Redox-active silica nanoparticles : Part 2. Photochemical hydrosilylation on a hydride modified silica particle surface for the covalent immobilization of ferrocene

The surface of spherical nonporous monodisperse silica particles (diameter: 200 nm) is covalently modified with hydride by means of hydrosilanization. A direct silicon-carbon link between the hydride modified silica surface and 10-undecylenic acid is obtained by photochemical radical initiated hydrosilylation. A ferrocene unit is attached through an amide link to the carboxylic acid modified surface. Solid-state NMR and DRIFT spectroscopy indicate success of the reactions. The electrochemical oxidation and reduction of the ferrocene modified silica particles are observed after their adsorption on a platinum electrode. The cyclic voltammograms indicate, in addition to the charge transfer between ferrocene units on the particle surface, an interparticle charge transfer.     

G-quadruplex recognition by quinacridines: a SAR, NMR, and biological study

The synthesis of a novel group of quinacridine-based ligands (MMQs) is described along with an evaluation of their G-quadruplex binding properties. A set of biophysical assays was applied to characterize their interaction with DNA quadruplexes: FRET-melting experiments and equilibrium microdialysis were used to evaluate their quadruplex affinity and their ability to discriminate quadruplexes across a broad panel of DNA structures. All data collected support the proposed model of interaction of these compounds with G-quadruplexes, which is furthermore confirmed by a solution structure determined by 2D NMR experiments. Finally, the activity of the MMQ series against tumor cell growth is reported, and the data support the potential of quadruplex-interactive compounds for use in anticancer approaches.     

Banded spherulites in poly(L‐lactic acid): Effects of the crystallization temperature and molecular weight

The spherulitic morphology of pure poly(L ‐lactide) (PLLA) was investigated with polarized optical microscopy as a function of the crystallization temperature and molecular weight. After being melted at 210°C for 3 min, samples were cooled quickly to designated temperatures for isothermal crystallization. It was shown for the first time that a clear banding‐to‐nonbanding morphological transition took place at a critical temperature for PLLA with a number‐average molecular weight of 86,000. With the increasing molecular weight of the material, the spherulite growth rates decreased notably, and the band spacing decreased significantly. On the basis of the main‐chain chirality in PLLA and the observation of a nonbanded spherulitic morphology in a certain temperature region, it was suggested that the crystallization temperature might have an effect on the relationship between the sense of lamellar twisting and the main‐chain chiral structure in PLLA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

X-ray microtomography: Application to microstructure analysis of a cementitious material during leaching process

Leaching of cementitious materials leads to an increase in porosity, which has important consequences on transport and mechanical properties. The present study outlines the characterization of microstructural evolution in a mortar subjected to a chemical attack, by means of a powerful non-intrusive experimental method, namely synchrotron X-ray computed microtomography. This innovative method is used to measure the variation of porosity in a leached mortar. Having described the effects of leaching in mortar (influences on microstructure and on mechanical properties), we present the accelerated leaching process and the microtomographic analysis that have been used to monitor it. We then investigate the capacity of this method to quantify the evolution of porosity during the leaching process. The method is validated by comparison of the results obtained with data available in the literature.     

Meeting application requirements with conductive carbon black

High structure or so‐called conductive carbon blacks (CCBs) constitute the major family of conductive additives. They enable one to make polymers permanently conductive at “low” to “very low” loadings, for technical applications involving, e.g., the transport of energy, protection against arcing or discharge. Designing conductive parts requires certain know‐how, as many parameters influence the final electrical conductivity, and many other requirements have to be fulfilled. This paper highlights the impact of the carbon black (CB) type and loading, the polymer type, and the compounding conditions on the conductive, mechanical, dispersion, and flow properties of various plastics compounds. The study especially positions a unique family of low surface area (LSA) CCBs, suggests means to make conductive parts with compounding and transformation ease, and points out that more conductive additive does not automatically give the best overall performance. J. VINYL. ADDIT. TECHNOL., 12:14–18, 2006. © 2006 Society of Plastics Engineers     

Diblock copolymer stabilization of multi-wall carbon nanotubes in organic solvents and their use in composites

A versatile method for the preparation of dispersed nanotubes using polystyrene-b-polyisoprene diblock copolymers in different selective organic solvents is presented. Stable dispersions have been obtained in polar (DMF) and apolar (heptane) media depending on the selectivity of the diblock copolymers. They have been characterized by means of optical microscopy, TEM imaging and dynamic light scattering, showing the first demonstration of multiwall carbon nanotubes (MWCNTs) solutions with in situ characterization of diblock copolymer stabilization. The most effectively stabilized dispersions have been used to make nanotube/polystyrene composites. We find that the coating of the nanotubes by the diblock polymer does not prevent electrical transport, so that the system can exhibit a relatively high surface conductivity above the percolation threshold. The low percolation threshold experimentally determined is presumably due to weak attractive interactions between the nanotubes as the composites are dried.     

Synthesis and Electrical Characterization of Thin Films of PT and PZT Made from a Diol-Based Sol-Gel Route

Lead titanate (PT) sols were prepared using propanediol, butanediol, or pentanediol solutions of lead acetate trihydrate and titanium diisopropoxide bis(acetylacetonate). Precursor sols for PbZr_0.53Ti_0.47O₃ (PZT) films were prepared from propanediol solutions, with zirconium tetrapropoxide being used as the zirconium source. Films were formed by spin-coating the sols onto silicon and platinized silicon substrates; the resulting gel layers were converted to ceramic films by adopting a two-stage heating schedule with final firing temperatures of 600–700°C. Information on film crystallization, microstructure development, and electrical properties is presented for both compositions. The limiting thickness of surface-smooth crack-free single-layer films was ∼1 μm. The PT films exhibited a "linear" polarization-electric field ( P-E ) response, while the PZT films gave rise to characteristic ferroelectric P-E hysteresis loops. A 0.5 μm thick single-layer PZT film exhibited remanent polarization (P_r) values of ∼34 μC·Cm⁻², with a coercive field ( E _c) of ∼45 kV·Cm⁻²; the relative permittivity (ɛ_r) and the dissipation factor ( D ) were ∼1250 and 0.07. For a 1 μm single-layer PZT film, the respective values were P _r∼19 μC∼Cm⁻², E _c∼40 kV∼Cm⁻², ɛ_r∼750, and D = 0.03.     

Nanometer-resolution electron microscopy through micrometers-thick water layers

Scanning transmission electron microscopy (STEM) was used to image gold nanoparticles on top of and below saline water layers of several micrometers thickness. The smallest gold nanoparticles studied had diameters of 1.4 nm and were visible for a liquid thickness of up to 3.3 μm. The imaging of gold nanoparticles below several micrometers of liquid was limited by broadening of the electron probe caused by scattering of the electron beam in the liquid. The experimental data corresponded to analytical models of the resolution and of the electron probe broadening as function of the liquid thickness. The results were also compared with Monte Carlo simulations of the STEM imaging on modeled specimens of similar geometry and composition as used for the experiments. Applications of STEM imaging in liquid can be found in cell biology, e.g., to study tagged proteins in whole eukaryotic cells in liquid and in materials science to study the interaction of solid:liquid interfaces at the nanoscale.     

Low wear metal sliding electrical contacts at high current density

Operation of a low wear (2 × 10^?5 mm³/(N-m)), low contact resistance copper sliding electrical contact was demonstrated. The wear rate of a lightly loaded copper–beryllium metal fiber sliding on a polished copper counterface was insensitive to (DC) current density values as great as 440 A/cm² (in a brush positive or anodic configuration). Low wear and relatively low friction (μ ～ 0.2 to 0.3) was achieved by operating the contact immersed in a liquid medium consisting of a hydrofluoroether with helium cover gas, inhibitingoxidationand providing cooling of the contact. Similar experiments performed in liquid mediums of ultrapure water and dilute (3%) hydrogen peroxide show an order of magnitude increase in wear rate and provide further insight on the role of electrochemically enhanced oxidation and the degraded contact resistance and tribological behavior of non-noble sliding electrical contacts in general. In contrast to high current density slidingin hydrofluoroether, an order of magnitude greater wear rate was observed for similar sliding conditionsin hydrogen peroxide or water without the aid of externally supplied electric potential. A conceptual model is proposed correlatingthe rate of brush wear to fatigue strength and electrochemically enhanced oxidation as a result of high current density transport through the contact. A mathematical expression was derived to calculate the approximate wear volume of a single fiber laterally contacting a slip-ring, based on direct measurement of the wear scar geometry.     

The magnetic-distortion probe: velocimetry in conducting fluids

A new type of velocimeter, capable of local velocity measurements in conducting fluids, is introduced. The principle of the "magnetic-distortion probe" is based on the measurement of the induced magnetic field by the flow of a conducting fluid in the vicinity of a localized magnetic field. The new velocimeter has no moving parts, and can be enclosed in a sealed cap, easing the implementation in harsh environments, such as liquid metals. The proposed method allows one to probe both the continuous part and fluctuations of the velocity, the temporal and spatial resolution being linked to the actual geometric configuration of the probe. A prototype probe has been tested in a gallinstan pipe flow and in a fully turbulent flow of liquid gallium generated by the counter rotation of two coaxial impellers in a cylinder. The signals have been compared to a reference potential probe and show very good agreement both for time-averaged velocities and turbulent fluctuations. The prototype is shown to detect motion from a few cm s(-1) to a few m s(-1). Moreover, the use of the magnetic-distortion probe with large-scale applied magnetic field is discussed.