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Abstract
This study demonstrated a method for toughening a highly crosslinked anhydride cured DGEBA epoxy using rubbery block copolymer grafted SiO2 nanoparticles. The particles were synthesized by a sequential reversible addition-fragmentation chain transfer (RAFT) polymerization. The inner rubbery block poly(n-hexyl methacrylate) (PHMA) had a glass transition temperature below room temperature. The outer block poly(glycidyl methacrylate) (PGMA) was matrix compatible. A rubbery interlayer thickness of 100% and 200% of the particle core radius was achieved by grafting a 20 kg/mol and a 40 kg/mol PHMA at a graft density of 0.7 chains/nm2 from the SiO2 surface. The 20 kg/mol rubbery interlayer transferred load more efficiently to the SiO2 cores than the 40 kg/mol rubbery interlayer and maintained the epoxy modulus up to a loading of 10 vol% of the rubbery interlayer. Both systems enabled cavitation or plastic dilatation. Improvement of the strain-to-break and the tensile toughness was found in both systems. We hypothesize that plastic void growth in the matrix is the primary mechanism causing the improvement of the ductility.
Highlights
Block-copolymer grafted SiO2 nanoparticles were synthesized by sequential reversible addition-fragmentation chain transfer (RAFT) polymerization from the surface of the nanoparticles [18]
poly(glycidyl methacrylate) (PGMA) outer layer were prepared by RAFT polymerization and were found to disperse well in an epoxy matrix
An improvement of the strain-to-break was found to be proportional to the volume percent of the rubbery interlayer
Summary
Rigid particles such as ceramic nanoparticles [10,11], clay [12], carbon nanotubes [13] and graphene [14], have been found to toughen epoxies by crack deflection, crack pinning, particle debonding, and plastic void growth, etc., while increasing the modulus and strength and maintaining the glass transition temperature They do not improve the matrix ductility and large volumetric loading of rigid fillers is required to improve the material toughness. The grafted copolymer consists of a thick rubbery inner block (poly(n-hexylmethacrylate), PHMA) and a matrix compatible outer block (poly(glycidylmethacrylate), PGMA) with well-controlled molecular weight and graft density This design aims at taking an advantage of a well bonded soft layer around the particle that can transfer tensile load to the silica core, maintaining the modulus and strength of the composites.
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