High strain electromechanical actuators based on electrodeposited polypyrrole doped with di-(2-ethylhexyl)sulfosuccinate

Sensors and Actuators B 155 (2011) 278–284

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

High strain electromechanical actuators based on electrodeposited polypyrrole doped with di-(2-ethylhexyl)sulfosuccinate Javad Foroughi, Geoffrey M. Spinks, Gordon G. Wallace ∗ ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia

a r t i c l e

i n f o

Article history: Received 12 July 2010 Received in revised form 6 December 2010 Accepted 21 December 2010 Available online 29 December 2010 Keywords: Polypyrrole Conducting polymer e-Textile Electropolymerization Actuators

a b s t r a c t The low-voltage electromechanical actuation of polypyrrole (PPy) doped with di-(2-ethylhexyl) sulfosuccinate (DEHS) has been investigated. The PPy-DEHS has been prepared both chemically (cast as films from solution) and by more conventional electrochemical polymerization. Very large strains of ∼30% were obtained during slow-scan redox cycling of the electrochemically prepared PPy-DEHS films. In constrast, PPy-DEHS films cast from solutions of the chemically polymerized polymer gave actuation strains of ∼2.5%. The polymerization method was also found to have a significant effect on the structure, conductivity and mechanical properties of the PPy-DEHS materials. The conductivity of the electrochemically polymerized PPy-DEHS was 75 S cm−1 , considerably higher than that found for the chemically derived polymer (7 S cm−1 ). The structure of the PPy-DEHS was further elucidated from UV–vis, Raman and FT-IR spectral studies which indicated that the conjugation length of the PPy could be increased significantly by varying the polymerization method. Films obtained by casting chemically prepared PPyDEHS showed higher modulus (2.3 GPa) than electropolymerized PPy-DEHS (0.6 GPa), but were more brittle. Both materials were electroactive in acetonitrile/water electrolyte. The higher actuation strain observed in the electrochemically prepared films was attributed to a more open molecular structure (as indicated by the lower modulus) allowing for easier ion diffusion and a higher conductivity allowing easier charge transfer. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Low voltage electromechanical actuators are attractive for a range of potential applications and may be suitable replacement materials for high voltage-low strain piezoelectric devices. Conducting polymers are potentially useful actuator materials because of their low voltage operation, relatively high strains [1,2] and reasonably fast response times [3]. A number of demonstration devices have been constructed using polypyrrole actuators [3–5]. Polypyrrole (PPy) is one of the most extensively investigated conducting polymer actuators, however its application is sometimes limited by its intractability. Polypyrrole is normally insoluble in ordinary organic solvents due to the presence of strong interchain interactions [2]. As such, PPy films are normally prepared by electrochemical polymerization of the monomer, which is a fast, simple and clean (usually most of the product is deposited on the working electrode surface) approach to producing highly conductive PPy films. However, due to the insolubility of such polypyrrole films, fabrication of the electropolymerized conductive polymers is

∗ Corresponding author. Tel.: +61 2 4221 3127; fax: +61 2 4221 3114. E-mail address: [email protected] (G.G. Wallace). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.12.035

limited and restricted to the electrode shape and area [2]. A small number of dopant ions are known to produce a form of polypyrrole that is soluble in organic solvents [6–9]. These solutions have then been used to prepare both films [8] and fibres [10,11]. The actuation behaviour of these materials have not previously been reported. In this paper, we specifically compare the actuation behaviour of chemically prepared PPy doped with the solubilising dopant di-(2ethylhexyl)sulfosuccinate (DEHS), and subsequently cast as films, with the equivalent electrochemically prepared PPy-DEHS. The latter was found to be insoluble in organic solvents, have a higher conductivity, lower modulus and significantly higher actuation strain. It is well known [1] that the electrochemical, mechanical, physical and other properties of conducting polymers such as polypyrrole depend upon the synthetic conditions employed during polymerization. In most early studies, conducting polypyrrole was prepared from acetonitrile solvent [12–15]. Later, it was found that pyrrole could be electrochemically polymerized in aqueous solution with various types of dopant counter anions (A− ) to give PPy-A films of good conductivity. With detergent anions such as n-alkyl sulfates, sulphate, alkylaryl sulfonates as dopants, or polymeric anions such as poly(vinylsulphate), poly(styrenesulfonate), and sulfonated poly(vinylalcohol), the PPy films obtained were poor in quality. However, Qian et al. [16] found that the use of

J. Foroughi et al. / Sensors and Actuators B 155 (2011) 278–284 − − SO2− 4 NO3 and tosylate (TSO ) as the counter anions provided good quality, compact conducting PPy films. Several studies have been carried out to compare the properties of chemically and electrochemically prepared polypyrrole. Song et al. [9] reported the effect of solvent on the properties of chemically synthesized PPy-DBSA (DBSA = dodecylbenzenesulfonate) that resulted in its UV–vis spectrum in DBSA/chloroform solution being very similar to that of an electrochemically prepared PPy-DBSA film except for a lower absorbance in the near-infrared region. Also, a PPy-DBSA film spin-cast from chloroform solution showed a Raman spectrum almost identical to that of electrochemically prepared PPy-DBSA [9]. Moreover, Lee et al. [6] reported that the S/N weight ratio of chemically prepared PPy-DBSA was almost the same as the ratio found for electrochemically prepared PPy-DBSA, indicating the same doping level (25%) for both materials. In addition, the FT-Raman and UV–vis-NIR spectra of chemically and an electrochemically prepared PPy-DBSA films confirmed the same chemical structure. The conductivities of the soluble PPy-DBSA cast as films from chloroform and m-cresol solutions were 5 and 0.5 S cm−1 , respectively, not far below the conductivity (15 S cm−1 ) of electrochemically polymerized PPy-DBSA [6]. The mechanical properties of the chemically prepared PPy-DBSA cast film were reported to be 17 MPa for the tensile strength at break, 1.94 GPa for the elastic modulus and 0.9% for the elongation at break. In related studies, an electrochemically prepared PPy film with dodecyl sulphate anions (DS− ) as the dopant (PPyDS) exhibited stress at break of 68 MPa with 7.7% strain [7,6]. The density of the cast PPy-DBSA film was 1.240 g cm−3 , which was higher than that of the electrochemically polymerized PPy-DBSA film, 1.201 g cm−3 [6]. The authors concluded that the greater density in the former case indicates that casting from the PPy-DBSA solution yields better packing of polypyrrole chains than achieved by electrochemical polymerization. The DEHS dopant (Scheme 1) has been used previously to prepare, by chemical polymerization, a “soluble” PPy that can be readily dispersed in organic solvents [8]. In subsequent studies we produced PPy-DEHS fibre for the first time using the soluble PPy-DEHS by wet-spinning [17]. We were also able to improve the mechanical properties of PPy-DEHS fibres by optimizing the chemical polymerization method to synthesize high molecular weight soluble PPy-DEHS [11].

279

To the best of our knowledge there have been no previous reports of using DEHS as a dopant to produce electropolymerized PPy-DEHS films. We report here the production of electropolymerized PPy-DEHS films and compare properties of these to films produced by casting the “soluble” form of the PPy produced chemically [11,17]. We are particularly interested in investigating the differences in actuation behaviour of these two types of PPy-DEHS. 2. Experimental 2.1. Materials Di-(2-ethylhexyl) sulfosuccinate sodium salt (Na+ DEHS− ), ammonium peroxydisulfate (APS) and dichloroacetic acid (DCAA, 98%) were supplied by Sigma Aldrich and used as received. Pyrrole monomer (95%, Aldrich) was used after distillation. 2.2. Preparation of cast films from chemically synthesized PPy-DEHS powder Chemically polymerized polypyrrole (PPy-DEHS) powder was synthesized as described previously [11]. This high molecular weight polymer can be dissolved in DCAA solvent up to 150 g l−1 . In order to prepare films, 1.50 g of the above chemically prepared PPy-DEHS powder was dissolved with magnetic stirring in 10 ml of DCAA, and the solution evaporatively cast onto glass. 2.3. Preparation of electropolymerized PPy-DEHS film Electropolymerized polypyrrole films were synthesized by anodic oxidation of pyrrole monomer (0.40 M) in aqueous 0.15 M Na+ DEHS− . These concentrations were chosen to be similar to those used in chemical polymerization. A stainless steel plate was used as the anode and stainless steel mesh as the cathode. The polymerization was carried out galvanostatically using a constant current of 0.2 mA cm−2 for 12 h at 0 ◦ C. Similar slow electropolymerization has been previously shown to produce dense, highly conducting films of PPy doped with hexafluorophosphate [18]. The resultant black film was peeled off the electrode and washed several times with water, and then allowed to dry for 24 h in air at room temperature.

Scheme 1. Schematic of interaction between DEHS− and oxidized polypyrrole.

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2.4. Instrumentation

3. Results and discussion Application of a constant current to the working electrode in a solution containing the pyrrole monomer as well as the Na+ DEHS− electrolyte resulted in immediate deposition of an insoluble conducting material. 3.1. UV–vis/NIR spectroscopy The electrodeposited material PPy-DEHS was obtained and its UV–vis/NIR spectrum (Fig. 1) was compared to that of a cast film obtained from soluble PPy-DEHS that was prepared via chemical oxidation. The films displayed strong absorption peaks at 430 and 465 nm, respectively, assigned in each case as transitions from the PPy valence band to an anti-bipolaron band. Intense, broad absorptions also occurred in the NIR region, with maxima at 970 and 1300 nm for the chemically and electrochemically prepared PPyDEHS films, respectively. These NIR absorptions are attributed to electron transitions from the polypyrrole valence band to a second bipolaron band in the band gap [19].

Electrochem. PPy-DEHS film

Abs

Tensile testing was carried out using a TA instrument Dynamic Mechanical Analyser. At 25 ◦ C, a 10 mm gauge length of film was stretched at a strain rate of 500 ␮m min−1 until the sample broke. To obtain accurate results for mechanical properties, 3 samples were tested. The electrical conductivity of the PPy-DEHS films was measured using an in-house built four point probe whose electrodes consisted of four parallel rods at a spacing of 0.33 cm. The films were connected to the parallel rods using silver paint (obtained from SPI) and a constant current applied between the two outer electrodes using a Potentiostat/Galvanostat (Princeton Applied Research Model 363). The potential difference between the inner electrodes was recorded using a digital multimeter 34401A (Agilent). Raman spectra were obtained with a Jobin Yvon Horiba Raman spectrometer model HR800. The spectra were collected with a spectral resolution of 1.8 cm−1 in the backscattering mode, using a 732.8 nm He/Ne laser. A Gaussian/Lorentzian-fitting function was used to obtain band positions and intensities. The incident laser beam was focused onto the specimen surface through a 50× objective lens, forming a laser spot ∼5 mm in diameter, using a capture time of 50 s. Ultra violet–visible-near infrared spectra were obtained using a Cary 5000 spectrophotometer, while Fourier Transform Infrared (FTIR) spectra of PPy-DEHS powder and films were measured with a Nicolet Avatar 360 spectrometer. Elemental analysis of samples was performed by Shimadzu IRPrestige-21. All samples were dried in oven at 70 ◦ C, for 24 h before analysis. Cyclic voltammetry studies were carried out using a three electrode electrochemical cylindrical cell (15 mm × 50 mm) coupled to a Bioanalytical Systems (Model CV27) potentiostat. A 1 cm2 PPy film was used as the working electrode with an Ag/AgCl reference electrode and a Pt mesh counter electrode. Electrochemical actuation studies of PPy-DEHS films were carried out using a three-electrode system. The working electrode was the PPy film, and Pt mesh was used as the auxiliary electrode. Ag/AgCl in 3 M NaCl was used as reference electrode. A Dual Mode Lever Arm System (Auroa Scientific Inc.) was used for the actuation tests. The sample extension/contraction was measured by the arm on which the sample was attached. The device is able to work in both mechanical modes: isotonic (control force, measure displacement) and isometric (control displacement, measure force). Two system models, 300B and 305B, were used with a maximum force output of 500 mN and 5000 mN, respectively.

Cast Chem. PPy-DEHS film

300

600

900

1200

1500

1800

2100

2400

Wavelength(nm) Fig. 1. UV–vis/NIR spectra of chemically (Chem) and electrochemically (Electrochem) prepared PPy-DEHS films.

The absorption in the NIR region, called a free carrier tail, is believed to reflect the delocalized conjugation length, i.e. the length of the intramolecular conduction path of a polymer chain [9,19]. The large differences observed between the NIR spectra of the chemically and electrochemically prepared PPy-DEHS films (max = 970 nm and 1300 nm) indicate that they possess significantly different structures, with the latter possessing a more “extended coil” conformation [20]. On the other hand, the localized nature of the 430 nm polaron band observed for the cast PPy-DEHS film is consistent with a more “compact coil” conformation. Its blue shift compared to the 465 nm polaron peak observed for the electrochemically derived film is also indicative of a shorter conjugation length, consistent with its lower observed electrical conductivity [21]. The shortening of conjugation length in the chemically prepared PPy-DEHS film may be caused by a conformational change from trans to gauche in the PPy backbone [22] and/or doping of the PPy with solvent DCAA. 3.2. Elemental analysis Results are summarised in Table 1. The electrochemically prepared material and the PPy-DEHS powder showed similar degrees of doping, with sulphur to nitrogen atomic ratios (S/N) of 0.33 and 0.30. However, casting a film of PPy-DEHS from a DCAA solution of the chemically prepared powder resulted in a decrease in the S level (from DEHS) to 0.17 and an increase in the Cl level (from DCAA). These results suggest that the DCAA solvent replaces some of the DEHS as co-dopant. In a previous study we also reported [11] that fibre spinning of the PPy-DEHS from DCAA solution results in a decrease in S content (from DEHS) and an increase in the Cl content (from DCAA). 3.3. Vibrational spectra Vibrational spectra provide useful information on the molecular structure of conducting polymers and their charge distribution. The spectra can probe the state of conjugation associated with the extent of electron delocalization thus giving rise to strong and characteristic infra-red and/or Raman activities [23]. 3.3.1. Raman spectra Figs. 2 and 3 show the Raman spectra of an electropolymerized PPy-DEHS film, chemically polymerized PPy-DEHS powder and a film cast from a DCAA solution of the latter. The peaks observed at 930 and 1086 cm−1 may be attributed to the bipolaronic form of PPy, while the peaks at 962 and at 1053 cm−1 are typical of the polaronic form [19]. The C C stretching peak at 1589 cm−1 is considered to arise from overlap of these bipolaron and polaron structures [24].

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281

Table 1 Comparison of the elemental analysis of PPy-DEHS samples: (a) chemically polymerized powder, (b) cast film of chemically prepared and (c) electropolymerized film. Sample

Elemental analysis (wt.%)

(a) PPy-DEHS powder (b) PPy-DEHS film (c) Elec. PPy-DEHS film

C

H

N

S

Cl

C/H

S/N

Cl/N

Doping (%)

59.9 58.3 59.4

7.1 5.5 6.7

7.3 9.1 6.7

5.6 3.5 4.6

– 4.8 –

0.70 0.88 0.74

0.33 0.17 0.30

– 0.21 –

33 37 30

Intensity (a. u.)

800

1000

1475

10531086 962

600

Table 2 Ratio of integrated Raman band intensities in PPy-DEHS samples.

1589

1380 1321

1233

930

400

Atomic ratio

1200

1400

1600

Wavenumber (cm -1)

1800

Fig. 2. Raman spectrum of an electropolymerized PPy-DEHS film.

The ratio of the intensity (I) of a band sensitive to the oxidation state of the polymer (1589 cm−1 ) to the intensity of the skeletal band (1475 cm−1 ) gives a quantitative measure of the conjugation length [24], i.e. the ratio I1589 /I1475 is proportional to the extent of delocalization. While some peak shifts occurred in the cast films, the peaks were assumed to be related to the same structural features and the peak intensities taken for further analysis. From Figs. 2 and 3, the I1589 /I1475 ratios were calculated as 21.6, 9.6 and 18.8 for the chemically polymerized PPy-DEHS powder, the corresponding PPy-DEHS cast film, and the electropolymerized PPy-DEHS film, respectively (Table 2). It is seen that the I1589 /I1475 ratio of the electrochemically prepared PPy-DEHS film falls between the values for the powder and cast film forms of chemically prepared PPy-DEHS and that the conjugation length increases along the series: Cast PPy-DEHS film < Electrochem- PPyDEHS film < Chem- PPy-DEHS powder. Similarly, the integrated absorption intensities of the bipolaron structure of chemically polymerized PPy-DEHS powder (peaks at I1086 + I930 = 3410) are higher than for the electrochemically poly-

Sample

I1086 + I930

I1053 + I962

I1589 /I1475

Chemically polymerized PPy-DEHS powder Chemically prepared PPy-DEHS film Electropolymerized PPy-DEHS film

3410

189

21.6

1455 3228

450 300

9.6 18.8

merized PPy film (I1086 + I930 = 3228), again indicating a greater extent of delocalization for the chemically polymerized PPy powder [23]. Furthermore, the integrated absorption intensities of the polaron structure in the chemically prepared PPy-DEHS film (peaks at I1053 + I962 = 450) are higher than for the corresponding powder (I1053 + I962 = 189) and the electropolymerized PPy film (I1053 + I962 = 300). 3.3.2. FT-IR spectroscopy Fig. 4 shows FT-IR spectra for the various PPy-DEHS samples. All spectra show a weak band at 1730 cm−1 . This band has been observed in electrochemically over-oxidized polypyrroles [21,25], and attributed to carbonyl groups. However, in the present samples there are carbonyl groups in the DEHS dopant so appearance of the band at 1730 cm−1 may not indicate over-oxidation. In fact, Jang and co-workers reported [26,27] that the peak at 1730 cm−1 corresponds to the C O vibration of the two ester groups in the dopant DEHS. Conjugation length related to molecular structure of conducting polymers could be estimated by measuring the degree of delocalization. Using calculations based on the concept of “effective conjugation coordinate”, Zerbi et al. [28] and Tian et al. [29] have successfully predicted the number and positions of the main IR bands in both neutral and doped PPy. This theory also predicts how the intensities and positions of these bands change with the extent of delocalization in the polymer chain. The bands at 1540 and 1460 cm−1 are especially affected by changes in the extent of delocalization. These changes can be most easily visualized from the ratio of the integrated absorption intensities of the

Chem. PPy-DEHS

Intensity (a. u.)

Absorbance (%)

Cast PPy-DEHS film

Chem. PPy-DEHS powder

Chemical polymerized PPy-DEHS powder Electrochem. PPy-DEHS film

400

600

800

1000

1200

1400

1600

1800

Wavenumber (cm-1) Fig. 3. Raman spectra of chemically polymerized PPy-DEHS powder and a PPy-DEHS film cast from a DCAA solution.

2000

1800

1600

1400

1200

1000

800

Wavenumber (cm-1) Fig. 4. FTIR spectra of an electropolymerized PPy-DEHS film, compared with a chemically polymerized PPy-DEHS powder and corresponding film.

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2

Table 3 Ratio of integrated IR band intensities in PPy-DEHS samples.

1.5

I1730

I1590 /I1490

I1730 /I1400

I1400

Chemically polymerized PPy-DEHS powder Chemically polymerized PPy-DEHS film Electropolymerized PPy-DEHS film

22.68

0.99

0.73

31.37

28.68

1.17

0.67

42.37

9.81

1.60

0.54

18.01

1540 cm−1

1460 cm−1

and bands (I1540 /I1460 ), which is inversely proportional to the extent of delocalization [21]. Experimental ratios of I1540 /I1460 values for chemically and electrochemically polymerized PPy-DEHS are shown in Table 3. The I1540 /I1460 ratio is seen to increase from 0.99 to 1.6 when the polymerization method was changed from chemical to electrochemical for PPy-DEHS films, indicating a decrease in conjugation length [25,28,30,31]. Thus, both the Raman and FTIR results imply that the polymer conjugation length dominates the change in conductivity observed for PPy-DEHS prepared by different polymerization methods. 3.4. Mechanical properties of PPy-DEHS films

A comparison of the mechanical properties of free-standing films prepared from the chemically polymerized or electropolymerized materials was carried out. Multiple tests were performed on each type of sample with small variations in properties observed as illustrated in Fig. 5. Typical stress–strain curves for the chemically polymerized PPy-DEHS free-standing films cast films (Fig. 5) indicate a stress at break point of ∼40 MPa with ∼2.2% strain. The Young’s modulus of the cast PPy-DEHS film was measured as ∼2.3 GPa. These results indicate that the cast PPy-DEHS film are more brittle than that previously found for corresponding undrawn PPy-DEHS fibres (65 MPa stress with 8% strain) [11]. This may be due to better orientation and morphology of the PPy fibres than in the cast PPy film. The stress–strain curves for electropolymerized PPy-DEHS films are shown in Fig. 5. The average stress at break point for two curves was ∼35 MPa, with ∼33% average strain respectively. The Young’s modulus was measured as 605 MPa. The Young’s modulus of electrochemically prepared PPy-DEHS films is about four times lower than that noted above for chemically prepared PPy-DEHS films. Possible reasons for this significantly different mechanical behaviour could include a higher crosslinking density, higher crystallinity or molecular alignment, lower porosity and lower solvent content in the chemically prepared polymer. X-ray diffraction studies showed 45 40

Chem. PPy-DEHS Film

Electrochem. PPy-DEHS Film

1

Current/ mA

Sample

0.5 0 -0.5 -1 -1.5 Chem. PPy-DEHS Film

-2 -2.5 -1

-0.5

0

0.5

1

E(vs. Ag/AgCl)/ V Fig. 6. Cyclic voltammograms obtained for chemically and electrochemically derived PPy-DEHS films (film thickness = 20 ␮m). Potential was scanned over the range shown in 0.10 M DEHS in acetonitrile/water (1:1) at 100 mV s−1 . The third cycle CV is shown for each sample.

that chemically prepared PPy-DEHS has an amorphous structure [11] and its solubility in DCAA indicates the absence of crosslinking. Furthermore, samples were carefully dried in a vacuum oven before mechanical testing and so the effect of solvent content would be insignificant. Therefore, a lower porosity (higher density) for chemically prepared PPy-DEHS film probably contributed to its higher Young’s modulus, although the difference in densities is small. 3.5. Electrochemical properties of PPy-DEHS The average electrical conductivity taken from 3 samples of electropolymerized PPy-DEHS film was measured to be 75 ± 5 S cm−1 . In contrast, the chemically polymerized film cast from material dissolved in DCAA exhibited a significantly lower electrical conductivity of only ∼7 ± 3 S cm−1 . The UV–vis spectra of the two polymers (Fig. 1) indicate a significantly larger free-carrier tail in the electrochemically prepared PPy-DEHS due to a more “extended coil” conformation. This conformation allows a greater overlap of ␲-orbitals and therefore greater intra-chain carrier mobility. Cyclic voltammograms recorded for the electropolymerized and the chemically polymerized materials (Fig. 6) showed that they both exhibited the expected electroactivity with oxidation and reduction of the polypyrrole backbone clearly observed. Redox couples were observed at −0.45/+0.20 V (chemically prepared film) and −0.60/−0.28 V (electrochemically prepared film) (vs. Ag/AgCl), respectively. The more positive oxidation potential and broader responses observed for the chemically derived material are consistent with a material having lower electronic conductivity.

35

3.6. Electromechanical actuation of PPy-DEHS films

Stress (Mpa)

30 25 20

Electrochem. PPy-DEHS Film

15 10 5 0

0

5

10

15

20

25

30

35

Strain (%) Fig. 5. Stress–strain curves for chemically and electrochemically prepared PPyDEHS films.

The electromechanical actuation properties of the chemically and electrochemically derived materials were investigated. The films were cut into 10 mm × 3 mm strips and used as the working electrode in a three-electrode cell in 0.10 M DEHS acetonitrile/water (1:1). The potential was then varied between −0.9 and +0.8 V (vs. Ag/AgCl) and the expansion–contraction ratio (the electromechanical strain) of the PPy-DEHS films monitored using a lever arm displacement meter. Electromechanical strain (%) was defined by (L/L0 ) × 100, where L and L0 were the change in length and the original length of the PPy strip, respectively. Fig. 7 shows both the cyclic voltammogram and the electrochemical actuation of the electropolymerized PPy-DEHS film, obtained using a scan rate of 1 mV/s. A large electrochemically induced strain of 30%

J. Foroughi et al. / Sensors and Actuators B 155 (2011) 278–284

20

0.08

15 10 5

0.04

0

0.02

-5

Strain(%)

Current (mA)

0.06

-10

0

-15 -0.02

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

-20

283

Young’s modulus was ∼0.60 and 2.30 GPa for the electrochemically and chemically prepared PPy-DEHS films, respectively. The electrochemically prepared PPy-DEHS film exhibited a better electrochemical actuation (∼30% strain) than the PPy-DEHS cast film (∼2.5% strain). The reduced actuation of the chemically prepared and cast film may be related to a more compact structure and lower electronic conductivity. Despite the smaller actuation strain, the improved processability of the soluble chemically prepared film allows the possibility of preparing actuating PPy structures from films, coatings, fibres and printed structures that would be difficult to prepare using conventional electrochemical methods.

E(vs.Ag/AgCl)/ V Fig. 7. Electrochemical actuation of an electropolymerized PPy-DEHS film. Potential was scanned between −0.9 and +0.8 V (vs. Ag/AgCl) in 0.10 M DEHS in acetonitrile/water (1:1) at 1 mV s−1 .

was observed over one redox cycle (first cycle). Expansion mainly occurred during oxidation of the polymer due to the incorporation of DEHS dopant anions and solvent. Large electrochemically induced strains have been observed previously in PPy doped with large anions [32,33]. In these studies, the oxidation of the PPy resulted in large expansions, as in the present study. Such large strains cannot be explained by ion movements alone, and it is likely that solvent influx driven by osmotic pressure contributes to the volume changes [34]. The electromechanical strain experiments were run for 10 cycles for each sample. Electromechanical actuation decreased significantly during these redox cycles; being only a 20% decrease after 5 cycles, then a 50% decrease after 10 cycles. In contrast, the cast film prepared from chemically produced PPy-DEHS film exhibited only ∼2.5% strain under the same test conditions. It therefore appears that for this chemically derived material it is difficult to facilitate ion movement during oxidation and reduction perhaps due to the more compact structure of this PPy film. The modulus of the cast PPy-DEHS film (2.3 GPa) was significantly higher than that found for electropolymerized PPy-DEHS film (0.6 GPa). The lower modulus of the electrochemically prepared polymer suggests a more open, porous or plasticized structure that may allow easier ion diffusion. The higher conductivity of the electrochemically prepared film also facilitates easier charge transfer. The result is an increased degree of actuation (∼30% strain) in the electrochemically-prepared film compared to the cast PPy-DEHS film (∼2.5% strain). 4. Conclusions PPy-DEHS films of polypyrrole doped with di-(2-ethylhexyl) sulfosuccinate have been prepared for the first time using electrochemical oxidation of pyrrole. The UV–vis/NIR spectrum is different to that of previously reported chemically polymerized PPy-DEHS films, indicating a different chemical structure. Elemental analyses show that the doping level of chemically polymerized PPy-DEHS powder is higher than for electrochemically prepared PPy-DEHS films. Raman and FTIR spectroscopy indicate that the polypyrrole in chemically prepared PPy-DEHS powder is more delocalized than electrochemically prepared PPy-DEHS films. However, the electrical conductivity of an electrochemically prepared PPy-DEHS film (75 ± 5 S cm−1 ) is significantly higher than for chemically prepared PPy-DEHS (7 ± 3 S cm−1 ). The mechanical properties of electrochemically prepared PPyDEHS films reveal a stress at break point of ∼35 MPa, with ∼33% strain. A stress at break point of ∼40 MPa with ∼2.5% strain was found for the corresponding chemically prepared film. The

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Biographies Javad Foroughi received his PhD degree in materials engineering from Wollongong University, Australia, in 2009. He is currently working as associate research fellow at the Intelligent Polymer Research Institute. His research interests include nanomaterials, electromechanical actuators (“artificial muscles”) using inherently conducting polymers and/or carbon Nanotube, bionics and novel fibres spinning and the use of these in the development of smart materials. Geoffrey M. Spinks is a Professor in the School of Mechanical, Materials and Mechatronic Engineering at the University of Wollongong and a Chief Investigator in the ARC Centre of Excellence for Electromaterials Science. His research interests relate to the mechanical behaviour of organic conductors and polymeric nanomaterials. A particular focus involves the development of actuator materials and systems, from the nano-scale to the macro-scale. Professor Spinks has published more than 120 refereed publications and a monograph (three editions) on inherently conducting polymers for intelligent material systems. He has supervised 20 PhD students to completion. Gordon G. Wallace received his PhD degree in chemistry from Deakin University, Australia, in 1983. He is currently Director of the Intelligent Polymer Research Institute and Executive Research Director of the ARC Centre of Excellence for Electromaterials Science. He was elected as a Fellow of the Australian Academy of Science (2007), the Australian Academy of Technological Sciences and Engineering (2003), and the Institute of Physics, UK (2004). He is a Fellow of the Royal Australian Chemical Institute (RACI). His research interests include organic conductors, nanomaterials and electrochemical probe methods of analysis and the use of these in the development of intelligent polymer systems.