Helical structures thermotropic

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... 

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 existence or nonexistence of mirror symmetry plays an important role in nature. The lack of mirror symmetry, called chirality, can be found in systems of all length scales, from elementary particles to macroscopic systems. Due to the collective behavior of the molecules in liquid crystals, molecular chirality has a particularly remarkable influence on the macroscopic physical properties of these systems. Probably, even the flrst observations of thermotropic liquid crystals by Planer (1861) and Reinitzer (1888) were due to the conspicuous selective reflection of the helical structure that occurs in chiral liquid crystals. Many physical properties of liquid crystals depend on chirality, e.g., certain linear and nonlinear optical properties, the occurrence of ferro-, ferri-, antiferro- and piezo-electric behavior, the electroclinic effect, and even the appearance of new phases. In addition, the majority of optical applications of liquid crystals is due to chiral structures, namely the ther-mochromic effect of cholesteric liquid crystals, the rotation of the plane of polarization in twisted nematic liquid crystal displays, and the ferroelectric and antiferroelectric switching of smectic liquid crystals. 

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. 

Fig. 14.6 SEM pictures of helical structures observed in electrospun APC thermotropic fibres. Suspended fibres show helices (a) or spirals (b) if suspended by two or one end, respectively. Reproduced in part liom Godinho MH, Canejo JP, Pinto LFV, Borges JP, Teixeira PIC (2009a), How to mimic the shapes of plant tendrils on the nano and microscale spirals and helices of electrospun liquid crystalline cellulose derivatives . Soft Matter 5 2772, with permission of The Royal Society of Chemistry...

If we approximate the liquid-crystalline molecules by rod-like particles, then the basic thermotropic mesophases can be visualized schematically as in Fig. 4.5. The nematic phase is the least organized of all liquid-crystalline phases and usually appears just below the isotropic phase. In this phase the centers of mass of the elongated molecules have three translational degrees of freedom and thus are distributed randomly as in an ordinary isotropic liquid. However, the long axes of the neighbouring molecules are preferentially aligned with respect to an axis which is called the director, usually denoted by a unit vector n. If the molecules have mirror symmetry, the ordinary nematic phase is observed. It is nearly always characterized by uniaxial symmetry and equivalence of n and — n. For molecules without mirror symmetry, however, the spatial variation of n leads to an helical structure. If one considers a plane perpendicular to the helix axis, then the direction of n is the same in this plane, but it... 

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

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). 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

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].

The other thermotropic mesophases (cholesteric and smectic) also result from an ordering of solvent molecules. The cholesteric occurring in solvents possessing a centre of asymmetry, so that the structure is characterised by a helical twist. The smectic mesophase consists of molecules parallel to each other with the extra order being in layers. They are generally viscous and are not so suitable for high resolution NMR spectroscopy. 

Samulski et al2D suggested that an asymetric helical molecular conformation may provide an explanation for the cholesteric structure, and very small oscillations of helices relative to their neighbors in the liquid crystal would be biased to one side of the parallel position because of the chirality of the van der Waals surface of the polypeptide molecules. Further, they found a linear relationship between the pitch and the reciprocal of temperature. This relation holds quite well for a number of thermotropic liquid crystal systems and was explained theoretically by Keating ), who treated the macroscopic twist as a rotational analogue of thermal expansion with the dominant anharmonic forces coming from nearest neigh-... 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

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