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Enhanced effect of dopant on polyaniline nanofiber based electrorheological response
Materials Chemistry and Physics 147 (2014) 843e849
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhanced effect of dopant on polyaniline nanofiber based electrorheological response Ying Dan Liu a, b, Ha Young Kim b, Ji Eun Kim b, In Gu Kim b, Hyoung Jin Choi b, *, Soo-Jin Park c a b c
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Department of Chemistry, Inha University, Incheon 402-751, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyaniline nanofibers were synthesized via an interfacial polymerization route. Polyaniline nanofibers were doped with indole-2-carboxylic acid. Typical electro-responsive characteristics were observed for polyaniline nanofibers.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2014 Received in revised form 20 May 2014 Accepted 10 June 2014 Available online 25 June 2014
As a potential candidate for electrorheological (ER) materials, polyaniline (PANI) nanofibers were synthesized via an interfacial polymerization route, because of their easy synthesis, high stability, controllable conductivity and good ER response. In this study, a new dopant, indole-2-carboxylic acid, was used to improve the ER performance. The synthesized PANI nanofibers were dispersed in silicone oil at 10 vol% and their rheological properties as an ER fluid were examined using a rotational rheometer under a variety of electric field strengths. In addition, their dielectric properties were studied using a LCR meter. The results showed that indole-2-carboxylic acid, which has high polarizability, enhanced the ER effect of PANI nanofibers, showing that polarizability is an important parameter affecting the ER behavior. © 2014 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Polymers Nanostructures Dielectric properties
1. Introduction Electrorheological (ER) fluids, which consist of polarizable solid particles dispersed in a non-conducting medium, have been studied extensively owing to the excellent tunable characteristics of
* Corresponding author. Tel.: þ82 32 860 7486; fax: þ82 32 865 5178. E-mail address: [email protected] (H.J. Choi). http://dx.doi.org/10.1016/j.matchemphys.2014.06.029 0254-0584/© 2014 Elsevier B.V. All rights reserved.
their states under an applied electric field strength. The rheological and mechanical properties of ER fluids are changed dramatically by an external electric field based on their phase transition from a liquid-like to solid-like state [1-5]. The changes induced by an external electric field are virtually instantaneous and reversible by controlling the electric field strength. As the beneficial effects with a rapid and reversible change, their potential applications have attracted considerable attention in a range of industrial [6] and scientific areas.
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The ER phenomenon results from the fibrillation chains in the presence of electric fields, where fibrillated particulate structures are induced by dielectric constant mismatch of the particles and insulating oil. In various theories to account for the ER mechanisms [7,8], a dielectric polarization theory has been widely adopted. According to the polarization model, the suspending particles are polarized under an applied electric field. The interactions among the polarized particles cause then them to form chains themselves along the electric field direction [9]. Therefore, the strength of the fibrillation chains is determined by the polarization force of the particles, which is affected by their dielectric constant [10e13]. In particular, because the interfacial polarization is one of the important factors in the performance of ER fluids, the polarizability of the particles in dry-base ER fluids is the most important factor, considering their higher ER performance. Under the present conditions, the ER effect still requires considerable improvement before high performance ER materials can be used in practical applications. Among the many anhydrous ER materials available, polyaniline (PANI) is frequently used as the dispersed solid particle owing to its environmental stability, simplicity, low cost of synthesis, and tunable conductivity with doping and dedoping processes [14e16]. In addition to conventional PANI, many other types of PANIs have been developed in ER research, which not only differed in structure or morphology but also exhibited different ER effects, such as oligomers and derivatives [14,17], modification [18], composites, coreeshell structured particles [19e21], nanospherical or fibrous PANI [22] and etc. Recently, Yin et al. [23] reported that PANI nanofiber-based ER fluids show higher yield stress, being more stable than granular PANI-based ER fluids. They also reported the effect of the particle morphology on the ER performance of the PANI suspension to achieve a better understanding of the morphological factors affecting the ER properties. The shape effect on the ER performance with a mainly anisotropically elongated or fibrous configuration has been studied widely for various particles ranging from whisker-like aluminum borate [24], titanate nanofibers [25], goethite nanorods [26] to polypyrrole nanofibers [27], all which show better ER properties. In the case of nanofiber-based ER fluids, improved suspension stability was also observed [9]. On the other hand, Hong et al. [28] examined the influences of the particle shape on the ER activity along with titania-coated silica nanomaterials. Above all, the polarizability of suspended particles as well as the particle shape affects the efficiency of ER fluids significantly. On the other hand, there still is a room for improvement on the polarizability of suspension particles. For example, less attention has paid to the effects of the dopants used in the PANI synthesis process on the ER performance. Because polarizability of the suspension particles is an important factor for high ER properties, dopants with high polarizability have the potential to enhance the ER effect. In this study, PANI nanofibers were synthesized using an interfacial polymerization method [29], which can produce uniform and template-free nanofibers. Furthermore, to improve the ER performance of the product, indole-2-carboxylic acid (ICA), which has high polarizability, was used as a dopant instead of common acids, such as hydrochloric, sulfuric, or nitric acid, during the synthesis process. The synthesized PANI nanofibers were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. The rheological and electrical properties were then examined to understand the role of particle polarizability. Conventional HCl-doped PANI nanofibers were also prepared for comparison.
2. Experimental 2.1. Materials and preparation of ICA-doped PANI fibers Aniline (DC Chemical, Korea), ammonium persulfate (APS) (98%, Daejung, Korea) and indole-2-carboxylic acid (ICA) (98%, SigmaeAldrich) were used as received. A 35 wt% HCl solution (DC Chemical, Korea) was diluted to 1 M for use as a dopant. In a typical synthesis route, the particular shape of PANI can be synthesized by the oxidative polymerization of aniline with APS in an acid aqueous solution. On the other hand, an interfacial polymerization method was adopted to obtain a fiber shape. The synthesis process was as follows. To prepare an organic phase, both the aniline monomer (0.05 mol) and ICA (0.005 mol) were dispersed in dichloromethane with energetic stirring. For the liquid phase, however, APS was dissolved in Di-water with agitation until a transparent solution was obtained. Subsequently, the liquid phase was added to the organic phase slowly and carefully along one side of the inner wall, and the obtained immiscible two phase system was kept in a refrigerator for 24 h. For comparison, HCl-doped PANI nanofibers were also synthesized in the same manner as above using the same reagents, but with HCl (0.005 mol) as a dopant instead of ICA. After polymerization, the product was washed with di-water and methanol from a reaction solution until the washing liquid became neutral to remove the unreacted reagents. These solid products were dried in a vacuum oven at 60 C. 2.2. Preparation of ER fluid To use the PANI nanofibers as a solid-phase of the ER fluid, the as-synthesized PANI nanofibers need to be dedoped by controlling the pH of the aqueous suspension of PANI with a 1 M NaOH solution. The electrical conductivity of the dry PANI powders was decreased from 101 to 1010 S cm1. Densities of the particles measured by a gas pycnometer (AccuPyc 1340, Micromeritics) were 1.50 and 1.51 g cm3 for ICA and HCl doped PANI, respectively. Subsequently, the ER fluids were prepared by dispersing the PANI nanofibers in silicone oil (r ¼ 0.96 g cm3, kinematic viscosity ¼ 50 cS at 25 C) under shaking and sonication, respectively. Two types of ER fluids were prepared containing 10 vol% of HCl-doped PANI and ICA-doped PANI, respectively. 2.3. Characterization The morphology of the PANI nanofibers was examined by field emission-scanning electron microscopy (FE-SEM) (Hitachi S-4300, Japan). Fourier transform infrared spectrometry (FT-IR) (Perkin Elmer System 2000, Norwalk, CT) was used to identify the chemical structure of the PANI nanofibers. The electrical conductivity was measured using a standard probe technique with a resistivity meter (Loresta-GP and Hiresta-UP, Mitsubishi Chemical Analytech CO., LTD, Japan). Optical microscopy (OM) (Olympus BX51, USA) equipped with a DC high voltage generator was used to examine the electro-responsive response of the ER fluid under an electric field. The ER properties of the PANI nanofibers suspension were characterized by steady shear experiments using a rotational rheometer (MCR 300, Physica, Austria) with a cup-bob system (CC17 ERD, the gap between the cup and bob was 0.71 mm) and a DC high voltage generator. In controlled shear rate (CSR) mode, the flow curves of each ER fluid were measured under a shear rate between 0.01 and 1000 s1 with and without an external electric field. In addition, dielectric relaxation spectra of the prepared ER fluids were analyzed using the HP 4284A precision LCR meter with
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Fig. 1. SEM images of PANI nanofibers: (a) ICA-doped PANI, (b) HCl-doped PANI.
HP 16452A liquid test fixture at room temperature. The frequency range for measuring the dielectric permittivity and loss factor was 20 ~ 106 Hz. 3. Results and discussion 3.1. Material characteristics
Transmittance
Fig. 1 shows SEM images of the synthesized PANI nanofibers. The morphology of PANI, which was synthesized using an interfacial polymerization method, typically has a nanofiber shape. From the SEM image, we can also observed that the morphology of PANI is not affected much by the dopant during the polymerization process. Although different dopants were used, both types of the synthesized PANI particles are consisted mainly of uniform nanofibers, about 50 nm in diameter and 1 mm in length. In the interfacial polymerization process, anilinium cations self-assemble and form micelles at the liquideliquid interface, which grow to be fibers and settle down to the bottom of the beaker by gravity. The morphology of fibers including diameter and shape was dependent on whether surfactant was used or not, molecular structure and concentration of dopants, etc [29,30]. While the effect of different dopants on influencing the diameter of the nanofibers was not so obvious when the fibers were prepared without templates [31]. The chemical structure of the PANI nanofibers was identified by an FT-IR spectroscopy. Fig. 2 shows the FT-IR spectra of the
HCI-doped PANI ICA doped PANI 4000
3500
3000
2500
2000
1500 -1
wave number (cm ) Fig. 2. FT-IR spectra of the PANI nanofibers.
1000
500
PANI nanofibers, in which the spectrum shows that two type of PANI nanofibers were synthesized like the typical PANI with an emeraldine base. As the analytical band, the characteristic peaks are summarized as follows. Absorption bands were observed at 820 cm1 (out-of-plane CeH deformation of the aromatic ring in the PANI unit sequence), 1305 cm1 (the CeN stretching of the secondary aromatic amine), 1240 cm-1 (the CeNþ stretching), 1145 cm1 (the benzenoid ring eNHþ ¼ quinonoid stretching), 1580 cm1 (the aromatic C]C stretching of the quinonoid ring), and 1500 cm1 (the aromatic C]C stretching of the benzenoid ring) [32]. The broad bands with multi peaks at 3500e2800 cm1 were assigned to NeH stretching vibration of the secondary amine in the PANI backbone and hydrogen-bonded NeH stretching [33]. However, it is difficult to distinguish sharply between HCl-doped PANI and ICA-doped PANI because the peaks of ICA overlap with those of PANI between 1720 and 1600 cm1. In the sodium ICA peak, two strong peaks at 1565 and 1409 cm1 were observed based on the asymmetric and symmetric stretching vibrations of COO, respectively [34]. As shown in Fig. 2, these characteristic peaks of ICA were overlapped by strong peaks for PANI. 3.2. Electrorheological property The ER fluids of both HCl-doped PANI nanofibers (a), (b) and ICA-doped PANI nanofibers (c), (d) dispersed randomly in silicone oil (50 Cs) were observed directly by an optical microscopy without and with an applied electric field (200 V) attached with a DC high voltage source, where a dilute ER fluid (10 vol % particle concentration) was dropped between two parallel electrodes of the glass slide. Both ER fluids exhibited a typical ER chain structure. Both HCl-doped PANI nanofibers of Fig. 3(a) and ICA-doped PANI nanofibers of Fig. 3(c) based ER materials in silicone oil without an electric field show a quite well dispersed liquid-like state. When the electric field was applied (Fig 3(b) and (d)), the particles begin to move form a chain-like structure with the adjacent particles. Generally, this ER phenomenon forms fibril chains as long as the electric field is applied. In this situation, the rheological properties of the ER fluids are being changed rapidly. The ER behavior of the PANI nanofiber-based ER fluids was compared. Fig. 4 presents the flow curves obtained from the CSR test for 10 vol% suspensions of the PANI nanofibers with various electric field strengths. In the absence of an electric field, the PANI nanofiber-based ER fluids all behave similarly to a Newtonian fluid, such that the shear stress increases in proportion to the shear rate. On the other hand, when an external electric field is applied, the ER fluids under an external electric field show non-Newtonian fluid characteristics, such as a Bingham fluid, which possesses a yield stress (ty), because of the generation of a particle fibrillar structure aligned with the applied electric field. The relationship between the
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Fig. 3. Optical microscopic images of HCl-doped PANI nanofibers (a), (b) and ICA-doped PANI nanofibers (c), (d) based ER fluids. The pictures were captured when the electric field (200 V) was off (a), (c) and on (b), (d).
_ for a Bingham fluid is expressed shear stress (t) and shear rate ðgÞ as follows:
_ t ty t ¼ ty þ hg;
(1)
g_ ¼ 0; t < ty
(2)
The Bingham behavior is related to the fact that the electrostatic force between the polarized particles becomes gradually stronger to resist the hydrodynamic force. In other words, the fibrillar structure will resist the flow until the shear stress approaches a critical value called the yield stress. When the shear rate increases, however, the flow curves of the ER fluid deviate from the Bingham model, showing a slight decrease in shear stress with increasing shear rate, as shown in Fig. 4(a). This can be explained by the equilibrium of the particle chains between the broken and reformed states [35] until the structures are totally destroyed by the high shear rate. A modified rheological equation of state was proposed for this behavior, the ChoeChoieJhon (CCJ) model, which is described by the equation below [35]:
! ty 1 t¼ þ h∞ 1 þ g_ _ a 1 þ ðt2 gÞ _ b ðt3 gÞ
(3)
where t2 and t3 are the time constants, h∞ is the shear viscosity at an infinite shear rate, and a and b are in charge of the decrease and increase in shear stress, respectively, in the low and high shear rate regions. b is in the range of 0 and 1 because dt/dg 0. The solid lines in Fig. 4(a) were generated from Eq. (3). It can be found that the CCJ model fitted the flow curves of the ICA-doped PANI nanofiber based ER fluid quite well in both the low and high shear rate regions with the optimal parameters of which are listed in Table 1. In addition to this CCJ model, there are other equations reported recently to describe the flow curves that deviate the Bingham model. A model equation with spring-damper theory [36] and a modified four parameter Papanastasiou equation [37] were examples adopted for fitting flow curves of various ER fluids. By comparing the shear stress curves in Fig. 4(a), the ICA-doped PANI nanofiber-based ER fluid is observed to have a larger ER effect than that of the HCl-doped PANI nanofibers. This shows that the ER effect depends not only on the electric field strength, but also on the constitution of the suspended particles. In the synthesis of PANI nanofibers, two different types of dopant were used, in which ICA has high polarizability. When an electric field is present, the ICA can help inducing an excellent ER effect through its high polarizability. Therefore, the ICA-doped PANI nanofiber-based ER fluid shows higher shear stress and enhanced ER performance. This confirms that the polarizability is an important physical parameter in the ER behavior. Furthermore, in the case of the field-off viscosity, the ICA-
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Fig. 5. Shear stress as a function of shear rate for both ER fluids (10 vol%) at an electric field strength of 3.0 kV mm1.
Fig. 4. (a) Shear stress vs. shear rate, and (b) Viscosity vs. shear rate for the PANI nanofibers based ER fluids (10 vol%) under various electric field strengths (closed: ICAdoped PANI nanofiber, open: HCl-doped PANI nanofiber). The solid lines in (a) are generated from the CCJ model.
increasing electric field strength. Note that a slight shear-thinning behavior of the shear viscosity was observed at a low shear rate region even at a zero applied electric field owing to its nonNewtonian suspension system with 10 vol% of the particle. The ER behavior of the ICA-doped PANI nanofiber-based ER fluid was enhanced over the entire shear rate range. On the other hand, it can be also noted that for quantifying the ER improvement, the ER efficiency has been also reported [38]. Fig. 5 shows the shear stress as a function of the shear rate for the two ER fluids at an electric field strength of 3.0 kV mm1. Despite the same applied electric field strength, the ICA-doped PANI nanofiber-based ER fluid has higher shear stress than that of the HCl-doped PANI nanofiber over the entire shear rate range. This means that the chains of the ICA-doped PANI nanofibers are hardly broken and stiff enough to withstand the shear in addition to their faster reformation rate once broken. Note that dynamic yield stress (ty), defined as the minimum shear stress required for maintaining flow is an important criteria to characterize an ER fluid. In the controlled shear rate mode, ty can be obtained by extrapolating shear stress of the flow curve to a zero
doped PANI nanofiber based ER fluid shows a slightly higher shear stress than that of the HCl-doped PANI nanofibers based ER fluid and this could be associated with slight different dispersion state of the dispersed particles due to different doping agents not only inducing potential aggregation but also different chemical affinity with a silicone oil despite well dispersion in a bulk state. These suspension behaviors of both ER fluids were also confirmed by Fig. 4(b), in which the shear viscosity increased with
Table 1 Optimal parameters in the CCJ model obtained by fitting the model to the flow curves of the ER fluid of ICA doped PANI nanofiber at various electric field strengths. Parameters
ty a t2
h∞
t3
В
Electric field strength (kV mm1) 0.5
1.0
1.5
2.5
3.5
15 0.4 1.1 0.15 0.01 0.75
30 0.7 1.4 0.15 0.004 0.80
50 0.9 1.5 0.14 0.002 0.80
110 0.9 2.0 0.16 0.0015 0.91
200 0.8 1.5 0.18 0.001 0.89
Fig. 6. Dynamic yield stress vs. electric field strengths for both ER fluids (10 vol%).
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Fig. 7. b t vs. b E for both HCl and ICA-doped PANI nanofiber ER fluids. The solid line is generated from Eq. (7).
shear rate limit. In general, the correlation of ty and electric field strength (E0) is presented as follows:
ty fE0m
(4)
The dependency of ty on the electric field strength differ from the E20 (m ¼ 2) dependency suggested by the polarization model depending on the particle concentration [39], particle shape, and applied electric field strength, in which the applied electric field induces electrostatic polarized interactions among the particles and also between the particles and electrodes. On the other hand, the polarization model does not describe fully the ER mechanism for all kinds of ER materials. Sometimes, the ER response is known to be influenced by the conductivity mismatch between the particles and medium and the non-linear electrical conduction effect, which leads to a lower m value. Thereby, a new equation was developed to derive a universal scaling equation for a wide range of ER fluids [40].
ty ðE0 Þ ¼ aE02
pffiffiffiffiffiffiffiffiffiffiffiffi! tanh E0 =Ec pffiffiffiffiffiffiffiffiffiffiffiffi E0 =Ec
(5)
Fig. 9. ColeeCole fitting curves for PANI nanofibers based ER fluids.
where Ec is the critical electric field strength, dividing the electric field into two regions with a different relationship between the yield stress and electric field strength: when E0 ≪Ec ; ty fE20 ; and 3=2 E0 ≪Ec ; ty fE0 . This means that the yield stress relationship depends on the E0 range, showing that m in Eq. (4) is 2.0 until the critical electric field strength is reached, and then generally becomes smaller than 2.0 when E0 > Ec. A single universal curve in Eq. (5) with Ec and ty (Ec) is normalized to collapse the data.
ty ðEc Þ ¼ aEc2 tanhð1Þ ¼ 0:726aEc2
(6)
resulting in
b t ¼ 1:313 b E
3=2
tanh
qffiffiffiffi b E
(7)
where b E ¼ E0 =Ec ; b t ¼ tðE0 Þ=tðEc Þ. This study estimated the dynamic yield stress of the PANI nanofiber based ER fluids from the CSR measurements in Fig. 4(a) [41], and the dynamic yield stress was then plotted as a function of the electric field strength. Fig. 6 shows the correlation between yield stress and electric field strength of the two PANI based ER fluids. The Ec originating from the nonlinear conductivity effect can be obtained by the crossover point of the slopes for all ranges of electric field strengths corresponding to the polarization model (slope ¼ 2.0) and conduction model (slope ¼ 1.5), respectively [42]. The estimated Ec was found to be 1.25 and 2.04 kV mm1 for the ICA-doped PANI nanofiber and HCl-doped PANI nanofiber-based ER fluids, respectively. Using the experimental data in Fig. 6, the newly defined b E and b t were plotted in Fig. 7 and fitted by Eq. (7). All the rearranged yield stresses for the two ER fluids collapsed to a single curve of Eq. (7) showing the good correlation between yield stress and electric field strength using the parameter, Ec.
Table 2 Parameters in Eq. (8) for two kinds of PANI nanofibers based ER fluids.
Fig. 8. Dielectric spectra of PANI nanofibers based ER fluids (close symbol and solid line for dielectric constant ε0 , open symbol and dash line for dielectric loss factor ε00 ).
No.a
30
3∞
D3 ¼ 3 0 3 ∞
l (s)
a
1 2
6.5 5.0
3.0 2.9
3.5 2.1
0.0056 0.006
0.47 0.35
a
1: ICA doped PANI, 2: HCl doped PANI.
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interpreted well by fitting the dielectric spectra to the ColeeCole formula.
3.3. Dielectric property Furthermore, the dielectric properties for the ER fluids were analyzed to further examine ER performance of the two ER fluids. Fig. 8 shows the frequency dependence of the dielectric constant (ε0 ) and dielectric loss factor (ε00 ), which are typical results for the interfacial polarization of suspensions including ER fluids. The interfacial polarization is considered to be the origin of the ER effect. The lines in Fig. 8 were fitted from the famous ColeeCole formula (Eq. (8)) [43e45] using these parameters.
ε ¼ ε∞ þ
Dε 1 þ ðiulÞ
(8)
1a
In Eq. (8), the dielectric constant and loss factor can be related to
ε0 ¼
ε0 ε∞ ð1 þ A cos qÞ 1 þ A2 þ 2A cos q
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00
ε ¼
Aðε0 ε∞ Þsin q 1 þ A2 þ 2A cos q
(9)
where the exponent (1 a) characterizes the broadness of the relaxation time distribution, l is the relaxation time, A ¼ ðulÞ1a , and q ¼ ð1 aÞp=2. When a ¼ 0, the ColeeCole formula reduces to the Debye's well-known single relaxation time model. Fig. 9 is the ColeeCole plot of ε00 as function of ε0 . The adopting ColeeCole plot not only fits the dielectric spectra of both ER fluids but also provides a better explanation of their polarizability. Table 2 lists the parameters in Eq. (8) for the two ER fluids. Here, Dε ¼ ε00 ε∞00 ' represents achievable polarizability in the ER fluids. Because large polarization will lead to a large ER effect, the Dε dependence on the ER properties has a positive effect on the better ER performance. The relaxation time l for the interfacial polarization of ER fluids is related to the yield stress and stress enhancement under an applied electric field. As shown in Table 2, Dε of the ICA-doped PANI nanofiber is slightly higher than that of the HCldoped PANI nanofiber, suggesting enhanced ER performance. In addition, the relaxation times for the ICA-doped PANI nanofiber and HCl-doped PANI nanofiber ER fluids were 5.6 ms and 6 ms, respectively. In general, a short relaxation time is related to a higher shear stress, in which the ICA-doped PANI nanofiber has a faster interfacial polarization mechanism. The above analysis of the dielectric spectra coincides with the results obtained from a comparison of their shear stresses. 4. Conclusions Indole-2-caboxylic acid-doped PANI nanofibers were synthesized using an interfacial polymerization process, and the influence of the dopant on their ER behavior was investigated. From the morphology analysis, the nanofiber morphology was unaffected by the two different kinds of dopant. The flow responses of the ER fluids were examined as a function of the shear rate under a range of electric field strengths, indicating that the ICA-doped PANI nanofiber-based ER fluid showed higher shear stress than the HCldoped PANI nanofiber-based ER fluid. This was attributed to the high polarizability of the ICA. Typical flow curves of the ER fluid were analyzed using a modified flow equation of state, the CCJ model. The yield stresses of both ER fluids all collapsed into a single line according to the suggested universal equation with the parameter Ec. Finally, the difference in the ER performance could be
Acknowledgment This work was supported by National Research Foundation of Korea (NRF-2013R1A1A2057955) and also by the Ministry of Knowledge Economy, Korea (2012).
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhanced effect of dopant on polyaniline nanofiber based electrorheological response Ying Dan Liu a, b, Ha Young Kim b, Ji Eun Kim b, In Gu Kim b, Hyoung Jin Choi b, *, Soo-Jin Park c a b c
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Department of Chemistry, Inha University, Incheon 402-751, Republic of Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Polyaniline nanofibers were synthesized via an interfacial polymerization route. Polyaniline nanofibers were doped with indole-2-carboxylic acid. Typical electro-responsive characteristics were observed for polyaniline nanofibers.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 March 2014 Received in revised form 20 May 2014 Accepted 10 June 2014 Available online 25 June 2014
As a potential candidate for electrorheological (ER) materials, polyaniline (PANI) nanofibers were synthesized via an interfacial polymerization route, because of their easy synthesis, high stability, controllable conductivity and good ER response. In this study, a new dopant, indole-2-carboxylic acid, was used to improve the ER performance. The synthesized PANI nanofibers were dispersed in silicone oil at 10 vol% and their rheological properties as an ER fluid were examined using a rotational rheometer under a variety of electric field strengths. In addition, their dielectric properties were studied using a LCR meter. The results showed that indole-2-carboxylic acid, which has high polarizability, enhanced the ER effect of PANI nanofibers, showing that polarizability is an important parameter affecting the ER behavior. © 2014 Elsevier B.V. All rights reserved.
Keywords: Electronic materials Polymers Nanostructures Dielectric properties
1. Introduction Electrorheological (ER) fluids, which consist of polarizable solid particles dispersed in a non-conducting medium, have been studied extensively owing to the excellent tunable characteristics of
* Corresponding author. Tel.: þ82 32 860 7486; fax: þ82 32 865 5178. E-mail address: [email protected] (H.J. Choi). http://dx.doi.org/10.1016/j.matchemphys.2014.06.029 0254-0584/© 2014 Elsevier B.V. All rights reserved.
their states under an applied electric field strength. The rheological and mechanical properties of ER fluids are changed dramatically by an external electric field based on their phase transition from a liquid-like to solid-like state [1-5]. The changes induced by an external electric field are virtually instantaneous and reversible by controlling the electric field strength. As the beneficial effects with a rapid and reversible change, their potential applications have attracted considerable attention in a range of industrial [6] and scientific areas.
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The ER phenomenon results from the fibrillation chains in the presence of electric fields, where fibrillated particulate structures are induced by dielectric constant mismatch of the particles and insulating oil. In various theories to account for the ER mechanisms [7,8], a dielectric polarization theory has been widely adopted. According to the polarization model, the suspending particles are polarized under an applied electric field. The interactions among the polarized particles cause then them to form chains themselves along the electric field direction [9]. Therefore, the strength of the fibrillation chains is determined by the polarization force of the particles, which is affected by their dielectric constant [10e13]. In particular, because the interfacial polarization is one of the important factors in the performance of ER fluids, the polarizability of the particles in dry-base ER fluids is the most important factor, considering their higher ER performance. Under the present conditions, the ER effect still requires considerable improvement before high performance ER materials can be used in practical applications. Among the many anhydrous ER materials available, polyaniline (PANI) is frequently used as the dispersed solid particle owing to its environmental stability, simplicity, low cost of synthesis, and tunable conductivity with doping and dedoping processes [14e16]. In addition to conventional PANI, many other types of PANIs have been developed in ER research, which not only differed in structure or morphology but also exhibited different ER effects, such as oligomers and derivatives [14,17], modification [18], composites, coreeshell structured particles [19e21], nanospherical or fibrous PANI [22] and etc. Recently, Yin et al. [23] reported that PANI nanofiber-based ER fluids show higher yield stress, being more stable than granular PANI-based ER fluids. They also reported the effect of the particle morphology on the ER performance of the PANI suspension to achieve a better understanding of the morphological factors affecting the ER properties. The shape effect on the ER performance with a mainly anisotropically elongated or fibrous configuration has been studied widely for various particles ranging from whisker-like aluminum borate [24], titanate nanofibers [25], goethite nanorods [26] to polypyrrole nanofibers [27], all which show better ER properties. In the case of nanofiber-based ER fluids, improved suspension stability was also observed [9]. On the other hand, Hong et al. [28] examined the influences of the particle shape on the ER activity along with titania-coated silica nanomaterials. Above all, the polarizability of suspended particles as well as the particle shape affects the efficiency of ER fluids significantly. On the other hand, there still is a room for improvement on the polarizability of suspension particles. For example, less attention has paid to the effects of the dopants used in the PANI synthesis process on the ER performance. Because polarizability of the suspension particles is an important factor for high ER properties, dopants with high polarizability have the potential to enhance the ER effect. In this study, PANI nanofibers were synthesized using an interfacial polymerization method [29], which can produce uniform and template-free nanofibers. Furthermore, to improve the ER performance of the product, indole-2-carboxylic acid (ICA), which has high polarizability, was used as a dopant instead of common acids, such as hydrochloric, sulfuric, or nitric acid, during the synthesis process. The synthesized PANI nanofibers were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. The rheological and electrical properties were then examined to understand the role of particle polarizability. Conventional HCl-doped PANI nanofibers were also prepared for comparison.
2. Experimental 2.1. Materials and preparation of ICA-doped PANI fibers Aniline (DC Chemical, Korea), ammonium persulfate (APS) (98%, Daejung, Korea) and indole-2-carboxylic acid (ICA) (98%, SigmaeAldrich) were used as received. A 35 wt% HCl solution (DC Chemical, Korea) was diluted to 1 M for use as a dopant. In a typical synthesis route, the particular shape of PANI can be synthesized by the oxidative polymerization of aniline with APS in an acid aqueous solution. On the other hand, an interfacial polymerization method was adopted to obtain a fiber shape. The synthesis process was as follows. To prepare an organic phase, both the aniline monomer (0.05 mol) and ICA (0.005 mol) were dispersed in dichloromethane with energetic stirring. For the liquid phase, however, APS was dissolved in Di-water with agitation until a transparent solution was obtained. Subsequently, the liquid phase was added to the organic phase slowly and carefully along one side of the inner wall, and the obtained immiscible two phase system was kept in a refrigerator for 24 h. For comparison, HCl-doped PANI nanofibers were also synthesized in the same manner as above using the same reagents, but with HCl (0.005 mol) as a dopant instead of ICA. After polymerization, the product was washed with di-water and methanol from a reaction solution until the washing liquid became neutral to remove the unreacted reagents. These solid products were dried in a vacuum oven at 60 C. 2.2. Preparation of ER fluid To use the PANI nanofibers as a solid-phase of the ER fluid, the as-synthesized PANI nanofibers need to be dedoped by controlling the pH of the aqueous suspension of PANI with a 1 M NaOH solution. The electrical conductivity of the dry PANI powders was decreased from 101 to 1010 S cm1. Densities of the particles measured by a gas pycnometer (AccuPyc 1340, Micromeritics) were 1.50 and 1.51 g cm3 for ICA and HCl doped PANI, respectively. Subsequently, the ER fluids were prepared by dispersing the PANI nanofibers in silicone oil (r ¼ 0.96 g cm3, kinematic viscosity ¼ 50 cS at 25 C) under shaking and sonication, respectively. Two types of ER fluids were prepared containing 10 vol% of HCl-doped PANI and ICA-doped PANI, respectively. 2.3. Characterization The morphology of the PANI nanofibers was examined by field emission-scanning electron microscopy (FE-SEM) (Hitachi S-4300, Japan). Fourier transform infrared spectrometry (FT-IR) (Perkin Elmer System 2000, Norwalk, CT) was used to identify the chemical structure of the PANI nanofibers. The electrical conductivity was measured using a standard probe technique with a resistivity meter (Loresta-GP and Hiresta-UP, Mitsubishi Chemical Analytech CO., LTD, Japan). Optical microscopy (OM) (Olympus BX51, USA) equipped with a DC high voltage generator was used to examine the electro-responsive response of the ER fluid under an electric field. The ER properties of the PANI nanofibers suspension were characterized by steady shear experiments using a rotational rheometer (MCR 300, Physica, Austria) with a cup-bob system (CC17 ERD, the gap between the cup and bob was 0.71 mm) and a DC high voltage generator. In controlled shear rate (CSR) mode, the flow curves of each ER fluid were measured under a shear rate between 0.01 and 1000 s1 with and without an external electric field. In addition, dielectric relaxation spectra of the prepared ER fluids were analyzed using the HP 4284A precision LCR meter with
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Fig. 1. SEM images of PANI nanofibers: (a) ICA-doped PANI, (b) HCl-doped PANI.
HP 16452A liquid test fixture at room temperature. The frequency range for measuring the dielectric permittivity and loss factor was 20 ~ 106 Hz. 3. Results and discussion 3.1. Material characteristics
Transmittance
Fig. 1 shows SEM images of the synthesized PANI nanofibers. The morphology of PANI, which was synthesized using an interfacial polymerization method, typically has a nanofiber shape. From the SEM image, we can also observed that the morphology of PANI is not affected much by the dopant during the polymerization process. Although different dopants were used, both types of the synthesized PANI particles are consisted mainly of uniform nanofibers, about 50 nm in diameter and 1 mm in length. In the interfacial polymerization process, anilinium cations self-assemble and form micelles at the liquideliquid interface, which grow to be fibers and settle down to the bottom of the beaker by gravity. The morphology of fibers including diameter and shape was dependent on whether surfactant was used or not, molecular structure and concentration of dopants, etc [29,30]. While the effect of different dopants on influencing the diameter of the nanofibers was not so obvious when the fibers were prepared without templates [31]. The chemical structure of the PANI nanofibers was identified by an FT-IR spectroscopy. Fig. 2 shows the FT-IR spectra of the
HCI-doped PANI ICA doped PANI 4000
3500
3000
2500
2000
1500 -1
wave number (cm ) Fig. 2. FT-IR spectra of the PANI nanofibers.
1000
500
PANI nanofibers, in which the spectrum shows that two type of PANI nanofibers were synthesized like the typical PANI with an emeraldine base. As the analytical band, the characteristic peaks are summarized as follows. Absorption bands were observed at 820 cm1 (out-of-plane CeH deformation of the aromatic ring in the PANI unit sequence), 1305 cm1 (the CeN stretching of the secondary aromatic amine), 1240 cm-1 (the CeNþ stretching), 1145 cm1 (the benzenoid ring eNHþ ¼ quinonoid stretching), 1580 cm1 (the aromatic C]C stretching of the quinonoid ring), and 1500 cm1 (the aromatic C]C stretching of the benzenoid ring) [32]. The broad bands with multi peaks at 3500e2800 cm1 were assigned to NeH stretching vibration of the secondary amine in the PANI backbone and hydrogen-bonded NeH stretching [33]. However, it is difficult to distinguish sharply between HCl-doped PANI and ICA-doped PANI because the peaks of ICA overlap with those of PANI between 1720 and 1600 cm1. In the sodium ICA peak, two strong peaks at 1565 and 1409 cm1 were observed based on the asymmetric and symmetric stretching vibrations of COO, respectively [34]. As shown in Fig. 2, these characteristic peaks of ICA were overlapped by strong peaks for PANI. 3.2. Electrorheological property The ER fluids of both HCl-doped PANI nanofibers (a), (b) and ICA-doped PANI nanofibers (c), (d) dispersed randomly in silicone oil (50 Cs) were observed directly by an optical microscopy without and with an applied electric field (200 V) attached with a DC high voltage source, where a dilute ER fluid (10 vol % particle concentration) was dropped between two parallel electrodes of the glass slide. Both ER fluids exhibited a typical ER chain structure. Both HCl-doped PANI nanofibers of Fig. 3(a) and ICA-doped PANI nanofibers of Fig. 3(c) based ER materials in silicone oil without an electric field show a quite well dispersed liquid-like state. When the electric field was applied (Fig 3(b) and (d)), the particles begin to move form a chain-like structure with the adjacent particles. Generally, this ER phenomenon forms fibril chains as long as the electric field is applied. In this situation, the rheological properties of the ER fluids are being changed rapidly. The ER behavior of the PANI nanofiber-based ER fluids was compared. Fig. 4 presents the flow curves obtained from the CSR test for 10 vol% suspensions of the PANI nanofibers with various electric field strengths. In the absence of an electric field, the PANI nanofiber-based ER fluids all behave similarly to a Newtonian fluid, such that the shear stress increases in proportion to the shear rate. On the other hand, when an external electric field is applied, the ER fluids under an external electric field show non-Newtonian fluid characteristics, such as a Bingham fluid, which possesses a yield stress (ty), because of the generation of a particle fibrillar structure aligned with the applied electric field. The relationship between the
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Fig. 3. Optical microscopic images of HCl-doped PANI nanofibers (a), (b) and ICA-doped PANI nanofibers (c), (d) based ER fluids. The pictures were captured when the electric field (200 V) was off (a), (c) and on (b), (d).
_ for a Bingham fluid is expressed shear stress (t) and shear rate ðgÞ as follows:
_ t ty t ¼ ty þ hg;
(1)
g_ ¼ 0; t < ty
(2)
The Bingham behavior is related to the fact that the electrostatic force between the polarized particles becomes gradually stronger to resist the hydrodynamic force. In other words, the fibrillar structure will resist the flow until the shear stress approaches a critical value called the yield stress. When the shear rate increases, however, the flow curves of the ER fluid deviate from the Bingham model, showing a slight decrease in shear stress with increasing shear rate, as shown in Fig. 4(a). This can be explained by the equilibrium of the particle chains between the broken and reformed states [35] until the structures are totally destroyed by the high shear rate. A modified rheological equation of state was proposed for this behavior, the ChoeChoieJhon (CCJ) model, which is described by the equation below [35]:
! ty 1 t¼ þ h∞ 1 þ g_ _ a 1 þ ðt2 gÞ _ b ðt3 gÞ
(3)
where t2 and t3 are the time constants, h∞ is the shear viscosity at an infinite shear rate, and a and b are in charge of the decrease and increase in shear stress, respectively, in the low and high shear rate regions. b is in the range of 0 and 1 because dt/dg 0. The solid lines in Fig. 4(a) were generated from Eq. (3). It can be found that the CCJ model fitted the flow curves of the ICA-doped PANI nanofiber based ER fluid quite well in both the low and high shear rate regions with the optimal parameters of which are listed in Table 1. In addition to this CCJ model, there are other equations reported recently to describe the flow curves that deviate the Bingham model. A model equation with spring-damper theory [36] and a modified four parameter Papanastasiou equation [37] were examples adopted for fitting flow curves of various ER fluids. By comparing the shear stress curves in Fig. 4(a), the ICA-doped PANI nanofiber-based ER fluid is observed to have a larger ER effect than that of the HCl-doped PANI nanofibers. This shows that the ER effect depends not only on the electric field strength, but also on the constitution of the suspended particles. In the synthesis of PANI nanofibers, two different types of dopant were used, in which ICA has high polarizability. When an electric field is present, the ICA can help inducing an excellent ER effect through its high polarizability. Therefore, the ICA-doped PANI nanofiber-based ER fluid shows higher shear stress and enhanced ER performance. This confirms that the polarizability is an important physical parameter in the ER behavior. Furthermore, in the case of the field-off viscosity, the ICA-
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Fig. 5. Shear stress as a function of shear rate for both ER fluids (10 vol%) at an electric field strength of 3.0 kV mm1.
Fig. 4. (a) Shear stress vs. shear rate, and (b) Viscosity vs. shear rate for the PANI nanofibers based ER fluids (10 vol%) under various electric field strengths (closed: ICAdoped PANI nanofiber, open: HCl-doped PANI nanofiber). The solid lines in (a) are generated from the CCJ model.
increasing electric field strength. Note that a slight shear-thinning behavior of the shear viscosity was observed at a low shear rate region even at a zero applied electric field owing to its nonNewtonian suspension system with 10 vol% of the particle. The ER behavior of the ICA-doped PANI nanofiber-based ER fluid was enhanced over the entire shear rate range. On the other hand, it can be also noted that for quantifying the ER improvement, the ER efficiency has been also reported [38]. Fig. 5 shows the shear stress as a function of the shear rate for the two ER fluids at an electric field strength of 3.0 kV mm1. Despite the same applied electric field strength, the ICA-doped PANI nanofiber-based ER fluid has higher shear stress than that of the HCl-doped PANI nanofiber over the entire shear rate range. This means that the chains of the ICA-doped PANI nanofibers are hardly broken and stiff enough to withstand the shear in addition to their faster reformation rate once broken. Note that dynamic yield stress (ty), defined as the minimum shear stress required for maintaining flow is an important criteria to characterize an ER fluid. In the controlled shear rate mode, ty can be obtained by extrapolating shear stress of the flow curve to a zero
doped PANI nanofiber based ER fluid shows a slightly higher shear stress than that of the HCl-doped PANI nanofibers based ER fluid and this could be associated with slight different dispersion state of the dispersed particles due to different doping agents not only inducing potential aggregation but also different chemical affinity with a silicone oil despite well dispersion in a bulk state. These suspension behaviors of both ER fluids were also confirmed by Fig. 4(b), in which the shear viscosity increased with
Table 1 Optimal parameters in the CCJ model obtained by fitting the model to the flow curves of the ER fluid of ICA doped PANI nanofiber at various electric field strengths. Parameters
ty a t2
h∞
t3
В
Electric field strength (kV mm1) 0.5
1.0
1.5
2.5
3.5
15 0.4 1.1 0.15 0.01 0.75
30 0.7 1.4 0.15 0.004 0.80
50 0.9 1.5 0.14 0.002 0.80
110 0.9 2.0 0.16 0.0015 0.91
200 0.8 1.5 0.18 0.001 0.89
Fig. 6. Dynamic yield stress vs. electric field strengths for both ER fluids (10 vol%).
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Fig. 7. b t vs. b E for both HCl and ICA-doped PANI nanofiber ER fluids. The solid line is generated from Eq. (7).
shear rate limit. In general, the correlation of ty and electric field strength (E0) is presented as follows:
ty fE0m
(4)
The dependency of ty on the electric field strength differ from the E20 (m ¼ 2) dependency suggested by the polarization model depending on the particle concentration [39], particle shape, and applied electric field strength, in which the applied electric field induces electrostatic polarized interactions among the particles and also between the particles and electrodes. On the other hand, the polarization model does not describe fully the ER mechanism for all kinds of ER materials. Sometimes, the ER response is known to be influenced by the conductivity mismatch between the particles and medium and the non-linear electrical conduction effect, which leads to a lower m value. Thereby, a new equation was developed to derive a universal scaling equation for a wide range of ER fluids [40].
ty ðE0 Þ ¼ aE02
pffiffiffiffiffiffiffiffiffiffiffiffi! tanh E0 =Ec pffiffiffiffiffiffiffiffiffiffiffiffi E0 =Ec
(5)
Fig. 9. ColeeCole fitting curves for PANI nanofibers based ER fluids.
where Ec is the critical electric field strength, dividing the electric field into two regions with a different relationship between the yield stress and electric field strength: when E0 ≪Ec ; ty fE20 ; and 3=2 E0 ≪Ec ; ty fE0 . This means that the yield stress relationship depends on the E0 range, showing that m in Eq. (4) is 2.0 until the critical electric field strength is reached, and then generally becomes smaller than 2.0 when E0 > Ec. A single universal curve in Eq. (5) with Ec and ty (Ec) is normalized to collapse the data.
ty ðEc Þ ¼ aEc2 tanhð1Þ ¼ 0:726aEc2
(6)
resulting in
b t ¼ 1:313 b E
3=2
tanh
qffiffiffiffi b E
(7)
where b E ¼ E0 =Ec ; b t ¼ tðE0 Þ=tðEc Þ. This study estimated the dynamic yield stress of the PANI nanofiber based ER fluids from the CSR measurements in Fig. 4(a) [41], and the dynamic yield stress was then plotted as a function of the electric field strength. Fig. 6 shows the correlation between yield stress and electric field strength of the two PANI based ER fluids. The Ec originating from the nonlinear conductivity effect can be obtained by the crossover point of the slopes for all ranges of electric field strengths corresponding to the polarization model (slope ¼ 2.0) and conduction model (slope ¼ 1.5), respectively [42]. The estimated Ec was found to be 1.25 and 2.04 kV mm1 for the ICA-doped PANI nanofiber and HCl-doped PANI nanofiber-based ER fluids, respectively. Using the experimental data in Fig. 6, the newly defined b E and b t were plotted in Fig. 7 and fitted by Eq. (7). All the rearranged yield stresses for the two ER fluids collapsed to a single curve of Eq. (7) showing the good correlation between yield stress and electric field strength using the parameter, Ec.
Table 2 Parameters in Eq. (8) for two kinds of PANI nanofibers based ER fluids.
Fig. 8. Dielectric spectra of PANI nanofibers based ER fluids (close symbol and solid line for dielectric constant ε0 , open symbol and dash line for dielectric loss factor ε00 ).
No.a
30
3∞
D3 ¼ 3 0 3 ∞
l (s)
a
1 2
6.5 5.0
3.0 2.9
3.5 2.1
0.0056 0.006
0.47 0.35
a
1: ICA doped PANI, 2: HCl doped PANI.
Y.D. Liu et al. / Materials Chemistry and Physics 147 (2014) 843e849
interpreted well by fitting the dielectric spectra to the ColeeCole formula.
3.3. Dielectric property Furthermore, the dielectric properties for the ER fluids were analyzed to further examine ER performance of the two ER fluids. Fig. 8 shows the frequency dependence of the dielectric constant (ε0 ) and dielectric loss factor (ε00 ), which are typical results for the interfacial polarization of suspensions including ER fluids. The interfacial polarization is considered to be the origin of the ER effect. The lines in Fig. 8 were fitted from the famous ColeeCole formula (Eq. (8)) [43e45] using these parameters.
ε ¼ ε∞ þ
Dε 1 þ ðiulÞ
(8)
1a
In Eq. (8), the dielectric constant and loss factor can be related to
ε0 ¼
ε0 ε∞ ð1 þ A cos qÞ 1 þ A2 þ 2A cos q
849
00
ε ¼
Aðε0 ε∞ Þsin q 1 þ A2 þ 2A cos q
(9)
where the exponent (1 a) characterizes the broadness of the relaxation time distribution, l is the relaxation time, A ¼ ðulÞ1a , and q ¼ ð1 aÞp=2. When a ¼ 0, the ColeeCole formula reduces to the Debye's well-known single relaxation time model. Fig. 9 is the ColeeCole plot of ε00 as function of ε0 . The adopting ColeeCole plot not only fits the dielectric spectra of both ER fluids but also provides a better explanation of their polarizability. Table 2 lists the parameters in Eq. (8) for the two ER fluids. Here, Dε ¼ ε00 ε∞00 ' represents achievable polarizability in the ER fluids. Because large polarization will lead to a large ER effect, the Dε dependence on the ER properties has a positive effect on the better ER performance. The relaxation time l for the interfacial polarization of ER fluids is related to the yield stress and stress enhancement under an applied electric field. As shown in Table 2, Dε of the ICA-doped PANI nanofiber is slightly higher than that of the HCldoped PANI nanofiber, suggesting enhanced ER performance. In addition, the relaxation times for the ICA-doped PANI nanofiber and HCl-doped PANI nanofiber ER fluids were 5.6 ms and 6 ms, respectively. In general, a short relaxation time is related to a higher shear stress, in which the ICA-doped PANI nanofiber has a faster interfacial polarization mechanism. The above analysis of the dielectric spectra coincides with the results obtained from a comparison of their shear stresses. 4. Conclusions Indole-2-caboxylic acid-doped PANI nanofibers were synthesized using an interfacial polymerization process, and the influence of the dopant on their ER behavior was investigated. From the morphology analysis, the nanofiber morphology was unaffected by the two different kinds of dopant. The flow responses of the ER fluids were examined as a function of the shear rate under a range of electric field strengths, indicating that the ICA-doped PANI nanofiber-based ER fluid showed higher shear stress than the HCldoped PANI nanofiber-based ER fluid. This was attributed to the high polarizability of the ICA. Typical flow curves of the ER fluid were analyzed using a modified flow equation of state, the CCJ model. The yield stresses of both ER fluids all collapsed into a single line according to the suggested universal equation with the parameter Ec. Finally, the difference in the ER performance could be
Acknowledgment This work was supported by National Research Foundation of Korea (NRF-2013R1A1A2057955) and also by the Ministry of Knowledge Economy, Korea (2012).
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