An alternative approach to evidence the structural conditioning in the dynamic slowdown in a polymer glass-former

Dynamic slowdown of liquids, leading to a breakdown of Arrhenius behavior of relaxation and Stokes-Einstein relationship (SER), as the glass transition is approached, is still not fully understood despite decades of study. They are usually associated to the emergence of dynamic heterogeneity, that is, regions or clusters of particles that have high or low mobilities. But the physical origin of these dynamic heterogeneity, and in particular, the question whether they have a structural origin or they are a purely dynamical phenomenon, is still under debate. In this work we study through molecular dynamics simulations in a polymer model the dynamic slowdown and the breakdown of SER, in connection with dynamic susceptibility calculated for an isoconfigurational ensemble, such that the effects of structure on dynamics can be discriminated. The onset of structure effects on dynamical behavior is found to be coincident with the onset of slow dynamics and SER breakdown.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Soulé, Ezequiel Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Static and dynamic correlation lengths in supercooled polymers

A key point to understand the glass transition is the relationship between structural and dynamic behavior experienced by a glass former when it approaches Tg. In this work, the relaxation in a simple bead-spring polymer system in the supercooled regime near its glass transition temperature was investigated with molecular dynamic simulations. We develop a new manner to look at the dynamic length scales in a supercooled polymeric system, focusing on correlated motion of particles in an isoconfigurational ensemble (that is, associated with the structure), as measured by Pearson’s correlation coefficient. We found that while the usual dynamic four-point correlation length deviates from the structural (mosaic or point-to-set) length scale at low temperatures, Pearson’s length behaves similarly to the static length in the whole temperature range. The results lead to a consensus of similar scaling of structural and dynamical length scales, reinforcing the idea of the theories of Adam-Gibbs and random first order transition.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Gianetti, Melisa Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Soulé, Ezequiel Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Análisis del contrato de concesión de la red vial N° 6, tramo vial puente Pucusana-Cerro Azul-Ica de la carretera Panamericana Sur

En el presente trabajo de investigación se analiza el contrato de concesión del tramo vial Puente Pucusana – Cerro Azul – Ica" (Red Vial No 6), suscrito el 20 de setiembre de 2005, así como los problemas que se han suscitado en el transcurso del proceso de promoción de la inversión y la ejecución contractual. El proceso de promoción del proyecto Red Vial N° 6 presentó problemas en la adjudicación de la buena pro, al tener que recurrirse a una segunda convocatoria y licitación, debido a que solo se presentó un postor, lo que representaba una limitación a la libre competencia. Luego de la primera licitación, las bases fueron modificadas para permitir la adjudicación. Asimismo, se analizó el factor de competencia aplicado y cuál fue la experiencia en otros proyectos

Mobile ions site identification through the isoconfigurational ensemble to reveal the ion dynamics diversity in a glass

When applying the Molecular Dynamics (MD) formalism to the short-time dynamics on a paradigmatic lithium metasilicate glass system, it reveals the major features of the mobility. Experimental evidence of the physical phenomenon known as the Nearly Constant Loss (NCL) can be attributed to the independent motion of a small proportion of uncorrelated lithium ions which can leave their cages. Dynamical and structural results in this work when applying the isoconfigurational ensemble method (IEM) to the lithium metasilicate MD simulation, reveal the existence of two different lithium environment, lithium ions with high and low propensity to move. The dynamics of each kind of lithium shows that those ions hosted in high-propensity sites displace continuously in time and are responsible for contributing to the slow Mean Square Displacement (MSD) increase in time due to a highly interconnected path. On the other side, the low-propensity lithium ions remain confined to their cages for a longer time until they leave them and jump to the next site. Low-propensity lithium ions only contribute to the MSD when they reach the pre-diffusive time, after 40 ps, even after the maximum dynamical heterogeneity time at 700 K. The results reported in this research evidence that the NCL microscopic origin is due to those highly-dynamically- connected system regions which do not confine the lithium ion, i.e. the so called high-propensity lithium ions in this work.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Frechero, Marisa Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentin

Structural signatures of (two) characteristic dynamical temperatures in lithium metasilicate

We report on the dynamic and structural characterization of lithium metasilicate Li2SiO3, a network-forming ionic glass, by means of molecular dynamics simulations. The system is characterized by a network of SiO4 tetrahedra disrupted by Li ions which diffuse through the network. Measures of mean square displacement and the diffusion constant of Si and O atoms allow us to identify the mode-coupling temperature, Tc ≈ 1500 K. At a much lower temperature, a change in the slope of the specific volume versus temperature singles out the glass transition at Tg ≈ 1000 K, the temperature below which the system goes out of equilibrium. We find signatures of both dynamical temperatures in structural order parameters related to the orientation of the tetrahedra. At lower temperatures we find that a set of order parameters which measure the relative orientation of neighbouring tetrahedra cease to increase and stay constant below Tc. Nevertheless, the bond orientational order parameter, which in this system measures local tetrahedral order, is found to continue growing below Tc until Tg, below which it remains constant. Although these structural signatures of the two dynamical temperatures do not imply any real thermodynamic transition in terms of the order parameters, they do give insight into the relaxation processes that occur between Tc and Tg, in particular they allow us to characterize the nature of the crossover happening around Tc.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; ArgentinaFil: Brito, Carolina. Universidade Federal do Rio Grande do Sul; BrasilFil: Stariolo, Daniel. Universidade Federal do Rio Grande do Sul; Brasi

Channel diffusion in a lithium–potassium metasilicate glass using the isoconfigurational ensemble: towards a scenario for the mixed alkali effect

Performing Molecular Dynamic simulations and using the isoconfigurational ensemble method, we studied the effect of the potassium cation replacing the half part of lithium ions in glassy Li2SiO3. This so-constructed glassy system has the main ingredients present in an immediate forthcoming definition of mixed alkali effect (MAE) in glasses. We show the existence of dynamic correlations among the cations of the same species, i.e. Li–Li and K–K, whereas a very weak correlation was observed between a distinct pair of cations. With this novel approach we can put into evidence that the alkali ion diffusion evolves in specific channels for the ions: a Li ion prefers the lithium ion channel and a K ion prefers the potassium ion channel. This result is coincident with previous simulational studies using the bond–valence technique to reverse Monte Carlo and recent experimental findings using quasielastic neutron scattering.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; ArgentinaFil: Frechero, Marisa Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; ArgentinaFil: Montani, Ruben Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentin

Looking at the dynamical heterogeneity in a supercooled polymer system through isoconfigurational ensemble

The dynamic correlations that emerge in a polymer system in supercooling conditions have been studied using molecular dynamic simulations. It is known that when a glass former approaches the glass transition temperature, the dynamics of the system (in terms of the mobilities of the particles) not only significantly slows down but also becomes more heterogeneous. Several theories relate this slowing down to increasing spatial (structural) correlations, for example, through the onset of cooperative relaxation regions in the Adam-Gibbs theory. In this work, we employ Pearson’s coefficient in the isoconfigurational ensemble (ICE) which allows us to study the dynamic correlations of the monomers in the ICE and establish the relation between the structure of the monomers and its dynamic behavior. Similar to what happens with mobility, monomers with highest correlation are clustered, and the clustering increases with decreasing temperature. An interesting result is that regions with high ICE dynamic correlation are not coincident with highly mobile or immobile regions. These results represent a new approach to the study of dynamic heterogeneity that emerges in glass forming liquids, complementing the more traditional characterization in terms of mobility. The methodology proposed in this work that characterize the connected dynamic regions to structural causes can represent an alternative way to observe the cooperative relaxation regions.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Gianetti, Melisa Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Soulé, Ezequiel Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Is ergodicity in an oxide glass ionic conductor a matter of time?

From the results of molecular dynamic simulations of lithium metasilicate glass - at temperatures above and below their transition temperature (Tg ) - we propose a simple graphical representation to search for the broken ergodicity in an ionic oxide glass. Knowing whenergodicity is lost is critical for the proper use of statistical mechanics as a tool for measuring dynamical and structural properties through molecular dynamic simulation. This work shows how an abrupt qualitative transformation occurs in the way the system explores itspossible states when it goes down below the glass transition temperature range. We revise the broken ergodicity phenomena through its relationship with the observation time and the dynamic diversity of their atoms.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Química del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Montani, Ruben Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Química del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Frechero, Marisa Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Química del Sur; Argentina. Universidad Nacional del Sur; Argentin

A structural study and its relation to dynamic heterogeneity in a polymer glass former

The relationship between structure and dynamical behavior (super-Arrhenius temperature dependence of relaxation time accompanied by heterogeneous dynamics) in glassy materials remains an open issue in the physics of condensed matter. The question of whether this dynamic phenomena have a thermodynamic origin or not still remains unanswered. In this work we analyze several dynamic and structural parameters in a polymer glass-former by means of molecular dynamics simulations. The results obtained in this work indicate that the structure does affect dynamic behavior, whereas structural conditioning becomes noticeable below the temperature at which the non-Arrhenius behavior manifests and increases as the system approaches the glass transition temperature. Moreover, we observed that the short-range order parameters are related to local dynamics at the single-particle level. These results reinforce the idea of a connection between the structure and dynamics and that could indicate the thermodynamic nature of glass transition.Fil: Balbuena, Cristian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Gianetti, Melisa Mariel. Università degli Studi di Milano; Italia. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Soulé, Ezequiel Rodolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin