The dynamics of field-induced director reorientation for a nematic phase comprised of flexible molecules

The effect of rotating the nematic phase of 4-n-pentyl-4'-cyanobiphenyl (5CB) at a frequency omega(r) about an axis perpendicular to an applied 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 independent of the site monitored. The change in the director orientation produced by stopping the rotation of a sample was followed by recording 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 reorientation rates by this method will produce the same result independently of which molecular site is monitored. This contrasts with recent reports of experiments which found that monitoring director reorientational dynamics via a biphenyl core observable gave a different result than when the process was followed by measuring one corresponding to a site in the chain. The possible reasons for the observation of a site dependence of director reorientational dynamics are discussed. (C) 1997 American Institute of Physics.