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Enthalpy-Driven Swelling of Photonic Block Polymer Films Atsushi Noro,*,† Yusuke Tomita,† Yushu Matsushita,† and Edwin L. Thomas‡ †

Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ‡ Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77251, United States S Supporting Information *

ABSTRACT: Nonvolatile, soft photonic films that reflect UV/vis light were prepared by enthalpy-driven swelling of lamellar-forming polystyrene-b-poly(2-vinylpyridine) (PS−P2VP) block copolymer thin films with a neat protic solvent. These films are very sensitive to further swelling with the addition of a small amount of acid. Transmission electron microscopy and ultrasmall-angle X-ray scattering of the films before and after the addition of the neat protic solvent revealed selective swelling of the P2VP phase while maintaining the lamellar morphology due to the presence of the layered glassy PS domains. P2VP is swollen due to the hydrogen bonding between a hydroxy group of a protic solvent and the pyridyl group of P2VP. The interdomain distance of the neat PS−P2VP film as measured by U-SAXS increased by about 200% and the films acted as a 1D photonic crystal reflecting UV light. Moreover, by exposing the neat PS−P2VP films to a mixture of a protic solvent and a small amount of sulfonic acid that can protonate the pyridyl groups of the P2VP block, the degree of swelling, therefore the interdomain distance and the wavelength of the reflection light, became significantly larger, resulting in color variations across the visible spectrum and suggesting that such a nonvolatile material system can be a sensor of the acid concentration in the millimolar regime for anhydrous solutions.

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INTRODUCTION Block copolymers are polymers with mutually incompatible polymer components connected to each other by covalent bonds, which spontaneously form periodic structures with typical dimensions on the size of 10−100 nm, i.e., nanophaseseparated structures, due to the repulsion between incompatible components.1 The interdomain distance of the nanophaseseparated structure varies with the molecular weight of the block copolymer,2,3 and various morphologies such as lamellae, double gyroid, cylinders, and spheres are formed depending on the relative volume fraction of components.4−6 Such nanophase-separated block copolymer structures can be used for many applications such as porous membranes,7−9 electronic device templates,10−13 and solar cells.14,15 Another application is fabrication of a self-assembled macromolecule-based photonic crystal.16−23 A photonic crystal is a structural array of two or more materials with different refractive indices arranged periodically.24−27 The simplest photonic crystal is a one-dimensional photonic crystal, also referred to as an optical multilayer stack.28 In accordance with the thickness di and the refractive index ni of the layers, incident light with a specific wavelength λ from the direction perpendicular to the film is reflected by the structure, where λ = 2(n1d1 + n2d2) for the alternating multilayer stack of components 1 and 2. Many applications in the field of optics such as mirrors, sensors, displays, laser devices, etc., are possible. © XXXX American Chemical Society

Since the refractive index of a typical organic material, in general, is approximately 1.5, the necessary interdomain distance D = d1 + d2 (or periodicity distance) is at least 130 nm for a one-dimensional photonic crystal with a lamellar structure formed by a block copolymer composed of components 1 and 2, reflecting the visible light in the violet region of the spectrum (λ ∼ 390 nm). On the other hand, D of a block copolymer with a volume ratio of 1:1 forming a lamellar structure is proportional to about 2/3 power of the numberaverage molecular weight Mn.2,3,29,30 Accordingly, a Mn of at least 400 000 g/mol is necessary to attain a D of 130 nm. However, the synthesis of such high molecular weight block copolymers is challenging but has been accomplished in a few studies.31,32 Recently, in order to attain the required large interdomain distances of nanophase-separated structures, comb-type block copolymers with large molecular weight have been used, where steric crowding between the comb arms causes strong elongation of the backbone chain.33−39 Alternative ways to attain the large interdomain distance from a typical molecular weight polymer having a total Mn of less than say 100 000 g/ mol are to swell the one or both of the respective nanodomains of a block copolymer by blending homopolymers,40−43 Received: August 26, 2016 Revised: November 9, 2016

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DOI: 10.1021/acs.macromol.6b01867 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules inorganic fillers,44−47 or especially solvents.48−52 Swelling a lamellar structure with a selective solvent for one component is a useful and easy way for attaining large interdomain distance since it does not cause undesired phase transformations if the other layered domain is glassy and maintains the lamellar morphology but now with a highly asymmetric composition.53 Polystyrene-b-poly(2-vinylpyridine) (PS−P2VP)3,54−56 is one of the most useful block copolymer systems for preparation of responsive photonic crystal films (hereafter simply referred to as photonic films) swollen with solvents. There are many studies concerning photonic films of such block copolymers.57−68 Use of the medium molecular weight block copolymers is sufficient for preparation of photonic films for visible light reflection using the solvent-swelling method. For example, swelling of the poly(2-vinylpyridine) (P2VP) phase can occur up to approximately 5 times, such that the corresponding lamellar period increases by a factor of ∼3× that of the neat block copolymer film. Although this solventswelling method is very useful, in order to use such films as for example, pressure sensors, there is the requirement to encapsulate the system to prevent solvent evaporation that causes deswelling of the films and loss of the photonic properties.52 Recently, we reported on the fabrication of nonvolatile photonic films that reflect light in the near-ultraviolet to nearinfrared range obtained by immersing the thin films of PS− P2VP block copolymer forming a lamellar nanophase-separated structure into a nonvolatile protic ionic liquid (IL),69 where the P2VP blocks are selectively dissolved in the IL.70−75 Here we define “nonvolatile character” as having a vapor pressure of 1 Pa or less at normal temperature and pressure. Selective swelling of the P2VP phase by IL could be directly observed by transmission electron microscopy (TEM) by imaging the lamellar domains both before and after the IL addition since the solvent is stable in a high vacuum of the TEM. The reflectance spectra corresponding to the appearance of the film with regard to color were also acquired. It has also been revealed that the photonic reflection can be further controlled by utilizing block copolymer blends. Note that the nonvolatility of the photonic film due to addition of a nonvolatile solvent is very useful for material applications. Here we focused on the features of our nonvolatile photonic film fabrication method, i.e., on the interaction between the block chain and the solvent. As mentioned in the previous report, in the case of adding a protic ionic liquid to a PS−P2VP film, it is anticipated that hydrogen bonds are formed between the pyridyl groups in a P2VP block and the hydrogens of the protic ionic liquid, and the hydrogen bonding serve as a driving force to promote the penetration of the protic ionic liquid into the P2VP phase to thereby intensively swell the P2VP phase and express photonic properties for the multilayer BCP for visible light. Therefore, if we could design a pair of polymer and solvent where the interaction is still stronger than the abovementioned interaction, it would be possible to achieve even greater penetration and swelling, which would also lead to the further control of the range of reflected wavelengths by adding the swelling inducer that generates stronger interactions between a swollen polymer and a solvent (Figure 1). Thus, in this study, by immersing the PS−P2VP thin film into a nonvolatile protic liquid containing a nonvolatile acid to partially protonate or ionize the pyridyl groups of P2VP, we successfully fabricated nonvolatile, anhydrous photonic films with a larger degree of swelling than those obtained by just

Figure 1. Schematic of the fabrication of nonvolatile photonic films highly swollen via a combination of a protic solvent and a small amount of acid. Note protic solvents and acids used in this study are almost nonvolatile; therefore, the swollen photonic films can keep the photonic properties for long times.

immersion in a neat protic solvent without nonvolatile acid. The photonic properties were maintained for long times due to the nonvolatile and protic nature of the solvent and the acid. Furthermore, we can conveniently control the degree of swelling and the optical properties by varying the concentration of the nonvolatile acid.

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EXPERIMENTAL SECTION

Materials. Polystyrene−poly(2-vinylpyridine) (PS−P2VP) diblock copolymer was synthesized by living anionic polymerization.3,55 The molecular weight of the polystyrene (PS) precursor and the molecular weight distribution of PS−P2VP were determined by GPC, and 1H NMR was used for the determination of the volume fraction of PS. The molecular weight Mn of PS−P2VP was 121 000 (70 500 and 50 500 for PS and P2VP, respectively), the PS volume fraction ϕPS was 0.60, and the dispersity Mw/Mn was 1.07 (see also Figures S1 and S2 in the Supporting Information). This PS−P2VP forms a lamellar structure in bulk. The nonvolatile acid added to the solvent was a disulfonic acid: 3,3′-(propane-1,3-diylbis(oxy))bis(propane-1-sulfonic acid) (hereafter this acid is referred to as SA). The synthesis was carried out by reacting 1,3-propanediol and propane sultone in a molar ratio of 1:2 in dioxane at 80 °C for 12 h (see previous literature76,77 and Scheme S1 in the Supporting Information). The solvents used for swelling were tetraethylene glycol (TEG), 1,5-pentanediol (PDO), and sulfolane (SL) (see also chemical structures of these solvents in Figure S3). All these species have negligible vapor pressure at room temperature and were purchased from TCI. The protic ionic liquid78,79 1-ethylimidazolium bis(trifluoromethanesulfonyl)imidide (EImTFSI) was synthesized by mixing 1-ethylimidazole and bis(trifluoromethanesulfonyl)imidide.80,81 The aprotic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imidide (EMImTFSI) as a reference was also synthesized according to the previous literature.82−84 Photonic Film Preparation. PS−P2VP thin films were fabricated on a glass substrate or a polyimide substrate by the spin-coating method. After spin-coating of a 5 wt % PS−P2VP solution in 1,4dioxane at 500 rpm for 60 s, solvent annealing was performed in chloroform vapor (45 °C) for 24 h. The PS−P2VP thin film was cut to rectangular pieces of 7.5 mm square with a thickness of approximately 2 μm and then immersed in 1 mL of a nonvolatile liquid (neat nonvolatile protic solvent or mixture of nonvolatile protic solvent/ nonvolatile acid) at 40 °C for 12 h to produce a visible light-reflecting photonic thin film. When the liquid is a TEG solution containing SA B

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Macromolecules and the sulfo group concentration in the solution is approximately 3 mM, the number of pyridyl groups in an PS−P2VP thin film and that of sulfo groups in the solution become nearly equivalent. Photonic films were prepared by immersing PS−P2VP films in solutions having seven different sulfonic acid concentrations (0, 1, 3, 5, 10, 30, and 50 mM). For TEM observations, PS−P2VP bulk films about 200 μm in thickness were also fabricated by the solvent casting method using THF, and the photonic film was prepared by the same manner as described above. The sample used in the U-SAXS measurements was a photonic thin film prepared on a polyimide substrate. Hereafter, the PS−P2VP film before swelling is referred to as neat PS−P2VP, the PS−P2VP film swollen with TEG solvent alone as PS−P2VP/TEG, and the PS−P2VP film swollen with TEG solution of SA as PS− P2VP/(SA/TEG) (Table 1).

addition of a particular solvent system were evaluated by the spectrophotometer. A reflection peak in the UV/vis region was only observed when adding EImTFSI, TEG, and PDO. That is, the solvents used when reflection was observed were protic while the other aprotic solvents caused no reflection in the UV/ vis region (Figure 2a), suggesting that the protic nature of the

Table 1. Sample Code, Interdomain Distance, and Peak Maximum Wavelength of Samples Prepared sample code

Da (nm)

λb (nm)

neat PS−P2VP PS−P2VP/TEG PS−P2VP(SA/TEG) 1 mM PS−P2VP(SA/TEG) 3 mM PS−P2VP (SA/TEG) 5 mM PS−P2VP (SA/TEG) 10 mM PS−P2VP (SA/TEG) 30 mM PS−P2VP (SA/TEG) 50 mM

55 114 126 178 NEc 212 203 210

NEd 341 399 522 566 620 615 624

Figure 2. (a) Reflectivity spectra of PS−P2VP/nearly nonvolatile solvents. (b) FT-IR spectra of P2VP/TEG, P2VP/TEGDME, neat P2VP, neat TEG, and neat TEGDME within a range of 970−1020 cm−1. Dashed line in (b) represents 993 cm−1.

a Interdomain distance determined by U-SAXS measurements. bPeak maximum wavelength determined by reflectivity measurements. cNot estimated because the sample was damaged unfortunately before the U-SAXS measurement in the synchrotron facility. dNot estimated because of no reflection in the UV/vis region. Note that experimental errors are included in values determined by U-SAXS/reflectivity measurements since photonic films were not completely homogeneous, and there should be some areas where the D/λ is larger/smaller than the average D/λ.

solvent induces photonic properties; i.e., this protic nature is essential for large swelling of the P2VP phase. Figure 2b shows the results of FT-IR measurements for neat P2VP homopolymer alone, a mixture of P2VP and TEG (weight ratio 1:1), and neat TEG. As a reference, measurements were also performed using an aprotic solvent tetraethylene glycol dimethyl ether (TEGDME) having a chemical structure where both terminal OH groups of the TEG are substituted with an OCH3 group. Furthermore, a mixture of TEGDME and P2VP was also measured. In the range shown in Figure 2b, there was essentially no absorption in the spectra of neat TEG and neat TEGDME. Neat P2VP homopolymer showed absorption at 993 cm−1, attributable to the free pyridine ring.85 In the mixture of aprotic TEGDME and P2VP, the position of the absorption peak from the pyridine ring remained the same, while the P2VP/TEG mixture also showed an absorption peak at 1003 cm−1, confirming the presence of hydrogen bonds between the pyridyl groups and the TEG hydroxyl groups. Therefore, it can be supposed that a protic solvent such as TEG with hydroxy groups can induce significant enthalpy-driven swelling of the P2VP phase by establishing hydrogen bonds with P2VP blocks, leading to a large interdomain distance and reflection of visible light. On the other hand, in the case of an aprotic solvent such as TEGDME, virtually no enthalpy-driven swelling occurs, and swelling as a whole is negligible in the system such that no reflection with respect to UV/vis light can be observed. Hereinafter, thin films swollen with TEG were used as a representative example of a nonvolatile protic solvent for the purpose of photonic characterizations and nanostructure observations. TEM Observations of Neat PS−P2VP and PS−P2VP/ TEG. For the examination of the internal structure of a PS− P2VP photonic film swollen with TEG, structural analysis was performed by TEM on a neat PS−P2VP film, and PS−P2VP/ TEG film. Because of the nonvolatile nature of TEG, TEM observations for those films could be carried out under vacuum conditions. Since the P2VP phase was stained with iodine

Measurements. The PS−P2VP bulk film and the PS−P2VP photonic film prepared by solvent swelling were embedded in an epoxy resin, and ultrathin sections having a thickness of about 100 nm were prepared by a Leica Ultracut UCT microtome. These ultrathin sections were exposed to iodine vapor for 40 min to stain the P2VP phase for TEM observations. A JEM-1400 electron microscope (JEOL, Japan) was used with an accelerating voltage of 120 kV. U-SAXS measurements were performed with BL-15A2 of the Photon Factory in Tsukuba, Japan. The camera length was 3.6 m with an X-ray wavelength of approximately 0.172 nm. Additionally, BL-40B2 of the SPring-8 in Harima, Japan, was also used for preliminary U-SAXS measurements of photonic films. The reflected light spectrum was measured using a deuterium−halogen lamp (Ocean Optics DH-2000BAL) as a light source and a spectrophotometer with optical fiber (Ocean Optics QE-65000). The direction of incident light was perpendicular to the film, and the reflected light was detected by the optical fiber. FT-IR measurements were performed using a Shimadzu IR Prestige-21 coupled with a Shimadzu AIM 8800. A thin film sample on an aluminum substrate was measured in a reflection mode. PS homopolymer with Mn = 37 000 and P2VP homopolymer with Mn = 34 000 were used for solubility tests and FT-IR measurements.

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RESULTS Photonic Films Swollen with Protic Solvents. We prepared photonic films by adding a variety of nonvolatile solvents to the PS−P2VP thin films. The solvents used were EImTFSI, EMImTFSI, TEG, PDO, and SL (see the chemical structures in Figure S3). Any of these solvents dissolves the P2VP block and is selective so it does not dissolve the PS block. The photonic properties of the thin film prepared by the C

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quantitatively, U-SAXS measurements were performed (Figure 3c). In the U-SAXS profile of the neat PS−P2VP film, integer order peaks were observed, suggesting formation of a lamellar structure, and D was estimated to be 55 nm from the relationship D = 2π/q1, where q1 is the scattering vector of the first-order peak. Integer order peaks were also observed in the U-SAXS profile of the PS−P2VP/TEG thin film, which proves the lamellar structure is maintained after TEG addition. As expected, the peak positions are shifted to lower q, demonstrating that D has become larger. D is estimated as 114 nm from D = 2π/q1, which means a 2.1× increase in the D value. The estimated D value was approximately the same (114 nm vs 120 nm) as that estimated from TEM images. Reflectivity Measurements of Photonic Films with Acid. We next focus attention on the interaction between P2VP chains and the protic solvent in order to produce protonated films that will swell to a greater extent and reflect light of increased wavelengths, i.e., to swell the structure to the desired size. As already mentioned with regard to FT-IR, the hydrogen bonding between P2VP and the solvent increased the driving force for swelling and large values of D can be attained for the swollen films. It is expected that greater extent of swelling could be attained by using a polymer and solvent pair where the interaction could even be much stronger. However, if the polymer chain−solvent pair has to be changed each time, the degree of freedom for tunability is clearly restricted. In this study the strength of interaction between the polymer chain and the solvent is modified by combining the nonvolatile protic solvent with a nonvolatile acid that protonates/ionizes the P2VP chain to control the degree of swelling, i.e., to also control the photonic properties. Photonic thin films were fabricated by swelling of P2VP phase with the liquid, i.e., by immersing the PS−P2VP thin film into TEG solution of SA. By fixing the overall amount of solution added to PS−P2VP thin films, PS−P2VP thin films of a predetermined area size and thickness were immersed into the solution and photonic thin films were prepared by swelling, where the concentration and the number of SA molecules ionizing the pyridyl groups via acid−base complexation86,87 were proportional to the concentration of the SA in the TEG. As shown in Figure 2a, no visible color appeared from the film swollen by TEG alone (0 mM) because of reflection in the UV. On the other hand, with the increase in acid concentration, reflections in the visible regime appeared in the order of blue, green, yellowish green, and red (Figure 4a). The reflectivity measurements demonstrate the peak wavelength of the reflected light shifts from the ultraviolet region to the red light region with increasing acid concentration (Figure 4b). The wavelength values of the reflected light became substantially constant above an SA concentration higher than approximately 10 mM. U-SAXS Measurements of Photonic Films. Some of these photonic films were subjected to U-SAXS measurements (Figure 5). In all the profiles, integer order peaks were observed, confirming a lamellar structure. Swelling occurred while the lamellar structure was maintained in spite of addition of SA/TEG. Furthermore, along with an increase in acid concentration up to around 10 mM, the q1 value of U-SAXS profile shifted to the lower q side, which means an increase in the interdomain distance as deduced from the relational expression of D = 2π/q1. For the sulfonic acid concentration of 10 mM and above, the position of the peak became substantially constant (Table 1).

vapor, the phase with brighter contrast is the PS component while the P2VP component appears darker. A lamellar structure with approximately symmetrical composition was observed for the neat PS−P2VP material (Figure 3a). From TEM, the

Figure 3. Nanostructural observation results of neat PS−P2VP and swollen PS−P2VP: (a) TEM image of neat PS−P2VP; (b) TEM image of PS−P2VP/TEG; (c) U-SAXS profiles of neat PS−P2VP and PS−P2VP/TEG. Note that the presence of a strong reflection of the second-order peak indicates that the composition is asymmetric.

average interdomain distance D, the average PS layer thickness dPS, and the average P2VP layer thickness dP2VP were estimated to be 59, 34, and 25 nm, respectively. The proportion of dPS/ (dPS + dP2VP), i.e., 34/(34 + 25) = 0.58, is approximately consistent with the PS volume fraction ϕPS = 0.60 of PS−P2VP, which confirms the expected lamellar nanodomain structure in accordance with the molecular characteristics. In the PS− P2VP/TEG, only the darker contrast layer became larger than the layers in the neat PS−P2VP; therefore, a lamellar structure with asymmetrical composition was formed since only the P2VP layer was swollen with TEG (Figure 3b). The vicinity of the interface in the darker layer domain is slightly darker and the center of the darker layer is slightly brighter. It can be assumed that the (iodine stained) P2VP blocks form a swollen brush and have a higher concentration in the vicinity of the interface and that TEG is present in higher concentration in the vicinity of the center of the darker layer. Since the bright layer is attributed to the PS component and the dark layer is attributed to the mixture of P2VP/TEG, the values of D, dPS, and dP2VP of PS−P2VP/TEG are 120, 37, and 83 nm, respectively, showing that the D value has approximately doubled after addition of TEG. The addition of TEG hardly affected dPS because the PS chains are not swollen by TEG. The P2VP layer thickness, dP2VP, showed an increase of 83/25 = 3.3× in thickness, which makes clear that the P2VP layer is selectively swollen by adding TEG to the PS−P2VP thin film, while the lamellar structure is maintained. It has previously been reported that PS−P2VP could be swollen at the nanoscale level with a protic IL,69 and it is now understood that similar behavior can be found as long as the solvent is protic without being limited to ILs. U-SAXS Measurements of Neat PS−P2VP and PS− P2VP/TEG. To determine the thickness of the films more D

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periods of alternating layers with estimated values of dPS, dP2VP, nPS, and nP2VP was assumed (see also Table S2 in the Supporting Information). Clearly, TMM results agreed well with experimental spectra, which proves that these photonic films swollen with neat TEG and SA/TEG are quantitatively 1D photonic crystals reflecting UV/vis light.

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DISCUSSION D and λ vs Acid Concentration. Figure 6 represents plots of interdomain distance and wavelength of reflection light

Figure 6. Plots of interdomain distance (red circles) and reflectivity wavelength (blue diamonds) vs molar concentration of sulfonic acid.

against sulfonic acid concentration. Until reaching a sulfonic acid concentration of approximately 10 mM, both D of the lamellar structure and λ of reflection light increased by increasing the concentration. This suggests that the degree of swelling increases until about 10 mM, indicating the protonation/ionization rate of the pyridyl group increases and the degree of interaction strength between the P2VP chains and TEG changes, resulting in larger D and λ. We have further seen that D takes a substantially constant value at a sulfonic acid concentration of about 10 mM or higher. This is probably because the degree of protonation/ionization of pyridyl groups attained approximately 100% above 10 mM, and both D and λ became constant. Note that the equivalent concentration of the sulfo groups to the pyridyl groups in this experimental system is about 3 mM if we take account of the amount of neat PS− P2VP on the substrate (7.5 mm square with a thickness of approximately 2 μm) and the P2VP volume fraction ϕP2VP = 0.4; however, since steric hindrance is present due to the fact that nitrogen atoms of the pyridyl groups in P2VP are oriented toward the main chain, 100% ionization rate cannot be attained even by adding the equivalent mole number of sulfonic acid. Therefore, we suggest an ionization rate of 100% was probably achieved around a concentration of 10 mM, i.e., 3 times higher than 3 mM. Estimation of the Degree of Swelling. With reference to Table 1, compared with a neat PS−P2VP film, D increased by approximately 2.0× in the case of a PS−P2VP/TEG film; on the other hand, in the case of PS−P2VP/(SA/TEG) films, it increased up to approximately 3.8× above 10 mM. The thickness of the P2VP layer dP2VP increased by 3.7× by addition of neat TEG and increased up to 8.1× by addition of SA/TEG solutions, if we assume that the PS layer thickness dPS after addition of TEG or SA/TEG is the same as dPS of the neat PS− P2VP (55 nm × 0.6 = 33 nm). It is known that swollen chains in the swollen phase cannot be fully extended into a complete all-trans state due to dissolution network pinning,61 but if we assume the P2VP chains in a P2VP layer were fully extended and each P2VP domain was consisting of a bilayer of P2VP chains, the maximum thickness of the P2VP layer is estimated

Figure 4. (a) Appearance of PS−P2VP/(SA/TEG) photonic films with various concentrations of SA, where a black cloth was used as background. (b) Experimental reflectivity spectra of PS−P2VP/(SA/ TEG) photonic films. (c) Calculated TMM spectra for PS−P2VP/ (SA/TEG) photonic films. Black, blue, green, yellow green, red, orange, and brown lines represent 0, 1, 3, 5, 10, 30, and 50 mM, respectively. Since U-SAXS of PS−P2VP/(SA/TEG) 5 mM was not measured as shown in the caption of Table 1, the TMM spectrum for PS−P2VP/(SA/TEG) 5 mM could not be calculated, either.

Figure 5. U-SAXS profiles of PS−P2VP photonic films swollen with TEG or SA/TEG. Profiles are displayed in the order of magnitude of SA concentration from bottom to top, i.e., 0, 1, 3, 10, 30, and 50 mM. Red, orange, green, blue, and purple arrows represent the first, second, third, fourth, and fifth order peaks, respectively.

TMM Spectra for Photonic Films. By using nanostructural results of U-SAXS, spectra from transfer matrix method (TMM) modeling of the films were also calculated and presented in Figure 4c, where a structure composed of N E

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measurements on the film before and after the addition of the solvent and confirmed selective swelling of the P2VP phase while the lamellar structure was maintained. The formation of a hydrogen bond between the proton of the protic solvent and the pyridyl group in the P2VP chain is attributed to the large osmotic pressure, i.e., large swelling. It has also been confirmed by transfer matrix calculations that the wavelengths of the reflected light measured by the spectroscopy measurements are in good agreement with the wavelengths estimated from the Bragg condition with using the interdomain distance obtained by U-SAXS measurements. Moreover, by immersing neat PS− P2VP films into the nonvolatile liquid containing a protic solvent and sulfonic acid molecules that also protonate the pyridyl groups, the degree of swelling of PS−P2VP became significantly higher than by the addition of neat protic solvent; in this way, photonic films that reflect the visible light of longer wavelengths were prepared. Combination of U-SAXS and reflectivity measurements also confirmed that by varying the degree of swelling via changing the concentration of sulfonic acid, the interdomain distance of internal structure could be tuned in a range of 114−210 nm as well as the wavelength of the reflection light in a range of 340−620 nm. The findings in this study facilitate the preparation of nonvolatile, anhydrous block copolymer photonic films without using water, and they might also be useful in the field of sensing acidic substances or substances with functional groups that generate secondary interactions with some molecules, including applications in the field of solvent-containing gel-like materials. It is also good to note that a study on the swelling behavior of acid/aprotic polar solvent is interesting, though we used acid/protic solvent for attaining large swelling in this study. Such study on acid/aprotic polar solvent will elucidate the swelling mechanism more clearly.

as ∼2 × 50500/105 × 0.254 nm = 244 nm.88 Therefore, about 33% (= 81 nm/244 nm) of the maximum elongation of P2VP chains was attained by addition of neat TEG and up to 73% (= 177 nm/244 nm) by addition of SA/TEG solutions. Thus, remarkable swelling can be achieved by the addition of the small amount of sulfonic acid, comparing above two cases, though there is also a maximum degree of swelling even by adding acid. This is mainly attributed to the length (molecular weight) of the P2VP chains. Another factor limiting the swelling would be the screw dislocation network that holds the layers together and prevents the individual layers from floating apart.61 Interactions between Swollen Chains and Solvents. Next we discuss interactions between chain molecules and solvents in a photonic thin film. In a PS−P2VP photonic film prepared with addition of neat TEG, if neglecting the dynamic nature of hydrogen bonds, one hydroxy group in a TEG molecule would be linked by a hydrogen bond to a pyridyl group (Figure 7a), possibly forming intra- and intermolecular

Figure 7. Interactions among P2VP, TEG, and SA in photonic films: (a) P2VP/TEG and (b) P2VP/(SA/TEG).

cross-links since there are two hydroxy groups in one TEG molecule. In contrast, when swelling a PS−P2VP thin film by an SA/TEG solution, ionization (protonation) occurs via a preferential reaction between sulfo groups in SA and pyridyl groups in the P2VP chain; furthermore, ionic hydrogen bonds are formed between sulfonate anions and pyridinium cations. Multiple hydroxy groups of TEG molecules are assumed to be solvated to the resulting pair of cations and anions (Figure 7b), which possibly also decreases intra- and intermolecular crosslinking between TEG and P2VP because the stoichiometric balance between hydroxy groups and pyridinium groups are inclined toward hydroxy groups. Consequently, it can be supposed that by addition of SA the amount of TEG penetrating and swelling into the P2VP layer dramatically increases, which in turn induces further increase in the values of dP2VP and D. Note that this phenomenon can also be understood conventionally by reducing charge repulsion between ionic complexes along a polymer chain with a lot of neutralizing solvents. Despite a quite small amount of SA added, significant red-shift of the reflectivity wavelength was observed, which means that the technique might also be useful for detecting acidic substances judging from the color and value of λ, also offering a powerful technique for photonic color tuning.

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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/acs.macromol.6b01867. NMR spectrum and GPC chromatogram of PS−P2VP, chemical structures of protic/aprotic solvents used, synthetic scheme of SA, and TMM calculation conditions for photonic films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (A.N.). ORCID

Atsushi Noro: 0000-0002-3336-763X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Joseph J. Walish at Massachusetts Institute of Technology for his kind advice on block copolymer photonic crystals. The authors also thank Prof. Nobutaka Shimizu and Prof. Noriyuki Igarashi for their kind assistance in U-SAXS measurements. This work was financially supported through KAKENHI grant numbers 24685035 (A.N.) and 15K13785 (A.N.) from JSPS, Japan. Use of synchrotron X-ray source was supported by Photon Factory, KEK, Japan (nos.

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SUMMARY By immersing a PS−P2VP thin film having a lamellar structure in a nonvolatile protic solvent (in this work: TEG) which interacts preferentially with the P2VP block, a block copolymerbased soft photonic film was prepared and that reflected ultraviolet light. We performed TEM observations and U-SAXS F

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Macromolecules

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2014G115 (A.N.) and 2016G149 (A.N.)) and SPring-8, JASRI, Japan (no. 2014A1486 (A.N.)). A.N. also expresses his gratitude for a research grant from Tatematsu Foundation, Japan.

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