Sensitive Stretchable and Pressable Mechanochromic Photonic Crystals for Dynamic Displays and Visual Sensing

Mechanochromic photonic crystals (MPCs) have shown wide applications in visual sensing, displays, anti‐counterfeiting, etc. However, most reported MPCs can only sense stretching or pressure and exhibit poor sensitivity, which greatly limits their applications. Here, sensitive MPCs with brilliant structural colors capable of sensing both stretching and pressures are fabricated by combining self‐assembly and photo‐polymerization processes. The non‐closely packed structures and high reflectance enable MPCs with outstanding mechanochormic performances. The photo‐initiator concentration and thickness have a dramatic influence on the reflectance, wavelength, and even color uniformity. Interestingly, the same MPC shows diverse sensitivity values under stretching or pressing, and the sensitivity of the MPC by pressing is much higher than that by stretching due to their different deformations along three‐dimensional directions. These results are helpful for gaining a deeper understanding of the mechanochromic mechanism and sensitivity. In addition, dynamic patterns with tailorable color contrasts based on pressure and visual sensing based on stretching in practical applications are realized by taking the above advantages of MPCs. This study offers new insight into understanding mechanochromic behavior and will advance their applications in sensing, printing, and information encryption.

Up to now, various MPCs including liquid MPCs, [28] MPC hydrogel, [29][30][31][32] and MPCs elastomers [33][34][35][36] were manufactured on the basis of self-assembly of colloids into desired solvents, solvents/polymers, and polymers, respectively. These MPCs can alter their reflectance or reflection wavelength under strain. For example, the liquid MPC consists of non-closely packed structures with each silica particle wrapped in solvents. Under pressure, the ordered structures of this liquid MPC disassemble into disordered structures, leading to a decrease in reflectance and fading of structural colors. Different from liquid MPCs, the MPC hydrogels and elastomers can regulate their reflection wavelengths and structural colors in response to strain through altering their lattice distances, suitable for visual strain sensing. An ideal MPC should possess the following merits: (1) a large interparticle distance (D id ) to gain a large sensitivity and tuning range of colors [37] ; (2) an intense reflectance to show brilliant colors during deformation [38] ; (3) good stability under normal conditions [39] ; and (4) more importantly, good responsiveness to both stretching and pressures to reveal the underlying deformation mechanism. To date, a variety of MPCs [35,[40][41][42][43][44] showing high sensitivities, fast responsiveness, and board tuning ranges of structural colors have been fabricated. Nevertheless, it is still a big challenge to reveal the relationship between the microstructure variation and optical properties under deformation since most MPCs are unfavorable hydrogels, not accessible for SEM observation. Moreover, the mechanochromic difference of MPCs caused by stretching and pressing remains unclear. In this regard, sensitive MPC solids can address these issues that are urgently desired, which will facilitate the applications of MPCs in the fields of optical devices, visual readout sensors, and wearable devices.
Here, sensitive MPCs with non-closely packed structures and large D id that can meet all the requirements were fabricated by self-assembly of silica particles in acrylates to generate ordered structures that were then fixed by photocuring. Both the photo-initiator content and thickness have great effects on the structure, reflectance, and reflection wavelength of MPCs. Benefiting from the non-closely packed and highly ordered structures, MPCs exhibit: (1) high reflective intensity (> 60%) and brilliant structural colors, (2) outstanding mechanochromic performance in response to stretching and pressure; (3) high sensitivity (maximal: 7.4 nm per %), a large tuning range of reflection wavelength (Δ = 160 nm), high reversibility (>50 cycles), and good stability (>7 days). More importantly, we found that the same MPC shows diverse sensitivity values under stretching or pressing, and the sensitivity corresponding to pressing is 4.4 times higher than stretching. Such differences can be attributed to their different deformations along three-dimensional directions. Based on these characteristics, MPCs have been used for dynamic displays with tunable color contrasts and visually sensing the strain of gloves during expansion, which will facilitate their applications in the fields of printing, [45] sensing, displays, and so on. Figure 1a, MPCs were fabricated 1) by non-closeassembling silica particles in acrylates and photo-initiator to obtain non-closely packed structures and 2) subsequently fixing the ordered structures by photo-polymerization. Briefly, silica particles (194 nm) were dispersed in ethanol, acrylates, and photoinitiator by sonication. After evaporating ethanol at 100°C for 1 h, a liquid PC with iridescent colors was obtained due to the nonclose-packing of silica particles. Finally, MPCs were fabricated by fixing the ordered structures through simply UV-curing. The volume fractions of silica particles ( s ), acrylates ( a ), and photoinitiator ( p ) are 25%, 71.25%, and 3.75%, respectively. The s , much smaller than that of the closely packed structure (74%), indicates its non-closely packed structures.

As shown in
This MPC possesses a narrow reflection wavelength at 620 nm ( Figure 1b) and a brilliant red color (Figure 1c). The D id and surface-to-surface distance (D s-s ) can be calculated by Bragg's law (Equation (1)) and Equation (2). m, , , and d s are the diffraction order, reflection wavelength, viewing angles, and particle diameter, respectively. n i and i are the refractive index and volume fraction of each component. Thus, the D id and D s-s are calculated to be 258 and 64 nm, respectively. The intense reflectance suggests the highly ordered structure of the MPC, which can be confirmed by the scanning electron microscope (SEM). Under SEM (Figure 1d), silica particles are non-closely packed into long-range order. The D id and D s-s were measured to be 257 and 66 nm, respectively, well consistent with the results calculated from spectra. In the meanwhile, the random two positions of the cross-sections of the MPC ( s = 25%) were observed by SEM. Apparently, the silica particles of both positions show non-closely packed and ordered structures, like those of the MPC surface, demonstrating the uniform structures of the MPC ( Figure S1, Supporting Information). The disordered regions may be caused by the cutting damage during the preparation of the SEM sample. For a PC, its reflectance depends on the thickness, refractive index contrast (Δn), and order degree. [19] A large value of thickness, Δn, and high order degree will lead to a high reflectance of a PC. For the MPC in this work, the decrease in reflectance by a small Δn can be offset by the large thickness (180 μm) and long-range order, leading to the high reflectance of the MPC.
The long-range ordered structures can be attributed to the intense electrostatic repulsions between silica particles. Silica particles are negatively charged on their surfaces (silica-OH → silica-O − + H + ), showing a -potential value of −46 mV. Such highly charged surfaces induce strong electrostatic repulsive forces between silica particles and thus lead to the ordered packing of silica particles at a long distance. The van der Waals attraction between silica particles can be neglected because the large D s-s (66 nm) of neighboring particles of MPC exceeds the effective distance (1-20 nm) of van der Waals attraction. [37] Therefore, the non-closely packed structures originate from the strong electrostatic repulsion between silica particles.
Like conventional opals, the MPCs display angle-dependent reflection wavelengths and colors. As presented in Figure 1e, the structure color of MPC changes from red to blue and the corresponding reflection wavelength blueshifts from 620 to 503 nm when increases from 0°to 60°. This can be easily explained by the inversely proportional relationship between and wavelength according to Bragg's law. Compared to MPC hydrogels, this MPC shows excellent stability due to the lack of solvents ( Figure 1f). The stress-strain curve of the MPC has been shown in Figure S2 (Supporting Information). The maximal stress and strain are 0.4 MPa and 247%, respectively, suggesting an excellent mechanical performance of the MPC. Thanks to the nonclosely packed structure, the colors and reflection wavelengths of MPCs can be adjusted by changing s with the same particle size. For example, orange, yellow and green MPCs corresponding to 600, 588, and 561 nm were prepared using s of 30%, 35%, and 40% ( Figure 1g and Figure S3a, Supporting Information), respectively. Alternatively, one can control the color and peak position of the MPC through tailoring particle sizes, similar to the traditional ways. Here, red, green, and blue MPCs corresponding to 620, 532, and 447 nm were prepared using sizes of 194, 162, and 136 nm ( Figure S3b, Supporting Information), respectively. These results demonstrated that MPCs with non-closely packed structures, bright colors, and tunable optical properties were successfully fabricated based on the self-assembly strategy.
Unlike conventional PCs, the MPC has no cracks. According to previous reports, [18,26] cracks usually occur in photonic crystals with closely packed stacked structures owing to the full evaporation of solvents of colloids/solvents solution during the self-assembly process. In this work, only ethanol was evaporated out from the silica/ethanol/acrylates solution, resulting in a silica/acrylates solution in which silica particles are uniformly packed in liquid acrylates. The ordered structure of silica/acrylates solution can be fixed by photo-polymerization. Unlike the conventional self-assembly methods (drop-casting, evaporation-induced self-assembly, etc.), the formation and fixation of the ordered structure by the self-assembly in this work can be independently controlled. In addition, the gaps between silica particles are filled with polymerized acrylates. These lead to the fabrication of crack-free MPCs in a more controllable and flexible way.

The Effect of Photo-Initiator and Thickness on MPCs
Photo-initiators, facilitating the conversion of monomers into polymers, usually have a neglectable effect on the ordered struc-ture of PCs. However, systematical investigation proves that the photo-initiator amount is an important parameter in the regulation of the structures and reflection signals of MPCs. Here, MPCs with different amounts (1-13%) of the photo-initiator were prepared (Figure 2a). When the amount is 1-5%, the reflective intensity of MPCs is similar and reaches ≈60%. However, the reflectance and color saturation decrease dramatically when the amount increases (7-13%), as shown in Figure 2b and Figure  S4 (Supporting Information). For the fabrication of MPCs, liquid PCs show brilliant colors and high reflectance before polymerization, suggesting the formation of highly ordered structures. The reflectance and color were almost retained after polymerization, suggesting the polymerization has a negligible effect on the long-range order. Thus, it is reasonable to infer that the polymerization speed should not directly influence the reflectance. Previous reports [37,39,[46][47][48] have demonstrated that silica particles can self-assemble into highly reflective PCs with a variety of acrylates, implying the molecule of polymerized acrylates is not a major reason for the dramatic change in reflectance.
In order to understand the functions of photo-initiator to the ordered structures, we try to self-assemble silica particles in pure photo-initiator with similar procedures. After evaporation of ethanol at high temperature, a silica/photo-initiator solution showing a white appearance rather than iridescent structural colors was obtained, indicating the disordered structures of silica particles in photo-initiator. In contrast, the silica/acrylates solution shows iridescent colors. These can be attributed to the structure difference between photo-initiator and acrylates. Compared to acrylates, the photo-initiator is less polar due to the lack of the ester group ( Figure S5, Supporting Information), which is unfavorable for the charge separation of silica particles and thus leads to disordered structures and negligible reflectance. It is therefore the reflectance of MPCs decreases as the content of photoinitiator increases. In addition, the reflection peak position is redshifted from 616 to 641 nm (Figure 2c) as the content of photoinitiator increases from 1% to 13%, due to the increase in the refractive index of the MPC along with the photo-initiator (Figure S6, Supporting Information), and the refractive index of the system increase with the addition of the photo-initiator. According to Equation (1), the reflected wavelength is proportional to the refractive index of the MPC system, which causes the redshift of wavelength.
Brilliant MPCs with different thicknesses can be easily achieved by altering the spacer thickness between two glasses. Here, MPCs with thicknesses (T) of 5-450 μm were prepared for further investigation (Figure 2d and Figure S7, Supporting Information). When T = 5 μm, the reflectance is as low as 4% and the corresponding color is weak, due to the small numbers (≈25 layers) of periodic structures. As the T increases from 5 to 180 μm, the reflection intensity and color saturation increase significantly due to the increase in the number of ordered layers (Figure 2e). However, the reflectance remains nearly constant when the T is further increased (>180 μm). This can be explained by the competition between the reflectance increase from increased ordered numbers and the reflectance decrease caused by the increased defects. In general, defects exist in colloidal crystals due to the imperfection packing of the particles. The number of defects increases significantly along with the T, leading to the decrease in reflectance. The increase in reflectance caused by increased www.advancedsciencenews.com www.advsensorres.com number of ordered structures will be countered by the reflectance decrease by defects at a threshold T (T th ). Here, T th of MPCs is 180 μm, much larger than the conventional silica opals (<10 μm, in most cases) due to the smaller refractive index contrast between silica particles and matrix of MPCs. Such a large T th allows us to prepare thick MPCs without high reflectance and bright colors, which is quite difficult for conventional opals. For instance, a silica opal film with a reflection wavelength locating at 620 nm and a T of 14 μm ( Figure S8a, Supporting Information) was fabricated through a simple drop casting method. As expected, this film shows white appearance ( Figure S8b,c, Supporting Information) rather than a vivid red color because of the strong incoherent scattering by the thick film. One may notice that this film also shows light blue, which might be caused by the Rayleigh scattering. Except for the high reflectance, these MPCs also show uniform colors. Here, the reflection spectra of ten random points of MPCs (90-450 μm) were collected and the results were presented in Figure 2f and Figure S9 (Supporting Information). Their reflection peak positions are all located at ≈620 nm, suggesting the color uniformity of these MPCs. Therefore, the MPC with the T of 180 μm should be the best choice.

Different Deformation Mechanisms by Stretching and Pressing
The as-fabricated MPCs can change their reflection wavelengths and structural colors in response to both stretching and pressing. Here, a MPC with s = 25%, T = 180 μm, and a reflection wavelength located at 620 nm is used in the following section. Considering the different shape changes by stretching and pressing, MPCs were cut into a rectangular shape for stretching and a cylindrical shape for pressing. As shown in Figure 3a, by stretching, the color of the MPC changes gradually from red to blue with the corresponding reflection peak position altering from 620 to 460 nm when the strain ( s , along the force direction) increases from 0% to 100% (Figure 3b). This can be attributed to the decrease in lattice distances according to Bragg's law (Equation (1)), as confirmed in Figure S10 (Supporting Information). The tuning range of reflection wavelength (Δ ) is 160 nm, and the sensitivity is 1.6 nm per %. After releasing, the MPC recovers to its pristine state and the switching between the = 0 and s = 100% is highly reversible (Figure 3c).
Except for stretching, this MPC also can adjust its optical performance under pressure. In order to compare the sensitivity between two different deformation approaches, the shift of the reflective peak position by pressure was controlled to match that by stretching and the corresponding strain along the vertical direction is recorded. As presented in Figure 3d, the variation in the reflection wavelength and color of the MPC is similar to those by stretching. Δ and p are 160 nm and 22%, resulting in a sensitivity of 7.3 nm per %, 4.6 times higher than that by stretching. As expected, this MPC can also show reversible changes in reflection wavelengths in the absence and presence ( p = 22%) of pressure. Moreover, one may notice that the colors of pressed MPCs are more uniform than those by stretching.
The large difference in sensitivity between stretching and pressing originates from their diverse deformation along the three-dimensional directions. Since Δ is the same by stretching and pressing, the sensitivity is determined by the value of p . In other words, the large change in thickness will induce a large sensitivity. After being pressed, the thickness of the MPC decreases but the diameter increases uniformly along all directions  ( Figure 4a). This means the decrease in thickness was supplied by the increase in other directions, considering the constant volume of the MPC, which leads to an effective deformation along the vertical direction and thus a large sensitivity. In striking contrast, when the MPC is stretched (Figure 4b), the length along the force direction increases, while the width and thickness decrease. The deformation in thickness is less effective than that by pressing. Therefore, the stretched MPC requires a much larger strain to achieve a similar wavelength shift to that by pressing and shows a small sensitivity value. To prove this, we use SEM to measure the thickness change by stretching and pressing. In the absence of stress, the thickness of MPC is 184 μm (Figure 4c). The thickness of the stretched and pressed MPC at maximal strain is 146 and 148 μm, respectively. The ratio of thickness between deformed and the pristine one is 0.79 (146/184), similar to that (0.74) of reflection wavelengths, demonstrating the change in thickness is the key to the blueshift of reflection wavelengths by stretching or pressing. These results prove that the pressed MPC need less strain to reach the same wavelength than the stretched MPC. Therefore, the sensitivity of the MPC by pressing is much higher than that by stretching.

Display and Sensing Application
MPCs can be used for dynamic display and visual sensing based on their capability of sensing both pressing and stretching, respectively. For display, stamps with any desired patterns are placed below MPCs (Figure 5a). Afterward, the MPC was pressed, which leads to a decrease in lattice distances and the color change of the patterned region, resulting in a large color contrast between the background and the pattern. As expected (Figure 5b), the color of the pattern can be dynamically controlled through simply adjusting the p , while the pattern can be easily changed by different stamps. The boundaries remain clear even under a large p , suggesting their high-resolution. When the pressure is released, the MPC returns to its original red color. According to our experiences, the patterns can be reversibly shown and hidden at least 100 times without apparent color fading. In addition, the MPC can be used as a visual strain sensor. The sensor with a butterfly shape was pasted on the sealed rubber glove (Figure 5c and Movie S1, Supporting Information). By pressing point A, point B and the MPC were stretched, causing the blueshift of structural colors. The edge shows negligible deformation during stretching so that it remains nearly unchanged pristine red.
For both applications, MPC sensors can be recycled and reused, which significantly lower the cost of use and is more environmentally friendly. At the same time, MPC sensors do not consume the electric energy usually required by commercial electronic sensors, which is more convenient for practical usage. In addition, by taking the advantage of the swellable polymers, MPCs even have potential in solvent sensing, multi-color displays, optical devices, anti-counterfeiting, etc.

Conclusion
In summary, the sensitive MPCs with the non-closely structure in response to both stretching and pressing were manufactured by self-assembly of silica particles in acrylates combined a photopolymerization process. The amount of photo-initiator and thickness have great influences on the reflection wavelength and reflectance of MPCs. A low-content photo-initiator and a large thickness are favorable for MPCs with brilliant and uniform structural colors. The photo-initiator of 5% and the thickness of 180 μm are the best choices for MPCs. By stretching and pressing, the MPC shows a large tuning range of reflection wavelength (Δ = 160 nm), a high sensitivity, good reversibility, and excellent reversibility. The same MPC shows much higher sensitivity (7.4 nm per %) by pressing than that (1.7 nm per %) by stretching and the color is more uniform during pressing, due to their diverse deformation along three-dimensional directions. Based on the supersensitive characteristics, we demonstrated that MPCs can be used for dynamic display with adjustable color contrasts and visual readout strain sensors. This work reveals the underlying deformation mechanism of MPCs by stretching and pressing and will facilitate their applications in printing, strain sensing, optical devices, and anti-counterfeiting.
Fabrication of MPCs: Silica particle powders (0.025 cm −3 , 194 nm) were dispersed in a mixture of ethanol (0.5 mL), PEGPEA (0.0375 mL), and DEGEEA (0.0375 mL). The content of the photo-initiator relative to DEGEEA/PEGPEA was 5%. The mixed solution was then heated in an oven at 100°C for 1 h to evaporate the ethanol and a concentrated solution was obtained. Finally, the precursor solution was sandwiched between two hydrophilic glasses with an interval of 180 μm, and then the silica particles self-assembled. The MPCs were obtained by exposing the precursor sample to UV light (365 nm, 4.8 mW cm −2 ) for 3 min. The distance between the UV light source and the sample was 15 cm. Brilliant red colors can be observed by the naked eye and the wavelength of MPC locate at around 620 nm.
Characterization: The reflectance spectra were measured by a NOVA spectrometer (Hamamatsu, S7031). The optical microscope images were obtained on an Olympus BXFM reflection-type microscope operated in the darkfield mode. The structures of MPCs were investigated using Hitachi SEM SU8010. Angle-resolved spectra were collected by mode 2 with reflection angles changes from 0°to 60°, using the angle-resolved spectrum system (R1, Ideaoptics, China) equipped with a highly sensitive spectrometer (NOVA, Ideaoptics, China).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.