The order parameters < P2 > and < P 4 > in nematic p-alkyl-p' -cyano-biphenyls : polarized Raman measurements and the influence of molecular association

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Abstract
Values for the order parameters < P2 > and < P4 > 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 » < P4 >-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 < P4 >-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 < P4 >