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     

High pressure isomerization of 1,4-bis(methylthio)hexafluoro-2-butene to-1-butene

The complete bond migration of the internal olefinic bond of trans-1,4-bis(methythio)hexafluoro-2-butene(I) under high pressure to the terminal olefinic bond to form cis- and trans-1,4-bis(methylthio)hexafluoro-1-butene(II) is described. The Teflon-lined high pressure cell maintains a constant pressure at 16,000 atm and 180 to 200°C for 24 hours. The cis- and trans-II are elucidated from gas chromatography-mass spectroscopy, which shows two identical parent ions at m/e value of 255(C₆H₆F₆S₂⁺), but at different elution times. The ¹⁹FNMR data of the isomeric products are summarized. Elemental analysis of II is verified by double focussing high resolution mass spectrometer.     

Evidence for Ps formation in spurs in high-density gaseous CO2

Measurements of Ps yields in mixtures of CO₂ and Ar with the electron scavengers CCl₄ and CCl₂F₂ are reported. It is shown that the CO₂ mixtures provide direct evidence for Ps formation in spurs in this gas. Further, some support is also given to recent models which consider Ps formation in gases by both Ore and spur processes. The results obtained for the Ar mixtures are consistent with the Ore model of Ps formation.     

Temperature dependence of the inhibition of positronium by chlorine-substituted hydrocarbons in non-polar liquids

Positron annihilation lifetime spectra were measured for solutions of 1,2,3,5-C₆H₂Cl₄ in hexane, toluene, m-xylene and mesitylene, CCl₄ in hexane and toluene, and C₂HCl₃ in n-hexane for concentrations below 1 M and at various temperatures between ?30°C and 67°C. The Ps inhibition by C₆H₂Cl₄ was roughly 14 times stronger at ?30°C than at 67°C in toluene, m-xylene, and mesitylene, while that of CCl₄ displayed hardly any temperature dependence in hexane and toluene. Previously, the Ps inhibition by C₆H₂Cl₄ in various liquids at 20°C was explained by a competition between electron pick off by the positron from C₆H₂Cl₄^? formed by electron scavenging in the positron spur, and dehalogenation of the same anion at picosecond times after the spur formation. Three effects might reasonably explain the strong temperature dependence of the Ps inhibition of C₆H₂Cl₄: (1) The dechlorination time of C₆H₂Cl₄^? might be much shorter at low than at high temperatures. (2) The positron mobility might decrease very strongly with decreasing temperature. (3) Positrons possibly trapped on C₆H₂Cl₄ might thermally detach at high but not at low temperatures.     

A Study of picosecond dehalogenation of chlorobenzene anions in liquids by positronium inhibition measurements

Radiation chemistry and results of Ps yields indicate that the following processes occur in the positron spur in solution of halogen-substituted hydrocarbons, RX_n: e⁺ + e^? → Ps, e^? + RX _n → (RX_n)^? → RX_n?1 + X^?, e⁺ + (RX_n)^? → Ps + RX_n, e⁺ + X^? → [X^?, e⁺]. Hence the trapped electron can form Ps only if (RX _n)^? is stable or has a lifetime that is longer than o comparable to the Ps formation time. Previous studies have shown that some of the strongly chlorinated benzenes (n = 4.5 give reasonable inhibition in benzene but not in linear hydrocarbons. The reason is very probably that the dechlorination time is much shorter in benzene than in saturated hydrocarbons because Cl^? is more strongly solvated in benzene than in non-aromatic hydrocarbons. To test those ideas further we have begun detailed studies of solutions of the possible “intermediate” inhibitors, viz. 1,2,3,5- and 1,2,4,5-C₆H₂Cl₄, in mixtures of C₆H₆/C₆H₁₄ different methyl-substituted benzene aniline, anisole, dioxane and ethylbenzene. The results are discussed and interpreted in terms of the spur model. The Ps inhibition efficiency of the two isomeric forms of tetrachlorobenzene studied, appears most probably to depend on intramolecular electron transfer with subsequent dehalogenation of the molecular anion on a picosecond timescale. The divergence in inhibitor efficiency obtained for the chlorobenzenes when dissolved in aromatic solvents compared to the same solutes when dissolved in a saturated alkane appears most probably to be caused by complex formation between the initially formed chlorobenzene anion and benzene molecules, which permits a rapid relaxation of the molecular anion with subsequent bond stretching and expulsion of the chloride anion.     

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition. IV.- etude cinetique de l'interconversion catalysee cis-trans du butene-2.

The cis-trans interconversion of olefins in the system W(CO)₆ + CCl₄ + 2-butene is studied, both with initial irradiation of a solution of W(CO)₆ in CCl₄ (photoinduction), and with continuous irradiation of the system, for cis- and trans-butene concentrations between 0.09 and 6.0 M. Analysis of the results of the photoinduction experiments (rate of conversion and kinetic law as a function of the initial concentration of the olefin) allowed us to propose a simple kinetic scheme comprising a cis-trans interconversion of 2-butene and olefin-catalyzed destruction of the catalytic entity (k₂ = (0.62 ± 0.06)x10^?4 M s^?1). In the continuous irradiation experiments the final distribution of the olefin was independent of the initial butene concentration (cis-2-butene/trans-2-butene 3.0) and the reaction kinetics are of first-order (k_obs = (3±1) x10^?4 s^?1. Comparison of the two experiments suggests continuous photochemical regeneration of the catalytically active entity. The first-order reaction kinetics are in agreement with a carbene-metal carbonyl structure of the intermediate     

卡宾与2-丁烯环加成反应机理的MNDO法研究

用MNDO法研究单线态和三线态卡宾与乙烯,顺式及反式-2-丁烯的反应途径。合理地解释了顺式和反式-2-丁烯与三线态卡宾都形成顺式和反式-2-二甲基环丙烷。用MNDOCI计算了有关反应的反应活化能。     

Temperature and composition dependence of viscosity. II. Temperature dependence of viscosity of propylene carbonate-dimethoxyethane mixtures

The application of the currently used equations for the reproduction of the temperature dependence of viscosity η of binary solvent mixtures of propylene carbonate (PC) and dimethoxyethane (DME) favors the choice of the Vogel-Fulcher-Tammann (VFT) equation using the ideal glass transition point as the reference temperature T_o for the estimation of free and blocked (inaccessible) volumes. Reduced plots show that the free volumes as obtained from the VFT equation mainly determine the viscosities of the liquid mixtures; the blocked volumes V_x(T_o)≡V_o (x: mole fraction of PC) show ideal behavior. The effect of negative excess volumes V ^E on viscosity, studied in the preceding paper, is examined in comparison with the effect of a temperature decrease. The equal signs of η ^E and V ^E can be explained by referring the viscosities to the reference temperature T_o.     

Solvated positron chemistry. II. The reaction of hydrated positrons with Cl−, Br− and I− ions

The reaction of the hydrated positron, e_aq⁺ with Cl^?, Br^?, and I^? ions in aqueous solutions was studied by means of positron The measured angular correlation curves for [Cl^?, e⁺], [Br^?, e⁺, and [I^?, e⁺] bound states were in good agreement with th Because of this agreement and the fact that the calculated positron wavefunctions penetrate far outside the X^? ions in the [X^?, e⁺] sta propose that a bubble is formed around the [X^?, e⁺] state, similar to the Ps bubble found in nearly all liquids. F^?ions did not react w Preliminary results showed that CN^? ions react with e_aq⁺ while OH^?ions are non reactive. The rate constants were 3.9 × 10¹⁰ M^?1 s^?1, 4.4 × 10¹⁰ M^?1 s^?1, and 6.3 × 10¹⁰ M^?1 s^?1 for Cl^?, Br^?, and I^?, respectively, at low (? 0.03 M) X^? concentrations. A 25% decrease in the rate constant caused by the addition of 1 M ethanol to the I^? solutions was i The influence of halide ions on the positronium (Ps) yields in pure water was studied by use of lifetime measurements. The Cl^?, Br^?, and I^? ions reduced the Ps yields at low concentrations (? 0.03 M), while F^? ions only reduced the Ps-yield However, the Ps yields saturated (e.g. at ≈ 21% ortho-Ps yield in the Cl^? case) at higher concentrations. This saturation and the high-concentration effects-in the angular correlation results were interpreted as caused by rather complicated spur effects, wh It is proposed that spur electrons may pick off the positron from the [X^?, e⁺ states with an efficiency which depends on the structure of the     

Alkenoxy radicals in gas-phase reactions of alkenes with oxygen atoms or ozone

Observations in the O₃ + trans-2-butene reaction system and in the O + trans-2-butene + O₂ reaction system suggest the intermediacy of alkenoxy radicals. A mechanism is proposed for the production of C_n and C_m (m <n) alkenoxy radicals by the reaction of C_nH_2n alkenes with oxygen atoms or with ozone.     

Inhibition of Ps formation in benzene and cyclohexane by CH3Cl and CH3Br. Correlation to radiation chemistry results

Positron-annihilation lifetime spectra have been measured for mixtures of CH₃Cl and CH₃Br in cyclohexane and of CH₃Cl in benzene. The ortho-positronium (Ps) yield decreased monotonically from 38% and 43% in cyclohexane and benzene respectively to 11% in pure CH₃Cl and 6% in pure CH₃Br. The strength of the inhibition of Ps formation by CH₃Br was ten times that of CH₃Cl in cyclohexane, because the CH₃Br^? anion debrominates rapidly, while CH₃Cl^? is long-lived (= 30 ns) compared to the maximum time of Ps formation of 400–500 ps. as shown in radiation chemistry. The positron can pick off the electron from the CH₃X^? anions to form Ps. while it forms a bound state with the halides. X^?. CH₃Cl was a roughly three times weaker Ps inhibitor in benzene than in cyclohexane, which shows that CH₃Cl^? does not dechlorinate in times comparable to or shorter than 400–500 ps in benzene. An improved model for the explanation of Ps formation in mixtures, where the Ps yield versus electron scavenger concentration has a minimum, is proposed and discussed.     

Vinyl hydrogen acidities of two stereoisomers

The gas-phase acidities of the vinyl hydrogens of cis- and trans-2-butene were measured by the silane kinetic method in a Fourier-transform ion cyclotron resonance spectrometer. The acidities of ethene and the secondary vinyl hydrogen of propene were measured by the same method. The method was calibrated using the known acidities of methane and benzene. The vinyl hydrogens of trans-2-butene are more acidic than the vinyl hydrogens of cis-2-butene by 4.5 kcal/mol; the acidities of ethene and the secondary vinyl hydrogen of propene are between those of the two butenes. The acidity of cis-2-butene is 409 +/- 2 kcal/mol, and the acidity of trans-2-butene is 405 +/- 2 kcal/mol. Density functional theory calculations are in good agreement with the experiments. The results are discussed in terms of steric interactions, polarizabilities, dipole-dipole interactions, and charge-dipole interactions.     

Kinetics, products, and stereochemistry of the reaction of chlorine atoms with cis- and trans-2-butene in 10-700 Torr of N2 or N2/O2 diluent at 297 K

The reactions of Cl atoms with cis- and trans-2-butene have been studied using FTIR and GC analyses. The rate constant of the reaction was measured using the relative rate technique. Rate constants for the cis and trans isomers are indistinguishable over the pressure range 10-900 Torr of N2 or air and agree well with previous measurements at 760 Torr. Product yields for the reaction of cis-2-butene with Cl in N2 at 700 Torr are meso-2,3-dichlorobutane (47%), DL-2,3-dichlorobutane (18%), 3-chloro-1-butene (13%), cis-1-chloro-2-butene (13%), trans-1-chloro-2-butene (2%), and trans-2-butene (8%). The yields of these products depend on the total pressure. For trans-2-butene, the product yields are as follows: meso-2,3-dichlorobutane (48%), dl-2,3-dichlorobutane (17%), 3-chloro-1-butene (12%), cis-1-chloro-2-butene (2%), trans-1-chloro-2-butene (16%), and cis-2-butene (2%). The products are formed via addition, addition-elimination from a chemically activated adduct, and abstraction reactions. These reactions form (1) the stabilized 3-chloro-2-butyl radical, (2) the chemically activated 3-chloro-2-butyl radical, and (3) the methylallyl radical. These radicals subsequently react with Cl2 to form the products via a proposed chemical mechanism, which is discussed herein. This is the first detailed study of stereochemical effects on the products of a gas-phase Cl+olefin reaction. FTIR spectra (0.25 cm(-1) resolution) of meso- and DL-2,3-dichlorobutane are presented. The relative rate technique was used (at 900 Torr and 297 K) to measure: k(Cl + 3-chloro-1-butene) = (2.1 +/- 0.4) x 10(-10), k(Cl + 1-chloro-2-butene) = (2.2 +/- 0.4) x 10(-10), and k(Cl + 2,3-dichlorobutane) = (1.1 +/- 0.2) x 10(-11) cm3 molecule(-1) s(-1).     

NaOH改性WO3/SiO2催化剂催化2-丁烯和乙烯复分解制丙烯SurasaMaksasithorn,DamienP.Debecker,PiyasanPraserthdam,JoongjaiPanpranot,KongkiatSuriye,SirachayaKunjaraNaAyudhya简

A WO₃/SiO₂ catalyst is used in industry to produce propylene from 2-butene and ethylene metathesis. Catalysts with various WO₃ loading (4% to 10%) were prepared by impregnation and tested for the metathesis of ethene and trans-2-butene. Ion exchange of NaOH onto the WO₃/SiO₂ catalyst was used to mitigate the acidity of the catalysts in a controlled way. At low WO₃ loading, the treatment with large amounts of NaOH resulted in a significant decrease in metathesis activity concomitant with significant W leaching and marked structural changes (XRD, Raman). At higher WO₃ loading (6% to 10%), the treatment with NaOH mainly resulted in a decrease in acidity. FT-IR experiments after adsorption of pyridine showed that the Lewis acidic sites were poisoned by sodium. Nevertheless, the metathesis activity remained constant after the NaOH treatment. This suggested that the remaining acidity on the catalyst was enough to ensure the efficient formation of the carbene active sites. Interestingly, Na poisoning resulted in some modification of the selectivity. The mitigation of acidity was shown to favor propene selectivity over the formation of isomerization products (cis-2-butene, 1-butene, etc.). Moreover, treatment with NaOH led to a shorter induction period and reduced coke formation on the WO₃/SiO₂ catalyst.     

The use of positron lifetime measurements in a study of electron scavenging by a range of radiosensitizer compounds

Positron lifetime measurements were made in aqueous solutions of a number of radiosensitizer compounds in order to measure the o-Ps inhibition and quenching constants. By making measurements in the presence and in the absence of perchloric acid, the inhibiting effects on the o-Ps yield due to epithermal and trapped electrons could be determined on the basis of the spur model. Inhibition constants relating to the scavenging by the sensitizer molecule of epithermal electrons. (K₁) and traped electrons, (K₂) were found for eight basic nitroimidazoles. The results are also discussed in terms of electron trap depth which is here taken to be equivalent to the one-electron redox potential, E¹₇. It is concluded that o-Ps lifetime measurements alone are not capable of determining the finer details of these spur reactions. Estimates of the respective yields of RS^- and OH^. species in oxygen and R—NO₂ compounds show that these are not able to explain the large differences in sensitizing efficiency.     

Addition of the phenoxathiin cation radical to alkenes and nonconjugated dienes. Formation of (E)- and (Z)-(10-phenoxathiiniumyl)alkenes and (E)- and (Z)-(10-phenoxathiiniumyl)dienes on basic alumina

Addition of phenoxathiin cation radical (PO*+) to acyclic alkenes in acetonitrile (MeCN) solution occurred stereospecifically to form bis(10-phenoxathiiniumyl)alkane adducts. Stereospecific trans addition is ascribed to the intermediacy of an episulfonium cation radical. The alkenes used were cis- and trans-2-butene, cis- and trans-2-pentene, cis- and trans-4-methyl-2-pentene, cis- and trans-4-octene, trans-3-hexene, trans-3-octene, trans-5-decene, cis-2-hexene, and cis-2-heptene. The erythro bisadducts (compounds 6) were obtained with trans-alkenes, while threo bisadducts (compounds 7) were obtained with cis-alkenes. The assigned structures of 6 and 7 were consistent with their NMR spectra and, in one case, 6c (the adduct of trans-4-methyl-2-pentene) was confirmed with X-ray crystallography. Additions of PO*+ to 1,4-hexa-, 1,5-hexa-, 1,6-hepta-, and 1,7-octadiene gave bis(10-phenoxathiiniumyl)alkenes (compounds 8), the assigned structures of which were consistent with their NMR spectra. Each of these adducts lost a proton and phenoxathiin (PO) when treated with basic alumina in MeCN solution. Compounds 6 (from trans-alkenes) gave mixtures of (Z)- (9) and (E)-(10-phenoxathiiniumyl)alkenes (10) in which the (Z)-isomers (9) were dominant. On the other hand, compounds 7 (from cis-alkenes) gave mixtures of 9 and 10 in which, with one exception (the adduct 7c of cis-4-methyl-2-pentene), compounds 10 were dominant. The path to elimination is discussed. The alkenes 9 and 10 were characterized with NMR spectroscopy and, in one case (9a), with X-ray crystallography. Reactions of 8b-d with basic alumina gave mixtures of (E)- (13) and (Z)-(10-phenoxathiiniumyl)dienes (14), in which compounds 13 were dominant. The configuration of the product from 8a (the adduct of 1,4-hexadiene) could not be settled. Noteworthy features in the coupling patterns and chemical shifts in the NMR spectra of some of the adducts and their products are discussed and related to adduct conformations.     

Intermolecular Interactions in Binary Liquid Mixtures of Styrene with m-, o-, or p-xylene

The densities (ρ), ultrasonic speeds (ν), and refractive indices (n) of binary mixtures of styrene (STY) with m-, o-, or p-xylene, including those of their pure liquids, were measured over the entire composition range at the temperatures 298.15, 303.15, 308.15, and 313.15 K. The excess volumes (V^E), deviations in isentropic compressibilities (Δks), acoustic impedances (ΔZ), and refractive indices (Δn) were calculated from the experimental data. Partial molar volumes (V⁰_?,2) and partial molar isentropic compressibilities (K⁰_?,2) of xylenes in styrene have also been calculated. The derived functions, namely, V^E, Δk_s, ΔZ, Δn, V⁰_?,2, and K⁰_?,2 were used to have a better understanding of the intermolecular interactions occurring between the component molecules of the present liquid mixtures. The variations of these parameters suggest that the interactions between styrene and o-, m-, or p-xylene molecules follow the sequences: p-xylene>o-xylene>m-xylene. Apart from using density data for the calculation of VE, excess molar volumes were also estimated using refractive index data. Furthermore, several refractive index mixing rules have been used to estimate the refractive indices of the studied liquid mixtures theoretically. Overall, the computed and measured data were interpreted in terms of interactions between the mixing components.     

Die Bildungsenthalpie von SnJ4

The changes of enthalpy for the reactions

1. Sn(c)+2I₂(c)+4165 CS₂(l)=[SnI₄; 4165 CS₂] (sol.),
2. SnI₄(c)+4223 CS₂(l)=[SnI₄; 4223 CS₂] (sol.)

At 298,15 K have been found by solution calorimetry to be ΔH ₁=(?46.7±0.3) and ΔH ₂=(+3.2±0.1) kcal Mol^?1, resp. Neglecting the heat of dilution which is approximately zero these values give ΔH _f ^o (SnI₄; c; 298 K)=9?49.9±0.4) kcal Mol^?1 for the enthalpy of formation of SnI₄. From existing literature data the standard entropy is calculated to beS ^o(SnI₄; c; 298 K)=69,7 cal Mol^?1 K^?1 giving ΔG _f ^o (SnI₄; c; 298 K)=?50,5 kcal Mol^?1 for the corresponding change in theGibbs free energy.     

Microstructures occurring in reduced vanadium dioxide

Slightly oxygen-deficient VO₂ contains small clusters of (121) CS planes which ultimately order into the V_nO_2n?1 series of oxides with increasing degrees of reduction. The fact that CS is limited to (121) planes within this system may be due to a specific degree of ionic relaxation within the CS plane. Nominal compositions near to M_xVO₂ in the AlVO, GaVO, and FeVO systems contain microstructures which are essentially similar to those of the binary system and no new CS plane types are found at temperatures of the order of 1400°K.