1,720,958 research outputs found
Morphological and Thermal Properties of Cellulose-Montmorillonite Nanocomposites
No full textCellulose-layered montmorillonite (MMT) nanocomposites were prepared by precipitation from N-methylmorpholine-
N-oxide (NMMO)/water solutions. Two hybrid samples were obtained to investigate the influence of the
reaction time on the extent of clay dispersion within the matrix. It was observed that longer contact times are
needed to yield nanocomposites with a partially exfoliated morphology. The thermal and thermal oxidative properties
of the hybrids, which might be of interest for fire-resistant final products, were investigated by thermogravimetry
and chemiluminescence (CL). The nanocomposites exhibited increased degradation temperatures compared to
plain cellulose, and the partially exfoliated sample showed the maximum stability. This result was explained in
terms of hindered transfer of heat, oxygen, and degraded volatiles due to the homogeneously dispersed clay filler.
Kinetic analysis of the decomposition process showed that the degradation of regenerated cellulose and cellulosebased
hybrids occurred through a multistep mechanism. Moreover, the presence of nanoclay led to drastic changes
in the dependence of the activation energy on the degree of degradation. CL analysis showed that longer permanence
in NMMO/water solutions brought about the formation of carbonyl compounds on the polymer backbone. Moreover,
MMT increased the rate of dehydration and oxidation of cellulose functional moieties. As a consequence, cellulose
was found to be less stable at temperatures lower than 100 °C. Conversely, at higher temperatures, the hindering
of oxygen transfer prevailed, determining an increase in thermo-oxidative stabilit
Thermal oxidation kinetics of additive free polyamide 6-6
Full text linkThermal aging of an additive free PA 6-6 has been elucidated at 90, 100, 120, 140, 150 and 160 C in airventiled ovens by Fourier transform infrared spectrophotometry, viscosimetry in molten state and uniaxial tensile testing. Oxidation of methylene groups starts after a considerably shorter induction period but reaches a lower maximal rate than in additive free PE. Cleavage of CeN bonds constitutes the main source of chain scissions. It leads to the formation of aldehyde chain-ends and a catastrophic decrease in molar mass. Embrittlement occurs at a very low conversion ratio of the oxidation process, in particular when the concentration of aldehyde chain-ends reaches a critical value of [PH¼O]F z 5.6 10 3 mol l 1, corresponding to a critical value of the number average molar mass ofMnFz17 kg mol 1. At this stage, the entanglement network in the amorphous phase is deeply damaged. A non-empirical kinetic model has been derived from the oxidation mechanistic scheme previously established for PE, but improved by adding elementary reactions specific to polyamides such as the rapid decomposition of unstable hydroxylated amide groups. This model describes satisfyingly the main features of the thermal oxidation kinetics of PA 6-6, but also of other types of aliphatic polyamides studied previously in the literature such as: PA 6, PA 12 and PA 4-6, as long as it is not controlled by oxygen diffusion. At the same time, it confirms the existence of an universal character for the thermal oxidation kinetics of aliphatic polyamides whatever their origin, i.e. their initial molar mass, crystallinity ratio, concentration of impurities, structural irregularities, etc
Easy estimation of the progress of artificial weathering of palm fiber-polypropylene composites by chemiluminescence
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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
Easy estimation of the progress of artificial weathering of palm fiber-polypropylene composites by chemiluminescence
Full text linkAppropriate 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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