Viscoelasticity of ambient-temperature nematic binary mixtures of bent-core and rodlike molecules

Abstract

We report measurements of the temperature variations of physical parameters in ambient-temperature nematic liquid crystal mixtures of bent-core (BC) and rodlike molecules (5CB): birefringence An; static dielectric constants E_{I I} and E⊥; splay K₁₁ and bend K₃₃ elastic constants; rotational viscosity Y1; and diffusion coefficients D_{I I} and D⊥ of a microsphere. Both An and E_{I I} decreases rapidly with increasing BC concentration, whereas E⊥ remains almost constant. At a shifted temperature (e.g., T−T_NI=−10°C), K₁₁ increases by ∼50% and K₃₃ decreases by ∼80% compared to pure 5CB when the BC concentration is increased to ∼43 mol in the mixture. Viscosities parallel and perpendicular to the director, n_{I I}, n⊥, which are nearly equal to the Miesowicz viscosities n₂ and n₃, respectively, were obtained by D_{I I} and D⊥ using the Stokes-Einstein relation. Both the viscosities at room temperature increase by 60 and 50 times, respectively, whereas Y1 increases by 180 times (at ∼43 mol %) compared to the corresponding values of pure 5CB. The stiffening of K₁₁ and exorbitantly large enhancement in all the viscosities at a higher mol % of BC indicate that the viscoelastic properties are highly impacted by the presence of smectic clusters of BC molecules that results from the restricted free rotation of the molecules along the bow axis in the nematic phase. A possible attachment model of smectic type clusters of BC molecules surrounding the microparticle is presented.