Effect of graft density and molecular weight on mechanical properties of rubbery block copolymer grafted SiO2 nanoparticle toughened epoxy

15 nm diameter SiO2 nanoparticles with a grafted block copolymer consisting of a 5–20 nm rubbery polyhexylmethacrylate (PHMA) inner block and a 30 nm outer block of matrix compatible polyglycidylmethacrylate (PGMA) were synthesized to toughen an epoxy. A systematic study of the effect of block copolymer graft density (from 0.07 to 0.7 chains/nm2) and block molecular weight (from 20 to 80 kg/mol) on the tensile behavior, fracture toughness, and fatigue properties was conducted. It was found that the copolymer grafted SiO2 nanoparticles enhanced the ductility (maximum 60% improvement), fracture toughness (maximum 300% improvement) and fatigue crack growth resistance of the epoxy matrix while maintaining the modulus at loadings of less than 2 vol% of silica core. The PHMA block induced plastic void growth and shear banding. At lower graft density and larger molecular weight of the PHMA block, the nanocomposites exhibited simultaneous improvements in fracture toughness and tensile modulus. The PGMA epoxy compatible block also contributed to the improved fracture energy of the nanocomposites.