Flexible and Patterned Thin Film Polarizer - ACS Publications

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Flexible and Patterned Thin Film Polarizer: Photopolymerization of Perylene-based Lyotropic Chromonic Reactive Mesogens Pureun Im,† Dong-Gue Kang,† Dae-Yoon Kim,† Yu-Jin Choi,† Won-Jin Yoon,† Myong-Hoon Lee,† In-Hwan Lee,‡ Cheul-Ro Lee,‡ and Kwang-Un Jeong*,† †

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

ABSTRACT: A perylene-based reactive mesogen (DAPDI) forming a lyotropic chromonic liquid crystal (LCLC) phase was newly designed and synthesized for the fabrication of macroscopically oriented and patterned thin film polarizer (TFP) on the flexible polymer substrates. The anisotropic optical property and molecular self-assembly of DAPDI were investigated by the combination of microscopic, scattering and spectroscopic techniques. The main driving forces of molecular self-assembly were the face-to-face π−π intermolecular interaction among aromatic cores and the nanophase separation between hydrophilic ionic groups and hydrophobic aromatic cores. Degree of polarization for the macroscopically oriented and photopolymerized DAPDI TFP was estimated to be 99.81% at the λmax = 491 nm. After mechanically shearing the DAPDI LCLC aqueous solution on the flexible polymer substrates, we successfully fabricated the patterned DAPDI TFP by etching the unpolymerized regions selectively blocked by a photomask during the photopolymerization process. Chemical and mechanical stabilities were confirmed by the solvent and pencil hardness tests, and its surface morphology was further investigated by optical microscopy, atomic force microscopy, and three-dimensional surface nanoprofiler. The flexible and patterned DAPDI TFP with robust chemical and mechanical stabilities can be a stepping stone for the advanced flexible optoelectronic devices. KEYWORDS: lyotropic chromonic liquid crystal, reactive mesogen, perylene-diimide, photopolymerization, polarizability

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INTRODUCTION When two or more ionic groups are chemically or physically connected at the periphery of disc-shaped chromonic cores, lyotropic chromonic liquid crystals (LCLC) can be prepared in aqueous solutions.1−3 At a low concentration, LCLC molecules are randomly dispersed without any order. Upon increasing the concentration of the aqueous solution, the disc-shaped LCLC molecules spontaneously self-assemble to nanocolumns due to the strong π−π interactions between disc-shaped aromatic cores and the nanophase separation between hydrophobic discshaped cores and hydrophilic ionic groups.4−8 When the aspect ratio and the volume concentration of nanocolumns in the aqueous solution are high enough, a partially ordered mesophase such as columnar nematic phase (ColN) can be formed.9,10 At a sufficiently high concentration, the selfassembled lyotropic nanocolumns self-organize parallel to each other with a quasi-long-range positional order perpendicular to the long axis of nanocolumns, resulting in the highly ordered two-dimensional (2D) structures, for instance, hexagonal, rectangular and oblique LC phases.11−14 If an aqueous solvent is completely evaporated, a columnar © XXXX American Chemical Society

crystalline phase can be formed with long-range positional orders in 3D. LCLC molecules at a low concentration have been used as pharmaceutical drugs and dyes.15,16 Because the self-assembled LCLC nanocolumns in LC phases can be uniaxially oriented in the macroscopic length scales by applying external forces, such as mechanical,17−19 electrical or magnetic forces20,21 with and without the support of surface alignment layers,22−24 their applications have been dramatically magnified and expanded in various fields, such as dichroic light-polarizing materials,25 biosensing active materials,4,26,27 and electro-optic devices.28−30 However, the water-based LCLC system can cause several drawbacks. First of all, it is difficult to control the coating and drying processes especially on the hydrophobic polymer substrates due to the high surface energy of the aqueous LCLC solution. Second, a lot of defects, such as macroscopic cracks, can be generated due to the significant volume Received: October 20, 2015 Accepted: November 30, 2015

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DOI: 10.1021/acsami.5b09995 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces shrinkage during the drying and recrystallization processes. Finally, the macroscopically oriented thin films fabricated with the water-based LCLC solutions exhibit poor mechanical and chemical stabilities. To overcome these drawbacks, our research group recently used the photopolymerizable ionic monomers as solvents rather than water for the fabrication of polymer-stabilized robust coatable thin film polarizers (TFP).25,31 The photopolymerization of ionic monomers right after the mechanical orientation of LCLC columns significantly increased mechanical stabilities, such as the adhesion strength with substrates and the surface hardness. Additionally, the polymer-stabilized LCLC TFP exhibited excellent chemical stabilities. However, the polarizability of polymer-stabilized LCLC TFP was not improved compared with that of water-based TFP. The low polarizability should be related with the stress relaxations during the drying and photopolymerization processes, which resulted in the disturbances of molecular orientations. Because more than 60 wt % ionic monomers were initially used to prepare a LCLC solution, the macroscopic phase-separations may occur between LCLC crystalline and photopolymerized polymeric domains during the drying and photopolymerization processes, which may cause the significant light scatterings. Additionally, the polymer-stabilized LCLC TFP should be thick enough to absorb all the light. Even though the adhesion strength with glass substrates was significantly improved in the ionic monomer-based LCLC system, the patterning on the hydrophobic polymer substrates was impossible due to the limited adhesion strength. In this research, the macroscopically oriented and patterned LCLC TFP with robust chemical and mechanical stabilities was successfully fabricated on the flexible polymer substrates. To get over the limitations of ionic monomer-based LCLC TFP, a programmed perylene-based reactive mesogenic molecule (abbreviated as DAPDI) was synthesized at first by chemically introducing the diacrylate functional groups at the both ends of lyotropic perylene-diimide (PDI). Phase diagram of DAPDIH2O was constructed based on the cross-polarized optical microscope (POM) observations at different compositions and temperatures. The photopolymerization after the mechanical shearing at the 25 wt % DAPDI aqueous solution and subsequent drying processes provided the macroscopically oriented film with the 99.81% polarizability. The molecular packing structure in the photopolymerized DAPDI TFP was investigated by the combination of POM and 2D wide-angle Xray diffraction (WAXD). The patterned LCLC TFP on the flexible poly(ethylene terephthalate) (PET) substrates was successfully fabricated by etching the unpolymerized regions which were selectively blocked by a photomask during the photopolymerization process. Chemical and mechanical properties were evaluated by solvent and pencil resistance (ASTM D3363) tests, respectively. Surface morphology of the patterned LCLC TFP was also investigated by optical microscopy (OM), atomic force microscope (AFM) and 3D nanoprofiler.

Scheme 1. Synthetic Procedure of DAPDI

Reagent and conditions: (i) 2-dimethylaminoethylamine, DMF, 130 °C, 5 h; (ii) 2-bromoethanol, 100 °C, 12 h; (iii) methacrylic anhydride, DMAP, DMF, 25 °C, 24 h.

paper. The DAPDI molecule is obtained by the acylation reaction of alcohol with anhydride in the presence of 4dimethylaminopyridine (DMAP) with high yield and purity. The perylene aromatic core is specifically selected as a chromonic group to induce strong intermolecular face-to-face π−π interactions. Generally, the perylene-based derivatives show poor solubility in general organic solvents and in aqueous environments.33−35 To enhance the solubility in aqueous solutions and to induce ionic interactions for the formation of LCLC phases, we chemically introduced amine-bromine ionic groups at the both ends of perylene aromatic core. When the ionic interactions are balanced with the strong π−π interactions, LCLC molecules are spontaneously self-assembled to nanocolumns which are the building blocks of LCLC phases.5,7 In spite of the outstanding optical property of perylene-based LCLC, the water-based LCLC system can cause several drawbacks, such as the poor coating and drying processabilities, the generations of defects, and the poor mechanical and chemical stabilities. To overcome these drawbacks, we chemically connected the photopolymerizable methacrylate functional groups at the both ends of lyotropic PDI. Chemical structure and purity of DAPDI molecule are confirmed by proton (1H) nuclear magnetic resonance (NMR) spectroscopy (Figure S1) and electrospray ionization mass spectrometer (ESI MS, Figure S2). The molecular weight of DAPDI is 758.6 g/mol. The detail synthesis and characterization of DAPDI are provided in the Supporting Information. Phase Transition Behaviors and Molecular Packing Structures. On the basis of POM texture observations (Figure 1a−d), we constructed the binary phase diagram36 of DAPDIH2O with respect to temperature and concentration, as shown in Figure 1e. The aqueous solutions are kept in the sandwiched LC test cells completely sealed with epoxy resin to maintain the initial concentration of DAPDI-H2O solution during the experiments. As represented in Figure 1, a typical columnar nematic (ColN) phase37,38 forms between 10 to 30 wt % at

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RESULTS AND DISCUSSION Perylene-based Programmed Reactive Mesogen. For the fabrication of macroscopically oriented and patterned TFP on the glass and polymer substrates, a perylene-based reactive mesogen (abbreviated as DAPDI) forming LCLC phases is synthesized32 as illustrated in Scheme 1. The detail synthetic procedures are represented in the Experimental Section of this B


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with and without a 530 nm retardation plate after the photopolymerization (Figure 2). As shown in Figure 2a,b, the

Figure 2. POM images of the macroscopically oriented and photopolymerized DAPDI TFP on the bare glass substrates: when the SD is (a) 45° and (b) parallel to the polarization axes. POM images with a 530 nm retardation plate: when the maximum refractive axis (nmax) of the retardation plate is (c) parallel and (d) perpendicular to the SD.

macroscopically oriented and photopolymerized DAPDI reveals the obvious optical property of E-type absorptive polarizer. Note that the E-type polarizers transmit extraordinary ray and absorb ordinary ray, as opposed to the conventional Otype polarizers that absorb the polarization parallel to the optic axis.22,42 When the shear direction (SD) is parallel to the polarizer axis (P), a perfectly dark image is obtained (Figure 2b), which means that the optic axis of the molecular aggregates is perpendicular to the analyzer axis (A). By rotating the SD of DAPDI TFP, the POM image is gradually bright and the maximum brightness is achieved when the SD is 45° to the polarization axes. From the POM observation, it is realized that the molecular arrangements and morphologies are robustly maintained even after the photopolymerization process without generating any macroscopic cracks. To confirm the orientation of the DAPDI molecules, a quarter-wave retardation plate (530 nm) is additionally inserted. When the SD is aligned parallel (Figure 2c) and perpendicular (Figure 2d) to the maximum refractive axis (nmax), the POM images are red- and blue-shifted, respectively. This result clearly indicates that the molecular long-axis of DAPDI is perpendicular to the SD. In other words, DAPDI molecules in the aqueous solution are self-assembled to nanocolumns with high aspect ratios and the long axis of selfassembled nanocolumns is oriented parallel to the SD under the mechanical shear forces. The schematic illustrations of molecular orientations are proposed in Figure 2c,d on the basis of the POM results. To understand the molecular packing structures in the macroscopically oriented and photopolymerized DAPDI TFP, the structure-sensitive X-ray diffraction techniques are applied. As represented in Figure 3a, the 2D WAXD pattern is obtained by irradiating X-ray perpendicular to the SD. Here, the SD of DAPDI TFP is parallel to the meridian direction, and diffraction arc positions and widths are calibrated based on the diffraction ring of the silicon powder crystal at 2θ = 28.466° (d-spacing = 0.314 nm). On the meridian, a pair of arc

Figure 1. POM images at 25 °C of (a) 10, (b) 15, (c) 20, and (d) 30 wt % aqueous solutions. (e) Phase diagram of DAPDI-H2O solution constructed based on the POM observations.

room temperature. Below 10 wt % at room temperature, a complete dark state is observed under POM, indicating the isotropic (I) phase. Even though the self-assembled nanocolumns exist below 10 wt %, the aspect ratio and the volume concentration of nanocolumns are not high enough for the formation of anisotropic ColN phase. At concentrations above 30 wt % at room temperature, on the other hand, the crystallized particles are precipitated out of the aqueous solution. Upon increasing the temperature of DAPDI-H2O solutions between 10 and 30 wt %, the ColN phase transforms to the I phase by passing the biphase (ColN + I) region, as shown in Figure 1e. Note that the formation of biphase between ColN and I phases is one of the typical characteristics of LCLC compounds.1,3,39 As expected, the clearing temperature and viscosity are increased with respect to the concentration of solution.34 For the fabrication of macroscopically oriented DAPDI TFP on the glass and polymer substrates by mechanical shear coating at room temperature, the concentration of DAPDI-H2O solution is optimized to be 25 wt %, which is determined based on the POM morphological observations and the degree of polarization (DOP). Additionally, for the subsequent photopolymerization40 of the macroscopically oriented DAPDI TFP, 1 wt % of water-soluble photoinitiator (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, Sigma-Aldrich) is added into the 25 wt % aqueous solution before the doctor-blade shear coating process.41 The detailed fabrication process is described in the Experimental Section of this paper. Morphology and molecular orientation of the uniaxially oriented DAPDI TFP are investigated by POM C


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the real and reciprocal spaces. Comparing the 1D WAXD pattern of DAPDI crystalline powder with the 2D WAXD pattern of photopolymerized DAPDI TFP, it is realized that the self-assembled nanocolumns do not have enough time for the crystallization during the drying and photopolymerization processes but remain as a metastable glassy ColN phase. The metastability of glassy ColN phase is not changed since the chemically cross-linked 3D anisotropic networks are created during the photopolymerization. On the basis of 2D WAXD results, the molecular packing structure in the macroscopically oriented and photopolymerized DAPDI TFP is proposed against the POM image, as schematically illustrated in Figure 3b. Degree of Polarization of the Photopolymerized DAPDI TFP. To estimate the molecular arrangement and the polarizability of the photopolymerized DAPDI TFP, the transmittance UV−vis spectra45,46 are obtained by rotating the UV−vis polarizer, as shown in Figure 4. Here, TTD and TMD

Figure 3. (a) 2D WAXD pattern of the uniaxially oriented and photopolymerized DAPDI TFP. (b) Schematic illustration of DAPDI molecular packing structures in the DAPDI TFP, which is fabricated by the mechanical shearing of the 25 wt % DAPDI-H2O solution.

diffractions is only detected at 2θ = 26.5° (d-spacing = 0.335 nm) which is a typical face-to-face π−π stacking distance between aromatic rigid cores of the perylene-based molecules.43,44 On the basis of POM and 2D WAXD results, it is understood that the self-assembled DAPDI nanocolumns are aligned parallel to the SD and the π−π stacking intermolecular interaction is one of the main driving forces for the molecular self-assembly of DAPDI molecule. On the equator, there are two pairs of broad weak diffractions at 2θ = 5.5 and 8.0°. This result indicates that the lateral correlations among nanocolumns are relatively weak. To obtain the all possible diffractions available in the DAPDI crystal, the 1D WAXD powder pattern is acquired from the unpolymerized crystal, as represented in Figure S3. Two strong diffractions are detected at the identical positions of 2D WAXD in the low-angle region below 2θ = 10°. Above 2θ = 10°, five diffraction peaks, including 2θ = 26.5°, are detected. Diffractions on the subnanometer length scale between 11 and 30° should be originated from the molecular packing structures inside of the self-assembled nanocolumns as well as from the molecular correlations between neighboring nanocolumns. To identify the detail crystalline structure of DAPDI, the X-ray diffraction results should be analyzed by the support of the electron diffractions of single crystals and the computer simulations in

Figure 4. (a) UV−vis spectra for the photopolymerized DAPDI TFP from the 25:75 solution of DAPDI-H2O by weight ratios and (b) its normalized value of transmittance and degree of polarization.

stand for the transverse and the mechanical directions when the azimuthal angle is 0° and 90° to the polarization axis, respectively. The SD of photopolymerized DAPDI TFP is aligned parallel to the polarization axis of the incident light. As shown in Figure 4a, the transmittance is maximized at 491 nm when the SD is parallel to the polarization axis of UV−vis polarizer, which clearly indicates that the transmission axis of the uniaxially oriented DAPDI TFP coincides with the mechanical SD. This conclusion is well matched with that extracted from the POM and 2D WAXD results. Recall that general LCLC molecules are flat and disc-shaped with a polyaromatic core that is absorbing the light.47,48 Therefore, the polarized light on the molecular plane is absorbed whereas the polarized light can transmit along the molecular plane normal axis. From the transmittance UV−vis spectra, the single transmittance (TS) is calculated to be 39.40% at the maximum D


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coating process.51−53 Note that MAPT monolayer is often used as a strong adhesive by creating chemical bonds between the two incompatible layers. The more detailed procedures of the surface modification with MAPT are illustrated in Experimental Section and Figure S5. Because the DAPDI TFP is covalently connected to the surface modified substrates by the photopolymerization process, the chemical stability is dramatically enhanced without any dissolution and delamination indications even on the PET substrates. The mechanical stability is also evaluated by the pencil scratch resistance test according to ASTM D3363,54 and the tested macroscopic images are represented in Figure 5. Film

absorption wavelength 491 nm, based on the definition of TS = (TTD + TMD)/2. Degree of polarization (DOP) defined as DOP = (TTD − TMD)/(TTD + TMD) is also calculated to be 99.80%.48−50 Because DOP is a function of TFP thickness, the DOP of DAPDI TFP must be normalized when the TS is 40% which is a standard transmission for common single polarizing filters. As represented in Figure 4b, the DOP of photopolymerized DAPDI TFPs at λmax = 491 nm is estimated to be 99.81% after the normalization. On the basis of our own knowledge, the DOP value is the highest one reported for the LCLC-based TFP. Note that the DOPs of water-based and ionic monomer-based LCLC TFPs are below 98%. The lower polarizability of TFP fabricated from water-based and ionic monomer-based LCLC solutions should be related with the stress relaxations during the drying and photopolymerization processes, which resulted in the disturbances of molecular orientations. This kind of molecular disturbances is relatively decreased by the fast glassification progress in the DAPDI system. The DOP of DAPDI TFP before and after the photopolymerization process is not changed. Chemical and Mechanical Stabilities of the DAPDI TFP. Chemical and mechanical stabilities of the anisotropic polymer films should be secured for practical optoelectronic applications. Chemical stabilities of the macroscopically oriented DAPDI TFPs on the substrate are estimated by immersing the film in the various solvents for 5 min. The experimental results of the chemical resistance test are compiled in Table 1 and Figure S4. The unpolymerized part Table 1. Chemical Stabilities of the Photopolymerized DAPDI TFP on the Various Substrates

0.1 N NaOH 0.1 N HCl H2O DMF MeOH acetone IPA chloroform THF EA toluene hexane

bare glass

MAPT-modified glass

bare PET

MAPT-modified PET

X X X △ △ △ △ △ O O O O

X △ O O O O O O O O O O

X X X X △ X X △ △ O O O

X △ O O O O O O O O O O

Figure 5. OM images after the scratch resistance test of the photopolymerized DAPDI films on (a) bare glass, (b) MAPTmodified glass, (c) bare PET, and (d) MAPT-modified PET substrates and (e) the corresponding quantitative pencil resistance of the films.

resistance against the pencil scratch is judged just before the surface of substrate is exposed and the results are summarized in the Figure 5e. The photopolymerized DAPDI TFP on the bare glass and PET substrates are bearable up to the soft pencils of HB and B, respectively. This difference should come from the adhesion strengths between the photopolymerized surface of the film and substrates. The hardness of unpolymerized DAPDI TFP on the bare PET withstanding to 2B is somewhat improved to B by the formation of 3D molecular network during the photopolymerization. However, the adhesion strength between photopolymerized film and bare PET substrate is still weak, resulting in a moderate pencil scratch resistance. After the surface modification with the reactive MAPT self-assembled monolayer, the pencil scratch resistances of photopolymerized DAPDI TFP are increased up to 2H on the both glass and PET substrates. This result indicates that the pencil scratch resistance depends both on the hardness of DAPDI TFP and on the adhesion strength with a substrate. Compared with the ionic monomer-based LCLC TFP system, the chemical and mechanical resistances after the photopolymerization are enhanced, which should be related with the higher cross-linking density and the strong interfacial adhesion strength in the DAPDI-based system. It is concluded that the surface modification of substrates with reactive MAPT self-

O: stable, △: partially peeled, X: completely peeled, dimethylformamide (DMF), tetrahydrofuran (THF), ethyl acetate (EA), and isopropyl alcohol (IPA).

of TFP are almost washed away by polar protic solvents (such as water, methanol and ethanol) as well as by a polar aprotic solvent (dimethylformanide, DMF). As represented in Table 1 and Figure S4, the chemical resistance is significantly enhanced by the photopolymerization. Even after the photopolymerization process, the DAPDI TFP is often partially delaminated from the substrates in the polar aprotic solvents, such as chloroform, isopropyl alcohol, and acetone. The delamination phenomena are more serious on the hydrophobic PET substrates. To achieve the strong adhesion between DAPDI TFP and substrates, the photopolymerizable methacryloxypropyl-trimethyoxysilane (MAPT) self-assembled monolayer is created on the oxygen plasma treated substrates before the E


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numerous macroscopic striped cracks along the SD, resulting in the fibrous morphology (Figure 6e). 58,59 The fibrous morphology is not observed after photopolymerization process, which means that the fibrous morphology is generated during the developing process in methanol. This phenomenon should be related with the unsatisfied adhesion strength between the photopolymerized DAPDI TFP and the bare glass substrate. After securing the strong adhesion between the photopolymerized LC surface and the glass substrate by introducing the photopolymerizable MAPT self-assembled monolayer before the coating process, the chemically and mechanically stable patterned DAPDI TFP is successfully fabricated without sacrificing the polarizability, as represented in Figure 6b,d. As shown in Figure 6f, the magnified OM image of fabricated film on the MAPT-modified glass substrate shows a relatively smooth morphology without the generation of any macroscopic defects. Additionally, the positively patterned DAPDI TFP on the MAPT-modified glass substrate is successfully fabricated and its OM morphology is represented in Figure S6. In addition to the OM observations, AFM analysis is conducted to investigate the surface topology of the patterned DAPDI TFP on the fine scale, as shown in Figure 7. Fibrous DAPDI aggregates on the bare glass substrate (Figure 7a) are lined up along the SD with the uniform height (150 nm) and width (2.88 μm). Its root-mean-square (RMS) roughness is estimated to be 61.8 nm from the cross-sectional information on height profile, as shown in Figure 7c. As expected, the patterned DAPDI TFP on MAPT-modified glass substrate shows a relatively smooth surface topology compared with that on the bare one. The RMS roughness is 4.5 nm for the patterned film on MAPT-modified glass substrate (Figure 7d). From the 50 nm length scale AFM image (Figure 7d and Figure S7), it is realized that many nanoprotrusions are generated during the etching process in methanol. Based on the OM and AFM observations, it is realized that there are neither the macroscopic striped cracks nor the indications of delamination after the etching process. At this circumstance, the macroscopically oriented and patterned DAPDI TFP with robust chemical and mechanical stabilities on the flexible PET polymer substrates was successfully fabricated. Remind that during the developing process in methanol, the patterned film on the bare PET substrate is delaminated from the substrate due to the poor adhesion strength between hydrophilic DAPDI surface and hydrophobic PET surface. To overcome this limitation, the photopolymerizable MAPT self-assembled monolayers are constructed before the coating process. As represented in Figure 8a,b, the positively and negatively patterned DAPDI TFPs on the MAPT-modified PET substrates are successfully fabricated and represented in Figure 8c, the patterned film on the MAPT-modified PET substrates are bendable without generating any damages. As shown in Figure 9, the 3D surface topology of positively patterned film on the MAPT-modified PET substrate is also investigated by utilizing 3D surface profiler Nanoview. The dotted patterns with 35 μm length and width are clearly observed at the top (Figure 9a) and side (Figure 9b) views. As represented in Figure 9, the average height of the positively dotted patterns is estimated to Rz = 1.03 μm with the maximum height of Rt = 1.16 μm. The average height of the patterns can be controlled by the thickness of DAPDI films and the developing time.

assembled monolayer and the photopolymerization after the coating process are very effective in enhancing the chemical and mechanical resistances. Flexible and Patterned DAPDI TFP on PET Substrate. To build the hierarchical structures on the different length scale, we applied the patterned photomasks between the UVlight source and the macroscopically oriented DAPDI TFP during the photopolymerization.23,55−57 The DAPDI reactive monomers located in the open silt are selectively initiated and polymerized by the UV irradiation and the unpolymerized parts blocked by the patterned photomask from the UV light are subsequently developed with methanol in 60 s. The thickness of the pattern can be controlled by the developing time. At the earlier stage, the thickness is rapidly decreased, while a linear relationship between thickness and developing time is maintained after 10 s. The patterned DAPDI TFP can be fabricated not only on the hydrophilic glass substrates but also on the hydrophobic flexible PET films. Optical microscopic (OM) technique with a single polarizer is employed at first to study the formation of patterned DAPDI TFP by a conventional lithographic process. The macroscopically patterned film is fabricated on the bare glass substrate after the developing process (Figure 6 and Figure S6), while that on

Figure 6. OM images of negatively patterned DAPDI TFP on the bare glass substrate when the polarizer axis is (a) parallel and (c) perpendicular to the SD and (e) its magnified image of dashed area in image c. (b, d, f) Corresponding OM images of patterned DAPDI TFP on the MAPT-modified glass substrate.

the bare PET substrate is peeled off by methanol etchant. As shown in Figure 6a,c, the light transmittance is maximized when the axis of polarizer is parallel to the SD of photopolymerized film. The molecular orientation and polarizability is not changed even after the developing process with methanol. However, the magnified OM of patterned DAPDI TFP on the bare glass substrate exhibit the generation of F


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Figure 7. 3D topographic AFM images of patterned DAPDI TFP after the photopolymerization and etching processes on the (a) bare glass and (b) MAPT-modified glass substrates. Corresponding height profiles for (c) panel a and (d) panel b.

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CONCLUSIONS For the fabrication of macroscopically oriented and patterned thin film polarizer (TFP) with robust mechanical and chemical stabilities, a perylene-based reactive mesogenic molecule (abbreviated as DAPDI) was newly synthesized by chemically connecting methacrylate photoreactive groups to the both sides of perylene-based LCLC molecule. Phase behaviors and molecular packing structures were systematically investigated on the basis of microscopic, scattering, and spectroscopic analyses. From the 2D WAXD and POM results, it was found that the self-assembly of DAPDI molecules in the aqueous solution is due to the face-to-face π−π intermolecular interaction among perylene cores, as well as the nanophase separation between hydrophilic ionic groups and hydrophobic perylene cores. Utilizing polarized UV−vis spectroscopy, we evaluated the normalized DOP of DAPDI TFP to be 99.81% at the maximum absorption wavelength 491 nm. To fabricate the patterned film, the uniaxially oriented film was first prepared by shearing the DAPDI-H2O LCLC solution on the glass and flexible polymer substrates. The photopolymerization was consecutively conducted at specific regions selected by a photomask and the unpolymerized regions were successfully developed in methanol. Solvent and scratch resistances were significantly enhanced after the surface treatment and photopolymerization. Furthermore, the surface morphology and topology were investigated by employing the OM, AFM, and 3D surface profiler. The fabrication of macroscopically oriented chromonic TFP on the flexible polymer films with robust chemical and mechanical stabilities may open new doors for the practical applications for the flexible devices.

Figure 8. Patterned DAPDI TFP on the MAPT-modified PET flexible substrate: OM images of the (a) positively and (b) negatively patterned DAPDI TFP, and (c) its corresponding macroscopic image of patterned DAPDI on the bendable polymer film.

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EXPERIMENTAL SECTION

Materials. Perylene-3,4,9,10-tetracarboxylic dianhydride (97%, Aldrich), N,N-dimethylethylene diamine (98%, TCI), 2-bromoethanol (95%, TCI), 4-(dimethylamino)pyridine (99%, Aldrich), and methacrylic anhydride (94%, Aldrich) were used as received without further purification. N,N-dimethylformamide (DMF) and tetrehydrofuran (THF) were refluxed with calcium hydride and distilled before use. Water (H2O) was deionized and distilled before use. DAPDI and its intermediates were synthesized according to the reference32 and the detail synthetic route was shown Scheme 1. Synthesis of N,N-Bis[diethylamine]-perylene-3,4,9,10-tetracarboxylic Dianhydride (PDI-1). A solution of perylene-3,4,9,10tetracarboxylic dianhydride (2.5 g, 6.4 mmol) and N,N-dimethyl-

Figure 9. 3D surface topological observation of the positively patterned DAPDI TFP on the MAPT-modified PET using Nanoview: (a) top-view and (b) side-view. G


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ACS Applied Materials & Interfaces ethylene diamine (4 mL, 20 mmol) in dried DMF (25 mL) was refluxed for 5 h at 130 °C. After the reaction, a suspension was cooling down to room temperature and poured into the 150 mL THF. The precipitate was filtered and washed with THF. After drying in the vacuum oven overnight, the resulting product (abbreviated as PDI-1) was obtained as a dark brown solid. The product was used for the next step without further purifications. (3.15 g, 92%). 1H NMR (400 MHz, TFA-d, δ): 8.66 (d, 8H, Ar H), 4.71 (t, 4H, 2CH2), 3.73 (d, 4H, 2CH2), 3.1 (s, 12H, 4CH3). PDI-2. A suspension of PDI-1 (1.5 g, 2.8 mmol) and 2bromoethanol (1.6 mL, 22 mmol) was refluxed for 12 h at 100 °C. After the reaction, the solution was cooled to room temperature and added to 100 mL of THF solvent. The resulting precipitates were filtered and washed several times with THF. After drying in the vacuum oven overnight, the crude product was dissolved in 100 mL of deionized water. After the solution was filtered with a glass-filter, the remains were freeze-dried, and reddish solids were obtained (2 g, 91%). 1H NMR (400 MHz, D2O, δ): 7.87 (bs, 4H, ArH), 7.4 (bs, 4H, ArH), 4.5 (t, 4H, 2CH2), 4.0 (t, 4H, 2CH2), 3.7−3.8 (t, 8H, 4CH2), 3.18 (s, 12H, 4CH3). DA-PDI. PDI-2 (0.5 g, 0.6 mmol) and DMAP (20 mg, 0.2 mmol) were dissolved in 50 mL of dried DMF and stirred for 15 min at room temperature. Methacrylic anhydride (1.0 mL, 6.4 mmol) was added dropwise under light protection and stirred at room temperature for 1 day. The crude product was poured into the 200 mL THF, and the resulting precipitates were filtered. The final reddish product was obtained after drying in the vacuum oven (0.52 g, 89%). 1H NMR (400 MHz, D2O, δ): 7.9 (bs, 4H, ArH), 7.42 (bs, 4H, ArH), 6.53 (s, 2H, CH2), 6.18 (s, 2H, CH2), 5.15 (t, 4H, 2CH2), 4.9 (t, 4H, 2CH2), 4.58 (t, 4H, 2CH2), 4.11 (t, 4H, 2CH2), 3.77 (s, 12H, 4CH3), 2.3 (s, 6H, 2CH3); ESI-MS m/z: [M]2+ calcd for C44H46N4O8, 758.3; found, 758.6. Equipment and Experiments. Chemical structure and purity of DAPDI molecule were confirmed by proton (1H) nuclear magnetic resonance (NMR, JNM-EX400, JEOL) in deuterated trifluoroacetic acid (TFA-d) and water (D2O). Chemical shifts were quoted in part per million (ppm) with a reference of solvent peak. Molecular weight of DAPDI was identified by electrospray ionization mass spectroscopy (ESI-MS, 3D CE/ESI-MS, Agilent). Optical textures at different concentrations were obtained by employing POM (Nikon ECLIPSE LV100POL) coupled with a METTLER FP 90 heating stage in order to observe the morphology on the micrometer length scale. To prevent the evaporation of solvent and keep the uniformity of solution, all edges of LC cells were sealed using epoxy resin before optical observations. The phase transition was monitored during the heating process at 1.0 °C/min. On the basis of the morphological observation, the composition of a solution was optimized to be 25:75 (DAPDI-H2O) in the presence of 1 wt % watersoluble photoinitiator (2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, Sigma-Aldrich). The uniaxially oriented DAPDI TFP was fabricated by the doctor blade coating of a LCLC solution on the glass or PET substrates. The uniaxially oriented film was dried at room temperature under light protection for 24 h to remove the residual solvent. By adjusting the height of the doctor blade, we controlled the film thickness between 1 and 10 μm. To photopolymerize the DAPDI reactive monomers, the anisotropic film was sequentially photoirradiated under the UV light with the intensity of 20 mW/cm2 for 30 min. The molecular packing structure of the oriented and photopolymerized DAPDI film was confirmed utilizing the POM and 2D WAXD (Bruker, D8 DISCOVER). For the POM and 2D WAXD measurement, the uniaxially oriented and photopolymerized sample was prepared.60,61 The thickness of film was controlled to be 5 μm. Additionally, a tint retardation plate (530 nm) was introduced to the orthogonally arranged polarizer and analyzer in POM. The slow axis (nmax) of the retardation plate was placed parallel or perpendicular to the SD. To obtain all diffractions available in the DAPDI crystal, was conducted the 1D WAXD experiment for the isotropic powder. The transmittances according to the angle between SD of anisotropic film and UV−vis polarizer were also evaluated by the polarized UV−vis

(Scinco S-3100) spectroscopy. For the measurement of the transmittances, the sample was prepared on the transparent quartz substrate. The wavelength of UV−vis polarizer was in the range of 250−3000 nm. The degree of polarization (DOP) and single transmittance (TS) were calculated from the measured transmittance spectra. Photolithographic patterning on the DAPDI TFP was carried out by irradiating UV light to the photomask-covered film. The photopolymerized film was developed in methanol for 10−80 s by controlling the thickness of the pattern. Note that the experiments were conducted both on the bare and surface modified substrates. The surface modifications were carried out via two steps: the generation of hydroxy functions by oxygen plasma and the grafting reaction of the reactive molecule (methacryloxypropyl-trimethoxysilane, MAPT). After washing and drying substrates, the oxygen plasma treatment was conducted to the substrates with plasma cleaner (PDC-32G, Harrick plasma) for 5 min. Subsequently, the substrates were immediately dipped into a solution with 10 wt % MAPT in ethanol for 1 h. The MAPT-modified substrates were dried in the vacuum oven for 12 h. The detail schematic procedures is shown Figure S5 of the Supporting Information. We conducted the chemical resistance test by dropping the film into the various solvents for 5 min including commonly used polar and nonpolar solvents and 0.1 N aqueous solutions of acid/base. The DAPDI-H2O solution was coated on the bare and modified glass and PET substrates and then dried at room temperature for 24 h. The mechanical stability was estimated by the pencil hardness test using the STAEDTLER Mars Lumograph 100-G12 (ASTM D3363) and the results were observed by OM. We observed the morphology of the patterned DAPDI TFP via OM by rotating the sample and changing the magnification of lens. Surface topology was additionally examined using AFM (Bruker, Nanoscope V multimode 8) and three-dimensional (3D) surface profiler (Nanosystem, NanoView NV-2000). In the AFM techniques, the tapping mode was applied to get the height information and to collect accurate surface features, and the forces used by the cantilever were sufficient but light enough to avoid unexpected damage. The scanning rate was manipulated to be 1.0 Hz.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09995. 1 H NMR, ESI-MS, 1D WAXD, POM, schematic procedure, AFM, photoimaged results and detailed experimental processes were represented and discussed. (PDF)

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by Basic Research Laboratory (2015042417), MOTIE-KDRC (10050334), and BK21 Plus program, Korea.

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REFERENCES

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