Influence of Side Chain Sizes on Dielectric and Electrorheological

Article pubs.acs.org/JPCB

Influence of Side Chain Sizes on Dielectric and Electrorheological Responses of Poly(ionic liquid)s Yuezhen Dong, Bo Wang, Liqin Xiang, Yang Liu, Xiaopeng Zhao, and Jianbo Yin* Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710129, China S Supporting Information *

ABSTRACT: Poly(ionic liquid)s (PILs) show potential as new anhydrous polyelectrolyte-based smart electrorheological (ER) materials. Understanding the structure−property relationship on a molecular level is very important for guiding the design of PIL-based ER materials. In this paper, a family of (pvinylbenzyl)trialkylammonium hexafluorophosphate-based PIL particles containing different length of substituent alkyl chains attached to immobile ammonium charged site is synthesized for especially understanding the size effect of side chains on ER property. To exclude the particle shape effect, the PIL particles are controlled to be monodisperse sphere-like morphology with a similar size. The ER property of PIL particles when dispersed in insulating oil is investigated and compared by temperature-modulated rheological test under external electric fields. The dielectric spectroscopy is finally performed to study the mechanism behind the size effect of side chains on the ER property of PIL particles. We demonstrate that the size of side chains on the charged site has a significant impact on the ER effect of PIL particles and the PIL particles with shorter side chains have stronger ER property but degraded temperature dependence, and this is related to the fact that the variation of side chain size alters the transport dynamic of mobile counterions and ion motion-induced interfacial polarization.

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INTRODUCTION Electrorheological (ER) fluid is a stimuli responsive smart suspension consisting of polarizable microparticles in insulating liquid carrier. Under electric fields, the particles can be polarized and attract each other to form fibrous structure spanning the gap of electrodes. This gap-spanning structure can dramatically increase the apparent viscosity of the suspension within several milliseconds.1 If electric fields are sufficiently strong, the suspension even becomes a gel-like solid, showing a significant yield stress. After electric fields are removed, the suspension can rapidly go back to its initial low viscous state. This rapid and reversible field-induced thickening makes ER fluid shows many potential applications in automotive, aerospace and medical fields.2−4 However, so far there is not an ER fluid with versatile performance for practical use and the problem involving either low yield stress or particle sedimentation or thermal instability still requires a more effective solution. Since ER fluid is a two-phase heterogeneous system whose electro-responsive behavior is related to the electric polarization processes of dispersed particles, many researches have focused on developing ER active particles in order to improve ER performance. The first generation ER fluid is based on watercontaining particles, such as starch, silica gel, zeolite, and so on.5,6 This system is also named as extrinsic ER system, in which the particles need to absorb small amount of water to activate ER property. The role of absorbed water is supposed to set up “water-bride” between particles or to promote ion © 2017 American Chemical Society

motion for ionic polarization in particles. If the absorbed water is removed, the ER effect of this system will disappear. Thus, the extrinsic ER system is largely limited in real applications due to temperature instability. Current researches have mainly focused on so-called second generation intrinsic ER fluid based on dry particles. Some inherently polarizable inorganics with high dielectric constant, such as rutile, perovskite, and so on, are typical intrinsic ER active system. In dry state, they can show strong ER response to AC electric fields.7 Some doped inorganics, such as zirconium-doped aluminosilicates, rare earth ion doped titania, etc., can also show strong ER property under DC electric fields.8 Because of inherently high density and stiffness, however, these inorganic ER particles have to be subjected to problems involving particle sedimentation and abrasiveness to devices. To get rid of these problems, more researches have concentrated on ER fluid with organic polymer particles as the dispersed phase due to their low density and suitable softness.9 Among various polymer particles, polyelectrolyte particles are considered to be the most of promising candidates due to their low-cost and relatively high ER activity.10 The classic polyelectrolyte ER particles include poly(lithium methacrylate), poly(sodium styrenesulfonate), and some ion-exchange resins.11 However, these particles still need to absorb small amount of water to promote the dissociation Received: March 13, 2017 Revised: June 1, 2017 Published: June 2, 2017 6226

DOI: 10.1021/acs.jpcb.7b02366 J. Phys. Chem. B 2017, 121, 6226−6237

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The Journal of Physical Chemistry B Scheme 1. Schematic Synthesis of PIL Particles

radical polymerization. Hexafluorophosphate (PF6−) is chosen as counterion not only because it is strong hydrophobic nature but also it can ensure all PIL particles are glassy state solid in wide temperature region. To exclude the external particle shape effect, all of the PIL particles are controlled to be monodisperse sphere-like morphology with a similar diameter by a microwave assisted dispersion polymerization. Under electric fields, the ER property of PIL particles when dispersed in insulating oil is investigated by temperature-modulated rheological test. By dielectric spectroscopy analysis, the mechanism behind the impact of side chains on ER property is studied. We demonstrate that the size of side chains has a significant impact on ER property and this is related to the fact that the size variation of side chains alters the transport dynamic of mobile counterions and ion motion-induced interfacial polarization.

and diffusion of ions and thus activate particle polarization and ER property. In the dry state, the metal ions in the polyelectrolytes are strongly bound by carboxylate or sulfonate groups on the charged sites. Therefore, the classic polyelectrolyte particles still belong to extrinsic ER system and the problems caused by the absorbed water need to overcome. Many efforts have been made to overcome these problems. One of interesting way is using polymer solid electrolyte based on poly(ethylene oxide) (PEO) salts as the dispersed phase.12 The PEO solid electrolyte is prepared by reacting poly(ethylene glycol) ethers with toluene diisocyanate to form cross-linked polyurethane elastomers which are dissolving salts (e.g., lithium chloride or zinc dichloride).7 In this system, the interaction between the metal cations and electron pairs of oxygen of PEO is weaker than the interaction between the metal cations and COO− and SO3− groups in classic polyelectrolyte. Thus, the particles contain mobile ions themselves in absence of water or other small molecule activators. Although the PEO salts show high ER activity in the dry state, they are subjected to problems including large leaking current density due to the facile leach of small size counter metal ions and moisture sensitivity due to the hydrophilic nature of PEO segments. In addition, it is often difficult to obtain products in a powder or particle form for the PEO solid electrolyte. Very recently, a new kind of anhydrous polyelectrolyte ER system based on PIL particles with bis(trifluoromethylsulfonyl) imide ((CF3SO2)2N−) as counterions has developed.13 Because of the hydrophobic nature of fluorinated counterions, the ion motion and ionic conductivity in the PILs are almost without affinity to water and the ER fluid based on PIL particles exhibits strong ER property in the absence of any external activators. Dielectric spectra analysis has indicated that the anhydrous ER property is associated with ion motion-induced interfacial polarization in PIL particles.13 Thus, the local microscopic surrounding or molecular structure affecting ion transport should be very important. However, a detailed study addressing the impact of microscopic structure on ion transport and ER property is still lacking. To guide the design of new PIL ER particles with improved performance, it would be worthwhile to deeply understand the relationship between microscopic structure and macroscopic ER property on the molecular level. Having considered the local microscopic surrounding affecting ion transport, in this paper, we synthesize a family of (p-vinylbenzyl)trialkylammonium hexafluorophosphatebased PIL particles containing different length of substituent alkyl chains attached to immobile ammonium charged site as new anhydrous ER particles for especially understanding the size effect of side chains on ER property. p-Vinylbenzyl quarterammonium modified with different lengths of alkyl side chains is chosen as the backbone because it is easy to polymerize into products with a particle form by simple free

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EXPERIMENTAL SECTION Synthesis of PIL Particles. First, the ionic liquid (IL) monomers were synthesized. Except for (p-vinylbenzyl)trimethylammonium hexafluorophosphate ([VBTMA][PF6]), other IL monomers ((p-vinylbenzyl)triethylammonium hexafluorophosphate ([VBTEA][PF 6 ]), (p-vinylbenzyl)tripropylammonium hexafluorophosphate ([VBTPA][PF6]), and (p-vinylbenzyl)tributylammonium hexafluorophosphate ([VBTBA][PF6])) were synthesized by two step procedure as shown in Scheme 1: the quaternization reaction of 4vinylbenzyl chloride with trialkylamine and the anion exchange reaction of halide ions with PF6− anions. The quaternization reaction was slow with a moderate yield, and the resulting salts were soluble in water for [VBTEA]Cl, but insoluble for [VBTPA]Cl and [VBTBA]Cl. Therefore, acetonitrile was used to dissolve [VBTPA]Cl and [VBTBA]Cl for the next anion exchange reaction. The anion exchange reaction of (pvinylbenzyl)alkylammonium chloride with PF6− produced resulting IL monomers as crystalline solids, which were insoluble in water but soluble in polar solvents such as DMF, DMSO, and so on. Silver nitrate tests indicated that no chloride was present in all monomers. Then, we used a microwaveassisted dispersion polymerization to polymerize IL monomers into PIL particles. Meanwhile, 2,2′-azobis(isobutyronitrile) (AIBN) was used as an initiator and poly(vinylpyrrolidone) (PVP) was used as the steric stabilizer. A mixture of DMF and ethanol (volume ratio of DMF to ethanol = 1:4) was used as solvent. During polymerization, the monomode microwave irradiation was used as heating source in order to induce fast and effective homogeneous heating and reduce side reactions. Finally, the products were separated, washed with ethanol and water, and vacuum-dried to obtain resulting monodisperse PIL particles. A more detailed description about synthesis is given in the Supporting Information. 6227


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The Journal of Physical Chemistry B

Figure 1. SEM images of PIL particles: (a) P[VBTMA][PF6]; (b) P[VBTEA][PF6]; (c) P[VBTPA][PF6]; (d) P[VBTBA][PF6].

Characterization. The particle morphology was observed by scanning electron microscopy (SEM, Hitachi TM-3000). 1H nuclear magnetic resonance (1H NMR) measurement was carried out using a Bruker DPX-400 spectrometer operating at 400 MHz. DMSO-d6 was used as solvent. The chemical group of samples was determined by the Fourier transform infrared spectra (FT-IR, JASCO FT/IR-470 Plus). The thermal transformation of samples was determined by the thermogravimetric analyzer (TGA, Netzsch STA449F3) with heating rate of 10 °C/min within 30−800 °C under air atmosphere. The glass transition temperature (Tg) of samples were estimated using a differential scanning calorimeter (DSC, Q200, TA Instruments) at a heating and cooling rate of 10 °C/min. The measurement temperature ranged from 0 to 280 °C. Tg values were calculated in the second scanning, taken from the midpoint of the total heat flow curve in the thermal transition region. Preparation of ER Fluid. The PIL particles were further dried in vacuum for 48 h and then mixed into dimethyl silicone oil (KF-96, Shin-Etsu Chemical Co. Ltd.) with a kinetic viscosity of 50 cSt and a density of ∼0.96 g/cm3 at 25 °C by mechanical stirring and ultrasonic to form uniform suspension. The volume fraction (ϕ) of particles in suspension was defined by the ratio of particle volume to total suspension volume. The density of PIL particles was measured by a pycnometer method. Because PIL particles are hydrophobic, we used silicone oil (KF-96, Shin-Etsu Chemical Co. Ltd.) with a density of ∼0.96 g/cm3 as the dispersing liquid. In the density measurement, the PIL particles were first added into the pycnometer (5 mL) containing oil. Then, the pycnometer was placed in an ultrasonic cleaning bath and connected to a vacuum pump. After ultrasonication under reduced pressure for 10 min to remove the air in the PIL particles, the pycnometer was filled with additional oil, and the density was measured. The density of P[VBTMA][PF6], P[VBTEA][PF6], P[VBTPA][PF6], and P[VBTBA][PF6] particles is ∼1.31, ∼1.35, ∼ 1.27, and ∼1.26 g/cm3, respectively. Although there is slight difference in

particle density, all of the PILs are hard solid due to their high glass transition temperature (over 150 °C). Rheological Measurement. The ER property of suspension was measured by a stress-controlled electrorheometer (Thermal-Haake RS600) with a parallel plate system with a diameter of 35 mm and gap of 1.0 mm, a DC high-voltage generator, an oil bath system, and a computer. The flow curves of the shear stress-shear rate were measured by the controlled shear rate mode within 0.1−1000 s−1 in the temperature range of 20−120 °C. Before each measurement, we presheared the suspension for 60 s at 300 s−1 and then applied electric fields for 30 s to ensure the formation of equilibrium chain structures before shearing. The static yield stress (τs), which is defined as the stress that allows a solidified ER fluid to start to flow, was approximately obtained from log−log plot rheograms by extrapolating the stress of the pseudoplateau region in low shear rate region to zero shear rate.14 The dynamic yield stress (τd), which is defined as the stress making ER fluid continuously flow, was approximately obtained by using Bingham function eq 1 to fit the flow curves in the high shear rate region and then extrapolating the fitting lines to zero shear rate15 τ = τd + ηp1γ ̇

(1)

where ηpl is the plastic viscosity and γ̇ is the shear rate. Conductivity and Dielectric Spectra Measurement. In order to avoid the influences of moisture and porosity on the conductivity and dielectric properties when using the powder method, we measured the electric properties of PIL particles in suspension. In the conductivity measurement, the leaking current density through suspensions under electric fields was detected by galvanometer. The conductivity of suspension was determined according to σ = j/E, where j is the current density through suspensions and E is the electric field strength. Because high electric field over 1.0 kV/mm was applied to suspension and, thus, the dispersed particles could form fibrous-like structure and this structured suspension was analogous to 6228


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Figure 2. TGA (A) and DSC (B) curves of PIL particles: (a) P[VBTMA][PF6]; (b) P[VBTEA][PF6]; (c) P[VBTPA][PF6]; (d) P[VBTBA][PF6].

Figure 3. Flow curves of shear stress vs shear rate for the suspensions of PIL particles under various DC electric fields: (A) P[VBTMA][PF6]; (B) P[VBTEA][PF6]; (C) P[VBTPA][PF6]; (D) P[VBTBA][PF6]. (T = 25 °C, ϕ = 20 vol %). Black dash lines show the fit by eq 1.

fibrous composite. Thereby, the conductivity of particles could be calculated by the following approximate mixture conductivity eq 2:16 σ = ϕσp + (1 − ϕ)σf

structure within suspension was induced, and as a result, we could well evaluate the interfacial polarization.

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RESULTS AND DISCUSSION Structure Characteristic. The successful polymerization of IL monomers to PILs could be verified by 1H NMR and FT-IR spectra as shown in Figure S1 and Figure S2. Figure 1 shows the SEM images of resulting PIL particles. It is found that the PIL particles with different length of substituent alkyl chains attached to immobile ammonium charged site are all monodisperse microspheres with smooth surface. The diameter of P[VBTMA][PF6], P[VBTEA][PF6], P[VBTPA][PF6] and P[VBTBA][PF6] particles is 1.41 ± 0.02, 1.43 ± 0.08, 1.52 ± 0.12, and 1.42 ± 0.08 μm, respectively. The particles are of similar size possibly due to the same backbone structure, same anion (PF6−), and synthetic conditions used. The employment

(2)

Here, σ is the conductivity of suspension, σp is the conductivity of particles, σf is the conductivity of liquid carrier, and ϕ is the volume fraction of particles in suspension. The dielectric properties of particles in suspension were measured by an impedance analyzer (HP 4284A) using a measuring fixture (HP 16452A) for liquids in the frequency range from 20 to 106 Hz. The liquid measuring fixture was heated by an oil bath. When the temperature reached the certain degree, we began the measurement. A 1 V bias electrical potential was applied during measurements. Thus, no fibrous 6229


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Figure 4. Static yield stress (A) and dynamic yield stress (B) as a function of electric field strengths for the suspensions of PIL particles. (T = 25 °C, ϕ = 20 vol %).

of the suspensions of PIL particles with different size of side chains in silicone oil under electric fields. Electrorheological Property. Figure 3 shows the rheological curves of shear stress as a function of shear rate for the suspensions of PIL particles with different length of substituent alkyl chains. Without electric fields, all suspensions are low viscous state with a slightly shear thinning behavior. Their off-field viscosities are very close, about 0.15 Pa·s calculated at 1000 s−1. This can be attributed to the fact that these PIL particles have similar particle size, morphology and density. With electric fields, all suspensions exhibit a significant increase in shear stress and behave like a plastic material with large yield stress, so-called ER effect. This field-induced thickening can be attributed to the formation of gap-spanning fibrous structure along the direction of electric fields due to interparticle electrostatic interaction. Meanwhile, the real-time switch response of shear stress to electric fields when alternately turned on and off shows the ER effect is rapid and reversible (see also Figure S3 in Supporting Information). As the electric field strength increases, the enhancement in yield stress is clearly visible in Figure 3. This is due to the increase of interparticle electrostatic interaction. Comparing the rheograms, however, we can see significant differences in the suspensions of PIL particles with different length of substituent alkyl chains, indicating that the ER property depends on the size of side chains. The first difference concerns the static yield stress that makes suspensions start to flow. It is well-known that, under electric fields, the ER fluid is solidified due to the formation of gapspanning fibrous structure. The critical stress that makes the solidified ER fluid start to flow is usually referred as static yield stress and it characterizes the rigidity of fibrous structure and the ER strength of suspension at the transformation point from solid into liquid or when it starts to flow.14,15 We approximately obtained the values of static yield stress from log−log plot rheograms like those in Figure 3 by extrapolating the stress of the pseudoplateau region in low shear rate region (0.1−1.0 s−1) to zero shear rate,15 and plotted them as a function of electric field strength in Figure 4A. It is seen that the static yield stress increases with the electric field strength. Meanwhile, the static yield stress of the suspension of P[VBTMA][PF6] particles is strongest compared to the other three suspensions at the same electric field strength. As the length of substituent alkyl chains attached to immobile ammonium charged site increases, the static yield stress decreases. However, the static yield stress of

of microwave irradiation with a suitable power as heating source can improve the monodisperse level of the PIL particles. This may be related to the fact that microwave irradiation can provide homogeneous heating and accelerate the reaction rate, which favors the homogeneous formation of growing centers of polymer chains in the initial stage and high yield. Other factors such as reactant and polymerization conditions can influence the particle diameter and distribution. For example, the diameter of PIL particles increases as the PVP concentration decreases. According to the previous reports, however, the large difference in particle size and morphology could affect the ER properties.17 Therefore, to exclude the external particle shape effect, we controlled the PIL particles to be monodisperse sphere-like morphology with a similar diameter in this study. In addition, the PIL particles were soluble in DMF and DMSO but insoluble in water due to the presence of hydrophobic fluorinated counterions. Figure 2 shows the TGA and DSC curves of PIL particles. The TGA curves in Figure 2A show that all PIL particles possess relatively high thermal stability and their temperature corresponding to 10% weight loss (Td10) exceeds 300 °C. Td10 also depends on the length of side chains and the order is P[VBTMA][PF6] (335 °C) > P[VBTEA][PF6] (330 °C) > P[VBTPA][PF6] (317 °C) > P[VBTBA][PF6] (305 °C). The DCS curves in Figure 2B show that Tg of PIL particles depends on the length of side chains and the order of Tg is P[VBTMA][PF6] (258 °C) > P[VBTEA][PF6] (205 °C) > P[VBTPA][PF6] (166 °C) > P[VBTBA][PF6] (156 °C). This may be attributed to the fact that the increase of the length of alkyl side chains can reduce the restriction of mobility of polymer chains, and, thus, enhance the flexibility of polymer.18−20 On the basis of the characterizations above, monodisperse PIL particles with different length of substituent alkyl chains attached to immobile ammonium charged site have been successfully obtained by a microwave-assisted dispersion polymerization. These PIL particles have high Tg and thermal stability, which can make the PIL particles maintain a glassy state during rheological measurement in the temperature range 25−120 °C. These particles have uniform morphology and similar diameter, which can provide convince for the comparable study of the influence of the size of side chains on ER property because of no the effect from particle shape. In the following, we systematically investigate the ER characteristic 6230


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Figure 5. Flow curves of shear stress vs shear rate under zero (solid symbol) and 3.0 kV/mm (open symbol) for the suspensions of PIL particles at various temperatures: (A) P[VBTMA][PF6]; (B) P[VBTEA][PF6]; (C) P[VBTPA][PF6]; (D) P[VBTBA][PF6] (ϕ = 20 vol %).

in particular in low shear rate region. To make a further comparison, however, we got the values of dynamic yield stress in the Bingham function by extrapolating the fitting lines to a zero shear rate as shown in Figure 3 and plotted them as a function of electric field strength in Figure 4B. It is seen that the dynamic yield stress of the suspension of P[VBTMA][PF6] particles is also strongest. As the length of substituent alkyl chains increases, the dynamic yield stress also decreases. When comparing dynamic yield stress with static yield stress, however, it can see that the decline of dynamic yield stress as a function of the length of substituent alkyl chains is more significant compared to the static yield stress. For example, the value of dynamic yield stress of the suspension of P[VBTMA][PF6] particles is close to the magnitude of its static yield stress at the same electric field strength. The dynamic yield stress of the suspension P[VBTEA][PF6] particles is slightly lower than the static yield stress. However, the dynamic yield stress of the suspension of P[VBTPA][PF6] and the suspension of P[VBTBA][PF6] particles is much lower than their static yield stress, in particular at electric fields higher than 1.0 kV/ mm. It is known that the dynamic yield stress is the stress making ER suspension continuously flow and it can characterize the ER strength of suspension in the flow regime.14 Therefore, the results in Figure 4 clearly indicate that, although the ER strength of suspensions when they start to flow seems to be less dependence on the length of substituent alkyl chains if the alkyl chains exceeds three carbons, the ER strength in flow regime and the flow stability are still further degraded with the increase of the length of substituent alkyl chains. This can be attributed to the fact that the size variation of side chains has influenced the transport dynamic of mobile counterions and the

the suspension of P[VBTPA][PF6] particles is close to that of the suspension of P[VBTBA][PF6] particles at the same electric field strength. These reveal that the rigidity of fibrous structure or the ER strength of suspension of PIL particles at the transformation point from solid into liquid tends to degrade with the increase of the length of substituent alkyl chains but it seems to be less dependence on the length of substituent alkyl chains when the alkyl chains exceed three carbons. The second difference can be observed from the flow curves of shear stress as a function of shear rate. From Figure 3A, it can see that the flow curves of the suspension of P[VBTMA][PF6] particles maintain a very stable level or a wide plateau region of shear stress as a function of shear rate and the Bingham fluid model (see eq 1) can well fit the flow curves at various electric fields. The suspension of P[VBTEA][PF6] particles also maintains a stable flow in wide shear rate region but the shear stress at 3.0 kV/mm tends to slightly decline to a minimum value after a critical shear rate (see arrow in Figure 3B) and then increases again. The suspension of P[VBTPA][PF6] particles only shows a stable flow when the electric field strength is lower than 2 kV/mm. When the electric field strength is higher than 2 kV/mm, the flow curves only maintain a narrow plateau region within 0.1−5.0 s−1 and then the shear stress declines to a minimum value after a critical shear rate (see arrows in Figure 3C). The plateau region of the suspension of P[VBTBA][PF6] particles becomes much narrower and the shear stress tends to decline to a minimum value after a lower critical shear rate compared to the suspension of P[VBTPA][PF6] particles (see arrows in Figure 3D). Meanwhile, the Bingham fluid model cannot well fit the flow curves of the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles, 6231


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Figure 6. Static yield stress (A) and dynamic yield stress (B) at 3.0 kV/mm plotted as a function of temperature for the suspensions of PIL particles (ϕ = 20 vol %).

temperature, also supporting the fact that the flow curves of the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles tend to become more stable with the increase of temperature. The rheological results above clearly show that the size of side chains, in particular the length of substituent alkyl chains attached on the immobile charged site, has a significant influence on the ER property of PIL particles. The reason for this may be ascribed to the fact the size variation of side chains alters the ion transport and the ion motion-induced interfacial polarization. To understand this, we conduct a dielectric relaxation study below. Dielectric Spectra Analysis. Figure 7 shows the dependence of dielectric constant (ε′) and loss factor (ε″) on angular

rate of ion motion-induced interfacial polarization. We will discuss this below. To understand the impact of the size of side chains on ER property more deeply, we also measure the ER property at various temperatures. Figure 5 shows typical rheological curves under zero and 3.0 kV/mm of electric fields. At zero electric field, the rheological curves of all suspensions of PIL particles show a similar collapse with increasing working temperature. This can be attributed to the decrease of viscosity of silicone oil. At 3.0 kV/mm of electric field, however, the PILs with different size of substituent alkyl chains exhibit different temperature dependence. It can be seen from Figure 5A that, although the yield stress and shear stress of the suspension of P[VBTMA][PF6] particles are highest at room temperature, they tend to decrease as the temperature increases. The yield stress and shear stress of the suspension of P[VBTEA][PF6] particles are almost independent of temperature within the investigated temperature region (see Figure 5B). In contrast, although the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles show low yield stress and flow instability at room temperature, their yield stress increases and the flow curves tend to become more and more stable with the increase of temperature as shown in Figure 5, parts C and D. In particular, the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles can maintain flow stability in wide shear rate region of 0.1−1000 s−1 when the temperature exceeds 100 °C. Parts A and B of Figure 6 plot the temperature dependence of static yield stress and dynamic yield stress under 3.0 kV/mm for different suspensions. It clearly shows that the suspension of P[VBTMA][PF6] particles has the highest static yield stress at low temperature but the value of static yield stress gradually decreases with elevated temperature. Similarly, the dynamic yield stress of the suspension of P[VBTMA][PF6] particles also declines with increasing temperature. Both static yield stress and dynamic yield stress of the suspension of P[VBTEA][PF6] particles seems to be insensitive to the temperature. In contrast, although the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles show low static yield stress and dynamic yield stress at room temperature, their yield stresses increase with temperature. When the temperature goes up to 100 °C, the yield stresses of the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles even exceed those of the suspension of P[VBTMA][PF6] particles. At the same time, the value of dynamic yield stress is close to that of static yield stress at high

Figure 7. Dielectric spectra of the suspensions of PIL particles (ϕ = 13 vol %, T = 25 °C). Solid lines show the fit of ε′ and ε″ by eqs 4 and 5.

frequency (ω) for the suspensions of PIL particles at 25 °C. We employ the following relaxation function eq 3 including a Cole−Cole’s term, a DC conductivity term and an electrode polarization (EP) term to analyze the dielectric characteristic:21 ′ + ε*(ω) = ε′ + iε″ = ε∞

Δε′ σ +i + A ω −n 1 + (iωτ )α ε0′ω (3)

The following expressions are the solutions of ε′ and ε″: 6232


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The Journal of Physical Chemistry B Table 1. Dielectric Characteristics of the Suspensions of PIL Particles (ϕ = ∼13 vol %, T = 25 °C) sample P[VBTMA][PF6] P[VBTEA][PF6] P[VBTPA][PF6] P[VBTBA][PF6] a

ε′∞ 2.92 3.04 2.95 2.90

Δε′ 1.83 2.05 2.11 2.13

τ (s)

σ (S/m) −3

1.74 × 10 0.03 0.12 0.19

∼1.88 ∼8.38 ∼2.80 ∼1.83

× × × ×

σp (S/m)a −10

10 10−11 10−11 10−11

∼5.0 ∼1.6 ∼1.0 ∼1.1

× × × ×

10−9 10−9 10−10 10−10

The DC conductivity of particles calculated by eq 2 approximately.

⎤−1/2 ⎡ ⎛ πα ⎞ ′ + Δε′⎢1 + 2(ωτ )α cos⎜ ⎟ + (ωτ )2α ⎥ ε′ = ε∞ ⎝ 2 ⎠ ⎦ ⎣ πα ⎡ ⎤ sin 2 ⎥ + A ω −n cos⎢tan−1 πα −α ⎢⎣ (ωτ ) + cos 2 ⎥⎦ (4) ε″ =

other three suspensions locates beyond the low limit of adequate frequency range. Therefore, the adequate polarization rate should be the origin of high ER property of the suspension of P[VBTMA][PF6] particles at room temperature, while the inadequate polarization rate results in the degradation of ER property of the PIL particles with longer length of substituent alkyl chains as shown in Figure 4. Certainly, it is also noted that Δε′ seems to increase with the increase of the length of substituent alkyl chains attached on the immobile charged site. This may be because longer substituent alkyl chains have made the more mobile PF6− be limited in its ability to show local motion in particles rather than long-range drift through particles by steric effect. However, the degradation of ER property of PIL particles with the length of substituent alkyl chains indicates that the adequate relaxation time or polarization rate is chiefly important for achievement of high ER property. On the other hand, the relaxation time can also explain the flow behavior of suspensions under electric fields. It is known that the flow behavior of ER fluid is associated with the change of fibrous ER structure under the simultaneous effect of both electric and shear fields.27,28 As the shear rate increases, the gap spanning fibrous structure will be gradually destroyed. To achieve the rapid reorganization of destroyed fibrous structures during flow and maintain a stable flow behavior, it needs ER particles possess enough polarization response. Too slow or too fast polarization rate is not favorable to the stability and the reorganization of fibril structures during flow because too slow polarization rate easily results in insufficient particle polarization during flow, while too fast polarization rate easily results in an increase of repulsive interaction between particles due to the difference between the polarization direction and the direction connecting two particles. It is proposed that ER fluid with a polarization rate corresponding to the relaxation frequency within 102−105 Hz is appropriate for achieving a stable flow behavior in wide shear rate region under DC electric fields.24 From Figure 7, we can see that the relaxation frequency of the suspension of P[VBTMA][PF6] particles is located in the adequate frequency range of 102−105 Hz and the corresponding polarization rate is appropriate. Therefore, the P[VBTMA][PF6] particles can maintain a sufficient polarizability and a rapid reorganization of destroyed fibrous structures and, as a result, the suspension shows a stable flow behavior in a wide shear rate region as shown in Figure 3A. As the length of substituent alkyl chains attached on the immobile charged site increases, the relaxation time or polarization rate becomes slow and this is not favorable to the rapid reorganization of destroyed fibrous structure by shearing fields and, as a result, the flow becomes instable or the shear stress declines with shear rate. In particular, the polarization rate of P[VBTPA][PF6] and P[VBTBA][PF6] particles further becomes slow and, thus, the destroyed structure is more difficult to rebuilt. Thus, the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles show deep decline in the flow curves as shown in Figure 3,

⎤1/2 ⎡ ⎛ πα ⎞ σ + Δε′⎢1 + 2(ωτ )α cos⎜ ⎟ + (ωτ )2α ⎥ ⎝ 2 ⎠ ⎦ ⎣ ε0′ω πα ⎡ ⎤ sin 2 ⎥ sin⎢tan−1 πα −α ⎢⎣ (ωτ ) + cos 2 ⎥⎦ (5)

Here, Δε′ = ε0′ − ε∞ ′ is the dielectric polarization or relaxation strength (ε′0 and ε′∞ are the limit values of the relative dielectric constant at the frequencies below and above the relaxation frequencies, respectively), ω = 2πf is the angular frequency, τ = 1/ωmax is the relaxation time (ωmax is the local angular frequency of the dielectric loss peak), α is the Cole−Cole parameter indicating the broadening factor for the spectrum, σ is the DC conductivity, and n is related to the slope of EP’s high frequency tail, A is related to the amplitude of EP. Although this simple treatment of EP by inclusion of Aω−n cannot describe the EP process that is not responsible for ER phenomenon, the Aω−n is helpful for the accurate fitting of the main relaxation process.22 The solid lines in Figure 7 shows the eq 4 and eq 5 can well fit the ε′ and ε″ data and the dielectric characteristics obtained from the fitting procedure are listed in Table 1. It can be seen that, in the measured frequency range from 20 to 106 Hz, only the suspension of P[VBTMA][PF6] particles show a clear dielectric relaxation with a maximum of loss factor at ∼100 Hz and the corresponding relaxation time is 1.74 × 10−3 s and the relaxation strength is ∼1.83. As the length of substituent alkyl chains attached on the immobile charged site increases, the dielectric relaxation peak shifts toward lower frequency (150 °C), and no segmental relaxation influences the ionic conductivity by coupling effect. According to the model proposed by Nakamura et al.,29 the mobile counterions in the glassy PIL matrix would be transported or diffused upon repetition of the ion-pair formation and dissociation processes to form conduction. Thus, the local microscopic surrounding, i.e., the side chains on the immobile charged sites, will influence the transport dynamic of mobile ions. For the present (pvinylbenzyl)trialkylammonium hexafluorophosphate-based PIL particles, the longer substituent alkyl chains attached on the immobile ammonium cation site may provide higher potential barrier or steric effects for mobile PF6− and thus decrease the mobility of PF6−.30,31 Therefore, the PIL particles with longer substituent alkyl chains have a lower conductivity and a slower relaxation time. The activation energy related to the mobility of

parts C and D. Therefore, the dependence of ER property on the size of side chains can be explained by the dielectric relaxation above. Since the dielectric constant and loss factor of silicone oil are almost independent of frequency and temperature within the investigated range, the dielectric relaxation of the suspensions should arise from the polarization of dispersed PIL particles. Also, because Tg of these PIL particles are all higher than 150 °C and the segmental relaxation of PIL particles is very weak at room temperature,29 the dielectric relaxation in the suspensions should mainly arise from the local ion motion-induced interfacial polarization of PIL particles in silicone oil. Thus, the conductivity from the motion of conducting ions is the key to the interfacial polarization. From Table 1, we can see that the conductivity gradually becomes low as the length of substituent alkyl chains increases, which is also in accordance with the fact that the relaxation time or polarization rate becomes slow with the increase of length of substituent alkyl chains. On the molecular level, the quaternary ammonium cations are attached on the polymer backbone in the family of (p-vinylbenzyl)trialkylammonium hexafluorophosphate-based PIL particles, the motion of conducting ions should originate from the mobile PF6− counteranions. Two reasons may be responsible for the change of ionic conductivity with the length of substituent alkyl chains attached on the ammonium cation site. One is the difference in the number density of mobile PF6− ions in the PIL particles with different length of substituent alkyl chains at the same particle volume fraction. Here, the 6234


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Figure 9. Temperature dependence of the reciprocal of relaxation time (A) and conductivity (B) for the suspensions of PIL particles. Solid lines are fits by the Arrhenius equation, shown in eq 6.

sensitive to the size of substituent alkyl chains attached on the immobile charged site when the alkyl chain length does not exceed three carbon atoms, and then, it becomes insensitive to the legnth of substituent alkyl chains when the alkyl chain length exceeds three carbon atoms. This change tendency is in accordance the change of ER property as a function of the length of substituent alkyl chains. On the molecular level, the increase of Ea or the decrease of mobility of mobile PF6− can be attributed to the fact that longer substituent alkyl chains has provided higher potential barrier or steric effect for mobile PF6−. According to the decreased glass transition temperature of PILs in Figure 2B, however, the longer substituent alkyl chains attached to immobile charged site may also increase the polymer flexibility which partly offsets the contribution of steric effect. Therefore, the change of Ea becomes insensitive to the length of substituent alkyl chains when the alkyl chain length exceeds three carbons. Anyway, for PIL particles in glassy state, the length of substituent alkyl chains attached to immobile charged site can influence the ionic mobility and the relaxation time, and this is responsible for the dependence of ER property on the size of side chains. In addition, the temperature-dependent dielectric relaxation and activation energy of relaxation time or ionic conductivity can also explain the different temperature dependence of ER property of the PIL particles with different size of side chains. As the temperature increases, the dielectric relaxation peak of the suspension of P[VBTMA][PF6] particles shifts toward higher frequency but the relaxation strength decreases as shown in Figure 10. As mention above, a good ER property requires ER fluid should first have a proper dielectric relaxation time or dielectric relaxation peak within an adequate frequency range of 102−105 Hz and then have a large relaxation strength or Δε′. Δε′ represents the achievable polarizability of ER particles. Thus, although the P[VBTMA][PF6] particles show appropriate polarization rate within an adequate frequency range of 102−105 Hz at various temperatures, the decrease of relaxation strength has led to the decline of achievable polarizability and, as a result, the ER strength of the suspension of P[VBTMA][PF6] particles degrades with temperature as shown in Figure 5A. We further consider that the decrease of relaxation strength may be ascribed to the low activation energy of ionic transport in the P[VBTMA][PF6] particles because the low activation energy is easy to result in thermally promoted long-range drift of more PF6− ions and large ion leakage through particles and, thus, weaken the interfacial polarization from the local motion

counterions, which is calculated by temperature-modulated dielectric relaxation measurements, can further support this point. Figure 8 shows the temperature-modulated dielectric relaxation spectra. As temperature increases, the dielectric relaxation peak of all of the suspensions shifts toward higher frequency and the remarkably sharp ramps in ε′ and ε″ are observed in the lower frequency regime. The shifts of dielectric relaxation peak toward higher frequency indicate that increasing temperature has increased the rate of interfacial polarization, while the sharp ramps in ε′ and ε″ in the low frequency regime reveal that the direct current conductivity through the samples has also been thermally promoted. The temperature dependence of the reciprocal of relaxation time and the conductivity obtained by fitting ε′ and ε″ in Figure 8 with the relaxation function eq 4 and 5 is shown in Figure 9. The reciprocal of relaxation time and the conductivity well agree with the following Arrhenius equation: τ −1 or σ ∝ e−Ea / RT

(6)

Here Ea is the activation energy, R is the molar gas constant, and T is the absolute temperature. This Arrhenius dependence also indicates that thermal diffusion of ions, rather than segmental relaxation, is dominating the process of ionic conductivity and polarization in the PIL particles. Thus, the ionic mobility is essentially important. To evaluate the mobility of the PF6− counteranions, Ea is calculated and the value of Ea for the relaxation mode and conductivity mode are shown in parts A and B of Figure 9, respectively. Note that the value of Ea for the relaxation mode is slightly smaller than that for ionic conductivity mode. This can be attributed to the fact that, although the relaxation mode and direct current conductivity derive from the same phenomenon (i.e., thermal diffusion of mobile ions), the formation of direct current conductivity in suspensions also requires the mobile ions to pass through the interface between particles and medium oil. It is seen that Ea depends on the length of substituent alkyl chains attached on the immobile charged site. The order of the value of Ea is P[VBTBA][PF6] > P[VBTPA][PF6] > P[VBTEA][PF6] > P[VBTMA][PF6]. This indicates that the potential energy necessary to activate the diffusion of counter PF6− increases with the increase of the length of substituent alkyl chains. However, it can be also noted that the value of Ea of P[VBTBA][PF6] and P[VBTPA][PF6] is relatively close. That is to say, the mobility of mobile PF6− in PIL particles is 6235


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temperature. As the length of substituent alkyl chains increases, the ER property of PIL particles at room temperature degrades but the temperature effect of ER property is enhanced. Dielectric spectroscopy analysis indicates that the size of side chains strongly influences the transport of mobile counterions by steric effect and the ion motion-induced interfacial polarization of PIL particles. The PIL particles with shortest substituent alkyl chains attached to immobile charged sites have the lowest activation energy of ionic conductivity and fastest relaxation time of interfacial polarization from local motion counterions. This should be responsible for the strongest ER property of PIL particles with shortest substituent alkyl chains at room temperature. As the length of substituent alkyl chains increases, the activation energy increases and the ion conductivity increases and, as a result, the rate of ion motioninduced interfacial polarization becomes slow and ER effect is degraded. As the temperature increases, however, the relaxation strength of PIL particles with shortest substituent alkyl chains is also easy to decrease, and this results in the degradation of ER property of PIL particles with shortest size of side chains at high temperature.

Figure 10. Temperature dependence of the relaxation strength of the suspensions of PIL particles.

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of ions. As the activation energy gets higher, the thermal promoted long-range drift of ions may be further restricted. For example, for the suspension of P[VBTEA][PF6] particles, Δε′ is only decreased from ∼2.05 to ∼2.00 when the temperature increases from 25 to 120 °C. Thus, the contribution from temperature-promoted polarization rate to the enhanced ER property will not be offset by the slight decrease of achievable polarizability. That is why the ER property of the suspension of P[VBTEA][PF6] particles has much less temperature dependence as shown in Figure 6. For the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles, the dielectric relaxation peak shifts toward higher frequency with the increase of temperature but the relaxation strength almost remains unchanged. In particular, it can be seen that the dielectric relaxation frequency of the suspension of P[VBTPA][PF6] particles starts to exceed 100 Hz at ∼80 °C and the dielectric relaxation frequency of the suspension of P[VBTBA][PF6] starts to exceed 100 Hz at 90 °C, which are located within an adequate frequency range of 102−105 Hz. At this temperature, their flow curves of shear stress as a function of shear rate have become very stable as shown in Figure 5, parts C and D. Therefore, the enhancement of ER property with temperature for the suspensions of P[VBTPA][PF6] and P[VBTBA][PF6] particles can be attributed to temperature-promoted polarization rate. However, it is also seen that the parameters such as relaxation time, conductivity, relaxation strength and activation energy are fairly close for both P[VBTPA][PF6] and P[VBTBA][PF6] and this can also explain why they have similar temperature dependence.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b02366. Description of the experimental details of synthesis procedures, the characterization about 1H NMR (Figure S1) and FT-IR (Figure S2), and the real-time switch response of shear stress of the suspensions to electric fields (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(J.Y.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Nos. 51572225 and 51502247).

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REFERENCES

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CONCLUSIONS A family of monodisperse (p-vinylbenzyl)trialkylammonium hexafluorophosphate-based PIL particles containing different length of substituent alkyl chains attached to immobile ammonium charged site has been prepared by a microwaveassisted dispersion polymerization for especially understanding the size influence of side chains of PILs on ER property. Under electric fields, temperature-modulated rheological measurements show that the size of side chains has a significant impact on ER property. The P[VBTMA][PF6] particles with shortest substituent alkyl chains have strongest ER property and stable rheological behavior in wide shear rate region at room 6236


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