In order to explore effective strategies to fabricate main-chain polybenzoxazine (PBZ) thermosetting resin for structural material applications, here, five PBZ with different molecular structures, that is, poly(Co-ddm), poly(Co-BPA-ddm-Co) main , poly(Ph-BPA-ddm-Co) main , poly(Ph-BPA-ddm-Ph) main and poly(Ph-ddm) were designed, synthesized and compared by introducing unsaturated fatty chains through the Mannich reaction with cardanol. The structure and properties of five PBZ with different structure were systematically investigated by using Fourier transform infrared (FTIR), NMR, water absorption and mechanical tests. FTIR and NMR tests indicated that five PBZ with different molecular structures were successfully synthesized. The mechanical tests indicated that the impact strength of the double-capped cardanol-based main-chain PBZ was 243% higher than that of the traditional PBZ. The water absorption of main-chain PBZ was higher than that of diamine PBZ, and the water absorption of poly(Ph-BPA-ddm-Co) main was the lowest in the main-chain PBZ. Based on the experimental results, five PBZ amorphous cells with different structures were successfully established. It indicated that the main chain PBZ showed better thermomechanical properties than the traditional diamine PBZ due to high crosslinking conversion and hydrogen bond density. Moreover, the modulus and glass transition temperature of the main chain structure containing unsaturated fatty chains did not decrease significantly.

Polyimide (PI) dielectric nanocomposites containing functional nanofillers based on layered structure (single-layer: BT@Al 2 O 3 @PI, double-layer: PI/B-T@Al 2 O 3 @PI, three-layer: PI/BT@Al 2 O 3 @PI/PI) were designed and prepared by using PI as matrix, barium titanate (BT)@alumina (Al 2 O 3 ) as nanofillers through in-situ polymerization compounding technology. FTIR tests indicated that PI and PI dielectric nanocomposites have been synthesized successfully.The molecular mass of BaTiO 3 @Al 2 O 3 @PAA oligomer was higher than that of pure PAA when BT and Al 2 O 3 nanofillers were incorporated simultaneously, as verified by GPC and intrinsic viscosity tests. XRD analysis showed that the addition of nanofillers destroyed the order of PI molecular structure and reduced the arrangement density of PI molecular chains. Both FESEM and HRTEM observations showed that the nanofillers were homogeneously dispersed in the PI matrix, contributing to the property improvements of PI dielectric nanocomposites. TGA results indicated that adding nanofillers improved the thermal stability and heat resistance of PI dielectric nanocomposites. The dielectric constant of PI/BT@Al 2 O 3 @PI double-layer nanocomposites was between the single-layer nanocomposites and pure PI. Due to the effective medium theory, the dielectric constant of three-layer PI/BT@Al 2 O 3 @PI/PI nanocomposites containing 5 wt% BT@Al 2 O 3 reached 5.43. This work can be expected to provide an effective strategy to fabricate PI dielectric nanocomposite films for energy storage applications. K E Y W O R D Sbarium titanate (BT), dielectric property, layered structure, polyimide (PI), thermal property | INTRODUCTIONRecently, the applications of high dielectric and high energy storage materials have attracted extensive attention. [1,2] Especially, with the rapid development of

Herein, multi‐walled carbon nanotubes (MWCNTs) with different surfacial structure were first designed and synthesized, and then incorporated into main‐chain polybenzoxazine (PBZ) to fabricate PBZ composites. Fourier transform infrared results showed that the ring‐opening crosslinking process of benzoxazine (BZ) oligomer was catalyzed due to the existence of COOH and NH2 groups on the surface of MWCNTs. The enhanced interface interactions between MWCNTs‐fBZ and PBZ contributed to the improvement of the mechanical properties. When the MWCNTs‐fBZ content was 1.5%, the MWCNTs‐fBZ/PBZ composites had excellent tensile strength of 102.64 MPa. The highest impact strength, which was 15.96 kJ/m2 and 64.15% higher than that of neat PBZ at 1.5% MWCNTs‐fBZ was achieved. Through the Halpin‐Tsai equation, combined with the experimental results, the correction factor was obtained to predict the mechanical properties. FESEM observations showed that the dispersion of MWCNTs in the PBZ matrix depended on the functional groups of MWCNTs. TGA tests and Horowitz‐Metzger calculation found that the thermal decomposition temperature of the composites reached 451.7°C and had the highest decomposition activation energy of 153.20 kJ/mol at 2% MWCNTs‐fBZ. In summary, the obtained MWCNTs‐fBZ/PBZ composite showed excellent comprehensive performance and can potentially be used as advanced engineering material in demanding environments.