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Electrochemically controlled swelling properties of nanoporous templated polypyrrole and layer by layer polypyrrole
Journal of Electroanalytical Chemistry 684 (2012) 47–52
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochemically controlled swelling properties of nanoporous templated polypyrrole and layer by layer polypyrrole Aysegul Aygun a,b,⇑, Joseph W. Buthker c, Larry D. Stephenson a, Ashok Kumar a, Thomas K. Mahle c, Andrew A. Gewirth c a b c
Construction Engineering Research Laboratory, US Army Corps of Engineers, Champaign, IL 61822, USA The Pertan Group, 44 Main Street, Suite 403, Champaign, IL 61820, USA University of Illinois at Urbana–Champaign, Department of Chemistry, 600 South Mathews Avenue Urbana, IL 61801, USA
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Article history: Received 1 May 2012 Received in revised form 24 August 2012 Accepted 28 August 2012 Available online 5 September 2012 Keywords: Polypyrrole EQCM Conducting polymers Layer-by-layer Nanoporous
a b s t r a c t Electrochemically controlled swelling properties of polypyrrole (PPy) are investigated by comparing the performance of layer-by-layer (LbL) assembled and nanoporous templated (NT) systems. The swelling behavior was improved for the NT and LbL systems over that of unmodified PPy. Reversible swelling of about 20% in 120 s on the first cycle was measured with the NT PPy via the electrochemical quartz crystal microbalance (EQCM). The changes in the morphology and roughness that occur with swelling and shrinking were measured using in situ electrochemical atomic force microscopy (EC-AFM). Electrochemical impedance spectroscopy (EIS) measurements showed that both the LbL and NT systems exhibited increased surface area relative to unmodified PPy thin films. The understanding of the changes occurring due to molecular and nanostructural reconfiguration in response to electrochemical stimuli may lead to potential applications in the development of a new generation of smart materials, in particular, more effective switchable materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Electroactive polymers (EAPs) have been studied extensively due to their ability to serve as both actuators and sensors [1–5]. These materials have gained particular attention due to their light weight, low cost, fracture tolerance and the ability to be tailored to specific applications [5,6]. EAPs are divided into two major groups: electronic and ionic. Electronic EAPs are driven by Coulomb forces under high applied voltages. Ionic EAPs rely on the transport or diffusion of ions, and include ionic polymer gels (IPGs), carbon nanotubes (CNTs) and conductive polymers (CPs) [7]. Interest in CPs has increased due to their biocompatibility, biodegradability, low actuation power, versatility and stability with volume change [1,3,8,9]. Polypyrrole (PPy) is a CP with the additional advantage of high conductivity [10]. Volume changes of PPy films during redox processes depend on many factors related to ion transport between the film and solution. Multiple studies have shown the ability of counterions and electrolyte ions to transfer in and out of PPy films depends on their mobility, size and valency [11–17]. The flow of ions can also result from salt draining, where an electroneutral ion pair drifts out of the reduced polymer ⇑ Corresponding author at: Construction Engineering Research Laboratory, US Army Corps of Engineers, Champaign, IL 61822, USA. Tel.: +1 217 352 6511x7441; fax: +1 217 373 6732. E-mail address: [email protected] (A. Aygun). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.08.034
[18]. In addition, the solvent has been shown to affect both the ion flux and the conformational relaxations that occur within the polymer film [19,20]. These conformational relaxations are a vital component to the actuation of PPy films and have been thoroughly investigated [12,13,19,21–24]. Improving the ionic conductivity of PPy films should improve their electromechanical actuation properties, since ion movement is vital to the swelling and contraction of the PPy film. Since ionic conductivity is dependent on the porosity of the material, increasing the porosity of the PPy films will allow for improved contact with the electrolyte and decreased resistance to mass transfer [25]. One method for increasing PPy film porosity is layer-by-layer (LbL) assembly, alternating PPy layers with layers of CNTs. The addition of CNTs can add an additional method of actuation that emanates from dimensional changes in the covalent bonds of the CNTs upon application of a potential [26]. There are only a few reports examining the swelling properties of LbL assembled polymer films. Forzani et al. studied polyallylamine multilayer films [27] that achieved a 10% thickness change due to an electrical potential fully oxidizing the film from a fully reduced state. The swelling was attributed to film swelling produced by the entrance of anions and water. Grieshaber et al. reported 5–10% expansion and contraction of polymer multilayer films that take up ferrocyanide ions from aqueous solution [28]. Recently, Schmidt et al. studied the mechanical properties and swelling in electrochemically activated nanocomposite thin films comprised of cationic linear
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poly(ethyleneimine) and anionic Prussian Blue nanoparticles using LbL assembly. Reversible swelling upon reduction of the Prussian Blue nanoparticles was on the order of 2–10%, and reversible changes in the Young’s elastic modulus of the hydrated composite film upon reduction were on the order of 50% [29]. Porosity can also be increased by creating a nanoporous templated (NT) PPy film. Luo and Cui developed an electrochemically controlled drug release system based on nanoporous template PPy films. They reported that due to the porous morphology and huge surface area, the as-prepared nanoporous PPy films were much more effective in electrochemically controlled drug release than the conventional PPy films [30]. However, this group did not look at the actuator response. The goal of this contribution is to use the electrochemical quartz crystal microbalance (EQCM) and electrochemical atomic force microscopy (EC-AFM) to determine how increasing the porosity of a PPy film will affect its swelling and contraction behavior. The EQCM technique is used to study ionic and molecular transport during oxidation and reduction processes in conductive polymers and allows for sensitive monitoring of the simultaneous mass changes [31–34]. Two PPy systems with increased porosity over traditional PPy films were examined: a PPy LbL construction utilizing multiwalled carbon nanotubes (MWCNTs) and a polystyrene (PS) NT PPy. 2. Experimental 2.1. Chemicals and materials All experiments utilized AR grade propylene carbonate (PC) and tetrabutylammonium hexafluorophosphate (TBAPF6) from Sigma– Aldrich and were used as received. Electrochemical control was provided with a CH Instruments Electrochemical Workstation 760 (unless otherwise noted). Electrochemical impedance spectroscopy (EIS) was carried out from 0.1 Hz to 1 MHz at the open circuit potential (OCP) and was analyzed using CHI version 11.08. Identical electrodes were used throughout. Pt gauze was flame annealed with research grade H2 and employed as a counter electrode. A Ag/AgCl (3.4 M KCl) ‘‘no leak’’ electrode (Dionex) was used for the reference electrode. The working electrodes were quartz crystals for the EQCM with a polished Au electrode surface (Inficon, catalog number 1492731). The crystals have a resonant frequency of 5 MHz and are optimized for room temperature operation. Prior to experimentation, the crystals were cleaned in a piranha solution of H2SO4 and 30% H2O2 (3:1) for 15 min, and subsequently rinsed with Milli-Q water and dried under Ar. Milli-Q water (18.2 MX cm) from a Millipore Reference (Millipore Corp.) water purification system was used throughout. 2.2. Polymerization of PPy Pyrrole monomer (Merck) was dried over CaH2 and distilled under N2 and stored at 18 °C. Electropolymerization of PPy was carried out galvanostatically in a water-cooled beaker kept at 30 °C using a Neslab RTE 10 Refrigerated Bath (Thermo Electron Corp.). The cell was maintained under a positive pressure of Ar during the PPy polymerization. The electrodes were immersed into the PC solution containing 0.06 M pyrrole and 0.05 M TBAPF6 and the PPy film was grown using a current density of 0.15 mA cm2 for 15 min. 2.3. LbL assembly of PPy incorporating MWCNTs The LbL growth of PPy/MWCNT films assembled on Au-coated quartz crystals is shown in Fig. 1a. MWCNTs (US Research
Nanomaterials, >95%, OD: 20–30 nm) were refluxed in dilute hydrochloric acid for 8 h., followed by treatment with concentrated sulfuric and nitric acid to provide carboxyl and hydroxyl functional groups [35]. Subsequently, the pretreated MWCNTs were washed with 0.1 M NaOH (to pH 7.0), rinsed with Milli-Q water, vacuum filtered, and dried [36]. A PPy film was first grown on a Au EQCM crystal as described in Section 2.2. Next, 0.5 mg ml1 of the functionalized MWCNTs was dispersed in Milli-Q water by sonicating for 15 min. A 400 lL aliquot of the MWCNT dispersion was cast on the Au electrode surface and dried overnight under ambient conditions (Fig 1b bottom). Each step was repeated until a LbL assembly of four layers was formed. 2.4. NT PPy The approach to synthesize NT PPy films is similar to a method described elsewhere [30]. A thin PPy film grown on a clean Au substrate at a current density of 0.15 mA cm2 for 100 s was used as an adhesion layer, shown in Fig. 2. The PS templates were prepared by dropping a 1.0% (w/v) PS nanobead suspension (Thermo Scientific, d = 46 ± 2.0 nm) onto the polymer surface using a micropipette. The electrodes were then placed vertically and covered, and the water was allowed to evaporate slowly over several hours. When the suspension was dry, the PS nanobead-modified electrodes were heated at 60 °C for 15 min to solidify the template. The PS nanobead-modified electrodes were then immersed into a PC solution containing 0.06 M pyrrole and 0.05 M TBAPF6 for polymerization of PPy using a current density of 0.15 mA cm2 for 15 min. To create the NT PPy film, the PS nanobeads were dissolved out of the film by soaking in toluene for 1 h. The electrode was then thoroughly rinsed with ethanol and water. 2.5. EQCM The frequency of the EQCM is monitored with a Maxtek PM-710 plating monitor. EQCM data were recorded using a home-built program written in LabVIEW (National Instruments). The EQCM cell includes a water-cooled beaker kept at 30 °C using a Neslab RTE 10 Refrigerated Bath (Thermo Electron Corporation) to minimize frequency changes due to temperature fluctuations. 2.6. EC-AFM AFM images were obtained using a PicoSPM 300 (Molecular Imaging) device controlled with a Nanoscope E controller (Digital Instruments). All experiments were conducted in contact mode at room temperature. Commercial Si3N4 V-shaped contact cantilevers with a gold reflective coating on the top were cleaned by rinsing successively with ethanol and Milli-Q water and placing under a 254 nm UV lamp for 20 min. Potential control was provided by a BASi CV-27. The PPy samples were allowed to equilibrate with the electrolyte for 30 min before OCP images were obtained and for 5 min with each additional potential change. 3. Results and discussion 3.1. Cyclic voltammetry (CV) of the PPy systems The CVs for a Au substrate (control), PPy, NT PPy, and LbL PPy/ MWCNT are presented in Fig. 3. The three PPy systems show prominent electrochemical processes compared to the Au sample. Degradation in polymer electroactivity will occur when the anodic potential range extends beyond +1.5 V [37]. This degradation is attributed to the irreversible ‘‘overoxidation’’ observed for PPy at
A. Aygun et al. / Journal of Electroanalytical Chemistry 684 (2012) 47–52
49
4th layer PPy 3rd MWCNT layer 3rd layer PPy 2 nd MWCNT layer 2nd layer PPy 1st MWCNT layer 1st layer PPy Gold QCM crystal substrate
(a)
(b)
Fig. 1. (a) Cartoon of LbL PPy/MWCNT system. The figure is exaggerated for the reader’s convenience. (b) QCM crystals: top: clean crystal, bottom: after LBL PPy/MWCNT casted on the surface.
(a)
(b)
(c)
(d)
Fig. 2. Cartoon of templated PPy exaggerated for the reader’s convenience. (a) A thin layer of PPy was grown on gold substrate. (b) Polystyrene (PS) beads were deposited onto the PPy layer. (c) A PPy film was grown on PS nanobeads. (d) The PS was dissolved in toluene (soaked 1 h) to leave a porous structure.
order to limit degradation of the PPy film during electromechanical actuation. 3.2. EIS estimation of surface area
Fig. 3. CV for Au, PPy, NT PPy, and LbL PPy/MWCNT in TBAPF6/PC solution at 50 mV s1.
anodic potentials, where the polymer backbone becomes susceptible to attack from nucleophiles in the electrolyte, either from solvent or electrolyte ions. When the limit was extended beyond +1.5 V, the breakdown was found to be rapid. The potential limits in this work were confined to the region between 1 V and +1 V in
EIS (Fig. 4) was carried out to compare the surface areas of the three PPy films estimated from their resistances. The overall resistance of the system is given by the value of Z0 at the region where the semi-circle closes at Z00 = 0 [38]. This resistance is assumed to be a combination of the solution resistance (RX) and the chargetransfer resistance (Rct). The LbL PPy/MWCNT system had the least overall resistance (0.82 kX), followed by the NT PPy (1.12 kX). The PPy had the highest overall resistance (1.20 kX). The inductive loops that appear in the EI spectra have been observed before and are attributed to crosstalk between the working and reference electrodes [39]. The data was further analyzed using the Randles equivalent circuit, which includes the two resistances mentioned above, as well as a capacitance (C) and the Warburg impedance (W). The calculated parameters are given in Table 1. Fitting the data in Fig. 4 to the Randles equivalent circuit revealed that the LbL PPy/MWCNT system had the highest capacitance (C) and lowest charge-transfer resistance (Rct). Based on this data, it can be concluded that the LbL PPy/MWCNT films have the highest surface area. Both the LbL
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Fig. 6. Change in frequency of a NT PPy vs. (untemplated/bare) PPy upon alternate application of 1 V and +1 V for 20 s (vs. Ag/AgCl electrode). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. EIS of PPy, LbL PPy/MWCNT and NT PPy in TBAPF6/PC.
Table 1 Data values for the fitting of the impedance data to the Randles equivalent circuit, where R (X) is the solution resistance, Rct (X) is the charge-transfer resistance, C (F) is the capacitance and W (X) is the Warburg impedance.
PPy LbL PPy/MWCNT NT PPy
RX (X)
Rct (X)
C (F)
W (X)
113 72.7 97.2
983 653 862
1.06 108 3.39 108 1.92 108
2.33 103 6.19 103 8.94 104
Fig. 5. The change in frequency of LbL PPy/MWCNT systems of different thicknesses on Au-coated QCM crystal upon alternate application of 1 V and +1 V (vs. Ag/ AgCl). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
PPy/MWCNT and NT PPy were found to have higher estimated surfaces areas than the unmodified PPy film. 3.3. Swelling behavior of PPy films under potentiodynamic conditions The EQCM was used to evaluate the films during swelling/ deswelling states and measure nanogram mass changes that occur due to insertion and removal of ions from the PPy films. The Sauerbrey equation can be applied to relate the mass change per unit area (Dm) to the frequency shift (Df) [40]:
Df ¼ C f Dm
ð1Þ
where Df is the observed frequency change (Hz), Dm is the change in mass per unit area (lg/cm2), and Cf is the sensitivity factor for the crystal (33.1 lg Hz1 cm2). Because of the viscoelastic changes in the polymer film due to swelling/deswelling, we have to assume that the Sauerbrey equation is not linear, but that the frequency changes will still depend monotonically on mass changes [41]. The EQCM data in Fig. 5
Fig. 7. Change in frequency of a NT PPy vs. (untemplated/bare) PPy upon alternate application of 1 V and +1 V for 120 s (vs. Ag/AgCl electrode). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shows the change in frequency of a PPy and LbL assembled PPy/ MWCNT systems. The swelling behavior is investigated by applying a square wave alternating between 1 V and +1 V (vs. Ag/AgCl) for 20 s in 0.05 M TBAPF6/PC. For a PPy film, the relationship between mass changes and the applied potential is fairly simple. As shown in Fig. 5, when the PPy film is held at 1 V, the mass of the film decreases (frequency increases), most likely due to the expulsion of anions from the film. Once the potential is changed to +1 V, the mass of the PPy film increases (frequency decreases), due to the anions being incorporated back into the film. Upon the addition of 2 and 4 PPy/MWCNT layers, the magnitude of the mass change decreases on the first cycle compared to the PPy film (Fig. 5). By the fifth cycle, the 4-layer PPy/MWCNT shows a change in the magnitude of the frequency of 225 Hz, which is nearly equal to the PPy film and almost twice the 120 Hz revealed by the 2-layer PPy/MWCNT. The speed of ion-movement is faster for the 4-layer PPy/MWCNT system, where the maximum change in frequency occurs at an earlier time when compared to the other two systems. The doubled peak for 4-layer PPy/MWCNT (Fig. 5) is due to the movement of both cations and anions to and from the film. The smaller PF anion (ionic radius, r = 0.254 nm, volume, V = 6 0.069 nm3) will move in and out of the film more easily than the bulkier TBA+ cation (r = 0.411 nm, V = 0.29 nm3) [42]. However, evidence of the simultaneous movement of both the cations and anions has been observed when a mobile counterion is used in the system [11]. Initially, the film is held at 1 V and the mass of PPy film decreased due to anion expulsion. However, a small mass increase is observed while the film is still being held at 1 V, most likely due to cations being incorporated back into the film. Upon applying a potential of +1 V the previous process occurs in reverse. The mass initially decreases due to cation expulsion. This is followed by a larger mass increase due to anion incorporation. This indicates that the flux of the anions in and out of the PPy films is the primary method for mass changes, since the changes in mass due to anion movement are much larger than those due to cation movement. It is expected that increasing the surface area of a PPy film by incorporating MWCNTs should provide for an improved swelling
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-1 V
+1 V
PPy
OCP
LbL PPy
z nm
NT PPy
0 nm
Fig. 8. In situ EC-AFM images of the three PPy systems under different potential conditions in 0.5 M TBAPF6 in PC. All images are 10 lm 10 lm. For PPy, z = 50 nm; for LbL PPy, z = 250 nm; for NT PPy, z = 50 nm at OCP and 350 nm at 1 V and +1 V. Voltages are reported vs. Ag/AgCl reference electrode.
response, but this appears to be offset by the increase in the thickness of the film that occurs from the 2-layer PPy/MWCNT system, where the magnitude of the frequency change is nearly half of what it was with the bare PPy system. However, upon adding four layers to the PPy/MWCNT system, the increase in surface area is large enough to compensate for a film that is approximately four times thicker, making the magnitude of the frequency response nearly equal to that of the bare PPy. We can conclude that the swelling properties are improved by the incorporation of the MWCNT into the PPy film. The comparison of NT PPy and untemplated (bare) PPy upon alternate application of 1 V and +1 V (vs. Ag/AgCl electrode) for 20 s is given in Fig. 6. For both systems, there is a significant net anion movement, as seen by the mass increase on oxidation and a mass decrease during subsequent reduction. The ion-movement plays a significant role in maintaining charge balance during untemplated (bare) PPy redox changes. As can be seen from Fig. 6, the magnitude of the mass change for the NT PPy (400 Hz) remains similar for each cycle, while the frequency change for untemplated (bare) PPy decreases from about 300 Hz to 250 Hz upon cycling. The increased stability can be attributed to the templating. EQCM results for the comparison of untemplated (bare) PPy and NT PPy for 120 s intervals is given in Fig. 7. For the longer interval, there is sufficient time to build up mass in material. While the NT PPy has 20% mass change, which is fades with time to 13% mass change, the untemplated (bare) PPy has 10% mass change which fades to 7% mass change. It is clearly seen that porous NT PPy structure has better swelling behavior. In order to directly investigate electrochemically triggered (active) swelling in situ, EC-AFM) was used as a complementary technique. Using EC-AFM allows imaging of the surface under electrochemical potential control. Morphology changes of the three surfaces are attributed to the swelling and contracting of the polymer film as the electrolyte is inserted and removed upon application of a potential. The first row in Fig. 8 shows AFM images of PPy, where no visible changes in the surface morphology occur at 1 V and +1 V. Only a small increase is observed in the RMS roughness
Fig. 9. The RMS values measured from the AFM image of each system. The error bars indicate the standard error of each measurement.
of the PPy surface from 4.0 ± 0.6 to 5.8 ± 0.1 (Fig. 9). The LbL PPy surface (Fig. 8) is visibly rougher than the PPy thin films under all conditions, and upon applying a potential, the surface morphology sees no significant changes from the surface held at the open circuit potential (OCP). As with the PPy surface, the RMS roughness of the LbL PPy film only increases slightly upon applying a potential from 29 ± 2 to 32 ± 3 (Fig. 9). The NT PPy film at OCP shows a smooth surface, similar to that of the PPy (Fig. 8). However, at 1 V, large features appear on the film. These features are not removed from the surface upon switching the voltage to +1 V. The appearance of these features also results in a large increase of the RMS roughness from 6.2 ± 0.5 to above 50 (Fig. 9), making it larger than that of the LbL PPy films when under potential control. 4. Conclusions The electrochemically triggered swelling behavior of the NT PPy and LbL PPy/MWCNT systems have been studied. The swelling
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phenomenon was characterized using EQCM and surface morphology changes were observed using EC-AFM. NT PPy and LbL PPy/ MWCNT films were found to provide improved swelling properties over PPy, which is attributed to the increased porosity of the LbL and NT systems, as revealed from the EIS measurements. Future direction will include an examination of the mechanical properties of these three systems, along with methods to increase the porosity of other polymer thin film systems. Acknowledgment This work was funded by USACE 6.1 funds. References [1] B. Gaihre, G. Alici, G.M. Spinks, J.M. Cairney, Sens. Actuators, A 165 (2011) 321– 328. [2] S.A. Wilson, R.P.J. Jourdain, Q. Zhang, R.A. Dorey, C.R. Bowen, M. Willander, Q.U. Wahab, S.M. Al-hilli, O. Nur, E. Quandt, C. Johansson, E. Pagounis, M. Kohl, J. Matovic, B. Samel, W. van der Wijngaart, E.W.H. Jager, D. Carlsson, Z. Djinovic, M. Wegener, C. Moldovan, R. Iosub, E. Abad, M. Wendlandt, C. Rusu, K. Persson, Mater. Sci. Eng. R56 (2007) 1–129. [3] T.F. Otero, M.T. Cortés, Sens. Actuators, B 96 (2003) 152–156. [4] Y. Bar-Cohen, Q. Zhang, MRS Bull. 33 (2008) 173–181. [5] Y. Bar-Cohen, P. I. Mech. Eng. G-J. Aer. 221 (2007) 553–564. [6] R. Pelrine, R. Kornbluh, Q. Pei, J. Joseph, Science 287 (2000) 836–839. [7] J.A. Trotter, J. Tipper, G. Lyons-Levy, K. Chino, A.H. Heuer, Z. Liu, M. Mrksich, C. Hodneland, W.S. Dillmore, T.J. Koob, M.M. Koob-Emunds, K. Kadler, D. Holmes, Biochem. Soc. Trans. 28 (2000) 357–362. [8] E. Smela, Adv. Mater. 15 (2003) 481–494. [9] L.-X. Wang, X.-G. Li, Y.-L. Yang, React. Funct. Polym. 47 (2001) 125–139. [10] Y.-J. Qui, J.R. Reynolds, Polym. Eng. Sci. 31 (1991) 417–421. [11] M.R. Gandhi, P. Murray, G.M. Spinks, G.G. Wallace, Synth. Met. 73 (1995) 247– 256. [12] J. Chengyou, Y. Fenglin, Sens. Actuators, B 114 (2006) 737–739. [13] T.F. Otero, H. Grande, J. Rodriguez, Synth. Met. 83 (1996) 205–208. [14] C. Weidlich, K.M. Mangold, K. Jüttner, Electrochim. Acta 50 (2005) 1547–1552.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Electrochemically controlled swelling properties of nanoporous templated polypyrrole and layer by layer polypyrrole Aysegul Aygun a,b,⇑, Joseph W. Buthker c, Larry D. Stephenson a, Ashok Kumar a, Thomas K. Mahle c, Andrew A. Gewirth c a b c
Construction Engineering Research Laboratory, US Army Corps of Engineers, Champaign, IL 61822, USA The Pertan Group, 44 Main Street, Suite 403, Champaign, IL 61820, USA University of Illinois at Urbana–Champaign, Department of Chemistry, 600 South Mathews Avenue Urbana, IL 61801, USA
a r t i c l e
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Article history: Received 1 May 2012 Received in revised form 24 August 2012 Accepted 28 August 2012 Available online 5 September 2012 Keywords: Polypyrrole EQCM Conducting polymers Layer-by-layer Nanoporous
a b s t r a c t Electrochemically controlled swelling properties of polypyrrole (PPy) are investigated by comparing the performance of layer-by-layer (LbL) assembled and nanoporous templated (NT) systems. The swelling behavior was improved for the NT and LbL systems over that of unmodified PPy. Reversible swelling of about 20% in 120 s on the first cycle was measured with the NT PPy via the electrochemical quartz crystal microbalance (EQCM). The changes in the morphology and roughness that occur with swelling and shrinking were measured using in situ electrochemical atomic force microscopy (EC-AFM). Electrochemical impedance spectroscopy (EIS) measurements showed that both the LbL and NT systems exhibited increased surface area relative to unmodified PPy thin films. The understanding of the changes occurring due to molecular and nanostructural reconfiguration in response to electrochemical stimuli may lead to potential applications in the development of a new generation of smart materials, in particular, more effective switchable materials. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Electroactive polymers (EAPs) have been studied extensively due to their ability to serve as both actuators and sensors [1–5]. These materials have gained particular attention due to their light weight, low cost, fracture tolerance and the ability to be tailored to specific applications [5,6]. EAPs are divided into two major groups: electronic and ionic. Electronic EAPs are driven by Coulomb forces under high applied voltages. Ionic EAPs rely on the transport or diffusion of ions, and include ionic polymer gels (IPGs), carbon nanotubes (CNTs) and conductive polymers (CPs) [7]. Interest in CPs has increased due to their biocompatibility, biodegradability, low actuation power, versatility and stability with volume change [1,3,8,9]. Polypyrrole (PPy) is a CP with the additional advantage of high conductivity [10]. Volume changes of PPy films during redox processes depend on many factors related to ion transport between the film and solution. Multiple studies have shown the ability of counterions and electrolyte ions to transfer in and out of PPy films depends on their mobility, size and valency [11–17]. The flow of ions can also result from salt draining, where an electroneutral ion pair drifts out of the reduced polymer ⇑ Corresponding author at: Construction Engineering Research Laboratory, US Army Corps of Engineers, Champaign, IL 61822, USA. Tel.: +1 217 352 6511x7441; fax: +1 217 373 6732. E-mail address: [email protected] (A. Aygun). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.08.034
[18]. In addition, the solvent has been shown to affect both the ion flux and the conformational relaxations that occur within the polymer film [19,20]. These conformational relaxations are a vital component to the actuation of PPy films and have been thoroughly investigated [12,13,19,21–24]. Improving the ionic conductivity of PPy films should improve their electromechanical actuation properties, since ion movement is vital to the swelling and contraction of the PPy film. Since ionic conductivity is dependent on the porosity of the material, increasing the porosity of the PPy films will allow for improved contact with the electrolyte and decreased resistance to mass transfer [25]. One method for increasing PPy film porosity is layer-by-layer (LbL) assembly, alternating PPy layers with layers of CNTs. The addition of CNTs can add an additional method of actuation that emanates from dimensional changes in the covalent bonds of the CNTs upon application of a potential [26]. There are only a few reports examining the swelling properties of LbL assembled polymer films. Forzani et al. studied polyallylamine multilayer films [27] that achieved a 10% thickness change due to an electrical potential fully oxidizing the film from a fully reduced state. The swelling was attributed to film swelling produced by the entrance of anions and water. Grieshaber et al. reported 5–10% expansion and contraction of polymer multilayer films that take up ferrocyanide ions from aqueous solution [28]. Recently, Schmidt et al. studied the mechanical properties and swelling in electrochemically activated nanocomposite thin films comprised of cationic linear
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poly(ethyleneimine) and anionic Prussian Blue nanoparticles using LbL assembly. Reversible swelling upon reduction of the Prussian Blue nanoparticles was on the order of 2–10%, and reversible changes in the Young’s elastic modulus of the hydrated composite film upon reduction were on the order of 50% [29]. Porosity can also be increased by creating a nanoporous templated (NT) PPy film. Luo and Cui developed an electrochemically controlled drug release system based on nanoporous template PPy films. They reported that due to the porous morphology and huge surface area, the as-prepared nanoporous PPy films were much more effective in electrochemically controlled drug release than the conventional PPy films [30]. However, this group did not look at the actuator response. The goal of this contribution is to use the electrochemical quartz crystal microbalance (EQCM) and electrochemical atomic force microscopy (EC-AFM) to determine how increasing the porosity of a PPy film will affect its swelling and contraction behavior. The EQCM technique is used to study ionic and molecular transport during oxidation and reduction processes in conductive polymers and allows for sensitive monitoring of the simultaneous mass changes [31–34]. Two PPy systems with increased porosity over traditional PPy films were examined: a PPy LbL construction utilizing multiwalled carbon nanotubes (MWCNTs) and a polystyrene (PS) NT PPy. 2. Experimental 2.1. Chemicals and materials All experiments utilized AR grade propylene carbonate (PC) and tetrabutylammonium hexafluorophosphate (TBAPF6) from Sigma– Aldrich and were used as received. Electrochemical control was provided with a CH Instruments Electrochemical Workstation 760 (unless otherwise noted). Electrochemical impedance spectroscopy (EIS) was carried out from 0.1 Hz to 1 MHz at the open circuit potential (OCP) and was analyzed using CHI version 11.08. Identical electrodes were used throughout. Pt gauze was flame annealed with research grade H2 and employed as a counter electrode. A Ag/AgCl (3.4 M KCl) ‘‘no leak’’ electrode (Dionex) was used for the reference electrode. The working electrodes were quartz crystals for the EQCM with a polished Au electrode surface (Inficon, catalog number 1492731). The crystals have a resonant frequency of 5 MHz and are optimized for room temperature operation. Prior to experimentation, the crystals were cleaned in a piranha solution of H2SO4 and 30% H2O2 (3:1) for 15 min, and subsequently rinsed with Milli-Q water and dried under Ar. Milli-Q water (18.2 MX cm) from a Millipore Reference (Millipore Corp.) water purification system was used throughout. 2.2. Polymerization of PPy Pyrrole monomer (Merck) was dried over CaH2 and distilled under N2 and stored at 18 °C. Electropolymerization of PPy was carried out galvanostatically in a water-cooled beaker kept at 30 °C using a Neslab RTE 10 Refrigerated Bath (Thermo Electron Corp.). The cell was maintained under a positive pressure of Ar during the PPy polymerization. The electrodes were immersed into the PC solution containing 0.06 M pyrrole and 0.05 M TBAPF6 and the PPy film was grown using a current density of 0.15 mA cm2 for 15 min. 2.3. LbL assembly of PPy incorporating MWCNTs The LbL growth of PPy/MWCNT films assembled on Au-coated quartz crystals is shown in Fig. 1a. MWCNTs (US Research
Nanomaterials, >95%, OD: 20–30 nm) were refluxed in dilute hydrochloric acid for 8 h., followed by treatment with concentrated sulfuric and nitric acid to provide carboxyl and hydroxyl functional groups [35]. Subsequently, the pretreated MWCNTs were washed with 0.1 M NaOH (to pH 7.0), rinsed with Milli-Q water, vacuum filtered, and dried [36]. A PPy film was first grown on a Au EQCM crystal as described in Section 2.2. Next, 0.5 mg ml1 of the functionalized MWCNTs was dispersed in Milli-Q water by sonicating for 15 min. A 400 lL aliquot of the MWCNT dispersion was cast on the Au electrode surface and dried overnight under ambient conditions (Fig 1b bottom). Each step was repeated until a LbL assembly of four layers was formed. 2.4. NT PPy The approach to synthesize NT PPy films is similar to a method described elsewhere [30]. A thin PPy film grown on a clean Au substrate at a current density of 0.15 mA cm2 for 100 s was used as an adhesion layer, shown in Fig. 2. The PS templates were prepared by dropping a 1.0% (w/v) PS nanobead suspension (Thermo Scientific, d = 46 ± 2.0 nm) onto the polymer surface using a micropipette. The electrodes were then placed vertically and covered, and the water was allowed to evaporate slowly over several hours. When the suspension was dry, the PS nanobead-modified electrodes were heated at 60 °C for 15 min to solidify the template. The PS nanobead-modified electrodes were then immersed into a PC solution containing 0.06 M pyrrole and 0.05 M TBAPF6 for polymerization of PPy using a current density of 0.15 mA cm2 for 15 min. To create the NT PPy film, the PS nanobeads were dissolved out of the film by soaking in toluene for 1 h. The electrode was then thoroughly rinsed with ethanol and water. 2.5. EQCM The frequency of the EQCM is monitored with a Maxtek PM-710 plating monitor. EQCM data were recorded using a home-built program written in LabVIEW (National Instruments). The EQCM cell includes a water-cooled beaker kept at 30 °C using a Neslab RTE 10 Refrigerated Bath (Thermo Electron Corporation) to minimize frequency changes due to temperature fluctuations. 2.6. EC-AFM AFM images were obtained using a PicoSPM 300 (Molecular Imaging) device controlled with a Nanoscope E controller (Digital Instruments). All experiments were conducted in contact mode at room temperature. Commercial Si3N4 V-shaped contact cantilevers with a gold reflective coating on the top were cleaned by rinsing successively with ethanol and Milli-Q water and placing under a 254 nm UV lamp for 20 min. Potential control was provided by a BASi CV-27. The PPy samples were allowed to equilibrate with the electrolyte for 30 min before OCP images were obtained and for 5 min with each additional potential change. 3. Results and discussion 3.1. Cyclic voltammetry (CV) of the PPy systems The CVs for a Au substrate (control), PPy, NT PPy, and LbL PPy/ MWCNT are presented in Fig. 3. The three PPy systems show prominent electrochemical processes compared to the Au sample. Degradation in polymer electroactivity will occur when the anodic potential range extends beyond +1.5 V [37]. This degradation is attributed to the irreversible ‘‘overoxidation’’ observed for PPy at
A. Aygun et al. / Journal of Electroanalytical Chemistry 684 (2012) 47–52
49
4th layer PPy 3rd MWCNT layer 3rd layer PPy 2 nd MWCNT layer 2nd layer PPy 1st MWCNT layer 1st layer PPy Gold QCM crystal substrate
(a)
(b)
Fig. 1. (a) Cartoon of LbL PPy/MWCNT system. The figure is exaggerated for the reader’s convenience. (b) QCM crystals: top: clean crystal, bottom: after LBL PPy/MWCNT casted on the surface.
(a)
(b)
(c)
(d)
Fig. 2. Cartoon of templated PPy exaggerated for the reader’s convenience. (a) A thin layer of PPy was grown on gold substrate. (b) Polystyrene (PS) beads were deposited onto the PPy layer. (c) A PPy film was grown on PS nanobeads. (d) The PS was dissolved in toluene (soaked 1 h) to leave a porous structure.
order to limit degradation of the PPy film during electromechanical actuation. 3.2. EIS estimation of surface area
Fig. 3. CV for Au, PPy, NT PPy, and LbL PPy/MWCNT in TBAPF6/PC solution at 50 mV s1.
anodic potentials, where the polymer backbone becomes susceptible to attack from nucleophiles in the electrolyte, either from solvent or electrolyte ions. When the limit was extended beyond +1.5 V, the breakdown was found to be rapid. The potential limits in this work were confined to the region between 1 V and +1 V in
EIS (Fig. 4) was carried out to compare the surface areas of the three PPy films estimated from their resistances. The overall resistance of the system is given by the value of Z0 at the region where the semi-circle closes at Z00 = 0 [38]. This resistance is assumed to be a combination of the solution resistance (RX) and the chargetransfer resistance (Rct). The LbL PPy/MWCNT system had the least overall resistance (0.82 kX), followed by the NT PPy (1.12 kX). The PPy had the highest overall resistance (1.20 kX). The inductive loops that appear in the EI spectra have been observed before and are attributed to crosstalk between the working and reference electrodes [39]. The data was further analyzed using the Randles equivalent circuit, which includes the two resistances mentioned above, as well as a capacitance (C) and the Warburg impedance (W). The calculated parameters are given in Table 1. Fitting the data in Fig. 4 to the Randles equivalent circuit revealed that the LbL PPy/MWCNT system had the highest capacitance (C) and lowest charge-transfer resistance (Rct). Based on this data, it can be concluded that the LbL PPy/MWCNT films have the highest surface area. Both the LbL
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Fig. 6. Change in frequency of a NT PPy vs. (untemplated/bare) PPy upon alternate application of 1 V and +1 V for 20 s (vs. Ag/AgCl electrode). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. EIS of PPy, LbL PPy/MWCNT and NT PPy in TBAPF6/PC.
Table 1 Data values for the fitting of the impedance data to the Randles equivalent circuit, where R (X) is the solution resistance, Rct (X) is the charge-transfer resistance, C (F) is the capacitance and W (X) is the Warburg impedance.
PPy LbL PPy/MWCNT NT PPy
RX (X)
Rct (X)
C (F)
W (X)
113 72.7 97.2
983 653 862
1.06 108 3.39 108 1.92 108
2.33 103 6.19 103 8.94 104
Fig. 5. The change in frequency of LbL PPy/MWCNT systems of different thicknesses on Au-coated QCM crystal upon alternate application of 1 V and +1 V (vs. Ag/ AgCl). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
PPy/MWCNT and NT PPy were found to have higher estimated surfaces areas than the unmodified PPy film. 3.3. Swelling behavior of PPy films under potentiodynamic conditions The EQCM was used to evaluate the films during swelling/ deswelling states and measure nanogram mass changes that occur due to insertion and removal of ions from the PPy films. The Sauerbrey equation can be applied to relate the mass change per unit area (Dm) to the frequency shift (Df) [40]:
Df ¼ C f Dm
ð1Þ
where Df is the observed frequency change (Hz), Dm is the change in mass per unit area (lg/cm2), and Cf is the sensitivity factor for the crystal (33.1 lg Hz1 cm2). Because of the viscoelastic changes in the polymer film due to swelling/deswelling, we have to assume that the Sauerbrey equation is not linear, but that the frequency changes will still depend monotonically on mass changes [41]. The EQCM data in Fig. 5
Fig. 7. Change in frequency of a NT PPy vs. (untemplated/bare) PPy upon alternate application of 1 V and +1 V for 120 s (vs. Ag/AgCl electrode). The red plot shows the applied square wave. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shows the change in frequency of a PPy and LbL assembled PPy/ MWCNT systems. The swelling behavior is investigated by applying a square wave alternating between 1 V and +1 V (vs. Ag/AgCl) for 20 s in 0.05 M TBAPF6/PC. For a PPy film, the relationship between mass changes and the applied potential is fairly simple. As shown in Fig. 5, when the PPy film is held at 1 V, the mass of the film decreases (frequency increases), most likely due to the expulsion of anions from the film. Once the potential is changed to +1 V, the mass of the PPy film increases (frequency decreases), due to the anions being incorporated back into the film. Upon the addition of 2 and 4 PPy/MWCNT layers, the magnitude of the mass change decreases on the first cycle compared to the PPy film (Fig. 5). By the fifth cycle, the 4-layer PPy/MWCNT shows a change in the magnitude of the frequency of 225 Hz, which is nearly equal to the PPy film and almost twice the 120 Hz revealed by the 2-layer PPy/MWCNT. The speed of ion-movement is faster for the 4-layer PPy/MWCNT system, where the maximum change in frequency occurs at an earlier time when compared to the other two systems. The doubled peak for 4-layer PPy/MWCNT (Fig. 5) is due to the movement of both cations and anions to and from the film. The smaller PF anion (ionic radius, r = 0.254 nm, volume, V = 6 0.069 nm3) will move in and out of the film more easily than the bulkier TBA+ cation (r = 0.411 nm, V = 0.29 nm3) [42]. However, evidence of the simultaneous movement of both the cations and anions has been observed when a mobile counterion is used in the system [11]. Initially, the film is held at 1 V and the mass of PPy film decreased due to anion expulsion. However, a small mass increase is observed while the film is still being held at 1 V, most likely due to cations being incorporated back into the film. Upon applying a potential of +1 V the previous process occurs in reverse. The mass initially decreases due to cation expulsion. This is followed by a larger mass increase due to anion incorporation. This indicates that the flux of the anions in and out of the PPy films is the primary method for mass changes, since the changes in mass due to anion movement are much larger than those due to cation movement. It is expected that increasing the surface area of a PPy film by incorporating MWCNTs should provide for an improved swelling
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-1 V
+1 V
PPy
OCP
LbL PPy
z nm
NT PPy
0 nm
Fig. 8. In situ EC-AFM images of the three PPy systems under different potential conditions in 0.5 M TBAPF6 in PC. All images are 10 lm 10 lm. For PPy, z = 50 nm; for LbL PPy, z = 250 nm; for NT PPy, z = 50 nm at OCP and 350 nm at 1 V and +1 V. Voltages are reported vs. Ag/AgCl reference electrode.
response, but this appears to be offset by the increase in the thickness of the film that occurs from the 2-layer PPy/MWCNT system, where the magnitude of the frequency change is nearly half of what it was with the bare PPy system. However, upon adding four layers to the PPy/MWCNT system, the increase in surface area is large enough to compensate for a film that is approximately four times thicker, making the magnitude of the frequency response nearly equal to that of the bare PPy. We can conclude that the swelling properties are improved by the incorporation of the MWCNT into the PPy film. The comparison of NT PPy and untemplated (bare) PPy upon alternate application of 1 V and +1 V (vs. Ag/AgCl electrode) for 20 s is given in Fig. 6. For both systems, there is a significant net anion movement, as seen by the mass increase on oxidation and a mass decrease during subsequent reduction. The ion-movement plays a significant role in maintaining charge balance during untemplated (bare) PPy redox changes. As can be seen from Fig. 6, the magnitude of the mass change for the NT PPy (400 Hz) remains similar for each cycle, while the frequency change for untemplated (bare) PPy decreases from about 300 Hz to 250 Hz upon cycling. The increased stability can be attributed to the templating. EQCM results for the comparison of untemplated (bare) PPy and NT PPy for 120 s intervals is given in Fig. 7. For the longer interval, there is sufficient time to build up mass in material. While the NT PPy has 20% mass change, which is fades with time to 13% mass change, the untemplated (bare) PPy has 10% mass change which fades to 7% mass change. It is clearly seen that porous NT PPy structure has better swelling behavior. In order to directly investigate electrochemically triggered (active) swelling in situ, EC-AFM) was used as a complementary technique. Using EC-AFM allows imaging of the surface under electrochemical potential control. Morphology changes of the three surfaces are attributed to the swelling and contracting of the polymer film as the electrolyte is inserted and removed upon application of a potential. The first row in Fig. 8 shows AFM images of PPy, where no visible changes in the surface morphology occur at 1 V and +1 V. Only a small increase is observed in the RMS roughness
Fig. 9. The RMS values measured from the AFM image of each system. The error bars indicate the standard error of each measurement.
of the PPy surface from 4.0 ± 0.6 to 5.8 ± 0.1 (Fig. 9). The LbL PPy surface (Fig. 8) is visibly rougher than the PPy thin films under all conditions, and upon applying a potential, the surface morphology sees no significant changes from the surface held at the open circuit potential (OCP). As with the PPy surface, the RMS roughness of the LbL PPy film only increases slightly upon applying a potential from 29 ± 2 to 32 ± 3 (Fig. 9). The NT PPy film at OCP shows a smooth surface, similar to that of the PPy (Fig. 8). However, at 1 V, large features appear on the film. These features are not removed from the surface upon switching the voltage to +1 V. The appearance of these features also results in a large increase of the RMS roughness from 6.2 ± 0.5 to above 50 (Fig. 9), making it larger than that of the LbL PPy films when under potential control. 4. Conclusions The electrochemically triggered swelling behavior of the NT PPy and LbL PPy/MWCNT systems have been studied. The swelling
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phenomenon was characterized using EQCM and surface morphology changes were observed using EC-AFM. NT PPy and LbL PPy/ MWCNT films were found to provide improved swelling properties over PPy, which is attributed to the increased porosity of the LbL and NT systems, as revealed from the EIS measurements. Future direction will include an examination of the mechanical properties of these three systems, along with methods to increase the porosity of other polymer thin film systems. Acknowledgment This work was funded by USACE 6.1 funds. References [1] B. Gaihre, G. Alici, G.M. Spinks, J.M. Cairney, Sens. Actuators, A 165 (2011) 321– 328. [2] S.A. Wilson, R.P.J. Jourdain, Q. Zhang, R.A. Dorey, C.R. Bowen, M. Willander, Q.U. Wahab, S.M. Al-hilli, O. Nur, E. Quandt, C. Johansson, E. Pagounis, M. Kohl, J. Matovic, B. Samel, W. van der Wijngaart, E.W.H. Jager, D. Carlsson, Z. Djinovic, M. Wegener, C. Moldovan, R. Iosub, E. Abad, M. Wendlandt, C. Rusu, K. Persson, Mater. Sci. Eng. R56 (2007) 1–129. [3] T.F. Otero, M.T. Cortés, Sens. Actuators, B 96 (2003) 152–156. [4] Y. Bar-Cohen, Q. Zhang, MRS Bull. 33 (2008) 173–181. [5] Y. Bar-Cohen, P. I. Mech. Eng. G-J. Aer. 221 (2007) 553–564. [6] R. Pelrine, R. Kornbluh, Q. Pei, J. Joseph, Science 287 (2000) 836–839. [7] J.A. Trotter, J. Tipper, G. Lyons-Levy, K. Chino, A.H. Heuer, Z. Liu, M. Mrksich, C. Hodneland, W.S. Dillmore, T.J. Koob, M.M. Koob-Emunds, K. Kadler, D. Holmes, Biochem. Soc. Trans. 28 (2000) 357–362. [8] E. Smela, Adv. Mater. 15 (2003) 481–494. [9] L.-X. Wang, X.-G. Li, Y.-L. Yang, React. Funct. Polym. 47 (2001) 125–139. [10] Y.-J. Qui, J.R. Reynolds, Polym. Eng. Sci. 31 (1991) 417–421. [11] M.R. Gandhi, P. Murray, G.M. Spinks, G.G. Wallace, Synth. Met. 73 (1995) 247– 256. [12] J. Chengyou, Y. Fenglin, Sens. Actuators, B 114 (2006) 737–739. [13] T.F. Otero, H. Grande, J. Rodriguez, Synth. Met. 83 (1996) 205–208. [14] C. Weidlich, K.M. Mangold, K. Jüttner, Electrochim. Acta 50 (2005) 1547–1552.
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