1,721,784 research outputs found
Simulation as a Tool for Feasibility Studies about PIP-SEC Experiments
No full textAdvanced homo- and copolymerization models have been used to perform a feasibility study on the potential of pulse-initiated polymerization (PIP) experiments for ethene (co)polymerizations. An application of PIP experiments directly to the ethene homo-polymerization appears not as a very promising strategy to derive the homo-propagation rate coefficient k(p) of ethene. This failure can be attributed to the special characteristics of high temperature size exclusion chromatographs, being required to determine the molecular weight distribution (MWD) of polyethylene. PI copolymerizations appear as an interesting alternative to provide access to the homo-propagation rate coefficient of ethene. Most advantageous in this strategy is the fact that even a simple convergence contemplation (using a variation in monomer composition) yields the ethene home-propagation rate coefficient k(p). Simply aiming at this coefficient, there is no necessity of knowing the detailed kinetic parameters of the copolymerization. In a further part, the extended kinetic information being available about branching processes in ethene polymerizations was used to test for the potential influence of a slower propagation rate of secondary macroradicals on the PIP structure in MWDs. Even at the significant level of branching present in ethene homopolymerizations still a PLP structure inside the MWD remains observable, assuming retardation up to an extend of almost two orders of magnitude. In order to perform these studies a kinetic model was designed explicitly accounting for the formation of secondary macroradicals by transfer. The kinetic; information about branching being available in literature, was adopted toward this scheme
Modeling Kinetics and Structural Properties in High-Pressure Fluid-Phase Polymerization
No full textThe present contribution provides an overview of actual applications in modeling free-radical polymerizations. Topics of interest are the simulation of pulsed laser polymerization experiments with subsequent analysis of the formed product by size exclusion chromatography (PLP-SEC), single pulse laser experiments, experimental techniques for determining rate coefficients of elementary reactions that control polymer properties, and technical applications. Aspects being investigated are model validation and testing predictive potential in polymerization models using well-defined experiments as well as developing and testing experimental strategies for deriving rate coefficients of elementary reactions that exist (especially when dealing with copolymerizations) within a network of complex coupled reactions, in any of these fields remarkable success in modeling can be achieved. This demonstrates the great potential that can grow from combining modem mathematical methods, computational power and detailed kinetic insights into the mechanism of polymerization It is the wide scope of applications, e.g. ranging from modeling kinetics to the investigation of termination processes being dependent on the chain-length of the macroradical (as an example of pure fundamental research) to modeling of technical reactors, that provides attractiveness and defines challenges. Especially, the success in transforming results directly from laboratory experiments into technical applications justifies laborious efforts in determining highly precise rate coefficients and proves the concept breaking down a complex process into elementary subparts. A necessary boundary condition for this is keeping in mind the demands along the whole scope of applications and avoiding simplifications that are only applicable for part of them. Although at a first glance this may appear to hinder fast progress in one discipline, it is the essential require ment for final success
Chain-length dependence of free-radical termination rate deduced from laser single-pulse experiments
No full textTermination rate coefficients of free-radical polymerization are accessible from SP-PLP studies where monomer conversion induced by a laser single pulse is measured with a time resolution of microseconds. Previous experiments with 2,2-dimethoxy-2-phenylacetophenone (DMPA) acting as the initiator revealed that upon variation of the DMPA concentration, the resulting monomer conversion vs. time traces intersect. A detailed kinetic analysis of this unexpected type of behavior is presented. It turns out that such crossings occur in situations Where k(t) is chain-length dependent and, at the same time. the primary initiator-derived free-radical species differ in reactivity toward the monomer. As is known from the literature, this difference in radical reactivity is particularly pronounced with DMPA, which photo-decomposes to a propagating and to a non-propagating free radical. Modeling of the crossing behavior opens a novel route for determining chain-length dependent k(t). Results for methyl acrylate (MA) and styrene homopolymerizations at low degrees of monomer conversion, to a maximum of are presented. The decrease of k(t) with chain length is modeled via an exponential function. The dependence is significantly larger for styrene. The exponents derived from simulation studies via PREDICI® are in excellent agreement with corresponding data reported by Olaj et al. for styrene and by de Kock for methyl acrylate
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
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
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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