Journal of Industrial and Engineering Chemistry 17 (2011) 49–55

Sulfonated polystyrene-based ionic polymer–metal composite (IPMC) actuator

Mohammad Luqman a, Jang-Woo Lee a,b, Kwang-Kil Moon a,b, Young-Tai Yoo a,b,*a Artificial Muscle Research Center, Seoul 143-701, Republic of Koreab Department of Materials Chemistry and Engineering, College of Engineering, Konkuk University, Seoul 143-701, Republic of Korea

A R T I C L E I N F O

Article history:

Received 21 December 2009

Accepted 10 March 2010

Available online 8 October 2010

Keywords:

Sulfonated polystyrene

Ionomers

Ionic polymer–metal composites (IPMCs)

Actuators

Artificial muscles

A B S T R A C T

Herein, we report the actuation performance of a cost-effective sulfonated polystyrene (sPS)-based

IPMC, and its comparison with that of a Nafion-based IPMC. It was observed that the current density (810

vs. 456 mA/cm2), the tip displacement (ca. 44 vs. 23 mm), the response rate (ca. 10.3 vs. 2.9 mm at

starting 3 s) and the blocking force (ca. 2.76 vs. 1.51 gf) were significantly higher for the sPS-IPMC

compared to those for the Nafion-IPMC. Additionally, the sPS-IPMC showed very slow back relaxation.

With the aid of the scanning electron microscopy for the morphological analysis and various methods for

quantitative analysis, we found that the excellent electromechanical response of the sPS-IPMC was due

to the smoother and thicker electrode layer, the higher tensile modulus, the enhanced hydraulic force

based on the higher water uptake and higher ion exchange capacity (IEC) value than those of the Nafion-

IPMC. The sPS-based IPMCs seem to be one of the promising alternatives of the conventional expensive

IPMCs.

� 2010 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering

Chemistry.

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1. Introduction

Organic polymers are increasingly being used in numerousmature and cutting-edge technologies owing to their robustbehavior, versatile properties, easy processability, etc. The poly-mers which can be stimulated to change shape and/or size havebeen known and studied for years. There are many ways by whichmaterials can be activated, for example, electrical, chemical,pneumatic, optical and magnetic mechanisms. Among thesemechanisms, electrical stimulation is one of the most promisingways to generate elastic deformation in either electronic or ionicpolymers. Ionic polymer metal composites (IPMCs) are one of themost promising electroactive polymeric (EAPs) materials whichcan be used to mimic the biological muscles, for their large bendingdeformation under low voltage of electricity, and are used asdynamic sensors, robotic actuators and artificial muscles [1–3].IPMCs are composed of swollen ionomer membrane having asolvent usually water for the migration of the free cations (in thecase of cation-exchange membranes), sandwiched between twolayers of electrodes made up of noble metals deposited to both ofthe faces of the membranes. Under low electric potential, themovable solvated (usually hydrated) cations migrate to the

* Corresponding author at: Department of Materials Chemistry and Engineering,

College of Engineering, Konkuk University, Seoul 143-701, Republic of Korea.

Tel.: +82 2 450 3207; fax: +82 2 444 0711.

E-mail address: ytyoo@konkuk.ac.kr (Y.-T. Yoo).

1226-086X/$ – see front matter � 2010 Published by Elsevier B.V. on behalf of The Ko

doi:10.1016/j.jiec.2010.10.008

opposite charged electrode, cathode, creating a volume differencebetween both sides of the electrodes, leading to a bendingdeformation of membrane towards the anode [4].

The electromechanical (actuation) and mechanoelectrical(sensing) responses of an IPMC membrane depend on manyfactors including types of the ionomer (the backbone, acidic co-monomer, immovable ion, and distance of the ion from thebackbone), counter ion, solvent, solvent uptake and electrode [5].As the ionomer membranes provide the pathways to the solvatedcations, the characteristics of the membrane material could beimportant factors to decide the fate of the IPMC performance. Asurvey of the latest open literature suggests that the polymermatrices for the IPMC applications are largely limited to a numberof perfluorinated polymers including DuPont’s Nafion1 and AsahiChemical’s Aciplex1, most probably for the reasons of theirexcellent mechanical strengths, chemical stability and high protonconductivity, and to avoid synthetic complexicities associated withnew polymers. At the same time, in general, a short operation time,a low generative blocking force, extreme expensiveness, and lessenvironment friendliness [6] of these conventional IPMCs havemotivated researchers to find easily available and synthesizable,affordable, high-performing, and environmentally more accept-able alternatives to these IPMCs.

Shahinpoor and Kim reported solid-state polymer actuatorsbased on poly(ethylene oxide) and poly(ethylene glycol). Thesematerials were capable of exhibiting large bending motion withconsiderable stress, fast responses and a stable operation over tenmillions of cycles in air with nearly no performance degradation

rean Society of Industrial and Engineering Chemistry.

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–5550

[7]. Akle et al. studied the effects of polymer structure andelectrode composition on the electromechanical actuation re-sponse of the IPMCs based on three families of ionomers: Nafion1,BPSH (sulfonated poly(arylene ether sulfone)) and PATS (poly(-arylene thioether sulfone)). They suggested a strong relationshipbetween charge accumulation at the polymer–metal interface andtransducer performance [8,9]. Kim et al. fabricated IPMCs based onchitosan-polyaniline ion-exchange membranes using freeze-dry-ing method. Under dc voltage, freeze-dried samples showed afaster and larger bending motion than non-freeze-dried samples,and no back relaxation was observed with time [10]. Phillips andMoore tried sulfonated ethylene vinyl alcohol copolymer mem-branes for IPMC applications. They showed that these membranesbehaved similar to the Nafion1, however, the actuation kineticswere significantly slower [11]. Han et al. reported IPMC actuatorsbased on fluoropolymers grafted with sulfonated polystyrene. Thebending displacements (with negligible back relaxation) of theseIPMCs although very small (ca. 3 mm at 2 V dc), were few timeshigher than that of the Nafion-IPMC of similar thickness [12].Sulfonated poly(styrene-b-ethylene-co-butylene-b-styrene) basedmembranes were tested for IMPC applications. The IPMC actuatorswere found to show very small (ca. 1 mm at 2 V dc) but a bit high-speed bending movements under dc voltage [13]. Very recently, Luet al. presented a study on fabrication and actuation of EAPactuators based on poly(styrene-alt-maleimide) (PSMI)-incorpo-rated poly(vinylidene fluoride) (PVDF) ionic networking polymers(INPs). They showed better mechanical displacement performanceof these non-conventional actuators as compared to theirtraditional widely used Nafion1 counterparts [14]. Few otherstudies have been performed to present alternatives to theconventional IPMCs. Interested readers are referred to theconcerned articles [15–19].

From the literature survey presented, we came to know that sofar there is only a limited progress in finding unique and effectivealternatives to conventional materials used for IPMC applications.Thus, it is highly desirable to fill this technological gap. To fulfillthis demand, herein, we report a high-performance IPMC based onsulfonated polystyrene ionomers (sPS). There are few EAP relatedpapers reporting the use of sPS for introducing ionic groups in non-ionic polymers other than polystyrene or polystyrene basedcopolymers where styrene may be the main monomer unit.However, to the best of our knowledge, the use of sPS itself as theion conducting membrane for IPMC applications has not beentried. The sPS has widely been used for many applicationsincluding as ion-exchange materials for waste water treatment,as proton-exchange membranes in fuel cells [20,21], and ashumidity sensors [22]. It is one of the affordable and commerciallyavailable organic polymers. It can also easily be tuned with desiredlevels of sulfonation for the improved water absorption and protonconductivity; important characteristics of ionic membranesneeded for high performance of an IPMC, and can be fabricatedconveniently for the desired applications. The comparison ofactuation performance of sPS based IPMC with that of Nafion-IPMCis presented.

2. Experimental

2.1. Materials

A Nafion dispersion (DuPont DE-2021, 20 wt.% in a mixture ofpropanol and water), polystyrene (PS) (Mw = 350,000,Mn = 170,000) and tetraamine platinum (II) chloride hydrate(98.0%) were purchased from Sigma–Aldrich, USA. Benzene(99.5%), 1,2-dicholoroethane (99.0%), hexane (95.0%), hydrochloricacid (HCl) (35.0–37.0%), methyl alcohol (99.5%) and sodiumhydroxide (98.0%) were purchased from Samchun Chemicals,

Korea. Ammonium hydroxide (28.0%) and sulfuric acid (95.0%)were from Duksan Reagents, Korea, and acetic anhydride (93.0%)and sodium borohydride (98.0%) were obtained from JunseiChemicals, Japan. All of the materials were used as receivedwithout further purification.

2.2. Partial sulfonation of polystyrene

Lightly sulfonated PS copolymer, poly(styrene-co-styrenesul-fonic acid) (sPS) was synthesized using a method analogous to thatreported by Makowski et al. [23]. The sulfonation reaction wascarried out in 1,2-dichloroethane at ca. 60 8C for 2 h. By controllingthe amount of the sulfonating agent (acetyl sulfate), the degree ofsulfonation can be adjusted as per need. The reaction wasterminated by the addition of methanol into the reaction mixture.The polymer solution was poured into de-ionized water (is written‘‘water’’ hereafter) at room temperature, resulting in whitish jellymass. It was followed by the addition of 35–37% HCl. The sPScopolymer was easily coagulated into a solid mass. The polymerwas cut into small pieces, put into excess of n-hexane to removethe solvents entrapped in the polymer mass, pulverized thepolymer mass into powder, followed by washing several timeswith n-hexane. The sample was finally dried at 60 8C under avacuum for 24 h.

2.3. Determination of ion-exchange capacity (IEC)

To determine the IEC (i.e. mmol of the ionic groups per g of thedry polymer) values of the sPS copolymers synthesized in differentbatches, the copolymer samples were dissolved in a benzene/methanol (1/1, v/v) mixture to make a 5% (w/v) solution andtitrated with methanolic NaOH solution to the phenolphthaleinend point. The IEC were found to be 1.75 and 2.23 meq/g,respectively. The IEC value of Nafion engaged in this study was0.95 meq/g.

2.4. Membrane preparation and water uptake

The sPS solution in a 5% (w/v) benzene/methanol (1/1, v/v)mixture, and the Nafion dispersion were casted into a Teflonmold, and dried at 40, 70 and 100 8C for 12 h each, followed byannealing at 120 8C for 3 h. As per our experience, if we startdrying the sPS membranes at high temperatures, it is difficult toget bubble free membranes. Thus, the drying temperatures wereslowly increased to ensure the formation of bubble freemembranes and complete removal of the solvents from themembranes. The sPS membranes (sPS1 and sPS2) were brittle inthe dry (dehydrated) state. To reduce the brittleness signifi-cantly, these were soaked into water at ca. 60 8C for ca. 6 and 4 h,respectively. Series of water soaking experiments at differenttime and temperature were performed. The mentioned temper-ature and time were found optimum to let the either membraneattain a considerable level of mechanical strength and waterabsorption for an expected optimum actuation performance andion conductivity. At lower temperatures, membranes either didnot absorb a considerable amount of water, and hence, it wasnot easy to get flexible membranes, or it took more time to getthe desirable results. On the other hand, the higher temperatureand time led to an excess water uptake by the membranes,which in turn, significantly decreased the mechanical stability.Nafion membrane was soaked into water at ca. 95 8C for 2 h tolet it absorb almost a maximum amount of water. The hydratedmembranes were kept in water at room temperature until used.There was negligible change in the wt.% of the soaked waterafter keeping the membranes in water at room temperature formany days.

Table 1Properties of sPS and Nafion membranes.

Sample IEC value

(meq/g)

Proton conductivity

(S/cm)

Water uptakea

(wt.%)

Water loss

5 min, 3 V dc (%)

Tensile modulus

(MPa)

Tensile strength

(MPa)

Elongation at

break (%)

sPS1 1.75 0.064 103 – 214 9.08 9.70

sPS2 2.23 0.069 113 33 154 6.18 10.85

Nafion 0.95 0.111 45 37 39 5.41 37.71

a The data were obtained after immersing in water: 60 8C (6 h) for sPS1, 60 8C (4 h) for sPS2, and 95 8C (2 h) for Nafion.

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55 51

2.5. Electroless plating of the membranes

IPMCs were fabricated using an analogous electroless platingmethod described in the literature [24,25]. Membranes wereroughened by sand paper on both sides, cleaned ultrasonically for30 min, treated with 2.0 N HCl aqueous (aq.) solution at 45 8C for6 h followed by washing with water at 45 8C for 1 h. To exchangethe protons of the membranes with platinum ions, the membraneswere stirred in 45 ml of 0.01 N (for 30 cm2 of the membrane) aq.solution of tetraammine platinum (II) chloride hydrate([Pt(NH3)4]Cl2�xH2O) and 1 ml of 5.0 wt.% of aq. NH4OH solutionat room temperature for 4 h. The membranes were rinsed with,and stirred in, 180 ml water at 45 8C. For reduction of platinum ionsinto metal, 2 ml of 5.0 wt.% aq. NaBH4 solution was added every30 min for 7 times, followed by finally adding 20 ml of the NaBH4

solution stirred at 45 8C for 1.5 h. To terminate the reductionprocess, the membranes were rinsed with water, and placed in0.1 N HCl solution at room temperature for 1 h to convert themembranes into an acid form.

2.6. Characterization of the membranes

Proton conductivity was measured in water at room tempera-ture using a complex impedance analyzer (IM6ex, Zahner) and acustom-made cell for the normal four-point probe technique.Mechanical properties of hydrated membranes were measuredwith a universal testing machine (Model 4468, Instron) with acrosshead speed of 1 mm/min. Surface and cross-sectionalmicrographs of the IPMC membranes were obtained using ascanning electron microscope (SEM) (JSM-6380, Jeol). For exam-ining the electrical characteristics of the IPMC samples, cyclicvoltammetry (CV) and potentiostatic analysis experiments wereperformed. CV curves were obtained with a potentiostat–galvanostat (WMPG-1000, Wonatech) in a water environmentafter 30 cycles of a triangle voltage input of �3 V with a step of100 mV/s. Potentiostatic analysis was carried out with a step voltageof �3 V and a frequency of 0.1 Hz. The blocking force and tipdisplacement of the IPMCs were measured at 3 V dc using a load cell(CB1-G150, Dacell), and a custom-made Pt clip and a CCD camera,respectively. The dimensions of the H+-form IPMC samples were5 mm (width) � 25 mm (length) � ca. 0.42 (thickness) mm size. Thesample were vertically supported by a gold grip in air and fixed to5 mm of length on both sides giving the effective length as 20 mm.

3. Results and discussion

3.1. Water uptake and proton conductivity

The performance of a conventional IPMC membrane depend onmany factors including types of the ionomer (e.g. the backbone, theacidic co-monomer, the immovable ion, and the distance of the ionfrom the backbone), counter ion, solvent, solvent uptake andelectrode. The deformation of an IPMC sample under an appliedvoltage is believed to be based on the movement of solvatedcations and free solvent molecules towards the cathode. Theimmovable ionic groups (sulfonic acid groups in the present case)

of the membranes, being polar in nature, absorb polar molecules,e.g. water, in this case. The higher the moles of the sulfonic acidgroups, the higher the water uptake, an important factor indetermining the performance of an IPMC sample. The IEC valuesand other properties of the membranes are mentioned in Table 1.The higher IEC values of the sPS membrane allow higher levels ofwater uptake (113 and 103 vs. 45 wt.% for sPS (2.23 meq/g) and sPS(1.75 meq/g) vs. Nafion). One of the two sPS samples, sPS2, having2.23 meq/g of IEC value, was selected for IPMC studies for itscomparably higher water uptake and proton conductivity, and thelower brittleness. The time and temperature of the water uptakemeasurement for sPS and Nafion membranes were different,however, it was adjusted so for an optimum level of water uptakeand dimensional stability of the membranes. At 60 8C, thetemperature used for water uptake measurement of sPS mem-brane, Nafion membrane does not absorb significant amount ofwater. The higher water uptake by Nafion membrane does notadversely affect its dimensional stability; rather it may increasethe proton conductivity, and hence, may lead in an expectedimprovement in its actuation performance. Although the IEC valuefor the selected sPS sample (2.23 meq/g) is significantly higherthan that for the Nafion dispersion (0.95 meq/g), the protonconductivity of the sPS membrane (0.069 S/cm) is lower than thatof the Nafion (0.111 S/cm) membrane. We are not exactly sure forthe reasons for a different behavior by both membranes. Mostprobably, it is due to the difference in types of the ionic clusteringin both systems. Well-defined hydrophilic ion-conducting chan-nels, a very unique morphology of fully hydrated perflourinatedmembranes only, are formed if the membranes are properly heat-treated, leading to a good ionic conductivity.

3.2. Factors affecting the performance of IPMCs

3.2.1. Morphology of electrode layers

Fig. 1(a)–(d) shows the surface SEM micrographs of IPMCsprepared from sPS and Nafion. (a) and (c) images are from asprepared IPMCs, while (b) and (d) images are from the IPMCsamples run in air for ca. 30 min under 3 V dc, for sPS- and Nafion-IPMCs, respectively. A smooth Pt surface with well inter-connectedlarge domains leaving behind negligible space at joints is seen inmicrograph ‘a’ (as prepared sPS-IPMC), while a good deposition of abit non-uniform sized Pt particles is observed in micrograph ‘c’ (asprepared Nafion-IPMC). The electrode surface of actuation testedsPS-IPMC damaged to some extent resulting in a few small inter-connected and comparably rough domains with considerablespace at joints. The electrode surface of Nafion-IPMC, however,seems negligibly affected.

The representative cross-sectional SEM micrographs of asprepared sPS- and Nafion-IPMCs are shown in Fig. 2(a) and (b). Thethickness of electrode layers seems to be 23–32 mm and 7–11 mmin sPS and Nafion-IPMCs, respectively. The observed thickness of Ptelectrode layers in these IPMCs is one of the highest reported so farfor other IPMCs including Nafion based ones [1–4]. The smooth-ness of the electrode surfaces and large thickness of electrodelayers are expected to contribute in enhancing the performance ofthese IPMCs by (1) exhibiting the so-called ‘‘granular damming

[()TD$FIG]

Fig. 1. Surface SEM micrographs of IPMCs before and after actuation of few cycles at 3 V dc: (a) and (b) for sPS-IPMC, and (c) and (d) for Nafion-IPMC.

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–5552

effect’’, making it more difficult for solvent molecules to passthrough, thus, is expected to check the water leakage at least tosome extent [1], and (2) improvement in the surface conductivityof the IPMCs [25].

3.2.2. Water loss from IPMCs

The water loss from these IPMCs was determined by weighingthe IPMCs after applying an electric potential of 3 V dc for 5 min.The data is given in Table 1. A water loss from IPMCs and damage ofthe electrode layers are the important reasons for their short life-time. The important mechanisms for loss of inner liquid (e.g.water) include (1) leakage from the damaged or porous electrodesurface, (2) natural evaporation, and (3) electrolysis. It is evidentthat the water loss in both IPMCs is almost same. As the loss ofwater from natural evaporation and electrolysis is applicable toboth IPMCs, only the water leakage from the electrode surfaceseems to matter. As evidenced from SEM micrographs, on theapplication of electric potential, the smooth and almost cavity freeelectrode surface of the sPS-IPMC turns into comparably rough and

[()TD$FIG]

Fig. 2. Cross-sectional SEM micrograph

fractured surface with few visible cavities. The electrode surface inNafion-IPMC before and after the application of electric potential,however, does not change significantly. Thus, one may speculatethat it may be comparably easier for water to leak from thefractured electrode layer of sPS-IPMC. However, as the thickness ofelectrode layer of sPS-IPMC is significantly higher, the twoopposing factors (fracture in and thickness of electrode layer)may annihilate the effects of each other, leading to a negligiblewater leakage from electrode layer. Therefore, if our interpretationwould be accepted, we believe that the factors applicable to bothsystems, i.e. natural evaporation and electrolysis, are the mainmechanisms for water loss in these systems.

3.2.3. Electrical properties of IPMCs

Cyclic voltammetry and potentiostats were used for analyzingelectrical properties of the prepared IPMCs. Shown in Fig. 3 iscurrent–voltage hysteresis curves recorded under a �3 V trianglevoltage input with a scan rate of 100 mV/s. It would be worthy toremind that the shape of the I–V hysteresis curves generally reflects

s of (a) sPS- and (b) Nafion-IPMCs.

[()TD$FIG]

Fig. 3. Cyclic voltammetric curves as a function of voltage for sPS and Nafion IPMCs,

obtained at �3 V (triangle) with a scan rate of 100 mV/s.

[()TD$FIG]

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55 53

the movement of hydrated ions induced by the applied voltage, withdecomposition profile of water resulting from its electrolysis at ca.�1.5 V. It is observed that the current-density of the sPS-IPMC issignificantly higher than that of the Nafion-based one. We proposethat the higher IEC value of sPS-IPMC in comparison to Nafion basedone, and the expected higher surface conductivity owing to thesmoothness of electrode surface and higher thickness of electrodelayer are two important contributing factors for considerably highercurrent density of sPS-IPMC. It may be interesting to note that incomparison to other IPMC systems reported so far, under comparableexperimental conditions, the observed current-densities for theseIPMCs are significantly higher. The smoothness of electrode surfacesand higher thickness of electrode layers are, again, important factorsfor the same. The current density generally reflects energy storageability of a system; the higher the current density, the higher theperformance of an IPMC, by a larger deformation under an electricfield.

The result of potentiostatic experiment is shown in Fig. 4. Thepotential was initially kept at 3 V for 30 s, during which the currentdecreased almost exponentially. Again, it is seen that the currentdensity is considerably higher for sPS-IPMC compared to its Nafion

[()TD$FIG]

Fig. 4. Potentiometric analysis curves as a function of time for sPS and Nafion IPMCs,

obtained at 3 V, 0.1 Hz.

counterpart, which is attributed to the reasons mentioned above.Another interesting feature lies in that the charge transfer undertime t, reflected by the area under curves, is seen to be significantlyhigher for sPS-IPMC. This implies a considerably higher chargeconsumption capacity of sPS-IPMCs compared to that of theNafion-IPMC, which is expected to lead an increase in the blockingforce and tip displacement under an applied voltage.

3.3. Tip blocking force and displacement

Few of the important desirable properties those may lead anIPMC to be applicable in real life applications are high tip blockingforce, large tip displacement and low or negligible back relaxation.The blocking force and the tip displacement were measured underan electric potential of 3 V dc to evaluate the actuationperformance of IPMCs. Shown in Fig. 5a is the representativeblocking force profiles for these IPMCs. The dimension of IPMCsamples was 5 mm � 20 mm with a thickness of ca. 420 mm. It wasobserved that the maximum blocking forces of 2.76 and 1.51 gf

after ca. 32 and 40 s were exhibited by sPS and Nafion IPMCs,respectively. It seems interesting to note that within ca. 7 s, a value(ca. 2.6 gf) similar to the maximum one was achieved by sPS-IPMC,reflecting a very fast response of this IPMC. An average force valueof ca. 2.6 gf from ca. 7 to 37 s for sPS-IPMC, and ca. 1.45 gf from ca.33 to 43 s for Nafion-IPMC is seen, followed by a fast relaxationcausing a significant decay in the blocking force is observed in bothsystems. This type of relaxation in IPMCs is believed due mainly to

Fig. 5. Time-dependent (a) blocking force and (b) tip displacement of sPS- and

Nafion-IPMCs obtained under 3 V dc. The inset in (b) is the plot in the shorter time

scale (0–7 s).

[()TD$FIG]

Fig. 6. Displacement images of IPMCs at varying times under 3 V dc.

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–5554

the residual stress induced by the difference in the stiffnessbetween the electrolyte and electrode layer, as well as to the loss ofwater [26–28].

There are fluctuations in the tip force profile of sPS-IPMC.Studies on this behavior are a part of another forthcoming article.We are, presently, not sure for the exact reason (s). We anticipate,however, that this may be due to the non-well defined andconnected randomly distributed ionic channels in sPS membrane.It would be worthy to remind that like Nafion, sPS also has randomdistribution of the ionic groups throughout the membrane. Despitethe random distribution of the ionic groups, the formation of well-connected ionic channels in Nafion based membranes takes place,and is well known and documented [29,30]. The flexibility of thelong side chains and randomness of ionic groups facilitates theformation of well-connected ionic channels in Nafion, if properlyheat treated and have sufficient water uptake. This is unique toperflourinated membranes. Formation of the ionic clusters in sPSbased materials is also well documented [29]. Ion-hopping i.e. themovement of the ionic groups from one cluster to another in sPSbased ionomeric membranes is responsible for the transportationof ions. Owing to the bulkiness and short side chain in sPS, thenumber of the well-connected ionic channels, and the level of theconnectivity in the channels might be lower as compared to that inNafion at same mol% of the ionic groups. Thus, keeping this factorin mind, we had synthesized sPS ionomers of significantly higherIEC value. As the IEC value is higher, the number and size ofmultiplets/ionic clusters will also be high, and thus the clusters areexpected to be comparably closer to each other. The higher numberof the clusters and their closeness is expected to compensate to alarge extent the effect of the non-well connected channels in sPSmembranes on its performance. Most probably, the reasons for thefaster response of the sPS-IPMC as compared to that based on theNafion-IPMC are due to the smoother and thicker electrode layer,and the higher hydraulic force due to the higher water uptakeowing to higher IEC value of the sPS-IPMC.

Under an applied potential of 3 V dc, the tip displacements,measured horizontally, vertically, and diagonally depending on thetype of the curvature generated from the tip movement betweentwo adjacent positions, are shown in Fig. 5b. It was observed thatwithin ca. 65 s, a displacement of ca. 41 mm was reached by thesPS-IPMC. There was an additional ca. 3 mm forward tip movementfor ca. 65 s, and the tip remained at more or less at the sameposition during this period. The back relaxation to the startingposition was very slow and was achieved in ca. 420 s. In the case of

Nafion-IPMC, however, a displacement of ca. 23 mm is achieved inca. 55 s, and back relaxation to the starting position was achievedin ca. 205 s. The difference in the tip displacement can be clearlyseen in Fig. 6.

A significantly higher blocking force (2.76 vs. 1.51 gf), a largerdisplacement (44 vs. 23 mm), a faster response rate (10.3 vs.2.9 mm at starting 3 s), and a slower back relaxation (420 vs. 205 s)in the sPS-IPMC compared to its Nafion counterpart seems verypromising. These interesting results seem, most probably, to bedue to the smoother and thicker electrode layer, the higher tensilemodulus, and the higher hydraulic force due to the higher wateruptake and IEC value of the sulfonated polystyrene-based IPMC,compared to those of the Nafion-IPMC.

Further studies on the effects of various factors on theimprovements in the performance of polystyrene and other ionicpolymers based IPMCs to provide inexpensive and efficientalternatives to conventional IPMCs for actuation, sensing androbotic applications, are currently underway in our lab, and resultswill be reported in forthcoming articles.

4. Conclusions

We synthesized poly(styrene-co-styrenesulfonic acid) s-PSrandom copolymer by Makowski method, prepared and comparedfor the first time, the actuation behaviors of pristine sPS-IPMC withthose of pristine Nafion-IPMC. An optimum amount of the wateruptake by sPS film turned it from highly brittle to a bit strong andflexible film suitable for IPMC applications. The current density, tipdisplacement, response rate and blocking force were significantlyhigher for sPS-IPMC compared to those for the Nafion-IPMC. ThesPS-IPMC also registered slower back relaxations than its Nafioncounterpart. These unexpected electromechanical behaviors wereattributed to the comparably thicker electrode layer, the highermechanical modulus, and the inherently higher IEC value of thesPS-IPMC. The present study demonstrated that an economicallyfeasible sulfonated PS could be applied for IPMC applications.

Acknowledgements

This work was supported by a grant from the Fundamental R&DProgram for Core Technology of Materials funded by the Ministryof Knowledge Economy, Republic of Korea and a grant (KRF-2006-005-J03302) from the Korea Research Foundation.

M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55 55

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