polyaniline composite particles and their electrorheological response under an applied electric field

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Fabrication of semiconducting graphene oxide/polyaniline composite particles and their electrorheological response under an applied electric field Wen Ling Zhang, Ying Dan Liu, Hyoung Jin Choi

*

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

Semiconducting graphene oxide/polyaniline (GO/PANI) composite particles for potential

Received 5 April 2011

electrorheological (ER) fluid applications were synthesized by the in situ dispersion poly-

Accepted 24 August 2011

merization of aniline in the presence of GO particles, which were prepared using a modified

Available online 30 August 2011

Hummers method. The electroresponsive ER characteristics of the composite when dispersed in silicone oil exhibited a phase transition from a liquid-like to solid-like state under an applied electric field. The morphology and composition of the composite particles were characterized by scanning and transmission electron microscopy and Raman spectroscopy. Its fibrillation phenomenon was observed by optical microscopy during the application of an external electric field. The bulk rheological characteristics of both the flow curve and yield stress were examined using a rotational rheometer equipped with a high voltage generator. The GO/PANI composite showed typical ER behavior, which demonstrated its potential applications as an ER smart material. Ó 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene is a single layer of carbon atoms with excellent thermal, electrical, structural and mechanical properties [1–3] that has attracted an enormous attention recently [4,5]. Its diverse applications, such as resonators, catalyst supports, electronic devices, supercapacitors, batteries, solar cell and composites materials [6–8], have been widened. Graphene oxide (GO) is an oxidation state of graphene with oxygen functional groups (epoxide, hydroxyl, carbonyl and carboxyl groups) on the basal planes and edges [9]. Although GO has the same lamellar structure as graphene, it possess expanded properties due to the functional groups, typically its good dispersion stability in aqueous and other organic solvents [10]. This hydrophilicity provides a facile way of synthesizing GO/polymer composites with simple chemical routes [11]. Polymers, such as polyaniline (PANI), poly(acrylonitrile) and

poly(methyl methacrylate) [12], have been reported as potential candidates for preparing GO/polymer composites for different applications [13]. As a conducting polymer, PANI in its emeraldine salt (ES) form possesses special favors because of its high conductivity, easy synthesis, high thermal stability and remarkable environmental stability [14,15], as well as its good activity as electrorheological (ER) particles when dedoped into the semiconducting state. ER particles are a type of semiconducting or polarizable material, forming field-induced reversible chain-like structures as a dispersed phase in a non-conducting liquid (mineral or silicone oil) (called ER fluid) when applied in an electric field [16–19]. The microstructural transition from a liquid-like to solid-like state of an ER fluid by the application of an electric field induces interesting changes in its rheological properties, including shear stress, shear viscosity, yield stress and storage/loss modulus [20–22]. Similarly,

* Corresponding author: Fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.08.049

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magnetically analogous magnetorheological fluids exhibit the same behavior under applied magnetic fields [23–25]. ER fluids, which are characterized by this reversible and tunable transition have potential applications for a range of electromechanical engineering devices including brake/clutch, vibration damper and shock absorbers etc. [26]. PANI and various electroresponsive materials, such as inorganics with high dielectric properties, organic or polymeric semiconducting materials and their composites have been used as ER particles [27]. Graphene and GO have also attracted attention as ER materials [28–30]. In this study, GO/PANI composite particles were synthesized with PANI, which was fabricated by in situ polymerization [31,32] in the GO dispersion. The low electrical conductivity and acidic groups of GO, which are disadvantages in many investigations, become positive and favorite factors in the present ER study. A low conductivity can reduce the possibility of short-circuits and acid groups can be used as dopant sites of PANI. ER fluids were prepared by dispersing the GO/ PANI composite particles in 30cS silicone oil. The ER responses were observed using a rotational rheometer under various electric field strengths.

2.

Experimental section

2.1.

Materials

Natural graphite (flake, 20 lm, 100 mesh (P75% min)) and the strong oxidant KMnO4 were purchased from Sigma– Aldrich. NaNO3 was purchased from Junsei Co. Ltd., Korea. Other reagents including aniline, 35% HCl and 98% H2SO4 were purchased from DC Chemical Korea. Ammonium per-

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sulfate (APS, Daejung Co. Ltd., Korea) was used as an initiator. All regents were used as received. Distilled water was used in all experimental processes. Initially, GO was synthesized from graphite using a modified Hummers method [33]. One gram graphite was added to 70 ml H2SO4 (98%) in an ice bath, which was followed by the addition of 3 g KMnO4 and 0.5 g NaNO3. After stirring for 4 h, 70 ml distilled water was slowly added and the mixture was maintained at that temperature for 30 min. Subsequently, a 30% H2O2 solution was added to the solution until the color turned a brilliant brown indicating fully oxidized graphite. The as-obtained graphite oxide slurry was exfoliated to generate GO nanosheets by sonication at 60 °C using a custom-made powerful ultrasonic generator (28 kHz, 600 W, Kyungil Ultrasonic Co., Korea) for 3 h. Finally, the mixture was separated by centrifugation, washed repeatedly with 5% HCl and distilled water, and dried in a vacuum oven at 60 °C for 24 h. At a second step, to synthesize the GO/PANI composite particles, GO (1 g) was dispersed in 500 ml distilled water using an ultrasonic generator for 1 h to produce a dark brownish colloidal system. 1 g aniline monomer and 100 ml distilled water containing 3.1 g ammonium persulfate (APS) were added to a three-neck round-bottom flask, in which the above GO system was predisposed. Polymerization was allowed to proceed for 24 h at room temperature with continuous stirring. The product obtained was separated by a centrifuge, washed with distilled water and dried at 60 °C under vacuum for 24 h. To prepare the ER fluid, the obtained GO/PANI composite particles were dispersed in silicone oil using a sonicator and stored in a vacuum oven at room temperature 24 h to remove air bubbles.

Fig. 1 – SEM and TEM images of GO (a), (c) and GO/PANI composite particles (b), (d), respectively.

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Transform Standalone Raman Breaker (FT-Raman, RFS-100/ S). The GO/PANI composite particle-based ER fluid was prepared by dispersing them in silicone oil (0.955 g/cm3, 30cS) by shaking and sonication. A fibrillation phenomenon was first observed by optical microscopy (OM, Olympus BX51). The ER performance of the fluid was tested using a stress controlled rotational rheometer (MCR 300, Physica, Germany), which was equipped with a high voltage power supply (Fug, HCN 7E-12 500, Germany) using a Couette-type sample loading geometry with a bob and cup (CC 17, gap distance is 0.71 mm). By changing the electric field strength, the flow curves were carried out above a wide shear rate range of 0.01–1000 s1.

Fig. 2 – The Raman spectra of (a) the pristine GO and (b) GO/ PANI composite particles.

2.2.

Measurement

The morphology of the pure GO and GO/PANI composite particles was observed by scanning electron microscopy (SEM, S-4300, Hitachi, Japan) and transmission electron microscopy (TEM, Philips CM200). To examine microstructure of the two samples of pure GO and GO/PANI composite particles, their Raman spectra were characterized using a Fourier

3.

Results and discussion

Both SEM and TEM provide evidence supporting the differences between GO and GO/PANI composite particles. As shown in Fig. 1a, the surface of GO is fairly smooth and the layered structure can be observed clearly. In contrast, the GO/PANI composite particles exhibited a much rougher surface due to the wrapping of PANI on two-dimensional GO sheets, as shown in Fig. 1b. TEM of GO and GO/PANI composite particles confirmed the SEM results. Fig. 1c shows that GO was exfoliated into a single or several-thick (two or three) multilayers by the ultrasonication. On the other hand, the GO/PANI composite particles were not as transparent as GO

Fig. 3 – XPS spectra in the C1s region of (a) GO and (b) GO/PANI composite particles, and the survey curves of (c) GO and (d) GO/ PANI composite particles.

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Fig. 4 – Schematic diagram of microstructure changes (1) and OM images (2) of GO/PANI composite particles (10 vol.% particle concentration) based ER fluid with and without the external electrical field (left and right). Very dilute ER fluid was dropped between two parallel electrodes (400 lm) and observed by OM. The OM pictures were captured when the electric field (300 V) is off (a) and on (b).

and were made up of several layers due to the existence of PANI on the GO surface, as shown in Fig. 1d. The structural changes to the GO/PANI composite particles from GO are also reflected in their Raman spectra. In the Raman spectrum of pristine GO (Fig. 2a), a broad D band at 1340 cm1 and a G band at 1600 cm1 are displayed, which are due to activation in the first order scattering process of sp3 carbon and sp2-bonded carbon atoms in graphene sheets, respectively [34]. The Raman spectrum of the GO/PANI composite particles (Fig. 2b) also contains both D and G bands, which shifted to lower Raman shift positions at 1306 and 1594 cm1, respectively. These shift-changes were attributed to the emergence of covalent bonds between the GO and PANI chains, as demonstrated by the FT-IR spectra in our previous work [29]. After chemical processing, the Raman shift changes of GO were also observed, as described elsewhere [35]. On the other hand, the two samples gave a similar Raman spectrum in terms of the shapes and positions of the Raman peaks, indicating that the structure of GO was maintained after PANI insertion. X-ray photoelectron spectroscopy (XPS) was used to confirm the difference between GO and GO/PANI composite particles. The C1s spectrum of GO (Fig. 3a) shows clearly binding energy features at 287.42 eV (C@O) (34.01%), 285.41 eV(C–OH) (29.37%), 288.27 eV(O–C@O) (22.15%) and 286.15 eV(C–O), (14.24%), which arise from epoxide, carboxyl

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and hydroxyl functional groups on the graphene edge [36,37]. The C1s spectrum of the GO/PANI composite particles (Fig. 3b), shows primary binding energy features at 284.53 eV (C@C) (60.31%). On the other hand, the intensities of the binding energy bands corresponding to epoxide, carboxyl and hydroxyl functional groups were reduced greatly due to the PANI coating. Compared to the survey curves of GO (Fig. 3c), the GO/PANI composite particles (Fig. 3d) exhibited a higher intensity of N1s, indicating the addition of PANI chains. The ER fluids are composed of semiconducting or polarizable materials dispersed in non-conducting liquid. A microstructural transition of the ER fluid from fluid-like to solid-like can be obtained under an electric field, as shown in Fig. 4(1). In this paper, the ER fluid was prepared by dispersing GO/PANI composite particles (10 vol.% particle concentration) in 30cS silicone oil. The microstructural changes were observed by OM under a DC applied electric field, as shown in Fig. 4(2). The gap between two parallel electrodes was fixed to 400 lm. When the electric field was absent, the particles were dispersed randomly in silicone oil with liquid-like state characteristic. On the other hand, when the electric field was present, the particles began to move and form chains in the direction perpendicular to the aluminum electrodes [38]. The typical ER fluid behavior of GO/PANI composite particles (10 vol.% particle concentration) dispersed in silicone oil from the controlled shear rate measurement (CSR) was tested using a rotational rheometer equipped with a high voltage generator under different electric field strengths, as shown in Fig. 5. The fluid behaved similarly to a Newtonian fluid without an external electric field, in which the shear stress increased linearly with increasing shear rate [39]. In contrast, when a high electric field was applied, the ER fluid exhibited a large increase in shear stress because the particles were polarized and formed chain-like structures, as shown in the OM images, which can be called Bingham-like behavior. In addition, Fig. 5 indicates that the shear stress initially exhibits a wide plateau region at a low shear rate region, then decreases slightly, and finally increases with increasing shear rate, which can be explained by the following mechanism.

Fig. 5 – Shear stress versus Shear rate for GO/PANI composite particles (10 vol.% particle concentration) based ER fluid under different electric field strengths. Solid, dotted lines are from CCJ model and Bingham model.

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Table 1 – The optimal parameters in each model equation obtained from the flow curves of GO/PANI composite particles (10 vol.% particle concentration) based ER fluid. Model

Parameter

Electric field strength 0.5 kV/mm

1 kV/mm

1.5 kV/mm

2 kV/mm

Bingham

s0 g0

50 0.15

120 0.16

200 0.08

240 0.28

Cho–Choi–Jhon

s0 g1 a b t1 t2

65 0.23 0.35 0.85 0.7 0.01

186 0.23 0.3 0.80 0.075 0.01

490 0.20 0.13 0.70 0.44 0.74

630 0.31 0.28 0.26 0.15 0.142

In a low shear rate region, the electrostatic interactions between particles arising from external electric fields are dominant compared to the hydrodynamic interactions. The aligned structures can break and reform the chain-like structure repeatedly. Hence, there is a wide plateau of shear stress with a low shear rate. In a high shear rate region, the broken-chain particles have no to time to reform and the shear stress tends to decreases until a high shear rate was obtained. The typical Bingham equation with two parameters has been used to illuminate this shear stress behavior, as given below [38]: s ¼ s0 þ g0 c_ ; c_ ¼ 0;

s  s0 s < s0

ð1Þ

where s is the shear stress, s0 is the yield stress, which is a function of an electric field, c_ is the shear rate, and g0 is the shear viscosity. Nevertheless, with the increasing number of ER fluids discovered, the simple Bingham model was found to be unable to describe the various ER fluids well through the whole shear rate region because not all of the ER fluid curves were smooth and steady. Therefore, another sixparameter model called the Cho–Choi–Jhon (CCJ) model [40] was suggested to fit these ER fluids, as follows: ! s0 1 s¼ þ g 1 þ c_ ð2Þ 1 1 þ ðt1 c_ Þa ðt2 c_ Þb where s0 is the static yield stress is defined as the extrapolated stress from the low shear rate region, g1 is the viscosity at the infinite shear rate and is interpreted as the viscosity in the absence of an electric field. The exponents a and b are related to the decrease and increase in shear stress; b has the range 0 < b 6 1, since ds=_c  0. The parameters t1 and t2 are time constants. Fig. 5 shows the fitting of the two model equations for the 10 vol% GO/PANI composite particles based an ER fluid. Table 1 summarizes the optimal fitting parameters used for these two models. Compared to the Bingham model, the six parameter equation of the CCJ model can cover the curves better through the entire shear rate range, particularly at a low shear rate. Fig. 6 shows the shear viscosity as a function of the shear rate. The ER fluid shows Newtonian fluid characteristics without an electric field and typical shear thinning behavior [41] under different electric fields.

Fig. 6 – Shear viscosity versus Shear rate for GO/PANI composite particles (10 vol.% particle concentration) based ER fluid under different electric strengths.

Fig. 7 – The dynamic yield stress versus electric field strength for GO/PANI composite particles (10 vol.% particle concentration) based ER fluid.

The dynamic yield stress was plotted as a function of the electric field strength in log–log scale curves. In general, the

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ER fluid showed typical ER behaviors. A suggested CCJ model was found to describe the ER behaviors more accurately than the Bingham fitting equation. The dynamic yield stress was also analyzed as a function of the electric field by plotting the data on a log–log scale. The critical electric field for this ER fluid was 0.7 kV/mm and the extrapolated data was found to lie on a single line after adopting a universal equation.

Acknowledgment This work was supported by National Research Foundation (#43450-1), Korea (2011).

R E F E R E N C E S

^ for GO/PANI composite particles (10 vol.% Fig. 8 – ^s versus E particle concentration) based ER fluid.

dependency of the dynamic yield stress on the electric field strength was presented by a power law relationship as follows: sy / Em

ð3Þ

where m = 2 is suggested by the polarization model, and m = 1.5 is suggested for the conduction model [42]. The ER response was affected by complicated factors, such as a conductivity mismatch, electrical breakdown, interaction between particles and medium. The only power index m appears deficiently. To correlate the dynamic yield stress for a broad range of electric field strengths, Choi et al. [43] introduced a simple hybrid universal yield stress equation (Eq. (4)) with the parameter named critical electric field strength Ec, which originated from the nonlinear conductivity effect. pffiffiffiffiffiffiffiffiffiffiffiffi! E0 =Ec 2 tan h pffiffiffiffiffiffiffiffiffiffiffiffi sy ðE0 Þ ¼ aE0 ð4Þ E0 =Ec Here, the parameter a depends on the dielectric properties of the fluid, particle volume fraction and critical electric field. Ec can be obtained by the crossover point of the slopes corresponding to the polarization model (slope = 2) and conductivity model (slope = 1.5) among all ranges of electric field strengths. Fig. 7 presents the dynamic yield stress as a function of the electric field strengths related to Eq. (2). The estimated Ec was 0.7 kV/mm. Eqs. (4) and (5) were normalized to collapse the data into a single curve shown in Fig. 8. pffiffiffi ^ ^ 3=2 tan h E ^s ¼ 1:313E ð5Þ ^  E0 =Ec and ^s  sy ðE0 Þ=sy ðEc Þ. The data obtained Here, E from Fig. 5 collapsed onto to a single curve using the normalized universal yield stress equation Eq. (5).

4.

Summary

GO/PANI composite particles were synthesized by in situ polymerization using stable GO. Raman spectroscopy, XPS, SEM and TEM confirmed the changes on the GO surface due to the addition of PANI. The GO/PANI composite particles-based

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