E. Ciampi et al., THE DYNAMICS OF FIELD-INDUCED DIRECTOR REORIENTATION FOR A NEMATIC PHASE COMPRISED OF FLEXIBLE MOLECULES, The Journal of chemical physics, 107(15), 1997, pp. 5907-5913
The effect of rotating the nematic phase of 4-n-pentyl-4'-cyanobipheny
l (5CB) at a frequency omega(r) about an axis perpendicular to an appl
ied magnetic field has been studied using deuterium NMR spectroscopy.
The alignment of the directors produced by the competing magnetic and
rotational torques was monitored by the effect produced on Delta v(i),
the deuterium quadrupolar splittings at all five alkyl chain and one
biphenyl core sites in 5CB. The director orientation was found to be i
ndependent of the site monitored. The change in the director orientati
on produced by stopping the rotation of a sample was followed by recor
ding deuterium spectra during the reorientation process. The ratios of
the quadrupolar splittings, Delta v(i)(t)/Delta v(i)(t = 0) are found
to be site independent, which shows that measuring the director reori
entation rates by this method will produce the same result independent
ly of which molecular site is monitored. This contrasts with recent re
ports of experiments which found that monitoring director reorientatio
nal dynamics via a biphenyl core observable gave a different result th
an when the process was followed by measuring one corresponding to a s
ite in the chain. The possible reasons for the observation of a site d
ependence of director reorientational dynamics are discussed. (C) 1997
American Institute of Physics.