Screw chiral nematics

This analogy was taken further by Renn and Lubensky [15] to incorporate chiral systems. At the smectic A to chiral nematic phase diverges and k becomes large, hence the formation of a dislocation-stabilized structure in chiral systems becomes possible, see Fig, 13, In the case of chiral phases, Renn and Lubensky showed that the intermediary phase would be stabilized by the incorporation of screw dislocations, see Fig, 14(a),... 

The through-space induction was also demonstrated in the polymerization of acetylene within a chiral nematic liquid crystalline field to yield achiral polyacetylene (PA) films that assume helical conformations with left- and right-handed screw... 

FIGURE 3.2 (See color insert following page 14-20.) Misdbilit)r test between the chiral nematic LC induced by (R)- or (S)-PCH506-Binol and the standard LC, cholesteryl oleyl carbonate of counterclockwise (left-handed) screw direction. 

Scanning electron microscope (SEM) images of polyacetylene films show that multidomains of spiral morphology are formed (Figure 3.4a), and each domain is composed of helical structure of bundle of fibrils with one-handed screwed direction (Figure 3.4b). The multidomain-type fibril morphology of polyacetylene seems to replicate the chiral nematic LC during interfacial acetylene polymerization. 

Closer observation of SEM images indicates that helical polyacetylenes synthesized in the (R)- and (S)-chiral nematic LCs form screwed bundles of fibrils and screwed fibrils with counterclockwise and clockwise directions, respectively (Figure 3.5). This result implies that the screw direction of helical polyacetylene is controllable by choosing the helicity, i.e., optical configuration of the chiral dopant, so far as the chiral nematic LC induced by the chiral dopant is employed as an asymmetric polymerization solvent. In addition, it is clear that the screw directions of bundle and fibrils are opposite to those of the (R-l)- and (S-1)-chiral nematic LCs used as solvents. This is an unexpected and even surprising result, requiring a sound interpretation that is to be discussed later. 

The bundles of fibrils for helical polyacetylenes synthesized in the (fi-2)- and (S-2)-chiral nematic LCs are screwed counterclockwise and clockwise, respectively The screw directions of helical polyacetylene are opposite to those of the corresponding (fi-2)- and (S-2)-chiral nematic LCs whose directions are confirmed to be clockwise and counterclockwise, respectively, through the miscibility test with choles-teryl oleyl carbonate. This is the same situation as that of the (fi-1)- and (S-l)-chiral nematic LCs including (fi)- and (S)-PCH506-Binol. 

In circular dichroism (CD) spectra of the polyacetylene thin films synthesized under the (fi-2)- and (S-2)-chiral nematic LCs, positive and negative Cotton effects are observed, respectively, in the region from 450 to 800 nm corresponding to tt — it transition of polyacetylene chain (Figure 3.8), despite the absence of chiroptical substituent in side chains. This indicates that the polyacetylene chain itself is helically screwed. It is evident that the above Cotton effect is not due to the chiral dopant [( )- or... 

From the above mentioned results, it can be remarked that counterclockwise and clockwise helical polyacetylene chains are formed in (R)- and (S)-chiral nematic LCs, respectively, and that these hehcal chains are bundled through van der Waals interactions to form hehcal fibrils with the opposite screw directions to those of the chiral nematic LCs. The bundles of fibrils further form the spiral morphology with various sizes of domains (Figure 3.9). 

FIGURE 3.7 Schematic representation of plausible mechanism for acetylene polymerization in the chiral nematic LC. The helical polyacetylene with counterclockwise (left-handed) screw direction blue arrow) grows up starting from catalytic species in the clockwise (right-handed) chiral nematic LC. 

It should be noted that cholesteric liquid crystals (chiral nematics) having point group symmetry Dqo are also periodic with flie pitch considerably exceeding a molecular size. The preferable direction of the local molecular orientatiOTi, i.e. the director oriented along the Coo axis, rotates additionally through subsequent infinitesimal angles in the direction perpendicular to that axis. Hence a helical structure forms with a screw axis and continuous translation group. 

Since chiral nematic liquid crystals phases have a very similar structural morphology i.e. rod-like molecules with a rotating director), the method developed to describe chiral nematic liquid crystals can be applied to the case of a solid helicoidal film. This method examines the electromagnetic response of materials with screw-like rotating electrical axes. The use of... 

As for achiral disclinations, the Volterra process may be used to create screw or edge disclinations by cutting parallel or perpendicular, respectively, to the chiral nematic twist axis. 

As a result of the layered nature of the chiral nematic structure, like the smectic A, it can also exhibit focal-conic textures [79] and both phases exhibit screw and edge dislocations. A dislocation corresponds to a displacement of the layered structure in a plane orthogonal to the layer and may be formed by the pairing of two disclinations of opposite sign. A screw dislocation has a singular line along the screw axis and is equivalent to a f-screw disclination in a chiral nematic. An edge dislocation corre-... 

Not only cellulose macromolecules are found to exhibit liquid crystalline phases but also suspensions of cellulose microfibrils can form cholesteric chiral nematic phases (Sugiyama et al. 1990), which are attributed to the chiral screw-like stmcture of the microfibrils (Fleming et al. 2001). 

Cholesteryl benzoate belongs to a special type of nematics because the molecule is chiral. Chiral means that the rod-like molecules have a handedness like a screw, which is usually right-handed but could be left-handed. Chiral molecules in a nematic phase can impart a gentle rotation on their... 

Among the several liquid crystal polymers that have been studied in recent years those containing intrinsically chiral elements with a prevalent chirality hold a particular position. Some of these, in fact, by virtue of their structural characteristics, assume a spatial array with nematic planes stacked in a superhelical structure characterized by a prevalent screw sense and are known as cholesteric phase. This kind of order can be controlled by either concentration in solution (lyotropic systems) or temperature in bulk (thermotropic systems). 

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