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.     

A note on a boundary-layer collision on a rotating sphere

A recent study of the unsteady boundary layer near the equator of a spinning sphere provided strong numerical evidence that the solution develops a singularity a finite time after the motion is initiated. In this paper we reformulate and complete the asymptotic structure proposed to describe this singularity. It is then self-consistent and provides convincing evidence of the accuracy of the computations and of the existence of the singularity.

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Zusammenfassung Eine neuere Untersuchung über die nicht-stationäre Grenzschicht nahe am Äquator einer rotierenden Kugel ergab starke numerische Evidenz für das Auftreten einer Singularität in der Lösung nach einer endlichen Zeit, vom Beginn der Bewegung gerechnet. In dieser Arbeit wird die asymptotische Struktur zur Beschreibung der Singularität neu formuliert und ergänzt. Sie wird selbstkonsistent und gibt ein wichtiges Zeugnis für die Genauigkeit der Berechnungen und für die Existenz der Singularität.

     

New photoluminescence features of cinnabar (α-HgS) crystals grown by chemical vapor transport

We report new broad bands and narrow lines in the photoluminescence of cinnabar (α-HgS) crystals grown by chemical vapor transport. For exciting light, ?ω > Eg, our spectra resemble those due to other workers, however, for ?ω < Eg we observe a new broad luminescence band and two sharp features separated by ～ 18 meV.     

Palladium, rhodium and platinum complexes of ortho-xylyl-linked bis-N-heterocyclic carbenes: Synthesis, structure and catalytic activity

New Pd(II), Pt(II) and Rh(I) N-heterocyclic carbene (NHC) complexes containing two NHC units linked by an ortho-xylyl group are described and structurally and spectroscopically characterised. The Pt(II) complexes represent the first examples of Pt-bis(NHC) complexes where the NHC units are linked by an ortho-xylyl group. Functionalisation of the bis(NHC) ligands with heptyl groups has been used as a means of enhancing the solubility of the complexes, in order to facilitate spectroscopic characterisation and catalytic studies. The catalytic activity of the palladium(II) complexes in Heck and Suzuki cross-coupling reactions has been examined to investigate any effects of the diverse structural changes, though these appear to be insignificant.     

Necessary conditions at the boundary for minimizers in finite elasticity

Dedicated to Bernard Coleman on the occasion of his 60^th birthday     

Chemical repercussions of orbital interactions through bond and through space. Effect of remote substituents on the addition of nitrenes to bicyclic olefins

The reactivity of a series of bicyclic olefins with nitrenes is profoundly influenced by the nature of remote functional groups. There is a marked lack of reactivity for reactions with carboethoxynitrene as compared to phthalimidonitrene which is distinctly nucleophilic in character. An explanation for the reluctance to form aziridines is offered in terms of orbital interactions between the distant groups and the olefinic bond, making the latter remarkably electron deficient as evidenced by UV-photoelectron spectrosoopy. Because of the complexity of the spectra, identification of the Ip associated with the reactive π-centre was made by recourse to ab initio configuration interaction calculations for key members of the series.     

A probe of the relative rate of interconversion of carbonium ion conformers vs proton loss in 9-hydroxy-10-methyldecahydronaphthalenes

Deutero-alcohol (2b) was prepared and dehydrated with H₂SO₄Ac₂OHOAc to give a kinetic isotope effect demonstrating anion mobility within the intermediate carbocation-anion ion pair.