Optical Gain of 1550-nm Range Multiple-Quantum-Well Heterostructures and Limiting Modulation Frequencies of Vertical-Cavity Surface-Emitting Lasers Based on Them

Results of investigation of 1550 nm range stripe semiconductor lasers fabricated from heterostructures with different designs of the gain medium are presented. It is shown that the proposed designs of the gain medium allow obtaining the effective lasing at high level of total optical losses, comparable with the typical optical losses in the vertical-cavity surface-emitting lasers. The evaluation of modal gain in different types of the gain mediummade it possible to estimate the possible frequencies of the small-signal modulation of vertically emitting lasers and proposed the ways to increase them up to 20 GHz or more.     

Small strain behavior of polyethylene: In situ SAXS measurements

Stack lamella deformation depends on their orientation with respect to the loading axis, the intrinsic properties of the lamellae, and the mechanical coupling between crystalline and amorphous phases. The aim of this work is to investigate the influence of the stress transmitter (ST) density and the crystallinity X_c on the local deformation. A wide experimental campaign has been undertaken on several polyethylenes with controlled molecular parameters and subjected to different thermal treatments. The ST density has been evaluated by the natural draw ratio and calculated by the Brown's model. The local deformation was measured by SAXS along a tensile test by using the long period stretching of the equatorial lamella stacks. The ratio ε_local/ε_macro was found to be a constant close to 0.5. This surprising low value has highlighted that the equatorial regions could be either the stiffest zone of the spherulite or submitted to a lower stress. It is proposed that the stability of the ratio ε_local/ε_macro is the result of two opposite phenomena: On one hand, the increase of X_c leads to unload the equatorial regions due to partial percolation of the crystalline phase and so decreases the stresses. On the other hand, when increasing X_c, the ST density decreases which causes the decrease in the local equatorial modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1535–1542, 2010     

Structure and mechanical behavior of nylon 6 fibers filled with organic and mineral nanoparticles. II. In situ study of deformation mechanisms

This study reports on the in situ characterization of the deformation mechanisms at room temperature of polyamide 6 (PA6) fibers filled with hyperbranched molecules or montmorillonite (MMT) platelets. A small‐angle X‐ray scattering study shows that the stretching and sliding of the microfibrils takes place concomitantly in the first stage of elastic loading of as‐spun and partially drawn fibers. In the second stage of loading, which is basically plastic, sliding turns out to be the main process of deformation, accompanied by a significant reduction in the microfibril radius. Fibers drawn close to their maximum draw ratio only display the deformation process of microfibril stretching. This in situ study also reveals subtle features of the reversible processes of deformation that could not be detected from ex situ experiments reported previously. A thickening of the crystal blocks in the microfibrils takes place under stress and disappears upon unloading, indicating that some reversible strain‐induced molecular ordering occurs in the amorphous layers close to the crystal surface. The tentative mechanical modeling enabled a characterization of the components of the fibers: the stiffness of the microfibrils appears to be insensitive to the presence of the particles that are excluded in the interfibrillar regions. The presence of HB molecules clearly increases the stiffness of the interfibrillar regions owing to a physical crosslinking effect. Moreover, it seems that the stiffness improvement of the drawn MMT‐PA6 fiber lies in a greater capability of chain unfolding in the interfibrillar amorphous region. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2633–2648, 2004     

Anomalous small angle x-ray scattering determination of ion distribution around a polyelectrolyte biopolymer in salt solution

The distribution of counterions in solutions of high molecular mass hyaluronic acid, in near-physiological conditions where mono- and divalent ions are simultaneously present, is studied by small angle neutron scattering and anomalous small angle x-ray scattering. The solutions contain either sodium or rubidium chloride together with varying concentrations of calcium or strontium chloride. The effects of monovalent-divalent ion exchange dominate the amplitude and the form of the counterion cloud. In the absence of divalent ions, the shape of the anomalous scattering signal from the monovalent ions is consistent with the distribution calculated from the Poisson-Boltzmann equation, as found by other workers. In mixtures of monovalent and divalent ions, however, as the divalent ion concentration increases, both the diameter and the amplitude of the monovalent ion cloud decrease. The divalent counterions always occupy the immediate neighborhood of the charged polyanion. Above a given concentration their anomalous scattering signal saturates. Even in a large excess of divalent ions, ion exchange is incomplete.     

Ordering in Bio-Polyelectrolyte Chitosan Solutions

The scaling of the polyelectrolyte scattering peak in chitosan solutions, as deduced from the relation q_max∼c_p^α was studied by synchrotron SAXS as a function of the charge density of the polymer. We observe a variation in the α exponent corresponding to the limit of the ionic condensation, by varying the degree of acetylation of the polymer. The nature of the solution medium also affects the polyelectrolyte peak, and it is shown that in alcoholic/water mixtures, the lower dissociation of the acid induces a lower charge density, thus influencing the polyelectrolyte ordering.     

Ternary complexes in gels from agarose and from chemically‐modified agarose

Thermodynamic data and mechanical measurements are shown for gels prepared in aqueous binary solvents (water/DMSO, water/DMF, water/methyl formamide and water/formamide). When electrostatic interactions, as opposed to hydrogen bonding, can be established with the cosolvent (DMSO, DMF, methyl formamide) we come to the conclusion that ternary complexes are formed (agarose/water/cosolvent). In the case of chemically‐modified agarose (OH groups replaced by OCH₃ groups) we suggest that these cosolvents are directly involved in the formation of the gel.     

Aqueous GPC of electrolytes and polyelectrolytes

The gel permeation chromatography (GPC) of polyelectrolytes is discussed. The electrostatic exclusion is demonstrated using simple electrolytes with different eluents. The data for aqueous free salt solutions of polyelectrolytes are interpreted. The osmotic coefficient is deduced and its dependence on molecular weight and polymer concentration reported. In salt eluents, the Donnan exclusion is discussed. The universal calibration obtained with different standards in aqueous solution is shown to be valid and identical to that obtained in an organic solvent using polystyrene standards.     

Conformational transition of kappa-carrageenan

The calorimetry of kappa carrageenan using differential scanning calorimetry and calorimetry at constant temperature is presented. Both sets of experimental results confirmed the formation of a helical dimer in the absence of gel formation. When gel is formed, the formation of a helical dimer is the major effect, the contribution of gelation represents only 20% of the enthalpy recorded.

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Zusammenfassung Es wird über die Kalorimetrie von Kappacarrageen mittels DSC und Kalorimetrie bei konstanter Temperatur berichtet. Die experimentellen Resultate bestätigen beim Ausbleiben der Gelbildung, daß ein helixartiges Dimer gebildet wird. Auch bei Gelbildung steuert die Bildung des helixartigen Dimers den Hauptanteil zum thermischen Effekt bei, der Beitrag der Gelierung stellt nur etwa 20% der registrierten Enthalpie dar.

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Influence of surface chemistry on the SAXS response of polymer-based activated carbons

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.     

Agarose Sols and Gels Revisited

Agarose sols have been seen for long as solutions of flexible chains that, on cooling, produce thermoreversible gels through double-helix formation. Investigations of the chain conformation in the sol state by small-angle neutron scattering reveals instead a rigid chain with a very large persistence length (l_p > 9 nm). The chain cross-section radius and mass per unit length correspond to characteristics of helices as those described by Foord and Atkins. These results lead one to a reappraisal of the occurrence of double helices in the gelation process, as they rather suggest a transition of the type loose-single helix⇒tight single helix. Studies of gels from agarose/water/cosolvent where the cosolvent is Dimethyl Sulfoxide (DMSO), Dimethyl Formamide (DMF), and Methyl Formamide (MF) have led one to conclude on the formation of agarose/water/ cosolvent ternary complexes. The contrast variation method by neutron scattering gives further support to this assumption. Finally, determination of the gel nanostructure allows one to account for the two regimes observed for the variation of the elastic modulus vs concentration.