Smectic 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). 

Two basically different types of mesophases have been observed. First, there are those that retain a 3-dimensional crystal lattice, but are characterized by substantial rotational disorder (i.e., disordered crystal mesophases), and second, there are those with no lattice, which are therefore fluid, but nevertheless exhibit considerable rotational order (i.e., ordered fluid mesophases). Molecular structure is in fact important and, generally speaking, molecules comprising one of these two types of mesophase are distinctly different in shape from molecules comprising the other. Indeed, with the possible exception of some polymorphous smectic materials, there are no known substances that show both disordered crystal and ordered fluid mesophases. 

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. 

The question of predicting the type of smectic polymorph that a given mesogen... 

The smectic phases are commonly divided into seven basic polymorphs 7 5S). They all show layered structures of the mesogens (see Fig. 10). The layers can slide... 

Fig. 11. DSC-trace of bis-(4 -n-octyloxybenzal)-l,4-phenylenediamine at 5 K/min. The lowest temperature transition is the fully ordered crystal-to-smectic transition, followed by four transitions between the five smectic polymorphs. Next is the smectic-nematic transition, followed by the nematic-to-isotropic liquid transition. Drawn after data by Petrie581...

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21>. 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

That is why here we describe two groups of polymers- those. forming an ordered smectic phase (of SB type) and polymers with mesogenic groups, for which the formation of crystalline structure is proposed (Table 5). Sometimes it is difficult to distinguish between crystalline and LC states of ordered smectic phases. Thus, the interpretation of data on crystalline phases of some polymers, listed in Table 5, is also possible from the viewpoint of smectic polymorphism. 

For polymeric smectic mesophases polymorphism is just as plausible as for low-molecular smectics. Examples of polymorphism of the type SB SA, discovered for polymeric smectics containing Schiff bases as mesogenic groups, are listed in Table 6 66,7S). 

One feature of the non-amphiphilic cubic mesophases is that they frequently show mutual miscibility even when constituted from dissimilar molecules. Such miscibility, which contrasts with the immiscibility between dissimilarly constituted solid crystals, is also found between nematic mesophases, between corresponding smectic polymorphs, and, of course, between amorphous liquids. This miscibility is important in its implication that in the cubic mesophases of the amphiphilic series there could well be an equilibrium of related globular micellar forms (Figures 1 and 5) rather than a single clearly defined form. 

Cyclic main-chain liquid crystal oligomers have been shown to display a number of crystalline polymorphs in addition to smectic and nematic phases.133 These collapsed rings have rigid mesogens in their stems while folds form selectively in the flexible spacer. Significant difference between even and odd-membered rings are seen, particularly where the flexible spacer is short. 

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]. 

Solid-state NMR spectroscopy has been available for the analysis of polymorphs for isotactic polypropylene (i-PP) for approximately 15 years [1-3]. By different sample preparation (e.g., the method of crystallization), i-PP forms a, /3 and smectic forms. Figure 11.1 shows the CP/MAS NMR spectra of these three forms of i-PP at 20°C. 

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