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Title: Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view

Abstract

Aging kinetics of a glass is currently modeled in terms of slowing of its α-relaxation dynamics, whose features are interpreted in terms of dynamic heterogeneity, i.e., formation and decay of spatially and temporally distinct nm-size regions. To test the merits of this view, we studied the calorimetric effects of aging an orientational glass of levoglucosan crystal in which such regions would not form in the same way as they form in liquids, and persist in structural glasses, because there is no liquid-like molecular diffusion in the crystal. By measuring the heat capacity, C{sub p}, we determined the change in the enthalpy, H, and the entropy, S, during two aging-protocols: (a) keeping the samples isothermally at temperature, T{sub a}, and measuring the changes after different aging times, t{sub a}, and (b) keeping the samples at different T{sub a}s and measuring the changes after the same t{sub a}. A model-free analysis of the data shows that as t{sub a} is increased (procedure (a)), H and S decrease according to a dispersive rate kinetics, and as T{sub a} is increased (procedure (b)), H and S first increase, reach a local maximum at a certain T{sub a}, and then decrease. Even though there ismore » no translational diffusion to produce (liquid-like) free volume, and no translational-rotational decoupling, the aging features are indistinguishable from those of structural glasses. We also find that the Kohlrausch parameter, originally fitted to the glass-aging data, decreases with decrease in T{sub a}, which is incompatible with the current use of the aging data for estimating the α-relaxation time. We argue that the vibrational state of a glass is naturally incompatible with its configurational state, and both change on aging until they are compatible, in the equilibrium liquid. So, dipolar fluctuations seen as the α-relaxation would not be the same motions that cause aging. We suggest that aging kinetics is intrinsically dispersive with its own characteristic rate constant and it does not yield the α-relaxation rate. In this view, thermodynamic and other properties define the fictive temperature; the real or imaginary components of a dynamic property do not define it. While particles’ overall motions may still play a crucial role in (structural) glass physics, we conclude that translational diffusion alone is not a requirement for structure stabilization on aging of a kinetically frozen state.« less

Authors:
;  [1];  [2]
  1. Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico-Fisici (CNR-IPCF), Via Moruzzi 1, 56124 Pisa (Italy)
  2. Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7 (Canada)
Publication Date:
OSTI Identifier:
22679027
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 145; Journal Issue: 5; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; DIFFUSION; DISPERSIONS; GLASS; LIQUIDS; MOLECULAR CRYSTALS; REACTION KINETICS; SPECIFIC HEAT; VIBRATIONAL STATES

Citation Formats

Righetti, Maria Cristina, Tombari, Elpidio, and Johari, G. P., E-mail: joharig@mcmaster.ca. Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view. United States: N. p., 2016. Web. doi:10.1063/1.4959806.
Righetti, Maria Cristina, Tombari, Elpidio, & Johari, G. P., E-mail: joharig@mcmaster.ca. Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view. United States. doi:10.1063/1.4959806.
Righetti, Maria Cristina, Tombari, Elpidio, and Johari, G. P., E-mail: joharig@mcmaster.ca. Sun . "Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view". United States. doi:10.1063/1.4959806.
@article{osti_22679027,
title = {Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view},
author = {Righetti, Maria Cristina and Tombari, Elpidio and Johari, G. P., E-mail: joharig@mcmaster.ca},
abstractNote = {Aging kinetics of a glass is currently modeled in terms of slowing of its α-relaxation dynamics, whose features are interpreted in terms of dynamic heterogeneity, i.e., formation and decay of spatially and temporally distinct nm-size regions. To test the merits of this view, we studied the calorimetric effects of aging an orientational glass of levoglucosan crystal in which such regions would not form in the same way as they form in liquids, and persist in structural glasses, because there is no liquid-like molecular diffusion in the crystal. By measuring the heat capacity, C{sub p}, we determined the change in the enthalpy, H, and the entropy, S, during two aging-protocols: (a) keeping the samples isothermally at temperature, T{sub a}, and measuring the changes after different aging times, t{sub a}, and (b) keeping the samples at different T{sub a}s and measuring the changes after the same t{sub a}. A model-free analysis of the data shows that as t{sub a} is increased (procedure (a)), H and S decrease according to a dispersive rate kinetics, and as T{sub a} is increased (procedure (b)), H and S first increase, reach a local maximum at a certain T{sub a}, and then decrease. Even though there is no translational diffusion to produce (liquid-like) free volume, and no translational-rotational decoupling, the aging features are indistinguishable from those of structural glasses. We also find that the Kohlrausch parameter, originally fitted to the glass-aging data, decreases with decrease in T{sub a}, which is incompatible with the current use of the aging data for estimating the α-relaxation time. We argue that the vibrational state of a glass is naturally incompatible with its configurational state, and both change on aging until they are compatible, in the equilibrium liquid. So, dipolar fluctuations seen as the α-relaxation would not be the same motions that cause aging. We suggest that aging kinetics is intrinsically dispersive with its own characteristic rate constant and it does not yield the α-relaxation rate. In this view, thermodynamic and other properties define the fictive temperature; the real or imaginary components of a dynamic property do not define it. While particles’ overall motions may still play a crucial role in (structural) glass physics, we conclude that translational diffusion alone is not a requirement for structure stabilization on aging of a kinetically frozen state.},
doi = {10.1063/1.4959806},
journal = {Journal of Chemical Physics},
number = 5,
volume = 145,
place = {United States},
year = {Sun Aug 07 00:00:00 EDT 2016},
month = {Sun Aug 07 00:00:00 EDT 2016}
}