Core–shell structured poly(2-ethylaniline) coated crosslinked poly(methyl methacrylate) nanoparticles by graft polymerization and their electrorheology

Abstract

This paper reports the synthesis of core–shell structured poly(2-ethylaniline) (PEAN) coated cross-linked poly(methyl methacrylate) (PEGDMA) particles and their electrorheological property under an applied electric field. Primarily, monodisperse poly(methyl methacrylate) nanoparticles (∼700 nm) were synthesized by dispersion polymerization. The PEAN–PEGDMA microspheres with an average diameter of 1.6 μm were then prepared by an oxidative polymerization process. The application of a suspension of these particles as an electrorheological ER fluid (10 vol%) was assessed using a rotational rheometer, and the effects of the electric field strength were examined. Direct observation of the fibrillation phenomenon of the ER fluid was also investigated using an optical microscope. The results showed that the performance of the ER fluids was enhanced by increasing the electric field strength. The dielectric spectra were further correlated with their ER effect using a LCR meter.

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1. Introduction

An electrorheological (ER) fluid is defined as a smart and intelligent material, in which the rheological properties of a suspension of particles dispersed in an insulating oil,¹ can change from a liquid-like state to a solid-like state within milliseconds by the application of an external electric field. In the absence of an external electric field, conducting particles are dispersed randomly and the ER fluid behaves simply as a Newtonian fluid. When an external electric field strength is applied, the dispersed particles immediately form chain-like structures and the ER fluid behaves as a Bingham fluid. Consequently, in the process, their rheological properties (yield stress, shear viscosity and modulus etc.) are being transformed rapidly.^2–6 The change in the rheological properties is induced by chain formation of the particles carrying dipole moments induced by an external electric field.⁷ This characteristic of ER fluids has been used in many engineering applications, for example dampers,⁸ torque transducers, clutches, breaks, actuator devices in orthotic devices,⁹ and haptic devices.¹⁰ Similarly, magnetically analogous magnetorheological (MR) fluids can be controlled under an external magnetic field.¹¹ Although ER fluids have similar properties, the application of ER suspensions has been limited compared to MR suspensions, mainly because of their lower yield stresses. On the other hand, ER fluids with designed electro-responsive particles can show many new potential applications.¹²

Electro-responsive ER fluids include electrically polarizable particles, such as inorganic non-metallic, organic and polymeric conducting materials.¹³ Conducting materials generally have a π-conjugated structure,¹⁴ e.g. in polyaniline (PANI),¹⁵ copolyaniline, polythiophene,¹⁶ poly(p-phenylene)¹⁷ and polymer–inorganic nanocomposites.^18–21

Core–shell structured particles are programmable smart materials with functionality and shape. For conducting polymers, the insulating morphological effects of core particles are generally combined with the electrical properties of conducting shell layers. Therefore, core–shell structured electroresponsive particles have advantages, such as an enhanced ER effect, thermal stability, improved dispersibility, rapid response to an electric fields and interesting designs.²² Furthermore, a coated conducting layer of a shell reduces the cost of conducting polymers and improves the transparency of the particles.²³

In the present study, a conducting polymer of poly(2-ethylaniline) (PEAN) was adopted as a new material of core–shell structured particles for ER materials without a dedoping process. Note that while the PANI is the most commonly used as a conducting polymer in the ER materials, it requires an additional de-doping process of an appropriate conductivity value for its ER application. To enhance the adhesion between the PEAN shell and poly(methyl methacrylate) (PMMA) core, grafting polymerization was introduced through the crosslinking agent of ethylene glycol dimethacrylate.^24,25 Core–shell particles synthesized by this process had a uniform thickness of the PEAN shell and the PEAN-cross-linked PMMA (PEGDMA) particles were adopted as the ER material.²⁶

2. Experimental

Materials

The monomer, methyl methacrylate (MMA) was used after purification. The initiator of α,α′-azobisisobutyronitrile (AIBN) was recrystallized in ethanol prior to use. The other materials, including poly(vinylpyridine) (PVP) (M_w = 360 000 g mol⁻¹), glycidyl methacrylate (GMA), ethylene glycol dimethacrylate (EGDMA, 98%), oxydianiline (ODA), ammonium persulfate (APS), 2-ethylaniline (EAN), poly(vinyl alcohol) (PVA) (M_w = 1700 g mol⁻¹), and sodium dodecyl sulfate (SDS) were used as received. Distilled water was used in all the experiments.

Synthesis of monodisperse PMMA seeds

Both the purified MMA monomer and the radical initiator of AIBN were dissolved in methanol containing PVP as a stabilizer at room temperature. The reaction mixture was heated to 65 °C using a circulator and polymerization was continued for 24 h. After the reaction, the fabricated monodispersed PMMA nanospheres were centrifuged several times with methanol and deionized water, and then dried using a freezing-drier for 48 h.

Synthesis of grafting PMMA

The monodispersed PMMA nanoparticles were then swollen by GMA with a radical initiator. The reaction system was kept at room temperature for 12 h with mild stirring. In the course of stirring, the added SDS was adsorbed on the surface of PMMA. Both EGDMA and AIBN were then placed in the reactor for 12 h. Here, the GMA was added to the reactor to be active for 5 h. The reaction system was heated to 65 °C and kept at that temperature for 24 h with rapid mechanical stirring. The PEGDMA particles were centrifuged and dried using a freezing-drier for 48 h.

PEGDMA modified with ODA (ODA–PEGDMA)

The ODA was dissolved separately in acetone, and the PEGDMA nanoparticles were then dispersed in the ODA solution. The epoxy–amine grafting reaction occurred between the glycidyl group in the GMA and the amine group in the ODA. The system was heated to 55 °C and maintained at that temperature for 12 h. The products were dried in a vacuum oven at 50 °C.

Conducting PEAN coating onto ODA–PEGDMA

The synthesized ODA–PEGDMA particles were dispersed in an acidic aqueous solution containing APS and PVA, in which the PVA was used as a stabilizer and APS was used as the initiator. Subsequently, the mixture was transferred to a 500 mL reactor, and polymerization was commenced by adding 2-ethylaniline dropwise. The reaction was stirred for 12 h at 0 °C. The final product was washed with methanol and deionized water to remove the unreacted initiator, monomer, oligomer, and PVA, and then vacuum-dried for 24 h at 50 °C. Fig. 1 presents a schematic diagram of these processes.

Fig. 1 Schematic diagram of the polymerization mechanism.

Preparation of ER fluid

The fabricated PEAN–PEGDMA nanoparticles were dispersed in silicone oil (kinematic viscosity = 100 cS) with a volume fraction of 10% by mechanical shaking and ultrasonication to achieve good dispersibility. The ER properties were examined using a rotational rheometer under a range of electric field strengths.

Characterization

Scanning electron microscopy (SEM, S-4300, Hitachi, Japan) and transmission electron microscopy (TEM, CM200, Philips, Holland) were used to observe the morphology of the PEAN–PEGDMA particles. Average particle size and size distribution are analyzed by dynamic light scattering apparatus (DLS, ELS-8000, Otsuka, Japan). Fourier transform infrared spectroscopy (FT-IR, Bruker, VERTEX 80V) using KBr pellets was performed to confirm the chemical structures. Thermogravimetric analysis (TGA, TA instrument Q50, USA) was carried out from a room temperature to 800 °C at a heating rate of 10 °C min⁻¹ in air to observe the thermal properties of the particles. The density of the particles was measured using a pycnometer (Accupyc 1330, Gas pycnometer, USA).

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 behavior was observed using a rotational rheometer (MCR 300, Anton Paar, Germany) equipped with a high DC voltage generator (HCN 7E-500, fug) and CC17 geometry. The electric spectra of the ER fluid were measured using a LCR meter (Hewlett-Packard HP 4284A) with a liquid measuring fixture (HP 16452A).

3. Results and discussion

The PEAN–PEGDMA particles were fabricated by chemical oxidative polymerization. Fig. 1 shows the entire process from the PMMA seeds to the PEAN–PEGDMA nanoparticles. In the first step, the PMMA was synthesized by dispersion polymerization. The PEGDMA particles were prepared by swelling PMMA seeds with a crosslinking agent (EGDMA) and swelling agent (GMA). This method allows the particles attach to the surface epoxide groups. The ODA was attached to react with the epoxide groups and aniline monomer. Finally, the anilines were adsorbed on the ODA–PEGDMA particle surface via a chemical oxidation reaction.

Fig. 2 shows SEM images indicating the size of the spheres as well as the surface morphology of the experimental steps from (a) to (d). For example, Fig. 2(a) represents the smooth surface and size of pure PMMA nanospheres synthesized. The seeds were approximately monodisperse and 700 nm in diameter. Once grafted and swelled by EGDMA and GMA, Fig. 2(b) indicates a lack of smoothness of the particle surface and average diameter is 1.26 μm. In the Fig. 2(c), after the amine group of ODA and the epoxy group of GMA reacted, average diameter became 1.42 μm, the surface of the ODA–PEGDMA particle became even rougher than the PEGDMA particles. The PEAN–PEGDMA particles (d) were prepared by chemical oxidation polymerization, and the surface became significantly irregular with their average diameter of about 1.6 μm.

Fig. 2 SEM images of the PMMA nanoseeds (a), PEGDMA particles (b), ODA–PEGDMA particles (c), and PEAN–PEGDMA particles (d).

Fig. 3 shows TEM images of the PEAN–PEGDMA particles. After coating with PEAN (a), the surface of the particle became rough. The particle surface became quite irregular with some small agglomerates attached. Fig. 3(b) shows a partially enlarged view. TEM and SEM confirmed that the PEAN–PEGDMA particle size was in principle monodisperse.

Fig. 3 TEM images of core–shell structured particles of PEAN–PEGDMA (a), high-magnification image of PEAN–PEGDMA (b).

Particle size and size distribution of pure PMMA, PEGDMA, ODA–PEGDMA and PEAN–PEGDMA particles determined by DLS are indicated in Fig. 4. Fabricated pure PMMA particles (Fig. 4(a)) distribute over a range of 650–770 nm, with the maximum intensity positioned at about 700 nm which is average particle size. PEGDMA particle size (Fig. 4(b)) is from 1.2 μm to 1.45 μm and having an average size of 1.3 μm. Fig. 4(c) shows distribution of the ODA–PEGDMA particles distribute over a range of 1–2 μm, average particle size is 1.34 μm. In the case of the PEAN–PEGDMA particles (Fig. 4(d)), particle size is 1.3–1.8 μm, average particle size is 1.6 μm. The particle size is similar to the value observed from the SEM image.

Fig. 4 Size distribution of pure PMMA (a), PEGDMA (b), ODA–PEGDMA (c), PEAN–PEGDMA (d) particles determined by DLS.

Fig. 5 shows the FT-IR spectra of the samples. FT-IR spectroscopy can confirm the presence of functional groups on the surface of the PMMA particles after coating. The FT-IR spectrum of pure PMMA (Fig. 5(a)) revealed a peak for C–H stretching at 2952 cm⁻¹, C=O bending at 1730 cm⁻¹, C–H bending at 1450 cm⁻¹ and C–O asymmetric stretching of the ester group at 1263 cm⁻¹ and 1148 cm⁻¹ were confirmed. In the spectrum of PEGDMA (Fig. 5(b)), the peak at 2948 cm⁻¹ was assigned to the C–H stretching of an epoxy group. Therefore, graft polymerization was confirmed. After the epoxy–amine reaction, the epoxy peak disappeared in ODA–PEGDMA particles because the ODA was adsorbed chemically onto the PEGDMA particle surface. In addition to ODA–PEGDMA (Fig. 5(c)), a new peak for the NH₂ stretch at 3473 cm⁻¹ was observed. Fig. 5(d), the absorption peaks at 3461 cm⁻¹ were assigned to the symmetric and asymmetric NH₂ stretching bands of aromatic amine. Bending vibrations of NH₂ in the aromatic ring can be observed at 720–445 cm⁻¹. Therefore, the successful coating of PEAN on the PMMA surface was confirmed.

Fig. 5 FT-IR spectra of pure PMMA, after adding EGDMA and GMA, after adding ODA, final PEAN–PEGDMA.

TGA curve of Fig. 6 shows thermal stability and weight composition of the obtained particles over the temperature range of 25–700 °C at a heating rate of 10 °C min⁻¹ in air. The curves marked from (a) to (d) were for pure PMMA, PEGDMA, ODA–PEGDMA and PEAN–PEGDMA, respectively. The pure PMMA nanoparticle (a) showed a rapid weight loss, beginning at 280 °C and finally ending at approximately 400 °C in a single rapid step.

Fig. 6 TGA curves of the samples at each step including pure PMMA, after adding EGDMA and GMA, after adding ODA, final PEAN–PEGDMA.

Subsequently, the PEGDMA particles (b) obtained by grafting polymerization showed two steps of weight loss regions. The major weight loss in curve (b) from 270 °C to 450 °C was assigned to the decomposition of PEGDMA. The starting temperature of the thermal degradation of the ODA–PEGDMA particles (c) was higher than those of both PMMA and PEGDMA. The first degradation step in the temperature range of 260–400 °C was attributed to the degradation of PMMA and PEGDMA, whereas the degradation of ODA occurred in the second range, 400–600 °C. In other words, the mass fraction of ODA was estimated to be 20%. In the case of the PEAN–PEGDMA (d), a second weight loss begins at 390 °C was attributed to the thermal degradation of grafted PMMA, ODA and coated PEAN shell.

The OM images in Fig. 7 explain the fibrillate formation phenomenon of the PEAN–PEGDMA particles under an applied electric field, where a dilute ER fluid (10 vol% particle concentration) was dropped between two parallel electrodes and observed by OM. Fig. 7(a) shows the PEAN–PEGDMA particle-based ER materials dispersed randomly in silicone oil without an electric field. After applying an electrical field (Fig. 7(b)), the PEAN–PEGDMA particles reshaped rapidly to connect the nearby particles to form a chain-like structure. Generally, this ER phenomenon remains stable as long as the electric field is applied. In this way, the rheological properties of the PEAN–PEGDMA particles are being altered rapidly.

Fig. 7 OM images of PEAN–PEGDMA particle-based ER fluid. The pictures were captured when the electric field (200 V) was off (a) and on (b).

The ER behaviors of the 10 vol% PEAN-PEGDMA particle-based ER fluid were examined through a controlled shear rate test under the electric field strength ranging from 0 to 3.5 kV mm⁻¹, in which the shear rate range was set up from 0.01 to 1000 (1/s) on a log–log scale. Fig. 8 shows the flow curves of the shear stress and shear viscosity, as a function of the shear rate for the PEGDMA suspension. As shown in Fig. 8(a), in the absence of an electric field, the ER fluid represents the characteristics of a typical Newtonian fluid behavior comparable to many other ER fluids, in which the slope of the shear stress increases linearly with increasing shear rate. On the other hand, when an external electric field is applied, the fluid exhibited non-Newtonian behavior as well as a wide plateau region with increased shear stress, showing the yield stress.^27,28

Fig. 8 Shear stress (a) and viscosity (b) curves of the 10 vol% PEAN–PEGDMA-based ER fluid as a function of the shear rate with increasing electric field strength. The solid lines in (a) were fitted using the suggested CCJ model.

This indicates that the application of an electric field induces the polarization of dispersed particles. As a result, the suspended particles form a chain-like structure, resulting in a phase change from a liquid-like to solid-like state. While ER fluids are known to follow a Bingham fluid model, the subsistence of a critical shear rate in the flow curve of various ER fluids was observed, such that below the shear rate, the shear stress decreased as a function of the shear rate, and then above the shear rate, the fluid exhibited pseudo-Newtonian behavior,^29–31 which could not be fitted using the simple Bingham fluid model. The Cho–Choi–Jhon (CCJ) model³² was applied to describe the flow curves of the PEAN–PEGDMA particle-based ER fluid, as shown in eqn (1):

This is a six-parameter model that is used widely to fit the experiment data for a range of ER suspensions. In eqn (1), τ_y is the dynamic yield stress, which is explained as the extrapolated stress from the low shear rate region and η_∞ is the viscosity at a high shear rate that is interpreted as the viscosity in the absence of an electric field. t₂ and t₃ are the time constants, α and β 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, 0 < β ≤ 1, because of dτ/d[small gamma, Greek, dot above] ≥ 0. The solid lines were generated by fitting the CCJ model to the special flow curves. Table 1 lists the fitting parameters.

Table 1 Fitting parameters in the equations of CCJ model obtained from the flow curve

Model	Parameters	Electric filed strength (kV mm^−1)
		0.5	1	1.5	2	2.5	3	3.5
CCJ	τ0	12.2	30.6	63.2	103	181	313	458
	t2	0.00001	0.001	0.001	0.002	0.001	0.006	0.007
	α	0.2	0.65	0.4	0.9	0.8	0.6	0.7
	η∞	0.206	0.221	0.251	0.298	0.354	0.419	0.488
	t3	0.1	1	0.5	0.2	3	1	1
	β	0.9	0.8	0.9	0.9	0.9	0.9	0.8

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Fig. 8(b) shows that the shear viscosities of the ER fluid at various electric strengths are aligned. Without an applied electric field, the ER fluid also showed Newtonian fluid behavior. After the electric field was applied, the shear viscosity decreased gradually with increasing shear rate,³³ showing shear thinning^34–36 behavior under an electric field. The shear thinning phenomenon increased with increasing electric field strength.

Fig. 9 shows the shear viscosity as a function of the shear stress measured using the controlled shear stress (CSS) method by applying a shear stress from below to above the critical stress. The static yield stress obtained from this CSS test can be compared with the dynamic yield stress obtained from Fig. 8(a). A graph of the shear viscosity as a function of the shear stress showed that the viscosity was constant at a low shear stress but decreased dramatically at the point of critical stress and finally approached a constant value at a high shear stress. A rapid decrease in viscosity occurred at the point of certain shear stress. Therefore, the value of this point can be considered to be the static yield stress, indicating that there is no flow below this.

Fig. 9 Shear viscosity curves of the PEAN–PEGDMA-based ER fluid as controlled shear stress (CSS) under different electric field strengths.

Fig. 10 shows the frequency sweep of the ER fluid at a fixed strain of 0.03% with the results of the storage and loss modulus as a function of the angular frequency. Without an electric field, the storage and loss modulus indicated a liquid-like state. Both the storage and loss modulus increase when the electric field is applied, indicating that the ER fluids show solid-like behavior. Furthermore, both G′ & G′′ increase with increasing electric field strength and showed a plateau-shaped region in all angular frequencies.

Fig. 10 Frequency sweep of the 10 vol% PEAN–PEGDMA-based ER fluid under different electric field strengths: frequency curves show storage modulus (a) and loss modulus (b) with a fixed strain amplitude of 0.03%.

In addition, the relaxation modulus (G(t)), examining a solid-like state of ER fluids, can be calculated from the values of G′ and G′′ in Fig 10 using a following eqn (2):³⁴

G(t) ≅ G′(ω) − 0.560G′′(ω/2) + 0.200G′′(ω) (2)

This equation is known as the Schwarzl equation, confirming the short-term relaxation behavior of the PEAN–PEGDMA, which is difficult to obtain experimentally. Generally, this is because of the limitation of the mechanical measurement, resulting from the equipment itself and the intrinsic properties of the polymeric materials. Fig. 11 confirmed that the relaxation modulus under an external electric field had a plateau region compared to the G(t) without an electric field. Therefore, the ER fluid of PEAN–PEGDMA indicates solid-like behavior under an external electric field because of the strong interactions among the particles.

Fig. 11 Relaxation modulus of the ER fluids calculated from G′ and G′′.

Fig. 12 shows the dependence of τ_y on various electric field strengths, in which dynamic and static τ_y are the extrapolated values obtained from the flow curve in Fig. 8 and the CSS curve in Fig. 10, respectively. The dynamic and static yield stress are plotted as function of the electric field, and from a power law equation τ_y ∝ H^α, for both cases the α equals 2.^35–37 The correlation between the electric field and yield stress were predicted using the classic polarization model,³⁸ in which the yield stress in proportional to the square of the electric strength, E².³⁹ This suggests that the electric field-induced a polarization force to form a chain-like structure. The polarization model depends on the particle concentration, particle shape and applied electric field strength. The applied electric field induces electrostatic polarized interactions between the particles and between the particles and the electrodes. A static yield stress higher than the dynamic yield stress for ER fluids because of the ER fluid at the static state contains aggregated particles without a shear rate compared to the fluid in the dynamic state.

Fig. 12 Dynamic and static yield stress as a function of the various electric field strengths for the 10 vol% PEAN–PEGDMA-based ER fluid.

To examine the ER properties of the PEAN–PEGDMA particle based ER fluid further, the dielectric properties⁴⁰ were analyzed using an LCR meter and the results are shown in Fig. 13. The magnitude of charge separation, which is responsible for particle polarization in the suspension, and the rate are related to the dielectric parameters. The dielectric constant and loss factor are the results for the interfacial polarization of suspensions including an ER fluid, which consists of a polarizable phase dispersed in insulating oil.⁴¹

Fig. 13 Dielectric spectra (a) and Cole–Cole fitting curves of the 10 vol% PEAN–PEGDMA particles based ER fluid.

Fig. 13(a) and (b) show the dielectric spectra as a function of the frequency and Cole–Cole plots for the 10 vol% PEAN–PEGDMA microsphere-based ER fluid. The experimental data in Fig. 13 were fitted to the well-known Cole–Cole eqn (4):^42,43

where Δε = ε₀ − ε_∞ is the polarizability of an ER fluid, which is related to the electrostatic interactions among particles. Here, ε₀ is the dielectric constant when the frequency ω is close to 0, ε_∞ is the dielectric constant at the high frequency limit, and the value of Δε is related to the ER performance of the ER fluids.⁴⁴ The relaxation time, λ = 1/2πf_max considers the rate of interfacial polarization when an electric field is applied, where f_max is defined by the maximum of the dielectric loss of an ER fluid, (1 − α) characterizes the broadness of the relaxation time distribution, and α is a value in the range 0–1. The higher stress enhancement is performed when the λ becomes smaller within the acceptable range and the large Δε is applied.⁴⁵

From the fitting result, the value of ε₀ and ε_∞ were 2.38 and 1.49, α (0.58) and λ (0.9 × 10⁻⁶ s⁻¹), respectively and indicate a more rapid response to an electric field in this PEAN–PEGDMA particle-based ER fluid. The value 0.89 of Δε estimated was much lower than those reported such as 1.53 for PANI coated PMMA and 1.56 for PANI coated snowman-like PMMA particle,^46,47 indicating that a weak ER effect of the PEAN–PEGDMA particle based ER fluid is related to the weaker polarizability due to its ethyl group in the shell layer.⁴⁸ Nonetheless, the merit of this new material is its fabrication method without a de-doping process compared to the PANI systems due to its intrinsically lower conductivity.

4. Conclusion

The core–shell structure of the PEAN–PEGDMA particles was synthesized using grafted and swollen PMMA as a core material and PEAN as a shell material. Both SEM and TEM revealed the expected core–shell morphology and particle size. The FT-IR spectra indicated the formation of new peaks or disappeared peaks between the particles. TGA confirmed the enhanced thermal stability. The PEAN–PEGDMA particle-based ER fluid exhibited typical ER characteristics in the presence of an electric field, showing that the shear stress, shear viscosity, storage modulus, loss modulus, relaxation modulus increases with increasing electric field strength.

Acknowledgements

This study was supported by a research grant from both National Research Foundation, Korea (NRF-2013R1A1A2057955), and Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea (2013).

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