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Abstract
We present a two-step in situ photoalignment method for achieving stable planar alignment of nematic liquid crystals. Monodomain and multidomain planar alignments were efficiently induced by employing photochromic side chain polyimides with linearly polarized visible light. Subsequently, the induced alignment was effectively stabilized by UV-curable reactive mesogens (RMs). For efficient alignment control and stabilization, two processes were performed separately by using different wavelengths of exposed light (i.e., dual wavelength in situ photoalignment). The polymerized RM-layers acted as new alignment layers, by overcoating the pristine PI-alignment layers. As a result, the reversible photochromic PI-layer lost its function as an alignment layer and thus the LC alignment became irreversible and stable. Therefore, the proposed dual wavelength in situ photoalignment can be beneficially adopted for practical device applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid crystal is a fascinating phase of matter with fluidic soft crystalline properties. This unique nature makes LC a very useful material for various applications. However, to use LCs in electro-optical devices, one of the most prerequisite is to set up boundary conditions to control the molecular alignment [1]. Various methods have been proposed and developed for either planar or vertical alignment of LCs [1–6]. The most typical technique adopted for mass production is to use polymeric alignment layers. Polyimides are solution coated, soft and hard baked, and gently rubbed along uniaxial direction for homogeneous planar alignment of LCs [1]. Such a contact method (i.e., rubbing technique) is extremely challenging for achieving uniform alignment for large area and high resolution displays. In particular, the current trend of small pixel size makes the rubbing method far more challenging due to the uneven surfaces. To overcome drawbacks of the contact method, the non-contact techniques such as the oblique evaporation [7,8], Langmuir–Blodgett films [9], ion-beam alignment [10–12], and photo-alignment [13–15] were proposed and have been a great topic of interest for last two decades.
The photoalignment was one of the key technology for a noncontact alignment method and provided crucial solution to overcome such problems, caused by the rubbing [13]. The photoalignment based on the photo-sensitive polymers and additives has been researched by many groups [13–24]. The dichroic chromophores selectively respond to the linearly polarized light and induce molecular orientational anisotropy at surfaces. As a result, LCs are aligned with respect to surface energy anisotropy. In general, the chromophores such as azobenzene, cinnamate, chalcone, and coumarin moieties are covalently introduced to polymers as either main chain or side chain. The substrates with a thin polymer layer are pretreated by the linearly polarized light and assembled to LC cells. Photochromic isomerization and/or photochemical [2 + 2] cycloaddition reaction is responsible for the induced surface energy anisotropy, resulting in LC alignment. In many cases, the alignment is reversible (i.e., unstable) and the anchoring of LC molecule is insufficient. Many attempts have been made to achieve irreversible and permanently stable alignment with strong anchoring [13–24].
The photoalignment based on photodegradation was also developed and recently employed for commercial production. Selective segmentation of the polymer chains induces surface energy anisotropy and therefore aligns LC molecules [24–26]. High quality alignment and sufficient anchoring have been accomplished. In this case, however, linearly polarized deep UV-light (LPUV, ~254 nm) with high energy is required for segmental degradation of polymer chains. Additional process, after LPUV treatment, is necessary to clean fragmented chains. Slow leaching decomposed segments beneath the surface may cause long term reliability of electro-optical properties.
On the other hand, reactive mesogens (RMs) can be beneficially used for stabilizing a specific configuration of LC alignment [27–31]. The efficient stabilization of LC alignment can be achieved by photopolymerizing RMs in the aligned LC medium. High quality LC alignment is stabilized and thus retained stable against heat, UV-Vis light, and applied electric field.
In this report, we present a two-step in situ photoalignment method for achieving stable planar alignment of nematic LCs. For in situ alignment control, the visible light sensitive azobenzene chromophores were covalently introduced to polyimide alignment layers. Photopolymerizable RMs with UV-light was doped to host LCs for the stabilization of LC alignment. For efficient alignment control and subsequent stabilization, the alignment and its stabilization processes were performed separately to avoid interference each other (i.e., dual wavelength in situ photoalignment). The efficient alignment control was achieved by using visible light (λ > 400 nm) and the induced alignment was effectively stabilized by polymerizing RMs with UV-light (350 nm ~380 nm). For the study, the soluble polyimides with azo-chromophore were designed and synthesized for dichroic visible-light sensitivity. Mono- and multi-domain planar alignments were obtained by the dual wavelength in situ photoalignment. The stabilities of LC alignment was investigated before and after RM-network stabilization by employing polarized optical microscopy (POM), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM).
2. Experimental section
2.1 Materials
For the dual wavelength alignment control, we employed visible-light responsive polymeric dyes and UV-light sensitive reactive mesogens. The Methyl Red and Disperse Red 1 were chosen due to their visible-light responsive photochromism. The dyes were chemically modified to covalently attach to the polymer. The soluble polyimide, containing dihydroxy biphenyl unit, was prepared from 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3′-dihydroxy-4,4’-diaminobiphenyl (DHDAB) by the solution imidization of polyamic acid in dry xylene at 180°C. The soluble polyimide (PI) was used as a polymer backbone. The modified Methyl Red and Disperse Red 1 were covalently introduced as a side chain by the Mitsunobu reaction and resulted in the corresponding soluble photochromic polyimides (PI-MR and PI-DR), respectively. As a photo-polymerizable monomer with the UV-light, the 2-methylbenzene-1,4-diyl bis{4-[3-(acryloyloxy)propoxy]benzoate} (also, known as RM-257) was used for stabilizing the induced LC alignment. The nematic LC mixture MLC-15600-100 (Merck), used as a host LC, exhibits a positive dielectric anisotropy and the nematic-to-isotropic transition temperature at 90.0 °C.
2.2 LC Cell fabrication
Plates of ITO-glass were cleaned with a soap solution in sonic bath and used as substrates to fabricate LC cells. The PI-MR and PI-DR solutions in γ-butyrolactone with varied concentrations (5.0 wt%, 3.0 wt%, 1.0 wt% and 0.5 wt%) were spin cast on the substrates. The PI-films were soft baked at 80 °C and hard baked at 220 °C, respectively. The photochromic films were also prepared by dissolving the PI-MR or PI-DR in the commercial polyimide solution (Sunever 6154, Nissan Chemical Industries, LTD.) used for conventional planar alignment layers. The ratio in solid content (PI-MR in Sunever 6154) was varied from 5.0 to 50.0 wt.%. Thin films of the mixture solutions were spin cast, soft baked at 80 °C, and hard baked at 220 °C. The substrates with a thin photochromic film were assembled to LC cells. No surface treatment was employed for the alignment of LCs. The cell gap was maintained using 10 µm tape spacer. Small amount (ca. 0.1 ~0.5 wt.%) of photopolymerizable monomer was dissolved in a LC host (MLC-15600-100) and injected to LC cells by capillary action in the isotropic phase (95.0 °C).
2.3 In situ alignment control
In situ photo-alignment was performed by using linearly polarized visible light (LPVL). The cells were illuminated with linearly polarized visible light in the isotropic phase at 110 °C. The LPVL treatment for LC alignment and monitoring LC texture were carried out using polarized optical microscope (POM, Nikon Eclipse LV100 POL) equipped with a Nikon DS-Ri1 CCD camera. The collimated LPVL irradiation was performed in the reflection mode with no objective lens and LC textures were observed in the transmission mode. Nikon LV-HL50W 12V 50W LONGLIFE halogen lamp was used as the light source for illuminating the cells. Instec STC 200 temperature controller with HCS 402 hot stage was used to perform the experiment at elevated temperatures. The metal halide lamp with a bandpass filter for 400 ~700 nm visible wavelength was also used for LPVL treatment. Illuminations were carried out with 0.9 J/cm2 at 450 nm. For the uniform monodomain alignment, the LC cell was treated by the one-step LPVL. The patterned LC alignment was achieved by the two-step LPVL treatment. After uniform LPVL treatment, the second LPVL illumination was carried out with the chessboard patterned mask with 400 μm pitch (200 x 200 μm2 square). The polarization axes for the first and second LPVL irradiation were made at 45°.
2.4 Stabilization of LC alignment
The stabilization of LC alignment was achieved by the photopolymerization of reactive mesogens. After completion of in situ photoalignment, the LC cells were cooled to room temperature and illuminated with UV-light. The Spot Cure Model SP-9 (Ushio Inc.) or metal halide lamp was used as UV-light source. The wavelength, ranging 350 ~380 nm, was selected using bandpass filter. The entire cell was exposed with UV-light at the intensity of 5 mW/cm2 for 10 minutes.
2.5 Characterization of the stabilization layer
The stabilization layers formed by polymerized RMs were characterized by the field emission scanning electron microscope (FE-SEM; S4700, Hitachi, Japan) and atomic force microscope (AFM; Multimode-8, Bruker, Germany) after complete removal of LCs in hexane/chloroform mixed solvent. All AFM images were obtained with software NanoScope V in air with tapping mode using silicon tips.
3. Results and discussion
Dual wavelength approach: Under polarized light irradiation, dichroic absorption and consequent photochromic isomerization of azo-dyes promote reorientation of surface molecules, inducing chemical anisotropy of the surfaces [13,15]. As a result, LC molecules are preferentially aligned along a specific direction. The alignment control is very efficient and useful for photo-addressable alignment change of LCs [13–15]. However, its reversibility critically limits practical device applications since most of LC devices require permanently stable LC alignment. In the meanwhile, RMs are extensively used for stabilization of various LC textures [27–31]. Polymerized RMs form thin surface layers and thus stabilize specific LC configurations at the surface.
By combining these processes, a permanently stable LC alignment can be effectively achieved by the noncontact in situ photoalignment method. For the best result, the LC alignment should be induced first and followed by the stabilization process, forming a thin polymer alignment layer. In general, both processes are performed using a similar wavelength range of electromagnetic radiation (i.e., 300 ~400 nm UV-light). For in situ photoalignment, however, LC loaded cells are treated at different temperatures for the isotropic and nematic phase, respectively [27–31]. In this case, the aligning and stabilizing processes may occur simultaneously and therefore result in a poor LC alignment and stability. To avoid this situation, we employed the dual wavelength approach. The aligning and stabilizing processes were executed by separate steps using different wavelength ranges of irradiating lights. To minimize the interference with each other, the LC cells were exposed to linearly polarized visible light (LPVL) first for LC alignment in the isotropic phase and followed by the UV-light irradiation for the stabilization of alignment in the nematic phase. Sequential visible- and UV-light treatments produced a high quality LC alignment with irreversible stability as discussed below.
Design and synthesis of photochromic polyimides: For dual wavelength approach, two different photochromic polyimides were designed and synthesized (see Fig. 1). Because the aligning process is executed as the first step, we designed the azo-dye sensitive to longer wavelength than UV-light used for stabilization. The Methyl Red and Disperse Red 1 were chosen as chromophores since their absorption peaks extend to visible range. It is known that the electron donor and acceptor in the conjugated π-electron system promote a red shift in absorption of azobenzene chromophores. Figure 1 shows the chemical structures of synthesized photochromic polyimides and their UV-Vis absorption spectra. As expected, both photochromic polyimides showed broad and intense absorption in the visible wavelengths. In addition to the absorption near 300 nm for the PI backbone, distinct absorption bands ranging 350 ~550 nm and 360 ~600 nm were observed for the PI-MR (in blue) and PI-DR (in red), respectively.
Fig. 1 Chemical structures and UV-Vis absorption spectra of the PI (black), photochromic polyimides PI-MR (blue) and PI-DR (red).
LPVL-induced In situ photo-alignment: Fig. 2 summarizes the principle and processes of in situ alignment control of LCs based on the proposed dual wavelength approach. It is well understood that the surface energy anisotropy and associated LC alignment is induced by the molecular reorientation of the azo-dye [13,15,17]. Due to the dichroic absorption of chromophores and polarized light irradiation, reversible photochromic trans-cis isomerization leads to the anisotropic surface in molecular orientation. The dye molecules are oriented perpendicular to the incident light polarization as illustrated in Fig. 2(a). As a result, LC molecules align perpendicular to the irradiated light polarization [13,15]. Fig. 2(b) corresponds to the LC cell prior to the LPVL irradiation. The LC host, mixed with RMs, is randomly aligned planar to the PI-surface due to the isotropic nature of the surface as illustrated. Upon LPVL irradiation in the isotropic phase (110 °C), molecular orientational anisotropy is induced at both the top and bottom surfaces as shown in Fig. 2(c). The LC director uniformly aligns perpendicular to the polarization direction, indicated by the arrow head and double ended arrow in red. The LC alignment is not permanent at this stage. Because photochromic trans-cis isomerization is reversible, the LC director can be realigned by changing the polarization direction of irradiating light (i.e., photo-addressable) [13,15]. Circular polarized light can completely randomize LC director. To make the induced alignment permanent, however, the uniform LC alignment can be stabilized by photopolymerization of RMs with UV-light. As illustrated in Fig. 2(d), the polymerized RMs form thin polymer layers (in red) at surfaces, permanently stabilizing the uniformly aligned LC director. The stabilization layers act as in situ produced alignment layers, replacing the reversible photochromic alignment layers by overcoating.
Fig. 2 Schematic illustration of the “dual wavelength in situ photoalignment”: (a) molecular reorientation through reversible photochromic isomerization with LPVL irradiation; (b) pristine LC cell with no alignment (isotropic surfaces denoted by the gray blocks); (c) LPVL-induced uniform planar alignment with anisotropic surfaces (yellow blocks); (d) permanent alignment stabilized by the polymerized RM-networks (red blocks); (e) multidomain alignment with patterned anisotropic surfaces (blue and yellow blocks) induced by the two-step LPVL irradiation with a photomask; (f) irreversible patterned alignment stabilized by the polymerized RM-networks (red blocks). The double-ended arrows and arrow head indicate the polarization direction of LPVL.
The patterned alignment can also be achieved by the two step LPVL-treatments with different polarization. As illustrated in Figs. 2(c) and 2(e), the second polarized irradiation with 45 degree from the first one by applying photomask can produce the patterned LC alignment. The alignment at both the top and bottom surfaces are patterned depending on the photomask with different polarization directions (blue and yellow) as shown in Fig. 2(e). This can be stabilized, as in Fig. 2(d), by polymerizing RMs with UV-light. The polymer layers, represented by the red blocks, at inner surfaces irreversibly stabilize the patterned alignment as shown in Fig. 2(f) and serve as permanent alignment layers. Figure 2 clearly demonstrates advantage of the dual wavelength approach for in situ photoalignment with irreversible stability.
The LC cells were fabricated by employing the photoresponsive polyimides PI-MR and PI-DR and used for in situ photoalignment by LPVL. For both PIs, essentially the same dual wavelength photoaligning and stabilizing effects were observed. However, the PI-MR exhibited somehow better quality for uniform alignment. In addition, alignment layers with 100%, 50%, and 5% PI-MR solid content in the commercial PI (Sunever 6154) showed efficient in situ photoalignment for the study. Therefore, we present the experimental results by focusing on the 5.0 wt.% PI-MR employed cells to demonstrate the dual wavelength approach below.
Figure 3 presents the results of in situ photo-controlled monodomain and multidomain LC alignment. Figures 3(a-i) and 3(a-ii) are the polarized optical images of the LC cell after LPVL treatment. The circled area was exposed with linearly polarized light (direction denoted by the single arrow). The POM image in Fig. 3(b) corresponds to the area outside of the exposed circle, where LC shows no uniform alignment. The POM images in Figs. 3(c-i) and 3(c-ii) represent uniform LC orientation as designated by the ellipses. The director was aligned perpendicular to the polarization direction (denoted by the single arrow). It was confirmed by using quarter-wave plate.
Fig. 3 In situ photo-controlled monodomain and multidomain LC alignment: (a-i)/(a-ii) macroscopic polarized optical images of monodomain uniform planar cell; POM images of (b) unexposed area and (c-i)/(c-ii) LPVL-exposed area with different orientations; (d-i)/(d-ii) POM images of the chessboard patterned planar alignment with different orientations. The single and crossed arrows indicate the direction of LPVL and crossed polarizers, respectively. The pitch of chessboard pattern corresponds to 400 μm in (d). The exposed circle corresponds to 1.2 cm in diameter. The microscopic images were taken with 200 times magnification.
The chessboard patterned alignment in Figs. 3(d-i)/3(d-ii) was obtained by the two-step LPVL treatment with a photomask. The first and second LPVL with 45° difference in polarization direction were consecutively illuminated without and with applying the chessboard patterned mask. Therefore, the bright and dark domains were formed by the first and second LPVL treatments, or vice versa. This result indicates that a variety of patterned alignments can be achieved by multistep process. In addition, with the application of patterned retarders, various patterned LC alignments can be obtained by a single step LPVL treatment.
The resolution of patterned alignment can be determined mainly by the lithographic or holographic resolution of polarized light used for photoalignment. We conjecture that the resolution may reach to the order of microns if the lithographic or holographic condition is satisfied. However, for the in situ LPVL treatment, the resolution can be further limited by the existence of the top substrate (i.e., distance between the mask and surface of alignment layers). The thermal randomization can affect the induced alignment during the second LPVL treatment. However, it was a slow process with the order of tens of minutes so that provided a stable window for the second masked irradiation. The alignment quality, induced by the first-step was stable enough during the second exposure process.
Although the process was very efficient, the alignment exhibited a poor stability against heat and light. Figure 4 discloses heat and light instability of the LPVL-induced LC alignment. As observed in Figs. 4(a-i), 4(a-ii) and 4(a-iii), the exposed area showed uniform alignment with no defects right after the LPVL treatment. However, the alignment was significantly degraded as shown in Figs. 4(b-i), 4(b-ii) and 4(b-iii) after the thermal treatment at 110 °C for 1 h. The degradation of LC alignment was much accelerated at the presence of unpolarized light. After irradiation of circularly polarized light at 10 mW/cm2 and 110 °C for 10 minutes (6.0 J/cm2 at 110 °C), the orientation of LC molecules was completely randomized as observed in Figs. 4(c-i), 4(c-ii) and 4(c-iii). The macroscopic (Fig. 4(c-i)) and microscopic (Figs. 4(c-ii)/4(c-iii)) images clearly demonstrate randomized director orientation.
Fig. 4 Polarized optical images of the in situ photo-aligned LC cells before stabilization: (a-i) macroscopic image of the cell and (a-ii)/(a-iii) microscopic images with different orientation after photoalignment; (b-i) macroscopic and (b-ii)/(b-iii) microscopic images after thermal treatment (110 °C for 1 h); (c-i) macroscopic and (c-ii)/(c-iii) microscopic images after thermal and circularly polarized visible-light treatment (6.0 J/cm2 at 110 °C). The single and crossed arrows indicate the direction of LPVL and crossed polarizers, respectively. The ellipses in (a) represent LC molecules. The exposed circle corresponds to 1.2 cm in diameter. The microscopic images were taken with 200 times magnification.
Stabilization for irreversible alignment: Although the process is well known and very efficient, it has shown a critical limitation in stability as discussed above. Because the stable alignment is an essential requirement for most LC devices, we introduced the stabilization layers, as discussed in Figs. 2(d) and 2(f), by photopolymerizing RMs with UV-light irradiation. Figure 5 demonstrates the effect of the RM-stabilization by showing the macroscopic and microscopic cell images. It showed a good stability of LC alignment against heat and light treatments. Figures 5(a-i), 5(a-ii) and 5(a-iii) correspond to the polarized optical images after the stabilization of the LPVL-induced alignment. It represents high quality alignment with no defect. After stabilization and subsequent heat treatment (110 °C for 3 days), the alignment in circled area was not degraded at all as shown in Fig. 5(b). Even after additional unpolarized light treatment (36.0 J/cm2 at 110 °C), it maintained uniform dark and bright states with no defect as shown in Figs. 5(c-i), 5(c-ii) and 5(c-iii). The cell images for a bright state, corresponding to Fig. 5(a-i), 5(b), 5(c-i), also showed uniformly aligned states. No meaningful change was observed before and after stability test. Figures 5(d-i) and 5(d-ii) correspond to the outside of the circle in Figs. 5(a-i) and 5(c-i), where no uniform alignment was induced and stabilized during the dual wavelength process. In this area, no alignment was induced by the LPVL treatment after stabilization. It was evident that both aligned and unaligned area were completely stabilized by the polymerized RMs, and therefore the LC alignment for both areas became irreversible and permanent.
Fig. 5 Polarized optical images of the in situ photo-aligned LC cells after stabilization: (a-i) macrograph and (a-ii)/(a-iii) micrographs of the cell after photoalignment; (b-i) macroscopic and (b-ii)/(b-iii) microscopic images after thermal treatment (110 °C for 3 days); (c-i) macroscopic and (c-ii)/(c-iii) microscopic images after thermal and circularly polarized visible-light treatment (36.0 J/cm2 at 110 °C). The single and crossed arrows indicate the direction of LPVL and crossed polarizers, respectively. The ellipses in (a) represent LC molecules. The exposed circle corresponds to 1.2 cm in diameter. The microscopic images were taken with 200 times magnification.
The photochromic PIs are also sensitive to the UV-light and thus the induced LC alignment can be affected by the UV-light irradiation. However, the alignment efficiency is highly dependent on temperature. It was very slow at room temperature, presumably due to slow thermal relaxation of cis- to trans-isomer. Therefore, the visible-light-induced alignment was effectively stabilized by polymerizing RMs at room temperature. As typically observed in the polymer stabilized LCs, the latter UV-process simply stabilized the aligned LC rather than randomizing alignment.
Characterization of stabilization layer: After complete removal of LCs from the stabilized cell, the inner surfaces were examined by FE-SEM and AFM. Figure 6 shows surface morphologies observed from the cells with 0.2 wt.% (Fig. 6(a)) and 0.5 wt.% (Fig. 6(b)) of RM-257. Figures 6(a-i) and 6(a-ii) correspond to the FE-SEM images of the top surface (i.e., near UV-side during stabilization) from the normal and 60° tilted from the surface normal, respectively. The thin stabilization layer, composed of small polymer aggregates, was formed on the PI-layer. No morphological anisotropy was evident in the plane of the film. No meaningful difference was observed between the top and bottom surfaces. The bottom surface also showed essentially the same morphology.
Fig. 6 The morphology of RM-stabilization layers observed by FE-SEM: (a-i) normal and (a-ii) 60 degree tilted views for the stabilized LC cell with 0.2 wt.% RMs; (b-i) normal and (b-ii) 60 degree tilted views for the 0.5 wt.% RM cell.
For higher RM concentration (0.5 wt.%), however, the morphological anisotropy was developed along the nematic director n (denoted by the arrow in (b-i)) of the host LC as observed in Figs. 6(b-i) and 6(b-ii). For both cases of RM concentration, the polymerized RMs formed thin films on top of PI-layers. Because both the cells were well stabilized and maintained good alignment, the distinct morphologies seemed not critical for the stabilization. Instead, it seemed that the uniform polymerized RM-layer on the surface was acting as a new alignment layer. Because the photoresponsive alignment layer no longer serves as an alignment layer, the reversible photochromic isomerization has no effect on LC alignment and therefore the stabilized cell maintains a stable LC alignment. This conclusion was corroborated by the result discussed in Fig. 5(d), where no new alignment was induced by the LPVL after RM-stabilization.
The surfaces were further investigated by using AFM. Figure 7(a) shows a topographic image of the pristine PI-MR surface prior to the stabilization. The surface exhibited relatively smoother nature with the RMS roughness (Rq ~0.57 nm) and average roughness (Ra ~0.44 nm). Figures 7(b) and 7(c) correspond to the surfaces of stabilization layer inside and outside the circle in Fig. 5, respectively. The LCs were uniformly aligned on the surface 7(b) whereas the surface 7(c) exhibited random alignment. However, the observed morphologies in these two regions were very similar and showed no clear distinction. The roughness was increased to inside (Rq ~0.97 nm and Ra ~0.75 nm) and outside (Rq ~0.90 nm and Ra ~0.71 nm) the circle, compared to the PI-MR surface (Fig. 7(a)). It was obvious that the polymerized RMs, composed of polymer beads, formed continuous stabilization layer on the PI-MR surface. The AFM images in Fig. 7 corroborate the formation of polymerized RM-layer on the PI-MR surface. As discussed in Fig. 6, the in situ formed polymerized RM-layer serves as a new stable alignment layer.
Fig. 7 3D AFM topography of the inner surfaces of the LC cell before and after RM-stabilization: (a) the pristine surface of the PI-MR layer with no stabilization layer; the surface of RM-stabilization layer, formed by 2.0 wt.% RMs, (b) inside and (c) outside the circular LPVL-exposed area. The images were taken for 1 µm × 1µm area.
The POM, SEM, and AFM could be rather qualitative and may not provide sufficient quantitative analysis for device applications. Although each step for LC alignment and its RM-stabilization process was well understood in the previous reports, it could be necessary to further investigate the characteristics of LC cells, such as V-T curve, residual DC, voltage holding ratio and image sticking parameter.
4. Conclusion
The dual wavelength approach for in situ photoalignment and stabilization was demonstrated for high quality planar alignment with good stability for device applications. The photoresponsive polyimides, used as alignment layers, were designed and synthesized for the dual wavelength in situ photoalignment. In situ LPVL treatment in the isotropic temperature efficiently induced uniform planar alignment. Multidomain patterned alignment was also achieved by employing chessboard-patterned photomask with two-step LPVL treatment. The LPVL-induced LC alignments were effectively stabilized by photopolymerizing RMs with the second wavelength (i.e., UV-light) in the aligned LC host. The alignments were stable against heat and light treatments. The investigation on the stabilization layers confirmed that the polymerized RM-layers act as new alignment layers, by overcoating the pristine PI-alignment layer. Therefore, reversible photochromic PI-layer lost its function as an alignment layer and thus the LC alignment became irreversible and stable, which is essential for practical device applications. It was clear that the proposed dual wavelength in situ photoalignment efficiently induced and stabilized high quality planar alignment, and therefore can be beneficially employed for practical device applications. It would be also meaningful to explore the proposed approach for other LC phases because the cholesteric and smectic LCs are of great significances for optical applications [32,33]. Since the proposed method adopts the aligning process performed in the isotropic phase of LC and exhibits stable anchoring after stabilization, it could be possible to control the alignment of other LC phases.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Funding
Samsung Display Company in Korea; BK21 Plus Project through the National Research Foundation of Korea
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