Conformation of main chain liquid crystal polymers

Abstract

The main chains are liquid crystalline polymers (LCP) [I], obtained from an alternative succession of liquid crystal segments (mesogens) and spacers (generally alkyl groups) (Figure 1). These polymers are of a great technical interest since they may have both the elastic properties of polymers and the liquid crystalline proper-ties of the mesogenic segments. The bulk of such polymers present the characteristic phases of liquid crystals as a function of the temperature: isotropic, nematic, or sometimes smectic. They exhibit very low solubilities in common organic solvents and very high nematicisotropic transition temperatures (22OOOC). These reasons have been major obstacles for the investigation of the chain conformation by the classical methods generally used to study polymers, and explain why such studies are rare. In the nematic phase, the mesogens are aligned along the nematic direction. The competition between the long-range nematic order and the tendency of the polymer to maximize its entropy by having the conformation of a gaussian coil leads to two models [2] (Figure 1). In the first one, the chain undulates around the nematic director. In the other one, the conformation presents hairpin defects where the chain executes abrupt reversals. Their existence can have an important role in the rheology, in the non linear dielectric and optical properties of main chain LCPs. Thus, the problem arising from this new class of polymers was the determination of the polymer chain conformation in the isotropic and nematic phases.