poly(diphenylamine) composite microparticles and their electro-response

Polymer 182 (2019) 121851

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Facile fabrication of core-shell typed silica/poly(diphenylamine) composite microparticles and their electro-response Hyo Seon Jang a, Seung Hyuk Kwon a, Jin Hyun Lee b, **, Hyoung Jin Choi a, * a b

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

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrorheological Core-shell Silica poly(diphenylamine)

Novel semi-conductively encapsulated microspheres were fabricated by coating poly(diphenylamine) (PDPA) onto spherical silica particles via chemical oxidative polymerization. Their nanoscale-encapsulated spherical structure and PDPA shell thickness of approximately 30 nm were observed by both SEM and TEM. Fourier transform infrared spectroscopy confirmed the successful surface-modification of silica particles with PDPA, which was probably obtained through π π* stacking interactions between the aromatic groups of PDPA and n[3-(trimethoxysilyl) propyl]aniline as a grafting agent attached to the silica surface. Thermogravimetric analysis revealed the reasonable thermal stability of the fabricated particles. The electrorheological (ER) suspension (10 vol%) with the silica/PDPA composite microspheres was fabricated without a typically used de-doping process. Their dynamic and elastic yield stresses increased with applied electric field strength, following the power-law model (~E1.5). Their shear stress and their solid-like behavior were explained using the Herschel-Bulkley model and Schwarzl equation, respectively. The immediate and reversible response of the ER fluids was observed using on-off tests.

1. Introduction

including ER dampers, haptic devices, ER tactile displays, actuators, and rehabilitation devices, owing to their quick response time to the appli­ cation of an electrical field, feasibly controllable characteristics, and low energy consumption [5–9]. For ER fluids, diverse semiconducting nano- and microparticles with controllable size/shape have been developed for electro-responsive smart systems using various materials such as conducting polymers, inorganic materials, and inorganic/organic composite particles [10,11]. Among them, a variety of conductive polymers with a π-conjugated structure, such as polyaniline (PANI) [12,13], poly(diphenylamine) (PDPA) [14], polypyrrole (PPy) [15,16], and polythiophene [17], have been used extensively. In addition, inorganic/polymer hybrid composite material-based particles with a core-shell structure (i.e., clay/polymer composites and metal/polymer composites) are attracting increasing attention compared to pure inorganic materials or organic polymers [18–22]. Furthermore, the size and morphology of the particles composing the ER fluids also affect significantly their ER characteristics. For core-shell structured particles, their suspension state can be tuned by the size, density, and surface state of the core particle. In addition, the synergic effects on their properties can be obtained from the advantages

Smart and intelligent soft materials, whose characteristics can be altered by a range of input stimuli, such as temperature, pH, stress, and electric or magnetic field, have drawn a huge attention because of their tunable properties depending on the desired applications [1–3]. Among them, electro-responsive electrorheological (ER) fluids are generally suspensions with semiconducting particles dispersed in an electrically insulating fluid, exhibiting promising rheological properties that can be altered immediately before and after employing an electric field, being influenced by the electrical field strength. With an input electrical field, the ER fluids transform rapidly and reversibly from an original liquid-like to a solid-like phase via forming chain-like or columnar structure of the randomly dispersed particles parallel to the input elec­ trical field. This is induced by a difference of the dielectric constant between the dispersed phase and the continuum phase [4]. With an off-electric field, however, the ER fluid with the particles suspended randomly exhibited a Newtonian fluid-like behavior. These structural changes have considerable effects on the rheological behaviors. There­ fore, they have been applied to a range of electro-responsive systems,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.H. Lee), [email protected] (H.J. Choi). https://doi.org/10.1016/j.polymer.2019.121851 Received 28 July 2019; Received in revised form 20 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.

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of both the core and shell [23]. In particular, the silica cores, used in this study, have several advantages. Because the silica surface can be func­ tionalized easily and have a spherical and monodispersed shape, it not only promotes the formation of the core-shell structure of particles but also simplifies the entire process [24–26]. Moreover, the silica core has reasonable mechanical strength and higher thermal stability than the organic core. It can be also noted that compared to the silica particles which are a wet-based ER material, the conducting polymer coated silica particles make the ER fluid dry-based. Among various conducting polymers, PANI has been tested exten­ sively for ER fluids because of its simple fabrication with cheap price, environmentally benign characteristics, and high conductivity. Although PANI has many advantages, it has drawbacks, such as poor mechanical strength and insolubility in common organic solvents because of hydrogen bonding between the amino groups. For these reasons, many other derivatives and new methods have been studied to solve these drawbacks. Recently, the PDPA used in this study, a PANI derivative, has attracted considerable interest for ER fluids. As reported by Kim et al. [27], PDPA has superior thermal stability, optical properties, and solu­ bility in several organic solvents including tetrahydrofuran and chlo­ roform compared to PANI [28]. Ozkan et al. [29] investigated that the thermal stability of PDPA was higher than that of PANI. Other con­ ducting polymers (i.e., PANI and polythiophene) generally have high conductivity, resulting in electrical short-circuits during the ER tests. To improve this problem, the electrical conductivity is typically adjusted by a doping/de-doping process using an acid or a base solution. On the other hand, because PDPA particle possesses relatively lower electrical conductivity than that of particles made from pure PANI, it can be used directly for the fabrication of an ER suspension without an additional doping/de-doping step [30]. In addition, several studies on the appli­ cation of PDPA-based composite to electrochemical rechargeable bat­ teries [31,32] and sensor have been reported [33,34]. This paper presents a facile fabrication of novel core/shell typed silica/PDPA composite microsphere through a chemical oxidative polymerization at low temperatures and the ER behaviors of the microparticle-based ER suspensions. Previously we have synthesized core-shell structured polystyrene (PS)/PDPA particles [35] and studied their ER properties. However, as an organic core, PS particle has a problem such that the flexibility of its chemical surface modification is limited. Since the shape and performance of the particles vary little, the most reliable way to introduce functional groups is known to grafting other monomers onto the precursor [36]. Therefore, silica particle was introduced as a core in this study. Compared to previously reported silica/PANI core-shell particles, the synthesized silica/PDPA core-shell particles in this study not only are more compatible with organic sol­ vents than the PANI used as the shell in silica/PANI core-shell particles but also possess the advantage such that they can be directly applied to ER applications without a dedoping process. Thus, the surface of one micron-sized spherical silica particles was first functionalized with n-[3-(trimethoxysilyl)propyl]aniline (TMSPA) as a grafting agent. The TMSPA-modified silica particles were encapsulated with PDPA (shell), which was probably obtained by π-π* stacking interactions between the aromatic rings of PDPA and TMSPA. The core-shell structure and ther­ mal stability of the PDPA-encapsulated silica composite particles were characterized. To examine the electro-responsive properties of the composite microspheres for smart fluids, ER suspensions (10 vol%) with the encapsulated microspheres in the silicone oil were prepared, and their ER behaviors, including the dynamic and elastic yield stresses, dynamic and relaxation moduli, and shear stress with shear rate were examined by varying the electric field strength, and their steady-shear flow curves were fitted using the Herschel-Bulkley equation. Their solid-like behavior was also confirmed by the shear relaxation moduli.

2. Experimental 2.1. Materials Silica particles (SiO2, 1 μm, 99.9%, Alfa Aesar, England) and diphe­ nylamine (DPA) (99% purity, Sigma-Aldrich) as a monomer were adopted for core and shell part of semiconducting spherical particles, respectively. Ammonium persulfate (APS) (�98.0% purity, Daejung Chem., Korea) and N-[3-trimethoxysilyl)-propyl]aniline (TMSPA) (Aldrich) were used as an initiator and a grafting agent, respectively. In addition, hydrochloric acid (Junsei Chemical, Japan), toluene (99.8% purity, Sigma-Aldrich, USA), and ethanol (95% purity, Samchun Pure Chem., Korea) were applied for polymerization. All these chemicals were adopted as received. 2.2. Sample preparation 2.2.1. Modification of silica surface One micron-sized silica particles (10 g) were added to an aqueous HCl solution (1 M, 150 mL) with 24 h stirring. The mixture was washed 5–6 times with deionized (DI) water and dried at 60 � C under vacuum condition. The resulting particles were suspended in toluene (150 mL), and TMSPA (10 mL) was added as a grafting agent and stirred for two days. The TMSPA-modified silica microspheres were washed several times using a 1:1 (v/v) mixture solution of toluene and ethanol. They were then dried in a vacuum oven (60 � C). 2.2.2. Fabrication of the silica/PDPA composite microsphere The surface-modified silica spherical particles (1.875 g) were dispersed in DI water (100 mL) under vigorous mechanical stirring for half an hour at room temperature. The DPA (1.875 g, the 1:1 w/w of DPA/silica), as a monomer dissolved in ethanol (10 mL), was added to the dispersion and ultrasonicated for 1 h. In this process, the π electrons in the aromatic rings of the aniline-functionalized silica and DPA monomers were considered to be bonded via π-π*stacking interactions, causing non-covalent functionalization of the core microspheres with PDPA. The resulting solution was moved to a three-neck flask and stirred at 5 � C, while the initiator APS (2.46 g) dissolved in a 3 M HCl aq. so­ lution (12 g) was added dropwise to the reactor. The chemical oxidation polymerization proceeded for 24 h with vigorous stirring to obtain a uniform PDPA coating on the core surface. The resulting suspension was filtered, cleaned several times with ethanol and DI-water, and then separated by ultracentrifuge. Finally, the obtained core-shell structured silica/PDPA composite microparticles were dried under vacuum condi­ tion (60 � C) for one day. 2.2.3. Fabrication of poly(diphenylamine) particles The DPA as a monomer (3 g) was dissolved in ethanol (100 mL) with stirring for several min, and 3 M HCl aq. solution (60 g) solution was added and then transferred to a three-neck flask reactor. The reaction solution was reacted at 5 � C while being stirred continuously. Subse­ quently, we slowly added drop-wise APS (1.5 g) solution dissolved in 3 M HCl aq. solution (6 g) over 30 min. Mechanical stirring was carried out for 12 h and then the resultant was mixed with ethanol and water, and washed several times. The washing process was performed for 20 min at 6400 rpm each time using a super-centrifuge. The final prod­ uct was dried in a 60 � C vacuum oven for one day. 2.2.4. Fabrication of ER fluids The electrical conductivity of the silica/PDPA composite micro­ spheres tested via a four-probe resistivity meter (MCP-HT450, Mitsu­ bishi Petrochem., Japan) was 3.0 � 0.3 � 10 11 S/cm. They were adopted directly without an extra doping/de-doping process which has been typically used for conducting particles such as PANI, because the conductivity of the particles was within the proper range of semi­ conductors that is suitable for the ER tests. The silica/PDPA composite 2

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Fig. 1. SEM images of a pure silica particle (a) and a silica/PDPA (core/shell) composite microsphere (b), and EDS spectrum of the core-shell structured silica/PDPA composite particles (c).

microsphere-based ER suspension was prepared by suspending the synthesized spheres (10 vol%) in non-conducting silicone oil (the den­ sity of 0.965 g/cm3, the shear viscosity of 0.1 Pa s, Shin-Etsu, Japan). 2.3. Characterization 2.3.1. Characterization of the core-shell typed silica/PDPA composite particles The microstructures of the synthesized silica/PDPA composite mi­ crospheres were examined by high-resolution scanning electron micro­ scope (SEM) (SU-8010, Hitachi Co., Japan) and transmission electron microscope (TEM) (CM200, Philips, Netherlands). Chemical composi­ tion of the microspheres was also examined by energy-dispersive X-ray spectroscope (EDS) and Fourier-transform infrared spectroscope (FT-IR) (VERTEX 80V, Bruker). Particle density of the composite microspheres was measured using a pycnometer (Accupyc 1330, Gas pycnometer, USA). Thermal properties of the microspheres and the mass percentage of PDPA coated on the silica surface were investigated by thermogra­ vimetric analysis (TGA) (TA Q50, USA) in an N2 atmosphere by heating at 10 � C/min from 25 to 800 � C. 2.3.2. Characterization of ER fluids After preparing the ER suspension with the semiconducting coreshell structured silica/PDPA composite microspheres (10 vol%), ultrasonication, and vortex mixing of the suspension proceeded for 2 h to improve the dispersion of the particles, prior to each ER measurement. The ER behaviors of the suspension such as shear stress, shear viscosity, dynamic and elastic yield stress, and dynamic and relaxation moduli were studied using a rotational rheometer (MCR 302, Anton-Paar GmbH, Austria) at room temperature. The geometry used in the mea­ surements was a Couette-type (CC 17, Anton-Paar, Austria), and the input electrical field was increased from 0.0 to 2.5 kV/mm by 0.5 kV/ mm.

Fig. 2. TEM images of the pure silica particles with smooth surface (a–b) and the core-shell structured silica/PDPA composite microspheres with rough sur­ face (c–d). The red dashed line describes the interface between silica core and PDPA shell (the thickness ~30 nm).

3. Results and discussion Fig. 1(a)–(b) present SEM photos of both pristine silica particle and silica/PDPA composite microparticles, respectively, proving the PDPA coating on the core surface. The pure silica particle had a smooth surface and a spherical shape with a mean diameter of 1 μm. On the other hand, the silica/PDPA composite microsphere appeared rougher than the pure 3

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Polymer 182 (2019) 121851

Weight [%]

100

80

60

40

20

PDPA silica/PDPA silica

0

200

400

600

800

Temperature [ C] Fig. 3. FT-IR absorption spectra of PDPA (black dotted line), the core-shell structured silica/PDPA composite microspheres (red solid line), modified sil­ ica (pink dash dotted line) and pure silica particles (blue dot-dot-dashed line).

Fig. 4. TGA curves of PDPA (black dotted line), the core-shell structured silica/ PDPA composite microspheres (red solid line), and pure silica particles (blue dot-dot-dashed line).

silica surface, meaning that the PDPA shell made the composite surface rough. The particle size of the silica/PDPA composite microsphere was slightly larger than that of the pure silica particle. All these findings show that the PDPA was coated successfully on the surface of core silica. Fig. 1(c) presents the EDS spectrum of the synthesized silica/PDPA composite particles. The spectrum shows the carbon and nitrogen ele­ ments of PDPA presented on the silica surface (without carbon and ni­ trogen), which also confirmed the successful coating of PDPA on the silica surface. Fig. 2 shows TEM photos of the pristine silica particle and silica/ PDPA composite particles; the PDPA shell surrounding the silica core can be observed clearly. Fig. 2(a) and (b) show TEM photos of pure silica particle and a four-fold magnification, respectively. In addition, Fig. 2(c) and (d) present an image of the core-shell silica/PDPA composite microsphere and a four-fold magnification, respectively. The rough gray outer shell part surrounding the black inner core part can be seen. The inner black part was found to be a silica particle, as shown in Fig. 2(a) and (b). The rough gray part was most probably a PDPA coating layer, and the thickness of the shell was determined to be ca. 30 nm and did not appear to be significantly affected by the size of the silica particle based on the observation of other composite particles. The density of the synthesized silica/PDPA composite particles decreased from 2.65 g/cm3 of that of silica to 2.04 g/cm3 after coating PDPA on silica particles. The chemical components of silica/PDPA composite microsphere were identified by FT-IR spectroscopy. Its measurements were per­ formed in the mid-infrared region (wavenumbers from 400 to 4000 cm 1) under ambient conditions. Fig. 3 shows the FT-IR spectra of PDPA, silica/PDPA microspheres, modified silica, and pristine silica particles. Characteristic peaks of the PDPA at 1591, 1311, and 1172 cm 1 are considered to be originated from N–H bending vibration, C–N stretching mode, and C–H in-plane bending vibration, respectively [31]. In addition, characteristic peaks at 807 and 472 cm 1 are related to the main peaks of the stretching vibration in the Si–O–Si. In addition, anti-symmetric stretching in the Si–O–Si and Si–OH bending were observed at 1108 and 948 cm 1, respectively. Finally, the red solid line in Fig. 3 indicates that the core-shell structured silica/PDPA composite microspheres exhibit absorption peaks that are also observed in both spectra of pure silica and PDPA. This confirms the successful preparation of the composite microsphere. Additionally, it can be seen from Fig. 3 that spectrum of the silica modified with N-[3-(trimethoxysilyl) propyl] aniline (pink dash dotted line) shows a peak similar to the red solid line. This demonstrates that the core and shell were combined by π-π* stacking interaction because no additional absorption peak was

observed after grafting onto the silica surface. Fig. 4 shows the thermal stability of the pristine silica, pure PDPA, and silica/PDPA composite microparticles, which was determined by TGA from 25 to 800 � C with 10 � C/min heating rate. In the three curves, the weight loss below 100 � C was induced by moisture loss. In the dotdot-dashed line (pure silica), the first weight loss was assigned to the elimination of water molecule up to 150 � C. The following small weight loss starting from 400 � C was associated to thermal decomposition of oxygen functional group on the surface of silica particle [37]. In the dotted line (pure PDPA), significant weight loss was observed in the range of 200–400 � C because of the thermal degradation of the main chain of PDPA [14]. In the solid line (silica/PDPA microparticles), the weight loss was indicated by the thermal decomposition of not only the PDPA chains but also the oxygen functional groups of the pure silica. Here, based on the TGA results of thermal decomposition from Fig. 4, the PDPA shell thickness could be calculated using its mass balance. The weight ratio of each component was calculated from (1.1) (1.3) as follows [38]: V2 p2 � 100% ¼ m V1 p1 þ V2 p2

(1.1)

4 V1 ¼ π R3 3

(1.2)

4 h V2 ¼ π ðR þ dÞ3 3

i R3

(1.3)

where V1 and ρ1 denote the volume and density of the silica part, and V2 and ρ2 are the volume and density of the PDPA shell, respectively. The R is the core radius, d is the PDPA shell thickness, and m is the mass fraction of PDPA shell part. The R of the pure silica spheres was calcu­ lated to be 1.0 μm from both SEM and TEM images. The measured densities of pristine silica and PDPA were 2.64 and 1.20 g/cm3, respectively. The calculated d and m values were 37 nm and 9.8%, respectively, which are the same as the values determined from the TGA curve. The TEM photos and thermal analysis data were compared with the calculated results. The results showed that the thickness obtained from the three different approaches were similar. The electric stimulus-response of the silica/PDPA ER suspension inserted between two aluminum electrodes was observed directly by an optical microscope (OM) in the presence of an input electrical field. Fig. 5(a) and (b) presents OM images showing the remarkable change in the microstructure of the silica/PDPA composite particles in the ER 4

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Fig. 5. Optical microscopy (OM) images of the ER suspension of the core-shell structured silica/PDPA particles dispersed in silicone oil; (a) without an electric field (b) with an electric field (applied voltage: 200 V). Table 1 Three parameters (τdy, K and n) of the Herschel-Bulkley (HB) model determined where the shear stress data of the core-shell structured silica/PDPA microspheres-based ER fluid as a function of shear rate for various electric field strengths are well fitted with HB model. Model Herschel-Bulkley

Parameters

τdy K n

Electric field strength [kV/mm] 0.5

1.0

1.5

2.0

2.5

3.058

4.438

6.679

8.638

10.456

0.071 0.971

0.074 0.976

0.059 0.978

0.045 0.980

0.04 0.985

fluids before and after applying an electrical field, in which the semi­ conducting silica/PDPA particles migrate immediately along the field direction due to the electrostatic attraction of the induced dipoles, forming a chain-like structure. In contrast, initially the microspheres were suspended uniformly between the two electrodes without an electric field. Owing to this chain-like structure, ER fluids exhibit a solidlike property that can sustain an external shear force until the shear stress stretches to a yield stress. The effect of the electrical field strength on the flow curve of an ER suspension (10 vol% silica/PDPA microspheres) was observed from a controlled shear rate (CSR) technique. Fig. 6 shows the shear-ratedependent shear stress (τ) (a) and shear viscosity (b) of the ER suspen­ sion in the shear rate (_γ ) range, 0.1–200 s 1, under different electrical field strengths. Its shear stress was linearly dependent on a shear rate without an electric field (Fig. 6(a)), indicating that the ER suspension behaved aslike a Newtonian fluid, while with an input electric field, the shear stress of the ER fluid demonstrated a wide plateau region due to the chain-like structure formation of the particles. Both plateau region and yield stress of the ER fluid expanded with input electrical field strengths, most probably due to the increased electrostatic attractions between microspheres, resisting a larger hydrodynamic force. The flow curve behavior with a yield stress and plastic behavior of the ER fluid were analyzed using a Herschel-Bulkley (HB) model as follows [39]:

τ ¼ τdy þ K γ_ n ;

Fig. 6. Shear stress, τ, (a) and shear viscosity, η, (b) of the ER fluid with the core-shell structured silica/PDPA particles (10 vol%) as a function of shear rate ​ γ_ , for electric field strengths from 0 to 2.5 kV/mm (The solid lines are fitting curves according to the Herschel-Bulkley model).

for

τ � τy

(2)

where τdy is the dynamic yield stress, K is the consistency index, and n is the flow index. The shear stress data points of the ER suspension ob­ tained for various electrical field strength were fitted well using the HB equation (Fig. 6(a)), indicating that the ER fluids can be explained using the HB model. Table 1 lists the three parameters (τdy, K, and n) describing the rheological behaviors of the ER fluid for various electric 5

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Polymer 182 (2019) 121851

Fig. 9. Elastic stress (G0 γ) of the ER fluid with the core-shell structured silica/ PDPA composite microspheres as a function of strain.

Fig. 7. ER efficiency, e, of the core-shell structured silica/PDPA composite microspheres-based ER fluid.

4

4

10

0 kV/mm 0.5 kV/mm 1.0 kV/mm 1.5 kV/mm 2.0 kV/mm 2.5 kV/mm

3

'

10

3

10

2

10

1

10

0

10

2

10

1

10

''

Loss modulus, G [Pa]

Storage modulus, G [Pa]

10

0

10

-1

10

-1

-4

10

-3

-2

10

10

-1

10

10

0

10

Strain Fig. 8. Storage modulus, G0 , (closed symbols) and loss modulus, G00 , (open symbols) of the core-shell structured silica/PDPA composite microspheresbased ER fluid as a function of strain.

Fig. 10. Yield stress, τy, (dynamic yield stress, τdy, and elastic yield stress, τey, of the 10 vol % of the core-shell structured silica/PDPA microspheres-based ER fluid as a function of electric field strength.

field strengths. All the n values obtained showed less than 1.0, which indicates the shear thinning behavior of the fluids with an applied electrical field. This concurred with the decrease in shear viscosity of the ER fluid with an increased shear rate for all applied electrical field (Fig. 6 (b)), and the ER suspension also exhibited a decrease in shear viscosity. With an input electric field, however, the magnitude of the shear vis­ cosity increased with increased applied field strength for an overall shear rate, which is in contrast to the constant fluid viscosity without an electric field. To estimate the electro response of the silica/PDPA particle ER suspension, its ER efficiency was calculated based on the following equation [40]: e¼

ðτE

τ0 Þ τ0

� 100ð%Þ or

ðηE

η0 Þ η0

� 100ð%Þ

ER fluid, formed by an input electrical stimulus, enhances the resistance to shear deformation by the shear stress as the field strength increases. The various viscoelastic behaviors of the ER suspensions were measured in the linear viscoelastic region (γLVE) by a dynamic oscillation test using a rheometer. First, the strain amplitude from 10 4 to 1 was input to the silica/PDPA particle ER suspension at a constant value of angular frequency (6.28 rad/s) to obtain the γ LVE. Fig. 8(a) shows the strain-dependent storage (G0 ) and loss modulus (G00 ) of the ER suspen­ sion. The G0 values were much higher than those of G00 in the plateau region in the initial strain at all electric field strengths. This plateau region represents the LVE region where the internal structure of the ER fluid is unbroken regardless of the applied strain, indicating that the structures are dominated by the elastic property. The critical strain value of the LVE region (γ LVE Þ, beyond where both G0 and G00 decrease rapidly due to the broken down of the internal structure in the fluid [41], was determined, and its value of 0.0003 was used as the set strain value for further measurements of viscoelastic properties of the ER fluid. Fig. 9 presents the elastic stress, τ0 , of the ER fluid, which was ob­ tained by reanalyzing the G0 data measured in the strain amplitude sweep test (Fig. 8) using the relationship: τ0 ¼ G0 γ [42]. The elastic stress of the ER suspension increased linearly with a strain until its linear viscoelastic limit was reached for each electrical field strength. The

(3)

where τE and ηE refer to the shear stress and shear viscosity of the ER fluids under an input electrical field, respectively, and τ0 and η0 corre­ spond to the cases where there is no electric field. Fig. 7 represents that the ER efficiency decreases sharply with an increased shear rate for all applied electrical field strengths, but they increased with increased applied field strength for a whole shear rate range. This is probably due to the fact that the chain-like structure of the silica/PDPA particles in the 6

Polymer 182 (2019) 121851

4

4

10

3

10

'

10

3

10

2

10

1

10

''

0 kV/mm 0.5 kV/mm 1.0 kV/mm 1.5 kV/mm 2.0 kV/mm 2.5 kV/mm

2

10

polarization model provides a slope m equal to 2.0, and the slope m is 1.5 from the conduction model due to conductivity difference between the particle and medium liquid. In this study, the power law index m was determined to be 1.5, implying that the silica/PDPA particles-based ER suspension is associated to the conduction model [45]. In addition, the dynamic yield stress was slightly larger than elastic yield stress at all electric field strengths. Dynamic yield stress was obtained under a sus­ tained shear strain, which is higher than that of the strain affecting the elastic yield stress [46]. The dynamic viscoelastic characteristics of the silica/PDPA ER sus­ pension were investigated by performing frequency sweep tests at the strain of 0.0003 in the γLVE for different electrical field strengths. As given in Fig. 11, without an electric field, the G0 and G00 of the ER sus­ pension increase with an angular frequency, reflecting the liquid-like behavior. On the other hand, with an electrical field, the G0 of the ER fluid increases with an electrical field strength, and a constant plateau of G0 is observed for a wide frequency range, which is due most probably to the solid-like behavior induced by the strong chain-like structure of the particles. In addition, the G0 values are generally higher than those of G00 , implying that the ER suspension tends to have more dominant elastic behavior than viscous behavior in the presence of electric field. Furthermore, the solid-like property of the ER suspensions was analyzed by studying their shear stress relaxation. Actually, the stress relaxation of the suspensions is generally not easy to obtain directly due to their inherent properties (i.e., liquid state) and the limitation of equipment to monitor the behavior. Therefore, to determine the relax­ ation characteristics of the silica/PDPA-based ER fluid, the stress relaxation modulus (G(t)) data as a function of time (Fig. 12) was calculated from the G0 (ω) and G00 (ω) values deduced from the frequency sweep tests (Fig. 11) via a following Schwarzl equation [47]:

Loss modulus, G [Pa]

Storage modulus, G [Pa]

H.S. Jang et al.

1

1

2

10

10

10

Angular frequency [rad/s] Fig. 11. Storage modulus, G0 , (closed symbols) and loss modulus, G00 , (open symbols) of the core-shell structured silica/PDPA microspheres-based ER fluid as a function of angular frequency at the set strain of 0.0003 in the linear viscoelastic region.

point where the elastic stress curve begins to deviate from the linearity because the internal structure of the semi-conducting silica/PDPA par­ ticles in the ER fluid begins to gradually be destructed or yield, referring to the elastic yield stress, τey, which is noted by the green circle. Both dynamic, τdy, and elastic yield stress, τey, of the ER suspension, which were estimated from the CSR and strain amplitude sweep tests, respectively, were plotted as a function of the input electrical field strength, as described in Fig. 10. The τdy was obtained from the intercept of the shear stress at a zero shear rate limit (Fig. 6(a)). According to the ER mechanism, both the dynamic and elastic yield stresses of the ER suspensions followed the power-law relationship:

0

GðtÞ ffi G ðωÞ

The power-law index, m, varies from 1.0 to 2.0, depending on the morphology of shape and size of particles along with particle concen­ tration in the fluids and the applied electric field strength [43,44]. The

Relaxation modulus [Pa]

(5)

Without an electric field, G(t) values of the fluid decrease with time, indicating a liquid-like behavior. At an electrical field strength of 0.5 kV/mm, G(t) slowly relaxes in the initial stages and then reaches a plateau, meaning that 0.5 kV/mm is not high enough to induce the strong chain-like structure of silica/PDPA particles in the fluid. When

(4)

τy ∝Em

0:566G00 ðω = 2Þ þ 0:203G00 ðωÞ

4

10

0 kV/mm 0.5 kV/mm 1.0 kV/mm 1.5 kV/mm 2.0 kV/mm 2.5 kV/mm

3

10

-1

10

Time [s] Fig. 12. Relaxation modulus, G(t), of the core-shell structured silica/PDPA microspheres-based ER fluid, as calculated with G0 (ω) and G00 (ω) obtained from the frequency sweep using the Schwarzl equation [G(t) ffi G0 (ω) 0.566G00 (ω/2) þ 0.203G00 (ω)]. 7

Shear stress, [Pa]

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viscoelastic materials experience time-dependent reversible behavior [48]. As described in Fig. 14, the silica/PDPA-based ER fluid exhibited viscoelastic characteristics with a time-dependent strain behavior in both creep and recovery processes. In addition, as the electric field strength increases, the ER fluid behaved more like a solid. The recovery rate (χ) was calculated to assess the elasticity of the ER fluid as a following equation (6):

0.5 kV/mm 1.0 kV/mm 1.5 kV/mm 2.0 kV/mm 2.5 kV/mm

10

χ¼ 5

γR γ max

(6)

where γR means the recoverable strain and γ max is the maximum strain. The calculated values χ were 0.55 at l.0 kV/mm and 0.87 at 2.0 kV/mm. As a result, a higher recovery rate was shown due to the elastic inter­ action of the solidified particles at higher electric field strength. 0

20

40

60

Time [s]

80

4. Conclusions

100

Semiconducting core-shell typed silica/PDPA composite micropar­ ticles were fabricated by chemical oxidative polymerization. The PDPA coating on the silica particles was obtained through π π* stacking interaction between the aromatic groups of PDPA and TMSPA as a grafting agent attached to the silica surface. A core-shell structure of the PDPA-encapsulated silica microspheres with a rough surface compared to the pure silica surface was identified by SEM and TEM, and its thickness (37 nm) were determined. The ER response of the silica/PDPA composite microsphere was examined by rheological measurements of steady shear, dynamic oscillation, and creep tests. The electric-field dependent flow curves were well analyzed using the Herschel-Bulkley model, while both dynamic and elastic yield stresses of the ER fluid were correlated with applied electrical field strengths, ~E1.5. The

Fig. 13. Shear stress of the core-shell structured silica/PDPA microspheres-ER fluid at a fixed shear rate of 1 s 1 on and off electric field, with applying a square voltage pulse (t ¼ 20 s).

the input electrical field strength was >0.5 kV/mm, the G(t) curves exhibited no stress relaxation, showing a plateau region, due most likely to the solid-like behavior resulting from the chain-like structure formed from the strong attractive forces between the well-polarized particles. In addition, the value of G(t) also increased with an applied electric field strength because of the larger number of polarized particles. The short time relaxation behavior of the silica/PDPA particle ER suspension could be predicted from the G(t) obtained using the Schwarzl equation. The sensitive and reversible ER response of the silica/PDPA microsphere-based ER suspension was examined by on-off tests, which were carried out by switching the voltage on and off (t ¼ 20 s) under a constant shear rate (1.0 s 1) with a periodically applied square voltage pulse. Fig. 13 presents the shear stress of the ER suspension determined from on-off tests. The shear stress increases momentarily immediately after applying the electric field (by voltage), and was maintained with the field applied. The shear stress decreased promptly to zero as soon as the field was removed. The immediate transformation in the shear stress of the silica/PDPA microspheres-based ER suspension without any hysteresis emphasizes the superior tune-ability and reversibility of the ER performance. Therefore, the silica/PDPA microsphere-based ER fluid exhibiting superior ER behaviors is a desirable candidate for ER systems. The core/shell structured silica/PDPA composite microsphere-based ER fluid, whose properties could be tuned by varying the electric field strength, can be applied to a range electro-responsive system, such as shock absorbers, vibration dampers, and actuators(see Scheme 1). Fig. 14 exhibits the creep and recovery test of the silica/PDPA-based ER fluid as a function of time. The test was performed under a constant shear stress (10 Pa) initially and then the stress applied was removed at a certain moment. The strain was measured over time. In general, when a constant shear stress is applied, the ideal elastic material deforms instantly and returns as soon as the stress is removed. On the other hand,

1.0 kV/mm 2.0 kV/mm

Strain (%)

0.06

0.04

0.02

0.00

10

20

30

40

Time (s)

50

60

70

Fig. 14. Strain as a function of time of the silica/PDPA microparticle-based ER fluid under a creep and recovery experiment. Shear stress ¼ 10 Pa was applied during the first 38 s, and then shear stress ¼ 0 Pa. Applied electric field strength: a black square line for 1 kV/mm and a red circle line for 2 kV/mm.

Scheme 1. Schematic diagram of the synthetic process of core-shell structured silica/PDPA composite microspheres. 8

H.S. Jang et al.

Polymer 182 (2019) 121851

superior sensitivity and reversibility of the ER fluid were also observed by the on-off test. Based on the experimental results of various ER tests, we observed that the core-shell typed silica/PDPA microparticles are suitable as for ER materials.

[22] N. Mohamadian, H. Ghorbani, D.A. Wood, M.A. Khoshmardan, A hybrid nanocomposite of poly(styrene-methyl methacrylate- acrylic acid)/clay as a novel rheology-improvement additive for drilling fluids, J. Polym. Res. 26 (2019) 14. [23] S. Goswami, T. Brehm, S. Filonovich, M.T. Cidade, Electrorheological properties of polyaniline-vanadium oxide nanostructures suspended in silicone oil, Smart Mater. Struct. 23 (2014), 105012. [24] S.H. Joo, J.Y. Park, C.K. Tsung, Y. Yamada, P. Yang, G.A. Somorjai, Thermally stable Pt/mesoporous silica core–shell nanocatalysts for high-temperature reactions, Nat. Mater. 8 (2009) 126. [25] S. Ciftci, A. Mikosch, B. Haehnle, Ł. Witczak, A.J. Kuehne, Silica core/conjugated polymer shell particles via seeded Knoevenagel dispersion polymerization–laser action in whispering gallery mode resonators, Chem. Commun. 52 (2016) 14222–14225. [26] L. Hao, C. Zhu, C. Chen, P. Kang, Y. Hu, W. Fan, Z. Chen, Fabrication of silica core–conductive polymer polypyrrole shell composite particles and polypyrrole capsule on monodispersed silica templates, Synth. Met. 139 (2003) 391–396. [27] M.H. Kim, D.H. Bae, H.J. Choi, Y. Seo, Synthesis of semiconducting poly (diphenylamine) particles and analysis of their electrorheological properties, Polymer 119 (2017) 40–49. [28] A.M. Showkat, K.-P. Lee, A.I. Gopalan, S.-H. Kim, Synthesis and chiro-optical properties of water processable conducting poly(diphenylamine) nanocomposites, Macromol. Res. 15 (2007) 575–580. [29] S.Z. Ozkan, G. Karpacheva, A. Orlov, M. Dzyubina, Thermal stability of polydiphenylamine synthesized through oxidative polymerization of diphenylamine, Polym. Sci. Ser. B 49 (2007) 36–41. [30] B. Wang, C. Liu, Y. Yin, S. Yu, K. Chen, P. Liu, B. Liang, Double template assisting synthesized core-shell structured titania/polyaniline nanocomposite and its smart electrorheological response, Compos. Sci. Technol. 86 (2013) 89–100. [31] A.I. Gopalan, K.-P. Lee, K.M. Manesh, P. Santhosh, Poly(vinylidene fluoride)– polydiphenylamine composite electrospun membrane as high-performance polymer electrolyte for lithium batteries, J. Membr. Sci. 318 (2008) 422–428. [32] M. Baibarac, I. Baltog, S. Lefrant, P. Gomez-Romero, Polydiphenylamine/carbon nanotube composites for applications in rechargeable lithium batteries, Mater. Sci. Eng. B 176 (2011) 110–120. [33] K.M. Manesh, A.I. Gopalan, K.P. Lee, P. Santhosh, K.D. Song, D.D. Lee, Fabrication of functional nanofibrous ammonia sensor, IEEE Trans. Nanotechnol. 6 (2007) 513–518. [34] T. Permpool, A. Sirivat, D. Aussawasathien, Polydiphenylamine/zeolite Y composite as a sensor material for chemical vapors, Sens. Actuators B Chem. 220 (2015) 91–100. [35] M.H. Kim, H.J. Choi, Core-shell structured semiconducting poly (diphenylamine)– coated polystyrene microspheres and their electrorheology, Polymer 131 (2017) 120–131. [36] A. Bhattacharya, B.N. Misra, Grafting: a versatile means to modify polymers Techniques, factors and applications, Prog. Polym. Sci. 29 (2004) 767–814. [37] J.Z. Zheng, X.P. Zhou, X.L. Xie, Y.W. Mai, Silica hybrid particles with nanometre polymer shells and their influence on the toughening of polypropylene, Nanoscale 2 (2010) 2269–2274. [38] D.E. Park, H.J. Choi, C.M. Vu, Stimuli-responsive polyaniline coated silica microspheres and their electrorheology, Smart Mater. Struct. 25 (2016), 055020. [39] X.J. Wang, F. Gordaninejad, Flow analysis of field-controllable, electro- and magneto-rheological fluids using Herschel-Bulkley model, J. Intell. Mater. Syst. Struct. 10 (1999) 601–608. [40] J.B. Yin, X.P. Zhao, Preparation and enhanced electrorheological activity of TiO2 doped with chromium ion, Chem. Mater. 16 (2) (2004) 321–328. [41] M.C. Yang, L. Scriven, C. Macosko, Some rheological measurements on magnetic iron oxide suspensions in silicone oil, J. Rheol. 30 (1986) 1015–1029. [42] M. Parthasarathy, D. Klingenberg, Electrorheology: mechanisms and models, Mater. Sci. Eng. R Rep. 17 (1996) 57–103. [43] J. Yin, X. Zhao, X. Xia, L. Xiang, Y. Qiao, Electrorheological fluids based on nanofibrous polyaniline, Polymer 49 (2008) 4413–4419. [44] J. Sung, D. Park, B. Park, H. Choi, M. Jhon, Phosphorylation of potato starch and its electrorheological suspension, Biomacromolecules 6 (2005) 2182–2188. [45] L.C. Davis, Time-dependent and nonlinear effects in electrorheological fluids, J. Appl. Phys. 81 (1997) 1985–1991. [46] H. Uematsu, Y. Aoki, M. Sugimoto, K. Koyama, Rheology of SiO2/(acrylic polymer/ epoxy) suspensions. I. Linear viscoelasticity, Rheol. Acta 49 (2010) 299–304. [47] F.R. Schwarzl, Numerical calculation of stress relaxation modulus from dynamic data for linear viscoelastic materials, Rheol. Acta 14 (1975) 581–590. [48] D. Chotpattananont, A. Sirivat, A.M. Jamieson, Creep and recovery behaviors of a polythiophene-based electrorheological fluid, Polymer 47 (2006) 3568–3575.

Acknowledgements This work was supported by National Research Foundation of Korea (2018R1A4A1025169). References [1] B. Wang, X. Tian, K. He, L. Ma, S. Yu, C. Hao, K. Chen, Q. Lei, Hollow PAQR nanostructure and its smart electrorheological activity, Polymer 83 (2016) 129–137. [2] S. Dai, P. Ravi, K.C. Tam, Thermo-and photo-responsive polymeric systems, Soft Matter 5 (2009) 2513–2533. [3] M. Wei, Y. Gao, X. Li, M.J. Serpe, Stimuli-responsive polymers and their applications, Polym. Chem. 8 (2017) 127–143. [4] H.J. Choi, M.S. Jhon, Electrorheology of polymers and nanocomposites, Soft Matter 5 (2009) 1562–1567. [5] M. Hoseinzadeh, J. Rezaeepazhand, Vibration suppression of composite plates using smart electrorheological dampers, Int. J. Mech. Sci. 84 (2014) 31–40. [6] Y.H. Hwang, S.R. Kang, S.W. Cha, S.B. Choi, An electrorheological spherical joint actuator for a haptic master with application to robot-assisted cutting surgery, Sens. Actuator A-Phys. 249 (2016) 163–171. [7] C.L. Li, J.K. Chen, S.K. Fan, F.H. Ko, F.C. Chang, Electrorheological operation of low-/high-permittivity core/shell SiO2/Au nanoparticle microspheres for display media, ACS Appl. Mater. Interfaces 4 (2012) 5650–5661. [8] K. Tsuda, Y. Hirose, H. Ogura, Y. Otsubo, Motion control of pattern electrode by electrorheological fluids, Colloids Surf., A 360 (2010) 57–62. [9] J. Nikitczuk, B. Weinberg, C. Mavroidis, Control of electro-rheological fluid based resistive torque elements for use in active rehabilitation devices, Smart Mater. Struct. 16 (2007) 418–428. [10] X.Y. Zhu, Q. Zhao, T. Zhang, X.F. Pang, Electrorheological response of novel polyaniline-Fe2O3 nanocomposite particles, Polym.-Plast. Tech. Mater. 58 (2019) 573–577. [11] K. Shikinaka, Design of stimuli-responsive materials consisting of the rigid cylindrical inorganic polymer 0 imogolite0 , Polym. J. 48 (2016) 689–696. [12] M. Sedlacik, V. Pavlinek, M. Mrlik, Z. Moravkova, M. Hajna, M. Trchova, J. Stejskal, Electrorheology of polyaniline, carbonized polyaniline, and their coreshell composites, Mater. Lett. 101 (2013) 90–92. [13] J. Santos, S. Goswami, N. Calero, M.T. Cidade, Electrorheological behaviour of suspensions in silicone oil of doped polyaniline nanostructures containing carbon nanoparticles, J. Intell. Mater. Syst. Struct. 30 (2019) 755–763. [14] T.-C. Wen, J.-B. Chen, A. Gopalan, Soluble and methane sulfonic acid doped poly (diphenylamine)—synthesis and characterization, Mater. Lett. 57 (2002) 280–290. [15] Y.D. Kim, D.J. Yoon, Electrorheological fluids of polypyrrole-tin oxide nanocomposite particles, Korea Aust. Rheol. J. 28 (2016) 275–279. [16] M. Mrlík, V. Pavlínek, P. S� aha, O. Quadrat, Electrorheological properties of suspensions of polypyrrole-coated titanate nanorods, Appl. Rheol. 21 (2011), 52365. [17] M. Cabuk, M. Yavuz, H.I. Unal, Electrokinetic, electrorheological and viscoelastic properties of Polythiophene-graft-Chitosan copolymer particles, Colloid. Surf. Physicochem. Eng. Asp. 510 (2016) 231–238. [18] S.Y. Oh, T.J. Kang, Electrorheological response of inorganic-coated multi-wall carbon nanotubes with core-shell nanostructure, Soft Matter 10 (2014) 3726–3737. [19] C.W. Wang, L.L. Ma, Q.K. Wen, B.X. Wang, R.J. Han, C.C. Hao, K.Z. Chen, Enhanced electrorheological characteristics of titanium oxide@H2Ti2O5 nanotube core/shell nanocomposite, Colloid. Surf. Physicochem. Eng. Asp. 578 (2019). [20] N. Roosz, M. Euvrard, B. Lakard, L. Viau, A straightforward procedure for the synthesis of silica@polyaniline core-shell nanoparticles, Colloid. Surf. Physicochem. Eng. Asp. 573 (2019) 237–245. [21] Q.K. Wen, L.L. Ma, C.W. Wang, B.X. Wang, R.J. Han, C.C. Hao, K.Z. Chen, Preparation of core-shell structured metal-organic framework@PANI nanocomposite and its electrorheological properties, RSC Adv. 9 (2019) 14520–14530.

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