Electrochemical Switching of Conformation of Random Polyampholyte

Article pubs.acs.org/Langmuir

Electrochemical Switching of Conformation of Random Polyampholyte Brushes Grafted onto Polypyrrole Yiwen Pei,† Jadranka Travas-Sedjic,†,‡ and David E. Williams*,†,‡ †

Polymer Electronic Research Centre, Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand

‡

S Supporting Information *

ABSTRACT: We demonstrate the possibilities for subtle control over redoxdriven surface switching that could be obtained by altering the balance between hydrophobic, ionic, and dipolar components of polymer brushes that are grafted onto electrochemically active conducting polymers (ECP). We extend our previous work1 on the conformation switching of polyzwitterionic brushes grafted onto ECP to the case of ABC random polyampholyte brushes: here, a statistical near-neutral copolymer of methyl methacrylate (MMA), methacryloxyethyltrimethylammonium chloride (METAC), and 3-sulfopropyl methacrylate potassium salt (SPMA). The statistical polyampholyte differs from the polyzwitterion in that it is not strictly neutral and the charges do not have a defined spatial relationship to one another. The consequence is a significant change in the switching behavior that is also dependent on the salt concentration in the electrolyte. The results can be understood by reference to the theory of polyampholyte conformation in solution given by Higgs and Joanny2 modified to apply to a surface-bound brush. Three states of the polyampholyte brush are deduced from electrochemical impedance spectroscopy (EIS): collapsed, partially collapsed, and expanded. At low salt concentration, the behavior was the opposite of that of the polyzwitterion: the brush switched between partially collapsed with the ECP reduced and expanded with the ECP oxidized. With increase of salt concentration, the switch changed, to collapsed with the ECP oxidized and partially collapsed with ECP reduced. At still higher salt concentration, the switch changed back again, to partially collapsed with the ECP reduced and expanded with the ECP oxidized. Measurements of surface wetting under electrochemical control supported the interpretation. The behavior can be contrasted with that of zwitterionic brushes, which show a switch between collapsed with ECP oxidized and expanded with ECP reduced, independent of salt concentration over the same range (10−3−2 M NaCl) as that studied here, and that of zwitterionic−hydrophobic block copolymers where the switch is suppressed at low salt concentration. The results illustrate the significant range of behavior that can be engineered into these electrochemically switchable systems.

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INTRODUCTION Polyampholytes are ionic polymers containing both positive and negative charges. The presence of ionic monomers of opposite charge leads to some interesting properties (such as condensation or swelling) which are essentially controlled by the net charge or the solution ionic strength.3−6 If the net charge of a polyampholyte in aqueous solution is near zero, then the polymer exhibits chain expansion upon the addition of salt due to the counterion screening effect, diminishing the attractive interaction of the opposing charges on the polymer. On the other hand, if there is an excess of one charge over the other, then these polymers demonstrate polyelectrolyte-like behavior−chain shrinkage with increasing ionic strength caused by screening of the repulsive electrostatic force between like charges.7−9 “Quenched” polyampholytes are defined as ionic polymers that contain strong acid and strong base segments. Since the charge is permanently fixed on certain chemical groups, the solution behavior of “quenched” polyampholytes is independent of pH. On the other hand, “annealed” polyampholytes consist of weak acid or base segments, and © 2012 American Chemical Society

their net charge can be easily tuned by changes of pH. In both cases, “quenched” and “annealed” polyampholyte, the observed effects of the interaction between the chain segments depend on the ionic strength of the solution. For “quenched” polyampholytes, up to five different chain conformations are predicted, depending on the chain length, net charge, and ionic strength.2 The synthesis and solution properties of ABC random10−16 and block17−19 polyampholytes that contain anionic, neutral, and cationic segments have been demonstrated. Surfaceinitiated atom transfer radical polymerization (SI-ATRP) has been proven to be an efficient approach to grow a polymer with well-controlled molecular weight and low polydispersity from a surface.20 The resultant properties of the polymer depend upon the surface density of the brushes and upon its chemical nature.21,22 There have been some studies of stimulusReceived: May 30, 2012 Revised: August 3, 2012 Published: August 27, 2012 13241

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Figure 1. (A) Cyclic voltammograms of poly(METAC-stat-MMA-stat-SPMA) grafted poly(Py-co-PBA) film in (a) 1 mM, (b) 10 mM, (c) 100 mM, and (d) 1 M NaCl aqueous solution at 19 °C. Scan rate is 0.01 V s−1. (B) Half-wave potential for oxidation with respect to Ag, AgCl, 3 M KCl reference electrode (◇) and the current on the reduction wave at −0.6 V Ag/AgCl (□) of poly(METAC-stat-MMA-stat-SPMA) grafted poly(Py-coPBA) film in the salt concentration range 0.1−0.5 M. before the polymerizations. METAC was purified by passing the solution through a basic alumina column. ATRP-initiator attached poly(Py-co-PBA) was synthesized as reported.1,29 The procedure was essentially as previously reported1 and is given in detail in the Supporting Information (SI). Trial polymerizations in solution were performed to determine the relative proportions of the three monomers that were required to obtain approximately equal molar ratios in the final polymer. This assessment can of course only be approximate because the kinetics of reaction at a surface, involving diffusion of the monomers toward the surface, will necessarily be different from the solution kinetics. The prepared films were characterized by XPS (see SI). Electrochemical characterization was performed in a conventional three-electrode cell (6 mL) with conducting polymer film-coated glassy carbon as working electrode, Pt wire as counter electrode, and Ag/AgCl (3 M KCl) as reference electrode. Cyclic voltammetry was recorded at 19 °C using a CH Instrument potentiostat (model 650). An EG&G potentiostat/ galvanostat (Princeton Applied Research, model 280) and an EG&G 1025 frequency response analyzer were employed for the impedance measurements. Impedance was performed with 5 mV sinusoidal modulation amplitude at an applied bias potential of +0.357 or −0.841 V at 19 °C. The impedance data were measured from 10 000 to 0.1 Hz at 10 points per decade. Cole−Cole diagrams were generated by subtracting from the impedance the series resistance Rs, determined as the high-frequency limit of the real part of the impedance: C*(ω)corr = 1/iw(Z*(ω) − Rs). The porous polymer film is best represented as a transmission line. This equivalent circuit model has been extensively developed by Bisquert et al.30−33 In the previous work,1 we reported a simplified version of Bisquert’s model that can sufficiently describe the main features of the impedance data. As shown in Figure 3B, the simplified impedance model involves a distributed constant-phase element (CPE) with fractional exponent 0 < α < 1 and prefactor q1, in parallel with a resistive element R1 which is in series with another CPE (0 < α < 1). These circuit elements can be interpreted as a double-layer capacitance associated with the conducting polymer film−solution interface (q1) and a type of surface-state capacitance (q2) that is charged through the conducting polymer or by ionic diffusion in the polymer brush layer-characterized by resistance R1. Fitting and error analysis was as previously described (see SI). Electrochemically induced wetting was demonstrated in a threeelectrode cell in which a chlorided silver wire as reference electrode and a Pt wire as counter electrode were inserted into a drop on the surface of the polymer-coated glassy carbon working electrode. The drop shape was photographed as the electrode potential was changed to change the film between oxidized and reduced states. The

responsive behavior of surface-grafted polyampholytes. Huck et al. have reported a salt-induced responsiveness of triblock copolymer brushes with cationic METAC, neutral MMA, and anionic sodium methacrylate.23 Tran and co-workers24,25 have investigated the pH-responsive behavior of random polyampholyte brushes. These authors showed that the structure adopted by the polymer brushes strongly depends on the balance between electrostatic repulsions of charges of the same sign and attractions of charges of opposite sign, which is further strongly governed by the net charge of the random polyampholyte. For random polyampholyte brushes, the overall charge, charge density, and distribution of charges along the polymer chains are significant factors that affect the stimulusresponsive behavior. A variety of electrochemical characterization approaches have been explored to investigate the counterion exchange and responsive behavior in polyelectrolyte brushes, including cyclic voltammetry (CV),26 electrochemical impedance spectroscopy (EIS),27 and scanning electrochemical microscopy.28 We recently reported reversible electrochemical switching of polymer brushes grafted onto the surface of electrochemically active conducting polymers (ECP). We demonstrated the collapse and expansion of poly(zwitterionic) brushes driven by the ion exchange that is associated with the oxidation and reduction of the ECP.1 Here, we extend this work, reporting synthesis and electrochemical switching of the statistical, nearneutral mixed hydrophobic−ionic ampholyte poly(METACstat-MMA-stat-SPMA) grafted poly(pyrrole-co-pyrrole-3-butyric acid) (Py-co-PBA) films. The random copolymer brushgrafted film was characterized by X-ray photoelectron spectroscopy (XPS), CV, EIS, and contact angle measurement. We demonstrate electrochemically controlled conformational changes of the random copolymer grafted films driven by the redox process of the conducting polymer, which are different from that of the zwitterionic grafts reported in our earlier studies.1,29 Three distinguishable surface states are demonstrated, with switches between them dependent on the salt concentration in solution.

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EXPERIMENTAL SECTION

All reagents were purchased from Sigma-Aldrich and used as received. Pyrrole and MMA were freshly distilled under a reduced pressure 13242

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Figure 2. Capacitance plots of (A, C) the unmodified and (B, D) poly(METAC-stat-MMA-stat-SPMA) grafted poly(Py-co-PBA) films in 0.01 and 0.5 M NaCl(aq) solution under an applied potential of (●) +0.357 V or (○) −0.841 V (vs Ag/AgCl, 3 M KCl) at 19 °C. (★) Fitting results for the proposed model are marked; labels are frequency in Hz. experiment was started from the state that was the most hydrophobic. The apparatus is shown in the SI. The experiments were repeated three times on different areas of the working electrode.

determined atomic proportions of S 2s and Br 3d were 0.51% and 0.11%, respectively. The surface density (σ ≈ 0.01 chains nm−2) and the average distance between anchor points (d ≈ 10 nm) were estimated based on these XPS results (see SI). Under the admittedly crude approximation that surface-bound polymer grows at the same rate as that of the solution polymerization using the same concentration of monomers, then the chain length of the polymer brushes (L ≈ 16 nm) is calculated from the molecular weight of random polyampholytes synthesized in solution (see SI). Electrochemical properties of the unmodified and the modified films were characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results showed that the redox process of the ECP could still drive through the grafted layer after surface modification and that the voltammetry was clearly different from that of the unmodified film, previously reported.1 Figure 1A shows the cyclic voltammogram of the modified film in various concentrations of the NaCl aqueous solutions (0.001, 0.01, 0.1, and 1 M) at a scan rate of 0.01 V s−1. The modified film in the anodic range (−0.2 to +0.2 V, Ag/AgCl) behaved like a salt-independent capacitor. The reduction wave appeared as a step that shifted anodic to a limit with increasing salt concentration, analogous to a salt-dependent increase in

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RESULTS AND DISCUSSION Random copolymer brushes were grafted from surface-initiated poly(Py-co-PBA) films via ATRP using a mixed monomer solution. We assumed that the graft would be weakly negatively charged for the relative amounts of the different monomers that we used because we had previously prepared the statistical copolymer by ATRP in solution and inferred its composition by elemental analysis and NMR (net charge = −0.19 mmol g−1: see SI). As in the previous work,1 XPS analysis (see SI) provided clear evidence of the successful growth of polymer brushes on the poly(Py-co-PBA) film, specifically the appearance of new sub-bands of the C 1s peak consistent with the new carbon environments introduced by the graft and the appearance of a new feature on the N 1s peak that can be attributed to quaternary ammonium (N+),34,35 which is indicative of the successful grafting of METAC. Other evidence for poly(METAC-stat-MMA-stat-SPMA) brushes was the observation of core level emissions due to S 2p (resulting from SPMA grafts) and Br 3d (as a consequence of ATRP initiator), which are absent in the unmodified film. The XPS13243

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Figure 3. (A) Chemical structure of poly(METAC-stat-MMA-stat-SPMA) grafted poly(Py-co-PBA) films; the polymer is assumed to be a pseudorandom sequence of the three different monomers. (B) Simplified equivalent circuit model for a thin porous conducting polymer film, following Bisquert et al.,30,32 used for fitting and interpretation of the impedance data. (C) Charging resistance R1 and (D) surface capacitance q2 of the unmodified (dotted line) and modified films (solid line) in the oxidized (diamonds, filled circles) and the reduced state (squares, open circles) as a function of salt concentration at 19 °C. Estimated standard deviation of the fitted parameters was (1/b)(f + g )3/2

(1)

3

where we here interpret b as the volume occupied by a single monomer averaged over the surface, b3 ∼ ad2 where a is the 13245

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monomer length and d the average spacing of the chain anchors on the surface. Although the theory is strictly applicable only in the dilute solution limit, where Debye−Huckel theory is valid, it is useful to characterize the different interactions in terms of screening lengths. Thus, the electrostatic interactions due to the charge on the polymer are characterized by a screening length, κp−1, where using dilute electrolyte theory as an approximation2 κp2 ∼

(f + g )N Ld 2

l

and expansion of a zwitterionic brush grafted onto the same ECP as used here by assuming that oxidation of the modified ECP involved just cation expulsion from the interface. That is, anions were trapped and immobile in the modified ECP. Although the ideas may be adapted to accommodate anion insertion as the oxidation process, again we have found that we can develop a self-consistent interpretation of the different surface states observed in the present work by using the same assumption, of cation expulsion on oxidation of the modified ECP. We also find we need to assume that the brush as synthesized carried a small net negative charge, consistent with the analysis of the analogous solution-synthesized polymer (see SI). We note here also an important detail that we have only superficially considered in our argument: an energy balance based on relatively long-range interactions, such as is implied by the use of dilute solution-theory screening lengths, should also include the electrostatic energy of interaction of the charges on the brush polymer chain with the charges trapped on the conducting polymer side of the interface. (1) Low concentration range of the electrolyte (10−3−2 × 10−1 mol dm−3): the electrolyte screening length, κ0−1 (eq 3), is large in comparison with the internal screening length of the brush, κp−1 (eq 2). There is no effective screening by the electrolyte, and the conformation is determined by the charge state of the brush (eq 1). If the polyampholyte brush carries a net charge, and inequality (1) is satisfied, then the brush will be swollen. This is the first state, corresponding to the oxidized form of the ECP in dilute electrolyte. With cation expulsion as the model for oxidation, the ECP side of the interface would be neutral in the oxidized state. (2) Reduction of the ECP is assumed to be associated with cation insertion to the ECP−graft interface. We can consider these ions to be essentially immobilized at the interface, and therefore they can be considered as part of the net charge associated with the brush, within a distance on the order of the screening length of the brush, κp−1. The effect would be to increase the fraction, f, of positive charges in this zone. If the brush had an excess of negative charges, then f − g would be diminished. If inequality (1) no longer held, then the brush would collapse near the interface but could remain expanded sufficiently far from the interface. This interpretation is consistent with the interpretation of the change of the experimental parameters. With cation insertion as the model for reduction, the ECP side of the interface would be negative in the reduced state: the repulsive energy associated with interaction of a net negative polymer with the ECP charge might act to help keep the outer part of the brush expanded. (3) Over the concentration range approximately 3 × 10−1−5 × 10−1 mol dm−3 the experimental parameters changed markedly. We can interpret the lower limit of this range as corresponding to a salt concentration that approaches the net charge concentration in the brush, (f − g)N/Ld2, or alternatively as the salt concentration such that the screening length associated with the electrolyte, κ0−1, becomes less than that associated with the brush, κp−1, and also less than κθ−1 (eq 5). Then, the electrostatic interactions that cause expansion of the brush (the polyelectrolyte term in eq 4) are screened so the polyampholyte term (eq 4), and the elastic energy of the

(2)

Here, l = e2/(4πεkT), L denotes the brush thickness, and N denotes the chain length of the brush. In the presence of a salt whose concentration is significant in comparison with the charge concentration within the polymer brush, then there is an additional screening of the electrostatic interactions. The salt concentration can also be characterized by a screening length, κ0−1, where for a 1:1 electrolyte κ0 2 ∼ 8πn0l

(3)

where n0 is the salt concentration. The electrostatic energy terms modify the excluded volume, v, giving an effective excluded volume, v*: v* = v −

πl 2(f + g )2 3

κ0b

+

4πl(f − g )2 κ0 2b3

(4)

The key point is that the second term, due to the polyampholyte effect, has a different dependence on the electrolyte screening length, κ0−1, than does the third term, due to the polyelectrolyte effect. The balance between these two terms defines a transition screening length, κθ−1, below which the chains tend to be collapsed, because the polyampholyte effect dominates, and above which the chains tend to be expanded, because the polyelectrolyte effect dominates. Here κθ =

4(f − g )2 l(f + g )2

(5)

The predicted effects have been observed in experiments on non-neutral polyampholyte solution36 and gels37,38 and also agree with results of molecular dynamic simulation.39 Pincus40 has used similar ideas in the theory of conformational changes in polyelectrolytes grafted onto surfaces. He considers that the polymers are extended in the form of a brush whose thickness, L, is determined by the balance between the osmotic pressure due to the polyelectrolyte, the elastic energy of the chain, and the free energy associated with the excluded volume. We can interpret our results using these ideas of a pressure balance along with different zones of behavior defined by the net charge on the brush and different relative values of the various screening lengths, altered by altering the electrolyte concentration and by ion insertion or ejection caused by altering the charge state of the ECP support. In our previous work, we assumed that the charge balancing ions exchanged upon reduction or oxidation of the surfacemodified electrochemically active conducting polymer (ECP) were adsorbed at the ECP−graft interface and that it was this ion exchange that drove the conformational switch of the graft. While the ECP oxidation could involve both anion insertion and cation expulsion, with anion insertion the dominant process for unmodified chloride-doped polypyrrole, rather we were able to interpret in a simple way the redox-driven collapse 13246

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active conducting polymers. We expect that these materials will be useful for electrochemical control of surface properties, such as wetting and adhesion,41 and will find applications in microfluidic and sensor devices.

brush will cause the brush to collapse. This would be the case with the ECP in the oxidized state and so with the interface ions expelled from the brush. Reduction of the ECP would, according to our hypothesis, cause insertion of ions into the brush−ECP interface. The osmotic pressure associated with this ion insertion could then cause the brush to partially expand, as implied by the observations. (4) The final set of conditions is marked by the change in experimental parameters for the ECP in the oxidized state, for NaCl concentration greater than 5 × 10−1−1 mol dm−3. The ideas of Higgs and Joanny2 indicate two possible cases for this range, according to whether κ0 is greater than or less than κθ (eq 5). If κ0 > κθ, or alternatively if the salt concentration approaches the total charge concentration in the brush, (f + g)N/Ld2, then all electrostatic interactions become screened and the brush can approach the ideal, fully extended state. This would correspond to the condition where the ECP is oxidized. If the ECP is reduced, then again the effect of excess cations inserted into the graft−ECP interface has to be considered. As before, the effect can be considered as a change in both the net charge on the brush, f − g, and the total charge near the interface, and hence an increase in κθ in this zone. The expected effect would be a partial collapse of the brush in the zone near the interface, which would indeed explain the experimental observation.

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ASSOCIATED CONTENT

S Supporting Information *

Details of preparation of the conducting polymer film, synthesis of the polymer brushes, XPS characterization of the unmodified and modified films, estimation of the grafting density and the average distance between anchor points, equivalent circuit fitting, estimation of chain length and net charge of statistical copolymers, and apparatus for observation of changes in contact angle under electrochemical control. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from the Ministry for Science and Innovation, New Zealand. We thank the Centre for Advanced Macromolecular Design (CAMD) group at the University of New South Wales, Australia, for advice on CRP techniques. The authors thank Dr. Colin Doyle for his help in XPS experiments.

CONCLUSIONS

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We have demonstrated the preparation of a poly(METAC-statMMA-stat-SPMA) grafted poly(Py-co-PBA) film. An electrochemically controlled conformational switch of the surfaceattached polyampholyte brushes has been demonstrated. We have used a straightforward adaptation of ideas about the effect of electrolyte on the conformation of polyampholytes in solution to explain the results. The switch results from redoxdriven ion insertion altering the balance between Coulombic repulsive and attractive interactions within the brush. The effects depend both on the net charge density and the total charge density along the polymer chains, both of which could in principle be easily changed by changing the proportions of the anionic, cationic, and nonionic monomers. The switch is dependent upon the oxidation state of the ECP, which alters the fixed ionic composition in the immediate vicinity of the interface. For the polyampholyte brushes the effects of electrolyte concentration were very marked, due to the effects of screening of the charges by the electrolyte. This behavior is in contrast to the behavior of zwitterionic brushes, which is determined by the dipolar interactions between the side chains of the brush, and to that of zwitterionic−hydrophobic block copolymers where the switch is suppressed at low salt concentration.1 We anticipate that the behavior could also be strongly influenced by the surface density of the polymer graft. We note that the nonionic monomer constituent not only acts as a charge diluent on the polymer brush but could also further modify the behavior as a result of hydrophobic interactions with the solvent. Taken together with our previously reported work,1,29 this study illustrates the subtle control over redoxdriven surface switching that could be obtained by altering the balance between hydrophobic, ionic, and dipolar components of polymer brushes that are grafted onto electrochemically

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dx.doi.org/10.1021/la302202k | Langmuir 2012, 28, 13241−13248