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Huang Y, Wang W, Whitmer JK, Zhang R. Structures, thermodynamics and dynamics of topological defects in Gay-Berne nematic liquid crystals. SOFT MATTER 2023; 19:483-496. [PMID: 36533944 DOI: 10.1039/d2sm01178f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Topological defects are a ubiquitous phenomenon across different physical systems. A better understanding of defects can be helpful in elucidating the physical behaviors of many real materials systems. In nematic liquid crystals, defects exhibit unique optical signatures and can segregate impurities, showing their promise as molecular carriers and nano-reactors. Continuum theory and simulations have been successfully applied to link static and dynamical behaviors of topological defects to the material constants of the underlying nematic. However, further evidence and molecular details are still lacking. Here we perform molecular dynamics simulations of Gay-Berne particles, a model nematic, to examine the molecular structures and dynamics of +1/2 defects in a thin-film nematic. Specifically, we measure the bend-to-splay ratio K3/K1 using two independent, indirect measurements, showing good agreement. Next, we study the annihilation event of a pair of ±1/2 defects, of which the trajectories are consistent with experiments and hydrodynamic simulations. We further examine the thermodynamics of defect annihilation in an NVE ensemble, leading us to correctly estimate the elastic modulus by using the energy conservation law. Finally, we explore effects of defect annihilation in regions of nonuniform temperature within these coarse-grained molecular models which cannot be analysed by existing continuum level simulations. We find that +1/2 defects tend to move toward hotter areas and their change of speed in a temperature gradient can be quantitatively understood through a term derived from the temperature dependence of the elastic modulus. As such, our work has provided molecular insights into structures and dynamics of topological defects, presented unique and accessible methods to measure elastic constants by inspecting defects, and proposed an alternative control parameter of defects using temperature gradient.
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Affiliation(s)
- Yulu Huang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
| | - Weiqiang Wang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
| | - Jonathan K Whitmer
- Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
| | - Rui Zhang
- Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China.
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Molecular Simulation Approaches to the Study of Thermotropic and Lyotropic Liquid Crystals. CRYSTALS 2022. [DOI: 10.3390/cryst12050685] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Over the last decade, the availability of computer time, together with new algorithms capable of exploiting parallel computer architectures, has opened up many possibilities in molecularly modelling liquid crystalline systems. This perspective article points to recent progress in modelling both thermotropic and lyotropic systems. For thermotropic nematics, the advent of improved molecular force fields can provide predictions for nematic clearing temperatures within a 10 K range. Such studies also provide valuable insights into the structure of more complex phases, where molecular organisation may be challenging to probe experimentally. Developments in coarse-grained models for thermotropics are discussed in the context of understanding the complex interplay of molecular packing, microphase separation and local interactions, and in developing methods for the calculation of material properties for thermotropics. We discuss progress towards the calculation of elastic constants, rotational viscosity coefficients, flexoelectric coefficients and helical twisting powers. The article also covers developments in modelling micelles, conventional lyotropic phases, lyotropic phase diagrams, and chromonic liquid crystals. For the latter, atomistic simulations have been particularly productive in clarifying the nature of the self-assembled aggregates in dilute solution. The development of effective coarse-grained models for chromonics is discussed in detail, including models that have demonstrated the formation of the chromonic N and M phases.
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Affiliation(s)
- Michael P. Allen
- Department of Physics, University of Warwick, Coventry, UK
- H. H. Wills Physics Laboratory, Royal Fort, Bristol, UK
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Zhu G, Kang X, Zhou S, Tang X, Sha M, Cui Z, Xu X. Molecular insight into the microstructure and microscopic dynamics of pyridinium ionic liquids with different alkyl chains based on temperature response. RSC Adv 2017. [DOI: 10.1039/c6ra26866h] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
High temperature is advantageous to the aggregation of the polar regions as well as the nonpolar regions of pyridinium ionic liquids.
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Affiliation(s)
- Guanglai Zhu
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
| | - Xianqu Kang
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
| | - Siwen Zhou
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
| | - Xiaoshuan Tang
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
| | - Maolin Sha
- Department of Chemistry and Chemical Engineering
- Hefei Normal University
- Hefei 230061
- China
| | - Zhifeng Cui
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
| | - Xinsheng Xu
- Institute of Atomic and Molecular Physics
- Anhui Normal University
- Wuhu 241000
- China
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Boyd NJ, Wilson MR. Optimization of the GAFF force field to describe liquid crystal molecules: the path to a dramatic improvement in transition temperature predictions. Phys Chem Chem Phys 2015; 17:24851-65. [DOI: 10.1039/c5cp03702f] [Citation(s) in RCA: 46] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
Systematic optimization of the General Amber Force Field (GAFF) for mesogenic fragments leads to a dramatic improvement in the modelling of liquid crystal clearing points.
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Sarman S, Laaksonen A. Twist viscosities and flow alignment of biaxial nematic liquid crystal phases of a soft ellipsoid-string fluid studied by molecular dynamics simulation. Phys Chem Chem Phys 2012; 14:11999-2013. [DOI: 10.1039/c2cp41456b] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Affiliation(s)
- Sten Sarman
- Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden.
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Investigating the Calculation of Rotational Viscosity of the Mixture Comprising Different Kinds of Liquid Crystals: Molecular Dynamics Computer Simulation Approach. CHINESE J CHEM 2011. [DOI: 10.1002/cjoc.201190059] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Xue XG, Zhao L, Lu ZY, Li MH, Li ZS. Molecular dynamics simulation study on the isomerization and molecular orientation of liquid crystals formed by azobenzene and (1-cyclohexenyl)phenyldiazene. Phys Chem Chem Phys 2011; 13:11951-7. [DOI: 10.1039/c0cp02094j] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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9
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Luckhurst GR, Satoh K. The director and molecular dynamics of the field-induced alignment of a Gay–Berne nematic phase: An isothermal-isobaric nonequilibrium molecular dynamics simulation study. J Chem Phys 2010. [DOI: 10.1063/1.3374692] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Molecular Dynamics Simulations of Liquid-Crystalline Dendritic Architectures. ADVANCES IN THE ATOMIC-SCALE MODELING OF NANOSYSTEMS AND NANOSTRUCTURED MATERIALS 2010. [DOI: 10.1007/978-3-642-04650-6_4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
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11
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Sarman S, Laaksonen A. Flow alignment phenomena in liquid crystals studied by molecular dynamics simulation. J Chem Phys 2009; 131:144904. [DOI: 10.1063/1.3238549] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
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Cacelli I, Lami CF, Prampolini G. Force-field modeling through quantum mechanical calculations: Molecular dynamics simulations of a nematogenic molecule in its condensed phases. J Comput Chem 2009; 30:366-78. [DOI: 10.1002/jcc.21062] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Cifelli M, De Gaetani L, Prampolini G, Tani A. Atomistic Computer Simulation and Experimental Study on the Dynamics of the n-Cyanobiphenyls Mesogenic Series. J Phys Chem B 2008; 112:9777-86. [DOI: 10.1021/jp802935q] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Affiliation(s)
- Mario Cifelli
- Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy
| | - Luca De Gaetani
- Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy
| | - Giacomo Prampolini
- Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy
| | - Alessandro Tani
- Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy
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Application and New Developments in Polymer-Dispersed Liquid Crystal Simulation Studies. MACROMOL THEOR SIMUL 2007. [DOI: 10.1002/mats.200700008] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Cacelli I, Gaetani LD, Prampolini G, Tani A. Liquid Crystal Properties of the n-Alkyl-cyanobiphenyl Series from Atomistic Simulations with Ab Initio Derived Force Fields. J Phys Chem B 2007; 111:2130-7. [PMID: 17288473 DOI: 10.1021/jp065806l] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4'-cyanobiphenyls (nCB) with n = 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336), successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here been extended to higher homologues. Nematic and isotropic phases were found for all members of the series, and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures reproduce the experimental values within +/-10 K. Also, structural properties as second and fourth rank orientational order parameters are in good agreement with the corresponding experimental quantities. This means that the well-known odd-even effect, observed for many properties along the nCB series, is well reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the correlation between molecular properties and odd-even effects is presented.
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Affiliation(s)
- Ivo Cacelli
- Dipartimento di Chimica e Chimica Industriale Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy.
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Peláez J, Wilson M. Molecular orientational and dipolar correlation in the liquid crystal mixture E7: a molecular dynamics simulation study at a fully atomistic level. Phys Chem Chem Phys 2007; 9:2968-75. [PMID: 17551620 DOI: 10.1039/b614422e] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Molecular dynamics simulations are reported for the four component nematic liquid crystal mixture E7, which is used commercially. We are able to show the growth of a nematic phase directly from an isotropic liquid over a 100 ns period for an all-atom model, and study orientational and dipole order within the nematic phase. The simulations show that the cyanoterphenyl component of the mixture, 5CT, is more ordered than the three cyanobiphenyl components. The simulations show also that both parallel and anti-parallel dipole correlation take place in E7 but that the strong anti-parallel dipole correlation is localised to particular arrangements of molecules. It is possible to identify two key preferred configurations for molecular pairs in the fluid, which explain the form of the dipole correlation function, g(1)(r).
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Affiliation(s)
- Jorge Peláez
- Department of Chemistry, Durham University, South Road, Durham, UKDH1 3LE.
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Peláez J, Wilson MR. Atomistic simulations of a thermotropic biaxial liquid crystal. PHYSICAL REVIEW LETTERS 2006; 97:267801. [PMID: 17280466 DOI: 10.1103/physrevlett.97.267801] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/11/2006] [Indexed: 05/13/2023]
Abstract
We have performed molecular dynamics simulations of a 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole mesogen (ODBP-Ph-C(7)) at a fully atomistic level for a range of temperatures within the region that has experimentally been assigned to a biaxial nematic phase. Analysis of the data shows that the simulated nematic phase is biaxial but that the degree of biaxiality is small. The simulations show also the formation of ferroelectric domains in the nematic where the molecular short axis is aligned with the oxadiazole dipoles parallel to each other. Removal of electrostatic interactions leads to destabilization of ferroelectric domains and destabilization of the biaxiality. An additional simulation shows the slow growth of a mesophase directly from the isotropic fluid over a period of approximately 50 ns. This is the first time this has been achieved within the framework of an all-atom model.
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Affiliation(s)
- Jorge Peláez
- Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, United Kingdom
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19
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Amovilli C, Cacelli I, Cinacchi G, De Gaetani L, Prampolini G, Tani A. Structure and dynamics of mesogens using intermolecular potentials derived from ab initio calculations. Theor Chem Acc 2006. [DOI: 10.1007/s00214-006-0209-y] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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20
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Capar MI, Cebe E. Molecular dynamic study of the odd-even effect in some 4-n-alkyl-4'-cyanobiphenyls. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2006; 73:061711. [PMID: 16906853 DOI: 10.1103/physreve.73.061711] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/19/2005] [Indexed: 05/11/2023]
Abstract
Using a united atom model, we performed molecular dynamics (MD) simulations on the 4-n-alkyl-4'-cyanobiphenyl (nCB) liquid crystal homologous series for at a constant reduced temperature in the nematic phase. To evaluate the rotational diffusion coefficient (RDC), the second- and fourth-rank order parameters (OP's) and correlation time were calculated from MD trajectories. We analyzed the rotational viscosity coefficient (RVC) by using the Nemtsov-Zakharov and Fialkowski methods based on statistical-mechanical approaches. The simulated RDC, RVC, and OP's were found to be reasonable when compared with previous theoretical and experimental data. These quantities were also calculated for the rigid core and alkyl chain segments separately for each mesogen, to take a closer look at the molecule tail-chain flexibility. The properties calculated for a rigid core were compared with those for an alkyl chain, and satisfactory results were obtained. Odd-even effects for RDF, RVC, and molecular ordering parameters were calculated for five mesogens.
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Affiliation(s)
- Mine Ilk Capar
- Dokuz Eylül University, Faculty of Arts and Science, Department of Physics, Buca-Izmir, Turkey
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Stimson LM, Wilson MR. Molecular dynamics simulations of side chain liquid crystal polymer molecules in isotropic and liquid-crystalline melts. J Chem Phys 2005; 123:34908. [PMID: 16080764 DOI: 10.1063/1.1948376] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
A detailed molecular dynamics simulation study is described for a polysiloxane side chain liquid crystal polymer (SCLCP). The simulations use a coarse-grained model composed of a combination of isotropic and anisotropic interaction sites. On cooling from a fully isotropic polymer melt, we see spontaneous microphase separation into polymer-rich and mesogen-rich regions. Upon application of a small aligning potential during cooling, the structures that form on microphase separation anneal to produce a smectic-A phase in which the polymer backbone is largely confined between the smectic layers. Several independent quenches from the melt are described that vary in the strength of the aligning potential and the degree of cooling. In each quench, defects were found where the backbone chains hop from one backbone-rich region to the next by tunneling through the mesogenic layers. As expected, the number of such defects is found to depend strongly on the rate of cooling. In the vicinity of such a defect, the smectic-A structure of the mesogen-rich layers is disrupted to give nematiclike ordering. Additionally, several extensive annealing runs of approximately 40 ns duration have been carried out at the point of microphase separation. During annealing the polymer backbone is seen to be slowly excluded from the mesogenic layers and lie perpendicular to the smectic-A director. These observations agree with previous assumptions about the structure of a SCLCP and with interpretations of x-ray diffraction and small angle neutron scattering data. The flexible alkyl spacers, which link the backbone to the mesogens, are found to form sublayers around the backbone layer.
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Cheung DL, Clark SJ, Wilson MR. Calculation of flexoelectric coefficients for a nematic liquid crystal by atomistic simulation. J Chem Phys 2004; 121:9131-9. [PMID: 15527381 DOI: 10.1063/1.1802231] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Equilibrium molecular dynamics calculations have been performed for the liquid crystal molecule n-4-(trans-4-n-pentylcyclohexyl)benzonitrile (PCH5) using a fully atomistic model. Simulation data have been obtained for a series of temperatures in the nematic phase. The simulation data have been used to calculate the flexoelectric coefficients e(s) and e(b) using the linear response formalism of Osipov and Nemtsov [M. A. Osipov and V. B. Nemtsov, Sov. Phys. Crstallogr. 31, 125 (1986)]. The temperature and order parameter dependence of e(s) and e(b) are examined, as are separate contributions from different intermolecular interactions. Values of e(s) and e(b) calculated from simulation are consistent with those found from experiment.
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Affiliation(s)
- David L Cheung
- Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, United Kingdom
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CUETOS ALEJANDRO, ILNYTSKYI JAROSLAVM, WILSON MARKR. Rotational viscosities of Gay-Berne mesogens. Mol Phys 2002. [DOI: 10.1080/0026897021000028410] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
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