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.