Colloid Polym Sci (2003) 281: 1196–1200 DOI 10.1007/s00396-003-0909-y

Jun H. Sung Hyoung J. Choi Jeong-In Sohn Myung S. Jhon

Received: 4 January 2003 Accepted: 19 March 2003 Published online: 17 May 2003
Springer-Verlag 2003

J.H. Sung Æ H.J. Choi (&) Department of Polymer Science and Engineering, Inha University, 402–751, Incheon, Korea E-mail: [email protected] J.-I. Sohn Department of Chemistry, Hallym University, 200–7021 Chunchon, Korea M.S. Jhon Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213-3890, USA

SHORT COMMUNICATION

Electrorheology of chitosan polysaccharide suspensions in soybean oil

Abstract Chitosan, a biodegradable polysaccharide composed of primarily D-glucosamine repeating units, was adopted as a dry-base electrorheological fluid using soybean oil as a suspending medium. The electrorheological properties were examined under various applied electric field strengths. We found that natural organic polymers such as chitosan possessing amino polar groups induce electrorheological behavior, and the yield stress, sy, of chitosan increases with the electric field strength, E , according to a power-law form, sy1 E a, which is consistent with the conduction model.

Introduction Electrorheological (ER) fluids are characterized by a rapid and reversible change in suspension structure under an applied electric field. This peculiar response is determined from the transient aggregation of the solidlike, or Bingham, behavior owing to the attractive forces among the dipolar moments induced on each particle by the external field [1, 2, 3,4]. The Bingham behavior usually possesses a yield stress, which is defined as a stress where the suspension property changes from solidlike to fluidlike in a zero shear rate limit. Thus, the Bingham fluid equation has been adopted as the suitable rheological model for the steady-shear behavior of many ER fluids as described later: s ¼ sy þ g_c s
sy ; c_ ¼ 0 s < sy ;

ð1Þ

Keywords Electrorheological fluid Æ Chitosan Æ Yield stress Æ Suspension

where sy is the yield stress, which is a function of the electric field, s is the shear stress, c is the shear rate, and g is the shear viscosity. Compared to conventional wet-base ER systems which require active substrates, such as water, ethylene glycol, and surfactant, to promote the ER effect, polymeric particles possessing either a polar group, such as amino ()NH2), hydroxyl ()OH), and amino-cyan ()NHCN), or electrical conductivity are known as drybase ER materials. Various polymer particles have been used as dry-base ER materials to avoid the thermal breaking, particle settling, irreversible clumping, and abrasiveness. Polyaniline [5, 6,7] and its derivatives [8], poly(acene quinone radicals) [9], poly(p-phenylene) [10], poly(phenylenediamine) [11], polypyrrole [12,13], and polymer/clay nanocomposites [14,15], which are thermally stable and relatively easy to synthesize, are representative of the conducting polymers used in ER fluids, and are polarizable under an electric field via

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electron movement through the polymer backbones. The typical conducting polymers studied are based on a p-conjugated electron system constituting the polymer backbone [16]. Polymers with conjugated p-electron systems display unusual electronic properties, including high electron affinities and low ionization potentials. The local electron distribution of particles induces the ER effect under the applied electric field [5,17]. On the other hand, chitosan and its derivatives, such as chitosan adipate [18] and dihydroxylpropyl chitosan [19], are also used as ER materials. The branched amino polar group from these natural organic polymers displays the ER property resulting from the electronic donor group ()NH2). Recently, these biopolymers have been applied for the feasibility of adopting ER fluids for a controllable drug delivery system [20]. In the absence of an electric field, the drug release occurs via diffusion across a mesh electrode. In the presence of an electric field, however, drug release may be controlled either by hindering or halting. The effect of different media on ER performance has also been examined for an anhydrous chitosan-based ER fluid [21]. Chitosan is obtained from N-deacetylation of chitin, which is a linear polysacharride consisting of poly-b(1– 4)- N -acetyl- D -glucosamine. As the degree of deacetylation of chitinous material exceeds 50%, it becomes soluble in acidic aqueous solution and is called chitosan. Chemically, chitosan is a natural organic random copolymer primarily composed of repeated sugar units with a structure of (1–4)-linked 2-amino-2-deoxy-b- D glucopyranose, as well as some units with the structure of (1–4)-linked 2-acetic-amino-2-deoxy-b- D -glucopyranose [22.23,24]. Chitosan has been previously modified and prepared to produce various chemical and biological properties, such as for pharmaceutical applications [25] and DNA–chitosan nanoparticles as gene carriers [26]. In this study, we investigated ER properties of chitosan particles suspended in soybean oil as a potential biomedical application.

with a conductivity cell. The conductivity, r, was then obtained from the relationship, r= d /( A · R), where d is the thickness, A is the surface area, and R is the resistance of the pellet. This conductivity is sufficient for an ER application without any posttreatment, in contrast to the semiconducting polyaniline particles where the conductivity of the particles is controlled by a doping process prior to usage. The ER properties of the chitosan-based ER fluids were examined via a commercially available rotational rheometer (Physica MC120, Stuttgart, Germany) with Couette geometry equipped with a high-voltage generator (VG 5000, Stuttgart, Germany). Several direct current electric field strengths (0–3.0 kV mm)1) were applied to the bob. Specifically, the flow curve was measured via a controlled shear rate (CSR) mode in which a shear rate was applied to the ER fluid and then the shear stress was measured. A static yield stress, however, was obtained by a controlled shear stress (CSS) mode. The ER fluid was stressed by an applied mechanical torque until the particle chains were broken to generate flow, and the stress at the onset of flow was recorded as a static yield stress [28].

Results and discussion The chitosan particle shape was identified as irregular with rough surfaces by a scanning electron microscope photograph, as shown in Fig. 1, with their particle size in the range 50–200 lm. Flow curves measured in the CSR mode for the 15 wt% chitosan-based ER fluid in the soybean oil at six different electric field strengths are shown in Fig. 2. The ER response exhibits Bingham behavior under an applied electric field. The interparticle forces (polarization forces) cause the dispersed particles to form chains or fibrils, which connect to the gap. This suspension structure leads to the enhancement of shear stress and viscosity because the particle chains formed the fibril structures and aligned along the electric field direction. The shear stress curve reveals a plateau region as the

Experimental Chitosan obtained from Samchully Pharm. (Korea) with a deacetylation degree of 95% is a relatively cheap, biocompatible, biodegradable, and nontoxic cationic linear polymer. The amino group in its backbone is protonated in acidic solution. The chemical reaction converting chitin to chitosan was performed using 50% NaOH solution at 100 C for 1 h. After removing any trace of moisture, the particle size of the chitosan was adjusted by using a 100-lm sieve. ER fluids were then prepared by dispersing chitosan particles in soybean oil. The density of the soybean oil was 0.916 g m)3 with a kinematic viscosity of 50 cS. All measurements were taken at 25 C unless otherwise specified. The conductivity of chitosan particles was 5.26·10)10 S cm)1 using a two-probe method with a pressed disk of polymer with silver electrodes on each side [27]. The pellets of dried chitosan particles were prepared using a 13-mm KBr pellet die, and the pellet resistance was measured using a picoammeter (Keithley model 487, Cleveland, USA)

Fig. 1 Scanning electron microscope photograph of chitosan particles

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Fig. 2 Shear stress versus shear rate for a 15 wt% chitosan– soybean oil suspension for six different electric field strengths

the shear stress shows a linear relation with the square of the electric field strength; the interaction force for dipoles in an electric field is proportional to the electric field strength on the basis of the polarization model [30]. This relation s¥ E 2.0 is different from the yield stress relation given in Fig. 5, since the polarization model is dominant in this shear rate range studied, while the conduction model is dominant at the zero shear rate limit. Meanwhile, Fig. 4 clearly shows the microscopic image (·400) of the 15 wt% chitosan-based ER fluid in the soybean oil with (at 2.0 kV mm)1) and without an applied electric field. As soon as the electric field is applied, the chitosan particles form fibril structures aligned in the direction of the applied electric field. This explains how the dielectric characteristic on the basis of the polarization model shows the particle interactions among the suspended particles in the nonconducting medium [31], and the degree of dipole response is considerably fast under the applied electric field in a shear flow. The shear viscosity for the 15 wt% chitosan-based ER fluid is presented in Fig. 5 at six different electric field strengths. The viscosity increment under the electric field results from the energy spent to break the chainlike or columnar structure of the particles. When a strain is applied to the aligned structures, it distorts and destroys the fibril chains, resulting in the decrease of the viscosity (or a shear-thinning behavior) [32]. Shear-thinning

Fig. 3 Shear stress versus electric field strength for a 15 wt% chitosan-–soybean oil suspension at the shear rate of 100 s)1

shear rate increases up to a critical value (e.g., 70 s)1 for 0.5 kV mm)1, 300 s)1 for 1.0 kV mm)1, and 600 s)1 for 1.5 kV mm)1). As the electric field strength increases the electrostatics become dominant over the hydrodynamic forces [4,29]. The ER response of the suspension indicates that the shear stress increases with the electric field strength owing to the enhancement of interparticle interactions. The shear stress of this ER suspension at the specific shear rate c=100 s)1 is plotted in Fig. 3 as a function of the applied electric field strength, showing that the shear stress at c=100 s)1 is proportional to E2.0. Although the chitosan particles are irregularly shaped,

Fig. 4 Microscope image (·400) of a chitosan electrorheological suspension before and after applying an electric field (2 kV mm)1)

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dependence suggested by the polarization model [31,33]. The fibril structures formed from the aligned particles enhanced the shear resistance with increasing electric field strength. The slow polarization model contributed to the ER effect and the fibril structures were formed by particle rotation under an electric field. The yield stress may be attributed to surface polarization, which is determined by electron movement within the chitosan particles and electron hopping between the chitosan particles under large electric field strengths [34]. Recently, Choi et al. [3] introduced the critical electric field strength, E c, into their universal scaling function to explain the deviation of the yield stress from the polarization model behavior. The proposed yield stress equation for a broad electric field strength range is pffiffiffiffiffiffiffiffiffiffiffi! E=Ec 2 tanh pffiffiffiffiffiffiffiffiffiffiffi sy ðEÞ ¼ jE ; ð3Þ E=Ec Fig. 5 Shear viscosity versus shear rate for a 15 wt% chitosan– soybean oil suspension for six different electric field strengths

where j depends on the dielectric constant of the fluid and the particle volume fraction. Equation (2) clearly demonstrates the two regimes on the basis of E c, showing different behavior at low and high electric field strengths, respectively: sy ¼ jE2 / E2 E << Ec ; pffiffiffiffiffiffiffiffiffiffi sy ¼ j Ec E3 / E3=2 E >> Ec :

Fig. 6 Static yield stress versus electric field strength for a chitosan–soybean oil suspension

behavior of the ER fluid is also clearly observed. Similar shear thinning properties have been reported for various ER fluids, such as zeolite [29] and polyaniline [5]. The static yield stress, sy, is represented in Fig. 6 as a function of electric field strength, E , for chitosan in soybean oil obtained from the CSS mode. The correlation of sy with E is represented in power-law form as sy / Ea :

ð2Þ

The a value could be calculated to be approximately 1.6 if we fitted all the data in Fig. 6 to a single straight line. This dependence of the sy on E differs from the E 2

ð4aÞ ð4bÞ

E c stemmed from the nonlinear conductivity model, and represented the crossover behavior. The exponent in the power-law expression varies with the electric field strength. In addition, E c appears to be proportional to the particle conductivity and is influenced by the conductivity mismatch between the suspended particle and liquid media. sy is proportional to E 2, that is a=2 for E << E c, while a=1.5 for E >> E c. E c is measured as 1 kV mm)1 for soybean oil suspensions. The yield stress originates from the attractive forces between particles and explains the polarization and conduction models. The conduction model is described by conductivity between the stationary adjacent particles, and is a static model rather than a dynamic one [35,36]. The nonlinear conductivity effect with the bulk conducting particle model and the exponent of the power law is 1.5 at high electric field strengths [37,38]. The chitosan-based ER fluid used in our study deviates from the polarization model, but obeys the conduction model. We assume that the electron movement within the chitosan particles plays an important role in the surface polarization, which results in increasing the yield stress.

Conclusion For potential applications in the biomedical area, a biocompatible chitosan-based ER suspension in soybean

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oil was prepared, and its ER characteristics were examined. The ER response of this suspension system upon the application of an electric field was observed to behave as Bingham flow behavior. The shear stress and shear viscosity increased with electric field strength. Furthermore, it was found that the measured yield stress was proportional to E 2 for E  E c and E 1.5 for E  Ec,

where E c is found to be 1 kV mm)1 for soybean oil suspension, which obeys the nonlinear conduction model. Acknowledgement This study was supported by research grants from the Korea Science and Engineering Foundation through the Applied Rheology Center at Korea University, Seoul, Korea (2003).

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