Poly(styrene–maleic anhydride) films as alignment layers for liquid crystal systems via ion-beam irradiation

Abstract

We report an investigation of poly(styrene–maleic anhydride) (SMA) films as liquid crystal (LC) alignment layers fabricated by ion-beam (IB) irradiation. We confirmed that SMA-deposited LC cells have superior optical properties by measuring the transmittance and phase differences. Using an IB-irradiated SMA film for the alignment layer, we could confirm that uniform and homogeneous LC alignment was achieved, yielding extremely high-performance nematic liquid crystal (NLC) systems. The LC alignment mechanism was determined by X-ray photoelectron spectroscopy. IB irradiation caused chemical modifications that led to strong van der Waals forces between LCs and the modified SMA surface, thereby inducing uniform LC alignment. Moreover, the IB-irradiated SMA films exhibited good electro-optical characteristics. Therefore, these IB-irradiated SMA films are suitable as alternative alignment layers for LC display applications.

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1 Introduction

The uniform alignment of liquid crystal (LC) molecules is an important prerequisite for high-quality liquid crystal display (LCD) systems. Conventional mechanical rubbing alignment processes have been widely used to align LC molecules uniformly on the polymer layer in LCD devices.^1–3 However, creating a rubbed polymer surface generates debris, local defects, and streaks, as well as static electricity during mechanical contact with the roller, resulting in the degradation of the display performance.^4,5 To address these problems, several noncontact techniques have been evaluated over the past several years, such as ultraviolet (UV) photoalignment,^6,7 SiO oblique evaporation,⁸ and ion-beam (IB) irradiation^9–11 methods. Among these methods, IB irradiation of the substrate surface is a noncontact alignment process in which the alignment properties can be easily changed by controlling the Ar⁺ ion-induced plasma. In addition, unlike other noncontact alignment techniques, this method has been used to align LC molecules on both organic and inorganic films. However, IB irradiation method has been mainly applied to inorganic materials such as ZnO,¹² Al₂O₃,¹³ and GaO.¹⁴ Recently, polymers such as polyimide,¹⁵ carbon like diamond,¹⁶ polystyrene,¹⁷ and polyvinylidenefluoride¹⁸ has been researched as suitable for LC alignment. Among them, polyimide seems to be one of the most suitable materials for LC alignment layer because it has high heat resistance, mechanical strength, chemical stability, and electrical properties.

In this study, we demonstrate the use of a styrene–maleic anhydride copolymer (SMA) film treated with IB irradiation as an LC alignment layer. SMA has a higher glass transition temperature than polystyrene and is chemically reactive because of its active functional groups. Furthermore, SMA is well suited for use in display devices because of its high transmittance, low phase difference, and good thermal properties.^19,20 Homogeneous and uniform LC alignment was achieved on SMA films by adjusting the annealing temperature. Peak shifts and bonding energy transitions of the IB-irradiated SMA films were investigated by X-ray photoelectron spectroscopy (XPS). The orientation order of LC molecules was induced by a chemical modification of the surface features of the SMA films by IB irradiation, which is related to the van der Waals forces between LCs and the modified SMA surface. Breaking the carbon–oxygen double bonds and phenyl ring by IB irradiation provided a uniform LC alignment and pretilt angle. A good threshold voltage was obtained using the LC alignment layer with IB-irradiated SMA films. The film alignment properties and EO properties of twisted nematic (TN) LC cells were measured using various methods.

2 Experiment

SMA (Sigma-Aldrich) polymers of various concentrations were prepared by mixing 5, 10, 20, or 30 wt% SMA in 10 mL acetone (Fig. 1). The obtained solution was stirred at 200 rpm for 1 h at room temperature and then aged for at least 1 d. A slab of indium tin oxide (ITO)-deposited glass was cleaned in an ultrasonic bath using acetone and isopropyl alcohol for 10 min and then rinsed using deionized water. Using a spin-coater, the prepared SMA solution was coated onto the ITO glass substrate for 30 s at 3000 rpm, achieving a thickness of 50 nm. Next, the coated substrates were annealed at 100, 200, or 300 °C for 1 h on a hot plate. The coated substrates were irradiated by Ar IB plasma for 2 min at an intensity of 1.8 keV and incident angle of 45° using an advanced DouPIGatrontype IB system. The density of the Ar plasma IB was from 10¹⁴ to 10¹⁵ ions per cm², and the IB current was from 1.0 to 1.2 mA cm⁻². The substrates were fabricated antiparallel with cell gaps of 60 μm to observe the LC alignment and measure the pretilt angle and thermal stability. Positive nematic liquid crystals (NLCs) (MJ001929, n_e = 1.5859, n_o = 1.4872, Δε = 8.2, T_c = 72 °C; Merck) were injected into empty cells in an isotropic state via capillary action. We confirmed the LC alignment characteristics and thermal stability. Additionally, the SMA-coated substrates were fabricated in twisted nematic (TN) cells with a cell gap of 4.25 μm for measurement of the electro-optical (EO) characteristics and anchoring energy. The transmittance of the SMA substrate at different annealing temperatures was measured using ultraviolet, visible, and near-infrared (UV-VIS-NIR) spectroscopy. The LC alignment state and thermal stability were observed using a polarized optical microscope (POM) (Olympus BXP 51). The pretilt angles of the LC cells were measured by the crystal rotation method (Autronic TBA 107). To evaluate the EO properties of the SMA-based TN cell, the voltage–transmittance (V–T) curves were obtained using an LCD evaluation system (Otsuka Electronics LCD-700). The polar anchoring energy of the TN cells was calculated on the basis of LCR meter (Agilent 4284A) measurements at room temperature.

Fig. 1 Chemical formula of the styrene–maleic anhydride copolymer (SMA) studied herein.

3 Results and discussion

The optical properties and pretilt angle of the LC alignment for the SMA layer were measured to confirm its suitability for LC applications. We performed naked-eye observations of the surface state of samples consisting of various concentrations of SMA (5, 10, 20, and 30 wt%) deposited on an ITO glass substrate. It can be seen that the surface of the substrate with 5 wt% SMA is clear and smooth, whereas that of the substrate with 10 wt% SMA is nontransparent and highly hazy. Additionally, the surfaces of the >20 wt% SMA layers are rough and feature edge burrs. Therefore, the 5 wt% SMA-deposited substrate is used in the following experiments.

Fig. 2 presents the X-ray diffraction (XRD) pattern of SMA deposited on an ITO glass substrate exposed to a 1.5 kW Cu radiation (λ = 1.5406 Å) sealed X-ray source. No distinguishable diffraction peaks were observed at any annealing temperature, indicating that the SMA film is amorphous. Moreover, the IB irradiation did not appear to affect the crystallinity of the SMA film.

Fig. 2 XRD patterns of IB-irradiated SMA films annealed at 100, 200, and 300 °C.

Field emission scanning electron microscopy (FESEM) images show that the surface morphology of the SMA films was dependent on the annealing temperature, as evident in Fig. 3. The surface morphology of the SMA layer increased with annealing temperature from 100 to 300 °C. This morphological change was due to an etching effect induced by the Ar⁺ ion-induced plasma.

Fig. 3 FESEM images of SMA layers deposited on ITO-coated glass at different annealing temperatures and irradiated with IB at an intensity of 1.8 keV.

Fig. 4 shows UV/VIS transmittance over the wavelength range of 250–900 nm for SMA layers deposited on the glass substrate at different annealing temperatures. The average transmittances of the SMA-deposited substrate in the visible region (420–780 nm) were 84.3–84.6%. This result indicates that the SMA layer has superior optical transmittance to pure glass, without loss of transparency.

Fig. 4 UV-visible transmittance spectra of IB-irradiated SMA-based TN LC cells annealed at 100, 200, and 300 °C (a pure NLC cell was used as reference).

The phase differences of the polyimide (PI) film and SMA film on the ITO glass are compared in Table 1. R_e and R_th represent the phase differences on the surface of the alignment layer on ITO glass and in the thickness direction of the alignment layer on ITO glass, respectively. We can confirm that the R_th value of the SMA-deposited substrate is much lower than that of the PI film. The diminishing phase difference in the thickness direction decreases the distortion of the screen display. Thus, it is expected that our proposed SMA films possess outstanding optical properties for display device applications.

Table 1 Phase difference between PI and SMA films on ITO glass

Sample	Refractive index	Re (nm)	Rth (nm)
ITO/PI	1.40	0.143	15.1
	1.45	0.143	16.2
	1.50	0.143	17.5
	1.55	0.143	18.8
	1.60	0.143	20.1
ITO/SMA	1.40	0.054	1.8
	1.45	0.054	2.0
	1.50	0.054	2.2
	1.55	0.054	2.3
	1.60	0.054	2.5

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Fig. 5 shows the LC alignment of LC cells based on SMA-deposited substrates prepared using different annealing temperatures. At an annealing temperature of 100 °C, the LC molecules were not oriented uniformly. The boiling point of acetone is 57 °C, which is less than the annealing temperature in this case. However, the dissolved SMA polymer could increase the boiling point of the SMA solution, causing some of the acetone solvent to remain on the surface rather than evaporating. In contrast, uniform and stable homogenous LC alignment was achieved by annealing above 200 °C.

Fig. 5 Microscope images of the IB-irradiated TN LC cells with SMA films annealed at 100, 200, and 300 °C treated with IB irradiation at 1.8 keV (“A” indicates the analyzer and “P” the polarizer).

The thermal stability of the films was also observed, as shown in Fig. 6. Heating studies were conducted from 90 to 210 °C in 30 °C increments for 10 min on a hot plate, and then the cells were cooled. Compared to LC cells prepared using the conventional rubbed PI, LC cells with IB-irradiated SMA films have better thermal properties. After annealing from 90 to 150 °C, the LC molecules maintain in their orientation, and the LC cells were transparent without any defects. At 180 °C and above, unstable and partially aligned LC molecules in the cracking state were observed and formed disclination lines. Finally, the LC orientation of the CeO₂-doped cells became unstable at an annealing temperature of 240 °C. These results indicate that LCs on the SMA layers have an excellent thermal budget, making these alignment layers competitive for LCD devices.

Fig. 6 Photomicroscopic images of (a) IB-irradiated cells with SMA films and (b) NLC Cells fabricated with rubbed PI at various temperatures as a function of heated temperature from 90 °C to 210 °C.

Fig. 7 shows the calculated pretilt angles, which are in the range of −70° to 70°, of the LC on the SMA layer at different annealing temperatures. The blue and red lines correspond to the simulation and experimental results, respectively. In these plots, if the difference between the simulated and experimental results is small, the LC molecules are uniformly aligned. Therefore, we confirmed that LCs were oriented with high uniformity on the SMA layer. The calculated pretilt angles for SMA-based LC cells at annealing temperatures of 200 and 300 °C were 0.51° and 0.67°, respectively.

Fig. 7 Transmittance of various SMA-based LC cells as a function of incidence angle (from −70° to 70°) for annealing temperatures of 200 and 300 °C.

To determine the mechanism of LC alignment, we investigated the chemical modification of IB-irradiated SMA film using XPS. The SMA polymer has two functional groups able to interact with hydroxyl functional groups: carbonyl oxygen and ether oxygen. As shown in Fig. 8, the XPS spectra for C 1s and O 1s core levels were analyzed at the surface for an annealing temperature of 200 °C before and after IB treatment of the SMA films. The C 1s spectra consist of four main peaks (obtained by fitting several Gaussians): C–C bonding centered at 284.8 eV, C–O bonding centered at 286.6 eV, C=O bonding centered at 289.0 eV, and a π–π* satellite centered at 291.3 eV.^21,22 The C–C peak intensity was dramatically decreased by IB irradiation, whereas the C=O peak intensity increased considerably after IB irradiation.

Fig. 8 (a) C 1s and (b) O 1s core-level XPS spectra of the surface after IB irradiation.

These results indicate that IB irradiation breaks C–C bonds and creates C=O bonds. Furthermore, the phenyl ring was broken after IB irradiation and could form new C–O and C=O bonds, leading to the reconstruction of the chemical bonds of the SMA films. The O 1s spectra can be fit by two Gaussian peaks centered at 531.6 and 533.0 eV.²² IB irradiation allowed for the selective breaking of the C–C bonds, some of which then formed C–O and C=O bonds. This phenomenon is reflected in the change in the atomic composition after IB irradiation, as shown in Table 2. Before IB irradiation, the atomic compositions of carbon and oxygen were 91.10% and 8.90%, respectively, whereas the values after IB irradiation were 77.15% and 22.85%, respectively. The carbon content decreased, but the oxygen content increased by 14%. Before IB irradiation, the molecular interaction between LCs was stronger than the van der Waals forces between the surface of LCs and SMA films; thus, the alignment on the surface was random. On the other hand, after IB irradiation, the surface of the SMA layer was modified such that the phenyl ring was destroyed, and the carbon atoms formed bonds with oxygen. This process increased the van der Waals forces between LCs and the reformed SMA surface because oxygen bonds provide the surface with a higher molecular weight and polarity. In summary, IB irradiation induces the breaking of chemical bonds on the SMA surface to give rise to new bonds between carbon and oxygen atoms, which induces a strong van der Waals interaction between LCs and the modified surface. This mechanism led to a uniform homogeneous LC alignment.

Table 2 Carbon and oxygen atomic compositions of SMA films before and after IB irradiation

Atoms	C	O
Before IB	91.10%	8.90%
After IB	77.15%	22.85%

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As shown Fig. 9(a), the EO properties of TN cells based on the SMA-deposited substrates prepared by IB irradiation were assessed on the basis of the V–T curve, which is related to power consumption. The threshold voltage is the voltage at 90% transmittance, corresponding to the minimum voltage required to turn the transistor ON and was 3.171 V for an IB energy of 1.8 keV. For comparison, the threshold voltage of the TN cell based on conventional rubbed PI was 3.072 V, indicating that the IB-irradiated SMA films could serve as an alternative alignment layer with lower power consumption for LC devices. In addition, we confirmed that the anchoring energy was increased by IB irradiation, being 6.34 × 10⁻⁵ and 3.15 × 10⁻⁴ J m⁻² for non-IB-treated and IB-treated TN cells, respectively, as shown Fig. 9(b). Upon IB irradiation of the SMA layer, the anchoring energy increased. The weak anchoring energy of the TN cells prevented the LC molecules from being properly anchored on the SMA layer, inducing a quasihomogeneous alignment.²³ Thus, the TN cell exhibited a nonuniform switching behavior. Moreover, we confirmed that as the anchoring energy increases, the threshold voltage decreases.

Fig. 9 (a) Comparison of the voltage–transmittance curves of TN cells fabricated with rubbed PI, non-IB-irradiated SMA, and IB-irradiated SMA. (b) Anchoring energy of the TN cells with non-IB-irradiated and IB-irradiated SMA.

4 Conclusion

We successfully demonstrated homogeneous liquid crystal alignment on IB-treated SMA films in an LC system. The superior optical properties of these films were confirmed by measuring the transmittance and phase difference of SMA films on ITO glass. POM images revealed a clear and homogeneous LC alignment without any defects. The alignment mechanism of LCs was investigated by XPS analysis, which indicated that IB irradiation of the SMA surface changed the intensity of C 1s and O 1s peaks. van der Waals interactions between LCs and the modified alignment layer were considered to be the main factors in the LC alignment. We also proved that LCs deposited on IB-irradiated SMA films exhibited a good threshold voltage similar to that of LCs deposited on a rubbed PI alignment layer, making them suitable for use in commercial LCDs.

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