1,720,955 research outputs found
A general model describing molecular weight distribution and branching indices in copolymerizations demonstrated by the high-pressure free-radical copolymerization of ethene and methyl acrylate
No full textA general copolymerization model is developed which incorporates various transfer reactions, like transfer to polymer and backbiting, and scission reactions. The potential of the model is investigated using the simulation package PREDIC(R) by performing a number of sensitivity analyses. The main focus of this work is directed toward the development of experimental strategies for,deriving proper kinetic parameters based on simulation;calculations. To provide a broad base for general application the model is designed for investigating fundamental aspects as well as providing the potential for use in technical production. This would mean dealing with complex reactor operation modes and dealing with multiple broad species distributions. At these simulations the high-pressure free-radical copolymerization of ethene CE) with methyl acrylate (MA) is used. This complex fluid-phase copolymerization system features all elementary reaction steps being discussed to occur at free-radical (co-polymerizations at the moment and is therefore self-suggesting for such investigations. The experimental strategy derived from the simulation study is verified by two experimental examples of E-MA copolymers both containing 15 mol-% methyl acrylate synthesized at 150 degreesC and 2000 bar featuring 22 mol-% and 32 mol-% acrylate conversion. It becomes obvious that the kinetic model is capable of well describing experimental molecular weight distributions and branching indices by one set of kinetic parameters. Successful experimental design and description of data demonstrate the usefulness of modeling for kinetic investigations. Moreover, they are the justification for a future application of models following strategies being proposed in this contribution for other copolymerization systems and more complex copolymerization applications. In this model the coupled implementation of the transfer to poly-mer and p-scission reaction is applied for the first time in copolymerizations (this description is close to the real process). Also the design of an experimental strategy showing singular sensitivities on the determination of rate coefficients for branching and scission in copolymerizations, is presented for the first time. An assumption that has still to be made within this model is the treatment of p-scission acting on a linear chain. However, as long as a macromolecule is not multiply long-chain branched there is no error introduced into modeling using this approximation
Long-chain and short-chain branches in ethene-methyl acrylate copolymers studied by quantitative C-13 NMR spectroscopy
No full textThe frequency of long-chain branches (LCBs) in ethene (E)-methyl acrylate (MA) copolymers is investigated for material that was synthesized at 150 degrees C and 2,000 bar at different levels of conversion but constant copolymer composition. The copolymers exhibit significantly more LCBs than found in LDPE synthesized under similar conditions. Moreover, it is found that the LCB frequency increases with conversion. At 0.3 mol-% conversion 0.3 LCBs and a 4.0 mol-% conversion 1.1 LCBs per 1,000 C atoms are observed. In addition, the frequency of short-chain branches (SCBs) in the copolymers is investigated. About 9 mol-% of the acrylate units are alkylated, which corresponds to numbers given in literature. As sensors, both LCB and SCB indices provide valuable information for simulation calculations to determine rate coefficients for inter- and intramolecular transfer reactions
Long-chain and short-chain branches in ethene-methyl acrylate copolymers studied by quantitative C-13 NMR spectroscopy
No full textThe frequency of long-chain branches (LCBs) in ethene (E)-methyl acrylate (MA) copolymers is investigated for material that was synthesized at 150 degrees C and 2,000 bar at different levels of conversion but constant copolymer composition. The copolymers exhibit significantly more LCBs than found in LDPE synthesized under similar conditions. Moreover, it is found that the LCB frequency increases with conversion. At 0.3 mol-% conversion 0.3 LCBs and a 4.0 mol-% conversion 1.1 LCBs per 1,000 C atoms are observed. In addition, the frequency of short-chain branches (SCBs) in the copolymers is investigated. About 9 mol-% of the acrylate units are alkylated, which corresponds to numbers given in literature. As sensors, both LCB and SCB indices provide valuable information for simulation calculations to determine rate coefficients for inter- and intramolecular transfer reactions
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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