1
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Merkel K, Welch C, Ahmed Z, Piecek W, Mehl GH. Dielectric response of electric-field distortions of the twist-bend nematic phase for LC dimers. J Chem Phys 2019; 151:114908. [PMID: 31542029 DOI: 10.1063/1.5114824] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023] Open
Abstract
Wide band dielectric spectroscopy of bent-shaped achiral liquid-crystal dimers 1″-n″-bis(4-cyanobiphenyl-4'-yl) n-alkanes (CBnCB n = 7, 9, 11) has been investigated in a frequency range 0.1 Hz-100 MHz using planar-aligned cells of sample thicknesses ranging from 2 to 10 (μm) over a temperature range that covers both nematic and twist bend nematic phases. Two peaks in the dielectric spectrum in the higher frequency range are assigned to the molecular relaxation processes. The peak at the highest frequency, ∼40 to 80 MHz, is assigned to an internal precessional rotation of a single unit of the dimer around the director. The mode in the next lower frequency range of 2-10 MHz is assigned to the spinning rotation of the dimer around its long axis. This involves fluctuations of the dipole moment of the bent-shaped conformation that is directed along its arrow direction of the bow shape formed by the dimer. The peak in the frequency range 100 kHz-1 MHz can be assigned to the collective fluctuations of the local director with reference to the helical axis of the NTB structure. The dependence of its frequency on temperature is reminiscent of the soft mode observed at the SmA* to SmC* phase transition. This result clearly corresponds to the electro-clinic effect-the response of the director to the applied electric field in an electro-optic experiment. The lowest frequency mode, observed in the frequency range of 0.1 Hz-100 Hz, is identified with the Goldstone mode. This mode is concerned with the long range azimuthal angle fluctuations of the local director. This leads to an alternating compression and expansion of the periodic structure of the NTB phase.
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Affiliation(s)
- K Merkel
- Faculty of Computer Science and Material Science, Institute of Technology and Mechatronics, University of Silesia in Katowice, Katowice, Poland
| | - C Welch
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Z Ahmed
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - W Piecek
- Faculty of Advanced Technologies and Chemistry, Military University of Technology, Warszawa, Poland
| | - G H Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
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2
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Hann JL, Mandle RJ. Pentaerythritol Derived Tetrapode Exhibiting a Nematic-Like Mesophase at Ambient Temperatures. Chemphyschem 2019; 20:1941-1945. [PMID: 31194279 DOI: 10.1002/cphc.201900585] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2019] [Indexed: 11/09/2022]
Abstract
The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'NX ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the NX phase to exhibit an electrooptic response consistent with a nematic-like phase.
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Affiliation(s)
- Jodie L Hann
- Department of Chemistry, University of York, York, UK, YO10 5DD
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3
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Merkel K, Kocot A, Welch C, Mehl GH. Soft modes of the dielectric response in the twist–bend nematic phase and identification of the transition to a nematic splay bend phase in the CBC7CB dimer. Phys Chem Chem Phys 2019; 21:22839-22848. [DOI: 10.1039/c9cp04952e] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Two collective processes resulting from distortion of the heliconical structure of the twist–bend nematic phase of an achiral dimer: one tilt mode due to distortions of the conical angle and second related to long range fluctuation of the cone phase.
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Affiliation(s)
- K. Merkel
- Faculty of Computer Science and Material Science
- Institute of Technology and Mechatronics
- University of Silesia in Katowice
- Poland
| | - A. Kocot
- Faculty of Computer Science and Material Science
- Institute of Technology and Mechatronics
- University of Silesia in Katowice
- Poland
| | - C. Welch
- Department of Chemistry
- University of Hull
- Hull HU6 7RX
- UK
| | - G. H. Mehl
- Department of Chemistry
- University of Hull
- Hull HU6 7RX
- UK
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4
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Bhattacharjee D, Alapati PR, Bhattacharjee A. Molecular polarization, order parameter and dielectric study of a dimeric compound. J Mol Liq 2016. [DOI: 10.1016/j.molliq.2016.10.029] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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5
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Wei N, Déjardin PM, Kalmykov YP, Coffey WT. External dc bias-field effects in the nonlinear ac stationary response of dipolar particles in a mean-field potential. Phys Rev E 2016; 93:042208. [PMID: 27176294 DOI: 10.1103/physreve.93.042208] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2016] [Indexed: 06/05/2023]
Abstract
External dc bias-field effects on the nonlinear dielectric relaxation and dynamic Kerr effect of a system of permanent dipoles in a uniaxial mean-field potential are studied via the rotational Brownian motion model postulated in terms of the infinite hierarchy of differential-recurrence equations for the statistical moments f_{n}(t)=〈P_{n}〉(t) (the expectation value of the Legendre polynomials P_{n}). By solving these equations, the nonlinear dielectric and Kerr-effect ac stationary responses are evaluated for arbitrary dc field strength via perturbation theory in the ac field. Simple analytic equations based on the large separation of the time scales of the fast intrawell and slow overbarrier (interwell) relaxation processes are also derived.
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Affiliation(s)
- Nijun Wei
- Department of Electronic and Electrical Engineering, Trinity College, Dublin 2, Ireland
| | - Pierre-Michel Déjardin
- Laboratoire de Mathématiques et de Physique (LAMPS, EA4217), Université de Perpignan Via Domitia, F-66860, Perpignan, France
| | - Yuri P Kalmykov
- Laboratoire de Mathématiques et de Physique (LAMPS, EA4217), Université de Perpignan Via Domitia, F-66860, Perpignan, France
| | - William T Coffey
- Department of Electronic and Electrical Engineering, Trinity College, Dublin 2, Ireland
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6
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7
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Balachandran R, Panov VP, Vij JK, Shanker G, Tschierske C, Merkel K, Kocot A. Dielectric and electro-optic studies of a bimesogenic liquid crystal composed of bent-core and calamitic units. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 90:032506. [PMID: 25314464 DOI: 10.1103/physreve.90.032506] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/03/2014] [Indexed: 06/04/2023]
Abstract
A bimesogen, BR1, composed of a bent-core and calamitic unit, linked laterally via a flexible spacer is investigated by dielectric and electro-optic techniques. X-ray results show the presence of clusters in the nematic phase, and the cluster size is of the order of the thickness of a single layer. The splitting of the small-angle scattering Δχ/2 is about 50°, which indicates SmC like clusters with a significant tilt of the molecules in the quasilayers. The sign reversal of the dielectric anisotropy Δε' is observed as a function of frequency; the behavior is rather similar to that exhibited by the conventional dual frequency nematics, composed of a calamitic mesogen, with the exception that it occurs at much lower frequencies in this material. Interestingly, as the bimesogen enters its nematic phase, the average permittivity decreases as the temperature is lowered. This indicates the onset of antiparallel association of some of the dipoles in the system, and this type of association is much more prominent in BR1 in comparison to other bent-core liquid crystalline systems composed of the same bisbenzoate core unit. The analysis of the dielectric spectra using the Maier-Meier model confirms the onset of an antiparallel correlation of dipoles occurring at the isotropic to nematic phase transition temperature. Additionally these results support a model of the cluster where the transverse dipole moments in the neighboring layers are antiparalleled to each other.
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Affiliation(s)
- R Balachandran
- Department of Electronic and Electrical Engineering, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
| | - V P Panov
- Department of Electronic and Electrical Engineering, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland and School of Physics and Astronomy, University of Manchester, M13 9PL Manchester, United Kingdom
| | - J K Vij
- Department of Electronic and Electrical Engineering, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
| | - G Shanker
- Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Strasse 2, 06120 Halle, Germany and BMS R&D Centre, BMS College of Engineering, Bull Temple Road, Bangalore-560019, India
| | - C Tschierske
- Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Strasse 2, 06120 Halle, Germany
| | - K Merkel
- Central Mining Institute, Katowice, Poland
| | - A Kocot
- Institute of Physics, Silesian University, Katowice, Poland
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8
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Sebastián N, de la Fuente M, López D, Pérez-Jubindo M, Salud J, Ros M. Overall Dielectric Study on the Odd Members of a Highly Nonsymmetric Pyrene-Based Series of Liquid Crystal Dimers. J Phys Chem B 2013; 117:14486-96. [DOI: 10.1021/jp406085r] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- N. Sebastián
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - M.R. de la Fuente
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - D.O. López
- Grup
de Propietas Físiques dels Materials (GRPFM), Departament de
Física i Enginyeria Nuclear, E.T.S.E.I.B., Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M.A. Pérez-Jubindo
- Departamento
de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - J. Salud
- Grup
de Propietas Físiques dels Materials (GRPFM), Departament de
Física i Enginyeria Nuclear, E.T.S.E.I.B., Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M.B. Ros
- Departamento
de Química Orgánica, Facultad de Ciencias, Instituto
de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
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9
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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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10
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Benzie PW, Corbett D, Elston SJ. Simulation of the viewing properties and optical compensation of the biaxial nematic in-plane switching mode. APPLIED OPTICS 2012; 51:75-83. [PMID: 22270415 DOI: 10.1364/ao.51.000075] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/07/2011] [Accepted: 09/02/2011] [Indexed: 05/31/2023]
Abstract
Using Berreman 4 × 4 optical methods and continuum theory, we investigate the theoretical viewing properties of a potential homeotropically aligned biaxial nematic display switched with in-plane fields. We determine the isocontrast, isotransmission viewing characteristics for wide-angle viewing for in-plane switching and consider the necessary requirements for optical compensation to produce a high transmission in the bright state and low transmission in the dark state. We show how compensation can be achieved with biaxial compensation layers using a homogeneous biaxial film or from birefringence.
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Affiliation(s)
- Philip W Benzie
- Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK.
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11
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Schubert CPJ, Tamba MG, Mehl GH. Chiral nematic organo-siloxane oligopodes based on an axially chiral binaphthalene core. Chem Commun (Camb) 2012; 48:6851-3. [DOI: 10.1039/c2cc33050d] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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12
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Pawlus S, Paluch M, Nagaraj M, Vij JK. Effect of high hydrostatic pressure on the dielectric relaxation in a non-crystallizable monohydroxy alcohol in its supercooled liquid and glassy states. J Chem Phys 2011; 135:084507. [DOI: 10.1063/1.3626027] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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13
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Sebastián N, de la Fuente MR, López DO, Pérez-Jubindo MA, Salud J, Diez-Berart S, Ros MB. Dielectric and Thermodynamic Study on the Liquid Crystal Dimer α-(4-Cyanobiphenyl-4′-oxy)-ω-(1-pyreniminebenzylidene-4′-oxy)undecane (CBO11O·Py). J Phys Chem B 2011; 115:9766-75. [DOI: 10.1021/jp202796y] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- N. Sebastián
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - M. R. de la Fuente
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - D. O. López
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M. A. Pérez-Jubindo
- Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, E-48080 Bilbao, Spain
| | - J. Salud
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - S. Diez-Berart
- Grup de Propietas Físiques dels Materials (GRPFM), Departament de Física i Enginyeria Nuclear, E.T.S.E.I.B. Universitat Politècnica de Catalunya, Diagonal, 647 08028 Barcelona, Spain
| | - M. B. Ros
- Departamento de Química Orgánica, Facultad de Ciencias, Instituto de Ciencia de Materiales de Aragón, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
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14
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Salamon P, Eber N, Buka A, Gleeson JT, Sprunt S, Jákli A. Dielectric properties of mixtures of a bent-core and a calamitic liquid crystal. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:031711. [PMID: 20365755 DOI: 10.1103/physreve.81.031711] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/29/2009] [Revised: 01/07/2010] [Indexed: 05/29/2023]
Abstract
Dielectric spectroscopy measurements have been performed on a bent-core nematic liquid crystal and on its binary mixtures with a calamitic nematic. We have detected more dispersions in the bent-core compound than in the calamitic one, including one at an unusually low frequency of a few kilohertz. The dispersions detected in the mixtures have been identified and the spectra have been split into contributions of the constituents. In order to connect the dielectric increment with the molecular dipole moment we have applied a sophisticated conformational calculation not performed before for a large, flexible mesogen molecule with numerous polar groups.
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Affiliation(s)
- Péter Salamon
- Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, PO Box 49, Hungary
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15
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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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16
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Cruz C, Figueirinhas JL, Filip D, Feio G, Ribeiro AC, Frère Y, Meyer T, Mehl GH. Biaxial nematic order and phase behavior studies in an organosiloxane tetrapode using complementary deuterium NMR experiments. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:051702. [PMID: 19113138 DOI: 10.1103/physreve.78.051702] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2006] [Revised: 07/10/2008] [Indexed: 05/27/2023]
Abstract
The biaxial nematic phase was recently observed in different thermotropic liquid crystals, namely bent-core compounds, side-chain polymers, bent-core dimers, and organosiloxane tetrapodes. In this work, a series of experiments with a nematic organosiloxane tetrapode where nuclear magnetic resonance (NMR) spectra are collected while the sample is continuously rotating around an axis perpendicular to the magnetic field, are discussed in conjunction with the analysis of a deuterium NMR experiment on the same system reported earlier. The sample used is a mixture of a deuterated probe with the tetrapode. The mixture exhibits a nematic range between -40 degrees C and 37 degrees C. The results of the two independent, but complementary deuterium NMR experiments confirm the existence of a biaxial nematic phase for temperatures below 0 degrees C with high values of the asymmetry parameter at low temperatures. The presence of slow movements of the tetrapode mesogenic units in the low-temperature regime could also be detected through the analysis of the NMR spectra. Simulations indicate that these movements are mainly slow molecular reorientations of the mesogenic units associated with the presence of collective modes in the nematic phases of this compound. In the case of tetrapodes, recent investigations attribute the origin of biaxiality to the hindering of reorientations of the laterally attached mesogenic units which constitute the tetrapode. This study relates the molecular movements with the nematic biaxial ordering of the system.
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Affiliation(s)
- C Cruz
- CFMC-UL, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal
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17
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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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18
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Filip D, Cruz C, Sebastião PJ, Ribeiro AC, Vilfan M, Meyer T, Kouwer PHJ, Mehl GH. Structure and molecular dynamics of the mesophases exhibited by an organosiloxane tetrapode with strong polar terminal groups. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2007; 75:011704. [PMID: 17358170 DOI: 10.1103/physreve.75.011704] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2006] [Indexed: 05/14/2023]
Abstract
The polymorphism of a new organosiloxane tetrapode compound with cyano terminal polar groups was characterized by means of polarizing optical microscopy and x-ray diffraction. The compound exhibits smectic- A and smectic- C phases with a partial bilayer arrangement due to a certain degree of head-to-head association of the mesogenic units through their cyano end groups. On the basis of x-ray diffraction results, evidencing the microsegregation of polyphilic molecules, packing models for the smectic- A and smectic- C phases are proposed. A high degree of smectic positional order and a relatively low value of the tilt angle in the smectic- C phase are indicated. Molecular dynamics of the studied compound was investigated by means of proton NMR relaxometry. The frequency dispersions of the spin-lattice relaxation time (T1) show that the relaxation is induced by three rotational modes of individual dendrimer arms with frequencies between 10;{6} and 10;{9}Hz . In the smectic phases, the effect of individual rotations is overwhelmed by a well expressed contribution of layer undulations at Larmor frequencies below approximately 10MHz . The appearance of this relaxation mechanism over the frequency range of three decades is so far unique in the case of thermotropic liquid crystals. The analysis of the layer undulations contribution supports the microsegregation model of the smectic phases by revealing a slowing-down of translational diffusion and the lack of interactions among the sublayers formed by the mesogenic groups.
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Affiliation(s)
- D Filip
- Centro de Física da Matéria Condensada, Av. Prof. Gama Pinto 2, 1649-003 Lisbon, Portugal
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19
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Kaznacheev K, Hegmann T. Molecular ordering in a biaxial smectic-A phase studied by scanning transmission X-ray microscopy (STXM). Phys Chem Chem Phys 2007; 9:1705-12. [PMID: 17396182 DOI: 10.1039/b616010g] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Results of STXM investigations of a binary mixture (-TNF = 2 : 1; SmA(b) 140 M 180 Iso) known to form a SmA(b) phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the -TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring pi* orbitals (284.5-286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the -TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive +/-(1/2) disclination theoretically predicted for the SmA(b) phase.
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Affiliation(s)
- Konstantin Kaznacheev
- Department of Physics & Engineering Physics and Canadian Light Source Inc., University of Saskatchewan, Saskatoon (SK), CanadaS7N 0X4.
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Apreutesei D, Mehl GH. The design and investigation of laterally functionalised oxadiazoles. ACTA ACUST UNITED AC 2007. [DOI: 10.1039/b708930a] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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