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A new approach in the modelling of molecular weight distribution in free radical polymerization.
No full textAnalysis of the molecular weight distribution in polymerization process
No full textIn this paper different techniques for the deconvolution of experimental GPC curves obtained from batch reactors are examined with the aim of improving the modelling of the polymerization process. The deconvolution of the experimental GPC curves gives insight into the effects of diffusive phenomena on the values of the average molecular weights (Mn and Mw) and the Molecular Weight Distribution (MWD) of the polymer product. Two methods, one based on a two-peak deconvolution and the other based on a multi-peak deconvolution, are compared.
The different characteristics and relative merits of the two methods are illustrated using several sets of experimental data. As conclusions, the first method is less accurate but allows a separation of the two contributions given by the chemical and diffusive control to the MWD; the second method has a sound theoretical background (confirms the prediction of the free volume theory) and is potentially much superior but results show it to be more sensitive to the quality of the experimental data
Study of diffusion controlled polymerization through the analysis of moleular weight distribution
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Optimal operating conditions of batch reactors for the polymerization of Methylmethacrylate.
No full textA model for the simulation of suspension polymerization of MMA has been developed, which allows to accounts for the effect of diffusive phenomena on reaction rate and to evaluate molecular weight distribution under different operation conditions. The model has been validated by experimental runs carried out in a laboratory reactor. Obtained results show that an increase of the reaction temperature during the gel effect time can have a favourable influence on the polymer quality. The control of temperature profiles in the reactor is indicated as an interesting operating strategy to be adopted in industrial unit
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Modelling of polymerization reactors The influence of diffusion phenomena on the choice of operating conditions.
No full textMany acrylic and vinylic monomers are often polymerized by a free radical chain addition mechanism. This process consists of a three main steps: initiation, propagation and termination. Free radicals are formed by the fragmentation of the initiator, once formed these radicals propagate by reacting with monomers to form long macroradical chains. In batch reactors, this reaction mechanism is often complicated by the fact that the physical properties of the reacting system change dramatically with conversion during the reaction; in concentrated systems a strong acceleration in the polymerization rate occurs along with the increase in the medium viscosity when the reactions become controlled by diffusion phenomena (gel and glass effect). Because the chain length of the polymer produced during polymerization is directly related to the reaction kinetics, as a consequence also the average molecular weights (Mn and Mw) and the Molecular Weight Distribution (MWD) are strongly affected by the diffusion phenomena.
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Sperimentazione di una nuova modalità attuativa per la realizzazione di una campagna di screening per la prevenzione dei tumori del colon-retto
No full textVariations on the Author
Get PDF“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
Get PDFWe 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
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