1,720,953 research outputs found
Crystallization Phenomena in Individualized Polymeric Molecules as Studied by Broadband Dielectric Spectroscopy
No full textThis work focuses on the crystallization behavior of polymers, especially under confined conditions. Polymers are large molecules of long chain-like architecture. Typically, they form amorphous (disordered) aggregates, and their tendency to entangle largely dictate the characteristic macroscopic properties. Nevertheless, also highly ordered crystalline structures are possible. Unlike small molecules, polymer chains must undergo substantial conformational rearrangements to form these which resemble lamellar arrangements. These crystalline structures are another key factor in their properties since they alter e.g. shear modulus and solubility, and thus strongly affect the performance of these materials in a wide range of applications, including food packaging, automotive parts, and drug delivery systems. Consequently, understanding polymer crystallization is not just a scholastic endeavor but also of technological importance.
Crystallization begins with nucleation, and according to classical nucleation theory (CNT) [1], heterogeneous and homogeneous nucleation are distinguished. Heterogeneous nucleation occurs at surfaces or impurities if the interfacial energy with the crystal is lower than that with the melt. In contrast, homogeneous nuclei emerge spontaneously when adjacent chains form lamellar aggregates which are stable. The stability is achieved when the aggregate is large enough for the volumetric enthalpy gain from the crystalline structure exceeds the energetic penalty of the melt-to-crystal surface. Clearly, homogeneous nucleation is governed by properties intrinsic to the molecular structure and inter-molecular interactions. To single out this process from heterogeneous nucleation, often finite size experiments were conducted to minimize the probability of an impurity in the system (provided that surface interactions with the confining system boundaries are high enough to prevent heterogeneous nucleation).
CNT predicts that the crystallization temperature (Tc) decreases with decreasing confinement size which can be attributed to the reduced volume-to-surface ratio that makes nucleation less favorable (and thus requires stronger supercooling to emphasize the enthalpy gain). This prediction is supported by several experimental observations e.g. in small polymer droplets or polymers in nanometric pores [2 - 15]. Furthermore, lamellar orientation exhibits peculiar effects in spatial confinement. On the one hand, in films the crystal orientation at the interface is controlled by the interfacial energy between solid surface and the different faces of a polymer crystal. On the other hand, it was shown that in wide pores, crystalline lamellae tend to align in perpendicular to the pore axis; while for pore diameter smaller than the contour length of the chains, parallel alignment becomes thermodynamically favored [11 - 13]. Consequently, crystal orientation may depend on both interfacial energy and dimensionality of confinement. Numerous experiments on crystallization in small polymer droplets reveal changes like reduced Tc. However, lamellar orientation in such droplets is still an open question in polymer crystallization. A reason for that is that past experiments either used optical microscopy which is limited to droplets above micrometer size or atomic force microscopy (AFM) which probes only surface topography. Furthermore, the droplets deposited in these experiments were prepared by spinodal dewetting, i.e. they strongly vary in size which complicates a reasonable analysis of the average Tc.
In the current thesis, broadband dielectric spectroscopy (BDS) was used as an alternative for investigating nanometer-sized polymer droplets, enabled by the combination with a nanostructured electrode arrangement. Although BDS is commonly used to detect molecular dynamics, crystallization can be probed indirectly through tracing the loss of mobility due to arrested motion in the crystal. Two types of droplets were studied here: (i) droplets deposited on a chemically repelling surface (i.e. also by dewetting), and (ii) droplets of polymer chains grafted to gold nanoparticles which were prepared in a regular pattern on the surface. The shape size and the size distribution of these droplets was determined by AFM yielding a much smaller size distribution for the latter ones.
The main findings of this work are summarized as follows:
• Nano-broadband dielectric spectroscopy has been refined to characterize the crystallization behavior of nanometric polymer droplets, including grafted aggregates containing fewer than 20 chains.
• A pronounced reduction in Tc was observed in the droplets on the repelling surface as qualitatively predicted by CNT.
• However, a quantitative description by CNT fails to explain the observed relationship between droplet size and Tc. This is likely due to the assumption of a spherical nucleus in CNT which does not seem to be fulfilled here.
• To explore the actual nucleus shape and possibly also crystal orientation, CNT was expanded using alternative geometrical nuclei shapes—hemisphere (modified CNT assumption), cylinder (disc-like lamellar sheet), and ellipse (entire droplet)—were tested. None of these shapes could explain the data entirely. Consequently, the crystal orientation and thus nucleus morphology in these droplets are still an open question.
• In contrast, polymer aggregates grafted onto gold nanoparticles show no signature of crystallization despite having the same average droplet volume as the deposited ones. Apparently, this arrangement imposes strong constraints which prevent crystallization. The precise mechanism is not clear; either a limited chain mobility or an inadequate shape is suspected.
This work provides insight into the constraints imposed by finite size, reduced dimensionality and their interplay on crystallization in polymer chains which alter the process from reduced Tc to complete absence. Despite numerous unanswered questions, it opens the possibility to finely tune confinement size and dimension as well as molecular mobility in polymer aggregates which paves the way for future investigations of the impact of these factors on the formation of polymer crystals
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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