Research Article www.acsami.org
Polymer Stabilization of Liquid-Crystal Blue Phase II toward Photonic Crystals Seong-Yong Jo,† Sung-Wook Jeon,† Byeong-Cheon Kim,† Jae-Hyun Bae,† Fumito Araoka,‡ and Suk-Won Choi*,† †
Department of Advanced Materials Engineering for Information and Electronics (BK21Plus), Kyung Hee University, Yongin-shi, Gyeonggi-do 446-701, Republic of Korea ‡ Physicochemical Soft Matter Research Unit, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *
ABSTRACT: The temperature ranges where a pure simple-cubic blue phase (BPII) emerges are quite narrow compared to the body-centered-cubic BP (BPI) such that the polymer stabilization of BPII is much more difficult. Hence, a polymer-stabilized BPII possessing a wide temperature range has been scarcely reported. Here, we fabricate a polymer-stabilized BPII over a temperature range of 50 °C including room temperature. The fabricated polymer-stabilized BPII is confirmed via polarized optical microscopy, Bragg reflection, and Kossel diagram observations. Furthermore, we demonstrate reflective BP liquid-crystal devices utilizing the reflectance−voltage performance as a potential application of the polymer-stabilized BPII. Our work demonstrates the possibility of practical application of the polymerstabilized BPII to photonic crystals. KEYWORDS: photonic crystals, liquid crystals, simple-cubic blue phase, polymer-stabilizing, photonic devices
1. INTRODUCTION Chiral liquid-crystal (LC) phases possess unique self-assembled periodic structures.1−3 For example, LC chiral nematic (N*) phases regarded as one-dimensional photonic crystals (PCs) have a periodic helical structure and accordingly lead to a Bragg reflection at the photonic band gap (PBG) wavelength.3 Similar PC phenomena have been also observed in a variety of different chiral LC phases3 such as chiral smectic phases,4 a twisted grain boundary,5 and LC cubic blue phases (BPs),3,6,7 which originate from their unique periodic structures. LC cubic BPs have attracted significant research interest because they have potential for application in advanced photonic devices as three-dimensional (3D) PCs.8 Cubic BPs have a double-twisted cylinder (DTC) structure, and the selfassembly of DTCs in 3D space determines their peculiar periodic lattice structures.8,9 Cubic BPs can be categorized into two types, namely, a simple-cubic BP (BPII) and a bodycentered-cubic BP (BPI), in the order of decreasing temperature.2,8,10,11 In general, each cubic BP makes a brief appearance, typically 0−2 °C, in the temperature range between the high-temperature isotropic (Iso) phase and the low-temperature N* phase. This narrow temperature range limits their utility in practical applications. To overcome this obstacle, polymer-stabilization methods have been introduced. A homogeneous mixture of photopolymerizable monomers and © 2017 American Chemical Society
low-molecular-weight LCs is irradiated with ultraviolet (UV) light in a temperature range where a BP emerges.12 Consequently, the specific polymer networks formed by in situ polymerization at the BPs can stabilize a disclination in a 3D cubic lattice. Using this method, Kikuchi et al. showed the polymer-stabilized BPI over a temperature range of more than 60 °C including room temperature (RT).12 Subsequently, this polymer-stabilization concept is presently recognized as a feasible method for extending the temperature range of a BP. Unfortunately, although the polymer-stabilization concept has received significant attention to overcome the practical limitation of BPs, most polymer-stabilized BPs reported to date have been targeted at BPI. To the best of our knowledge, although polymer-stabilized BPI systems with broad temperature ranges have been presented by several researchers,12−16 such a broad range for polymer-stabilized BPII has been scarcely reported. Even if there were a few reports on polymerstabilized BPII,17,18 in-depth studies about stabilizing BPII via polymer networks have hardly been performed. This is because the temperature ranges where a pure BPII emerges are narrow compared to those for BPI such that the in situ polymerization Received: January 30, 2017 Accepted: February 20, 2017 Published: February 20, 2017 8941
DOI: 10.1021/acsami.7b01502 ACS Appl. Mater. Interfaces 2017, 9, 8941−8947
Research Article
ACS Applied Materials & Interfaces in BPII is much more difficult than that in BPI, namely, BPII possesses a very tight process window; there is only a brief allowance for the temperature range, typically 0−1 °C,19 for stabilizing BPII by photopolymerization. Hence, a broad temperature range where a pure BPII emerges is favorable for the preparation of polymer-stabilized BPII. It is expected that polymer-stabilized BPII exhibits a higher elastic resistance against lattice deformation by an applied electric field compared to that for BPI.20 This can be explained by the influence of the structural differences between BPI and BPII,20 as illustrated in Figure 1. It is considered that the BP is
Table 1. Constituent Fractions of the HNM, HN*M, and Precursor Prepared in This Work (wt %) photopolymerizable monomer
HNM HN*M precursor
NLC
bent-core molecule
chiral dopant (S811)
TMPTA
RM257
83 58 52
17 12 11
30 27
5
5
POM images, reflection spectra, and Kossel diagrams according to the phase sequences of the HN*M. The Iso phase progressed to a relatively uniform fluid phase with a green color upon cooling at a rate of 0.5 °C/min, as shown in Figure 2a. Upon further cooling, the uniform fluid phase changed to a phase exhibiting small platelets (Figure 2b), followed by an N* phase with oily streaks19 (Figure 2c). In the uniform fluid phase in the higher temperature range between the Iso and the N* phase upon cooling, a strong reflection peak appeared at approximately 520 nm, corresponding to the colors observed by POM, as shown in Figure 2d. In contrast, for the phase in the lower temperature range, two peaks were observed with relatively low reflectance profiles originating from different platelet domains19 (Figure 2e). The Kossel diagram of the uniform fluid phase in Figure 2f reveals a typical diffraction pattern corresponding to the lattices of BPII.21,22 On the other hand, in Figure 2g the detected pattern for the phase in the lower temperature range corresponds to the lattices of BPI,21,22 namely, both the small platelets and the two reflection peaks for the phase in the lower temperature range originate from the (200) and (110) lattices of BPI. Figure 2h displays the temperature dependence of the Bragg reflection wavelength of the HN*M for the peak wavelength collected during cooling (0.5 °C/min). Consequently, BPI and BPII in HN*M could be clearly distinguished. Our BPII in the higher temperature range exhibited uniform alignment under the cells with unidirectional rubbing on both glass substrates. However, BPI in the lower temperature range showed random alignment with small platelets in the same cells. On the basis of the POM, Bragg reflection, and Kossel diagram, we confirmed that the temperature ranges of both pure BPI and BPII were about 5 and 4 °C, respectively, during the cooling process, which are broader than those of the typical pure BPI and BPII. These expansions of the temperature ranges of the BPs are attributed to the doping effect of the bent-core molecules.25,26 2.2. BPII in a Precursor. Next, we prepared a precursor (HN*M-blended photopolymerizable monomers) to fabricate a polymer-stabilized BP. Here, two types of photopolymerizable monomers were used. The details of the mixing ratios of the precursor are described in Table 1. Figure 3a and 3b shows two typical POM images of the precursor during cooling (0.5 °C/ min). From the Iso phase, an unknown uniform phase with a green color appeared as shown in Figure 3a. Upon further cooling, the unknown phase changed to an N* (Figure 3b). The unknown phase appearing in a higher temperature range can be clearly distinguished from the N* phase with oily streaks. Figure 3c shows the temperature dependence of the Bragg reflection wavelength of the prepared precursor and clearly displays a discontinuous change at the unknown phase− N* transition during cooling (0.5 °C/min). In the unknown phase, the Bragg reflection wavelength moved toward slightly shorter wavelengths with decreasing temperature.19 This trend
Figure 1. Unit lattices of BPI, BPII, polymer-stabilized BPI, and polymer-stabilized BPII.
stabilized when the polymers formed within the BP are condensed into disclinations, and the disclinations are then thermally stabilized.8,9,12 In the case of polymer-stabilized BPI, the seven independent disclination lines stabilized by polymers do not contact one another inside the unit lattice. In contrast, the four disclination lines of BPII have an intersection point at the center of the unit lattice,20,21 as shown in Figure 1. Hence, it is anticipated that polymer-stabilized BPII will be more robust to lattice deformation compared with polymer-stabilized BPI. In addition, BPII tends to easily achieve a uniform alignment compared to BPI.21−24 Hence, a polymer-stabilized BPII that possesses a wide temperature range has greater potential for practical applications compared to polymer-stabilized BPI. Herein, we report the polymer stabilization of BPII over a temperature range of 50 °C including RT. This polymerstabilized BPII was confirmed via polarized optical microscopy (POM), Bragg reflection, and Kossel diagrams. The potential application of the polymer-stabilized BPII in PC devices was also demonstrated.
2. RESULTS AND DISCUSSION 2.1. Pure BPs in a Host N* Mixture. A host N mixture (HNM) consisting of conventional NLCs and bent-core molecules was prepared. The evaluated birefringence (Δn) and dielectric anisotropy (Δε) of HNM consisting of 83 wt % NLCs and 17 wt % bent-core molecules were, respectively, about 0.15 and 15 at RT. Then a host N* mixture (HN*M) composed of the HNM blended with a chiral dopant was prepared. The details of the blending fractions of the HNM and HN*M are summarized in Table 1. Figure 2 shows typical the 8942
DOI: 10.1021/acsami.7b01502 ACS Appl. Mater. Interfaces 2017, 9, 8941−8947
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Figure 2. Typical POM images of (a) BPII, (b) BPI, and (c) the N* phase of the HN*M. Typical reflection peaks of (d) BPII and (e) BPI of the HN*M. Kossel diagrams of (f) BPII and (g) BPI of the HN*M. (h) Temperature dependence of the Bragg reflection wavelength of the HN*M.
2.3. Polymer-Stabilized BPII. A polymer-stabilized “supercooled” BP method was proposed and reported by the Kikuchi group.27 Their proposed method directly induces metastable BPs through the supercooling effect, and they presented the feasibility of this method using supercooled BPI. Here, we attempt to stabilize the supercooled BPII via polymer networks. The precursor blended with a small amount (0.6 wt %) of photoinitiator was injected into a sandwich cell at an Iso temperature (40 °C). Then the cell was slowly cooled (0.5 °C/ min) to 30 °C, where BPII was maintained. At this BPII state, UV light irradiation was used to fabricate a polymer-stabilized BPII. Figure 4a and 4b shows a POM image and Kossel diagram of the polymer-stabilized phase at RT, respectively. The stabilized phase exhibited a uniform texture. The POM image and Kossel diagram strongly indicate that the polymer-stabilized phase is
was similar to that of BPII observed in the aforementioned BPII in host N* mixture (see Figure 2h). In addition, the Kossel diagram also indicates that the unknown phase is not BPI but BPII (see inset of Figure 3a). Intriguingly, BPI disappeared as a result of the supercooling of BPII. By introducing the photopolymerizable monomers, BPII in the higher temperature range could be expanded owing to the supercooling effect. This is because the viscosity of the precursor was increased by adding the photopolymerizable monomers. In general, the supercooling effect is more noticeable in higher viscosity mixtures. The estimated temperature range of the supercooled BPII of the prepared precursor was about 10 °C, from 35 to 25 °C, and the BPII lasted more than 48 h. We strategically utilize this supercooled BPII with a wide process window to realize the preparation of polymer-stabilized BPII. 8943
DOI: 10.1021/acsami.7b01502 ACS Appl. Mater. Interfaces 2017, 9, 8941−8947
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controller. Then the reflection spectra were measured during heating. As depicted in Figure 4c, relatively constant reflection peaks around 540 nm were observed from 10 to 60 °C. The lattices of BPII were stabilized by the polymer network; thus, there is barely any shift in the Bragg reflection wavelengths. These results support the successful stabilization of BPII over a range of 50 °C including RT. To our knowledge, such a wide temperature range for polymer-stabilized BPII has not yet been reported. By controlling the number of chiral dopants doped in the host materials, polymer-stabilized BPII with red, green, and blue colors can be realized. In this present work, polymerstabilized BPII exhibiting green color was successfully fabricated. Now, we attempted to fabricate polymer-stabilized BPII showing blue and red colors. The details will be reported in another paper soon. 2.4. Reflective PC Device Using the Polymer-Stabilized BPII. Considering the potential application of the polymer-stabilized BPII as PC devices, we fabricated a reflective-type device utilizing the reflectance−voltage performance of the polymer-stabilized BPII. Typical reflective images of our sample (at RT) are depicted in Figure 5a. The reflected green color finally turned dark owing to the electric field. Upon elimination of the electric field, the sample reverted to reflect a
Figure 3. Typical POM images of (a) an unknown uniform phase and (b) the N* phase of the precursor during cooling. (c) Temperature dependence of the Bragg reflection wavelength of the prepared precursor. Kossel diagram of the unknown uniform phase is show in the inset in a. From the POM image, Bragg reflection, and Kossel diagram, the unknown uniform phase is BPII.
Figure 4. (a) POM image and (b) Kossel diagram of the polymerstabilized BP II at RT. (c) Bragg reflection spectra at various temperatures. Temperature dependence of the reflection-peak wavelength for a range of temperatures of polymer-stabilized BPII is also shown in the inset.
BPII. Figure 4c shows the dependence of the Bragg reflection spectra on the temperature, and the temperature dependence of the reflection-peak wavelength for a range of temperatures for polymer-stabilized BPII is also in the inset. We heated the polymer-stabilized phase into the Iso state once and cooled it to 10 °C, which is the lowest temperature of our temperature
Figure 5. (a) Typical reflective images, corresponding to the extremes of the applied electric fields, of our sample at RT. (b) Measured reflectance spectra of the polymer-stabilized BPII sample at different voltages at RT. (c) Image showing the mechanism of the reflectance− voltage performance of the polymer-stabilized BPII. 8944
DOI: 10.1021/acsami.7b01502 ACS Appl. Mater. Interfaces 2017, 9, 8941−8947
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Figure 6. (a) Measured reflectance−voltage curve for our polymer-stabilized BPII sample. (b) Typical response profile for our polymer-stabilized BPII.
green color. Figure 5b exhibits the detected reflectance spectra of the polymer-stabilized BPII sample for different voltages at RT. As the voltage increases, the LC molecules are reoriented along the electric field and the DTC structures are gradually unwound; therefore, the reflectance decreases. When most of the LC molecules are reoriented parallel to the electric field, the polymer-stabilized BPII loses its unique periodic DTC structures,28,29 and the reflectance is reduced to the minimum value, as shown in Figure 5c. The lattices of BPII are stabilized by the polymer network; thus, there is barely any shift in the Bragg reflection wavelengths. The small blue shifts are due to the electric-field-induced change in the refractive index.28 Figure 6a depicts the measured reflectance−voltage curves for our polymer-stabilized BPII sample. Interestingly, the evaluated hysteresis is ∼0.25% (ΔV/Vmax; ΔV is the voltage difference at 50% reflectance, and Vmax is the maximum driving voltage). This evaluated hysteresis was about 1/30 of that of the previously reported polymer-stabilized BPI.28 These results strongly indicate that the polymer-stabilized BPII exhibits a higher elastic resistance against lattice deformation by an applied electric field than that for the polymer-stabilized BPI reported previously.28 Unfortunately, our precursor before polymer stabilization shows only BPII. Thus, only polymerstabilized BPII could be fabricated. Thus, we cannot compare the performance between BPII and BPI possessing the same LC host, polymer concentration, and curing conditions in our case. The measured rise and decay times were ∼0.8 and ∼0.6 ms, respectively, for an applied electric field of 25 V/μm at RT, as shown in Figure 6b. The fast decay time also supports the successful polymer stabilization of BPII, and both rise and decay times fascinate as high-performance PC devices. 2.5. Morphological Study. To confirm the polymer network structures of our polymer-stabilized BPII, a morphological study was performed by extracting BPII from the cell. The cell was placed in hexane for 24 h. The cell was opened and dried. Then the polymer network was sputtered with a thin layer of Pt and examined with a scanning electron microscope (SEM). As described in the Introduction, it is considered that the BP is stabilized when the polymers formed within the BP condense into disclinations, which are then thermally stabilized.8,9,12 In the case of polymer-stabilized BPII, the four disclination lines stabilized by polymers have an intersection point at the center of the unit lattice.19,20 As shown in Figure 7, the SEM image shows that the polymer networks exhibiting a
Figure 7. SEM image of polymer-stabilized BPII showing close-woven polymer networks with a width of around 80−100 nm.
close-woven structure with a width of around 80−100 nm were interconnected with each other throughout the cell. These close-woven networks stabilized the lattice structures of BPII and were more robust to lattice deformation by an applied electric field.
3. CONCLUSION We succeeded in fabricating polymer-stabilized BPII over a temperature range of 50 °C including RT. Our approach was as follows. First, we prepared an HN*M consisting of conventional NLCs and bent-core molecules. In the HN*M, the temperature ranges of both pure BPI and BPII were about 5 and 4 °C, respectively, during the cooling process. Then we prepared a mixture precursor consisting of the HN*M and photopolymerizable monomers. The mixture precursor exhibited supercooled BPII from 35 to 25 °C. Using this wide process window, the polymer-stabilized BPII was successfully fabricated and confirmed via POM, Bragg reflection, and Kossel diagrams. Furthermore, the potential application of the polymer-stabilized BPII to PC devices was also demonstrated. The fabricated reflective-type device utilizing the reflectance− voltage performance of the polymer-stabilized BPII showed very small hysteresis and fast response times. 4. EXPERIMENTAL SECTION The LC materials for the HNM were an NLC mixture (HTW109100100, HCCH) and a bent-core molecule first synthesized by Weissflog et al.30 A chiral dopant, S811 (Merck), was used to prepare the 8945
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HN*M. In order to prepare the precursor for stabilizing BPII, two types of photopolymerizable monomersTMPTA (Aldrich) and RM257 (Merck)were used, which are generally employed to fabricate polymer-stabilized BPs. The chemical structures of the bentcore molecule, TMPTA, and RM257 used in this work are illustrated in Figure S1 in the Supporting Information. A glass cell filled with the precursor was irradiated with UV light (365 nm) at approximately 400 mW/cm2 for 1200 s at 30 °C to induce polymerization. The constituent fractions of the HNM, HN*M, and prepared precursor are summarized in Table 1. For optical observation, the blended samples infiltrated a sandwich cell (cell gaps 5 μm) composed of two glass substrates coated with a unidirectionally rubbed, polyimide-based alignment layer (AL22620, JSR). Measurements of the optical spectra were performed with a spectrometer (USB2000, Ocean Optics) coupled to a polarized optical microscope equipped with a temperature controller. The Kossel diagram is a diffraction pattern for converging monochromatic light incident upon the BP crystal and provides information about its periodicity and symmetry. Kossel diagrams were observed at λ = 450 nm using a ×100 objective with a numerical aperture of 0.95. To evaluate the electro-optical performance, the polymer-stabilized BPII sample was injected into LC cells (cell gaps 6 μm) made of two ITO glass substrates with rubbed polyimide. A 1 kHz square-wave root-mean-square (rms) voltage was employed to test the samples.8 The reason why we apply the voltage with frequency f = 1 kHz was to compare the results reported in the previous work by Yan et al., which presented the performance of polymer-stabilized BPI.28 The reflectance was normalized to that of the mirror. The response time was measured using a 1 kHz square-wave between 0 and 25 Vrms/μm.
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ASSOCIATED CONTENT
* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01502. Chemical structures of the bent-core molecule, TMPTA, and RM257 used in this work (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Suk-Won Choi: 0000-0001-7160-7801 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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REFERENCES
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ACKNOWLEDGMENTS
This work was supported by the Basic Science Research Program (NRF-2016R1D1A1A09917580) in the Republic of Korea.
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ABBREVIATIONS LC, liquid crystal; N, nematic; N*, chiral nematic; PC, photonic crystal; PBG, photonic band gap; BP, blue phase; DTC, doubletwisted cylinder; 3D, three-dimensional; BPI, body-centeredcubic BP; BPII, simple-cubic BP; UV, ultraviolet; RT, room temperature; POM, polarized optical microscopy; HNM, host N mixture; HN*M, host N* mixture 8946
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ACS Applied Materials & Interfaces (23) Kim, S.; Kim, K.; Jo, S.-Y.; Choi, S.-W. Uniform Alignment of Liquid Crystalline Cubic Blue Phase II via Rubbing Treatment. Mol. Cryst. Liq. Cryst. 2015, 611, 186−191. (24) Kim, K.; Hur, S.-T.; Kim, S.; Jo, S.-Y.; Lee, B. R.; Song, M. H.; Choi, S.-W. A Well-aligned Simple Cubic Blue Phase for a Liquid Crystal Laser. J. Mater. Chem. C 2015, 3, 5383−5388. (25) Lee, M.; Hur, S.-T.; Higuchi, H.; Song, K.; Choi, S.-W.; Kikuchi, H. Liquid Crystalline Blue Phase I observed for a Bent-core Molecule and its Electro-optical Performance. J. Mater. Chem. 2010, 20, 5813− 5816. (26) Hur, S.-T.; Gim, M.-J.; Yoo, H.-J.; Choi, S.-W.; Takezoe, H. Investigation for Correlation between Elastic Constant and Thermal Stability of Liquid Crystalline Blue Phase I. Soft Matter 2011, 7, 8800− 8803. (27) Choi, H.; Higuchi, H.; Ogawa, Y.; Kikuchi, H. Polymerstabilized Supercooled Blue Phase. Appl. Phys. Lett. 2012, 101, 131904. (28) Yan, J.; Wu, S.-T.; Cheng, K.-L.; Shiu, J.-W. A Full-color Reflective Display using Polymer-stabilized Blue Phase Liquid Crystal. Appl. Phys. Lett. 2013, 102, 081102. (29) Fukuda, J.-I.; Yoneya, M.; Yokoyama, H. Simulation of Cholesteric Blue Phases using a Landau−de Gennes theory: Effect of an Applied Electric Field. Phys. Rev. E 2009, 80, 031706. (30) Kovalenko, L.; Schröder, M. W.; Reddy, R. A.; Diele, S.; Pelzl, G.; Weissflog, W. Unusual Mesomorphic Behaviour of New Bent-core Mesogens derived from 4-Cyanoresorcinol. Liq. Cryst. 2005, 32, 857− 865.
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