Engineering two nitrogen-containing polyhedral oligomeric silsesquioxanes (N-POSSs) to enhance the fire safety of epoxy resin endowed with superior thermal stability

•Two nitrogen-containing polyhedral oligomeric silsesquioxanes (N-POSSs) are synthesized via one-pot method.•N-POSSs endow with ultra-high thermal stability and are suitable for industrialized production.•Adding 4 wt% of AP-POSS reduces the peak heat release rate of epoxy resin by 60.6%.•N-POSSs apparently reduce fire hazards of the formed EP composites.•The flame retardant mechanism of N-POSSs for EP is fully illustrated.