameters was different during the heating and cooling cycles. These observations are correlated with observations of the electrical conductivity. The free volume hole and the fractional free volume were also derived from the positron annihilation parameters. A correlation between the fractional free volume and conductivity above Tg was experimentally confirmed using measured positron annihilation results. The value of the critical volume for doped PVC is found to be larger than that for pure PVC. This larger difference might be the result of the existence of large ionic charge carriers.

Studies of the Positron Lifetime and Doppler Broadening Annihilation Radiation of Polyvinyl Chloride Doped with Al2O3

Positron annihilation lifetime and Doppler broadening of annihilation radiation studies of pure and doped polyvinyl chloride (PVC) with Al2O3 reflect the effect of concentration on free volume. Therefore, variations of the relative fractional free volume, which is extracted from positron annihilation parameters, have been correlated with changes in the dielectric properties (dielectric constant and a.c. conductivity) of the pure and doped PVC with Al2O3 from 10 to 50 %. The o-Ps lifetime and its intensity show a linear dependence with a discontinuity at 30 % concentration of Al2O3. A correlation between the relative fractional free volume and the dielectric parameters has been achieved.

Study of sorption in poly(vinyl alcohol) using the positron annihilation technique

The vacancy states of fresh and annealed poly(vinyl alcohol) polymers sorbing CO2 gas at various pressures have been investigated by the positron annihilation lifetime (PAL) technique. The change of the bulk volume and the amount of sorbed CO2 gas were measured simultaneously to compare the free volume fraction with the size of the ortho-positronium holes. The PAL data indicate that the size of the ortho-positronium hole increases with the progress of the sorption while the sorption and the bulk volume data show that the free volume fraction becomes constant. Also, it supports the existence of local plasticization and the improvement of the dual mechanism model of sorption of gas.

Per-fluorinated sulfonic acid/PTFE copolymer studied by positron annihilation lifetime and gas permeation techniques

The mechanism of gas permeation in per-fluorinated sulfonic acid/PTFE copolymer Fumapem® membranes for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Three different samples, Fumapem® F-950, F-1050 and F-14100 membranes with ion exchange capacity (IEC) = 1.05, 0.95 and 0.71 meq/g, respectively were used after drying. Free volume was quantified using the positron annihilation lifetime (PAL) technique and gas permeabilities were measured for O2 and H2 as function of temperature. Good linear correlation between the logarithm of the permeabilities at different temperatures and reciprocal free volume indicate that gas permeation in dry Fumapem® is governed by the free volume. Nevertheless permeabilities are much smaller than the corresponding flexible chain polymer with a similar free volume size due to stiff chains of the perfluoroethylene backbone.

Positron annihilation study of acryl amide/poly (metha acrylic acid) membrane

Gamma irradiation posses a serious role for casting the membranes. Acryl amide /poly (methacrylic acid) membrane was synthesized under ?-radiation effect. The structure of the membrane was characterized by FTIR, thermo-gravimetric analysis and the scanning electron microscope. The properties of the membranes were also investigated in terms of proton conductivity and positron annihilation lifetime (PAL) parameters. On the basis of the values of the long-lived components in the lifetime spectra, the size of the free volume and their intensity were calculated. The positron lifetime study on these irradiated casted membranes shows that the cross-linking and degradation within the membrane matrix affect the free volume content and hence the microstructure.

Miscibility and crystallization behavior of poly (3-hydroxybutyrate) and poly (ethylene glycol) blends studied by positron annihilation spectroscopy

Positron annihilation Lifetime (PAL) spectroscopy has been used to study the effect of PEG concentrations on the free volume properties of PHB. The data revealed that the ortho-positronium (o-Ps) lifetime τPs increases with 20% increase in concentration, decrease as the concentration increases to 40%, then rapid increase at 50% concentration of PEG. The o-Ps intensity, I3, shows a linear dependence as the concentration increases with a discontinuity at 20% concentration of PEG. Furthermore, the results presented and discussed in this work show that the PHB and PEG are miscible up to 40% of PEG but greater than 40%, the blend is immiscible. In addition, the mechanical properties of PHB are well improved by the addition of PEG with a low concentration up to 20%, while at higher concentration the blend becomes waxy.

Positron annihilation in polyvinylalcohol doped with CuCl2

Positron annihilation lifetime spectra were measured forpolyvinylalcohol (PVA) doped with CuCl2 (0.5 to 5.0 wt%) at temperature range from room temperature to 160°C. For a fresh pure PVA (without annealing) τ3 belowTg was larger in the heating runs than in the cooling runs, but aboveTg, τ3 was the same for both runs. The larger τ3 values in the heating run were considered to be due to the existence of strain brought forth in the process of preparation. For annealed pure PVA τ3 was the same for the heating and the cooling runs andTg was shifted to lower temperature, 88 and 80°C for fresh and annealed samples, respectively. This shift was attributed to loss of H-bonded water in the annealed pure PVA by heating above 100°C. BothI30 and τ3 were decreased by the added CuCl2, being interpreted as the result of inhibition and quenching, respectively. The results show that both the inhibition coefficient α and the quenching rate constantk were smaller than the corresponding values in liquids. The small diffusion constant ofo-Ps estimated fromk implies thato-Ps is not very mobile in the polymer.

Cross-linked PVA / SSA proton exchange membranes: correlation of physiochemical properties with the free volume determined by positron annihilation spectroscopy

Two processes for crosslinking polyvinyl alcohol (PVA) with sulfosuccinic acid (SSA) and thermal crosslinking were used to fabricate a proton exchange membrane (PEM). Such PEMs are used in different fields involving fuel cell applications. The crosslinking reaction between PVA and SSA was confirmed using Fourier-transform infrared (FTIR) spectroscopy. The characterization of the prepared membranes, namely, ion exchange capacity (IEC), thermal analyses, water uptake, and ionic conductivity, was carried out. The IEC of the prepared membranes was found to be between 0.084 and 2.086 mmol g-1, resulting in an essential increase in the ionic conductivity. It was observed that the ionic conductivity was in the range of 0.003-0.023 S cm-1, depending on both temperature and SSA content. From the thermogravimetric analysis (TGA) results, it was revealed that the thermal stability of the crosslinked membranes improved. Moreover, water uptake decreased with increasing SSA content. Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the PVA/SSA membranes and their distribution at different ambient temperatures and relative humidity (RH) values. At room temperature, no significant change was observed in the free-volume holes up to 15 wt% SSA; thereafter, the size of the free-volume holes increased with the SSA content. The PALS results show that at different humidity values, the size of the free-volume holes for crosslinked PVA/SSA membranes is lower than those for Nafion membranes, i.e., the gas permeability for the prepared PVA/SSA membranes is less than that for the Nafion membrane. In addition, a strong correlation between the water uptake, ionic conductivity, tensile strength, and free-volume holes was observed.

The effect of wheat hay on carbon black loaded EPDM composite studied by positron annihilation lifetime spectroscopy

Abstract. The positron annihilation lifetime (PAL) technique has been applied to investigate the behavior of free volume hole in carbon black loaded ethylene propylene diene monomer (EPDM) composite as a function of the wheat hay concentration from 10 to 50 phr. The macroscopic studies including stress-strain, hardness and swelling behavior were measured too. An attempt is done to establish a correlation between the macroscopic measurements and the microscopic properties of the free volume holes. The ortho-positronium (o-Ps) parameters show a compact phase around 20 phr concentration of wheat hay. Such a phase is thought to be responsible for the maximum values of hardness, rupture stress, rupture strain and the minimum in the degree of swelling. Introduction Today, a renaissance in the use of natural fibers as reinforcements in technical applications is taking place mainly in the automobile and packaging industries [1, 2]. The availability of large qualities of such fibers with well-defined mechanical properties is a general prerequisite for the successful use of these materials. Coupled with the growing concern for disposal of agricultural and industrial waste and low cost fillers, considerable studies have been carried out on the utilization of some of these materials as fillers with elastomers and plastics as well as with thermoplastic elastomers. The PAL technique has been used to study the microstructure change in polymers and gives direct information about the free volume size and concentration [3, 4]. It was applied to investigate the behavior of the free volume (size and concentration) in carbon black loaded EPDM composites as a function of the concentration of wheat hay. Macroscopic studies including stress-strain behavior, hardness and swelling behavior were performed as an attempt to find a correlation between the positron annihilation parameters and the macroscopic results.