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Lehmann M, Maisch S, Scheuring N, Carvalho J, Cruz C, Sebastião PJ, Dong RY. From molecular biaxiality of real board-shaped mesogens to phase biaxiality? On the hunt for the holy grail of liquid crystal science. SOFT MATTER 2019; 15:8496-8511. [PMID: 31633147 DOI: 10.1039/c9sm01007f] [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 the search of the predicted biaxial nematic phase, a series of shape-persistent board-shaped mesogens with maximum molecular biaxiality and a dipole along the minor molecular axis were designed to form nematic (N) mesophases. One compound exhibits a wide nematic temperature range, which can be supercooled to room temperature. A comprehensive variable temperature X-ray study on aligned samples reveals patterns being dominated by the form factor of very small aggregates, from which the aspect ratio of the lead compound with length (L) : breadth (B) : width (W) of 10.73 : 3.16 : 1.23 could be obtained. The ratio is close to the predicted optimum molecular biaxiality by Straley's hard particle model. Hence variable temperature proton relaxation studies on this mesogen were carried out over a wide frequency range. The global fit of the frequency dispersions at five temperatures with a motional model requires in addition to the usual rotation/reorientation contribution, two independent director fluctuations contributions: one for the conventional nematic order director (n) fluctuations and the other for the minor director (m) fluctuations (normal to n). The correlation length of the minor directors determined by NMR could extend to 5-8 molecules in the W direction, but only to the nearest neighbour in the B direction, as found by X-ray diffraction. Both X-ray and NMR studies indicate that these new types of lead structure are extremely promising to find the long sought-after biaxial N mesophase.
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Affiliation(s)
- Matthias Lehmann
- Institute of Organic Chemistry, University of Würzburg, Germany.
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2
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D'Urso C, Celebre G, Cinacchi G. Phase behavior of hard C_{2h}-symmetric particle systems. Phys Rev E 2019; 100:012709. [PMID: 31499787 DOI: 10.1103/physreve.100.012709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2019] [Indexed: 06/10/2023]
Abstract
Using Monte Carlo numerical simulation, this work sketches the phase diagram of systems of certain hard C_{2h}-symmetric particles, formed by gluing two aligned and displaced hard spherocylinders with a cylindrical-length-to-diameter ratio realistically, if viewed not only from the lyotropic colloidal liquid-crystal side but also from the thermotropic low-molecular-mass liquid-crystal side, equal to 5, as a function of the displacement. Several distinctive phases are observed, such as a nonperiodic smectic-B-like phase, a nonperiodic smectic-H-like phase, a smectic-C phase, and a short-layer-spacing uniaxial smectic-A phase but no biaxial nematic phase.
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Affiliation(s)
- Christian D'Urso
- Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, via P. Bucci, I-87036 Arcavacata di Rende (CS), Italy
| | - Giorgio Celebre
- Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, via P. Bucci, I-87036 Arcavacata di Rende (CS), Italy
| | - Giorgio Cinacchi
- Departamento de Física Teórica de la Materia Condensada, Instituto de Física de la Materia Condensada (IFIMAC), Instituto de Ciencias de Materiales "Nicolás Cabrera," Universidad Autónoma de Madrid, Ciudad Universitaria de Cantoblanco, E-28049 Madrid, Spain
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3
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Chrzanowska A. Mean-field model of boomerang nematic liquid crystals with diminished coupling of molecular uniaxial and biaxial susceptibilities. Phys Rev E 2019; 99:062703. [PMID: 31330725 DOI: 10.1103/physreve.99.062703] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2018] [Indexed: 06/10/2023]
Abstract
The mean-field theory approach has been applied to the boomerang type particles from P. I. C. Teixeira, A. Masters, and B. Mulder [Mol. Cryst. Liq. Cryst. 323, 167 (1998)MCLCE91058-725X10.1080/10587259808048440] but with diminished strength of the interaction coefficient responsible for the coupling between molecular uniaxial and biaxial susceptibilities. For the rodlike particles, when the apex boomerang angle is larger than 107.35^{∘}, the stable uniaxial rodlike phase occurs. For smaller angles, beyond the point where the transition is of the second order (the Landau point) and for diminished parameter of molecular biaxial-uniaxial coupling, a biaxial phase is observed with the transition undergoing directly from the isotropic phase. According to the order parameters the character of this transition is of the first order. Such behavior is in accordance with the Sonnet-Durand-Virga model of the biaxial phases. The change in the type of the phase transition order is also illustrated by the changes in the equations of state and the changes in second and third derivatives of the free energy. The possibilities to tailor interaction coefficients of real molecules to obtain such a phase transition scenario are discussed.
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Affiliation(s)
- Agnieszka Chrzanowska
- Institute of Physics, Kraków University of Technology, ul. Podchorążych 1, 30-084 Kraków, Poland
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4
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Beira MJ, Silva MP, Condesso M, Cosme P, Almeida PL, Corvo MC, Sebastião PJ, Figueirinhas JL, de Pinho MN. Molecular order and dynamics of water in hybrid cellulose acetate–silica asymmetric membranes. Mol Phys 2018. [DOI: 10.1080/00268976.2018.1537526] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
Affiliation(s)
- M. Jardim Beira
- Centro de Física e Engenharia de Materiais Avançados, Insiituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - M. P. Silva
- Centro de Física e Engenharia de Materiais Avançados, Insiituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - M. Condesso
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - P. Cosme
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - P. L. Almeida
- CENIMAT/i3N- Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal
- ISEL, ADF, Rua Conselheiro Emídio Navarro 1, Lisboa, Portugal
| | - M. C. Corvo
- CENIMAT/i3N- Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal
| | - P. J. Sebastião
- Centro de Física e Engenharia de Materiais Avançados, Insiituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - J. L. Figueirinhas
- Centro de Física e Engenharia de Materiais Avançados, Insiituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
| | - M. N. de Pinho
- Centro de Física e Engenharia de Materiais Avançados, Insiituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
- Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
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Kamala Latha B, Murthy KPN, Sastry VSS. Complex free-energy landscapes in biaxial nematic liquid crystals and the role of repulsive interactions: A Wang-Landau study. Phys Rev E 2018; 96:032703. [PMID: 29346959 DOI: 10.1103/physreve.96.032703] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/16/2017] [Indexed: 11/07/2022]
Abstract
General quadratic Hamiltonian models, describing the interaction between liquid-crystal molecules (typically with D_{2h} symmetry), take into account couplings between their uniaxial and biaxial tensors. While the attractive contributions arising from interactions between similar tensors of the participating molecules provide for eventual condensation of the respective orders at suitably low temperatures, the role of cross coupling between unlike tensors is not fully appreciated. Our recent study with an advanced Monte Carlo technique (entropic sampling) showed clearly the increasing relevance of this cross term in determining the phase diagram (contravening in some regions of model parameter space), the predictions of mean-field theory, and standard Monte Carlo simulation results. In this context, we investigated the phase diagrams and the nature of the phases therein on two trajectories in the parameter space: one is a line in the interior region of biaxial stability believed to be representative of the real systems, and the second is the extensively investigated parabolic path resulting from the London dispersion approximation. In both cases, we find the destabilizing effect of increased cross-coupling interactions, which invariably result in the formation of local biaxial organizations inhomogeneously distributed. This manifests as a small, but unmistakable, contribution of biaxial order in the uniaxial phase. The free-energy profiles computed in the present study as a function of the two dominant order parameters indicate complex landscapes. On the one hand, these profiles account for the unusual thermal behavior of the biaxial order parameter under significant destabilizing influence from the cross terms. On the other, they also allude to the possibility that in real systems, these complexities might indeed be inhibiting the formation of a low-temperature biaxial order itself-perhaps reflecting the difficulties in their ready realization in the laboratory.
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Affiliation(s)
- B Kamala Latha
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - K P N Murthy
- School of Physics, University of Hyderabad, Hyderabad 500046, India
| | - V S S Sastry
- School of Physics, University of Hyderabad, Hyderabad 500046, India.,Centre for Modelling, Simulation and Design, University of Hyderabad, Hyderabad 500046, India
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To TBT, Sluckin TJ, Luckhurst GR. Molecular field theory for biaxial nematics formed from liquid crystal dimers and inhibited by the twist-bend nematic. Phys Chem Chem Phys 2017; 19:29321-29332. [PMID: 29072729 DOI: 10.1039/c7cp04350c] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Liquid crystal dimers with odd spacers are good candidates as materials for biaxial nematic phases (NB). The dimers are flexible molecules sustaining biaxial conformations, and couplings between the conformational and orientational distributions could be expected to stabilise NB. We apply a molecular field theory for flexible molecules developed elsewhere to study a simple system made up of dimers composed of two cylindrically symmetric mesogenic groups. Our model allows for two idealised conformations: one linear and one bent at a tetrahedral angle. For a restricted set of chain lengths, the model predicts a first-order reentrant phase transition from the NB phase into a low temperature uniaxial nematic phase (NU). However the formation of the biaxial nematic could be blocked by the appearance of a twist-bent nematic.
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Affiliation(s)
- T B T To
- Mathematical Sciences, University of Southampton, Southampton, SO17 1BJ, UK.
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Querciagrossa L, Ricci M, Berardi R, Zannoni C. Can multi-biaxial mesogenic mixtures favour biaxial nematics? A computer simulation study. Phys Chem Chem Phys 2017; 19:2383-2391. [DOI: 10.1039/c6cp05117k] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A mixture of mesogens with different anisotropies favours the biaxial nematic phase.
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Affiliation(s)
- Lara Querciagrossa
- Department of Industrial Chemistry “Toso Montanari”
- University of Bologna
- 40136 Bologna
- Italy
| | - Matteo Ricci
- Department of Industrial Chemistry “Toso Montanari”
- University of Bologna
- 40136 Bologna
- Italy
| | - Roberto Berardi
- Department of Industrial Chemistry “Toso Montanari”
- University of Bologna
- 40136 Bologna
- Italy
| | - Claudio Zannoni
- Department of Industrial Chemistry “Toso Montanari”
- University of Bologna
- 40136 Bologna
- Italy
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8
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Aliev MA, Ugolkova EA, Kuzminyh NY. The Landau-de Gennes free energy expansion of a melt of V-shaped polymer molecules. J Chem Phys 2016; 145:084908. [PMID: 27586947 DOI: 10.1063/1.4961662] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The phase behavior of a monodisperse melt of polymer molecules consisting of two rod-like segments joined at an angle α has been inspected within the Landau theory of phase transitions. The interactions between monomer units were assumed to be of the Maier-Saupe form. The Landau-de Gennes expansion of the free energy of the melt has been obtained up to the sixth order in powers of the nematic order parameter, the coefficients of this expansion have been calculated from the microscopic model of polymer molecule. The phase diagram contains the regions of stability of isotropic, prolate uniaxial, oblate uniaxial, and biaxial nematic phases. The isotropic-uniaxial nematic and uniaxial-biaxial nematic transitions are of the first and second order, respectively. We found two Landau points in the phase diagram at which continuous transition from biaxial nematic state to isotropic phase occurs.
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Affiliation(s)
- M A Aliev
- Institute of Biochemical Physics, 4, Kosygina St., 119334 Moscow, Russia
| | - E A Ugolkova
- Institute of General and Inorganic Chemistry, 31 Leninsky Pr., 119991 Moscow, Russia
| | - N Yu Kuzminyh
- Institute of Biochemical Physics, 4, Kosygina St., 119334 Moscow, Russia
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9
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Teixeira PIC, Masters AJ. Phase behavior of shape-changing spheroids. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2015; 92:062506. [PMID: 26764710 DOI: 10.1103/physreve.92.062506] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/10/2015] [Indexed: 06/05/2023]
Abstract
We introduce a simple model for a biaxial nematic liquid crystal. This consists of hard spheroids that can switch shape between prolate (rodlike) and oblate (platelike) subject to an energy penalty Δε. The spheroids are approximated as hard Gaussian overlap particles and are treated at the level of Onsager's second-virial description. We use both bifurcation analysis and a numerical minimization of the free energy to show that, for additive particle shapes, (i) there is no stable biaxial phase even for Δε=0 (although there is a metastable biaxial phase in the same density range as the stable uniaxial phase) and (ii) the isotropic-to-nematic transition is into either one of two degenerate uniaxial phases, rod rich or plate rich. We confirm that even a small amount of shape nonadditivity may stabilize the biaxial nematic phase.
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Affiliation(s)
- P I C Teixeira
- ISEL-Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Rua Conselheiro Emídio Navarro 1, 1959-007 Lisbon, Portugal
- Centro de Física Teórica e Computacional, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, Edifício C8, 1749-016 Lisbon, Portugal
| | - A J Masters
- School of Chemical Engineering and Analytical Science, University of Manchester, Manchester M60 1QD, United Kingdom
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10
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Statistical thermodynamics of thermotropic biaxial nematic liquid crystals: An effective, molecular-field based theoretical description by means of a closed approximate form of the orientational partition function. J Mol Liq 2015. [DOI: 10.1016/j.molliq.2015.05.009] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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11
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Kamala Latha B, Jose R, Murthy KPN, Sastry VSS. Reexamination of the mean-field phase diagram of biaxial nematic liquid crystals: Insights from Monte Carlo studies. Phys Rev E 2015; 92:012505. [PMID: 26274193 DOI: 10.1103/physreve.92.012505] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2014] [Indexed: 11/07/2022]
Abstract
Investigations of the phase diagram of biaxial liquid-crystal systems through analyses of general Hamiltonian models within the simplifications of mean-field theory (MFT), as well as by computer simulations based on microscopic models, are directed toward an appreciation of the role of the underlying molecular-level interactions to facilitate its spontaneous condensation into a nematic phase with biaxial symmetry. Continuing experimental challenges in realizing such a system unambiguously, despite encouraging predictions from MFT, for example, are requiring more versatile simulational methodologies capable of providing insights into possible hindering barriers within the system, typically gleaned through its free-energy dependences on relevant observables as the system is driven through the transitions. The recent paper from this group [Kamala Latha et al., Phys. Rev. E 89, 050501(R) (2014)], summarizing the outcome of detailed Monte Carlo simulations carried out employing an entropic sampling technique, suggested a qualitative modification of the MFT phase diagram as the Hamiltonian is asymptotically driven toward the so-called partly repulsive regions. It was argued that the degree of (cross) coupling between the uniaxial and biaxial tensor components of neighboring molecules plays a crucial role in facilitating a ready condensation of the biaxial phase, suggesting that this could be a plausible factor in explaining the experimental difficulties. In this paper, we elaborate this point further, providing additional evidence from curious variations of free-energy profiles with respect to the relevant orientational order parameters, at different temperatures bracketing the phase transitions.
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Affiliation(s)
- B Kamala Latha
- School of Physics, University of Hyderabad, Hyderabad 500046, Telangana, India
| | - Regina Jose
- School of Physics, University of Hyderabad, Hyderabad 500046, Telangana, India
| | - K P N Murthy
- School of Physics, University of Hyderabad, Hyderabad 500046, Telangana, India
| | - V S S Sastry
- School of Physics, University of Hyderabad, Hyderabad 500046, Telangana, India
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Kamala Latha B, Jose R, Murthy KPN, Sastry VSS. Detection of an intermediate biaxial phase in the phase diagram of biaxial liquid crystals: entropic sampling study. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:050501. [PMID: 25353730 DOI: 10.1103/physreve.89.050501] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2014] [Indexed: 06/04/2023]
Abstract
We investigate the phase sequence of biaxial liquid crystals, based on a general quadratic model Hamiltonian over the relevant parameter space, with a Monte Carlo simulation which constructs equilibrium ensembles of microstates, overcoming possible (free) energy barriers (combining entropic and frontier sampling techniques). The resulting phase diagram qualitatively differs from the universal phase diagram predicted earlier from mean-field theory (MFT), as well as the Monte Carlo simulations with the Metropolis algorithm. The direct isotropic-to-biaxial transition predicted by the MFT is replaced in certain regions of the space by the onset of an additional intermediate biaxial phase of very low order, leading to the sequence N(B)-N(B1)-I. This is due to inherent barriers to fluctuations of the components comprising the total energy, and may explain the difficulties in the experimental realization of these phases.
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Affiliation(s)
- B Kamala Latha
- School of Physics, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India
| | - Regina Jose
- School of Physics, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India
| | - K P N Murthy
- School of Physics, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India
| | - V S S Sastry
- School of Physics, University of Hyderabad, Hyderabad 500046, Andhra Pradesh, India
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Kim YK, Senyuk B, Shin ST, Kohlmeier A, Mehl GH, Lavrentovich OD. Surface alignment, anchoring transitions, optical properties, and topological defects in the thermotropic nematic phase of organo-siloxane tetrapodes. SOFT MATTER 2014; 10:500-509. [PMID: 24651889 DOI: 10.1039/c3sm52249k] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
We perform optical, surface anchoring, and textural studies of an organo-siloxane "tetrapode" material in the broad temperature range of the nematic phase. The optical, structural, and topological features are compatible with the uniaxial nematic order rather than with the biaxial nematic order, in the entire nematic temperature range -25 °C < T < 46 °C studied. For homeotropic alignment, the material experiences surface anchoring transition, but the director can be realigned into an optically uniaxial texture by applying a sufficiently strong electric field. The topological features of textures in cylindrical capillaries, in spherical droplets and around colloidal inclusions are consistent with the uniaxial character of the long-range nematic order. In particular, we observe isolated surface point defects - boojums and bulk point defects - hedgehogs that can exist only in the uniaxial nematic liquid crystal.
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Affiliation(s)
- Young-Ki Kim
- Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent, OH 44242, USA.
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Bisi F, De Matteis G, Romano S. Calamitic and antinematic orientational order produced by the generalized Straley lattice model. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 88:032502. [PMID: 24125280 DOI: 10.1103/physreve.88.032502] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2013] [Indexed: 06/02/2023]
Abstract
We consider here a classical model, consisting of D_{2h}-symmetric particles in a three-dimensional simple-cubic lattice; the pair potential is isotropic in orientation space, and restricted to nearest neighbors. The simplest potential model is written in terms of the squares of the scalar products between unit vectors describing the three interacting arms of the molecules, as proposed in previous literature. Two predominant antinematic couplings of equal strength (+1) are perturbed by a comparatively weaker calamitic one, parameterized by a coupling constant -z ranging in [-1,0]. This choice rules out thermodynamically stable phases endowed with macroscopic biaxiality. The antinematic terms favor states with the corresponding molecular axes mutually orthogonal. Although the low-temperature phase of the special case with null calamitic term (PP0) is uniaxial and antinematically ordered, in the general case presented here both Monte Carlo and molecular-field approaches show that, for z close to zero, the models exhibit a low-temperature uniaxial nematic phase, followed by an antinematic one, and finally by the orientationally disordered one. On the other hand, for sufficiently large values of z, we only find evidence of uniaxial calamitic behavior, as expected by following the limiting cases.
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Affiliation(s)
- Fulvio Bisi
- Dipartimento di Matematica "F. Casorati", Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy
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Cifelli M, Domenici V, Veracini CA. Recent advancements in understanding thermotropic liquid crystal structure and dynamics by means of NMR spectroscopy. Curr Opin Colloid Interface Sci 2013. [DOI: 10.1016/j.cocis.2013.03.003] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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16
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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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Matsuyama A. Novel biaxial nematic phases of side-chain liquid crystalline polymers. J Chem Phys 2012; 137:224906. [PMID: 23249031 DOI: 10.1063/1.4771592] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We present a mean field theory to describe biaxial nematic phases of side-chain liquid crystalline polymers, in which rigid side-chains (mesogens) and rigid-backbone chains favor mutually perpendicular orientations. Taking into account both excluded volume and attractive interactions between rigid rods, novel biaxial nematic phases are theoretically predicted. We calculate uniaxial and biaxial orientational order parameters as a function of temperature and the length of backbone chains. We find a first-order biaxial-biaxial phase transition and a first (or second)-order uniaxial-biaxial one, depending on the length of mesogens and backbone chains.
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Affiliation(s)
- Akihiko Matsuyama
- Department of Bioscience and Bioinformatics, Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, Kawazu 680-4, Iizuka, Fukuoka 820-8502, Japan.
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Aluculesei A, Vaca Chávez F, Cruz C, Sebastião PJ, Nagaveni NG, Prasad V, Dong RY. Proton NMR Relaxation Study on the Nematic–Nematic Phase Transition in A131 Liquid Crystal. J Phys Chem B 2012; 116:9556-63. [DOI: 10.1021/jp305064x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- A Aluculesei
- Department of Physics, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
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19
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Osipov MA, Pająk G. Influence of dipole-dipole correlations on the stability of the biaxial nematic phase in the model bent-core liquid crystal. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:142201. [PMID: 22388661 DOI: 10.1088/0953-8984/24/14/142201] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
A molecular theory of biaxial nematic ordering in the system of bent-core molecules has been developed in the two-particle cluster approximation which enables one to take into account short-range polar correlations determined by both electrostatic dipole-dipole interaction and polar molecular shape. All orientational order parameters and short-range correlation functions are calculated numerically as functions of temperature in the uniaxial and in the biaxial nematic phases, and the results are compared with the ones obtained in the mean-field approximation and in the cluster approximation but without taking into consideration the dipole-dipole interaction. It is shown that short-range polar correlations and, in particular, the dipole-dipole correlations dramatically increase the temperature of the transition into the biaxial nematic phase and enhancing its stability range. The results are also very sensitive to the value of the opening angle of a model bent-core molecule.
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Affiliation(s)
- Mikhail A Osipov
- Department of Mathematics and Statistics, University of Strathclyde, Livingstone Tower, Richmond Street, Glasgow, G1 1XH, UK.
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Merkel K, Nagaraj M, Kocot A, Kohlmeier A, Mehl GH, Vij JK. Biaxial order and a rotation of the minor director in the nematic phase of an organo-siloxane tetrapode by the electric field. J Chem Phys 2012; 136:094513. [PMID: 22401458 DOI: 10.1063/1.3690108] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Biaxiality in the nematic phase for a liquid crystalline tetrapode made up of organo-siloxanes mesogens is investigated using polarized infrared spectroscopy. An ordering of the minor director for the homeotropically aligned sample is found to depend on the amplitude of the in-plane electric field. On increasing the in-plane electric field, the minor director, lying initially along the rubbing direction, rotates to the direction of the applied field. The scalar order parameters of the second rank tensor are found to depend significantly on the strength of the electric field. A most significant increase is found in the nematic order parameter and in the parameter that characterizes the phase biaxiality.
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Affiliation(s)
- K Merkel
- Central Mining Institute, Plac Gwarkow 1, Katowice, Poland
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21
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Speetjens F, Lindborg J, Tauscher T, LaFemina N, Nguyen J, Samulski ET, Vita F, Francescangeli O, Scharrer E. Low nematic onset temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bent-core mesogens possessing lateral methyl groups. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c2jm33705c] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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22
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Hernández-Ainsa S, Barberá J, Marcos M, Serrano JL. Liquid Crystalline Ionic Dendrimers Containing Luminescent Oxadiazole Moieties. Macromolecules 2011. [DOI: 10.1021/ma202051c] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Affiliation(s)
- Silvia Hernández-Ainsa
- Departamento
de Química
Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales
de Aragón, Universidad de Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
| | - Joaquín Barberá
- Departamento
de Química
Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales
de Aragón, Universidad de Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
| | - Mercedes Marcos
- Departamento
de Química
Orgánica, Facultad de Ciencias-Instituto de Ciencia de Materiales
de Aragón, Universidad de Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
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23
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Berardi R, Lintuvuori JS, Wilson MR, Zannoni C. Phase diagram of the uniaxial and biaxial soft–core Gay–Berne model. J Chem Phys 2011; 135:134119. [DOI: 10.1063/1.3646310] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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24
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Tallavaara P, Kantola A, Jokisaari J, Meyer T, Wilson D, Mehl G. 129Xe and 2H nuclear magnetic resonance (NMR) of xenon and deuterated-chloroform solutes in a thermotropic biaxial nematic liquid crystal. CAN J CHEM 2011. [DOI: 10.1139/v11-062] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Properties of a thermotropic nematic liquid crystal (LC) TM35 (organosiloxane tetrapode) were investigated by means of nuclear magnetic resonance (NMR) spectroscopy of probe atoms and molecules. 129Xe isotope enriched xenon gas and 2H isotope enriched chloroform were used as probes. Both 129Xe and 2H NMR measurements were carried out over a wide temperature range, covering isotropic, nematic, and glass phases. A clear change in the behavior of the LC is observed within the nematic phase at 289 K by changing the temperature from low to high and inspecting the spectral line width. The derivative curve of the 129Xe chemical shift also reveals a change at 289 K. These changes may be an indication of hindered molecular rotation around the long axis, and the appearance of a biaxial nematic phase.
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Affiliation(s)
- P. Tallavaara
- NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland
| | - A.M. Kantola
- NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland
| | - J. Jokisaari
- NMR Research Group, Department of Physics, University of Oulu, P.O. Box 3000, FI-90014 University of Oulu, Finland
| | - T. Meyer
- Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
| | - D.A. Wilson
- Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
| | - G.H. Mehl
- Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
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25
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Jokisaari JP, Kantola AM, Lounila JA, Ingman LP. Detection of phase biaxiality in liquid crystals by use of the quadrupole shift in 131Xe NMR spectra. PHYSICAL REVIEW LETTERS 2011; 106:017801. [PMID: 21231771 DOI: 10.1103/physrevlett.106.017801] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/06/2010] [Indexed: 05/30/2023]
Abstract
An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in 131Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the 131Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the 131Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases.
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Affiliation(s)
- Jukka P Jokisaari
- NMR Research Group, Department of Physics, University of Oulu, Finland.
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26
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Figueirinhas JL, Feio G, Cruz C, Lehmann M, Köhn C, Dong RY. Nuclear magnetic resonance spectroscopic investigations of phase biaxiality in the nematic glass of a shape-persistent V-shaped mesogen. J Chem Phys 2010; 133:174509. [DOI: 10.1063/1.3496491] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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27
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Osipov MA, Gorkunov MV. Ferroelectricity in low-symmetry biaxial nematic liquid crystals. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:362101. [PMID: 21386515 DOI: 10.1088/0953-8984/22/36/362101] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Order parameters and phenomenological theory for both high- and low-symmetry biaxial nematic phases are presented and it is predicted that the chiral low-symmetry biaxial phase must be ferroelectric. This conclusion is based on general symmetry arguments and on the results of the Landau-de Gennes theory. The microscopic mechanism of the ferroelectric ordering in this chiral biaxial phase is illustrated using a simple molecular model based on dispersion interactions between biaxial molecules of low symmetry. Similar to the chiral smectic C* phase, the ferroelectricity in the chiral biaxial nematic phase is improper, i.e., polarization is not a primary order parameter and is not determined by dipolar interactions. Ferroelectric ordering in biaxial nematics may be found, in principle, in materials composed of chiral analogues of the tetrapod molecules which are known to exhibit biaxial phases.
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Affiliation(s)
- Mikhail A Osipov
- Department of Mathematics and Statistics, University of Strathclyde, Glasgow G1 1XH, UK.
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28
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Park MS, Yoon BJ, Park JO, Prasad V, Kumar S, Srinivasarao M. Raman scattering study of phase biaxiality in a thermotropic bent-core nematic liquid crystal. PHYSICAL REVIEW LETTERS 2010; 105:027801. [PMID: 20867740 DOI: 10.1103/physrevlett.105.027801] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2009] [Indexed: 05/29/2023]
Abstract
Polarized Raman spectroscopy was used to investigate the development of orientational order and the degree of phase biaxiality in a bent-core mesogenic system. The values of the uniaxial order parameters <P200> and <P400>, and biaxial order parameters <P220>, <P420>, and <P440>, and their evolution with temperature were determined. The temperature dependence of almost all order parameters reveals a second order transition from the uniaxial to biaxial nematic phase with <P220> increasing to ∼0.22 before a first order transition to the smectic-C phase, upon cooling.
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Affiliation(s)
- Min Sang Park
- School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
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29
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Lehmann M, Köhn C, Figueirinhas J, Feio G, Cruz C, Dong R. Biaxial Nematic Mesophases from Shape-Persistent Mesogens with a Fluorenone Bending Unit. Chemistry 2010; 16:8275-9. [DOI: 10.1002/chem.201001214] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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30
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Gorkunov MV, Osipov MA, Kocot A, Vij JK. Molecular model of biaxial ordering in nematic liquid crystals composed of flat molecules with four mesogenic groups. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:061702. [PMID: 20866427 DOI: 10.1103/physreve.81.061702] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/19/2010] [Revised: 05/17/2010] [Indexed: 05/29/2023]
Abstract
Relative stability of uniaxial and biaxial nematic phases is analyzed in a model nematic liquid crystal composed of flat molecules of C2h symmetry with four mesogenic groups rigidly linked to the same center. The generalized effective quadrupole mean-field potential is proposed and its constants are evaluated numerically for the pair intermolecular potential based on Gay-Berne interaction between mesogenic groups. The dependencies of the constants on molecular shape parameters are systematically analyzed. Order parameters of the uniaxial and biaxial nematic phases are evaluated by direct minimization of the free energy at different temperatures. The corresponding phase diagrams are obtained enabling one to study the effects of molecular model parameters on the stability regions of uniaxial and biaxial phases. The results are used to clarify the nature of experimentally observed biaxial ordering in nematic liquid crystals composed of tetrapode molecules with the same symmetry.
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Affiliation(s)
- M V Gorkunov
- Department of Electronic and Electrical Engineering, Trinity College Dublin, Dublin 2, Ireland
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31
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Yoon HG, Kang SW, Dong RY, Marini A, Suresh KA, Srinivasarao M, Kumar S. Nematic biaxiality in a bent-core material. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2010; 81:051706. [PMID: 20866246 DOI: 10.1103/physreve.81.051706] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2009] [Revised: 02/21/2010] [Indexed: 05/29/2023]
Abstract
The results of a recent investigation of the nematic biaxiality in a bent-core mesogen (A131) are in apparent disagreement with earlier claims. Samples of mesogen A131 used in the two studies were investigated with polarized optical microscopy, conoscopy, carbon-13 NMR, and crossover frequency measurements. The results demonstrate that textural changes associated with the growth of biaxial nematic order appear at ∼149 °C. The Maltese cross observed in the conoscopic figure gradually splits into two isogyres at lower temperatures indicating phase biaxiality. Presence of the uniaxial to biaxial nematic phase transition is further confirmed by temperature trends of local order parameters based on 13C chemical shifts in NMR experiments. Frequency switching measurements also clearly reveal a transition at 149 °C. Differences between the two reports appear to be related to the presence of solvent, impurities, and/or adsorbed gases in samples of A131 used in the study of Van Le [Phys. Rev. E 79, 030701 (2009)].
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Affiliation(s)
- Hyung Guen Yoon
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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32
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Van-Quynh A, Sebastião PJ, Wilson DA, Mehl GH. Detecting columnar deformations in a supermesogenic octapode by proton NMR relaxometry. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2010; 31:275-283. [PMID: 20300797 DOI: 10.1140/epje/i2010-10575-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/15/2009] [Revised: 01/18/2010] [Accepted: 02/01/2010] [Indexed: 05/29/2023]
Abstract
We used proton ( (1)H nuclear magnetic relaxation (NMR) dispersions to study the molecular dynamics in the isotropic phase and mesophases (nematic and columnar hexagonal) of a supermesogenic octapode formed by laterally connecting calamitic mesogens to an inorganic silsesquioxane cube through flexible spacers. The dispersions of the spin-lattice relaxation time (T(1)) are interpreted through relaxation mechanisms used for the study of molecular dynamics in low-molar-mass liquid crystals but adapted to the case of liquid crystalline supermolecules. At high frequencies (above 10MHz) the behaviour of the T(1) with the Larmor frequency is similar for all phases and is ascribed to local reorientations and/or rotations. At intermediate and low frequencies (below 10MHz) our results show notable differences in the T(1) behaviour with respect to the mesophases. The nematic (N) and isotropic (Iso) phases' low-frequency results are similar and are interpreted for both phases in terms of order director fluctuations (ODF), revealing that even in the isotropic phase local nematic order is detected by proton NMR relaxometry. Local nematic order in the Iso phase is interpreted in terms of the presence of nematic cybotactic clusters induced by the interdigitation of mesogens that is promoted by the silsesquioxane octapode molecular structure. In the columnar hexagonal (Col (h) phase, the T(1) dispersions show that elastic columnar deformations (ECD) dominate the nuclear magnetic relaxation below 10MHz. This result shows that the columnar packing of the octapode clearly restricts the collective fluctuations of the mesogenic units inspite of their local nematic order.
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Affiliation(s)
- A Van-Quynh
- Centro de Física da Matéria Condensada, Universidade de Lisboa, Av. Gama Pinto, 1649-003, Lisboa, Portugal.
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Lebar A, Cordoyiannis G, Kutnjak Z, Zalar B. The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers. LIQUID CRYSTAL ELASTOMERS: MATERIALS AND APPLICATIONS 2010. [DOI: 10.1007/12_2010_103] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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34
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35
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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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36
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Lehmann M, Seltmann J. Low temperature enantiotropic nematic phases from V-shaped, shape-persistent molecules. Beilstein J Org Chem 2009; 5:73. [PMID: 20300458 PMCID: PMC2839554 DOI: 10.3762/bjoc.5.73] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2009] [Accepted: 11/17/2009] [Indexed: 11/30/2022] Open
Abstract
A series of V-shaped, shape-persistent thiadiazole nematogens, based on an oligo(phenylene ethynylene) scaffold with ester groups connected via alkyloxy spacers, was efficiently prepared by a two-step procedure. Phase engineering results in an optimum of the mesophase range and low melting temperature when the nematogens are desymmetrised with a butoxy and a heptyloxy spacer. The mesophases are enantiotropic and over the whole temperature range nematic. For the optimised mesogen structure, optical investigations by conoscopy monitored a uniaxial nematic phase upon cooling from the isotropic phase to room temperature (ΔT = 150 °C). X-ray studies on magnetic-field-aligned samples of this mesogen family revealed a general pattern, indicating the alignment of two molecular axes along individual directors in the magnetic field. These observations may be rationalised with larger assemblies of V-shaped molecules isotropically distributed around the direction of the magnetic field.
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Affiliation(s)
- Matthias Lehmann
- Institute of Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany.
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37
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Karahaliou PK, Vanakaras AG, Photinos DJ. Symmetries and alignment of biaxial nematic liquid crystals. J Chem Phys 2009; 131:124516. [DOI: 10.1063/1.3226560] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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38
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De Matteis G, Romano S. Mesogenic lattice models with partly antinematic interactions producing uniaxial nematic phases. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2009; 80:031702. [PMID: 19905126 DOI: 10.1103/physreve.80.031702] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/18/2009] [Indexed: 05/28/2023]
Abstract
The present paper considers nematogenic lattice models, involving particles of D_{2h} symmetry, whose centers of mass are associated with a three-dimensional simple cubic lattice; the pair potential is isotropic in orientation space and restricted to nearest neighbors. Let two orthonormal triads define orientations of a pair of interacting particles; the simplest potential models proposed in the literature can be reduced to a linear combination involving the squares of the scalar products between corresponding unit vectors only and depending on three parameters. By now, various sets of potential parameters have been proposed and studied in the literature, some of which capable of producing biaxial orientational order at sufficiently low temperature. On the other hand, in experimental terms, mesogenic biaxial molecules mostly produce uniaxial mesophases; thus we address here two very simple cases, involving a nematic (calamitic) term as well as one (model P0M) or two (model PPM) antinematic ones, whose coefficients are set equal in magnitude; when only one antinematic coefficient is used, the third one is set to zero. The calamitic term favors the alignment of two corresponding molecular axes, whereas antinematic terms or geometric constraints tend to keep two other pairs of axes mutually orthogonal. The models were investigated by molecular-field treatments and Monte Carlo simulation and found to predict a first- or second-order transitions between uniaxial nematic and isotropic phases; the molecular-field treatments yielded results in reasonable agreement with simulation.
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Affiliation(s)
- Giovanni De Matteis
- Centro di Ricerca Matematica Ennio De Giorgi, Collegio Puteano, Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 3, I-56100 Pisa, Italy.
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Cuetos A, Galindo A, Jackson G. Thermotropic biaxial liquid crystalline phases in a mixture of attractive uniaxial rod and disk particles. PHYSICAL REVIEW LETTERS 2008; 101:237802. [PMID: 19113595 DOI: 10.1103/physrevlett.101.237802] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/04/2008] [Indexed: 05/27/2023]
Abstract
We present the first examples of thermotropic biaxial mesophases in mixtures of model attractive uniaxial molecules. Nematic and smectic biaxial phases are identified. The nematic biaxial phase has remained a key challenge in the science of liquid crystals since it was first proposed almost 40 years ago. Very recently the first experimental evidence of stable biaxial nematic phases has been obtained in thermotropic liquid crystals of single component biaxial mesogens. Still elusive is the possibility of stabilizing biaxial nematic phases in mixtures of uniaxial particles. We use Monte Carlo molecular simulations to study model mixtures of rodlike and disklike molecules interacting through two intermolecular potential models: one incorporating spherically symmetric (isotropic) attractive interactions; another with anisotropic attractive interactions. These models exhibit nematic and smectic biaxial phases.
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Affiliation(s)
- Alejandro Cuetos
- Complex Fluid Physics Group, Departamento de Física Aplicada, Universidad de Almería, 04120 Almería, Spain
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Berardi R, Muccioli L, Orlandi S, Ricci M, Zannoni C. Computer simulations of biaxial nematics. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2008; 20:463101. [PMID: 21693834 DOI: 10.1088/0953-8984/20/46/463101] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Biaxial nematic (N(b)) liquid crystals are a fascinating condensed matter phase that has baffled, for more than thirty years, scientists engaged in the challenge of demonstrating its actual existence, and which has only recently been experimentally found. During this period computer simulations of model N(b) have played an important role, both in providing the basic physical properties to be expected from these systems, and in giving clues about the molecular features essential for the thermodynamic stability of N(b) phases. However, simulation studies are expected to be even more crucial in the future for unravelling the structural features of biaxial mesogens at the molecular level, and for helping in the design and optimization of devices towards the technological deployment of N(b) materials. This review article gives an overview of the simulation work performed so far, and relying on the recent experimental findings, focuses on the still unanswered questions which will determine the future challenges in the field.
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Affiliation(s)
- Roberto Berardi
- Dipartimento di Chimica Fisica e Inorganica, and INSTM-CRIMSON, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
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41
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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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De Matteis G, Romano S. Biaxial and uniaxial phases produced by partly repulsive mesogenic models involving D2h molecular symmetries. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:021702. [PMID: 18850847 DOI: 10.1103/physreve.78.021702] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/24/2007] [Revised: 04/22/2008] [Indexed: 05/26/2023]
Abstract
The present paper considers biaxial nematogenic lattice models, involving particles of D2h symmetry, whose centers of mass are associated with a three-dimensional simple-cubic lattice. The pair potential is isotropic in orientation space and restricted to nearest neighbors. Let two orthonormal triads define orientations of a pair of interacting particles. The investigated potential models are quadratic with respect to the nine scalar products between the two sets of unit vectors. Actually, based on available geometric identities, these expressions can be reduced to diagonal form containing only the scalar products between corresponding unit vectors and depending on three parameters. Over the years, this comparatively simple functional form has also proven to be rather versatile. By now, various sets of potential parameters capable of producing mesogenic behavior of some kind have been proposed and studied in the literature. A new and simplified form was recently proposed and investigated by Sonnet, Virga, Durand, and De Matteis [A. M. Sonnet, E. G. Virga, and G. E. Durand, Phys. Rev. E 67, 061701 (2003); G. De Matteis and E. G. Virga, Phys. Rev. E 71, 061703 (2005)] and is known to support a biaxial phase at sufficiently low temperature. Following the idea of the above authors, we have studied a more extended range of parameters, including cases where biaxiality cannot be sustained in the pair ground state. In cases where a biaxial phase survives, an appropriate mean-field analysis may predict the existence of a direct second-order transition to the isotropic phase as well as a second-order sequence isotropic-to-uniaxial-to-biaxial. A second-order phase transition is also predicted, which involves isotropic and uniaxial phases only. Monte Carlo simulations have been carried out as well, for a few points in the parameter space, and found to produce results which partly confirm mean-field predictions.
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Affiliation(s)
- Giovanni De Matteis
- Centro di Ricerca Matematica Ennio De Giorgi, Collegio Puteano, Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 3, I-56100 Pisa, Italy.
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43
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Bisi F, Luckhurst GR, Virga EG. Dominant biaxial quadrupolar contribution to the nematic potential of mean torque. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:021710. [PMID: 18850855 DOI: 10.1103/physreve.78.021710] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2008] [Indexed: 05/26/2023]
Abstract
Within the general quadrupolar model for biaxial nematic liquid crystals, whose potential of mean torque extends that in the Maier-Saupe theory with two extra interaction terms, we propose a quantitative criterion to identify the dominant biaxial interaction. We show that the ratio of the biaxial-to-uniaxial and uniaxial-to-isotropic transition temperatures is almost independent of one interaction parameter, thus indicating the other as dominant. We also show that there is a significant mismatch between the principal orientational order parameters predicted by the theory and those measured for the biaxial phase of a tetrapode.
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Affiliation(s)
- Fulvio Bisi
- Dipartimento di Matematica and CNISM, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy
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44
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Cordoyiannis G, Apreutesei D, Mehl GH, Glorieux C, Thoen J. High-resolution calorimetric study of a liquid crystalline organo-siloxane tetrapode with a biaxial nematic phase. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2008; 78:011708. [PMID: 18763974 DOI: 10.1103/physreve.78.011708] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2008] [Indexed: 05/26/2023]
Abstract
High-resolution adiabatic scanning calorimetry and differential scanning calorimetry have been employed to study the thermal behavior of an organo-siloxane tetrapode reported to exhibit a biaxial nematic phase. No signature of the uniaxial to biaxial nematic phase transition could be retraced in sequential heating and cooling runs under different scanning rates, within the experimental resolution. The results obtained reveal that an extremely small heat should be involved in the uniaxial to biaxial nematic phase transition. The isotropic to uniaxial nematic transition at 318+/-0.01 K is very stable, and it is weakly first order with a rather small latent heat of 0.20+/-0.02 J/g .
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Affiliation(s)
- George Cordoyiannis
- Laboratorium voor Akoestiek en Thermische Fysica, Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, Leuven, Belgium
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Karahaliou PK, Kouwer PHJ, Meyer T, Mehl GH, Photinos DJ. Long- and Short-Range Order in the Mesophases of Laterally Substituted Calamitic Mesogens and their Radial Octapodes. J Phys Chem B 2008; 112:6550-6. [DOI: 10.1021/jp712182v] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Panagiota K. Karahaliou
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Paul H. J. Kouwer
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Thomas Meyer
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Georg H. Mehl
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - Demetri J. Photinos
- Department of Materials Science, University of Patras, 265 04 Patras, Greece, and Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
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46
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Vanakaras AG, Photinos DJ. Thermotropic biaxial nematic liquid crystals: Spontaneous or field stabilized? J Chem Phys 2008; 128:154512. [DOI: 10.1063/1.2897993] [Citation(s) in RCA: 107] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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47
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Skacej G, Zannoni C. Biaxial liquid-crystal elastomers: a lattice model. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2008; 25:181-186. [PMID: 18357411 DOI: 10.1140/epje/i2007-10280-6] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2007] [Accepted: 01/28/2008] [Indexed: 05/26/2023]
Abstract
We present a simple coarse-grained lattice model for monodomain biaxial liquid-crystal elastomers and perform large-scale Monte Carlo simulations in the proposed model system. Orientational ordering--uniaxial or biaxial--reflects in sample deformations on cooling the system. The simulation output is used to predict calorimetry data and deuterium magnetic resonance spectra.
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Affiliation(s)
- G Skacej
- Fakulteta za matematiko in fiziko, Univerza v Ljubljani, Jadranska 19, SI-1000, Ljubljana, Slovenia.
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Berardi R, Muccioli L, Zannoni C. Field response and switching times in biaxial nematics. J Chem Phys 2008; 128:024905. [DOI: 10.1063/1.2815804] [Citation(s) in RCA: 93] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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49
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High nematic ordering in a bent-core smectogen showing a biaxial nematic phase: A 13C NMR study. Chem Phys Lett 2007. [DOI: 10.1016/j.cplett.2007.09.070] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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50
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Cseh L, Mehl GH. Structure–property relationships in nematic gold nanoparticles. ACTA ACUST UNITED AC 2007. [DOI: 10.1039/b614046g] [Citation(s) in RCA: 76] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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