Article pubs.acs.org/Langmuir
Electrowetting of Weak Polyelectrolyte-Coated Surfaces Vincent Sénéchal,†,‡ Hassan Saadaoui,†,‡ Juan Rodriguez-Hernandez,§ and Carlos Drummond*,†,‡ †
CNRS, Centre de Recherche Paul Pascal (CRPP), UPR 8641, F-33600 Pessac, France Centre de Recherche Paul Pascal, Université de Bordeaux, F-33600 Pessac, France § Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain ‡
S Supporting Information *
ABSTRACT: Polymer coatings are commonly used to modify interfacial properties like wettability, lubrication, or biocompatibility. These properties are determined by the conformation of polymer molecules at the interface. Polyelectrolytes are convenient elementary bricks to build smart materials, given that polyion chain conformation is very sensitive to different environmental variables. Here we discuss the effect of an applied electric field on the properties of surfaces coated with poly(acrylic acid) brushes. By combining atomic force microscopy, quartz crystal microbalance, and contact angle experiments, we show that it is possible to precisely tune polyion chain conformation, surface adhesion, and surface wettability using very low applied voltages if the polymer grafting density and environmental conditions (pH and ionic strength) are properly formulated. Our results indicate that the effective ionization degree of the grafted weak polyacid can be finely controlled with the externally applied field, with important consequences for the macroscopic surface properties.
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INTRODUCTION Adjusting wettability, friction, and adhesion of surfaces is often crucial for the precise operation of many processes in both industry and nature. The increasing miniaturization of engineering devices, which augments the relative importance of surfaces and interfaces, has further accelerated the relevance of these surface properties. Sophisticated strategies, based in chemistry, formulation, or materials science, have been developed to address these challenges. However, these approaches are inherently limited; a material is formulated to be hydrophilic or hydrophobic, adhesive or not. Considering the surfaces as active components may open avenues for novel functions; appealing applications can be foreseen if the control of the composition and morphology of the surfaces can be tuned in a reasonable time scale. For this reason, the development of stimuli-responsive materials is a rapidly evolving field of research. The most common strategy to generate adaptive (“smart”) surfaces involves employing molecules with sensitive moieties which are in contact with the external environment. Molecular state or conformations can be controlled by external stimuli or environmental changes, inducing reversible modifications of the structure of the surface. Polyelectrolytes (PE) have been often proposed as basic ingredients of switchable surfaces:1 as has been extensively discussed in the literature, a polyion grafted onto a surface can adopt different conformations, resulting from the balance between electrostatic, steric, and conformational forces.2−4 Formulation and environmental variables will determine the preferred molecular conformation. The for© 2017 American Chemical Society
mulation variables comprise chemical composition and polymer molecular weight, monomer charging density, and grafting density. The second group of variables (environmental) is related to solvent quality, pH, temperature, or ionic strength, which can be used to achieve a dynamic control of properly designed materials. Depending on the grafting density, σ, polymer surface layers are commonly categorized as “mushrooms” or “brushes”, the former applying to relatively isolated polymer molecules and the second to the case of large σ that results in important interaction between tethered molecules, which affects their conformation.5 The transition between the two regimes can be loosely defined as the density when the typical distance between tethered chains, D, is shorter than twice the unperturbed gyration radius of the polymer chain, Rg. The final configuration of tethered PE will be determined by the interplay between Coulombic and interchain steric forces, favoring chain extension, and elastic entropic forces, preventing it. Thus, the extension of a polyelectrolyte brush (the brush thickness, h) can be varied by changing σ and the fraction of charged monomers. In the design of polymer-based responsive materials, σ needs to be precisely controlled. Often, a large grafting density is desired to maximize durability and reactiveness, as the responsive moieties are present on the polymer chain. Nevertheless, a large σ may favor steric hindrance or specific Received: February 10, 2017 Revised: April 26, 2017 Published: May 12, 2017 4996
DOI: 10.1021/acs.langmuir.7b00473 Langmuir 2017, 33, 4996−5005
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Langmuir chain−chain interactions (e.g., interchain hydrogen bonding) which may restrain or completely deter surface response. It is also important to distinguish between quenched (permanently charged) and annealed (weak polyions) polyelectrolytes.6 Annealed PE, with variable and reversible ionization degree, are clearly more versatile for smart-surface applications, given that their molecular conformation depends on the physicochemical environment (i.e., pH or ionic strength). Weak PE brushes present a rich diagram of states that depends on the variables described above;2−4 previous studies have addressed this structural complexity. By using a molecular densityfunctional-self-consistent-field approach, Szleifer and co-workers showed that the ionization degree of tethered weak polyelectrolytes and the local pH can vary significantly inside the grafted chains and differ from the corresponding values for the same polymer chains in bulk solution.7 Thus, by changing grafting density or the local ionic strength or pH, the polymeric conformation can be affected. These conformational changes may have important consequences for the properties of the material. Chemical variables are not always easy to control; their manipulation may be sterically hindered or limited by species diffusion. Fine spatial-temporal control of pH or salinity is often out of reach. In addition, the characteristic time of response of molecules to changes of these parameters can be long. On the contrary, active control of molecular conformation by externally applied electromagnetic fields may allow direct fine functional control of surface properties. Tuning the strength of Coulomb interactions opens pathways to manipulate polymer conformation and to trigger its self-organization. A number of experimental and theoretical studies have shown that polyion configuration can be manipulated by the application of an external field. Weir et al. used ellipsometry to show that the conformation of a weak polybase brush can be remotely controlled using an applied voltage.8 Borisova and co-workers reached similar conclusions in a study of poly(acrylic acid) copolymer brushes using a quartz crystal microbalance (QCM).9 By studying the deflection of asymmetrically polyelectrolyte-coated microcantilevers, Zhou and co-workers found that significant surface stress can be generated by the effect of an electrical potential bias on the polyelectrolyte and associated counterions.10 For the most part, theoretical and numerical studies of field-responsive polyelectrolytes have investigated quenched polyions. In a theoretical description, Yamamoto and Pincus showed that the thickness of polymer brushes can be dramatically reduced in the presence of an externally applied field; the nature of this collapse is determined by the salt concentration.11 Several molecular dynamic (MD) simulations and mean-field models have shown that the response of a quenched polyelectrolyte brush to an externally applied field is a strong function of grafting density.12−15 Fieldinduced complete or partial collapse of the grafted polymers is predicted depending on σ. At large σ, the resulting bimodal state (collapsed/stretched) under field can be interpreted as a renormalized grafting density.12 Tong reported on a numerical study of the behavior of a weak polybase brush under an applied electric field.16 A complex field-induced response was found, even though only voltages smaller than 0.1 V were explored. A field-dependent brush height was observed at relatively low grafting densities. On the contrary, no major field-induced changes of the polymer brushes were observed at higher densities due to the competing effects of Coulombic repulsion and reduced ionization under field.16
Few studies have addressed the possibility of tuning macroscopic material properties by controlling the configuration of polyelectrolyte brushes by means of an applied electric field. We have recently shown that the conformation of adsorbed polyelectrolytes can be manipulated to dynamically control its lubrication properties by using an oscillatory external electric field.17 This method was based on the reduction of chain interpenetration between opposite rubbing brushes. Using MD simulations, Cao and co-workers suggested a method of controlling electroosmotic flow in polyelectrolytecoated nanochannels.18 We show here how the macroscopic surface wettability can be readily adjusted as a consequence of the conformational changes of grafted polyions. The modification of the wettability of a surface by means of an externally applied electric field is commonly termed electrowetting. This effect has been extensively discussed in the literature and is used in a number of applications, like adjustable liquid lenses, drop sorting, nucleation or deposition in microfluidic devices, electronic paper, or displays, among others.19 More than a century ago, Lippmann20 reported the first experimentally supported descriptions of electrocapillarity. Beni and Hackwood21 were probably the first to propose that the contact angle of a conductive liquid could be reduced by applying an electric field between the liquid and the supporting electrode. In that report and others that follow, the liquid was put in contact with a metallic electrode; thus, electrochemical reaction (e.g., water electrolysis) limits the practical application of the technique. The idea of using a thin insulating dielectric layer between the liquid and the electrode in order to avoid redox processes, preconized by Berge and co-workers,22 stimulated a large number of studies on what is now called electrowetting-on-dielectrics (EWOD). The wettability of a solid surface is usually described by using the Young equation cos θ0 = (γSV − γSL)/γLV
(1)
where θ0 is the contact angle and γSV, γSL, and γLV are the solid− vapor, solid−liquid, and liquid−vapor interfacial energies. When an external voltage V is applied between the liquid and the electrode, the apparent (experimentally measured) contact angle, θapp, is reduced from θ0 due to the electrostatic energy injected to the system. In the usual descriptions of EWOD experiments the liquid drop is considered as a perfect conductor. The apparent contact angle is modified because of the effect of the electrostatic energy stored (mostly) in the dielectric. Descriptions of EWOD based on a voltage-induced reduction γSL or as a purely electromechanical effect have also been reported in the literature.19,23−25 Both approaches lead to similar results, mainly that cos θapp = cos θ0 +
εε0 2 V 2dγLV
(2)
where ε is the dielectric constant of the conductive liquid, ε0 is the permittivity of free space, and d is the dielectric film thickness. In a conventional EWOD setup, the actual (as opposite to the effective) solid−liquid interfacial energy26 and the “real” contact angle (θ0, measured sufficiently close to the surface) remain unchanged upon application of the external voltage, as illustrated by Mugele and co-workers;25 thus, the Young angle is strictly preserved very close to the substrate. On the contrary, if a field-responsive coating is used, γSL and θ0 can be modified 4997
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Langmuir by the applied external field. As we show in this work, it is then possible to achieve a significant EWOD response at very low voltages.
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per molecule. Before any measurement, the solvent was allowed to evaporate for 30 min. The surface pressure was measured using a Wilhelmy plate. Isotherms were measured at a compression rate of 5 mm/min before the PS36-b-PAA125 molecules were transferred on the PS-coated surfaces at Π = 0.2 mN m−1 (corresponding 19 nm2 per diblock molecule). The surfaces were then dried using filtered nitrogen and heated at 110 °C for 10 min to improve the anchoring of the PS36b-PAA125 molecules on the PS film.29 Atomic Force Microscopy (AFM). Morphology of the PS36-bPAA125 films in air was determined by atomic force microscopy in tapping mode (Icon, Bruker) using Si tips on rectangular Si cantilevers. Morphology of the PS36-b-PAA125 films immersed in water was determined by atomic force microscopy in soft-contact mode (Multimode, Veeco) using Si tips on triangular silicon nitride cantilevers. Force versus separation curves were measured with 2 μm SiO2 colloidal probes on silicon nitride triangular cantilevers 200 μm long (sQUBE CP-PNPL-SiO-A-2). We measured the interaction between the AFM tip and PS36-b-PAA125 films as a function of the relative tip− substrate distance under different applied voltages between the gold substrate and the bulk solution. The cantilever deflection due to the tip−substrate interaction was determined from the voltage measured on the AFM sectored photodiode. As customary, we considered the tip to be in contact with the surface when the voltage vs displacement response varied steeply (and linearly): we have used this data to calibrate the response of the photodiode for each force curve. We used a single probe for the force curves reported; thus, the cantilever deflection is an accurate representation of the relative force measured for the different conditions investigated. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). pH-induced conformational changes of grafted PS36-bPAA125 coating on gold was measured in a commercial quartz crystal microbalance (QCM-D E1, Q-Sense). The principles of the technique have been extensively described in the literature.30 Briefly, the resonance frequency f of a quartz resonator is measured; a change in the effective mass of the resonator due to material adsorption translates into a variation in the resonance frequency, Δf. In addition, the damping of the oscillation of the crystal was measured and used to calculate the “dissipation factor”, D, defined as the inverse of the quality factor of the resonance peak.31 A large value of D indicates quickly decaying oscillations of the crystal, which is observed for thicker and nonrigidly attached layers. In a QCM-D experiment, the measured Δf and ΔD can be related to the thickness and viscoelastic properties of the material adsorbed on the quartz crystal by using adequate models.30 We used 5.0 MHz quartz resonators with gold electrodes, which were coated with PS36-b-PAA125 as described above. At the beginning of each experiment the coated resonator was placed in the cell and immersed in Milli-Q water adjusted to the pH and salt concentration to be studied for at least 30 min, until a stable baseline was established. This enabled the system to thermally equilibrate. The effect of pH on polymer conformation was investigated by measuring the response of the odd harmonics (n = 3 to n = 13). Contact Angle. Surface wettability was studied by using an automated contact angle goniometer (Teclis Tracker). Static, advancing, and receding contact angle were investigated. The initial drop diameter was fixed at 3 mm. In the dynamic mode the volume of the drop was changed by using a motorized syringe (Hamilton) at a fixed rate of 0.1 mm3/s. Stages of increasing and decreasing drop volume were alternated to measure advancing and receding contact angles. The motion direction was reversed after a change of 3% of the liquid−solid contact radii diameter; at least two advancing/receding cycles were measured for each condition investigated. The contact angle of aqueous drops in dry and 100% RH atmosphere was measured. By working at high relative humidity, stable and reproducible values of advancing and receding angle are commonly observed after the first cycle of increasing−decreasing drop volume. In addition, some measurements in liquid−liquid/solid were also performed, using drops of 1-bromodecane in water. 1-Bromodecane has been used before in electrowetting experiments; we have chosen to
EXPERIMENTAL SECTION
Materials. Styrene (St) (Aldrich, 99%) and tert-butyl acrylate (tBA) (Sigma-Aldrich, 98%) were distilled under reduced pressure over calcium hydride prior to use. Copper(I) bromide (CuBr) (SigmaAldrich, 98%), 2,2′-bipyridyl (bipy) (Sigma-Aldrich, 99+%), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (SigmaAldrich, 99%), phenylethyl bromide (PhEBr) (Sigma-Aldrich, 97%), and other solvents were used as received. Synthesis of Polystyrene-block-Poly(acrylic acid) (PS-b-PAA). Controlled radical polymerization was employed in order to prepare a block copolymer with precise composition. In particular, the diblock copolymers were prepared by ATRP in two steps following previously reported procedures which are briefly described below. Synthesis of Polystyrene (PS) Macroinitiator by ATRP. All polymerizations were performed in Schlenk flasks previously flamed and dried under vacuum. ATRP was carried out using the following stoichiometry [M]:[I]:[CuBr]:[L] = 250:1:1:2, where M = styrene, I = initiator (PhEBr), and L = ligand (bipy). The reactants were added under an N2 atmosphere. The reaction mixtures were then degassed by three freeze−pump−thaw cycles and placed in a thermostated oil bath at 110 °C. After the polymerization, the mixtures were cooled to room temperature, diluted with dichloromethane (CH2Cl2), and passed through a neutral alumina column to remove the copper salt. After evaporation, the polymers were precipitated in ethanol, filtered, washed, and dried under vacuum. Synthesis of PS-b-PtBA by ATRP. The macroinitiator PS-Br and 5 mL of degassed acetone were added to the mixture ([M]:[I]:[CuBr]: [L] = 400:1:1:1). Acetone enhanced the solubility of the CuBr/ PMDETA complex. The tBA polymerizations were carried out at 65 °C. Hydrolysis of the PtBA block to produce PS-b-PAA copolymers. Copolymers were first dissolved in CH2Cl2. Trifluoroacetic acid (TFA) was then added (10 equiv to tert-butyl ester units), and the mixture was stirred at room temperature for 3 days. The deprotected polymers, precipitated in the reaction media, were filtered and washed with CH2Cl2 and finally dried under vacuum. The number-average molecular weight (Mn) and polydisperstiy index (PD = Mw/Mn) were measured with a chromatographic system (Waters Division Millipore) equipped with a Waters model 2414 refractive index detector. Tetrahydrofuran (Multisolvent HPLC, Scharlau) was used as the eluent at a flow rate of 0.7 mL min−1 at 40 °C. Styragel packed columns (HR2 and HR5, Waters Division Millipore) were used. Polystyrene standards (Polymer Laboratories, Laboratories, Ltd.) between 5.7 × 105 and 5.8 × 102 g mol−1 were used to calibrate the columns. A copolymer PS36-b-PAA125, with PD = 1.45, was used for all the results presented in this work. Substrates. Silicon surfaces of about 1 cm2 were rinsed with ethanol (VWR Chemicals), cleaned with an ultraviolet ozone cleaning system (UVOCS INC, model T0606E), rinsed again with ethanol, and dried with filtered nitrogen gas. A 10 nm thick primer chromium film and a 50 nm thick gold film were evaporated on the surfaces. Then the surfaces were coated by thin polystyrene PS (Acros Organics, Mw 250 kDa) films by spin-coating (SCS Specialty Coating System, model G38). The PS films were annealed overnight at 95 °C before use. PS films were 50 nm thick (determined by ellipsometry, NanoFilm). Polymer Brush Assembly. Poly(acrylic acid) brushes were deposited using the Langmuir−Schaefer approach, as described by Currie and co-workers.27,28 5 mg of diblock copolymer polystyrene-bpoly(acrylic acid) (PS36-b-PAA125) was dissolved at 60 °C in 3 mL of dioxane (Sigma-Aldrich) for 2 days. Then, 2 mL of dry toluene was added to the solution to obtain a 1 g L−1 PS36-b-PAA125 solution in a 60%/40% dioxane/toluene mixture.27−29 A 243 cm2 Teflon Langmuir through was filled with 0.1 M NaCl solution in Milli-Q water. 24 μL of the PS36-b-PAA125 solution was spread at the air/water interface using a Hamilton precision syringe. This corresponds to an area of 21.4 nm2 4998
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Figure 1. (a) Compression isotherm of the PS36-b-PAA125 monolayer at the air−water interface. (b−e) Tapping mode height AFM micrographs measured in air of PS36-b-PAA125 films deposited by the Langmuir−Schaefer method at (b) 0.2, (c) 0.4, (d) 0.6, and (e) 1 mN/m (as indicated by the arrows in part a). (f) Soft-contact mode height AFM micrograph of a PS36-b-PAA125 film deposited at 0.2 mN/m, measured in a solution 2 mM KCl in water, pH = 4 (with HCl) The scale bars correspond to 200 nm.
Figure 2. (a) QCM-D: thickness variation of a PS36-b-PAA125 film, calculated from changes in Δf and ΔD of the coated quartz crystal, as described in the text. PS36-b-PAA125 monolayer deposited at Π = 0.2 mN/m. [KCl] = 2 mM. (b) Advancing (blue squares) and receding (red circles) water contact angle on a silicon wafer coated with 50 a nm PS thin film and a PS36-b-PAA125 monolayer deposited at Π = 0.2 mN/m as a function of pH. pH was adjusted by adding HCl or KOH as necessary. use this liquid in reason for its low viscosity and because it is denser than water. All measurements were conducted between 25 and 30 °C on polyelectrolyte-coated surfaces prepared as described above. Ionic strength of the aqueous phase was modified using KCl. pH was adjusted by using HCl or KOH. In the electrowetting experiments, voltages between −1 and +1 V were applied between metallic gold electrode and the aqueous drop (or aqueous solution in the liquid/ liquid case), which was grounded.
elaboration of polymer brushes. The subphase was a neutral 0.1 M NaCl solution. PAA, a weak polyelectrolyte, is expected to be mostly ionized under these conditions: the acid dissociation constant pKa of the AA monomer is close to 4.7. (As has been discussed in the literature, the pKa of the confined polymer is likely to be a function of the monomer position in the chain and probably several units of pH larger than the value found for the monomer;32 however, at neutral pH most of the AA monomers are expected to be ionized.7) In agreement with a number of earlier reports in the literature for this copolymer, the measured Langmuir isotherms of compression were reversible and reproducible, indicating that there was not substantial copolymer migration to the subphase under compression. A typical Langmuir isotherm is presented in Figure 1. AFM micrographs taken under dry
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RESULTS Polyelectrolyte Brushes: Preparation and Structural Characterization. We used the Schaefer variant of the Langmuir−Blodgett technique to deposit monolayers of PS36b-PAA125 on PS-coated gold electrodes. This copolymer has been extensively studied in the literature, in particular for the 4999
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Figure 3. Advancing (squares) and receding (circles) water contact angle on a PS36-b-PAA125 monolayer deposited at Π = 0.2 mN/m as a function of the applied external voltage. Dashed lines: variation of contact angle predicted by the Young−Lippmann equation (eq 2). (a) pH = 4. [KCl] = 2 mM. (b) pH = 4; no added salt. Inset: representation of the experimental setup.
conditions (after film deposition and annealing) are also presented in the figure. It has been shown before that under surface compression micelles are reversibly formed at the air− water interface. Thus, when the copolymer is deposited at very low surface pressure Π values, islands of PS surrounded by PAA coronas can be observed on the surface.33 An example of this condition can be observed in Figure 1b. The PS islands get increasingly closer as Π is increased; at sufficiently large pressure a micelle-to-brush transition is observed.33 When the coated surface is immersed in water, the PAA coronas are swollen while the hydrophobic PS moieties remain collapsed on the substrate (Figure 1f). Most of the results presented in this work correspond to polymer coatings prepared at Π = 0.2 mN/ m, corresponding to a polymer surface density σ of 0.05 chain nm−2. At these deposition conditions, the gyration radius RG of the PAA chains can be estimated to be between 4 and 5 nm.34,35 Thus, σ was close to the expected mushroom-to-brush transition density at the relatively low Π applied. Polyelectrolyte Brushes: pH Responsiveness. We have used a QCM-D to characterize the response of the PAA coating to a changing environment. As has been extensively documented in the literature, changes in the thickness and viscoelastic properties of a surface adlayer can be readily characterized using this technique.36 Changing the pH of the solution in contact with the adsorbed layer induces important changes in the adsorbed polymer chains; significant reduction in resonance frequency (negative Δf) and increase in energy dissipation (positive ΔD) are observed with increasing basicity. A typical example of the dependence on pH of Δf and ΔD for the different harmonics measured is presented in the Supporting Information (Figure S1). The changes observed were completely reversible when a number pH cycles were studied. The QCM-D response can be explained by a substantial increase of the effective hydrated polymer coating thickness (Figure 2a) and effective viscoelastic modulus at larger pH values. The degree of ionization of the polyelectrolyte increases under more basic conditions, increasing the electrostatic repulsion between the ionized monomers of the polymer chain and enhancing polymer swelling. On the contrary, the solubility of PAA in water is reduced in the neutral state; a conformational change from expanded coil to more compact globule is attained at more acidic pH values. Similar results
were recently reported in studies of grafted PAA by QCM-D9 or ellipsometry.37 The pH-induced configurational change of the grafted polymer translates into an important change in wetting condition. The effect of pH on the water wettability of copolymer-coated surfaces is presented in Figure 2b; these results evidence the responsive character of the material. A large difference between advancing and receding angle (contact angle hysteresis, Δθ) can also be appreciated, which can be deleterious for some applications. At first sight, this hysteresis could be attributed to surface heterogeneity. However, the PS bumps observed in dry conditions by AFM are much less significant under wet conditions, as can be appreciated in the AFM micrograph of the surface measured under water (Figure 1f). More importantly, a larger Δθ is observed when the copolymer grafting density increases, i.e., when the coating was less heterogeneous. For instance, the copolymer coating deposited at Π = 1 mN/m was very smooth, yet Δθ measured on this substrate was 10° larger than the one measured in the rougher surface coated at Π = 0.2 mN/m (cf. Figure 2b). Thus, the large observed Δθ is not due to surface roughness; it is likely to be related to the significant configurational change suffered by the polyelectrolyte molecules from dry to wet conditions, as will be discussed below. Polyelectrolyte Brushes: Electroresponsiveness. Changing pH is often a difficult route to trigger material response. Many systems are pH-sensitive (e.g., in biologicalrelated applications), so changes in this variable are restricted. In addition, local control of this variable is rather difficult, deterring a number of important applications. However, polyelectrolyte configuration can potentially be modified by a number of different parameters. Interestingly, we observed an important wettability change induced by the application of a small voltage (smaller than 1 V) in PS/PS-b-PAA coated surfaces, using EWOD configuration. Figure 3a shows the water wettability of a PS surface coated at Π = 0.2 mN/m. The pH of the water drop is adjusted to 4 by adding HCl; in this condition, the (unperturbed) polymer molecules are expected to be mostly in the uncharged configuration. The ionic strength of the water was adjusted to 2 mM by using KCl. As can be observed in the figure, substantial changes in receding contact angle are observed when negative voltages smaller than 1 V (in absolute value) are applied. Many cycles of voltage-induced 5000
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Figure 4. (a) Normal interaction force measured by AFM between a silica colloidal probe and a PS36-b-PAA125 monolayer deposited at Π = 0.2 mN/ m with (closed red squares) or without (black circles) application of a voltage bias of −800 mV between the gold electrode and the bulk aqueous solution. Only data measured upon approaching are shown. Inset: log−linear representation of the measured data. Dashed lines: exponential decay fit of the measured cantilever deflection. (b) Characteristic decay length of the measured repulsive tip−substrate interaction force for different applied voltage bias. Inset: representation of the experimental setup. (c) Maximum cantilever deflection upon retraction for different applied voltage bias. The adhesive pull-off force vanishes at sufficiently large negative voltages. pH = 4.5. [KCl] = 2 mM.
basic condition using larger applied ac voltages, which were significant enough to modify the lubrication properties of the copolymer-coated surfaces.17 However, at basic pH, PAA is mostly dissociated, acting as a quenched PE; the voltages explored in this study are probably not large enough to trigger conformational changes of the PE coating which are substantial enough to increase the contact angle of water on the surface. Thus, from the different experimental conditions evaluated in this work, we found a more significant voltage-induced response for polymer swelling accompanied by increasing wettability (at negative voltages) than for polymer collapse. Analogous trends in electroresponsiveness were found by Borisova and co-workers in a QCM-D study of PAA grafted brushes. However, in contrast to our results, they found only a weak response (polymer swelling) at acid pH. This difference is probably due to the different pH in both studies (3.3 in their work vs 4 in our case). It seems reasonable to expect that for more acidic pH values (farther away from pKa) a larger stimulus would be necessary to trigger the polyion conformational change. Using PE coatings as electrowetting actuator offers a number of advantages. It is possible to produce PE coatings that are resistant to abrasion and chemical degradation. The method is based on molecular reorganization, with no chemical reaction taking place. A longer lifetime of the adaptive layer may be anticipated. It is also remarkable that a continuous variation of θ is obtained. As opposite to the bistable response reported with self-assembled monolayers SAM,38 a real control of wettability and a large variation in θ can be obtained with the electroresponsive PE coatings reported here. Our experimental results also point out a number of disadvantages. The most important is related to Δθ. As can be observed in Figure 3, we observe a large voltage-induced variation in the receding contact angle; on the contrary, the changes on the advancing angle are less significant; similar results were observed with SAM coatings.38 We believe the reason for this hysteresis seems related to the important configurational change that the polymer chains undergo when passing from dry to wet conditions and not to the heterogeneity of the coated surface. Δθ is substantially reduced (to less than 10°) when a water/1bromododecane (instead of water/air) interface is studied. Because of the lower hysteresis observed in this configuration, the voltage-induced change of the surface from oil to water
wettability change were performed in a single drop/surface pair with identical results, ruling out the occurrence of voltageinduced polymer desorption. The possibility of applying larger voltages was severely restricted by breakdown of the dielectric layer that could be anticipated for voltages much larger than 1 V. However, the fact that voltages lower than 1 V are enough to trigger the PE response opens the possibility to dispense with the dielectric layer and grafting the PE coating directly on the metallic electrode, which will further enhance the responsive character of the coating. Interestingly, no effect was observed in the absence of salt: as can be observed in Figure 3b, in this case the measured contact angle remains unchanged when an external field is applied. As mentioned in the Introduction, the degree of ionization α of brushes of weak polyions is a strong function of ionic strength. In particular, it has been proposed theoretically2,37 and verified experimentally that α is substantially reduced at low ionic strengths. Thus, in the absence of added salt the ionic character (and electroresponsiveness) of the polyions is severely hindered. No significant electrowetting effect was observed in the range of voltage explored (−1 to +1 V) for the bare PS surface; a much lesser change was observed when the pH of the drop was higher than 8 and the PE chains were mostly ionized. As discussed below, we did not observe worsening water wetting upon applying a positive bias to the gold electrode. Thus, the effective wettability variation achievable by electrical stimulation was determined by the difference between the unperturbed state of the PAA chains, regulated by pH and ionic strength, and the maximum water-wetting condition achieved at the highest negative bias applied. Results illustrating the variation of electrowetting response at different pHs are presented in the Supporting Information (Figure S2). Several aspects of these wettability results deserve to be highlighted. First, negligible changes in θ would be anticipated from the Young−Lippmann equation at the experimental conditions investigated, as can be observed in Figure 3. Second, a different response is observed under positive or negative voltages, in contradiction with what is typically observed in EWOD (and can be readily inferred from the variation of θ on V2 in eq 2). Finally, a less significant electrowetting effect was observed when the PE layer was in the ionized state (basic pH) or at very low ionic strengths. We have shown in the past that voltage-induced conformational changes can be obtained in 5001
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and a bare surface is the Alexander and de Gennes AdG theory,5 which considers the balance between the osmotic and elastic forces (first two terms in the brackets on the right-hand side of eq 4) when monodisperse polymer brushes with a boxlike density profiles are compressed. For the asymmetric (tipbare surface) discussed in this work, the resulting steric force Fsteric is given by
wetting condition is readily observed even in static conditions. Pictures illustrating this transformation are presented in Figure S3. However, triggering the reverse transition (water-wet to oilwet) was out of reach within the range of voltages used in this work. Once again, it appears simpler to trigger polymer swelling than polymer dehydration. The marked reduction in advancing contact angle induced by the application of the field strongly suggests that the process enhances PE ionization, as can be inferred from the behavior of θ as a function of pH (Figure 2b). Weir and co-workers suggested a similar hypothesis from their ellipsometric study of a grafted weak polybase, although higher voltages (of the order of +3 V) were required to trigger the transition in their work.8 To explore this hypothesis, we have measured the interaction force between a 2 μm silica colloidal probe and a copolymercoated surface under the influence of an external electric field (similarly to the setup for the EWOD experiment) by using an AFM. Typical results are presented in Figure 4a. As can be observed in the figure, a long-range repulsive force between the probe and the substrate is observed. However, the repulsive interaction is significantly enhanced when a negative bias is applied to the gold electrode. Several facts indicate that these changes are due to voltage-induced configurational changes on the grafted copolymers and not to the enhanced direct electrostatic repulsion between the substrate and the (negatively charged) silica probe. First, the observed changes in repulsive forces are much more significant that the ones observed in the absence of the polymer coating (bare PS substrate). More importantly, the characteristic decay length of the repulsive force λ significantly changes with the applied voltage (Figure 4b). Additionally, the onset of the measured repulsive force (the distance at which a repulsive force was detectable) is substantially augmented when the negative bias is applied (inset Figure 4a). These bias-induced changes of the tip−substrate interaction force clearly indicate that polymer conformational changes are occurring when the voltage is applied. By tuning the magnitude of this repulsion, a precise control of the jump-in probability (and the mean adhesion force) can be achieved, as can be observed in Figure 4c. Two main contributions can be expected for the tip− substrate interaction: electrostatic and steric (polymer mediated). At the pH investigated, the colloidal probe is negatively charged, as the isoelectric point of silica is smaller than 3. Likewise, the PAA-coated surface will carry a small negative charge. Thus, a long-range electrostatic repulsive interaction can be anticipated. At sufficiently large separations (typically larger than the Debye screening length, λD) the electrostatic interaction Fele can be described by ⎛ D⎞ Fele = C exp⎜ − ⎟ ⎝ λD ⎠
⎤ Fsteric 8k T ⎡ ⎛ L ⎞5/4 ⎛ D ⎞7/4 = B 3 ⎢7⎜ ⎟ + 5⎜ ⎟ − 12⎥ ⎝L⎠ 2πR 35s ⎣ ⎝ D ⎠ ⎦
(4)
where D is the tip−substrate separation and L and s are the brush thickness and the typical distance between grafted polymer chains.39−41 This expression can be approximated by a single-exponential decay for D/L between 0.2 and 0.9 as39 ⎛ 2πD ⎞ ⎟ Fsteric = C′ exp⎜ − ⎝ L ⎠
(5)
To describe the interaction between polyelectrolyte brushes, macromolecular and electrostatic effects must be considered, making the problem much more involved. Different interaction regimes can be observed, depending on the relevant parameters of the system: L, s, degree of ionization, and λD. It is particularly relevant to compare the brush thickness L and the relevant Debye length. When L < λD, and for D greater than L, the interaction between the surfaces will be an exponentially decaying function, F ∼ exp(−D/λD). On the contrary, if L ≫ λD, the interaction will be determined by the elasticity of polymer chains and the osmotic contribution of confined ions, but no purely long-range electrostatic repulsion is expected. A complete description of the measured interaction force profiles is beyond the reach of this work. However, the significance of the different contributions to the measured interaction (steric and electrostatic) can be assessed by fitting the long-range repulsive force curves measured by singleexponential decay functions; typical results for the observed decay length λ are presented in Figure 4b. As can be observed in the figure, λ deviates significantly from λD when sufficiently large negative voltages were applied. On the contrary, λ was very close to the expected λD value at low applied voltages or in the absence of the electroactive copolymer coating for the whole range of applied potentials investigated. These results strongly suggest that the polyelectrolyte molecules undergo a significant conformational transformation due to the applied electric field. At the pH of the study, the ionization degree of the polyions is low in the absence of external bias; in this state, the PAA chains will adopt a compact conformation. On the basis of previous studies, we can estimate a radius of gyration between 3 and 4 nm for the PAA globule.34,35 The long-range repulsion between the charged tip and the substrate is mainly of electrostatic origin, and it decays as an exponential with characteristic decay length λD. The significant increment in strength and range of the repulsive force upon application of the negative bias lead us to believe that increasing ionization and conformational change of the grafted polyelectrolytes are taking place. A similar transit from electrostatic-dominated to steric-dominated interaction forces has been documented in several studies of pH-triggered conformational change of PAA brushes.42,43 The onset of the observed repulsion under negative voltages (ca. 40 nm) is larger than the expected contour length for PAA chains of 125 monomers (ca. 35 nm), which is the natural cutoff for polymer mediated steric forces. Moreover, we
(3)
where C is determined by the charge density (or potential) of both surfaces, the size of the colloidal sphere, and other experimental conditions. Thus, a quantitative description of Fele requires the knowledge of C and a precise determination of the size of the colloidal sphere and the spring constant of the cantilever, which we will not attempt here. On the contrary, the Debye screening length that characterizes the decay of Fele can be accurately calculated for the 1:1 salt investigated in this work from the salt concentration39 as λD = 0.304/[KCl]1/2 nm. One commonly used model to describe the interaction between neutral polymer brushes or between a polymer brush 5002
DOI: 10.1021/acs.langmuir.7b00473 Langmuir 2017, 33, 4996−5005
Article
Langmuir estimate a large L (ca. 50 nm) by using eq 5. We believe this apparent contradiction is most likely due to the large chain polydispersity: the long-range interaction forces will be dominated by the longest PAA chains present.44
Besides the ions chemically bounded to the polymer chain at pH < pKawhich, as discussed above, varies along the polyion chaina fraction of counterions may remain closely associated with the polymer backbone when the chain charge density is sufficiently large. If the energetic cost of complete ionization is large compared with the entropic advantage of counterion release, the state of bounded (condensed) counterion is favored.53 This condition, often called Manning condensation, has implications on a number of properties of polyelectrolyte solutions which have been largely described in the literature. Several authors have extended Onsager’s theory to the case of polyions showing how an external electric field can promote the dissociation of counterions, given that the free energy of the condensed state increases as a consequence of the applied field.54,55 Using Brownian dynamic simulations, Netz has shown that this process may trigger stretching and unfolding of polyelectrolytes at sufficiently large electric fields; similar results have been reported by other groups.56,57 This unfolding could be the reason for the voltage-triggered changes in wettability an interaction forces observed in this work. One significant difference between dissociating chemical bonds and condensed counterions is the behavior at high dilution. In saltfree media and at low concentration of small ionic-bonded molecules the law of mass action promotes full dissociation. On the contrary, in the same conditions, the fraction of condensed counterions will remain finite. Conversely, as the electrostatic screening is less significant at low ionic strength, a larger applied field will be necessary to achieve a similar reduction in the fraction of condensed counterions when the salt concentration is reduced. The increasing free energy cost of counterion release at low ionic strengths may play a role in the observed absence of electrowetting effect in the absence of salt (cf. Figure 3).
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DISCUSSION Externally applied electric fields can influence macromolecular conformation at different levels. First, direct electrostatic forces often play an important role, inducing movement of charges, alignment of permanent dipoles, and deformation (and orientation) of polarizable moieties or groups. For instance, directly controlling Coulombic forces allows manipulating the orientation of double-stranded DNA.45 Polarizability of counterions clouds associated with polyions often determines dielectric46 and electro-optical47,48 properties of polyelectrolyte solutions. A more subtle (and often more dramatic) effect of applied fields can exist if the stimulated macromolecule exhibits sufficiently different conformational states, like the coil−globule or helix−coil transitions of weak polyelectrolytes. The applied field may force a conformational transition by direct electrostatic effect (for instance by promoting dissociation of weak acid or basis) or by changing the physicochemical environment of the macromolecule (local ionic environment or pH). These effects can be put to good use in the design of macromolecularbased sensors, actuators, and smart surfaces. A number of examples of field-induced conformational change of macromolecules (e.g., grafted oligonucleotides,45 bovine serum albumin,49 poly(2-vinylpyridine),50 or (Na,K)-ATPase51) have been reported. The field intensities required to induce a significant change in macromolecular conformation can often be large (of the order of few hundred kV cm−1), which may limit its use in aqueous solutions. The situation is different at surfaces and interfaces: significant potential changes are the rule when an interface is crossed. The transition is very sharp, implying the existence of a large electric field within a few Debye lengths from the interface. This field may be significantly altered by an externally applied voltage drop through the interface. Thus, it is apparent that more important electroconformation effects can be expected on grafted or adsorbed macromolecules compared to the same molecules in solution. Our results suggest that the application of the external electric field promotes an important conformational change of the PE chains at sufficiently large negative voltages. Previous experimental studies have also shown the possibility of triggering analogous changes of weak-polyelectrolyte brushes by the application of low voltages.8−10 At the pH used in this work, and in the absence of any applied field, the PAA chains are close to the globule-to-coil transition, as can be observed in Figure 2. However, the equilibrium condition can be altered if the physicochemical environment (ionic strength, pH, and grafting density) is changed. It is conceivable that the external field triggers the ionization of the grafted weak polyions, resulting in enhanced intramolecular repulsion between the charged monomers which can promote the conformational change of the tethered polyelectrolytes and the reduction in contact angle. It is well-known that strong electric fields can promote dissociation of weak electrolytes, by stretching and weakening the ionic bond (second Wien effect52), as described by the theory developed by Onsager.52 However, we observe PAA swelling only upon application of a negative bias to the supporting electrode. As discussed by Borisov and co-workers, this bias will necessarily diminish the local pH, decreasing polyanion dissociation.2
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CONCLUSIONS We have shown how electric-field-induced conformational changes of a weak polyelectrolyte brush can be used to tune surface wettability and adhesion. Active and precise control of the wettability or adhesion at local or global scale is possible by adjusting the local molecular conformation of a responsive layer via the application of an electric field. In the experiments described in this work a very thin dielectric film was used, which would allow a local control of wetting with high resolution. In addition, as low bias voltages are required, the dielectric layer can potentially be left aside by directly grafting the polymer on the electrode substrate, enhancing the robustness and easiness of the proposed method of control of surface properties.
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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.7b00473. Figures S1−S3 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(C.D.) E-mail
[email protected]. ORCID
Carlos Drummond: 0000-0003-4834-3259 5003
DOI: 10.1021/acs.langmuir.7b00473 Langmuir 2017, 33, 4996−5005
Article
Langmuir Notes
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The authors declare no competing financial interest.
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