Application of the random break theory to thermal degradation of polymers

Thermal degradation of high-molecular-weight polyethylene is considered from the standpoint of the random break theory. The quantitative estimations made indicate that the vaporization conditions realized in the experiment exert a decisive effect on the total conversion rate. The heat of vaporization of macromolecular fragments formed by cleavage of C-C bonds amounts to approximately a half of the total thermal effect recorded by thermal analysis. The results obtained make it possible to rationalize some phenomena that were observed experimentally but have not been satisfactorily interpreted previously, and also to predict the influence of experimental conditions on the course of thermal degradation of polymers.     

Regioselectivity in the Addition of 1,3-Dipolarophiles to 6-Aryl-1,5-diazabicyclo[3.1.0]hexanes

Thermolysis of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes in the presence of 1,3-dipolarophiles having an unsymmetrically substituted double C=C bond (such as N-arylimides derived from 2-aryl-substituted maleic, citraconic, and itaconic acids, ethyl propynoate, aryl isocyanates, and aryl isothiocyanates) leads to formation of the corresponding 1,3-dipolar cycloaddition products. The reaction is regioselective, and in most cases only one regioisomer is obtained. The addition direction depends on the 1,3-dipolarophile structure, i.e., electronic and steric factors determining the most effective orbital interaction upon approach of the reagent to substrate.     

Stereoselectivity in the Addition of 1,3-Dipolarophiles to 6-Aryl-1,5-diazabicyclo[3.1.0]hexanes

Thermolysis of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes in the presence of N-arylmaleimides having a substituent in the ortho position of the aromatic ring leads to predominant formation of the corresponding trans-9-arylperhydropyrazolo[1,2-a]pyrrolo[3,4-c]pyrazole-1,3-diones. 6-Aryl-1,5-diazabicyclo[3.1.0]hexanes react with fumaric acid derivatives in a stereoselective fashion, affording perhydropyrazolo[1,2-a]pyrazoles with a trans,trans configuration.     

Stochastic void coarsening

The random nature of diffusing jumps and cascade occurrence produce stochastic fluctuations of the point-defect fluxes. The effect of such fluctuations on the kinetics of void growth is investigated in the present paper. It is found that the non-linear coupling between the stochastic fluctuations and the void sizes may lead to the instability of void evolution within the mean-field theory, when the sizes of voids and their growth rates are both relatively small. The growth rate of voids becomes dominated by the stochastic component, causing the smaller voids to shrink away. This effect is investigated in terms of a non-equilibrium phase transition induced by a purely random stochastic noise. The derived conditions for this non-equilibrium transition are compared favourably with experimental observations. Received: 5 June 2000 / Accepted: 9 October 2000 / Published online: 21 March 2001     

Cross section for the subthreshold fission of 236U

The cross section for ²³⁶U fission in the neutron-energy range E _n = 0.001–20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the ²³⁸U(n, f) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of ²³⁶U was confirmed.     

Potential of thermal analysis as applied to studying the kinetics of thermal degradation of polymers

The kinetics of thermal degradation of low-density polyethylene was studied by TGA and DSC at heating rates from 0.5 to 40 deg min^–1. Causes of significant discrepancies in the published effective kinetic constants of the overall reaction of thermal degradation of the polymers, determined using different experimental methods and different data treatment procedures, were analyzed. The possibility of using random break model as an alternative approach to describing polymer thermal degradation curves obtained by thermal analysis methods was demonstrated by the example of polyethylene.     

Thermal opening of the diaziridine fragment in 1-methyl-and 1,3,3-trimethyl-1,3,4,8b-tetrahydro-[1,2]diazirino[3,1-<Emphasis Type="Italic">a</Emphasis>]isoquinolines in the presence of n-arylmaleimides

The thermal opening of the diaziridine ring in 1-methyl-and 1,3,3-trimethyl-1,3,4,8b-tetra-hydro[1,2]diazirino[3,1-a]isoquinolines in the presence of N-arylmaleimides leads to the predominant or exclusive formation of the trans isomers of the products of 1,3-dipolar cycloaddition. In the absence of dipolarophile, the conversion of the starting diaziridines is incomplete over the same time period, while the thermolysis products are N-[3,4-dihydro-2(1H)-isoquinolyl]-and N-[3,3-dimethyl-3,4-di-hydro-2(1H)-isoquinolyl]-N-methyleneamines formed as the result of isomerization of intermediate labile azomethineimines. Dedicated to the memory of A. A. Potekhin. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1071–1079, July, 2008.     

Multifunctional pulsed neutron spectrometer IN-06 of INR RAS

The elements of multifunctional neutron spectrometer designed to simultaneously determine the parameters of atomic and supra-atomic structure and the dynamic parameters of the substance in a condensed state were tested. The neutron spectrum was measured by a direct-beam detector placed in the sample position on a 20-meter sample-source path length. Diffraction patterns of industrial diamond powder were obtained in a wide range of transmitted pulses. The resolutions of back-scattering and high-intensity diffractometers were determined.