Supermolecular structures in nematic-cholesteric mixtures

A phenomenological free-energy density for a mixture of a nematic with a ch olesteric liquid crystal is constructed on the basis of symmetry considerat ions. The possible spatially nonuniform distribution of the individual comp onents of the medium is taken into account. From a microscopic description of a mixture of two cholesteric liquid crystals and a nematic with a choles teric liquid crystal, the dependence of the reciprocal of the pitch of the cholesteric spiral on the concentration of the cholesteric liquid crystal i s determined, and the results agree with all experimental data. It is shown that at mesophase temperatures a spinodal decomposition, or stratification , of the mixture is possible and the possible resulting supermolecular stru ctures are determined. Specifically, it is found that the spinodal decompos ition in nematic-cholesteric mixtures could be responsible for the distorti on of the ideal spiral structure and for the appearance of a second period in the director distribution. (C) 1999 American Institute of Physics. [S106 3-7761(99)01208-1].