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Visualizing Dynamic Actuation of Ultrathin Polypyrrole Films Michael J. Higgins,* Scott T. McGovern, and Gordon G. Wallace* ARC Centre of Excellence for Electromaterials Science (ACES), Intelligent Polymer Research Institute (IPRI), AIIM Facility, InnoVation Campus, UniVersity of Wollongong, Squires Way, Fairy Meadow, NSW, 2519, Australia ReceiVed NoVember 24, 2008. ReVised Manuscript ReceiVed January 12, 2009 We report on the use of electrochemical atomic force microscopy (EC-AFM) to visualize the dynamic actuation of ultrathin polypyrrole films doped with polystyrene sulfonate. By varying the film thickness over 3 orders of magnitude from the micrometer to the nanometer range and measuring their actuation height displacement as a function of the applied potential and its change in frequency, we are able to differentiate between diffusion and current limiting processes that determine the rate at which charge balancing ions move in and out of the polymer during actuation. In particular, we observe a 100-350% increase in strain and strain rate when the film thickness is reduced below 100 nm and provide unique insight into how the nanoscale architecture of these ultrathin films is correlated to their actuation performance.
Introduction Conducting polymers are recognized as “intelligent materials” for their potential to transmit signals while sensing incoming information in order to generate a desired response to the external environment. External stimuli from the polymer can be generated electrochemically with the application of a voltage potential that removes or adds electrons within the conjugated backbone of the polymer. This electrochemical process is accompanied by the insertion and ejection of charge balancing dopant ions and induces unique electrical, chemical, and mechanical property changes that can elicit color, volume, and conductivity changes in the polymer.1 These properties can also be reversibly switched to dynamically stimulate the surrounding environment. At present, there is immense interest in utilizing the dynamic properties of conducting polymers for a range of applications including chemical sensing, mechanical actuators, and tissue bioengineering.2 They are particularly desirable as components in biomedical devices (e.g., nanoelectrodes, neural probes, implants, cell scaffolds) due to their biocompatible properties, potential for controlled drug release, and ability to deliver direct electrical stimulation to promote the growth of excitable cells such as nerve and muscle cells.3-5 In many of these applications, the ability to understand the technology at nanometer dimensions is viewed as an important step forward in generating improvements in the electrochemical behavior and performance of autonomous nanosized conducting polymer structures. Characterizing nanoscale variations in the interfacial properties such as topography, adhesion, and elasticity will also be important factors when considering their possible influence on interactions with biological systems, for example, protein films and cell membranes, whose surfaces vary over similar nanometer length scales. * Corresponding authors. (M.J.H.) E-mail:
[email protected]. Telephone: +61-2-4298-1441. Fax: +61-2-4221-3114. (G.G.W.) E-mail:
[email protected]. Telephone: +61-2-4221-3127. Fax: +61-2-42213114. (1) Wallace, G. G.; Innis, P. C.; Moulton, S. E. Handbook of Conducting Polymers; CRC Press: Boca Raton, 2006. (2) Wallace, G. G.; Spinks, G. M.; Kane-Maquire, L. A. P.; Teasedale, P. R. ConductiVe ElectroactiVe Polymers - Dynamic Properties and Intelligent Material Systems; CRC Press: Boca Raton, 2002. (3) Wallace, G. G.; Spinks, G. M. Chem. Eng. Prog. 2007, 103, S18–S24. (4) Smela, E. AdV. Mater. 2003, 15, 481–494. (5) Guimard, N.; Gomez, N.; Schmidt, C. E. Prog. Polym. Sci. 2007, 32, 876–892.
A key approach for the characterization of conducting polymers is the ability to probe their electrochemical surface properties at the nanoscale level during electrical stimulation. In order to undertake such measurements, previous studies have probed the electroactive interface of conducting polymers using electrochemical atomic force microscopy (EC-AFM) or related scanning tunneling microscopy (STM) techniques that typically involve implementing a three electrode electrochemical cell under the AFM with the polymer coated working electrode as the sample substrate.6,7 This approach enables imaging of the polymer surface while simultaneously applying a potential to switch the electrochemical properties of the polymer. For example, previous studies have measured the process of film formation or nanoscale degradation of the film8,9 as a function of cyclic voltammetry (CV) measurements. Changes observed in the polymer film are caused by the electrochemical doping/dedoping processes and can be explained using eqs 1 and 2, where P+ and Po are the doped (oxidized, removal of electrons) and dedoped (reduced, addition of electrons) state of the polymer, respectively. A- is the dopant anion incorporated into the polymer upon oxidation of the monomer, and C+ is the cation in the surrounding electrolyte medium.
P+(A-) + C+ + e- T Po + A- + C+ +
-
+
-
P (A ) + C + e T P (AC) o
(1) (2)
Typically, upon reduction of the polymer (eq 1), the ejection of anions causes the polymer to contract; however, if the anion is immobile (eq 2), cations will instead be inserted into the polymer to balance the loss in charge causing it to expand. In each case, the process is generally reversible and forms the basis of conducting polymer actuators for which there are numerous applications10 and advantages over competing technologies, as they can be packed at high densities and are readily fabricated at sub-micrometer scales. In terms of mechanical actuation, most studies have focused on measuring various actuation related (6) Chainet, E.; Billon, M. J. Electroanal. Chem. 1998, 451, 273–277. (7) Nyffenegger, R.; Ammann, E.; Siegenthaler, H.; Ko¨tz, R.; Hass, O. Electrochim. Acta 1995, 40, 1411–1415. (8) Li, J.; Wang, E.; Green, M.; West, P. E. Synth. Met. 1995, 74, 127–131. (9) Ibanez, J. G.; Alatorre-Ordaz, A.; Gutierrrez-Granados, S.; Batina, N. Polym. Degrad. Stab. 2008, 93, 827–837. (10) Smela, E. MRS Bull. 2008, 33, 197–204.
10.1021/la803874r CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
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parameters, including volume changes, strain, strain rates, and work efficiency, of macroscopic polymer samples.11 These measurements have primarily been conducted on macrosized free-standing films,12-14 while similar studies on supported thin films and nanoscale structures have been done to a much lesser extent.15,16 In EC-AFM studies on supported films, most systems have reported dynamic roughness or height changes as a function of the electrical stimulus.7,16-18 For the few studies that have determined film thickness, there has been significant variability in reported strain values from a surprisingly high value of 36% for a 1-1.5 µm thick polypyrrole(PPy)/dodecylbenzenesulfonate (DBS-) system18 down to 5-7% for differently doped submicrometer PPy films.16 In addition, morphology changes associated with the doping/dedoping process have revealed distinct lateral reorganization of the film;19 however, some investigations have shown no changes at all.18 Although it is suspected that these properties vary depending on conditions such as film thickness, type of polymer/dopant combination, and electrochemical conditions, the molecular and nanoscale origin for the predicted improved strain and strain rate performance for nanoscale thin films (i.e.,
