A COMPARATIVE-STUDY OF DYNAMICS IN THE NEMATIC AND REENTRANT-NEMATIC PHASES OF 60CB AND 60CB 80CB MIXTURE BY DEUTERON NUCLEAR-MAGNETIC-RESONANCE RELAXATION/
Xd. Shen et Ry. Dong, A COMPARATIVE-STUDY OF DYNAMICS IN THE NEMATIC AND REENTRANT-NEMATIC PHASES OF 60CB AND 60CB 80CB MIXTURE BY DEUTERON NUCLEAR-MAGNETIC-RESONANCE RELAXATION/, The Journal of chemical physics, 108(21), 1998, pp. 9177-9185
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].