Alignment of unconventional nematic liquid crystals

Abstract

Planar (homogeneous) and homeotropic alignment of some unconventional nematic liquid crystals with both positive (Δε > 0) and negative (Δε < 0) dielectric anisotropies are studied on planar (AL-1254) and homeotropic (JALS-204, SE-1211) alignment layers. Irrespective of the sign of dielectric anisotropy and molecular structure, uniform planar alignment of the director is observed in Al-1254 coated cells whereas no stable homeotropic alignment of bent-core compounds with Δε < 0 is observed in cells coated with JALS-204 or SE-1211. Alignment properties of these two types (Δε > 0 and Δε < 0) of unconventional nematic liquid crystals are explained considering the steric, short-range dipolar and long-range van der Waals interactions at the interface. A bent-core compound (Δε > 0) which has no higher-order liquid crystalline phase except for the nematic exhibits a narrow temperature range of homeotropic state (on JALS-204) while cooling from the isotropic phase followed by an anchoring transition to a planar state as the temperature is further lowered. The measured polar anchoring energy of this compound on JALS-204 is significantly lower than many known calamitic liquid crystals and is responsible to the anchoring transition observed.