Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rodlike and bent-core molecules

Abstract

We report on two anomalous trends in the temperature dependences of the splay (K₁₁) and bend (K₃₃) elastic constants in the nematic (N) phase of mixtures of compounds with rodlike (R) and bent-core (BC) molecules: As the sample is cooled from the isotropic to N transition point, (i) K₃₃ increases, attains a maximum value and then decreases, and (ii) close to the N to smectic A (SmA) transition point, K₁₁ decreases sharply. At higher temperatures the bow axes of BC molecules are aligned along the director n^{^}, strongly favoring a bend distortion of n^{^} as the orientational order parameter is increased. Close to the N-SmA transition point the smecticlike short-range order builds up, and the arrow axes of BC molecules are aligned along n^{^}, facilitating a splay distortion of n^{^}. A simple model calculation brings out the anomalous trend in K₃₃.