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Photo-controllable Wrinkle Morphology Evolution on Azo-Based Multilayers for Hierarchical Surface Micropatterns Fabrication Chuanyong Zong, Umair Azhar, Chunhua Zhou, Juanjuan Wang, Luqing Zhang, Yanping Cao, Shuxiang Zhang, Shichun Jiang, and Conghua Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04237 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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Photo-controllable Wrinkle Morphology Evolution on Azo-Based Multilayers for Hierarchical Surface Micropatterns Fabrication Chuanyong Zong1, Umair Azhar1, Chunhua Zhou1, Juanjuan Wang2, Luqing Zhang1*, Yanping Cao3, Shuxiang Zhang1, Shichun Jiang2*, and Conghua Lu2* –––––––– Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials,

1

School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P. R. China. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P.

2

R. China. AML, Department of Engineering Mechanics, Tsinghua University, Beijing, 100084,

3

P. R. China. E-mail: [email protected]; [email protected]; [email protected] ––––––––– Abstract: Inspired by nature, comprehensive understanding and ingenious utilization of the self-organized wrinkling behaviors of the sandwiched multilayer bonded on substrates are important for engineering and/or functional laminated devices design. Herein, we report a facile and effective strategy to regulate the wrinkles morphology evolution and the resultant hierarchical surface micropatterns on azobenzene-based laminated multilayers by visible-light irradiation. Revealed by systematic experiments, the photo-controlled dynamic wrinkle evolutions are triggered by the reversible photoisomerization of azobenzene in the top azopolymer film and strongly dependent on the intermediate photoinert layers (e.g., polystyrene (PS) and OP-induced SiOx layer) with the wrinkle reinforcing effect or the stress relaxation acceleration effect. Interestingly, large area well-defined hierarchical surface wrinkle patterns could be 1 ACS Paragon Plus Environment

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fabricated on the multilayers upon selective exposure. In unexposed region, the wrinkles were evolved into highly oriented patterns, while in exposed region, they were fully erased or evolved into smaller wavelength of wrinkles. This study not only sheds light on the morphological evolution of the wrinkling laminated composites in engineering and nature but also paves a new avenue to conveniently and controllably realize the hierarchical stimulus-responsive surface patterns. Keywords: stimuli-responsive; wrinkling; multilayer; surface patterns; evolution

Introduction As a spontaneous phenomenon, surface wrinkling of different scales is ubiquitous in engineering and nature.1,2 As a representative research model, a bi-layer wrinkling system with a rigid skin resting on an elastomeric substrate has been well studied theoretically and experimentally.2-4 Owing to mechanical instabilities driven selforganizing characteristic and quantitative relationship between wrinkle morphologies (e.g., wavelength  and amplitude A) and the mechanical properties of the films, surface wrinkling has been ingeniously utilized as a simple versatile patterning method as well as a buckling-based metrology, which has found broad applications ranging from creating functional surfaces,2,5 designing stretchable electronics,6 advanced optical devices,7,8 to measuring thin-film properties9,10 and/or quantifying residual stress in the films.11 More importantly, based on the wrinkles mechanical instability, various wrinkling-based adaptive systems have been developed via introducing different stimuli-responsive materials to regulate the as-formed localized stress field.12 Such fascinating feature allows us to realize the reversibility of the stress-relief wrinkling morphologies, in combination with smart surfaces with tunable

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adhesion,13 friction,14,15 wettability,16 optical properties,17,18 and/or constructing abundantly hierarchical wrinkling patterns via post-modification.19,20 However, it is pointed out that surface wrinkling also frequently occurred in the multilayer systems for some biological systems and engineering devices. Till now, although there is flourishing interest in utilizing surface wrinkling on multilayered films for applications in epidermal electronics,6,21,22 photoelectric devices,23,24 and high-throughput film metrology,25-28 etc., the underlying wrinkling mechanisms and corresponding instability responsible for the multi-layered systems remain elusive.29 Driven by this need, a series of theoretical models have been proposed for interpreting geometric instabilities in multilayered thin films and further guiding the multi-layer engineering model design. Recently, systematic theoretical studies have indicated that in comparison with the bi-layer system, the multi-layered one exhibits various buckling modes along with intriguing surface pattern transformations via changing Young’s modulus of the intermediate layer.28,30 Lejeune et al.31 have designed novel comprehensive tri-layer/quad-layer models to provide physical insights into the influence of the interfacial layers on the wrinkling and/or delamination behavior in multi-layer systems. Furthermore, these sophisticated models show broad applications ranging from buckling-based metrology in ultrathin films to interpreting topology formation in novel engineering systems. Subsequently, motivated by the development of epidermal electronics, the above-mentioned tri-layer analytical solution has been extended to the systems with unrestricted number of layers, limitless layer thicknesses, and material properties.32 By adopting this model, we can conveniently capture and control the wrinkling behavior in stretchable multi-layer electronics,21,22 along with elaborating biological pattern formation33 and inspiring biomimetic design.34 Further, a modified tri-layer model with the inner and outer layer of similar stiffness has been 3 ACS Paragon Plus Environment

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proposed to investigate the self-organizing process of the hierarchical cerebellar microstructures from a physics perspective.35 It is worth noticing that all the abovementioned solutions of the multi-layer theoretical models are based on a hypothesis that the wrinkling behavior of the system is independent of the substrate thickness.35 Just recently, Yoo et al. have analyzed the thermally-induced surface wrinkling instability in multi-layer (tri- or quad-layer) systems by using a cumulative energy balance analysis, considering variation in the substrate thickness and the contribution of each layer.36 Equipped with this improved individual layer model, it is now possible to estimate the wrinkling wavelengths of the heterogeneously stacked multilayers more accurately, as well to sharply monitor the delicate change in the thermomechanical properties of polymers near their glass transition temperature. Besides, wrinkling of interfacial stiff layers sandwiched between two different compliant layers has also been theoretically investigated.37,38 However, the experimental studies in the controlled multi-layers wrinkling especially for the wrinkles morphology transformation of heterogeneously laminated multilayer films on a compliant substrate have been rarely studied. In our previous work, we have incorporated a photosensitive interlayer into the multilayer system to conveniently fabricate various hierarchically functional surface patterns over a large area with controllable photo-modulated wrinkle morphological evolution.39 In this paper, we use an azo-based poly(disperse orange 3) (PDO3) film as the top stiff layer to fabricate light-responsive multi-layer wrinkling systems with different intermediate stiff layer bonded to a poly(dimethylsiloxane) (PDMS) substrate. Surprisingly, the heating/cooling-triggered surface wrinkles in the multilayer systems present distinct morphology transformation behavior upon visible-light irradiation. The existence of laminated stiff intermediate layer demonstrates dual character on the 4 ACS Paragon Plus Environment

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photo-modulated surface wrinkle evolution induced by the photoisomerizationtriggered photo-softening and the stress redistribution in the top azopolymer film. The compressive stress release of the as-wrinkled multilayer system was suppressed to some extent, and consequently the surface wrinkles exhibited an obvious decrease in the wrinkle wavelength. That resulted from the light-irradiation triggered photosoftening effect of the PDO3 film when using photoinert polystyrene (PS) film as the intermediate layer. However, when a stiff silica-like (SiOx) intermediate layer was introduced via oxygen plasma (OP) oxidation of the top PDMS layer, the surface wrinkles of the multilayer system could be accelerated to erase through visible-light exposure compared to the corresponding PDMS/PDO3 bilayer system. Especially, upon selective exposure, different hierarchical surface patterns were obtained with the initially labyrinth wrinkles evolved to form highly oriented ones in unexposed region, and the wrinkles in exposed region were erased or wavelength decreased evidently for different multilayer wrinkling systems. The photo-modulated wrinkles micropatterns with controlled multiscale topography features have been well regulated by the laminated stiff layers thickness and the photomask patterns. This study not only provides experimental insight into the underlying mechanisms of surface morphology evolution in wrinkling multilayer systems but also presents a versatile design strategy to precisely regulate the localized stress/strain of wrinkling systems for hierarchical surface patterning.

Experimental Section Fabrication of poly(dimethylsiloxane) (PDMS) sheet. PDMS sheet with a thickness of ~ 2 mm was prepared by mixing the base/curing agent (Sylgard 184, Dow Corning) at the weight ratio of 10:1. After degassing for 1 h, the mixed 5 ACS Paragon Plus Environment

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base/curing agent was poured into a culture dish and baked at 70 ºC for 4 h to complete the curing. Synthesis of poly(disperse orange 3). Disperse orange 3 (95%, Acros Organics) and bisphenol A diglycidyl ether (Sigma-Aldrich) were used directly without further purification. Poly(disperse orange 3) (PDO3) was synthesized by one-step solid state polymerization.40 The weight-average molecular weight and dispersity (Đ) of the obtained PDO3 based on the gel permeation chromatography result were estimated to be ~ 10,000 and 2.62, respectively. The glass transition temperature was ~ 120 °C with the maximum UV-vis absorption peak at 478 nm. Fabrication of PDMS/PS/PDO3 multilayer. The detailed procedure is as follows. First, the PDMS/PS bilayer was prepared by spin-coating PS solution in toluene onto a PDMS substrate. Note, before spin-coating, the free hydrophobic substrate was exposed to oxygen plasma (OP) (Harrick PDC 32 G) with a high power for 30 s. The obtained PDMS/PS bilayer was dried in a vacuum oven at room temperature for 2 h. The PDO3 stiff layer was obtained by spin-coating PDO3 solution in tetrahydrofuran (THF) onto a glass substrate. Then, the PDO3-coated side of the glass slide was contacted with the PDMS/PS sample carefully, followed by immersing into deionized water. Subsequently, the PDO3 film was slowly detached from the glass slide yielded the PDMS/PS/PDO3 sample. The thickness of deposited PS film and PDO3 film were adjusted via the variation of the spin-coating speed (e.g., 1500, 2500, 3500 rpm) and/or the solution concentration (e.g., 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%). Fabrication of PDMS/SiOx/PDO3 multilayer. The detailed procedure is as follows. First, the PDMS/SiOx bilayer was prepared by treating the free PDMS substrate with OP oxidation at a pressure of 0.02 mbar, which converted the PDMS surface into a thin glassy silica-like (SiOx) film. The thickness of SiOx layer was 6 ACS Paragon Plus Environment

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controlled by the variation of OP treatment time (e.g., 30 s, 5 min, 10 min, 15 min, 20 min, 30 min, 40 min and 50 min, respectively). Then, PDO3 solution in THF was spin-coated onto the PDMS/SiOx bilayer to obtain the PDMS/SiOx/PDO3 multilayer. The obtained multilayer was dried in a vacuum oven at room temperature for 2 h to completely remove the residual solvent. Exposure of the wrinkled PDO3-based multilayer to visible light. The asprepared samples were heating at 90 °C (1 h), followed by cooling to room temperature induced surface wrinkling on the PDO3-based multilayers. A halogen lamp (CEL-HXF300) with full spectrum was used to irradiate the as-prepared wrinkled multilayers. The light intensity was adjusted by the input voltage. To exclude the influence of photothermal effect on surface wrinkles, we used 30 mW/cm2 as the exposure light intensity to avoid sample heating. For the selective exposure, the wrinkled samples were covered by a fresh copper grid (KYKY Technology Development Ltd.) in a conformal way. Characterization. The light intensity was measured with optical power meter (Zhongjiao Aulight, Beijing, China). Optical images were recorded using an inverted Observer A1 microscope (Zeiss, Germany) equipped with a charge coupled device camera. Films thickness and atomic force microscope (AFM) images were obtained by an Agilent 5500 AFM/SPM microscope in tapping mode. Differential scanning calorimetry (DSC) measurement was recorded on a DSC-Q2000 (TA instruments Co., America). The scanning temperature ranges were from 20 ⁰C to 250 ⁰C with a heating rate of 10 ⁰C/min in N2 atmosphere.


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Results and Discussion The key steps to fabricate different light-responsive multilayers and subsequent photo-operated tailoring of surface wrinkle morphology are depicted in Figure 1. Here azo-based PDO3 film is used as the photo-responsive skin layer with PS film or stiff silica-like (SiOx) layer as the laminated photoinert intermediate layer. Firstly, the film/substrate bilayers are prepared through spin-coating of PS film on OP-exposed (30 s) PDMS substrate (Figure 1 a→I b1) or converting the PDMS surface into a thin glassy SiOx layer via oxygen plasma treatment for a variable duration (tOP) (Figure 1a→II b2). After that, transfer the PDO3 film onto the PS-coated PDMS substrate results in formation of the PDMS/PS/PDO3 multilayer (Figure 1 I b1→c1), or spincoating

PDO3/THF

solution

onto

the

PDMS/SiOx

bilayer

yields

the

PDMS/SiOx/PDO3 multilayer (Figure 1 II b2→c2). Homogeneous labyrinth wrinkles are emerged on the multilayers owing to the surface instability after heating/cooling processing (Figure 1 I c1→d1, II c2→d2). Upon the selective visible-light irradiation via the mask-projection system, distinct morphology transformation occurs on the wrinkling multilayer systems. Surface wrinkles in unexposed area are transformed into highly oriented ones and those in exposed area are modulated with obviously decreased wavelength (Figure 1 I d1→e1) or completely erased (Figure 1 II d2→e2), respectively, which is related to the employed photoinert intermediate layer.


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Figure 1. Schematic illustration of the procedure to realize photo-modulated wrinkle morphology transformation for controlled fabrication of different hierarchically patterned wrinkles on the multilayer films. The as-prepared PDMS/PS/PDO3 multilayer shows a smooth flat surface with the surface roughness similar to the corresponding PDMS/PS bilayer (Figure S1, Supporting Information). As a kind of amorphous polymer, the PS and PDO3 films have the similar material properties19,26,41 and exist a considerable interlaminar adhesion possibly owing to the π-π interaction of the aromatic rings present in both.42 When external stimuli were applied, i.e., 90 °C heating/cooling, they triggered the compressive stress (). When the obtained  exceeded the multilayer-defined critical wrinkling value (c), homogeneous labyrinth wrinkles are induced over the large area on the PDMS/PS/PDO3 multilayer (Figure 2a). This wrinkling behavior of the current multilayer is very similar to the corresponding PDMS/PS and PDMS/PDO3 bilayers with the wrinkle amplitude-to-wavelength ratio of ~1:20 (Figure 2a-c and Figure S2), which is consistent with previous reports.25-28,39 As shown in Figure 2d, both of the films thickness of PDO3 (hPDO3) and PS (hPS) have relatedness with the wrinkle wavelength (λ) of the PDMS/PS/PDO3 multilayers, which reveals the thin laminated PS/PDO3 films wrinkle jointly at the same wavelength. When the PDMS/PS/PDO3 multilayer was cooled from 90 °C to room temperature, the resulted equi-biaxial compressive strain is less than 5% (the coefficient of thermal 9 ACS Paragon Plus Environment

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expansion for PDMS used here is about 3.0 × 10-4 °C-1).43,44 By assuming the laminated PS/PDO3 layers as a homogeneous film, the wrinkle wavelength (λ) of the multilayer is determined by Equation (1)25-28,45

 (1  v s ) E eff    2 h f    3(1  v f ) E s  2

1 3

2

(1)

where hf is the total thickness of surface stiff films (hf = hPS + hPDO3).25-27 vf and vs denote the Poisson’s ratios of the glassy polymer film and rubbery substrate with their values generally assumed to be 0.33 and 0.5, respectively.45 Eeff is the effective modulus of the laminated stiff composite layers and can be calculated according to Equation (2)25:

1  m 2 n 4  2 mn ( 2 n 2  3n  2 ) E eff  E PDO3 (1  n ) 3 (1  mn )

(2)

where m and n are the modulus ratio (i.e., EPS/EPDO3) and the thickness ratio (i.e., hPS/hPDO3) of the surface layers, respectively. The theoretical wrinkle wavelengths of the present multilayer calculated via above equations are consistent with the experimental values, when ES, EPS, and EPDO3 are assumed to be 1.32 ± 0.04 MPa,46 3.4 ± 0.2 GPa,26 and 1.2 ± 0.3 GPa,39 respectively. This result confirms that the laminated PS/PDO3 layers give rise to the same wrinkling with their homogeneous counterparts, which is similar to the other reported multilayer systems.27,39,45


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Figure 2. (a) Optical image, (b) AFM height image, (c) cross-section profile of the surface wrinkles of the PDMS/PS/PDO3 multilayer (hPS = 130 nm, hPDO3 = 140 nm), and (d) the dependence of the wrinkle wavelength on hPDO3 (hPS = 70 nm) and hPS (hPDO3 = 295 nm), respectively. Hollow symbols show theoretical data as the function of hPDO3 and hPS, respectively. Based on the intrinsic mechanical instability of surface wrinkling, the as-prepared multilayer surface wrinkles can be modulated via regulating the as-formed localized stress field.47 As a pseudo-stilbene-type azo molecule,48 the pendant azobenzene group of PDO3 has the overlapped trans and cis absorption bands with the maximal peak at ~ 478 nm.19 In other words, during the visible-light exposure, the π–π* and n– π* transitions of the azobenzene chromophore occurs simultaneously yielded a rapid “trans/cis/trans” cycling isomerization48 that results in a significant optomechanical stress49-52 and a photosoftening effect19,53 in the PDO3 film. Consequently, the above azo-containing multilayer surface wrinkles become visible-light responsive.


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Figure 3. Optical images of the wrinkles on the multilayer system (hPS = 70 nm, hPDO3 = 172 nm) before (a) and after blanket irradiate with visible light (b), selective irradiate with visible light by 100 mesh-sized hexagonal holes (H100) (c) and 300 mesh-sized square holes (S300) (d), respectively. Scale bar: 50 m. As shown in Figure 3a,b, with blanket exposure to visible light, the wrinkle wavelength on the as-wrinkled PDMS/PS/PDO3 multilayer decreased obviously, while the wrinkle orientation is retained. Under the same conditions, selective irradiation through the mask-projection system (e.g. different copper grids) (Figure S3, Supporting Information) leads to well-defined hierarchical micropatterns on the multilayer surface (Figure 3c,d). Same as blanket exposure, the wrinkles in the exposed part retain their primary labyrinth morphology, and wrinkle wavelength decreases considerably after the selective exposure (Figure 3c,d). In-situ optical microscopy observation explicitly indicates that the wrinkle wavelength reduces gradually during the selective exposure and finally arrives at a saturated value, regardless of the initial wrinkle morphology and the dimensions of the applied copper grids (Figure 4, Figure 12 ACS Paragon Plus Environment

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5a and Figure S4, Supporting Information). Noteworthy, the achieved ultima wavelength has a approximately positive linear correlation with hPDO3 (Figure 5b) and hPS (Figure 5c), which enabled us to prepare various hierarchical surface wrinkle micopatterns via simply changing the stiff layer thickness (hPDO3 and/or hPS) and/or the copper grid types (Figures S5-S8, Supporting Information). However, the initial disordered wrinkles in unexposed part are dynamically transformed into highly ordered ones (Figure 3c,d) with almost unchanged wrinkle wavelengths (Figure 5b,c).

Figure 4. In-situ optical microscopy observation of the optical modulates of the wrinkles during selective irradiate with visible light by S300: (a) the initial labyrinth wrinkles on the multilayer system; (b-h) the copper grid was still on the exposed bilayer surface; and (i) the copper grid was removed from the exposed bilayer surface. Scale bar: 50 m. According to Equation (1) and (2), the decrease in the wrinkle wavelength of the exposed region is primarily attributed to the light irradiation-induced modulus 13 ACS Paragon Plus Environment

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reduction (i.e., photosoftening effect) of the photoresponsive PDO3 film, which has been deeply discussed and verified in previous works.19,39,53,54 The wrinkle wavelength dynamic evolution of the wrinkling PDMS/PS/PDO3 multilayer upon visible-light exposure is very similar to the case of the UV-exposure caused photosoftening of the wrinkling azo-containing liquid crystalline polymers,54 and the reversible photo-dimerization induced modulus change of the wrinkling polymers.20 In unexposed region, the wrinkle orientation evolution is attributed to the boundary effect1,19,55,56 and the consequence of the redistribution of the local stress field resulted from the successive rapid “trans/cis/trans” photoisomerization of azobenzene moieties.19,49-52 In our case, the photoisomerization-induced stress release of the wrinkling PDMS/PS/PDO3 system not only occurred in the exposed region but also spread to the adjacent unexposed region slightly, in where the wrinkle amplitude-towavelength ratio decreased from the initial ~1:20 (Figure 2a-c) to ~1:24 (Figure S9, Supporting Information). And the existence of the photoinert intermediate PS layer could inhibited the photoisomerization-induced stress release in the as-wrinkled PDMS/PS/PDO3 multilayer. Especially, when the multilayer systems had a thick PS intermediate layer and/or a thin PDO3 top film, the stress release of the as-wrinkled multilayer induced via light irradiation is nearly completely suppressed (Figure 5b and Figure S10, Supporting Information). Conversely, at the other extreme, the stress release-inhibiting effect of the photoinert layer for the PDMS/PS/PDO3 multilayer could be ignored in the case of a ultrathin PS film employed. Namely, the PDMS/PS/PDO3 multilayer with ultrathin PS film shows the similar wrinkle evolution behavior as that of the counterpart of PDMS/PDO3 system, in which the wrinkles in the exposed region could be completely erased (Figures S11 and S12, Supporting Information). This may be attributed to the significant effective modulus decrease 14 ACS Paragon Plus Environment

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with decreasing PS film thickness and less compressive stress retained in thin PS film,11,26 which could not effectively inhibit the photoisomerization-induced stress release of the as-wrinkled multilayer. In order to further assert the inhibiting effect of the photoinert PS film on the stress release of the multilayer system, a tetra-layer PDMS/PDO3/PS/PDO3 system was prepared. The light irradiation-induced wrinkle dynamic evolution in both exposed and unexposed regions of the wrinkling PDO3based tetra-layer system is roughly in accord with that in as-wrinkled PDO3-based trilayer system, which corroborates the above-mentioned results (Figure S13, Supporting Information).

Figure 5. Frame a shows the dependence of the wrinkle wavelength of the multilayers (hPS = 70 nm, hPDO3 = 172 nm) in exposed regions on the selective exposure time. Frames b and c show the dependence of the wrinkle wavelength of the multilayers on (b) hPDO3 (hPS = 70 nm) and (c) hPS (hPDO3 = 295 nm), before and after selective light exposure, respectively. It has been recognized that the long-period oxygen plasma (OP) treatment can lead to convert the top PDMS layer into a stiff silica-like (SiOx) layer, which has been well investigated and characterized in the previous reports.43,57,58 Interestingly, with an introduction of the stiff SiOx intermediate layer instead of the above PS intermediate layer (Figure 1 II), the as-prepared PDMS/SiOx/PDO3 multilayer system shows the similar heating/cooling-induced wrinkling behavior, while distinct different wrinkles 15 ACS Paragon Plus Environment

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morphology evolution upon visible-light irradiation compared to the PDMS/PS/PDO3 system. As shown in Figure 6a-e, large-area labyrinth wrinkles could also be triggered on the PDMS/SiOx/PDO3 system by cooling the 90 ℃-heated sample. From the plots of the surface wrinkle wavelength (λ) of PDMS/SiOx/PDO3 multilayers as a function of the applied OP oxidation time (tOP) with the fixed PDO3 top-layer thickness, we can clearly see that recorded λ demonstrate approximately linear trend with tOP in our case (30 s ≤ tOP ≤ 40 min) (Figure 6f). Based on the functional relationship between λ and the surface stiff layer thickness (i.e., hSiOx and hPDO3) of the PDMS/SiOx/PDO3 system according to Equation (1) and (2),25-28,45 we can deduce that the thickness of the asformed stiff SiOx layer hSiOx increases gradually with tOP, which is in accord with the previous results.43,58 It can also be confirmed from the mentioned results that the laminated SiOx/PDO3 stiff layers give rise to wrinkling jointly, which is similar to the above PDMS/PS/PDO3 multilayer system.

Figure 6. Optical images of the initial disorder wrinkles on the PDMS/SiOx/PDO3 multilayer system with a fixed PDO3 film thickness and different oxygen plasma oxidation time (tOP) (a-e). Plot of the wrinkle wavelength of the as-wrinkled


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PDMS/SiOx/PDO3 system as a function of tOP in the case of the fixed PDO3 thickness (f). Scale bar: 50 m. Likewise,

the

as-wrinkled

PDMS/SiOx/PDO3

multilayer

can

be

easily

photomodulated based on the facile spatiotemporal control of light. Surprisingly, after selective visible-light exposure, the initial labyrinth wrinkles are evolved into highly ordered ones in unexposed region, while the wrinkles in exposed region have been completely erased, resulting in the well-defined hierarchical surface wrinkle patterns (Figure 7). The in-situ optical observation shows that for the wrinkles in the exposed region, the wrinkle amplitude gradually decreases but the wrinkle wavelength remains roughly unchanged during the visible light irradiation (Figure 8). These observations are similar to the analogous photodegradable-polymer containing bilayer systems.59 And it is to be noted that the existence of photoinert SiOx intermediate layer induces an effect of “wrinkles erasing acceleration” (Figure 7f), which is different from the stress release inhibiting effect of photoinert PS intermediate film in PDMS/PS/PDO3 multilayers.

Figure 7. Optical images of the surface wrinkle micropatterns on the multilayers with different tOP after selective light-irradiation by different copper grids. tOP =5 min, 100 17 ACS Paragon Plus Environment

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mesh-sized square hole (S100) (a); tOP =5 min, 200 mesh-sized square hole (S200) (b); tOP =5 min, 400 mesh-sized round hole (R400) (c); tOP =10 min, graded copper grid simultaneously containing S100, S200, S300 and R400 (d); tOP =23 min, H100 (e). Frame d shows the dependence of the optical erasure time of the multilayer surface wrinkles as a function of tOP. Scale bar: 100 m. For the PDMS/PS/PDO3 system, the 90℃-heating/cooling results in surface wrinkling on the corresponding PDMS/PS bilayer system (Figure S2, Supporting Information), which means the residual localized compressive stress could be restrained in the wrinkling PS film. However, there are no wrinkles observed on the PDMS/SiOx bilayer after the same heating/cooling process in the absence of PDO3 film as the top layer (Figure S14a,b, Supporting Information). Simultaneously, the ethanol-swelling-driven surface wrinkling on the PDMS/SiOx bilayer can be reversibly released with ethanol evaporation (Figure S14c,d, Supporting Information), which is consistent with the previous report.7 This is an indication that the localized compressive stress resulted from the heating/cooling or solvent-induced swelling/deswelling processing cannot be restrained in the SiOx layer for the stress-relief wrinkling formation of the PDMS/SiOx bilayer. We infer that during cooling of the 90 ℃-heated PDMS/SiOx/PDO3 multilayers, the vitrified PDO3 top film suppressed the stress release in the underlying SiOx layer and then gave rise to surface wrinkling jointly. In this case, the SiOx layer with increased thickness behaves just like a slightly deformed spring and shares an increased amount of residual compressive stress applied to the top PDO3 film from the PDMS substrate. When exposed to visible light, photosoftening and stress release effect co-exist in the exposed PDO3 film, resulting in invalidation of the stress release inhibiting effect of the PDO3 film for the aswrinkled SiOx layer in exposed area. Consequently, the wrinkling PDMS/SiOx/PDO3 18 ACS Paragon Plus Environment

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multilayer in exposed region exhibits an enhanced stress releasing rate due to the simultaneous stress release occurred in the as-wrinkled PDO3 and SiOx layer during light

irradiation.

And

the

residual

compressive

stress

of

the

wrinkling

PDMS/SiOx/PDO3 multilayer in exposed region have been rapidly released to lower than the multilayer-defined critical wrinkling value (c) during light irradiation, which resulted in the unobserved wrinkles wavelength evolution of the wrinkling multilayer. Ultimately, the accelerated stress release dominates the PDMS/SiOx/PDO3 multilayer wrinkle evolution along with the decreasing amplitude until the wrinkles in exposed region have been fully erased. For the PDMS/PS/PDO3 multilayer, stress release is suppressed and hence photosoftening effect plays a dominant role in the wrinkle evolution, leading to decrease in the wavelength in exposed regions. Furthermore, the selective

exposure

yields

hierarchical

surface

wrinkling

patterns

on

the

PDMS/SiOx/PDO3 multilayer, which possess a wide range of dynamically and optically tuned wrinkle wavelength, compared to the corresponding PDMS/PDO3 bilayer system upon regulating the stiff layers thickness and/or the photomasks. (Figures S15 and S16, Supporting Information). Most importantly, this stress relaxation acceleration effect resulted from the existence of laminated SiOx interlayer in the PDO3-based multilayer system, may be applied for regulating the surface wrinkle morphology of other wrinkling-based adaptive systems with shorter response times and/or smaller stimulus.


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Figure 8. Morphological evolution on the PDMS/SiOx/PDO3 system during the selective exposure via H100 (tOP = 23 min). (a-i) In-situ optical microscopy observation of the optical erasing of the wrinkles during selective light-irradiation; (bh) the copper grid was still on the exposed multilayer surface; and (i) the copper grid was removed from the exposed multilayer surface.

Conclusions In summary, we have studied the azo-containing film as a stimuli-responsive surface to regulate the localized stress and the accompanying wrinkle evolution of the as-wrinkled multilayer systems with the different intermediate photoinert layer. The existence of laminated stiff photo-inert interlayer induces stress relaxation inhibiting/acceleration effect on the photo-modulated wrinkle transformation of the designed multilayer systems. This phenomenon strongly depends on the material properties of the photoinert intermediate layer. As a result, upon selective exposure, the initially disordered wrinkles are evolved into highly oriented ones in unexposed 20 ACS Paragon Plus Environment

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regions, while the wrinkles in exposed regions are erased or retained while the wavelength obviously decreases. Light-induced stress-release or photosoftening of the wrinkling azo-containing film plays a dominant role in this wrinkle evolution. Undoubtedly, this outcome not only promotes the understanding of the process that how the intermediate layer influences global wrinkle evolution behavior of the multilayer systems, but also offers a new avenue to realize desirable hierarchical surface patterns on multilayer functional devices for the applications such as epidermal electronics, photoelectric devices, advanced optical devices, sensors and so on.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures for Optical images and/or AFM height images of the wrinkle patterns on the multilayer systems.

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID Shichun Jiang: 0000-0003-3818-0230 21 ACS Paragon Plus Environment

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Conghua Lu: 0000-0003-3995-8067 Notes The authors declare no competing financial interest.

Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of China (Nos. 21704033, 21574099).

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Table of Contents A wrinkling-based photoresponsive multilayer system has been developed for harnessing surface morphology evolution upon visible-light irradiation. Different laminated intermediate layer results in yield diverse hierarchical surface micropatterns on the multilayers via selective exposure. This study sheds light on the self-organized wrinkling behaviors of laminated composites in engineering and nature and paves a new avenue to realize the hierarchically patterned surfaces. Keywords: stimuli-responsive, wrinkling, multilayer, surface patterns, evolution

Chuanyong Zong1, Umair Azhar1, Juanjuan Wang2, Luqing Zhang1*, Yanping Cao3, Shichun Jiang2*, and Conghua Lu2* Photo-controllable Wrinkle Morphology Evolution on Azo-Based Multilayers for Hierarchical Surface Micropatterns Fabrication


Photocontrollable Wrinkle Morphology Evolution ... - ACS Publications

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