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  • 1
    Digitale Medien
    Digitale Medien
    Lattice models for the MSXα description of transition metal impurities in ionic crystals (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 9065-9078 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The development of a near MSXα description of transition metal impurities in polar solids is described. Non-muffin-tin corrected MTXα calculations are performed for clusters comprising the impurity together with one or two shells of adjacent lattice ions stabilized by potentials calculated from lattice models of increasing sophistication. The method is illustrated by its application to Cr3+@B:MgO and Cu+@B:NaX (X=Cl,F). The calculations are much faster than Hartree–Fock (+configuration interaction) techniques and the method offers an attractive route for computing the potential surfaces and wave functions which determine the impurity related properties of the doped host. The representation of the confining lattice potential is shown to be of paramount importance. Clusters comprising the impurity plus the two nearest shells of lattice ions embedded in a lattice of adjacent Hartree–Fock–Slater ions and remote point charges are shown to give good quality potential surfaces, but only when lattice–cluster exchange effects are included.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461811
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  • 2
    Digitale Medien
    Digitale Medien
    Transition matrix elements from multiple scattering wave functions for LiH (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 9079-9085 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We outline the theory for computing radiative and nonradiative transition rates from MSXα wave functions. The electronic matrix elements involved are identified, and evaluated for LiH. Some radiative and nonradiative rates for this molecule are estimated within the framework of first-order time-dependent perturbation theory. The results suggest that the method has a general utility.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461186
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  • 3
    Digitale Medien
    Digitale Medien
    The effect of chain structure on the annealing and deformation behaviour of polymers (1980)
    Springer
    Colloid & polymer science 258 (1980), S. 643-662 
    Details
    ISSN: 1435-1536
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Chemie und Pharmazie , Maschinenbau
    Beschreibung / Inhaltsverzeichnis: Zusammenfassung Beim Zug-Test erfolgt bei dehnbaren Thermoplasten entweder eine gleichmäßige Deformation oder eine Einschnürung. Erstere tritt auf bei Vorliegen ausgestreckter Kettenkonfigurationen in Lösung oder bei Makromolekülen linearer Struktur und relativ unbiegsamer Bindungen. Die beobachteten Unterschiede wurden anhand eines ViskositätsModells mit verminderter Gummi-Elastizität unter Zugrundelegung von Literaturdaten diskutiert. Die Kettensteifheit wird rechnerisch durch Einfügung einer Konstanten berücksichtigt; sie steht in Konkurrenz mit der Lokalisierung der plastischen Dehnung und korreliert mit dem Auftreten einer Spannungsriß-Bildung im Polyäthylen hoher Dichte. Es wird darauf hingewiesen, daß sich die spezifische Wärme von Polymeren mit ausgedehnter Kettenkonfiguration beiTg nur wenig ändert; dieser Effekt steht mit dem Dehnungsverhältnis verschiedener Polymerer in Einklang. Bei PVC kann jedoch durch Abschrecken des heißen Materials in Eiswasser ein Übergang vom normalen Einschnürungsverhalten zu einer gleichmäßigen Deformation erzielt werden; dieser Effekt ist durch die Unterdrückung einer speziellen Struktur bedingt und kann auch bei der Temperung anderer thermoplastischer Polymerer nahe beiTg auftreten. Die sich bei der physikalischen Alterung und bei der Temperung abspielenden Vorgänge lassen sich noch nicht erklären. Sie umfassen sicherlich Effekte des freien Volumens und die Rückverteilung von Rotations-Isomeren. Beobachtungen dieser Art sind von grundsätzlicher Bedeutung für die Physik glasartiger Polymerer.
    Notizen: Summary In a tensile test ductile thermoplastics may give either uniform deformation or necking. Recently it has been found that those giving uniform deformation either, are known to have extended chain configurations in solution, or have chemical formulae with linear structures and relatively few flexible bonds. The observed differences in behaviour can be predicted from a viscosity retarded rubber elasticity model in which a constant related to chain flexibility is introduced, which quantifies the strain hardening of the material. It is argued that the early development of strain hardening generally interferes with the localization of plastic strain in shear bands or crazes (as well as in a neck) and correlates with the stress cracking performance of high density polythenes. There is now also some evidence that polymers with extended chain configurations have small values ofΔcp (the change in specific heat atTg) and that this figure has an apparent correlation with draw ratio for different polymers. In the case of P.V.C. however it is possible to cause a changeover from the normal necking behaviour to uniform deformation by quenching the hot material in ice water. This process is believed to eliminate a structure which develops slowly in P.V.C. and most other thermoplastics when they are annealed at temperatures near toTg. The elimination of this structure during yielding causes strain softening and also promotes plastic strain localisation. This physical ageing or annealing process is still not well understood. It almost certainly includes free volume effects but recent studies have shown that a redistribution of rotational isomers also occurs. These observation, if correct, have fundamental implications for the physics of glassy polymers.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01384357
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  • 4
    Digitale Medien
    Digitale Medien
    Transition properties from multiple scattering X-Alpha Calculations: A study of NO2 (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 39 (1991), S. 71-91 
    Details
    ISSN: 0020-7608
    Schlagwort(e): Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The usefulness of the MS-Xα method for calculating molecular transition properties is investigated in a study of NO2. Two-dimensional potential surfaces are obtained in C2v symmetry for the X̃2A1/Ã2B2 system. Potential curves that approximately follow the asymmetric stretch coordinate are also presented: On investigation, no symmetry-broken solutions were found. At all nuclear geometries investigated, the muffin-tin sphere radii were chosen by an energy-minimization technique. The inclusion of corrections to the muffin-tin form of the Xα total energy is shown to be essential, if the computed surfaces are to be qualitatively correct. The transition dipole moment connecting the two states is evaluated as a function of nuclear coordinates, using the transition-state approximation. Finally, the radiative decay of a single vibronic level in NO2 is modeled and the relevance to experiment is discussed.
    Zusätzliches Material: 8 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/qua.560390109
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  • 5
    Digitale Medien
    Digitale Medien
    Characterization of novel modified amorphous poly (ether sulphone)s (1994)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 32 (1994), S. 3171-3182 
    Details
    ISSN: 0887-624X
    Schlagwort(e): high-temperature poly (ether sulphone) ; amine-ended polymer ; maleimide-ended polymer ; Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: New high-temperature amorphous polymers with chlorine, amine, and maleimide chain-ends have been synthesized by nucleophilic polycondensation and fully characterized by 13C-NMR, 1H-NMR, and potentiometric titration. From chain-end determination, number average molecular masses were calculated. It was confirmed that transetherification during the synthesis led to a randomized polymer of the monomer residues. For nominally amine-ended polymers obtained by addition of m-aminophenol at the end of the synthesis, a small amount of hydroxyl chain-ends was observed. This is ascribed also to transetherification. Complete reaction of the amine chain-ends with maleic anhydride was demonstrated. Reaction of hydroxyl chain-ends with acetic anhydride was also observed. The thermal stability of these different polymers was investigated; lower thermal stability was observed for amine and maleimide-ended polymers. By two different methods, a Tg around 270d°C was determined for these novel amorphous aromatic polymers. © 1994 John Wiley & Sons, Inc.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pola.1994.080321618
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  • 6
    Digitale Medien
    Digitale Medien
    Curing of a maleimide model compound (1995)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 33 (1995), S. 1943-1955 
    Details
    ISSN: 0887-624X
    Schlagwort(e): maleimide ; size exclusion chromatography ; mass spectroscopy ; thermal stability ; Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A monofunctional maleimide-ended model compound was synthesized and purified. The thermal curing of this compound was followed by size exclusion chromatography and the molecular masses of the oligomers formed during the curing reaction were determined by mass spectroscopy. It was found that high molecular mass species were not thermally stable. At high temperature and after enough time, a strong decrease in the polydispersity of the oligomers and a significantly preferential formation of trimer were observed. From 13C-NMR characterization, a five membered ring structure was found for the trimer, which explains the thermal stability of this species. © 1995 John Wiley & Sons, Inc.
    Zusätzliches Material: 13 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pola.1995.080331201
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  • 7
    Digitale Medien
    Digitale Medien
    The properties of some poly(arylene ether sulfones) (1980)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 25 (1980), S. 821-828 
    Details
    ISSN: 0021-8995
    Schlagwort(e): Chemistry ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie , Maschinenbau , Physik
    Notizen: Some 2,4′-substituted poly(arylene ether sulfones) have been prepared and their properties compared with those of the conventional 4,4′ homopolymer. The introduction of the comonomer leads to a small increase in yield stress and to a greater tendency to brittle fracture, which can be demonstrated from fracture surface micrographs.
    Zusätzliches Material: 6 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/app.1980.070250510
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  • 8
    Digitale Medien
    Digitale Medien
    The polymerization of n-butyl isocyanate at low catalyst concentrations (1980)
    New York : Wiley-Blackwell
    Journal of Polymer Science: Polymer Chemistry Edition 18 (1980), S. 371-372 
    Details
    ISSN: 0360-6376
    Schlagwort(e): Physics ; Polymer and Materials Science
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/pol.1980.170180139
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