Raman studies of &lt;<i>P</i>₂&gt; and &lt;<i>P</i>₄&gt; of some homologous nematic liquid crystals with small optical anisotropy

Abstract

We present results of Raman studies of the order parameters <P 2> and <P 4> for some liquid crystalline cyanophenylcyclohexanes (PCH-3, PCH-5 and PCH-7), which, in contrast to most data available in the literature, are in remarkably close accordance with predictions by the mean-field theories for the nematic order. Furthermore our measurements do not reveal a significant dependence on the molecular length. The PCHs differ from materials for which the theory is not capable of reproducing the experimental findings on <P 4>, by their rather low optical and dielectrical anisotropy. In our opinion, this difference seems to be the key for the solution of the <P 4> problem of the Raman scattering technique. Due to the lack of a proper theory, the influence of the anisotropic local electric field on the depolarization ratios of the Raman intensities has been neglected in the past in the formalism developed to evaluate the order parameters from the measurements. It will be shown that this disregard cannot be correct generally, with the error becoming larger with increasing anisotropy. In the light of this concept our verification of the theoretical predictions is a consequence of the very small local field anisotropy in the PCH series. The magnitude of the local field effect is estimated by means of a two-dimensional model.