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Kubala P, Tomczyk W, Cieśla M. In silico study of liquid crystalline phases formed by bent-shaped molecules with excluded volume type interactions. J Mol Liq 2022. [DOI: 10.1016/j.molliq.2022.120156] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/15/2022]
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2
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Vita F, Adamo FC, Pisani M, Francescangeli O. Nanostructure of Unconventional Liquid Crystals Investigated by Synchrotron Radiation. NANOMATERIALS (BASEL, SWITZERLAND) 2020; 10:E1679. [PMID: 32859117 PMCID: PMC7558469 DOI: 10.3390/nano10091679] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Revised: 07/27/2020] [Accepted: 08/12/2020] [Indexed: 11/30/2022]
Abstract
The macroscopic properties of novel liquid crystal (LC) systems-LCs with unconventional molecular structure as well as conventional LCs in unconventional geometries-directly descend from their mesoscopic structural organization. While X-ray diffraction (XRD) is an obvious choice to investigate their nanoscale structure, conventional diffractometry is often hampered by experimental difficulties: the low scattering power and short-range positional order of the materials, resulting in weak and diffuse diffraction features; the need to perform measurements in challenging conditions, e.g., under magnetic and/or electric fields, on thin films, or at high temperatures; and the necessity to probe micron-sized volumes to tell the local structural properties from their macroscopic average. Synchrotron XRD allows these problems to be circumvented thanks to the superior diffraction capabilities (brilliance, q-range, energy and space resolution) and advanced sample environment available at synchrotron beamlines. Here, we highlight the potentiality of synchrotron XRD in the field of LCs by reviewing a selection of experiments on three unconventional LC systems: the potentially biaxial and polar nematic phase of bent-core mesogens; the very high-temperature nematic phase of all-aromatic LCs; and polymer-dispersed liquid crystals. In all these cases, synchrotron XRD unveils subtle nanostructural features that are reflected into macroscopic properties of great interest from both fundamental and technological points of view.
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Affiliation(s)
| | | | | | - Oriano Francescangeli
- Dipartimento di Scienze e Ingegneria della Materia, dell’Ambiente ed Urbanistica, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy; (F.V.); (F.C.A.); (M.P.)
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3
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Lehmann A, Alaasar M, Poppe M, Poppe S, Prehm M, Nagaraj M, Sreenilayam SP, Panarin YP, Vij JK, Tschierske C. Stereochemical Rules Govern the Soft Self-Assembly of Achiral Compounds: Understanding the Heliconical Liquid-Crystalline Phases of Bent-Core Mesogens. Chemistry 2020; 26:4714-4733. [PMID: 31859404 PMCID: PMC7186843 DOI: 10.1002/chem.201904871] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2019] [Revised: 12/15/2019] [Indexed: 11/16/2022]
Abstract
A series of bent-shaped 4-cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short-pitch heliconical lamellar liquid-crystalline phase with incommensurate 3-layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric-(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field-induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non-tilted and ferreoelectric switching lamellar phase, providing an alternative non-chiral way for the transition from anticlinic to synclinic tilt.
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Affiliation(s)
- Anne Lehmann
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
| | - Mohamed Alaasar
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
- Department of ChemistryCairo University12613GizaEgypt
| | - Marco Poppe
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
| | - Silvio Poppe
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
| | - Marko Prehm
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
| | - Mamatha Nagaraj
- Department of Electronic and Electrical EngineeringTrinity College, Dublin, The University of DublinDublin2Ireland
| | - Sithara P. Sreenilayam
- Department of Electronic and Electrical EngineeringTrinity College, Dublin, The University of DublinDublin2Ireland
| | - Yuri P. Panarin
- Department of Electronic and Electrical EngineeringTrinity College, Dublin, The University of DublinDublin2Ireland
| | - Jagdish K. Vij
- Department of Electronic and Electrical EngineeringTrinity College, Dublin, The University of DublinDublin2Ireland
| | - Carsten Tschierske
- Department of ChemistryMartin Luther University Halle-WittenbergKurt Mothes Str. 206120Halle (Saale)Germany
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4
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Patranabish S, Wang Y, Sinha A, Majumdar A. One-dimensional theoretical analysis of coupling and confinement effects on the cybotactic clusters of bent-core nematic liquid crystals. Phys Rev E 2019; 99:012703. [PMID: 30780335 DOI: 10.1103/physreve.99.012703] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2018] [Indexed: 11/07/2022]
Abstract
The bent-core liquid crystals (LCs) are highly regarded as the next-generation materials for electro-optic devices. The nematic (N) phase of these LCs possesses highly ordered smecticlike cybotactic clusters which are promising for electro-optic applications. We have studied a one-dimensional Landau-de Gennes model of spatially inhomogeneous order parameters for the N phase of bent-core LCs. We investigate the effects of spatial confinement and coupling (between these clusters and the surrounding LC molecules modeled by a coupling parameter γ) on the order parameters. The coupling is found to increase the cluster order parameter significantly, suggesting enhancement in cluster formation, and also predicts a transition to a phase with weak nematiclike ordering above the nematic supercooling temperature.
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Affiliation(s)
- Sourav Patranabish
- Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
| | - Yiwei Wang
- School of Mathematical Sciences, Peking University, Beijing 100871, China
| | - Aloka Sinha
- Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
| | - Apala Majumdar
- Department of Mathematical Sciences, University of Bath, Bath BA2 7AY, England, United Kingdom
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6
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Madhusudana NV. Two-state model for nematic liquid crystals made of bent-core molecules. Phys Rev E 2017; 96:022710. [PMID: 28950498 DOI: 10.1103/physreve.96.022710] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2017] [Indexed: 11/07/2022]
Abstract
Nematic (N) liquid crystals made of bent-core molecules exhibit unusual physical properties such as an intermediate phase between the N and isotropic (I) phases, a very weak NI transition as inferred from magnetic birefringence measurements in a low field, which is apparently incompatible with a large shift in the NI transition temperature (T_{ni}) measured under a high field. Using our conformational studies on the aromatic cores, we propose that only conformers which are more straightened than those in the ground state (GS) form clusters with a few layers, which persist even in the isotropic phase, as inferred from x-ray and rheological experiments. We present a Landau-de Gennes theory of the medium, including an orientational coupling between the clusters and the GS molecules, which accounts for all the unusual properties. The intermediate phase to isotropic transition is predicted to exhibit critical behavior at a very low magnetic field of <1kG.
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Affiliation(s)
- N V Madhusudana
- Raman Research Institute, C. V. Raman Avenue, Bengaluru 560080, India
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7
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Ivšić T, Vinković M, Baumeister U, Mikleušević A, Lesac A. Towards understanding the NTB phase: a combined experimental, computational and spectroscopic study. RSC Adv 2016. [DOI: 10.1039/c5ra26078g] [Citation(s) in RCA: 38] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Combined studies support the hierarchical model for the NTB phase that involves formation of embryonic self-assembly of the propeller-shaped dimeric molecules with syn-parallel orientation in the isotropic melt.
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Affiliation(s)
- Trpimir Ivšić
- Division of Organic Chemistry and Biochemistry
- Ruđer Bošković Institute
- 10000 Zagreb
- Croatia
| | | | - Ute Baumeister
- Institute of Chemistry
- Physical Chemistry
- Martin Luther University Halle-Wittenberg
- 06120 Halle
- Germany
| | - Ana Mikleušević
- Division of Organic Chemistry and Biochemistry
- Ruđer Bošković Institute
- 10000 Zagreb
- Croatia
| | - Andreja Lesac
- Division of Organic Chemistry and Biochemistry
- Ruđer Bošković Institute
- 10000 Zagreb
- Croatia
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8
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Peroukidis SD, Vanakaras AG, Photinos DJ. Molecular simulation study of polar order in orthogonal bent-core smectic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 91:062501. [PMID: 26172725 DOI: 10.1103/physreve.91.062501] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/03/2014] [Indexed: 06/04/2023]
Abstract
We explore the phase behavior and structure of orthogonal smectic liquid crystals consisting of bent-core molecules (BCMs) by means of Monte Carlo molecular simulations. A simple athermal molecular model is introduced that describes the basic features of the BCMs. Phase transitions between uniaxial and biaxial (antiferroelectric) orthogonal smectics are obtained. The results indicate the presence of local in-plane polar correlations in the uniaxial smectic phase. The macroscopic uniaxial-biaxial transformation is rationalized in terms of local polar correlations giving rise to polar domains. The size of these polar domains grows larger under the action of an external vector field and their internal ordering is enhanced, leading to field-induced biaxial order-disorder transitions.
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Affiliation(s)
| | | | - Demetri J Photinos
- Department of Materials Science, University of Patras, Patras 26504, Greece
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9
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Hoffmann A, Vanakaras AG, Kohlmeier A, Mehl GH, Photinos DJ. On the structure of the Nx phase of symmetric dimers: inferences from NMR. SOFT MATTER 2015; 11:850-855. [PMID: 25571939 DOI: 10.1039/c4sm02480j] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
NMR measurements on a selectively deuterated liquid crystal dimer CB-C9-CB, exhibiting two nematic phases, show that the molecules in the lower temperature nematic phase, N(X), experience a chiral environment and are ordered about a uniformly oriented director throughout the macroscopic sample. The results are contrasted with previous interpretations that suggested a twist-bend spatial variation of the director. A structural picture is proposed wherein the molecules are packed into highly correlated chiral assemblies.
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Affiliation(s)
- Anke Hoffmann
- Department of Inorganic and Analytical Chemistry, University of Freiburg, Albertstr. 21, 79104 Freiburg, Germany
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10
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Ilnytskyi JM, Trokhymchuk A, Schoen M. Topological defects around a spherical nanoparticle in nematic liquid crystal: Coarse-grained molecular dynamics simulations. J Chem Phys 2014; 141:114903. [DOI: 10.1063/1.4894438] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
Affiliation(s)
- Jaroslav M. Ilnytskyi
- Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1, Svientsitskii Str., 79011 Lviv, Ukraine
| | - Andrij Trokhymchuk
- Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1, Svientsitskii Str., 79011 Lviv, Ukraine
| | - Martin Schoen
- Stranski-Laboratorium für Physikalische und Theoretische Chemie, Fakultät für Mathematik und Naturwissenschaften, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany and Department of Chemical and Biomolecular Engineering, Engineering Building I, Box 7905, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, USA
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11
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Hoang DT, Diep HT. Phase transition in dimer liquids. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2014; 26:035103. [PMID: 24285401 DOI: 10.1088/0953-8984/26/3/035103] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
We study the phase transition in a system composed of dimers interacting with each other via a nearest-neighbor (NN) exchange J and competing interactions taken from a truncated dipolar coupling. Each dimer occupies a link between two nearest sites of a simple cubic lattice. We suppose that dimers are self-avoiding and can have only three orientations, which coincide with the x, y or z direction. The interaction J is attractive if the two dimers are parallel to each other at the NN distance, zero otherwise. The truncated dipolar interaction is characterized by two parameters: its amplitude D and the cutoff distance rc. Using the steepest descent method, we determine the ground-state (GS) configuration as functions of D and rc. We then use Monte Carlo simulations to investigate the nature of the low-temperature phase and to determine characteristics of the phase transition from the ordered phase to the disordered phase at high temperatures at a given dimer concentration. We show that as the temperature increases, dimers remain in the compact state and the transition from the low-T compact phase to the disordered phase where dimers occupy the whole space is of second order when D is small, but becomes of first order for large enough D, for both polarized and nonpolarized dimers. This transition has a resemblance to the unfolding polymer transition. The effect of rc is discussed.
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Affiliation(s)
- Danh-Tai Hoang
- Asia Pacific Center for Theoretical Physics, POSTECH, San 31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk 790-784, Korea
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12
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Vita F, Sparnacci K, Panzarasa G, Placentino IF, Marino S, Scaramuzza N, Portale G, Di Cola E, Ferrero C, Torgova SI, Galli G, Laus M, Francescangeli O. Evidence of Cybotactic Order in the Nematic Phase of a Main-Chain Liquid Crystal Polymer with Bent-Core Repeat Unit. ACS Macro Lett 2014; 3:91-95. [PMID: 35651116 DOI: 10.1021/mz400518x] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We report the synthesis and structural characterization of a main-chain liquid crystal polymer constituted by a 1,2,4-oxadiazole-based bent-core repeat unit. For the first time, a liquid crystal polymer made of bent mesogenic units is demonstrated to exhibit cybotactic order in the nematic phase. Coupled with the chain-bond constraints, cybotaxis results in maximized molecular correlations that make this material of great potential in the search for the elusive biaxial and ferroelectric nematic phases. Indeed, repolarization current measurements in the nematic phase hint at a ferroelectric-like switching response (upon application of an electric field of only 1.0 V μm-1) that, albeit to be definitely confirmed by complementary techniques, is strongly supported by the comparative repolarization current measurements in the nematic and isotropic phases. Finally, the weak tendency of this polymer to crystallize makes it possible to supercool the cybotactic nematic phase down to room temperature, thus, paving the way for a glassy phase in which the biaxial (and possibly polar) order is frozen at room temperature.
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Affiliation(s)
- Francesco Vita
- Dipartimento
SIMAU and CNISM, Università Politecnica delle Marche, via Brecce
Bianche, 60131 Ancona, Italy
| | - Katia Sparnacci
- Dipartimento
DISIT, Università del Piemonte Orientale “A. Avogadro”, INSTM, UdR Alessandria, Via T. Michel 11, 15121 Alessandria, Italy
| | - Guido Panzarasa
- Dipartimento
DISIT, Università del Piemonte Orientale “A. Avogadro”, INSTM, UdR Alessandria, Via T. Michel 11, 15121 Alessandria, Italy
| | - Immacolata F. Placentino
- Dipartimento
SIMAU and CNISM, Università Politecnica delle Marche, via Brecce
Bianche, 60131 Ancona, Italy
| | - Salvatore Marino
- Dipartimento
di Fisica, Università della Calabria, CNR-IPCF UoS Cosenza, Licryl Laboratory, and Centro di Eccellenza CEMIF.CAL, Via Pietro
Bucci, Cubo 31C, 87036 Rende, Cosenza, Italy
| | - Nicola Scaramuzza
- Dipartimento
di Fisica, Università della Calabria, CNR-IPCF UoS Cosenza, Licryl Laboratory, and Centro di Eccellenza CEMIF.CAL, Via Pietro
Bucci, Cubo 31C, 87036 Rende, Cosenza, Italy
| | - Giuseppe Portale
- Netherlands Organization
for Scientific Research (NWO), DUBBLE CRG at the ESRF, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France
| | - Emanuela Di Cola
- European
Synchrotron
Radiation Facility, Boîte Postale
220, 38043 Grenoble
Cedex, France
| | - Claudio Ferrero
- European
Synchrotron
Radiation Facility, Boîte Postale
220, 38043 Grenoble
Cedex, France
| | - Sofia I. Torgova
- P.N. Lebedev Physical
Institute of the Russian Academy of Sciences, Leninsky Pr. 53, Moscow, 119991 Russia
| | - Giancarlo Galli
- Dipartimento
di Chimica e Chimica Industriale, Università di Pisa, INSTM, UdR Pisa, Via Risorgimento 35, I-56126 Pisa, Italy
| | - Michele Laus
- Dipartimento
DISIT, Università del Piemonte Orientale “A. Avogadro”, INSTM, UdR Alessandria, Via T. Michel 11, 15121 Alessandria, Italy
| | - Oriano Francescangeli
- Dipartimento
SIMAU and CNISM, Università Politecnica delle Marche, via Brecce
Bianche, 60131 Ancona, Italy
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13
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Peroukidis SD, Vanakaras AG, Photinos DJ. Supramolecular nature of the nematic-nematic phase transitions of hard boardlike molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:062508. [PMID: 24483469 DOI: 10.1103/physreve.88.062508] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/18/2013] [Indexed: 06/03/2023]
Abstract
The phase behavior of hard boardlike biaxial particles of relative dimensions close to the clamitic to discotic crossover is explored by means of Monte Carlo molecular simulations. Transitions between two distinct biaxial nematic phases as well as transitions from a biaxial nematic to a uniaxial Sm-A phase are obtained. The formation of anisotropic supramolecular assemblies is demonstrated and is quantified by means of rotationally invariant pair correlation functions.
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Affiliation(s)
| | | | - Demetri J Photinos
- Department of Materials Science, University of Patras, Patras 26504, Greece
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14
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To TBT, Sluckin TJ, Luckhurst GR. Biaxiality-induced magnetic field effects in bent-core nematics: molecular-field and Landau theory. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:062506. [PMID: 24483467 DOI: 10.1103/physreve.88.062506] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/12/2013] [Revised: 11/15/2013] [Indexed: 06/03/2023]
Abstract
Nematic liquid crystals composed of bent-core molecules exhibit unusual properties, including an enhanced Cotton-Mouton effect and an increasing isotropic (paranematic)-nematic phase transition temperature as a function of magnetic field. These systems are thought to be good candidate biaxial liquid crystals. Prompted by these experiments, we investigate theoretically the effect of molecular biaxiality on magnetic-field-induced phenomena for nematic liquid crystals, using both molecular field and Landau theory. The geometric mean approximation is used in order to specify the degree of molecular biaxiality using a single parameter. We reproduce experimental field-induced phenomena and predict also an experimentally accessible magnetic critical point. The Cotton-Mouton effect and temperature dependence of the paranematic-nematic phase transition are more pronounced with increased molecular biaxiality. We compare our theoretical approaches and make contact with recent relevant experimental results on bent-core molecular systems.
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Affiliation(s)
- T B T To
- School of Mathematics, University of Southampton, Southampton, SO17 1BJ, United Kingdom
| | - T J Sluckin
- School of Mathematics, University of Southampton, Southampton, SO17 1BJ, United Kingdom
| | - G R Luckhurst
- School of Chemistry, University of Southampton, Southampton, SO17 1BJ, United Kingdom
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15
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Emsley JW, Lelli M, Lesage A, Luckhurst GR. A comparison of the conformational distributions of the achiral symmetric liquid crystal dimer CB7CB in the achiral nematic and chiral twist-bend nematic phases. J Phys Chem B 2013; 117:6547-57. [PMID: 23634827 DOI: 10.1021/jp4001219] [Citation(s) in RCA: 76] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The sets of residual dipolar couplings between carbon and hydrogen nuclei obtained from the proton-encoded (13)C 2D NMR experiment are used to investigate the conformational changes which occur when the achiral symmetric liquid crystal dimer CB7CB changes from the achiral nematic to the chiral twist-bend nematic phase. It is found that these changes are a consequence of the chirality of the twist-bend nematic phase, rather than being the driving force for the stability of this phase.
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Affiliation(s)
- James W Emsley
- Centre RMN à Très Hauts Champs, CNRS/ENS Lyon/UCB-Lyon 1, Université de Lyon, 5 rue de la Doua, 69100 Villeurbanne, France.
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16
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Trojanowski K, Pająk G, Longa L, Wydro T. Tetrahedratic mesophases, chiral order, and helical domains induced by quadrupolar and octupolar interactions. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 86:011704. [PMID: 23005434 DOI: 10.1103/physreve.86.011704] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/21/2012] [Revised: 05/22/2012] [Indexed: 06/01/2023]
Abstract
We present an exhaustive account of phases and phase transitions that can be stabilized in the recently introduced generalized Lebwohl-Lasher model with quadrupolar and octupolar microscopic interactions [L. Longa, G. Pająk, and T. Wydro, Phys. Rev. E 79, 040701(R) (2009)]. A complete mean-field analysis of the model, along with Monte Carlo simulations allows us to identify four distinct classes of the phase diagrams with a number of multicritical points where, in addition to the standard uniaxial and biaxial nematic phases, the other nematic like phases are stabilized. These involve, among the others, tetrahedratic (T), nematic tetrahedratic (N(T)), and chiral nematic tetrahedratic (N(T)(*)) phases of global T(d), D(2d), and D(2) symmetry, respectively. Molecular order parameters and correlation functions in these phases are determined. We conclude with generalizations of the model that give a simple molecular interpretation of macroscopic regions with opposite optical activity (ambidextrous chirality), observed, e.g., in bent-core systems. An estimate of the helical pitch in the N(T)(*) phase is also given.
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Affiliation(s)
- Karol Trojanowski
- Marian Smoluchowski Institute of Physics, Department of Statistical Physics and Mark Kac Center for Complex Systems Research, Jagiellonian University, Reymonta 4, Kraków, Poland
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Osipov MA, Pająk G. Influence of dipole-dipole correlations on the stability of the biaxial nematic phase in the model bent-core liquid crystal. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:142201. [PMID: 22388661 DOI: 10.1088/0953-8984/24/14/142201] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
A molecular theory of biaxial nematic ordering in the system of bent-core molecules has been developed in the two-particle cluster approximation which enables one to take into account short-range polar correlations determined by both electrostatic dipole-dipole interaction and polar molecular shape. All orientational order parameters and short-range correlation functions are calculated numerically as functions of temperature in the uniaxial and in the biaxial nematic phases, and the results are compared with the ones obtained in the mean-field approximation and in the cluster approximation but without taking into consideration the dipole-dipole interaction. It is shown that short-range polar correlations and, in particular, the dipole-dipole correlations dramatically increase the temperature of the transition into the biaxial nematic phase and enhancing its stability range. The results are also very sensitive to the value of the opening angle of a model bent-core molecule.
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Affiliation(s)
- Mikhail A Osipov
- Department of Mathematics and Statistics, University of Strathclyde, Livingstone Tower, Richmond Street, Glasgow, G1 1XH, UK.
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18
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Jang Y, Panov VP, Kocot A, Lehmann A, Tschierske C, Vij JK. Short-range correlations seen in the nematic phase of bent-core liquid crystals by dielectric and electro-optic studies. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:060701. [PMID: 22304031 DOI: 10.1103/physreve.84.060701] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/18/2011] [Indexed: 05/31/2023]
Abstract
Three bent-core nematic liquid crystals having the same core but with different terminal groups, short (C4) and long (C7,C9) tails, are investigated by dielectric and electro-optic contrast spectroscopic techniques. C4 shows sign reversal in the dielectric anisotropy Δε' as a function of both temperature and frequency, whereas C9 shows only negative Δε' in the entire mesophasic region. The behavior of C7 is intermediate of the two. Results of a dielectric study show that both C7 and C9 exhibit strong short-range polar correlations normal to the director. The correlation lengths of these interactions are found to be similar to those from the x-ray scattering. An increased hindered rotation for C9 compared to C4 moves the dielectric dispersion for ε(|') to much lower frequencies, such that C9 shows only negative Δε' over the entire temperature range.
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Affiliation(s)
- Yun Jang
- Department of Electronic and Electrical Engineering, Trinity College Dublin, Dublin 2, Ireland
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