A deuterium nuclear magnetic resonance investigation of the director distribution in a thin nematic liquid crystal slab

The director distribution in a thin nematic liquid crystal (NLC), 4-pentyl-d(2)-4'-cyanobiphenyl (5CB-d(2)) deuteriated in the ex-position of the pentyl chain, confined between two glass plates, with untreated and treated anchoring conditions, has been investigated using a deuterium nuclear magnetic resonance (NMR). The NMR spectra have been measured as a function of the applied electric field. In the absence of surface forces it is found that the director aligns parallel to the magnetic field at relatively low values of the electric field as Delta<(<chi>)over tilde> is positive for 5CB. Then as the electric field is increased the director remains parallel to the magnetic field until some critical value of the electric field, at which point the director is aligned parallel to the electric field and hence perpendicular to the magnetic field, since Delta<(<epsilon>)over tilde> is positive for 5CB. In the presence of surface forces with increasing electric field the quadrupolar splitting decreases, passes through zero and then increases again to a value which is essentially half of that at zero electric field. That is, the director orientation changes more or less continuously from being parallel to the magnetic field to being orthogonal to it, as the electric field grows.