Preparation of high molecular weight polyurethane particles by nonaqueous emulsion polyaddition

A versatile nonaqueous emulsion polyaddition process for the one-step fabrication of spherical polyurethane nanoparticles is presented. Three different emulsion systems were used consisting of N,N′-dimethylformamide (DMF) dispersed in n-hexane, acetonitrile dispersed in cyclohexane, and acetonitrile dispersed in tetradecane. After successful stabilization of the emulsion systems by using a poly(isoprene)-poly(methylmethacrylate) block copolymer, the fabrication of the polyurethanes was carried out within the dispersed polar phase. The polyurethane particles showed average diameters as small as 35 nm. Additionally, infrared (IR) characterization revealed that the formation of any urea, which decreases the mechanical properties of the polyurethanes, was prevented during the polyaddition. This was attributed to the anhydrous reaction conditions. Gel permeation chromatography (GPC) analysis demonstrated the average molecular weights (M _n) of the polyurethanes to be as high as 16,500 g/mol, corresponding to conversions of 0.98. Comparable molecular weights and conversions have not previously been achieved without the formation of urea.     

2,6-diaza-4,8-dicyanosemibullvalene. A short lived intermediate?

Reductive dehalogenation of dichloride with magnesium affords the new stable diazacyclooctatetraene (1,5-diazocine) . There is strong evidence for the intermediate formation of a 2, 6-diaza-4,8 - dicyanosemibullvalene .     

Hydrogen-induced reconstruction of transition metal surfaces

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. It particularly deals with the structural aspect of these low energy electron diffraction (LEED) investigations and correlates them, if available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually lift the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent periodicity of these adlayer structure elements together with the respective shift buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption it should be generally considered in all adsorption systems.     

Certification of reference materials and accreditation of reference material producers: Questionable terminology leads to confusion

In the view of the Deutscher Kalibrierdienst (DKD) , a certifying body for reference materials can be considered to be a calibration laboratory. Therefore, accreditation of calibration laboratories in accordance with ISO/IEC 17025 is the most appropriate way to establish confidence in certificates for reference materials. If necessary, the criteria of ISO/IEC 17025 can be tailored to specific cases. There is no need to provide any new kind of reference-material specific accreditation. However, in view of the variety of reference materials and the practice existing in other countries, accreditation of testing laboratories and product certification bodies may optionally be acceptable as long as the same stringent principles with respect to traceability and measurement uncertainty are applied. Such accreditations but not accreditations of reference material producers (ISO Guide 34) are also covered by existing international mutual recognition arrangements (MRA).     

PROTECTION OF ESCHERICHIA COLI CELLS AGAINST THE LETHAL EFFECTS OF ULTRAVIOLET AND X IRRADIATION BY PRIOR X IRRADIATION: A GENETIC AND PHYSIOLOGICAL STUDY

Abstract— When log phase cells of wild-type E. coli K-12 were maintained in growth medium after X irradiation, they became progressively more resistant to a subsequent exposure to UV or X radiation. The time to achieve maximum resistance was about 60 min. The uvrB, uvrD, polA and certain exrA strains (W3110 background) also demonstrated this X ray-induced resistance to subsequent UV or X irradiation but recA, recB, lex (AB1157 or W3110 backgrounds) and other exrA strains (AB1157 background) did not. The resistance induced in wild-type, uvrB and uvrD cells was characterized by the production or enhancement of a shoulder on the survival curves obtained for the second irradiation, while the resistance induced in the W3110 exrA strains was expressed only as a change in slope. The induction of resistance in the W3110 exrA strain was not inhibited by the presence of chloramphenicol, but that in the wild-type cells appeared to be. The production or enhancement of a shoulder on the survival curves of the rec ⁺ lex ⁺ exr ⁺ cells is consistent with the concept of the radiation induction of repair enzymes. Alternative explanations, however, are discussed.     

Divergencies in a vector model with hyperbolic symmetry on a chain

A chain of interacting vectors (, ) withn components subject to the hyperbolic conditions ²–²=1 is considered. The interaction possesses a global non-compactO(1, n–1)/O(1)×O(n–1) symmetry. Symmetries of this kind are known from the study of disordered electronic systems. The non-compactness of the symmetry group causes divergencies such that a symmetry breaking term is necessary. Asymptotic behaviour of the expectation values and long-ranged correlation functions are calculated for a small symmetry breaking field.Supported in part by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 123 at the University of Heidelberg     

T-Periodic solutions of time dependent Hamiltonian systems with a potential vanishing at infinity

We are discussing existence and multiplicity of T-periodic solutions of the time dependent Hamiltonian system = U(x, t) under the assumption that U(x, t) approaches 0 for large |x| meaning that the force U is concentrated in a finite region. Our method is variational and it is shown how the difficulties with the Palais-Smale condition can be overcome in this case.     

über eine Klasse Riemannscher Mannigfaltigkeiten mit raumartigen pkf-ZusammenhÄngen

One considers m-dimensional Riemannian manifoldsM with tangent spaces T_p(M), p M that are a direct sum of a spacelike m-2 planeR _p and a 2-planeH _p. It is supposed that onM there exists a connection whose space-like components are parallel conformal flat (pkf). These components are generated by a vector field X. Assuming that X belongs to a pair X,Y of reciprocal quasi-cocircular vector fields and that the Pfaffian of this pair is the 1-form associated with the connection, the following results are derived: 1. X and Y are of equal constant length (This is true for all Riemannian manifolds). 2. The immersion of the integral manifold ofH _p intoM is cylindrical and the normal connection is flat. 3. The immersion of any space-like submanifold intoM is cylindrical with respect to the sections inH _p and umbilical with respect to all spacelike sections. 4. If m 4, the integral manifoldP _p is flat.

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Herrn Prof. Dr. WERNER BURAU zum 70.Geburtstag Klaus Buchner und Radu Rosca     

A simple tree lemma and its application to a counterexample of philips

We prove a tree lemma on perfect trees to obtain an ultrafilter of definable subsets of , which is Ramsey but not conservative (see [Phi 2]).