Genaue Bestimmungen der Viskosität von Polystyrollösungen

Zusammenfassung Es wurde die Herstellung von polymerem Styrol und seine im Thermostaten ausgeführte Fraktionierung beschrieben. Aus den hergestellten Fraktionen wurden drei Polystyrole ausgew?hlt: ein nieder-, mittel- und hochmolekulares Produkt, bei deren L?sungen in Benzol die Viskosit?t in Abh?ngigkeit von Konzentration und Temperatur gemessen wurde. Der Konzentrationsbereich erstreckt sich bis zu den verdünntesten, eben noch genügend genau (bei einer Genauigkeit von 0,1‰) me?baren L?sungen. Die Me?ergebnisse zeigen, da? weder die einfache Gleichung von Staudinger noch die von Arrhenius-Duclaux, die Abh?ngigkeit der Viskosit?t von der Konzentration genügend genau ausdrücken, wobei die Abweichungen die Me?fehler weit überschreiten. Diese Arbeit ist zum Teile im Laboratorium der Kolloid-Abteilung des Physikalisch-chemischen Institutes der Leipziger Universit?t ausgeführt worden. Herrn Prof. Wo. Ostwald erlaube ich mir für die Anregung zu dieser Arbeit sowie für den mir immer freundlichst erteilten Rat meinen verbindlichsten Dank auszusprechen. Der Alexander von Humboldt-Stiftung m?chte ich an dieser Stelle danken für die Errn?glichung eines Studienaufenthaltes in Deutschland.     

Elektrolyse

Ohne Zusammenfassung     

Autocatalytic dehydrogenation of propane

Homogeneous gas-phase pyrolysis of propane was performed by using continuous CO₂ laser irradiation for bulk heating of the reaction mixture. Laser energy was absorbed by ethylene, the main product of propane dehydrogenation, and transferred to the reaction medium via collisional relaxation. A mechanism of propane dehydrogenation is suggested to describe the pyrolysis process. The mechanism involves autocatalysis by ethylene and includes propane–ethylene interaction with the formation of ethyl and propyl radicals.     

Subcritical catalytic branching random walk with finite or infinite variance of offspring number

Subcritical catalytic branching random walk on the d-dimensional integer lattice is studied. New theorems concerning the asymptotic behavior of distributions of local particle numbers are established. To prove the results, different approaches are used, including the connection between fractional moments of random variables and fractional derivatives of their Laplace transforms. In the previous papers on this subject only supercritical and critical regimes were investigated under the assumptions of finiteness of the first moment of offspring number and finiteness of the variance of offspring number, respectively. In the present paper, for the offspring number in the subcritical regime, the finiteness of the moment of order 1 + δ is required where δ is some positive number.     

Features of Superradiance in a Cyclotron Quantum-Dot Heterolaser Under Continuous Pumping

We study the generation of femtosecond superradiance (SR) pulses under continuous pumping, caused by the collective recombination of electron-hole (eh) pairs in quantum wells placed into a strong magnetic field that is perpendicular to the quantum-well plane. Such a superradiant semiconductor laser can operate even at room temperature owing to the complete quantization of the moving particles, the maximum possible spectral density of carrier states, the high volume density of effective cyclotron quantum dots, and the partial suppression of intraband scattering. In a multilayer laser heterostructure having an optical confinement factor of the order of 0.2 and located in a magnetic field of 10-50 T, generation of a quasi-periodic or chaotic sequence of coherent pulses with a peak power about 1 W, pulse duration about 100 fs, and a period-to-duration ratio of order 10 is expected. It is shown that dichromatic superradiant generation of a pair of modes resonant to two neighboring transitions between the corresponding electron and hole Landau levels is possible over a broad range of pumping powers. We performed numerical and analytical studies of monochromatic and dichromatic superradiance thresholds including studies with allowance for their modification caused by inhomogeneous broadening due to thickness fluctuations of the quantum wells and barriers in actual heterostructures.     

Anionic Polymerization of Methyl Methacrylate in the Presence of Piperidine

Anionic polymerization of methyl methacrylate (MMA) initiated by lithium tert-butoxide (t-BuOLi) was investigated in different mixtures of benzene and piperidine. The latter compound activates the associated alkoxide, as evidenced by the observed increase in the rate of polymerization, proportional to piperidine concentration and also by the lowering of the overall kinetic order of the polymerization reaction with respect to initial monomer concentration. However, at higher piperidine concentration the rate of the polymer growth significantly decreases after a short period of time; a probable reason for this retardation or termination effect is the decay of active growth centers by a termination reaction with the methacrylate carbonyl group. The molecular weight of the polymer is significantly lowered by even a small addition of piperidine as a result of increased initiator efficiency that leads to a higher absolute concentration of active centers and approaches the theoretical limit given by the stoichiometric ratio of monomer and initiator concentrations. The microstructure of the product is affected by the presence of the polar solvent to a considerably lesser degree than in the case of classical alkylmetal initiators. The isotacticity slowly decreases with piperidine concentration over the whole investigated range. The high stability of the complex active center of growth formed by the alkoxide initiator and also the gradual change in the character of the ionic pair at the end of the growing polymer chain are responsible for the relatively small changes of the microstructure.     

Weibel Mechanism of Magnetic-Field Generation in the Process of Expansion of a Collisionless-Plasma Bunch with Hot Electrons

Radiophysics and Quantum Electronics - We perform qualitative physical analysis and particle-in-cell two-dimensional numerical simulation of the Weibel mechanism of magnetic-field generation due to...