1,720,986 research outputs found
Structure and properties of melt processed polypropylene/layered silicate nanocomposites
No full textCe travail de thèse porte sur les liens entre les conditions opératoires du procédé de mise en œuvre par mélange à l'état fondu et la structure de nanocomposites polypropylène/argile, ainsi que sur l'influence de l'état de dispersion de l'argile sur les propriétés mécaniques des matériaux obtenus. L'étude est basée sur des essais expérimentaux. Les analyses structurales sont réalisées en s'appuyant sur la rhéologie, la diffraction de rayons X ainsi que sur des observations en microscopie électronique.Les études en mélangeur interne ont montré, d'une part, que l'augmentation de la concentration en agent compatibilisant (PP-g-MA) favorise la dispersion de l'argile à l'échelle manométrique tout en augmentant la fragilité des nanocomposites, et d'autre part, que le mélange par voie mélange maître permet d'améliorer considérablement l'état de dispersion comparativement à la voie directe. Les essais réalisés en extrusion bivis corotative ont permis de mettre en évidence l'impact de la vitesse de rotation des vis (N), du débit d'alimentation (Q) et de la température de régulation (Trég) sur l'état de dispersion. L'influence de ces trois variables peut être décrite à l'aide d'un paramètre unique : l'énergie mécanique spécifique (EMS). L'accroissement de l'EMS entraîne une augmentation du niveau d'exfoliation jusqu'à une valeur critique au-delà de laquelle les conditions opératoires ne semblent plus influencer l'état de dispersion. Une relation entre le module de Young des nanocomposites et le niveau d'exfoliation a été établie. Le logiciel LUDOVIC© nous a permis de montrer que l'EMS permet également une bonne description de la progression de l'état de dispersion le long du profil d'extrusion. Enfin, l'étude du comportement thixotrope des nanocomposites à l'état fondu a notamment permis de comprendre que le principe de superposition temps-température ne s'applique pas systématiquement aux nanocomposites étant donné leur caractère évolutif.The present PhD work deals with the relationships between melt processing conditions and the structure of polypropylene-layered silicate nanocomposites, as well as the influence of the dispersion state on the mechanical properties of nanocomposites. This study is based on experimental results. Structural analysis are performed using rheology, X-ray diffraction and electron microscopy.Internal mixer studies revealed that the increase of compatibilizer (PP-g-MA) content simultaneously leads to enhanced nanoscale dispersion and increased brittleness of the nanocomposites. A significantly higher dispersion was stated using a masterbatch method, as opposed to direct blending method. Co-rotating twin screw extrusion experiments allowed to highlight the effect of screw rotation speed (N), feed rate (Q) and barrel temperature (TrÈg) on the degree of dispersion. The influence of these three parameters on the nanocomposites structure can be described using the specific mechanical energy (SME) as a single processing parameter. Exfoliation is clearly promoted by the increase of the SME until it reaches a critical value. Above that threshold, the degree of exfoliation levels off and improvement of the dispersion state cannot be obtained through the optimization of processing conditions anymore. A relationship between the Youngís modulus of the nanocomposites and the exfoliation level has been established. Twin screw extrusion simulation software (LUDOVIC©) was used to calculate the evolution of processing data along the screw profile. From these results, a correlation between the progression of the dispersion state along the extrusion profile and the SME was found. Eventually, rheological investigation on the thixotropic behaviour of nanocomposites enabled to emphasize that time-temperature superposition principle doesn't systematically hold true for nanocomposites because of their structure aging
Structure et propriétés de nanocomposites polypropylène/argile lamellaire préparés par mélange à l'état fondu
No full textThe present PhD work deals with the relationships between melt processing conditions and the structure of polypropylene-layered silicate nanocomposites, as well as the influence of the dispersion state on the mechanical properties of nanocomposites. This study is based on experimental results. Structural analysis are performed using rheology, X-ray diffraction and electron microscopy.Internal mixer studies revealed that the increase of compatibilizer (PP-g-MA) content simultaneously leads to enhanced nanoscale dispersion and increased brittleness of the nanocomposites. A significantly higher dispersion was stated using a masterbatch method, as opposed to direct blending method. Co-rotating twin screw extrusion experiments allowed to highlight the effect of screw rotation speed (N), feed rate (Q) and barrel temperature (TrÈg) on the degree of dispersion. The influence of these three parameters on the nanocomposites structure can be described using the specific mechanical energy (SME) as a single processing parameter. Exfoliation is clearly promoted by the increase of the SME until it reaches a critical value. Above that threshold, the degree of exfoliation levels off and improvement of the dispersion state cannot be obtained through the optimization of processing conditions anymore. A relationship between the Youngís modulus of the nanocomposites and the exfoliation level has been established. Twin screw extrusion simulation software (LUDOVIC©) was used to calculate the evolution of processing data along the screw profile. From these results, a correlation between the progression of the dispersion state along the extrusion profile and the SME was found. Eventually, rheological investigation on the thixotropic behaviour of nanocomposites enabled to emphasize that time-temperature superposition principle doesn't systematically hold true for nanocomposites because of their structure aging.Ce travail de thèse porte sur les liens entre les conditions opératoires du procédé de mise en œuvre par mélange à l'état fondu et la structure de nanocomposites polypropylène/argile, ainsi que sur l'influence de l'état de dispersion de l'argile sur les propriétés mécaniques des matériaux obtenus. L'étude est basée sur des essais expérimentaux. Les analyses structurales sont réalisées en s'appuyant sur la rhéologie, la diffraction de rayons X ainsi que sur des observations en microscopie électronique.Les études en mélangeur interne ont montré, d'une part, que l'augmentation de la concentration en agent compatibilisant (PP-g-MA) favorise la dispersion de l'argile à l'échelle manométrique tout en augmentant la fragilité des nanocomposites, et d'autre part, que le mélange par voie mélange maître permet d'améliorer considérablement l'état de dispersion comparativement à la voie directe. Les essais réalisés en extrusion bivis corotative ont permis de mettre en évidence l'impact de la vitesse de rotation des vis (N), du débit d'alimentation (Q) et de la température de régulation (Trég) sur l'état de dispersion. L'influence de ces trois variables peut être décrite à l'aide d'un paramètre unique : l'énergie mécanique spécifique (EMS). L'accroissement de l'EMS entraîne une augmentation du niveau d'exfoliation jusqu'à une valeur critique au-delà de laquelle les conditions opératoires ne semblent plus influencer l'état de dispersion. Une relation entre le module de Young des nanocomposites et le niveau d'exfoliation a été établie. Le logiciel LUDOVIC© nous a permis de montrer que l'EMS permet également une bonne description de la progression de l'état de dispersion le long du profil d'extrusion. Enfin, l'étude du comportement thixotrope des nanocomposites à l'état fondu a notamment permis de comprendre que le principe de superposition temps-température ne s'applique pas systématiquement aux nanocomposites étant donné leur caractère évolutif
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
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
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