Difference in Steady Shear Flow Viscosity between Polar and Nonpolar Nematic Liquid Crystals in Aromatic Polyesters Derived from VECTRA

Abstract

Steady shear flow viscosities was measured for nematic liquid crystals (LCs) in the aromatic copolyesters composed of 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), telephthalic acid, and biphenol whose molar contents are 0.73 − x, 0.27 − x, x, and x, respectively. Second harmonic generation (SHG) measurements showed that a SHG-active polar nematic LC formed by poly(HBA/HNA) with x = 0 is altered to a conventional nonpolar nematic LC with an increase in x more than 0.07. The shear-rate dependence of melt viscosity measured for nematic LCs of all the copolymers showed four characteristic regions; a Newtonian plateau in lowest shear-rate region 0 in addition to the well-known three regions, low-shear-rate shear-thinning region I, intermediate Newtonian plateau region II and high-shear-rate shear-thinning region III. The SHG-activity in nematic LCs reflected on the Newtonian plateau region 0. The viscosity of the SHG-active nematic LCs in this region 0 was 10 times higher than that of the non-SHG-active nematic ones. The higher viscosity of the SHG-active nematic LC was connected to the smaller domain size (or larger number of disclinations) estimated by small-angle light scattering. In the nonpolar nematic LC, many disclinations initially formed were easily annihilated because of a coalescence of two disclinations with opposite signs, whereas in the polar nematic LC, such an annihilation hardly occurred because of the polar packing symmetry.