Polymers, liquid crystalline main chain type

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. 

Another interesting phenomenon found by Stevens et al. is the monotropic mesophase formed by the dimer and trimer of the polymer with m = 6. The dimer has a monotropic nematic phase, the trimer has a monotropic smectic phase. These metastable monotropic phases become stable enantiotropic phases with the increase of n by 1. At about the same time, Blumstein et al. (1984) found the low mass model compound of a main-chain type liquid crystalline polymer was monotropic while the mesophase of the polymer was enantiotropic. 

Zhou et al. (1986) studied the transformation of monotropic to enantiotropic mesophase of the main-chain type liquid crystalline polymer, (3.15), in terms of the molecular weight effect. 

While main-chain type liquid crystalline polymers are made by step-wise polymerizations, side-group type polymers have been synthesized by both step and chain polymerizations of monomers with desired mesogenic side groups. In addition, mesogenic side groups can be introduced to polymers... 

The mesogenic structure of a benzoic acid dimer has been introduced as a noncovalent cross-linker for polysiloxanes [79]. Polymer 57 exhibits a smectic C phase due to the dynamics of H-bonding. In contrast, mesomorphic order is locked in the solid state of poly[(4-acryloyl)benzoic acid] by polymerization in its mesophase [128]. No liquid-crystalline state is observed for this material because of the lack of flexibility of the structures. Main-chain-type polymeric liquid-crystal associates are formed from carboxyl-bifunctionalized aromatic compounds [129]. 

The liquid crystalline material in the LCD is not necessarily a LCP. However, in order to fix the liquid crystalline material conventional polymers are used for embedding and LCP are also used. Side chain LCPs combine the properties of liquid crystals and polymers with flexible main chains. Since the liquid crystal moieties are fixed on the backbone of the main chains of the polymer, this type of liquid crystals may not flow away, as it may be the case in monomeric liquid crystals. 

Liquid crystals (LCs) are described as a fluid phase that flows like a liquid and is oriented in a crystalline manner. LCs are divided into two types thermotropic LCs, where the LC phase transition is dependent on temperature or lyotropic LCs, where the LC phase transition occurs as a function of solvent concentration. To introduce liquid crystallinity to conjugated polymers, LC moieties can be introduced to the polymer side chains for side chain-type liquid crystallinity. On the other hand, designing conjugated polymers with rigid main chain structures with flexible alkyl side chains for solubility enables main chain-type liquid crystallinity. 

Fig. 10. Cartoons depicting situations in which a spin-labeled polymer may yield the types of local director distribution shapes shown in Fig. 9. (a) Director perpendicular to polymer chain direction, determined by interactions between spin-label and liquid-crystalline side chains. Different ordering around two axes leads to the elongated distributions shown at the top of Fig. 9b Director parallel to main-chain direction, with spin label held at a fixed angle with respect to the chain, leading to the conical distributions shown at the bottom of Fig. 9.

Yamamoto, T., Muramatsu, Y., Lee, B.L., Kokubo, H., Sasaki, S., Hasegawa, M., Yagi, T., Kubota, K. Preparation of new main-chain type polyanthraquinones. chemical reactivity, packing stmcture, piezochromism, conductivity, and liquid crystalline and photonic properties of the polymers. Chem. Mater. 15, 4384 393 (2003)... 

Thermotropic liquid crystalline polymers, like polyesters containing mesogenic units on the main chain, may not be described by the wormlike chain model (cf. Sect. 1.2). The present article does not consider this type of polymers. 

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