PVDF/BaTiO3/carbon nanotubes ternary nanocomposites: Effect of nanofillers and processing

Ternary thermoplastic systems based on poly(vinylidene fluoride), PVDF, filled with barium titanate, BaTiO3, submicrometric particles and carbon nanotubes, CNT, were prepared. Their structure and morphology were studied as a function of composition and finally correlated with thermal and mechanical properties. High energy ball milling, HEBM, under cryogenic conditions and subsequent hot pressing were used to obtain films with quite uniform dispersion of the nanofillers. The presence of BaTiO3 particles and CNT did not modify the thermodegradation mechanism of the PVDF. However, enough amount of BaTiO3 seemed to inhibit the volatility of the products of pyrolysis, hindering the decomposition of PVDF. The presence of CNT favored the PVDF thermodegradation probably due to improved heat transmission by an increase in the thermal conductivity. Variations in PVDF thermal transitions were more dependent of processing conditions. Improvements in the mechanical properties of PVDF were ascribed to a reinforcing effect of the fillers. This effect only happened below the fraction of percolation of CNT, pointing out that CNT reinforce through an optimum load transfer from the PVDF matrix to the nanofillers

Using the equivalent material concept and the average strain energy density to analyse the fracture behaviour of structural materials

ABSTRACT: This paper provides a complete overview of the applicability of the Equivalent Material Concept in conjunction with the Average Strain Energy Density criterion, to provide predictions of fracture loads in structural materials containing U-notches. The Average Strain Density Criterion (ASED) has a linear-elastic nature, so in principle, it does not provide satisfactory predictions of fracture loads in those materials with nonlinear behaviour. However, the Equivalent Material Concept (EMC) is able to transform a physically nonlinear material into an equivalent linear-elastic one and, therefore, the combination of the ASED criterion with the EMC (EMC?ASED criterion) should provide good predictions of fracture loads in physically nonlinear materials. The EMC?ASED criterion is here applied to different types of materials (polymers, composites and metals) with different grades of nonlinearity, showing the accuracy of the corresponding fracture load predictions and revealing qualitatively the limitations of the methodology. It is shown how the EMC?ASED criterion provides good predictions of fracture loads in nonlinear materials as long as the nonlinear behaviour is mainly limited to the tensile behaviour, and how the accuracy decreases when the nonlinear behaviour is extended to the material behaviour in the presence of defectsFunding: This research was funded by the Spanish Ministry of Science, Innovation and Universities, grant number PGC2018-095400-B-I00 (MCIU/AEI/FEDER, UE)

Escalas de respuesta tipo Likert. ¿Es relevante la alternativa 'indiferente'?

En tests de actitudes y de personalidad es frecuente utilizar escalas de respuesta tipo Likert. En este tipo de escalas es frecuente también que se presente una catego-ría intermedia. Esta categoría representa una posición intermedia a lo largo del continuo y entre las categorías adyacentes. El presente estudio evalúa si las categorías de las escalas de respuesta de una serie de cuestionarios frecuentemente utilizados en la evaluación organizacional se presentan realmente ordenadas, tal y como asume el sistema de puntuación empleado. Asimismo, se evalúa si la categoría intermedia ("indiferente") es relevante en términos de probabilidad de respuesta. Los cuestionarios analizados presentan una escala de respuesta tipo Likert de cinco puntos. Para lograr los objetivos propuestos, se ha aj ustado el modelo nominal de Bock (1972). Los resultados obtenidos indican que si bien las categorías de respuesta aparecen ordenadas, la categoría intermedia"indiferente" no es relevante, es decir, en ningún intervalo de valores del rasgo latente aparece como la alternativa que presenta una mayor probabilidad de respuesta. Estos resultados sugieren eliminar la citada categoría de respuesta de las escalas de respuesta empleadas

A near-infrared spectroscopy study on thermal transitions of PMMA and PMMA/SiO2 nanocomposites

In this work infrared spectroscopy in the near infrared (NIR) range is used to study the dynamics of polymethylmethacrylate (PMMA) as a pure polymer and when it is modified with silica nanoparticles. Different ways of analyzing FT-NIR spectra were explored taking the temperature as the variable of the systems: i) bands shifts in terms of the first moment of the wavenumber; ii) bands shifts in terms of an absorbance ratio method and iii) absorbance variations in terms of the integrated absorbance. The spectroscopic analysis allowed to study thermal transitions in PMMA and PMMA based nanocomposites. In particular the method based on the use of an absorbance ratio is the most sensible to analyze the thermal transitions. All bands considered could be used to detect thermal transitions reflecting how the vibrations of the groups CH2; CH3 and C¼O are affected by changes in the free volume of the PMMA. It seems that the PMMA dynamics is more driven by the lateral groups than the backbone of the polymer. The presence of silica nanoparticles, at least when 1% by weight is used, restrains the PMMA motion increasing the relaxation temperatures that can be associated with the glass transitions

Understanding the behavior of stimuli-response ionogels for microfluidic applications

Part of special issue: Proceedings of the 30th anniversary Eurosensors Conference – Eurosensors 2016, 4-7. Sepember 2016, Budapest, HungaryElectrochemical Impedance Spectroscopy (EIS) studies of two different ionogels (IOs) are performed using gold interdigitated electrodes (Au-IDEs). Poly(N isopropylacrylamide) (pNIPAAM) is polymerized in the presence of two ionic liquids (ILs), ethyl- 3-methylimidazolium ethyl sulfate [C2mIm][EtSO2] (1) or trihexyltetradecyl-phosphonium dicyanamide [P6,6,6,14][DCA] (2). The Nyquist diagrams of the IOs reflect significant differences due to their polarity and porosity dissimilarities. This fact is supported with the study of the diffusion of water through the polymer matrix, which is faster for IO-1, the most porous IO. Moreover, the ability of IO-1 and IO-2 to conduct current is measured and the sheet resistance of IO-2 is two orders of magnitude higher than IO-1. Finally, the swelling/drying properties of the IOs are monitored exposing them to several vacuum and rehydration cycles. The Nyquist plot of IO-1 shows faster diffusion and recovery of the original properties. Both hydrophilicity and porosity are in the basis of these results.This work was supported by the Basque Government under the Etortek Program (Grant No. IE14–391). N.G-G. was supported by a PhD fellowship from the University of Navarra

A little garbage in, lots of garbage out: Assessing the impact of careless responding in personality survey data

[EN]In self-report surveys, it is common that some individuals do not pay enough attention and effort to give valid responses. Our aim was to investigate the extent to which careless and insufficient effort responding contributes to the biasing of data.We performed analyses of dimensionality, internal structure, and data reliability of four personality scales (extroversion, conscientiousness, stability, and dispositional optimism) in two independent samples. In order to identify careless/insufficient effort (C/IE) respondents, we used a factor mixture model (FMM) designed to detect inconsistencies of response to items with different semantic polarity. The FMM identified between 4.4% and 10% of C/IE cases, depending on the scale and the sample examined. In the complete samples, all the theoretical models obtained an unacceptable fit, forcing the rejection of the starting hypothesis and making additional wording factors necessary. In the clean samples, all the theoretical models fitted satisfactorily, and the wording factors practically disappeared. Trait estimates in the clean samples were between 4.5% and 11.8% more accurate than in the complete samples. These results show that a limited amount of C/IE data can lead to a drastic deterioration in the fit of the theoretical model, produce large amounts of spurious variance, raise serious doubts about the dimensionality and internal structure of the data, and reduce the reliability with which the trait scores of all surveyed are estimated. Identifying and filtering C/IE responses is necessary to ensure the validity of research result

The spatially resolved star formation history of CALIFA galaxies: Cosmic time scales

This paper presents the mass assembly time scales of nearby galaxies observed by CALIFA at the 3.5m telescope in Calar Alto. We apply the fossil record method of the stellar populations to the complete sample of the 3rd CALIFA data release, with a total of 661 galaxies, covering stellar masses from 10$^{8.4}$ to 10$^{12}$ M$_{\odot}$ and a wide range of Hubble types. We apply spectral synthesis techniques to the datacubes and process the results to produce the mass growth time scales and mass weighted ages, from which we obtain temporal and spatially resolved information in seven bins of galaxy morphology and six bins of stellar mass (M$_{\star}$) and stellar mass surface density ($\Sigma_{\star}$). We use three different tracers of the spatially resolved star formation history (mass assembly curves, ratio of half mass to half light radii, and mass-weighted age gradients) to test if galaxies grow inside-out, and its dependence with galaxy stellar mass, $\Sigma_{\star}$, and morphology. Our main results are as follows: (a) The innermost regions of galaxies assemble their mass at an earlier time than regions located in the outer parts; this happens at any given M$_{\star}$, $\Sigma_{\star}$, or Hubble type, including the lowest mass systems. (b) Galaxies present a significant diversity in their characteristic formation epochs for lower-mass systems. This diversity shows a strong dependence of the mass assembly time scales on $\Sigma_{\star}$ and Hubble type in the lower-mass range (10$^{8.4}$ to 10$^{10.4}$), but a very mild dependence in higher-mass bins. (c) All galaxies show negative $\langle$log age$\rangle_{M}$ gradients in the inner 1 HLR. The profile flattens with increasing values of $\Sigma_{\star}$. There is no significant dependence on M$_{\star}$ within a particular $\Sigma_{\star}$ bin, except for the lowest bin, where the gradients becomes steeper.Comment: 15 pages, 13 figures, 3 tables, accepted for publication in Astronomy & Astrophysics. *Abridged abstract

La eclosión de la opinión del alumnado sobre la enseñanza de la Geografía. El geoforo iberoamericano en 2015

Los datos que se aportan en esta síntesis anual nos indican que existe un crecimiento muy relevante no sólo en las visitas sino, sobre todo, en la participación del Geoforo Iberoamericano a través de las opiniones de alumnos y docentes. La didáctica de la Geografía se transforma en educación ciudadana por medio de las redes sociales, una aspiración que se ha gestado desde el año 2009. La colaboración de las personas de un lado y otro del Océano Atlántico nos permitirá definir con más rigor y precisión los desafíos de la formación docente en los próximos años