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Vita F, Adamo FC, Campana M, Bordokas B, Ciuchi F, De Santo MP, Hermida-Merino D, Lisovsky A, Pisani M, Pontoni D, Scharrer E, Francescangeli O. Macroscopic Biaxial Order in Multilayer Films of Bent-Core Liquid Crystals Deposited by Combined Langmuir-Blodgett/Langmuir-Schaefer Technique. NANOMATERIALS (BASEL, SWITZERLAND) 2024; 14:357. [PMID: 38392730 PMCID: PMC10892925 DOI: 10.3390/nano14040357] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/31/2023] [Revised: 01/29/2024] [Accepted: 02/09/2024] [Indexed: 02/24/2024]
Abstract
Bent-core liquid crystals, a class of mesogenic compounds with non-linear molecular structures, are well known for their unconventional mesophases, characterized by complex molecular (and supramolecular) ordering and often featuring biaxial and polar properties. In the nematic phase, their unique behavior is manifested in the formation of nano-sized biaxial clusters of layered molecules (cybotactic groups). While this prompted their consideration in the quest for nematic biaxiality, experimental evidence indicates that the cybotactic order is only short-ranged and that the nematic phase is macroscopically uniaxial. By combining atomic force microscopy, neutron reflectivity and wide-angle grazing-incidence X-ray scattering, here, we demonstrate that multilayer films of a bent-core nematic, deposited on silicon by a combined Langmuir-Blodgett and Langmuir-Schaefer approach, exhibit macroscopic in-plane ordering, with the long molecular axis tilted with respect to the sample surface and the short molecular axis (i.e., the apex bisector) aligned along the film compression direction. We thus propose the use of Langmuir films as an effective way to study and control the complex anchoring properties of bent-core liquid crystals.
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Affiliation(s)
- Francesco Vita
- Department of Science and Engineering of Matter, Environment and Urban Planning (SIMAU), Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy; (F.C.A.); (M.P.)
| | - Fabrizio Corrado Adamo
- Department of Science and Engineering of Matter, Environment and Urban Planning (SIMAU), Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy; (F.C.A.); (M.P.)
| | - Mario Campana
- ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, UK;
| | - Blake Bordokas
- Department of Chemistry, University of Puget Sound, Tacoma, WA 98416, USA; (B.B.); (A.L.); (E.S.)
| | - Federica Ciuchi
- CNR-Nanotec c/o Physics Department, University of Calabria, Ponte Bucci, Cubo 31C, 87036 Arcavacata di Rende, Italy; (F.C.); (M.P.D.S.)
| | - Maria Penelope De Santo
- CNR-Nanotec c/o Physics Department, University of Calabria, Ponte Bucci, Cubo 31C, 87036 Arcavacata di Rende, Italy; (F.C.); (M.P.D.S.)
- Physics Department, University of Calabria, Ponte Bucci, Cubo 31C, 87036 Arcavacata di Rende, Italy
| | - Daniel Hermida-Merino
- DUBBLE@ESRF BP CS40220, 38043 Grenoble, France;
- Departamento de Física Aplicada, Centro de Investigación en Nanomateriais e Biomedicina (CINBIO), Universidade de Vigo, Campus Lagoas-Marcosende, E36310 Vigo, Spain
| | - Angela Lisovsky
- Department of Chemistry, University of Puget Sound, Tacoma, WA 98416, USA; (B.B.); (A.L.); (E.S.)
| | - Michela Pisani
- Department of Science and Engineering of Matter, Environment and Urban Planning (SIMAU), Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy; (F.C.A.); (M.P.)
| | - Diego Pontoni
- ESRF—The European Synchrotron, 71 Avenue des Martyrs, 38043 Grenoble, France;
| | - Eric Scharrer
- Department of Chemistry, University of Puget Sound, Tacoma, WA 98416, USA; (B.B.); (A.L.); (E.S.)
| | - Oriano Francescangeli
- Department of Science and Engineering of Matter, Environment and Urban Planning (SIMAU), Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy; (F.C.A.); (M.P.)
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Photosensitive Bent-Core Liquid Crystals with Laterally Substituted Azobenzene Unit. CRYSTALS 2021. [DOI: 10.3390/cryst11101265] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables fast (E)-to-(Z)-isomerization upon irradiation with UV-light and visible light, while the substitution facilitates the high stability of the photochemically formed (Z)-isomer. The effectiveness of the irradiation and the composition of photostationary states was determined by UV/Vis and 1H NMR spectroscopy. A nematic phase formed by the materials was characterized by differential scanning calorimetry and optical polarizing microscopy. We show that the materials easily change their relative configuration of the N=N double bond not only in solution, but also in the mesophase, which leads to fast isothermal phase transition from the nematic phase to isotropic liquid.
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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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Šmahel M, Poryvai A, Xiang Y, Pociecha D, Troha T, Novotná V, Svoboda J, Kohout M. Photosensitive bent-core nematic liquid crystals with various linking units in the side arms: Structure-properties relationships. J Mol Liq 2020. [DOI: 10.1016/j.molliq.2020.112743] [Citation(s) in RCA: 14] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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D'Urso C, Celebre G, Cinacchi G. Phase behavior of hard C_{2h}-symmetric particle systems. Phys Rev E 2019; 100:012709. [PMID: 31499787 DOI: 10.1103/physreve.100.012709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2019] [Indexed: 06/10/2023]
Abstract
Using Monte Carlo numerical simulation, this work sketches the phase diagram of systems of certain hard C_{2h}-symmetric particles, formed by gluing two aligned and displaced hard spherocylinders with a cylindrical-length-to-diameter ratio realistically, if viewed not only from the lyotropic colloidal liquid-crystal side but also from the thermotropic low-molecular-mass liquid-crystal side, equal to 5, as a function of the displacement. Several distinctive phases are observed, such as a nonperiodic smectic-B-like phase, a nonperiodic smectic-H-like phase, a smectic-C phase, and a short-layer-spacing uniaxial smectic-A phase but no biaxial nematic phase.
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Affiliation(s)
- Christian D'Urso
- Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, via P. Bucci, I-87036 Arcavacata di Rende (CS), Italy
| | - Giorgio Celebre
- Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, via P. Bucci, I-87036 Arcavacata di Rende (CS), Italy
| | - Giorgio Cinacchi
- Departamento de Física Teórica de la Materia Condensada, Instituto de Física de la Materia Condensada (IFIMAC), Instituto de Ciencias de Materiales "Nicolás Cabrera," Universidad Autónoma de Madrid, Ciudad Universitaria de Cantoblanco, E-28049 Madrid, Spain
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6
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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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8
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Panarin YP, Sreenilayam SP, Vij JK, Lehmann A, Tschierske C. Formation and development of nanometer-sized cybotactic clusters in bent-core nematic liquid crystalline compounds. BEILSTEIN JOURNAL OF NANOTECHNOLOGY 2018; 9:1288-1296. [PMID: 29765807 PMCID: PMC5942366 DOI: 10.3762/bjnano.9.121] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/01/2017] [Accepted: 03/28/2018] [Indexed: 05/23/2023]
Abstract
Two homologue achiral bent-core liquid crystals (LCs), BCN66 and BCN84, in their nematic phases are studied by dielectric spectroscopy in the frequency range 10 Hz-10 MHz. In each of these compounds, two relaxation processes are identified and assigned to (i) collective dynamics of molecules in nanometer-sized cybotactic clusters and (ii) individual molecular relaxations, in the ascending order of frequency of the probe field. The temperature and the bias electric field dependence of the dielectric strength and relaxation frequency for these processes are shown to give rise to sharpness in cluster boundaries, increased size and volume fraction in the LC nematic phase. The effect of the bias field on the LC cell is similar to reducing its temperature; both variables increase the cluster size and volume fraction and give rise to sharp cluster boundaries. The findings confirm that dielectric spectroscopy is a powerful and an extremely useful technique to provide a deeper understanding of the mechanism of cybotactic cluster formation in the isotropic liquid and the nematic phase of LCs as a function of temperature and the bias field.
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Affiliation(s)
- Yuri P Panarin
- Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland
- School of Electrical and Electronic Engineering, Dublin Institute of Technology, Dublin 8, Ireland
| | - Sithara P Sreenilayam
- Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland
| | - Jagdish K Vij
- Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland
| | - Anne Lehmann
- Institute of Chemistry, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany
| | - Carsten Tschierske
- Institute of Chemistry, Martin Luther University Halle-Wittenberg, 06120 Halle, Germany
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9
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Solodkov NV, Nagaraj M, Jones JC. Effects of monoclinic symmetry on the properties of biaxial liquid crystals. Phys Rev E 2018; 97:042702. [PMID: 29758680 DOI: 10.1103/physreve.97.042702] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2017] [Indexed: 06/08/2023]
Abstract
Tilted smectic liquid crystal phases such as the smectic-C phase seen in calamitic liquid crystals are usually treated using the assumption of biaxial orthorhombic symmetry. However, the smectic-C phase has monoclinic symmetry, thereby allowing disassociation of the principal optic and dielectric axes based on symmetry and invariance principles. This is demonstrated here by comparing optical and dielectric measurements for two materials with highly first-order direct transitions from nematic to smectic-C phases. The results show a high difference between the orientations of the principal axes sets, which is interpreted as the existence of two distinct cone angles for optical and dielectric frequencies. Both materials exhibit an increasing degree of monoclinic behavior with decreasing temperature. Due to fast switching speeds, ferroelectric smectic-C* materials are important for fast modulators and LCoS devices, where the dielectric biaxiality influences device operation.
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Affiliation(s)
- Nikita V Solodkov
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
| | - Mamatha Nagaraj
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
| | - J Cliff Jones
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
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Querciagrossa L, Berardi R, Zannoni C. Can off-centre mesogen dipoles extend the biaxial nematic range? SOFT MATTER 2018; 14:2245-2253. [PMID: 29485161 DOI: 10.1039/c8sm00067k] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
We have investigated the possibility of extending the stability range of the biaxial nematic phase by adding an off-centre dipole of various strengths and orientations to elongated biaxial Gay-Berne (GB) mesogens yielding a relatively narrow biaxial nematic (Nbx) phase, and a smectic (Sbx) phase when dipole-less. The effect of dipoles is not easy to predict, and our previous investigations have shown the limited benefits of having a central dipole. Here we show, employing molecular dynamics (MD) simulations, that a not too strong off-centre dipole positioned along the longest axis of the nematogen can extend the temperature range of stability of the biaxial nematic phase, also shifting it towards lower temperatures.
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Affiliation(s)
- Lara Querciagrossa
- Dipartimento di Chimica Industriale "Toso Montanari" and INSTM, Università di Bologna, Viale Risorgimento 4, IT-40136 Bologna, Italy.
| | - Roberto Berardi
- Dipartimento di Chimica Industriale "Toso Montanari" and INSTM, Università di Bologna, Viale Risorgimento 4, IT-40136 Bologna, Italy.
| | - Claudio Zannoni
- Dipartimento di Chimica Industriale "Toso Montanari" and INSTM, Università di Bologna, Viale Risorgimento 4, IT-40136 Bologna, Italy.
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11
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Kumar J, Prasad V. Ferroelectric Nematic and Ferrielectric Smectic Mesophases in an Achiral Bent-Core Azo Compound. J Phys Chem B 2018; 122:2998-3007. [PMID: 29485879 DOI: 10.1021/acs.jpcb.7b11733] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Here, we report the observation of ferroelectric nematic and ferrielectric smectic mesophases in an achiral bent-core azo compound consisting of nonsymmetrical molecules with a lateral fluoro substitution on one of the wings. These mesophases are enantiotropic in nature with fairly low transition temperatures and wide mesophase ranges. The liquid crystalline properties of this compound are investigated using polarizing optical microscope, differential scanning calorimeter, X-ray diffraction, and electro-optical studies. As revealed by X-ray diffraction measurements, the nematic mesophase is composed of skewed cybotactic clusters and, in the smectic mesophase, the molecules are tilted with respect to the layer normal. The polar order in these mesophases was confirmed by the electro-optical switching and dielectric spectroscopy measurements. The dielectric study in the nematic mesophase shows a single relaxation process at low frequency ( f < 1 kHz) measured in the range 10 Hz to 5 MHz, which is attributed to the collective motion of the molecules within cybotactic clusters. The formation of local polar order in these clusters leads to a ferroelectric-like polar switching in the nematic mesophase. Of particular interest is the fact that the smectic phase exhibits a field induced ferrielectric state, which can be exploited for designing of the potential optical devices due to multistate switching.
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Affiliation(s)
- Jitendra Kumar
- Centre for Nano and Soft Matter Sciences , P.B. No. 1329, Jalahalli, Bengaluru - 560 013 , India
| | - Veena Prasad
- Centre for Nano and Soft Matter Sciences , P.B. No. 1329, Jalahalli, Bengaluru - 560 013 , India
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12
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Kamala Latha B, Murthy KPN, Sastry VSS. Complex free-energy landscapes in biaxial nematic liquid crystals and the role of repulsive interactions: A Wang-Landau study. Phys Rev E 2018; 96:032703. [PMID: 29346959 DOI: 10.1103/physreve.96.032703] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2017] [Indexed: 11/07/2022]
Abstract
General quadratic Hamiltonian models, describing the interaction between liquid-crystal molecules (typically with D_{2h} symmetry), take into account couplings between their uniaxial and biaxial tensors. While the attractive contributions arising from interactions between similar tensors of the participating molecules provide for eventual condensation of the respective orders at suitably low temperatures, the role of cross coupling between unlike tensors is not fully appreciated. Our recent study with an advanced Monte Carlo technique (entropic sampling) showed clearly the increasing relevance of this cross term in determining the phase diagram (contravening in some regions of model parameter space), the predictions of mean-field theory, and standard Monte Carlo simulation results. In this context, we investigated the phase diagrams and the nature of the phases therein on two trajectories in the parameter space: one is a line in the interior region of biaxial stability believed to be representative of the real systems, and the second is the extensively investigated parabolic path resulting from the London dispersion approximation. In both cases, we find the destabilizing effect of increased cross-coupling interactions, which invariably result in the formation of local biaxial organizations inhomogeneously distributed. This manifests as a small, but unmistakable, contribution of biaxial order in the uniaxial phase. The free-energy profiles computed in the present study as a function of the two dominant order parameters indicate complex landscapes. On the one hand, these profiles account for the unusual thermal behavior of the biaxial order parameter under significant destabilizing influence from the cross terms. On the other, they also allude to the possibility that in real systems, these complexities might indeed be inhibiting the formation of a low-temperature biaxial order itself-perhaps reflecting the difficulties in their ready realization in the laboratory.
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Affiliation(s)
- B Kamala Latha
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - K P N Murthy
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - V S S Sastry
- School of Physics, University of Hyderabad, Hyderabad 500046, India.,Centre for Modelling, Simulation and Design, University of Hyderabad, Hyderabad 500046, India
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13
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Kumar A, Vanakaras AG, Photinos DJ. Polar Molecular Ordering in the N X Phase of Bimesogens and Enantiotopic Discrimination in the NMR Spectra of Rigid Prochiral Solutes. J Phys Chem B 2017; 121:10689-10703. [PMID: 29064695 DOI: 10.1021/acs.jpcb.7b08319] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The potential of mean torque governing the orientational ordering of prochiral solutes in the two nematic phases (N and NX) formed by certain classes of symmetric achiral bimesogens is formulated and used for the analysis of existing NMR measurements on solutes of various symmetries dissolved in the two phases. Three distinct attributes of the solvent phase, namely polarity of the orientational ordering, chirality of the constituent molecules, and spatial modulation of the local director, are identified as underlying three possible mechanisms for the generation of chiral asymmetry in the low temperature nematic phase (NX). The role and quantitative contribution of each mechanism to enantiotopic discrimination in the NX phase are presented and compared with the case of the conventional chiral nematic phase (N*). It is found that polar ordering is essential for the appearance of enantiotopic discrimination in small rigid solutes dissolved in the NX phase and that such discrimination is restricted to solutes belonging to the point group symmetries Cs and C2v.
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Affiliation(s)
- Anant Kumar
- Department of Materials Science, University of Patras , Patras 26504, Greece
| | | | - Demetri J Photinos
- Department of Materials Science, University of Patras , Patras 26504, Greece
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14
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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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15
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Kim YK, Cukrov G, Vita F, Scharrer E, Samulski ET, Francescangeli O, Lavrentovich OD. Search for microscopic and macroscopic biaxiality in the cybotactic nematic phase of new oxadiazole bent-core mesogens. Phys Rev E 2016; 93:062701. [PMID: 27415328 DOI: 10.1103/physreve.93.062701] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2016] [Indexed: 06/06/2023]
Abstract
The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales. Locally, at the scale of a few molecules, x-ray-diffraction data demonstrate biaxial packing. However, above the mesoscopic scale, the global orientational order in all three compounds is uniaxial, as evidenced by uniform homeotropic alignment of the nematic phase which is optically tested over the entire temperature range and by the observations of topological defects induced by individual and aggregated colloidal spheres in the nematic bulk.
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Affiliation(s)
- Young-Ki Kim
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
| | - Greta Cukrov
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
| | - Francesco Vita
- Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy
| | - Eric Scharrer
- Department of Chemistry, University of Puget Sound, 1500 N. Warner, Tacoma, Washington 98416, USA
| | - Edward T Samulski
- Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA
| | - Oriano Francescangeli
- Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy
| | - Oleg D Lavrentovich
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
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Lucchetti L, Vita F, Scharrer E, Francescangeli O, Simoni F. Optical nonlinearity in the nematic phase of bent-core mesogens. OPTICS LETTERS 2015; 40:2953-2956. [PMID: 26125340 DOI: 10.1364/ol.40.002953] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Nonlinear optical response of the cybotactic nematic phase of a bent-core mesogen has been investigated for the first time through self-phase modulation induced by a Gaussian beam. The material exhibits a high nonlinear response achieving a nonlinear index n(2)≈5×10(-5) cm(2)/W and an unconventional behavior characterized by two different regimes. While the high-intensity regime can be easily explained in terms of a thermal indexing effect, the low-intensity regime is metastable and characterized by an unusual dependence on the irradiation energy. It is suggested that a change of the director configuration, possibly due to a light-induced modification of surface anchoring, is responsible for the observed behavior.
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17
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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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18
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Ahmed Z, Welch C, Mehl GH. The design and investigation of the self-assembly of dimers with two nematic phases. RSC Adv 2015. [DOI: 10.1039/c5ra18118f] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Non-symmetric nematic dimers are designed and investigated by OPM, DSC and XRD; assembly models for the Nx phase are developed.
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Affiliation(s)
- Z. Ahmed
- Department of Chemistry
- University of Hull
- UK
| | - C. Welch
- Department of Chemistry
- University of Hull
- UK
| | - G. H. Mehl
- Department of Chemistry
- University of Hull
- UK
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19
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Francescangeli O, Vita F, Samulski ET. The cybotactic nematic phase of bent-core mesogens: state of the art and future developments. SOFT MATTER 2014; 10:7685-7691. [PMID: 25133629 DOI: 10.1039/c4sm01256a] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase. The mounting experimental evidence points to a converging picture of the microscopic nature of this relatively new class of liquid crystals.
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Affiliation(s)
- Oriano Francescangeli
- Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica delle Marche, Via Brecce Bianche, 60131, Ancona, Italy.
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20
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Figueirinhas JL, Dong RY. NMR of bent-core nematogens: a mini-review. MAGNETIC RESONANCE IN CHEMISTRY : MRC 2014; 52:614-624. [PMID: 24975632 DOI: 10.1002/mrc.4097] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2014] [Accepted: 05/26/2014] [Indexed: 06/03/2023]
Abstract
This review focuses on two of our studied bent-core liquid crystals 2-methyl-3-[4-(4-octylbenzoyloxy)-benzylidene]-amino-benzoic acid 4-(4-dodecylphenylazo)-phenyl ester A131, and a shape-persistent fluorenone mesogen, F1a by means of temperature-dependent (13)C NMR spectra and deuterium NMR spectra, respectively. Orientational order parameters, molecular packing, and/or conformations in the nematic phases or nematic glass can be extracted from the observed NMR spectra. These will be discussed in terms of certain motional models. In particular, F1a has been found to form a biaxial nematic glass. The derived orientational order parameters would put the biaxial system of phase symmetry lower than orthorhombic.
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Affiliation(s)
- João L Figueirinhas
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal; CFMC, Universidade de Lisboa, Lisboa, Portugal
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21
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1,2,4-Oxadiazole-Based Bent-Core Liquid Crystals with Cybotactic Nematic Phases. Chemphyschem 2014; 15:1323-35. [DOI: 10.1002/cphc.201301070] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2013] [Indexed: 11/07/2022]
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22
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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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Kim YK, Senyuk B, Shin ST, Kohlmeier A, Mehl GH, Lavrentovich OD. Surface alignment, anchoring transitions, optical properties, and topological defects in the thermotropic nematic phase of organo-siloxane tetrapodes. SOFT MATTER 2014; 10:500-509. [PMID: 24651889 DOI: 10.1039/c3sm52249k] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
We perform optical, surface anchoring, and textural studies of an organo-siloxane "tetrapode" material in the broad temperature range of the nematic phase. The optical, structural, and topological features are compatible with the uniaxial nematic order rather than with the biaxial nematic order, in the entire nematic temperature range -25 °C < T < 46 °C studied. For homeotropic alignment, the material experiences surface anchoring transition, but the director can be realigned into an optically uniaxial texture by applying a sufficiently strong electric field. The topological features of textures in cylindrical capillaries, in spherical droplets and around colloidal inclusions are consistent with the uniaxial character of the long-range nematic order. In particular, we observe isolated surface point defects - boojums and bulk point defects - hedgehogs that can exist only in the uniaxial nematic liquid crystal.
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Affiliation(s)
- Young-Ki Kim
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent, OH 44242, USA.
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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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25
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Golo VL, Kats EI, Sevenyuk AA, Sinitsyn DO. Twisted quasiperiodic textures of biaxial nematic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:042504. [PMID: 24229198 DOI: 10.1103/physreve.88.042504] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/22/2013] [Revised: 09/18/2013] [Indexed: 06/02/2023]
Abstract
Textures (i.e., smooth space nonuniform distributions of the order parameter) in biaxial nematics turned out to be much more complex and interesting than expected. Scanning the literature we find only a very few publications on this topic. Thus, the immediate motivation of the present paper is to develop a systematic procedure to study, classify, and visualize possible textures in biaxial nematics. Based on the elastic energy of a biaxial nematic (written in the most simple form that involves the least number of phenomenological parameters) we derive and solve numerically the Lagrange equations of the first kind. It allows one to visualize the solutions and offers a deep insight into their geometrical and topological features. Performing Fourier analysis we find some particular textures possessing two or more characteristic space periods (we term such solutions quasiperiodic ones because the periods are not necessarily commensurate). The problem is not only of intellectual interest but also of relevance to optical characteristics of the liquid-crystalline textures.
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Affiliation(s)
- V L Golo
- Department of Mechanics and Mathematics, the Lomonosov Moscow State University, Lenin Hills, GSP-1 119991, Moscow, Russia and National Research University Higher School of Economics, Myasnitskaya 20, 101978 Moscow, Russia
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26
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Bisi F, De Matteis G, Romano S. Calamitic and antinematic orientational order produced by the generalized Straley lattice model. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:032502. [PMID: 24125280 DOI: 10.1103/physreve.88.032502] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2013] [Indexed: 06/02/2023]
Abstract
We consider here a classical model, consisting of D_{2h}-symmetric particles in a three-dimensional simple-cubic lattice; the pair potential is isotropic in orientation space, and restricted to nearest neighbors. The simplest potential model is written in terms of the squares of the scalar products between unit vectors describing the three interacting arms of the molecules, as proposed in previous literature. Two predominant antinematic couplings of equal strength (+1) are perturbed by a comparatively weaker calamitic one, parameterized by a coupling constant -z ranging in [-1,0]. This choice rules out thermodynamically stable phases endowed with macroscopic biaxiality. The antinematic terms favor states with the corresponding molecular axes mutually orthogonal. Although the low-temperature phase of the special case with null calamitic term (PP0) is uniaxial and antinematically ordered, in the general case presented here both Monte Carlo and molecular-field approaches show that, for z close to zero, the models exhibit a low-temperature uniaxial nematic phase, followed by an antinematic one, and finally by the orientationally disordered one. On the other hand, for sufficiently large values of z, we only find evidence of uniaxial calamitic behavior, as expected by following the limiting cases.
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Affiliation(s)
- Fulvio Bisi
- Dipartimento di Matematica "F. Casorati", Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy
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27
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Polineni S, Figueirinhas JL, Cruz C, Wilson DA, Mehl GH. Capacitance and optical studies of elastic and dielectric properties in an organosiloxane tetrapode exhibiting a N(B) phase. J Chem Phys 2013; 138:124904. [PMID: 23556748 DOI: 10.1063/1.4795582] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
Abstract
Biaxial (N(B)) and uniaxial nematic (N(U)) phase behavior was detected and confirmed for an organosiloxane tetrapode material using capacitance and birefringence measurements. Elastic constants, permittivities at two distinct low frequencies, and birefringencies were determined as a function of temperature over both the N(U) and the N(B) phase ranges. The N(U)-N(B) transition is clearly observed in the birefringencies and conoscopy data. A temperature dependent cross-over frequency is also detected in this material for the permittivities, allowing the electrical switching of both planar and homeotropic aligned samples.
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Affiliation(s)
- S Polineni
- IST-Technical University of Lisbon, Dept. de Física, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal
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28
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Querciagrossa L, Ricci M, Berardi R, Zannoni C. Mesogen polarity effects on biaxial nematics. Centrally located dipoles. Phys Chem Chem Phys 2013; 15:19065-72. [DOI: 10.1039/c3cp52970c] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
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29
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Kaur S, Addis J, Greco C, Ferrarini A, Görtz V, Goodby JW, Gleeson HF. Understanding the distinctive elastic constants in an oxadiazole bent-core nematic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 86:041703. [PMID: 23214599 DOI: 10.1103/physreve.86.041703] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/12/2012] [Indexed: 06/01/2023]
Abstract
The splay and bend elastic constants of the bent-core oxadiazole material [C5-Ph-ODBP-Ph-OC12] have been investigated as a function of temperature across the nematic phase. The bend constant K(33) is found to take values of ~3.0 pN and to be almost temperature independent, whereas, the splay constant K(11) increases monotonically from ~3.5 pN close to the isotropic phase transition to values of ~9 pN deep in the nematic phase. No pretransitional divergence is observed in either K(11) or K(33) at temperatures approaching the underlying phase. This behavior of the elastic constants is distinct from that observed in rodlike liquid crystal systems but appears to share characteristics with the few other bent-core nematic systems studied to date. We discuss the interdependence of the elastic constants, the birefringence, and the order parameter to allow a comparison of the observed behavior with theory. We show that calculations of the elastic constants via molecular-field theory and atomistic modeling are in excellent qualitative as well as good quantitative (within 2 pN) agreement with the measurements across the temperature range, offering a deeper understanding of the elasticity in bent-core nematic materials than has been, hitherto, available.
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Affiliation(s)
- S Kaur
- School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom
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Shanker G, Prehm M, Tschierske C. Laterally connected bent-core dimers and bent-core-rod couples with nematic liquid crystalline phases. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c1jm13649f] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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32
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Speetjens F, Lindborg J, Tauscher T, LaFemina N, Nguyen J, Samulski ET, Vita F, Francescangeli O, Scharrer E. Low nematic onset temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bent-core mesogens possessing lateral methyl groups. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c2jm33705c] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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33
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Francescangeli O, Vita F, Fauth F, Samulski ET. Extraordinary magnetic field effect in bent-core liquid crystals. PHYSICAL REVIEW LETTERS 2011; 107:207801. [PMID: 22181777 DOI: 10.1103/physrevlett.107.207801] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/28/2011] [Indexed: 05/31/2023]
Abstract
A bent-core mesogen that forms a cybotactic nematic phase exhibits a giant magnetic field-induced shift of its nematic-isotropic and smectic-C-nematic transition temperatures: ΔT(H) = 4 K for H = 10 kOe. In contrast with molecular nematics, in cybotactic nematics the field couples with the anisotropic susceptibility of clusters containing several hundred partially ordered molecules. X-ray diffraction data corroborate a quantitative estimate of inferred cluster size (∼300 molecules). The results represent an unequivocal demonstration of the cluster picture of the nematic phase of this class of nonlinear liquid crystals.
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Affiliation(s)
- Oriano Francescangeli
- Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica delle Marche, Ancona, Italy.
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Romano S, De Matteis G. Orientationally ordered phase produced by fully antinematic interactions: a simulation study. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:011703. [PMID: 21867192 DOI: 10.1103/physreve.84.011703] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/10/2010] [Revised: 04/22/2011] [Indexed: 05/31/2023]
Abstract
We consider here a classical model, consisting of D(2h) symmetric particles, whose centers of mass are associated with a three-dimensional simple-cubic lattice; the pair potential is isotropic in orientation space, and restricted to nearest neighbors. Two orthonormal triads define orientations of a pair of interacting particles; the simplest potential models proposed in the literature can be written as a linear combination involving the squares of the scalar products between corresponding unit vectors only, thus depending on three parameters, and making the interaction model rather versatile. A coupling constant with negative sign tends to keep the two interacting unit vectors parallel to each other, whereas a positive sign tends to keep them mutually orthogonal (antinematic coupling). We address here a special, extreme case of the above family, involving only antinematic couplings: more precisely, three antinematic terms whose coefficients are set to a common positive value (hence the name PPP model). The model under investigation produces a doubly degenerate pair ground state; the nearest-neighbor range of the interaction and the bipartite character of the lattice can propagate the pair ground state and increase the overall degeneracy, but without producing frustration. The model was investigated by a simplified molecular field treatment as well as by Monte Carlo simulation, whose results suggested a second-order transition to a low-temperature biaxially ordered phase; ground-state configurations producing orientational order have been selected by thermal fluctuations. The molecular field treatment also predicted a continuous transition, and was found to overestimate the transition temperature by a factor 2.
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Affiliation(s)
- Silvano Romano
- Dipartimento di Fisica A Volta, Università di Pavia, Via A Bassi 6, I-27100 Pavia, Italy.
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35
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Peroukidis SD, Vanakaras AG, Photinos DJ. Molecular simulation of hierarchical structures in bent-core nematic liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:010702. [PMID: 21867106 DOI: 10.1103/physreve.84.010702] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/08/2011] [Indexed: 05/31/2023]
Abstract
The structure of nematic liquid crystals formed by bent-core mesogens (BCMs) is studied in the context of Monte Carlo simulations of a simple molecular model that captures the symmetry, shape, and flexibility of achiral BCMs. The results indicate the formation of (i) clusters exhibiting local smectic order, orthogonal or tilted, with strong in-layer polar correlations and antiferroelectric juxtaposition of successive layers and (ii) large homochiral domains through the helical arrangement of the tilted smectic clusters, while the orthogonal clusters produce achiral (untwisted) nematic states.
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Nagaraj M, Lehmann A, Prehm M, Tschierske C, Vij JK. Evidence of a polar cybotactic smectic A phase in a new fluorine substituted bent-core compound. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c1jm13140k] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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37
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Shanker G, Prehm M, Nagaraj M, Vij JK, Tschierske C. Development of polar order in a bent-core liquid crystal with a new sequence of two orthogonal smectic and an adjacent nematic phase. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c1jm14653j] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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38
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Senyuk B, Wonderly H, Mathews M, Li Q, Shiyanovskii SV, Lavrentovich OD. Surface alignment, anchoring transitions, optical properties, and topological defects in the nematic phase of thermotropic bent-core liquid crystal A131. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:041711. [PMID: 21230299 DOI: 10.1103/physreve.82.041711] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/03/2010] [Revised: 09/29/2010] [Indexed: 05/30/2023]
Abstract
We study optical, structural, and surface anchoring properties of thermotropic nematic bent-core material A131. The focus is on the features associated with orientational order as the material has been reported to exhibit not only the usual uniaxial nematic but also the biaxial nematic phase. We demonstrate that A131 experiences a surface anchoring transition from a perpendicular to tilted alignment when the temperature decreases. The features of the tilted state are consistent with surface-induced birefringence associated with smectic layering near the surface and a molecular tilt that changes along the normal to the substrates. The surface-induced birefringence is reduced to zero by a modest electric field that establishes a uniform uniaxial nematic state. Both refractive and absorptive optical properties of A131 are consistent with the uniaxial order. We found no evidence of the "polycrystalline" biaxial behavior in the cells placed in crossed electric and magnetic fields. We observe stable topological point defects (boojums and hedgehogs) and nonsingular "escaped" disclinations pertinent only to the uniaxial order. Finally, freely suspended films of A131 show uniaxial nematic and smectic textures; a decrease in the film thickness expands the temperature range of stability of smectic textures, supporting the idea of surface-induced smectic layering. Our conclusion is that A131 features only a uniaxial nematic phase and that the apparent biaxiality is caused by subtle surface effects rather than by the bulk biaxial phase.
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Affiliation(s)
- B Senyuk
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
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40
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Lehmann M, Köhn C, Figueirinhas J, Feio G, Cruz C, Dong R. Biaxial Nematic Mesophases from Shape-Persistent Mesogens with a Fluorenone Bending Unit. Chemistry 2010; 16:8275-9. [DOI: 10.1002/chem.201001214] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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41
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Sathyanarayana P, Mathew M, Li Q, Sastry VSS, Kundu B, Le KV, Takezoe H, Dhara S. Splay bend elasticity of a bent-core nematic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:010702. [PMID: 20365313 DOI: 10.1103/physreve.81.010702] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/06/2009] [Indexed: 05/29/2023]
Abstract
We measured the splay (K11) and bend (K33) elastic constants in the nematic phase of a bent-core liquid crystal. In the vicinity of the nematic-isotropic transition temperature K33 is proportional to the square of the order parameter. In the nematic range K11 increases monotonically with decreasing temperature, whereas K33 is practically independent of temperature and is smaller than K11 . K33 exhibits a pretransitional slow divergence toward the transition temperature to the smectic phase and becomes slightly larger than K11. The small K33 is explained on the basis of strong coupling of the bent shape of the molecules with the bend distortion.
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Filip D, Cruz C, Sebastião PJ, Cardoso M, Ribeiro AC, Vilfan M, Meyer T, Kouwer PHJ, Mehl GH. Phase structure and molecular dynamics of liquid-crystalline side-on organosiloxane tetrapodes. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:011702. [PMID: 20365386 DOI: 10.1103/physreve.81.011702] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/07/2009] [Indexed: 05/29/2023]
Abstract
X-ray diffraction and proton NMR relaxation measurements were carried out on two liquid-crystalline organosiloxane tetrapodes with side-on mesogenic groups, exhibiting nematic and smectic- C phases, and on a monomeric analog. Packing models for the mesophases exhibited by these systems are proposed on the basis of x-ray diffraction data. As a consequence of microsegregation, the aromatic cores are packed in between two sublayers formed by a mixture of interdigitated aliphatic and siloxane chains. The mixed sublayers are characteristic for the tetrapodes with side-on mesogenic groups presented in this work and have not been observed in tetrapodes with terminally attached mesogens. The tilt angle in the smectic- C phase is found very large, i.e., approximately 61 degrees -62 degrees . Notably, smectic- C clusters are present also in the whole temperature range of the nematic phase. NMR relaxometry yields T(1)-1 dispersions clearly different from those of conventional calamitics. The influence of molecular tendency to form interdigitated structures is evidenced by frequency-dependent relaxation rate in the isotropic phase-indicating the presence of ordered clusters far above the phase transition-and by the diminished role of molecular self-diffusion in ordered phases. Nematiclike director fluctuations are the dominating relaxation mechanism whereas the translational displacements are strongly hindered by the interdigitation of dendrimer arms.
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Affiliation(s)
- D Filip
- Centro de Física da Matéria Condensada, Universidade de Lisboa, Lisboa, Portugal
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Lehmann M, Seltmann J. Low temperature enantiotropic nematic phases from V-shaped, shape-persistent molecules. Beilstein J Org Chem 2009; 5:73. [PMID: 20300458 PMCID: PMC2839554 DOI: 10.3762/bjoc.5.73] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2009] [Accepted: 11/17/2009] [Indexed: 11/30/2022] Open
Abstract
A series of V-shaped, shape-persistent thiadiazole nematogens, based on an oligo(phenylene ethynylene) scaffold with ester groups connected via alkyloxy spacers, was efficiently prepared by a two-step procedure. Phase engineering results in an optimum of the mesophase range and low melting temperature when the nematogens are desymmetrised with a butoxy and a heptyloxy spacer. The mesophases are enantiotropic and over the whole temperature range nematic. For the optimised mesogen structure, optical investigations by conoscopy monitored a uniaxial nematic phase upon cooling from the isotropic phase to room temperature (ΔT = 150 °C). X-ray studies on magnetic-field-aligned samples of this mesogen family revealed a general pattern, indicating the alignment of two molecular axes along individual directors in the magnetic field. These observations may be rationalised with larger assemblies of V-shaped molecules isotropically distributed around the direction of the magnetic field.
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Affiliation(s)
- Matthias Lehmann
- Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany.
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Karahaliou PK, Vanakaras AG, Photinos DJ. Symmetries and alignment of biaxial nematic liquid crystals. J Chem Phys 2009; 131:124516. [DOI: 10.1063/1.3226560] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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De Matteis G, Romano S. Mesogenic lattice models with partly antinematic interactions producing uniaxial nematic phases. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 80:031702. [PMID: 19905126 DOI: 10.1103/physreve.80.031702] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/18/2009] [Indexed: 05/28/2023]
Abstract
The present paper considers nematogenic lattice models, involving particles of D_{2h} symmetry, whose centers of mass are associated with a three-dimensional simple cubic lattice; the pair potential is isotropic in orientation space and restricted to nearest neighbors. Let two orthonormal triads define orientations of a pair of interacting particles; the simplest potential models proposed in the literature can be reduced to a linear combination involving the squares of the scalar products between corresponding unit vectors only and depending on three parameters. By now, various sets of potential parameters have been proposed and studied in the literature, some of which capable of producing biaxial orientational order at sufficiently low temperature. On the other hand, in experimental terms, mesogenic biaxial molecules mostly produce uniaxial mesophases; thus we address here two very simple cases, involving a nematic (calamitic) term as well as one (model P0M) or two (model PPM) antinematic ones, whose coefficients are set equal in magnitude; when only one antinematic coefficient is used, the third one is set to zero. The calamitic term favors the alignment of two corresponding molecular axes, whereas antinematic terms or geometric constraints tend to keep two other pairs of axes mutually orthogonal. The models were investigated by molecular-field treatments and Monte Carlo simulation and found to predict a first- or second-order transitions between uniaxial nematic and isotropic phases; the molecular-field treatments yielded results in reasonable agreement with simulation.
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Affiliation(s)
- Giovanni De Matteis
- Centro di Ricerca Matematica Ennio De Giorgi, Collegio Puteano, Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 3, I-56100 Pisa, Italy.
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Vaupotic N, Szydlowska J, Salamonczyk M, Kovarova A, Svoboda J, Osipov M, Pociecha D, Gorecka E. Structure studies of the nematic phase formed by bent-core molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 80:030701. [PMID: 19905050 DOI: 10.1103/physreve.80.030701] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/10/2009] [Indexed: 05/28/2023]
Abstract
In recent years there are several reports showing that bent-core mesogenic molecules are able to form biaxial nematic phase in which molecular rotation around the long molecular axis is strongly hindered. The x-ray pattern with azimuthally split signals at low angle region of diffraction is usually given as evidence for the biaxial nematic phase. We show experimentally and theoretically that such x-ray pattern is due to the local smectic- C fluctuations ("cybotactic" groups) in the uniaxial nematic phase.
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Affiliation(s)
- N Vaupotic
- Department of Physics, Faculty of Natural Sciences and Mathematics, University of Maribor, Koroska 160, 2000 Maribor, Slovenia
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Berardi R, Muccioli L, Orlandi S, Ricci M, Zannoni C. Computer simulations of biaxial nematics. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2008; 20:463101. [PMID: 21693834 DOI: 10.1088/0953-8984/20/46/463101] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Biaxial nematic (N(b)) liquid crystals are a fascinating condensed matter phase that has baffled, for more than thirty years, scientists engaged in the challenge of demonstrating its actual existence, and which has only recently been experimentally found. During this period computer simulations of model N(b) have played an important role, both in providing the basic physical properties to be expected from these systems, and in giving clues about the molecular features essential for the thermodynamic stability of N(b) phases. However, simulation studies are expected to be even more crucial in the future for unravelling the structural features of biaxial mesogens at the molecular level, and for helping in the design and optimization of devices towards the technological deployment of N(b) materials. This review article gives an overview of the simulation work performed so far, and relying on the recent experimental findings, focuses on the still unanswered questions which will determine the future challenges in the field.
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Affiliation(s)
- Roberto Berardi
- Dipartimento di Chimica Fisica e Inorganica, and INSTM-CRIMSON, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
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Cordoyiannis G, Apreutesei D, Mehl GH, Glorieux C, Thoen J. High-resolution calorimetric study of a liquid crystalline organo-siloxane tetrapode with a biaxial nematic phase. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:011708. [PMID: 18763974 DOI: 10.1103/physreve.78.011708] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2008] [Indexed: 05/26/2023]
Abstract
High-resolution adiabatic scanning calorimetry and differential scanning calorimetry have been employed to study the thermal behavior of an organo-siloxane tetrapode reported to exhibit a biaxial nematic phase. No signature of the uniaxial to biaxial nematic phase transition could be retraced in sequential heating and cooling runs under different scanning rates, within the experimental resolution. The results obtained reveal that an extremely small heat should be involved in the uniaxial to biaxial nematic phase transition. The isotropic to uniaxial nematic transition at 318+/-0.01 K is very stable, and it is weakly first order with a rather small latent heat of 0.20+/-0.02 J/g .
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Affiliation(s)
- George Cordoyiannis
- Laboratorium voor Akoestiek en Thermische Fysica, Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, Leuven, Belgium
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Karahaliou PK, Kouwer PHJ, Meyer T, Mehl GH, Photinos DJ. Long- and Short-Range Order in the Mesophases of Laterally Substituted Calamitic Mesogens and their Radial Octapodes. J Phys Chem B 2008; 112:6550-6. [DOI: 10.1021/jp712182v] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Panagiota K. Karahaliou
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Paul H. J. Kouwer
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Thomas Meyer
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Georg H. Mehl
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Demetri J. Photinos
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
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