A large number of binary mixtures of 4-pentyloxy-49-cyanobiphenyl (5OCB) and 4,49-di(alkoxy)azoxybenzenes (nOAB) with n55, 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 50.50 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 A 1 phase with increasing concentrations.
Static dielectric properties of 18 binary mixtures of 4,4 0 -pentyloxy cyanobiphenyl (5OCB) with 4,4 0 -bis(alkoxy)azoxybenzenes (nOAB) for n ¼ 5, 6 and 7 have been studied in detail. Seventeen mixtures show induced smectic-A (SmA) phase; a few mixtures exhibit nematic phase as well. Pure 5OCB has a large positive dielectric anisotropy while nOAB members have small but negative anisotropy. However, all the mixtures exhibit positive anisotropy. In the mixtures, except those with x 5OCB ! 0.85, the values of " k , " ? and "" " increase smoothly with temperature. The values of " iso are found to be higher than the "" " values within the mesophases, with a jump at T NI or T SI . Effective values of the dipole moments in the mixtures are found to be less than the values observed in the pure compounds. This suggests that anti-parallel homo-dimers of 5OCB exist in the mixtures as observed in the pure compound. Moreover, at the onset of SmA phase, values of " " " are found to decrease from those in the nematic phase. This implies that additional anti-ferroelectric type of ordering is developed in the induced SmA phase. In all the mixtures the anisotropy parameter Á" decreases with concentration, becomes minimum at x 5OCB ¼ 0.5, and then increases quite fast. Kirkwood correlation factors have also been calculated to find the nature and extent of the molecular associations.
Eighteen binary mixtures of 4-pentyloxy-4′-cyanobiphenyl (5OCB) and 4, 4′-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6, and 7 at different concentrations have been studied in detail. Over a wide concentration range, induced SmA phase was observed in all the mixtures except in one. In most of the mixtures, the refractive indices no, ne, and Δn lie between the values of pure components. The optical birefringence of the mixtures is found to decrease non-additively with concentration. The molecular polarizability anisotropy and the orientational order parameter <P2> of the mixtures were calculated. Nature of variation of polarizability anisotropy with concentration is similar to that of Δn. In allmixtures <P2> is more in smectic phase than in nematic phase and it also shows non-additive behaviour. Observed <P2> values of the mixtures were compared with McMillan’s and Maier-Saupe theoretical values and good agreement is found in some mixtures. The transition temperatures obtained by fitting <P2> data to McMillan’s theory agree with the observed values within a few degrees. Ratio of elastic constants K33 and K11 is determined for mixtures having nematic phase and found to diverge at TNA transition point.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.