1,721,279 research outputs found
The Effects of Temperature and Humidity on the Microstructure of Sulfonated Syndiotactic–polystyrene Ionic Membranes
No full textPolymeric membranes based on the semi-crystalline syndiotactic–polystyrene (sPS) become hydrophilic, and therefore conductive, following the functionalization of the amorphous phase by the solid-state sulfonation procedure. Because the crystallinity of the material, and thus the mechanical strength of the membranes, is maintained and the resistance to oxidation decomposition can be improved by doping the membranes with fullerenes, the sPS becomes attractive for proton-exchange membranes fuel cells (PEMFC) and energy storage applications. In the current work we report the micro-structural characterization by small-angle neutron scattering (SANS) method of sulfonated sPS films and sPS–fullerene composite membranes at different temperatures between 20 °C and 80 °C, under the relative humidity (RH) level from 10% to 70%. Complementary characterization of membranes was carried out by FTIR, UV-Vis spectroscopy and prompt–γ neutron activation analysis in terms of composition, following the specific preparation and functionalization procedure, and by XRD with respect to crystallinity. The hydrated ionic clusters are formed in the hydrated membrane and shrink slightly with the increasing temperature, which leads to a slight desorption of water at high temperatures. However, it seems that the conductive properties of the membranes do not deteriorate with the increasing temperature and that all membranes equilibrated in liquid water show an increased conductivity at 80 °C compared to the room temperature. The presence of fullerenes in the composite membrane induces a tremendous increase in the conductivity at high temperatures compared to fullerenes-free membranes. Apparently, the observed effects may be related to the formation of additional hydrated pathways in the composite membrane in conjunction with changes in the dynamics of water and polymer
Phase Behaviour of CoC12-MnC12 Mixed Crystals
No full textThermal properties of CoCl2, MnCl2 and their mixed crystals are investigated by means of temperature dependent neutron powder diffraction and time-resolved small angle neutron scattering. The coefficients of thermal expansion of these crystals have been determined as a function of temperature. The temperature-induced structural variations can be explained by distortions of CoCl5/MnCl6 octahedra and changes in Co/Mn-Cl bond lengths. CoCl2 and MnCl2 form homogeneous solid solutions in the entire temperature range between the solidus and 50 K. There is no indication of any miscibility gap as predicted from thermodynamic calculations
Poly-ethylene-vinyl alcohol microgels prepared through salting out: Rationalizing the aggregation process and tuning the microstructural properties
No full textThe ion-specific effect on the solubility of macromolecules in water, rationalized through the Hofmeister series, can be used as an effective method to obtain polymer particles with desired microstructural properties thanks to the so-called salting out effect. By choosing the proper salt and optimizing the polymer and the salt concentration, it is possible to tailor the particle structural features like the radius and the degree of compactness and to speed up the aggregation process until complete conversion from free chains to aggregates. Here we rationalize the segregation of poly-ethylene-vinyl alcohol (EVOH) chains in the presence of sodium chloride (NaCl). The effect of polymer and salt concentrations on the kinetics and the thermodynamics of the aggregation process is considered. In particular, we show how such parameters influence the microstructural properties of the particles through a combination of light and neutron scattering techniques as well as microscopy. We found that by increasing sodium chloride concentration it is possible to speed up the aggregation process, obtaining a complete conversion from free chains to aggregates at shorter times, whereas increasing poly-ethylene-vinyl alcohol concentration from 0.70% w/w to 1.30% w/w determines an increase of the microgel size of a two-times factor. Moreover, NaCl concentration directly affects both the packing degree of the polymer chains within the aggregate and the morphology of the polymer particle, leading to a more compact structure at higher NaCl concentrations. Understanding how it is possible to tune both aggregation kinetics and EVOH particle microstructure is fundamental in order to give a general picture which could be broadened to the class of vinyl alcohol-based polymers
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
A Small-Angle Neutron Scattering Study of a Soft Model Nanofiller in an Athermal Melt
No full textWe present a detailed small-angle neutron scattering investigation of mixtures of poly(ethylene oxide) (PEO)/polyhedral oligomeric silsesquioxane (POSS) hybrids, a popular nanofiller, addressing the conformation of the model-grafted nanoparticles as well as their influence on the linear polymer matrix in which they are embedded. The nanofiller has a well-defined graft arm length and is monomolecular. Because of the identical chemical nature of the grafted arm and matrix chains, it can be considered as an ideal athermal mixture. We have introduced a random phase approximation approach with general application possibilities for other grafted nanofillers, combining the star-like and particle-like characteristics of the POSS nanofiller. The size of the nanofiller remained unaffected and Gaussian upon mixing with linear chains of various lengths, and no aggregation of the nanofiller was observed. This turns the POSS-based nanocomposite into an interesting model compound. The mixtures of PEO chains with the octafunctional PEO–POSS nanofiller were found to resemble those of a micellar system whereas within experimental uncertainty the radius of gyration of the embedding matrix remained basically unaltered despite some tendency toward shrinking
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
Microstructure of sulfonated syndiotactic-polystyrene model polyelectrolyte membranes resolved by contrast variation small-angle neutron scattering
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