1
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Lynch ST, De Francesco A, Scaccia L, Suvorov A, Cai YQ, Agra-Kooijman DM, Sharpnack LL, Kumar S, Cunsolo A. Shear wave propagation in a liquid crystal: An inelastic X-ray scattering study. J Chem Phys 2024; 160:234505. [PMID: 38904943 DOI: 10.1063/5.0212229] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2024] [Accepted: 05/24/2024] [Indexed: 06/22/2024] Open
Abstract
We investigated the spectrum of density fluctuations of a liquid crystal, CB7CB, in two different orientations by using high-resolution inelastic x-ray scattering. Our analysis, based on Bayesian principles, revealed that high-frequency collective excitations propagate through this mesoscale-ordered sample in a peculiar manner that lies somewhere between those observed in liquids and crystalline systems. Interestingly, when we probed longer length scales, a more pronounced solid-like response emerged. This was mainly characterized by anomalously sharp inelastic excitations and the onset of shear mode propagation. Comparison with previous x-ray diffraction results suggests a correlation between the observed behavior and the mesogen arrangement.
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Affiliation(s)
- S T Lynch
- Department of Physics, University of Wisconsin - Madison, 1150 University Avenue, Madison, Wisconsin 53706, USA
| | - A De Francesco
- CNR - Istituto Officina dei Materiali (IOM), Grenoble, INSIDE@ILL, 71 Avenue des Martyrs, Grenoble CEDEX 9, France
- Institut Laue Langevin, 72 Avenue des Martyrs, F-38042 Grenoble, France
| | - L Scaccia
- Dipartimento di Economia e Diritto, Università di Macerata, Via Crescimbeni 20, 62100 Macerata, Italy
| | - A Suvorov
- National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA
| | - Y Q Cai
- National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA
| | | | - L L Sharpnack
- Earth Science and Environmental Studies, University of California, Santa Barbara, California 93106, USA
| | - Satyendra Kumar
- Department of Physics and Division of Research and Economic Development, University at Albany, Albany, New York 12222, USA
| | - A Cunsolo
- Department of Physics, University of Wisconsin - Madison, 1150 University Avenue, Madison, Wisconsin 53706, USA
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2
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Kocot A, Czarnecka M, Arakawa Y, Merkel K. Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases. J Phys Chem B 2023; 127:7082-7090. [PMID: 37526147 PMCID: PMC10424235 DOI: 10.1021/acs.jpcb.3c03496] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2023] [Revised: 07/02/2023] [Indexed: 08/02/2023]
Abstract
Dielectric spectroscopy in frequencies that range from 10 Hz to 1 GHz has been used to study the molecular orientational dynamics of the two types of dimers that form the twist-bend nematic phase over a wide range of temperatures for both nematic and twist-bend nematic phases. The symmetrical and asymmetrical liquid crystal dimers with the cyanobiphenyl mesogenic groups were investigated. The results were analyzed within the framework of the molecular theory of dielectric permittivity for nematogens. The two molecular processes can be assigned to the reorientation of the monomeric unit: the high frequency one to the precessional rotation of the longitudinal components of the cyanobiphenyl groups (CN) and the second (low frequency) to the end-over-end rotation of the CN dipole around the short molecular axis. The precession mode, which is determined by the local order and is almost unaffected by the phase transition from the nematic to the twist-bend phase, while the end-over-end rotation clearly slowed down at the transition, as it is affected by the growth of the intermolecular interactions. The latter corresponds well to the fact revealed by IR spectroscopy about the longitudinal correlation of the molecular dipoles.
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Affiliation(s)
- Antoni Kocot
- Institute
of Materials Engineering, Faculty of Science and Technology, University of Silesia, 75 Pułku Piechoty 1a, Chorzów 41-500, Poland
| | - Małgorzata Czarnecka
- Faculty
of Electrical Engineering, Automatics, Computer Science and Biomedical
Engineering, AGH University of Science and
Technology, al. Adama Mickiewicza 30, Cracow 30-059, Poland
| | - Yuki Arakawa
- Department
of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan
| | - Katarzyna Merkel
- Institute
of Materials Engineering, Faculty of Science and Technology, University of Silesia, 75 Pułku Piechoty 1a, Chorzów 41-500, Poland
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3
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Sellarès J, Diego JA, López DO, Salud J, Robles-Hernández B, de la Fuente MR, Cañadas JC, Mudarra M, López de Rioja V, Levit R, Diez-Berart S. Comparative dielectric and thermally stimulated-depolarization-current studies of the liquid crystal dimers 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane and heptane and a binary mixture between them, close to the glass transition. Phys Rev E 2022; 106:054702. [PMID: 36559473 DOI: 10.1103/physreve.106.054702] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2022] [Accepted: 10/27/2022] [Indexed: 06/17/2023]
Abstract
We have performed dielectric spectroscopy and thermally stimulated-depolarization-current experiments to study the molecular dynamics of the twist-bend nematic phase close to the glass transition of two members of the 1″,7'-bis(4-cyanobiphenyl-4'-yl)alkane homologous series (CBnCB): the liquid crystal (LC) dimers CB9CB and CB7CB, as well as a binary mixture of both. By doping CB9CB with a small quantity of CB7CB, the crystallization is inhibited when cooling the sample down, while the bulk properties of CB9CB are retained and we can investigate the supercooled behavior close to the glass transition. The study reveals that the inter- and intramolecular interactions of the mixture are similar to those of pure CB9CB and confirms that there is a single glass transition in symmetric LC dimers.
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Affiliation(s)
- Jordi Sellarès
- DILAB, Departament de Física, E.S.E.I.A.A.T. Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
| | - José Antonio Diego
- DILAB, Departament de Física, E.S.E.I.A.A.T. Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
| | - David O López
- Grup de les Propietas Físiques del Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Avinguda Diagonal, 647, 08028 Barcelona, Spain
| | - Josep Salud
- Grup de les Propietas Físiques del Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Avinguda Diagonal, 647, 08028 Barcelona, Spain
| | - Beatriz Robles-Hernández
- Donostia International Physics Center, Manuel Lardizabal Ibilbidea 4, 20018 Donostia, Spain
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - María Rosario de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - Juan Carlos Cañadas
- DILAB, Departament de Física, E.S.E.I.A.A.T. Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
| | - Miguel Mudarra
- DILAB, Departament de Física, E.S.E.I.A.A.T. Universitat Politècnica de Catalunya, Colom 11, 08222 Terrassa, Spain
| | - Victor López de Rioja
- Grup de les Propietas Físiques del Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Avinguda Diagonal, 647, 08028 Barcelona, Spain
| | - Rafael Levit
- Caracterització Elèctrica dels Materials i Dispositius (CEMAD), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Avinguda Diagonal, 647, 08028 Barcelona, Spain
| | - Sergio Diez-Berart
- Grup de les Propietas Físiques del Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Avinguda Diagonal, 647, 08028 Barcelona, Spain
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4
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Photo-driven effects in twist-bend nematic phases: Dynamic and memory response of liquid crystalline dimers. J Mol Liq 2021. [DOI: 10.1016/j.molliq.2021.117680] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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5
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Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen. CRYSTALS 2020. [DOI: 10.3390/cryst10121110] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.
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6
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Longa L, Tomczyk W. Twist-Bend Nematic Phase from the Landau-de Gennes Perspective. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2020; 124:22761-22775. [PMID: 33193939 PMCID: PMC7662770 DOI: 10.1021/acs.jpcc.0c05711] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/23/2020] [Revised: 08/18/2020] [Indexed: 06/11/2023]
Abstract
Generalized Landau-de Gennes theory is proposed that comprehensively explains currently available experimental data for the heliconical twist-bend nematic (NTB) phase observed in liquid crystalline systems of chemically achiral bent-core-like molecules. A bifurcation analysis gives insight into possible structures that the model can predict and guides in the numerical analysis of relative stability of the isotropic (I), uniaxial nematic (NU), and twist-bend nematic phases. An estimate of constitutive parameters of the model from temperature variation of the nematic order parameter and the Frank elastic constants in the nematic phase enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the pitch and conical angle in NTB. Properties of order parameters also explain a puzzling lack of a half-pitch band in resonant soft X-ray scattering. Other key findings of the model are predictions of I-NTB and NU-NTB tricritical points and insight into biaxiality of NTB.
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7
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Merkel K, Welch C, Ahmed Z, Piecek W, Mehl GH. Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers. J Chem Phys 2019; 151:114908. [PMID: 31542029 DOI: 10.1063/1.5114824] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1″-n″-bis(4-cyanobiphenyl-4'-yl) n-alkanes (CBnCB n = 7, 9, 11) has been investigated in a frequency range 0.1 Hz-100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (μm) over a temperature range that covers both nematic and twist bend nematic phases. Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ∼40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director. The mode in the next lower frequency range of 2-10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz-1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electro-clinic effect-the response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in the frequency range of 0.1 Hz-100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.
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Affiliation(s)
- K Merkel
- Faculty of Computer Science and Material Science, Institute of Technology and Mechatronics, University of Silesia in Katowice, Katowice, Poland
| | - C Welch
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Z Ahmed
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - W Piecek
- Faculty of Advanced Technologies and Chemistry, Military University of Technology, Warszawa, Poland
| | - G H Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
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8
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Merkel K, Kocot A, Welch C, Mehl GH. Soft modes of the dielectric response in the twist–bend nematic phase and identification of the transition to a nematic splay bend phase in the CBC7CB dimer. Phys Chem Chem Phys 2019; 21:22839-22848. [DOI: 10.1039/c9cp04952e] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Two collective processes resulting from distortion of the heliconical structure of the twist–bend nematic phase of an achiral dimer: one tilt mode due to distortions of the conical angle and second related to long range fluctuation of the cone phase.
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Affiliation(s)
- K. Merkel
- Faculty of Computer Science and Material Science
- Institute of Technology and Mechatronics
- University of Silesia in Katowice
- Poland
| | - A. Kocot
- Faculty of Computer Science and Material Science
- Institute of Technology and Mechatronics
- University of Silesia in Katowice
- Poland
| | - C. Welch
- Department of Chemistry
- University of Hull
- Hull HU6 7RX
- UK
| | - G. H. Mehl
- Department of Chemistry
- University of Hull
- Hull HU6 7RX
- UK
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9
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Arakawa Y, Komatsu K, Tsuji H. Twist-bend nematic liquid crystals based on thioether linkage. NEW J CHEM 2019. [DOI: 10.1039/c8nj06456c] [Citation(s) in RCA: 40] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
“Thioether”-based twist-bend nematogens.
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Affiliation(s)
- Yuki Arakawa
- Department of Applied Chemistry and Life Science
- Graduate School of Engineering
- Toyohashi University of Technology
- Toyohashi
- Japan
| | - Kenta Komatsu
- Department of Applied Chemistry and Life Science
- Graduate School of Engineering
- Toyohashi University of Technology
- Toyohashi
- Japan
| | - Hideto Tsuji
- Department of Applied Chemistry and Life Science
- Graduate School of Engineering
- Toyohashi University of Technology
- Toyohashi
- Japan
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10
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López DO, Salud J, de la Fuente MR, Sebastián N, Diez-Berart S. Cooperative behavior of molecular motions giving rise to two glass transitions in the same supercooled mesophase of a smectogenic liquid crystal dimer. Phys Rev E 2018; 97:012704. [PMID: 29448339 DOI: 10.1103/physreve.97.012704] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2017] [Indexed: 06/08/2023]
Abstract
In the present work, a detailed analysis of the glassy behavior and the relaxation dynamics of the liquid crystal dimer α-(4-cyanobiphenyl-4'-yloxy)-ω-(1-pyrenimine-benzylidene-4'-oxy) heptane (CBO7O.Py) throughout both nematic and smectic-A mesophases by means of broadband dielectric spectroscopy has been performed. CBO7O.Py shows three different dielectric relaxation modes and two glass transition (T_{g}) temperatures: The higher T_{g} is due to the freezing of the molecular motions responsible for the relaxation mode with the lowest frequency (μ_{1L}); the lower T_{g} is due to the motions responsible for the two relaxation modes with highest frequencies (μ_{1H} and μ_{2}), which converge just at their corresponding T_{g}. It is shown how the three modes follow a critical-like description via the dynamic scaling model. The two modes with lowest frequencies (μ_{1L} and μ_{1H}) are cooperative in the whole range of the mesophases, whereas the highest frequency mode (μ_{2}) is cooperative just below some crossover temperature. In terms of fragility, at the glass transition, the ensemble (μ_{1H}+μ_{2}) presents a value of the steepness index and μ_{1L} a different one, meaning that fragility is a property intrinsic to the molecular motion itself. Finally, the steepness index seems to have a universal behavior with temperature for the dielectric relaxation modes of liquid crystal dimers, being almost constant at high temperatures and increasing drastically when cooling the compound down to the glass transition from a temperature about 3/4T_{NI}.
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Affiliation(s)
- David O López
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
| | - Josep Salud
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
| | - María Rosario de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - Nerea Sebastián
- Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia
| | - Sergio Diez-Berart
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
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11
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Trbojevic N, Read DJ, Nagaraj M. Dielectric properties of liquid crystalline dimer mixtures exhibiting the nematic and twist-bend nematic phases. Phys Rev E 2017; 96:052703. [PMID: 29347724 DOI: 10.1103/physreve.96.052703] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2017] [Indexed: 11/07/2022]
Abstract
A detailed investigation of the thermal and dielectric properties of a series of binary mixtures exhibiting the nematic (N) and twist-bend nematic (N_{TB}) liquid crystal phases is presented. The mixtures consist of an achiral, dimeric liquid crystal CB7CB, which forms the nematic and twist-bend nematic phases, and a calamitic liquid crystal 5CB, which shows the nematic phase. As the concentration of the calamitic liquid crystal is increased, the transition temperatures decrease linearly, and the width of the nematic phase increases. The enthalpies of phase transitions obtained from DSC measurements show that on increasing the concentration of 5CB in the binary mixtures, the enthalpy associated with the N-N_{TB} phase transitions reduces considerably compared to a clear first-order N-N_{TB} transition in pure CB7CB. The real and imaginary parts of the dielectric permittivity are measured as a function of frequency from 100 Hz to 2 MHz in the nematic and twist-bend nematic phases in planar and homeotropic devices. A significant decrease in the average dielectric permittivity as a function of temperature for mixtures forming the N_{TB} phase is observed. Measurements of the imaginary part of the dielectric permittivity show a relaxation peak in the measured frequency window for all of the mixtures exhibiting the N_{TB} phase. The activation energy associated with this relaxation process is calculated and is shown to remain constant irrespective of the composition of the mixtures.
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Affiliation(s)
- Nina Trbojevic
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
| | - Daniel J Read
- School of Mathematics, University of Leeds, Leeds LS2 9JT, United Kingdom
| | - Mamatha Nagaraj
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
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12
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Salili SM, Xiang J, Wang H, Li Q, Paterson DA, Storey JMD, Imrie CT, Lavrentovich OD, Sprunt SN, Gleeson JT, Jákli A. Magnetically tunable selective reflection of light by heliconical cholesterics. Phys Rev E 2016; 94:042705. [PMID: 27841485 DOI: 10.1103/physreve.94.042705] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2016] [Indexed: 06/06/2023]
Abstract
We present studies of chiral nematic liquid crystals composed of flexible dimer molecules subject to large dc magnetic fields between 0 and 31 T. We observe that these fields lead to selective reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-C band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation in accordance with early theoretical predictions. The use of magnetic field here instead of electric field allows precise measurements of some material constants and holds promise for wireless tuning of selective reflection.
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Affiliation(s)
- S M Salili
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - J Xiang
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - H Wang
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - Q Li
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - D A Paterson
- Department of Chemistry, University of Aberdeen, AB24 3UE Scotland, United Kingdom
| | - J M D Storey
- Department of Chemistry, University of Aberdeen, AB24 3UE Scotland, United Kingdom
| | - C T Imrie
- Department of Chemistry, University of Aberdeen, AB24 3UE Scotland, United Kingdom
| | - O D Lavrentovich
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - S N Sprunt
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | - J T Gleeson
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | - A Jákli
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
- Complex Fluid Group, Wigner Research Centre, Budapest, Hungary
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13
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Mandle RJ. The dependency of twist-bend nematic liquid crystals on molecular structure: a progression from dimers to trimers, oligomers and polymers. SOFT MATTER 2016; 12:7883-7901. [PMID: 27722733 DOI: 10.1039/c6sm01772j] [Citation(s) in RCA: 59] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
This article gives an overview on recent developments concerning the twist-bend nematic phase. The twist-bend nematic phase has been discussed as the missing link between the uniaxial nematic mesophase (N) and the helical chiral nematic phase (N*). After an introduction discussing the key physical properties of the NTB phase and the methods used to identify the twist-bend nematic mesophase this review focuses on structure property relationships and molecular features that govern the incidence of this phase.
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Affiliation(s)
- Richard J Mandle
- Department of Chemistry, University of York, York, YO10 5DD, UK.
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14
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Robles-Hernández B, Sebastián N, Salud J, Diez-Berart S, Dunmur DA, Luckhurst GR, López DO, de la Fuente MR. Molecular dynamics of a binary mixture of twist-bend nematic liquid crystal dimers studied by dielectric spectroscopy. Phys Rev E 2016; 93:062705. [PMID: 27415332 DOI: 10.1103/physreve.93.062705] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2016] [Indexed: 06/06/2023]
Abstract
We report a comprehensive dielectric characterization of a liquid crystalline binary mixture composed of the symmetric mesogenic dimer CB7CB and the nonsymmetric mesogenic dimer FFO9OCB. In addition to the high-temperature nematic phase, such a binary mixture shows a twist-bend nematic phase at room temperature which readily vitrifies on slow cooling. Changes in the conformational distribution of the dimers are reflected in the dielectric permittivity and successfully analyzed by means of an appropriate theoretical model. It is shown that the dielectric spectra of the mixture reflect the different molecular dipole properties of the components, resembling in the present case the characteristic dielectric spectra of nonsymmetric dimers. Comparison of the nematic and twist-bend nematic phases reveals that molecular dynamics are similar despite the difference in the molecular environment.
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Affiliation(s)
- Beatriz Robles-Hernández
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - Nerea Sebastián
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
- Otto-von-Guericke Universitat Magdeburg, Institute for Experimental Physics, ANP, 39106 Magdeburg, Germany
| | - Josep Salud
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain
| | - Sergio Diez-Berart
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain
| | - David A Dunmur
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom
| | - Geoffrey R Luckhurst
- Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
| | - David O López
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E-08028 Barcelona, Spain
| | - M Rosario de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
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15
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Zhu C, Tuchband MR, Young A, Shuai M, Scarbrough A, Walba DM, Maclennan JE, Wang C, Hexemer A, Clark NA. Resonant Carbon K-Edge Soft X-Ray Scattering from Lattice-Free Heliconical Molecular Ordering: Soft Dilative Elasticity of the Twist-Bend Liquid Crystal Phase. PHYSICAL REVIEW LETTERS 2016; 116:147803. [PMID: 27104729 DOI: 10.1103/physrevlett.116.147803] [Citation(s) in RCA: 66] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/31/2015] [Indexed: 05/16/2023]
Abstract
Resonant x-ray scattering shows that the bulk structure of the twist-bend liquid crystal phase, recently discovered in bent molecular dimers, has spatial periodicity without electron density modulation, indicating a lattice-free heliconical nematic precession of orientation that has helical glide symmetry. In situ study of the bulk helix texture of the dimer CB7CB shows an elastically confined temperature-dependent minimum helix pitch, but a remarkable elastic softness of pitch in response to dilative stresses. Scattering from the helix is not detectable in the higher temperature nematic phase.
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Affiliation(s)
- Chenhui Zhu
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Michael R Tuchband
- Department of Physics and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0390, USA
| | - Anthony Young
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Min Shuai
- Department of Physics and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0390, USA
| | - Alyssa Scarbrough
- Department of Chemistry and Biochemistry and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0215, USA
| | - David M Walba
- Department of Chemistry and Biochemistry and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0215, USA
| | - Joseph E Maclennan
- Department of Physics and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0390, USA
| | - Cheng Wang
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Alexander Hexemer
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
| | - Noel A Clark
- Department of Physics and Soft Materials Research Center, University of Colorado Boulder, Boulder, Colorado 80309-0390, USA
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16
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López DO, Robles-Hernández B, Salud J, de la Fuente MR, Sebastián N, Diez-Berart S, Jaen X, Dunmur DA, Luckhurst GR. Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition. Phys Chem Chem Phys 2016; 18:4394-404. [DOI: 10.1039/c5cp07605f] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We have developed a Landau model that predicts a first order twist-bend nematic–nematic phase transition.
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Affiliation(s)
- D. O. López
- Grup de Propietats Físiques dels Materials (GRPFM-GCMM)
- Departament de Física
- E.T.S.E.I.B
- Universitat Politècnica de Catalunya
- 647 08028 Barcelona
| | - B. Robles-Hernández
- Grup de Propietats Físiques dels Materials (GRPFM-GCMM)
- Departament de Física
- E.T.S.E.I.B
- Universitat Politècnica de Catalunya
- 647 08028 Barcelona
| | - J. Salud
- Grup de Propietats Físiques dels Materials (GRPFM-GCMM)
- Departament de Física
- E.T.S.E.I.B
- Universitat Politècnica de Catalunya
- 647 08028 Barcelona
| | - M. R. de la Fuente
- Departamento de Física Aplicada II
- Facultad de Ciencia y Tecnología
- Universidad del País Vasco
- E-48080 Bilbao
- Spain
| | - N. Sebastián
- Institute for Experimental Physics
- Otto-von-Guerike Universität Magdeburg
- ANP
- 39106 Magdeburg
- Germany
| | - S. Diez-Berart
- Grup de Propietats Físiques dels Materials (GRPFM-GCMM)
- Departament de Física
- E.T.S.E.I.B
- Universitat Politècnica de Catalunya
- 647 08028 Barcelona
| | - X. Jaen
- Grup de Propietats Físiques dels Materials (GRPFM-GCMM)
- Departament de Física
- E.T.S.E.I.B
- Universitat Politècnica de Catalunya
- 647 08028 Barcelona
| | - D. A. Dunmur
- School of Physics and Astronomy
- University of Manchester
- Manchester M13 9PL
- UK
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17
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Robles-Hernández B, Sebastián N, de la Fuente MR, López DO, Diez-Berart S, Salud J, Ros MB, Dunmur DA, Luckhurst GR, Timimi BA. Twist, tilt, and orientational order at the nematic to twist-bend nematic phase transition of 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane: A dielectric, (2)H NMR, and calorimetric study. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:062505. [PMID: 26764709 DOI: 10.1103/physreve.92.062505] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/06/2015] [Indexed: 06/05/2023]
Abstract
The nature of the nematic-nematic phase transition in the liquid crystal dimer 1″,9″-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB) has been investigated using techniques of calorimetry, dynamic dielectric response measurements, and (2)H NMR spectroscopy. The experimental results for CB9CB show that, like the shorter homologue CB7CB, the studied material exhibits a normal nematic phase, which on cooling undergoes a transition to the twist-bend nematic phase (N(TB)), a uniaxial nematic phase, promoted by the average bent molecular shape, in which the director tilts and precesses describing a conical helix. Modulated differential scanning calorimetry has been used to analyze the nature of the N(TB)-N phase transition, which is found to be weakly first order, but close to tricritical. Additionally broadband dielectric spectroscopy and (2)H magnetic resonance studies have revealed information on the structural characteristics of the recently discovered twist-bend nematic phase. Analysis of the dynamic dielectric response in both nematic phases has provided an estimate of the conical angle of the heliconical structure for the N(TB) phase. Capacitance measurements of the electric-field realignment of the director in initially planar aligned cells have yielded values for the splay and bend elastic constants in the high temperature nematic phase. The bend elastic constant is small and decreases with decreasing temperature as the twist-bend phase is approached. This behavior is expected theoretically and has been observed in materials that form the twist-bend nematic phase. (2)H NMR measurements characterize the chiral helical twist identified in the twist-bend nematic phase and also allow the determination of the temperature dependence of the conical angle and the orientational order parameter with respect to the director.
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Affiliation(s)
- Beatriz Robles-Hernández
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - Nerea Sebastián
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
- Otto-von-Guericke Universitat Magdeburg, Institute for Experimental Physics, ANP, 39106 Magdeburg, Germany
| | - M Rosario de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - David O López
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
| | - Sergio Diez-Berart
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
| | - Josep Salud
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, E- 08028 Barcelona, Spain
| | - M Blanca Ros
- Departamento de Química Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
| | - David A Dunmur
- Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
| | - Geoffrey R Luckhurst
- Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
| | - Bakir A Timimi
- Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom
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18
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Diez-Berart S, López DO, Salud J, Diego JA, Sellarès J, Robles-Hernández B, de la Fuente MR, Ros MB. Two Glass Transitions Associated to Different Dynamic Disorders in the Nematic Glassy State of a Non-Symmetric Liquid Crystal Dimer Dopped with g-Alumina Nanoparticles. MATERIALS 2015. [PMCID: PMC5455711 DOI: 10.3390/ma8063334] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
In the present work, the nematic glassy state of the non-symmetric LC dimer α-(4-cyanobiphenyl-4′-yloxy)-ω-(1-pyrenimine-benzylidene-4′-oxy) undecane is studied by means of calorimetric and dielectric measurements. The most striking result of the work is the presence of two different glass transition temperatures: one due to the freezing of the flip-flop motions of the bulkier unit of the dimer and the other, at a lower temperature, related to the freezing of the flip-flop and precessional motions of the cyanobiphenyl unit. This result shows the fact that glass transition is the consequence of the freezing of one or more coupled dynamic disorders and not of the disordered phase itself. In order to avoid crystallization when the bulk sample is cooled down, the LC dimer has been confined via the dispersion of γ-alumina nanoparticles, in several concentrations.
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Affiliation(s)
- Sergio Diez-Berart
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, Barcelona 08028, Spain; E-Mails: (D.O.L.); (J.S.)
- Author to whom correspondence should be addressed; E-Mail: ; Tel.: +34-934-054-107; Fax: +34-934-011-839
| | - David O. López
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, Barcelona 08028, Spain; E-Mails: (D.O.L.); (J.S.)
| | - Josep Salud
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal 647, Barcelona 08028, Spain; E-Mails: (D.O.L.); (J.S.)
| | - José Antonio Diego
- Dielectric Materials Physics Laboratory (DILAB), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Colom 11, Terrassa 08222, Spain; E-Mails: (J.A.D.); (J.S.)
| | - Jordi Sellarès
- Dielectric Materials Physics Laboratory (DILAB), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Colom 11, Terrassa 08222, Spain; E-Mails: (J.A.D.); (J.S.)
| | - Beatriz Robles-Hernández
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV-EHU, Apartado 644, Bilbao 48080, Spain; E-Mails: (B.R.-H.); (M.R.F.)
| | - María Rosario de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV-EHU, Apartado 644, Bilbao 48080, Spain; E-Mails: (B.R.-H.); (M.R.F.)
| | - María Blanca Ros
- Instituto de Ciencia de Materiales de Aragón (ICMA), Departamento de Química Orgánica-Facultad de Ciencias, Universidad de Zaragoza-CSIC, Zaragoza 50009, Spain; E-Mail:
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19
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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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20
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Sebastián N, López DO, Robles-Hernández B, de la Fuente MR, Salud J, Pérez-Jubindo MA, Dunmur DA, Luckhurst GR, Jackson DJB. Dielectric, calorimetric and mesophase properties of 1′′-(2′,4-difluorobiphenyl-4′-yloxy)-9′′-(4-cyanobiphenyl-4′-yloxy) nonane: an odd liquid crystal dimer with a monotropic mesophase having the characteristics of a twist-bend nematic phase. Phys Chem Chem Phys 2014; 16:21391-406. [DOI: 10.1039/c4cp03462g] [Citation(s) in RCA: 73] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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21
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Chen D, Nakata M, Shao R, Tuchband MR, Shuai M, Baumeister U, Weissflog W, Walba DM, Glaser MA, Maclennan JE, Clark NA. Twist-bend heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core mesogen. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:022506. [PMID: 25353488 DOI: 10.1103/physreve.89.022506] [Citation(s) in RCA: 85] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/14/2013] [Indexed: 06/04/2023]
Abstract
The chiral, heliconical (twist-bend) nematic ground state is reported in an achiral, rigid, bent-core mesogen (UD68). Similar to the nematic twist-bend (N(TB)) phase observed in bent molecular dimers, the N(TB) phase of UD68 forms macroscopic, smecticlike focal-conic textures and exhibits nanoscale, periodic modulation with no associated modulation of the electron density, i.e., without a detectable lamellar x-ray reflection peak. The N(TB) helical pitch is p(TB) ∼ 14 nm. When an electric field is applied normal to the helix axis, a weak electroclinic effect is observed, revealing 50-μm-scale left- and right-handed domains in a chiral conglomerate.
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Affiliation(s)
- Dong Chen
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Michi Nakata
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Renfan Shao
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Michael R Tuchband
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Min Shuai
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Ute Baumeister
- Institut für Chemie, Physikalische Chemie, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz, D-06120 Halle (Saale), Germany
| | - Wolfgang Weissflog
- Institut für Chemie, Physikalische Chemie, Martin-Luther-Universität Halle-Wittenberg, von-Danckelmann-Platz, D-06120 Halle (Saale), Germany
| | - David M Walba
- Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309-0215, USA
| | - Matthew A Glaser
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Joseph E Maclennan
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
| | - Noel A Clark
- Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, USA
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22
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Sebastián N, de la Fuente M, López D, Pérez-Jubindo M, Salud J, Ros M. Overall Dielectric Study on the Odd Members of a Highly Nonsymmetric Pyrene-Based Series of Liquid Crystal Dimers. J Phys Chem B 2013; 117:14486-96. [DOI: 10.1021/jp406085r] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- N. Sebastián
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - M.R. de la Fuente
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - D.O. López
- Grup
de Propietas Físiques dels Materials (GRPFM), Departament de
Física i Enginyeria Nuclear, E.T.S.E.I.B., Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M.A. Pérez-Jubindo
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - J. Salud
- Grup
de Propietas Físiques dels Materials (GRPFM), Departament de
Física i Enginyeria Nuclear, E.T.S.E.I.B., Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M.B. Ros
- Departamento
de Química Orgánica, Facultad de Ciencias, Instituto
de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
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23
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Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers. Proc Natl Acad Sci U S A 2013; 110:15931-6. [PMID: 24006362 DOI: 10.1073/pnas.1314654110] [Citation(s) in RCA: 239] [Impact Index Per Article: 21.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called "twist-bend" nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ~ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ~ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ~ in CB(CH2)7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θ(TB) ~ 25° and the full pitch of the director helix to be p(TB) ~ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend.
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