Sulfonated polystyrene nanoparticles coated with conducting polyaniline and their electro-responsive suspension characteristics under electric fields

Polymer 127 (2017) 174e181

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Sulfonated polystyrene nanoparticles coated with conducting polyaniline and their electro-responsive suspension characteristics under electric fields Shang Hao Piao, Chun Yan Gao, Hyoung Jin Choi* Department of Polymer Science and Engineering, Inha University, Incheon 22212, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 June 2017 Received in revised form 27 August 2017 Accepted 2 September 2017 Available online 5 September 2017

Core-shell structured conducting polyaniline (PANI)-coated polystyrene (PS) composite nanospherical particles were synthesized and assessed as a candidate for electrorheological (ER) materials. Monodisperse PS nanospheres were initially synthesized by a dispersion polymerization process and their surface was modified using concentrated sulfuric acid. This allowed the aniline monomer to be adsorbed easily on the sulfonated PS nanosphere and polymerized further by forming a PS/PANI nanosphere. The ER performance of their suspension dispersed in silicone oil was examined using a rotational rheometer under a variety of electric field strengths. The Bingham fluid model was used to analyze their flow curves to understand the interrelation between shear stress and shear rate. The relationship between the yield stress and electric field strength showed a slope of 2.0 following a polarization model. The dielectric spectra showed a good correlation with the ER performance of the ER fluid. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Core-shell Electrorheological Polyaniline Polystyrene Sulfonated

1. Introduction Electrorheological (ER) fluids are a family of intelligent material systems that are typically composed of semiconducting or polarizable particles suspended in insulating liquids, where their rheological characteristics can be controlled by an electric field [1e4]. Under the application of an electric field, the particles dispersed in the ER fluids are polarized and connected with each other to construct chain-like structures with a strong dipoleedipole interaction along the applied electrical field direction. Therefore, ER fluids undergo a phase transition from liquid-like to solid-like under the application of an electric field, and return reversibly to a liquid state when the electric field is removed [5e9]. Through this rapidly controllable process, the rheological properties, such as shear stress, shear viscosity, and dynamic modulus, can be changed simultaneously. Therefore, ER fluids have attracted considerable attention in many areas with a wide range of industrial applications based on their phase-transitions, such as automobile components, mechanical polishing, damper systems, tactile displays, and efficient energy production [10e13]. In recent years, there has been extensive research on the

* Corresponding author. E-mail address: [email protected] (H.J. Choi). http://dx.doi.org/10.1016/j.polymer.2017.09.004 0032-3861/© 2017 Elsevier Ltd. All rights reserved.

fabrication of electro-responsive materials. A variety of inorganic (e.g. zeolite, TiO2, silica, etc.) [14e16] and organic materials have been proposed to be applicable in ER fluids. Conducting polymers, whose electrical conductivity can be adjusted by doping/dedoping processes, such as polyaniline (PANI) and its derivatives [17e21], poly (3, 4-ethylenedioxythiophene) [22], polythiophene [23], and polypyrrole [24,25], are also applied as anhydrous ER materials. Among them, PANI has been applied widely in ER fluids owing to its low cost, easy synthesis, and remarkable thermal and chemical stability [26e28]. In addition, core-shell structured conductingpolymer-incorporated particles have been reported to be good ER candidates via facile physical adsorption or chemical interactions to control the composite particle size, shape (sphere, rod, tube and even urchin-like form), and density [29e32]. In the present study, PANI-coated polystyrene (PS) core-shell nanosphere particles were synthesized. A better PANI coating onto PS was expected by sulfonating the surface of PS spheres using concentrated sulfuric acid than the previously reported technique of the simple pep stacking interactions [33]. This could allow the aniline monomer to be adsorbed easily on the sulfonated PS nanosphere and polymerized further by forming a better PS/PANI nanosphere. These monodispersed PS/PANI core/shell particles were dispersed in silicone and their ER performance was investigated under various electric field strengths using a rotational

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rheometer. The Bingham fluid model was used to analyze the resulting flow curves to understand the interrelationship between the shear stress and shear rate. The relationship between the yield stress and electric field strength was investigated further using a power-law relation. 2. Experimental 2.1. Preparation of PS and sulfonated PS nanospheres Monodisperse PS nanospheres were synthesized using a typical dispersion polymerization technique. A 15 g sample of styrene monomer (99% purity, Sigma-Aldrich) was dissolved in 270 mL of methanol containing 3 g of polyvinylpyrrolidone (PVP) (Mw ¼ 360,000 g/mol, Sigma-Aldrich) as a stabilizer with stirring under a N2 atmosphere and heated to 60  C. The reaction was initiated by adding the solution into the reactor, in which 0.25 g of 2,20 -azobisisobutyronitrile (AIBN, 98% purity, Junsei Chemical Co.) was dissolved in 30 mL of methanol, and polymerization continued for 24 h. Following the reaction, the PS nanospheres were centrifuged with methanol and deionized water, and then dried under vacuum. For their surface treatment, according to the procedures reported by Dai et al. [34], 3 g of the resulting PS nanospheres were dispersed in 200 mL of concentrated sulfuric acid and warmed to 40  C, the sulfonation reaction was allowed to proceed for 6 h with stirring at 150 rpm. The sulfonated PS nanospheres were collected by repeated centrifuging and washing 3 times. 2.2. Synthesis of PANI coated PS The conducting PANI shell was coated on the sulfonated PS surface using the following procedure. The resulting sulfonated PS nanospheres (1 g) were dispersed in water (150 mL) with vigorous stirring of 200 rpm. Subsequently, 0.75 g of aniline monomer (DC Chemical, Korea) in 20 mL of a 1 M HCl (35%, OCI Co., Korea) solution was added to the above suspension. The aniline monomer can be adsorbed on the sulfonated PS nanosphere and polymerized further on the PS surface to form a PS/PANI nanosphere [35,36]. An aqueous solution containing 2.3 g of ammonium persulfate (APS) (Daejung Co., Korea) was added, and the mixture was allowed to react at 0  C for 24 h. The PS/PANI nanospheres obtained were collected by washing with ethanol and water, and finally dried using a freezing-drier with 24 h. Fig. 1 presents a schematic diagram of the entire experimental process of PS/PANI. 2.3. Preparation of ER fluid The electrical conductivity of the PS/PANI hybrid nanospheres fabricated was measured to be 103 S/cm using a resistivity meter. Because this electrical conductivity is too high for direct application to ER materials, it is important to tune the electrical conductivity. This is because a high electrical conductivity would lead to damages to the rheometer by an electric short circuit. The obtained PS/ PANI nanospheres were dispersed in di-water, and their pH in the dispersion was maintained at 9 by adding a 1 M NaOH solution. The dedoped PS/PANI nanospheres were then filtered and dried. The electrical conductivity of the final obtained PS/PANI nanospheres was decreased to 108 S/cm. As for the relationship between electrical conductivity and ER performance, Plachy et al. [37] recently reported the higher ER effect for the samples with higher conductivity. But the critical conductivity for the ER effect was 107 S/cm. For the ER fluid in this study, the PS/PANI nanospheres (10 vol% particle concentration) were milled carefully in a mortar and added to 50 cSt silicone oil for the ER test.

175

2.4. Characterization The morphology of the PS/PANI nanospheres was observed by scanning electron microscopy, (SEM, SU 8010, Hitachi) and transmission electron microscopy (TEM, CM200, Philips). The chemical structure of the prepared samples was examined by Fourier transform-infrared (FT-IR) vacuum spectrometry (VERTEX 80V, Bruker). The thermal properties were analyzed by thermogravimetric analysis (TGA, TG209F3, Tarsus) in air. The ER behaviors of the PS/PANI nanosphere-based ER fluid were investigated using a rotational rheometer (Physica MCR 300, Stuttgart) equipped with a DC high voltage generator. To examine the interfacial polarization of the ER fluids, the dielectric spectra were analyzed using a LCR meter (HP 4284A Precision) with a Liquid Test Fixture (HP 16452A) for liquids, in which the frequency of the AC electric fields was varied from 20 Hz to 1 MHz. 3. Results and discussion The morphologies of both PS and PS/PANI nanospheres were observed by SEM. As shown in Fig. 2(a), the spherical pure PS particles had relatively smooth surface and a uniform size with a mean particle diameter of 360 nm. In contrast, the PS/PANI particles have a much rougher surface than the PS particles, as shown in Fig. 2(b). The PS/PANI particles maintained a spherical shape but particles were larger than the PS particle diameter with a mean particle size of approximately 500 nm. This indicates that the coating thickness of the PANI layer is approximately 70 nm. Fig. 3 presents TEM images of pure PS and core-shell structured PS/PANI nanospheres at different magnifications. Compared to pure PS (Fig. 3 (a), (b)), a dense and rough PANI shell wrapped the PS surface, in which the internal black core (PS) was coated with an external gray shell (PANI), as shown in Fig. 3 (c), (d). The mean thickness of the PANI shell was approximately 70 nm. Fig. 4 shows the thermal stability of the pure PS and PS/PANI nanospheres. TGA was carried out from 25  C to 800  C at a heating rate of 10  C/min in air. The TGA curve of the pure PS revealed a sharp weight loss at approximately 280  C and continuing to 430  C due to the large-scale thermal degradation of PS. In the case of the PS/PANI particles, the slight weight loss before 100  C was possibly attributed to the adsorbed moisture in the particles before the TGA measurement even though the trace water in the particles has been removed completely during the particle synthesis. The PS/PANI showed a second weight loss of approximately 23% between 280  C and 410  C which is in accordance with the complete decomposition of the pure PS particles. Furthermore, the 70% weight loss from 410  C to 650  C corresponds to the thermal degradation of PANI. The thickness of the PANI shell was calculated based on the weight loss of PANI using the following equations [38]:

V2 r2  100 ¼ m V1 r1 þ V2 r2

(1.1)

V1 ¼

4 3 pR 3

(1.2)

V2 ¼

i 4 h p ðR þ dÞ3  R3 3

(1.3)

where m is the PANI weight ratio and R is the radius of the PS sphere; d is the thickness of the PANI shell. V1 and V2 represent the volume of the PS and PANI; r1 and r2 represent the density, respectively. The density of PS increased after the PANI coating to 1.13 g/cm3 from 0.98 g/cm3 of pure PS while the density of PANI was 1.36 g/cm3, which was measured using a pycnometer. The

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Fig. 1. Schematic diagram of the experimental route to synthesize PS/PANI particles.

calculated thickness is about 73 nm which is similar to the thickness observed from the SEM and TEM images. Furthermore, the compositions of the PS/PANI core-shell nanospheres were confirmed by FT-IR analysis. Fig. 5 compares the FT-IR spectra of PS, sulfonated PS, and PS/PANI nanospheres. At about 3400 cm1, all samples show one typical peak coming from either the -OH or the absorbed bound water. The spectrum of PS showed CeC out-of-plane bending, CeH out-of-plane bending, CH2 bending, and CeH stretching vibrations at 700 cm1, 765 cm1, 1454 cm1, and 3000 cm1, respectively. The tertiary CeH stretching and aromatic CeC stretching bands of PS were visible at 1470 cm1. The spectrum of sulfonated PS revealed a sulfuric acid (SO3H) peak at 1209 and 1006 cm1. For PS/PANI, the spectrum showed similar peaks to that of PS, as well as C¼N and C¼C stretching vibrations at 1580 cm1 and 1490 cm1, respectively. The strong peak at 1145 cm1 and 1310 cm1 was assigned to the

aromatic amine stretching band of PANI. This shows that the PS sphere had been coated successfully with the PANI shell. The ER performance was examined using a rotational rheometer in a typical controlled shear rate mode. Fig. 6 presents the flow curves of the shear stress (a) and shear viscosity (b) as a function of the shear rate for the PS/PANI-based ER fluid under a range of electric field strengths. The shear rate range was set from 0.1 to 1000 s1 on a log-log scale and the electric field strength ranged from 0 to 2.5 kV/mm. As shown in Fig. 6(a), in the absence of an electric field, the shear stress of the ER fluid increased linearly with gradually increasing shear rate, similar to a Newtonian fluid. In the presence of the electric field, however, the fluid exhibited enhancement in shear stress and showed a plateau region due to the formation of chain-like structures, and showed a higher value with increasing electric field. When the ER performance of this new PS/PANI-based ER fluid is compared with that

Fig. 2. SEM images of PS (a) and PANI coated PS (b).

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Fig. 3. TEM images of PS (a), (b) and PANI coated PS (c), (d).

of previous system from differently prepared PS/PANI microspheres [33], new PS/PANI-based ER fluid shows higher shear stress at a given electric field at the same particle concentration. Nonetheless slight different particle size needs to be considered. These typical ER behaviors can be explained by the Bingham fluid model, which is a general model used to describe the shear stress behavior, as expressed in Eq. (2),

Fig. 4. TGA analysis of both PS and PANI coated PS particles.

t ¼ ty þ hg_ ; t  ty g_ ¼ 0; t < ty

(2)

Here, t is the shear stress; g_ is the shear rate; h is the shear

Fig. 5. FT-IR spectra of pure PS, PS/PANI particles.

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Fig. 6. Shear stress (a) and viscosity (b) vs. shear rate curves of the PS/PANI particlebased ER fluid with different electric fields. The solid line in (a) is generated from Eq. (2).

thinning behavior [39,40] where the particle chains began tilting and breaking with the increased shear rate. A dynamic oscillatory test was used to examine the viscoelastic properties of the ER fluids. Before the main dynamic oscillation test, the amplitude sweep test was performed to verify the linear viscoelastic range (gLVE), in which the strain ranged from 1  105 to 1 at a fixed angular frequency of 6.28 rad/s. Fig. 7 presents both the storage modulus (filled plots: G0 ), which is the deformation energy stored as the elastic part and the loss modulus (blank plots: G00 ) as the viscous part, which is the deformation energy consumed in the sample as function of the strain ranging from 1  103 to 1  102%. G0 was much higher than G00 at all electric field strengths, showing a constant plateau in the low amplitude region from 1  103% to 0.1%, which is the so-called gLVE. In the gLVE region, the elasticity response was dominant over the viscous part because the deformation of the chain-like structures was considered to be reversible. Therefore, the mean gLVE was selected as 0.01% for the frequency sweep test. In zero electric field, G0 was higher than G00 due to the particles agglutination phenomenon. When the strain amplitude exceeded gLVE, both G0 and G00 were observed to decrease because the chain-like structure had broken irreversibly. Thus, the G00 value became larger than G0 in the high strain region. The frequency sweep was measured based on the chosen mean gLVE value, and models the behavior of G0 (filled plots) and G00 (blank plots) as a function of the angular frequency, ranging from 1 to 200 rad/s, as shown in Fig. 8. The storage and loss moduli increased gradually with increasing electric field strength. In addition, the storage modulus was significantly higher than the loss modulus over a wide range of angular frequencies, indicating that the elastic response was higher than the viscous behavior in the PS/PANI particle-based ER fluid. Even in the absence of the electric field, the particles could possibly agglutinate, resulting in a higher value of storage modulus. The ER fluid changes its state from liquid-like to solid-like under an applied electric field because the PS/PANI particles formed chain-like structures due to the polarized conducting polymer [41]. To effectively study the progressive structural breakdown with the increased strain (g), the value of the elastic stress (t' ¼ G'g) was calculated [42]. The elastic stress increased with increasing strain at the low strain region, meaning that the ER fluid responds to the deformation elastically (Fig. 9). After a critical strain, the elastic

viscosity; and ty is the yield stress, which is dependent on the electric field strength. Fig. 6 (a) shows that the Bingham fluid model fitted the flow curves of the PS/PANI-based ER fluid quite well. Optimal values of the yield stress and shear viscosity under different electric field strength in Eq. (2) are given in Table 1. In the absence of the electric field, similar to the shear stress curve, the ER fluid behaved like a Newtonian fluid with a constant shear viscosity with increasing shear rate (Fig. 6(b)). When the electric field was applied, a sudden increase in the starting shear viscosity and stepwise enhancement with increasing electric field strength were observed because of the formation of a chain structure and stronger chains formed with a higher electric field strength. In addition, the ER fluid also displayed strong shearTable 1 Optimal values to fit Bingham fluid model of Eq. (2) for the flow curve of ER fluid (10 vol %) under different electric field strength. Model

Bingham

Parameters

t0 h

Electric field strength (kV/mm) 0.5

1

1.5

2

2.5

15.5 0.07

60 0.06

115 0.04

200 0.02

420 0.01

Fig. 7. Storage modulus (filled curves) and loss modulus (blank curves) vs. strain curves of the PS/PANI particle-based ER fluid (10 vol% particle concentration) with different electric fields.

S.H. Piao et al. / Polymer 127 (2017) 174e181

Fig. 8. Storage modulus (filled curves) and loss modulus (blank curves) vs. angular frequency curves of the PS/PANI particle-based ER fluid (10 vol% particle concentration) with different electric fields.

stresses stop increasing at a high strain value due to the destruction of chain-like structures in the ER fluid. Fig. 10 shows the dependence of the dynamic and elastic yield stresses on different applied electric field strengths on a log-log scale. In this case, the dynamic and elastic yield stresses were obtained by extrapolating the shear stress at a zero shear rate in Fig. 6(a) and elastic stress at a strain value of approximately 1% from Fig. 9, respectively. The dynamic yield stress was higher than the elastic yield stress. The interrelationship between the yield stress and electric field can be represented using a power-law equation as follows:

ty fEm

(3)

The index m ¼ 2.0 corresponds to the polarization model and m ¼ 1.5 is suggested by the conduction model. For both yield stress cases, as shown in Fig. 10, m is approximately 2.0; this behavior complies with the polarization model [43,44]. Note that this polarization model generally shows an excellent agreement with the experimental data for small volume fraction and low electric field

Fig. 10. Yield stress curves various electric field strengths fit PS/PANI based ER fluid.

strength compared to the conduction model. The stress relaxation behavior measurements were conducted to analyze the solid-like phenomenon of the ER fluids. Fig. 11 shows the calculated stress relaxation modulus G(t) from the values of G0 and G00 with the frequency data from Fig. 8, using the Schwarzl equation [45] as follows: 0

00

00

GðtÞyG ðuÞ  0:560G ðu=2Þ þ 0:200G ðuÞ

(4)

Under an external electric field, G(t) exhibits plateau behavior with an increase in the values with increasing electric field strength as a function of time, indicating that the PS/PANI-based ER fluid exhibits solid-like behavior due to the strong attractive interactions existing in the particles under the external electric field. To examine the reliability and sensitivity of the PS/PANI particlebased ER fluid, the shear stress was measured at a constant shear rate of 1 s1 under a voltage pulse (the applied electric field was sustained for 20 s and then in the off state for 20 s), as shown in Fig. 12. As the electric field was applied, the shear stress jumped immediately to higher levels, and decreased to a zero-field when the electric field was removed. The turning points of each transformation of the shear stress were quite sharp, and the shear stress was similar under the same electric field. This shows that the PS/ PANI particle-based ER fluid exhibits reversible and sensitive properties with and without an external electric field due to the formation of chain-like structures. To better understand the ER properties of the PS/PANI-based ER fluids, their dielectric properties were measured. A LCR meter was used to measure both the dielectric constant (ε0 ) and loss factor (ε00 ) over a large frequency range from 20 Hz to 1 MHz to obtain the dielectric spectra (Fig. 13(a)) and a Cole-Cole plot for the PS/PANI based-ER fluid is expressed using Eq. (4) as follows [46,47]: 0

00

ε* ¼ ε þ iε ¼ ε∞ þ

Fig. 9. Elastic stress of PS/PANI nanoparticles plotted as a function of strain amplitude.

179

ε0  ε∞

1a

1 þ ðiulÞ

! (5)

where u is the angular frequency, ε* is a complex dielectric constant, ε∞ represents the dielectric constant at a high-frequency limit, and ε0 is the dielectric constant when the frequency is 0. The difference (Dε ¼ ε0  ε∞ ) in the dielectric strength is related to the polarizability of the ER fluids. The ε∞ and ε0 obtained were 1.71 and 2.95, respectively, in which the value of Dε equals to 1.24 for the ER fluid. Under an applied electric field, the dielectric relaxation

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Fig. 11. Relaxation modulus G(t) of PS/PANI based ER fluid, as calculated from storage modulus and loss modulus.

time (l) for an interfacial polarization is related to the yield stress and stress enhancement. Here, the l of the PS/PANI-based ER fluids was 0.03 m s, which is very faster than other PANI systems due to the core effect [48,49]. The (1ea) represents the distribution of the relaxation time, in which the exponent parameter a, a value in the range from 0 to 1.0, is related to explain different spectral shapes. For a value larger than 0, the relaxation is stretched, extending for the entire frequency range. The a value of the ER fluid was 0.38 and note that for the Debye model, a value is zero.

4. Conclusion Core-shell structured PS/PANI nanospherical particles with a uniform PANI shell coating onto the monodispersed PS were synthesized by sulfonating the surface of the PS spheres to adsorbed aniline monomer and further polymerized on the surface of PS. The coreeshell structure with a uniform PANI shell thickness and the particle size of the PS/PANI nanospherical particles were confirmed by SEM and TEM. The weight percentage of PANI in the product

Fig. 13. (a) Permittivity and loss factor as a function of the frequency, (b) Cole-Cole fitting curve.

examined by TGA was approximately 23%. FT-IR spectroscopy confirmed the chemical structures and that the PANI shell had been coated successfully on the PS sphere with the help of sulfonation. The PS/PANI nanosphere-based ER fluid showed a typical ER behavior, in which the flow curves of PS/PANI-based ER fluid were fitted quite well to a Bingham fluid model. Both the dynamic and elastic yield stresses of the ER fluid showed a dependence on the electric field strength according to the power law of tyfE2.0, which followed the polarization model. From the on-off test, i.e., a shear stress under a pulsed voltage, the ER fluid exhibited reversible and sensitive properties with and without an external electric field. Furthermore, the dielectric characteristics of the ER fluid correlated well with the ER effects. Acknowledgements This research was supported by National Research Foundation, Korea (2016R1A2B4008438). References

Fig. 12. Shear stress of the PS/PANI particle-based ER fluid (10 vol% particle concentration) in the electric field with a voltage pulse (t ¼ 20 s) at a fixed shear rate of 1 s1.

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