Article pubs.acs.org/Macromolecules
Enhanced Electromechanical Response of Ionic Polymer Actuators by Improving Mechanical Coupling between Ions and Polymer Matrix Yang Liu,†,§ Mehdi Ghaffari,‡,§ Ran Zhao,†,§ Jun-Hong Lin,‡,§ Minren Lin,§ and Q. M. Zhang*,†,‡,§ †
Department of Electrical Engineering, ‡Department of Materials Science and Engineering, and §Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Poly[(vinylidene difluoride)-co-(chlorotrifluoroethylene)] (P(VDF−CTFE)) and P(VDF−CTFE)/poly(methyl methacrylate) (PMMA) cross-linked blends were exploited as ionic electroactive polymer (i-EAP) actuators with electrolyte ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2mim][TfO] for the first time. Compared to the traditional i-EAPs that are based on Nafion or Aquivion (Hyflon) ionomers, i-EAP actuators made with P(VDF−CTFE) and P(VDF−CTFE)/PMMA polymers exhibited remarkably enhanced actuation strain and significantly reduced electrical capacitance. The increase in the strain−charge ratio ε/C indicates that this improvement is a direct result of improved elastic coupling between the [C2mim][TfO] electrolyte and the polymer matrix. As a result, an order-of-magnitude improvement in electromechanical energy conversion efficiency is achieved for both P(VDF− CTFE) and P(VDF−CTFE)/PMMA polymer-based actuators, which makes them promising polymer matrices for i-EAP actuators.
I. INTRODUCTION Electroactive polymers (EAPs) have many attractive attributes for electromechanical applications, especially as actuators and sensors.1 A broad range of polymers such as ferroelectric polymers, conducting polymers, elastomers, and ionic polymers are commonly used as EAPs.2−13 Ionic electroactive polymers (i-EAPs) and their composites, such as ionic polymer metal composites (IPMCs), have attracted a great attention in the past years due to their low operation voltages and large strain.8−13 Since the polymer matrix in IPMCs plays an important role in the electromechanical coupling process, the selection of polymer materials is crucial to the device performance. Up to now, Nafion has been widely used as the polymer matrix in fabricating IPMC actuators,8−15 and recently, Aquivion is also introduced and has demonstrated improved performance.16 Both Nafion and Aquivion are perfluorosulfonic acid ionomer membranes that exhibit high ionic conductivity, good mechanical strength, and high chemical resistance. As displayed in Figure 1, these ionomers consist of a polytetrafluoroethylene (PTFE) backbone, which serves as the high elastic modulus phase, and a double ether perfluoro side chain terminating in a sulfonic acid group, which form a separated low modulus phase with a lower glass transition temperature (Tg) than that of the backbone.15,17−19 The low Tg phase when plasticized by the electrolyte molecules facilitates fast ion transport;15,19,20 however, they may damp the strain coupling between the mobile ions and Teflon backbones. One critical question is how the i-EAP actuators work if these side chains are removed from the polymer matrix. © 2012 American Chemical Society
Figure 1. Molecular structures of Nafion and Aquivion.
In this paper, we investigate polymer matrices without side groups, such as fluoropolymer poly[(vinylidene difluoride)-co(chlorotrifluoroethylene)] (P(VDF−CTFE)), for i-EAP actuators. P(VDF−CTFE) is known as a ferroelectric polymer EAP (dielectric constant ∼12) that can be operated under high voltage, and its Young’s modulus (∼500 MPa) is comparable to that of Nafion.21−24 It has been reported that poly(methyl methacrylate) (PMMA) is miscible with PVDF-based polymers in the melt state due to the hydrogen bonding between the carbonyl groups of PMMA and the hydrogens in the Received: March 22, 2012 Revised: June 2, 2012 Published: June 8, 2012 5128
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25 μm thick films of P(VDF−CTFE), cross-linked P(VDF− CTFE)/PMMA, Nafion (Dupont, 211), Aquivion (Hyflon) (Solvay Solexis, 7903), and ionic liquid [C2mim][TfO] (Aldrich) were dried in vacuum at 80 °C to remove moisture before processing. Polymer membranes swollen with 40 wt % [C2mim][TfO] were prepared by soaking the membranes in [C2mim][TfO] at 60 °C on a VWR hot plate. Earlier studies in the IPMC and ionic membrane actuators have shown that in Nafion and Aquivion there is a critical uptake of [C2mim][TfO] (∼30 wt %), above which the actuator performance does not change with the ionic liquids uptake.14,16 Hence, 40 wt % of [C2mim][TfO] uptake was chosen in this study. The [C2mim][TfO] uptake within the membranes was calculated by measuring the weight gain after swelling. 50 nm thick gold foils (L.A. Gold Leaf) were then hot-pressed at 90 °C on both sides of the membranes as electrodes. The final four ionic membrane actuators had a thickness of 31−33 μm. The densities of the polymers were calculated from their weights and volumes. The storage moduli of the membranes were measured by a dynamic mechanical analyzer (TA DMA 2980), and the heat flow characteristics of the samples were measured by differential scanning calorimetry (TA DSC Q100). The electrical measurement was carried out in a sealed box with desiccant inside to prevent the absorption of moisture, and the electrical characteristics were measured by a potentiostat (Princeton Applied Research 2273). The electromechanical response was recorded by using a probe station (Cascade Microtech M150) equipped with a Leica microscope and a CCD camera (Pulnix 6740CL).36−40
fluoropolymers.25−33 PMMA has a Tg ∼ 105 °C, which is much higher than that of P(VDF−CTFE) ( 10 s). The strain ε even reverses the sign in the case of ionomers. There are several possibilities that may lead to the observed phenomenon. For example, as has been demonstrated earlier, the strains between the cations and anions can cancel each other when substantial anion diffusion occurs which can lead to the reverse of the actuation direction.45 The mechanical energy density of the i-EAP membrane actuators can be calculated from the bending curvature κ and the Young’s moduli of the ionic polymer layer Yi and the Au layer Ym by the equation
intrinsic property of the i-EAP actuators that caused by the two mobile ions in the actuation process.45 From Figure 5a,b, both P(VDF−CTFE) and P(VDF−CTFE)/PMMA possess enhanced electromechanical responses compared to Aquivion and Nafion. P(VDF−CTFE) exhibits maximum bending magnitude of 0.45 mm−1 and a relatively faster initial strain (t < 10 s) response compared to all other samples, which is an indication of better ac actuation for this material which will be discussed in the next section. P(VDF−CTFE)/PMMA displayed the highest bending response (0.75 mm−1) at longer actuation time scale (t > 12 s). At shorter times, P(VDF− CTFE) and P(VDF−CTFE)/PMMA display nearly the same bending magnitude. As has been shown earlier, the strain response at large time scale is mainly determined by diffusion process of anions near the electrodes (due to the ion concentration gradient).16 To quantitatively characterize the strain generation capability of stored ions at the electrodes, the strain in the polymer matrix is deduced from the bending curvature by using a bimorph model13,46 t ε= (1) 2R where ε is the strain, t is the actuator thickness, and R is the radius of curvature. The ratio of strain to unit charge is calculated and presented in Figure 6a. Figure 6a reveals that
um =
1 [ t i + 2tm
∫0
t i /2
Yi(κy)2 dy +
t i /2 + t m
∫t /2
Ym(κy)2 dy]
i
(2)
where y is the distance from the middle plane of the actuator. 5131
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Meanwhile, the electrical energy density ue could be calculated as follows ue = QV /At
(3)
where Q/A is the charge density, V is the applied voltage, and t is the total sample thickness. Hence, the electromechanical energy efficiency of the actuators can be deduced by the relation η = um /ue × 100%
(4)
As illustrated in Figure 7, P(VDF−CTFE) and P(VDF− CTFE)/PMMA achieved an order of magnitude higher energy
Figure 8. (a) Picture and (b) time-dependent diagram of the bending actuation of P(VDF−CTFE) membrane actuator under 4 V 0.1 Hz triangle wave. (c) Comparison of peak-to-peak bending magnitude and peak-to-peak current consumption of four types of i-EAP membrane actuators driven by 4 V 0.1 Hz triangle stimulus.
Nafion and Aquivion perfluorosulfonate ionomers. The improvement in the strain−charge ratio ε/C illustrates that the electromechanical coupling between the ionic liquids and the polymer matrix is enhanced by selecting i-EAP polymers without flexible sulfonic acid side chains. Under both dc and ac stimulus, the reduction in charge consumption and the increase in mechanical energy output of the P(VDF−CTFE) and P(VDF−CTFE)/PMMA membrane actuators lead to an orderof-magnitude higher electromechanical energy efficiencies, which indicates that they are valuable i-EAP actuator materials.
Figure 7. Device electromechanical energy conversion efficiencies of the four types of i-EAP membrane actuators.
efficiencies than Nafion and Aquivion, which makes them preferred materials for i-EAP actuators. Although the electromechanical coupling and efficiency are significantly improved by utilizing P(VDF−CTFE)-based materials, the efficiencies of these membrane actuators are still very low (η < 0.1%). i-EAP actuators (e.g., IPMCs) are complex systems, consisting of many components, such as the electrolyte, polymer matrix, and composite electrode. By adding conductor network composite (CNC) and varying the electrolyte, the electromechanical transduction efficiency may be improved. 3.3. Electromechanical Properties of the Actuators under Alternating Current (AC) Stimulus. Since the electromechanical coupling between ionic liquids and the polymer matrix is stronger at a short charging time (t < 10 s) than long time (Figure 6a), a comparison of the four i-EAP membrane actuators under ac stimulus is performed at 0.1 Hz. The bending actuation of the P(VDF−CTFE) i-EAP membrane actuator under 4 V 0.1 Hz triangle wave is presented in Figure 8a,b, and a comparison of the four i-EAP membrane actuators studied is displayed in Figure 8c. Again, despite their large Young’s moduli, P(VDF−CTFE)-based iEAP actuators have larger bending curvatures, while consuming less driving current than Nafion and Aquivion perfluorosulfonate ionomers, which indicate that P(VDF−CTFE) and P(VDF−CTFE)/PMMA are better i-EAP actuator materials.
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ASSOCIATED CONTENT
S Supporting Information *
Table S1 and Figures S1−S3. 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 This material is based upon work supported in part by NSF under Grant CMMI-1130437 and by the U.S. Army Research Office under Grant W911NF-07-1-0452 Ionic Liquids in Electro-Active Devices (ILEAD) MURI. The authors thank Prof. Ralph H. Colby for many stimulating discussions regarding the work.
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IV. CONCLUSIONS In conclusion, i-EAP actuators based on P(VDF−CTFE) and cross-linked P(VDF−CTFE)/PMMA polymers with 40 wt % of [C2mim][TfO] are investigated. These actuators exhibit much better electromechanical responses in comparison with
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