DSC And X-Ray Diffraction Coupling

A new technique, that allows simultaneous time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) and high sensitivity DSC to be carried out in the same apparatus, has been developed. Microcalorimetry and XRDT scans can be performed at any rate between 0.01 and 10°C min^?1 with a 0.01°C temperature resolution in the temperature range, 30–130°C and at lower cooling rates but the same heating rates in the ?30–+30°C range. The use of a single and very small sample (1 to 20 μl) contained in a thin glass capillary for both measurements and simultaneous data collection prevents any temperature shift between recordings and any possible difference in the thermal histories of the samples.     

DSC and high resolution X-ray diffraction coupling

Adaptive or smart hybrid composites consisting of a polymer matrix reinforced by aramid fibres and incorporating pre-strained Shape Memory Alloy (SMA) wires are able to tune some of their properties, such as their shape, the natural vibration frequency or the damping coefficient, in response to an external stimulus. The functional properties of these systems are directly related to the reversible martensitic transformation in the SMA elements. In this work the transformational behaviour of both free SMA wires and SMA wires embedded in polymer matrix is investigated by means of DSC. The martensitic transformation of the constrained wires is impeded by the polymer matrix, while the interface integrity plays a crucial role.     

Understanding coherent vortices through computational fluid dynamics

A new definition of coherent vortices in turbulence is proposed, where the vorticity equation reduces to a cyclostrophic balance. Afterward, we describe five fundamental vortex interactions, the sheet, the spiral, the pairing, the even longitudinal, and the odd longitudinal modes. Numerous examples of these interactions are provided from direct numerical or large-eddy simulations. The resulting vortices are responsible for the internal intermittent character of turbulence, with highly nongaussian tails for the probability density functions of vorticity, passive scalar, and low pressure. In a mixing layer, the combination of the odd longitudinal and the pairing modes (helical pairing) is inhibited by compressibility, above a convective Mach number of 0.7. When turbulence is submitted to a solid-body rotation, anticyclonic vortices of local Rossby number of the order of 1 transform into intense perpendicular Görtler-type alternate longitudinal vortices.The support of CCVR, CEA, CNRS, Dassault/CNES, DRET, LHF, and Région Rhône-Alpes is acknowledged. This paper is the text of an invited lecture given at the IUTAM Symposium on Eddy Structure Identification in Free Turbulent Shear Flows, Poitiers, 12–14 October 1992.     

Regioselective Synthesis of Benzimidazolic Bioisosteres of Melatonin

A convenient synthesis of benzimidazolic bioisosteres of melatonin is described through a regioselective synthesis of 1,6-disubstituted benzimidazoles.     

Rapid cooling of lipid in a prilling tower Theoretical considerations and consequences on the structure of the microspheres

Two lipid binders, glyceryl behenate and paraffin wax, were examined regarding their ability to be used in a prilling process. Prilling has the advantage to produce microgranules very reproducible in size and shape but involves ultrafast cooling of liquid droplets. The different steps to produce solid micropheres from the molten state were successfully modelled to predict crystallisation time as a function of the binder used. Bulk versus microgranules characterization by differential scanning calorimetry, X-ray diffraction and microscopies showed the peculiar suitability of the 50/50 mixture of the two lipid binders for prilling, in agreement with the theoretical approach.     

Marie-Louise Dubreil-Jacotin, 1905–1972

Sans résumé

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Marie-Louise Dubreil-Jacotin, 1905–1972

     

Thermal and structural behavior of milk fat. 3. Influence of cooling rate and droplet size on cream crystallization

Crystallization of triacylglycerols (TG) within milk fat globules of creams is studied with an instrument coupling time-resolved synchrotron X-ray diffraction (XRDT) at both small and wide angles and high-sensitivity differential scanning calorimetry (DSC) at cooling rates of -3 and -1 degrees C/min from 60 to -10 degrees C and compared to that of the anhydrous milk fat (AMF). Simultaneous thermal analysis permits correlation of the formation of the different crystalline species monitored by XRDT to the DSC events. Under the above cooling conditions, milk fat TG sequentially crystallize, within the globules, from about 19 degrees C, in three different lamellar structures with double-chain length (2L) stackings of 47 and 42 A and a triple-chain length (3L) stacking of 71 A, all of alpha type, which are correlated to two or three overlapped exothermic peaks recorded by DSC. Compared to what is observed for AMF, TG crystallization in emulsion (i) favors sub-alpha formation at low temperature and (ii) induces layer stacking defects in 3L crystals. Subsequent heating at 2 degrees C/min shows numerous structural rearrangements before final melting, confirming that (i) cooling at -1 degrees C/min leads to the formation of unstable crystalline varieties in the dispersed state and (ii) a monotropic transition alpha-->beta' takes place. Similar behavior is observed for cooling at -3 degrees C/min and subsequent heating. An overall comparison of the thermal and structural properties of the crystalline species formed as a function of the cooling rate, between >1000 and 0.15 degrees C/min, and stabilization time at 4 degrees C is given. Depending on the cooling rate, at least five crystalline subcell species are observed at wide angles, alpha and sub-alpha, two beta' and one beta. At small angles, at least six lamellar stackings are identified, three 3L and three 2L. However, a single subcell packing (e.g., alpha) might correspond to several longitudinal chain stackings, demonstrating the usefulness of the small-angle XRD technique. Reconstituted emulsions homogenized under different pressures are used to determine the influence of droplet size on crystallization. The decrease of droplet size induces (i) a higher supercooling/supersaturation and (ii) a higher disorder and/or a smaller size of TG crystals within the emulsion droplets. At the supramolecular scale, polarized light microscopy shows that various cooling rates applied in situ using a temperature-controlled stage directly influence crystal sizes and their type of organization within milk fat globules. The faster the cooling rate, the smaller the size of the crystals within the globules.     

A Versatile Palladium Synthon: [Pd(NHC)(PhC≡CPh)]**

The synthesis and isolation of [Pd(NHC)(PhC≡CPh)] complexes are reported. These new 14-electron Pd(0)-complexes are key synthons leading to known palladium(0) and palladium(II) species, as well as permitting access to unprecedented mixed NHC-phosphite palladium(0) complexes. This motif permits the facile catalytic hydrosilylation of allenes. DFT calculations have allowed the characterization of the relatively weak interaction between the metal and the diphenylacetylene ligand, with a comparison with a series of ligands with more or less coordinating power, bearing varied structural and electronic properties.