Mesomorphic behaviour of N,N′-dialkanoyl-2,3,5-trimethylbenzene-1,4-diamines and 2,3,5,6-tetramethylbenzene-1,4-diamines

DSC and microscopic studies on several series of N,N'-dialkanoylbenzene-1,4-diamines are presented. The isotropization enthalpy is drastically reduced by the introduction of methyl groups into the aromatic nucleus. The trimethyl derivatives carrying octanoyl and nonanoyl groups generate metastable mesophases exhibiting a spherulitic texture; those carrying undecanoyl-to-octadecanoyl groups generate metastable or stable nematic phases exhibiting schlieren textures. The addition of the fourth methyl group to the nucleus results in an increase in the order of molecular arrangement and the enhancement of the thermal stability. A fingerprint texture is observed for many members of the tetramethyl derivatives and a transient schlieren texture for the octadecanoyl member.