Mechanism of vinyl chloride polymerization

An overall mechanistic scheme for the suspension polymerization of vinyl chloride is presented. The process can be resolved into five discrete stages, each of which presents a unique environment for the interaction of the systems parameters. It is shown that the surface area of the polymer formed during the reaction is not a major factor in autoacceleration and that the increase of kinetic chain length with conversion is due to a radical dilution effect. The latter is a direct result of the difference in rates between polymerization and radical formation, the former being greater. The increase of the initial polymerization rate and the reduction of autoacceleration brought about by chain transfer agents can be explained by the lower diffusion rate and greater bulkiness of the chain transfer agent radical relative to that of the monomer radical. The chaintransfer agent CBr₄ is preferentially absorbed by PVC from solution in vinyl chloride. With lauryl peroxide as initiator it is shown that the “hot spot” is the result of a build-up of initiator in the monomer caused by its exclusion from the polymer phase. Vinyl chloride was found to dissolve 0.03% PVC at ambient temperature and to have no effect on the decomposition rate of lauryl peroxide.     

Engineered protein cages for nanomaterial synthesis

Self-assembled particles of genetically engineered human L subunit ferritin expressing a silver-binding peptide were used as nanocontainers for the synthesis of silver nanoparticles. The inner cavity of the self-assembled protein cage displays a dodecapeptide that is capable of reducing silver ions to metallic silver. This chimeric protein cage when incubated in the presence of silver nitrate exhibits the growth of a silver nanocrystal within its cavity. Our studies indicate that it is possible to design chimeric cages, using specific peptide templates, for the growth of other inorganic nanoparticles.     

Peroxides of higher aliphatic ethers

The oxidation of higher aliphatic ethers with oxygen at 50° yields two peroxides as the main oxidation products for each ether, namely a dihydroperoxy-dialkyl ether and a dihydroperoxy-dialkyl peroxide. The mass spectra of these compounds are recorded and the main fragmentation pattern is discussed.     

Spatially resolved characterization of PEFCs using simultaneously neutron radiography and locally resolved impedance spectroscopy

A method for performing neutron radiography and locally resolved impedance spectroscopy simultaneously in situ in an operating polymer electrolyte fuel cell (PEFC) is presented. The new method provides concurrently spatially resolved information about the local cell performance, the locally limiting processes, and the liquid water distribution. Information about the impact of water on cell performance and limiting processes can be gained in situ on a local scale in an operating PEFC. The method was applied to a PEFC operated on pure H₂/O₂ in co-flow mode under low humidity operating conditions. The results show that in co-flow mode strong flooding and severe drying can occur at the very same time in different sections of a PEFC.     

Fluoroscopic MR imaging at 0.064 Tesla

The authors developed a system for ultrafast magnetic resonance imaging (MRI) protocols at low field. The system design permits the acquisition of the raw data in the background while the reconstruction and display steps repeat as fast as they can in the foreground. The performance speeds depends partly on the desired use. By collecting raw data at a rate of 20 ms per echo with an echo delay of 9 ms, a complete data cycle for a 128x64 image takes 1.28 s. However, once half of that data is incorporated into the reconstruction, the image appears complete. Using this set of parameters the authors were able to get the rate of the recon/display loop to paint about two times per completed raw data cycle, showing an entirely new image at least once per second with an apparent frame rate of two per second. Interleaving of two or three orthogonal scans reduces the speed of update but provides better information. The authors discuss the system design for rapid scan/recon/display and demonstrate the image quality available at low field strength with scan times below one second.     

Low-field 3-DFT MRI: conceptual, analytical, and experimental aspects

Three-dimensional Fourier transform (3-DFT) magnetic resonance imaging (MRI) offers advantages in terms of signal-to-noise (S/N) per unit of time for the case where a large number of slices is desired. This advantage is enhanced when the relaxation time, T(1 ), is short. Because time limitations in 3-DFT imaging force the use of short time intervals, TR, between excitations of a slice, lesion contrast is often undesirable at mid-and high-field strength even when the S/N is good. At low fields, where T(1) values are short, high S/N and contrast can both be achieved with 3-DFT MR images. The conceptual and analytical aspects of low-field 3-DFT MRI are presented and demonstrated at 640 G.     

Photoproduction of jets at HERA: Comparison of next-to-leading order calculation with ZEUS data

We have calculated inclusive single-jet production in lowQ ² ep-collisions at next-to-leading order superimposing direct and resolved contributions. The results are compared with recent experimental results from the ZEUS Collaboration at HERA.Supported by Graduiertenkolleg der DFG Erzeugung und Zerfälle von Elementarteilchen an der Universität Dortmund     

Next-to-leading order fragmentation functions for pions and kaons

We present new sets of fragmentation functions for charged pions and kaons, both at leading and next-to-leading order. They are fitted to TPC data taken at energy =29 GeV and describe excellently a wealth of othere ⁺ e ^– data on charged-hadron production, ranging from =5.2 GeV way up to LEP 1 energy. They also agree with data on the production of neutral pions and kaons, if one makes the natural assumption that the respective fragmentation functions are related to the charged counterparts bySU(2) symmetry. We also list simple parameterizations of thex andQ ² dependence of our results, which may be implemented conveniently in applications.Supported by Bundesministerium für Forschung und Technologie, Bonn, Germany, under Contract 05 6 HH 93P (5), and by EEC ProgramHuman Capital and Mobility through NetworkPhysics at High Energy Colliders under Contract CHRX-CT93-0357 (DG12 COMA)