Field Responsive Polymers - American Chemical Society

Chapter 7

The Electrorheological Properties of Chitosan Sulfate Suspensions

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch007

Shuizhu Wu and Jiarui Shen Department of Polymer Science and Engineering, South China University of Technology, Guangzhou 510641, Peoples Republic of China

The activator-free electrorheological suspensions based on chitosan sulfate particles exhibit significant electrorheological effect under the applied electric field, and have good thermal stability and low conductivity. The suspension's electrorheological effect increases with the increasing field strength and suspension concentration but decreases with the increasing shear rate, and the dynamic yield stress of the suspension increases with the increasing concentration.

Electrorheology is the term applied to the phenomenon in which the fluidity of suspensions is modified by the application of electric fields(i). This phenomenon concerns the formation of a fibrilated microstructure in dense suspension due to dipole interactions. These interparticle forces result in a fluid with an enhanced viscosity and that is capable of sustaining a large yield stress(2). The magnitude of these stress, and the rapid time scales of the structure formation make these systems ideal working fluids in electromechanical applications(i). E R devices currently being developed^, 5), including engine mounts and shock absorbers, require large field-induced viscosities as well as rapid responses, with times scales on the order of milliseconds. However, there are still problems to be solved before ER fluids find extensive commercial applications. It has been long observed that wet particulates are most ER active. But these moist fluids are limited to a narrow temperature range (< 70°C) and show undesirable levels of conductance arising from mobile i o n s « 6 ) . Solution of these problems and the development of better ER fluids depend on improving our understanding of how the phenomenon depends upon the properties of the materials which make up ER fluids. Recent development of anhydrous suspensions

104

© 1999 American Chemical Society In Field Responsive Polymers; Khan, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


105 based on conducting materials seems to have overcome some of these problems, however they show undesirable levels of conductance as well(3,46). Natural polymers such as cellulose have been used in E R fluid preparation, but, these fluids usually require water or other polar liquids as activator(7,#). In this article, we made use of the unique features of chitosan, such as it contains lots of polar groups along its molecular chain, thus it has relatively higher dielectric constant, and is easy to go through modification reactions including sulfation. In present study, the activator-free chitosan sulfate-silicone oil suspensions are prepared, these suspensions appear to be able to avoid the disadvantages of the moist ER suspensions. On the other hand, the biodegradable feature of chitosan might be good for their future applications in terms of environmental protection. The electrorheological properties of these suspensions are investigated for a range of field strengths, particle concentrations and shear rates. The conductance of ER fluid is believed to be an important parameter in ER effect especially when it comes to actual applications. Large increase in the conductance of the fluid would result in excessive power demands with possible serious implications in terms of power supply and energy dissipation in the ER devices, it could even cause dielectric breakdown(7,P). For this reason, low conductance is an important goal for future E R fluids. The chitosan sulfate — silicon oil suspensions prepared in the present study have extremely low conductance. The relationship between the suspension's dynamic yield strength and particle concentration is determined experimentally as well. Experimental Synthesis of Chitosan Sulfate. The deacetylation of chitin (Katakura Chikkarin Co., Japan) was carried out according to literature(70), namely, the chitin samples were treated with 50wt% NaOH at 100°C for 1 hour to produce chitosan. The sulfation of chitosan was then conducted with the method of Wolfrom et al(77): First, chitosan sample was dissolved in dilute acetic acid solution; after the undissovled part being filtered out, chitosan was reprecipitated with NaOH solution, then washed with distilled water till the washings tested neutral with pH paper. After that, it was washed with ethanol, absolute ethanol, diethyl ether and dimethylformide (DMF). Then chitosan sample was suspended in D M F and treated with a S 0 (fuming sulfuric acid) - DMF mixture for 12h at 20°C. After neutralization, the reaction mixture then underwent the purification process by using the dialysis membrane. After drying, the sulfur content of the final product was determined by elemental analysis, the product has 13.68% S. The degree of sulfation (DS) is calculated as follows(77): 3

p

s

_

S(%)xl61 32xl00-S(%)xl03

where S is the sulfur content of the sample, 161 is the mole molecular weight of

In Field Responsive Polymers; Khan, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

106


the repeating unit of chitosan, 103 is the mole molecular weight of SC^Na, and 32 is the molecular weight of S. Therefore, the DS of the product is 1.23. The sulfation of chitosan is shown schematically as follows:

Suspension Preparation. The preparation for the activator-free suspension: After drying, chitosan sulfate samples were dispersed in a certain amount of silicone oil and ball-milled until microscopic examination indicated a mean particle size of 10 urn and the absence of particles > 20 |im. Particles were irregular in shape but without any tendency to anisometry. The silicone oil used is a colorless oil with the following physical properties: density 0.97 g/cm , viscosity 100 mPa-s at 20°C, dielectric constant 2.8, and boiling temperature 300°C. There was little tendency for these dispersions to separate in the short term, and such dispersions that had separated after lengthy standing readily redispersed on agitation. The preparation for the activator-containing suspension: The chitosan sulfate samples were immersed into the glycerin-methanol solution for 72 hours, to make sure the samples adsorb a certain amount of glycerin. The fraction of the adsorbed glycerin is determined by the weight method, and the glycerin content is 5wt%. After the removal of methanol, the chitosan sulfate suspension was made with the same procedures as the above-mentioned. 3

Methods of Measurement. For electrorheological measurement, a concentric cylinder rheometer was used. To apply large electric field strength across the concentric cylinders, each cylinder was insulated from the rest of the rheometer. The inner cylinder has an outer diameter of 14.6 mm and height of 30mm. The outer cylinder has an inner diameter of 20 mm and height of 35 mm. The annular gap is 2.7 mm. The electric field strength was applied to the gap by grounding the outer cylinder and connecting the inner cylinder to a high-voltage source. The DC voltage of 100 - 2500 V were used in the experiments. The current passing the suspension was monitored by using a multimeter attached in series to the ground wire of the circuit. A l l measurements were carried out at room temperature. Results and Discussion Effects of Field Strength and Particle Concentration. The shear stress of the activator-free suspensions containing chitosan sulfate particles at different


107


2500

200

400 600 E ( V/mm )

800

Figure 1 The suspension's shear stress versus field strength curve, (y = 1.441s" ) 1

concentrations is shown in Figure 1, and the ratio of the suspension's shear stress under the applied electric field over the zero-field shear stress is plotted as a function of particle concentration in Figure 2. It can be seen that, the suspension's shear stress increases with the increasing field strength, i.e., the suspension's ER effect increases with the field strength. There exists a critical concentration (about 3wt%), under which the suspension barely displays any E R effect. While over the critical concentration, the suspension's ER effect increases with the increasing concentration.

10 15 C (wt%) Figure 2 Relative shear stress versus concentration curve. ( E = 800 V/mm,

17s ) 1


108 These experimental phenomena can be understood by considering the polarization forces between the suspended particles. Since the polarization forces scale as 7ia eo8iPE , where a is the radius of particle, eo is the permittivity of free space, 61 is the dielectric constant of the dispersing medium, and p is the polarization coefficient of particle: P = (62 - 6i)/(82 + 2ei), where 82 is the dielectric constant of particle(72). Therefore, with the enhancing field strength, the polarization forces between particles increases, as a result, more and stronger particle chains or strands form, hence the suspension exhibits more obvious ER effect. On the other hand, the formation of particle chains is a percolation process(75). Only when there are enough amount of particles in the fluid, could the particle chains or strands span the gap of the electrodes, this is why there exists a critical concentration. Under the critical concentration, particles are simply dispersing in the continuous medium or form short chains, which couldn't cause significant ER effect. Over the critical concentration, the ER effect of the suspension increases with the concentration. It is because, with the increasing concentration, there are more particles dispersed in the continuous medium, therefore, more particle chains can form under the applied electric field, and the suspension exhibits much stronger ER effect. At the same time, the suspension's thermal stability was examined as well, the results are shown in Table I.


2

Table I.

2

The shear stress of the suspension before and after thermal treatment (C= 19wt%, E= 800V/mm, y = 1.441s' ). 1

T(°C)

r(Pa)

20 50 80 110 130

2031 2058 2064 2066 2071

The suspension was placed in an oven at 130°C for 72 hours; then its ER effect was examined again, the suspension's shear stress didn't decrease but increase a little, as shown in Table I. This indicates that activator-free suspension has quite good thermal stability. Effects of Shear Rate. The suspension viscosity at different shear rates are shown in Figure 3. As shown in this Figure, the suspension viscosity decreases with the increasing shear rate. According to the experimental facts, it is considered that, under the applied electric field, the interparticle polarization forces lead to the aggregation of


109 1600


—•—1.4s"

1

400

600

800

E (V/mm )

Figure 3 Suspension viscosity versus field strength at different shear rates. ( C = 19wt% )

particles or even fibril formation between the electrodes. Such a structural skeleton is across the direction of the shear field and leads to an increased suspension viscosity. In the presence of a shear field simultaneously, the particles are also acted on by the viscous forces, which is modulated by hydrodynamic interactions with other particles in the suspension. These viscous forces are in proportion with the shear rate y, and intend to disrupt the suspension structure^ 2). As y is increased, the viscous forces increase, so that the tendency to break down the structural skeleton of the suspension is increased; Therefore, the suspension structure is much easier to damage and the increment of the viscosity is much smaller, while at high enough shear rate, the suspension viscosity becomes almost independent of the electric field. This suggests that, at high enough shear rate, the viscous forces are dominant, and the suspension structure does not vary appreciably with the field strength. The characteristic parameter describing the interplay between dipole forces and flow, the Mason number, M n = 6x\\y /[eoei(PE) ], is the ratio of the viscous forces tending to disrupt the structure and the polarization forces responsible for the structure, where r)i is the viscosity of the dispersing medium(72). Based on the above analysis, it is expected that the dimensionless suspension viscosity x\lr\\ depends on two parameters, suspension concentration C and Mn. At a given concentration and temperature, a plot of suspension viscosity against M n should reduce the data at different field strengths and shear rates to a single curve. Figure 4 shows the dimensionless suspension viscosity as a function of the Mason number for different concentrations studied. A l l the results show an identical form; at low M n values, the curve is almost linear; while at high M n values, the curve approaches a constant viscosity. This behavior suggests that, at a given 2



110

Figure 4 Relative viscosity as a function of Mason number

concentration, the shear and electric field dependence of E R suspensions can be expressed as a single function of Mn. That the data collapse onto a single curve in the transition from polarization-controlled structures to the domination of the viscous forces suggests that the suspension structure does not vary appreciable with field strength. Suspension Current Density. In order to assess the suspension's conductivity, we measure the current passing the suspension, and the current density (j) is calculated by dividing the current over the surface area of the electrode. The current densities of the suspensions with or without activator are shown in Table II.

Table II. Relationship between the suspension current density and electric field strength. (C = 19wt%, y = 0 s") E (V/mm) j (fiA/cm ) j (/jA/cm ) (suspension with activator) (suspension without activator) — 500 1.84 — 1000 2.57 — 1500 3.18 0.07 2000 4.78 1

2

2

As shown in Table II, the current density of the suspension with activator increases with the increasing electric field strength. The current density of the suspension with activator is much larger than that of the suspension without activator. These phenomena can be explained as follows: The adsorption of



Ill glycerin will enhance the surface conductance of chitosan sulfate particles; on the other hand, glycerin as an impurity, will also enhance the system's volume conductance; and both these conductances increases with the increasing field strength(V4). So the suspension's current density increases with the field strength. In addition, The field-induced particle chains or strands bridging the electrodes could provide conducting pathways as well. Furthermore, under higher field strength, the space charge current could occur due to the electrons discharged into the fluid from the electrode. The space charge current is non-ohmic, and proportional to the square of the field strength(/4). Under higher field strength, the space charge current could contribute to the current density of the suspension. Due to the instrumental limitations, the current density of the suspension without activator under the field strength less than 2 0 0 0 V/mm couldn't be obtained. These experimental results indicate that the activator-free chitosan sulfate suspension has very low conductivity, compared with other fluids (6, 15). Rheograms.

The suspensions' shear stress (x) as a function of shear rate (y) is

shown in Figure 5 and 6. In the absence of an electric field, the dependence of T on

60

50

100

1!

200

Figure 5 Shear stress versus shear rate under zero field strength.

y is almost linear. Under the applied electric field, at lower concentration, the x ~ y curves are similar to the zero-field curves, and the suspension almost behaves as a Newtonian fluid; and the higher the concentration is, the greater the anomaly of the viscoplastic behavior is. In the presence of the electric field, at higher concentration range, there appears a yield limit (xa), which represents the limiting value of the shear stress as the shear rate approaches zero. The value of the dynamic yield stress is a function of concentration and increases with the increasing concentration (see Figure 6). This supports the assumption about a


112

2500 2000 • 1500 Q.

1000


500 °1

11 21

31 41

51 61

71 81 91

y (f1)

Figure 6 Shear stress versus shear rate curves. ( E = 800 V/mm )

growth of the interpaticle interaction forces and, therefore, strengthening of the system skeleton with the increasing concentration. Acknowledgments The authors gratefully acknowledge the support of Guangdong Natural Science Foundation and Guangdong High Education Bureau of China. Literature Cited (1). Block, H.; Kelly, J. P.; Qin, A.; Watson, T. Langmuir 1990, 6, 6. (2) . Winslow, W. M. J. Appl. Phys. 1949, 20, 1137. (3) . Block, H.; Kelly, J. P. J. Phys. D 1988, 21, 1661. (4) . Scott, D.; Yamaguchi, J. Automotive Engineering 1983, 91, 61. (5) . Jordan, T. C.; Shaw, M. T. IEEE Transactions on Electrical Insulation 1989, 24, 849. (6) . Block, H.; Kelly, J. P. U.S. Patent 4,687,589 1987 (7) . Yoshimura, R. JP 04,25,596 1992 (8) . Marakami, K. JP 02,255,798 1990 (9) . Klass, D. L.; Martinek, T. W.; J. Appl. Phys. 1967, 38, 75. (10) . Roberts, G. A. F. Chitin Chemistry; Macmillan Press: Hampshire, 1992; ppl21-130 (11). Wolfrom, M. L.; Han, Shen T. M. J. Amer. Chem. Soc. 1959, 81, 1764. (12). Gast, A. P.; Zukoski, C. F. Adv. Colloid Interface Sci. 1989, 30, 153. (13). Klingenberg, D. J.; Zukoski, C. F. J Chem. Phys. 1989, 91,7888. (14). Coelho, R. Physics of Dielectrics for the Engineer; Elsevier: London, 1979; pp153-200. (15). Gow, C. J.; Zukoski, C. F. J. Colloid Interface Sci. 1990, 36, 175.


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