Piezoelectric polymers polyimide

Park, C., Ounaies, Z., Wise, K.E., Harrison, J.S., 2004. In situ poUng and imidization of amorphous piezoelectric polyimides. Polymer 45,5417-5425. 

Nitrile-Substituted Polyimide. Amorphous Polsrmers that contain polar fimctional groups were synthesized (61-63) and investigated for use as high temperature piezoelectric sensors. (jS-CN) APB/ODPA polyimide is one such system. The (jS-CN) APB/ODPA polyimide possesses the three dipole functionalities shown in Table 5. Typically, the functional groups in amorphous polymers are pendent to the main chain. The dipoles, however, may also reside within the main chain of the polsrmer, such as the anhydride imits in the ( -CN) APB/ODPA polyimide. The nitrile dipole is pendent to a phenyl ring in = 4.2 D), and the two anhydride dipoles (ft = 2.34 D) are within the chain, resulting in a total dipole moment of 29.4 X 10 ° C m (8.8 D) per repeat unit. 

The piezoelectricity in amorphous polymers differs from that in semi-crystalline polymers and inorganic crystals in that the polarization is not in a state of thermal equilibrium, but rather a quasi-stable state due to the ffeezing-in of molecular dipoles. As mentioned by Broadhurst and Davis (55), four criteria are essential to make an amorphous polymer exhibit piezoelectric behavior. First, molecular dipoles must be present. As seen in Table 1, these dipoles are typically pendant to the polymer backbone as are the nitrile groups in PAN, PVDCN-VAC, and (p-CN) APB/ODPA. However, the dipoles may also reside within the main chain of the polymer such as the anhydride units in the (P-CN) APB/ODPA polyimide. In addition to a dipole moment X, the dipole concentration N (number of dipoles per unit volume) is also important in determining the ultimate polarization, P , of a polymer. 

---