Liquid Crystal Orientation Mechanism: Competition Between Rubbing and Ion-beam Method

Abstract

The effect of liquid crystal (LC) alignment on a homeotropic polyimide (PI) surface induced by ion beam (IB) irradiation and rubbing process was studied. LC alignment was not affected by IB irradiation with an exposure time of 10 s, and an IB irradiation with an exposure time of 60 s more effectively oriented the LCs on the PI layer than the rubbing process. It was assumed that the LC alignment depended on the C-O bonds created from the C=O bonds on the PI surface broken by IB irradiation after an exposure time of 60 s, which resulted in a strong surface energy that transformed the homeotropic LC alignment to homogeneous states.

1. Introduction

Recently, thin-film-transistor (TFT) liquid crystal displays (LCDs) have been frequently utilized in cellular phones, wide LC monitors, and large LCD-TVs. The alignment mechanism on substrate surfaces is a key issue in LCD applications. The rubbing method for nematic LCs (NLCs) on PI surfaces is most widely used to achieve uniform alignment for fabricating LCDs, and the resulting LC alignment properties have been reported by many researchers [1-11]. The rubbing treatment has a few advantages, such as uniform alignment and a stable pretilt angle; however, the treatment introduces debris and electrostatic discharge. Various alternative LC alignment methods for substrates surfaces have been proposed. Ion beam (IB) irradiation of a substrate surface is a noncontact alignment processes that uses Ar+ ion-induced plasma to provide controllability in the manufacturing of highresolution displays with a nonstop process. IB irradiation has been used to align LC molecules on deposited inorganic thin films [12-15] and PI film [16-21]. LC orientation via the rubbing method uses the microgroove effect and oriented polymer chains near the film surface [1- 11,22,23]. The pretilt angle of NLC on the PI surface may be due to the microscopic asymmetric triangles that are formed by rubbing [6]. Huang et al. [25] examined the alignment characteristics of multidirectionally rubbed LC cells. The direction of the final rubbing was observed to initially dominate the orientation of the LC molecules in the multidirectionally rubbed cell because the final rubbing obliterates some of the grooves produced by the first rubbing. Also, a dipole field is generated through the reformation of C–O bonds on the PI surface by IB irradiation, and Lee et al. [19] reported that this may favor LC alignment via dipole-dipole interactions. Most recently, the effects of IB irradiation and rubbing on the directional behavior and alignment mechanism of LCs on homogeneous PI surfaces have been reported by Lee and Seo et al [26]. The LC direction was shown to follow the IB irradiation alignment direction on the PI surface, regardless of the irradiation occurring before or after rubbing [26]. It was assumed that the LC direction depends strongly on the C–O bonds created from C=O bonds on the PI surface that were broken by IB irradiation, and an investigation of the chemical bonding state of the PI surface was conducted by X-ray photoelectron spectroscopy (XPS) [26].

In this study, we describe the effect of rubbing and IB irradiation on the behavior of LC molecules on the homeotropic PI surface as measured by photomicrography, pretilt angle, and voltage-transmittance (V-T) characteristics of vertical alignment (VA) and twisted nematic (TN) cells. The LC orientation mechanism of these alignment processes was investigated with XPS analysis.

 

2. Experiments

ITO-coated glass was supersonically cleaned in a tricloroethyl-acetone-methanol-deionized water solution for 15 min and dried by N2 gas. The homeotropic polymer (AL1H659) was uniformly spin-coated on indium-tinoxide (ITO) electrodes and imidized at 200 ℃ for 1 h. The PI layers were exposed to IB and rubbed in different LC alignment directions. The PI films were rubbed by a drum wrapped with a nylon cloth. The rubbing strength (RS) was defined by the following Eq. [9].

where N is the number of the times for rubbing, M is the depth of the fibers (mm), n is the rotation rate of the drum, V is the translating speed of the substrate, and r is the radius of the drum. An RS of 300 mm was used on the PI surface, which was strongly rubbed for LC alignment [9]. The PI films were irradiated with an Ar IB plasma using a DuoPIGaton-type IB system [17]. The IB chamber was initially evacuated to a base pressure of approximately 10-6 Torr, and the working pressure was maintained at approximately 10-4 Torr with an Ar gas flow of 1.4 SCCM (SCCM denotes cubic centimeter per minute at STP). The dosages of Ar+ IB plasma were 1014-1015 ions/cm2 for an IB incident energy of 0.8 keV, at an angle of 45° and an exposure time of 10 and 60 s. The current density in the beam of positively charged particles was measured from a Faraday cup system to be 1.84 mA/cm2, and the plasma ion density was measured by the double Langmuir probe tips and was approximately 1011 cm-3.

Fig. 1 shows the LC alignment sequence and describes the LC ordering directions. Because LC alignment is mainly effected by the final process in the multidirectionally aligned cell as mentioned above [25], the cells shown in Figs. 1(a) and (b) were fabricated with different sequences of IB irradiation and the rubbing process. To obtain an exact measurement of the LC orientation, the IB irradiation and rubbing directions were separated by 45°. To observe the pretilt angles and microphotographs, the glass substrates were fabricated in an antiparallel configuration with a cell gap of 60 μm. For the measurement of V-T characteristics, the VA and TN cells were prepared with a cell gap of 4.5 μm. The negative LC (Tc = 75℃, Δε = -4; MJ98468, Merck) was then injected into the cell and assembled. The state of the LC alignment was observed using a photomicroscope (Olympus BXP51) with a cross polarizer. The pretilt angle of the NLC was measured by the crystal rotation method (TBA107 device, Autronic). The chemical bonding states of the film surface were analyzed using XPS (PHI 5800). In addition, the V-T characteristics of the LC cell were obtained with a LCD evaluation system.

Fig. 1.Two LC alignment methods on a PI surface were used: (a) IB irradiation after rubbing, and (b) rubbing after IB irradiation.

 

3. Results and Discussion

Fig. 2 shows the photomicrograph of NLCs on homeotropic PI surfaces with the two alignment methods (under crossed nichols). The preparation conditions for the LC alignment were IB irradiation after rubbing and rubbing after IB irradiation. The IB exposure times were 10 and 60 s. Figs. 2(a) and (b) show stable homeotropic LC alignment on the PI surface observed in both the direction of IB irradiation with an exposure time of 10 s and in the rub direction. In Figs. 2(c) and (d), we note that the effect of the LC alignment is mainly dependent on IB irradiation with an exposure time of 60 s. And, we can see the LC alignment effect of rubbing direction was disappeared. It was indicated that the LC orientation direction is mainly determined by IB irradiation with exposure time of 60 s and not by rubbing.

Fig. 2.Microphotographs of LCs on PI surfaces subjected to two alignment methods (under crossed nichols): (a) IB irradiation with an exposure time of 10 s after rubbing; (b) rubbing after IB irradiation with an exposure time of 10 s; (c) IB irradiation with an exposure time of 60 s after rubbing and (d) rubbing after IB irradiation with an exposure time of 60 s.

Fig. 3 shows the transmittance characteristics as a function of the incident angle for measuring the pretilt angles of NLC on the homeotropic PI surface from two alignment methods. Figs. 3(a) and (b) show a stable graph of the pretilt angle of both the IB irradiation directions with an exposure time of 10 s and the rubbing direction, regardless of the alignment sequence. The pretilt angle was 78-80°, with a low error ratio that represents well aligned homeotropic LC orientation. Figs. 3(c) and (d) show a 1-2° pretilt angle with a low error ratio obtained in the IB irradiation direction using an exposure time of 60 s, not in the rubbing direction. It means that the IB irradiation with an exposure time of 60 s induced a strong NLC surface energy on the PI layer, which eliminates the effect of LC alignment on the rubbing direction and transforms the homeotropic LC alignment to the homogeneous states.

Fig. 3.Transmittance as a function of incident angle on the PI surface for two alignment methods to measure the pretilt angle: (a) IB irradiation with an exposure time of 10 s after rubbing; (b) rubbing after IB irradiation with an exposure time of 10 s; (c) IB irradiation with an exposure time of 60 s after rubbing and (d) rubbing after IB irradiation with an exposure time of 60 s.

The V-T characteristics of the VA and TN cells on the homeotropic PI surfaces produced by the two alignment methods are shown in Fig. 4. Figs. 4(a) and (b) showed good V-T characteristics of VA cells observed with the rubbing direction, while a poor transmittance characteristic was measured in the direction of IB irradiation with an exposure time of 10 s. This result is different from the stable homeotropic LC alignment of the IB irradiation direction with an exposure time of 10 s shown in Figs. 2 and 3. In the case of homeotropic LC alignment, the directional differences of LC orientation cannot be estimated from the photomicrograph and measuring pretilt angles. From directional differences of V-T characteristics, we can determine precise directional behavior of homeotropic LC. Therefore, we assume that LC alignment was not affected by IB irradiation with an exposure time of 10 s, but by the rubbing process. The TN cells in Figs. 4(c) and (d) are similar to the trends in Figs. 2 and 3, in which the direction of the IB irradiation for 60 s was the dominant LC alignment influence. The trends in Figs. 2, 3, and 4 confirm that the IB irradiation with an exposure time of 60 s predominates over the rubbing method in the LC orientation process, and has a strong surface energy.

Fig. 4.V-T characteristics of VA and TN cells on the PI surface for: (a) IB irradiation with an exposure time of 10 s after rubbing; (b) rubbing after IB irradiation with an exposure time of 10 s; (c) IB irradiation with an exposure time of 60 s after rubbing and (d) rubbing after IB irradiation with an exposure time of 60 s.

XPS analysis was conducted to clarify the properties of LC alignment on the homeotropic PI surface via IB irradiation.

Fig. 5 shows the XPS spectra for the C1s peak of the PI surface by IB irradiation with exposure time of 10 and 60 s.

Fig. 5XPS spectra for the C1s peak of the homeotropic PI (AL1H659) surface for (a) IB irradiation with an exposure time of 10 s and (b) IB irradiation with an exposure time of 60 s.

The peak at 286.3 eV corresponds to the C–O single bond, and the peak at 288.6 eV corresponds to carbonyl carbon (C=O) in imide rings. The intensity of the C=O bonding peak decreases and the intensity of the C–O peak increases with the increasing exposure time of IB irradiation, indicating that LC orientation was created by the breakdown of π bonds in the PI [18]. It can be suggested that the LC alignment of NLCs on the PI layer by IB irradiation using dipole-dipole interactions [19] between LC molecules and broken bonds on the homeotropic PI surface is more significant than the LC alignment induced by the microgroove effect or stretched polymer chain with the rubbing process. It was also assumed that the surface energy of NLCs on the PI layer with an IB irradiation time of 60 s was stronger than that of the PI surface with rubbing process.

 

5. Conclusion

In this study, we investigated the LC alignment characteristics on homeotropic PI via IB irradiation with exposure times of 10 and 60 s and a rubbing process with different alignment directions. The LC orientation was not affected by IB irradiation with an exposure time of 10 s, and IB irradiation with an exposure time of 60 s more significantly affected the LC orientation on the homeotropic PI surface than the rubbing process. XPS analysis illustrated that the LC alignment direction depends strongly on C–O bonds created from the C=O bonds on the PI surface that were broken by IB irradiation. We assumed the LC alignment mechanism was more affected by chemical bonding between LC molecules and broken chains on the PI layer than by physical processes, such as the microgroove effect or the stretched polymer chain. Furthermore, the surface energy of NLCs on the homeotropic PI layer with IB irradiation with an exposure time of 60 s was stronger than that of the PI surface with rubbing, as observed by photomicrograph analysis, pretilt angle measurement, and VT characteristics.