Catalysis in the synthesis of hyperbranched polyesters

An AB2 monomer, dimethyl 5-(2-hydroxyethoxy)isophthalate was prepared on a scale of 3kg. It was successfully polymerised in the melt and the influence of reaction conditions and catalysts was studied. Structural and physical characterisation of these materials was performed using variety of analytical techniques, including SEC and MALDI-TOF MS. The polymerisations yielded high molecular weight materials with a broad polydispersity. All polymers were cyclised, the proportion of cyclised species increased with time, generally until complete cyclisation was achieved. The polymerisation process conditions were varied and a reaction protocol establishing a degree of reproducibility was achieved. Catalysts were found to promote polycondensation, cyclisation and transesterification reactions. The trends of increase in molecular weights were similar to those observed in uncatalysed reactions. Catalysts based on divalent metals showed a higher activity compared to those based on tri- and tetravalent metals. Qualitatively alcoholysis and cyclisation occurred at about the same rate whether catalysed or uncatalysed and ester-ester interchange was significantly slower under all circumstances. There were only relatively small differences between the effects of the catalysts investigated, apart from Vertec 400AC, which caused very fast reaction and yielded an insoluble product of unidentified structure. A new route to the synthesis of an A(_2)B monomer, methyl 3,5-bis(2-hydroxyethoxy)benzoate, was also reported. The monomer was synthesised on a 10g scale and successfully polymerised. The polymer samples were functionalised with acetoxy groups and characterised by SEC and MALDI-TOF MS. Evidences for the presence of polycondensation, cyclisation and ester-ester interchange reactions were observed. Growth in molecular weights and in cyclisation for poly(methyl 3,5-bis(2-hydroxyethoxy)benzoate)s was faster than that for poly(dimethyl 5-(2-hydroxyethoxy)isophthalate)s prepared under same conditions

Imagerie d'objets mobiles à l'aide d'un radar bande étroite multistatique

Cet article traite de l'imagerie de cibles mobiles à l'aide d'un radar multistatique (dans notre cas plusieurs émetteurs et un seul récepteur). Tout d'abord, nous développons un algorithme original multistatique basé sur les méthodes spatio-temporelles de Radar à Synthèse d'Ouverture (RSO). Comme le signal émis est bande étroite et que sa fréquence centrale est faible, les résolutions finales de l'image dépendent principalement de deux paramètres : le nombre d'émetteurs et la longueur de l'antenne synthétique. La fonction d'ambiguïté du système est calculé numériquement pour étudier l'influence de ces deux paramètres. Ensuite, l'algorithme développé est testé sur des cibles réalistes. Les images sont intéressantes et permettent des premiers résultats de classification. A l'aide des modèles numériques des cibles, nous montrons aussi l'importance du placement des émetteurs ainsi que la nécessité d'un second récepteur

Multistatic and Multiple Frequency Imaging Resolution Analysis-Application to GPS-Based Multistatic Radar

International audienceThis paper focuses on the computation of the generalized ambiguity function (GAF) of a multiple antennas multiple frequencies radar system (MAMF). This study provides some insights into the definition of resolution parameters of a MAMF radar system. It turns out that the range and azimuth resolutions are not the most suitable criteria to specify the MAMF radar resolution. Therefore a new set of resolution parameters is introduced like the resolution ellipse which expresses the resolution anywhere in the image plane or δ→max, (δ→min) which expresses the highest (lowest) bound of the spatial radar resolution. To point out the pertinence of our study, we illustrate it with a MAMF radar system built around GPS satellites. The effect of the radar system geometry on resolution is investigated. For several scenarios, the GAF and its numerical form, the point spread function (PSF), are computed and their results are compared

Le laboratoire d’aérothermique

The Laboratory of Aerothermics In the period between the two wars, aviation was one of the priorities of the policy for the technological and industriel development in France This priority led to the creation of research teams whose purpose was to study various problems. The team formed around Edmond Brun had as its objective the problem of the icing of airplanes. Once this problem was solved, the team was made permanent with the nomination of Brun at the University of Paris. Located at Meudon, the research group became a laboratory of CNRS in 1945. Indeed, icing in planes is just one aspect of thermal exchanges in the atmosphere. The support of the Center enabled the team to get involved in research concerning multiple facets of the core subject – phenomena belonging to a field of research combining the equations of fluid dynamics and those of thermodynamics. Alter trying out various different names for the discipline, the laboratory settled on Aerothermics. The activity of the laboratory was structured around diverse wind tunnels, instruments that can be called heavy equipment. The laboratory was able to develop several lines of research that had immediate results applicable to technology while retaining the characteristics of basic research. These research directions were developed empirically using the heavy equipment so that researchers from CNRS, the universities but also industry, did not slip into this or that disciplinary field. Thus, the diversity of a spectrum ranging from the study of aerosols to the analysis of the thermal phenomena that accompany the reentry of spacecraft into the atmosphere was justified by the unity conferred by experimental practice together with theoretical analysis. At the beginning of the 1970s, the Laboratory for Aerothermics underwent an important evolution. The development of computers made simulation possible, thus reducing the role of observation through traditional means. Transition coincided with the retirement of the “founding father'" and the appointment of anew director. His first task was to reorganize the work of the research teams, who could no longer define their work purely in relation to heavy machinery

Statistic of simulated ocean clutter

La pression diffusée par une surface rugueuse est calculée par l'intermédiaire d'un programme de simulation pour une large gamme d'angles d'incidence et d'observation. La théorie utilisée repose sur le modèle de Kirchhoff et intègre l'ensemble des caractéristiques d'une chaîne de mesure ainsi que les interactions complexes de l'onde avec la surface (phénomène d'ombre et de diffusion multiple). Les données obtenues permettent de déterminer la densité de probabilité (pdf) de l'intensité diffusée pour différentes géométries. Celle-ci est ensuite comparée avec les lois habituelles (Weibull, Lognormal et distribution K) dont les vecteurs de paramètres sont calculés avec des estimateurs du maximum de vraisemblance. La distance de Kolmogorov-Smimov est calculée afin de déterminer la loi qui semble la plus pertinente. Il est apparu que la distribution exponentielle est une bonne modélisation pour des incidences faibles. Toutefois lorsque l'incidence devient rasante et au voisinage du spéculaire, la pdf de l'intensité reçue s'écarte notablement de cette description. Dans ce cas une description de Weibull avec un paramètre de forme égale à 4 semble plus appropriée. Une étude de l'intensité diffusée dans la direction spéculaire pour une large gamme d'incidences, montre que la distribution exponentielle fournit de bons résultats tant que les phénomènes d'ombre et de diffusion multiple n'apparaissent pas. Dés lors, la distribution est alors correctement modélisée par une loi de Weibull dont le paramètre de forme dépend de l'angle d'observation

Interseismic and Postseismic Shallow Creep of the North Qaidam Thrust Faults Detected with a Multitemporal InSAR Analysis

Understanding the mechanisms by which earthquake cycles produce folding and accommodate shortening is essential to quantify the seismic potential of active faults and integrate aseismic slip within our understanding of the physical mechanisms of the long-term deformation. However, measuring such small deformation signals in mountainous areas is challenging with current space-geodesy techniques, due to the low rates of motion relative to the amplitude of the noise. Here we successfully carry out a multitemporal Interferometric Synthetic Aperture Radar analysis over the North Qaidam fold-thrust system in NE Tibet, where eight Mw> 5.2 earthquakes occurred between 2003 and 2009. We report various cases of aseismic slip uplifting the thickened crust at short wavelengths. We provide a rare example of a steep, shallow, 13-km-long and 6-km-wide afterslip signal that coincides spatially with an anticline and that continues into 2011 in response to a Mw 6.3 event in 2003. We suggest that a buried seismic slip during the 2003 earthquake has triggered both plastic an-elastic folding and aseismic slip on the shallow thrusts. We produce a first-order two-dimensional model of the postseismic surface displacements due to the 2003 earthquake and highlight a segmented slip on three fault patches that steepen approaching the surface. This study emphasizes the fundamental role of shallow aseismic slip in the long-term and permanent deformation of thrusts and folds and the potential of Interferometric Synthetic Aperture Radar for detecting and characterizing the spatiotemporal behavior of aseismic slip over large mountainous regions

Imagerie radar multistatique utilisant des émetteurs d'opportunité GPS

Cet article présente l'utilisation des satellites du système GPS comme émetteurs radar d'opportunité pour détecter et imager des objets en mouvement sur le sol terrestre. Avec l'approche multistatique, il est possible d'avoir des images de la cible avec une résolution correcte, y compris avec des signaux bande étroite. La nature multistatique du système (plusieurs émetteurs pour un récepteur) oblige à faire évoluer les techniques Radar à Synthèse d'Ouverture Inverse (RSOI, inverse car la cible est en mouvement) jusqu'à maintenant monostatiques. Nous étudierons ici la faisabilité de ce concept

Interseismic Strain Accumulation on Faults Beneath Los Angeles, California

Geodetic data show that the Los Angeles metropolitan area is undergoing 8–9 mm/year of north‐south tectonic shortening associated with the Big Bend of the San Andreas Fault. This shortening has been linked to multiple damaging twentieth century thrust earthquakes as well as possible Mw ≥ 7.0 Holocene thrust events beneath central Los Angeles. To better characterize this seismic hazard, we assess how this shortening is being accommodated by interseismic strain accumulation on subsurface faults, incorporating detailed seismology‐ and geology‐based models of fault geometry and the low‐stiffness Los Angeles sedimentary basin. We find that strain accumulation on local strike‐slip faults likely contributes no more than 1–2 mm/year of the shortening. We formally invert the geodetic data for the pattern of interseismic strain accumulation on the north dipping Sierra Madre, Puente Hills, and Compton thrust faults and a master decollement. We explore the impact of the assumed material model, strain accumulation on faults to the west and east, and other model assumptions. We infer that the three faults slip at 3–4 mm/year over the long term and are currently partially or fully locked and accruing interseismic strain on their upper sections. This locking implies an annual deficit of seismic moment, 1.6 + 1.3/−0.5 × 1017 Nm/year in total, which is presumably balanced over the long‐term average by the moment released in earthquakes. The depth distribution of moment deficit accumulation rate matches that of seismicity rates in Los Angeles to first order, in part, because the models incorporate the blind nature of the Puente Hills and Compton Faults