Cellulose-chitosan blend electroactive paper actuator

Cellulose-chitosan blend electroactive paper actuator Zhijiang Cai, Yi Chen, Jaehwan Kim* Center for EAPap Actuator, Dept of Mechanical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, Korea Tel: 82-32-8747325 Fax: 82-32-8327325 Email: [email protected] ABSTRACT Cellulose based Electro-Active Paper (EAPap) has been reported as a smart material that has merits in terms of lightweight, dry condition, biodegradability, sustainability, large displacement output and low actuation voltage. However, its actuator performance is sensitive to humidity: its maximum bending performance was shown at high humidity condition. To overcome this drawback, we introduce an EAPap made with cellulose and chitosan blend. Cellulsoe-chitosan blend films with varied mixing ratio were prepared by dissolving the polymers in trifluoroacetic acid as a co-solvent followed by spincoating onto glass substrates. A bending EAPap actuator is made by depositing thin gold electrodes on both sides of the cellulose-chitosan films. The performance of the EAPap actuator is evaluated in terms of free bending displacement with respect to the actuation frequency, activation voltage, humidity level and content of chitosan. The actuation principle is also discussed. Keywords: Electro-Active Paper (EAPap) , Cellulsoe-chitosan blend, trifluoroacetic acid，Bending Actuator

1. Introduction Cellulose is an environmentally friendly, renewable and inexhaustible biomaterial. Cellulose has basic molecular unit of C6H10O5 and is linked in the form of β-1,4-glucan. Numerous new applications of cellulose take advantage of its biocompatibility and chirality for the immobilization of proteins and antibodies for the separation of enantiomeric molecules as well as the formation of cellulose composite with synthetic polymers and biopolymer. As one of new applications, cellulose paper has been discovered as a smart material that can be used as sensors and actuators [1]. This smart material is termed as Electro-Active Paper (EAPap) [2]. Cellulose EAPap has merits as a smart material in terms of lightweight, dryness, biodegradability, abundance, low price, large displacement output and low actuation voltage. Possible application areas of this material are micro-insect robots, flying objects, flying magic paper, flower-robots, smart wallpaper, e-papers and microelectro-mechanical systems (MEMS) sensors. However, this material is very sensitive to humidity, its maximum bending performance was shown at high humidity condition, and its performance is degraded with time [3]. Chitosan, a copolymer of glucosamine and N-acetylglucosamine unit linked by 1,4-glucosidic bonds, is obtained by Ndeacetylation of chitin. Chitosan is a biocompatible polymer reported to exhibit a great variety of useful biological properties. Chitosan has amino groups and hydroxyl groups on its backbone, which on the one hand makes chitosan itself hydrophilic and on the other hand brings chitosan a polycationic property [4]. The molecular structures of cellulose and chitosan are very similar seen from the Fig.1. In this paper, an EAPap actuator based on cellulose-chitosan blend is prepared to improve the electromechanical performance of cellulose based EAPap at room humidiy condition. Since the molecular structures of cellulose and chitosan are very similar, it is expected to give good miscibility between them, and so blending deformation of cellulose and chitosan is expected to be useful. Details of the cellulose-chitosan blend EAPap actuator fabrication are delineated. The electromechanical performance test of the EAPap actuator is performed in terms of free bending displacement with respect to the actuation frequency, voltage, humidity level and chitosan content. The actuation principle is also discussed.

2. Experiments 2.1 Materials Cotton cellulose (MVE, DPw 7450) was purchased from Buckeye Technologies Co., USA. Chitosan, medium molecular weight, Brookfield viscosity 200,000 cps, was purchased from Aldrich. Hydrochloric acid (36.5~38%) was purchased from Sigma-Aldrich, USA. Trifluoroacetic acid (>99%) was purchased from DAEJUNG CHEMICAL & METALS Electroactive Polymer Actuators and Devices (EAPAD) 2008, edited by Yoseph Bar-Cohen Proc. of SPIE Vol. 6927, 69271H, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.776518

Proc. of SPIE Vol. 6927 69271H-1 2008 SPIE Digital Library -- Subscriber Archive Copy

CO.LTD. Sodium hydroxide (bead, 98%) was purchased from SAMCHUN PURE CHEMICAL CO.LTD. 2.2 Preparation of cellulose-chitosan blend films Cotton cellulose was cut into small pieces, soaked into water overnight, squeezed and filtered to remove the water. The same process was performed four times with methanol and one time with acetone. The treated cotton cellulose and chitosan powder were heated under reduced pressure at 110℃ for 2 hours. Then cotton cellulose was mixed with chitosan powder and dissolved in trifluoroacetic acid at room temperature. The transparent cellulose-chitosan blend solution was kept in sealed glass bottle for future use. The clear mixture solution was spincoated on wafer and cured at room temperature for 12 hours. To ensure complete elimination of the solvent, the films were then dried at 60 ℃ for 6 hours. Transparent films can be obtained by peeling them off from the wafer. After that, the films were soaked in 1 N NaOH at room temperature for 1 day to remove the acids. They were then washed with running tap water for 8 hours, and then immersed in deionized water for 24 hours. Since sodium ions were entirely removed by running water, its concentration in the film was negligible. And then the films were immersed in hydrogen chloride aqueous solution (the concentration of hydrogen chloride is 1%) for 2 hours, then washed with top water and deionized water for 12 hours to delete little ionic molecules. And then the wet films were taken out from water and laid in air for 24 hours. The thickness of the blend films was 15±5µm. According to the percent of chitoan in the blend films 0%, 20%, 40%, they were coded as CH-0, CH-20, CH-40, respectively. 2.3 Gold electrode coating To make an EAPap actuator, gold electrodes were deposited on both sides of the cellulose-chitosan blend films using a physical vapor deposition system. The size of the EAPap sample was 10 mm × 40 mm. The thickness of the gold electrodes was so thin (0.1 µm) that the gold electrodes did not significantly affect the bending stiffness of the cellulose paper. 2.4 UV-visible spectra The optical properties of the cellulose-chitosan blend films were measured with a HP UV-8452A DIODE ARRAY SPECTROPHOTOMETER (wavelength range, 200–850 nm). 2.5 Scanning electron microscope Scanning electron microscopy images of the blend films were taken with a scanning electron microscope (Hitachi S4200, Japan) to study the morphology of the films. The surface and cross-section of the films were sputtered with gold, and then observed and photographed. 2.6 Bending displacement measurement The electromechanical performance was conducted in an environmental chamber that can control temperature and humidity. The bending displacement measurement system consists of a high precision laser doppler vibrometer (LDV) (Brüel&Kjær, 8336), an environmental chamber (KMS, CTH3-2S), a current probe (Tektronix, TCP 300), Labview software on a personal computer and a function generator (Agilent, 33220A) (Fig. 2). An EAPap actuator is supported vertically in air. Function generator controlled by computer sends out the excitation AC voltage. The input signal generated from the function generator is applied to the EAPap actuator and it produces a bending deformation. The bending displacement of the EAPap sample is measured by the high precision LDV mounted on an optical table and the LDV signal is converted to the displacement through the Labview software. OH

OH

c11,1: OH

— HO

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/

Chit: Figure 1. The molecular structure of cellulose and chitosan

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Figure 2. Computerized setup for bending displacement measurement of EAPap actuator

3. Results and discussions 3.1 UV-visible spectra Fig. 3 showed the transmittance of the cellulose-chitosan blend films as a function of chitosan mixing ratio, with a blend film thickness of 15 µm. The mixing ratio of 0 % meant that the chitosan was not mixed into the cellulose. Increasing the chitosan mixing ratio caused a increase in transmittance of the blend film. Blend films with chitosan mixing ratios of 20 % showed less than 8% transmittance in the ultraviolet region (300 nm) and greater than 5% transmittance in the visible wavelength region (400–700 nm). When the chitosan mixing ratio increased to 40%, the transmittance in the visible wavelength region (400–700 nm) was 90%. 3.2 Morphology of cellulose-chitosan blend films Figure 4 showed the scanning electron microscope images at the surface and cross-section of cellulose-chitosan blend EAPap actuator. As we expected, the cellulose-chitosan blend films showed very good miscibility from the SEM images. The surface morphology was very smooth and uniform, no particles and phase separation can be observed with chitosan


introduction. The cross-section images showed the typical regenerated cellulose’s layer-by-layer structure. All these results indicated that the cellulose and chitosan were blended very well and stable for all mixing ratio.

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I ___ Cross-section CH-0 CH-20 CH-40 Figure 4. Scanning electron microscope images at the surface and cross-section of cellulose-chitosan EAPap actuator

3.3 Actuation behavior under the DC voltage The movement direction of cellulose-chitosan EAPap actuator in the presence of DC voltage is shown in Fig. 5. Upon the DC voltage (4 V) the tip of the EAPap sample moved from the vertical position to the negative electrode (cathode) direction. The moving speed was rather slow. When the power is turned off, the tip stopped and returned to original place after a long period of time. The actuation principle is explained in Fig. 5. The cations (―+NH3) connected with polymer molecular chain could not move freely, but the anions (Cl-) are free to move approximately. On the low DC


0 C)

—o

o

—o

C)

=

I—t—I =

I

C) —o

r=

I_=_I

I

Q

=

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voltage condition, the cations cannot nearly move to negative electrode, while the anions can move to positive electrode. As the anions move and assemble at anode, the repelling force between the anions (Cl-) makes the film bend to negative electrode.

(a) (b) (c) Figure 5. Actuation behavior of cellulose-chitosan blend EAPap actuator under DC voltage: (a) chitosan chains, (b) when hydrogen chloride acid is added, and (c) when a DC electric field is applied. 3.4 Actuation behavior under AC voltage The bending displacements of cellulose-chitosan blend based actuator were measured at the tip of the samples with the laser vibrometer. The test was performed as a function of frequency under different activation voltage at room condition (25±0.5 °C & 30±2% RH) and relative humidity (RH) as 40%, 50% and 70%, respectively. 3.4.1 Effect of frequency The bending displacements of the EAPap actuator with frequency variation were shown in Fig. 6. As the frequency increased, the bending displacement was increased first and then decreased. When the frequencies were 7.5, 5.5 and 5 Hz, the bending displacements of the CH-0, CH-20 and CH-40 EAPap actuator were the maximum, at which resonant frequencies. 3.4.2 Effect of voltage As seen from Fig.6, with the actuation voltage increased from 3V to 7V, the maximum bending displacement was increased. This may be due to the fact that as the voltage increased, the anions moved to positive electrode quickly, resulting in the increased repulsive force between the anions on the positive electrode. Consequently, the bending displacement was increased. When the actuation voltage was 7V, the maximum bending displacements were 0.51, 0.94 and 3.72 mm for CH-0, CH-20 and CH-40 EAPap actuator, respectively. 3.4.3 Effect of chitosan content As shown in Fig.6, the maximum bending displacement of the cellulose-chitosan blend EAPap actuator was increased from 0.51mm to 3.72mm with the chitosan content increasing from 0% to 40%. On the other hand, the resonant frequency of the cellulose-chitosan blend EAPap actuator was decreased from 7.5Hz to 5Hz with the chitosan content increasing from 0% to 40%. 4.5

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Figure 6. Bending displacement of cellulose-chitosan EAPap actuator with voltage and frequency variation.

3.4.4 Effect of humidity


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Figure 7. Humidity effect on the bending displacement of cellulose-chitosan EAPap actuator (actuation voltage: 5V)

Figure 7 showed the measured maximum bending displacements at different relative humidity levels using CH-40 as a sample. At low humidity level (below 50% RH), the bending displacement was increased with the humidity level increasing, and its maximum value was reached at 50% RH. After that, the bending displacement decreased as the humidity increasing. At 50% RH the maximum bending displacement was 6.3 mm while the maximum bending displacement dropped to 5.62 mm at 70% RH under the same electric field. This behavior was quite different from our previous results [3]. As we known, the ions migration effect is strongly associated with the ion concentration and ion mobility in the actuator. When the humidity increases, the water molecules in the actuator increase, and it results in softening the actuator and making the anions easy to move. For cellulose-chitosan blend EAPap actuator, its water content is far higher than that of cellulose EAPap actuator. Since the chitosan is more hydrophilic polymer, the water content in cellulose EAPap actuator can be improved by blending chitosan with cellulose. Moreover, with the chitosan content increasing, the free moving anions also increase greatly. These effects make the cellulose-chitosan blend EAPap actuator show a large bending displacement at low humidity condition.

4. Conclusions Cellulose-chitosan blend EAPap actuator was prepared by dissolving these two polymers with different weight ratio using trifluoroacetic acid as a co-solvent. With chitosan mixing into the blends, the optical properties of the cellulose films were improved. The electromechanical performance of these cellulose-chitosan blend EAPap actuators was evaluated in term of free bending displacement with respect to the actuation frequency, voltage, and humidity level. On the DC voltage actuation, the tip of the film moved to negative electrode, which indicated the movement of anions (Cl-) to the positive electrode. When the cellulose-chitosan blend EAPap actuator was actuated with AC voltage, the bending displacement output was increased with actuation voltage and chitosan content. These cellulose-chitosan blend EAPap actuators showed high bending displacement at room condition humidity, especially for high chitosan content samples. The maximum value of 4.1 mm was achieved at 6 V and 4 Hz for CH-40 at room humidity condition. At high humidity condition (over 50%), the bending displacement was decreased with humidity level increasing for CH-40. In conclusion, this cellulose-chitosan blend EAPap actuator is more suitable to be used as an air working actuator under low humidity condition, which is promising for many smart actuator applications

Acknowledgements This work was supported by Creative Research Initiatives (EAPap Actuator) of MOST/KOSEF.


REFERENCES 1

J. Kim, S. –R. Yun and Z. Ounaies, “Discovery of cellulose as a smart material,” Macromolecules. 39, 4202-4206 (2006). 2 J. Kim and Y. B. Seo, “Electro-active paper actuators,” Smart Mate. Struct. 11, 355-360 (2002). 3 J. Kim, C. Song and S. –R. Yun, “Cellulsoe based electro-active paper: performance and environmental effects,” Smart Mate. Struct. 15, 719-723 (2006). 4 B. Krajewska, “Diffusional properties of chitosan hydrogel membranes,” J. Chem. Technol. Biotechnol. 76, 636-642 (2001).
