COMPARISON OF THE MESOMORPHIC PROPERTIES BETWEEN SUBSTITUTED SCHIFFS BASES INCORPORATING A LINEAR HYDROCARBON OR FLUORINATED TAIL

We have developed new compounds within the perfluorinated series incor porating a Schiffs base as the connector between the aliphatic chain a nd the 4-biphenyl used as mesogen group. All members of the series are thermally stable and exhibit enantiotropic smectic behavior. A smecti c polymorphism is observed only with F-butyl and F-hexyl tails. They s how an oscillation in the smectic-isotropic transition temperature. Th e influence of the fluorinated chain and the shape of the connector on the stability or the reversibility of mesophases has been investigate d by comparison with their hydrocarbon analogues and with a compound o btained by condensation of 2-(F-alkyl)ethylamine with 4-acetylbiphenyl . The fluorinated moiety strongly promotes enhanced smectic phases and the presence of a methyl group on the connector strongly decreases th e stability of the mesophase. Exceptionally, it would seem that the fl uorinated chain generates a liquid crystal behavior which can not be o bserved with the hydrocarbon homologous structures. The physical prope rties of these novel fluorinated materials are reported and the effect of fluorination on these properties is discussed. All these points ar e commented on and illustrated by the study realized from the differen tial thermal analysis and the light microscopy.