TGB<sub>A</sub> and TGB<sub>C</sub> phases in some chiral tolan derivatives

Nguyen, H. T.; Bouchta, A.; Navailles, Laurence; Barois, P.; Isaert, N.; Twieg, Robert J.; Maaroufi, A.; Destrade, C.

doi:10.1051/jp2:1992242

Cited by 122 publications

(70 citation statements)

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“…However, in all these experiments l b and l d were estimated rather than measured. Recently an extensive structural study of the TGB A phase occurring in a series of chiral tolane derivatives [14] was undertaken by Navailles and coworkers [15]. They observed an X-ray diffraction pattern in the transverse direction to the pitch axis that consisted of discrete Bragg spots, which provided a direct way to measure the rotation angle ∆Θ of smectic blocks.…”

Section: Introductionmentioning

confidence: 99%

Dislocation geometry in theTGBAphase: Linear theory

Bluestein¹,

Kamien²,

Lubensky³

2001

Phys. Rev. E

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We demonstrate that an arbitrary system of screw dislocations in a smectic-A liquid crystal may be consistently treated within harmonic elasticity theory, provided that the angles between dislocations are sufficiently small. Using this theory, we calculate the ground state configuration of the TGBA phase. We obtain an estimate of the twist-grain-boundary spacing and screw dislocation spacing in a boundary in terms of the macroscopic parameters, in reasonable agreement with experimental results. I. INTRODUCTIONCondensed matter systems offer a vast stage for the intricate interplay between order and disorder. A beautiful example of such interplay is the twist-grain-boundary phase (TGB) of chiral smectics, which is the liquidcrystalline analog of the Abrikosov vortex state of type II superconductors [1,2]. Morphologically, the phase consists of blocks of pure smectic (which can be either smectic-A or smectic-C) separated by parallel, regularly spaced twist grain boundaries, where each boundary is formed by a periodic array of screw dislocations. The direction of dislocation lines rotates by a constant angle from one grain boundary to the next. Such a dislocation arrangement causes the smectic blocks to rotate about the axis perpendicular to the grain boundaries dragging the nematic director along. Thus the TGB structure combines the properties of smectics and cholesterics: the nematic director twists on average as in cholesterics while the lamellar structure of a smectic is preserved. In this paper only the TGB A phase will be considered. As suggested by its name, the smectic blocks in TGB A are smectic-A. The analogy between the TGB A phase and the Abrikosov vortex lattice is based on the mathematical similarity of the Gibbs free energies for metals in a magnetic field and for chiral smectics [1][2][3][4]. Their respective forms, known as the Ginzburg-Landau free energy and the de Gennes free energy, areandwhere ψ is a complex order parameter, A the vector potential, H the magnetic intensity, h a chiral field determined by molecular structure [5], and n the unit director, often decomposed as n = n 0 + δn. The Ginzburg-Landau free energy (1.1) defines two characteristic length scales: the order parameter coherence length ξ = (h 2 /2m * |r|) 1/2 and the magnetic field penetration depth λ = (m * c 2 g/4πµ e * 2 |r|) 1/2 . Their ratio κ = λ/ξ, called the Ginzburg parameter, controls the phase diagram as a function of temperature and external magnetic field. When the Ginzburg parameter is less than the critical value κ c = 1/ √ 2, the system is type I, and there is a first order transition between a normal metal in a field and the Meissner phase with ψ = constant = 0 and magnetic field B = 0. When κ > κ c , the system becomes type II, and a new phase intervenes between the normalmetal state and the Meissner state. This new phase, the Abrikosov flux phase, is characterized by a proliferation of linear topological defects of the complex order parameter field ψ. The defects are magnetic flux lines, and they form a two-dim...

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Section: Introductionmentioning

confidence: 99%

Dislocation geometry in theTGBAphase: Linear theory

Bluestein¹,

Kamien²,

Lubensky³

2001

Phys. Rev. E

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“…All the detail of a TGBA mesophase were experimentally observed and confirmed by Ihn et al [5] using freeze fracture electron micrographs. The TGBC phase was discovered by Nguyen et al [6]. However, the TGBC structure proposed by them was found to differ from the picture originally proposed by RL [2].…”

Section: Introductionmentioning

confidence: 90%

Twist Grain Boundary Phases Giving Developable Domain Textures as Those Exhibited by Columnar Phases

Khandelwal¹,

Yadav²,

Bawa

2008

Molecular Crystals and Liquid Crystals

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This paper reports experimental observation of TGBA, TGBCÃ phases in a mixture of Liquid Crystals. The phases exhibit developable domain textures as those observed for columnar systems. Under polarizing microscope they appear as cylindrical and cone-like domain texture. In such domains the smectic blocks, the grain boundaries and the layers of helical structure of TGB phase are coiled in a double twist manner around an axis. A peculiar texture is obtained in the intermediate region between the TGBA and Cholesteric phase while cooling the sample from isotropic phase. This phase is identified as 'Twist Grain Boundary phase of Hexatics' (TGBH) proposed theoretically by Kamien. The phase reappears as TGBA is heated. The Twist grain boundary phase of Hexatics consists of twisted N þ 6 regions separated by grain boundaries. Within each region the bond order twists with pitch 2p=q and the director is aligned in a nematic state. The director is rotated by some finite angle across a grain boundary moving along the pitch axis that is perpendicular to the nematic director.

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“…This has been demonstrated by Small Angle X-ray Scattering on oriented samples. The structure of individual blocks can be of the smectic A type or the smectic C type [73][74][75]. In the former case one speaks of the TGBA* phase, while for the latter structure, several different types of TGBC* phases have been proposed: (1) the smectic layer normal is perpendicular to the helical axis and the director is tilted with respect to the screw dislocations.…”

Section: Twist Grain Boundary Phasesmentioning

confidence: 99%

Chiral Liquid Crystals: Structures, Phases, Effects

2014

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The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.

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TGB_A and TGB_C phases in some chiral tolan derivatives

Cited by 122 publications

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Dislocation geometry in theTGBAphase: Linear theory

Dislocation geometry in theTGBAphase: Linear theory

Twist Grain Boundary Phases Giving Developable Domain Textures as Those Exhibited by Columnar Phases

Chiral Liquid Crystals: Structures, Phases, Effects

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TGBA and TGBC phases in some chiral tolan derivatives

Cited by 122 publications

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Dislocation geometry in theTGBAphase: Linear theory

Dislocation geometry in theTGBAphase: Linear theory

Twist Grain Boundary Phases Giving Developable Domain Textures as Those Exhibited by Columnar Phases

Chiral Liquid Crystals: Structures, Phases, Effects

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TGB_A and TGB_C phases in some chiral tolan derivatives