Comparative Study of Individual and Collective Rotational Motion in Mixtures of Liquid Crystalline Side Group Polymers and Low Molecular Weight Mesogens

Abstract

Relations between the time scales of individual and collective rotational motion of mesogenic groups were analyzed in a combined dielectric and viscosimetric study. Measurements were conducted on mixtures of liquid crystalline (LC) side group polysiloxanes and the corresponding low molecular weight compounds. The rotational diffusion constant D_r, was derived from the frequency f_δ of the dielectric δ process, taking into account the retarding force of the nematic field. The time scale of collective rotation processes is set by the rotational viscosity γ₁; γ₁ was determined by a director reorientation experiment in a magnetic field. Experiments were performed under variation of the temperature and the composition of the mixture. The temperature dependencies of D_r and γ₁ are both mainly determined by that of the rotational friction coefficient ζ_r(T). ζ_r(T) is given by a Vogel-Fulcher formula. The effect of the nematic mean field on f_δ and γ₁ is correctly described by theories. In the polymer coupling of the LC groups leads to an enhanced increase of the viscosity, which is stronger than the decrease of D_r.