International Workshop SMART MATERIALS AND STRUCTURES

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1 Cansart 2009 International Workshop SMART MATERIALS AND STRUCTURES October 2009, Montreal, Quebec, Canada MECHANICAL CHARACTERIZATION OF POROUS MEMBRANE CORE MORPHOLOGIES FOR CONDUCTIVE POLYMER TRILAYER ACTUATORS A.D. Price, V.C. Kao, J.X. Zhang, H.E. Naguib Sart and Adaptive Polyers Lab, Departent of Mechanical and Industrial Engineering University of Toronto, Toronto ABSTRACT Multilayer electroactive polyer actuators consisting of polypyrrole fils electropolyerized on a passive polyer ebrane core have been harnessed as a source of siple actuation. As an integral coponent of the actuator, the ebrane plays a vital role in the transport of ionic species and largely dictates the stiffness of the layered configuration, yet in past studies the specification of the ebrane has reained largely arbitrary. In this investigation we review the pertinent conductive polyer odels to identify the properties of the ebrane that play a vital role in the actuation response of polypyrrole-based trilayer bending actuators. Candidate aterials with distinctly varied icrocellular orphologies are identified and include polyvinylidene difluoride, nylon, and nitrocellulose. The quasi-static stress-strain response and the frequency dependant viscoelastic nature of the candidates is then evaluated. On the basis of echanical properties these results indicate that polyvinylidene difluoride ebranes are superior to the other candidates for application as trilayer actuator cores. Keywords: Conductive polyer, polypyrrole, trilayer actuator, porous ebrane. INTRODUCTION Conductive polyer (CP) actuators possess any favourable advantages over conventional echanical actuator technologies [1]. The range of otion of these actuators has been aplified by fabricating layered configurations which bend upon the application of an external electric field due to the net otion of ions. Trilayer actuators (as illustrated in Figure 1) are typically eployed when the CP actuator is required to operate in the absence of an electrolytic environent. In this arrangeent encapsulation of the device is achieved 247

2 through the electropolyerization of a conductive polyer such as polypyrrole onto opposing sides of a porous ebrane core. The core thus serves as an electrolyte reservoir. The ebrane aterial and its corresponding pore structure influence the transport properties of the ionic species within the electrolyte and also heavily dictate the overall stiffness of the coposite; however existing studies involving CP trilayers and their application have ignored these contributions. Thus the specification of the ebrane aterial and structure reains a subjective process. In this study we review the pertinent diffusion and electrocheoechanical odels to identify the characteristics of the ebrane that play a vital role in the actuation response of polypyrrole-based trilayer bending actuators. Candidate ebrane aterials with distinctly varied icrocellular orphologies are then identified and characterized. Finally, the quasi-static stress-strain response and the frequency dependant viscoelastic nature of the candidates are evaluated to quantitatively deterine the ost suitable core ebrane candidate. Fig. 1 The trilayer bending actuator consists of a porous ebrane core with a conductive polyer such as polypyrrole deposited on opposing faces. The actuator is shown (a) at rest and (b) in the defored state. Underlying Actuation Mechanis THEORETICAL BACKGROUND The echanis responsible for the actuation behaviour of conductive polyers is coplex and is a result of several interrelated phenoena [2]. When an electric potential is applied to the polyer the priary source of actuation is attributed to dilation of the polyer upon the incorporation of ions fro the neighbouring electrolyte (or conversely the polyer ay contract upon expulsion of ions). On a olecular level, the ions interact with the polyer backbone in order to aintain charge neutrality upon odification in the latter s oxidation state. In a trilayer actuator one CP layer swells while the opposite CP layer 248

3 contracts. This induced differential strain results in a acroscopic bending effect which ay be harnessed to perfor echanical work. Prospective Applications Conductive polyer actuators have been successfully deonstrated in several prototypes. More recent innovations include active catheters for bioedical use [3, 4] and fins for aquatic robots [5]. As response ties for conductive polyer actuators are diffusion dependant, iniaturization is expected to greatly iprove actuation bandwidth beyond currently achievable levels [6]. Nanoscale lithography techniques are currently being explored to develop high-speed CP actuator technology that will be beneficial for a broad range of applications. Conductive Polyer Synthesis and Trilayer Fabrication Polyerization of the pyrrole onoer is achieved through a ulti-step electrocheical oxidation reaction [7, 8]. Several alternative growth echaniss have been proposed, and in all cases the reaction product is highly dependant on paraeters such as solvent choice, teperature, and electrolyte dopant [8]. The trilayer arrangeent is fabricated using a three electrode electrocheical cell as shown in Figure 2 that is connected to a potentiostat. This apparatus perits the siultaneous coating of both sides of a centrally fixed ebrane core under identical environental conditions. The ebranes are sputter coated with a thin layer of platinu prior to installation in the cell to facilitate conduction. Fig. 2 A polyerization vessel with opposing counter electrodes is utilized to siultaneously electropolyerize both faces of a porous ebrane core. Modeling The interdisciplinary nature of these actuators consisting of electrical, echanical, and cheical phenoena has resulted in the suggestion of analytical odels with varying foundations. Two such approaches include diffusion based odels such as those proposed by 249

4 Wang, Shapiro, and Sela [9, 10] and electrocheoechanical odels such as those proposed by Alici and Fang [11-13]. (a) (b) Fig. 3 (a) The idealized polyer-electrolyte interface geoetry for the one diensional charge transport odel proposed by Wang et al. can be extended to (b) the trilayer case. As the actuation echanis ultiately depends on ion otion, odels based on diffusion and igration of charge carriers have been established for a single layer of conductive polyer in electrolyte as shown in Figure 3(a) [10]. Extending this scenario to the trilayer configuration results in the odified geoetry indicated in Figure 3(b). The basis for this odeling approach is to apply the Nernst-Planck equation to each obile charge carrier in the electrolyte and conductive polyer to for a syste of partial differential equations. These equations are then coupled together using Poisson s equation which deterines the electric potential for the instantaneous charge distribution. A suary of the governing equations is provided in Table 1 and the odel paraeters are defined in Table 3(a). Solutions of the syste have been deterined using finite eleent analysis when the boundary (Dirichlet) and flux (Neuann) conditions outlined in Table 2 are enforced. In the trilayer case, a odification to the diffusion coefficient for the ebrane core is required to account for the open area and tortuosity inherent to the porous structure [7]. The odel paraeters indicate the relative iportance of the diffusivity and obility of the obile species in each layer. A systeatic investigation of these paraeters will be conducted in the future to assess the ost favourable cobination of ebrane aterial, electrolyte, and solvent. The electrocheoechanical odels are generally extensions of the Diffuse Elastic Metal (DEM) odel proposed by J. Madden [6, 14] in which an abstraction of the ionic response is achieved through an equivalent electrical adittance circuit. This abstraction is then coupled with the principle that the polyer dilation is a function of the ionic charge transferred to it, and thus an epirically deterined strain-to-charge ratio is necessary. Figure 4 indicates that the functional eleents of the odel proposed by Fang et al. include an adittance odule, an electroechanical coupling odule, and a echanical output odule. The corresponding paraeter definitions are suarized in Table 3(b). This odel 250

5 outputs the free tip displaceent of a trilayer actuator when an input potential is prescribed. The C coefficient captures aterial effects such as viscoelasticity. The viscoelastic nature of the actuator is not negligible, and a linear viscoelastic odel with epirically tuned paraeters has been deonstrated with liited success [11]. Thus we are otivated to quantify the viscoelastic behaviour of ebrane aterials in order to iprove odeling accuracy and objectively deterine the ost suitable ebrane aterial. This forulation also indicates that the actuator tip deflection is inversely proportional to the ebrane stiffness. Table 1 Charge diffusion and igration as odeled by the Wang diffusion odel. Region Phenoenon Governing Equation Polyer: Cation diffusion Hole diffusion Poisson s equation Electrolyte: Cation diffusion Anion diffusion Poisson s equation Table 2 Boundary conditions corresponding to the 1D geoetry illustrated in Figure 3(a). Location Phenoenon Boundary Condition Reference electrode: Initial concentration Reference voltage Electrolyte-polyer interface: Zero anion flux Zero hole flux (electrolyte side) (polyer side) Working electrode: Zero cation flux Hole flux Applied voltage 251

6 Table 3 Definition of noenclature eployed for each of the priary odeling approaches. (a) Diffusion based odel Designation Definition Units C cation ol/c 3 concentration t tie s D diffusion c 2 /s coefficient µ obility c 2 /(V s) electric V potential H hole ol/c 3 concentration ε 0 vacuu perittivity F/ 3 ε dielectric constant Q net charge C/c 3 density N electron ol/c 3 concentration J species flux ol/(c 2 s) L layer thickness A anion concentration ol/c 3 Subscripts: C H e p cation hole electron polyer ebrane (b) Electrocheoechanical odel Designation Definition Units s coplex variable Y adittance Ω -1 I electric current A V electric potential V D diffusion c 2 /s coefficient of ion in polyer R electrolyte/contact Ω resistance C double layer F capacitance δ double layer thickness σ induced stress Pa α strain-to-charge 3 /C ratio E elastic odulus Pa W conductive polyer width L conductive polyer length h conductive polyer layer thickness (each) C aterial and ( C) -1 geoetry coefficient y actuator tip displaceent 252

7 Fig. 4 The coplete electroechanical odel proposed by Fang et al. is coprised of three functional coponents. EXPERIMENTAL METHODOLOGY AND RESULTS Morphology of Candidate Mebrane Materials Several candidate ebrane aterials have been selected based on the diversity of their properties and icrostructure. Characterization of the echanical properties of ebranes will objectively indicate which ebrane is best suited as a trilayer core based on their static and viscoelastic response (the identification of diffusion characteristics is ongoing). The candidates are coercially available and are all rated for a retention size of 0.45 µ to facilitate coparison. SEM iages of the ebranes are shown in Figure 5 as follows: (a) nitrocellulose (Hybond ECL, RPN203D, GE Healthcare), (b) polyvinylidene difluoride (PVDF) (Westran CS, # , Whatan), and (c) nylon (RPN303B, GE Healthcare). These iages depict the variety of icrostructures available despite their equivalent retention size. The nitrocellulose and nylon ebranes are fibrous in nature and their pore orphologies consist of a wide distribution of pore sizes. In contrast, the PVDF orphology is relatively hoogeneous exhibiting an average cell density of cells per cubic 253

8 centieter. A separate study to investigate how the nature of these orphologies affects the ion obility ust be undertaken; however it is apparent that both the constituent aterial itself as well as its associated pore structure will ipact the actuator perforance. (a) Nitrocellulose (b) PVDF (c) Nylon Fig. 5 SEM iages reveal the distinct porous orphologies of the (a) nitrocellulose, (b) PVDF, and (c) nylon ebranes. Fig. 6 The stress-strain response of the ebranes illustrates their diverse echanical behaviour. The curves depict a single representative saple of each aterial. Quasi-static response of candidate ebrane aterials The stress-strain behaviour was deterined using a TA Instruents Q800 dynaic echanical analyzer (DMA) in tensile stress-strain ode at abient teperature. A strain rate of 1000 µ/in was prescribed to obtain the elastic response. As indicated in Figure 6, the nylon ebrane exhibits the highest strength and stiffness (E nylon = 1.90 GPa), followed by the PVDF ebrane (E PVDF = 1.69 GPa), and finally although nitrocellulose was nearly as stiff as the PVDF (E nitrocellulose = 1.58 GPa), it was found to be extreely brittle. Hence 254

9 these results eliinate nitrocellulose as a practical candidate aterial since the liited failure strain ay result in localized failure of the ebrane upon bending. Viscoelastic response of candidate ebrane aterials Deterination of the viscoelastic properties of the PVDF and nylon ebranes was perfored using the TA Instruents Q800 dynaic echanical analyzer (DMA) in tensile ode at abient teperature. An oscillating tensile strain of 1% was prescribed over a frequency range of 0.1 Hz to 10 Hz under a 1 N preload. The results indicated in Figure 7 confir the increased stiffness of the nylon ebrane with respect to the PVDF. Furtherore, the tan δ easureents indicate that the PVDF has a larger degree of daping (especially in the low frequency range). These results indicate that PVDF is the best candidate if tip-deflection is to be axiized, but the increased tip deflection coes at the expense of echanical efficiency since the PVDF dissipates ore energy than the nylon. (a) PVDF (b) Nylon Fig. 7 Plots of the storage odulus (left axis), loss odulus (outer right axis), and tan δ (inner right axis) convey the viscoelastic nature of the (a) PVDF and (b) nylon polyer ebranes. CONCLUSIONS This paper reviewed odels pertaining to the actuation behaviour of conductive polyers, and it was shown that ion diffusion rates, ebrane core stiffness, and ebrane core viscoelasticity were particularly vital paraeters in the optiization of the response. This review elucidated the iportance of a deliberate ebrane selection process. A odification to an existing diffusion based odel was also suggested to extend the odel to the trilayer actuator configuration. The echanical properties of several alternative core aterials were subsequently characterized based on their porous orphology, quasi-static stress strain response, and viscoelastic response. Fro these results it was deterined that PVDF is the ost favorable candidate aterial if tip displaceent is to be axiized. ACKNOWLEDGEMENTS Financial support was provided by NSERC Canada, Canada Foundation for Innovation, Canada Research Chairs Progra, and the Governent of Ontario: Ministry of Research and Innovation. 255

10 REFERENCES 1. Sela, E., "Conjugated polyer actuators for bioedical applications," Advanced Materials, 15, pp , Price, A. D., Liu, C. C., O'Shaughnessy, C. A. and Naguib, H. E., "Characterization of conductive polyer trilayer actuators for bioietic robots," in Cansart 2008: International Workshop on Sart Materials and Structures, Shoa, T., Munce, N. R., Yang, V. and Madden, J. D., "Conducting polyer actuator driven catheter: Overview and applications," in Electroactive Polyer Actuators and Devices (EAPAD) 2009, 2009, pp J Mirfakhrai, T., Shoa, T., Fekri, N. and Madden, J. D., "Electrically-activated catheter using polypyrrole actuators: Cycling effects," in Electroactive Polyer Actuators and Devices (EAPAD) 2009, 2009, pp I McGovern, S. T., Spinks, G. M., Xi, B., Alici, G., Truong, V. and Wallace, G. G., "Fast bender actuators for fish-like aquatic robots," in Proceedings of SPIE - the International Society for Optical Engineering, Shoa, T., Madden, J. D., Fok, C. E. and Mirfakhrai, T., "Rate Liits in Conducting Polyers," Advances in Science and Technology, 61, pp , Price, A. D. and Naguib, H. E., "Optiization of porous ebrane core orphology for polypyrrole trilayer actuators," in Electroactive Polyer Actuators and Devices (EAPAD) 2009, 2009, pp P Sadki, S., Schottland, P., Brodie, N. and Sabouraud, G., "The echaniss of pyrrole electropolyerization," Cheical Society Reviews, 29, pp , Wang, X., Shapiro, B. and Sela, E., "Modeling charge transport in conjugated polyers," in Sart Structures and Materials 2006: Electroactive Polyer Actuators and Devices (EAPAD), 2006, pp U Wang, X., Shapiro, B. and Sela, E., "Developent of a odel for charge transport in conjugated polyers," Journal of Physical Cheistry C, 113, pp , Fang, Y., Tan, X., Shen, Y., Xi, N. and Alici, G., "A scalable odel for trilayer conjugated polyer actuators and its experiental validation," Materials Science and Engineering C, 28, pp , Alici, G. and Huynh, N. N., "Predicting force output of trilayer polyer actuators," Sensors and Actuators, A: Physical, 132, pp , Fang, Y., Pence, T. J. and Tan, X., "Nonlinear elastic odeling of differential expansion in trilayer conjugated polyer actuators," Sart Mater Struct, 17, Madden, P. G. A., Madden, J. D. W., Anquetil, P. A., Vandesteeg, N. A. and Hunter, I. W., "The relation of conducting polyer actuator aterial properties to perforance," IEEE Journal of Oceanic Engineering, 29, pp ,