Cholesteric modifications

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

Copolymerization thus presents an effective method for modification of LC polymer properties aimed at diversification of their thermal characteristics as well as at the creation of new types of LC polymers, cholesteric LC polymers in particular. 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

Fig. 15 Schematic representation of the photoisomerization effect on a cholesteric meso-phase based on azo-chromophore-containing cellulose derivative molecules. (Quoted from [233] with an adequate modification)...

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

The following protocols (6-10) describe the synthesis of some cholesterol-based acrylates and their photopolymerization in an aligned cholesteric phase. The protocols utilize a modification of a system previously described by Shannon. 5 6 ip ie absence of a diacrylate comonomer, the cholesteric phase produced initially on copolymerization is not stable and reverts to a smectic phase on a single cycle of heating and cooling. In the presence of the diacrylate the first-formed phase is stable. This is one example of how crosslinking can stabilise the liquid crystal phase in liquid crystalline elastomers, others include, the so-called, polymer-stabilized liquid crystals and those described in the later protocols. 

The finding that the PEIs of 27b and monosubstituted hydro quinones form broad nematic phases, but show little propensity to crystallize, has prompted various modifications of their structures and properties. In this connection it should be stated that non-crystalline LC-polymers have found little interest in the past decades, but they may be attractive for various applications provided that the Tg can be varied between 90 and 250 °C. For instance, the absence of crystallinity has the advantage that the mechanical properties do not depend on the thermal history, and thus on the processing conditions. The temperature allowing a convenient processing may be reduced below 200 °C, which is of interest for the processing of LC-polymer reinforced blends and composites. Furthermore, non-crystalline nematic FC-polyesters are a useful basis for the synthesis of cholesteric lacquers, films or pigments (Sect. 5). 

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Ruslim C, Ichimura K. 2000. Conformational effect on macroscopic chirality modification of cholesteric mesophases by photochromic azobenzene dopants. J Phys Chem B 104(28) 6529 6535. 

The cholesteric nature of the cellulose derivatives and its optical, thermal and mechanical properties can be tuned by the chemical modification of the original molecular structure, giving rise to a high brand of thermotropic materials. These new features impart specific properties that make them of high interest for photonics, electronics, stimuli-responsive devices as well as biomedical applications. 

An increasing phase biaxiality obviously disturbed the smooth optical periodicity of the cholesteric phase structure. Whether these defects were due to a modification of the continuous twist of the main director or to the biaxial optics... 

Fig. 1.8 The twisted nematic N (cholesteric) structure, formed by chiral modifications of nematic compounds tmd by non-chiral nematagens doped with chiral solutes. Where the pitch of this helicoidal stiucture is comparable with the wavelength of light, the phase gives iridescent reflections. Larger pitch structures, where the repeat distance can be resolved in the optical microscope, appear as fingerprint textures with bands separated by half a pitch. Helicoidal structures of this type are common in biological material, most obviously in the carapaces of iridescent beetles. Their electron microscope pictures show a characteristic pattern of nested arcs, known as Bouligand patterns [4]...

As already mentioned, mesogens embrace a diversity of structure. We shall not, however, be concerned here with mesophases of disc-, pyramid-and phasm-like molecules, but shall confine attention to thermotropic mesomorphism exhibited by LMM compounds with elongated and relatively rigid lath-like molecules. Mesophases formed on heating such compounds are classified into three types nematic, cholesteric, and smectic. There are more than ten recognized smectic modifications and these are denoted by Sa, Sb, Sc-.-Sl- A description of the structural features of these phases may be found in standard books and reviews " and it is my intention in this section to bring the information on this subject up to date and to present it in a more concise form. 

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