Positron annihilation lifetime measurements of mechanically fatigued polystyrene samples

Early stages of cyclic fatigue‐loaded polystyrene (PS) specimens were investigated by positron annihilation lifetime spectroscopy (PALS) at a maximum stress amplitude of 15 MPa. PALS yields information about the average unoccupied hole volume. A linear increase in the ortho‐positronium (o‐Ps) lifetime was observed in a range from 0 to 50,000 cycles. This increase occurs homogeneously distributed at different positions along a sample of 170 mm. The average unoccupied void volume increases by 1.2%. On the other hand, the o‐Ps intensity shows no systematic change upon cycling. The results suggest a homogeneous and linear increase in free volume prior to craze formation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1991–1995, 2008     

Positron annihilation lifetime studies of free volume in gelatin

The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.     

Positron annihilation lifetime study of PTFE/silica composites

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of PTFE/silica composites. The positron lifetimes (τ_n) and intensities (I_n) of PTFE and the composites (30-62% silica) were measured at room temperature as a function of specimen thickness. Four lifetime components were found in PTFE and the composites. The longer lifetime components, τ₃ = 1.4 ns and τ₄ = 4.4 - 4.1 ns, were interpreted to be due to the presence of two different sized free volume cavity distributions within the PTFE/silica composites. A strong silica concentration dependence was found in the bulk intensities (I_3b and I_4b). The I_3b value increased from 13.0% in PTFE to 28.2% in the 62% composite, while the I_4b value decreased from 17.5% in PTFE to 4.5% in the 62% composite. The smaller-void size, free volume fraction (τ₃I_3b) values increased linearly between 0 and 100% silica concentration, while the larger void size, free volume fraction (τ₄I_4b) values decreased nonlinearly with silica concentration. Since silica has a long lifetime component (τ₃ = 1.6 ns), this behavior is ascribed to silica particles occupying the large free volume cavities (370 ų) in the PTFE/silica composites. © 1996 John Wiley & Sons, Inc.     

Positron annihilation lifetime studies of free volume in ovalbumin gels

The dependence upon concentration of sodium chloride of free volume in ovalbumin gels, a main component of an egg white, is studied by the positron annihilation lifetime technique. The average free-volume radius in ovalbumin gels was about 0.27 nm at 298 K, smaller than those of organic polymers such as low-density polyethylene (0.34 nm at 300 K) and polystyrene (0.29 nm at 300 K). These differences suggest that the positronium annihilates in free volumes located close to hydrogen bonds, thus decreasing average free-volume radius. Free-volume content decreased with the increase of NaCl concentration up to 30 mM. At lower concentrations of NaCl, it seems that a correlation exists between microscopic free volume and macroscopic hardness of the ovalbumin.     

Positron annihilation lifetime evaluation of thermal cycling effects in atactic polystyrene

Positron annihilation lifetime spectroscopy (PALS) was used to investigate the thermal cycling characteristics of two different molecular weight atactic polystyrene resins. Annealed compression molded samples were thermally cycled over the temperature range 253 K to 393 K with PAL spectra taken in 10 K increments upon both cooling and heating. The longest lived component lifetime and intensity, indicative of orthopositronium pick-off, exhibit thermal dependencies that can be interpreted in a manner consistent with anticipated free volume changes associated with structural transitions.     

Positron annihilation lineshape studies as a supplement method to lifetime measurement

We studied positron annihilation lineshapes by a Ge(Li) γ-ray spectrometer for CCl₄ solution in benzene and two monomer crystals which are polymerized by γ-ray irridation. From the lineshapes we evaluated the para-positronium intensity, and the results agree well with those of lifetime measurements in the cases of CCl₄ solutions in benzene and solid acrylamide. But in solid dimethylitaconate the results from the two different methods have not agreed.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

Positron annihilation lifetime and differential scanning calorimetric study of immiscible NBR/PE blends

The temperature dependence of the mean size of nanoscale free‐volume holes, 〈V_h〉, in polymer blend system consisting of polar and nonpolar polymers has been investigated. The positron lifetime spectra were measured for a series of polymer blends between polyethylene (PE) and nitrile butadiene rubber (NBR) as a function of temperature from 100 to 300 K. The glass transition temperatures (T_g) for blends were determined from the ortho‐positronium (o‐Ps) lifetime τ₃ and the mean size of free‐volume holes 〈V_h〉 versus temperature as a function of wt % of NBR. The T_gs estimated from the PALS data agree very well with those estimated from DSC in view of different time scales involved in the two measurements. Both DSC and PALS results for the blends showed two clear T_gs of a two‐phase system. Furthermore, from the variation of thermal expansivity of the nanoscale free‐volume holes, the thermal expansion coefficients of glass and amorphous phases were estimated. Variations of the o‐Ps formation probability I₃ versus temperature for pure PE and blends with low wt % of NBR were interpreted on the basis of the spur reaction model of Ps formation with reference to the effects of localized electrons and trapping centers produced by positron irradiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 227–238, 2009     

Positron annihilation in amino acids

Positron lifetime measurements were performed on eight different amino acids, τ₁, τ₂ and I₂ values were observed at around 0.35 nsec, 0.7–1.2 nsec and 20–30%, respectively. The long-lived component seems to be produced by positronium states trapped in the crystalline lattice or by positronium atoms bound to molecules.     

Positron annihilation in hypercrosslinked polystyrenes

Positron annihilation in hypercrosslinked polystyrene, called CPS(0.3)150E, has been studied as a function of temperature in the range from 35 to 370 K. The positron lifetime spectra were resolved into five components using the PATFIT and MELT programs. The annihilation rate constant of the longest-lived component was found to increase linearly with increasing temperature. Remarkably, the distribution of o-Ps lifetimes resolved by MELT was shown to broaden with decreasing temperature. Moreover, an effect of the oxygen pressure on the positronium characteristics was also investigated in another hypercrosslinked polystyrene, CPS(0.3)100E. The oxygen pressure varied from vacuum to 6 atm. The annihilation rate constants of the long-lived components were observed to increase linearly as a function of the oxygen pressure, which demonstrates a dependence of the kinetic of o-Ps quenching on the pore size.     

Positron annihilation in ionic conductivity polymers

Positron annihilation lifetime spectra and ionic conductivity have been measured for poly(etherurethane)-LiClO₄ as a function of temperature. The effects of Li salt on glas transition free volume and ionic conductivity have been discussed. A correlation between fractional free volume and ionic conductivity was first experimentally confirmed by using the free volume theory.Supported by the National Natural Science Foundation of China.     

Positron annihilation in solid charge-transfer complexes

Positron lifetime and angular correlation measurements have been carried out on 1:1 charge-transfer complexes, on their pure donor and acceptor components and on the 1:1 M mechanical mixtures of these components. Complex formation reduced the intensity of the long-lifetime component of the donor compounds nearly to the low level of the acceptors. The angular correlation curves obtained for the pure acceptor and the complex were practically identical and were substantially broadened as compared to that of the donor.     

Positron annihilation lifetime studies of free volume in a polymer electrolyte PEO complexed with NH4I

Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer–salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH₄I, with NH ≈ 0.076) in the temperature range of 298–353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, τ₃, was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, τ₃, and its intensity, I₃, both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of τ₃ and I₃ were observed at T ≈ 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 969–976, 1998     

Positron annihilation and phase transitions in n-octadecanol

The positronium formation probability and the mean lifetime in n-octadecanol-1 were observed as a function of temperature. The changes of that probability are discussed in the terms of the spur reaction model, suggesting the residual effect of dielectric constant increase in the rotational phase.     

Positron annihilation in liquids and complex formation

The bubble model, which is conventionally used to fit the observed characteristics of the pickoff component of ortho-Positronium decay in liquids, is subjected in this study to a critical assessment. It is demonstrated that in its present form (namely that of a bubble with a sharp boundary) the model is untenable when confronted jointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model, free from such shortcomings, is presented. Further, the dramatic turnover in the temperature dependence of the rate for the formation of positronium–acceptor complex in the case of nitro-benzene in different solvents is interpreted as a Kramers’ turnover in the context of the improved Bubble Model.     

Positron annihilation in irradiated and unirradiated polyethylenes

Positron lifetimes were measured for four kinds of polyethylene samples and were resolved into four components. The temperature dependence of the two longlived components was examined in detail. In agreement with other results, the longest lived component could be reasonably assigned to ortho-positronium located in amorphous regions. This component was shown to be sensitive to the defects in high-density polyethylene introducted in the course of its production. Both the intensity and the lifetime of the second longest lived component were structure insensitive, i.e., they did not change even on passing through the melting point. This component has been tentatively assigned to a positronium compound state. The effect of gamma-ray irradiation was also examined. Although the intensity (I₄) of the longest lived component was reduced by the irradiation, correlation between I₄ and the free radical concentration was poor, and the reduction in I₄ caused by the irradiation is considered to be due to structure change and not to chemical reasons.     

Positron annihilation studies of chromophore-doped polymers

Doppler-broadening energy spectra and positron annihilation lifetime have been measured as a function of positron implantation energy in pure and chromophore Disperse Red 1 (DR1)-doped poly(methyl methacrylate) (PMMA) polymers. In pure PMMA, the S parameter increases at very short range (<0.02 μm) from the surface to the bulk, while the S parameter of doped PMMA varies with a decrease from a depth of >0.02 μm to about 0.5 μm after an increase, a short distance from the surface. The o-Ps lifetime of the polymers is found to increase from the bulk to the surface, which indicates that the hole size expands near the surface. The o-Ps lifetime distribution becomes broader near the surface. The change of the o-Ps intensity shows the same trend as the change of the S parameter. These results are interpreted as a gradient of DR1 concentration in PMMA, as a function of the depth from the surface to the bulk in the chromophore-doped polymers.     

Positron annihilation spectroscopy and micellar systems

Positron annihilation spectroscopy (PAS) has emerged as a powerful technique for investigating structural changes, phase transitions and microenvironmental transformations in a variety of systems. The process of molecular aggregation in micellar systems is known to be cooperative and size limited; it shows features similar to that of a classical phase transition. Similarly, the changes in the concentration of surfactant and the solubilizate bring about several microstructural and conformational transformations in these systems. High sensitivity of positron annihilation parameters to such changes makes it a potential candidate for investigating micellar and microemulsion systems. This paper deals with this aspect of positron annihilation spectroscopy. Applications of this technique to investigate conformational, structural and microenvironmental transformations in micellar and microemulsion systems are discussed. Its superiority over the conventional techniques in such investigations is demonstrated. It is shown that this technique reveals finer details of otherwise considered to be single phase regions in a phase diagram. Its usefulness in delineating phase boundaries and hence in mapping of phase diagrams is also discussed.     

Positron annihilation in non-simple liquids. Normal hexane

Positron lifetime measurements were carried out in normal hexane vs. temperature in the range of 12.5–62.5°C. The evaluation of the lifetime spectra indicates the presence of four lifetime components, as published first by Jacobsen et al. An attempt is made to interprete the properties of the longest-living two components in terms of the microscopic structure of the liquid.