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 >

The order parameters

and

of three nematic compounds have been determined by polarized Raman scattering. Results obtained with p-pentyl-p'-cyanobiphenyl (5CB) agree well with published data. Next the influence of a strongly polar cyano end group on the order parameters was investigated. This was done by comparing two nematic liquid crystals of the tolane class, namely p-heptyl-p' -cyanotolane and p-hepty1-p'-methoxyto1ane, which have an almost identical geometrical shape. In spite of the difference in polarity hardly any difference in the order parameters and of both compounds couId be observed. of both tolanes show approximately mean field behavior in contrast to the low of SCB.

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