Enhanced Stimuli-Responsive Electrorheological Property of Poly(ionic liquid)s-Capsulated Polyaniline Particles

We used inherently conducting polyaniline as a core to develop a type of poly(ionic liquid)s-capsulated polyaniline composite particles in order to both overcome the surface charged character of pure poly(ionic liquid)s particles prepared by post ion-exchange procedure, and enhance electrorheological (ER) effect. The structure was characterized by different techniques and the electrorheological suspension was prepared by dispersing the composite particles in silicone oil. Under electric fields, the electrorheological properties of the suspensions of poly(ionic liquid)s-capsulated polyaniline composite particles were measured and compared with their single forms. It is demonstrated that the composite particles have distinctly enhanced electrorheological effect compared with the pure poly(ionic liquid)s and polyaniline particles under electric stimuli. At 4 kV/mm of electric field, the yield stress of the suspension of poly(ionic liquid)s-capsulated polyaniline composite particles in silicone oil is about 2.3 kPa, which is twice as high as 1.2 kPa stress of the suspension of poly(ionic liquid) particles and 2.5 times as high as 0.9 kPa stress of the suspension of polyaniline particles. By using dielectric spectroscopy, microscopic observation, and oscillation rheology, we studied the origin of this enhanced electrorheological effect. The results indicated that wrapping polyaniline into poly(ionic liquid)s could partly suppress the positively charged surface state of poly(ionic liquid)s particles prepared by post ion-exchange procedure and improve the column-like electrorheological structure. This suppression should be responsible for the enhanced electrorheological effect of poly(ionic liquid)s-capsulated polyaniline composite particles.


Introduction
Smart materials have attracted much attention because they can produce an adaptive response to external stimuli [1,2]. Electrorheological (ER) suspension, which is composed of polarizable particles with a high relative dielectric constant, and an insulating liquid medium with a low relative dielectric constant, is one of the most important electroresponsive smart materials [3]. Under external electric fields, ER suspension can rapidly transfer from a low viscous Newtonian fluid into a thickening Bingham fluid due to the electrostatic interactions of polarized particles [4]. Especially, the viscosity change or phase transition is reversible and requires low energy consumption. Therefore, ER suspension has attracted significant attention as electrical-mechanical interfaces in many potential technical applications, such as automotive, aerospace, food processing, robotic, and so on [5][6][7].
To promote the real utilization of ER technology, intensive research on the development of new effective ER materials has been continuously done since Winslow first discovered ER behavior [8]. Many kinds of ER materials, such as inorganic ion compounds, inorganic semiconductors, natural ion motion and conductivity of the PILs are without affinity to moisture or water, and as a result, the ER fluid of PIL particles exhibits ER effect in the absence of any activators [22]. Two methods can prepare hydrophobic PIL particles: the direct polymerization of hydrophobic IL monomers by different techniques, and the polymerization of hydrophilic IL monomers followed by post ion-exchange treatment [21,23,24]. When compared with direct polymerization, the post ion-exchange procedure is easier and produces a higher yield for industrial mass production. However, it is also found that PIL particles prepared by post ion-exchange procedure are easily surface charged, which sometimes results in the unexpected dependence of ER effect on electrode polarity. In addition, it is also necessary for PILs to improve ER effect for practical applications.
In this paper, we used inherently conducting polyaniline as a core to develop a type of poly(ionic liquid)s-capsulated polyaniline (PIL-c-PANI) composite particles in order to overcome the surface charged character of pure PIL particles prepared by post ion-exchange procedure and enhance ER effect. To form the capsulated composite, an aqueous dispersion was firstly prepared by polymerizing aniline in the presence of poly(p-vinylbenzyl trimethylammonium chloride) as steric stabilizer. By the addition of hexafluorophosphate potassium salt, the polymeric stabilizer became hydrophobic, precipitating in water and trapping polyaniline inside poly(p-vinylbenzyl trimethylammonium hexafluorophosphate) to form PIL-c-PANI composite particles. The ER suspension was prepared by dispersing the composite particles in silicone oil. Under electric fields, we measured and compared the ER effect of the PIL-c-PANI particles with their single forms. It was found that the PIL-c-PANI particles have a distinct enhancement in ER effect when compared with that of pure PIL particles and PANI particles, which is twice as high as the suspension of PIL particles and 2.5 times as high as the suspension of PANI particles. By using dielectric spectroscopy, microscopic observation, and oscillation rheology, we studied the origin of enhanced electrorheological effect. The results indicated that wrapping PANI into PILs could suppress the positively charged nature of present polycation-type PIL particles and improve the column-like ER structure, and this should be responsible for the enhanced ER effect of PIL-c-PANI particles.

Preparation of PIL-c-PANI Particles
PIL-c-PANI particles were prepared by the method of Rebeca Marcilla et al. with a slight modification [25]. First, poly(vinylbenzyl)trimethylammonium chloride (P[VBTMA]Cl) was prepared by radical polymerization of 2.00 g [VBTMA]Cl in 20 mL ethanol with 0.03 g AIBN as initiator under nitrogen atmosphere. The reaction temperature was 70 • C. After reaction for 12 h, a large amount of acetone was added into this mixture to form precipitate. The precipitate was washed with acetone and vacuum dried to obtain white P[VBTMA]Cl solid. Then, 1.00 g P[VBTMA]Cl and 0.10 g aniline were dissolved in 30 mL distilled water. We added 0.25 g APS dissolved in 10  Pure PANI particles were prepared as follows: 1.00 g aniline was dissolved in 30 mL distilled water, and then an aqueous solution containing 2.50 g APS in 10 mL water was added dropwise into the aniline solution. After reaction for 24 h at 10 • C under stirring, a dark green PANI precipitate was obtained. The precipitate was washed with DI water and further soaked in 60 mL of NH 3 ·H 2 O (3 wt %) for 5 min to obtain the final PANI particles.

Preparation of ER Suspensions
The P[VBTMA][PF 6 ], P[VBTMA][PF 6 ]-c-PANI, and PANI particles were further washed with water and ethanol, and dried in a vacuum at 100 • C for 48 h. The dry particles were mixed into dimethyl silicone oil (KF-96, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan,) with a kinetic viscosity of 50 cSt at 25 • C with a particle concentration of 27 vol %. The density of particles was measured by a pycnometer method.

Characterization and Measurements
The morphology and structure of samples were characterized by different techniques. Optical microscopy (OM, Nikon ALPHAPHOT-2, Tokyo, Japan) and scanning electron microscopy (SEM, Hitachi TM3000, Tokyo, Japan) were used to observe the morphology of samples. Fourier transform-infrared spectrum (FT-IR, JASCO FT/IR-470 Plus, Tokyo, Japan) was used to analyze the chemical groups of samples. A thermogravimetric analyzer (TGA, Netzsch STA449F3, Selb, Germany) test was conducted with a heating rate of 20 • C min −1 within 30-800 • C in flow air to determine the thermal decomposition. A particle analyzer (Malvern Zetasizer Nano, Malvern, UK) was used to measure the Zeta potentials of samples. In the measurement, the particles were dispersed in ethanol to form dilute solution.
The ER effect of suspensions was measured by a stress-controlled rheometer (Thermal-Haake RS600, Karlsruhe, Germany) with a parallel plate measurement system at room temperature. The diameter of the plate was 35 mm, and the gap between two plates was 1.0 mm. A DC high-voltage generator (WYZ-010, Beijing, China) applied the electric field through the two plates. The flow curves of the shear stress as a function of shear rate were measured by the controlled shear rate mode in the range of 0.1-1000 s −1 . In the dynamic oscillatory test, the modulus curves of storage modulus (G ) & loss modulus (G ) as a function of shear stress amplitude were obtained by the controlled stress mode at a constant oscillation frequency of 1.0 Hz. After one test, we exchanged the polarity of the rotation side of an electrode: the previous positive electrode would become a negative electrode at the next test, and this cycle repeated. Before every test started, the suspensions were pre-sheared for 60 s at 300 s −1 to remove structure history, and then the electric field was applied for 30 s in order to form an equilibrium gap-spanning ER structure.
The conductivity (σ p ) of P  6 ]-c-PANI, and PANI particles was measured by suspension method. In this method, the leaking current density (j) through suspension under the electric field (E) s was firstly detected by galvanometer, and then the conductivity (σ) of suspension was calculated by σ = j/E. Because the structure form in ER suspensions under a high electric field is similar to that of anisotropic fibrous composite, σ p can be calculated by the approximate mixture equation σ = ϕσ p + (1 − ϕ)σ f (where σ f is the conductivity of silicone oil, and ϕ is the volume fraction of particles in suspension.).
The dielectric property of suspensions was measured at room temperature by an impedance analyzer (HP 4284A, Santa Clara, CA, USA), with a liquid measuring fixture (HP 16452A, Santa Clara, CA, USA) in the frequency range of 20-10 6 Hz. The bias electrical potential was 1 V, which could not induce fibrous-like structure formation; thus, we could well compare the different polarization characteristics of different samples. The dielectric characteristic was obtained by the Cole-Cole function described as Dong et al. [22].
The ER structure formed between two electrodes was observed by an optical microscope (Nikon ALPHAPHOT-2). The gap of electrodes was 1.0 mm. Before microscopic observation, 2 kV/m of electric field was applied to the suspension in the gap for 30 s in order to form an equilibrium gap-spanning ER structure.

Synthesis of PIL-c-PANI Particles
The preparation process of P     6 ]-c-PANI are dark particles encapsulated by a transparent shell. More detailed morphology can be observed by SEM images. As shown in Figure 1D,F, P[VBTMA][PF 6 ] particles are irregular, with a size distribution of 2~5 µm, and PANI particles are also irregular but they seem to be composed of smaller primary particles. The morphology and size of P[VBTMA][PF 6 ]-c-PANI particles are obviously different from PANI particles, but they are very similar to those of P[VBTMA][PF 6 ] particles (see Figure 1E).      [PF6] particles show a two-step weight loss. The first one can be attributed to the thermal degradation of the polymeric backbone with an onset temperature of 300 °C, and the second one can be attributed to the thermal degradation of hexafluorophosphate counterions with an onset temperature at 500 °C [26]. PANI particles show a one-step weight loss with an onset temperature of around 400 °C, corresponding to the thermal degradation of the  6 ] particles show a two-step weight loss. The first one can be attributed to the thermal degradation of the polymeric backbone with an onset temperature of 300 • C, and the second one can be attributed to the thermal degradation of hexafluorophosphate counterions with an onset temperature at 500 • C [26]. PANI particles show a one-step weight loss with an onset temperature of around 400 • C, corresponding to the thermal degradation of the polymeric backbone. P[VBTMA][PF 6 ]-c-PANI particles show a three-step weight loss, with an onset temperature of 290, 420 and 500 • C, which can respectively be attributed to the thermal degradation of P   [PF6]-c-PANI, and PANI particles. Here, due to no relative material parameter of silicone oil, we used ethanol as the medium to measure the Zeta potentials. In addition, the particles can be dispersed in ethanol better than other solvents and, thus, it can provide a high measurement precision. The Zeta potential of P[VBTMA][PF6] particles prepared by post ion-exchange procedure is ~+134 mV, which indicates a positive charged nature. The positive charges may be attributed to the fact that most anions are outside during the post ion-exchange procedure, and it is easy for them to adsorb other cations. The Zeta potential of PANI particles is significantly smaller (~+39 mV [PF6]-c-PANI, and PANI particles in silicone oil is measured by a stress-controlled rheometer with a parallel plate system. Figure 4 shows the flow curves of shear stress as a function of shear rate for suspensions at different electric fields. It is seen that without electric fields, these suspensions exhibit a low viscous state.   6 ]-c-PANI, and PANI particles. Here, due to no relative material parameter of silicone oil, we used ethanol as the medium to measure the Zeta potentials. In addition, the particles can be dispersed in ethanol better than other solvents and, thus, it can provide a high measurement precision. The Zeta potential of P[VBTMA][PF 6 ] particles prepared by post ion-exchange procedure is~+134 mV, which indicates a positive charged nature. The positive charges may be attributed to the fact that most anions are outside during the post ion-exchange procedure, and it is easy for them to adsorb other cations. The Zeta potential of PANI particles is significantly smaller (~+39 mV  6 ]-c-PANI, and PANI particles in silicone oil is measured by a stress-controlled rheometer with a parallel plate system. Figure 4 shows the flow curves of shear stress as a function of shear rate for  6 ] particles, while the PANI particles are composed of smaller primary particles. After the electric field is applied, three suspensions exhibit an obvious increase in shear stress or viscosity and behave like a Bingham plastic material, with large yield stress at various electric field strengths. This is due to the polarization of the dispersed particles and the formation of chain or column-like ER structures under electric fields [27]. Interestingly, it can be seen that the suspension of P[VBTMA][PF 6 ]-c-PANI particles shows much larger shear stress and yield stress compared with the suspensions of P[VBTMA][PF 6 ] particles and PANI particles, in particular at high electric field strength. At 4 kV/mm of electric field, the yield stress of the suspension of PIL-c-PANI composite particles in silicone oil is about 2.3 kPa, which is twice as high as the 1.2 kPa stress of the suspension of PIL particles and 2.5 times as high as 0.9 kPa stress of the suspension of PANI particles. This indicates that the PIL-c-PANI particles have a distinct enhancement in ER effect. particles, while the PANI particles are composed of smaller primary particles. After the electric field is applied, three suspensions exhibit an obvious increase in shear stress or viscosity and behave like a Bingham plastic material, with large yield stress at various electric field strengths. This is due to the polarization of the dispersed particles and the formation of chain or column-like ER structures under electric fields [27]. Interestingly, it can be seen that the suspension of P       To clarify this enhanced ER effect more clearly, we further compare the ER effect of P[VBTMA][PF6]-c-PANI particles with that of a simple mixture of PANI and P[VBTMA][PF6] particles at the same PANI content. Figure 6 presents the yield stress as a function of PANI content at 3.0 kV/mm electric field strength. Note that no data over 40% PANI content is presented; we cannot obtain a good sample because P   To clarify this enhanced ER effect more clearly, we further compare the ER effect of P[VBTMA][PF 6 ]-c-PANI particles with that of a simple mixture of PANI and P[VBTMA] [PF 6 ] particles at the same PANI content. Figure 6 presents the yield stress as a function of PANI content at 3.0 kV/mm electric field strength. Note that no data over 40% PANI content is presented; we cannot obtain a good sample because P  To clarify this enhanced ER effect more clearly, we further compare the ER effect of P[VBTMA][PF6]-c-PANI particles with that of a simple mixture of PANI and P[VBTMA][PF6] particles at the same PANI content. Figure 6 presents the yield stress as a function of PANI content at 3.0 kV/mm electric field strength. Note that no data over 40% PANI content is presented; we cannot obtain a good sample because P

Dielectric Properties
We firstly measure the dielectric properties of suspensions, because it has been accepted that ER effect is related to the interfacial polarization of particles in suspensions, and the dielectric properties play an important role in ER effect [29]. Table 2 generalizes the dielectric characteristic of suspensions obtained by the Cole-Cole function fit [22]. It is seen that all suspensions show a dielectric relaxation that can be attributed to the interfacial polarization of dispersed particles in suspension, because the dielectric constant and loss factor of used silicone oil are almost independent of frequency within the measured frequency range. The relaxation time of the suspension of P[VBTMA][PF 6 ] particles is the fastest, and that of the suspension of PANI particles is the slowest. However, they are still in the same order, and the difference is small, because the conductivity of P[VBTMA][PF 6 ] particles is only slightly higher than that of PANI particles.  6 ]/PANI mixture, but the difference in their relaxation times is small. As a result, the overlapping of two relaxation times results in the broadened scattering degree. However, it is worthy to note that the relaxation time of the suspension of P[VBTMA][PF 6 ]/PANI mixture is only slightly slower than that of the suspension of P[VBTMA][PF 6 ]-c-PANI particles, and the relaxation strength of the former is also slightly smaller than that of the latter. Thus, although it has been proposed that a good ER effect requires that the ER suspension should first have a faster dielectric relaxation time within an appropriate frequency range of 10 2 -10 5 Hz, and then have a large dielectric relaxation strength, the differences in the dielectric properties of these samples, in particular for the [VBTMA][PF 6 ]/PANI mixture and P[VBTMA][PF 6 ]-c-PANI, are so small that they may not result in a large change in ER effect. Therefore, we consider that the small differences in dielectric properties should not be the main reason for the significantly enhanced ER effect of P[VBTMA][PF 6 ]-c-PANI particles, and that some other reasons may play a more dominant role.

Microscopic ER Structures
Besides the dielectric property, knowing the microscopic ER structure under electric fields is also essential in order to understand ER effect [30,31]. By optical microscopy, we observed the ER structure of diluted suspensions of the above mentioned particles under electric fields. As shown in Figure 7, all the particles can form a column-like ER structure spanning electrodes due to the interparticle attraction of polarized particles under electric fields, but there are differences in the column structures  6 ] particles, more particles tend to be attracted or aggregated towards the cathode side and, thus, the columns near the anode side are thinner than those near the cathode side. This may be because PIL particles prepared by post ion-exchange procedure are more easily surface positively charged, according to the Zeta potential analysis. For PANI particles, the particles tend to be attracted toward two electrodes and the central part of the columns seem to be the thinnest. In contrary, the columns formed by P[VBTMA][PF 6 ]-c-PANI particles seem to become uniform, and no obvious weak point can be observed. This may be because wrapping PANI into PIL particles has partly suppressed the positively charged surface state, according to the Zeta potential analysis. The columns formed by P[VBTMA][PF 6 ]/PANI mixture particles are also uniform, but more transparent P[VBTMA] [PF 6 ] particles tend to be attracted towards the cathode side, and more black PANI particles tend to be attracted towards the anode side. Based on these observations, we can preliminarily conclude that the formation of P[VBTMA][PF 6 ]-c-PANI particles might have to some extent improved the uniformity of the column ER structure and decreased thin column regions. It is known that the shear stress of ER suspension is always dominated by the weakest point of the ER structure. Therefore, the enhanced ER effect of P[VBTMA][PF 6 ]-c-PANI particles probably arises from the improvement of the ER structure. In order to clarify this more precisely, we further conduct an oscillation rheological measurement for different samples by the controlled shear stress mode.  Figure 7A,D, it is seen that the columns formed by P[VBTMA][PF6] and PANI particles seem to be not uniform. For P[VBTMA][PF6] particles, more particles tend to be attracted or aggregated towards the cathode side and, thus, the columns near the anode side are thinner than those near the cathode side. This may be because PIL particles prepared by post ion-exchange procedure are more easily surface positively charged, according to the Zeta potential analysis. For PANI particles, the particles tend to be attracted toward two electrodes and the central part of the columns seem to be the thinnest. In contrary, the columns formed by P[VBTMA][PF6]-c-PANI particles seem to become uniform, and no obvious weak point can be observed. This may be because wrapping PANI into PIL particles has partly suppressed the positively charged surface state, according to the Zeta potential analysis. The columns formed by P[VBTMA][PF6]/PANI mixture particles are also uniform, but more transparent P[VBTMA][PF6] particles tend to be attracted towards the cathode side, and more black PANI particles tend to be attracted towards the anode side. Based on these observations, we can preliminarily conclude that the formation of P[VBTMA][PF6]-c-PANI particles might have to some extent improved the uniformity of the column ER structure and decreased thin column regions. It is known that the shear stress of ER suspension is always dominated by the weakest point of the ER structure. Therefore, the enhanced ER effect of P[VBTMA][PF6]-c-PANI particles probably arises from the improvement of the ER structure. In order to clarify this more precisely, we further conduct an oscillation rheological measurement for different samples by the controlled shear stress mode.

Oscillatory Study
Since the ER suspension usually shows a solid-like state due to the formation of the ER structure under the electric field, the viscoelastic moduli as a function of applied shear stress presents an initial plateau corresponding to the linear viscoelastic behavior. Meanwhile, the storage

Oscillatory Study
Since the ER suspension usually shows a solid-like state due to the formation of the ER structure under the electric field, the viscoelastic moduli as a function of applied shear stress presents an initial plateau corresponding to the linear viscoelastic behavior. Meanwhile, the storage modulus is always higher than the loss modulus, and the ER suspension has a dominant elastic response. As the stress amplitude increases and exceeds a critical value, the viscoelastic moduli exhibit a sharp decrease that represents the rapture of the ER structure and the beginning of flow. The characteristic stress as this change happens is named the critical stress (τ c ), which is very sensitive to the weakest point in the ER structure [30]. Therefore, we can precisely understand the ER structure by the evolution of the viscoelastic moduli with the amplitude of the oscillatory stress. Figure 8 shows storage modulus (G ) and loss modulus (G ) as a function of stress amplitude at the frequency of 1 Hz and at 2.0 kV/mm electric field strength for the suspensions of pure P  6 ]/PANI mixture, and pure PANI particles. Here, having considered the distribution of shear deformation rate and to clarify the influence of the ER structure on properties, we carried out a comparable measurement by exchange the polarity of the rotation side of the electrode. As shown in Figure 8, under the electric field, the suspensions do show a solid-like state according to the fact that G is larger than G , and G remains constant before the stress exceeds the yield value, which is called linear viscoelastic behavior. As the stress amplitude increases and exceeds a critical value, the viscoelastic moduli exhibit a sharp decrease and the suspensions start to flow. However, there are significant differences in the amplitude of the storage modulus and the critical stress for different samples.
The  6 ]-c-PANI particles is the strongest. However, the most interesting aspect is the difference in the amplitude of critical stresses obtained in the case of the cathode as the rotation side, and in the case of the anode as the rotation side, for different samples. To show this difference, the values of τ c as a function of PANI content are plotted in Figure 9. It is seen that, for the suspension of P[VBTMA][PF 6 ] particles, the value of critical stress is smaller when the rotation side of the electrode is anode, compared with when the rotation side of the electrode is cathode. According to the optical microscopic observations before, we consider this should be related to the fact that the thinnest column region or the weakest point in the columns formed by P[VBTMA][PF 6 ] particles is located near the anode side. As a result, the rapture of the columns along the surface of the electrode easily occurs when the rotation side of the electrode is anode, because the value of the shear rate is at its maximum at the surface of the rotation side. For the suspension of PANI particles, the value of critical stress obtained in the case of cathode as rotation side is almost same with that obtained in the case of anode as rotation side. This is also in accordance with the fact that PANI particles tend to be attracted toward two electrodes, and the thinnest column region or the weakest point is located in the central part of the columns. For the suspension of P[VBTMA][PF 6 ]-c-PANI particles, the difference in the amplitude of critical stresses obtained in the case of the cathode as rotation side, and in the case of anode as rotation side, gradually becomes smaller with the increase of PANI content. When the PANI content is close to 40 wt %, the value of critical stress obtained in the case of cathode as rotation side is very close to that obtained in the case of anode as rotation side. The similar phenomenon is also observed for the suspension of P[VBTMA][PF 6 ]/PANI mixture. According to the optical microscopic observation, we can also consider this is related to the fact that According to the optical microscopic observation, we can also consider this is related to the fact that the columns formed by particles become more and more uniform with the addition of PANI. However, it can also be noted that both the critical stress obtained in the case of cathode as rotation side, and the critical stress obtained in the case of anode as rotation side, increase with the increase of PANI content for the suspension of P Since there are not significant differences in the dielectric properties, we consider that the improvement of the column-like ER structure may be associated with the fact that wrapping PANI into P[VBTMA][PF6] particles has partly suppressed the positively charged surface state.

Conclusions
By using intrinsically conducting PANI as a core, we prepared a type of PILs-capsulated PANI composite particles in order to both overcome the surface charged character of pure PILs particles prepared by post ion-exchange procedure and enhance ER effect. Under electric fields, the PILs-capsulated PANI particles were found to show an enhanced ER effect when compared with that of pure PIL particles, PANI particles, and their simple mixture. By using dielectric spectroscopy, microscopic observation, and oscillation rheology, we studied the origin of enhanced ER effect. The results indicated that wrapping PANI into PILs could partly suppress the positively charged state of PILs particles prepared by post ion-exchange procedure and improve the ER structure. This suppression should be responsible for the enhanced ER effect of PILs-capsulated PANI composite particles.