Raman studies of and of some homologous nematic liquid crystals with small optical anisotropy. Mol Phys 2006. [DOI: 10.1080/00268978300101761]

Orientational order parameters in liquid crystals: a comparative study of x-ray diffraction and polarized Raman spectroscopy results

The orientational order parameters <P(2)> and <P(4)> in the nematic liquid crystal 4-pentyl-4'-cyanobipheny (5CB) have been determined by means of the two completely different techniques: polarized Raman spectroscopy and x-ray scattering. In particular, the values of <P(2)> and <P(4)> obtained using two different Raman methods, proposed by Jen [J. Chem. Phys 66, 4635 (1977)] and Jones [J. Mol. Struct. 708, 145 (2004)], respectively, are compared with the results of x-ray measurements. A good agreement between the experimental values of <P(2)> and <P(4)> and the results of the Humphries-James-Luckhurst mean field theory has been found whenever they were determined using either x-ray measurement or following the Jen method. In addition, a study of the influence of the intensity of the exiting laser source on the apparent values of the order parameters has been performed. Therefore, it was found that the discrepancies of the value of <P(4)> determined following Jen and Jones could be determined by nonlinear effects.

Calculation of flexoelectric coefficients for a nematic liquid crystal by atomistic simulation

Equilibrium molecular dynamics calculations have been performed for the liquid crystal molecule n-4-(trans-4-n-pentylcyclohexyl)benzonitrile (PCH5) using a fully atomistic model. Simulation data have been obtained for a series of temperatures in the nematic phase. The simulation data have been used to calculate the flexoelectric coefficients e(s) and e(b) using the linear response formalism of Osipov and Nemtsov [M. A. Osipov and V. B. Nemtsov, Sov. Phys. Crstallogr. 31, 125 (1986)]. The temperature and order parameter dependence of e(s) and e(b) are examined, as are separate contributions from different intermolecular interactions. Values of e(s) and e(b) calculated from simulation are consistent with those found from experiment.

Investigations of orientational order for an antiferroelectric liquid crystal by polarized Raman scattering measurements

The orientational ordering of the antiferroelectric liquid crystal molecules, 4-(1-methyl-heptyloxycarbonyl)phenyl 4(')-octyloxybiphenyl-4-carboxylate was investigated in the series of the successive smectic phases by means of polarized Raman scattering measurement without any external field. An improved equation for the analysis of the polarized Raman intensity was derived as a function of an incident laser polarization and the orientational order parameters. Even in the chiral smectic phases, some apparent orientational order parameters could be defined by the proper corrections for the smectic layer structure and an optical disturbance. An unusual change of the orientational order parameters was observed with decrease in temperature. It was concluded that the irregular variation of the order parameter stemmed from the biaxiality of the molecular orientational distribution, which was attributed to the hindrance of molecular rotation around its long axis.