1,720,981 research outputs found
Single step in situ formation of porous zinc oxide/PMMA nanocomposites by pulsed laser irradiation: Kinetic aspects and mechanisms
No full textThe simultaneous in situ formation of zinc oxide nanoparticles and controlled porosity in poly(methyl methacrylate) films is presented herein as an innovative method for the preparation of polymer based porous nanocomposites. The process is based on the conversion of zinc acetate in spherical zinc oxide nanoparticles of 9 nm on average, by pulsed laser activated precursor decomposition. The kinetics of the nanoparticles formation, followed by UV-visible spectroscopy, show a remarkable increase of the reaction rate with respect to the conventional thermally activated process. Most importantly, as revealed by scanning electron microscopy investigations, the laser treatment leads to the simultaneous formation of a porous structure in the polymer matrix, which can be ascribed to the yielded gaseous by-products during the zinc oxide nanoparticle formation. The combination of all these characterizations allowed a deeper insight in the kinetic aspects and mechanisms involved in the single step formation of porous poly(methyl methacrylate)/zinc oxide nanocomposites with tailored characteristics that cannot be achieved by the conventional thermal treatment
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
Laser-induced in situ synthesis of Pd and Pt nanoparticles on polymer films
No full textAbstract: We present the localized in situ formation of Pd and Pt nanoparticles embedded in chitosan solid films. This is achieved by the photo-induced reduction of metallic precursors, previously incorporated in chitosan films, through controlled UV pulsed laser irradiation. Interestingly, at high number of laser pulses, Pd and Pt follow different formation pathways, contrary to their common photoreduction mechanism occurring at low irradiation pulses. Specifically, in the case of the Pd, a photofragmentation process takes place fracturing the previously formed nanoparticles into smaller ones; whereas in the case of Pt, the prolonged irradiation promotes the nanoparticles agglomeration. The combination of both precursors in a binary solid system results in the combined formation of both Pd and Pt nanoparticles in the polymer film upon laser irradiation. The herein reported approach is an efficient and precise tool to generate size- and density-controlled Pd and Pt nanoparticles in desired areas of polymeric films, rendering this method a potential candidate for the fabrication of flexible polymeric devices for gas-sensing or electro-catalysis applications. Graphical Abstract: Schematic representation of Pd, Pt and Pd–Pt combined NPs formation mechanisms under pulsed laser irradiation [Figure not available: see fulltext.
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
Spontaneous liquid flow in microfluidic systems by UV irradiation of the hybrid polymer/TiO2 nanorods channels
No full textA method for enhancing the surface wettability of PDMS microchannels used in microfluidic devices, is presented. Colloidal TiO2 nanorods are mixed in a PDMS solution that was used subsequently to realize parallel microchannels, through a replica molding procedure. TiO2 has the intrinsic capability of increasing dramatically its hydrophilicity upon UV irradiation. Due to this property, incorporating TiO2 nanofillers into the PDMS microchannels we manage to induce their hydrophobic-to-hydrophilic conversion upon UV irradiation. This conversion is essential for water to fill the microfluidic channels, in contrast to what happens for the non-irradiated nanocomposite or pure PDMS channels, which are characterized by high hydrophobicity and do not allow water to penetrate. The reversibility of the wettability changes permits to the microchannels to recover their original hydrophobicity
Thermally-induced in situ growth of ZnO nanoparticles in polymeric fibrous membranes
No full textWe present a two-step process to obtain PMMA fibrous membranes with homogeneously in situ synthesized zinc oxide nanoparticles of defined shape and size, both on the fibers' surface and bulk. The method is based on the electrospinning of PMMA/precursor solutions and the subsequent thermally activated in situ conversion of zinc acetate to zinc oxide nanoparticles directly in the solid polymeric fibers. X-ray diffraction measurements ensure the formation of crystalline ZnO nanoparticles. Scanning and transmission electron microscopy prove that homogeneously distributed nanoparticles with two different morphologies and size distributions are obtained, depending on the area of the fibers where the nanoparticles are nucleated and on the initial precursor's content. In particular, branched nanoparticles homogeneously decorate the fibers' surface with their dimensions that range from 50 nm to 140 nm for initial precursor contents from 23 to 40 wt%, respectively. On the other hand, small spherical nanoparticles of ca. 7 nm are mainly observed in the bulk of the fibers. Unlike the branched nanoparticles, the dimensions of the spherical nanoparticles are practically unaffected by the precursor amount initially loaded. The homogeneously distributed nanoparticles both on the surface and in the bulk of the polymeric fibers, combined with the high surface area provided by the fibrous structure, result in a multifunctional material characterized by reversible UV-induced wettability and water permeability, improved thermal stability and antibacterial activity particularly promising for diverse applications such as filtration, wound management, photocatalysis, antibacterial and UV-shielding textiles
Nanochains Formation of Superparamagnetic Nanoparticles
No full textWe present simulations on the aggregation of nanometer sized polydispersed superparamagnetic particles under the application of an external magnetic field. We make use of a Monte Carlo method, using a cluster moving approach as previously used in literature for ferrofluids van der Waals attraction and magnetic anisotropy are taken into account in the simulations. Chains elongated in the field direction are formed. The results are in good agreement with recent experimental results on nanochains made of iron oxide nanoparticles into polymer matrix, obtained with the application of a magnetic field during film deposition. The magnetization anisotropy of the nanocomposite film under dc magnetic field can be predicted within this simple model
Nanoporous PMMA foams with templated pore size obtained by localized in situ synthesis of nanoparticles and CO2 foaming
No full textPolymer foams with controlled and templated pore size have been obtained for the first time by CO2 gas dissolution foaming from poly(methyl methacrylate) (PMMA) films. This kind of materials, with a variable porous structure, mimic some high-performance natural materials and could present significant interest in many applications. However, up to now their controlled fabrication has not been successfully achieved. Herein, we present a method to achieve a fine control in the production of such materials. Thermal in situ synthesis of ZnO nanoparticles from Zn(OAc)2 was proposed to obtain PMMA nanocomposites, in which the ZnO nanoparticles induce heterogeneous nucleation that leads to formation of pores with size below the micron, upon CO2 foaming. Starting from templated solid PMMA samples with well-differentiated regions, presenting or not ZnO nanoparticles, it was possible to obtain PMMA-based foams with well-defined areas of different pore sizes
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
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