Why the glass transition problem remains unsolved?

Ngai, K. L.

doi:10.1016/j.jnoncrysol.2006.12.033

Cited by 99 publications

(95 citation statements)

References 75 publications

(165 reference statements)

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“…The fact that intermolecular constraints rule the dynamics is supported by results of systematic changes of dynamics with increase of intermolecular constraints and hence the presence of correlations between n, m, and the ratio τ α /τ β . These results from MD simulations are fully compatible with the experimental data of mixtures of small molecule glassformers [38][39][40][41], and polymer blends [42][43][44], which have been explained by the CM [37,38,[43][44][45] 4 .…”

Section: Dynamic Propertiessupporting

confidence: 86%

Guides to solving the glass transition problem

Ngai¹,

Prevosto²,

Capaccioli³

et al. 2008

J. Phys.: Condens. Matter

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Relaxation in glass-forming substances is necessarily a many-body problem because of intermolecular interactions and constraints. Results from molecular dynamics simulations and experiments are used to reveal the critical elements and general effects originating from many-body relaxation, but not dealt with in conventional theories of the glass transition. Although many-body relaxation is still an unsolved problem in statistical mechanics, these critical elements and general effects will serve as guides to the construction of a satisfactory theory of the glass transition. This effort is aided by concepts drawn from the coupling model, whose predictions have been shown to be consistent with experimental facts.

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Section: Dynamic Propertiessupporting

confidence: 86%

Guides to solving the glass transition problem

Ngai¹,

Prevosto²,

Capaccioli³

et al. 2008

J. Phys.: Condens. Matter

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show abstract

“…18 The glass transition temperature was calculated as the temperature at which the Arrhenius Eq. 2 gave a value of τ α equal to 100 s. Starting from the fitting parameters of the α relaxation feature, one may employ the so-called Coupling Model (CM) 32 to calculate the relaxation time τ CM of the precursor relaxation associated with the glass transition dynamics, i.e., the characteristic time of the (single-molecule) Johari-Goldstein relaxation. 33,34 According to the CM, the relaxation time τ CM of the precursor relaxation should be related with that of the primary process (τ α ) as:…”

Section: Resultsmentioning

confidence: 99%

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

Tripathi

Mitsari

Romanini

et al. 2016

The Journal of Chemical Physics

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We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156±1 K, corresponds to a cooperative motion by which each molecule rotates by 180º around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids. 2 INTRODUCTIONWhile conventional (atomic) solids are made of atomic constituents with only translational degrees of freedom, so that their structure is totally determined by translation symmetry and fundamental excitations are vibrational in character, in molecular solids the constituent molecules possess also orientational (as well as internal) degrees of freedom, which lead to a richer variety of possible solid phases and to the existence of rotational excitations such as librations and orientational relaxations. A molecular solid can display complete translational and rotational order, as in a molecular crystal, or complete rototranslational disorder, as in a molecular glass. In between these two extremes, molecular solids also display phases (known as Orientationally disordered (OD) phases are generally formed by relatively small globular molecules such as derivatives of methane, 1,2,3 neopentane, 4 adamantane 5 or fullerene, 6 or by small linear ones such as ethane derivatives 7,8,9 and dinitriles. 10 OD solids exhibit many of the phenomenological features of glass formers, displaying in particular a cooperative rotational motion, called α relaxation, that undergoes a continuous, dramatic slow-down upon cooling, 11,12 leading in some cases to a glass-like 3 transition associated with rotational freezing. 13,14 Contrary to structural glasses, which do not exhibit any long-range order, OD phases are characterized by a translationally ordered structure and can therefore be more thoroughly characterized with the help of methods that exploit the translational symmetry such as Bragg diffraction, lattice models, or solid-state NMR spectroscopy. Even more importantly, since as mentioned OD phases are generally formed by molecular species with a simple structure ...

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“…(Kremer and Schönhals, 2003;Ngai and Paluch, 2004) The latter type of secondary relaxations are termed Johari-Goldstein relaxations and are directly correlated with the primary α dynamics (Johari and Goldstein, 1970;Ngai, 1998). For example, according to the so-called Coupling Model (Ngai, 2007), the relaxation time τ CM of the JohariGoldstein relaxation should be related with that of the primary process (τ α ) as:…”

Section: Resultsmentioning

confidence: 99%

“…Here t c is a cross-over time, whose typical value is 2 × 10 -12 s for molecular and polymeric glass formers (Colmenero et al, 1997;Ngai, 2007), and p is a shape parameter of the α relaxation given by ( Alvarez et al, 1991;Alvarez et al, 1993). The theoretical precursor time τ CM calculated using Eq.…”

Section: Resultsmentioning

confidence: 99%

Collective relaxation dynamics and crystallization kinetics of the amorphous Biclotymol antiseptic

Tripathi

Romanini

Tamarit

et al. 2015

International Journal of Pharmaceutics

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We employ dielectric spectroscopy to monitor the relaxation dynamics and crystallization kinetics of the Biclotymol antiseptic in its amorphous phase. The glass transition temperature of the material as determined by dielectric spectroscopy is T g = 290±1 K. The primary (α) relaxation dynamics is observed to follow a Vogel-Fulcher-Tammann temperature dependence, with a kinetic fragility index m = 86±13, which classifies Biclotymol as a relatively fragile glass former. A secondary relaxation is also observed, corresponding to an intramolecular dynamic process of the non-rigid Biclotymol molecule. The crystallization kinetics, measured at four different temperatures above the glass transition temperature, follows an Avrami behavior with exponent virtually equal to n = 2, indicating one-dimensional crystallization into needle-like crystallites, as experimentally observed, with a time-constant nucleation rate. The activation barrier for crystallization is found to be E a = 115±22 kJ mol -1 .

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Why the glass transition problem remains unsolved?

Cited by 99 publications

References 75 publications

Guides to solving the glass transition problem

Guides to solving the glass transition problem

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

Collective relaxation dynamics and crystallization kinetics of the amorphous Biclotymol antiseptic

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