1,720,967 research outputs found
The Morphology and Surface Area of Emulsion derived (PolyHIPE) Foams Prepared with Oil-phase Solubile Porogenic Solvents 1: Span 80 as Surfactant
No full textPoly(divinylbenzene) emulsion-derived (PolyHIPE) solid foams prepared with porogens (toluene, chlorobenzene, (2-chloroethyl)benzene, 1,2-dichlorobenzene, and 1-chloro-3-phenylpropane) in the oil phase have morphologies and surface areas that are strongly influenced by the nature of the porogen. For the case where the surfactant employed is Span 80, we show that the solid foam structure depends on (i) the ability of the solvent to swell the growing network, (ii) the solvent polarity, and (iii)
the ability of the solvent to adsorb at the emulsion interface. In particular, relatively polar solvents that are able to transport water through the emulsion continuous phase (Ostwald ripening) are shown to produce much lower surface areas than analogous resins prepared by homogeneous solution polymerization of divinylbenzene in the presence of the solvent in question alone. The influence of Ostwald ripening is further suggested by the observation that surface area decreases with increasing emulsion aqueous phase content for relatively polar solvents whereas little variation in surface area with aqueous phase content is observed for more hydrophobic solvents. All PolyHIPEs prepared were characterized by SEM, TEM, N2 sorption analysis, and mercury intrusion porosimetry. The relative merits of TEM and mercuryintrusion porosimetry as techniques for the reliable characterization of the solid foams are discussed
The influence of porogen type on the porosity, surface area and morphology of poly(divinylbenzene) polyHIPE foams
No full textThe type of porogen added to the continuous phase of HIPEs containing divinylbenzene strongly in ̄uences the morphology of the resulting PolyHIPE foam. The cell size was reduced as the solvent became a better cosurfactant, as inferred from surface pressure measurements of ®lms representative of each HIPE continuous phase. In addition, this caused the windows connecting adjacent cells to increase, to such an extent in two cases that the cellular morphology was apparently lost. The surface area increased as the solubility parameter of the solvent approached that of the polymer, however the materials with highest surface areas also had a noncellular morphology and were very weak mechanically. This could be recti®ed by the use of mixtures of the solvents investigated, producing materials with surface areas up to 554 m2 g21, a cellular morphology and good mechanical properties
The Morphology and Surface Area of Emulsion derived (PolyHIPE) Foams Prepared with Oil-phase Solubile Porogenic Solvents 2: Three-component Surfactant System
No full textThe influence of organic-soluble porogenic solvents (toluene, chlorobenzene, (2-chloroethyl)-benzene, 1,2-dichlorobenzene, and 1-chloro-3-phenylpropane) on the morphology and microstructure of poly(divinylbenzene) emulsion-derived (PolyHIPE) solid foams prepared with a three-component surfactant system is described. It is found that all solvents employed apart from 1,2-dichlorobenzene (C2B) lead to much higher surface areas than corresponding solid foams prepared with SPAN 80 as surfactant.
Experiments where the emulsion droplet phase volume is varied indicate that Ostwald ripening is much reduced with the three-component surfactant system compared to SPAN 80, and this is the likely cause of the higher surface areas. The lesser extent of Ostwald ripening is also suggested by NMR water selfdiffusion experiments, which indicate a lower diffusion coefficient when the mixed surfactant is used.
The peculiar behavior of C2B is ascribed to its presumed effect on the packing of surfactant molecules at the interface; the 1,2-substitution pattern may produce a strong steric effect, thus allowing greater contact
between the aqueous and organic phases
High internal phase emulsions (HIPEs) containing divinylbenzene and 4-vinylbenzyl chloride and the morphology of the resulting polyHIPE materials
No full textThe type of porogen added to the continuous phase of HIPEs containing divinylbenzene strongly in ̄uences the morphology of the resulting PolyHIPE foam. The cell size was reduced as the solvent became a better cosurfactant, as inferred from surface pressure measurements of ®lms representative of each HIPE continuous phase. In addition, this caused the windows connecting adjacent cells to increase, to such an extent in two cases that the cellular morphology was apparently lost. The surface area increased as the solubility parameter of the solvent approached that of the polymer, however the materials with highest surface areas also had a noncellular morphology and were very weak mechanically. This could be recti®ed by the use of mixtures of the solvents investigated, producing materials with surface areas up to 554 m2 g21, a cellular morphology and good mechanical properties
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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