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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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Fractional Coupling of Primary and Johari-Goldstein Relaxations in a Model Polymer. Polymers (Basel) 2022; 14:polym14245560. [PMID: 36559927 PMCID: PMC9787821 DOI: 10.3390/polym14245560] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2022] [Revised: 12/14/2022] [Accepted: 12/15/2022] [Indexed: 12/23/2022] Open
Abstract
A polymer model exhibiting heterogeneous Johari−Goldstein (JG) secondary relaxation is studied by extensive molecular-dynamics simulations of states with different temperature and pressure. Time−temperature−pressure superposition of the primary (segmental) relaxation is evidenced. The time scales of the primary and the JG relaxations are found to be highly correlated according to a power law. The finding agrees with key predictions of the Coupling Model (CM) accounting for the decay in a correlation function due to the relaxation and diffusion of interacting systems. Nonetheless, the exponent of the power law, even if it is found in the range predicted by CM (0<ξ<1), deviates from the expected one. It is suggested that the deviation could depend on the particular relaxation process involved in the correlation function and the heterogeneity of the JG process.
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Inter-enantiomer conversion dynamics and Johari-Goldstein relaxation of benzophenones. Sci Rep 2021; 11:20248. [PMID: 34642356 PMCID: PMC8511015 DOI: 10.1038/s41598-021-99606-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2021] [Accepted: 09/23/2021] [Indexed: 11/09/2022] Open
Abstract
We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari-Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.
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Moura Ramos JJ, Diogo HP. Orientational glass, orientationally disordered crystal and crystalline polymorphism: A further study on the thermal behavior and molecular mobility in levoglucosan. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.110914] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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Affiliation(s)
- G. P. Johari
- Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada
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Yu HB, Yang MH, Sun Y, Zhang F, Liu JB, Wang CZ, Ho KM, Richert R, Samwer K. Fundamental Link between β Relaxation, Excess Wings, and Cage-Breaking in Metallic Glasses. J Phys Chem Lett 2018; 9:5877-5883. [PMID: 30240226 DOI: 10.1021/acs.jpclett.8b02629] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
In glassy materials, the Johari-Goldstein secondary (β) relaxation is crucial to many properties as it is directly related to local atomic motions. However, a long-standing puzzle remains elusive: why some glasses exhibit β relaxations as pronounced peaks while others present as unobvious excess wings? Using microsecond atomistic simulation of two model metallic glasses (MGs), we demonstrate that such a difference is associated with the number of string-like collective atomic jumps. Relative to that of excess wings, we find that MGs having pronounced β relaxations contain larger numbers of such jumps. Structurally, they are promoted by the higher tendency of cage-breaking events of their neighbors. Our results provide atomistic insights for different signatures of the β relaxation that could be helpful for understanding the low-temperature dynamics and properties of MGs.
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Affiliation(s)
- Hai-Bin Yu
- Wuhan National High Magnetic Field Center and School of Physics , Huazhong University of Science and Technology , WuHan , Hubei 430074 , China
| | - Meng-Hao Yang
- Ames Laboratory, U.S. Department of Energy , Ames , Iowa 50011 , United States
- Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering , Tsinghua University , Beijing 100084 , China
| | - Yang Sun
- Ames Laboratory, U.S. Department of Energy , Ames , Iowa 50011 , United States
| | - Feng Zhang
- Ames Laboratory, U.S. Department of Energy , Ames , Iowa 50011 , United States
| | - Jian-Bo Liu
- Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering , Tsinghua University , Beijing 100084 , China
| | - C Z Wang
- Ames Laboratory, U.S. Department of Energy , Ames , Iowa 50011 , United States
- Department of Physics , Iowa State University , Ames , Iowa 50011 , United States
| | - K M Ho
- Ames Laboratory, U.S. Department of Energy , Ames , Iowa 50011 , United States
- Department of Physics , Iowa State University , Ames , Iowa 50011 , United States
| | - Ranko Richert
- School of Molecular Sciences , Arizona State University , Tempe , Arizona 85287 , United States
| | - Konrad Samwer
- I. Physikalisches Institut, Universität Göttingen , D-37077 Göttingen , Germany
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Ransom TC, Fragiadakis D, Roland CM. The α and Johari–Goldstein Relaxations in 1,4-Polybutadiene: Breakdown of Isochronal Superpositioning. Macromolecules 2018. [DOI: 10.1021/acs.macromol.8b00664] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- T. C. Ransom
- Chemistry Division, Naval Research Laboratory, Code 6105, Washington, D.C. 20375-5342, United States
| | - D. Fragiadakis
- Chemistry Division, Naval Research Laboratory, Code 6105, Washington, D.C. 20375-5342, United States
| | - C. M. Roland
- Chemistry Division, Naval Research Laboratory, Code 6105, Washington, D.C. 20375-5342, United States
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9
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Yu HB, Richert R, Samwer K. Structural rearrangements governing Johari-Goldstein relaxations in metallic glasses. SCIENCE ADVANCES 2017; 3:e1701577. [PMID: 29159283 PMCID: PMC5693560 DOI: 10.1126/sciadv.1701577] [Citation(s) in RCA: 55] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2017] [Accepted: 10/20/2017] [Indexed: 05/10/2023]
Abstract
The Johari-Goldstein secondary (β) relaxations are an intrinsic feature of supercooled liquids and glasses. They are crucial to many properties of glassy materials, but the underlying mechanisms are still not established. In a model metallic glass, we study the atomic rearrangements by molecular dynamics simulations at time scales of up to microseconds. We find that the distributions of single-particle displacements exhibit multiple peaks, whose positions quantitatively match the pair distribution function. These are identified as the structural signature of cooperative string-like excitations. Furthermore, the most probable time of the string-like motions coincides with the β-relaxation time as probed by dynamical mechanical simulations over a wide temperature range and is consistent with a theoretical model. Our results provide insights into the long-standing puzzle regarding the structural origin of β relaxations in glassy metallic materials.
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Affiliation(s)
- Hai-Bin Yu
- Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, Wuhan 430074 Hubei, China
- Corresponding author. (H.-B.Y.); (R.R.); (K.S.)
| | - Ranko Richert
- School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA
- Corresponding author. (H.-B.Y.); (R.R.); (K.S.)
| | - Konrad Samwer
- I. Physikalisches Institut, Universität Göttingen, D-37077 Göttingen, Germany
- Corresponding author. (H.-B.Y.); (R.R.); (K.S.)
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10
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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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11
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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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12
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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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13
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Tripathi P, Mitsari E, Romanini M, Serra P, Tamarit JL, Zuriaga M, Macovez R. Orientational relaxations in solid (1,1,2,2)tetrachloroethane. J Chem Phys 2016; 144:164505. [PMID: 27131555 DOI: 10.1063/1.4947477] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
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 180° 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.
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Affiliation(s)
- P Tripathi
- Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain
| | - E Mitsari
- Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain
| | - M Romanini
- Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain
| | - P Serra
- Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba and IFEG-CONICET, Ciudad Universitaria, X5016LAE Córdoba, Argentina
| | - J Ll Tamarit
- Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain
| | - M Zuriaga
- Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba and IFEG-CONICET, Ciudad Universitaria, X5016LAE Córdoba, Argentina
| | - R Macovez
- Grup de Caracterització de Materials, Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain
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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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15
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Qiao J, Casalini R, Pelletier JM. Effect of physical aging on Johari-Goldstein relaxation in La-based bulk metallic glass. J Chem Phys 2014; 141:104510. [DOI: 10.1063/1.4895396] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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16
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Souillard C, Cavaillé JY, Chazeau L, Schach R. Dynamic mechanical relaxation of cross-linked styrene-butadiene polymers containing free chains: Possibility of reptation. POLYMER 2014. [DOI: 10.1016/j.polymer.2014.08.016] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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17
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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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18
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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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