Properties of aqueous salt solutions of poly(ethylene oxide)

The determination of the properties of aqueous salt solutions of poly(ethylene oxide) has been extended to cloud-point and θ-temperature measurements in sodium acetate, potassium fluoride, sodium thiosulfate, and potassium phosphate. The Hofmeister series for the decreasing effect of anion species in salting out the polymer is accordingly extended. However, the order of the effect depends on whether it is made on the basis of molar anion concentration the molar concentration of unit anion charge, or the ionic strength. Viscosity measurements on θ and non-θ solutions containing zinc sulfate, potassium fluoride, and potassium phosphate gave polymer dimensions (in addition to limiting viscosity numbers etc.), and characteristic ratios in good agreement with theoretical predictions (Abe and Mark), and enthalpy and entropy parameters χ_H and χ_s; the latter values, nominally ?0.14 and 0.63, are identical at 298 K for the three salt species.     

A nonlinear circuit model for studying the frequency response in buried heterostructure lasers

An equivalent-circuit model of a semiconductor laser is developed. The model includes diffusion and uses the parabolic gain approximation. It can be applied to various buried heterostructure (BH) devices to simulate optical response characteristics. The values of the circuit elements versus injection current clearly indicate that the parabolic gain approximation and carrier diffusion play an important role in obtaining a more accurate frequency response, especially for (BH) lasers with stripe widths greater than the diffusion length.     

RCM of tripeptide dienes containing a chiral vinylcyclopropane moiety: impact of different Ru-based catalysts on the stereochemical integrity of the macrocyclic products

[structures: see text] Tripeptide dienes containing an (1R,2S)-vinyl aminocyclopropylcarboxylate residue were cyclized to beta-strand scaffolds under ring-closing metathesis (RCM). Conformational factors, ligand effects, and reaction conditions were evaluated. A protocol was developed for the efficient synthesis of 15-membered ring peptides in high diastereomeric purity. These peptides are key synthetic precursors to antiviral agents that target the hepatitis C virus and represent the first class of clinically validated pharmaceutical agents that are synthesized in large scale using RCM.     

Solitons in the one-dimensional antiferromagnet TMMC

It is shown that magnetic solitons are expected to occur when a magnetic field is applied to a planar antiferromagnetic chain. Neutron inelastic scattering measurements have been performed at low energy on the one dimensional antiferromagnetic compound TMMC. A new fluctuation is observed, around q = π and ω = 0 which is well interpreted in terms of magnetic solitons.     

Transverse spin correlations in a random anisotropy system: Amorphous ErCo2

We provide a coherent interpretation of early small angle scattering experiments performed by some of us on amorphous ErCo₂ [9]. At low temperature the zero field transverse spin-spin correlation function is found to fit a simple exponential for large length scales (l >l_c), supporting the lower critical dimensionality d_c=4. For shorter length scales (l<l_c) the correlation function is of the Ornstein-Zernike type. These results are physically understood in terms of the breaking of ferromagnetism into Imry and Ma domains.A further physical interpretation leads us to consider the localization of ferromagnetic spin waves within Imry and Ma domains in zero field, and their delocalization by application of an external field.     

Electron energy-loss spectroscopy in the low-loss region as a characterization tool of electrode materials

Energy losses below 100 eV are by far less exploited than higher losses in electron energy-loss spectroscopy. Two new examples are given to illustrate the characterization possibilities offered by spectra in this energy range. Typical materials that could be used as electrodes in electrochemical cells were chosen as application cases. Through the use of calculations based on density functional theory, we first demonstrate that the first peak present at the lithium K edge in Li _x TiP₄ can give access to the precise localisation of inserted Li atoms in the f.c.c. structure. In particular, different tetrahedral sites could be differentiated according to their distance to the Ti site. Secondly, calculations of valence electron energy-loss spectra of perovskite materials indicate that a characteristic peak for regular perovskite (Pm m space group) exists in the 10–15 eV range. The high sensitivity of this peak to the distortion of the octahedron arrangements is also demonstrated. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Convex solubility parameters for polymers

Solubility parameters are certain measurable quantities that are observed to influence the ability of a solvent to fully dissolve a polymer. Current theory partitions the intermolecular forces between dispersion, polar, and hydrogen bonding interactions, thereby generating a three‐dimensional solubility parameter space. The Hansen solubility parameters of a polymer are taken to be the center of a sphere obtained from the best fit of the coordinates of good solvents in the parameter space. Investigations of several polymers (lignin, polyethersulfone, and bitumen) show that the convex hull of all known good solvents in the three‐dimensional parameter space also gives a meaningful interpretation of the solubility region. Several methods for computing the convex solubility parameters of a polymer from the convex solubility region are described. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1089–1097     

Revisiting lithium K and iron M₂,₃ edge superimposition: the case of lithium battery material LiFePO₄

Experimental electron energy-loss spectra are presented for FePO₄, LiFePO₄ and NaFePO₄ from 0 to 80 eV. With the help of the NaFePO₄ spectrum in the 50-80 eV range, the double peak observed in LiFePO₄ could be ascribed to the presence of Fe^II and not to the Li K edge, contrary to what was thought previously. Crystal field multiplet calculations confirm this attribution. Using VASP programme based on density functional theory, dielectric response calculations including local field effects in the Hartree approximation are then proven to properly simulate the fine structures due to the lithium K edge. By comparing absolute spectrum intensities, it is shown that the lithium K edge cannot be used to quantify lithium in such compounds. This detailed comparison between theoretical calculations and experimental spectra helps defining the relevant parameters governing intensities in the 50-80 energy range.     

Interface induced crystal structures of dioctyl-terthiophene thin films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.