Rotational viscosity measurements of bent-core nematogens

Abstract

Two simple but precise probing methods have been utilized to investigate the influence of structure, properties and molecular association on the director response time (τ0) and hence on the rotational viscosity (γ1) in the nematic phase of three 4-cyanoresorcinol bent-core derivatives (1/7, 1/9 and 1/10) having different terminal chain lengths. The results obtained from the two methods are in good agreement with a small deviation of about 10–15%. The γ1 values are more than an order of magnitude higher compared to the conventional calamitic nematics. Measurements of birefringence, dielectric anisotropy, splay and bend elastic constants and relaxation time of these compounds have also been carried out. For the lower homologues (1/7, 1/9) an inversion in dielectric anisotropy (Δe) is observed in the nematic phase, while the higher homologue (1/10) exhibits negative Δe throughout the entire mesomorphic range. The splay elastic constant (K11) is found to be comparatively greater than the bend elastic constant (K33); however, on approaching the low temperature CybC phase (a phase comprising elongated cybotactic clusters) K33 exhibits a small pretransitional divergence. For all three compounds the magnitude of the rotational viscosity coefficient (γ1) is found to exhibit a strong enhancement on approaching the CybC phase, while in the nematic phase activation energies are found to be nearly an order of magnitude higher than the normal calamitics. The observed behaviours are explained by considering the strong dipolar correlation of the bent-core molecules and the existence of the short range cybotactic clusters, their gradual growth and uniform alignment under the presence of strong surface anchoring.