Anomalous Temperature Dependence of Elastic Constants in the Nematic Phase of Binary Mixtures Made of Rodlike and Bent-Core Molecules

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.