Electrically Tunable Soft-Solid Block Copolymer Structural Color - ACS

Oct 27, 2015 - One-dimensional photonic crystals based on the periodic stacking of two different dielectric layers have been widely studied, but the f...
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Electrically Tunable Soft-Solid Block Copolymer Structural Color Tae Joon Park, Sun Kak Hwang, Sungmin Park, Sung Hwan Cho, Tae Hyun Park, Beomjin Jeong, Han Sol Kang, Du Yeol Ryu, June Huh, Edwin L. Thomas, and Cheolmin Park ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05234 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on November 2, 2015

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Electrically Tunable Soft-Solid Block Copolymer Structural Color Tae Joon Park,† Sun Kak Hwang,† Sungmin Park,‡ Sung Hwan Cho, † Tae Hyun Park, † Beomjin Jeong, † Han Sol Kang, † Du Yeol Ryu, ‡ June Huh, § Edwin L. Thomas # and Cheolmin Park†,* †

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,

Seoul, 120-749 (Republic of Korea)

‡

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-

gu, Seoul 120-749, Republic of Korea §

Department of Chemical and Biological Engineering, Korea University, Anam-dong, Seongbuk-gu,

Seoul 136-713, Korea #

Department of Materials Science and Nano Engineering, Rice University, Houston, TX , 77005 , USA

*

Corresponding author, E-mail [email protected].

Phone numbers to Cheolmin Park Office phone: +82-2-2123-2833 Fax:+82-2-312-5375

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Abstract 1D photonic crystals based on the periodic stacking of two different dielectric layers have been widely studied, but the fabrication of mechanically flexible polymer structural color (SC) films, with electroactive color switching, remains challenging. Here, we demonstrate free-standing electric field tunable ionic liquid (IL) swollen block copolymer (BCP) films. Placement of a polymer/ionic liquid filmreservoir adjacent to a self-assembled poly(styrene-block-quaternized 2-vinylpyridine) (PS-b-QP2VP) copolymer SC film allowed the development of red (R), green (G) and blue (B) full-color SC block copolymer films by swelling of the QP2VP domains by the ionic liquid associated with water molecules. The IL-polymer/BCP SC film is mechanically flexible with excellent color stability over several days at ambient conditions. The selective swelling of the QP2VP domains could be controlled by both the ratio of the IL to a polymer in the gel-like IL reservoir layer and by an applied voltage in the range of -3 V to +6 V using a metal/IL reservoir/SC film/IL reservoir/metal capacitor type device.

Keywords: Soft-Solid Structural Colored Film, Flexible Photonic Crystals, Block Copolymers, Electrically Switchable Structural Color, Low Operation Voltage

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Structural colors (SCs) of photonic crystals (PCs)1-5 arising from constructive interference of light with a periodic structure of two different dielectric constituents are of great interest due to their potential use in low-power reflective mode displays, e-books and sensors.6-20 PCs based on self-assembled block copolymers (BCP) 21 are additionally beneficial as their periodicities and dielectric constants are readily altered by various external stimuli such as electric,22-26 magnetic, thermal,27-30 solvating31-33 and mechanical forces.34 This allows for facile stimuli-dependent color change, making these PCs suitable for a variety of emerging sensors or display elements. However, a critical drawback of BCP PCs is that the maximum reflection of a BCP PC frequently occurs at UV or higher photon energies due to the relatively small period of normal molecular weight BCPs, typically having domain dimensions of tens of nanometers.35 Visible-color BCP PCs with ultra-high molecular weight and carefully designed molecular architecture require careful processing and have an inherently slow response to external stimuli due to extremely slow molecular kinetics.36-38 Alternatively, a solvent capable of swelling the domains is often employed to increase the domain spacing of a BCP and to impart structural color in the visible range.39 However, the use of a volatile solvent significantly restricts the advantages of polymeric BCP PCs in the use of flexible, foldable and stretchable devices. Recently, by employing a nonvolatile ionic liquid as a swelling solvent, photonic BCP films with colors spanning the visible were constructed using medium molecular weight (78 – 158 kg mol-1) BCPs.40 Most of the recent electrically-tunable PCs based on both BCPs and self-assembled colloids operate in a liquid environment.12-15,22-26 An early demonstration of an electrically switchable SC device utilized colloidal self-assembly followed by cross-linking to form a gel-polymer matrix that could be efficiently tuned by electrically controlled diffusion of a liquid electrolyte.12 The device exhibited reliable multiple color switching with an operating voltage and switching time of 2 V and 15 seconds, respectively. Recent work showed improvement of multiple operation stability of a colloidal SC device by employing an indium tin oxide electrode modified with an ion exchange layer.13 Electrically switchable liquid-type 4 ACS Paragon Plus Environment

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BCP devices have also been demonstrated. SC switching arising from effective swelling and deswelling of block copolymer domains occurred by electrochemical stimulation.22 Furthermore, the detailed switching dynamics of an electrochemical BCP SC cell was examined as a function of the types of anions in electrolytes.23 Fast switching of a liquid-type BCP SC cell was developed due to the rapid charging of quaternized 2vinyl pyridine domains, giving rise to a device operating at a few volts with a sub-second response.25 However, most of the electrically switchable SC devices must be tightly sealed to avoid solvent loss. Thus, it is important to demonstrate an electro-active, highly stable, soft solid-like BCP PC film40 with mechanical flexibility in which the SC of the PC can be readily reversibly switched under a modest electric field. This study demonstrates free-standing BCP PC films with an electrically-switchable structural color. Placement of a reservoir of an electrically tunable swelling agent comprised of a mixture of polymer and ionic liquid41 onto a self-assembled lamellar poly(styrene-block-quaternized 2-vinylpyridine) (PS-bQP2VP) copolymer SC film results in red (R), green (G) and blue (B) full-color SC block copolymer films in which selective swelling of QP2VP domains is controlled by the amount of IL and water in the top reservoir and the applied field. For constant humidity conditions, the IL-polymer/BCP SC film is mechanically flexible with excellent color stability. Furthermore, the film is electro-responsive and allows reproducible facile color switching between red and green over multiple switching cycles when a voltage of ±3 V is applied to the capacitor-type device consisting of a top metal electrode/IL-polymer reservoir/SC BCP film/IL-polymer reservoir/bottom transparent electrode.

Results and Discussion A PS-b-P2VP film was spin-coated from solution in propylene glycol monomethyl ether acetate (PGMEA) and then treated with chloroform vapor for 12 hours to facilitate formation of ordered inplane lamellae of alternating PS and P2VP domains. The subsequent preferential doping of bromoethane to the P2VP domains produced quaternized P2VP units (QP2VP) that allowed for easy access of IL 5 ACS Paragon Plus Environment

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molecules to the positively charged pyridine rings when exposed to an IL-polymer film that was spincoated onto the BCP film as shown in Figure 1a (Supporting Information, S1). The IL-polymer reservoir was comprised of a hygroscopic lithium bis-trifluoromethanesulfonimide (LiTFSI) with a 1:1 ratio of Li+ to TFSI- and poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE). The strong molecular attraction between QP2VP and Li+ as compared to P2VP-Li+TFSI- and PS-Li+TFSIinteraction was also confirmed by density functional theory (DFT) calculation, which suggests that the IL diffuses into the PS-b-QP2VP film more effectively than for the PS-b-P2VP film due to the stronger electrostatic attraction between QP2VP and Li+TFSI-. (Supporting Information, S2) When a blend of LiTFSI/ PVDF-TrFE containing 100 wt% IL with respect to the polymer was spin-coated onto a BCP film using acetonitrile as the spinning solvent, the sample immediately became bluish due to the redshift of the wavelength at maximum reflection of the BCP film as shown in Figure 1b. The color of the two layer IL-polymer/BCP SC film could be further changed from blue, to green to red with an increase in the amount of IL in the blend top film, as shown in the photographs of Figure 1c. The higher the concentration of IL vs the PVDF-TrFE, the more IL diffuses into the QP2VP domains. Visible-range BCP SCs with a IL-polymer top layer were obtained with other polymers in the reservoir such as poly(styrene-block-methyl methacrylate-block-styrene) (PS-b-PMMA-b-PS) and poly(vinylidene fluoride-co-hexafluore propylene) (PVDF-HFP) (Supporting Information, S3). In general, fluorinated copolymers were more useful and PVDF-TrFE was chosen for detailed investigation. The effect of the type of IL with different cations and anions was also examined on the swelling of BCP (Supporting Information, S4). The Li+TFSI- IL was the most successful. The typical reflectance we obtained in our IL-polymer/BCP SC films on glass substrates was approximately 25 %. The rather small reflectance value of our BCP SC film is mainly ascribed to the structural defects of the BCP film. The finite-difference time-domain (FDTD) simulation for the reflectance of BCP layers shows that almost 100 % reflectance can be obtained with perfectly ordered alternating PS/QP2VP-IL in-plane lamellae. (Supporting Information, S5) The efforts have been made to enhance the reflectance 6 ACS Paragon Plus Environment

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of our BCP SC. We have recently observed that a thin polymer film with preferential interaction with QP2VP domains was effective to induce the well-aligned in-plane lamellae, giving rise to the higher reflectance of the BCP film over 35 %. It should be, however, noted that the relatively small contrast is sufficient to distinguish two colored states. A plot of the values of the maximum in the reflectivity as measured by UV-vis spectroscopy versus the weight concentration of the IL to the polymer in the reservoir is shown in Figure 1b. The Q2VP domains swell with increasing IL content in the reservoir and become saturated at high IL concentration, which shows that the structural color can be controlled over the full visible range by choosing the concentration of IL in the reservoir film. It should be noted that all the two layer IL-polymer/BCP SC films were fabricated at a constant humidity of approximately 40 %. Representative R, G and B two layer IL-polymer/BCP SC films were easily fabricated by varying the amount of IL in the reservoir, as shown in Figure 1c (Supporting Information, S6). Facile color tuning of the BCP layer was attributed to the preferential swelling of QP2VP domains by IL molecules readily coordinating with quaternized amines of pyridines of P2VP, as schematically shown in Figure 1a. The FDTD simulation for the reflectance of BCP layers shows that the wavelength at maximum reflection increases linearly with the swelling ratio of QP2VP domains. (Supporting Information, S5) As is well known, the wavelength at the reflectivity maximum is linearly proportional to the thickness of the swelling layer (equivalently linear with the overall period). The width of the reflectivity peak depends on the magnitude of the difference in the index of refraction of the PS domain vs that of the swollen domain. To confirm the increase in domain size due to the preferential absorption of IL molecules, grazing incident small angle X-ray scattering (GISAXS) experiments were performed as shown in Figure 1d. First, we examined the domain structure of the neat PS-b-P2VP copolymer with its number average molecular weight of 81 kg mol− 1 (PS ~ 40 kg mol− 1, P2VP ~ 41kg mol− 1). Multiple out-of-plane (00l) reflections with a qn/q1 ratio of 1, 2, 3 or 4, which arises from the in-plane alternating PS and P2VP lamellae, are visible from a thin PS-b-P2VP film with a lamellae periodicity of approximately 30 nm 7 ACS Paragon Plus Environment

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(dPS = 15 nm, dP2VP = 15 nm) (Supporting Information, S7). Although only the odd orders of reflection should appear for a neat sample of a 50/50 diblock copolymer, our GISAXS results show all the integer reflections. When a blended IL/polymer layer was deposited onto the neat BCP film after quaternization, the set of reflections shifted to the lower q values with increased amount of IL, which confirms the preferential swelling of QP2VP domains, leading to an increase in the periodicity of BCP films, as shown in Figure 1d. It should be noted that first order reflections of the swollen films are near the incident beam at very low q regimes, and are not observed due to parasitic scattering. The inset of Figure 1d shows that the domain spacing increases rapidly with the IL content and approaches an asymptotic value, consistent with the reflectance results in Figure 1c. Since our approach uses nonvolatile IL provided from the IL/polymer reservoir, IL molecules absorbed into the QP2VP domains maintain the SC colors over time at a constant humidity, as shown in Figure 1e. Exposure to higher humidity conditions can also further swell the IL-QP2VP domains due to the hygroscopic properties of the IL, resulting in a red-shift with increased humidity, as shown in Figure 2a.FT-IR spectroscopy revealed that water was present in an IL-polymer/BCP SC film, and the amount of absorbed water at fixed humidity increased with the concentration of IL in the blended layers, as shown in Figure 2b (Supporting Information, S8). The characteristic absorption at approximately 3500 ~ 3700 cm-1 from the vibration of hydroxyl groups in water molecules increased with IL concentration. We also examined a neat BCP film with and without quarternization for comparison. Water absorption in both a PS-b-P2VP and a PS-b-QP2VP film was negligible, compared with the absorption in a BCP film with a 200 wt% IL/polymer reservoir (Supporting Information, S8), which confirms the role of IL in the BCP film. The FT-IR results that while water absorption in both a PS-b-P2VP and a PS-b-QP2VP film was negligible, the significant absorption in a BCP film with a 200 wt% IL/polymer reservoir clearly indicate that IL molecules coordinated in QP2VP domains are pre-requisite for swelling the domains and the water molecules which can be strongly associated with Li+ ions render the swelling of the domains occur more easily. In other words, water itself cannot swell the domains. 8 ACS Paragon Plus Environment

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The enhanced water adsorption in the presence of Li+TFSI- ions was also evidenced by water sorption simulations for model sorbent systems using Grand Canonical Monte Carlo method (the computational details are documented in Experimental section), as shown in Figure 2c and 2d which compares the extent of water adsorption of model sorbent systems for PVP, P2VP/Li+TFSI- and QP2VP/Li+TFSI- at 25 °C and 1 bar. Water absorption was further enhanced when Li+TFSI- molecules are present in QP2VP domains. Figure 2e shows the pair correlation functions between water molecules and various components (Li+, TFSI-, QP2VP+, Br-) in the model system of QP2VP+Br-/Li+TFSI-/water, calculated from NPT molecular dynamics (MD) simulation at 25 °C and 1 bar. The pair correlation functions suggest that the high hygroscopic property of a QP2VP/Li+TFSI- layer is attributed mainly to the water-Li+ attraction, consistent with the previous experimental study in which water uptake in the IL was monitored with a quartz crystal microbalance.42 In the radial distribution functions shown in Figure 2e, the first and second hydration shell radius for a Li+ ion, which is dominantly responsible for highly hygroscopic properties of SC films, are 2.3 and 3.2 Å, respectively. (the first and second water coordination number are 4.27 and 9.34.) Given that the volume of Connolly surface for Li+ ion is ~ 19.5 Å3, this estimations suggest that strongly attracting water-Li+ interactions could cause approximately 7 times volume increase from a neat Li+ ion to fully water-coordinated Li+ ion in principle. The results suggest that the hygroscopic control (domain swelling/shrinking) of BCP PC layer can be possible by switching an electric field that alters the transport direction of Li+ ions between BCP and the IL-polymer reservoir. Our soft solid-type BCP SC films are highly deformable and can readily undergo bending when placed on a flexible substrate, as shown in Figures 3a and 3b. The IL-polymer/BCP SC films prepared on poly(ethylene terephthalate) substrates (thickness, 125 µm) were bendable and maintained their characteristic reflective colors of R, G and B up to a bending radius of approximately 3 mm, as shown in Figure 3a. The BCP SC films are durable even after multiple bending cycles. The wavelength at maximum reflection corresponding to R, G and B was well preserved even after 1000 bending cycles, as 9 ACS Paragon Plus Environment

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shown in Figure 3b (Supporting Information, S9). Furthermore, by employing a conventional reactive ion etching process with a metal mask, we readily fabricated a micropatterned BCP layer. The subsequent deposition of an IL-polymer gel film on the BCP produced a micropatterned SC film, as shown in Figure 3c. When the IL-polymer/BCP SC film was peeled off the substrate, the resulting freestanding film could be easily transferred to another substrate, as shown in Figure 3d. For instance, a BCP SC film was transferred to a patterned electrode for successful integration into an electrically switchable soft solid-type BCP SC device. Since the visible-range of SC of an IL-polymer/BCP film is ascribed to the preferential swelling the QP2VP domains by Li+TFSI- associated with water, control of the reversibility into and out of these charged species by the electric field from the swollen QP2VP domains can enable an electricallyswitchable BCP SC film. An IL/polymer reservoir in contact with the BCP film is important because it supplies and stores ion species injected/extracted from the QP2VP domains by the electric field. In order to create an electrically switchable soft solid SC device, we designed the BCP SC device architecture and a novel capacitor-type device containing the BCP film sandwiched between two IL-polymer reservoir layers is shown in Figure 4a. First, an IL-polymer/BCP two layer film was prepared on a Si substrate, followed by physical transfer of the film onto a glass substrate previously patterned with indium tin oxide (ITO) electrodes as shown in Figure 4a. The IL-polymer/BCP film detached from the Si substrate was placed upside down on the ITO/glass substrate. Another IL-polymer layer was spincoated onto the transferred film, followed by deposition of a patterned top Al electrode by thermal evaporation through a metal mask. The cross-sectional view of the device in Figure 4b shows the BCP SC film located between the upper and lower IL/polymer reservoir layers with top, Al and bottom, ITO electrode layers. A magnified SEM cross section image of the central BCP region shows alternating (dark) PS and (bright) QP2VP lamellae (see inset of Figure 4b). It should be noted that screw dislocations in the lamellae shown in the inset are essential to vertical transport of IL associated with water across the PS layers (Supporting Information, S10).43 10 ACS Paragon Plus Environment

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Specific color reflection arising from the BCP film in the capacitor-type device as schematically shown in Figure 4c was then examined as a function of electric field. In order to further increase the dissociation of IL ion pairs, a poly(vinylidene fluoride−trifluoroethylene−chlorofloroethylene) (PVDFTrFE-CFE) polymer was used instead of PVDF-TrFE. The greater dielectric constant of the PVDFTrFE-CFE polymer (k ~ 30) than that of PVDF-TrFE (k ~ 12) lead to better dissociation of ion pairs, thus, the ionic conductivity is enhanced with the dielectric constant of polymer matrix.44 Note that the periodicity of PS and swollen QP2VP domain prepared with 500 wt% IL-PVDF-TrFE-CFE blend film is approximately 85 nm in the SEM cross-sectional view while the periodicity of the BCP with 200 wt% IL-PVDF-TrFE film was approximately 150 nm (Figure 1d). The smaller periodicity in Figure 4b likely arises due to evaporation of water associated with IL under the vacuum during focused ion beam milling process for the preparation of a cross-sectioned specimen as well as under the SEM vacuum. A blue shift of the maximum reflection wavelength was observed due to the electric field-dependent de-swelling of QP2VP domains, as shown in Figure 5a. The initially orange red-colored tri-layered BCP SC film turned green when a voltage of +3 V was applied to the top electrode and became blue with an applied voltage of +6 V, as shown in the inset of Figure 5a. (Supporting Information, S11) Significantly, the color change as a function of voltage was reversible. For example, a green-colored cell biased with +3 V (Step 2 of Figure 5b) returned to red when the voltage was removed (Step 3 of Figure 5b). Subsequently applying a voltage of -3 V to the top electrode again resulted in a green color, as shown in Step 4 of Figure 5b. The results imply that Li+ ions, water molecules and/or TFSI- ions, are extracted from QP2VP domains to de-swell the QP2VP domain, producing a blue shift in wavelength at maximum reflection. On the contrary, a device with only a top IL/polymer layer did not show any color change with an applied voltage of +3 V on the top electrode, which suggests that TFSI- ions are not responsive to the electric field, and that Li+ ions associated with water molecules are responsible for the color change. We propose that the electric field driven swelling/de-swelling mechanism involves the transport of Li+ ions 11 ACS Paragon Plus Environment

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and water. At zero bias, IL associated with water exists not only in the upper and lower reservoirs but also QP2VP domains of a BCP SC film as shown in the first scheme of Figure 5c. Some of Li+ ions associated with water in the QP2VP domains were drifted to the lower reservoir when +3 V was applied to the top electrode, making the QP2VP domains de-swollen, as shown in the second scheme of Figure 5c. The de-swelling of the QP2VP domains gave rise to the device with green SC. When the voltage was removed, Li+ ions with water which had been drifted to the lower reservoir moved back to the QP2VP domains due to their charge-charge attraction with TFSI- ions, rendering the QP2VP domains swollen as shown in the third scheme of Figure 5c. Next the -3 V applied to the top electrode attracted Li+ ions with water in this case to the upper reservoir, again resulting in the de-swollen QP2VP domains (the last scheme of Figure 5c). The molecular species location and domain thickness variation of the device during the sequential color switching from Step 1 to 4 of Figure 5b is schematically illustrated in Figure 5c. The device architecture with upper and lower IL-polymer layers was the most effective for switchable solid-type BCP SCs. Both reservoir layers effectively provide a source and a storage space for ions injected and extracted from the QP2VP domains by the electric field. In addition, the device configuration enabled the use of both polarities of applied voltage and thus minimized charges accumulating at the interface between the BCP SC and IL-polymer layer. The speed and reproducibility of the SC switching of the device were quantitatively examined with UV-vis spectroscopy. Since color switching of our BCP SC film is based on ion and water diffusion driven by an electric field, the switching time of a device depends strongly on the diffusivity of Li+ ions with water molecules in both BCP and the PVDF-TrFE-CFE matrix. A color change to either green or blue occurred in approximately 30 sec, as shown in Figure 5d. Since the diffusion time is proportional to the distance, faster switching can be obtained with a smaller device. In fact, we fabricated a SC device with its cell dimension of approximately 5 mm2 5 times smaller than the device shown in Figure 5d. The switching of the device occurred in a few seconds. Another evidence of the color switching of our device based on the molecular diffusion of ions can be found with the micropatterned SC film 12 ACS Paragon Plus Environment

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shown in Figure 3c. We observed that the color of the micropatterned film was altered much faster than that of a flat film when both films were exposed to high humidity. To investigate repetitive switching of the device, square-shape multiple voltage trains of 0 to +6 V with 1 min on and 2 min off were applied as shown in Figure 5e. The device exhibited 10 cycles of reliable color switching from red to blue and vice versa without significant variation in the initial transmittance, as shown in Figure 5f. The color switching from red to green also reliably occurred when 0 to +3 V trains were applied (Supporting Information, S12). We observed that reasonable switching can occur more than 20 times above which the ON state transmittance (the low T/T0 value) gradually increased, making the transmittance difference between OFF and ON state smaller. The color of our device switched by an applied voltage stayed at least for an hour upon the electric field. We believe that the degradation of the switching properties may arise from water evaporation and more care should be taken to avoid the water evaporation. Encapsulating the device with a proper resin may be a way. In addition, the fabrication of a smaller cell with the faster switching time can be an appropriate route.

Conclusions In this work, we demonstrated highly flexible, mechanically robust and electrically tunable soft solidtype block copolymer SC films. The pivotal step to realize full color R, G and B SCs of BCPs in a soft solid device was to employ a several micron-thick gel layer containing highly hygroscopic IL molecules that were able to readily swell the quaternized vinyl pyridine chains of a PS-b-QP2VP BCP film. The IL-polymer/BCP SC films were air-stable at ambient and constant humidity conditions for 25 days and mechanically durable even after 1000 bending events at a sharp bending radius. More importantly, a capacitive-type device comprised of a BCP SC film sandwiched between two IL-polymer layers exhibited reliable and repetitive color switching at a few volts. The electrically-switchable, free-standing BCP SC films offer great potential in the realization of solid-type, BCP-based reflective mode pixels potentially integrated in microcircuits for the future development of foldable and stretchable organic 13 ACS Paragon Plus Environment

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electronic sensors and displays. Our electrically switchable soft solid-type SC films can be useful for active humidity sensor where SC color can be controlled not only by electric field but also by humidity.

Experimental Section Material: Poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) block copolymer was synthesized via living anionic polymerization. The number average molecular weight of the PS-b-P2VP block copolymer was 81 kg mol-1 (Φps = 0.5, PDI = 1.04). PVDF-TrFE with 25 wt% TrFE was purchased from MSI Sensors. A

poly(styrene-b-methylmethacrylate-b-styrene)

(PS-b-PMMA-b-PS)

triblock

copolymer

was

purchased from Polymer Source Inc., Canada. The molecular weights of PS and PMMA were 6 and 34 kg mol-1, respectively (PDI = 1.5). Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) was purchased from Aldrich, Korea. The number average molecular weight of PVDF-HFP copolymer was 130 kg mol-1. Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) with a ratio of 59.4:33.4:7.3 was purchased from PIEZOTECH, France. All other materials, including various ionic liquids of lithium tetrafluoroborate, lithium trifluoroacetate, lithium hexafluorophosphate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethylsulfony)imide, and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, were purchased from Aldrich, Korea. Preparations of polymer SC films and a flexible device: Block copolymer SC films were prepared by spin-coating PS-b-P2VP block copolymer solutions (7 wt%, in propylene glycol monomethyl ether acetate) onto a silicon substrate or PET. These prepared films were annealed in chloroform vapor at 50 ° C for 12 hours. P2VP layers were selectively quaternized using 1-bromoethane in hexane at 50 ° C for 12 hours (20 vol% wrt PS-b-P2VP). Free-standing SC polymer films were prepared by spin-coating with IL-polymer solutions. The weight ratio between the polymer, ionic liquid, and solvent was 1:5:9. Flexible devices were fabricated on PET (thickness, 125 µm). Switchable BCP SC device fabrication: Free-standing IL-polymer covered SC BCP films were peeled from the silicon substrate and transferred upside down to pre-patterned, 3-mm-wide strips of transparent 14 ACS Paragon Plus Environment

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indium tin oxide (ITO). Pre-patterned ITO was purchased from Freemteck Inc., South Korea. Top blended IL/polymer layers were prepared by spin-coating the blended IL/polymer solutions at 2000 rpm. 100-nm-thick metal electrodes were thermally deposited using pre-patterned 3-mm-wide strips of shadow Sus-Mask (Thermal Evaporator: MEP 5000, SNTEK Co., Ltd.). Characterization: The nanostructures of the three-layered BCP SC films were analyzed using focused ion beam scanning electron microscopy (FIBSEM) (JIB-4601F). SAXS measurements were performed on a switchable BCP SC device using the PLS-II 3C U-SAXS beamline at the Pohang Accelerator Laboratory. Reflective spectra were recorded using a UV-vis spectrometer (LAMBDA 750, PerkinElmer). Simulations: We briefly summarize the computation methods used as follow. The details of each method were documented in Supporting Information. The binding energies and optimized geometries of the model complex systems, QP2VP-Li+TFSI-, P2VP-Li+TFSI-, and P2VP-Li+TFSI- (Figure S2), were obtained by density functional theory (DFT). The reflectance of the model BCP SC (Figure S4) was computed by the finite difference time domain (FDTD) method. Water sorption calculation for model sorbent systems (P2VP, P2VP/Li+TFSI- and QP2VP/Li+TFSI-) (Figure 2c, 2d) were obtained using Grand Canonical Monte Carlo (GCMC) method and NPT molecular dynamics simulation. The pair correlation functions in in the QP2VP/Li+TFSI-/water system ((Figure 2c, 2d)) were obtained from NPT MD simulation. (Supporting experiments)

Acknowledgements This research was supported by the Samsung Research Funding Center of Samsung Electronics under project number SRFC-MA1301-03

Supporting Information Available: Supporting results are available free of charge via the Internet at http://pubs.acs.org. 15 ACS Paragon Plus Environment

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1976, 46, 157-170.

Figure Legends Figure 1. (a) Schematic illustration of a two layer IL-polymer/BCP SC structure on a silicon substrate. The molecular level structures of a monomer repeat of the PVDF-TrFE polymers, and PS-b-QP2VP are also shown along with the ionic liquid components. (b) A plot of the equilibrium maximum reflectivity wavelength values of IL-polymer/BCP SC films as a function of IL concentration in the top layer. (c) UV-vis spectra showing the red shift in the maximum reflectivity of three swollen films with increasing IL content in the top reservoir layer. Photographs of the swollen films are shown in the inset. (d) GISAXS profiles of the two layer structures after equilibrium swelling with 200 wt%, 300 wt%, and 1200 wt% IL concentration with respect to polymer. The inset shows that the periodicities of lamellae of BCP films with neat, 200wt%, 300wt% and 1200 wt% IL-polymer layers are 30, 150, 178 and 212 nm, respectively. (e) The variation of wavelength values at maximum reflection of the two layer ILpolymer/BCP SC films swelling with different IL concentrations versus time. The films were placed in ambient conditions with the relative humidity of approximately 40 %. Figure 2. (a) Photographs of a two layer IL-polymer/BCP SC film with 500 wt% IL-polymer layer successively exposed to five different relative humidities. (b) The transmittance ratio at 3500 ~ 3700 cm− 1 of two layer IL-polymer/BCP SC films versus IL concentration (normalized with respect to that of a structure swollen with 200 wt% IL content, T200wt% at 3500 ~ 3700 cm− 1 = I200wt% = I0) (c) Water adsorption as a function of Monte Carlo steps for model sorbent systems (P2VP, P2VP/Li+TFSI- and PQVP+Br-/Li+TFSI-) 25 °C and 1 bar and (d) color mapping of high density region of adsorbed water (red region) in the model sorbent layer. (e) Pair correlation function between water molecules and various components (Li+, TFSI-, QP2VP+, Br-) in the model system of QP2VP/Li+TFSI-/water. Figure 3. (a) Photographs of flexible blue (top), green (middle) and red (bottom) colored bi-layered ILpolymer/BCP SC films as a function of bending radius. (b) The variation of wavelength values at 20 ACS Paragon Plus Environment

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maximum reflection of bi-layered IL-polymer/BCP SC films with different IL concentrations as a function of number of bendings at a bending radius of 2.5 mm. (c) Photographs of green colored SC films with BCP layers micropatterned by reactive ion etching with metal masks, followed by the deposition of the IL/polymer reservoir. The squares of the left and right images are 200 x 200 µm2 and 500 x 500 µm2 in area, respectively. (d) Photographs of green colored two layer IL-polymer/BCP SC film peeled off from a substrate and subsequently transferred to another substrate by hand. Figure 4. (a) Schematic illustration for fabricating an electrically switchable capacitor type soft SC device. (b) Cross-sectional SEM image of switchable solid-type BCP SC device. The device was directly milled by focused ion beam to have smooth and clean cross-section. The inset shows a magnified SEM image of BCP layer with alternating in-plane lamellae. (c) Schematics of switchable solid-type BCP SC device comprised of five layers: top Al electrode, BCP SC film sandwiched with top and bottom IL-polymer reservoir layers/an ITO bottom electrode on a glass substrate. The schematics are not to scale. Figure 5. (a) A plot of the maximum reflectivity wavelength as a function of applied voltage for the switchable solid-type BCP SC device. The inset of (a) shows that the color of device with 0, +3 and +6 applied voltage on top electrode are red, green and blue, respectively. Each voltage was applied for 30 seconds. (b) Photographs of switchable solid-type BCP SC device upon sequential application of voltage from 0, +3, 0 and -3 V (top) with 30 second time period for each voltage. (c) Schematic illustration of SC switching mechanism of a soft solid-type BCP SC device. The schematics are not to scale. (d) Evolution of green and blue from red of a switchable device monitored by decrease of characteristic 550 nm and 500 nm with time when +3 V and +6 V were was applied, respectively. (e) The square wave voltage profile. (f) Reversible switching of switchable solid-type BCP SC device from red to blue upon sequential application of voltage shown in (e).

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