987 research outputs found
Organic glass-forming liquids and the concept of fragility
No full textInternational audienceAn important category of glass-forming materials is organic; it includes molecular liquids, polymers, solutions, proteins that can be vitrified by cooling the liquid under standard conditions or after special thermal treatments. The range of applications is large from materials to life sciences and recently to electronics. To distinguish them from other systems described in this issue, some specific properties such as the range of their glass transition temperature (), their ability to vitrify and some rules of thumb to locate are presented. The most remarkable property of these liquids is how fast in temperature their viscosity or structural relaxation time increases as approaching . To characterize this behavior and rank the liquids of different strength, C.A. Angell introduced the concept of Fragility nearly 40 years ago. He proposed to classify liquids as fragile or strong in an Arrhenius plot with scaling (the strongest ones have never being observed in organic glasses, except for water under specific conditions). The value and the fragility of a given liquid can be changed by applying pressure, i.e. changing the density. One can then explore the properties of the supercooled/overcompressed liquid and the glass in a phase diagram. The line corresponds to an isochronic line, i.e. a line at constant relaxation time with different pairs of density-temperature. We observe that all data can be placed on master-curves that depend only on a single density-and species-dependent and T-independent effective interaction energy, (ρ). An isochoric fragility index is defined as an intrinsic property of a given liquid, that can help in rationalizing all the correlations between the glass properties below T g and the viscous slowing down just above from which they are made. Geometrical confinement of liquids is also a way to modify the dynamics of a liquid and the properties of a glass; it corresponds to a large number of situations encountered in nature. Another phase diagram (d= pore diameter) can be defined with a non-trivial pore size dependence of the glass transition, which is also strongly affected by surface interactions
Glassy properties and viscous slowing down: an analysis of the correlation between nonergodicity factor and fragility
No full textLa traducción de la fraseología del siglo XIX hasta nuestros días. Teoría y práctica
No full textThe phraseology is one of the language parts more difficult to teach, to learn and to translate. When, in the 19th century, the first phraseologists and the paremiologists studying proverbs started to think about the translation of phraseologisms and proverbs, the techniques that one uses nowadays for the correspondences search were not developped yet. In my article I will see how it goes from the first attempts, not always corrects, of phraselogisms and proverbs translation (19th century and beyond) to the application of the mentioned techniques of "proverbs translation" for the preparation of the reference works for the investigation that come close to the compared study of proverbs, as well as the translators who need a reliable consult instrument with correspondences in different languages.INGLESE
La fraseología es una de las parcelas de la lengua más difíciles de enseñar/aprender y traducir. Cuando, en el siglo XIX, los primeros fraseólogos y paremiólogos empezaron a reflexionar sobre la traducción de fraseologismos y paremias, todavía no se habían desarrollado las técnicas que se emplean hoy en día para la búsqueda de correspondencias. En nuestro artículo veremos cómo se pasa de los primeros intentos, no siempre acertados, de traducción de fraseologismos y paremias (siglo XIX y siguientes) a la aplicación de las mencionadas técnicas de 'traducción paremiológica' para la confección de obras de referencias para los investigadores que se acerquen a la paremiología comparada, así como para los traductores que necesiten un instrumento fiable de consulta de paremias con correspondencias en varias lenguas.SPAGNOL
Properties of a confined molecular glass-forming liquid Properties of a confined molecular glass-forming liquid
No full textInternational audienceWe use molecular dynamics (MD) simulations to investigate the modification of the dynamic and static properties of liquid toluene when confined in cylindrical mesopores a few molecular diameters across. Due to the strong influence of the substrate on the dynamics of the confined liquid, we choose a procedure where no additional thermal interactions between the wall and the liquid are taken into account. We observe the characteristic oscillations of molecular density profiles (layering) when temperature and pore size are changed. Mean square displacements and intermediate incoherent scattering functions of the centre of mass of the molecules are calculated as functions of different distances from the wall along the principal axis of the pore z and along the perpendicular x-and y-directions. At 200 K the relaxations of the two correlation functions slow down by one order of magnitude as compared to the bulk, with a slightly more pronounced slowing down in the x-direction. This slowing down increases strongly when the wall is approached. However, we do not observe any layer-specific dependence of the dynamics, but instead a continuous change. When the molecules are arrested near the wall in the time window (1 ns) of the simulations, we find hopping processes
Liquids in confined geometry: How to connect changes in the structure factor to modifications of local order
No full textEffects of density and temperature on correlations between fragility and glassy properties
No full textUdgivelsesdato: JU
Temperature versus density effects in glassforming liquids and polymers: A scaling hypothesis and its consequences
No full textDoes Molecular Self-Association Survive in Nanochannels?
No full textInternational audiencetert-Butanol spontaneously forms supermolecular clusters in the liquid state. This phenomenon results from the balance between hydrogen(H)-bonded self-association and steric repulsion, which is ubiquitous in complex supermolecular assemblies. Neutron scattering provides direct insight into this mesoscopic order through a distinct signature (prepeak) in the structure factor. Using MCM-41 molecular sieves, we show that the spatial correlations associated with micelle-like clusters are totally suppressed in nanoconfined geometry. This is direct evidence of a considerable variation of the structure at the mescoscopic scale of an associated liquid during its manipulation in nanostructured materials and illustrates to what extent nanoconfinement affects the formation of the highly directional hydrogen-bonded systems and the properties of confined liquids in general
A perspective on the fragility of glass-forming liquids
No full textInternational audienceWe discuss possible extraneous effects entering in the conventional measures of “fragility” at atmospheric pressure that may obscure a characterization of the genuine super-Arrhenius slowdown of relaxation. We first consider the role of density, which increases with decreasing temperature at constant pressure, and then the potential influence of the high-temperature dynamical behavior and of the associated activation energy scale. These two effects involve thermodynamic parameters and the strength of the “bare” activation energy reflecting the specific bonding between neighboring molecules. They vary from system to system with, most likely, little connection with any putative collective behavior associated with glass formation. We show how to scale these effects out by refining the definition of fragility and modifying the celebrated Angell plot. We dedicate this note to our great and so inspiring friend, Austen Angell, and associate in this tribute another dear colleague who died too soon, Daniel Kivelson
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