Thermotropic liquid crystals optical anisotropy

The main part of the side group in these macromolecules consists of the alkoxy-benzoic acid moiety. This acid may form thermotropic liquid crystals. The investigation of the hydrodynamic properties of PPhEAA molecules in dilute solutions has revealed that the equilibrium rigidity of their main chains is relatively low (Table 12). Hence, since for all flexible-chain polymers, the shear optical coefficient An/Ar in PPhEAA solutions is independent of molecular weight the segmental anisotropy - tt2 and the anisotropy of the monomer unit Aa may be determined by use ofEq. (67). 

Recent optical experiments have probed the effect of flexible polymeric solutes in both lyotropic [36] and thermotropic liquid-crystal [37, 38] environments. Long-chain solutes feel an average effective anisotropy, and are found to exhibit much stronger ordering than smaller solutes. The synthesis of the bent-core mesogens ( banana molecules) has resulted in a novel biaxial smectic-A (both pure phase [39] and solute-induced [40]) as well as nematic phases [41,42]. 

Nematic liquid crystals as thermotropic systans are also employed as orienting medium in the IR-LD analysis [65,66]. Thermotropy means that the liquid-crystal phase (mesophase) is implemented within a certain interval between the melting temperature and the point of clarification. Within these temperature limits, the liquid crystal molecules are located toward a certain direction, inducing orientation without any positional order (Figure 1.9). In their mesomorphic interval, the nematic liquid crystals are characterized by their lower fluidity and transparency as well as anisotropy in their optical properties snch as the double-beam refraction and dichroism. 

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