Enhancing electrical energy storage using polar polyimides with nitrile groups directly attached to the main chain

Abstract

A set of 12 new polyimides (PIs) with one or three polar CN dipoles directly attached to the aromatic diamine part were synthesized and their electric energy storage properties were studied using broadband dielectric spectroscopy (BDS) and electric displacement–electric field (D–E) loop measurements to determine their potential for high temperature film capacitors for aerospace applications. It was found that adding highly polar nitrile groups to the PI structure increased permittivity and thus electrical energy storage, especially at high temperatures, and 3 CN dipoles were better than 1 CN dipole. Below the glass transition temperature (T_g), a weak γ transition was observed around −100 °C and a broad β transition was observed between 100 and 150 °C. It was the β (i.e., precursor dipolar motion before long-range segmental motion, or glass transition), rather than the γ sub-T_g transition that substantially increased the permittivity of PIs. From the BDS results on PIs having 3 nitrile groups, the enhancement in permittivity from permanent dipoles decreased with dianhydride in the order of pyromellitic dianhydride (PMDA) > 4,4′-oxydiphthalic dianhydride (OPDA) > 1,1,1,3,3,3-hexafluoropropane dianhydride (6FDA) > 4,4′-benzophenonetetracarboxylic dianhydride (BTDA). Meanwhile, the increase in permittivity also decreased in the order of para–para, meta–para, and meta–meta linkage in the diamine, suggesting that the para–para linkage favored easier dipole rotation than the meta–meta linkage. From the D–E loop study, the PIs with a combination of PMDA dianhydride and a para–para linkage exhibited the highest discharged energy density and a reasonably low loss.