Nematic polar anchoring strength measured by electric field techniques

Abstract

We analyze the high-electric-field technique designed by Yokoyama and van Sprang ͓J. Appl. Phys. 57, 4520 ͑1985͔͒ to determine the polar anchoring coefficient W of a nematic liquid crystal-solid substrate. The technique implies simultaneous measurement of the optical phase retardation and capacitance as functions of the applied voltage well above the threshold of the Frederiks transition.We develop a generalized model that allows for the determination of W for tilted director orientation. Furthermore, the model results in a new high-field technique, ͑referred to as the RV technique͒, based on the measurement of retardation versus applied voltage. W is determined from a simple linear fit over a well-specified voltage window. No capacitance measurements are needed to determine W when the dielectric constants of the liquid crystal are known. We analyze the validity of the Yokoyama-van Sprang ͑YvS͒ and RV techniques and show that experimental data in real cells often do not follow the theoretical curves. The reason is that the director distribution is inhomogeneous in the plane of the bounding plates, while the theory assumes that the director is not distorted in this plane. This discrepancy can greatly modify the fitted value of 1/W, and even change its sign, thus making the determination of W meaningless. We suggest a protocol that allows one to check if the cell can be used to measure W by the YvS or RV techniques. The protocol establishes new criteria that were absent in the original YvS procedure. The results are compared with other data on W, obtained by a threshold-field technique for the same nematic-substrate pair.

The domain mixing effect on the electro-optical properties of liquid crystals using polyimide doped with reactive mesogen

“…Moreover, as shown in Figure 4(a), the slope of the EO curve of the LC cell with a 2.5 wt% RM concentration was quite steep, indicating the weak-anchoring case [24]. Now, the polar anchoring energy is determined using a high-electric-field method [25][26][27][28]. The polar anchoring energy (W) can be deduced from the following relationship between (R/R 0 − 1)CV and CV [27]:…”

Liquid Crystal Alignment Layer With Controllable Anchoring Energies

“…Electric field techniques are used to measure the polar anchoring energies [11]. This technique is based on the measurement of retardation versus applied voltage.…”

“…Polar anchoring energy W can be obtained by using the in Fig. 2 [11]. is the splay elastic constant of LC and is the cell gap of the sample cell.…”

Polyhedral Oligomeric Silsesquioxane Films for Liquid Crystal Alignment

“…The voltage-dependent optical transmittance of LC cells placed between crossed polarizers was measured using a diode laser (~635 nm), and the applied voltage was a 1 kHz square wave. The pretilt angles of the LC cells were measured by the modified crystal rotation method [19], and the polar anchoring energy of the POSS alignment films was measured by using the high electric field method [20], where a heterodyne interferometer was used to determine phase retardation [21].…”