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Polymeric nanoparticle-coated Pickering emulsion-synthesized conducting polyaniline hybrid particles and their electrorheological study Chan Soo Jun, Seung Hyuk Kwon, Hyoung Jin Choi, and Yongsok Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13808 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Materials & Interfaces

Polymeric

nanoparticle-coated

Pickering

emulsion-synthesized

conducting polyaniline hybrid particles and their electrorheological study Chan Soo Juna, Seung Hyuk Kwona, Hyoung Jin Choia,*, and Yongsok Seob,* a

Department of Polymer Science and Engineering, Inha University, Incheon 22212, Korea

b

RIAM, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

Abstract To produce an electric stimuli-responsive electrorheological (ER) material, semiconducting core/shell typed polyaniline (PANI) hybrid particles were fabricated through Pickering emulsion-type polymerization, using poly(divinylbenzene-alt-maleic anhydride) (PDVMA) particles as a solid surfactant. The PDVMA nanoparticles were initially polymerized using a self-stable precipitation method. The fabricated PANI/PDVMA composite particles were subjected to various chemical characterizations; further, they were suspended in silicone oil at 10 vol% to prepare an ER fluid, and their viscoelastic behaviors were scrutinized using a rheometer under various input electric fields. We also adopted an LCR meter to evaluate its dielectric characteristics. Our results showed that the PANI/PDVMA composite particles display typical ER performance, such that both dynamic and elastic yield stresses follow a polarization mechanism with a slope of 2.0.

Keywords: polyaniline; nanoparticle; electrorheology; core–shell, Pickering emulsion polymerization

---------------------------------------------------------------------------------------------*Corresponding authors: [email protected] (HJC) and [email protected] (YS)

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1. Introduction Pickering emulsion systems have become some of the best-studied emulsion processes owing to their interesting properties from a scientific perspective as well as their industrial applications based on their functional nano- and microstructures. The solid particles employed in Pickering emulsion-typed states can cover from inorganic particles such as silica, clay, or magnetic particles to organic polymeric particles1. In addition to the traditional Pickering emulsion systems of either oil in water or water in oil2 drops covered with solid particles, the Pickering emulsion-based polymerization technique has recently become one of the most important polymerization tools 3. This method has many industrial advantages in various polymerization processes such as emulsion 4, suspension

5,6

, mini-emulsion

7,8

, and

9

even inverse emulsion polymerizations, using various types of solid particles as solid surfactants. While conventional emulsion polymerization techniques require relatively large amounts of organic surfactants, solid particles can be used instead of molecular organic emulsifiers. These solid particles have the advantage of being partially wettable by both the water and the oil phases in Pickering emulsions, due to their amphiphilic affinity, and can therefore attach to the interface as a steric wall preventing coalescing tendency10. Thus, rigid particles aid the formation of the shell in Pickering emulsion polymerization and simultaneously act as surfactants. The fine, rigid particles play important roles in the production of core/shell typed hybrid particles, or of hollow spheres that are produced by extracting core 11,12. Fabrication of functional polymeric particles with multiple morphologies 13

using various nanoparticles1, 14 from Pickering emulsion process is possible. In addition to use in synthetic processes, Pickering emulsions have potential

applications in foods, cosmetics, petrochemicals, paints15, fuel cells, and environmental technologies

16,17

. Furthermore, Pickering emulsion-polymerized field-responsive particles

have recently been introduced as smart materials, in the form of both electrorheological (ER) and magnetorheological (MR) fluids. These two fluids are families of intelligently smart structural suspensions. The ER fluid in general consists of dielectrically polarizable particle suspended in a non-conducting medium liquid, while the MR suspension is typically a dispersion of soft- or para- magnetic particle in a magnetically insulating liquid. The ER fluids exhibit a structural change from a liquid- to a solid-like phase with external electrical fields

18-20

, in which the suspended particles came to be polarized instantly right after the

external electrical field was employed, and then arrange in the direction parallel to an electric field. Meanwhile, without an applied electric input, ER fluids display Newtonian-like fluid


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property 21. Among diverse ER particles, conductive polyaniline (PANI)

22-24

is regarded the

most fascinating owing to its various benefits including thermal and environmental stabilities, easy fabrication, easily tunable conductivity via a simple doping/de-doping process, and good ER response. In addition, different material variants of PANI such its oligomers, carbonized analogues and its effect of temperature on the ER performance are also interesting to be noted 25-27

. Concurrently, different core/shell typed electro-responsive particles have been

considered as one of the promising ER materials, such as inorganic/inorganic28, inorganic/organic29, and organic/organic core-shell typed particles. Pickering emulsion polymerization has also been adopted to synthesize core/shell-typed PANI composite particles using different solid emulsifiers such as clay nanosheets, ZnO30, SiO2

31

, and

32

magnetite . Nonetheless, surface properties between inorganics and polymer become different. Inorganic particles, when used as solid stabilizers, need to be chemically modified for use in a Pickering polymerization system. Such modifications, which include surface grafting modification and assembly of block copolymers33, are important to overcome the differences in surface properties. However, these modification processes are extremely complicated and requiring a lot of reaction time. To make a simpler fabrication process and lower the costs

34

, polymeric nanoparticles are introduced as potential stabilizer particles to

replace inorganic materials, as they do not require surface modification, owing to their surface similarity with the final product polymer. Therefore, it is possible to use polymeric nanoparticles as the stabilizer in Pickering polymerization. Recent research has focused on such Pickering emulsion polymerization 35 stabilized with polymeric nanoparticles. The polymeric nanoparticles represent promising stabilizers because of their organic compatibility, selective chemical compositions, and wide range of potential applications. For example, functional polymeric nanoparticles with reactive surface groups have been successfully used in the coatings industry

36

. Polymeric nanoparticles

possessing functional groups, with sizes ranging from 100 to 800 nm, have been directly synthesized through a self-stable precipitation polymerization process without a stabilizer 37. In this study, we fabricated a PANI/poly(divinylbenzene-alt-maleic anhydride) (PDVMA) core/shell typed hybrid particle as a novel ER particle via a Pickering emulsion technique38. Reactive co-polymeric nanoparticles were fabricated on the basis of self-stable precipitation polymerization of divinylbenzene (DVB) and maleic anhydride (MAH) in a low-toxicity solvent. The hydrolyzed PDVMA particles can function as surfactant-like



conventional inorganic particles without modification, as they disperse well in an aqueous salt solution. Therefore, we synthesized the PANI/PDVMA core/shell particle from a Pickering emulsion polymerization technique using PDVMA as a solid surfactant. The synthesized PANI/PDVMA had a hydrophobic surface, which improved their suspension stability and compatibility in the organic liquid suspending

39

. The ER system was obtained by

PANI/PDVMA hybrid particles into a silicone oil, and its electro-response was

tested with a rotation-type rheometer under different electric fields input.

2. Experimental section 2.1 Materials All the chemicals of MAH (Sigma Aldrich, USA), DVB (Sigma Aldrich, USA), NaHCO3 (OCI, Korea), toluene (Sigma Aldrich, USA), aniline (DC Chemical, Korea), butyle acetate (Junsei, Japan), HCl (Junsei, Japan), 2,2'-azobis-isobutyronitrile (AIBN) (Daejung, Korea), ammonium persulfate (APS) (Daejung), silicone oil (KF-96, Shin Etsu, Japan, 50cS) and petroleum ether (Showa Chemicals, Japan) were utilized without further treatment. Distilled (DI) water was also adopted throughout a whole process.

2.2 Fabrication of PDVMA as a surfactant First, the reactive PDVMA nanoparticles were fabricated through a self-stable precipitation method 40,41, using both MAH (1.18 g) and DVB (2.8 g) monomers with AIBN (0.04 g, 1 wt% of the monomer mixture) as an initiator, and butyl acetate (250 mL) as a nontoxic solvent. Once all chemicals input dissolved, the reactor was filled with nitrogen for about 15 min. After polymerization with a nitrogen environment for 2 h at 90 °C, petroleum ether was applied as a precipitant to accelerate the aggregation of the synthesized polymeric particles. The particle suspension was under centrifugation (4000 rpm, 10 min) to take out excess initiator, and unreacted DVB and MAH. The nanoparticle produced was dried with vacuum condition at room temperature for 2 days.

2.3 Synthesis of PANI/PDVMA composite materials Synthesized PDVMA (0.5 g) nanoparticles were dispersed in 0.1 M NaHCO3 (35 ml) under sonication for 5.0 min. Separately, a monomer of aniline (0.5 g) was mixed in toluene (5 ml). These two systems were blended with agitation using a homogenizer (Ultra Turrax). After mixing for 5 min, until no oil phase was observed, the emulsion reaction started at 0 °C


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under magnetic-stirring for 2 h. The 1.225 g of APS in HCl (0.5 M) was input drop-wise into the emulsion as the initiator for polymerization, and left for a further 24 h. The PANI particles coated with PDVMA were finally produced by centrifugation, and cleaned with DIwater and ethanol.

2.4 Characterization Morphological image of the synthesized PANI/PDVMA particle was studied using a scanning electron microscope (SEM) (HR-SEM, SU-8010 Hitachi), attached by an energy dispersive X-ray spectroscope (EDS) (Horiba, Japan). Molecular structure of the products was examined using Fourier transform-infrared spectroscope (FT-IR) (Perkin Elmer System 2000). The ER fluid was produced by suspending PANI/PDVMA particles in a silicone oil (50 cSt) (10 vol% particle concentration). To determine the ER characteristics, an optical microscopy (Olympus BX-51, U.S.A.) attached with a voltage generating apparatus was adopted to confirm the chain-like formation of the dispersed ER particles with an external electric field. The ER behavior was analyzed with a rotation-type rheometer (Anton-Paar MCR 300, Graz, Austria) supplied with a voltage (DC) generating apparatus. Density of the product hybrid particle was investigated by a gas pycnometer, and dielectric property of the ER fluid was studied by an LCR tester in a wide frequency window of 20–106 Hz.

3. Results and discussion As presented briefly in Scheme 1, Pickering emulsion polymerization of PANI was carried out using PDVMA as a solid stabilizer, with APS as a water-soluble initiator. The PDVMA nanoparticles were hydrolyzed in NaHCO3 aqueous solution initially. The hydrolyzed PDVMA nanoparticles could then be adhered between the aqueous and oil phases using a homogenizer, owing to their vinyl groups, which were anchored on the PANI particle and confer water affinity. PANI particles coated by PDVMA (a very effective stabilizer) were produced. The morphology of PANI/PDVMA composite is shown in Fig. 1. The PANI particles were wrapped by granules of similar size to the PDVMA nanoparticles (about 30 nm). Therefore, we could confirm that the PDVMA nanoparticles covered the surface of the PANI particles. The EDAX data for PANI/PDVMA hybrid particles are also given in Figure 1. The synthesized PANI/PDVMA nanoparticles are composed of the elements C, N, and O, with



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the following weight and atomic percentages: C, 41.57 wt%, 46.42 at%; N, 38.57 wt%, 36.93 at%; and O, 19.86 wt%, 16.65 at%. The O component could only be from the PDVMA, as PANI is composed of only C and N, indicating that the PDVMA nanoparticle is adhered well into the surface of PANI via physical adsorption. As for the chemical characterization from the FT-IR of pure PDVMA nanoparticles, the particular peaks at 1855 cm-1 and 1780 cm-1 in the FT-IR spectrum could be ascribed to a stretching vibration of C=O, while a peak at 1087 cm-1 was ascribed to that of C−O.

A red

dotted line in Fig. 2 indicates FT-IR spectra of the pure PANI. The particular peaks at 1586 and 1498 cm-1 in its typical spectra indicate C═C stretching vibration bands of quinonoid and benzenoid units, respectively, and can be also observed in the PANI/PDVMA composite 42. The peak position of 1304 cm-1 is related to stretching vibration of C–N in of the second aromatic amine. A peak related to a vibration mode of the N=Q=N ring was shown at 1143 cm-1. The peaks at 1142 and 823 cm-1 were associated to an out-of-plane vibration of its 1–4 substituted aromatic rings

43,44

. These characteristic peaks of PANI were also detected in the

PANI/PDVMA hybrid particles. For the PANI/PDVMA composite, the intensities of the characteristic peaks of PDVMA at around 1855 cm-1 and 1780 cm-1 were significantly reduced, while three new, strong absorption peaks emerged at 1546 cm-1, 1413 cm-1, and 1444 cm-1. These new peaks were assigned to the homogenization of C=O and C−O,45 resulting in the forming of COOwhen the H+ ion (or H atom) was replaced by the Na+. These results suggest that the alkali salt can hydrolyze the PDVMA nanoparticles well. The IR peak of PANI/PDVMA indicates the existence of hydrolyzed PDVMA nanoparticles and PANI. Thus, the IR spectrum shows that the PDVMA nanoparticles were not simply mixed with PANI, but attached to the PANI particles through physical adsorption. The electric conductivity of PANI/PDVMA particles examined by a four-probe tester was 2.78 × 10-8 S·cm-1; which is within the suitable range (10-8–10-10 S·cm-1) for an ER material to prevent electrical short circuits. The PANI/PDVMA particle based ER suspension was fabricated by mixing PANI/PDVMA particles in silicone oil (10 vol%). Prior to the ER test using a rotational rheometer, the ER suspension was placed between two parallel aluminum electrodes (a gap size of 100 µm) to observe the ER phenomenon of the fabricated particles. When a high-voltage direct current (300 V) was supplied to the sample, chain-like structures were examined by an optical microscope, as presented in Fig. 3. While a fluid-like property was measured in the absence of an external


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electrical field (Fig. 3(a)), the dispersed particles exposed to its field (Fig. 3(b)) rapidly started to migrate and produce a chain-like structural form owing to an attractive polarization force between the PANI/PDVMA particles, thereby demonstrating the good electro-response of the ER suspension. Bulk ER characteristics were analyzed using a rotational rheometer for both a steady shear and a dynamic oscillation mode.

In the steady shear test, a controlled shear rate (CSR)

method with a shear rate ranging from 0.1 to 900 s-1 was applied. The flow curves in a log-log scale provide data on a shear rate-dependent shear stress (Fig. 4a) and shear viscosity (Fig. 4b) of the ER suspension at various input electrical fields. The ER suspension exhibited Newtonian fluid-like behaviors in the absence of an input electrical field, similar to many other ER fluids 46, 47. On the contrary, under applied electric field strengths, a solid-like Bingham property was observed, as evidenced by both increased shear stress and a long plateau area, due to the fact that the suspended PANI/PDVMA particles dielectrically polarized by an input electrical field structured chains between the electrodes resisting the flow. When a shear rate became higher, the formed chain-like structures started to be destroyed close to a critical shear rate. Above this value, the structures were completely destroyed, and linearity between the shear rate and shear stress was observed. To analyze the typical ER fluids, the simplest rheological equation of state on ER fluids of Bingham fluid model possessing two parameters of yield stress and shear viscosity has been extensively adopted. Generally, Bingham fluid equation is presented in a following Eq. (1)48: τ = τ + η γ

τ ≥ τ

γ = 0

τ < τ

(1)

in which τ is a yield stress, estimated from an extrapolated shear stress at an extremely low shear rate range; and η is a shear viscosity at an infinite limiting shear rate, indicated by a shear viscosity without an applied electrical field. These τ and η are measurable. Figure 4(a) presents the fitted curves based on Eq. (1) as a dotted line. Table 1 indicates the optimal parameters from Eq. (1). Nonetheless, for many of the ER systems, this simplest rheological equation of state could not fit well the experimental results of shear stress in a wide shear rate range. To complement the limitations of the Bingham equation, we suggested the Cho–



Choi–Jhon (CCJ) equation

49

Page 8 of 29

possessing six physical parameters accurately described the

shear stress trend. The equation is as follows: τ=

+ η 1 + ∞

γ

(2)

The other additional four physical parameters compared to the Bingham fluid model could be deduced from a fitted curve using Eq. (2). The constants t1 and t2 represent the time constant, and the exponents, α and β, are the decline and increment in the shear stress, respectively; β possesses a range of 0 < β ≤ 1, as dτ/dγ ≥ 0. Terms in Eq. (2) describe each shear stress development in two different shear rate areas comprising the total range. The first term relates to the flow characteristics at a lower shear rate area, in which the shear stress declines or remains constant with an increased shear rate. The second portion implies increased shear stress in a higher shear rate area, as the data of shear stress increase toward their peak at the highest shear rate. Therefore, the front portion in the right-hand side of Eq. (2) is dominant at a low shear rate area. Furthermore, the decline of shear stresses can be fitted well by an exponent α

50

. Solid lines generated using CCJ model fitted quite well flow

curves throughout the whole shear rate area when compared to the dotted lines obtained from Bingham model. Table 2 indicates the physical parameters estimated using Eq. (2). Compared with the constant viscosity observed at E = 0 kV/mm, the ER suspension exhibited the shear-thinning characteristics for various electrical field strengths (Fig. 4(b)). The shear-thinning phenomenon results in decreasing shear viscosity as the shear rate increases 51. Figure 5 gives an electric field-dependent dynamic yield stress for the PANI/PDVMA particle based ER suspension plotted in a log scale, in which the yield stress was deduced by extrapolating the shear stress from Fig. 4(a) to a limiting zero shear rate. A power-law correlation exists between a yield stress and the electrical field strength as a following Eq. (3): τ ∝ E$

(3)

The relationship between a dynamic yield stress and an input electrical field could be estimated through a dielectric polarization model 52, where both a dynamic yield stress from


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flow curves and an elastic yield stress from elastic stress correspond to E2 (Fig. 5) with the outstanding ER performance of the PANI/PDVMA particles. The dielectric polarization mechanism is known to be dependent on

both concentration

53

and

shape of the

dispersed particles, and magnitude of the electric field applied. The external electric field produces a response in which dielectrically polarized attractive forces occur among the dispersed particles in addition between suspended particles and electrode. Dynamic oscillation tests are an important tool because they allow us to examine the viscoelastic characteristics of the ER suspensions for different electrical field strengths

54

.

Limits of the linear viscoelastic region (γ%&' ) was analyzed using an amplitude sweep test at a constant rotational frequency (6.28 rad·s-1) before a dynamic shearing oscillation measurement. Figure 6 represents experimental data of a storage (G′) and loss (G″) moduli as a function of

an input strain amplitude from 10-5 to 1

55

, in which the G′ implies a

deformation energy preserved in sample (elastic part), and G″ corresponds to the deformation loss energy in the sample (viscous property). Here, the values of G′ are much larger than those of the G″ at different electrical field strengths as presented in Fig. 6. This indicates that the PANI/PDVMA particle based ER suspension behaves exactly like an elastic solid, that is, it has a certain rigidity. Both G′ and G″ plots display constant plateau values in a low amplitude range of the γ%&', in which its structural deformation is regarded to be reversible. The strain value of 0.00025 in the γ%&' was adopted in an angular frequency sweep test. In the region where a strain passes through the γ%&' region, the values of G′ and G″ drop rapidly owing to an irreversible deformation of the formed structures. Using the experimental results from an amplitude sweep test of a strain, we analyzed the values of an elastic stress part, τ( = G′γ as a function of the strain amplitude γ, to understand the gradual structural breakdown 56. The change of ER suspension was observed from a liquid- to a viscoelastic solid-like state with exposed electrical fields

57

. The ER

particle structure may have been in an unstable state during this deformation; therefore, the dynamic modulus was affected by the amplitude of strain (Fig. 7). The maximum value of τ( is the elastic yield stress τ+ , shown as vertical lines in Fig. 7. The elastic stress close to the equilibrium or maximum value signifies yielding or structural breakdown. There is a two-step yielding behavior that is associated with the composition of a secondary structure. The first yield point was observed at ~0.001–0.01 of strain under various external electric fields, owing to breakdown of the secondary structure. The primary order of the chain structure was completely destroyed at the second yield point,



at about ~0.1–1 of strain. The elastic yield stress increased as a function of an electrical field input. In Fig. 5, the fitting slope of the elastic yield stresses is estimated to be 2.0, which is identical to that from dynamic yield stress given by the power-law relation. Under different electric fields, both G′ and G″ values were examined over a wide angular frequency area (1 to 100 rad·s-1) in the frequency sweep test, with an applied certain value of γ%&' (0.00025). The G′ and G′′ data increased with an external electric field strength as presented in Fig. 8. The experimental result of G′ became significantly higher than that of G″, as the elastic property is greater than the viscosity in the structure of the ER fluid. Therefore, along with input electric field strengths, an increase in G′ demonstrated the enhanced solidification characteristics of the ER suspension of PANI/PDVMA composite particles. To analyze response sensibility of the ER fluid from PANI/PDVMA particles, the shear stresses were examined at a fixed shear rate (1.0

1/s) under an applied square-typed

voltage pulse (t = 20.0 s), as presented in Fig. 9. In presence of an input electrical field, shear stresses of the PANI/PDVMA based ER fluid increased; stronger electric fields resulted in higher shear stress levels. However, as soon as we removed the electric field, shear stress of the ER suspension immediately dropped. The transformation of its shear stress at every turning point was prompt, without a typical hysteretic behavior, and the shear stress remained relatively stable under a given electric field, suggesting that there was a rapid and reversible change to a chain-like structural formation of the suspended PANI/PDVMA particles in the ER fluid, responding to the input electrical field 58. Dielectric characteristics were investigated using an LCR meter to determine the electro-responsive characteristics of the PANI/PDVMA composite particle-based ER suspension, as given in Fig. 10(a) and Fig. 10(b). The polarizability of PANI/PDVMA based ER fluid is related to their dielectric characteristics, in which the interfacial polarization also plays a major part in demonstrating ER efficiencies. Both permittivity (or dielectric constant, ε') and dielectric loss factor (ε'') were functions of frequency (ω), and the line shown in Fig. 10 was fitted by a dielectric relaxation behavior from Cole–Cole model, which was employed to obtain the correlation between

dielectric characteristics and ER performances of the ER

suspension 46, 47. The Cole–Cole equation is presented using a complex dielectric constant in a following Eq. (4):


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∆0

ε∗ = ε′ + .ε′′ = ε + .123α ∞

(4)

Where the ε∗ implies a complex dielectric constant. ∆ε (= ε0 – ε∞) is defined as difference between dielectric constants at ε (zero frequency) and ε (infinite frequency) 50, indicating distribution curves through a wide frequency region. λ represents a relaxation time at a frequency where the dielectric loss reaches its maximum point. On the other hand, an exponent, (1 − α), implies the width of a relaxation time distribution. When α is zero, Eq. (4) is identical to a single relaxation time equation from Debye model. ∆ε signifies the attainable polarizability of ER suspensions, as a large ER effect results from high polarization. The results are shown in Table 3. A short relaxation time generally means a higher shear stress, as a result of the PANI/PDVMA having a faster interfacial polarization process.

4. Conclusion PANI/PDVMA hybrid particles were fabricated by a novel and environmentally benign Pickering emulsion process, using synthesized PDVMA nanoparticles as a polymeric surfactant. SEM images showed that PDVMA nanoparticles effectively covered the surface of PANI. Rheological characteristics of the ER fluid containing PANI/PDVMA composites were observed through rotation and oscillation measurements with external electric fields; flow curve was fitted using CCJ model, and the correlation between yield stresses and applied electric fields exhibited a typical polarization mechanism with the outstanding ER efficiency of the PANI/PDVMA based ER suspension. Furthermore, dielectric analysis indicated that the polarizability of PANI/PDVMA-based ER suspension is correlated with its ER behavior well.

Acknowledgements One of the authors (HJC) appreciates financial support of this work from NRF, Korea (2016R1A2B4008438).

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Scheme 1. Schematic illustration of PANI particles stabilized with PDVMA and initiated by APS through Pickering emulsion polymerization



Fig. 1. SEM images of PDVMA NPs (a), PANI/PDVMA composites (b) and EDAX data of PANI/PDVMA composites (c).


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Fig. 2. FT-IR spectra of PDVMA NPs, PANI, PANI/PDVMA composites.



Fig. 3. OM images of PANI/PDVMA composites (10 vol%, 50cSt silicone oil) based ER fluid with and without an external electrical field (a) and with the electric field (300V) (b).


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Fig. 4. Flow curves of shear stress (a) and shear viscosity as a function of shear rate (b) of synthesized PANI/PDVMA composite based ER fluids. The lines in (a) are fitted from the Bingham (dotted line) and CCJ (solid line) models.



Fig. 5. Dynamic and elastic yield stress as a function of the electric field strength for the ER fluid of PANI/PDVMA composites.


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Fig. 6. Amplitude sweep (G′: closed symbols; G″: open symbols) of the PANI/PDVMA composite based ER fluid with a fixed angular frequency of 6.28 rad s-1 under various electric field strengths.



Fig. 7. Variation of the elastic stress component G′γ with strain amplitude γ for the PANI/PDVMA composite based ER fluid.


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Fig. 8. Storage modulus G′ and loss modulus G″ under various electric field strengths (a) and without electric field strengths (b) in a frequency sweep test for the PANI/PDVMA composite based ER fluid.



Fig. 9. Shear stress of PANI/PDVMA composite based ER fluids at a fixed shear rate (constant shear rate = 1 1/s) in the electric field with a square voltage pulse (t = 20 s).


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Fig. 10. (a) Permittivity and loss factor as a function of the frequency; (b) Cole-Cole plot for PANI/PDVMA composites based ER fluid (10 vol %, 50cS silicone oil). Eq. (4) generates the fitting line.



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Table 1. Fitting parameters of Bingham model equations to the flow curves of PANI/PDVMA composites based ER fluid. Electric field strength (kV/mm) Parameters 0.5

1.0

1.5

2.0

2.5

3.0

τ0

7.47

23.1

44.2

77.4

133

186

η∞

0.0838

0.0914

0.101

0.122

0.153

0.179

Table 2. Fitting parameters of CCJ model equations to the flow curves of PANI/PDVMA composite based ER fluid. Electric field strength (kV/mm) Parameters 0.5

1.0

1.5

2.0

2.5

3.0

τ0

7.47

23.1

44.2

77.4

133

186

t1

0.01

0.009

0.015

0.01

0.009

0.007

α

0.6

0.3

0.5

0.5

0.7

0.5

β

0.7

0.9

0.99

0.8

0.9

0.99

η∞

0.0838

0.0914

0.101

0.122

0.153

0.179

t2

0.99

0.98

0.8

0.9

0.99

0.9

Table 3. Fitting parameters of the Cole-Cole equation to the dielectric spectra of PANI/PDVMA composite based ER fluids. Sample

ε

ε∞

PANI/PDVMA

1.211

0.76

∆ε 0.451


λ 0.0015

α 0.6

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Table of Contents Electro-responsive smart core/shell structured polyaniline

composite particles were

synthesized through Pickering emulsion polymerization, using poly(divinylbenzene-altmaleic anhydride) nanoparticles as a solid surfactant and then adopted as an electrorheological (ER) material when dispersed in silicone oil. Their rheological properties measured using a rotational rheometer under various electric field strengths demonstrated a tunable and reversible phase transition from liquid-like to viscoelastic solid-like.


Polymeric Nanoparticle-Coated Pickering ... - ACS Publications

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