Polymer characterization: Quasi-elastic and elastic light scattering

Dynamic or quasi-elastic light scattering (LS) from polymers in solution arises from concentration fluctuations. With the aid of modern photomultipliers these can be followed as a function of time. A proper evaluation allows to study the center of mass motion and the dynamics of individual chains. The relevance of simultaneous recording of static and dynamic LS is emphazised. Various aspects are discussed in three main sections. In the first part basic relationships are reviewed. The particle scattering factor P(8), structure factor S(q,c) and osmotic compressibility RT(∂c/∂T) occuring in static LS are defined. The time correlation functions (TCF) in dynamic LS are described. The TCF of the scattered electric field contains the time dependent structure factor S(q,t) and the static structure factor S(q). The initial part of the TCF (short delay times) can be approximated by a cumulant expansion; the first cumulant is related to the translational diffusion coefficient D. The concentration dependence of D contains a thermodynamic and a hydrodynamic contribution where the thermodynamic part is identical with the osmotic compressibility in static LS. The second part deals with the behaviour of various polymeric architectures in dilute solutions. Two new structure sensitive parameters, C and = Rg/Rh, are introduced. Chain stiffness and branching are extensively discussed. In the third part properties of different macromolecular architectures in semi-dilute solution are considered. The inverse osmotic compressibility = osmotic modulus and the concentration dependence of the translational diffusion coefficient are discussed in the light of re-normalization group and scaling theories.     

Mycotoxins from Mold Fungi — Weapons of Uninivited Fellow-Boarders of Man and Animal: Structures,Biological Activity,Biosynthesis, and Precautions

In addition to the antibiotics, mycotoxins represent a second large group of biologically active substances of microbial origin. Over 300 mycotoxins are so far known and can be subdivided into 25 structural types. They are formed by approximately 350 molds of which there are 10 000 different strains. As mycotoxin-producing molds tend to spread aggressively on foodstuffs, they can cause epidemic mycotoxicoses.—The mycotoxins have been isolated, their structures determined, and their biological activities investigated; their biosynthesis has now also largely been elucidated. Accordingly, mycotoxins are derived from only three groups of key biogenetic building blocks: polyketides, isopentenyl pyrophosphate, and amino acids. Mycotoxins are mainly non-polar, chemically stable, are formed in high concentrations, and do not contain—unlike antibiotics—any sugars. Mycotoxins display an enormous variety of toxic-pharmacological activities, e.g. liver degradation, hemorrhages, carcinomas. The identification of mycotoxicoses is impeded by their long incubation periods and the fact that the mycotoxins tend to accumulate in the mammalian organism, which means that even very low concentrations in food are a potential danger. The particularly dangerous aflatoxin B₁ 1 may not exceed a limit in food of 5 ppb (5 μg/kg), which is difficult to control.     

Competitions for gold medals and prizes named after outstanding scientists held by the Russian Academy of Sciences in the field of chemical sciences in 1996

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Competitions for gold medals and prizes named after outstanding scientists held by the Russian Academy of Sciences in the field of chemical sciences in 1996     

Shape-persistent macrocycles with intraannular polar groups: synthesis, liquid crystallinity, and 2D organization

The synthesis of macrocycles with intraannular polar ester groups and extraannular oligo-alkyl groups is described. The compounds exhibit stable liquid crystalline phases showing fan-shaped textures under the polarizing microscope, typical for a columnar order of the molecules. X-ray powder diffraction data of the LC phase indicate that the unit cell contains two symmetry-related units, a feature pointing most probably to a restricted rotation of the macrocycles within a stack. The X-ray data were further supported by solid-state NMR experiments, showing that the rigid core of the compounds does not rotate with kHz or higher frequencies within the column in the LC phase. Apart from the organization of the molecules in the LC phase, the 2D organization of the macrocycles at the solvent-highly oriented pyrolytic graphite (HOPG) interface was investigated and showed that these compounds are capable of nanofunctionalizing the HOPG surface in the multinanometer regime.     

Zur Frage nach der Existenz eines kubischen Pyrochlors Pb2Sb2O7

On the Problem of the Existence of a Cubic Pyrochlore Pb₂Sb₂O₇ The reaction of PbO, PbO₂, PbCO₃ or Pb(CH₃CO₂)₂ · 3 H₂O with Sb₂O₃ (ratio 2:1 Mol.) gives up to 700°C a cubic pyrochlore together with PbSb₂O₆. The pyrochlore contains Pb(IV) and has the approximate composition Pb[PbSb]O_6,75 with a = 1066–1069 pm. After heating up to 900°C rhombohedral Pb₂Sb₂O₇ is formed. There is no evidence for the existence of a cubic Pb₂Sb₂O₇ with Pyrochlore structure.