Superhydrophobic Microporous Substrates via Photocuring: Coupling

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Superhydrophobic Microporous Substrates via Photocuring: Coupling Optical Pattern Formation to Phase Separation for ProcessTunable Pore Architectures Saeid Biria† and Ian D. Hosein*,† †

Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States S Supporting Information *

ABSTRACT: We present a new approach to synthesize microporous surfaces through the combination of photopolymerization-induced phase separation and light pattern formation in photopolymer−solvent mixtures. The mixtures are irradiated with a wide-area light pattern consisting of high and low intensity regions. This light pattern undergoes selffocusing and filamentation, thereby preserving its spatial profile through the mixture. Over the course of irradiation, the mixture undergoes phase separation, with the polymer and solvent located in the bright and dark regions of the light profile, respectively, to produce a binary phase morphology with a congruent arrangement as the optical pattern. A congruently arranged microporous structure is attained upon solvent removal. The microporous surface structure can be varied by changing the irradiating light profile via photomask design. The porous architecture can be further tuned through the relative weight fractions of photopolymer and solvent in the mixture, resulting in porosities ranging from those with discrete and uniform pore sizes to hierarchical pore distributions. All surfaces become superhydrophobic (water contact angles >150°) when spraycoated with a thin layer of polytetrafluoroethylene nanoparticles. The water contact angles can be enhanced by changing the surface porosity via the processing conditions. This is a scalable and tunable approach to precisely control microporous surface structure in thin films to create functional surfaces and antiwetting coatings. KEYWORDS: superhydrophobicity, microporous, phase separation, photopolymerization, PTFE nanoparticles

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INTRODUCTION Living systems abound1 with examples of surface anatomies that possess porous, textured, or roughened surface designs that confer special interfacial properties. Examples include the lotus leaf,2−5 insect legs6 and wings,7 bird feathers,8 mosquito eyes,9 fish scales,10 and even human bone.11 These structures have inspired creation of their biomimetic counterparts12 as a critical aspect of material surface design for numerous applications, including functional surfaces, water collection, antifouling, selfhealing, tissue engineering, and regenerative medicine, among many others.13−27 Extensive effort has focused on developing methods to fabricate surfaces, particularly with tailored porosity, and include electrospinning,28 templating,29 differential etching,30 photolithography,31 replication,32 treated fabrics,33 meshes,34,35 coatings,19 filter paper,36 and nanoparticle layers.37 Such porous structures provide the necessary microscopic surface porosity and roughness to induce hydrophobicity,38 which can be enhanced through additional coating or surface functionalization.14,39 However, while significant progress has been achieved with such methods, all suffer from their inherent trade-off between precise control over structure and scalability. For example, lithography is the most precise but least scalable; deposition methods are quite scalable but least © XXXX American Chemical Society

precise. Precise, scalable processing is important for applications in which not simply porosity but specific pore geometries and arrangements are necessary. Hence, such a synthetic approach is highly desirable to tune the structure and functionality for large-scale applications. Photopolymerization is an attractive approach to materials synthesis owing to its low-energy input, scalability, and ease in controlling the reaction via irradiation intensity. It is widely used to develop materials for applications in thin films, coatings, printing, artwork, dental materials, contact lenses, and electronics.40 To create microporous materials, several groups have employed photopolymer−solvent mixtures that upon irradiation undergo a polymerization-induced phase separation (PIPS),41−44 where after removing the solvent produces a microporous surface structure. While characteristically straightforward and scalable, presently only random porous structures are attained, and this inhibits their engineering to a degree at which finely tuned structure−property relations may be established. In terms of surface design, Received: October 22, 2017 Accepted: December 26, 2017

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DOI: 10.1021/acsami.7b16003 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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microporous substrates with precise, regular structures and is attractive for large-scale control over surface porosity.

concurrent control particularly over pore size, spacing, and arrangement remain elusive. Hence, the combination of PIPS with a mechanism that enables a high degree of control over structured porosity would realize a process in which precision and scalability are mutually compatible. Herein, we present a new strategy to scalably create precise, periodic microporous patterned surfaces by combining PIPS with the transmission of an incoherent light source under nonlinear optical conditions.45 Optical nonlinearity is characterized by an intensity dependent refractive index,46,47 which can be elicited in photopolymerizable media owing to polymerization-induced changes in refractive index associated with the increase in polymer molecular weight.48 When a widearea light source is transmitted through such a medium, it experiences a self-focusing effect that counters its natural tendency to diverge in space. As a result, the transmitted light undergoes spontaneous division into a multitude of densely packed, microscale filaments (i.e., filamentation).49 A filament is a nonlinear waveform characterized by divergence-free propagation, traveling in its own self-induced waveguide.49,50 Transmitted light becomes characteristically “self-trapped”, thereby preserving its intensity pattern over the depth of the medium via containment within this ensemble of filaments. Toward materials synthesis, spatial patterns programed into the transmitted light (i.e., via a photomask) in turn imprint a congruent pattern in the photopolymerizable medium, and this process thus far has been exploited to create microstructured patterns51−56 and microscale components.57 We recently showed that in the case of a binary mixture of photopolymers, this patterning mechanism can induce spatially local PIPS in the regions of the self-trapped light or filaments, where the resultant binary phase morphology possesses a congruent pattern as the light profile; namely, one polymer component located in the bright regions and the other in the dark regions.58,59 Hence, the combination of PIPS with light selffocusing enables optical patterns to be precisely programmed into binary phase morphology over large areas. To produce microporous surfaces, we examined PIPS and light self-focusing in a photopolymer−solvent mixture, where the solvent acts as a sacrificial phase (i.e., a porogen). We irradiated the mixture with a uniform, visible light source that is modulated by a photomask consisting of absorbing and nonabsorbing locations to produce a periodic array of dark regions in the profile of transmitted light. Self-focusing helps retain the profile over the depth of the sample to allow photopolymerization to occur predominately in the bright regions. As a result, PIPS leads to solvent being expelled into the dark regions. Basker and coworkers observed solvent located in the dark regions as a result of filamentation in an epoxide−solvent mixture;60 however, porous structures were not pursued. Herein, we removed the solvent to reveal pores on the material surface that have relatively uniform size and spacing that faithfully replicate the light pattern created by the photomask. We tuned the structures through variation in the photomask pattern and formulation of the mixture. To demonstrate their capability in antiwetting, as an example of one critical application, the microporous structures were converted to superhydrophobic surfaces via coverage with a thin, spray-coated layer of polytetrafluoroethylene (PTFE) nanoparticles. The PTFE nanoparticle coating aids in achieving strong hydrophobicity owing to it fulfilling the two key requirements61 of low surface energy17 and enhanced surface roughness.62 This is a new, straightforward approach to

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EXPERIMENTAL SECTION

Materials. Trimethylolpropane triacrylate (TMPTA) and polytetrafluoroethylene (Teflon, PTFE) nanoparticles (200−300 nm) were purchased from Sigma-Aldrich. The visible-light photoinitiator system consisted of free-radical initiator camphorquinone (CQ) purchased from Sigma-Aldrich, and cationic initiator (4-octyloxyphenyl) phenyliodonium hexafluoroantimonate (OPPI) purchased from Hampford Research Inc. Dimethyl sulfoxide (DMSO) solvent was purchased from Sigma-Aldrich. All chemicals were used as received. Preparation of Photopolymerizable Mixtures. Photopolymerizable mixtures were prepared by mixing TMPTA (the trifunctional, cross-linking photopolymer) and DMSO (the porogen) of different relative weight fractions and dissolving in it CQ (2.5 wt % of total mixture) and OPPI (1.5 wt % of total mixture). Mixtures were continuously stirred for 24 h while being protected from exposure to ambient light. CQ sensitizes the photoreactive blend to blue light (λmax = ∼470 nm), initiates the free-radical polymerization of TMPTA, and facilitates free-radical decomposition of OPPI,63 which accelerates the photopolymerization.64 From here on, mixtures are referred to by the ratio of their relative weight fraction of TMPTA to DMSO. Specifically, DMSO in the mixture is employed as an inert, nonreacting solvent that serves as a porogen, namely, the component that enables pore formation in the material (i.e., TMPTA) via phase separation and its subsequent removal. Photopolymerization of Mixtures. The mixture was poured into an open well consisting of a Teflon ring placed on top of a transparent plastic substrate. Mixture heights ranged from 100 to 400 μm. The mixture contained in the well was placed onto a homemade stage and irradiated from below with collimated blue light from a light-emitting diode (LED) (λmax = 470 nm, Thorlabs Inc.) at an irradiation intensity within 1−20 mW/cm2. We selected LED light because it is a simple, cost-effective irradiation source whose emission wavelength may be selected to correspond to the peak absorbance of CQ to attain efficient and maximal photon absorbance. LED light was first passed through a photomask (Photosciences Inc.) consisting of a square array of chrome circles with different diameters (D) and spacing (S) and subsequently transmitted through the mixture. Hereon, the masks are referred to by their diameter to spacing ratio, D/S. D values explored were 10, 20, and 40 μm; S values were 50, 100, 200, and 400 μm, and D/S ratios generally either 1:5 or 1:10. These ranges were sufficient to draw conclusions on the effect of the mask pattern on the resultant structure. The photopolymerized samples were subsequently washed with water and vacuum-dried overnight at 70 °C. In Situ Spectroscopy of Photopolymerization. In situ Raman spectroscopy of the mixture during irradiation was carried out with a setup described previously.59 Decreases over time in the integrated peak intensity of the CO bond were used to calculate the local conversion (p) (i.e., reaction yield) and degree of polymerization (i.e., number-average molecular weight, N = Xn). The conversion was determined by p=

2(I0 − I ) If

(1)

where I0 is the integrated peak area at t = 0 min, I is the integrated peak area any time thereafter, and f is the number of functions on TMPTA (i.e., f = 3). The degree of polymerization (N) was determined from p using Carothers’ equation.65 Imaging of Transmitted Light. To observe light self-trapping in the samples during irradiation, the transmitted light was passed through imaging optics and then focused onto a charge-coupled device (CCD) camera with pixel resolution of 3.2 × 3.2 μm (Dataray, WinCAMD-XHR). The imaging setup captured the spatial intensity profile of the transmitted beam at the top surface of the mixture, from which light exits. Surface Coating. The microporous surfaces were coated with a thin layer of PTFE nanoparticles using a commercial spray coater. B

DOI: 10.1021/acsami.7b16003 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Overview of the synthesis of microporous surfaces through irradiation of a photopolymer−solvent mixture with a light profile consisting of bright and dark regions.

Figure 2. Temporal evolution of spontaneous filamentation in the bright regions of transmitted blue LED light (15 mW/cm2) in a TMPTA/DMSO mixture (90/10) for masks (a) 40/100 and (b) 40/200. Insets show close-up images that reveal the self-trapped beams (arrows indicating one example in each) in their respective samples. Scale bar = 500 μm. PTFE nanoparticles (5 wt %) were dispersed in ethanol through ultrasonication (5 min) then loaded into the spray gun. Spray coating was applied onto the samples using nitrogen as the carrier gas and an applied flow pressure of 40 psi. We found that two generous coats, with drying of the solvent in between, was sufficient to confer samples with thorough coverage of a thin layer of PTFE nanoparticles. A scotch tape test over the coated surface confirmed excellent adhesion of the PTFE nanoparticles, as no discernible residue was observed on the peeled-off tape. Materials Characterization. Electron microscopy was carried out with a scanning electron microscope (Joel 5600) under an accelerating voltage of 5 keV. Samples were first coated with a thin layer of gold for imaging purposes. Compositional mapping was carried out with a confocal Raman spectrometer (InVia, Renishaw) using a 785 nm laser excitation. Details of the Raman mapping procedure can be found in our previous work.58,59 Briefly, a three-dimensional (3D) volume map of a final cured mixture was obtained from collected spectra at multiple positions in a sample at a 10 μm step size. Raman volumes were generated by mapping the intensity peak of TMPTA (carbonyl peak, 1720 cm−1) over the mixture depth to identify regions rich in TMPTA. Wettability Measurements. Static contact angle measurements were performed via the sessile drop method using a Ramé-Hart 250 F1 contact angle goniometer. A water droplet was placed onto the substrate, and a circular drop profile from the camera was used to determine the contact angles using DROP Image Advanced software. Contact angles reported herein are the average of 10 measurements.

the intensity profile of which is modulated by a photomask. We expect this modulated light pattern to undergo self-focusing over the course of irradiation and, with increased polymerization, for DMSO to phase separate out from the bright regions (where polymerization is predominately occurring) into the dark regions. Samples were irradiated for an extended period (∼1 h) to ensure maximal curing of the TMPTA, whereafter DMSO is removed. We expect the pores to form at the top of the mixture where light exits the sample and for the sample to be nonporous at the bottom owing to the depth dependence of the photo-cross-linking rate (vide infra). Finally, the structures are spray-coated with Teflon nanoparticles (100−300 nm in diameter) to generate superhydrophobic surfaces. The result is a solid, mildly flexible, and transparent thin film, wherein TMPTA forms a cross-linked polymer. The final sample appears slightly yellow due to remnant blueabsorbing photoinitiator. We were able to process films with areas as large as our LED light source (∼2″ diameter). Transmitted light undergoes spontaneous filamentation over the course of irradiation, as shown in Figure 2. The filamentation is characterized by the emergence of a multitude of self-trapped beams, indicated by their high intensity spots in the transverse profile of transmitted light leaving the sample (i.e., at the top of the mixture). The individual beam sizes are ∼80 μm in diameter, which corroborates with measurements from previous studies.49,54 In the case of the 40/100 mask, for example, the limited spacing allows only a single self-trapped beam to form locally in each of the open areas of the bright

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RESULTS The general approach to fabricate microporous surfaces is schematically shown in Figure 1. We first irradiate a mixture of photoinitiated TMPTA and DMSO with collimated LED light, C

DOI: 10.1021/acsami.7b16003 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces region, resulting in their periodic arrangement forced to have the same symmetry and spacing as the mask (Figure 2a, at 20 min). Visual examination of the profile reveals that, in the case of the 40/100, the beams are centered within their respective bright region, i.e., offset from the dark regions by 1/√2a diagonally, where a is the lattice spacing of the mask (i.e., 100 μm in this case). Whereas, in the case of 40/200 mask, the bright region provides irradiated areas larger than a single selftrapped beam, such that a multitude of self-trapped beams emerge (Figure 2b, 20 min). Concurrently, the dark regions in both cases are also retained on a lattice dictated by the mask and, after pattern formation, the optical profile consists of bright and dark regions that are fixed over the entire period of irradiation. Self-trapping was not observed in uniformly irradiated (i.e., no mask) samples (see Supporting Information), which is corroborated by previous work that showed that some modulation to the light is required to seed the pattern formation process, which can be achieved by using an optical mask.49,54,66 The results presented in Figure 2, namely the self-trapping in the bright regions as well as the increase in the diameter of the dark regions (∼66%), indicate the concurrent formation of both self-trapped bright and “black” beams, respectively. The confinement of light through self-trapping has been well studied in photopolymerizable media for light patterns of various arrangements and symmetries53,67 as well as the observation of concurrent formation of bright and black selftrapped beams that preserve the spatial intensity of a light profile.68 Particularly, the increase in width of the dark region is a result of light leaking into the bright regions owing to its preference to propagate in regions of higher refractive index, which in this case is caused by the photopolymerizationinduced rise in refractive index (Δn) in the bright regions.46,54,69 Overall, this light patterning process is what preserves the optical profile, consisting of dark regions with light confined to the interstitial spaces surrounding them, either occupied within a single or multitude of self-trapped beams. We performed in situ Raman spectroscopy over the course of irradiation to track the polymer conversion yield as well as to assess the mixing thermodynamics. Figure 3a shows a typical contour plot that maps the yield over the depth of the sample in an irradiated region (i.e., in between dark spots generated by the mask) over time. After a short residence time, the yield dramatically increases, then plateaus at an approximate value of 0.5. We used the degree of polymerization, N, to calculate and track over time the critical reaction parameter, χc, which indicates the onset of mixing instability (i.e., immiscibility, and thus phase separation). The value of χc may be calculated based on the conditions that yield spontaneous phase separation (i.e., spinodal decomposition):70 χc =

⎛ ⎞ φs2/3 1⎜ 2 1 ⎟ + + 2 ⎜⎝ fNφ (1 − φ) ⎟⎠ Nφ5/3

Figure 3. In situ measurements of polymerization over the depth of the mixture over time revealed by (a) the polymerization yield and (b) the value of χc. Sample measured was produced from an 80/20 mixture using a 40/200 mask. The dashed line indicates the contour level of χc = χFH, below which the blend is unstable. A depth = 0 μm corresponds to the top of the sample.

χFH =

Vr [δ TMPTA − δ DMSO]2 RT

(3)

Based on Hildebrand solubility parameters for TMPTA and DMSO of 9.02 and 6.32,66,72 respectively, values for χFH are 1.531, 1.41, 1.288, and 1.166 for 90/10, 80/20, 70/30, and 60/ 40 mixtures, respectively, as determined using calculation methods described previously.66 During polymerization when N is increasing, χc decreases, and when χc < χFH, phase separation can proceed71 until the mixture solidifies. Figure 3b shows a contour map of χc for an 80/20 mixture and 40/200 mask, yet it is exemplary of the changing thermodynamics in all mixtures and reveals a dramatic decrease in χc below χFH that corresponds to the rise in the yield and signals mixture instability and an expected phase separation. Overall, the kinetics and thermodynamics shown in Figure 3 reveal the rapid curing of TMPTA and inducing of phase separation of the mixture in the region of irradiation. The plots also reveal how the instability occurs predominately at the top of the sample. Despite low conversion at greater depths, the sample can still solidify in the form of a gel−sol, which is characterized as a mildly cross-linked polymer swollen with the remaining monomer.73 This gel−sol can form at reaction yields as low as ∼0.2, which corresponds to the yields observed at greater depths. Also, the variations in the yield at greater depths is most likely due to scattering of the Raman laser beam during phase separations, which can induce corresponding variations in the calculated values of the conversion. Figure 4 shows an exemplary, resultant morphology revealed through confocal Raman imaging and SEM, particularly at the surface and subsurface of the material. Raman volume maps of the composition of TMPTA reveal that, after irradiation and prior to solvent removal, a sample possesses a binary phase morphology, wherein the TMPTA occupies regions corre-

(2)

where φ is the volume fraction of TMPTA, φs is the network volume fraction, N is the degree of polymerization for TMPTA, f is the TMPTA functionality, and χc is the critical interaction parameter. As TMPTA and DMSO have similar densities (∼1.1 g/cm3), the weight fractions employed herein reasonably equate to their respective volume fractions. For miscible mixtures χc > χFH, where χFH is the Flory−Huggins interaction parameter of the mixture determined by the Hildebrand solubility parameters (δ) for TMPTA and DMSO:71 D

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that is spatially congruent to the profile of transmitted light. Figure 4b reveals the structure of an individual pore present after solvent removal that spatially corresponds to a dark region. Additionally, smaller pores and a porous network are present in the polymer matrix itself (what we term interstitial pores/porosity), indicating that random phase separation also occurred in the bright regions. This smaller scale, random phase separation resembles what has been observed for uniformly cured photopolymer−solvent mixtures.41 Thus, the surface may be classified as possessing a hierarchical pore structure consisting of the main pores dictated by the dark regions induced by the photomask and randomly sized but generally smaller pores in the surroundings. SEM images also confirm the thorough coverage and nanoscale surface roughness provided by the PTFE nanoparticle coating, both on the surface and within the pores. Evidence of phase separation of DMSO from TMPTA is observed only near the top surface of the sample; in contrast, the bottom of the film showed no evidence of pore formation (vide infra). By irradiating a pure TMPTA sample (i.e., no DMSO solvent), we confirmed that pore formation was not, to any significant extent, a result of uncured photopolymer in the dark regions that might have been removed during the washing. Small depressions do appear in the surface (see Supporting Information), but not to an extent as the significant pore sizes achieved by using a photopolymer−solvent mixture. Figure 5 shows examples of the final film structures that form using different photomask parameters. Varying the parameters (i.e., magnitude of S and D) yields microporous surface structures consisting of different pore sizes and pore-to-pore spacing. The position and size of the pores always correlates to the size and spacing of the mask, with pore size being slightly greater than its corresponding chrome diameter, D. Periodic (square symmetry) porous structures were attained over the entire 2″ area film under irradiation. Structures could also be produced over the entire a range of mixture heights explored (see Supporting Information). Hereon, we show data for samples that are 100−200 μm thick, which is a common thickness for such microstructured thin films. We also investigated a 5/25 photomask and found that pores remained ∼15 μm in size (see Supporting Information), similar to

Figure 4. Phase morphology and structure of the microporous surface. (a) Raman volume map of the cured photopolymer−solvent mixture. Insets show xy (z = 0 μm) and xz (y = 0 μm) slices of the volume. (b) SEM images of a single pore and surrounding microporous TMPTA matrix. Magnified images of the pore and surface reveal the PTFE nanoparticle coating. Conditions are a 90/10 mixture, 40/100 mask, and 15 mW/cm2 irradiation intensity.

sponding to the irradiated areas, and DMSO in the dark regions. In other words, the irradiation process has induced the mixture at the surface to form a phase separated morphology

Figure 5. Final microporous surface structures for TMPTA/DMSO mixtures of 90/10 for different mask patterns. (a) 10/50, (b) 10/100, (c) 40/ 100, (d) 40/200, (e) 20/400, and (f) 40/400. Irradiation intensity: 15 mW/cm2. E

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Figure 6. Microporous surface structures formed from a 90/10 mixture, 40/100 mask, and irradiation intensities of (a) 1, (b) 5, (c) 10, (d) 15, and (e) 20 mW/cm2. Insets show close-up images of a single pore. (f) Plot of the average pore size against irradiation intensity.

structures produced from 10/50, indicating that there is the minimum attainable pore size. Irradiation with appropriate intensity was critical to pore formation, as revealed in Figure 6. Formation of larger diameter pores occurs with an increase in intensity. The variation in pore size was unaffected by irradiation time; namely, employing longer irradiation times to account for lower intensities (i.e., to attain the same total energy flux) did not further increase the pore size. All results shown hereafter employed an irradiation intensity of 15 mW/ cm2. Figure 7 shows perspective SEM images of the microporous surfaces, which reveal their structure over both their surface and depth, mapped over different mask parameters and DMSO weight fractions. We produced a spectrum of structures that ranged from rough surfaces with no pores, to relatively smooth surfaces with individual main pores, to roughened surfaces with main pores, to extremely rough surfaces in which the main and interstitial pores have similar sizes. Randomly structured, porous surfaces were obtained with uniform irradiation (see Supporting Information), confirming the main pores form via the combination of PIPS and light patterning. Surface structures with more densely packed pores (e.g., 40/200 vs 40/400) or those created with less DMSO fraction visually consist of rougher surfaces. Also, in all samples, the pore depth did not permeate the entire thickness of the film, leaving the bottom side as nonporous. High resolution SEM imaging revealed that samples with interstitial porosity also possessed main pores whose walls were porous, as opposed to being smooth in the case when no interstitial porosity was present (see Supporting Information). All spray-coated samples showed superhydrophobicity, as evidenced by their large static water contact angles (WCAs) (Figure 8a). Contact angles above 150° were attained for all samples. Generally, for all mask patterns, there was a decrease in the WCA with an increase in DMSO weight fraction in mixtures. Owing to the hierarchy in the porous structure, we used the measured contact angles to back-calculate a value of the surface porosity to understand the relation between the photomask and mixture to the WCA. The wetting of water on

these porous surfaces can be described by the Cassie−Baxter (CB) equation:74 cos(θr) = f1 cos(θs) − f2

(4)

where θr and θs are the WCAs of a rough surface and a smooth surface, respectively, and f1 and f 2 are the fractional interfacial areas of a solid and air in contact with a liquid droplet, respectively ( f1 + f 2 = 1). f 2 was calculated based on the WCA for a flat PTFE-coated TMPTA surface (90°) and the measured values for θr. Plots of the WCA versus surface pore fraction (i.e., f 2, what we term porosity) are shown in Figure 8b. All samples, regardless of mask or DMSO weight fraction, fall on the same curve dictated by the CB relation shown in eq 1. However, the plots reveal the spread and locations of f 2 and the range of attainable WCAs for each mask. Lower DMSO weight fractions are located higher on the curve, which correlates to higher surface porosity observed in samples, as revealed in Figure 7. Instructively, for a desired main pore size (dictated by the mask), the DMSO fraction can be tuned to attain a greater porosity (main pores + interstitial pores), whereby the WCA is increased. Specifically, for any mask, employing a mixture with a lower DMSO content increases the WCA.

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DISCUSSION We can infer from the structures created, our previous studies,59 and the known contributions of photo-cross-linking and phase separation to morphology75 that the mechanism for formation is as follows and as schematically shown in Figure 9. Light is concentrated and confined to the bright regions due to the self-trapping effect. The intense irradiation close to the bottom of the sample, through which light enters, causes this region to rapidly cure into a continuous, cross-linked solid (i.e., no significant phase separation). Because light intensity decreases over the depth, owing to absorbance of the photoinitiator,76 the photo-cross-linking kinetics is slower at the top of the sample, thereby allowing the phase separation to proceed. This gradient in the reaction kinetics over mixture depth, whereby porosity appears at the top and a continuous solid at the bottom (Figure 9a), is emphasized by both diffusion F

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Figure 7. Perspective images of microporous surfaces mapped over a range of photomask parameters (rows) and DMSO weight fractions (columns). Irradiation intensity: 15 mW/cm2.

of oxygen (which operates as a polymerization inhibitor) from the ambient environment across the air−mixture interface (thereby lowering kinetics of the top) and diffusion of generated radicals into the dark regions (thereby driving photopolymerization at the bottom where oxygen does not reach a significant level given our mixture heights).64 Owing to the increased viscosity and elasticity of the polymerizing TMPTA cross-linked matrix, phase separation is facilitated by DMSO being expelled from the bright regions. If the drive for

phase separation is high, it is possible that the TMPTA and DMSO may also separate in the bright regions, which results in pores in the matrix. Otherwise, samples will consist of only the main pores. The depth-dependence of the photopolymerization kinetics, as described, also explains why the pores do not permeate over the entire thickness of the material: close to the bottom of the sample, eventually the photo-cross-linking rate is sufficiently high to inhibit the dynamics of phase separation. While calculations of the thermodynamics of mixing indicate G

DOI: 10.1021/acsami.7b16003 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 8. Contact angle measurements for water droplets on the microporous surfaces. (a) Measured contact angles for samples fabricated with different masks, over DMSO weight fraction. The dotted line demarcates contact angles above which samples are characteristically superhydrophobic. Error bars show the standard deviation for each measurement. Insets show water droplets for their respective samples and a macro-lens image of a larger blue-dyed droplet overlaying the surface, corresponding to one of the higher contact angles achieved. (b) Contact angles plotted against surface pore fraction ( f 2). Inset (bottom) shows data for uniformly irradiated samples (i.e., no photomask). The arrow indicates the direction of increased DMSO fraction.

mechanically stronger and resists distortions. This leads to welldefined pores (e.g., 70/30 and 80/20 samples) (Figure 9c, exemplified by weight fraction of 0.2−0.3). With even less DMSO, photo-cross-linking is so abundant that it can inhibit DMSO diffusion over long distances, resulting in DMSO locally separating from TMPTA both in the bright and dark regions, which leads to a hierarchical porous surface (Figure 9c, exemplified by weight fraction of 0.1−0.2). These effects explain, and are revealed by, the transitions in the structures with decreased DMSO fraction, as shown in Figure 7 (columns right to left). We observed no difference in morphology based on whether a single self-trapped beam or a multitude of them formed in the bright region, nor did the interstitial porosity correlate to the spatial profile of the light pattern. Rather, both nonporous and porous interstitial regions were attained whether a single selftrapped beam or a multitude formed. The key aspect and advantage of the self-trapping of transmitted light is the ability to preserve the optical profile over the depth of the sample. Indeed, the formation of the black beams in the dark regions is critical to providing low-intensity regions in which minimal polymerization occurs to facilitate the diffusion of DMSO into them. The increase in diameter of the black beams, to this end, is helpful in keeping these regions sufficiently large to establish a space for phase separation and pooling of the solvent and thereby eventual pore formation. The increase in the pore size with irradiation intensity can be explained by the intensity dependence of photopolymerization.77 Namely, the increase in molecular weight of TMPTA is what initially drives the mixture to become unstable and unmix. Higher intensities result in greater changes to molecular weight, and thus a stronger drive for phase separation. The mixture is also more responsive to light at higher intensities; namely, it can attain greater, faster changes in refractive index to significantly modulate light transmission.54 This leads to stronger self-focusing, which spatially confines light more tightly in the (smaller) bright regions, thereby allowing larger pores to open upon phase separation. Within our available intensity range, we observed that at a sufficient irradiation intensity the pore formation process can be elicited and that higher intensities assist with pore formation. It is possible that at significantly greater intensities, photopolymerization is so strong that the samples cure without any formation of main pores (i.e., only random small pores, or a pore network),

where and when the mixture is unstable, the occurrence of phase separation depends on whether the degree of crosslinking inhibits diffusion of solvent (i.e., kinetically trapped). Hence, despite being thermodynamically unstable, we can infer that cross-linking was too strong for any significant phase separation to occur at the bottom of the samples. The sample being solid at the bottom can be beneficial for applications because it renders the materials nonpermeable, provides a solid continuous support of the surface structure, and enables maximal contact to an applied surface. The different morphologies that form by using different DMSO fractions can be understood by considering the evolution of the spinodal line in the two-phase diagram (Figure 9b). With increased degree of polymerization over time (N(t)), the spinodal curve moves upward in temperature and expands laterally. Mixtures that are most symmetric (i.e., closest to φ = 0.5) will enter the spinodal region earlier (i.e., at smaller N(t)), allowing for spontaneous phase separation earlier relative to mixtures closer to either end of the phase diagram. An increase in N also results in higher levels of viscosity and elasticity in the TMPTA matrix owing to a greater corresponding degree of cross-linking, which slows or can even inhibit the dynamics of phase separation. The mixtures explored spanned the range from close to being symmetric (60/40) and far away from it (90/10), and Figure 9c schematically demonstrates their phase evolution based on the thermodynamic arguments illustrated in Figure 9b. Figure 9c illustrates the posited binary phase evolution for mixtures with different DMSO fractions. The observation that porosity increases with less DMSO can be explained by the fact that higher fractions of DMSO cause the earlier onset of phase separation, which may occur before the TMPTA matrix possesses the necessary connectivity via largescale cross-linking to significantly inhibit phase separation. Hence, DMSO can diffuse uninhibited until it eventually pools into larger phases (located in the dark regions). This will lead to samples with main pores and a relatively smooth TMPTA matrix (Figure 9c, exemplified by weight fraction of 0.4). Also, because the TMPTA matrix is not sufficiently rigid, it is possible to attain samples with no replication of the main pores or with main pores that are barely deciphered (e.g., 60/40 samples), owing to distortions in morphology during phase separation. With less DMSO, photo-cross-linking is more abundant (owing to a greater concentration of TMPTA), and phase separation occurs under conditions in which TMPTA is H

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possibly because significant free-radical diffusion into the dark regions causes the entire sample to undergo large-scale crosslinking.54 Nevertheless, the results demonstrate the ability to tune the pore size via irradiation intensity, which is beneficial because it relieves the need for a photomask for every specific pore size desired. A general relationship may be established between photomask pattern and final microporous structure based on the mask’s three independent parameters S, D, and D/S, the latter of which is scale invariant. For the same value of D, an increase in S yields more random structures (e.g., 90/10:10/50 → 10/ 100 and 80/20:40/400 → 40/400). For the same D/S ratio, a smaller scale mask (e.g., 10/50 vs 40/200) results in a greater porosity because the main pores grow to a larger extent relative to their spacing. For the same S but greater D, the pores can form more easily (e.g., 10/100 → 40/100); however, at a larger S there may be no significant difference (e.g., 20/400 → 40/ 400). These characteristics can be explained, once again, by the dynamics of the process: greater S values (fixed D) results in longer travel distances of DMSO to the dark regions, which gives a greater likelihood of DMSO separating out in the bright regions. Greater D values (fixed S) gives DMSO a greater likelihood of pooling into a correspondingly larger dark region. At a larger scale, the system favors a porous TMPTA matrix, owing to the associated larger diffusion distance for solvent to the nearest pore, regardless of pore size. Furthermore, despite employing a smaller D (i.e., 5 μm), the final pore size remains limited, most likely because the growth of the DMSO phase can allow pore formation to extend beyond the size dictated by the mask. Surfaces of uniformly irradiated samples also show the same trend as what is formed in the interstitial regions of the fabricated structures. Namely, high fraction DMSO samples (i.e., 60/40 and 70/30) show rough surfaces but no pores, and low fraction DMSO samples (i.e., 80/20 and 90/100) show porous structures (see Supporting Information). However, as uniformly irradiated samples do not attain WCAs above 150°, it can be inferred that the use of an optical mask and its generation of main pores on the surface is critical to transitioning samples into the superhydrophobic regime. Superhydrophobicity is achieved even in such instances as with the use of a 40/400 mask, for which the main pores are significantly separated as to have expected the surface porosity be similar to those that employed no mask. Indeed, comparison of the calculated surface porosities of uniformly irradiated samples with structures formed with a mask indicate that the mask has a marked effect on increasing the porosity, whereby stronger hydrophobicity is achieved. Close examination of the surfaces of the structures compared to the uniformly irradiated samples indicate that the former visually possess rougher surfaces, and it is possible that this is caused by the modulation of the light pattern provided by the mask via such effects as differential polymerization rates and locally induced phase separation. Additional explanations for the mask having a marked effect on the wetting is that, while uniformly irradiated samples visually appear porous, the fraction of the surface occupied by TMPTA may still be significant; also, their porosity may not be uniform, but rather possess local regions that consists of more TMPTA, which lowers the local fraction of the surface occupied by pores. Therefore, the use of a mask has a synergistic effect, inducing enhanced porosity while also imposing the formation of large, regularly spaced pores,

Figure 9. Schemes illustrating the formation of microporous structure and its dependence on different parameters. (a) Schematic of the depth profile of the microporous structure in relation to the light intensity (irradiation from the bottom) and the consequent crosslinking, whereby the upper region along the depth is permitted to phase separate due to low cross-linking. Locations and directions of the input of oxygen (green) from the ambient atmosphere and the diffusion of free radicals (R) generated in the irradiated regions are indicated. (b) Schematic of a phase diagram for a photopolymer− solvent mixture illustrating the evolution of the spinodal curve. The temperature−composition coordinates (T−φ, with T = 300 K) for the 4 mixture fractions investigated are shown, and all enter the spinodal region at different degrees of polymerization over time, N(t). The dashed arrow shows the movement of the spinodal curve along its region that crosses the T−φ coordinates of the mixtures. Four spinodal lines are numbered to correlate with the expected morphology. (c) Illustration of the expected binary phase morphology for different fractions of solvent as a function of the position of the spinodal curve. The cross-link degree expected to increase over time is illustrated by the density (i.e., fineness) of the hash grid (a square symmetry is drawn for simplicity). The arrows indicate the general direction of diffusion and are shown at the instance when phase separation is expected to begin, according to when the mixture enters the spinodal region. I

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ACS Applied Materials & Interfaces which allows the surface to attain the porous structure necessary for superhydrophobicity. Within the superhydrophobic regime, attained by using a mask, varying DMSO weight fraction tunes the hydrophobicity. That lower DMSO fractions yield greater WCAs can be explained by the emergence of the hierarchical porosity. Namely, the smaller interstitial pores located in the TMPTA matrix, with its associated fine-scale surface roughness and porosity, provide the predominant contribution to surface hydrophobicity, and whose presence is controlled by DMSO weight fraction. Nevertheless, based on the measured WCAs, all samples, even those that visually appear to be smooth, have sufficient surface roughness or porosity to attain superhydrophobicity. On the basis of this discussion of the data, a processingstructure−property relation emerges that offers rational and predictive control over the surface structure and properties. Eliciting self-trapping and consequent phase separation to produce microporous surfaces is attractive for several reasons. First, the process is relatively straightforward, entailing visible light irradiation with a mask pattern. The symmetry of the mask pattern can also be changed (e.g., hexagonal); herein, we employed square symmetries to demonstrate the proof-ofconcept for this process. Second, it is scalable by using larger incoherent light sources (e.g., LED arrays, incandescent lamps, even sunlight). Third, self-trapping can be achieved in wide range of polyfunctional free-radical systems. Fourth, the regular pore structure is attractive over randomly forming pore structures, specifically for tailoring pore size, spacing, and arrangement for applications. Employing a photopolymer− solvent mixture is preferable over irradiating a photopolymer alone because the latter poses the challenge of needing to take extensive measures to inhibit polymerization in the dark regions and also necessitates immediate, subsequent washing to remove unreacted resin. Whereas, with a photopolymer−solvent mixture, the phase separation concurrently reduces the concentration of the monomer in the DMSO-rich regions (i.e., pore forming locations), thereby inhibiting polymerization.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ian D. Hosein: 0000-0003-0317-2644 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding supported by the American Chemical Society under the Grant PRF# 57332DNI7 as well as support from the College and Engineering and Computer Science at Syracuse University.

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REFERENCES

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CONCLUSIONS In summary, we presented a new approach to create the microporous surfaces through the combination of photopolymerization in a photopolymer−solvent mixture with light self-focusing. Self-focusing maintains the patterned bright and dark regions, programmed using a photomask, in a light profile over the depth of the mixture through which it is transmitted, and with sustained irradiation phase separation occurs with the solvent separating into the dark regions. A microporous structure is attained upon solvent removal with an arrangement of pores spatially congruent to the mask pattern. A range of porous structures can be attained, which can be explained by the formation mechanisms during polymerization. This approach can be leveraged to tune pore size and spacing, to provide a range of porous surface structures, and is attractive for the rational control of surface structure of relevance to antiwetting properties and surface-related applications.

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Optical light pattern produced with uniform irradiation and additional SEM images of the microporous structures made from mixtures of (1) different thickness, (2) no DMSO, (3) additional mask configurations, and (4) with no mask employed (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16003. J

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