1,720,959 research outputs found
Mechanical properties of spider dragline silk: Humidity, hysteresis, and relaxation
No full textSpider silk is well-known for its outstanding mechanical properties. However, there is a significant variation of these properties in literature and studies analyzing large numbers of silk samples to explain these variations are still lacking. To fill this gap, the following work examines the mechanical properties of major ampullate silk based on a large ensemble of threads from Nephila clavipes and Nephila senegalensis. In addition, the effect of relative humidity (RH) on the mechanical properties was quantified. The large effect of RH on the mechanical properties makes it plausible that the variation in the literature values can to a large extent be attributed to changes in RH. Spider silk's most remarkable property-its high tenacity-remains unchanged. In addition, this work also includes hysteresis as well as relaxation measurements. It is found that the relaxation process is well described by a stretched exponential decay
Strain dependent structural changes of spider dragline silk
No full textSpider dragline silk is a natural semicrystalline polymer that is well-known for its unusual combination of tensile strength and extensibility. However, the correlation between structure and mechanical properties is still largely unknown. We have investigated the changes of the fiber's structure as probed by single fiber X-ray diffraction using a microfocus synchrotron beam, under controlled simultaneous mechanical load and variation of the humidity. Crystallite orientation, size, and lattice constants have been monitored while pulling the fibers
Spider silk softening by water uptake: an AFM study
No full textWe have investigated the mechanical properties of spider dragline fibers of three Nephila species under varied relative humidity. Force maps have been collected by atomic force microscopy. The Young's modulus E was derived from the indentation curves of each pixel by the modified Hertz model. An average decrease in E by an order of magnitude was observed upon immersion of the fiber in water. Single fiber stretching experiments were carried out for comparison, and also showed a strong dependence on relative humidity. However, the absolute values of E are significantly higher than those obtained by indentation. The results of this work thus show that the elastic properties of spider silk are highly anisotropic, and that the silk softens significantly for both tensile and compressional strain (indentation) upon water uptake. In addition, the force maps indicate a surface structure on the sub-micron scale
Simulations of charge transport in organic compounds
Full text linkTo aid the design of organic semiconductors, we study the charge transport properties of organic liquid crystals, i.e. hexabenzocoronene and carbazole macrocycle, and single crystals, i.e. rubrene, indolocarbazole and benzothiophene derivatives (BTBT, BBBT). The aim is to find structure-property relationships linking the chemical structure as well as the morphology with the bulk charge carrier mobility of the compounds. To this end, molecular dynamics (MD) simulations are performed yielding realistic
equilibrated morphologies. Partial charges and molecular orbitals are calculated based on single molecules in vacuum using quantum chemical methods. The molecular orbitals are then mapped onto the molecular positions and orientations, which allows calculation of the transfer integrals between nearest neighbors using the molecular orbital overlap method. Thus we obtain realistic transfer integral distributions and their autocorrelations. In case of organic crystals the differences between two descriptions of charge transport, namely semi-classical dynamics (SCD) in the small polaron limit and kinetic Monte Carlo (KMC) based on Marcus rates, are studied. The liquid crystals are investigated solely in the hopping limit. To simulate the charge dynamics using KMC, the centers of mass of the molecules are mapped onto lattice sites and the transfer integrals are used to compute the hopping rates. In the small polaron limit, where the electronic wave function is spread over a limited number of neighboring molecules,
the Schroedinger equation is solved numerically using a semi-classical approach. The results are compared for the different compounds and methods and, where available, with experimental data. The carbazole macrocycles form columnar structures arranged on a hexagonal lattice with side chains facing inwards, so columns can closely approach each other allowing inter-columnar and thus three-dimensional transport. When taking only intra-columnar transport into account, the mobility is orders of magnitude lower than in the three-dimensional case. BTBT is a promising material for solution-processed organic field-effect transistors. We are able to show that, on the time-scales of charge transport, static disorder due to slow side chain motions is the main factor determining the mobility. The resulting broad transfer integral distributions modify the connectivity of the system but sufficiently many fast percolation paths remain for the charges. Rubrene, indolocarbazole and BBBT are examples of crystals without
significant static disorder. The high mobility of rubrene is explained by two main features: first, the shifted cofacial alignment of its molecules, and second, the high center of mass vibrational frequency. In comparsion to SCD, only KMC based on Marcus rates is capable of describing neighbors with low coupling and of taking static disorder into account three-dimensionally. Thus it is the method of choice for crystalline systems dominated by static disorder. However, it is inappropriate for the case of strong coupling and underestimates the mobility of well-ordered crystals. SCD, despite its one-dimensionality, is valuable for crystals with strong coupling and little disorder. It also allows correct treatment of dynamical effects, such as intermolecular vibrations of the molecules. Rate equations are incapable of this, because simulations are performed on static snapshots. We have thus shown strengths and weaknesses of two state of the art models used to study charge transport in organic compounds,
partially developed a program to compute and visualize transfer integral distributions and other charge transport properties, and found structure-mobility relations for several promising organic semiconductors.Um die Herstellung organischer Halbleiter zu erleichtern, untersuchen wir den Ladungstransport in organischen Fluessigkristallen, wie Hexabenzocoronen und Carbazolringen, und Einkristallen, wie Rubren, Indolocarbazol und Benzothiophenderivaten. Zielsetzung ist es, Zusammenhaenge zwischen der Struktur und der Ladungstraegermobilitaet zu finden. Zu diesem Zweck werden Molekulardynamiksimulationen (MD) durchgefuehrt, welche realistische, equilibrierte Morphologien liefern. Partialladungen und Elektronenorbitale werden an
Einzelmolekuelen im Vakuum mit quantenchemischen Methoden berechnet. Die Orbitale werden dann auf die Positionen und Orientierungen der Molekuele abgebildet, was die Berechnung von Transferintegralen zwischen naechsten Nachbarn mit der Molecular-Orbital-Overlap-Methode ermoeglicht. Dies ergibt realistische Transferintegralverteilungen und dazugehoerige Autokorrelationen. Fuer organische Kristalle untersuchen wir zusaetzlich den Unterschied zwischen zwei Methoden zur Simulation von Ladungstransport, naemlich Semi-Classical-Dynamics (SCD) und Kinetic-Monte-Carlo (KMC) mit Marcusraten. Um den Ladungstransport mit KMC zu simulieren, werden die Schwerpunkte der Molekuele mit Gitterpunkten identifiziert, waehrend die Transferintegrale zur Berechnung der Sprungraten dienen. Im Grenzfall kleiner Polaronen (SCD), indem die Wellenfunktion der Elektronen ueber mehrere Molekuele ausgedehnt ist, wird die Schroedingergleichung semi-klassisch geloest. Die Ergebnisse fuer die unterschiedlichen Materialien und Methoden
werden verglichen, sofern vorhanden auch mit experimentellen Daten. Die Carbazolringe bilden hexagonal angeordnete Saeulen, deren Seitenketten nach innen gerichtet sind, was eine dichte Annaeherung und somit interkolumnaren Ladungstransport ermoeglicht. Beruecksichtigt man nur intrakolumnaren Transport, ist die Mobilitaet um mehrere Groeßenordnungen niedriger als im dreidimensionalen Fall. BTBT ist ein vielversprechendes Material fuer in Loesung hergestellte Feldeffekttransistoren. Wir zeigen, dass auf der Zeitskala von Ladungstransport statische Unordnung aufgrund langsam fluktuierender Seitenketten die Mobilitaet erheblich beeinflusst. Die resultierenden Transferintegralverteilungen modifizieren die Konnektivitaet des Systems, jedoch verbleiben genug Perkolationswege fuer schnellen Ladungstransport. Rubren, Indolocarbazol und BBBT sind Kristalle, in denen statische Unordnung eine untergeordnete Rolle spielt. Die hohe Mobilitaet in Rubren wird durch zwei Eigenschaften erklaert: erstens die planare,
verschobene Anordnung der Molekuele, und zweitens deren hohe Schwingungsfrequenz. Im Vergleich zu SCD ist KMC basierend auf Marcusraten in der Lage, Nachbarn mit niedrigen Transferraten und dreidimensionale statische Unordnung zu beschreiben. Diese Methode ist daher fuer Systeme, die von statischer Unordnung bestimmt werden, zu bevorzugen. Allerdings ist sie ungeeignet fuer den Fall hoher Transferraten und unterschaetzt die Mobilitaet in gut geordneten Kristallen. SCD ist zwar eindimensional, aber dennoch gut geeignet, um stark gekoppelte Materialien mit wenig Unordnung zu beschreiben. Zusaetzlich ermoeglicht es die Beschreibung dynamischer Effekte, wie intermolekularer Schwingungen. Ratenbasierende Methoden sind dazu nicht in der Lage, da sie auf statischen Momentaufnahmen basieren. Wir haben damit die Vor- und Nachteile zweier aktueller Modelle zur Untersuchung von Ladungstransport in organischen Molekuelen gezeigt, an einem Programm zur Berechnung und Visualisierung von Transferintegralen und
Transporteigenschaften mitgearbeitet und Zusammenhaenge zwischen Struktur und Eigenschaften vielversprechnder organischer Halbleiter gefunden.132 S
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
- …
