Molecular modeling of the poling of piezoelectric polyimides

The computational method described in this paper allows the calculation of the dielectric relaxation strength of an amorphous polymer based solely upo n its chemical structure. The 4,4' oxydiphthalic anhydride (ODPA) dianhydri de and bis-aminophenoxybenzene (APB) diamine based polyimides, (beta-CN) AP B-ODPA and APB-ODPA were studied. Amorphous cells were constructed and then poled using molecular dynamics. Dielectric relaxation strengths of Delta e psilon = 17.8 for (P-CN) APB-ODPA and Delta epsilon = 7.7 for APB-ODPA were predicted. These values are in excellent agreement with the experimental v alues. It was found that both the pendant nitrile dipole and the backbone a nhydride residue dipole make significant contributions to the polyimides' d ielectric response. Specifically, it was shown that the difference in the m agnitude of the dielectric relaxations is directly attributable to the nitr ile dipole. The size of the relaxations indicate an absence of cooperative dipolar motions. The model was used to explain these results in terms of th e average orientation of the nitrile and anhydride dipoles to within 51 deg rees and 63 degrees, respectively, of the applied electric field. (C) 1999 Elsevier Science Ltd. All rights reserved.