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Several Interesting Fields Exploited through Understanding of Polymeric Effects on Liquid Crystals of Main-Chain Polyesters

We review our experimental results on the structural characteristics of polymeric liquid crystals collected through our studies on the main-chain liquid crystalline polyesters with mesogenic biphenyl groups connected by flexible alkyl spacer. The polyesters studied, designated as BB-n where n is the number of carbon atoms in the alkyl spacer unit, form smectic mesophases. Of interest is that the type of smectic phase alters depending on the odd-even parity of n. An ordinal smectic A phase is formed by BB-n with even n, whereas BB-n with odd n forms a new type of smectic phase, so called smectic CA phase, in which the long axis of the mesogen is tilted against the layer normal and the tilting direction is opposite between neighboring layers. Finding of smectic CA phase with specific packing symmetry leads to a distinct discovery of ferroelectric and antiferroelectric phases in achiral molecular systems. In these smectic phases, the polymer chain assumes more extended conformation. Such an extended conformation, however, does not continue over the whole length of the polymer and hairpin folding arises to gain entropy. In the smectic phases, the hairpin foldings are located at a limited space, forming chain folded lamellae. Unusual shear flow orientation and anomalous crystallization behavior are explained based on the chain folded lamellar structure. Solid state morphology of liquid crystalline polymer is also presented. Since the degree of liquid crystallinity is almost 100%, the solid phase cooled through the formation of liquid crystals is composed of crystal and liquid crystal glass. Liquid crystal glass is studied and properties are discussed in a comparison with those of conventional isotropic glass.