A radio-tracer study of self-diffusion in the smectic phases iso-butyl 4-(4′-phenylbenzylideneamino) cinnamate [IBPBAC]

Average translational self-diffusion coefficients, , have been determined in the smectic E, B and A phases of unaligned samples of iso-butyl 4-(4′-phenylbenzylideneamino) cinnamate. A radiotracer technique was employed which was based on the serial sectioning method used in the study of diffusion in solids. In the smectic E phase is about 1 × 10^-14 m² s^-1. The self-diffusion in the smectic B phase fits reasonably well to an Arrhenius law with an activation enthalpy of 82 kJ mol^-1. The transition from the smectic B to A phase results in a dramatic rise in . In the smectic A phase the results fit an activation enthalpy for self-diffusion of about 168 kJ mol^-1. Similarities between the diffusion in the smectic E and B phases and in plastic solids are observed.     

Restricting the High-Temperature Growth of Nanocrystalline Tin Oxide

The sensitivity of tin oxide is dependent on various factors, one of which is the grain size. Three methods have been investigated with the aim of stabilising the grain size in the nanometer range, namely; (i) encapsulation within a silica matrix, (ii) coating the crystallites with hexamethyldisilazane and (iii) pinning the grain boundaries with a second metal oxide nanocrystal. The resulting materials have been characterised by X-ray powder diffraction (XRPD), Extended X-ray absorption fine structure (EXAFS) and conductivity measurements.     

Limits on an energy dependence of the speed of light from a flare of the active galaxy PKS 2155-304

In the past few decades, several models have predicted an energy dependence of the speed of light in the context of quantum gravity. For cosmological sources such as active galaxies, this minuscule effect can add up to measurable photon-energy dependent time lags. In this Letter a search for such time lags during the High Energy Stereoscopic System observations of the exceptional very high energy flare of the active galaxy PKS 2155-304 on 28 July 2006 is presented. Since no significant time lag is found, lower limits on the energy scale of speed of light modifications are derived.     

The applications of X-ray absorption spectroscopy in the study of solid state ionics

X-Ray absorption spectroscopy is a powerful method of determining local structures in condensed matter. This article reviews the applications of this technique in the study of solid state ionic materials. It begins with an outline of the principles and the experimental procedures followed by a brief overview of some of the recent developments. The bulk of the article is focused on a discussion of examples chosen to emphasise the kinds of problem that can be resolved using the technique. These include studies of local structure in perovskites-structured materials (BaLiF₃), proton conducting ceramics (CaZrO₃), oxygen ion conductors (BIMEVOX) and semiconducting gas sensors (SnO₂). Each system has been selected to show various facets of the technique in terms of the experimental method and data analysis. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Characteristics of Bimodal Polyethylene Prepared via Co‐Immobilization of Chromium and Iron Catalysts on an MgCl2‐Based Support

Bimodal polyethylenes comprising varying proportions of high‐ and low‐molecular‐weight fractions are synthesized in a single polymerization stage, via the co‐immobilization of a chromium and an iron catalyst on an MgCl₂/AlEt_n(OEt)_3?n support. Changes observed in the viscoelastic response of the polymer melt with increasing content of the high‐molecular‐weight fraction indicate effective mixing in the bimodal blend. In flow, chains in the high‐molecular weight fraction tend to orient and stretch under shear. Due to the longer relaxation time of the high‐molecular‐weight component, X‐ray diffraction and scattering reveal that shear‐induced crystallization takes place at temperatures close to the equilibrium melting point of linear polyethylene. The so‐crystallized high‐molecular‐weight component suppresses the nucleation barrier for further crystallization, leading to the formation of a “shish‐kebab” polymer morphology.

     

Synthesis of ordered mesoporous Fe3O4 and gamma-Fe2O3 with crystalline walls using post-template reduction/oxidation

Ordered mesoporous Fe(3)O(4) with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous alpha-Fe(2)O(3) (corundum structure) to Fe(3)O(4) spinel then to gamma-Fe(2)O(3) by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of alpha-Fe(2)O(3) (corundum structure) to the cubic close packed subarray of Fe(3)O(4) spinel and gamma-Fe(2)O(3). Preliminary magnetic measurements reveal that the spins in both Fe(3)O(4) and gamma-Fe(2)O(3) are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (>8 nm).     

Click Functionalization of a Dibenzocyclooctyne‐Containing Conjugated Polyimine

A conjugated poly(phenyl‐co‐dibenzocyclooctyne) Schiff‐base polymer, prepared through polycondensation of dibenzocyclooctyne bisamine (DIBO‐(NH₂)₂) with bis(hexadecyloxy)phenyldialdehyde, is reported. The resulting polymer, which has a high molecular weight (M_n>30 kDa, M_w>60 kDa), undergoes efficient strain‐promoted alkyne–azide cycloaddition reactions with a series of azides. This enables quantitative modification of each repeat unit within the polymer backbone and the rapid synthesis of a conjugated polymer library with widely different substituents but a consistent degree of polymerization (DP). Kinetic studies show a second‐order reaction rate constant that is consistent with monomeric dibenzocyclooctynes. Grafting with azide‐terminated polystyrene and polyethylene glycol monomethyl ether chains of varying molecular weight resulted in the efficient syntheses of a series of graft copolymers with a conjugated backbone and maximal graft density.     

A Preparation Method for Well‐Defined Crystallites of Mgcl2‐Supported Ziegler‐Natta Catalysts and their Observation by AFM and SEM

A method for the preparation of well‐defined crystallites of MgCl₂‐supported Ziegler‐Natta catalysts on Si wafers has been developed. This has been achieved by the spin‐coating of a MgCl₂ solution onto a flat Si wafer, followed by controlled crystal growth to give well‐defined MgCl₂ · nEtOH crystallites. The growth of the crystallites on the flat silica facilitates their characterization using electron and scanning probe microscopy. The relative proportions of 120° and 90° edge angles indicate the preference for the formation of a particular crystallite face for the MgCl₂. Polyethylene has been identified to be formed on the lateral faces of the crystallite.

     

Local atomic and electronic structure in nanocrystalline Sn-doped anatase TiO2.

Tin-doped anatase TiO(2) nanopowders and nanoceramics with particle sizes between 12 and 30 nm are investigated by X-ray absorption fine-structure (EXAFS) and M?ssbauer spectroscopies. Furthermore, ab initio calculations based on the density functional theory are performed to analyze changes in the electronic structure due to Sn doping. The three approaches consistently show that Sn is dissolved on substitutional bulk sites with a slight increase of the bond lengths of the inner coordination shells. The Debye-Waller factors show that the nanocrystallites are highly ordered. There is no indication of defect states or bandgap changes with Sn doping.