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

L.G.P. Dalmolen , S.J. Picken , A.F. de Jong , W.H. de Jeu

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 >.

Mots clés: association of liquids; nematic liquid crystals; organic compounds; Raman spectra of organic molecules and substances

Domaines: Articles anciens

L.G.P. Dalmolen, S.J. Picken, A.F. de Jong, W.H. de Jeu. The order parameters < P2 > and < P 4 > in nematic p-alkyl-p' -cyano-biphenyls : polarized Raman measurements and the influence of molecular association. Journal de Physique, 1985, 46 (8), pp.1443-1449. ⟨10.1051/jphys:019850046080144300⟩. ⟨jpa-00210089⟩