Relationship between the polymer/silica interaction and properties of silica composite materials

Cast film composites have been prepared from aqueous polymer solutions containing nanometric silica particles. The polymers were polyvinyl alcohol (PVA), hydroxypropylmethylcellulose (HPMC) and a blend of PVA‐HPMC polymers. In the aqueous dispersions, the polymer–silica interactions were studied through adsorption isotherms. These experiments indicated that HPMC has a high affinity for silica surfaces, and can adsorb at high coverage; conversely, low affinity and low coverage were found in the case of PVA. In the films, the organization of silica particles was investigated through transmission electron microscopy (TEM) and small‐angle neutron scattering (SANS). Both methods showed that the silica particles were well‐dispersed in the HPMC films and aggregated in the PVA films. The mechanical properties of the composite films were evaluated using tensile strength measurements. Both polymers were solid materials, with a high‐elastic modulus (65 MPa for HPMC and 291 for PVA) and a low‐maximum elongation at break (0.15 mm for HPMC and 4.12 mm for PVA). In HPMC films, the presence of silica particles led to an increase in the modulus and a decrease in the stress at break. In PVA films, the modulus decreased but the stress at break increased upon adding silica. Accordingly, the polymer/silica interaction can be used to tune the mechanical properties of such composite films. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1134–1146, 2006     

Erratum to Laplacian Instability of Planar Streamer Ionization Fronts—An Example of Pulled Front Analysis

Streamer ionization fronts are pulled fronts that propagate into a linearly unstable state; the spatial decay of the initial condition of a planar front selects dynamically one specific long-time attractor out of a continuous family. A stability analysis for perturbations in the transverse direction has to take these features into account. In this paper we show how to apply the Evans function in a weighted space for this stability analysis. Zeros of the Evans function indicate the intersection of the stable and unstable manifolds; they are used to determine the eigenvalues. Within this Evans function framework, we define a numerical dynamical systems method for the calculation of the dispersion relation as an eigenvalue problem. We also derive dispersion curves for different values of the electron diffusion constant and of the electric field ahead of the front. Numerical solutions of the initial value problem confirm the eigenvalue calculations. The numerical work is complemented with an analysis of the Evans function leading to analytical expressions for the dispersion relation in the limit of small and large wave numbers. The paper concludes with a fit formula for intermediate wave numbers. This empirical fit supports the conjecture that the smallest unstable wave length of the Laplacian instability is proportional to the diffusion length that characterizes the leading edge of the pulled ionization front. G. Derks acknowledges a travel grant of the Royal Society, which initiated this research, and a visitor grant of the Dutch funding agency NWO and the NWO-mathematics cluster NDNS⁺ to finish the work. The work was also supported by a CWI PhD grant for B. Meulenbroek.     

Inherently gain flattened L band TDFA based on W-fiber design

We present here a detailed theoretical analysis for realizing an inherently gain flattened L⁺ band thulium doped fiber amplifier (TDFA), based on a depressed inner-clad (W-fiber) design, wherein the inherent gain flattening is achieved by an optimized bend induced leakage loss. The leakage loss characteristics of W-fiber have been used to suppress higher wavelength amplified stimulated emission (ASE) in the designed TDFA, which otherwise depletes the population inversion in the amplifier, making it almost impossible to obtain high gain for wavelengths in and close to the conventional L-band. It has been shown through simulations that 20 dB net gain (±0.3 dB ripple) is achievable over 32 nm bandwidth (1604-1636 nm), using this design pumped with 160 mW of power. We also show that inherent gain flattening leads to redistribution of power among signal wavelengths, and hence an inherently gain-flattened TDFA is much more efficient as compared to a configuration that uses discrete filters for gain flattening. The net gain value and gain flattening of the designed TDFA module have been tested against tolerance with respect to fiber parameters as well as bend radius. This is for the first time to the authors knowledge that inherently gain flattened L⁺ band operation has been shown using TDFAs.     

Differences in affinity of arylstilbazolium derivatives to tetraplex structures

Research into the interactions of small molecules (ligands) with DNA is a very important field of biochemistry. A ligand interacts with a DNA structure in many ways, depending on the structural features of the ligand (the presence of rings, substituent groups, length of bonds, etc.) or nucleic acid (number and association of strands, base sequence etc.). This study reports on an investigation of the preferential binding of arylstilbazolium ligands to a four-stranded DNA. For this purpose, an equilibrium dialysis was used. Equilibrium dialysis is a versatile method which enables many DNA structures to be investigated at the same time. A dozen different DNA structures of (single-stranded, double-stranded (duplex), triple-stranded (triplex), and four-stranded (tetraplex)) were involved in experiments with each ligand. Following the dissociation of DNA-ligand complexes by SDS, the concentration of the ligand bound was calculated from fluorescence and absorbance calibration curves. As a result, the amount of the ligand bound was directly related to the ligand-binding affinity. Equilibrium dialysis was used as a powerful tool to indicate which of the arylstilbazolium ligands investigated was the best therapeutic agent targeting G-quadruplex. Arylstilbazolium derivatives demonstrated strong interactions with the DNA samples used in the assay. The most interesting finding was a selective, preferential binding of anthryl derivative to c-MYC DNA (c-MYC is a DNA sequence that appears in an oncogene). Furthermore, as this derivative binds preferentially to one of the triplexes investigated, it can find an application in the TFO-triplex forming oligonucleotides which are used in gene therapy.     

Photo‐induced surface transformations of silica nanocomposites

The photo‐induced, physicochemical surface transformations to silica nanoparticle (SiNP) ‐ epoxy composites have been investigated. The silica nanocomposites (SiNCs) were prepared using a two‐part epoxy system with a 10% mass fraction of SiNPs and exposed to varying doses of high intensity, ultraviolet (UV) radiation at wavelengths representative of the solar spectrum at sea level (290 nm to 400 nm) under constant temperature and humidity. Visibly apparent physical modifications to the SiNC surface were imaged with scanning electron microscopy. Surface pitting and cracking became more apparent with increased UV exposure. Elemental and surface chemical characterization of the SiNCs was accomplished through X‐ray energy dispersive spectroscopy and X‐ray photoelectron spectroscopy, while attenuated total reflectance Fourier transform infrared spectroscopy revealed changes to the epoxy's structure. During short UV exposures, there was an increase in the epoxy's overall oxidation, which was accompanied by a slight rise in the silicon and oxygen components and a decrease in overall carbon content. The initial carbon components (e.g. aliphatic, aromatic and alcohol/ether functionalities) decreased and more highly oxidized functional groups increased until sufficiently long exposures at which point the surface composition became nearly constant. At long exposure times, the SiNC's silicon concentration increased to form a surface layer composed of approximately 75% silica (by mass). Published 2012. This article is a U.S. Government work and is in the public domain in the USA.     

Phosphahelicenes: From Chiroptical and Photophysical Properties to OLED Applications

Herein, the experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism, optical rotations, and circularly polarized luminescence. Furthermore, detailed analysis of absorption and ECD spectra performed with the help of quantum-chemical calculations allowed us to highlight general features of these helicenic phosphines. Finally, due to well-suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light-emitting diodes.     

Real Time Imaging of Quantum and Thermal Fluctuations: The Case of a Two-Level System

A quantum system in contact with a heat bath undergoes quantum transitions between energy levels upon absorption or emission of energy quanta by the bath. These transitions remain virtual unless the energy of the system is measured repeatedly, even continuously in time. Isolating the two indispensable mechanisms in competition, we describe in a synthetic way the main physical features of thermally activated quantum jumps. Using classical tools of stochastic analysis, we compute in the case of a two-level system the complete statistics of jumps and transition times in the limit when the typical measurement time is small compared to the thermal relaxation time. The emerging picture is that quantum trajectories are similar to those of a classical particle in a noisy environment, subject to transitions à la Kramer in a multi-well landscape, but with a large multiplicative noise.