New Ionic Polymer Metal Composite Actuators Based on PVDF/PSSA/PVP Polymer Blend Membrane

Transcription

1 New Ionic Polymer Metal Composite Actuators Based on PVDF/PSSA/PVP Polymer Blend Membrane Varij Panwar, Bong-Sik Kang, Jong-Oh Park, Suk-Ho Park School of Mechanical Systems Engineering, Chonnam National University, Gwangju , Republic of Korea Ionic polymer metal composite (IPMC) actuators that display continuously large actuation displacements without back relaxation and with large blocking force at low direct current (DC) voltages are used as biomimetic sensors, actuators and biomedical devices. This article reports the preparation and actuation performance of new IPMC actuators based on the polyvinylidene fluoride (PVDF)/polystyrene sulfonic acid (PSSA)/ polyvinyl pyrrolidone (PVP) polymer blend membrane, which requires low voltage DC. The performance results of the proposed IPMC actuators are compared with Nafion-based IPMC actuators. In the blend membrane, PVDF is the hydrophobic polymer, PSSA is the polyelectrolyte, and PVP is the hydrophilic basic polymer. The proposed IPMC actuators based on the PVDF/PSSA/PVP blend membrane of polymer mixture ratios of 60/15/25 and 50/25/25 gave higher actuation displacement and higher blocking force at low DC voltages than the Nafion-based IPMC actuator. POLYM. ENG. SCI., 51: , ª 2011 Society of Plastics Engineers INTRODUCTION Ionic polymer metal composite (IPMC) actuators can be fabricated by plating metal particles on an ionexchange polymer membrane. IPMCs have received increasing attention in recent years due to their potential applications in various areas such as biomimetic sensors, actuators, transducers, and artificial muscles and biomedical devices [1, 2]. The most common ion-exchange polymer membrane used in the fabrication of IPMCs is the perfluorinated polymer membrane, e.g., the Nafion series from DuPont. This membrane consists of a large hydrophobic fluorocarbon backbone and short hydrophilic sulfonic pendant side chains. The hydrophobic fluorocarbon Correspondence to: Suk-Ho Park; spark@jnu.ac.kr or Jong-Oh Park; jop@jnu.ac.kr Contract grant sponsor: Korea Ministry of Knowledge Economy (Strategy Technology Development Programs); contract grant number: DOI /pen Published online in Wiley Online Library (wileyonlinelibrary.com). VC 2011 Society of Plastics Engineers backbone of the membrane governs the mechanical strength of the membrane and the short hydrophilic sulfonic pendant side chains provide the ionic groups that interact with water and control the passage of appropriate ions [3, 4]. IPMC actuators can create a large displacement under relatively low direct current (DC) voltage (\5 V) and has good flexibility, which make IPMC actuators attractive for various applications [1, 2]. For practical application, the IPMC produce large actuation displacements and large blocking forces. When a watersaturated cantilevered strip of IPMC in an alkali metal cation form is subjected to a small DC voltage, it undergoes a fast bending deformation toward the anode, due to the movement of the hydrated ions and cations toward the cathode. The bending deformation of the IPMC toward the anode is called actuation displacement. The blocking force of the IPMC actuator depends on the actuation displacement of the IPMC actuator and the stiffness of the membrane. Nafion-based actuators [5 7] initially bend toward the anode, and then bend back toward the cathode (back relaxation) when subjected to DC voltage for long periods due to the leakage of water molecules to the anode side. This is the major drawback of Nafion-based IPMC actuators. Therefore, this problem has to be resolved for various potential applications such as biomimetic sensors, actuators, artificial muscles, and biomedical devices. The other drawbacks of Nafion-based IPMC actuators are the high cost and low blocking force. Recently, a new IPMC actuator that can improve the actuation performance and overcome the drawbacks of the Nafion-based IPMC actuators has been fabricated [8 12]. Han et al. [8] used fluoropolymers grafted with polystyrene sulfonic acid (PSSA) to fabricate a new ionexchange membrane for new IPMC actuators and investigated the actuation of the middle parts of the IPMC actuators and the blocking force of the new IPMC under DC voltage of 2 V. They concluded that the newly prepared IPMCs based on the PSSA-grafted fluoropolymers can produce at least several times larger displacements than the Nafion-based IPMC with a similar thickness, and no back relaxation was found, unlike the Nafion-based POLYMER ENGINEERING AND SCIENCE -2011

2 IPMC, which exhibited back relaxation. The blocking force of the IPMCs based on the PSSA-grafted fluoropolymers was found to be lower than that of Nafion-based IPMCs. They concluded that the larger displacements of the IPMC actuators based on the PSSA-grafted fluoropolymers were due to the higher concentration of sulfonyl groups, larger ion exchange capacity (IEC), and consequent larger volume of moving water than those of the Nafion-based IPMC. Jeon et al. [9] fabricated new IPMC actuators based on sulfonated poly(ether ether ketone) (SPEEK) and SPEEK/polyvinylidene fluoride (PVDF) membranes and actuated both the actuators under DC voltages of 2.5 and 3 V. The harmonic responses of both types of actuators were examined for various sinusoidal voltages and excitation frequencies. They reported that the IPMC actuator based on the SPEEK/PVDF membrane yielded larger actuation than that of IPMC actuator based on the SPEEK membrane under the applied DC voltages, harmonic sinusoidal voltages, and excitation frequencies. At 3 V DC, the blocking force of the SPEEK/PVDF-based IPMC actuator was higher than that of the SPEEK-based IPMC actuator and lower than that of the Nafion-based actuator. They concluded that the good actuation performance of the IPMC actuator based on SPEEK/PVDF membrane may be due to the unique microstructure within the original membrane of SPEEK-incorporated PVDF, such as the hierarchical and striated structure. This structure is formed by the unique hydrophilic nanochannels of the ionic networking membrane between SPEEK and PVDF. They reported that the bending performance of the SPEEK/PVDF actuator is not superior over that of the Nafion-based actuator. Phillips and Moore [10] fabricated new IPMC actuators based on a sulfosuccinic acid-modified ethylene vinyl alcohol copolymer membrane and actuated the new IPMC actuators and the Nafion-based actuators for various square wave exciting signals of 0.1 Hz. The displacement and blocking force of their new IPMC were lower than those of the Nafion-based IPMC. They summarized that the actuation kinetics of the modified ethylene-co-vinyl alcohol (EVOH)-based IPMC actuators were significantly slower than that of the Nafion-based IPMC actuators because of the slow diffusion of water through the disorganized ionomer morphology present in the sulfonated EVOH matrix. Jeong and Kim [11] fabricated new IPMC actuators based on various compositions of perfluoroalkylacrylate (FA) and acryl acid (AA) copolymer (FA/AA ¼ 80/20, 75/25, 70/30, and 65/ 35), which were synthesized by a radical copolymerization of FA and AA. The actuation of the IPMC actuators were checked in air under DC voltage of 8 V and step voltage of 68 V after immersing the actuators in different types of counter cations such as Li þ,na þ, and Li þ. They concluded that the membrane prepared from 75/25 (FA/ AA) gave adequate flexibility and proper hydrated state, which allowed easy deflection toward the direction of the applied voltage. The IPMC actuator based on membrane 80/20 (FA/AA) had higher blocking force than those of the IPMCs based on other compositions due to its higher rigidity. Jun et al. [12] fabricated actuators based on poly(styrene-alt-maleimide) (PSMI)-incorporated PVDF ion exchange membrane and investigated the actuation of the PSMI-PVDF-based actuators under different DC excitation voltages (0.5 to 2 V) and a sinusoidal wave signal of 1.0 V amplitude at 2 Hz. They reported that the PSMI- PVDF based actuators overcame the relaxation of the IPMC actuator under constant voltage and showed larger tip displacement than that of the Nafion-based actuator. The PSMI-PVDF-based actuators yielded larger actuation displacement than the Nafion-based actuators under the sinusoidal wave signal of 1.0 V amplitude at 2 Hz. They concluded that the large tip displacement is attributed due to the large IEC and the unique hydrophilic nanochannels of the PSMI-PVDF membrane. The blocking force of PSMI-PVDF-based actuators was found to be lower than that of Nafion-based actuator under DC excitation of 2 V. The blocking forces of the new actuators proposed by [8 12] were lower than that of Nafion-based actuators, whereas the new actuators proposed by [10] showed back relaxation in actuation, and the new actuators proposed by [11] required high voltage [8 V] for actuation, which could damage the electrode of the IPMC actuators. Therefore, further research is necessary for resolving these issues. In this article, we propose new polymer blend membranes for the fabrication of IPMC actuators. The polymer blend membranes were fabricated by the solvent casting technique. The blend membranes consisted of three polymers: PVDF, PSSA, and PVP, all of which are biocompatible polymers. The PVDF is the hydrophobic polymer consisting of a repeat unit ( CH 2 CF 2 ) with a dipole moment l ¼ cm (2.1 D) associated with the positively charged H-atoms and negatively charged F- atoms [13]. The PSSA is a strong polyelectrolyte, which exhibits high charge-carrier concentration (IEC 4.4 mequiv. g 21 ). The polyvinyl pyrrolidone (PVP) is a basic hydrophilic polymer (polymeric nitrogen-containing bases). In a polar circumstance, such as water, PVP [14] shows more anionic properties as the atom N locates inside the PVP molecule and forms a saturated covalent bond, whereas the atom O locates outside the molecule. Chen and Hong [15] fabricated PVDF/PSSA/PVP blend membranes as the electrolyte in hydrogen fuel cells application. They reported that in the PVDF/PSSA/PVP blend membrane, PVP is miscible with PSSA and PVDF and makes two types of Lewis acid-base pairs in its matrices: PSSA-PVP and PVDF-PVP. They concluded that the acid-base interactions play a role in facilitating proton transport as well as maintaining the mechanical strength of the membrane in hydrated state. Until now, PVDF/ PSSA/PVP blend membranes have not been used for the IPMC actuation purpose. To use a blend membrane for the IPMC actuator, blend membranes should have high water uptake (WUP), high IEC, and proper mechanical strength in dry and hydrated DOI /pen POLYMER ENGINEERING AND SCIENCE

3 FIG. 1. Schematic diagram of fabrication of IPMC. conditions. In this research, the PVDF/PSSA/PVP blend membranes in blend ratios of 60/20/20, 60/15/25, and 50/ 25/25 were fabricated as the base blend membranes for IPMC actuators due to their high WUP, high IEC, and high mechanical strength and adequate flexibility in dry and wet conditions. To fabricate the new IPMC actuator based on the PVDF/PSSA/PVP blend membrane, Pt particles were deposited on the PVDF/PSSA/PVP-based blend membrane by the electroless plating method. The PVDF/PSSA/PVP-based polymer blend membrane and the new IPMC actuators were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) analyses were used to check the compatibility of the polymer mixture of the blend membrane. The dynamic mechanical properties of the membrane were analyzed. Finally, the actuation displacements and blocking forces of the new IPMC actuators were analyzed at a low DC voltage and compared with the results of Nafionbased actuators. MATERIALS AND METHODS Base Polymer Materials PVDF (M w ¼ 534,000) in powder form, and N, N- dimethylformamide (DMF) were purchased from Aldrich. PVP (M n ¼ 360,000) was purchased from IPS Technologies, in powder form. The amount of PSSA (Aldrich, M w ¼75,000, liquid form) used as a polyelectrolyte and IEC of PSSA was determined to be 4.4 mequiv. g 21. Tetraammineplatinum (II) chloride hydrate was purchased from Aldrich and molecular formula of Tetraammineplatinum (II) chloride hydrate is [Pt(NH 3 ) 4 ]Cl 2 3 H 2 O. Sodium borohydride, lithium chloride, hydroxylammonium chloride, hydrochloric acid, and ammonium hydroxide were obtained from DAE-JUNG, Korea, and hydrazine monohydrate was obtained from YAKURI, Japan. Nafion from Dupont with a thickness of 0.18 mm was used as the standard for comparison in this study. Fabrication of Blend Membrane To prepare the blend membrane, the different polymeric components (PVDF, PSSA, and PVP) were dissolved in DMF to form a 10% solution. Polymer blend solutions were prepared by 10% solute concentration with 90% DMF solvent. The membrane was prepared by casting the blend solution of all the three polymers on a Teflon mold and then drying the blend solution in the mold in vacuum at 808C for 48 hr. The average thickness of the blend solution inside the mold was 5 mm. Then, the mold was allowed to cool and membrane was stripped off carefully from the mold and submerged in water for 24 hr to saturate the matrix with water, after which the water content of the membrane was evaluated gravimetrically. To use the blend membrane for the IPMC for actuation, the blend membranes should have WUP, IEC, and high mechanical strength as well as flexibility in dry as well as hydrated conditions. For the dry condition, the membrane is roughened by dry sandblasting to increase the surface area (Fig. 1a) for exchange of protons for Pt ions; therefore, flexibility is essential for a blend membrane in the dry condition. PSSA and PVP are water soluble polymers and both are brittle in dry conditions. To make a flexible membrane, a higher ratio of PVDF was mixed with PSSA and PVP. Therefore, the PVDF/PSSA/ PVP blend membranes in blend ratios of 60/20/20, 60/15/ 25, and 50/25/25 were fabricated as the base blend membranes for the IPMC actuators. If the content of PVDF is decreased, the blend membrane will become brittle in dry 1732 POLYMER ENGINEERING AND SCIENCE DOI /pen

4 conditions. We named these membranes as S 1,S 2, and S 3 with PVDF/PSSA/PVP blend ratios of 60/20/20, 60/15/25, and 50/25/25, respectively. IPMC Fabrication Platinum particles were embedded in the blend membrane by the electroless plating process. This method is shown by the schematic diagram in Fig. 1. We followed the same process as reported in the previous reports [6] and [16, 17]. First, the surface of the dry blend membrane was sandblasted to increase the surface area density in preparation for platinum penetration and reduction and was cleaned ultrasonically and then cleaned chemically [(1) in Fig. 1]. In the second step, the ion exchanging process was carried out. In this process, the blend membrane was immersed in 100 ml solution containing 200 mg tetraammineplatinum (II) chloride hydrate for 8 hr to carry out the Pt ion exchange process [(2) in Fig. 1]. In the third step (initial platinum compositing process), platinum complex cations were reduced to their metallic state in the form of nanoparticles by using reducing agents such as an aqueous solution of sodium borohydride (5%) at temperature of 608C. The Pt black like layers were deposited near the surface of the blend membrane; this process is known as the first plating [(3) in Fig. 1]. In the final step (surface electroding process), Pt was grown effectively on top of the initial Pt surface to reduce the surface resistivity [(4) in Fig. 1]. For this process, a 240 ml aqueous solution containing 400 mg of tetraammineplatinum (II) chloride hydrate was prepared and 1 ml of the 5% ammonium hydroxide solution was added. Then, a 5% aqueous solution of hydroxylamine hydrochloride and a 20% solution of hydrazine monohydrate were prepared, and the polymer blend membrane was immersed in the stirred Pt solution at 408C. Six milliliters of the hydroxylamine hydrochloride solution and 3 ml of the hydrazine solution were added every 30 min. In the sequence of the platinum addition, the temperature was raised up to 608C gradually for 4 hr. After this process, grey metallic layers of Pt were formed on the surface of the blend membrane; this process is known as the second plating. At the end of this process, the blend membrane was rinsed by distilled water and was heated in HCl solution for 30 min and was again rinsed with distilled water. Then, the membrane was immersed in 1.5 N LiCl overnight for the ion exchange process [(5) in Fig. 1]. After immersing in 1.5 N LiCl, samples were stored in deionized water for actuation purpose. Therefore, for making S 1,S 2, and S 3 based-ipmc actuator, all three membranes underwent five cycles of the first plating. After completing five cycles of the first plating, all three samples underwent one cycle of the second plating. With the increase of the number of the first plating, more Pt layers, which are responsible for the actuation of the membrane, were adsorbed in the blend membrane. The Nafion-based IPMC actuators were also been fabricated using the same method and same number of cycles of the first plating and second plating. Measurement and Characterization WUP. WUP is determined by the difference between the weights of the vacuum-dried blend membrane and the fully water-equilibrated membrane. The surface of the blend membrane was quickly wiped with absorbent paper to remove any adhering excess water, and the sample was then weighed. The WUP of the membranes was determined from the following Eq. 1: WUP ¼ W w W d W d (1) where W w and W d denote the weights of the wet and dried membranes, respectively. IEC. For measuring IEC of the prepared membranes, we followed same method as reported Kang et al. [18]. The blend membranes were soaked in a large volume of 1 mol dm 23 NaCl solution to change the blend membranes into Na þ form for 24 hr. They were washed with distilled water to remove excess NaCl on the membrane surface and then equilibrated with exactly 50 ml of 1 mol dm 23 CaCl 2 solutions for 24 hr. The IEC values of all the membranes were evaluated from the quantitative analysis of Na þ ions via ion chromatography (Metrohm, Model No. 861) by the following equation: IECð mequiv: = g Þ¼ C Na þv vol W dry (2) where C Na þ is the concentration of Na þ ions (mmol cm 23 ¼ mequiv. cm 23 ) in the extraction solution (i.e., CaCl 2 solution) determined by IC analysis, V vol the volume of extraction solution (cm 3 ), and W dry is the weight of dry membrane (g). All data were averaged for at least three samples. SEM and EDX. SEM imaging of all the samples was performed using a Hitachi SEM (Model No. S-4700), attached with an EDX. Before the SEM measurement, all the samples were treated by gold sputtering. FTIR. FTIR spectrum was obtained on IR Prestige - 210, Shimadzu, Japan infrared spectroscopy. DSC. A DSC machine (Mettler Toledo DSC-823) was used to investigate the thermal behavior of all the samples at a heating rate of 108C min 21 under nitrogen atmosphere. Surface Resistance. The surface resistance between two points was tested with a digital multimeter, and the distance between the two points was about 1 cm. The DOI /pen POLYMER ENGINEERING AND SCIENCE

5 schematic diagram of the experimental setup for measuring the blocking force of the IPMC actuators. The force probe of the load cell is bonded on the tip of the actuator. The output signal of the load cell indicates the blocking force of the IPMC actuator. Therefore, the output signal of the load cell was obtained using an oscilloscope through the transducer and the blocking force data were recorded directly from the oscilloscope. The blocking force was measured for four specimens of each IPMC actuator. RESULTS AND DISCUSSION FIG. 2. Schematic diagram of the experimental setup for measuring the actuation displacements of the IPMC actuators. average value of the 10 measurements was taken and reported. Dynamic Mechanical Analysis (DMA). The dynamic properties (storage modulus and loss factor) of the blend membranes were measured by a dynamic mechanical analyzer (Instrument: 2980 DMA) using a rectangular specimen (height 15 mm, width 5 mm, and thickness 180 lm) in the tensile mode. Frequency sweep was carried out at room temperature. The storage modulus (E 0 ) and loss factor (tan d) (tan d the ratio of the loss modulus to the storage modulus) were measured in the frequency range of Hz in the dry and hydrated conditions. For the hydrated condition, the membranes were hydrated by immersing them in distilled water for 24 hr before starting measurement. Displacement Measurement of IPMC Actuators. In this article, the displacement measurements of all the actuators were executed in air. Figure 2 shows the schematic diagram of the experimental setup for measuring the actuation displacement of the IPMC actuators. The computer was linked with dspace 1103 data acquisition system, which was attached to the laser vibrometer (Polytec, Model No.-OFV-2510) and the electrode clamp of the IPMC. The displacement of the IPMC was measured using a laser vibrometer. Data was acquired by MAT- LAB, a real-time workshop of dspace, throughout the experiment. The displacement of the IPMC was measured from the middle part of the IPMC, a relatively distant point from the tip, to avoid exceeding the range of the laser vibrometer. Therefore, the laser beam was set on the middle part of the IPMC, where the reflecting tape was attached. For the displacement measurement, specimen samples with dimensions of 40 mm length, 5 mm width and 0.2 mm thickness were prepared. Blocking Force of IPMC Actuators. The blocking force was measured by a load cell (Model No.-TMO-2) based on the transducer technique. Figure 3 shows the SEM and EDX Figure 4a shows the SEM image of the surface part the PVDF membrane. From Fig. 4a, the spherical form of the PVDF particles and some hydrophobic pores of micron size are observed. Figure 4b d shows the SEM images of the cross sections of the S 1,S 2, and S 3 blend membranes. From the Fig. 4b d, it was clear that small and big pores seemed to have formed in the PVDF/PSSA/ PVP blend membranes. Figure 4e shows the cross-sectional image of the Nafion membrane. The Pt particles in the ionic membrane divide into two layers: the deposition layer with a depth of 1 2 lm and the diffusion layer with a depth of 1 20 lm [6, 18]. Figure 5a c depicts the SEM images of the upper part of the cross sections of the S 1 -, S 2 -, and S 3 - based IPMC actuators at magnification, respectively. In the upper part of the cross-sectional images of the S 1 -, S 2 -, and S 3 -based IPMC actuators, thick layers of Pt particles are seen within the surface of the blend membrane up to 1 4 lm, which is called the deposition layer, and below the deposition layer, Pt particles are dispersed in the blend membrane, which is called the diffusion layer. For the surface morphology of IPMC actuator, the SEM image of the surface part of the S 3 -based IPMC actuator was executed at and 350,000 magnification. Figure 5d shows the SEM image of the S 3 -based IPMC actuator at magnification. This image shows that Pt particles completely cover the surface morphology of the S 3 -based polymer blend membrane. From the SEM FIG. 3. Schematic diagram of the experimental setup for measuring the blocking forces of the IPMC actuators POLYMER ENGINEERING AND SCIENCE DOI /pen

6 images of the IPMC, it is clear that Pt particles form a uniform structure after coagulation. Figure 5e shows the SEM image of the S 3 -based IPMC actuator at the higher magnification of 350,000. The higher magnification surface of Pt electrode layers shows the coagulated Pt particles and the granular Pt particles. From the SEM images, the size of the Pt particles was determined to be in the range of nm. Figure 6a shows the EDX analysis of the cross-sectional parts of the SEM images of the S 1 (Fig. 4b), S 2 (Fig. 4c), S 3 (Fig. 4d), and Nafion membranes (Fig. 4e). The EDX spectrum illustrates various big peaks of C at 0.277, O at 0.522, F at 0.677, and S at 2.03 and 2.464, confirming the presence of these particles in the blend membrane. The larger peaks of C, O, and S are observed for S 1,S 2, and S 3 samples than those of the Nafion membrane. The Pt particle distribution in the deposition layer and diffusion layer of the blend membrane was supported by the EDX analysis. Figure 6b d depicts the EDX line scan of the Pt particles in cross-sectional parts of the S 1 - (Fig. 5a), S 2 - (Fig. 5b), and S 3 (Fig. 5c)-based IPMC actuators as a function of the depth of the IPMC actuator. The EDX line scan was carried out to a depth of 1 20 lm from the upper parts of the cross-sectional images of the S 1,S 2, and S 3 -based IPMC actuators. From the EDX line scan of Pt particles, it is clear that the concentration of Pt particles is high, up to a depth of 4 lm, in the blend membrane, which confirms the deposition layer of Pt particles and above the depth of 4 lm in the blend membrane, the concentration of the Pt particles decreases, but it is found up to 20 lm, and this result confirms the diffusion layer of Pt particles in the blend membrane. It was reported [19] that an electromechanical response of an IPMC actuator depends on the base membrane (ionomer) electrodes morphology. In addition, EDX was also performed on the surface area of the S 1 (Fig. 5d) based IPMC actuator, and the result is shown in Fig. 6e. The EDX graph illustrates various big peaks of Pt particles at 1.592, 2.33, 2.05, 9.442, and KeV and some small peaks at 8.268, 9.975, , , and KeV, confirming the penetration of Pt particles in the blend membrane. Various peaks of carbon at 0.277, O at 0.522, F at 0.677, and S at 2.03, are also observed, confirming the presence of these particles in the blend membrane. FIG. 4. SEM images of the (a) surface of PVDF, (b) cross section of S 1, (c) cross section of S 2, (d) cross section of S 3, and (e) cross section of Nafion membrane. Analysis of FTIR The infrared spectra of pure polymer and their blends can be compared to evaluate intermolecular interactions. The miscibility and compatibility of polymers is promoted by intermolecular interactions such as ion ion, ion dipole, dipole dipole, donor acceptor, and hydrogen-bonding interactions [20 22]. Bareck et al. [20] used FTIR analysis to compare the wavenumber of hydroxyl ( OH), carbonyl (C ¼O), sulfonic acid ( SO 3 H), and carboxylic ( COOCH 3 ) groups of poly (acrylic acid)/poly (sodium a) blends to those of pure polymer. They reported that the wavenumber of OH, C ¼O, SO 3 H, COOCH 3 groups in polymer blends shifted from that of the pure polymer due to the hydrogen bonding and FIG. 5. SEM images of the (a) cross section of S 1 -based IPMC, (b) cross section of S 2 -based IPMC, (c) cross section of S 3 -based IPMC, (d) and (e) surface of S 1 -based IPMC actuator at magnification of and 50,0003, respectively. DOI /pen POLYMER ENGINEERING AND SCIENCE

7 FIG. 6. EDX analysis of the (a) cross section of S 1,S 2,S 3, and Nafion membrane, (b) cross section of S 1 - based IPMC for Pt particles, (c) cross section part of S 2 -based IPMC for Pt particles, (d) cross section part of S 3 -based IPMC actuator for Pt particles, (e) surface part of S 1 -based IPMC. dipole-ion interactions in the blends. Daniliuc et al. [21] used FTIR analysis to compare the wavenumber of OH, C ¼O and C O groups of poly(vinyl alcohol)/ poly(acrylic acid) blends to those of pure polymer. They also reported a shift of the wavenumber of OH, C ¼O, and C O groups in polymer blends from that of the pure polymer due to the intermolecular hydrogen bonding in the blends. In PVDF/PSSA/PVP polymer blends, Chen and Hong [15] mentioned that both PSSA and PVDF are miscible with PVP via hydrogen bonding. Also, they reported that PVDF and PVP are miscible via quasihydrogen bonding between the C ¼O groups of PVP and the methyl group of PVDF [23]. To check the compatibility and miscibility of S 1,S 2, and S 3, the FTIR spectra analysis of the OH, C ¼O, and SO 3 H groups for PVDF, PSSA, and PVP, and samples S 1,S 2, and S 3 was carried out. The FTIR spectra of these blends between the wavenumber of 3600 to 3300 cm 21, 1800 to 1600 cm 21, and 1000 to 1300 cm 21 were used to check the vibrating spectra of OH, C ¼O, and SO 3 H groups, respectively. The OH group came from the SO 3 H group of PSSA, and the vibration bands between 1400 to 1700 cm 21 can be attributed to the stretching vibration C ¼O group, which is considered to be the combination of the stretching vibration band of the PVP molecule s C ¼O group and the resonant structure ¼N þ ¼C O 21 [14]. Figure 7a shows FTIR transmittance spectra of PVDF, PSSA, PVP, S 1,S 2, and S 3 samples between the wavenumber of 3600 to 3300 cm 21. From this figure, it was found that the OH band shifted from cm 21 (higher wavenumber) in pure PSSA to cm 21 (lower wavenumber) for S 1,S 2, and S 3 samples, and the vibration peak of the OH band broadened and increased in its intensity for S 1,S 2, and S 3 blends from those of pure polymer, which indicates the formation of hydrogen bonding [22] between PSSA and PVP. Figure 7b shows FTIR transmittance spectra of PVDF, PSSA, PVP, S 1,S 2, and S 3 samples between the wavenumber of 1800 to 1600 cm 21. This figure shows that the C ¼O 1736 POLYMER ENGINEERING AND SCIENCE DOI /pen

8 band shifted from cm 21 (higher wavenumber) in the pure PVP to for S 1 sample (lower wavenumber) and cm 21 (higher wavenumber) for S 2 and S 3 samples, and the vibration peak of the C ¼O groups broadened and increased in its intensity for S 1,S 2, and S 3 blends from those of pure polymer, which indicates the formation of the hydrogen bond between PVDF-PVP [23] and PSSA- PVP. The shifting of the vibration spectra of the OH and C ¼O bands and the broadening and increases of their intensities for the samples S 1,S 2, and S 3 in comparison to those of pure polymer, respectively, indicates the compatibility and miscibility of the polymer blends [20 22]. Figure 7c shows FTIR transmittance spectra of PVDF, PSSA, PVP, S 1,S 2, and S 3 samples between the wavenumber of 1000 and 1300 cm 21, which were associated with the sulfonated benzene group [24]. From the data, it was found that the peaks near to and cm 21 were attributed to the symmetric and asymmetric vibration of the SO 3 H group for pure PSSA, and the peak near to 1131 cm 21 to the plane skeleton stretching vibration of the substituted benzene ring with strong participation from the SO 3 H group [20, 25]. The intensity of the peaks for the SO 3 H group changed for the PVDF/PSSA/PVP blends in comparison to those of pure PSSA, which might be due to the interaction of SO 3 H of PSSA with the pyrrolidone group of PVP in which the tertiary amide forms positive charge after being protonized. Therefore, the intermolecular proton donor acceptor interaction between PSSA and PVP indicates the miscibility and compatibility of polymer blends. It was reported [26, 27] that the formation and properties of the acid-base membrane are based on the interaction between the sulfonic group acids and the N-bases which may lead to the formation of hydrogen bonds or the protonation of the basic N-sites. In conclusion, from the FTIR analysis of the PVDF/PSSA/PVP blend membranes, it can be concluded that PVDF and PSSA are miscible with PVP. FIG. 7. FTIR for PVDF, PSSA, PVP, S 1,S 2, and S 3 blends between the wavenumber of (a) 3600 to 3300 cm 21, (b) 1800 to 1600 cm 21, and (c) 1000 to 1300 cm 21. Analysis of DSC Here, the compatibility of samples S 1,S 2, and S 3 were checked by DSC. The decreases of the melting temperature of PVDF in a polymer blend membrane indicate the compatibility of the polymers of the blend [28, 29]. We also checked the melting temperatures of pure polymer and blends. Figure 8a shows the DSC graphs for PVDF, and Fig. 8b shows the DSC graphs for S 1,S 2, and S 3 between 25 and 2008C. From Fig. 6a, a big endothermic drop was observed at 1668C, which is the melting temperature of PVDF. With the addition of PSSA and PVP to the PVDF, one big endothermic drop was obtained for the S 1, S 2, and S 3 samples. The big endothermic drop was observed in all the samples below the melting temperature of pure PVDF. This shows the decreases of the melting temperature of PVDF with the addition of PSSA and PVP. Therefore, the melting temperature of PVDF was lowered in S 1,S 2, and S 3 blend membranes, and this lowering indicated the intermolecular interaction between PVDF with PSSA and PVP. These data suggest that PVDF, PSSA, and PVP can be mixed together with good compatibility. For PVDF/PSSA/PVP, below 1008C, a small endothermic drop was observed below 1008C, possibly due to the hygroscopic property of the blend membranes [30, 31], which forces the membranes to absorb water. Analysis of WUP and IEC The WUP and IEC of S 1,S 2, and S 3 blend membranes and Nafion membrane are given Table 1. The WUPs and IECs of S 1,S 2, and S 3 are higher than those of Nafion. DOI /pen POLYMER ENGINEERING AND SCIENCE

9 FIG. 8. DSC graphs of (a) PVDF and (b) S 1,S 2, and S 3 samples. The IEC and WUP of the S 3 blend membrane are higher than those of the S 1 and S 2 blend membranes may be due to the high quantities of PSSA and PVP in the S 3 membrane. The high IEC of the PVDF/PSSA/PVP blend membranes might be due to the high charge carrier concentration of PSSA (IEC 4.4 mequiv. g 21 ). In addition, the high WUP of the PVDF/PSSA/PVP might be due to the hydrophilic nature of PSSA and PVP. In addition, our proposed membranes for the IPMC have higher WUP than those of the new ion exchange membranes proposed by other research groups [9 12], excluding the membranes proposed by Han et al. [8], which gave higher WUP. Our proposed membranes for the IPMC have higher IEC than that of the membrane proposed by [10], and lower than those of the membranes proposed by [8, 9] and [12]. The base blend membrane for the IPMC needs to have high WUP and IEC to use this type of IPMC as an actuator. With high WUP of the blend membrane, more hydrated cations can move through the membrane to actuate the IPMC. With high IEC, Pt particles can be more easily embedded in the pores of the blend membrane by the electroless plating technique. Therefore, due to their high WUP and high IEC, the S 1,S 2, and S 3 blend membranes were utilized as the base blend membranes for IPMC actuators. Surface Resistance of IPMC The surface resistance of IPMC actuators is shown in Table 2. The surface resistances of the S 1, S 2, S 3, and Nafion-based IPMC actuators, which underwent the first Pt plating five times and the second plating one time, were measured to be 3.0, 2.0, 1.5, and 2.5 O, respectively. It should be mentioned that with increasing number of cycles of the first Pt plating, the surface resistance decreased. After the third cycle of the first Pt plating, the surface resistances of S 1,S 2, and Nafion were 20, 15, 7, and 10 O, respectively. Low surface resistance was necessary to actuate the IPMC actuators. Dynamic Analysis of Mechanical Properties For the dynamic analysis of the mechanical strength of the polymer blend membranes, E 0 and tan d of S 1,S 2, and S 3 blend membranes and Nafion membrane were analyzed as a function of frequency in a dry condition as well as a hydrated condition. The E 0 is the measure of the elasticity of a material and tan d is the measure of the damping ability of a material. The E 0, as a function of the frequency, of the S 1, S 2, and S 3 blend membranes and Nafion membrane are shown in Tables 3 and 4 in dry and hydrated conditions. In the dry condition, the E 0 of the S 1, S 2, and S 3 blend membranes were higher than those of the Nafion membrane with respect to frequency. Similarly, in the hydrated condition, the values of E 0 of the S 1, S 2, and S 3 blend membranes were higher than those of the Nafion membrane with respect to frequency. The S 1, S 2, and S 3 blend membranes were utilized as the base blend membranes for the IPMC actuators due to high mechanical strength and adequate flexibility in dry and wet conditions. It was reported [11] that the IPMC actuator displayed easy actuation due to the flexibility and proper hydrated state of the membrane. The values of E 0 of all the membranes were found to be higher in the dry condition than in the hydration condition. In the hydration condition, the E 0 of the all the mem- TABLE 1. Basic properties of polymer blend membrane. Membranes Ratio of PVDF Ratio of PSSA Ratio of PVP Thickness (mm) IEC (meq./g) WUP PVDF/PSSA/PVP (S 1 ) PVDF/PSSA/PVP (S 2 ) PVDF/PSSA/PVP (S 3 ) Nafion POLYMER ENGINEERING AND SCIENCE DOI /pen

10 IPMC TABLE 2. Surface resistance of IPMC actuators. Surface resistance (O) PVDF/PSSA/PVP (S 1 ) 3.0 PVDF/PSSA/PVP (S 2 ) 2.0 PVDF/PSSA/PVP (S 3 ) 1.5 Nafion 2.5 branes diminished by the disconnection of the polymer chains, the volume change of the base material, and the plasticizer effect of water [32]. The improvement in the mechanical stability of the S 1, S 2, and S 3 blend membranes (PVDF/PSSA/PVP blend membranes) [33 36] can be attributed to the entanglement of these acid-base polymers and possible mixing due to a specific interaction (polyelectrolyte complexes), e.g., proton transfer (Acidic polymer to basic polymer) for PVDF/PSSA/PVP blend membranes of various blend ratios. The tan d, as a function of frequency of S 1,S 2, and S 3 blend membranes of various blend ratios and Nafion membrane are shown in Tables 3 and 4 in dry and hydrated conditions. In the dry condition, the tan d of all the samples decreased continuously with frequency except at the high frequency, and the highest value of tan d was obtained for Nafion membrane at 100 Hz. In the hydrated condition, the tan d of all the samples decreased continuously with frequency up to 50 Hz after its value increased, and the highest value of tan d was obtained for the Nafion membrane. The values of tan d of all the membranes were found to be higher in the hydrated condition than in the dry condition. Analysis of Actuation Displacement of IPMC Actuators Figure 9a shows the actuation displacements of the S 1, S 2,S 3, and Nafion-based IPMC actuators as a function of time at 2.5 V DC voltages. The S 2,S 3, and Nafion-based IPMC actuator immediately actuated toward the anode side and S 1 -based actuators actuated slowly toward the anode at 2.5 V DC voltages due to the movement of the Li and hydrated cations toward the cathode. After 10 sec, the Nafion-based IPMC actuator started to slowly relax back toward the cathode, whereas the S 1 -, S 2 -, and S 3 - based actuators actuated toward the anode at 2.5 V DC voltages without back relaxation. The Nafion-based IPMC TABLE 3. Storage modulus and loss factor of blend membranes and Nafion membrane as a function of frequency in dry condition. Frequency (dry condition) Storage modulus (MPa) Loss factor S 1 S 2 S 3 Nafion S 1 S 2 S 3 Nafion actuator depicted back relaxation toward the cathode due to the lower IEC and lower WUP of the Nafion membrane. The S 3 -based IPMC actuator attained the highest value of actuation displacement with time among the four types of actuators. Figure 9b shows the actuation displacement of the S 1 -, S 2 -, S 3 -, and Nafion-based IPMC actuators as a function of time at 3.0 V DC voltages. At 3.0 V DC voltages, the S 1 -, S 2 -, S 3 -, and Nafion-based IPMC actuators depict fast actuation displacement in the first 20 sec. After 20 sec, the Nafion-based IPMC actuator shows back relaxation toward the cathode side. The actuation displacement of S 1 -, S 2 -, and S 3 -based IPMC actuators increased with time and did not show back relaxation. The actuation displacements of S 2,S 3, and, Nafion-based IPMC actuators were found to be larger than that of the S 1 -based actuator. At 3.0 V DC voltages, the S 2 -based IPMC actuator showed the best actuation performance among the four actuators: large and fast actuation displacement without back relaxation. The large actuations of S 2 - and S 3 -based IPMC actuators might be due to the higher IEC and higher WUP of the blend membranes. It was reported [11, 12] that large actuation of IPMC actuators depends on high IEC and high WUP. However, our IPMC actuators showed some drawbacks such as slow response time of IPMC at lower voltages (\2.5 V DC voltages). Therefore, research to overcome the given drawbacks is underway. The comparison of the actuation displacement results of S 1,S 2, and S 3 -based IPMC actuators showed that the S 2 and S 3 -based IPMC actuators gave larger actuation displacements with time than the S 1 -based IPMC actuator. This might be due to higher WUP of the S 2 and S 3 samples, which provide the favorable condition of continuous movement of Li and hydrated cations toward the cathode side, and therefore, the S 2 - and S 3 -based IPMC actuators continuously actuated with more speed. The higher WUP of the S 2 - and S 3 -based actuators may be due to the high quantity of PVP, which makes the hydrophilic network in the membrane with PSSA and PVDF, and this hydrated hydrophilic network allows for the fast exchange between the mobile counter ions (sulfonic acid groups of PSSA) and O or N sites of PVP via ion complexes [15]. Therefore, the new IPMC actuator based on PVDF/PSSA/PVP blend membrane of polymer ratios of 60/15/25 and 50/25/ 25 had the largest actuation displacement at 2.5 and 3.0 V DC voltages, respectively, with time among the four TABLE 4. Storage modulus and loss factor of blend membranes and Nafion membrane as a function of frequency in hydrated condition. Frequency (hydrated condition) Storage modulus (MPa) Loss factor S 1 S 2 S 3 Nafion S 1 S 2 S 3 Nafion DOI /pen POLYMER ENGINEERING AND SCIENCE

11 FIG. 9. DC. Actuation displacements of S 1 -, S 2 -, S 3 -, and Nafion-based IPMC at (a) 2.5 V DC and (b) 3.0 V types of actuators. In addition, our new IPMC actuators based on PVDF/PSSA/PVP blend membranes yielded larger actuation displacement than the IPMC actuators proposed by Jeon et al. [9] at 2.5 and 3.0 V DC voltages. The repeatability of the actuation displacements of the new PVDF/PSSA/PVP IPMC actuators was checked several times, and was confirmed. For further actuation uses, the samples were stored in deionized water. Analysis of Blocking Force of IPMC Actuators The blocking force was measured with a load cell at the tip of the IPMC beam. The blocking forces of the S 1 -, S 2 -, S 3 -, and Nafion-based IPMC actuators at 2.5 and 3.0 V DC voltages are given in Table 5. The blocking forces of the S 2 - and S 3 -based IPMC actuators were higher than those of the S 1 - and Nafion-based IPMC actuators. At 2.5 V DC, the blocking force of the S 3 based IPMC actuator was the highest, whereas at 3.0 V DC, the blocking force of the S 2 -based IPMC actuator was the highest; these results may be due to the higher mechanical strength and larger displacement of the S 2 and S 3 -based IPMC actuators than those of S 1 - and Nafion-based IPMC actuators. It should be mentioned that the blocking force of IPMC actuators depends on the actuator s stiffness and displacement. The S 1 -based IPMC actuator had a lower blocking force than the S 2 -, S 3 -, and Nafion-based actuators. This may have been due to the lower displacement of the S 1 -based IPMC actuator. Consequently, the new IPMC actuators based on TABLE 5. Blocking forces of S 1,S 2,S 3 and Nafion-based IPMC at 2.5 and 3 V DC voltages. IPMC Blocking force at 2.5 V (gf) Blocking force at 3 V (gf) PVDF/PSSA/PVP (S 1 ) PVDF/PSSA/PVP (S 2 ) PVDF/PSSA/PVP (S 3 ) Nafion the PVDF/PSSA/PVP blend membranes of polymer ratios of 60/15/25 and 50/25/25 had a higher blocking force than the Nafion-based actuator, whereas the blocking force of the actuators proposed by [8 12] was found to be lower than that of the Nafion-based actuator. CONCLUSION In this article, we proposed a PVDF/PSSA/PVP-based new blend membrane for ionic polymer metal composite actuators. The proposed blend membrane gave higher IEC, higher WUP capacity and higher storage modulus than the Nafion membrane. For the fabrication of IPMCs, the Pt particles were embedded into the blend membrane by the electroless plating method. The morphology of the blend membrane and IPMC were characterized by SEM images. The miscibility and compatibility of the blend membrane were verified by FTIR and DSC. The IPMC actuators based on the PVDF/PSSA/PVP blend membranes of polymer ratio of 60/15/25 and 50/25/25 had larger actuation displacement and blocking force at 2.5 and 3.0 V DC voltages than the Nafion-based actuator. The large actuation displacement of the PVDF/PSSA/ PVP-based IPMC actuator was due to the high IEC and high WUP of the blend membrane. The PVDF/PSSA/ PVP-based IPMC actuators did not show back relaxation, whereas the Nafion-based actuator showed back relaxation. The blocking forces of the PVDF/PSSA/PVP-based IPMC actuators were higher than that of the Nafion-based IPMC actuator; this higher blocking force is the advantage of our proposed actuators over the actuators proposed by [8 12]. Therefore, the PVDF/PSSA/PVP-based IPMC actuator with these characteristics can be utilized as biomimetic sensors, actuators, transducers, artificial muscles, and biomedical devices. REFERENCES 1. M. Shahinpoor, Y. Bar-Cohen, T. Xue, J.O. Simpson, and J. Smith, Ionic Polymer-Metal Composites (IPMC) as Biomi POLYMER ENGINEERING AND SCIENCE DOI /pen

12 metic Sensors and Actuators Proceedings of SPIE s 5th Annual International Symposium on Smart Structures and Materials, San Diego, CA, 3324 (1998). 2. M. Shahinpoor and K.J. Kim, Smart Mater. Struct., 14, 197 (2005). 3. M. Shahinpoor, K.J. Kim, and D.J. Leo, Polym. Compos., 24, 24 (2003). 4. K.J. Kim and M. Shahinpoor, Smart Mater. Struct., 12, 65 (2003). 5. M. Shahinpoor and K.J. Kim, Smart Mater. Struct., 13, 1362 (2004). 6. M. Shahinpoor and K.J. Kim, Smart Mater. Struct., 14, 197 (2005). 7. M. Shahinpoor and K.J. Kim, Smart Mater. Struct., 10, 819 (2001). 8. M.J. Han, J.H. Park, J.Y. Lee, and J.Y. Jho, Macromol. Rapid. Commun., 27, 219 (2006). 9. J.H. Jeon, S.P. Kang, S. Lee, and I.K. Oh, Sen. Actuator B, 143, 357 (2009). 10. A.K. Phillips and R.B. Moore, Polymer, 46, 7788 (2005). 11. H.Y. Jeong and B.K. Kim, J. Appl. Polym. Sci., 99, 2687 (2006). 12. J. Lu, S.G. Kim, S. Lee, and I.K. Oh, Adv. Funct. Mater., 18, 1290 (2008). 13. T. Furukawa, Adv. Colloid Interface Sci., 183, 71 (1997). 14. X. Sui, Y. Liu, C. Shao, Y. Liu, and C. Xu, Chem. Phys. Lett., 424, 340 (2006). 15. N. Chen and L. Hong, Solid State Ionics, 146, 377 (2002). 16. D.Y. Lee, I.S. Park, M.H. Lee, K.J. Kim, and S. Heo, Sens. Actuators A, 133, 117 (2007). 17. M. Shahinpoor and K.J. Kim, Smart Mater. Struct., 9, 543 (2000). 18. M.S. Kang, J. H. Kim, J. Won, S.H. Moon, and Y.S. Kang, J. Memb. Sci., 247, 127 (2005). 19. S.N. Nasser and W. Yongxian, J. Appl. Phys., 93, 5255 (2003). 20. C.O.M. Bareck, Q.T. Nguyen, M. Metayer, J.M. Saiter, and M.R. Garda, Polymer, 45, 4181 (2004). 21. L. Daniliuc, C.D. Kesel, and C. David, Eur. Polym. J., 28, 1365 (1992). 22. Y. He, B. Zhu, and Y. Inoue, Prog. Polym. Sci., 29, 1021 (2004). 23. N. Chen and L. Hong, Polymer, 43, 1429 (2002). 24. D. Dieter and O. Hummel, Atlas of Polymer Analysis, VCH, Weinheim, 42 (1988). 25. J.M. Amarilla, R.M. Rojas, J.M. Rojo, M.J. Cubillo, A. Linares, and J.L. Acosta, Solid State Ionics, 127, 133 (2000). 26. H.L. Wu, C.C.M. Ma, H.C. Kuan, C.H. Wang, C.Y. Chen, and C.L. Chiang, J. Polym. Sci. Part B: Polym. Phys., 44, 565 (2006). 27. W. Jang, S. Sundar, S. Choi, Y.G. Shul, and H. Han, J. Memb. Sci., 280, 321 (2006). 28. S. Xue and G. Yin, Polymer, 47, 5044 (2006). 29. G. Bauduin, B. Boutevin, P. Gramain, and A. Malinova, Eur. Polym. J., 35, 285 (1999). 30. Y.W. Kim, J.K. Choi, J.T. Park, and J.H. Kim, J. Memb. Sci., 313, 315 (2008). 31. L.F. Malmonge and L.H.C. Mattoso, Polymer, 41, 8387 (2000). 32. I.S. Park, S.M. Kim, and K.J. Kim, Smart Mater. Struct., 16, 1090 (2007). 33. B. Smitha, S. Sridhar, and A.A. Khan, Macromolecules, 37, 2233 (2004). 34. J. Kerres, A. Ullrich, F. Meier, and T. Haring, Solid State Ionics, 125, 243 (1999). 35. J. Efthimiadis, G. Annat, J. Efthimiadis, D.R. Macfarlane, and M. Forsyth, Solid State Ionics, 177, 95 (2006). 36. L. Wang, B.L. Yi, H.M. Zhang, and D.M. Xing, J. Phys. Chem. B, 112, 4270 (2008). DOI /pen POLYMER ENGINEERING AND SCIENCE