Smectic phase, calamitic mesogens

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Calamitic mesogens in general form two main types of mesophase the nematic phase (N) and the smectic (S) phases. The nematic phase is the most disordered of the mesophase tjpes in it, the molecules possess one-dimensional orientational order and no positional order (Fig. 22). 

Laschat and coworkers have investigated the mesomorphic behavior of guanidinium salts [38, 52], When calamitic mesogenic groups were attached to the guanidinium head, such as 31, SmA phases were observed chloride salts showed a smectic phase from above room temperature up to more than 200°C, depending on the chain length... 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

Whereas monophilic liquid crystals can show a high diversity of smectic phases (SmA-SmQ), the amphotropic liquid crystals normally exhibit only the SmA phase. Tilted smectic phases are only observed in a few cases. The first indication of possibly tilted phases was given in 1933 for thallium stearate [ 170]. A disordered SmC phase was also clearly deseribed for mesogens containing a classical calamitic core aside to their amphiphilic structure [171]. Monophilic liquid crystals can show various ordered tilted smectic phases, for example, smectic I, F, G, J, H, and K. In the case of lipids only one mesophase, the j8 phase,... 

In the case of copolymer LCs there is speculation that the alternating primary structure. .. core-spacer-core-spacer... restricts inter-core translations and thereby stabilizes smectic phases. While we remain cognizant of the potentially important role that primary and secondary molecular structure of mesogens can play, in the remainder of this chapter, however, we will focus on aspects of mesomorphism that, for the most part, can he described in terms of the idealized prolate (oblate) shape of calamitic (discotic) mesogens (Fig. 5.1). We consider the nature and implications of the local molecular order in the nematic state after briefly reviewing the molecular crystal and the isotropic liquid. 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

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