Macroscopic Geometry-Dominated Orientation of Symmetric

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23741−23749

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Macroscopic Geometry-Dominated Orientation of Symmetric Microwrinkle Patterns Han-Yu Hsueh,*,†,‡ Ming-Shiung Chen,† Chya-Yan Liaw,§ Yu-Cheng Chen,§ and Alfred J. Crosby*,§ Department of Materials Science and Engineering and ‡Innovation and Development Center of Sustainable Agriculture, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China § Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 20, 2019 at 23:46:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Orientated wrinkle patterns with controlled microarchitectures are highly attractive because of their potential and broad application in technologies ranging from flexible electronic devices to smart windows. Here, we demonstrate a macroscopic, geometry-dominated strategy to fabricate symmetric microwrinkles with precisely controllable pattern dimensions and orientations through a dynamic interfacial release process. The release-induced approach is based on the release of multilayer elastomer composites from polymeric sacrificial layers in solutions combined with crosslinking-induced contraction of the elastomer substrates. Crosslinking-induced contraction provides the driving force for developing and stabilizing surface wrinkle formation, whereas the polymeric sacrificial layer provides a mild and simultaneous release process to form orientated wrinkles through kinetic control of local strain development. The macroscopic shape of the composite controls release kinetics, hence strain history, leading to the generation of photonic reflective surfaces. Moreover, stable wrinkles fabricated from various materials including metals, ceramics, and carbons can be achieved. This versatile, mold-free, and cost-effective platform technology demonstrates how global strain distributions can be harnessed through kinetics to drive local pattern development. KEYWORDS: wrinkle, surface, release-induced, hybrid materials, pattern

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INTRODUCTION

An attractive attribute of surface wrinkles, which has motivated much of the research on this topic, is that the wavelength (λ) and amplitude (A) are predictably defined by the intrinsic properties of the materials, the film thickness (tf), and the applied in-plain strain (ε0). For small applied strains, the wavelength follows this well-established relationship18

Surface patterns at the microscale and nanoscale are of great importance from scientific and technological perspectives. Recently, mechanical instabilities in soft materials, specifically surface wrinkling, have received significant attention across various fields of research because of their simplicity, ubiquity, and applicability to arbitrary surface morphologies.1−3 In a bilayer system with a stiff film attached to a compliant substrate, surface wrinkling occurs when an in-plane compressive strain exceeds a critical value. The adopted outof-plane sinusoidal deformation, or wrinkle, minimizes the total potential energy of the system, a mechanistic process that has been studied extensively.1−15 In-plane compressive strains have been shown to be induced by a variety of stimuli, including thermal changes,4−6 swelling,7−9 mechanical stretching/ compression,10−13 capillarity-induced stretching,14,15 and cross-linked gels.16,17 Unless permanent material changes occur, surface wrinkles disappear after the compressive strain is removed. © 2019 American Chemical Society

ij E ̅ yz λ = 2πtf jjj f zzz j 3Es̅ z k {

1/3

(1)

where E̅ = E/(1 − ν2) is the plane-strain modulus, E is Young’s modulus, and ν is Poisson’s ratio. The subscripts “f” and “s” refer to the top hard thin layer and the bottom soft substrate, respectively. The wrinkle amplitude can be expressed as Received: March 27, 2019 Accepted: June 4, 2019 Published: June 4, 2019 23741

DOI: 10.1021/acsami.9b05264 ACS Appl. Mater. Interfaces 2019, 11, 23741−23749

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Formation of Symmetric Wrinkles on an Elastomer Substrate after CrosslinkingContraction of PDMS and Releasing of Water-Soluble Polymers as Sacrificial Layersa

a (a) Casting the liquid PDMS precursor [light green, purple, pink, and gray represent liquid PDMS precursor, PS, polymeric sacrificial layer (e.g., PEG), and substrate, respectively]; (b) thermal curing to solidify liquid PDMS at 70 °C; (c) dissolve the polymeric sacrificial layer to release the bilayer composites, and (d) formation of symmetric wrinkle patterns.

ij ε yz A = tf jjj 0 − 1zzz j εc z k {

pre-patterning is a significant disadvantage. To find a way of dealing with the difficulty for the limitation, we propose a simple, yet general, interfacial-release process that generates symmetric microwrinkle patterns governed by macroscopic shape-controlled kinetics of strain release. This process provides new lessons of how kinetics can guide the ordering of equilibrium elastic structures, leading to the fabrication of large-area surfaces with predictable and advanced properties. In this study, by combining crosslinking-induced biaxial compression and guided interfacial release from a sacrificial layer, highly ordered lamellar wrinkles are successfully fabricated on a wide range of soft elastomers with different material characteristics. We performed MATLAB calculations to analyze the angles between wrinkle wavefronts for identification of their geometry-dependent characteristic patterns. Furthermore, wrinkled structures composed of different materials, including block copolymers (BCPs), metals, ceramics, and carbons are fabricated, showing the feasibility of using this method for a diverse range of applications. To the best of our knowledge, this study is the first to describe the use of macroscopic edge shapes to extensively order elastically generated surface wrinkles. We believe that this approach will find broad use in many practical applications, such as smart windows, elastomeric displays, and flexible electronics.

1/2

(2)

where εc is the critical strain given by 1 ij 3E ̅ yz εc = − jjj s zzz 4 jk Ef̅ z{

2/3

(3)

Because of this predictable control, as well as the ability for wrinkles to organize into predictable patterns, wrinkles have been proposed and demonstrated for a diverse range of advanced applications, such as tunable optical devices,19−21 responsive microfluidic channels,22 microlens arrays,22,23 thinfilm metrology,24−26 dry adhesion,27,28 particle sorting,29,30 marine antifouling,31,32 twisted nematic liquid crystal displays,33 cell alignment,34 flexible electronics,35−39 actuatedreversible devices,40−42 and sensors.43,44 Fabricating wrinkles on polymer surfaces has been achieved by a wide variety of strategies, each with their own advantages and drawbacks. Mechanical stretching has been demonstrated extensively to fabricate two-dimensional surface wrinkles.10−13 Whereas orientated wrinkles are easily generated in the direction perpendicular to the principal compressive stress, scaled production by applying mechanical stretching is inefficient because of the difficulty in applying extensive and uniform stresses. Differential expansion, because of swelling or thermal change, is another common strategy for producing wrinkles. With this strategy, the most commonly achieved morphology is a labyrinth of wrinkles locally arranged in a herringbone pattern.45,46 Although visually interesting, these globally uncontrolled patterns are not practical for the control of properties, such as surface photonic effects. To overcome this limitation, numerous research groups have demonstrated the use of pre-applied microscale patterns to template the alignment of wrinkles.4,47−53 Although this can lead to globally aligned surface wrinkle patterns, which can be either fixed or responsive to environmental changes, the cost of microscale

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RESULTS AND DISCUSSION Fabrication and Identification of Polymeric Wrinkle Morphologies. As illustrated in Scheme 1a, glass substrates were first exposed to ultraviolet ozone irradiation (UV−O3) at room temperature to create clean and hydrophilic substrates. Water-soluble polymer thin films, such as polyethylene glycol (PEG), poly(acrylic acid), or poly(vinylpyrrolidone), were then spin-coated onto the glass substrates to serve as sacrificial layers. Subsequently, a stiff polymer film [e.g., polystyrene (PS)] was spin-coated on to the sacrificial layer, and then liquid poly(dimethyl siloxane) (PDMS) precursors (e.g., 23742


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Figure 1. (a) Optical microscopy images (scale bar = 50 μm) of various buckle patterns with different PDMS elastic moduli (E) (curing agent ratio ranging from 10:1 to 30:1) and PS film thicknesses (PS solution concentrations ranging from 1 to 5 wt %). Areas surrounded by orange lines indicate period-doubles structures, and white arrows identify folds. (b) Relationships between wavelength and amplitude of the PDMS elastomers (20:1) with varying PS top layer thickness after curing at 70 °C for 2 h. (c) Relationships between wavelength and amplitude of the PDMS elastomers (20:1) covered with PS top layers from 2 wt % PS solution concentration after curing at 70 °C for varying post-annealed time. The insets show AFM images of buckle structure after 1 and 2.5 h of post-annealing time, respectively.

theoretical (Figure S3) and experimental analyses agree well with each other, indicating that the theoretical model for bilayer systems was applicable to our system. In addition, the characteristics of wrinkles can be modified by exposing the bilayer composite films to a post-annealing step. A longer post-annealing time enhances the cross-linking degree of PDMS elastomers, resulting in a harder PDMS substrate (i.e., increase of modulus) so as to decrease their wrinkle wavelengths. As the post-annealing temperature was increased, the wrinkle wavelengths of the composites decreased from 13.14 to 9.63 μm as shown in Figure 1c. In general, for a bilayer sample with fixed top layer thickness, changes in sample amplitude are directly proportional to the corresponding wavelength. Nevertheless, in this case, the amplitudes first increased slightly and then decreased with the post-annealing time. The nonproportional changes in amplitudes resulted from the changes in the corresponding buckle waveforms; the sample surface shows period-doubled structures after shortterm post-annealing time and then becomes wrinkles after long-term post-annealing time. Table S2 summarizes the average wavelengths, amplitudes, and aspect ratios. Beyond the control of wrinkle wavelength and amplitude, certain combinations of film and substrate properties led to the generation of surface structures from secondary elastic instabilities, including period-doubling (highlighted by orange lines in Figure 1a) and folding structures (Figure 1a, white arrows). Figure S4a,b displays the corresponding AFM images of the period-doubling structures and folds, respectively. The formation of these structures is associated with materials that have a higher degree cross-linking (e.g., high modulus elastomers) or decreased bending resistance (e.g., ultrathin stiff films).

uncured, mixed components of Dow Corning Sylgard 184) were cast on to the stiff polymer film. The multilayer composite was then heated at 70 °C for several hours to cross-link the PDMS elastomer layer (Scheme 1b). Once cross-linked, the composite samples were immersed in deionized water (DI water) to dissolve the sacrificial layer (Scheme 1c), guiding release from the glass substrate. Upon release, symmetric wrinkles with geometry-dominated photonic reflective surfaces were spontaneously generated (Scheme 1d). As expected, the relationships in eqs 1 and 2 accurately predict the wrinkle wavelength and amplitude based on the dimensions of the stiff upper layer and the elastic modulus of the elastomer. Figure 1a shows optical microscopy images of the buckled surface morphologies created by the control of the thickness of the stiff polymer layer and the elastic modulus of the PDMS elastomers. The PDMS elastic modulus was varied by controlling the cross-link density (Figure S1), such that the modulus ranged from 0.05 to 1.67 MPa for samples discussed here. Figure 1b shows the relationships between the wavelength and amplitude of the wrinkled structures, as measured by atomic force microscope (AFM). The thickness of the polymeric top layer is controlled by the polymer solution concentration and spin-coating speed, such that the polymer film thickness increases with increasing solution concentration at a constant rotational speed.54 With the increase of the top layer thickness, the wavelength of the bilayer composites increased from 4.38 to 40.35 μm, and their amplitudes increased from 85.09 to 1140.66 nm. The corresponding AFM images can be seen in Figure S2. Additionally, Tables S1 summarizes the achieved average wavelengths and amplitudes. The average aspect ratio (A/λ) ranged from 0.019 to 0.03. The wavelengths obtained from 23743


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Figure 2. (a) Optical microscopy image of square-shaped PS/PDMS bilayer composites shows mutually perpendicular wrinkled structures. (b) AFM image of the corresponding wrinkle morphologies and (c) weighted histogram distribution of characteristic angles.

Control of Symmetric Wrinkle Surfaces. As mentioned above, although various wrinkle fabrication approaches have been developed, it is still a challenge to produce wrinkles with stable order and controlled pattern distribution. In general, surface patterns obtained by instability-mediated approaches afford features with well-defined mean length scales, but they are often randomly distributed. The orientation of wrinkles depends on the direction of applied forces; therefore, substrates that shrink in an uncontrolled manner lead to disordered wrinkle patterns. In our process, well-defined and symmetric wrinkle patterns can be generated spontaneously through the use of a sacrificial layer. To demonstrate the significant advantage of the interfacial release process, squareshaped samples (30 × 30 × 5 mm3) were made. As shown in Figure 2a, mutually perpendicular wrinkles (90° included angle between two wrinkle wavefronts) with long-range order were observed. Because of their order and dimensional regularity, regularly repeated and orientated wrinkled structures, the wrinkled bilayer composites serve as photonic crystal reflectors, displaying a striking “X” reflection across the sample surface (see inset in Figure 3a). AFM measurements reveal that the wavelength and amplitude are ∼10.5 μm and ∼195 nm, respectively (Figure 2b). For quantitative analysis of the wrinkle orientations, the dominant orientations of characteristic angles in the images were determined using a self-made computational code written in MATLAB. The Canny edge detector was first applied to an input image in grayscale to trace along the edges. Second, edge pixels were linked with their neighbors to generate lists of edge contours, where edge contours less than 10 pixels were discarded. The edge contours were then fitted with discrete line segments. The dominant wrinkle angles were highlighted in different colors as depicted in Figure S5. The length and angle of each segment were calculated and used to plot a histogram, showing the distribution of characteristic angles between 0° and 180°. Histogram distributions were normalized with the sum of segment lengths. Figure 2c shows the corresponding weighted histogram distribution of characteristic angles, with the angle defined clockwise from the horizontal line. It is clear that the dominant angle populations were observed at ∼90° and ∼0°. Symmetric and oriented wrinkles with geometry-dominated photonic patterns (i.e., external shape-driven structural reflection) can be generated

Figure 3. Optical microscopy images of square-shaped PS/PDMS bilayer composites released (a) with and (b) without a polymeric sacrificial layer. The insets above and below show the corresponding FFTs and photographs, respectively.

spontaneously through the interfacial release approach. The periodicity and organization of the surface structures can also be measured by a numerical two-dimensional fast Fourier transform (FFT) analysis. The insets shown in Figure 3 are calculated FFT patterns of the tested samples, which indicate lamellar wrinkles with characteristic average wavelength and preferred orientation.9,55 Mechanism for Interfacial Release-Induced Formation of Symmetric Wrinkles. To demonstrate the importance of the release kinetics for the generation of longrange ordered wrinkle structures, we released bilayer composites from substrates without a solvent-soluble sacrificial layers by using specific substrate materials, such as mica or polyethylene terephthalate in place of glass. Under these conditions, as shown in Figure 3b, wrinkled surfaces were formed with significant disorder. The FFT patterns shown in the Figure 3b inset further support this finding of long-range disorder. Furthermore, compared with the photonic films produced with controlled sacrificial layer release kinetics, the films produced without sacrificial layers often had damaged and cracked surfaces. These undesirable traits are associated with the inhomogenous and rapid release of the crosslinking23744


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Figure 4. Optical micrographs of the diverse wrinkle orientations generated over all the surface regions from the sample edge to the center. The scale bar is 100 μm. The white arrows in zones I and III indicate the formation of zigzag-based herringbone patterns and folds, respectively.

Figure 5. Optical micrographs and their corresponding histograms of characteristic angles obtained from the PS/PDMS bilayer composites with various external sample shapes, including triangle, pentagon, and hexagon. The scale bars are 50 μm and the insets show photographs of the centimeter-sized samples with geometry-dominated photonic reflection.

induced biaxial strain in the elastomer layer. Comparing the wrinkled structures released with and without sacrificial layers indicates the key to generating symmetric wrinkle patterns is the sequential and oriented release process provided by the mild dissolution of the sacrificial layer. To further understand the mechanism for interfacial releaseinduced formation of symmetric wrinkles, sample surfaces were examined from the sample edges to their center. As shown in Figure 4, three different zones are clearly visible. Zone I is close to the sample edges; zone III is at the center of the sample; and zone II is located at the region between zone I and III. As depicted in zone I, regular wrinkles perpendicular to the

sample edges formed spontaneously after release and propagated toward the center region. When moving further away from the edges, straight and regular wrinkles slightly distort and form zigzag-based herringbone patterns (see zone I in Figure 4, white arrow) because of the release of neighboring edges giving rise to equibiaxial compression locally. With further increase of the distance, zigzag-forming wrinkles parallel to the sample edges start to occur. The orientation of the zigzag-forming wrinkles is slightly parallel to the two mutually perpendicular sample edges, and the geometrical parameters, including length, width, and height of the zigzags, are uniform. After passing the transition zone, the zigzag23745


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Figure 6. (a) Photograph of a centimeter-sized sample with the letters of our school name abbreviation “NCHU”. Optical micrographs of wrinkled surfaces composed of (b) PS-b-P2VP BCPs, (c) SiO2, (d) Au, and (e) carbon materials fabricated via the interfacial release-induced wrinkles formation process.

examples. More complex sample designs via laser cutting or mask printing could be made to achieve complex surface wrinkle orientations for specific applications. As an example of arbitrary shapes and the feasibility of applying this interfacial release process to large area patterns, a sample with the letters “NCHU” was created (Figure 6a). A sacrificial layer was covered with a mask with letters “NCHU” before spin-coating PS solutions on it. After wrinkling, only the regions covered with the PS film showed photonic reflection owing to the oriented wrinkle arrangement. The square-shaped testing samples were 10 × 10 × 0.5 cm3 (length × width × height), showing the potential to use this process for large-area device fabrication. Furthermore, in this study, we used the spincoating process to achieve uniform thin films with various thicknesses; however, different coating processes, such as slot coating and gravure coating, should allow for similar control. The formation of surface wrinkle morphology is a thermodynamically driven process via kinetically controlled mechanical release, thus providing rich potential for broad applications. Interfacial Release-Induced Symmetric Wrinkles with Various Materials. Furthermore, to demonstrate the versatility of the dynamic interfacial release method for the creation of well-defined patterns, wrinkled surfaces were prepared using various types of materials. Figure 6b shows the wrinkled surface in poly(styrene-b-2-vinylpyridine) (PS-bP2VP) BCPs. BCPs can self-assemble into a variety of highly ordered nanostructures including sphere, cylinder, gyroid, and lamellae, owing to the incompatibility of the constituted blocks with a readily adjustable size scale depending on their molecular weight.56 By taking advantage of BCP self-assembly and surface wrinkling, hierarchical structures (nanometer-sized BCPs’ microphase-separated domains within a micrometersized wrinkled surface) can be fabricated. In addition to the structural hierarchy, the surface properties can be tailored by changing different polymer-constituted components. For instance, PS-b-P2VP BCP wrinkles could be employed as a novel platform for aligning metallic nanoparticles (NPs)

forming wrinkles transform into mutually perpendicular wrinkle patterns, as shown in zone II in Figure 4. We hypothesize that the formation of perpendicular kinks is due to interacting uniaxial strain fields associated with the release of neighboring edges, and the interaction leads to a biaxial state of strain. The confirmation of these processes and the development of a model is a current research focus. In the center region, the wrinkles become disordered and form a pattern of multiple concentric circles (zone III). The zigzag-forming concentric wrinkles are distributed randomly throughout the center region (see dashed circles in zone III in Figure 4), likely associated with the lack of controlled strain release as all edges propagate simultaneously at the final moments of release. Also, the height of whole sample surface gradually rises from the edges to the center region [height (h)/radius (r) ≈ 1:100] determined by AFM as shown in Figure 2b. According to the above discussion, the distribution of the lateral strain in the symmetric wrinkle area (zone II) identified in Figure 4 should be highly dependent on the external shape of the samples. To test this hypothesis, various external shapes were fabricated to control the resulting wrinkle orientations. As shown in Figure 5, wrinkles with specific shape-aligned orientations, including triangle, pentagon, and hexagon, were created for corresponding external sample shapes. Quantitative analysis of the wrinkled surface characteristic angles (Figure 5) indicates that the dominant populations occurred at ∼60°/ 120°, ∼108°/72°, and ∼120°/60°, matching the included angles of the triangle, pentagon, and hexagon, respectively. These results evidently show that the orientation of wrinkle patterns can be successfully controlled by the kinetic release of strains, as dictated by the interfacial release of bilayer composites. Because of the release kinetics being dictated by the enhanced dissolution of a sacrificial layer along the composite edges, the macroscopic shape of the composite templates results in wrinkle ordered morphology. A geometric sample shape at the centimeter scale dictates the arrangement of wrinkle patterns at the micron scale. The samples with different geometric shapes shown above are only simple 23746


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with Mn = 260 kg mol−1 and Mn/Mw = 1.05. PEG was purchased from Aldrich Inc. with Mw = 20 000 g mol−1 and diluted into 1.5 wt % solution in water. Before spin-coating of PS solutions, the glass slide was rinsed with toluene and isopropanol, followed by cleaning with UV−ozone for 20 min. Afterwards, the PEG solution was spin-coated on a UV−ozone-treated glass slide with 3000 rpm for 30 s. The glass slide with a polymeric sacrificial layer (i.e., PEG) was prepared. Subsequently, the PS was dissolved in toluene to concentrations ranging from 1 to 5 wt % and then spin-coated on the polymeric sacrificial layer/glass slide with 3000 rpm for 60 s. After that, the PS film thickness was measured by the interferometry profilometer (MP100-ME thin film analyzer) and an AFM (Veeco Dimension 3100). The PS elastic modulus E = 3.3 GPa was obtained from literature and their Poisson’s ratios (υPS) were assumed to be 0.33.60 The film should be used immediately after being prepared. Wrinkle Formation Process. Elastomeric substrates were made of cross-linked PDMS, Dow Corning Sylgard 184 elastomer kit. The mixing ratios of liquid PDMS precursor/cross-linker included 5:1, 10:1, 20:1, and 30:1 by weight in order to vary the substrate modulus. The average PDMS elastic moduli (E) are 0.05, 0.18, 0.88, and 1.67 MPa, corresponding to the mixing ratios given as 5:1, 10:1, 20:1, and 30:1. The mixed liquid PDMS (30 g) was cast on the PS/PEG/glass slide in a plastic Petri dish (10 cm2) and degassed in a moderate vacuum for 40 min. Subsequently, the sample was cured in a 70 °C oven for 1−3 h and then cooled to room temperature (∼25 °C). The prepared PDMS thickness was approximately 3 mm in this study. Part of the cured PDMS elastomer without PS and PEG was reserved and punched into a dogbone shape (20 mm × 5 mm) for modulus measurement. The elastic modulus, E, for each sample was measured using tensile tests on a Shimadzu’s AGS-X with a 50 N load cell, with a stretching rate of 100 mm/min. A stress−strain curve for each sample was fitted with the neo-Hookean model and their elastic modulus could be determined. The solidified samples after crosslinking were carefully shaped into specific shapes (30 × 30 × 5 mm in length × width × height), including square, triangle, pentagon, and hexagon, by using blades. The sample edges should be sharp and smooth for following formation of ordered wrinkle patterns. After that, the shaped composite samples were immersed into DI water for 12 h to dissolve the PEG layers and then dried in a vacuum oven at 50 °C for 3 days. As a result, symmetric wrinkles with geometrydominated photonic reflection form spontaneously. All the experiments of wrinkle formation process were repeated five times under each condition. Wrinkles Form in Various Materials. Before spin-coating, all glass slides were rinsed with toluene and isopropanol, followed by UV−ozone cleaner for 20 min. Afterwards, PEG was spun on them to form PEG/glass substrates. For PS-b-P2VP BCP wrinkles, the PS-bP2VP was purchased from Polymer Source Inc. with Mn = 66 kg mol−1, Mn/Mw = 1.05, and f vP2VP = 0.28. The PS-b-P2VP was dissolved in toluene with a weight concentration of 1% and then spin-coated on the PEG/glass slides with 3000 rpm for 60 s. For ceramic wrinkles, a sol−gel reaction was carried out for SiO2 wrinkles. The silica precursor mixtures were prepared by mixing TEOS, HCl(aq) (0.1 M), and methanol [weight fraction of TEOS/HCl(aq) (0.1 M)/methanol = 1/1/1.5], stirring at room temperature for 7 days to form transparent solutions and then spin-coated on the PEG/glass slides with 1000 rpm for 30 s. Sequentially, the samples with silica precursors were treated under controlled humidity at 40 °C for 48 h for aging and drying. For Au wrinkles, organosoluble Au NPs (i.e., dodecanethiolfunctionalized Au NPs) were purchased from Sigma-Aldrich. The Au NPs were purified by washing with mixtures of CHCl3 and acetone, followed by centrifugation (sedimentation and re-dispersion) cycles. The Au NPs were dispersed in toluene [2% (w/v)] to form a light purple and transparent solution, and then spin-coated on the PEG/ glass slides with 3000 rpm for 60 s. For carbon wrinkles, PCBM was purchased from Sigma-Aldrich and dispersed in chlorobenzene (1.0 wt %), followed by spin-coating on the PEG/glass slides. Subsequently, after casting liquid PDMS precursors, curing, and then releasing the samples in water, wrinkled structures composed with different materials were fabricated.

because the electron-donating nitrogen atoms can coordinate with the metal ions;57 polystyrene-b-polylactide58 or polystyrene-b-poly(L-lactide)59 BCP wrinkles would show hierarchical nanoporous surfaces after hydrolysis of the ester groups in polylactide chains. For ceramic wrinkled surfaces, the sol−gel process is an ideal method to form stiff ceramic top layers for subsequent surface wrinkling. Functional ceramic oxides including the oxides of Al, Si, Ti, Zn, and Zr can be synthesized through sol−gel processes. Figure 6c demonstrates silicon dioxide (SiO2) wrinkles synthesized using a sol−gel reaction, which involves a mixture of tetraethoxysilane (TEOS), HCl (0.1 M), and methanol at room temperature. Nevertheless, cracks appear during the release process because of the inherent shrinkage of sol−gel films and high stiffness of ceramics. Suitable surfactants such as diethylolamine could be used to inhibit the formation of cracks. We are currently in the process of improving the stability and quality of the ceramic films. Stretchable conductive wrinkled structures have promising applications in many areas, including stretchable electronics, precision metrology, optical gratings, surface engineering, packaging, energy harvesting, and storage. For homogeneity of the dispersed particles, conductive NPs should be surfacemodified with ligands or polymer brushes. Herein, organosoluble Au NPs (i.e., dodecanethiol-functionalized Au NPs) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were used as conductive precursors. Figure 6d,e presents the formation of gold and carbon wrinkled surfaces, respectively. Because of the significant differences in the mechanical strengths between gold and carbon, their wrinkle morphologies and wavelengths are extremely different. In addition to inorganic conductive materials, organic conductive polymers could also be adopted for fabrication of conductive polymeric wrinkles and the work is still in progress. Therefore, this dynamic interfacial release approach for wrinkled surfaces formed by a variety of materials offers great promise for the industry and nanotechnologies.

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CONCLUSIONS In conclusion, we report a simple and versatile interfacial release-controlled approach for preparing stable and ordered wrinkled surfaces with predictable pattern dimensions and orientations. This method relies upon the kinetic release of local strain, enabled by the enhanced dissolution of a sacrificial layer along the edges of a multilayer composite. Thermally induced cross-linking causes the volumetric shrinkage of the composite, driving the development and stabilization of the surface wrinkling morphology. The arrangement of edges that define the macroscopic shape of the composite is found to dictate the internal orientation of micrometer-sized wrinkle patterns. Thus, complex sample designs were used to achieve specific surface microwrinkle orientations. Additionally, stable wrinkled structures composed of various materials, including metals, ceramics, and polymers, were demonstrated. To the best of our knowledge, this novel facile way is the first surface wrinkling method to demonstrate microwrinkle pattern control by macroscopic geometry. This approach has potential beyond the fabrication of desirable photonic surfaces, such as the spatial control of adhesion, surface wetting, or cell growth.

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EXPERIMENTAL SECTION

Preparation of Glassy Top Films. Glassy top films were composed of PS purchased from Scientific Polymer Source Inc. 23747


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ACS Applied Materials & Interfaces Characterization. Images of wrinkled surfaces were examined through a tapping-mode scanning probe microscope. A Dimension3100 AFM with an Olympus AC200TS microcantilevers was utilized at room temperature. A rectangular silicon tip was used in dynamic force mode experiments with a spring force of 5 N m−1 and a scan rate of 1 Hz. Quantitative Analysis by MATLAB (R2014a). This script was used for determining the dominant orientations of strokes present in an input image. It constructed a histogram showing the amount of strokes in a given orientation. The code consists of three main parts. (1) stats: the outlines of the wrinkles were detected by the Canny edge detection method. After edge detection, disconnected pixels were linked together into lists of connected edge points (edgelink.m). The outlines were further simplified by replacing the connected edge points by fitted line segments, based on a specified, maximum allowable fitting tolerance (lineseg.m). The script then calculates the lengths and angles of each line segment. The files (edgelink.m, findendsjunctions.m, lineseg.m, maxlinedev.m, drawedgelist.m) were obtained from Prof. Peter Kovesi’s website (http://www.peterkovesi. com/matlabfns/) (2) histwc: counts the cumulative lengths in specified bins. Each bin contains a range of angles determined by the number of bins and the upper and lower limits of the bin. The file, histwc.m, is modified from the source code written by Mehmet Suzen (http://goo.gl/j8tAZD) (3) showhist: generates a normalized weighted histogram by dividing the cumulative lengths within each bin by the sum of lengths. More details can be found on the following website: https://github.com/dupypy/stroke-orientation.

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MY2 and 107-2221-E-005-021-. This work is also supported in part by the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. A.J.C. acknowledges the financial support of the Human Frontier Science Program (HFSP).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05264. PDMS elastic solid modulus (E) for the testing samples with varying curing agents; AFM images of the surface buckle morphologies resulted from PS top layers with different thicknesses (i.e., PS solution concentrations); scaling relationship of buckling wavelength versus PS solution concentration; AFM images of the surface buckle morphologies of period-doubled structures and folds; the dominant orientations of wrinkle angles determined by self-made computational code written in MATLAB and highlighted in different colors; a summary of the average wrinkle wavelength, amplitude, and aspect ratio of PDMS elastomers (20:1) covered with different PS top layer thicknesses; a summary of the average wrinkle wavelength, amplitude, and aspect ratio of bilayer composites for varying post-annealing times (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 886-4-22840500 ext. 506. Fax: 886-4-22857017 (H.-Y.H.). *E-mail: [email protected], [email protected]. Phone: 413-577-1313 (A.J.C.). ORCID

Han-Yu Hsueh: 0000-0003-2850-7279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology of the Republic of China, Taiwan, for financial support under contract no. Grant MOST 106-2218-E-005-00923748


Research Article

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Macroscopic Geometry-Dominated Orientation of Symmetric

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