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  • Physics  (73)
  • STRUCTURAL MECHANICS  (57)
  • CHEMISTRY  (55)
  • ASTROPHYSICS
  • 1970-1974  (185)
  • 1960-1964
  • 1972  (185)
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Keywords
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  • 1970-1974  (185)
  • 1960-1964
Year
  • 1
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    Dynamics of the molecular and atomic mechanisms for the hydrogen-iodine exchange reaction. (1972)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The molecular and atomic mechanisms for the hydrogen-iodine exchange reaction are treated theoretically by means of extensive classical trajectories calculated on a reasonable potential energy surface on which the single adjustable parameter is the iodine-core effective charge. The analysis shows the molecular mechanism to be dynamically forbidden, but gives an over-all rate constant for the atomic mechanism that is in agreement with the experimental values. It is indicated that the formation of a weak H2I complex plays an important dynamical role if the atomic mechanism is limited to reactions with collision complexes involving no more than two hydrogen atoms and two iodine atoms. Excellent agreement with experiment is obtained for the rate constant for the recombination I+I+H2 yields I2+H2 and its negative temperature coefficient.
    Keywords: CHEMISTRY
    Type: Journal of Chemical Physics; 56; June 15
    Format: text
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  • 2
    Electronic Resource
    Electronic Resource
    EPR studies of mechanically induced bond rupture in elastomersPaper presented at the Symposium on Mechano-Chemistry of Polymers, American Chemical Society National Meeting, Washington, D. C., September 16, 1971. (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-2: Polymer Physics 10 (1972), S. 599-606 
    Details
    ISSN: 0449-2978
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Electron paramagnetic resonance (EPR) spectroscopy was used to compute the surface bond rupture density in polyurethane and to determine the phase experiencing fracture in styrene-butadiene block copolymers when these elastomers are subjected to mechanical degradation by grinding. The polyurethane grinding was done at temperatures above and below the glass transition Tg; 0.155 × 1013 radicals/cm2 of fracture surface area were formed above the Tg and 4.42 × 1013 radicals/cm2 for grinding below the Tg. These values are essentially equal to those found earlier for spherulitic polymers. In all cases the fracture appears able to progress along preferential paths so as to rupture significantly fewer molecular chains than one would expect on the basis of calculations of the number of chains passing through each square centimeter of cross section. Comparison of EPR spectra formed by grinding styrene-butadiene copolymer with those of styrene and butadiene above indicated that at cryogenic temperature, the fracture in the copolymer takes place in the butadiene phase.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.160100403
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  • 3
    Electronic Resource
    Electronic Resource
    Fracture behavior in nylon 6 fibers (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-2: Polymer Physics 10 (1972), S. 1415-1445 
    Details
    ISSN: 0449-2978
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A reaction rate model of fracture in polymer fibers is described. This model assumes that bond rupture is governed by absolute reaction rate theory with a stress-aided activation energy. It is demonstrated that the key in obtaining good agreement between the model and experiment lies in taking proper account of the variation of stress on the tie-chain molecules. The more taut chains rupture first, and the load is redistributed among the remaining unruptured tie chains. The effect of varying the temperature both in the model and in experiments on fracture in fibers is explored. Good agreement between predictions of the model and experiment is possible only with an undeterstanding of the distribution in stress on the tie chains. The distribution in stress on the chains was experimentally determined by monitoring the kinetics of bond rupture with electron paramagnetic resonance (EPR) spectroscopy. Temperature is found to have two effects on macroscopic strength. (1) The thermal energy aids the atomic stress in breaking the atomic bonds; as a consequence the rate of bond rupture of a family of bonds under a given molecular stress is increased. In this respect temperature might be viewed as decreasing the “strength” of a bond. (2) Temperature also serves to “loosen” the molecular structure and in this way modify the distribution in stress on the tie chains. To explain bond rupture and macroscopic fracture behavior quantitatively, account must be taken of both effects.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.160100802
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  • 4
    Electronic Resource
    Electronic Resource
    (Diethoxyphosphato)- and (π-cyclopentadienyl)-(diethoxyphosphato)-oxotitanium (IV) polynuclear complexes (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-1: Polymer Chemistry 10 (1972), S. 1559-1562 
    Details
    ISSN: 0449-296X
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.150100527
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  • 5
    Electronic Resource
    Electronic Resource
    Chromocene catalysts for ethylene polymerization: Scope of the polymerization (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-1: Polymer Chemistry 10 (1972), S. 2621-2637 
    Details
    ISSN: 0449-296X
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chromocene deposited on silica supports of high surface area forms a highly active catalyst for polymerization of ethylene. Polymerization is believed to occur by a coordinated anionic mechanism previously outlined. The catalyst formation step liberates cyclopentadiene and leads to a new divalent chromium species containing a cyclopentadienyl ligand. The catalyst has a very high chain-transfer response to hydrogen which permits facile preparation of a full range of molecular weights. Catalyst activity increases with an increase in silica dehydration temperature, chromium content on silica, and ethylene reaction pressure. The temperature-activity profile is characterized by a maximum near 60°C, presumably caused by a deactivation mechanism involving silica hydroxyl groups. A value of 72 was estimated for the ethylene-propylene reactivity ratio (r1). Linear, highly saturated polymers are normally prepared below 100°C. By contrast with other commercial polyethylenes, the chromocene catalyst produces polyethylenes of relatively narrow molecular weight distribution. Above 100°C, unsaturated, branched polymers or oligomers are formed by a simultaneous polymerization-isomerization process.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.150100910
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  • 6
    Electronic Resource
    Electronic Resource
    Ethylene polymerization with supported bis(triphenylsilyl) chromate catalysts (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-1: Polymer Chemistry 10 (1972), S. 2609-2620 
    Details
    ISSN: 0449-296X
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bis(triphenylsilyl) chromate is an active catalyst for ethylene polymerization without further treatment or additives. Catalytic activity is markedly increased when the compound is deposited on silica-alumina and is further increased if it is deposited on silica and then treated with an aluminum alkyl. Polymer molecular weight can be controlled by reaction temperature, hydrogen addition, support type, and reducing agent structure to give polymers ranging in melt index from essentially zero to 〉 100. In the supported catalysts the bis(triphenylsilyl) chromate appears to be bound to the support and to undergo a reduction step either by reaction with ethylene or with aluminum alkyl prior to polymerization. The active site is envisioned as chromium alkyl, bound to the support, with propagation occurring by insertion of the monomer into a Cr—C bond. Chain termination is by chain transfer to monomer.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.150100909
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  • 7
    Electronic Resource
    Electronic Resource
    Stereochemistry of alternating isobutene-maleic anhydride, isobutene-dimethyl fumarate, and isobutene-dimethyl maleate copolymersPresented in part at the XXIIIrd International Congress of Pure and Applied Chemistry, Boston, Massachusetts, July 1971. (1972)
    New York : Wiley-Blackwell
    Journal of Polymer Science Part A-1: Polymer Chemistry 10 (1972), S. 1297-1310 
    Details
    ISSN: 0449-296X
    Keywords: Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereochemical composition of the free radical alternating isobutene-maleic anhydride (IB/MA), isobutene-dimethyl fumarate (IB/DMF), and isobutene-dimethyl maleate (IB/DMM) copolymers was investigated by proton magnetic resonance. In contrast to the singlet gem-dimethyl resonance found in polyisobutene or in the alternating isobutene/acrylonitrile copolymer, the gem-dimethyl resonance of IB/MA, hydrolyzed IB/MA, and esterified IB/MA is a quadruplet with peaks of approximately equal intensity. The multiplicity of the spectra is consistent with the presence of equal amounts of threo-di-isotactic and threo-di-syndiotactic triads, disproving previous claims that such copolymers are predominantly threo-di-isotactic. The spectrum of the analogous IB/DMF indicates that the copolymer is composed entirely of erythro-di- isotactic and erythro-di-syndiotactic triads. This result is consistent with the exclusive trans opening of the dimethyl fumarate double bond and provides the first example for the stereospecific double bond opening of a noncyclic monomer in free radical polymerization. In contrast, the spectrum of IB/DMM shows that the dimethyl maleate double bond opens approximately 93% cis and 7% trans during copolymerization. Since the stereochemical composition of IB/DMF and IB/DMM is not the same, it is concluded that the radicals formed from dimethyl maleate and/or dimethyl fumarate do not equilibrate freely among all the possible configurations before isobutene addition.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pol.1972.150100502
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  • 8
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    The electron heating rate and ion chemistry in the thermosphere above Wallops Island during the solar eclipse of 7 March 1970. (1972)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Measurement of the N2 concentration and temperature, the ion composition and concentrations, and the electron temperature up to 290 km about 30 and 5 min before totality during the Mar. 7, 1970, eclipse. The rockets traveled similar trajectories, thus permitting the purely temporal changes between flights to be resolved. The neutral temperature and N2 concentration changed little, but the electron temperature decreased by as much as 20% in the lower F region. The ion concentration decreased by about 30% in the F region and about 50% in the E region, with little change in relative ion composition. The electron cooling rates decreased by a factor of 6 in the lower F region, approximately in proportion to the change in the visible solar disk. A smaller than expected decrease in the cooling rate below 150 km between the two flights indicates a hardening of the solar spectrum and suggests a significant heat contribution from the solar corona near totality. The ion composition measurements were consistent with solutions of the ion continuity equations. A proper fit required a factor-of-three enhancement of the flux below 200 A, an amount also consistent with the electron heat balance analysis. Reactions involving the minor ions N(+) and N2(+) were found to be important for the ion chemistry of the major ions O2(+) and NO(+), especially at the time of eclipse.-
    Keywords: CHEMISTRY
    Type: Journal of Atmospheric and Terrestrial Physics; 34; Apr. 197
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  • 9
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    Purification, crystallization, and subunit structure of allosteric adenosine 5'-monophosphate nucleosidase. (1972)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Keywords: CHEMISTRY
    Type: Biochemistry; 11; 15, 1; 1972
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  • 10
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    EPR investigation of Ti2+ in SrCl2 single crystals. (1972)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: The observation of 'double quantum' transitions which made it possible to determine the charge state of Ti as 2+ is reported. The EPR spectrum observed at 1.2 K is presented in a graph. The first derivative of the absorption is shown vs the magnetic field. The hyperfine patterns for the Ti-47 and Ti-49 isotopes are identified. Spin-Hamiltonian parameters for Ti(2+) in various cubic hosts are listed.
    Keywords: CHEMISTRY
    Type: Journal of Chemical Physics; 57; Oct. 15
    Format: text
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