Induced Smectic Mesomorphism by Incompatible Nematogens

Non-linear behaviors of nematic-isotropic transition temperatures and the appearance of the “solid-like” phase found in the mixtures between MBBA and 4-cyano-4-n-pentylbiphenyl (CPB)1 are now found to be quite general among many other nematogens. The cyano-group containing nematogens do not have to be biphenyl derivatives, nor do the “donner” type molecules need to be limited to Schiff Bases and azo-type compounds. This type of phase diagram has also been obtained in mixtures of CPB with phenyl benzoates, azoxy and tolan nematogens. Other cyano-group containing nematogens, such as substituted 4′-cyanophenyl benzoates and 4′alkoxybenzylidene-4-aminobenzonitriles gave rise to behavior identical with the above nematogens. A detailed study on the phase diagram of mixtures containing 4-cyano- 4′-n-octyloxybiphenyl and 4′n-hexyloxy-phenyl-4-n-butyl blenzoate has yielded further insight into the nature of the “solid-like” phase. The smectic thermal stability of the alkoxycyanobiphenyl decreased sharply as the concentration of the phenyl benzoate nematogen increased, while the thermal stability of the “solid-like” phase increased, drawing the typical conical shape, to such an extent that the mixtures of certain concentration ranges failed to exhibit any nematic phase. Careful observation of the “solid-like” phase, as it appeared from its isotropic phase, under a polarizing microscope, revealed many characteristics of the classical smectic mesophase, e.g. bPtonnets, focal conic fan textures, and Grandjean terraces. The mixtures outside of this range exhibited a series of phase transitions; isotropic to nematic, isotropic and nematic to solid-like phase, and nematic to true solid. All these phase transitions, except for the last one, were reversible without super-cooling. These results cast doubts over the nature of this solid-like phase which has been claimed to be due to formation of a charge transfer type compound in the literature.1 A substructure between the two incompatible nematogens, such as a dipole induced lamellar structure, is proposed as an alternative model to account for the results.