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Sulfonated polystyrene-based ionic polymer–metal composite (IPMC) actuator
Journal of Industrial and Engineering Chemistry 17 (2011) 49–55
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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 b
Artificial Muscle Research Center, Seoul 143-701, Republic of Korea 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
A B S T R A C T
Article history: Received 21 December 2009 Accepted 10 March 2010 Available online 8 October 2010
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 NafionIPMC. The sPS-based IPMCs seem to be one of the promising alternatives of the conventional expensive IPMCs. ß 2010 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Sulfonated polystyrene Ionomers Ionic polymer–metal composites (IPMCs) Actuators Artificial muscles
1. Introduction Organic polymers are increasingly being used in numerous mature and cutting-edge technologies owing to their robust behavior, versatile properties, easy processability, etc. The polymers which can be stimulated to change shape and/or size have been known and studied for years. There are many ways by which materials can be activated, for example, electrical, chemical, pneumatic, optical and magnetic mechanisms. Among these mechanisms, electrical stimulation is one of the most promising ways to generate elastic deformation in either electronic or ionic polymers. Ionic polymer metal composites (IPMCs) are one of the most promising electroactive polymeric (EAPs) materials which can be used to mimic the biological muscles, for their large bending deformation under low voltage of electricity, and are used as dynamic sensors, robotic actuators and artificial muscles [1–3]. IPMCs are composed of swollen ionomer membrane having a solvent usually water for the migration of the free cations (in the case of cation-exchange membranes), sandwiched between two layers of electrodes made up of noble metals deposited to both of the faces of the membranes. Under low electric potential, the movable 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: [email protected] (Y.-T. Yoo).
opposite charged electrode, cathode, creating a volume difference between both sides of the electrodes, leading to a bending deformation of membrane towards the anode [4]. The electromechanical (actuation) and mechanoelectrical (sensing) responses of an IPMC membrane depend on many factors including types of the ionomer (the backbone, acidic comonomer, immovable ion, and distance of the ion from the backbone), counter ion, solvent, solvent uptake and electrode [5]. As the ionomer membranes provide the pathways to the solvated cations, the characteristics of the membrane material could be important factors to decide the fate of the IPMC performance. A survey of the latest open literature suggests that the polymer matrices for the IPMC applications are largely limited to a number of perfluorinated polymers including DuPont’s Nafion1 and Asahi Chemical’s Aciplex1, most probably for the reasons of their excellent mechanical strengths, chemical stability and high proton conductivity, and to avoid synthetic complexicities associated with new polymers. At the same time, in general, a short operation time, a low generative blocking force, extreme expensiveness, and less environment friendliness [6] of these conventional IPMCs have motivated researchers to find easily available and synthesizable, affordable, high-performing, and environmentally more acceptable alternatives to these IPMCs. Shahinpoor and Kim reported solid-state polymer actuators based on poly(ethylene oxide) and poly(ethylene glycol). These materials were capable of exhibiting large bending motion with considerable stress, fast responses and a stable operation over ten millions of cycles in air with nearly no performance degradation
1226-086X/$ – see front matter ß 2010 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2010.10.008
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[7]. Akle et al. studied the effects of polymer structure and electrode composition on the electromechanical actuation response 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 relationship between charge accumulation at the polymer–metal interface and transducer performance [8,9]. Kim et al. fabricated IPMCs based on chitosan-polyaniline ion-exchange membranes using freeze-drying method. Under dc voltage, freeze-dried samples showed a faster and larger bending motion than non-freeze-dried samples, and no back relaxation was observed with time [10]. Phillips and Moore tried sulfonated ethylene vinyl alcohol copolymer membranes for IPMC applications. They showed that these membranes behaved similar to the Nafion1, however, the actuation kinetics were significantly slower [11]. Han et al. reported IPMC actuators based on fluoropolymers grafted with sulfonated polystyrene. The bending displacements (with negligible back relaxation) of these IPMCs although very small (ca. 3 mm at 2 V dc), were few times higher than that of the Nafion-IPMC of similar thickness [12]. Sulfonated poly(styrene-b-ethylene-co-butylene-b-styrene) based membranes were tested for IMPC applications. The IPMC actuators were found to show very small (ca. 1 mm at 2 V dc) but a bit highspeed bending movements under dc voltage [13]. Very recently, Lu et al. presented a study on fabrication and actuation of EAP actuators based on poly(styrene-alt-maleimide) (PSMI)-incorporated poly(vinylidene fluoride) (PVDF) ionic networking polymers (INPs). They showed better mechanical displacement performance of these non-conventional actuators as compared to their traditional widely used Nafion1 counterparts [14]. Few other studies have been performed to present alternatives to the conventional IPMCs. Interested readers are referred to the concerned articles [15–19]. From the literature survey presented, we came to know that so far there is only a limited progress in finding unique and effective alternatives to conventional materials used for IPMC applications. Thus, it is highly desirable to fill this technological gap. To fulfill this demand, herein, we report a high-performance IPMC based on sulfonated polystyrene ionomers (sPS). There are few EAP related papers reporting the use of sPS for introducing ionic groups in nonionic polymers other than polystyrene or polystyrene based copolymers where styrene may be the main monomer unit. However, to the best of our knowledge, the use of sPS itself as the ion conducting membrane for IPMC applications has not been tried. The sPS has widely been used for many applications including as ion-exchange materials for waste water treatment, as proton-exchange membranes in fuel cells [20,21], and as humidity sensors [22]. It is one of the affordable and commercially available organic polymers. It can also easily be tuned with desired levels of sulfonation for the improved water absorption and proton conductivity; important characteristics of ionic membranes needed for high performance of an IPMC, and can be fabricated conveniently for the desired applications. The comparison of actuation performance of sPS based IPMC with that of Nafion-IPMC is presented. 2. Experimental 2.1. Materials A Nafion dispersion (DuPont DE-2021, 20 wt.% in a mixture of propanol 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%), hydrochloric acid (HCl) (35.0–37.0%), methyl alcohol (99.5%) and sodium hydroxide (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 Junsei Chemicals, Japan. All of the materials were used as received without further purification. 2.2. Partial sulfonation of polystyrene Lightly sulfonated PS copolymer, poly(styrene-co-styrenesulfonic acid) (sPS) was synthesized using a method analogous to that reported by Makowski et al. [23]. The sulfonation reaction was carried out in 1,2-dichloroethane at ca. 60 8C for 2 h. By controlling the amount of the sulfonating agent (acetyl sulfate), the degree of sulfonation can be adjusted as per need. The reaction was terminated 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 jelly mass. It was followed by the addition of 35–37% HCl. The sPS copolymer was easily coagulated into a solid mass. The polymer was cut into small pieces, put into excess of n-hexane to remove the solvents entrapped in the polymer mass, pulverized the polymer mass into powder, followed by washing several times with n-hexane. The sample was finally dried at 60 8C under a vacuum 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 the dry polymer) values of the sPS copolymers synthesized in different batches, the copolymer samples were dissolved in a benzene/ methanol (1/1, v/v) mixture to make a 5% (w/v) solution and titrated with methanolic NaOH solution to the phenolphthalein end point. The IEC were found to be 1.75 and 2.23 meq/g, respectively. The IEC value of Nafion engaged in this study was 0.95 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 Teflon mold, and dried at 40, 70 and 100 8C for 12 h each, followed by annealing at 120 8C for 3 h. As per our experience, if we start drying the sPS membranes at high temperatures, it is difficult to get bubble free membranes. Thus, the drying temperatures were slowly increased to ensure the formation of bubble free membranes and complete removal of the solvents from the membranes. The sPS membranes (sPS1 and sPS2) were brittle in the dry (dehydrated) state. To reduce the brittleness significantly, these were soaked into water at ca. 60 8C for ca. 6 and 4 h, respectively. Series of water soaking experiments at different time and temperature were performed. The mentioned temperature and time were found optimum to let the either membrane attain a considerable level of mechanical strength and water absorption for an expected optimum actuation performance and ion conductivity. At lower temperatures, membranes either did not absorb a considerable amount of water, and hence, it was not easy to get flexible membranes, or it took more time to get the desirable results. On the other hand, the higher temperature and 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 to let it absorb almost a maximum amount of water. The hydrated membranes were kept in water at room temperature until used. There was negligible change in the wt.% of the soaked water after keeping the membranes in water at room temperature for many days.
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Table 1 Properties 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 sPS2 Nafion
1.75 2.23 0.95
0.064 0.069 0.111
103 113 45
– 33 37
214 154 39
9.08 6.18 5.41
9.70 10.85 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.
2.5. Electroless plating of the membranes IPMCs were fabricated using an analogous electroless plating method described in the literature [24,25]. Membranes were roughened by sand paper on both sides, cleaned ultrasonically for 30 min, treated with 2.0 N HCl aqueous (aq.) solution at 45 8C for 6 h followed by washing with water at 45 8C for 1 h. To exchange the protons of the membranes with platinum ions, the membranes were 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]Cl2xH2O) and 1 ml of 5.0 wt.% of aq. NH4OH solution at room temperature for 4 h. The membranes were rinsed with, and stirred in, 180 ml water at 45 8C. For reduction of platinum ions into metal, 2 ml of 5.0 wt.% aq. NaBH4 solution was added every 30 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 reduction process, the membranes were rinsed with water, and placed in 0.1 N HCl solution at room temperature for 1 h to convert the membranes into an acid form. 2.6. Characterization of the membranes Proton conductivity was measured in water at room temperature using a complex impedance analyzer (IM6ex, Zahner) and a custom-made cell for the normal four-point probe technique. Mechanical properties of hydrated membranes were measured with a universal testing machine (Model 4468, Instron) with a crosshead speed of 1 mm/min. Surface and cross-sectional micrographs of the IPMC membranes were obtained using a scanning electron microscope (SEM) (JSM-6380, Jeol). For examining the electrical characteristics of the IPMC samples, cyclic voltammetry (CV) and potentiostatic analysis experiments were performed. CV curves were obtained with a potentiostat– galvanostat (WMPG-1000, Wonatech) in a water environment after 30 cycles of a triangle voltage input of 3 V with a step of 100 mV/s. Potentiostatic analysis was carried out with a step voltage of 3 V and a frequency of 0.1 Hz. The blocking force and tip displacement 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 were 5 mm (width) 25 mm (length) ca. 0.42 (thickness) mm size. The sample were vertically supported by a gold grip in air and fixed to 5 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 on many factors including types of the ionomer (e.g. the backbone, the acidic co-monomer, the immovable ion, and the distance of the ion from the backbone), counter ion, solvent, solvent uptake and electrode. The deformation of an IPMC sample under an applied voltage is believed to be based on the movement of solvated cations and free solvent molecules towards the cathode. The immovable 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 acid groups, the higher the water uptake, an important factor in determining the performance of an IPMC sample. The IEC values and other properties of the membranes are mentioned in Table 1. The higher IEC values of the sPS membrane allow higher levels of water 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, having 2.23 meq/g of IEC value, was selected for IPMC studies for its comparably higher water uptake and proton conductivity, and the lower brittleness. The time and temperature of the water uptake measurement for sPS and Nafion membranes were different, however, it was adjusted so for an optimum level of water uptake and dimensional stability of the membranes. At 60 8C, the temperature used for water uptake measurement of sPS membrane, Nafion membrane does not absorb significant amount of water. The higher water uptake by Nafion membrane does not adversely affect its dimensional stability; rather it may increase the proton conductivity, and hence, may lead in an expected improvement in its actuation performance. Although the IEC value for the selected sPS sample (2.23 meq/g) is significantly higher than that for the Nafion dispersion (0.95 meq/g), the proton conductivity of the sPS membrane (0.069 S/cm) is lower than that of the Nafion (0.111 S/cm) membrane. We are not exactly sure for the reasons for a different behavior by both membranes. Most probably, it is due to the difference in types of the ionic clustering in both systems. Well-defined hydrophilic ion-conducting channels, a very unique morphology of fully hydrated perflourinated membranes only, are formed if the membranes are properly heattreated, 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 IPMCs prepared from sPS and Nafion. (a) and (c) images are from as prepared IPMCs, while (b) and (d) images are from the IPMC samples run in air for ca. 30 min under 3 V dc, for sPS- and NafionIPMCs, respectively. A smooth Pt surface with well inter-connected large domains leaving behind negligible space at joints is seen in micrograph ‘a’ (as prepared sPS-IPMC), while a good deposition of a bit non-uniform sized Pt particles is observed in micrograph ‘c’ (as prepared Nafion-IPMC). The electrode surface of actuation tested sPS-IPMC damaged to some extent resulting in a few small interconnected and comparably rough domains with considerable space at joints. The electrode surface of Nafion-IPMC, however, seems negligibly affected. The representative cross-sectional SEM micrographs of as prepared sPS- and Nafion-IPMCs are shown in Fig. 2(a) and (b). The thickness of electrode layers seems to be 23–32 mm and 7–11 mm in sPS and Nafion-IPMCs, respectively. The observed thickness of Pt electrode layers in these IPMCs is one of the highest reported so far for other IPMCs including Nafion based ones [1–4]. The smoothness of the electrode surfaces and large thickness of electrode layers are expected to contribute in enhancing the performance of these IPMCs by (1) exhibiting the so-called ‘‘granular damming
[()TD$FIG]
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Fig. 1. Surface SEM micrographs of IPMCs before and after actuation of few cycles at 3 V dc: (a) and (b) for sPS-IPMC, and (c) and (d) for Nafion-IPMC.
effect’’, making it more difficult for solvent molecules to pass through, thus, is expected to check the water leakage at least to some extent [1], and (2) improvement in the surface conductivity of the IPMCs [25].
[()TD$FIG]
3.2.2. Water loss from IPMCs The water loss from these IPMCs was determined by weighing the 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 of the electrode layers are the important reasons for their short lifetime. The important mechanisms for loss of inner liquid (e.g. water) include (1) leakage from the damaged or porous electrode surface, (2) natural evaporation, and (3) electrolysis. It is evident that the water loss in both IPMCs is almost same. As the loss of water from natural evaporation and electrolysis is applicable to both IPMCs, only the water leakage from the electrode surface seems to matter. As evidenced from SEM micrographs, on the application of electric potential, the smooth and almost cavity free electrode surface of the sPS-IPMC turns into comparably rough and
fractured surface with few visible cavities. The electrode surface in Nafion-IPMC before and after the application of electric potential, however, does not change significantly. Thus, one may speculate that it may be comparably easier for water to leak from the fractured electrode layer of sPS-IPMC. However, as the thickness of electrode layer of sPS-IPMC is significantly higher, the two opposing factors (fracture in and thickness of electrode layer) may annihilate the effects of each other, leading to a negligible water leakage from electrode layer. Therefore, if our interpretation would be accepted, we believe that the factors applicable to both systems, i.e. natural evaporation and electrolysis, are the main mechanisms for water loss in these systems. 3.2.3. Electrical properties of IPMCs Cyclic voltammetry and potentiostats were used for analyzing electrical properties of the prepared IPMCs. Shown in Fig. 3 is current–voltage hysteresis curves recorded under a 3 V triangle voltage input with a scan rate of 100 mV/s. It would be worthy to remind that the shape of the I–V hysteresis curves generally reflects
Fig. 2. Cross-sectional SEM micrographs of (a) sPS- and (b) Nafion-IPMCs.
[()TD$FIG]
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counterpart, which is attributed to the reasons mentioned above. Another interesting feature lies in that the charge transfer under time t, reflected by the area under curves, is seen to be significantly higher for sPS-IPMC. This implies a considerably higher charge consumption capacity of sPS-IPMCs compared to that of the Nafion-IPMC, which is expected to lead an increase in the blocking force and tip displacement under an applied voltage. 3.3. Tip blocking force and displacement
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.
the movement of hydrated ions induced by the applied voltage, with decomposition profile of water resulting from its electrolysis at ca. 1.5 V. It is observed that the current-density of the sPS-IPMC is significantly higher than that of the Nafion-based one. We propose that the higher IEC value of sPS-IPMC in comparison to Nafion based one, and the expected higher surface conductivity owing to the smoothness of electrode surface and higher thickness of electrode layer are two important contributing factors for considerably higher current density of sPS-IPMC. It may be interesting to note that in comparison to other IPMC systems reported so far, under comparable experimental conditions, the observed current-densities for these IPMCs are significantly higher. The smoothness of electrode surfaces and higher thickness of electrode layers are, again, important factors for the same. The current density generally reflects energy storage ability of a system; the higher the current density, the higher the performance of an IPMC, by a larger deformation under an electric field. The result of potentiostatic experiment is shown in Fig. 4. The potential was initially kept at 3 V for 30 s, during which the current decreased almost exponentially. Again, it is seen that the current density is considerably higher for sPS-IPMC compared to its Nafion
Few of the important desirable properties those may lead an IPMC to be applicable in real life applications are high tip blocking force, large tip displacement and low or negligible back relaxation. The blocking force and the tip displacement were measured under an electric potential of 3 V dc to evaluate the actuation performance of IPMCs. Shown in Fig. 5a is the representative blocking force profiles for these IPMCs. The dimension of IPMC samples was 5 mm 20 mm with a thickness of ca. 420 mm. It was observed 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 value of 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 relaxation causing a significant decay in the blocking force is observed in both systems. This type of relaxation in IPMCs is believed due mainly to
[()TD$FIG]
[()TD$FIG]
Fig. 4. Potentiometric analysis curves as a function of time for sPS and Nafion IPMCs, obtained at 3 V, 0.1 Hz.
Fig. 5. Time-dependent (a) blocking force and (b) tip displacement of sPS- and Nafion-IPMCs obtained under 3 V dc. The inset in (b) is the plot in the shorter time scale (0–7 s).
[()TD$FIG]
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Fig. 6. Displacement images of IPMCs at varying times under 3 V dc.
the residual stress induced by the difference in the stiffness between the electrolyte and electrode layer, as well as to the loss of water [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 and connected randomly distributed ionic channels in sPS membrane. It would be worthy to remind that like Nafion, sPS also has random distribution of the ionic groups throughout the membrane. Despite the random distribution of the ionic groups, the formation of wellconnected ionic channels in Nafion based membranes takes place, and is well known and documented [29,30]. The flexibility of the long side chains and randomness of ionic groups facilitates the formation of well-connected ionic channels in Nafion, if properly heat treated and have sufficient water uptake. This is unique to perflourinated membranes. Formation of the ionic clusters in sPS based materials is also well documented [29]. Ion-hopping i.e. the movement of the ionic groups from one cluster to another in sPS based ionomeric membranes is responsible for the transportation of ions. Owing to the bulkiness and short side chain in sPS, the number of the well-connected ionic channels, and the level of the connectivity in the channels might be lower as compared to that in Nafion at same mol% of the ionic groups. Thus, keeping this factor in mind, we had synthesized sPS ionomers of significantly higher IEC value. As the IEC value is higher, the number and size of multiplets/ionic clusters will also be high, and thus the clusters are expected to be comparably closer to each other. The higher number of the clusters and their closeness is expected to compensate to a large extent the effect of the non-well connected channels in sPS membranes on its performance. Most probably, the reasons for the faster response of the sPS-IPMC as compared to that based on the Nafion-IPMC are due to the smoother and thicker electrode layer, and the higher hydraulic force due to the higher water uptake owing 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 the type of the curvature generated from the tip movement between two adjacent positions, are shown in Fig. 5b. It was observed that within ca. 65 s, a displacement of ca. 41 mm was reached by the sPS-IPMC. There was an additional ca. 3 mm forward tip movement for ca. 65 s, and the tip remained at more or less at the same position during this period. The back relaxation to the starting position 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 in ca. 55 s, and back relaxation to the starting position was achieved in ca. 205 s. The difference in the tip displacement can be clearly seen in Fig. 6. A significantly higher blocking force (2.76 vs. 1.51 gf), a larger displacement (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 very promising. These interesting results seem, most probably, to be due to the smoother and thicker electrode layer, the higher tensile modulus, and the higher hydraulic force due to the higher water uptake 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 the improvements in the performance of polystyrene and other ionic polymers based IPMCs to provide inexpensive and efficient alternatives to conventional IPMCs for actuation, sensing and robotic applications, are currently underway in our lab, and results will be reported in forthcoming articles. 4. Conclusions We synthesized poly(styrene-co-styrenesulfonic acid) s-PS random copolymer by Makowski method, prepared and compared for the first time, the actuation behaviors of pristine sPS-IPMC with those of pristine Nafion-IPMC. An optimum amount of the water uptake by sPS film turned it from highly brittle to a bit strong and flexible film suitable for IPMC applications. The current density, tip displacement, response rate and blocking force were significantly higher for sPS-IPMC compared to those for the Nafion-IPMC. The sPS-IPMC also registered slower back relaxations than its Nafion counterpart. These unexpected electromechanical behaviors were attributed to the comparably thicker electrode layer, the higher mechanical modulus, and the inherently higher IEC value of the sPS-IPMC. The present study demonstrated that an economically feasible sulfonated PS could be applied for IPMC applications. Acknowledgements This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and a grant (KRF-2006005-J03302) from the Korea Research Foundation.
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References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Shahinpoor, K.J. Kim, Smart Mater. Struct. 10 (2001) 819. K.J. Kim, M. Shahinpoor, Smart Mater. Struct. 12 (2003) 65. M. Shahinpoor, K.J. Kim, Smart Mater. Struct. 14 (2005) 197. S.N. Nasser, J.Y. Li, J. Appl. Phys. 87 (2000) 3321. J.D.W. Madden, A.N. Vandesteeg, P.A. Anquetil, P.G.A. Madden, A. Takshi, R.Z. Pytel, S.R. Lafontaine, P.A. Wieringa, I.W. Hunter, IEEE J. Oceanic Eng. 20 (2004) 706. S. Holmberg, P. Holmlund, R. Nicolas, C.E. Wilen, T. Kallio, G. Sundholm, F. Sundholm, Macromolecules 37 (2004) 9909. M. Shahinpoor, K.J. Kim, Appl. Phys. Lett. 80 (2002) 3445. B.J. Akle, D.J. Leo, M.A. Hickner, J.E. McGrath, J. Mater. Sci. 40 (2005) 3715. B.J. Akle, M.D. Bennett, D.J. Leo, Proc. SPIE 5759 (2005) 153. S.J. Kim, M.S. Kim, S.R. Shin, I.Y. Kim, S.I. Kim, S.H. Lee, T.S. Lee, G.M. Spinks, Smart Mater. Struct. 14 (2005) 889. A.K. Phillips, R.B. Moore, Polymer 46 (2005) 7788. M.J. Han, J.H. Park, J.Y. Lee, J.Y. Jho, Macromol. Rapid Commun. 27 (2006) 219. X.L. Wang, I.K. Oh, J. Lu, J. Ju, S. Lee, Mater. Lett. 61 (2007) 5117. J. Lu, S.G. Kim, S. Lee, I.K. Oh, Smart Mater. Struct. 17 (2008) 045002. T. Zeng, R. Claus, F. Zhang, W. Du, K.L. Cooper, Smart Mater. Struct. 10 (2001) 780. H.M. Jeong, S.M. Woo, S. Lee, G.C. Cha, M.S. Mun, J. Appl. Polym. Sci. 99 (2005) 1732.
55
[17] C. Hong, J.D. Nam, Y. Tak, J. Korean Electrochem. Soc. 9 (2006) 137. [18] H.Y. Jeong, B.K. Kim, J. Appl. Polym. Sci. 99 (2006) 2687. [19] C. Bartholome, A. Derre´, O. Roubeau, C. Zakri, P. Poulin, Nanotechnology 19 (2008) 325501. [20] V. Neburchilov, J. Martin, H. Wang, J. Zhang, J. Power Sources 169 (2007) 221. [21] N. Carretta, V. Tricoli, F. Picchioni, J. Membr. Sci. 166 (2000) 189. [22] C.P.L. Rubinger, C.R. Martins, M.A.D. Paoli, R.M. Rubinger, Sens. Actuators B 123 (2007) 42. [23] H.S. Makowski, R.D. Lundberg, G.H. Singhal, Flexible polymeric compositions comprising a normally plastic polymer sulfonated to about 0.2 to about 10 mol% sulfonate, U.S. Patent 3870841, 1975. [24] K. Asaka, K. Oguro, Y. Nishimura, M. Mizuhata, H. Takenaka, Polym. J. 27 (1995) 436. [25] V.K. Nguyen, J.W. Lee, Y.T. Yoo, Sens. Actuators B 120 (2007) 529. [26] J.H. Lee, J.D. Nam, Sens. Actuators A 118 (2005) 98. [27] S.N. Nasser, Y. Wu, J. Appl. Phys. 93 (2003) 5255. [28] H. Tamagawa, K. Yagasaki, F. Nogata, J. Appl. Phys. 92 (2002) 7614. [29] A. Eisenberg, J.S. Kim, Introduction to Ionomers, first ed., John Wiley & Sons, Inc., New York, 1998. [30] M.R. Tant, K.A. Mauritz, G.L. Wilkes, Ionomers: Synthesis, Structure, Properties and Applications, first ed., Blackie Academic Professional, New York, 1996 .
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
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 b
Artificial Muscle Research Center, Seoul 143-701, Republic of Korea 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
A B S T R A C T
Article history: Received 21 December 2009 Accepted 10 March 2010 Available online 8 October 2010
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 NafionIPMC. The sPS-based IPMCs seem to be one of the promising alternatives of the conventional expensive IPMCs. ß 2010 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Sulfonated polystyrene Ionomers Ionic polymer–metal composites (IPMCs) Actuators Artificial muscles
1. Introduction Organic polymers are increasingly being used in numerous mature and cutting-edge technologies owing to their robust behavior, versatile properties, easy processability, etc. The polymers which can be stimulated to change shape and/or size have been known and studied for years. There are many ways by which materials can be activated, for example, electrical, chemical, pneumatic, optical and magnetic mechanisms. Among these mechanisms, electrical stimulation is one of the most promising ways to generate elastic deformation in either electronic or ionic polymers. Ionic polymer metal composites (IPMCs) are one of the most promising electroactive polymeric (EAPs) materials which can be used to mimic the biological muscles, for their large bending deformation under low voltage of electricity, and are used as dynamic sensors, robotic actuators and artificial muscles [1–3]. IPMCs are composed of swollen ionomer membrane having a solvent usually water for the migration of the free cations (in the case of cation-exchange membranes), sandwiched between two layers of electrodes made up of noble metals deposited to both of the faces of the membranes. Under low electric potential, the movable 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: [email protected] (Y.-T. Yoo).
opposite charged electrode, cathode, creating a volume difference between both sides of the electrodes, leading to a bending deformation of membrane towards the anode [4]. The electromechanical (actuation) and mechanoelectrical (sensing) responses of an IPMC membrane depend on many factors including types of the ionomer (the backbone, acidic comonomer, immovable ion, and distance of the ion from the backbone), counter ion, solvent, solvent uptake and electrode [5]. As the ionomer membranes provide the pathways to the solvated cations, the characteristics of the membrane material could be important factors to decide the fate of the IPMC performance. A survey of the latest open literature suggests that the polymer matrices for the IPMC applications are largely limited to a number of perfluorinated polymers including DuPont’s Nafion1 and Asahi Chemical’s Aciplex1, most probably for the reasons of their excellent mechanical strengths, chemical stability and high proton conductivity, and to avoid synthetic complexicities associated with new polymers. At the same time, in general, a short operation time, a low generative blocking force, extreme expensiveness, and less environment friendliness [6] of these conventional IPMCs have motivated researchers to find easily available and synthesizable, affordable, high-performing, and environmentally more acceptable alternatives to these IPMCs. Shahinpoor and Kim reported solid-state polymer actuators based on poly(ethylene oxide) and poly(ethylene glycol). These materials were capable of exhibiting large bending motion with considerable stress, fast responses and a stable operation over ten millions of cycles in air with nearly no performance degradation
1226-086X/$ – see front matter ß 2010 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2010.10.008
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[7]. Akle et al. studied the effects of polymer structure and electrode composition on the electromechanical actuation response 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 relationship between charge accumulation at the polymer–metal interface and transducer performance [8,9]. Kim et al. fabricated IPMCs based on chitosan-polyaniline ion-exchange membranes using freeze-drying method. Under dc voltage, freeze-dried samples showed a faster and larger bending motion than non-freeze-dried samples, and no back relaxation was observed with time [10]. Phillips and Moore tried sulfonated ethylene vinyl alcohol copolymer membranes for IPMC applications. They showed that these membranes behaved similar to the Nafion1, however, the actuation kinetics were significantly slower [11]. Han et al. reported IPMC actuators based on fluoropolymers grafted with sulfonated polystyrene. The bending displacements (with negligible back relaxation) of these IPMCs although very small (ca. 3 mm at 2 V dc), were few times higher than that of the Nafion-IPMC of similar thickness [12]. Sulfonated poly(styrene-b-ethylene-co-butylene-b-styrene) based membranes were tested for IMPC applications. The IPMC actuators were found to show very small (ca. 1 mm at 2 V dc) but a bit highspeed bending movements under dc voltage [13]. Very recently, Lu et al. presented a study on fabrication and actuation of EAP actuators based on poly(styrene-alt-maleimide) (PSMI)-incorporated poly(vinylidene fluoride) (PVDF) ionic networking polymers (INPs). They showed better mechanical displacement performance of these non-conventional actuators as compared to their traditional widely used Nafion1 counterparts [14]. Few other studies have been performed to present alternatives to the conventional IPMCs. Interested readers are referred to the concerned articles [15–19]. From the literature survey presented, we came to know that so far there is only a limited progress in finding unique and effective alternatives to conventional materials used for IPMC applications. Thus, it is highly desirable to fill this technological gap. To fulfill this demand, herein, we report a high-performance IPMC based on sulfonated polystyrene ionomers (sPS). There are few EAP related papers reporting the use of sPS for introducing ionic groups in nonionic polymers other than polystyrene or polystyrene based copolymers where styrene may be the main monomer unit. However, to the best of our knowledge, the use of sPS itself as the ion conducting membrane for IPMC applications has not been tried. The sPS has widely been used for many applications including as ion-exchange materials for waste water treatment, as proton-exchange membranes in fuel cells [20,21], and as humidity sensors [22]. It is one of the affordable and commercially available organic polymers. It can also easily be tuned with desired levels of sulfonation for the improved water absorption and proton conductivity; important characteristics of ionic membranes needed for high performance of an IPMC, and can be fabricated conveniently for the desired applications. The comparison of actuation performance of sPS based IPMC with that of Nafion-IPMC is presented. 2. Experimental 2.1. Materials A Nafion dispersion (DuPont DE-2021, 20 wt.% in a mixture of propanol 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%), hydrochloric acid (HCl) (35.0–37.0%), methyl alcohol (99.5%) and sodium hydroxide (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 Junsei Chemicals, Japan. All of the materials were used as received without further purification. 2.2. Partial sulfonation of polystyrene Lightly sulfonated PS copolymer, poly(styrene-co-styrenesulfonic acid) (sPS) was synthesized using a method analogous to that reported by Makowski et al. [23]. The sulfonation reaction was carried out in 1,2-dichloroethane at ca. 60 8C for 2 h. By controlling the amount of the sulfonating agent (acetyl sulfate), the degree of sulfonation can be adjusted as per need. The reaction was terminated 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 jelly mass. It was followed by the addition of 35–37% HCl. The sPS copolymer was easily coagulated into a solid mass. The polymer was cut into small pieces, put into excess of n-hexane to remove the solvents entrapped in the polymer mass, pulverized the polymer mass into powder, followed by washing several times with n-hexane. The sample was finally dried at 60 8C under a vacuum 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 the dry polymer) values of the sPS copolymers synthesized in different batches, the copolymer samples were dissolved in a benzene/ methanol (1/1, v/v) mixture to make a 5% (w/v) solution and titrated with methanolic NaOH solution to the phenolphthalein end point. The IEC were found to be 1.75 and 2.23 meq/g, respectively. The IEC value of Nafion engaged in this study was 0.95 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 Teflon mold, and dried at 40, 70 and 100 8C for 12 h each, followed by annealing at 120 8C for 3 h. As per our experience, if we start drying the sPS membranes at high temperatures, it is difficult to get bubble free membranes. Thus, the drying temperatures were slowly increased to ensure the formation of bubble free membranes and complete removal of the solvents from the membranes. The sPS membranes (sPS1 and sPS2) were brittle in the dry (dehydrated) state. To reduce the brittleness significantly, these were soaked into water at ca. 60 8C for ca. 6 and 4 h, respectively. Series of water soaking experiments at different time and temperature were performed. The mentioned temperature and time were found optimum to let the either membrane attain a considerable level of mechanical strength and water absorption for an expected optimum actuation performance and ion conductivity. At lower temperatures, membranes either did not absorb a considerable amount of water, and hence, it was not easy to get flexible membranes, or it took more time to get the desirable results. On the other hand, the higher temperature and 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 to let it absorb almost a maximum amount of water. The hydrated membranes were kept in water at room temperature until used. There was negligible change in the wt.% of the soaked water after keeping the membranes in water at room temperature for many days.
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Table 1 Properties 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 sPS2 Nafion
1.75 2.23 0.95
0.064 0.069 0.111
103 113 45
– 33 37
214 154 39
9.08 6.18 5.41
9.70 10.85 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.
2.5. Electroless plating of the membranes IPMCs were fabricated using an analogous electroless plating method described in the literature [24,25]. Membranes were roughened by sand paper on both sides, cleaned ultrasonically for 30 min, treated with 2.0 N HCl aqueous (aq.) solution at 45 8C for 6 h followed by washing with water at 45 8C for 1 h. To exchange the protons of the membranes with platinum ions, the membranes were 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]Cl2xH2O) and 1 ml of 5.0 wt.% of aq. NH4OH solution at room temperature for 4 h. The membranes were rinsed with, and stirred in, 180 ml water at 45 8C. For reduction of platinum ions into metal, 2 ml of 5.0 wt.% aq. NaBH4 solution was added every 30 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 reduction process, the membranes were rinsed with water, and placed in 0.1 N HCl solution at room temperature for 1 h to convert the membranes into an acid form. 2.6. Characterization of the membranes Proton conductivity was measured in water at room temperature using a complex impedance analyzer (IM6ex, Zahner) and a custom-made cell for the normal four-point probe technique. Mechanical properties of hydrated membranes were measured with a universal testing machine (Model 4468, Instron) with a crosshead speed of 1 mm/min. Surface and cross-sectional micrographs of the IPMC membranes were obtained using a scanning electron microscope (SEM) (JSM-6380, Jeol). For examining the electrical characteristics of the IPMC samples, cyclic voltammetry (CV) and potentiostatic analysis experiments were performed. CV curves were obtained with a potentiostat– galvanostat (WMPG-1000, Wonatech) in a water environment after 30 cycles of a triangle voltage input of 3 V with a step of 100 mV/s. Potentiostatic analysis was carried out with a step voltage of 3 V and a frequency of 0.1 Hz. The blocking force and tip displacement 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 were 5 mm (width) 25 mm (length) ca. 0.42 (thickness) mm size. The sample were vertically supported by a gold grip in air and fixed to 5 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 on many factors including types of the ionomer (e.g. the backbone, the acidic co-monomer, the immovable ion, and the distance of the ion from the backbone), counter ion, solvent, solvent uptake and electrode. The deformation of an IPMC sample under an applied voltage is believed to be based on the movement of solvated cations and free solvent molecules towards the cathode. The immovable 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 acid groups, the higher the water uptake, an important factor in determining the performance of an IPMC sample. The IEC values and other properties of the membranes are mentioned in Table 1. The higher IEC values of the sPS membrane allow higher levels of water 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, having 2.23 meq/g of IEC value, was selected for IPMC studies for its comparably higher water uptake and proton conductivity, and the lower brittleness. The time and temperature of the water uptake measurement for sPS and Nafion membranes were different, however, it was adjusted so for an optimum level of water uptake and dimensional stability of the membranes. At 60 8C, the temperature used for water uptake measurement of sPS membrane, Nafion membrane does not absorb significant amount of water. The higher water uptake by Nafion membrane does not adversely affect its dimensional stability; rather it may increase the proton conductivity, and hence, may lead in an expected improvement in its actuation performance. Although the IEC value for the selected sPS sample (2.23 meq/g) is significantly higher than that for the Nafion dispersion (0.95 meq/g), the proton conductivity of the sPS membrane (0.069 S/cm) is lower than that of the Nafion (0.111 S/cm) membrane. We are not exactly sure for the reasons for a different behavior by both membranes. Most probably, it is due to the difference in types of the ionic clustering in both systems. Well-defined hydrophilic ion-conducting channels, a very unique morphology of fully hydrated perflourinated membranes only, are formed if the membranes are properly heattreated, 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 IPMCs prepared from sPS and Nafion. (a) and (c) images are from as prepared IPMCs, while (b) and (d) images are from the IPMC samples run in air for ca. 30 min under 3 V dc, for sPS- and NafionIPMCs, respectively. A smooth Pt surface with well inter-connected large domains leaving behind negligible space at joints is seen in micrograph ‘a’ (as prepared sPS-IPMC), while a good deposition of a bit non-uniform sized Pt particles is observed in micrograph ‘c’ (as prepared Nafion-IPMC). The electrode surface of actuation tested sPS-IPMC damaged to some extent resulting in a few small interconnected and comparably rough domains with considerable space at joints. The electrode surface of Nafion-IPMC, however, seems negligibly affected. The representative cross-sectional SEM micrographs of as prepared sPS- and Nafion-IPMCs are shown in Fig. 2(a) and (b). The thickness of electrode layers seems to be 23–32 mm and 7–11 mm in sPS and Nafion-IPMCs, respectively. The observed thickness of Pt electrode layers in these IPMCs is one of the highest reported so far for other IPMCs including Nafion based ones [1–4]. The smoothness of the electrode surfaces and large thickness of electrode layers are expected to contribute in enhancing the performance of these IPMCs by (1) exhibiting the so-called ‘‘granular damming
[()TD$FIG]
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M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55
Fig. 1. Surface SEM micrographs of IPMCs before and after actuation of few cycles at 3 V dc: (a) and (b) for sPS-IPMC, and (c) and (d) for Nafion-IPMC.
effect’’, making it more difficult for solvent molecules to pass through, thus, is expected to check the water leakage at least to some extent [1], and (2) improvement in the surface conductivity of the IPMCs [25].
[()TD$FIG]
3.2.2. Water loss from IPMCs The water loss from these IPMCs was determined by weighing the 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 of the electrode layers are the important reasons for their short lifetime. The important mechanisms for loss of inner liquid (e.g. water) include (1) leakage from the damaged or porous electrode surface, (2) natural evaporation, and (3) electrolysis. It is evident that the water loss in both IPMCs is almost same. As the loss of water from natural evaporation and electrolysis is applicable to both IPMCs, only the water leakage from the electrode surface seems to matter. As evidenced from SEM micrographs, on the application of electric potential, the smooth and almost cavity free electrode surface of the sPS-IPMC turns into comparably rough and
fractured surface with few visible cavities. The electrode surface in Nafion-IPMC before and after the application of electric potential, however, does not change significantly. Thus, one may speculate that it may be comparably easier for water to leak from the fractured electrode layer of sPS-IPMC. However, as the thickness of electrode layer of sPS-IPMC is significantly higher, the two opposing factors (fracture in and thickness of electrode layer) may annihilate the effects of each other, leading to a negligible water leakage from electrode layer. Therefore, if our interpretation would be accepted, we believe that the factors applicable to both systems, i.e. natural evaporation and electrolysis, are the main mechanisms for water loss in these systems. 3.2.3. Electrical properties of IPMCs Cyclic voltammetry and potentiostats were used for analyzing electrical properties of the prepared IPMCs. Shown in Fig. 3 is current–voltage hysteresis curves recorded under a 3 V triangle voltage input with a scan rate of 100 mV/s. It would be worthy to remind that the shape of the I–V hysteresis curves generally reflects
Fig. 2. Cross-sectional SEM micrographs of (a) sPS- and (b) Nafion-IPMCs.
[()TD$FIG]
M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55
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counterpart, which is attributed to the reasons mentioned above. Another interesting feature lies in that the charge transfer under time t, reflected by the area under curves, is seen to be significantly higher for sPS-IPMC. This implies a considerably higher charge consumption capacity of sPS-IPMCs compared to that of the Nafion-IPMC, which is expected to lead an increase in the blocking force and tip displacement under an applied voltage. 3.3. Tip blocking force and displacement
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.
the movement of hydrated ions induced by the applied voltage, with decomposition profile of water resulting from its electrolysis at ca. 1.5 V. It is observed that the current-density of the sPS-IPMC is significantly higher than that of the Nafion-based one. We propose that the higher IEC value of sPS-IPMC in comparison to Nafion based one, and the expected higher surface conductivity owing to the smoothness of electrode surface and higher thickness of electrode layer are two important contributing factors for considerably higher current density of sPS-IPMC. It may be interesting to note that in comparison to other IPMC systems reported so far, under comparable experimental conditions, the observed current-densities for these IPMCs are significantly higher. The smoothness of electrode surfaces and higher thickness of electrode layers are, again, important factors for the same. The current density generally reflects energy storage ability of a system; the higher the current density, the higher the performance of an IPMC, by a larger deformation under an electric field. The result of potentiostatic experiment is shown in Fig. 4. The potential was initially kept at 3 V for 30 s, during which the current decreased almost exponentially. Again, it is seen that the current density is considerably higher for sPS-IPMC compared to its Nafion
Few of the important desirable properties those may lead an IPMC to be applicable in real life applications are high tip blocking force, large tip displacement and low or negligible back relaxation. The blocking force and the tip displacement were measured under an electric potential of 3 V dc to evaluate the actuation performance of IPMCs. Shown in Fig. 5a is the representative blocking force profiles for these IPMCs. The dimension of IPMC samples was 5 mm 20 mm with a thickness of ca. 420 mm. It was observed 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 value of 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 relaxation causing a significant decay in the blocking force is observed in both systems. This type of relaxation in IPMCs is believed due mainly to
[()TD$FIG]
[()TD$FIG]
Fig. 4. Potentiometric analysis curves as a function of time for sPS and Nafion IPMCs, obtained at 3 V, 0.1 Hz.
Fig. 5. Time-dependent (a) blocking force and (b) tip displacement of sPS- and Nafion-IPMCs obtained under 3 V dc. The inset in (b) is the plot in the shorter time scale (0–7 s).
[()TD$FIG]
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M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55
Fig. 6. Displacement images of IPMCs at varying times under 3 V dc.
the residual stress induced by the difference in the stiffness between the electrolyte and electrode layer, as well as to the loss of water [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 and connected randomly distributed ionic channels in sPS membrane. It would be worthy to remind that like Nafion, sPS also has random distribution of the ionic groups throughout the membrane. Despite the random distribution of the ionic groups, the formation of wellconnected ionic channels in Nafion based membranes takes place, and is well known and documented [29,30]. The flexibility of the long side chains and randomness of ionic groups facilitates the formation of well-connected ionic channels in Nafion, if properly heat treated and have sufficient water uptake. This is unique to perflourinated membranes. Formation of the ionic clusters in sPS based materials is also well documented [29]. Ion-hopping i.e. the movement of the ionic groups from one cluster to another in sPS based ionomeric membranes is responsible for the transportation of ions. Owing to the bulkiness and short side chain in sPS, the number of the well-connected ionic channels, and the level of the connectivity in the channels might be lower as compared to that in Nafion at same mol% of the ionic groups. Thus, keeping this factor in mind, we had synthesized sPS ionomers of significantly higher IEC value. As the IEC value is higher, the number and size of multiplets/ionic clusters will also be high, and thus the clusters are expected to be comparably closer to each other. The higher number of the clusters and their closeness is expected to compensate to a large extent the effect of the non-well connected channels in sPS membranes on its performance. Most probably, the reasons for the faster response of the sPS-IPMC as compared to that based on the Nafion-IPMC are due to the smoother and thicker electrode layer, and the higher hydraulic force due to the higher water uptake owing 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 the type of the curvature generated from the tip movement between two adjacent positions, are shown in Fig. 5b. It was observed that within ca. 65 s, a displacement of ca. 41 mm was reached by the sPS-IPMC. There was an additional ca. 3 mm forward tip movement for ca. 65 s, and the tip remained at more or less at the same position during this period. The back relaxation to the starting position 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 in ca. 55 s, and back relaxation to the starting position was achieved in ca. 205 s. The difference in the tip displacement can be clearly seen in Fig. 6. A significantly higher blocking force (2.76 vs. 1.51 gf), a larger displacement (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 very promising. These interesting results seem, most probably, to be due to the smoother and thicker electrode layer, the higher tensile modulus, and the higher hydraulic force due to the higher water uptake 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 the improvements in the performance of polystyrene and other ionic polymers based IPMCs to provide inexpensive and efficient alternatives to conventional IPMCs for actuation, sensing and robotic applications, are currently underway in our lab, and results will be reported in forthcoming articles. 4. Conclusions We synthesized poly(styrene-co-styrenesulfonic acid) s-PS random copolymer by Makowski method, prepared and compared for the first time, the actuation behaviors of pristine sPS-IPMC with those of pristine Nafion-IPMC. An optimum amount of the water uptake by sPS film turned it from highly brittle to a bit strong and flexible film suitable for IPMC applications. The current density, tip displacement, response rate and blocking force were significantly higher for sPS-IPMC compared to those for the Nafion-IPMC. The sPS-IPMC also registered slower back relaxations than its Nafion counterpart. These unexpected electromechanical behaviors were attributed to the comparably thicker electrode layer, the higher mechanical modulus, and the inherently higher IEC value of the sPS-IPMC. The present study demonstrated that an economically feasible sulfonated PS could be applied for IPMC applications. Acknowledgements This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea and a grant (KRF-2006005-J03302) from the Korea Research Foundation.
M. Luqman et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 49–55
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
M. Shahinpoor, K.J. Kim, Smart Mater. Struct. 10 (2001) 819. K.J. Kim, M. Shahinpoor, Smart Mater. Struct. 12 (2003) 65. M. Shahinpoor, K.J. Kim, Smart Mater. Struct. 14 (2005) 197. S.N. Nasser, J.Y. Li, J. Appl. Phys. 87 (2000) 3321. J.D.W. Madden, A.N. Vandesteeg, P.A. Anquetil, P.G.A. Madden, A. Takshi, R.Z. Pytel, S.R. Lafontaine, P.A. Wieringa, I.W. Hunter, IEEE J. Oceanic Eng. 20 (2004) 706. S. Holmberg, P. Holmlund, R. Nicolas, C.E. Wilen, T. Kallio, G. Sundholm, F. Sundholm, Macromolecules 37 (2004) 9909. M. Shahinpoor, K.J. Kim, Appl. Phys. Lett. 80 (2002) 3445. B.J. Akle, D.J. Leo, M.A. Hickner, J.E. McGrath, J. Mater. Sci. 40 (2005) 3715. B.J. Akle, M.D. Bennett, D.J. Leo, Proc. SPIE 5759 (2005) 153. S.J. Kim, M.S. Kim, S.R. Shin, I.Y. Kim, S.I. Kim, S.H. Lee, T.S. Lee, G.M. Spinks, Smart Mater. Struct. 14 (2005) 889. A.K. Phillips, R.B. Moore, Polymer 46 (2005) 7788. M.J. Han, J.H. Park, J.Y. Lee, J.Y. Jho, Macromol. Rapid Commun. 27 (2006) 219. X.L. Wang, I.K. Oh, J. Lu, J. Ju, S. Lee, Mater. Lett. 61 (2007) 5117. J. Lu, S.G. Kim, S. Lee, I.K. Oh, Smart Mater. Struct. 17 (2008) 045002. T. Zeng, R. Claus, F. Zhang, W. Du, K.L. Cooper, Smart Mater. Struct. 10 (2001) 780. H.M. Jeong, S.M. Woo, S. Lee, G.C. Cha, M.S. Mun, J. Appl. Polym. Sci. 99 (2005) 1732.
55
[17] C. Hong, J.D. Nam, Y. Tak, J. Korean Electrochem. Soc. 9 (2006) 137. [18] H.Y. Jeong, B.K. Kim, J. Appl. Polym. Sci. 99 (2006) 2687. [19] C. Bartholome, A. Derre´, O. Roubeau, C. Zakri, P. Poulin, Nanotechnology 19 (2008) 325501. [20] V. Neburchilov, J. Martin, H. Wang, J. Zhang, J. Power Sources 169 (2007) 221. [21] N. Carretta, V. Tricoli, F. Picchioni, J. Membr. Sci. 166 (2000) 189. [22] C.P.L. Rubinger, C.R. Martins, M.A.D. Paoli, R.M. Rubinger, Sens. Actuators B 123 (2007) 42. [23] H.S. Makowski, R.D. Lundberg, G.H. Singhal, Flexible polymeric compositions comprising a normally plastic polymer sulfonated to about 0.2 to about 10 mol% sulfonate, U.S. Patent 3870841, 1975. [24] K. Asaka, K. Oguro, Y. Nishimura, M. Mizuhata, H. Takenaka, Polym. J. 27 (1995) 436. [25] V.K. Nguyen, J.W. Lee, Y.T. Yoo, Sens. Actuators B 120 (2007) 529. [26] J.H. Lee, J.D. Nam, Sens. Actuators A 118 (2005) 98. [27] S.N. Nasser, Y. Wu, J. Appl. Phys. 93 (2003) 5255. [28] H. Tamagawa, K. Yagasaki, F. Nogata, J. Appl. Phys. 92 (2002) 7614. [29] A. Eisenberg, J.S. Kim, Introduction to Ionomers, first ed., John Wiley & Sons, Inc., New York, 1998. [30] M.R. Tant, K.A. Mauritz, G.L. Wilkes, Ionomers: Synthesis, Structure, Properties and Applications, first ed., Blackie Academic Professional, New York, 1996 .