Influence of the viscosity of poly(methyl methacrylate) on the cellular structure of nanocellular materials

Three different grades of poly(methyl methacrylate) (PMMA) with different rheological properties are used for the production of nanocellular materials using gas dissolution foaming. The influences of both the viscosity of the different polymers and the processing parameters on the final cellular structure are studied using a wide range of saturation and foaming conditions. Foaming conditions affect similarly all cellular materials. It is found that an increase of the foaming temperature results in less dense nanocellular materials, with higher cell nucleation densities. In addition, it is demonstrated that a lower viscosity leads to cellular polymers with a lower relative density but larger cell sizes and smaller cell nucleation densities, these differences being more noticeable for the conditions in which low solubilities are reached. It is possible to produce nanocellular materials with relative densities of 0.24 combined with cell sizes of 75 nm and cell nucleation densities of 1015 nuclei cm−3 using the PMMA with the lowest viscosity. In contrast, minimum cell sizes of around 14 nm and maximum cell nucleation densities of 3.5 × 1016 nuclei cm−3 with relative densities of 0.4 are obtained with the most viscous one. © 2019 Society of Chemical Industr

Key Production Parameters to Obtain Transparent Nanocellular PMMA

Transparent nanocellular polymethylmethacrylate (PMMA) with relative density around 0.4 is produced for the first time by using the gas dissolution foaming technique. The processing conditions and the typical characteristics of the cellular structure needed to manufacture this novel material are discovered. It is proved that low saturation temperatures (−32 °C) combined with high saturation pressures (6, 10, 20 MPa) allow increasing the solubility of PMMA up to values not reached before. In particular, the highest CO2 uptake ever reported for PMMA, (i.e., 48 wt%) is found for a saturation pressure of 20 MPa and a saturation temperature of −32 °C. Due to these processing conditions, cell nucleation densities of 1016 nuclei cm−3 and cell sizes clearly below 50 nm are achieved. The nanocellular polymers obtained, with cell sizes ten times smaller than the wavelength of visible light and very homogeneous cellular structures, show a significant transparency

Overcoming the Challenge of Producing Large and Flat Nanocellular Polymers: A Study with PMMA

Although nanocellular polymers are interesting materials with improved properties in comparison with conventional or microcellular polymers, the production of large and flat parts of those materials is still challenging. Herein, gas dissolution foaming process is used to produce large and flat nanocellular polymethylmethacrylate samples. In order to do that, the foaming step is performed in a hot press. The methodology is optimized to produce flat samples with dimensions of 100 × 100 × 6 mm3, relative densities in the range 0.25–0.55 and cell sizes around 250 nm. Additionally, foaming parameters are modified to study their influence on the final cellular structure, and the materials produced in this paper are compared with samples produced by using a most conventional approach in which foaming step is conducted in a thermal bath. Results obtained show that an increment in the foaming temperature leads to a reduction in relative density and an increase of cell nucleation density. Moreover, differences in the final cellular structure for materials produced by both foaming routes are studied, proving that although there exist some differences, the mechanisms governing the nucleation and growing are the same in both processes, leading to the production of homogeneous materials with very similar cellular structures

The role of parenting styles on behavior problem profiles of adolescents

Parental behavior is one of the most influential factors on the development of adolescent externalizing and internalizing behavior problems. These behavioral problems are closely related and often co-occur. The objectives of this work were: (i) to identify adolescents profiles according to their behavior problems; (ii) to explore individual, family, and social characteristics associated with these profiles; and (iii) to analyze the potential role of parenting styles in belonging to adolescents’ profiles. A total of 449 Spanish adolescents (223 from families declared at-risk and enrolled in Child Welfare Services and 226 from families from the general population) participated in this study. The analyses revealed three profiles of adolescents based on external and internal behavior problems (adjusted, external maladjustment, and internal maladjustment). Parenting styles explained the adolescents’ belonging to different profiles, in which the indulgent style was the most favorable in general terms. The distinctive role of parenting styles on two types of maladjustment profiles was confirmed. The relationship between parenting styles and adolescent adjustment is a key component that should be included in interventions according to adolescents’ behavior problem profiles. Furthermore, the results shed light on the need that family interventions are complemented with individualized interventions with adolescents that accumulate stressful life events.Ministerio de Economía y Competitividad EDU2013-41441-

Transparent nanocellular PMMA: Characterization and modeling of the optical properties

In this work, the optical properties of transparent nanocellular polymethylmethacrylate (PMMA) have been studied, experimental and theoretically. Transmittance measurements of samples presenting different cell sizes (14, 24, 39 and 225 nm) and a constant relative density of around 0.45 have been carried out obtaining values as high as 0.94 for the sample with the smaller cell size and a thickness of 0.05 mm. In addition, the light absorption coefficient has been measured as a function of cell size and wavelength. It has been found that the transmittance has a strong dependence with the wavelength, presenting these transparent materials Rayleigh scattering. On the other hand, the transmission of visible light through these nanocellular materials has been modelled for the first time. The developed model reproduces with good accuracy the trends observed in the experimental results and provides remarkable insights into the physics mechanisms controlling the optical behavior of these materials

Modeling the heat transfer by conduction of nanocellular polymers with bimodal cellular structures

Nanocellular polymers are a new generation of materials with the potential of being used as very efficient thermal insulators. It has been proved experimentally that these materials present the Knudsen effect, which strongly reduces the conductivity of the gas phase. There are theoretical equations to predict the thermal conductivity due to this Knudsen effect, but all the models consider an average cell size. In this work, we propose a model to predict the thermal conductivity due to the conduction mechanisms of nanocellular materials with bimodal cellular structures, that is, with two populations of cells, micro and nanocellular. The novelty of our work is to consider not only the average cell size, but the cell size distribution. The predictions of the model are compared with the experimental conductivity of two real bimodal systems based on poly(methyl methacrylate) (PMMA), and it is proved that this new model provides more accurate estimations of the conductivity than the models that do not consider the bimodality. Furthermore, this model could be applied to monomodal nanocellular polymers. In particular, for monomodal materials presenting a wide cell size distribution and at low densities, the model predicts important variations in comparison with the current models in the literature. This result indicates that the cell size distribution must be included in the estimations of the thermal conductivity of nanocellular polymer

Perfil social del youtuber adulto en España

El presente trabajo va a estudiar la figura del youtuber de videojuegos español en la edad adulta, es decir jugadores de videojuegos mayores de 30 años desde el punto de vista social. La figura del youtuber del videojuego adulto en nuestro país, no está recogida en estudios relevantes que puedan darnos datos significativos del porqué se comparten las partidas, con que finalidad y sobre todo ser reconocido dentro de una red social como es Youtube. Para la recogida de datos hemos aplicado una metodología mixta, tanto cualitativa, a través de interpretaciones a nivel social de los datos recogidos en los cuestionarios realizados por los youtuber que han participado en la investigación, como cualitativa. El trabajo de investigación tiene como objetivos: Conocer el porqué de la necesidad de compartir partidas con otros jugadores a través de un canal, dentro de una red social. Descubrir los motivos o motivo que llevan a un adulto(personas mayores de 30 años) a ser youtuber dentro de una red social, dotando de notoriedad al personaje. La metodología adecuada para esta tesis sería la metodología mixta, cualitativa y cuantitativa incluyendo análisis de contenidos, entrevistas personalizadas y grupos focales Sobre la investigación realizada se esperan las siguientes conclusiones: El incremento del youtuber del videojuego en la edad a adulta en España ha supuesto una nueva forma de comunicación entre personas con gustos similares que se produce a través de una red social, Youtube. Las motivaciones sociales que llevan a los jugadores a compartir sus partidas Radiografía social del youtuber español, que relacione lo que esconden este tipo de personalidades. Descubrimiento del mundo youtuber a través del videojuego

Nanocellular Polymers with a Gradient Cellular Structure Based on Poly(methyl methacrylate)/Thermoplastic Polyurethane Blends Produced by Gas Dissolution Foaming

Graded structures and nanocellular polymers are two examples of advanced cellular morphologies. In this work, a methodology to obtain low-density graded nanocellular polymers based on poly(methyl methacrylate) (PMMA)/ thermoplastic polyurethane (TPU) blends produced by gas dissolution foaming is reported. A systematic study of the effect of the processing condition is presented. Results show that the melt-blending results in a solid nanostructured material formed by nanometric TPU domains. The PMMA/ TPU foamed samples show a gradient cellular structure, with a homogeneous nanocellular core. In the core, the TPU domains act as nucleating sites, enhancing nucleation compared to pure PMMA and allowing the change from a microcellular to a nanocellular structure. Nonetheless, the outer region shows a gradient of cell sizes from nano- to micron-sized cells. This gradient structure is attributed to a non-constant pressure profile in the samples due to gas desorption before foaming. The nucleation in the PMMA/ TPU increases as the saturation pressure increases. Regarding the effect of the foaming conditions, it is proved that it is necessary to have a fine control to avoid degeneration of the cellular materials. Graded nanocellular polymers with relative densities of 0.16–0.30 and cell sizes ranging 310–480 nm (in the nanocellular core) are obtained

Herramientas de gestión de la información para el desarrollo de las habilidades de aprendizaje autorregulado en la formación inicial del profesorado

En aquest article presentem una estratègia didàctica per desenvolupar els entorns personals d?aprenentatge (PLEs) dels estudiants universitaris amb un enfocament d'aprenentatge autorregulat (SRL). L'estratègia s'emmarca en el model de Dabbagh and Kitsantas (2012) el qual connecta les fases d'aprenentatge autorregulat de Zimmerman (planificació, realització, auto-reflexió) i tres nivells d'interacció que la tecnologia permet (gestió personal de la informació, interacció social i col·laboració, recuperació i gestió d'informació). L'estratègia s'implementa amb estudiants de tercer curs del Grau d'Educació Primària de la Universitat de les Illes Balears i les dades es recullen mitjançant l'aplicació d'un qüestionari sobre l'ús de la tecnologia i les percepcions dels estudiants sobre l?efectivitat de les eines utilitzades en la de gestió d'informació. L'anàlisi dels resultats permet esbossar alguns patrons en l'ús d'eines de gestió de la informació en els diversos escenaris d'aprenentatge. En les conclusions identifiquem desafiaments relacionats amb la resistència dels estudiants i l'enfocament per a l'avaluació tradicional; destaquem possibilitats de transferència dels processos de gestió d'informació desenvolupats a altres contextos i suggerim noves implementacions educatives i d'investigació. Amb aquest treball s'aporta un model aplicable a altres contextos, i es presenta una proposta didàctica per a la gestió de la informació basada en el PLE i el SRL