Liquid crystal polymer banana-shape

Examples of (a) calamitic [65], (b) discotic [28], and (c) banana-shape [30] mesogenic structures of liquid crystal polymers. 

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],... 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

However, it should be noticed here that the introduction of chirality is one of the simplest ways to reduce the packing symmetry. The research of banana-shaped liquid crystal is actually bom from this fact. In other words, the attachment of a bent shape into a molecule could give rise to a smectic (Sm) stmcture which has the polar twofold axis along the bent direction and thus to ferroelectric and antiferro-electric Sm LCs without a chiral center. This was firstly suggested in the polymer molecules, successfully demonstrated in bent dimers and then in banana molecules. 

Niori T, Sekine T, Watanabe J, Furukawa T, Takezoe H (1996) Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules. J Mater Chem 6 1231-1233 Noel C, Navarad P (1991) Liquid crystalline polymers. Prog Polym Sci 16 55-110 Ortega J, Folcia CL, Etxebarria J, Gimeno N, Ros MB (2003) Interpretation of unusual textures in the B2 phase of a liquid crystal composed of hent-core molecules. Phys Rev E 68 011707-011710... 

An accepted experimental proof for the existence of a biaxial nematic phase in a thermotropic liquid crystal remained missing for a very long time. However, in recent years, biaxial nematic phases have been found in liquid crystalline polymers as well as in liquid crystals made of rod-disc mesogens, banana-shaped (bent-core) molecules, and organo-siloxane tetrapodes. Here, some characteristics of these systems and the corresponding experimental procedure for the investigation of phase biaxiality will be introduced. Further details for the individual systems can be found in the cited literature. 

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