1,721,015 research outputs found
Conformation and crystallinity in polymers : a study using novel Roman techniques
No full textCrystallinity and conformation of biopolymers, polyamides and even numbered normal paraffins were studied using novel Raman techniques. The newly developed Fourier transform Raman method was used for characterization of biopolymers and polyamides. Low frequency modes of vibration by conventional Raman spectroscopy were investigated in terms of structure for the paraffins. The temperature dependence of the low frequency and internal mode regions were studied for even n-paraffins from C22H46 to C36H74 and for the binary mixture C28H58 + C30H62. Particular emphasis was placed on temperatures just below the melting point. The lattice packing in the crystals was found to depend on the history of the sample crystallization. As the temperature was raised up to the solid-solid phase transition, a change from two chains per unit cell to one chain was observed. The degree of order was found to decrease as the temperature increased but no evidence of the presence of defects was detected. The study of the binary mixture suggests that the chains in the mixture do not vibrate as isolated rods but rather like the longest hydrocarbon component. The FT Raman spectra of three solid homopolypeptides were acquired for the first time. These spectra show mainly sharp bands arising from the characteristic side group vibrations. The corresponding infrared spectra exhibit predominantly bands arising from secondary amide vibrations broadened by OH species absorptions. Thus identification was more readily achieved with the Raman technique. Studies in the liquid state led to the conclusion that considerable improvements in sensitivity are needed before the method becomes useful to biochemists. Aliphatic polyamides of the type nylon X and nylon X,Y were characterized by FT Raman. The spectra obtained tended towards that of polyethylene as the number of methylene group per repeat unit increases. The FT Raman study of samples of nylon 6 with different crystallinity led to the conclusion that this technique has the potential to give a quantitative measure of percentage crystallinity. Evidence is presented that FT Raman spectroscopy may be capable of distinguishing between various nylon copolymers. In contrast, it was confirmed that the infrared study of aliphatic polyamides does not provide an easy means to their identification since the strongest bands arise from the secondary amide vibrations and sample preparation in the form of film was required.</p
Study of the degradation of the insulation of HV cables with PVC insulation
No full textUne baisse constatée de la résistivité de l'isolation en PVC de certains câbles haute tension unipolaires conduit à s'interroger sur leur aptitude à assurer leur fonction. Cette étude avait pour objectif de fournir des éléments de réponse concernant en particulier l'origine de cette variation de résistivité et ses conséquences sur la tenue diélectrique. Les caractérisations ont porté sur des câbles usagés dont les propriétés avaient évolué au cours de leur utilisation. Les caractérisations physico-chimiques (spectroscopie et microscopie IR, spectroscopie UV, MEB-EDX et coulométrie) ont mis en évidence que le vieillissement du câble résultait d'un mécanisme de déshydrochloruration. La présence de deux modes de conduction dans le matériau a été observée : une conduction électronique à basse température (< -10°C) et une conduction ionique à température ambiante et au-delà. La présence de ces deux modes de conduction est compatible avec le mécanisme de déshydrochloruration. Au contraire d'une loi d'Arrhénius, les vieillissements artificiels ont mis en évidence un effet de seuil de température dans l'activation du mécanisme à l'origine de la chute de résistivité. La tenue diélectrique des câbles a été confortée par des essais à des tensions ou à des températures bien au-delà des valeurs nominales. Les mesures de calorimétrie différentielle à balayage (DSC) ont mis en évidence des surchauffes ponctuelles plus ou moins prononcées qui se corrèlent aux chutes de résistivités constatées, et peuvent donc être considérées comme étant à l'origine des évolutions constatées.The observed decrease in the resistivity of the PVC insulation of some high voltage unipolar cables led to question their ability to perform their function. Provide answers concerning in particular the origin of the variation in resistivity and the impact on the dielectric strength were the objectives of this study. The characterizations were carried on cables withdrawn from service whose properties had changed during their use. Physico-chemical characterization (IR microscopy, UV spectroscopy, SEM - EDX and coulometry) showed that aging of the cable resulted from a mechanism of dehydrochlorination. The presence of two modes of electric conduction in the material was observed: electronic conduction at a low temperature (< -10 ° C) and ionic conduction at room temperature and beyond. The presence of these two modes of conduction is consistent with the mechanism of dehydrochlorination. In contrast to an Arrhenius law, artificial aging showed a threshold effect in the thermal activation of the mechanism at the origin of the resistivity drop. The dielectric strength of the cable has been confirmed by tests at voltages or temperatures well beyond the nominal values. Measurements of differential scanning calorimetry (DSC) showed occasional more or less pronounced overheatings that correlate with the resistivity drops, and can therefore be considered as being at the origin of the observed evolutions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Study of the deformation mechanisms of porous polymer membranes for biomedical applications
No full textLe «pancréas bioartificiel» (ou MAILPAN pour Macro-encapsulation d’ILots PANcréatiques) en développement par la start-up Defymed est un implant médical destiné aux patients atteints de diabète de type I. L’élément-clé de cet implant est une membrane poreuse qui a pour fonction d’assurer une certaine sélectivité moléculaire. De ce fait, une fissuration ou rupture de cette membrane entrainerait la perte de ses fonctionnalités. Il est par conséquent indispensable d’analyser et de comprendre le comportement mécanique de ce matériau afin de garantir son intégrité tout au long de la période d’implantation. Cette thèse s’inscrit dans le projet FUI MECABARP regroupant plusieurs PMEs et laboratoires lorrains et alsaciens.La membrane est un matériau unique obtenu par lamination de plusieurs matériaux polymères poreux. Elle se compose de films rendus poreux par le procédé de «track-etching» ainsi que de non-tissé consolidé par calandrage à picots. Cette thèse a pour objectif d’en étudier les mécanismes de déformation par l’utilisation de techniques de caractérisation et d’imagerie in situ à un essai de traction. Des campagnes expérimentales de micro-tomographie à rayons X et de diffusion de rayons X aux grands angles et petits angles ont été menées sur lignes haute énergie. Ces résultats sont complétés par des essais en laboratoire de microscopie électronique et spectroscopie Raman in situ à un essai de traction. La complémentarité des techniques choisies permet une approche multi-échelles (du millimètre à l’angström) dans le but d’obtenir l’étude la plus complète possible. Les faiblesses de la membrane d'un point de vue mécanique sont mises en avant et des solutions sont proposées. En parallèle, un essai mécanique en gonflement («bulge test») est développé dans l’optique de fournir un chemin de déformation équibiaxial plus proche des sollicitations réellesThe "bioartificial pancreas" (named MAILPAN for Macro-encapsulation d’ILots PANcréatiques) developed by the startup company Defymed is an implantable device for patients diagnosed with type I diabetes. The core element of the device is a porous membrane providing molecular selectivity. The emergence of cracking in this membrane would lead to the loss of its selective properties. As a consequence, it is crucial to study and understand the mechanical behavior of this material in order to ensure its integrity during the lifetime of the device. This thesis is a part of the FUI MECABARP project, gathering together SMEs and laboratories from the Lorraine and Alsace regions. The membrane is a unique material obtained by laminating several porous polymer materials. It is made of porous “track-etched” films as well as thermal-spot bonded nonwovens. The objective is to study its deformation mechanisms using time-resolved imaging and characterization techniques during a tensile test. X-ray micro-tomography and wide- and small-angle X-ray scattering experiments were performed on high energy beamlines. These results were supplemented with time-resolved scanning electron microscopy and Raman spectroscopy experiments during a tensile test. The synergy of the chosen techniques enables a multi-scale approach (from millimeter to angström) in order to obtain the most comprehensive analysis. Solutions are suggested to improve the mechanical properties of the membrane. Besides, a mechanical testing device by inflation (“bulge test”) has been designed to provide an equibiaxial mechanical path that is closer to the actual demand
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Elaboration and characterization of a chemically modified polypropylene matrix re-inforced with functionalized GNP / graphene
No full textAu cours de la dernière décennie, la prise de conscience écologique a impacté les domaines de la recherche. Pour diminuer la production de gaz à effet de serre de nombreux changements sont à apporter aux modes de consommations. Dans le domaine des transports, plusieurs pistes de recherche sont possibles comme l’allégement des structures. L’une des pistes pour permettre cet allégement est l’élaboration de nouveaux matériaux composites métal/polymère. Le polymère de ce composite doit présenter une bonne adhésion aux surfaces métalliques, de bonnes propriétés mécaniques, et de conductivité thermoélectrique. L’objectif de ce travail de thèse est l’élaboration d’une telle matrice par l’emploi de procédé facilement transposable à l’échelle industrielle. Afin d’assurer l’adhésion aux surfaces métalliques, la matrice sélectionnée est le polypropylène greffé anhydride maléique. Cette matrice grâce à ses greffons MAH présente des propriétés chimiques intéressantes au détriment de ses propriétés mécaniques. Pour contourner ce problème, une méthode de réticulation du PP-g-MAH par polyéther amine a été développée. Plusieurs configurations de réticulation ont été réalisées par extrusion réactive. L’évolution de la réticulation suivant le type d’élaboration et son impact sur les matrices ont été déterminés. Le principal changement est le passage d’un comportement mécanique fragile à ductile. Ce changement a été analysé de manière post mortem et in situ à un essai de traction uniaxial, ce qui a permis de mettre en évidence l’impact de la réticulation sur les micro-mécanismes de déformation des matrices. Les propriétés de conductivité thermoélectrique souhaitées sont assurée l’ajout d’une nano-charge de graphite lors de l’extrusion. Afin d’améliorer la dispersion du carbone et d’assurer une percolation mécanique et électrique à faible concentration, une nouvelle méthode d’exfoliation/fonctionnalisation du graphite par décharges plasma a été mise au point. Cette méthode de fonctionnalisation a permis la diminution de l’épaisseur des nanocharges ainsi que l’oxydation de leurs surfaces sans impliquer la création de défaut de structure. Deux gammes de nanocomposites ont été réalisées à l’aide de la nanocharge initiale et traitée. Bien que de hautes teneurs en carbone aient pu être atteintes, il n’a pas été possible d’atteindre la percolation électrique. Malgré cela, une meilleure dispersion de la charge traitée a été observée.In the last decade, the environmental awareness of the need to reduce human impact on the environment affects the research sector. Reducing the fuel consumption and the human gas emissions became a priority, the humanity needs to switch to more eco-efficient patterns of consumption. In the transport field, numerous new lines of research are possible, like lighter structure. Composite materials, like multi-layered steel/polymer, appeared to be the perfect candidates to reach that objective. The polymer part of such material needs to present good shock absorption and thermo-electrical properties, and good compatibility with a metallic surface. The objective of this thesis is the elaboration and characterization of a polymer material with such properties that can be produced at industrial scale. To obtain good compatibility with the metal surfaces, the isotactic polypropylene-grafted-Maleic anhydride was selected as the initial material. This matrix presents good chemical property, its major drawback is its brittleness. A crosslinking reaction based on the reaction between MAH and amine groups was used to modify the mechanical properties of the matrix. The PP-g-MAH were crosslinked by twin-screw reactive extrusion with different configurations. The reaction affects the microstructure and the mechanical behavior of the materials, with a switch from brittle to ductile. The switch is explained by the variation of micro-mechanism of deformation implied by the crosslinked network. The solution used to modify the thermo-electrical properties of the material is the dispersion of different carbon nano-fillers such as graphite. To raise the dispersion quality of the fillers, and the compatibility between the carbon filler and the polymer matrix, a new method of functionalization of the graphite, by plasma discharge was developed. This new method implied a diminution of the filler thickness and an oxidation of the graphene surface, without any significative degradation of the fillers quality. Two types of nanocomposites were produced, one with an unmodified graphite filler and another one with a graphite modified by plasma treatment. Even with high filler concentration no electrical percolation was obtained, a high fillers exfoliation process need to be obtained. The functionalization of the graphite implied a carbon dispersion improvement, thus a better compatibility between the filler and the matrix
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