Liquid crystals banana-like

Molecular shape anisotropy plays a very important role in determining whether or not liquid crystal phases will be formed, and indeed, when they are formed, which particular modification will be generated in preference to other mesophases [3, 4]. Molecules that form liquid crystals are usually carbon based, but they need not be. They often have preferred shapes, based on rods, bananas, boards or discs. In the following account, the properties of some unusual rod-like organic systems which have asymmetric structures thereby making them optically active and chiral will be described. In particular, the unusual self-organizing properties and the structures of the frustrated phases of the (R), (S), and racemic forms of 1-methylheptyl 4 -(4-n-... 

L.M. Blinov, M. Kozlovsky, T. Nagata, M. Ozaki and K. Yoshino, Influence of guest conformation (rod- or banana-like photo-isomers) on flexoelectric coefficients in nematic liquid crystals, Jpn. J. Appl. Phys. 38(9A/B), L1042-L1045, (1999). 

Many other types of liquid crystalline compounds exist besides those with rod-like molecules e.g. compounds with disk-shaped, banana-shaped, and bowlshaped molecules. However, over 80% of all liquid crystals have a rod-like form (e.g. the molecule shown in Fig. 5.1-3). 

Thermotropic liquid crystals are most often composed of elongated rod-like or plane disc-like organic molecules cf. Fig. 3.1, top part). However, the molecules may also take other geometries as long as they are anisotropic, e.g. a banana-like shape as found for bent-core molecules [1]. This anisotropic shape is essential, as orientational order cannot be defined for building blocks with an isotropic shape. 

The ferroelectric properties of the chiral smectic-C phase and the electroclinic effect of the smectic- phase appeared as a result of the symmetry breaking caused by the presence of chiral molecules. One can think of smectic phases in which nonchiral molecules arrange themselves in a polar order [86], and it seems that such phases were recently observed, indeed experimentally. The molecules which establish these phases are not chiral but possess a bent core resembling a bow- or banana-like shape [87] a second class of nonchiral liquid crystals showing polar ordering consists of certain polymer-monomer mixtures [88],... 

Thermotropic liquid crystals are composed of moderate-size ( 2-5nm) organic molecules, which are strongly anisometric elongated and shaped like a cigar (so-called calamitic liquid crystals), disc-shape (discotic LCs), or bent-shape (pyramidal or banana-shape). 

The Bj textures show characteristic helical filamentary growth in cooling from isotropic melt. i- 2 However they denote at least two distinct phases. X-ray measurements clarified that the original By materials, which form freestanding strands, - just like columnar liquid crystals of disc-shape molecules (see Chapter 2), indeed have a columnar phase (hereafter we will label them as Bj j). Other By-type materials (we call them Byu) were foimd to have modulated layer structures. We note that the By and B2 type banana-smectics also form strands of fibers. ... 

In the effort to make free-standing films of the first B1 material it was found that instead of films they form free-standing strands [25] just like columnar liquid crystals of disc-shape molecules [95], Such observation was confirmed with other B7 materials [98], and later it was found that the B7 and B2 type banana-smectics also form strands of fibers [99]. Although the most stable fibers are the B7 materials with slenderness ratio as large as 5000, the B7 and B2 fibers also have aspect ratios over 1000 and 100, respectively. The values are orders of magnitudes larger than of the Rayleigh-Plateau limit [100] of Newtonian isotropic fluids and of nematic and smectic liquid crystals of rod-shape molecules [96]. 

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