Effect of smectic short-range order on the discontinuous anchoring transition in nematic liquid crystals.

We report studies on the temperature dependent alignment behavior of a homologous series of trans, trans- 4, 4'-dialkyl-(1α,1' α-bicyclohexyl)-4ß-carbonitrile (CCNs) on a perfluoropolymer coated cells. Among six compounds in the series, one (CCN-35) has only nematic phase and the remaining five have either smectic-A or smectic-B in addition to the nematic phase. We simultaneously performed temperature dependent dielectric measurements and optical polarising microscope observation. It is found that except for CCN-35 and CCN-73, the remaining four compounds exhibit discontinuous anchoring transition from planar to homeotropic and vice versa with increasing thermal hysteresis. We developed a simple theory taking into account the effect of smectic short-range order at the substrates to explain the experimental observations.