Highly heat-resistant and mechanically strong co-crosslinked polyimide/bismaleimide rigid foams with superior thermal insulation and flame resistance

The polyimide (PI) foams possess outstanding properties such as light weight, high temperature resistance, sound absorption and noise reduction, thermal insulation and flame retardant, etc. However, the currently reported PI rigid foams (PIRFs) exhibited poor mechanical properties and heat resistance due to their low crosslinking density, and could not be used for structural materials in areas such as aerospace and military ships, etc. Herein, the highly heat resistant and mechanically strong co-crosslinked polyimide/bismaleimide (PI/BMI) rigid foams were fabricated by the feasible and scalable capping precursor powder foaming method. The norbornene-end-capped polyimide oligomer (NEPIO) was melt copolymerized with 4,4′-bismaleimide diphenylmethane (BDM), incorporating the maleimide structure into the PI molecular chain and constructing the high-density PI/BMI co-crosslinking networks. Four different diamine monomers were used for the fabrication of co-crosslinked PI/BMI rigid foams. The results showed that the PI/BMI rigid foams using m-phenylenediamine (MPD) as diamine monomer exhibited the superior comprehensive performances, including outstanding heat resistance with high glass transition temperature (T_g) of 386.1 °C and thermal degradation temperature (T_{10 wt%}) of 534.9 °C, as well as excellent compression strength of 6.58 MPa at room temperature and 6.32 MPa at high temperature of 200 °C with a mass density of 290 kg/m³, which surpassed all the previously reported PI rigid foams. Moreover, the PI/BMI rigid foams showed superior thermal insulation properties with ultra-low thermal conductivities at room temperature and 200 °C, as well as outstanding flame resistance with high limited oxygen index (LOI) of above 45%. The obtained co-crosslinked PI/BMI rigid foams showed promising application potentials as structural materials in aerospace, military ships, rail transportation and other high temperature resistant special fields.