1,721,183 research outputs found
Changes of the primary and secondary relaxation of sorbitol in mixtures with glycerol
No full textThe dielectric relaxation experimental data on the alpha- and Johari-Goldstein beta-relaxation dynamics of sorbitol in the neat state and in mixtures with glycerol published by Duvvuri and Richert (J. Phys. Chem. B, published online Apr 23, http://dx.doi.org/10.1021/jp031366b) is challenging to explain. Not only does one need a theory or model that can address both the alpha- and Johari-Goldstein beta-relaxation dynamics in a neat glassformer, but one also a theory or model that can address them in a mixture. The coupling model fulfills these requirements. Naturally, we employ the coupling model to interpret the experimental data. Fair agreements are found between the predictions of the model and data of neat sorbitol and mixtures of sorbitol with glycerol
Relation between the alpha-relaxation and Johari-Goldstein beta-relaxation of a component in binary miscible mixtures of glass-formers
No full textThe coupling model was applied to describe the alpha-relaxation dynamics of each component in perfectly miscible mixtures A(1)-(x)B(x) of two different glass-formers A and B. An important element of the model is the change of the coupling parameter of each component with the composition, x, of the mixture. However, this change cannot be determined directly from the frequency dispersion of the alpha-relaxation of each component because of the broadening caused by concentration fluctuations in the mixture, except in the limits of low concentrations of either component, x -> 0 and x -> 1. Fortunately, the coupling model has another prediction. The coupling parameter of a component, say A, in the mixture determines tau(alpha)/tau(JG), the ratio of the alpha-relaxation time, tau(alpha), to the Johari-Goldstein (JG) secondary relaxation time, tau(JG), of the same component A. This prediction enables us to obtain the coupling parameter, (n) over cap (A), Of component A from the isothermal frequency spectrum of the mixture that shows both the alpha-relaxation and the JG beta-relaxation of component A. We put this extra prediction into practice by calculating (n) over cap (A) of 2-picoline in binary mixtures with either tri-styrene or o-terphenyl from recently published broadband dielectric relaxation data of the alpha-relaxation and the JG beta-relaxation of 2-picoline. The results Of (n) over cap (A) obtained from the experimental data show its change with composition, x, follows the same pattern as assumed in previous works that address only the alpha-relaxation dynamics of a component in binary mixtures based on the coupling model. There is an alternative view of the thrust of the present work. If the change of (n) over cap (A) with composition, x, in considering the alpha-relaxation of component A is justified by other means, the theoretical part of the present work gives a prediction of how the ratio tau(alpha)/tau(JG) of component A changes with composition, x. The data of tau(alpha), and tau(JG) of 2-picoline mixed with tri-styrene or o-terphenyl provide experimental support for the prediction
Dispositivo di misura delle proprietà elettriche di campioni geologici solidi e liquidi
No full textRelation between the activation energy of the Johari-Goldstein beta relaxation and T(g) of glass formers
Full text linkFor glass-forming substances, we show that the ratio E(beta)/RT(g) can be predicted quantitatively from the coupling model. Here E(beta) is the glassy state activation enthalpy of the Johari-Goldstein beta relaxation, T(g) is the glass transition temperature of the alpha relaxation, and R is the gas constant. The calculated value is in good agreement with the experimental value in many glass formers. The results locate the origin of this cross correlation between E(beta) of the Johari-Goldstein beta relaxation and T(g) of the alpha relaxation, although there are some notable exceptions to this cross correlation
Reconsidering the Dynamics in Mixtures of Methyltetrahydrofuran with Tristyrene and Polystyrene
Full text linkMixtures of methyltetrahydrofuran (MTHF) with tristyrene and high molecular weight polystyrene involve an exceptionally large difference in the glass transition temperatures of the two components not realized in other binary mixtures studied before. The extensive study of the molecular dynamics of these mixtures by various experimental techniques by Blochowicz et al. has revealed the presence of a new Α'-relaxation not found before in other mixtures and also the more familiar Α- and Β-relaxations, but their properties are more extreme. Attention was focused on the new Α'-relaxation by Blochowicz et al. in interpreting it to originate from MTHF in confinement and explaining its properties by the Mode Coupling Theory. In a different direction, we concentrate on the highly unusual properties of the Α- and Β-relaxations. Earlier, we had success in explaining the properties of these two relaxations and their connection in other mixtures by the coupling model. In this paper, we apply the same model to explain the highly unusual dynamics of the Α- and Β-relaxations found in the mixtures of MTHF with tristyrene and polystyrene. Possible relation between the Α'- and the Β-relaxations also is explored
Surface diffusion of polymer glasses redux
Full text linkMeasurements of the surface self-diffusion coefficients of glass-forming small molecules, indomethacin (IMC), nifedipine (NIF),2 and o-terphenyl (OTP),3 by Yu and coworkers using the method of surface grating decay have found exceedingly large enhancement of molecular mobility at the surface. The decay of surface grating in all three molecules occurs by viscous flow at high temperatures, but by surface diffusion at lower temperatures starting at about 10 deg above Tg
The challenging problem of glass transition
No full textThe glass transition phenomena are observed in many different classes of materials and have been studied in various disciplines. It is a long-standing problem due to the increasing number of
important and general experimental facts that challenge conventional theories to explain. A collection of such experimental facts on the structural relaxation is given here, together with the observation
that they are either governed by or correlated with the time/frequency dispersion of the structural relaxation. There is a class of secondary relaxations (named after Johari–Goldstein
(JG)) that are well-connected to the structural relaxation in many ways. Thus, a fully successful theory of glass transition must take into consideration the roles played by the dispersion of
the structural relaxation and the JG secondary relaxation. The results from our study of mixtures of van der Waals liquids are presented to illustrate these points, and a satisfactory explanation
of the data is given. The understanding of the component dynamics gained from the study of these ideal systems is used to elucidate and interpret the experimental data of aqueous mixtures and hydrated proteins, which are more complicated systems. This exercise illustrates the benefit of broadening the study of one class of glass-forming materials to another class normally investigated by workers in another disciplin
On the relevance of the coupling model to experiments
No full textThe research field of the glass transition is experimentally driven. There is an abundance of experimental data and facts accumulated in the past as well as the emergence of important new results every now and then. Therefore, when judging theories and models of the glass transition, it is fair to ask the question of their relevance to experiments. This question was one of the themes of a round-table discussion session entitled An assessment of current theories: interconnections and relevance to experiments, organized at the 4th Workshop on Non-equilibrium Phenomena in Supercooled Fluids, Glasses and Amorphous Materials (4th WNEP). The coupling model has a history of making connections of its theoretical results with many experimental facts on dynamics of glassforming substances. This characteristic of the coupling model is demonstrated herein by using it to explain or rationalize some of the new experimental data reported at the 4th WNEP and published in this volume and elsewhere
Comment on "A Generalized Rouse Incoherent Scattering Function for Chain Dynamics of Unentangled Polymers in Dynamically Asymmetric Blends"
No full textImpact of the application of pressure on the fundamental understanding of glass transition
No full textSeveral remarkable dynamic properties of glass-forming materials have recently been discovered experimentally by the application of pressure. These properties have had a great impact on the research field of glass transition because they are general and fundamental, and not easy to explain. We review some of these experimental facts and show that they originate from the intermolecular interactions and many-body relaxation dynamics of the structural α-relaxation. While these properties are either not explained or not explainable by conventional theories and models, they can be rationalized by the coupling mode
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