The order parameters and in nematic p-alkyl-p' -cyano-biphenyls : polarized Raman measurements and the influence of molecular association

Values for the order parameters and from polarized Raman measurements are reported for the homologous series of nematic alkylcyanobiphenyls. With increasing chain length a change of behaviour is observed from « low » -values to higher values corresponding to the Maier-Saupe mean-field theory. In addition, correlation factors describing the anti-parallel molecular association in these compounds have been derived from permittivity measurements. A tentative qualitative model is given in which the low -values are attributed to this association. This leads to a certain proportion of « dimers », for which the long molecular axis is not parallel to the direction of the Raman-vibration anymore. In addition, steric interactions can be expected to lead to non-parallelism of the long axis of a dimer and of a neighbouring monomer. For specific parameters of the dimer, the combination of these two effects can lead to a marked broadening of f(β), and thus to low or negative values for .