Toughening of Infusible Epoxy Resins by Core/Shell Nanoparticles Plus a Soluble Thermoplastic Polymer and Their Synergistic Mechanism at the Mesoscopic Morphological Level

To develop high-performance epoxy resins (EP) that can be used to produce aircraft primary structure composite parts via vacuum-assisted resin infusion technology (VARI), low resin viscosity and high fracture toughness requirements must be met as well as maintaining the usual thermomechanical properties. Polymeric core/shell nanoparticles have demonstrated effectiveness in achieving these objectives, but their use at high level causes reduction of composites’ glass transition temperature and modulus. By investigating the fracture toughness of 180 °C-cured epoxy resins containing poly(2-ethylhexyl acrylate) core/poly(methyl methacrylate) shell nanoparticles (E/M), together with a poly(ether sulfone) (PES) thermoplastic polymer, the synergistic toughening effect is obtained and high fracture toughness is achieved, which is an over 101% increase in K_IC over the untoughened resin, without lowering the resin properties and still having viscosities suitable for resin infusion. Morphological studies using scanning electron microscopy (SEM) led to a mesoscopic toughening model comprising macroscale “core/shell particles” formed with thermoplastic PES domains as “cores” and the polyacrylate core/shell nanoparticles as the “shells”, resulting in much more effective functioning of common toughening mechanisms, i.e., crack deflection, bridging, and pinning, plastic deformation, and shear banding.