Dynamic Interpenetrating Polymer Network (IPN) Strategy for

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Applications of Polymer, Composite, and Coating Materials

Dynamic Interpenetrating Polymer Network (IPN) Strategy for Multi-responsive Hierarchical Pattern of Reversible Wrinkle Liangwei Zhou, Tianjiao Ma, Tiantian Li, Xiaodong Ma, Jie Yin, and Xuesong Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22216 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Dynamic Interpenetrating Polymer Network (IPN) Strategy for Multi-responsive Hierarchical Pattern of Reversible Wrinkle Liangwei Zhou, Tianjiao Ma, Tiantian Li, Xiaodong Ma, Jie Yin, Xuesong Jiang* L. Zhou, T. Ma, T. Li, X. Ma, Prof. J. Yin, Prof. X. Jiang School of Chemistry & Chemical Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, P.R. China E-mail: [email protected] Prof. J. Yin School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, P.R. China Keywords: interpenetrating polymer network, surface pattern, wrinkle, multiresponsive, dynamic chemistry Abstract Dynamic micro/nano wrinkle patterns with response to multi-environmental stimuli can offer a facile method for on-demand regulation of surface properties, thus allowing for generation of a smart surface. Here a practical yet robust strategy is described to fabricate redox, light and thermal responsive wrinkle by building dynamic double interpenetrating polymer network (IPN) as the top layer for a typical bilayer system. IPNs were constructed through the photochemical reaction of a mixture comprised of light-sensitive anthracene-containing polymer (PAN) and redox-sensitive disulfide-containing diacrylate monomer (DSDA). Thanks to the dynamic covalent reversible c-c bond in PAN and s-s bond in DSDA, the morphology of wrinkled surface not only can be reversibly and precisely (micrometer scale) tailored to all kinds of complicated hierarchical pattern permanently, but also can be controlled temporarily by irradiation of near-infrared light (NIR). A sine wave model is proposed to investigate

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the dynamics of real-time reversible wrinkle evolution. This general approach based on IPN allows independent multi-stimuli control over wettability and optical properties on the wrinkled surface, and thus presents a considerable alternative to implement a smart surface. 1. Introduction Wrinkle is ubiquitous in many biological systems and artificial materials that brings different surface structures with various functions.1 In nature, the wrinkling of human brain makes neurons the optimal way stacked in limited space giving human high intelligence.2 Gastrointestine covered by mucosa with buckling essential enhances the surface area for absorbing nutrients and water, and also promotes the transport of substances in the cavity without generating high pressure that may damage the mucosa.3 Furrows generate on animal skin or plant surface to accommodate the aging process.1 A variety of methods such as mechanical pre-stretching-compression,4-7 thermal expansion,8,9 solvent swelling,10-12 and capillarity13 have been developed for wrinkle fabrication. Furthermore, the dynamic wrinkles with responsive surface topography that can be adjusted by external stimuli, may enable the on-demand regulation of the surface characteristic, and might be essential and intriguing for a variety of applications, including information security,14,15 tunable optical devices,16-18 flexible electronics,19-22 energy storage,23,24 membrane separation,25 switchable wettability26,27 and smart adhesion.28 For the bilayer structure materials consisting of a stiff top film bonded on a soft substrate, when giving the driving stress F exceeds a critical value, wrinkles occur from


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the strain mismatch to minimize the total potential energy. Predicted by linear buckling theory,29-32 the driving stress F is: 2

F =E f w

h3 31

Es 2 f

4

1

2 s

(1)

Ef

Where h and w are the thickness and width of the top film, respectively, wavelength of wrinkle, Ef ,

f

and Es ,

s

is the

are the Young’s modulus and the Poisson’s

ratio of the top film and the substrate layers, respectively. The corresponding critical wavelength of the wrinkle, obtained from dF d

2 h

where E

1 31 E 1

1

2 s

Ef 2 f

Es 2

1

3

2 h

Ef 3 Es

0 , is:

3

(2)

is the plane-strain modulus, and the subscripts ‘f ’and ‘s’ are

assigned to the top film and the substrate, respectively. Equation (2) shows that the surface morphology of wrinkle can be controlled by the modulus of the top film ( E f ). Considerable research efforts have been devoted to bring in various single stimulus responses to the buckling bilayer system for expanding new functions, such as thermal response,33 water response,34 light response,35 electrical response,36 and redox response.37 Despite the notable progress, fabrication of wrinkle pattern with orthogonal responses38,39 to multi-stimuli still keeps challenging because it is difficult to introduce multi-responses independently to the ultra-thin (~100 nm) top layer without interfering with each other. Building dynamic interpenetrating polymer network (IPN) in the top layer may be a facile and robust method to bring in multi independent responses and can be easily employed to the ultra-thin film. IPNs are unique “alloys” of crosslinked polymers that provide a strategy of ‘‘forcing’’ mixing between two otherwise



immiscible materials, and there are almost endless arrays of polymers worthy to explore for a variety of practical applications. IPNs are constructed to confer key attributes of one of the components while retaining the critical attributes of another, thus, some completely new and unexpected properties are emerged by the interpenetrating network that are not obtained in either of the two single networks alone.40,41 Here, we provide a versatile and robust strategy for the preparation of multiresponsive wrinkle with the surface geometry that can be dynamically formation and elimination in situ easy by multi-physical or chemical stimuli, respectively, such as ultraviolet light (UV), DL-1,4-dithiothreitol aqueous solution (DTT) and heat (Figure 1). Dynamic IPN containing light and redox-sensitive covalent bonds was designed as the top layer, which was constructed through the photochemical reaction of anthracenecontaining polymer (PAN) and disulfide-based diacrylate monomer (DSDA) on the top film. Compared with existing responsive wrinkle systems, thanks to the dynamic covalent reversible c-c bond in PAN and s-s bond in DSDA,42,43 the wrinkle surface topological not only can be reversible precise (micrometer scale) fixed and tailored to all kinds of complicated hierarchical pattern permanently, but also can be a fast realtime dynamic switch between wrinkled state and dewrinkled state temporarily. A sine wave model was proposed to investigate the dynamics of real-time reversible wrinkle evolution. The unique orthogonal approach based on dynamic IPN allows independent multi-stimuli control over wettability and optical properties on the wrinkling surface of materials, and thus can be used for sensor, information storage, smart display and anticounterfeiting.


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2. Results and discussion 2.1. Strategy for the reversible multi-responsive wrinkle Figure 1 illustrates the total strategy to fabricate the dynamic IPN for reversible multi-responsive wrinkle. A mixture containing PAN and DSDA was spin-coated onto the PDMS elastomer. The detailed synthesis process of these materials PAN and DSDA is described in the Supporting Information (Figure S1-S3). PAN was selected as one component of the top film material due to its high reversibility and fast light response rate for photo-dimerization of anthracene (Figure 1c).44-46 DSDA was selected as another component of the skin layer film due to its redox response to a variety of oxidizers and reducing agent and thus might have potential applications in the biomedical field (Figure 1c).47-49 Upon irradiation by UV light of 365 nm with an intensity of 15 mW/cm-2, the photo-dimerization of AN in PAN and radical photopolymerization of double-bond in DSDA result in the formation of double interpenetrating network as the top layer (Figure 1b). Then after heating at 80 °C for 3 min to make the thermal dilation of the PDMS substrate, a compressive stress was emerged upon cooling the system to room temperature, which caused wrinkle formation due to the mismatch of modulus and thermal dilation coefficient between the top stiff film and the soft substrate (Figure 1a). The wrinkle morphology could be erased into smooth by 254 nm UV light irradiation or heating at 150 °C, owing to the double interpenetrating network of the film transforming into a single network by the depolymerization of PAN. Furthermore, wrinkled surface could be regenerated due to the recovery of IPN by 365 nm UV light irradiation again. Another network of disulfide



bond could be broken or re-combined by reducing agent DTT or heating at 80 °C in the air, respectively, leading to the surface reversible transformation into a smooth or wrinkled state.

Figure 1. Schematic diagram of the fabrication and transformation process of IPN on the top film and the corresponding wrinkle generation/extinction process. (a) Schematic diagram of wrinkle generation/extinction process of the bilayer by multi-stimuli, respectively. (b) Schematic diagram of IPN formation/disconnection in the top film by multi-stimuli, respectively. (c) Schematic diagram of the reversible chemical reaction of PAN and DSDA in the skin layer. 2.2. Regulation of labyrinth wrinkle We first investigated the effect of light radiation on the formation and extinction of labyrinth wrinkle pattern. A certain ratio of PAN and DSDA (the mass ratio of 1:2) was chosen to explore the effects of UV light irradiation time on wrinkle. As shown from atomic force microscopy (AFM) characterization in Figure 2a, no wrinkle could be seen without 365 nm UV light irradiation on the initial film because the low Young’s


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modulus of top film can not aggregate the compressive stress to induce the surface mechanical instability for wrinkle generation. Upon irradiation by 365 nm UV for 10 min (Figure 2b), shallow wrinkles were emerged due to the formation of IPN with increasing Ef. As the irradiation time increased to 30 min, wrinkles became much more obvious. The wavelength , amplitude A of the wrinkles were enhanced from 9.5 µm, 131 nm to 11.5 µm, 510 nm, respectively (Figure 2b-2e, 2g). Then converting the bilayer to 254 nm UV light irradiation, wrinkles disappeared rapidly and were completely erased in only 4 min (Figure 2f, 2g). The corresponding AFM & O

of

wrinkles under different UV irradiation time in Figure S4 fit well with the shape of sine wave. Thanks to the highly reversible property of the dynamic chemical reaction, the film surface contains tunable wettability by UV light (Figure 2h). The water contact angle can be transformed from 92° (smooth surface) to 113° (wrinkled surface), and the surface free energy also changed from 35.7 N/m to 26.8 N/m. To acquire deep comprehending into the influence of the wrinkle on the formation of dynamic IPN, we traced the kinetics on the photo-dimerization/depolymerization of PAN and photocrosslinking of DSDA by ultraviolet-visible spectroscopy (UV-Vis) and real-time Fourier transform infrared spectroscopy (FT-IR). (Figure S5, S6) When exposed to 365 nm UV, 254 nm UV, or heating at 150 °C, individually, a significant decrease or increase in the characteristic absorption peak of anthracene (AN) at 320-400 nm was observed, which confirms the reversible photo-dimerization process of PAN to regulate Young’s modulus for fast and reversible wrinkle morphology transformation. The conversion rate of PAN and DSDA was calculated by peak intensity at 370 nm assigned



to AN in UV-Vis spectra and that at 1636 cm-1 assigned to C=C in FT-IR spectra, respectively, indicating that acrylate can undergo faster photo-crosslinking than photodimerization of AN. Young’s modulus mapping of film (Figure 2i, 2j) was monitored by atomic force microscopy nano-mechanical mapping (AFM-NM) to confirm the wrinkle formation mechanism. When half of the film was blocked by a black light baffle, only after 5 min light irradiation, an obvious modulus gradient region with color of Young’s modulus mapping changing from red to purple could be seen on the lighting interface (Figure 2i, 2j). When continuing to light irradiation, Young’s modulus of the lighting area enhanced remarkably from the original 114 MPa to 603 MPa in 30 min (Figure S7), which confirms that the morphology of wrinkles can be controlled by the modulus of the top layer.

Figure 2. Evolution process of formation/ elimination of wrinkles irradiated by UV light. (a-e) 3D AFM images of the wrinkles formation process when flat surface was irradiated by 365 nm UV light for 0, 10, 15, 20, and 30 min, respectively. The sample was heated at 80 °C for 3 min and cooled to room temperature after UV irradiation to


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induce the generation of wrinkles. (The intensity of 365 nm UV light was approximately 15 mW/cm2). (f) 3D AFM image of wrinkle irradiated by 254 nm UV light for 2 min. (The intensity of 254 nm UV light was approximately 3.5 mW/cm2). AFM images size: 90 90 µm. (g) Wavelength and amplitude of wrinkles as a function of UV light irradiation time. (h) Transformation in water contact angle (WCA) and surface free energy of the reversible wrinkle with sequential control of the ongoing photo-crosslinking under 365 nm UV irradiation. (i, j) 2D colorized Young’s modulus mapping across the lighting interface after 5 min UV irradiation and the corresponding modulus & O across the interface. As the formulation of top layer determines its Young’s modulus, we then investigated its effect on the formation and morphology of wrinkles. A series of mixture of DSDA/PAN with mass ratio range from 0:1 to 2:1 was spin-coated on PDMS substrate, and then irradiated by UV light of 365 nm. As shown in AFM images (Figure 3a-3f, S8), the wavelength and amplitude of wrinkles decreased significantly with the increasing mass ratio of DSDA/PAN, suggesting that the network formed by PAN is stiffer than that formed by DSDA. When any single network in the IPN is destroyed, the wrinkle will be erased triggered by the reduction of Young’s modulus of the top film. The redox regulation of the resulting wrinkle in 10% (mass ratio) DTT aqueous solution was investigated, and its morphology was traced by an optical microscope (Figure 3g, 3h and S9). The morphology of wrinkles can remain stable in pure water for over one week, while the wrinkles generated by radiation of 365 nm UV light for 10 min can be erased in 40 min by immersing in DTT aqueous solution due to the cleavage of disulfide bond. When irradiated for 30 min, the erasure time of wrinkle increases to 3.1 h due to the high crosslinking density of top layer caused by the enhanced irradiation of 365 nm light (Figure 3g). The erasure time of wrinkle will increase to 18.3 h with the decreasing DTT concentration to 2% (Figure 3h). Compared



with erasure of wrinkle by exposure of 254 nm light (Figure 2), the erasure rate by DTT is much lower, which may be ascribed to the following reasons: The IPN is highly crosslinked and hydrophobic resulting in the slow DTT diffusion in IPN, and DTT only can react with disulfide bond at the interface between the top layer and water, consequently leading to a low reaction rate. After heating at 80 °C in the air for 1h and then cooling to room temperature, the smooth surface becomes wrinkled again due to recovery of the disulfide bond. We think that the redox responsive wrinkles may have the potential application for molecular recognition, such as DTT, glutathione, tris(2carboxyethyl)phosphine, and 2-mercaptoethanol.

Figure 3. Regulation of wrinkle morphology by DSDA network. (a-e) 3D AFM images of the wrinkles with different content of DSDA (mass ratio of DSDA:PAN range from 0:1 to 2:1). The samples were irradiated by 365 nm UV light for 20 min. AFM images


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size: 90 90 µm. (f) The corresponding AFM & O of wrinkles. (g) The erasure time of wrinkle samples prepared by different 365 nm UV irradiation time in 10% DTT aqueous solution. (h) The erasure time of wrinkle samples in DTT aqueous solution with different concentrations. The samples were irradiated by 365 nm UV light for 20 min. 2.3. Regulation of ordered hierarchical wrinkle Thanks to the contactless spatial and temporal control of photo-chemical reaction, we can produce not only the labyrinth wrinkle, but also a library of ordered hierarchical wrinkle using light-induced crosslinked IPN as the top layer. By using a photomask, the wrinkles are fabricated and oriented to the light exposed boundary due to an inplane asymmetric distribution of stress generated by a physical confinement effect.50-52 Figure 4a is the schematic illustration for the formation of hierarchical wrinkles and their response process. Strip and circular patterns of wrinkles were fabricated by radiation of 365 nm UV light through the photomasks with 200 µm strip array and 500 µm circular array, respectively (Figure 4b, c, e, f, i, j). The strip wrinkles can be reversibly eliminated to a flat state by UV-light with different wavelengths over multiple cycles, and then turned into circular wrinkle patterns by 365 nm light irradiation. Any letters like “SJTU”, can be written directly through selective formation of wrinkle under the corresponding photomask (Figure 4d, 4h). Figure 4g is the enlarged view on the upper left of “S”. The characteristic wavelength of wrinkle is fixed on the whole pattern, but the radius of curvature at the edge of “S” is different, thus leading to a split of the single wrinkle wave (Figure 4g, S10). All these wrinkle patterns can be reversibly displayed or erased by various stimuli such as light, heat and chemicals. By taking these advantages, we fabricated a QR code with wrinkle pattern, which is viewing to the naked eye due to light scattering of wrinkle (Figure 4k). It can



be scanned and decoded by QR code software to get the information of “SJTU”. The QR code could be erased reversibly by 254 nm UV, heating at 150 °C or DTT aqueous solution, respectively. When part of the wrinkle pattern is destroyed, the QR code will not be recognized and decoded by QR code software for the sake of security and anticounterfeiting in some fields of practical application. On the contrary, when improving the error correction level of QR code, it can be recognized and decoded even if the whole pattern is destroyed by 30% (Figure S11), suggesting that the multi-responsive pattern of wrinkle might find potential application in some fields such as information storage and smart display.

Figure 4. The preparation and regulation of hierarchical wrinkle patterns. (a) Schematic illustration of hierarchical wrinkle formation and response process. (b, c, e, f, i, j) The


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microscopic optical photos of strip wrinkle patterns and circular wrinkle patterns with different scale bars (195 µm and 621 µm) were fabricated by a 200 µm strip array photomask and a 500 µm circular array photomask, respectively. f is the corresponding macrophotographs by camera (The sample size: 1 1 cm). The wrinkle patterns can be reversible generation/extinction by 365 nm UV irradiation/254 nm UV irradiation. (d, h) “SJTU” shape wrinkle pattern photo by optical microscope and camera, respectively. (g) The partially enlarged photo on the upper left corner of “S” shape wrinkle pattern. Scale bar: 100 µm. (k) Demonstration of the macrophotograph of QR code wrinkle pattern reversible fabricated by 365 nm UV irradiation and erased by 254 nm UV, 150 °C and DTT aqueous solution, respectively. The sample size: 2 2 cm. Transforming disordered wrinkled surface into 2D ordered wrinkle pattern represents a significant experimental and theoretical challenge, but it will open up a number of interesting applications. There is thereby an urgent need to achieve largearea, highly controlled, multi-directional ordered wrinkle patterns in a top-down manner.53 Through sequential exposure with a photomask, we can fabricate orthogonal wrinkle patterns by a two-step light irradiation method (Figure 5b, 5c). As illustrated in Figure 5a, firstly, the strip wrinkle pattern

also shown in Figure 4b,4c was fabricated

by 365 nm UV irradiation for 20 min under a strip photomask. Secondly, the photomask was rotated 90° horizontally, and the sample yield the wrinkle pattern

was then further exposed for 20 min to

. What is surprising is that the obtained wrinkle is not a

continuous crisscross pattern as predicted, but a discontinuous orthogonal wrinkle pattern. The yellow zones in wrinkle pattern

are the two-step exposure overlaps and

had been illuminated twice. According to the traditional buckling theory, the yellow area should generate wrinkles with the increased wavelength due to the higher Young’s modulus. On the contrary, the wrinkles in yellow area generated in the first step were then erased by the second irradiation. It may be the twice physical confinement effect5456

by photomask counteracting the compressive stress in the yellow area. Future studies



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both by experiments and finite element analysis in our next work will give detailed mechanism exploration of the striking phenomenon. To further demonstrate the versatility and multi-responsitivity of the dynamic wrinkle pattern based on our IPN strategy, we introduced the IPN film on top of a photothermally responsive platform using a carbon nanotubes (CNT, 0.05% wt) containing CNT-PDMS composite substrate according to our previous research.57 Instead of controlling reversible chemical reaction of the top layer, the thermal expansion of CNT-PDMS substrate can be reversibly regulated by 808 nm NIR irradiation due to the high photon-to-thermal energy conversion efficiency of CNT, 58 and the resulting wrinkle can be erased in-situ and real-time by NIR, endowing an additional response for IPN based dynamic wrinkle pattern (Figure S12). Due to the intrinsic photochemical sensitivity of our IPN film, we can obtain a series of hierarchical wrinkle patterns using programmable selective light exposure, which are multiplex sensitive to UV, chemicals and NIR independently (Figure 5). Moreover, with NIR response, IPN based reversible wrinkle pattern can exhibit real-time dynamic transformation of surface properties, such as surface topography and optical property. NIR response in the wrinkle bilayer confirms the universality and superiority of IPN strategy and allows the system to have more potential applications. The NIR-driven hiericahable pattern can be switched between orthogonal wrinkled state state

and smooth

(Figure 5a). Figures 5c, 5d are the optical images of wrinkle morphology

transition by NIR. The amplitude of wrinkle (A) dropped rapidly to 0 in 20s by NIR irradiation and restored to the original value in 30s after removing NIR (Movie S1 and


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S2). At the same time, the wavelength of the wrinkle remained almost unchanged. This is consistent with Equation (2) that only the amplitude of wrinkle is dependent on the practical strain. Owing to the suitable modulus ratio, the small strain and the high adhesion energy in the bilayer,59 the dynamic process of wrinkle extinction and formation can cycle more than 1000 times without delamination or other damage to the bilayer system (Figure S13). According to linear buckling theory, the detailed calculation process for wrinkle formation is shown in Section S2.4. When the practical strain is fixed to 1.8% (Due to the heat treatment temperature for each sample to induce the wrinkle formation being fixed to 80 °C), a top film with Young’s modulus lower than 310 MPa will have a large critical strain higher than the practical strain. Thus wrinkle cannot be generated. On the contrary, when the surface is exposed to 365 nm UV light for enough time (over 9 min), the Young’s modulus of the film will be increased higher than the critical Young’s modulus (310 MPa), and the critical strain will be decreased lower than the practical strain. Therefore, the wrinkle can be generated. Although the critical formation condition of wrinkle can be calculated by linear buckling theory, surprisingly little attention has been devoted to the critical elimination condition of wrinkle, since the wrinkle elimination process is not simply a reverse process of wrinkle formation. The waveform of wrinkle matches well with the sine wave according to the AFM & O

of wrinkles in Figure 3f and Figure S4. We

try to give the sine wave model for the calculation of the critical elimination condition of wrinkle (the detailed calculation process for wrinkle elimination is shown in Section S2.5). The critical elimination strain !ce is calculated according to the equation below:



0

1

A

cos

2 x

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2

dx (3)

ce

When the wrinkle was eliminated via radiation by NIR for 20 s, the temperature variation is approximately 20 °C and the corresponding practical strain !2 is approximately 0.60%, which has exceeded the critical elimination strain !ce and consequently causes wrinkle elimination (Figure S16).

Figure 5. Fabrication of orthogonal wrinkle pattern by sequential irradiation and its NIR-driven real-time dynamics. (a) - Schematic illustration of orthogonal wrinkle preparation and its NIR-driven reversible extinction and formation. (b) Large area optical photo of orthogonal wrinkle pattern. Scale bar: 621 µm. (c, d) Optical images of orthogonal wrinkle morphology change in extinction/formation cycle by NIR. The NIR intensity is 1.5 W/cm2. Scale bars: 195 µm. The reversible wrinkle pattern might be suitable for developing a new class of smart materials that can adjust its surface geometry and property in response to additional stimuli, which may have potential applications in the smart display and switchable optical devices. By taking advantage of the reversible chemical reaction in the top film of dynamic IPN. We demonstrated a smart display with the dynamic wrinkle pattern


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(Figure 6). The initial bilayer (a) was irradiated by 365 nm UV light for 20 min under a photomask to produce “SJ” letter with wrinkle pattern (b), which can be recognized by naked eye due to light scattering. Only letter “S” (c) could be seen after selective elimination wrinkle in the location of “J” under 254 nm UV light. The surface with wrinkles in the region of “S” turned completely smooth (d) after heating at 150 °C, and then became fully wrinkled (e) after irradiation of 365 nm UV light again. A negative wrinkle pattern “SJ” was obtained by selective irradiation of 254 nm UV light (f), and could be restored to the initial smooth state (a) by DTT aqueous solution. It may be used for smart information display, dynamic no-ink printing, and anti-counterfeiting. Owing to the highly reversible property of the dynamic bond in the top layer of IPN, the wrinkle pattern can be self-healed by light, heat or chemicals. As shown in Figure 7, the wrinkle pattern scratched by a spindle probe could become flat and wrinkle-free by heating at 150 °C for 60 min, and then irradiated by 365 nm UV light for 15 min resulted in an intact and identically sized wrinkled surface again.



Figure 6. Schematic representation of multi-responsive wrinkle patterns in smart display. (a) The initial smooth surface. (b) “SJ” wrinkle pattern acquired by selective exposure to 365 nm UV light for 20 min. (c) Erasure of “J” by selective exposure to 254 nm UV light for 4 min. (d) Wrinkle pattern was erased by heat at 150 °C. (e) Regeneration of wrinkles by 365 nm UV light radiation. (f) Selective erasure wrinkle pattern by 254 nm UV light radiation. The pattern then transformed into a wrinkle-free state after immersed in DTT aqueous solution. Sample: 1 1 cm.

Figure 7. (a) Schematic representation the self-healing properties of reversible wrinkle pattern. (b) 3D AFM images illustrating the self-healing process. The wrinkled surface


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scratched by a spindle probe can become flat by heating at 150 °C for 60 min, and then irradiated by 365 nm UV light for 15 min resulted in a wrinkled surface again. (DSDA/PAN with mass ratio 2:1) AFM images size: 90 90 µm. 3. Conclusion We demonstrated a simple and universal strategy to fabricate reversible and multi-responsive labyrinth or ordered wrinkle patterns based on dynamic IPN, whose morphology can be controlled by various physical or chemical stimuli such as light, heat and reductant. The fabrication and erasure of wrinkle is owing to the dynamical crosslinking of the IPN through the reversible photo-dimerization of AN and redox reaction of disulfide group or simply the consequence of thermal expansioncompression by NIR. It is believed that this strategy for the preparation of reversible wrinkle pattern is a general and promising method to obtain multifunctional smart surfaces. To expand response to environmental stimuli, versatile dynamic chemistry such as carbene coupling reaction, pH-responsive imine covalent bonds and glucoseresponsive boronic ester bonds can be used for constructing the dynamic IPN as top layer, while other kinds of functional materials such as graphene, Ag nanowires, Fe particles, Fe3O4 particles, organic dyes, and conjugated polymers can be introduced into the substrate to provide new functions to the bilayer. The reversible multi-responsive wrinkle patterns may have a variety of applications in information security, smart display, optical devices and flexible electronics. 4. Experimental Section Preparation of PDMS Substrate Elastomeric PDMS sheets were prepared using silicone elastomer (Sylgard 184, Dow



Corning). The silicone base and curing agents were mixed at 10:1 mass ratio and stirred at room temperature. Then the mixture was kept at 70 °C for 4 h after degassed for 0.5 h to yield a crosslinked PDMS elastomer substrate. Preparation of CNT-PDMS Substrate Fifteen grams of silicone elastomer base agent, 0.05% wt of multi-walled carbon nanotubes (CNT), and 15 ml of toluene were mixed in a flask and then were ultrasonic treatment for 8 h. The mixture was dried in an oven at 80 °C for 4 h to completely remove toluene. Then, the obtained CNT-containing PDMS base was added in 1.5 g of curing agent. The mixture was poured into a petri dish before degassed for 0.5 h, and then cured at 70 °C for 4 h to yield the CNT-PDMS elastomer substrate. Fabrication of multi-responsive wrinkle pattern The 5% toluene solution of a mixture of PAN and DSDA with a certain ratio (mixed with 1% wt photoinitiator I907 of DSDA) was firstly spin-coated on a PDMS substrate and then irradiated by 365 nm UV light for the desired time resulting a stiff top-layer film. The coated PDMS substrate was heated to 80 °C for 3 min and cooled down to room temperature to generate the wrinkle patterns. Characterization Wrinkle patterns and their elimination/ reappeared process were obtained by a profile measurement microscope (VF-7501, KEYENCE). AFM was also used to record wrinkles morphology in tapping mode by using silicon cantilevers with a force constant of 40 N/mv ((Dimension Icon & FastScan Bio, Bruker). Young’s modulus of top film was measured by atomic force microscopy nanomechanical mapping, the oscillation


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frequency of the Z-piezo was 1.0 kHz, and the peak force amplitude was set at 150 nm. The samples were scanned using Olympus micro cantilevers with a spring constant of 3 N/m. The NIR was produced by a laser diode controller (l = 808 nm; LE-LS-8081000TFCB, LEO Photonics).1H NMR spectra was measured on a Varian Mercury Plus spectrometer (400 MHz) with deuterated chloroform (CDCl3) as the solvent and tetramethylsilane as an internal standard at room temperature. The photochemical reaction dynamics of PAN and DSDA were investigated through a TU-1091 spectrophotometer (Persee) and Fourier transform infrared spectrometer (FT-IR, Nicolet IS10). Average molecular weight was recorded by gel permeation chromatography (LC-20A, Shimadzu) using THF as an eluent. Contact angle measurements were investigated on an interfacial tensiometer (SL200C, USA KINO Industry). Surface free energy was calculated from the OWRK equation by measuring the contact angle of water and diiodomethane on the samples. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of materials, the synthetic route schemes, NMR, UV-Vis spectra, FT-IR spectra; The calculation and analysis of wrinkle patterns’ critical formation and elimination condition.

Movie S1: micro zone of the orthogonal wrinkle pattern’s elimination/regeneration behavior in microscope. Movie S2: large areas of the orthogonal wrinkle pattern’s elimination/regeneration



behavior in microscope. Acknowledgements The authors thank the National Nature Science Foundation of China (51773114, 21704062) and the Shanghai Municipal Government (17JC1400700, 15SG13) for their financial support.

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