Polymer with cholesteric side groups, liquid

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

Liquid Crystalline Order in Polymers and Copolymers with Cholesteric Side Groups... 

Several hundred LC polymers with mesogenic side groups which model the structure of nematic, smectic, and cholesteric liquid crystals have now been synthesized with these methods. Polymers of the acrylic and methacrylic series and polysiloxanes containing different types of widely distributed mesogenic... 

The information on the structure of the cholesteric mesophase of polymers is currently limited to data on cholesteric polymers of the comb-shaped type. The comb-shaped structure of macromolecules with mesogenic side groups determines their tendency to form layered structures. In this respect, the question arises of how the helical supermolecular structure is formed in such a system and what its features are in comparison to the cholesteric structure of low-molecular-weight liquid crystals. The answer to this question is given in [81, 82], where the structure of homopolymers and copolymers forming the cholesteric mesophase was studied. 

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... 

Cholesteric liquid crystal pol5miers are commercially available, such as the HELICONE series. This type of polymer has been described in detail (42). Cholesteric siloxanes are liquid-crystaUine side chain polymers (41). Also, compoimds based on cyclic organo-siloxanes with side chains containing cholesterol and methacryloyl groups have been described (43). 

Polymers may exhibit liquid crystalline, ordered structures similar to those of small molecular liquid crystals. As polymer chains are very long, containing about 10 -10 monomer units, only parts of them or side groups can be ordered in oeir tic. cholesteric, and different kinds of smectic structures. As in small-molecular nematic liquid crystals, in nematic polymer structures groups of cylindrical-like symmetry are arranged statistically with their long axes along a certain direction. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

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... 

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