Ring structures nematics

Lyotropic polymers consisting solely of ring structures are also known, and are exemplified by poly(p-phenylene benzobisthiazole) (IV), which forms a nematic phase in several strongly protonating acids including polyphosphoric acid. Its synthesis and application, like those of the aromatic amide polymers, are discussed later. 

The temperature of the sample had a substantial effect. The ring structure was observed only in the nematic mesophase, and arose the more readily (the threshold power was lower and the number of rings at a given power was larger) the closer the temperature of the sample to the nematic— isotropic-liquid phase transition temperature. The ring structure was not observed in the smectic mesophase or in the isotropic liquid anywhere in the investigated interval of laser powers. 

Most often the lyotropic LC polymers form nematic mesophases. Most of the polymers in this class are aromatic polyamides with aromatic ring structures, as shown in Thble 7.1 (7). Several of the polymers in Table 7.1 form very high-modulus fibers see Chapter 11. The fibers are crystalline after formation. 

The crystal structure of 4-butylphenyl-4 -butylbenzoyloxybenzoate was determined by Haase et al. [101]. The compound forms a nematic phase. The neighbouring phenyl rings of the molecule are twisted by 49 and 62°. The dipole moments of the carbonyloxy groups perpendicular to the long molecular axis are compensated to each other as much as possible. 

Perez et al. [128] investigated the crystal structure of 4-(4 -ethoxybezoy-loxy)-2-butoxy-4 -(4-butoxysalicylaldimine)azobenzene. This compound contains four rings in the main core and a lateral alkoxy branch on one of the inner rings. The lateral butoxy chain is nearly perpendicular to the long axis of the main core. This molecular conformation induces the molecules to make a very complex network in the solid. The crystal cohesion is due to van der Waals interactions. The change of the lateral chain conformation in the solid and nematic phases is discussed. 

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

A NMR study of trans -PtCl2(C2H4)py in a nematic solution in CD2CI2 shows the ratio of dgem dCis to correspond with the structure obtained by neutron diffraction.697 The plane of the pyridine ring is inclined at an angle to the PtCl2 plane with rapid reorientation between the symmetry-related forms.698... 

Pc macrocycle. Some of them are mesomorphic and show a Colh phase (Fig. 79 I-Gl G 115 Coin 270 I II-Gl G <-20 Colh > 320 h H-G2 G 115 Colh 250 I II-G3 94 Colh 108 I) for which the structure of the columnar mesophase is frozen at room temperature (anisotropic glasses). The functionalization of these dendrons in the 3 and 5 position of the terminal rings by oligo(ethyleneoxy) chains (0CH2CH2)30CH3, and their subsequent grafting onto the phthalocyanine of type I (Fig. 79) led to amphiphiUc materials [333]. In concentrated ethanol solutions ( 20-40% by mass), the compound with the fimctionaUzed G1 dendron behaves as a discotic amphiphUe forming a coliunnar nematic lyotropic phase. In addition, it possesses a columnar mesophase stable from room temperature up to 260 °C. Systems of type III are not mesomorphic. 

The last example in this section is the fluorinated derivative 6 [12] (Fig. 9-3), which exhibited an enantiotropic nematic phase with relatively low melting (114.9 °C) and clearing (128.9 °C) points. Its molecular structure revealed an eclipsed conformation for the two eyclopentadienyl rings. 

Structurally similar polymers with azo links, Polymers 9 and 10, formed no meso-phases when the rigid aromatic unit contained terminal carbonyl groups, but these formed nematic mesophases when the ester link was reversed The transition temperatures of the azo and azoxy polymers with the same spacer were quite close to each other. Substitution of methyl groups on the phenylene rings of these polymers reduced melting temperatures without adversely affecting nematic mesophase stability... 

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