Second harmonic generation (SHG) measurements were performed on the nematic liq. crystal formed from polar arom. polyester which comprises 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid in a molar ratio of 73/27. The polymer is com. available and called Vectra. The shear-oriented nematic liq. crystal of Vectra shows strong SHG mostly along the n-director. Since no elec. poling is made, the obsd. SHG activity indicates that the polar ordering is spontaneously formed in the nematic liq. crystal. From detailed analyses of the SHG intensity profiles measured with combinations of polarizer and analyzer directions to the n-director, the nematic liq. crystal is found to possess the Cs packing symmetry; in other words, it is not uniaxial but biaxial, and the polarization arises in the symmetry plane. To search the origin of the spontaneous polarization, a series of Vectra polymers were prepd. with various d.p. (DP) and two chem. modified Vectras. Of interest is that the DP of polymer significantly affects the polar structure. When DP is decreased, the SHG activity in the nematic liq. crystal disappears completely. Since the dipole moment increases with DP, the huge dipole moment of polymer is considered to be responsible for the polar ordering. In one of the two chem. modified Vectras, terephthalic acid (TA) and biphenol (BP) in an equimolar content are introduced. As expected from the dissipation of head-tail character in constituent polymer, the nematic liq. crystals lose completely the SHG activity only with an incorporation of 5 mol % of TA and BP. In another modified Vectra including 3-hydroxybenzoic acid (m-HBA), the head-tail character is not lost, but their liq. crystallinity is destabilized because of kink conformation induced by the m-HBA unit. In this system, the SHG is invariably obsd. for the nematic liq. crystals, but as one of the significant effects, polar biaxial nematic liq. crystal of Vectra is altered to the polar uniaxial one by the introduction of m-HBA above 5 mol %.
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