Properties of cerium-zirconium mixed oxides partially substituted by neodymium: Comparison with Zr-Ce-Pr-O ternary oxides

CeO₂ doped with praseodymium, neodymium and/or zirconium atoms were prepared by coprecipitation and by the sol-gel method. Structural properties were investigated by in situ XRD and Raman spectroscopy while oxygen storage capacity (OSC) was measured by transient CO oxidation. All the compounds, except pure Nd₂O₃, have a fluorite-type structure as well as a Raman band at 560 cm⁻¹ characteristic of the oxygen vacancies involving non-stoichiometric oxides. The lattice parameter under hydrogen, being dependent on the temperature, revealed two reduction mechanisms: one at a low temperature at the surface and another at a high temperature in the bulk. Ce-Nd binary oxides show a strong tendency towards crystallite aggregation, which reduces accessibility to gases and OSC properties. Zirconium improves the thermal resistance to sintering of both Ce-Nd and Ce-Pr oxides. The Zr-Ce-Pr-O followed by Zr-Ce-Nd-O compounds displaying high oxygen mobility at a low temperature, appear to be very promising for practical applications such as OSC materials.     

Determination of polyethylene glycols by precipitation with iodine

Two methods for the determination of polyethylene glycols (PEGs) in aqueous solution by precipitation with iodine have been developed. For PEGs with molecular weight 4 x 10(3)-2 x 10(4) the excess of iodine is titrated with thiosulphate, and for PEGs with average m.w. > 2 x 10(4) turbidimetric measurement is used. Both methods are relatively simple and give accurate and reproducible results.     

Encapsulation of porphyrins and chlorins in biodegradable nanoparticles: the effect of dye lipophilicity on the extravasation and the photothrombic activity. A comparative study

In the present work, we performed a preclinical inter-comparison study using several photosensitizers with the goal of optimizing photodynamic therapy (PDT) for the treatment of choroidal neovascularization (CNV) associated with age-related macular degeneration. The tested molecules were the porphyrins meso-tetraphenylporphyrin (TPP) and meso-tetra-(4-carboxyphenyl)-porphyrin (TCPP), and the chlorins pheophorbide-a (Pheo-a) and chlorin e(6) (Ce(6)). Each of these molecules was entrapped in biodegradable nanoparticles (NP) based on poly(d,l-lactic acid). The influence of the degree of lipophilicity on the incorporation efficiency of the drug in the NPs, and on the dye leakage from blood vessels as well as on the photothrombic efficiency was investigated using the chick chorioallantoic membrane (CAM) as in vivo model. NP characterization showed that the dye was more effectively entrapped in the polymeric matrix when its degree of lipophilicity increased. While less lipophilic compounds (TCPP, Ce(6)) extravasate rather easily, the more lipophilic dyes (TPP, Pheo-a) tend to remain inside the blood vessels. After injection of a drug dose of 1 mg/kg body weight and a drug-light application interval of 1 min, irradiation with light doses ranging from 5 to 20 J/cm(2) led to the highest photothrombic efficiency when using the NPs loaded with the most lipophilic molecule (TPP). The latter induced vascular damage, which was significantly higher than that observed with the other molecules tested. Thus, in addition to minimal leakage from blood vessels, the TPP in NP formulation exhibited photothrombic efficiency similar to Visudyne which was also tested in the CAM model.     

Polymerization of mono- and α,ω-dicarbanionic polystyryl-barium—II. Effect of added lithium salt in THF

The propagation rate of polystyryl-barium was studied in THF at 20°, in the presence of small amounts of lithium chloride. These kinetic results furnish a new method for the determination of triple ion formation in both mono- and α,ω-dicarbanionic polystyryl-barium. The constant of triple ion association of monocarbanionic (PS^?)₂Ba, K_T, was found to be 3.7 × 10⁵ l M^?1, close to the value calculated from published data.

The triple ion association constant of α,ω-dicarbanionic PS^2?Ba²⁺, K_DT, is about 1.2 × 10⁶ l M^?1.

Taking into account the cyclic structure of α,ω-dicarbanionic PS^2?Ba²⁺ and a statistical factor 3 between K_T and K_DT, it is concluded that mono- and dicarbanionic polystyryl-barium have similar abilities for triple ion formation. Nevertheless, stronger associations are observed for dicarbanionic oligomers with a degree of polymerization lower than 5–7.     

Anderson localization for multi-dimensional systems at large disorder or large energy

We prove that discrete Schrödinger operators on ^d with a random-potential have almost-surely only pure point spectrum and exponentially decaying eigenfunctions for large disorder or large energy. This is the first proof of localization for multi-dimensional Anderson models.Groupe de recherche 048 du CNRS     

Poly(hexamethylene terephthalate)—I. Interpretation of its diffraction patterns

X-ray diffraction, electron microscopy and electron diffraction studies of poly(hexamethylene terephthalate) reveal a variety of structures and morphologies. Polymer micro-single crystals, grown from nitrobenzene-hexane or nitrobenzene-decane solutions exist in three crystal structures. Two forms are triclinic, the third is monoclinic. The single chain triclinic unit-cell (form I) has dimensions $\text{a = 5.217, b = 5.284, c = 15.738}\text{A}\text{?}\text{(}\text{fibre axis}\text{) α = 129.4, β = 97.6, γ = 95.6°}$. The double chain triclinic unit-cell, (form II), has all dimensions and angles equal to those of form I but for b which is twice as large. There are six chains in the monoclinic unit-cell (form III) with dimensions $\text{a = 9.100, b = 17.560, c = 15.74}\text{A}\text{?}\text{(}\text{fibre axis}\text{), α = 127.8°}$. All three forms have a common value of the fibre repeat, $\text{c = 15.74}\text{A}\text{?}$, indicating that in all three forms the polyester presents itself in a fully extended conformation. The differences between the various polymorphs arise from different packing modes of the chains. In the fibre, both the monoclinic and the triclinic forms coexist.