Rigid and crosslinkable polyimide curing epoxy resin with enhanced comprehensive performances

The flammability and hydrophilicity of epoxy resins hinder their applications in the high-tech fields of aerospace and electronics. To break these limitations, a highly rigid and hydrophobic polyimide structure has been designed as the harder for epoxy resins. This diamine monomer polyimide (PI–OH) featured a new spiral fluorene-xanthene core doubly grafted phenolic hydroxyl units, to match the commercial bi-trifluoromethyl dianhydride monomer. The curing performance of the polyimide PI-OH was studied in a concentration gradient fashion with the epoxy resin N, N, N′, N′-tetraglycidyl-4, 4′-diaminodiphenylmethane (TGDDM), and compared to the benchmark curer 4, 4′-diaminodiphenyl sulfone (DDS). Due to the unique structural rigidity of spiral crosslinks and the compatibility of the polyimide, the cured PI-OH/TGDDM exhibited Young's modulus of 6.18 GPa and a hardness of 480 MPa, 27.7% and 42.3% higher than that of DDS/TGDDM, demonstrating excellent comprehensive performance. The introduction of PI-OH also reduced the curing temperature with higher curing activity. Resin with 40% loading of the polyimide reached a residual carbon rate of 49.0% at 700 °C (under nitrogen), presenting flame retardancy superior to DDS/TGDDM. Furthermore, the involvement of trifluoromethyl groups reduced the surface free energy and hydrophilicity, lowering the water absorption rate for the modified epoxy resin. This study opens an avenue for comprehensively improving the performance of epoxy resins at lower curing temperatures, which has broad application prospects in aerospace and motor vehicle fields.