Orientational order in liquid crystals exhibited by some binary mixtures of rod-like and bent-core molecules

Abstract

We report measurements of the temperature variations of the optical birefringence in the nematic (N) and partial bilayer SmA (SmAd) phases in 4-n-octyloxy 4' -cyanobiphenyl made of rod-like (R) molecules and five mixtures of this compound with 1,3-phenylene bis[4-(3-methylbenzoyloxy)] 4' -n- dodecylbiphenyl 4' -carboxylate, made of bent-core (BC) molecules. The birefringence decreases with the concentration x of the BC molecules but the macroscopic order parameter initially decreases upto 11mol% of BC molecules and subsequently increases with x . This is attributed to the possible formation of polar clusters of BC molecules. Orientation of BC molecules changes between the N and SmAd phases and the birefringence data in the two phases imply that the kink angle of the BC molecules is ~90° rather than ~110° as obtained from calculations which minimize the energy of the molecule. IR spectroscopic measurements on the mixture with 11mol% of BC molecules have been used to estimate the molecular order parameter S of the R molecules, and to provide additional support for a relatively small kink angle of BC molecules.