Smectic A polymorphism

Another example shows the smectic A polymorphism. Fig. 1.17 [43]. In mixtures of 80CB (l.xv) and 4-n-heptyloxyphenyl-4 -cyanobenzoyloxyben-zoate (DB70CN), in addition to the bilayer 5a,2 and the partially bilayer 5a,d phases, a novel incommensurate phase 5a,ic appears. Its X-ray diffraction pattern consists of both 5a,u and 5a,2 periodicities which are incommensurate. The SA2 SAd transition line (for x < 0.24) corresponds to a continuous transition between the two phases. Other examples of the incommensurate and modulated smectic A phases are given in [4, 44]. 

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

They observed that the sample with a polyion backbone formed a smectic A phase with a focal conic fan texture and a perpendicular structure. On the other hand, the material based on the neutral backbone formed a nematic phase with a schlieren texture. Once again, the presence of charges in the polymer severely influenced the polymorphism of the compounds. The authors showed the potential of these LC systems in the fields of photomemory, optical storage, and light drive display, especially because these amphiphilic polymers yield excellent azodense LC thin films [96]. 

Monosubstitution, as in the case of conventional calamities, is expected to lead to a predominance of nematic and smectic A phases, with possibly nematic phases being favored because of the repulsive steric effects of the ferrocene unit causing a reduced ability of the molecules to pack in layers. Disubstitution, on the other hand, may be expected to lead to smectic polymorphism because the overall shape of the molecule can be extended and varied from being linear to being slightly bent. For instance, let us first consider disubstitution in one of the ferrocene rings. In... 

Liquid-like layers with the molecules upright on the average (fig. 1.1.5(a)) negligible in-plane and interlayer positional correlations. Thus the structure may be described as an orientationally ordered fluid on which is superimposed a onedimensional density wave. A number of polymorphic types of smectic A have been discovered (see 5.6). 

As previously stated in Chapter 3, isotaetie polypropylene is a polymorphic material with four basic crystalline forms, namely the monoelinie (a), trigonal (P), orthorhombic (y) and mesomorphic smectic (intermediate state between ordered and amorphous phase) forms. The a-form exhibits excellent modulus and tensile strength but poor fracture toughness. ... 

The smectic mesophase is more ordered than the nematic phase and furthermore, whereas only one nematic phase exists, the smectic phase exhibits polymorphism (see Figure 3. 2), i.e., there are many different types of smectic phases. As for the nematic phase, the smectic phases can be identified by optical polarising microscopy (see Chapter 9). In 1917, Grandjean was studying (by microscopy) a sample of smectic liquid crystal (later classified as smectic A) which showed stepped edges, indicating that the smectic phase was lamellar in natnre. The lamellar nature of smectic phase allows various combinations of molecular correlations both within the layers and between the layers , each of which constitutes a different type of smectic mesophase. 

Unfortunately, most of the liquid crystalline mixtures are far from being ideal solutions. Intermolecular interactions result in the deviation of experimental data from theoretical data. Moreover, new phases can occur in mixtures of nematogens. For example, a binary mixture may be more preferable for the formation of the smectic A phase, due to a specific packing of different-type molecules with each other [35]. In addition, many mixtures display a so-called reentrant polymorphism. 

In addition to containing a wealth of information on microscopic techniques and materials, Friedel s article represented in 1922 the first classification of liquid crystals into types, i.e., nematic, smectic and cholesteric. Today, of course, cholesterics are known simply as chiral nematics with no need that they be derived from cholesterol, and we recognize the existence of several polymorphic smectic forms, whereas Frie-del allowed for only one (today s smectic A SmA). 

Most mesogenic salts derived from aliphatic acids, (R-COO) M (R = alkyl (normal and branched) or alkenyl M=Li, Na, K, Rb, Cs, NH4, Tl, Pb or other metal) form layered structures (lamellar phases, neat phases) that are similar to smectic A phases. However, mesogenic salts form double layers and are not miscible with smectic A phases [276]. Some of the materials show very complicated polymorphism with a large number of mesophases [276-279]. In general, the transition temperatures of the salts are quite high compared with those of nonpolar liquid crystals. Most of the salts can also form lyotropic liquid crystals. 

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