Article pubs.acs.org/Langmuir
Real-Time Monitoring of Chemical and Topological Rearrangements in Solidifying Amphiphilic Polymer Co-Networks: Understanding Surface Demixing Gustavo Guzman,† Turgut Nugay,§ Joseph P. Kennedy,‡ and Mukerrem Cakmak*,† †
Polymer Engineering Department and ‡Polymer Science Department, The University of Akron, Akron, Ohio 44325, United States § Chemistry Department, Polymer Research Center, Boğaziçi University, 34342 Bebek, Istanbul, Turkey S Supporting Information *
ABSTRACT: Amphiphilic polymer co-networks provide a unique route to integrating contrasting attributes of otherwise immiscible components within a bicontinuous percolating morphology and are anticipated to be valuable for applications such as biocatalysis, sensing of metabolites, and dual dialysis membranes. These co-networks are in essence chemically forced blends and have been shown to selectively phaseseparate at surfaces during film formation. Here, we demonstrate that surface demixing at the air−film interface in solidifying polymer co-networks is not a unidirectional process; instead, a combination of kinetic and thermodynamic interactions leads to dynamic molecular rearrangement during solidification. Time-resolved gravimetry, low contact angles, and negative out-of-plane birefringence provided strong experimental evidence of the transitory trapping of thermodynamically unfavorable hydrophilic moieties at the air−film interface due to fast asymmetric solvent depletion. We also find that slow-drying hydrophobic elements progressively substitute hydrophilic domains at the surface as the surface energy is minimized. These findings are broadly applicable to common-solvent bicontinuous systems and open the door for process-controlled performance improvements in diverse applications. Similar observations could potentially be coupled with controlled polymerization rates to maximize the intermingling of bicontinuous phases at surfaces, thus generating true three-dimensional, bicontinuous, and undisturbed percolation pathways throughout the material.
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contact with that surface.6 For co-network films cast in dry air, the higher hydrophobicity moiety is found at the surface.7 The bulk of polymer co-networks is prevented from demixing by chemical cross-linking in a common solvent, but inhibiting demixing at surfaces has proven to be particularly difficult.6,7,11 The presence of both phases at the surface is relevant for applications in which bicontinuous, undisturbed pathways must be ensured, especially if selective swelling of both polymer phases is required. Fast UV photocuring reactions are able to partially or totally halt demixing6 but fluctuating morphologies between the bulk and surfaces have been observed, and the number of available chemistries for co-network formation under this strategy is considerably reduced. As surface demixing is thermodynamically favorable, it has been assumed that is dependent only on the effective rate of polymerization/cross-linking, i.e., the time required for gelation to take place.6,12 However, the process has not been observed in real time, and the complexity of solidification of a multicomponent system with phases of contrasting of proper-
INTRODUCTION Amphiphilic polymer co-networks (APCNs) are networks of immiscible hydrophilic and hydrophobic moieties and have received increasing amounts of attention in the past 10−15 years.1−7 Unlike block copolymers, where self-assembly into useful geometries is a product of thermodynamic equilibrium under rigorously controlled conditions,8 in polymer conetworks an otherwise nonequilibrium morphology is trapped by chemical cross-linking.4,5 Their main advantage over block copolymers is their ability to enable, in a wide range of compositions, the three-dimensional percolation of two different phases throughout the material. Applications are found in biocatalysis, sensing of metabolites, and dual dialysis membranes (oxygen + hydrophilic metabolites).1−3 In most of these applications, the co-network acts as a matrix for interfacial interactions in which the two reactants are located in opposite co-network phases, but their contact is enabled by a huge interfacial area.9 Thus far, research efforts have been focused on establishing synthetic routes that efficiently prevent macroscopic phase separation during cross-linking and on increasing the long-range order of the co-networks.4,8,10 Even so, it is well known that polymer blends and mixtures selectively separate on surfaces depending on the chemical nature of the material in © XXXX American Chemical Society
Received: February 15, 2016 Revised: March 21, 2016
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DOI: 10.1021/acs.langmuir.6b00587 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Real-time monitoring of solidifying polymer co-networks. (a) Bimodal amphiphilic co-network model system. (b) Parallel collection of real-time data; solidification is carried out until constant weight and thickness under controlled conditions are achieved, and a temperature decrease is observed due to evaporative cooling. (c) Real-time monitoring system (sample top view): four pyrometers measure the local surface temperature, three laser micrometers measure the local thickness, and two light sources (at 0 and 45°) are used for the birefringence determination. The sample is located on top of a precision balance inside a wind tunnel. (d) Time-resolved weight fraction for polymer co-networks with different cross-linker ratios. Solidification is divided into three stages: equal drying rate (I), composition-dependent diffusion-controlled dying (II), and constant weight (III). As the cross-linker is PDMS-based, an increased cross-linker ratio also increases the hydrophobic fraction. Grafts of 1, 2, and 5% HMW-PDMS (0.9 g) were mixed with crosslinker in three mole ratios (allyl chain end/hydrosiloxane = 1:5, 1:10, and 1:25) and Karstedt’s catalyst (25 μL) in THF (8 mL). The bAPGs were mixed with PHMS-co-PDMS cross-linker by strong stirring in THF for 10 min. Films of controlled thicknesses (∼150 μm final thickness) were prepared by blade casting. Cross-linker is added in order to adequately emulate the composition of the original films and to evaluate its effect on physical parameters during drying. Relevant progression of the cross-linking reaction is not expected to take place during the drying process (Supporting Information). Real-Time Weight, Thickness, Temperature, and Birefringence Measurements during Film Formation. Films were dried in a custom-built real-time measurement system detailed elsewhere.13 Drying was carried out for several minutes at room temperature until constant weight and thickness were achieved. Drying parameters were controlled by the air heater/blower upstream of the airflow tunnel. Films with 1, 2, and 5% HMW-PDMS and 1:5, 1:10, and 1:25 crosslinker ratios were tested. The control was a vinyl-terminated PDMS cast and dried under the same conditions. Time-Resolved Refractive Index Measurements through the Abbe Refractometer. Principal refractive index measurements during the drying of bAPCN films were performed using a Bellingham + Stanley limited 60/HR Abbe refractometer with eyepiece polarizer. To track refractive indices against substrate surfaces, the film was cast on the Abbe refractometer prism and the in- and out-of-plane refractive indices (MD and ND) at the film−substrate interface were determined as a function of time with an eyepiece polarizer and without additional immersion contact liquid. Measurements were carried out using a white light source with a band-pass filter (633 nm) to generate monochromatic light. Films with 1, 2, and 5% HMWPDMS and 1:5, 1:10, and 1:25 cross-linker ratios were tested. Contact Angles during Drying. Contact angles of drying samples were tracked by a contact angle KRUSS DSA100 analyzer equipped with a camera, homogeneous LED lighting, and automatic baseline
ties has not been explored. Here, time-resolved monitoring of solidifying amphiphilic polymer co-networks by noncontact techniques13−15 is performed for the first time. Real-time localized optical anisotropy and coupled real-time gravimetric and contact angle data are employed as an indicator of molecular motions in fast processes at the film−air interface. We chose the bimodal amphiphilic polymer co-network model system11 presented in Figure 1a in view of the strong contrasting properties of its components and its slow crosslinking kinetics. Bimodal amphiphilic grafts (Figure 1a, left) are poly(N,N-dimethylacrylamide) (PDMAAm) main chains carrying vinyl-terminated poly(dimethylsiloxane) (PDMS) branches of two molecular weights. Polyhydrosiloxane-co-PDMS (PHMS-co-PDMS) serves as a cross-linker.
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EXPERIMENTAL SECTION
Materials. The synthesis and characterization of bimodal amphiphilic grafts and co-networks were reported.11 Polyhydrosiloxane-PDMS copolymer (PHMS-co-PDMS) containing 30% PHMS and Karstedt’s catalyst (3% Pt in xylene, low color) were purchased from Gelest and used without further purification. Tetrahydrofuran (THF) was obtained from Sigma-Aldrich. Sample Preparation. The synthesis of bimodal amphiphilic grafts was reported before.11 Briefly, the synthesis involves the free radical terpolymerization of DMAAm with a statistical mixture of macronomers of low- and high-molecular-weight (1, 2, or 5%) PDMS, carrying either vinylsilyl (-V) or methacrylate (-MA) terminations: MA-PDMS-V, V-PDMS-V, and MA-PDMS-MA. The reaction yields a bimodal amphiphilic graft (bAPG) consisting of PDMAAm main chains carrying -PDMS-V branches. Because of the presence of MAPDMS-MA, the graft is slightly cross-linked and of high molecular weight. The arm number is kept in the 2−5 range so that network formation is still possible but without gel forming on the graft. B
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Figure 2. Time-resolved molecular shifts via optical anisotropy and contact angle. Dotted lines (I−III) illustrate simultaneous phenomena. (a) Realtime monitoring of average out-of-plane birefringence (optical anisotropy) of model co-networks of varying cross-linker ratio (1) and %HMWPDMS (2), as measured by our custom-built system. (b) Time-resolved optical anisotropy at the substrate−solution interface varying cross-linker ratio (1) and %HMW-PDMS (2). (c) Time-resolved contact angles. Again, a varying cross-linker ratio (1) and %HMW-PDMS (2) are tested. detection. Films were cast onto glass surfaces ,and their contact angles were determined at fixed time intervals using water. To avoid surface modification during the measurement, the examination area was changed at each time interval. Time-Resolved Surface Roughness Determination by Specular Reflection. Films were cast on a black painted glass substrate and immediately placed on a platform while a HeNe laser (1 mW, at 632.8 nm) shone on the surface of the drying film at a specular angle. Light reflected from the surface was projected onto a screen and recorded with an HD camera. Images were analyzed by ImageJ software. Samples of 5% HMW-PDMS and a 1:25 cross-linker ratio were tested. Small-Angle X-ray Scattering. Fully cured films with 1, 2 and 5% HMW-PDMS with a 1:25 cross-linker ratio and uncured 5% HMWPDMS with a 1:25 cross-linker ratio were tested in a Rigaku MicroMax 002+High Intensity Microfocus sealed tube X-ray generator. SAXS data were collected for exposures of 1000 s at room temperature.
support stresses and relaxation times increases, leading to larger out-of-plane birefringence as the chain axes preferentially orient randomly in the film plane, maintaining zero in-plane birefringence.13−16 The sign of this birefringence depends upon the orientation of the dominating polarizable group relative to the chain backbone axis set at the time of polymerization. During drying, time-dependent optical gradients are known to occur due to one-directional mass flux16 so that optical anisotropy measured at early drying times originates mainly at the air−film interface of the solidifying film. We take advantage of this occurrence to monitor the nature of the co-network surface during solidification. Figure 1d shows the weight fraction vs time for different conetworks and hydrophobic PDMS as a control. During the early stages, the film weight decreases rapidly, exhibiting the same rate of drying for all samples (stage I). At this stage, the surface is solvent-rich, and solidification is controlled by external mass-transfer resistance, i.e., solvent mass- and heattransfer coefficients, air temperature, solvent partial pressure in the gas phase, and latent heat of vaporization.17,18 In stage II, the solvent concentration decreases at the film−air interface and a “skin” is formed.18−20 The skin acts as a diffusion barrier slowing the solidification. Consequently, the drying rate in stage II is composition-dependent, and the nature of the skin layer dictates mass-transfer resistance. Figure 1d shows that increasing the amount of cross-linker, i.e., hydrophobic PDMScontaining compounds, decreases the rate of drying, with PDMS exhibiting the slowest rate.
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RESULTS AND DISCUSSION Effect of Composition on the Rate of Drying. Real-time monitoring of the solidification kinetics and molecular orientation of model amphiphilic polymer co-networks (bAPCNs) of varied composition was carried out in a custom-built real-time measurement system detailed elsewhere.13 Figure 1b presents an example of parallel real-time data collection, and Figure 1c presents an illustration of the instrumentation. As shrinkage imposes compressive stresses on the solidifying film,16 most polymer solutions exhibit a slow increase in out-of-plane birefringence while in-plane birefringence remains zero.14 During later stages, the film starts to C
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Optical birefringence, as measured by our custom-designed system, represents an average of chain orientation across the film’s thickness, and as such, it does not distinguish the orientation at the surface from the orientation occurring in the bulk of the solution. To make this distinction, we employed Abbe refractometry,15 which directly measures principal refractive indices and thus birefringence at the substrate− solution interface. Our refractometry results (Figure 2b1,b2) suggest that orientation does occur at the substrate/film interface; however, the observed smaller and later maxima of the OP birefringence indicate a faster and more pronounced orientation at the air−film interface. For the first ∼250 s of solidification (dotted line I in Figure 2), optical anisotropy originates at the surface of the film. We indicated how a rapid decrease in thickness causes a preferential orientation of chains in the film plane and established that solvent preferentially leaves the hydrophilic phase first. The combination of these results, along with the sharp negative OP birefringence increase originating at the film surface, strongly suggests the formation of a hydrophilically oriented PDMAAm skin during the early stages of solidification. In this manner, asymmetric solvent depletion is able to provisionally confine thermodynamically unfavorable hydrophilic domains at the surface. Because of the massive phase segregation of chemically forced blends containing relatively large phases, these amphiphilic polymer co-networks exhibit two distinct glasstransition temperatures.4 While the film is solvent-rich, PDMAAm remains rubbery as the solvent depresses the glass-transition temperature.25 With progress in solidification, a rubber-to-glass transition occurs when the glass-transition temperature matches the environmental temperature. Removal of solvent from the PDMAAm phase leads to structural rearrangements in the polymer matrix as it progressively moves toward the unrelaxed glassy state.25 The observed fast solvent depletion of the PDMAAm phase (activity >1) quickly induces glass transition of hydrophilic chains close to the air−film interface and in turn increases the chain orientation and OP birefringence. This quick transition to the glassy state explains the uncommonly sharp birefringence slope at the beginning of drying. This point was further confirmed by a strong change in the birefringence behavior when a small amount (>1%) of water is added (Figure 2S in the Supporting Information), as bound water plasticizes the hydrophilic phase, preventing their rubber-to-glass transition and reducing relaxation times and orientation. Contact Angle Measurements during Solidification. Evidence so far points to the formation of oriented hydrophilic PDMAAm skin during the early stages of solidification. Nonetheless, previous XPS data suggested the presence of only PDMS at the surface11 of solid films, which was attributed to favorable energy minimization.7 Contact angle measurements during the solidification of amphiphilic co-networks may indicate surface chemical composition due to the contrasting nature of its components. Figure 2c1,c2 present time-resolved measurements of contact angles during solidification. Data collection started at 300 s, as only beyond this point a strongenough skin was formed to support liquid droplets for contact angle measurements. The water droplet did not dissolve on the film during measurement. The trend of time-resolved variations in contact angles is analogous to that of OP birefringence. During early drying stages, the contact angles decrease and reach a minimum at ∼600 s. After this point, contact angles increase and reach a maximum between 800 and 1000 s. Dotted
Amphiphilic solvents are commonly used to prevent phase separation during the formation of bicontinuous morphologies.4 As co-network phases are assumed to percolate throughout the material, they can interact with solvents independently, for example, individually swelling in solvents of different polarity. As a result, there is no reason that the solvent should leave phases of contrasting properties at the same rate. An approximate calculation of solvent activity (Supporting Information) validates the gravimetric data of Figure 1d, as solvent activity in the hydrophilic PDMAAm phase is found to be much larger (>1) than in the hydrophobic PDMS phase, indicating the dominance of repulsive forces and increasing the solvent vapor pressure/drying rate.18 The wide difference in the solvent quality of the bicontinuous phases effectively induces asymmetric solidification because solvent leaves the hydrophilic PDMAAm phase at a faster rate. At the same time, rapid solvent evaporation from the hydrophilic phase is consistent with hydrophilic PDMAAm at the surface, at least during the early stages of solidification. Time-Resolved Interfacial Molecular Shifts via Optical Anisotropy. The alignment of polymer chain axes randomly in the plane of the film during solidification from solution gives rise to a difference between the refractive indices between the film plane and the film normal directions (out-of-plane birefringence). The sign and magnitude of this birefringence depend upon the orientation of the dominating polarizable group relative to the chain backbone.16,19 Hydrophobic PDMS has a small optical anisotropy20 of 0.96 × 10−25cm3, and the value for PDMAAm was theoretically calculated19 (Supporting Information) to be −9.16 × 10−25 cm3. Having opposite signs of their intrinsic optical anisotropies, the orientation of PDMS and PDMAAm chains would be in competition in regard to the total birefringence of the co-network. The orientation of PDMS chains with chain axes in the film plane would lead to a positive increase in OP birefringence, while the same for PDMAAm would represent a negative increase. Figure 1S in the Supporting Information presents a theoretical weight-fractionaveraged intrinsic anisotropy for different co-network compositions. Thus, the fast negative increase in out-of-plane (OP) birefringence at the start of drying for all compositions, observed in Figure 2a1,a2 (stage I), corresponds to the orientation of hydrophilic PDMAAm chains with chain axes preferentially in the film plane. An increased amount of crosslinker sharpens the initial birefringence slope and increases its maximum but at the same time shifts the slope sign change to earlier times. Increasing %HMW-PDMS has a similar but more pronounced effect. Unoriented biphasic materials may still exhibit birefringence if the comprising phases are assembled into anisotropic structures and possess a significant difference in their refractive indices.21 This phenomenon has been reported for block copolymers,22 and it is known as form birefringence. In most amphiphilic polymer co-networks, domains are not arranged in a regular lattice as a result of random cross-linking in the network.6 Nonetheless, ordered bicontinuos systems have recently been reported.10 The co-network under study lacks long-range order after casting, as evidenced by small-angle Xray scattering experiments (Supporting Information). This fact, coupled with a relatively small refractive index difference (∼1.42 for PDMS and ∼1.47 for PDMAAm),20 indicates that form birefringence is unlikely to occur. D
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Figure 3. Real-time optical determination of roughness. (a) Roughness parameters based on image analysis of the reflected beam. The blue and red dotted lines indicate the time at which the film exhibits the lowest and highest contact angles, respectively. (b) Percentage of light transmission. As the film roughness increases or decreases, the light transmitted through the film mirrors the roughness trend. (c) Illustration of the wrinkling instability. (d) Experimental setup for the roughness determination based on the specular reflection of laser light.
Time-Resolved Roughness by Specular Reflection. We used the specular reflection technique for the roughness determination, which is based on measurements of a scattered light pattern projected onto a screen and a statistical analysis of its light intensity distribution.25,26 Figure 3d presents a schematic of the experimental setup. Images of the timeresolved roughness of solidifying bAPCN films were analyzed by the methods reported by Brodmann27 and Luk,25,28 who characterized surface roughness by the variance of the scattering angle (S) and the frequency distribution of the scattered light intensity (R), respectively. Both methods depend on standards of known roughness for calibration.26 In the present instance, calibration is unnecessary as we are concerned only with identifying the precise time when roughness increases during drying and its correlation with contact angles. Figure 3a presents S and R as a function of drying time. Figure 3b shows the percent light transmitted through the sample. Both roughness parameters show similar trends over time. During early drying there is a peak at ∼200 s and another at ∼500 s. The latter is the highest roughness during drying, but interestingly, the film becomes smooth again and the peak disappears in about 100 s. This peak is mirrored by a dip in light transmission (Figure 3b), as rougher surfaces scatter light more effectively. After a minimum at ∼500 s, light transmission increases again, reaching almost 100% after about 100 s. Romdhane and co-workers reported the formation of wavefront-like perturbations perpendicular to the air flow direction during the initial stages of drying. The waves relaxed and eventually disappeared.23 Powers and Collier29 attributed this phenomenon to flow instabilities caused by large differences in viscosity and diffusivity between the “skinned” surface and the bulk of the coating. They also reported that the initial instability disappeared. Notably, roughness peaks at 200 and 500 s do not directly correlate with increasing contact angle, as the maxima occur later during drying. From 800 to
lines in Figure 2a−c illustrate synchronicity between different time-resolved measurements. The minima in contact angles and OP birefringence do not temporally match precisely, but the similarity in trends is significant. Consistent with previous observations, the initially low contact angles suggest a hydrophilic surface during the early stages of solidification. At the same time, the lowest contact angle (∼89°) indicates significant hydrophobicity and suggests that although PDMAAm dries faster and its orientation dominates OP birefringence, demixing at the surface is neither immediate nor complete. Fast asymmetric solvent depletion kinetically traps hydrophilic PDMAAm domains at the surface, but the low surface energy of PDMS and the huge interfacial area between moieties characteristic of bicontinuous systems keep it from completely demixing. As drying proceeds and more solvent leaves the slower-drying hydrophobic PDMS domains, thermodynamics takes over, i.e., PDMS migrates to the surface toward a more energetically favorable state and renders it gradually more hydrophobic. We must clarify that these surface rearrangements do not imply a massive migration of hydrophobic chains from the bulk to the surface but rather a gradual replacement of the PDMAAm phase that initially occupied the surface. In amphiphilic polymer co-networks, the phases are vast but still covalently linked. A caveat for this analysis is that changes in surface topology would affect contact angle measurements as rougher surfaces have higher contact angles.24 Previously, we demonstrated a composition-dependent wrinkling instability in bAPCN films, which arises during solidification. Using AFM imaging, the instability parameters (wavelength and amplitude) were shown to be compositiondependent.11 The two major effects on contact angle, i.e., hydrophilicity and topology, needed to be decoupled by identifying the onset of the wrinkling instability during solidification. E
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Figure 4. Overview of real-time evolution of surface demixing. (i) Optically isotropic film with the drying kinetics controlled by the vapor/liquid equilibrium at the film/air surface. (ii) Transitory surface hydrophilic entrapment, with increases in the negative OP birefringence (Δn = (−)) and decreases in the contact angle. (iii) Hydrophobic PDMS surface migration (minimizing free energy), with increases in the contact angle and changes (Δn = (+)) in the sign of the slope of OP birefringence. (iv) Final hydrophobic surface (PDMS domains) and PDMS surface wrinkles under compressive stress as solidification is completed.
solvent quality and drying conditions play fundamental roles in controlling surface demixing. These findings expand the fundamental understanding of interfacial phenomena in amphiphilic polymer co-networks and are broadly applicable for common-solvent bicontinuous systems. Observations similar to the ones presented in this article could be coupled with controlled polymerization/curing rates to maximize the intermingling of co-continuous phases at surfaces, thus generating true three-dimensional, bicontinuous, and undisturbed percolation pathways throughout the material.
1000 s, the roughness increases again and remains high, followed by a small decline. In this case, an increase in roughness occurs after the contact angle has reached its final value. As the later increase in roughness is final, it can safely be assumed that it corresponds to the onset of the wrinkling instability seen in the final form of the film.11 Figure 4 summarizes a comprehensive view of the solidification process by combining drying rates, OP birefringence, contact angle, and roughness data: During stage (i), the film is solvent-rich and optically isotropic, and drying kinetics are controlled by the vapor/liquid equilibrium at the film/air surface. During stage (ii), asymmetric fast solvent evaporation creates a partially hydrophilic skin of kinetically trapped hydrophilic chains at the air−film interface. As the hydrophilic chains orient themselves in the plane of the film and undergo the glass transition, negative OP birefringence increases (Δn = (−)) and the contact angle decreases. Solidification is diffusion-controlled during this stage. During the third stage (iii), thermodynamics takes over as the hydrophobic PDMS migrates to the surface (minimizing the free energy), the contact angle increases, and the sign of the slope of OP birefringence changes (Δn = (+)). During the final stage (iv), the surface is completely hydrophobic, consisting of only PDMS domains. The PDMS skin wrinkles under compressive stresses as solidification is completed. The process is similar for every composition with variations in timing but not in trend.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00587. Theoretical calculation of poly(N,N- dimethylacrylamide)’s optical anisotropy, birefringence development in samples with added water, structural characterization by small-angle X-ray scattering, activity calculation, crosslinking kinetics, and video feed of the drying process (PDF) Movie of the drying process (ZIP)
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AUTHOR INFORMATION
Corresponding Author
CONCLUSIONS We report for the first time the temporal evolution of surface structuring during the film formation of amphiphilic polymer co-networks from solution. We have demonstrated that demixing at surfaces of this set of chemically forced blends during film formation is not unidirectional; instead, we revealed an active process in which the drying kinetics and thermodynamics have a complex interplay and induce dynamic molecular rearrangements to take place. Specifically, we presented strong experimental evidence of the temporary trapping of thermodynamically unfavorable hydrophilic moieties at the surface during early drying times due to asymmetric solvent depletion. Such moieties are progressively replaced by slow-drying hydrophobic domains. The hydrophilic-to-hydrophobic surface rearrangement was observed for co-networks within a considerable range of cross-linker ratios and PDMS molecular weights, indicating that our conclusions are relevant beyond the example discussed here. We also proved that
*Tel: 330-972-6928. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Our research was made possible by Ohio Third Frontier funding of a Wright Center program under CMPMD. REFERENCES
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DOI: 10.1021/acs.langmuir.6b00587 Langmuir XXXX, XXX, XXX−XXX
