Smectic systems

Fig. 29. Observed and calculated 2H NMR spectra for the mesogenic groups of a) the nematic (m = 2), b) the smectic (m = 6) liquid crystalline polymer in the glassy state, showing the line shape changes due to the freezing of the jump motion of the labelled phenyl ring. The exchange frequency corresponds to the centre of the distribution of correlation times. Note that the order parameters are different, S = 0.65 in the frozen nematic, and S = 0.85 in the frozen smectic system, respectively...

The synthesis described in this paper renders possible the preparation of block copolymers of uniform molecular weight composed of amorphous and LC side chain blocks. Beside the specific cholesterol mesogen introduced by carbonate linkages leading to a smectic system other mesogens with various spacer lengths can be introduced, e.g. by esterification. 

The various orientations and conformations mesogenic groups can adopt in a layer has been studied extensively for low molar mass smectic materials, and the classification and terminology of smectic systems is entirely based on these studies. However, low molar mass smectic compound or smectic LC-side chain polymers do, of course, not allow one to elucidate the role spacers play in the layer structures of LC-main chain polymers. Therefore, poly(ester-imide)s, po-... 

The molecules arrange themselves in parallel layers in smectic systems. In these cases, the molecular axis is perpendicular to the layer plane. Within the layers, the molecule may be arranged randomly or ordered with respect to other molecules. The molecules are also arranged parallel to each other but not in layers in nematic systems. The cholesteric state is midway between nematic and smectic the molecules are arranged in layers, but with the molecular axis being parallel to the plane of the layer. 

Numerical Simulation of X-ray Diffraction Patterns of Smectic Systems... 

NUMERICAL SIMULATION OF X-RAY DIFFRACTION PATTERNS OF SMECTIC SYSTEMS... 

The equations presented above are derived based on the least ordered smectic liquid crystalline polymers, i.e., the smectic A liquid crystals. The more ordered smectic systems may generate X-ray scattering patterns similar to those of crystal structures. 

The symmetry approach to ferroelectricity in liquid crystals can be realized not only for individual substances but also for multicomponent systems. For low-molar-mass ferroelectric liquid crystals, most applications use LC mixtures with two main components a nonchiral matrix providing the tilted smectic structure and a chiral dopant [7]. As for the preparation of FLCPs, mixing of a smectic C polymer with a chiral dopant also results in a ferroelectric chiral smectic system [74]. Japanese authors [75,76] have carried out systematic studies on mixing tilted smectic polymers with low-molar-mass ferroelectric liquid crystals. 

The effect of the chemical ermstitution of the crosslinker oti the local topology of the network is the second new feature to be considered. If the crosslinker molecule is flexible it can behave like an isotropic solvent. In that case, essentially only the phase transition and phase transformatiOTi temperatures of the LC phase are affected [90]. If, however, the chemical constitution resembles that of a mesogen of the constituent polymer backbone, the history of the crosslinking process becomes important. Under these conditions the crossUnker adopts the state of order in which the final crosslink process of the network occurs and thus determines the local topology of the crosslink [120,121]. The mechanical properties and the reorientational behavior are considerably modified for networks with the same chemical constitution but crosslinked either in the isotropic or in the liquid crystalline state [122-124]. Other important aspects of the local topology at the crosslink concern the phase transformation behavior [125] as well as the positional ordering in smectic systems [126]. 

Order and Disorder in Smectic Systems 4.1 iMndau-Peierls InstabiHty... 

Fig. 21 (a) Molecular structure of the main-chain smectic system TR5 with pentaphenyl transverse rods, (b) X-ray picture of the smectic-A structure of the elastomer stretched at room temperature, (c) Structural model of the elastomer [160]... 

Dynamic processes that can be investigated by NMR include both the motions of individual molecules, e.g., conformational dynamics and molecular rotations, and collective motions, e.g., director fluctuations in nematic systems, layer undulations in smectic systems, or density modulations in columnar phases of discotic systems. Self-diffusion can be measured by NMR relaxation or field gradient methods, as discussed in Sec. 13 of Chap. VII of this Volume. Table 1 gives an overview of the time scales accessible by the most common experimental techniques and examples of the type of motion that can be studied. 

The synthesis of nonchiral smectic liquid crystals is a broad topic for discussion, however, it can be divided into subsections in two different ways. For example, smectic systems can be split into metallomesogens and nonmetallomesogens, alternatively, they can be divided into materials for (1) meso-phase structure elucidation and classification [ 1 ], (2) property-structure correlations [2] and (3) host systems for ferroelectric and antiferroelectric mixtures. In the following sections template structures used for the synthesis of smectic materials will be described, followed by discussions of the syntheses of materials that have extensive histories in the elucidation of smectic phase structures, and finally of the syntheses of smectogens that are useful in applications. 

The FTIR spectra of Cop-D co-oIigomers contain the spectral band at 1682 cm that represents C=0 bond (vco) in the cyclic dimers of carboxylic groups shown in smectic systems we have discussed before (Shandryuk et al. 2002 Shatalova et al. 2003 Vasilets et al. 2004). It means that COOH-containing mraiomer units exist in macromolecules of co-oUgomers in the form of cyclic dimers even if their content does not exceed 5 mol.%. With the increase in their content the intensity of i/co band increases. We may assume that monomers form cyclic dimers even in... 

The variety of columnar structures is every bit as complex as that for smectic systems and a number of similar themes occur. X-ray diffraction patterns indicate, that even a simple hexagonal columnar phase can have columns, which are can be ordered or disordered along the column axis see Fig. 1.13, and molecules can be normal or tilted within a column. Some of the possible patterns of lateral ordering are shown in Fig. 1.14 with the columns lying on hexagonal, rectangular, and oblique lattices. The way in which the symbols summarise the geometry of a columnar phase is illustrated in the example for Colrd shown in Fig. 1.15. 

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