Ribbon mesophase

Note 4 The SmC mesophase is also known as an oblique or ribbon mesophase. The dimensional space group ispmg in the International System. 

From the earliest days of X-ray scattering studies (e.g. Luzzati and co-workers), there have been reports of variations on the hexagonal mesostructure. Many modifications are not seen regularly, although some distinct packings of parallel deformed rods are. The (anisotropic) optical textures of ribbon mesophases vary, sometimes resembling the fan texture of hexagonal mesophases, and sometimes not. 

Figure 16.13. Idealized mesostructures of (Type 2) ribbon mesophases (a) centred (cmm) phase (b) primitive (pgg) phase...

The only mesomorphic phases observed are columnar or ribbon mesophases. 

Edie, D. D., Robinson, K. E., Fleurot, O., Jones, S. P. and Fain, C. C., High thermal conductivity ribbon fibers from naphthalene-based mesophase. Carbon, 1994, 32(6), 1045 1054. 

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

Figure 4.31 (Left) Schematic view of the relative arrangement of chiral molecules (extended lozenges) in the cholesteric liquid crystalline mesophase (after [59]. The twist between layers is greatly exaggerated. In reality approximately 10 layers lie between equally inclined layers. (Right ) Helical arrangement of molecules, with a relative twist between molecules along one direction only the axis of the helical ribbon.

Ribbonlike parallel-ordered anisotropic domains that can also occur as folded structures Lenticular anisotropic domains of various sizes that are not aligned parallel to the particle surface Ribbon and lenticular anisotropic domains of various sizes in curved and irregular layered arrangements Porous microstructure with walls that are generally anisotropic but with pores and walls that vary in size Ribbon and lenticular anisotropic domains arranged in concentric patterns to form shothke coke Isotropic carbon form closely associated with parent hquor. Higher in volatile matter than incipient mesophase... 

Interesting examples of biomolecules that show lyotropic liquid crystallinity by association of identical molecules are guanosines and folic acids [168-171]. These molecules exhibit columnar mesophases due to the formation of hydrogen-bonded complexes in the presence of water. For example, folic acid 80 sodium salt exhibits cholesteric and hexagonal phases with water due to the formation of a tetramer, which stacks as shown in Fig. 23. The introduction of long alkyl chains into the guanosines leads to the assembly of ribbon-like structures in organic solvents [172]. 

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