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Mechanically Induced Hydrophobic Recovery of Polydimethylsiloxane (PDMS) for the Generation of Surfaces with Patterned Wettability Ali Mazaltarim, Jay M. Taylor, Abhiteja Konda, Michael Stoller, and Stephen A. Morin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10454 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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ACS Applied Materials & Interfaces

Mechanically Induced Hydrophobic Recovery of Polydimethylsiloxane (PDMS) for the Generation of Surfaces with Patterned Wettability Ali J. Mazaltarim,a Jay M. Taylor,a Abhiteja Konda,a Michael A. Stoller,a Stephen A. Morin†ab a Department

of Chemistry, University of Nebraska-Lincoln, 409C Hamilton Hall, P.O. Box 880304, Lincoln, NE 68588 Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, NE KEYWORDS Materials Science, Surface Chemistry, Elastomer, Polydimethylsiloxane, Chemical gradients. b Nebraska

ABSTRACT: Silicone elastomers are used in a variety of “stretchable” technologies (e.g., wearable electronics and soft robotics) that require the elastomeric components to accommodate varying magnitudes of mechanical stress during operation; however, there is limited understanding of how mechanical stress influences the surface chemistry of these elastomeric components despite the potential importance of this property with regards to overall function. In this study, plasma oxidized silicone (polydimethylsiloxane; PDMS) films were systematically subjected to various amounts of tensile stress while the resulting surface chemical changes were monitored using contact angle measurements, X-ray photoelectron spectroscopy, and gas chromatographymass spectrometry. Understanding the influence of mechanical stress on these materials made possible the development of a facile method for the rapid, on-demand switching of surface wettability, and the generation of surface wettability patterns and gradients. The use of mechanical stress to control surface wettability is broadly applicable to the fields of microfluidics, soft robotics, printing, and to the design of adaptable materials and sensors.

INTRODUCTION Silicone elastomers are central to many technologies including stretchable/wearable electronic devices,1 soft robots,2 microfluidic reactors,3,4 and smart materials.5 In these technologies, surface chemical changes (e.g., hydrophobic recovery) to the silicone components is often undesirable, and the surface chemistry (especially wettability) has been extensively investigated.6–10 Despite the prevalence of such reports, there remains limited literature focused on the effects of mechanical stress on the surface chemistry of silicone elastomers. Recognizing the importance of silicone surface chemistry to “stretchable” technologies, we worked to elucidate the surface-chemical changes to plasma-oxidized silicone elastomers that are associated with mechanical stress. These efforts will enhance our ability to design silicone-based technologies with functionality optimized to use and/or mitigate mechano-chemical changes, which can be localized to regions of high stress or distributed over large areas. In these investigations, we chemically modified the surface of PDMS films, and subjected them to controlled amounts of tensile stress while monitoring the surface chemical changes using contact angle measurements (CA), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GC-MS). We discovered that the application of mechanical stress to oxygenplasma-treated PDMS resulted in the “on-demand” restoration of the hydrophobicity of the films. We utilized these understandings to develop a simple method based on the rational application of mechanical stress that enabled the rapid manipulation of surface wettability and thus the generation of precise patterns of wettability and wettability gradients on

elastomeric films. Unlike traditional methods, mechanical stress enables the use of a range of new process variables to control surface wettability (e.g., the number of stress cycles, magnitude of strain, and duration of the applied stress). Furthermore, mechanical stress is a method that does not rely on surface-chemical reactions or additional vacuum processing technologies (e.g., plasma or reactive ion etching) to alter surface chemistry. Controlling wettability is important to a range of applications including: surface coatings for thermal management,11 printing media for controlling the spreading of ink,12 microfluidic devices,13 biomedical devices,14 anti-icing coatings,15,16 selfcleaning technologies,17,18 adhesives,17 and more generally, processes that depend on heterogeneous chemical interactions at the surface (e.g., the assembly and manipulation of droplets).17–20 Traditionally, patterning wettability is produced using soft lithography, chemical modifications, microcontact printing, etc.12 Techniques that make controlling the wettability of elastomers easier or more robust will have significant implications to many technologies including: stretchable/wearable electronics and sensors,1 soft robotics,2 microfluidic devices,3,4 smart mechano-sensing materials,5 etc. The surface wettability of silicone elastomers (e.g., PDMS) has been extensively investigated.6,7,10,21 CA measurements have been correlated to surface free energies, and by extension chemistry, through Young’s equation.22 These understandings have been used, for example, to optimize the bonding strength of plasma oxidized PDMS to glass and silicon in microfluidic devices.23 Generally, PDMS surfaces are rendered hydrophilic using plasma or UVO oxidation schemes, but these surfaces slowly (on the order of hours to days, depending on conditions,

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Figure 1. Mechanically induced hydrophobic recovery of oxidized PDMS (PDMS-OH) films. (a) Schematic illustration of the oxidation and mechanical deformation of PDMS films. (b-c) Contact angle of PDMS-OH and monomer-extracted PDMS-OH (ePDMSOH) films following cycles of mechanical stress (ε = 1.0 at 1,000 mm/min). (d-e) Contact angle of PDMS-OH and ePDMS-OH films following a single cycle of mechanical stress (ε = 1.0 at 1,000 mm/min) where the films were held at maximum strain for increasing amounts of time. (f-g) XPS analysis of PDMS-OH and ePDMS-OH films following cycles of mechanical stress (ε = 1.0 at 1,000 mm/min).

EXPERIMENTAL DESIGN

temperature, atmosphere, etc.) undergo hydrophobic recovery.6–8,10 This hydrophobic recovery has been attributed to the migration of uncured monomers from the bulk to the surface,7,10 surface rearrangement of chemical functional groups,6 surface silanol condensation,6 and surface cracking of the brittle oxide layer.7,8 Despite the range of reports on this topic, the influence of mechanical stress on hydrophobic recovery of oxidized PDMS has been relatively unexplored.9 Considering that mechanical stress in silicone elastomers is known to influence the arrangement and packing densities of self-assembled monolayers,24 modulate wettability,25 and tune reactivity,26 we hypothesized that mechanical stress would be very important to the process of hydrophobic recovery. Previously, we utilized the mechano-chemical properties of surface functionalized silicone elastomers to assemble liquid droplets into ordered arrays of picoliter volume droplets and to fabricate arrays of microgels.27,28 More recently, we demonstrated the ability to create surface wettability patterns on PDMS by a templated vapor-phase reaction procedure, thus enabling the control and manipulation of liquid droplets and gases.29 In this report, we investigate mechanically-induced hydrophobic recovery of oxygen plasma treated PDMS, and demonstrate the utility of this method for the rapid tuning, switching, and patterning of surface wettability.

Oxygen plasma is commonly used to generate oxidized (hydrophilic) PDMS (PDMS-OH) in microfluidics, and is generally the first step in surface derivatization schemes.27–30 In this investigation, we systematically applied mechanical stress to PDMS-OH films while monitoring the chemical changes of the surface using CA (Figure 1a-e), XPS (Figure 1f, g), and GCMS (Figure 2). Specifically, we used CA measurements, a method for the characterization of surface wettability and by extension surface energy,22,23 to assess the properties of these films in a high throughput fashion. We used water as the probing liquid in CA as it is a common solvent in the surface derivatization of silicone elastomers and ubiquitous in microfluidic devices. Furthermore, water does not swell silicone elastomers significantly.31 We used XPS, a surfacesensitive technique for quantitative elemental analysis, to measure chemical changes in the PDMS-OH surface that CA measurements do not provide. Uncured monomers have previously been shown to contribute in the hydrophobic recovery of aged silicone elastomers.7,10 To elucidate the role of uncured monomers in the hydrophobic recovery of mechanically stressed PDMS-OH films, we used GC-MS to detect trace amounts of uncured monomers that migrated from the bulk to the film’s surface as

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ACS Applied Materials & Interfaces

Figure 2. Mechanically induced migration of free monomers. (a) Schematic illustration of the sampling scheme. (b) Gas chromatogram of an unstressed and stressed (ε = 1.0, rate = 1000 mm/min) PDMS-OH film (inset are representative mass analyses). (c) Gas chromatogram of an unstressed and stressed (ε = 1.0, rate = 1000 mm/min) ePDMS-OH film.

the greatest magnitude of hydrophobic recovery of the PDMSOH films as measured by a change in CA of 44° (5 ± 3° to 49 ± 3°; Figure 1b). After 50 stress cycles, a wettability “plateau” was observed at a water CA of 90 ± 3° for PDMS-OH. This mechano-activated hydrophilic to hydrophobic switching occurred in minutes (a rate that is orders of magnitude faster than prior reports of aged PDMS),8,10 and it could be faster if more rapid extension rates were used. To probe the influence of uncured monomers which, as noted previously, play a role in the hydrophobic recovery of aged silicone films, uncrosslinked components were extracted from cured PDMS films using triethylamine, ethyl acetate, and acetone, in series (see Supporting Information, SI, text).23 Hydrophobic recovery of oxidized monomer-extracted PDMS (ePDMS-OH) persisted with mechanical stress although the observed wettability plateau was lower than that for PDMS-OH (75 ± 3° versus 90 ± 3° for ePDMS-OH and PDMS-OH, respectively), suggesting that uncured monomers play a minor role in the rapid, mechano-induced hydrophobic recovery of PDMS (Figure 1c). We thus believed that surface cracking of the brittle oxide layer, as observed in previous reports,7–9 must be the primary reason for the observed mechano-induced hydrophobic recovery while migration of uncured monomers through cracks in the brittle over-layer played a minor role (as will be supported further below). We also investigated the effect of the duration of stress on hydrophobic recovery of PDMS-OH and ePDMS-OH films. We stretched the films to ε = 1.0 and held strained for increasing

a result of mechanical stress.32 We conducted these measurements on as-prepared PDMS films as well as films that were cleared of uncrosslinked monomers using an extraction procedure (extracted PDMS, ePDMS).33 We created wettability patterns to demonstrate the applicability of mechanical stress to the generation of surfaces with heterogeneous wettability. More broadly, since surface chemistry/microstructure influences various interfacial properties, such as adhesivity, reactivity, permeability, etc.,17,24,25 the generation of local variations in surface characteristics could represent new opportunities in surfacechemical/structural transformations. Specifically, we utilized 3D-printed relief structures to selectively stress the films, and create wettability patterns. We visualized these patterns using surface wettability visualization (SWAV), a technique for visualizing wettability patterns and gradients using microdroplets.29 Furthermore, we used mechanical stress to fabricate wettability gradients. Again, we used 3D printed relief patterns to control stress on the films and by extension the steepness of the gradients. Wettability gradients are known to induce droplet motion,34,35 and are therefore important due to their potential application in surface fluidics,29 transport devices,29 and moisture management.36

RESULTS AND DISCUSSION We first demonstrated the ability to switch the surface wettability of PDMS-OH films homogeneously using mechanical stress (Figure 1a, b). The first stress cycle induced



Figure 4. Mechanically generated wettability patterns on silicone films. (a) Schematic illustration of the use of vacuum forming to pattern wettability on PDMS-OH films. (b) Optical micrograph of a 3×3 array of 2 mm “N” patterns visualized using SWAV (scale bar = 5 mm). (c) Stitched optical micrograph of the area indicated by the zoom box in panel d (scale bar = 1 mm).

cracking does not occur, and we measured no changes in surface wettability or chemical composition. The applied tensile stress resulted in orthogonal compressivestress on the elastomer due to the Poisson effect, presumably driving displacement of uncured monomers from the bulk towards the film’s surface where they could contribute to hydrophobic recovery.7 To further support our assertion that this effect was a minor contributor to hydrophobic recovery (compared to cracking) we measured the amount of uncured monomers at the surface of stressed PDMS-OH and ePDMSOH films after one stress cycle using a GC-MS procedure (Figure 2a). We detected a 2.5 fold increase in siloxane monomer concentration on the surface of the mechanically stressed PDMS-OH substrate when compared to unstrained PDMS-OH (Figure 2b). The majority component (6.49 min. retention time) was identified as dodecamethylpentasiloxane (Figure 2b and Figure S9).32 The remaining peaks can be characterized as siloxane monomers according to their common ionic fragments, consistent with prior reports (Figure 2b and Figure S9).32 In contrast, for ePDMS-OH, we did not detect uncured monomers on the surface (where the limit of detection was 25 ng/cm2, Figure S10) following mechanical stress (Figure 2c). Despite the lack of detectable monomers on the ePDMSOH surfaces, a significant amount of hydrophobic recovery was still observed in mechanically stressed ePDMS-OH films (Figure 1c, e), indicating that the wettability change must be mainly due to the presence of the observed surface cracks (an Ra increase from 0.417 nm to 1.485 nm was observed using AFM and confocal microscopy again showed significant crack formation Figure S8 and S11-S13). We therefore believe surface cracking of the brittle oxide layer to be the principal cause for the observed mechano-induced hydrophobic recovery of oxidized silicone elastomers. We do not rule out migration of uncured monomers (at levels below our limits of detection in GC-MS) from the bulk towards the surface as a contributing factor, but our results indicate that it is a minor effect compared to the influence of cracking. More specifically, as surface crack density dramatically increased during the first stress cycle, remaining virtually unchanged from cycles 20 to 50 (Figure S7), we can attribute the increased hydrophobicity for films subjected to several (>20) stress cycles to these time-dependent processes (monomer diffusion and surface rearrangement), which are enhanced by mechanical stress (Figure 1d, e). We

Figure 3. Mechanically generated heterogeneous wettability zones on silicone films. (a) Schematic illustration for the generation of multiple zones of distinct wettability on PDMS-OH films. (b,c) Optical micrographs of representative films produced using different variations of the procedure shown in panel a. (b) (zone 1, unstressed; zone 2, ε = 1.0 for 1 cycle; zone 3, ε = 1.0 for 20 cycles). (c) (zone 1, unstressed; zone 2, ε = 1.0 for 5 cycles; zone 3, unstressed) (scale bars 1 mm).

amounts of time (Figure 1d, e) observing a rapid change in wettability within 60 s. After 1200 s, we measured the CA for PDMS-OH and ePDMS-OH at 95 ± 2° and 87 ± 3°, respectively (values that were comparable to 20 or more consecutive stress cycles, ε = 1.0). The magnitude of strain also influenced hydrophobic recovery of PDMS-OH films—larger magnitudes of strain enhanced hydrophobic recovery (Figure S1). The rate of strain did not change the general trends in hydrophobic recovery (Figure S2). Additionally, hydrophobic recovery of mechanically stressed films over time was enhanced within the first two hours following stress when compared to unstressed films (Figure S3 and S4). As has been previously reported,9,27 mechanical stress induces surface cracking in PDMS-OH films. Both confocal microscopy and AFM analysis confirm the formation of cracks in the PDMS-OH films (Figure S5-S8). Specifically, confocal microscopy showed that cracks formed within the first stress cycle, remaining relatively constant with time/subsequent cycles (Figure S7), and AFM showed a distinct change in surface morphology and an increase in surface roughness (Ra) from 0.545 nm to 2.108 nm following mechanical stress cycling of PDMS-OH films (50 cycles at  = 1.0, Figure S8). Further, XPS analysis of the substrates following increasing numbers of stress cycles showed an increase in the surface carbon content and a corresponding decrease in surface oxygen content while the silicon content remained relatively unchanged (Figure 1f, g). A significant change in the surface atomic composition occurred within 10 stress cycles (for both PDMS-OH and ePDMS-OH) which can also be attributed to surface cracking of the brittle oxide carbon while decreasing in oxygen following mechanical stress. This observation was expectedly different than that of unoxidized PDMS (Table S1), where surface


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ACS Applied Materials & Interfaces mechanical stress cycling by treating the films with oxygen plasma (Figure S18). We repeated this hydrophobic to hydrophilic switching for a total of 5 cycles at low and high strain (0.2 and 1.0, respectively; Figure S18). We observed no significant wettability hysteresis after each cycle. We applied our understandings of mechano-activated hydrophobic recovery to generate wettability patterns (heterogeneous patterns and gradients) using rationally controlled stress fields. To illustrate this concept, small regions of PDMS-OH films were mechanically stressed to different magnitudes using a mechanical tensioning device, inducing a controlled wettability change in the strained region (Figure 3a). We demonstrated the ability to generate various wettability profiles using different strain magnitudes/cycles indicating the simplicity of rapidly switching the wettability of subsections of the PDMS-OH films (Figure 3b, c). Additionally, by 3D printing different relief structures we generated millimeter to micrometer scale wettability patterns on the surface of the oxidized silicone elastomer using vacuum forming techniques (Figure 4a). Briefly, we placed 100 µm thick PDMS-OH films on 3D-printed relief structures and deformed the substrate by applying vacuum which induced localized mechanical strain, resulting in the formation of different wettability zones that we visualized using SWAV (Figure 4b, c and Figure S19). The surface wettability patterns were also visualized by depositing dyed microdroplets which color the PDMS films following evaporation (Figure S20-S21). These wettability patterns were clearly visible up to 24 hrs. after fabrication, and their stability was prolonged when stored in water (Figure S19). Furthermore, it is worth noting that surface cracks generated by stress scatter visible light and result in translucent PDMS-OH and e-PDMS-OH films, consistent with previous observations.37 Incremental changes in ε from 0 to 1.0 of a PDMS-OH film resulted in gradual hydrophobic recovery, providing a means to generate wettability gradients (Figure 5a). To achieve this outcome in one step using the vacuum forming procedure described above, we 3D-printed two “ramps” with different horizontal lengths (L), heights (H), and ramp angles (Φ), and deformed PDMS-OH substrates into the relief structures inducing gradual increases in strain of the film (Figure 5b). Using this procedure, we synthesized a 10 mm wettability gradient (ramp dimensions: L = 10 mm, H = 4 mm, Φ = 21.8°; Figure 5c). Additionally, we produced a sharper wettability gradient over a shorter distance (L = 6.0 mm) which were capable of transporting droplets (Figure 5d, Video S1). The water droplets moved from an area of high strain (low wettability) to an area of low strain (high wettability) at a rate of 14 ± 1 mm/s.

Figure 5. Mechanically generated wettability gradients. (a) Contact angle of PDMS-OH films after 1 stress cycle at increasing magnitudes of strain (rate = 1000 mm/min). (b) Schematic illustration of the generation of wettability gradients on PDMS-OH surfaces using vacuum forming. (c) Optical micrograph of 1 µL water droplets on the surface of a mechanically generated wettability gradient (ramp dimensions used in forming: L = 10 mm, H = 4 mm, and Φ = 21.8°). (d) Optical micrographs of a 3 µL H2O droplet traveling along a mechanically generated wettability gradient (ramp dimensions used in forming: L =6.0 mm, H = 4 mm, and Φ = 21.8°).

note that it is well known that surface microtopography, such as microcracks, heavily influences the hydrophobicity of PDMS films.17 It is possible to etch away the brittle silica layer that is formed during plasma oxidation via chemical dissolution.27 Hydrophobic recovery of mechanically stressed, etched PDMSOH substrates was still observed (Figure S14), albeit to a lesser degree than non-buffer treated PDMS-OH (CA changes of 47 ± 4° versus 85 ± 3°, respectively). In this case, hydrophobic recovery must be attributed to the migration of uncured monomers from the bulk to the surface as well as surface rearrangement (Figure S15-S17), which has previously been suggested.7 Furthermore, we demonstrated the ability to revert the film’s wettability to its original state after undergoing

CONCLUSIONS In summary, we investigated the effects of mechanical stress on oxidized silicone films and demonstrated that these surfaces undergo immediate and irreversible hydrophobic recovery upon mechanical stress. We attribute hydrophobic recovery to three processes: surface cracking of the brittle oxide layer, displacement of uncured monomers from the bulk to the surface, and rearrangement of surface functional groups, where surface cracking is most significant. We used this understanding to develop a new method to control and switch



the wettability of oxidized silicone elastomeric films rapidly, enabling the simple, rapid, and rational generation of surface wettability patterns and wettability gradients of various dimensions that can control droplet motion. Following this methodology we can tune surface wettability using a range of new processing parameters related to mechanical stress (e.g., the number of stress cycles, magnitude of strain, and duration of stress). Although oxidized PDMS films undergo hydrophobic recovery over time, this effect can be minimized with proper storage considerations along with extraction of uncured siloxane monomers.38 This work focused primarily on the hydrophobic recovery of uniaxially stressed, plasma oxidized silicone films, but other oxidation procedures (e.g., UVO) could be applicable, and more complicated stress profiles (e.g., biaxial, equiaxial, or combinations thereof) could be employed. These findings directly impact researchers working in soft robotics, stretchable electronics, surface coatings, microfluidics, and smart and adaptive materials, where plasmabased oxidation schemes are commonly used. Furthermore, the ability to control and manipulate surface wettability using mechanical stresses could potentially lead to the design of novel adaptive materials such as switchable sensors, mechanoactivated adhesives, switchable water repellent surfaces, etc.

Coordinated Infrastructure and NCMN, which are supported by the National Science Foundation under Award ECCS: 1542182, and the Nebraska Research Initiative.

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ASSOCIATED CONTENT SUPPORTING INFORMATION. XPS and CA of unmodified PDMS and ePDMS with mechanical stress; CA of PDMS-OH at varying strain magnitudes and rates; Optical imaging of surface cracks for PDMS and ePDMS; Uncured monomers mass analysis; GC calibration and LOD; Hydrophobic recovery and GC-MS of buffer treated PDMS/ePDMS; Patterining PDMS surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author † E-mail: [email protected] † Fax: 402-472-9402; † Tel: 402-472-4608

Funding Sources This work was supported by the National Science Foundation under Grant No. 1555356.

Notes The authors declare no conflicts of interest.

ACKNOWLEDGMENTS We would like to acknowledge Professor Barry Cheung for giving access and assistance with AFM. We thank the Department of Chemistry and the Nebraska Center for Materials and Nano Science (NCMN) at the University of Nebraska–Lincoln for start-up funds. S.A.M. thanks 3M for support through a Non-Tenured Faculty Award. This work was supported by the National Science Foundation under Grant No. 1555356. This research was performed in part at the Nebraska Nanoscale Facility: National Nanotechnology


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