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 (K(11)) and bend (K(33)) 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) K(33) increases, attains a maximum value and then decreases, and (ii) close to the N to smectic A (SmA) transition point, K(11) decreases sharply. At higher temperatures the bow axes of BC molecules are aligned along the director (n) over cap, strongly favoring a bend distortion of (n) over cap 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) over cap, facilitating a splay distortion of (n) over cap. A simple model calculation brings out the anomalous trend in K(33).