1,720,956 research outputs found
Influence of Molecular Structure of Polybutadienes in Polymer-Solvent Interactions
No full textProceedings (senza ISBN) del ongresso “XIV Congreso Interamericano – III Congreso Argentino de Ingenieria Quimica”, Buenos Aires (RA), 15-18 settembre 1991.
In polymer processing there is a growing need of knowledge about phase bahavior of polymeric mixtures. In particular activity coefficients at infinite dilution of solvents in polymer are useful for evaluating the last step of volatilization of solvents in the drying process.
Correlative and predictive methods are available for the evaluation of activity of solvents in polymeric solutions. Correlative models, such as the well known Flory-Huggins, need specific experimental data for the estimation of parameters. Predictive methods, on the other hand, are based in group contribution and need no specific information about the system. Despite the generality of these methods, they are not able to differentiate among different isomers of the same polymers. In the present work three well characterized polybutadienes differing in structure and in molecular weight were studied. Activity coefficients at infinite dilution of 20 organic compounds of variate polarity were determined at different temperatures by inverse gas chromatography in the three polymers. Flory parameters and infinite dilution partial molar enthalpies were calculated. All the results evidenced the structure differences. Among the polymers these differences were put in evidence also by fitting activity coefficients using the PHCT equation of state
Polymer Characterization by GLC: Influence of Polymer Structure (Polybutadienes)
No full textProceedings (senza ISBN) del Congresso “Fourth Asian Chemical Congress”, Pechino (CN), 26-30 agosto 1991
The infinite dilution activity coefficient of a solvent in a polymer is a thermodynamic property which can given useful information about the interactions between the components. This property can be determined by inverse gas chromatography, which enables to obtain reliable and accurate data in a relatively short time.
In this work activity coefficients at infinite dilution of various solvents, representative of the different classes of organic compounds, are determined in different polybutadienes.
The polymers were well characterized on the basis of structure and molecular weight. The influence of structure and of molecular weight of the polymer on the compatibility with the solvents is pointed out.
Comparison with available data obtained by different authors in similar polymers is reported. The results are qualitatively interpreted by using the Perturbed Hard Chain E0S developed by Cottermann and Prausnitz
Infinite Dilution Activity Coefficients of Aromatic and Aliphatic Ethers
No full textActivity coefficients at infinite dilution of several ethers and organics in different solvents were measured by gas-liquid chromatography. Tetrahydrofurane, anisole, diethyl, dipropyl and dibutyl ethers, n-pentane, n-hexane, methanol, chloroform and 1-chloropentane, were studied. The data were compared, where possible, with infinite dilution activity coefficients from the literature. The UNIFAC model was used to correlate the experimental data. Two definitions of the ether group were considered for anisole (methyl-phenyl-ether) the classical CH2O and an aromatic ACOCH2 group. Binary and ternary vapour-liquid equilibria of anisole with several organic compounds at finite concentrations were predicted with the parameters obtained from the infinite dilution data. The introduction of the ACOCH2 group does not improve predictions on these systems
Solvent-Polymer Interactions in Polybutadienes
No full textInfinite-dilution activity coefficients of 26 solvents in three polybutadienes were determined by inverse gas chromatography in the temperature range 40-100°C. The polybutadienes have well-characterized structure and different molecular weights. The data are useful for the development of thermodynamic models for polymer-solvent interaction and for characterization. The results were fitted to the PHCT equation of state. The data indicate that activity coefficients depend on polymer molecular weight and structure
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
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