Characterization of linear polyethylene fractions obtained by preparative GPC

A new preparative liquid-phase chromatograph, operating at a high temperature (160°C), allowed us to fractionate a linear polyethylene sample in narrow fractions (polydispersity about 1.1). These fractions, characterized by viscosity, light scattering, osmometry, and GPC measurements, can be used to calibrate any analytical GPC, i.e., to determine the variation of molecular weight and axial dispersion versus the elution volume.     

Effect of ionizing radiation on the properties of arc-induced long-period fiber gratings

We experimentally study the effect of ionizing radiation on the properties of long-period gratings fabricated in two pure-silica-core fibers with the arc-discharge technique. It is observed that the spectra of the gratings remain almost unchanged after being subjected to doses in excess of 0.5 MGy. The results also show that the gratings' temperature and strain sensitivities are not affected by gamma radiation.     

Exploration of the retinal nerve fiber layer thickness by measurement of the linear dichroism

An electro-optic device mounted on a slit lamp to assess the degree of polarization of a light beam that has double passed through the retina about the optic-nerve head in the living human eye is described. The asymmetric structure of the retinal nerve's fiber layer possesses a linear-form dichroism and will partially polarize an unpolarized light beam that is scattered at the fundus of the eye and has double passed the ocular media (cornea, lens, retina). This partial polarization is a function of the retinal nerve's fiber layer thickness, and its measurement may be used for exploring glaucoma and other retinal neuropathies. Experimental conditions allow us to neglect corneal dichroism. The first clinical measurements show a different degree of polarization between normal and glaucomatous eyes and a good correlation with the results obtained by optical coherence tomography.     

Phase contrast and DIC illumination for AFM hybrids

High-resolution optical microscopy is an essential pre-requisite for life science force microscopy, particularly for applications in cell biology and medicine. Identification and validation of cells is typically established with techniques like phase contrast microscopy or differential interference contrast microscopy. The option to select or monitor individual cells online with such light microscopy techniques while performing atomic force microscopy (AFM) measurements is therefore extremely beneficial. Here, we report two conceptually different strategies to implement these light microscopy techniques in a fully functional AFM head at the ultimate resolution of the Abbe diffraction limit.     

4C1--Nonlinear effects in an NMR experiment

We have studied multiple-photon transition in a low-field nuclear magnetic resonance (NMR) experiment. It is well known that a saturation of the NMR occurs when using a sufficiently high alternating magnetic field which induces multiple-photons transitions. We show theoretically that these multiphoton transitions are accompanied by an harmonic generation. For an-photon transition, this generation occurs mostly on thenth and (n pm 1)th harmonic. For an irradiating field2H_{1} cos omegat of pulsation ω such thatnomega= gamma H_{0}=omega_{0}where H0is the steady field and θ is the angle between H0and H1the amplitudesSmin{n}max{n}andSmin{n}max{n+1}of these components are given bySmin{n}max{n} = f_{n}(theta) (H_{1})^{n},Smin{n}max{n+1} = cos theta f_{n}(theta) (H_{1})^{n+1}. We have verified these equations in a low-field NMR experiment (H_{0} = 0.7Gs/s), polarizing first a flowing liquid in a high magnetic field. The liquid then flows in a modified Bloch spectrometer. The receiving coil is still perpendicular to the steady field but it is possible to adjust the angle θ between the axis of the emission coils (H1field) and the steady-field H0. Using a synchronous detection at the output of the receiving coil on the pulsation(n - 1) omega, nomega, or(n + 1) omega, we have directly detected multiphoton transistions. The above equations have been verified forn = 2ton = 5. There are slight discrepancies at high excitation which may be explained taking into account a large saturation.     

Émission jaune dans la cathodoluminescence des couches minces de ZnS/Cu, Cl

We have observed a yellow emission in the cathodoluminescence spectrum of ZnS/Cu, Cl thin films annealed at 250°C. The association Cu-Cl responsible for this emission is transient and unstable, since annealing at 400°C transforms the yellow centre into blue and green centres.

Thin films appear to be a useful form in which to study and follow the formation of luminescent centres.     

Segregation and conformational transitions in triblock copolymers in dilute solution: 3. Viscometric investigations in solvent mixtures

Viscometric measurements were carried out on a triblock copolymer PMMA/PS/PMMA at two temperatures in two binary solvent mixtures, in order to establish to what extent segregation between the two kinds of blocks is maintained as the composition of the solvent mixture i.e. its preferential affinity to one of the blocks, changes. From the variation of the limiting viscosity number of the polymer versus composition of the solvent mixture, taking into account the corresponding plots for the two homopolymers, it was established: that at relatively low temperature the molecular dimensions of the block copolymer are very close to those calculated neglecting the heterocontact interactions, i.e. assuming segregation, and that at higher temperatures the dimensions observed for the block copolymer are higher than the values calculated by assuming segregation, thus showing that the heterocontacts exert some influence, inducing the chain to expand. A comparison with a PS/PMMA random copolymer of the same composition showed, however, that in the same solvent mixtures the number of heterocontacts was far lower in the case of the block copolymer than in the case of the random copolymer, even at elevated temperatures.     

Internal friction and microdeformation due to the intrinsic properties of dislocations: The Bordoni relaxation

A review is presented of the relaxation phenomena due to the intrinsic properties of dislocations. The Bordoni peaks of internal friction were first observed in f.c.c. metals. Their essential features (primary and secondary characteristics) have been deduced from the ensemble of experimental results in the literature. The stability of these peaks during annealing after cold work strongly suggests that the Bordoni relaxation is linked to the intrinsic properties of dislocations. Thus, we present the principal theoretical models proposed to explain intrinsic dislocation relaxations. There are: the relaxation models based upon the thermally-activated generation of double-links, models based upon the migration of dislocation dipoles and jogs. A detailed comparison of the predictions of these models with the published experimental results leads us to the conclusion that the double-kink relaxation model is the most suitable.

The experimental results obtained on deformed h.c.p. and b.c.c. metals, ionic and covalent crystal-line materials are also reviewed. In these materials there are many relaxation phenomena attributed to dislocations, and there is a good deal of variability in the results reported from one laboratory to another. A coherent picture appears to be emerging in the case of h.c.p. and b.c.c. metals; but at present, it is difficult to attribute a precise mechanism to each internal friction peak.

In addition, studies are reviewed of microplasticity in the temperature range of the Bordoni relaxation. A correlation between the mechanisms of microplasticity and internal friction is demonstrated. This analysis yields an estimate of the Peierls stress in f.c.c. metals of about 10⁻⁴ μ.