Surface and disorder effects in aerosil dispersed liquid crystals

Abstract

Aerosil nanoparticles are dispersed in liquid crystals to introduce disorder. The particles form a hydrogen bonded network, which interferes with the normal liquid crystalline order. Among other techniques, broadband dielectric spectroscopy has been applied to these systems, and three main differences have been observed. First, a new slow relaxation process has been found, about two decades slower than the bulk rotation around the short axis. Its strength increases with aerosil concentration and it is interpreted as a retarded relaxation due to surface interaction with the aerosil particles. In the particular case of cyanobiphenyl liquid crystals, a hydrogen bond is involved. Second, an acceleration of the rotation around the short molecular axis is observed in the ordered nematic phase. The liquid crystalline phase becomes subdivided into domains by the aerosil network. The presence of such domains disturbs the long-range orientational order, and the sample appears more isotropic-like. Therefore the relaxation shifts towards the values in the isotropic phase, and for the rotation around the short axis this means an acceleration. Third, in the isotropic phase a similar acceleration is observed, but only for the associating cyanobiphenyl molecules. These molecules have a strong terminal dipole moment, and form dynamically antiparallel correlated pairs. The aerosil particles disturb the normal pair formation.