Preparation of aromatic polyamide with ultra-high intrinsic breakdown strength via layered stacking structure induced by coplanar monomer

Abstract

Dielectric polymers with high breakdown strength (Eb) and high retention rate of breakdown strength at elevated temperature have important application potential in advanced electrical insulation devices. Herein, the aromatic heterocyclic diamine monomer, 5-amino-2-(2-hydroxy-4-aminobenzene)-benzoxazole (HBOA), was synthesized. Theoretical calculation and single crystal date demonstrated fully the formation of intramolecular H-bond of OH⋯N]C between benzoxazole and benzene ring in HBOA, which endows the monomer a coplanar geometry. Moreover, the aromatic polyamide films were prepared by polycondensation of HBOA, and the in-plane orientation of films increases with increasing the coplanar HBOA content, which reduces the orientation confusion and cavity of chains packing. When the HBOA content is over 70%, the films exhibit dense-layered stacking structure with high crystallinity. It is found that the dense-layered stacking structure can prevent the films breakdown and failure effectively, which endows the homopolymerization (HBOA-100) film with Eb of 771 kV/mm. Moreover, the Eb of the HBOA-100 film is still as high as 634 kV/mm at 150 °C, and its retention rate of Eb reaches 82% in high temperature environment. In addition, tensile strength of the HBOA-100 film is nearly 343 MPa, glass transition temperature is about 334 °C and the thermal stability up to 487 °C. • HBOA and BOA single crystals were obtained, suggesting that the intramolecular hydrogen bond leads to coplanar geometry. • The HBOA monomer and its model compound present layered stacking pattern arising from the coplanar structure. • The dense-layered stacking structure can be constructed in the aromatic polyamide film by the coplanar monomer. • The intrinsic breakdown strength of the homopolymerization film reaches 771 kV/mm and still as high as 634 kV/mm at 150 °C. • This work provides a promising strategy of molecular structure design for high-temperature dielectric polymers.