Optical-field-induced refractive indices and orientational relaxation times in a homologous series of isotropic nematic substances

We have measured the optical Kerr constant and the orientational relaxation times of seven $p$,$p^{\prime }-\mathrm{di}-n-\mathrm{alkoxyazoxybenzene}$ homologous compounds as functions of temperature in the isotropic phase. The observed critical behaviors near the isotropic-nematic transitions agree well with the predictions from the Landau-de Gennes model. Various characteristic parameters of the homologs are deduced from the experiment. Their variations with increase of methylene groups in the alkyl chain are discussed. Our results suggest that neither the mean-field theory of Maier and Saupe nor the Landau expansion of free energy is a good approximation for quantitative description at the isotropic-nematic transition.