Voltage-Induced Swelling and Deswelling of Weak Polybase Brushes

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Voltage-Induced Swelling and Deswelling of Weak Polybase Brushes Michael P. Weir,*,†,§ Sasha Y. Heriot,† Simon J. Martin,† Andrew J. Parnell,† Stephen A. Holt,‡,§ John R. P. Webster,‡ and Richard A. L. Jones† †

Department of Physics and Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom ‡ ISIS, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom

bS Supporting Information ABSTRACT: We have investigated a novel method of remotely switching the conformation of a weak polybase brush using an applied voltage. Surface-grafted polyelectrolyte brushes exhibit rich responsive behavior and show great promise as “smart surfaces”, but existing switching methods involve physically or chemically changing the solution in contact with the brush. In this study, high grafting density poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grown from silicon surfaces using atom transfer radical polymerization. Optical ellipsometry and neutron reflectivity were used to measure changes in the profiles of the brushes in response to DC voltages applied between the brush substrate and a parallel electrode some distance away in the surrounding liquid (water or D2O). Positive voltages were shown to cause swelling, while negative voltages in some cases caused deswelling. Neutron reflectometry experiments were carried out on the INTER reflectometer (ISIS, Rutherford Appleton Laboratory, UK) allowing time-resolved measurements of polymer brush structure. The PDMAEMA brushes were shown to have a polymer volume fraction profile described by a Gaussian-terminated parabola both in the equilibrium and in the partially swollen states. At very high positive voltages (in this study, positive bias means positive voltage to the brush-bearing substrate), the brush chains were shown to be stretched to an extent comparable to their contour length, before being physically removed from the interface. Voltage-induced swelling was shown to exhibit a wider range of brush swelling states in comparison to pH switching, with the additional advantages that the stimulus is remotely controlled and may be fully automated.

’ INTRODUCTION Polymer brushes hold great promise as smart surfaces1 and as components for soft nanotechnology2 due to the rich variety of responsive behavior they exhibit when exposed to external stimuli, and due to their robust nature when covalently grafted to surfaces. The exploitation of conformational changes in macromolecules to produce useful work on the nanoscale is a common motif in cellular biology, and biomimetic systems made from synthetic polymers could offer a route to intelligent manmade materials that are biocompatible and highly functional. The development of versatile controlled surface-initiated polymerization methods,3 particularly in the past two decades, has allowed for significant development and progress in the field of polymer brush research. This research covers the synthesis, characterization, and a number of novel applications including switchable surfaces, nonfouling surfaces, sensors, and actuators.48 Polymer brushes are formed when polymer chains are tethered at one end to an interface at such high grafting density (σg, chains per nm2) that neighboring chains are forced to overlap. The competition between the entropic elasticity of the chains, which resists stretching of each chain from its equilibrium coil conformation in solution, and the repulsive excluded volume interactions between the chains (described by the excluded volume parameter r 2011 American Chemical Society

ν), leads to an equilibrium brush height L, which scales with the degree of polymerization N as L≈Nðνσ g Þ1=3

ð1Þ

The linear scaling of L with N1 (as compared to L ≈ N3/5 for a neutral polymer chain in a good solvent) is a key expression that reflects the profound effect of confinement upon the polymer chains in the brush regime.9 Equation 1 also illustrates the responsive nature of polymer brushes, through the dependence of the height upon the excluded volume parameter ν, which allows the brush height to be carefully tuned through control of the solvent quality.10 Homopolymer brushes formed from weak polyelectrolyte molecules have rich responsive behavior as they carry a degree of charge that depends upon the local pH via an equilibrium between association and dissociation of charges upon monomer groups along the chain. Furthermore, due to Debye screening, the electrostatic contributions to brush swelling from the monomers and counterions are dependent upon salt concentration. Received: April 13, 2011 Revised: July 10, 2011 Published: July 28, 2011 11000

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Langmuir Weak polyelectrolyte brushes have been of experimental interest for a number of years, with numerous examples of pH switching of weak polyacid11,12 and weak polybase13,14 brushes in the literature. The equilibrium conformation of a high grafting density weak polyelectrolyte brush such as poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA, a weak polybase) is dictated by an equilibrium between the osmotic pressure of counterions, which acts to stretch the brush, and the entropic elasticity of the chains, which resists stretching. This is the so-called “annealed osmotic brush” regime in which the brush height has the predicted scaling dependence:15  1=3 Rb L≈Nσ g 1=3 ðCHþ þ CS Þ ð2Þ 1  Rb which describes the dependence of the brush height upon pH and salt concentration and is valid when the counterion osmotic pressure exceeds the osmotic pressure of monomers due to Flory excluded volume interactions. Here, Rb is the charge fraction of an isolated chain in the bulk solution at a given pH value, and CH+ and CS are the respective concentrations of H+ ions and salt. At low pH where there is an abundance of H+ ions, the chains are ionized and trap a layer of counterions, which causes swelling of the brush. At high pH where the degree of ionization upon the chains is low, the brush is “quasi-neutral” because there may still be charge present, but brush swelling is dominated by the excluded volume interactions. The counterions remain bound in the vicinity of the brush due to their electrostatic interaction with the charged groups upon the brush. Counterion binding is described by the GouyChapman length scale16 within which the counterions are largely confined to the charged surface. The presence of charged monomers and counterions in polyelectrolyte brush systems leads naturally to the possibility of controlling their structure using externally applied electric fields. Recently, molecular dynamics simulations have shown that partially charged strong polyelectrolyte brushes may be reversibly swollen and deswollen by the application of electric fields.17 Experimental work has also shown that strong polyelectrolyte brushes grafted from the surface of microcantilevers can produce reversible deflection by deformation of the substrate as they swell and deswell.18 In this Article, we investigate the use of an applied electrical potential as an external stimulus to control the conformation of a weak polybase brush. This facile electrical switching method allows a well-defined, versatile, and highly reproducible stimulus to be remotely applied, removing the need to physically access or chemically change the liquid phase in contact with the brush. This offers the possibility of controlling the conformation of a responsive layer, with the thickness of a single macromolecule, via simple digital electronic circuits. The work described here explores the changes in conformation within weak polybase brushes that occur when a voltage is applied between the brush-bearing substrate and another parallel electrode some distance away in the surrounding solution. It is assumed that in the absence of solvent, that is, when the brush is measured in air, the brush is fully collapsed and assumes a slab-like conformation, with a “dry” thickness γ. For a uniform brush layer, this dry thickness is equal to the volume of polymer per unit area covering the substrate and is proportional to the mass of grafted material. The volume fraction of polymer within the brush is denoted ϕ and is assumed equal to 1 in the dry brush. Because the chains are anchored to the surface, this amount of

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Figure 1. Illustration of the step-like, parabolic, and Gaussian-terminated parabolic volume fraction profiles for polymer brushes.

material is conserved, and thus the swollen brush layer can be thought of as a mixture of polymer and water, where the integral of the volume fraction of the polymer within the brush is equal to the dry thickness, that is: Z ∞ ϕðzÞ dz ¼ γ ð3Þ 0

The dry thickness is proportional to the mass of polymeric material within the brush, and calculation of γ for a brush in any state (swollen or unswollen) allows a determination of the percentage of the polymer that remains attached to the interface compared with the pristine brush. This becomes useful when polymer brushes are damaged by the physical removal of chains from the interface. For any polymer brush volume fraction profile ϕ(z), the thickness L may be defined as the normalized first moment: Z 2 ∞ zϕðzÞ dz ð4Þ L¼ γ 0 which allows meaningful comparison to be made between brushes whose profiles have different functional forms. The degree of swelling in a polymer brush may be defined13 as a dimensionless swelling ratio S between the swollen thickness and the dry thickness. S¼

L γ

ð5Þ

The simplest model of the swollen brush layer is a box model where the brush is treated as a slab of material with volume fractions of polymer and water labeled ϕP and ϕW, respectively. This can serve as a first approximation for data modeling (i.e., for ellipsometry). In reality, the confinement of chains near the grafting substrate is greater than that at the exterior of the brush, resulting in the parabolic brush profiles predicted by selfconsistent field theory for densely grafted brushes of neutral19 and strong polyelectrolyte20 chains. Figure 1 shows the simple parabolic volume fraction profile along with the Gaussianterminated parabolic function (GTP) function. The latter includes a Gaussian “tail” extending beyond the classical cutoff of the parabolic shape, for which there is strong experimental evidence.13,21,22 Furthermore, the Gaussian “tail” is predicted by self-consistent field theory where it is believed to arise chiefly from the translational entropy of chain ends.23 11001

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brushes both in air and in deionized water using a Woollam M2000 V rotating compensator ellipsometer on a custom-built frame at a fixed angle of incidence of 70° to the sample surface normal. The brush samples were mounted in a custom-built PTFE liquid cell with quartz windows mounted at 20° from the vertical to allow normal transmission of the ellipsometry beam. Data analysis and modeling was performed using the Woollam CompleteEase instrument software. The refractive index np of dry PDMAEMA layers was described by a Cauchy approximation: Figure 2. Schematic diagram of the experimental setup used to measure polymer brush height as a function of applied voltage. For example, the circuit is shown in the negative bias configuration (negative voltage terminal connected to the brush).

Figure 2 shows a schematic diagram of the experimental layout, showing the silicon grafted polymer brush acting as an electrode, and a second electrode some distance away in the surrounding solution. A voltage was applied between the two electrodes, and the response of the brush was monitored using ellipsometry or neutron reflectometry.

’ EXPERIMENTAL SECTION Brush Synthesis. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) weak polybase brushes were grown from silicon (100) surfaces (Prolog Semicor Ltd., Kiev, Ukraine) via atom transfer radical polymerization.24,25 20 mL of anhydrous toluene (Sigma Aldrich UK) was placed in a sealed container at 20 °C for >18 h prior to deposition. 50 μL of 60 μL/mL solution of (11-(2-bromo-2-methyl) propionyloxy) undecyl trichlorosilane (BMPUS-TCS, Department of Chemistry, University of Sheffield26,27) in dry toluene was added to form a solution of 2.5 μL/mL BMPUS-TCS/dry toluene. Clean silicon wafers were placed in the solution, sealed, and stored at 20 °C for a further 6 h. On removal, the wafers were rinsed immediately with toluene and dried under N2. The samples were placed in a custom glass sample cell, which was purged with N2 for ∼1 h. The glass cell was sealed air- and watertight and submerged in a heat bath preheated to 37 °C. A Schlenk reaction and storage tube containing a small magnetic stirrer was purged with N2 for ∼1 h. 25 mL of acetone was sparged with N2 for at least 20 min using a converted HPLC filter. 0.078 g of bipyridyl (bpy), 0.023 g of CuCl, and 0.00325 g of CuBr2 were added, followed by 5.3 mL of N2sparged acetone to dissolve the solids and form a deep red-brown solution. 0.5 mL of 0.2 μm filtered, deionized water was added to the solution before the reaction tube was temporarily sealed. After ∼5 min, the magnetic stirrer bar was turned off, and the flask was sonicated for ∼5 s to help dissolve any remaining powders. Twenty-five milliliters of 2-(dimethylamino)ethyl methacrylate monomer (DMAEMA, Sigma Aldrich UK) was sparged with N2 for at least 20 min. 3.6 mL of DMAEMA was added to the reopened flask before it was resealed and sonicated for a further 5 s. The sealed flask containing the ATRP solution was left for at least 2 h and sonicated once more if necessary. Approximately 6 mL of the ATRP solution was added to the sealed cell containing the initiator-covered sample(s), which was incubated typically for ∼5 h in a heat bath at 35 °C to allow growth of a poly(2-(dimethlyamino)ethyl methacrylate) brush of the required thickness. On removal, the samples were rinsed with copious amounts of methanol and acetone and dried under flowing N2. Spectroscopic Ellipsometry. Spectroscopic ellipsometry measures changes in polarization that occur upon reflection of a light beam from a sample, typically a thin film (11000 nm) deposited upon a reflective substrate such as silicon. The changes in polarization depend upon the thickness and refractive index of the composite layers. Spectroscopic ellipsometry measurements were performed on the PDMAEMA

np ðλÞ ¼ An þ

Bn Cn 2 þ 4 λ λ

ð6Þ

with An = 1.49, Bn = 0.003 μm2, and Cn = 0. PDMAEMA brushes swollen by immersion in water were treated using a simple effective medium approximation with water and PDMAEMA (Cauchy) as the two constituent layers. Spectroscopic ellipsometry was used to measure the dry thickness γ of each sample prior to immersion in solvent, assuming that while in air the brush adopts a fully collapsed, slab-like conformation. In water, the brushes become swollen as the chains dissolve in the liquid and begin to stretch away from the surface into an extended layer with an equilibrium thickness L. The swelling ratio S = L/γ is related to the volume fractions of water (ϕW) and polymer (ϕP) in the swollen layer by the expression 1/S = ϕP = 1  ϕW, such that the refractive index of the composite layer is described by the expression n = ϕPnP + ϕWnW = nP/S + (1  1/S)nW. The CompleteEase software fits values of the ellipsometric angles Ψ and Δ28 calculated from this threelayer model of the system to the experimentally measured values of Ψ and Δ. Neutron Reflectivity. Neutron reflectivity is sensitive to the neutron scattering length density profile normal to the reflecting interface and is therefore ideally suited to measuring the volume fraction profiles of polymer brushes swollen in solvents provided that there is sufficient contrast between the scattering length densities (SLDs) of polymer and solvent. In this case, the necessary contrast was produced by using normal hydrogenous (nondeuterated) PDMAEMA brushes with deuterium oxide (D2O) as the solvent, having SLDs of 7.6  107 and 6.3  106 Å2, respectively. The neutron reflectivity R of the brush-covered surfaces was measured as a function of the neutron momentum transfer Q = 4π sin θ/λ (where θ is the grazing angle of incidence) resolved using neutron time-of-flight. The INTER neutron reflectometer is located on the second target station at ISIS, RAL, UK. A white neutron beam (wavelength range 1.516 Å) was incident on the sample surface at θ = 0.8°. The data were corrected for detector efficiency, air transmission, and scaled such that the intensity at total reflection was equal to 1 prior to data fitting. The results were fitted to simulated scattering length density profiles SLD(z), and hence polymer volume fraction profiles ϕ(z), using the slab fit fitting routine29 or a Gaussian-terminated parabolic function fitted using a downhill simplex fitting routine.30

’ RESULTS AND DISCUSSION To illustrate the pH response of a weak polybase brush, a PDMAEMA brush sample with a dry thickness of 15 nm was placed in a liquid cell containing 0.2 μm-filtered deionized water, and the thickness was measured using spectroscopic ellipsometry as the liquid was altered from pH 7 to 2 by the addition of HCl. Figure 3 shows the swollen thickness L as a function of pH indicating a linear increase in brush thickness with decreasing pH corresponding to the increasing charge fraction upon the brush. At neutral pH, the brush is swollen relative to its dry thickness, and the additional swelling caused by ionization due to the pH change induces a further increase in brush thickness of about 15%. 11002

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Figure 3. Thickness (nm) and change in thickness (%) as a function of pH for a γ = 15 nm PDMAEMA brush in deionized water as the pH was altered from pH 7 to pH 2, as measured using spectroscopic ellipsometry.

Figure 5. (a) Change in PDMAEMA brush thickness, expressed as a percentage of the original wet thickness, for applied voltages of +5, +3, +1, 1, 3, and 5 V, as determined from spectroscopic ellipsometry; (b) summary of the data shown in part (a) at t = 7.5 min, as compared to the range of swelling accessible via pH variation (blue bar).

Figure 4. Brush thickness L (black, nm), water content 100ϕW(blue, %), and calculated dry thickness γcalc = LϕW (green, nm) as a function of time for γ = 15 nm PDMAEMA brushes under applied voltages of (a) +3 V and (b) +5 V in deionized water, as determined using spectroscopic ellipsometry.

Identical brushes (grown from pieces of the same silicon wafer and under the same polymerization conditions to ensure consistency) were used for the following experiments. A gold wire was attached to the rear of the brush-bearing silicon substrate using silver epoxy (Circuit Works), and the sample was placed in the liquid cell. Twenty milliliters of deionized water filtered with a 0.2 μm pore size was added. A counter-electrode consisting of a 6 mm length of gold wire (Agar Scientific) was placed parallel to the sample surface at a distance of 3 mm and aligned with the long axis of the elliptical ellipsometry light spot. In the convention used throughout, positive bias refers to the positive terminal of the voltage supply being connected to the brush substrate and vice versa. Using a pristine sample each time, voltages of 1, 3, and 5 V were applied between the brush and the counter electrode in both directions of bias. Each sample was allowed to sit in the liquid for at least 10 min, before the voltage was applied for 60 min. A further 30 min of recovery time was allowed after the instant when the voltage was switched off. In situ spectroscopic ellipsometry measurements were made with 0.5 s time

Figure 6. Electrical current versus time for PDMAEMA brush samples on Si wafers, showing constant currents for applied voltages of (1 and (3 V and anomalous currents for (5 V.

resolution. This excellent time resolution is a result of using an ellipsometer equipped with a CCD detector (Woollam M-2000 V), which allows a minimum acquisition time of 0.02 s for measurement of Ψ and Δ across the entire available spectrum 3502000 nm. The data were fitted in real time using the instrument software CompleteEase, and the brush behavior was monitored on plots of the brush thickness L and water content expressed as a percentage (=100ϕW). In addition, the calculated dry thickness of the brush ϕPL was plotted to monitor the integrity of the polymer brush layers. An applied voltage of +3 V (Figure 4a) produced an initial increase in thickness and water content of the brush that reached 11003

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Figure 7. (a) Fitted neutron reflectivity data (top) and the corresponding polymer volume fraction profiles (bottom) for (a) +5 V, (b) +3 V, and (c) 3 V. Fits to the reflectivity data are shown by the solid black line. Data taken before and after the voltage were applied are labeled in blue and red, respectively. Data are selected to show (a) loss of material, (b) swelling, and (c) no response or response smaller than experimental uncertainty.

Figure 8. Summary of degree of swelling measured in the neutron reflectometry before/after voltage studies (blue markers) and ellipsometry measurements after 60 min (red markers) for PDMAEMA brushes at applied voltages of +3, +1, 1, 3, and 5 V. Data from experiments that caused damage to the polymer brush are excluded.

a steady state after approximately 5 min, indicating a stable swelling of the brush. At a voltage of +5 V (Figure 4b), it was observed that the brush layer suffered physical damage after a few minutes of exposure to the applied voltage. In the first 5 min of exposure to +5 V, the brush layer was swollen (corresponding increases in L and ϕW). However, after 7.5 min, the brush thickness began to decrease in a manner inconsistent with a straightforward deswelling of the brush, and the calculated dry thickness decreased steadily after this point, suggesting physical loss of material from the brush into solution. Voltages of +1 or 1 V produced no discernible response, while applied voltages of 3 and 5 V produced simultaneous decreases in the thickness and water content of the brush, consistent with deswelling of the brush. Figure 5a shows the change in thickness expressed as a percentage of the original wet

thickness at 0 V for the range of applied voltages, over the first 15 min. Figure 5b is a summary of the thickness data shown in part (a) at t = 7.5 min. In the ellipsometry cell, the electrical current in the electrode circuit was measured as a function of time using a calibrated digital multimeter (Keithley 2000) operating in ammeter mode. Although it is understood that an electrochemical cell complete with reference electrode and potentiostat is required to perform conclusive electrochemical studies on this system, a number of important observations can be made from this simple current data. The current versus time data are plotted in Figure 6 for applied voltages of (1, ( 3, and (5 V. First, at voltages of (1 and (3 V, the current varies over the first 5 min of applied voltage before it reaches an approximately constant value. This corresponds to a situation where the brush exhibits either no response or a stable response to the applied voltage. Second, at +5 V, there is an anomalously high current, which reproducibly becomes unstable at around 12 min, with an onset of noise in the current signal. In all except the +5 V case, the current was similar to the control measurement (Supporting Information), but in the +5 V case, the current flowing is much larger with the presence of a PDMAEMA brush than without. At 5 V, the magnitude of the current increases linearly after 5 min. Neutron reflectometry experiments were performed upon the same range of applied voltages (+5, +3, +1, 1, 3, and 5 V). The reflectometry profiles and fits before and after the applied voltage are presented in Figure 7 along with the corresponding volume fraction profiles. The data from +5 V corroborate the loss of material measured in the ellipsometry experiments, showing a marked depletion in the mass of the brush. At +3 V, the PDMAEMA brush is swollen but retains the Gaussianterminated parabolic volume fraction profile. At 3 V, there is a slight deswelling of the brush visible from the fitted volume 11004

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Figure 9. Neutron reflectivity data for a PDMAEMA brush measured at voltages from 0 V to +5 V in increments of 0.5 V: (a) neutron reflectivity curves and fits; (b) polymer volume fraction profiles associated with fits shown in (a); and (c) summary of calculated dry thickness γc, brush thickness L, and swelling ratio S, as a function of applied voltage.

fraction profile, but this is small as compared to the experimental error in the brush height. The data are summarized in Figure 8. Incremental Study of Brush Swelling. To study the swelling effect in greater detail, a PDMAEMA brush was measured for 20 min at applied voltages from 0 V to +5 V in steps of 0.5 V. Figure 9a shows the reflectivity curves at each applied voltage with associated fits. Reflectivity data from 0 to 3.5 V were fitted using the Gaussian-terminated parabolic profile and downhill simplex fitting routine, while data from 4.0 to 5.0 V were fitted using the slab fit fitting routine. Figure 9b shows the polymer volume fraction profiles associated with the fits to the data. Figure 9c shows the thickness L, calculated dry thickness γc, and swelling ratio S extracted from the volume fraction profiles shown in Figure 9b. In all cases, the Gaussian-terminated

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parabolic profile was preferred unless a fit with χ2 of less than 10 could not be reached. The data in Figure 9a appear to show that the brush was not perturbed significantly from its equilibrium conformation at voltages up to 2.5 V, apart from small changes in the brush mass and in the size of the Gaussian tail. At +3 V, there is a transition to a swollen state where the brush conformation maintains its parabolic nature but becomes thicker (extending up to 800 Å from the grafting interface) and less concentrated near the interface. The reflectivity data for +3 V are rather noisy (there was partial loss of neutron beam during this measurement), whereas the +3.5 V data show a very good fit (χ2 = 2.18) to the Gaussian terminated parabolic profile, suggesting that the brush structure was unchanging during the 20 min duration of the measurement at 3.5 V. At 4.0 V and above, however, the Gaussian-terminated parabolic volume fraction profile no longer provides a meaningful fit to the data. Therefore, the slab fit routine was used to fit reflectivity data for 4.0, 4.5, and 5.0 V. The slab fit profiles (green, orange, and sky blue dashed lines in Figure 9b) show that the brush chains are strongly stretched and become comparable to the estimated contour length of the PDMAEMA chains (1130 Å) calculated from the dry thickness and estimated grafting density. Subsequently, material is removed from the brush, suggesting that the chains become fully stretched before they are pulled from the interface by the breaking of a backbone bond or detachment of the initiator layer. It is unclear whether physical stretching of the chain or chemical cleavage (not requiring the chain to be fully stretched) is the dominant effect in brush damage. A thin layer (∼3 nm) of material remains upon the substrate as measured using spectroscopic ellipsometry on the dried brush after the experiment. At high positive bias, the pH in the vicinity of the electrode is increased by the attraction of OH counterions, and this high pH could lead to cleavage of the ester group within the ATRP initiator molecule.31 The thin layers that remain after brush damage are similar to the expected thickness of the residual part of the initiator molecule, comprising the alkyl chain and silane linkage. The voltage response of the brush is complex, particularly given the weak polyelectrolyte nature, and could occur by a number of possible mechanisms: pH effects, counterion effects, monomer effects, electrochemical effects, or a combination thereof. The induced changes in pH caused by the applied voltages are in the wrong sense to explain swelling and deswelling effects via the pH response of PDMAEMA brushes alone. An applied voltage in positive bias is expected to raise the pH near the electrode, and likewise a negative bias should lower the pH. These two situations normally cause respective deswelling and swelling within a PDMAEMA brush, but this is the opposite of what is observed in voltage experiments. To maintain charge neutrality, when a voltage is applied the total number of counterions must equal the brush charge plus or minus an adjustment due to the surface charge induced upon the brush substrate. Neglecting changes in the brush charge due to pH effects, this would lead to an increase in the number of counterions at positive bias, and a decrease in the number of counterions at negative bias. These changes, if significant, would cause swelling at positive bias and deswelling at negative bias. However, the charge induced upon the substrate is orders of magnitude smaller than the brush charge fraction of 0.5 at pH 7 estimated by Sanjuan and Tran13 and does not account for a significant change in the counterion osmotic pressure. 11005

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Langmuir Finally, the charges upon segments of the brush are able to interact with the electric field due to the applied voltage. This is naïvely expected to cause a swelling at positive bias and a deswelling at negative bias, through the respective repulsion and attraction of charged monomer units from the brush substrate. It is clear that a dedicated theoretical study is required to fully understand the response of the weak polyelectrolyte brush to an applied voltage, due to the complex interdependencies present. Migliorini20 has used self-consistent field theory to predict the monomer and counterion density profiles of semidilute strong polyelectrolyte brushes grafted from charged interfaces. Although not directly comparable to this study, mainly due to the difference in charge mechanism and grafting density, a swelling of the polymer brush was predicted at positive bias. Zhou et al.18 made predictions of changes due to applied voltages in the volume fraction profile of strong polyelectrolytes near the grafting interface, but did not extend their discussion to overall brush conformation or swelling. Molecular dynamics simulations could show intriguing insights into the system. Simulations by Ouyang17 based on partially charged strong polyelectrolyte brushes grafted from a planar interface showed that reversible swelling and deswelling could be achieved using applied electric fields. Importantly, the study showed both partial and strong swelling of the polymer chains dependent upon the swelling conditions. This appears to corroborate the data shown in Figure 9 where partial and fully stretched states are seen depending upon the strength of the applied voltage, although again the differences in charging mechanisms make a direct comparison between the studies difficult.

’ CONCLUSION We have investigated the use of applied voltages to control the conformation of weak polybase brushes. Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) polybase brushes were grown from silicon wafers using atom transfer radical polymerization. DC voltages were applied between the brush substrate and a second electrode placed parallel to the brush some distance away in the surrounding water or D2O. Spectroscopic ellipsometry and neutron reflectometry were used to study the brush conformation as a function of the applied voltage. Positive biases (positive terminal connected to brush) resulted in swelling of the brush for voltages g3 V, while voltages g5 V stripped material from the brush into solution. Negative voltages appeared to produce deswelling in ellipsometry measurements, but only slight deswelling (that was smaller than the experimental error) was observed in neutron reflectometry experiments. Kinetic neutron reflectometry experiments at a single angle and with 20 min time resolution were conducted on the new INTER reflectometer (ISIS, RAL, UK). Neutron reflectometry revealed that the brushes exhibited a polymer volume fraction profile described by a Gaussian-terminated parabolic density profile both in the equilibrium and in the partially swollen state. At high positive voltages, the brush chains were shown to stretch to an extent near to their contour length before being physically removed from the substrate. This study should provide a platform for further polyelectrolyte brush research and for research in the field of electronically addressed smart surfaces. Future interest may lie in investigating the control of polymer brush structure with spatial resolution, and in the investigation of the holding, release, and transport of colloidal, macromolecular, and cellular cargoes upon surfaces.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Diagram showing the comparison between electrical current measurements made with PDMAEMA brushes on Si and the control measurement with a clean Si wafer for applied voltages of (a) +1 V, (b) +3 V, (c) +5 V, (d) 1 V, (e) 3 V, and (f) 5 V. Part (g) shows a summary of the data in parts (a)(f), and the crude electric field strength E between the electrodes is plotted to emphasize the nonohmic behavior exhibited in the system. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses §

Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia

’ ACKNOWLEDGMENT We acknowledge the EPSRC (UK) for funding Michael P. Weir via a University of Sheffield Doctoral Training Account. Experiments at the ISIS Pulsed Neutron and Muon Source were supported by a beamtime allocation (RB910277) from the STFC (UK). We acknowledge Andrew Dennison, Stephen Ebbens, and Hiroshi Hamamatsu for assistance with neutron reflectometry experiments and David Mohamad for help with electrical current measurements. ’ REFERENCES (1) Zhou, F.; Huck, W. T. S. Phys. Chem. Chem. Phys. 2006, 8, 3815. (2) Ryan, A. J.; Crook, C. J.; Howse, J. R.; Topham, P.; Jones, R. A. L.; Geoghegan, M.; Parnell, A. J.; Ruiz-Perez, L.; Martin, S. J.; Cadby, A.; Menelle, A.; Webster, J. R. P.; Gleeson, A. J.; Bras, W. Faraday Discuss. 2005, 128, 55. (3) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14. (4) Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, I.; Szleifer, G. B.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101. (5) Tokarev, I.; Motornov, M.; Minko, S. J. Mater. Chem. 2009, 19, 6932. (6) Ducker, R.; Garcia, A.; Zhang, J.; Chen, T.; Zauscher, S. Soft Matter 2008, 4, 1774. (7) Ballauff, M.; Borisov, O. Curr. Opin. Colloid Interface Sci. 2006, 11, 316. (8) Advincula, R. C.; Brittain, W. J.; Caster, K. C.; R€uhe, J. Polymer Brushes; Wiley: New York, 2004. (9) Alexander, S. J. Phys. (Paris) 1977, 38, 983. (10) Bunjes, N.; Paul, S.; Habicht, J.; Prucker, O.; Ruhe, J.; Knoll, W. Colloid Polym. Sci. 2004, 282, 939. (11) Parnell, A. J.; Martin, S. J.; Dang, C. C.; Geoghegan, M.; Jones, R. A. L.; Crook, C. J.; Howse, J. R.; Ryan, A. J. Polymer 2009, 50, 1005. (12) Parnell, A. J.; Martin, S. J.; Jones, R. A. L.; Vasilev, C.; Crook, C. J.; Ryan, A. J. Soft Matter 2009, 5, 296. (13) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Langmuir 2007, 23, 5769. (14) Geoghegan, M.; Ruiz-Perez, L.; Dang, C. C.; Parnell, A. J.; Martin, S. J.; Howse, J. R.; Jones, R. A. L.; Topham, P. D.; Crook, C. J.; 11006

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