JOURNAL OF APPLIED PHYSICS 103, 083301 共2008兲
Alignment of cellulose chains of regenerated cellulose by corona poling and its piezoelectricity Sungryul Yun,a兲 Jung Hwan Kim,b兲 Yuanxie Li,c兲 and Jaehwan Kimd兲 Center for EAPap Actuator, Department of Mechanical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, South Korea
共Received 4 October 2007; accepted 22 February 2008; published online 21 April 2008兲 Cellulose based electroactive paper has been developed as smart material. In this paper, corona poled cellulose films were prepared to improve their piezoelectricity and the influence of grid voltage to the corona poling was investigated. Cellulose was regenerated by dissolving cellulose natural fibers using a solvent, and removing it. During the regeneration process, alignment of cellulose was attempted by applying corona electrical poling. These characteristics were investigated by field emission scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. As increasing the grid voltage of the corona poling, the generation of cellulose nanofibers in the cellulose layered structures was observed, which influenced the increased crystallinity resulting in improved piezoelectric charge constant of cellulose films. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2908883兴 I. INTRODUCTION
Piezoelectric response of polymers has been used for sensor and actuator applications since the discovery of polyvinylidene fluoride 共PVDF兲.1 Several polymers, such as polyvinyl fluoride,2 polyvinyl chloride,3 and polyimide4 have shown their piezoelectric response. The piezoelectric properties are originated from aligned dipoles in uniaxial direction. To achieve high dipolar orientation in polymers, mechanical drawing, conventional poling, and corona poling techniques have been used. However, piezoelectric polymers treated with these techniques have not been shown outstanding properties comparable to PVDF and PVDF copolymers.2,4,5 Cellulose is one of the most naturally abundant polymers. Cellulose is suitable material for its purpose of demand for environmentally friendly and biocompatible products.6,7 Cellulose has been utilized in many fields due to its biocompatibility and chirality for the immobilization of proteins, antibodies, as well as the formation of cellulose composites with synthetic polymers and biopolymers. Recently, cellulose paper has been discovered as a smart material that can be used as sensor or actuator material.8,9 This smart material is termed as electroactive paper 共EAPap兲.10 EAPap has many advantages in terms of lightweight, dryness, low cost, biodegradability, large deformation, low actuation voltage, and low power consumption. EAPap actuator produces a large deformation in the presence of electric field due to a combination of ion migration and piezoelectric effect. Piezoelectric effect in cellulose has been reported long time ago, although its effect was small.11 Piezoelectricity in cellulose is originated from dipolar orientation and monoclinic crystal structure of cellulose. However, people have tested its piezoeleca兲
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tricity on naturally owned or factory manufactured cellulose samples. They have not attempted to align cellulose to improve the piezoelectricity. Of course, many researchers have given several efforts to align cellulose with electric field, magnetic field, and drawing.12–15 However, they have not clearly investigated the piezoelectric response of artificially aligned cellulose samples. This paper reports a preliminary investigation of piezoelectricity in electrically aligned cellulose, especially regenerated cellulose. Cellulose can be regenerated by dissolving natural cellulose fibers using a special solvent, and removing it. There have been many processes that can regenerate cellulose.16 During the regeneration process, the crystallinity as well as alignment of cellulose chains can be easily controlled. To effectively align cellulose chains, corona electrical poling technique was utilized. The corona poling treatment might improve the piezoelectric property. The piezoelectric response of corona poled cellulose samples was evaluated with different poling conditions. To find an optimal poling condition, different grid voltages were investigated. Characteristics associated with piezoelectricity of cellulose were studied by scanning electron microscope 共SEM兲, transmission electron microscope 共TEM兲, and x-ray diffractogram 共XRD兲. The piezoelectric response of cellulose samples was evaluated by measuring the piezoelectric charge constant 关d31兴. II. EXPERIMENTAL DETAILS A. Material preparation
Cotton pulp 共Buckeye兲, which has degree of polymerization of 4500 was torn in pieces and dried with LiCl 共Junsei Chemical兲 in heating oven at 100 ° C. Water contamination in DMAc 共Aldrich兲 was eliminated with aluminum sodium silicate molecular sieve 共effective pore size, 0.4 nm兲 at room temperature. Torn cotton pulp was mixed with LiCl/ anhydrous DMAc in proportion of cotton cellulose pulp/
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FIG. 1. 共Color online兲 Schematic of corona poling system.
LiCl/DMAc to 2 / 8 / 90. The cellulose was dissolved in the solvent by heating at 155 ° C with mechanical stirring as solvent exchange process. To dissolve remnant cellulose fibers, additional heating process was made at 100 ° C for 6 h. Cellulose solution, approximately 20 m in thickness, was spin coated on indium tin oxide 共ITO兲 coated glass by a spin coater. B. Corona poling process
In this experiment, electrical poling of cellulose under solution state should be made. The allowable direct dc poling field for cellulose solution was very limited to 2.5 V / m mainly due to limited electrical breakdown strength of the cellulose solution state. Furthermore, the dc poling process is difficult for the curing process of cellulose solution with deionized 共DI兲 water and isopropyl alcohol 共IPA兲 mixture. Therefore, we decided to utilize corona poling technique as noncontact poling method. Corona poling has many advantages compared to direct contact poling.17 In corona poling, the surface voltage of sample is about the same as the grid voltage. Schematic of corona poling system is described in Fig. 1. The cellulose coated ITO glass was laid on hotplate 共Corning, PC 420兲 and 10 kV dc electric voltage was applied between the ITO glass surface and tungsten wires for positive corona discharge on the cellulose layer. A thick copper plate was used to uniformly heat the cellulose coated ITO glass. Corona discharge source was provided by eight tungsten wires 共40 m diameter兲. The wires were powered by high dc voltage amplifier 共TREK, 20/20C兲. To apply a uniform electric field on sample, stainless steel grid was placed 2.5 cm below the tungsten wires and different grid voltages were applied to the grid by using dc voltage amplifier 共TREK, PZD 350M/S兲. The grid voltage can control the surface voltage level generated on the sample by limiting amount of ions passing through the grid holes. The hole diameter of grid was 1 mm and the hole density was 36 holes/ cm2. To hold aligned cellulose chains in the coated cellulose layer, a slow coagulation process of regenerating cellulose was made by dropping the solvent mixture after exposing corona discharge for 3 h. The solvent mixture composed of DI water and IPA with 5:95 ratio, was dropped on the coated cellulose layer in the presence of corona discharge, and heat treatment was made at 50 ° C for 2 h. The use of highly concentrated IPA in solvent mixture can retard the fast coagulation of cellulose caused by DI water. The
FIG. 2. 共Color online兲 Experimental setup for induced charge measurement.
corona poled film was rinsed twice by another solvent mixture composed of IPA:DI water with 50:50 ratio for 1 day, and it was dried on the hotplate for 1 h. C. Characterization
Characteristics of corona poled cellulose films were analyzed by thin film x-ray diffractometer 共X’Pert MPD, Philips兲, field emission scanning electron microscopy 共S-4200, Hitachi兲, and transmission electron microscopy 共Philips, CM200兲. X-ray diffraction pattern was taken on flat the cellulose films using nickel-filtered Cu K␣ radiation supplied by x-ray generator at 40 kV and 40 mA. The pattern was recorded at 2 from 5° to 40° under scanning rate of 1 ° / min. Surface and cross-sectional morphologies of corona poled films were studied by field emission SEM. Cellulose chain alignment due to corona poling was observed by TEM. To prepare the TEM sample, cellulose film was molded in epoxy, and cured at 60 ° C for 12 h. Ultramicrotome 共MTX Ultramicrotome, RMC兲 with diamond knife 共PE-4006-B, elementsix™兲 was used to prepare epoxy-embedded thin membrane 共⬍70 nm兲 at room temperature. The membrane floated on de-ionized water were attached on a carbon coated TEM grid 共IGC 200, PELCO®兲, and dried in vacuum oven at 40 ° C for 12 h. Cross-sectional image of TEM sample was taken by a GATAN model 794 camera. D. Piezoelectric charge constant measurement
To characterize the piezoelectricity, induced charge of the cellulose EAPap was measured by quasi-static method. Figure 2 shows the experimental setup for induced charge measurement that consists of a pull test machine, an environmental chamber and piccoammeter 共Keithley, 6485兲. In the pull test machine, load cell 共Daecell Korea, UU-K010兲 and linear stage 共Sony Japan, GB-BA/SR128-015兲 were used to measure the applied load and displacement. Cellulose film on which gold electrodes were coated on both sides was installed on the pull test machine by gripping the sample with the ASTM standard grip for polymer film tests. The electrodes were wired to picoammeter and LABVIEW was used to acquire the displacement, force, and induced charge data. The induced charge during the pull test was measured by the picoammeter. Strain rate was designated at 0.005 mm/ s and the test temperature and humidity conditions were 24 ° C and 20%–25%RH. Once the induced charge is measured, in-
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FIG. 3. SEM cross-sectional images of corona poled cellulose: 共A兲 without grid and 共B兲 with grid voltage.
plane piezoelectric charge constant can be found as18 d31 =
冉 冊 D3 T1
E
Induced charge per unit electrode area 关C/N兴 = Applied in-plane normal stress
FIG. 5. TEM image of corona poled cellulose.
共1兲
III. RESULTS AND DISCUSSION A. Characteristics
In this research, corona poling was used to align cellulose for improving piezoelectric response of cellulose films. To find an optimal corona poling condition, at first, the grid effect was investigated. Figures 3共a兲 and 3共b兲 show SEM cross-sectional images of corona poled samples 共A兲 without grid and 共B兲 with grid. As shown in Fig. 3共a兲, some diffused layers were randomly generated between the layered structures when the grid was not used. It might be due to the fact that a locally formed strong electric field disturbed the generation of cellulose layered structures. When the cellulose film was corona poled without grid, positively charged ions were unevenly distributed on the sample surface, so as to cause an inhomogeneous electric field distribution. On the other hand, when the corona poling was made with a grid voltage, diffused layers were disappeared and nanofibers were uniformly generated 关Fig. 3共b兲兴. Corona poling treatment induced the generation of cellulose nanofibers 共about 100 nm diameter兲 in layered structures. To investigate the influence of grid voltage, different grid voltages were applied during the corona poling. Figure 4
FIG. 4. SEM surface images of corona poled cellulose according to grid voltage: 共A兲 100 V, 共B兲 200 V, 共C兲 300 V and 共D兲 400 V.
shows SEM images taken on the surface of cellulose samples with grid voltage. As increasing the grid voltage from 100 to 400 V, more craters were generated on the discharged surface of cellulose film, meanwhile, the average diameter of craters gradually decreased from 871⫾ 90 to 72⫾ 14 nm. It reveals that high grid voltage produces a uniform electric field distribution during the corona poling. To confirm the cellulose chain alignment by the corona poling, the TEM image was taken at the cross section of the corona poled cellulose sample without grid voltage 共Fig. 5兲. Distributed black dots in this figure represent crystalline cellulose parts and the rest areas are amorphous cellulose parts, which show cellulose chains are aligned along the electric field direction. This TEM image of the aligned cellulose chains is quite similar to the configuration aligned by wet drawing process.15,19 XRD patterns of corona poled cellulose samples with different grid voltages were investigated 共Fig. 6兲. Originally, cellulose II has three XRD peaks of 12.1°, 19.8°, and 22° for ¯ 0兲, and 共200兲, respectively. Cellulose II formation 共110兲, 共11 is easily generated by fast coagulation of regenerated cellulose with DI water.20 However, as shown in Fig. 6, the re-
FIG. 6. 共Color online兲 XRD patterns of corona poled cellulose according to grid voltage.
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TABLE I. Characteristics of corona poled cellulose according to applied grid voltage.
Grid voltage 共V兲
Crystallinity index
Piezoelectric charge constant 共pC/N兲
100 200 300 400
0.21 0.24 0.28 0.32
2.3⫾ 0.9 3.5⫾ 0.4 5.4⫾ 0.3 16.7⫾ 0.2
generated cellulose films exhibited different diffraction profiles with cellulose II. This was due to increased amorphous cellulose regions caused by slow coagulation in the regenerating cellulose process.15,21 Especially, corona poled cellulose films increased its crystallinity by increasing the grid voltage from 100 to 400 V. The crystallinity of the samples was evaluated by calculating crystallinity index according to Ref. 22: CI 共crystallinity index兲 = 1 −
Imin , Imax
共2兲
where Imin is the minimum intensity between 2 = 18° and 19°, Imax is the maximum intensity between 2 = 22° and 23°. First column of Table I shows the calculated CI of corona poled cellulose samples with different grid voltages. As grid voltage increased, CI increased in the corona poled cellulose samples. This result might be associated with the generation of cellulose nanofibers after corona poling with grid voltage. B. Piezoelectric charge constant
Piezoelectric behavior of the corona poled cellulose samples was investigated according to the test method shown in Sec. II D with different grid voltages. Second column of Table I shows the measured in-plane piezoelectric charge constant d31. Error range of the piezoelectric constant was found from the test results. Generally, grid voltage is an important parameter to control surface voltage on the samples of corona poling. According to the grid voltage, extent of ions is bombarded on grid and ions passing through the grid can be changed. As increasing the grid voltage, ions can easily passing through the grid holes, but it causes weak ion bombardment on the grid due to relatively reduced electrical potential difference between grid and corona discharger.23 Therefore, there might be an optimum grid voltage of the corona poling that can eventually improve the piezoelectric constant of cellulose films. When the grid voltage increased, the piezoelectric charge constant of cellulose gradually increased, and the value reached to 16.7⫾ 0.2 pC/ N when the grid voltage was 400 V. However, it decreased to 12.5⫾ 0.3 pC/ N when grid voltage was 600 V. Based on the result, we confirmed that 400 V is an optimum for corona poling of cellulose, which effectively aligned the cellulose chains so as to improve its piezoelectricity. This d31 value is higher than 7.2⫾ 1.1 pC/ N of stretched and corona poled polyvinyl chloride films,5 Once the cellulose films are well aligned by combining mechanical stretching and corona poling, its piezoelectricity will be comparable
to PVDF piezoelectric polymer. Since cellulose is a nature material, it has many merits in terms of biodegradability, biocompatibility, and low price. IV. CONCLUSIONS
In this paper, corona poled cellulose films were prepared and the influence of grid voltage was investigated by taking SEM, TEM, and XRD. Based on morphological study by SEM, corona poling with grid voltage provided a uniform electric field distribution so as to generate uniform cellulose nanofibers. According to the XRD investigation, as the grid voltage increased, the CI increased in the corona poled cellulose samples. TEM analysis of corona poled cellulose showed a distribution of cellulose nanofibers and revealed aligned cellulose chains with electric field direction. The generation of cellulose nanofibers in the presence of grid voltage increased crystallinity of cellulose. The piezoelectric characteristics of corona poled cellulose films were evaluated by measuring piezoelectric charge constant. The increased crystallinity associated with the alignment of cellulose chains improved the piezoelectric response of cellulose films. By applying an optimum grid voltage for corona poling of cellulose 16.7⫾ 0.2 pC/ N of piezoelectric charge constant was obtained. The piezoelectric behavior of regenerated cellulose films allows potential applications of cellulose EAPap such as biodegradable, flexible, and low price sensors as well as paper speakers. ACKNOWLEDGMENTS
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