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Imrie CT, Paterson DA, Storey JMD, Chamignon C, Lelli M, Emsley JW, Luckhurst GR. Phase transitions in a high magnetic field of an odd, symmetric liquid crystal dimer having two nematic phases, N_{U} and N_{TB}, studied by NMR spectroscopy. Phys Rev E 2020; 102:042706. [PMID: 33212702 DOI: 10.1103/physreve.102.042706] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2019] [Accepted: 07/31/2020] [Indexed: 11/07/2022]
Abstract
Both ^{1}H and ^{13}C NMR spectra have been obtained in a static magnetic field of 23.5 T on a bent-shaped dimer molecule, 1^{''},7^{''}-bis(4-cyanobiphenyl-4'-yl) nonane (CB9CB), which shows the sequence of liquid crystal phases twist-bend nematic, N_{TB}, and uniaxial nematic, N_{U}, before entering the isotropic phase. The ^{1}H spectra are used to locate the temperature at which the sample melts to form a twist-bend nematic, T_{CrN_{TB}}, and then T_{N_{U}I} when the isotropic phase is entered, both in a magnetic field of 23.5 T, and to compare these with those measured at the Earth's field. The differences between these transition temperatures are found to be zero within the error in their measurement, in stark contrast to previous measurements by Salili et al. [Phys. Rev. Lett. 116, 217801 (2016)10.1103/PhysRevLett.116.217801]. In the isotropic phase in the presence of the field the sample exists in a paranematic phase in which the molecules of CB9CB are partially ordered. The ^{1}H and ^{13}C NMR spectra in the paranematic phase are used to measure the critical temperature T* below which this phase is unstable. The spectra are also used to study the structure, molecular orientational order, and distribution of molecular conformations in the paranematic phase.
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Affiliation(s)
- C T Imrie
- Department of Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen AB24 3FX, United Kingdom
| | - D A Paterson
- Department of Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen AB24 3FX, United Kingdom
| | - J M D Storey
- Department of Chemistry, School of Natural and Computing Sciences, University of Aberdeen, Aberdeen AB24 3FX, United Kingdom
| | - C Chamignon
- Centre de RMN à Très Hauts Champs de Lyon (FRE 2034-CNRS, UCB Lyon 1, ENS Lyon), 5 rue de la Doua, 69100 Villeurbanne, France
| | - M Lelli
- Department of Chemistry "Ugo Schiff," Center for Magnetic Resonance, University of Florence, Sesto Fiorentino (FI) 50019, Italy
| | - J W Emsley
- School of Chemistry, Faculty of Engineering and Physical Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom
| | - G R Luckhurst
- School of Chemistry, Faculty of Engineering and Physical Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom
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Paladugu S, Kaur S, Mohiuddin G, Pujala RK, Pal SK, Dhara S. Microrheology to probe smectic clusters in bent-core nematic liquid crystals. SOFT MATTER 2020; 16:7556-7561. [PMID: 32706008 DOI: 10.1039/d0sm00796j] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Many bent-core nematic liquid crystals exhibit unusual physical properties due to the presence of smectic clusters, known as "cybotactic" clusters, in the nematic phase. Here, we investigate the effect of these clusters on the complex shear modulus (G*(ω)) of two asymmetric bent-core liquid crystals using a microrheological technique. The compound with a shorter hydrocarbon chain (8OCH3) exhibits only a nematic (N) phase whereas the compound with a longer chain (16OCH3) exhibits both nematic (N) and smectic-A (SmA) phases. The rheological results are correlated with the measurements of curvature elastic constants. Our results show that the directional shear modulus of 16OCH3, just above the SmA to N phase transition temperature, is strikingly different than that of 8OCH3, owing to the smectic clusters. An approximate size of the clusters is estimated using a simple model. Therefore, microrheological studies on bent-core nematic liquid crystals are very useful in extracting information about underlying smectic clusters.
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Affiliation(s)
- Sathyanarayana Paladugu
- School of Physics, University of Hyderabad, Hyderabad, India. and Soft and Active Matter Group, Department of Physics, Indian Institute of Science Education and Research (IISER), Tirupati, Andhra Pradesh 517507, India
| | - Supreet Kaur
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Golam Mohiuddin
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Ravi Kumar Pujala
- Soft and Active Matter Group, Department of Physics, Indian Institute of Science Education and Research (IISER), Tirupati, Andhra Pradesh 517507, India
| | - Santanu Kumar Pal
- Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, Knowledge City, Manauli 140306, India
| | - Surajit Dhara
- School of Physics, University of Hyderabad, Hyderabad, India.
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Magnetic Field and Dilution Effects on the Phase Diagrams of Simple Statistical Models for Nematic Biaxial Systems. CRYSTALS 2020. [DOI: 10.3390/cryst10080632] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
We use a simple statistical model to investigate the effects of an applied magnetic field and of the dilution of site elements on the phase diagrams of biaxial nematic systems, with an emphasis on the stability of the Landau multicritical point. The statistical lattice model consists of intrinsically biaxial nematogenic units, which interact via a Maier–Saupe potential, and which are characterized by a discrete choice of orientations of the microscopic nematic directors. According to previous calculations at zero field and in the absence of dilution, we regain the well-known sequence of biaxial, uniaxial, and disordered structures as the temperature is increased, and locate the Landau point. We then focus on the topological changes induced in the phase diagram by the application of an external magnetic field, and show that the Landau point is destabilized by the presence of an applied field. On the other hand, in the absence of a field, we show that only a quite strong dilution of nematic sites is capable of destabilizing the Landau point.
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Aliev M, Ugolkova E, Kuzminyh N. Effect of an external magnetic field on the phase behavior of the thermotropic melt of V-shaped molecules. J Mol Liq 2019. [DOI: 10.1016/j.molliq.2019.111395] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Chiappini M, Drwenski T, van Roij R, Dijkstra M. Biaxial, Twist-bend, and Splay-bend Nematic Phases of Banana-shaped Particles Revealed by Lifting the "Smectic Blanket". PHYSICAL REVIEW LETTERS 2019; 123:068001. [PMID: 31491177 DOI: 10.1103/physrevlett.123.068001] [Citation(s) in RCA: 31] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/11/2019] [Revised: 06/20/2019] [Indexed: 06/10/2023]
Abstract
We perform an extensive computational study on the phase behavior of hard banana-shaped particles, and show that biaxial, twist-bend, and splay-bend nematic phases are metastable with respect to a smectic phase for a system of hard bent spherocylinders. However, if the smectic phase is destabilized-either by polydispersity in the particle length or by curvature in the particle shape-stable biaxial, twist-bend, and splay-bend nematic phases are obtained. This provides a unified and consistent picture on the subtle role of particle shape on the phase behavior of bent rods.
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Affiliation(s)
- Massimiliano Chiappini
- Soft Condensed Matter, Debye Institute for Nanomaterials Science, Department of Physics, Utrecht University, Princetonplein 1, Utrecht 3584 CC, The Netherlands
| | - Tara Drwenski
- Institute for Theoretical Physics, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
| | - René van Roij
- Institute for Theoretical Physics, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
| | - Marjolein Dijkstra
- Soft Condensed Matter, Debye Institute for Nanomaterials Science, Department of Physics, Utrecht University, Princetonplein 1, Utrecht 3584 CC, The Netherlands
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Ghoshal N, Pramanick S, DasGupta S, Roy SK. Monte Carlo study with reweighting of uniaxial nematic liquid crystals composed of biaxial molecules. Phys Rev E 2019; 99:022703. [PMID: 30934352 DOI: 10.1103/physreve.99.022703] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2018] [Indexed: 06/09/2023]
Abstract
We present a high accuracy Monte Carlo simulation study of the uniaxial nematic (N_{U}) to isotropic (I) phase transition of a lattice dispersion model of uniaxial nematics composed of biaxial molecules. The N_{U}-I coexistence curve terminating at the Landau critical point has been determined using the multiple histogram reweighting technique. A close investigation reveals a sharp departure in the nature of the N_{U}-I coexistence curve in the temperature-biaxiality parameter phase diagram in comparison to the earlier theoretical (either mean-field or computer simulation) predictions. The coexistence curve shows a change in curvature with increasing value of the degree of molecular biaxiality.
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Affiliation(s)
- Nababrata Ghoshal
- Department of Physics, Mahishadal Raj College, Mahishadal, Purba Medinipur, West Bengal, India
| | | | - Sudeshna DasGupta
- Department of Physics, Lady Brabourne College, Kolkata 700 017, India
| | - Soumen Kumar Roy
- Department of Physics, Jadavpur University, Kolkata 700 032, India
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Madhusudana NV. Two-state model for nematic liquid crystals made of bent-core molecules. Phys Rev E 2017; 96:022710. [PMID: 28950498 DOI: 10.1103/physreve.96.022710] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2017] [Indexed: 11/07/2022]
Abstract
Nematic (N) liquid crystals made of bent-core molecules exhibit unusual physical properties such as an intermediate phase between the N and isotropic (I) phases, a very weak NI transition as inferred from magnetic birefringence measurements in a low field, which is apparently incompatible with a large shift in the NI transition temperature (T_{ni}) measured under a high field. Using our conformational studies on the aromatic cores, we propose that only conformers which are more straightened than those in the ground state (GS) form clusters with a few layers, which persist even in the isotropic phase, as inferred from x-ray and rheological experiments. We present a Landau-de Gennes theory of the medium, including an orientational coupling between the clusters and the GS molecules, which accounts for all the unusual properties. The intermediate phase to isotropic transition is predicted to exhibit critical behavior at a very low magnetic field of <1kG.
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Affiliation(s)
- N V Madhusudana
- Raman Research Institute, C. V. Raman Avenue, Bengaluru 560080, India
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Salili SM, Tamba MG, Sprunt SN, Welch C, Mehl GH, Jákli A, Gleeson JT. Anomalous Increase in Nematic-Isotropic Transition Temperature in Dimer Molecules Induced by a Magnetic Field. PHYSICAL REVIEW LETTERS 2016; 116:217801. [PMID: 27284674 DOI: 10.1103/physrevlett.116.217801] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2016] [Indexed: 06/06/2023]
Abstract
We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15 K when subjected to a 22 T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.
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Affiliation(s)
- S M Salili
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - M G Tamba
- Department of Nonlinear Phenomena, Institute for Experimental Physics, Otto von Guericke University Magdeburg, Magdeburg 39106, Germany
| | - S N Sprunt
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
| | - C Welch
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - G H Mehl
- Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
| | - A Jákli
- Chemical Physics Interdisciplinary Program & Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
| | - J T Gleeson
- Department of Physics, Kent State University, Kent, Ohio 44242, USA
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Ghoshal N, Mukhopadhyay K, Roy SK. Effect of an external magnetic field on the nematic-isotropic phase transition in mesogenic systems of uniaxial and biaxial molecules: a Monte Carlo study. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:042505. [PMID: 24827267 DOI: 10.1103/physreve.89.042505] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/20/2013] [Indexed: 06/03/2023]
Abstract
We determine the nematic-isotropic coexistence curve terminating at the critical point in a temperature-external field phase diagram for nematic liquid crystals with positive diamagnetic anisotropy, where the molecules are either perfectly uniaxial or biaxial using computer simulation of a lattice model. The coexistence curve is much steeper than that predicted by the standard Landau-de Gennes and Maier-Saupe mean-field theories. For the uniaxial system the critical magnetic field is estimated to be one order of magnitude lower than the mean-field estimate but of the same order of magnitude as the experimental measurement. Our study shows that molecular biaxiality could reduce the critical field strength significantly.
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Affiliation(s)
- Nababrata Ghoshal
- Department of Physics, Mahishadal Raj College, Mahishadal, Purba-Medinipur, West Bengal, India
| | - Kisor Mukhopadhyay
- Department of Physics, Sundarban Mahavidyalaya, Kakdwip, South 24 Parganas, West Bengal, India
| | - Soumen Kumar Roy
- Department of Physics, Jadavpur University, Kolkata 700 032, India
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1,2,4-Oxadiazole-Based Bent-Core Liquid Crystals with Cybotactic Nematic Phases. Chemphyschem 2014; 15:1323-35. [DOI: 10.1002/cphc.201301070] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2013] [Indexed: 11/07/2022]
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