Enhanced Electrorheological Properties of Elastomers Containing

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Enhanced electrorheological properties of elastomers containing TiO2/urea core-shell particles Chenguang Niu, Xufeng Dong, and Min Qi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08127 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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ACS Applied Materials & Interfaces

Enhanced electrorheological properties of elastomers containing TiO2/urea core-shell particles Chenguang Niu, Xufeng Dong*, Min Qi** School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China KEYWORDS: electrorheological materials, elastomer, TiO2/urea core-shell particles, storage modulus, polar molecules

ABSTRACT. Polar molecules coated core-shell particles have been used to prepare electrorheological (ER) fluids with high performance. Inspired by those studies, TiO2/urea coreshell structured particles were fabricated and used to prepare novel ER elastomers, whose properties were compared with the ER elastomers with bare TiO2 particles. Particles characterization results illustrate the TiO2/urea particles present little change in size, morphology and crystal structure with respect to the bare amorphous TiO2 particles, while clear core-shell structure is observed. Compared with the bare TiO2 particles filled elastomer, the TiO2/urea particles filled elastomer presents higher dielectric constant, indicating enhanced polarization. The viscoelastic properties of the two elastomers under different strain amplitude, frequency and electric field were tested. The results indicate that the TiO2/urea particles filled elastomer shows higher storage modulus G' and higher relative ER effect within low field strength region from 0

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to 2 kV/mm. Coating polar molecules is an effective method to improve the ER performance for ER elastomers.

1. Introduction Electrorheological (ER) materials are a kind of intelligent soft materials. Their rheological properties can be changed continuously by an externally applied electric field. The special properties make ER materials have many promising applications in actuators, dampers, torque transducers, artificial muscle, etc.1-4 One branch of these materials, ER fluids, is a complex system which is composed of dielectric particles and non-conducting liquid.5-10 When an electric field is applied, the rheological properties (viscosity, shear stress, yield stress, etc.) of ER fluids can be switched reversible and fast (within a millisecond). In spite of the strong ER effect, ER fluids still have several impediments in practical application, such as instability caused by the aggregation of particles and the leakage of liquid medium.11,12 ER elastomers composed by polarizable particles and dielectric polymers are another kind of ER materials. By applying an electric field during vulcanization process, the particles form an anisotropic structure like chains or columns that are locked in the matrix. In comparison with ER fluids, ER elastomers are used in their pre-yield region, and have electric field-responsive viscoelastic properties. Besides, the particles in ER elastomers do not undergo sedimentation. Due to those advantages, ER elastomers have been extensively studied in recent years.13-15 The electric field-induced storage modulus, the absolute change of storage modulus by applying an electric field, is an important parameter to evaluate ER elastomers. However, for most applications, the relative ER effect, as defined by the following equation, is a more important parameter,


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G '−G0 ' ∆G ' = × 100% G0 ' G0 '

(1)

where G' and G0' stand for the storage modulus with and without an electric field, respectively. It is conspicuous that higher relative ER effect will enable the elastomers attain a larger adjusting range of storage modulus, which is quite meaningful when they are used in stiffness tunable devices. Sakurai et al. found that the ER elastomer with low elasticity showed the most significant increment in viscoelastic properties under electric fields.16 Besides the influence of matrix’s modulus, the elastomer with higher weight fractions of particles shows stronger ER effect. Sirivat et al. experimentally investigated the ER properties of polythiophene/polyisoprene elastomers, which attained the maximum relative ER effect of 110% at 2kV/mm.17 Although several ER elastomers with high field-induced storage modulus have been prepared, their relative ER effect is still lower than their magnetorheological counterparts, magnetorheological (MR) elastomers. The reported relative MR effect was in the range of 100~800%.18-21 Therefore, efforts must be done to break through this bottleneck of ER elastomers for practical applications. Even though the properties of ER elastomers greatly depend on the magnitude of elastic modulus of polymer matrix, the ER activities of the suspended particles still occupy a vital position. In previous reports, a series of inorganic or organic particles, such as titanium dioxide, silica, barium titanate, carbonaceous particles, polymethacrylic acid cobalt(Ⅱ) salt, starch, and polyaniline have been used as the dispersed phase of ER elastomers22-29. However, owning to the restriction of intrinsic ER activities of these particles, the problem for increasing the relative ER effect keeps unsolved. In 2003, Wen et al. prepared a giant ER fluid, the yield stress of which is much larger than traditional ER fluids. It indicated that the polar molecules covering the particles


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play an important role in improving the ER activities of ER fluid.30,31 Inspired by this study, the elastomers containing polar-molecule modified particles are expected to have much enhanced ER activities, which needs experimental evidence. In this work, urea-coated titanium dioxide (TiO2/urea) particles with complete core-shell structure were fabricated with a hydrolysis method. Pre-structured ER elastomer specimens with these particles dispersed in silicon rubber were prepared under a fixed DC electric field. The morphology and composition of the particles as well as the microstructure and dielectric spectra of the elastomers were characterized. The influences of strain amplitude, shear frequency, and electric field strength on viscoelastic properties of the ER elastomers were analyzed with a rotational rheometer. Correspondingly, the relationship between the particle composition and the ER properties of the elastomers were further discussed according to the experimental results.

2. Experimental 2.1 Materials Tetrabutyl titanate (TBT, CP) was purchased from Shanghai Kefeng Industry Co., Ltd. Urea (H2NCONH2, AP) was obtained from Tianjin Bodi Chemical Co., Ltd. Ethanol (C2H5OH, AP) was supplied by Tianjin Fuyu chemical Co., Ltd. Silicon RTV 615 and catalyst (dibutyltin dilaurate, CP) were obtained from Shenzhen Hongyejie Technology Co., Ltd. Dimethyl silicon oil (η=100 mPa·s, 25℃) was from Beijing Hangping silicon and chemical Co., Ltd. Deionized water was used in all the experimental processes. All chemicals were used as received without further purification. 2.2 Synthesis of particles


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Typical preparation process of TiO2/urea particles is as follows: Firstly, 3 g TBT was mixed with 60 mL of ethanol and stirred for 2 h at room temperature to get uniform solution A. Secondly, 1 g deionized water and 0.3 g urea were dissolved in 100 mL and 40 mL ethanol, respectively. Both of the mixtures were vigorously stirred at room temperature for 30 min. Thirdly, solution A was poured into the deionized water ethanol solution prepared previously to get a white emulsion and stirred for 15 min, which was named as suspension B. Then the urea ethanol solution was added to suspension B and kept stirring for another 3 h. Finally, the mixed suspension was filtrated and washed with ethanol for several times, and dried under vacuum at 60 ℃ for 24 h to get the end-product. Bare TiO2 particles were also obtained from suspension B for comparison. 2.3 Preparation of ER elastomers Silicon RTV 615 was mixed with silicon oil and stirred for about 15 min firstly. Then, the particles were introduced in the mixture and mechanically stirred for another 30 min. The asobtained ER mixture contained 35 % silicon RTV, 35 % silicon oil, and 30% dielectric particles in volume (the true densities of TiO2 and TiO2/urea particles are 2.04 g·cm-3 and 1.96 g·cm-3, respectively). Subsequently, the catalyst, which accounted for 3 % of the weight of silicon rubber, was added to the mixture above and vigorously stirred with motor stirrer at 500 r min-1for 5 min. The rubber mixture was transferred into a vacuum case to remove the entrapped air to eliminate voids in the final product, and then it was filled in the mold made by polymethyl methacrylate before vulcanizing. Two electrode plates were inserted into the mold and the rubber mixture was subjected to a constant electric field of 1 kV/mm at room temperature during the curing process (Figure 1). The curing time was 24 hours.


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Figure 1. Schematic of customized rubber vulcanizing mold with DC electric field.

2.4 Characterization All the particles were carefully grinded before characterization. The morphology of the particles was observed by a field emission scanning electron microscope (FSEM) (FEI NOVA NanoSEM 450) with an applied voltage of 3kV at a work distance of 4.8 mm and a transmission electron microscope (TEM) (FEI Tecnai F30). The X-ray diffraction (XRD) pattern of the particles was obtained with a powder X-ray diffractometer (EMPYREAN), using a Cu Kα radiation. Fourier transform infrared (FTIR) (EQUINOX55) spectra were employed to determine the chemical structure with a KBr disk method. Thermogravimetric analysis (TGA) of the particles was conducted by a TGA/DSC simultaneous thermal analyzer (TGA/SDTA851e), in which the samples were heated from room temperature to 700℃ at 10 ℃/min under nitrogen flow. The true densities and the element content of the particles were measured with a full-automatic density analyzer (G-DenPyc2900) and an element analyzer (Vario ELⅢ), respectively.

The microstructure of the brittle fracture section of the ER elastomers was observed by SEM (ZEISS SUPRA 55) with an acceleration voltage of 10 kV. All the elastomer samples were cryogenically fractured in liquid nitrogen and coated with a thin layer of gold before the SEM


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observation. The dielectric spectra of all prepared elastomers were examined by an impedance analyzer (WK 6500B). The voltage of applied AC electrical field was 1 V and the frequency of which ranged from 300 Hz to 1 MHz. The viscoelastic properties of these elastomer samples were measured with a rotational rheometer (Physica MCR 301, Anton Paar) equipped with an electrorheological cell with parallel plates (PP15-E). The cylindrical samples with the diameter of 15 mm and 1 mm thickness were placed on the stationary bottom plate. The upper plate would press downward to the elastomer sample controlled by fixed normal force mode for insuring the sample against sliding between two plates. The electric field was perpendicular to the shear direction and the temperature was set as 22 ± 0.5 ℃ during the measurement. The influence of strain amplitude on dynamic viscoelasticity was measured by setting a constant oscillation shear frequency at 10 Hz with electric field strength of 0, 1, 2, and 3kV/mm. The oscillation shear frequency dependence of storage modulus G' and loss modulus G" was measured at constant strain amplitude of 0.01% and fixed electric field strength, and the frequency f was varied in the range of 0.1~100 Hz.

3. Results and discussion 3.1 Characterization of particles The morphologies of the TiO2 and TiO2/urea particles are shown in figure 2. It can be seen from figure 2a that the TiO2 particles are spherical and uniform. Figure 2b indicates that the morphology of the TiO2/urea particles have no significant change compared with the TiO2 particles. The surfaces of these two kinds of particles are smooth without obvious defects. The average diameters of the TiO2 and the TiO2/urea particles were measured in the SEM images of low magnification (figure S1 in the Supporting Information). The average sizes of the TiO2 and the TiO2/urea particles are 565.3 and 546.7 nm, respectively. However, the two-sample t test


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result indicates there is no statistically significant difference between the diameters of the two kinds of particles. The TEM image of the TiO2 particles (Figure 2c) shows that the microspheres are well dispersed. As shown in figure 2d, the TiO2/urea particles have a surface coating about 10 nm in thickness, which should be the urea shell. The results indicate the TiO2 particles have been successfully coated by urea layers and the core-shell structure is complete.

Figure 2. Typical SEM images of TiO2 particles (a) and TiO2/urea particles (b); TEM images of TiO2 particles (c) and TiO2/urea particles (d).


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The possible forming procedure of urea shell of the TiO2/urea particles is shown in figure 3. The dash lines represent the hydrogen bonds between the TiO2 particles and urea molecule. The synthesis of TiO2 particles consists of hydrolysis and condensation of TBT. Plenty of hydroxyl groups could be left on the particle surface due to the incomplete condensation. The formation of hydrogen bonds between amino groups of urea molecule and hydroxyl groups on TiO2 particles begins with adding of the urea ethanol solution. Thus, dense shells composed by urea molecules are formed on the surface of the particles.

Figure 3. Schematic of preparation process of TiO2/urea particles.

The X-ray diffraction spectra of the TiO2 and TiO2/urea particles are illustrated in figure S2. As shown in figure S2 in the Supporting Information, there is only a weak broad peak around 25 °and no other peaks are found. The results reveal that the TiO2 and TiO2/urea particles are both amorphous. In addition, no diffraction peaks of urea crystal are observed for the TiO2/urea particles. It indicates the urea layers on TiO2 particles are uniform, and no stacked layers of urea are formed.


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Figure 4 shows the FTIR spectra of the TiO2 (a) and TiO2/urea (b) particles. For the bare TiO2 particles, the broad band around 3420 cm-1 is both asymmetric and symmetric stretching vibrations of -OH. The band around 1620 cm-1 is assigned to the H-O-H bending vibration of physically absorbed water. The bands at 1120 and 1039 cm-1 correspond to the stretching vibration of C-O, which indicates the hydrolysis of TBT is incomplete and some butyl ester is left in the product. The absorption band of 400~800 cm-1 is attributed to Ti-O vibration. These results indicate that the TiO2 particles have been successfully fabricated with abundant hydroxyl groups on surfaces. Comparing to the bare TiO2 particles, some differences are found in the spectrum of the TiO2/urea particles. The -OH absorption band around 3420 cm-1 shifts to 3320 cm-1, which is due to the emergence of stretching vibration of -NH2. Besides, the band at 1446 cm-1 stands for the C-N vibration, and the absorption band at 1630 cm-1 corresponds to the N-H stretching vibration. Those differences with respect to the spectra of the bare TiO2 particles confirm the existence of urea molecule on the TiO2/urea particles.

Figure 4. FT-IR patterns of TiO2 particles (a) and TiO2/urea particles (b).


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Figure 5 shows the TGA curves of the TiO2 and TiO2/urea particles measured from room temperature to 700 ℃ at a heating rate of 10 ℃ min−1 under nitrogen atmosphere. The TGA curves indicate the two kinds of particles both experience two stages of weight loss. The initial weight loss up to 180 ℃ for the TiO2 and TiO2/urea particles is attributed to the removal of physically absorbed water and decomposition of urea. The second stage of weight loss is observed in the range of 200~300 ℃, which can be assigned to the pyrolysis of organic groups on the particles surface. It’s worth noting that the total weight loss of the TiO2/urea particles is 1.03 % higher than that of the bare TiO2 particles. It can be attributed to the decomposition of urea on the TiO2/urea particles. The accurate contents of nitrogen, carbon, and hydrogen measured by the element analyzer are shown in Figure S3 in the Supporting Information. The weight fraction of nitrogen in the TiO2/urea particles is 0.601%, which is much higher than that in the TiO2 particles (0.0565%). Meanwhile, the weight fractions of carbon and hydrogen also show an increment when the TiO2 particles are covered by urea. Urea molecule is the main source of nitrogen, carbon, and hydrogen in the TiO2/urea particles, and the content of element is basically in conformity with the result of TGA analysis.

Figure 5. TGA curves of TiO2 particles (dash line) and TiO2/urea particles (solid line).


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3.2 Characterization of elastomers The brittle fracture section of the elastomers filled with the TiO2 and TiO2/urea particles are shown in figure 6. The TiO2 and TiO2/urea particles are both well dispersed in the silicon rubber. The two kinds of particles are arranged into aligned chains paralleling to the direction of electric field applied during the curing process. As a result, anisotropic structures are formed in the two elastomers.

Figure 6. Typical section SEM microphotograph of TiO2 particles filled elastomer (a) and TiO2/urea particles filled elastomer (b).

The dependence of dielectric constant and dielectric loss on electric field frequency for the elastomers filled with the TiO2 and TiO2/urea particles is presented in figure 7. It is clearly found in figure 7a that the dielectric constant ε' in low frequency is obviously enhanced for the elastomer containing the TiO2/urea particles with respect to that containing the TiO2 particles. The main mechanisms relevant to ER activities include interfacial and dipolar polarizations, which are located in the low frequency range.32 Urea molecule is known to have a high dipole moment of µ=4.4957 Debye,31 and has been utilized as the polar groups adding onto the particles for increasing ER activities. The interfacial polarization of the TiO2/urea particles filled elastomers is strengthened owning to the presence of urea molecules at the interfaces between particles and rubber matrix.


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The relationships between the dielectric loss ε" and AC frequency for the two samples are shown in figure 7b. The ER effect of the ER elastomers is related to the polarization of the filling particles. There are four kinds of polarization, which includes interfacial, dipolar, atomic and electronic polarizations. Compared with the latter two polarizations, the interfacial and dipolar polarizations are rather slow, which are responsible for the ER effect. The two polarizations dominate the response time of the ER elastomers by the following equation, t = 1 / 2πf max

(2)

where fmax is the frequency of the first ε" peak.5 Considering it takes 10-5~10-2 s for the interfacial and dipolar polarizations process, the ER materials with notable ER activities should present ε" peak in the range of 102~105 Hz. As shown in figure 7b, the elastomers with the TiO2 particles and TiO2/urea particles both present a ε" peak at ~103 Hz


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Figure 7. Dielectric constant (a) and dielectric loss (b) as a function of the electric field frequency of TiO2 particles and TiO2/urea particles filled elastomers.

Figure 8 illustrates the possible microstructure of the elastomers filled with the TiO2/urea particles. The left side of figure 8 shows a representative unit of elastomer containing the TiO2/urea particles. Some amino groups of urea molecule covered on the particles form hydrogen bonds with hydroxyl groups at the end of the silicon rubber molecules, and some amino groups form hydrogen bonds with the oxygen atom in the backbone of the rubber molecules. The red dashed lines in figure 8 represent the hydrogen bonds formed between the particles and the matrix. In contrast, O-H···O hydrogen bonds are formed between the hydroxyl groups on the bare TiO2 particles and the rubber molecules. The bond energy of O-H···N hydrogen bond is 29 kJ/mol, which is stronger than that of O-H···O hydrogen bond (21 kJ/mol). Therefore, the interfacial strength between the TiO2/urea particles and the rubber matrix may be higher than that between the bare TiO2 particles and the rubber matrix. It is also beneficial to the improvement of the ER effect of the ER elastomers.


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Figure 8. Sketch of the structure of TiO2/urea particles filled elastomers and connection between TiO2/urea particles and silicon chains.

3.3 Viscoelastic properties The dependence of storage modulus G' of the elastomers filled with the TiO2 particles and TiO2/urea particles on strain amplitude under different electric field strength is demonstrated in figure 9a. The storage modulus of the two elastomers shows nearly linear relation up to a critical strain, beyond which a drop in the magnitude of the storage modulus is observed. The electric interaction forces between the adjacent particles along the field direction, which mainly contribute to the strength of storage modulus, are inversely proportional to their distance. Hence the decrement of storage modulus can be attributed to the increasingly larger inter-filler distance and the destruction of particle chains due to the increasing strain amplitude. This phenomenon is called as the Payne effect.33,34 When the strain is smaller than the critical value, the microstructure of elastomers are not damaged and the storage modulus keeps unchanged with the increasing strain amplitude. This region is defined as the linear viscoelastic (LVE) range. Under any electric field strength, the storage modulus of the TiO2/urea particles filled elastomer is higher than the corresponding elastomer containing the bare TiO2 particles. The possible reason


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for the enhanced ER activity by using the TiO2/urea particles is that the obtained stronger interfiller interaction forces deriving from the strengthened polarization of the particles. Besides, the enhanced interfacial strength by using the TiO2/urea particles also contribute to the higher storage modulus of the TiO2/urea particles filled elastomers. Figure 9b shows the loss modulus G" of the elastomers filled with the TiO2 and TiO2/urea particles as a function of strain amplitude under stationary electric field strength. It is found that the loss modulus curves of the two elastomers can be divided into three stages: the constant loss modulus in LVE range (γ

Enhanced Electrorheological Properties of Elastomers Containing

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