Nematic phase, main-chain

Fig. 14. Main-chain polymer Hquid crystal phases (a) nematic, (b) smectic.

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

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. 

NMR spectra and Tj measurements at different temperatures. The local polymer chain motion varies over a frequency range of 104-106 Hz in the nematic phase. The activation energy of this motion is found to increase with decreasing number ( ) of methylene units in the spacer, and exhibits odd-even fluctuations. In a study of a homologous series of main-chain LC polyesters, 13C CP/MAS and variable-temperature experiments reveal a conformation-ally more homogeneous and a less dynamic nature for the even-chained than for the odd-chained polymer structures.300... 

The polyamides are soluble in high sirength sulfuric acid or in mixtures of hexamelhylphosphoiamide. AMV-diinethylaeciamidc. and l.ifT. The liquid-crystal phase is optically anisotropic and the texture is nematic. The nematic texture can he transformed lo a chiral nematic texture by adding chiral species us a dopant or incorporating a chiral unit in the main chain as a copolymer. 

The development is reviewed of liquid-crystalline polymers whose mesophase formation derives from the nature of the chemical units in the main chain. The emphasis lies primarily on highly aromatic condensation polymers and their applications. The general properties of nematic phases formed by such polymers are surveyed and some chemical structures capable of producing nematic phases are classified in relation to their ability to form lyotropic and thermotropic systems. The synthesis, properties, physical structure and applications of two of the most important lyotropic systems and of a range of potentially important thermotropic polymers are discussed with particular reference to the production and use of fibres, films and anisotropic mouldings. 

Academic and industrial interest in liquid-crystalline polymers of the main-chain type has been stimulated by certain special properties shared by lyotropic and thermotropic systems that exhibit a nematic phase. Although these special properties affect both the processing into fibres and other shaped articles and the physical behaviour of the products, the product behaviour is at least partly attributable to the novel processing behaviour. 

In addition to their unusual rheological properties, the nematic phases of polymers, like those of simple compounds, can be oriented by the application of magnetic or electrical fields. These properties have been more fully examined for comb-type polymers with mesogenic side-chains than for polymers with the mesogenic groups in the main chain, since in the comb polymers it is possible to influence the side-chain orientation independently of the main-chain orientation. 

So far we have not considered the influence of the constitution of the polymer main chain on the formation of the nematic phase. If the same mesogenic group is linked to different backbones, the nematic phase can be preserved, as shown for one example in Table 3. Owing to the different flexibilities of the backbones, the nematic state is shifted with respect to the temperature. With falling flexibility of the main chain, as indicated by the increasing glass transition temperature, the phase transformation temperatures nematic to isotropic are shifted towards higher temperatures. This clearly indicates that the restriction of motions, due to the polymer-fixation, directly reflects on the phase transformation temperature. If this restriction... 

Table 3. Phase behavior of polymers having the same mesogenic group linked to the different main chains (AT = extent of the nematic phase)...

In any case, both models have in common that owing to the positional ordering of the mesogenic side chains, the polymer backbone no longer exhibits a statistical three dimensional coil conformation. Therefore at the phase transformation isotropic to smectic or nematic (cholesteric) to smectic, in addition to the change of the anisotropic packing of the side chains, the main chain has to change its conformation, which must be consistent with the layered smectic structure. A direct interaction... 

The only general statement that might be made is that the specific features of the structure of polymeric nematics are provided for by the ordering effect of the main chain on the packing of mesogenic groups in LC phase. 

The fusion of LC phases above Tcl causes a sharp change in the character of flow and the values of Ef for nematic and smectic polymers become closer. In an isotropic phase Ef for a polymeric smectic ( 140 kJ/mole) is only twice as large as Ef for a polymeric nematic (70—80 kJ/mole). In other words, the transition from LC phase to isotropic melt, accompanied by the liberation of mesogenic groups from the mesophase levels the differences in the character of flow of smectic and nematic polymers. The differences in Ef for isotropic phase are determined only by the differences in chemical nature of the main chain of smectic and nematic polymers. The values of Ef, in this case, are close to the Ef values for poly(butylmethacrylate) and poly(butylacrylate), respectively, which are structurally similar to polymers XI and XII except that they do not contain mesogenic groups. 

A number of characteristic temperatures are important in LC polymer work. The glass transition temperature, Tg, is that temperature below which segmental motion of the main chain of the polymer does not occur, although motions (e.g., rotation) of side-groups may occur. The isotropization or clearing temperature, 7j, is the temperature at which the polymer enters the isotropic melt from one of its mesophases and the birefringence of the mesophase disappears. Temperatures are often quoted more specifically defining where phase transitions occur. For example, 7, would be the temperature where the nematic phase enters the isotropic melt. In this case, of course, TNI is the same as Tr... 

Lor polymers in which the mesogen is separated by a spacer of six methylenes units from the polymer backbone, it is obvious that the more rigid poly(norbornene)s favor nematic liquid crystalline phases. Poly-(VI-6) with the rigid and bulky 2,5-dimethine oxacyclopentane-3,4-dicarboximide unit in the main chain does not show liquid crystalline behavior (Table 6, entry 12). The more flexible poly-(II-6) backbone allowed the formation of a nematic mesophase. If the mesogen density was increased, as realized in poly-(IV-6), the isotropization temperature was found to be 26 °C higher than that for poly-... 

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

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