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Enhanced interfacial polarization and electro-responsive characteristic of di-ionic poly(ionic liquid)s
Polymer 182 (2019) 121847
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Enhanced interfacial polarization and electro-responsive characteristic of di-ionic poly(ionic liquid)s Zhengyu Wang, Jia Zhao, Chen Zheng, Yang Liu, Xiaopeng Zhao, Jianbo Yin * Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710129, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Di-ionic poly(ionic liquid)s Interfacial polarization Electrorheological effect
Poly(ionic liquid)s are providing potential platforms to develop new generation of water-free polyelectrolytebased electorheological materials due to strong polarizability and intrinsic hydrophobicity. To enhance polari zation and electro-response by employing combined contribution of ion number density and ion transport dy namic, we here have developed a kind of di-ionic poly(ionic liquid)s for use as new electorheological materials. It has found that di-ionic poly(ionic liquid)s have enhanced electorheological effect compared with common monoionic poly(ionic liquid)s but enhancement degree depends on counterion type. The enhancement of di-ionic poly (ionic liquid)s with PF6 as counterions is more significant than those with TFSI as counterions. By combining dielectric spectra with Raman and X-ray scattering analyses, it has demonstrated that enhanced electorheological effect of di-ionic poly(ionic liquid)s is because di-ionic structure can increase interfacial polarization by com bined contribution of enhanced ion number density and transport dynamic. However, the enhancement degree is more significant for PF6 than TFSI due to its small size and low plasticizing capacity.
1. Introduction Electrorheological fluids (ERFs) are a class of stimuli-responsive colloid suspensions whose rheological properties can be quickly and reversibly modulated by an external electric field [1]. When the electric field strength is enough, ERFs can even solidify into viscoelastic solids. When the electric field is turned off, ERFs can go back to viscous fluids. This electrically tunable rheology makes ERFs have many potential applications in automotive, aerospace and medical fields [2–4]. To achieve the real applications, ERFs having versatile performances including high electro-response, low particle sedimentation, and ther mal ability are specially desired [5]. In the past decades, ERFs based on various materials including inorganic, organic, and composite have been developed. Among these systems, ERFs with polyelectrolyte particles as the dispersed phase have received wide attentions because of advan tages, such as the softness of particles and therefore reduced abrasion, the low density and therefore reduced particle sedimentation, and the high density of ion pair and therefore large polarizability for ER effect [6]. The earliest polyelectrolyte-based ERFs have been invented based on classic polyelectrolyte system in 1980s. The typical materials include poly(lithium methacrylate), poly(sodium styrene sulfonate), ion-
exchange resin, and so on [6]. Among them, poly(lithium methacry late) is the most documented. However, the ER effect of the classic polyelectrolyte ERFs is only active when the particles absorb small amount of water. In dry state, their ER effect is very weak. This is pre sumably because untethered counterions in the classic polyelectrolytes are strongly bounded by ion-ion electrostatic interaction and the inter facial polarization related to ER effect cannot be induced [7]. Because the absorbed water also has a polarization function, however, it is difficult to disclose the origin of ER effect of classic polyelectrolytes and formulate the structure-property relationship. In addition, the absorbed water can also cause problems, such as dielectric breakdown, high current leakage, and thermal instability. In 1993, the scientists in Bayer AG have invented new ERFs based on polyether-salt solid electrolytes by dissolving an alkali metal salt (e.g. lithium chloride or zinc dichloride) in polyethylene oxide (PEO) poly mer [8]. Compared to the strong electrostatic interaction between metal cations and COO or SO3 groups in the classic polyelectrolytes, the interaction between metal cations and electron pairs of oxygen of PEO in the polymer electrolytes is much weaker. Thus, the polyether-salt par ticles can provide mobile ions to induce interfacial polarization and ER effect without water or other small molecule solvents. In despite of without affinity to extrinsic water, the polyether-salt ERFs are still
* Corresponding author. E-mail address: [email protected] (J. Yin). https://doi.org/10.1016/j.polymer.2019.121847 Received 3 July 2019; Received in revised form 30 August 2019; Accepted 27 September 2019 Available online 27 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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sensitive to moisture due to the hydrophilic nature of PEO. In addition, it is difficult to fabricate products in a powder or particle form due to the low glass transition temperature of polyether. To address these problems, we have developed a new class of poly electrolyte ERFs based on poly(ionic liquid) (PIL) particles in 2014 [9]. Different from the previous polyelectrolyte ER materials, the PILs are composed of polyatomic fluorinated component ions (e.g. (CF3SO2)2N , PF6 , etc.). These component ions make the PILs have not only weaker ion-ion electrostatic interaction compared to monoatomic component ions but also intrinsically hydrophobic nature [10–14]. As a result, the PILs are insensitive to moisture and possess high density of mobile ions to induce strong interfacial polarization in dry and glassy state. In particular, because the water function is completely excluded, the origin of ER effect and the link of structure-property can be well clarified. For example, it has found that the size of alkyl substituents on the tethered charged pendant groups has a significant impact on the ER effect of PILs because the variation of alkyl substituent size can alter the transport dynamic of mobile ions and ion motion-induced interfacial polarization [15]. The type of component ions also has a significant influence on the ER effect of PILs and the component ions with smaller size and larger plasticization effect can enhance the interfacial polarization and ER ef fect due to enhanced ion transport dynamic [16]. However, these studies have mainly focused on the influence of ion transport or mobility on the interfacial polarization and ER effect of PILs. According to the Nernst–Einstein equation, σ ¼ nqμ (where q is the elementary charges. n is the number density of delocalized or mobile charges, and μ is the mobility of mobile charges), not only the mobility but also the number of delocalized charges are important to the conductivity and interfacial polarization. However, the strategy of enhancing the ER effect of PIL-based ERFs by employing the combined contribution from the number density and mobility of mobile ions has not been considered. In this paper, we have synthesized a kind of di-ionic PILs (D-PILs) for ERFs. Compared to common mono-ionic PILs (M-PILs), the di-ionic structure is expected to not only intensify the number density of mo bile ions but also modify the ion transport dynamic for the improvement of interfacial polarization and ER effect of PILs. We here present the synthesis, structure characterization, and ER effect of D-PIL particles by comparing with M-PIL particles. Through dielectric spectroscopy and Xray scattering measurements, we also analyze the mechanism behind the enhancement of ER effect of D-PILs compared to common M-PILs.
2. Experimental section 2.1. Chemicals 1-Vinylimidazole (VIm, 99%) was purchased from Aldrich. 1-bromo butane (99%), KPF6 (99%), (2-bromoethyl)trimethylammonium bro mide ([BETA]Br, 99%), lithium bis(trifluoromethane sulfonylimide) (Li [TFSI], 99%) were purchased from Alpha Chemical Co. Ltd.. Dime thylformamide (DMF), acetone, methanol, ether, alcohol were pur chased from Sinopharm Chemical Reagent Co. Ltd. of China. These chemicals were used as received. Reagent grade 2,20 -azobis(iso butyronitrile) (AIBN) was purchased from Sinopharm Chemical Reagent Co. Ltd. of China and was purified by recrystallization in methanol. 2.2. Synthesis of D-PILs and M-PILs The synthesis route of D-PILs was presented in Scheme 1A. First, VIm and AIBN (AIBN/VIm ¼ 1 wt%) were dissolved in DMF and the solution was heated to 70 � C under N2 protection. After further stirring for 8 h at 70 � C, the white solid precipitate was formed. The precipitate was washed several times with acetone and vacuum dried at 70 � C to obtain poly(vinyl imidazole) (PVIm). Then, PVIm and [BETA]Br were mixed in ethanol and the mixture was refluxed under stirring for 6 d at 75 � C to form precipitate. The precipitate was washed several times with ethanol at 75 � C and further vacuum dried to get P[VIm][BETA]Br [17]. Finally, the P[VIm][BETA]Br and KPF6 or Li[TFSI] were dissolved in deionized water and stirred to form precipitate. The precipitate was washed several times with deionized water and vacuum dried to obtain the final D-PIL product P[VIm][BETA][PF6] or P[VIm][BETA][TFSI]. The synthesis route of M-PILs was presented in Scheme 1B: First, VIm and AIBN (AIBN/VIm ¼ 1 wt%) were dissolved in DMF and the solution was heated to 70 � C under N2 protection. After further stirring for 8 h at 70 � C, the white solid precipitate was formed. The precipitate was washed several times with acetone and vacuum dried at 70 � C to obtain poly(vinyl imidazole) (PVIm). Then, the PVIm and 1-bromobutane were mixed in ethanol and the mixture was refluxed for 6 d at 75 � C to form precipitate. The precipitate was washed several times with ether and dried in vacuum to obtain P[C4VIm]Br. Finally, P[C4VIm]Br and KPF6 or Li[TFSI] were dissolved in deionized water and stirred to form precipi tate. The precipitate was washed several times with deionized water and vacuum dried to obtain the final M-PIL product P[C4VIm][PF6] or P [C4VIm][TFSI].
Scheme 1. Synthesis routes of P[VIm][BETA][PF6] or P[VIm][BETA][TFSI] D-PILs (A) and P[C4VIm][PF6] or P[C4VIm][TFSI] M-PILs (B). 2
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2.3. Preparation of ERFs
3. Results and discussion
First, the D-PILs and M-PILs were ground and sieved to form fine particles with mean size of 5–15 μm. Then, the particles were further vacuum dried for 48 h at 50 � C. Finally, the dried particles were dispersed into methyl silicone oil (KF-96, kinetic viscosity ¼ 50 cSt, Shin-Etsu Chemical Co. Ltd) by mechanical stirring to get ERFs. The volume fraction of particles in ERFs is 25%. The density of particles is measured by pycnometer method. The density of P[C4VIm][PF6] parti cles is 1.76 g/cm3 and that of P[VIm][BETA][PF6] particles is 1.74 g/ cm3. The density of P[C4VIm][TFSI] particles is 1.78 g/cm3 and that of P [VIm][BETA][TFSI] particles is 1.80 g/cm3.
Fig. 1 presents the 1H NMR spectra of D-PILs and M-PILs. It can be seen the 1H NMR of P[VIm][BETA][PF6] is similar to that of P[VIm] [BETA][TFSI] and the 1H NMR of P[C4VIm][PF6] is similar to that of P [C4VIm][TFSI] due to their same main chains and the absence of hydrogen atoms in counterions. For example, both P[VIm][BETA][PF6] and P[VIm][BETA][TFSI] show one broad peak at ~2.10 ppm corre sponding to the hydrogens at the alkyl backbone that does not connect with imidazole ring, one peak at 3.22 ppm corresponding to the hy drogens of three methyl groups on the ammonium group, one peak at 3.79 ppm corresponding to the hydrogens of the methylene group con nected with the ammonium group, one broad peak at 4.56 ppm corre sponding to the hydrogens of the methylene group and the hydrogen on the alkyl backbone that connects with imidazole ring, and three broad peaks at 6.39–9.11 ppm corresponding to the hydrogens of the imid azole ring, respectively [17]. Both P[C4VIm][PF6] and P[C4VIm][TFSI] show one peak at 0.93 ppm corresponding to the hydrogens at the end of the alkyl side chain, one peak at 1.28 ppm corresponding to the hydrogen of the methylene connected with the end of the alkyl side chain, one peak alone with a broad peak at 1.70 ppm corresponding to the hydrogens of the methylene in the middle of the alkyl chain and on the alkyl backbone that does not connect with imidazole ring, one broad peak at 4.03 ppm corresponding to the hydrogens at methylene group connected with imidazole ring and at the alkyl backbone connect with imidazole ring, and three broad peaks at 6.39 ppm–9.11 ppm corre sponding to the hydrogens of imidazole ring, respectively. Two sharp peaks at 2.50 ppm and 3.39 ppm are from DMSO‑d6 and H2O in solvents. In addition, by the integral of peaks in 1H NMR, the percentage of the quaternized units on the polymer chains is estimated. The quaternized grafting efficiencies of P[VIm][TMEN][PF6] and P[C4VIm][PF6] are ~63% and ~62%, respectively. The grafting efficiencies of P[VIm] [TMEN][TFSI] and P[C4VIm][TFSI] are ~62% and ~60%, respectively. Although it is difficult to achieve higher grafting efficiencies, the values of the grafting efficiencies are close for D-PILs and M-PILs. So, it will not influence ER effect comparison. Fig. 2 shows the FTIR spectra of D-PILs and M-PILs. It can be seen that P[VIm][BETA][PF6] and P[C4VIm][PF6] show many similar adsorption bands due to their same main chains and counterions except for the end of pendant groups, such as 3149 cm 1 and 2967 cm 1 bands corresponding to C–H stretching vibration from imidazole ring and alkyl – N stretching vibration of chain, 1632 cm 1 band corresponding to C– 1 imidazole ring, 1131 cm band corresponding to C–N stretching vi bration of imidazole ring, 1490 cm 1 band corresponding to C–N vi bration of ammonium cation, and 840 cm 1 band corresponding to P–F stretching vibration of PF6 [18]. Compared to those of P[C4VIm][PF6], however, the intensity of 1487 cm 1 band corresponding to C–N
2.4. Characterization and measurements 1
H nuclear magnetic resonance (1H NMR) spectra of samples were carried out using a Bruker DPX-400 spectrometer operating at 400 MHz. DMSO‑d6 was used as solvent. Fourier transform infrared (FT-IR) spectra of samples were determined by a JASCO FT/IR-470 Plus Fourier trans form infrared spectrometer. The glass transition temperature (Tg) of samples was estimated using a PerkinElmer DSC8500 differential scan ning calorimeter (DSC) at a heating and cooling rate of 10 � C/min in nitrogen. Tg value was taken from the midpoint of total heat flow curves in the second heating run. Scanning electron microscopy (SEM) of samples was observed on a Hitachi TM-3000 scanning electron micro scopy. Wide-angle X-ray scattering (WAXS) spectra of samples were conducted on a SAXS/WAXS SYSTEM at 50 kV and 0.6 mA, XENOCS instrument at the Shared Materials Instrument Facility at Nanjing Uni versity of Science and Technology. The Raman spectra of samples were measured using a Witec Alpha 300 R spectrometer with an integral Zeiss Axio Series microscope equipped with a 50x/0.80 POL objective with a resolution of 1.6 cm 1. A 532 nm TEM00 laser source was used for excitation with the power of 15 mW. The ER effect of ERFs was measured by a Thermal-Haake RS600 rheometer with a parallel plate system. The diameter of plate was 35 mm and the gap between upper plate and bottom plate was 1.0 mm. The plate system was connected with a DC high-voltage generator and the temperature of bottom plate was controlled by an oil bath. During test, the ERFs were added into the gap of between upper plate and bottom plate and then pre-sheared for 60 s at 300 s 1. After that, the electric field was applied and the rheological curves of shear stress-shear rate were measured by the controlled shear rate mode within 0.1–1000 s 1. The dielectric spectra of ERFs were measured by an impedance analyzer (Agilent 4284A) using a liquid fixture (Agilent 16452A) within frequency range of 20–106 Hz and temperature range of 20–120 � C.
Fig. 1. 1H NMR spectra: (A) P[VIm][BETA][PF6] and P[C4VIm][PF6]; (B) P[VIm][BETA][TFSI] and P[C4VIm][TFSI]. 3
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(a) 2879
1544
3162 2967
1159 555 838
T (%)
(b) 1490
(c) 2879
31622967
(d) 1056
653 571
13501195
4000
3500
3000
2500
2000
1500
1000
500
Fig. 3. SEM images: (a) P[C4VIm][PF6]; (b) P[VIm][BETA][PF6]; (c) P[C4VIm] [TFSI]; (d) P[VIm][BETA][TFSI].
-1
Wavenumber (cm ) Fig. 2. FTIR spectra: (a) P[C4VIm][PF6]; (b) P[VIm][BETA][PF6]; (c) P[C4VIm] [TFSI]; (d) P[VIm][BETA][TFSI].
vibration and the intensity of 2933 cm 1 band corresponding to C–H stretching vibration in CH3 of P[VIm][BETA][PF6] are significantly stronger due to the presence of ammonium cation. P[VIm][BETA][TFSI] and P[C4VIm][TFSI] also show many similar adsorption bands due to their same main chains and counterions except for the end of pendant group, such as 3162 cm 1 and 2967 cm 1 bands corresponding to C–H stretching vibration from imidazole ring and alkyl chain, 1632 cm 1 – N stretching vibration of imidazole ring, band corresponding to C– 1 1131 cm band corresponding to C–N stretching vibration of imidazole ring, 1487 cm 1 band corresponding to C–N vibration of ammonium cation, and characteristic bands corresponding to TFSI (S(¼O)2 asymmetrical stretching vibration at 1350 cm 1, S(¼O)2 symmetrical stretching vibration at 1195 cm 1, CF3 symmetrical stretching vibration – O stretching vibration at 1056 cm 1). Compared to at 1138 cm 1, S– those of P[C4VIm][TFSI], however, the intensity of 1487 cm 1 band corresponding to C–N vibration and the intensity of 2933 cm 1 band corresponding to C–H stretching vibration in CH3 of P[VIm][BETA] [TFSI] are also significantly stronger due to the presence of ammonium cation. Tg of P[C4VIm][PF6], P[VIm][BETA][PF6], P[VIm][BETA][TFSI], and P[C4VIm][TFSI] is 151 � C, 135 � C, 108 � C, and 75 � C, respectively. It is seen that the PILs containing inorganic PF6 counterions have higher Tg than the PILs containing organic TFSI counterions. This should be due to the larger plasticized effect of TFSI . In addition, compared to M-PILs, D-PILs have higher Tg. This should be due to the stronger polar effect of di-ionic structure. Because Tg of all PILs is higher than room tempera ture, the appearance of these PILs is hard glassy solid at room temper ature. To prepare ERFs, the PILs are milled and sieved into fine particles. Fig. 3 shows the morphology of particles. It is seen that the particles are irregular and the size is mainly distributed within 5–15 μm. Compared to P[C4VIm][TFSI] and P[VIm][BETA][TFSI] particles, the edge of P [C4VIm][PF6] and P[VIm][BETA][PF6] particles seems to be much sharper. This may be the PILs with PF6 as counterions are more brittle due to their higher Tg. Fig. 4 shows the optical microscopy photo of the ERFs of D-PIL and M-PIL particles dispersed in silicone oil. It is seen that, the PIL particles are randomly suspended in silicone oil before the electric field is applied, whereas the particles rapidly connect and form a gap-spanning fibrous-like structure between electrodes after the electric field is turned on. This fibrous-like structure originates from the polarization of parti cles and particle-particle electrostatic interaction, which can hinder the shear flow perpendicular to the electric field direction and enhance the
0.3 mm (A)
(B)
(C)
(D)
Fig. 4. Optical microscopy photo of ERFs without and with electric fields: (A) P [C4VIm][PF6],(B) P[VIm][BETA][PF6], (C) P[C4VIm][TFSI], (D) P[VIm] [BETA][TFSI] (ϕ ¼ ~3 vol%, T ¼ 23 � C).
shear stress or viscosity of ERFs [19]. When comparing these photos, however, we can find the morphology of fibrous-like structure of different samples seems to be different. From Fig. 4A and B, it can be observed that the fibrous-like structure of P[VIm][BETA][PF6] particles is thicker than that of P[C4VIm][PF6] particles. From Fig. 4C and D, however, it is observed that the fibrous-like structure of P[VIm][BETA] [TFSI] particles seems to be similar to that of P[C4VIm][TFSI] particles. This hints that the ER effect of P[VIm][BETA][PF6] may be stronger than P[C4VIm][PF6], whereas the ER effect of P[VIm][BETA][TFSI] may be close to that of P[C4VIm][TFSI]. To disclose the difference quantita tively, we measure their rheological curves below. Fig. 5 presents the rheological curves of shear stress a function of shear rate of ERFs at room temperature. Under zero electric field, all ERFs are in a low viscous state and their off-field viscosities are close, about 0.27 Pa at the shear rate of 1000 s 1. This is because the PIL particles have similar shape, size, and density. Under the electric field, the shear stress increases obviously and the ERFs behave like a plastic material showing an obvious yield stress. This is so called ER effect, which can be attributed to the formation of fibrous-like structure as shown in Fig. 4. At the same electric field, however, it is clearly seen that the magnitude of yield stress or shear stress of D-PIL ERFs is larger than that of M-PIL ERFs, in particular for D-PILs with PF6 as counterions. Fig. 6 plots the dependence of yield stress of ERFs on electric field strength. Here, the yield stress is so-called static yield stress (τs ) that can be obtained by extrapolating the shear stress corresponding to the low shear rate (1.0 s 1) pseudo-plateau to the zero shear rate in the above log-log rheological curves in Fig. 5 [20]. Obviously, at the same field strength, the yield stress of D-PIL ERFs is higher than that of M-PIL ERFs. But the enhancement degree of yield stress for P[VIm][BETA][PF6] ERF is more significant compared to P[VIm][BETA][TFSI] ERF. For example, at 3 kV/mm, the yield stress of P[VIm][BETA][PF6] ERF is 980 Pa, 4
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1200
103
P[C4VIm][PF6]
Yield stress (Pa)
Shear stress (Pa)
1000
102
0 kV/mm 0.5 1.0 2.0 3.0
101
100 10-1
100
101
102
P[C4VIm][TFSI]
P[VIm][BETA][PF6]
800
P[VIm][BETA][TFSI]
600 400 200 0
103
0
-1
1
2
3
4
Electric field (kV/mm)
Shear rate (s )
Fig. 6. Yield stress of ERFs as a function of electric field strength (ϕ ¼ 25 vol %,T ¼ 23 � C).
[TFSI] ERF is significantly broadened, the shear tress still shows a lightly decrease to a minimum value at high shear rate under high electric fields. This further reveals that the introduction of di-ionic structure into PILs can enhance the ER effect but the enhancement degree depends on the type of counterions. According to the proposed ER mechanisms, ER effect originates from the electric field-induced interparticle interaction, and the polarization, particularly interfacial polarization of ER particles is very important [21]. To understand the ER enhancement of di-ionic structured PILs, we investigate the polarization characteristic via a dielectric spectroscopy method below. Fig. 7 shows the dielectric spectra of ERFs at room temperature. It is seen that the four ERFs show a strong dielectric dispersion. Because the permittivity of carrier liquid (silicone oil) is independent of frequency within the measured temperature range, the dielectric dispersion should be associated with the polarization of dispersed PIL particles. However, it is noted that the intensity and rate of polarization is different among these ERFs. To make a quantitative comparison, we employ the following Cole–Cole’s equation to approximately fit the permittivity data [22,23].
Shear stress (Pa)
(B) 103
102
0 kV/mm 0.5 1.0 2.0 3.0
101
100 10-1
100
101
102
103
Shear rate (s-1)
ε* ðωÞ ¼ ε’ þ iε’’ ¼ ε’∞ þ
Fig. 5. Rheological curves of shear stress as a function of shear rate of ERFs under different electric field strengths: (A) P[VIm][BETA][PF6] (solid points) and P[C4VIm][PF6] (open points); (B) P[VIm][BETA][TFSI] (solid points) and P [C4VIm][TFSI] (open points) (ϕ ¼ 25 vol%,T ¼ 23 � C).
Δε’ 1 þ ði2πf λÞα
(1)
In Eq. (1), Δε0 ¼ ε0 0 - ε0 ∞ is the dielectric intensity (ε0 0 and ε0 ∞ are the limit values of ε0 at the frequency below and above the relaxation fre quency, respectively), ω is the angular frequency, λ ¼1/2πfmax is the relaxation time (fmax is the local frequency at the peak of ε00 ), α is the Cole–Cole parameter indicating the distribution of λ [24]. Table 1 lists the dielectric characteristic parameters. Among these parameters, Δε0 and λ are two important ones for ER effect according to the proposed ER mechanisms [21,25]. Δε0 reveals the intensity of po larization of particles, which is related to the magnitude of particle-particle electrostatic interaction induced by electric field. λ re veals the rate of polarization of particles, which is related to the rebuilding rate of particle-particle interaction or fibrous-like structure destroyed by shearing field [26,27]. As λ gets smaller within 102–105 Hz and a larger Δε0 is demonstrated, a stronger ER effect can be achieved. From Table 1, it can be seen that Δε0 and λ of the P[VIm][BETA][PF6] ERF is 5.42 and 0.0165 s, which are much larger and faster than those (Δε0 ¼ 3.70 and λ ¼ 0.09 s) of the P[C4VIm][PF6] ERF. Therefore, compared to the mono-ionic P[C4VIm][PF6], the enhanced polarization intensity and polarization rate of di-ionic P[VIm][BETA][PF6] should be
which is 1.5 times as high as 650 Pa of P[C4VIm][PF6] ERF. At 3 kV/mm, however, the yield stress of P[VIm][BETA][TFSI] ERF is 940 Pa, which is 1.2 times as high as 800 Pa of P[C4VIm][TFSI] ERF. These indicate that the introduction of di-ionic structure into PILs can enhance the ER effect but the enhancement degree depends on the type of counterions. Besides the yield stress, the flow behavior of D-PIL ERFs is also more stable than that of M-PIL ERFs. From Fig. 5A, it can be seen that, after the appearance of yield stress, the shear stress of P[VIm][BETA][PF6] ERF shows a plateau and then increases with the increase of shear rate, whereas the shear stress of P[C4VIm][PF6] ERF tends to decrease to a minimum and then climb with the increase of shear rate again. Simi larly, it can be seen from Fig. 5B that the shear stress as a function of shear rate for P[VIm][BETA][TFSI] ERF is also more stable compared to P[C4VIm][TFSI] ERF. Compared to P[VIm][BETA][TFSI] ERF, however, the enhancement degree in flow stability of P[VIm][BETA][PF6] ERF is more significant. Although the plateau region of the P[VIm][BETA] 5
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(A) 7.50
Dielectric constant, '
P[C4VIm][PF6]
2.50
P[VIm][BETA][PF6]
2.00
4.50
1.50 3.00
1.00
1.50
Dielectric loss factor, ''
' '' ' ''
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ion polarization caused by the shift between cations and anions under electric fields. The other is so-called interfacial polarization induced by the local diffusion of dissociated ions under electric fields. The former belongs to fast polarization mode and the latter belongs to slow polar ization mode. In Table 1, the high frequency dielectric constant ε0 ∞ depends on the fast polarization mode. Here, ε0 ∞ of the di-ionic PILs is much larger than that of mono-ionic PILs. This should be attributed to higher number density of ion pair in the di-ionic PILs. However, λ and Δε0 should be resulted from the slow interfacial polarization mode ac cording to the frequency range of relaxation peak appearance in Fig. 7. For the PILs, the positively charged imidazole ring or ammonium group are covalently attached to the polymer chain [28], the mobile ions should be from untethered counterions (PF6 or TFSI ). Thus, the num ber and mobility of mobile PF6 or TFSI are important for polarization intensity and polarization rate. Since the values of the grafting efficiencies are close for D-PILs and M-PILs, the number ratio of untethered PF6 or TFSI in D-PILs and MPILs can be roughly calculated according to the molecular weight of monomer, and particle density. Here, the number ratio of untethered PF6 is P[C4VIm][PF6]: P[VIm][BETA][PF6] ¼ 1: 1.32, that of untethered TFSI is P[C4VIm][TFSI]: P[VIm][BETA][TFSI] ¼ 1: 1.18. The number ratio of mobile TFSI between P[VIm][BETA][TFSI] and P[C4Vim] [TFSI] should be in accordance with this calculated ratio because most of TFSI tends to dissociate and move due to its low binding energy and plasticization function [29]. To support this, we measure the number proportion of dissociated or “free” TFSI ions to total TFSI ions in the PILs by Raman spectroscopy because the S–N stretching of (tri fluoromethylsulfonyl)imide at ~740 cm 1 is sensitive to the coordina tion environment [30,31]. Usually, the dissociated or “free” TFSI can produce S–N stretching peak at ~740 cm 1, whereas the S–N stretching peak in associated TFSI appears at higher frequency shift (>746 cm 1) [30]. Thus, as shown in Fig. 8, the peak at 741 cm 1 can be attributed to the dissociated or “free” TFSI , whereas the peak at 748 cm 1 is the characteristic peak of the associated TFSI in P[VIm][BETA][TFSI] and P[C4VIm][TFSI]. By a convolution of Voigt profiles, we can calculate the proportion of dissociated or “free” TFSI in P[VIm][BETA][TFSI] is ~84%, which is only slightly smaller than ~88% of P[C4VIm][TFSI]. However, the proportion of mobile PF6 between P[VIm][BETA][PF6] and P[C4Vim][PF6] should be not in accordance with the calculated proportion because PF6 tends to incompletely dissociate due to its relatively large binding energy with cation part [32]. Although we cannot directly measure the number proportion of mobile PF6 , the number proportion of mobile PF6 between P[VIm][BETA][PF6] and P [C4Vim][PF6] should be larger than the above calculated proportion because the binding energy of PF6 with ammonium cation part is much weaker compared to that with imidazole ring cation part due to the unique molecular structure of the ammonium cation, which provides a more effective shielding of electrostatic interaction with the anion compared to the imidazolium cation [33]. As the number of mobile ions increases, the intensity of interfacial polarization is usually improved. Therefore, the fact that Δε0 of P[VIm][BETA][PF6] is larger than that of P[C4Vim][PF6] should be because it has larger number of mobile PF6 , whereas the fact that Δε0 of P[VIm][BETA][TFSI] is close to that of P [C4Vim][TFSI] should be because they have close number of mobile TFSI . After dissociation, the transport of mobile PF6 or TFSI would be follow the repetition of the ion-pair formation and dissociation process, i.e. hopping mode, in the glassy polyelectrolyte matrix, where the ion motion is not aided by polymer segmental dynamic [34]. The mobility of mobile PF6 or TFSI is usually controlled by the characteristic energy barrier (Ea) separating ion sites. Ea can be obtained from temperature dependence of the reciprocal of relaxation time (λ 1) using the Arrhe nius equation λ 1∝exp(-Ea/RT), where R is the gas constant and T is the absolute temperature [34]. Fig. 9 plots the temperature dependence of λ 1. Before Tg, λ 1 as a function of temperature follows the Arrhenius dependence. The value of
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Dielectric loss factor, ''
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Fig. 7. Temperature-modulated dielectric spectra of ERFs: (A) P[C4VIm][PF6] and P[VIm][BETA][PF6]; (B) P[C4VIm][TFSI] and P[VIm][BETA][TFSI] (φ ¼~25 vol% T ¼ 25 � C). Solid lines are the fitting curves using Eq. (1). Table 1 Dielectric characteristic parameters of ERFs at room temperature (φ ¼ ~25 vol %, T ¼ 25 � C). Sample
ε0 ∞
Δε0
λ (s)
P[C4VIm][PF6] P[VIm][BETA][PF6] P[C4VIm][TFSI] P[VIm][BETA][TFSI]
2.60 2.95 2.80 3.07
3.70 5.42 4.33 4.25
9.00 � 10 2 1.65 � 10-2 4.70 � 10 3 1.80 � 10 3
responsible for the enhancement of ER effect and flow stability as shown Figs. 5 and 6. The λ of P[VIm][BETA][TFSI] is 0.0018 s, which is also faster than that (0.0047 s) of P[C4VIm][TFSI]. But Δε0 (4.25) of P[VIm] [BETA][TFSI] is close to that (4.33) of P[C4VIm][TFSI]. This indicates that compared to the mono-ionic P[C4VIm][TFSI], the polarization rate of di-ionic P[VIm][BETA][TFSI] is enhanced, whereas the polarization intensity is not enhanced. This is also in accordance with the fact that the enhancement degree of ER effect of P[VIm][BETA][TFSI] is not as sig nificant as that of P[VIm][BETA][PF6] as shown in Figs. 5 and 6. Let’s further analyze the dielectric property in Fig. 7 on a molecular level. Since the main chain of the PILs is nonpolar, the contribution from main chain to dielectric polarization is small. At the same time, the in fluence from segmental motion on dielectric dispersion in Fig. 7 can also be neglected at room temperature because their Tg is higher than room temperature. Thus, the dielectric polarization and observed dielectric dispersion should be closely related to the ion pair part. It’s well known that the ion pair can contribute two polarizations [22]. One is so-called 6
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Fig. 9. Temperature dependence of the reciprocal of relaxation time for ERFs: (A) P[C4VIm][PF6] and P[VIm][BETA][PF6]; (B) P[C4VIm][TFSI] and P[VIm] [BETA][TFSI] (φ ¼~25 vol%). Solid lines are the fitting curves using Arrhenius equation and dot lines are the fitting curves using Vogel-Fulcher-Tammann (VFT) equation.
Fig. 8. Raman spectra of P[VIm][BETA][TFSI] (A) and P[C4VIm][TFSI] (B). Voigt fitted curves are plotted in red (Right Peak) and green (Left Peak) lines.
Ea is 92.6 kJ/mol for P[C4VIm][PF6] and 83.2 kJ/mol for P[VIm][BETA] [PF6], respectively. The value of Ea is 108.6 kJ/mol for P[C4VIm][TFSI] and 103.5 kJ/mol for P[VIm][BETA][TFSI], respectively. It can be seen that the value of Ea for PILs with TFSI as counterions is larger than that of PILs with PF6 as counterions. This is because the size of TFSI is larger than that of PF6 . However, it can also be seen that the value of Ea of DPILs is lower than that of M-PILs. This indicates the energy barrier to activate the local motion of mobile counterions is decreased after the introduction of di-ionic structure into PILs. It has known that, in glassy PILs, two factors are controlling the energy barrier of ion transport. One is ion-ion electrostatic interaction [35]. Because the molecular structure of the ammonium cation can provide a better shielding of electrostatic interaction with the anion compared to the imidazolium cation, the contribution of electrostatic ion-ion interaction to energy barrier in D-PILs should be smaller than that in M-PILs. In addition, from Table 1, it can be seen the polarity of D-PILs is much higher than that of M-PILs, which can also decrease the electrostatic ion-ion interaction and thus decrease the energy barrier [35]. This is one of the reasons of that Ea of P [VIm][BETA][PF6] or P[VIm][BETA][TFSI] is lower than that of P [C4VIm][ PF6] or P[C4VIm][TFSI].
The other factor is elastic force, which depends on the local envi ronment of PIL matrix, ion jump length, and the radius of mobile ions. To understand these, we conduct a WAXS measurement as shown in Fig. 10. It is seen that both D-PILs and M-PILs display three scattering peaks at 3-4 nm 1 (qb), 8-10 nm 1 (qi) and 13-14 nm 1 (qp), which can be attributed to the spacing between ionic aggregates, the correlation between anions (PF6 -to-PF-6 or TFSI -to-TFSI-), and the amorphous halos, respectively [36]. The position of highest qp due to the amorphous halos is similar for both D-PILs and M-PILs. However, the intermediate qi in D-PILs shifts towards lower value compared to that in M-PILs and the intensity of qb in D-PILs is obviously lower than that in M-PILs. This indicates that the anion-to-anion (PF6 -to-PF-6 or TFSI -to-TFSI-) corre lation distance in D-PILs is shorter than that in M-PILs and the ionic aggregation in D-PILs is obviously weakened compared to that in M-PILs. Because the transport of mobile PF6 or TFSI in glassy PILs follows the hopping mode, the ion jump length or hopping distance is related to the anion-to-anion correlation distance [37]. The shorter PF6 -to-PF-6 or TFSI -to-TFSI- correlation distance means the ion jump length or hopping distance is shorter in D-PILs than that in M-PILs. Thus, 7
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in Fig. 5 according to the proposed ER mechanisms. Due to the different dissociation degree of counterions, however, the enhancement degree in polarization and ER effect depends on the type of counterions. The enhancement in polarization and ER effect of D-PILs is more significant when PF6 as counterions compared to TFSI with large size and plas ticizing capacity.
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Di-ionic PILs have been synthesized for use as new PIL-based ER materials. Under electric field, compared with common mono-ionic PILs, the D-PILs have enhanced ER effect but the enhancement degree depends on the type of counterions. At the same electric field, the ER enhancement of D-PILs with PF6 as mobile counterions is more signifi cant compared to D-PILs with TFSI as mobile counterions. Dielectric spectra and X-ray scattering measurements have indicated that the enhanced ER effect is because D-PILs possess higher interfacial polari zation intensity and rate compared to M-PILs due to the combined contribution of enhanced charge density and ion transport dynamicfrom high polarity di-ionic structure. However, the enhancement degree in polarization and ER effect of D-PILs is more significant when PF6 as counterions compared to TFSI with large size and plasticizing capacity.
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q (nm ) Fig. 10. X-ray scattering spectra: (A) P[C4VIm][PF6] (a) and P[VIm][BETA] [PF6] (b); (B) P[C4VIm][TFSI] (a) and P[VIm][BETA][TFSI] (b).
the energy barrier for ion transport in D-PILs should be lower than that for ion transport in M-PILs. In addition, compared to di-ionic P[VIm] [BETA][PF6] or P[VIm][BETA][TFSI], the alkyl chain of neighboring pendant group in mono-ionic P[C4VIm][PF6] or P[C4VIm][TFSI] can form the nonpolar regions resulting in aggregation more easily. The ionic aggregation morphology in mono-ionic P[C4VIm][PF6] or P [C4VIm][TFSI] can result in hindrance of the ion diffusion paths by allowing the formation of restrictive nonpolar regions between polymer chains, causing an increase of energy barrier and a decrease in the ion mobility [38]. Therefore, compared to mono-ionic P[C4VIm][PF6] or P [C4VIm][TFSI], the shorter PF6 -to-PF-6 or TFSI -to-TFSI- correlation distance and the weakened ion aggregation also can decrease the energy barrier of mobile ions and increase the ion mobility in di-ionic P[VIm] [BETA][PF6] or P[VIm][BETA][TFSI]. On the basis of the above analysis, the D-PILs can have not only higher number density of mobile ions but also faster mobility of mobile ions. Since the interfacial polarization originates from the transport and aggregation of mobile charges, the higher the number of mobile ions and the faster the mobility of mobile ions are, the greater polarization in tensity and the faster polarization rate will be achieved. Therefore, the D-PILs have larger polarization intensity and faster polarization rate compared to the M-PILs as shown in Table 1. As a result, the D-PILs show larger and more stable ER effect in the wide shear rate region as shown 8
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Enhanced interfacial polarization and electro-responsive characteristic of di-ionic poly(ionic liquid)s Zhengyu Wang, Jia Zhao, Chen Zheng, Yang Liu, Xiaopeng Zhao, Jianbo Yin * Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an, 710129, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Di-ionic poly(ionic liquid)s Interfacial polarization Electrorheological effect
Poly(ionic liquid)s are providing potential platforms to develop new generation of water-free polyelectrolytebased electorheological materials due to strong polarizability and intrinsic hydrophobicity. To enhance polari zation and electro-response by employing combined contribution of ion number density and ion transport dy namic, we here have developed a kind of di-ionic poly(ionic liquid)s for use as new electorheological materials. It has found that di-ionic poly(ionic liquid)s have enhanced electorheological effect compared with common monoionic poly(ionic liquid)s but enhancement degree depends on counterion type. The enhancement of di-ionic poly (ionic liquid)s with PF6 as counterions is more significant than those with TFSI as counterions. By combining dielectric spectra with Raman and X-ray scattering analyses, it has demonstrated that enhanced electorheological effect of di-ionic poly(ionic liquid)s is because di-ionic structure can increase interfacial polarization by com bined contribution of enhanced ion number density and transport dynamic. However, the enhancement degree is more significant for PF6 than TFSI due to its small size and low plasticizing capacity.
1. Introduction Electrorheological fluids (ERFs) are a class of stimuli-responsive colloid suspensions whose rheological properties can be quickly and reversibly modulated by an external electric field [1]. When the electric field strength is enough, ERFs can even solidify into viscoelastic solids. When the electric field is turned off, ERFs can go back to viscous fluids. This electrically tunable rheology makes ERFs have many potential applications in automotive, aerospace and medical fields [2–4]. To achieve the real applications, ERFs having versatile performances including high electro-response, low particle sedimentation, and ther mal ability are specially desired [5]. In the past decades, ERFs based on various materials including inorganic, organic, and composite have been developed. Among these systems, ERFs with polyelectrolyte particles as the dispersed phase have received wide attentions because of advan tages, such as the softness of particles and therefore reduced abrasion, the low density and therefore reduced particle sedimentation, and the high density of ion pair and therefore large polarizability for ER effect [6]. The earliest polyelectrolyte-based ERFs have been invented based on classic polyelectrolyte system in 1980s. The typical materials include poly(lithium methacrylate), poly(sodium styrene sulfonate), ion-
exchange resin, and so on [6]. Among them, poly(lithium methacry late) is the most documented. However, the ER effect of the classic polyelectrolyte ERFs is only active when the particles absorb small amount of water. In dry state, their ER effect is very weak. This is pre sumably because untethered counterions in the classic polyelectrolytes are strongly bounded by ion-ion electrostatic interaction and the inter facial polarization related to ER effect cannot be induced [7]. Because the absorbed water also has a polarization function, however, it is difficult to disclose the origin of ER effect of classic polyelectrolytes and formulate the structure-property relationship. In addition, the absorbed water can also cause problems, such as dielectric breakdown, high current leakage, and thermal instability. In 1993, the scientists in Bayer AG have invented new ERFs based on polyether-salt solid electrolytes by dissolving an alkali metal salt (e.g. lithium chloride or zinc dichloride) in polyethylene oxide (PEO) poly mer [8]. Compared to the strong electrostatic interaction between metal cations and COO or SO3 groups in the classic polyelectrolytes, the interaction between metal cations and electron pairs of oxygen of PEO in the polymer electrolytes is much weaker. Thus, the polyether-salt par ticles can provide mobile ions to induce interfacial polarization and ER effect without water or other small molecule solvents. In despite of without affinity to extrinsic water, the polyether-salt ERFs are still
* Corresponding author. E-mail address: [email protected] (J. Yin). https://doi.org/10.1016/j.polymer.2019.121847 Received 3 July 2019; Received in revised form 30 August 2019; Accepted 27 September 2019 Available online 27 September 2019 0032-3861/© 2019 Elsevier Ltd. All rights reserved.
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sensitive to moisture due to the hydrophilic nature of PEO. In addition, it is difficult to fabricate products in a powder or particle form due to the low glass transition temperature of polyether. To address these problems, we have developed a new class of poly electrolyte ERFs based on poly(ionic liquid) (PIL) particles in 2014 [9]. Different from the previous polyelectrolyte ER materials, the PILs are composed of polyatomic fluorinated component ions (e.g. (CF3SO2)2N , PF6 , etc.). These component ions make the PILs have not only weaker ion-ion electrostatic interaction compared to monoatomic component ions but also intrinsically hydrophobic nature [10–14]. As a result, the PILs are insensitive to moisture and possess high density of mobile ions to induce strong interfacial polarization in dry and glassy state. In particular, because the water function is completely excluded, the origin of ER effect and the link of structure-property can be well clarified. For example, it has found that the size of alkyl substituents on the tethered charged pendant groups has a significant impact on the ER effect of PILs because the variation of alkyl substituent size can alter the transport dynamic of mobile ions and ion motion-induced interfacial polarization [15]. The type of component ions also has a significant influence on the ER effect of PILs and the component ions with smaller size and larger plasticization effect can enhance the interfacial polarization and ER ef fect due to enhanced ion transport dynamic [16]. However, these studies have mainly focused on the influence of ion transport or mobility on the interfacial polarization and ER effect of PILs. According to the Nernst–Einstein equation, σ ¼ nqμ (where q is the elementary charges. n is the number density of delocalized or mobile charges, and μ is the mobility of mobile charges), not only the mobility but also the number of delocalized charges are important to the conductivity and interfacial polarization. However, the strategy of enhancing the ER effect of PIL-based ERFs by employing the combined contribution from the number density and mobility of mobile ions has not been considered. In this paper, we have synthesized a kind of di-ionic PILs (D-PILs) for ERFs. Compared to common mono-ionic PILs (M-PILs), the di-ionic structure is expected to not only intensify the number density of mo bile ions but also modify the ion transport dynamic for the improvement of interfacial polarization and ER effect of PILs. We here present the synthesis, structure characterization, and ER effect of D-PIL particles by comparing with M-PIL particles. Through dielectric spectroscopy and Xray scattering measurements, we also analyze the mechanism behind the enhancement of ER effect of D-PILs compared to common M-PILs.
2. Experimental section 2.1. Chemicals 1-Vinylimidazole (VIm, 99%) was purchased from Aldrich. 1-bromo butane (99%), KPF6 (99%), (2-bromoethyl)trimethylammonium bro mide ([BETA]Br, 99%), lithium bis(trifluoromethane sulfonylimide) (Li [TFSI], 99%) were purchased from Alpha Chemical Co. Ltd.. Dime thylformamide (DMF), acetone, methanol, ether, alcohol were pur chased from Sinopharm Chemical Reagent Co. Ltd. of China. These chemicals were used as received. Reagent grade 2,20 -azobis(iso butyronitrile) (AIBN) was purchased from Sinopharm Chemical Reagent Co. Ltd. of China and was purified by recrystallization in methanol. 2.2. Synthesis of D-PILs and M-PILs The synthesis route of D-PILs was presented in Scheme 1A. First, VIm and AIBN (AIBN/VIm ¼ 1 wt%) were dissolved in DMF and the solution was heated to 70 � C under N2 protection. After further stirring for 8 h at 70 � C, the white solid precipitate was formed. The precipitate was washed several times with acetone and vacuum dried at 70 � C to obtain poly(vinyl imidazole) (PVIm). Then, PVIm and [BETA]Br were mixed in ethanol and the mixture was refluxed under stirring for 6 d at 75 � C to form precipitate. The precipitate was washed several times with ethanol at 75 � C and further vacuum dried to get P[VIm][BETA]Br [17]. Finally, the P[VIm][BETA]Br and KPF6 or Li[TFSI] were dissolved in deionized water and stirred to form precipitate. The precipitate was washed several times with deionized water and vacuum dried to obtain the final D-PIL product P[VIm][BETA][PF6] or P[VIm][BETA][TFSI]. The synthesis route of M-PILs was presented in Scheme 1B: First, VIm and AIBN (AIBN/VIm ¼ 1 wt%) were dissolved in DMF and the solution was heated to 70 � C under N2 protection. After further stirring for 8 h at 70 � C, the white solid precipitate was formed. The precipitate was washed several times with acetone and vacuum dried at 70 � C to obtain poly(vinyl imidazole) (PVIm). Then, the PVIm and 1-bromobutane were mixed in ethanol and the mixture was refluxed for 6 d at 75 � C to form precipitate. The precipitate was washed several times with ether and dried in vacuum to obtain P[C4VIm]Br. Finally, P[C4VIm]Br and KPF6 or Li[TFSI] were dissolved in deionized water and stirred to form precipi tate. The precipitate was washed several times with deionized water and vacuum dried to obtain the final M-PIL product P[C4VIm][PF6] or P [C4VIm][TFSI].
Scheme 1. Synthesis routes of P[VIm][BETA][PF6] or P[VIm][BETA][TFSI] D-PILs (A) and P[C4VIm][PF6] or P[C4VIm][TFSI] M-PILs (B). 2
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2.3. Preparation of ERFs
3. Results and discussion
First, the D-PILs and M-PILs were ground and sieved to form fine particles with mean size of 5–15 μm. Then, the particles were further vacuum dried for 48 h at 50 � C. Finally, the dried particles were dispersed into methyl silicone oil (KF-96, kinetic viscosity ¼ 50 cSt, Shin-Etsu Chemical Co. Ltd) by mechanical stirring to get ERFs. The volume fraction of particles in ERFs is 25%. The density of particles is measured by pycnometer method. The density of P[C4VIm][PF6] parti cles is 1.76 g/cm3 and that of P[VIm][BETA][PF6] particles is 1.74 g/ cm3. The density of P[C4VIm][TFSI] particles is 1.78 g/cm3 and that of P [VIm][BETA][TFSI] particles is 1.80 g/cm3.
Fig. 1 presents the 1H NMR spectra of D-PILs and M-PILs. It can be seen the 1H NMR of P[VIm][BETA][PF6] is similar to that of P[VIm] [BETA][TFSI] and the 1H NMR of P[C4VIm][PF6] is similar to that of P [C4VIm][TFSI] due to their same main chains and the absence of hydrogen atoms in counterions. For example, both P[VIm][BETA][PF6] and P[VIm][BETA][TFSI] show one broad peak at ~2.10 ppm corre sponding to the hydrogens at the alkyl backbone that does not connect with imidazole ring, one peak at 3.22 ppm corresponding to the hy drogens of three methyl groups on the ammonium group, one peak at 3.79 ppm corresponding to the hydrogens of the methylene group con nected with the ammonium group, one broad peak at 4.56 ppm corre sponding to the hydrogens of the methylene group and the hydrogen on the alkyl backbone that connects with imidazole ring, and three broad peaks at 6.39–9.11 ppm corresponding to the hydrogens of the imid azole ring, respectively [17]. Both P[C4VIm][PF6] and P[C4VIm][TFSI] show one peak at 0.93 ppm corresponding to the hydrogens at the end of the alkyl side chain, one peak at 1.28 ppm corresponding to the hydrogen of the methylene connected with the end of the alkyl side chain, one peak alone with a broad peak at 1.70 ppm corresponding to the hydrogens of the methylene in the middle of the alkyl chain and on the alkyl backbone that does not connect with imidazole ring, one broad peak at 4.03 ppm corresponding to the hydrogens at methylene group connected with imidazole ring and at the alkyl backbone connect with imidazole ring, and three broad peaks at 6.39 ppm–9.11 ppm corre sponding to the hydrogens of imidazole ring, respectively. Two sharp peaks at 2.50 ppm and 3.39 ppm are from DMSO‑d6 and H2O in solvents. In addition, by the integral of peaks in 1H NMR, the percentage of the quaternized units on the polymer chains is estimated. The quaternized grafting efficiencies of P[VIm][TMEN][PF6] and P[C4VIm][PF6] are ~63% and ~62%, respectively. The grafting efficiencies of P[VIm] [TMEN][TFSI] and P[C4VIm][TFSI] are ~62% and ~60%, respectively. Although it is difficult to achieve higher grafting efficiencies, the values of the grafting efficiencies are close for D-PILs and M-PILs. So, it will not influence ER effect comparison. Fig. 2 shows the FTIR spectra of D-PILs and M-PILs. It can be seen that P[VIm][BETA][PF6] and P[C4VIm][PF6] show many similar adsorption bands due to their same main chains and counterions except for the end of pendant groups, such as 3149 cm 1 and 2967 cm 1 bands corresponding to C–H stretching vibration from imidazole ring and alkyl – N stretching vibration of chain, 1632 cm 1 band corresponding to C– 1 imidazole ring, 1131 cm band corresponding to C–N stretching vi bration of imidazole ring, 1490 cm 1 band corresponding to C–N vi bration of ammonium cation, and 840 cm 1 band corresponding to P–F stretching vibration of PF6 [18]. Compared to those of P[C4VIm][PF6], however, the intensity of 1487 cm 1 band corresponding to C–N
2.4. Characterization and measurements 1
H nuclear magnetic resonance (1H NMR) spectra of samples were carried out using a Bruker DPX-400 spectrometer operating at 400 MHz. DMSO‑d6 was used as solvent. Fourier transform infrared (FT-IR) spectra of samples were determined by a JASCO FT/IR-470 Plus Fourier trans form infrared spectrometer. The glass transition temperature (Tg) of samples was estimated using a PerkinElmer DSC8500 differential scan ning calorimeter (DSC) at a heating and cooling rate of 10 � C/min in nitrogen. Tg value was taken from the midpoint of total heat flow curves in the second heating run. Scanning electron microscopy (SEM) of samples was observed on a Hitachi TM-3000 scanning electron micro scopy. Wide-angle X-ray scattering (WAXS) spectra of samples were conducted on a SAXS/WAXS SYSTEM at 50 kV and 0.6 mA, XENOCS instrument at the Shared Materials Instrument Facility at Nanjing Uni versity of Science and Technology. The Raman spectra of samples were measured using a Witec Alpha 300 R spectrometer with an integral Zeiss Axio Series microscope equipped with a 50x/0.80 POL objective with a resolution of 1.6 cm 1. A 532 nm TEM00 laser source was used for excitation with the power of 15 mW. The ER effect of ERFs was measured by a Thermal-Haake RS600 rheometer with a parallel plate system. The diameter of plate was 35 mm and the gap between upper plate and bottom plate was 1.0 mm. The plate system was connected with a DC high-voltage generator and the temperature of bottom plate was controlled by an oil bath. During test, the ERFs were added into the gap of between upper plate and bottom plate and then pre-sheared for 60 s at 300 s 1. After that, the electric field was applied and the rheological curves of shear stress-shear rate were measured by the controlled shear rate mode within 0.1–1000 s 1. The dielectric spectra of ERFs were measured by an impedance analyzer (Agilent 4284A) using a liquid fixture (Agilent 16452A) within frequency range of 20–106 Hz and temperature range of 20–120 � C.
Fig. 1. 1H NMR spectra: (A) P[VIm][BETA][PF6] and P[C4VIm][PF6]; (B) P[VIm][BETA][TFSI] and P[C4VIm][TFSI]. 3
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(a) 2879
1544
3162 2967
1159 555 838
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(c) 2879
31622967
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653 571
13501195
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Fig. 3. SEM images: (a) P[C4VIm][PF6]; (b) P[VIm][BETA][PF6]; (c) P[C4VIm] [TFSI]; (d) P[VIm][BETA][TFSI].
-1
Wavenumber (cm ) Fig. 2. FTIR spectra: (a) P[C4VIm][PF6]; (b) P[VIm][BETA][PF6]; (c) P[C4VIm] [TFSI]; (d) P[VIm][BETA][TFSI].
vibration and the intensity of 2933 cm 1 band corresponding to C–H stretching vibration in CH3 of P[VIm][BETA][PF6] are significantly stronger due to the presence of ammonium cation. P[VIm][BETA][TFSI] and P[C4VIm][TFSI] also show many similar adsorption bands due to their same main chains and counterions except for the end of pendant group, such as 3162 cm 1 and 2967 cm 1 bands corresponding to C–H stretching vibration from imidazole ring and alkyl chain, 1632 cm 1 – N stretching vibration of imidazole ring, band corresponding to C– 1 1131 cm band corresponding to C–N stretching vibration of imidazole ring, 1487 cm 1 band corresponding to C–N vibration of ammonium cation, and characteristic bands corresponding to TFSI (S(¼O)2 asymmetrical stretching vibration at 1350 cm 1, S(¼O)2 symmetrical stretching vibration at 1195 cm 1, CF3 symmetrical stretching vibration – O stretching vibration at 1056 cm 1). Compared to at 1138 cm 1, S– those of P[C4VIm][TFSI], however, the intensity of 1487 cm 1 band corresponding to C–N vibration and the intensity of 2933 cm 1 band corresponding to C–H stretching vibration in CH3 of P[VIm][BETA] [TFSI] are also significantly stronger due to the presence of ammonium cation. Tg of P[C4VIm][PF6], P[VIm][BETA][PF6], P[VIm][BETA][TFSI], and P[C4VIm][TFSI] is 151 � C, 135 � C, 108 � C, and 75 � C, respectively. It is seen that the PILs containing inorganic PF6 counterions have higher Tg than the PILs containing organic TFSI counterions. This should be due to the larger plasticized effect of TFSI . In addition, compared to M-PILs, D-PILs have higher Tg. This should be due to the stronger polar effect of di-ionic structure. Because Tg of all PILs is higher than room tempera ture, the appearance of these PILs is hard glassy solid at room temper ature. To prepare ERFs, the PILs are milled and sieved into fine particles. Fig. 3 shows the morphology of particles. It is seen that the particles are irregular and the size is mainly distributed within 5–15 μm. Compared to P[C4VIm][TFSI] and P[VIm][BETA][TFSI] particles, the edge of P [C4VIm][PF6] and P[VIm][BETA][PF6] particles seems to be much sharper. This may be the PILs with PF6 as counterions are more brittle due to their higher Tg. Fig. 4 shows the optical microscopy photo of the ERFs of D-PIL and M-PIL particles dispersed in silicone oil. It is seen that, the PIL particles are randomly suspended in silicone oil before the electric field is applied, whereas the particles rapidly connect and form a gap-spanning fibrous-like structure between electrodes after the electric field is turned on. This fibrous-like structure originates from the polarization of parti cles and particle-particle electrostatic interaction, which can hinder the shear flow perpendicular to the electric field direction and enhance the
0.3 mm (A)
(B)
(C)
(D)
Fig. 4. Optical microscopy photo of ERFs without and with electric fields: (A) P [C4VIm][PF6],(B) P[VIm][BETA][PF6], (C) P[C4VIm][TFSI], (D) P[VIm] [BETA][TFSI] (ϕ ¼ ~3 vol%, T ¼ 23 � C).
shear stress or viscosity of ERFs [19]. When comparing these photos, however, we can find the morphology of fibrous-like structure of different samples seems to be different. From Fig. 4A and B, it can be observed that the fibrous-like structure of P[VIm][BETA][PF6] particles is thicker than that of P[C4VIm][PF6] particles. From Fig. 4C and D, however, it is observed that the fibrous-like structure of P[VIm][BETA] [TFSI] particles seems to be similar to that of P[C4VIm][TFSI] particles. This hints that the ER effect of P[VIm][BETA][PF6] may be stronger than P[C4VIm][PF6], whereas the ER effect of P[VIm][BETA][TFSI] may be close to that of P[C4VIm][TFSI]. To disclose the difference quantita tively, we measure their rheological curves below. Fig. 5 presents the rheological curves of shear stress a function of shear rate of ERFs at room temperature. Under zero electric field, all ERFs are in a low viscous state and their off-field viscosities are close, about 0.27 Pa at the shear rate of 1000 s 1. This is because the PIL particles have similar shape, size, and density. Under the electric field, the shear stress increases obviously and the ERFs behave like a plastic material showing an obvious yield stress. This is so called ER effect, which can be attributed to the formation of fibrous-like structure as shown in Fig. 4. At the same electric field, however, it is clearly seen that the magnitude of yield stress or shear stress of D-PIL ERFs is larger than that of M-PIL ERFs, in particular for D-PILs with PF6 as counterions. Fig. 6 plots the dependence of yield stress of ERFs on electric field strength. Here, the yield stress is so-called static yield stress (τs ) that can be obtained by extrapolating the shear stress corresponding to the low shear rate (1.0 s 1) pseudo-plateau to the zero shear rate in the above log-log rheological curves in Fig. 5 [20]. Obviously, at the same field strength, the yield stress of D-PIL ERFs is higher than that of M-PIL ERFs. But the enhancement degree of yield stress for P[VIm][BETA][PF6] ERF is more significant compared to P[VIm][BETA][TFSI] ERF. For example, at 3 kV/mm, the yield stress of P[VIm][BETA][PF6] ERF is 980 Pa, 4
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Electric field (kV/mm)
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Fig. 6. Yield stress of ERFs as a function of electric field strength (ϕ ¼ 25 vol %,T ¼ 23 � C).
[TFSI] ERF is significantly broadened, the shear tress still shows a lightly decrease to a minimum value at high shear rate under high electric fields. This further reveals that the introduction of di-ionic structure into PILs can enhance the ER effect but the enhancement degree depends on the type of counterions. According to the proposed ER mechanisms, ER effect originates from the electric field-induced interparticle interaction, and the polarization, particularly interfacial polarization of ER particles is very important [21]. To understand the ER enhancement of di-ionic structured PILs, we investigate the polarization characteristic via a dielectric spectroscopy method below. Fig. 7 shows the dielectric spectra of ERFs at room temperature. It is seen that the four ERFs show a strong dielectric dispersion. Because the permittivity of carrier liquid (silicone oil) is independent of frequency within the measured temperature range, the dielectric dispersion should be associated with the polarization of dispersed PIL particles. However, it is noted that the intensity and rate of polarization is different among these ERFs. To make a quantitative comparison, we employ the following Cole–Cole’s equation to approximately fit the permittivity data [22,23].
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ε* ðωÞ ¼ ε’ þ iε’’ ¼ ε’∞ þ
Fig. 5. Rheological curves of shear stress as a function of shear rate of ERFs under different electric field strengths: (A) P[VIm][BETA][PF6] (solid points) and P[C4VIm][PF6] (open points); (B) P[VIm][BETA][TFSI] (solid points) and P [C4VIm][TFSI] (open points) (ϕ ¼ 25 vol%,T ¼ 23 � C).
Δε’ 1 þ ði2πf λÞα
(1)
In Eq. (1), Δε0 ¼ ε0 0 - ε0 ∞ is the dielectric intensity (ε0 0 and ε0 ∞ are the limit values of ε0 at the frequency below and above the relaxation fre quency, respectively), ω is the angular frequency, λ ¼1/2πfmax is the relaxation time (fmax is the local frequency at the peak of ε00 ), α is the Cole–Cole parameter indicating the distribution of λ [24]. Table 1 lists the dielectric characteristic parameters. Among these parameters, Δε0 and λ are two important ones for ER effect according to the proposed ER mechanisms [21,25]. Δε0 reveals the intensity of po larization of particles, which is related to the magnitude of particle-particle electrostatic interaction induced by electric field. λ re veals the rate of polarization of particles, which is related to the rebuilding rate of particle-particle interaction or fibrous-like structure destroyed by shearing field [26,27]. As λ gets smaller within 102–105 Hz and a larger Δε0 is demonstrated, a stronger ER effect can be achieved. From Table 1, it can be seen that Δε0 and λ of the P[VIm][BETA][PF6] ERF is 5.42 and 0.0165 s, which are much larger and faster than those (Δε0 ¼ 3.70 and λ ¼ 0.09 s) of the P[C4VIm][PF6] ERF. Therefore, compared to the mono-ionic P[C4VIm][PF6], the enhanced polarization intensity and polarization rate of di-ionic P[VIm][BETA][PF6] should be
which is 1.5 times as high as 650 Pa of P[C4VIm][PF6] ERF. At 3 kV/mm, however, the yield stress of P[VIm][BETA][TFSI] ERF is 940 Pa, which is 1.2 times as high as 800 Pa of P[C4VIm][TFSI] ERF. These indicate that the introduction of di-ionic structure into PILs can enhance the ER effect but the enhancement degree depends on the type of counterions. Besides the yield stress, the flow behavior of D-PIL ERFs is also more stable than that of M-PIL ERFs. From Fig. 5A, it can be seen that, after the appearance of yield stress, the shear stress of P[VIm][BETA][PF6] ERF shows a plateau and then increases with the increase of shear rate, whereas the shear stress of P[C4VIm][PF6] ERF tends to decrease to a minimum and then climb with the increase of shear rate again. Simi larly, it can be seen from Fig. 5B that the shear stress as a function of shear rate for P[VIm][BETA][TFSI] ERF is also more stable compared to P[C4VIm][TFSI] ERF. Compared to P[VIm][BETA][TFSI] ERF, however, the enhancement degree in flow stability of P[VIm][BETA][PF6] ERF is more significant. Although the plateau region of the P[VIm][BETA] 5
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Dielectric constant, '
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P[VIm][BETA][PF6]
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Dielectric loss factor, ''
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ion polarization caused by the shift between cations and anions under electric fields. The other is so-called interfacial polarization induced by the local diffusion of dissociated ions under electric fields. The former belongs to fast polarization mode and the latter belongs to slow polar ization mode. In Table 1, the high frequency dielectric constant ε0 ∞ depends on the fast polarization mode. Here, ε0 ∞ of the di-ionic PILs is much larger than that of mono-ionic PILs. This should be attributed to higher number density of ion pair in the di-ionic PILs. However, λ and Δε0 should be resulted from the slow interfacial polarization mode ac cording to the frequency range of relaxation peak appearance in Fig. 7. For the PILs, the positively charged imidazole ring or ammonium group are covalently attached to the polymer chain [28], the mobile ions should be from untethered counterions (PF6 or TFSI ). Thus, the num ber and mobility of mobile PF6 or TFSI are important for polarization intensity and polarization rate. Since the values of the grafting efficiencies are close for D-PILs and M-PILs, the number ratio of untethered PF6 or TFSI in D-PILs and MPILs can be roughly calculated according to the molecular weight of monomer, and particle density. Here, the number ratio of untethered PF6 is P[C4VIm][PF6]: P[VIm][BETA][PF6] ¼ 1: 1.32, that of untethered TFSI is P[C4VIm][TFSI]: P[VIm][BETA][TFSI] ¼ 1: 1.18. The number ratio of mobile TFSI between P[VIm][BETA][TFSI] and P[C4Vim] [TFSI] should be in accordance with this calculated ratio because most of TFSI tends to dissociate and move due to its low binding energy and plasticization function [29]. To support this, we measure the number proportion of dissociated or “free” TFSI ions to total TFSI ions in the PILs by Raman spectroscopy because the S–N stretching of (tri fluoromethylsulfonyl)imide at ~740 cm 1 is sensitive to the coordina tion environment [30,31]. Usually, the dissociated or “free” TFSI can produce S–N stretching peak at ~740 cm 1, whereas the S–N stretching peak in associated TFSI appears at higher frequency shift (>746 cm 1) [30]. Thus, as shown in Fig. 8, the peak at 741 cm 1 can be attributed to the dissociated or “free” TFSI , whereas the peak at 748 cm 1 is the characteristic peak of the associated TFSI in P[VIm][BETA][TFSI] and P[C4VIm][TFSI]. By a convolution of Voigt profiles, we can calculate the proportion of dissociated or “free” TFSI in P[VIm][BETA][TFSI] is ~84%, which is only slightly smaller than ~88% of P[C4VIm][TFSI]. However, the proportion of mobile PF6 between P[VIm][BETA][PF6] and P[C4Vim][PF6] should be not in accordance with the calculated proportion because PF6 tends to incompletely dissociate due to its relatively large binding energy with cation part [32]. Although we cannot directly measure the number proportion of mobile PF6 , the number proportion of mobile PF6 between P[VIm][BETA][PF6] and P [C4Vim][PF6] should be larger than the above calculated proportion because the binding energy of PF6 with ammonium cation part is much weaker compared to that with imidazole ring cation part due to the unique molecular structure of the ammonium cation, which provides a more effective shielding of electrostatic interaction with the anion compared to the imidazolium cation [33]. As the number of mobile ions increases, the intensity of interfacial polarization is usually improved. Therefore, the fact that Δε0 of P[VIm][BETA][PF6] is larger than that of P[C4Vim][PF6] should be because it has larger number of mobile PF6 , whereas the fact that Δε0 of P[VIm][BETA][TFSI] is close to that of P [C4Vim][TFSI] should be because they have close number of mobile TFSI . After dissociation, the transport of mobile PF6 or TFSI would be follow the repetition of the ion-pair formation and dissociation process, i.e. hopping mode, in the glassy polyelectrolyte matrix, where the ion motion is not aided by polymer segmental dynamic [34]. The mobility of mobile PF6 or TFSI is usually controlled by the characteristic energy barrier (Ea) separating ion sites. Ea can be obtained from temperature dependence of the reciprocal of relaxation time (λ 1) using the Arrhe nius equation λ 1∝exp(-Ea/RT), where R is the gas constant and T is the absolute temperature [34]. Fig. 9 plots the temperature dependence of λ 1. Before Tg, λ 1 as a function of temperature follows the Arrhenius dependence. The value of
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Fig. 7. Temperature-modulated dielectric spectra of ERFs: (A) P[C4VIm][PF6] and P[VIm][BETA][PF6]; (B) P[C4VIm][TFSI] and P[VIm][BETA][TFSI] (φ ¼~25 vol% T ¼ 25 � C). Solid lines are the fitting curves using Eq. (1). Table 1 Dielectric characteristic parameters of ERFs at room temperature (φ ¼ ~25 vol %, T ¼ 25 � C). Sample
ε0 ∞
Δε0
λ (s)
P[C4VIm][PF6] P[VIm][BETA][PF6] P[C4VIm][TFSI] P[VIm][BETA][TFSI]
2.60 2.95 2.80 3.07
3.70 5.42 4.33 4.25
9.00 � 10 2 1.65 � 10-2 4.70 � 10 3 1.80 � 10 3
responsible for the enhancement of ER effect and flow stability as shown Figs. 5 and 6. The λ of P[VIm][BETA][TFSI] is 0.0018 s, which is also faster than that (0.0047 s) of P[C4VIm][TFSI]. But Δε0 (4.25) of P[VIm] [BETA][TFSI] is close to that (4.33) of P[C4VIm][TFSI]. This indicates that compared to the mono-ionic P[C4VIm][TFSI], the polarization rate of di-ionic P[VIm][BETA][TFSI] is enhanced, whereas the polarization intensity is not enhanced. This is also in accordance with the fact that the enhancement degree of ER effect of P[VIm][BETA][TFSI] is not as sig nificant as that of P[VIm][BETA][PF6] as shown in Figs. 5 and 6. Let’s further analyze the dielectric property in Fig. 7 on a molecular level. Since the main chain of the PILs is nonpolar, the contribution from main chain to dielectric polarization is small. At the same time, the in fluence from segmental motion on dielectric dispersion in Fig. 7 can also be neglected at room temperature because their Tg is higher than room temperature. Thus, the dielectric polarization and observed dielectric dispersion should be closely related to the ion pair part. It’s well known that the ion pair can contribute two polarizations [22]. One is so-called 6
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Fig. 9. Temperature dependence of the reciprocal of relaxation time for ERFs: (A) P[C4VIm][PF6] and P[VIm][BETA][PF6]; (B) P[C4VIm][TFSI] and P[VIm] [BETA][TFSI] (φ ¼~25 vol%). Solid lines are the fitting curves using Arrhenius equation and dot lines are the fitting curves using Vogel-Fulcher-Tammann (VFT) equation.
Fig. 8. Raman spectra of P[VIm][BETA][TFSI] (A) and P[C4VIm][TFSI] (B). Voigt fitted curves are plotted in red (Right Peak) and green (Left Peak) lines.
Ea is 92.6 kJ/mol for P[C4VIm][PF6] and 83.2 kJ/mol for P[VIm][BETA] [PF6], respectively. The value of Ea is 108.6 kJ/mol for P[C4VIm][TFSI] and 103.5 kJ/mol for P[VIm][BETA][TFSI], respectively. It can be seen that the value of Ea for PILs with TFSI as counterions is larger than that of PILs with PF6 as counterions. This is because the size of TFSI is larger than that of PF6 . However, it can also be seen that the value of Ea of DPILs is lower than that of M-PILs. This indicates the energy barrier to activate the local motion of mobile counterions is decreased after the introduction of di-ionic structure into PILs. It has known that, in glassy PILs, two factors are controlling the energy barrier of ion transport. One is ion-ion electrostatic interaction [35]. Because the molecular structure of the ammonium cation can provide a better shielding of electrostatic interaction with the anion compared to the imidazolium cation, the contribution of electrostatic ion-ion interaction to energy barrier in D-PILs should be smaller than that in M-PILs. In addition, from Table 1, it can be seen the polarity of D-PILs is much higher than that of M-PILs, which can also decrease the electrostatic ion-ion interaction and thus decrease the energy barrier [35]. This is one of the reasons of that Ea of P [VIm][BETA][PF6] or P[VIm][BETA][TFSI] is lower than that of P [C4VIm][ PF6] or P[C4VIm][TFSI].
The other factor is elastic force, which depends on the local envi ronment of PIL matrix, ion jump length, and the radius of mobile ions. To understand these, we conduct a WAXS measurement as shown in Fig. 10. It is seen that both D-PILs and M-PILs display three scattering peaks at 3-4 nm 1 (qb), 8-10 nm 1 (qi) and 13-14 nm 1 (qp), which can be attributed to the spacing between ionic aggregates, the correlation between anions (PF6 -to-PF-6 or TFSI -to-TFSI-), and the amorphous halos, respectively [36]. The position of highest qp due to the amorphous halos is similar for both D-PILs and M-PILs. However, the intermediate qi in D-PILs shifts towards lower value compared to that in M-PILs and the intensity of qb in D-PILs is obviously lower than that in M-PILs. This indicates that the anion-to-anion (PF6 -to-PF-6 or TFSI -to-TFSI-) corre lation distance in D-PILs is shorter than that in M-PILs and the ionic aggregation in D-PILs is obviously weakened compared to that in M-PILs. Because the transport of mobile PF6 or TFSI in glassy PILs follows the hopping mode, the ion jump length or hopping distance is related to the anion-to-anion correlation distance [37]. The shorter PF6 -to-PF-6 or TFSI -to-TFSI- correlation distance means the ion jump length or hopping distance is shorter in D-PILs than that in M-PILs. Thus, 7
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Polymer 182 (2019) 121847
in Fig. 5 according to the proposed ER mechanisms. Due to the different dissociation degree of counterions, however, the enhancement degree in polarization and ER effect depends on the type of counterions. The enhancement in polarization and ER effect of D-PILs is more significant when PF6 as counterions compared to TFSI with large size and plas ticizing capacity.
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Di-ionic PILs have been synthesized for use as new PIL-based ER materials. Under electric field, compared with common mono-ionic PILs, the D-PILs have enhanced ER effect but the enhancement degree depends on the type of counterions. At the same electric field, the ER enhancement of D-PILs with PF6 as mobile counterions is more signifi cant compared to D-PILs with TFSI as mobile counterions. Dielectric spectra and X-ray scattering measurements have indicated that the enhanced ER effect is because D-PILs possess higher interfacial polari zation intensity and rate compared to M-PILs due to the combined contribution of enhanced charge density and ion transport dynamicfrom high polarity di-ionic structure. However, the enhancement degree in polarization and ER effect of D-PILs is more significant when PF6 as counterions compared to TFSI with large size and plasticizing capacity.
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q (nm ) Fig. 10. X-ray scattering spectra: (A) P[C4VIm][PF6] (a) and P[VIm][BETA] [PF6] (b); (B) P[C4VIm][TFSI] (a) and P[VIm][BETA][TFSI] (b).
the energy barrier for ion transport in D-PILs should be lower than that for ion transport in M-PILs. In addition, compared to di-ionic P[VIm] [BETA][PF6] or P[VIm][BETA][TFSI], the alkyl chain of neighboring pendant group in mono-ionic P[C4VIm][PF6] or P[C4VIm][TFSI] can form the nonpolar regions resulting in aggregation more easily. The ionic aggregation morphology in mono-ionic P[C4VIm][PF6] or P [C4VIm][TFSI] can result in hindrance of the ion diffusion paths by allowing the formation of restrictive nonpolar regions between polymer chains, causing an increase of energy barrier and a decrease in the ion mobility [38]. Therefore, compared to mono-ionic P[C4VIm][PF6] or P [C4VIm][TFSI], the shorter PF6 -to-PF-6 or TFSI -to-TFSI- correlation distance and the weakened ion aggregation also can decrease the energy barrier of mobile ions and increase the ion mobility in di-ionic P[VIm] [BETA][PF6] or P[VIm][BETA][TFSI]. On the basis of the above analysis, the D-PILs can have not only higher number density of mobile ions but also faster mobility of mobile ions. Since the interfacial polarization originates from the transport and aggregation of mobile charges, the higher the number of mobile ions and the faster the mobility of mobile ions are, the greater polarization in tensity and the faster polarization rate will be achieved. Therefore, the D-PILs have larger polarization intensity and faster polarization rate compared to the M-PILs as shown in Table 1. As a result, the D-PILs show larger and more stable ER effect in the wide shear rate region as shown 8
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