Cholesteric homopolymers

Figure 6.28. The side chain cholesteric homopolymers. (From Chielhni et al., 1983.)...

More recently it has been proved that it is possible to prepare cholesteric homopolymers.Krigbaum et al. observed fan-like texture for polyester synthesized from 4,4 -dihydroxy-a-methylstilbene and (+)-3-methyladipic acid. These authors suggested the formation of a strongly twisted cholesteric. Indeed, in low molecular weight cholesterics with high twist, the defects and textures resemble those of smectics, especially smectic A. Such cholesteric substances may yield non-planar textures. They may appear in fan-shaped, focal conic or polygonal textures. In the case of... 

Strzelecki and coworkersfound that the use of (+) 3-methyladipic acid in the synthesis of liquid crystal polyesters led to a cholesteric polymer. They prepared a homopolymer and a series of copolyesters of the following structures ... 

Blumstein and coworkers studied the cholesteric behavior of polyesters with azoxybenzene mesogenic units and the same chiral spacer, (+) 3-methyladipic acid. They could clearly observe oily streak textures, which are typical of low molecular weight cholesterics, for the following homopolymer and copolymers ... 

Recently Krigbaum and coworkers prepared cholesteric polyesters from the reaction of 4,4 -dihydroxy-a-methylstilbene with mixtures of (-f) 3-methyladipic acid and adipic acid. They could change the morphology of the cholesteric state formed by the (4-) 3-methylapidic acid homopolymer by mixing it either with a low molecular... 

Starting from these polymers it is possible to introduce the chiral acids known from low molar mass liquid crystals (12) and to obtain the chiral homopolymers presented in Scheme III and Table III. These polymers show a high spontaneous polarization in the chiral smectic C phase (14) (see polymer 7, Table III) and selective reflection of visible light in the cholesteric phase (see polymer 9, Table III) (13). 

In the side chain liquid crystalline polymers the nematogenic side groups and chiral side groups are attached respectively to a polymer backbone to form a copolymer. They exhibit the cholesteric phase. A typical example is shown below. One is the homopolymer (shown in Figure 6.28) and the other is the copolymer (Figure 6.29). 

LCs were the earliest studied structures, in which polypeptide homopolymer rods pack in an ordered manner to form smectic, nematic, and cholesteric phases. The smectic LCs are mainly formed by polypeptide homopolymers with identical polymer length. The cholesteric phase can be prepared by synthetic polypeptides with polydisperse chain length. The nematic phase can be regarded as a special example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric pitch and chirahty in the polypeptide LCs are dependent on many factors, such as temperature, polymer concentration, solvent nature, and polypeptide cOTiformation. Deep understanding of such phenomena is necessary for preparation of ordered polypeptide assembles with delicate stmctures. The addition of denaturing solvent to polypeptide solution can lead to an anisotropic-isotropic reentrant transition at low temperatures where the intramolecular helix-coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to the conformational ordering effect. 

In addition to the homopolymers also copolymers were studied, in which the comonomers were characterized by spacer groups of different length. The combinations studied were n = 2/6 n = 6/12 and n 2/12. Smectic polymers were observed by copolymerization of monomers with n = 2/6 and n = 6/12, whereas cholesteric polymers were obtained by copolymerization of monomers with n = 2/12, if the composition was approximately 1 1. Apparently large differences in spacer... 

A specific feature of ch selective light reflection an trophotometric study of the f homopolymers in the 20-150 C ing temperatures of homopolym the existence of a broad refl 300 nm region. Circular dich same wavelength region (Fig. homopolymers produce a choles sic selective UV-light reflec molecular cholesterics, chole play a tendency to layer pack the structuring effect of the... 

Figure 22 Photoinduced untwisting of (a) the cholesteric helix in the films of (b) an LC copolymer containing chiral-photochromic monomer units and (c) a mixture of nematic homopolymer and chiral-photochromic...

Figure 22 schematically represents (a) light control over the helical stmcture of (b) a cholesteric photochromic polymer through the use of a copolymer containing the combined chiral-photochromic group, and (c) a mixture of a nematic homopolymer with low-molecular-mass chiral-photochromic dopant. 

These results indicate that copolymerization of small amounts of a chiral monomer with a nematogenic monomer results in the formation of a cholesteric mesophase even in cases where the homopolymer of the chiral monomer has an amorphous structure. 

Similar results were also obtained for copolymers I (Table 7.5). Homopolymer A forms smectic and nematic mesophases, and homopolymer B has an amorphous structure. Copolymers containing up to 25 mole % chiral units form the cholesteric mesophase. 

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. 

We will first examine the structure of homopolymers which form cholesteric and smectic mesophases (the polymers in series 2, Table 7.4). At room temperature, these polymers form the smectic phase in which the cholesterol groups form layers with liquid intralayer order, and the long axes of these groups... 

The cholesteric mesophase of low-molecular-weight liquid crystals is usually represented as a twisted nematic phase. In the homopolymers examined, pronounced layered order is observed in the cholesteric mesophase, and this does not allow considering them as a twisted nematic phase. The cholesteric mesophase of polymers can apparently be represented as follows (see Fig. 7.10d, e). The smectic mesophase 5 is formed at low temperatures, and twisting of the smectic layers takes place in the region of the S -Ch transition so that any section perpendicular to the axis of twisting is a structure characteristic of die phase. 

The phenomenon of untwisting of the cholesteric helix near the transition into the smectic phase is especially clearly manifested in homopolymer 2.3 (Table 7.4) with a decrease in the temperature the temperature region of existence of the cholesteric mesophase in this polymer does not exceed 5 C. For this polymer, the values of the S -Ch and CA-rf transition temperatures increase with an increase in the molecular weight [84], which results in a shift in the region of the existence of the cholesteric mesophase for the different firactions along the temperature scale. A sharp difference in the pitch of the cholesteric helix for fractions with a different molecular weight is the consequence of this (Fig. 7.17). 

Analogous to the described homopolymers, the copolymers are mainly based on stereoirregular methacryloyl or, less frequently, acryloyl derivatives containing cholesterol as the intrinsically chiral component. A series of linear and crosslinked (elastomeric)" copolymers based on the highly flexible siloxane backbone has been investigated in great detail and in all cases the copolymers have been found to assume a cholesteric structure. 

Another possibility is offered by introducing chirality into the molecular structure. Copolymerization or copolycondensation of a monomer, capable of forming a thermotropic nematic homopolymer, with a chiral compound yields cholesteric copolymers. 5,25,32,41 -47 optical properties of these cholesteric copolymers resemble those of conventional cholesteric compounds. ... 

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