Alignment of unconventional nematic liquid crystals

Planar (homogeneous) and homeotropic alignment of some unconventional nematic liquid crystals with both positive (Delta epsilon 0) and negative (Delta epsilon<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 Delta epsilon<0 is observed in cells coated with JALS-204 or SE-1211. Alignment properties of these two types (Delta epsilon>0 and Delta epsilon<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 (Delta epsilon<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.