Dielectric Studies of Bicyclohexylcarbonitrile Nematogens with Large Negative Dielectric Anisotropy

Abstract

Molecular dynamics of a homologous series of trans, trans-4,4′-dialkyl-(1α,1′-bicyclohexyl)-4β-carbonitrile(CCNs) with a lateral cyano group has been studied by dielectric relaxation spectroscopy. The high frequency relaxation process connected with the molecular reorientation around the long molecular axis was investigated in the isotropic and nematic phases. Due to the negligible value of the longitudinal dipole moment, the relaxation process around the short axis is inactive in dielectric spectroscopy. The dielectric relaxation time changes smoothly at the phase transition isotropic/nematic. The activation energy obtained (EA ∼ 30 kJ mol−1) is typical for a relaxation process around the molecular long axis. For planar alignment the high frequency limit of the dielectric permittivity ˜⊥∞ is distinctly higher than no2. The difference ˜⊥∞−no2 ≈ 1.2 is most probably connected with Poley absorption due to librational motion of molecules around the long axis.