Victor P. Shantarovich

Inhibition of positronium formation by polar groups in polymers: relation with TSL experiments

Abstract Positronium (Ps) formation in polymers is affected by polar groups, contained in polymer chains and having dipole distribution of electric charge. Oxygen, double-bonded to carbon (–CO), is one of the examples of the group with dipole moment. Inhibition of Ps formation can be partly explained in this case by the trapping of positrons (e+) into negatively charged part of the dipole –C+O−. A proof of this explanation is based on the results of coincidence Doppler broadening spectroscopy (CDBS) and obtained in this paper for the copolymer of ethylene (E) and methylmethacrylate (MMA). The same trapping was demonstrated also for polymethylmethacrylate (PMMA) and for one of polyimides (Kapton). However, part of the inhibition can be due to trapping of the spur electrons. A role of this process can be found from a comparison of the effects of Ps inhibition and suppression of thermostimulated luminescence (TSL). In some of the described cases, inhibition is very effective but limited: it starts at very low concentrations of inhibitors, but saturates at a certain level. This fact is interpreted as a result of incomplete slowing down of positrons inside of the spur in the moment of Ps formation.

Mechanism of enhanced positronium formation in low-temperature polymers

An enhanced positronium (Ps) formation in low-temperature polymers has been widely observed. The additional positronium formation is due to shallow trapped electrons in them. Positron annihilation lifetime spectroscopy was applied to investigate the Ps formation in a series of polymers, such as low-density polyethylene, ethylene-methyl methacrylate copolymers with various methyl methacrylate contents, and pure poly(methyl methacrylate) at low temperature. An analysis of the experimental data based on simple kinetic equations enables one to understand the Ps formation mechanism in polymers during low-temperature positron annihilation experiments as functions of the temperature and elapsed time. Good fittings of the experimental data were obtained. The fitting parameters seemed to show clear physical meanings.

Positron annihilation in polypropylene studied by lifetime and coincidence Doppler-broadening spectroscopy

Abstract The momentum density distributions (MDDs) of electrons taking part in the annihilation processes in polypropylene (PP) have been measured by coincidence Doppler-broadening spectroscopy. MDDs at the beginning of measurements to those at the saturation level of Ps formation have been compared in order to follow the possible changes in concentration of carbonyl groups (CG). A high initial CG concentration in PP has been observed, while for antioxidant-containing PP no significant presence of CG has been detected, and no changes have been observed during positron irradiation.

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.

PIM-1/MIL-101 hybrid composite membrane material: Transport properties and free volume

A hybrid composite membrane material based on the PIM-1 polymer of intrinsic microporosity and MIL-101 nanoparticles of mesoporous chromium terephthalate has been prepared and tested. Fourier-transform IR spectroscopy has revealed the presence of interaction between the polymer matrix and the nanoparticles introduced into it. The addition of MIL-101 nanoparticles leads to an increase in the permeability and diffusion coefficients for gases (He, O2, N2, CO2) compared to the original polymer. Using positron annihilation lifetime spectroscopy (PALS), it has been shown that these changes result from an increase in the free volume in the polymer composite material.

The influence of radiation on Ps formation in PE studied by coincidence Doppler-broadening spectroscopy

Abstract The high-resolution Doppler-broadening spectroscopy of positron-annihilation gamma rays emitted during an experiment on positron annihilation lifetime spectroscopy (PALS) has been applied to study the radiation and oxygen effects on positronium (Ps) formation in polyethylene (PE). Using two Ge detectors, the DB spectra were obtained by measuring the coincidences of two photons. While the Ps formation rate decreased with elapsed time during the PALS experiment, the increase of annihilation on oxygen-containing groups was monitored by coincidence Doppler-broadening spectroscopy (CDBS). This suggested that the decrease in Ps formation was associated with the carbonyl groups (–CO) formed by radiation emitted from the positron sources.

Positronium formation in low-density polyethylene (LDPE)

Positron annihilation lifetime spectroscopy has been applied to low-density polyethylene (LDPE) as a function of the temperature and also the elapsed time in the dark as well as in the light. Positronium formation below, around and above the glass transition temperature of LDPE was investigated. It was found that far below the glass transition temperature, Tg, the o-Ps intensity increased with the elapsed time in the dark. It was interesting to observe an initial rapid increase in the o-Ps intensity, followed by a continuous decrease with the elapsed time, especially at around Tg. Above Tg, the o-Ps intensity decreased very slowly with the elapsed time at 250 K and almost did not decrease at room temperature. Furthermore, no increase, but only a decrease, in the o-Ps intensity with the elapsed time was seen in light. Possible explanations of these effects were discussed. A simple calculation of the activation energy of the trapped electrons was suggested.

Effect of supercritical carbon dioxide on nanoporous polyhexafluoropropylene

The effect of supercritical CO2 (sc-CO2) on the size distribution of free volume holes (nanopores) in a polyhexafluoropropylene (PHFP) polymer matrix has been studied. Residual (after CO2 release) swelling improves gas transport properties of the material. Relaxation of these properties over time has been compared with changes in permeability and nanoporosity. For PHFP samples with different histories, the data on nanoporosity have been obtained using positron annihilation lifetime spectroscopy (micropores) and the lowtemperature gas sorption technique (mesopores and part of micropores). Matching of the data is intended to reveal the role of pores of different sizes in permeability of membrane materials to different gases and determine the specifics of application of the positron annihilation technique to studying sc-CO2-modified objects.

Positron Annihilation in Polymers with Highly Developed Specific Surface

PAL spectroscopy and a technique for low-temperature gas sorption together with Brunauer-Emett-Teller (BET) theory (1) were used for the studies of elementary free volumes (EFV) in super-cross-linked polystyrene networks (2,3). The ranges of effective sizes observed by PAL and BET overlap partly confirming and complementing each other very effectively. I. Samples Polystyrene (PS) networks were prepared by the cross-linking of linear PS swollen in a solvent. Cross-agents of different length p-xylylene dichloride (X) and monochloro-dimethylether (E) were used After removing a solvent, the samples had extremely high specific surface up to 1600 m 2 /g

Positronium annihilation data and actual free-volume distribution in polymers

Determination of the size distribution of free-volume holes in solids, in particular, polymers, is an important physicochemical problem. The positron annihilation technique has been proposed for this purpose. The central point in this technique is the quantitative interpretation of data, especially, for substances with a high specific surface area. A developed free-volume system in open-pore membrane materials, such as poly(trimethylsilylpropyne) PTMSP and the spirocyclically bound benzodioxane polymer PIM-1, and polymeric sorbents (hypercrosslinked polystyrenes) makes it possible for the first time to compare the sorption characteristics and positron annihilation data on the character of size distribution of nanopores in these polymers. In combination with the results of mathematical simulation of the structure and radiothermoluminescence measurements, the array of data indicate the structural inhomogeneity of the test amorphous materials. It was shown that this inhomogeneity in relation to the positron annihilation technique is expressed in the insufficiency of the representation of the orthopositronium decay curve by one component that takes into account the Gaussian lifetime distribution (symmetrical pore size distribution) and in the necessity of use of several decay components. The feasibility of revealing a nonrandom character of pore size distribution gives the positron annihilation technique an advantage over other approaches (inverse gas chromatography, 129Xe NMR) to investigation of nanopores in polymers.