Rheology of nematic liquid crystals with highly polar molecules

We report experimental studies on the rheology of a few nematic liquid crystals with highly polar molecules (CCH-7, PCH-7, CB-7). The selected molecules have the same alkyl chain ($\text{$-${}}{\text{C}}_{7}{\text{H}}_{15}$) and cyano (-CN) end group. In the core part of the molecule, CCH-7 has two cyclohexane rings, PCH-7 has one cyclohexane and one aromatic ring, and CB-7 has two aromatic rings. Two viscosities were measured as a function of temperature, namely, ${$\eta${}}_{2}$ (director parallel to the shear direction) and ${$\eta${}}_{1}$ (director perpendicular to the shear direction). The orientation of the director was studied using small angle light scattering techniques. ${$\eta${}}_{2}$ was measured in presheared sample, whereas the electrorheological technique was used to measure ${$\eta${}}_{1}$. We show that both viscosities of the liquid crystals depend on the number of aromatic rings and Kirkwood correlation factor. The temperature dependent viscosities can be understood based on the intramolecular $$\pi${}$-electron conjugation and intermolecular association of highly polar molecules.