Nematogenic polymers

FIGURE 19 Schematic phase diagram of a nematogen-polymer solution. 

Bianchi E, Ciferri A and Tealdi A (1982) Liquid crystal solutions. Ternary systems involving a nematogenic and a non-nematogenic polymer, Macromolecules 15 1268-1272. 

Actually, these considerations are confirmed by experiments 82). The systems investigated are shown in Table 8, No. 4, which are induced cholesteric polymer systems. The nematogenic host molecules of benzoic acid phenyl esters are linked via spacers of different length m (m = 3, 4, 5, 6) to the polymer backbone. The polymers are converted to polymers having a cholesteric phase by the chiral cholesteryl derivative, which is also linked to the polymer backbone (copolymer). 

Copolymerization of two mesogenic monomers is, at the present time, the only pathway to obtain polymers with cholesteric mesophase (see Part 4.4). On the other hand, only by copolymerizing smectogenic and nematogenic monomers and investigating the properties of copolymers in a broad interval of compositions, is it possible to establish the principles of formation of each type of mesophase. We demonstrate... 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 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). 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

Polymer chains with fully rigid conformation are rare in nature. Usually, the nematogenic chain molecules are able to adopt a variety of conformations, some more extended and rodlike than others. Occurrence of liquid crystallite in such... 

The applicability of the rule of selective miscibility has been also examined for mixturg gf nematic side chain polymers with conventional nematogens In contrast to linear polymers, onli-... 

Fig, 20. Isobaric phase diagram of a LC side chain polymer and a small molecule nematogen. From ref. 6h. 

In LMLC the ratio ASNi/AStotal is traditionally used to visualize the partitioning of order between the meso-phase and the crystalline phase. For LMM nematogens this ratio is typically. 03-0.05. In the case of nematic main chain polymers, ratios of 0 07 to 0.17 have been re-ported i, and these relatively high values have been interpreted to indicate a relatively high level of order in the polymeric nematic phase. 

If nematogenic groups are linked via short flexible spacers to the polymer backbone, the macroscopic properties of nematic polymers may deviate from conventional nematic systems, o i/ing to a disturbing effect of the polymer main chain. This is indicated e.g. by the absence of typical nematic textures and some anomalous optical properties The symmetry of the phase structure, how-... 

Ballauff M (1989) Networks composed of rigid rod polymers, Angew Chem Int Ed 28 1130-1131. Krigbaum W R, Hakemi H and Kotek R (1985) Nematogenic poljmers having rigid chains. 1. Substituted poly(p-phenylene terephthalates), Macromolecules 18 965-973. 

A necessary condition for the formation of the cholesteric mesophase, or chiral nematic phase, is the presence of a chiral center chemically bound to the molecules or an optically active additive. Such a centre determines the helical twisting meso-gens in both low-molecular-mass and polymeric liquid crystals. Wide opportunities for the synthesis of cholesteric polymers are provided by the copolymetization of nematogenic and chiral monomers (Figure 19). 

The orientation of the segments of a macromolecule in an electric field is cooperative in nature, where the orientation of each chain element causes the orientation of neighboring elements. This means that increases in the cooperative effect can be expected with an increase in the molecular weight. This hypothesis is confirmed by the dependence of the Kerr constant on the composition of the copolymer (cf. Fig. 3.9) and the different character of the dependence of K on M in the two compositions of the copolymer (cf. Fig. 3.13). The absolute values of K decrease with a decrease in the proportion of the nematogenic component (polymer PM-16 in the copolymer with polycetyl methacrylate), i.e., with a decrease in the intramolecular interaction (copolymer of 1 1 composition), and they are not dependent on the molecular weight (line 2 in Fig. 3.13) [81], which indicates attenuation of the cooperative character of the orientation of the individual kinetic units in an electric field. Such a decrease in the orientational order reflects a decrease in the absolute values of K (curve 2 in Fig. 3.12) with an increase in the temperature [79]. 

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