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Highly soluble phenylethynyl-endcapped imide oligomers derived from thioetherdiphthalic anhydride isomers
No full textNovel imide oligomers (calculated molecular weights: 1250-5000 g mol(-1)) based on the mixture of thioetherdiphthalic anhydride isomers (m-TDPA) with 2-phenyl-4,4-diaminodiphenyl ether (p-ODA) in the presence of 4-phenylethynylphthalic anhydride (PEPA) as reactive endcapping agent were prepared. Then pyromellitic dianhydride (PMDA) was introduced to the polymerization of m-TDPA/p-ODA (mole(m-TDPA):mole(PMDA) = 1:1). The effect of molecular weights and polymer chemical structures of the aromatic oligomers on their processability and solubility as well as the thermal and mechanical properties of the thermal-cured polyimides (PIs) was systematically investigated. All the oligomers showed good solubility (more than 30 wt%) in N-methyl-2-pyrrolidone and very low melt viscosities. The minimum melt viscosities of Oligo-5 was 61.5 Pa s at 321 degrees C. The melt viscosity of the oligomers increased with the increase of molecular weight. After thermally curing at 370 degrees C for 1 h, the thermoset PIs exhibited good thermal properties. The glass transition temperatures of oligomers measured by differential scanning calorimetry were in the range of 264-337 degrees C, and the temperature of 5% weight loss was higher than 505 degrees C. The cured films also demonstrated good mechanical properties with tensile strength and modulus greater than 52 MPa and 2.9 GPa, respectively
Preparation, characterization, and properties of poly(thioether ether imide)s from isomeric bis (chlorophthalimide)s and 4,4 '-thiobisbenzenethiol
No full textPreparation, characterization, and properties of poly(thioether ether imide)s from isomeric bis(chlorophthalimide)s and bis(4-mercaptophenyl) ether
No full textA series of isomeric poly(thioether ether imide)s (PTEIs) containing both thioether and ether linkages were prepared by aromatic nucleophilic substitution reaction of isomeric bis(chlorophthalimide)s (BCPIs) with bis(4-mercaptophenyl) ether (BMPE). The inherent viscosities of synthesized polymers were found in the range of 0.41-0.86 dL g(-1) in N-methyl-2-pyrrolidone at 30 degrees C. The glass transition temperature (T-g) of the isomeric PTEIs were 210-242 degrees C and increased by increasing the content of 3-substituted phthalimide unit in the polymer backbone. The 5% weight loss temperature values reached up to 525-539 degrees C under nitrogen and 523-534 degrees C in air atmospheres, respectively, which indicated this kind of polyimide possessed excellent thermal stability. Flexible films could be cast from the polymer solution. The PTEI films exhibited moderate mechanical properties with tensile strengths of 106-127 MPa, elongations at break of 8.6-11.5%, and tensile moduli of 2.2-2.8 GPa, respectively. Dynamic mechanical thermal analysis results illustrated that the storage moduli (E) of PTEI (a-e) almost completely maintained at about 2.3 GPa before reaching the corresponding T-g. It is noted that the minimum melt viscosity of isomeric PTEIs (a-e) decreased by increasing the content of unsymmetrical 3,4-substituted phthalimide unit in the polymer main chain
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
Variations 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
Synthesis of organosoluble and light-colored cardo polyimides via aromatic nucleophilic substitution polymerization
No full textA series of cardo polyimides (PIs) containing large-volume cardo groups were prepared via aromatic nucleophilic substitution polymerization of bisphenols and bis(fluorophthalimide)s. This series of cardo PIs show excellent solubility in a variety of organic solvents. The glass transition temperatures (T-g(s)) of these PIs are recorded from 217 degrees C to 293 degrees C, the 5% weight loss temperatures (T-5%) reach up to 495-536 degrees C in air and 487-531 degrees C in nitrogen, respectively. Flexible films, which could be cast from the polymer solutions, exhibit good mechanical properties with tensile strengths of 118-127 MPa, elongations at break of 10.6-15.4% and tensile moduli of 2.3-2.8 GPa. Their films with cut-off wavelengths from 342 to 423 nm are essentially colorless, and the transmittance of these cardo polyimide films is over 80% between 450 and 800 nm. It is noted that cardo PIs possess better solubility, higher glass transition temperature, and better optical transparency than linear PIs. Copyright (C) 2015 John Wiley & Sons, Ltd
Synthesis and Characterization of Sulfonated Block Copolyimides Derived from 4,4 '-Sulfide-bis(naphthalic anhydride) for Proton Exchange Membranes
No full textA series of sulfonated copolyimides (SPIs) with hydrophilic segment length of 20-60 based on 4,4-sulfide-bis(naphthalic anhydride) (SBNA) have been successfully synthesized to improve hydrolytic stability and proton conductivity. The SPI membranes were cast from their m-cresol solutions, and they were characterized by determining the water uptake, water swelling ratio, mechanical properties, hydrolytic stability, oxidative stability, and proton conductivity. It was found that the water uptake of SPI membranes was low and decreased as the hydrophilic segment length increased, which led to good dimensional stability. In addition, the SPI membranes with low ion-exchange capacity (IEC) value displayed excellent hydrolytic stability and retained good mechanical properties even after harsh hydrolysis testing, in which the block SPI with hydrophilic segment length of 40 had the best hydrolytic stability, while those with high IEC value showed an apparent decrease. All of the block SPI membranes show better conductivity than the random ones at the temperature range from 30 to 70 degrees C. Interestingly, the proton conductivities of random SPI membranes were higher than that of corresponding block ones at 90 degrees C. The block SPI with hydrophilic segment length of 40 gave the highest proton conductivity as the temperature increased among the block SPIs. (c) 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41501
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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