corn oil suspensions

Carbohydrate Research 345 (2010) 672–679

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Carbohydrate Research journal homepage: www.elsevier.com/locate/carres

Investigation of electrorheological properties of biodegradable modified cellulose/corn oil suspensions Tahir Tilki a,*, Mustafa Yavuz a, Çig˘dem Karabacak a, Mehmet Çabuk b, Mehmet Ulutürk a a b

Süleyman Demirel University, Faculty of Arts and Sciences, Department of Chemistry, Isparta, Turkey Musß Alparslan University, Faculty of Arts and Sciences, Department of Chemistry, Musß, Turkey

a r t i c l e

i n f o

Article history: Received 3 November 2009 Received in revised form 10 December 2009 Accepted 22 December 2009 Available online 6 January 2010 Keywords: Modification of cellulose ER fluids Biodegradable material

a b s t r a c t Considerable scientific and industrial interest is currently being focused on a class of materials known as electrorheological (ER) fluids, which display remarkable rheological behaviour, being able to convert rapidly and repeatedly from a liquid to solid when an electric field (E) is applied or removed. In this study, biodegradable cellulose was modified and converted to their carboxyl salts. Modified cellulose is characterised by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA) and conductivity measurements. Suspensions of cellulose (C) and modified cellulose (MC) were prepared in insulated corn oil (CO). The effects of electric field strength, shear rate, shear stress, temperature, etc. of these suspensions onto ER activity were determined. Rheological measurements were carried out via a rotational rheometer with a high-voltage generator to investigate the effects of electric field strength and particle concentration on ER performance. The results show that the ER properties are enhanced by increasing the particle concentration and electric field strength. Also the cellulose-based ER fluids exhibit viscoelastic behaviour under an applied electric field due to the chain formation induced by electric polarization between particles. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction A wide variety of particulates or solid particles, such as cellulose, starch, flour, silica, alumina, titania, zeolite and dielectric powders can be dispersed in low-conductivity, nonpolar matrices such as silicone oil, hydrocarbon oils and vegetable oils. Those suspensions, whose rheological properties can change abruptly on application of an external electric field, are commonly known as electrorheological (ER) fluids. The typical characteristics of ER fluids, which include a reversible and swift transition between the liquid state and the solid state, potentially provide the most efficient approach for the control of mechanical responses by adjusting electric field strengths. The electric field-induced interaction, arising from particle polarization, is commonly believed to be responsible for ER behaviour.1 Because of their rapid response time and controllable shear viscosity, engineering design based on ER fluids has facilitated the development of specifications for a broad range of applications, such as dampers, clutches and adaptive structures.2 Most applications that require fluids that possess a large field-induced yield stress are stable in settling and irreversible aggregation, are environmentally benign and draw a limited current. Dry-based systems * Corresponding author. Tel.: +90 246 211 40 84; fax: +90 246 237 11 06. E-mail address: [email protected] (T. Tilki). 0008-6215/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2009.12.025

with anhydrous particles have been investigated in an attempt to overcome the limitations (thermal stability and corrosion) of wet-based systems. Recently, anhydrous ER suspensions using polymer particles, inorganic–organic nanocomposite particles and semiconducting polymer particles have been reported.3 Corn starch, silica, cellulose and zeolite4 have been widely used as the dispersed phase in the formulation of hydrous (wet-base) ER fluids, which have several problems related to durability, limited temperature range and colloidal instability. Special attention has been paid to anhydrous (dry-base) ER fluids which do not contain any polar solvent in the dispersed phase. Examples include polymers,5 polymer composites,6 ionomers7 and similar organic materials. The carrier species of the polarised particles is the main difference between dry- and wet-base ER fluid systems. In wet ER fluids, the fibrillar structure is formed as a result of the migration of ionic particles in the absorbed water, whereas in dry ER fluids, electronic migration inside the molecules of the dispersed particles is the driving force behind the particle—chain structure. Both affect the ER and the magnitude of shear stress under an applied electric field. To overcome some of the limitations mentioned above, ioncontaining polymers (ionomers) are chosen as a dispersed phase, because they receive ever-increasing attention attributed to the dramatic effect that small amounts of ionic groups exert on ultimate polymer properties. After the incorporation of ionic groups into the polymer backbone, the tensile strength, melting solution

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viscosity, glass transition temperature and rheological properties of the main polymer change, and this makes them useful for ER studies.8 Electrorheological (ER) fluids composed of a suspension of micron-sized particles in a non-conducting fluid form fibrillated particle structures, which are caused by the dielectric constant mismatch of the particles and the insulating oil, in strong electric fields. The chemical structure of the organic materials is, therefore, an important factor in the ER performance.9 Cellulose, as a natural polymer, has received great deal of attention recently as a possible alternative to petroleum-based polymers, and cellulose possess branched polar groups such as hydroxy (–OH) groups. The polar groups may affect the ER behaviour under the imposed electric field. Cellulose has the advantages of being renewable, biodegradable, abundantly available and low in cost. Native cellulose, irrespective of its source, is undesirable for many applications because of its inability to withstand processing conditions such as extreme temperature, diverse pH and high shear rate. In order to improve desirable functional properties, native celluloses are often modified. The most common type of cellulose modification is the treatment of native cellulose with small amounts of approved chemical reagents. Chemical modification of cellulose changes the functionality of the cellulose. The chemistry involved in the modification of cellulose is quite straightforward and involves primarily reactions associated with the hydroxyl groups of the cellulose polymer. Saito et al. reported10 that TEMPO-mediated oxidation of native cellulose, which is one of the most promising methods for surface modifications of native celluloses, effectively introduces carboxylate and aldehyde functional groups into solid native celluloses under mild aqueous conditions. Kim et al.,11 reported that cellulose phosphate particles were synthesized via the phosphoric ester reaction of cellulose particles and dispersed in silicone oil to produce anhydrous dry-base ER fluids. In this study, we investigate C and MC as a vigorous nominee for anhydrous particles in high-performance systems by analysing the effect of particle concentration and electric field strength via shear tests. An ER material, a wet-base microcrystalline cellulose with a small amount of adsorbed water, and the ER behaviour of modified cellulose suspensions have been recently studied. Therefore, the unmatched feature of the modified cellulose-based ER fluid is a dry-base system, possessing similar shear stress compared to other high-performance ER fluids such as the polyaniline system.12 The ER properties of the C/CO and MC/CO suspensions were then investigated by examining the effects of electric field strength and particle concentration on shear stress. 2. Results and discussion 2.1. Characterization of C and MC The FTIR spectrum of C showed the expected distinctive absorptions.13 The absorptions are O–H stretching at 3346 cm1, aliphatic C–H stretching at 2899 cm1, aliphatic C–H bending at 1429 cm1, C–H bending at 1170 cm1 and C–O–C symmetric bending at 1100 cm1. MC also gave an FTIR spectrum similar to that of C. In addition to this, MC gave the absorption of C@O stretching at 925 cm1. The reason for this is that C was partially hydrolyzed to MC. Also, because of the carbonyl group, the absorption at 3346 cm1 at C is broader at MC (Fig. 1). The 1H NMR spectra of C and MC gave the expected distinctive chemical shifts. The signals at 3.35, 3.57, 3.65 and 4.58 ppm were successfully ascribed to the ring protons of H-2, H-4, H-3 and H5, respectively, of both C and MC. The integral ratio of the new peak appearing at 4.37 ppm of MC showed that the C was successfully

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partially hydrolysed to a degree of 7.5%, which was targeted to be 8% for ER purposes (Scheme 1). MC showed similar 13C spectra to that of C, indicating that modification did not have an effect on the molecular packing of the double helices in the crystalline regions, but a new additional signal of the carbonyl carbons in ester groups formed by the formylation at 172.1 ppm is clearly visible, and some differences exist also in the C6 region. EDS photographs of native C and MC are displayed in Figure 2a and b. The C granules appear elliptic, break-like or irregular.4 After modification, cellulose granules become smaller and show features that are more regular, homogeneous and harmonious. On the other hand, the EDS image of MC (Fig. 2b) showed an extra Li peak besides the O and C atoms, indicating that partial conversion to the lithium salt was successful.14 When the TGA curves of C and MC samples were examined, removal of adsorbed H2O was detected at 40–105 °C. The thermal stability of the MC particles approached 275 °C as can be seen from the TGA, shown in Figure 3. In the case of MC particles, no chemical reaction or decreasing weight was found below 275 °C. The decomposition temperature of C is between 290 and 320 °C. The first degradation peaks corresponded to the removal of ions in the structure. However, the maximum temperature of the degradation of MC (330 °C) was higher than that of C (320 °C). The second degradation peaks corresponded to the degradation of polymeric chains of cellulose. Second decomposition temperature of MC was higher than that of C. Similar results were observed by Ko et al.15 in TGA studies of chitosan. From the TGA analysis, it was concluded that MC is thermally more stable, without any degradation below 275 °C, indicating that the MC-based ER fluids can be used safely up to 275 °C. 2.2. Electrorheology 2.2.1. Sedimentation stability The gravitational stability of C/CO and MC/CO suspensions against sedimentation was determined at constant temperature (20 °C). When the density of the particles is not the same as that of the medium, the particles of micron size settle down according to Stoke’s law.16 Figure 4 shows the change of sedimentation ratio with time for C/CO and MC/CO ER-active suspensions. C/CO and MC/CO suspensions exhibited the same amount of colloidal stability against sedimentation, with a sedimentation ratio of 56% at the end of 30 days. These sedimentation stability results are satisfactory and meet the industrial requirements.17 2.2.2. Effect of concentration Suspension concentration exerts a major effect on the ER activity. The change in viscosity with suspension concentrations of dispersed particles at constant shear rate (c_ ¼ 0:2 s1 ) and temperature (T = 20 °C) is depicted in Figure 5. The viscosity of cellulose derivative/CO suspensions was observed to increase with increasing suspended particle concentration. This tendency may be attributed to the effects of increased polarization forces between particles with increasing suspension concentration. Up to a 20 wt % particle concentration, the viscosity ratio increases with increasing particle concentration. This trend is due to the polarization forces acting between the ionic particles. The magnitude of this polarization force (F) in the direction of applied electric field (E) is,

F ¼ 62  r 6  E2 =q4

ð1Þ

where e2 is the dielectric constant of the particle, q is the distance between particles, E is electric field strength and r is the radius of the particle.

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75

4000

3600

4000

3600

669,30 2

46,50

30

1317,38

1429,25

2899,01

45

1373,32

1624,06

60

896,90

%T

3200

2800

2400

2000

1800

2000

1800

1600

1400

1200

800

1000

600

400 1/cm

600

400 1/cm

45 %T

3200

2800

2400

1600

1400

925,83

765,74

860,25

1200

3

0

7,29

2,79

2929,87

15

1369,46

1647,21

2360,87

30

1000

800

Figure 1. FTIR spectra of (a) cellulose and (b) modified cellulose.

Scheme 1. The modification reaction mechanism of native cellulose. [k = (m + n); m = % 92.5, n = % 7.5 from NMR data].

As shown by this equation, an increased suspension concentration will decrease the distance between the particles, which will result in an increased polarization force. When the particle concentration further increases above 20 wt %, the viscosity reaches a constant level with increasing particle concentration. This may be attributed to the following reasons: at higher suspension concentrations, the particles are close to each other, and the electric double layers particles overlap. The mutual action between particles increases, and the electric double layers may drop out of the particles. As a result, the viscosity of the suspension levelled off. Similar results were observed by Yavuz and Unal18 in ER studies of PI-coPTBMA-Li/silicone oil systems.

2.2.2. Effects of electric field strength Figure 6 shows the change in the electric field viscosity with electric field strength at constant conditions: c_ ¼ 0:2 s1 , c = 20 wt % and T = 20 °C. As seen in the graph, electric field viscosity (gE) increases with increasing electric field strength and nearly reaches to gE = 6.2 kPa s for C/CO and to gE = 10.5 kPa s for MC/CO. Under applied electric field strength, the magnitude of the polarization forces between particles increases, and in turn, the particles rapidly aggregate into the chain length (formed by the polarized particles) perpendicular to the electrodes, hence resulting in the improvement of the viscosity. Under an applied shearing force, particles are also affected by the effects of viscous

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Figure 2a. (a) EDS analysis of energy positions of cellulose (E = 20 kV, particle size scale 100 lm).

forces, which is due to the hydrodynamic interactions of the particles in the suspension. The magnitude of these viscous forces is

F ¼ 6p  gs  c_ 6

ð2Þ

where gs is the viscosity of suspension, r is the radius of particle and c_ is the average shear rate. The viscosity of the MC suspension is higher than that of C. Similar results were observed by Kim et al.4 in ER studies of cellulose phosphate/silicone oil systems and by Zhao et al.19 in ER studies of nano titanium oxide/silicone oil systems. Shear stress is one of the critical design parameters in the ER phenomenon and has attracted considerable attention both theoretically and experimentally. Figure 7 shows the change in shear stress as a function of external E, keeping the rest of the conditions constant: c_ ¼ 0:2 s1 , c = 20 wt %, T = 20 °C. Electric field-induced shear stress (sE) is linearly related to E2 as mentioned by Conrad20 and Davis:21

sE / uK f E2 b2

ð3Þ

where u is the volume fraction of particles, Kf is the dielectric permittivity of the base fluid, E is the electric field strength and b is the relative polarizability at dc or low frequency ac fields. This trend, induced by E and also in accordance with theoretical predictions proposed by Klingenberg and Zukoski,22

s / En

ð4Þ

originate from the stronger interaction between C- and MC-particles dispersed in corn oil.

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Figure 2b. (b) EDS analysis of energy positions of modified cellulose (E = 20 kV, particle size scale 100 lm).

Figure 7 presents the dependence of static yield stress on electric field strength for C and MC-based ER fluid from the controlled shear stress modes. We proposed a power law relationship between s and the E. The n values in Eq. 5 for C- and MC-based ER fluids were 1.76 and 1.69, which is similar to those for many other ER fluids. The C/CO and MC/CO ER systems also possesses the property that yield stress increases as electric field strength increases as a result of an increase in the polarization forces between particles. The shape of our C and MC particles was irregular and break-like (through SEM). Thus the dipole moment of the particles is not uniform, and it may have given the observed n <2. Figure 7 represents the change in shear stress with electric field strength which was obtained at constant suspension concentration (c = 20 wt %), shear rate ðc_ ¼ 0:2 s1 ) and temperature (T = 20 °C). As reflected by the graph, shear stress sharply increases with increasing field strength, which indicates that the ER suspension becomes more stable under strong electric field strength.23 The shear stress of the MC suspension is about five times higher than that of the C suspension with electric field strength. This result shows that the ER strength of the MC/CO suspension systems is 5.5 times stronger after the modification. Besides, leaking current density in these experiments is limited to <30 lA/cm.2 In the literature, Gerçek et al. also reported for polyaniline derivatives, namely: poly(o-toluidine) (POT), poly(N-methylaniline) (PNMAn), poly(N-ethylaniline) (PNEAn) and poly(2-ethylaniline) (P2EAn), that shear stress is linearly related to the square of the electric field strength (E2). This trend originates from the stronger interaction between particles induced by the electric field strength.24

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Figure 3. TGA of (a) cellulose and (b) modified cellulose.

Figure 4. Sedimentation stability of C and MC. c = 20% by wt, T = 20 °C.

Figure 6. The change in viscosity with electric field strength. T = 20 °C, c = 20% by wt, c_ ¼ 0:2 s1 .

1000 C

Shear stress (Pa)

MC

100

10

1 1

10

100

2

E (kV/mm) Figure 5. The change in viscosity with concentration. T = 20 °C and E = 0 and 2 kV/ mm, c_ ¼ 0:2 s1 .

Figure 7. The change of shear stress with electric field strength. T = 20 °C, c = 20% by wt, c_ ¼ 0:2 s1 .

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2.2.3. Effect of shear rate Change in the viscosity of the suspension with shear rate at optimum suspension concentration (20 wt %, T = 20 °C, E = 0) is shown in Figure 8. As is evident, the viscosity of C/CO and MC/ CO suspensions decreases sharply with increasing shear rate, giving a typical curve of shear-thinning non-Newtonian viscoelastic behaviour.25 Change of shear stress with shear rate at constant conditions (c = 20 wt %, T = 20 °C, E = 0 and E = 2 kV/mm) is shown in Figure 9. Shear stress of the suspension both increased with increasing c_ (between c_ ¼ 0:01—20:0 s1 ) and showed a Newtonian flow behaviour in the absence of E (E = 0 kV/mm). However, Bingham plastic behaviour was observed under the external influence E (E = 2 kV/mm). This is caused by the role of induced polarization forces, which is a typical rheological characteristic of ER fluids under the influence of external E.26 Change of s with c_ is also shown in Figure 9. Shear stress of C/ CO and MC/CO suspensions was observed to increase with increasing c_ (between c_ ¼ 0:01—20:0 s1 ), the s values of C increased from 175 Pa (E = 0 kV/mm) to 200 Pa (E = 2.0 kV/mm), and s values of MC increased from 158 Pa (E = 0 kV/mm) to 288 Pa (E = 2.0 kV/ mm). Effect of E on s for both materials is clearly seen. Both C and MC showed a Newtonian flow behaviour in the absence of E (E = 0 kV/mm). However, Bingham plastic behaviour was observed under E = 2 kV/mm condition with yield stresses (sy) of 15 Pa and 21.83 Pa for C and MC. 2.2.4. Effect of temperature Figure 10 shows the changes in the shear stress of C and MC suspensions under various temperatures and at constant conditions (E = 2 kV/mm, c_ ¼ 0:2 s1 , c = 20 wt %). It was observed that the shear stresses of all the suspensions examined in this work decrease with increasing temperature. Generally, temperature has two effects on ER fluids: one is on the polarization forces and another one is on the Brownian motion. The increase in temperature results both in decreased activation energy of polarization of suspended particles and in the polarizability of particles, which results in a decrease in shear stress. On the other hand, Brownian motion does not contribute to the chain formation of the suspended particles. Although shear stress increases with increasing temperature as reported in the literature by Choi27 and Lu and Zhao,28 Unal et al.29 and Liu and Shaw30 reported that shear stress decreases with increasing temperature. In our study, the shear stress of C and MC decreases with increasing temperature. Probably the first factor mentioned in the paragraph above is valid. Losses of shear stress in materials go as follows: DsE = 130 Pa for C and DsE = 87 Pa for MC. MC is of

Figure 9. The change of shear stress with shear rate. T = 20 °C, c = 20% by wt (a) E = 0, (b) E = 2 kV/mm.

Figure 10. The change of shear stress with temperature. c = 20% by wt, E = 2 kV/ mm, c_ ¼ 0:2 s1 .

greater thermal stability than C. These results are in harmony with the TGA results.

Figure 8. The change in viscosity with shear rate. T = 20 °C, c = 20% by wt, E = 0.

2.2.5. Effect of frequency The external stress frequency (f ) is an essential factor for characterizing the dynamic viscoelastic properties of ER fluids.31 Figure 11 demonstrates the change of electric field-induced complex shear modulus (G0 ) with f at c = 20 wt %, E = 2.0 kV/mm and T = 20 °C under constant conditions. The shear stress set for this

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(g) Cellulose suspensions exhibited Newtonian flow in the absence of E and Bingham plastic fluid behaviour upon application of the electric field. It might have been caused by the polarizability of the branched polar group of the cellulose particles. (h) Complex shear modulus of C and MC suspensions was observed to increase with increasing external frequency and show a typical characteristic of a viscoelastic material and a potential for vibration damping.

4. Experimental 4.1. Materials

Figure 11. The change of G0 with frequency. c = 20% by wt, T = 20 °C, s = 10 Pa, E = 2 kV/mm.

experiment was s = 10 Pa, which can ensure that the measurements are conducted in the small strain region. For C/CO and MC/CO suspension systems, the G0E remained unchanged in the linear viscoelastic region up to f = 46.4 Hz, then a sharp increase with the further increase in frequency after f = 70 Hz was observed. Because the relaxation of the internal chain structures of the ER fluids was very slow, they exhibited the rubber-like behaviour in the linear region. The increase in G0 with frequency indicates that the ER fluid becomes more elastic with the electric fields under the linear viscoelastic conditions. The increase in G0 with increasing external frequency was also reported in the literature32,33 as the typical characteristic of a viscoelastic material. 3. Conclusions In this paper, modified cellulose was obtained with the reaction of native cellulose and TEMPO. MC was characterised by FTIR, NMR, EDS, TGA and conductivity measurements. The ER properties of the C/CO and MC/CO suspensions were then investigated by examining the effects of sedimentation stability, concentration, electric field strength, shear rate, temperature, frequency and shear stress. The following is a summary of the results (a) We showed that the native cellulose can be partially modified and converted to the Li salt. (b) Sedimentation stabilities of C/CO and MC/CO suspensions were found to be 56% and suitable for potential industrial applications. Colloidal stability of the polymeric salt in corn oil was found to be 56% at a suspension concentration of 20% by wt. (c) Optimum particle concentration of the both suspensions was determined to be 20% by wt. (d) MC/CO suspension systems showed 5.5 times stronger ER strength after the modification. The ER strength of the suspensions was observed to be slightly sensitive at high temperature. (e) The ER activity of the suspensions increased with increasing field strength and decreasing shear rate. (f) It was observed that the viscosity of suspensions decreased sharply with increasing shear rate, causing typical shearthinning non-Newtonian viscoelastic behaviour.

All chemicals [amorphous cellulose, 2,2,6,6-tetramethyl-1piperidinyloxy (TEMPO), LiBr] were products of analytical grade from Acros Organics and were used as received. Methanol, ethanol and acetone were used as solvents throughout the experimental procedure. Hydrochloric acid and sodium hydroxide were J.T. Baker products, and sodium hypochlorite was an E. Merck product; all chemicals were used as received. The host oil employed was food-grade corn oil produced by Luna and had the following physical properties at 25 °C: density (q) = 0.936 g/cm3, viscosity (g) = 45 mPa s, dielectric constant (e0029 = 3.34 and conductivity (r) = 4  1011 S/m. 4.2. Modification of cellulose Oxidation experiments were carried out under the following conditions. Amorphous cellulose samples (0.648 g, 4 mmol of anhydroglucose units) were dispersed in distilled water (80 mL) for 1 min with an Ultra-Turrax homogenizer. TEMPO (10 mg, 0.065 mmol) and LiBr (0.20 g, 1.9 mmol) were added to the suspension, which was maintained at 4 °C. The sodium hypochlorite solution (13%, 4.88 mL, 8.8 mmol) with pH adjusted to 10 by the addition of 0.5 M aq HCl was maintained at 4 °C by means of an ice bath and was added four times (every 30 min) to the suspension, which was stirred mechanically. The pH was maintained at 10 during the reaction by adding a 0.5 M NaOH solution. The temperature of the suspension was maintained at 4 °C by means of an ice bath during the oxidation reaction. When the solution became hazy, almost all the cellulose samples had disappeared, and the reaction was stopped by adding 10 mL of methanol. The reaction mixture was neutralised to pH 7 with 0.5 M HCl and centrifuged to remove the residual insoluble material. The oxidised cellulose sample in the supernatant was precipitated by adding an excess of ethanol (5–10 volumes), followed by centrifugation. The precipitate was washed with 9:1 ethanol–water and centrifuged several times and finally washed with acetone. The precipitate was then redissolved in distilled water, dialysed and freeze-dried. After this process, we partially hydrolyzed C to MC, and then we converted the active functional group of COOH to its Li salt. Also, NMR results showed that the C was successfully partially hydrolyzed at a degree of 7.5%, which was aimed to be 8% for ER purposes. 4.3. Characterization FTIR spectra of C and MC were recorded on a Mattson Model 1000 instrument (UK) as KBr discs. The 1H NMR and 13C NMR spectra were obtained in DMSO-d6 or D2O at ambient temperatures using a 400-MHz Bruker DPX Avance Nuclear Magnetic Resonance Spectrometer at the Scientific and Technical Research Council of Turkey (TUBITAK), Ankara Test and Analysis Laboratory (ATAL), respectively.

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TGA was carried out using a Setaram 8ET8 V8 Evolution 1760 model thermogravimetric analyzer in the presence of nitrogen atmosphere up to 600 °C, at a heating rate of 10 °C min1. Energy dispersive spectroscopy (EDS) of the samples was recorded using a Jeol JSM-6360 LV scanning electron microscope (Japan). The particle size of the samples was determined using a Malvern Mastersizer E, version 1.2b particle size analyzer (UK). During the particle size measurements, samples were dispersed in distilled water and stirred at a constant temperature of 20 °C. The data collected were evaluated according to Fraunhofer diffraction theory by the Malvern software computer. Average particle diameters d(0, 5) of the C and MC were determined as 23.48 and 46.30 lm, respectively. Conductivity measurements were determined using the fourprobe technique at the Suleyman Demirel University Chemistry Research Laboratory. MC’s conductivity (r = 1.38 104 S/cm, d = 1.355 g/cm3) was higher than native C’s conductivity (r = 7.28 105 S/cm, d = 1.350 g/cm3). 4.4. Preparation of suspensions Suspensions of cellulose derivative particles were prepared in corn oil at a series of concentrations (c = 5–25% m/m), by dispersing a definite amount of dispersed phase in calculated amount of continuous phase according to the formula:

ðm=m; %Þ ¼

mdispersedphase  100 mdispersedphase þ moil

ð5Þ

4.5. Electrorheological measurements Suspensions were mechanically stirred before each measurement against sedimentation. Rheological properties of the suspensions were determined with a Termo-Haake RS600 parallel plate Electro-rheometer (Germany). The gap between the parallel plates was 1.0 mm and the diameters of the upper and lower plates were 35 mm. All the experiments were carried out at a controlled rate (CR) mode [except for the shear modulus (G0 ) versus frequency (f) graph, which was carried out at controlled stress (CS) mode] and at various temperatures (25–125 °C, with 25 °C increments.). The voltage used in these experiments was also supplied by a 0– 12.5 kV (with 0.5 kV increments) dc electric field generator (Fug Electronics, HCL 14, Germany), which enabled resistivity to be created during the experiments.

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