EAPap

Center for EAPap Actuator, Department of Mechanical Engineering, Inha University, 253 Yonghyun-Dong, Nam-Ku, Incheon 402-751, South Korea... 

The environmental effects of humidity and temperature on the performance of EAPap are studied. As humidity increases, the displacement increases due to the increase of mobihty and the decrease of stiffness. At 30 C, the best performance is observed. This temperature may be associated with the ion mobility of EAPap samples. The mechanical properties of EAPap are tested for cellulose-based papers. In the mechanical test, the typical Young s modulus of EAPap ranges from 4 to 9 GPa, which is quite high compared to other EAP materials. 

The role of inherent polarization and ionic transport effects in actuation mechanism of EAPap actuators are investigated. To physically investigate the actuation mechanism, several tests are performed. X-ray diffraction (XRD) spectra are compared before and after electrical activation and the possibility of crystalline structure change is observed. Dielectric property measurement indicates a dependence of the dielectric constant on fiber direction, as well as on frequency, humidity, and temperature. Thus, we conclude that piezoelectric effect and ionic migration effect are in the EAPap at the same time associated with dipole moment of cellulose paper ingredients. The amount of these effects may depend on environmental condition. 

Since cellulose-based EAPaps are quite simple to fabricate, lightweight, and utilize low-excitation voltage, a number of appUcations including flapping wing for flying... 

Figure 18-1. Configuration of EAPap bending actuator (Kim et al. 2006c) [Courtesy of ACS] (See Color Plate of this figure beginning on page 355)...

When the samples were actuated by applying electric field across the electrodes, small bending displacements were observed. Especially, the cellophane EAPap exhibited the best displacement output. Thus, we have focused on regenerated cellulose. For further study, the fabrication process of regenerated cellulose EAPap was setup at our laboratory. Figure 18-3 shows the xanthate cellulose... 

Table 18-1 represents the tip displacement of EAPap actuator made with six different papers. Here, means that no significant displacement was observed. Hardwood and softwood paper exhibited a small displacement less than 1 mm. Meanwhile, cellophane EAPap exhibited very large displacement output. Detailed displacement measurements were conducted on the cellophane EAPap actuator. 

Figure 18-5 shows the displacement output of the actuator as a function of voltage (Vp p) and relative humidity (%) at 4Hz. At 5 V, it exhibited the maximum displacement of 4.3 mm. This value corresponds to the electric field strength of 0.25 V/pm, which is very low comparing with other electronic electroactive polymers (150 V/pm) (Su et al. 1999). This low actuation voltage is an advantage of the EAPap actuautor. The tip displacement result shows that the displacement increases with the humidity. Normally, there is a decrease in the elastic modulus of paper when the... 

Table 18-1. Tip displacement output of EAPap actuator with different papers...

To successfully transit cellulose EAPap actuators into these applications, it is crucial to ascertain the actuation principle responsible for the performance parameters. Based on the cellulose structure and our processing of the cellulose-based EAPap, we believe that the actuation is due to a combination of two mechanisms ion migration and piezoelectric effect associated with dipolar orientation. In the remainder of this report, we present experimental evidence of both. 

Cellulose EAPap material is composed of molecular chains with a dipolar nature. In particular, the crystal structure of cellulose II is monoclinic, which is noncentro-symmetric and exhibits piezoelectric and pyroelectric properties. To investigate the dipole effects in EAPap, thermally stimulated current (TSC) measurement was conducted (Hongo et al. 1996). The classical procedure in TSC includes (1) heating the sample to a given temperature (200°C) (2) applying... 

Further investigation of the actuation behavior of EAPap material, x-ray diffraction (XRD) was tested on the cellophane EAPap sample before and after the electrical actuation. The cellophane EAPap actuator was activated for several hours, then it was removed from the power source and XRD was performed on the surface of the EAPap sample. XRD was measured with an x-ray diffractometer (D/MAX-2500, Rigaku). XRD patterns using Cu-Ka radiation at 40 kV and 30 mA were recorded using 20 = 5-80°. Eigure 18-7 shows the XRD results. Table 18-2 summarizes the XRD peaks before and after the actuation. After actuation, the (110) peak at 12.26° decreased to 12.08° while the (200) peak at 21.64° increased slightly to 22.02°. It is clear that the (110) peak sharpened after the actuation while the (200) peak was changed to blunt. This confirms that the first peak increased and the second peak decreased. 

Notice that the small peak at 16.78° started to appear after the electrical activation. This means that some structural change took place during the electrical actuation, which might be associated with the crystallization of disordered region. In other words, recrystallization of disordered cellulose of EAPap sample could be accelerated by molecular rearrangement during the electrical... 

For the dielectric constant measurement of cellophane EAPap, LCR (inductance, capacitance and resistance) meter (HP4282A) was used, and the frequency ranged from 20 Hz to 10 MHz. EAPap samples were prepared parallel (mechanical), perpendicular (transverse), and 45° to the fiber orientation. Figure 18-8 shows... 

Table 18-2. X-ray peaks of cellulose EAPap material before and after electrical actuation (the last two columns show the peaks for Cellulose I and II for comparison (Klemm et al. 2005))...

In the presence of an electric field, the sodium ions surrounded with free water molecules can move to the cathode. Selective ionic and water transport across the polymer under an electric field results in volumetric changes, which in turn lead to bending. When a DC electric field was applied, the cellulose EAPap actuator was bent to the positive electrode, which confirmed the above explanation. The ambient humidity effect on the EAPap actuator performance is further evidence, where ion transport is facilitated when humidity intake is higher. Thus, the actuation principle of cellulose EAPap might be a combination of piezoelectric and ionic migration effects associated with the dipole moment of cellulose material. 

The cellulose film used for EAPap is anisotropic, decreasing its strength from the mechanical direction to the transverse direction. 

Table 18-4. Representative applications for microwave-driven EAPap...

Application Potential advantages of EAPap Current EAP materials and disadvantages... 

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