Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

Phys. Chem. Chem. Phys.

Rams-Baron

et al. 2018

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Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via the Coupling Model, secondary relaxations having such connection have been identified by properties similar to the primitive relaxation of the Coupling Model and are called the Johari-Goldstein (JG) β-relaxations. They involve the motion of the entire molecule and act as the precursor of the structural α-relaxation. The change in dynamics of the secondary relaxation by aging an ordinary glass is one way to understand the connection between the two relaxations, but the results are often equivocal. Ultrastable glasses, formed by physical vapour deposition, exhibit density and enthalpy levels comparable to ordinary glasses aged for thousands of years, as well as some particular molecular arrangement. Thus, ultrastable glasses enable the monitoring of the evolution of secondary processes in case aging does not provide any definitive information. Here, we study the secondary relaxation of several ultrastable glasses to identify different types of secondary relaxations from their different relationship with the structural relaxation. We show the existence of two clearly differentiated groups of relaxations: those becoming slower in the ultrastable state and those becoming faster, with respect to the ordinary unaged glass. We propose ultrastability as a way to distinguish between secondary processes arising from the particular microstructure of the system and those connected in properties to and acting as the precursor of the structural relaxation in the sense of the Coupling Model.

Section: Resultsmentioning

confidence: 57%

Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses

Rodríguez-Tinoco

Phys. Chem. Chem. Phys.

Rams-Baron

et al. 2018

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“…The explanation is beneficial to complete a previous rationalization of another remarkable experimental finding, 29 which is approximately the same surface diffusion coefficient D S in USG, OG, and nanometer thin films of the same glassformer, [30][31][32] while their structural a-relaxation times differ by many orders of magnitude. The rationalization is based on the relation D S (T) E d 2 /4t(T), where d is the size of the molecule, used previously to account for the enhancement of the surface diffusion 29 (further support for the use of this expression is shown in the ESI, † Fig. S5), and the experimental evidence showing that t b (T) is approximately the same in USG, aged OG, OG, and ultrathin films.…”

Section: Discussionmentioning

confidence: 64%

“…The explanation also applies to another remarkable finding of approximately the same surface diffusion coefficient D S in USG, OG, and nanometer thin films of the same glass former. [29][30][31][32]…”

Section: Discussionmentioning

confidence: 99%

“…The explanation is also relevant in achieving a better understanding than before 29 of another remarkable experimental finding, which is about the same surface diffusion coefficient, D S , in USG and OG with and without physical aging, and ultrathin films of the molecular glass, N,N 0 -bis(3-methylphenyl)-N,N 0 -diphenylbenzidine (TPD) by Fakhraai and coworkers. [30][31][32]…”

Section: Introductionmentioning

confidence: 86%

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Why is the change of the Johari–Goldstein β-relaxation time by densification in ultrastable glass minor?

Phys. Chem. Chem. Phys.

Paluch

Rodríguez-Tinoco

2018

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“…The ubiquitous presence of the JG -relaxation and the inseparable relations of its relaxation times to that of the -relaxation (Eq.3) supported by many corroborative evidences such as given in Refs. [ 33,78,79,80,81,82,83 ] are critical in restoring the verity of the dynamics obtained by using dielectric permittivity spectroscopy of polar glass-formers. On the other hand, the presence of the JG -relaxation and its relations to the -relaxation were not recognized in the papers of Körber et al [ 19,84 ], Gabriel et al [ 43 ], and Pabst et al [ 46 ], and the consequence is that they were not able to reach the same conclusion.…”

Section: Discussionmentioning

confidence: 99%

Is there Universality in the Dielectric Response of Polar Glass Formers?

Wojnarowska

Paluch

2021

Preprint

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The frequency dispersion of structural α-relaxation obtained from broadband dielectric spectroscopy measurements is relatively narrow in many polar glass-formers. On the other hand, it becomes much broader when probed by other techniques, including photon correlation spectroscopy (PCS), nuclear magnetic resonance (NMR), and mechanical shear modulus. Therefore, the dynamics of glass-formers observed by dielectric permittivity spectroscopy (DS) is called into question. Herein we propose a way to resolve this problem. First, we point out an unresolved Johari-Goldstein (JG) β-relaxation is present nearby the α-relaxation in these polar glass-formers. The dielectric relaxation strength of the JG β-relaxation is sufficiently weak compared to the α-relaxation so that the narrow dielectric frequency dispersion faithfully represents the dynamic heterogeneity and cooperativity of the α-relaxation. However, when the other techniques are used to probe the same glass-former, there is a reduction of relaxation strength of α-relaxation relative to that of the JG β-relaxation. Additionally, the separation between the α and the JG β relaxations in dielectric permittivity) decreases when probed by mechanical shear modulus. These changes in relation of α- to JG β-relaxation, when examined by the other techniques, engender the non-negligible contribution of the latter to the former. Hence the apparent α-relaxation is broader than observed by the dielectric permittivity. The broadening is artificial because it is due to a confluence of the α and JG β relaxations with a disparity in their relaxation strengths much less when the other techniques than by dielectric permittivity are used. This explanation is supported by showing the α-relaxation of polar glass-formers becomes broader when the dielectric data are represented in terms of the electric modulus instead of permittivity. The broadening, in this case, is again due to a reduction of the relaxation strength of the α-relaxation relative to that of the JG β-relaxation in the electric modulus representation. A corollary of the explanation applicable to weakly polar glass-formers having JG β-relaxation widely separated from the α-relaxation is the prediction that the frequency dispersion of dielectric α-relaxation is nearly the same as that of the electric modulus, and there is no significant additional broadening when probed by the other techniques. A host of experimental data from the literature and our new measurements are given to support the explanation for polar glass-formers and the ancillary prediction for weakly polar glass-formers. Thus the narrow frequency dispersion of the intense relaxation in polar glass-formers observed by dielectric permittivity is real and genuinely represents the dynamically heterogeneous and cooperative dynamics of α-relaxation. By contrast, the broad dispersion found by the other techniques is artificial and misleading.