Enhanced Electrorheological Response of Cellulose: a Double Effect of Modiﬁcation by Urea-Terminated Silane

polymers

Article Enhanced Electrorheological Response of Cellulose: A Double Effect of Modiﬁcation by Urea-Terminated Silane

Zhao Liu 1, Panpan Chen 1, Xiao Jin 1, Li-Min Wang 1, Ying Dan Liu 1,* and Hyoung Jin Choi 2,* ID 1 State Key Lab of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, Hebei, China; [email protected] (Z.L.); [email protected] (P.C.); [email protected] (X.J.); [email protected] (L.-M.W.) 2 Department of Polymer Science and Engineering, Inha University, Incheon 402751, Korea * Correspondence: [email protected] (Y.D.L.); [email protected] (H.J.C.)

Received: 30 June 2018; Accepted: 2 August 2018; Published: 4 August 2018

Abstract: As a natural polymer with abundant sources, cellulose was one of the earliest applied electrorheological (ER) materials. However, cellulose-based ER materials have not attracted much attention because of their relatively low ER effect and sensitivity to water. In this study, cellulose rods were decorated with a urea-terminated silane, 1-(3-(trimethoxysilyl) propyl) urea, after being swelled in sodium hydroxide solution. The morphologies and structures of the cellulose particles were investigated using scanning electron microscopy, Fourier-transform infrared spectroscopy and X-ray diffraction, confirming the dramatic differences of the treated cellulose particles from the pristine cellulose. Rheological behaviors of the pristine and modified cellulose particles in silicone oil were observed using a rotational rheometer. It was found that the silane-modified cellulose showed higher ER effect and higher dielectric properties than the pristine cellulose particles, which was not only related to the grafted polar molecules but may also be associated with the porous morphologies of the treated cellulose particles.

Keywords: electrorheological ﬂuid; cellulose; swelling; silane; yield stress; dielectric spectra

1. Introduction Cellulose is the most abundant biopolymer, and comes from the cell wall of plants and bacteria, having an annual production of 1.5 × 1012 tons [1,2]. The current environmental and energy-related problems have been motivating the development of novel production approaches that are both sustainable and eco-friendly. As a renewable, biodegradable and inexpensive material, cellulose has attracted much attention since it was initially characterized in 1838 [3]. It is well known that cellulose is composed of three basic elements of C, H and O, being a typical carbohydrate. D-pyranose, which has three –OH groups as the repeating units linked by the glycosidic bond, forms the chain conformation with successive glucose rotated through an angle of 180◦ about the molecular axis and hydroxyl groups in an equatorial position [4–7]. The physical and chemical properties of cellulose are different from those of other polymers and are largely dependent on its supramolecular structure [8–11]. The supramolecular structure of cellulose is not simply based on the gathering of the six-membered rings held together by physical interaction [12]. Rather, the anhydroglucose units (AGUs) are linked by β-1,4-glycosidic linkages, a kind of covalent bond, considered to be the specific structure. The large cohesive energy in cellulose obviously reveals extensive hydrogen networks between intermolecular and intramolecular interactions formed by three –OH groups [13,14]. The ability of these hydroxyl groups to form hydrogen bonds plays a major role in the formation of fibrillar and semicrystalline packing, hence cellulose has at least five allomorphic forms [1,15–17].

Polymers 2018, 10, 867; doi:10.3390/polym10080867 www.mdpi.com/journal/polymers Polymers 2018, 10, 867 2 of 16

Both physical and chemical methods can be used to change the morphology and properties of cellulose. A high-pressure homogenization method [18,19] and ball-milling [20,21] are used to make the cellulose smaller and amorphous. Noncovalent modification [22], sulfonation [23], TEMPO-mediated oxidation [24], esterification [25–27], etherification [28] and silylation [29,30] are chemical approaches applied to change the properties of cellulose. These effective methods allow cellulose to have more extensive applications, such as in composite materials, optical films, pharmaceuticals and foodstuffs, as well as in the dispersed phase of electrorheological fluids [26,27,31]. Electrorheological (ER) fluids are a form of smart colloidal suspensions, whose rheological properties will change significantly upon the application of an electric field [32–34]. The ER fluids consist of dielectric or semiconducting particles as the dispersed phase and insulating liquids as the continuous phase. When an electrical field stimulus is applied, the dispersed particles are able to be polarized and form clusters, chains or column structures. This makes the ER fluids transform from a liquid-like state to a solid-like state possessing a typical yield stress. The rheological properties of an ER fluid, including flow curves of shear stress or shear viscosity and dynamic modulus, are mainly dependent on the field-induced structures formed by the dispersed particles. To date, a variety of inorganic and organic materials that have specific ER effects when dispersed in insulating liquids have been investigated. Some compounds are attracting attention, such as titanium oxide with different morphologies, and calcium and strontium titanates, because of their high dielectric polarizability. The yield stress of these kinds of inorganic-based ER fluids can be over 130 kPa or higher when modified by polar molecules [35–38]. The high yield stress is actually enough for applications, however, inorganics always result in serious abrasion to devices and have poor suspension stability. Polymer-based ER materials are attractive candidates as they can overcome some drawbacks of inorganics, but they have relatively lower ER effects. Chemical modification and composite methods have been applied to enhance the ER effects of polymer-based ER materials [39–42]. Cellulose particles, as well as other biopolymers such as starch and chitosan, have been applied as ER materials because of their electro-responsive polar groups [43–45]. However, the pristine forms of these biopolymers are very sensitive to water due to their hydroxyl groups, which reduces the thermal stability and long-term stability of the ER fluid. Therefore, anhydrous biopolymers were introduced via various chemical modification methods including phosphorate and carbamate esterification. Consequently, outstanding ER effects have not been observed from these cellulose esters and microcrystalline cellulose particles. In the current study, urea-terminated silane was applied to modify cellulose particles via a hydrolysis process [46], before which the cellulose particles were swelled in sodium hydroxide solution. It was found that the chemically treated cellulose particles were extremely different from pristine cellulose in both morphology and structure. Rheological analysis indicated that the modified cellulose showed higher ER effect, which was attributed to the porous morphologies and decorated urea groups caused by the chemical modification process.

2. Materials and Methods

2.1. Materials Cellulose powders (C6288) used in this study were purchased from Sigma Aldrich (Shanghai, China). 1-(3-(trimethoxysilyl) propyl) urea (T1915, TCI, Shanghai, China), silicone oil (viscosity: 50 cSt; density: 0.96 g/mL) (Beijing Hangping Guichuang Chemical Co., Ltd., Beijing, China), sodium hydroxide (NaOH, analytical purity) (Tianjin Kaitong Chemical Reagent Co., Ltd., Tianjin, China), and hydrochloric acid (HCl, analytical purity) (Tianjin Jindongtianzheng Chemical Reagent, Tianjin, China) were used without any further puriﬁcation.

2.2. Preparation of Silane-Modiﬁed Cellulose (Cel-Si-Urea) 5.0 g of cellulose powders were ﬁrstly dispersed in an aqueous solution of NaOH (10 wt %) and the suspension was precooled to −13 ◦C in a refrigerator to make the cellulose particles swell in the alkali Polymers 2018, 10, 867 3 of 16

Polymers 2018, 10, x FOR PEER REVIEW 3 of 15 solution. Then, the suspension was stirred vigorously until the suspension became semitransparent. 0.1alkali mol/L solution. HCl solution Then, was the dropped suspension into the was above stirred suspension vigorously until until the pH the value suspension reached became7.0. In this processsemitransparent. the gelatinous 0.1 mol/L cellulose HCl precipitated solution was and dropped the suspension into the above became suspension opaque white.until the Another pH value liquid reached 7.0. In this process the gelatinous cellulose precipitated and the suspension became opaque mixture was then prepared by adding 40 mL alcohol to 160 mL of deionized water, into which the white. Another liquid mixture was then prepared by adding 40 mL alcohol to 160 mL of deionized cellulose suspension was poured. 1.0 g of 1-(3-(trimethoxysilyl) propyl) urea, pre-hydrolyzed in a water, into which the cellulose suspension was poured. 1.0 g of 1-(3-(trimethoxysilyl) propyl) urea, small amount of water, was then dropped into the cellulose suspension. The suspension was stirred pre-hydrolyzed in a small amount of water, was then dropped into the cellulose suspension. The at room temperature for 4 h, then the precipitates were washed with deionized water several times suspension was stirred at room temperature for 4 h, then the precipitates were washed with anddeionized freeze-dried. water several A schematic times reactionand freeze process-dried. betweenA schematic the cellulosereaction process and silane between molecules the cellulose is shown inand Scheme silane1. molecules is shown in Scheme 1.

Scheme 1. Schematic interaction process of silane with cellulose particles via a hydrolysis process. Scheme 1. Schematic interaction process of silane with cellulose particles via a hydrolysis process.

2.3. Characterization 2.3. Characterization Fourier-transform infrared spectroscopy (FT–IR) (E55+FRA106, Bruker, Karlsruhe, Germany) wasFourier-transform applied to examine infrared the chemical spectroscopy structures (FT–IR) of the cellulose (E55+FRA106, after modification. Bruker, Karlsruhe, Morphologies Germany) of wastheapplied cellulose to particles examine before the chemical and after structures treatment ofwere the observed cellulose by after scanning modification. electron Morphologiesmicroscopy of(SEM) the cellulose (S4800, Hitachi particles, Tokyo, before Japan and). after The treatmentcrystal structures were observed of the cellulose by scanning were characterized electron microscopy using (SEM)a powder (S4800, X- Hitachi,ray diffraction Tokyo, Japan).pattern The (XRD) crystal (MAX structures-2500PC, of Rigaku, the cellulose Tokyo, were Japan characterized) with a Cu using-Kα a powderradiation X-ray source. diffraction The thermal pattern stability (XRD) (MAX-2500PC,of the samples was Rigaku, determined Tokyo, Japan)using witha thermogravimetric a Cu-Kα radiation source.analyzer The (TGA) thermal (STA4993 stability, Netzsch of the samples, Selb, Germany was determined) with a heatingusing a thermogravimetricrate of 10 °C/min in analyzer the (TGA)temperature (STA4993, range Netzsch, of 25 to Selb, 800 ° Germany)C in air atmosphere. with a heating rate of 10 ◦C/min in the temperature range of 25 to 800 ◦C in air atmosphere. 2.4. Preparation of ER Fluids and Rheological Measurements 2.4. Preparation of ER Fluids and Rheological Measurements Two ER fluids were prepared using pristine and modified cellulose (Cel-Si-Urea) particles, respectively.Two ER fluids The particles were prepared were dispersed using pristinein silicone and oil modified and treated cellulose by ultrasonication (Cel-Si-Urea) to form particles, a respectively.uniform suspension, The particles with a were mass dispersed fraction of in 15%. silicone The rheological oil and treated properties by ultrasonication of the ER fluids to were form a uniformmeasured suspension, using a rotational with a mass rheometer fraction (MCR of 15%.502, TheAnton rheological Paar, Graz, properties Austria)of with the ERa concentric fluids were measuredcylinder using(CC) geometry a rotational and rheometer a DC power (MCR502, supply. AntonA schematic Paar, Graz,sectional Austria) diagram with of a the concentric CC geometry cylinder (CC)composed geometry of a and bob a and DC a power cup is supply. shown Ain schematicScheme 2. sectionalIn the measurement diagram of state, the CC the geometry bob was dipped composed ofinto a bob the and cup aat cup a fixed is shown position. in SchemeThe ER fluid2. In was the measurementfilled in the gap state, of the the bob bob and was the dippedcup (gap into distance the cup

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Polymers 2018, 10, x FOR PEER REVIEW 4 of 15 at a fixed position. The ER fluid was filled in the gap of the bob and the cup (gap distance = 0.7 mm). When= 0.7 mm). the DC When power the supplyDC power was supply turned was on, turned an electric on, an field electric formed field between formed thebetween bob and the thebob cup,and andthe c thenup, and the particulatethen the particulate chain-like chain structures-like structures formed in formed the direction in the direction of the electric of the field. electric Ifthe field. bob If wasthe rotated, bob was the rotated, torque (theM) fromtorque the (M ER) fluidfrom could the ER be fluid sensed could and measured be sensed by and the measuredrheometer. by In the the . steadyrheometer. shear In mode, the steady the shear shear rate mode, (γ) theof theshear bob rate was (훾̇) set of in the the bob range was ofset 0.01–1000 in the range 1/s. of The 0.01 shear–1000 stress1/s. The (τ) shear and shear stress viscosity (τ) and shear (η) were viscosity calculated () were using calculated the measurement using the softwaremeasurement using software the equations using asthe follows, equations as follows, 1 + δ2 M =1+훿2 · 푀 τ 2 2 , (1) 휏 = 2δ∙ 2 L·,r ·C (1) 2훿 2휋퐿2∙푟π푖 ∙퐶퐿 i L τ 휏 휂 =η =, . , (2) 훾̇ γ (2) 2 . 1+1훿2+ δ 푟̇ =r =휔 ∙ω· , ,(3 (3)) 훿2δ−21 − 1 wherewhere δδ= =r ree//rii representsrepresents the the radius radius ratio ratio of of the cup (re)) and the bob (rii), L isis thethe gapgap lengthlength ofof thethe measurementmeasurement system, system, ωω representsrepresents the the angularangular velocity, velocity, and and CCLL iiss an end-effectend-effect correction factor. OscillatoryOscillatory strainstrain sweepsweep waswas performedperformed inin thethe strainstrain rangerange ofof 0.001%–10%0.001%–10% atat aa constantconstant frequencyfrequency ofof 1010 rad/s.rad/s. The frequency sweep waswas performedperformed inin thethe frequencyfrequency rangerange ofof 1–1001–100 rad/s rad/s withwith aa constantconstant strainstrain ofof 0.003%.0.003%. For For all all the the measurement measurement processes, processes the, the electric electric field field strength strength was was set set individually individually by theby the measurement measurement software. software.

Scheme 2. Schematic sectional diagram of the concentric cylinder (CC) geometry connected with a Scheme 2. Schematic sectional diagram of the concentric cylinder (CC) geometry connected with a DC DC power supply. The electrorheological (ER) fluid is filled in the gap of the CC geometry. power supply. The electrorheological (ER) fluid is filled in the gap of the CC geometry. 3. Results and Discussion 3. Results and Discussion 3.1. Morphologies and Structures 3.1. Morphologies and Structures The morphologies of the cellulose particles, including particulate shape, size and surface The morphologies of the cellulose particles, including particulate shape, size and surface condition, condition, were observed by SEM and are shown in Figure 1. The pristine cellulose particles (Figure were observed by SEM and are shown in Figure1. The pristine cellulose particles (Figure1a) had 1a) had irregular shapes amongst which some were rod-like with particle sizes ranging from 20 to irregular shapes amongst which some were rod-like with particle sizes ranging from 20 to 100 µm. 100 μm. Particles treated in NaOH solution and modified by urea-terminated silane (Figure 1b) Particles treated in NaOH solution and modified by urea-terminated silane (Figure1b) showed a showed a similar size but became porous with pore sizes of 5–20 μm, which can be seen clearly from similar size but became porous with pore sizes of 5–20 µm, which can be seen clearly from the the inset of Figure 1b. Consequently, the surfaces of the particles become rougher than the pristine inset of Figure1b. Consequently, the surfaces of the particles become rougher than the pristine cellulose particles. These morphological characteristics of the treated cellulose either relate to the low- cellulose particles. These morphological characteristics of the treated cellulose either relate to the temperature swelling process in the NaOH solution or modification by silane. A comparative low-temperature swelling process in the NaOH solution or modification by silane. A comparative cellulose sample (Figure S1), prepared via swelling in NaOH only, showed that the particles become cellulose sample (Figure S1), prepared via swelling in NaOH only, showed that the particles become much smaller with no obvious porous structure, even though the alkaline treatment can change the much smaller with no obvious porous structure, even though the alkaline treatment can change the size, crystal structure and remove some impurity of cellulose [47,48]. These observations indicate that size, crystal structure and remove some impurity of cellulose [47,48]. These observations indicate the modification by urea-terminated silane maintains the rough and porous structures of the cellulose particles.

Polymers 2018, 10, 867 5 of 16 that the modiﬁcation by urea-terminated silane maintains the rough and porous structures of the cellulose particles. Polymers 2018, 10, x FOR PEER REVIEW 5 of 15 Polymers 2018, 10, x FOR PEER REVIEW 5 of 15

Figure 1. SEM images of the pristine (a) and urea-terminated silane-modified (b) cellulose particles. Figure 1. SEM images of the pristine (a) and urea-terminated silane-modified (b) cellulose particles. Figure 1. SEM images of the pristine (a) and urea-terminated silane-modified (b) cellulose particles. Figure 2 shows the FT–IR spectra (a) and XRD patterns (b) of the pristine and the chemically modifiedFigure 2cellulose shows the(Cel FT–IR-Si-Urea) spectra samples. (a) In and Figure XRD 2a, patterns a broad (b) band of thearound pristine 3400 andcm−1 thewas chemicallyobserved Figure 2 shows the FT–IR spectra (a) and XRD patterns (b) of the pristine and the chemically modifiedin the two cellulose cellulose (Cel-Si-Urea) samples owing samples. to both In asymmetric Figure2a, a broadand symmetric band around stretching 3400 cmvibrations−1 was of observed –OH modified cellulose (Cel-Si-Urea) samples. In Figure 2a, a broad band around 3400 cm−1 was observed ingroups, the two which cellulose is a samplestypical characteristic owing to both band asymmetric of cellulose. and Another symmetric special stretching band of cellulose, vibrations the of C –OH– in the two cellulose samples owing to both asymmetric and symmetric stretching vibrations of –OH groups,O–C asymmetric which is a stretchingtypical characteristic vibration, appe bandared of at cellulose. 1164 cm− Another1. The band special around band 2900 of cmcellulose,−1 can be the groups,attributed which to stretchingis a typical vibration characteristic of C–H. band The bandof cellulose. at 1431 cmAnother−−1 in the special spectrum band of ofpristine cellulose, cellulose−1 the C– C–O–C asymmetric stretching vibration, appeared at 1164 cm −1 . The band around 2900 cm −1 can be O–representsC asymmetric bending stretching vibration vibration of C–H [49,50, appe]. aredThe similar at 1164 band cm −for.1 The Cel - bandSi-Urea around is shifted 2900 to 1420 cm cmcan−1 be attributed to stretching vibration of C–H. The band at 1431 cm −1 in the spectrum of pristine cellulose attributed to stretching vibration of C–H. The band at 1431 cm in−1 the spectrum of pristine −cellulose1 representsbecause of bending the stretching vibration of C of–N C–H generally [49,50 ].located The similar at 1420 band–1400 forcm Cel-Si-Urea. A strong band is shifted at 1640 to cm 1420 may cm −1 representsbe attributed bending to the vibration absorbed ofwater, C–H which [49,50 is]. shifted The similar to 1650 band cm− 1for for CelCel-Si-Urea dueis shifted to the stretchingto 1420 cm −1 because of the stretching of C–N generally located at 1420–1400 cm−1. A strong band at 1640 cm−1 becausevibration of theof C=O. stretching Thus, ofmodification C–N generally by urea located-term inatedat 1420 silane–1400 was cm− confirmed.1. A strong band at 1640 cm−1 may may be attributed to the absorbed water, which is shifted to 1650 cm−1 for Cel-Si-Urea due to the be attributed to the absorbed water, which is shifted to 1650 cm−1 for Cel-Si-Urea due to the stretching stretching vibration of C=O. Thus, modification by urea-terminated silane was confirmed. vibration of C=O. Thus, modification by urea-terminated silane was confirmed.

Figure 2. FT–IR spectra (a) and XRD patterns (b) of the pristine cellulose (Cel) and the urea-terminated silane-modified cellulose (Cel-Si-Urea).

FigFigureure 2. FT 2b– IR shows spectra the (a) and XRD XRD patterns patterns of (b ) the of the pristine pristine and cellulose modified (Cel) and cellulose the urea sam-terminatedples. The Figure 2. FT–IR spectra (a) and XRD patterns (b) of the pristine cellulose (Cel) and the urea-terminated characteristicsilane-modified peaks cellulose of pristine (Cel cellulose-Si-Urea). are located around 15.6° and 22.4° [51], corresponding to the silane-modiﬁed cellulose (Cel-Si-Urea). diffraction pattern of cellulose I. For the decorated cellulose, two peaks at 12° and 20° were observed, in accordanceFigure 2b with shows the thecharacteristi XRD patternsc peaks of cellulose the pristine II. From and previous modified studies, cellulose the treatment samples. by The characteristicNaOHFigure always2b showspeaks results of the pristine XRD in crystal patterns cellulose transformation of theare pristinelocated from andaround cellulose modified 15.6° I celluloseand to cellulose 22.4° samples. [51], II. corresponding These The results characteristic also to the ◦ ◦ peaksdiffractionconfirm of pristine thepattern effect cellulose of of cellulose swelling are locatedI. inFor NaOH. the around decorated The 15.6 degree cellulose,and of 22.4 crystallinity two[51 peaks], corresponding of at the 12° cellulose and 20° to sampleswere the diffraction observed, was ◦ ◦ patternin calculatedaccordance of cellulose using with I.Jade the For 6.characteristi the0 ( decoratedMDI, Livermore,c cellulose,peaks ofCA cellulose two, USA peaks), the II. at Fromvalue 12 and ofprevious 20whichwere wasstudies, observed, 32.37% the infortreatment accordance pristine by NaOHcellulose always and 22.33% results for in the crystal decorated transformation cellulose. These from values cellulose indicate I to that cellulose the physical II. These and resultschemical also confirmtreatments the effectnot only of change swelling the in crystal NaOH. form The but degree also reduce of crystallinity the crystallinity of the of cellulose. cellulose samples was calculated using Jade 6.0 (MDI, Livermore, CA, USA), the value of which was 32.37% for pristine

cellulose and 22.33% for the decorated cellulose. These values indicate that the physical and chemical treatments not only change the crystal form but also reduce the crystallinity of cellulose.

Polymers 2018, 10, 867 6 of 16 with the characteristic peaks of cellulose II. From previous studies, the treatment by NaOH always results in crystal transformation from cellulose I to cellulose II. These results also conﬁrm the effect of swelling in NaOH. The degree of crystallinity of the cellulose samples was calculated using Jade 6.0 (MDI, Livermore, CA, USA), the value of which was 32.37% for pristine cellulose and 22.33% for the Polymers 2018, 10, x FOR PEER REVIEW 6 of 15 decorated cellulose. These values indicate that the physical and chemical treatments not only change the crystalThe thermogravimetric form but also reduce analysis the crystallinity (TGA) and ofderivative cellulose. thermogravimetric analysis (DTG) curves of theThe cellulose thermogravimetric samples are analysisshown in (TGA) Figure and 3. derivativeIn Figure 3a, thermogravimetric the pristine cellulose analysis has (DTG)a weight curves loss of the cellulose samples are shown in Figure3. In Figure3a, the pristine cellulose has a weight loss before reaching 100◦ °C, which is caused by the minute amount of absorbed water because of the high beforehydrophilicity reaching of 100 cellulose.C, which There is caused is only by one the mai minuten weight amount-loss ofstep absorbed of almost water 70% because in the temperature of the high hydrophilicity of cellulose. There is only one main weight-loss step of almost 70% in the temperature range of 270–450 ◦°C . This change relates to the decomposition of the polymer chains of cellulose. rangeCorrespondingly, of 270–450 aC. sharp This DTG change peak relates was toobserved the decomposition in Figure 3b. of The the weight polymer loss chains before of 100 cellulose. °C , for ◦ Correspondingly,the silane-decorated a sharp cellulose, DTG peak is negligible. was observed Then in, Figure three3 b. weight The weight-loss steps loss before are observed 100 C, for in the the silane-decoratedtemperature ranges cellulose, of 120 is–220 negligible., 220–300 Then, and three300–450 weight-loss °C , attributed steps areto the observed decomposition in the temperature of silane, ◦ rangeshydrolyzed of 120–220, cellulose 220–300 chains and and 300–450 the initialC, attributed cellulose chains, to the decomposition respectively. Therefore, of silane, hydrolyzed the thermal cellulosedecomposition chains process and the of initial the treated cellulose cellulose chains, becomes respectively. complicated Therefore, and the different thermal from decomposition that of the processpristine of cellulose. the treated cellulose becomes complicated and different from that of the pristine cellulose.

Figure 3. Thermogravimetric analysis (TGA) (a) and derivative thermogravimetric analysis (DTG) (b) Figure 3. Thermogravimetric analysis (TGA) (a) and derivative thermogravimetric analysis (DTG) curves of the pristine cellulose (Cel) and the urea-terminated silane-modified cellulose (Cel-Si-Urea). (b) curves of the pristine cellulose (Cel) and the urea-terminated silane-modiﬁed cellulose (Cel-Si-Urea).

3.2. ER Properties 3.2. ER Properties From the above analysis, it was found that the morphological, chemical and condensed From the above analysis, it was found that the morphological, chemical and condensed structures structures of the silane-modified cellulose are different from the pristine cellulose. All these of the silane-modified cellulose are different from the pristine cellulose. All these differences are differences are considered to have major effects on the ER properties of cellulose. Two ER fluids based considered to have major effects on the ER properties of cellulose. Two ER fluids based on pristine on pristine and modified cellulose were prepared with the same weight percent (15%). Dynamic and modified cellulose were prepared with the same weight percent (15%). Dynamic oscillation tests oscillation tests were used to predict the electric field-induced structures in the ER fluids. Amplitude were used to predict the electric field-induced structures in the ER fluids. Amplitude oscillatory shear oscillatory shear was firstly applied to determine the linear viscoelastic (LVE) region of the ER fluids was firstly applied to determine the linear viscoelastic (LVE) region of the ER fluids under an applied under an applied electric field. electric field. Figure 4 illustrates the storage modulus (G′) and loss modulus (G″) of the two ER fluids Figure4 illustrates the storage modulus (G 0) and loss modulus (G”) of the two ER fluids measured measured with amplitude (strain) sweeps ranging from 0.001% to 10% at a constant frequency of 10 with amplitude (strain) sweeps ranging from 0.001% to 10% at a constant frequency of 10 rad/s. rad/s. In Figure 4a, at a zero field, the value of G′ is firstly higher than G″ and then becomes lower In Figure4a, at a zero field, the value of G 0 is firstly higher than G” and then becomes lower than G” at than G″ at a very small strain value of 0.03%, indicating that the ER fluid containing rod-like cellulose a very small strain value of 0.03%, indicating that the ER fluid containing rod-like cellulose particles is particles is a viscoelastic solid at low oscillation strain but becomes a viscoelastic liquid when the a viscoelastic solid at low oscillation strain but becomes a viscoelastic liquid when the strain is over a strain is over a critical value. When an electric field is applied, the cellulose particles form chain-like critical value. When an electric field is applied, the cellulose particles form chain-like structures. It was structures. It was observed that G′ becomes higher than G″ before a critical strain, indicating that the ER fluid has a broad solid-like region at applied electric field. In Figure 4b, both G′ and G″ of the Cel- Si-Urea ER fluid were more stable than those shown by the pristine cellulose ER fluid. When an electric field was applied, there was a stable plateau in G′, which is called the LVE region, caused by the recoverable deformation of the solid-like ER fluid. The value of the modulus and scope of the LVE increased with the electric field. More importantly, both G′ and G″ for the ER fluid of Cel-Si- Urea were higher than those of the pristine cellulose. These data indicate that after being decorated by urea-terminated silane, the particles have stronger interactions than the pristine ones.

Polymers 2018, 10, 867 7 of 16 observed that G0 becomes higher than G” before a critical strain, indicating that the ER fluid has a broad solid-like region at applied electric field. In Figure4b, both G 0 and G” of the Cel-Si-Urea ER fluid were more stable than those shown by the pristine cellulose ER fluid. When an electric field was applied, there was a stable plateau in G0, which is called the LVE region, caused by the recoverable deformation of the solid-like ER fluid. The value of the modulus and scope of the LVE increased with the electric field. More importantly, both G0 and G” for the ER fluid of Cel-Si-Urea were higher than those of the pristine cellulose. These data indicate that after being decorated by urea-terminated silane, Polymers 2018, 10, x FOR PEER REVIEW 7 of 15 the particles have stronger interactions than the pristine ones. InIn addition, addition, porous porous morphology morphology is is also also considered considered to to have have a a positive positive effect effect on on ER ER performance. performance. ThisThis may be due to the porous structurestructure beingbeing ableable toto supportsupport aa largerlarger surface surface area area and and thus thus having having a alarger larger interface interface with with the the silicone silicone oil oil to improveto improve the interfacialthe interfacial polarization polarization of the of particles. the particles. This pointThis pointof view of hasview been has proposedbeen proposed in previous in previous studies studies [52,53 ].[52,53].

Figure 4. Strain amplitude sweep of the ER fluids at different electric field strengths: (a) pristine Figure 4. Strain amplitude sweep of the ER fluids at different electric field strengths: (a) pristine cellulose; (b) urea-terminated silane-modified cellulose (G′: closed symbol; G″: open symbol). cellulose; (b) urea-terminated silane-modified cellulose (G0: closed symbol; G”: open symbol). Frequency sweep tests were based on the LVE region measured in the strain amplitude sweeps and conductedFrequency in sweep the angular tests were frequency based on range the LVEof 1 region–100 rad/s measured with the in thestrain strain amplitude amplitude of 0.003%. sweeps Figureand conducted 5 shows inthe the G′ angularand G″ frequencyof the two rangeER fluids of 1–100 measured rad/s during with the the strain frequency amplitude sweeps. of 0.003%.In the 0 absenceFigure5 ofshows an electric the G field,and G G”″ is of slightly the two higher ER fluids than G measured′ within a duringcertain thefrequency frequency range. sweeps. Both G In′ and the 0 0 Gabsence″ increase of an with electric frequency, field, G” confirming is slightly the higher viscoelastic than G within liquid acharacteri certain frequencystics of the range. ER fluids. Both G Withand theG” increaseapplication with of frequency, an electric confirming field, G′ is thehigher viscoelastic than G″ liquid at all characteristicsthe applied electric of the field ER fluids. strengths With and the 0 bothapplication remain ofstable an electric in the measured field, G is frequency higher than range. G” at The all themain applied difference electric between field strengths the two ER and fluids both isremain that the stable modulus in the de measuredmonstrated frequency by the range. ER fluid The of main Cel- differenceSi-Urea (Figure between 5b) theis higher two ER than fluids those is that of thethe ERmodulus fluid of demonstrated pristine cellulose by the (Figure ER fluid 5a). of Cel-Si-UreaThis means (Figure that the5 b) chain is higher structures than those formed of the by theER decoratedfluid of pristine cellulose cellulose particles (Figure are 5stiffera). This than means those that formed the chain by the structures pristine formed cellulose by, theand decorated a higher moduluscellulose particlescan be sensed are stiffer in the than oscillatory those formed deformation by the pristineprocess. cellulose, and a higher modulus can be sensed in the oscillatory deformation process.

Figure 5. Frequency sweep of the ER fluids at different electric field strengths: (a) pristine cellulose; (b) urea-terminated silane-modified cellulose (G′: closed symbol; G″: open symbol).

Polymers 2018, 10, x FOR PEER REVIEW 7 of 15

In addition, porous morphology is also considered to have a positive effect on ER performance. This may be due to the porous structure being able to support a larger surface area and thus having a larger interface with the silicone oil to improve the interfacial polarization of the particles. This point of view has been proposed in previous studies [52,53].

Figure 4. Strain amplitude sweep of the ER fluids at different electric field strengths: (a) pristine cellulose; (b) urea-terminated silane-modified cellulose (G′: closed symbol; G″: open symbol).

Frequency sweep tests were based on the LVE region measured in the strain amplitude sweeps and conducted in the angular frequency range of 1–100 rad/s with the strain amplitude of 0.003%. Figure 5 shows the G′ and G″ of the two ER fluids measured during the frequency sweeps. In the absence of an electric field, G″ is slightly higher than G′ within a certain frequency range. Both G′ and G″ increase with frequency, confirming the viscoelastic liquid characteristics of the ER fluids. With the application of an electric field, G′ is higher than G″ at all the applied electric field strengths and both remain stable in the measured frequency range. The main difference between the two ER fluids is that the modulus demonstrated by the ER fluid of Cel-Si-Urea (Figure 5b) is higher than those of the ER fluid of pristine cellulose (Figure 5a). This means that the chain structures formed by the decoratedPolymers 2018 cellulose, 10, 867 particles are stiffer than those formed by the pristine cellulose, and a higher8 of 16 modulus can be sensed in the oscillatory deformation process.

FigFigureure 5.5. FrequencyFrequency sweep sweep of of the the ER ER fluids fluids at at different different electric electric field field strengths: strengths: (a ()a pristine) pristine cellulose; cellulose; (b()b urea) urea-terminated-terminated silane silane-modified-modified cellulose cellulose (G (G′: 0closed: closed symbol; symbol; G G”:″: open open symbol). symbol).

Shear stress (τ) and shear viscosity (η), which can be obtained in the steady shear process, are important rheological properties of ER fluids. In this study, both shear stress and shear viscosity . curves of the two ER fluids were measured in a controlled shear rate mode (γ = 0.01–500 1/s). Figure6 shows the shear viscosity curves of the two ER fluids. At zero electric field, both ER fluids show slight shear-thinning behavior in the low-shear-rate region, and then Newtonian-like behavior with shear-rate-independent shear viscosity in the high-shear-rate region. These data indicate that the cellulose particles have better dispersibility in silicone oil than many other nanostructured ER materials [54,55]. In addition, the values of the shear viscosity of the two ER fluids are similar. This means that the change in particle morphology does not have a significant influence on the zero-field shear viscosity of the ER fluid. When the electric field was applied, obvious shear-thinning behaviors were observed in the two ER fluids. The shear viscosity increases gradually with the electric field strength. More importantly, the Cel-Si-Urea ER fluid (Figure6b) exhibits higher shear viscosity than the pristine cellulose ER fluid (Figure6a). For example, when the electric field strength is 4.0 kV/mm and the shear rate is 10−2 1/s, the shear viscosity of the Cel-Si-Urea ER fluid is 105 Pa·s. However, it is only 3 × 104 Pa·s for the pristine cellulose ER fluid. Considering the similar zero-field shear viscosity of the two ER fluids, the Cel-Si-Urea ER also presents higher ER efficiency (I), which is defined by the equation as follows:

η − η I(%) = E 0 × 100, (4) η0

where η0 and ηE are the viscosities of the ER fluid at zero and nonzero electric field, respectively. The ER efficiency of the two ER fluids is presented in Figure7. It is obvious that the Cel-Si-Urea ER fluid shows higher ER efficiency than the pristine cellulose ER fluid in the whole shear rate range. The ER properties of the cellulose particles treated by NaOH only are shown in Figure S2. It is found that at a certain electric field strength, the NaOH-treated cellulose shows a slightly higher shear stress than pristine cellulose, but it is not as high as that of Cel-Si-Urea. Therefore, chemical modification by polar molecules is very important for the enhancement of the ER properties of cellulose. Polymers 2018, 10, x FOR PEER REVIEW 8 of 15

Shear stress (τ) and shear viscosity (η), which can be obtained in the steady shear process, are important rheological properties of ER fluids. In this study, both shear stress and shear viscosity curves of the two ER fluids were measured in a controlled shear rate mode (훾̇ = 0.01–500 1/s). Figure 6 shows the shear viscosity curves of the two ER fluids. At zero electric field, both ER fluids show slight shear-thinning behavior in the low-shear-rate region, and then Newtonian-like behavior with shear-rate-independent shear viscosity in the high-shear-rate region. These data indicate that the cellulose particles have better dispersibility in silicone oil than many other nanostructured ER materials [54,55]. In addition, the values of the shear viscosity of the two ER fluids are similar. This means that the change in particle morphology does not have a significant influence on the zero-field shear viscosity of the ER fluid. When the electric field was applied, obvious shear-thinning behaviors were observed in the two ER fluids. The shear viscosity increases gradually with the electric field strength. More importantly, the Cel-Si-Urea ER fluid (Figure 6b) exhibits higher shear viscosity than the pristine cellulose ER fluid (Figure 6a). For example, when the electric field strength is 4.0 kV/mm and the shear rate is 10−2 1/s, the shear viscosity of the Cel-Si-Urea ER fluid is 105 Pa·s. However, it is only 3 × 104 Pa·s for the pristine cellulose ER fluid. Considering the similar zero-field shear viscosity of the two ER fluids, the Cel-Si-Urea ER also presents higher ER efficiency (I), which is defined by the equation as follows:

휂퐸 − 휂0 퐼(%) = × 100, (4) 휂0 where 휂0 and 휂퐸 are the viscosities of the ER fluid at zero and nonzero electric field, respectively. The ER efficiency of the two ER fluids is presented in Figure 7. It is obvious that the Cel-Si-Urea ER fluid shows higher ER efficiency than the pristine cellulose ER fluid in the whole shear rate range. The ER properties of the cellulose particles treated by NaOH only are shown in Figure S2. It is found that at a certain electric field strength, the NaOH-treated cellulose shows a slightly higher shear stress thanPolymers pristine2018, 10 cellulose,, 867 but it is not as high as that of Cel-Si-Urea. Therefore, chemical modification9 of 16 by polar molecules is very important for the enhancement of the ER properties of cellulose.

Figure 6. Shear viscosity vs shear rate at different electric field strengths for the ER fluids of pristine Figure 6. Shear viscosity vs. shear rate at different electric ﬁeld strengths for the ER ﬂuids of pristine cellulose (a) and Cel-Si-Urea (b). cellulose (a) and Cel-Si-Urea (b). Polymers 2018, 10, x FOR PEER REVIEW 9 of 15

Figure 7. ER efficiency of the two ER fluids: closed symbol for pristine cellulose and open symbol for Figure 7. ER efficiency of the two ER fluids: closed symbol for pristine cellulose and open symbol Cel-Si-Urea. for Cel-Si-Urea. Figure 8 shows the shear stress curves of the ER fluids as a function of shear rate. Without electric field Figureapplied,8 shows both ER the fluids shear perform stress curves like a of Newtonian the ER fluids fluid, as ashowing function a of linear shear increase rate. Without in shear electric stress fieldwith applied,shear rate both, and ER both fluids have perform similar like shear a Newtonian stress values. fluid, When showing an electric a linear field increase is applied, in shear the stress ER withfluids shear exhibit rate, non and-Newtonian both have behavior similar shear with stressfield-dependent values. When yield an stresses electric which field is can applied, be attributed the ER fluidsto the f exhibitield-induced non-Newtonian liquid-to-solid behavior phase with transition. field-dependent In the solid yield state, stresses yield whichstress is can needed be attributed to deform to thethe field-inducedsolid-like structures liquid-to-solid of the ER phase fluid. transition. At each electric In the field solid strength, state, yield the stresspristine is neededcellulose to ER deform fluid the(Figure solid-like 8a) shows structures a slight of thedecrease ER fluid. in shear At each stress electric with fieldshear strength, rate. The the Cel pristine-Si-Urea cellulose ER fluid ER (Figure fluid (Figure8b) presents8a) shows similar a slight behavior decrease but with in shear a less stress obvious with reduction shear rate. of The shear Cel-Si-Urea stress with ER shear fluid rate (Figure. Similar8b) presentsto the shear similar viscosity, behavior the but Cel with-Si-Urea a less ERobvious fluid reduction also shows of shear higher stress shear with stress shear than rate. the Similar pristine to thecellulose shear viscosity,ER fluid. theThe Cel-Si-Urea differences ERin shear fluid alsostress shows of the higher two solid shear-like stress ER than fluids the are pristine mainly cellulose due to the ER fluid.robustness The differences of the chains in shearformed stress by the of the cellulose two solid-like particles. ER The fluids more are robust mainly the due chains, to the the robustness higher the of theshear chains stress formed that is by needed the cellulose to deform particles. or destr Theoy more the chains robust under the chains, the same the highershear rate. the shear To analyze stress that the isshear needed stress to deformcurves further, or destroy a modified the chains constitutive under the sameequation shear for rate. non To-Newtonian analyze the fluids shear proposed stress curves by Choi et al. [56,57] was used to fit the flow curves. The equation, named the CCJ model, is shown as follows:

휏푦 1 휏 = 훼 + 휂∞(1 + 훽)훾̇, (5) 1+(푡2훾̇ ) (푡3훾̇ )

where τy and η∞ denote the dynamic yield stress and viscosity at an infinite shear rate, respectively, t2 and t3 are time constants, and α and β are fitting parameters related to the shape of flow curves. The fitting values of the parameters are shown in Table 1. The modified model is able to fit the shear stress curves very well in the measured shear rate range.

Polymers 2018, 10, 867 10 of 16 further, a modified constitutive equation for non-Newtonian fluids proposed by Choi et al. [56,57] was used to fit the flow curves. The equation, named the CCJ model, is shown as follows:

τy 1 . τ = + η (1 + )γ, (5) . α ∞ . β 1 + t2γ t3γ where τy and η∞ denote the dynamic yield stress and viscosity at an infinite shear rate, respectively, t2 and t3 are time constants, and α and β are fitting parameters related to the shape of flow curves. The fitting values of the parameters are shown in Table1. The modified model is able to fit the shear stress curves very well in the measured shear rate range. Polymers 2018, 10, x FOR PEER REVIEW 10 of 15

Figure 8. Shear stress vs shear rate at different electric field strengths for the ER fluids of pristine Figure 8. Shear stress vs. shear rate at different electric field strengths for the ER fluids of pristine cellulose cellulose (a) and Cel-Si-Urea (b) (symbols: experimental, lines: fitting of CCJ model to experimental (a) and Cel-Si-Urea (b) (symbols: experimental, lines: fitting of CCJ model to experimental points). points).

Table 1. Parameters of CCJ model (Equation (5)) obtained from the ﬁtting of shear stress curves of the Table 1. Parameters of CCJ model (Equation (5)) obtained from the fitting of shear stress curves of the ER ﬂuids. ER fluids.

sampleSample E (kV/mm) E (kV/mm) 0.5 0.51.0 1.0 2.02.0 3.03.0 4.0 4.0

y τ τy 7.997.99 85.7 85.7 271271 532532 817 817 α α 1.141.14 0.17 0.17 0.310.31 0.320.32 0.79 0.79 β β 0.980.98 0.30 0.30 0.500.50 0.240.24 0.90 0.90 Cel-SiCel-Si-Urea-Urea t2 t2 1.491.49 0.05 0.05 0.010.01 0.010.01 8.37 8.37 t3 t3 0.010.01 0.47 0.47 0.060.06 0.040.04 0.0020.002 η∞ η∞ 0.070.07 0.08 0.08 0.090.09 0.130.13 1.49 1.49 τy τy 2.722.72 11.7 11.7 36.236.2 173173 214 214 α α 1.811.81 1.25 1.25 1.781.78 0.280.28 0.36 0.36 β 1.00 0.90 1.09 0.18 1.44 Pristine cellulose β 1.00 0.90 1.09 0.18 1.44 Pristine cellulose t t2 2 2.932.93 2.86 2.86 10.410.4 0.300.30 0.07 0.07 t 0.02 0.004 0.002 0.30 0.04 t3 3 0.02 0.004 0.002 0.30 0.04 η∞ 0.06 0.52 0.08 0.09 0.17 η∞ 0.06 0.52 0.08 0.09 0.17

The dynamic yield stresses obtained from the above fitting process were re-plotted as a function The dynamic yield stresses obtained from the above fitting process were re-plotted푚 as a function of electric field strength, as shown in Figure 9. A power-law equation, 휏푦 ∝ 퐸m, was used to fit the of electric field strength, as shown in Figure9. A power-law equation, τy ∝ E , was used to fit the dynamic yield stress of each ER fluid. The value of the power exponent m is in the range of 1.0 to 2.0, dynamic yield stress of each ER fluid. The value of the power exponent m is in the range of 1.0 to according to the characteristics of the particles [58,59]. In this situation, it is 1.85 and 2.0 for the pristine 2.0, according to the characteristics of the particles [58,59]. In this situation, it is 1.85 and 2.0 for the cellulose and Cel-Si-Urea, respectively, indicating that the decorated cellulose has a higher field- dependent increase in dynamic yield stress.

Polymers 2018, 10, 867 11 of 16 pristine cellulose and Cel-Si-Urea, respectively, indicating that the decorated cellulose has a higher ﬁeld-dependent increase in dynamic yield stress. Polymers 2018, 10, x FOR PEER REVIEW 11 of 15

Polymers 2018, 10, x FOR PEER REVIEW 11 of 15

Figure 9. Dynamic yield stress of the ER fluids as a function of electric field strength (symbols: Figure 9. Dynamic yield stress of the ER fluids as a function of electric field strength (symbols: dynamic dynamic yield stress obtained from CCJ model, lines: fitting of power-law equation). yield stress obtained from CCJ model, lines: fitting of power-law equation). SquareFigure 9wave. Dynamic pulse yield voltage stress measurements of the ER fluids were as used a function to observe of electric the fieldinstantaneous strength (s responseymbols: of Squarethe ERdynamic fluid wave to yield pulsean electricstress voltage obtained field. measurements Figure from CCJ 10 model,shows werelines: the shear fitting used stress of to power observe of the-law two equation) the ER instantaneous fluids. measured response at a of the ERfixed fluid shear to anrate electric of 1 1/s field. and a Figure square 10 pulse shows electric the shearfield (0.5 stress–4 kV/mm). of the two Once ER the fluids electric measured field is at a fixedapplied, shearSquare rate the shear ofwave 1 1/s stresspulse and voltageof athe square Cel measurements-Si- pulseUrea ER electric fluidwere jumps fieldused (0.5–4torapidly observe kV/mm). to thea higher instantaneous Once value the and response electric reaches fieldofa is plateau. However, the shear stress of the pristine cellulose ER fluid grows gradually until reaching a applied,the ER the fluid shear to stressan electric of the field. Cel-Si-Urea Figure 10 shows ER fluid the shear jumps stress rapidly of the to two a higher ER fluids value measured and reaches at a a criticalfixed shear value rate at an of applied 1 1/s and electric a square field. pulseThis implies electric that field the (0.5 Cel–4-Si kV/mm).-Urea particles Once thecan electricrespond field to the is plateau. However, the shear stress of the pristine cellulose ER fluid grows gradually until reaching a electricapplied, field the rapidlyshear stress and form of the stable Cel- chainSi-Urea structures ER fluid within jumps a rapidly very short to atime. higher In contrast,value and the reaches pristine a critical value at an applied electric field. This implies that the Cel-Si-Urea particles can respond to the celluloseplateau. However,needs a period the shear of time stress to arrangeof the pristine the chain cellulose structures. ER fluid It was grows observed gradually that theuntil Cel reaching-Si-Urea a electricERcritical fieldfluid value rapidlyshows at ana and higher applied form shear electric stable stress chainfield. than This structures the implies pristine withinthat cellulose the aCel very- SiER-Urea short fluid particles time.at the Insame can contrast, respond electric the tofield pristinethe cellulosestrength.electric needs field At a therapidly period switched and of timeform-off point, tostable arrange bothchain ER thestructures fluids chain cannot within structures. immediately a very It short was time.transform observed In contrast, from that theathe solid pristine Cel-Si-Urea-like ER fluidstatecellulose shows to liquid needs a- higherlike a period state. shear of The time stress reason to arrange than for the thethe delayedchain pristine structures. transition cellulose It was is ER tha observed fluidt when at thethat electricthe same Cel electric- fieldSi-Urea is field strength.removed,ER fluid At the showsthe switched-offpolarized a higher particles shear point, stress need both athan longer ER the fluids time pristine cannotto relax cellulose to immediately their ER original fluid transformat unpolarized the same fromelectric states a solid-likewithfield statemechanical tostrength. liquid-like At shear the state. switched as the The only reason-off driving point, for bothforce. the delayedER Therefore, fluids transition cannot when immediatelythe is square that when wave transform the electric electric from field field ais solid applied, is removed,-like the polarizedthestate ER to fluids liquid particles show-like need state. different a The longer response reason time for behaviors to the relax delayed to [60]. their Nevertheless, transition original is unpolarized tha thet Celwhen-Si - statestheUrea electric ER with fluid fieldmechanical still is responds more rapidly than the pristine cellulose ER fluid. shearremoved, as the only thedriving polarized force. particles Therefore, need a longer when thetime square to relax wave to their electric original field unpolarized is applied, states the ERwith fluids mechanical shear as the only driving force. Therefore, when the square wave electric field is applied, show different response behaviors [60]. Nevertheless, the Cel-Si-Urea ER fluid still responds more the ER fluids show different response behaviors [60]. Nevertheless, the Cel-Si-Urea ER fluid still rapidly than the pristine cellulose ER fluid. responds more rapidly than the pristine cellulose ER fluid.

Figure 10. Shear stress of the ER fluids based on pristine cellulose (square) and decorated cellulose (circle) at the square voltage pulse electric field.

TheFigure rheological 10. Shear stress properties of the ER of fluids the ER based fluids on pristine under cellulose electric fields(square) are and closely decorated related cellulose to their Figure 10. Shear stress of the ER ﬂuids based on pristine cellulose (square) and decorated cellulose dielectric(circle) characteristics at the square ,voltage which pulse indicate electric the field. field -induced polarization dynamics of the ER fluids. In (circle)this study, at the the square dielectric voltage spectra pulse of electric the two ﬁeld. ER fluids were measured by a broadband dielectric The rheological properties of the ER fluids under electric fields are closely related to their

dielectric characteristics, which indicate the field-induced polarization dynamics of the ER fluids. In this study, the dielectric spectra of the two ER fluids were measured by a broadband dielectric

Polymers 2018, 10, 867 12 of 16

The rheological properties of the ER fluids under electric fields are closely related to their dielectric characteristics, which indicate the field-induced polarization dynamics of the ER fluids. In this study, thePolymers dielectric 2018, 10 spectra, x FOR PEER of the REVIEW two ER fluids were measured by a broadband dielectric spectrometer12 of at 15 a frequency range of 0.03–107 Hz. In Figure 11, the dielectric constant (ε0) and the dielectric loss factor spectrometer at a frequency range of 0.03–107 Hz. In Figure 11, the dielectric constant (ε′) and the (ε”) of the two ER fluids are presented and analyzed by the Havriliak–Negami (HN) equation [61,62], dielectric loss factor (ε″) of the two ER fluids are presented and analyzed by the Havriliak–Negami modified by a conductivity term and an electrode polarization term [63]: (HN) equation [61,62], modified by a conductivity term and an electrode polarization term [63]: ∆휀′∆ε0 휎 σ 휀∗∗((휔) = 휀0′ ++ 푖휀00′′ == 휀0′ ++ + 푑푐++ 퐴dcω−+푛, ω−n ε ω ε iε ε ∞∞ (1+(푖휔휏)훼)훾 α푖휀γ′ 휔 A (6(6)) 1 + (iωτ) 0 iε00ω

where Δε′ = ε′∞ − ε′0 is the dielectric strength and represents the polarizability of the dispersed phase, where ∆ε0 = ε0 − ε0 is the dielectric strength and represents the polarizability of the dispersed phase, ε′0 and ε′∞ are ∞the dielectric0 constants at low and high frequency, respectively, τ = 1/(2πfmax) denotes ε0 and ε0 are the dielectric constants at low and high frequency, respectively, τ = 1/(2πf ) denotes the0 dielectric∞ relaxation time, α and γ are the profile shape factors of the relaxation dispersion,max σdc therepresents dielectric the relaxation dc-conductivity time, α atand lowγ frequency,are the profile and n shape is associated factors ofwith the the relaxation slope at high dispersion, frequency.σdc representsThe parameters the dc-conductivity obtained from atfitting low frequency,by the HN andequationn is associated are shown with in Table the slope 2. at high frequency. The parametersIn Figure 11a, obtained the Cel from-Si-Urea fitting ER byfluid the shows HN equation higher ε are′ than shown the pristine in Table cellulose2. ER fluid. Figure 0 11b representsIn Figure 11thea, ε the″ spectra Cel-Si-Urea of the ER ER fluids. fluid There shows is higher an obviousε than dielectric the pristine relaxation cellulose for the ER Cel fluid.-Si- FigureUrea ER 11 bfluid represents and the the preε”-increase spectra ofin theε″ is ER caused fluids. by There dc-conductivity. is an obvious For dielectric the pristine relaxation cellulose for theER Cel-Si-Ureafluid, no dielectric ER fluid relaxation and the pre-increase appears in intheε ”measured is caused frequency by dc-conductivity. range. In Fora previous the pristine study, cellulose it was ERconfirmed fluid, no that dielectric both Δ relaxationε′ and τ are appears crucial inparameters the measured in determining frequency range.ER properties In a previous of an ER study, fluid it [61]. was 0 confirmedTherefore, thatthe larger both ∆ Δε εand′ andτ obviousare crucial dielectric parameters loss infor determining the Cel-Si-Urea ER properties ER fluid are of anconsistent ER fluid with [61]. 0 Therefore,the excellent the ER larger performance.∆ε and obvious The larger dielectric Δε′ of loss the for Cel the-Si Cel-Si-Urea-Urea ER fluid ER fluidmay arebe associated consistent with thethe 0 excellentporous morphology ER performance. of the The treated larger ∆ celluloseε of the Cel-Si-Urea particles, which ER fluid could may beresult associated in a larger with the interfacial porous morphologypolarizability. of In the addition, treated cellulosethe urea polar particles, molecule which is could considered result into acontribute larger interfacial to the polarization polarizability. of Incellulose. addition, the urea polar molecule is considered to contribute to the polarization of cellulose.

Figure 11. Dielectric spectra of the ER fluids: dielectric constant ε′ (a) and dielectric loss factor ε″ (b), Figure 11. Dielectric spectra of the ER fluids: dielectric constant ε0 (a) and dielectric loss factor ε”(b), Cel represents the pristine cellulose ER fluid and Cel-Si-Urea is for the urea-terminated silane- Cel represents the pristine cellulose ER fluid and Cel-Si-Urea is for the urea-terminated silane-modified modified cellulose ER fluid. The solid lines were fitted using the modified HN equation. cellulose ER fluid. The solid lines were fitted using the modified HN equation.

Table 2. Parameters in Equation (6) fitted from the dielectric spectra of the Cel-Si-Urea ER fluid. Table 2. Parameters in Equation (6) ﬁtted from the dielectric spectra of the Cel-Si-Urea ER ﬂuid.

Δε’ ε’0 ε’∞ α γ τ σ A n Cel-Si-Urea ∆ε’ ε’ ε’∞ α γ τ σ A n Cel-Si-Urea 1.38 3.630 2.25 0.65 0.82 0.0098 0.048 0.19 −0.1 1.38 3.63 2.25 0.65 0.82 0.0098 0.048 0.19 −0.1 4. Conclusions The pristine cellulose particles were modified using a urea-terminated silane, 1-(3- (trimethoxysilyl) propyl) urea, before which the particles were pretreated in NaOH solution at low temperature. The morphology and crystal structures of the cellulose particles dramatically changed after physical swelling and chemical modification. Compared to the ER effects of the pristine cellulose, the decorated cellulose showed very similar rheological properties without the stimulus of an electric field, but presented higher dynamic yield stress and dynamic modulus when an electric field was applied, and higher sensitivity to electric field. Dielectric analysis indicated that the silane- decorated cellulose with porous morphology had higher dielectric strength with an obvious dielectric

Polymers 2018, 10, 867 13 of 16

4. Conclusions The pristine cellulose particles were modified using a urea-terminated silane, 1-(3-(trimethoxysilyl) propyl) urea, before which the particles were pretreated in NaOH solution at low temperature. The morphology and crystal structures of the cellulose particles dramatically changed after physical swelling and chemical modification. Compared to the ER effects of the pristine cellulose, the decorated cellulose showed very similar rheological properties without the stimulus of an electric field, but presented higher dynamic yield stress and dynamic modulus when an electric field was applied, and higher sensitivity to electric field. Dielectric analysis indicated that the silane-decorated cellulose with porous morphology had higher dielectric strength with an obvious dielectric loss which was not observed in the pristine cellulose. It confirmed that the double effect of silane modification, maintenance of the porous structure and introduction of highly polar groups, had a positive effect on the ER properties of cellulose particles. The modification by silane could also reduce the sensitivity of cellulose to water, which is crucial for the commercial application of cellulose-based ER fluid.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/8/867/s1, Figure S1: SEM of physically treated cellulose particles in NaOH solution, Figure S2: Shear stress and shear viscosity curves of the ER fluid containing physically treated cellulose in NaOH solution. Author Contributions: Conceptualization, H.C.; Methodology, Z.L.; Formal analysis, Z.L., Y.L., H.C. and L.W.; Data Curation, X.J.; Writing-Original Draft Preparation, Z.L.; Writing-Review & Editing, P.C. and Y.L.; Supervision, Y.L. Funding: This research was funded by National Natural Science Foundation of China, grant number 21403186; the National Basic Research Programme of China, grant number 2015CB856805; and the Natural Science Foundation of Hebei Province, grant number E2015203257. Conflicts of Interest: The authors declare no conflict of interest.

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