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Grzybowski A, Koperwas K, Paluch M. Role of anisotropy in understanding the molecular grounds for density scaling in dynamics of glass-forming liquids. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2024; 87:084501. [PMID: 38861964 DOI: 10.1088/1361-6633/ad569d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2024] [Accepted: 06/11/2024] [Indexed: 06/13/2024]
Abstract
Molecular Dynamics (MD) simulations of glass-forming liquids play a pivotal role in uncovering the molecular nature of the liquid vitrification process. In particular, much focus was given to elucidating the interplay between the character of intermolecular potential and molecular dynamics behaviour. This has been tried to achieve by simulating the spherical particles interacting via isotropic potential. However, when simulation and experimental data are analysed in the same way by using the density scaling approaches, serious inconsistency is revealed between them. Similar scaling exponent values are determined by analysing the relaxation times and pVT data obtained from computer simulations. In contrast, these values differ significantly when the same analysis is carried out in the case of experimental data. As discussed thoroughly herein, the coherence between results of simulation and experiment can be achieved if anisotropy of intermolecular interactions is introduced to MD simulations. In practice, it has been realized in two different ways: (1) by using the anisotropic potential of the Gay-Berne type or (2) by replacing the spherical particles with quasi-real polyatomic anisotropic molecules interacting through isotropic Lenard-Jones potential. In particular, the last strategy has the potential to be used to explore the relationship between molecular architecture and molecular dynamics behaviour. Finally, we hope that the results presented in this review will also encourage others to explore how 'anisotropy' affects remaining aspects related to liquid-glass transition, like heterogeneity, glass transition temperature, glass forming ability, etc.
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Affiliation(s)
- A Grzybowski
- Institute of Physics, University of Silesia in Katowice, 41-500 Chorzów, Poland
| | - K Koperwas
- Institute of Physics, University of Silesia in Katowice, 41-500 Chorzów, Poland
| | - M Paluch
- Institute of Physics, University of Silesia in Katowice, 41-500 Chorzów, Poland
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2
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Shiraishi K, Mizuno H, Ikeda A. Johari-Goldstein β relaxation in glassy dynamics originates from two-scale energy landscape. Proc Natl Acad Sci U S A 2023; 120:e2215153120. [PMID: 36989301 PMCID: PMC10083593 DOI: 10.1073/pnas.2215153120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2022] [Accepted: 02/17/2023] [Indexed: 03/30/2023] Open
Abstract
Supercooled liquids undergo complicated structural relaxation processes, which have been a long-standing problem in both experimental and theoretical aspects of condensed matter physics. In particular, past experiments widely observed for many types of molecular liquids that relaxation dynamics separated into two distinct processes at low temperatures. One of the possible interpretations is that this separation originates from the two-scale hierarchical topography of the potential energy landscape; however, it has never been verified. Molecular dynamics simulations are a promising approach to tackle this issue, but we must overcome laborious difficulties. First, we must handle a model of molecular liquids that is computationally demanding compared to simple spherical models, which have been intensively studied but show only a slower process: α relaxation. Second, we must reach a sufficiently low-temperature regime where the two processes become well-separated. Here, we handle an asymmetric dimer system that exhibits a faster process: Johari-Goldstein β relaxation. Then, we employ the parallel tempering method to access the low-temperature regime. These laborious efforts enable us to investigate the potential energy landscape in detail and unveil the first direct evidence of the topographic hierarchy that induces the β relaxation. We also successfully characterize the microscopic motions of particles during each relaxation process. Finally, we study the correlation between low-frequency modes and two relaxation processes. Our results establish a fundamental and comprehensive understanding of experimentally observed relaxation dynamics in supercooled liquids.
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Affiliation(s)
- Kumpei Shiraishi
- Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo153-8902, Japan
| | - Hideyuki Mizuno
- Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo153-8902, Japan
| | - Atsushi Ikeda
- Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo153-8902, Japan
- Research Center for Complex Systems Biology, Universal Biology Institute, University of Tokyo, Komaba, Tokyo153-8902, Japan
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3
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Mutual Information in Molecular and Macromolecular Systems. Int J Mol Sci 2021; 22:ijms22179577. [PMID: 34502480 PMCID: PMC8430596 DOI: 10.3390/ijms22179577] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2021] [Revised: 08/24/2021] [Accepted: 08/28/2021] [Indexed: 11/22/2022] Open
Abstract
The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari–Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.
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Holt AP, Fragiadakis D, Roland CM. Dynamic Properties of Supercooled Chlorinated Biphenyls. J Phys Chem B 2020; 124:5073-5078. [PMID: 32432473 DOI: 10.1021/acs.jpcb.0c02601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A study of the dynamics of a series of biphenyl compounds having varying chlorine levels was carried out. Increasing the chlorine content increases the glass transition temperature and makes the dynamics substantially more sensitive to density changes. Nonetheless, in the vicinity of their respective glass transitions, the different liquids display very similar extents of dynamic correlation and dynamic heterogeneity. The slight narrowing of the relaxation peak with increasing chlorine follows the general trend of the effect of increasing molecular polarity. This relationship between the peak breadth and dipole moment was reproduced in molecular dynamics simulations of a simplified model of the Aroclor molecule.
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Affiliation(s)
- A P Holt
- Chemistry Division, Code 6105, Naval Research Laboratory, Washington, D.C. 20375-4032, United States
| | - D Fragiadakis
- Chemistry Division, Code 6105, Naval Research Laboratory, Washington, D.C. 20375-4032, United States
| | - C M Roland
- Chemistry Division, Code 6105, Naval Research Laboratory, Washington, D.C. 20375-4032, United States
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Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation. Polymers (Basel) 2020; 12:polym12040761. [PMID: 32244537 PMCID: PMC7240390 DOI: 10.3390/polym12040761] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2020] [Revised: 03/19/2020] [Accepted: 03/24/2020] [Indexed: 11/24/2022] Open
Abstract
The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.
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6
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Niss K, Hecksher T. Perspective: Searching for simplicity rather than universality in glass-forming liquids. J Chem Phys 2018; 149:230901. [PMID: 30579292 DOI: 10.1063/1.5048093] [Citation(s) in RCA: 34] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
This article gives an overview of experimental results on dynamics in bulk glass-forming molecular liquids. Rather than looking for phenomenology that is universal, in the sense that it is seen in all liquids, the focus is on identifying the basic characteristics, or "stylized facts," of the glass transition problem, i.e., the central observations that a theory of the physics of glass formation should aim to explain in a unified manner.
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Affiliation(s)
- Kristine Niss
- Glass and Time, IMFUFA, Department of Science and Environment, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark
| | - Tina Hecksher
- Glass and Time, IMFUFA, Department of Science and Environment, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark
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Pötzschner B, Mohamed F, Bächer C, Wagner E, Lichtinger A, Bock D, Kreger K, Schmidt HW, Rössler EA. Non-polymeric asymmetric binary glass-formers. II. Secondary relaxation studied by dielectric, 2H NMR, and 31P NMR spectroscopy. J Chem Phys 2018; 146:164504. [PMID: 28456197 DOI: 10.1063/1.4980085] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We investigate the secondary (β-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; Tg = 356 K) as the high-Tg component and the low-Tg component tripropyl phosphate (TPP; Tg = 134 K). The main relaxations are studied in Paper I [B. Pötzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high Tg contrast of ΔTg = 222 K is put into effect in a non-polymeric system. Component-selective studies are carried out by combining results from dielectric spectroscopy (DS) for mass concentrations cTPP ≥ 60% and those from different methods of 2H and 31P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ cTPP ≤ 100%) is covered. The neat components exhibit a β-relaxation (β1 (SBC) and β2 (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the β-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below ±10° encompassing all molecules is involved. In the low temperature range, where TPP shows the typical 31P NMR echo spectra of the β2-process, very similar spectral features are observed for the (deuterated) SBC component by 2H NMR, in addition to its "own" β1-process observed at high temperatures. Apparently, the small TPP molecules enslave the large SBC molecules to perform a common hindered reorientation. The temperature dependence of the spin-lattice relaxation time of both components is the same and reveals an angular displacement of the SBC molecules somewhat smaller than that of TPP, though the time constants τβ2 are the same. Furthermore, T1(T) of TPP in the temperature region of the β2-process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the β-process introduced by one component are essentially independent of the second component. Finally, at cTPP ≤ 20% one finds indications that the β2-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing cTPP. We conclude that the β-process is a cooperative process.
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Affiliation(s)
- B Pötzschner
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - F Mohamed
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - C Bächer
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - E Wagner
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - A Lichtinger
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - D Bock
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - K Kreger
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - H-W Schmidt
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
| | - E A Rössler
- Experimentalphysik II, University of Bayreuth, 95447 Bayreuth, Germany and Macromolecular Chemistry I, University of Bayreuth, 95447 Bayreuth, Germany
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8
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Fragiadakis D, Roland CM. A test for the existence of isomorphs in glass-forming materials. J Chem Phys 2017; 147:084508. [PMID: 28863541 DOI: 10.1063/1.4986774] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We describe a method to determine whether a material has isomorphs in its thermodynamic phase diagram. Isomorphs are state points for which various properties are invariant in reduced units. Such materials are commonly identified from strong correlation between thermal fluctuations of the potential energy, U, and the virial W, but this identification is not generally applicable to real materials. We show from molecular dynamic simulations of atomic, molecular, and polymeric materials that systems with strong U-W correlation cannot be pressure densified, that is, the density obtained on cooling to the glassy state and releasing the pressure is independent of the pressure applied during cooling.
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Affiliation(s)
- D Fragiadakis
- Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342, USA
| | - C M Roland
- Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342, USA
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9
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Fragiadakis D, Roland CM. Participation in the Johari–Goldstein Process: Molecular Liquids versus Polymers. Macromolecules 2017. [DOI: 10.1021/acs.macromol.7b00621] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- D. Fragiadakis
- Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5342, United States
| | - C. M. Roland
- Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5342, United States
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10
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Fragiadakis D, Roland CM. Role of structure in the α and β dynamics of a simple glass-forming liquid. Phys Rev E 2017; 95:022607. [PMID: 28297980 DOI: 10.1103/physreve.95.022607] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2016] [Indexed: 06/06/2023]
Abstract
The elusive connection between dynamics and local structure in supercooled liquids is an important piece of the puzzle in the unsolved problem of the glass transition. The Johari-Goldstein β relaxation, ubiquitous in glass-forming liquids, exhibits mean properties that are strongly correlated to the long-time α dynamics. However, the former comprises simpler, more localized motion, and thus has perhaps a more straightforward connection to structure. Molecular dynamics simulations were carried out on a two-dimensional, rigid diatomic molecule (the simplest structure exhibiting a distinct β process) to assess the role of the local liquid structure on both the Johari-Goldstein β and the α relaxation. Although the average properties for these two relaxations are correlated, there is no connection between the β and α properties of a given (single) molecule. The propensity for motion at long times is independent of the rate or strength of a molecule's β relaxation. The mobility of a molecule averaged over many initial energies, a measure of the influence of structure, was found to be heterogeneous, with clustering at both the β and α time scales. This heterogeneity is less extended spatially for the β than for the α dynamics, as expected; however, the local structure is the more dominant control parameter for the β process. In the glassy state, the arrangement of neighboring molecules determines entirely the relaxation properties, with no discernible effect from the particle momenta.
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Affiliation(s)
- D Fragiadakis
- Naval Research Laboratory, Chemistry Division, Washington, DC 20375-5342, USA
| | - C M Roland
- Naval Research Laboratory, Chemistry Division, Washington, DC 20375-5342, USA
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11
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Seguin A, Dauchot O. Experimental Evidence of the Gardner Phase in a Granular Glass. PHYSICAL REVIEW LETTERS 2016; 117:228001. [PMID: 27925738 DOI: 10.1103/physrevlett.117.228001] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/03/2016] [Indexed: 06/06/2023]
Abstract
Analyzing the dynamics of a vibrated bidimensional packing of bidisperse granular disks below jamming, we provide evidence of a Gardner phase deep into the glass phase. To do so, we perform several compression cycles within a given realization of the same glass and show that the particles select different average vibrational positions at each cycle, while the neighborhood structure remains unchanged. The separation between the cages obtained for different compression cycles plateaus with an increasing packing fraction, while the mean square displacement steadily decreases. This phenomenology is strikingly similar to that reported in recent numerical observations when entering the Gardner phase, for a mean-field model of glass as well as for hard spheres in finite dimension. We also characterize the distribution of the cage order parameters. Here we note several differences from the numerical results, which could be attributed to activated processes and cage heterogeneities.
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Affiliation(s)
- A Seguin
- Laboratoire FAST, Université Paris-Sud, CNRS, Université Paris-Saclay, F-91405, Orsay, France
- SPEC, CEA, CNRS, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France
| | - O Dauchot
- EC2M, UMR Gulliver 7083 CNRS, ESPCI ParisTech, PSL Research University, 10 rue Vauquelin, 75005 Paris, France
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12
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Fragiadakis D, Roland CM. Rotational dynamics of simple asymmetric molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 91:022310. [PMID: 25768508 DOI: 10.1103/physreve.91.022310] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/11/2014] [Indexed: 06/04/2023]
Abstract
Molecular dynamic simulations were carried out on rigid diatomic molecules, which exhibit both α (structural) and β (secondary) dynamics. The relaxation scenarios range from onset behavior, in which a distinct α process emerges on cooling, to merging behavior, associated with two relaxation peaks that converge at higher temperature. These properties, as well as the manifestation of the β peak as an excess wing, depend not only on thermodynamic conditions, but also on both the symmetry of the molecule and the correlation function (odd or even) used to analyze its dynamics. These observations help to reconcile divergent results obtained from different experiments. For example, the β process is more intense and the α-relaxation peak is narrower in dielectric relaxation spectra than in dynamic light scattering or NMR measurements. In the simulations herein, this follows from the weaker contribution of the secondary relaxation to even-order correlation functions, related to the magnitude of the relevant angular jumps.
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Affiliation(s)
- D Fragiadakis
- Naval Research Laboratory, Chemistry Division, Code 6120, Washington DC 20375-5342, USA
| | - C M Roland
- Naval Research Laboratory, Chemistry Division, Code 6120, Washington DC 20375-5342, USA
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13
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Abstract
Abstract
Focusing on metallic glasses as model systems, we review the features and mechanisms of the β-relaxations, which are intrinsic and universal to supercooled liquids and glasses, and demonstrate their importance in understanding many crucial unresolved issues in glassy physics and materials science, including glass transition phenomena, mechanical properties, shear-banding dynamics and deformation mechanisms, diffusion and the breakdown of the Stokes–Einstein relation as well as crystallization and stability of glasses. We illustrate that it is an attractive prospect to incorporate these insights into the design of new glassy materials with extraordinary properties. We also outline important questions regarding the nature of β-relaxations and highlight some emerging research directions in this still-evolving field.
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Affiliation(s)
- Hai Bin Yu
- Physikalisches Institut, Universität Göttingen, D-37077 Göttingen, Germany
| | - Wei Hua Wang
- Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
| | - Hai Yang Bai
- Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
| | - Konrad Samwer
- Physikalisches Institut, Universität Göttingen, D-37077 Göttingen, Germany
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14
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Samanta S, Richert R. Nonlinear Dielectric Behavior of a Secondary Relaxation: Glassy d-Sorbitol. J Phys Chem B 2014; 119:8909-16. [DOI: 10.1021/jp506854k] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Subarna Samanta
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States
| | - Ranko Richert
- Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States
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15
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Casalini R, Roland CM. Effect of Regioisomerism on the Local Dynamics of Polychlorostyrene. Macromolecules 2014. [DOI: 10.1021/ma5008495] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Affiliation(s)
- R. Casalini
- Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, United States
| | - C. M. Roland
- Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, United States
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16
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Fragiadakis D, Roland CM. Dynamic correlations and heterogeneity in the primary and secondary relaxations of a model molecular liquid. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:052304. [PMID: 25353797 DOI: 10.1103/physreve.89.052304] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/06/2014] [Indexed: 06/04/2023]
Abstract
Molecular dynamics simulations were carried out on a series of Lennard-Jones binary mixtures of rigid, asymmetric, dumbbell-shaped molecules. Below an onset temperature, the rotational and translational dynamics split into the slow structural α relaxation and a higher-frequency Johari-Goldstein β relaxation. Both processes are dynamically heterogeneous, having broad distributions of relaxation times. However, only the α relaxation shows strong dynamic correlations; correlations at the β time scale are weak, in particular for molecules having shorter bonds. Despite the close connection between the two processes, we find no correlation between the α and β relaxation times of individual molecules; that is, a molecule exhibiting slow β motion does not necessarily undergo slow α dynamics and likewise for fast molecules. However, the single-molecule α relaxation times do correlate with both the α and β relaxation strengths.
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Affiliation(s)
- D Fragiadakis
- Naval Research Laboratory, Chemistry Division, Code 6120, Washington, DC 20375-5342, USA
| | - C M Roland
- Naval Research Laboratory, Chemistry Division, Code 6120, Washington, DC 20375-5342, USA
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17
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Fragiadakis D, Roland CM. Characteristics of the Johari-Goldstein process in rigid asymmetric molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:042307. [PMID: 24229172 DOI: 10.1103/physreve.88.042307] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2013] [Indexed: 06/02/2023]
Abstract
Molecular dynamics simulations were carried out on a Lennard-Jones binary mixture of rigid (fixed bond length) diatomic molecules. The translational and rotational correlation functions, and the corresponding susceptibilities, exhibit two relaxation processes: the slow structural relaxation (α dynamics) and a higher frequency secondary relaxation. The latter is a Johari-Goldstein (JG) process, by its definition of involving all parts of the molecule. It shows several properties characteristic of the JG relaxation: (1) merging with the α relaxation at high temperature; (2) a change in temperature dependence of its relaxation strength on vitrification; (3) a separation in frequency from the α peak that correlates with the breadth of the α dispersion; and (4) sensitivity to volume, pressure, and physical aging. These properties can be used to determine whether a secondary relaxation in a real material is an authentic JG process, rather than more trivial motion involving intramolecular degrees of freedom. The latter has no connection to the glass transition, whereas the JG relaxation is closely related to structural relaxation, and thus can provide new insights into the phenomenon.
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Affiliation(s)
- D Fragiadakis
- Naval Research Laboratory, Chemistry Division, Code 6120, Washington, DC 20375-5342, USA
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18
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Johari GP, Khouri J. Non-exponential nature of calorimetric and other relaxations: Effects of 2 nm-size solutes, loss of translational diffusion, isomer specificity, and sample size. J Chem Phys 2013; 138:12A511. [DOI: 10.1063/1.4770056] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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19
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Casalini R, Snow AW, Roland CM. Temperature Dependence of the Johari–Goldstein Relaxation in Poly(methyl methacrylate) and Poly(thiomethyl methacrylate). Macromolecules 2012. [DOI: 10.1021/ma3021322] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- R. Casalini
- Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, United States
| | - A. W. Snow
- Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, United States
| | - C. M. Roland
- Chemistry Division, Naval Research Laboratory, Code 6120, Washington, D.C. 20375-5342, United States
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