Enhancing Electroresponsive Electrorheological Effect and

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Enhancing Electroresponsive Electrorheological Effect and Temperature Dependence of Poly(ionic liquid) Particles by Hard Core Confinement Qi Lei, Chen Zheng, Fang He, Jia Zhao, Yang Liu, Xiaopeng Zhao, and Jianbo Yin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03508 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Enhancing Electroresponsive Electrorheological Effect and Temperature Dependence of Poly(ionic liquid) Particles by Hard Core Confinement

Qi Lei, Chen Zheng, Fang He, Jia Zhao, Yang Liu, Xiaopeng Zhao, Jianbo Yin*

Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710129, P. R. China.

1

ACS Paragon Plus Environment

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KEYWORDS：Poly(ionic liquid); Electrorheology; Dielectric spectroscopy; Confinement effect.

ABSTRACT: Monodisperse core-shell structured SiO2@poly(ionic liquid) (SiO2@PIL) particles are prepared

by

the

polymerization

methacryloxypropyltrimethoxysilane

of

ionic

modified

liquid SiO2

monomer particles.

on The

the

surface

of

electroresponsive

electrorheological (ER) effect of SiO2@PIL particles when dispersed in insulating carrier liquid is investigated

and

compared

with

pure

poly(ionic

liquid)

(PIL)

particles

based

on

temperature-modulated rheology under electric fields. It demonstrates that hard SiO2 core not only enhances the ER effect of PIL particles but also improves the temperature dependence of ER effect. By dielectric spectroscopy analysis, the mechanism behind the property enhancement was discussed. It indicates that the hard SiO2 core can not only increase the interfacial polarization strength of SiO2@PIL particles by core-shell architecture but also restrain the segment relaxation or softening of PIL shell and influence the ion dynamics above the calorimetric glass transition of PILs by so called “substrate confinement effect”, and this should be responsible for the enhanced electroresponsive ER effect and temperature stability of SiO2@PIL particles.

2


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 INTRODUCTION As a new type of functional polyelectrolytes, poly(ionic liquid)s (PILs) have attracted significant interest in polymer and material science fields because they combine the unique properties of ionic liquids (e.g., high ionic conductivity, negligible vapor pressure, nonﬂammability, and good chemical and thermal stability) and the mechanical stability of polymers into the same substances.1 This special mix of properties endorse PILs with many potential applications not only in traditional fields such as electrolytes for batteries and fuel cells, but also in interdiscipline fields such as drug delivery and biosensors.2 On the other hand, as “designable and tunable” substances, PILs also have attracted significant attention as suitable candidates for the development of smart materials with enhanced stimulus-responsiveness, such as PIL membrane-based soft actuators with thermal and pH dual-responsive characteristics,3 porous PIL membrane-based actuators with acetone vapor response,4, 5 novel electroactive PIL actuator with low-voltage actuation response, and so on.6 Recent research of using PIL particles as electroactive component has opened a way to develop new generation of polyelectrolyte-based electrorheological (ER) fluid, a smart suspension whose rheological properties can be rapidly and reversibly adjusted by an external electric field. Different from conventional polyelectrolyte particles, such as poly(lithium methacrylate), poly(sodium styrene sulfonate), and some kinds of ion-exchange resins, which need absorb small amount of water to activate ER effect,7 the PIL particles can exhibit strong ER effect in dry state.8 Thus, the problems caused by external adsorbed water, such as dielectric breakdown, high leakage current and thermal instability, can be effectively eliminated and the practical use of PIL-based ER fluid can be largely broaden in different fields such as automotive, aerospace, food processing, and biomedical fields.9-11 The origin of anhydrous ER effect of PILs is related to the presence of polyatomic fluorinated counterions (e. g. bis(trifluoromethylsulfonyl)imide ((CF3SO2)2N-)) that endorse PILs with not only intrinsically hydrophobic nature but also weaker electrostatic interaction or lower dissociation energy 3


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of ion pairs compared to classic polyelectrolytes containing monoatomic counterions. Thus, the ion transport, conductivity and polarization related to ER effect are without affinity to extrinsic water but depend on the inherent chemistry and physics of PILs.12 However, the polyatomic fluorinated counterions also have plasticizing effect and this inevitably decreases the glass transition temperature (Tg) of PILs. Thus, the PIL particles are also found to be easy to soften and stick together under the simultaneous effect of high temperature and high electric fields and, as a result, the ER effect tends to degrade or become instable. To address this problem, we recently have developed cross-linked PIL particles and broaden the working temperature region.13 However, the ER effect of cross-linked PIL particles has been found to decrease with the increase of crosslink level. Using small size of inorganic polyatomic counterions to replace organic polyatomic counterions can also increase Tg of PILs, but the polarization and ER effect of PILs have also been found to degrade possibly because of increased ion aggregation.14, 15 Therefore, despite the ER effect of PILs is without affinity to water, the ER effect and its temperature dependence still need to be effectively improved in order to be satisfied with the real application. The fabrication of polymer thin film on hard substrate with a strong interfacial interaction has been demonstrated to be a very effective way to reinforce mechanical strength, rigidity, and thermal properties of polymer.16-18 For example, poly(methyl methacrylate) (PMMA) thin film on a native silicon or silica surface has been found to exhibit an elevated Tg when there is a strong interaction with the substrate by hydrogen bonding.19, 20 Thiolated polystyrene (PS-SH) on gold nanoparticles has shown an increased Tg.21 This is known as so called “substrate confinement effect”. However, using “substrate confinement effect” to enhance physical or chemical properties of PILs has still received less investigation though it has been demonstrated that the ion dynamic of PILs can be changed when PILs have been confined within nanopores.22

4


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In this paper, we use hard SiO2 particles as core to prepare monodisperse SiO2@PIL core-shell particles for stimuli-responsive ER fluid application. By the formation of PIL shell on the surface of hard SiO2 particles with covalent interaction, it is expected to not only enhance the interfacial polarization of particles by core-shell architecture but also restrain the segment relaxation or thermal deformation of PIL shell above the calorimetric glass transition of bulk PILs by “hard substrate confinement effect” and, thereby, improve the macroscopic ER effect and its temperature dependence. A simple method involving surface modification of SiO2 particles and polymerization of ionic liquid monomer on the surface of modified SiO2 particles is used to prepare the SiO2@PIL core-shell particles. Scanning electron microscopy and transmission electron microscopy are used to characterize the morphology of particles, while thermogravimetric analysis and Fourier transform infrared spectroscopy are used to analyze the chemical structure. Under electric field, temperature-modulated rheology is used to characterize the ER effect of the core-shell particles when dispersed in insulating carrier liquid. It shows that using hard SiO2 particles as core not only enhances the ER effect of PILs but also largely enhances the temperature stability of ER effect at high temperature. To understand the mechanism behind the property enhancement, dielectric spectroscopy is finally conducted and analyzed.

 EXPERIMENTAL SECTION Chemicals.

Reagent

grade

tetraethylorthosilicate

(TEOS),

ammonia,

and

2,2’-azobis(isobutyronitrile) (AIBN) were purchased from Sinopharm Chemical Reagent Co. Ltd. China. [2-(methacryloyloxy)ethyl] trimethylammonium chloride solution ([MTMA]Cl, 80 wt % in water),

Lithium

bis(trifluoromethane

sulfonyl)imide

(Li[TFSI],

99%),

and

3-Methacryloxypropyltrimethoxy-silane (MPS) were purchased from Aldrich. AIBN was purified by recrystallization in methanol. Other chemicals were used without further purification. 5


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Preparation of SiO2 Particles. First, 100 mL of ethanol and 19 mL of ammonia were added into a 250 mL three-necked flask. Then, 60 mL of ethanol containing 11 mL of TEOS was injected into the flask by micro flow pump at a speed of 0.25 mL/min under stirring. After that, the mixture was further stirred for 12 h at 30 oC to form white precipitate. Finally, the precipitate was separated by centrifugation, and washed with ethanol several times to obtain final monodisperse SiO2 particles. Preparation of Ionic Liquid Monomer. The ionic liquid monomer [2-(methacryloyloxy)ethyl] -trimethylammonium bis(trifluoromethanesulfonyl)amide ([MTMA][TFSI]) was prepared by mixing the aqueous solutions of 4.40 g of [MTMA]Cl and 4.60 g of Li[TFSI] at room temperature. After stirring for 10 min, a phase separation occurred. The bottom oily layer was collected, washed several times with distilled water, and vacuum dried to obtain the final [MTMA][TFSI] monomer. Preparation of SiO2@P[MTMA][TFSI] Core-shell Particles. First, 0.50 g of dry SiO2 particles was dispersed into 15 mL of ethanol with ultrasonic agitation in a 50 mL flask. The flask was degassed by purging with nitrogen for 30 min. Then, 300 μL of MPS and 600 μL of ammonia were added in turn and the dispersion was magnetically stirred for 30 min at room temperature. After that, 5 mL of ethanol containing 1.00 g of [MTMA][TFSI] and 0.01 g of AIBN was dropped into the dispersion within 60 min under stirring at room temperature. Finally, the flask was placed in an oil bath and treated at a temperature of 70 oC, a stirring rate of 2000 rpm for 6 h to form white precipitate. The precipitate was separated and washed several times with ethanol and deionized water to obtain resulting SiO2@P[MTMA][TFSI] core-shell particles. For comparison, pure P[MTMA][TFSI] microspheres with similar size were also prepared via dispersion polymerization without SiO2 particles. Characterization. The morphology of particles was observed by scanning electron microscopy (SEM, FEI Quanta 600 FEG) and transmission electron microscopy (TEM, FEI Talos F200X). The chemical structure was determined by the Fourier transform infrared spectroscopy (FT-IR, JASCO 6


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FT/IR-470 Plus). The surface chemical composition was analyzed by X-ray photoelectron spectrometer (XPS, Kratos Axis Supra). The crystal structure was determined by the powder X-ray diffraction pattern (XRD, D2 PHASER X). The thermal behavior of particles was determined by the thermogravimetric analyzer (TGA, Netzsch STA449F3) with heating rate of 10 oC/min within 30– 800 oC in air. The glass transition temperature (Tg) of particles was estimated using a differential scanning calorimeter (DSC, Q200, TA Instruments) at a heating and cooling rate of 10 oC/min in nitrogen. The measurement temperature ranged from 30 to 200 oC. Tg values were taken from the midpoint of the total heat flow curve in the thermal transition region. Measurements. The particles were vacuum dried for 48 h and then mixed into silicone oil with a kinetic viscosity of 50 mPa·s at 25 oC by mechanical stirring to form uniform suspension. The volume fraction of particles in suspension was defined as the ratio of particle volume to total fluid volume. The density of PIL particles was measured by a pycnometer filled with silicone oil (density of 0.96 g/cm3 at 25 oC). The ER effect of suspension was measured by electrorheometer (Thermal-Haake RS600) with a parallel plate system having diameter of 35 mm and gap of 1.0 mm, a DC high-voltage generator, and an oil bath system. The flow curves of shear stress vs. shear rate were measured by the controlled shear rate mode within 0.1-1000 s−1 at different working temperatures. Prior to the test, the suspension was heated by the oil bath to the working temperature and was pre-sheared for 1 min at 300 s−1 to remove the structure history, and then the electric field was applied for 30 s to ensure the formation of equilibrium gap-spanning fibrous structure. The conductivity of particles was detected by a liquid method in order to avoid the influence of porosity and moisture. Simply, the leaking current density through suspension under electric fields was detected by Ampere meter and then the conductivity of suspension was determined according to σs= j/E, where j is the current density through suspension and E is the applied electric field strength. Because the applied electric field is over 1.0 kV/mm, the dispersed particles could form fibrous 7


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structure and this structured suspension is analogous to a fibrous-containing composite. Thus, the conductivity of particles could be approximately calculated by the following equation: 𝜎s = 𝜙𝜎p + (1 − 𝜙)𝜎f

(1)

where σs is the conductivity of suspension, σp is the conductivity of dispersed particles, σf is the conductivity of carrier liquid (silicone oil), and 𝜙 is the volume fraction of particles in suspension. The polarization characteristic of particles in suspension was determined by an impedance analyzer (HP 4284A) with a liquid measuring fixture (HP 16452A) in the angular frequency range of 1.26×102-6.28×106 rad/s. The liquid measuring fixture was heated by an oil bath. When the temperature reached to the certain degree, we began the measurement. 1 V of bias electrical potential was applied during measurement that was insufficient to form fibrous structure and, as a result, we could well understand the interfacial polarization characteristic of particles in carrier liquid.

 RESULTS AND DISCUSSION Scheme 1 shows the preparation process of SiO2@P[MTMA][TFSI] core-shell particles. The process involves two procedures. The first one is the surface modification of SiO2 particles with MPS molecule by the hydrolyzation reaction of coupling agent MPS with the hydroxyl group on the surface of SiO2 particles. In this procedure, very small amount of ammonia needs to be added in order to ensure a thin layer of MPS to be modified on the surface of SiO2 particles and the flask needs to be fully degassed by purging with nitrogen to avoid the loss of reaction activity of the vinyl group of modified MPS. Figure 1 shows the FT-IR spectra of SiO2 particles after modification with MPS. It is seen that MPS-modified SiO2 particles have not only the characteristic bands corresponding to SiO2 at 1103 cm-1 (Si–O-Si stretching), 474 cm-1 and 808 cm-1 (Si–O bending) but also the characteristic bands corresponding to MPS at 1703 cm-1 (C=O stretching), and 1460 cm-1 (C-H bending). Figure 2 shows the high-resolution XPS spectra for C 1s and O 1s of samples. The 8


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neat SiO2 particles exhibit C1s peaks at 284.58, 285.85, and 288.82 eV corresponding to C–C/C–H, C–O and C=O from hydrocarbon contamination,23 and O1s peals at 532.50 eV and 533.40 eV corresponding to Si–O–Si and Si–O–H.24 After MPS-modification, the C1s peaks significantly become strong and a new signal corresponding to O–C=O appears at 532.05 eV in O1s peaks.25 Meanwhile, the O atom ratio decreases from 58.04 % of SiO2 to 45.86 % of MPS-modified SiO2, while the C atomic ratio increases from 6.91 % of SiO2 to 24.24% of MPS-modified SiO2. These support that the surface of SiO2 particles has been functionalized by MPS molecules. The second procedure in Scheme 1 concerns the polymerization of [MTMA][TFSI] monomer on the surface of MPS-modified SiO2 particles. In the polymerization procedure, the vinyl groups in modified MPS molecules react with the vinyl groups of [MTMA][TFSI] monomer under AIBN initialization, leading to a coating of P[MTMA][TFSI] on the surface of MPS-modified SiO2 particles. The FT-IR spectra in Figure 1 shows the resulting SiO2@P[MTMA][TFSI] core-shell particles possess not only the characteristic bands of SiO2 but also the characteristic bands of P[MTMA][TFSI] shell at 3057 cm-1 (CH3 stretching), 2992 cm-1 (CH2 stretching), 1483 cm-1 (CH2 variable-angle), 1353 cm-1 (S(=O)2 asymmetrical stretching), 1735 cm-1 (C=O stretching), 1198 cm-1 (CF3 stretching), 1139 cm-1 (S=O symmetrical stretching), 1055 cm-1 (S=O stretching), etc.8 The XPS spectra of SiO2@P[MTMA][TFSI] particles shows C1s peaks at 284.75, 286.35, 288.67 and 292.48 eV corresponding to C–C/C–H, C–O/C–N, C=O and C–F,26 and O1s peaks at 531.90 and 532.90 eV corresponding to O–C=O and O=S.27 However, the characteristic peaks corresponding to Si–O–Si and Si–O–H have disappeared. Meanwhile, the O atom ratio further decreases to 22.37 % but the C atom ratio increases to 61.86 %. These indicate that the SiO2 cores have been completely covered by P[MTMA][TFSI] shell.23

9


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Scheme 1. Schematic preparation of SiO2@P[MTMA][TFSI] core-shell particles.

Transmittance (%)

(d)

(c) (b)

(a)

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber(cm )

Figure 1. FT-IR spectra of SiO2 (a), MPS-modified SiO2 (b), SiO2@P[MTMA][TFSI] (c), and P[MTMA][TFSI] particles (d).

SiO2 C-O C=O

4k 3k

50.0k

Si-O-H

0.0 MPS-modified SiO2

MPS-modified SiO2

C-C/C-H/C=C

100.0k

15k

CPS

C-O

10k

C=O

Binding Energy (eV) O-Si

50.0k

O C-O

Si-O-H

5k

=

CPS

O 1s O-Si

C-C/C-H

5k

20k

SiO2

100.0k

6k

CPS

(b)

C 1s

0.0 SiO2@P[MTMA][TFSI]

30k

C-C/C-H

CPS

20k C-O/C-N

10k

C=O

C-F

292

290 288 286 Binding Energy (eV)

20k

O C-O

O=S

10k

0 294

SiO2@P[MTMA][TFSI]

=

7k

CPS

(a)

CPS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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284

282

0 538

280

536

534

532

530

528

Binding Energy (eV)

Figure 2. High-resolution XPS spectra for (a) C1s and (b) O1s of SiO2, MPS-modified SiO2, and SiO2@P[MTMA][TFSI] particles. 10


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Figure 3 shows the morphology of neat SiO2 and resulting SiO2@P[MTMA][TFSI] particles. From SEM images, it can be seen that SiO2 and SiO2@[MTMA][TFSI] particles are uniform spheres with a smooth surface. However, the mean diameter of SiO2@[MTMA][TFSI] particles is increased to about 580 nm from 410 nm of neat SiO2 particles. The broken particle as shown in the inset of Figure 3b and the TEM image in Figure 3d clearly show the SiO2@[MTMA][TFSI] particles have core-shell architecture. The diameter of particles in TEM observation is close to that in SEM observation. The thickness of P[MTMA][TFSI] shell is about 80 nm. At the same time, the interaction between SiO2 core and P[MTMA][TFSI] shell is strong. After washing with N-dimethylformamide (DMF) five times, the mass of SiO2@P[MTMA][TFSI] particles is decreased, but the IR absorption bands and XPS peaks from P[MTMA][TFSI] are still strong (not shown here). This also reflects that modified MPS molecule might have been interacted with P[MTMA][TFSI] by covalent band rather than simple physical absorption. In addition, the pure P[MTMA][TFSI] particles having a similar diameter with the SiO2@[MTMA][TFSI] particles are also synthesized by dispersion polymerization (not shown here). The XRD patterns show both [MTMA][TFSI] and SiO2@[MTMA][TFSI] particles are amorphous structure.

Figure 3. SEM images of SiO2 (a) and SiO2@P[MTMA][TFSI] particles (b); TEM images of SiO2 (c) and SiO2@P[MTMA][TFSI] particles (d). 11


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Figure 4a is TGA trace of particles under an air atmosphere. The neat SiO2 particles show a weight loss of ~14 % up to 800 oC, which can be attributed to the evaporation of water or solvent and hydroxyl groups as suggested in the literature.28 The pure P[MTMA][TFSI] particles show a three-step weight loss including ~20 % within 300-380 oC, ~60 % within 380-450 oC, and ~20 % within 450-600 oC. These should be attributed to the degradation of pendant groups on the polymer backbone, the cleavage of counter [TFSI] anions, and combustion of the polymer backbone.29,30 After heating at 600 oC, no residual is found, indicating that P[MTMA][TFSI] was fully thermal degradation. Similar to pure P[MTMA][TFSI] particles, SiO2@P[MTMA][TFSI] particles also exhibit a large weight loss after 320 oC due to the thermal degradation of P[MTMA][TFSI] shell. But the weight loss does not reach 100% and the residual weight is ~40 % that can be attributed to the residual SiO2 core. Thus, according to the intimal added amount of [MTMA][TFSI] monomer and the TGA result, we can approximately evaluate the yield of P[MTMA][TFSI] shell is close to 60 %, which is lower than that (~80 %) of pure P[MTMA][TFSI] particles. Figure 4(b) shows the DSC curves of particles under a nitrogen atmosphere. It is seen that the glass transition temperature (Tg) of pure P[MTMA][TFSI] particles is ~82 oC. However, SiO2@P[MTMA][TFSI] particles don’t show obvious glass transition phenomenon. It has been widely accepted that when there is an attractive interaction between polymer chains on hard substrate, the chains will become less mobile.31 Therefore, no obvious glass transition phenomenon reflects that the segment motion of PIL shell should be restrained by SiO2 hard core by the formation of a strong interfacial interaction between PIL shell and SiO2 core.

12


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2.0

(a)

(b)

100 SiO2

1.6

SiO2@P[MTMA][TFSI]

40

0.8

0.4

20

o

Deriv. Weight(%/ C)

1.2 60

Exothermal (mW)

80

TG(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SiO2@P[MTMA][TFSI]

P[MTMA][TFSI]

o

82 C P[MTMA][TFSI]

0.0

0 100

200

300

400 500 o Temperature( C)

600

700

20

800

40

60

80

100 120 140 o Temperature( C)

160

180

200

220

Figure 4. (a) TGA (solid line) and DTG (dash line) curves of SiO2, P[MTMA][TFSI] and SiO2@P[MTMA][TFSI]

particles

in

air;

(b)

DSC

curves

of

P[MTMA][TFSI]

and

SiO2@P[MTMA][TFSI] particles in nitrogen.

Under DC electric fields, the electroresponsive ER effect of P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] core-shell particles when dispersed in insulating carrier liquid (silicone oil) was observed by optical microscopy and measured by electrorheometer. Figure 5 shows the optical microscopy photo of suspensions without and with an external electric field. It is seen that when the electric field is applied, the randomly dispersed particles can rapidly contact each other to form a gap-spanning fibrous structure between electrodes, indicating that P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles have a good electroresponsive ER characteristic. This gap-spanning fibrous structure, which is dominated by sufficient electrostatic interaction between particles, will provide a large resistance to the shear flow perpendicular to the electric field, and thus enhance the shear stress or viscosity of suspensions.

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Figure

5.

Optical

microscopy

photo

of

suspensions

Page 14 of 48

of

P[MTMA][TFSI]

(a)

and

SiO2@P[MTMA][TFSI] particles (b) without and with an electric field of 1.5 kV/mm (T=25 °C, 𝜙 =12 vol %).

Figure 6 shows the rheological curves of shear stress and shear viscosity versus shear rate under different electric field strengths at room temperature. It is seen that, without electric fields, the suspensions of P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles show a shear thinning behavior and a weak yield stress. This behavior is often observed in suspension systems due to the presence of interparticle interaction before shear. Compared to the suspension of P[MTMA][TFSI] particles, however, the yield stress and shear thinning characteristic of the suspension of SiO2@P[MTMA][TFSI] particles are obviously weaker. This may be because the rigidity of P[MTMA][TFSI] shell has been increased after coating onto hard SiO2 core and thus the colloid interaction among SiO2@P[MTMA][TFSI] particles is decreased. We will verify this later. With electric fields, both suspensions exhibit a significantly increase in the shear stress and behave like a plastic material with a large yield stress, so called ER effect.32 This can be attributed to the fact that under electric fields the particles are polarized and attracted each other to form gap-scanned fibrous 14


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structure between electrodes as shown in Figure 5. Meanwhile, the rheological curves under electric fields can be well fitted by the following Bingham fluid equation, 𝜏 = 𝜏0 + 𝜂0 𝛾̇

(2)

where 𝜏 is shear stress, 𝛾̇ is shear rate, and 𝜂0 is the plastic viscosity, and 𝜏0 is the electric field induced yield stress.

10

10

10

3

(b) 10

Shear viscosity (Pas)

Shear stress (Pa)

(a) 10

2

Bingham Fitting 0.0 kV/mm 0.5 1.0 2.0 3.0

1

0

10

0

10

1

10

2

10

10

2

10

1

10

0

10

3

3

0.0 kV/mm 0.5 1.0 2.0 3.0

-1

10

0

10

-1

(d) 10

10


10

2

Bingham Fitting 0.0 kV/mm 0.5 1.0 2.0 3.0

1

0

10

0

10

1

10

2

10

3

Shear rate (s )

(c) 103

10

1 -1

Shear rate (s )

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

2

10

10

2

10

1

10

0

10

3

3

0.0 kV/mm 0.5 1.0 2.0 3.0

-1

10

0

10

1

10

2

10

3

-1

-1

Shear rate (s )

Shear rate (s )

Figure 6. Rheological curves of shear stress and shear viscosity vs shear rate of suspensions of P[MTMA][TFSI] (a, b) and SiO2@P[MTMA][TFSI] particles (c, d) (T=25 °C, 𝜙 =12 vol %).

𝜏0 is an important parameter to characterize the magnitude of ER effect. As the electric field strength increases, 𝜏0 increases by following the power law relation 𝜏0 ∝ 𝐸 α as shown in Figure 7. 15


Langmuir

Compared to the suspension of P[MTMA][TFSI] particles, however, the magnitude of 𝜏0 of the suspension of SiO2@P[MTMA][TFSI] particles is higher. This indicates that SiO2@P[MTMA][TFSI] particles have stronger ER effect than pure P[MTMA][TFSI] particles. Such result is interesting because SiO2 is usually low ER activity but using it as core seems to enhance the ER effect of P[MTMA][TFSI]. In addition, ER effect of the poly(ionic liquid)-coated SiO2 particles is also higher compared to some ionic liquids-modified composites.33

10

3



Fitted by 0 ∝E

Yield stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 48

P[MTMA][TFSI] SiO2@P[MTMA][TFSI]

10

2

10

1

10

-1

0

10 Electric field stenghth (kV/mm)

10

1

Figure 7. Yield stress as a function of electric field strengths for suspensions of P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles (T=25 °C, 𝜙=12 vol %).

More interesting is the result displayed in Figure 8 and Figure 9, in which the temperature dependence of rheological properties are displayed. It is seen that the temperature dependence of ER effect of the suspension of SiO2@P[MTMA][TFSI] particles is significantly different from that of the suspension of P[MTMA][TFSI] particles. Consisted with the previous report, the working temperature range of ER effect of the suspension of P[MTMA][TFSI] particles is relatively narrow.34 When the external temperature exceeds 80 oC, not only the ER effect cannot be measured at even 1.0 kV/mm (see Figure 9a) because the leaking current density through suspension has exceeded the upper limitation of rheometer (200 μA/cm2), but also the off-field viscosity shows a reverse increase 16


Page 17 of 48

as shown in Figure 8a and Figure 9a. This should be related to the fact that the P[MTMA][TFSI] particles start to swell and become soft when the temperature is close to Tg (~82 oC). As soon as the particles swell and become soft, the relative particle volume fraction in suspension increases and the particles are easy to cohere to each other and, as a result, the shear viscosity of the suspension shows a reverse increase. At the same time, as the particles become soft and swell, the ion motion is accelerated and the leaking current density rapidly increases. We will discuss this based on dielectric analysis and experimental observation later. Different from the suspension of P[MTMA][TFSI] particles, however, the work temperature range of the suspension of SiO2@P[MTMA][TFSI] particles is significantly broadened (see Figure 8b and Figure 9b). When temperature reaches 120 oC, the ER effect is still sound and the current density at 3.0 kV/mm is lower than 50 μA/cm2. At the same time, the off-field viscosity continuously declines with temperature like a hard sphere suspension (see the inset in Figure 9b). This indicates that using SiO2 as core seems to well restrain the softening of PILs and influence the ion motion above Tg of PILs.

(a) 103

10

2

10

1

10

0

10

(b) 103

o

Shear stress (Pa)

Shear stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

o

25 C 40 60 80 100 0

10

1

10

2

10

3

10

2

10

1

10

0

10

25 C 40 60 80 100 120

0

-1

10

1

10

2

10

3

-1

Shear rate (s )

Shear rate (s )

Figure 8. Temperature dependence of rheological curves of shear stress vs. shear rate of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) at 0 kV/mm (solid point) and 3 kV/mm (open point) electric field (𝜙 =12 vol %).

17


Langmuir

(b) 800 0.20

400

0.15

600 0.10

0.05

300

0.5 kV/mm 1.0 2.0 3.0

0 kV/mm

20

40 60 80 100 o Temperature ( C)

Yield stress (Pa)

500


0.5 kV/mm 1.0 2.0 3.0

120

200

0.35 Shear viscosity (Pas)

(a) 600

Yield stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 48

0.25 0.20 0.15 0.10 0.05

400

0 kV/mm

0.30

20

40 60 80 100 o Temperature ( C)

120

200 100 0

0 20

40

60

80

100

120

20

o

Temperature ( C)

40

60 80 o Temperature ( C)

100

120

Figure 9. Temperature dependence of yield stress at different electric fields and off-field viscosity (inset) of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) (𝜙 =12 vol %).

The above rheological results clearly show that using SiO2 as core not only enhances the ER effect of P[MTMA][TFSI] particles but also significantly improves the temperature dependence of ER effect. According to the proposed mechanisms,35,36 ER effect is closely associated with particle polarization, in particular interfacial polarization of particles. For polyelectrolyte particles, the interfacial polarization is further related to the local movement of ion charges on particles.37 In P[MTMA][TFSI] particles, the positive charged quaternary ammonium ions (MTMA+) are attached to polymer backbone and thus the particle polarization should be mainly originated from the movement of mobile TFSI- counterions. In SiO2@P[MTMA][TFSI] particles, however, the movement of mobile TFSI- counterions and corresponding particle polarization might have been largely influenced by the presence of hard SiO2 core. Therefore, investigating the polarization characteristic of particles in suspensions is an effective way to understand the mechanism behind the enhanced ER effect and temperature dependence of SiO2@P[MTMA][TFSI] particles. In the following section, we conduct a dielectric spectra study.

18


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Langmuir

Figure

10

shows

the

dielectric

spectra

of

suspensions

of

P[MTMA][TFSI]

and

SiO2@P[MTMA][TFSI] particles. The following relaxation function including the Cole–Cole's term, a dc conductivity term and an electrode polarization (EP) term is used to fit the dielectric data.38 ∆𝜀 ′

′ 𝜀 ∗ (𝜔) = 𝜀 ′ + 𝑖𝜀 ′′ = 𝜀∞ + 1+(𝑖𝜔𝜏)𝛼 + 𝑖 𝜀

𝜎 0𝜔

+ 𝐴𝜔−𝑛

(3)

′ is the dielectric relaxation strength (𝜀0′ and where, 𝜀 ∗ is the complex permittivity, ∆𝜀 ′ = 𝜀0′ − 𝜀∞ ′ 𝜀∞ are the limit values of the relative permittivity at the frequencies below and above the relaxation

frequencies, respectively), 𝜔 is the angular frequency, 𝜏 = 𝜔

1 𝑚𝑎𝑥

represents the relaxation time

(𝜔𝑚𝑎𝑥 is the local angular frequency of dielectric loss peak), 𝛼 is the Cole–Cole parameter indicating the distribution of relaxation time, 𝜎 is the dc conductivity, n is related to the slope of EP’s high frequency tail and A is related to the amplitude of EP. Although the simple treatment of EP by inclusion of 𝐴𝜔−𝑛 cannot describe the EP process, 𝐴𝜔−𝑛 is helpful for the accurate fitting of the main relaxation process.39 From Figure 10, it is seen that eq 3 can well fit the dielectric data and both suspensions show a clear dielectric relaxation in the measured frequency range. Because the permittivity of carrier liquid (silicone oil) is almost independent of frequency within the measured range, the observed dielectric relaxation process should be originated from the polarization of suspended particles. But there are differences in the dielectric characteristics and their temperature dependence between P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles.

19


Langmuir

(b) 1.60

Dielectric loss factor, ''

Dielectric constant, '

(a) 7.00

6.00

5.00

o

4.00

T C

1.20

0.80

0.40 o

T C

o

 C

o

 C

3.00 2 10

10

3

10

4

10

5

10

6

10

0.00 2 10

7

10

3

10

4

10

5

10

6

10

7

 (rad/s)

 (rad/s)

(d) 2.00

(c) 9.00

o

T C

o

T C

8.00

o

 C

o

Dielectric loss factor, ''

 C

Dielectric constant, '

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 48

7.00 6.00 5.00

1.50

1.00

0.50

4.00 3.00 2 10

10

3

10

4

10

5

10

6

10

0.00 2 10

7

10

3

10

4

10

5

10

6

10

7

 (rad/s)

 (rad/s)

Figure 10. Dielectric spectra of suspensions of P[MTMA][TFSI] particles (a, b) and SiO2@P[MTMA][TFSI] particles (c, d) at different temperatures. The lines represent the best fit of data by eq 3 (𝜙 =12 vol %).

Table 1. Dielectric characteristics of suspensions of P[VBTMA][TFSI]

particles and

SiO2@P[MTMA][TFSI] particles (T=25 °C, 𝜙=12 vol %) Sample

ε0′

ε∞′

Δε′

ε′′a

τ (s)

σ (S/m)

P[VBTMA][TFSI]

5.42

3.13

2.29

0.67

8.80×10-5

~7.08×10-10 ~2.7×10-8

SiO2@P[MTMA][TFSI] 7.66

3.11

4.55

1.13

5.10×10-3

~2.66×10-12 ~2.3×10-9

a

σpb (S/m)

The value of dielectric loss factor at the relaxation peak. b The approximate dc conductivity of the

particles calculated using eq 1.

20


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Langmuir

The first difference concerns the magnitude of τ and Δ𝜀 ′ . Table 1 lists the dielectric parameters at room temperature. It is seen that, τ of the suspension of P[MTMA][TFSI] particles is ~ 8.80×10-5 s and Δ𝜀 ′ is ~2.29. Since Tg of P[MTMA][TFSI] particles is ~82 oC, the segmental relaxation is frozen at room temperature.40 In addition, the dipole polarization from ion pairs is usually located at higher frequency region. Thus, the observed dielectric relaxation process in the measured frequency region should originate from the interfacial polarization of P[MTMA][TFSI] particles in carrier liquid due to the movement of charges. For P[MTMA][TFSI], the positive charged quaternary ammonium (MTMA+) units are attached on polymer backbone and, thus, the movement of charges should be associated with the mobile TFSI- counteranions.41 Under electric fields, the movement and aggregation of TFSI- ions at the interface between particles and insulating carrier liquid can induce interfacial polarization as shown in Figure 11a. Compared to P[MTMA][TFSI] particles, however, τ of the suspension of SiO2@P[MTMA][TFSI] particles is slowed down but Δ𝜀 ′ is increased. This should be attributed to the presence of SiO2 core which provides not only two interfaces but also might have influenced the ion dynamic in P[MTMA][TFSI] shell by the interaction between shell and core.42 As shown by the scheme in Figure 11b, due to the insulating effect of SiO2 core and silicone oil, the mobile TFSI- counteranions will aggregate at not only the interface between PIL shell and SiO2 core but also the interface between PIL shell and silicone oil. Thus, compared to neat P[MTMA][TFSI] particles, the strength of interfacial polarization in SiO2@P[MTMA][TFSI] particles should be stronger. Some literatures have also observed core/shell structure is benefit for the enhancement of polarization, especially for interfacial polarization of particles.43,44 On the other hand, although the interfacial polarization of P[MTMA][TFSI] particles is closely related to the movement of mobile TFSI-, the immobile polycation part also needs to orientate under electric field and attend interfacial polarization process of particles. Thus, the interaction between P[MTMA][TFSI] shell and SiO2 core is expected to reduce the rate of ion dynamics by blocking 21


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Page 22 of 48

polycation orientation. Therefore, the relaxation time of SiO2@P[MTMA][TFSI] particles is slowed down. According to the ER mechanism, τ is related to the stability of interparticle interaction while Δε′ is related to the intensity of interparticle interaction under the simulate effect of electric and shear fields.35,36 A good ER effect often requires ER particles to possess large Δε′ and suitable τ. Here, although the value of τ of SiO2@P[MTMA][TFSI] particles becomes slow, it is still located in the accepted range of 6.18×101 rad/s to 6.18×106 rad/s for available ER effect. Compared to P[MTMA][TFSI] particles, however, the value of Δε′ of SiO2@P[MTMA][TFSI] particles is significantly higher. Therefore, SiO2@P[MTMA][TFSI] particles show enhanced ER effect as shown in Figure 6.

Figure

11.

Schematic

interfacial

polarization

of

P[MTMA][TFSI]

particles

(a)

and

SiO2@P[MTMA][TFSI] particles (b) under electric fields.

The second difference is the temperature dependence of dielectric characteristics. It is seen from Figure 10 that when the temperature is lower than 80 oC, the dielectric relaxation peak of both suspensions shifts toward higher frequency and the values of 𝜀 ′ and 𝜀 ′′ in the low frequency region increase with elevated temperature. This reflects that the rate of interfacial polarization is increased with temperature and the DC conductivity through suspensions is thermally promoted. When the temperature exceeds 80 oC, however, there appears difference between two suspensions. 22


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Langmuir

As shown in Figure 10d, the dielectric relaxation peak of the suspension of SiO2@P[MTMA][TFSI] particles still maintains and continually shifts towards higher frequency. But 𝜀 ′ and 𝜀 ′′ of the suspension of P[MTMA][TFSI] particles show a sharp increase and the dielectric relaxation peak is covered (see Figure 10a,b). This means the DC conductivity of the suspension of P[MTMA][TFSI] particles increases more quickly than that of the suspension of SiO2@P[MTMA][TFSI] particles when the temperature exceeds 80 oC. To show this difference more clearly, we plot the temperature dependence of reciprocal of relaxation time (𝜏 −1 ) and conductivity (𝜎) in Figure 12. It is found that when the temperature is lower than 80 oC, 𝜏 −1 and 𝜎 of both suspensions as a function of temperature follow the Arrhenius equation below:45 −𝐸𝑎

−𝐸𝑎

𝜏 −1 ∝ 𝑒 𝑅𝑇 or 𝜎 ∝ 𝑒 𝑅𝑇

(4)

where, Ea is the activation energy, R is the molar gas constant, and T is the degree Kelvin. The Arrhenius dependence indicates that the thermal diffusion of ions, rather than segment relaxation, is dominating the process of conductivity and polarization of P[MTMA][TFSI] particles. This is in accordance with the fact that P[MTMA][TFSI] is in glassy state below 80 oC. The values of Ea of SiO2@P[MTMA][TFSI] particles for the relaxation mode and conductivity mode are 82.2 kJ/mol and 103.2 kJ/mol, respectively (Note that the value of Ea for 𝜎 is higher than that for 𝜏 −1 . This is because the formation of ionic conductivity in suspensions requires the mobile ions to go through not only the potential energy barrier in polymer matrix but also the potential energy barrier of interface between particles and carrier liquid.). They are very close to those of P[MTMA][TFSI] particles. It means the presence of hard SiO2 core does not affect the energy dissipation characteristics before 80 o

C and this may be because the ion transport is hopping mode in glassy polymer matrix.41

23


Langmuir

(a) 10

7

(b) 10

-4

10

-5

SiO2@P[MTMA][TFSI]

10

-6

P[MTMA][TFSI] Fitted by ∝exp(-Ea/RT)

10

-7

10

-8

10

-9

SiO2@P[MTMA][TFSI]

10

5

P[MTMA][TFSI] -1 Fitted by  ∝exp(-Ea/RT)

(S/m)

10

6

85.9 kJ/mol



-1

 (s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

10

4

10

3

10

82.2 kJ/mol 2

2.4

2.6

2.8

3.0

3.2

3.4

105.0 kJ/mol

10

-10

10

-11

10

-12

103.2 kJ/mol

2.4

2.6

2.8

3.0

3.2

3.4

-1

-1

1000/T (K )

1000/T (K )

Figure 12. Temperature dependence of reciprocal of relaxation time (a) and direct current conductivity (b) for suspensions of P[MTMA][TFSI] particles and SiO2@P[MTMA][TFSI] particles. The solid lines represent the best fit curves of eq 4 to data at temperature below Tg.

As temperature increases and exceeds 80 oC, 𝜏 −1 and 𝜎 of the suspension of P[MTMA][TFSI] particles depart from the Arrhenius line (see Figure 12). This can be attributed to the activation of segment relaxation of P[MTMA][TFSI] when the temperature exceeds Tg (~82 oC). However, unlike bulk

PILs

whose

𝜏 −1

and

𝜎

as

a

function

of

temperature

usually

follows

Vogel−Fulcher−Tammann (VFT) dependence above Tg,41,46 𝜏 −1 and 𝜎 of P[MTMA][TFSI] particles also depart from the VFT dependence above Tg. As shown in Figure 12, 𝜏 −1 of P[MTMA][TFSI] particles become insensitive to temperature but 𝜎 increase intensively above Tg. We consider this may be because the PIL particles are in suspension state. Compared to bulk state, the PIL particles in suspension state are easy to become much softer above Tg due to the additional swelling effect of carrier liquid for PIL particles. As a result, the diffusion of TFSI- in particles becomes easier and even some counterions can enter into carrier liquid. Thereby, the conductivity and leaking current density of suspension rapidly increase, which results in the failure of ER measurement at high temperature as shown in Figure 8a and Figure 9a. To further support the significant softening of P[MTMA][TFSI] particles above Tg, we also observed the appearance of the 24


Page 25 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

suspension of P[MTMA][TFSI] particles after ER measurement at temperature lower and higher than Tg as shown in Figure 13a. It is seen the suspension can maintain original state after ER measurement at room temperature, whereas the suspension state loses and the particles even stick together to form glutinous precipitate at the bottom of bottle after ER measurement (0.5 kV/mm) at temperature higher than Tg. After cooling, the glutinous precipitate solidifies into a transparent P[MTMA][TFSI] bulk. It clearly reveals P[MTMA][TFSI] particles do become very soft above Tg and they are easy to stick together under electric field. This significant softening of P[MTMA][TFSI] particles above Tg can also explain the inverse increase of the off-field viscosity of the suspension of P[MTMA][TFSI] particles in Figure 8a and Figure 9a. Different from P[MTMA][TFSI] particles, SiO2@P[MTMA][TFSI] particles can well maintain dispersed state after ER measurement at temperature higher than Tg (see Figure 13b). At the same time, 𝜏 −1 and 𝜎 still follow the Arrhenius line at high temperature (see Figure 12). These clearly reveal using SiO2 as core has well restrained the softening of P[MTMA][TFSI] shell and suppressed the segment relaxation promoted ion dynamics of TFSI- above Tg. It has been reported that the polymer thin film fabricated on hard substrates with a strong interfacial interaction can largely reinforce the rigidity and thermal properties of polymer by so called “substrate confinement effect”.47, 48

According to the structure characterization, the P[MTMA][TFSI] shell with nanometer thickness is

attached on the surface of SiO2 core by a strong covalent bond with the help of MPS. Therefore, it can be believed that “substrate confinement effect” from hard SiO2 core should be responsible for the suppression of softening and segment mobility of P[MTMA][TFSI] shell above Tg. Thus, SiO2@P[MTMA][TFSI] particles can maintain hard particle state at high temperature and the movement of TFSI- or ionic conductivity is not largely promoted by segmental dynamic. As a result, the working temperature range of the suspension of SiO2@P[MTMA][TFSI] particles is largely broadened as shown in Figure 8b and Figure 9b. 25


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Page 26 of 48

Figure 13. The appearance of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) after ER measurement at temperature lower and higher than Tg. The inset is the schematic representation of particle deformation under electric fields.

 CONCLUSIONS Core-shell structured SiO2@P[MTMA][TFSI] particles have been prepared by the polymerization of ionic liquid monomer on the surface of MPS-modified SiO2 particle. The characterization shows that the SiO2@P[MTMA][TFSI] particles have monodisperse morphology and enhanced thermal property compared to P[MTMA][TFSI] particles with the help of hard SiO2 core. The rheological measurements under electric fields have demonstrated that hard SiO2 core not only enhances the ER effect of P[MTMA][TFSI] particles but also improves the temperature dependence of ER effect. The dielectric spectroscopy analysis and optical observation have indicated that the hard SiO2 core can not only increase the interfacial polarization strength of SiO2@P[MTMA][TFSI] particles by core-shell architecture but also restrain the segment relaxation or softening of P[MTMA][TFSI] shell and influence the ion dynamics above the calorimetric glass transition of P[MTMA][TFSI] by 26


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Langmuir

“substrate confinement effect”, and these are responsible for the enhanced electroresponsive ER effect and temperature stability of SiO2@P[MTMA][TFSI] particles.

 AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. ORCID Jianbo Yin: 0000-0003-0578-6463 Notes The authors declare no competing financial interest



ACKNOWLEDGEMENTS

This study was supported by the National Natural Science Foundation of China (no. 51572225; 51872243) and the Natural Science Foundation of Shaanxi province (No.2017JM5062).



REFERENCES

(1). Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An update. Prog. Polym. Sci. 2013, 38 (7), 1009-1036. (2). Mecerreyes, D. Polymeric ionic liquids: Broadening the Properties and Applications of Polyelectrolytes. Prog. Polym. Sci. 2011, 36 (12), 1629-1648. (3). Chen, F.; Guo, J.; Xu, D.; Yan, F. Thermo- and pH-Responsive Poly(ionic liquid) Membranes. Polym. Chem. 2016, 7 (6), 1330-1336. (4). Zhao, Q.; Heyda, J.; Dzubiella, J.; Täuber, K.; Dunlop, J. W.; Yuan, J. Sensing Solvents with Ultrasensitive Porous Poly(ionic liquid) Actuators. Adv. Mater. 2015, 27 (18), 2913-2917. 27


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Scheme 1. Schematic preparation of SiO2@P[MTMA][TFSI] core-shell particles.


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Figure 1. FT-IR spectra of SiO2 (a), MPS-modified SiO2 (b), SiO2@P[MTMA][TFSI] (c), and P[MTMA][TFSI] particles (d). 165x88mm (300 x 300 DPI)


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Figure 2. High-resolution XPS spectra for (a) C1s and (b) O1s of SiO2, MPS-modified SiO2, and SiO2@P[MTMA][TFSI] particles. 165x88mm (300 x 300 DPI)


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Figure 3. SEM images of SiO2 (a) and SiO2@P[MTMA][TFSI] particles (b); TEM images of SiO2 (c) and SiO2@P[MTMA][TFSI] particles (d).


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Figure 4. (a) TGA (solid line) and DTG (dash line) curves of SiO2, P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles in air; (b) DSC curves of P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles in nitrogen. 165x88mm (300 x 300 DPI)


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Figure 5. Optical microscopy photo of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) without and with an electric field of 1.5 kV/mm (T=25 °C, ϕ =12 vol %). 165x88mm (300 x 300 DPI)


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Figure 6. Rheological curves of shear stress and shear viscosity vs shear rate of suspensions of P[MTMA][TFSI] (a, b) and SiO2@P[MTMA][TFSI] particles (c, d) (T=25 °C, ϕ =12 vol %). 165x88mm (300 x 300 DPI)


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Figure 7. Yield stress as a function of electric field strengths for suspensions of P[MTMA][TFSI] and SiO2@P[MTMA][TFSI] particles (T=25 °C, ϕ=12 vol %).


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Figure 8. Temperature dependence of rheological curves of shear stress vs. shear rate of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) at 0 kV/mm (solid point) and 3 kV/mm (open point) electric field (ϕ =12 vol %). 165x88mm (300 x 300 DPI)


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Figure 9. Temperature dependence of yield stress at different electric fields and off-field viscosity (inset) of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) (ϕ =12 vol %). 165x88mm (300 x 300 DPI)


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Figure 10. Dielectric spectra of suspensions of P[MTMA][TFSI] particles (a, b) and SiO2@P[MTMA][TFSI] particles (c, d) at different temperatures. The lines represent the best fit of data by eq 3 (ϕ =12 vol %).


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Figure 11. Schematic interfacial polarization of P[MTMA][TFSI] particles (a) and SiO2@P[MTMA][TFSI] particles (b) under electric fields. 165x88mm (300 x 300 DPI)


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Figure 12. Temperature dependence of reciprocal of relaxation time (a) and direct current conductivity (b) for suspensions of P[MTMA][TFSI] particles and SiO2@P[MTMA][TFSI] particles. The solid lines represent the best fit curves of eq 4 to data at temperature below Tg.


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Figure 13. The appearance of suspensions of P[MTMA][TFSI] (a) and SiO2@P[MTMA][TFSI] particles (b) after ER measurement at temperature lower and higher than Tg. The inset is the schematic representation of particle deformation under electric fields.


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