Radiation-induced crosslinking of polyethylene in the presence of acetylene: A gel fraction,UV-visible,and ESR spectroscopy study

Electron-spin resonance (ESR) and altraviolet (UV) visible spectroscopic evidence has been found for the formation of diene, triene and tetraene, following the irradiation of polyethylene in the presence of acetylene. The polyenes are formed by a mechanism which is different from that observed under vacuum or with inert gas saturation. The sum of the G (polyene) values obtained by UV spectroscopy is almost half that of initial radical formation. It is concluded that polyene bridges, predominantly diene, form crosslinks between radical pairs. G (X) values determined from gel fraction data, using Saito-Kang-Dole theory, are found to be greatly in error and misrepresentative of crosslink changes. © 1993 John Wiley & Sons, Inc.     

Identification of the relative distribution of rare-earth ions in phosphate glasses

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl(0.30)P(3.05)O(9.62) was measured by employing the method of isomorphic substitution in neutron diffraction. It is found that 7.9(7) R-R nearest neighbors reside at 5.62(6) A in a network made from interlinked PO4 tetrahedra. Provided that the role of Al is explicitly considered, a self-consistent account of the local matrix atom correlations can be developed in which there are 1.68(9) bridging and 2.32(9) terminal oxygen atoms per phosphorus.     

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction φ = 20% and a concentrated system with φ = 75%. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data, thus, raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in the light of recent works on soft glassy materials. Received 1 November 2002 and Received in final form 8 January 2003 / Published online: 1 April 2003 RID="a" ID="a"e-mail: salmon@crpp.u-bordeaux.fr     

Magnetic properties of the uranium sesquichalcogenides

Magnetic measurements on the orthorhombic uranium sesquichalcogenides and orthorhombic U₃Te₄ were performed in the temperature range 4.2–1000 K and in magnetic fields up to 140 kOe. The temperature dependence of the reciprocal susceptibility was found to be curvilinear for all the investigated compounds. At low temperatures U₂Te₃ and U₃Te appeared to be antiferromagnetic below 106 K. U₂Se₃ and U₂S₃ seem to have a more considerable ferromagnetic contribution below 60 and 80 K, respectively. The magnetic properties are discussed in terms of crystal structure and related properties of the investigated compounds.     

Integral method for the Stokes problemMéthode intégrale pour le problème de Stokes

We consider the bidimensional Stokes problem for incompressible fluids in stream function-vorticity formulation. For this problem, the classical finite elements method of degree one converges only in $\text{O}\text{(}\text{h}\text{)}$ for the quadratic norm of the vorticity, if the domain is convex and the solution regular. We propose to use harmonic functions obtained by a simple layer potential to approach vorticity along the boundary. Numerical results are very satisfying and we prove that this new numerical scheme leads to an error of order $\text{O}\text{(h)}$ for the natural norm of the vorticity and under more regularity assumptions from $\text{O}\text{(h}{}^{\mathrm{3/2}}\text{)}$ to $\text{O}\text{(h}{}^2\text{)}$ for the quadratic norm of the vorticity. To cite this article: T. Abboud et al., C. R. Acad. Sci. Paris, Ser. I 334 (2002) 71–76     

The NMR spectra and conformations of cyclic compounds?V: Proton couplings and chemical shifts in bridged cyclobutanes

The PMR spectra of twelve pinene derivatives are reported, analysed and assigned. The proton couplings in the bridged cyclobutane group are compared with those of other strained cyclobutanes, and the relationship between ²J_HH and the C.CH₂.C angle is shown to be anomalous in these systems, suggesting unusually small H. C. H. angles in cyclobutanes. The very large values of ⁴J_HH (eq-eq) in buckled cyclobutanes are interpreted in terms of current M. O. theory and also given a simple geometric rationalisation based on the direct mechanism. The various couplings in the pinene skeleton are discussed in terms of present theories and minor conformational effects in these molecules. Substituent chemical shift (SCS) values for Me and OH groups around the pinene skeleton are obtained, and shown not to agree with calculations based on present theories of chemical shifts.     

Multiple filamentation of terawatt laser pulses in air

The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW. Beam propagation is shown to be driven by the interplay between intense, robust spikes created by the defects of the input beam and random nucleation of light cells. Evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time (2D+1)-dimensional model derived from the propagation equations for ultrashort pulses.