The effect of molecular association and tube dilation on the rotational viscosity and rotational diffusion in nematic liquid crystals

A combination of director reorientation experiments with dielectric relaxation spectroscopy was used to study the relation between the rotational viscosity γ1 and the rotational diffusion constants in nematic compounds with different association tendency. Increased values for γ1 were found for compounds showing strong association. This is explained by increased rotational friction of the associates due to the necessary translational motion of the molecules within these groups. For nonassociating compounds, γ1 was found to be described quantitatively by Marrucci’s theory, when, in addition, the tube dilation effect described by Doi for the rotational diffusion in an orientationally ordered environment is taken into account.