Dielectric Properties of Piezoelectric Polyimides
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'National Research Council, NASA Langley Research Center, Hampton, VA 23681 2 University of Virginia, Department of Materials Science and Engineering, Charlottesville, VA 22903 3 Composites and Polymers Branch, NASA Langley Research Center, Hampton, VA 23681 ABSTRACT Molecular modeling and dielectric measurements are being used to identify mechanisms governing piezoelectric behavior in polyimides such as dipole orientation during poling, as well as degree of piezoelectricity achievable. Molecular modeling on polyimides containing pendant, polar nitrile (CN) groups has been completed to determine their remanent polarization. Experimental investigation of their dielectric properties evaluated as a function of temperature and frequency has substantiated numerical predictions. With this information in hand, we are then able to suggest changes in the molecular structures, which will then improve upon the piezoelectric response. INTRODUCTION This investigation is motivated by NASA's interest in developing high performance piezoelectric polymers for a variety of high temperature aerospace applications. Reported herein are numerical calculations and experimental results which are used to characterize and understand the poling of a piezoelectric, nitrile-substituted aromatic polyimide [1, 2]. Molecular modeling provides fundamental understanding of the polyimide's response to temperature and electric field. The experimental studies are used to evaluate the accuracy of the model. Molecular structures of the polyimides investigated are given in Figure 1 below. Dianhydride
N0O Q II..
ZP 0
APB/ODPA Tg = 1850C
Dianhydride
Nitrile
0N
0
(P3-CN)-APB/ODPA Tg = 220°C
Figure 1. Molecular structures of polyimides studied. 53 Mat. Res. Soc. Symp. Proc. Vol. 459 0 1997 Materials Research Society
METHODS In order to induce a piezoelectric response in the polyimide systems shown above, they are poled by applying a strong electric field (Ep) at an elevated temperature (T > T ) which produces orientation of the molecular dipoles. Partial retention of this orientation is achieved by lowering the temperature below Tg in the presence of Ep. This is known as orientation polarization. In order to maximize piezoelectricity, the remanent polarization (P,) must be maximized. Pr is related to the poling field by Pr = eo Ae Ep
(1)
where eo is the permittivity of space, and Ae is the change in dielectric constant upon traversing the glass transition. As pointed out by Furukawa [3], Ar is the parameter of greatest interest in designing amorphous polymers with large piezoelectric activity. Due to dielectric breakdown of the polymeric materials, Ep is limited to approximately 100 MV/m. Hence, molecular design must be implemented to increase the Ar and consequently Pr. Computational Molecular modeling of high temperature polyimides is done within the BIOSYM molecular modeling package. Initial quantum mechanical calculations using MOPAC have been done on segments of the polyimide to assign force field parameters and partial atomic charges for t
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