Mesogen

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. 

It is, however, important to note tliat individual columns are one-dimensional stacks of molecules and long-range positional order is not possible in a one-dimensional system, due to tlieniial fluctuations and, therefore, a sliarji distinction between colj. and colj. g is not possible [20]. Phases where tlie columns have a rectangular (col. ) or oblique packing (col j of columns witli a disordered stacking of mesogens have also been observed [9, 20, 25,... 

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. 

Flardouin F, Levelut A M, Aohard M F and Sigaud G 1983 Polymorphism in polar mesogens. I—Physioo-ohemistry and struotural aspeots J Chem.Phys. 80 53... 

Prost J and Barois P 1983 Polymorphism in polar mesogens. II—Theoretioal Aspeots J.Chem.Phys 80 65-81... 

Veerman J A C and Frenkei D 1992 Phase-behavior of disk-iike hard-oore mesogens Phys.Rev. A 45 5632-48... 

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. 

Fig. 18. Single steps of a write/read/erase cycle for an LC side-chain polymer (134). (—), mesogenic group (0), dyestuff. transition temperature...

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. 

Chiral nematic Hquid crystals are sometimes referred to as spontaneously twisted nematics, and hence a special case of the nematic phase. The essential requirement for the chiral nematic stmcture is a chiral center that acts to bias the director of the Hquid crystal with a spontaneous cumulative twist. An ordinary nematic Hquid crystal can be converted into a chiral nematic by adding an optically active compound (4). In many cases the inverse of the pitch is directiy proportional to the molar concentration of the optically active compound. Racemic mixtures (1 1 mixtures of both isomers) of optically active mesogens form nematic rather than chiral nematic phases. Because of their twist encumbrance, chiral nematic Hquid crystals generally are more viscous than nematics (6). 

Most polymeric Hquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

Mesogenic diols, such as 4,4 -bis( CO-hydtoxyaLkoxy)biphenyls, ate used with 2,4-TDI or 1,4-diisocyanatobenzene (PPDI) to constmct Hquid crystalline polyurethanes (7). Partial replacement of the mesogenic diols by PTMG shows that the use of lower molecular weight flexible spacers form polymers that have a more stable mesophase and exhibit higher crystallinity (8). Another approach to Hquid crystal polyurethanes involves the attachment of cholesterol to the polyurethane chain utilizing the dual reactivity in 2,4-TDI (9). 

The thermotropic aromatic main chain liquid crystalline polymers are also prepared by the phase transfer catalyzed aromatic nucleophilic polymerization [87]. Polyetherification of bis(4-chloro-3-nitrophenyl) sulfone with mesogenic aromatic diols is shown below ... 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

Polybibenzoates are a kind of thermotropic polyesters obtained by polycondensation of 4,4 -biphenyldicar-boxylic acid (p,p -bibenzoic acid) with a diol. These polyesters contain the biphenyl group, which is one of the simplest mesogens. They are synthesized by melt transesterification of the dimethyl or diethyl ester of p,p -bibenzoic acid and the corresponding diol, using a titanium compound as catalyst, according to the following scheme ... 

Figure 7 Integrated distribution curves for the angle 0 between two successive mesogenic groups calculated [14,15] for P7MB (continuous line) and P8MB (broken line).

M. Saminathan, PhD. Thesis, Synthesis and Characterization of Novel Liquid Crystalline Polymers Containing Azobenzene Mesogen, Regional Research Laboratory,... 

Shibaev, V. P. and Plate, N. A. Thermotropic Liquid-Crystalline Polymers with Mesogenic Side Groups, Vol. 60/61, pp. 173—252. 

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