Helical structures lyotropic

A variety of chiral alignment media for organic solvents is known with the most widely used and best characterized being the poly(amino acids) PBLG and PCBLL. Both polymers form lyotropic mesophases and possess ot-helical structures for which many examples of enantiomeric differentiation have been shown. In addition to chiral poly(amino acids), it was demonstrated that achiral media with chiral cages like cyclodextrines serve as alignment media with the potential of chiral discrimination.121... 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

The solution properties of these materials are unusual. They form optically anisotropic solutions in both amide and acid solvent systems over quite wide ranges of concentration and polymer molecular weight. In other words they are among the few known examples of synthetic polymers which can form lyotropic liquid crystals. (That is to say liquid crystals formed by the action of a solvent.) The usual example quoted in this context is poly(y-benzyl-L-glutamate) which forms cholesteric mesomorphic solutions in certain organic solvents. The helical structure adopted by the polypeptide in these solvents behaves as a rigid rod and it is... 

It has been shown that thermotropic N -LC materials demonstrate supramo-lecular helical ordering that leads to CPL with high dissymmetry factors [49-51], On the other hand, it would be appealing to investigate lyotropic N -LC systems as alternative circularly polarized optical materials for use in optoelectronic devices and displays. The effects of the solvent, solution concentration, and chiral dopant employed in the lyotropic N -LC system would be of particular interest, especially in relation to the helical structure of the polymer LC phase and its chiroptical properties. It was reported that di-substituted polyacetylene (di-PA) adopting a poly(diphenylacetylene) (PDPA) structure with alkyl side chains exhibits lyotropic LC behavior [18, 19]. The PDPA structure, with phenyl moieties... 

The inverse of the pitch which corresponds to the helical twist of the lamellas against each other, is plotted in Fig. 5.35 for different concentrations of formamide. The values shown in the upper part of Fig. 5.35 were determined with the Cano method, while the bottom part shows the results obtained by the direct method. The two plots in Fig. 5.35 basically show the same behavior. In both plots no clear temperature dependence of the helical twist can be found. Right after the phase transition into the lyotropic SmC analog phase, the helical structure is... 

Cellulose and its derivatives can form liquid crystalline solutions in a variety of organic solvents. Most of the lyotropic liquid crystalline phases derived from these compoxmds are cholesteric. Since the flow occurs in a shear field, the chiral nematic structure is transformed into a nematic phase. Nevertheless, shear phase orientation can be destroyed when the applied force is removed. This phenomenon is caused by the driving force that makes the liquid crystal form a supramolecular helical structure with thermodynamic stability [70]. The mesophase has a supramolecular helical structure, whose cellulose molecules are inclined at a small angle, which varies from one layer to another. 

As a result of almost 40 years of comprehensive studies of the physicochemical behavior of the LC phase of polypeptides, it was thus possible to formulate the basic conditions of the formation of the cholesteric mesophase, to reveal a series of factors which affect the stability and paramet s of the supermolecular helical structure, and even to learn to control this structure with electric and magnetic fields. Polypeptides were excellent models in the construction and verification of different theoretical models of the formation of the LC phase from rod-like rigid-chain macromolecules. With respect to the possibilities for the practical use of the optical properties of lyotropic and thermotropic systems based on polypeptides, these questions have still not been answered. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Finally, there is considerable interest in polymeric assemblies both in solution and in liquid crystalline phases [87]. In a seminal report, Meijer and co-workers [49] have synthesized dimers of module 75 (e.g. 101) and shown that its solutions have rheological properties similar to those shown by normal polymer solutions (Fig. 25). In this regard, the high dimerization constant of 75 allows a high degree of polymerization at accessible concentrations. Likewise, Lehn has shown that 1 1 mixtures of 102 103 and 33 104 form supramolecular, polymeric, liquid crystalline phases (Fig. 25). The structure of 102 103 is believed to contain a triple helical superstructure [88], whereas rigid assembly 33 104 forms a lyotropic mesophase [89]. 

Figure 7. Liquid crystal abstractions (left to right and top to bottom) thermotropics and lyotropics with structural visualizations of formerly alien discoid [28 a, b], the well-known calamitic artificial liquid crystal l-[/rani-4-(alk-3-en-l-yl)cyclohex-l-yl]-4-cyanobenzene [28 c], and long disregarded native helical DNA [28d].

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