Very strong azimuthal anchoring of nematic liquid crystals on uv-aligned polyimide layers.

The azimuthal anchoring energy of the nematic liquid crystal 4- n -pentyl- 4' -cyanobiphenyl (5CB) on a uv-aligned polyimide substrate with in-plane order parameter S'=0.2 is measured. The measurements are performed at temperature T=24 degrees C using simultaneously a high accuracy reflectometric method and a high accuracy transmitted light method. With both the methods, we observe an apparent surface director rotation opposite to the orienting torque that would correspond to a negative extrapolation length. It is shown that this unusual behavior is due to the relatively high birefringence of the uv-aligned polyimide layers. Taking into account for this birefringence, we find a small but positive extrapolation length. The experimental results are interpreted in terms of a simple mesoscopic model where the nematic molecules are assumed to be rigidly attached on the polymer surface and the measured extrapolation length is entirely due to the order parameter variation in a thin interfacial layer where the nematic order parameter passes from the surface value to the bulk value within a few nematic correlation lengths. Assuming the surface order parameter is S(0)=0.37 , the correlation length of the nematic liquid crystal is estimated to be xi'(c)=2.4+/-1 nm . The corresponding thermodynamic extrapolation length is de=2.8+/-1.2 nm that corresponds to a very strong azimuthal anchoring.

Strong azimuthal anchoring energy at a nematic-polyimide interface.

Some years ago we proposed an automated reflectometric method to measure the director azimuthal angle at the interface between a nematic liquid crystal and another medium. The method ensures a great accuracy and sensitivity and is virtually unaffected by the presence of a director bulk distortion. This latter property makes it possible to measure strong anchoring energies. In the present experiment, we use this method to measure the azimuthal anchoring energy at the interface between the nematic liquid crystal 4-pentyl-4-cyanobiphenyl (5CB) and a rubbed polyimide layer. This kind of interface is characterized by a strong azimuthal anchoring and, thus, it represents a good test for the proposed reflectometric method. An ac planar electric field is applied to a nematic layer and the consequent azimuthal rotation of the director at the interface is measured. The anchoring energy coefficient Wa at room temperature is strong (Wa=0.33 x 10(-3) J/m2) and decreases greatly as the clearing temperature is approached. The time response of the azimuthal surface director angle to a stepwise electric field evidences the characteristic slow dynamics which is currently observed for weak anchoring substrates.

Light transmission from a twisted nematic liquid crystal: accurate methods to measure the azimuthal anchoring energy.

In this paper, we analyze the light transmission from a twisted nematic liquid crystal (NLC) and we propose two accurate and very direct optical methods to measure the azimuthal anchoring energy. In both of them, a monochromatic beam of wavelength lambda with a polarization vector that rotates at an angular frequency omega impinges on a twisted nematic liquid crystal. The intensity of the transmitted beam is modulated at angular frequency 2omega with a phase shift beta, which is related to the surface azimuthal director angle phi(1) at the investigated interface. It is shown that there exists a special geometry where the simple adiabatic relation phi(1)=beta/2 is satisfied up to second order in the small perturbative parameter alpha=lambda/(2piDelta(n)xi), where Delta(n) is the anisotropy of the refractive indices of the NLC and xi is the twist distortion length. Furthermore, the small residual higher order correction terms can be greatly reduced by choosing a proper geometry for the experiment. With this choice, the range of validity of the adiabatic theory is greatly extended. The perturbative theoretical results are fully confirmed by numerical calculations. The azimuthal anchoring energy coefficient can be obtained by measuring phase shift beta versus the intensity of an applied magnetic field. These methods greatly improve the accuracy of the previous transmitted light techniques and also provide accurate measurements of strong azimuthal anchoring energies.

Azimuthal anchoring energy at the interface between a nematic liquid crystal and a PTFE substrate.

The azimuthal anchoring energy of the nematic liquid crystal 4-pentyl-4'-cyanobiphenil (5CB) on a poly(tetrafluoroethylene) (teflon, PTFE) film is measured for the first time. The PTFE film is deposed using the Wittmann and Smith technique which consists on rubbing a bar of this polymer against a glass substrate at a controlled temperature and pressure. Measurements of the azimuthal anchoring energy are made with a reflectometric technique which provides high accuracy and sensitivity. The dependence of the azimuthal anchoring energy on temperature and on the rubbing pressure is investigated. The extrapolation length remains virtually constant in the whole temperature range of the nematic phase except for an increase of 25% close to the clearing temperature. The azimuthal anchoring energy is somewhat strong and increases with increasing the deposition pressure of PTFE. The observation of a relevant pre-transitional anisotropy of the reflection coefficients in the isotropic phase proves that the surface interactions favor an excess of orientational order. Ageing of the anchoring energy and gliding of the easy axis are experimentally observed. Both these phenomena suggest the presence of an anisotropic adsorbed layer of nematic molecules on the PTFE film.

An improved reflectometric method to measure the azimuthal anchoring energy of nematic liquid crystals.

Some years ago we developed an automatized reflectometric method to measure the surface azimuthal anchoring energy of nematic liquid crystals on an optically isotropic substrate. This method provides a high accuracy and sensitivity but requires the use of wedge glass plates and a sufficiently high anisotropy of the intensity reflectivity coefficients. This latter condition restricts greatly the number of possible substrates that can be investigated with this technique. Here we develop a new reflectometric method which offers comparable or better accuracy and sensitivity but does not require wedge plates and high anisotropy of the reflectivity coefficients. The method is fully automated and provides a direct measurement of the azimuthal director angle. The experimental procedure exploits the dependence of the reflectivity tensor on the surface director orientation. The measurement of the azimuthal angle does not require any knowledge of the optical parameters of the nematic material and of the optically isotropic substrate, and provides an absolute accuracy better than 0.2( degrees ) in the whole range 0-360( degrees ) and a sensitivity better than 0.1( degrees ). This reflectometric method can be also used with weakly anisotropic substrates as well as thin rubbed polymeric layers. In this latter case, the effective uncertainty in the measurement of the director azimuthal angle depends on the substrate anisotropy. A simple and direct experimental procedure to estimate this uncertainty is proposed.