Cluster smectic

Wlren a nematic phase is cooled towards a smectic A phase, fluctuations of smectic order build up. These fluctuations were called cybotactic clusters in tire early literature. Regardless of tire physical picture of such fluctuations. 

Chiralsil-val, 6 96-97 Chiral smectic C liquid crystals, 15 106-107 Chiral stationary phases, 6 79-82 Chiral supramolecular clusters, 24 61 Chiral synthons, 11 5 Chiral titanium complexes, 25 98—99 Chirobiotic phases, for chiral separations, 6 90-91... 

Liquid crystals can be in the smectic, nematic, or isotropic states. In the smectic liquid crystalline state there is a long-range order in the direction of the long axis of the molecules. These molecules may be in single- or bilayer conformation, have molecular axis normal or tilted to the plane of the layer, and frozen or melted chains. In the nematic liquid crystalline state the molecules are aligned side by side but not in specific layers. The isotropic liquid crystalline state is more or less a liquid state, but where clusters with short-range order persist (Small, 1986, pp. 49-51). 

The fact that the structure of a solid monomer influences its polymerization substantially now seems obvious. It is not as clear whether structural phenomena can effect polymerization if the monomer is a liquid. It has long been known that ordered regions or clusters exist in liquids, and several years ago it was assumed that in some cases these regions in liquid monomers can influence the polymerization. One of the most vivid examples—namely, polymerization in the liquid-crystalline state—was accomplished by Krentzel and co-workers (I, 2, 3). The object of their study was p-methacrylylhydroxybenzoic acid, which forms conventional crystals in the pure state and does not polymerize in the solid state. However, when mixed with alkoxybenzoic acid, it forms liquid crystals of both smectic and nematic forms. Polymerization of p-meth-acryllylhydroxybenzoic acid in various forms of liquid crystals was compared with polymerization of the same substance dissolved in dioxane and dimethylformamide (DMF). 

Xj < 2A0Z3 and attractive for x > 2AoZ,. Thus, as in the case of smectic A (see fig. S.4.7), there can result a clustering of like edge disclinations to form a grain boundary . Such clustering is often observed in fingerprint textures (fig. 4.2.7). 

Fig. 5.4.7. Clustering of like edge dislocations in smectic A to form a domain wall or grain boundary . (After Pershan. )...

Kundler I, Finkelmann H. 1998. Director reorientation via stripe domains in nematic elastomers influence of cross link density, anisotropy of the network and smectic clusters. Macromol Chem Phys 199 677 686. 

Summarizing, experimental observations suggest that the giant (direct or converse) flexoelectricity of bent-core nematics is related to the polar smectic clusters occurring in them. In order to explore the exact mechanism for how clusters contribute to the flexoelectric response, further experimental and theoretical studies are needed. 

C. Zhang, N. Diorio, M. Gao, S. Sprrmt, J.T. Gleeson and A. Jakli, Direct observation of the smectic clusters in the nematic phase of bent core liquid crystals, to be published. 

Non-standard EC has also been observed in bent-core nematics, where < 0, Ca < 0 is also realized and where flexoelectricity is strong. There is still no detailed theoretical description, which would have to include the strong frequency dependence of as well as the unusual viscosity and elastic properties these might indicate smectic cluster formations not only in the nematic but even in the isotropic phase. 

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