Letter pubs.acs.org/Langmuir
Core−Shell-Structured Monodisperse Copolymer/Silica Particle Suspension and Its Electrorheological Response Ying Dan Liu,†,‡ Xuemei Quan,‡ Bora Hwang,‡ Yong Ku Kwon,‡ and Hyoung Jin Choi*,‡ †
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, Korea
‡
S Supporting Information *
ABSTRACT: Monodisperse core−shell-structured poly(styrene-co-butyl acrylate-co-[2-(methacryloxy)ethyl] trimethylammonium chloride)/silica (PSBM/ SiO2) nanoparticles were applied as new electrorheological (ER) materials in which the particles were dispersed in an insulating oil. These nanoparticles were prepared by the consecutive precipitation of cetyltrimethylammonium bromide and negatively charged tetraethylorthosilicate onto the cationic surfaces of PSBM colloidal particles. The successful deposition of the shell phase of the particles and their morphology was examined by transmission and scanning electron microscopy. Their ER properties were studied with a rotational rheometer under different shear modes: controlled shear rate, steady shear under constant shear rate, and creep test. The silica shell allowed the PSBM/ SiO2 particles to exhibit typical ER performance under an applied electric field. The dielectric spectra of the PSBM/SiO2-based ER fluid were also recorded using an LCR meter, which was correlated to the ER performance of the ER fluid.
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INTRODUCTION Core−shell-structured functional particles have attracted considerable attention because of their synergistic combination of different materials in special structures, which makes the core or shell materials more effective for targeted applications such as sensors, drug delivery, organic field-effect transistors,1 and so forth. Electric field-responsive materials incorporated with core−shell particles have been applied as the active phases of electrorheological (ER)2−4 fluids to achieve advanced properties, such as reduced density, enhanced polarizability,5 and special particle shapes.6 ER fluids are smart suspensions of electric-field-responsive particles in insulating liquids, in which their shear viscosity can be controlled by an electric field, realizing a reversible liquidlike to solidlike phase transition.7−12 Owing to the controllable properties of the ER fluids, they have considerable potential in a range of applications,13 including dampers, optical finishing, and haptic devices. The active materials used in ER fluids are mostly semiconducting or polarizable particles, such as conducting polymers with a suitable conductivity range of 10−6−10−9 S/ m and dielectric inorganics (e.g., SiO2, TiO2, and BaTiO3) and their composites. In an applied electric field, the particles are polarized and arrange into interconnected chain structures in the direction of the applied electric field. Hence, the rheological properties of the ER fluid can be changed significantly by the application of an electric field. Research on ER-active materials raises several areas of study, including the ER effects of different materials,14−17 particle shapes,18 and structures.19,20 A study of core−shell-structured ER particles is an interesting topics with © 2014 American Chemical Society
respect to the ER effect of the individual core or shell part or both.21 Polyaniline (PANI), polypyrrole-coated polystyrene (PS), or poly(methyl methacrylate) (PMMA) particles were synthesized to examine core−shell particles with an active shell.6,22 Melamine-formaldehyde was used to encapsulate the conducting PANI particles.23 In addition, the conductingpolymer-coated silica particles were fabricated as core−shell coactive candidates.24 As mentioned above, the conducting materials were applied mainly as the shell material in preparing core−shell-structured ER particles because of the relatively easy synthesis on either polymeric or inorganic core particles. The conducting shell, however, always generates a high current density in an ER fluid, which impedes its use in practical applications. In this study, silica was selected as an electroresponsive shell material because of its easy and controllable synthesis method, good ER response, and density lower than that of another common inorganic ER material, titania,25 whereas a polystyrene core, a commonly applied core material with good monodispersity and a controllable diameter, was modified to be a copolymer of poly(styrene-co-butyl acrylate-co-[2-(methacryloxy)ethyl]trimethylammonium chloride) (PSBM) fo easy coating with silica. Therefore, the core−shell-structured PSBM/SiO2 spheres were synthesized and applied as active ER particles. The PSBM/SiO2 ER particles had a much lower density than pure Received: June 23, 2013 Revised: February 7, 2014 Published: February 10, 2014 1729
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Scheme 1. Schematic Diagram of the Synthesis Procedure for Monodisperse PSBM Core/Silica Shell Nanoparticles
increasing the shear rate, ranging from 0.01 to 500/s at various electric field strengths (0, 0.3, 0.5, 1.0, 2.0, and 3.0 kV/mm) controlled by Rheoplus measurement software. A high dc voltage generator (HVG 5000) was applied to support the electric field of the measurement system. In addition, a switch test was also applied using a square voltage pulse under a fixed shear rate (1/s). For the creep test, a constant shear stress (10 Pa) was applied for 20 s and then removed and tested for another 20 s along with the costimulation of an electric field. The dielectric properties of the ER fluid were measured using an LCR meter (Agilent HP 4284A) over the frequency range of 20−106 Hz.
SiO2 and a lower conductivity than the conducting polymerincorporated particles. Owing to the silica shell, the PSBM/ SiO2 particles respond to an electric field and exhibit typical ER behavior as observed with a rotational rheometer under an applied electric field.
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EXPERIMENTAL SECTION
Materials. Styrene (99%), butyl acrylate (BA) (99+%), [2(methacryloxy)ethyl]trimethylammonium chloride (MOTAC) (75 wt % in water), 2,2′-azobis-(2-methyl propionamidine)dihydrochloride (AIBA) (97%), tetraethylorthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB) [C16H13(CH3)3NBr], and ethanol were purchased from Sigma-Aldrich. The AIBA was recrystallized from a 50/50 wt % water/acetone mixture. Styrene was purified with a NaOH solution to remove the inhibitor. The styrene monomer (50 mL) was washed with a 5 wt % NaOH solution three times and then washed with deionized water several times in a separation funnel. The purified monomer was stored in a refrigerator before use. Other reagents were used as received. Furthermore, silicone oil (X-22-170BX, kinematic viscosity = 40 mm2/s, density = 0.97 g/cm3, Shinetsu, Japan) was used as the carrier liquid for the ER fluid. Fabrication of PSBM/SiO2. Initially, cationic polystyrene nanospheres were prepared by a soap-free emulsion copolymerization method using AIBA (0.25 g) as the initiator. Both MOTAC (0.1 g) and BA (1 g) were added as comonomers with styrene (10 g) to facilitate rapid adsorption and the formation of a uniform coating on the PSBM nanospheres with a negatively charged silica precursor and to obtain a better dispersion in water. Polymerization was carried out in nitrogen at 70 °C for 20 h, and the stirring speed was adjusted to 450 rpm. Four milliliters of 0.1 M NaCl was added to control the ionic strength of the polymeric nanospheres. They were then purified by three consecutive centrifugation/redispersion cycles in distilled water. The final product of the nanospheres was obtained using a freezedrying process. To synthesize monodisperse core−shell-structured PSBM/SiO2 nanospheres, 0.5 g of charged PSBM nanospheres was dispersed in 34 g of water. The CTAB, ammonium hydroxide, and 14 mL of ethanol were added and stirred at room temperature. TEOS was then added dropwise with stirring at room temperature. The mixture was kept at room temperature for 72 h. The resulting mixture was centrifuged and washed three times each with ethanol and distilled water. Characterization. Transmission electron microscopy (TEM, Phillips CM-220) and scanning electron microscopy (SEM, Hitachi S-4200) were conducted on the as-received powders. The as-prepared powders of the specimens were used as received, and their density was measured with a gas pycnometer (AccuPyc 1340, Micrometitics) at room temperature. For the ER measurements, dry powders of PSBM/ SiO2 (density = 1.20 g/cm3) were mixed with silicone oil with a volume fraction of 7% by mechanical shaking and sonication. The suspension, called a PSBM/SiO2 ER fluid, was loaded into the cup of a concentric cylinder cell (CC17/E, inside cup diameter = 18.07 mm, gap size distance between cup and bob = 0.71 mm). The rotor (or bob) of the concentric cylinder was dipped in the ER fluid and driven by the motor of a rotational rheometer (MCR300, Physica, Austria). A controlled shear rate (CSR) mode measurement was performed by
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RESULTS AND DISCUSSION Synthesis Mechanism and Structures. Scheme 1 shows a schematic diagram of the synthesis procedure for the monodisperse PSBM core/silica shell nanoparticles. The particle size and distribution of the PSBM and PSBM/SiO2 nanospheres were determined by SEM and TEM. Figure 1a,b presents SEM and TEM images of the PSBM particles, respectively. Monodisperse polymeric nanospheres with a diameter of approximately 80 nm were observed. The diameter of these particles decreased with increasing concentration of
Figure 1. (a) SEM and (b) TEM images of the PSBM nanospheres. 1730
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MOTAC. The addition of MOTAC also led to a better dispersion of emulsions, compared to that prepared using a conventional emulsion polymerization method. SEM indicated that these particles were highly monodisperse with clear surfaces because this technique avoids the use of a stabilizer during polymerization. The growth of a silica shell was carried out by the consecutive precipitation of CTAB and negatively charged, hydrolyzed silica precursors on the positively charged surfaces of the PSBM nanospheres. During the adsorption of the silica precursor on the surfaces of the polymer cores, CTAB was added to prevent the particles from coagulating and to facilitate the adsorption of the silica precursor. By exploring the positively charged nanospheres, negatively charged silica precursors can be adsorbed rapidly to form silica shells with a uniform thickness. Figure 2a,b shows SEM and TEM images of Figure 3. TGA data of the PSBM/SiO2 nanospheres, measured during heating from room temperature to 600 °C.
stepwise rapid thermal decomposition observed upon further heating from 120 to 600 °C was attributed to the consecutive removal of the oligomeric materials including CTAB and the PSBM polymeric core. In this figure, the final weight loss of the silica-coated nanoparticles was approximately 74%, and the weight ratio of SiO2 to the polymeric cores was 0.26. Electrorheological Analysis. After dispersion in silicone oil, the rheological behavior of the PSBM/SiO2 particles was measured using a rotational rheometer. Before the ER fluid was injected, free rotation was carried out to measure the shear stress generated by the connection between the rotational bob and the pin of the high voltage generator. The real shear stress of an ER fluid should be the result of the measured value subtracted from the shear stress of the free rotation (Figure S1). Figure 4a shows the flow curves of the ER fluid, indicating a relationship between the shear stress and shear rate, and the effect of the electric field strength on the shear stress. Before the electric field is applied, the shear stress of the ER fluid grows linearly with increasing shear rate. Although an electric field was applied, the yield stress, which is caused by the formation of particle chains over the electrodes of the concentric cylinder cell, appeared at each electric field strength.26 A yield stress is needed to break the chain structures and start flow in an ER fluid. The shear stress was stable in the low-shear-rate region where there was equilibrium in the chain structures: two states of break and reformation. When the shear rate was greater than 1/s, the shear stress was not as stable as it was in the low-shear-rate region. The particle chains could not reform promptly because of the high shear rate. The particle chains were then destroyed so that a slight increase in the shear stress as the hydrodynamic force domains was observed in this region. In addition, the shear stress (or yield stress) increased with increasing electric field strength. This suggests that the interaction between the PSBM/SiO2 particles is enhanced by increasing the electric field strength. The same enhancement in the shear viscosity of the ER fluid was observed (Figure 4b). For each electric field strength, the shear viscosity exhibits a shear-thinning phenomenon resulting from the deformation and breaking of the particle chains in the shear flow at a controlled shear rate. As mentioned, because the ER effect of the core−shellstructured PBSM/SiO2 particles was generated from the SiO2 shell, we also measured the ER performance of pure SiO2
Figure 2. (a) SEM and (b) TEM images of the PSBM/SiO2 nanospheres.
the silica-coated polymer nanospheres, respectively. The final size of the coated spheres was ∼100 nm, and the average thickness of the silica shell was estimated to be approximately 10 nm. Figure 3 shows the TGA data of silica-coated PSBM nanoparticles as measured during heating from room temperature to 800 °C, where the core−shell nanoparticles decompose in two steps. In this figure, a small weight loss (about 5%) of water was found in the temperature range of 100 ≤ T(°C) ≤ 120, followed by rapid weight loss upon further heating up to approximately 400 °C. The initial small weight loss was caused by the removal of the bound water attached to the silica shells of core−shell nanoparticles during the freeze-drying process. In the case of silica particles, the small amount of water is thought to be the source of the electroresponsive ER property. The 1731
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to the electric field strength (E), which is described as a power law τy ∝ Em
(1)
where the exponent m can be obtained by fitting the yield stresses over a broad electric field range on a logarithmic scale. For the polarization model, m = 2.0 by employing the pointdipole approximation.28 However, in a practice study, m deviates from the empirical value and goes to 1.5 or even lower, particularly at high electric field, as a result of the properties of the particles such as conductivity, particle shape, and size.8,29 As shown in Figure 5, the dynamic yield stress of the PSBM/SiO2
Figure 5. Dynamic yield stress of the ER fluid as a function of the electric field. The fitting line is generated from the power law equation τy ∝ E1.5.
ER fluid follows a power law relationship with the electric field strength, where the exponent m = 1.5 (i.e., τy ∝ E1.5). The lower field dependence may be influenced by the higher surface conductivity of the silica shell organized by CTAB, following a conductivity model with a slope of 1.5. Note that the slope change from 2.0 to 1.5 has also been observed for magnetorheological fluids under an applied magnetic field.30 One of the important properties of an ER fluid in application is its ability to switch (i.e., its ability to respond to a voltage pulse signal). Figure 6 shows the shear stress of the ER fluid measured under a voltage pulse and a constant shear rate of 1/ s. The electric field was applied and sustained for 10 s after stirring each time for 10 s in the off state. Each experimental point was recorded every 0.1 s. When the electric field was switched on, the shear stress jumped to a higher level and became stable in less than 1 s by showing several points on the off−on (or on−off) boundaries. In other words, the particle structures could not be formed instantaneously but need time to arrange and build up. On the off−on boundaries, the first point always has the same value with the last operation, which means that the response time of the system is 0.1 to 0.2 s. In other words, the increase (or decrease) in shear stress cannot be finished in 0.1 s. However, as the electric voltage increases, the shear stress increases and fewer points are observed on the off−on boundary, whereas more points are found on the on− off boundary. A creep and recovery test was carried out to observe the deformation of the particle structures as a function of time, and the results are shown in Figure 7. The deformation (or strain)
Figure 4. (a) Shear stress and (b) viscosity of the ER fluid as a function of the shear rate, measured at various electric field strengths.
particles (average diameter = 2 μm). From the composition of the core−shell particles (PSBM core, 80 nm in diameter; SiO2 shell, 10 nm in thickness), the volume fraction of the SiO2 shell can be estimated to be 3.5%. An ER fluid containing the same percentage of pure SiO2 particles was prepared separately. Figure S2 is the shear stress versus shear rate of the pure SiO2 ER fluid measured at various electric field strengths, to be compared with the PBSM/SiO2-based ER fluid. The pure SiO2 particles, without the insulating PSBM core, exhibit a slightly greater shear stress than do the PBSM/SiO2 core−shell particles. However, the difference almost disappears when the electric field becomes higher than 0.5 kV/mm, meaning that a higher electric field overcomes the PSBM core block to induce the polarization of the SiO2 shell. In addition, the shell thickness was thought to have a significant effect on the ER properties of the core−shell particles, as has been studied by other researchers.27 However, in this system, another core− shell PBSM/SiO2 with a larger shell thickness of 30 nm did not show much difference in the flow curves compared to the sample of 10 nm shell thickness (Figure S3). The dynamic yield stress of the PSBM/SiO2 ER fluid was obtained by extrapolating the shear stress to a zero shear rate limit, which was also considered to be the shear stress at the starting shear rate. The yield stress (τy) of an ER fluid is related 1732
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χ=
γR γmax
(2)
where γR is the recoverable strain. χ was calculated to be 0.60 and 0.93 for 1.0 and 2.0 kV/mm, respectively, indicating a higher elastic interaction of the particles under a higher electric field strength. Dielectric Analysis. Generally, an ER fluid displays dielectric properties corresponding to its rheological behavior under an electric field.32 Figure S4 shows the dielectric properties of both the PSBM/SiO2 and the pure SiO2 ER fluids, including the dielectric spectra and Cole−Cole plot of the ER fluids. The dielectric spectra were recorded over a broad frequency range of 20−106 Hz, which was analyzed by the Cole−Cole equation8 ε0 − ε∞ ε* = ε′ + iε″ = ε∞ + 1 + (iωλ)1 − α (3)
Figure 6. Shear stress of the ER fluid measured under a constant shear rate (1/s) of the electric field with a square voltage pulse (t = 10 s).
where ε0 and ε∞ are the dielectric constants (ε′) at zero and infinite frequency, respectively. The dielectric strength Δε = ε0 − ε∞ indicates the polarizability of the ER fluid. ω = 2πf is the frequency and λ = 1/2πf max is the relaxation time in which f max is the frequency where the dielectric loss (ε″) reaches a maximum. A short relaxation time also contributes to the good ER effect. The exponent (1 − α) characterizes the broadness of the relaxation time distribution. When α = 0, eq 3 reduces to the Debye well-known single relaxation time model. Table S1 in the Supporting Information lists the above parameters of the PSBM/SiO2 ER fluid compared to those from the pure SiO2 ER fluid. Δε of the PSBM/SiO2 ER fluid was only 0.27, which is lower than 0.80 for the pure SiO2 ER fluid, and the λ is shorter for PSBM/SiO2 than for SiO2. This means that the solid SiO2 particles have a higher polarizability but take a longer time to be polarized than the SiO2 shell. The combined effect of Δε and λ reduces the difference in the ER effect of the two ER fluids. However, it is inevitable that the PSBM/SiO2 ER fluid presents lower shear stress and shear viscosity, which is consistent with its lower Δε.
Figure 7. Deformation of the ER fluid as a function of time under a creep and recovery test: shear stress = 10 Pa for the first 20 s and then shear stress = 0 Pa. Two different electric field strength are applied: a solid line for 1 kV/mm and a dashed line for 2 kV/mm.
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CONCLUSIONS Monodisperse PSBM/SiO2 core−shell nanospheres were fabricated using a consecutive precipitation method in which positively charged PSBM nanoparticles were first synthesized via soap-free emulsion polymerization and coated with inorganic silica using negatively charged precursors. Because of the silica shell, the PSBM/SiO2 core−shell particles exhibited typical ER performance, as indicated by their flow curves of shear stress and viscosity, on−off switching, and creep tests. The ER fluid exhibited electric-field-enhanced shear stress and elastic recovery. The field dependence followed the power law τy ∝ E1.5. The dielectric measurements indicated the relatively low dielectric constant of the ER fluid corresponding to its lower ER effects. Nevertheless, the fabrication of PSBM/SiO2 particles shows that the inorganic shell on the nanometer scale can respond to an electric field and suggests a new methodology for preparing inorganic-material-based ER particles with a low density.
was recorded as a function of time: first under a load of constant shear stress (10 Pa) and then with no load. Under a shear stress, an ideal elastic material will deform immediately and then reform after the shear stress is removed. However, viscoelastic materials represent time-dependent nonlinear deformation under sustained constant stress and subsequent time-dependent reformation after the shear stress is removed.31 Therefore, as shown in Figure 7, the PSBM/SiO2 ER fluid is like a viscoelastic fluid by showing time-dependent deformation in both creep and recovery processes. The two experimental curves were measured under different electric field strengths, 1.0 kV/mm (solid line) and 2.0 kV/mm (dashed line). Once the shear stress is applied, an instantaneous strain appears, which then increases with time until it reaches a maximum (γmax,1 for 1.0 kV/mm and γmax,2 for 2.0 kV/mm). Under a higher electric field strength (E = 2 kV/mm), the ER fluid becomes more solidlike (i.e., γmax,2 ≪ γmax,1). In the recovery process, the deformation of the ER fluid cannot be recovered completely. The recovery ratio (χ) was defined to evaluate the elasticity of the ER fluid
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ASSOCIATED CONTENT
S Supporting Information *
Shear stress and shear viscosity measured without any ER fluid sample. Shear stress of the pure SiO2 ER fluid compared to that of the PSBM/SiO2 ER fluid at various electric field strengths. 1733
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Flow curves of the ER fluid of PSBM/SiO2 (shell thickness = 30 nm). Dielectric spectra and Cole−Cole plot of the PSBM/ SiO2 and the pure SiO2 ER fluids. Optical parameters obtained by fitting the data to the dielectric spectra of the ER fluid. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF-2013R1A1A2057955). Y.K.K. is grateful for support from a National Research Foundation of Korea grant funded by the Korean Government (MEST) (NRF-2009C1AAA001-2009-0093204).
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