Explanation of the static and dynamic director orientation in thin nematic liquid crystal films using deuterium NMR spectroscopy

摘要

In this chapter, we describe some of our studies of the static director distribution in thin nematic liquid crystal cells with different film thicknesses and different surface anchoring strengths using a combination of deuterium NMR spectroscopy and continuum theory. The nematic liquid crystal, 4-pentyl-d{sub}2-4'-cyanobiphenyl (5CB-d{sub}2) deuteriated in the α-position of the pentyl chain, was confined between two glass plates with both weak and strong anchoring conditions; the anchoring strengths were measured by using a saturation voltage method [21]. A series of deuterium NMR spectra was acquired as a function of the applied electric field, which can be used to explore the director deformation. We also describe the application of deuterium NMR spectroscopy to investigate the director dynamics in the same nematic liquid crystal (5CB-d{sub}2) confined between two glass plates and subject to magnetic, B, and AC electric, E, fields. The cell was set in the NMR probe with the electric field, whose direction is normal to the substrate surface, making an angle of about 45°, with the magnetic field. This experimental geometry allows a unique director motion in the alignment process. In the absence of the electric field the director for 5CB will align parallel to the magnetic field because the diamagnetic anisotropy, △X{top}～, is positive. When an electric field, which is strong enough to overcome the magnetic torque, is applied then the director will make an angle with the electric field since the dielectric anisotropy, △ε{top}～, is also positive. After the electric field is switched off, the director will then move from being at an angle to the magnetic field to being parallel to it. The dynamics of the director relaxation can be followed by monitoring the NMR spectrum during this alignment process, as a function of time. That is, the time dependence of the director orientation during and after the application of an electric field is studied. This is possible because the NMR spectrum for a monodomain sample with one group of equivalent deuterons, having a negligible dipolar interaction, contains a simple quadrupolar doublet whose separation is determined by, among other things, the angle, θ, made by the director, n, with the magnetic field.