Increased fracture toughness in nanoporous silica-polyimide matrix composites

Silica-polyimide nanocomposites were prepared by hot-pressing mixtures of p olyimide and highly porous silica powder. The silica powder was produced us ing a sol-gel process that generates pores as small as 15 Angstrom. The eff ects of loading, cure, and post-cure temperature on fracture toughness were investigated, The addition of silica particles improved the fracture tough ness from 0.5 to a maximum of 1.9 MPa m(0.5) However, fracture toughness dr opped at silica weight percentages greater than or equal to 30%. The cure a nd post-curing temperatures have a strong influence on toughness, post-curi ng exposure greater than or equal to 400 degreesC reduced toughness. Transm ission electron microscopy examination of the fracture surfaces indicated t hat the toughness improvements may occur at the nanometer scale due to crac k pinning and branching induced by the nanoporous silica particles.