Smectic D phase

The dimers of benzoic and cinnamic acid were probably the first hydrogen-bonded liquid crystals [16], Since this report early in the 20th century, a wide variety of aromatic acid dimers such as 55 have provided information on the structure and properties of liquid crystals [125-127], A notable example is compound 56 which shows an optically isotropic smectic D phase [127],... 

Sc Sa, Sa->Ch, and Ch->I respectively. The smectic D phase is observed in the hexyl series but not in the decyl analogs. This observation suggests that the increased flexibility provided by the longer alkyl group destabilizes the higher temperature smectic phases, while stabilizing the smectic C. Optical micrographs of the PDBPB polymer exhibit a fine broken focal conic fan texture for Sq phase, and focal conic fan textures for the Sa phase and cholesteric texture. 

There is ample evidence that small cycloalkanes and many of their derivatives with similar sphere-like shapes can exist as plastic crystals [21,22]. They are not birefringent and exhibit fewer X-ray diffraction lines in the plastic phases than in the corresponding (lower temperature) normal crystalline phases. Many plastic phases have cubic lattices like those of the closely related smectic D phases [33] which are comprised of aggregates of rod-like molecules. In both cases, more than one molecule can constitute the unit cell or fundamental lattice unit so that several molecules may tumble in a correlated motion. A similar microscopic model explains the lack of optical birefringence in the related cholesteric blue liquid crystalline phases from some chiral... 

To conclude this discussion of the thermotropic mesophases composed of rod-like molecules we would like to compile Table 1.1 from [18], where the structural classification of all known phases is given. Among them, the cubic smectic D phase is mentioned which was not discussed above. It is formed by rod-like molecules with specific intermolecular interactions between them [6]. Other types of cubic mesophases were discussed theoretically by Katz [19]. 

In the smectic-d phase there is usually a complete rotational symmetry around the long molecular axis. As described in Section 8.3.1, the tilt of the molecules in the smectic-C phase breaks this symmetry and leads to a polar... 

It is now known that the rarely observed smectic D phase does not have a layered structure rather it is thought to be a bicontinuous cubic phase. This structure may result from three-dimensional labyrinths formed by molecular heads and tails , both of which are continuous. Thus it is analogous to the bicontinuous cubic phases formed by amphiphiles in solution (see Fig. 4.25d). This phase is thus most suitably classified as a non-smectic phase and has been renamed cubic D. 

The smectic phases Ai, A2 and A have the same macroscopic symmetry, differing from each other in the wavelength of spacing. Hence it is possible to go from Ai to Aa or from Aa to A2 by varying only the layer periodicity in a continuous or discontinuous way(with the jump in the layer spacing d). Smectic-smectic transition lines of first order may terminate at a critical point, where the differences between the periodicities of the smectic A phases vanish, providing a continuous evolution from an Aa to bilayer A2 phase [12]. 

If Lp = Lh, the monolayer smectic Ai phase is formed. In the case that Lp > Lh, the layer spacing d exceeds the molecular length L, which corresponds to the formation of the smectic A phase (Fig. lla,b). In the opposite case, Lp < Lh, the layer spacing is less than L, and the flexible hydrocarbon chains just All the space which is determined by the fluorinated tails (Fig. 10c). 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

A large number of low molecular weight smectogens have been prepared and studied by X-ray diffraction methods At this time relatively few smectic polymers have been prepared and identified, and the smectic D, G, and H phases have not, as yet, been reported in polymers. 

In addition to the homopolyesters 80a-i, 81a-i and 82a-i, three classes of co PEIs were studied 83a-g [24], 84a-d and 85a-f [84]. The combination of two different alkane spacers did not significantly change the properties of the co PEIs 83a-d, when compared to 82i. However, when the difference on their length increased, a destabilization of the smectic layer-structures became evident, with the consequence that a nematic phase was formed on top of the smectic-C phase... 

The properties of the co PEIs 84a-d agree largely with those of their parent homo PEIs. They form a smectic solid phase and a smectic LC-phase. A differentiation between smectic-A and smectic-C has not been reported yet [83]. The co PEIs 83a-f and 84a have in common that when fibres were drawn from the melt the chain axes may be arranged parallel to the fibre axis as usual but also in a perpendicular array (Fig. 14). A detailed discussion of the reasons underlying this unusual behavior is presented in [83]. 

In the smectic A phase, molecules tend to be perpendicular to the smectic layers. The layer thickness d is roughly the same as the molecular length l. The thickness of the layers in the case of liquid crystalline polymers is about the order of the monomer s length. But in the smectic C phase, the molecules in the layers are parallel and tilted in arrangement with respect to the normal of the layers by a tilt angle 0. The layer thickness of the smectic C phase is d = l cos 6. The ordering of the smectic A and C phases are both higher than the nematic phase so that they appear at a lower temperature than the nematics do. The smectic A phase appears first as the temperature decreases if a compound shows both the smectic A and C phases. 

The local symmetry of smectic A phase is the same as that of the nematics, be., its point group is D h, while the symmetry of the smectic C phase is ( b/, (a ( 2 symmetry axis plus a reflection plane perpendicular to the axis). In addition, both smectic phases exhibit a one-dimension translational order. Owing to the difference in symmetry, the smectic phases show different optical properties. The smectic A phase is optically uniaxial, but the smectic C phase is optically biaxial. 

The helical structure of the c-director in the smectic C phase makes the defects different from those in the smectic C phase. As the Volterra process produces a screw dislocation, for example, along the z axis and the Burger vector b = d, it must be accompanied by a parallel wedge disclination in the c-director, in the form... 

Figure 4.31. Schematic X-ray diffraction patterns of liquid crystalline phases (a) unoriented nematic (b) oriented nematic (c) unoriented smectic A or C (d) oriented smectic A (e) oriented smectic C (f) unoriented smectic B phase.

Figure 6.38. The chiral smectic C phase (a and b) and the ferroelectric liquid crystal display (c and d).

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