1,721,280 research outputs found
Block copolymer template-directed synthesis of well-ordered metallic nanostructures
Get PDFNanoporeuze metallische schuimen fabriceren met blokcopolymeren Het proefschrift van Ivana Vukovic gaat over metallische nanoschuimen, een nieuwe klasse van 3D-nanomaterialen waarin eigenschappen van metalen en nanoporeuze materialen worden gecombineerd. De unieke combinatie zorgt ervoor dat nanoporeuze metallische schuimen kunnen worden gebruikt voor waterstofopslag, actuatoren, nieuwe 3D-gestructureerde batterijen, vervangers voor platinum-katalysatoren, etc. Veelgebruikte technieken, zoals het scheiden van legeringen, sol-gel synthese, nanosmelten, verbrandingssynthese, etc., leveren over het algemeen materialen op met een ongeordende architectuur, iets wat een nadelig effect kan hebben op de mechanisch eigenschappen. Blokcopolymeren daarentegen kunnen spontaan overgaan in bi-continue geordende nanostructuren. Vervolgens kunnen ze worden toegepast als een soort mal (op nanoschaal) voor de het maken van goedgeordende metallische nanoschuimen. Het idee om deze blokcopolymeren te gebruiken voor de fabricatie van deze nanoschuimen wordt verder uitgewerkt en beschreven in dit proefschrift, met de daarbij behorende experimentele resultaten en toekomstperspectieven. Daarnaast wordt het fasegedrag onderzocht van supramoleculaire complexen die op blokcopolymeren zijn gebaseerd en die worden gebruikt als voorlopers voor goedgeordende metallische nanoschuimen.
Biobased Networks from Lignin/Cellulose via Diels-Alder Click Chemistry
Get PDFThermosets play a key role in modern plastic and coating industries. However, the cross-links responsible for their outstanding properties, such as improved tensile strength and chemical and thermal resistance, render them unable to undergo reprocessing or recycling. Herein, biobased thermosets were prepared from lignin and cellulose via the Diels-Alder (DA) click reaction, thereby providing fully biobased polymer thermosets while envisioning the properties of dynamic networks. Furan and maleimide moieties were attached to soda lignin and cellulose. DA reactions were then carried out using different combinations of functionalized lignins and bismaleimide, forming a cross-linked network while simultaneously enabling a reversible control over thermally induced de-cross-linking and re-cross-linking. In contrast, thermoreversibility was not achieved when associating both lignin and cellulose via DA-based cross-links, even though mild reaction temperatures and lower degrees of cross-linking were considered. Ultimately, the study contributed to widening the use of furan-maleimide DA click chemistry for the preparation of sustainable materials, providing insights into a green and versatile platform that may be considered for future applications.</p
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
Polymerization of hyperbrached polysaccharides by combined biocatalysis
Get PDFIn this thesis it is shown that enzymes are essential laboratory tools for the in vitro synthesis of polysaccharides with control over macromolecular properties. Here we present a method to enzymatically polymerize hyperbranched polysaccharides with control over stereoregularity, degree of branching and molecular weight. Moreover, the possibility to construct hybrid materials consisting of a hyperbranched polyglucan part connected to a synthetic substrate (e.g. polymer, surface, etc) is shown.
Using an enzymatic catalyzed tandem polymerization in which the unique properties of the enzymes potato phosphorylase and glycogen branching enzyme (GBEDG; from Deinococcus geothermalis) are combined, a hyperbranched polyglucan was polymerized consisting of (1→4) linked alfa-D-glucose residues with branches at the glucose C6 hydroxy group. In this tandem polymerization, phosphorylase catalyzes the addition of (1→4) linked alfa-D-glucose residues from a short oligosaccharide, using glucose-1-phosphate (G-1-P) as donor substrate (monomer). GBEDG introduces in situ branch points at the growing polymer chain. More specifically, GBEDG catalyzes the formation of alfa(1→6) branch points by the hydrolysis of an alfa(1→4) linked glycosidic linkage and subsequent inter- or intra-chain transfer of the non reducing terminal fragment to the C6 hydroxyl position of an alfa-glucan.
A property of phosphorylase, essential for the research as outlined in this thesis, is the donor substrate (primer) dependency. Polymerization is impossible without an oligomeric alfa(1→4) linked D-glucose primer of at least 3 glucose residues. By taking advantage of this property hybrid materials were constructed by anchoring the oligomeric primer to a substrate prior to the enzymatic tandem polymerization with. This resulted in hyperbranched multi arm structures, diblock copolymers consisting of a hyperbranched polyglucan part and hyperbranched brush polymers anchored to Si wafers.
Self-assembly of binary block copolymers and supramolecular copolymer complexes
Get PDFDankzij de vaak repulsieve interacties tussen de verschillende blokken treedt er in blokcopolymeren een spontane fasescheiding op. Dit resulteert in de formatie van een reeks geordende structuren op nanometerschaal. Dergelijke nanostructuren zijn zeer interessant vanwege de mogelijke toepassingen in de nanotechnologie. Subtiele verschillen in de chemische structuur van de gebruikte monomeren en in de architectuur van de blokcopolymeren kan leiden tot een verbeterde controle over de gevormde morfologie of kan zelfs resulteren in de formatie van geheel nieuwe structuren. Het is daarom zeer belangrijk om goed te begrijpen welke parameters van invloed zijn en in welke mate deze veranderd kunnen worden om zo unieke structuren mogelijk te maken. In dit proefschrift wordt het onderzoek naar de synthese en zelf-assemblage van verschillende soorten blokcopolymeren beschreven. Hierbij ligt de nadruk op binaire blokcopolymeren en supramoleculaire complexen. De supramoleculaire complexen bestaan uit laagmoleculaire amfifielen gebonden aan een polymeerketen met behulp van waterstofbruggen.
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