Polarization and Piezoelectric Properties of a Nitrile Substituted Polyimide
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chemically imidized polyimide has a slightly lower Tg and thermal stability than the thermally imidized films.
APB/ODPA
(P-CN)-APB/ODPA Figure 1. Chemical structures of polyimides studied. Table 1. Thermal properties of olyimide films. (5-CN)-APB/ODPA Standard thermal cure Relative thermal cure Chemical imidization
Tý (OC)I 225 222 199
TGA, (C) 480 484 459
2
431 185 APB/ODPA ITg measured by DSC at 20'C/min. 2 Temperature of 5%weight loss by TGA in air at 2.50C/min. Poling A silver electrode is evaporated onto each side of the film for electrical contact. For this study all polymers are poled at the polymer's T at a field strength of 80 MV/m for 30 minutes unless otherwise indicated. The 80 MV/m field strength is used because it yields films which are best suited for piezoelectric measurements. Characterization Three techniques are used to characterize the dielectric and piezoelectric properties of the polymers. To evaluate the Pr in the poled samples, a thermally stimulated current (TSC) method is used. A Keithley 6517 electrometer is used to measure the discharged current as temperature is ramped at 1lC/min from room temperature to Tg +20'C. The d31 is measured by mounting the polymer sample on a load frame and applying a stress parallel to the electrodes. The charge (Q) is measured using an electrometer and d 31 is given by: d 31 = Q/AX 1
(1)
where A is the area of the electrodes and X1 is the applied stress. A linear response is obtained for each of the polymers in the range from 2 to 10 newtons of force. The g33 coefficient is given by:
g33 = Vo/X 3t, 60
(2)
where Vo is the open circuit voltage, X3 is the applied stress normal to the surface, and t is the film thickness. A ceramic piezoelectric actuator applies a 10 Hz force to the polymer film sample normal to the surface. A calibrated piezoelectric load cell located beneath the sample measures the force applied to the sample. An electrometer is used to measure the voltage developed across the sample in response to the applied force. RESULTS A summary of the remanent polarization and piezoelectric properties for the polyimides studied is presented in Table 2. Each of the materials demonstrates a piezoelectric response although lower than previously reported [6]. The P, values and room temperature piezoelectric coefficients are low relative to other known piezoelectric polymers. Table 2. Polarization and piezoelectric properties of polyimide films. (5-CN)-APB/ODPA Standard thermal cure Relative thermal cure Chemical imidization
APB/ODPA
P (mC/m 2) 9.6 10.8 11.5
3.7
-dA,(pC/N)1 0.12 0.15 0.14
0.01
-g,, (mVm/N)' 16 20 14
6
'Measured at 25'C. The influence of cure temperature on the piezoelectric properties is investigated for the nitrile substituted polyimide. Previous work [7] indicates that optimum mechanical film properties are obtained by limiting the ultimate cure temperature to Tg + 20'C. Based on the measurements completed in this study, there is no significant difference in the piezoelectric response for the different (standard or rela
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