The relationship between the second and fourth rank order parameters for liquid crystals composed of non-rigid molecules

Abstract

Abstract The values of the fourth rank order parameter P4 determined by Raman scattering experiments are invariably lower than those predicted by the Maier-Saupe theory of nematic liquid crystals. However this theory is only applicable to rigid particles whereas the nematogenic molecules studies contain flexible alkyl chains. We have estimated the influence of the alkyl chain on P4 with the aid of a model which account successfully for the variation of the second rank order parameter P2 along the chain. It is found that the values of P2 and P4 calculated for the CN direction in the 4-n-alkyl-4′-cyanobiphenyls are indistinguishable from those predicted by the Maier-Saupe theory. It would appear therefore that the low values observed for P4 cannot be attributed to the non-rigidity of the alkyl chain.