2.1 Liquid Crystalline Polymers with a Helical Superstructure

Several liquid crystalline phases - for example nematic and tilted smectic phases - develop a helical superstructure if a chiral dopant is added or the liquid crystalline compound itself has a chiral molecular structure [1, 2]. One enantiomer forms a lefthanded and the other a right-handed helical structure with the same pitch ( P). The twisted chiral form of the nematic phase isA commonly denoted as the cholesteric phase, whereas the twisted chiral forms of all other phases are just marked by an asterisk, e.g., SC * for the chiral form of the SC phase. The helical superstructure is a periodical structure on which incident light satisfying the Bragg condition is reflected. If the wavelength ( ?) of the reflected light is in the visible region the liquid crystal becomes intensely coloured. Pitches of the helical superstructure of chiral liquid crystalline phases ranging between about 100 nm and infinite have been observed so far. Besides synthetic compounds, numerous cholesteryl and cellulose derivatives showing in certain temperature ranges selective reflection in the visible region are described in the literature. Generally, the pitch depends on temperature and can betuned by an external electric field. Thus, these compounds possess thermochromic and also electrochromic properties.

Chiral liquid crystalline polymers with thermochromic properties are also described in numerous publications. Most of them are synthetic polymers. Within the group of biopolymers, especially, several cellulose derivatives have been found to possess thermotropic and lyotropic cholesteric mesophases with...