Preparation et caracterisation des polymeres tete-a-tete—VI. Proprietes physicochimiques du polystyrene tete-a-tete en solution diluee. Comparaison avec des polystyrenes de structure differente

The dilute solution properties of head-to-head polystyrene are reported. Determination of the “θ” temperature in cyclohexane has been made and compared with the “θ” temperature of the head-to-tail polystyrene (polyprotostyrene and poly-deuteriostyrene). The viscometric behaviour in a good solvent has been also examined and compared with the properties of head-to-tail polystyrenes (atactic, isotactic and deuterated).     

Role of Zr4+ cations in NO2 adsorption on Ce(1-x)Zr(x)O2 mixed oxides at ambient conditions

Mixed oxides Ce(1-x)Zr(x)O(2) prepared by slow coprecipitation in NaOH were tested for NO(2) adsorption in dynamic conditions at room temperature. The samples were characterized before and after exposure to NO(2) by XRD, N(2)-adsorption, thermal analysis, potentiometric titration, and FT-IR. Mixed oxides show a better NO(2) adsorption capacity than the parent materials (CeO(2) and Zr(OH)(4)). This effect is linked to the presence of reduced cerium and oxygen vacancies induced by the addition of Zr(4+) cations to the structure. The results indicate that NO(2) reacts with Ce(3+) to form nitrite and nitrate species on the surface. The NO retention increases with an increase in the Zr(OH)(4) content. A decrease in the density of -OH groups on the surface after the exposure to NO(2), suggests their involvement in reactive adsorption of NO and/or NO(2). From the structural point of view, no real difference was observed on the Ce(1-x)Zr(x)O(2) materials before and after exposure to NO(2).     

Self-induced transparency in diblock copolymer dispersions

We report on the versatile effect of weak red laser light impinging on diblock copolymer [poly(isoprene-b-styrene)] dispersions in two selective solvents for each block. In the strongly scattering but transparent micellar solutions in hexane (a good solvent for polyisoprene), higher refractive index copolymer-rich fibers were formed. In the turbid dispersions of the same copolymer in ethyl acetate (a good solvent for polystyrene), the effect of self-induced transparency was observed. A two-step patterning mechanism caused the generation of a transparent microchannel, increasing light transmission. The analogy between the current effect and that observed in homopolymer polyisoprene solutions in different solvents is discussed toward an understanding of the unanticipated light-soft-matter interaction.     

Effect of carbon nanotubes on the crystallization and properties of polypropylene

The effect of different concentrations of single‐walled carbon nanotubes (SWNTs) on the nonisothermal crystallization kinetics, morphology, and mechanical properties of polypropylene (PP) matrix composites obtained by melt compounding was investigated by means of X‐ray diffraction, differential scanning calorimetry, optical and scanning electron microscopy, and dynamic mechanical thermal analysis. Microscopy showed well‐dispersed nanotube ropes together with small and large aggregates. The modulus was found to increase by about 75% at a level of 0.5 wt % nanotubes. The SWNTs displayed a clear nucleating effect on the PP crystallization, favoring the α crystalline form rather than the β form. The crystallization kinetics analysis showed a significant increase in activation energy on incorporating nanotubes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2445–2453, 2005     

Hydrogen migrations in mass spectrometry. I—The loss of olefin from phenyl-n-propyl ether following electron impact ionization and chemical ionization

Using specifically labelled compounds we have made a detailed study of the source of the hydrogen transferred in the elimination of C₃H₆ from the molecular ion of phenyln-propyl ether following electron impact ionization and from the protonated (and ethylated) molecule following chemical ionization. The migrating hydrogen originates from all three positions of the npropyl group but not in the ratio expected for randomization of the alkyl hydrogens prior to transfer. The source of the migrating hydrogen is similar for both electron impact ionization and chemical ionization, indicating that the factors governing the rearrangement are the same for both modes of ionization. From a comparison of the results for labelled 2,6-dimethyl phenyl n-propyl ethers with the results for the unsubstituted ether it is concluded that hydrogen transfer occurs only to the ether oxygen and not to the phenyl ring. A two-step mechanism involving a set of competing reversible hydrogen transfer reactions followed by C? O bond cleavage is proposed to explain the results.     

Dual Electroactivity in a Covalent Organic Network with Mechanically Interlocked Pillar[5]arenes

The synthesis and characterization of a polyrotaxanated covalent organic network (CON) based on the association between the viologen and pillar[5]arene (P[5]OH) units are reported. The mechanical bond allows for the irreversible insertion of n-type redox centers (P[5]OH macrocycles) within a pristine structure based on p-type viologen redox centers. Both redox units are active on a narrow potential range and, in water, the presence of P[5]OH greatly increases the electroactivity of the material.     

4‐Benzoyl‐3,4‐dihydro‐2H‐1,4‐benzoxazine‐2‐carbonitrile: refinement using a multipolar atom model

The structural model for the title compound, C₁₆H₁₂N₂O₂, was refined using a multipolar atom model transferred from an experimental electron‐density database. The refinement showed some improvements of crystallographic statistical indices when compared with a conventional spherical neutral‐atom refinement. The title compound adopts a half‐chair conformation. The amide N atom lies almost in the plane defined by the three neighbouring C atoms. In the crystal structure, molecules are linked by weak intermolecular C—H...O and C—H...π hydrogen bonds.     

State of the art of the sphere method, a unique characterization technique for non-linear crystals

We report the sphere method as a unique characterization technique for the complete study of non-linear optical properties for frequency conversion in new materials belonging to the uniaxial or biaxial optical class. It relies on the use of a single crystal with millimetre dimensions cut as a sphere, combined with a tuneable laser source. With the sphere method we perform direct measurements of phase-matching angles and associated conversion efficiencies for second harmonic, sum- and difference-frequency generation. Furthermore, we follow the orientation of the dielectric frame as a function of the wavelength for monoclinic and triclinic crystals. It also allows the determination of the magnitude of the principal refractive indices in biaxial crystals based on the study of the double refraction affect at the exit of a sphere. By combining the analysis of all these data simultaneously, we determine Sellmeier equations reliable over the whole transparency domain and we are able to get the non-zero elements of the second-order susceptibility tensor of uniaxial or biaxial crystals. Finally, the sphere method is completely self-sufficient for the study of biaxial crystals.     

Retractions onto series-parallel posets

The poset retraction problem for a poset P is whether a given poset Q containing P as a subposet admits a retraction onto P, that is, whether there is a homomorphism from Q onto P which fixes every element of P. We study this problem for finite series-parallel posets P. We present equivalent combinatorial, algebraic, and topological charaterisations of posets for which the problem is tractable, and, for such a poset P, we describe posets admitting a retraction onto P.