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

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 Delta n; static dielectric constants epsilon(parallel to) and epsilon(perpendicular to); splay K-11 and bend K-33 elastic constants; rotational viscosity gamma(1); and diffusion coefficients D-parallel to and D-perpendicular to of a microsphere. Both Delta(n) and epsilon(parallel to) decreases rapidly with increasing BC concentration, whereas epsilon(perpendicular to) remains almost constant. At a shifted temperature (e. g., T - T-NI = -10 degrees C), K-11 increases by similar to 50% and K-33 decreases by similar to 80% compared to pure 5CB when the BC concentration is increased to similar to 43 mol % in the mixture. Viscosities parallel and perpendicular to the director, eta(parallel to), eta(perpendicular to), which are nearly equal to the Miesowicz viscosities eta(2) and eta(3), respectively, were obtained by D-parallel to and D-perpendicular to using the Stokes-Einstein relation. Both the viscosities at room temperature increase by 60 and 50 times, respectively, whereas gamma(1) increases by 180 times (at similar to 43 mol %) compared to the corresponding values of pure 5CB. The stiffening of K-11 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.