Electrorheological properties of poly(Li-2-hydroxyethyl methacrylate) suspensions

Colloids and Surfaces A: Physicochem. Eng. Aspects 274 (2006) 77–84

Electrorheological properties of poly(Li-2-hydroxyethyl methacrylate) suspensions Halil Ibrahim Unal a , Oya Agirbas a , Hasim Yilmaz b,∗ a b

Gazi University, Science Faculty, Chemistry Department, Teknikokullar, 06500 Ankara, Turkey Harran University, Science Faculty, Chemistry Department, Osmanbey, 63190 Sanliurfa, Turkey

Received 26 December 2004; received in revised form 17 August 2005; accepted 25 August 2005 Available online 29 September 2005

Abstract In this study, poly(2-hydroxyethly methacrylate), poly(hema), was synthesized by free radical polymerization using K2 S2 O8 as an initiator. The polymer was characterized by FTIR, 1 H NMR and elemental analysis measurements. The molecular mass of poly(hema) was determined by osmometry, viscometry and end group analysis techniques approximately as 6 × 103 g/mol. Poly(hema) was partially hydrolyzed and converted to a lithium salt, poly(Li-hema) before the electrorheological (ER) measurements carried out. A series of particle size of poly(Li-hema) polymeric salt were prepared and average particle diameters were determined by dynamic light scattering (DLS) as 8, 13, 19 and 25 ␮m. Suspensions of poly(Li-hema) polymeric salts were prepared in four insulating oils namely silicone oil (SO), mineral oil (MO), dioctylphthalete (DOP) and trioctyltrimellitate (TOTM). ER properties of poly(Li-hema)/silicone oil suspensions were studied as a function of electric field strength, particle size, dispersed phase concentration, shear rate, shear stress, temperature, frequency and promoter. For these ER suspensions, yield stresses and excess shear stresses were determined. Further dielectric properties of poly(Li-hema) ionomer were also investigated. © 2005 Elsevier B.V. All rights reserved. Keywords: Electrorheological fluids; Poly(Li-2-hydroxyethyl methacrylate) ionomer; Colloidal suspensions

1. Introduction 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 [1,2]. Thus, it is quite natural that dielectric polarization theory appeared, because ER behavior was closely related to dielectric phenomena, and among various polarizations, interfacial polarization is assumed to be responsible for ER phenomena [3]. To overcome the shortcomings (thermal instability and corrosion) that wet-base systems possess, various dry-base systems have been investigated with anhydrous particles, including zeolite [4], carbonaceous particle, and intrinsically polarizable semi-conducting polymers [5]. Special attention has been paid to the polymer-based ER materials. Examples include: acene quinone radical polymers [6], polyaniline [7], copolyaniline

∗

Corresponding author. fax: +90 414 344 00 51. E-mail address: [email protected] (H. Yilmaz).

0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.08.037

[8], polyphenylenediamine [9], poly(2-acrylamido-2-methyl1-propane sulfonic acid) [10], polypyrrole [11], polystyreneblock-polyisoprene [12] and polymer–diatomite composites with polyacrylonitrile [13] or polyaniline [14]. The difference between dry-base and wet-base systems is the carrier species for particle polarization. The particle chain structure is formed by the migration of ions in the absorbed water in wet-base ER fluids, whereas the electrons move inside the molecules of the particles in the dry-base ER fluids. There is also a need for fluids with enhanced colloidal stability against sedimentation and sludge deposits formation [15]. Most of the studies on the literature are focused on the ER activity of acrylate salts and zeolitic materials, and very few of these researchers have investigated the influence of colloidal stability of suspensions. Another target is, for ER fluids, with long service stabilities, particularly at high temperatures and rigid environmental conditions [16]. Electrorheological active materials possess either branched polar groups such as amine ( NH2 ), hydroxyl ( OH) and aminocyano ( NHCN), or semi-conducting repeated groups. The polar groups may affect the ER behavior by playing a role of the elec-

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tronic donor under imposed electric field. The chemical structure of the organic materials is, therefore, an important factor in the ER performance. There are very wide range of potential applications for ER fluids in such areas as vibration damping, robotics, hydraulics, couplings and automotive [17]. The patent literature on the subject suggests a growing interest in such devices after a period of research and assessment. A major limiting factor is still the need for fluids with better overall performance. Important factors influencing the ER effect are electric field strength, field frequency, shear rate, fluid composition, temperature, colloidal stability and presence of a polar promoter [18]. In this research, poly(Li-hema), as a new organic dispersed phase, was chemically synthesized; characterization, salt formation, and ER and dielectrical properties, pertaining to the ER behavior of poly(Li-hema) suspensions, in four insulating oil media, were investigated. 2. Experimental 2.1. Materials The monomer (2-hydroxy ethyl methacrylate) was purified by vacuum distillation and the initiator (K2 S2 O8 ) was used as received. The insulating oils (silicone oil, SO, dioctaylphthalate, DOP, trioctyltrimellitate, TOTM and mineral oil, MO) were used after drying at 130 ◦ C for 3 h in a vacuum oven, to remove any moisture present. The physical properties of four insulating oils are given in Table 1 [19]. All the chemicals were Aldrich (Aldrich Chemicals, Steinheim, Germany) products, with analytical grade. 2.2. Preparation of an ionomer from poly(hema) Poly(2-hydroxyethyl methacrylate) was synthesized with suspension polymerization by radicalic mechanism at 60 ◦ C, using K2 S2 O8 as an initiator. K2 S2 O8 was dissolved in distilled water and 2-hydroxyethyl methacrylate monomer was added drop-wise into this solution (containing poly(vinyl alcohol) as stabilizer) under N2(g) atmosphere. The polymerizing solution was kept stirring for 4h. Then poly(hema) was recovered by freeze-drying and vacuum dried at 25 ◦ C for 48 h under 15 mmHg, and kept in a desiccator until use. To prepare an ER active ionomer from poly(hema), it needs to be partially hydrolyzed and converted to a lithium salt, poly(Lihema), by washing with 10% LiOH(aq) solution. Poly(Li-hema) was separated from the solvent by freeze-drying. It was then dried in a vacuum oven less than 15 mmHg pressure for 24 h

Scheme 1.

at 50 ◦ C. The reaction mechanism for the formation of ionomer (poly(Li-hema)) is described in Scheme 1. 2.3. Characterization Poly(hema) and poly(Li-hema) were characterized before ER measurements to be carried out by elemental analysis, FTIR spectroscopy (Mattson Model-1000 FTIR spectrometer), 1 H NMR spectroscopy (400 MHz Brooker 400 DPX Avonce Spectrometer), intrinsic viscosity measurements (in N,N-dimethylformamide (DMF) using a Ubbelohde capillary flow viscometer at 25.0 ◦ C ± 0.1 ◦ C), and end-group analysis (by titrating acid units with 0.1 M KOH(aq) solution). Molecular mass of poly(hema) was also determined by a vapor pressure osmometer (Vapro Model 5520) in DMF at 90 ◦ C. Poly(Li-hema) ionomers were ground milled in a threedimensional turbula shaker for 4, 8, 12 and 16 h and a series of various particle size of samples were prepared. Particle sizes of poly(Li-hema) were determined using a Malvern Mastersizer E, version 1.2b particle size analyzer according to Fraunhofer scattering. Some poly(Li-hema) samples were dispersed in ethanol and stirred at a constant temperature of 20 ± 0.1 ◦ C. The data collected were evaluated according to Fraunhofer diffraction theory by the Malvern software computer [20]. The current–potential measurements were performed on an ionomeric salt disc (20 mm long, 5 mm wide, and 1 mm thick) with a Keithley 220 programmable current source and a Keithley 199 digital multimeter (Ohio) at the ambient temperature. The capacitance, C, of ER particles was measured with an HP 4192 A LF Impedance Analyzer at frequency of 1.0 MHz at constant temperature (20 ± 0.1 ◦ C). 2.4. Preparation of suspensions Suspensions of poly(Li-hema) ionomers were prepared in four insulating oils (SO, MO, TOTM and DOP) at a series of particle concentrations (c = 5–30 m/m, %), by dispersing definite amount of ionomers in calculated amount of insulating oils

Table 1 Physical properties of insulating oils Oil

IUPAC name

Boiling point (◦ C)

Density (g/mL)

Viscosity (Pas)

DOP TOTM Silicone oil Mineral

Bis(2-ethylhexyl ftalate) Tris(2-ethylhexyl trimellitate) Poly(dimethyl-siloxane) –

384 163–165 >140 >110

0.981 0.821 0.963 0.862

0.04 0.08 0.08 0.04

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according to the formula: 
mpolymer × 100 (m/m, %) = (mpolymer + moil )

(1)

2.5. Sedimentation stability measurements Sedimentation stabilities of poly(Li-hema) suspensions prepared in four insulating oils (SO, MO, TOTM and DOP) were determined at 25 ± 0.1 ◦ C. Glass tubes containing the suspensions, prepared at a series of ionomer concentrations (c = 5–30 m/m, %), were immersed into a constant temperature water bath and formation of first precipitates was recorded to be the indication of colloidal instability. 2.6. Rheological and electrical tests 2.6.1. Flow rate measurements Since good sedimentation stability results were obtained for poly(Li-hema)/SO system, rheological experiments were carried out for the ionomer suspensions prepared in just silicone oil. The experimental determination of flow behavior and viscoelastic material properties, which influence processing technology and polymers stability and consistency, were aimed. Flow rate measurements of the suspensions were carried out between two brass electrodes. The gap between the electrodes was 0.5 cm, the width of the electrodes was 1.0 cm and the height of liquid on the electrodes was 5.0 cm. During the measurements these electrodes were connected to an external high voltage dc electric source (0–12.5 kV, with 0.5 kV increments, Fug electronics HCL-14, Germany) and a digital voltmeter. The electrodes were dipped into a vessel containing the ionomer suspension and after a few seconds the vessel was removed and the flow time for complete drainage measured, using a digital stop-watch under E = 0 kV/mm and E = 0 kV/mm conditions. This procedure was repeated for each ionomer suspension concentration under various field strengths. 2.6.2. Electrorheological measurements Ionomers having 8, 13, 19 and 25 ␮m particle sizes were dispersed in silicone oil and stirred fully (weight fractions of 5–30%) to prepare all the ER fluid samples. The ER experiments were performed using a Thermo-Haake RS600 Torque Rheometer, equipped with an ER adapter. The apparatus can work under a given temperature and plate clearance to measure the shear stress and apparent viscosity of a fluid at various shear rates, and it has the function of controlled rate, controlled stress and oscillation operating modes. In this study, measurements were carried out using a PP35 ER sensor system, which is a pair of parallel palates and the gap between the plates was 1.000 mm. For each measurement, a suspension sample was placed between the lower and upper plates at constant temperature and the upper plate was rotated at a predetermined shear rate, while the down plate kept stationary. The applied shear rate ranges were 0–1000 s−1 . In this study, shear stresses and viscosities of the samples were determined under various external applied dc electric field strengths (0–2 kV/mm) at various temperatures (25–125 ◦ C). The voltage

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used in these experiments was supplied by a 0–12.5 kV dc electric field generator (Fug electronics HCL-14, Germany), which enabled resistivity to be created during the experiments. 3. Results and discussion 3.1. Characterization Poly(Li-hema) was characterized by a series of methods before ER measurements to be carried out. End group analysis was first performed to determine the number average molecular mass of poly(Li-hema), and from the acid number of poly¯ n , was calculated to be mer, number average molecular mass, M ¯ n estimated from vapor pressure osmometry 6000 g/mol. The M for poly(hema) was 6300 g/mol. From capillary viscometry measurements, intrinsic viscosity of poly(hema) was determined to be 2.7 dL/g and the corresponding viscosity average molecular ¯ v ) was calculated to be 6300 g/mol. mass (M Elemental analysis results were used as a check for purity and percentage conversion of ester groups into lithium salt by comparison with calculated composition. It was obtained from the elemental analysis results that, the amount of lithium ion in poly(hema) was 0.57% by mass within the experimental error. The poly(hema) and its Li ionomer poly(Li-hema) were assumed to have the chemical structure given in Scheme 1. FTIR spectra of poly(hema) and poly(Li-hema) (Fig. 1) were shown the expected distinctive absorptions. The absorptions of poly(hema) at 3200–3400 cm−1 , 1700 cm−1 , 3000–3100 cm−1 , and 1100 cm−1 are typical of O H, C O, C H, and C O stretching vibrations, respectively. O H band of poly(hema) at 3200–3400 cm−1 became narrower at the FTIR spectrum of poly(Li-hema). 1 H NMR was used to determine the structure of poly(hema) and the expected characteristic peaks observed are tabulated in Table 2. In this study crude poly(Li-hema) was ground-milled in a three-dimensional mill for various hours (4, 8, 12 and 16 h), and a series of particle sizes were prepared. Particle sizes of the ionomers were determined from particle size measurements as

Fig. 1. FTIR spectrum obtained from poly(Li-hema).

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Table 2 Chemical shifts obtained for poly(hema) by 1 H NMR spectroscopy in CDCl3 Assignment

Chemical shifts (1 H, δ, ppm)

3H, CH3 2H, CH2 C 4H, O (CH2 )2 O 1H, CH2 OH

1.17–1.30 (s) 1.62–1.85 (m) 3.98–4.04 (m) 4.01 (s)

s: Singlet, m: multiplied.

8, 13, 19, and 25 ␮m according to Fraunhofer diffraction theory [20]. 3.2. Conductivity and dielectric measurements The conductivity of poly(Li-hema) was 1.0 × 10−7 S/m. That is well in the range of conductivity of ER particles [21]. The low conductivity value was due to the low ion content of the ionomer (0.57% Li+ ). As expected, the driving force behind the ER activity of this ionomer/insulating oil system is the polarization of ionomeric particles suspended inside the suspensions [22]. The dielectric constant was derived from the measured C according to the conventional relation, ε = εCd , where ε0 is the dielectric 0S constant of the vacuum (i.e., 8.85 × 10−12 F/m), d is the distance of the gap between the electrodes, and S is the contact area of the electrodes. The permittivity of poly(Li-hema) was found to be 1774. As suggested by Hao [21] the permittivity of an ER suspension should lie between 2 × 104 and 1 × 104 . The permittivity of poly(Li-hema) is well within the range expected from an ER fluid. 3.3. Sedimentation stability of suspensions Despite the recent activities surrounding ER fluids and ER effect, little efforts have focused on the colloidal stability of these suspensions. Few investigations probe the colloid chemistry of ER fluids [15,22,23]. When the density of particles is not as same as that of medium, the particles with micron order size will settle down according to the Stoke’s law [24]. In order to solve the traditional problem of particle sedimentation, several researchers have developed different solutions [25]. Density mismatch between dispersed and continuous phase in the suspension plays an important role in sedimentation stability of an ER fluid. Sedimentation stabilities of poly(Li-hema) suspensions, prepared in those four insulating base fluids (SO, DOP, TOTM, and MO) in a series of concentrations (c = 5–30%), were determined at 25 ◦ C and results obtained are tabulated in Table 3. As reflected from the table, sedimentation stability of suspensions was increased with decreasing particle contents of the suspensions. Maximum colloidal stability was observed in silicone oil suspensions for c = 5% concentration as 60 days. This is between the colloidal stability value of poly(Li-styrene)-block-polyisoprene copolymer [12] (52 days) and polypyrrole [11] (63 days), both prepared in silicone oil at c = 5% particle concentration. Since the maximum colloidal stability was achieved in silicone oil, ER measurements

Table 3 Sedimentation stability results of poly(Li-hema) suspensions (T = 20 ◦ C) Oils

Silicone Mineral TOTM DOP

Concentrations (m/m, %) 5

10

15

20

25

30

60 days 47 days 11 days 8 days

56 days 32 days 8 days 7 days

53 days 26 days 7 days 5 days

44 days 21 days 6 days 4 days

28 days 19 days 4 days 3 days

17 days 12 days 3 days 1 day

were also carried out for the suspensions prepared in silicone oil. 3.4. Electrorheology Since the ER phenomena are widely attributed to the chaining of micron-sized polarisable particles, when subjected to an external electric field, flow rate and ER studies are conducted to observe the viscosity change at flow and ER response of poly(Lihema) suspensions. 3.4.1. Flow rate measurements To observe the effect of dc electric field on the ER activity; flow rate measurements are carried out on the poly(Li-hema) suspensions. For this purpose, poly(Li-hema) suspensions were prepared at a series of particle sizes (8, 13, 19, 25 ␮m) in four insulating oils (SO, DOP, TOTM, MO); and flow times measured under E = 0 kV and E = 0 kV conditions. Results obtained just from silicone oil suspensions are depicted in Fig. 2. As reflected from the graph, flow times of suspensions were first shown little increase with increasing electric field strength up to threshold energies (Et = 1.5 kV/mm), and then sharp increases were observed for all the particle sizes studied. Minimum and maximum flow times were obtained for poly(Li-hema)/SO system as 450 and 650 s for 25 and 8 ␮m particle sizes, respectively. Flow times given for the suspensions are the maximum flow times, which could be observed under the external applied electric field. When electric field was further increased, a highly

Fig. 2. Effect of electric field strength on flow time: (
) 8 ␮m; (䊉) 13 ␮m; () 19 ␮m; () 25 ␮m.

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stronger bridge formation was occurred for all the suspensions and no flow was observed. Similar behaviors were reported for sepiolite [26], diatomite and diatomite/polyacrylonitrile composite [13] suspensions in silicone oil. 3.4.2. Effect of particle size on ER effect Polarisability, particle size and aggregation ability of suspended particles are important factors which influence the efficiency of ER fluids. Thus, when choosing a suitable dispersed phase, all these factors have to be taken into account in order to obtain a material with enhanced ER performance suitable for potential industrial applications. The effect of the particle size distribution on shear stress was investigated using four different sized (d = 8, 13, 19 and 25 ␮m) poly(Li-hema) samples. The results of shear stress measurements against particle size, at a constant electric field strength (E = 2.0 kV/mm), shear rate (γ˙ = 1.0 s−1 ) concentration (c = 20%) and temperature (T = 25 ◦ C) are given in Fig. 3. Considering the ER activity of poly(Li-hema) particles, when particle sizes get smaller the ER performance of poly(Li-hema)/SO suspensions increase. As seen from the graph, as the particle size of ionomer decreases from 25 to 8 ␮m, the electric field induced shear stress of the suspensions increases from 10.40 to 13.75 kPa. Similar behavior was reported for polyaniline particles dispersed in silicone oil [27]. This is in contrast to the results observed in the study of ER performance of nano-sized particle materials containing ZrO2 by Liao et al. [28]. In this study they reported that nanosized particle materials containing ZrO2 as doping agent shown decreasing ER performance, as the particle sizes were further decreased.

Fig. 4. The change of electric field viscosity with concentration. T = 25 ◦ C, E = 2 kV/mm; (
) 8 ␮m; () 13 ␮m; () 19 ␮m; (䊉) 25 ␮m.

apparent viscosity sharply increases. The higher ionomer concentration results in a denser particle structure organized in the electric field with a higher resistance against flow. Also, as a result of polarization forces acting between the poly(Li-hema) ionic particles, electric field viscosity (or ER activity) increases with increasing poly(Li-hema) concentration in the suspensions. Similar behavior was observed in the studies of polyaniline suspensions [29].

3.4.3. Effect of concentration on electric field viscosity The change in electric field viscosity with suspension concentration of poly(Li-hema) particles dispersed in silicone oil at various particle sizes, constant shear rate (γ˙ = 1.0 s−1 ), electric field strength (E = 2 kV/mm) and temperature (T = 25 ◦ C) is depicted in Fig. 4. When increasing the poly(Li-hema) ionomer concentration, the flow curves of suspensions in the presence of electric field become more pseudo-plastic and the low-shear

3.4.4. Effects of electric field strength on viscosity and shear stress Fig. 5 illustrates the characteristic flow behavior of poly(Li-hema)/SO suspension (c = 20%) under applied external electric field. In the presence of electric field strength (E = 0.5–2.0 kV/mm), the flow curves become non-Newtonian and their pseudo plastic characteristic increases with the increasing electric-field strength. Various polymer salts had been developed in the literature as ER dispersed phase. But when the strength of these ER fluids are compared with our ER fluid system, it is obvious that poly(Li-hema)/SO system shows very high ER strength

Fig. 3. Effect of particle size distribution on shear stress. γ˙ = 1.0 s−1 , c = 20%, T = 25 ◦ C; (
) 8 ␮m; () 13 ␮m; () 19 ␮m; (䊉) 25 ␮m.

Fig. 5. The change of apparent viscosity and shear stress with electric field strength. γ˙ = 1.0 s−1 , T = 25 ◦ C, c = 20 %, d = 8 ␮m; (
) shear stress () apparent viscosity.

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with shear stress of τ = 14 kPa, which a very important property for potential industrial applications [30]. For example, for the ER study of polyisoprene-co-poly(tert-butyl methacrylateLi) suspensions [22] τ = 25 Pa, for the study of ER fluid based on inorganic/polymer blend particles and its adaptive viscoelastic properties, τ = 1.9 kPa [31]; for the study of synthesis and properties of ionic conducting cross linked polymer and copolymer based on dimethacryloyl poly(ethylene glycol), τ = 2.3 kPa, [32]; for the study on conductivity of two kinds cross-linked polyether solid electrolytes and ER properties of unhydrous suspensions, τ = 2.8 kPa [33]; and for the study of electrorheological characterization of polyaniline-coated poly(methyl methacrylate) suspensions, τ < 1 kPa [34] shear stresses were reported. 3.4.5. Change in shear stress with shear rate Change of shear stress of poly(Li-hema)/SO suspension with shear rate at constant conditions (c = 20%, E = 2.0 kV/mm, T = 25 ◦ C) is shown in Fig. 6. As seen from the figure, shear stress of poly(Li-hema)/SO suspension system increases with increasing shear rate as typical of non-Newtonian fluids for all the particle sizes investigated. The shear stress increase was highest for the smallest particle size and for d = 8 ␮m particle size we obtained to τ = 12.0 kPa shear stress with τ 0 = 6.0 kPa yield stress. Similar non-Newtonian flow behaviors were reported in the literature [10,22]. 3.4.6. Change of ER efficiency with shear rate For the possible industrial application of ER suspensions, it is necessary to attain the highest possible electric field viscosity (ηE ) at the lowest field-off viscosity (η0 ) of the suspension. It is clear that the ER efficiency expressed as the ratio (ηE − η0 )/η0 can be significantly affected by the flow properties of each ionomer suspension particles in addition to particle polarisability. Change of ER efficiency of poly(Li-hema)/SO suspensions with shear rate at constant suspension concentration (c = 20%) is

Fig. 6. The change of shear stress with shear rate. T = 25 ◦ C, E = 2 kV/mm, c = 20 %; (
) 8 ␮m; () 13 ␮m; () 19 ␮m; (䊉) 25 ␮m.

Fig. 7. The dependence of (a) the electric field viscosity, ηE , at E = 2.0 kV/mm; (b) the field-off apparent viscosity, η0 , at E = 0.0 kV/mm; and (c) the ER effi˙ (
) 8 ␮m, () 13 ␮m, () 19 ␮m, (䊉) ciency (ηE − η0 )/η0 , on shear rate, γ. 25 ␮m.

shown in Fig. 7. As is evident, with an applied E (Fig. 7a), viscosity of suspensions decreases sharply with increasing shear rate up to γ˙ = 10 s−1 , gives a typical curve of shear thinning non-Newtonian visco-elastic behavior, and then become shear rate independent (Newtonian). For example, for d = 8 ␮m average particle sized poly(Li-hema)/SO suspension system, the on field viscosity was observed to be ηE = 134 Pas and the off field viscosity (Fig. 7b) was η0 = 4 Pas, which is a desired property in terms of potential industrial applications. The ER efficiency of poly(Li-hema)/SO suspension system, which is an extremely important parameter for future design of highly effective ER fluids, characterized by a pseudoplastic decrease of the viscosity with the shear rate is given in Fig. 7c [35]. 3.4.7. Effect of temperature on electric field viscosity The temperature dependence of the shear stress is shown in Fig. 8. The results were obtained at temperatures of 25, 50, 75,

Fig. 8. The variation of viscosity with temperature. γ˙ = 1.0 s−1 , E = 2 kV/mm, c = 20 %; (
) 8 ␮m; () 13 ␮m; () 19 ␮m; (䊉) 25 ␮m.

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Table 4 Effect of temperature on shear stress loss. (γ˙ = 1.0 s−1 , E = 2.0 kV/mm and c = 20%) Particle size (␮m)

Shear stress loss (kPa)

8 13 19 25

1.40 1.40 1.75 1.50

100 and 125 ◦ C. As shown in Fig. 8, the shear stress decreases with increasing the particle size and the temperature. The effect of temperature is one of the most important parameter to evaluate ER effect [36]. Generally, the temperature has two effects on the ER fluid: one is the effect on the polarization intensity of particle and another one is Brownian motion. The increase of the temperature results, both in decreased activation energy of polarization of suspended particles and, also on the polarizability of the particles, which results with a decrease in the ER strength of the suspension. On the other hand, Brownian motion does not contribute to chain formation for poly(Li-hema) particles. The shear stress losses between 25 and 125 ◦ C are also calculated and given in Table 4. As reflected from the table, minimum shear stress loss (τ = 1.4 kPa) was obtained from the suspension prepared with 8 ␮m poly(Li-hema) ionomers, which may be attributed to the stronger electric field induced fibrillar structure formation in the suspension. These observations are consistent with the results reported by Choi for chitosan/silicone oil suspensions [37]. 3.4.8. Effect of frequency on ER activity The external stress frequency is an important factor for characterizing the dynamic visco-elastic properties of ER fluids [38]. Fig. 9 shows the relationship of electric field (E = 2.0 kV/mm) induced complex shear modulus and frequency. The setting shear stress for this measurement was 10 Pa, which can ensure the measurements are conducted in the small strain region. As seen from the figure, ER activity of the poly(Li-hema)/SO system decreases with increasing frequency, which may be

Fig. 10. The change of viscosity with promoter. T = 25 ◦ C, d = 8 ␮m, E = 2 kV/mm, c = 20%; (
) 1000 ppm ethanol promoted; () promoter free.

attributed to the dielectric loss and low polarizability of the ionomer suspension [37]. 3.4.9. Effect of promoter on ER activity Alcohol, ethylene glycol, dimethylamine, formamide and water have been reported in the literature as polar promoters [18]. In this study, the influence of moisture on the ER activity was also investigated by adding polar promoters, such as ethanol, water and glycerol at c = 1000 ppm concentration into poly(Li-hema)/SO suspensions. Since the three promoted suspensions shown similar trend, the data obtained just from ethanol promoted suspension is presented in Fig. 10. The ER effect of these samples can be compared by their yield stresses measured at different electric field strengths (Fig. 10). It was observed that, ER activity of suspensions was insensitive to the presence of moisture and addition of promoters caused extremely small increase at the electric field induced viscosity (ER activity) of the suspensions. Electric field induced viscosity of ethanol promoted suspension was observed to increase from 128 to 134 Pas. The observation that the poly(Li-hema)/SO system investigated in this work is not affected by the addition of a polar promoter could prove particularly important for industrial applications. Similar behaviors were reported in the study of saponite suspensions [39] by D¨urrschmidt and Hoffmann. 4. Conclusions

Fig. 9. Effect of electric field frequency on storage shear modulus. T = 25 ◦ C, d = 8 ␮m, E = 2 kV/mm, c = 20%.

Colloidal stability of poly(Li-hema) ionomer in silicone oil was found to be 60 days (c = 5%). Flow times of suspensions observed to increase with increasing electric field strength and suspension concentration. ER activity of all the suspensions was observed to increase with increasing electric field strength, concentration and decreasing shear rate and decreasing particle size. Shear stress was found to sharply increase with increasing field strength, suspension concentration and ionomer’s particle size. The viscosity of all the suspensions was decreased sharply with increasing shear rate and showing a typical shear-thinning non-Newtonian visco-elastic behavior. The addition of polar

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