Liquid crystals mesogens

Keywords Dendrimer Dendritic Liquid crystal Mesogen Mesophase Polypede... 

A thermotropic liquid crystal (mesogen) is a compound that, on heating the crystal or on cooling the isotropic liquid, gives rise to mesomorphism. Liquid crystallinity occurs between the crystal and isotropic liquid states. The intermediate phases, or mesophases, can be either cnantiotropic, i.e., thermodynamically stable, or monotropic, i.e., thermodynamically unstable. The solid to mesophase transition is referred to as the melting point, while the mesophase to isotropic liquid transition is referred to as the clearing point. 

Fig. 1.7 Actuation mechanism of a liquid crystal elastomer. The application of an electric field results in the realignment of intrinsically polarized liquid crystal mesogens. The mesogens are either grafted to elastomer chains or incorporated within them. The elastomer chains prevent the free flow of the mesogens and couple their motion to bulk stresses and strain...

Solutions of rod-like entities in a normally isotropic solvent often form liquid-crystal phases for sufficiently high solute concentration. These anisotropic solution mesophases are called lyotropic liquid crystals". Although the rod-like entities are usually quite large compared with typical thermotropic liquid-crystal mesogens, their axial ratios are seldom greater than 15. Deoxyribonucleic acid (DNA), certain viruses (e.g., tobacco mosaic virus (TMV)), and many synthetic poly-... 

Kim N, Huang TM, Kyu T, Nosaka M, Kudo H, Nishikubo T. Induced smectic phase in mixtures of hyperbranched polyester and liquid crystal mesogens. J Phys Chem B 2008 112 13225-30. 

These examples involve pendant groups that interact with each other. There are, of course, several examples of polymers containing pendant chromophores, particularly as liquid crystal mesogens, that have been investigated as nonlinear optical matoials. In these materials, the electronic interaction between chromophores does not generally contribute to the material s nonlinear optical susceptibility. ... 

Molecules tliat are capable of fonning liquid crystal phases are called mesogens and have properties tliat are mesogenic. From the same root, tire tenn mesophase can be used instead of liquid crystal phase. A substance in a liquid crystal phase is tenned a liquid crystal. These conventions follow tliose in tire Handbook of Liquid Crystals, [4, 5 and 6] tire nomenclature of which [7] for various liquid crystal phases is adopted elsewhere in tliis section. 

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

Parameters (ii)-(vii) depend on the dielectric, mechanical and optical properties of the mesogens. To optimize a dis compromise between different molecular characteristics is often required and mixtures of liquid crystals are usually commercial displays. 

Penner T L, Schiidkraut J S, Ringsdorf H and Schuster A 1991 Oriented films from polymeric amphiphiles with mesogenic groups—Langmuir-Blodgett liquid-crystals/Macromo/ecu/es 24 1041-9... 

Kurst G R, R A Stephens and R W Phippen 1990. Computer Simulation Studies of Anisotropic iystems XIX. Mesophases Formed by the Gay-Berne Model Mesogen. Liquid Crystals 8 451-464. e F J, F Has and M Orozco 1990. Comparative Study of the Molecular Electrostatic Potential Ibtained from Different Wavefunctions - Reliability of the Semi-Empirical MNDO Wavefunction. oumal of Computational Chemistry 11 416-430. 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

In the case of liquid crystals in particular, vibrational properties reflect very directly the complex hierarchy of the structure and bonding problem in these materials. For example, in a single mesogenic molecule vibrational frequencies range from about 10 cm to over 3000 cm which arise from the very wide range of force constants present [79]. 

The interaction of complex liquid crystal molecules with realistic surfaces is an area which is currently unexplored using electronic structure methods though, as stated earlier, the problem of surface-induced control of molecular orientation remains at the forefront of liquid crystal device technology. This problem is currently at the limits of practical capability of the most powerful computer systems. However treatment of a single mesogenic molecule on a... 

Fig. 7. Snapshots from a smectic-A phase of liquid crystal dimer molecules composed of two Gay-Berne mesogens linked by a flexible alkyl chain. Data from [112] supplied by Dr M R Wilson, University of Durham...

Computer Simulation of Liquid Crystal Phases Formed by Gay-Berne Mesogens... 

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