Measurements of the ratio of the Frank constants for splay and bend in nematics of disc-like molecules

Abstract

We report measurements of the magnetic Fredericks transition in nematics consisting of disc-like molecules (2,3,7,8,12,13-hexa(alkanoyloxy)truxenes). We find that in these materials the splay elastic constant K 1 is larger than the bend elastic constant K 3 throughout the nematic regime, in contrast to what is commonly observed in fluids of rodlike mesogens. This behaviour is in agreement with the behaviour predicted by mean-field theory [1]. At the transition from the nematic to the high-temperature columnar phase the ratio K 1 /k 3 appears to approach unity. We find no evidence for the predicted pretransitional enhancement of the ratio K 1 /K 3 due to columnar order parameter fluctuations [2] Etude de la transition magnetique de Freedericks dans les hexaalcanoyloxytriuxenes. Mesure de la constante elastique K 1 et de la constante de flexion K 3