Trapping Structural Coloration by a Bioinspired Gyroid Microstructure

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Trapping Structural Coloration by a Bioinspired Gyroid Microstructure in Solid State En-Li Lin, Wei-Lun Hsu, and Yeo-Wan Chiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07017 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Trapping Structural Coloration by a Bioinspired Gyroid Microstructure in Solid State

En-Li Lin, Wei-Lun Hsu and Yeo-Wan Chiang*

Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan

KEYWORDS: block copolymers; photonic crystals; trapping; structural coloration; gyroid

* To whom all correspondence should be addressed. E-mail: [email protected] Tel: +886-7-5252000 ext 4081; Fax: +886-5-5254099 1

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ABSTRACT In theory, gyroid photonic crystals in butterfly wings exhibit advanced optical properties as a result of their highly interconnected microstructures. Because of the difficulties in synthesizing artificial gyroid materials having periodicity corresponding to visible wavelengths, human-made visible gyroid photonic crystals are still unachievable

by

self-assembly.

In

this

study,

we

develop

a

physical

approach—trapping of structural coloration (TOSC)—through which the visible structural coloration of an expanded gyroid lattice in a solvated state can be preserved in the solid state, thereby allowing the fabrication of visible-wavelength gyroid photonic crystals. Through control over the diffusivity and diffusive distance for solvent evaporation, the single-molecular-weight gyroid block copolymer photonic crystal can exhibit desired structural coloration in the solid state without the need to introduce any additives, namely, evapochromism. Also, greatly enhanced reflectivity is observed arising from the formation of porous gyroid nanochannels, similar to those in butterfly wings. As a result, TOSC facilitates the fabrication of the human-made solid gyroid photonic crystal featuring tunable and switchable structural coloration without the synthesis to alter the molecular weight. It appears to be applicable in the fields of optical communication, energy, light-emission, sensor and display.

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for Table of Contents use only

Trapping Structural Coloration by a Bioinspired Gyroid Microstructure in Solid State En-Li Lin, Wei-Lun Hsu and Yeo-Wan Chiang*

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Photonic crystals with brilliant structural colorations were found in self-assembled biomaterials composed of long- or short-range ordered microstructures that had formed more than 500 million years ago.1-10 Network-structured photonic crystals

(e.g.,

single

and

double

gyroids;

single and

double

diamonds;

three-dimensional (3-D) rod-connected amorphous diamond microstructures) have drawn much attention because of their interesting optical properties such as linear dichroism, circular dichroism, and noniridescent structural colors.11-14 Difficulties in fabricating network microstructures with unit lattice sizes in the visible wavelength range

have

prompted

scientists

to

replicate

the

self-assembled

network

microstructures found naturally. For example, butterfly wings and macaw feather bards have been used as templates to prepare network-structured photonic crystals through infiltration and backfilling.15-19 Photonic crystals prepared through self-assembly of soft matter are readily functionalizable, well scalable and highly flexible, being potential in displays, energy fields, waveguides, sensors and metamaterials.20-28 Block copolymers (BCPs) consisting of chemically distinct block chains are able to self-organize into various ordered microstructures, including body-centered cubic spheres, hexagonally-packed cylinders, lamellae, and gyroid phases. They are, therefore, good candidate materials for fabricating polymer-based photonic crystals, especially, 3-D gyroid photonic crystals.29,30 However, techniques 4


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that do not utilize ultrahigh-molecular-weight (ultrahigh-Mw) BCPs (overall Mw >1000 kg/mol), as well as those that do not use additives (e.g., solvents, ionic liquids, homopolymers) to expand the unit lattice size of microstructures,31-37 do not yield solid-state BCPs exhibiting the visible-wavelength structural coloration or photonic bandgap (PBG). Because of challenges in synthesis and processing of ultrahigh-Mw BCPs, the fabrication of self-assembled gyroid microstructures with periodicity corresponding to visible wavelengths has remained extremely difficult.38 Only the study, by Urbas and coworkers in 2002, led to the preparation of a gyroid BCP photonic crystal (overall Mw is 750 kg/mol) in the bulk with the near-ultraviolet (near UV) structural coloration at 327 nm.39 Apart from specific top-down lithography,40,41 human-made gyroid photonic crystals having visible or near-infrared (NIR) reflectance have not been accomplished previously using bottom-up methods. Consequently, the fabrication of gyroid photonic crystals with tunable visible structural colorations and PBGs in the solid state remains an emerging field of research. Different from chemical approaches, we developed a physical means—trapping of structural coloration (TOSC)—to prepare a gyroid-structured photonic crystal exhibiting tunable visible structural coloration in the solid state (Figure 1). We first completely transferred the bulk-state gyroid microstructure of an amphiphilic 5


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polystyrene-block-poly(2-vinylpyridine) (PS-P2VP) BCP into the film state (Figure 1a). Because of the insufficient lattice spacing, the gyroid-structured PS-P2VP film is initially colorless in the solid state. Upon expanding the gyroid unit lattice by introducing an external solvent (ethanol), we observed a significant red shift of the structural coloration to the visible range (red hue) in the solvated-state gyroid photonic crystal film (Figure 1b). Surprisingly, after complete evaporation of the solvent, the dried gyroid-structured film retained its visible structural coloration in the solid state (Figure 1c). This is due to the immobilization of the expanded gyroid lattice through vitrification of stretched P2VP chains, namely, TOSC. Meanwhile, the nanoporous gyroid channels that formed upon removal of the solvent give rise to markedly enhanced reflectivity. By heating the TOSC-featured film at temperature above the glass transition temperatures, the gyroid lattice spacing was recovered and the intrinsic colorless film was reformed (Figure 1a). In addition, controlling the diffusivity (D) and diffusive distance (x) allowed us to further manipulate the structural coloration of the gyroid photonic crystal in the solid state. Accordingly, a human-made solid gyroid photonic crystal can exhibit tunable visible-wavelength structural coloration without the need to alter the molecular weight, modify functionalities, or introduce additives. This approach appears to be an efficient means of readily controlling the optical properties of solid-state gyroid photonic crystal films 6


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for advanced optical applications.

Figure 1. Schematic representation of the TOSC process for fabrication of a solid gyroid PS-P2VP photonic crystal film exhibiting the visible-wavelength structural coloration. (a) The initial gyroid-structured film is colorless (i.e., a featureless optical image) because of its small gyroid unit lattice with respect to visible wavelengths. (b) After immersing in ethanol, the solvated gyroid-structured film exhibits red structural coloration (i.e., a human-made gyroid-structured butterfly with red structural coloration in ethanol) as a result of expansion of the gyroid unit lattice. After complete evaporation of ethanol, instead of reverting to the initial colorless state (a), the solid gyroid-structured film still displays the visible structural coloration (i.e., a butterfly with green structural coloration in the solid state) with a slight blue shift in comparison with the state (b). The structural coloration in the solid (c) is attributed to immobilization of the expanded gyroid lattice in (b) during evaporation of the ethanol. Removal of the ethanol also drives the formation of porous gyroid nanochannels in (c), resulting in strong reflectivity in contrast to that in the solvated state (b). After heating the material in the state (c) at 110 °C, the green structural coloration disappears, due to reversion of the gyroid lattice size to that in the initial state (a), exhibiting reversible and rapid structurally color changes.

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RESULTS AND DISCUSSION Self-Assembled

Gyroid

Microstructures.

In

Figure 2a,

single

gyroid

microstructures are frequently observed in butterflies such as the green-colored wing scale of Teinopalpus imperialis observed through field emission scanning electron microscopy (FESEM). Bioinspired from Nature, a well-defined double gyroid microstructure of the high-Mw PS-P2VP cast from dichloromethane (DCM) solution (5 wt%) is observed in the transmission electron microscopy (TEM) micrograph (Figure 2b), which displays the characteristic projection image from the [111] direction of the gyroid. The corresponding small-angle X-ray scattering (SAXS) profile also exhibits the characteristic reflections of the gyroid phase at relative q* ratios of √8: √6: √14 (Figure S1). To transfer the bulk-state gyroid microstructure into the film state, various solvents including PS-selective, P2VP-selective and neutral solvents were employed for spin casting on glass substrates (Figure S2). The miscibility between polymer and solvent molecules can be determined by the polymer–solvent interaction parameter, χP/S, where P and S are denoted as the polymer and solvent, respectively. For a polar system, χP/S can be calculated by42,43

χP/S=VS(δS-δP)2/RT+0.34 where VS is the molar volume of the solvent; R is the gas constant; T is temperature;

δS and δP are the solubility parameters for the solvent and polymer, respectively. According to Flory-Huggins theory, the polymer and solvent are miscible or 8


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compatible as χP/S0.5. As reported, δ values for PS and P2VP are 18.7 and 20.6 MPa1/2, respectively.44 After calculation, the χP/S values at 25 °C (298K) for various solvent– polymer pairs are listed in Table S1. In Figure S2a, the cross-sectional TEM micrograph of the as-spun film from propylene glycol monomethyl ether acetate (PGMEA) displayed the P2VP micelles in the PS matrix due to poor solubility for the P2VP block (χPGMEA/P2VP=0.61). Inversely, spin casting using N, N-dimethylformide (DMF), a poor solvent for the PS block (χDMF/PS=1.46), resulted in the formation of the PS micelles (Figure S2b). When a neutral DCM (χDCM/P2VP=0.37 and χDCM/PS=0.4) was utilized for spin casting, the as-spun film revealed a disordered network microstructure (Figure S2c), due to fast evaporation of DCM (VDCM=435 mmHg at 25 °C). To extend the self-assembled time period during the spin casting process, low-vapor-pressure neutral 1,1,2-trichloroethane (TCE) (VTCE = 23.3 mm Hg at 25 °C) was employed for spin-casting. As revealed in Figure 2c, a low-order gyroid microstructure was obtained in the as-spun film, consistent with the corresponding SAXS profile (Figure S3). After solvent annealing with TCE at 25 °C for 0.5 h, a well-order gyroid microstructure was observed (Figures 2d and S4). Because of the as-spun

low-order

gyroid

morphology

from

TCE,

the

well-order

gyroid

microstructure can thus be achieved by the short period of solvent annealing. 9


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Therefore, the low evaporation rate of solvent combined with subsequent solvent annealing can overcome the slow kinetics of microphase separation of the high-Mw BCP.

Figure 2. (a) FESEM micrograph of the green-colored wing scale of Teinopalpus imperialis (red circle). (b) Cross-sectional TEM micrograph of the high-Mw PS-P2VP bulk sample cast from DCM, revealing the (111) projection images of the gyroid microstructures. (c) Low-order gyroid microstructures with many small grains in the high-Mw PS-P2VP film as-spun from a TCE solution. (d) Well-order gyroid microstructures with (111) projection images in the high-Mw PS-P2VP film after solvent annealing with TCE at 25 °C for 0.5h. After I2 staining, P2VP regions appear dark and PS regions are bright.

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Trapping of Structural Coloration. The reflectivity spectrum of the solid gyroid-structured film is featureless in the visible-wavelength range because of the insufficient microstructural periodicity; the gyroid has a unit lattice size of 247.4 nm, as determined using SAXS (Figure S4). As calculated, the solid PS-P2VP film could exhibit the reflectance peak at 305 nm based on the (211) plane (101.0 nm) of the gyroid lattice. This behavior is similar to the ultraviolet–visible (UV–Vis) spectrum, which featured a transmittance trough at 298 nm (Figure S5). A typical approach to achieve visible-wavelength structural coloration is the use of additives (e.g., solvents) that can expand the periodicity of the BCP microstructure. Nevertheless, when the solvent molecules are evaporated completely from the BCP, the visible structural coloration has previously always reverted to the colorless UV range. As expected, the solvated gyroid-structured film exhibits a red reflectance peak at 664 nm (Figure 3a) after it had been immersed in ethanol (a P2VP-selective solvent). This is due to expansion of the gyroid unit lattice through swelling of the P2VP microdomains. After the ethanol had been evaporated completely, instead of reverting to the colorless state, the dried solid film displayed a visible green hue accompanied by strong reflectivity (ca. 60%) (Figure 3a), namely, trapping of structural coloration (TOSC). A video recording (Supplementary Movie S1) revealed the in situ TOSC process during the process of solvent evaporation. In Figure 3a, the UV-Vis spectrum exhibits 11


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two reflectance peaks at 481 and 427 nm, with a ratio of 1.13 similar to the spacing ratio of √8 ∶ √6 (1.15) belonging to the (220) and (211) planes of the gyroid lattice.39 The TEM micrograph of the TOSC-featured PS-P2VP film, recorded after drying, also revealed the retained gyroid microstructure (Figure 3b). In contrast to the well-order gyroid microstructure in Figure 2d, a low-order gyroid microstructure was observed in Figure 3b, due to the large osmotic pressure caused by the presence of the ethanol in the solvated state. The TOSC-featured film is significantly thicker (2.9 µm, Figure 3b) than the initial film (1.6 µm, Figure 2d); that is, the gyroid lattice is expanded by 1.81-fold in the TOSC-featured film after complete evaporation of ethanol. From the effective refractive index of 1.31 (measured using ellipsometry), the reflectance peak wavelength of the TOSC-featured film was calculated to be 479 nm, consistent with the experimental value of 481 nm determined from the (211) plane in Figure 3a. We suspected that the TOSC is strongly dependent on the conformational stretching or relaxing of the high-Mw P2VP chains in the highly interconnected gyroid network. To test this hypothesis, we examined the behavior of a PS-P2VP film having highly-aligned parallel lamellar microstructures for comparison. In Figure 3c, the lamellar PS-P2VP film exhibits reversible solvatochromism in which a reflectance peak at 646 nm for the solvated film in ethanol rapidly blue shifts to a colorless hue 12


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for the solid-state film after drying without any morphological change (Figure 3d). The disappearance of the visible structural coloration in the lamellar photonic crystal after drying is evident in a video recording (Supplementary Movie S2). As a result, without the highly interconnected 3-D gyroid microstructure, TOSC could not be achieved in the one-dimensional (1-D) lamellar PS-P2VP film.

Figure 3. (a) UV–Vis spectra of the gyroid-structured PS-P2VP film in ethanol (dashed line) and after complete evaporation of the ethanol (solid line). (b) Cross-sectional TEM micrograph of the gyroid-structured PS-P2VP film after complete evaporation of the ethanol. The inset is taken under high magnification. (c) UV–Vis spectra of the lamella-structured PS-P2VP film in ethanol (dashed line) and 13


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after complete evaporation of the ethanol (solid line). (d) Cross-sectional TEM micrograph of the lamella-structured PS-P2VP film after complete evaporation of the ethanol. Inserts are the corresponding optical photographs taken in the solvated and dried states.

The mean square of the diffused distance (x) of the inclusive solvent in the film can be described using the equation: ~ D • t where D is the diffusivity of the inclusive solvent and t is time. D is assumed to be constant in the gyroid- and lamella-structured PS-P2VP films at the same temperature because of their identical P2VP/ethanol interactions As a result of the extremely large value of x for the interconnected gyroid-forming P2VP microdomains, the t value for complete evaporation of ethanol increased markedly, in contrast to that for the lamella-forming P2VP microdomains. As illustrated in Figure 4a, the P2VP chains are compact and relaxed in the initial film. After the film is immersed in ethanol, stretching of the P2VP chains occurs in the solvated state (due to swelling induced by ethanol), resulting in a red-shifting reflectance peak at 664 nm (Figure 3a). With the gradual decrease in the concentration of ethanol during drying, the swollen and stretched P2VP chains assumed a relaxed and compact conformation because of entropic consideration. When the ethanol concentration was sufficiently low, the P2VP chains reached the glass transition temperature (Tg) and formed a thin glassy 14


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P2VP layer covering the film surface. Meanwhile, the metastable stretched P2VP chain conformation, which is between the completely stretched and relaxed conformations, became vitrified and immobilized at a particular instant. Accordingly, the expanded gyroid unit lattice could be thus maintained such that it exhibited the reflectance peak at 481nm in the dried solid film. Also, porous gyroid structures formed in the dried solid film. The presence of the glassy P2VP layer on the film surface was verified using scanning probe microscopy (SPM) in Figures 4b and 4c. Distinct from the gyroid (110) morphology observed in the film prior to TOSC treatment, a featureless SPM surface image was obtained in the film after TOSC treatment, indicating the coverage of the glassy P2VP layer. The nature of this P2VP layer was confirmed through water contact angle (WCA) measurement (Figure S6). After removing the glassy P2VP layer through oxygen reactive ion etching (RIE), the porous gyroid microstructure was evident (Figure 4d), whereas no such porous structure was found after etching of the film that had not been subjected to TOSC treatment (Figure 4e). Notably, because of large osmotic pressure, many cracks were present in the PS region (inset in Figure 4d), suggesting the strong expansion force driven by ethanol and the resultant increase in gyroid period. Consequently, the solid human-made gyroid photonic crystal exhibiting the visible-wavelength structural coloration was fabricated. Using the TOSC technique can, therefore, overcome the 15


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limited gyroid lattice sizes of self-assembled polymer materials in the solid state arising from insufficient molecular weights. Notably, the TOSC-featured solid film had reflectivity (~60%) higher than that (ca. 16%) of the solvated film in ethanol (Figure 3a)—an important property for self-assembled materials intended to exhibit direct and vivid visualization in sensor and display applications. As noted in Figure 4d, we attribute this property to the large refractive index contrast, due to the formation of porous structures within the gyroid-shaped P2VP microdomains after evaporation of the ethanol. This phenomenon was confirmed in the cross-sectional FESEM micrograph of the TOSC-featured film fractured in liquid nitrogen (Figure S7a). As a result, the gyroid-forming porous nanochannels cannot only enhance the reflectivity but also function as a nanoreactor in the template synthesis of various organic and inorganic materials. Therefore, the mechanical properties, refractive-index contrast, and bandgap quality of such a gyroid-structured photonic crystal can be widely manipulated in visible-wavelength range.

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Figure 4. (a) Schematic representation of the TOSC mechanism involving the expansion and contraction of the gyroid lattice accompanied by a series of conformational changes of the P2VP chains in the as-prepared initial film, the solvated gel film in ethanol, and the dried solid film after complete removal of the 17


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ethanol. The black and blue lines represent P2VP and PS chains, respectively. SPM images of the surface morphologies of the PS-P2VP film (b) before and (c) after TOSC treatment. The gyroid microstructure is not evident in (c) after TOSC treatment because of the coverage of a glassy P2VP layer. (d) FESEM micrograph of the sample in (c) after removal of the surficial P2VP layer by RIE, revealing the porous gyroid microstructure. (e) FESEM micrograph of the sample in (b) after RIE treatment identical to that in (d), revealing a featureless morphology. Insets are corresponding optical

photographs,

indicating

the

identical

structural

coloration

of

the

TOSC-featured PS-P2VP film before and after RIE treatment.

The structural coloration of the TOSC-featured PS-P2VP film was very stable at room temperature and even at 70 °C for more than six months because of the high Tg values of the PS and P2VP blocks (Tg,P2VP = 92 °C and Tg,PS = 102 °C in Figure S8). After heating the nanoporous gyroid structures at a temperature above Tg,P2VP and Tg,PS (e.g., at 110 °C), they collapsed because of softening of the PS and P2VP chains (Figure S7b). Meanwhile, the thickness of the TOSC-featured film (2.9 µm; Figure S7a) decreased back to that of the initial film (1.6 µm; Figure S7b). This shrinkage results in contraction of the gyroid unit lattice and, hence, colorless hue. Over 10 cycles of entrapment and reversal of the structural coloration, the reflectance wavelength remained almost unchanged, indicating excellent stability and a rapidly reversible colour change upon thermal switching (Figure 5a). The human-made butterfly-shaped photonic crystal film made from this TOSC-featured PS-P2VP film with porous gyroid nanochannels also exhibited high-strength mechanical (bending) properties (Figure 5b and Supplementary Movie S3). Consequently, this technique 18


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appears to be promising for the fabrication of flexible optical materials (e.g., flexible photonic crystal reflectors, waveguides, and displays).

Figure 5. (a) Results of 10 cycles of entrapment and reversal of structural coloration in the gyroid-structured PS-P2VP film. The visible structural coloration driven by TOSC can be erased by heating the film at 110 °C for 30s. (b) Optical micrographs of the bending of a gyroid-structured photonic crystal thin film featuring a butterfly contour on a soft and transparent poly(ethylene terephthalate) (PET) substrate.

Tunable Visible Structural Coloration by Evapochromism. Control over the D and x through TOSC enabled modification of the structural coloration of the gyroid photonic crystal. Upon decreasing the temperature at which the ethanol was evaporated, the resulting lower D value of ethanol leads to red shifting of the TOSC in the gyroid-structured PS-P2VP film from 438 (40 °C) to 458 (30 °C) to 482 (25 °C) to 492 nm (10 °C) (Figure 6a). The higher evaporation rate at higher temperature leads to more rapid removal of large quantities of ethanol molecules and, thus, faster 19


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shrinkage of the film. This phenomenon gives rise to the rapid contraction of the gyroid unit lattice to smaller dimensions and shifts the reflectance peak to shorter wavelengths. Conversely, lower temperatures lead to smaller quantities of evaporated ethanol, resulting in slower shrinkage of the film. Accordingly, the longer-wavelength reflectance peak was observed due to lower degrees of contraction of the film and the gyroid unit lattice. We also varied the x value of the gyroid microstructure for ethanol evaporation by changing the film thickness via controlling the BCP concentration for spin-casting. With a fixed evaporation temperature of 25 °C, increasing the film thickness from 0.6 to 0.8 to 1.6 to 2.4 to 3.2 µm causes the reflectance peak wavelength to red shift markedly from 442 to 462 to 481 to 548 to 600 nm, respectively (Figure 6b). Consistent with the temperature effect, the time required for ethanol evaporation increased upon increasing x value, leading to the longer-wavelength reflectance peak. Notably, the 3.2-µm-thick film exhibiting TOSC displayed almost complete reflectivity (ca. 98%) at 600 nm. Further increases in the film thickness cause the reflectance peak to remain localized near 600 nm. This is attributed to the competitive effects of the shrinkage rate of the film and the timing of the formation of the glassy P2VP layer. Figure 6c displays the widely tunable reflectance peak wavelengths of the TOSC-featured PS-P2VP films obtained after manipulating the values of D and x. 20


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Figure 6d presents the corresponding chromaticity chart. Thus, the TOSC technique allows the visible-wavelength structural colorations to be controlled to a high degree. As a result, by controlling both parameters D and x influencing the rate of ethanol evaporation, we could rapidly fabricate self-assembled gyroid-structured photonic crystal films with full-visible-wavelength structural colorations in the solid state—that is, through a process of evapochromism.

Figure 6. UV–Vis spectra of the TOSC-featured gyroid photonic crystals film (a) having a thickness of 1.6 µm after evaporation of ethanol at various temperatures and (b) of various thicknesses (0.6, 0.8, 1.6, 2.4, and 3.2 µm) after evaporation of ethanol at 25 °C. (c) Variations in reflectance peak wavelength plotted with respect to the film thickness and evaporation temperature in the TOSC-featured gyroid photonic crystal films. (d) The corresponding CIE diagram. 21


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CONCLUSIONS Polymer-based photonic crystals are promising materials that exhibit tunable structural colorations. Their unit lattice size can be altered by taking advantage of the high flexibility of the polymer chain dimensions. Through a TOSC process, we have prepared the solid single-molecular-weight gyroid photonic crystal exhibiting full-visible-wavelength structural colorations without the need for additives. The high interconnectivity of the gyroid network microstructure is the key property for achieving TOSC such that the expanded gyroid unit lattice in the solvated state became immobilized in the solid state with visible structural coloration after drying. Because of the enhanced refractive index contrast, the TOSC-featured PS-P2VP film having porous gyroid nanochannels also exhibited strong reflectivity. The optical properties of the gyroid-structured films obtained through TOSC were strongly affected by the x value of the gyroid microstructure and the D value of ethanol. Thus, the structural color changes, which were reversible and rapid, resulted from control over the chain conformation of the P2VP block chains. Close control over the TOSC process

allowed

full-visible-wavelength

structural

coloration

in

the

solid

gyroid-structured photonic crystals. This concept for the design of solid gyroid-structured photonic crystals with tunable optical properties, without the need for ultra-high-Mw BCPs, should enable the mass production of large-area, visible-wavelength gyroid photonic crystal films. We anticipate that TOSC-featured 22


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photonic crystal films having interconnected gyroid nanochannels will be of value in controlling mechanical properties, refractive index contrast, and band gap quality. Such

control might be achieved through

solvent-resistant

cross-linkable

polymers,

infiltration and backfilling of

high-refractive-index

metamaterials with good mechanical and thermal properties.

23


ceramics,

or

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METHODS Materials. The high-Mw PS-P2VP BCP was used as received from Polymer Source, Inc. (Doval, Canada.) The number-average molecular weights of PS and P2VP blocks are 248000 and 195000 g/mol, respectively. The volume fraction of the PS block was calculated to be 0.58, based on densities for PS and P2VP of 1.05 and 1.15 g/cm3, respectively..

Sample Preparation. The PS − P2VP bulk samples with double gyroid microstructures were prepared by solution casting onto glass substrates from dichloromethane (5 wt %) at room temperature over 2 days. The bulk samples were then dried in a vacuum oven at 50 °C for 3 days to remove the residual solvent. The film samples of the high-Mw PS-P2VP BCPs were spin-cast at a rate of 1000 rpm onto glass substrates from 1,1,2-trichoroethane (TCE) at room temperature. The film thickness was control by varying the concentration of the solution for spin casting. Well-order gyroid-structured films were obtained via subsequent solvent annealing through exposure to TCE vapor at 25 °C for 0.5 h. In contrast, the highly parallel lamellar-structured film was obtained after subsequent solvent annealing under chloroform vapor at 50 °C for 12 h.

Transmission

Electron

Microscopy.

To

observe

the

cross-sectional

morphologies, the PS-P2VP films were embedded within epoxy resin for subsequent 24


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microsection. To prevent the films from dissolution in the epoxy resin, the film surfaces were coated with carbon films using a high-vacuum evaporator. The epoxy-embedded PS-P2VP films were microtomed using a Reichert ultracut microtome (Reichert EM FC7). The microsectioned slices were then stained with I2 for 60 min. Morphological observation was performed using a JEOL JEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Staining with I2 vapor caused the PS microdomains to appear bright and the P2VP microdomains to appear dark, due to the enhanced difference in the mass–thickness contrast.

X-ray Experiment. Small-angle X-ray scattering (SAXS) experiments were performed using the synchrotron X-ray beamline 25A at the National Synchrotron Research Center in Taiwan. The wavelength (λ) of the X-ray was 0.0827 nm. A two-dimensional (2-D) Pilatus-1MF pixel detector (169 × 179 mm2) with a pixel resolution of 172 mm was used to observe the textures. 1-D SAXS profiles were obtained through azimuthal integration of the corresponding 2-D SAXS textures. The scattering angle in the SAXS pattern was calibrated using the first-order scattering vector of silver behenate: that is, q* = 1.076 nm–1 (q* = 4π λ–1 sinθ, where 2θ is the scattering angle enclosed by the transmitting and scattering beams.

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Field Emission Scanning Electron Microscopy. Topographical images were explored using a JEOL JSM-6700F field emission scanning electron microscope operated with an accelerating voltage of 5 keV. The sample surface was sputter-coated with 2–3-nm layer of gold. To remove the glassy P2VP layer from the PS-P2VP film after TOSC treatment, oxygen reactive ion etching (RIE) was performed with an RF-power of 100 W at a pressure of 75 mtorr for 80s.

Scanning Probe Microscopy. Tapping-mode SPM images of PS-P2VP films were recorded at room temperature using a HITACHI AFM 5000 instrument. A silicon probe having a spring force constant of 10 N/m was employed in dynamic force mode at a scan rate of 0.5 Hz.

Water

Contact

Angle

Measurements.

Water

contact

angle

(WCA)

measurements were conducted at room temperature using 6 uL droplets of deionized water by Contact Angle Dataphysics OCA 20 (Dataphysics, Germany).

Reflectivity Measurements. Reflectivity spectra were measured using a UV–Vis spectrometer (Jasco J-1700 circular dichroism spectroscopy) and an optical microscope (ESPA N-800M) equipped with a fiber-optic spectrometer (B&W Teki-trometer). For microscopic spectroscopy, an Ag mirror was employed as a standard of 100% reflectivity. The measurement of refractive index of the film was conducted by ellipsometer (SOPRA GES5E) at 70°. The refractive index can be 26


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obtained by data fitting using a Cauchy model in the wavelength range from 350 to 850 nm.

ASSOCIATED CONTENT Supporting information Polymer-solvent interaction parameters; the corresponding SAXS profile of the PS-P2VP bulk sample; cross-sectional TEM micrographs of the as-spun PS-P2VP films from PGMEA, DMF and DCM; the SAXS profile of the as-spun PS-P2VP film from TCE solvent; the SAXS profile of the PS-P2VP film after solvent annealing by TCE at 25ºC for 0.5 h; the calculation of the unit lattice size of the gyroid structure; the UV-Vis spectrum of the PS-P2VP film; water contact angle measurements for the neat P2VP film and PS-P2VP film before and after TOSC treatment; the cross-sectional FESEM micrographs of the TOSC-featured PS-P2VP films before and after thermal treatment at 110 ºC; and differential scanning calorimetry profile of the PS-P2VP BCP film

Movie of in situ trapping of the structural coloration using a gyroid-structured film during the evaporation of ethanol; movie of absence of trapping of the structural coloration using a lamellar-structured film during the evaporation of ethanol; and movie of bending behavior of a large-area butterfly-shaped gyroid film after TOSC treatment 27


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

ACKNOWLEDGMENTS

We thank Mr. H.-T. Lin of the Regional Instruments Center of National Sun Yat-Sen University for the TEM experiments; Drs. Y. S. Huang and J. M. Lin of the National Synchrotron Radiation Research Center (Taiwan) for the synchrotron SAXS experiments; and Ministry of Science and Technology of the Republic of China, Taiwan, for financial support (MOST 105-2628-E-110-002-MY3).

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Trapping Structural Coloration by a Bioinspired Gyroid Microstructure

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