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Aya S, Tang W, Kong X, Arakawa Y, Komatsu K, Tsuji H. Nontrivial ultraslow dynamics under electric-field in nematics of bent-shaped molecules. Phys Chem Chem Phys 2022; 25:297-303. [PMID: 36477024 DOI: 10.1039/d2cp02407a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
For over decades, nematic liquid crystals have been recognized as highly fluidic materials that respond to electric field on the millisecond scale. In contrast to traditional nematics with fast responsivity, we herein report nontrivial ultraslow electric-driven dynamics in bent-shaped nematic materials. Varying the alkyl chain spacers of bent-shaped cyanobiphenyl dimers (COOm and OCOm) shows a 'transition' in the dynamics behavior between the bent-dimeric and bent-core materials. Interestingly, with short alkyl chain spacers, COO2 exhibits unexpected ultra-slow dynamic pathways, i.e., "quasi-static" electrohydrodynamic convection. A significant observation is that the on/off-electro-switching time of COO2 is 10 000 times higher than that of typical nematic materials, which is the largest value reported ever in the kilo-second range. In addition, the threshold voltage for inducing the reorientation of the nematic director for COO2 is higher than 5 V, which is uncommon in traditional N materials. These properties are distinct from those of traditional nematic materials and discussed in terms of dielectric constants and electrohydrodynamic convection.
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Affiliation(s)
- Satoshi Aya
- South China Advanced Institute for Soft Matter Science and Technology (AISMST), School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640, China. .,Guangdong Provincial Key Laboratory of Functional and Intelligent Hybrid Materials and Devices, South China University of Technology, Guangzhou 510640, China
| | - Wentao Tang
- South China Advanced Institute for Soft Matter Science and Technology (AISMST), School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640, China.
| | - Xian Kong
- South China Advanced Institute for Soft Matter Science and Technology (AISMST), School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640, China.
| | - Yuki Arakawa
- Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, 441-8580, Japan.
| | - Kenta Komatsu
- Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, 441-8580, Japan.
| | - Hideto Tsuji
- Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, Toyohashi, 441-8580, Japan.
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2
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Kaur S, Barthakur A, Mohiuddin G, Gupta SP, Dhara S, Pal SK. Observation of “de Vries-like” properties in bent-core molecules. Chem Sci 2022; 13:2249-2257. [PMID: 35310491 PMCID: PMC8864698 DOI: 10.1039/d1sc06629c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2021] [Accepted: 01/17/2022] [Indexed: 11/21/2022] Open
Abstract
“de Vries” liquid crystals, defined by a maximum layer shrinkage of ≤1% from the smectic A to C phase transition, are an integral component of ferroelectric liquid crystal (FLC) displays. Bona fide de Vries materials described in the literature are primarily perfluorinated, polysiloxane and polysilane-terminated rod-like (or calamitic) LCs. Herein, for the first time, we report a series of newly designed achiral unsymmetrical bent-core molecules with terminal alkoxy chains exhibiting similar properties to “de Vries” LCs. The new molecular structure is based on the systematic distribution of four phenyl rings attached via ester and imine linkers having 3-amino-2-methylbenzoic acid as the central core with a bent angle of 147°. Detailed microscopic investigations in differently aligned (planar as well as homeotropic) cells along with SAXS/WAXS studies revealed that the materials exhibited a SmA–SmC phase sequence along with the appearance of the nematic phase at higher temperatures. SAXS measurements divulged the layer spacings (d-spacings) and hence, the layer shrinkage was calculated ranging from 0.19% to 0.68% just below the SmA–SmC transition. The variation of the calculated molecular tilt angle (α) derived from the temperature-dependent SAXS data, followed the power law with exponent values 0.29 ± 0.01 and 0.25 ± 0.01 for compounds 1/10 and 1/12, respectively. The experimental values obtained were very close to the theoretically predicted values for the materials with de Vries-like properties. The analysis of temperature-dependent birefringence studies based on the prediction of the Landau theory, showed a dip across the SmA–SmC phase transition typical of compounds exhibiting the de Vries characteristics. The collective results obtained suggest “de Vries” SmA as a probable model for this bent-core system which may find applications in displays. A simple molecular design of unsymmetrical bent-core molecules exhibiting low layer shrinkage and a dip in the birefringence at the SmA–SmC phase transition, typical characteristics of “de Vries” liquid crystals.![]()
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Affiliation(s)
- Supreet Kaur
- Indian Institute of Science Education and Research (IISER) Mohali Sector-81, Knowledge City Manauli 140306 India
| | | | - Golam Mohiuddin
- Indian Institute of Science Education and Research (IISER) Mohali Sector-81, Knowledge City Manauli 140306 India
| | | | - Surajit Dhara
- School of Physics, University of Hyderabad Hyderabad-500046 India
| | - Santanu Kumar Pal
- Indian Institute of Science Education and Research (IISER) Mohali Sector-81, Knowledge City Manauli 140306 India
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3
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Revignas D, Ferrarini A. Interplay of Particle Morphology and Director Distortions in Nematic Fluids. PHYSICAL REVIEW LETTERS 2020; 125:267802. [PMID: 33449752 DOI: 10.1103/physrevlett.125.267802] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/10/2020] [Revised: 10/16/2020] [Accepted: 10/26/2020] [Indexed: 06/12/2023]
Abstract
The existing microscopic theories for elasticity of nematics are challenged by recent findings on systems, whether bent molecules or semiflexible polymers, which do not comply with the model of rigid rodlike particles. Here, we propose an extension of Onsager-Straley second-virial theory, based on a model for the orientational distribution function that, through explicit account of the director profile along a particle, changes in the presence of deformations. The elastic constants reveal specific effects of particle morphology, which are not captured by the existing theories. This paves the way to microscopic modeling of the elastic properties of semiflexible liquid crystal polymers, which is a longstanding issue.
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Affiliation(s)
- Davide Revignas
- Università di Padova, Dipartimento di Scienze Chimiche, via Marzolo 1, 35131 Padova, Italy
| | - Alberta Ferrarini
- Università di Padova, Dipartimento di Scienze Chimiche, via Marzolo 1, 35131 Padova, Italy
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4
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Morris R, Jones JC, Nagaraj M. Variable pitch hydrodynamic electro-optic gratings utilising bent liquid crystal dimers. SOFT MATTER 2020; 16:10439-10453. [PMID: 33057533 DOI: 10.1039/d0sm01425g] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Electrohydrodynamic instabilities (EHDI) in liquid crystals form uniform and continuously variable diffractive structures when subject to certain material and geometry determined conditions. A one-dimensional grating is one such diffractive structure, where the refractive index changes periodically in a direction parallel to the initial liquid crystal director. The period of this structure has been shown previously to vary continuously between the values of the cell gap and half-cell gap approximately, allowing continuous angular modulation of optical beams but with a limited angular range. In this work, the lower pitch limit is shown to also be governed in part by the ratio of the splay and bend elastic constants (k11/k33) of the liquid crystal. A host nematic liquid crystal with standard elastic constant ratios (k11/k33 < 1) is doped with odd-alkyl-spaced dimeric liquid crystal CB7CB, to create a liquid crystal mixture with a far higher elastic constant ratio (k11/k33 > 5) than for those previously used in literature EHDI studies. The EHDI gratings formed in this new mixture exhibit pitch lengths significantly below half-cell gap, allowing up to 50% wider angle continuous steering of light. This improves the potential for application in beamsteering and diffractive optical devices.
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Affiliation(s)
- R Morris
- School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK.
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5
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Paladugu S, Kaur S, Mohiuddin G, Pujala RK, Pal SK, Dhara S. Microrheology to probe smectic clusters in bent-core nematic liquid crystals. SOFT MATTER 2020; 16:7556-7561. [PMID: 32706008 DOI: 10.1039/d0sm00796j] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Many bent-core nematic liquid crystals exhibit unusual physical properties due to the presence of smectic clusters, known as "cybotactic" clusters, in the nematic phase. Here, we investigate the effect of these clusters on the complex shear modulus (G*(ω)) of two asymmetric bent-core liquid crystals using a microrheological technique. The compound with a shorter hydrocarbon chain (8OCH3) exhibits only a nematic (N) phase whereas the compound with a longer chain (16OCH3) exhibits both nematic (N) and smectic-A (SmA) phases. The rheological results are correlated with the measurements of curvature elastic constants. Our results show that the directional shear modulus of 16OCH3, just above the SmA to N phase transition temperature, is strikingly different than that of 8OCH3, owing to the smectic clusters. An approximate size of the clusters is estimated using a simple model. Therefore, microrheological studies on bent-core nematic liquid crystals are very useful in extracting information about underlying smectic clusters.
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Affiliation(s)
- Sathyanarayana Paladugu
- School of Physics, University of Hyderabad, Hyderabad, India. and Soft and Active Matter Group, Department of Physics, Indian Institute of Science Education and Research (IISER), Tirupati, Andhra Pradesh 517507, India
| | - Supreet Kaur
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Golam Mohiuddin
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Ravi Kumar Pujala
- Soft and Active Matter Group, Department of Physics, Indian Institute of Science Education and Research (IISER), Tirupati, Andhra Pradesh 517507, India
| | - Santanu Kumar Pal
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Surajit Dhara
- School of Physics, University of Hyderabad, Hyderabad, India.
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Chakraborty S, Das MK, Weissflog W. Measurement of visco-elastic properties in a binary system of hockey stick-shaped and calamitic mesogens. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2019; 42:138. [PMID: 31654238 DOI: 10.1140/epje/i2019-11902-0] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/18/2019] [Accepted: 10/08/2019] [Indexed: 06/10/2023]
Abstract
Temperature variation of the splay elastic modulus (K11) and rotational viscosity ([Formula: see text]) have been measured throughout the entire nematic (N phase of a binary liquid crystal system comprising of host calamitic compound, 4'-octyloxy-4-cyanobiphenyl (8OCB) and small amount of a hockey stick-shaped mesogen, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). Two different structural conformations play a significant role in the phase behavior leading to a noticeable shrinkage in the nematic width. During cooling in the nematic phase, both K11 and [Formula: see text] enhance monotonically and divulge an evidence of pretransitional fluctuation near the Smectic-A (Sm-A) transition. With increase in the dopant concentration, the slope of the temperature-dependent K11 and [Formula: see text] becomes uniformly steeper. The observed variations are explained in accordance with the formation of smectic-like clusters in the nematic phase. A representative model of the smectic-like clusters in the nematic phase is presented. However, the obtained values of [Formula: see text] have found to be between that of conventional calamitics and typical bent-core mesogens. Measurement of the temperature-dependent orientational order parameter (〈P2〉 ) and activation energy (Ea) has also been carried out for all the studied mixtures, exhibiting a small augmentation due to the addition of a hockey stick-shaped compound. This indicates an enhancement in intermolecular packing within the mixtures which further influences the molecular motions.
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Affiliation(s)
| | - Malay Kumar Das
- Department of Physics, University of North Bengal, 734 013, Siliguri, India.
| | - Wolfgang Weissflog
- Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Physikalische Chemie, Halle, Germany
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7
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Sofi JA, Barthakur A, Dhara S. Whispering gallery mode lasing in mesomorphic liquid crystal microdroplets. SOFT MATTER 2019; 15:7832-7837. [PMID: 31528973 DOI: 10.1039/c9sm01132c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
In recent years, investigation on the non-display applications of liquid crystals has increased considerably. One of the emerging applications is whispering gallery mode (WGM) lasing. Here, we report experimental studies on the morphology and WGM lasing in nematic (N), smectic-A (SmA) and smectic-C (SmC) microdroplets dispersed in a highly transparent and low refractive index perfluopolymer. The mesomorphic microdroplets, obtained by varying the temperature, exhibit radial director configuration. The SmA microdroplets are found to be highly stable and robust against mechanical stress compared to the N and SmC microdroplets. We study lasing properties such as intensity, threshold pump energy and linewidth, and show that overall the SmA microdroplets are superior to the N and SmC microdroplets. The experimental results are discussed based on the orientation of the dye molecules, director fluctuations and tilting at the interface.
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Affiliation(s)
- Junaid Ahmad Sofi
- School of Physics, University of Hyderabad, Hyderabad-500046, India.
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8
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Eremin A, Nádasi H, Kurochkina M, Haba O, Yonetake K, Takezoe H. Light-Responsive Microstructures in Droplets of the Twist-Bend Nematic Phase. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:14519-14527. [PMID: 30253102 DOI: 10.1021/acs.langmuir.8b02823] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
We report on the structure and optical manipulation of the director configurations in emulsions of liquid-crystalline droplets of a compound exhibiting the nematic (N) and the twist-bend nematic (NTB) phases. We demonstrate a decrease in the ratio of the bent elastic constant K33 to the splay constant K11 by nearly 2 orders of magnitude with decreasing temperature in the N phase. The director structures in liquid-crystal droplets doped with a photoswitchable surfactant without and under ultraviolet (UV) light are discussed in light of the strong elastic anisotropy of the investigated compound. We also compare our findings with the results obtained in doped nematic droplets of a conventional 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal. The dynamics of droplets in the NTB phase by UV light irradiation are also studied using polarizing and confocal microscopies.
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Affiliation(s)
- Alexey Eremin
- Otto von Guericke University, Institute of Physics , 39016 Magdeburg , Germany
| | - Hajnalka Nádasi
- Otto von Guericke University, Institute of Physics , 39016 Magdeburg , Germany
| | | | - Osamu Haba
- Department of Organic Materials Science , Yamagata University , 4-3-16 Jonan , Yonezawa , Yamagata 992-8510 , Japan
| | - Koichiro Yonetake
- Department of Organic Materials Science , Yamagata University , 4-3-16 Jonan , Yonezawa , Yamagata 992-8510 , Japan
| | - Hideo Takezoe
- Toyota Physical and Chemical Research Institute , 41-1 Yokomichi , Nagakute , Aichi 480-1192 , Japan
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9
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Chakraborty A, Chakraborty S, Das MK, Weissflog W. Visco-elastic properties of multi-component mixtures of hockey stick-shaped liquid crystal compounds. J Mol Liq 2018. [DOI: 10.1016/j.molliq.2018.06.020] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/14/2022]
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10
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Khan RK, Turlapati S, Begum N, Mohiuddin G, Rao NS, Ghosh S. Impact of terminal polar substitution on elastic, electro-optic and dielectric properties of four-ring bent-core nematic liquid crystals. RSC Adv 2018; 8:11509-11516. [PMID: 35542808 PMCID: PMC9079133 DOI: 10.1039/c8ra00575c] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/19/2018] [Accepted: 03/18/2018] [Indexed: 11/21/2022] Open
Abstract
Here we report the influence of terminal –F, –Cl and –NO2 substitution on the elastic, dielectric and polar switching behavior of four-ring bent-core liquid crystals (LCs).
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Affiliation(s)
- R. K. Khan
- Department of Physics
- University of Calcutta
- Kolkata 700 009
- India
| | - S. Turlapati
- Chemistry Department
- Assam University
- Silchar 788011
- India
| | - N. Begum
- Chemistry Department
- Assam University
- Silchar 788011
- India
| | - G. Mohiuddin
- Chemistry Department
- Assam University
- Silchar 788011
- India
| | - N. V. S. Rao
- Chemistry Department
- Assam University
- Silchar 788011
- India
| | - S. Ghosh
- Department of Physics
- University of Calcutta
- Kolkata 700 009
- India
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11
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Babakhanova G, Parsouzi Z, Paladugu S, Wang H, Nastishin YA, Shiyanovskii SV, Sprunt S, Lavrentovich OD. Elastic and viscous properties of the nematic dimer CB7CB. Phys Rev E 2017; 96:062704. [PMID: 29347367 DOI: 10.1103/physreve.96.062704] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2017] [Indexed: 05/16/2023]
Abstract
We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4'-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, N_{TB}, and the oblique helicoidal cholesteric, Ch_{OH}, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the N-N_{TB} and N-to-isotropic (I) transitions. The splay constant K_{11} is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K_{33} and twist K_{22} constants are measured by DLS. K_{33}, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and N-N_{TB} pretransitional regions. The measured ratio K_{11}/K_{22} is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the N-N_{TB} transition.
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Affiliation(s)
- Greta Babakhanova
- Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
- Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
| | - Zeinab Parsouzi
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | | | - Hao Wang
- Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
- Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
| | - Yu A Nastishin
- Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
- Hetman Petro Sahaidachnyi National Army Academy, 32, Heroes of Maidan Street, Lviv 79012, Ukraine
| | | | - Samuel Sprunt
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | - Oleg D Lavrentovich
- Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
- Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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12
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Khan RK, Turlapati S, Rao NVS, Ghosh S. Elastic and dielectric properties of ferroelectric nanoparticles/bent-core nematic liquid crystal blend. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2017; 40:75. [PMID: 28861775 DOI: 10.1140/epje/i2017-11564-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/10/2017] [Accepted: 08/18/2017] [Indexed: 06/07/2023]
Abstract
Bent-core liquid crystals present the first evidence of forming polar superstructures from achiral molecules. The nematic phase is the newest member of the bent-core family and turns out to be extremely interesting owing to its distinct features compared to its calamitic counterpart. Here the investigation of one achiral unsymmetrical 2-methyl-3-amino-benzoic acid (2,6-substituted toluene)-derived four-ring bent-core nematic (BCN) liquid crystals (11-2M-F) is presented after nanodispersion. Ferroelectric nanoparticles significantly affect the phase transition temperature, threshold voltage, dielectric permittivity, elastic constants and splay viscosity of the pristine BCN. In most bent-core nematic liquid crystals the bent elastic constant (K33) is usually lower than the splay elastic constant (K11) owing to the presence of short-range smectic-C-like correlations in the nematic phase. Thus the elastic anisotropy ([Formula: see text]) is usually negative in bent-core nematics unlike in rod-like nematic liquid crystals where K33 is always greater than K11. Here we report a short-core bent-shaped nematic liquid crystal whose negative elastic anisotropy was turned to positive by minute addition of ferroelectric nanoparticles.
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Affiliation(s)
- Raj Kumar Khan
- Department of Physics, University of Calcutta, 92 Acharya Prafulla Chandra Road, 700 009, Kolkata, India
| | | | | | - Sharmistha Ghosh
- Department of Physics, University of Calcutta, 92 Acharya Prafulla Chandra Road, 700 009, Kolkata, India.
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Gupta M, Gupta SP, Mohapatra SS, Dhara S, Pal SK. Room-Temperature Oligomeric Discotic Nematic Liquid Crystals over a Wide Temperature Range: Structure-Property Relationships. Chemistry 2017; 23:10626-10631. [PMID: 28509394 DOI: 10.1002/chem.201701578] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2017] [Indexed: 11/10/2022]
Abstract
The design and synthesis of three room-temperature discotic nematic (ND ) liquid crystals (LCs) is presented. The LC consists of an azobenzene core attached to which are four pentaalkynylbenzene (PA) units through flexible alkyl spacers. The presence of a short azo linking group provides more disorder in the system, thereby reducing the packing efficiency among the PA units and resulting into the formation of a room-temperature ND phase over a wide temperature range. Dielectric constant and birefringence studies were performed to gain further insights into the physical properties of the mesophase.
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Affiliation(s)
- Monika Gupta
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli, 140306, India
| | - Santosh Prasad Gupta
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli, 140306, India
| | | | - Surajit Dhara
- School of Physics, University of Hyderabad, Hyderabad, 500046, India
| | - Santanu Kumar Pal
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli, 140306, India
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Huang CC, Wu ZY, Sie BH, Chou WH, Huang YC, Yu MC, Chen BH, Hsu IJ, Wu LC, Lee JJ. Effect of the Functional Groups of Racemic Rodlike Schiff Base Mesogens on the Stabilization of Blue Phase in Binary Mixture Systems. J Phys Chem B 2016; 120:12736-12754. [PMID: 27973846 DOI: 10.1021/acs.jpcb.6b09823] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Four series of rodlike racemic Schiff base mesogens possessing different alkyl chains and two types of linkages, ester and alkynyl linkages, were synthesized and applied to induce cubic blue phases (BPs) in simple binary mixture systems. The mesophases of these Schiff base mesogens were confirmed by variable-temperature X-ray diffraction and the characteristic texture from polarized optical microscopy (POM). In general, when chiral additive S-(+)-2-octyl 4-(4-hexyloxybenzoyloxy)benzoate (S811; 20-40 wt %) is added into the rodlike racemic salicylaldimine-based mesogens, the cubic BPs could be observed and its temperature range is larger than 20 K. The widest temperature range of the cubic BP (35 K) can be observed in the blending mixture composed of rodlike racemic salicylaldimine-based mesogen OH-TIn possessing alkynyl linkage and 35-40 wt % S811. However, Schiff base mesogens possessing alkynyl linkage show a direct isotropic to chiral nematic transition when equal amount of chiral dopant is added. Notably, the termination temperature of BPs is very close to room temperature (ca. 35 °C) after 40.0 wt % S811 is added into the salicylaldimine-based mesogens possessing terminal alkyl chains and ester linkage. Interestingly, wide BPs (>30 K) can also be induced by adding chiral additive 1,4:3,6-dianhydro-2,5-bis[4-(n-hexyl-1-oxy)benzoic acid]sorbitol (ISO(6OBA)2) with a high helical twisting power into the racemic Schiff base mesogen possessing ester linkage. Cubic BPI and BPII can be confirmed by reflectance spectra and POM. The results of reflectance spectra indicate that the binary mixture composed of salicylaldimine-based mesogens and S811 easily exhibits a supercooling effect and induces BPI. However, only BPII can be observed in all binary mixtures containing Schiff base mesogens. On the basis of our experimental results and molecular modeling, we suppose that the values of biaxiality, polarizability, and the dipole moment of molecular geometry are the main factors that affect BP stabilization.
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Affiliation(s)
- Chiung-Cheng Huang
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - Zong-Ye Wu
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - Bing-Han Sie
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - We-Hao Chou
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - Yu-Chang Huang
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - Mei-Ching Yu
- Department of Chemical Engineering, Tatung University , Taipei 104, Taiwan
| | - Bo-Hao Chen
- Department of Molecular Science and Engineering, National Taipei University of Technology , Taipei 106, Taiwan
| | - I-Jui Hsu
- Department of Molecular Science and Engineering, National Taipei University of Technology , Taipei 106, Taiwan
| | - Lai-Chin Wu
- National Synchrotron Radiation Research Center , Hsinchu 300, Taiwan
| | - Jey-Jau Lee
- National Synchrotron Radiation Research Center , Hsinchu 300, Taiwan
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15
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Effect of the hockey-shaped mesogen as an additive on the critical behaviour at the smectic A to nematic phase transition. J Mol Liq 2016. [DOI: 10.1016/j.molliq.2016.04.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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16
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De Gregorio P, Frezza E, Greco C, Ferrarini A. Density functional theory of nematic elasticity: softening from the polar order. SOFT MATTER 2016; 12:5188-5198. [PMID: 27198729 DOI: 10.1039/c6sm00624h] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Recent experiments have evidenced some unconventional features in the elasticity of nematics, which cannot be explained by standard microscopic theories. Here, in the framework of a second-virial density functional theory, we have developed a general approach, relaxing the usual assumption that the angular distribution of particles with respect to their local director is unaffected by the deformation. We show that, for particles with polar symmetry, a new contribution to the splay and bend deformation free energy arises, associated with the onset of polar order. Calculations for conical and bent-shaped particles reveal dramatic softening of the splay and the bend mode, respectively, which eventually may lead to spontaneous deformation.
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Affiliation(s)
- Paolo De Gregorio
- Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, I-35131 Padova, Italy.
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17
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Parthasarathi S, Rao DSS, Palakurthy NB, Yelamaggad CV, Krishna Prasad S. Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants. J Phys Chem B 2016; 120:5056-62. [PMID: 27181926 DOI: 10.1021/acs.jpcb.6b03048] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N-NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.
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Affiliation(s)
| | - D S Shankar Rao
- Centre for Nano and Soft Matter Sciences , Bengaluru 560013, India
| | | | - C V Yelamaggad
- Centre for Nano and Soft Matter Sciences , Bengaluru 560013, India
| | - S Krishna Prasad
- Centre for Nano and Soft Matter Sciences , Bengaluru 560013, India
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18
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Le KV, Hafuri M, Ocak H, Bilgin-Eran B, Tschierske C, Sasaki T, Araoka F. Unusual Electro-Optic Kerr Response in a Self-Stabilized Amorphous Blue Phase with Nanoscale Smectic Clusters. Chemphyschem 2016; 17:1425-9. [PMID: 26910727 DOI: 10.1002/cphc.201501206] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2016] [Revised: 02/15/2016] [Indexed: 11/08/2022]
Abstract
We investigated the electro-optic response in the "foggy" amorphous blue phase (BPIII) as well as in the isotropic phase. To the best of our knowledge, such a study has not yet been performed due to the very limited thermal range of BPIII. In this study, we used a single-component chiral bent-core liquid crystal with a self-stabilized BPIII, which is stable over a wide temperature range. The results show that the response time is on the order of hundreds of microseconds in the isotropic phase and increases to 1-2 ms in the BPIII (at TI-BP -T <1), then drastically increases up to a few tens of milliseconds upon further cooling in BPIII. Such an unusual behavior was explained on the basis of the high rotational viscosity and/or the existence of nanoscale smectic (Sm) clusters. The Kerr constant was also measured and found to be ∼500 pm V(-2) , which is the largest among bent-core BP systems reported so far and comparable with that of polymer-stabilized BPs.
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Affiliation(s)
- Khoa V Le
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan. .,Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan.
| | - Miho Hafuri
- Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan
| | - Hale Ocak
- Department of Chemistry, Yildiz Technical University, Davutpasa Yerlesim Birimi, 34210, Esenler, Istanbul, Turkey.,Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle/Saale, Germany
| | - Belkız Bilgin-Eran
- Department of Chemistry, Yildiz Technical University, Davutpasa Yerlesim Birimi, 34210, Esenler, Istanbul, Turkey
| | - Carsten Tschierske
- Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt Mothes Str. 2, 06120, Halle/Saale, Germany
| | - Takeo Sasaki
- Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan
| | - Fumito Araoka
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
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19
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Kim H, Zep A, Ryu SH, Ahn H, Shin TJ, Lee SB, Pociecha D, Gorecka E, Yoon DK. Linkage-length dependent structuring behaviour of bent-core molecules in helical nanostructures. SOFT MATTER 2016; 12:3326-3330. [PMID: 26876379 DOI: 10.1039/c5sm03100a] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction were used to elucidate the linkage length-dependent formation of HNFs.
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Affiliation(s)
- Hanim Kim
- Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Republic of Korea.
| | - Anna Zep
- Department of Chemistry, University of Warsaw, Warsaw, 02-089, Poland.
| | - Seong Ho Ryu
- Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Republic of Korea.
| | - Hyungju Ahn
- Pohang Accelerator Laboratory, POSTECH, Pohang, 790-784, Republic of Korea
| | - Tae Joo Shin
- UNIST Central Research Facilities, UNIST, Ulsan 689-798, Republic of Korea
| | - Sang Bok Lee
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - Damian Pociecha
- Department of Chemistry, University of Warsaw, Warsaw, 02-089, Poland.
| | - Ewa Gorecka
- Department of Chemistry, University of Warsaw, Warsaw, 02-089, Poland.
| | - Dong Ki Yoon
- Graduate School of Nanoscience and Technology, KAIST, Daejeon 305-701, Republic of Korea.
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20
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Madhuri PL, Hiremath US, Yelamaggad CV, Madhuri KP, Prasad SK. Influence of virtual surfaces on Frank elastic constants in a polymer-stabilized bent-core nematic liquid crystal. Phys Rev E 2016; 93:042706. [PMID: 27176365 DOI: 10.1103/physreve.93.042706] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2015] [Indexed: 06/05/2023]
Abstract
Effect of a polymer network on the threshold voltage of the Fréedericksz transition, Frank elastic constants, switching speed, and the rotational viscosity are investigated in a polymer-stabilized bent-core nematic liquid crystal with different polymer concentrations. These polymer networks form virtual surfaces with a finite anchoring energy. The studies bring out several differences in comparison to similar studies with a calamitic liquid crystal as the nematic host. For example, on varying the polymer content the threshold voltage decreases initially, but exhibits a drastic increase above a critical concentration. A similar feature-reaching a minimum before rising-is seen for the bend elastic constant, which gets enhanced by an order of magnitude for a polymer content of 2.5 wt %. In contrast, the splay elastic constant has a monotonic variation although the overall enhancement is comparable to that of the bend elastic constant. The behavior changing at a critical concentration is also seen for the switching time and the associated rotational viscosity. The presence of the polymer also induces a shape change in the thermal dependence of the bend elastic constant. We explain the features observed here on the basis of images obtained from the optical and atomic force microscopy.
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Affiliation(s)
- P Lakshmi Madhuri
- Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
| | - Uma S Hiremath
- Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
| | - C V Yelamaggad
- Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
| | - K Priya Madhuri
- Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
| | - S Krishna Prasad
- Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
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21
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Vaupotič N, Curk S, Osipov MA, Čepič M, Takezoe H, Gorecka E. Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals. Phys Rev E 2016; 93:022704. [PMID: 26986385 DOI: 10.1103/physreve.93.022704] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2015] [Indexed: 06/05/2023]
Abstract
We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters.
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Affiliation(s)
- Nataša Vaupotič
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška 160, 2000 Maribor, Slovenia
- Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
| | - Samo Curk
- Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška 160, 2000 Maribor, Slovenia
| | - Mikhail A Osipov
- Department of Mathematics and Statistics, University of Strathclyde, 26 Richmond Street, G1 Glasgow, United Kingdom
| | - Mojca Čepič
- Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
- Faculty of Education, University of Ljubljana, Kardeljeva ploščad 16, 1000 Ljubljana, Slovenia
| | - Hideo Takezoe
- Toyota Physical and Chemical Research Institute, 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan
- Department of Chemistry, University of Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland
| | - Ewa Gorecka
- Department of Chemistry, University of Warsaw, Zwirki i Wigury 101, 02-089 Warsaw, Poland
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22
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Kos Ž, Ravnik M. Relevance of saddle-splay elasticity in complex nematic geometries. SOFT MATTER 2016; 12:1313-1323. [PMID: 26610395 DOI: 10.1039/c5sm02417j] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
We demonstrate the relevance of saddle-splay elasticity in nematic liquid crystalline fluids in the context of complex surface anchoring conditions and the complex geometrical confinement. Specifically, nematic cells with patterns of surface anchoring and colloidal knots are shown as examples where saddle-splay free energy contribution can have a notable role which originates from nonhomogeneous surface anchoring and the varying surface curvature. Patterned nematic cells are shown to exhibit various (meta)stable configurations of nematic field, with relative (meta)stability depending on the saddle-splay. We show that for high enough values of saddle-splay elastic constant K24 a previously unstable conformation can be stabilised, more generally indicating that the saddle-splay can reverse or change the (meta)stability of various nematic structures affecting their phase diagrams. Furthermore, we investigate saddle-splay elasticity in the geometry of highly curved boundaries - the colloidal particle knots in nematic - where the local curvature of the particles induces complex spatial variations of the saddle-splay contributions. Finally, a nematic order parameter tensor based saddle-splay invariant is shown, which allows for the direct calculation of saddle-splay free energy from the Q-tensor, a possibility very relevant for multiple mesoscopic modelling approaches, such as Landau-de Gennes free energy modelling.
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Affiliation(s)
- Žiga Kos
- Faculty of Mathematics and Physics, University of Ljubljana, Slovenia.
| | - Miha Ravnik
- Faculty of Mathematics and Physics, University of Ljubljana, Slovenia. and Josef Stefan Institute, Ljubljana, Slovenia
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23
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Aya S, Ogino S, Hayashi Y, Okano K, Pociecha D, Le KV, Araoka F, Kawauchi S, Gorecka E, Vaupotič N, Takezoe H, Ishikawa K. Structure-sensitive bend elastic constants between piconewton and subnanonewton in diphenylacetylene-core-based liquid crystals. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 90:042506. [PMID: 25375514 DOI: 10.1103/physreve.90.042506] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/20/2014] [Indexed: 06/04/2023]
Abstract
Elastic constants in liquid crystals are known to be in the range of pico- and several-tens piconewton (pN). We report herein that a bend elastic constant, K33, remarkably varies depending on a slight modification of the chemical structure in an analogous series of calamitic liquid crystals. In contrast to the record-high bend elastic constants (hundreds pN or sub-nN) reported previously in a compound with an azo linkage, analogous compounds with tolan and ester linkages show several-tens pN and pN, respectively. X-ray diffraction studies of these compounds reveal that smectic-like layer structures (cybotacticclusters) are formed in the nematic phase of only the homologous compounds with an azo linkage, certifying the idea that the existence of cybotactic clusters strongly enhances K33. Two theoretical considerations were made: (1) Based on molecular conformation calculation, flat molecules that have high torsional potential energy, such as the one with an azo linkage, easily pack to form cybotactic clusters. (2) Theoretical estimation was made of how much cluster volume ratio is necessary to give about 100-times-larger K33s.
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Affiliation(s)
- Satoshi Aya
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
| | - Shohei Ogino
- Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan
| | - Yoshihiro Hayashi
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
| | - Kunihiko Okano
- Department of Pure and Applied Chemistry, Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba, 278-8510, Japan and Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku, Yokohama, Kanagawa 223-8522, Japan
| | - Damian Pociecha
- University of Warsaw, Department of Chemistry, ul. Zwirki i Wigury 101, Warsaw, 02-089, Poland
| | - Khoa V Le
- RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
| | - Fumito Araoka
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan and RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
| | - Susumu Kawauchi
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
| | - Ewa Gorecka
- University of Warsaw, Department of Chemistry, ul. Zwirki i Wigury 101, Warsaw, 02-089, Poland
| | - Nataša Vaupotič
- Department of Physics, Faculty of Natural Sciences and Mathematics, University of Maribor, Koroška 160, Maribor, Slovenia and Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
| | - Hideo Takezoe
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
| | - Ken Ishikawa
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan
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24
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Gleeson HF, Kaur S, Görtz V, Belaissaoui A, Cowling S, Goodby JW. The nematic phases of bent-core liquid crystals. Chemphyschem 2014; 15:1251-60. [PMID: 24700653 PMCID: PMC4257080 DOI: 10.1002/cphc.201400014] [Citation(s) in RCA: 72] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2014] [Indexed: 11/07/2022]
Abstract
Over the last ten years, the nematic phases of liquid crystals formed from bent-core structures have provoked considerable research because of their remarkable properties. This Minireview summarises some recent measurements of the physical properties of these systems, as well as describing some new data. We concentrate on oxadiazole-based materials as exemplars of this class of nematogens, but also describe some other bent-core systems. The influence of molecular structure on the stability of the nematic phase is described, together with progress in reducing the nematic transition temperatures by modifications to the molecular structure. The physical properties of bent-core nematic materials have proven difficult to study, but patterns are emerging regarding their optical and dielectric properties. Recent breakthroughs in understanding the elastic and flexoelectric behaviour are summarised. Finally, some exemplars of unusual electric field behaviour are described.
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Affiliation(s)
- Helen F Gleeson
- [a] Prof. H. F. Gleeson, Dr. S. Kaur School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL (UK) E-mail:
| | - Sarabjot Kaur
- [a] Prof. H. F. Gleeson, Dr. S. Kaur School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL (UK) E-mail:
| | - Verena Görtz
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
- Department of ChemistryLancaster University, Lancaster LA1 4YB (UK)
| | - Abdel Belaissaoui
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
| | - Stephen Cowling
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
| | - John W Goodby
- Department of ChemistryUniversity of York, Heslington, York YO10 5DD (UK)
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25
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Aya S, Obara H, Pociecha D, Araoka F, Okano K, Ishikawa K, Gorecka E, Yamashita T, Takezoe H. Highly elastic liquid crystals with a sub-nanonewton bending elastic constant mediated by the resident molecular assemblies. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2014; 26:1918-1922. [PMID: 24449478 DOI: 10.1002/adma.201304171] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/19/2013] [Revised: 10/29/2013] [Indexed: 06/03/2023]
Abstract
A surprisingly high bending elastic constant (K33 ) is obtained in a newly synthesized compound shown here. Mixtures containing a few percent of this compound confirm the influence of the unusual K33 values, and show an improved electro-optic response. The origin of the huge K33 is discussed based on the formation of a smectic cluster structure formed in the nematic phase.
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Affiliation(s)
- Satoshi Aya
- Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo, 152-8552, Japan
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26
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Ananthaiah J, Sahoo R, Rasna MV, Dhara S. Rheology of nematic liquid crystals with highly polar molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:022510. [PMID: 25353492 DOI: 10.1103/physreve.89.022510] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/04/2013] [Indexed: 06/04/2023]
Abstract
We report experimental studies on the rheology of a few nematic liquid crystals with highly polar molecules (CCH-7, PCH-7, CB-7). The selected molecules have the same alkyl chain (-C(7)H(15)) and cyano (-CN) end group. In the core part of the molecule, CCH-7 has two cyclohexane rings, PCH-7 has one cyclohexane and one aromatic ring, and CB-7 has two aromatic rings. Two viscosities were measured as a function of temperature, namely, η(2) (director parallel to the shear direction) and η(1) (director perpendicular to the shear direction). The orientation of the director was studied using small angle light scattering techniques. η(2) was measured in presheared sample, whereas the electrorheological technique was used to measure η(1). We show that both viscosities of the liquid crystals depend on the number of aromatic rings and Kirkwood correlation factor. The temperature dependent viscosities can be understood based on the intramolecular π-electron conjugation and intermolecular association of highly polar molecules.
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Affiliation(s)
- J Ananthaiah
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - Rasmita Sahoo
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - M V Rasna
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - Surajit Dhara
- School of Physics, University of Hyderabad, Hyderabad 500046, India
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27
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Greco C, Marini A, Frezza E, Ferrarini A. From the Molecular Structure to Spectroscopic and Material Properties: Computational Investigation of a Bent-Core Nematic Liquid Crystal. Chemphyschem 2014; 15:1336-44. [DOI: 10.1002/cphc.201301030] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2013] [Indexed: 11/12/2022]
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28
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Balachandran R, Panov VP, Vij JK, Lehmann A, Tschierske C. Effect of cybotactic clusters on the elastic and flexoelectric properties of bent-core liquid crystals belonging to the same homologous series. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:032503. [PMID: 24125281 DOI: 10.1103/physreve.88.032503] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2013] [Revised: 07/29/2013] [Indexed: 06/02/2023]
Abstract
We report results of the splay (K_{11}) and bend (K_{33}) elastic constants and the effective flexoelectric coefficient of three bent-core liquid crystals belonging to a homologous series of 4-cyanoresorcinol bisbenzoates with varying chain lengths. Based on the results of x-ray scattering studies, one of the three compounds with a shorter chain length (C4) has few, if any, clusters present in its nematic phase and behaves quite normally, whereas the others two with longer chain lengths (C6 and C7) show the presence of cybotactic nematic phase with smectic C type clusters. These grow in size with a reduction in temperature. K_{33} is found to be the least for C7, whereas it is weakly dependent on temperature. K_{33} is somewhat higher for C4 and C6 and is almost independent of temperature. K_{11} for C6 and C7 is higher by 20% to 50% than C4 depending on the temperature. K_{11} increases linearly with a reduction in temperature for the three compounds. For C6 K_{11}>K_{33} by a factor up to ∼2 depending on the temperature, for C4 it is greater by a factor up to 1.3, and for C7 it is greater by a factor of ∼2.5. These results suggest that the clusters do not have any effect on K_{11}. The magnitude of the effective flexoelectric coefficient e=(|e_{1}-e_{3}|) is measured by creating a uniform lying helix (ULH) configuration in a planar cell. By doping the bent-core system with a small wt% of a chiral dopant, the ULH is obtained by cooling planar cells to the cholesteric phase under weak electric field. The effective flexoelectric coefficient is greater for the bent-core systems than for calamatics but it is much lower than would otherwise have been expected for such systems. |e_{1}-e_{3}| for C4 > C6 ≈ C7 is greater by 20% to 25% than C6 and C7 at the same reduced temperature. These differences in the effective flexoelectric coefficient can easily arise from a difference in the chain lengths among the members of the series but if the presence of clusters were to have an influence on |e_{1}-e_{3}|, then these would reduce it, contrary to the expectations for the bent-core systems.
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Affiliation(s)
- R Balachandran
- Department of Electronic and Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland
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29
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Lejček L, Glogarová M, Novotná V. Model of dark conglomerate structure in the B2 phase of bent-shape molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 87:042503. [PMID: 23679432 DOI: 10.1103/physreve.87.042503] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/01/2012] [Indexed: 06/02/2023]
Abstract
Texture observations in B2 phase of bent-shape molecules showed a coexistence of so-called dark conglomerate (DC) structure with fan-shaped texture composed of focal conic domains (FCDs). A model of DC structure based on grains of dimensions lower than visible wavelengths is proposed and used to compare the energies of DC and FCD structures. The comparison of energies of both structures enables the estimation of approximate model parameters. Reorientation of smectic layers in grains under an electric field and transformation into the system of FCD structure are discussed.
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Affiliation(s)
- Lubor Lejček
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic.
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30
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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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31
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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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Salamon P, Eber N, Seltmann J, Lehmann M, Gleeson JT, Sprunt S, Jákli A. Dielectric technique to measure the twist elastic constant of liquid crystals: the case of a bent-core material. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 85:061704. [PMID: 23005111 DOI: 10.1103/physreve.85.061704] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/21/2012] [Indexed: 06/01/2023]
Abstract
The effect of director pretilt on the twist magnetic Fréedericksz transition of nematics was investigated in a planar cell. The director configuration was calculated as a function of magnetic inductance. The dielectric and optical response of the nematic liquid crystal was numerically modeled. A dielectric measurement method for determining the elastic constant K_{22} is presented. The influence of the conditions for the Mauguin effect is discussed. The theoretical predictions were confirmed by our experiments. Experimental data for all elastic constants of a bent-core nematic material are presented and discussed.
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Affiliation(s)
- P Salamon
- Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B. 49, Hungary
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33
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Fukuda JI. Stabilization of blue phases by the variation of elastic constants. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 85:020701. [PMID: 22463144 DOI: 10.1103/physreve.85.020701] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/05/2011] [Indexed: 05/31/2023]
Abstract
We study theoretically the effect of the variation of the elastic constants on the stability of cholesteric blue phases (BPs). We demonstrate that the stability of BPs is greatly enhanced when the bend elastic constant K(33) is smaller, in agreement with recent experimental findings. Larger splay (K(11)) and twist (K(22)) elastic constants also stabilize BPs. Our argument sheds light on the subtle effect of the variation of the elastic constants on the stability of BPs, and could guide the synthesis of liquid crystals with a wider temperature range of stable BPs.
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Affiliation(s)
- Jun-ichi Fukuda
- Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan.
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Tadapatri P, Krishnamurthy KS. Competing Instability Modes in an Electrically Driven Bent-Core Nematic Liquid Crystal. J Phys Chem B 2012; 116:782-93. [DOI: 10.1021/jp210383p] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Pramod Tadapatri
- Centre for Soft Matter Research, P.O. Box 1329, Jalahalli, Bangalore 560013, India
| | - K. S. Krishnamurthy
- Centre for Soft Matter Research, P.O. Box 1329, Jalahalli, Bangalore 560013, India
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Sathyanarayana P, Jampani VSR, Skarabot M, Musevic I, Le KV, Takezoe H, Dhara S. Viscoelasticity of ambient-temperature nematic binary mixtures of bent-core and rodlike molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2012; 85:011702. [PMID: 22400578 DOI: 10.1103/physreve.85.011702] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/21/2011] [Indexed: 05/31/2023]
Abstract
We report measurements of the temperature variations of physical parameters in ambient-temperature nematic liquid crystal mixtures of bent-core (BC) and rodlike molecules (5CB): birefringence Δn; static dielectric constants ε(||) and ε(⊥); splay K(11) and bend K(33) elastic constants; rotational viscosity γ(1); and diffusion coefficients D(||) and D(⊥) of a microsphere. Both Δn and ε(||) decreases rapidly with increasing BC concentration, whereas ε(⊥) remains almost constant. At a shifted temperature (e.g., T-T(NI)=-10 °C), K(11) increases by ~50% and K(33) decreases by ~80% compared to pure 5CB when the BC concentration is increased to ~43 mol % in the mixture. Viscosities parallel and perpendicular to the director, η(||), η(⊥), which are nearly equal to the Miesowicz viscosities η(2) and η(3), respectively, were obtained by D(||) and D(⊥) using the Stokes-Einstein relation. Both the viscosities at room temperature increase by 60 and 50 times, respectively, whereas γ(1) increases by 180 times (at ~43 mol %) compared to the corresponding values of pure 5CB. The stiffening of K(11) and exorbitantly large enhancement in all the viscosities at a higher mol % of BC indicate that the viscoelastic properties are highly impacted by the presence of smectic clusters of BC molecules that results from the restricted free rotation of the molecules along the bow axis in the nematic phase. A possible attachment model of smectic type clusters of BC molecules surrounding the microparticle is presented.
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Affiliation(s)
- P Sathyanarayana
- School of Physics, University of Hyderabad, Hyderabad 500046, India
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Jeong HC, V. Le K, Gim MJ, Hur ST, Choi SW, Araoka F, Ishikawa K, Takezoe H. Transition between widened BPs by light irradiation using photo-active bent-core liquid crystal with chiral dopant. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c2jm16650j] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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37
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Lejček L, Novotná V, Glogarová M. Stripe and line textures in the B2 phase of bent-shape molecules in samples with polar surface anchoring. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 84:061701. [PMID: 22304103 DOI: 10.1103/physreve.84.061701] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/23/2011] [Indexed: 05/31/2023]
Abstract
In the B2 phase formed by bent-shaped molecules a dense line texture is frequently observed. For the texture description a model is proposed consisting of a periodic system of anticlinic antiferroelectric bulk domains with opposite chiralities separated by π walls in which polarization rotates. The bulk domains are situated between layers of synclinic ferroelectric phase near the upper and lower surfaces. In the surface layers induced by polar anchoring domains of opposite chirality are separated by defect lines. Under the electric field the ferroelectric layer near one sample surface is growing against the antiferroelectric structure in the sample bulk and the stripe texture is fading out, but still in the saturated field surface lines or walls persist separating ferroelectric domains of opposite chirality. The proposed model is adapted also for a case when the anticlinic antiferroelectric structure in the sample bulk is sandwiched between anticlinic ferroelectric layers near the surfaces. In that case the applied electric field eliminates all the lines because the resulting anticlinic ferroelectric structure is racemic, with no chiral domains.
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Affiliation(s)
- Lubor Lejček
- Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-182 21 Prague 8, Czech Republic.
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Majumdar M, Salamon P, Jákli A, Gleeson JT, Sprunt S. Elastic constants and orientational viscosities of a bent-core nematic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2011; 83:031701. [PMID: 21517512 DOI: 10.1103/physreve.83.031701] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/23/2010] [Indexed: 05/30/2023]
Abstract
Using a combination of dynamic light scattering and Freedericksz transitions induced in applied magnetic and electric fields, we have determined the absolute magnitudes of the Frank elastic constants and effective orientational viscosities of the bent-core nematic liquid crystal, 4-chloro-1,3-phenylene bis 4-[4'-(9-decenyloxy)benzoyloxy] benzoate. At a fixed temperature 2 °C below the isotropic-nematic transition, we find K₁₁ = 3.1 x 10⁻¹² N, K₂₂ = 0.31 x 10⁻¹² N, K₃₃ = 0.88 x 10⁻¹² N, η{splay}=1.1 Pa s, η{twist}=0.37 Pa s, and η{bend}=1.2 Pa s. The unusual anisotropies of these parameters are discussed in terms of short-range, smectic-C-like correlations among molecules in the nematic phase.
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Affiliation(s)
- M Majumdar
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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Cestari M, Frezza E, Ferrarini A, Luckhurst GR. Crucial role of molecular curvature for the bend elastic and flexoelectric properties of liquid crystals: mesogenic dimers as a case study. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c1jm12233a] [Citation(s) in RCA: 106] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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40
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Le KV, Aya S, Sasaki Y, Choi H, Araoka F, Ema K, Mieczkowski J, Jakli A, Ishikawa K, Takezoe H. Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c0jm04009f] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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41
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Takezoe H. Spontaneous Achiral Symmetry Breaking in Liquid Crystalline Phases. Top Curr Chem (Cham) 2011; 318:303-30. [DOI: 10.1007/128_2011_242] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
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42
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Sathyanarayana P, Varia MC, Prajapati AK, Kundu B, Sastry VSS, Dhara S. Splay-bend elasticity of a nematic liquid crystal with T-shaped molecules. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:050701. [PMID: 21230425 DOI: 10.1103/physreve.82.050701] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/14/2010] [Revised: 09/14/2010] [Indexed: 05/30/2023]
Abstract
We measured the splay (K11) and bend (K33) elastic constants in the nematic phase of a liquid crystal with T-shaped molecules. We find that the ratio, K33/K11 ≃1 in the entire nematic range except very close to the nematic to Sm-A (SN) transition. Both K33 and K11 show pretransitional divergence as the SN transition is approached from higher temperature. The ratio, K33/K11 suggests that the length (L) to effective width (D) ratio (i.e., L/D ) is significantly smaller due to the presence of long and flexible lateral group, compared to that of rigid rodlike molecules. It is argued that apart from the extra contribution to the elasticity the long and flexible lateral group also has a significant contribution to the suppression of the splay fluctuations in the onset of smectic short-range fluctuation. The structure of the Sm-A phase is investigated by using small angle x-ray diffraction, and a possible arrangement of the molecules in the Sm-A layer is proposed.
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Affiliation(s)
- P Sathyanarayana
- School of Physics, University of Hyderabad, Hyderabad 500046, India
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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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Tadapatri P, Krishnamurthy KS, Weissflog W. Multiple electroconvection scenarios in a bent-core nematic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 82:031706. [PMID: 21230090 DOI: 10.1103/physreve.82.031706] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/29/2010] [Indexed: 05/30/2023]
Abstract
We report on the anisotropic electrohydrodynamic states formed over a wide temperature range (∼45 °C) in a planarly aligned bent-core nematic liquid crystal driven by fields of frequency in the range 0.1 Hz-1 MHz. Three different primary bifurcation scenarios are generated in the voltage-frequency (V-f) plane, depending on the temperature T. These, under increasing T, are characterized by the pattern sequences (i) in-plane longitudinal rolls (ILR)→in-plane normal rolls 1 (INR1), (ii) Williams rolls (WR)→ILR→INR1, and (iii) WR→INR2→INR1. Temperature-induced ILR→INR2 transition, the first example of its kind, points to elastic anisotropy as possibly the determining factor in wave vector selection. In the ILR and INR states, at threshold, the director modulations are predominantly azimuthal, and the streamlines, mainly normal to the wave vector, lie in the sample plane. Well above threshold, growing director deviations lead to narrow disclination loops that evolve in regular arrays, with their area density being exponential in voltage. The defects drift in a coordinated manner along the flow lines with a speed that scales nonlinearly with voltage; they mediate in the eventual occurrence of turbulence. The current theories of anisotropic convection based on static electrical parameters fail to account for the observed high-frequency instabilities. The study includes (i) a quantitative characterization of the critical parameter functions V(c)(f), V(c)(T), q(c)(f), and q(c)(T), with q(c) denoting the critical pattern wave number, and (ii) measurement of electrical and elastic parameters of relevance to electroconvection; the latter show anomalous features supporting the cluster hypothesis.
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Affiliation(s)
- Pramod Tadapatri
- Centre for Soft Matter Research, P.O. Box 1329, Jalahalli, Bangalore 560 013, India
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