Dynamic mechanical, electrical, and actuation properties of ionic polymer metalcomposites using PVDF/PVP/PSSA blend membranes

Varij Panwar, Cheong Lee, Seong Young Ko, Jong-Oh Park*, Sukho Park*

School of Mechanical Systems Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea

h i g h l i g h t s

< PVDF/PVP/PSSA based IPMC depicted higher tip displacement and higher blocking force.< Under DC voltage, PVDF/PVP/PSSA based IPMC was actuated without back-relaxation.< PVDF/PVP/PSSA based IPMC showed higher actuation bandwidth and lower current density.< PVDF/PVP/PSSA (the ratio of 60/15/25) based IPMC showed higher storage modulus.

a r t i c l e i n f o

Article history:Received 14 December 2011Received in revised form9 May 2012Accepted 31 May 2012

Keywords:IPMCStorage modulusCurrent densityTip displacementBlocking force

a b s t r a c t

This paper reports the dynamic mechanical, electrical, and actuation properties of ionic polymer metalcomposite (IPMC) actuators that use polyvinylidene fluoride (PVDF)/polyvinyl pyrrolidone (PVP)/poly-styrene sulfonic acid (PSSA) blend membranes. In this research, two new IPMC actuators using PVDF/PVP/PSSA blend membranes in the blend ratios of 60/15/25 and 40/30/30 were fabricated. All of thePVDF/PVP/PSSA-based IPMC actuator properties were compared with Nafion-based IPMC. The storagemoduli of the IPMC using 60/15/25 blend membrane were higher than those of the IPMCs using the 40/30/30 blend membrane and the Nafion membrane. The current density of the IPMCs increased withincreasing alternating current (AC) voltage and increasing frequencies. The current density of the Nafion-based IPMC was found to be higher than those of the PVDF/PVP/PSSA-based IPMCs. The actuationperformances of the IPMCs were tested under direct current (DC) voltages of 1e3 V and AC voltages of2e4 V. Under DC voltage, the PVDF/PVP/PSSA-based IPMC was actuated toward the anode side withoutback-relaxation, while the Nafion-based IPMC depicted back-relaxation. The blocking forces of all theIPMCs were evaluated as a function of time under DC voltages of 2 and 3 V. The tip displacements of theIPMCs increased with increasing AC voltages and decreased with increasing frequency. The bandwidth ofthe PVDF/PVP/PSSA-based IPMC was slightly higher than that of the Nafion-based IPMC.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Electro-active polymers (EAP) comprise an emerging class ofactuation materials that can be used as biomimetic sensors andactuators in medical devices, engineering devices, and roboticsapplications. EAP actuators can be divided into two major cate-gories, ionic EAP and electronic EAP actuators, based on theiractivationmechanism. Ionic EAP actuators include polymer gels [1],ionic polymer metal composites (IPMCs) [2e9], conducting poly-mer actuators [10], and carbon nanotubes [11], all of which exhibitshape or volume changes in response to low voltage. Electronic EAPactuators include dielectric, electrostrictive, electrostatic, piezo-electric, and ferroelectric EAP actuators [12e14], which shrink andexpand in response to applied voltage. These EAPs require high

voltage (1 kV) at very low currents for operation and can be oper-ated in air without major constraints.

Among EAPs, IPMCs are considered most suitable for artificialmuscles since they exhibit a large bending displacement under lowapplied voltage and have biocompatibility [15e17], flexibility, andlightweight properties. An IPMC consists of an ion-exchangemembrane sandwiched between two noble metal plates [5]. Inpractical application, the IPMC produces large actuation and largeblocking forces. When a voltage is applied to an IPMC, the IPMCstarts bending towards the anode direction due to themovement ofthe cations along with water molecules towards the cathodedirection [18]. The blocking force of the IPMC depends on theactuation of the IPMC and stiffness of the IPMC membrane.

Perfluorinated polymer membranes, e.g., Nafion, are the mostcommon ion exchange polymer membranes used in the fabricationof IPMCs. However, Nafion is a fluorinated polymer, which is notvery environmentally friendly and very costly for practical use [19].

* Corresponding authors.E-mail addresses: jop@jnu.ac.kr (J.-O. Park), spark@jnu.ac.kr (S. Park).

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Materials Chemistry and Physics 135 (2012) 928e937

Although Nafion-based IPMC actuators have shown good actuationperformance, they have problems such as the “straightening-back”phenomenon under a DC electric field [5e7].

Over the past few years, some research groups have proposeda cheaper and more ecologically acceptable ion-exchange polymermembrane to replace Nafion polymer membranes [7,9,20e24]. Hanet al. [7] fabricated polystyrene sulfonic acid (PSSA) grafted fluo-ropolymers-based IPMCs. They reported that the actuationdisplacements and blocking forces of the PSSA/fluoropolymers-based IPMCs were found to be larger and lower than the Nafion-based IPMC, respectively. They summarized that the larger actuationdisplacements of the PSSA/fluoropolymers-based IPMCswere due tohigher water uptake (WUP) and higher high ion-exchange capacity(IEC) of the PSSA/fluoropolymers membrane than the Nafionmembrane. In addition, they concluded that the lower blockingforces of the PSSA/fluoropolymers-based IPMCs were due to thelower stiffness of the PSSA/fluoropolymers membranes than theNafionmembrane. They reported that therewas a trade-off betweenactuation displacements and blocking forces in terms ofWUP. Jun etal. [22] fabricated poly(styrene-alt-maleimide) (PSMI)/PVDF-basedIPMCs. They reported that the actuation displacements and blockingforces of the PSMI/PVDF-based IPMCs were larger and lower thanthe Nafion-based IPMC, respectively. They concluded that the largeractuation displacements of the PSMI/PVDF-based IPMCswere due tohigher WUP and higher IEC of the PSMI/PVDF membrane than theNafion membrane. In addition, the lower blocking forces of thePSMI/PVDF-based IPMCswere due to the lower stiffness of the PSMI/PVDF membrane than the Nafion membrane.

Dai et al. [24] fabricated and actuated an IPMC based on poly-vinyl alcohol/poly(2-acrylamido-2-methyl-1-propanesulfonic acid)ion-exchange membrane. Previous IPMC actuators have shownlower mechanical strengths and lower blocking forces than theNafion-based actuators [7,9,20e24]. In addition, some actuatorshave depicted lower actuation than the Nafion-based actuators[9,20]. Therefore, ongoing research has attempted to identify a newion-exchange membrane for IPMCs.

In this paper, we try to findnew ion-exchangemembranes (PVDF/PVP/PSSA blend membranes) for IPMCs and analyze the followingpropertiesofPVDF/PVP/PSSA-based IPMCs. First, the storagemodulusand loss factors of IPMCs have been analyzed with respect tofrequency. Second, the current density of PVDF/PVP/PSSA-basedIPMCs has been analyzed under AC voltages of 2e4 V at 0.1e5 Hz. Inaddition, the cyclic voltammetry analysis of the proposed IPMCs hasalso been reported. Third, the tip displacements of IPMCs using PVDF/PVP/PSSA (60/15/25 and 40/30/30) blend membranes have beenanalyzedunder AC voltages of 2e4V at 0.1e5Hz. Finally, the blockingforces of IPMCs have been analyzed as a function of time under DCvoltages of 2 and 3 V. The different compositions used for makingIPMC in the current studywere comparedwithearlier studies [18,25].The PVDF in the blend membranes is a hydrophobic polymer [25].PSSA is a strong polyelectrolyte, while PVP is a basic hydrophilicpolymer [25]. The IPMCs were characterized by scanning electronmicroscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX),and the blend membranes were characterized by SEM, EDX, DMA(storage modulus and loss factor) and Fourier transform infraredspectroscopy (FTIR). In addition, the WUP, IEC, and proton conduc-tivity of the membranes were also analyzed.

2. Materials and methods

2.1. Fabrication of blend membranes and IPMCs

The details of the materials, blend membrane fabrication, andIPMCwere reported in an earlier paper [25]. In this paper, the PVDF/PVP/PSSA blend membranes in blend ratios of 60/15/25 and 40/30/

30 (named S1 and S2, respectively)were fabricated as the base blendmembranes for the IPMCs. The S1 and S2 blend membranes had thesame thickness of 0.18 mm as the Nafion membrane, and the blendsolutions of 20 mL and 25 mL were used, respectively. The PVDF/PVP/PSSA- and Nafion-based IPMCs were fabricated using an elec-troless plating procedure with four first-plating cycles and onesecond-plating [25]. The IPMC samples were immersed in 1.5 N LiClfor 48 h to facilitate the ion exchange process and then stored indeionizedwater for at least 48 h prior to the actuation experiments.

2.2. Measurement and characterization

For the characterization of the blend membranes and thefabricated actuators, various material properties were measuredand analyzed as follows:

� The IECs of the prepared membranes were determined usingan ion chromatography method as reported elsewhere [25,26].Three specimens each of the S1, S2, and Nafion membraneswere used for the IEC test.

� The proton conductivities of the hydrated membranes weremeasured using a complex impedance analyzer (1252 A) con-nected with a Solartron SI 1287 electrochemical interface overa frequency range of 1e100 kHz. The 180-mm-thick PVDF/PVP/PSSA membrane was sandwiched between two stainless steelelectrodes, towhich anACperturbation of 10mVwas applied. Themembranes were immersed in deionized water for 24 h beforetheir proton conductivities were measured. The proton conduc-tivity (s) was measured at room temperature using Eq. (1):

s ¼ LR� A

; (1)

where s is the proton conductivity (S cm�1), L is the measuredthickness of the polymer membrane (cm), R is the measuredimpedance of the membrane (U), and A is the cross-sectional areaof the membrane (cm2). Three specimens each of the S1, S2, andNafion membranes were used for the proton conductivity test.

� TheWUPwas determined by taking the difference between theweights of fully water-equilibrated membranes and vacuum-dried blend membranes. The membranes were first vacuum-dried at 80 �C for 24 h and then soaked for >24 h in deion-ized water. The surface of each blend membrane was quicklywiped with absorbent paper to remove any excess water, andthe fully hydrated membranes were immediately weighed. Themembranes were then dried overnight in a vacuum at 60 �C,and the weights of the dried membranes were then measured.The WUP of the membranes was determined using Eq. (2):

WUP ¼ Ww �WdWd

; (2)

where Ww and Wd denote the weights of the wet and drymembranes, respectively. Three specimens each of the S1, S2, andNafion membranes were used for the WUP test.

� SEM imaging of the blend membranes and the IPMCs wasperformed using a Hitachi SEM (Model No. S-4700) with anattached EDX. Before the SEM measurement, all of the sampleswere treated with gold sputtering. Two specimens of eachmembrane were used for the SEM analysis.

� The pure and blendmembraneswere examined by FTIR spectrato characterize the molecular structure after the chemicalmodifications. The FTIR spectra were obtained using an IR

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937 929

Prestige-21 (Shimadzu, Japan) using transmittance mode and2 cm�1 resolution. All of the FTIR specimens were prepared inKBr (Potassium Bromide). Two blend solution specimens ofeach membrane were used for the FTIR analysis.

� The surface resistance between two points at a distance ofabout 1 cm was tested using a digital multimeter. The averagevalue of ten measurements was taken and reported. Threespecimens each of the S1, S2 and Nafion membranes were usedfor the surface resistance test.

� The dynamic mechanical properties (storage modulus and lossfactor) of the blend membranes and the IPMC were measuredusing a dynamic mechanical analyzer (DMA 2980, TA Instru-ments, DE) in tensile mode. The rectangular specimens (length,15 mm; width, 5 mm; thickness, 0.18 mm) were used formeasuring the dynamic mechanical properties. The frequencysweep was conducted at room temperature. For the hydratedcondition, the membranes and actuators were hydrated byimmersion in distilled water for 24 h before the measurement.Two specimens of each membrane and IPMC were used for thedynamical mechanical properties test.

2.3. Actuation displacement measurements of IPMC

In this study, the tip displacements of the IPMC samples weremeasured in air and defined as the actuation of the actuator. Fig. 1shows the schematic diagram of the experimental setup formeasuring the IPMC actuation. A computer was linked to a dSPACE1103 data acquisition system attached to a laser vibrometer (ModelOFV-2510, Polytec, DE) and the IPMC’s electrode clamp. The IPMCdisplacement was measured using the laser vibrometer. Data wereacquired using MATLAB and a real-time workshop of dSPACEthroughout the experiment. The IPMC actuation was measuredfrom the IPMC tip. Therefore, the laser beam was set on the IPMCtip, to which a reflecting tape was attached. For the actuationmeasurements, 40-mm-long, 5-mm-wide, and 0.18-mm-thicksamples were prepared. Three specimens of each IPMC were usedfor the actuation displacement test.

2.4. Blocking force measurement of IPMC

The blocking force was measured by a load cell (Model TMO-2,Transduce Techniques, USA) based on the transducer technique. Theforce probe of the load cell was bonded to the IPMC tip. The blocking

force of the IPMC was indicated by the output signal of the load cell,which was obtained using an oscilloscope through a transducer. Theblocking force data were recorded directly from the oscilloscope.Three specimens of each IPMCwere used for the blocking force test.

2.5. Current measurement of IPMC under AC voltages

Fig. 2 shows a schematic diagram of the experimental setup formeasuring the IPMC actuator current. Sinusoidal wave inputs withvarious driving voltages and frequencies were applied to the IPMCsamples by a single-channel arbitrary function generator (Tektronix3251). The IPMC voltage and current values were evaluated usinga passive probe (P6139 A) and a current probe (TCP 0030 Active) ofa Digital Phosphor oscilloscope (Tektronix DPO 4104), respectively.Two specimens of each IPMC were used for the current measure-ment under AC voltage test.

2.6. Cyclic voltammetry of IPMC

Cyclic voltammetry (CV) experiments were executed for evalu-ation of cyclic currentevoltage analysis of IPMC samples. The CVmeasurements were taken using a potentiostat (Solartron SI 1287)electrochemical interface in an air environment after 30 cycles ofa triangle voltage input of �2 V with a scan rate of 100 mV s�1. Acyclic currentevoltage test using a two-electrode system was per-formed by connection of the counter-electrode leads to one side ofthe IPMC and connection of theworking-electrode lead to the otherside. The experimental test was performed without the use ofa reference electrode [17]. Two specimens of each IPMC were usedfor the CV test.

3. Experimental results and discussion

3.1. SEM and EDX

Fig. 3 (a)e(c) shows SEM images of the surfaces of the S1, S2blend membranes and the Nafion membrane, respectively. Theimages show the formation of small and large pores in the PVDF/PVP/PSSA blend membranes. The increased porosity and pore sizeled to an increase in the liquid electrolyte trapped within the pores,which increased the carrier ion content and enhanced the elec-trolyte conductivity accordingly [18]. Fig. 4 (a) and (b) shows SEMimages of the cross-sections of the S1- and S2-based IPMC,respectively, which depict the Pt particle deposition layers on both

Fig. 1. Schematic diagram of the experimental setup for measuring ionic polymer metal composite (IPMC) actuation.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937930

sides of the blend membranes (white arrows) [27]. Fig. 5 (a) showsthe EDX analysis of the PVDF, PVP, PSSA, S1, S2, and Nafion samplesurfaces that were used to investigate the carbon (C), fluorine (F),oxygen (O), and sulfur (S) elements of the pure polymer and blendmembranes. It has been reported that the EDX analyses of elements(C, F, O and S) of blend membranes could be related to the IEC andWUP of the blend membranes (ionic membrane) [18].

The EDX spectrum of the pure polymer revealed that PVDFdepicts a C peak at 0.277 and F peak at 0.677, PVP depicts a C peak at0.277 and O peak at 0.522 keV. PSSA depicts a C peak at 0.277, Opeak at 0.522 keV, and S peak at 2.3 keV. In the graph, PVDF haselements of C and F; PVP has elements of C and O; and PSSA haselements of C, O, and S.

The EDX spectrum of the ionic membrane reveals that the S2sample yielded higher peaks of C, S, and O than those of the S1 andNafion samples, whereas the S1 sample yielded higher peaks of F, C,O, and S than did the Nafion sample. Therefore, the S2 samples had

larger amounts of S and O elements than did the S1 and Nafionsample, which proves that it had larger numbers of sulfonic groupsðSO3

�1Þ than did the S1 and Nafion membranes. The high quantityof S and O elements in the S2 might be due to the high quantity ofPSSA in the S2 membrane as evidenced by the EDX spectrum ofPSSA, which revealed that PSSA has highest S and O peaks, con-firming that PSSA has the highest S and O element content (i.e.,highest amount of SO3

�1).Fig. 5 (b) shows the EDX analysis of the cross-sectional images of

the S1- and S2-based IPMCs. An EDX line scan was carried out toa depth of 1e20 mm from the upper parts of the cross-sections ofthe S1- and S2-based IPMCs. From the EDX line scan of the Ptparticles, the Pt particle concentration was found to be high, up toa depth of 6 mm in the blend membrane, which confirmed thedeposition layer of the Pt particles. Over a depth of 6 mm in theblend membrane, the concentration of the Pt particles decreased,but Pt particles were found up to 20 mm deep, which confirms thepresence of a Pt particle diffusion layer [27]. Therefore, EDX analysisconfirmed that the morphology of the Pt electrode played animportant role in IPMC actuation performance [28].

3.2. FTIR analysis

Fig. 6 (a) shows the FTIR transmittance spectra of the PVDF, PSSA,PVP, S1, and S2 blends wavenumbers between 1300 and 1000 cm�1

that were associated with the sulfonated benzene group [29]. Thesedata revealed that the peaks near 1037.70 and 1186.22 cm�1 wereattributed to the symmetric and asymmetric vibration of the eSO3Hgroup in pure PSSA, while the peak near 1131 cm�1 to the planeskeleton stretching vibration of the substituted benzene ring hadstrong participation from the eSO3H group [21,30]. The graphdemonstrates that the wavenumber peak and the intensity peak ofthe eSO3H group were changed in the PVDF/PVP/PSSA blendscompared to thoseofpurePSSA. These changesofeSO3Hgroup in theblend membranes might be due to the interaction between theeSO3H of PSSA with the pyrrolidone group of PVP in which thetertiary amide forms a positive charge after protonization. Therefore,

Fig. 3. Scanning electron microscopy images of the surfaces of the (a) S1 sample, (b) S2 sample, and (c) Nafion samples.

Fig. 2. Schematic diagram of the experimental setup for measuring ionic polymermetal composite (IPMC) absorbed current.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937 931

the intermolecular protondonoreacceptor interactionbetweenPSSAandPVP indicates themiscibility and compatibility of polymerblends[31e33]. The formation and properties of the acidebase membranewere reportedly based on the interaction between the SO3

�1 acidsand theN-bases,whichmay lead tohydrogenbond formationorbasicN-site protonation [34,35].

Fig. 6 (b) shows the FTIR transmittance spectra of the PVDF,PSSA, PVP, S1, and S2 blends with wavenumbers between 1800 and1500 cm�1 attributed to C]O group, which is considered to be thecombination of the stretching vibration band of the PVP molecule’sC]O group and the resonant structure >Nþ]CeO�1 [36]. Thisfigure shows that the C]O band shifted from 1674.21 cm�1 (lowerwavenumber) in the pure PVP to 1676.14 cm�1 (higher wave-number) in the S1 sample and 1678.06 cm�1 (higher wavenumber)in the S2 samples. The vibration peak of the C]O groups broadenedand increased in intensity for the S1 and S2 blends compared tothose of pure polymer, indicating the formation of a hydrogen bondbetween PVDFePVP [37] and PSSAePVP, which in turn indicates anacidebase interaction in the PVDF/PVP/PSSA blend membranesthat is responsible for miscibility [31e33].

3.3. WUP, IEC, and proton conductivity

The WUP, IEC, and proton conductivity of the S1, S2, and Nafionmembrane are given Table 1. The data in the table demonstrate thatthe WUPs and IECs of the S1 and S2 membranes have higher valuesthan those of the Nafion membrane, whereas the proton conduc-tivities of the S1 and S2 membranes have lower values than those ofthe Nafion membrane. It has been reported that the high WUP andIEC values of the blendmembrane (ionicmembrane)might be due tothe high concentration of SO3

�1 availability in the blendmembrane,which contains free charge carriers [18]. From the EDX analysis, itwas proven that the S1 and S2 membranes have higher numbers ofSO3

�1 group. The IEC andWUP values of the S2 blendmembrane are

higher than those of the S1 blend membrane, which may be due tothe high quantities of PSSA and PVP in the S2 blend membrane.

Due to the high WUP value of the blend membrane, morehydrated cations can move through the membrane to actuate theIPMC. With a high IEC value, Pt particles can be more easilyembedded in the pores of the blend membrane by the electrolessplating technique. The high IEC value of the blendmembranemightbe due to the high concentration of SO3

�1 availability in the ionicmembrane in which the SO3

�1 bonds to the large number of Ptcations [18,38]. During the reduction process, large numbers of Ptparticles can penetrate the porous surface of the blend membrane.As a result, the layer resistance can be decreased, enabling large tipdisplacements under AC and DC voltages. Therefore, due to theirhigh WUP and IEC values, the S1 and S2 blend membranes wereutilized as the base blend membranes for the IPMCs.

3.4. Dynamic mechanical properties of polymer blend membraneand IPMCs

For determination of the dynamic mechanical properties of thepolymer blend membranes and IPMCs, the storage modulus (E0)and loss factor (tan d) of all samples were analyzed as functions ofoscillation frequency in the hydrated condition. E0 is the measure ofmaterial elasticity, while tan d is the material’s damping ability.Fig. 7 (a) shows E0 as a function of frequency of the pure S1, S2, andNafionmembranes and of the S1-, S2-, and Nafion-based IPMCs. TheE0 values of the S1membranewith respect to frequencywere higherthan those of the S2 and Nafion membranes. The E0 values of the S2membrane were lower than those of the S1 and Nafionmembranes,possibly due to the higher water uptake of the S2 membrane.Comparison of mechanical strength among IPMCs, pure blendmembranes, and Nafion membrane revealed that the E0 values ofthe S1-, S2-, and Nafionmembranes were higher than those of the E0

values of the pure S1, S2, and Nafion membranes without the Pt

Fig. 4. Scanning electron microscopy images of cross-sections of the (a) S1-based ionic polymer metal composite (IPMC) and (f) S2-based IPMC.

Fig. 5. Energy-dispersive X-ray spectroscopy analysis of (a) surface parts of the polyvinylidene fluoride (PVDF), polyvinyl pyrrolidine (PVP), polystyrene sulfonic acid (PSSA), andpure S1 and S2 samples and (b) cross-sections of S1- and S2-based ionic polymer metal composite (IPMC) for Pt particles.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937932

coating. The E0 value of the S1-based IPMC was the highest, indi-cating high elasticity. The S1 and S2 (PVDF/PVP/PSSA) blendmembranes can be utilized as the basemembranes for IPMCs due totheir adequate mechanical strengths under wet conditions (highWUP) [39e42] that can be attributed to the entanglement of theseacidebase polymers and possible mixing due to the specific inter-actions, e.g., ionedipole, dipoleedipole, and proton transfer forPVDF/PSSA/PVP blend membranes of various blend ratios. TheIPMC was reported [43] to display easy actuation due to themembrane flexibility and proper hydrated state.

Fig. 7 (b) shows tan d as a function of oscillation frequency of thepure S1, S2, and Nafion membranes and S1-, S2-, and Nafion-basedIPMCs. The tan d of all of the samples decreased continuously upto a frequency of 50 Hz. Beyond that point, the tan d value increasedwith increasing frequency, and the highest tan d valuewas obtainedfor the S2 blend membrane, indicating high damping properties ofthe S2 blend membrane that decrease its elastic properties. Thetan d values for the IPMCs were lower than those of the puremembranes (S1, S2, and Nafion). The high tan d values of the S2-based IPMC indicated higher damping behavior than those of theS1- and Nafion-based IPMCs.

3.5. Electrical properties of IPMCs

3.5.1. Surface resistance of IPMCsThe surface resistances of the IPMCs are shown in Table 2. Those

of the S1-, S2-, and Nafion-based IPMCs, each of which underwentfour first Pt platings and 1 s plating, were 10, 2.5, and 4 U,respectively. The S2-based IPMC had the lowest resistance valuedue to its highest IEC value. With a high IEC, more Pt particles canbe more easily embedded in themembrane pores by the electrolessplating technique, reducing the resistance of the ionic polymerlayer. It should be noted that surface resistance decreased as thenumber of cycles of the first Pt plating increased. After the secondcycle of the first Pt plating, the average surface resistances of the S1-, S2-, and Nafion-based IPMCs were 25, 10, and 15 U, respectively.Low surface resistance is necessary to actuate IPMCs.

3.5.2. Current density of IPMC under AC voltages at differentfrequencies

The current densities of the S1-, S2-, andNafion-based IPMCswereinvestigated under sinusoidal wave input at various driving voltagesand frequencies. Fig. 8 (a) and (b) shows the current density of theIPMCs under an AC voltage of 4 V at 0.5 and 5 Hz, respectively. Thegraph clearly demonstrates that the current density of the Nafion-based IPMC is higher than those of the S1- and S2-based IPMCs.Fig. 8 (c) shows the current density of the S1-, S2-, and Nafion-basedIPMCs as a function of frequency under AC voltages of 2e4 V. Themaximumcurrentmeasured at thepoint of the polarity change of theinput slope (i.e., from plus to minus) [44] is reported in this figure.These data clearly show that the current density increased withincreasing AC voltages and frequencies. This result can be used topredict the current consumption of the IPMC actuator under variousvoltages at various frequencies [44]. Therefore, from this analysis, theamount of current consumed during actuation under a particular ACvoltage and frequency can be easily estimated. Themaximumcurrentdensityof theS1-, S2-, andNafion-based IPMCsweredetermined tobe0.25, 0.40, and 1 A cm�2, respectively.

3.5.3. CV analysis of IPMCsThe electrical properties of IPMCs have also been investigated

using CV. Fig. 9 shows the currentevoltage hysteresis curves of IPMCsrecorded under a �2 V triangle voltage input with a scan rate of100 mV s�1. The graph shows that the current density of the Nafion-based IPMC was highest, followed by the S2-based IPMC and finallythe S1-based IPMC. The higher current density of the Nafion-basedIPMC sample might be due to its higher ionic and surface conduc-tivities. It was reported in cyclic currentevoltage analysis that anincrease in current density usually implies a greater energy storageability, which determines an IPMC’s actuation performance [17,45].

3.6. Actuation properties of IPMCs

3.6.1. Tip displacement of IPMCs under DC and AC voltagesIn this paper, displacement of the IPMC tip was defined as IPMC

actuation. Fig. 10 (a) and (b) shows the tip displacements of the S1-

Fig. 6. Fourier transform infrared spectroscopy analysis of the polyvinylidene fluoride (PVDF), polyvinyl pyrrolidine (PVP), polystyrene sulfonic acid (PSSA), S1, and S2 blends withwavenumbers of (a) 1300e1000 cm�1 and (b) 1800e1500 cm�1.

Table 1Basic properties of the polymer blend membranes.

Membrane PVDF ratio PVP ratio PSSA ratio Thickness (mm) IEC (meq g�1) WUP Proton conductivity (S cm�1)

PVDF/PVP/PSSA (S1) 60 15 25 0.18 1.35 � 0.1 0.44 � 0.02 0.030 � 0.005PVDF/PVP/PSSA (S2) 40 30 30 0.18 1.56 � 0.08 0.82 � 0.02 0.049 � 0.004Nafion� e e e 0.18 0.98 � 0. 09 0.17 � 0.01 0.090 � 0.002

PVDF, polyvinylidene fluoride; PVP, polyvinyl pyrrolidine; PSSA, polystyrene sulfonic acid; IEC, ion-exchange capacity; WUP, water uptake.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937 933

and S2-based IPMCs under DC voltages of 1, 2, and 3 V for 60 s. Withthe application of DC voltages, the S1- and S2-based IPMCs imme-diately actuated toward the anode side due to movement of the Liand hydrated cations toward the cathode.When the DC voltagewasonly 1 V, the responsewas weak. Once the DC voltage exceeded 1 V,the bending response increased and then continued to increasewith increasing voltage. The tip displacement was greatly improved

when the applied voltage reached 3 V. The tip displacement of theS1- and S2-based IPMCs increased with time and did not show back-relaxation, whereas the Nafion-based IPMC displayed back-relaxation under a DC voltage of 3 V (Fig. 10 (c)). Back-relaxationby an IPMC refers to the IPMC moving back in the opposite direc-tion (cathode) after it reaches its largest tip displacement after DCvoltage application. The slow relaxation of the tip displacement forthe Nafion-based IPMC may be caused by the diffusion of watermolecules to the anode side. The PVDF/PVP/PSSA-based IPMCs didnot show back-relaxation, a finding that might be due to the highWUP and IEC of the PVDF/PVP/PSSA blend membranes, whichprovide a small amount of water for the movement of IPMCs withtime after diffusion of water back to the anode side [20,46]. All ofthe IPMCs depicted different time constants, which might be duethe different motions of IPMCs with different voltages according tothe actuator’s WUP, IEC, and proton conductivity. Under DC volt-ages, the S2-based IPMC displayed larger tip displacement withtime, followed by the S1- and then the Nafion-based IPMCs. This

Fig. 7. (a) E0 and (b) tan d values as a function of frequency of pure S1, S2, and Nafion samples and S1-, S2-, and Nafion-based ionic polymer metal composites (IPMC).

Table 2Surface resistance of the ionic polymer metal composites (IPMC).

IPMC Surface resistance (U)

PVDF/PVP/PSSA (S1) 10 � 5PVDF/PVP/PSSA (S2) 2.5 � 1Nafion� 4 � 2

PVDF, polyvinylidene fluoride; PVP, polyvinyl pyrrolidine; PSSA, poly-styrene sulfonic acid.

Fig. 8. (a) and (b) Absorbed current of S1-, S2-, and Nafion-based ionic polymer metal composites (IPMC) under an alternating current (AC) voltage of 4 V at 0.5 Hz and 5 Hz,respectively; (c) Maximum absorbed current of S1-, S2-, and Nafion-based IPMCs with frequencies under AC voltages of 2e4 V.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937934

finding might be due to the higher WUP, higher IEC, and lowstorage modulus of the S2-based actuator. It has been reported thatlarge IPMC actuation is are dependent on high IEC and WUP values[10,11].

Fig.11 (a) and (b) depicts the tip displacement of the S1-, S2-, andNafion-based IPMCs under AC voltages of 4 V at 0.5 Hz and 5 Hz,respectively. The figure demonstrates that the Nafion-based IPMCdepicted larger tip displacement than did the S1- and S2-basedIPMCs at 0.5 Hz. The S2-based IPMC depicted larger tip displace-ment than did the Nafion- and S1-based IPMCs at 5 Hz. The S1-based IPMC depicted the smallest tip displacements at 0.5 Hz and5 Hz. Analysis of these findings demonstrated that the bandwidthsof the S1- and S2-based IPMCs were slightly higher than that of theNafion-based IPMC.

Fig. 12 depicts the mean tip displacements of the S1-, S2-, andNafion-based IPMCs under AC (sinusoidal) voltages of 2e4 V in the

frequency ranges of (a) 0.1e0.7 Hz and (b) 1e5 Hz. The mean tipdisplacement (d) value in one circle was calculated using Eq. (3):

d ¼ dmax � dmin2

; (3)

where dmax and dmin respectively represent the maximum andminimum tip displacement values in one cycle recorded in thesteady-state response. The mean values were calculated from atleast three experimental results measured from various IPMCs. Thefigures clearly demonstrate that the tip displacement of the IPMCsincreased with increasing AC voltage. From Fig. 12 (a) and (b), it wasfound that the tip displacement of the IPMCs decreased withincreasing frequency under various AC voltages, except for thefluctuation in the tip displacement at 0.2 Hz under an AC voltage of2 V for all IPMCs. From Fig. 12 (a), it was found that the Nafion-based IPMC depicted larger tip displacement with various ACvoltages than the S1- and S2-based IPMCs below 1 Hz except at thefrequencies of 0.2 Hz and 0.4 Hz. All three IPMCs showed highest tipdisplacement under the AC voltage of 4 V at 0.1 Hz. From Fig. 12 (b),it was found that the S2-based IPMC depicted larger tip displace-ment with various AC voltages than the Nafion- and S1-basedIPMCs over 1 Hz frequencies. Consequently, the S1-based IPMCdepicted lower tip displacement under various AC voltages than theS2- and Nafion-based IPMC actuators. At lower frequencies, IPMCsshow larger tip displacements according to voltage because lowerfrequencies allow sufficient time for charge accumulation andhence produce higher actuation performance. At higher frequen-cies, the IPMCs show lower tip displacements according to voltagebecause higher frequencies do not allow for charge accumulation.

3.6.2. Blocking force of the IPMC actuator under various DCvoltages

The blocking force is the maximum force of the IPMC that isproduced at the tip of the actuator. This force is measured by theforce probe of the load cell that is attached to the tip. The blocking

Fig. 9. Cyclic currentevoltage graph of ionic polymer metal composites (IPMCs).

Fig. 10. (a) and (b) Tip displacement as a function of time under direct current (DC) voltages of 1e3 V of S1- and S2-based ionic polymer metal composites (IPMC), respectively; (c)Tip displacement as a function of time under a DC voltage of 3 V of S1-, S2-, and Nafion-based IPMC.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937 935

force was measured with a load cell at the tip of the IPMC beam.Fig. 13 (a) shows the blocking force of the S1-, S2-, and Nafion-basedIPMCs as a function of time at the DC voltages of 2 and 3 V,respectively. At 2 V, the blocking forces of the S1- and S2-basedIPMCs increased slowly over time and attained higher values thanthat of the Nafion-based IPMC. The Nafion-based actuatorincreased sharply with time but decreased after 40 s.

Fig. 13 (b) shows the blocking force of the S1-, S2-, and Nafion-based IPMCs as functions of time at a DC voltage of 3 V. Theblocking force of the IPMC at 3 Vwas found to be higher than that ofthe actuator at 2 V. The blocking forces of the S1-based IPMC

increased slowly over time and increased sharply at 20e50 s. After50 s, it decreased, which might be due to the decreased motion ofthe S1-based IPMC in this period of time, yet it remained higherthan those of the S2- and Nafion-based IPMCs. The blocking force ofthe S2-based IPMC increased with time and remained higher thanthat of the Nafion-based IPMC at 40e60 s. The blocking force of theNafion-based IPMC increased sharply after giving voltage. Theblocking force of the Nafion-based IPMC was higher than the S1-and S2-based IPMCs in the first 15 and 33 s, respectively, and itsvalue decreased after 33 s. The data analysis clearly shows that theblocking force of the S1-based IPMC is higher than those of the S2-

Fig. 11. Tip displacement of S1-, S2-, and Nafion-based ionic polymer metal composites (IPMC) as a function of time under alternating current voltage of 4 V at (a) 0.5 Hz and (b)5 Hz.

Fig. 12. The mean tip displacements of the S1-, S2-, and Nafion-based IPMCs as functions of frequency between (a) 0.1e0.7 Hz and (b) 1e5 Hz under AC (sinusoidal) voltages of2e4 V.

Fig. 13. Blocking force of S1-, S2-, and Nafion-based ionic polymer metal composite (IPMC) as a function of time under (a) direct current (DC) of 2 V and (b) DC of 3 V.

V. Panwar et al. / Materials Chemistry and Physics 135 (2012) 928e937936

and Nafion-based IPMCs, a finding that might be due to the higherE0 value (storage modulus) of the S1-based IPMC than those of theS2- and Nafion-based IPMCs.

4. Conclusion

The dynamic mechanical, electrical, and actuation properties ofPVDF/PVP/PSSA blend membrane-based IPMCs were presented inthis paper. The storage modulus of the S1-based IPMC was deter-mined to be higher than those of the S2- and Nafion-based IPMCs.The current densities of the S1- and S2-based IPMCs were lowerthan that of the Nafion-based IPMC. Under DC voltage, the S1- andS2-based IPMCs actuated toward the anode over time withoutdisplaying back-relaxation, whereas the Nafion-based IPMCshowed back-relaxation. Under DC voltages, the largest tipdisplacement over time was obtained for the S2-based IPMC, whilethe highest blocking force was obtained for the S1-based IPMC. Thetip displacements of the IPMCs increased with increasing AC volt-ages of 2e4 V and decreased with increasing frequencies. Below1 Hz, the largest tip displacement was obtained for the Nafion-based IPMC under an AC voltage of 4 V at 0.1 Hz, whereas above1 Hz, the S2-based IPMC depicted the largest tip displacement at5 Hz under an AC voltage of 4 V. The bandwidths of the S1- and S2-based IPMCs were slightly higher than that of the Nafion-basedIPMC. Finally, the S2-based (PVDF/PVP/PSSA in the ratio of 40/30/30) IPMC showed larger tip displacement and higher blocking forcethan the Nafion-based IPMC under DC voltage, higher actuationbandwidth and lower current density than the Nafion-based IPMCunder AC voltage, and a lower storage modulus than that of theNafion-based IPMC. Due to these characteristics, the S2-based(PVDF/PVP/PSSA in the ratio of 40/30/30) IPMC can be utilized forrobotics applications.

Acknowledgment

This work was supported by a Grant-in-Aid for Strategy Tech-nology Development Programs (No. 10030037) from the KoreaMinistry of Knowledge Economy.

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