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Precision-Trimming 2D Inverse-Opal Lattice on Elastomer to Ordered Nanostructures with Variable Size and Morphology Haoran Zhan,†,‡,# Yanqiu Chen,*,†,# Yu Liu,*,§ Woonming Lau,*,∥ Chao Bao,† Minggan Li,⊥ Yunlong Lu,† Jun Mei,† and David Hui¶ †

Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu, 610207, China Beijing Computational Science Research Center, Beijing, 100093, China § School of Mechanical Engineering, Jiangnan University, Wuxi, 214122, China ∥ Center for Green Innovation, School of Mathematics and Physics, University of Science & Technology Beijing, Beijing, 100083, China ⊥ Department of Chemical Engineering, Ryerson University, Toronto, Ontario M5B 2K3, Canada ¶ Department of Mechanical Engineering, The University of New Orleans, New Orleans, Louisiana 70148, United States ‡

S Supporting Information *

ABSTRACT: A low-cost and scalable method is developed for producing large-area elastomer surfaces having ordered nanostructures with a variety of lattice features controllable to nanometer precision. The method adopts the known technique of molding a PDMS precursor film with a close-packed monolayer of monodisperse submicron polystyrene beads on water to form an inverse-opal dimple lattice with the dimple size controlled by the bead selection and the dimple depth by the molding condition. The subsequent novel precision engineering of the inverse-opal lattice comprises trimming the PDMS precursor by a combination of polymer curing temperature/time and polymer dissolution parameters. The resultant ordered surface nanostructures, fabricated with an increasing degree of trimming, include (a) submicron hemispherical dimples with nanothin interdimple rims and walls; (b) nanocones with variable degrees of tip-sharpness by trimming off the top part of the nanothin interdimple walls; and (c) soup-plate-like submicron shallow dimples with interdimple rims and walls by anisotropically trimming off the nanocones and forming close-packed shallow dimples. As exemplars of industrial relevance of these lattice features, tunable Young’s modulus and wettability are demonstrated. monolayer having the hexagonal 2D crystal structure.7−21 Subsequently, the resultant 2D colloidal crystal is used as a template to mold thermally or UV-curable low-density polymer, like polydimethylsiloxane (PDMS),14 with the viscous precursor solution forming a thin layer infiltrating into the close-packed monolayer of beads floating on water. An orderedsurface nanostructured polymer film having an array of dimples with nanothick interdimple rims and walls is finally produced by subsequent curing of the precursor and preferential dissolution of the beads.15−21 The center-to-center separation of the lattice is controlled by the size of the bead, down to a few tens of nanometers. Current research focuses on further development and optimization of this self-assembled technology, as well as novel applications of the resultant orderedsurface nanostructures.17−21 In one example of such current research, Kamperman and co-workers17 showed that by adjusting the pH and salt

1. INTRODUCTION Ordered surface nanostructures are known to offer the versatility of tuning surface energy1 for adjusting adhesion, friction, and wettability, as well as inventing new optical properties for realizing invisibility2 and inventing new electronic properties for producing nanoplasmonics.3 The versatility is accomplished by controlling the lattice attributes such as the separation, size, and morphology of the lattice constituents. Although ordered surface nanostructures can be precisely engineered and reproduced by advanced lithographic techniques1−4 for producing integrated circuits with nanofeatures, these preparation methods are inferior to self-assembled techniques5,6 due to the overall consideration of cost control and scale-up production. Among the prevailing self-assembled techniques for fabricating ordered-surface nanostructures, the technology of forming 2D colloidal crystals7−13 is particularly attractive because of its simplicity. In one simple approach of this technology, monodisperse submicron beads, such as the commercially available polystyrene (PS) beads or poly(methyl methacrylate) (PMMA) beads, are placed on water and the suspensions self-assemble themselves into a close-packed © XXXX American Chemical Society

Received: December 8, 2016 Revised: April 21, 2017 Published: May 1, 2017 A

DOI: 10.1021/acs.langmuir.6b04409 Langmuir XXXX, XXX, XXX−XXX

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schematically depicted in Figure 1a. Briefly, in the first step, a selfassembled monolayer of monodisperse submicron PS beads is formed

concentration of the water and by surface-engineering the hydrophilicity of the polystyrene beads, they could tune the details of the ordered-surface nanostructure of the PDMS films thus produced. Particularly, their engineering of the relative stability of the three interfaces of bead/water, bead/PDMSprecursor, and PDMS-precursor/water constitutes a novel means of controlling the degree of immersion of the PDMS precursor into the bead monolayer. This control consequentially determines the resultant size and depth of the dimples formed on the PDMS film surface, as well as the resultant dimension of the nanothick interdimple rims and walls. Most importantly, the controlled combination of these surface features of hollow dimples and flat interdimple rims was shown to yield outstanding adhesion and friction performance. Coincidentally, our research group has also adopted a similar approach to fabricate surface nanostructures with ultrahigh sensitivity in surface-enhanced Raman scattering.18,20 Further, a variation of this replication technique was reported by Kamperman and co-workers19 for the fabrication of polypyrrole with an ordered-surface nanostructure having peculiar overhang features; these overhang features were shown to be suitable for engineering nonwettability even with a hydrophilic polymer like polypyrrole. In short, nanoengineering of the morphology of the lattice elements of the inverse-opal-like lattice produced by molding a polymer with a 2D colloidal crystal promises a practical way of developing innovative nanodevices of industrial and market relevance. We report here a simple, low-cost, and scalable method of precision engineering of the common hemisphere-dimple morphology of an inverse-opal 2D lattice into a variety of surface nanostructures. The method is derived from a combination of controlling the degree of curing of the molded polymer precursor by accurately setting the appropriate curing temperature and time, and controlling the subsequent degree of dissolution of the incompletely cured polymer by precisely setting the dissolution parameters. By doing so, we trim the common submicron hemispherical dimples having nanothin interdimple rims and walls with nanoprecision. In this sequential trimming process, the nanothin walls are the first topological feature being consumed, and this trimming step yields an array of sharp pillars having triangular-like shape when the dimple depth is slightly larger than the dimple radius. The next critical morphological change takes place at the neck of each pillar, and the consumption of this leads to the production of an array of nanocones. Fine-tuning of the next phase of trimming gives nanocones with different degrees of tip sharpness, from round to needle-like. Further trimming leads to the consumption of all nanocones and the near-isotropic dissolution induced etching gives a close-packed array of soupplate-like shallow dimples. In the entire process of precisionengineering such a vast variety of lattice morphologies, the lattice constant remains unchanged and loyal to that of the original colloidal crystal template. To show examples of possible applications of the resultant polymer film having various ordered-surface nanostructures, we track the tunability of Young’s modulus and wettability of some of these nanostructures, and elucidate the basic physics of the experimentally observed tunability.

Figure 1. Schematics of the fabrication process for producing and precision-trimming PDMS 2D inverse-opal. (a) Five basic steps: forming a close-packed monodisperse PS-bead monolayer on water → dispensing PDMS precursor on the PS-bead monolayer → preliminarily curing the PDMS and lifting the PDMS/PS film from the water bath → thermally curing PDMS to adjust its degree of crosslinking → dissolving/removing PS beads with a solvent and partially dissolving incompletely cured PDMS; (b) top and cut-away views of PDMS 2D inverse-opal with no trimming (top panel), mild trimming (second panel), strong trimming (third panel), and severe trimming (bottom panel); the corresponding key topographic feature of the lattice element in each case is a dimple with an overhanging rim (top panel), sharp pyramid with a triangular base (second panel), dull pyramid (third panel), and soup-plate-like shallow-dimple (bottom panel). at the air−water interface of a bath of pure water at room temperature (Figure 1a(1)). Our previously reported method18,20 of 3D-printing is then adopted to dispense a thin layer of PDMS-precursor solution onto the monolayer PS bead suspension (Figure 1a(2)). Subsequently, the PDMS/PS film is lifted from the water bath after preliminary curing of the PDMS precursor at room temperature (Figure 1a(3)). Following this, the PDMS/PS film is subjected to an additional PDMS curing process with curing temperature and time to control the degree of cross-linking (Figure 1a(4)); this step is the key innovation in this work because it leads to a precise control of the solubility of PDMS in

2. EXPERIMENTAL DETAILS 2.1. Precision-Trimming PDMS Inverse-Opal 2D Lattice into a Variety of Ordered Arrays of Nanostructures. 2.1.1. Concept of the Process Design. The fabrication process in this work is B

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Langmuir an appropriate solvent used in the next step for precision-trimming of PDMS. Finally, the PDMS/PS film with a controlled degree of PDMS cross-linking is soaked in an appropriate solvent, such as gammabutyrolactone (GBL), which dissolves PS readily and slowly dissolves partially cured PDMS but does not dissolve fully cured PDMS (Figure 1a(5)); this dissolution-induced etching step is another key innovation of this work because it transforms the ordinary inverse-opal 2D lattice of ordered dimples into a variety of ordered nanostructures depicted in Figure 1b. The resultant transformation of the ordinary inverse-opal 2D lattice of ordered dimples into a variety of arrays of ordered nanostructures is schematically depicted in Figure 1b. The top panel of Figure 1b reveals that when the PDMS 2D inverse-opal film is fully cured, the dissolution-induced trimming step yields no trimming at all, and thus retains the ordinary close-packed dimples of a 2D inverse-opal morphology. In this panel, the dimple depth is set to be more than that of a hemispherical dimple, in order to show the possibility of engineering dimples with overhanging rims. From the right side of this panel, one sees from this top view a hexagonal array of circular dimple openings (pale blue representing exposed dimple surface, and dark blue representing the structural PDMS). From the left side of this panel, one sees from the cut-away view of the PDMS 2D inverse-opal with the following main features: (a) the side view of an array of closepacked cut-open dimples (painted pale blue); (b) the overhanging rims and walls of these cut-open dimples (painted dark blue); and (c) the overhanging rims and walls of the dimple row immediately behind the cut-open dimples (decorated by very pale blue dots). From these comprehensive views, one finds that the dimple diameter is considerably larger than the diameter of a top dimple-opening, and that overhanging rims can be engineered. In addition, while the dimple diameter is typically more than 100 nm and thus cannot be considered as a nanostructure, the dimension of the rims, walls, and other interdimple structures are all on the nanoscale. The most interesting nanostructure is the pillar separating three adjacent dimples; the bottom of this pillar comprises a pyramid-like topography with a triangular-like base and the top is an inverse version of this. The top and bottom pyramid-like objects are connected with a nanothin neck. Indeed, the key novelty of this work is the practical and low-cost exploitation of a full variety of these ordered nanostructures, particularly those nanostructures derived from such pillars. The second panel from the top of Figure 1b is designed to illustrate the results of mild dissolution-induced trimming to the extent that the nanothin walls and the necks of the pyramid-like pillars are consumed. As such, this critical stage of nanostructure transformation yields an array of nanostructures, each of which gives a protrusion from the PDMS film; each protrusion comprises a pyramid-like nanostructure with a triangular-like base. From the top view of this configuration, one sees an array of triangular-like projection images of the protrusion. From the cut-away view, one sees an array of pyramid-like projection images of the protrusion. The third panel from the top of Figure 1b illustrates the results of further dissolution-induced trimming. In this case, the stage of nanostructure transformation is a slight variation of the preceding stage of revealing an array of pyramid-like protrusion. Basically, the sharpness of the pyramid-tips is fine-tuned. Typically, the aspect ratio is down-tuned in this stage. The bottom panel of Figure 1b represents the configuration when severe trimming leads to the result of rounding off all protrusion topographies. Basically, a close-packed array of soupplate-like shallow dimples is formed. On the whole, the precision-trimming control is accomplished by setting the dissolution characteristics of PDMS with appropriate curing at a specific combination of temperature and time, and setting the dissolution-induced trimming condition with appropriate solvent, temperature, time, and agitation. In the absence of any agitation, the dissolution-induced trimming process is very much diffusion-limited because without diffusion of the dissolved polymer molecules away from the solvent−polymer interface, saturation of the solution at the interface stalls the dissolution to a halt. The coupling of this nature of diffusion-limited dissolution with the 3D microstructural topography of the original dimple-array leads to some considerable deviation of

topographic evolution from the near-ideal situations depicted in Figure 1b. Especially, the difficulty in getting the dissolved macromolecules diffused from the dimple bottom is expected to slow down the dissolution-induced trimming of the dimple bottom considerably more than other upper parts of the dimple. Because of this, the precisiontrimming process tends to transform the bottom part of each dimple with a certain degree of flattening. The conceptual design illustrates that a variety of ordered nanostructures with different size and morphology can be precisionengineered. However, the critical lattice parameter of the original 2D inverse-opal, namely, the center-to-center separation of a nearest pair of dimples in the hexagonal dimple-array, is retained in the precisiontrimming process. This retention separates the relatively welldeveloped precision engineering of 2D inverse-opal from the currently innovated precision-trimming of the lattice element of the 2D inverse opal, which very much ease the development of a set of fabrication procedures for the production of an array of nanostructures with the size and morphology specifically designed for a certain engineering application. 2.1.2. Fabrication Recipes. For the fabrication of the PDMS 2D inverse opal, a colloid of monodisperse polystyrene (PS) submicron suspensions (2.5 wt % in water, surfactant-free, diameters of 500 nm/ 750 nm/1000 nm, Alfa Aesar Corporation), was diluted into 1.25 wt % by mixing with an equal volume of ethanol. Then, the colloid was added dropwise onto a bath of water (DI water, 18 MΩ·cm, NANO pure Diamond system, Thermo Scientific). A few drops of 1 wt % sodium dodecyl sulfate (SDS, Sigma-Aldrich) aqueous solution were added to the bath to force the formation of a close-packed PS bead monolayer. Although the density of PS is 1.05 g/cm3, PS beads float on the water because of surface tension. After the PS bead monolayer template was formed on the water surface, a PDMS precursor solution comprising a mixture of PDMS monomers and a cross-linking-agent (Sylgard 184 of Dow Corning), with a weight ratio of prepolymer/ cross-linking-agent equal to 10:1, was gently added dropwise onto the top of the PS monolayer by using a home-built 3D automated fluid dispenser system (equipped with an Nordson EFD Ultimus V dispenser and a customized 3D motion platform). Each PDMS droplet was squeezed out of a syringe tip with nozzle diameter 0.84 mm by using the EFD dispenser, with a dispensing time 0.70 s and a dispensing pressure 12.0 psi. Hexane was added to the PDMS precursor solution to fine-tune the flowability of the solution through the syringe tip. When the PDMS precursor solution reached the PS bead monolayer, the solution wetted the PS surface and displaced the air in the monolayer. Since the density of the PDMS precursor mixture is about 0.8 g/cm3, which is slightly smaller than water density, the PDMS precursor mixture infiltrates through the PS bead monolayer and replaces the air−water interface in the monolayer with a stable PDMS/water interface. The process led to the formation of a triplephase system with PS beads, PDMS precursor, and water in a new interfacial balance for facilitating the practical room-temperature curing and molding of PDMS with the PS bead monolayer on water. After 48 h partial curing of the PDMS, the PDMS/PS film became rigid enough to be lifted off the water bath and subsequently processed. For the generation of a PDMS film having a 2D inverseopal lattice of close-packed hollow dimples, typically the PDMS/PS film was 100 °C for 2 h for fully curing the PDMS and was then soaked in a solvent such as gamma-butyrolactone (GBL) for dissolving/removing the PS beads. In this process, the key features of the resultant 2D inverse-opal lattice, including the size, depth, and center-to-center separation of a pair of adjacent dimples, were controlled by the selections of PS bead, the condition of forming the colloidal crystal, and the condition of molding PDMS. In this work, the nominal sizes of the monodisperse PS beads were 500, 750, and 1000 nm. The actual dimple rim openings and dimple depths were measured by AFM. With these data, the dimple sizes and the thickness of the thinnest interdimple walls were deduced. For the realization of the precision trimming process, the degree of cross-linking of PDMS was controlled in this work by setting the additional thermal curing temperature and time after lifting the PDMS/PS film from the water bath. In this work, the curing C

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Langmuir temperatures were 60, 80, and 100 °C. The curing time was typically 2 h but other settings were also used. For the dissolution-induced trimming steps, the PDMS/PS films were soaked in gammabutyrolactone (GBL from Sigma-Aldrich). In this work, soaking was done at room temperature. The typical soaking time was 4 h. A small set of soaking experiments were performed with different time settings, supports, and with sonication. We did not find out significant differences for whether sonication was being used. For others, the soaking step led to the quick dissolution and removal of PS beads and a controllable trimming of PDMS. After the GBL soaking step, the precision-trimmed film was cleaned with several cycles of washing with water and drying with N2. 2.2. Morphology and Elastic Modulus Measurements. The surface morphologies of the precision-trimmed PDMS films were characterized with the tapping mode of an AFM (MFP-3D, Asylum Research). Silicon tips (OLYMPUS, AC240TS-R3) with 10 nm radius of curvature at the tip, and with a nominal cantilever spring constant of 2 N/m and resonant frequency of 70 kHz, were employed in the measurements. The spring constant was calibrated by using the standard thermal method. The AFM images with a scan size of 5 μm × 5 μm and pixel density of 256 × 256 were obtained with a scan rate of 0.5 Hz in air. For the pore depth measurements, the data collected at 10 distinct positions were averaged with standard error included. For each modulus measurements, 5 spots which randomly selected on each PDMS sample were scanned by AFM to obtain morphological images and 30 force−displacement curves were recorded at different locations of each morphological image by forwarding the AFM tip to the sample surface until reaching the load force of 40 nN. A modulus value was fitted from each force−displacement curve by the Hertz model and the measured data were statistically analyzed with standard error included. 2.4. Contact Angle Measurements. The measurements of water contact angles were performed using a contact angle goniometer (Kruss DSA25). A droplet with 5 μL DI water was used in each measurement. Five different positions were probed on each sample and an average contact angle with standard error was reported.

Figure 2. Average pore depth (or protrusion height) vs thermal curing time, to validate that surface features can be controllably trimmed by tuning the degree of PDMS cross-linking with curing time, when the curing temperature at 80 °C and dissolution-induced trimming in GBL for 4 h are fixed; the PDMS 2D inverse-opal was molded by 750 nm PS beads.

“pore” is chosen only because the original protrusions are always partition walls and pillars associating with an array of dimple pores. In short, the results in Figure 2 demonstrate that trimming can indeed be controlled as a function of thermalcuring time, with the curing temperature and dissolutioninduced trimming conditions being appropriately set. In another validation test, PDMS 2D inverse-opal films formed with monodisperse PS beads having different nominal bead sizes (500, 750, and 1000 nm) were tested in Figure 3. In this set of tests, the thermal curing time of PDMS was set at 2 h and the dissolution-induced trimming condition was set as GBL for 4 h at room temperature with no agitation; the only variable was curing temperature, and the settings of 60, 80, and 100 °C were chosen. The results show that full curing was achieved with the setting of 100 °C, the pore depth was 486 ± 15 nm for

3. RESULTS AND DISCUSSION 3.1. Validation of the Proposed Precision Trimming Method. A series of measurements were performed to validate the proposed precision trimming method for transforming a PDMS 2D inverse-opal lattice of dimples to a variety of nanostructures. First, the controllability of dimple depths is shown in Figure 2 where a PDMS 2D inverse-opal lattice of dimples formed by a PS bead monolayer with a nominal bead size of 750 nm was tested. In this test, the degree of PDMS cross-linking was tuned by the curing time with a constant curing temperature of 80 °C, and this degree of PDMS crosslinking revealed itself by the resistance of dissolution-induced trimming of the protruding features of the tested PDMS sample. AFM was used to determine the height of the surface features on each tested sample. For the sample receiving a prolonged curing for 10 h, its resistance to trimming was very high, so after the dissolution process, PS beads were removed and AFM revealed merely a minor change from the original dimple lattice with a pore depth of 420 ± 25 nm. Reducing the curing time led to a reduction of trimming resistance so the residual pore depth dropped. With no thermal curing after the PDMS/PS film was lifted from the water bath, the PDMS was only very weakly cross-linked and was effectively etched in the GBL soaking process. The dimple feature of the 2D inverseopal lattice was virtually completely trimmed, and the measured pore depth reached zero. Note that the trimming process actually gave a variety of ordered nanostructures as shown in Figure 1b; “pore depth” actually refers to the height of the highest protrusion feature of the tested PDMS sample, and

Figure 3. Average pore depth (protrusion height) vs thermal curing temperature to validate that precision trimming of PDMS 2D inverseopal can be engineered by controlling thermal curing temperature when curing time of 2 h and dissolution in GBL for 4 h are fixed. D

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AFM data are benchmarks for surface features with no trimming. A thorough analysis of the left-most image (PS beads of 500 nm as template, the AFM height imaging and line profiling data can be found in Figure S2d) shows that the dimple opening is 471 ± 20 nm, the dimple depth is 275 ± 15 nm, and the shortest center-to-center dimple separation is 510 ± 17 nm. Assuming that the dimple is spherical, we calculate from the opening and depth data the dimple diameter and find a value of 477 nm, larger than the dimple opening. Hence, the presence of a small overhanging rim is evident. From the center-to-center dimple separation and the calculated dimple diameter, we infer that the interdimple wall separating a pair of close-packed dimples is 33 nm, which is very thin. As such, when PDMS was dispensed onto a PS bead monolayer, PDMS wetted the PS beads and formed an ultrathin wall between a pair of closed-packed PS beads, without significantly dilating the PS bead lattice. We note that Kamperman and coworkers17,19 innovated a technique to control the dilation of the PS bead lattice in the process of fabricating elastomer 2D inverse-opal films. Hence, it is evident that the thickness of the interdimple walls and the cross-sectional size of the interdimple pillars can be further tuned for specific functional applications demanding such tunability. Another similar thorough analysis of the central top image and the right-most top image also reveals that the PDMS 2D inverse-opal structures molded by PS beads of 750 and 1000 nm also comprised close-packed dimples with nanothin walls separating each pair of close-packed dimples. The results also reveal that the specific fabrication condition for the production of 2D inverse-opal with 1000 nm dimples yielded very deep dimples (847 ± 22 nm). In fact, the finding of a large dimple depth is also consistent with the fact that the dimple openings are relatively small and the interdimple rims are relative large, as shown in the rightmost top image in Figure 4. The degree of immersion of the PS beads into PDMS is typically controlled by the density of PDMS, and the relative stability of the PDMS/ PS, PDMS/water, and PS/water interfaces. AFM is known to give false images of pores, particularly for spherical pores such as the case in this work; this deficiency is indeed clearly demonstrated by the inverse-cone-like pores of the AFM images on the top row of Figure 4. However, the protrusion features after trimming off the interdimple walls and rims are well revealed by AFM imaging, as shown by the images of the PDMS samples cured at 80 °C. These protrusion features are predicted in the conceptual design of precision trimming illustrated by Figure 1b (the cases of mild and strong trimming). The AFM images of the PDMS samples cured at 60 °C suggest that these samples were merely weakly cross-linked. As such, the trimming of them in GBL for 4 h was sufficient to consume those sharp pyramid-like protrusions. The surface nanostructures resumed the dimple-like topography, except that the dimples were shallow with no overhanging rims. This scenario is also adequately predicted by the conceptual design of precision trimming illustrated by Figure 1b (the cases of severe trimming). 3.2. Evidence of Variations in Young’s Modulus and Cross-Link Density. Since the transformation of the PDMS 2D inverse-opal lattice of dimples into a variety of ordered nanostructures is controlled by the thermal curing conditions to tune the degree of cross-linking and resistance of dissolutioninduced trimming, we expect that the variation of the resultant ordered nanostructures in topography is also accompanied by a

the case of PS bead size of 750 nm. This pore depth is indeed larger than the pore depth of 420 ± 25 nm for the case of curing the same sample at 80 °C for 10 h (as shown in Figure 2 and discussed earlier). The observed difference shows that curing PDMS at 100 °C for 2 h is sufficient to fully cure PDMS for resisting any trimming effect and preserving the deepest dimple pore of the original 2D inverse-opal lattice of dimples (486 nm). As for the case of curing PDMS at 80 °C for 10 h, PDMS was not yet fully cured and was slightly trimmed by the GBL soaking treatment, with the result of a slightly shallower dimple pore (420 nm). The results in Figure 3 further show that when the curing condition is set to 80 °C for 2 h, the degree of PDMS cross-linking drops and the pore depth (or protrusion height) also drops to about 295 ± 7 nm. For the curing condition of 60 °C for 2 h, PDMS is only weakly crosslinked and the pore depth drops to 108 ± 10 nm. Hence, the test examples with PDMS molded by PS beads of 750 nm demonstrate clearly that precision-trimming can be engineered by appropriately setting the curing temperature, when curing time and the dissolution-induced trimming conditions are fixed. The results in Figure 3 confirm that this conclusion of validation also applies to PDMS molded by PS beads of other sizes. The most important perspective of the validity of the proposed precision-trimming method of transforming a PDMS 2D inverse-opal lattice of dimples to a variety of ordered nanostructures is shown in Figure 4 where AFM images of the

Figure 4. AFM images of PDMS samples with curing temperature as a control variable to set the degree of trimming for transforming the untrimmed surface feature of an array of dimples (top row) to pyramidal nanoprotrusions with different degree of tip sharpness (second row), and then to shallow dimples (third row); the case of too much trimming (fourth row) and the case of PDMS retaining the PS bead monolayer are shown as references.

trimmed samples are given. In this set of results, the degree of PDMS cross-linking (and thus the resistance to trimming) is varied by the thermal curing temperature (60, 80, or 100 °C), with the curing time of 2 h and the dissolution-induced trimming condition of soaking in GBL for 4 h being fixed. Three templating conditions, with PS beads of 500, 750, and 1000 nm, are included. On the top row of AFM images, the presence of close-packed dimple arrays can be inferred. In this work, curing at 100 °C for 2 h was found enough for PDMS to resist any trimming by the GBL soaking treatment; hence the E

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Figure 5. Variations in Young’s modulus and cross-link density of the precision-trimmed PDMS 2D inverse-opal samples, as a function of thermal curing temperature when the curing time was fixed at 2 h.

modulus of the treated PDMS. In this work, PDMS cured at 100 °C showed no more changes in cross-link density and Young’s modulus by the GBL treatment; this implies the complete curing of PDMS with a curing temperature of 100 °C, and justifies our experimental design of the proper curing condition of PDMS. 3.3. Evidence of Variations in Wettability. The results from the conceptual design (Figure 1b) and the actual experimental measurements (Figure 4) of the proposed variety of ordered nanostructures, particularly those related to the production of ordered pyramid-like protrusions, are well correlated to the engineering of variable wettability. In this section, we articulate this issue with water contact angle (CA) measurements of the precision-trimmed PDMS 2D inverseopal samples. The results are summarized in Figure 6 and Table 1. Briefly, an average CA of 117.4° ± 1.1° is recorded on ordinary flat PDMS surfaces in this work; the CA measurements taken from the large variety of ordered nanostructures

variety in Young’s modulus (E) because the degree of crosslinking correlates well with the mechanical property. In this work, we probed this issue with the method of AFM nanoindentation under the Hertz model22 F = kd = k(z − δ) =

2 E δ 2 tan(α) π 1 − ν2

(1)

where the loading force F was set to 40 nN for assuring the condition of elastic deformation, the spring constant k is equated to 2 N/m, the cantilever deflection d is the difference between the vertical movement of the piezo (z) and the sample indentation (δ), the Poisson’s ratio ν is set to 0.5 for incompressible material, and the half-opening angle of the AFM tip-cone α was 36°. By fitting the measured AFM force curves,23 we found Young’s modulus data in the range of 250 to 860 kPa, and confirmed that these variations correlate well with the thermal curing conditions. These results are summarized in Figure 5. In addition, it is also relevant to determine the crosslink density νc of the nanostructures prepared by different thermal curing conditions, with νc from the following correlation with Young’s modulus, E24,25

νc =

E 3RT

(2)

where R is the gas constant and T is 298 K in the present case. The cross-link density data are also included in the plot of the Young’s modulus data (Figure 5). The results sumarized in Figure 5 clearly show a large variation in Young’s modulus and cross-link density. For applications specifically requiring nanostructures with relatively low Young’s modulus, the as-prepared nanostructures produced by the precision-trimming process with thermal curing at 60− 80 °C are adequate. For those applications requiring similar nanostructures with high Young’s modulus and cross-link density, a post-preparation curing step can be applied to further tune these properties. It should also be noted that the GBL treatment is another means to influence the cross-link density and Young’s modulus of PDMS. In this work, we observed that for incompletely cured PDMS, the cross-link density increased remarkably after the GBL treatment. This improvement is due to the dissolution of the low-molecular-weight components in the incompletely cured PDMS in the GBL treatment.26−28 The extraction and removal of such low-molecular-weight components lead to an increase in cross-link density and Young’s

Figure 6. Water CA results of the PDMS 2D inverse-opal lattice of close-packed dimples trimmed to a variety of ordered nanostructures, with the PS bead size as a variable to control the dimple size, and with the thermal curing temperature as another variable to control of the degree of dissolution-induced trimming of the dimples. Inset images present the water contact angle test on samples using 500 nm PS beads. F

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Table 1. Experimental and Calculated Theoretical CA Results of a Variety of PDMS Surface Structures Prepared by Precision Trimming PDMS 2D Inverse-Opal Films samplea

rb

fb

500−no 500−60 500−80 500−100 750−no 750−60 750−80 750−100 1000−no 1000−60 1000−80 1000−100

1.02 1.08 1.46 1.69 1.01 1.07 1.45 1.85 1.01 1.07 1.45 2.17

31.90% 48.40% 13.40% 62.50% 41.90% 51.70% 37.10% 66.90% 30.60% 70.10% 55.90% 67.20%

θexp [°] 121.3 128.9 143.9 131.2 122.3 130.9 138.2 131.3 121.3 129.8 134.3 131.6

± ± ± ± ± ± ± ± ± ± ± ±

1.7 0.8 0.7 1.2 0.4 1.5 1.6 3.1 1.4 1.4 1.6 1.5

θw [°]c

θc [°]c

118.3 120.3 132.3 140.6 117.6 119.6 131.6 147.3 117.6 119.6 131.6 170.1

145.6 137.4 158.1 131.3 140.3 136.1 142.8 129.1 146.3 127.9 134.1 128.9

model Wenzel metastable metastable Cassie Wenzel metastable metastable Cassie Wenzel Cassie Cassie Cassie

Cassie Cassie

Cassie Cassie

a

The labels of xxx−yy denote the size (xxx in nm) of the PS-beads used to mold the PDMS 2D inverse-opal, and the thermal curing temperature (yy in °C) in treating the PDMS before it was soaked in GBL for 4 h. bExperimental surface roughness factor (r) and fraction of polymer above half of maximum height ( f) were obtained through analyzing the images by the AFM software developed on Igor Pro 6.35. cTheoretical contact angles satisfying the Wenzel model (θw) were calculated by eq 3 in the text; theoretical contact angles satisfying the Cassie model (θc) were calculated by eq 4 in the text.

Figure 7. Theoretical CA results calculated by the Wenzel model (a) and Cassie model (b), together with the experimental CA data measured from different samples. Inset images present different nanostructures as prepared based on different process parameters.

prepared in this work can readily reach 143.9° ± 0.7°, with the highest values from sharp protrusions. To understand the CA variations with different ordered nanostructures such as those displayed in Figure 4, we first consider the basic Wenzel model29 cos θw = r cos θe

conditions of thermal curing do not follow the Wenzel model. They can only be understood by the concept of the Cassie model30 cos θc = f cos θe + f − 1

(4)

where θc is the theoretical CA from the Cassie model, f is the fraction of the polymer sample with heights above the half of the maximum height obtained from the AFM data measurements.31,32 The calculated theoretical CA result θc and f are listed in Table 1. As an illustrative example, our AFM image of the PDMS sample processed with the 750 nm PS beads and a thermal curing temperature of 100 °C (central image at the top row of Figure 4) was scanned over a total area of 25 μm2, the half of maximum height was 223 nm, and 66.9% of the image pixels sitting above 223 nm. As such, f is equal to 0.669. The experimental CA was 131.3° ± 3.1° which is close to the theoretical CA from the Cassie model of 129.1°. In this particular situation, the experimental condition matched the Cassie model well and the water droplet in the CA measurement was in a stable Cassie state. The theoretical CA data complying to the Cassie model are represented by the green curve in Figure 7b, and with this benchmark, one can see that the experimental CA data of

(3)

where θw is the theoretical CA obtained from the Wenzel model, r is the surface roughness factor obtained from AFM data and is equal to the ratio of actual surface area to projected geometric surface area, θe is equilibrium CA on a flat PDMS film without any surface nanostructure. The experimental CA result, calculated theoretical CA results θw and r are listed in Table 1 and the table shows that r increases with an increase of the size of the PS beads used as the 2D inverse-opal template. As shown in Figure 7a, the blue curve which follows the Wenzel model is only matched by the experimental data measured from the PDMS samples with no thermal curing and thus with shallow surface roughness shown in the second bottom row of Figure 4. In this case of CA measurements, the water droplets were in the Wenzel state. However, Figure 7a also clearly shows that the experimental CA data measured from the PDMS samples with various G

DOI: 10.1021/acs.langmuir.6b04409 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir PDMS samples with thermal curing at 100 °C follow the Cassie model well. These data points were all measured from those nanostructures of the top row in Figure 4, nanostructures which have overhanging rims around relatively deep dimple pores. These features are known to hold water droplets in a stable Cassie state. At the far left away from the Cassie curve in Figure 7, there are CA data points reaching from 128.9° ± 0.8° to 143.9° ± 0.7°; these data points were measured from nanostructures of sharp protrusions, such as those in the second and third rows in Figure 4. These are typically known to hold water droplets in a metastable Cassie state. Our nanostructured surfaces prepared with different degree of thermal curing (different cross-link density) have topographic features to hold water droplets in three different wetting states. For no thermal curing, nearly all protruding features of the 2D inverse-opal lattice were trimmed to give merely low-lying surface roughness. The measured CA 121.3° ± 1.4° is merely slightly higher than that of the flat PDMS surface 117.4° ± 1.1°, and such a surface structure can only hold water droplets in a Wenzel state. With an increase in degree of thermal curing, more and more protruding features were retained from the trimming treatment; this leads to an increase in surface roughness but a drastic reduction of the f factor. Although relative high CA, up to 143.9° ± 0.7°, were accomplished but these were qualified as metastable Cassie state. For the nearly fully cured samples, the flat top rims of the dimple pores were retained and this led to a large factor f of 60−70%, and the stable Cassie state was satisfied.

ORCID

Yu Liu: 0000-0002-7945-7462 Author Contributions

# Haoran Zhan and Yanqiu Chen contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grants No. 51475484) and National Key Research and Development Program of China (No. 2016YFB0700700). The support from the Beijing Computational Science Research Center and the University of Science & Technology Beijing are greatly appreciated.

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4. CONCLUSION This work outlines the concept, with supportive experimental data, on transforming the ordinary elastomer 2D inverse-opal lattice of close-packed submicron dimples to a variety of ordered nanostructures. The transformation technique is derived from controlling the degree of thermal curing of the elastomer 2D inverse-opal film by a combination of curing temperature and time. The degree of curing determines the cross-link density, Young’s modulus, and solubility. With simple dissolution-induced trimming the nanothin walls and nanopillars separating the submicron dimples, ordered nanoprotrusions with a range of sharpness can be precisionengineered. Severe trimming, however, leads to the consumption of all these protrusions and gives back ordered shallow dimples. This method is simple, low-cost, and scalable; it thus holds relevance in developing practical applications such as engineering wettability, adhesion, friction, and other surface functionalities.

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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/acs.langmuir.6b04409. Details of experiments and characterizations, AFM and optical images (PDF)

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REFERENCES

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Corresponding Authors

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DOI: 10.1021/acs.langmuir.6b04409 Langmuir XXXX, XXX, XXX−XXX