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) ₃ belowT _g was larger in the heating runs than in the cooling runs, but aboveT _g, ₃ was the same for both runs. The larger ₃ 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 ₃ was the same for the heating and the cooling runs andT _g 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. BothI ₃ ⁰ and ₃ were decreased by the added CuCl₂, being interpreted as the result of inhibition and quenching, respectively. The results show that both the inhibition coefficient 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.     

A positron lifetime and Doppler-broadening study of positron states and free volume in polyethylene

Positron lifetime (LT) and Doppler-broadening (DB) studies of polyethylene have been performed simultaneously in the temperature range between 80 and 300 K. The LT spectra have been analysed assuming four exponential components. Two long-lived components appear, which were attributed too-Ps pick-off annihilation in crystalline regions (₃ = 0.9 to 1.2 ns) and at free-volume holes in the amorphous phase ( to 2.8), The variation in ₄ correponds to an increase of the mean hole size from 0.053 nm³ at 80 K to 0.188 nm³ at 300 K. From the data the glass transition temperature (T _g=195 K), the coefficient of thermal expansion of holes in the glassy and rubbery phase ( _{h, g} = 14.5 · 10^–4 K^–1 and _{h, r} = 189 · 10^–4 K^–1) and the fractional free volume (2.8% to 10.4%) were estimated. The DB curves were fitted by a sum of three Gaussians, the narrowest of which is assumed to represent the self-annihilation ofp-Ps localised at holes. The intensity of the narrow component,I _n, varies between 0 and 7.3% in a similar way as the LT intensityI _4/3 varies. From this it was concluded that other Ps reactions beside pick-off are not important. Further, it was shown that the average positron lifetime is dominated by theo-Ps component,T _{4 g}, while the behaviour of the DB peak height is mainly affected by thep-Ps narrow componentI _n .     

Effect of electric field strength and exposure time on epoxy and high density polyethylene measured by positron annihilation lifetime

Using the positron annihilation lifetime technique, the annihilation parameters have been measured for epoxy and high density polyethylene (HDPE) as a function of AC electric field strength and the exposure time. The lifetime spectra have been resolved into three components, the longest component (I₃₃) is attributed to the pick-off annihilation of o-Ps in the amorphous regions. The intermediate one (I₂₂) is due to the annihilation of free positrons, while the shorter component (I₁₁) stems from self annihilation of p-Ps. In HDPE, the o-Ps parameters ₃ andI ₃ are measured as a function of electric field strengths in the range from 10 to 100 kV/cm exposed for 24 hours. A decrease inI ₃ of 8% is observed from zero to 50 kV/cm followed by an increase of the same order from 50 to 100 kV/cm. By investigating the effect of the exposure time from 2 to 24 hours at 16 and 50 kV/cm, the effect is confirmed and is attributed to the inhibition of o-Ps formation at lower field strength. In epoxy, the effect or exposure time onI ₃ at 166 and 133 kV/cm shows a similar behavior as in HDPE. At 133 kV/cmI ₃ decreases by only 2.5%. On the other hand, the changes in ₃ occur at short exposure times. Again at large times the saturation is obtained. These effects are attributed to the expansion of free volume (increase of ₃) competing at longer exposure times with other phenomena, such as liberation of free radicals, which reduce the o-Ps intensityI ₃ through the conversion to p-Ps. The reactions between o-Ps and free radicals might also lead to free positrons, which could explain the increase ofI ₂ and the decrease of ₃ at longer exposure times.     

Sorption of CO2 in polymers observed by positron annihilation technique

Positron annihilation lifetimes were measured for several polymers in the atmosphere of high pressure CO₂ gas. At low CO₂ pressured both ₃ andI ₃ decreased due to the Langmuir-type sorption, and at higher pressures their values recovered because the Henry-type sorption takes over. The amount of sorbed CO₂ and dilation of the bulk volume were measured simultaneously, and the free volume fraction was determined at each CO₂ pressure. The free volume fraction became smaller (for polyimide and polycarbonate) or slightly larger (for polyethylene) with the progress of sorption. However, the size of the o-Ps hole estimated from the ₃ value increased regardless of the change of the free volume fraction. It appears that o-Ps is selectively looking at larger holes or expanding the holes in which it is accommodated. For polycarbonate, which remains to be glassy even at the largest CO₂ sorption attained in the experiment, the o-Ps hole size became larger than that before sorption. This implies that, even if the polymer is glassy as bulk, the sorption site is strongly prone to molecular displacement by the pressure of the penetrating Ps. Cautious consideration is evoked about directly correlating the o-Ps lifetime and intensity with the free volume in general.     

Transference numbers of aqueous HCl up to 15 mol-kg−1 at 25°C: A systematization and reassessment

Ion-transfer parameters of aqueous hydrochloric acid over a wide molality range have been re-investigated taking the concurrent solventtransfer into due account. In this context, the transference numbers for the hydrogen ion _H and for water _w have been determined from 10^–2 to 15 mol-kg^–1, together with the corresponding infinite-dilution values _H ^o and _w ^o , by processing a set of nearly a hundred emf measurements (some twenty of them new for this work) on HCl concentration cells with transference with both cation-reversible and anion-reversible electrode pairs.     

Zur Bereehnung des Behinderungspotentials der inneren Rotation von Wasserstoffperoxid

Zesummenfassung Auf der Grundlage des Hellmann-Feynman-Theorems wird das Behinderungspotential der inneren Rotation von H₂0₂ berechnet unter Verwendung einer genäherten Elektronendichteverteilung, in welcher zweizentrige Bond-Orbitale die Bindungselektronen undsp ³-Hybride die einsamen Elektronenpaare beschreiben. Eine dreitermige Fourier-Approximation des erhaltenen Potentialverlaufs hat die Gestalt:U() = const. + 5,248 · cos + 2,592 · cos 2 + 0,142 · cos 3 [kcal · Mol^–1] .Für die PotentialschwellenU _cis undU _trans ergeben sich 11,76 bzw. 0,98 kcal · Mol^–1, dem Minimum der Potentialkurve entspricht ein Torsionswinkel von 120,5°.

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Using the Hellmann-Feynman-theorem the potential of internal rotation of H₂0₂ is calculated, the electronic charge distribution being represented by two-center bond orbitals andsp ³ hybrid orbitals (for the lone pairs). Developing the calculated potential in a Fourier series leads to the above-mentioned formula. The potential barries are 11.76 and 0.98 kcal/Mole, the angle of twist of equilibrium is 120.5°.

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Resume On calcule à l'aide du théorème de Hellmann Feynman le potentiel de rotation interne de H₂O₂. La distribution de charge électronique de la liaison OH nécessaire pour ce calcul est représentée par une fonction de liaison à 2 centres tirée d'un calcul d'orbitales de liaison de l'eau. Le développement en série de Fourier du potentiel donne:U() = 3.780 + 5.248 cos + 2.592 cos 2 + 0.142 cos 3 [kcal · Mol^–1] .Pour les barrières de potentiel on obtient les valeursU _cis = 11.76 et Urane = 0.98 kcal/Mole; l'angle d'équilibre est trouvé égal à 120° 5.

     

Vibrational isotope effect of linear and planar molecules: Deuterated bromoethenes

Linear (planar) molecules A and B which are identical except for isotopic substitutions at the atomic sites are considered. Stretching (bending, out-of-plane) frequencies _k and normal modes _k of the isotopically perturbed molecule B are expressed in terms of stretching (bending, out-of-plane) frequencies _i and the corresponding normal modes _i of the unperturbed molecule A. Complete specification of the unperturbed normal modes is not required. All that is needed are stretching (bending, out-of-plane) amplitudes | _i of the normal modes _i at those sites that are affected by isotopic substitution. The rule which interlaces frequencies _k of molecule A with frequencies _i of molecule B is derived. Given two isotopic molecules A and B that differ by a single isotopic substitution at site , the inversion relation is derived. This relation expresses unperturbed stretching (bending, out-of-plane) amplitudes at the site in terms of stretching (bending, out-of-plane) frequencies of molecules A and B . As an example, out-of-plane vibrations of deuterated bromoethene were considered. In the simplest method 12 out-of-plane frequencies of four polydeuterated bromoethenes were calculated from 12 out-of-plane frequencies of bromoethene and three monodeuterated bromoethenes. Standard deviation of thus calculated frequencies from experimental frequencies is =2.74 cm^–1. In another method, 15 out-of-plane frequencies of four polydeuterated bromoethenes and selected monodeuterated bromoethene are calculated from 9 out-of-plane frequencies of bromoethene and the remaining two monodeuterated bromoethenes. Depending on which monodeuterated bromoethene is selected (1-, cis- or trans-), standard deviation of thus obtained frequencies from experimental frequencies is 1=2.84 cm^–1, c=2.96 cm^–1 and t=2.72 cm^–1.     

Temperature Dependence of Ligand Exchange in the Coordination Sphere of Tb(ClO4)3 · nH2O in the Presence of Adamantylideneadamantane-1,2-dioxetane and Benzophenone in Acetonitrile Solutions

The processes of complexation and solvation in the Tb(ClO₄)₃ · nH₂O –adamantylideneadamantane-1,2-dioxetane (I)–benzophenone (II) system in acetonitrile solutions were studied at 280–320 K. The complexation of Tb(ClO₄)₃ · nH₂O in ground and electronically excited states with I and II was found out. An anomalous increase in the lifetime of Tb(ClO₄)^* ₃ · nH₂O with temperature was observed; this anomalous increase is indicative of a structural change in the environment of the terbium ion in solution. It was found that of Tb(III)* increased because of rearrangement of the inner sphere of solvation aqua complexes toward the replacement of H₂O molecules by solvent molecules that exhibit a lower quenching ability.     

Correlation between the rotational mobility and the kinetics of the intramolecular exchange of the unpaired electron and bond in a tin-containing free radical with two pyrocatechol ligands

EPR spectroscopy was used to study the effect of the mobility of the medium on the dynamics of the interligand exchange of the unpaired electron and bond in the free radical formed upon the reaction of stannous bis-3,6-di-tert-butyl-o-semiquinolate with 3,6-di-tert-butyl-2-hydroxyphenoxyl in the presence of triphenylphosphine. A dependence exists between the characteristic exchange time _ex and the correlation time for the rotational mobility of the radical _c; _ex= _c. A temperature dependence was found for coefficient .Translated from Izvestiya Akademii Nauk SSSR. Seriya Khimicheskaya, No. 7, pp. 1663–1666, July, 1990.     

Orientational dynamics of C70 molecules in chlorobenzene

Dynamics of anisotropy relaxation of C₇₀ singlet excited molecules in chlorobenzene was measured at room temperature by the picosecond transient grating technique. The time-ependent diffraction efficiency exhibits a two-stage decay: a fast component (₁ = 12±5 ps), which is comparable with the corresponding signal of C₆₀ in chlorobenzene ( = 8±2 ps), and a slow one (₂ = 30±5 ps). It is supposed that relaxation of anisotropy is related to the orientational mobility of excited C₇₀ molecules relative to two axes of the molecular framework. The results obtained cannot be described by the Einstein-Stokes-Debye theory. The Hynes-Kapral-Weinberg theory, which takes into account microscopic interactions between molecules upon collisions, agrees satisfactorily with the experiment. The influence of dielectric friction on the orientational mobility of C₇₀ in chlorobenzene was estimated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 601–604, March, 1996.     

Accurate lifetime measurements of the 2p 3d 1 D 0 and 2p 3d 3 P 2 0 levels in N II with the beam-foil-laser method

We have obtained accurate values for the radiative lifetimes of the 2p 3d ¹ D ⁰ and 2p 3d ³ P ₂ ^o levels in NII by the cascade-free beam-foil-laser spectroscopy method. Our results are (2p 3d ¹ D ⁰)=0.346±0.012 ns and (2p 3d ³ P ₂ ^o )=0.457±0.020 ns. Comparison of these results with experimental and recent theoretical lifetimes reported previously is also made.Senior Research Associate of the Belgian FNRS     

A simplified model of the ECD

A simplified model of the ECD is presented, which is based on the assumption that only a change in the concentration of electron is generating a signal. The model allows to determine four different time constants related to: the collection of electrons (₁), the loss of electrons in the capture process by the impurity molecules (₂), the loss of sample molecules by electron capture (_n) and the removing rate of molecules from the detector volume by the carrier gas (_v). The values of these time constants have been estimated to be in the range of s for ₁, ms for ₂, a part of a second for _n and a few seconds for _v. The electron capture efficiency coefficient (p) and the detection coefficient (S_d) have been defined. These coefficients serve in the model for the coulometric calculation of the mass of analyzed compounds, if the detector works using the conditions described.     

Investigation of transitions and relaxation processes in polystyrene by using positron annihilation

The glass transition and relaxation processes in polystyrene resins with the number average molecular weight ranging from 7.0·10² to 9.8·10⁴ were studied with the positron annihilation technique. The pick-off annihilation lifetime of ortho-positronium (₃) and its intensity (I ₃) were measured in the temperature range from 20 to 430 K. The glass transition temperature (T _g) was determined as an onset temperature coefficient of ₃.T _g shows the molecular weight dependence in these samples. BelowT _g, local motions were detected by measurements ofI ₃. The local motions could be observed above 100 K in this experiment.I ₃ show the minimum at around 250 K and it does not show molecular weight dependence.     

Correlation between solvent-reorganization dynamics and the kinetics of metallotropy in polar media

The kinetics of metallotropy of the 3,6-di-tert-butyl-2-(triphenylstannyl)oxyphenoxyl radical has been studied by ESR. The correlation between chemical exchange time _ex, solvent polarity, and longitudinal relaxation time of the solvent corresponds to the model of a process controlled by molecular dynamics. A linear relation between _ex and correlation time _c for reorientation movements of the radical is found: _ex = · _c. The coefficient depends on the dielectric properties of the solvent.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 410–413, March, 1994.This work was supported by the Russian Foundation for Basic Research, project No. 93-03-4372.     

Transient photocapacitance study in 4-nitro-4′-aminodiphenyl layers

In 4-nitro-4-aminodiphenyl (NAD) layers the photocapacitance (PhC) transients were investigated. PhC spectral distribution as well as the temperature dependences of time constants (under/_on/or after/_off/illumination) have been measured. The photocapacitance spectral distribution possesses maxima at 350, 500, and 750 nm. It was shown that _on (T) is of exponential form while _off (T) is more complicated. The values of thermal activation energies (0.65±0.05) eV of both time constants coincide. The mechanism of photocapacitance process has been disclosed in NAD layers.     

Intramolecular Mobility in the Complexes 2SbCl3· p-C6H4(CH3)2and SbCl3· C6H5COCH3according to 35Cl NQR Data

Temperature dependences of the ³⁵Cl and ¹²¹Sb spin-lattice relaxation time ₁in antimony trichloride complexes with p-xylene and acetophenone were studied using NQR spectroscopy. The activation energies of the retarded motion (similar to that discovered in some v complexes of SbCl₃) of chlorine atoms were determined from the ³⁵Cl ₁(T) function. New data on the shape of the ¹²¹Sb ₁(T) function are discussed.     

Mechanical properties and molecular mobility in oriented polyolefines

The static modulus of elasticity (E) and the correlation time of rotation ( _c ) of 2,-2,6,6-tetramethylpiperidine-1-oxyl are studied as a function of the temperature (210<T <350°K) for oriented films of isotactic polypropylene and polyethylene of high and low density.E and _c change both upon heating and polymer orientation; this result indicates that sample properties are influenced by the microstructure of the amorphous phase where probes are localized.     

Effect of the Reaction Products on the Rate of Oxidation of Crotonaldehyde

Study of the oxidation of crotonaldehyde revealed an appreciable inhibitory effect of the products on the process. Analysis of the kinetic data obtained over a wide range of reaction conditions (c ₀ 1.5-3.3 M, pO₂ 1-16 atm, T 293-309 K) showed that the overall oxidation process (with account taken of the inhibitory effect of the products) is described by the equation: W _CA = k*_ap c _CA (pO₂)^0.6 (1 + 0.17c _CA)^{- 1}, where k*_ap is the apparent rate constant, and c _CA is the decrease of the aldehyde concentration by a moment .     

Temperature deactivation of excited Tb3+ in the presence of 1,2-dioxetane in acetonitrile

Quenching the fluorescence (FL) of terbium perchlorate by 2,2-adamantane-2,2-dioxide (1) was shown to have a chemical character and was caused by the formation of the [1...Tb³⁺] complex. The dependence of the lifetime () of FL of Tb^*3+ in acetonitrile on the temperature and concentration of1 has been studied. The temperature dependence of is caused by a rearrangement of the inner sphere of the aquasolvate complexes of Tb³⁺, which leads to the replacement of H₂O with MeCN and1. The energy of replacing the H₂O molecule in the inner sphere of complexes with a solvent molecule has been calculated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1762–1766, October, 1994.     

Solvent effect on intensities off-ftransitions in lanthanide(III) complexes

A general theory of the solvent effect on the intensities of f-f; transitions of lanthanide complexes based on static and dynamic coupling between metal ion and ligands and solvent molecules is presented. New expressions are found correlating the intensity parameters with physical characteristics of the solvent. It is shown that the solvent effect influences only in the parameter ₂.