Optical microscopy, DSC and X-ray diffraction studies in binary mixtures of 4-pentyloxy-4'-cyanobiphenyl with three 4,4'-di(alkoxy)azoxybenzenes

A large number of binary mixtures of 4-pentyloxy-4'-cyanobiphenyl (5OCB) and 4,4'-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6 and 7 have been studied by optical microscopy, DSC and X-ray diffraction methods. Over a wide concentration range the smectic A (SmA) phase is induced and the nematic (N) phase of the parent compounds is absent. Instead of the tilted smectic C phase of 7OAB an orthogonal SmA phase is observed when only 12% of the host molecules are replaced by 5OCB molecules. Moreover, in some mixtures the thermal stability of the induced SmA phase is found to be more than 1.6 times the stability of the mesophses in the pure compounds. The N-isotropic or Sm-I transition temperatures slowly increases with increasing concentrations, reaching a maximum at x _5OCB = 0.50 and then falling quite rapidly. In all the mixtures the enthalpy changes at the N-I transitions follow the simple additivity rule. In no mixture, except mixture C2, could the SmA-N transition be detected by DSC, although in all the mixtures the ratio T _NA/T _NI is found to be more than 0.95, which is in contradiction to McMillan's theory. Average intermolecular distances are found to have a minimum value near the equimolar concentration. From the concentration dependence of the smectic layer spacings it also appears that in all the mixtures the smectic A_d phase evolves from the smectic A₁ phase with increasing concentrations.