Tunable electrorheological characteristics and mechanism of a series of graphene-like molybdenum disulfide coated core–shell structured polystyrene microspheres

Abstract

Core–shell structured molybdenum disulfide (MoS₂) coated polystyrene (PS) microspheres are synthesized with the help of hexadecyl trimethyl ammonium bromide (CTAB) through negative–positive electrostatic attraction. The morphology of the composite particles is studied by scanning electron microscopy (SEM), which apparently provides evidence of MoS₂-coated PS. The microspheres’ structure and chemical components are investigated by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy, respectively. The MoS₂/PS composite particles show better thermal stability than PS according to thermogravimetric analysis (TGA). Novel electrorheological (ER) fluids based on the MoS₂/PS composite dispersed in silicone oil are prepared and further examined by a rotational rheometer in a controlled shear rate mode under various electric field strengths. The influence of factors such as the electric field strength, the particle sizes, the proportions of the MoS₂/PS composite and the functional groups on the surface of PS on ER properties is investigated. The related mechanism of these effects on ER behaviors is also analyzed in detail, aiming to find whether the graphene analogue MoS₂ is superior to graphene when making them into ER fluids. MoS₂ has reversible and tunable electrorheological characteristics and can transform its phase from a liquid-like to a solid-like state when exposed to an external electric field.

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Introduction

Considerable research on electrorheological (ER) fluids composed of layered graphene or graphene oxide combined with conducting polymers or other organic materials has been reported over the past few years.^1–4 The two-dimensional (2D) materials graphene or graphene oxide, can be applied widely in the fields of electronic, optical, mechanical, and electrochemical materials owing to their unique laminar structures that can provide high specific surface areas, and thus they also have a dominant position in the area of ER fluids.⁵ An ER fluid always consists of polarizable particles with a high dielectric constant and low conductivity dispersed in an insulating liquid medium. The polarizable particles’ rheological properties can rapidly and reversibly change upon application of an external electric field. It is found that graphene oxide has already been applied to prepare polarizable composite particles such as in graphene oxide (GO)–PS, and GO–TiO₂ nanocomposites.^6–8 However, graphene based ER fluids have not got the expected ER behaviour, which must be attributed to the fact that layered graphene does not have a band gap and there is always a low current on/off ratio in graphene-based field effect transistors, limiting graphene’s ER application.⁹ The graphene analogue molybdenum disulfide (MoS₂) not only has most of the advantages of graphene or graphene oxide, but unlike graphene it could also be prepared to produce a high current on/off ratio above 10⁸ which is significantly higher than that of graphene and its band gap could be altered from an indirect band gap of 1.06 eV to a direct band gap of 1.9 eV after exfoliation to layered MoS₂.^10–13 It has more remarkable performance and more powerful competitiveness than graphene, so it will replace graphene,^14–17 and there are few studies about the ER properties of MoS₂-based composite materials. Thus, MoS₂ has been chosen as a component of ER particles with the aim of researching the ER characteristics of MoS₂-based composite materials and whether MoS₂ behaves better than graphene in ER fluids.

ER liquids, in a general situation, are typically composed of polarizable particles dispersed in insulating oil media.^18,19 These particles can become polarized and exhibit field-induced chain or fibril structures by connecting with each other in the gap of the field-generating electrodes, thus exhibiting solid-like properties within a millisecond.²⁰ The transformation will disappear as soon as the electric field is removed. Given its advantages, ER fluid has got tremendous mechanical engineering applications including in engine mounts, brakes/clutches, vibration dampers, torque transducers, shock absorbers, control systems, ER valves, robotic arms, and human muscle stimulators.²¹ Although various studies have been performed on ER materials until now, the experimental results are still far from those required for ER material commercialization because of insufficient yield stress and large amounts of sedimentation. As a consequence, much effort must be devoted to developing effective ER fluids after overcoming such limitations.

Following this purpose, many advanced materials such as inorganic nanomaterials, polymers, conducting polymers and their nanocomposites, as well as novel nanomaterials with core–shell structure have been developed to improve the ER performance. Among these materials, core–shell structured composites composed of insulating polymers and inorganic semiconductor additives are always considered as excellent ER material candidates because we can control the dielectric constant and conductivity of the particles by altering the proportion of the materials’ core and shell.²² Despite many studies on the preparation of core–shell structured composites and their electrorheological characteristics, only a few studies investigating factors influencing the ER properties have been reported so far, and despite much research on the modification of PS, this research has not studied the ER characteristics of these modified PSs, even though the modifications influence the mechanism of the ER properties.²³

In this study, we synthesized MoS₂ using a literature method and the core–shell structured PS/MoS₂ nanocomposite was successfully synthesized with the help of the cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB). In this nanocomposite, PS has been chosen as a component of the ER particles because PS is known to be one of the most technologically important materials with many advantages such as it could be prepared easily and very cheaply, its degree of crosslinking could be controlled easily, and most importantly, the benzene ring of PS could be functionalized easily to prepare good materials with higher activity and selectivity. We made these core–shell structured nanocomposites as electrorheological (ER) polarizable particles, and we studied some factors which probably influence the performance of the ER particles aiming to research how these factors impact the ER character and to discover how to improve the yield stress. It could be possible to prepare a better ER fluid whose properties could be controlled with more flexibility by considering most of those factors. To obtain the appropriate conductivity and better ER performance, the factors that need to be handled are the particle size, particle shape, appropriate adjustment of the mass of semiconducting MoS₂ adsorbed on the PS surface, along with the groups on the surface of the PS. The aim of this paper is to prepare a novel ER material based on MoS₂ analogues of graphene and research the influencing mechanism of the electric field strength, particle size, and especially, proportion of MoS₂ to PS and groups on the surface of PS on the ER properties, which have rarely been studied before.

Characterization

Scanning electron microscopy (JSM-5600LV, JEOL, Japan) was used for the morphology study of PS and MoS₂/PS particles. FT-IR spectra were recorded on a Nicolet AVATAR 360 E.S.P spectrometer using KBr pellets. An X-ray diffractometer [X’Pert Pro MPD, Holland] was used to study the crystal structures. The ER properties were measured by dispersing the MoS₂/PS particles in silicone oil then using a rotational rheometer (RS 6000, HAAKE, Germany) which was equipped with a high DC voltage generator, forming an electric field whose direction was perpendicular to the concentric cylinder (CC 15) geometry (gap size: 1 mm) and the surface of the ER fluid. The shear stress of the fluid was measured by increasing the shear rate from 0.01 to 1000 (s⁻¹) under various electric field strengths (0, 1, 2, 3 kV mm⁻¹) on a log–log scale at room temperature.

Experimental section

Materials

Styrene monomer (Aladdin Reagent Co., China, chemically pure grade) was purified briefly by washing (three times) with aqueous NaOH (5 wt%), followed by distillation under reduced pressure to remove the inhibitor. 2,2′-Azobisiso-butyronitrile (AIBN, Aladdin Reagent Co., China, analytical grade) was purified by recrystallization in ethanol. KPS (Tianjin Jinke Fine Chemical Plant, Tianjin, China, chemically pure grade) and PVP ([–CH₂CH(NCH₂CH₂CH₂CO)–]_n, PVP, K-30, M_w ca. 10 000–70 000, Aladdin Reagent Co., China, analytical grade) were used immediately without further purification. Ammonium molybdate ([(NH₄)₆Mo₇O₂₄·4H₂O], Aladdin Reagent Co., China, analytical grade), elemental sulfur (Aladdin Reagent Co., China, analytical grade), and hydrazine monohydrate (86%, Aladdin Reagent Co., China, analytical grade) were used as precursors to the preparation of MoS₂.

Preparation of layered MoS₂

The preparation of MoS₂ has been described in the literature using a hydrothermal process, which was carried out as follows.³⁷ 3 g of ammonium molybdate [(NH₄)₆Mo₇O₂₄·4H₂O], 1.05 g of elemental sulfur, and 24 mL of hydrazine monohydrate (86%) were put in Teflon-lined stainless steel autoclaves of capacity 100 mL. Then the autoclaves were filled to 80% of the total volume with distilled water and sealed tightly; they were maintained at 165 °C for 48 h and then were cooled naturally. The resulting dark-gray powders were filtered and washed with distilled water, diluted hydrochloric acid and ethanol, successively. The final products were dried under vacuum at 40 °C for 3 h. In the end, a surfactant-free liquid-exfoliation method was carried out by putting the final prepared MoS₂ solids into a 45 vol% ethanol/water mixture and sonicating for 8 h, then the dispersion was centrifuged at 3000 rpm for 20 minutes to remove aggregates.^38,39

Preparation of PS with different sizes

Emulsion polymerization, soap-free emulsion polymerization and dispersion polymerization described in the literature^40–43 have been used to prepare different sizes of PS.

The nanometer-sized PS spheres were prepared by emulsion polymerization: typically, under gentle magnetic stirring, 20 mL of styrene and 0.8 g of sodium dodecyl sulfate (SDS) were added at room temperature with 180 mL of deionized water in a 250 mL three-neck round bottom flask. After 15 min, 0.3 g of KPS dissolved in 20 mL of deionized water was added, and the temperature was gradually increased to 78 °C, and maintained for 24 h. The mixture was finally allowed to cool to room temperature. The obtained latex spheres remained suspended in their mother liquor until being used.

The micrometer-sized PS spheres were prepared by soap-free emulsion polymerization: the process was carried out in a four-neck round bottom flask equipped with a nitrogen bubbler, an overhead D-shaped mechanical stirrer, and a condenser. Deionized water (100 mL) was firstly introduced into the flask which was purged with nitrogen for 10 min. Then, styrene (20 mL) was added into the flask, and the mixture was stirred vigorously at 330 rpm for another 10 min under a nitrogen atmosphere while the temperature was raised to 70 °C. After that, 0.2 g of potassium persulfate (KPS) dissolved in a small amount of water was added, and the polymerization reaction was carried out at 70 °C for 20 h under a nitrogen atmosphere. After the polymerization, the resultant product was cooled to room temperature and filtered to remove coagulum. The filtrate was directly used for the following experiment.

The large-sized monodisperse PS microspheres were prepared by dispersion polymerization with a dropwise monomer feeding procedure: a 250 mL three-neck round-bottom flask was employed, which was equipped with a reflux condenser, a nitrogen gas inlet, a Teflon stirrer paddle, and a funnel with pressure compensation involving a governing valve. A solution containing PVP dissolved in ethanol was poured into the reaction flask with magnetic stirring at 300 rpm under a nitrogen atmosphere at 70 °C. Subsequently, a solution composed of styrene and AIBN in the constant voltage funnel was introduced into the flask at a controllable rate by adjusting the valve. The dispersion polymerization was carried out for 24 h and the nitrogen atmosphere was maintained throughout this period. The resulting PS microspheres were purified by the means of centrifugation, decantation, and redispersion in sequence to remove the residual styrene and PVP.

Preparation of nitro-PS particles

The PS particles prepared by dispersion polymerization were dispersed in deionized water. Then the mixture of sulfuric acid and nitric acid was poured into the PS emulsion with a proportion of 3 : 2 under stirring at 45–50 °C. The reaction was carried out for 2 h. The final products were purified and dried under vacuum.

Preparation of amino-PS particles

The nitro-PS particles were put into a 2 mol L⁻¹ NaOH aqueous solution under drastic stirring at 75 °C. Then some reductant, Na₂S₂O₄, was added and the reaction was carried out for 2 h. The final products were purified and dried under vacuum.

Preparation of carboxyl-PS particles

The PS particles prepared by dispersion polymerization, a certain amount of styrene, AIBN, and methacrylic acid were put into a mixed solution of deionized water and ethanol, and was sonicated homogeneously. The reaction was carried out for 24 h under a nitrogen atmosphere. The final products were purified and dried under vacuum.

Preparation of sulfo-PS particles

The PS particles prepared by dispersion polymerization were dispersed in an ethanol aqueous solution and sonicated for 5 min. Sodium p-styrenesulfonate, AIBN, and styrene were put into the mixture and sonicated for 5 min. The reaction was carried out at 30 °C, swelling for 12 h. After swelling, the reaction was carried out at 70 °C for 24 h under a nitrogen atmosphere. The final products were purified by ethanol with centrifugation 3 times and dried under vacuum.

Preparation of ER fluids based on the MoS₂/PS particles

The PS particles were first dispersed in the cationic surfactant CTAB solution with mild stirring. Then the residual CTAB was removed with centrifugation, and the obtained layered MoS₂ was poured into the modified PS aqueous solution (MoS₂ can be adsorbed on the cationic surfactant CTAB modified PS particles’ surface through a strong negative–positive electrostatic attraction because the layered MoS₂ in an ethanol aqueous solution has negative charge). The reaction was carried out for 24 h. The final products were purified and dried under vacuum at 40 °C for 6 h. The MoS₂/PS particles with different sizes, four kinds of MoS₂/PS with four proportions of MoS₂ and MoS₂/group modified PS were synthesized, then we put them into insulating silicone oil, seperately, and dispersed them uniformly with sonication for the next ER tests.

Results and discussion

The SEM and TEM images of the PS particles, MoS₂/PS particles of different size, and functionalized PS particles of the same size are shown in Fig. 1. As shown in Fig. 1(d–h), MoS₂ was adsorbed on the surface of the PS particles forming an expected core–shell structured microspherical nanocomposite with the modification of CTAB to PS particles through the wrinkles on the PS surface that were smooth originally without the participation of MoS₂. And as shown in Fig. 1(c), after the grafting of functional groups onto the PS, the surface of the PS was pock marked or flocculent instead of smooth.

Fig. 1 SEM images of (a and b) pure PS with a smooth surface, (c) the same sized PS with functional groups and (d–f) MoS₂/PS with wrinkles on the surface of the particles. TEM images of (g and h) MoS₂/PS.

The FT-IR spectra of the pure PS particles and the MoS₂/PS composites are shown in Fig. 2. For the pure PS particles, the very strong peaks observed at 2919 and 2849 cm⁻¹ arise from the attachment of additional methylene groups. Further evidence of the MoS₂ coating of PS was shown in the MoS₂/PS spectrum. The peaks at 1451 and 2917 cm⁻¹ correspond to the absorptions of the benzene ring of PS segments. However, the peaks at 1590 and 2918 cm⁻¹ are much weaker than those of PS, which is possibly caused by the layered MoS₂ on the PS surface. In addition, no neonatal peaks are observed in the nanocomposite, therefore, there are no significant chemical bonds between PS and MoS₂.^24–26

Fig. 2 FT-IR spectra of the PS particles and MoS₂/PS with much weaker peaks than those of PS.

The XRD patterns of PS, MoS₂/PS and MoS₂ obtained by the hydrothermal synthesis method are shown in Fig. 3. For MoS₂, the pattern indicates that the MoS₂ aggregates as S–Mo–S layers, and that the stacking of these layers has not taken place. After attachment with PS, the MoS₂/PS composite shows both a weaker peak at 2θ = 11.27° and a broad peak at 2θ = 18.69°, demonstrating the expected coexisting structures of MoS₂ and PS.^27–29

Fig. 3 XRD patterns of (a) PS, (b) MoS₂/PS composite, and (c) MoS₂, showing the formation of core–shell structured MoS₂/PS microspheres.

The thermal stability curves (Fig. 4) of pure PS and the MoS₂/PS composite were measured by TGA under nitrogen at a heating rate of 10 °C min⁻¹ from room temperature to 600 °C. Compared to the low decomposition temperature of 300 °C for pure PS, the decomposition temperature of the MoS₂/PS composite increased to 400 °C. This phenomenon that the MoS₂/PS composite showed a better thermal stability than pure PS may be caused by the adsorption of MoS₂ on the PS surface by simple negative–positive electrostatic attraction. About 9.27% of the weight of the MoS₂/PS particles remained when heated up to 600 °C.

Fig. 4 TGA curves of the MoS₂/PS and PS particles, with MoS₂/PS showing a better thermal stability than pure PS.

The flow curves for the ER characteristics were measured by a rotational rheometer equipped with a high voltage generator through the conventional controlled shear rate (CSR) tests for different types of core–shell structured MoS₂/PS-based ER fluid (20 wt%) under different electric field strengths. The shear stress and shear viscosity versus shear rate curves are shown in Fig. 5 for the ER fluid based on same sized (7 μm) particles with comparatively a high content of MoS₂ attachment under different electric field strengths, Fig. 6 for the different sized particle-based ER fluid under the same electric field strength, and Fig. 7 for the particles formed by different MoS₂ proportions in the MoS₂/PS-based ER fluid under the same electric field strength.

Fig. 5 Flow curves of 20 wt% core–shell structural MoS₂/PS composite based ER fluid under different electric strength, showing the rising trend of (a) shear stress and (b) shear viscosity with increase of electric field strength.

Fig. 6 Flow curves of different sizes of PS core coated with the same amount of MoS₂ under electric fields of (a and b) 1 kV, and (c and d) 3 kV, and (e) the variation trend of shear stress with different particle sizes under an electric field of 1 kV and 3 kV, showing the rising trend of shear stress and shear viscosity with increase of electric field strength and particle size.

Fig. 7 Flow curves of particles formed by ER fluids based on different proportions of MoS₂ to PS under electric fields of (a) 1, (b) 2, and (c) 3 kV, and (d) the variation trend of shear stress in ER fluids with different proportions of MoS₂ to PS under electric fields of 1 kV, 2 kV and 3 kV, showing an apparent rising trend of shear stress when the content of MoS₂ changes from C1 to C2 under electric fields of 1 and 2 kV, and C3 to C4 under an electric field of 3 kV.

As shown in Fig. 5(a), the flow curve with the absence of an electric field shows behaviour similar to that of a Newtonian-like fluid, in which the shear stress increases monotonically with the shear rate except at a very low shear rate region. However, the shear stress presented a progressively increasing tendency when being exposed to an escalated electric field, as a result of the chain-like structural changes among the polarized particles which showed a typical Bingham fluid behavior. When under the electric field at a low shear rate, there appeared a plateau region because the electric fields are dominant compared to the hydrodynamic interactions. After the plateau region, the ER fluid behaved like a fluid because the chain-like structures were deformed and began to break down, and the destruction rate of the fibrillar structures was faster than the reforming rate.

In Fig. 5(b), the ER fluid displayed Newtonian fluid-like characteristics with a constant viscosity at a low shear rate range without an electric field. Then the shear viscosity decreased quite rapidly with an increasing shear rate under an applied electric field, which is called a shear thinning behavior, a phenomenon of a solid-like property.

The shear stress and shear viscosity increased observably under an external applied electric field, but the increase will be inconspicuous when the electric field reaches a special point. This is probably because the amount of electric charge is limited and almost all of the particles became chain-like solid.^30,31 The initial obvious improvement must be because the power charged by the electric field increased and particles in increasing numbers became chain-like, which are both factors influencing the ER properties, simultaneously.

As shown in Fig. 6, we studied the influence factor of different sizes of particles to the ER characteristics under the same electric field. The ER fluids based on different sizes of PS cores coated with the same amount of MoS₂ were measured under the electric fields of 1 kV and 3 kV. It can be found that on increasing the size of the particles, the rise of the shear stress and shear viscosity is not apparent under an electric field of 1 kV when the size class of the particles is micrometer. At the same time, the rise of the shear stress and shear viscosity is indistinctive under an electric field of 3 kV. When changing the size of the particles though, what could be affirmed is that not only the shear stress but also the shear viscosity will increase on increasing the size of the particles. This is probably because the larger torque and larger effective surface area of the larger sized PS spheres can provide a stronger attraction between particles than small ones and show better improvement of ER properties³² (as we all know, spherical particles would produce induced charge at their surface area due to the existence of electrostatic screening).³³ There is an obvious increase of yield stress when the micrometer-sized particles are substituted for nanometer-sized particles under relatively low electric fields, which shows an excellent response to changing the size class of the particles. The same as with the 7 μm-sized particles shown in Fig. 5(a–c), the shear stress and the shear viscosity won’t increase when the electric field reaches a certain degree, which could be explained as follows: at first, the ER properties must be influenced by the proportion of MoS₂, the reverse was also true, the size of PS becomes large when the content of MoS₂ remains consistent meaning that the capacity of the collect electrics is constant.^34,35 In other words, the electricity produced by the electric field is one of the important factors affecting the ER properties when the MoS₂ content is invariable; what’s more, the power charged by the electric field also affects the ER properties. Of course, the ER properties will improve on increasing the extent of the electric field, however, it won’t change the ER properties any more when the content of MoS₂ is changeless.

As shown in Fig. 7(a–d), the electric field was set as 1 kV, 2 kV, and 3 kV, to show the influence of the content of MoS₂ in coated PS, and the proportion of MoS₂ to PS is gradually increased from C1 to C4 (C1 is 250 mL ethanol solution of 1 g of PS coated with 0.5 g of MoS₂, and C4 is 1000 mL ethanol solution with 1 g of PS coated with 2 g of MoS₂). It is found clearly that the increase of shear stress shows a non-monotonic relationship with the increase of the content of MoS₂. In fact, the rising tendency of shear stress is not obvious in comparison with the influence of the particle size on the ER properties. In Fig. 7(a), the shear stress showed a significant increase when the content of MoS₂ changed from C1 to C2. A similar situation also appeared when the content increased from C1 to C2 (in Fig. 7(b)) and from C3 to C4 (in Fig. 7(c)). Therefore, the shear stress could show a transformation from the minimum to maximum on changing the content of MoS₂ under different electric fields. However, this kind of transformation is special as the shear stress will sharply increase if the content of MoS₂ increases from C1 to C2 under the electric field of 1 kV, from C1 to C2 under the electric field of 2 kV, and from C3 to C4 under the electric field of 3 kV. This is mainly because the shear stress will present a different ascent trend under different electric fields. The detailed reason is as follows: the coated structure will change the electrical conductivity through changing the proportion of MoS₂ to PS, thus it will present a different response under different electric fields. As is mentioned above, the content of MoS₂ increases while the size of the particles is invariable, so the proportion of PS declines. In fact, the ER properties will be poor, accordingly, but the capacity for collecting electric charge will improve.³⁶ As a result, these effects counteract each other, and the properties will not improve on increasing the content of MoS₂ with a constant size of PS. What is interesting is that the shear stress will sharply improve in different proportions of MoS₂ to PS under various electric fields. Compared to graphene, the graphene analogue is a semiconductor that has a band gap and exhibits an excellent on/off current ratio and a high carrier mobility. Its electrical conductivity will sharply improve when electrons are stimulated by enough energy for promotion from the valence band to the conduction band across the band gap, which is always interpreted in the theory of the energy of the band gap. Consequently, MoS₂ becomes a conductor and has a better electrical conductivity when the stimulation is powerful enough, which could bring more electric charges that can attract particles under an electric field than ever before. We could produce tunable ER fluids and use them for applications in electronic products such as transistors and mechanical products such as motor elements.

As shown in Fig. 8(a–e), the effects of different groups of (a) sulfonyl, (b) carboxyl, (c) amino and (d) nitryl on the surface of the PS core coated with the same amount of MoS₂ under electric fields of 1 kV, 2 kV and 3 kV on the ER characteristics was studied. It is found that the shear stress varies from the maximum to minimum when the groups on the surface of the PS are changed from sulfonyl to nitryl. The subtle relationship between these four groups could perfectly explain the reason why the shear stress decreases when changing the groups on the surface of the PS. As we all know, the most important factors that influence the ER properties are the conductivity and dielectric constant of ER materials: the ER properties will be enhanced with the increase of the dielectric constant when the dielectric loss and dielectric constant are at a high point; regarding conductivity, high conductivity of the ER materials will lead to the ER materials responding to the electric field quickly, but the ER materials will be broken down by the electric field, however, the ER characteristic will not be obvious because of the low conductivity of the ER materials. Conductivity and dielectric constant are related to the polarity of the groups on the surface of PS, the polarity of these four groups comply with the order: (a) sulfonyl > (b) carboxyl > (c) amino > (d) nitryl, the sulfonyl group increases the polarity of the PS on a large scale, and thus enhances the ER properties most.

Fig. 8 Flow curves of different functional groups (a) sulfonyl, (b) carboxyl, (c) amino and (d) nitryl on the surface of the PS core coated with the same amount of MoS₂ under electric fields of 1 kV, 2 kV and 3 kV, and (e) the variation trend of the shear stress with different functional groups on the surface of PS under electric fields of 1 kV, 2 kV and 3 kV, showing the apparent rising trend of shear stress if the functional groups on PS are sulfonyl compared with carboxyl, amino and nitryl when the electric field strengths are 1 kV, 2 kV and 3 kV.

Conclusions

The novel MoS₂/PS particle-based ER fluid was synthesized through a series of processes in this study. The morphology of the particles tested by SEM presented the structure well, and FT-IR spectra confirmed the coexistence of MoS₂ and PS in the MoS₂/PS particles. The thermal stability of the particles tested by TGA was enhanced due to the better stability of MoS₂-improved PS. Most importantly, the ER properties tested by the rotational rheometer presented excellent behaviors. Even though a MPa grade of metal materials could not be reached, there was still a good improvement among inorganic/polymer particle-based ER fluids. The reason why MoS₂-based ER fluid showed a better performance in terms of yield stress than GO must be that the interlayer separation is larger in MoS₂ than in GO, which means it could gather more charge and consequently, the stress among the chain-like particles becomes more powerful than that in GO. The response of MoS₂ to low electric fields must be due to the band gap of MoS₂ that is absent in GO. Moreover, a different content of MoS₂ under the same electric field will produce different extents of electron transition, which leads to different ER properties. Through changing some factors that affect the ER properties, more efforts should be made to acquire better ER behavior in order to reach higher grades of yield stress in future work.

Acknowledgements

The work was financially supported by National Natural Science Foundation of China (No. 21376054).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25467a

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