1,721,052 research outputs found
SP-PLP-EPR study of chain-length-dependent termination in free-radical polymerization of n-dodecyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate: Evidence of "composite" behavior
No full textThe chain-length dependence of the termination rate coefficient in n-dodecyl methacrylate (DMA), cyclohexyl methacrylate (CHMA), and benzyl methacrylate (BzMA) bulk free-radical homopolymerizations at ambient pressure and at temperatures from -20 to 0 degrees C is deduced using the recently developed technique of SP-PLP-EPR: pulsed-laser polymerization (PLP) in which time-resolved EPR measurement of radical concentration, c(R), is made following each single pulse (SP) of an excimer laser. The decay of c(R) results from termination of radicals of almost identical size. Their chain length, i, increases linearly with time, t, after applying a SP. The rate coefficient, k(t)(i,i), for termination of two radicals of size i is determined by fitting the experimental c(R) vs t data. This process demonstrates that (at least) two power-law exponents are necessary to describe kti,i over the extended chain-length range of i = 1 to 1000. This is consistent with the so-called "composite model", which uses power-law exponents alpha(S) and alpha(L) to describe termination of radicals either shorter or longer, respectively, than a crossover chain length, i(c). The fourth parameter obtained from fitting the SP-PLP-EPR data with this model is k(t)(1,1), the termination rate coefficient for two radicals of degree of polymerization 1. Previous DMA experiments are reanalyzed while new experimental results are reported and analyzed for CHMA and BzMA. The parameter values for CHMA and BzMA termination at 0 degrees C are almost identical - k(t)(1,1) approximate to 3 x 10(7) L mol(-1) s(-1), alpha(s) approximate to 0.50, i(c) approximate to 90, and alpha(L) approximate to 0.21-and they are close to those for DMA at 0 degrees C: k(t)(1,1) approximate to 1 x 10(7) L mol(-1) s(-1), alpha(s) approximate to 0.64, i(c) approximate to 50, and alpha(L) approximate to 0.18. The results fully support the composite model in that the chain-length dependence is more pronounced for shorter than for longer radicals, i.e., alpha(S) > alpha(L). Moreover, the power-law exponent that characterizes termination of long-chain radicals is close to the theoretical value of alpha(L) = 0.16. In fact all parameter values-including the small differences between DMA and CHMA/BzMA-are more-or-less in accord with expectations based on polymer dynamics. Furthermore, our results suggest that termination of methacrylate radicals with large cyclic or long n-alkyl substituents may be affected by steric shielding of the radical functionality
Detailed analysis of termination kinetics in radical polymerization
Get PDFStudies into the termination step of radical polymerization have mostly been carried out via chain-length-averaged properties. With the advent of pulsed laser polymerization (PLP) induced by single pulses (SPs), the analysis of termination rate coefficients kt has significantly improved. In conjunction with highly time-resolved near-infrared (NIR) detection of monomer consumption, SP-PLP-NIR allows for the measurement of chain-length-averaged termination rate coefficients as a function of temperature, pressure and degree of monomer conversion. With the addition of a suitable reversible addition−fragmentation chain−transfer agent, this technique even allows for the determination of chain-length-dependent termination rate coefficients kti,i, where i refers to the chain length of two terminating radicals of identical size. The decay of kti,i with chain length is more pronounced at small radical size below a crossover chain length ic. At larger chain lengths, kti,i decreases to a weaker extent. Thus, the so-called composite model is perfectly suited for representing kti,i behaviour across the entire chain-length range. A further clear improvement of kti,i determination is provided by the SP-PLP-EPR method, which directly measures the type and concentration of radicals via microsecond time-resolved electron paramagnetic resonance (EPR) spectroscopy. This EPR-assisted technique is particularly valuable for polymerizations where secondary radicals undergo backbiting and thus two types of radicals, secondary and tertiary, are simultaneously present, as is the case with acrylic monomers. © 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry
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
Determination of the Mode of Termination in Radical Polymerization via Mass Spectrometry
No full textWe comprehensively explore the use of mass spectrometry (MS) for quantitative determination of the mode of termination, i.e., the extent of disproportionation versus combination, in radical polymerization. Development of the underlying kinetic theory forms a major portion of our endeavors. What emerges from this theory is that the ratio of polymer from disproportionation to combination is a function of chain length and of fundamental kinetic parameters such as the rate of initiation and the rate coefficients for propagation and termination. Therefore, all this information must be at hand in order to determine lambda, the fraction of termination by disproportionation. We implement our findings using electrospray-ionization MS results for poly(methyl methacrylate) that we synthesized at 85 degrees C in benzene employing bis(3,5,5-trimethylhexanoyl) peroxide as initiator, chosen because it has the advantage of yielding only one primary radical species. Our results are well fitted by theory and are found to vary with chain length and initiator concentration as predicted. We also address the issue that signal intensities from macromolecular MS techniques may not give the true molecular weight distribution of a sample because of variation of ionization efficiency with species mass. We devise a way of obtaining lambda from data for hypothetical combination and disproportionation species of identical mass. Our final result is lambda = 0.63 +/- 0.10, suggesting that disproportionation is not as rampant in methyl methacrylate systems as commonly thought.Deutsche Forschungsgemeinschaft [BU 426/10
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
The Impact of Band Broadening on Molar-Mass Determination of Narrow-Distribution Polymer by Size-Exclusion Chromatography
No full textA systematic study of the effects of band broadening (BB) in size-exclusion chromatography (SEC) on Poisson molar-mass distributions (MMDs) is carried out. Such distributions apply for polymer standards used for SEC calibration, and they are a good model for the MMD from any quasi-living polymerization, including reversible-deactivation radical polymerization (RDRP). BB is simulated using the exponentially modified Gaussian model, which is shown to work excellently for actual polymer standards. A series of simulations with this model revealed how BB parameter values affect number-average degree of polymerization and polydispersity index from SEC. It is shown that the skewness parameter is particularly important in these considerations. Many of the obtained trends echo ones from RDRP, suggesting that BB has been making a hitherto unsuspected-indeed uninvestigated-contribution to deviations from ideal behavior in such systems
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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