THE RELATIONSHIP BETWEEN THE CHEMICAL-STRUCTURE OF NEMATIC LIQUID-CRYSTALS AND THEIR PRETILT ANGLES

About 50 different additives in one or more of three different nematic mixtures have been investigated to clarify the relationship between t he chemical structure of the liquid crystal and the pretilt angle on a polyimide surface. The pretilts found for cells have been explained w ithin our recently proposed population distribution model. For compoun ds with cyano-groups at one end, we find that the in-plane order is go verned both by the surface-mesogen interaction and by the relative str ength of the intermolecular interactions in the nematic phase. This st rength is nearly linear in alkyl chain length for the compounds invest igated. Changes in the strength due to variations in the core of the m olecules can be calculated easily by using group contributions from th e known Parachors. The in-plane order can be treated as a simple produ ct of the contributions from the liquid crystal and from the polyimide . Different polar end groups will give different angles between the su rface and the optical axis of the individual mesogens in the first mon olayer. The cyano-group gives the highest angle and alkyl groups the l owest. For nitro-compounds the dimers formed are so strongly bound tha t they do not break up at the surface. Nitro-compounds will thus act a s dialkyl compounds. For dialkyl compounds the pretilt angles are domi nated by the difference between the chain lengths at the two ends of t he molecule.