On the correlation between fragility and stretching in glass-forming liquids

Niss, Kristine; Dalle-Ferrier, Cécile; Tarjus, Gilles; Alba‐Simionesco, Christiane

doi:10.1088/0953-8984/19/7/076102

Cited by 69 publications

(58 citation statements)

References 71 publications

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“…On the other hand this effect can be compensated by heating up the liquid. Different combinations of p and T can result in the same relaxation dynamics at same τ or materials obey the temperature-pressure superposition at the same relaxation times (TTPS) [6][7][8][9][10][11]. There are differnet kinds of secondary (β) relaxation including the Johari-Goldstein (JG) and these of intra-molecular motions [11][12][13][14][15][16] or any excess wings.…”

Section: To Cite This Versionmentioning

confidence: 99%

Pressure dependence of the dielectric loss minimum slope for ten molecular liquids

Nielsen

Pawlus

Paluch

et al. 2008

Philosophical Magazine

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To cite this version:Albena Ivanova Nielsen, Sebatian Pawlus, Marian Paluch, Jeppe Dyre. Pressure dependence of the dielectric loss minimum slope for ten molecular liquids. Philosophical Magazine, Taylor & Francis, 2008, 88 (33-35) We present a comprehensive study of data for the dielectric relaxation of ten glass-forming organic liquids at high pressure along isotherms, showing that the primary (α) high frequency relaxation is characterized well by the minimum slope and the width of the loss peak. The advantage of these two parameters is that they are model independent. For some materials with β processes in mHz and kHz range the high frequency slope tends to be −1/2 with pressure increase. Besides the two parameters captures the relaxation shape invariance at given relaxation time but different combinations of pressure and time [6].Glass may be regarded as the fourth state of conventional matter, isotropic as the liquid state, but solid as the crystalline state. With the notable exception of helium, any liquid may be turned into glass by cooling it fast enough to avoid crystallization [1][2][3][4][5].Physical systems usually relax following perturbations forced upon them. The relaxation of the systems consists of processes going on different time scales. The dominant and slowest relaxation process of a glass-forming liquid is the so-called α process. The α process defines the liquid relaxation time, an important quantity because the glass transition takes place when the relaxation time significantly exceeds the inverse relative cooling rate. Compression of supercooled liquid slows down the α relaxation (increases the characteristic relaxation time, τ ). On the other hand this effect can be compensated by heating up the liquid. Different combinations of p and T can result in the same relaxation dynamics at same τ or materials obey the temperature-pressure superposition at the same relaxation times (TTPS) [6][7][8][9][10][11]. There are differnet kinds of secondary (β) relaxation including the Johari-Goldstein (JG) and these of intra-molecular motions [11][12][13][14][15][16] or any excess wings.In a paper from 2001 it was shown that the high frequency slope of the dielectric loss for a group of materials tends to be −1/2 as the temperature approaches the glass transition temperature, T → T g , and this was linked to time-temperature superposition (TTS) [17], but there is found a general prevalence of the −0.5 for the slope in highly viscous liquids no matter if TTS is obeyed [18]. In those papers the dielectric scans were taken at ambient pressure. In the following is investigated: if this result holds for dielectric frequency scans for ten liquids along isotherms with increasing pressure (p → p g ) ; and if the used model-independent shape quantities, minimum slope and half loss peak width capture TTPS. The minimum slope, of course, may well be affected by secondary processes, the well expressed in the experimental frequency window as well the underlying low-frequency β relaxation * Corresponding author. Emai...

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Section: To Cite This Versionmentioning

confidence: 99%

Pressure dependence of the dielectric loss minimum slope for ten molecular liquids

Nielsen

Pawlus

Paluch

et al. 2008

Philosophical Magazine

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“…These phthalates are good glassformers and extensively used in industry for various purposes. Their dynamic properties over broad frequency/time range have been studied by several experimental techniques including optical Kerr effect [31], dynamic light scattering [32], dielectric relaxation measurements at ambient and elevated pressures [30,33,34], and inelastic X ray scattering [35]. The results of these studies have impact on theoretical models of glass transition [36--38].…”

Section: Introductionmentioning

confidence: 99%

Is the Johari-Goldstein β-relaxation universal?

Thayyil

Capaccioli

Prevosto

et al. 2008

Philosophical Magazine

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“…Not all transition points are equal: MCT predicts nonuniversal amplitudes and shapes for relaxation spectra that change along the transition line, invalidating a 'temperature-pressure' superposition principle. But the changes are small enough to make it appear to work, explaining why some experiments find it [12,13,14,15,24,45], some with restrictions [27,54,55,56], some not at all [25,57].…”

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Idealized Glass Transitions under Pressure: Dynamics versus Thermodynamics

Voigtmann

2008

Phys. Rev. Lett.

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The interplay of slow dynamics and thermodynamic features of dense liquids is studied by examinining how the glass transition changes depending on the presence or absence of Lennard-Jones-like attractions. Quite different thermodynamic behavior leaves the dynamics unchanged, with important consequences for high-pressure experiments on glassy liquids. Numerical results are obtained within mode-coupling theory (MCT), but the qualitative features are argued to hold more generally. A simple square-well model can be used to explain generic features found in experiment. The quest for identifying the physical mechanism behind the dynamical transformation of a liquid into an amorphous solid, the glass transition, has prompted many studies aiming to disentangle the dominant control variables involved. It is recognized that the slow dynamics connected with the glass transition is universal, but its connection to the underlying liquid structure is highly debated [1,2,3,4,5]. Some argue in terms of a density effect called free or excluded volume; others attribute the main physics to energetic interactions and thermally activated processes.Experiments changing both temperature T and pressure P close to the transition are emerging to resolve such issues, but have brought contradictory results. Some find that temperature dominates glassy dynamics by far [6,7,8,9]; some that it does not [10,11,12]. Others find both variables to exert equal influence [13,14,15,16,17,18,19,20,21,22], some with temperature [23,24,25,26,27], some with density ̺ being more relevant [28,29,30,31]. The difficulty of obtaining data over wide pressure ranges might be impeding: few studies, pioneered only in the 1990's [28,32], go beyond 1 GPa.Here we propose that to resolve the apparent contradictions, one needs to separate non-universal thermodynamic aspects, namely the equation of state (EOS) of the system, from universal dynamical features, viz. the slow relaxation. Specifically, we show how the presence or absence of attractive interactions affects the glass transition, and how this emerges in different pairs of variables linked by the EOS: (̺, T ) (preferred by theory) vs. (P, T ) (more amenable to experiments), yielding a transition that appears 'temperature-driven' in the latter.The Lennard-Jones (LJ) potential serves as a realistic interaction model: V LJ (r) = 4ǫ[(r/σ) −12 −(r/σ) −6 ], with dimensionless parameters ̺ * = ̺σ 3 , T * = 1/(βǫ), P * = P σ 3 /ǫ; β = 1/(k B T ) with Boltzmann's constant k B . To study relative effects of entropy and energy, we compare the LJ glass transition with that of its purely repulsive (LJR) counterpart, V LJR (r) = V LJ (r) + ǫ ≥ 0 for r ≤ 2 1/6 σ, V LJR (r) = 0 else. For the transition lines, modecoupling theory (MCT) [33,34] provides a reasonable qualitative description. Its transition temperature T c is systematically above the experimental (calorimetric) one, T g , but nevertheless serves as a good indicator for the change from high-T liquid-like dynamics to low-T glasslike one [35,36]. In MCT, T c is the...

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On the correlation between fragility and stretching in glass-forming liquids

Cited by 69 publications

References 71 publications

Pressure dependence of the dielectric loss minimum slope for ten molecular liquids

Pressure dependence of the dielectric loss minimum slope for ten molecular liquids

Is the Johari-Goldstein β-relaxation universal?

Idealized Glass Transitions under Pressure: Dynamics versus Thermodynamics

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