The influence of oxygen on radiation crosslinking and degradation of polymers at low irradiation doses

The influence of oxygen on radiation cross-linking and scission at low doses has been followed in order to explain anomalous drop of molecular weight found in polyethylene at low irradiation doses. The G - values and yields of crosslinking and scission estimated on the basis of molecular weight measurements (by GPC) are much higher than those obtained through gel content determinations at higher doses. The possible explanations of such a peculiar behavior are suggested and discussed. The influence of oxygen, even when present in the form of air absorbed in the sample, was found to be an important factor of scission process in irradiated polymer.     

The effects of temperature on ESR spectrum of gamma-irradiated ammonium tartrate

Unirradiated ammonium tartrate (AT) samples do not exhibit any ESR signal. However, irradiation produces an unresolved singlet at g=2.0034±0.0006 with two shoulders at g₁=2.0093±0.0006 and g₂=2.0048±0.0006. The dose–response curve was found to increase linearly with the applied radiation doses in the range of 0.1–2.0 kGy and the slope of this curve was increased as the modulation amplitude increased. The activation energy value E_a=69.0±1.2 kJ/mol was calculated from Arrhenius plot for the radical species responsible from ESR spectrum of irradiated AT.     

The influence of the temperature on the formation of samarium nitrato complexes

The stability constants of the Sm(NO₃)²⁺ complex were determined at three temperatures, using the solvent extraction method. It was found that:K ₁ ⁰ =63.6 at 17°C, 30.3 at 35°C, 20.1 at 50°C. This corresponds with the formation of a Sm(H₂O)(NO₃)²⁺ complex at 17°C and a Sm(H₂O)₂(NO₃)²⁺ complex at 50°C.

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Der Einfluß der Temperatur auf die Bildung von Samarium Nitrato Komplexen

Zusammenfassung Die Stabilitätskonstanten von Sm(NO₃)²⁺ Komplexen wurden mittels der Lösungsmittelextraktionsmethode bei drei Temperaturen bestimmt. Dabei ergab sichK ₁ ⁰ =63.6 bei 17°C, 30.3 bei 35°C und 20.1 bei 50°C. Das entspricht der Bildung eines Sm(H₂O)(NO₃)²⁺ Komplexes bei 17°C und eines Sm(H₂O)₂(NO₃)²⁺ Komplexes bei 50°C.

     

Fixed crosslink formation and viscoelasticity of polystyrene networks

The crosslinking formation in the 8/92 mol % random copolymer of ethynyl- styrene and styrene is clarified quantitatively from the IR spectrum analysis on the basis of the thermally homogeneous reaction of ethynylstyrenes between the chains. The viscoelastic properties of the crosslinked polystyrene, CL-P(ESt/St), are investigated based on the temperature and frequency dependencies of the dynamic Young modulus. The relaxation spectrum of CL-P(ESt/St) drops deeply like the Wedge type with a slope of −½ in the glass transition region, and became a plateau without a terminal zone in the rubbery elasticity region. The effects of the ethynyl crosslinked junctions for the micro-Brownian motions of the segments in the strand are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3319–3327, 1999     

Electric field effect on positronium formation in gamma-irradiated polypropylene and polyethylene

The effect of an external electric field (up to 48 kV cm⁻¹) on positronium (Ps) formation in γ-irradiated polypropylene and polyethylene has been studied by means of positron lifetime spectroscopy. The application of an electric field to unirradiated polypropylene and polyethylene considerably decreased the Ps formation. For example, the intensity of the longest-lived component (I₃) for one of the polypropylene samples was reduced from 26% to 18% by a field of 48 kV cm⁻¹. It was found that the electric field effect for the irradiated samples becomes weaker when increasing the irradiation dose. This result is discussed on the basis of the spur reaction model of Ps formation.     

The influence of crystal formation on segmental mobility in polymers: hints from a statistical mechanical relaxation model

A statistical mechanical model is used to analyze literature data regarding the restricted segmental dynamics of a number of crystallized polymers, as observed by means of broadband dielectric spectroscopy. A relationship between well defined physical quantities and the width parameter in the Havriliak–Negami representation of symmetric processes is established. It is found that, for materials crystallized from an isotropic amorphous state, the segmental relaxation process is associated to conformational changes within cooperatively rearranging regions of ~1 nm diameter. In case of chain orientation, the dimension of the rearranging regions along the chain direction increases up to 3–5 nm. It is argued that the average size of the rearranging regions may influence the thickness of the amorphous interlamellar layers in the stacks. It is also found in all cases that, at the end of the crystallization process, the average fluctuation component of the chemical potential within the confined amorphous regions, $\overline{\Delta\mu}A statistical mechanical model is used to analyze literature data regarding the restricted segmental dynamics of a number of crystallized polymers, as observed by means of broadband dielectric spectroscopy. A relationship between well defined physical quantities and the width parameter in the Havriliak–Negami representation of symmetric processes is established. It is found that, for materials crystallized from an isotropic amorphous state, the segmental relaxation process is associated to conformational changes within cooperatively rearranging regions of ~1 nm diameter. In case of chain orientation, the dimension of the rearranging regions along the chain direction increases up to 3–5 nm. It is argued that the average size of the rearranging regions may influence the thickness of the amorphous interlamellar layers in the stacks. It is also found in all cases that, at the end of the crystallization process, the average fluctuation component of the chemical potential within the confined amorphous regions, [`(Dm)]\overline{\Delta\mu}, is of the same order of the chemical potential drop Δμ <sub>cryst</sub> associated to crystallization from the undercooled, relaxed melt. Except in one among the cases considered, it is found that [`(Dm)] ? - Dm_cryst\overline{\Delta\mu}\approx - \Delta\mu_{\rm cryst}, which is a hint towards the formalization of a thermodynamic criterion for crystallization arrest.     

Inert sampling and sample preparation - the influence of oxygen on heavy metal mobility in river sediments

Two approaches have been used to investigate changes in the nature of metal binding in river sediments caused by atmospheric oxygen. Firstly, non-inert and inert sample preparation were applied, in combination with sequential extraction, to determine for which metals inert sample preparation is necessary for correct determination of metal mobility under environmental conditions. Secondly, the metal contents of sediments sampled before and after a river weir were fractionated by sequential extraction to study the effect of the oxygen impact at the weir on heavy metal mobility in the sediments. Different grain-size fractions from one sample were also extracted, to enable selection of the upper grain-size limit most suitable for answering this analytical question. The results showed the need for the inert sample preparation technique for Cd, Zn, Pb, Mn, and Fe, but not for Co, Ni, Cu, and Cr. No significant change of heavy metal mobility at the weir could be proved, although the mobilization behavior of some elements was different. The optimum upper grain-size limit was 63 microm.     

Conductive properties of sintered zinc oxide surface. The influence of electron irradiation and oxygen adsorption

The influence of irradiation with 5 MeV electrons on charge transport phenomena in the surface region of sintered ZnO was studied in the temperature range 200–600 K. Mobility and concentration of electrons were measured in the presence and absence of oxygen as a function of temperature both for irradiated and unirradiated samples. The influence of electron beam processing was found most pronounced for ZnO samples with absorbed oxygen. Radiation induced desorption of O₂ was assumed to be responsible for enhanced effect of irradiation. Increased sensitivity of irradiated ZnO as an oxygen sensor has been observed and attributed to the formation of additional adsorption centres. A correlation between mobility of conduction electrons and their concentration in the surface region of irradiated ZnO with subsequently adsorbed oxygen was observed.     

Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.     

The effect of stoichiometry on chain segment and ion mobility in partially polymerized epoxy systems

The temperature dependence of steady-shear viscosity and ionic conductivity were measured for a series of unreacted mixtures and partially cured, ungelled samples of diglycidyl ether of bisphenol-A (DGEBA) and an amine cross-linking agent, diamino diphenyl sulfone (DDS). Six stoichiometric ratios of epoxide groups to amine hydrogens were examined. Free volume expressions were used to model the temperature dependence of the conductivity and viscosity for the unreacted DGEBA-DDS mixtures. In addition, these expressions were combined to successfully correlate changes in viscosity and conductivity during the DGEBA-DDS polymerization prior to gelation. It also was demonstrated that the change in weight average molecular weight during polymerization could be interpreted from the dielectric data. Through studying variations in the stoichiometry, it was possible to examine the effects of changes in chemical structure and ion concentration on the fitted parameters in the free volume models. The inherent ion transport factor (ζ₀) was found to be inversely proportional to the concentration of ions in the test samples. The fractional free volume for segmental motion (B) was found to increase with an increase in the glass transition temperature and to be a function of the rigidity of the polymer. ©1995 John Wiley & Sons, Inc.     

The influence of surface phenomena on molecular mobility in glassy polymers

Literature data on molecular mobility in glassy polymers have been analyzed. It has been shown that, in the temperature range corresponding to the glassy state of a polymer, a large-scale (segmental) molecular motion is possible, with this motion being responsible for the physical (thermal) aging of the polymer. Heating of an aged polymer restores its initial state, and the aging process begins again (effect of “rejuvenation”). At the same time, aging processes may be initiated by a mechanical action on a glassy polymer. It is sufficient to subject an aged polymer to a mechanical action to transfer it to a state characteristic of a polymer heated above the glass-transition temperature. It should be noted that deformation of a glassy polymer is nonuniform over its volume and occurs in local zones (shear bands and/or crazes). It is of importance that these zones contain an oriented fibrillized polymer with fibril diameters of a few to tens of nanometers, thereby giving rise to the formation of a developed interfacial surface in the polymer. The analysis of the published data leads to a conclusion that the aging of a mechanically “rejuvenated” polymer is, as a matter of fact, the coalescence of nanosized structural elements (fibrils), which fill the shear bands and/or crazes and have a glasstransition temperature decreased by tens of degrees.     

The sulfur reversion process in natural rubber in terms of crosslink density and crosslink density distribution

Unfilled natural rubber compounds composed of conventional (CV), semi-efficient (SEV), efficient (EV) and sulfur donor (SD) vulcanization systems were heat aged to promote sulfur reversion. Rheometry, hardness, strain-strain characteristics including Mooney-Rivlin analysis, equilibrium solvent swell and Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) were used to monitor crosslink density changes. A loss of crosslink density was observed by rheometry, C₁, equilibrium swelling and by DQ NMR as a function of cure extent. No chain scission reactions were operating in the time/temperature conditions used. All crosslink distributions were unimodal and the network homogeneity followed the order of EV > SD > SEV > CV. The crosslink distribution narrowed during the curing process for the CV and SEV systems. Non-oxidative maturation reactions were advantageous in promoting a more random distribution of crosslinks in the polymer matrix.     

Ultra-weak photon emission from human hand: influence of temperature and oxygen concentration on emission

We have studied ultra-weak photon emission (UPE) from living organisms. We report here some features of the UPE from human hand by means of photon counting techniques. The intensity of the UPE depended on the position of human hand; nail>finger>palm. As the temperature declined, the intensity of the UPE from the palm decreased. Further, as oxygen concentration around the palm was lowered, the intensity of the UPE from the palm decreased. These results show the UPE from the palm partly contains emissions based on oxidation reaction on skin surface as a potential. When we used mineral oil between the photomultiplier tube and the palm, the intensity of the UPE increased twice as much, which indicates the UPE from the inside of the skin certainly exists. The fact may be explained by refractive index matching. As mentioned above, we considered the generation mechanism of photons emitted from the human hand.     

The role of oxygen in structure formation on metal surfaces

Russian Chemical Bulletin -     

Advancements in accuracy of the alanine dosimetry system. Part 2. The influence of the irradiation temperature

Systematic measurements of the temperature coefficient for alanine electron paramagnetic resonance (EPR) response have been performed for irradiation in the temperature range (10–50)°C and in the absorbed dose range (1–100) kGy at the dose rate 9.5 kGy/h. During the ⁶⁰Co-ray irradiation, -l-alanine dosimeters were kept in a sealed aluminum holder that provided an effective heat exchange with the temperature-controlled environment. The time between the irradiation and signal measurements was standardized, and a reference sample fixed in the resonant cavity was used to correct the signals for small variations in the spectrometer sensitivity. The temperature coefficient for each dose was determined from approximately 30 experimental points processed by the weighted least-squares technique after the necessary statistical tests were done. The temperature coefficients thus determined were considerably lower than previously reported. The dose dependence of the temperature coefficient features a minimum at (20–30) kGy (about 0.135%/K) with higher values at 1 kGy (0.17%/K) and at 100 kGy ((0.175–0.19) %/K). With the exception of very high doses, no significant distinction was found between the temperature coefficients of Bruker and NIST dosimeters, which differ in shape and binder content.     

Influence of hydrothermal synthesis temperature on the redox and oxygen mobility properties of manganese oxides in the catalytic oxidation of toluene

A series of MnO_x samples synthesized by hydrothermal methods at different temperatures were investigated as catalysts for the oxidation of toluene. The optimum oxidation performance was achieved with the catalyst prepared at 120 °C (Mn-120), for which complete conversion of toluene was attained at 250 °C. The Mn-120 sample possessed the highest concentration of Mn³⁺ and the highest initial H₂ consumption rate, which are indicative of abundant crystal defects and superior reducibility. In addition, Mn-120 exhibited excellent oxidation ability due to the abundance of lattice oxygen species and excellent oxygen mobility. Therefore, the superior catalytic performance of Mn-120 could be attributed mainly to its redox performance and abundant crystal defects, both of which are determined by the temperature of the hydrothermal synthesis of MnO_x.

     

Induction of pigment formation in phenolic compounds by gamma-irradiated sodium chloride

A spectrophotometric study is reported of the pigment formation from free radicals produced from o-dianisidine dihydrochloride and pyrogallol by the action of -irradiated sodium chloride. The species liberated during dissolution of the -irradiated salt also greatly enhance the rate of catalytic formation of the pigment due to peroxidase enzyme in the presence of hydrogen peroxide. The G values for these systems are compared.     

The temperature dependence of the electron mobility in molecular crystals

We show that a hopping model may explain the experimental temperature dependence of the electron mobility in molecular crystals. The flat, higher temperature portion is obtained if anharmonic effects are included in a simple manner. The low temperature part is obtained if coherent effects are added to the diffusional ones.