Photonic Block Copolymer Films Swollen with an Ionic Liquid

Jun 3, 2014 - Department of Materials Science and Engineering, Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology,. Cambrid...
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Photonic Block Copolymer Films Swollen with an Ionic Liquid Atsushi Noro,*,† Yusuke Tomita,† Yuya Shinohara,‡ Yoshio Sageshima,† Joseph J. Walish,§ Yushu Matsushita,† and Edwin L. Thomas*,§,∥ †

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ‡ Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan § Department of Materials Science and Engineering, Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ∥ Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77251, United States S Supporting Information *

ABSTRACT: Nonvolatile solvent swollen 1D periodic films were fabricated from lamellae-forming block copolymers with medium molecular weight by infiltrating an ionic liquid. A mixture of imidazole and imidazolium bis(trifluoromethanesulfonyl)imide as a room temperature ionic liquid was added after spin-coating of thin films of polystyrene-b-poly(2-vinylpyridine) (PS−P2VP) block copolymers having an approximately 50/50 composition to create photonic films reflecting in the visible regime. Under normal conditions of temperature and humidity, the films maintained their photonic properties for more than 100 days without perceptible change, stemming from the nonvolatility of the ionic liquid. Transmission electron microscopy revealed the selective swelling of the P2VP nanodomains by the IL and ultrasmall angle X-ray scattering measurements provided quantitative nanostructure information on the periodicities of the films. The wavelength of reflected light from photonic films was tunable by using different molecular weight block copolymers as well as by employing blends of two block copolymers. The experimental wavelength of the reflected light, detected by a fiber-optic spectrophotometer, agreed with values estimated from the Bragg condition and was able to be controlled from about 380 to 620 nm.

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mol) BCPs are typically required;37−39 however, such large MW BCPs are not easy to synthesize, which limits the photonic applications of BCPs. A recent approach to attain BCP domains with large spacings utilized comb-type BCPs with large MW (up to 6400 kg/mol), where steric crowding between the comb arms caused the backbone to elongate, leading to formation of large domains.40 Similarly, Grubbs et al. and Rzayev et al. developed high-MW brush BCPs by grafting-through41−45 and grafting-from46,47 procedures, respectively, which also leads to formation of large (greater than 100 nm) interdomain spacings suitable for photonic applications. The responsiveness or dynamic tunability of photonic properties of large-MW brush BCPs should be generally rather slow because of the high viscosity associated with the large total MW.48 Alternatively, the respective nanodomains of BCPs can be swelled by blending low-MW homopolymer,36,37 where the band gap can be readily tuned by varying the amount of homopolymer added. To prepare the photonic films by adding homopolymer, however, requires the homopolymer to be

INTRODUCTION Periodic nanophase-separated structures of block copolymers (BCPs) are promising as high performance functional nanomaterials in a wide variety of applications from nanoporous membranes1−4 to nanoelectronic device templates,5−10 solar cells,11−14 and soft photonic crystals.15−19 BCP morphologies of thin films20−24 have been extensively investigated for applications.25 The principal nanodomain structures of simple A/B diblock copolymers include spheres, cylinders, gyroid, and lamellae, depending on molecular characteristics such as composition,26 molecular weight,27,28 interaction parameter,29,30 polydispersity,31 etc. Photonic crystals made from nanophase-separated BCPs are an attractive application of molecular soft nanomaterials. Photonic crystals are periodic structures composed of components with different refractive indices.32−34 The simplest photonic crystal is a one-dimensional stack with alternating layers of differing index of refraction. The constructive and destructive interference due to light reflecting off the interfaces between the layers gives rise to a partial photonic band gap (stop band) depending on the relative layer thicknesses and respective refractive indices.35,36 To employ nanophaseseparated BCPs as one-dimensional photonic crystals for visible light or NIR, large molecular weight (Mn > 300 kg/ © 2014 American Chemical Society

Received: March 10, 2014 Revised: May 21, 2014 Published: June 3, 2014 4103

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dissolved along with the BCP in a volatile common solvent(s), typically followed by the solvent casting or spin-coating. Swelling the film with homopolymer after film formation is not possible since the homopolymer is usually in the solid state or at least very viscous liquid state (e.g., polydimethylsiloxane), which cannot be easily imbibed into the films. The addition of homopolymer into the BCP−solvent solution can also cause structural phase transformations during the subsequent selfassembly, so the amount of homopolymer that can be added to a lamellar BCP is limited. Another approach to achieving large domain spacings is to swell the respective layer of nanostructures of BCPs with either a nonpreferential or preferential solvent. Systems explored include polystyrene-bpoly(ethylene−propylene copolymer) with the neutral solvent dioctyl phthalate15 and polystyrene-b-quaternized poly(2-vinylpyridine) (PS-b-QP2VP), swollen with water by creating a highly responsive polyelectrolyte (PE) block via quaternization (e.g., iodomethane treatment of P2VP to give a QP2VP block) that can be preferentially swollen by water.49,50 Especially for the latter case, use of the medium molecular weight (∼50−100 kg/mol per block) BCPs may be sufficient for photonic film preparation since the strong affinity of a polyelectrolyte block for solvents provides for very large, upward of 600% swelling of the PE layers while the glassy PS layers maintain the lamellar morphology even for highly asymmetric compositions. A major limitation of the solvents thus far used in creating photonic films is their volatility, wherein the photonic features are lost as the solvent evaporates and deswelling occurs. Here we report fabrication of BCP photonic films using a nonvolatile ionic liquid,51−54 which selectively swells one component of a lamellar BCP. Recently, imidazolium bis(trifluoromethanesulfonyl)imide (ImTFSI), the 1:1 mixture of imidazole and bis(trifluoromethanesulfonyl)imide (HTFSI),55 was used as a nonvolatile ionic liquid for phase behavior studies of PS−P2VP/ImTFSI, where ImTFSI preferentially swells the P2VP block.56−60 Since the 1:1 ImTFSI mixture is a solid at room temperature with the melting point around 70 °C, we used a 7:3 mixture of imidazole and HTFSI, which results in a room temperature liquid of neural imidazole and ImTFSI.61 The schematic illustration of fabrication of a BCP photonic film is depicted in Figure 1. Photonic features under normal conditions of temperature and humidity were investigated by reflectivity measurements. Tunability of the stop band was demonstrated by choice of BCP molecular weight and with finer control by the simple blending of two PS−P2VP copolymers having different molecular weights.62 The nanostructures of the ionic liquid-added films could be directly examined by transmission electron microscopy due to the nonvolatile nature of the IL solvent. Ultrasmall angle X-ray scattering provided quantitative measure of domain periodicities of the films.

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Figure 1. Schematic of the fabrication of a nonvolatile IL photonic film from a lamellae-forming BCP film. The red domains are PS, the blue domains are P2VP, and the light blue domains are the IL swollen P2VP layers. Ionic liquids are composed of blue circle cations and light green triangle anions, both of which disperse in the P2VP domains. After spin-coating and annealing the PS−P2VP film, the ionic liquid preferentially swells the P2VP layers, which causes reflection of light with a specific wavelength (blue light in this cartoon) depending on the overall structural periodicity and the refractive indices of components.

Table 1. Molecular Characteristics of Neat PS−P2VP Copolymers code e

SV-1 SV-2f SV-3e

Mn(PS)a

Mnb

ϕPSc

PDId

38 000 55 000 77 000

78 000 105 000 158 000

0.50 0.54 0.50

1.07 1.05 1.14

a

Average molecular weight of a precursor PS block determined by GPC. bNumber-average molecular weight of a BCP calculated by using a molecular weight of precursor PS from GPC and the molar composition of a BCP from 1H NMR. cVolume fraction of PS blocks calculated by using molar composition of a BCP from 1H NMR and the room temperature bulk densities of component polymers, i.e., 1.05 for PS and 1.14 for P2VP. dPolydispersity index determined by GPC. Calibration was done by using polystyrene standards. eAnionically synthesized under high vacuum. fProvided by Polymer Source Inc. be noted that all three PS−P2VP samples have nearly symmetric composition (ϕPS ∼ 0.5), which assures formation of lamellar nanophase-separated structures for each PS−P2VP BCP as well as for their blends.63,64 Because the PS layers are glassy, the IL swollen structures also remain as 1D periodic layer structures.65 A liquid mixture composed of neural imidazole and imidazolium cation/bis(trifluoromethanesulfonyl)imide anion, which was termed hereafter as IL, was made for preparation of the photonic films by mixing imidazole and bis(trifluoromethanesulfonyl)imide (HTFSI) at the molar ratio of 7:3 under an inert atmosphere in dichloromethane, followed by complete evaporation of dichloromethane. A molar ratio of 7:3 provides a liquid at room temperature whereas a 1:1 mixture is a solid.61 Note 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI), while a very common ionic liquid, did not provide sufficient swelling, even though EMImTFSI dissolves P2VP to some degree. This is probably because EMImTFSI is an aprotic ionic liquid, which does not have an attractive interaction with P2VP, while the IL, composed of imidazole and HTFSI, is a protic ionic liquid, likely providing an attractive hydrogen bonding interaction in the presence of P2VP. Photonic Film Preparation. PS−P2VP thin films were prepared on glass or polyimide substrates by spin-coating of approximately 5 wt % polymer dioxane solutions at 500 rpm for 60 s, followed by solvent vapor annealing with a tetrahydrofuran (THF)/chloroform (1/1 by vol) mixture for 24 h. By depositing several drops of IL onto the vapor-annealed PS−P2VP films, followed by heating the films to 40 °C

EXPERIMENTAL SECTION

Materials. Polystyrene-b-poly(2-vinylpyridine) (PS−P2VP) BCPs were synthesized via sequential two-step living anionic polymerization in tetrahydrofuran at −78 °C.28 To facilitate the characterization of PS−P2VP, small amounts of living PS solutions were separated as a precursor from the main fraction of solutions in the course of polymerization. Molecular weights of PS precursors were determined by gel permeation chromatography (GPC). Polydispersity of PS− P2VP was also measured by GPC. The composition of PS in PS− P2VP was analyzed by a nuclear magnetic resonance (NMR) spectrometer. Three PS−P2VP BCPs were coded as SV-1, SV-2, and SV-3. Table 1 summarizes the molecular characteristics. It should 4104

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for 120 min, films reflective for UV/vis light (Figure 2) were fabricated, where the reflective properties were maintained for more

Figure 2. Normal incidence optical images of nonvolatile BCP photonic films: (a) SV-1/IL, (b) SV-2/IL, and (c)SV-3/IL. than 100 days without perceptible change. Note that P2VP homopolymer dissolves in the IL while PS homopolymer is insoluble. IL-containing photonic films are denoted as SV-X/IL or (SV-X:SV-Y)/ IL for blends (X or Y = 1, 2, and 3). Measurements. Ultrathin sections with a thickness of approximately 100 nm were prepared by using a Leica Ultracut UCT microtome from thin films of specimens on polyimide film substrate which were embedded in epoxy resin. The thin sections of neat PS− P2VP and PS−P2VP/IL were subsequently exposed to iodine vapor in order to selectively stain the P2VP layers. Transmission electron micrographs were taken of these sections using a JEM-1400 at an acceleration voltage of 120 kV. Ultrasmall angle X-ray scattering from the edge of thin film specimens on a polyimide film substrate was conducted in conditions of a camera length of 4.01 m and an X-ray wavelength of 0.150 nm by using the instrument installed in the BL40B2, SPring-8, Japan. Reflectivity spectra were measured at room temperature on a fiber-optic spectrophotometer (QE-65000, Ocean Optics) combined with a light source (DHL-2000-BAL, Ocean Optics) in reflection mode.

Figure 3. TEM images of neat BCP thin films and nonvolatile ILswollen BCP photonic films: (a) neat SV-1, (b) SV-1/IL, (c) neat SV3, and (d) SV-3/IL. All scale bars are 100 nm. Note the neat BCPs exhibit approximately equal dark/light layer thickness and uniform contrast across the domains, whereas the IL swollen BCP films display thin light regions (similar to the PS layers in the neat BCPs) but the thicker layers have variable darkening with the midlayer regions appearing brighter due to the lower P2VP segment density.

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Table 2. Estimated Domain Periodicity and Thickness of Each Component Layer

RESULTS AND DISCUSSION Transmission electron microscopy (TEM) was carried out on microtomed sections of the IL containing films to understand the detailed microstructure. Two samples, SV-1/IL and SV-3/ IL, were used as a representative for TEM observation. A polyimide film was employed as the substrate instead of a glass substrate to enable microtoming. Neat PS−P2VP films were also observed by TEM for comparison. It should be noted that the nonvolatile nature of the IL enables high resolution and direct observation of the films under the vacuum at room temperature in the TEM.66−68 After microtomy, the thin sections of neat PS−P2VP and PS−P2VP/IL were subsequently exposed to iodine vapor in order to selectively stain the P2VP layers; hence, the P2VP layers should appear both darker and much thicker than the PS layers. TEM observations revealed the neat films of SV-1 and SV-3 are composed of nearly symmetric lamellar structures composed of two microphases (Figure 3), consistent with the expected equilibrium morphologies of the BCPs. Based on TEM images, the estimated average domain periodicity distance (D), the average thickness of a PS layer (LPS), and the average thickness of a P2VP layer (LP2VP) are 33, 16, and 17 nm for neat SV-1, respectively, whereas SV-3 was estimated to have D of 51 nm, LPS of 24 nm, and LP2VP of 27 nm (see Table 2). TEM images of SV/IL films are also displayed in Figure 3. As previously mentioned, the morphology remained lamellar since the PS layers lock in the layered morphology originating from the nanostructure of neat PS−P2VP.65 Though a similar phenomenon of selective swelling of PI with volatile tetradecane in a PS−PI lamellar structure was directly observed by cryo-TEM, the nonvolatile nature of the IL in this study enables clearer and easier observation of images with conventional TEM as well as easier preparation of specimens.69 The lamellar periodicity D in

sample code SV-1 SV-1 SV-1/IL SV-1/IL SV-2 SV-2/IL SV-3 SV-3 SV-3/IL SV-3/IL

measurement

domain periodicity, D/nm

PS layer thickness, LPS/nm

P2VP or swollen layer thickness, LP2VP/nm

TEM U-SAXS TEM U-SAXS U-SAXS U-SAXS TEM U-SAXS TEM U-SAXS

33 37 106 137 43 153 51 57 164 202

16 18.5 18 18.5 23 23 24 28.5 31 28.5

17 18.5 88 118.5 20 130 27 28.5 133 173.5

the SV/IL films is much larger than for neat SV films (for example, 33 nm for neat SV-1 vs 106 nm for SV-1/IL). It should also be noted that the lamellar layers are now highly asymmetric in thickness and exhibit larger nanodomains for the darker phase, as anticipated for the IL swollen P2VP blocks, yielding a metastable nonequilibrium lamellar structure. This selective swelling at the nanometer scale without a morphological transition is due to the pinning of the lamellar morphology by the edge and screw dislocations that together create a PS sheet network that prevents total micellization of the lamellar layers,65 unlike the equilibrium morphological transitions that occur for micelles caused by addition of a selective solvent for one block.70−73 The TEM results, the dissolution of P2VP homopolymer in the IL, and the insolubility of PS homopolymer in the IL all point to the P2VP nanodomains in the BCP are selectively swollen with IL. As will be evident in the next section, the values for the P2VP/ IL layer thicknesses measured from the TEM images are 4105

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Figure 4. U-SAXS profiles of neat BCP films and BCP/IL photonic films: (a) neat SV-1 (top) and SV-1/IL (bottom); (b) neat SV-2 (top) and SV2/IL (bottom); (c) neat SV-3 (top) and SV-3/IL (bottom). The periodicity is 37 nm for neat SV-1, 137 nm for SV-1/IL, 43 nm for SV-2, 153 nm for SV-2/IL, 57 nm for SV-3, and 202 nm for SV-3/IL.

ionic solvents in P2VP domains. These multiple integer order peaks at the lower q region correspond to lamellar structures with larger D and a composition different from 50/50 with D calculated as 137 nm for SV-1/IL, 153 nm for SV-2/IL, and 202 nm for SV-3/IL. The swelling ratios of the P2VP layers by the addition of the IL are 6.6×, 6.5×, and 6.1× for the SV-1, SV-2, and SV-3 samples as determined by the U-SAXS experiments and composition of BCPs (see also Table 2). It should also be noted that respective total film thickness of SV-1 and SV-1/IL was estimated to be 2.3 and 7.9 μm by SEM (Figure S5); therefore, a SV-1 film swelled 7.9/2.3 (= 3.4) times. This roughly agrees with nanoscopic observations by TEM (106/33 = 3.2 times) and SAXS (137/37 = 3.7 times). Photonic properties of SV/IL films were investigated using a fiber-optic spectrophotometer. Incident light was directed normal to the SV/IL films, and the specular reflected light was detected. Because the IL is nonvolatile, the measurements are much easier than for volatile solvent-swollen photonic films which, in general, possess time-dependent photonic properties unless placed in an enclosure to prevent solvent loss. A reflectivity spectrum of SV-1/IL film on a glass slide is shown in Figure 5a. A peak occurs at 379 nm, corresponding to the purple color of SV-1/IL (Figure 2a). The low intensities in the under-300 nm portion of the spectrum are due to the strong absorption of PS, P2VP, and IL in the UV regime. (Figure S6 in the Supporting Information provides absorbance spectra of the components on a quartz substrate.) A peak can also be seen at 469 nm on the reflectivity spectrum of SV-2/IL (Figure 5b), which is also consistent with the sky blue color of the film (Figure 2b). The reflectivity spectrum of SV-3/IL shows peaks at 618 and 317 nm (Figure 5c). The peak at the higher wavelength agrees with the orange-red appearance of the SV-3/ IL film (Figure 2c). The two peaks are at a ratio of approximately 1/2, indicating Bragg reflection from the multiple layer film. It should also be noted that the first stop band peak position of visible light reflection from these photonic films systematically increases with increase in the molecular weight of BCP, indicating the peak position is tunable by varying the BCP molecular weight at photonic film preparation. According to the Bragg condition for a simple, 1D periodic two-component, dielectric stack, the following equation holds35

somewhat smaller than those derived from U-SAXS measurements (see Table 2). This is likely due to a combination of microtome deformation, iodine staining, and electron beam damage to the TEM sections. The variation of iodine staining (darkness) across the P2VP/IL layers with darker regions at the PS interface and a brighter midplane region of the layer is quite interesting. This effect corresponds to the lower P2VP chain segment density (and hence lower iodine staining) toward the midlayer due to the very large (>600% swelling of the layers). Prior work with swelling and freezing via sol−gel conversion of the swollen layers in a PS-b-P2VP system indicated a low P2VP segment density toward the midplane of the swollen P2VP layers by cross-sectional SEM imaging but suffers from void formation and fracture of the sol−gel during SEM sample preparation.74 Quantitative estimates of swelling for SV/IL films were also carried out using ultrasmall-angle X-ray scattering (U-SAXS). U-SAXS profiles of the neat SV and the SV/IL samples are presented in Figure 4. Samples SV-1 and SV-3 display U-SAXS spectra with Bragg peaks at the relative scattering vector (q) of 1 and 3, suggesting a structure composed of alternating layers with almost equal layer thickness (for precisely equal thickness, only odd order peaks 001, 003, ... occur, while the even order peaks are forbidden), which is consistent with the molecular characteristics of both neat BCPs (ϕPS = 0.5) and the TEM results. On the other hand, a U-SAXS profile of SV-2 displays three Bragg peaks with a small peak at the relative q of 2, indicating an off-symmetric structure, consistent with the composition (ϕPS = 0.54). The lamellar periodicity was calculated as 37 nm for SV-1, 43 nm for SV-2, and 57 nm for SV-3 using D = q1/2π, where q1 represents the scattering vector of the first peak based on assuming integer peak ratios for all the peaks. D of SV-2 and SV-3 are larger than that of SV1 due to the larger molecular weight, since the periodicity of neat PS-P2VP scales28 with molecular weight as D ∼ Mn0.64. The measured periodicity ratios, D(SV-2)/D(SV-1) and D(SV3)/D(SV-1), are 43 nm/37 nm = 1.16 and 57 nm/37 nm = 1.54, respectively, quite close to the ratios of 1.20 and 1.57 calculated based on a 0.64 power scaling of the respective molecular weights. As expected from TEM images in Figure 3, the scattering profiles of SV-1/IL, SV-2/IL, and SV-3/IL are much different from those of the neat BCPs with many peaks at the lower q region, and once again the relative q positions are integer multiples, which might be induced by improved longer range ordering of the nanostructures resulting from stronger segregation between phases because of the incorporation of

mλ = 2(n1d1 + n2d 2)

where λ is the wavelength of reflected light, m is an integer number, ni is the refractive index of component i, di is the 4106

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transfer matrix method (TMM) modeling of the films are presented in Figure 5, where a structure composed of 50 periods of alternating layers with the above estimated values of d1, d2, n1, and n2 was assumed. As can be seen in the TMM calculation in Figure 5 (dashed lines), the calculated peak position(s) are 399 nm for SV-1/IL, 447 nm for SV-2/IL, and 589 and 296 nm for SV-3/IL. These peak position values are similar to those provided by calculation based on the simple Bragg condition. The small differences between the calculated values by TMM, the Bragg equation, and those of the experimental spectra likely originate from slightly different sample swelling properties due to variable defect densities of edge and screw dislocations65 in the samples used for the USAXS experiments and for the reflectivity measurements. Precise tunability of the stop band is also straightforward by solution blending two BCPs, spin-coating, annealing, and then adding the ionic liquid. Figure 6 shows reflectivity vs

Figure 5. Experimental and TMM calculated reflectivity spectra of (a) SV-1/IL, (b) SV-2/IL, and (c) SV-3/IL. The solid lines represent experimental spectra, whereas the dashed lines represent calculated spectra based on TMM modeling. The wavelength of the peak position on the experimental spectrum for SV-1/IL is 379 nm, the experimental peak position for SV-2/IL is 469 nm, and the experimental peak positions for SV-3/IL are 618 and 317 nm. The peak positions based on TMM are 399 nm for SV-1/IL, 447 nm for SV-2/IL, and 589 and 296 nm for SV-3/IL.

Figure 6. (a) Normal incidence optical images and (b) experimental reflectivity spectra of IL-swollen photonic films of blends of SV-1 and SV-3. Both images and spectra are displayed in the order of the amount of SV-3 added from left to right, i.e., SV-1/IL, 3:1 (SV-1:SV3)/IL, 1:1 (SV-1:SV-3)/IL, 1:3 (SV-1:SV-3)/IL, and SV-3/IL.

thickness of component i, and the subscripts (i) represent the layer components. The refractive indices of PS and P2VP at λ = 589 nm are 1.59 and 1.62, respectively, while the refractive index of the IL was determined to be 1.443 using an Abbe refractometer at λ = 589 nm. On the basis of U-SAXS results and the composition, the size of nonswollen PS nanodomain and that of swollen P2VP nanodomain in SV-1/IL are calculated to be 18.5 nm (corresponding to d1) and 118.5 nm (corresponding to d2), respectively, assuming that only the P2VP nanodomains are swollen by the IL and that the thickness of the glassy PS nanodomains is the same in the neat and IL swollen films. The TEM images and the insolubility/ solubility of the respective homopolymers of PS and P2VP for the IL support the above assumption. The refractive index of the swollen (6.6×, 6.5×, and 6.1×) P2VP layers corresponding to n2 was estimated to be 1.45 for all three P2VP/IL layers using the Bruggemann effective medium approximation75 and the refractive indices of the neat P2VP and neat IL. Using these parameters gave a calculated reflectivity peak at λ = 402 nm for SV-1/IL if m is unity. Similarly, d1, d2, and n2 were estimated to be 23 nm, 130 nm, and 1.45 for SV-2/IL, resulting in a calculated peak at λ = 450 nm for m = 1. Also, d1, d2, and n2 for SV-3/IL are calculated to be 28.5 nm, 173.5 nm, and 1.45, leading to peaks at λ = 594 and 297 nm for m = 1 and 2 (see Table S1 in the Supporting Information.). In addition to the above estimation using the Bragg condition, results from

wavelength for SV-1/IL, SV-3/IL, and 3 intermediate blends at weight ratios of 3:1, 1:1, and 1:3 along with images of the film color at normal incidence. The peak reflectivities range from 379 nm (SV-1/IL) to 442 nm at 3:1 (SV-1:SV-3)/IL to 503 nm for 1:1 (SV-1:SV-3)/IL to 558 nm for 1:3 (SV-1:SV3)/IL to 618 nm for SV-3/IL.

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CONCLUSIONS One dimensional periodic nonvolatile photonic films were fabricated from lamellae-forming PS−P2VP BCPs using an ionic liquid. Anionically synthesized PS−P2VP (SV) samples, with ϕPS ∼ 0.5 were spin-coated onto either glass or polyimide substrates, followed by solvent vapor annealing. By placing a drop of a 7:3 liquid mixture of imidazole and bis(trifluoromethanesulfonyl)imide (HTFSI), the films reflected a particular band of visibile light dependent on the degree of swelling and the block copolymer molecular weight for long periods (>100 days) under normal conditions of temperature and humidity, originating from the nonvolatility of the solvent. Transmission electron microscopy imaging revealed that only P2VP nanodomains in the neat films were swollen with the nonvolatile IL, consistent with the solubility tests of PS and P2VP homopolymers with the IL. U-SAXS provided 4107

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quantitative nanostructural information for the neat SV and SV/IL and established that the P2VP layers swelled approximately 6 times. UV/vis reflectivity measurements of specular reflection from the SV/IL films revealed the colors of SV/IL films corresponded to a peak wavelength consistent with that calculated based on the sample structural parameters and the Bragg condition for peak reflectivity. The wavelength of the reflected light can be tuned by using different molecular weight BCPs and making BCP-BCP/IL blends. Fabrication of nonvolatile, photonic materials from common BCPs with medium-size molecular weights also offers good potential for future mechano-, electro-, and thermochromic applications.

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra of PS−P2VP and the IL, DSC thermogram of the IL, FE-SEM images of neat and swollen thin films, UV−vis spectra of polymers or the IL on a glass substrate, plot of wavelength vs SV-3 fraction for blend photonic films, TMM modeling results based on U-SAXS data and composition of BCP, and fitting for reflectivity spectra with TMM modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.N.). *E-mail: [email protected] (E.L.T.). Notes

The authors declare the following competing financial interest(s): Patent pending (#JP2013-101409).

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ACKNOWLEDGMENTS This work was financially supported through KAKENHI grant no. 24685035 (A.N.), no. 25620172 (A.N), and no. 25248048 (Y.M.) by JSPS, Japan. The work was also supported by the US Air Force through AOARD FA238611-1-4095 (E.L.T.). A.N. thanks the financial support of JSPS Institutional Program for Young Researcher Overseas Visits to Massachusetts Institute of Technology from Nagoya University. The authors also thank the Program for Leading Graduate Schools at Nagoya University entitled “Integrate Graduate Education and Research Program in Green Natural Sciences” at Nagoya University. Use of synchrotron X-ray source was supported by SPring-8, JASRI, Japan (no. 2013B1432 (A.N.)). The authors also thank Dr. Noboru Ohta for his kind support on U-SAXS measurements at SPring-8.

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