Elastic constants of nematic liquid crystalline Schiff’s bases

The three elastic constants of a number of structurally related nematic liquid crystalline Schiff’s bases have been measured as a function of temperature. The results are new in the sense that they exhibit a clear distinction in behavior between compounds with a rigid molecular structure and compounds with a molecular structure consisting of a rigid core and a flexible alkyl chain. In particular, it has been found that the ratio of the bend and splay elastic constants K3/K1 increases for compounds with a rigid molecular structure with increasing length to width ratio. This trend is the reverse of that observed wihin homologous series, for which K3/K1 has been found to decrease with increasing length of the alkyl chain. New is the experimental finding that the twist elastic constant K2 of the homologs studied is independent of the length of the alkyl chain. Finally, the experiments show that the average elastic constant K̄1+K2+K3)/3 is proportional to the smectic–isotropic transition temperature. The results can only be partly understood within the framework of existing theoretical models.