Effect of multiwalled carbon nanotube (M-CNT) loading on M-CNT distribution behavior and the related electromechanical properties of the M-CNT dispersed ionomeric nanocomposites

Surface & Coatings Technology 200 (2005) 1920 – 1925 www.elsevier.com/locate/surfcoat

Effect of multiwalled carbon nanotube (M-CNT) loading on M-CNT distribution behavior and the related electromechanical properties of the M-CNT dispersed ionomeric nanocomposites Deuk Yong Lee a,*, Myung-Hyun Lee b, Kwang J. Kim c, Seok Heo c, Bae-Yeon Kim d, Se-Jong Lee e a Department of Materials Engineering, Daelim College of Technology, Anyang 431-715, South Korea Next Generation Enterprise Group, Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, South Korea Active Materials and Processing Laboratory, Department of Mechanical Engineering, University of Nevada, Reno, NV 89557, USA d Department of Materials Science and Engineering, University of Incheon, Incheon 402-749, South Korea e Department of Advanced Materials Engineering, Kyungsung University, Busan 608-736, South Korea b

c

Available online 15 September 2005

Abstract The multiwalled carbon nanotube (M-CNT)/Nafion nanocomposites were prepared by a method of solution casting and then characterized by using X-ray diffraction (XRD), thermogravimetry/differential scanning calorimetry (TG/DSC), scanning and transmission electron microscopy (SEM/TEM) to evaluate the effect of M-CNT loading in the range of 0 to 7 wt.% on M-CNT distribution behavior and the related electromechanical properties of the composites. The M-CNT bundles induced by the Nafion polymer was uniformly distributed for the 1 wt.% M-CNT/Nafion nanocomposites, exhibiting the highest elastic modulus and improved electromechanical properties. However, further M-CNT loading caused a heterogeneous distribution of M-CNT bundles and a negative impact on the connectivity within the Nafion matrix, giving rise to poor actuation properties. An appropriate equivalent circuit model was proposed to evaluate the effect of capacitance and resistance of M-CNT/polymer nanocomposites. In conclusion, it is found that the actuation properties of the nanocomposites are primarily governed by the M-CNT distribution behavior within the polymer matrix. D 2005 Elsevier B.V. All rights reserved. Keywords: Multiwalled carbon nanotube (M-CNT); Nafion; Ionic polymer – metal composite (IPMC); M-CNT bundle; Actuation property; Equivalent RC circuit

1. Introduction The increased demand for highly active, flexible and miniaturized sensors and actuators producing high power density and large force generation capabilities in biomedical applications and micro-robotics has fueled the development and introduction of carbon nanotube (CNT)/polymer nanocomposites [1,2]. The CNT/ionic polymer –metal composite (IPMC) nanocomposites have been prepared widely by using * Corresponding author. Department of Materials Engineering, Daelim College of Technology, Anyang 431-715, Korea. Tel.: +82 31 467 4835; fax: +82 31 467 4830. E-mail address: [email protected] (D. Yong Lee). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.024

Nafion as a flexible polyelectrolyte matrix and a network of mobile positive and negative ions for the optimization of the CNT actuators [3]. IPMC consisted of a perfluorinated polymer electrolyte (Nafion) sandwiched by electrolessplated platinum (Pt) on both sides. The soft-actuation of IPMCs was restricted by mechanical and electrochemical fatigue associated with the underlying faradic intercalation – deintercalation process, resulting in the limitation of lifetime [3 – 7]. Single wall carbon nanotube (S-CNT) sheets have been studied for electromechanical actuators such as robotics, artificial muscles and microscopic pumps because of their excellent mechanical properties, electrical conductivity and charge transfer properties [1,2]. The effectiveness of S-CNT could be extended to the efficient and direct conversion of

D. Yong Lee et al. / Surface & Coatings Technology 200 (2005) 1920 – 1925

electrical energy to mechanical energy due to a combination of quantum chemical and double-layer electrostatic effects. However, S-CNT sheets lead to a decrease in net surface area available for double layer charging, and electrochemical creep and a low elastic modulus because they are consisted of mats of nanotube bundles of 100 – 1000 nm joined by mechanical entanglements and van der Waals forces [2]. To alleviate the aforementioned drawbacks, an S-CNT powder dispersed Nafion solution was cast to obtain SCNT/Nafion composite actuators. Successful dispersion of purified S-CNT into Nafion demonstrated the efficient actuation behavior [3]. Although inferior stress generation capability and reduction in elastic modulus of the composites in comparison with pure S-CNTs were found, actuation threshold of the S-CNT/IPMC nanocomposites at low doping levels (less than 5 wt.%) of S-CNTs was reported [3]. However, no systematic study was conducted for the correlation between the distribution behavior of S-CNT bundles and the related electromechanical properties of the composites throughout the process. In the present study, multiwalled carbon nanotubes (MCNTs)/Nafion nanocomposites were studied because the synthesis of M-CNT was highly advanced to improve its quality and quantity [8], and to satisfy many needs such as robotics, optical fiber switches, optical displays, prosthetic devices, sonar projectors, and microscopic pumps [1]. The M-CNT/IPMC nanocomposites were prepared by a method of solution casting and then characterized by using XRD, TG/DSC, SEM and TEM to elucidate the relationship between the distribution behavior of M-CNT bundles and the actuation properties of the composites. The electromechanical properties were further analyzed by incorporating an equivalent circuit model because the percolation of nanotubes in a polymer matrix was observed with very small loadings (less than 1 wt.%).

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Appropriate amount of M-CNTs and 50 ml of 5% Nafion solution were dispersed in 50 ml methanol by sonicating for 24 h, followed by high shear stirring for 72 h at room temperature. The solutions were then cast onto a teflon trough. After drying in stagnant air and subsequently invacuo for 24 h, respectively, the M-CNT/Nafion film was peeled off. This film was annealed for 2 h at 70 -C to induce Nafion resistance to water and alcohol [5]. Final thickness of the composite film was about 150 Am. The M-CNT/Nafion nanocomposites were platinized by a method of initial compositing process (surface roughening + adsorption + reduction) [6] and then characterized by using SEM and TEM. SEM was performed using a Jeol model JSM 6700F operating at 10 kV. All specimens were coated with Au to ensure higher conductivity, necessary for the Nafion composites that otherwise exhibit significant image distortion due to charging. Microstructure was also observed by TEM (Jeol 2000EX, Japan) using 200 kV beam energy. TEM samples were prepared by sonicating M-CNTs in ethanol for 10 min and drop drying them onto a TEM (Atype) mesh grid. The performance of M-WNT/Nafion composites was characterized by measuring the blocking force using a small load-cell (0.098 N) in a cantilever configuration under a voltage of 2.5 V across the sample [7]. A computer based test platform was built for actuation tests of all samples. Data acquisition was achieved via Matlab, a real-time workshop of d-Space, throughout the experiment. For modulus experiment, the experimental apparatus was slightly modified to use the load cell and the laser displacement sensor simultaneously to obtain applied force and displacement. Using the standard beam theory, the elastic modulus of the nanocomposites was determined [7].

3. Results and discussion

2 100

TG 0

DSC

80

-2

60

-4

40

-6

20 o

0

DSC (mW/mg)

Liquid Nafion solution (5 wt.%) and M-CNTs (> 99.5% purity) were purchased from Aldrich and Catalytic Materials, respectively. M-CNT powders were boiled for 15 min at 120 -C in a mixture of concentrated sulfuric acid and concentrated nitric acid (3:1 volume ratio) for better dispersion in either water or alcohol. After filtration and washing, the M-CNTs were debundled mechanically using an agate mortar and pestle, followed by a ball mill in a polyethylene jar with zirconia beads for 72 h [5]. The particle size and distribution of M-CNT were evaluated by a laser particle size analyzer. The purity of the M-CNTs was determined by SEM equipped with energy dispersive spectroscopy (EDS) and TG/DSC. TG and DSC were carried out with M-CNTs in the form of powders with a heating rate of 10 -C/min to 1000 -C under 70 sccm flowing air atmosphere. The M-CNT/Nafion composites were prepared by the dispersion of the as-treated MCNTs in a Nafion solution in the range of 0 to 7 wt.%.

Properties of as-received M-CNTs were investigated by SEM/EDS, TG/DSC and TEM, which revealed that the raw

Weight (%)

2. Experimental

0

200

400

517.7 C

894.1 C

600

800

o

-8 1000

o

Temperature ( C) Fig. 1. TG/DSC result of M-CNTs purchased from Catalytic Materials Inc.

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D. Yong Lee et al. / Surface & Coatings Technology 200 (2005) 1920 – 1925 12 as-received as-treated

10

Volume (%)

8

6

4

2

0 0.01

0.1

1

10

100

1000

Particle diameter (µm) Fig. 2. Particle size distributions of as-received and as-treated M-CNT powders.

M-CNTs had the purity of more than 99.8% and a dimension of 10 to 20 nm in diameter and a few micron upwards in length. TG/DSC results of highly entangled M-CNTs indicated that M-CNTs lost their weight linearly with increasing temperature. They started to burn at 500 -C in air, however, the burning was completed at 900 -C. It was higher than the previous work (600 -C) [8] probably due to severe entanglements (bundling) of M-CNTs (Fig. 1). Average bundle size of M-CNT powders after mechanical treatments decreased significantly 25.87 Am to 11.12 Am, as depicted in Fig. 2. The debundled M-CNTs may be

advantageous for uniform dispersion and distribution of MCNTs throughout the Nafion polymer matrix [3], however, mechanical processes including mortar and pestle and ball milling were estimated to be insufficient to refine the bundle size of M-CNT more than a factor of 2. XRD peak patterns of M-CNTs, Nafion, M-CNT/Nafion composites are shown in Fig. 3. Typical (002) and (100) peaks of M-CNTs (Fig. 3(a)) were observed at 2h of ¨ 26and ¨43-, respectively, which was in good agreement with the previous results [9]. These peaks were highly attenuated when M-CNTs were dispersed in Nafion matrix. The

(002)

(100)

7000

6000

Relative intensity (a.u.)

5000

(e) 4000

(d) 3000

(c) 2000

(b) 1000

(100)

(a) 0 20

30

40

50

60

70

2θ Fig. 3. XRD patterns of (a) M-CNTs, (b) Nafion, (c) 1 wt.% M-CNT/Nafion, (d) 3 wt.% M-CNT/Nafion and (e) 5 wt.% M-CNT/Nafion nanocomposites. Note that the (002) and (100) reflections are for M-CNTs (a) and the (100) reflection for the Nafion membrane (b).

D. Yong Lee et al. / Surface & Coatings Technology 200 (2005) 1920 – 1925

enhanced asymmetric broadening of the peak at 2h of ¨ 18in Fig. 3(b) was detected for the Nafion membrane, which was equivalent to the (100) reflection of p-TFE (tetrafluoroethylene) [10]. The asymmetric (100) peak can be decomposed into two peaks: a broad peak at 2h = 16- and a sharp peak around 2h = 18-, implying noncrystalline and crystalline regions of the membranes, respectively. The ionic clustering [10], as demonstrated in Fig. 4, may not disturb the crystalline portion of the matrix completely but may change the degree of crystallinity moderately because the clustering perturbs crystallization. As the M-CNT content rose from 0 to 5 wt.%, the intensity of the Nafion (100) peak was enhanced as a consequence of increasing crystallinity with the M-CNT addition, as illustrated in Fig. 3. The increase in peak intensity may be due to the synergic combination of Nafion ionic clustering and M-CNT’s debundling as a result of charge balance [2]. Charge injection at the surface of CNT bundle caused by an applied potential is known to be balanced by the outer surface layer of opposite electrolyte ions [2]. Fig. 5 suggested that the great affinity of Nafion with M-CNTs resulted in efficient percolation of high aspect ratio of MCNT and enhanced dispersion as a result of chemical debundling of M-CNTs. The average bundle size for the treated M-CNTs was 11.2 Am, while the average bundle size of the M-CNTs (5 wt.%) within the Nafion matrix was 1.6 Am, resulting in a factor of 7 reduction in bundle size. TEM observation indicated that the degree of debundling and distribution within the polymer matrix was more pronounced for the composite containing small amount of M-CNTs (1 wt.%). A homogeneous distribution of M-CNT caused by the bundles size reduction (Fig. 5(a)) may enhance membrane conductivity. However, the degree of the M-CNT bundle distribution became heterogeneous (Fig. 5(b)) as the M-CNT content rose above 1 wt.%. The inhomogeneous distribution of the M-CNTs impeded the connectivity within the polymer matrix, resulting in the partial disruption of the percolated structure of the M-CNT bundles from Nafion. In the present 800

Fig. 5. TEM images of (a) 1wt.% and (b) 5 wt.% M-CNT/Nafion nanocomposites. Note that grey-shaded background and dark shaded region caused by the difference in M-CNT distribution represent Nafion and M-CNTs, respectively.

study, it is conceivable that the extent of CNT bundle size reduction is more pronounced during the casting due primarily to Nafion affinity and electrochemical double-layer charging rather than mechanical refinements. After an initial Pt compositing process on both surfaces of the M-CNT/Nafion nanocomposites, maximum stress and elastic modulus of the M-CNT/IPMC nanocomposites were measured to elucidate the dependence of the M-CNT bundle distribution within the polymer matrix on the electromechanical performance of the composites. The electromechanical results clearly show that the elastic modulus of M-CNT/ IPMC nanocomposite increases drastically even with a small M-CNT loading (1 wt.%) and then decreases slightly with increasing the M-CNT content as shown in Fig. 6. The maximum value of elastic modulus of 0.485 GPa was obtained for the 1% M-CNT/IPMC nanocomposite, which had the uniform M-CNT distribution within the polymer matrix. This nearly doubles the elastic modulus of M-CNT/ IPMC composite relative to pure Nafion. The actuation performance of M-CNT/IPMC nanocomposites is given in

600

0.55

Elastic modulus (GPa)

Relative intensity (a.u.)

1923

400

200

0

0.50 0.45 0.40 0.35 0.30 0.25 0.20

-200 15

20

2θ Fig. 4. An XRD pattern of Nafion membrane.

25

0

1

2

3

4

5

6

7

Content of M-CNT (wt.%) Fig. 6. Variation of elastic modulus of the M-CNT/IPMC nanocomposites having various amount of M-CNT.

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D. Yong Lee et al. / Surface & Coatings Technology 200 (2005) 1920 – 1925

Blocking force (x9.8 kPa)

0.3

0% 1% 5% 7%

0.2

0.1

0.0

-0.1

-0.2

0

1

2

3

4

5

using four RC circuits as shown in Fig. 8. In these RC circuits, we include four resistors (R1, R3, R6 and R8) and four capacitors (C1, C2, C3, and C4), which account for the effective electrodes on the surface of the IPMC and added MCNTs. In this way, we consider embedding M-CNTs into the polymer matrix being represented by two RC circuits (R6/C3 and R8/C4). This approach was based upon the fact that the percolation of nanotubes in a polymer matrix has been observed with very small loadings (less than a few %). Also, there is a resistor placed between the RC circuits to account for polymeric material between the electrodes (R2 and R7). The differential equation describing a single RC circuit with a step function input is shown in Eq. (1) (t
0 s) [13].

Time (second) Fig. 7. Time response of blocking force of the M-CNT/IPMC nanocomposite actuators.

Fig. 7 in terms of the measured blocking forces as a function of the M-CNT loading under a DC voltage (2.5 V). Interestingly, the composite having an 1% M-CNT loading shows the best performance in terms of both maximum stress (2.3 kPa) and stress rate as expected. Therefore, it is believed that the uniformity of the M-CNT distribution within the polymer matrix is attributed to the actuation performance of the M-CNT/IPMC nanocomposites. An equivalent circuit model was incorporated to address the mechanoelectric behavior of IPMCs [11,12]. The equivalent circuit model used in this study applied the previously developed circuit-analysis [12]. IPMCs are, in general, both inherently resistive and capacitive under electrical field. This allows for the material to be modeled

RT C

dvðt Þ þ vðt Þ ¼ VA dt

ð1Þ

where RT, v(t) and VA are the thevenin resistance of the proposed circuit, the step response at time t and the applied step voltage of the IPMC, respectively. By solving Eq. (1) using Laplace transform method for the proposed circuit, the equivalent current for each capacitor is shown to have the form of equation as, I ðsÞ ¼ 

VA =S  : 2R1 R2 þ 2R4 þ 2 R1 C1 S þ 1

ð2Þ

The resulting inverse Laplace transform of Eq. (2) is shown in Eq. (3). For the case of single force input such as a step input or impulse,  1 iðt Þ ¼ 2VA ð2R1 þ R2 þ 2R4 Þ
 2R1 þR2 þ2R4 2R1  t þ e R1 R2 C1 þ2R1 R4 C1 : ð2R1 þ R2 þ 2R4 ÞðR2 þ 2R4 Þ ð3Þ By applying Kirchhoff’s voltage law across each current loop as shown in Fig. 8, the following expression 3.0

Blocking force (x9.8 kPa)

Model Experiment

2.5 2.0 1.5 1.0 0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Time (second) Fig. 8. An equivalent circuit model used in this study.

Fig. 9. Comparison of time response of blocking force of M-CNT/IPMC nanocomposite actuators for 1 wt.%.

D. Yong Lee et al. / Surface & Coatings Technology 200 (2005) 1920 – 1925 Table 1 Parameters of the equivalent circuit model

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4. Conclusions

VA

R1

C1

R2

R4

Applied voltage

Resistor of RC circuit

Capacitor of RC circuit

Resistance for the center of the unit cell

Resistance of the surface interface

2.5 V

1V

0.01 F

1V

32 V

for the voltage across either capacitor, V C, is found to be,   1 VC ðt Þ ¼ VA  2VA R2 ð2R1 þ R2 þ 2R4 Þ
 2R þR2 þ2R4 2R1  R R 1C þ2R R4 C1 t 1 2 1 1 þ e 2: ð2R1þR2 þ2R4 ÞðR2 þ2R4 Þ ð4Þ From Fig. 8 the capacitance C 1, C 2, C 3, and C 4 were set equal to each other by assuming that the IPMC electrode surfaces were prepared identically throughout the manufacturing process. The value of R 1 = R 3 = R 6 = R 8 and R 4 =R 5 = R 9 = R 10 were also set each other for the same reason and will remain constant, along with the value for the resistor across the middle, R 2 = R 7, throughout (Table 1). The typical sample size is 0.5 cm width, 1.5 cm length, and 0.1 mm thickness. Experimental results in Fig. 7 were compared with the model output based upon the equivalent circuit model. In Fig. 9, experiment and model data match before reaching the peak (where capacitive behavior is dominating) but show different paths of relaxation. The rapid decrease in the relaxation area may be due to the fast movement of the cations towards cathode together with associated water molecules. Also, an increase in resistance is caused by a high-pressure layer near the cathode towards the anode through channels present in the polymer backbone. Electrochemical effects occurring at the metal electrode (platinum) excluded in the equivalent circuit model may attribute to the deviation in the relaxation region. Although the use of M-CNT/IPMC nanocomposite at a small loading (uniform distribution of M-CNT) was very promising, its electromechanical coupling decreased as the M-CNT loading rose. This is probably due to the low conductivity or enhanced electrochemical effects caused by partial disruption of the percolated structure of M-CNT bundles at higher loading. In addition, it can be envisioned that the combination effect of double-layer electrostatic and quantum chemical effects lead to enhanced electro-osmotic effects.

The M-CNT/IPMC nanocomposites were prepared to evaluate the influence of M-CNT loading on the M-CNT distribution behavior and the related electromechanical actuation performance of the composites. As the M-CNT loading rose above 1 wt.%, the uniformly distributed MCNT bundles induced by the Nafion polymer were perturbed. This change was confirmed by TEM. The heterogeneously dispersed M-CNT bundles may provide a negative impact on the connectivity within the Nafion membrane, leading to the poor actuation properties. An equivalent RC circuit model was studied to compare with the experimental output. The initial fast response of experimental and model data were well agreed due to the capacitive behavior, however, following relaxation behavior was slightly different because the RC circuit model did not consider the electrochemical reaction occurring at the Pt metal electrodes. The mechanical properties of M-CNT/ IPMC nanocomposites were superior to unloaded IPMC actuators approximately doubling the values, suggesting that considerably low doping level (¨ 1 wt.%) of M-CNTs into the polymer matrix was highly effective for the actuation properties of the M-CNT/IPMC nanocomposites.

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