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 (-C(7)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, η(2) (director parallel to the shear direction) and η(1) (director perpendicular to the shear direction). The orientation of the director was studied using small angle light scattering techniques. η(2) was measured in presheared sample, whereas the electrorheological technique was used to measure η(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 π-electron conjugation and intermolecular association of highly polar molecules.