Rheo-dielectric behavior of low molecular weight liquid crystals. 1. Behavior of nematic 5CB and 7CB

Dielectric relaxation behavior was examined for 4-4'-n-pentyl-cyanobiphenyl (5CB) and 4-4'-n-heptyl-cyanobiphenyl (7CB) under flow. In quiescent states at all temperatures examined, both 5CB and 7CB exhibited dispersions in their complex dielectric constant epsilon* (omega) at characteristic frequencies omega(c) above 10(6) rad s(-1). This dispersion reflected orientational fluctuation of individual 5CB and 7CB molecules having large dipoles parallel to their principal axis (in the direction of C = N bond). Zn the isotropic state at high temperatures, these molecules exhibited no detectable changes of epsilon* (omega) under flow at shear rates (gamma) over dot much less than omega(c). In contrast, in the nematic state at lower temperatures the terminal relaxation intensity of epsilon* (omega) as well as the static dielectric constant epsilon' (0) decreased under flow at (gamma) over dot much less than omega(c). This rheo-dielectric change was discussed in relation to the flow effects on the nematic texture (director distribution) and anisotropy in motion of individual molecules with respect to the director.