Strong azimuthal anchoring energy at a nematic-polyimide interface

Abstract

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.

Azimuthal director gliding at a strongly anchoring interface of polyimide

“…The azimuthal anchoring energy coefficient W a is obtained by substituting in equation (9) K 22 53.93610 212 N [22], e a 513.1 [7] and the best fit coefficient d52.836u mm V 21 54.95610 22 rad mm V 21 . We find W a5 0.43610 23 J m 22 , which is comparable but slightly higher than the value W a 50.33610 23 J m 22 that we measured on the same substrate and at the same temperature using the rotating polarizer method [16]. The corresponding extrapolation length [1,2] is d e 5K 22 / W a 59.1 nm.…”

“…In the experiment discussed in [16], the system was made by polarizer P 1 and analyser A 1 rotated with the angular velocity v. Thus, Q p is a linear function of time, and intensity I in equation (5) becomes a sinusoidal function of time with pulsation 4v and with a phase which is simply related to the surface azimuthal angle Q s . Therefore, Q s can be obtained by measuring the phase of the oscillating intensity signal.…”

“…Very recently [16], using a new high accuracy reflectometric method, we have investigated the azimuthal anchoring at the interface between a rubbed polyimide layer and the NLC 5CB. It was found that rubbed polyimide has a strong azimuthal anchoring energy (W a 50.33610 23 J m 22 at T525uC) and exhibits the characteristic slow dynamics related to director gliding.…”

“…These two outputs were connected to the two brass electrodes of the nematic cell. Using a special experimental procedure [16,20] we measured the space dependence of the electric field in the region between the two electrodes. The electric field increases close to the electrodes but is virtually uniform in a central stripe of width 0.6 mm between them.…”

Measurement of anchoring coefficient of homeotropically aligned nematic liquid crystal using a polarizing optical microscope in reflective mode

“…5 There are several methods for measuring the anchoring coefficients in homeotropic alignment. [6][7][8][9][10][11][12][13] The reflectance at the interface of interest was used in these methods. A wedge cell was used to remove the effect of the unimportant substrate in one method.…”