Crystallization kinetics of isotactic polystyrene. II. Influence of thermal history on number of nuclei

The influence of various thermal pretreatments on the nucleation of isotactic polystyrene has been studied quantitatively by dilatometry. A distinction can be made between nuclei still present above the melting point (“resistant” nuclei) and nuclei created by severe supercooling (“induced” nuclei). The number of spherulites formed has been determined for different combinations of supercooling and crystallization temperatures. The results are interpreted in a satisfactory manner by assuming that in severe supercooling induced nuclei are created, which may grow into effective nuclei at higher temperatures. The crystallization of a severely supercooled polymer is completely governed by these induced nuclei, because they outnumber the resistant nuclei by some orders of magnitude. The number of induced nuclei can be decreased by purifying the polymer (removing catalyst residues). When cooled polymer is heated to temperatures just above the melting point, the induced nuclei are destroyed (“reversible melting”), so that only the resistant nuclei, which are few in number, remain. These resistant nuclei govern the crystallization behavior of a polymer which has not previously been cooled. Their number decreases on heating to temperatures far above the melting point.     

Hyperelektronische Polarisierbarkeit und elektrische Leitfähigkeit in konjugierten Makromolekülen

Ohne Zusammenfassung     

Influence of the structural rearrangement on the thermal and electrical conductivities of nonequilibrium thin copper films evaportated on cooled mica substrate

Using photon correlation spectroscopy we have studied the Brownian motion of polystyrene balls. The results of the spectral analysis are consistent with the predicted $\text{t}{}^{\mathrm{<mtext>-</mtext><mtext>3</mtext><mtext>2</mtext>}}$ effect for the long time behaviour of the velocity auto-correlation function.     

Multiplicative groups used for coding QAM signals

Recently, some multiplicative groups of algebraic integers wereused to obtain quadrature amplitude modulation (QAM) signalspaces and to design error-correcting codes. This paper showsthat one subgroup of the multiplicative group of units in thealgebraic integer ring of each quadratic number field with uniquefactorization property Q(m), modulo the ideal (2ⁿ), can be usedto obtain a QAM signal space of 2^2n–2 points with goodgeometrical properties, where n 3, m 1 (mod 8) and m is asquare-free rational integer. These QAM signals can be codedsuch that a differentially coherent method can be applied todemodulate the QAM signals. The multiplicative subgroups canalso be used to construct block codes over Gaussian integerswhich are able to correct some error patterns.     

Zero-Dimensional Nanodots on One-Dimensional Nanowires: Reductive Deposition of Metal Nanoparticles on Silicon Nanowires

We review herein the surface chemical properties of silicon nanowires (SiNWs) and show how SiNWs can be used as platforms in doing chemistry in the nanorealm. In particular, the surfaces of HF-treated SiNWs (which are H-terminated) exhibit interesting chemical reactivities towards reductive deposition of metal ions such as silver, copper, palladium, etc., giving rise to metal particles or aggregates on the SiNW surfaces. By varying the concentration of the metal ions in solution, nanostructures of these metals of different shapes, sizes, and morphologies can be fabricated. The reductive growth of ligated Au–Ag clusters of single size, shape, composition, and structure, on the SiNWs was also investigated. Two interesting phenomena, the sinking cluster and the cluster fusion processes, were observed by TEM. These assemblies of metal nanoparticles on silicon nanowires may be considered as zero-dimensional nanodots, on one-dimensional nanowires. It is hoped that fabrication of these metallic nanodots on silicon nanowires will lead to new and novel composite materials of importance in nanotechnology.     

Selective phosphatidylethanolamine translocation across vesicle membranes using synthetic translocases

Two sulfonamide derivatives of tris(aminoethyl)amine selectively facilitate the translocation of a fluorescent phospholipid probe containing the phosphoethanolamine head-group across vesicle membranes.     

Crystallization kinetics of polymer–diluent mixtures. Influence of benzophenone on the spherulitic growth rate of isotactic polystyrene

As an extension of earlier work on the crystallization kinetics of isotactic polystyrene, the spherulite growth rate in mixtures of isotactic polystyrene and benzophenone has been measured over a concentration range extending from pure polymer to a mixture containing about 30% benzophenone. The glass transition temperature has been measured over the entire range from pure polymer to pure benzophenone. For the mixtures the dependence of the growth rate on temperature is similar to that of the undiluted polymer. The addition of benzophenone causes a shift of the crystallization range to lower temperatures. For mixtures containing up to about 20% benzophenone, the maximum in the growth rate increases with increasing content of benzophenone. On addition of more benzophenone, the maximum rate is depressed. Taking into account the glass transition temperature of the mixtures, the influence of benzophenone on the melting point of isotactic polystyrene, and the volume fraction of polymer, we can describe the influence of benzophenone on the growth rate in a semiquantitative way.     

Modeling evolving spatial choice patterns

Over recent years the ideas emerging from the discovery of dissipative structures have been applied to the problem of modeling the dynamic evolution of urban systems. A dynamic model of a central place system was developed and tested, as well as a preliminary intraurban model involving six urban actors. Here we present a further development of this latter model, where an initial investigation is made of the evolving structures which characterize the interaction of seven types of urban actor. A transportation network of both public and private modes is introduced explicity into the model, and a combined transportation—land-use model capable of exploring structural urban changes is presented, which will be used to explore the possible effects of changing circumstances and tastes on urban structure and organization.     

Anomalous thermopower of some liquid chalcogenide semiconductors

In this paper the model used to explain the linear negative magnetoresistance of some amorphous semiconductors, in which scattering takes place as a result of interaction between electrons in extended and localised states is used to account for the differences in activation energy between conductivity and thermopower.