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Sulfonated copper phthalocyanine/sulfonated polysulfone composite membrane for ionic polymer actuators with high power density and fast response time Taehoon Kwon, Hyeongrae Cho, Jang-Woo Lee, Dirk Henkensmeier, Youngjong Kang, and Chong Min Koo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07572 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Sulfonated copper phthalocyanine/sulfonated polysulfone composite membrane for ionic polymer actuators with high power density and fast response time Taehoon Kwon, †, ‡ Hyeongrae Cho, § Jang-Woo Lee, ∥ Dirk Henkensmeier, §, ⊥, #, * Youngjong Kang, ‡ and Chong Min Koo †, #, ¶, *

†

Materials Architecturing Research Center, Korea Institute of Science and Technology

(KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea ‡

Department of Chemistry, College of Natural Sciences, Hanyang University, Wangsimni-ro

222, Seongdong-gu, Seoul 04763, Republic of Korea §

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarang-ro

14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea ∥

Future Technology R&D Team, Petrochemical Division, Daelim Industrial Co., Ltd.,

Sinsungnam-ro 106, Yuseong-gu, Daejeon 34108, Republic of Korea ⊥

ET-GT, KIST School, University of Science and Technology, Hwarang-ro 14-gil 5,

Seongbukgu, Seoul 02792, Republic of Korea #

KU-KIST Graduate School of Science and Technology, Korea University, Anam-ro 145, 1

ACS Paragon Plus Environment


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Seongbuk-gu, Seoul 02841, Republic of Korea ¶

Nanomaterials Science and Engineering, University of Science and Technology, Gajeong-ro

217, Yuseong-gu, Daejeon 34113, Republic of Korea

KEYWORDS: ionic polymer actuator, organic composite, polymer electrolyte, sulfonated copper phthalocyanine, response rate, mechanical power density

2


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ABSTRACT Ionic polymer composite membranes based on sulfonated poly(arylene ether sulfone) (SPAES) and copper(II) phthalocyanine tetrasulfonic acid (CuPCSA) are assembled into bending ionic polymer actuators. CuPCSA is an organic filler with very high sulfonation degree (IEC = 4.5 mmol H+/g) that can be homogeneously dispersed on the molecular scale into the SPAES membrane, probably due to its good solubility in SPAES containing solutions. SPAES/CuPCSA actuators exhibit larger ion conductivity (102 mS cm–1), tensile modulus (208 MPa) and strength (101 MPa), strain (1.21 %), exceptional faster response to electrical stimuli, and larger mechanical power density (3028 W m–3) than ever reported for ion conducting polymer actuators. This outstanding actuation performance of SPAES/CuPCSA composite membrane actuators makes them attractive for next generation transducers with high power density, which are currently developed e.g. for underwater propulsion and endoscopic surgery.

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1. INTRODUCTION Ionic polymer actuators convert electric power into a mechanical force. Ionic polymer ctuators have gained great interest for applications in robotics and underwater propulsion owing to their unique advantages, including their large and fast bending actuation under operation voltages of 1–5 V.1–7 Ionic polymer actuators usually contain mobile cations (mainly lithium ions) and an inner solvent (typically water) for dissociating the cations from the sulfonate groups covalently bonded to the polymer macromolecular chains.8,9 The actuators bend under electric field because the hydrated mobile cations migrate towards the cathode, causing the cathode side to swell and the anodic side to shrink. Nafion, commercialized by DuPont, is the most popular ionic polymer in the actuators field because of its high proton conductivity and robust thermochemical and mechanical stabilities. However, because of severe drawbacks of Nafion-based actuators, including early dehydration and fast back-relaxation behavior,10–18 many researches attempted to develop new ionic polymer actuators based on perfluorosulfonate polymers,1–3 fluoropolymers

(including

poly(vinylidene

fluoride)

(PVDF)-based

copolymers),3,9

fluoropolymers grafted with sulfonated polystyrene,19 and hydrocarbon polymers (such as sulfonated poly(vinyl alcohol),13 sulfonated polystyrene,14 fluorinated acrylic copolymers,20 semi-interpenetrating polymer networks,21 anion-conducting polymers,22,23 and ionconducting block copolymers).11,24–29 Nanocomposite approaches have been also widely attempted to improve the performance of ionic polymer actuators. Ionic composite membranes containing surfacefunctionalized inorganic fillers, such as sulfonated-silica and sulfonated montmorillonite (MMT), revealed enhanced actuation performances in terms of displacement, blocking force, 4


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and response rate.26,29 Because of strong interactions between mobile ions and the ionic groups on the surface of filler particles, mobile ions can diffuse efficiently along the surface of these particles. The addition of silica or MMT to the polymer matrix also contributes to maintaining the bending shape during dc operation. However, despite the surface functionalization of fillers, enhancement of ionic conductivity is limited for ionic polymer nanocomposites. One reason for this issue is that even surface functionalized fillers reduce the overall concentration of mobile ions when their ion exchange capacity (IEC) values are smaller than that of the polymer.30 Another reason is that inorganic fillers usually have much larger particles size (e.g., 10–100 nm) than ionic channels (< 10 nm) in polymer electrolyte membranes. As a result, the mobile ions cannot penetrate the gigantic inorganic fillers but should travel around them, increasing the pathway between the electrodes. However, at low particle size and low filler contents (e.g., 1%–3%) the surface of the filler particles probably enhances conductivity by connecting the channels.31 Herein, we demonstrate ionic bending actuators based on an ionic polymer composite membrane consisting of a sulfonated poly(arylene ether sulfone) (SPAES) hydrocarbon polymer filled with copper(II) phthalocyanine tetrasulfonic acid (CuPCSA) as shown in Figure 1a–1c. Because of the increased concentration of mobile ions and a uniform distribution of CuPCSA molecules throughout the SPAES membrane, probably as part of the hydrophilic channels, SPAES/CuPCSA composite membranes reveal not only large ion conductivity values but also superior actuation performances in aspect of bending strain, response rate, blocking force, and mechanical power density. 2. EXPERIMENTAL SECTION 2.1. Materials 5



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SPAES (SES0005, ion exchange capacity (IEC) 2.08 meq g–1, Aquafone™) (Figure 1b) was purchased from Yanjin Technology, China. CuPCSA organic filler (IEC 4.46 meq g–1) (Figure 1c) was purchased from Sigma-Aldrich. N,N’-dimethylacetamide (DMAc) (SigmaAldrich), Nafion solution (DE-2021, IEC 0.95 meq g–1, DuPont), hydrochloric acid (HCl, Daejung Chemical, China). The reagents were used as received. 2.2. Preparation of SPAES/CuPCSA composite membranes Composite membranes were prepared following the procedure as reported before in literature.32 Firstly, acid-form SPAES was prepared by dissolving salt-form of SPAES polymer resin in anhydrous DMAc. Then, the polymer was converted to the corresponding acid-form by pouring the solution into a mixture of methanol and concentrated hydrochloric acid. The ion-exchanged SPAES precipitates were collected through filtration, washed with deionized water, and then dried under vacuum at 100 °C for 24 h. Next, the required amount of CuPCSA was dissolved in DMAc and the required amount of acid-form SPAES was added to the CuPCSA solution. The mixtures were stirred to get a homogeneous solution at room temperature. After that, the solvent was evaporated from the mixture in a petri dish at 60 °C for 24 h, initially without vacuum and then under vacuum at 60 °C for 24 h, and eventually at 80 °C for 24 h. The membranes were soaked in deionized water to delaminate them from the glass surface. The fully acid-form composite membranes were obtained through soaking the membranes in 2 M HCl solution at room temperature. After 24 h acidification, the membranes were washed with deionized water until the washing solution reached a neutral pH. The membranes were stored in deionized water at room temperature before use. A pristine SPAES membrane was also prepared through the same procedure for comparison. The thickness of the membranes was determined as approximately 250 µm. Although SPAES/CuPCSA membranes were made with the various CuPCSA ratio (2, 5, 10 and 15 6


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wt%), the SPAES/CuPCSA 10 and 15 wt% membranes were not suitable for the actuator application since they showed the over-swelling behavior that was caused poor dimension stability and mechanical strength. Hence, the experiment for actuator was performed with SPAES/CuPCSA 2% and 5% membranes. 2.3. Preparation of Nafion membranes 15 g DE-2021 Nafion dispersion were cast into a 10-cm diameter Petri dish with a lid. After solvent evaporation under ambient conditions, the membrane was carefully removed from the dish, exchanged into the sodium form through immersion in 0.5 M NaCl solution for 24 h, followed by immersion in water for another 24 h at room temperature. Then, the dry membrane was annealed at 130 °C for 2 h, followed by ion exchange back into the acid form through immersion in 1 M HCl solution. The thickness of the membranes determined as approximately 250 µm, which was the same as the thickness of SPAES/CuPCSA membranes. 2.4. Fabrication of SPAES/CuPCSA actuators SPAES/CuPCSA composite membrane actuators were fabricated through electroless plating of platinum electrodes on the SPAES/CuPCSA membrane as described in the literature.11,26

A

surface-roughened

membrane

was

immersed

in

0.01

M

tetraammineplatinum(II) chloride solution for 24 h at room temperature. The immersed membranes were rinsed and placed in deionized water at 40 °C. Then, 2 mL of 5% ammonium hydroxide solution and 2 mL of 5% sodium borohydride solution were added to the bath every 30 min to reduce Pt ions on the surface layer of the membranes. After the addition of reducing agents several times, the membranes were rinsed and soaked into 0.1 M HCl solution for 2 h at room temperature to remove the excess reducing agents. This process was repeated three times until the surface resistance (measured using a multimeter) dropped below 10 Ω. Finally, the composite membranes with Pt electrodes were immersed in 1 M 7



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lithium hydroxide solution for 12 h before the actuator experiment. The actuator samples had a length of 23 mm and a width of 2.5 mm. Nafion and SPAES actuators were also fabricated through the same process for the comparison. 2.5. Characterization The morphologies of SPAES/CuPCSA composite membrane actuators were observed with a scanning electron microscope (SEM, Inspect™ F50, FEI) equipped with an energydispersive X-ray spectrometer (EDS). Water uptake values of membranes were calculated through measurement of the weight of dried and swollen membranes with the following equation: Uptake (wt%) =

௪౩ ି௪ౚ ௪ౚ

× 100

(1)

where ws and wd are weights of swollen and dried membrane, respectively. The weight of dried membrane was measured at room temperature at first. Then the membrane was soaked in water for 24 h at room temperature. After swelling the membrane, the surface water was removed by blotting with a tissue paper, followed by immediate measurement of the wet weight. IEC values of membranes were evaluated through back titration method. Briefly, a fully proton-exchanged membrane was soaked in 0.1 M NaOH aqueous solution for 24 h at room temperature and then removed. The remaining NaOH solution was titrated with 0.1 M HCl solution after adding a few drops of 0.1% phenolphthalein indicator. The IEC values were obtained by subtracting the added volume of HCl solution from the initial NaOH volume. In-plane ionic conductivities of the membranes were measured with a potentiostatgalvanostat electrochemical impedance spectroscopy (EIS) instrument (VMP3, BioLogic Science Instruments) combined with a custom-made four-probe cell in water at room temperature. The mechanical properties were examined with a universal testing machine 8


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(H5KT, Tinius Olsen, USA) at a cross header speed of 20 mm min−1 in ambient condition (25 °C, 50% relative humidity). The wet membrane samples were used for the mechanical tests. The wet membrane was prepared by swelling in water for 24 h at room temperature. For the electromechanical actuation measurements, bipolar voltages on the ionic polymer actuators were applied using EIS instrument. Displacement was measured as tip-to-tip displacement from initial tip position to final tip position by using a CCD camera connected to a computer with a data acquisition (DAQ) system (SCB-68, National Instruments) under dc voltage and horizontal displacement at 10 mm distance from the grip with laser sensor (LB-11, Keyence) under ac voltage. Strain was calculated from the displacement of the actuator following the equation:33,34 ଶ஽ௐ

ε (%) = ௅మ ା஽మ × 100

(2)

where ε is the strain of actuator, D is the measured displacement, W is the thickness of actuator, and L is the free length from the grip, respectively. Blocking force generated by the tip of actuators was measured with a load cell (CB1-G150, Dacell) and DAQ system. All the electrochemical actuation tests were carried out in ambient condition (25 °C, 50% relative humidity). 3. RESULTS AND DISCUSSION SPAES/CuPCSA composite membranes were fabricated through solution casting process. While SPAES membranes are practically colorless, SPAES/CuPCSA composite membranes are transparent and blue.32 With increasing thickness and CuPCSA content, the color intensifies and membranes appear dark blue. As shown in Figure 1d and 1e, CuPCSA and mixtures of CuPCSA and SPAES were well dispersed in DMAc solvent and membranes casted from the latter solution appeared fully homogeneous, suggesting that the CuPCSA 9



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does not form large agglomerates. The cross-sectional SEM micrograph of a SPAES/CuPCSA composite membrane actuator with Pt electrodes electroless-plated on both surfaces is shown in Figure 1f. The thickness of the SPAES/CuPCSA membrane was measured as ca. 250 µm in which 8–9 µm thick Pt electrodes were uniformly coated on both surfaces. The magnified interfacial region between membrane and electrode and the surface morphology of the Pt electrode are displayed in the insets of Figure 1f. The electrode surface was observed to be very smooth all over the SPAES/CuPCSA membrane and revealed a low surface resistance (ca. 2–5 Ω). The cross-sectional SEM micrograph of SPAES/CuPCSA membrane with higher magnification and its EDS mapping of Cu atoms are shown in Figure 1g and 1h. In the SEM micrographs, also no CuPCSA agglomerates were observed, indicating that CuPCSA is well distributed in the membrane on the molecular scale. TEM and EDS mapping results of a SPAES/CuPCSA 5% membrane sample further support this. (Supporting Information Figure S1.) The IEC values for SPAES/CuPCSA membranes were examined through back titration (Figure 2a). SPAES has a larger theoretical IEC value (2.08 meq g–1) than Nafion (0.95 meq g–1). On the other hand, IEC values of SPAES/CuPCSA membranes increased with the increase in CuPCSA content as CuPCSA has a larger IEC value (4.46 meq g–1) than the polymer matrix. Interestingly, experimental IEC values of SPAES/CuPCSA membranes are very close to the theoretical values, as shown in Figure 2a, indicating that all sulfonic acid groups in the composite membranes are freely accessible and therefore contribute to the experimentally observed IEC value.

10


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Figure 1. (a) Schematic illustration of ion transport in SPAES/CuPCSA membrane. Chemical structure of: (b) salt form SPAES polymer, (c) CuPCSA. (d) Photographs of SPAES (left), CuPCSA (center), and SPAES/CuPCSA (right) solutions in DMAc. (e) Photographs of SPAES (left) and SPAES/CuPCSA 5% (right) membranes (60 µm thickness). (f) SEM 11



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micrographs of cross-sectioned SPAES/CuPCSA ionic polymer actuator. The two inset images are magnified SEM micrographs of membrane/platinum electrode interface (top) and surface (bottom) of Pt electrode coated membrane. (g) Cross-sectional SEM micrographs with higher magnification, and (h) its EDS mapping with Cu element.

Table 1 Physical and mechanical properties of ionic polymer membranes and their actuators Membrane

Thickness [µm]

IEC value (experimental) [meq g–1]

Water uptake [wt %]

Tensile modulus [MPa]

Blocking force (maximum) [mN]

Nafion

242 ± 15

0.95 ± 0.08

26.2 ± 1.5

55.0 ± 2.8

1.67 ± 1.13

SPAES

252 ± 13

2.08 ± 0.11

70.2 ± 2.1

282.6 ± 14.1

2.16 ± 1.74

SPAES/CuPCSA 2%

251 ± 16

2.12 ± 0.10

73.6 ± 2.5

193.4 ± 15.2

2.74 ± 1.82

SPAES/CuPCSA 5%

250 ± 17

2.18 ± 0.12

71.8 ± 2.2

208.3 ± 14.0

3.15 ± 1.59

Figure 2b shows the ionic conductivities of Nafion, SPAES, and SPAES/CuPCSA membranes. The SPAES sample revealed a larger ionic conductivity value (93 mS cm–1) than Nafion (76 mS cm–1). The ion conductivity of SPAES/CuPCSA increased further with increasing the CuPCSA content. The SPAES/CuPCSA membrane containing 5 wt% CuPCSA revealed an ionic conductivity of 102 mS cm–1. Water uptake values of membranes are listed in Table 1. The SPAES and SPAES/CuPCSA membrane samples revealed 2.8 times larger water uptakes than Nafion, but only about 34% higher conductivity. This different response to the water uptake for Nafion and hydrocarbon based membranes was already described by Harrison et al. for a very similar membrane (BPSH-40).35 In general, the larger ionic conductivity and water uptake of SPAES/CuPCSA membranes are attributed to larger IEC values compared with Nafion membranes. Tensile stress-strain curves and the resulting tensile properties of hydrated Nafion, SPAES, 12


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and SPAES/CuPCSA membranes are plotted in Figure 2c and 2d. The pure SPAES membranes revealed superior mechanical properties to Nafion membranes in terms of modulus and ultimate stress. SPAES/CuPCSA membranes showed slightly smaller tensile modulus and strength than SPAES membrane due to larger water uptake, but those were still stronger than Nafion membrane. The good mechanical properties of SPAES and SPAES/CuPCSA membranes are ascribed to the intrinsic rigidity of their aromatic backbones.

Figure 2. (a) IEC values, (b) proton conductivities (at room temperature, in water), (c) stressstrain curves, and (d) tensile moduli and strength of hydrated SPAES/CuPCSA ionic polymer. All examinations were performed at room temperature. 13



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Figure 3. (a) Actuation photographs of SPAES/CuPCSA 5% actuator under 5 V dc with the definition of tip-to-tip displacement in this study. (b) Strain vs time curves under 2 V dc voltage and (c) initial displacement rate vs ionic conductivity for SPAES/CuPCSA actuators compared with various other actuators. Displacement rate was acquired from the initial slope in displacement vs time plots. (d) Retention of peak-to-peak displacement of Nafion, SPAES and SPAES/CuPCSA 5% actuators for 10,000 cycles under 2 V ac voltage with the frequency of 0.5 Hz with the strain of first 10 cycles inset.

14


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Figure 3a shows photographs of the SPAES/CuPCSA composite actuators during actuation under dc voltage. As defined in Figure 3a, tip-to-tip displacement was used for the displacement measure of dc actuation. The bending deformation of SPAES/CuPCSA actuators was so large and circular that the conventional horizontal tip displacement was not able to describe the actuation properly. (See also Supporting Information Movie S1–S3). The strain of Nafion, SPAES, and SPAES/CuPCSA actuators under 2 V dc voltage and their initial displacement rates vs ionic conductivity plot compared with various actuators are shown in Figure 3b and 3c. Nafion actuators followed the known actuation pattern; they quickly reached the maximum strain but then experienced back-relaxation. In contrast, all SPAESbased actuators showed a larger maximum strain and a much faster deformation rate than Nafion; the values increased with increasing CuPCSA content. Furthermore, SPAES and SPAES/ 2% CuPCSA actuators exhibited less or no back-relaxation (SPAES/ 5% CuPCSA) after reaching the maximum strain within the observed period of 60 sec. It is known that the back-relaxation is attributed to the back diffusion of water to the anode after full deformation of actuator under dc voltage.36,37 The reduced back-relaxation behavior in SPAES/CuPCSA actuators, unlike Nafion, may be described by lower acidity (pKa of Nafion: approximately 6; benzenesulfonic acid as an analog of sulfonate group in SPAES: 0.7; sulfonate group in CuPCSA: about 1.1).26,38 Low acidity could reduce the back-relaxation through less rearrangement of cations after full charging in the cathode boundary layer.39 Besides, hydrocarbon membranes have higher tortuosity than Nafion, which may slow down diffusion of water in SPAES/CuPCSA composite membranes.40 To our knowledge, Nafion-based composite polyelectrolytes have the fastest initial 15



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displacement rate among the reported materials, as shown exemplary in Figure 3c.3,9,14,26,41–45 Even materials for which comparable or better ionic conductivity or actuation displacement were reported show smaller displacement rates.3 The huge scattering of literature data reflects their dependency on the membrane thickness, electrode quality, actuation potential, and some other factors. In this work, examined under the same conditions, SPAES and SPAES/CuPCSA membranes revealed not only larger bending deformation but also faster response to electrical stimuli than Nafion. Especially, the SPAES/CuPCSA 5 wt% membrane revealed a 2.5 times faster initial response rate (see Supporting Information, Figure S2). The bending strain and response rate values of Nafion, SPAES, and SPAES/CuPCSA actuators were also observed in the actuation under ac voltage (Supporting Information Figure S3). The ac actuations were examined at 1.5 V at a frequency of 1 Hz. The SPAES/CuPCSA 5% actuator showed the largest strain and fastest response rate. Finally, the durability test for 10,000 cycles in water was performed with Nafion, SPAES, and SPAES/CuPCSA 5% actuator, as shown in Figure 3d. After 10,000 cycles, 99% of the strain of a SPAES/CuPCSA 5% actuator was retained, which was higher retention of displacement than Nafion (about 96%) and SPAES actuators (98%).

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Figure 4. (a) Blocking force vs time plots. (b) Mechanical power density of Nafion and SPAES/CuPCSA actuators. Mechanical power density was calculated from blocking force and displacement rate. (c–e) Arm-wrestling force comparison of SPAES/CuPCSA 5% and Nafion actuator under 5 V dc voltage (c) before actuation, (d) at the moment when both contact each other, and (e) after 20 s.

Figure 4a and 4b show the variations of blocking force and the values of mechanical power density for Nafion-, SPAES- and SPAES/CuPCSA-based actuators. The maximal blocking force for Nafion was observed as 1.7 mN; whereas, SPAES and SPAES/CuPCSA revealed values of 2.16 and 3.14 mN, respectively. The mechanical power density (Pout) can 17



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be estimated from the following equation:

ܲ୭୳୲ = ‫ݒ ୲ܨ‬/ܸ (W mିଷ )

(3)

where Ft and v are the blocking force and the velocity of the actuator under the applied voltage, respectively, and V is the volume of the actuator. It assumes that the load is uniformly distributed over the actuator.2 Actuators based on SPAES and SPAES/CuPCSA revealed much larger mechanical power density values than Nafion-based actuators. SPAES/CuPCSA 5% actuators, especially, exhibited the largest mechanical power density, which was more than six times larger than that of Nafion actuators. Arm wrestling challenge between a SPAES/CuPCSA 5% and a Nafion actuator was performed to visualize the difference in mechanical power density, as shown in Figure 4c–4e (See also Supporting Information

Movie

S4).

SPAES/CuPCSA

5%

actuator

defeats

Nafion

actuator

overwhelmingly due to the much larger mechanical power density. Thus, SPAES/CuPCSA composite membranes revealed much superior actuation performance for bending strain, response rate, blocking force, and mechanical power density in comparison with SPAES and Nafion membranes. The excellent actuation performance of SPAES/CuPCSA actuators can be understood by molecular structure and microstructure of their composite membranes, as shown in Figure 1a–1c. Organic CuPCSA molecules have the molecular size of ca. 2 nm, which is smaller than the size of hydrated hydrophilic ionic clusters in typical polyelectrolyte membranes (ca. 4–5 nm, see Supporting Information Figure S4 for SAXS analysis for the measurement of ionic domain spacing). Considering these dimensions and good miscibility with the matrix, it can be assumed that the CuPCSA molecules with very high IEC degree are molecularly dispersed in the membranes as part of the ionic channels. Therefore, the solvated mobile cations can be quickly transported in the hydrophilic channels of SPAES/CuPCSA membranes; as a result, SPAES/CuPCSA actuators 18


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achieve much larger strain, faster response rate, superior blocking force, and mechanical power density than SPAES and Nafion actuators. 4. CONCLUSIONS Ionic polymer composite membranes consisting of SPAES ionic hydrocarbon polymer and CuPCSA metalorganic filler were fabricated for high-performance ionic bending actuators. The prepared SPAES/CuPCSA actuators exhibited larger ion conductivity, mechanical properties and bending strain, with exceptionally faster response to electrical stimuli and larger mechanical power density than ever reported to the best of our knowledge. These excellent performances are attributed to the fact that the CuPCSA organic fillers with very high sulfonation degree were homogeneously dispersed as part of the ionic channels, enhancing the ion transport of mobile ions in the composite membrane actuators. The outstanding actuation performance of SPAES/CuPCSA actuators makes this class of products attractive for applications in biomimetic devices requiring high power output, like underwater propulsion or medical tools for endoscopic surgery.

ASSOCIATED CONTENT Supporting Information TEM analysis, initial displacement rate, actuation test under ac voltage, SAXS analysis (Figures S1-S4), actuation movies under dc voltage and arm-wrestling force comparison movie (Movies S1-S4). 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]; Phone +82-2-958-5298 (D. H.). *E-mail [email protected]; Phone +82-2-958-6872; Fax +82-2-958-5309 (C.M.K.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by Fundamental R&D Program for Core Technology of Materials and the Industrial Strategic Technology Development Program funded by the Ministry of Trade, Industry and Energy, Republic of Korea. It was also partially funded by Korea Institute of Science and Technology through Young Fellow program. The authors are grateful for the support by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

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