Nematic order structure

Thermotropic liquid crystalline PPV derivatives 43 were prepared by the coupling of dihalodialkoxybenzene and divinylbenzene in the presence of a palladium catalyst, as outlined in Scheme 47 [133]. Polarized light microscopy, as a function of temperature, showed evidence of a nematically ordered structure in the material. X-ray diffraction analysis of the pristine polymers showed them to be semi-crystalline in nature, although the crystallinity of the polymer changed dramatically upon heating above 100 °C. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

Figure 10.13. (a) SEM image of ZnO nanorods coated with octylamine. Scale bar, 200 nm. (b) Uniform nanorod film fabricated by spin coating of ZnO nanorods. Scale bar, 500 nm. The nanorods assemble into domains with nematic ordering, (c) Saturated transfer characteristics for a thin-film transistor fabricated by spin coating of ZnO nanorods with different ligands octylamine (solid line), butylamine (dashed line). Vi = 60V. (d) Output characteristics of a spin-coated device made from octylamine-stabilized ZnO nanorods.The device structure is shown in the inset in (c). Reproduced from Ref. 83, Copyright 2006, with permission from the American Chemical Society. 

Mesophase with a helicoidal supramolecular structure of blocks of molecules with a local smectic C structure. The layer normal to the blocks rotates on a cone to create a helix-like director in the smectic C. The blocks are separated by plane boundaries perpendicular to the helical axis. At the boundary, the smectic order disappears but the nematic order is maintained. In the blocks the director rotates from one boundary to the other to allow the rotation of the blocks without any discontinuity in the thermomolecular orientation. 

Liquid crystals represent a transition between solid crystalline substances and isotropic liquids. On heating, mesophases are formed that have ordered structures which can be nematic, smectic or cholesteric. On further heating, the orientation is disturbed and the phases are converted into an isotropic liquid. The long structure of liquid crystals causes isomers with more drawn-out shapes to be readily dissolved in the ordered liquid crystal substrate ( mesophase ) thus yielding stronger sor-bat-sorbent interactions,... 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

Cholesteric liquid crystals are similar to smectic liquid crystals in that mesogenic molecules form layers. However, in the latter case molecules lie in two-dimensional layers with the long axes parallel to one another and perpendicular or at a uniform tilt angle to the plane of the layer. In the former molecules lie in a layer with one-dimensional nematic order and the direction of orientation of the molecules rotates by a small constant angle from one layer to the next. The displacement occurs about an axis of torsion, Z, which is normal to the planes. The distance between the two layers with molecular orientation differing by 360° is called the cholesteric pitch or simply the pitch. This model for the supermolecular structure in cholesteric liquid crystals was proposed by de Vries in 1951 long after cholesteric liquid crystals had been discovered. All of the optical features of the cholesteric liquid crystals can be explained with the structure proposed by de Vries and are described below. 

Rod—coil block copolymers have both rigid rod and block copolymer characteristics. The formation of liquid crystalline nematic phase is characteristic of rigid rod, and the formation of various nanosized structures is a block copolymer characteristic. A theory for the nematic ordering of rigid rods in a solution has been initiated by Onsager and Flory,28-29 and the fundamentals of liquid crystals have been reviewed in books.30 31 The theoretical study of coil-coil block copolymer was initiated by Meier,32 and the various geometries of microdomains and micro phase transitions are now fully understood. A phase diagram for a structurally symmetric coil—coil block copolymer has been theoretically predicted as a... 

As early as 1938, Langmuir observed the phase separation of clay suspensions into an isotropic phase and a birefringent gel at the macroscopic level in test-tubes [9]. However, in the same report, he noted that this property of phase separation was gradually lost with time, which he tentatively explained by the incorporation of impurities diffusing from the glass tubes. He also compared this system to normal liquid crystals. Later, in 1956, Emerson observed a banded texture similar to that displayed by the Tobacco Mosaic Virus [48]. The investigation of clay suspensions from the structural point of view has been recently resumed. However, the study of the nematic order of suspensions of montmorillonite clays is in fact complicated by their gel properties. In spite of sustained efforts to understand its nature, the gelation mechanism has not yet been fully elucidated [49]. 

Flexibihty of the moieties is another important parameter because it was recently shown that flexible objects require larger volume fractions to undergo nematic ordering. Flexibihty also reduces the nematic order parameter at the transition. Intuitively, very flexible mineral polymers should not show any orientational order at rest, but may display a strong flow birefringence. Thus, any soluble system where the structural unit in the solid state is anisotropic may not necessarily be a lyotropic Hquid crystal. For example, in solution a polymer is much less constrained than in the soHd state, and hence one must consider the elastic properties of the polymer chain and whether the anisotropic units still exist in solution. As shown recently for the case of the complex fluid with a min-... 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

For commercial TLCPs having their mesogenic moieties contained within the main chain of the polymer, this structure is often nematic. Nematic order in the liquid crystalline phase is shown schematically in Figure 3. The nematic phase is the LCP phase of lowest order, consisting of aggregates of rod-like molecules. Within each aggregate, or domain, the molecules have a distribution about a common axis, denoted by n.. No order exists, however. 

---