Phase equilibrium and dynamics of 5CB mixed with dimethyl terephthalate: coupling of orientation and composition fluctuations in isotropic phase†

4-Cyano-4′-pentylbiphenyl (5CB) is a representative liquid crystalline compound undergoing isotropic-to-nematic phase transition at a critical temperature, . This phase behavior significantly changes on addition of a small amount of a second compound, and the dynamics of 5CB molecules changes accordingly. In this study, these changes were examined for mixtures of 5CB with dimethyl terephthalate (DMT), the latter being in the crystalline state at T < 415 K if not mixed with 5CB, and the results were utilized to discuss dynamic properties of the mixture, the kinematic viscosity and dielectric relaxation time. Experiments showed that the mixtures with low DMT content w_DMT (<3.2 wt%) exhibited the isotropic-to-nematic transition at T_IN and this T_IN gradually decreased to 305 K on an increase of w_DMT up to 3.2 wt%. On further cooling to T_NC (<T_IN), pure DMT crystallite precipitated from the nematic phase. On the other hand, for the mixture with larger w_DMT (>3.2 wt%), the DMT crystallite precipitated from the isotropic phase on cooling to T_IC, and this T_IC strongly increased on an increase of w_DMT (which reflected a finite solubility of DMT in the isotropic phase). On further cooling to (= 305 K), the liquid phase being in equilibrium with the DMT crystallite changed from the isotropic phase to the nematic phase. This was independent of w_DMT and coincided with the other transition temperatures T_IC, T_IN, and T_NC merging at w_DMT = 3.2 wt%. All these features, cast in a phase diagram, were well described by a simple free energy model incorporating the Flory–Huggins type mixing free energy, the Landau–de Gennes type nematic free energy, and the crystal dissolution energy. Corresponding to this phase behavior, the kinematic viscosity ν and the dielectric relaxation time τ_ε of the 5CB/DMT mixtures exhibited Eyring–Andrade type temperature dependence in the high-T asymptote (in the isotropic one phase state) but positively deviated from respective high-T asymptotes on cooling to T_IN (for w_DMT = 1.5 wt%) and/or to (for w_DMT = 4.5 wt%). Pure 5CB exhibited similar positive deviation but in a much narrower range of T (just in a close vicinity of ). This difference between the 5CB/DMT mixture and pure 5CB was attributed to the composition fluctuation (missing in pure 5CB) strongly coupled with the orientation fluctuation in the isotropic phase of the mixture, and this coupling was assigned as the origin of the gradual/broad deviation observed for both ν and τ_ε. On the basis of this assignment, the fluctuation of the free energy was analyzed with the aid of the model explained above. This analysis specified a relationship between the deviations of ν and τ_ε, and the ν and τ_ε data well obeyed this relationship. These results demonstrate the importance of the coupling of the composition and orientation fluctuations in the dynamic properties of 5CB/DMT mixtures.