1,721,139 research outputs found
Termination kinetics of tert-butyl methacrylate and of n-butyl methacrylate free-radical bulk homopolymerizations
No full textTermination kinetics in tert-butyl methacrylate (tert-BMA) and n-butyl methacrylate (n-BMA) bulk homopolymerizations has been studied via the single pulse-pulsed laser polymerization-near infrared (SP-PLP-NIR) method between 40 and 80 degrees C at pressures from 500 to 2 250 bar. Toward increasing monomer conversion, the chain-length averaged termination rate coefficient, [k(t)], for both monomers exhibits the methacrylate-specific sequence of an initial plateau region, assigned to control by segmental diffusion, followed by a steep decrease of ([k(t)] at intermediate conversion, which is assigned to translational diffusion control, and a weaker decrease of (k(t)), associated with reaction-diffusion control, at still higher degrees of monomer conversion. Despite this similarity, the two isomeric monomers clearly differ in absolute size of [k(t)] and in the monomer concentration ranges where the transitions between the different types of diffusion control occur. The differences are assigned to effects of chain mobility which is hindered to a larger extent in tert-BMA than in n-BMA. As a consequence, the [k(t)] behavior of tert-BMA at 80 degrees C is close to the one of n-BMA at 40 degrees C. Investigations into the chain-length dependence of k(t), in particular into k(t)(i,i), the rate coefficient for termination of two radicals of identical size, support the evidence on the different types of diffusion control that operate as a function of monomer conversion. In the initial conversion range, the power-law exponent which characterizes the chain-length dependence of larger (entangled) radicals, is found for both monomers to be close to the theoretical value of alpha = 0.16
Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques
No full textThe propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k(p), is deduced for temperatures from 20 to 90 C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E(A)(k(p))=(34.2 +/- 1.0) kJ mol(-1) and A(k(p))/L mol(-1) s(-1) = (1.08 +/- 0.49) x 10(7) L mol(-1) s(-1). The termination rate coefficient, k(t), has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range -15 to +30 degrees C are: E(A)()=(22.8 +/- 3.7) kJ mol(-1) and log(A/L mol(-1) s(-1)) = 10.6 +/- 1. The chain-length dependence of k(t) was studied at 30 degrees C. For short chains a significant dependence was found which may be represented by an exponent alpha=0.79 in the power-law expression k(t)(i)=k(t)(0)i(-alpha). (C) 2009 Elsevier Ltd. All rights reserved
PLP Labeling in ESR spectroscopic analysis of secondary and tertiary acrylate propagating radicals
No full textChain-length-dependent termination in acrylate radical polymerization studied via pulsed-laser-initiated RAFT polymerization
No full textThe chain-length dependence of the termination rate coefficient, k(t), in methyl acrylate ( MA) and dodecyl acrylate (DA) radical polymerization has been determined via the single pulse pulsed-laser polymerization near-infrared reversible addition-fragmentation chain transfer (SP-PLP-NIR-RAFT) technique. Polymerization is induced by a laser SP and the resulting decay in monomer concentration, c(M), is monitored via NIR spectroscopy with a time resolution of microseconds. A RAFT agent ensures the correlation of radical chain length and monomer-to-polymer conversion. The obtained rate coefficients for termination of two radicals of approximately the same chain length, i, are represented by power-law expressions, k(t)(i, i)alpha i(-alpha). For both monomers, composite model behaviour of k(t)(i, i) showing two distinct chain length regimes is observed. The exponent as referring to short chain lengths is close to unity, whereas the exponent alpha(1), which characterizes the chain-length dependency of large radicals, is slightly above the theoretical value for coiled chain-end radicals. The crossover chain length, i(c), which separates the two regions, decreases from MA(i(c) = 30) to DA(i(c) = 20). The results for MA and DA are consistent with earlier data reported for butyl acrylate. There appears to be a correlation of as and ic with chain flexibility
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
Critically evaluated rate coefficients in radical polymerization - 8. propagation rate coefficients for vinyl acetate in bulk
No full textPropagation rate coefficient values, <i>k</i><sub>p</sub>, reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed-laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in <i>k</i><sub>p</sub> is observed as laser pulse repetition rate is increased from low (25–100 Hz) to high (300–500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5–70°C. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 1.35 × 10<sup>7</sup> L mol<sup>−1</sup> s<sup>−1</sup> and an activation energy of 20.4 kJ mol<sup>−1</sup>, with 95% confidence ellipsoids for the parameters also presented
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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