Article pubs.acs.org/Langmuir
Tunable Wrinkle and Crease Surface Morphologies from Photoinitiated Polymerization of Furfuryl Alcohol James G. Gaillard,† Chelsea Hendrus,‡ and Bryan D. Vogt*,§ †
Department of Biomedical Engineering, University of Akron, Akron, Ohio 44325, United States Department of Physics, University of Akron, Akron, Ohio 44325, United States § Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States ‡
ABSTRACT: Addition of a small fraction of hydrophobic photoacid generator (PAG) to furfuryl alcohol provides a facile route to generate wrinkle topology by acid-catalyzed polymerization that is induced by ultraviolet (UV) light. Here, we describe how the primary characteristic parameters, wavelength and amplitude, of these periodic wrinkles can be tuned through control of the thickness of this furfuryl alcohol− PAG solution prior to UV exposure and the environmental humidity. As the initial coating thickness is increased, the wavelength remains unchanged at fixed temperature and PAG concentration, but the amplitude of the wrinkles increases exponentially with increased coating thickness. A wrinkle to crease transition is observed in some cases as the thickness of the solution coating is increased; this behavior is dependent on the PAG selection. Conversely, variation in relative humidity does not significantly impact the amplitude of the wrinkles, but there is a step change in the wavelength of the wrinkles near approximately 45% relative humidity with a factor of 3 decrease in the wavelength at high humidity. Through this knowledge, we have been able to fabrication wrinkles with an aspect ratio greater than 0.7 in a single step by UV exposure. These simple processing parameters to independently control wavelength and amplitude provide a facile route to systematically examine the role of aspect ratio of wrinkles on physical properties.
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the PDMS provides the requisite force to induce wrinkling.20 The formation of a silicate glass (SiOx) at the surface dramatically increases the modulus compared to the bulk of the PDMS, and it is this mechanical mismatch in modulus that leads to the wrinkling instability in this case.18,20 The sinusoidal wrinkles can be characterized by their wavelength and amplitude.21 In the linear elastic limit for a semi-infinite substrate, the wavelength, λ, and amplitude, A, can be calculated analytically as12
INTRODUCTION Mechanical instabilities, such as wrinkling, buckling, and creasing,1−3 lead to ubiquitous features in nature from a human frown to the diffraction component of structural color and water repellency associated with Morpho butterfly wings.4 Although nature has exploited these instabilities for evolutionary advantages, wrinkling is generally an undesired phenomenon for man-made products; buckling instabilities are often associated with failure of structures.5−7 However, in the past decade, there has been tremendous interest in taking advantage of this instability to create self-organized microstructures in a cost-effective manner.8,9 These structures have been demonstrated to be useful for generation of microlens arrays,10 tunable diffraction gratings,11 metrology of thin films,12,13 arranging colloidal crystals,14 controlled wetting surfaces,15 antifouling in marine environments,16 and enhanced adhesion,17 among many other applications. Advantages to these wrinkled structures over classic lithographic patterns are the ability to pattern over large areas, easily tune the feature sizes without generation of a new master, and the low cost associated with pattern formation.18 This facile manipulation of feature size in terms of both wavelength and amplitude has enabled improved understanding of touch that was not possible to unambiguously explain from lithographic fabrication.19 One common method to generate wrinkles is through oxidation and cross-linking of a prestretched poly(dimethylsiloxane) (PDMS) slab; release of the prestress in © 2013 American Chemical Society
⎛ E ̅ ⎞1/3 λ = 2πh⎜ f ⎟ ⎝ Es̅ ⎠
(1)
⎛ε ⎞1/2 A = h⎜ − 1⎟ ⎝ εc ⎠
(2)
where h is the thickness of the film or coating, E̅f is the planestrain modulus for the film (i = f) or substrate (i = s), and εc is the critical strain threshold to induce wrinkling. One issue with this commonly utilized approach is the need for an elastic substrate in order to apply the requisite strain to impart wrinkling, which limits the applicability to hard substrates typically encountered in microfabrication such as glass or Received: October 2, 2013 Revised: November 7, 2013 Published: November 8, 2013 15083
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silicon. Although pattern transfer is possible,20,22 this additional step complicates the simple wrinkling process. There have been a number of alternative processes developed and described to generate wrinkles that do not rely on PDMS that can be divided into two broad classes. The first involves an exchange of the PDMS for alternative elastomers; these have included hydrogels,23 shape memory polymers,24 and liquid crystal elastomers.25 Additionally, the strain to induce wrinkling can be applied by solvent swelling of the elastomer instead of mechanical stretching.26,27 An alternative approach is to induce the wrinkles on a rigid substrate through careful manipulation of the characteristics of a coating and its processing. One early example of controlled wrinkling instability on a rigid substrate involved the partial cross-linking of an elastomer film on a glass substrate; subsequent swelling of the film with a monomer solvent through sorption from the vapor into the elastomer and its polymerization leads to controllable wrinkles on the surface.10,28 Similarly, the alignment of liquid crystals and their polymerization/cross-linking on a rigid surface provide a route to directional wrinkles over large areas.29 However, the fabrication in wrinkles in these cases requires several processing steps. More recently, Crosby and co-workers reported a selfwrinkling system based upon polymerization inhibition at the surface by atmospheric O2;30 these polymeric wrinkles can be formed on glass slides in a single exposure step with swelling of the cured resin by the uncured liquid surface producing sufficient stress to induce wrinkling. In this case, the thickness of the monomer coating (typically on order of 102 μm) provides a facile handle to tuning the wavelength and amplitude of the periodic wrinkles with both initially increasing for films 45%) is shown to effectively reduce the wavelength of the wrinkles. These results demonstrate the potential for facile tuning of wrinkle morphology on hard surfaces with commercially available components and a standard UV source for curing of polymer resins.
(3)
where Δε = ε − εc. Using common fabrication conditions in the literature, the aspect ratio for the wrinkles is typically approximately 0.1;31 the maximum aspect ratio for oxidized PDMS obtainable at 20% strain is approximately 0.25.22 For standard oxidation conditions even at 40% strain, the aspect ratio is still less than 0.2, and significant cracking occurs at higher strains.15 Crosby and co-workers have systematically examined how the stress state impacts the wrinkle morphology for oxidized PDMS with the stress applied via solvent swelling;32 in these cases, dimple and nonequilibrium morphologies such as zipper and weave patterns are observed. As the simple single self-swelling approach relies on monomer induced swelling, these alternative structures may be obtainable by this approach; however, it does not appear to be able to effectively modulate the aspect ratio over a wide range as both wavelength and amplitude initially increase as the thickness of the coating increases without modification of the resin formulation.30 Based upon their previous report,30 the aspect ratio in this self-swelling case appears to be limited to 0.3. We have recently developed an alternative self-wrinkling ap-
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EXPERIMENTAL SECTION
Materials. Furfuryl alcohol (FA, 98%) and triphenylsulfonium triflate (TPS-Tf) were purchased from Sigma-Aldrich. Rhodorsil PI2074 was obtained from Promerus, LLC. The precursor solutions utilized consisted of either 0.05 wt % Rhodorsil in FA or TPS-Tf at 0.1 or 0.25 wt % in FA. Solutions were stirred in the dark at room temperature to ensure that the photoacid generator (PAG) was fully dissolved prior to use. Silicon wafers (University Wafers, ⟨100⟩, 500 μm, test grade) were used as substrates in approximately 1 cm by 1 cm pieces. These silicon squares were cleaned using ultraviolet-ozone (UVO) (model 42, Jelight Company, Inc.) prior to use. Figure 1 illustrates the general procedure associated with the generation of wrinkles in this work. Briefly, the PAG-FA solution was placed on a cleaned silicon wafer that was preheated to 100 °C (constant temperature for these studies) on a hot plate, following the procedure 15084
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Figure 2. Topology of wrinkled poly(furfuryl alcohol) surfaces from UV-induced polymerization of furfuryl alcohol at 100 °C for coatings that are initially (A) 200 μm (20 μL) and (B) 500 μm (50 μL) thick using 0.05 wt % Rhodorsil. From characterization across the samples, the wavelength is statistically similar (147 ± 5.6 and 139 ± 9.4 μm in terms of standard error), while the amplitude increases from 13 ± 0.8 to 22 ± 0.8 μm for 200 and 500 μm thick coatings, respectively. The color scale for height is identical between samples and extends 0−100 μm.
Figure 1. Schematic of wrinkling protocol and resulting wrinkles in poly(furfuryl alcohol). reported previously.33 The solution volume was selected to yield an average thickness of 200, 300, 400, 500, 600, or 700 μm on the silicon substrate. It is important to note that the volume was sufficient to mostly flatten the drop through the middle of the substrate from gravity. In order to induce polymerization, the FA solution was exposed to broadband UV (Spectroline, 4500 μW/cm) with a constant source to substrate distance of 32 cm. The solutions were exposed continuously until the coating had hardened. During the polymerization of the FA, periodic wrinkles spontaneously developed in most cases. Characterization. The surface morphology of the polymerized FA coating was elucidated by optical profilometry (Zeta-20, NanoScience Instruments). The micrographs were obtained from the middle of the substrate to avoid edge artifacts.37 A minimum of five images was collected for each wrinkled coating. In many cases, multiple images were stitched together to provide images that were many wrinkle wavelengths wide for improved accuracy in assessing the wavelength. To obtain the wavelength of the wrinkles formed with Rhodorsil, the 3D image obtained from optical profilometry was flattened; application of fast Fourier transform provides a characteristic peak associated with the wavelength. Coatings fabricated with TPS-Tf tended to exhibit lower contrast between peaks and valleys. As such, line sections were analyzed to determine the wavelength from the average peak-to-peak distance. To obtain the amplitude of the wrinkles for both PAGs, line sections through the 3D image were also used with the peak-to-valley height determined for a minimum of 20 points on each image. The amplitude reported here is the wave amplitude and is one-half of the peak-to-valley height (as denoted in Figure 1). Both the wavelength and amplitudes reported herein are the average of multiple images. The error bars represent one standard deviation from this analysis.
respectively. The wavelength for both coatings is approximately 150 μm (statistically identical), but the amplitudes are quite different as can be seen from Figure 2 with the thicker coating resulting in substantially larger wrinkle amplitude (13 ± 0.8 μm vs 22 ± 0.8 μm). This difference is intriguing as it appears that the wavelength and amplitude of the wrinkles can be adjusted independently unlike other wrinkling methods, where both parameters typically vary.15,30 For high aspect ratios using oxidized PDMS, the PDMS must be stressed beyond its linear elastic limit and the wavelength becomes strain dependent.20 As the aspect ratio appears to be a critical quantity for a number of technologically important properties,15−17 the ability to tune the aspect ratio independent of the wrinkle wavelength could provide a simple route to easily tune physical properties. In order to better illustrate this ability to tune the amplitude without impacting the wavelength, Figure 3 shows both the wavelength and amplitude as a function of the initial solution thickness using either Rhodorsil or TPS-Tf as the PAG catalyst. In both cases, the wavelength is statistically independent of the solution volume used, but the amplitude increases as the solution thickness increases. For Rhodorsil (Figure 3A), the wavelength remains relatively constant at approximately 150 μm for coatings that are 200−700 μm thick. In contrast, the amplitude of the associated wrinkles increases from 13 ± 3.8 to 61 ± 6.3 μm over the same thickness range. This results in a variation in the aspect ratio by more than a factor of 5. In most cases, the aspect ratio for wrinkles is limited to between 0.1 and 0.25 for oxidized PDMS.22,27,38 In this case, the aspect ratio is varied from 0.08 to 0.50 simply by variation of the volume of solution placed on the substrate. For TPS-Tf, the data also suggest that increasing the thickness of the solution leads a larger average amplitude without significantly impacting the wavelength of the surface wrinkles; however, the variance in the range of the wavelength and amplitude for a given processing condition increases significantly as the thickness increases. Examination of the surface morphology provides one explanation for the very large variance with a transition from very regular, periodic wrinkle structures to structures more consistent with creasing,39,40 as shown in Figure 4. Figure 4A illustrates the periodic wrinkles with an aspect ratio of 0.1 that occur for the 300 μm coating using TPS-Tf. As the thickness of the sample increases (Figure 4B), the periodic wrinkles still occur over regions of the coating, but wide plateaus between wrinkles and sharper peaks are present; we attribute this change to a “precreasing” structure, which coexists with periodic wrinkles. When the
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RESULTS AND DISCUSSION The wrinkling process begins with a liquid precursor consisting of FA (monomer) and a small fraction of PAG (polymerization catalyst precursor). The high molecular mobility in this system allows for some of the fluorinated PAG to segregate34 at the air−liquid interface in a wetting layer to decrease the surface tension; this concentration gradient provides a source for differential polymerization rates between the surface and bulk of the solution upon UV exposure that generates the photoacid catalyst for polymerization of FA. Polymerization of FA leads to a substantial decrease in the specific volume (ca. 20−30%) in comparison to the monomer that provides the strain to induce wrinkling. Figure 2 illustrates the wrinkled structure obtained from the photocatalyzed polymerization of FA using Rhodorsil for a single frame using optical profilometry. A well-defined sinusoidal pattern is clearly evident for both of these coatings. The only difference in the processing between Figures 2A and 2B is the solution thickness (volume) on the silicon wafer prior to UV exposure: 200 μm (20 μL) and 500 μm (50 μL), 15085
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Figure 3. Influence of thickness of furfuryl alcohol layer on wrinkle wavelength (●) and amplitude (□) using (A) Rhodorsil and (B) TPS-Tf as the photoacid generator to catalyze polymerization. The polymerization was conducted at 100 °C in both cases. The lines are drawn as guides to the reader.
the concentration of TPS-Tf in solution is increased to 0.25 wt %. The transition from wrinkled to highly creased morphologies associated with coatings using 0.25 wt % TPS-Tf is illustrated in Figures 4D and 4E. At 300 μm (Figure 4D), a wrinkling morphology similar to that for the same thickness coating with 0.10 wt % TPS-Tf (Figure 4A) is found, but the wrinkles are better defined with the higher TPS-Tf concentration. Increasing the thickness to 500 μm with 0.25 wt % TPS-Tf eliminates the wrinkles, and deep creases are now found in the coatings (Figure 4E). This shift toward creasing at higher TPS-Tf concentrations is consistent with our previous report.33 However, the complexities associated with the process limit the quantitative mechanistic modeling. First, the polymerization pathways for FA associated with acid catalysis are complex.41,42 The polymerization does not proceed by only a simple condensation reaction mechanisms, but additional pathways associated with chain branching and cross-linking lead to the formation of conjugated moieties. Second, most models assume two distinct layers of differing moduli that provides the mechanical contrast required for the wrinkling instability; in this case, the layers are not well-defined with a gradient between the more rigid (cross-linked) surface and gelled bulk of the coating when the instability occurs. This gradient is dependent upon not only the initial PAG gradient profile but also the diffusivity of the photoacid that is generated on UV exposure. Photolysis of the PAG generates a more hydrophilic molecule, and thus there will be a chemical potential gradient associated with the initial PAG gradient that will promote diffusion of the photoacid to the bulk. This process occurs as the furfuryl alcohol is polymerizing, so the diffusion of the photoacid will be varying with the local extent of polymerization. Third, the strain (stress) imparted in the system to generate the wrinkles is from the polymerization. Initially, the solution is a viscous fluid that cannot transfer stress, so the strain developed from volumetric contraction during initial stages of polymerization is not transferred into the strain responsible for generating the wrinkles. Near the degree of polymerization of FA when the bulk of the coating reaches the gel point, the elastic modulus exceeds the loss modulus and stress is transferred to the more rigid surface as polymerization of the bulk proceeds. This limits the knowledge associated with the strain and stress applied to the system during wrinkle formation. Finally, the mechanical properties (E̅f and E̅ s) are
Figure 4. Instability using TPS-Tf as the PAG produces a transition from periodic wrinkles to creases. Using 0.1 wt % TPS-Tf solution in FA, well-defined wrinkles are observed for (A) 300 μm, while a mixed wrinkle and crease morphology occur at (B) 500 μm with a transition to predominately creases at (C) 800 μm. This transition occurs for thinner coatings using 0.25 wt % TPS-Tf in FA with thicknesses of (D) 300 μm and (E) 500 μm illustrated. Note the change in height scale and the loss of periodicity in the structures as the thickness of the solution increases.
initial coating thickness increases to 800 μm, the periodic wrinkle structure no longer dominates the surface morphology and creasing structures are clearly observed in the coating (Figure 4C). These crease structures tend to be associated with flat plateau regions that are taller than surrounding areas of the coating with the crease near the middle of these plateaus. The transition from wrinkles to creases occurs more abruptly when 15086
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Figure 5. Comparison of wrinkle morphology for (A) high humidity (54.7%) and (B) low humidity (35.6%) using 0.05 wt % Rhodorsil with a solution thickness of 500 μm. The average amplitude is 65 ± 4.2 μm for both samples, while the average wavelength increased from 95 ± 35 to 165 ± 46 μm.
evolving during the polymerization that induces the wrinkles. Thus, the real process associated with the formation of the wrinkles is quite complex. In addition to the solution thickness (volume), other environmental factors impact the wrinkle wavelength as illustrated previously for temperature and PAG concentration.33 However, as the condensation reaction of FA is acid catalyzed,41 the extent of hydration of the FA will impact the transport properties of the generated acid and presumably the wrinkle formation. The humidity in the environment should rapidly equilibrate with these relatively thin coatings of liquid FA to provide a facile route to tuning the polymerization reaction within the film. In the prior studies (including Figures 2 and 3), the ambient humidity was low (
