A comparative study of dynamics in the nematic and reentrant-nematic phases of 60CB and 60CB/80CB mixture by deuteron nuclear magnetic resonance relaxation

Abstract

Deuteron longitudinal (T1Z) and quadrupolar (T1Q) spin-lattice relaxation times and quadrupolar splittings were measured over all the stable mesophases in a mixture of perdeuterated 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) at 15.1 and 46 MHz, and compared with those reported previously for a pure 6OCB sample. The 6OCB/8OCB mixture has 28 wt. % of 6OCB and shows a nematic, smectic A and reentrant-nematic (RN) phases. We have carried out data analyses for both samples in order to achieve a consistent physical picture. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings in these samples. A decouple model is used to describe correlated internal motions of the end chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a master rate equation. In comparing the NMR results of the pure 6OCB sample and of the 6OCB/8OCB mixture, both the dynamic and static behaviors appear to be similar, and there are no dramatic changes upon entering the RN phase of 6OCB/8OCB, supporting the belief that the effects driving the reentrancy in this mixture are very subtle. The tumbling motion of 6OCB molecules, however, shows quite different behaviors in the two studied samples. Both 6OCB and 8OCB possess a strong terminal dipole and tend to form "loose" dimers. The degree of dimerization can be inferred from the tumbling motion of 6OCB molecules and their internal chain dynamics. © 1998 American Institute of Physics.