Type I Chiral Nematic Liquid Crystals

As the electro-optic display industry developed, great interest was centred on the development of suitable materials for use in twisted nematic or phase change displays, particularly as many of the first generation of materials frequently presented problems in practical usage. For instance, cholesteryl esters were deemed to be unsuitable for such applications for many of the reasons already outlined in Sec. 1.4 of this chapter, and also because of difficulties associated with optimising physical properties such as birefringence, viscosity, and dielectric anisotropy. Consequently, novel chiral materials based on known nematogenic materials 

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Miscellaneous Type I Chiral Nematic Liquid Crystals.323... 

Despite the wide variety of chiral nematic liquid crystals (and of course low molar mass chiral liquid crystals in general), it is possible to sub-divide them into three class types, according to the relationship of the chiral moiety and the liquid-erystalline core [29]. Firstly, we consider type I here the chiral centre (or multiplieity of chiral centres) is situated in a terminal alkyl chain attached to the effective liquid-crystalline core. As will be seen later, it is entirely possible that the compound can have two or more chiral terminal groups associated with the molecule. Type I materials are probably the most frequently encountered chiral nematic liquid-crystalline compounds, often... 

Cholesteric Liquid Crystals. These crystals are often called twisted nematic or chiral nematic, because only chiral molecules possess this special type of liquid crystalline phase. These liquid crystals are similar to the nematic liquid crystals, but along the director, each molecule is twisted as compared to the previous molecule by a certain angle. Similar to (i.e., in much the same way as do) nematic liquid crystals, cholesteric liquid crystals become isotropic liquids at the clearing point. 

In addition to containing a wealth of information on microscopic techniques and materials, Friedel s article represented in 1922 the first classification of liquid crystals into types, i.e., nematic, smectic and cholesteric. Today, of course, cholesterics are known simply as chiral nematics with no need that they be derived from cholesterol, and we recognize the existence of several polymorphic smectic forms, whereas Frie-del allowed for only one (today s smectic A SmA). 

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