1,721,058 research outputs found
Molecular mobility in halogen-containing side-chain liquid-crystalline polymers studied by dielectric spectroscopy
No full textMolecular mobility in polymethacrylate-based thermotropic side-chain liquid-crystalline polymers (SCLCPs) of the chemical structure -CH2-C(CH3)-COO-(CH2)5-COO-C6H4-OOC-C6H4-X (where X = F, Cl, Br, I) has been investigated by means of broadband dielectric relaxation spectroscopy (DRS, 10-2-106 Hz) and thermally stimulated depolarization current (TSDC; -180 to 30 C) techniques. The process, with a significant contribution being provided by the segmental main-chain motions, has been studied in detail. The temperature dependence of the relaxation rate of the approximately process shows a crossover at a characteristic temperature Tc from the Arrhenius type at high temperatures to the Vogel-Tammann-Fulcher-Hesse type at lower temperatures. The magnitude and the shape of the relaxation show characteristic changes in the liquid-crystalline phases, in particular close to Tc. These results are discussed in the framework of cooperativity and of the influence of the liquid-crystalline layers
Effect of the Structure of Reactor Poly(propylene-co-ethylene) Blends on the Diffusion Coefficient and Activation Energy of a Conventional Antioxidant
No full textThe diffusion of a thermal stabilizer, i.e., Irganox 1010, has been studied in three different reactor poly(propylene-co-ethylene) blends, namely HecoQ, ExpSS, and Super Soft. The diffusion experiments were carried out by using Roe's method based on a stack of several polymer films where each of them had 60-70 μm thickness. The concentration profile of the stabilizer in every film has been monitored by the absorbance at 282 nm of the recorded UV spectra. Under our experimental conditions, the diffusion of Irganox 1010 for all polymer materials can be correctly interpreted by using Fick's second law resolved under certain boundary conditions. Best-fitting the experimental data with the equation model also allows calculation of the diffusion coefficient, D, for these polymer materials at 40, 70 and 80 °C. By applying an Arrhenius-type equation to the calculated D coefficients, an estimation of activation energies of the diffusion process has been achieved. An interpretation, in terms of free volume theory, has been applied to explain the observed variation of D which depends mostly on the amount of amorphous phase of the blends
Diffusion coefficient and activation energy of Irganox 1010 in poly(propylene-co-ethylene) copolymers
No full textThe diffusion of a thermal stabiliser, Irganox 1010, has been studied both in a commercially available polypropylene homopolymer (HomoQ) and in a poly(propylene-co-ethylene) copolymer (RacoS). The diffusion experiments were carried out by using the Roe method formed by a stack of several polymer films having 60-70 μm as thickness under nitrogen atmosphere. The concentration profile of stabiliser in every film has been monitored by the absorbance at 282 nm in UV spectra recorded for every film forming the stack assembly. Under our experimental conditions, the diffusion of Irganox 1010 in each film stack for both polymer materials can be correctly interpreted by using the Fick's second law. The best fit of the experimental data with this equation allows calculation of the diffusion coefficients, D, for these polymer materials at 40, 70 and 80°C. By applying an Arrhenius equation to the calculated D coefficients, an estimation of activation energies of the diffusion process has been also achieved
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
Poly(1-vinylindole) and some of its methyl derivatives as substrates for photorefractive materials: their synthesis, optical and electrical characterization
No full textThe best conditions for the synthesis of poly(1-vinylindole) (PVI) and some of its methyl derivatives have been investigated. The aim of the research was to verify if PVI could be used instead of poly(1-vinylcarbazole) (PVK) in polymeric blends having photoconductive and/or photorefractive behavior. All synthesized polymers are characterized by a glass transition temperature that is lower than that of PVK. It has been verified that the indole ring system, inclusive of its methyl substituted derivatives, lacking of the symmetry characteristics of carbazole, possesses an electric dipole moment that is constantly higher than that of PVK. A higher dipole moment can be an advantageous feature to improve the solubility within the polymeric matrix of the optically non-linear molecule, necessary for photorefractivity. Charge-transfer complexes with a fluorenone-like photosensitizer are efficiently formed by both PVI and its derivatives. A direct current measuring apparatus has been assembled by which photocurrents of few picoamperes can be detected and recorded. Preliminary reported results clearly show that PVI is a photoconductive material. Its photoconductivity is compared with that of PVK and of hybrid materials containing mixtures of carbazole and indole moieties, present in the measuring cell as pendant groups on a macromolecular chain, as single molecules, or both
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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