Cholesteric phase mesogenic groups

A large number of azobenzene-based amorphous and liquid crystalline polymers, particularly polyacrylates and polymethacrylates with chiral azobenzene pendants, have been prepared for the development of data storage and photonic devices [1-3,11-14]. For instance, the introduction of optically active mesogenic azobenzene residues into the side groups of the polymers produces chiral nematic and cholesteric phases, which are regulated by photoisomerization of the azobenzene units [10,14]. In most cases, however, the optical activity and chiroptical... 

There have been many papers concerning liquid-crystalline polymers having azobenzenes in their side chains (Figure 14) because of the mesogenic nature of the azo chromophore. Liquid-crystalline polymers incorporating azobenzene moieties in their main chains (30) have also been prepared.102 A family of poly(acrylates) substituted with azobenzenes having chiral alkyl groups (31) showed smectic A as well as cholesteric phases.103 The photochromic behavior... 

In contrast to the extensive experimental investigations of nematic, cholesteric and chiral SmC phases, comparatively little work has been done on tbe characterization of the physical properties of SmA and SmC elastomers. The elongation, A, has been measured as a function of temperature for constant external load for a number of different loads in SmA sidechain elastomers [6]. It was found that A increases monotonically as a function of temperature for a material, which has a SmA-I transition. In addition it was shown that the elasticity modulus, E, decreases monotonically with temperature. X-ray investigations on SmA phases in side-chain LCE have been performed [70]. It was found that for the family of compounds studied, the orientation of the mesogenic groups was always perpendicular to the direction of stretching. 

This chapter highlights the photomodulation of CLCs, including light-induced phase transition related to cholesteric mesophase, helix inversion, reflection color control, and creation of mechanical motion in photoresponsive CLCs. The material systems introduced in this chapter are classified into two main groups based on chiral meso-gens and induced CLCs with several subsystems in each. Although the cholesteric phase was initially found with pure mesogenic compounds and the early quest for... 

Aromatic imide groups are known to be nearly planar, rigid, polar, and thermostable. However, aromatic imide structures are also known to be non-mesogen in nature. Poly(ester-amide) (PEI) derived from N- (4 -carboxyphenyl) trimellitimide and aliphatic spacers are not thermotropic, whether the spacer used is chiral or not. Semi-aliphatic spacers are observed to exhibit both a smectic and a nematic LC phase in the resultant thermotropic PEIs. The semi-aliphatic chiral spacers exhibit both chiral smectic phase (A or C ) and cholesteric phase. Such a chiral smectic LC-phase, which may be ferroelectric in nature, is extremely rare for LC-main-chain polymers [27]. It is a particular advantage of polar imide mesogens to favor the formation of layer-structures when combined with non-polar species [28]. 

Polymers can have similar chiral effects as the corresponding monomers. Chiral centers in the wing groups or in the mesogenic groups can cause cholesteric and ferroelectric phases very similar to Section 4.3.1. However, polymers can have additional positions and effects of chirality. Chirality can be located in the backbone or in spacer groups, chiral dopants can be fixed by copolymerization, and cholesteric networks can be filled with guest compounds, etc. 

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