A COMPARATIVE-STUDY OF DYNAMICS IN THE NEMATIC AND REENTRANT-NEMATIC PHASES OF 60CB AND 60CB 80CB MIXTURE BY DEUTERON NUCLEAR-MAGNETIC-RESONANCE RELAXATION/

Deuteron longitudinal (T-1Z) and quadrupolar (T-1Q) spin-lattice relax ation times and quadrupolar splittings were measured over all the stab le mesophases in a mixture of perdeuterated 4-n-hexyloxy-4'-cyanobiphe nyl (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, smecti c A and reentrant-nematic (RN) phases. We have carried out data analys es 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 de couple model is used to describe correlated internal motions of the en d chain, which are independent of the molecular reorientation. The lat ter motion is treated using the small-step rotational diffusion model of Tarroni and Zannoni, while the former motion is described using a m aster rate equation. In comparing the NMR results of the pure 6OCB sam ple and of the 6OCB/8OCB mixture, both the dynamic and static behavior s appear to be similar, and there are no dramatic changes upon enterin g the RN phase of 6OCB/8OCB, supporting the belief that the effects dr iving the reentrancy in this mixture are very subtle. The tumbling mot ion of 6OCB molecules, however, shows quite different behaviors in the two studied samples. Both 6OCB and 8OCB possess a strong terminal dip ole and tend to form ''loose'' dimers. The degree of dimerization can be inferred from the tumbling motion of 60CB molecules and their inter nal chain dynamics. (C) 1998 American Institute of Physics. [S0021-960 6(98)50421-7].