Controlling pre-tilt angles of liquid crystal using mixed polyimide alignment layer

In this article, control of the pre-tilt angle in a LC cell, which is defined as the angle made between the LC director with respect to a substrate surface is reported. This technique allows us to precisely control the pre-tilt angle in LC cells, and has a high potential for practical application. The pre-tilt angle was measured by fitting the data obtained using the crystal rotation method with self-developed Labview software. The measured angle was doubly checked by fitting the transmission versus voltage (T-V) curve using Dimos software.

In liquid crystal displays (LCDs), a uniform LC pre-tilt angle is very important for the proper functioning of the display, because it has a marked influence on the display quality and the response time of LCD devices.In addition, a nonzero pre-tilt angle is required to avoid the formation of the reverse tilt domains (Fig 1 .).
In this article, control of the pre-tilt angle in a LC cell, which is defined as the angle made between the LC director with respect to a substrate surface is reported.This technique allows us to precisely control the pre-tilt angle in LC cells, and has a high potential for practical application.The pre-tilt angle was measured by fitting the data obtained using the crystal rotation method with self-developed Labview software.The measured angle was doubly checked by fitting the transmission versus voltage (T-V) curve using Dimos software.LC devices are currently being applied for uses for many electro-optic devices, including display, phase modulator,…, etc. due to their low operating voltage, low power consumption.In particular, the optically compensated bend (OCB) mode LCDs designed with bend alignment has recently attracted much attention because it possesses excellent features of fast response and wide viewing-angle.Operationally, the bright and dark states are the bend state (B-state) and homeotropic state, respectively.However, a typical OCB LCD with a low pre-tilt angle is actually stable in the splay state (S-state).Thus, an OCB display has to be switched to the B-state first by applying a bias voltage to maintain the LCD in the B-state to avoid a very slow response from the S-state to B-state.It is found that OCB LSDs with a pre-tilt angle higher than ~47˚ can be operated without applying a bias voltage [1][2][3].
In this letter, we demonstrate simple approaches, based on a Horizontal (H) + Vertical (V) Polyimide (PI) mixture to obtain an arbitrary pre-tilt angle by controlling the H-to V-PI concentration ratio, the baking temperature and the rubbing strength.The pre-tilt angle can be varied in a wide range from 85° to ~15° by three approaches.
The materials adopted herein were H-PI (AL-1426B; from Daily Polymer Corporation) and V-PI (AL-00010; from Japan Synthetic Rubber Company).The homogeneously mixed PI compound was coated on an indium-tin-oxide (ITO) glass substrate by spin coating.After they had been baked and rubbed, two substrates that had been treated identically were assembled to produce an empty anti-parallel LC cell with a ~ 12 um gap.An empty cell was finally filled with K15 liquid crystal (Merck) to form a LC cell.
The first approach is to vary concentration ratios of V-to H-PI.The PI-coated substrate was treated at a baking temperature of 200 ˚C for 1 h and rubbed with a pile impression of ~ 100 μm , which represented the distance from the flannel to the surface of the substrate measured using the scale provided in the rubbing machine, was produced to fabricate an empty LC cell.Figure 2(a) plots the pre-tilt angle increases (~ 20˚ to 60˚) monotonically with the increasing concentration of the V-PI (~ 3.57 to 4.55 wt%).
The second approach is to vary the baking temperature ranging from 180 to 240 ˚C for 1 hour.A substrate was coated with a mixed PI layer with a V-PI concentration of ~ 4.55 wt% and rubbed with a pile impression of ~ 0.1 μm.The third approach is to change the rubbing strength with various pile impressions.The substrate coated with a mixed PI layer, with a concentration of V-PI ~ 4.55 wt%, was baked at 200 ˚C for 1 h. Figure 2(c) indicates that the pre-tilt angle declines (90˚ to 15˚) monotonically as the rubbing strength increases, since the liquid crystals align in a direction that is determined by the equilibrium between the two orthogonal easy axes.For weak rubbing, the V-PI side chains dominate, and the formed LC cell is homeotripical, θ ~ 90˚.
Using the techniques obtained from these studies, an LC cell was fabricated for use as a polarization converter, as presented in Fig. 3. Notably, LCs on the bottom substrate of the cell are aligned at continuously varying tilt angles, while those on the top substrate are vertically aligned.This cell was fabricated as follows.The top substrate was coated with a vertical layer, and the bottom substrate was coated with a mixed PI layer with a V-PI concentration of ~ 4.55 wt%, and baked at 200 ˚C for 1 h.It was then rubbed along the x axis with increasing rubbing strength with pile impressions from 0 to 350 μm.  Figure 4 presents images of the LC cell under an optical polarized microscope (OPM) with a white light source.In the figure, β is the angle made between the rubbing direction and the polarization of the incident beam.Fig. 4(a) obtained the cell image under a parallel-polarizer OPM is fully bright, since the polarization of emergency beam is the same as the incident beam, and is transmitted through the analyzer.As expected, the cell is completely dark observed under the crossed-polarizer OPM, as presented in Fig. 4(b).Figures 4(c) and 4(d), the color image show the effects of phase retardation when white light passes the cell with β=45˚ between the parallel-polarizer and the crossed-polarizer conditions, respectively.
To verify the results presented in Fig. 4, the cell with the structure in Fig. 4 was simulated using Jones Matrix method.
The pre-tilt angle on the bottom substrate is assumed to be 15o on one side, increasing continuously to 90˚ on the other side.The conditions presented in Figs.4(c) and 4(d) were used to simulate the incidence of light of three wavelengths -450, 550 and 650 nm onto the cell.The reason to perform simulations with three different wavelengths of incident light instead of using white light is for simplifying the calculation.
Figure 5 presents the comparisons between experimental (Fig. 4(c)) and simulated results under P‖A and β=45˚ conditions.As seen, region 1 shows reddish orange color.It results from the mixture of rich red with weak green color.Similar argument is applied to regions 2 -5.The experimental results are consistent with the simulated ones.
The light transmittance of the LC cell that was placed between two crossed polarizers was measured using a He-Ne laser (λ=632.8nm) to further confirm that the cell indeed functions as a variable polarization converter.The dots in Fig. 6 plot the measured position-dependent transmittance of the LC cell.The position is defined from the left to the right side of the sample, as presented in Fig. 4(d).Because continuously varying tilt angles induced the varying LC birefringence ∆n, the transmittance through the analyzer changes with the phase retardation along the x axis.An ideal transmittance curve for tilt angles from ~15˚ to 90˚ was simulated using the Jones Matrix formulism.Figure 7 gives the results, and clearly indicates that the experimental results are consistent with the simulated results.The error is attributed to the finite spot size (~ 1 mm) of the probe laser beam.In conclusion, we demonstrate three approaches to controlling the LC pre-tilt angles (~ 15˚ to 85˚) in a cell by varying the concentration ratio of H-to V-PI, the baking temperature and the rubbing strength.Notably, the technique developed in this thesis utilizes a conventional rubbing machine and mixture of commercial PIs.Thus, it is compatible with the existing manufacturing processes.Additionally, a variable-polarization converter, based on the LC cell that is presented in Fig. 4 was fabricated with one substrate rubbed with increasing strength.The polarization of a polarized beam incident on the cell can be converted continuously upon emergence from the device.

Fig. 1 .
Fig. 1.LC director configurations of (a) pre-tilt angle is zero and (b) pre-tilt angle is nonzero before and after electrical switching.

Figure 2 (Fig. 2 .
Figure 2(b) shows the pre-tilt angle declines (75˚ to ~15˚) monotonically as the baking temperature increases, since over-baking in the polyimide has two effects; 1) it causes the further imidization of the backbones of V-PI, promoting the planar alignment; 2) it cleaves away a proportion of the side chains of the V-PI component, weakening the vertical alignment.

Fig. 3 .
Fig. 3. LC director configurations in cell with one substrate rubbed with increasing strength.

Fig. 5 .
Fig. 5. Simulation of LC polarization converter cell with structure presented in Fig. 4 at P‖A, β=45˚, with probe beams at wavelengths of 450, 550 and 650 nm.

Fig. 6 .
Fig. 6.The transmittance of LC polarization converter, as a function of LC cell position.The dots and the dotted line represent the experimental and simulated results, respectively.