Photopolymerization of Reactive Amphiphiles: Automatic and Robust

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Photopolymerization of Reactive Amphiphiles: Automatic and Robust Vertical Alignment Layers of Liquid Crystals with a Strong Surface Anchoring Energy Won-Jin Yoon,† Yu-Jin Choi,† Dae-Yoon Kim,† Jin Soo Kim,‡ Yeon-Tae Yu,‡ Hyojin Lee,§ Ji-Hoon Lee,§ and Kwang-Un Jeong*,† †

Polymer Materials Fusion Research Center & Department of Polymer-Nano Science and Technology, ‡Division of Advanced Materials Engineering, and §Division of Electronics Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Korea S Supporting Information *

ABSTRACT: A photopolymerizable itaconic acid-based amphiphile (abbreviated as Ita3C12) consisting of a hydrophilic carboxylic acid, three alkyl tails, and a reactive vinyl function was newly designed and synthesized for the formation of automatic and robust vertical alignment (VA) layer of nematic liquid crystals (NLC). Since a hydrophilic carboxylic acid was chemically attached to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium were migrated toward the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer directly interacted with host NLC molecules and made them to automatically align vertically. Because of the reactive vinyl functions of Ita3C12 amphiphiles, it was possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsesquioxane (MAPOSS) cross-linkers. The polymer-stabilized robust Ita3C12 VA layer exhibited a strong surface anchoring energy without generating any light scatterings. The automatic fabrication of robust LC alignment layers can allow us to reduce the manufacturing cost and to open new doors for electro-optical applications.

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INTRODUCTION Liquid crystal displays (LCDs) have been widely used because they have many advantages such as low power consumption, light weight, thinness, and low manufacturing cost.1 One of the key technologies in LCDs is the fabrication of LC alignment layer.2−4 Depending on the types of LC alignment and electric field application, LCD mode can be classified as twisted nematic (TN),5,6 vertical alignment (VA),7,8 in-plane switching (IPS),9 and fringe-field switching (FFS)10,11 modes. Among them, the VA mode has some advantages including a high contrast ratio without the mechanical rubbing process.12−16 In the VA mode, the nematic (N) LC directors are aligned to the surface normal at zero field state and the transmittance becomes the minimum (dark state). When a sufficiently strong electric field (threshold voltage, Vth) is applied, the LC directors are reoriented to the planar state showing the maximum transmittance (white state).16−18 Thermally and chemically stable polymers such as polyimides have been generally used as planar as well as vertical alignment materials. However, the polyimide-type alignment materials exhibit several drawbacks such as long process time, low yield, low processability, and high cost.1,5,19 Without applying the conventional polyimide-based alignment layers, the LC alignment direction and the surface anchoring energy can be modified by directly introducing lecithins, silanes, or nanomaterials into the host NLC media.20−24 Especially, the nanoparticle-induced vertical LC alignment layers are very attractive © XXXX American Chemical Society

because of low driving voltage, memory effect, and frequency modulation response.25−29 However, most nanoparticle-induced VA materials can generate serious problems because of poor interactions with NLC molecules and strong aggregations among themselves. As an example, by directly introducing 3 wt % pristine polyhedral oligomeric silsequioxane (POSS) nanoparticles in NLC host media, the automatic VA layer was constructed in the macroscopic area.30−32 However, because of strong interactions among the pristine POSS nanoparticles, a lot of POSS clusters on the micrometer sizes were formed in the bulk NLC medium before the formation of monolayered VA layer on the substrates. The macroscopically grown POSS aggregates resulted in the severe light scatterings.33−39 To overcome these drawbacks, our research group recently proposed the cyanobiphenyl monosubstituted POSS giant LC surfactant (POSS-CBP1).30 The cyanobiphenyl moiety chemically attached to the pristine POSS with an alkyl chain improved the initial solubility and interaction with LC media. The finely tuned POSS-CBP1 giant LC surfactants gradually diffuse onto the substrate of LC test cell for the formation of VA layer without forming the macroscopic aggregations. However, the homogeneous POSS-CBP1 monolayer can be only constructed by carefully optimizing the concentration of Received: October 19, 2015 Revised: November 23, 2015

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DOI: 10.1021/acs.macromol.5b02296 Macromolecules XXXX, XXX, XXX−XXX

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temperature to 70 °C, n-hexane (20 mL) was added to the mixture and reacted for 3 h. A white crystal (yield: 1.2 g, 57%) was obtained when the temperature was decreased to 5 °C. 1H NMR (400 MHz, CDCl3, TMS): δ = 6.56 (s, 2H), 6.52 (s, H), 5.88 (s, H), 3.97−3.93 (t, 6H), 1.84−1.71 (m, 6H), 1.50−1.43 (m, 6H), 1.26 (m, 48H), 0.89−0.86 ppm (t, 9H). 13C NMR (120 MHz, CDCl3, TMS): δ = 14.1, 22.7, 26.1, 29.3, 29.7, 31.9, 37.3, 67.1, 69.1, 73.4, 106.8, 130.5, 130.7, 132.9,153.2, 168.1, 173.7 ppm. FT-IR (KBr, cm−1): 3000−2770 (OH), 1740 (conjugation CO), 1592 (conjugation CC), 1457 (CO). Preparation of the Vertically Aligned LC Test Cells. Vertically aligned LC test cells were fabricated by sandwiching two ITO-coated glasses. The ITO-coated glasses were carefully washed with distilled water, isopropyl alcohol, and acetone several times and then dried in a vacuum oven at 80 °C for 3 h. The cell gap (d) was maintained to be 5 or 10 μm by introducing bead spacers. A typical dimension of LC test cell was 1.5 cm × 1.5 cm × 5 μm. NLC (Δε = −3.4, Δn = 0.101) mixture (NLC/Ita3C12/MAPOSS) was injected into the test cell by capillary forces. Homogeneous NLC mixtures with Ita3C12 and MAPOSS (99.5/0.4/0.1, 99.62/0.3/0.07, 99.75/0.2/0.05, and 99.87/ 0.1/0.025) were prepared by stirring at 60 °C for 24 h. Measurement of LC Surface Anchoring Energy. To measure the voltage−transmittance (TR) curve of the LC cell, a white light generated from a halogen lamp was consecutively passed through a polarizer, the LC test cell with a LC mixture (NLC/Ita3C12/MAPOSS weight ratio from 99.875/0.1/0.025 to 99.5/0.4/0.1 and to 97.5/2/ 0.5), an analyzer, and a detector. The polarizing direction of the polarizer and the analyzer was crossed. The TR data were then converted to the retardation (R), and (R/R0 − 1)(V − V′) was plotted as a function of (V − V′), where R0 is the maximum retardation under 50 V and V′ stands for σβVth. Here, σ = (1/π)∫ 10[(1 + β)(1 + km)/(1 + βm)m]1/2 dm, β = (ε⊥/ε∥) − 1, k = (K11/K33) − 1, and Vth is the threshold voltage of the cell. The K11 and K33 are the splay and the bend elastic constants of the LC, respectively. The fitted slope of the graph corresponds to 2K33/Wd. By substituting the K33 and the cell gap (d), the surface anchoring energy (W) was obtained. Characterization. Proton (1H) and carbon (13C) nuclear magnetic resonance (NMR) spectra were recorded on a JNM-EX400 spectrometer in deuterated chloroform (CDCl3) at room temperature. Chemical shifts were quoted in parts per million (ppm) and referenced to tetramethylsilane (TMS). Polarized optical microscopic (POM) and conoscopic textures were taken by using cross-polarized POM (Nikon ECLIIPSE E600POL) with a LINKAM LTS 350 heating stage. To monitor the thermal behaviors of Ita3C12 amphiphile, a PerkinElmer PYRIS Diamond DSC equipped with an Intra cooler 2P apparatus was utilized. To determine the thermal transition temperatures, the onset temperature was measured during the cooling and heating scans. Small-angle X-ray scattering (SAXS) on a Siemens (Bruker AXS, Madison, WI) M18HF22 rotating anode power supply with a 0.3 × 3 mm2 filament and Cu target operated at 80 mA, 40 kV. Atomic force microscopy (AFM) images were taken on an Agilent 550 AFM (Agilent Technologies) using silicon cantilevers with spring constants of 20−30 N/m. The scan speed was varied from 0.5 to 1.0 line/s, and the resonance frequencies were set as 140−160 kHz. Contact angle was measured by contact angle analyzer (Phoenix-300, SEO). Electrooptical switching behaviors were measured by LCMS-200 (Sesim Photonics Technology, Korea).

POSS-CBP1 in the LC mixture as well as by finely tuning the rates of phase separation from the NLC medium and diffusion of POSS-CBP1 onto the substrates. Additionally, it is difficult and expensive to synthesize the asymmetric POSS giant LC surfactants. Furthermore, since there is no reactive functional group in POSS-CBP1, the LC test cell cannot be stabilized against the thermal, electrical, and chemical attacks, and it is hard to control the electro-optical characteristics, such as the threshold voltage, the alignment direction, the surface anchoring energy, and response time.30 To get over these limitations, we newly proposed and synthesized a photopolymerizable reactive amphiphilic surfactant (Ita3C12) in this research. We programmed the Ita3C12 molecule with three alkyl tails for controlling the interaction with LC molecules, a hydrophilic carboxylic acid for automatically forming the VA alignment layer on the substrates, and a photopolymerizable vinyl function for stabilizing the system as well as for controlling the electro-optical characteristics such as anchoring energy and response time.40−42 It was possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsequioxane (MAPOSS) cross-linker. By optimizing the content of Ita3C12 and MAPOSS in the NLC host medium, we successfully fabricated the automatic and robust VA layer with a strong surface anchoring energy, which can allow us to reduce the manufacturing cost and to open new doors for electro-optical applications.

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EXPERIMENTAL SECTION

Materials. 1-Bromododecane (97%, Aldrich), methyl 3,4,5trihydroxybenzoate (98%, Aldrich), itaconic anhydride (95%, TCI), methacryl polyhedral oligomeric silsequioxane (Hybrid Plastics), lithium aluminum hydride (95%, Aldrich), anhydrous chloroform (99%, Sigma-Aldrich), acetone (99.5%, Showa), n-hexane (99.5%, Showa), and nematic liquid crystal (NLC, ZSM-7232XX, LIXON) were used as received. Synthesis. Methyl 3,4,5-Tris(dodecyloxy)benzoate (1). 1-Bromododecane (22.8 mL, 95.2 mmol) was added to a solution of methyl 3,4,5-trihydroxybenzoate (5.0 g, 27.2 mmol) and potassium carbonate (18.8 g, 136.0 mmol) in dried acetone (70 mL). After reacting the mixture for 24 h at 60 °C under a nitrogen atmosphere, the reactant was extracted with water and chloroform. The organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. The resulting crude product was purified by column chromatography on silica gel using ethyl acetate/n-hexane = 1:20 (v/v) and washed with methanol. The resulting product was obtained as a white solid by drying under vacuum (yield: 82%, 15.4 g). 1H NMR (400 MHz, CDCl3, TMS): δ = 7.24 (s, 2H), 4.02−3.99 (m, 6H), 3.88 (s, 3H), 1.84−1.71 (m, 6H), 1.50−1.43 (m, 6H), 1.26 (m, 48H), 0.89−0.86 ppm (t, 9H). (3,4,5-Tris(dodecyloxy)phenyl)methanol (2). Lithium aluminum hydride (LiAlH4, 1.1 g, 29.02 mmol) was suspended in dried CHCl3 (30 mL). The solution of methyl 3,4,5-tris(dedecyloxy)benzoate (2.0 g, 2.90 mmol) in dried CHCl3 was dropped slowly into the refluxed suspension and reacted for 12 h. At ambient temperature, the excess hydride was destroyed by the careful addition of moist CHCl3, and the resulting slurry was acidified by introducing 2 M H2SO4. After extracting the CHCl3−water mixture, the organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. The resulting product was white crystal (yield: 1.79 g, 93%). 1H NMR (400 MHz, CDCl3, TMS): δ = 6.56 (s, 2H), 4.59 (d, 2H), 3.97−3.93 (t, 6H), 1.84−1.71 (m: 6H), 1.50−1.43 (m: 6H), 1.26 (m: 48H), 0.89− 0.86 ppm (t: 9H). 4-(3,4,5)-Tris(dodecyloxy)benzyloxy)-2-methylene-4-oxibutanoic Acid (3). (3,4,5-Tris(dodecyloxy)phenyl)methanol (1.8 g, 2.73 mmol) and itaconic anhydride (0.31 g, 2.73 mmol) were mixed and reacted without any additional solvent at 110 °C for 1 h. After reducing the

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RESULTS AND DISCUSSION Construction of Automatic VA Layer by Directly Doping Ita3C12 Amphiphiles into the NLC Medium. As illustrated in Scheme 1, the photopolymerizable amphiphile (Ita3C12) is newly designed and successfully synthesized for the formation of VA in the NLC cell. The Ita3C12 amphiphile consisted of a hydrophilic carboxylic acid, three alkyl tails, and a reactive vinyl function. The detailed synthetic procedures of Ita3C12 are described in the Experimental Section. Chemical structures and purities of Ita3C12 and intermediates are identified by 1H NMR, 13C NMR, and FT-IR, and their B

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Figure 1. Orthoscopic POM images of circled (a′−e′) and squared (a″−e″) parts in the macroscopic photographs (a−e) of NLC/Ita3C12filled LC test cells with different Ita3C12 contents: 0.01 (a), 0.1 (b), 1 (c), 5 (d), and 10 wt % (e). The inset images are the corresponding conoscopic POM images.

observed when the sample is truly or accidently isotropic (isotropic effect). This isotropic effect is detected at the corners of LC test cell which are filled with the UV-cured isotropic adhesive (Figure 1a−e). Second, if the optic axes of materials are parallel to one of the polarizers (zero amplitude effect), we can see the dark POM image. Finally, the dark image appears when the optic axis of materials is parallel to the propagation direction of light (zero birefringence effect). Therefore, the dark orthoscopic POM images of circled (Figure 1a′−e′) and squared (Figure 1d″,e″) parts in the macroscopic photographs of NLC/Ita3C12-filled LC test cells cannot convince us the formation of automatic VA layer. To elucidate other possibilities of dark POM images and to confirm the LC orientation, the conoscopic POM images are additionally measured and represented as the insets of Figure 1. The formation of a Maltese cross in the conoscopic POM image is a direct evidence of the VA of NLC. When the theoretical concentration of Ita3C12 in the NLC/ Ita3C12 mixture is considered for the formation of Ita3C12 perfect monolayers on the ITO-coated glass substrates, the ideal Ita3C12 concentration is roughly calculated to be 0.1 wt % in the case of the 5 μm cell gap. At the 0.01 wt % of Ita3C12 (10 times lower than the ideal concentration), NLC molecules are vertically aligned on the 40% area of the LC test cell, as shown in Figure 1a. From this result, it is realized that the perfect formation of Ita3C12 monolayer is not a necessary condition for the vertical alignment of NLC molecules, which have been also proved in previous reports.30 As increasing the concentration of Ita3C12 to 0.1 wt % (the ideal concentration for the perfect Ita3C12 monolayers), the vertically aligned area is expanded up to 85% (Figure 1b). Even though this LC test cell is annealed at room temperature for 24 h, the vertically aligned area is not much changed. This means that at least more than 15% of Ita3C12 molecules are maintained in the NLC bulk state. As exhibited in Figure 1c, when the concentration of Ita3C12 is increased up to 1 wt % (10 times higher than the ideal concentration), NLC molecules are vertically aligned on the 88% area of the LC test cell. NLC molecules in the 12% area of the LC test cell near to the boundary of UV-cured adhesive are not aligned yet, but the whole area of LC test cell turns to the complete VA state by annealing the cell at room temperature for 12 h. Above the 5 wt % of Ita3C12, the perfect VA state on

experimental results are provided in the Supporting Information (Figures S1−S5). Since a hydrophilic carboxylic acid is chemically attached to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium can migrate onto the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer are programmed to be directly interacted with host NLC molecules and induce the vertical orientation.1,41,42 Because of the reactive vinyl functions of the Ita3C12 amphiphiles, it is possible to stabilize the automatic VA layer by the photopolymerization with methacryl polyhedral oligomeric silsequioxane (MAPOSS) cross-linkers. To realize this proposal, a series of Ita3C12/NLC mixtures (NLC, ZSM-7232XX, LIXON) are first prepared. The Ita3C12/ NLC mixtures are mechanically stirred for 24 h at 60 °C (in the N phase) to be homogeneous. The homogeneously mixed LC mixtures with different contents of Ita3C12 are injected into the LC test cell fabricated by sandwiching two ITO-coated glass substrates. The LC test cell gap is maintained with 5 μm spacers which are used together with the UV-curable adhesive between ITO-coated glass substrates. The fabricated Ita3C12/ NLC-filled LC test cells are further annealed at 70 °C (5 °C below the TNI = 75 °C) for 10 min and cooled down to room temperature at different cooling rates. The macroscopic photographs of NLC/Ita3C12-filled LC test cells with different Ita3C12 contents are taken at room temperature by a common digital camera and shown in Figure 1a−e. Since the NLC/Ita3C12-filled LC test cells are located between two polaroid films which linearly polarized optic axes are at a right angle and the macroscopic photographs are taken in the transmission mode, the dark domain of the macroscopic photograph tentatively informs us that the Ita3C12 molecules in the NLC/Ita3C12 mixtures are phase-separated from the NLC medium, diffuse onto the hydrophilic ITO-coated glass substrates, and form the Ita3C12 monolayers which can induce the vertically oriented NLC molecules. However, the dark images under the cross-polarized microscopy can be observed in the different situations. First, the dark POM image can be C

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Since the molar ratio of functional groups is 8 (MAPOSS):1 (Ita3C12) and the molecular weight of MAPOSS is twice of Ita3C12, the relative amount of MAPOSS cross-linker compared with that of Ita3C12 reactive amphiphile is fixed to be 1 (MAPOSS):4 (Ita3C12) in weight. Because the theoretical concentration of Ita3C12 in the NLC/Ita3C12/MAPOSS mixture for the formation of perfect monolayers on the LC test cell is roughly calculated to be 0.04 wt % in the case of the 5 μm cell gap, a series of homogeneous NLC/Ita3C12/ MAPOSS mixtures are prepared by varying the content of Ita3C12 from 0.1 to 0.4 wt %. Note that the automatic VA layer is formed on the macroscopic area with 1 wt % Ita3C12 in the NLC/Ita3C12 system (Figure 1), in which the amount of Ita3C12 is 10 times higher than the ideal concentration. The NLC/Ita3C12/MAPOSS-filled LC test cells are fabricated by injecting the LC mixtures into the sandwiched LC test cell. The cell gap of LC test cells are controlled to be 5 μm by introducing spacers into the UV-curable adhesive between ITO-coated glass substrates. The prepared NLC/Ita3C12/ MAPOSS-filled LC test cell are additionally annealed at 70 °C (5 °C below the TNI of NLC = 75 °C) for 10 min and cooled down to room temperature at 10 °C/min. As shown Figure 3, the automatically constructed VA region is expanded

the whole area of the LC test cell is spontaneously obtained at room temperature without any annealing process, as shown in Figures 1d and 1e. Optimized Conditions for the Automatic VA Layer. Even though the perfect VA state on the whole area of the LC test cell is spontaneously obtained by directly introducing the proper amount of Ita3C12 to the NLC media, the system should be stabilized by the polymerization of self-assembled Ita3C12 monolayers. From the experimental results (Figure 1), it is realized that at least more than 15% of Ita3C12 molecules are dissolved in the NLC bulk state as monomeric or dimeric formations. The remain Ita3C12 in the NLC bulk state can influence and fluctuate the physical properties of NLC medium at different situations. Additionally, the NLC/Ita3C12-filled LC test cell can provide unstable transmittance information and low polar-anchoring energy under electric fields. To protect the automatic VA LC test cell from the thermal, electrical, and chemical attacks, the NLC/Ita3C12-filled LC test cell is irradiated by UV light for the photopolymerization. However, the Ita3C12 in the NLC medium is not efficiently photopolymerized even with enough amounts of initiator and Ita3C12 monomer. The reason can be found from the chemical structure of Ita3C12 molecule. Three alkyl tails chemically connected to one corner of Ita3C12 can generate steric hindrances and interrupt the close interactions between reactive vinyl functions attached to the other side of Ita3C12. Therefore, we need reactive cross-linkers which can physically fill the gaps between Ita3C12 molecules and chemically connect the reactive vinyl functions. Additionally, the cross-linker with a proper initial solubility in the NLC medium can be phase-separated from the NLC bulk and diffused toward the surface to fill the gaps between Ita3C12 molecules before the photopolymerization process. Among many cross-linking candidates, a commercially available polyhedral oligomeric silsequioxane cross-linker with eight methacrylate reactive functions (MAPOSS) is selected because the butyl or phenyl functionalized POSS nanoparticles in NLC test cells can be used as the automatic VA formation materials.30−33 To check the automatic VA formation with MAPOSS, a series of homogeneous NLC/ MAPOSS mixtures with different MAPOSS contents are prepared and injected into the sandwiched ITO-coated LC test cells. As shown in Figure 2, however, neither a dark orthoscopic POM image nor a Maltese cross in the conoscopic POM image is detected even with 10 wt % MAPOSS. Therefore, it is concluded that MAPOSS itself cannot form the automatic VA layer.

Figure 3. Materials information: host nematic liquid crystal (NLC, ZSM-7232XX), photoreactive amphiphile (Ita3C12), and cross-linker (MAPOSS) and their weight ratios in the mixture.

from 67% to 82% by increasing the Ita3C12 content from 0.1 to 0.3 wt %. Note that the automatically constructed VA region is not changed by further annealing at room temperature. When the concentration of Ita3C12 in the mixture is 0.4% (10 times higher than the ideal Ita3C12 concentration for constructing monolayers), the whole regions are vertically oriented without any light leakage, and the formation of Maltese crosses is obvious in the conoscopic POM images (Figure 4). This result indicates that a significant amount of Ita3C12 is dissolved in the bulk NLC medium, but they do not generate micelles on a micrometer length scale which can generate light scatterings. Polymer-Stabilized Automatic and Robust VA Layer. After confirming the construction of automatic VA layer in the NLC/Ita3C12/MAPOSS-filled LC test cell, the self-assembled Ita3C12 and MAPOSS are photopolymerized by irradiating UV light with 365 nm wavelength for 30 min. Orthoscopic and the corresponding conoscopic POM images of the LC test cells are not much changed before and after the photopolymerizations. To investigate the surface morphology of the photopolymerized

Figure 2. Macroscopic photographs (a−e) and their corresponding orthoscopic (a′−e′) and conoscopic (insets) POM images of NLC/ MAPOSS-filled LC test cells with different MAPOSS contents: 0.01 (a), 0.1 (b), 1 (c), 5 (d), and 10 wt % (e). D

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Figure 4. Orthoscopic POM images of circled (a′−d′) and squared (a″−d″) parts in the macroscopic photographs (a−e) of NLC/ Ita3C12/MAPOSS-filled LC test cell with a weight ratio of 99.875/0.1/ 0.025 (a), 99.75/0.2/0.05 (b), 99.625/0.3/0.075 (c), and 99.5/0.4/0.1 (d). The inset images are the corresponding conoscopic POM images.

Ita3C12/MAPOSS-filled NLC test cells, a series of polymerstabilized NLC/Ita3C12/MAPOSS-filled test cell are prepared by varying the contents of Ita3C12 in the LC mixture from 0.1 to 2 wt %. After disassembling the LC test cells, the unpolymerized LC mixtures are removed by washing with methyl chloride/hexane mixture and dried for the investigation of surface with atomic force microscopy (AFM) and deionized (DI) water contact angle (CA) analyzer. As represented in Figure 5, the topographic AFM images and their corresponding height profiles of the polymer-stabilized Ita3C12/MAPOSS VA layers on the ITO-coated glass substrates clearly indicate that the VA is constructed more than an ideal single monolayer. Note that the length of Ita3C12 molecule is 2.12 nm with the extended chain conformation (Figure S6). With the 0.1 wt % content of Ita3C12, the polymer-stabilized Ita3C12/MAPOSS VA layer exhibits nanosized protrusions with 130 nm width and 13 nm height in averaged values. The measured root-means-square (RMS) roughness and CA of the polymer-stabilized Ita3C12/MAPOSS VA layer are 5.7 nm and 99.2°, respectively. The nanoprotrusions are grown up to 220 nm width and 30 nm height in averaged values by increasing the content of Ita3C12 up to 2 wt %. From this result, it is understood that during the polymer-stabilization process of Ita3C12/MAPOSS VA layer the dissolved Ita3C12/MAPOSS molecules in the NLC medium split up with NLC due to the polymerization-induced phase separation and migrate toward the Ita3C12/MAPOSS VA layer and then participate to the photopolymerization with the previously formed Ita3C12/ MAPOSS VA layer. As illustrated in Figure 5, both RMS and CA of the polymer-stabilized Ita3C12/MAPOSS VA layer are increased with the amount of Ita3C12 in the LC mixtures.43−45 To investigate the correlation of vertical alignment with roughness, the polar surface anchoring energy (W) of the NLC test cells is measured according to the roughness.46 The transmittances (TR) of NLC/Ita3C12/MAPOSS-filled LC test cells with the different Ita3C12 contents are monitored by increasing the voltage (V) from 0 to 22 V. As shown in Figure 6, (R/R0 − 1)(V − V′) is plotted with respect to (V − V′). Here, R0 is the maximum retardation under 50 V, and V′ =

Figure 5. Topographic AFM images (a, c, e) of Ita3C12/MAPOSS VA layer on the ITO glass substrates and their corresponding height profiles (b, d, f): 0.1/0.025 wt % (a, b), 0.4/0.1 wt % (c, d), and 2/0.5 wt % (e, f). The corresponding roughness and contact angle with respect to the Ita3C12/MAPOSS content are also summarized in (g).

σβVth. As described in the Experimental Section, the fitted slope of the graph corresponds to 2K33/Wd. By substituting the bend elastic constant of NLC (K33) and the cell gap (d), the W of the alignment layer can be obtained.47−49 The W value in the LC test cell with 0.4 wt % Ita3C12 increases from 9.071 × 10−5 to 1.404 × 10−4 J/m2 during the polymer-stabilization process. The increase of W is presumably originated from the increased hardness of the surface alignment layer and the growth of nanosized protrusions during the polymer-stabilization process. Note that the unpolymerized Ita3C12/MAPOSS might be dragged by the LC molecules near the surface in the presence of strong electric field, resulting in the greater change of R. Upon increasing the content of Ita3C12/MAPOSS from 0.1/ 0.025 to 0.4/0.05 and to 2/0.5 wt %, the W value is accompanied by the increased value from 8.730 × 10−5 to 1.404 × 10−4 and to 1.559 × 10−4 J/m2, respectively. This result indicates that the W of the alignment layer is increased with a greater fraction of the Ita3C12/MAPOSS. This is apparently E

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CONCLUSIONS For the formation of automatic and robust vertical alignment (VA) layer of nematic liquid crystals (NLC), a novel photopolymerizable itaconic acid-based amphiphile (Ita3C12) was programmed and successfully synthesized. Since a hydrophilic carboxylic acid was chemically connected to the end of Ita3C12, the Ita3C12 amphiphiles initially dissolved in the host NLC medium were migrated onto the substrates for the construction of VA layer of NLC. The alkyl tails of Ita3C12 in the VA layer directly interacted with host NLC molecules and made them to automatically align in vertical. During the polymer-stabilization process of automatic VA layer, the width and height of nanosized protrusions were grown by increasing the content of Ita3C12. By considering polar surface anchoring energy and transmittance, the contents of Ita3C12 and MAPOSS cross-linker were optimized to be 0.4 and 0.1 wt %, respectively. The polymer-stabilized VA layer exhibited a strong surface anchoring energy (1.404 × 10−4 J/m2) without generating any light scatterings. Furthermore, it was realized that the physical properties of polymer-stabilized VA layer were stable against chemical, thermal, and electrical attacks. The automatic fabrication of robust LC alignment layers can allow us to reduce the manufacturing cost and to open the new doors for electro-optical applications.

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Figure 6. Dependence of (R/R0 − 1)(V − V′) vs (V − V′). Here, the solid lines represent the best linear fit between 10 and 22 V. The slope of the graph corresponds to 2K33/Wd(R/R0 − 1)(V − V′) vs (V − V′) before and after the polymer-stabilized LC test cells (a) and about comparison 0.1/0.4/2.0 wt % (b).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02296. 1 H NMR, 13C NMR, DSC, POM, and 1D SAXS results and detailed experimental processes (PDF)

related to the greater density of the protruded domains of the Ita3C12/MAPOSS layer by increasing its concentration (Figure 5). It is worthy to note that the conventional polyimide-based VA layers show the W values between 1.0 × 10−4 and 3.5 × 10−4 J/m2.47,48 Even though the TR of NLC/Ita3C12/ MAPOSS-filled LC test cells is somewhat decreased after the photopolymerization, the increase of anchoring energy is dramatically increased up to the 0.4 wt % Ita3C12 (S7). The polymer-stabilized LC test cell with the optimized content of Ita3C12/MAPOSS (0.4/0.1 wt %) exhibits a perfect vertical alignment of NLC without any light scatterings at room temperature, as shown in Figure 7. To confirm the thermal

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.-U.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by BRL (2015042417), MOTIE-KDRC (10051334), and BK21 Plus program of Korea. REFERENCES

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Figure 7. Thermal stabilities of 0.4/0.1 wt % of Ita3C12/MAPOSS after the polymer-stabilization process. The inset images are the corresponding conoscopic POM images.

stability of LC test cells, the temperature is increased up to the 80 °C (5 °C above the TNI = 75 °C of NLC). After annealing the LC test cell at 80 °C for 12 h, the LC test cell is quenched to RT directly from 80 °C. As represented in Figure 7, the quenched LC cell spontaneously form the VA of NLC without any defect of LC alignment. This result clearly indicates that the thermal stability of polymer-stabilized LC test cell is significantly enhanced by the photopolymerization process. F

DOI: 10.1021/acs.macromol.5b02296 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02296 Macromolecules XXXX, XXX, XXX−XXX